Patent Application
20100330420
The present invention relates to an electrode for an electrochemical device including a current collector and an active material carried by the current collector, and a method for producing the same.
Recently, along with the development of portable devices such as personal computers, mobile phones and the like, there is increasing demand for cells as power supplies of these devices. Cells for such uses are desired to have a high energy density and a superb cycle characteristic.
As a response to such a desire, nonaqueous electrolytic lithium secondary cells are now a target of attention, which use any of various types of nonaqueous electrolysis such as, for example, an organic electrolytic solution or a gel polymeric electrolysis obtained by defluidizing an organic electrolytic solution by use of a polymer or a gelating agent as an electrolysis, and use lithium ions as a charge-migrating medium. Usable as a positive electrode material of such a nonaqueous electrolytic lithium secondary cell are materials which are capable of reversibly occluding and releasing lithium ions with any of various types of electrolysis and exhibiting a high reversible potential, for example, LiCoO2, LiNiO2 and LiMn2O4. Usable as a negative electrode material are materials exhibiting a low reversible potential, for example, various carbon materials such as graphite, carbon and the like.
As the functions of the portable devices have been improving, the power supplies are now desired to have a still higher energy density. In such circumstances, it has been proposed to use, as the negative electrode material, a large capacity negative electrode material which is capable of forming an inter-metal compound with Li and providing a very large capacity by occluding and releasing lithium, in place of a carbon material such as graphite, carbon or the like. Usable as such as large capacity negative electrode material are, for example, Si (silicon), Sn (tin) and alloys containing Si or Sn as a main component. Si has a theoretic discharge capacity of about 4199 mAh/g, which is about 11 times the theoretic discharge capacity of graphite.
Such a large capacity negative electrode material (hereinafter, also referred to simply as the “negative electrode active material”) significantly expands and contracts when occluding and releasing lithium ions for charge and discharge. Therefore, where a thin film containing a negative electrode active material (hereinafter, referred to as the “active material layer”) is deposited on a current collector to form a negative electrode, the active material layer expands and contracts by the lithium ions being occluded and released, whereas the current collector does not expand or contract almost at all. As a result, the negative electrode including the current collector is distorted because of generation of a large stress. This may undesirably wrinkle or rupture the current collector or delaminate the active material layer from the current collector. When the negative electrode is distorted or wrinkled, a gap is made between the negative electrode and the separator to cause a non-uniform charge/discharge reaction, which may undesirably cause a local decline of cell characteristics. In addition, when the active material layer is delaminated from the current collector, the electron conductivity of the negative electrode may be decreased and as a result a sufficiently high charge/discharge cycle characteristic may possibly not be provided.
In order to solve these problems caused by the expansion and contraction of the negative electrode active material, it has been proposed to locate a plurality of pillar-like bodies containing a negative electrode active material (hereinafter, referred to as the “active material body”) on the current collector and thus provide a space between each two adjacent active material bodies, so as to alleviate the expansion stress of the negative electrode active material.
For example, Patent Document No. 1 proposes the following. On a surface of the current collector, a negative electrode active material is vapor-deposited in a direction inclined with respect to the normal direction to the current collector (oblique vapor deposition). Thus, using the shadowing effect described later, a plurality of active material bodies inclined with respect to the normal direction to the current collector are grown at certain intervals.
Patent Document No. 2 proposes the following. A regular ruggedness pattern is formed in a surface of the current collector. On each of the bumps, an active material body is grown by oblique vapor deposition. With this structure, the size of the space between the active material bodies can be controlled by the size or the pitch of the ruggedness pattern. Therefore, the expansion stress of the active material bodies can be effectively decreased. Patent Document No. 2 also discloses a method and an apparatus for moving a sheet-like current collector while continuously forming active material bodies on a surface thereof by oblique vapor deposition.
In order to alleviate the expansion stress of the active material applied on the current collector more effectively, it has been proposed to perform oblique vapor deposition a plurality of times while switching the vapor deposition direction, so as to grow active material bodies in a zigzag manner (for example, Patent Documents Nos. 2 through 4). The zigzag active material bodies are formed, for example, as follows.
First, on a current collector having projections (bumps) on a surface thereof, vapor deposition is performed in a first direction inclined with respect to the normal direction to the current collector to form a first layer on each of the bumps. Next, vapor deposition is performed in a second direction inclined, with respect to the normal direction to the current collector, oppositely to the first direction to form a second layer on the first layer. Then, vapor deposition is performed in the first direction to form a third layer. Vapor deposition is repeated while switching the vapor deposition direction until an arbitrary number of layers are formed. Thus, the active material bodies are obtained.
For forming such active material bodies, a vapor deposition apparatus as described below is used. In a vacuum chamber of the vapor deposition apparatus, a vaporization source and a fixing table for fixing the current collector are located. The fixing table is located such that a surface thereof is inclined with respect to a plane parallel to a vaporization surface of the vaporization source (top surface of the vapor deposition material). Owing to this, the vapor deposition material can be incident on the surface of the current collector in a direction inclined, at an arbitrary angle, with respect to the normal direction to the current collector. By switching the inclination direction of the fixing table, the direction of incidence of the vapor deposition material (vapor deposition direction) can be switched. Accordingly, by repeating vapor deposition a plurality of times while the inclination direction of the fixing table is switched, the zigzag active material bodies as described above can be provided.
Patent Document No. 4 proposes, in order to grow the active material bodies in a zigzag manner, switching the direction of incidence of the vapor deposition material by changing the position of the vaporization source or by using a plurality of vaporization sources alternately.
Patent Document No. 1: Japanese Laid-Open Patent Publication No. 2005-196970
Patent Document No. 2: International Publication WO2007/094311 pamphlet
Patent Document No. 3: International. Publication WO2007/086411 pamphlet
Patent Document No. 4: International Publication WO2007/052803 pamphlet
According to the structures of Patent Documents Nos. 1 through 4, the active material bodies can be located on bumps formed on the surface of the current collector. Therefore, a space in which the active material bodies are allowed to expand can be obtained more certainly.
However, as a result of the study performed by the present inventors, the following was found. For example, when the active material bodies are continuously formed using the apparatus disclosed in Patent Document No. 2, there may be cases where the vapor deposition particles enter the space between the active material bodies and so the active material is grown also on the dents of the current collector (areas between each two adjacent bumps, i.e., grooves) when the vapor deposition particles for oblique vapor deposition are incident at a certain angle or the bumps have a certain size. The reason for this will be described later. When the amount of the active material grown on the dents increases and a thick active material film is formed, the current collector may undesirably be wrinkled or ruptured easily by the expansion stress of the active material film.
When the active material bodies are grown on the bumps, there are problems that the size of contact area between the active material bodies and the current collector decreases and so the adhering strength between the current collector and the active material bodies decreases. Therefore, although the electrode is suppressed from being wrinkled or ruptured by the expansion stress of the active material at the time of charge/discharge, the active material bodies may undesirably be delaminated from the current collector and so the charge/discharge cycle characteristic may be deteriorated.
In addition, when the active material bodies are formed by any of the methods described in Patent Documents Nos. 1 through 4, there may be cases where the width of the active material bodies increases and so it becomes difficult to certainly obtain a sufficiently large space, between each two adjacent active material bodies, in which the active material bodies are allowed to expand. Herein, the “width of the active material bodies” means the maximum possible width of each active material body in a plane parallel to the surface of the current collector. The “plane parallel to the surface of the current collector” means a plane parallel to a virtual plane obtained by averaging the bumps and dents on the surface of the current collector.
For forming the above-described zigzag active material bodies, any of the vapor deposition apparatuses disclosed in Patent Documents Nos. 2 through 4 is usable. In these vapor deposition apparatuses, vapor deposition is performed on a current collector which has been cut into a size corresponding to the size of the fixing table. Therefore, the productivity of these apparatuses is low and it is difficult to use the apparatuses for mass production processes.
The present invention made in light of the above-described situation has an object of producing an electrode, for an electrochemical device, which has a large capacity and a superb charge/discharge cycle characteristic by a highly productive method.
A method for producing an electrode for an electrochemical device according to the present invention includes the steps of preparing a sheet-like current collector having a plurality of bumps on a surface thereof; and forming an active material body having a stacked structure on each of the bumps of the current collector. The step of forming the active material body includes a first layer vapor deposition step of causing a vaporized vapor deposition material to be incident on the surface of the current collector in a direction inclined with respect to the normal to the current collector to form a first layer of the active material body on each bump, the first layer being located closest to the current collector; and a second layer vapor deposition step of causing the vaporized vapor deposition material to be incident on the surface of the current collector in a direction inclined, with respect to the normal to the current collector, opposite to the incidence direction of the vapor deposition material in the first layer vapor deposition step to form a second layer on at least a part of the first layer. In the first layer vapor deposition step, vapor deposition is performed while moving the current collector in a direction in which the incidence angle of the vapor deposition material with respect to the normal to the current collector is decreased.
In a preferable embodiment, in the second layer vapor deposition step, vapor deposition is performed while moving the current collector in a direction in which the incidence angle of the vapor deposition material with respect to the normal to the current collector is increased.
According to the present invention, each active material body is formed on a bump of the current collector. Therefore, a space for alleviating the expansion stress of the active material body can be formed more certainly between the active material bodies, and so the electrode can be suppressed from being deformed or wrinkled due to the expansion stress of the active material body.
Specifically, in the first layer vapor deposition step, while the amount of the active material deposited on the dents of the current collector (the parts in which the bumps are not formed) is suppressed to be low, the active material can be deposited selectively on a side surface and a top surface of each bump of the current collector. Next, in the second layer vapor deposition step, vapor deposition is performed in a direction inclined, with respect to the normal to the current collector, oppositely to the incidence direction of the vapor deposition material in the first layer vapor deposition step. Therefore, the active material can be deposited on a side surface of the bump which is not covered with the first layer. In this manner, the first and second layers can be formed so as to cover the entire surface of the bump, which can increase the adhering strength between the active material bodies including the first and second layers and the bump of the current collector. Therefore, the deterioration of the charge/discharge characteristic which would otherwise be caused by the delamination of the active material can be suppressed. In addition, the amount of the active material deposited on the dents of the current collector can be decreased, and so the expansion stress applied on the current collector can be alleviated.
In the second layer vapor deposition step, it is preferable that vapor deposition is performed while the current collector is moved in a direction in which the incidence angle of the vapor deposition material with respect to the normal to the current collector is increased. Owing to this, the active material can be deposited in a sufficient thickness on the side surface of the bumps while the deposition of the active material on the dents of the current collector is suppressed. Therefore, the adherence between the active material bodies and the side surface of the bumps, as well as the top surface of the bumps, can be effectively improved. In addition, the first layer and the second layer can be continuously formed in the chamber, which improves the productivity.
Accordingly, an electrode, for an electrochemical device, which provides a large capacity and is highly reliable can be produced by a simple and highly productive process.
According to the present invention, a space for alleviating the expansion stress of the active material layer is certainly provided between active material bodies, while the contact area size is increased, and so the adherence can be improved, between the active material bodies and the surface of the bumps of the current collector. In addition, the amount of the active material deposited on the dents of the current collector can be decreased.
Accordingly, the electrode can be suppressed from being distorted, wrinkled or ruptured due to the repetition of charge/discharge, and also the active material bodies can be suppressed from being delaminated from the current collector due to the repetition of charge/discharge. Thus, an electrode for an electrochemical device having highly reliable charge/discharge cycle characteristic and other characteristics can be provided using an active material largely expandable and contractable.
Also according to the present invention, an electrode as described above can be produced by a simple and highly productive method.
a) through 1(e) are cross-sectional views showing steps of a method for producing an electrode in Embodiment 1 according to the present invention.
a) through 2(d) are cross-sectional views showing steps of the method for producing the electrode in Embodiment 1 according to the present invention.
a) through 3(d) are cross-sectional views showing steps of the method for producing the electrode in Embodiment 1 according to the present invention.
a) through 4(c) are cross-sectional views showing steps of the method for producing the electrode in Embodiment 1 according to the present invention.
a) is a schematic cross-sectional view of a vapor deposition apparatus usable for the method in Embodiment 1 according to the present invention; and
a) is a schematic cross-sectional view of another vapor deposition apparatus usable for the method in Embodiment 1 according to the present invention; and
a) and 8(b) are cross-sectional views showing another structure of an active material body formed by the method for producing the electrode in Embodiment 1 according to the present invention.
a) and 10(b) are respectively a cross-sectional view and a top view of a current collector used in an example and a comparative example.
a) is a cross-sectional view of active material bodies in Example 1, and 11(b) is a top view of an active material layer in Example 1.
a) is a schematic cross-sectional view of a vapor deposition apparatus usable for the method in Reference Embodiment according to the present invention; and
a) is a cross-sectional view of active material bodies in Comparative Example 1, and 13(b) is a top view of an active material layer in Comparative Example 1.
a) through 14(c) are cross-sectional views showing steps of a method for producing an electrode in Reference Embodiment according to the present invention.
a) and 15(b) are cross-sectional views showing steps of the method for producing the electrode in Reference Embodiment according to the present invention.
a) through 21(c) are schematic cross-sectional views showing steps of a first layer vapor deposition step in an embodiment according to the present invention.
The present invention relates to a method for producing an electrode for an electrochemical device including a current collector having a plurality of bumps having a height of 3 μm or grater and 10 μm or less on a surface thereof and also including an active material body which is formed on each of the bumps and has a stacked structure. According to the method, a first layer of each active material body which is located closest to the current collector is formed by performing oblique vapor deposition while moving the current collector in a direction in which the angle made by the normal to the current collector and a direction in which a vapor deposition material is incident on the surface of the current collector is decreased (hereinafter, the angle will be referred to as the “incidence angle of the vapor deposition material”; and the direction in which the vapor deposition material is incident will be referred to as the “incidence direction”). Also according to the method, a second layer is formed on at least a part of the first layer by causing the vapor deposition material to be incident in a direction inclined, with respect to the normal to the current collector, oppositely to the incidence direction for the first layer.
First, with reference to the drawings, advantages of the method will be described.
a) through 21(c) are schematic enlarged cross-sectional views illustrating an example of a vapor deposition step for forming a first layer of an active material body in a preferable embodiment according to the present invention. The vapor deposition step proceeds in the order of
According to this embodiment, during an initial period of the vapor deposition step, as shown in
Next, vapor deposition is performed while the current collector is moved in a direction in which the incidence angle ω is decreased (ωc<ωb<ωa). As a result, the active material 101 is deposited so as to cover the surface of the bump 4A, and thus the first layer 101a of the active material body is obtained.
During the formation of the first layer 101a of the active material body, the incidence angle ω of the vapor deposition material is changed as described above. Owing to this, the vapor deposition material can be selectively deposited so as to cover the surface of the bump 4A (the top surface and the vapor deposition material incidence-side side surface 4AS1). Therefore, as compared with the case where the first layer is formed with a fixed incidence angle or the first layer is formed while the current collector is moved in a direction in which the incidence angle ω is increased, the contact area size between the surface of the current collector 4 and the first layer 101a of each active material body can be increased and so the adherence thereof can be more improved.
By contrast, although not shown, in the case where the first layer is formed while the current collector is moved in a direction in which the incidence angle ω is increased, the incidence angle ω is small during the initial period of the vapor deposition step, and so the shadowing effect is not sufficiently provided. As a result, the vapor deposition material is likely to be deposited on the dents of the current collector as well as on the bumps. By contrast, according to the method of this embodiment, as described above with reference to
In this embodiment, the growth direction of the first layer 101a changes from a direction in which the inclination angle with respect to the normal H to the current collector is larger to a direction in which the inclination angle is smaller in accordance with the change of the incidence angle ω Therefore, even when the vapor deposition time is set to be long, the first layer 101a extends upward (a direction in which the active material body rises). For this reason, as compared with the case where the first layer is formed at a fixed incidence angle or the first layer is formed while the current collector is moved in a direction in which the incidence angle ω is increased, a width w of the first layer 101a in a plane parallel to the surface of the current collector 4 can be suppressed to be small, which allows a sufficiently large space to be made between adjacent first layers 101a more easily.
Since the first layer 101a is directed upward, the vapor deposition material for a second layer can be easily deposited on a part of the surface of the bump 4A which is not in contact with the first layer 101a (especially, a side surface 4As2 of the bump 4A opposed to the side surface 4As1). Owing to this, the contact area size can be increased, and so the adherence can be more improved, between the active material body and the surface of the bump 4A. It is preferable that the incidence direction of the vapor deposition material for forming the second layer is inclined, with respect to the normal H to the current collector, oppositely to the incidence direction thereof for forming the first layer 101a. Owing to this, the vapor deposition material can be deposited on the side surface 4As2 of the bump 4A more certainly. The first layer and the second layer are formed so as to cover the surface of the bump, and then the active material is deposited in further layers when necessary to form an active material body. The obtained active material body has the first and second layers as an underlying layer, and so has a stronger adherence to the bump 4 of the current collector.
For forming the second layer, the incidence angle ω of the vapor deposition material may be kept the same or changed to be increased or decreased. Preferably, the second layer is formed while the current collector 4 is moved in a direction in which the incidence angle ω is increased. This allows a plurality of layers to be formed continuously on the current collector 4, which is sheet-like, in a chamber of the vapor deposition apparatus. The above manner of forming the second layer also has the following advantages.
Before the second layer is formed, the first layer is already formed on the bump of the current collector. Therefore, the shadowing effect during the formation of the second layer is larger than that in the vapor deposition step for forming the first layer. For this reason, when the second layer is formed while the incidence angle is decreased, the active material may not be deposited, undesirably, on a part of the side surface of the bump. This makes it difficult to cover the entirety of the surface of the bump with the active material, which may weaken the adherence. By contrast, when the second layer is formed while the incidence angle is increased, the active material can be deposited in a sufficient thickness on the side surface of the bump. Therefore, the adherence between the active material body and the side surface of the bump, as well as the top surface of the bump, can be effectively improved.
Regarding the first layer vapor deposition step, there is a problem that when vapor deposition is performed while the current collector is moved in a direction in which the incidence angle is increased, the amount of the active material deposited on the dent increases. By contrast, regarding the second layer vapor deposition step, the first layer is already formed on the bump of the current collector and so the shadowing effect is increased. Therefore, even though vapor deposition is performed while the current collector is moved in a direction in which the incidence angle is increased, the active material is unlikely to be deposited on the dent of the current collector. Accordingly, when the first layer is formed by vapor deposition while the current collector is moved in a direction in which the incidence angle is decreased and the second layer is formed by vapor deposition while the current collector is moved in a direction in which the incidence angle is decreased, the first and second layers can be continuously formed while the active material is suppressed from being deposited on the dent in both of the vapor deposition steps.
For forming the second and subsequent later layers, vapor deposition may be performed in a direction inclined, with respect to the normal to the current collector 4, in the same direction as the first layer 101a to form an active material body having a growth direction inclined in one direction. Alternatively, vapor deposition may be performed in oppositely inclined directions alternately to form an active material body having a growth direction extending in a zigzag manner in a direction away from the surface of the current collector.
Hereinafter, embodiments according to the present invention will be described more specifically.
A method for producing an electrode for an electrochemical device (hereinafter, referred to simply as the “electrode”) in Embodiment 1 according to the present invention will be described. The electrode for an electrochemical device in this embodiment is a negative electrode for a lithium secondary cell using a silicon oxide as the active material.
<Structure of the Vapor Deposition Apparatus>
First, a vacuum vapor deposition apparatus used in this embodiment will be described. As shown in
The vaporization source 9 includes a container 10 such as, for example, a crucible for accommodating the vapor deposition material and a heating device 32 for vaporizing the vapor deposition material. The vapor deposition material and the container 10 are structured to be detachable when necessary. As the heating device 32, for example, a resistance heating device, an induction heating device, an electron beam heating device or the like is usable. For performing vapor deposition, the vapor deposition material accommodated in the crucible 10 is heated by the heating device 32 to be vaporized from a top surface (vaporization surface) 9S of the crucible 10 and supplied to the surface of the current collector 4.
A part of a transportation path for transporting the current collector 4 which is between the cooling supports 6a and 6b, and a part thereof between the cooling supports 6b and 6c, respectively act as a first vapor deposition zone 35 and a second vapor deposition zone 36 in which vapor deposition is performed on the current collector 4. Similarly, a part between the cooling supports 7a and 7b and a part between the cooling supports 7b and 7c respectively act as a third vapor deposition zone 37 and a fourth vapor deposition zone 38 in which vapor deposition is performed on the current collector 4. In the vacuum vapor deposition apparatus 50, vapor deposition is performed on one surface of the current collector 4 in the first and second vapor deposition zones 35 and 36, and on the other surface of the current collector 4 in the third and fourth vapor deposition zones 37 and 38. Both ends of the vapor deposition zones 35, 36, 37 and 38 are defined by masks 33a through 33d and 34a through 34d.
In the vacuum vapor deposition apparatus 50, the current collector 4 is fed out from one of the first and second rollers 3 and 8, guided along the transportation path by the transportation rollers 5a through 5c and the cooling supports 6a through 6c and 7a through 7c, and taken in by the other of the first and second rollers 3 and 8. The current collector 4 which has been taken in is further fed out from the roller, which has taken in the current collector 4, when necessary and transported along the transportation path in the opposite direction to the above. In this manner, in this embodiment, the first and second rollers 3 and 8 may act either as a feeding roller or a take-in roller depending on the transportation direction. By repeating the transportation in a forward direction and a reverse direction, the vapor deposition step can be continuously performed a desired number of times.
<Method for Forming the Electrode>
First, as shown in
Next, the active material bodies are formed on the current collector 4 using the vacuum vapor deposition apparatus 50 described above. The active material bodies are formed by causing the current collector 4 to run in the vapor deposition zones 35, 36, 37 and 38 in the chamber 1 and causing the vaporized vapor deposition material to be incident on the surface of the running current collector 4. In this embodiment, silicon, for example, is used as the vapor deposition material, and oxygen gas is supplied from the nozzles 31a through 31d to the corresponding vapor deposition zones 35, 36, 37 and 38 during the vapor deposition. This causes silicon to be reacted with oxygen gas, and the active material bodies formed of a silicon oxide are formed on the bumps 4A and 4B of the current collector 4.
Hereinafter, the step for forming the active material bodies will be described in more detail with an example of the active material bodies each formed of four layers.
First, the current collector 4 is set as being wound around the first roller 3. Then, the current collector 4 is fed out from the first roller 3 and transported to the vapor deposition zone 35. In the vapor deposition zone 35, while the current collector 4 is moved in a direction approaching the vaporization source 9 (vaporization surface 9S), the vaporized vapor deposition material (for example, silicon) is caused to be incident on the surface of the current collector 4 (first layer vapor deposition step). Thus, as shown in
The first layer vapor deposition step will be described in more detail. When the current collector 4 passes in the vicinity of a top end of the vapor deposition zone 35, as shown in
As the current collector 4 is moved from the vicinity of the top end toward a bottom end of the vapor deposition zone 35, the incidence angle ω of the vapor deposition material decreases (see
When the current collector 4 passes in the vicinity of the bottom end of the vapor deposition zone 35, as shown in
Then, the current collector 4 is transported to the vapor deposition zone 36. In the vapor deposition zone 36, while the current collector 4 is moved in a direction away from the vaporization source 9 (vapor deposition surface 9S), the vaporized vapor deposition material is caused to be incident on the surface of the current collector 4 (second vapor deposition step). Thus, as shown in
When the current collector 4 passes in the vicinity of a bottom end of the vapor deposition zone 36, as shown in
As the current collector 4 is moved from the vicinity of the bottom end toward a top end of the vapor deposition zone 36, the incidence angle ω of the vapor deposition material increases (see
Next, the current collector 4 is transported such that the vapor deposition material is incident on the opposite surface thereof, and the current collector 4 is transported to the vapor deposition zones 37 and 38 in this order.
In the vapor deposition zone 37, while the current collector 4 is moved in a direction approaching the vaporization source 9, the vapor deposition material is caused to be incident on the surface of the current collector 4 opposite to the surface on which the first layers 101a and the second layers 102a are formed (hereinafter, referred to simply as the “bottom surface”) (first layer vapor deposition step). This step is substantially the same as the first layer vapor deposition step for forming the first layers 101a. At a top end of the vapor deposition zone 37, as shown in
In the vapor deposition zone 38, while the current collector 4 is moved in a direction away from the vaporization source 9, the vapor deposition material is caused to be incident on the bottom surface of the current collector 4 (second vapor deposition step). This step is substantially the same as the second vapor deposition step for forming the second layers 102a. At a bottom end of the vapor deposition zone 38, as shown in
In this manner, on both surfaces of the current collector 4, the first and second layers 101a, 101b, 102a and 102b are formed. Then, the current collector 4 is once taken in by the second roller 8.
Next, the current collector 4 is fed out from the second roller 8 and transported in a direction opposite to the direction in the first and second vapor deposition steps described above with reference to
In the vapor deposition zone 38, while the current collector 4 is moved in a direction approaching the vaporization source 9, the vapor deposition material is caused to be incident on the bottom surface of the current collector 4. Thus, as shown in
In the vapor deposition zone 37, while the current collector 4 is moved in a direction away from the vaporization source 9, the vapor deposition material is caused to be incident on the bottom surface of the current collector 4. As shown in
After the fourth layers 104b are formed, the current collector 4 is transported such that the vapor deposition material is incident on the opposite surface thereof, and the current collector 4 is transported to the vapor deposition zone 36. In the vapor deposition zone 36, while the current collector 4 is moved in a direction approaching the vaporization source 9, the vapor deposition material is caused to be incident on the top surface of the current collector 4. As shown in
In the vapor deposition zone 35, while the current collector 4 is moved in a direction away from the vaporization source 9, the vapor deposition material is caused to be incident on the top surface of the current collector 4. As shown in
In this manner, an electrode 300 in which the active material bodies 110 and 120 each formed of four layers are located at certain intervals on both surfaces of the current collector 4 is produced.
With the above-described method, the active material bodies 110 and 120 each formed of four layers are formed. Alternatively, active material bodies each formed of five or more layers may be formed by transporting the current collector 4 in the forward direction and the reverse direction a plurality of times alternately. Still alternatively, active material bodies each formed of two layers may be formed by transporting the current collector 4 once in the forward direction. In this specification, the “layer” of the active material bodies 110 and 120 is a portion formed by causing the vapor deposition material to be incident in a direction inclined in one direction with respect to the normal H to the current collector 4 while the current collector 4 passes one vapor deposition zone.
The vapor deposition apparatus usable in this embodiment is not limited to the vapor deposition apparatus 50 shown in
a) is a schematic cross-sectional view showing a structure of another vapor deposition apparatus usable in the method of this embodiment.
A vapor deposition apparatus 51 includes four vapor deposition zones 45, 46, 47 and 48 between the first and second rollers 3 and 8 on the transportation path. In all of these vapor deposition zones, vapor deposition is performed on one surface of the current collector 4. Accordingly, after the current collector 4 is fed out from the first roller 3 until the current collector 4 is taken in by the second roller 8, vapor deposition step (i) of performing vapor deposition on the surface of the current collector 4 while the incidence angle ω is decreased, and vapor deposition step (ii) of performing vapor deposition on the surface of the current collector 4 while the incidence angle ω is increased, are repeated twice alternately. Thus, active material bodies each formed of four layers can be formed. Then, the current collector 4 may be fed out from the second roller 8 in the opposite direction to form active material bodies formed of a greater number of layers. With a structure for causing the steps (i) and (ii) to be repeated twice or a greater number of times while the current collector 4 is transported once in the forward direction, active material bodies formed of a plurality of stages having alternately opposite growth directions with respect to the normal H to the current collector 4 can be formed. The vapor deposition zones 45 through 48 are structured such that thicknesses tp1 and tp4 of the active material bodies formed in the vapor deposition zones 45 and 48 are respectively ½ of thicknesses tp2 and tp3 of the active material bodies formed in the vapor deposition zones 46 and 47. In addition, for example, soon after the second vapor deposition step, the third vapor deposition step is performed. Therefore, a surface oxide film or the like is prevented from being formed on the second layers. In this manner, a big change of the composition of the silicon oxide (SiOx, 0<x<2) is suppressed at an interface of two layers which form one stage. Therefore, the active material can be suppressed from being delaminated from the interfaces by the expansion and contraction at the time of charge/discharge. Thus, the reliability can be improved.
It is preferable that the vapor deposition apparatus used in this embodiment has a vapor deposition zone including a planar transportation zone in which the current collector 4 is transported such that the vapor deposition surface of the current collector 4 is planar, such as the vapor deposition zones 35 through 38 or 45 through 48 in the vapor deposition apparatus 50 or 51. Namely, in such a vapor deposition apparatus, in the cross-sections shown in the figures, the current collector 4 is transported linearly. With such a structure, the number of vaporized and flying particles and the amount of oxygen supplied from the nozzle change in accordance with the distance between the vapor deposition zone and the vaporization surface 9S. More specifically, at a bottom end of the vapor deposition zone which has a short distance from the vaporization surface 9S, SiOx having a low ratio of oxygen x is formed. As the distance from the vaporization surface 9S increases, SiOx having a higher ratio of oxygen x is formed. Therefore, the oxygen ratio x of obtained active material bodies 201a is high on the side of the current collector and is gradually decreased along the thickness direction. As the oxygen ratio x is lower, the lithium occluding amount is larger and so the volume expansion ratio by occlusion of lithium is higher. Owing to this, on the current collector side of the active material bodies 201a, the volume expansion can be suppressed to prevent the reduction of the adherence; whereas on the top surface side of the active material bodies 201a, a certain lithium occluding amount can be obtained with certainty. This is advantageous because the charge/discharge capacity can be kept large while the active material bodies 201a can be suppressed from being delaminated.
The structure of the electrode in this embodiment is not limited to the structure of the electrode 300. For example, in this embodiment, it is sufficient that each active material layer in the electrode has a stacked structure including the first layers 101a and 101b formed by performing vapor deposition while the incidence angle w is decreased and at least one layer formed thereon. Owing to this, as described above with reference to
It is preferable that the active material bodies have a structure in which a plurality of portions having alternately opposite growth directions with respect to the normal H to the current collector 4 (referred to as the “stages”) are stacked. Hereinafter, with reference to the figures, a structure of the active material bodies will be described with an example of an active material body formed of four layers and an active material body formed of six layers.
a) is a schematic cross-sectional view showing an active material body 111 formed of four layers, which is formed by substantially the same method as described above using the vapor deposition apparatus 51.
As shown in
As shown in
There is no specific limitation on the number of portions included in the active material body. For example, it is preferable that an active material body having a height of 10 μm or greater along the normal H to the current collector 4 includes at least four portions. For example, it is preferable that an active material body having a height of 20 μm or greater along the normal H to the current collector 4 includes at least eight portions. This can suppress the width (diameter) of the active material body from increasing in a direction away from the current collector 4, and so a space between the active material bodies in which the active material bodies are allowed to expand can be obtained more certainly.
Where the thickness of each portion is large, there may be cases where a bent portion is formed at the border between adjacent portions of the active material body in correspondence with the change of the growth direction, depending on the incidence angle ω. Where the number of portions is N, the number of the bent portions is N−1. In the case of an active material body formed of four stages P1 through P4, a bent portion is formed between adjacent stages of the four stages included in the active material body. Therefore, it is preferable that an active material body having a height of, for example, 10 μm or greater along the normal H to the current collector 4 has at least three bent portions. It is preferable that an active material body having a height of, for example, 20 μm or greater along the normal H to the current collector 4 has at least seven bent portions.
In the vapor deposition apparatus 50, the first and third vapor deposition zones 35 and 37 respectively for forming the first layers 101a and 101b on the top and bottom surfaces of the current collector 4 are located while facing each other above the vaporization surface 9S. It is preferable that the first and third vapor deposition zones 35 and 37 are inclined oppositely to each other with respect to the normal direction to the vaporization surface 9S, and that the inclination angles have generally equal absolute values. With such a structure, the first layers 101a and 101b are formed on the top and bottom surfaces of the current collector 4 at the same time, and the growth direction of the first layer 101a and the growth direction of the first layer 101b are generally symmetrical to each other. For example, the first layer 101a grows in a direction inclined rightward with respect to the normal H to the current collector 4, whereas the first layer 101b grows in a direction inclined leftward with respect to the normal H to the current collector 4. Accordingly, the expansion stresses generated on the top and bottom surfaces of the current collector 4 can cancel each other, and so the current collector 4 can be suppressed from being deformed by charge/discharge.
The vapor deposition apparatus 50 includes a single vaporization source 9, and a plurality of vapor deposition zones are located in a range in which the vapor deposition material can reach from the vaporization source 9. This is advantageous in realizing a highly productive step with a simple structure of the apparatus.
Hereinafter, with reference to
<Production of the Current Collector>
First, as shown in
The obtained current collector 4 is set to the first roller 3 in the vapor deposition apparatus 50 shown in
Next, in the vapor deposition zone 35 located between the cooling supports 6a and 6b, the current collector 4 is moved in a direction approaching the vaporization source 9. At this point, an active material such as, for example, silicon (scrapped silicon; having a purity of 99.999%) or tin is heated by the heating apparatus (for example, an electron beam) 32 to be vaporized from the vaporization source 9.
At the top end of the vapor deposition zone 35, namely, in the vicinity of the mask 1a, as shown in
Then, along with the movement of the current collector 4, the silicon oxide 101 grows while the incidence angle ω of the vaporized particles is decreased from all (75°). At the bottom end of the vapor deposition zone 35, as shown in
In the vapor deposition zone 35, the number of vaporized and flying particles and the amount of oxygen supplied from the nozzle 31a change in accordance with the distance from the vaporization surface 9S. Accordingly, in a part of the vapor deposition zone 35 which has a short distance from the vaporization surface 9S (in the vicinity of the bottom end), a silicon oxide (SiOx) having a low oxygen ratio x is formed. As the distance from the vaporization surface 9S increases, a silicon oxide (SiOx) having a higher oxygen ratio x is formed. As a result, the oxygen ratio x of the obtained first layers 101a is high on the side of the current collector and is decreased along the thickness direction.
At the same time as the step of forming the first layers 101a is performed, the following is performed in the vapor deposition zone 37 located to be generally symmetrical to the vapor deposition zone 35 with respect to the normal to the vaporization surface 9S. The current collector 4 is moved in a direction approaching the vaporization source 9, and thus the first layers (thickness: for example, 0.2 μm) 101b are formed on the bumps 4B formed on the bottom surface of the current collector 4 as shown in
In the initial period of the first layer vapor deposition step of forming the first layers 101a and 101b, the incidence angles ω11 and ω21 of the vapor deposition material are each preferably 60 degrees or greater and 75 degrees or less, and more preferably 70 degrees or greater and 75 degrees or less. When each of the incidence angles ω11 and ω21 exceeds 75 degrees, the inclination from the normal H to the current collector 4 is too large and so an underlying layer for covering the side surfaces of the bumps 4A and 4B is unlikely to be formed by the shadowing effect. As a result, the contact area size between the current collector 4 and the first layers 101a and 101b is small, and so the adherence therebetween is reduced. When each of the incidence angles ω11 and ω21 is smaller than 60 degrees, the silicon oxide 101 may undesirably be grown also on the dents between the bumps 4A and 4B. For this reason, a sufficiently large space cannot be obtained between the active material bodies, and so the current collector may undesirably be wrinkled by the expansion at the time of charge. By contrast, in the final period of the first layer vapor deposition step, the incidence angles ω12 and ω22 of the vapor deposition material are each preferably 45 degrees or greater and 70 degrees or less, and more preferably 60 degrees or greater and 70 degrees or less. When each of the incidence angles ω12 and ω22 is smaller than 45 degrees, the active material bodies grow to become thicker along the thickness direction (as being away from the current collector 4), which decreases the size of the space between the bumps. When each of the incidence angles ω12 and ω22 exceeds 70 degrees, the size of the area in which vapor deposition material can be deposited is decreased to reduce the productivity of the electrode. The incidence angles ω11, ω12, ω21 and ω21 are adjusted in accordance with the positions of the masks located between the vaporization surface 9S and the vapor deposition zones 35 and 37, and the inclination angles of the vapor deposition zones 35 and 37.
Next, in the vapor deposition zone 36 located between the cooling supports 6b and 6c, the current collector 4 is moved in a direction away from the vaporization source 9.
At the bottom end of the vapor deposition zone 36, namely, in the vicinity of the mask 11c, as shown in
Then, along with the movement of the current collector 4, the silicon oxide 102 grows while the incidence angle ω of the vaporized particles is increased from ω13 (62°). At the top end of the vapor deposition zone 36, as shown in
In the vapor deposition zone 36 also, the number of vaporized and flying particles and the amount of oxygen supplied from the nozzle 31b change in accordance with the distance from the vaporization surface 9S. Namely, as the distance from the vaporization surface 9S increases, a silicon oxide (SiOx) having a higher oxygen ratio x is formed. As a result, the oxygen ratio x of the obtained second layers 102a is low on the side of the current collector and is increased along the thickness direction.
At the same time as the step of forming the second layers 102a is performed, the following is performed in the vapor deposition zone 38 located to be generally symmetrical to the vapor deposition zone 36 with respect to the normal to the vaporization surface 9S. The current collector 4 is moved in a direction away from the vaporization source 9, and thus the second layers (thickness: for example, 0.2 μm) 102b are formed on the bumps 4B formed on the bottom surface of the current collector 4 as shown in
In this manner, an underlying layer 110u formed of the first layer 101a and the second layer 102b is formed on each bump 4A on the top surface of the current collector 4. An underlying layer 120u formed of the first layer 101b and the second layer 102b is formed on each bump 4B on the bottom surface of the current collector 4. A thickness tu of each of the underlying layers 110u and 120u (the thickness along the normal H to the current collector 4) is preferably 0.1 μm or greater and 3 μm or less. When the thickness tu of each of the underlying layers 110u and 120u is 0.1 μm or greater, the adherence between the layer deposited on the underlying layer 110u, 120u and the current collector 4 can be obtained more effectively. When the thickness tu of each of the underlying layers 110u and 120u is 3 μm or less, the width of each active material body can be suppressed from increasing in the thickness direction. Therefore, the rupture or delamination of the active material, which would be otherwise caused due to the expansion at the time of charge, can be prevented.
Then, the current collector 4 provided with the underlying layers 110u and 120u is once taken in by the second roller 8.
Next, the current collector 4 is transported from the second roller 8 toward the first roller 3. In the vapor deposition zone 38 located between the cooling supports 7d and 7c, the current collector 4 fed out from the second roller 8 is moved in a direction approaching the vaporization source 9.
At the top end of the vapor deposition zone 38, namely, in the vicinity of the mask 34d, as shown in
Then, along with the movement of the current collector 4, the silicon oxide 103 grows while the incidence angle ω of the vaporized particles is decreased from ω24 (75°). At the bottom end of the vapor deposition zone 38, as shown in
As described above, in the vapor deposition zone 38, the number of vaporized and flying particles and the amount of oxygen supplied from the nozzle 31d change in accordance with the distance from the vaporization surface 9S. As the distance from the vaporization surface 9S increases, a silicon oxide (SiOx) having a higher oxygen ratio x is formed. As a result, the oxygen ratio x of the obtained third layers 103b is high on the side of the current collector and is decreased along the thickness direction.
During the above step, the following is performed in the vapor deposition zone 36. The current collector 4 is moved in a direction approaching the vaporization source 9, and thus the third layers (thickness: for example, 0.2 μm) 103a are formed also on the underlying layer 110u covering the bumps 4A on the top surface of the current collector 4 as shown in
Next, in the vapor deposition zone 37 located between the cooling supports 7b and 7a, the current collector 4 is moved in a direction away from the vaporization source 9.
At the bottom end of the vapor deposition zone 37, namely, in the vicinity of the mask 34b, as shown in
Then, along with the movement of the current collector 4, the silicon oxide 104 grows while the incidence angle ω of the vaporized particles is increased from ω22 (62°). At the top end of the vapor deposition zone 37, as shown in
In the vapor deposition zone 37 also, the number of vaporized and flying particles and the amount of oxygen supplied from the nozzle 31d change in accordance with the distance from the vaporization surface 9S. As the distance from the vaporization surface 9S increases, a silicon oxide (SiOx) having a higher oxygen ratio x is formed. As a result, the oxygen ratio x of the obtained fourth layers 104b is low on the side of the current collector and is increased along the thickness direction.
During the above step, the following is performed in the vapor deposition zone 35. The current collector 4 is moved in a direction away from the vaporization source 9, and thus the fourth layers (thickness: for example, 0.2 μm) 104a are formed on the bumps 4A formed on the top surface of the current collector 4 as shown in
In this manner, the active material bodies 110 and 120 each formed of four layers are formed as shown in
In this embodiment, there is no specific limitation on the material of the current collector 4. A metal material containing at least one element selected from copper, nickel and titanium is preferable. An alloy material containing any of these elements as a main component is also usable. It is especially preferable to use copper or a copper alloy, which is highly bendable and expandable and is not reactable with lithium. For example, a metal foil such as an electrolytic copper foil, an electrolytic copper alloy foil, an electrolytic copper foil treated with roughening, a rolled copper foil treated with roughening or the like is usable. Preferably, such a metal foil is a ruggedness foil having an arithmetic average roughness Ra of about 0.3 to 5.0 μm. The arithmetic average roughness Ra is defined by the Japanese Industrial Standards (JIS B0601-1994), and can be measured by, for example, a surface roughness meter or the like. When the current collector 4 is produced by forming bumps 4A and 4B on such a ruggedness foil, the surface roughness of the bumps 4A and 4B is increased, and so the adhering strength between the current collector 4 and the active material bodies 110 and 120 can be further increased.
The height of the bumps 4A and 4B formed on the current collector 4 is preferably 3.0 μm or greater and 10 μm or less. With such a height, a sufficiently large space can be provided between the active material bodies 110 and 120 more certainly. The bumps 4A and 4B may be formed by providing a regular ruggedness pattern on a surface of the metal foil by a resist method, a plating method or a machining method. There is no specific limitation on the thickness of a part of the current collector 4 on which the bumps 4A and 4B are not formed, but is, for example, 6 μm or greater and 50 μm or less.
Regarding the bumps 4A and 4B regularly formed on the surfaces of the current collector 4, the interval between adjacent bumps is preferably 5 μm or greater and 50 μm or less. When the interval is 50 μm or less, the active material can be selectively vapor-deposited on the surface of the bumps 4A and 4B more certainly, using the shadowing effect. When the interval is 5 μm or greater, the active material can be vapor-deposited on the entirety of the side surface of the bumps 4A and 4B as well as on the top surface thereof. Therefore, the contact area size between the active material bodies and the bumps 4A and 4B can be sufficiently large, and so the adhering strength between the bumps 4A and 4B and the active material bodies can be increased more effectively.
In order to increase the contact area size between the electrolysis and the active material as well as to alleviate the stress caused by the expansion of the active material, it is desirable that the active material layer formed of the active material bodies 110 or 120 has a predetermined porosity. The porosity P of the active material layer can be easily found from the weight of the active material layer deposited on a certain area size of the current collector 4, the thickness of the active material layer, and the density of the active material. In this embodiment, the “thickness t of the active material layer” is, as shown in
In this embodiment, the porosity P of the active material layer is preferably 10% or greater and 70% or less, and more preferably 30% or greater and 60% or less. When the porosity P is 10% or greater, the stress caused by the expansion and contraction of the pillar-like particles can be sufficiently alleviated. Therefore, the contact area size between each active material body and the electrolysis can be sufficiently large. Even an active material layer having the porosity P exceeding 70% can be preferably used as the active material of a negative electrode depending on the use of the cell. However, in such a case, the energy density of the negative electrode is reduced, which requires the active material layer to be thicker. This may undesirably wrinkle the current collector due to the expansion and contraction at the time of charge/discharge.
When the active material body contains lithium of an amount corresponding to an irreversible capacity, the thickness t of the active material layer is preferably, for example, 1 μm or greater and 100 μm or less, and more preferably 8 μm or greater and 30 μm or less. When the thickness t of the active material layer is 1 μm or greater, a certain level of energy density can be obtained certainly. Therefore, the large capacity characteristic of the active material containing silicon can be fully utilized. When the thickness t of the active material layer is 100 μm or less, the ratio of the area of the active material body to which lithium does not reach as a result of each active material body being shielded by an adjacent active material body, with respect to the entire surface of the active material body, can be suppressed to be low. The collector resistance of each active material body can also be suppressed to be low. These are advantageous for high-rate charge/discharge.
The incidence angle ω of the vapor deposition particles vaporized from the center of the vaporization surface 9S of the vaporization source 9 with respect to the surface of the current collector 4 is preferably 45 degrees or greater and 75 degrees or less. When the incidence angle ω is less than 45 degrees, the active material bodies are curved to protrude in the direction of rising. This provides the problems that it is made difficult to form a vapor deposition film (active material layer) having a space between adjacent active material bodies 110 or 129, and that the current collector 4 is wrinkled due to the expansion and contraction of the active material bodies 110 or 120 at the time of charge/discharge. When the incidence angle ω is 75 degrees or greater, the growth direction of the active material bodies 110 and 120 is significantly inclined with respect to the normal H to the current collector 4, and so the adhering strength of the active material bodies 110 and 120 to the surface of the current collector 4 is decreased. For this reason, the obtained vapor deposition film has a low adherence with the current collector 4, which provides, for example, the problem that the active material is delaminated from the surface of the current collector 4 by charge/discharge.
There is no specific limitation on a width D of the active material bodies 110 and 120. The width D may change in the thickness direction of the active material bodies 110 and 120. In order to prevent the active material bodies 110 and 120 from being ruptured or delaminated from the current collector 4 (for example, copper foil) by the expansion at the time of charge, the width D of the active material bodies 110 and 120 is preferably 100 μm or less, and more preferably 3 to 30 μm. The width D of the active material bodies 110 and 120 can be found as, for example, the width of the active material bodies 110 and 120 at the center of the height thereof provided in any number of 2 to 10 (in the case where the active material bodies 110 and 120 are generally cylindrical, the average value of the diameters of the active material bodies at the center of the height thereof). Herein, the “center of the height” is the center (t/2) of the thickness (height) t of the active material bodies 4 along the normal H to the current collector 4. The “width D” or the “diameter” is, as shown in
It is sufficient that the active material bodies 110 and 120 grow to be stacked in a direction away from the current collector 4 from the bumps 4A and 4B of the current collector 4. The shape thereof is not limited to a cylindrical shape or a polygonal column shape.
It is desirable that the measurement of the porosity P of the active material layer, the thickness t of the active material layer, and the width D of the active material bodies 110 and 120 is performed in a state where the active material contains lithium of an amount corresponding to an irreversible amount and does not contain lithium of an amount corresponding to a reversible capacity (in the state where the reversible capacity is 0), namely, in a completely discharged state. The completely discharged state corresponds to a state of a finished cell in which the volume of the active material layer of the negative electrode is minimum.
The porosity P of the active material layer, the thickness t of the active material layer, and the width D of the active material bodies may be measured in a state where the active material does not contain lithium of an amount corresponding to an irreversible capacity. In such a case, by correcting the measured value, the value in the completely discharged state can be obtained. For example, the porosity P of the active material layer which does not contain lithium at all may be measured using a mercury porosimeter. In this case, the value of the porosity P is corrected using a volume difference ΔV between the volume of the active material layer in the completely discharged state containing lithium of an amount corresponding to an irreversible capacity and the volume of the active material layer not containing lithium at all. Porosity P′ of the active material layer containing lithium of an amount corresponding to an irreversible capacity is found by P′=P−ΔV.
There is no specific limitation on the material of the active material bodies 110 and 120, and the active material bodies 110 and 120 may contain at least one active material selected from the group consisting of silicon, tin, silicon oxides, silicon nitrides, tin oxides and tin nitrides. For increasing the capacity of the electrode, it is preferable that the active material bodies 110 and 120 contain a silicon element. More preferably, the active material bodies 110 and 120 are formed of, for example, at least one selected from the group consisting of silicon, silicon alloys, compounds containing silicon and oxygen, and compounds containing silicon and nitrogen. The active material layer may be formed of a single material or a plurality of materials among these. The compounds containing silicon and nitrogen may further contain oxygen. An example of the active material formed of a plurality of materials is formed of a compound containing silicon, oxygen and nitrogen. Another example of such an active material is formed of a composite of a plurality of types of silicon oxides having different ratios of silicon and oxygen. The compound containing silicon and oxygen preferably has a composition represented by general formula: SiOx (0<x<2). More preferably, the value of “x” representing the content of the oxygen element is 0.1≦x≦1.2.
The electrode 300 in this embodiment is applicable to various shapes of lithium ion secondary cells such as a cylindrical shape, a flat shape, a coin shape, a polygonal shape and the like. A lithium ion secondary cell may be produced by a known method. Specifically, the electrode 300 is slit to have a prescribed electrode width and wound or stacked as facing a positive electrode plate with a separator being located between the electrode 300 and the positive electrode. As the separator, a polypropylene separator (produced by Celgard; thickness: 20 μm) or the like is usable. The positive electrode plate may be produced as follows. Powder of LiCoO2, LiNiO2, LiMn2O4 or the like as an active material and acetylene black (AB) are kneaded together with an organic binder such as polyvinylidene difluoride (PVDF) or the like. The resultant substance is applied, dried and rolled on a current collector of, for example, a rolled Al foil having a thickness of 15 μm. Then, the electrolytic solution is injected thereto, and thus a wound cell, a stacked cell or the like is produced.
Alternatively, the electrode in this embodiment may be applied to a coin-shaped cell as shown in
Electrodes and electrodes using the same in an example and a comparative example were produced and characteristics thereof were evaluated. This will be described below.
First, an electrode in Example 1 according to the present invention will be described.
1. Production of a Current Collector
A copper alloy foil having a thickness of 18 μm (produced by Hitachi Cable Ltd.; Zr amount: 0.02% by weight) was processed with roller press at a cable pressure of 1 ton/cm using a roller having a generally diamond-shaped bottom surface and a plurality of dents. Thus, a current collector having a plurality of bumps on a surface thereof was formed.
a) and 10(b) are respectively a schematic cross-sectional view and a schematic top view of a current collector in Example 1. As shown in the figures, each of bumps 4A was like a rectangular column (average height: 6 μm) having a diamond-shaped top surface (lengths of diagonal: 10 μm×20 μm). A pitch Px of the bumps 4A in an X direction along the shorter diagonal of the diamond shape was 30 μm. Lines of the bumps 4A along the X direction, and another lines of the bumps 4A shifted from the above lines by ½ of the pitch PX in the X direction, were alternately arranged at a pitch Py of 25 μm in a Y direction perpendicular to the X direction.
2. Formation of the Active Material Layer
An active material layer was formed on each of the two surfaces of the current collector using the vapor deposition apparatus shown in
In this example, in the chamber 1 of the vapor deposition apparatus 50, the rate of the current collector 4 running between the first roller 3 and the second roller 8 was 11 cm/min. As the vaporization source 9, the carbon crucible 10 located below the vapor deposition zones 35 through 38 and holding silicon having a purity of 99.9999% was used. For vapor deposition, an electron beam set to have an accelerating voltage of −10 kV and an emission current of 450 mA was output from an electron gun 32 to heat the vaporization material.
The nozzles 31a through 31d were connected to a gas pipe (not shown), and the gas pipe was connected to an oxygen tank via a mass flow controller. The nozzles 31a through 31d were located to be directed to the vapor deposition surface of the current collector 4 running in the vapor deposition zones 35 through 38, respectively. From the nozzles 31a through 31d, oxygen gas was supplied to the chamber 1 at a flow rate of 71 sccm. At this point, the inside of the chamber 1 was exhausted by the evacuation pump 2 down to a vacuum degree of 0.034 Pa. In this state, the vapor deposition steps for forming the first layers and the second layers of the active material bodies were performed.
First, as described above with reference to
The thickness of each of the first layers 101b and the second layers 102b was 0.2 μm or greater and 0.3 μm or less. The second layers 102a and 102b were grown in a direction inclined, with respect to the normal H to the current collector 4, oppositely to the growth direction of the first layers 101a and 101b. The growth directions (inclination directions) of the first layers 101a and 101b formed on the top and bottom surfaces of the current collector 4 were generally symmetrical to each other with respect to the normal H to the current collector 4. The growth directions (inclination directions) of the second layers 102a and 102b were generally symmetrical to each other with respect to the normal H to the current collector 4. In addition, in this example, the position and the power at which the electron beam was output from the electron gun 32 were adjusted such that the thickness of the first layers 101a and 101b and the thickness of the second layers 102a and 102b would be generally equal to each other.
Next, the flow rate of the oxygen gas was switched to 70 sccm, and the inside of the chamber 1 was exhausted by the evacuation pump 2 down to a vacuum degree of 0.033 Pa. In this state, the vapor deposition steps for forming the third layers and the fourth layers of the active material bodies were performed.
The current collector 4 taken in by the second roller 8 was transported in the opposite direction toward the first roller 3. As described above with reference to
The inclination directions (growth directions) of the third layers 103b and 103a were the same as the inclination directions (growth directions) of the second layers 102b and 102a, and each second layer and a corresponding third layer formed one pillar-like portion (second portion). The fourth layers 104b grew in a direction inclined, with respect to the normal H to the current collector 4, oppositely to the growth directions of the second and third layers 102b and 103b. The fourth layers 104a grew in a direction inclined, with respect to the normal H to the current collector 4, oppositely to the growth directions of the second and third layers 102a and 103a.
Although the steps after this are not shown, the flow rate of the oxygen gas was switched to 67 sccm, and the inside of the chamber 1 was exhausted by the evacuation pump 2 down to a vacuum degree of 0.032 Pa. In this state, the transportation direction of the current collector 4 was switched. While the current collector 4 was moved in the forward direction (the same direction as the transportation direction in the vapor deposition step for forming the first layers), vapor deposition steps for forming the fifth and sixth layers were performed. The inclination directions (growth directions) of the fifth layers were the same as the inclination directions (growth directions) of the fourth layers, and each fourth layer and a corresponding fifth layer formed one pillar-like portion (third portion). The sixth layers grew in directions inclined, with respect to the normal H to the current collector 4, oppositely to the growth directions of the fourth and fifth layers.
The flow rate of the oxygen gas was switched to 63 sccm, and the inside of the chamber 1 was exhausted by the evacuation pump 2 down to a vacuum degree of 0.030 Pa. In this state, the transportation direction of the current collector 4 was switched. While the current collector 4 was moved in the reverse direction (the opposite direction to the transportation direction in the vapor deposition step for forming the first layers), vapor deposition steps for forming seventh and eighth layers were performed. The inclination directions (growth directions) of the seventh layers were the same as the inclination directions (growth directions) of the sixth layers, and each sixth layer and a corresponding seventh layer formed one pillar-like portion (fourth portion). The eighth layers were grown in directions inclined, with respect to the normal H to the current collector 4, oppositely to the growth directions of the sixth and seventh layers.
After this, vapor deposition steps were performed while switching the transportation direction in a similar manner. The vapor deposition conditions of the steps are shown in Table 1. As shown in Table 1, vapor deposition steps for forming ninth and 10th layers were performed while the current collector 4 was moved in the forward direction with the flow rate of the oxygen gas of 56 sccm and the vacuum degree of the inside of the chamber 1 of 0.028 Pa. Vapor deposition steps for forming 11th and 12th layers were performed while the current collector 4 was moved in the reverse direction with the flow rate of the oxygen gas of 46 sccm and the vacuum degree of the inside of the chamber 1 of 0.022 Pa. Vapor deposition steps for forming 13th and 14th layers were performed while the current collector 4 was moved in the forward direction with the flow rate of the oxygen gas of 33 sccm and the vacuum degree of the inside of the chamber 1 of 0.014 Pa. Vapor deposition steps for forming 15th and 16th layers were performed while the current collector 4 was moved in the reverse direction without introducing the oxygen gas into the chamber 1.
The current collector 4 was transported 27 more times in repetition alternately in the forward and reverse directions without introducing the oxygen gas into the chamber 1 to form 17th through 70th layers. Thus, the active material bodies were obtained. The growth direction of each active material body extends in a zigzag manner in a direction away from the surface of the current collector 4. Each active material body has a structure in which 36 portions, having growth directions inclined in opposite directions alternately with respect to the normal H to the current collector 4, are stacked.
In this manner, the current collector 4 with a plurality of active material bodies formed thereon was obtained. In this example, at one end of the sheet-like current collector 4, the active material bodies were formed only on one surface thereof, and at the other end thereof, the active material bodies were formed only on the opposite surface (bottom surface) thereof. In a central area of the current collector 4 except for the two ends, the active material bodies were formed on both surfaces thereof. The reason for this is, as described below in more detail, vapor deposition performed on the top and bottom surfaces of the sheet-like current collector 4 is started and finished at positions shifted from the ends of the current collector 4.
In the vapor deposition apparatus 50, the vapor deposition zones 35 through 38 are located in a part of the transportation path between the first roller 3 and the second roller 8. Especially, there is a part in which the vapor deposition is not performed between the vapor deposition zone 36 and the vapor deposition zone 37. Therefore, when the current collector 4 starts to be transported from one of the rollers and then is taken in by the other roller, a leading part and a trailing part of the sheet-like current collector 4 do not pass the vapor deposition zones 35/36, or do not pass the vapor deposition zones 37/38. Accordingly, even when the vapor deposition steps are repeated, one end of the current collector 4 never passes the vapor deposition zones 35/36, and the other end of the current collector 4 never passes the vapor deposition zones 37/38. As a result, as described above, the active material bodies are formed only on one surface at the ends of the current collector 4, and on both surfaces in the central area thereof.
After this, a part of the current collector 4 having the active material bodies formed only on one surface (the top or bottom surface) was cut out, and thus a negative electrode 1A was produced.
3. Composition of the Active Material Bodies
The oxygen ratio (the x value in SiOx) of the active material bodies (a silicon oxide) in the negative electrode 1A in Example 1 was found by processing the active material bodies by line analysis measurement in a cross-sectional direction thereof using EPMA. Here, the line analysis measurement of the active material bodies was performed on a cross-section perpendicular to the surface of the current collector 4 and including the growth direction of the active material.
The measurement results are shown in Table 1. As shown in Table 1, the x value in the first layers 101a and 101b (first portion) was 1.2. The oxygen ratio (x value) of the active material bodies was continuously decreased from the first layers 101a and 101b (first portion) toward the 15th layers (eighth portion). The x value of the 15th and subsequent layers (eighth and subsequent portions) was 0.2. It was confirmed from the measurement results that the ratio of the oxygen element varies in the thickness direction of the active material layer. This is considered to be caused by the vapor deposition condition (flow rate of the oxygen gas) for forming each layer of the active material bodies.
The oxygen ratio (x value) of the active material layer formed of a plurality of active material bodies was measured by a combustion method. The result was 0.27.
4. Observation of the Cross-Section of the Active Material Bodies
A cross-section of the negative electrode 1A was observed at various angles by a scanning electron microscope (SEM).
As a result of the observation, the following was confirmed: each of active material bodies 80 is deposited so as to cover the bump 4A of the current collector 4; and the active material body 80 has a structure in which a portion deposited from the left in the sheet of
5. Porosity P of the Active Material Layer
The porosity P of the active material layer in the negative electrode 1A was measured using a mercury porosimeter (Autopore III9410 produced by Shimadzu Corporation).
First, a current collector having active material bodies formed on one surface thereof in substantially the same conditions as above was produced as a porosity measurement sample. The size of the porosity measurement sample was 3 cm×3 cm. The weight of the current collector (copper foil) was subtracted from the weight of the sample to find the weight of the active material layer. Next, the true volume (VT) of the active material layer was found from the density of SiO0.27. Then, mercury was caused to enter the spaces of the sample, and the volume (VH) of the mercury which entered was found by the mercury porosimeter. The porosity P, found from the true volume (VT) of the active material layer and the volume (VH) of the mercury which entered the spaces of the sample, was 50%.
6. Measurement of the Adhering Force of the Active Material Layer
The adhering strength of the active material to the current collector 4 was measured using a tacking tester (TAC-II produced by Rhesca Corporation).
First, a both-sided adhesive tape (No. 515 produced by Nitto Denko Corporation) was attached to a tip of a measuring head (having a tip diameter of 2 mm) of the tacking tester. A piece of 2 cm×3 cm of the negative electrode 1A was cut out to prepare an adhesive strength measurement sample. The both-sided adhesive tape was attached and fixed to a position of the active material layer of this sample which faced the measurement head. The measurement was performed under the conditions of a pressing rate of 30 mm/min., a pressing time of 10 seconds, a load of 400 gf, and a pulling rate of 600 mm/min. As a result, it was confirmed that the adhering strength of the active material was 30 kgf/cm2 or greater (the measurement limit or greater because delamination was not confirmed).
7. Production of a Negative Electrode for a Test Cell
The negative electrode 1A was cut into a size of 31 mm×31 mm, and Li metal of 15 μm was vapor-deposited on a surface of the negative electrode by a vacuum vapor deposition method to obtain a negative electrode for a cell. An exposed portion of the current collector (Cu foil) was provided at a position which was on the inner side of the negative electrode for the cell and did not face the positive electrode, and a Cu negative electrode lead was welded thereto.
8. Production of a Positive Electrode for a Test Cell
10 g of lithium cobalt oxide (LiCoO2) powder having an average particle diameter of 10 μm as a positive electrode active material, 0.3 g of acetylene black as a conductor, 0.8 g of polyvinylidene difluoride (PVDF) powder as a binder, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) solution (#1320 produced by Kureha Corporation) were fully mixed to prepare a positive electrode compound paste. The obtained positive electrode compound paste was applied to one surface of a positive electrode current collector (thickness: 15 μm) formed of a 20 μm-thick aluminum (Al) foil by a doctor blade method. Next, the positive electrode compound paste was rolled so as to have a thickness of 70 μm and fully dried at 85° C. to form a positive electrode active material layer. Then, the positive electrode current collector having the positive electrode active material layer formed thereon was cut into a size of 30 mm×30 mm to obtain a positive electrode for a cell. An exposed portion of the current collector (Al foil) was provided at a position which was on the inner side of the positive electrode for the cell and did not face the negative electrode, and an Al positive electrode lead was welded thereto.
9. Production of a Test Cell
A coin-shaped cell as shown in
First, a positive electrode 68 for the test cell and the negative electrode 64 for the test cell were located such that the positive electrode active material layer 65 and the negative electrode active material layer 62 would face each other. The separator 69 formed of a microporous polyethylene layer having a thickness of 20 μm produced by Asahi Kasei Corporation was located between the positive electrode active material layer 65 and the negative electrode active material layer 62. Thus, a thin electrode plate assembly was obtained. This electrode plate assembly was inserted together with an electrolysis into the external case formed of an aluminum laminate seal. Used as the electrolysis was a nonaqueous electrolytic solution obtained by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 1:1 and dissolving LiPF6 in this mixture at a concentration of 1.0 mol/L. The positive electrode active material layer 65, the negative electrode active material layer 62, and the separator 69 were impregnated with the nonaqueous electrolytic solution. Then, ends of the external case 70 were welded while vacuum pressure reduction was performed in a state where the positive electrode lead and the negative electrode lead (not shown) were drawn out externally. Thus, the test cell A was obtained.
For comparison, a negative electrode 1B in Comparative Example 1 was produced by forming active material bodies in contact with the bumps of the current collector while moving the current collector in a direction in which the incidence angle ω of the vapor deposition material was increased.
In Comparative Example 1, a current collector substantially the same as that of Example 1 was used. For forming the active material layer, a vapor deposition apparatus 52 as shown in
As the vaporization source 9, the carbon crucible 10 holding silicon having a purity of 99.9999% was used. An electron beam set to have an accelerating voltage of −10 kV and an emission current of 600 mA was output from the electron gun 32 to heat and vaporize silicon. The nozzle 31a was connected to a gas pipe (not shown), and the gas pipe was connected to an oxygen tank via a mass flow controller. The nozzles 31a and 31b were located respectively at the bottom end and the top end of the vapor deposition zone 91 to be directed to the vapor deposition surface of the current collector 4 running in the vapor deposition zone 91. From the nozzle 31a located at the bottom end of the vapor deposition zone 91, oxygen gas was supplied to the chamber 1 at a flow rate of 40 sccm. From the nozzle 31b located at the top end of the vapor deposition zone 91, oxygen gas was supplied to the chamber 1 at a flow rate of 20 sccm. At this point, the inside of the chamber 1 was exhausted by the evacuation pump 2 down to a vacuum degree of 0.02 Pa.
In the vapor deposition zone 91, silicon vaporized from the vaporization source 9 reacted with the oxygen gas supplied from the nozzle 31a to deposit a compound containing silicon and oxygen (SiOx) selectively on the bumps of the current collector 4. At this point, the power of the electron gun 2 was appropriately adjusted such that the thickness of the active material layer formed on the current collector 4 running in the vapor deposition zone 91 would be generally uniform. In this manner, the negative electrode 1B in Comparative Example 1 including an active material layer formed of a plurality of active material bodies was produced.
The oxygen ratio (x value) of the active material layer of the obtained negative electrode 1B was measured by a combustion method. The result was 0.6.
Next, a cross-section of the negative electrode 1B was observed by a scanning electron microscope (SEM).
As a result of the observation, the following was confirmed: each of active material bodies 85 has a growth direction inclined in one direction with respect to the normal H to the current collector 4; and each active material body 85 covers the incidence-side side surface of the bump 4A of the current collector 4 (on the silicon incidence side in the vapor deposition step), but does not cover the side surface opposed thereto and so the contact area size between the active material body 85 and the bump 4A of the current collector 4 is smaller than that in Example 1. As understood from
The porosity P of the negative electrode 1B, which was found using a mercury porosimeter, was 42%.
A tacking test was performed in substantially the same manner as in Example 1. The adhering strength of the negative electrode 1B was 17 kgf/cm2, and a part of the negative electrode active material was delaminated from the surface of the current collector 4 by the tacking test.
A coin-shaped cell was produced using the negative electrode 1B in Comparative Example 1 in substantially the same manner as in Example 1 to obtain a test cell B.
Evaluation of the charge/discharge characteristic was performed on the test cells A and B in the following manner.
First, the test cells A and B were each accommodated into a thermostat oven of 20° C. and charged by a constant current/constant voltage method. Here, each cell was charged at a constant current of 1 C rate (1 C is a current value at which the entirety of the cell capacity can be used up within 1 hour) until the cell voltage became 4.2 V, and then charged at a constant voltage until the current value became 0.05 C.
After the charge, the operation was paused for 20 minutes. Then, the cell was discharged at a constant current as high as 1 C rate until the cell voltage became 2.5 V. After the high-rate discharge, the cell was re-discharged at a constant current of 0.2 C until the cell voltage became 2.5 V. After the re-discharge, the operation was paused for 20 minutes.
The above-described charge/discharge was repeated by 300 cycles.
In an initial period of the cycles, the ratio of the total discharge capacity (the sum of the high-rate discharge capacity and re-discharge capacity) with respect to the charge capacity was obtained with the percentage as the charge/discharge efficiency. In the initial period of the cycles, the ratio of the high-rate discharge capacity with respect to the total discharge capacity was obtained with the percentage as the high-rate ratio. The ratio of the total discharge capacity at the 300th cycle with respect to the total discharge capacity in the initial period of the cycles was obtained with the percentage as the capacity maintenance ratio.
The measurement results are shown in Table 2.
From Table 2, it was found that the test cells A and B exhibit generally equal and high charge/discharge efficiency and high-rate ratio in the initial period of the cycles. This is considered to occur for the following reasons. One reason is that active material bodies in these test cells each have a part inclined with respect to the current collector and so the area size of a part of the surface of the active material bodies which is in contact with the electrolysis is larger than that in the conventional cells. Another reason is that parts of the negative electrode active material and the positive electrode active material which face each other are increased and so the charge/discharge reaction is made uniform, which suppresses lithium deposition reaction and excessive local charge/discharge of the positive electrode.
However, at the 300th cycle, the capacity maintenance ratio of the test cell A is about 80% whereas the capacity maintenance ratio of the test cell B drops to about 50%. This is considered to occur for the following reasons. One reason is that the active material bodies of the test cell A are structured to have a plurality of stages so as to cover the bumps of the current collector, and so the adhering force between the surface of the current collector and the active material bodies is increased to suppress the delamination, rupture and the like of the active material bodies. Another reason is that in the test cell A, a sufficiently large space is made between adjacent active material bodies, and as a result, the contact between the adjacent active material bodies at the time of charge/discharge is reduced and so the current collector can be suppressed from being wrinkled, distorted or the like.
From the above evaluation results, it was confirmed that by providing the active material bodies which have an underlying layer formed so as to cover the bumps of the current collector and are formed of a plurality of stages, the cycle characteristic of the secondary cell can be improved more effectively.
Hereinafter, a method for producing an electrode in Reference Embodiment according to the present invention will be described. In this embodiment, active material bodies inclined in one direction with respect to the normal to the current collector are formed unlike in Embodiment 1.
First, as shown in
Next, the active material bodies are formed on both surfaces of the current collector 4 using the vacuum vapor deposition apparatus 52 shown in
First, the current collector 4 is set to the first roller 3 of the vapor deposition apparatus 52. The vaporization source 10 contains silicon or the like. The vaporization source 10 is heated by a heating device (not shown) such as a resistance heating device, an induction heating device, an electron beam heating device or the like to vaporize silicon. Oxygen gas is introduced into the chamber 1 while the inside of the chamber 1 is exhausted by the evacuation pump 2. In this state, the current collector 4 is fed out from the first roller 3, caused to run on a circumferential surface of a substrate cooling support 6 rotating in a direction of the arrow, and taken in by the second roller 8. On the circumferential surface of the substrate cooling support 6, the vapor deposition zone 91 is provided, and vapor deposition is performed on the current collector 4 in the vapor deposition zone 91.
In the vapor deposition zone 91, while the current collector 4 is moved in a direction approaching the vaporization surface 9S, the vaporized particles are selectively incident on the bumps 4A on the surface of the current collector 4. Specifically, as shown in
Next, the current collector 4 is fed out from the second roller 8, caused to run on the circumferential surface of the substrate cooling support 6 rotating in the direction of the arrow, and taken in by the first roller 3. During this operation, vapor deposition is performed on the current collector 4 in the vapor deposition zone 91.
In the vapor deposition zone 91, while the current collector 4 is moved in a direction approaching the vaporization surface 9S, the vaporized particles are selectively incident on the bumps 4B on the surface of the current collector 4. Specifically, as shown in
Hereinafter, with reference to
First, as shown in
Next, the current collector 4 is set to the first roller 3 shown in
Then, as shown in
The vapor deposition zone 91 in this embodiment includes a plane transportation zone on which the current collector 4 is transported such that the vapor deposition surface of the current collector 4 is planar. Namely, in the shown cross-section, the current collector 4 is transported linearly. With such a structure, as described in Embodiment 1 in detail, the number of vaporized and flying particles and the amount of oxygen supplied from the oxygen introduction nozzles (31a, 31b) continuously change in accordance with the distance from the vaporization surface 9S. Namely, in the vicinity of the bottom end of the vapor deposition zone 91 having a short distance from the vaporization surface 9S, an SiOx having a low oxygen ratio x is formed. As the distance from the vaporization surface 9S increases, an SiOx having a higher oxygen ratio x is formed. As a result, the oxygen ratio x of the obtained active material bodies 201a is high on the side of the current collector and is gradually decreased along the thickness direction. As the oxygen ratio x is lower, the lithium occluding amount is larger and so the volume expansion ratio by occlusion of lithium is larger. Owing to this, on the current collector side of the active material bodies 201a, the volume expansion can be suppressed to prevent the reduction of the adherence; whereas on the top surface side of the active material bodies 201a, a certain lithium occluding amount can be obtained with certainty. This is advantageous because the charge/discharge capacity is kept large while the active material bodies 201a are suppressed from being delaminated.
The active material bodies 201a in this embodiment are formed on the bumps 40A and are inclined at an angle of θ1 with respect to the normal H to the current collector 4. The thickness t of the active material bodies 201a along the normal H to the current collector 4 (the thickness from the top surface of the bumps 4A) is, for example, 18 μm. Between the bumps 4A (dents) on the surface of the current collector 4, the active material 202 having a thickness T of 0.5 μm along the normal H to the current collector 4 is deposited.
The thickness t of the active material bodies 201a is preferably, for example, 3 μm or greater and 30 μm or less, and more preferably 5 μm or greater and 20 μm or less. When the thickness t of the active material bodies 201a is less than 3 μm, a sufficiently large charge capacity may not be obtained undesirably. The width of the active material bodies 201a increases as the thickness t thereof increases, namely, toward the tip of the active material bodies 201a. Therefore, when the thickness t is excessively large (for example, greater than 30 μm), a sufficiently large space cannot be obtained between the active material bodies 201a, which may undesirably cause the active material to be ruptured or delaminated by the expansion stress at the time of charge. The thickness T of the active material 202 on the dents of the current collector 4 is, for example, 0.1 μm or greater and 2 μm or less. As described above, the active material 202 does not need to be deposited (T=0). The thickness T of the active material 202 is preferably 3 μm or less, and more preferably 1 μm or less. When the thickness T of the active material 202 exceeds 3 μm, the current collector 4 may be wrinkled by the expansion stress at the time of charge, which may deteriorate the cycle characteristic.
Then, the current collector 4 having the active material bodies 201a formed thereon is once taken in by the second roller 8. In the vapor deposition apparatus 52, the current collector 4 is taken in such that the surface of the current collector 4 having the active material bodies 201a formed thereon is external to the bobbin.
Next, the inside of the chamber 1 is once made non-vacuum, and the current collector 4 taken in by the second roller 8 is re-attached to the first roller 3. At this point, the current collector 4 is attached such that the surface opposite to the surface having the active material bodies 201a formed thereon acts as the vapor deposition surface (such that the bottom surface acts as the vapor deposition surface).
Next, the current collector 4 is fed out from the first roller 3 and transported to the second roller 8 via the vapor deposition zone 91. At this point, an active material such as, for example, silicon (scrapped silicon; having a purity of 99.999%) or tin is heated by the heating apparatus 32 such as an electron beam heating device or the like to be vaporized from the vaporization source 9. The inside of the chamber 1 is an oxygen atmosphere having a pressure of 0.02 Pa. Thus, as shown in
In this manner, the electrode 400 in which the plurality of active material bodies 201a and 201b are formed on both surfaces of the current collector 4 is obtained.
With the above-described method, the active material bodies 201a and 201b formed of one layer are formed. Alternatively, the second layer may be formed in the vapor deposition zone 91 by, after the first layer is formed by vapor deposition, causing the current collector 4 once taken in by the second roller 8 to pass the vapor deposition zone 91 again to the first roller 3. Still alternatively, the active material bodies having a desired number of layers may be formed by causing the current collector 4 to run between the first roller 3 and the second roller 8 a desired number of times continuously while the transportation direction is switched. Even in such cases, the second and subsequent layers have a growth direction inclined, with respect to the normal H to the current collector 4, in the same side as the first layer. Therefore, the active material bodies shaped to be inclined in one direction with respect to the normal H to the current collector 4 are obtained.
With the above-described method, the vapor deposition apparatus 52 is used. Instead, a vapor deposition apparatus 53 shown in
Alternatively, as shown in
In this embodiment, the material, surface roughness Ra and thickness of the current collector 4 and the material of the active material bodies 201a and 201b may be substantially the same as those of Embodiment 1. The preferable range of the porosity P of the active material layers formed of the active material bodies 201a and 201b, and the method for measuring the same, may be substantially the same as those of Embodiment 1.
When the active material contains lithium of an amount corresponding to an irreversible capacity, the thickness t of the active material layers, namely, the thickness of the active material bodies 201a and 201b is preferably, for example, 3 μm or greater and 30 μm or less, and more preferably 5 μm or greater and 20 μm or less. When the thickness t of the active material layer is 3 μm or greater, a sufficient level of energy density can be obtained with certainty. Therefore, the large capacity characteristic of the active material containing silicon can be fully utilized. When the thickness t of the active material layer is 30 μm or less, the ratio of the area of the active material body to which lithium does not reach as a result of each active material body being shielded by an adjacent active material body, with respect to the entire surface of the active material body, can be suppressed to be low. The collector resistance of each active material body can also be suppressed to be low. These are advantageous for high-rate charge/discharge.
In this embodiment, the incidence angle ω in the vapor deposition zone 91 is preferably 45 degrees or greater and 75 degrees or less. When the incidence angle ω is less than 45 degrees, the active material bodies 201a and 201b grow in the direction of rising, which makes it difficult to form a sufficiently large space between the active material bodies 201a and 201b. For this reason, when the cell is charged/discharged, the current collector 4 may be undesirably wrinkled due to the expansion stress of the active material. By contrast, when the incidence angle ω exceeds 75 degrees, the inclination of the incidence angle ω16 with respect to the normal H to the current collector 4 is excessively large and so the active material is not deposited on the side surface of the bumps 4A. This may undesirably reduce the adherence between the current collector and the active material.
The initial incidence angle w16 defined by the mask 33a is preferably 60 degrees or greater and 75 degrees or less, and more preferably 70 degrees or greater and 75 degrees or less. When the initial incidence angle ω16 is less than 60 degrees, the active material 202 deposited between the bumps 4A of the current collector (dents) is excessively thick, and so the current collector 4 may undesirably be wrinkled by the expansion of the active material 202 at the time of charge. On the other hand, the final incidence angle ω17 defined by a mask 33b is preferably 45 degrees or greater. As long as the final incidence angle ω17 is smaller than the initial incidence angle w16, the effect of the present invention is provided. When the difference between these incidence angles (ω16−ω17) is 15 degrees or greater, the adherence between the active material bodies 201a and 201b and the current collector 4 can be improved more certainly.
The preferable range of the width D of the active material bodies 201a and 201b is the same as the range described in Embodiment 1. The definition of the width D and the measuring method thereof are substantially the same as those in Embodiment 1. The shape of the active material bodies 201a and 201b is not limited to a cylindrical shape, or a polygonal column shape.
The thickness t and the width D of the active material bodies 201a and 201b may be measured by substantially the same methods as those described in Embodiment 1.
In this embodiment, the active material bodies 201a and 201b are formed so as to contact the current collector 4. Alternatively, an underlying film may be formed on the current collector 4 before forming the active material bodies 201a and 201b. Still alternatively, the current collector 4 may be surface-treated. The underlying film may contain the same material as that of the active material bodies 201a and 201b. When the active material bodies 201a and 201b contain a silicon oxide (SiOx (0<x<2)), the underlying film may contain a silicon oxide having the same composition as, or a different x value from, the active material bodies 201a and 201b. In order to provide the effect of the present invention with a structure having the underlying film, a surface of the underlying film needs to have a shape reflecting the ruggedness pattern of the surface of the current collector 4. Such an underlying film may be formed by, for example, causing vapor deposition particles to be incident perpendicularly to the surface of the current collector 4. The underlying film is, for example, a continuous film.
A lithium ion secondary cell can be produced using the electrode 400 produced as described above, by substantially the same method as described in Embodiment 1. The electrode 400 is applicable to various shapes of lithium ion secondary cells such as a cylindrical shape, a flat shape, a coin shape, a polygonal shape and the like.
Electrodes and electrodes using the same in an example and a comparative example were produced and characteristics thereof were evaluated. This will be described below.
First, an electrode in Reference Example 2 according to the present invention will be described.
1. Production of a Current Collector
A current collector substantially the same as used in Example 1 was produced by substantially the same method as described above with reference to
2. Formation of the Active Material Layer
An active material layer was formed on both surfaces of the current collector using the vapor deposition apparatus 52 shown in
In this example, in the chamber 1 of the vapor deposition apparatus 52, the rate of the current collector 4 running between the first roller 3 and the second roller 8 was 1 cm/min. As the vaporization source 9, the carbon crucible 10 located below the vapor deposition zone 91 and holding silicon having a purity of 99.9999% was used. For vapor deposition, an electron beam set to have an accelerating voltage of −10 kV and an emission current of 600 mA was output from the electron gun 32 to heat the vaporization material.
The nozzles 31a and 31b were connected to a gas pipe (not shown), and the gas pipe was connected to an oxygen tank via a mass flow controller. Oxygen gas was supplied from the nozzle 31a located at the top end (on the side on which the incidence angle w was larger) of the vapor deposition zone 91 at a flow rate of 40 sccm, and from the nozzle 31b located at the bottom end (on the side on which the incidence angle ω was smaller) of the vapor deposition zone 91 at a flow rate of 20 sccm. At this point, the inside of the chamber 1 was exhausted by the evacuation pump 2 down to a vacuum degree of 0.02 Pa. In this state, the vapor deposition steps for forming the active material bodies 201a were performed.
First, as described above with reference to
Next, the inside of the chamber 1 was once made non-vacuum, and the current collector 4 taken in by the second roller 8 was re-attached to the first roller 3. At this point, the current collector 4 was attached such that the surface opposite to the surface having the active material bodies 201a formed thereon would act as the vapor deposition surface (such that the bottom surface would act as the vapor deposition surface).
Then, the active material bodies b were formed on the bottom surface of the current collector 4 in substantially the same conditions as for forming the active material bodies 201a. Specifically, oxygen gas was introduced into the chamber 1 from the oxygen nozzle 31a at a flow rate of 40 sccm and from the oxygen nozzle 31b at a flow rate of 20 sccm, and the inside of the chamber 1 was exhausted by the evacuation pump 2 down to a vacuum degree of 0.02 Pa.
Next, as described above with reference to
In the electrode 2C in Reference Example, the growth directions of the active material bodies 201a and 201b were generally symmetrical to each other with respect to the normal H to the current collector 4.
3. Composition of the Active Material Bodies
The oxygen ratio (the x value in SiOx) of the active material bodies (a silicon oxide) 201a and 201b in the negative electrode 2C in Reference Example 2 was found by processing the active material bodies by line analysis measurement in a cross-sectional direction using EPMA. Here, the line analysis measurement of the active material bodies was performed on a cross-section perpendicular to the surface of the current collector 4 and including the growth direction of the active material.
As a result, the x value in the vicinity of the interface between the active material bodies 201a and 201b and the current collector 4 was 0.8. The oxygen ratio (x value) of the active material bodies 201a and 201b was decreased in a direction away from the current collector 4, and the x value on the top surface of the active material bodies 201a and 201b was 0.4. It was confirmed that the ratio of the oxygen element varies in the thickness direction of the active material bodies 201a and 201b.
The oxygen ratio (x value) of the active material layer formed of a plurality of active material bodies was measured by a combustion method. The result was 0.6.
4. Observation of the Cross-Section of the Active Material Bodies
Next, a cross-section of the negative electrode 2C was observed at various angles by a scanning electron microscope (SEM).
As a result of the observation, it was confirmed that each of active material bodies 87 was selectively deposited on the bump 4A of the current collector 4. The thickness t of the active material bodies 87 was 18 μm. On the dents of the current collector 4, the active material was deposited in a small thickness, and the thickness T thereof was 0.5 μm. It was found that the width of the active material bodies 87 was large on the side of the current collector and small at the tip thereof. The inclination angle of the active material bodies 87 with respect to the normal H to the current collector 4 was 47 degrees.
5. Porosity P of the Active Material Layer
The porosity P of the active material layer in the negative electrode 2C was measured using a mercury porosimeter (Autopore III9410 produced by Shimadzu Corporation).
First, a current collector having an active material layer formed on one surface thereof in substantially the same conditions as above was produced as a porosity measurement sample. The size of the porosity measurement sample was 3 cm×3 cm. The weight of the current collector (copper foil) was subtracted from the weight of the sample to find the weight of the active material layer. Next, the true volume (VT) of the active material layer was found from the density of SiO0.27. Then, mercury was caused to enter the spaces of the sample, and the volume (VH) of the mercury which entered was found by the mercury porosimeter. The porosity P, found from the true volume (VT) of the active material layer and the volume (VH) of the mercury which entered the spaces of the sample, was 48%.
6. Production of a Test Cell
Using the negative electrode 2C, a negative electrode for a test cell was produced by substantially the same method as described in Example 1. A positive electrode for a test cell was produced by substantially the same method as described in Example 1. Using these, a coin-shaped cell was obtained as a test cell C.
For comparison, an electrode 2D in Comparative Example 2 was produced by forming active material bodies so as to be in contact with the bumps of the current collector while moving the current collector in a direction in which the incidence angle ω of the vapor deposition material was increased. A method for producing this will be described.
In Comparative Example 2, a current collector substantially the same as that in Reference Example 2 and the vapor deposition apparatus 52 were used. In the chamber 1 of the vapor deposition apparatus 52, the current collector 4 was fed out from the second roller 8, caused to pass the vapor deposition zone 91 and transported to the first roller 3 at a rate of 1 cm/min. In the vapor deposition zone 91, vapor deposition was performed while the current collector 4 was moved in a direction away from the vapor deposition surface 9S, namely, in a direction in which the incidence angle ω of the vapor deposition material was increased. The incidence angle ω of silicon was 62 degrees at the bottom end of the vapor deposition zone 91 and 75 degrees at the top end thereof.
As the vaporization source 9, the carbon crucible 10 holding silicon having a purity of 99.9999% was used. An electron beam set to have an accelerating voltage of −10 kV and an emission current of 600 mA was output from the electron gun 32 to heat and vaporize silicon. The nozzles 31a and 31b were connected to a gas pipe (not shown), and the gas pipe was connected to an oxygen tank via a mass flow controller. From the nozzle 31b located at the bottom end of the vapor deposition zone 91, oxygen gas was supplied to the chamber 1 at a flow rate of 20 sccm. From the nozzle 31a located at the top end of the vapor deposition zone 91, oxygen gas was supplied to the chamber 1 at a flow rate of 4 40 sccm. At this point, the inside of the chamber 1 was exhausted by the evacuation pump 2 down to a vacuum degree of 0.02 Pa.
In the vapor deposition zone 91, silicon vaporized from the vaporization source 9 reacted with the oxygen gas supplied from the nozzles 31a and 31b to deposit a compound containing silicon and oxygen (SiOx) selectively on the bumps of the current collector 4. Thus, the active material bodies were formed. At this point, the power of the electron gun 2 was appropriately adjusted such that the thickness of the active material layer formed on the current collector 4 running in the vapor deposition zone 91 would be generally uniform.
Next, the inside of the chamber 1 is once made non-vacuum, and the current collector 4 taken in by the second roller 8 was re-attached to the first roller 3 such that the bottom surface of the current collector 4 would act as the vapor deposition surface. Then, the active material bodies were formed also on the bottom surface of the current collector 4 in the vapor deposition zone 91 by substantially the same method as described above. Thus, the negative electrode 2D in Comparative Example 2 was obtained. The growth directions of the active material bodies formed on the top and bottom surfaces of the current collector 4 were generally symmetrical to each other with respect to the normal H to the current collector 4.
The oxygen ratio (the x value in SiOx) of the active material bodies (a silicon oxide) in the obtained negative electrode 2D was found by processing the active material bodies by line analysis measurement in a cross-sectional direction using EPMA. As a result, the x value in the vicinity of the interface between the active material bodies and the current collector 4 was 0.4. The x value was decreased in a direction away from the current collector 4, and was 0.8 on the top surface (head) thereof.
Next, the cross-section of the negative electrode 2D was observed by a scanning electron microscope (SEM).
As a result of the observation, it was confirmed that active material bodies 88 were formed on the dents as well as on the bumps 4A of the current collector 4. The thickness t of the active material bodies 88 was 18 μm. On a part of the dents of the current collector 4 on which the active material bodies 88 were not formed, the active material layer was deposited in a small thickness, and the thickness T thereof was 3 μm.
The width of each active material body 88 was large on the side of the current collector and was decreased toward the tip thereof. The inclination angle of the active material bodies 88 with respect to the normal H to the current collector 4 was 42 degrees.
From the observation results of the negative electrodes 2C and 2D in Reference Example 2 and Comparative Example 2, it was confirmed that when vapor deposition is performed while the current collector is moved in a direction in which the incidence angle ω is decreased (Reference Example 2), the amount of the active material deposited on the dents of the current collector 4 can be decreased than when vapor deposition is performed while the current collector is moved in a direction in which the incidence angle ω is increased (Comparative Example 2). It was also found that in Reference Example 2, the active material bodies can be formed selectively on the bumps more certainly and a sufficiently large space can be secured between the active material bodies. It was further found that in Reference Example 2, the active material bodies which are thin on the side of the current collector and thick at the tip thereof are formed and so the current collector can be suppressed more effectively from being deformed due to the expansion stress of the active material.
The porosity P of the negative electrode 2D, which was found using a mercury porosimeter, was 46%.
A coin-shaped cell was produced using the negative electrode 2D in Comparative Reference Example 2 in substantially the same manner as in Example 2 as a test cell D.
Evaluation of the charge/discharge characteristic was performed on the test cells C and D in the following manner.
First, the test cells C and D were each accommodated into a thermostat oven of 20° C. and charged by a constant current/constant voltage method. Here, each cell was charged at a constant current of 1 C rate (10 is a current value at which the entirety of the cell capacity can be used up within 1 hour) until the cell voltage became 4.2 V, and then charged at a constant voltage until the current value became 0.05 C.
After the charge, the operation was paused for 20 minutes. Then, the cell was discharged at a constant current as high as 1 C rate until the cell voltage became 2.5 V. After the high-rate discharge, the cell was re-discharged at a constant current of 0.2 C until the cell voltage became 2.5 V. After the re-discharge, the operation was paused for 20 minutes.
The above charge/discharge was repeated by 200 cycles.
In an initial period of the cycles, the ratio of the total discharge capacity (the sum of the high-rate discharge capacity and re-discharge capacity) with respect to the charge capacity was obtained with the percentage as the charge/discharge efficiency. In the initial period of the cycles, the ratio of the high-rate discharge capacity with respect to the total discharge capacity was obtained with the percentage as the high-rate ratio. The ratio of the total discharge capacity at the 200th cycle with respect to the total discharge capacity in the initial period of the cycles was obtained with the percentage as the capacity maintenance ratio.
The measurement results are shown in Table 3.
From Table 3, it was found that the test cells C and D exhibit generally equal and high charge/discharge efficiency and high-rate ratio in the initial period of the cycles. This is considered to occur for the following reasons. One reason is that active material bodies in these test cells have a part inclined with respect to the current collector and so the area size of a part of the surface of the active material bodies which is in contact with the electrolysis is larger than that in the conventional cells. Another reason is that parts of the negative electrode active material and the positive electrode active material which face each other are increased and so the charge/discharge reaction is made uniform, which suppresses lithium deposition reaction and excessive local charge/discharge of the positive electrode.
However, at the 200th cycle, the capacity maintenance ratio of the test cell C is about 85% whereas the capacity maintenance ratio of the test cell D drops to about 73%. This is considered to occur for the following reason.
The active material bodies of the test cell C are formed while the current collector is moved in a direction in which the incidence angle ω is decreased, and so the active material bodies can be selectively located on the bumps. Therefore, the amount of the active material deposited on the dents of the current collector can be decreased, and a sufficiently large space can be obtained with certainty between the active material bodies. As a result, the stress applied on the current collector at the time of charge/discharge by the expansion and contraction of the active material on the dents of the current collector and the contact between the active material bodies can be decreased. For this reason, the current collector can be suppressed from being wrinkled or distorted, and the active material bodies can be suppressed from being delaminated or ruptured.
From the above evaluation results, it was confirmed that by forming the active material bodies selectively on the bumps of the current collector and decreasing the amount of the active material deposited on the dents of the current collector, the charge/discharge cycle characteristic can be improved.
The present invention is applicable to the production of cells for various forms of electrochemical devices. The present invention is especially advantageous when being applied to the production of a lithium secondary cell required to have a large capacity and a good cycle characteristic. There is no specific limitation on the shape of the lithium secondary cell to which the present invention is applicable, and the lithium secondary cell may have any of various shapes including a coin shape, a button shape, a sheet shape, a cylindrical shape, a flat shape, a polygonal shape and the like. The electrode plate assembly including the positive electrode, the negative electrode and the separator may of either a wound type or a stacked type. The cell may be small and used for compact mobile devices or the like, or large and used for electric vehicles or the like. A lithium ion secondary cell produced by a production method according to the present invention is usable as a power supply of, for example, mobile information terminals, mobile electronic devices, home-use compact power storage devices, motorcycles, electric vehicles, hybrid electric vehicles and the like, but the use of the cell is not specifically limited.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-017757 | Jan 2008 | JP | national |
| 2008-139780 | May 2008 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2008/003792 | 12/16/2008 | WO | 00 | 7/28/2010 |
