Patent Application
20240429561
The present disclosure relates to a lithium secondary battery.
Non-aqueous electrolyte secondary batteries are used for ICTs for personal computers and smart phones, in-vehicle applications, power storage applications, etc. There is demand for further increase in the capacity of non-aqueous electrolyte secondary batteries in these applications. Lithium ion batteries are known as high-capacity non-aqueous electrolyte secondary batteries. It is possible to increase the capacity of a lithium ion battery by using a combination of graphite and an alloy active material such as a silicon compound, as a negative electrode active material, for example. However, the increase in the capacity of lithium ion batteries is approaching the limit.
Lithium secondary batteries (lithium metal secondary batteries) are promising as non-aqueous electrolyte secondary batteries superior in capacity to lithium ion batteries. In a lithium secondary battery, lithium metal is deposited on the negative electrode during charging, and the lithium metal dissolves in the non-aqueous electrolyte during discharging. Various proposals have been conventionally made on lithium secondary batteries.
Patent Literature 1 (WO 2020/066254) discloses “a lithium secondary battery, comprising: a positive electrode including a positive electrode current collector, and a positive electrode mixture layer containing a positive electrode active material; a negative electrode including a negative electrode current collector that faces the positive electrode; a separator disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte having lithium ion conductivity, wherein the positive electrode active material includes a composite oxide containing lithium and a metal M other than lithium, the metal M containing at least a transition metal, lithium metal deposits on the negative electrode during charge, and the lithium metal dissolves from the negative electrode during discharge, the positive electrode current collector has a first length in a first direction D1 and a second length in a second direction D2 intersecting the first direction, the first length being shorter than the second length, a spacer is provided between the positive electrode and the separator so as to form a space for storing the lithium metal, between the positive electrode and the negative electrode, and a straight line SL is drawable along the first direction D1 so as to pass through three or more points of the spacer.”
PTL1: WO 2020/066254
Nowadays, further improvement in characteristics of lithium secondary batteries are required. An object of the present disclosure is to provide a lithium secondary battery that has improved characteristics (e.g., discharge capacity retention rate).
One aspect of the present disclosure relates to a lithium secondary battery. The lithium secondary battery includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte having lithium ion conductivity, wherein the negative electrode is an electrode on which lithium metal is deposited during charging and from which the lithium metal dissolves during discharging, the separator includes a substrate and a spacer, the spacer is disposed on the positive electrode side with respect to the substrate, the spacer includes line-shaped protrusions arranged in a mesh pattern, and the mesh pattern includes a lacking part connecting regions within adjacent meshes.
Another aspect of the present disclosure relates to a lithium secondary battery. The lithium secondary battery includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte having lithium ion conductivity, wherein the negative electrode is an electrode on which lithium metal is deposited during charging and from which the lithium metal dissolves during discharging, the separator includes a substrate and a spacer, the spacer is disposed on the positive electrode side with respect to the substrate, the spacer includes line-shaped protrusions arranged in a mesh pattern, and the separator includes a first region in which the mesh pattern is formed and a second region in which the mesh pattern is not formed.
According to the present disclosure, it is possible to obtain a lithium secondary battery having improved characteristics (e.g., discharge capacity retention rate). Although novel features of the present invention are described in the appended claims, the following detailed description referring to the drawings will facilitate the understanding of both the configuration and content of the present invention together with other objects and features of the present invention.
The following describes examples of embodiments according to the present disclosure, but embodiments according to the present disclosure are not limited to the following examples. In the following description, specific numerical values and materials are described as examples, but it is possible to apply other numerical values and materials as long as the invention according to the present disclosure can be implemented. In the present specification, the wording “from a numerical value A to a numerical value B” refers to a range that includes the numerical values A and B, and can be read as “the numerical value A or more and the numerical value B or less”. When examples of a lower limit value and examples of an upper limit value are described below regarding a specific physical property or condition, any of the examples of the lower limit value and the examples of the upper limit value can be combined unless the lower limit value is larger than or equal to the upper limit value.
The following describes lithium secondary batteries according to embodiments of the present disclosure. Hereinafter, the lithium secondary batteries may each be referred to as a “lithium secondary battery (L)”. The following describes two lithium secondary batteries as the “lithium secondary battery (L)”. In the following description, the two lithium secondary batteries may be referred to as a “lithium secondary battery (L1)” and a “lithium secondary battery (L2)”.
The lithium secondary battery (L1) includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode. and a non-aqueous electrolyte having lithium ion conductivity. The negative electrode is an electrode on which lithium metal is deposited during charging and from which the lithium metal dissolves during discharging. The separator includes a substrate and a spacer. The spacer is disposed on the positive electrode side with respect to the substrate. The spacer includes line-shaped protrusions arranged in a mesh pattern. The mesh pattern includes lacking parts connecting regions within adjacent meshes.
The lithium secondary battery (L2) includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte having lithium ion conductivity. The negative electrode is an electrode on which lithium metal is deposited during charging and from which the lithium metal dissolves during discharging. The separator includes a substrate and a spacer. The spacer is disposed on the positive electrode side with respect to the substrate. The spacer includes line-shaped protrusions arranged in a mesh pattern. The separator includes a first region in which the mesh pattern is formed and a second region in which the mesh pattern is not formed.
The lithium secondary battery (L1) and the lithium secondary battery (L2) differ from each other only in the arrangement pattern of the spacer (the shape of the spacer in a plan view). Accordingly, the following description is applicable to both the lithium secondary battery (L1) and the lithium secondary battery (L2) unless otherwise specified or contradiction arises. Also, matters described for the lithium secondary battery (L) are applicable to both the lithium secondary battery (L1) and the lithium secondary battery (L2).
In the present specification, a configuration in which an element B is formed on an element A encompasses a configuration in which the element B is formed directly on the element A and a configuration in which the element B is formed on the element A with another element arranged between the elements A and B.
The lithium secondary battery (L) is also called a lithium metal secondary battery. In this type of battery, lithium metal is deposited on the negative electrode during charging, and the lithium metal dissolves from the negative electrode during discharging. Specifically, the negative electrode includes at least a negative electrode current collector, and the lithium metal is deposited on the negative electrode current collector.
In the lithium secondary battery (L), for example, 70% or more of the rated capacity is exhibited through deposition and dissolution of the lithium metal. Movement of electrons at the negative electrode during charging and discharging occurs mainly due to the lithium metal being deposited on the negative electrode and dissolving from the negative electrode. Specifically, 70% to 100% (e.g., 80% to 100% or 90% to 100%) of the movement of electrons (from a different viewpoint, a current) that occurs at the negative electrode during charging and discharging is due to deposition and dissolution of the lithium metal. That is, the negative electrode according to the present disclosure differs from a negative electrode at which movement of electrons during charging and discharging occurs mainly due to lithium ions being absorbed and released by a negative electrode active material (e.g., graphite).
In the following description, the positive electrode, the negative electrode, and the separator may be collectively referred to as an “electrode group”. The positive electrode, the negative electrode, and the separator may be wound with the separator arranged between the positive electrode and the negative electrode. In the formation of a wound electrode group, a belt-like positive electrode, a belt-like negative electrode, and a belt-like separator are used. Alternatively, the positive electrode, the negative electrode, and the separator may be stacked on each other. For example, a flat positive electrode, a flat negative electrode, and a flat separator may be stacked on each other. That is, the electrode group may be a wound electrode group or a stacked electrode group.
In the lithium secondary battery (L), spaces are secured between the positive electrode and the negative electrode by the spacer. The spacer is disposed on the positive electrode side of the separator and forms the spaces on a surface of the positive electrode. Lithium metal is deposited on a surface of the negative electrode during charging. Due to the deposited lithium metal, the separator moves toward the positive electrode in portions that are not provided with the spacer. Regions in which the spacer is not present are spaces, and therefore, the separator (the substrate and a heat-resistant layer) can move toward the positive electrode in those regions. Accordingly, expansion of the electrode group can be suppressed when charging and discharging are repeated. Also, in the lithium secondary battery (L), the substrate or a heat-resistant layer is present on the surface of the negative electrode. Therefore, it is possible to suppress coarse deposition of lithium metal. Accordingly, expansion of the negative electrode can be suppressed.
Furthermore, in the lithium secondary battery (L), the spacer included in the separator includes a mesh pattern. Therefore, when the electrode group is stacked or wound, it is possible to suppress the occurrence of a situation in which the spacer is displaced and electrode plates are curved in an undesirable manner. Accordingly, in the lithium secondary battery (L), a space in which lithium metal is deposited can be appropriately formed by the spacer.
In the separator included in the lithium secondary battery (L1), the mesh pattern includes lacking parts. The non-aqueous electrolyte can move through the lacking parts. When there are no lacking parts, flowability of the non-aqueous electrolyte decreases and characteristics of the battery may deteriorate. In the lithium secondary battery (L1), the non-aqueous electrolyte can move through the lacking parts, and therefore, such deterioration in characteristics can be suppressed. Consequently, it is possible to obtain a high discharge capacity retention rate.
The separator included in the lithium secondary battery (L2) includes the second region in which the mesh pattern is not formed. The non-aqueous electrolyte can move freely in the second region. When the second region is not provided, flowability of the non-aqueous electrolyte decreases and characteristics of the battery may deteriorate. In the lithium secondary battery (L2). the non-aqueous electrolyte can move through the second region, and therefore, such deterioration in characteristics can be suppressed. Consequently, it is possible to obtain a high discharge capacity retention rate.
Also, if the spacer is disposed on the positive electrode, the spacer covers the surface of the positive electrode, and therefore, transfer of lithium ions by a positive electrode active material is partially hindered. Consequently, the cell capacity may decrease. However, in the lithium secondary battery (L), the spacer is formed on the separator, and therefore, the above problem can be avoided. That is, in the lithium secondary battery (L), it is possible to suppress expansion of the negative electrode and maintain a high cell capacity at the same time, and furthermore, it is possible to suppress an increase in the temperature of the battery in the event of an abnormality.
If a space is formed on the surface of the negative electrode by the spacer, lithium metal is freely deposited on the surface of the negative electrode and dissolves from the surface during charging and discharging. Consequently, a short circuit due to dendrites may be likely to occur or the occurrence of a side reaction may be likely to increase due to an increase in the surface area of the negative electrode. An increase in the occurrence of a side reaction leads to a reduction in the capacity retention rate. On the other hand, in the lithium secondary battery (L), the spacer is disposed on the positive electrode side, and accordingly, no space is formed on the surface of the negative electrode by the spacer. Therefore, the lithium secondary battery (L) can have high reliability and a high capacity retention rate.
The lithium secondary battery (L2) preferably satisfies at least one of the following conditions (J1) to (J3), and may satisfy two or all of the conditions. For example, the lithium secondary battery (L2) may satisfy the following conditions (J2) and (J3).
(J1) The first region in which the mesh pattern is formed and the second region in which the mesh pattern is not formed are alternately arranged along a longitudinal direction LD of the separator.
(J2) A length L1 of the first region in the longitudinal direction LD of the separator is longer than a length L2 of the second region in the longitudinal direction LD. A ratio L1/L2 between the length L1 and the length L2 is larger than 1, and may be 2 or more, or 3 or more, and 10 or less, or 5 or less.
(J3) The length L2 of the second region in the longitudinal direction LD of the separator is 1 mm or more and 15 mm or less. The length L2 may be 1 mm or more, 3 mm or more, or 5 mm or more, and 15 mm or less, or 10 mm or less.
An average height Hs of the spacer is larger than an average thickness Tb of the substrate. A ratio Hs/Tb between the average height Hs and the average thickness Tb is larger than 1, and may be 1.5 or more, 2 or more, or 3 or more. The ratio Hs/Tb may be 10 or less, 8 or less, 5 or less, or 4 or less. When the ratio is set to 1.5 or more, the expansion rate of the electrode group can be particularly reduced. Also, when the spacer is made high, the strength of the spacer increases, and accordingly, an effect of suppressing contraction of the substrate when the substrate is excessively heated increases. Consequently, an excessive increase in the temperature of the electrode group can be particularly suppressed.
The average height Hs can be measured using the following method. First, an image of a cross section of the separator along the thickness direction of the separator is captured with use of an electron microscope. Next, 20 points on the spacer are arbitrarily selected in the image, and the height of the spacer is measured at the selected points. Next, the arithmetic average of the heights measured at the 20 points is calculated, and the obtained average is taken as the average height Hs. The average thickness Tb and an average thickness Tt of a heat-resistant layer, which will be described later, can be measured in the same manner.
The average thickness Tb of the substrate may be 5 μm or more, or 10 μm or more, and 30 μm or less, or 20 μm or less. The average thickness Tt of the heat-resistant layer, which will be described later, may be 1 μm or more, or 2 μm or more, and 5 μm or less, or 3 jam or less. The average height Hs of the spacer may be 10 μm or more, or 20 μm or more, and 100 μm or less, 50 μm or less, 40 μm or less, or 30 μm or less. These height and thicknesses vary depending on the configurations of the positive electrode and the negative electrode, and may take values that are out of the ranges described above as examples. Note that the spacer is commonly formed so as to have a constant height as far as possible to make the distance between the electrode plates secured by the spacer constant as far as possible.
In an example of the lacking parts, the spacer is not formed at all in the lacking parts. However, it is sufficient that the lacking parts are formed in such a manner that, when the electrode group is formed, the non-aqueous electrolyte present in a space (region) within a mesh constituted by the spacer can move to an adjacent space. Accordingly, the lacking parts may be formed by reducing the height of a portion of the spacer. For example, the lacking parts may be formed by setting the height of a portion of the spacer to 50% or less (e.g., 30% or less, 20% or less, or 10% or less) of the average height Hs of the spacer. An average height Hd of the lacking parts may be 0% or more, 3% or more, 5% or more, 10% or more, or 20% or more of the average height Hs. When the lacking parts are formed while leaving a portion of the spacer, it is possible to allow movement of the non-aqueous electrolyte between adjacent meshes while maintaining the continuous mesh pattern. By maintaining the continuous mesh pattern, it is possible to particularly increase the effect of suppressing contraction of the substrate when the temperature of the electrode group has increased. It is possible to reduce the height of a portion of the spacer by changing an application amount of a slurry (or a coating liquid) for forming the spacer depending on positions, for example.
It is preferable that the spacer includes a non-porous structure through which lithium ions do not pass. Such a spacer can be realized by forming the spacer under conditions in which the spacer does not become porous. Note that, in the present specification, the description “lithium ions do not pass through” means that lithium ions do not pass through in an amount that will affect characteristics and the shape of the battery, and encompasses a case in which lithium ions move through the spacer but it can be considered that lithium ions do not substantially pass through the spacer.
An area S1 of the spacer may be 30% or less of an area S0 of the separator. When the area S1 is within this range, it is possible to secure sufficient spaces for deposition of lithium metal. Note that the area S1 and the area S0 are areas of the spacer and the separator when the separator is viewed from the spacer side. A ratio S1/S0 between the area S1 and the area S0 may be 0.20 or less (20% or less), or 0.10 or less, and 0.03 or more (3% or more), or 0.05 or more. By setting the ratio to 0.05 or more (5% or more), it is possible to increase the effect of suppressing an excessive increase in the temperature of the electrode group.
The separator may include at least one heat-resistant layer. The heat-resistant layer may be formed on the positive electrode side main surface of two main surfaces of the substrate, or on the negative electrode side main surface, or on each of the two main surfaces. The spacer may be formed on the heat-resistant layer, or on the substrate without the heat-resistant layer being arranged between the substrate and the spacer. For example, the separator may have a configuration constituted of the substrate/the heat-resistant layer/the spacer, a configuration constituted of the heat-resistant layer/the substrate/the spacer, or a configuration constituted of the heat-resistant layer/the substrate/the heat-resistant layer/the spacer. In these configurations, the spacer is disposed on the positive electrode side. That is, the separator is disposed in such a manner that the spacer faces the positive electrode. In an example, the heat-resistant layer is formed on the positive electrode side main surface of the two main surfaces of the substrate, and the spacer is formed on the heat-resistant layer. When the spacer is formed on the heat-resistant layer, a particularly high effect of suppressing thermal contraction of the substrate can be obtained.
It is preferable that the average height Hs of the spacer is larger than a sum Tw of the average thickness Tb of the substrate and the average thickness Tt of the heat-resistant layer. A ratio Hs/Tw between the average height Hs and the sum Tw may be more than 1, 1.5 or more, 2 or more, or 3 or more. The ratio Hs/Tw may be 10 or less, 8 or less, 5 or less, or 4 or less.
When the separator includes a heat-resistant layer formed on a surface of the substrate, the heat-resistant layer can suppress contraction of the substrate when the temperature of the electrode group has excessively increased. If the substrate contracts, a short circuit is likely to occur between the positive electrode and the negative electrode due to the contraction of the substrate, and therefore, the temperature of the electrode group is likely to further increase. When the separator includes a heat-resistant layer, contraction of the substrate can be suppressed, and accordingly, a further increase in the temperature of the electrode group can be suppressed.
Moreover, the spacer not only secures spaces between the electrode plates, but also significantly increases the effect of suppressing an excessive increase in the temperature of the electrode group when the spacer is used in combination with the beat-resistant layer.
At present, it is not clear why the above effect can be obtained by using the heat-resistant layer and the spacer in combination. However, the effect of suppressing thermal contraction of the substrate, which is obtained with use of the heat-resistant layer, may be significantly increased by the spacer.
By disposing the heat-resistant layer on the positive electrode side, it is possible to suppress deterioration of the substrate due to an oxidation reaction. On the other hand, by disposing the heat-resistant layer on the negative electrode side, it is possible to suppress deterioration of the substrate due to a reduction reaction.
A porous sheet that has ion permeability and insulating properties is used as the substrate. Examples of the porous sheet include a porous film, a woven fabric, and a nonwoven fabric. The material of the separator is not particularly limited, and may be a polymer material. Examples of the polymer material include a polyolefin resin, a polyamide resin, and cellulose. Examples of the polyolefin resin include polyethylene, polypropylene, and a copolymer of ethylene and propylene. The substrate may contain an additive as necessary. Examples of the additive include an inorganic filler. A sheet that is used as a separator in a lithium secondary battery may be used as the substrate.
The spacer may contain a resin (e.g., an insulating resin), or contain a resin and particles. The spacer may be constituted of only a resin, or constituted of a resin and particles. The percentage of resin contained in the spacer may be 10 volume % or more, 30 volume % or more, or 50 volume % or more, and 100 volume % or less, or 80 volume % or less. The content percentage of particles contained in the spacer may be lower than a content percentage of particles contained in the heat-resistant layer.
Examples of the resin material include: fluorine-containing resin such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene; fluorine-containing rubber such as vinylidene fluoride-tetrafluoroethylene copolymer and ethylene-tetrafluoroethylene copolymer; rubbers such as styrene-butadiene copolymer and hydride thereof, acrylonitrile-butadiene copolymer and hydride thereof, methacrylic ester-acrylic ester copolymer, styrene-acrylic ester copolymer, acrylonitrile-acrylic ester copolymer, ethylene propylene rubber, polyvinyl alcohol, and polyvinyl acetate; cellulose derivatives such as ethyl cellulose, methyl cellulose, hydroxyethyl cellulose, and carboxymethyl cellulose; acrylic resin such as acrylic acid-methacrylic acid copolymer; polyphenylene ether; polysulfone; polyethersulfone; polyphenylene sulfide; polyether imide; polyimide; polyamide such as wholly-aromatic polyamide (aramid); polyamide imide; polyacrylonitrile; polyvinyl alcohol; polyether; polyacrylic acid; polymethacrylic acid; polyester; polyolefin; silicone resin; urethane resin; melamine resin; urea resin; and epoxy resin.
Out of the resin materials described above, polyimide, polyvinylidene fluoride, an acrylonitrile-acrylic ester copolymer, and the like are preferably used as materials that do not allow lithium ions to pass therethrough, and polyimide may be used. A non-porous spacer made of any of these resin materials and having a height larger than or equal to a certain height is a layer through which lithium ions do not pass. When the separator includes a heat-resistant layer, the average height of the spacer may be 3 times or more, 5 times or more, or 10 times or more of the average thickness of the heat-resistant layer, and 100 times or less, 30 times or less, or 20 times or less of the average thickness of the heat-resistant layer.
The particles may be inorganic particles or organic particles. In particular. the particles may be inorganic particles having insulating properties such as particles of a metal oxide, a metal hydroxide, a metal nitride, a metal carbide, or a metal sulfide. Examples of preferable metal oxides include aluminum oxide (alumina or boehmite), magnesium oxide, titanium oxide (titania), zirconium oxide, and silicon oxide (silica). Examples of the metal hydroxide include aluminum hydroxide. Examples of the metal nitride include silicon nitride, aluminum nitride, boron nitride. and titanium nitride. Examples of the metal carbide include silicon carbide and boron carbide. Examples of the metal sulfide include barium sulfide. It is also possible to use minerals such as aluminosilicate, layered silicate, barium titanate, and strontium titanate. In particular, it is preferable to use alumina, silica, titania, or the like.
An average particle diameter of the particles is not particularly limited, and may be 0.1 μm or more, or 0.5 μm or more, and 10 μm or less, 5 μm or less, or 2 μm or less. The average particle diameter can be measured using the following method. First, an image of a cross section of the spacer along the thickness direction of the separator is captured with use of an electron microscope. Next, portions corresponding to particles are identified in the image by performing image processing such as binarization on the image. Next, diameters (equivalent circle diameters) of circles having the same areas as cross-sectional areas of the particles are determined, and an arithmetic average of the determined equivalent circle diameters is taken as the average particle diameter. The arithmetic average can be obtained based on 100 or more particles, for example. Note that average particle diameters of other particles included in the electrode plates and the separator can be determined in the same manner.
When the spacer contains a resin material and particles, the content percentage of the particles in the spacer is preferably 50 volume % or less. In this case, the spacer is likely to have sufficiently high strength.
The area S1 of the spacer may be 30% or less of the area S0 of the separator. When the area S1 is within this range, it is possible to secure sufficient spaces for deposition of lithium metal. Note that the area S1 and the area S0 are areas of the spacer and the separator when the separator is viewed from the spacer side. The ratio S1/S0 between the area S1 and the are S0 may be 0.20 or less (20% or less), or 0.10 or less, and 0.03 or more (3% or more), or 0.05 or more. By setting the ratio to 0.05 or more (5% or more), it is possible to increase the effect of suppressing an excessive increase in the temperature of the electrode group.
The spacer may include dot-shaped protrusions in addition to the line-shaped protrusions. From a point of view. the line-shaped protrusions are ridge-shaped protrusions. In terms of increasing the effect of suppressing an excessive increase in the temperature of the electrode group, it is preferable that the spacer includes line-shaped protrusions, and may be constituted of only line-shaped protrusions.
The width of the line-shaped protrusions may be 100 μm or more, or 200 μm or more, and 2000 μm or less, or 1000 μm or less.
The mesh pattern may be a pattern obtained by combining polygons. Examples of the mesh pattern include a pattern obtained by combining polygons in such a manner that the polygons share their sides. Examples of the polygons include triangles. rectangles, and hexagons. It is also possible to combine different types of polygons. The mesh pattern may be a honeycomb pattern.
There is no particular limitation on positions of the lacking parts in the lithium secondary battery (L1). The lacking parts may be provided at positions corresponding to sides of polygons constituting the mesh pattern. Alternatively, the lacking parts may be provided at positions corresponding to vertexes of the polygons constituting the mesh pattern. Alternatively, the lacking parts may be provided at positions corresponding to sides and vertexes of the polygons. A total length of the lacking parts along the line-shaped protrusions may be 1% or more, 5% or more, or 10% or more of a total length of the line-shaped protrusions, and 80% or less, 65% or less, or 50% or less of the total length of the line-shaped protrusions. By setting this percentage high, it is possible to facilitate circulation of the non-aqueous electrolyte. On the other hand, by setting this percentage low. it is possible to increase the effect of suppressing an increase in the temperature of the electrode group when an abnormality occurs in the battery. From these viewpoints, the percentage may be set so as to fall within a range from 10% to 50%. When the spacer is not formed at all in the lacking parts, a total length of the lacking parts along the line-shaped protrusions may be 1% or more, 5% or more, or 10% or more of the total length of the line-shaped protrusions, and may be 50% or less, 30% or less, or 20% or less of the total length of the line-shaped protrusions.
Note that lacking parts may be formed in a portion of the mesh pattern in the lithium secondary battery (L2), similarly to the spacer included in the lithium secondary battery (L1). From another viewpoint, the lithium secondary battery (L1) may include a first region in which the mesh pattern is formed and a second region in which the mesh pattern is not formed.
A preferable example of the spacer satisfies the following condition (14) and/or the following condition (J5). When the following conditions are satisfied, it is possible to secure spaces in which lithium metal is deposited and to suppress an excessive increase in the temperature of the electrode group, in a well-balanced manner. (J4) The ratio S1/S0 between the area S1 and the area S0 is 0.30 or less. The ratio S1/S0 may be within any of the ranges described above.
(J5) The width of the line-shaped protrusions constituting the spacer is 2000 μm or less. The width of the protrusions may be within any of the ranges described above.
It is preferable that a first resin constituting the spacer has a higher heat resistance than a second resin constituting the substrate. Here, having a higher heat resistance means that the decomposition temperature or melting point of the first resin is higher than the decomposition temperature or melting point of the second resin. Note that each of the first resin and the second resin may include a plurality of types of resins.
The heat-resistant layer may contain a polymer (hereinafter may be referred to as a “polymer (PL)”) and inorganic particles. The inorganic particles may include first particles made of a phosphate containing lithium, and may further include second particles made of a material other than a phosphate. The heat-resistant layer is a layer that allows lithium ions to pass therethrough.
The phosphate constituting the first particles may be at least one selected from the group consisting of lithium phosphate (Li3PO4), dilithium hydrogen phosphate (Li2HPO4), and lithium dihydrogen phosphate (LiH2PO4). Among these, lithium phosphate is preferable because it has a high effect of suppressing heat generation from the battery in the event of an abnormality.
An average particle diameter of the first particles may be within a range from 0.1 μm to 1.0 μm (e.g., from 0.1 μm to 0.5 μm, from 0.1 μm to 0.2 μm, or from 0.1 μm to 0.19 μm). The average particle diameter of the first particles may be 0.1 μm or more, or 0.15 μm or more. The average particle diameter of the first particles may be 1.0 μm or less, 0.5 μm or less, 0.3 μm or less, or 0.2 μm or less. By setting the average particle diameter to 0.1 μm or more, it is possible to secure necessary and sufficient voids for allowing the electrolyte solution to permeate. From the viewpoint of forming a dense layer of the first particles, it is preferable to set the average particle diameter to 1.0 μm or less.
A polymer that has a higher heat resistance than a main component of the substrate of the separator can be used as the polymer (PL). It is preferable that the polymer (PL) includes at least one selected from the group consisting of aromatic polyamides, aromatic polyimides, and aromatic polyamide imides. These are known as polymers (from other viewpoints, macromolecules or resins) having high heat resistance. From the viewpoint of heat resistance, aramids, i.e., meta-aramids (meta-type wholly-aromatic polyamides) and para-aramids (para-type wholly-aromatic polyamides) are preferable. A preferable example of the polymer (PL) is a meta-aramid. It is possible to use, as the polymer (PL), a known aromatic polyamide, a known aromatic polyimide, and a known aromatic polyamide imide.
Examples of the aromatic polyamide (polymer (PL)) include a polymer that is formed through condensation polymerization of monomers having an aromatic skeleton and includes an amide bond in a repeating unit. Examples of the aromatic polyamide (e.g., wholly-aromatic polyamide) include meta-type aromatic polyamides (e.g., meta-type wholly-aromatic polyamides) and para-type aromatic polyamides (e.g., para-type wholly-aromatic polyamides). Wholly-aromatic polyamides are also called aramids.
A preferable example of the second particles (inorganic particles) are particles constituted by an inorganic compound that has insulating properties and does not melt and decompose when abnormal heat generation from the battery occurs. The second particles may be inorganic particles that are commonly used as an inorganic filler. Examples of the material of the second particles include an oxide, a hydrate of an oxide, a hydroxide, a nitride, a carbide, and a sulfide, and these may contain metal elements. An average particle diameter of the second particles may be 0.2 μm or more and 2 μm or less.
Examples of the oxide and the hydrate of an oxide include aluminum oxide, boehmite, magnesium oxide, titanium oxide, zirconium oxide, silicon oxide, yttrium oxide, and zinc oxide. Examples of the nitride include silicon nitride, aluminum nitride, boron nitride, and titanium nitride. Examples of the carbide include silicon carbide and boron carbide. Examples of the sulfide include barium sulfide. Examples of the hydroxide include aluminum hydroxide.
Alternatively, the material of the second particles may be a porous aluminosilicate such as zeolite, a layered silicate such as talc, barium titanate (BaTiO3), strontium titanate (SrTiO3), or the like. From the viewpoints of insulating properties, heat resistance, etc., it is possible to use, as the material of the second particles, at least one selected from the group consisting of aluminum oxide, boehmite, tale, titanium oxide, and magnesium oxide.
The average particle diameter of the second particles may be within any of the ranges described as examples for the average particle diameter of the first particles.
The inorganic particles may include the above-described first particles and the above-described second particles made of a material other than a phosphate. In this case, the heat-resistant layer may include a first layer containing the first particles and a second layer containing the second particles. With this configuration, the effect of suppressing an excessive increase in the temperature of the electrode group can be particularly increased. Note that the heat-resistant layer may be constituted of only the first layer or only the second layer.
The first layer and the second layer may be provided on the positive electrode side main surface of the two main surfaces of the substrate, or on the negative electrode side main surface, or may be respectively provided on different main surfaces. For example, the separator may have a layered structure constituted of the substrate/the first layer/the second layer/the spacer, the substrate/the second layer/the first layer/the spacer, the first layer/the second layer/the substrate/the spacer, or the second layer/the first layer/the substrate/the spacer. Alternatively, the first layer and the second layer may be respectively provided on different main surfaces of the substrate. For example. the separator may have a layered structure constituted of the first layer/the substrate/the second layer/the spacer or the second layer/the substrate/the first layer/the spacer. The separator is disposed in such a manner that the spacer faces the positive electrode. That is, the spacer is disposed on the positive electrode side with respect to the substrate and the heat-resistant layer.
The first layer may contain the first particles as a main component. The content percentage of the first particles in the first layer may be within a range from 50 mass % to 99 mass %, a range from 85 mass % to 99 mass %, or a range from 90 mass % to 98 mass %. The content percentage may be 50 mass % or more, 70 mass % or more, 85 mass % or more, or 90 mass % or more. The content percentage may be 99 mass % or less, 98 mass % or less, or 95 mass % or less. When the content percentage is within any of these ranges, the first particles have a sufficient surface area, and it is easy to inactivate lithium when the temperature is high.
The first layer may contain a solid component other than the first particles. For example, the first layer may contain a binder, an inorganic component (e.g., inorganic particles) other than the first particles, the polymer (PL), or the like.
When the first layer contains a binder, the content percentage of the binder in the first layer may be within a range from 1 mass % to 15 mass %, or a range from 2 mass % to 10 mass %. The content percentage of the binder in the first layer may be 1 mass % or more, or 2 mass % or more. The content percentage of the binder in the first layer may be 15 mass % or less, or 10 mass % or less.
There is no particular limitation on the binder contained in the first layer, and examples of the binder include a polyolefin (e.g., polyethylene, polypropylene, or a copolymer of ethylene and a-olefin), a fluorine-containing resin (e.g., polyvinylidene fluoride, polytetrafluoroethylene, or polyvinyl fluoride), fluorine-containing rubber (e.g., a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer or an ethylene-tetrafluoroethylene copolymer), a styrene-butadiene copolymer and a hydride thereof, an acrylonitrile-butadiene copolymer and a hydride thereof, an acrylonitrile-butadiene-styrene copolymer and a hydride thereof, and N-vinyl acetamide.
The second layer contains the second particles other than the first particles (particles of a phosphate) as described above. Note that the second layer may contain the first particles, but does not necessarily need to contain the first particles.
It is preferable that the second layer contains the polymer (PL). The content percentage of the polymer (PL) in the second layer may be within a range from 50 mass % to 100 mass % (e.g., from 80 mass % to 100 mass %, or from 90 mass % to 100 mass %). The second layer may be constituted of only the polymer (PL).
The second layer may contain the second particles as a main component. The content percentage of the second particles in the second layer may be within a range from 50 mass % to 99 mass % (e.g., from 85 mass % to 99 mass %). In this case, the second layer may also contain a binder. Any of the examples of binder described for the first layer may be used as the binder. The content percentage of the second particles in the second layer may be 50 mass % or more, 70 mass % or more, 85 mass % or more, or 90mass % or more. The content percentage may be 99 mass % or less, 98 mass % or less, or 95 mass % or less.
Thicknesses of the first layer and the second layer may be independently set so as to fall within a range from 0.2 μm to 10 μm (e.g., from 1 μm to 8 μm, from 2 pun to 4 μm, or from 4 μm to 10 μm). For example, the thickness of the first layer may be within a range from 0.2 μm to 10 μm, and the thickness of the second layer may be within a range from 0.2 μm to 10 μm. The thickness of the first layer may be 0.2 μm or more, 0.3 μm or more, or 0.5 μm or more, and is preferably 1 μm or more, more preferably 2 μm or more, further preferably 3 μm or more, and yet more preferably 4 μm or more. The thickness of the first layer may be 10 μm or less, 8 μm or less, 7 μm or less, or 5 μm or less. The thickness of the second layer may be 0.2 μm or more, 0.3 μm or more, or 0.5 μm or more, and is preferably 1 μm or more, more preferably 2 μm or more, further preferably 3 μm or more, and yet more preferably 4 μm or more. The thickness of the second layer may be 10 μm or less, 8 μm or less. 7 μm or less, or 5 μm or less. A configuration in which at least one of the first layer and the second layer has a thickness of 0.2 μm or more is advantageous in terms of suppressing an increase in the temperature of the battery in the event of an abnormality. From the viewpoint of electrical characteristics of the battery, a configuration in which at least one of the first layer and the second layer has a thickness of 10 μm or less is advantageous.
When the heat-resistant layer contains the first particles, an increase in the temperature of the battery in the event of an abnormality can be particularly suppressed. At present, it is unclear by what mechanism such an increase can be suppressed with this configuration. One possibility is that, when the temperature of the battery has increased abnormally, the first particles react with lithium metal on the negative electrode and reactivity of the surface of the lithium metal decreases.
A preferable example of the separator may satisfy the following condition (K1), and may further satisfy the following condition (K2) and/or the following condition (K3). When the following conditions are satisfied, an excessive increase in the temperature of the electrode group can be particularly suppressed.
(K1) The spacer contains at least one resin selected from the group consisting of polyvinylidene fluoride, an acrylonitrile-acrylic ester copolymer, and polyimide. (K2) The heat-resistant layer contains at least one resin selected from the group consisting of wholly-aromatic polyamide, polyvinylidene fluoride, and N-vinyl acetamide. In this case, it is preferable that the heat-resistant layer contains particles made of a phosphate. (K3) The substrate contains a polyolefin (polyethylene, polypropylene, etc.) as a main component (content percentage: 50 mass % or more).
There is no particular limitation on the method for forming the separator, and the separator may be formed using the following method. First, a substrate is prepared. A commercially-available substrate may be used. Next, a heat-resistant layer is formed on the substrate.
There is no particular limitation on the method for forming the heat-resistant layer, and the heat-resistant layer may be formed using the following method. First, components of the heat-resistant layer are mixed with a liquid component (dispersion medium) to form a slurry (or a coating liquid). Next, the slurry (or the coating liquid) is applied to the substrate to form a coating film, and then the coating film is dried. Thus, the heat-resistant layer can be formed. There is no particular limitation on the liquid component used to form the slurry (or the coating liquid). Examples of the liquid component include N-methyl-2-pyrrolidone. When the heat-resistant layer includes a first layer and a second layer. each of the first layer and the second layer can be formed using the above-described method.
There is no particular limitation on each step included in the formation of the heat-resistant layer, and known methods can be applied. For example, the slurry (or the coating liquid) may be applied using a known method such as a method in which a bar coater is used. Also, drying may be performed using a known method such as heating or natural drying.
The spacer is formed on the heat-resistant layer or on the substrate. There is no particular limitation on the method for forming the spacer, and the spacer may be formed using the following method. First, components of the spacer and a liquid component are mixed to prepare a slurry or a coating liquid. Next, the slurry or the coating liquid is applied to portions in which the spacer is to be formed, and then the slurry or the coating liquid is dried. Thus, the spacer can be formed. Examples of the liquid component include N-methyl-2-pyrrolidone. The slurry or the coating liquid may be applied with use of a dispenser or the like, or a known printing method such as gravure printing. ink jet printing, or screen printing. Also, drying may be performed using a known method such as heating or natural drying. The separator is obtained as described above. The lacking parts can be formed by providing portions to which the slurry (or the coating liquid) is not applied, or reducing the application amount of the slurry (or the coating liquid) in portions corresponding to the lacking parts, when compared with the other portions.
The following specifically describes examples of components of the lithium secondary battery. Note that the components described below are examples, and components of the lithium secondary battery according to the present embodiment are not limited to the following components. Known components may be used as components other than components that are characteristic to the present embodiment. The separator has been described above, and a redundant description thereof is omitted.
The negative electrode includes a negative electrode current collector. In the lithium secondary battery (L), lithium metal is deposited on the negative electrode current collector during charging. More specifically, lithium ions contained in the non-aqueous electrolyte receive electrons on the negative electrode current collector during charging and become lithium metal, which is deposited on the negative electrode current collector. The lithium metal deposited on the negative electrode current collector dissolves as lithium ions in the non-aqueous electrolyte during discharging. The lithium ions contained in the non-aqueous electrolyte may be either derived from a lithium salt added to the non-aqueous electrolyte or supplied from a positive electrode active material during charging, or both. An electrically conductive sheet can be used as the negative electrode current collector. When the electrode group is a wound electrode group. a belt-shaped conductive sheet is used. Examples of the conductive sheet include a conductive film and a metal foil.
The conductive sheet may have a smooth surface. In this case, lithium metal derived from the positive electrode tends to be uniformly deposited on the conductive sheet during charging. The smooth surface means that the conductive sheet has a maximum height roughness Rz of 20 μm or less. The conductive sheet may have a maximum height roughness Rz of 10 μm or less. The maximum height roughness Rz is measured in accordance with JIS (Japanese Industrial Standard) B 0601:2013.
The negative electrode current collector (conductive sheet) may be made of an electrically conductive material other than lithium metal and lithium alloys. The conductive material may be a metal material such as a metal or an alloy. The conductive material preferably causes no reaction with lithium. Specifically, a material that forms neither an alloy nor an intermetallic compound with lithium is preferred. Examples of such a conductive material include copper (Cu), nickel (Ni), iron (Fe), an alloy of one or more of these metal elements, and graphite having a basal plane predominately exposed on its surface. Examples of the alloy include a copper alloy and stainless steel (SUS). Among these, copper and/or a copper alloy is preferred because of its high electrical conductivity. The thickness of the negative electrode current collector is not particularly limited, and may be within a range from 5 to 300 μm.
The negative electrode current collector may have a negative electrode mixture layer on its surface. The negative electrode mixture layer is formed by, for example, applying a paste including a negative electrode active material, such as graphite, to at least a part of the surface of the negative electrode current collector. In view of achieving a lithium secondary battery superior in capacity to lithium ion batteries, however, the negative electrode mixture layer is formed sufficiently thin so that lithium metal can be deposited on the negative electrode.
The negative electrode may include the negative electrode current collector and sheet-shaped lithium metal or a lithium alloy on the negative electrode current collector. That is, it is also possible to provide the negative electrode current collector with a primer layer (a layer of lithium metal or a lithium alloy) containing lithium metal in advance. The lithium alloy may contain elements such as aluminum, magnesium, indium, or zinc in addition to lithium. By providing the primer layer in advance and causing lithium metal to be deposited on the primer layer during charging, it is possible to more effectively suppress dendritic deposition of lithium metal. The thickness of the primer layer is not particularly limited, and may be within a range from 5 μm to 25 μm, for example.
The positive electrode includes, for example, a positive electrode current collector and a positive electrode mixture layer supported on the positive electrode current collector. The positive electrode mixture layer contains, for example, a positive electrode active material, an electrically conductive material, and a binder. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode current collector. The positive electrode can be obtained by, for example, applying a positive electrode mixture slurry including the positive electrode active material, the electrically conductive material, and the binder to both surfaces of the positive electrode current collector, followed by drying and then rolling the dry applied film.
The positive electrode active material absorbs and releases lithium ions. Examples of the positive electrode active material include a lithium-containing transition metal oxide, a transition metal fluoride, a polyanion, a fluorinated polyanion, and a transition metal sulfide. Preferred among them is a lithium-containing transition metal oxide in terms of its low production cost and high average discharge voltage.
Examples of a transition metal element contained in the lithium-containing transition metal oxide include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, and W. The lithium-containing transition metal oxide may contain one transition metal element or two or more transition metal elements. The transition metal element may be Co, Ni, and/or Mn. The lithium-containing transition metal oxide can contain one or more typical elements, as needed. Examples of the typical element include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, and Bi. The typical element may be Al, for example.
The lithium-containing transition metal oxide is preferably a composite oxide that contains Co, Ni, and/or Mn as a transition metal element(s) and Al as an optional component and has a crystal structure like that of rock-salt having a layered structure, in view of achieving a high capacity. In this case, in the lithium secondary battery, a molar ratio mLi/mM of a total amount mLi of lithium present in the positive and negative electrodes to an amount mM of metal M other than lithium present in the positive electrode is set to 1.1 or less, for example.
The conductive material is a carbon material, for example. Examples of the carbon material include carbon black, acetylene black, Ketjen Black, carbon nanotube, and graphite.
Examples of the binder include a fluorocarbon resin, polyacrylonitrile, a polyimide resin, an acrylic resin, a polyolefin resin, and a rubbery polymer. Examples of the fluorocarbon resin include polytetrafluoroethylene and polyvinylidene fluoride.
It is sufficient that the positive electrode current collector is an electrically conductive sheet. The conductive sheet may be a foil, film, or the like. The positive electrode current collector may be coated with a carbon material.
The positive electrode current collector (conductive sheet) may be made of a metal material including Al, Ti, Fe, or the like, for example. The metal material may be Al, an Al alloy, Ti, a Ti alloy, a Fe alloy, or the like. The Fe alloy may be stainless steel (SUS). The thickness of the positive electrode current collector is not particularly limited, and may be within a range from 5 to 300 μm.
The non-aqueous electrolyte having lithium ion conductivity includes, for example, a non-aqueous solvent, and lithium ions and anions dissolved in the non-aqueous solvent. The non-aqueous electrolyte may be a liquid or a gel.
A liquid non-aqueous electrolyte is prepared by dissolving a lithium salt in a non-aqueous solvent. When the lithium salt is dissolved in the non-aqueous solvent, lithium ions and anions are produced.
A gel non-aqueous electrolyte includes a lithium salt and a matrix polymer, or includes a lithium salt, a non-aqueous solvent, and a matrix polymer. The matrix polymer is a polymer material that forms a gel by absorbing the non-aqueous solvent, for example. Examples of the polymer material include a fluorocarbon resin, an acrylic resin, and a polyether resin.
The lithium salt or anions may be any known lithium salt or anions that are used for non-aqueous electrolytes for lithium secondary batteries. Specific examples thereof include BF4—, ClO4—, PF6—, CF3SO3—, CF3CO2—, imide anions, and oxalate complex anions. Examples of the imide anions include N(SO2CF3)2— and N(CmF2m+1SO2)x(CnF2n+1SO2)y—, where m and n are independently 0 or an integer of 1 or more, x and y are independently 0, 1. or 2, and x+y=2. The oxalate complex anions may contain boron and/or phosphorus. Examples of the oxalate complex anions include bis(oxalato) borate anions, BF2(C2O4)—, PF4(C2O4)—, and PF2(C2O4)2—z. The non-aqueous electrolyte may include one of these anions, or two or more of then.
From the viewpoint of suppressing dendritic deposition of lithium metal, the non-aqueous electrolyte preferably includes at least oxalate complex anions. The oxalate complex anions interact with lithium to make lithium metal more likely to be deposited uniformly in a fine particulate state. Therefore, local deposition of lithium metal is unlikely to occur. The oxalate complex anions may be used in combination with other anions. The other anions may be PF6— and/or imide anions.
The non-aqueous electrolyte may include LiBF2(C2O4) (lithium difluoro (oxalato) borate) as a solute (lithium salt).
Examples of the non-aqueous solvent include an ester, an ether, a nitrile, an amide, and halogen substituted derivatives of these. The non-aqueous electrolyte may include one of these non-aqueous solvents, or two or more of them. Examples of the halogen substituted derivatives include fluorides.
Examples of the ester include a carbonic ester and a carboxylic acid ester. Examples of a cyclic carbonic ester include ethylene carbonate, propylene carbonate, and fluoroethylene carbonate (FEC). Examples of a chain carbonic ester include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate. Examples of a cyclic carboxylic acid ester include γ-butyrolactone and γ-valerolactone. Examples of a chain carboxylic acid ester include ethyl acetate, methyl propionate, and methyl fluoropropionate.
Examples of the ether include a cyclic ether and a chain ether. Examples of the cyclic ether include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of the chain ether include 1,2-dimethoxyethane, diethyl ether, ethyl vinyl ether, methyl phenyl ether, benzyl ethyl ether, diphenyl ether, dibenzyl ether, 1,2-diethoxyethane, and diethylene glycol dimethyl ether.
The lithium salt concentration in the non-aqueous electrolyte is 0.5 mol/L or more and 3.5 mol/L or less, for example. The anion concentration in the non-aqueous electrolyte may be set to 0.5 mol/L or more and 3.5 mol/L or less. The oxalate complex anion concentration in the non-aqueous electrolyte may be set to 0.05 mol/L or more and 1 mol/L or less.
The non-aqueous electrolyte may contain an additive. The additive may form a surface film on the negative electrode. When the surface film derived from the additive is formed on the negative electrode, the dendrite formation and growth tend to be suppressed. Examples of the additive include vinylene carbonate, FEC, and vinyl ethyl carbonate (VEC).
The following specifically describes an example of the lithium secondary battery according to the present embodiment with reference to the drawings. The above-described components can be applied to components of the following example of the lithium secondary battery. Also, the components of the following example can be changed based on the above description. Also, matters described below may be applied to the embodiments described above. In the lithium secondary battery described below, components that are not essential for the lithium secondary battery according to the present disclosure may be omitted. In the drawings referred to below, the scale of the components is changed to facilitate understanding.
The case body 15 has a step portion 21 formed by partially pressing a sidewall of the case body 15 from outside, for example. The step portion 21 may be formed as an annular step portion extending along the circumferential direction of the case body 15 in the sidewall of the case body 15. In this case, the sealing body 16 is supported on the opening-side surface of the step portion 21.
The sealing body 16 includes a filter 22, a lower valve body 23, an electrically insulating member 24, an upper valve body 25, and a cap 26. These members are layered in this order in the sealing body 16. The sealing body 16 is attached to the opening of the case body 15 in such a manner that the cap 26 is located outside the case body 15 and the filter 22 is located inside the case body 15. The above-described members constituting the sealing body 16 each have a circular plate shape or a ring shape, for example. Central portions of the lower valve body 23 and the upper valve body 25 are connected to each other, and the insulating member 24 is interposed between peripheral portions of the lower valve body 23 and the upper valve body 25. Central portions of the filter 22 and the lower valve body 23 are connected to each other. Central portions of the upper valve body 25 and the cap 26 are connected to each other. That is, the members other than the insulating member 24 are electrically connected to each other.
The lower valve body 23 includes an air vent hole (not shown). Accordingly, when the internal pressure of the battery case rises due to abnormal heat generation or other events, the upper valve body 25 bulges toward the cap 26 and is spaced away from the lower valve body 23. Thus, the electrical connection between the lower valve body 23 and the upper valve body 25 is cut off. When the internal pressure further rises, the upper valve body 25 ruptures to let gas escape through an opening (not shown) provided in the cap 26.
The electrode group 14 includes a positive electrode 11, a negative electrode 12. and a separator 50. The positive electrode 11, the negative electrode 12, and the separator 50 each have a belt-like shape. The belt-shaped positive electrode 11 and the belt-shaped negative electrode 12 are wound spirally together with the separator 50 interposed therebetween, such that the width direction of each of the positive electrode 11 and the negative electrode 12 is parallel with the winding axis. In a cross section of the electrode group 14 perpendicular to the winding axis, the positive electrode 11 and the negative electrode 12 are alternately layered in a radial direction of the electrode group 14, with the separator 50 interposed between the positive electrode 11 and the negative electrode 12. That is, the longitudinal direction of each electrode is a winding direction, and the width direction of each electrode is an axial direction.
The positive electrode 11 includes a positive electrode current collector 11a and positive electrode mixture layers 11b. The positive electrode current collector 11a is electrically connected to the cap 26, which serves as a positive electrode terminal, via a positive electrode lead 19.
As shown in
The spacer 53 is formed on the heat-resistant layer 52. In the electrode group 14. the spacer 53 is in contact with the negative electrode 12. Spaces 14s are formed on the surface of the positive electrode 11 (between the heat-resistant layer 52 and the positive electrode 11) by the spacer 53. On the other hand, the substrate 51 is present on the surface of the negative electrode 12. The height of the spacer 53 is indicated by h in
In the lithium secondary battery 10, lithium metal is deposited on the negative electrode 12 during charging. The lithium metal deposited on the negative electrode 12 presses portions of the separator 50 (the substrate 51 and the heat-resistant layer 52) in which the spacer 53 is not provided, toward the positive electrode 11. Consequently, the lithium metal is deposited in the spaces 14s. The deposited lithium metal dissolves in the non-aqueous electrolyte during discharging. Since the lithium metal deposited on the surface of the negative electrode 12 is stored in the spaces 14s, changes in the volume of the electrode group 14 associated with the deposition of lithium metal are suppressed, and cycle characteristics are improved.
If the spacer 53 allows lithium ions to pass therethrough, lithium ions pass through the spacer 53 during charging. Lithium ions that have passed through the spacer 53 are deposited between the substrate 51 and the negative electrode 12 in portions in which the spacer 53 is provided. Consequently, the thickness of the electrode group 14 in a layered direction increases. Therefore, it is preferable that the spacer 53 does not allow lithium ions to pass therethrough. In the case of the wound electrode group, the layered direction is the radial direction of the wound electrode group.
The spacer 53 is constituted by line-shaped protrusions 53a. The line-shaped protrusions 53a are arranged in a mesh pattern. More specifically, the line-shaped protrusions 53a are unifornily formed in a honeycomb pattern. The honeycomb pattern is a pattern obtained by arranging a plurality of hexagons in such a manner that the hexagons share their sides. Regions in which the line-shaped protrusions 53a are not formed constitute the spaces 14s.
The honeycomb pattern (mesh pattern) shown in
The spacer 53 shown in
Regions (spaces 14s) within adjacent meshes are connected by the lacking parts 53b. This allows the non-aqueous electrolyte to move between adjacent spaces 14s. Note that
In Embodiment 2, an example of the lithium secondary battery (L2) will be described. Embodiment 2 differs from the lithium secondary battery according to Embodiment 1 only in the arrangement pattern of the spacer, and therefore, redundant descriptions are omitted.
The separator 50 shown in
In Embodiments 1 and 2, cylindrical lithium secondary batteries including wound electrode groups have been described. However, the lithium secondary battery according to the present embodiment is not limited to the form described in Embodiment 1, and can also be applied to other forms. The shape of the lithium secondary battery can be selected according to its use and the like from various shapes such as cylindrical, coin, square, sheet, and flat shapes. The electrode group also may be in any form, and may be in the form of a stack.
The lithium secondary battery according to the embodiments of the present disclosure will be described specifically with reference to Examples and Comparative Examples. However, the present disclosure is not limited to the following Examples.
A lithium secondary battery A1 was produced as described below.
A lithium-containing transition metal oxide (NCA: positive electrode active material) containing Li, Ni. Co, and Al (the molar ratio of Li to the total of Ni, Co, and Al was 1.0) and having a layered rock-salt structure was prepared. A positive electrode mixture slurry was prepared by mixing the lithium-containing transition metal oxide (NCA), acetylene black (AB; conductive material), and polyvinylidene fluoride (PVdF; binder) at a mass ratio of NCA:AB:PVdF=95:2.5:2.5, adding an appropriate amount of N-methyl-2-pyrrolidone (NMP) to the mixture, and stirring the mixture. Next, the obtained positive electrode mixture slurry was applied to both surfaces of a belt-shaped Al foil (positive electrode current collector), and then dried to form coating films of the positive electrode mixture. Next, the coating films were rolled with use of rollers. Lastly, the resultant stack of the positive electrode current collector and the positive electrode mixture was cut in a predetermined electrode size, and thus a positive electrode including positive electrode mixture layers on both surfaces of the positive electrode current collector was produced.
A belt-shaped copper foil (thickness: 12 μm) provided with rolled layers of lithium metal (thickness: 25 μm) on both surfaces thereof was used as a negative electrode.
First, a belt-shaped porous polyethylene film (average thickness: 10 μm) was prepared as a substrate. Next, a heat-resistant layer (average thickness: 2 μm) was formed on a surface of the substrate. The heat-resistant layer was formed by forming a second layer and a first layer in this order on the substrate.
The second layer was formed as described below. First, N-methyl-2-pyrrolidone (NMP) and calcium chloride were mixed at a mass ratio of 94.2:5.8. The mixture was heated to about 80° C. to completely dissolve calcium chloride. Then, the solution was cooled to room temperature, 2200 g of the solution was collected, and 0.6 mol of p-phenylenediamine (PPD) was added thereto and completely dissolved. In a state where the solution was kept at about 20° C., 0.6 mol of terephthalic acid dichloride (TPC) was added thereto little by little. The obtained solution was aged at about 20° C. for 1 hour to obtain a polymerization solution. Next, 100 g of the polymerization solution was mixed with an N-methyl-2-pyrrolidone solution in which 5.8 mass % of calcium chloride had been dissolved to obtain a solution (coating liquid) containing p-phenylene terephthalamide (PPTA), which is an aromatic polyamide (aramid), at a concentration of 2 mass %.
Next, the coating liquid was applied to the substrate using a slot-die method to form a coating film. Next, the substrate provided with the coating film was left to stand in an atmosphere having a relative humidity of 70% at a temperature of 25° C. for 1 hour to cause deposition of the aromatic polyamide. Next, NMP and calcium chloride contained in the coating film were removed by washing the substrate with water. Next, the coating film was dried at 60° C. for 5 minutes, and thus the second layer was formed.
The first layer was formed as described below. First, particles of lithium phosphate (Li3PO4) and poly(N-vinylacetamide) (PNVA) were mixed at a mass ratio of 100:8 to obtain a mixture. The particles of lithium phosphate used had a volume-based median diameter of 0.19 μm. Water (ion exchanged water) was added to the obtained mixture, and the mixture was stirred to obtain a slurry (coating liquid) having a solid content concentration of 12 mass %. Next, the slurry was applied to the second layer using a micro gravure coating method to form a coating film. Next. the coating film was dried in a drying furnace provided beside the coating machine. Thus, the first layer was formed. Thus, the heat-resistant layer was formed.
Next, a coating liquid containing polyvinylidene fluoride and alumina particles (inorganic filler) was ejected toward the heat-resistant layer with use of a dispenser to form the pattern shown in
Meshes formed by the spacer were regular hexagons including lacking parts. The height of line-shaped protrusions was set to 30 μm (average height: 30 μm). The distance between two sides facing each other out of the sides constituting each regular hexagonal mesh was set to about 2.25 mm. The width of the line-shaped protrusions was set to 0.25 mm. The lacking parts each had a shape with a size of 0.25 mm×0.25 mm in a plan view. The area S1 of the spacer was 16.7% of the area S0 of the separator.
(4) Preparation of non-Aqueous Electrolyte
Ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of EC:DMC=30:70. LiPF6 and LiBF2(C2O4) were dissolved in the obtained solvent mixture at concentrations of 1 mol/L and 0.1 mol/L, respectively, to prepare a liquid non-aqueous electrolyte.
An A1 tab was attached to the positive electrode obtained as described above. An Ni tab was attached to the negative electrode described above. The positive electrode and the negative electrode were spirally wound in an inert gas atmosphere with the separator interposed therebetween to form a wound electrode group. At this time, the separator was disposed in such a manner that the spacer faced the positive electrode.
The obtained electrode group was housed in a pouch-like outer case formed of a laminate sheet including an A1 layer, the above-described non-aqueous electrolyte was then poured into the outer case, and thereafter, the outer case was sealed. Thus, the lithium secondary battery A1 was produced.
A battery A2 was produced under the same conditions using the same method as those used to produce the battery A1, except that the pattern of the spacer was changed. In the battery A2, a spacer having the arrangement pattern shown in
A battery A3 was produced under the same conditions using the same method as those used to produce the battery A1, except that the pattern of the spacer was changed. In the battery A3, a spacer having the arrangement pattern shown in
A battery C1 was produced under the same conditions using the same method as those used to produce the battery A1. except that the pattern of the spacer was changed. In the battery C1, a spacer constituted of a plurality of line-shaped protrusions arranged in a stripe pattern was used. Each line-shaped protrusion was arranged so as to extend along the width direction (a direction orthogonal to the longitudinal direction LD) of the separator. The distance between adjacent line-shaped protrusions was set to 2.25 mm. The height and width of the line-shaped protrusions were the same as those in the separator included in the battery A1. The area S1 of the spacer was 20% of the area S0 of the separator.
A battery C2 was produced under the same conditions using the same method as those used to produce the battery A1, except that the pattern of the spacer was changed. A spacer used in the battery C2 was the same as the spacer included in the battery A1, except that the lacking parts were not formed. That is, the spacer used in the battery C2 had a honeycomb shape. The area S1 of the spacer was 21% of the area S0 of the separator.
In a battery C3, a spacer was formed on the positive electrode, rather than being formed on the separator. The spacer was formed using the same method as that used to form the spacer included in the battery A1. The arrangement pattern and size of the spacer were the same as those of the spacer included in the battery C1.
In a battery C4, a spacer was formed on the positive electrode, rather than being formed on the separator. The spacer was formed using the same method as that used to form the spacer included in the battery A1. The arrangement pattern and size of the spacer were the same as those of the spacer included in the battery C2.
The plurality of batteries produced as described above were evaluated using the following methods.
A charge-discharge test was performed on the obtained batteries. In the charge-discharge test, each battery was charged under conditions described below, and after left to stand for 20 minutes, discharged under conditions described below, in a constant-temperature bath at 25° C.
Constant-current charging was performed until the battery voltage reached 4.1 V at a current of 2.15 mA per unit area (square centimeters) of the electrodes, and then constant-voltage charging was performed at the voltage of 4.1 V until the current value per unit area of the electrodes reached 0.54 m.A.
Constant-current discharging was performed at a current of 2.15 mA per unit area of the electrodes until the battery voltage reached 3.75 V.
The charging and discharging described above were taken as one cycle, and the produced batteries were subjected to the first cycle and charging in the second cycle. Each of the batteries prior to being subjected to the charging and discharging and the batteries subjected to the first cycle and charging in the second cycle was disassembled to take out a stack constituted of the positive electrode, the negative electrode, and two separators on both sides of the negative electrode. The batteries were disassembled in an inert gas atmosphere. The obtained stack constituted of the positive electrode, the negative electrode, and the separators was washed with dimethyl carbonate and then dried, and the thickness of the stack was measured. The thickness of the stack was measured with use of a Peacock digital thickness gauge G2-205M. The thickness was measured at five arbitrarily selected points on the stack, and an arithmetic average of the five measured values was taken as an average thickness. Then, thicknesses of the two substrates and thicknesses of the two heat-resistant layers were subtracted from the average thickness to obtain an average thickness X. The percentage (%) of the average thickness X in the second cycle to the average thickness X prior to the charging and discharging was taken as an expansion rate of the electrode group. That is, the expansion rate (%) of the electrode group is the percentage of the average thickness X in the second cycle when the average thickness X prior to the charging and discharging is taken as 100%.
The charging and discharging described above were taken as one cycle, and each of the produced batteries was subjected to the 1st to 50th cycles of the charging and discharging. The percentage (%) of a discharge capacity at the 50th cycle to a discharge capacity at the Ist cycle was determined as the capacity retention rate (%). Also, the discharge capacity at the 1st cycle was taken as an initial discharge capacity.
Table 1 shows some of the production conditions and evaluation results of the batteries. In Table 1, initial discharge capacities are shown as relative values when the initial discharge capacity of the battery C3 is taken as 100. The higher the initial discharge capacity, the better, and the higher the discharge capacity retention rate, the better. The lower the expansion rate of the electrode group, the better.
The batteries A1 to A3 are batteries according to the present disclosure, and the batteries C1 to C4 are batteries of Comparative Examples. The batteries A1 to A3 had high initial discharge capacities, high discharge capacity retention rates, and low expansion rates of the electrode group. In contrast, the battery C1, in which the spacer was formed in a stripe pattern, had a high expansion rate of the electrode group. This is presumably because, when the electrode plates and the separator were wound, the spacer was displaced and a sufficient space could not be formed between the electrode plates. The battery C2, in which a spacer not including lacking parts was used, had a low discharge capacity retention rate. This is presumably because movement of the non-aqueous electrolyte was hindered by the spacer. The batteries C3 and C4, in which the spacer was formed on the positive electrode, had low initial discharge capacities. This is presumably because a portion of the positive electrode active material was not effectively used due to the presence of the spacer.
The present disclosure is applicable to a lithium secondary battery. For example, a lithium secondary battery according to the present disclosure is applicable to electronic devices such as cellular phones, smart phones, and tablet terminals, electric vehicles including hybrids and plug-in hybrids, and home storage battery systems combined with a solar cell.
Although the present invention has been described in terms of the presently preferred embodiments, such a disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains, after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications that fall within the true spirit and scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-162246 | Sep 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/035301 | 9/22/2022 | WO |
