Patent Application
20150004474
1. Field
The described technology generally relates to a secondary battery.
2. Description of the Related Technology
Batteries are widely used for mobile devices as sources of power. Secondary batteries can be reversibly charged and discharged multiple times. Secondary batteries can also be recycled, and thus may be used efficiently. Also, secondary batteries may have various shapes depending on the external electronic devices employing the secondary batteries. Since secondary batteries can effectively store energy considering their volume and mass, secondary batteries are widely used as sources of power for mobile electronic devices.
Due to the development of mobile communication devices, the need for secondary batteries employed in these communication devices is increasing. Accordingly, research on improving the reliability of secondary batteries such as life characteristics, etc. is being conducted.
The above information is designed to assist in understanding the disclosed technology and therefore it may contain information that does not constitute prior art.
One inventive aspect is a secondary battery which may have an improved reliability.
Another aspect is a method of manufacturing a secondary battery including: assembling a secondary battery by storing an electrode assembly in a battery case with an electrolyte including lithium salt, the electrode assembly consisting of a first pole plate, a second pole plate and a separator between the first pole plate and the second pole plate; precharging the secondary battery; storing the precharged secondary battery at room temperature; and performing a first charging and a first discharging of the secondary battery multiple times.
The precharging step includes charging with a constant current-constant voltage (CC-CV) to a range of about 3.5V to about 3.8V at about 0.2 C to about 1.0 C. Here, 1.0 C refers to charging with a current such as the rated power of the secondary battery.
The ceiling voltage of the positive active material may be about 4.2V.
The first charging step may include charging with a CC-CV to a range of about 3.5V to about 3.8V at about 0.2 C to about 1.0 C. Here, 1.0 C refers to charging with a current such as the rated power of the secondary battery.
Also, the first charging step may include charging with a CC-CV up to the range of about SOC 10% to about SOC 30% at about 0.2 C to about 1.0 C.
In the first discharging step, the secondary battery may be discharged with a constant current (CC) to the range of about 2.6V to about 2.8V at about 0.5 C. Here, 1.0 C refers to discharging with a current such as the rated power of the secondary battery.
The first charging and a first discharging may each be performed three times or more. Also, the first charging and the first discharging may each be performed three times to five times.
The first charging may be performed to a SOC in the range of about 20% to about 30%. The first charging and a first discharging may be performed three times or more.
Another aspect is a secondary battery including a first pole plate at least partially coated with a first active material over a first substrate; a second pole plate at least partially coated with a second active material over a second substrate; an electrode assembly consisting of a separator between the first pole plate and the second pole plate; an electrolyte including lithium salt; and a battery case storing the electrode assembly and the electrolyte, wherein the lithium salt is chemically bonded to an outermost side of the second pole plate.
An SEI film may be provided on a surface of the second active material of the second pole plate, and the lithium salt may be bonded on an outer side of the SEI film.
The lithium salt may include at least one of LiPF6, LiBF4, LiBETI, LiBOB, LiFAP or LiTFSI.
The lithium salt may be LiPF6.
The SEI film may include reduced LiPF6 including a P—F bonded specie having a binding energy in the range of about 686 eV to about 688 eV and LiPF6 including a P—F bonded specie having a binding energy in the range of about 688 eV to about 690 eV.
The first active material may be a positive active material including a lithium compound, and the second active material may be a negative active material including carbon.
The accompanying drawings, together with the specification, illustrate exemplary embodiments of the described technology, and, together with the description, serve to explain the principles of the described technology.
a is a schematic diagram illustrating a cross section of a typical negative active material.
b is a schematic diagram illustrating a cross section of a negative active material according to an embodiment.
a and 4b are SEM pictures of SEI films that are SOC 10% charged at about 0.5 C.
a and 5b are SEM pictures of SEI films that are SOC 20% charged at about 0.5 C.
a and 6b are SEM picture of SEI films that are SOC 30% charged at about 0.5 C.
a is an XPS graph illustrating the composition of an SEI film that is charged one time at about 0.5 C.
b is an XPS graph illustrating the composition of an SEI film charged three times at about 0.5 C.
a is an XPS graph illustrating the composition of an SEI film charged one time at about 0.2 C.
b is an XPS graph illustrating the composition of an SEI film charged three times at about 0.2 C.
In the following detailed description, only certain exemplary embodiments of the described technology have been shown and described, simply by way of illustration. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the described technology. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. In addition, when an element is referred to as being “on” another element, it can be directly on the other element or be indirectly on the other element with one or more intervening elements interposed therebetween. Also, when an element is referred to as being “connected to” another element, it can be directly connected to the other element or be indirectly connected to the other element with one or more intervening elements interposed therebetween. Hereinafter, like reference numerals refer to like elements.
In the embodiment illustrated in
In the assembling of the secondary battery (S1), the secondary battery may include the first and second pole plates having different polarities and the separator may be configured to prevent shorting from occurring between the first and second pole plates due to direct contact between the first and second pole plates. The first pole plate may be formed by coating a first active material on a metal substrate. The first active material may be a positive active material, and the positive active material may be a lithium compound including lithium cobalt oxide LiCoO2. The second pole plate may be formed by coating a second active material on a metal substrate. The second active material may be a negative active material, and the negative active material may include carbon, etc. The separator may be an electrically insulative thin film having high ion permeability and mechanical strength. The separator may include a porous film, felt, etc. including polyethylene, polypropylene or poly vinylindene fluoride.
The secondary battery may be manufactured by receiving the electrode assembly and the electrolyte in the battery case with the battery case being sealed. The first and second pole plates and the separator may be wound or stacked to form the electrode assembly. The electrolyte may enable ion (such as lithium ions, etc.) mobility. The electrolyte may further include lithium salt or an additive that acts as a supply source of lithium ions in the secondary battery. The electrolyte may be a nonaqueous organic solvent, and the nonaqueous organic solvent may be at least one solvent selected from the group of a linear ester and a ring-shaped ester. The linear ester may be at least one linear ester selected from the group of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, dipropyl carbonate, dibutyl carbonate, methyl acetate, ethyl acetate, methyl hexanoate, and methyl formate or any combination thereof. In addition, the ring-shaped ester may be selected from the group of ethylene carbonate, propylene carbonate, butylene carbonate, γbutyrolactone, γ-valerolactone, γ-carprolactone, δ-valerolactone and ε-carprolactone. The lithium salt in the electrolyte may include at least one lithium salt selected of LiPF6, LiBF4, LiBETI, LiBOB, LiFAP and LiTFSI. The additive may be added to improve the reliability or safety, etc. of the secondary battery.
The precharging of the secondary battery (S2) may be performed after the assembly of the secondary battery (S1). Precharging may be the first step in charging the assembled secondary battery, and a solid electrode interface (SEI) film may be formed for the first time on the surface of the second pole plate. The SEI film may prevent copper dissolution from the metal plate of the second pole plate, which may be, for example, a metal substrate formed of copper. The precharging may CC-CV charge the secondary battery to a voltage of about 3.5V to 3.8V at a current of about 0.2 C to 1.0 C. Here, 1.0 C refers to charging with the same current as the rated power of the secondary battery. For example, in the case the rated power of the secondary battery is 200 mAh, 1.0 C refers to 200 mA, and 0.2 C refers to 40 mA. That is, in the case the secondary battery is rated at 200mAh, the precharging may be performed by applying a charging current in the range of about 40 mA to about 200 mA. However, the SEI film that is formed in the precharging step (S2) is not completely formed. While the incomplete SEI may not be able to perform all of the substantive functions of a complete SEI film which will be described later, the incomplete SEI film prevents metal from flowing out into the electrolyte from the metal substrate of the second pole plate.
After precharging is completed in the secondary battery, the secondary battery may go through the step of being stored at room temperature (S3). In the step of storing the secondary battery at room temperature (S3), the secondary battery may be left for a predetermined time at room temperature. The electrolyte in the secondary battery may be absorbed by and disposed on the first and second pole plates, allowing lithium ions to move effectively. Also, defects in the secondary battery, such as a tear fluid of the electrolyte or a thickness defect due to a swelling phenomenon, etc., may be sorted out during the predetermined storage time.
The secondary battery that has been stored at room temperature may go through a first charge and a first discharge step (S4). In this step, the secondary battery that has been stored at room temperature may be first charged and first discharged multiple times. The first charge may include CC-CV charging to a voltage of about 3.5V to 3.8V with a current of about 0.2 C to 1.0 C, and the first discharge may include CC discharging to a voltage about 2.6V to 2.8V at a current of about 0.5 C. 1.0 C refers to charging with a current such as the rated power of the secondary battery.
The SEI film may be formed as the negative active material of the second pole plate and the electrolyte react. The precharging step (S2) may be terminated before the negative active material having a complex structure and the electrolyte including a plurality of lithium salts or additives are sufficiently absorbed in the negative active material. As a result, the SEI film formed in the precharging step (S2) may not be completely formed. As such, the incompletely formed SEI film may have a negative impact on the characteristics of the secondary battery and may cause problems to the secondary battery life, capacity, reliability, and safety due to overcharging, etc. Accordingly, since the incomplete formation of the SEI film may be prevented by the steps of storing the secondary battery at room temperature (S3) and first charging and first discharging after the electrolyte is sufficiently absorbed in the negative active material of the second pole plate (S4), performance of the secondary battery may be improved.
Generally, after leaving or storing the secondary battery at room temperature, a formation process is performed in which the SEI film is additionally formed by fully charging up to the ceiling voltage, for example, about 4.2V, of the positive active material, at about 1 C. The second pole plate of the secondary battery may be formed structurally adjacent to the first pole plate and the separator, and the negative active material of the second pole plate may include a conductive agent and a binder, formed to have a complex and random network structure. Therefore, it may be difficult to form a substantially uniform SEI film on the surface of the second pole plate with only a single charge for the negative active material to react with the electrolyte. In addition, it may take a long time to charge the secondary battery up to about 4.2V, which reduces the productivity of the manufacturing of the secondary battery.
In a method of manufacturing a secondary battery according to an embodiment, the SEI film of the secondary battery may be formed by going through the first charge and first discharge step after the step of storing the secondary battery at room temperature. By performing the step of the first charge and first discharge multiple times, the SEI film of the secondary battery may be made substantially uniform. In addition, if the first charge is performed up to about 3.5V to about 3.8V and is CC-CV charged at about 0.2 C to 1.0 C, the charging time of the secondary battery may be reduced. Furthermore, the first discharge may be performed to about 2.6V to about 2.8V and may be CC discharged at about 0.5 C. The first charge and the first discharge may be each performed three times or more, and the first charge and the first discharge may be each performed three times to five times. In the case that the first charge and the first discharge are performed fewer than three times, the SEI film may not be made substantially uniform, which reduces the performance of the secondary battery. Accordingly, the first charge and the first discharge may be performed three times or more. However, since the SEI film may not go through a great change after only the first charge and the first discharge an unnecessary film deterioration may occur in extreme cases, thus the first charge and the first discharge may be performed between three and five times, in consideration of the performance and productivity of the secondary battery.
The secondary battery according to an embodiment may include an electrode assembly including a first pole plate coated with a first active material at least partially on a first substrate, a second pole plate coated with a second active material at least partially on a second substrate and a separator between the first and second pole plates; an electrolyte including lithium salt; and a battery case may be configured to receive the electrode assembly and the electrolyte, wherein at an outermost surface of the second pole plate may include the lithium salt chemically bonded thereto. In addition, an SEI film is provided on a surface of the second active material of the second pole plate, and the lithium salt may be bonded to an outer surface of the SEI film.
The first pole plate may be a positive plate, the first substrate may be a metal substrate acting as a positive current collector, and the first active material may be a positive active material. In addition, the second pole plate may be a negative plate, the second substrate may be a metal substrate acting as a negative current collector, and the second active material may be a negative active material.
a and 2b are schematic diagrams illustrating, using the same secondary battery, the surface A of the negative active material that has been fully charged using a different charging method after the secondary battery has been stored or left at room temperature.
Furthermore, the outermost surface of the SEI film B that has been charged three times up to about 3.5V to about 3.8V may be modified (B′). In the case that the lithium salt C is LiPF6, the lithium salt C may exist as Li+ and PF6− ions in the electrolyte, and the Li+ and PF6− ions may be provided on the surface of the SEI film B due to a chemical bond therebetween. The bond strength between P—F for Li+ and PF6− ions on the surface of the SEI film B is used in binding some of the ions to the SEI film. Therefore, the bond strength may have a binding energy weaker than the bond strength between P—F of Li+ and PF6− ions that are not bonded to the SEI film. Here, in order to distinguish the bonded lithium salt over ordinary lithium salt, the lithium salt that is chemically bonded to the SEI film B is referred to as reduced lithium salt. For example, generally, LiPF6 (normal) is referred to as LiPF6, but when it is bonded to the SEI film, it is referred to as reduced LiPF6.
The SEI film may be a nonconductor. After it is formed, the SEI film may act to prevent a side reaction between lithium ions and other material at the surface of the negative active material when being charged. The SEI film may act as a type of an ion tunnel and may selectively allow only lithium ions to pass. Accordingly, a reaction due to contact between an organic solvent having a great molecule weight which is a type of electrolyte that aids in lithium ion mobility and the negative active material may be prevented, and therefore, destruction of the structure of the negative active material may be avoided. That is, the SEI film may prevent a side reaction between the negative active material and materials other than lithium ions at the surface of the negative active material, and thus the effectiveness of lithium ions may be enhanced. Also, since the structure of the negative active material formed of carbon, etc. may be substantially maintained, the life of the secondary battery may be increased by effectively reversibly charging and discharging the secondary battery. By modifying the SEI film in this way, uniformity and physical properties of the SEI film may be improved.
Referring to
As previously described, only charging the secondary battery one time in order to form the SEI film may result in an incomplete formation of an SEI film due to the structural characteristics of the secondary battery, for example, the structure of the negative active material and the complex structure of the electrolyte. Therefore, in the charging for forming the SEI film, the secondary battery may be repeatedly charged up to about 3.5V to about 3.8V three times or more, thereby improving the uniformity of the SEI film, and by modifying the outermost side of the SEI film, the performance of the secondary battery may be improved.
The charged voltage of the secondary battery may be expressed as a state of charge (SOC) which is the remaining capacity, i.e. the charge state of the secondary battery. In the case that the secondary battery is charged with substantially the same voltage as the ceiling voltage of the positive active material, it may be referred to as SOC 100% of the secondary battery. For instance, in a secondary battery where a lithium compound is used as the positive active material, the ceiling voltage may be about 4.2V. Here, when the secondary battery is charged to about 4.2V, the SOC of the secondary battery is 100%. Furthermore, SOC 0% may mean the lowest voltage from the range of voltages actually used in the secondary battery. In the case that the secondary battery is discharged down to about 2.75V, the voltage of the secondary battery may be restored to a voltage higher than about 2.75V due to a spring back phenomenon. As such, the voltage of the secondary battery, after restoration of the voltage due to the spring back phenomenon immediately after the discharge by a charge and discharge system, is SOC 0%. As such, there are differences in the characteristics of some of the secondary batteries with respect to a voltage of SOC 0%. For example, in a secondary battery that uses a lithium compound as the positive active material and has a ceiling voltage of about 4.2V, the voltage values of the secondary battery for SOC 10%, SOC 20% and SOC 30% may be about 3.5V, about 3.7V and about 3.8V, respectively. Accordingly, the first charge may be CC-CV charged to the range of about SOC 10% to SOC 30% with a current in the range of about 0.2 C to about 1.0 C.
In the present disclosure, the terms “Example,” and “Comparative Example” are used arbitrarily to simply identify a particular example or experimentation and should not be interpreted as admission of prior art. While this disclosure has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the present embodiments is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The comparative example will be compared with the embodiments of the described technology in each of the following sections.
1. Assembling, precharging and leaving or storing the secondary battery at room temperature.
The positive plate was manufactured by making the positive active material including the lithium compound in a slurry state and coating the slurry on the metal substrate including an aluminum (Al) sheet. The negative plate was manufactured by making the negative active material including carbon in a slurry state and coating the slurry on the metal substrate including a copper (Cu) sheet. The electrode assembly was manufactured by interposing the separator between the positive plate and the negative plate and winding it. The manufactured electrode assembly was put in the battery case, the electrolyte and the lithium salt were injected and the battery case was sealed. Here, a mixture of ethylene carbonate (EC), ethylene propylene (EP) and diethyl carbonate (DEC) at a volume ratio of 3:5:2 was used as the electrolyte, and 1.0M of LiPF6 of lithium salt was used.
Using a charge and discharge regulator, the secondary battery was charged up to about 3.5V at about 0.5 C using the CC-CV method. Here, 1.0 C is a current that is substantially the same as the rated power of the secondary battery, and the precharging was performed by charging with such a current. The secondary battery for which the precharging was completed was left at room temperature for one day. Hereinafter, the manufacturing process may be performed by performing the first charge and the first discharge and varying the SOC and the number of cycles of first charging and discharging of the secondary battery. The secondary battery according to the comparative example A and embodiments B to M may be manufactured as shown in Table 1. Each of A to M may include a plurality of secondary batteries, and the following experiment was carried out.
In Table 1 above, A is the comparative example, B to M are the results obtained by performing formation charging and discharging which is the first charge and the first discharge by varying the charging current, the SOC and the number of charge cycles. Table 1 lists the charge current (C-rate), the SOC and the number of cycles (where {circle around (1)} refers to one cycle, and {circle around (3)} refers to three cycles). That is, a different name for B is (0.5 C) SOC10-{circle around (1)}. B is SOC 10% charged, one cycle only, with a 0.5 C charging current.
2. Determination of SEI film surface from repeated SOC and charge and discharge of the secondary battery.
a and 4b are SEM pictures of SEI films that are SOC 10% charged at about 0.5 C.
In
3. Determination of the organic matter composition of the SEI film according to repeated SOC and charge and discharge of the secondary battery.
a is an XPS graph illustrating the composition of an SEI film charged one time at about 0.5 C.
a illustrates XPS (C1s, C1s peak) of the SEI film of A, B, D and F in Table 1.
a illustrates the XPS (C1s, C1s peak) of the SEI film of A and H, J and L in Table 1, and
4. Determination of reduced LiPF6 of the SEI film according to repeated SOC and charge and discharge of the secondary battery.
Referring to
Referring to
5. Content analysis of (normal) LiPF6 and reduced LiPF6 in the SEI film of the secondary battery.
On the other hand, in the case of A that is charged to SOC 100% at about 1 C, there was a similar ratio of about 8:2 of reduced LiPF6: (normal) LiPF6 as in C, E and G and I, K and M which were repeatedly charged three times. However, A was similar to C, E and G and I, K and M which were repeatedly charged three times at a low voltage only in terms of the ratio of 8:2 of reduced LiPF6: (normal) LiPF6. As shown in
Referring to
Referring to
4. Determination of life characteristics according to repeated SOC and charging and discharging of the secondary battery.
Referring to
That is, as described earlier, by performing the first charge and the first discharge three times or more, the SEI film characteristics as well as the life characteristics can be improved. Also, in formation charging, the secondary battery is charged with a low voltage, low SOC, and consequently the time for formation process can be reduced, which improves productivity of the manufacturing.
By way of summation and review, the secondary battery in various embodiments has improved life characteristics and reliability.
Also, the secondary battery has an improved SEI film.
Furthermore, a formation process used in manufacturing the secondary battery is disclosed.
While the present invention has been described in connection with certain exemplary embodiments, it will be appreciated by those skilled in the art that various modifications and changes may be made without departing from the scope of the present disclosure. It will also be appreciated by those of skill in the art that parts included in one embodiment are interchangeable with other embodiments; one or more parts from a depicted embodiment can be included with other depicted embodiments in any combination. For example, any of the various components described herein and/or depicted in the Figures may be combined, interchanged or excluded from other embodiments. With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity. Thus, while the present disclosure has described certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof.
Any and all applications for which a foreign or domestic priority claim is identified in the Application Data Sheet as filed with the present application, are hereby incorporated by reference under 37 CFR 1.57. For example, this application claims priority to and the benefit of Provisional Patent Application No. 61/841,851 filed on Jul. 1, 2013 in the U.S Patent and Trademark Office, the disclosure of which is incorporated in its entirety herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61841851 | Jul 2013 | US |
