Patent Application
20240387871
This nonprovisional application is based on Japanese Patent Application No. 2023-082005 filed on May 18, 2023, with the Japan Patent Office, the entire contents of which are hereby incorporated by reference.
The present disclosure relates to a non-aqueous electrolyte secondary battery, and it also relates to a battery pack and a battery module including the same.
Aiming at improving post-endurance input-output properties and impedance properties of non-aqueous electrolyte secondary batteries, Japanese Patent Laying-Open No. 2013-152956 suggests a non-aqueous electrolyte solution that includes LiPF6 and LiFSO3, and Japanese Patent Laying-Open No. 2011-187440 suggests limiting the ratio of the molar content of FSO3 to the molar content of PF6 to a certain range.
When a non-aqueous electrolyte secondary battery that includes a wide electrode assembly (for instance, one that has an active material layer with a longitudinal dimension of 15 cm or more) is applied with pressure in the electrode plate stacking direction of the electrode assembly, salt concentrations during high-rate cycles tend to be nonuniform, the electric potential of the positive electrode tends to rise greatly, and capacity degradation tends to occur. When an electrolyte solution that includes LiFSO3 is used, SO3F− tends to be adsorbed on the surface of the positive electrode active material, and formation of a film of LiF derived from electrolyte salts tends to be inhibited: and when the cell size is large, salt concentrations during high-rate charging cycles tend to be markedly nonuniform and salt concentrations at an end of an electrode plate tend to be decreased. As a result, the electric potential at an end of the positive electrode plate rises to reach an electric potential at which the positive electrode active material reacts with strong-acid SO3F−, and thereby, from the positive electrode active material which is not sufficiently covered with a LiF film, transition metal elutes to be deposited on the negative electrode plate.
An object of the present disclosure is to provide a non-aqueous electrolyte secondary battery that comprises a wide electrode assembly to which pressure is applied in the electrode plate stacking direction of the electrode assembly and that is low in output resistance and reduced in cycling performance degradation, as well as a battery pack and a battery module including the same.
The present disclosure provides a non-aqueous electrolyte secondary battery, a battery pack, and a battery module as described below.
[1] A non-aqueous electrolyte secondary battery comprising:
[2] The non-aqueous electrolyte secondary battery according to [1], wherein the A/B ratio is from 6.7 to 10.
[3] The non-aqueous electrolyte secondary battery according to [1] or [2], wherein the total concentration of LiPF6 and LiBF4 in the electrolyte solution defined as A is from 1 to 1.5 mol/L.
[4] The non-aqueous electrolyte secondary battery according to any one of [1] to [3], wherein the concentration of LiFSO3 in the electrolyte solution defined as B is from 0.1 to 0.25 mol/L.
[5] The non-aqueous electrolyte secondary battery according to any one of [1] to [4], wherein a length of the electrode assembly in a direction parallel to the axis of winding is 180 mm or more.
[6] A battery pack comprising the non-aqueous electrolyte secondary battery according to any one of [1] to [5].
[7] A battery module comprising the non-aqueous electrolyte secondary battery according to any one of [1] to [5].
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.
Next, a description will be given of embodiments of the present invention with reference to drawings, but the below embodiments do not limit the scope of the present invention. In each drawing below, the scale has been changed as appropriate for the purpose of assisting the understanding of the components; therefore, the scale of the components in the drawings may not necessarily coincide with the actual scale of the components.
Herein, a singular form also includes its plural meaning, unless otherwise specified. For example, “a particle” may mean not only “one particle” but also “a group of particles (powder, particles)”.
Battery 100 includes an electrode assembly 50 and an electrolyte solution. As illustrated in
Exterior package 90 may include a sealing plate 91 and an exterior container 92, for example. Sealing plate 91 closes the opening of exterior container 92. Sealing plate 91 and exterior container 92 may be bonded together by laser processing and/or the like, for example. The configuration of exterior package 90 is not particularly limited. For example, exterior package 90 may be in the shape of a pouch. More specifically, exterior package 90 may be a pouch made of Al-laminated film, and/or the like.
To sealing plate 91, a positive electrode terminal 81 and a negative electrode terminal 82 are provided. To sealing plate 91, an inlet (not illustrated), a gas-discharge valve (not illustrated), and/or the like may further be provided. Through the inlet, the electrolyte solution may be injected into exterior package 90. The inlet may be closed with a plug and/or the like, for example. Positive electrode terminal 81 is connected with electrode assembly 50 by a positive electrode current-collecting member 71. Positive electrode current-collecting member 71 may be an Al plate and/or the like, for example. Negative electrode terminal 82 is connected with electrode assembly 50 by a negative electrode current-collecting member 72. Negative electrode current-collecting member 72 may be a copper (Cu) plate and/or the like, for example.
Electrode assembly 50 is a wound-type electrode assembly that is formed by winding a positive electrode plate and a negative electrode plate together with a separator interposed therebetween. The positive electrode plate, the negative electrode plate, and the separator may form a stack that has a belt-like planar shape, for example. This belt-like stack may be wound spirally to form a wound assembly. The wound assembly may be tubular, for example. This tubular wound assembly may be compressed in a radial direction to form a flat electrode assembly 50. The dimension of electrode assembly 50 in the direction of the axis of winding (hereinafter also called the widthwise direction) (the W-axis direction in
The number of positive electrode plates 10 stacked in electrode assembly 50 is not particularly limited. The number of the stacked positive electrode plates 10 refers to the number of times a straight line crossing electrode assembly 50 in the stacking direction crosses positive electrode plates 10. The stacking direction refers to the direction in which positive electrode plate 10, negative electrode plate 20, and separator 30 are stacked in electrode assembly 50. The stacking direction in wound-type electrode assembly 50 is parallel to the direction of the thickness of positive electrode plate 10, negative electrode plate 20, and separator 30 at flat portion 52 (the D-axis direction in
Electrode assembly 50 can be produced in the following way: positive electrode plate 10 and negative electrode plate 20 are stacked with separator 30 interposed therebetween in such a manner that an aluminum foil of the positive electrode plate and a copper foil of the negative electrode plate are exposed at respective ends, as illustrated in
The number of the stacked positive electrode plates 10 may be from 2 to 100, for example. The number of the stacked negative electrode plates 20 may be from 2 to 100, for example. The number of the stacked separators 30 may be from 4 to 200, for example. The number of the stacked negative electrode plates 20 and the number of the stacked separators 30 may be counted in the same manner as in the counting of the number of the stacked positive electrode plates 10.
Positive electrode plate 10 comprises a positive electrode active material layer. In
Negative electrode plate 20 comprises a negative electrode active material layer. In
Positive electrode plate 10 includes a positive electrode core material 11 and a positive electrode active material layer 12 (see
The thickness of positive electrode active material layer 12 refers to the total thickness of positive electrode active material layer(s) 12 included in stack 40. For example, when positive electrode active material layer 12 is formed on both sides of positive electrode plate 10, the thickness of positive electrode active material layer 12 refers to the total thickness of positive electrode active material layers 12 on both sides (namely, two of them). The thickness of positive electrode active material layer 12 may be from 10 to 260 μm, or may be from 20 to 60 μm, or may be from 30 to 50 μm, for example. It should be noted that the thickness of positive electrode active material layer 12 on one side (namely, one piece of it) may be from 5 to 130 μm, or may be from 10 to 30 μm, or may be from 15 to 25 μm, for example.
Positive electrode active material layer 12 can include a lithium-(transition metal) composite oxide. The lithium-(transition metal) composite oxide includes, for example, at least one selected from the group consisting of LiCoO2, LiNiO2, LiMnO2, LiMn2O4, Li (NiCoMn) O2, Li (NiCoAl) O2, and LiFePO4. In a composition formula such as “Li (NiCoMn)O2”, for example, the constituents within the parentheses are collectively regarded as a single unit in the entire composition ratio. That is, the relationship of “CNi+CCo+CMn=1” is satisfied. For example, “CNi” refers to the composition ratio of Ni. As long as (NiCoMn) is collectively regarded as a single unit in the entire composition ratio, the amounts of individual constituents are not particularly limited. Positive electrode active material layer 12 can include positive electrode active material particles. The positive electrode active material particles may include any component. The positive electrode active material particles may include the above-described lithium-(transition metal) composite oxide.
For example, positive electrode active material layer 12 may further include a conductive material, a binder, and/or the like, in addition to the positive electrode active material particles. For example, positive electrode active material layer 12 may consist essentially of a conductive material in an amount from 0.1 to 10% and a binder in an amount from 0.1 to 10% in terms of mass fraction, with the remainder being made up of the positive electrode active material particles. The conductive material may include a carbon material and/or the like, for example. The binder may include any component. The binder may include polyvinylidene difluoride (PVdF) and/or the like, for example. The packing density of positive electrode active material layer 12 (after compression) may be from 2.0 g/cm3 to 4.0 g/cm3, for example.
Negative electrode plate 20 includes a negative electrode core material 21 and a negative electrode active material layer 22 (see
The thickness of negative electrode active material layer 22 refers to the total thickness of negative electrode active material layer(s) 22 included in stack 40. For example, when negative electrode active material layer 22 is formed on both sides of negative electrode plate 20, the thickness of negative electrode active material layer 22 refers to the total thickness of negative electrode active material layers 22 on both sides (namely, two of them). The thickness of negative electrode active material layer 22 may be from 10 to 260 μm, or may be from 40 to 80 μm, or may be from 50 to 70 μm, for example. It should be noted that the thickness of negative electrode active material layer 22 on one side (namely, one piece of it) may be from 5 to 130 μm, or may be from 20 to 40 μm, or may be from 25 to 35 μm, for example. The packing density of negative electrode active material layer 22 (after compression) may be from 1.0 g/cm3 to 1.8 g/cm3, for example.
Negative electrode active material layer 22 may include, for example, at least one selected from the group consisting of graphite, silicon, silicon oxide, tin, tin oxide, and Li4Ti5O12, as a negative electrode active material. Negative electrode active material layer 22 can include negative electrode active material particles. The negative electrode active material particles may include the above-described negative electrode active material. Negative electrode active material layer 22 may consist essentially of the negative electrode active material particles. The negative electrode active material particle may be a composite particle, for example. The negative electrode active material particle may include a base material particle and a film, for example. The surface of the base material particle may be covered with the film. The base material particle may include graphite and/or the like, for example. The film may include amorphous carbon and/or the like, for example.
Negative electrode active material layer 22 may further include a conductive material, a binder, and/or the like, in addition to the negative electrode active material particles. For example, negative electrode active material layer 22 may consist essentially of a conductive material in an amount from 0 to 10% and a binder in an amount from 0.1 to 10% in terms of mass fraction, with the remainder being made up of the negative electrode active material particles. The conductive material may include any component. The conductive material may include a carbon material and/or the like, for example. The binder may include any component. The binder may include at least one selected from the group consisting of carboxymethylcellulose (CMC) and styrene-butadiene rubber (SBR), for example.
Separator 30 includes a resin film. Separator 30 may consist essentially of a resin film. Separator 30 may have a monolayer structure consisting of a single resin film, or may have a multilayer structure consisting of two or more resin films. When separator 30 has a multilayer structure, the resin films may be of different types from each other. The resin film may consist essentially of a polyolefin-based material, for example. The polyolefin-based material may include, for example, at least one selected from the group consisting of polyethylene (PE) and polypropylene (PP). When separator 30 has a multilayer structure, separator 30 may have a three-layer structure consisting of a resin film made of PP, a resin film made of PE, and a resin film made of PP, for example. The resin film may have a thickness from 10 to 50 μm, or may have a thickness from 10 to 30 μm, or may have a thickness from 10 to 20 μm, for example. The resin film may be porous.
The electrolyte solution is a liquid electrolyte. The electrolyte solution includes an electrolyte salt and LiFSO3. In the electrolyte solution, the electrolyte salt may be dissolved in a solvent which is described below.
The electrolyte salt includes at least one of LiPF6 and LiBF4. The electrolyte salt may include LiPF6 alone, or may include LiBF4 alone, or may include both LiPF6 and LiBF4. When the total concentration of LiPF6 and LiBF4 in the electrolyte solution is defined as A (mol/L) and the concentration of LiFSO3 in the electrolyte solution is defined as B (mol/L), the ratio of A to B which is expressed as an A/B ratio is from 5 to 12. When the A/B falls within the above-described range, which means that at least one of LiPF6 and LiBF4 is present in a sufficient amount relative to LiFSO3, it is conjectured that a LiF film is sufficiently formed on the surface of the positive electrode active material and, thereby, transition metal elution from the active material due to LiFSO3 tends not to occur, making it possible to obtain a battery that is capable of exhibiting low output resistance and good cycling performance. The A/B ratio is preferably from 5 to 10, more preferably from 6.7 to 10, from the viewpoints of output resistance and cycling performance.
The total concentration of LiPF6 and LiBF4 in the electrolyte solution, which is defined as A, may be from 1.0 to 1.5 mol/L, for example. The concentration of LiFSO3 in the electrolyte solution, which is defined as B, may be from 0.1 to 0.25 mol/L, for example.
The electrolyte solution can include a solvent. The solvent is aprotic. The solvent may include any component. The solvent can include at least one selected from the group consisting of carbonate-based solvent, 1,2-dimethoxyethane (DME), methyl formate (MF), methyl acetate (MA), methyl propionate (MP), and γ-butyrolactone (GBL). Preferably, the solvent includes a carbonate-based solvent. Examples of the carbonate-based solvent include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and the like.
The battery according to the present disclosure can exhibit a capacity retention of 90% or more in cycling performance evaluation. When the cycle capacity retention is 90% or more, the battery can exhibit good cycling performance. The cycling performance evaluation is carried out by a method that is described below in the Examples section.
A method of producing a battery can include, for example, a placement step that involves placing an electrode assembly in an exterior package, and an injection step that involves injecting an electrolyte solution. In the placement step, an aluminum foil of a positive electrode plate current-collecting member and an aluminum plate of the electrode assembly for external current collection can be welded together, and a copper foil of a negative electrode plate current-collecting member and a copper plate of the electrode assembly for external current collection can be welded together, and then the resultants can be inserted into an exterior package made of an aluminum laminated film. In the injection step, the above-described electrolyte solution can be injected. After injection, sealing and welding can be carried out to obtain a battery.
Batteries 100 are prismatic battery cells, and a plurality of them are provided in the Y-axis direction. Batteries 100 are electrically connected to each other via a bus bar (not illustrated).
Each inter-cell separator 201 is provided between batteries 100. Inter-cell separators 201 prevent unintended electrical connection between batteries 100 that are located adjacent to each other. Inter-cell separators 201 ensure electrical insulation between batteries 100 that are located adjacent to each other.
Batteries 100 and inter-cell separators 201 that are alternately aligned in the Y-axis direction (the first direction) are pressed by end plates 302 and restrained between two end plates 302.
End plates 302 are placed at both ends in the Y-axis direction. End plates 302 are secured to a platform, such as a case, that accommodates battery module 300. Restraint member 301 connects two end plates 302 to one another and restrains batteries 100 and inter-cell separators 201 in the Y-axis direction.
Restraint member 301 is secured to end plates 302 while compression force is being applied in the Y-axis direction to a stack formed of batteries 100, inter-cell separators 201, and end plates 302, and, then, the compression force is relieved, which causes tensile force to be applied to restraint member 301 that connects two end plates 302 to each other. As a reaction to this, two end plates 302 are pressed toward each other by restraint member 301. As a result, battery module 300 is formed.
Battery module 300 is placed inside a pack case to form a battery pack (Cell-Module-Pack structure). Alternatively, a structure in which battery pack 200 illustrated in
In the following, the present invention will be described in further detail by way of Examples. “%” and “part(s)” in Examples refer to mass % and part(s) by mass, respectively, unless otherwise specified.
A composite material for a positive electrode active material layer having a composite material composition of LiNiCoMnO2:AB:pVdF=100:1:1 (wt %) was mixed with NMP to produce a positive electrode composite material slurry. The resultant was applied to an aluminum foil of a positive electrode current collector, dried, and then compressed to a certain thickness and cut into a certain width, and thereby a positive electrode plate was produced that had two portions aligned in the widthwise direction, namely, a portion where a positive electrode active material layer was formed on the aluminum foil and a portion where an active material layer was not formed. The width of the formed positive electrode active material layer was 150 mm.
As a negative electrode active material, graphite was used. A composite material for a negative electrode active material layer having a composite material composition of graphite: SBR:CMC=100:1:1 (wt %) was mixed with water to produce a negative electrode composite material slurry. The resulting negative electrode composite material slurry was applied to a copper foil of a negative electrode current collector, dried, and then compressed to a certain thickness and cut into a certain width, and thereby a negative electrode plate was produced that had a portion where a negative electrode active material layer was formed on the copper foil and a portion where an active material layer was not formed. The width of the formed negative electrode active material layer was 154 mm.
An electrode assembly having a composition specified in
The aluminum foil of the positive electrode plate current collector and an aluminum plate for external current collection were welded together, and the copper foil of the negative electrode plate current collector and a copper plate for external current collection were welded together, and then the resultants were inserted into an exterior package made of an aluminum laminated film, into which an electrolyte solution 1 was injected, followed by sealing the laminated film.
Solvent: EC/EMC (volume ratio, 1:3)
LiPF6 concentration (concentration A): 1 mol/L
LiFSO3 concentration (concentration B): 0.1 mol/L
Then, as illustrated in
A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the width to form the positive electrode active material layer in [Production of Positive Electrode Plate] above was changed to 100 mm, the width to form the negative electrode active material layer in [Production of Negative Electrode Plate] above was changed to 104 mm, and electrolyte solution 1 was replaced by an electrolyte solution 2 described below.
Solvent: EC/EMC (volume ratio, 1:3)
LiPF6 concentration (concentration A): 1 mol/L
LiFSO3: Not used
A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the width to form the positive electrode active material layer in [Production of Positive Electrode Plate] above was changed to 100 mm, the width to form the negative electrode active material layer in [Production of Negative Electrode Plate] above was changed to 104 mm, and electrolyte solution 1 was replaced by an electrolyte solution 3 described below.
Solvent: EC/EMC (volume ratio, 1:3)
LiPF6 concentration (concentration A): 1 mol/L
LiFSO3 concentration (concentration B): 0.08 mol/L
A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the width to form the positive electrode active material layer in [Production of Positive Electrode Plate] above was changed to 100 mm, the width to form the negative electrode active material layer in [Production of Negative Electrode Plate] above was changed to 104 mm, and electrolyte solution 1 was replaced by an electrolyte solution 4 described below.
Solvent: EC/EMC (volume ratio, 1:3)
LiPF6 concentration (concentration A): 1 mol/L
LiFSO3 concentration (concentration B): 0.1 mol/L
A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the width to form the positive electrode active material layer in [Production of Positive Electrode Plate] above was changed to 100 mm, the width to form the negative electrode active material layer in [Production of Negative Electrode Plate] above was changed to 104 mm, and electrolyte solution 1 was replaced by an electrolyte solution 5 described below.
Solvent: EC/EMC (volume ratio, 1:3)
LiPF6 concentration (concentration A): 1 mol/L
LiFSO3 concentration (concentration B): 0.2 mol/L
A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the width to form the positive electrode active material layer in [Production of Positive Electrode Plate] above was changed to 100 mm, the width to form the negative electrode active material layer in [Production of Negative Electrode Plate] above was changed to 104 mm, and electrolyte solution 1 was replaced by an electrolyte solution 6 described below.
Solvent: EC/EMC (volume ratio, 1:3)
LiPF6 concentration (concentration A): 1 mol/L
LiFSO3 concentration (concentration B): 0.25 mol/L
A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that electrolyte solution 1 was replaced by electrolyte solution 2.
A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that electrolyte solution 1 was replaced by electrolyte solution 3.
A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that electrolyte solution 1 was replaced by an electrolyte solution 7 described below.
Solvent: EC/EMC (volume ratio, 1:3)
LiPF6 concentration (concentration A): 1 mol/L
LiFSO3 concentration (concentration B): 0.15 mol/L
A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that electrolyte solution 1 was replaced by electrolyte solution 5.
A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that electrolyte solution 1 was replaced by electrolyte solution 6.
A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that electrolyte solution 1 was replaced by an electrolyte solution 8 described below.
Solvent: EC/EMC (volume ratio, 1:3)
LiPF6 concentration (concentration A): 1.5 mol/L
LiFSO3 concentration (concentration B): 0.25 mol/L
A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that electrolyte solution 1 was replaced by an electrolyte solution 9 described below.
Solvent: EC/EMC (volume ratio, 1:3)
Concentration A=1.0 mol/L
LiPF6 concentration: 0.9 mol/L
LiBF4 concentration: 0.1 mol/L
LiFSO3 concentration (concentration B): 0.2 mol/L
A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that electrolyte solution 1 was replaced by electrolyte solution 7 and battery restraining was not performed.
After the non-aqueous electrolyte secondary battery was produced in the manner described in the Examples, initial activation of the battery was carried out by charging at 4.2 Vcccv in an environment at 25°° C. at a value of current of C/10, storing at 60° C. for 24 hours, and discharging at a value of current of C/10 to 3 V.
In an environment at 25° C., charging was performed at a value of current of C/3 up to SOC of 50%, followed by 30 minutes of rest, and the voltage of the battery was measured (=V0). Subsequently, discharging was performed in an environment at 25°° C. at a value of current of 2 C for 10 seconds. After a lapse of 10 seconds, the voltage of the battery was measured (=V1), and the output resistance of the battery was calculated by the following equation.
The output resistance is shown in Table 1, where the values of Comparative Examples 1 to 5 are relative to the value of Comparative Example 1 which is regarded as 100%, and the values of Examples 1 to 5 and Comparative Examples 6 to 9 are relative to the value of Comparative Example 6 which is regarded as 100%.
After the measurement of output resistance as described above, cycle testing was carried out. The conditions for the cycle testing were as follows: multiple cycles of charging and discharging were carried out, where a single cycle consisted of charging at 4.2 Vcccv at a value of current of 2 C and discharging at a value of current of C/2 to 3 V in an environment of 25° C. The ratio of the discharged capacity at the 500th cycle to the discharged capacity at the first cycle was defined as cycle capacity retention.
Results are shown in Table 1.
In Comparative Examples 1 to 5 where the width of the positive electrode active material layer was 100 mm and thus the width of the electrode assembly was relatively small, a decrease of output resistance and an enhancement of cycle capacity retention were observed as the LiFSO3 concentration increased, but no specific influence of the A/B ratio was observed. It is conjectured that, due to the relatively small width of the electrode assembly, salt concentrations tend to be uniform in the widthwise direction of the electrode assembly even during the charging cycle at a relatively high rate of 2 C, and, as a result, the electric potential of the positive electrode tends not to rise and transition metal elution from the positive electrode active material tends to be low, so, thereby, not only the resistance-lowering effect attributed to LiFSO3 addition was exhibited but also the cycle capacity retention was slightly enhanced. To say the least, in Comparative Examples 1 to 5, a decrease of cycle retention was not observed.
In Comparative Examples 6 to 8 and Examples 1 to 5, the width of the positive electrode active material layer was 150 mm; in Comparative Examples 6 to 8 and Examples 1 to 5 where the width of the electrode assembly was relatively large, a decrease of output resistance and an enhancement of cycle capacity retention tended to be obtained as the LiFSO3 concentration increased. In Examples 1 to 3 where the A/B ratio fell within the range of 5 to 10, the specific effect of lowering output resistance as well as a cycle capacity retention of 90% or more were obtained. Within the A/B ratio range in Examples 1 to 3, a decrease of output resistance attributed to LiFSO3 and an enhancement of cycle capacity retention were obtained. In Comparative Example 8 where the LiFSO3 concentration was relatively high and the A/B ratio was 4.0, cycle capacity retention decreased to a great extent. It is conjectured that because the width of the positive electrode active material layer was as large as 150 mm, charging cycles at a relatively high rate of 2 C caused a marked level of salt concentration nonuniformity in the widthwise direction of the cell, resulting in an increase of the electric potential of the positive electrode during charging. In contrast, in Examples 1 to 3 where the LiPF6 concentration was sufficient relative to the LiFSO3 concentration, it is conjectured that LiF film formation on the surface of the positive electrode active material was sufficient, and, as a result, transition metal elution from the positive electrode active material was reduced and thereby cycle capacity retention was maintained high. In Comparative Example 8 where the LiPF6 concentration was low relative to the LiFSO3 concentration, it is conjectured that LiF film formation on the surface of the positive electrode active material was insufficient, and, as a result, the electric potential rose enough for a reaction to occur between the positive electrode active material and strong-acid SO3F− to allow transition metal to elute from the positive electrode active material at a portion with insufficient LiF film formation and to be deposited on the negative electrode, causing a decrease of capacity retention. In Examples 1 to 3 where the A/B ratio fell within the range of 5 to 10, a decrease of output resistance and a cycle capacity retention of 90% or more were obtained. Good results were also obtained in Example 4 and Example 5.
In Comparative Example 9 where the battery was not restrained, the A/B ratio was 6.7 but output resistance was high and cycle capacity retention was low. It is conjectured that because the battery was not restrained, the flow of electrolyte solution should have been smooth and thereby salt concentrations tended to be uniform during cycle testing, but also because the battery was not restrained, the electrode-to-electrode distance tended to be large, resulting in high resistance and also in a great extent of capacity degradation during cycle testing.
Although the embodiments of the present invention have been described, the embodiments disclosed herein are illustrative and non-restrictive in any respect. The scope of the present invention is defined by the terms of the claims, and is intended to encompass any modifications within the meaning and the scope equivalent to the terms of the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2023-082005 | May 2023 | JP | national |
