Negative electrode sheet, secondary battery including negative electrode sheet, and method for producing negative electrode sheet

Abstract

To suppress reductive decomposition of a nonaqueous solvent and / or a binder in initial charging of a secondary battery using a nonaqueous electrolyte.SOLUTION: A negative electrode sheet for a secondary battery using a nonaqueous electrolyte solution includes a negative electrode active material, a binder, and an SEI forming agent. A standard electrode potential of the SEI-forming agent is higher than a standard electrode potential of a nonaqueous solvent contained in the nonaqueous electrolyte solution.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The technology disclosed in this specification relates to a negative electrode sheet and a method for manufacturing a negative electrode sheet. [Background technology]

[0002] Patent Document 1 describes a negative electrode sheet for a battery cell and a method for manufacturing the negative electrode sheet. This negative electrode sheet includes a negative electrode active material and a binder. The method for manufacturing the negative electrode sheet includes a step of preparing a negative electrode composite mixture by mixing the negative electrode active material and the binder, and a step of preparing a negative electrode sheet from the negative electrode composite mixture. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Special Publication No. 2021-504877 Summary of the Invention [Problem to be solved by the invention]

[0004] The above-described negative electrode sheet is used, for example, in the negative electrode of a lithium-ion secondary battery using a nonaqueous electrolyte. It is known that during initial charging of this secondary battery, the nonaqueous solvent and / or binder in the nonaqueous electrolyte undergo reductive decomposition, forming a solid electrolyte (Solid Electrolyte Interphase: SEI) on the surface of the negative electrode active material. When the SEI is formed on the surface of the negative electrode active material, subsequent reductive decomposition of the nonaqueous solvent and / or binder is suppressed. However, because the formation of the SEI by reductive decomposition of the nonaqueous solvent and / or binder is an irreversible reaction, lithium ions are consumed in the reductive decomposition, resulting in a decrease in the initial charge / discharge efficiency of the secondary battery.

[0005] In view of the above circumstances, the present specification provides a technique for suppressing reductive decomposition of a non-aqueous solvent and / or a binder during initial charging of a secondary battery using a non-aqueous electrolyte. [Means for solving the problem]

[0006] The technology disclosed in this specification is embodied in a negative electrode sheet for a secondary battery using a nonaqueous electrolyte. In a first aspect, the negative electrode sheet includes a negative electrode active material, a binder, and an SEI forming agent. The SEI forming agent has a standard electrode potential higher than the standard electrode potential of a nonaqueous solvent contained in the nonaqueous electrolyte.

[0007] The above-mentioned negative electrode sheet includes an SEI-forming agent, and the standard electrode potential of the SEI-forming agent is higher than the standard electrode potential of the nonaqueous solvent contained in the nonaqueous electrolyte. An SEI-forming agent with a high standard electrode potential is more easily reduced than a nonaqueous solvent with a low standard electrode potential. Therefore, for example, during the initial charge of a secondary battery using the above-mentioned negative electrode sheet as a negative electrode, charging at a relatively low voltage can reductively decompose the SEI-forming agent without reductively decomposing the nonaqueous solvent. By preferentially forming an SEI derived from the SEI-forming agent on the surface of the negative electrode active material, it is possible to suppress the formation of an SEI due to reductive decomposition of the nonaqueous solvent. This can suppress a decrease in the initial charge / discharge efficiency of the secondary battery.

[0008] In a second aspect, the nonaqueous solvent in the first aspect may be ethylene carbonate (EC). The standard electrode potential of ethylene carbonate is higher than the standard electrode potential of polytetrafluoroethylene (PTFE), which can be used as a binder. Therefore, by charging a secondary battery using the above-mentioned negative electrode sheet as a negative electrode at a relatively low voltage during initial charging, the SEI forming agent can be reductively decomposed without reductively decomposing the nonaqueous solvent and the binder. This suppresses a decrease in the initial charge / discharge efficiency of the secondary battery.

[0009] In a third aspect, in the first or second aspect, the binder may contain at least polytetrafluoroethylene (PTFE). PTFE can be fibrillated by applying a shear force. Therefore, with the above configuration, the tensile strength of the negative electrode sheet can be improved.

[0010] In a fourth aspect, in any one of the first to third aspects, the SEI-forming agent may be a lithium-containing compound. In order to suppress the reductive decomposition of the nonaqueous solvent and / or binder, it is conceivable to reduce the exposed area of ​​the negative electrode active material by coating the surface of the negative electrode active material with a binder. However, coating the surface of the negative electrode active material with a binder may increase the electrical resistance of the negative electrode sheet due to the electrical resistance of the binder. In this regard, when the SEI-forming agent is a lithium-containing compound, the SEI formed by the reduction reaction of the SEI-forming agent has a characteristic of being lower in electrical resistance than the binder coating the surface of the negative electrode active material. Therefore, with the above-described configuration, during initial charging of a secondary battery using the negative electrode sheet as a negative electrode, it is possible to suppress the reductive decomposition of the nonaqueous solvent and / or binder and to suppress an increase in the electrical resistance of the secondary battery.

[0011] In a fifth aspect, in any one of the first to fourth aspects, the SEI forming agent may be at least one selected from the group consisting of lithium bis(oxalato)borate, lithium difluorooxalatoborate, and 1,3-propane sultone. These SEI forming agents are water-soluble and therefore suitable for use when producing a negative electrode sheet by a so-called dry process.

[0012] The technology disclosed in this specification is also embodied in a secondary battery. In a sixth aspect thereof, the secondary battery includes a positive electrode, a negative electrode including the negative electrode sheet according to any one of the first to fifth aspects, and a nonaqueous electrolyte solution containing ethylene carbonate. With this configuration, as described above, a secondary battery using the negative electrode sheet of the present technology as the negative electrode can be prevented from experiencing a decrease in initial charge / discharge efficiency.

[0013] The technology disclosed in this specification is also embodied in a method for manufacturing a negative electrode sheet for a secondary battery using a nonaqueous electrolyte. In a seventh aspect, the manufacturing method includes the steps of mixing a negative electrode active material, a binder, and an SEI forming agent to prepare a negative electrode composite mixture, and preparing the negative electrode sheet from the negative electrode composite mixture. The standard electrode potential of the SEI forming agent is higher than the standard electrode potential of the nonaqueous solvent contained in the nonaqueous electrolyte.

[0014] When the negative electrode sheet produced by the above-described production method is used, for example, as the negative electrode of a secondary battery, the decrease in the initial charge / discharge efficiency of the secondary battery can be suppressed, as described above.

[0015] In an eighth aspect, in the seventh aspect, the binder may include polytetrafluoroethylene. In this case, the polytetrafluoroethylene may be at least partially fibrillated in the process of preparing the negative electrode composite mixture. This configuration can improve the tensile strength of the electrode sheet. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a diagram schematically illustrating the configuration of a battery 100. FIG. [Figure 2] FIG. 2 is a diagram schematically showing the configuration of an electrode body 108. [Figure 3] FIG. 2 is a diagram schematically illustrating the configuration of a negative electrode 114. [Figure 4] FIG. 4 is an enlarged view of part IV shown in FIG. 3. [Figure 5] 4 is a flowchart illustrating a method for manufacturing the negative electrode 114. [Figure 6] FIG. 2 is a diagram illustrating a process of preparing a negative electrode composite mixture by mixing graphite, PTFE, and LiBOB using a mixer 200. [Figure 7] FIG. 2 is a diagram illustrating a process of forming the negative electrode composite mixture into a sheet using a roll press device 206. [Figure 8]10 is a diagram illustrating a step of pressing a negative electrode active material sheet 10 and a negative electrode current collector 122 together using a flat plate press device 210. FIG. [Figure 9] 1 is a flowchart illustrating a method for manufacturing the negative electrode active material sheets of Examples 1 to 3 and Comparative Example 1 and the negative electrode using the same. [Figure 10] The amounts of each raw material and LiBOB in the negative electrode active material sheets of Examples 1 to 3 and Comparative Example 1 are shown. [Figure 11A] 1 is a graph showing the relationship between differential capacity (dQ / dV) and voltage of small cells for each of Examples 1 to 3 and Comparative Example 1. [Figure 11B] The results of evaluation of the initial charge-discharge efficiency of small cells for each of Examples 1 to 3 and Comparative Example 1 are shown. [Figure 11C] For each of Examples 1 to 3 and Comparative Example 1, the evaluation results of the electrical resistance (ie, cell resistance) of the small cells are shown. DETAILED DESCRIPTION OF THE INVENTION

[0017] An embodiment of a negative electrode active material sheet 10 will be described with reference to the drawings. The negative electrode active material sheet 10 of this embodiment, together with a negative electrode current collector 122, constitutes a negative electrode 114 of a battery 100. The battery 100 is, for example, a lithium ion secondary battery.

[0018] <Battery> As shown in FIGS. 1 and 2, the battery 100 includes a housing 102, a positive electrode terminal 104, a negative electrode terminal 106, an electrode assembly 108, and a non-aqueous electrolyte 110. The housing 102 accommodates the electrode assembly 108 and the non-aqueous electrolyte 110. The housing 102 includes a housing main body 102a and a cover plate 102b. The housing main body 102a is a housing having an opening 102c at its top. The cover plate 102b is a plate-shaped member. The cover plate 102b is attached to the opening 102c and closes the opening 102c. The housing main body 102a and the cover plate 102b are made of a metal such as aluminum.

[0019] The positive electrode terminal 104 and the negative electrode terminal 106 are each attached to a cover plate 102b. One end 104a of the positive electrode terminal 104 is exposed to the outside of the housing 102, and the other end 104b of the positive electrode terminal 104 is connected to a positive electrode connection portion 112a of the electrode assembly 108 within the housing 102. One end 106a of the negative electrode terminal 106 is exposed to the outside of the housing 102, and the other end 106b of the negative electrode terminal 106 is connected to a negative electrode connection portion 114a of the electrode assembly 108 within the housing 102.

[0020] <Electrode body> As shown in FIG. 2, the electrode assembly 108 includes a positive electrode 112, a negative electrode 114, and a separator 116. The positive electrode 112, the negative electrode 114, and the separator 116 are each in the form of a long sheet. The positive electrode 112 and the negative electrode 114 are stacked with the separator 116 interposed therebetween, and the stack is wound to form the electrode assembly. That is, the electrode assembly 108 in this embodiment is a so-called wound electrode assembly. However, the electrode assembly 108 does not necessarily have to be a wound electrode assembly. The electrode assembly 108 may be, for example, a so-called stacked electrode assembly in which a plurality of positive electrodes 112 and a plurality of negative electrodes 114 are alternately stacked with the separator 116 interposed therebetween.

[0021] ≪Positive electrode≫ As shown in FIG. 2, the positive electrode 112 includes a positive electrode current collector 118 and a positive electrode active material sheet 120. The positive electrode current collector 118 is a conductive sheet. The positive electrode current collector 118 is, for example, aluminum foil. The positive electrode active material sheet 120 is disposed on the surface of the positive electrode current collector 118. One widthwise edge of the positive electrode 112 is free of the positive electrode active material sheet 120, and instead has a positive electrode exposed portion 118a where the positive electrode current collector 118 is exposed. When forming the electrode assembly 108, the positive electrode exposed portion 118a is wound and overlapped in a state where it protrudes from the negative electrode 114, thereby forming the positive electrode connection portion 112a. The thickness of the positive electrode current collector 118 is, for example, 5 μm to 50 μm. The thickness of the positive electrode active material sheet 120 is, for example, 10 μm to 500 μm.

[0022] The positive electrode active material sheet 120 includes a positive electrode active material. Examples of the positive electrode active material include lithium composite oxides. Examples of the lithium composite oxides include lithium nickel composite oxides, lithium cobalt composite oxides, lithium manganese composite oxides, and lithium nickel manganese composite oxides (e.g., LiNi 1 / 2 Mn 3 / 2 O4), lithium nickel manganese cobalt composite oxides (e.g., LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2). The positive electrode active material may be composed of a single material or a plurality of materials. The positive electrode active material sheet 120 may further include a binder, a conductive additive, etc.

[0023] ≪Negative electrode≫ As shown in FIG. 2, the negative electrode 114 includes a negative electrode current collector 122 and a negative electrode active material sheet 10. The negative electrode current collector 122 is a conductive sheet. The negative electrode current collector 122 is, for example, copper foil. The negative electrode active material sheet 10 is disposed on the surface of the negative electrode current collector 122. One widthwise edge of the negative electrode 114 is free of the negative electrode active material sheet 10, and instead has a negative electrode exposed portion 122a where the negative electrode current collector 122 is exposed. When forming the electrode body 108, the negative electrode exposed portion 122a is wound and overlapped in a state where it protrudes from the positive electrode 112, thereby forming a negative electrode connection portion 114a. The thickness of the negative electrode current collector 122 is, for example, 5 μm to 50 μm. The thickness of the negative electrode active material sheet 10 is, for example, 10 μm to 500 μm.

[0024] 3 and 4, the negative electrode active material sheet 10 includes a negative electrode active material 12, a binder 14, and a solid electrolyte (Solid Electrolyte Interphase: SEI) 16. Examples of the negative electrode active material 12 include carbon materials such as graphite, hard carbon, and soft carbon, materials that form alloys with lithium such as silicon (Si), and lithium alloys thereof (for example, Li XExamples of the negative electrode active material 12 include M (where M is C, Si, Sn, Sb, Al, Mg, Ti, Bi, Ge, Pb, P, or the like, and X is a natural number). The negative electrode active material 12 may be composed of a single material or a plurality of materials. The average particle size of the negative electrode active material 12 is not particularly limited, but is, for example, 5 μm or more and 50 μm or less. The average particle size here refers to the particle size at 50% cumulative value (D50) in a volume-based particle size distribution measured by a laser diffraction / scattering method.

[0025] Examples of the binder 14 include carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), polyvinylidene fluoride (PVdF), and polytetrafluoroethylene (PTFE). The binder 14 may be composed of a single material or multiple materials. The binder 14 in this embodiment is PTFE, which can be fibrillated by applying a shear force. Therefore, in the negative electrode active material sheet 10, the PTFE serving as the binder 14 is fibrillated. This can improve the tensile strength of the negative electrode active material sheet 10.

[0026] 4, the SEI 16 is formed on the surface of the negative electrode active material 12. The SEI 16 is formed on the surface of the negative electrode active material 12 during the initial charge of the battery 100.

[0027] The negative electrode active material sheet 10 may further include a conductive additive or the like.

[0028] <Separator> The separator 116 is configured to allow charge carriers (here, lithium ions) to pass through. Examples of the separator 116 include porous polymer membranes such as porous polyethylene membranes, porous polypropylene membranes, porous polyolefin membranes, and porous polyvinyl chloride membranes, and lithium ion conductive polymer electrolyte membranes. These separators 116 may be used alone or in combination of two or more types.

[0029] <Nonaqueous electrolyte> The nonaqueous electrolyte 110 permeates the inside of the electrode assembly 108. The nonaqueous electrolyte 110 includes a nonaqueous solvent and a supporting salt. Examples of the nonaqueous solvent include carbonates, ethers, esters, nitriles, sulfones, and lactones. Examples of carbonate solvents include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and monofluoroethylene carbonate (MFEC). Examples of the supporting salt include fluorine-containing lithium salts. Examples of fluorine-containing lithium salts include LiPF6, LiBF4, and LiCF3SO3. Thus, the nonaqueous electrolyte 110 includes a lithium compound. Each of the nonaqueous solvent and the supporting salt may be composed of a single material or multiple materials. The concentration of the supporting electrolyte in the nonaqueous electrolyte 110 is, for example, 0.75 mol / L or more and 1.5 mol / L or less.

[0030] It is known that in the above-described battery 100, during initial charging, the nonaqueous solvent and / or binder 14 in the nonaqueous electrolyte solution 110 are reductively decomposed, and an SEI 16 is formed on the surface of the negative electrode active material 12. When the SEI 16 is formed on the surface of the negative electrode active material 12, the subsequent reductive decomposition of the nonaqueous solvent and / or binder 14 is suppressed. However, since the formation of the SEI 16 by the reductive decomposition of the nonaqueous solvent and / or binder 14 is an irreversible reaction, lithium ions are consumed in the reductive decomposition, which reduces the initial charge / discharge efficiency of the battery 100.

[0031] In this regard, before the above-described battery 100 is initially charged, the negative electrode active material sheet 10 includes an SEI-forming agent instead of the SEI 16. The standard electrode potential of the SEI-forming agent is higher than the standard electrode potential of the nonaqueous solvent contained in the nonaqueous electrolyte solution 110. An SEI-forming agent with a high standard electrode potential is more easily reduced than a nonaqueous solvent with a low standard electrode potential. Therefore, during the initial charge of the battery 100, charging at a relatively low voltage allows the SEI-forming agent to be reductively decomposed without reductively decomposing the nonaqueous solvent. By preferentially forming the first SEI 16a derived from the SEI-forming agent on the surface of the negative electrode active material 12, the formation of the second SEI 16b due to reductive decomposition of the nonaqueous solvent is suppressed, and a decrease in the initial charge / discharge efficiency of the battery 100 can be suppressed. Thus, the SEI 16 of this embodiment includes a first SEI 16a formed by reductive decomposition of the SEI-forming agent, and a second SEI 16b formed by reductive decomposition of something other than the SEI-forming agent (for example, a non-aqueous solvent) (see FIG. 4).

[0032] Examples of SEI forming agents include lithium bis(oxalato)borate (LiBOB), lithium difluorooxalatoborate (LPFO), and 1,3-propane sultone. These SEI forming agents are water-soluble and therefore suitable for use when the negative electrode active material sheet 10 is produced by a so-called dry process. The SEI forming agent may be composed of a single material or multiple materials.

[0033] In addition, the standard electrode potential of the SEI-forming agent is preferably higher than the standard electrode potential of the material constituting the binder 14. In this case, the SEI-forming agent is more easily reduced than the binder 14. Therefore, by charging the battery 100 at a relatively low voltage during initial charging, the SEI-forming agent can be reductively decomposed without reductively decomposing the binder 14. By preferentially forming the first SEI 16a derived from the SEI-forming agent on the surface of the negative electrode active material 12, the formation of the second SEI 16b due to reductive decomposition of the binder 14 is also suppressed, and a decrease in the initial charge / discharge efficiency of the battery 100 can be suppressed.

[0034] As an example, in the above-described embodiment, the nonaqueous solvent may be ethylene carbonate (EC). The material constituting the binder 14 may be polytetrafluoroethylene (PTFE). The standard electrode potential of EC is higher than the standard electrode potential of PTFE. Therefore, the standard electrode potential of the SEI-forming agent only needs to be higher than the standard electrode potential of EC. By charging the battery 100 at a relatively low voltage during the initial charge, the SEI-forming agent can be reductively decomposed without reductively decomposing the nonaqueous solvent and the binder 14. This can prevent a decrease in the initial charge / discharge efficiency of the battery 100.

[0035] Furthermore, the standard electrode potential of EC is higher than the standard electrode potential of other nonaqueous solvents (e.g., dimethyl carbonate, ethyl methyl carbonate) contained in the nonaqueous electrolyte solution 110. Therefore, when the standard electrode potential of the SEI-forming agent is higher than the standard electrode potential of EC, it is considered that the standard electrode potential of the SEI-forming agent will be higher than all of the nonaqueous solvents contained in the nonaqueous electrolyte solution 110. It is considered that by charging the battery 100 at a relatively low voltage during the initial charge described above, the SEI-forming agent can be reductively decomposed without reductively decomposing all of the nonaqueous solvents contained in the nonaqueous electrolyte solution 110.

[0036] As an example, in the present embodiment described above, a lithium-containing compound such as lithium bis(oxalato)borate (LiBOB) or lithium difluorooxalatoborate (LPFO) may be used as the SEI-forming agent. To suppress the reductive decomposition of the nonaqueous solvent and / or the binder 14, it is conceivable to reduce the exposed area of ​​the negative electrode active material 12 by coating the surface of the negative electrode active material 12 with the binder 14. However, coating the surface of the negative electrode active material 12 with the binder 14 may increase the electrical resistance of the negative electrode active material sheet 10 due to the electrical resistance of the binder 14. In this regard, when the SEI-forming agent is a lithium-containing compound, the first SEI 16a formed by the reduction reaction of the SEI-forming agent has a characteristic of being lower in electrical resistance than the binder 14 that coats the surface of the negative electrode active material 12. Therefore, with the above-described configuration, the reductive decomposition of the nonaqueous solvent and / or the binder 14 can be suppressed during initial charging of the battery 100, and an increase in the electrical resistance of the battery 100 can be suppressed.

[0037] Next, a method for manufacturing the negative electrode 114 including the negative electrode active material sheet 10 of this embodiment will be described with reference to Figures 5-8. This manufacturing method allows the negative electrode 114 to be produced without using a solvent. In other words, this manufacturing method is a so-called dry process.

[0038] As shown in FIG. 5, the manufacturing method includes a step of mixing the negative electrode active material 12, the binder 14, and the SEI forming agent (S10). In this step, for example, a mixer 200 is used, as shown in FIG. 6. The mixer 200 mixes the negative electrode active material 12, the binder 14, and the SEI forming agent introduced into a container 204 by rotating a blade 202. This produces a negative electrode composite mixture. Note that the mixer 200 does not necessarily have to be used in S10. In other embodiments, other mixers such as a blender or a mill may be used instead of the mixer 200.

[0039] As shown in FIG. 5, the manufacturing method further includes a step of fibrillating the binder 14 by applying a shear force to the negative electrode composite mixture (S12). In this step, for example, a mixer is used, as shown in FIG. 6. The mixer applies a shear force to the negative electrode composite mixture placed in a container by rotating blades. As described above, since the binder 14 in this embodiment is PTFE, the PTFE is fibrillated by applying a shear force to the binder 14 (i.e., PTFE) constituting the negative electrode composite mixture. This improves the tensile strength of the negative electrode active material sheet 10. Note that a mixer does not necessarily have to be used in S12. In other embodiments, other mixers such as a blender, mill, or kneader may be used instead of the mixer 200.

[0040] As shown in FIG. 5, the manufacturing method further includes a step of producing a negative electrode active material sheet 10 from the negative electrode composite mixture (S14). In this step, for example, a roll press device 206 is used, as shown in FIG. 7. The roll press device 206 includes a pair of rollers 208 and is configured to roll the negative electrode composite mixture passing between the pair of rollers 208. Therefore, the negative electrode composite mixture is formed into a sheet by being rolled by the pair of rollers 208. In this way, the negative electrode active material sheet 10 is produced.

[0041] As shown in FIG. 5, the manufacturing method further includes a step of pressing the negative electrode active material sheet 10 and the negative electrode current collector 122 together (S16). In this step, as shown in FIG. 8, for example, a flat plate press 210 is used. The flat plate press 210 includes a lower mold 212 and an upper mold 214, and the upper mold 214 can be lowered toward the lower mold 212. The negative electrode active material sheet 10 and the negative electrode current collector 122 are placed in a stacked state on the lower mold 212, and the upper mold 214 is lowered, thereby pressing the negative electrode active material sheet 10 and the negative electrode current collector 122 together. This produces the negative electrode 114. Note that, although not particularly limited, the process of S16 may be performed in a state in which the lower mold 212 and the upper mold 214 are heated to a predetermined temperature.

[0042] The content of the negative electrode active material 12 in the negative electrode mixture is, for example, 90% by weight to 99% by weight, or for example, 95% by weight to 98.5% by weight, based on the total weight (100% by weight) of the negative electrode active material 12 and the binder 14. The content of the binder 14 in the negative electrode mixture is, for example, 0.5% by weight to 5% by weight, or for example, 1% by weight to 4% by weight, based on the total weight (100% by weight) of the negative electrode active material 12 and the binder 14. The content of the SEI forming agent in the negative electrode mixture is, for example, 0.25% by weight to 3% by weight, or for example, 0.5% by weight to 2% by weight, based on the total weight (100% by weight) of the negative electrode active material 12 and the binder 14.

[0043] The negative electrode active material sheet 10 produced by the above-described production method contains an SEI-forming agent, and the standard electrode potential of the SEI-forming agent is higher than the standard electrode potential of the non-aqueous solvent contained in the non-aqueous electrolyte solution 110. Therefore, in a battery 100 using the negative electrode active material sheet 10 in the negative electrode 114, a decrease in initial charge efficiency can be suppressed. Note that the negative electrode active material sheet 10 in this specification is an example of the negative electrode sheet in the present technology.

[0044] Examples of the present technology will be described below, but it is not intended that the present technology be limited to those shown in these examples.

[0045] Example 1 <Preparation of positive electrode> LiCo as positive electrode active material particles 1 / 3 Ni 1 / 3 Mn 1 / 3O2 (hereinafter referred to as NCM, product name: NCM811), carbon tube (CNT, manufactured by LG Chem) powder as a conductive additive, and polyvinylidene fluoride (PVdF, manufactured by Arkema) powder as a binder were charged into a mixer (manufactured by Nippon Coke Company, MP5B) in a weight ratio of NCM:CNT:PVdF = 98.3:0.7:1.0 and mixed with a solvent. This produced a paste for preparing a positive electrode composite mixture. The paste for preparing a positive electrode composite mixture was applied to the surface of aluminum foil (thickness: 12 μm) as a positive electrode current collector 118 and dried to produce a positive electrode 112.

[0046] <Preparation of negative electrode> First, graphite powder (average particle size: 20 μm) as the negative electrode active material 12, polytetrafluoroethylene (PTFE, manufactured by Chemours) powder as the binder 14, and lithium bis(oxalato)borate (LiBOB) powder as the SEI forming agent were placed in a mixer (MP5B, manufactured by Nippon Coke Company) and mixed at 300 rpm for 180 seconds. This produced a negative electrode composite mixture. As shown in FIG. 10 , the weight ratio of graphite:PTFE:LiBOB was 97:3:0.5, based on the total weight of graphite and PTFE (100 wt %). This negative electrode composite mixture was mixed in the mixer at 3000 rpm for 8 minutes. This applied shear force to the PTFE, fibrillating it. The negative electrode composite mixture was then rolled at a linear pressure of 0.4 t / cm using a roll press (SA-602, manufactured by Tester Sangyo Co., Ltd.) to produce a negative electrode active material sheet 10. The thickness of this negative electrode active material sheet 10 was 110 μm. The negative electrode active material sheet 10 and copper foil (thickness: 8 μm) serving as a negative electrode current collector 122 were pressed together under a load of 5 tons while being heated to 160° C. in a flat plate press (H300-05K, manufactured by As-one Corporation), to produce a negative electrode 114.

[0047] <Preparation of non-aqueous electrolyte> Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed as a non-aqueous solvent in a volume ratio of EC:DMC:EMC = 30:30:40. LiPF as a supporting electrolyte was dissolved in the non-aqueous solvent to a concentration of 1.1 mol / L to prepare a non-aqueous electrolyte solution 110.

[0048] <Production of small cells> The positive electrode 112, the negative electrode 114, and a separator for a lithium ion battery as the separator 116 were assembled into a coin-type small cell, and the non-aqueous electrolyte solution 110 was poured into the cell to prepare a small cell.

[0049] Example 2 Example 2 was similar to Example 1, except that the weight ratio of graphite:PTFE:LiBOB was 97:3:1.0, based on the total weight of graphite and PTFE (100% by weight).

[0050] Example 3 Example 3 was similar to Example 1, except that the weight ratio of graphite:PTFE:LiBOB was 97:3:1.5, based on the total weight of graphite and PTFE (100% by weight).

[0051] (Comparative Example 1) Example 1 was the same as Example 1, except that the weight ratio of graphite:PTFE was 97:3, based on the total weight of graphite and PTFE (100% by weight).

[0052] (Differential capacitance (dQ / dV)-voltage characteristic curve) Each small cell fabricated was initially charged by CCCV charging. Specifically, constant current (CC) charging was performed at a current rate of 0.1 C until the voltage between the positive electrode 112 and the negative electrode 114 reached 4.25 V, and then constant voltage (CV) charging was performed at the same voltage. The voltage between the positive electrode 112 and the negative electrode 114 at the start of charging was 3.0 V. From the charge curve for this initial charge, the differential capacity (dQ / dV) was calculated by differentiating the charge capacity with respect to the voltage. The results are shown in Figure 11A. The CCCV charging conditions were a CC current of 0.1 C and a CV voltage of 4.25 V. The current rate unit "C" indicates the current rate at which the rated capacity of the small cell is fully discharged in one hour.

[0053] The differential capacity (dQ / dV) is an index showing the reaction amount of SEI formation, and it can be said that the larger the differential capacity, the more progressed the SEI formation. As shown in FIG. 11A, in the dQ / dV-voltage characteristic curves of the small cells in Examples 1-3 during the initial charge, two peaks mainly due to SEI formation were observed. The two peaks are referred to as the first peak and the second peak, in order from the lowest voltage. A sub-peak was observed between the first and second peaks. In the dQ / dV-voltage characteristic curves of the small cells in Comparative Example 1 during the initial charge, the first peak was not observed, and only the second peak and the sub-peak were observed.

[0054] The standard electrode potential increases in the order of PTFE, EC, and LiBOB. The higher the standard electrode potential, the more likely a material is to be reductively decomposed during the initial charge of a small cell, i.e., it is thought that reductive decomposition occurs at a lower potential. Therefore, it is thought that the first peak is due to the formation of an SEI by the reductive decomposition of LiBOB, the sub-peak is due to the formation of an SEI by the reductive decomposition of EC, and the second peak is due to the formation of an SEI by the reductive decomposition of PTFE.

[0055] From the results shown in FIG. 11A, it was found that the first peak became higher as the LiBOB content increased. This indicates that the formation of the first SEI 16a derived from LiBOB progresses as the LiBOB content increases. It was also found that the second peak became lower as the LiBOB content increased. This indicates that the formation of the second SEI 16b derived from PTFE is suppressed as the LiBOB content increases. This is thought to be because the formation of the first SEI 16a derived from LiBOB preferentially progresses, suppressing the formation of the second SEI 16b derived from PTFE. Furthermore, it can be said that the formation of the SEI due to the reductive decomposition of EC hardly progressed in any of the small cells of Examples 1-3 and Comparative Example 1.

[0056] (Initial charge / discharge efficiency) Each small cell was initially charged and discharged at a constant current (CC) rate of 0.1 C to 4.25 V, followed by discharging at a 0.1 C rate to 3.0 V. The ratio of the discharge capacity to the charge capacity, i.e., the initial charge / discharge efficiency, was calculated. The results are shown in Figure 11B.

[0057] The results shown in FIG. 11B indicate that the initial charge-discharge efficiency of each small cell in Examples 1-3 was higher than that of the small cell in Comparative Example 1. Furthermore, comparing the initial charge-discharge efficiency of each small cell in Examples 1-3, it was found that the initial charge-discharge efficiency of the small cells increased as the LiBOB content increased. This is thought to be because the first SEI 16a formed by the reductive decomposition of LiBOB became thicker as the LiBOB content increased, thereby suppressing the reductive decomposition of PTFE that occurred after the reductive decomposition of LiBOB. This likely resulted in a decrease in the irreversible capacity of the small cells and an increase in the initial charge-discharge efficiency.

[0058] (cell resistance) The electrical resistance (i.e., cell resistance) of each small cell was measured. Specifically, after adjusting the SOC to 50%, the cell was discharged at a current rate of 0.3 C for 10 seconds, and the cell resistance was calculated from the voltage drop (ΔV). The results are shown in Figure 11C.

[0059] The results shown in FIG. 11C indicate that the cell resistance is almost constant regardless of the LiBOB content. Meanwhile, the inventors have confirmed that coating the surface of the negative electrode active material 12 with polyvinylidene fluoride (PVdF) as the binder 14 increases the electrical resistance of the negative electrode active material sheet 10. These results suggest that the first SEI 16a derived from the LiBOB has lower electrical resistance than, for example, PVdF, which coats the surface of the negative electrode active material 12. Thus, it is believed that the use of a lithium-containing compound as an SEI-forming agent can suppress an increase in cell resistance.

[0060] Although specific examples of the present invention have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples exemplified above. Modifications of the above embodiments are listed below.

[0061] In the above-described embodiment, the present technology has been described by taking the case where the battery 100 is a lithium-ion secondary battery as an example. However, the battery 100 does not necessarily have to be a lithium-ion secondary battery. The battery 100 may be any secondary battery that uses a nonaqueous electrolyte solution 110. In other words, the battery 100 may be any battery that uses a nonaqueous electrolyte solution as an electrolyte and can be repeatedly charged and discharged.

[0062] In the above-described embodiment, the negative electrode active material sheet 10 constitutes the negative electrode 114 together with the negative electrode current collector 122. However, the negative electrode active material sheet 10 is a free-standing electrode sheet. The free-standing electrode sheet here refers to an electrode sheet that is self-supporting without requiring a support such as the negative electrode current collector 122. Therefore, the negative electrode 114 does not necessarily need to include the negative electrode current collector 122. That is, in another embodiment, the negative electrode active material sheet 10 may constitute the negative electrode 114 by itself. With such a configuration, the electrode energy density of the negative electrode 114 can be improved.

[0063] 5, the method for manufacturing the negative electrode 114 includes a step of fibrillating the binder 14 (S12). However, in one variation, the step of fibrillating the binder 14 may be omitted, and in this case, a material that cannot be fibrillated may be used for the binder 14.

[0064] Furthermore, the technical elements described in this specification or drawings exhibit technical utility either alone or in various combinations, and are not limited to the combinations set forth in the claims at the time of filing. Furthermore, the technologies illustrated in this specification or drawings can achieve multiple objectives simultaneously, and achieving one of those objectives is itself technically useful. [Explanation of symbols]

[0065] 10: negative electrode active material sheet, 12: negative electrode active material, 14: binder, 16: SEI, 100: battery, 102: housing, 104: positive electrode terminal, 106: negative electrode terminal, 108: electrode body, 110: non-aqueous electrolyte, 112: positive electrode, 114: negative electrode, 116: separator, 118: positive electrode current collector, 120: positive electrode active material sheet, 122: negative electrode current collector, 200: mixer, 206: roll press device, 210: plate press device

Claims

1. A negative electrode sheet for a secondary battery using a non-aqueous electrolyte, a negative electrode active material, a binder, and an SEI forming agent; a standard electrode potential of the SEI forming agent is higher than a standard electrode potential of a non-aqueous solvent contained in the non-aqueous electrolyte solution; Negative electrode sheet.

2. The negative electrode sheet according to claim 1 , wherein the non-aqueous solvent is ethylene carbonate.

3. The negative electrode sheet according to claim 1 , wherein the binder contains at least polytetrafluoroethylene.

4. The negative electrode sheet according to claim 1 , wherein the SEI forming agent is a compound containing lithium.

5. 2. The negative electrode sheet according to claim 1, wherein the SEI forming agent is at least one selected from the group consisting of lithium bis(oxalato)borate, lithium difluorooxalatoborate, and 1,3-propane sultone.

6. A positive electrode and A negative electrode comprising the negative electrode sheet according to any one of claims 1 to 5; a non-aqueous electrolyte containing ethylene carbonate; A secondary battery comprising:

7. A method for manufacturing a negative electrode sheet for a secondary battery using a non-aqueous electrolyte, comprising: mixing a negative electrode active material, a binder, and an SEI forming agent to prepare a negative electrode composite mixture; preparing the negative electrode sheet from the negative electrode composite mixture; Equipped with The standard electrode potential of the SEI forming agent is higher than the standard electrode potential of the non-aqueous solvent contained in the non-aqueous electrolyte solution. Manufacturing method.

8. the binder includes polytetrafluoroethylene; The manufacturing method according to claim 7 , wherein the polytetrafluoroethylene is at least partially fibrillated in the step of preparing the negative electrode mixture.

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