Secondary battery

Abstract

The cycle characteristic is improved. The secondary battery includes a positive electrode, a negative electrode, a separator positioned between a main surface of the positive electrode and a main surface of the negative electrode, and an electrolyte solution. The positive electrode has a positive electrode active material layer. The positive electrode active material layer contains a positive electrode active material and a first polymer compound. The negative electrode has a negative electrode active material layer. The negative electrode active material layer contains a negative electrode active material and a second polymer compound. The negative electrode active material includes a first negative electrode active material containing silicon. The second polymer compound contains an NVA polymer, and the NVA polymer contains N-vinyl acetamide as a monomer. The separator has a base material, a positive electrode-side coating layer positioned on the positive electrode side of the base material, and a negative electrode-side coating layer positioned on the negative electrode side of the base material. The positive electrode-side coating layer contains a first polymer compound. The negative electrode-side coating layer contains an NVA polymer.

Description

Technical Field

[0001] This invention relates to a secondary battery. Background Technology

[0002] Patent document 1 discloses a lithium-ion battery comprising a modified polymer as a binder, the modified polymer comprising a structure derived from an N-alkyl unsaturated carboxylic acid amide as a monomer unit.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: International Publication No. 2021 / 006198 Summary of the Invention

[0006] The technical problem that the invention aims to solve

[0007] However, in the secondary battery shown in Patent Document 1, there is a possibility of reduced cycle performance due to decreased adhesion between the separator and the negative electrode.

[0008] The present invention was made in view of the above-mentioned technical problems, and its purpose is to improve the cycle characteristics.

[0009] Technical solutions for solving technical problems

[0010] One aspect of the present invention relates to a secondary battery comprising a positive electrode, a negative electrode, a separator located between the main surfaces of the positive electrode and the negative electrode, and an electrolyte. The positive electrode has a positive electrode active material layer comprising a positive electrode active material and a first polymeric compound. The negative electrode has a negative electrode active material layer comprising a negative electrode active material and a second polymeric compound. The negative electrode active material comprises a first negative electrode active material containing silicon, and the second polymeric compound comprises an NVA polymer containing N-vinylacetamide as a monomer. The separator has a substrate, a positive electrode side coating located on the positive electrode side of the substrate, and a negative electrode side coating located on the negative electrode side of the substrate. The positive electrode side coating comprises the first polymeric compound, and the negative electrode side coating comprises the NVA polymer.

[0011] Invention Effects

[0012] According to the present invention, the cycle characteristics can be improved. Attached Figure Description

[0013] Figure 1 This is a perspective view showing an example of a secondary battery according to the first embodiment.

[0014] Figure 2 It is shown Figure 1An enlarged cross-sectional view of a portion of the cross-section of the electrode involved.

[0015] Figure 3 This is a cross-sectional view showing different examples of the secondary battery involved in the first embodiment.

[0016] Figure 4 yes Figure 3 A schematic diagram of the cross section of line IV-IV. Detailed Implementation

[0017] The embodiments of the present invention will now be described. However, the present invention is not limited to these embodiments.

[0018] (Secondary battery)

[0019] Figure 1 This is a perspective view showing an example of a secondary battery according to the first embodiment. Figure 1 The secondary battery 1 shown is a laminated lithium-ion secondary battery. For example... Figure 1 As shown, the secondary battery 1 includes a battery element 20, an outer packaging component 30, and a sealing material 32.

[0020] The battery element 20 is located inside the outer packaging component 30. For example... Figure 1 As shown, the battery element 20 includes an electrode body 200, a positive electrode lead 21, and a negative electrode lead 22. The positive electrode lead 21 is a terminal that extends from the positive electrode 210 (described later) to the outside of the outer packaging component 30. That is, the positive electrode lead 21 is the terminal that serves as the positive (+) electrode of the secondary battery 1. Figure 1 In this configuration, the positive electrode lead 21 is disposed on the end face of the electrode body 200. The negative electrode lead 22 is a terminal that extends from the interior of the negative electrode 220 (described later) to the exterior of the outer packaging component 30. That is, the negative electrode lead 22 is the terminal that serves as the negative electrode (- electrode) of the secondary battery 1. Figure 1 In this configuration, the negative electrode lead 22 is disposed on the end face of the electrode body 200. Details of the electrode body 200 will be described below.

[0021] The outer packaging component 30 is a shell that houses the battery element 20. The outer packaging component 30 includes two outer packaging sheets 30a and 30b. The outer packaging sheets 30a and 30b have an insulating layer, a metal layer, and an outermost layer. Figure 1 In the example, a recess 31 is provided in the outer packaging sheet 30a. Thus, the battery element 20 is accommodated in the recess 31, and the periphery of the outer packaging sheets 30a and 30b is bonded to it, thereby accommodating the battery element 20 within the outer packaging component 30.

[0022] The outer packaging sheets 30a and 30b are structured by laminating and bonding them together from the inside, i.e., the side where the battery element 20 is disposed, in the order of insulating layer, metal layer, and outermost layer, and then through lamination or other processes. The insulating layer of the outer packaging sheets 30a and 30b is, for example, made of a resin containing polyethylene, polypropylene, modified polyethylene, modified polypropylene, ethylene, or propylene as monomers, such as a polyolefin resin. Therefore, the outer packaging sheets 30a and 30b can reduce the moisture permeability of the secondary battery 1 and improve airtightness. The metal layer of the outer packaging sheets 30a and 30b is made of metal sheets or foils such as aluminum, stainless steel, nickel, or iron. The outermost layer can be made of any material, but it is preferably made of a high-strength material such as the same resin as the insulating layer, nylon, or other materials with high resistance to cracking or puncture.

[0023] The sealing material 32 is a component used to make the outer packaging component 30 airtight. The sealing material 32 is disposed between the outer packaging component 30 and the positive electrode lead 21 and the negative electrode lead 22. The material of the sealing material 32 is preferably one that has a sealing effect on the positive electrode lead 21 and the negative electrode lead 22. For example, when the positive electrode lead 21 and the negative electrode lead 22 are made of a metallic material, the sealing material 32 is made of a polyolefin resin such as polyethylene, polypropylene, modified polyethylene, or modified polypropylene. Thus, the sealing material 32 can seal the gap between the outer packaging component 30 and the positive electrode lead 21 and the negative electrode lead 22, thereby making the interior of the outer packaging component 30 airtight.

[0024] Figure 2 It is shown Figure 1 An enlarged cross-sectional view of a portion of the cross-section of the electrode involved. More specifically, Figure 2 This is a cross-sectional view showing a portion of a positive electrode 210 and a negative electrode 220 in the electrode body 200. (See diagram below.) Figure 2 As shown, the electrode body 200 includes a positive electrode 210, a negative electrode 220, and a separator 230. In the secondary battery 1, the electrode body 200 is a structure in which the positive electrode 210 and the negative electrode 220 are stacked in the thickness direction with the separator 230 in between. The positive electrode 210 and the negative electrode 220 included in the electrode body 200 are layered components used for the charge and discharge reaction of the secondary battery according to the first embodiment.

[0025] The positive electrode 210 has a positive current collector 211 and a positive active material layer 212. In the positive electrode 210, the positive current collector 211 is stacked between the positive active material layers 212.

[0026] The positive current collector 211 is a conductor layer, such as aluminum foil or stainless steel foil. Figure 1 In the example, the positive current collector 211, when viewed from above in the thickness direction, is a rectangular sheet with a protrusion on the side of the positive lead 21. The protrusion of the positive current collector 211 is connected to the positive lead 21.

[0027] The positive electrode active material layer 212 is a layer containing a positive electrode active material. The positive electrode active material layer 212 contains the positive electrode active material, a first polymer compound, and a conductive agent. The positive electrode active material layer 212 is not limited to the materials listed above; for example, it may also contain a dispersant.

[0028] The preferred positive electrode active material is a lithium-containing compound such as a lithium-containing composite oxide or a lithium-containing phosphate compound. A lithium-containing composite oxide is an oxide containing lithium and one or more elements other than lithium as constituent elements. For example, lithium-containing composite oxides have a layered rock salt type or spinel type crystal structure. A lithium-containing phosphate compound is a phosphate compound containing lithium and one or more elements other than lithium as constituent elements. For example, lithium-containing phosphate compounds have a olivine type crystal structure. Specific examples of lithium-containing composite oxides are LiNiO2, LiCoO2, and LiCo... 0.98 Al 0.01 Mg 0.01 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.8 Co 0.15 Al 0.05 O2, LiNi 0.33 Co 0.33 Mn 0.33 O2, Li 1.2 Mn 0.52 Co 0.175 Ni 0.1 O2, Li 1.15 (Mn 0.65 Ni 0.22 Co 0.13 Examples of lithium phosphate compounds include O2, LiMn2O4, etc. Specific examples of lithium phosphate compounds include LiFePO4, LiMnPO4, and LiFe... 0.5 Mn 0.5 PO4, LiFe 0.3 Mn 0.7 PO4, etc.

[0029] The first polymer compound contained in the positive electrode active material layer 212 is a binder. The first polymer compound can be any material, including, for example, any one or more of synthetic rubber and polymer compounds. Examples of synthetic rubbers include styrene-butadiene rubber, fluorinated rubber, and ethylene propylene diene monomer (EPDM) rubber. Examples of polymer compounds include polyvinylidene fluoride (PVdF) and polyimide.

[0030] The conductive agent contained in the positive electrode active material layer 212 can be any material, such as carbon. Carbon can be, for example, graphite, carbon black, acetylene black, Ketjen black, etc. However, the conductive agent contained in the positive electrode active material layer 212 is not limited to these materials as long as it is a conductive material; it can also be a metallic material, a conductive polymer, etc.

[0031] The negative electrode 220 has a negative electrode current collector 221 and a negative electrode active material layer 222. In the negative electrode 220, the negative electrode current collector 221 is stacked between the negative electrode active material layers 222.

[0032] The negative current collector 221 is a conductor, such as copper foil. Figure 1 In this example, the negative current collector 221, when viewed from above in the thickness direction, is a rectangular sheet with protrusions on the side of the negative lead 22. The protrusions of the negative current collector 221 are connected to the negative lead 22.

[0033] The negative electrode active material layer 222 is a layer containing negative electrode active material. The negative electrode active material layer 222 contains negative electrode active material and a second polymer compound.

[0034] The negative electrode active material includes a first negative electrode active material, and preferably also includes a second negative electrode active material. Here, the negative electrode active material refers to a substance that undergoes a reduction reaction during charging of the secondary battery 1 and an oxidation reaction during discharging of the secondary battery 1; it refers to a reducing agent that can reversibly undergo these redox reactions; and it refers to a reducing agent that can perform a reaction that absorbs charge carriers that have been removed from the secondary battery 1.

[0035] Therefore, for example, carbon nanotubes and other fibrous carbon, carbon black and other carbon particles do not substantially absorb lithium ions as charge carriers in lithium-ion secondary batteries, and are therefore not included in the negative electrode active material of this disclosure.

[0036] The first negative electrode active material is a silicon-containing negative electrode active material. Examples of silicon-containing negative electrode active materials include monomeric silicon, silicon alloys, and silicon compounds. Examples of silicon alloys that can be used as the first negative electrode active material include alloys containing at least one of the following as a second constituent element other than silicon: tin (Sn), nickel (Ni), copper (Cu), iron (Fe), cobalt (Co), manganese (Mn), zinc (Zn), indium (In), silver (Ag), titanium (Ti), germanium (Ge), bismuth (Bi), antimony (Sb), and chromium (Cr). Furthermore, examples of silicon compounds that can be used as the first negative electrode active material include silicon oxide (SiO₂). x Compounds containing oxygen (O) or carbon (C), such as silicon carbide (SiC), may also contain the aforementioned second constituent element in addition to silicon. Furthermore, the first negative electrode active material may also be doped with Li. When the first negative electrode active material is SiO...x In this case, it is preferable to pre-dope Li by doping it with Li during the fabrication process of the negative electrode. This reduces the SiO₂ content as the active material of the negative electrode. x The irreversible capacity is reduced. Furthermore, the first negative electrode active material can also be a composite of Si and other materials such as carbon, or a composite of Si alloy and other materials such as carbon. In this case, the irreversible capacity can be reduced. Additionally, the particle surface of the first negative electrode active material is preferably partially or completely covered by carbon. This improves the electronic conductivity of the particle surface of the first negative electrode active material.

[0037] The second negative electrode active material is a negative electrode active material containing carbon as a constituent element. Materials that can be used as second negative electrode active materials include, for example, MCMB (MesoCarbon MicroBeads), artificial graphite, natural graphite, difficult-to-graphitize carbon, and easily-graphitize carbon. More specifically, materials that can be used as second negative electrode active materials include thermally decomposable carbon, coke, glassy carbon fibers, sintered organic polymer compounds, activated carbon, and carbon black. Coke includes pitch coke, needle coke, and petroleum coke. Here, sintered organic polymer compounds are produced by sintering and carbonizing polymer compounds such as phenolic resin and furan resin at an appropriate temperature.

[0038] Furthermore, the negative electrode active material is not limited to the first and second negative electrode active materials, but may also include other negative electrode active materials, such as alloys or compounds of metals or half-metals, alloys or compounds of tin (Sn), and other materials capable of lithium insertion and extraction. Examples of metals and half-metals that can be used as negative electrode active materials include tin (Sn), lead (Pb), aluminum (Al), indium (In), zinc (Zn), antimony (Sb), bismuth (Bi), cadmium (Cd), magnesium (Mg), boron (B), gallium (Ga), germanium (Ge), arsenic (As), silver (Ag), zirconium (Zr), yttrium (Y), and hafnium (Hf). Among these, germanium, tin, and lead are preferred. Moreover, tin has a high capacity for lithium insertion and extraction, resulting in high energy density, and is therefore preferred.

[0039] Here, the first negative electrode active material preferably comprises 30% or more by mass in the negative electrode active material. That is, the mass fraction of the first negative electrode active material relative to the negative electrode active material is preferably 30% or more. This improves the negative electrode capacity. The mass fraction of the first negative electrode active material relative to the negative electrode active material can be calculated by distinguishing between silicon (Si) particles and carbon (C)-only particles in an EDX (Energy Dispersive X-ray Spectroscopy) mapping image of a cross-section along the thickness direction of the negative electrode active material layer 222. More specifically, the particle size distribution is obtained using image analysis software such as ImageJ for both Si-containing particles (particles of the first negative electrode active material) and C-only particles (particles of the second negative electrode active material). The volume distribution is calculated based on the obtained particle size distribution, and the total volume of Si-containing particles and the total volume of C-only particles are calculated by summing the volumes of each particle. The density of particles containing Si and the density of particles containing only C can be approximated as the same, so the mass fraction of the first negative electrode active material can be calculated using the following formula (1).

[0040] (Mass fraction of the first negative electrode active material (%)) = (Total volume of particles containing Si) / [(Total volume of particles containing Si) + (Total volume of particles containing only C)]...(1)

[0041] Here, if the area for obtaining the particle size distribution is too narrow, the error will increase. Therefore, the particle size distribution is obtained in a region with a maximum width of 100 μm or more. Furthermore, the aforementioned particle size distribution is obtained by determining the particle size distribution using image analysis software such as ImageJ, based on the particle sizes of multiple particles measured using ImageJ.

[0042] Examples of tin alloys that can be used as negative electrode active materials include alloys containing at least one of the following as a second constituent element other than tin: nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony, and chromium. Furthermore, examples of tin compounds that can be used as negative electrode active materials include compounds containing oxygen or carbon, which may also contain the aforementioned second constituent element in addition to tin.

[0043] The second polymer compound contained in the negative electrode active material layer 222 is a binder containing a polymer (NVA polymer) that includes N-vinylacetamide (NVA) as a monomer. The NVA polymer that can be used as the second polymer compound includes a copolymer that includes poly-N-vinylacetamide (PNVA) and NVA as monomers. The copolymer containing NVA as a monomer that can be used as the second polymer compound also includes at least one of an alkali metal salt of acrylate, an alkali metal salt of methacrylate, and derivatives of an alkali metal salt of acrylate and an alkali metal salt of methacrylate as monomers. Here, the alkali metal salt refers to a salt containing alkali metals such as lithium (Li), sodium (Na), and potassium (K). Therefore, the decrease in electronic conductivity between negative electrode active material particles caused by the expansion and contraction of the negative electrode active material layer 222 during charge-discharge cycles can be suppressed, and the cycle retention rate can be improved.

[0044] The negative electrode active material layer 222 on the membrane 230 side has a larger NVA polymer mass fraction than the negative electrode active material layer 222 on the negative electrode current collector 221 side. Specifically, for example, the portion of the negative electrode active material layer 222 in contact with the membrane 230 has a larger NVA polymer mass fraction than the portion of the negative electrode active material layer 222 in contact with the negative electrode current collector 221. In the first embodiment, the negative electrode active material layer 222 has a first layer 222a on the negative electrode current collector 221 side and a second layer 222b on the membrane 230 side. The second layer 222b has a larger NVA polymer mass fraction than the first layer 222a. This further improves the adhesion between the membrane 230 and the negative electrode 220, thus further improving cycle characteristics. Furthermore, methods for differentiating the NVA polymer mass fraction are not limited to the methods described above. For example, the manufacturing conditions of the negative electrode active material layer 222, such as the drying rate of the negative electrode slurry, can be changed to make the mass fraction of NVA polymer in the negative electrode active material layer 222 on the diaphragm 230 side greater than that on the negative electrode current collector 221 side. The distribution of the mass fraction of NVA polymer in the negative electrode active material layer 222 along the thickness direction can be determined by quantifying the amount of N (nitrogen) contained in the NVA polymer in the EDX mapping image of a cross-section along the thickness direction of the negative electrode active material layer 222. The negative electrode active material layer 222 is divided into half regions along the thickness direction, and the mass concentration of N contained in the first layer 222a on the negative electrode current collector 221 side and the mass concentration of N contained in the second layer 222b on the diaphragm 230 side are measured. Based on the measured mass concentration of N, the difference in the mass concentration of N between the first layer 222a and the second layer 222b is calculated using the following formula (2). If the difference in the mass concentration of N is greater than 10%, it can be considered that the mass fraction of NVA polymer is larger on the negative electrode active material layer 222 on the diaphragm 230 side compared to the negative electrode active material layer 222 on the negative electrode current collector 221 side.

[0045] (The difference in mass concentration of N between the first layer 222a and the second layer 222b) (%) = [(Mass percentage of N contained in the second layer 222b) - (Mass percentage of N contained in the first layer 222a)] / [(Mass percentage of N contained in the second layer 222b) + (Mass percentage of N contained in the first layer 222a)] × 100… (2)

[0046] Furthermore, the negative electrode active material layer 222 is not limited to containing only the negative electrode active material and the second polymer compound; for example, it may also contain a negative electrode conductive agent. The negative electrode conductive agent includes at least one of carbon materials, metallic materials, and conductive polymer compounds. Specific examples of carbon materials used as negative electrode conductive agents are particulate carbon materials such as carbon black, acetylene black, and Ketjen black, and fibrous carbon materials such as carbon nanotubes. Carbon nanotubes are, for example, single-walled carbon nanotubes (SWCNTs). This improves the electronic conductivity of the particle surface of the first negative electrode active material. The mass fraction of the negative electrode conductive agent relative to the negative electrode active material layer 222 is preferably 5% or less, more preferably 2% or less. This allows for good coatability of the negative electrode slurry.

[0047] The separator 230 is a membrane that insulates the positive electrode 210 and the negative electrode 220. The separator 230 is disposed between the main surface of the positive electrode 210 and the main surface of the negative electrode 220, so that the positive electrode 210 and the negative electrode 220 do not directly contact each other. Figure 1 In this example, the diaphragm 230 is a rectangular sheet when viewed from above in the thickness direction. The diaphragm 230 includes a substrate 231, a positive electrode side coating 232, and a negative electrode side coating 233.

[0048] The material of substrate 231 is preferably electrically stable, chemically stable relative to the positive electrode active material, the negative electrode active material, and the electrolyte, and has insulating properties. Substrate 231 can be, for example, a layer composed of polymeric nonwoven fabric, porous membrane, glass, or ceramic fibers. More preferably, the material of substrate 231 comprises a porous polyolefin membrane. Therefore, battery safety can be improved through short-circuit prevention and shut-off effects.

[0049] The positive electrode side coating 232 is a layer covering the positive electrode 210 side of the substrate 231. The positive electrode side coating 232 contains a first polymer compound. As a result, the adhesion between the separator 230 and the positive electrode 210 is improved, thus enhancing the cycle performance.

[0050] The negative electrode side coating 233 is a layer covering the negative electrode 220 side of the substrate 231. The negative electrode side coating 233 contains NVA polymer. As a result, the adhesion between the separator 230 and the negative electrode 220 is improved, thus enhancing the cycle performance.

[0051] The positive electrode side coating 232 and the negative electrode side coating 233 preferably further comprise particles composed of inorganic materials. Examples of such inorganic materials include metals, semiconductors, oxides of metals and semiconductors, and nitrides of metals and semiconductors. Examples of metals used as inorganic materials include aluminum (Al) and titanium (Ti). Examples of semiconductors used as inorganic materials include silicon (Si) and boron (B). Furthermore, examples of oxides and nitrides used as inorganic materials include aluminum oxide (Al₂O₃), boron nitride (BN), aluminum nitride (AlN), titanium dioxide (TiO₂), and silicon dioxide (SiO₂). Moreover, the inorganic material preferably possesses excellent insulation properties, high availability, and high heat capacity. The particle size of the inorganic material is not particularly limited; for example, when using aluminum oxide as the inorganic material, inorganic material particles with an average particle size of 0.5 μm can be used.

[0052] The electrolyte is impregnated into the diaphragm 230. Figure 1 In this example, the electrolyte fills the space within the outer packaging component 30. The electrolyte is a non-aqueous electrolyte containing an electrolyte salt and a solvent for dissolving the electrolyte salt.

[0053] Electrolyte salts include lithium salts such as lithium perchlorate (LiClO4), lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2), lithium bis(pentafluoroethanesulfonyl)imide (LiN(SO2C2F5)2), and lithium hexafluoroarsenate (LiAsF6).

[0054] Solvents include, for example, ester solvents such as γ-butyrolactone, γ-valerolactone, δ-valerolactone, and ε-caprolactone; carbonate solvents such as ethylene carbonate, propylene carbonate, butyl carbonate, vinylene carbonate, dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate; ether solvents such as 1,2-dimethoxyethane, 1-ethoxy-2-methoxyethane, 1,2-diethoxyethane, tetrahydrofuran, and 2-methyltetrahydrofuran; nitrile solvents such as acetonitrile; sulfolane solvents; phosphoric acid solvents; phosphate solvents; pyrrolidone solvents; and other non-aqueous solvents.

[0055] The electrolyte preferably contains at least one of fluorinated carboxylic acid esters, sulfonates, sulfonic anhydrides, and carboxylic anhydrides as additives. This promotes the formation of a low-resistivity SEI (Solid Electrolyte Interphase), thereby improving charge load characteristics. Examples of fluorinated carboxylic acid esters include fluoroethylene carbonate. Examples of sulfonic anhydrides include propanedisulfonic acid anhydride (PSAH). Examples of sulfonates include 1,3-propanesulfonyl lactone. Examples of carboxylic anhydrides include 1,4-dioxane-2,6-dione.

[0056] The battery according to the first embodiment has been described above, but the secondary battery according to the first embodiment is not limited to... Figure 1 The secondary battery shown is illustrated below. Other examples will be described using the accompanying drawings, and will be compared with... Figure 1 as well as Figure 2 The same structural labels are used in the attached diagrams, but the explanations are omitted.

[0057] Figure 3 This is a cross-sectional view showing different examples of the secondary battery involved in the first embodiment. Figure 4 yes Figure 3 A schematic diagram of the cross section of line IV-IV. Figure 3 as well as Figure 4 The secondary battery 1A shown is Figure 1 The difference in the examples is that the electrode body 200 is a structure wound around the positive electrode lead 21A and the negative electrode lead 22A.

[0058] Battery element 20A is disposed inside the outer packaging component 30. For example... Figure 4 As shown, the battery element 20A includes an electrode body 200A, a positive electrode lead 21A, a negative electrode lead 22A, and a protective material 23. The positive electrode lead 21A is a terminal extending from the inside of the battery element 20A to the outside of the outer packaging component 30, and is located near the center of the battery element 20A. The negative electrode lead 22A is a terminal extending from the inside of the battery element 20A to the outside of the outer packaging component 30, and is located near the center of the battery element 20A. The protective material 23 is a component that protects the outside of the battery element 20A. The protective material 23 is wound around the electrode body 200A. The protective material 23 is, for example, an insulating strip.

[0059] exist Figure 4In this example, the electrode body 200A is a laminate used for the charge-discharge reaction of the secondary battery according to the first embodiment. The electrode body 200A includes: a positive electrode 210A, having a positive current collector 211A and a positive active material layer 212A; a negative electrode 220A, having a negative current collector 221A and a negative active material layer 222A; and a separator 230A. The electrode body 200A has a structure wound around the positive electrode lead 21A and the negative electrode lead 22A as the center, and is laminated from the outside, i.e., the protective material 23 side, in the following order: negative current collector 221A, negative active material layer 222A, separator 230A, positive active material layer 212A, positive current collector 211A, positive active material layer 212A, separator 230A, and negative active material layer 222A. The electrode body 200A does not have any layers other than the negative current collector 221A, the separator 230A, and the positive current collector 211A near the positive lead 21A and the negative lead 22A. Through this structure, the positive current collector 211A is connected to the positive lead 21A, and the negative current collector 221A is connected to the negative lead 22A.

[0060] As described above, the secondary battery 1 according to the first embodiment includes a positive electrode 210, a negative electrode 220, a separator 230 located between the positive electrode 210 and the negative electrode 220, and an electrolyte. The positive electrode 210 has a positive electrode active material layer 212. The positive electrode active material layer 212 includes a positive electrode active material and a first polymer compound. The negative electrode 220 has a negative electrode active material layer 222. The negative electrode active material layer 222 includes a negative electrode active material and a second polymer compound. The negative electrode active material includes a first negative electrode active material containing silicon. The second polymer compound includes an NVA polymer, which contains N-vinylacetamide as a monomer. The separator 230 has a substrate 231, a positive electrode side coating 232 located on the positive electrode 210 side of the substrate 231, and a negative electrode side coating 233 located on the negative electrode 220 side of the substrate 231. The positive electrode side coating 232 includes the first polymer compound. The negative electrode side coating 233 includes the NVA polymer.

[0061] Therefore, during charge-discharge cycles, the adhesion between the diaphragm 230 and the electrode can be maintained. In addition, the reduction in electronic conductivity between negative electrode active material particles caused by the expansion and contraction of the negative electrode active material layer 222 during charge-discharge cycles can be suppressed, thereby improving cycle characteristics.

[0062] Preferably, the negative electrode active material layer 222 on the side of the separator 230 has a larger mass fraction of NVA polymer compared to the negative electrode active material layer 222 on the opposite side of the separator 230. This maintains the adhesion between the separator 230 and the negative electrode 220, further improving cycle performance.

[0063] As a preferred embodiment, the negative electrode active material further includes a second negative electrode active material containing carbon, and the mass fraction of the first negative electrode active material relative to the negative electrode active material is 30% or more. This further improves the cycle performance.

[0064] (Example)

[0065] The embodiments will be described below. Table 1 is a table showing the embodiments and comparative examples. Furthermore, the present invention is not limited to these embodiments.

[0066] Table 1

[0067] (Example 1)

[0068] The positive electrode involved in Example 1 was prepared by the following method. 95% by mass of lithium cobalt oxide (LiCoO2) as the positive electrode active material, 2% by mass of amorphous carbon powder (Ketjen Black) as the conductive agent, and 3% by mass of PVdF as the first polymer compound were mixed and dispersed in N-methyl-2-pyrrolidone (NMP) to obtain a positive electrode slurry. The prepared positive electrode slurry was uniformly coated on both sides of a 10 μm thick strip of aluminum foil, serving as the positive electrode current collector, to prepare a positive electrode coating. After hot air drying, the obtained positive electrode coating was compressed and molded using a roller press to form a positive electrode sheet. The formed positive electrode sheet was then cut into 70 mm × 800 mm strips to prepare the positive electrode involved in Example 1. In the prepared positive electrode, a positive electrode lead was installed on the exposed portion of the positive electrode current collector.

[0069] The negative electrode involved in Example 1 was fabricated by the following method. Li-doped SiO₂ was used as the first negative electrode active material. x 47.5% by mass of MCMB (as the second negative electrode active material), 47.5% by mass of PNVA (as the second polymer compound), 0.7% by mass of carbon black (as the negative electrode conductive agent), and 0.3% by mass of SWCNT were added to an appropriate amount of ion-exchange water, and the mixture was kneaded and stirred using a rotary mixer to obtain the first layer of negative electrode slurry. Similarly, Li-doped SiO₂ was used as the first negative electrode active material. x46.5% by mass of MCMB (as the second negative electrode active material), 46.5% by mass of PNVA (as the second polymer compound), 0.7% by mass of carbon black (as the negative electrode conductive agent), and 0.3% by mass of SWCNT were added to an appropriate amount of ion-exchange water and mixed and stirred using a rotary mixer to obtain the second layer of negative electrode slurry. This slurry was uniformly and continuously coated on both sides of an 8μm thick copper foil (as the negative electrode current collector) in the order of the first and second layers of negative electrode slurry, forming a negative electrode coating. After hot air drying, the resulting negative electrode coating was compressed and molded using a roller press to form a negative electrode sheet. The negative electrode sheet was cut into 72mm × 810mm strips to fabricate the negative electrode described in Example 1. In the fabricated negative electrode, negative electrode leads were installed on the exposed portion of the negative electrode current collector.

[0070] In the following explanation, we will describe the case where the first negative electrode active material in the first layer is A1 by mass% and the second negative electrode active material is B1 by mass%. In this case, the mass fraction w1 (%) of the first negative electrode active material in the first layer relative to the negative electrode active material has the following relationship as shown in equation (3). Furthermore, we will describe the case where the first negative electrode active material in the second layer is A2 by mass% and the second negative electrode active material is B2 by mass%. In this case, the mass fraction w2 (%) of the first negative electrode active material in the second layer relative to the negative electrode active material has the following relationship as shown in equation (4).

[0071] w1=A1 / (A1+B1)×100…(3)

[0072] w2=A2 / (A2+B2)×100…(4)

[0073] In Example 1, in the first and second layers, the first negative electrode active material is a silicon compound (SiO2). x The second negative electrode active material is MCMB. As shown in Table 1, the mass fraction w1 of Example 1 is 50% (=47.5 mass% / (47.5 mass%+47.5 mass%)). The mass fraction w2 of Example 1 is 50% (=46.5 mass% / (46.5 mass%+46.5 mass%)).

[0074] The membrane involved in Example 1 was manufactured by the following method. In a solution prepared by mixing and fully dissolving PVdF (with an average molecular weight of 150,000) as a binder and NMP (as a solvent) at a mass ratio of 10:90, alumina (Al2O3) micro-powder, consisting of inorganic particles with an average particle size of 0.5 μm, was added at twice the mass relative to the PVdF, and the mixture was stirred thoroughly to prepare a coating slurry for the positive electrode side layer. In a solution prepared by mixing PNVA (with an average molecular weight of 1,000,000) as a binder and ion-exchanged water as a solvent at a mass ratio of 3:97, alumina (Al2O3) micro-powder, consisting of inorganic particles with an average particle size of 0.5 μm, was added at twice the mass relative to the PNVA, and the mixture was stirred thoroughly to prepare a coating slurry for the negative electrode side layer. The coating slurry for the prepared positive and negative electrode side sheets was applied to the respective surfaces of a 15 μm thick microporous polyethylene membrane, forming positive and negative electrode side sheets with a thickness of 3 μm, thereby producing a separator sheet. The separator sheet was cut into strips of 74 mm × 860 mm to produce the separator involved in Example 1.

[0075] The electrolyte involved in Example 1 was prepared by dissolving lithium hexafluorophosphate (LiPF6) as an electrolyte salt in a solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a mass ratio of 5:5, with a molar concentration of 1 mol / L.

[0076] The secondary battery described in Example 1 was manufactured using the following method. The positive and negative electrodes were stacked and sealed together with the prepared separator in between, and wound along their long sides. A protective tape, serving as a protective material, was attached to the outermost periphery of the wound stack, thereby creating an electrode body. The manufactured electrode body was then placed in an outer packaging component, and three sides of the outer packaging component were heat-sealed, while one side remained open without heat-sealing. The outer packaging component used a laminated film, which consisted of a 25μm thick nylon film as the outermost layer, a 40μm thick aluminum foil as the metal layer, and a 30μm thick polypropylene film as the insulating layer. Subsequently, the prepared electrolyte was injected through the opening in the outer packaging component, and the remaining side of the outer packaging component was heat-sealed under reduced pressure, thereby sealing the outer packaging component and manufacturing the secondary battery described in Example 1.

[0077] In Example 1, the negative electrode capacity was calculated using the following method. Here, negative electrode capacity refers to the capacity of the negative electrode active material.

[0078] Using a positive electrode with a positive electrode active material layer as described in Example 1 formed only on one side of the positive electrode current collector, a Li button cell with a counter electrode is fabricated. The cell is charged and its capacity is measured under the following conditions to determine the charging capacity of the positive electrode active material layer at each thickness.

[0079] Charging rate: 0.1C

[0080] Charging method: CCCV

[0081] Charging control voltage: 4.45V

[0082] Charging termination current: 0.01C

[0083] Similarly, using a negative electrode with the negative electrode active material layer of Example 1 formed only on one side of the negative electrode current collector, a counter electrode Li button cell was fabricated. The cell was charged and its capacity was measured under the following conditions to determine the charging capacity of the negative electrode active material at each thickness.

[0084] Charging rate: 0.1C

[0085] Charging method: CCCV

[0086] Charging control voltage: 0V

[0087] Charging termination current: 0.01C

[0088] Using the charging capacity of the positive electrode active material at each thickness and the charging capacity of the negative electrode active material at each thickness as measured above, the thicknesses of the positive and negative electrode active material layers were adjusted by changing the solid content of the positive and negative electrode slurries, coating speed, etc., so that the charging capacity of the positive electrode relative to the charging capacity of the negative electrode was 0.9, thereby fabricating a test battery. The fabricated test battery was discharged under the following conditions and its capacity was measured to calculate the negative electrode capacity. Here, the negative electrode capacity is calculated as a relative value to the value in Example 6 described later.

[0089] Discharge rate: 0.1C

[0090] Discharge method: CC

[0091] Discharge termination voltage: 1.0V

[0092] In Example 1, the cyclic characteristic test was conducted using the following method. The cyclic characteristic test was performed at 23°C.

[0093] The battery prepared above was subjected to its first charge-discharge cycle under the following conditions.

[0094] Charging rate: 0.2C

[0095] Charging method: CCCV

[0096] Charging control voltage: 4.40V

[0097] Charging termination current: 0.025C

[0098] Discharge rate: 0.1C

[0099] Discharge method: CC

[0100] Discharge termination voltage: 3.0V

[0101] Subsequently, the second to the 100th cycles were performed under the following conditions: the discharge capacity of the second cycle and the discharge capacity of the 100th cycle were measured, and the ratio of the discharge capacity of the 100th cycle to the discharge capacity of the second cycle was calculated as the cycle retention rate.

[0102] Charging rate: 1.0C

[0103] Charging method: CCCV

[0104] Charging control voltage: 4.40V

[0105] Charging termination current: 0.025C

[0106] Discharge rate: 1.0C

[0107] Discharge method: CC

[0108] Discharge termination voltage: 3.0V

[0109] (Example 2)

[0110] In Example 2, as shown in Table 1, the battery was fabricated in the same manner as in Example 1, except that the second polymer compound and the binder of the negative electrode side coating were changed to a copolymer of NVA and sodium acrylate (NVA-AANa), and the measurements were performed.

[0111] (Example 3)

[0112] In Example 3, as shown in Table 1, the battery was fabricated in the same manner as in Example 1, except that the second polymer compound and the binder of the negative electrode side coating were changed to a copolymer of NVA and lithium acrylate (NVA-AALi), and the measurements were performed.

[0113] (Example 4)

[0114] In Example 4, as shown in Table 1, the battery was fabricated in the same manner as in Example 1, except that the second polymer compound and the binder of the negative electrode side coating were changed to a copolymer of NVA and potassium acrylate (NVA-AAK), and the measurements were performed.

[0115] (Example 5)

[0116] In Example 5, as shown in Table 1, the battery was fabricated in the same manner as in Example 1, except that the mass fraction of the second polymer compound in the first layer and the second layer was set to the same 5%. The measurements were then performed.

[0117] (Example 6)

[0118] In Example 6, as shown in Table 1, the battery was manufactured and measured in the same manner as in Example 1, except that the mass fraction w1 of the first negative electrode active material in the first layer relative to the negative electrode active material was changed to 10% (=9.5 mass% / (9.5 mass%+85.5 mass%)) and the mass fraction w2 of the second negative electrode active material in the second layer relative to the negative electrode active material was changed to 10% (=9.3 mass% / (9.3 mass%+83.7 mass%)).

[0119] (Example 7)

[0120] In Example 7, as shown in Table 1, the battery was manufactured and measured in the same manner as in Example 1, except that the mass fraction w1 of the first negative electrode active material in the first layer relative to the negative electrode active material was changed to 30% (=28.5 mass% / (28.5 mass%+66.5 mass%)) and the mass fraction w2 of the second negative electrode active material in the second layer relative to the negative electrode active material was changed to 30% (=27.9 mass% / (27.9 mass%+65.1 mass%)).

[0121] (Example 8)

[0122] In Example 8, as shown in Table 1, the battery was manufactured and measured in the same manner as in Example 1, except that the mass fraction w1 of the first negative electrode active material in the first layer relative to the negative electrode active material was changed to 70% (=66.5 mass% / (66.5 mass%+28.5 mass%)) and the mass fraction w2 of the second negative electrode active material in the second layer relative to the negative electrode active material was changed to 70% (=65.1 mass% / (65.1 mass%+27.9 mass%)).

[0123] (Example 9)

[0124] In Example 9, as shown in Table 1, the battery was manufactured and measured in the same manner as in Example 1, except that the mass fraction w1 of the first negative electrode active material in the first layer relative to the negative electrode active material was changed to 100% (=91.8 mass% / (91.8 mass%+0 mass%)) and the mass fraction w2 of the second negative electrode active material in the second layer relative to the negative electrode active material was changed to 100% (=88.2 mass% / (88.2 mass%+0 mass%)).

[0125] (Example 10)

[0126] In Example 10, as shown in Table 1, the battery was fabricated and measured in the same manner as in Example 9, except that the first negative electrode active material in the first layer and the second layer was changed to elemental silicon (Si), the mass fraction w1 of the first negative electrode active material in the first layer relative to the negative electrode active material was changed to 100% (=91.8 mass% / (91.8 mass%+0 mass%)), and the mass fraction w2 of the second negative electrode active material in the second layer relative to the negative electrode active material was set to 100% (=88.2 mass% / (88.2 mass%+0 mass%)).

[0127] (Example 11)

[0128] In Example 11, as shown in Table 1, except that the first negative electrode active material of the first layer and the second layer is changed to a silicon-containing alloy (SiTi) 0.01 The battery was manufactured in the same manner as in Example 9, except that the mass fraction w1 of the first negative electrode active material in the first layer relative to the negative electrode active material was changed to 100% (=91.8 mass% / (91.8 mass%+0 mass%)) and the mass fraction w2 of the second negative electrode active material in the second layer relative to the negative electrode active material was set to 100% (=88.2 mass% / (88.2 mass%+0 mass%)).

[0129] (Comparative Example 1)

[0130] In Comparative Example 1, as shown in Table 1, the battery was manufactured in the same manner as in Example 1, except that the binder of the positive electrode side coating of the separator was changed to PNVA, and the measurements were performed.

[0131] (Comparative Example 2)

[0132] In Comparative Example 2, as shown in Table 1, the battery was manufactured in the same manner as in Example 1, except that the binder of the negative electrode side coating of the separator was changed to PVdF, and the measurements were performed.

[0133] (Comparative Example 3)

[0134] In Comparative Example 3, as shown in Table 1, the battery was prepared in the same manner as in Example 1, except that the second polymer compound was changed to polyacrylic acid (PAA), and the results were measured.

[0135] (Comparative Example 4)

[0136] In Comparative Example 4, as shown in Table 1, the battery was prepared in the same manner as in Example 1, except that the second polymer compound and the binder of the negative electrode side coating were changed to PAA, and the measurements were performed.

[0137] As shown in Table 1, in Examples 1 to 11, by making the binder contained in the positive electrode side coating of the separator the same as the first polymer compound, the cycle retention rate can be improved compared to Comparative Example 1, where the binder contained in the positive electrode side coating of the separator is different from the first polymer compound.

[0138] As shown in Table 1, in Examples 1 to 11, by making the binder contained in the negative electrode side coating of the diaphragm the same as the second polymer compound, the cycle retention rate can be improved compared to Comparative Example 2, where the binder contained in the negative electrode side coating of the diaphragm is different from the second polymer compound.

[0139] As shown in Table 1, in Examples 1 to 11, by making the second polymer compound an NVA polymer, the cycle retention rate can be improved compared to Comparative Examples 3 and 4, where the second polymer compound is not an NVA polymer.

[0140] As shown in Table 1, in Examples 1 to 4, by making the mass fraction of the second polymer compound in the second layer greater than that in the first layer, the cycle retention rate can be further improved compared to Example 5, where the mass fraction of the second polymer compound in the second layer is equal to that in the first layer.

[0141] As shown in Table 1, in Examples 1 to 5 and Examples 7 to 9, by making the mass fraction w1 and w2 of the first negative electrode active material in the first layer and the second layer relative to the negative electrode active material 30% or more, the negative electrode capacity can be improved compared with Example 6, where the mass fraction w1 and w2 of the first negative electrode active material in the first layer and the second layer relative to the negative electrode active material is less than 30%.

[0142] The above embodiments are provided for easy understanding of the present invention and are not intended to limit or explain the embodiments of the present invention. The present invention can be modified / improved without departing from its spirit, and equivalents are also included in the present invention.

[0143] Explanation of reference numerals in the attached figures

[0144] 1, 1A: Secondary battery; 20, 20A: Battery element; 21, 21A: Positive electrode lead; 22, 22A: Negative electrode lead; 23: Protective material; 30: Outer packaging component; 30a, 30b: Outer packaging sheet; 31: Recess; 32: Sealing material; 200, 200A: Electrode body; 210, 210A: Positive electrode; 211, 211A: Positive electrode current collector; 212, 212A: Positive electrode active material layer; 220, 220A: Negative electrode; 221, 221A: Negative electrode current collector; 222, 222A: Negative electrode active material layer; 222a: First layer; 222b: Second layer; 230, 230A: Separator; 231: Substrate; 232: Positive electrode side coating; 233: Negative electrode side coating.

Claims

1. A secondary battery comprising a positive electrode, a negative electrode, a separator located between the main surface of the positive electrode and the main surface of the negative electrode, and an electrolyte. The positive electrode has a positive electrode active material layer. The positive electrode active material layer comprises a positive electrode active material and a first polymer compound. The negative electrode has a negative electrode active material layer. The negative electrode active material layer comprises a negative electrode active material and a second polymer compound. The negative electrode active material includes a first negative electrode active material containing silicon. The second polymeric compound comprises an NVA polymer, wherein the NVA polymer contains N-vinylacetamide as a monomer. The diaphragm has a substrate, a positive electrode side coating located on the positive electrode side of the substrate, and a negative electrode side coating located on the negative electrode side of the substrate. The positive electrode side coating contains the first polymer compound. The negative electrode side coating contains the NVA polymer.

2. The secondary battery according to claim 1, wherein, The NVA polymer has a larger mass fraction on one side of the membrane compared to the negative electrode active material layer on the opposite side of the membrane.

3. The secondary battery according to claim 1 or 2, wherein, The negative electrode active material also includes a second negative electrode active material containing carbon. The mass fraction of the first negative electrode active material relative to the negative electrode active material is 30% or more.

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