Negative electrode for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery

Abstract

The present invention provides: a negative electrode for a nonaqueous electrolyte secondary battery, the negative electrode having high capacity and excellent high-rate charge cycle characteristics; and a nonaqueous electrolyte secondary battery. A negative electrode for a nonaqueous electrolyte secondary battery, which is an example of the embodiment of the present invention, includes: a negative electrode current collector; and a negative electrode mixture layer that is disposed on the surface of the negative electrode current collector. The surface roughness Rz of the negative electrode current collector is 1.0 μm or more and 2.5 μm or less. The negative electrode mixture layer contains a negative electrode active material that contains graphite and a silicon-containing material, and carboxymethyl cellulose or a similar derivative thereof. The degree of etherification of the carboxymethyl cellulose or the similar derivative thereof is 0.8 or more and 1.5 or less.

Description

Negative electrode for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery

[0001] The present disclosure relates to a negative electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery, and more particularly to a negative electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery that have high capacity and good rapid charge cycle characteristics.

[0002] Electrodes, which are major components of non-aqueous electrolyte secondary batteries such as lithium ion batteries, have a significant effect on battery performance such as battery capacity and cycle characteristics, and therefore, electrodes have been the subject of much research. For example, Patent Document 1 discloses a negative electrode in which a negative electrode mixture layer containing silicon oxide in an amount of 0.5% by mass to 20% by mass and containing a carboxymethyl cellulose-ammonium salt is disposed on the surface of a negative electrode current collector having a thickness of 5.9 μm to 8.1 μm and a surface roughness Rz of 0.8 μm to 1.5 μm, with the aim of obtaining a non-aqueous electrolyte secondary battery with high capacity and excellent cycle characteristics.

[0003] International Publication No. 2015 / 146079

[0004] In recent years, non-aqueous electrolyte secondary batteries have been widely used in applications requiring high capacity, such as in-vehicle applications and power storage applications, and there is a strong demand for high capacity and small capacity loss during repeated rapid charging, i.e., excellent rapid charging cycle characteristics. Patent Document 1 does not consider achieving both high capacity and rapid charging cycle characteristics, and there is still room for improvement.

[0005] An object of the present disclosure is to provide a negative electrode for a non-aqueous electrolyte secondary battery that has high capacity and good rapid charge cycle characteristics, and a non-aqueous electrolyte secondary battery.

[0006] The negative electrode for a non-aqueous electrolyte secondary battery according to the present disclosure includes a negative electrode current collector and a negative electrode mixture layer disposed on a surface of the negative electrode current collector, wherein the surface roughness Rz of the negative electrode current collector is 1.0 μm or more and 2.5 μm or less, and the negative electrode mixture layer includes a negative electrode active material containing graphite and a silicon-containing material, and carboxymethyl cellulose or an analogous derivative thereof, and the degree of etherification of the carboxymethyl cellulose or an analogous derivative thereof is 0.8 or more and 1.5 or less.

[0007] A non-aqueous electrolyte secondary battery according to the present disclosure is characterized by including the above-described negative electrode for a non-aqueous electrolyte secondary battery, a positive electrode, and a non-aqueous electrolyte.

[0008] The negative electrode for a non-aqueous electrolyte secondary battery according to the present disclosure can achieve both high capacity and rapid charge cycle characteristics.

[0009] 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment;

[0010] As described above, improving rapid charge cycle characteristics while increasing the capacity of batteries is an important issue. In particular, for automotive batteries, high capacity and minimal capacity loss during repeated rapid charging, i.e., excellent rapid charge cycle characteristics, are strongly desired. The use of carboxymethyl cellulose with a high degree of etherification as a binder for the negative electrode mixture layer improves the permeability of the non-aqueous electrolyte into the negative electrode mixture layer, enabling rapid charging. However, the inventors' studies have revealed that, because negative electrodes containing silicon-containing materials undergo a large rate of expansion and contraction during charge and discharge, the use of carboxymethyl cellulose with a high degree of etherification may cause the negative electrode mixture layer to peel off from the negative electrode current collector.

[0011] The present inventors further conducted extensive research into the above-mentioned problems and found that by using a negative electrode in which a negative electrode mixture layer containing a negative electrode active material containing graphite and a silicon-containing material, and a carboxymethylcellulose having a degree of etherification of 0.8 or more and 1.5 or less, is disposed on the surface of a negative electrode current collector having a surface roughness Rz of 1.0 μm or more and 2.5 μm or less, it is possible to uniquely achieve both high capacity and rapid charge cycle characteristics.

[0012] Hereinafter, an example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail with reference to the drawings. Note that configurations obtained by selectively combining the respective components of the multiple embodiments and variations described below are included within the scope of the present disclosure.

[0013] In the following, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom is exemplified as a nonaqueous electrolyte secondary battery, but the outer can of the battery is not limited to a cylindrical outer can. The nonaqueous electrolyte secondary battery according to the present disclosure may be, for example, a prismatic battery having a prismatic outer can, a coin battery having a coin-shaped outer can, or a laminate battery having an outer can made of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode assembly is not limited to a wound type, and may be a laminated type electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0014] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown in FIG. 1 , the nonaqueous electrolyte secondary battery 10 is a cylindrical battery including an electrode assembly 14, a nonaqueous electrolyte, a cylindrical outer can 16 with a bottom that accommodates the electrode assembly 14 and the nonaqueous electrolyte, and a sealing member 17 that closes the opening of the outer can 16. The electrode assembly 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The outer can 16 has a groove 22 formed in its sidewall, and the sealing member 17 is supported by the groove 22 and closes the opening of the outer can 16. Hereinafter, for convenience of explanation, the sealing member 17 side of the nonaqueous electrolyte secondary battery 10 is referred to as the top, and the bottom side of the outer can 16 is referred to as the bottom.

[0015] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents of these. The non-aqueous solvent may contain a halogen-substituted compound (e.g., fluoroethylene carbonate) in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine. Examples of the electrolyte salt include LiPF ６ Lithium salts such as

[0016] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all long, strip-like bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium precipitation. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the longitudinal and width directions. The separator 13 is formed to be at least slightly larger than the positive electrode 11, and, for example, two separators 13 are arranged to sandwich the positive electrode 11. The electrode assembly 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0017] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0018] The negative electrode 12 is disposed on the outer peripheral surface of the electrode body 14, and an exposed portion may be provided in which the surface of the negative electrode current collector (see FIG. 2 described later) constituting the negative electrode 12 is exposed. In this case, the exposed portion may be in contact with the inner peripheral surface of the outer can 16, and the negative electrode 12 and the outer can 16 may be electrically connected.

[0019] A gasket 28 is provided between the outer can 16 and the sealing body 17, ensuring airtightness inside the battery and preventing electrical contact between the outer can 16 and the sealing body 17. The outer can 16 has a groove 22 formed on its side surface that protrudes inward and supports the sealing body 17. The groove 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the outer can 16 by the groove 22 and the open end of the outer can 16 that is crimped to the sealing body 17.

[0020] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0021] The positive electrode 11, the negative electrode 12, and the separator 13 that constitute the electrode assembly 14, with the negative electrode 12 being particularly described in detail below.

[0022] [Positive Electrode] The positive electrode 11 includes a positive electrode current collector and a positive electrode mixture layer disposed on the surface of the positive electrode current collector. The positive electrode current collector can be a foil of a metal stable within the potential range of the positive electrode 11, such as aluminum, an aluminum alloy, stainless steel, or titanium, or a film with such a metal disposed on the surface. The positive electrode mixture layer contains a positive electrode active material, a conductive agent, and a binder, and is preferably provided on both sides of the positive electrode current collector except for the portion where the positive electrode lead 20 is connected. A protective layer containing inorganic particles and a binder may be provided between the positive electrode current collector and the positive electrode mixture layer, or on the positive electrode mixture layer. The positive electrode 11 can be fabricated, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder to the positive electrode current collector, drying the coating, and then compressing it to form a positive electrode mixture layer on both sides of the positive electrode current collector.

[0023] The positive electrode active material uses a lithium-containing composite oxide containing transition metal elements such as Ni, Co, and Mn. Examples of metal elements contained in the lithium-containing composite oxide include Ni, Co, Mn, Al, Be, B, Na, Mg, Si, K, Ca, Sc, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, Ta, W, Pb, and Bi. Among these, it is preferable to contain at least one of Ni, Co, and Mn. Examples of suitable composite oxides include lithium-containing composite oxides containing Ni, Co, and Mn, and lithium-containing composite oxides containing Ni, Co, and Al. One type of lithium-containing composite oxide may be used alone, or multiple types may be used in combination.

[0024] The lithium-containing composite oxide has, for example, a layered rock salt structure. Examples of the layered rock salt structure include a layered rock salt structure belonging to the space group R-3m and a layered rock salt structure belonging to the space group C2 / m. Among these, a layered rock salt structure belonging to the space group R-3m is preferred from the viewpoints of increasing capacity and stability of the crystal structure.

[0025] Examples of the conductive agent contained in the positive electrode mixture layer include carbon black such as acetylene black and ketjen black, graphite, carbon nanotubes (CNT), carbon nanofibers, graphene, metal fibers, metal powder, conductive whiskers, etc. One type of conductive agent may be used alone, or multiple types may be used in combination.

[0026] Examples of binders contained in the positive electrode mixture layer include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), olefin resins such as polyethylene, polypropylene, ethylene-propylene-isoprene copolymer, and ethylene-propylene-butadiene copolymer, and acrylic resins such as polyacrylonitrile (PAN), polyimide, polyamide, and ethylene-acrylic acid copolymer. These resins may also be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), or the like. One type of binder may be used alone, or multiple types may be used in combination.

[0027] [Negative Electrode] The negative electrode has a negative electrode current collector and a negative electrode mixture layer disposed on the surface of the negative electrode current collector. The negative electrode current collector can be a foil of a metal that is stable within the potential range of the negative electrode 12, such as copper, a copper alloy, stainless steel, nickel, or a nickel alloy, or a film having such a metal disposed on its surface. The negative electrode current collector is preferably a copper foil.

[0028] The surface roughness Rz of the negative electrode current collector is 1.0 μm or more and 2.5 μm or less. If the surface roughness Rz of the negative electrode current collector is less than 1.0 μm, the negative electrode mixture layer is likely to peel off from the negative electrode current collector. If the surface roughness Rz of the negative electrode current collector is more than 2.5 μm, the negative electrode current collector may break during battery manufacture or use. The surface roughness Rz of the negative electrode current collector is preferably 1.5 μm or more and 2.0 μm or less. The thickness of the negative electrode current collector is, for example, 5 μm or more and 30 μm or less.

[0029] The negative electrode mixture layer is provided, for example, on both sides of the negative electrode current collector excluding the portion to which the negative electrode lead 21 is connected. A protective layer containing inorganic particles and a binder may be provided between the negative electrode current collector and the negative electrode mixture layer or on the negative electrode mixture layer. The negative electrode 12 can be produced, for example, by applying a negative electrode mixture slurry containing a negative electrode active material and a binder onto the negative electrode current collector, drying the coating, and then rolling the coating to form a negative electrode mixture layer on both sides of the negative electrode current collector.

[0030] The negative electrode mixture layer includes, for example, a negative electrode active material and a binder. The content of the negative electrode active material in the negative electrode mixture layer is, for example, 90% by mass or more and 99.5% by mass or less, or 95% by mass or more and 99% by mass or less, relative to the mass of the negative electrode mixture layer. The negative electrode mixture layer may also include a conductive agent. As the conductive agent, the same conductive agent as in the case of the positive electrode 11 can be used.

[0031] The negative electrode active material contained in the negative electrode mixture layer includes graphite and a silicon-containing material. Graphite is, for example, artificial graphite such as massive artificial graphite (MAG) or graphitized mesophase carbon microbeads (MCMB), natural graphite such as flake graphite, massive graphite, or earthy graphite, or a mixture thereof. The volume-based D50 of the graphite is, for example, 1 μm or more and 30 μm or less. The D50 of the negative electrode active material particles refers to the particle size at which the cumulative frequency in the volume-based particle size distribution is 50% from the smallest particle size. The particle size distribution of the lithium-containing composite oxide can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II manufactured by Microtrack Bell Co., Ltd.) using water as a dispersion medium.

[0032] The silicon-containing material functioning as the negative electrode active material may be any material containing Si, and examples thereof include silicon alloys, silicon compounds, and Si-containing composite materials. Among these, Si-containing composite materials are preferred. The D50 of silicon-containing materials is generally smaller than the D50 of graphite. The volume-based D50 of silicon-containing materials is, for example, 1 μm or more and 20 μm or less.

[0033] A suitable silicon-containing material (composite material) is a composite particle containing an ion-conducting phase and Si particles dispersed in the ion-conducting phase. An example of a suitable ion-conducting phase is at least one selected from the group consisting of a silicon oxide phase, a silicate phase, and a carbon phase. The ion-conducting phase is a continuous phase composed of an aggregate of particles finer than Si particles.

[0034] The composite material may have a conductive layer covering the surface of the ion-conducting phase. The conductive layer is made of a material with higher conductivity than the ion-conducting layer and forms a good conductive path in the negative electrode mixture layer. The conductive layer is, for example, a carbon coating made of a conductive carbon material. Examples of the conductive carbon material include carbon black such as acetylene black and ketjen black, graphite, and amorphous carbon (amorphous carbon) with low crystallinity.

[0035] An example of a suitable Si-containing composite material has a sea-island structure in which fine Si particles are dispersed uniformly in an amorphous silicon oxide phase, and the overall structure is represented by the general formula SiO ｘ(0.5≦x≦1.6). The main component of the silicon oxide may be silicon dioxide. The silicon oxide phase may be doped with Li.

[0036] Another example of a suitable Si-containing composite material is a composite particle having a sea-island structure in which fine Si particles are substantially uniformly dispersed in an amorphous silicate phase. The silicate phase contains at least one element selected from an alkali metal element and a Group 2 element. A suitable silicate phase is a lithium silicate phase containing Li. The lithium silicate phase can be, for example, a compound represented by the general formula Li ２ｚ SiO （２＋ｚ） (0<z<2). The lithium silicate phase contains Li ４ SiO ４ It is preferable that (Z=2) is not included. ４ SiO ４ is an unstable compound and reacts with water to become alkaline, which may cause Si to change and lead to a decrease in charge / discharge capacity. The lithium silicate phase is considered to be a suitable phase for Li, from the viewpoints of stability, productivity, Li ion conductivity, etc. ２ SiO ３ (Z=1) or Li ２ Si ２ O ５ It is preferable that (Z=1 / 2) is used as the main component.

[0037] Another example of a suitable Si-containing composite material is a composite particle having a sea-island structure in which fine Si particles are substantially uniformly dispersed in a carbon phase. The carbon phase is preferably an amorphous carbon phase. The carbon phase may contain a crystalline phase component, but preferably contains a larger amount of amorphous phase components. The composite material may contain particles whose ion-conducting phase is an amorphous carbon phase. The amorphous carbon phase is, for example, composed of a carbon material having an average interplanar spacing of (002) planes of more than 0.34 nm as measured by X-ray diffraction. The composite material containing a carbon phase may or may not have a conductive layer separate from the carbon phase.

[0038] The negative electrode active material contained in the negative electrode mixture layer may include a material other than graphite and a silicon-containing material. For example, it may include a carbon material other than graphite, such as soft carbon or hard carbon, or an element other than Si that alloys with Li, such as tin (Sn), or a material containing such an element.

[0039] The ratio of the silicon-containing material to the total mass of the negative electrode active material is preferably 0.1% by mass or more and 10% by mass or less, more preferably 1% by mass or more and 10% by mass or less, and even more preferably 5% by mass or more and 8% by mass or less. This allows a high-capacity secondary battery to be obtained. Furthermore, since the silicon-containing material has a higher rate of expansion and contraction during charge and discharge than graphite, the higher the ratio of the silicon-containing material, the more likely the rapid charge cycle characteristics are to deteriorate. Therefore, the effect of the present disclosure, which is to achieve both high capacity and rapid charge cycle characteristics, becomes more pronounced as the ratio of the silicon-containing material increases.

[0040] The negative electrode mixture layer contains carboxymethyl cellulose (CMC) or a derivative analogous thereto. The content of carboxymethyl cellulose (CMC) or a derivative analogous thereto in the negative electrode mixture layer is, for example, 0.1% by mass or more and 5% by mass or less, or 0.5% by mass or more and 3% by mass or less, relative to the mass of the negative electrode mixture layer. Examples of CMC-analogous derivatives include sodium salts, lithium salts, ammonium salts, potassium salts, rubidium salts, and cesium salts of CMC, with sodium salts (CMC-Na) and lithium salts (CMC-Li) being preferred. CMC-Na is generally a partially neutralized salt in which a portion of the carboxyl groups is neutralized. The weight-average molecular weight of CMC-Na is, for example, 200,000 to 1,000,000, and preferably 200,000 to 500,000.

[0041] The CMC or its analogous derivative contained in the negative electrode mixture layer functions as a binder and also as a thickener for imparting an appropriate viscosity to the negative electrode mixture slurry. The negative electrode mixture layer preferably contains a binder other than CMC or its analogous derivative. Each negative electrode mixture layer may contain the same binder as that used in the positive electrode 11, but preferably contains styrene-butadiene rubber (SBR). Furthermore, from the perspective of suppressing swelling and shrinkage of silicon, it is more preferable to add a polyacrylic acid (PAA)-based material. Not only the adhesion of the SBR binder, but also the toughness of the PAA binder suppresses swelling and shrinkage of silicon, contributing to improved cycle life.

[0042] The degree of etherification of CMC or its analogous derivative is 0.8 or more and 1.5 or less. If the degree of etherification is less than 0.8, the liquid absorption of the negative electrode decreases, resulting in poor rapid charge cycle performance. If the degree of etherification is more than 1.5, even if the surface roughness Rz of the negative electrode current collector is within a predetermined range, the peel strength of the negative electrode mixture layer decreases, resulting in poor rapid charge cycle performance. The degree of etherification of CMC or its analogous derivative is preferably 1.0 or more and 1.3 or less.

[0043] CMC or its analogous derivatives are water-soluble polymers in which some of the hydroxyl groups in the cellulose backbone have been substituted with carboxymethyl groups. The degree of substitution by carboxymethyl groups is expressed as the degree of etherification. The degree of etherification of CMC or its analogous derivatives can be measured as follows.

[0044] [Method for measuring the degree of etherification of CMC or its analogous derivatives] Approximately 1 g of sample is weighed into a flask and dissolved in an appropriate amount of pure water. Approximately 5 mL of 0.05 M (N / 10) aqueous sulfuric acid is added using a pipette, boiled for several minutes, and cooled. After this, phenolphthalein indicator is added and titration is performed with 0.1 M (N / 10) aqueous sodium hydroxide. Calculate the alkalinity using the following formula: Alkalinity = 5 x F' - mL of 0.1 M (N / 10) aqueous sodium hydroxide x F / sample (g of anhydrous equivalent), where F is the factor of the 0.1 M (N / 10) aqueous sodium hydroxide solution, F' is the factor of the 0.05 M (N / 10) aqueous sulfuric acid solution, and approximately 1 g of sample is weighed out and incinerated at 500°C or higher. After cooling, the crucible is transferred to a beaker, an appropriate amount of pure water is added, and then 50 mL of 0.05 M (N / 10) aqueous sulfuric acid is accurately added and boiled for several minutes. After cooling, add phenolphthalein indicator and titrate with 0.1M (N / 10) aqueous sodium hydroxide solution. A blank titration is carried out in the same manner using this method. The degree of etherification is calculated using the following formula: A = (blank titration mL - sample titration mL) x F / sample (anhydrous equivalent g) - alkalinity F: Factor of 0.1M (N / 10) aqueous sodium hydroxide solution The above A refers to the number of mL of 0.05M (N / 10) aqueous sulfuric acid consumed to neutralize the bound sodium in 1 g of sample. The above A is used to calculate the degree of etherification as follows: Degree of etherification (DS) = 162A / (10000 - 80A)

[0045] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. The separator 13 may have, for example, a multi-layer structure including a thermoplastic resin layer such as polyolefin and a cellulose fiber layer, a two-layer structure of polyethylene (PE) / polypropylene (PP), or a three-layer structure of PE / PP / PE. A filler layer including an inorganic filler or a layer of a highly heat-resistant resin such as aramid resin may be provided on the surface of the separator 13.

[0046] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0047] Experimental Example 1 [Fabrication of Positive Electrode] LiCo was used as the positive electrode active material. ０．９７９ Zr ０．００１ Mg ０．０１ Al ０．０１ O ２ A lithium-containing composite oxide represented by the formula (I) was used. The positive electrode active material, carbon black, and polyvinylidene fluoride were mixed in a solids mass ratio of 95:2.5:2.5, and a positive electrode mixture slurry was prepared using N-methyl-2-pyrrolidone (NMP) as a dispersion medium. Next, the positive electrode mixture slurry was applied to both sides of a positive electrode current collector made of aluminum foil (thickness 15 μm), and the coating film was dried. The coating film was rolled using a rolling roller and cut to a predetermined electrode size, producing a positive electrode in which a positive electrode mixture layer was formed on both sides of the positive electrode current collector. Note that an exposed portion was provided in part of the positive electrode, exposing the surface of the positive electrode current collector.

[0048] [Preparation of Negative Electrode] The negative electrode active material was a mixture of artificial graphite and SiO2 in a mass ratio of 94:6. This negative electrode active material was mixed with a sodium salt of carboxymethylcellulose (CMC-Na) having an etherification degree of 1.0 and styrene butadiene rubber (SBR) in a mass ratio of 100:1:1, and water was used as the dispersion medium to prepare a negative electrode mixture slurry. Copper foil having a thickness of 8 μm and a surface roughness Rz of 1.0 μm was used as the negative electrode current collector. The negative electrode mixture slurry was applied to both sides of this negative electrode current collector, and the coating was dried. The coating was rolled using a rolling roller and cut to a predetermined electrode size to prepare a negative electrode in which a negative electrode mixture layer was formed on both sides of the negative electrode current collector. An exposed portion in which the surface of the negative electrode current collector was exposed was provided in a portion of the negative electrode.

[0049] [Preparation of non-aqueous electrolyte] Ethylene carbonate and ethyl methyl carbonate were mixed in a volume ratio of 3:7 (at 25°C and 1 atmosphere), and then 2 mass% of vinylene carbonate and LiPF 6 were added to the mixed solvent. ６ was added to give a concentration of 1 mol / L to obtain a non-aqueous electrolyte.

[0050] [Preparation of Test Cell (Non-Aqueous Electrolyte Secondary Battery)] A positive electrode lead was attached to the exposed portion of the positive electrode, and a negative electrode lead was attached to the exposed portion of the negative electrode. The positive and negative electrodes were spirally wound with a separator made of a polyethylene microporous membrane interposed therebetween. Polypropylene tape was attached to the outermost surface of the wound body, and the wound body was then pressed radially to produce a flat wound electrode body. Under an argon atmosphere, the electrode body and non-aqueous electrolyte were placed in a cup-shaped container of an outer casing composed of a laminate sheet having a five-layer structure of a polypropylene layer / adhesive layer / aluminum alloy layer / adhesive layer / polypropylene layer. The pressure inside the outer casing was then reduced, allowing the electrode body to be impregnated with the non-aqueous electrolyte. The opening of the outer casing was sealed, producing a non-aqueous electrolyte secondary battery measuring 62 mm in height, 35 mm in width, and 3.6 mm in thickness.

[0051] [Measurement of Peel Strength] The negative electrode was cut to a predetermined size to prepare a test specimen. Using Nitto Denko double-sided tape #515, the negative electrode mixture layer on one side of the test specimen was attached to a stainless steel substrate with a smooth surface, and the stainless steel substrate to which the test specimen was fixed was positioned horizontally. One end of the negative electrode current collector in the longitudinal direction of the test specimen was fixed to a movable jig of a tensile tester (A&D Corporation's Tensilon Universal Tester RTC1210). The negative electrode current collector was set to peel off in a direction 90° relative to the substrate surface of the stainless steel substrate, and the movable jig was then moved to peel off the negative electrode mixture layer of the test specimen from the negative electrode current collector at a speed of 100 mm / min. The tensile direction was always maintained at 90° relative to the substrate surface of the stainless steel substrate to which the test specimen was fixed. A stable tensile strength value was read when 30 mm or more of the test specimen had peeled off. The above measurements were performed on five test specimens, and the average of the measured values ​​was taken as the peel strength (N / m). By increasing the peel strength, it is possible to suppress the deterioration of the rapid charging cycle characteristics.

[0052] [Measurement of Liquid Absorption Time] The negative electrode was dried for 10 hours in a thermostatic chamber heated to 200°C under a nitrogen atmosphere and then cut into a 5 cm x 5 cm test piece. 3.0 μL of polypropylene carbonate (PC) was dropped vertically onto the surface of the test piece, and the time until the PC was absorbed into the sample was measured visually. Six measurements were performed, and the average value was taken as the liquid absorption time. The shorter the liquid absorption time, the smoother the exchange of non-aqueous electrolyte from the surface of the negative electrode mixture layer to the current collector side, so that the non-aqueous electrolyte discharged from the negative electrode mixture layer during discharge is quickly absorbed into the negative electrode mixture layer during charge. Therefore, the shorter the liquid absorption time, the more suppressed the deterioration of rapid charge cycle characteristics.

[0053] [Evaluation of Initial Discharge Capacity and Rapid Charge Cycle Characteristics (Capacity Retention)] In a temperature environment of 25°C, the test cell was charged at a constant current of 1.0 C until the cell voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V down to 0.05 C. Thereafter, the test cell was discharged at a constant current of 1.0 C until the cell voltage reached 2.75 V, and the discharge capacity at this time was taken as the initial discharge capacity. The above charge / discharge cycle was repeated 100 times, and the capacity retention was calculated using the following formula: Capacity retention (%) = (Discharge capacity at 100th cycle / Discharge capacity at 1st cycle) × 100

[0054] Example 2 A negative electrode and a test cell were produced and evaluated in the same manner as in Example 1, except that a copper foil having a surface roughness of 2.0 μm was used as the negative electrode current collector in the production of the negative electrode.

[0055] Example 3 A negative electrode and a test cell were prepared and evaluated in the same manner as in Example 1, except that CMC-Na having a degree of etherification of 1.3 was used in the preparation of the negative electrode.

[0056] Example 4 A negative electrode and a test cell were produced and evaluated in the same manner as in Example 1, except that in the production of the negative electrode, a copper foil having a surface roughness of 1.5 μm was used as the negative electrode current collector and CMC-Na having a degree of etherification of 1.3 was used.

[0057] Example 5 A negative electrode and a test cell were produced and evaluated in the same manner as in Example 1, except that in the production of the negative electrode, a copper foil having a surface roughness of 2.0 μm was used as the negative electrode current collector and CMC-Na having a degree of etherification of 1.3 was used.

[0058] Comparative Example 1 A negative electrode and a test cell were produced and evaluated in the same manner as in Example 1, except that in the production of the negative electrode, artificial graphite alone was used as the negative electrode active material without using SiO, copper foil with a surface roughness of 0.5 μm was used as the negative electrode current collector, and CMC-Na with a degree of etherification of 0.7 was used.

[0059] Comparative Example 2 A negative electrode and a test cell were produced and evaluated in the same manner as in Example 1, except that in the production of the negative electrode, a copper foil having a surface roughness of 0.5 μm was used as the negative electrode current collector and CMC-Na having a degree of etherification of 0.7 was used.

[0060] Comparative Example 3 A negative electrode and a test cell were produced and evaluated in the same manner as in Example 1, except that in the production of the negative electrode, a copper foil having a surface roughness of 0.5 μm was used as the negative electrode current collector and CMC-Na having a degree of etherification of 1.3 was used.

[0061] Comparative Example 4 A negative electrode and a test cell were produced and evaluated in the same manner as in Example 1, except that in the production of the negative electrode, a copper foil having a surface roughness of 2.0 μm was used as the negative electrode current collector and CMC-Na having a degree of etherification of 0.7 was used.

[0062] Comparative Example 5 A negative electrode and a test cell were produced and evaluated in the same manner as in Example 1, except that in the production of the negative electrode, artificial graphite alone was used as the negative electrode active material without using SiO, copper foil having a surface roughness of 2.0 μm was used as the negative electrode current collector, and CMC-Na having a degree of etherification of 1.3 was used.

[0063] The evaluation results of the negative electrodes and test cells in the examples and comparative examples are shown in Table 1. The negative electrode peel strength, negative electrode liquid absorption time, initial discharge capacity, and capacity retention rate shown in Table 1 are relative values ​​when the negative electrode peel strength, negative electrode liquid absorption time, initial discharge capacity, and capacity retention rate of Example 1 are each set to 100.

[0064]

[0065] As shown in Table 1, all of the test cells of the examples were able to achieve both initial discharge capacity and capacity retention. The test cells of the examples had high peel strength and short liquid absorption time at the negative electrode, allowing for a high capacity retention. On the other hand, the test cells of Comparative Examples 2 to 4 had significantly lower capacity retention than the examples because at least one selected from the surface roughness Rz and the degree of etherification of CMC-Na did not satisfy the specified range. Furthermore, the test cells of Comparative Examples 1 and 5 did not contain SiO, resulting in a lower initial discharge capacity than the examples. Comparing the results of the test cells of Comparative Examples 1 and 5 with the results of the other examples and comparative examples reveals that while it is relatively easy to increase the capacity retention if the initial discharge capacity is small, achieving both the initial discharge capacity and the capacity retention is difficult. The test cells of the examples uniquely achieved both.

[0066] The present disclosure is further described by the following embodiments. Aspect 1: A negative electrode for a non-aqueous electrolyte secondary battery, comprising a negative electrode current collector and a negative electrode mixture layer disposed on a surface of the negative electrode current collector, wherein the surface roughness Rz of the negative electrode current collector is 1.0 μm or more and 2.5 μm or less, and the negative electrode mixture layer comprises a negative electrode active material containing graphite and a silicon-containing material, and carboxymethyl cellulose or an analogous derivative thereof, and the carboxymethyl cellulose or analogous derivative thereof has a degree of etherification of 0.8 or more and 1.5 or less. Aspect 2: The negative electrode for a non-aqueous electrolyte secondary battery according to Aspect 1, wherein a ratio of the silicon-containing material to the total mass of the negative electrode active material is 0.1 mass% or more and 10 mass% or less. Aspect 3: The negative electrode for a non-aqueous electrolyte secondary battery according to Aspect 1 or 2, wherein the silicon-containing material comprises an ion-conducting phase and Si particles dispersed in the ion-conducting phase, the ion-conducting phase comprising at least one phase selected from a silicon oxide phase, a silicate phase, and a carbon phase, and the silicate phase comprising at least one phase selected from an alkali metal element and a Group 2 element. Aspect 4: A non-aqueous electrolyte secondary battery comprising the negative electrode for a non-aqueous electrolyte secondary battery according to any one of Aspects 1 to 3, a positive electrode, and a non-aqueous electrolyte.

[0067] REFERENCE SIGNS LIST 10 Non-aqueous electrolyte secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer can, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Grooved portion, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket

Claims

1. A negative electrode for a non-aqueous electrolyte secondary battery having a negative electrode current collector and a negative electrode mixture layer disposed on the surface of the negative electrode current collector, wherein the surface roughness Rz of the negative electrode current collector is 1.0 μm or more and 2.5 μm or less, the negative electrode mixture layer contains a negative electrode active material including graphite and a silicon-containing material, and carboxymethyl cellulose or a derivative similar thereto, and the degree of etherification of the carboxymethyl cellulose or a derivative similar thereto is 0.8 or more and 1.5 or less.

2. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the ratio of said silicon-containing material to the total mass of said negative electrode active material is 0.1 mass % or more and 10 mass % or less.

3. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the silicon-containing material comprises an ion-conducting phase and Si particles dispersed in the ion-conducting phase, the ion-conducting phase comprises at least one phase selected from a silicon oxide phase, a silicate phase, and a carbon phase, and the silicate phase comprises at least one phase selected from an alkali metal element and a Group 2 element.

4. A non-aqueous electrolyte secondary battery comprising the negative electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, a positive electrode, and a non-aqueous electrolyte.

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