Non-aqueous electrolyte secondary battery

Abstract

This non-aqueous electrolyte secondary battery has a high capacity and demonstrates good rapid charge cycle characteristics. A nonaqueous electrolyte secondary battery according to one embodiment includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. The negative electrode includes a negative electrode current collector and a negative electrode mixed agent layer disposed on a surface of the negative electrode current collector. The negative electrode mixed agent layer includes a silicon-containing material, a first carboxymethyl cellulose having an etherification degree of 0.7 or less, and a second carboxymethyl cellulose having an etherification degree of more than 0.7 but not more than 1.8. The content of the silicon-containing material in the negative electrode mixed agent layer is 1 mass% or more with respect to the total mass of a negative electrode active material.

Description

Nonaqueous electrolyte secondary battery

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery.

[0002] In recent years, non-aqueous electrolyte secondary batteries such as lithium ion batteries have been widely used in applications requiring high capacity, such as automotive applications and power storage applications. Furthermore, non-aqueous electrolyte secondary batteries are required to have various performance characteristics, such as rapid charging performance and good cycle characteristics. Because electrodes, which are key components of batteries, have a significant impact on these performance characteristics, many studies have been conducted on electrodes. For example, Patent Document 1 discloses a negative electrode in which a negative electrode mixture layer contains a first carboxymethyl cellulose with a low viscosity and a high degree of etherification and a second carboxymethyl cellulose with a high viscosity and a low degree of etherification in a predetermined ratio, with the aim of improving cycle characteristics and safety.

[0003] JP 2012-59488 A

[0004] In recent years, the use of silicon-containing materials with high theoretical capacity densities as negative electrode active materials has been proposed as a means of increasing the capacity of batteries. However, batteries using such methods may experience significant capacity loss during rapid charge cycles. Achieving both high capacity and good rapid charge cycle characteristics is an important challenge, and conventional technologies, including those described in Patent Document 1, still have significant room for improvement in achieving both high capacity and good rapid charge cycle characteristics.

[0005] The non-aqueous electrolyte secondary battery according to the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. 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 mixture layer contains a silicon-containing material, a first carboxymethyl cellulose having a degree of etherification of 0.7 or less, and a second carboxymethyl cellulose having a degree of etherification of more than 0.7 and 1.8 or less. The content of the silicon-containing material in the negative electrode mixture layer is 1 mass% or more relative to the total mass of the negative electrode active material.

[0006] According to the nonaqueous electrolyte secondary battery according to the present disclosure, the capacity retention rate during rapid charge cycles can be improved, and for example, both high capacity and good rapid charge cycle characteristics can be achieved.

[0007] 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment;

[0008] As described above, it is an important challenge to increase the capacity of a battery while improving its capacity retention rate during rapid charge cycles. In particular, for automotive batteries, there is a strong demand for high capacity and minimal capacity loss during repeated rapid charge, i.e., excellent rapid charge cycle characteristics. 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 charge. 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 can sometimes cause the negative electrode mixture layer to peel off from the negative electrode current collector.

[0009] The present inventors further conducted extensive research into the above-mentioned problems and found that by including a negative electrode mixture layer containing 1% by mass or more of a silicon-containing material, as well as a first carboxymethyl cellulose having a degree of etherification of 0.7 or less and a second carboxymethyl cellulose having a degree of etherification of more than 0.7 and 1.8 or less, it is possible to specifically achieve both high capacity and good rapid charge cycle characteristics.

[0010] 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.

[0011] 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.

[0012] 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.

[0013] 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

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] [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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] [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 stable within the potential range of the negative electrode 12, such as copper, copper alloy, stainless steel, nickel, or nickel alloy, or a film having such a metal disposed on its surface. The negative electrode mixture layer contains a negative electrode active material and a binder, and is preferably provided on both sides of the negative electrode current collector except for the portion where 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 to the negative electrode current collector, drying the coating, and then compressing it to form a negative electrode mixture layer on both sides of the negative electrode current collector.

[0026] The negative electrode active material generally uses a carbon material that reversibly absorbs and releases lithium ions. The negative electrode active material may be an element that alloys with Li, such as silicon (Si) or tin (Sn), or a material containing such an element. Among these, a silicon-containing material containing Si is preferred. Furthermore, lithium titanate, which has a higher charge / discharge potential relative to metallic lithium than carbon materials, may also be used as the negative electrode active material. One type of negative electrode active material may be used alone, or multiple types may be used in combination. The negative electrode mixture layer may contain a conductive agent such as CNT. The conductive agent may be the same as that used in the positive electrode 11.

[0027] The carbon material functioning as the negative electrode active material is, for example, at least one selected from the group consisting of natural graphite, artificial graphite, soft carbon, and hard carbon. Among them, graphite is preferably used as the carbon material. The graphite may be artificial graphite such as massive artificial graphite (MAG) or graphitized mesophase carbon microbeads (MCMB), natural graphite such as flake graphite, massive graphite, or amorphous graphite, or a mixture thereof.

[0028] The volume-based D50 of graphite is, for example, 1 μm or more and 30 μm or less, preferably 5 μm or more and 25 μm or less. The D50 of the negative electrode active material particles means 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 measuring device (e.g., MT3000II manufactured by Microtrac-Bell Co., Ltd.) using water as a dispersion medium.

[0029] 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, or 1 μm or more and 15 μm or less.

[0030] 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 silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase. The silicide phase is a phase of a compound consisting of Si and an element more electropositive than Si, such as NiSi, Mg ２ Si, TiSi ２ The ion-conducting phase is a continuous phase composed of an aggregate of particles that are finer than Si particles.

[0031] 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.

[0032] 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.

[0033] Another example of a suitable Si-containing composite material is a composite particle having a sea-island structure in which fine Si particles are uniformly dispersed in an amorphous silicate phase. A suitable silicate phase is a lithium silicate phase containing Li. The lithium silicate phase can be, for example, 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 ４is 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.

[0034] 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.

[0035] The content of the silicon-containing material in the negative electrode mixture layer is 1% by mass or more relative to the total mass of the negative electrode active material. This allows a high-capacity secondary battery to be obtained. The content of the silicon-containing material is preferably 2% by mass or more, more preferably 3% by mass or more. The upper limit of the content of the silicon-containing material is not particularly limited, but is, for example, 10% by mass. 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.

[0036] The negative electrode mixture layer contains a first carboxymethyl cellulose (hereinafter referred to as the first CMC) having a degree of etherification of 0.7 or less and a second carboxymethyl cellulose (hereinafter referred to as the second CMC) having a degree of etherification of more than 0.7 and less than 1.8. When the negative electrode mixture layer contains the first CMC, the adhesion between the negative electrode current collector and the negative electrode mixture layer is improved. Furthermore, when the negative electrode mixture layer contains the second CMC, the non-aqueous electrolyte can easily penetrate into the negative electrode mixture layer, improving rapid charging cycle characteristics. The lower limit of the degree of etherification of the first CMC is not particularly limited, but is, for example, 0.1. The degree of etherification of the second CMC is preferably 1.0 or more and 1.8 or less, more preferably 1.2 or more and 1.7 or less.

[0037] Carboxymethyl cellulose is a water-soluble polymer in which some of the hydroxyl groups in the cellulose backbone are substituted with carboxymethyl groups. The degree of substitution by carboxymethyl groups is expressed as the degree of etherification. The degree of etherification of carboxymethyl cellulose can be measured as follows.

[0038] [Method for measuring the degree of etherification of carboxymethyl cellulose] 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. The alkalinity is calculated 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 placed in a crucible 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 tens of 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)

[0039] The ratio of the content of the first CMC in the negative electrode mixture layer to the content of the second CMC in the negative electrode mixture layer is preferably in the range of 1:9 to 9:1 by mass. This range more significantly improves the rapid charge cycle characteristics. The ratio of the content of the first CMC in the negative electrode mixture layer to the content of the second CMC in the negative electrode mixture layer is more preferably in the range of 3:7 to 7:3, and particularly preferably in the range of 4:6 to 6:4.

[0040] The sum of the content of the first CMC in the negative electrode mixture layer and the content of the second CMC in the negative electrode mixture layer is preferably 0.2% by mass or more and 3% by mass or less, more preferably 0.5% by mass or more and 3% by mass or less, and particularly preferably 1% by mass or more and 3% by mass or less, based on the total mass of the negative electrode active material. Within this range, the rapid charge cycle characteristics are more significantly improved. Note that if the sum of the content of the first CMC in the negative electrode mixture layer and the content of the second CMC in the negative electrode mixture layer exceeds 3% by mass, the battery capacity may decrease.

[0041] The first CMC and the second CMC comprise carboxymethyl cellulose (CMC) or a similar derivative thereof. Examples of similar derivatives of CMC include sodium salt, lithium salt, ammonium salt, potassium salt, rubidium salt, and cesium salt of CMC, with the sodium salt (CMC-Na) or lithium salt (CMC-Li) being preferred. CMC-Na is generally a partially neutralized salt in which some of the carboxyl groups are neutralized. The weight-average molecular weight of CMC-Na is, for example, 200,000 or more and 1,000,000 or less, preferably 200,000 or more and 500,000 or less.

[0042] 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.

[0043] The first CMC and the second CMC each preferably have a 1% aqueous solution viscosity of 500 mPa·s or more. The 1% aqueous solution viscosity of the first CMC is more preferably 1000 mPa·s or more and 9000 mPa·s or less. The 1% aqueous solution viscosity of the second CMC is more preferably 1000 mPa·s or more and 5000 mPa·s or less.

[0044] The ratio of the 1% aqueous solution viscosity of the first CMC to the 1% aqueous solution viscosity of the second CMC (1% aqueous solution viscosity of the first CMC / 1% aqueous solution viscosity of the second CMC) is preferably 0.5 or more and 9.0 or less, more preferably 1.0 or more and 3.0 or less. This further improves the adhesion between the negative electrode mixture layer and the negative electrode current collector. When a silicon-containing material with a large expansion and contraction rate during charge and discharge is used as the negative electrode active material, the negative electrode mixture layer may peel off from the negative electrode current collector, adversely affecting the rapid charge cycle characteristics. Furthermore, by improving the adhesion between the negative electrode mixture layer and the negative electrode current collector, it is possible to thicken the negative electrode mixture layer while maintaining high rapid charge cycle characteristics.

[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] Example 1 [Fabrication of Positive Electrode] LiCo was used as the positive electrode active material. ０．９７９ Zr ０．００１ Mg０．０１ 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) by the doctor blade method, and the coating was dried and compressed using a rolling roller. Thereafter, the positive electrode current collector was cut to a predetermined electrode size, and a positive electrode in which a positive electrode mixture layer was formed on both sides of the positive electrode current collector was obtained. Note that an exposed portion in which the surface of the positive electrode core was exposed was provided in a part of the positive electrode.

[0048] [Fabrication of Negative Electrode] As a negative electrode active material, SiO Ｘ (X=1) and graphite with a median diameter of 22 μm were used. Furthermore, as the first CMC, a sodium salt of carboxymethyl cellulose (CMC-Na) with a degree of etherification (DS) of 0.7 and a 1% aqueous solution viscosity of 2500 mPa·s was used, and as the second CMC, a sodium salt of carboxymethyl cellulose (CMC-Na) with a degree of etherification (DS) of 1.2 and a 1% aqueous solution viscosity of 1500 mPa·s was used. SiO Ｘ (X = 1), graphite, a first CMC, a second CMC, and styrene butadiene rubber (SBR) were mixed in a solids mass ratio of 3.0:97.0:0.5:0.5:1.5, and a negative electrode mixture slurry was prepared using water as a dispersion medium. Next, the negative electrode mixture slurry was applied to both sides of a negative electrode current collector made of copper foil (thickness 10 μm) by a doctor blade method, the coating was dried, and compressed using a rolling roller. Thereafter, the negative electrode current collector was cut to a predetermined electrode size, and a negative electrode in which a negative electrode mixture layer was formed on both sides of the negative electrode current collector was obtained. In addition, an exposed portion in which the surface of the negative electrode core was exposed was provided in a part 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 rapid charge cycle characteristics (capacity retention)] In a temperature environment of 25°C, the test cell was subjected to constant current charging at 0.7 C until the cell voltage reached 4.2 V, and then constant voltage charging at 4.2 V down to 0.05 C. Thereafter, constant current discharging was performed at 1.0 C until the cell voltage reached 2.75 V. This 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 In the production of the negative electrode, SiO Ｘ (X = 1), graphite, the first CMC, the second CMC, and styrene butadiene rubber (SBR) were mixed in a ratio of 3.0:97.0:0.25:0.25:2.0. Except for this, a negative electrode and a test cell were prepared in the same manner as in Example 1 and evaluated.

[0055] Example 3 In the production of the negative electrode, SiO Ｘ (X = 1), graphite, the first CMC, the second CMC, and styrene butadiene rubber (SBR) were mixed in a ratio of 3.0:97.0:1.0:1.0:0.5. Except for this, a negative electrode and a test cell were prepared in the same manner as in Example 1 and evaluated.

[0056] Example 4 In the production of the negative electrode, SiO Ｘ(X = 1), graphite, the first CMC, the second CMC, and styrene butadiene rubber (SBR) were mixed in a ratio of 3.0:97.0:0.9:0.1:1.5. Except for this, a negative electrode and a test cell were prepared in the same manner as in Example 1 and evaluated.

[0057] Example 5 In the preparation of the negative electrode, a sodium salt of carboxymethyl cellulose (CMC-Na) having a degree of etherification (DS) of 1.0 and a viscosity of a 1% aqueous solution of 2000 mPa·s was used as the second CMC, and SiO Ｘ (X = 1), graphite, the first CMC, the second CMC, and styrene butadiene rubber (SBR) were mixed in a ratio of 3.0:97.0:0.1:0.9:1.5. Except for this, a negative electrode and a test cell were prepared in the same manner as in Example 1 and evaluated.

[0058] Example 6 A negative electrode and a test cell were produced and evaluated in the same manner as in Example 1, except that in producing the negative electrode, a sodium salt of carboxymethyl cellulose (CMC-Na) having a degree of etherification (DS) of 1.7 and a 1% aqueous solution viscosity of 1000 mPa·s was used as the second CMC.

[0059] Example 7 A negative electrode and a test cell were produced and evaluated in the same manner as in Example 1, except that in producing the negative electrode, a sodium salt of carboxymethyl cellulose (CMC-Na) having a degree of etherification (DS) of 0.2 and a 1% aqueous solution viscosity of 2500 mPa·s was used as the first CMC.

[0060] Example 8 In producing the negative electrode, a lithium salt of carboxymethyl cellulose (CMC-Li) having a degree of etherification (DS) of 0.7 and a 1% aqueous solution viscosity of 2000 mPa·s was used as the first CMC, and a lithium salt of carboxymethyl cellulose (CMC-Li) having a degree of etherification (DS) of 1.2 and a 1% aqueous solution viscosity of 1000 mPa·s was used as the second CMC. Except for this, a negative electrode and a test cell were produced and evaluated in the same manner as in Example 1.

[0061] <Comparative Example 1> In the preparation of the negative electrode, a sodium salt of carboxymethyl cellulose (CMC-Na) having a degree of etherification (DS) of 0.7 and a 1% aqueous solution viscosity of 2000 mPa·s was used as the first CMC, and a second CMC was not used, but SiO Ｘ A negative electrode and a test cell were prepared in the same manner as in Example 1, except that the mixing ratio of (X = 1), graphite, the first CMC, and styrene butadiene rubber (SBR) was changed to 3.0:97.0:1.0:1.5. Evaluation was performed.

[0062] Comparative Example 2 In the production of a negative electrode, the first CMC was not used, and SiO Ｘ A negative electrode and a test cell were prepared in the same manner as in Example 1, except that the mixing ratio of (X = 1), graphite, the second CMC, and styrene butadiene rubber (SBR) was changed to 3.0:97.0:1.0:1.5. Evaluation was performed.

[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, and capacity retention rate shown in Table 1 are relative values ​​when the negative electrode peel strength, negative electrode liquid absorption time, and capacity retention rate of Example 1 are each set to 100.

[0064]

[0065] As shown in Table 1, the examples used negative electrodes with peel strength and liquid absorption time within appropriate ranges, and therefore the test cells of the examples all had high capacity retention rates. On the other hand, the negative electrode of Comparative Example 1 did not contain the second CMC, and therefore the liquid absorption time was long, and the capacity retention rate of the test cell of Comparative Example 1 was poorer than that of the examples. Furthermore, the negative electrode of Comparative Example 2 did not contain the first CMC, and therefore the peel strength was low, and the capacity retention rate of the test cell of Comparative Example 2 was poorer than that of the examples.

[0066] The present disclosure is further described by the following embodiments. Configuration 1: A non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein 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, wherein the negative electrode mixture layer contains a silicon-containing material, a first carboxymethyl cellulose having a degree of etherification of 0.7 or less, and a second carboxymethyl cellulose having a degree of etherification of more than 0.7 and 1.8 or less, and wherein the content of the silicon-containing material in the negative electrode mixture layer is 1 mass % or more relative to the total mass of the negative electrode active material. Configuration 2: The non-aqueous electrolyte secondary battery according to Configuration 1, wherein the ratio of the content of the first carboxymethyl cellulose in the negative electrode mixture layer to the content of the second carboxymethyl cellulose in the negative electrode mixture layer is in the range of 1:9 to 9:1 by mass. Configuration 3: The nonaqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the first carboxymethyl cellulose and the second carboxymethyl cellulose comprise carboxymethyl cellulose or a derivative similar thereto.Configuration 4: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the sum of the content of the first carboxymethyl cellulose in the negative electrode mixture layer and the content of the second carboxymethyl cellulose in the negative electrode mixture layer is 0.2 mass % or more and 3 mass % or less, relative to the total mass of the negative electrode active material.Configuration 5: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the first carboxymethyl cellulose and the second carboxymethyl cellulose both have a 1% aqueous solution viscosity of 500 mPa s or more.Configuration 6: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 5, wherein the silicon-containing material includes an ion-conducting phase and Si particles dispersed in the ion-conducting phase, and the ion-conducting phase is at least one phase selected from a silicon oxide phase, a silicate phase, and a carbon phase. Configuration 7: The nonaqueous electrolyte secondary battery of any one of Configurations 1 to 5, wherein the silicon-containing material is SiOx (0.5≦x≦1.6).

[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 non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode has a negative electrode current collector and a negative electrode mixture layer disposed on a surface of the negative electrode current collector, the negative electrode mixture layer comprises a negative electrode active material, a first carboxymethyl cellulose having a degree of etherification of 0.7 or less, and a second carboxymethyl cellulose having a degree of etherification of more than 0.7 and 1.8 or less, the negative electrode active material comprises a silicon-containing material, and the content of the silicon-containing material in the negative electrode mixture layer is 1 mass% or more relative to the total mass of the negative electrode active material.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein a ratio of a content of the first carboxymethyl cellulose in the negative electrode mixture layer to a content of the second carboxymethyl cellulose in the negative electrode mixture layer is in a range of 1:9 to 9:1 in terms of mass ratio.

3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the first carboxymethyl cellulose and the second carboxymethyl cellulose include carboxymethyl cellulose or a derivative similar thereto.

4. The nonaqueous electrolyte secondary battery according to claim 1, wherein a sum of a content of the first carboxymethyl cellulose in the negative electrode mixture layer and a content of the second carboxymethyl cellulose in the negative electrode mixture layer is 0.2 mass % or more and 3 mass % or less with respect to a total mass of the negative electrode active material.

5. The nonaqueous electrolyte secondary battery according to claim 1, wherein the first carboxymethyl cellulose and the second carboxymethyl cellulose each have a 1% aqueous solution viscosity of 500 mPa·s or more.

6. The nonaqueous 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, and the ion-conducting phase is at least one selected from a silicon oxide phase, a silicate phase, and a carbon phase.

7. The nonaqueous electrolyte secondary battery according to claim 1, wherein the silicon-containing material is SiOx (0.5≦x≦1.6).

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