Non-aqueous electrolyte secondary battery
Patent Information
- Application Number
- JP2025524077
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Filing Date
- 2025-11-11
- Publication Date
- 2026-03-04
Abstract
Description
Nonaqueous electrolyte secondary battery
[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery including a wound electrode assembly.
[0002] Conventionally, nonaqueous electrolyte secondary batteries have been widely known, each including a wound electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween. Generally, an electrode has active material layers provided on both sides of a current collector. In a wound electrode assembly, however, the first active material layer facing the inside of the electrode assembly and the second active material layer facing the outside of the electrode assembly have different degrees of curvature. Therefore, nonaqueous electrolyte secondary batteries have been proposed in which the layer structure of each active material layer is modified. For example, Patent Document 1 discloses a nonaqueous electrolyte secondary battery including a negative electrode in which the binder concentration distribution is modified in the first and second active material layers.
[0003] International Publication No. 2019 / 230296
[0004] While various approaches to increasing the capacity of batteries have been proposed, such as using a silicon-containing material with a high theoretical capacity density as the negative electrode active material or increasing the packing density of the negative electrode active material, batteries employing such approaches tend to experience significant capacity loss during rapid charge cycles. As a result of research conducted by the present inventors, capacity loss during rapid charge cycles was confirmed in high-capacity nonaqueous electrolyte secondary batteries equipped with wound electrode assemblies. Conventional technologies, including those described in Patent Document 1, still have room for improvement in achieving both high capacity and good rapid charge cycle characteristics.
[0005] The nonaqueous electrolyte secondary battery according to the present disclosure is a nonaqueous electrolyte secondary battery including a wound electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, and a nonaqueous electrolyte, wherein the negative electrode has a negative electrode current collector, a first negative electrode active material layer provided on a first surface of the negative electrode current collector facing the inside of the electrode assembly, and a second negative electrode active material layer provided on a second surface of the negative electrode current collector facing the outside of the negative electrode assembly, and the first and second negative electrode active material layers have a density of 1.3 g / cm ３The negative electrode active material layer is characterized in that it contains silicon or a silicon-containing material, and carboxymethyl cellulose or a salt thereof, and a first degree of etherification (DS1) of the carboxymethyl cellulose or the salt thereof contained in the first negative electrode active material layer is greater than a second degree of etherification (DS2) of the carboxymethyl cellulose or the salt thereof contained in the second negative electrode active material layer.
[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 non-aqueous electrolyte secondary battery according to an embodiment of the present invention;
[0008] As described above, improving the capacity retention rate during rapid charge cycles while increasing the capacity of batteries is an important challenge. In particular, for automotive batteries, high capacity and minimal capacity loss during repeated rapid charge cycles, i.e., excellent rapid charge cycle characteristics, are strongly desired. As a result of the inventors' studies, it was found that when the density of the negative electrode active material layer is high and silicon or a silicon-containing material is used as the negative electrode active material, a difference in electrolyte permeability occurs between the first negative electrode active material layer facing the inside of the electrode body and the second negative electrode active material layer facing the outside of the electrode body. This is thought to be due to increased bending stress in the first negative electrode active material layer, which reduces electrolyte permeability. It is assumed that the inhibition of uniform electrolyte permeation facilitates deterioration of the negative electrode, resulting in significant capacity loss during charge and discharge.
[0009] As a result of extensive research into the above-mentioned problems, the inventors have discovered that excellent rapid charge cycle characteristics can be obtained by making the first degree of etherification (DS1) of the carboxymethyl cellulose or its salt contained in the first negative electrode active material layer greater than the second degree of etherification (DS2) of the carboxymethyl cellulose or its salt contained in the second negative electrode active material layer (DS1 > DS2). This configuration is believed to effectively improve the liquid circulation in the negative electrode, thereby enabling uniform electrolyte permeation in each negative electrode active material layer. As a result, rapid charge cycle characteristics can be improved far beyond expectations.
[0010] The effect of improving the rapid charge cycle characteristics due to DS1>DS2 is particularly exhibited when a high-capacity negative electrode is used, which contains silicon or a silicon-containing material as the negative electrode active material and has a density of 1.3 g / cc or more in the negative electrode active material layer.
[0011] 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 components of the multiple embodiments and variations described below are included within the scope of the present disclosure.
[0012] 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 battery outer can is not limited to a cylindrical outer can. The nonaqueous electrolyte secondary battery according to the present disclosure may be a prismatic battery including a prismatic outer can or a laminated battery including an outer can made of a laminate sheet including a metal layer and a resin layer, as long as it includes a wound electrode assembly. However, the configuration of the present disclosure is particularly suitable for cylindrical batteries in which there are many areas where there is a difference in electrode plate density between the first negative electrode active material layer facing the inside of the electrode assembly and the second negative electrode active material layer facing the outside of the electrode assembly.
[0013] 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 houses 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.
[0014] 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
[0015] 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.
[0016] 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.
[0017] 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 30 (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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] [Positive Electrode] The positive electrode 11 includes a positive electrode current collector and a positive electrode active material layer provided on 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 its surface. The positive electrode active material 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 21 is connected. A protective layer containing inorganic particles and a binder may be provided between the positive electrode current collector and the positive electrode active material layer, or on the positive electrode active material 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 positive electrode active material layers on both sides of the positive electrode current collector.
[0022] The positive electrode active material uses a lithium transition metal composite oxide containing transition metal elements such as Ni, Co, and Mn. Examples of metal elements contained in the lithium transition metal 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 transition metal composite oxides containing Ni, Co, and Mn, and lithium transition metal composite oxides containing Ni, Co, and Al. One type of lithium transition metal composite oxide may be used alone, or multiple types may be used in combination.
[0023] The lithium transition metal 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 viewpoint of high capacity and stability of the crystal structure. The density of the positive electrode active material layer is 3.3 g / cm from the viewpoint of high capacity. ３ An example of the upper limit of the density of the positive electrode active material layer is 3.8 g / cm ３ is.
[0024] Examples of the conductive agent contained in the positive electrode active material 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.
[0025] Examples of binders contained in the positive electrode active material 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.
[0026] [Negative Electrode] FIG. 2 is a cross-sectional view of the negative electrode 12 constituting the wound electrode assembly 14. As shown in FIG. 2, the negative electrode 12 includes a negative electrode current collector 30 and negative electrode active material layers provided on both sides of the negative electrode current collector 30. As will be described in detail later, the negative electrode 12 includes a first negative electrode active material layer 31 provided on a first surface 30a of the negative electrode current collector 30 facing inward of the electrode assembly 14, and a second negative electrode active material layer 32 provided on a second surface 30b of the negative electrode current collector 30 facing outward of the negative electrode current collector 30. The degrees of etherification of carboxymethyl cellulose (CMC) or a salt thereof differ between the first and second negative electrode active material layers. This prevents differences in the permeability of the electrolyte solution to the respective negative electrode active material layers, significantly improving the capacity retention rate during rapid charging cycles.
[0027] The negative electrode current collector 30 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 first negative electrode active material layer 31 and the second negative electrode active material layer 32 contain a negative electrode active material and a binder, and are provided, for example, on the entire negative electrode current collector 30 except for 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 30 and the negative electrode active material layer, or on the negative electrode active material layer.
[0028] 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 surface of the negative electrode current collector 30, drying the coating, and then compressing it to form a negative electrode active material layer on each side of the negative electrode current collector 30. At this time, first and second negative electrode mixture slurries containing CMC or a salt thereof with different degrees of etherification are applied to each side of the negative electrode current collector 30. The first surface 30a, which is the inner surface of the negative electrode current collector 30, is coated with the first negative electrode mixture slurry containing CMC or a salt thereof with a high degree of etherification, and the first surface 30b, which is the outer surface of the negative electrode current collector 30, is coated with the second negative electrode mixture slurry containing CMC or a salt thereof with a low degree of etherification.
[0029] The negative electrode 12 contains at least silicon (Si) or a Si-containing material as the negative electrode active material. Among these, a Si-containing material is preferred. While it is possible to use only a Si-containing material as the negative electrode active material, it is preferable to use a carbon material that reversibly absorbs and releases lithium ions in combination with the Si-containing material in order to improve cycle characteristics. The negative electrode active material layer may contain a conductive agent such as CNT. The conductive agent may be the same as that used in the positive electrode 11.
[0030] 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.
[0031] 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 of the particles in the volume-based particle size distribution is 50% from the smallest particle size. The particle size distribution of the lithium transition metal composite oxide can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II manufactured by Microtrac-Bell Co., Ltd.) with water as the dispersion medium.
[0032] The Si-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 the Si-containing material is generally smaller than the D50 of graphite. The volume-based D50 of the Si-containing material is, for example, 1 μm or more and 20 μm or less, or 1 μm or more and 15 μm or less.
[0033] A suitable Si-containing material (composite material) is a composite particle containing an ion-conducting phase and a Si phase 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 Si phase is formed by dispersing Si in the form of fine particles, and the ion-conducting phase is a continuous phase formed by an aggregation of particles that are finer than the Si phase.
[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 active material layer. The conductive layer is, for example, a carbon coating made of a conductive carbon material. Examples of the conductive carbon material that can be used 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 is dispersed almost uniformly in an amorphous silicon oxide phase, and the overall structure is represented by the general formula SiO ｘ The silicon oxide may be mainly composed of silicon dioxide. The silicon oxide phase may be doped with Li. The oxygen to Si content (x) is, for example, 0.5≦x<2.0, and preferably 0.8≦x≦1.5.
[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 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, 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 composite particles 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 first negative electrode active material layer 31 and the second negative electrode active material layer 32 have a density of 1.3 g / cm ３ or more, and includes CMC or a salt thereof. As described above, each negative electrode active material layer preferably contains graphite and a Si-containing material as negative electrode active material particles. The graphite content is, for example, 80% by mass to 99% by mass, 85% by mass to 98% by mass, or 90% by mass to 97% by mass of the total mass of the negative electrode active material. The Si-containing material content is, for example, 1% by mass to 20% by mass, 2% by mass to 15% by mass, or 3% by mass to 10% by mass of the total mass of the negative electrode active material.
[0039] The CMC or a salt thereof contained in the first negative electrode active material layer 31 and the second negative electrode active material layer 32 functions as a binder and also functions as a thickener for imparting an appropriate viscosity to the negative electrode mixture slurry. Each negative electrode active material layer preferably contains a binder other than CMC or a salt thereof. Each negative electrode active material layer may contain the same binder as that used in the positive electrode 11, but preferably contains styrene-butadiene rubber (SBR).
[0040] The content of the negative electrode active material is, for example, 90% by mass to 99.5% by mass, or 95% by mass to 99% by mass, relative to the mass of the negative electrode active material layer. The content of CMC or a salt thereof is, for example, 0.1% by mass to 2.5% by mass, or 0.5% by mass to 1.5% by mass, relative to the mass of the negative electrode active material layer. In this case, it is easy to achieve both high capacity and good cycle characteristics.
[0041] The thickness of the negative electrode active material layer is determined by observing the cross section of the active material layer using a scanning electron microscope (SEM). The thickness of the negative electrode active material layer is generally uniform, but unless otherwise specified, the thickness herein refers to the average thickness. The thicknesses of the first negative electrode active material layer 31 and the second negative electrode active material layer 32 may be different from each other, but are preferably approximately the same from the viewpoint of achieving both high capacity and good cycle characteristics.
[0042] The density of the first negative electrode active material layer 31 and the second negative electrode active material layer 32 is set to 1.3 g / cm3 from the viewpoint of increasing capacity. ３ More preferably, 1.4 g / cm ３The upper limit of the density is, for example, 2.0 g / cm ３ and preferably 1.8 g / cm ３ From the viewpoint of achieving both high capacity and good cycle characteristics, an example of a suitable range of the density of each negative electrode active material layer is 1.3 g / cm ３ 1.8g / cm or more ３ The reason is as follows: If the density of the negative electrode active material layer is reduced, the liquid circulation is improved and the rapid charge cycle characteristics are enhanced, but in that case, the capacity is significantly reduced. With negative electrode 12, excellent rapid charge cycle characteristics can be achieved without significantly reducing the density.
[0043] [Method for calculating density of negative electrode active material layer] The thickness of each of the first negative electrode active material layer 31 and the second negative electrode active material layer 32 is measured from a cross-sectional SEM image of the negative electrode active material layer, the mass of the negative electrode active material layer per unit area is determined, and the density (mass of the negative electrode active material layer per unit area × 100 / (unit area + thickness of the negative electrode active material layer)) is calculated.
[0044] As described above, the CMC or its salt contained in the first negative electrode active material layer 31 and the second negative electrode active material layer 32 have different degrees of etherification (DS1, DS2), satisfying the condition of DS1 > DS2. In this case, the difference in the permeability of the electrolyte between the first negative electrode active material layer 31 and the second negative electrode active material layer 32 is suppressed, and the capacity retention rate during rapid charging cycles can be significantly improved. CMC is a water-soluble polymer in which some of the hydroxyl groups in the cellulose skeleton are substituted with carboxymethyl groups. The degree of substitution by carboxymethyl groups is expressed as the degree of etherification.
[0045] The difference (DS1-DS2) between the degree of etherification (DS1) of the CMC or salt thereof contained in the first negative electrode active material layer 31 and the degree of etherification (DS2) of the CMC or salt thereof contained in the second negative electrode active material layer 32 is preferably 0.3 or more, more preferably 0.4 or more. In this case, the effect of improving the rapid charge cycle characteristics becomes significant. The upper limit of DS1-DS2 is not particularly limited, but is preferably 1.0, more preferably 0.9, and particularly preferably 0.8.
[0046] The degrees of etherification (DS1, DS2) are preferably 0.5 or more, more preferably 0.6 or more. In this case, the rapid charge cycle characteristics can be more effectively improved. An example of a suitable degree of etherification (DS1) is 0.7 to 1.8, 0.8 to 1.5, or 1.0 to 1.3. An example of a suitable degree of etherification (DS2) is 0.4 to 0.7, or 0.5 to 0.7. If the degrees of etherification (DS1, DS2) are too high, the adhesion of the negative electrode active material layer to the negative electrode current collector 30 is thought to decrease, and although the permeability of the electrolyte is improved, the effect of improving the cycle characteristics may not be obtained.
[0047] The first negative electrode active material layer 31 and the second negative electrode active material layer 32 preferably contain a salt in which the carboxyl groups of CMC have been neutralized. Examples of CMC salts include sodium salt, ammonium salt, lithium salt, potassium salt, rubidium salt, and cesium salt, with sodium salt (CMC-Na) being preferred. CMC-Na is generally a partially neutralized salt in which some of the carboxyl groups have been neutralized. The weight-average molecular weight of CMC-Na is, for example, 200,000 or more and 1,000,000 or less, and preferably 200,000 or more and 500,000 or less.
[0048] When the density of the negative electrode active material layer is increased for the purpose of increasing capacity, the first negative electrode active material layer 31 provided on the inside of the negative electrode current collector 30 becomes more difficult for the electrolyte to permeate than the second negative electrode active material layer 32 provided on the outside of the negative electrode current collector 30. By making DS1 > DS2, the negative electrode 12 achieves uniform liquid permeability in each negative electrode active material layer, thereby achieving both high capacity and excellent rapid charge cycle characteristics. Note that the effect of the DS1 > DS2 configuration is that when the density of the negative electrode active material layer is 1.3 g / cm ３ It is specifically expressed when
[0049] [Method for measuring the degree of etherification] 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) F: Factor of 0.1 M (N / 10) aqueous sodium hydroxide F': Factor of 0.05 M (N / 10) aqueous sulfuric acid Approximately 1 g of sample is weighed out, 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)
[0050] [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.
[0051] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.
[0052] Experimental Example 1 [Fabrication of Positive Electrode] Aluminum-containing lithium nickel cobalt oxide (LiNi) was used as the positive electrode active material. ０．８８ Co ０．０９ Al ０．０３ O ２ ) was used. A positive electrode active material, graphite, and polyvinylidene fluoride were mixed in a solid content mass ratio of 100:1:0.9, 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 a 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 active material layer was formed on both sides of the positive electrode current collector was obtained.
[0053] [Preparation of Negative Electrode Mixture Slurry A] Graphite and SiO Ｘ A mixture was used in which the negative electrode active material was mixed with a Si-containing material represented by (X = 1) at a mass ratio of 95:5. The negative electrode mixture slurry A was prepared by mixing the negative electrode active material, a dispersion of styrene butadiene rubber (SBR), and a sodium salt of carboxymethyl cellulose (CMC-Na) having an etherification degree of 1.3 at a solids mass ratio of 100:1:1, using water as a dispersion medium.
[0054] [Preparation of Negative Electrode Mixture Slurry B] A negative electrode mixture slurry B was prepared by mixing graphite and SiO Ｘ A mixture of the negative electrode active material and a Si-containing material represented by (X = 1) in a mass ratio of 95:5 was used. The negative electrode mixture slurry B was prepared by mixing the negative electrode active material, a dispersion of styrene butadiene rubber (SBR), and CMC-Na having an etherification degree of 0.6 in a solid content mass ratio of 100:1:1 using water as a dispersion medium.
[0055] [Fabrication of Negative Electrode] Negative electrode mixture slurry A was applied to the first surface of a negative electrode current collector made of copper foil, and negative electrode mixture slurry B was applied to the second surface of the negative electrode current collector by the doctor blade method, and each coating was dried. At this time, the coating amount of each negative electrode mixture slurry per unit area was the same. Thereafter, the coating film was compressed using a rolling roller, and the negative electrode current collector was cut to a predetermined electrode size, thereby obtaining a negative electrode in which a first negative electrode active material layer made of negative electrode mixture slurry A and a second negative electrode active material layer made of negative electrode mixture slurry B were formed on the negative electrode current collector. The density of each negative electrode active material layer measured by the above method was 1.5 g / cm. ３ It was.
[0056] [Preparation of non-aqueous electrolyte] Ethylene carbonate and dimethyl carbonate were mixed at a volume ratio of 1:3 (at 25°C), and then 5 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 solution.
[0057] [Preparation of Test Cell (Non-Aqueous Electrolyte Secondary Battery)] The positive electrode, in which the positive electrode lead was welded to the positive electrode current collector, and the negative electrode, in which the negative electrode lead was welded to the negative electrode current collector, were spirally wound with a separator interposed therebetween to prepare a wound electrode assembly. The negative electrode was positioned so that the first negative electrode active material layer faced the inside of the electrode assembly. Insulating plates were placed above and below the electrode assembly, and the negative electrode lead was welded to the inner bottom of a cylindrical outer can with a bottom, and the positive electrode lead was welded to a sealing member, and the electrode assembly was then housed in the outer can. The non-aqueous electrolyte was poured into the outer can under reduced pressure, and the opening of the outer can was sealed with a sealing member via a gasket to obtain a test cell.
[0058] Example 2 A negative electrode and a test cell were produced in the same manner as in Example 1, except that negative electrode mixture slurry C containing CMC-Na having a degree of etherification of 1.0 was used instead of negative electrode mixture slurry A.
[0059] Example 3 A negative electrode and a test cell were produced in the same manner as in Example 1, except that negative electrode mixture slurry D containing CMC-Na having a degree of etherification of 1.0 was used instead of negative electrode mixture slurry A, and negative electrode mixture slurry E containing CMC-Na having a degree of etherification of 0.5 was used instead of negative electrode mixture slurry B.
[0060] Comparative Example 1 A negative electrode and a test cell were fabricated in the same manner as in Example 2, except that only the negative electrode mixture slurry C was used to form the negative electrode active material layers on both sides of the negative electrode current collector.
[0061] Comparative Example 2 A negative electrode and a test cell were fabricated in the same manner as in Example 1, except that only negative electrode mixture slurry B was used to form the negative electrode active material layers on both sides of the negative electrode current collector.
[0062] Reference Example 1 In the production of the negative electrode, the density of each negative electrode active material layer was set to 1.2 g / cm ３ A negative electrode and a test cell were prepared in the same manner as in Example 1, except for the above change.
[0063] Reference Example 2 In the production of the negative electrode, the density of each negative electrode active material layer was set to 1.2 g / cm ３ A negative electrode and a test cell were prepared in the same manner as in Comparative Example 2, except that the above-mentioned change was made.
[0064] The performance of each test cell of the Examples, Comparative Examples, and Reference Examples was evaluated by the following method, and the evaluation results, along with the degree of etherification of CMC-Na contained in each negative electrode active material layer, are shown in Tables 1 and 2. The capacity retention rates shown in Table 1 are relative values when the capacity retention rate of Example 1 is set to 100, and higher values indicate better rapid charge cycle characteristics. Furthermore, the capacity retention rates shown in Table 2 are relative values when the capacity retention rate of Reference Example 1 is set to 100.
[0065] [Evaluation of rapid charge cycle characteristics (capacity retention)] In a temperature environment of 25°C, each test cell of the Examples, Comparative Examples, and Reference Examples was subjected to constant current charging at 1 C (4600 mA) until the cell voltage reached 4.2 V, followed by constant voltage charging at 4.2 V up to C / 50. Subsequently, constant current discharging was performed at 0.5 C until the cell voltage reached 2.5 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
[0066]
[0067]
[0068] As shown in Table 1, the test cells of the examples had higher capacity retention rates during rapid charge cycles than the test cells of the comparative examples. By increasing the degree of etherification (DS) of the CMC-Na contained in the first negative electrode active material layer facing the inside of the electrode assembly relative to the DS of the CMC-Na contained in the second negative electrode active material layer facing the outside of the electrode assembly, it is believed that differences in electrolyte permeability between the negative electrode active material layers can be effectively suppressed, resulting in improved rapid charge cycle characteristics. In particular, when the difference in DS of the CMC-Na contained in the first and second negative electrode active material layers is 0.3 or more (Examples 1 and 2), the rapid charge cycle characteristics are significantly improved.
[0069] As shown in Table 2, when the density of the negative electrode active material layer is 1.2 g / cm ３ In this case, even if the DS of the CMC-Na contained in the first negative electrode active material layer is greater than the DS of the CMC-Na contained in the second negative electrode active material layer, the effect of improving the capacity retention rate during rapid charging cycles cannot be obtained. When the density of the negative electrode active material layer is low, the permeability of the electrolyte is originally high, and therefore, even if the configuration of the Example is adopted, it is thought that no effect sufficient to affect the cycle characteristics is obtained. As a result of the investigations by the present inventors, the effect of the Example is not obtained when the density of the negative electrode active material layer is 1.3 g / cm ３ It was revealed that it is specifically expressed when
[0070] The present disclosure will be further described by the following embodiments. Configuration 1: A non-aqueous electrolyte secondary battery including a wound electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, and a non-aqueous electrolyte, wherein the negative electrode has a negative electrode current collector, a first negative electrode active material layer provided on a first surface of the negative electrode current collector facing inward of the electrode assembly, and a second negative electrode active material layer provided on a second surface of the negative electrode current collector facing outward of the negative electrode assembly, the first and second negative electrode active material layers containing silicon or a silicon-containing material, and carboxymethyl cellulose or a salt thereof, and a first degree of etherification (DS1) of the carboxymethyl cellulose or the salt thereof contained in the first negative electrode active material layer is greater than a second degree of etherification (DS2) of the carboxymethyl cellulose or the salt thereof contained in the second negative electrode active material layer. Aspect 2: The nonaqueous electrolyte secondary battery according to Aspect 1, wherein the difference between the first degree of etherification (DS1) and the second degree of etherification (DS2) is 0.3 or more. Aspect 3: The density of the first and second negative electrode active material layers is 1.3 g / cm ３ 1.8g / cm or more ３ The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 2, wherein the weight-average molecular weight of the carboxymethyl cellulose or salt thereof contained in the first and second negative electrode active material layers is 200,000 or more and 500,000 or less. Configuration 5: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the carboxymethyl cellulose salt contained in the first and second negative electrode active material layers contains at least one of sodium salt, ammonium salt, lithium salt, potassium salt, rubidium salt, and cesium salt as a salt. Configuration 6: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 5, is a cylindrical battery provided with a cylindrical outer can with a bottom.
[0071] 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, 30 Negative electrode current collector, 30a First surface, 30b Second surface, 31 First negative electrode active material layer, 32 Second negative electrode active material layer
Claims
1. A non-aqueous electrolyte secondary battery including a wound electrode assembly in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, and a non-aqueous electrolyte, the negative electrode includes a negative electrode current collector, a first negative electrode active material layer provided on a first surface of the negative electrode current collector facing the inside of the electrode body, and a second negative electrode active material layer provided on a second surface of the negative electrode current collector facing the outside of the electrode body; The first and second negative electrode active material layers have a density of 1.3 g / cm ３ The composition contains silicon or a silicon-containing material and carboxymethyl cellulose or a salt thereof. a first degree of etherification (DS1) of the carboxymethyl cellulose or the salt thereof contained in the first negative electrode active material layer is greater than a second degree of etherification (DS2) of the carboxymethyl cellulose or the salt thereof contained in the second negative electrode active material layer.
2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein a difference between the first degree of etherification (DS1) and the second degree of etherification (DS2) is 0.3 or more.
3. The density of the first and second negative electrode active material layers is 1.8 g / cm ３ 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein:
4. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the weight average molecular weight of the carboxymethyl cellulose or salt thereof contained in the first and second negative electrode active material layers is 200,000 or more and 500,000 or less.
5. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the carboxymethyl cellulose salt contained in the first and second negative electrode active material layers includes, as a salt, at least one of a sodium salt, an ammonium salt, a lithium salt, a potassium salt, a rubidium salt, and a cesium salt.
6. The nonaqueous electrolyte secondary battery according to any one of claims 1 to 5 is a cylindrical battery provided with a cylindrical outer can with a bottom.