Negative electrode for secondary battery and secondary battery

Abstract

This negative electrode for secondary batteries is provided with a negative electrode mixture that contains a negative electrode active material, an additive and a conductive agent. The negative electrode active material contains an Si-containing material; the additive contains an alkali metal sulfate salt; and the conductive agent contains carbon nanotubes. The content of the alkali metal sulfate salt in the negative electrode mixture is from 0.0025% by mass to 0.1% by mass relative to the total mass of the negative electrode active material.

Description

[Technical Field]

[0001] The present disclosure relates to a negative electrode for a secondary battery and a secondary battery. [Background technology]

[0002] Si-containing materials are alloying materials that can be alloyed with lithium, and are known to be able to store more lithium ions per unit volume than carbon-based active materials such as graphite, and are expected to be used as negative electrode active materials in secondary batteries.

[0003] However, Si-containing materials undergo large volume changes (expansion and contraction) during charge and discharge, which tends to reduce the conductivity of the negative electrode, resulting in a problem of reduced charge and discharge cycle characteristics.

[0004] To address these issues, there is a technology that adds carbon nanotubes to the negative electrode containing a Si-containing material to prevent the decrease in conductivity of the negative electrode due to the expansion and contraction of the Si-containing material, thereby suppressing the deterioration of charge-discharge cycle characteristics. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-310760 Summary of the Invention

[0006] Because carbon nanotubes have a large specific surface area, they react with the electrolyte during the initial charge / discharge of a secondary battery, resulting in a decrease in the initial charge / discharge efficiency, which is defined as the ratio of the initial discharge capacity to the initial charge capacity.

[0007] A negative electrode for a secondary battery according to one embodiment of the present disclosure includes a negative electrode mixture containing a negative electrode active material, an additive, and a conductive agent, wherein the negative electrode active material contains a Si-containing material, the additive contains an alkali metal sulfate, and the conductive agent contains carbon nanotubes, and the content of the alkali metal sulfate in the negative electrode mixture is 0.0025 mass% or more and 0.1 mass% or less with respect to the total amount of the negative electrode active material.

[0008] A secondary battery according to one aspect of the present disclosure includes the above-described negative electrode for secondary batteries, a positive electrode, and a non-aqueous electrolyte solution.

[0009] According to the present disclosure, it is possible to suppress a decrease in the initial charge / discharge efficiency. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view of a secondary battery according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] A negative electrode for a secondary battery according to one embodiment of the present disclosure includes a negative electrode mixture containing a negative electrode active material, an additive, and a conductive agent. The negative electrode active material includes a Si-containing material, the additive includes an alkali metal sulfate, and the conductive agent includes carbon nanotubes. The content of the alkali metal sulfate in the negative electrode mixture is 0.0025% by mass or more and 0.1% by mass or less, based on the total amount of the negative electrode active material. According to the present disclosure, the alkali metal sulfate is present in large amounts specifically on the carbon nanotubes and has the property of suppressing a reaction between the carbon nanotubes and an electrolyte solution, which is thought to suppress a decrease in the initial charge / discharge efficiency of the secondary battery. However, to achieve this effect, the content of the alkali metal sulfate in the negative electrode mixture must be within the above range. If the content of the alkali metal sulfate is less than 0.0025% by mass based on the total amount of the negative electrode active material, for example, the reaction between the carbon nanotubes and the electrolyte solution may not be sufficiently suppressed, and the effect of suppressing a decrease in the initial charge / discharge efficiency may not be achieved. Furthermore, if the content of the alkali metal sulfate exceeds 0.1 mass% with respect to the total amount of the negative electrode active material, for example, the alkali metal sulfate will be present in large amounts in places other than the carbon nanotubes, increasing the influence of side reactions caused by the alkali metal sulfate, and the effect of suppressing a decrease in the initial charge / discharge efficiency will not be obtained.

[0012] Furthermore, the carbon nanotubes contained in the negative electrode mixture follow the expansion and contraction of the Si-containing material that occurs during charge and discharge, thereby preventing the Si-containing material from becoming isolated from the conductive path in the negative electrode mixture. Therefore, the carbon nanotubes suppress the decrease in the conductivity of the negative electrode that occurs due to the expansion and contraction of the Si-containing material, and contribute to the effect of suppressing the deterioration of charge-discharge cycle characteristics.

[0013] Hereinafter, embodiments of the negative electrode for a secondary battery and the secondary battery according to the present disclosure will be described in detail with reference to the drawings. In this specification, the expression "numerical value (1) to numerical value (2)" means numerical value (1) or more and numerical value (2) or less.

[0014] FIG. 1 is a cross-sectional view of a secondary battery according to an embodiment. The secondary battery 10 shown in FIG. 1 includes a wound electrode assembly 14 formed by winding a positive electrode 11 and a negative electrode 12 with a separator 13 interposed therebetween, a nonaqueous electrolyte, insulating plates 18 and 19 disposed above and below the electrode assembly 14, respectively, and a battery case 15 for accommodating the above components. The battery case 15 is composed of a cylindrical case body 16 with a bottom and a sealing member 17 that closes the opening of the case body 16. Note that, instead of the wound electrode assembly 14, an electrode assembly of another shape may be used, such as a laminated electrode assembly formed by alternately stacking positive and negative electrodes with separators interposed therebetween. Examples of the battery case 15 include a cylindrical, prismatic, coin-shaped, or button-shaped metal case, and a resin case (laminated battery) formed by laminating a resin sheet.

[0015] Case body 16 is, for example, a cylindrical metal container with a bottom. A gasket 28 is provided between case body 16 and sealing body 17 to ensure airtightness inside the battery. Case body 16 has, for example, a protruding portion 22, which is a portion of the side surface that protrudes inward and supports sealing body 17. Protruding portion 22 is preferably formed in an annular shape along the circumferential direction of case body 16, and supports sealing body 17 on its upper surface.

[0016] The sealing body 17 has a structure in which, in order from the electrode body 14 side, a filter 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 to one another at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the secondary battery 10 increases due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 deforms and ruptures, 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 ruptures, and gas is discharged from the opening of the cap 27.

[0017] In the secondary battery 10 shown in Fig. 1, a positive electrode lead 20 attached to the positive electrode 11 passes through a through-hole in an insulating plate 18 and extends toward the sealing body 17, and a negative electrode lead 21 attached to the negative electrode 12 passes outside an insulating plate 19 and extends toward the bottom side of the case body 16. The positive electrode lead 20 is connected to the underside of a filter 23, which is the bottom plate 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 filter 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the case body 16 by welding or the like, and the case body 16 serves as the negative electrode terminal.

[0018] The positive electrode 11, the negative electrode 12, the separator 13, and the non-aqueous electrolyte that constitute the secondary battery 10 will be described in detail below.

[0019] [Positive electrode] The positive electrode 11 includes, for example, a positive electrode current collector and a positive electrode mixture layer formed on the positive electrode current collector. The positive electrode current collector can be a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode, or a film with such a metal disposed on its surface. The positive electrode mixture layer is composed of, for example, a positive electrode mixture containing a positive electrode active material, a binder, a conductive agent, etc. The positive electrode mixture layer is preferably formed on both sides of the positive electrode current collector. The positive electrode 11 can be manufactured, for example, by applying a slurry of a positive electrode mixture containing a positive electrode active material, a binder, a conductive agent, etc., onto the positive electrode current collector, drying and rolling the coating, and forming a positive electrode mixture layer on both sides of the positive electrode current collector.

[0020] The positive electrode active material may be, for example, a lithium transition metal composite oxide. Metal elements contained in the lithium transition metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, and W. 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.

[0021] Examples of conductive agents contained in the positive electrode mixture layer include carbon materials such as carbon black, acetylene black, ketjen black, graphene, carbon nanotubes, graphite, etc. Examples of binders contained in the positive electrode mixture layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, carboxymethyl cellulose (CMC) or a salt thereof, and polyethylene oxide (PEO).

[0022] [Negative electrode] The negative electrode 12 includes, for example, a negative electrode current collector and a negative electrode mixture layer formed on the current collector. The negative electrode current collector can be, for example, a foil of a metal stable within the potential range of the negative electrode, such as copper or a copper alloy, or a film with such a metal disposed on its surface. The negative electrode mixture layer is composed of a negative electrode mixture containing a negative electrode active material, an additive, a conductive agent, etc. The negative electrode mixture preferably contains a binder in addition to the above materials. The negative electrode 12 can be manufactured, for example, by applying a slurry of a negative electrode mixture containing a negative electrode active material, an additive, a conductive agent, a binder, etc. to the negative electrode current collector, drying and rolling the coating, and forming a negative electrode mixture layer on both sides of the negative electrode current collector.

[0023] The negative electrode active material includes a Si-containing material. The Si-containing material may be any material capable of absorbing and releasing lithium ions, but from the viewpoint of increasing the capacity of the secondary battery, the Si-containing material preferably includes a lithium ion conductive phase and Si particles dispersed in the lithium ion conductive phase, the lithium ion conductive phase being at least one phase selected from a silicon oxide phase, a silicate phase, and a carbon phase.

[0024] The silicate phase preferably contains at least one element E1 selected from lithium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, and radium, for example, in view of high lithium ion conductivity.

[0025] It is preferable that the silicate phase further contains at least one element E2 selected from zirconium, niobium, tantalum, lanthanum, vanadium, titanium, phosphorus, bismuth, zinc, tin, lead, antimony, cobalt, fluorine, tungsten, aluminum, and boron, for example, in order to suppress a decrease in the initial charge / discharge efficiency.

[0026] Preferably, a conductive coating made of a highly conductive material is formed on the particle surface of the Si-containing material. Examples of materials constituting the conductive coating include at least one selected from carbon materials, metals, and metal compounds. Among these, carbon materials such as amorphous carbon are preferred. The carbon coating can be formed, for example, by a CVD method using acetylene, methane, or the like, or by a method in which coal pitch, petroleum pitch, phenolic resin, or the like is mixed with a silicon-based active material and heat-treated. Alternatively, the conductive coating may be formed by adhering a conductive filler such as carbon black to the particle surface of the Si-containing material using a binder.

[0027] Specific examples of the Si-containing material include composite material A containing a silicate phase and Si particles dispersed in the silicate phase, composite material B containing a silicon oxide phase and Si particles dispersed in the silicon oxide phase, and composite material C containing a carbon phase and Si particles dispersed in the carbon phase. These may be used alone or in combination of two or more.

[0028] The silicate phase of composite material A preferably contains the aforementioned element E1, and more preferably further contains the aforementioned element E2. Among these, a silicate phase containing lithium (hereinafter sometimes referred to as a lithium silicate phase) is preferred from the viewpoints of high lithium ion conductivity and suppression of a decrease in initial charge / discharge efficiency. That is, composite material A preferably contains a lithium silicate phase and Si particles dispersed within the lithium silicate phase (hereinafter sometimes referred to as LSX).

[0029] The content of silicon particles in composite material A is preferably 30% by mass or more and 80% by mass or less, more preferably 35% by mass or more and 75% by mass or less, and more preferably 55% by mass or more and 70% by mass or less, from the viewpoints of increasing capacity and improving charge-discharge cycle characteristics.

[0030] The content of silicon particles can be measured by Si-NMR. Desirable measurement conditions for Si-NMR are shown below.

[0031] Measurement equipment: Varian solid-state nuclear magnetic resonance spectrometer (INOVA-400) Probe: Varian 7mm CPMAS-2 MAS:4.2kHz MAS speed: 4kHz Pulse: DD (45° pulse + signal acquisition time 1H decoupled) Repeat time: 1200 seconds Observation width: 100kHz Observation center: Around -100 ppm Signal acquisition time: 0.05 sec Accumulation count: 560 Sample amount: 207.6 mg The Si particles dispersed in the silicate phase have a particulate phase of simple Si and are composed of single or multiple crystallites. The crystallite size of the Si particles is preferably 30 nm or less, for example, in terms of improving charge / discharge cycle characteristics. The lower limit of the crystallite size of the Si particles is not particularly limited, but is, for example, 5 nm.

[0032] The crystallite size of the Si particles is more preferably 10 nm or more and 30 nm or less, and even more preferably 15 nm or more and 25 nm or less. The crystallite size of the Si particles is calculated by the Scherrer equation from the half-width of the diffraction peak assigned to the Si(111) plane in the X-ray diffraction (XRD) pattern of the Si particles.

[0033] The average particle size of the Si particles is preferably 500 nm or less, more preferably 200 nm or less, and even more preferably 50 nm or less before the first charge, from the viewpoint of suppressing cracks in the Si particles themselves, etc. After the first charge, the average particle size of the Si particles is preferably 400 nm or less, and more preferably 100 nm or less.

[0034] The average particle size of the Si particles is measured by observing a cross-sectional SEM (scanning electron microscope) photograph of the composite material A. Specifically, the average particle size of the Si particles is obtained by averaging the maximum diameters of any 100 Si particles.

[0035] The lithium silicate phase is represented by, for example, the formula: Li 2z SiO 2+z (0 < z < 2). From the viewpoints of stability, ease of production, lithium ion conductivity, etc., z preferably satisfies the relationship of 0 < z < 1, and more preferably z = 1 / 2.

[0036] The composite material B in which Si particles are dispersed in the silicon oxide phase is represented by, for example, the general formula SiO x (a range of 0 < x < 2 is preferred, and a range of 0.5 ≤ x ≤ 1.6 is more preferred). The composite material C in which Si particles are dispersed in the carbon phase is represented by, for example, the general formula SixC1y (a range of 0 < x ≤ 1 and 0 < y ≤ 1 is preferred, and a range of 0.3 ≤ x ≤ 0.45 and 0.7 ≤ y ≤ 0.55 is more preferred). The content, crystallite size, and average particle size of the Si particles in the composite materials B and C may be the same as those in the composite material A.

[0037] The content of the Si-containing material in the negative electrode active material is preferably 1% by mass or more and 15% by mass or less based on the total amount of the negative electrode active material, from the viewpoints of increasing the capacity of the secondary battery and suppressing the deterioration of the charge-discharge cycle characteristics, etc.

[0038] The negative electrode active material preferably further contains a carbon material that electrochemically absorbs and desorbs lithium ions as another negative electrode material that expands and contracts less during charge and discharge than the Si-containing material. The content of the carbon material in the negative electrode active material is preferably 85% by mass or more and 99% by mass or less, based on the total amount of the negative electrode active material, in order to, for example, suppress deterioration of the charge and discharge cycle characteristics of the secondary battery.

[0039] Examples of carbon materials include graphite, easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon). Among them, graphite is preferred because it has excellent charge / discharge stability and low irreversible capacity. Graphite refers to a material having a graphite-type crystal structure, and examples include natural graphite, artificial graphite, and graphitized mesophase carbon particles.

[0040] The content of the negative electrode active material in the negative electrode mixture is, for example, preferably 85 mass % or more, more preferably 90 mass % or more, and even more preferably 95 mass % or more, relative to the total amount of the negative electrode mixture.

[0041] Examples of carbon nanotubes used as conductive agents in the negative electrode mixture include single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes. Single-walled carbon nanotubes (SWCNTs) are carbon nanostructures in which one layer of graphene sheets forms a cylindrical shape. Double-walled carbon nanotubes are carbon nanostructures in which two layers of graphene sheets are concentrically stacked to form a cylindrical shape. Multi-walled carbon nanotubes are carbon nanostructures in which three or more layers of graphene sheets are concentrically stacked to form a cylindrical shape. A graphene sheet refers to a layer in which carbon atoms in sp2 hybrid orbitals that form graphite crystals are located at the vertices of a regular hexagon. The shape of the carbon nanotubes is not limited. Examples of such shapes include needles, cylindrical tubes, fishbone-shaped (fishbone or cup-stacked), playing card shapes (platelets), and coil shapes.

[0042] The fiber length of the carbon nanotubes is preferably 500 nm or more and 200 μm or less, and more preferably 1 μm or more and 100 μm or less, in order to suppress deterioration of charge / discharge cycle characteristics, etc. The fiber length of the carbon nanotubes can be determined by measuring the lengths of 50 random carbon nanotubes using a field emission scanning electron microscope (FE-SEM) and taking the arithmetic average.

[0043] The outermost diameter of the carbon nanotubes (i.e., fiber diameter) is preferably 0.5 nm or more and 20 nm or less, and more preferably 1 nm or more and 10 nm or less, from the viewpoint of suppressing deterioration of charge-discharge cycle characteristics, etc. The outermost diameter of the carbon nanotubes can be determined by measuring the outer diameters of 50 arbitrary carbon nanotubes using a field emission scanning electron microscope (FE-SEM) or a transmission electron microscope (TEM) and taking the arithmetic average.

[0044] The content of carbon nanotubes in the negative electrode mixture is preferably 0.01 mass% or more and 1.0 mass% or less, and more preferably 0.1 mass% or more and 0.8 mass% or less, relative to the total amount of the negative electrode active material, for example, in terms of suppressing a deterioration in charge-discharge cycle characteristics.

[0045] The conductive agent contained in the negative electrode mixture may contain a particulate conductive agent in addition to carbon nanotubes. Examples of the particulate conductive agent include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. When a particulate conductive agent is used, it is preferable that the primary particle diameter of the particulate conductive agent is 5 nm or more and 100 nm or less, and that the aspect ratio is less than 10.

[0046] Examples of the alkali metal sulfate as an additive contained in the negative electrode mixture include lithium sulfate, sodium sulfate, potassium sulfate, rubidium sulfate, cesium sulfate, francium sulfate, etc. Among these, the alkali metal sulfate preferably includes at least one selected from lithium sulfate, sodium sulfate, and potassium sulfate, in order to further suppress a decrease in the initial charge / discharge efficiency of the secondary battery.

[0047] The content of the alkali metal sulfate in the negative electrode mixture may be 0.0025% by mass or more and 0.1% by mass or less, preferably 0.01% by mass or more and 0.1% by mass or less, and more preferably 0.02% by mass or more and 0.08% by mass or less, relative to the total amount of the negative electrode active material, in order to suppress a decrease in the initial charge / discharge efficiency of the secondary battery.

[0048] The binder contained in the negative electrode mixture may be the same as the binder used in the positive electrode 11. The content of the binder in the negative electrode mixture is, for example, preferably 0.5% by mass to 10% by mass, and more preferably 1% by mass to 5% by mass, relative to the total amount of the negative electrode active material.

[0049] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include olefin-based resins such as polyethylene, polypropylene, and copolymers containing at least one of ethylene and propylene, and cellulose. The separator 13 may have either a single-layer structure or a laminated structure. A heat-resistant layer or the like may be formed on the surface of the separator 13.

[0050] [Non-aqueous electrolyte] The nonaqueous electrolyte includes a nonaqueous solvent and an electrolyte salt. The nonaqueous electrolyte is not limited to a liquid electrolyte, and may be a solid electrolyte using a gel-like polymer or the like. Examples of the electrolyte salt include lithium salts such as LiFSI, LiTFSI, LiBF4, and LiPF6. Examples of the solvent include esters such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl acetate (MA), and methyl propionate (MP), ethers, nitriles, amides, and mixtures of two or more of these. The nonaqueous solvent may contain a halogen-substituted product in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine.

[0051] Examples of halogen-substituted compounds include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylates such as methyl fluoropropionate (FMP).

[0052] <Example> The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0053] Example 1 [Adjustment of Si-containing material (LSX)] Silicon dioxide and lithium carbonate were mixed so that the atomic ratio Si / Li was 1.05, and the mixture was calcined in air at 950°C for 10 hours to obtain lithium silicate represented by the formula: Li2Si2O5. The obtained lithium silicate was then pulverized to an average particle size of 10 μm.

[0054] The lithium silicate and raw silicon (average particle size 10 μm) were mixed in a mass ratio of 70:30. This mixture was loaded into a stainless steel pot (500 mL volume) of a commercial ball mill (Fritsch, P-5). 24 stainless steel balls (20 mm diameter) were placed in the pot, the lid was closed, and the mixture was pulverized at 200 rpm for 50 hours in an inert atmosphere. The powdered mixture was then removed from the inert atmosphere and sintered at 800°C for 4 hours under pressure from a hot press to obtain a sintered compact (LSX) of the mixture.

[0055] The resulting LSX was crushed and passed through a 40 μm mesh. The resulting LSX particles were then mixed with coal pitch, and the mixture was fired at 800°C in an inert atmosphere to coat the surfaces of the LSX particles with conductive carbon to form a conductive layer. The amount of the conductive layer was 5% by mass based on the total mass of the LSX particles and conductive layer. Subsequently, LSX particles with an average particle size of 5 μm and a conductive layer were obtained using a sieve. The Li2Si2O5 content measured by Si-NMR was 70% by mass (the Si particle content was 30% by mass).

[0056] [Preparation of negative electrode] The LSX particles with a conductive layer and graphite were mixed in a mass ratio of 5:95 to prepare a negative electrode active material. The negative electrode active material, sodium sulfate (additive), carbon nanotubes (conductive agent), sodium carboxymethylcellulose, and styrene-butadiene rubber were mixed in a mass ratio of 100:0.0025:0.3:1.3:1.0, and an appropriate amount of water was added to prepare a negative electrode mixture slurry.

[0057] The above-described slurry of the negative electrode mixture was applied to both surfaces of a negative electrode current collector made of copper foil, the coating film was dried, and then the coating film was rolled using a roller to prepare a negative electrode in which a negative electrode mixture layer was formed on both surfaces of the negative electrode current collector.

[0058] [Nonaqueous electrolyte] A non-aqueous electrolyte solution was prepared by dissolving LiPF6 at a ratio of 1.2 mol / L in a mixed solvent prepared by mixing ethylene carbonate (EC), dimethyl carbonate (DMC), and methyl acetate (MA) in a volume ratio of 20:40:40.

[0059] [Test cell] The positive electrode and the negative electrode were stacked facing each other with a separator interposed therebetween and wound up to prepare an electrode assembly. The electrode assembly and the nonaqueous electrolyte solution were then housed in a cylindrical battery case body with a bottom, and after the electrolyte solution was poured into the battery case body, the opening of the battery case body was sealed with a gasket and a sealing member to prepare a test cell.

[0060] <Example 2> A test cell was prepared in the same manner as in Example 1, except that in preparing the negative electrode, the negative electrode active material, sodium sulfate (additive), carbon nanotubes (conductive agent), sodium carboxymethyl cellulose, and styrene butadiene rubber were mixed in a mass ratio of 100:0.025:0.3:1.3:1.0.

[0061] Example 3 A test cell was prepared in the same manner as in Example 1, except that in preparing the negative electrode, the negative electrode active material, sodium sulfate (additive), carbon nanotubes (conductive agent), sodium carboxymethyl cellulose, and styrene butadiene rubber were mixed in a mass ratio of 100:0.05:0.3:1.3:1.0.

[0062] Example 4 A test cell was prepared in the same manner as in Example 1, except that in preparing the negative electrode, the negative electrode active material, sodium sulfate (additive), carbon nanotubes (conductive agent), sodium carboxymethyl cellulose, and styrene-butadiene rubber were mixed in a mass ratio of 100:0.1:0.3:1.3:1.0.

[0063] <Comparative Example 1> A test cell was prepared in the same manner as in Example 1, except that in preparing the negative electrode, the additive sodium sulfate was not used, and the negative electrode active material, carbon nanotubes (conductive agent), sodium carboxymethyl cellulose, and styrene-butadiene rubber were mixed in a mass ratio of 100:0.3:1.3:1.0.

[0064] <Comparative Example 2> A test cell was prepared in the same manner as in Example 1, except that in preparing the negative electrode, the negative electrode active material, sodium sulfate (additive), carbon nanotubes (conductive agent), sodium carboxymethyl cellulose, and styrene-butadiene rubber were mixed in a mass ratio of 100:0.3:0.3:1.3:1.0.

[0065] [Evaluation of initial charge / discharge efficiency] The test cell was charged at a constant current of 0.3 C in a temperature environment of 25°C until the battery voltage reached 4.2 V, and then discharged at a constant current of 0.3 C until the battery voltage reached 2.5 V. The initial charge capacity and initial discharge capacity were measured, and the initial charge / discharge efficiency was calculated using the following formula.

[0066] Initial charge / discharge efficiency = (initial discharge capacity / initial charge capacity) x 100 Table 1 shows the evaluation results of the initial charge-discharge efficiency for each example and comparative example. The evaluation results in Table 1 are based on the initial charge-discharge efficiency of Comparative Example 1, and the initial charge-discharge efficiencies of the other examples and comparative examples are shown as an increase rate relative to the above-mentioned standard. A positive increase rate indicates that the decrease in the initial charge-discharge efficiency was suppressed.

[0067] [Table 1]

[0068] The initial charge-discharge efficiencies of Examples 1 to 4, in which the content of alkali metal sulfate in the negative electrode mixture was 0.0025 mass % or more and 0.1 mass % or less with respect to the total amount of the negative electrode active material including the Si-containing material, were higher than the initial charge-discharge efficiency of Comparative Example 1, which did not contain any alkali metal sulfate. In other words, it can be said that Examples 1 to 4 suppressed the decrease in the initial charge-discharge efficiency. [Explanation of symbols]

[0069] 10 Secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode body 15 Battery case 16 Case body 17 Sealing body 18,19 Insulating plate 20 Positive lead 21 Negative lead 22 Overhang 23 Filters 24 Lower valve body 25 Insulating material 26 Superior valve 27 Cap 28 Gasket

Claims

1. A negative electrode for a secondary battery including a negative electrode mixture containing a negative electrode active material, an additive, and a conductive agent, the negative electrode active material includes a Si-containing material, the additive includes an alkali metal sulfate, and the conductive agent includes carbon nanotubes; the content of the alkali metal sulfate in the negative electrode mixture is 0.0025% by mass or more and 0.1% by mass or less with respect to the total amount of the negative electrode active material, The negative electrode for a secondary battery, wherein the alkali metal sulfate is unevenly distributed more on the carbon nanotubes than on the negative electrode active material.

2. 2. The negative electrode for a secondary battery according to claim 1, wherein the alkali metal sulfate comprises at least one selected from the group consisting of sodium sulfate, lithium sulfate, and potassium sulfate.

3. 3. The negative electrode for a secondary battery according to claim 1, wherein the content of the carbon nanotubes in the negative electrode mixture is 0.01% by mass or more and 1.0% by mass or less with respect to the total amount of the negative electrode active material.

4. 4. The negative electrode for a secondary battery according to claim 1, wherein the outermost diameter of the carbon nanotubes is 0.5 nm or more and 20 nm or less.

5. 5. The negative electrode for a secondary battery according to claim 1, wherein the carbon nanotubes have a fiber length of 500 nm or more and 200 μm or less.

6. the Si-containing material includes a lithium ion conductive phase and Si particles dispersed in the lithium ion conductive phase; 6. The negative electrode for a secondary battery according to claim 1, wherein the lithium ion conductive phase is at least one selected from a silicon oxide phase, a silicate phase, and a carbon phase.

7. 7. The negative electrode for a secondary battery according to claim 6, wherein the silicate phase contains at least one element E1 selected from lithium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, and radium.

8. 8. The negative electrode for a secondary battery according to claim 7, wherein the silicate phase contains at least one element E2 selected from zirconium, niobium, tantalum, lanthanum, vanadium, titanium, phosphorus, bismuth, zinc, tin, lead, antimony, cobalt, fluorine, tungsten, aluminum, and boron.

9. A secondary battery comprising the negative electrode for secondary batteries according to any one of claims 1 to 8, a positive electrode, and a non-aqueous electrolyte solution.

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