Secondary battery uses negative electrode and secondary battery

Abstract

The negative electrode for a secondary battery comprises a negative electrode compound, which includes: a negative electrode active material, an additive, and a conductive agent. The aforementioned negative electrode active material includes a Si-containing material, the aforementioned additive includes an alkali metal sulfate, and the aforementioned conductive agent includes carbon nanotubes. The content of the aforementioned alkali metal sulfate in the aforementioned negative electrode compound is more than 0.0025% by mass and less than 0.1% by mass relative to the total amount of the aforementioned negative electrode active material.

Description

Technical Field

[0001] This disclosure relates to a negative electrode for secondary batteries and secondary batteries. Background Technology

[0002] Si-containing materials are alloyed materials with lithium. Compared with carbon-based active materials such as graphite, they are known to be able to absorb a large number of lithium ions per unit volume, and can be expected to be used in the negative electrode active materials of secondary batteries.

[0003] However, Si-containing materials exhibit large volume changes (expansion / contraction) during charge and discharge, which can easily reduce the conductivity of the negative electrode, resulting in a decrease in charge-discharge cycle characteristics.

[0004] To improve this problem, the following technology exists: adding carbon nanotubes into the negative electrode containing Si material to suppress the decrease in conductivity of the negative electrode accompanied by the expansion / contraction of the Si-containing agent, and to suppress the decrease in charge-discharge cycle characteristics.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2005-310760 Summary of the Invention

[0008] Carbon nanotubes have a large specific surface area, therefore, they react with the electrolyte during the initial charge and discharge of a secondary battery, resulting in a decrease in initial charge and discharge efficiency. It should be noted that the initial charge and discharge efficiency refers to the ratio of the initial discharge capacity to the initial charge capacity.

[0009] As one aspect of this disclosure, the negative electrode for a secondary battery includes a negative electrode mixture comprising: a negative electrode active material, an additive, and a conductive agent. The negative electrode active material comprises a Si-containing material, the additive comprises an alkali metal sulfate, and the conductive agent comprises 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 relative to the total amount of the negative electrode active material.

[0010] As one aspect of this disclosure, a secondary battery comprises: a negative electrode, a positive electrode, and a non-aqueous electrolyte.

[0011] According to this disclosure, the reduction in initial charge and discharge efficiency can be suppressed. Attached Figure Description

[0012] Figure 1 This is a cross-sectional view of a secondary battery as an example of an implementation method. Detailed Implementation

[0013] As one aspect of this disclosure, the negative electrode for a secondary battery includes a negative electrode mixture comprising: a negative electrode active material, an additive, and a conductive agent. The negative electrode active material comprises a Si-containing material, the additive comprises an alkali metal sulfate, and the conductive agent comprises 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 relative to the total amount of the negative electrode active material. According to this disclosure, the alkali metal sulfate is specifically abundant on the carbon nanotubes and has the property of inhibiting the reaction between the carbon nanotubes and the electrolyte. Therefore, it is believed that the reduction in the initial charge-discharge efficiency in the secondary battery is suppressed. However, to achieve the above effect, the content of the alkali metal sulfate in the negative electrode mixture must be within the above range. When the content of the alkali metal sulfate relative to the total amount of the negative electrode active material is less than 0.0025% by mass, for example, the reaction between the carbon nanotubes and the electrolyte cannot be sufficiently suppressed, and the effect of suppressing the reduction in the initial charge-discharge efficiency cannot be obtained. In addition, when the content of alkali metal sulfate exceeds 0.1% by mass relative to the total amount of negative electrode active material, for example, when a large amount of alkali metal sulfate exists in parts other than carbon nanotubes, the influence of side reactions originating from alkali metal sulfate becomes greater, and the effect of suppressing the reduction in initial charge and discharge efficiency cannot be obtained.

[0014] It should be noted that the carbon nanotubes contained in the negative electrode mixture follow the expansion / contraction of the Si-containing material during charge / discharge. Therefore, the increase of Si-containing material isolated from the conductive pathways in the negative electrode mixture is suppressed. Thus, the carbon nanotubes suppress the decrease in conductivity of the negative electrode accompanied by the expansion / contraction of the Si-containing material, which is beneficial for suppressing the deterioration of charge / discharge cycle characteristics.

[0015] Hereinafter, embodiments of the negative electrode for a secondary battery and the secondary battery of the present disclosure will be described in detail with reference to the accompanying drawings. It should be noted that in this specification, the description of "value (1) to value (2)" refers to values ​​(1) and above and values ​​(2) and below.

[0016] Figure 1 This is a cross-sectional view of a secondary battery as an example of an implementation method. Figure 1 The secondary battery 10 shown includes: a wound electrode body 14 formed by winding a positive electrode 11 and a negative electrode 12 with a separator 13; a non-aqueous electrolyte; insulating plates 18 and 19 respectively disposed above and below the electrode body 14; and a battery casing 15 housing the above components. The battery casing 15 is composed of a bottomed cylindrical casing body 16 and a sealing body 17 that blocks the opening of the casing body 16. It should be noted that other forms of electrode bodies, such as a laminated electrode body in which the positive and negative electrodes are alternately stacked with a separator, can be used instead of the wound electrode body 14. In addition, examples of battery casing 15 include cylindrical, square, coin-shaped, button-shaped, and other metal casings, as well as resin casings formed by laminating resin sheets (laminated batteries).

[0017] The outer casing 16 is, for example, a bottomed cylindrical metal container. A gasket 28 is provided between the outer casing 16 and the sealing body 17 to ensure the airtightness of the battery interior. The outer casing 16 has, for example, a bulge 22 that protrudes inward from a portion of its side surface to support the sealing body 17. The bulge 22 is preferably formed in a ring shape along the circumference of the outer casing 16, and its upper surface supports the sealing body 17.

[0018] The sealing body 17 has a structure in which a partially open metal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cover 27 are stacked sequentially from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a circular or annular shape, and all components except the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective central portions, and the insulating member 25 is sandwiched between their respective peripheral portions. When the internal pressure of the secondary battery 10 rises due to heat generated by internal short circuits, for example, the lower valve body 24 deforms and breaks by pushing the upper valve body 26 towards the cover 27, thus blocking the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure rises further, the upper valve body 26 breaks, and gas is discharged from the opening of the cover 27.

[0019] Figure 1 In the secondary battery 10 shown, the positive electrode lead 20, installed on the positive electrode 11, extends towards the sealing body 17 through the through hole in the insulating plate 18, and the negative electrode lead 21, installed on the negative electrode 12, extends towards the bottom of the outer casing 16 through the outer side of the insulating plate 19. The positive electrode lead 20 is connected by welding or the like to the lower surface of the metal plate 23, which serves as a partial opening in the bottom plate of the sealing body 17. The top plate of the sealing body 17, i.e., the cover 27, which is electrically connected to the metal plate 23 with the partial opening, becomes the positive terminal. The negative electrode lead 21 is connected by welding or the like to the inner bottom surface of the outer casing 16, making the outer casing 16 the negative terminal.

[0020] The following is a detailed description of the positive electrode 11, negative electrode 12, separator 13, and non-aqueous electrolyte that constitute the secondary battery 10.

[0021] [positive electrode]

[0022] The positive electrode 11, for example, includes a positive current collector and a positive electrode additive layer formed on the positive current collector. The positive current collector can use a foil of a metal stable within the potential range of the positive electrode, such as aluminum or an aluminum alloy, or a thin film of the metal disposed on its surface. The positive electrode additive layer is, for example, formed by a positive electrode additive containing a positive active material, a binder, a conductive agent, etc. The positive electrode additive layer is preferably formed on both sides of the positive current collector. The positive electrode 11 can be manufactured, for example, by coating a slurry containing a positive electrode additive containing a positive active material, a binder, a conductive agent, etc., onto the positive current collector, drying the coating, and calendering it to form a positive electrode additive layer on both sides of the positive current collector.

[0023] Positive electrode active materials may include, for example, lithium transition metal composite oxides. Examples of metal elements contained in lithium transition metal composite oxides include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, and W. Preferably, at least one of Ni, Co, and Mn is included. 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.

[0024] Examples of conductive agents contained in the positive electrode binder layer include carbon materials such as carbon black, acetylene black, Ketjen black, graphene, carbon nanotubes, and graphite. Examples of binders contained in the positive electrode binder layer include fluoropolymers such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resins, polyolefins, carboxymethyl cellulose (CMC) or its salts, and polyethylene oxide (PEO).

[0025] [negative electrode]

[0026] The negative electrode 12, for example, includes a negative electrode current collector and a negative electrode binder layer formed on the current collector. The negative electrode current collector may use a foil of a metal stable within the negative electrode's potential range, such as copper or a copper alloy, or a thin film of that metal disposed on its surface. The negative electrode binder layer is constructed using a negative electrode binder comprising a negative electrode active material, additives, a conductive agent, etc. In addition to the aforementioned substances, the negative electrode binder suitably also includes a binder. The negative electrode 12 can be manufactured, for example, by coating a slurry comprising a negative electrode active material, additives, a conductive agent, a binder, etc., onto the negative electrode current collector, drying and calendering the coating, thereby forming a negative electrode binder layer on both sides of the negative electrode current collector.

[0027] The negative electrode active material includes a Si-containing material. The Si-containing material can be any material that can absorb / release lithium ions. From the viewpoint of increasing the capacity of secondary batteries, the following Si-containing material is preferred: it includes a lithium-ion conductive phase and Si particles dispersed in the lithium-ion conductive phase, wherein the lithium-ion conductive phase is selected from at least one of silicon oxide phase, silicate phase and carbon phase.

[0028] For example, silicate phases, from the perspective of high lithium-ion conductivity, preferably contain at least one element E1 selected from lithium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, and radium.

[0029] The silicate phase, for example, preferably includes 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, from the perspective of suppressing the reduction in initial charge and discharge efficiency.

[0030] Preferably, a conductive coating made of a highly conductive material is formed on the surface of Si-containing material particles. Examples of materials for the conductive coating include at least one selected from carbon materials, metals, and metal compounds. Amorphous carbon and other carbon materials are preferred. The carbon coating can be formed, for example, by a CVD method using acetylene, methane, etc.; or by a method of mixing coal tar pitch, petroleum tar pitch, phenolic resin, etc., with a silicon-based active material and then heat-treating the mixture. Alternatively, a conductive coating can also be formed by using a conductive filler such as carbon black as a binder, which is then fixed to the surface of the Si-containing material particles.

[0031] Specific examples of Si-containing materials include composite material A, which contains a silicate phase and Si particles dispersed in the silicate phase; composite material B, which contains a silicon oxide phase and Si particles dispersed in the silicon oxide phase; and composite material C, which contains a carbon phase and Si particles dispersed in the carbon phase. One type of material can be used alone, or two or more types can be used in combination.

[0032] The silicate phase of composite material A preferably contains the aforementioned element E1, and more preferably also contains the aforementioned element E2. From the perspectives of high lithium-ion conductivity and suppression of initial charge / discharge efficiency reduction, a silicate phase containing lithium (hereinafter, sometimes referred to as the lithium silicate phase) is preferred. That is, composite material A preferably contains a lithium silicate phase and Si particles (hereinafter, sometimes referred to as LSX) dispersed within the lithium silicate phase.

[0033] In terms of high capacity and improved charge-discharge cycle characteristics, the content of silicon particles in composite material A is preferably 30% by mass or more and 80% by mass or less, 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.

[0034] The content of silicon particles can be determined by Si-NMR. The ideal measurement conditions for Si-NMR are shown below.

[0035] Measurement Apparatus: Solid-state nuclear magnetic resonance spectrometer (INOVA-400) manufactured by Varian.

[0036] Probe: Varian 7mm CPMAS-2

[0037] MAS: 4.2kHz

[0038] MAS speed: 4kHz

[0039] Pulse: DD (45° pulse + 1-hour signal collection time, decoupling)

[0040] Repeat time: 1200 seconds

[0041] Observation amplitude: 100kHz

[0042] Observation center: around -100ppm

[0043] Signal acquisition time: 0.05 seconds

[0044] Total number of times: 560

[0045] Sample volume: 207.6 mg

[0046] Si particles dispersed within a silicate phase have a granular phase of elemental Si, consisting either individually or in the form of multiple crystallites. For example, in terms of improving charge-discharge cycle characteristics, the crystallite size of the Si particles is preferably below 30 nm. There is no particular limitation on the lower limit of the crystallite size of the Si particles, for example, 5 nm.

[0047] Furthermore, 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 from the half-width of the diffraction peaks attributable to the Si (111) plane in the X-ray diffraction (XRD) pattern of the Si particles according to the Scherer formula.

[0048] Regarding the average particle size of Si particles, for example from the perspective of suppressing cracking of the Si particles themselves, it is preferably 500 nm or less, more preferably 200 nm or less, and even more preferably 50 nm or less before the first charge. After the first charge, the average particle size of Si particles is preferably 400 nm or less, more preferably 100 nm or less.

[0049] The average particle size of Si particles was determined by observing a cross-sectional SEM (scanning electron microscope) image of composite material A. Specifically, the average particle size of Si particles was calculated by averaging the maximum diameters of any 100 Si particles.

[0050] The lithium silicate phase is represented, for example, by 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.

[0051] The composite material B in which Si particles are dispersed in the silicon oxide phase is represented, for example, by the general formula SiO x (preferably in the range of 0 < x < 2, more preferably in the range of 0.5 ≤ x ≤ 1.6). The composite material C in which Si particles are dispersed in the carbon phase is represented, for example, by the general formula SixC1y (preferably in the ranges of 0 < x ≤ 1 and 0 < y ≤ 1, more preferably in the ranges of 0.3 ≤ x ≤ 0.45 and 0.7 ≤ y ≤ 0.55). The content, crystallite size, and average particle size of the Si particles in the composite materials B and C can be the same as in the case of the composite material A.

[0052] Regarding the content of the Si-containing material in the negative electrode active material, for example, in terms of increasing the capacity of the secondary battery and suppressing the deterioration of the charge-discharge cycle characteristics, etc., it is preferably 1% by mass or more and 15% by mass or less relative to the total amount of the negative electrode active material.

[0053] The negative electrode active material preferably further contains a carbon material that electrochemically stores / releases lithium ions as another negative electrode material with a smaller degree of expansion and contraction during charge and discharge compared to the Si-containing material. Regarding the content of the carbon material in the negative electrode active material, for example, in terms of suppressing the deterioration of the charge-discharge cycle characteristics of the secondary battery, etc., it is preferably 85% by mass or more and 99% by mass or less relative to the total amount of the negative electrode active material.

[0054] Examples of the carbon material include graphite, graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), etc. Among them, graphite with excellent charge-discharge stability and little irreversible capacity is preferred. Graphite refers to a material having a graphite-type crystal structure, and examples include natural graphite, artificial graphite, graphitized mesophase carbon particles, etc.

[0055] The content of the negative electrode active material in the negative electrode binder is, for example, preferably 85% by mass or more, more preferably 90% by mass or more, and still more preferably 95% by mass or more relative to the total amount of the negative electrode binder.

[0056] The carbon nanotubes contained in the negative electrode mixture as conductive agents can be single-layer carbon nanotubes, two-layer carbon nanotubes, or multi-layer carbon nanotubes. A single-layer carbon nanotube (SWCNT) is a carbon nanostructure consisting of one layer of graphene sheets forming a cylindrical shape. A two-layer carbon nanotube consists of two layers of graphene sheets stacked concentrically to form a cylindrical shape. A multi-layer carbon nanotube consists of three or more layers of graphene sheets stacked concentrically to form a cylindrical shape. It should be noted that a graphene sheet refers to a layer in which the carbon atoms of the sp2 hybrid orbitals constituting the graphite crystal are located at the vertices of a hexagonal shape. The shape of the carbon nanotubes is not limited. Examples of the aforementioned shapes include needle-like, cylindrical, fishbone-like (fishbone or cup stacked), playing card-like (thin sheet), and coil-like forms.

[0057] For the fiber length of carbon nanotubes, for example, in suppressing the degradation of charge-discharge cycle characteristics, a length of 500 nm or more and 200 μm or less is preferred, and a length of 1 μm or more and 100 μm or less is even more preferred. It should be noted that the fiber length of carbon nanotubes can be calculated as follows: using a field emission scanning microscope (FE-SEM), the lengths of 50 arbitrary carbon nanotubes are measured, and the fiber length is calculated by arithmetic mean.

[0058] For the outermost diameter of carbon nanotubes (i.e., fiber diameter), for example, in suppressing the degradation of charge-discharge cycle characteristics, a diameter of 0.5 nm or more and 20 nm or less, more preferably 1 nm or more and 10 nm or less, is preferred. The outermost diameter of carbon nanotubes can be determined as follows: by measuring the outer diameter of any 50 carbon nanotubes using a field emission scanning microscope (FE-SEM) or a transmission electron microscope (TEM), and then calculating the outer diameter using an arithmetic mean.

[0059] Regarding the content of carbon nanotubes in the negative electrode compound, for example in suppressing the reduction of charge-discharge cycle characteristics, it is preferably 0.01% by mass or more and 1.0% by mass or less, more preferably 0.1% by mass or more and 0.8% by mass or less, relative to the total amount of negative electrode active material.

[0060] In addition to carbon nanotubes, the conductive agent contained in the negative electrode mixture may also include particulate conductive agents. Examples of particulate conductive agents include carbon materials such as carbon black, acetylene black, Ketjen black, and graphite. When using particulate conductive agents, it is preferable that their primary particle size is 5 nm to 100 nm, and more preferably that their aspect ratio is less than 10.

[0061] Examples of alkali metal sulfates used as additives in the negative electrode mixture include lithium sulfate, sodium sulfate, potassium sulfate, rubidium sulfate, cesium sulfate, and francium sulfate. Among these, the alkali metal sulfate preferably includes at least one selected from lithium sulfate, sodium sulfate, and potassium sulfate to further suppress the reduction in the initial charge-discharge efficiency of the secondary battery.

[0062] Regarding the content of alkali metal sulfates in the negative electrode mixture, in order to suppress the reduction of the initial charge and discharge efficiency of the secondary battery, it is acceptable to have a content of 0.0025% by mass or more and 0.1% by mass or less relative to the total amount of negative electrode active material, preferably 0.01% by mass or more and 0.1% by mass or less, more preferably 0.02% by mass or more and 0.08% by mass or less.

[0063] The binder contained in the negative electrode mixture can be the same as the binder used in the positive electrode 11. The content of the binder in the negative electrode mixture is preferably 0.5% to 10% by mass, more preferably 1% to 5% by mass, relative to the total amount of the negative electrode active material.

[0064] [Separator]

[0065] The separator 13 is a porous sheet with ion permeability and insulation. Specific examples of porous sheets include microporous films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 13 include polyethylene, polypropylene, olefin resins such as copolymers containing at least one of ethylene and propylene, and cellulose. The separator 13 can be a single-layer structure or a multilayer structure. A heat-resistant layer can be formed on the surface of the separator 13.

[0066] [Non-aqueous electrolytes]

[0067] Non-aqueous electrolytes comprise non-aqueous solvents and electrolyte salts. Non-aqueous electrolytes are not limited to liquid electrolytes and can also be solid electrolytes using gel polymers, etc. Examples of lithium salts used in electrolyte salts include LiFSI, LiTFSI, LiBF4, and LiPF6. Examples of solvents used include esters, ethers, nitriles, amides, and mixtures of two or more of these, 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). Non-aqueous solvents may also contain halogen-substituted derivatives formed by replacing at least a portion of the hydrogen atoms in these solvents with halogen atoms such as fluorine.

[0068] Examples of halogenated substitutes include fluorocyclic carbonates such as fluoroethylene carbonate (FEC), fluorochain carbonates, and fluorochain carboxylic acid esters such as fluoropropionate (FMP).

[0069] <Example>

[0070] The present disclosure will be further described below with reference to embodiments, but the present disclosure is not limited to these embodiments.

[0071] <Example 1>

[0072] [Adjustments to Si-containing materials (LSX)]

[0073] Silica and lithium carbonate were mixed with an atomic ratio of Si / Li of 1.05. The mixture was then calcined in air at 950°C for 10 hours to obtain lithium silicate with the formula Li₂Si₂O₅. The resulting lithium silicate was then pulverized to an average particle size of 10 μm.

[0074] The lithium silicate and raw silicon (average particle size 10 μm) were mixed at a mass ratio of 70:30. This mixture was then filled into a 500 mL SUS ball mill (P-5, Fritsch) and 24 SUS balls (20 mm in diameter) were placed inside. 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 in an inert atmosphere and calcined at 800 °C for 4 hours under pressure using a hot press in an inert atmosphere to obtain a sintered body (LSX).

[0075] The obtained LSX was pulverized and passed through a 40 μm sieve. The resulting LSX particles were then mixed with coal tar pitch and calcined at 800 °C in an inert atmosphere. The surface of the LSX particles was then coated with conductive carbon to form a conductive layer. The coverage of the conductive layer was 5% by mass relative to the total mass of the LSX particles and the conductive layer. Subsequently, using a sieve, LSX particles with an average particle size of 5 μm and a conductive layer were obtained. The Li₂Si₂O₅ content, as determined by Si-NMR, was 70% by mass (Si particle content was 30% by mass).

[0076] [Making the negative electrode]

[0077] LSX particles with a conductive layer were mixed with graphite at a mass ratio of 5:95, and this mixture was used as the negative electrode active material. The negative electrode active material was mixed with sodium sulfate (additive), carbon nanotubes (conductive agent), sodium carboxymethyl cellulose, and styrene-butadiene rubber at a mass ratio of 100:0.0025:0.3:1.3:1.0, and an appropriate amount of water was added to adjust the slurry of the negative electrode mixture.

[0078] The above-mentioned negative electrode mixture slurry is coated on both sides of the negative electrode current collector formed of copper foil. After the coating film dries, the coating film is rolled with a roller to form a negative electrode with a negative electrode mixture layer on both sides of the negative electrode current collector.

[0079] [Non-aqueous electrolyte]

[0080] A non-aqueous electrolyte was prepared by dissolving LiPF6 at a ratio of 1.2 mol / L in a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and methyl acetate (MA) in a volume ratio of 20:40:40.

[0081] [Test Battery Cell]

[0082] The positive and negative electrodes are stacked opposite each other with a separator between them and then wound up to form an electrode body. The electrode body and the aforementioned non-aqueous electrolyte are then housed in a bottomed cylindrical battery casing. After injecting the electrolyte, the opening of the battery casing is sealed with a gasket and a sealing body to create a test battery cell.

[0083] <Example 2>

[0084] In the fabrication of 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. Otherwise, the test battery cell was fabricated in the same manner as in Example 1.

[0085] <Example 3>

[0086] In the fabrication of 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. Otherwise, the test battery cell was fabricated in the same manner as in Example 1.

[0087] <Example 4>

[0088] In the fabrication of 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. Otherwise, the test battery cell was fabricated in the same manner as in Example 1.

[0089] <Comparative Example 1>

[0090] In the fabrication of the negative electrode, sodium sulfate is not used as an additive. The negative electrode active material, carbon nanotubes (conductive agent), sodium carboxymethyl cellulose, and styrene-butadiene rubber are mixed in a mass ratio of 100:0.3:1.3:1.0. Otherwise, the test battery cell is fabricated in the same manner as in Example 1.

[0091] <Comparative Example 2>

[0092] In the fabrication of 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. Otherwise, the test battery cell was fabricated in the same manner as in Example 1.

[0093] [Evaluation of initial charge and discharge efficiency]

[0094] For the test battery cell, it was charged at a constant current of 0.3C at 25°C until the battery voltage reached 4.2V, and then discharged at a constant current of 0.3C until the battery voltage reached 2.5V. The initial charge capacity and initial discharge capacity were measured, and the initial charge-discharge efficiency was calculated using the following formula.

[0095] Initial charge / discharge efficiency = (Initial discharge capacity / Initial charge capacity) × 100

[0096] Table 1 shows the evaluation results of the initial charge-discharge efficiency in each embodiment and comparative example. The evaluation results in Table 1 are as follows: using the initial charge-discharge efficiency of Comparative Example 1 as a baseline, the initial charge-discharge efficiency of other embodiments and comparative examples is expressed as the rate of increase relative to the baseline. A positive rate of increase indicates that the decrease in initial charge-discharge efficiency has been suppressed.

[0097] [Table 1]

[0098]

[0099] The initial charge-discharge efficiencies of Examples 1-4, in which the content of alkali metal sulfate in the negative electrode mixture is 0.0025% by mass or more and 0.1% by mass or less relative to the total amount of negative electrode active material containing Si material, show higher values ​​than those of Comparative Example 1, which does not contain alkali metal sulfate. That is, according to Examples 1-4, it can be said that the decrease in initial charge-discharge efficiency is suppressed.

[0100] Explanation of reference numerals in the attached figures

[0101] 10 Secondary batteries

[0102] 11 Positive electrode

[0103] 12 Negative electrode

[0104] 13. Separators

[0105] 14 Electrode bodies

[0106] 15 Battery casing

[0107] 16. Outer shell body

[0108] 17 Sealing body

[0109] Insulation boards 18 and 19

[0110] 20 Positive lead

[0111] 21 Negative lead

[0112] 22. Drum protrusion

[0113] 23. Metal plate with partial openings

[0114] 24 Lower valve body

[0115] 25 Insulating components

[0116] 26 Upper valve body

[0117] 27. Lid

[0118] 28 gaskets

Claims

1. A negative electrode for a secondary battery, comprising a negative electrode mixture, said negative electrode mixture comprising: a negative electrode active material, an additive, and a conductive agent. The negative electrode active material contains Si, the additive contains alkali metal sulfate, and the conductive agent contains carbon nanotubes. The content of the alkali metal sulfate in the negative electrode mixture is more than 0.0025% by mass and less than 0.1% by mass relative to the total amount of the negative electrode active material. The Si-containing material comprises: a lithium-ion conductive phase and Si particles dispersed in the lithium-ion conductive phase. The lithium-ion conductive phase is selected from at least one of silicon oxide phase, silicate phase and carbon phase.

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

3. The negative electrode for a secondary battery according to claim 1 or 2, wherein, The content of carbon nanotubes in the negative electrode mixture is more than 0.01% by mass and less than 1.0% by mass relative to the total amount of the negative electrode active material.

4. The negative electrode for a secondary battery according to claim 1 or 2, wherein, The carbon nanotubes have a fiber diameter of 0.5 nm or more and 20 nm or less.

5. The negative electrode for a secondary battery according to claim 1 or 2, wherein, The carbon nanotubes have a fiber length of 500 nm or more and 200 μm or less.

6. The negative electrode for a secondary battery according to claim 1, 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.

7. The negative electrode for a secondary battery according to claim 6, 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.

8. The negative electrode for a secondary battery according to claim 1 or 2, wherein, The alkali metal sulfate is more concentrated on the carbon nanotubes than on the negative electrode active material.

9. A secondary battery comprising: a negative electrode, a positive electrode, and a non-aqueous electrolyte as described in any one of claims 1 to 8.

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