Secondary battery

Abstract

Provided is a secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, the negative electrode including a carbon material, a first silicon-containing material, and a second silicon-containing material, wherein the first silicon-containing material contains a silicate phase and a first silicon phase dispersed in the silicate phase, and the second silicon-containing material contains a carbon phase and a second silicon phase dispersed in the carbon phase.

Description

Technical Field

[0001] The present invention relates to a secondary battery.

Background Art

[0002] In recent years, secondary batteries such as non-aqueous electrolyte secondary batteries have high voltage and high energy density, and thus are expected to be used as power sources for small consumer applications, power storage devices, and electric vehicles. As the demand for higher energy density of batteries increases, the use of silicon-containing materials containing silicon (silicon) alloyed with lithium is expected as a negative electrode active material with a high theoretical capacity density.

[0003] In Patent Document 1, a non-aqueous electrolyte secondary battery using a composite material containing a lithium silicate phase represented by Li 2z SiO 2+z (0 < z < 2) and silicon particles dispersed in the lithium silicate phase has been proposed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Since the silicon-containing material has large expansion and contraction during charge and discharge, voids are likely to be formed around the silicon-containing material when it contracts. In addition, the silicon-containing material cannot withstand the stress caused by expansion and contraction, and cracks may occur in the silicon-containing material. As a result, when the charge and discharge cycles are repeated, the contacts between a part of the silicon-containing material and its surroundings gradually decrease, a part of the silicon-containing material becomes isolated, and the capacity decreases.

Means for Solving the Problems

[0006] In view of the above, one aspect of the present invention relates to a secondary battery including a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode includes a carbon material, a first silicon-containing material, and a second silicon-containing material, the first silicon-containing material includes a silicate phase and a first silicon phase dispersed in the silicate phase, and the second silicon-containing material includes a carbon phase and a second silicon phase dispersed in the carbon phase.

Advantages of the Invention

[0007] According to the present invention, the cycle characteristics of the secondary battery can be improved.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0009] The secondary battery according to the embodiment of the present invention includes a positive electrode, a negative electrode, and an electrolyte (or electrolytic solution). The negative electrode includes, as an active material, a carbon material (hereinafter also referred to as a first carbon material), a first silicon-containing material, and a second silicon-containing material. Here, the first silicon-containing material includes a silicate phase and first silicon particles or a silicon phase dispersed in the silicate phase. Further, the second silicon-containing material includes a carbon phase and second silicon particles or a silicon phase dispersed in the carbon phase. Hereinafter, the silicon phase may be silicon particles or may not have a particle form.

[0010] The first silicon-containing material and the second silicon-containing material each have a so-called sea-island structure. The first or second silicon phase (island) is dispersed in a matrix (sea) of a silicate phase or a carbon phase (hereinafter, the silicate phase and the carbon phase are collectively also referred to as a lithium-ion conductive phase), and is covered with the lithium-ion conductive phase. In the sea-island structure, since the contact between the first or second silicon phase and the electrolyte is restricted, side reactions are suppressed. Also, the stress generated by the expansion and contraction of the silicon phase is relaxed by the matrix of the lithium-ion conductive phase.

[0011] The first silicon-containing material can contain a substantial amount of the first silicon phase, has few sites for capturing lithium ions that cause irreversible capacity, and is less likely to cause side reactions. However, the silicate phase of the first silicon-containing material does not have electronic conductivity. Therefore, if voids are formed around the first silicon-containing material due to expansion and contraction during charge and discharge, or if cracks occur in the first silicon-containing material due to stress caused by expansion and contraction, a part of the first silicon-containing material becomes isolated and the contact points between a part of the first silicon-containing material and its surroundings decrease, so the capacity is likely to decrease.

[0012] In contrast, the second silicon-containing material includes a carbon phase and a second silicon phase dispersed in the carbon phase. Since the carbon phase of the second silicon-containing material has electronic conductivity, even if voids are formed around the second silicon-containing material or cracks occur in the first silicon-containing material, a part of the second silicon-containing material is less likely to become isolated and the contact points between the second silicon-containing material and its surroundings are likely to be maintained. Therefore, by replacing a part of the first silicon-containing material with the second silicon-containing material, the contact points with the surroundings of the entire silicon-containing material are likely to be maintained, and a decrease in capacity when repeating charge and discharge cycles is likely to be suppressed.

[0013] The carbon phase can be composed of, for example, amorphous carbon (that is, non-crystalline carbon). The amorphous carbon may be, for example, hard carbon, soft carbon, or the like. Amorphous carbon (non-crystalline carbon) generally refers to a carbon material having an average interplanar spacing d002 of the (002) plane measured by X-ray diffraction method exceeding 0.34 nm.

[0014] The first carbon material included in the negative electrode as the active material is graphite, graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), etc., and may be a composite of these. The carbon material may be used alone or in combination of two or more. Among them, graphite, which is excellent in charge-discharge stability and has little irreversible capacity, is preferable. 50% by mass or more, and further 80% by mass or more of the carbon material may be graphite.

[0015] Graphite means a material in which a graphite-type crystal structure is developed, and generally refers to a carbon material having an average interplanar spacing d002 of the (002) plane measured by X-ray diffraction method of 0.340 nm or less. For example, natural graphite, artificial graphite, graphitized mesophase carbon particles, etc. are typical graphites.

[0016] The ratio of the mass B of the second silicon-containing material to the mass A of the first silicon-containing material: B / A may satisfy 0.2 < B / A < 10. The control of the B / A ratio is extremely important for improving the cycle characteristics of the secondary battery. By controlling the B / A ratio within the range of 0.2 < B / A < 10, and further within the range of 0.5 ≤ B / A ≤ 3 or 0.5 ≤ B / A ≤ 2, the effect of improving the cycle characteristics by the second silicon-containing material containing a carbon phase and the second silicon phase dispersed therein becomes remarkable. The second silicon-containing material is considered to play a role of entering the voids generated by the shrinkage after the expansion of the first silicon-containing material and the voids formed by the cracks generated in the first silicon-containing material, and maintaining the electrical connection between the first silicon-containing material and its surroundings. For that purpose, it is considered desirable that the second silicon-containing material is included in the negative electrode in an amount sufficient to act on the entire first silicon-containing material included in the negative electrode. The closer the B / A ratio is to 1, the more desirable it is. For example, it may satisfy 0.8 ≤ B / A ≤ 1.2.

[0017] The ratio of the total mass of the first silicon-containing material (A) and the second silicon-containing material (B) to the total mass of the first silicon-containing material (A), the second silicon-containing material (B), and the first carbon material (C) (A + B + C) may be 5% by mass or more and 30% by mass or less, may be 5% by mass or more and 20% by mass or less, may be 5% by mass or more and 15% by mass or less, may be 5% by mass or more and 12% by mass or less, or may be 5% by mass or more and 10% by mass or less. In such a range, it is considered that the expansion and contraction of the entire negative electrode can be controlled within a more suitable range, and the high-capacity merits of the first and second silicon-containing materials can be maximally enjoyed.

[0018] The relative values of the mass A of the first silicon-containing material, the mass B of the second silicon-containing material, and the mass C of the first carbon material, each contained in the negative electrode, can be determined by cross-sectional SEM-EDX analysis. First, the particles of the first silicon-containing material (particle A), the particles of the second silicon-containing material (particle B), and the particles of the first carbon material (particle C) are discriminated. The observation magnification is preferably 2000 to 20000 times.

[0019] To perform cross-sectional SEM-EDX analysis, for example, the battery is disassembled, the negative electrode is taken out, and a cross-section of the negative electrode is obtained using a cross-section polisher (CP). The cross-section of the negative electrode is observed using a scanning electron microscope (SEM). Element mapping analysis by energy-dispersive X-ray (EDX) is performed on the cross-sectional image of the backscattered electron image of the negative electrode. Using image analysis software, the total areas A to C occupied by particles A to C are calculated. The area ratios of the total areas A to C may be regarded as the volume ratios of particles A to C.

[0020] In SEM-EDX analysis, it is also possible to quantify the elements in particles A to C. From the cross-sectional images of the backscattered electron images of the negative electrode, 10 particles each of particles A to C with a maximum diameter of 5 μm or more are randomly selected, and for each of them, elemental mapping analysis is performed by energy-dispersive X-ray (EDX). The area of the target element is calculated using image analysis software. The measured values of the area of the predetermined element for 10 particles are averaged. The composition is calculated by converting the area into the number of atoms. The specific gravity of each of particles A to C is determined from the composition of particles A to C. Next, using the total areas A to C and the specific gravities A to C, B / A and (A + B) / (A + B + C) are calculated.

[0021] It is desirable to perform the above analysis and the analysis of the negative electrode described below using the negative electrode in the discharged state. Also, from the viewpoint of excluding the influence of the decomposition products of the electrolyte, it is desirable to analyze the sample of the negative electrode in the battery before or at the initial stage of the charge-discharge cycle.

[0022] Note that during the charge-discharge process, a film is formed on the surface of the silicon-containing material due to decomposition of the electrolyte or the like. Also, the silicon-containing material may have a conductive layer on its surface. Therefore, the mapping analysis by EDX is performed on a region 1 μm or more inside from the peripheral edge of the cross section of the silicon-containing material so that the film and the conductive layer are not included in the measurement range.

[0023] The measurement conditions for the desirable cross-sectional SEM-EDX analysis are shown below. <SEM-EDX Measurement Conditions> Processing device: JEOL, SM-09010 (Cross Section Polisher) Processing conditions: Acceleration voltage 6 kV Current value: 140 μA Vacuum degree: 1×10 -3 ~2×10 -3 Pa Measurement device: Electron microscope SU-70 manufactured by HITACHI Acceleration voltage during analysis: 10 kV Field: Free mode Probe current mode: Medium Probe current range: High Anode Ap.: 3 OBJ Ap.: 2 Analysis area: 1 μm square Analysis software: EDAX Genesis CPS: 20500 Lsec: 50 Time constant: 3.2

[0024] The average particle size Da of the first silicon-containing material may be 2 μm or more and 15 μm or less, may be 3 μm or more and 12 μm or less, or may be 5 μm or more and 10 μm or less. In such a range, voids that may be generated due to the expansion and contraction of the first silicon-containing material are appropriately suppressed, and cracks in the first silicon-containing material that may occur due to expansion and contraction are also considered to be easily suppressed.

[0025] The average particle size Db of the second silicon-containing material may be 3 μm or more and 18 μm or less, may be 6 μm or more and 15 μm or less, or may be 8 μm or more and 12 μm or less. In such a range, even if cracks are generated in the second silicon-containing material, a part of the second silicon-containing material easily penetrates into the voids generated due to the shrinkage and cracks after expansion of the first silicon-containing material, and the effect of maintaining the electrical connection between the first silicon-containing material and its surroundings is considered to be remarkable.

[0026] The average particle size Da or Db of the first or second silicon-containing material is measured by observing a cross-section of the negative electrode composite material layer using SEM or TEM. Specifically, it is obtained by averaging the maximum diameters of any 100 particles of the first or second silicon-containing material.

[0027] When the first and second silicon-containing particles can be separated, the volume-based particle size distributions of the first and second silicon-containing materials may be measured using a laser diffraction particle size distribution analyzer, respectively, and the particle sizes at a cumulative volume of 50% may be used as the average particle sizes Da and Db.

[0028] The content of the first silicon phase in the first silicon-containing material is, for example, 30% by mass or more and 80% by mass or less, and may be 40% by mass or more (or 50% by mass or more) and 70% by mass or less. Within such a range, not only can a sufficient high capacity of the negative electrode be achieved, but also side effects due to the expansion and contraction of the first silicon phase are limited, so that the cycle characteristics are also likely to be improved. This is because while the first silicon-containing material contains a sufficient amount of the first silicon phase, the proportion of the silicate phase in the first silicon-containing material does not become too small. By maintaining a considerable proportion of the silicate phase, the contact between the first silicon phase and the electrolyte is significantly limited, and side reactions are also significantly suppressed. In addition, the stress generated by the expansion and contraction of the first silicon phase is easily relaxed by the matrix of the silicate phase.

[0029] The content of the second silicon phase in the second silicon-containing material is, for example, 30% by mass or more and 80% by mass or less, and may be 40% by mass or more and 70% by mass or less. Within such a range, similar to the case of the first silicon-containing material, a sufficient high capacity of the negative electrode is achieved, and the cycle characteristics are also likely to be improved. In addition, by maintaining a considerable proportion of the carbon phase, the carbon phase easily penetrates into the voids generated subsequently due to charge and discharge, and for example, the electrical connection between the first silicon-containing material and its surroundings is likely to be maintained.

[0030] The average particle diameter Dc of the first carbon material contained as an active material in the negative electrode is desirably larger than the average particle diameter Da and the average particle diameter Db, and is 13 μm or more and 25 μm or less. Within such a range, voids are formed between the particles of the first carbon material with a relatively large size, and the first and second silicon-containing materials are easily accommodated in the voids. Therefore, it is easy to increase the filling rate of the active material in the negative electrode, and it is easy to obtain a higher-capacity negative electrode. In addition, the first and second silicon-containing materials present in the voids contribute to maintaining the electronic contact between the particles of the first carbon material. On the other hand, even if the first and second silicon-containing materials present in the voids expand and contract, the expansion and contraction of the entire negative electrode are less likely to occur, so that deterioration due to charge and discharge cycles is less likely to occur.

[0031] The average particle size Dc of the first carbon material is measured by observing the cross-section of the negative electrode composite layer using SEM or TEM. Specifically, it is obtained by averaging the maximum diameters of any 100 particles of the first carbon material.

[0032] When the first carbon material can be separated, the volume-based particle size distribution of the first carbon material may be measured using a laser diffraction particle size distribution measuring device, and the particle size at a cumulative volume of 50% may be defined as the average particle size Dc.

[0033] The average particle size of the first silicon phase may be, for example, 1 nm or more. Also, the average particle size of the first silicon phase may be 1000 nm or less, 500 nm or less, 200 nm or less, or 100 nm or less (more preferably 50 nm or less). The finer the first silicon phase, the smaller the volume change of the first silicon-containing material during charge and discharge, and the better the structural stability of the first silicon-containing material.

[0034] The average particle size of the second silicon phase may be, for example, 1 nm or more. Also, the average particle size of the second silicon phase may be 1000 nm or less, 500 nm or less, 200 nm or less, or 100 nm or less (more preferably 50 nm or less). The finer the second silicon phase, the smaller the volume change of the second silicon-containing material during charge and discharge, and the better the structural stability of the second silicon-containing material. Since the second silicon-containing material has a larger amount of side reactions than the first silicon-containing material, it is desirable that the average particle size of the second silicon phase is larger than the average particle size of the first silicon phase, and may be, for example, 1.1 to 2 times. By slightly increasing the average particle size of the second silicon phase, the contact area with the electrolyte can be reduced, and the amount of side reactions can be reduced.

[0035] The average particle size of the first or second silicon phase is measured by observing the cross-section of the first or second silicon-containing material using SEM or TEM. Specifically, it is obtained by averaging the maximum diameters of any 100 first or second silicon phases.

[0036] The crystallite size of the first or second silicon phase is preferably 30 nm or less. When the crystallite size is 30 nm or less, the volume change amount of the first or second silicon-containing material due to the expansion and contraction of the first or second silicon phase accompanying charge and discharge can be made smaller. The crystallite size is more preferably 30 nm or less, and still more preferably 20 nm or less. When the crystallite size is 20 nm or less, the expansion and contraction of the first or second silicon phase are made uniform, the fine cracks in the first or second silicon phase are reduced, and the cycle characteristics can be further improved.

[0037] The crystallite size of the first or second silicon phase is calculated by the Scherrer formula from the half-value width of the diffraction peak attributed to the Si(111) plane of the X-ray diffraction (XRD) pattern of the first or second silicon phase.

[0038] The silicate phase may contain at least one selected from the group consisting of alkali metal elements and Group II elements. By containing such elements, the irreversible capacity of the silicate phase is more significantly reduced. As the alkali metal elements and Group II elements, for example, Li, K, Na, Mg, Ca, Sr, Ba, etc. can be used.

[0039] The silicate phase may further contain an element M other than alkali metal elements and Group II elements. Here, the element M can be, for example, at least one selected from the group consisting of B, Al, Zr, Nb, Ta, La, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, and W.

[0040] The silicate phase preferably contains a lithium silicate phase with a small irreversible capacity and a high initial charge-discharge efficiency. Lithium silicate is lightweight and has excellent lithium ion conductivity. Lithium silicate may be an oxide phase containing Li, Si, and O, and may also contain other elements. The atomic ratio of O to Si in the lithium silicate phase: O / Si is, for example, greater than 2 and less than 4. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase: Li / Si is, for example, greater than 0 and less than 4.

[0041] The lithium silicate phase preferably contains a lithium silicate represented by the formula: Li 2z SiO 2+z (0 < z < 2). It is preferable that z satisfies the relationship of 0 < z < 1, and z = 1 / 2 (that is, Li2Si2O5) is more preferable.

[0042] The composition of the silicate phase can be analyzed by the following method. It is desirable to perform the analysis using the first silicon-containing material or the negative electrode composite layer in the discharged state. Also, from the viewpoint of excluding the influence of the decomposition products of the electrolyte, it is desirable to analyze a sample of the first silicon-containing material in the battery before or at the initial stage of the charge-discharge cycle.

[0043] The contents of B, Na, K, and Al contained in the silicate layer are determined, for example, by quantitative analysis in accordance with JIS R3105 (1995) (Method for Analyzing Borosilicate Glass). Also, the Ca content is determined by quantitative analysis in accordance with JIS R3101 (1995) (Method for Analyzing Soda-Lime Glass).

[0044] The content of each element contained in the silicon-containing material can be measured, for example, by inductively coupled plasma atomic emission spectrometry (ICP-AES). Specifically, a sample of the first silicon-containing material is completely dissolved in a heated acid solution, the carbon in the solution residue is filtered off, and then the resulting filtrate is analyzed by ICP-AES to measure the spectral intensity of each element. Subsequently, a calibration curve is created using standard solutions of each element available on the market, and the content of each element is calculated.

[0045] When analyzing the composition of the silicate phase, the first silicon-containing material and the second silicon-containing material may be taken out of the battery, for example, by the following method. Specifically, the battery is disassembled to take out the negative electrode, and the negative electrode is washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove the electrolyte. Next, the negative electrode composite layer is peeled off from the negative electrode current collector and ground in a mortar to obtain sample powder. Next, the sample powder is dried in a dry atmosphere for 1 hour and immersed in gently boiling 6M hydrochloric acid for 10 minutes to remove alkali metals such as Na and Li that may be contained in the binder or the like. Next, the sample powder is washed with ion-exchanged water, filtered, and dried at 200 °C for 1 hour.

[0046] In the first silicon-containing material, a silicate phase, a silicon oxide phase, a first silicon phase, etc. may be present. By using Si-NMR, these can be distinguished and quantified. The Si content obtained by ICP-AES as described above is the sum of the amount of Si constituting the first silicon phase, the amount of Si in the silicate phase, and the amount of Si in the silicon oxide phase. On the other hand, the amount of Si constituting the silicon phase and the amount of Si in the silicon oxide phase can be quantified separately using Si-NMR. Therefore, the amount of Si in the silicate phase can be quantified by subtracting the amount of Si constituting the silicon phase and the amount of Si in the silicon oxide phase from the Si content obtained by ICP-AES. For the standard substance required for quantification, a mixture containing a silicate and a silicon phase with a known Si content in a predetermined ratio may be used.

[0047] The following shows the desirable measurement conditions for Si-NMR. <Si-NMR Measurement Conditions> Measuring device: manufactured by Varian, solid nuclear magnetic resonance spectrometer (INOVA-400) Probe: Varian 7mm CPMAS-2 MAS: 4.2 kHz MAS speed: 4 kHz Pulse: DD (45° pulse + signal acquisition time 1H decoupling) Repetition time: 1200 sec to 3000 sec Observation width: 100 kHz Observation center: around -100 ppm Signal acquisition time: 0.05 sec Number of integrations: 560 Sample amount: 207.6 mg

[0048] In addition, the quantification of each element in the first silicon-containing material is also possible by SEM-EDX analysis, Auger electron spectroscopy (AES), laser ablation ICP mass spectrometry (LA-ICP-MS), X-ray photoelectron spectroscopy (XPS), etc.

[0049] At least a part of the surface of the first silicon-containing material may be coated with a conductive layer. By forming a conductive layer on the surface of the first silicon-containing material, the conductivity of the first silicon-containing material can be dramatically increased. As the conductive material constituting the conductive layer, a carbon material is preferable. The carbon material preferably contains at least one selected from the group consisting of carbon compounds and carbonaceous substances.

[0050] The thickness of the conductive layer is preferably thin to such an extent that it does not substantially affect the average particle size of the first silicon-containing material. Considering the ensuring of conductivity and the diffusibility of lithium ions, the thickness of the conductive layer is preferably 1 to 200 nm, more preferably 5 to 100 nm. The thickness of the conductive layer can be measured by observing the cross-section of the silicon-containing material using SEM or TEM (transmission electron microscope).

[0051] Examples of carbon compounds include compounds containing carbon and hydrogen, and compounds containing carbon, hydrogen, and oxygen. As the carbonaceous material, amorphous carbon with low crystallinity, highly crystalline graphite, etc. can be used. Examples of amorphous carbon include carbon black, coal, coke, charcoal, activated carbon, etc. Examples of graphite include natural graphite, artificial graphite, graphitized mesophase carbon particles, etc. Among them, amorphous carbon is preferred because it has low hardness and a large buffering effect on the silicon phase that changes in volume during charge and discharge. The amorphous carbon may be easily graphitizable carbon (soft carbon) or hardly graphitizable carbon (hard carbon). Examples of carbon black include acetylene black, ketjen black, etc.

[0052] The particle fracture strength Ma of the first silicon-containing material is, for example, 300 MPa < Ma < 1000 MPa, and may be 300 MPa < Ma < 800 MPa. Also, the particle fracture strength Mb of the second silicon-containing material is, for example, 300 MPa < Mb < 1000 MPa, and may be 300 MPa < Ma < 800 MPa. The first silicon-containing material and the second silicon-containing material having a particle fracture strength within the above range are difficult to be pulverized during the manufacturing process of the electrode plate, during charge and discharge cycles, etc., and are advantageous for suppressing the deterioration of cycle characteristics.

[0053] The particle fracture strength can be determined, for example, by the following method. As the particles for measurement (the first silicon-containing material or the second silicon-containing material), particles with a maximum particle size of 5 μm or more and 20 μm or less obtained from a photographed image are prepared. While gradually increasing the load, the particles are compressed with a indenter. The load when the particles reach fracture is defined as the particle fracture strength of the particles. The particle fracture strength can be measured using a commercially available micro compression tester (for example, MCT-211 manufactured by Shimadzu Corporation). For example, using a flat indenter with a tip diameter of 50 μm, a displacement speed of 5 μm / sec, the particle fracture strength of 10 particles is measured, and the average value is obtained.

[0054] The ratio of the particle breaking strength Mb of the second silicon-containing material to the particle breaking strength Ma of the first silicon-containing material: Mb / Ma may satisfy 0.5 < Mb / Ma < 2. In this case, neither of the first silicon-containing material and the second silicon-containing material is extremely hard or extremely soft compared to the other. Therefore, for example, during compression in the production process of the electrode plate, a phenomenon in which particles with a large particle breaking strength crush particles with a small particle breaking strength is less likely to occur, and the cycle characteristics are less likely to deteriorate. The closer the particle breaking strengths of the first silicon-containing material and the second silicon-containing material are, the more easily the effect of suppressing the deterioration of the cycle characteristics is exerted.

[0055] Next, an example of the manufacturing method of the first silicon-containing material will be described in detail. Here, the case of dispersing the first silicon phase in the lithium silicate phase will be explained.

[0056] Step (i) As the raw material of lithium silicate, a raw material mixture containing a Si raw material and a Li raw material in a predetermined ratio is used. The raw material mixture may contain the above-mentioned alkali metal element, Group II element, and / or element M. The raw material mixture is dissolved, and the melt is passed through a metal roll to be flaked to produce lithium silicate. Then, the flaked silicate is crystallized by heat treatment at a temperature above the glass transition point and below the melting point in an air atmosphere. Note that the flaked silicate can also be used without crystallization. It is also possible to produce silicate by solid-phase reaction by firing at a temperature below the melting point without dissolving the raw material mixture.

[0057] Silicon dioxide can be used as the Si raw material. As the Li raw material, for example, lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, etc. can be used. These may be used alone or in combination of two or more. Examples of the raw materials of the alkali metal element, Group II element, and element M include oxides, hydroxides, carbonate compounds, hydrides, nitrates, sulfates, etc. of each element.

[0058] Step (ii) Next, raw material silicon is blended with lithium silicate for compounding. For example, through the following steps (a) to (c), a first silicon-containing material which is composite particles of lithium silicate and a first silicon phase (hereinafter also referred to as silicate composite particles) is produced.

[0059] Step (a) The powder of raw material silicon and the powder of lithium silicate are mixed, for example, at a mass ratio of 20:80 to 95:5. As the raw material silicon, silicon coarse particles with an average particle size of about several μm to several tens of μm may be used.

[0060] Step (b) Next, using a pulverizing device such as a ball mill, the mixture of raw material silicon and lithium silicate is pulverized and compounded while being made into fine particles. At this time, an organic solvent may be added to the mixture for wet pulverization. The organic solvent serves to prevent adhesion to the inner wall of the pulverization container of the object to be pulverized.

[0061] As the organic solvent, alcohol, ether, fatty acid, alkane, cycloalkane, silicate ester, metal alkoxide, etc. can be used.

[0062] Note that the raw material silicon and lithium silicate may be separately made into fine particles and then mixed. Also, without using a pulverizing device, silicon nanoparticles and amorphous lithium silicate nanoparticles may be produced and mixed. For the production of nanoparticles, known methods such as a vapor phase method (for example, a plasma method) or a liquid phase method (for example, a liquid phase reduction method) may be used.

[0063] Step (c) Next, the mixture is pressurized and sintered while being heated to 600°C to 1000°C in, for example, an inert gas atmosphere (such as an atmosphere of argon, nitrogen, etc.). For sintering, a sintering apparatus capable of applying pressure under an inert atmosphere, such as a hot press, can be used. During sintering, the silicate softens and flows to fill the gaps between the first silicon phases. As a result, a dense block-shaped sintered body with a silicate phase as the continuous phase and the first silicon phase as the dispersed phase can be obtained. If the obtained sintered body is pulverized, silicate composite particles can be obtained.

[0064] Step (iii) Subsequently, at least a part of the surface of the composite particles may be coated with a conductive material to form a conductive layer. As a method of coating the surface of the composite particles with a conductive carbon material, a CVD method using a hydrocarbon gas such as acetylene or methane as a raw material, a method of mixing coal pitch, petroleum pitch, phenol resin, etc. with the composite particles and heating and carbonizing them at 700°C to 950°C in an inert atmosphere (such as an atmosphere of argon, nitrogen, etc.), etc. can be exemplified. Also, carbon black may be attached to the surface of the composite particles.

[0065] Step (iv) A step of washing the composite particles (including the case where they have a conductive layer on the surface) with an acid may be performed. For example, by washing the composite particles with an acidic aqueous solution, trace amounts of alkali components that may be generated when the raw material silicon and lithium silicate are compounded can be dissolved and removed. As the acidic aqueous solution, an aqueous solution of an inorganic acid such as hydrochloric acid, hydrofluoric acid, sulfuric acid, nitric acid, phosphoric acid, carbonic acid, etc. or an aqueous solution of an organic acid such as citric acid, acetic acid, etc. can be used.

[0066] FIG. 1 schematically shows a cross-section of silicate composite particles 20 coated with a conductive layer as an example of the first silicon-containing material.

[0067] The silicate composite particles (mother particles) 23 include a lithium silicate phase 21 and a silicon phase 22 dispersed within the lithium silicate phase 21. The silicate composite particles (mother particles) 23 have a sea-island structure in which fine silicon phases 22 are dispersed in the matrix of the lithium silicate phase 21. The surface of the silicate composite particles (mother particles) 23 is coated with a conductive layer 26.

[0068] In the lithium silicate phase 21, a silicon oxide phase (not shown) may be dispersed. The SiO2 content in the silicate composite particles (mother particles) 23 measured by Si-NMR is preferably, for example, 30% by mass or less, and more preferably less than 7% by mass.

[0069] In addition to the above, the silicate composite particles (mother particles) 23 may contain other components. For example, a reinforcing material such as a carbon material, an oxide such as ZrO2, or a carbide may be contained in an amount of less than 10% by mass based on the mother particles 23.

[0070] Next, a method for producing the second silicon-containing material will be exemplified. (i) First method Mix the raw material silicon and the carbon source, and use a pulverizing device such as a ball mill to pulverize and composite the mixture of the raw material silicon and the carbon source while atomizing it. An organic solvent may be added to the mixture for wet pulverization. At this time, the raw material silicon is finely pulverized to form a second silicon phase. The second silicon phase is dispersed in the matrix of the carbon source.

[0071] As the carbon source, for example, water-soluble resins such as carboxymethyl cellulose (CMC), hydroxyethyl cellulose, polyacrylate, polyacrylamide, polyvinyl alcohol, polyethylene oxide, polyvinyl pyrrolidone, saccharides such as cellulose and sucrose, petroleum pitch, coal pitch, tar, etc. can be used, but it is not particularly limited.

[0072] As the organic solvent, alcohol, ether, fatty acid, alkane, cycloalkane, silicate ester, metal alkoxide, etc. can be used.

[0073] Next, the composite of the second silicon phase and the carbon source is heated in an inert gas atmosphere (such as an atmosphere of argon, nitrogen, etc.) at 700°C to 1200°C to carbonize the carbon source and generate amorphous carbon. Thereby, a second silicon-containing material in which the second silicon phase is dispersed in the carbon phase containing amorphous carbon is obtained.

[0074] (ii) Second method Raw material silicon and a carbon material (hereinafter, also referred to as a second carbon material) are mixed, and using a pulverizing device such as a ball mill, the mixture of the raw material silicon and the second carbon material is pulverized and compounded while being made into fine particles. An organic solvent may be added to the mixture for wet pulverization. At this time, the raw material silicon is finely pulverized to generate a second silicon phase. The second silicon phase is dispersed in the matrix of the second carbon material.

[0075] By the compounding of the raw material silicon and the second carbon material as described above, a second silicon-containing material in which the second silicon is dispersed in the carbon phase of amorphous carbon is obtained. Thereafter, the second silicon-containing material may be heated at 700°C to 1200°C in an inert gas atmosphere.

[0076] As the second carbon material, amorphous carbon is preferable, and easily graphitizable carbon (soft carbon), hardly graphitizable carbon (hard carbon), carbon black, etc. can be used. Examples of carbon black include acetylene black and ketjen black. Even when graphite is used as the second carbon material, the crystal structure of graphite is almost lost when obtaining the composite of the second silicon and the carbon material using a pulverizing device, and a carbon phase of amorphous carbon is formed.

[0077] Next, the secondary battery according to the embodiment of the present invention includes a positive electrode, a negative electrode, an electrolyte, and a separator interposed between the positive electrode and the negative electrode. The negative electrode includes a first carbon material, a first silicon-containing material, and a second silicon-containing material. Hereinafter, the negative electrode, positive electrode, electrolyte, and separator included in the secondary battery according to the embodiment of the present invention will be described.

[0078] [Negative electrode] The negative electrode includes, for example, a negative electrode current collector and a negative electrode composite material layer formed on the surface of the negative electrode current collector and containing a negative electrode active material. The negative electrode composite material layer can be formed by applying a negative electrode slurry in which a negative electrode composite material is dispersed in a dispersion medium onto the surface of the negative electrode current collector and drying it. The dried coating film may be rolled if necessary.

[0079] The negative electrode composite material contains a negative electrode active material as an essential component and may contain, as optional components, a binder, a conductive agent, a thickening agent, and the like. The negative electrode active material includes a first carbon material and first and second silicon-containing materials.

[0080] As the negative electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh body, a net body, a punching sheet, etc.) is used. Examples of the material of the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, copper alloy, and the like.

[0081] Examples of the binder include fluororesin, polyolefin resin, polyamide resin, polyimide resin, vinyl resin, styrene-butadiene copolymer rubber (SBR), polyacrylic acid and its derivatives, and the like. These may be used alone or in combination of two or more.

[0082] Examples of the conductive agent include carbon black, conductive fiber, carbon fluoride, organic conductive material, and the like. These may be used alone or in combination of two or more.

[0083] Examples of the thickening agent include carboxymethyl cellulose (CMC), polyvinyl alcohol, and the like. These may be used alone or in combination of two or more.

[0084] Examples of the dispersion medium include water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), or a mixed solvent thereof.

[0085] [Positive Electrode] The positive electrode includes, for example, a positive electrode current collector and a positive electrode composite material layer formed on the surface of the positive electrode current collector. The positive electrode composite material layer can be formed by applying a positive electrode slurry in which a positive electrode composite material is dispersed in a dispersion medium to the surface of the positive electrode current collector and drying it. The dried coating film may be rolled if necessary.

[0086] The positive electrode composite material contains a positive electrode active material as an essential component, and may contain, as optional components, a binder, a conductive agent, and the like.

[0087] As the positive electrode active material, a lithium composite metal oxide can be used. Examples of the lithium composite metal oxide include Li a CoO2, Li a NiO2, Li a MnO2, Li a Co b Ni 1-b O2, Li a Co b M 1-b O c 、Li a Ni 1-b M b O c 、Li a Mn2O4, Li a Mn 2-b M b O 4、 LiMePO 4、 Li2MePO4F. Here, M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. Me contains at least a transition element (for example, contains at least one selected from the group consisting of Mn, Fe, Co, and Ni). Here, 0 ≦ a ≦ 1.2, 0 ≦ b ≦ 0.9, and 2.0 ≦ c ≦ 2.3. The a value indicating the molar ratio of lithium is the value immediately after the production of the active material and increases or decreases by charge and discharge.

[0088] As the binder and the conductive agent, the same ones as those exemplified for the negative electrode can be used. As the conductive agent, graphite such as natural graphite and artificial graphite may be used.

[0089] For the positive current collector, a conductive substrate similar to the negative current collector can be used. Examples of the material of the positive current collector include stainless steel, aluminum, aluminum alloy, titanium, and the like.

[0090] [Electrolyte] The electrolyte (or electrolytic solution) contains a solvent and a lithium salt dissolved in the solvent. The concentration of the lithium salt in the electrolyte is, for example, 0.5 to 2 mol / L. The electrolyte may contain known additives.

[0091] As the solvent, an aqueous solvent or a non-aqueous solvent is used. Examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and the like. Examples of the cyclic carbonate include propylene carbonate (PC), ethylene carbonate (EC), and the like. Examples of the chain carbonate include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and the like. Examples of the cyclic carboxylic acid ester include γ-butyrolactone (GBL), γ-valerolactone (GVL), and the like. The non-aqueous solvent may be used alone or in combination of two or more.

[0092] Examples of the lithium salt include lithium salts of chlorine-containing acids (such as LiClO4, LiAlCl4, LiB 10 Cl 10 etc.), lithium salts of fluorine-containing acids (such as LiPF6, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (such as LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (such as LiCl, LiBr, LiI, etc.), and the like. The lithium salt may be used alone or in combination of two or more.

[0093] [Separator] It is desirable to interpose a separator between the positive electrode and the negative electrode. The separator has a high ion permeability and appropriate mechanical strength and insulation properties. As the separator, a microporous thin film, a woven fabric, a non-woven fabric, etc. can be used. As the material of the separator, for example, polyolefins such as polypropylene and polyethylene can be used.

[0094] As an example of the structure of the secondary battery, there is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and an electrolyte are housed in an exterior body. Alternatively, instead of the wound electrode group, other forms of electrode groups such as a laminated electrode group formed by laminating a positive electrode and a negative electrode with a separator interposed therebetween may be applied. The secondary battery may be in any form such as a cylindrical shape, a rectangular shape, a coin shape, a button shape, a laminate shape, etc.

[0095] FIG. 2 is a schematic perspective view of a part of a rectangular secondary battery according to an embodiment of the present invention, with a cutout.

[0096] The battery includes a bottomed rectangular battery case 4, an electrode group 1 and an electrolyte (not shown) housed in the battery case 4, and a sealing plate 5 that seals the opening of the battery case 4. The electrode group 1 has a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed therebetween. The sealing plate 5 has a liquid injection port sealed with a seal 8 and a negative electrode terminal 6 insulated from the sealing plate 5 by a gasket 7.

[0097] One end of the negative electrode lead 3 is attached to the negative electrode current collector by welding or the like. One end of the positive electrode lead 2 is attached to the positive electrode current collector by welding or the like. The other end of the negative electrode lead 3 is electrically connected to the negative electrode terminal 6. The other end of the positive electrode lead 2 is electrically connected to the sealing plate 5.

[0098] Hereinafter, the present invention will be specifically described based on examples and comparative examples, but the present invention is not limited to the following examples.

[0099] <Example 1> [Preparation of the First Silicon-Containing Material] 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 (z = 0.5). The obtained lithium silicate was pulverized so as to have an average particle size of 10 μm.

[0100] Lithium silicate (Li2Si2O5) with an average particle size of 10 μm and raw material silicon (3N, average particle size 10 μm) were mixed at a mass ratio of 70:30. The mixture was filled into a pot (made of SUS, volume: 500 mL) of a planetary ball mill (manufactured by Fritsch, P-5), 24 SUS balls (diameter 20 mm) were put into the pot, the lid was closed, and the mixture was pulverized at 200 rpm for 50 hours in an inert atmosphere.

[0101] Next, the powdery mixture was taken out in an inert atmosphere and calcined at 800 °C for 4 hours under pressure applied by a hot press machine in an inert atmosphere to obtain a sintered body (silicon-silicate composite) of the mixture.

[0102] Thereafter, the silicon-silicate composite was pulverized, passed through a 40-μm mesh, and the obtained silicate composite particles were mixed with coal pitch (MCP250 manufactured by JFE Chemical Corporation), and the mixture was calcined at 800 °C in an inert atmosphere to coat the surface of the silicate composite particles with conductive carbon to form a conductive layer. The coating amount of the conductive layer was 5% by mass based on the total mass of the silicate composite particles and the conductive layer. Thereafter, using a sieve, silicate composite particles (first silicon-containing particles) having an average particle size of 10 μm and having a conductive layer were obtained.

[0103] Here, the volume-based particle size distribution of the first silicon-containing particles was measured using a laser diffraction particle size distribution measuring device (MT3300EXII manufactured by Microtrac Inc.), and the particle size at a cumulative volume of 50% was taken as the average particle size.

[0104] The crystallite size of the first silicon phase calculated by Scherrer's formula from the diffraction peak attributed to the Si(111) plane by XRD analysis of the silicate composite particles was 15 nm.

[0105] When the composition of the lithium silicate phase was analyzed by the above method (ICP - AES), the Si / Li ratio was 1.0, and the content of Li2Si2O5 measured by Si - NMR was 70 mass% (the content of the first silicon phase was 30 mass%).

[0106] [Preparation of the second silicon - containing material] Coal pitch as a carbon source (manufactured by JFE Chemical Corporation, MCP250) and raw silicon (3N, average particle size 10 μm) were mixed at a mass ratio of 50:50. The mixture was filled into a pot (made of SUS, volume: 500 mL) of a planetary ball mill (manufactured by Fritsch, P - 5). Twenty - four SUS balls (diameter 20 mm) were put into the pot and the lid was closed. The mixture was milled at 200 rpm for 50 hours in an inert atmosphere to obtain a composite of the second silicon phase and the carbon source.

[0107] Next, the composite of the second silicon phase and the carbon source was fired in an inert gas atmosphere to carbonize the carbon source, and a second silicon - containing material in which the second silicon phase was dispersed in a carbon phase containing amorphous carbon was obtained. Then, using a jet mill, second silicon - containing particles with an average particle size of 10 μm were obtained.

[0108] Here, the volume - based particle size distribution of the second silicon - containing particles was measured using a laser diffraction particle size distribution analyzer (MT3300EXII manufactured by Microtrac, Inc.), and the particle size at a cumulative volume of 50% was taken as the average particle size.

[0109] The crystallite size of the second silicon phase calculated by Scherrer's formula from the diffraction peak attributed to the Si(111) plane by XRD analysis of the second silicon - containing particles was 15 nm.

[0110] <Particle fracture strength> The particle crushing strength of the obtained first silicon-containing material or second silicon-containing material was measured. Ten particles each with a particle size of 5 μm or more and 20 μm or less obtained from the photographed images were prepared. While gradually increasing the load, with a displacement rate of 5 μm / sec, the particles were compressed with a flat indenter having a tip diameter of 50 μm, and the load at which the particles reached fracture was determined. MCT-211 manufactured by Shimadzu Corporation was used as the micro compression tester. The average value of the particle crushing strengths of the ten particles was determined.

[0111] [First Carbon Material] Spherical graphite with an average particle diameter Dc of 20 μm was prepared. Here, the volume-based particle size distribution of the graphite was measured using a laser diffraction particle size distribution measuring device (MT3300EXII manufactured by Microtrac Inc.), and the particle diameter at a cumulative volume of 50% was defined as the average particle diameter.

[0112] [Fabrication of Negative Electrode] The first silicon-containing particles having a conductive layer, the second silicon-containing particles, and the graphite were mixed at a mass ratio of 3:3:94 and used as the negative electrode active material. The negative electrode active material, sodium carboxymethyl cellulose (CMC-Na), and styrene-butadiene rubber (SBR) were mixed at a mass ratio of 97.5:1:1.5, and after adding water, they were stirred using a mixer (T.K. High Bismix manufactured by Primix Corporation) to prepare a negative electrode slurry.

[0113] Next, on the surface of the copper foil, 1 m 2 The negative electrode slurry was applied so that the mass of the negative electrode composite material per unit area was 190 g, and after drying the coating film, it was rolled to form a negative electrode having a negative electrode composite material layer with a density of 1.5 g / cm 3 on both sides of the copper foil.

[0114] [Fabrication of Positive Electrode] Lithium nickel composite oxide (LiNi 0.8 Co 0.18 Al 0.02Oxygen (O2), acetylene black, and polyvinylidene fluoride were mixed at a mass ratio of 95:2.5:2.5. After adding N-methyl-2-pyrrolidone (NMP), the mixture was stirred using a mixer (manufactured by Primix Corporation, T.K. High Bismix) to prepare a positive electrode slurry. Next, the positive electrode slurry was coated on the surface of an aluminum foil, and after drying the coating film, it was rolled to form a positive electrode composite layer with a density of 3.6 g / cm 3 on both sides of the aluminum foil to produce a positive electrode.

[0115] [Preparation of Electrolyte Solution] An electrolyte solution was prepared by dissolving a lithium salt in a non-aqueous solvent. As the non-aqueous solvent, a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and methyl acetate (MA) in a volume ratio of 20:40:40 was used. The concentration of LiPF6 in the electrolyte solution was set to 1.0 mol / L.

[0116] [Fabrication of Secondary Battery] Tabs were attached to each electrode, and an electrode group was fabricated by winding the positive electrode and the negative electrode spirally through a separator so that the tabs were located at the outermost peripheral part. The electrode group was inserted into an exterior body made of an aluminum laminate film, vacuum-dried at 105°C for 2 hours, then the electrolyte solution was injected, and the opening of the exterior body was sealed to obtain Battery A1.

[0117] [Examples 2 to 4 and Comparative Examples 1 to 3] Batteries A2, A3, and A4 of Examples 2, 3, and 4 and Batteries B1, B2, and B3 of Comparative Examples 1, 2, and 3 were fabricated in the same manner as in Example 1, except that the negative electrode active materials having the compositions shown in Table 1 were used. In Table 1 (and Tables 2 and 3 described later), the first silicon-containing material is denoted as the first Si material, and the second silicon-containing material is denoted as the second Si material. In Battery A4, the particle destruction strength Mb was controlled by changing the firing temperature.

[0118] [Evaluation] Each of the batteries fabricated above was evaluated by the following method. For each battery after fabrication, constant current charging was performed at a current of 0.3It until the voltage reached 4.2V, and then constant voltage charging was performed at a constant voltage of 4.2V until the current reached 0.015It. Thereafter, constant current discharging was performed at a current of 0.3It until the voltage reached 2.75V. The rest period between charging and discharging was set to 10 minutes. Charging and discharging were performed in an environment at 25°C.

[0119] Note that (1 / X)It represents current, and (1 / X)It (A) = rated capacity (Ah) / X (h), where X represents the time required to charge or discharge the electricity corresponding to the rated capacity. For example, 0.5It means X = 2, indicating that the current value is rated capacity (Ah) / 2 (h). Charging and discharging were repeated under the above charging and discharging conditions. The ratio (percentage) of the discharge capacity of the 300th cycle to the discharge capacity of the first cycle was determined as the capacity retention rate in the charge-discharge cycle. The evaluation results are shown in Table 1 together with the information on the particle destruction strength.

[0120]

Table 1

[0121] From Table 1, it can be seen that when the first silicon-containing material is used alone in combination with graphite (Batteries B2 and B3), the amount of the first silicon-containing material used is limited to a small amount in order to maintain the cycle characteristics. Even when the first silicon-containing material accounts for 9% by mass in the negative electrode active material, the cycle life is lower than that in the case of 6% by mass (B2). On the other hand, when graphite is used in combination with the first and second silicon-containing materials, the cycle characteristics are improved compared to the case where the first silicon-containing material is used alone (B2). Furthermore, even when the total amount of the first and second silicon-containing materials in the negative electrode active material is 9% by mass (A2 and A3), the cycle characteristics are improved compared to the case where the first silicon-containing material is used alone (B2). And the best cycle characteristics are obtained in Battery A1 with a B / A ratio of 1.0.

[0122] <Examples 5 to 7 and Comparative Examples 4 and 5> Except for using the negative electrode active material having the composition shown in Table 2, in the same manner as in Example 1, batteries A5, A6, and A7 of Examples 5, 6, and 7 and batteries B4 and B5 of Comparative Examples 4 and 5 were fabricated and evaluated in the same manner. Here, the particle destruction strength Ma was controlled by the applied pressure of hot pressing, and Mb was controlled by the firing temperature.

[0123]

Table 2

[0124] <Examples 8 and 9 and Comparative Examples 6 and 7> Except for using the negative electrode active material having the composition shown in Table 3, in the same manner as in Example 1, batteries A8 and A9 of Examples 8 and 9 and batteries B6 and B7 of Comparative Examples 6 and 7 were fabricated and evaluated in the same manner. Here, the particle destruction strength Ma was controlled by the applied pressure of hot pressing, and Mb was controlled by the firing temperature.

[0125]

Table 3

[0126] It can be understood from Tables 2 and 3 that good cycle characteristics can be obtained even when the composition of the negative electrode active material is varied, and when graphite and the first and second silicon-containing materials are used in combination.

Industrial Applicability

[0127] The secondary battery according to the present invention is useful as a main power source for mobile communication devices, portable electronic devices, and the like.

Explanation of Reference Numerals

[0128] 1: Electrode group, 2: Positive electrode lead, 3: Negative electrode lead, 4: Battery case, 5: Sealing plate, 6: Negative electrode terminal, 7: Gasket, 8: Plug, 20: Silicate composite particles coated with a conductive layer, 21: Lithium silicate phase, 22: First silicon phase, 23: Silicate composite particles, 26: Conductive layer

Claims

1. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode contains a carbon material, a first silicon-containing material, and a second silicon-containing material, the first silicon-containing material includes a silicate phase and a first silicon phase dispersed in the silicate phase, and the second silicon-containing material includes a carbon phase and a second silicon phase dispersed in the carbon phase.

2. The secondary battery according to Claim 1, wherein the ratio of the mass B of the second silicon-containing material to the mass A of the first silicon-containing material: B / A satisfies 0.2 < B / A < 10.

3. The secondary battery according to Claim 1 or 2, wherein the ratio of the total of the mass A of the first silicon-containing material, the mass B of the second silicon-containing material, and the mass C of the carbon material to the total of the mass A of the first silicon-containing material and the mass B of the second silicon-containing material is 5% by mass or more and 30% by mass or less.

4. The secondary battery according to any one of Claims 1 to 3, wherein the silicate phase contains at least one selected from the group consisting of an alkali metal element and a Group II element.

5. The silicate phase further contains an element M, and the element M is at least one selected from the group consisting of B, Al, Zr, Nb, Ta, V, La, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, and W. The secondary battery according to Claim 4.

6. The silicate phase is represented by the formula: Li 2z SiO 2+z The secondary battery according to claim 4 or 5, comprising a lithium silicate represented by (0 < z < 2).

7. The secondary battery according to any one of Claims 1 to 6, wherein the particle crushing strength Ma of the first silicon-containing material satisfies 300 MPa < Ma < 1000 MPa.

8. The secondary battery according to any one of Claims 1 to 7, wherein the particle crushing strength Mb of the second silicon-containing material satisfies 300 MPa < Mb < 1000 MPa.

9. The secondary battery according to any one of Claims 1 to 8, wherein the ratio of the particle crushing strength Mb of the second silicon-containing material to the particle crushing strength Ma of the first silicon-containing material: Mb / Ma satisfies 0.5 < Mb / Ma < 2.

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