Secondary battery

Abstract

A secondary battery includes a positive electrode, a negative electrode, and an electrolyte. 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. The second silicon-containing material includes 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 can be expected as power sources for small consumer applications, power storage devices, and electric vehicles. Since high energy density of the battery is required, as a negative electrode active material having a high theoretical capacity density, a silicon-containing material containing silicon alloyed with lithium can be expected to be used.

[0003] Patent Document 1 proposes a non-aqueous electrolyte secondary battery in which a negative electrode active material uses a composite material including a lithium silicate phase represented by Li 2z SiO 2+z (0 < z < 2) and silicon particles dispersed in the lithium silicate phase.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Pamphlet of International Publication No. 2016 / 035290 Summary of the Invention

[0007] Problems to be Solved by the Invention

[0008] The silicon-containing material has large expansion and contraction due to charge and discharge, and thus pores are likely to be formed around the silicon-containing material when it shrinks. In addition, the silicon-containing material cannot withstand the stress generated by expansion and contraction, and the silicon-containing material may crack. As a result, if the charge-discharge cycle is repeated, the contact points between a part of the silicon-containing material and its surroundings gradually decrease, isolating a part of the silicon-containing material and reducing the capacity.

[0009] Solution to the Problem

[0010] In view of the above circumstances, 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.

[0011] Effects of the Invention

[0012] According to the present invention, the cycle characteristics of the secondary battery can be improved. Brief Description of the Drawings

[0013] Figure 1 It is a cross-sectional view schematically showing a first silicon-containing material according to an embodiment of the present invention.

[0014] Figure 2 This is a schematic perspective view showing a portion of a secondary battery according to one embodiment of the present invention cut away. Detailed Implementation

[0015] The secondary battery according to embodiments of the present invention includes a positive electrode, a negative electrode, and an electrolyte (or liquid electrolyte). The negative electrode comprises a carbon material (hereinafter also referred to as a first carbon material) as an active material, a first silicon-containing material, and a second silicon-containing material. Here, the first silicon-containing material comprises a silicate phase and first silicon particles or a silicon phase dispersed within the silicate phase. Furthermore, the second silicon-containing material comprises a carbon phase and second silicon particles or a silicon phase dispersed within the carbon phase. Hereinafter, the silicon phase may be silicon particles or may have a particulate morphology.

[0016] Both the first and second silicon-containing materials possess a so-called island structure. The first or second silicon phase (island) is dispersed within a matrix (sea) of silicate or carbon phases (hereinafter collectively referred to as the lithium-ion conducting phase), and is covered by the lithium-ion conducting phase. In the island structure, the contact between the first or second silicon phase and the electrolyte is limited, thus suppressing side reactions. Furthermore, the stress generated by the expansion and contraction of the silicon phase is mitigated by the matrix of the lithium-ion conducting phase.

[0017] The first silicon-containing material can contain a large amount of the first silicon phase and has fewer sites for capturing lithium ions that lead to irreversible capacity, and is less prone to side reactions. However, the silicate phase of the first silicon-containing material is not electronically conductive. Therefore, when pores form around the first silicon-containing material due to expansion and contraction caused by charging and discharging, or when the first silicon-containing material cracks due to stress generated by expansion and contraction, a portion of the first silicon-containing material becomes isolated, reducing the number of contacts between that portion and its surroundings, thus easily leading to a decrease in capacity.

[0018] In contrast, the second silicon-containing material comprises a carbon phase and a second silicon phase dispersed within the carbon phase. The carbon phase of the second silicon-containing material is electronically conductive; therefore, even if pores form around the second silicon-containing material or cracks appear in the first silicon-containing material, a portion of the second silicon-containing material is not easily isolated, thus maintaining the contact points between the second silicon-containing material and its surroundings. Therefore, by replacing a portion of the first silicon-containing material with the second silicon-containing material, it is easier to maintain the contact points around the entire silicon-containing material, and capacity reduction during repeated charge-discharge cycles is easily suppressed.

[0019] The carbon phase can be composed of amorphous carbon, for example. Amorphous carbon can be hard carbon, soft carbon, or other types of carbon. Amorphous carbon generally refers to carbon materials with an average interplanar spacing d002 of the (002) plane exceeding 0.34 nm as measured by X-ray diffraction.

[0020] As the active material, the first carbon material contained in the negative electrode is graphite, easily graphitized carbon (soft carbon), difficult-to-graphitize carbon (hard carbon), or a combination thereof. One type of carbon material can be used alone, or two or more types can be used in combination. Graphite, which exhibits excellent charge-discharge stability and low irreversible capacity, is preferred. Graphite can comprise 50% or more, and further 80% or more, of the carbon material.

[0021] Graphite refers to materials with a well-developed graphitic crystal structure, typically carbon materials with an average interplanar spacing d002 of (002) plane less than 0.340 nm as measured by X-ray diffraction. Examples of representative graphite materials include natural graphite, artificial graphite, and graphitized mesophase carbon particles.

[0022] 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.

[0023] ​The ratio of the total mass A of the first silicon-containing material and the mass B of the second silicon-containing material (A+B) to the total 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 (A+B+C) can be 5% by mass or more and 30% by mass or less; 5% by mass or more and 20% by mass or less; 5% by mass or more and 15% by mass or less; 5% by mass or more and 12% by mass or less; or 5% by mass or more and 10% by mass or less. Within this range, it is considered that the overall expansion and contraction of the negative electrode can be controlled within a further suitable range, and the high capacity advantage brought by the first and second silicon-containing materials can be maximized.

[0024] The relative values ​​of the masses A of the first silicon-containing material, B of the second silicon-containing material, and C of the first carbon material contained in the negative electrode can be determined by cross-sectional SEM-EDX analysis. First, identify 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). The expected magnification is 2000–20000 times.

[0025] In cross-sectional SEM-EDX analysis, for example, the battery is disassembled, the negative electrode is removed, and a cross-section polisher (CP) is used to obtain the cross-section of the negative electrode. The cross-section of the negative electrode is then observed using a scanning electron microscope (SEM). For the cross-sectional image of the reflected electron image of the negative electrode, elemental mapping analysis based on energy-dispersive X-rays (EDX) is performed. Using image analysis software, the total areas A through C occupied by each particle are calculated. The area ratio of the total areas A through C can be considered as the volume ratio of particles A through C.

[0026] In SEM-EDX analysis, elemental quantification of particles A through C can also be performed. Ten particles A through C with a maximum diameter of 5 μm or more are randomly selected from the cross-sectional image of the negative electrode reflected electron image. Elemental mapping analysis based on energy-dispersive X-ray (EDX) is performed on each particle. The area containing the target element is calculated using image analysis software. The measured areas of the specified elements in the ten particles are averaged. The area is converted to the number of atoms to calculate the composition. The specific gravity of particles A through C is calculated based on their composition. Then, using the total area A through C and the specific gravity A through C, B / A and (A+B) / (A+B+C) are calculated.

[0027] The above analysis and the subsequent analysis of the negative electrode are intended to be performed using the negative electrode in a discharged state. Furthermore, from the viewpoint of eliminating the influence of electrolyte decomposition products, it is desirable to analyze samples of the negative electrode within the battery before or at the beginning of a charge-discharge cycle.

[0028] Note that during charge and discharge, a coating film is formed on the surface of the silicon-containing material due to decomposition of the electrolyte or the like. In addition, the silicon-containing material sometimes has a conductive layer on its surface. Therefore, the mapping analysis based on EDX is performed on the region more than 1 μm inside from the peripheral edge of the cross section of the silicon-containing material so that the coating film and the conductive layer are not included in the measurement range.

[0029] The measurement conditions for the desired cross-sectional SEM-EDX analysis are shown below.

[0030] <SEM-EDX Measurement Conditions>

[0031] Processing device: JEOL, SM-09010 (Cross Section Polisher)

[0032] Processing conditions: acceleration voltage 6 kV

[0033] Current value: 140 μA

[0034] Vacuum degree: 1×10 -3 ~2×10 -3 Pa

[0035] Measurement device: electron microscope SU-70 made by HITACHI

[0036] Acceleration voltage during analysis: 10 kV

[0037] Field: Free mode

[0038] Probe current mode Medium

[0039] Probe current range: High

[0040] Anode Ap.: 3

[0041] OBJ Ap.: 2

[0042] Analysis area: 1 μm square ​​​​​​​​​​​​​The average particle size Da of the first silicon-containing material can be 2 μm or more and 15 μm or less, 3 μm or more and 12 μm or less, or 5 μm or more and 10 μm or less. Within such ranges, it can be considered that porosity that may be generated due to the expansion and contraction of the first silicon-containing material can be moderately suppressed, and cracking of the first silicon-containing material that may be generated due to expansion and contraction can also be easily suppressed.

[0048] The average particle size Db of the second silicon-containing material can be 3 μm or more and 18 μm or less, 6 μm or more and 15 μm or less, or 8 μm or more and 12 μm or less. Within such ranges, it is believed that even if cracks occur in the second silicon-containing material, a portion of the second silicon-containing material can easily penetrate into the pores that may be generated due to the contraction and cracking caused by the expansion of the first silicon-containing material, thus significantly maintaining the electrical connection between the first silicon-containing material and its surroundings.

[0049] The average particle size Da or Db of the first or second silicon-containing material was determined by observing the cross-section of the negative electrode composite layer using SEM or TEM. Specifically, it was calculated by averaging the maximum diameter of any 100 particles of the first or second silicon-containing material.

[0050] When the first and second silicon-containing particles can be separated, the volume-based particle size distribution of the first and second silicon-containing materials is measured using a laser diffraction particle size distribution measuring device. The particle size at 50% of the cumulative volume can be used as the average particle size Da and Db.

[0051] 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, or 40% by mass or more (or 50% by mass or more) and 70% by mass or less. Within this range, not only can sufficient high capacity of the negative electrode be achieved, but the side effects caused by the expansion and contraction of the first silicon phase are also limited, thus easily improving cycle characteristics. This is because the first silicon-containing material contains a sufficient amount of the first silicon phase, while 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 mitigated by the silicate phase matrix.

[0052] 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, or 40% by mass or more and 70% by mass or less. Within this range, similar to the case of the first silicon-containing material, sufficiently high capacity of the negative electrode can be achieved, and cycle characteristics are easily improved. In addition, by maintaining a considerable proportion of carbon phase, the carbon phase can easily penetrate into the pores subsequently generated due to charge and discharge, for example, easily maintaining the electrical connection between the first silicon-containing material and its surroundings.

[0053] The average particle size Dc of the first carbon material included as the active material in the negative electrode is preferably greater than the average particle sizes Da and Db, and is between 13 μm and 25 μm. Within this range, pores are formed between the relatively large particles of the first carbon material, making it easier to contain the first and second silicon-containing materials. Therefore, it is easier to increase the filling rate of the active material in the negative electrode, and a negative electrode with higher capacity can be obtained. In addition, the first and second silicon-containing materials present in the pores help maintain the electrical 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 pores expand and contract, the expansion and contraction of the negative electrode as a whole is less likely to occur, thus reducing the risk of degradation caused by charge-discharge cycles.

[0054] The average particle size Dc of the first carbon material was determined by observing the cross-section of the negative electrode composite layer using SEM or TEM. Specifically, it was calculated by averaging the maximum diameter of any 100 particles of the first carbon material.

[0055] When the first carbon material can be separated, the volume-based particle size distribution of the first carbon material is measured using a laser diffraction particle size distribution measuring device, and the particle size at 50% of the cumulative volume can be used as the average particle size Dc.

[0056] The average particle size of the first silicon phase can be, for example, 1 nm or more. Alternatively, the average particle size of the first silicon phase can be less than 1000 nm, less than 500 nm, less than 200 nm, or less than 100 nm (and further less than 50 nm). The finer the first silicon phase, the smaller the volume change of the first silicon-containing material during charging and discharging, and the more the structural stability of the first silicon-containing material is improved.

[0057] The average particle size of the second silicon phase can be, for example, 1 nm or more. Alternatively, the average particle size of the second silicon phase can be less than 1000 nm, less than 500 nm, less than 200 nm, or less than 100 nm (and further, less than 50 nm). The finer the second silicon phase, the smaller the volume change of the second silicon-containing material during charging and discharging, and the better the structural stability of the second silicon-containing material. It should be noted that since the amount of side reactions in the second silicon-containing material is greater than that in the first silicon-containing material, it is desirable that the average particle size of the second silicon phase be greater than that of the first silicon phase, for example, by 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.

[0058] The average particle size of the first or second silicon phase is determined by observing the cross-section of the first or second silicon-containing material using SEM or TEM. Specifically, it is calculated by averaging the maximum diameter of any 100 first or second silicon phases.

[0059] 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 caused by the expansion and contraction of the first or second silicon phase accompanying charge and discharge can be further reduced. The crystallite size is more preferably 30 nm or less, and further 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, and the microcracks of the first or second silicon phase can be reduced, and the cycle characteristics can be further improved.

[0060] The crystallite size of the first or second silicon phase is calculated by the Scherrer formula using the half-value width of the diffraction peak attributed to the Si(111) plane in the X-ray diffraction (XRD) pattern of the first or second silicon phase.

[0061] 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.

[0062] The silicate phase may further contain an element M other than the alkali metal element and the Group II element. 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.

[0063] The silicate phase preferably contains a lithium silicate phase with a small irreversible capacity and a high initial charge-discharge efficiency. Lithium silicate is light in weight and excellent in lithium ion conductivity. Lithium silicate may be an oxide phase containing Li, Si, and O, and may also contain other elements. The atomic ratio O / Si of O to Si in the lithium silicate phase is, for example, greater than 2 and less than 4. O / Si is preferably greater than 2 and less than 3. The atomic ratio Li / Si of Li to Si in the lithium silicate phase is, for example, greater than 0 and less than 4.

[0064] The lithium silicate phase preferably contains lithium silicate represented by the formula Li 2z SiO 2+z (0 < z < 2). z preferably satisfies the relationship 0 < z < 1, and more preferably z = 1 / 2 (that is, Li2Si2O5).

[0065] The composition of the silicate phase can be analyzed by the following method. It is desirable to analyze the composition using the first silicon-containing material or the negative electrode composite material layer in the discharged state. In addition, from the viewpoint of eliminating the influence of the decomposition products of the electrolyte, it is desirable to analyze the sample of the first silicon-containing material in the battery before or at the initial stage of the charge-discharge cycle.

[0066] The contents of B, Na, K, and Al contained in the silicate layer can be determined, for example, by quantitative analysis in accordance with JIS R3105 (1995) (Method of Analysis of Borosilicate Glass). In addition, the Ca content can be determined by quantitative analysis in accordance with JIS R3101 (1995) (Method of Analysis of Soda-Lime Glass).

[0067] 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, the sample of the first silicon-containing material is completely dissolved in a heated acid solution, the carbon in the solution residue is filtered and removed, and then the resulting filtrate is analyzed by ICP-AES to measure the spectral intensity of each element. Next, a calibration curve is prepared using commercially available standard solutions of each element, and the content of each element is calculated.

[0068] When analyzing the composition of the silicate phase, the first silicon-containing material and the second silicon-containing material can be taken out of the battery, for example, by the following method. Specifically, the battery is disassembled and the negative electrode is taken out, and the negative electrode is washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove the electrolyte. Next, the negative electrode composite material layer is peeled off from the negative electrode current collector and pulverized with a mortar to obtain a sample powder. Next, the sample powder is dried in a dry atmosphere for 1 hour, immersed in boiling 6M hydrochloric acid for 10 minutes to remove alkali metals such as Na and Li that may be contained in the binder, etc. Next, the sample powder is washed with ion-exchanged water, filtered, and dried at 200 °C for 1 hour.

[0069] 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. As described above, the Si content obtained by ICP-AES is the total of the Si amount constituting the first silicon phase, the Si amount in the silicate phase, and the Si amount in the silicon oxide phase. On the other hand, the Si amount constituting the silicon phase and the Si amount in the silicon oxide phase can be quantified separately using Si-NMR. Therefore, by subtracting the Si amount constituting the silicon phase and the Si amount in the silicon oxide phase from the Si content obtained by ICP-AES, the Si amount in the silicate phase can be quantified. It should be noted that a mixture containing a silicate and a silicon phase with a known Si content in a specified ratio, etc., can be used as the standard substance required for quantification.

[0070] The following shows the desired measurement conditions for Si-NMR.

[0071] <Si-NMR Measurement Conditions>

[0072] Measurement device: Solid nuclear magnetic resonance spectrometer (INOVA-400) manufactured by Varian

[0073] Probe: Varian 7mm CPMAS-2

[0074] MAS: 4.2kHz

[0075] MAS speed: 4kHz

[0076] Pulse: DD (45° pulse + signal acquisition time 1H decoupling)

[0077] Repeat time: 1200 seconds to 3000 seconds

[0078] Observation width: 100kHz

[0079] Observation center: around -100ppm

[0080] Signal acquisition time: 0.05 seconds

[0081] Total number of times: 560

[0082] Sample volume: 207.6 mg

[0083] In addition, the quantification of each element in the first silicon-containing material can be achieved through SEM-EDX analysis, Auger electron spectroscopy (AES), laser ablation ICP mass spectrometry (LA-ICP-MS), X-ray photoelectron spectroscopy (XPS), etc.

[0084] At least a portion of the surface of the first silicon-containing material can be covered by 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 significantly improved. Carbon materials are preferably used as the conductive material constituting the conductive layer. The carbon material preferably comprises at least one material selected from the group consisting of carbon compounds and carbon-containing substances.

[0085] The thickness of the conductive layer is preferably thin enough not to substantially affect the average particle size of the first silicon-containing material. Considering both conductivity and lithium-ion diffusion, the thickness of the conductive layer is preferably 1–200 nm, more preferably 5–100 nm. The thickness of the conductive layer can be measured by cross-sectional observation of the silicon-containing material using SEM or TEM (transmission electron microscopy).

[0086] As carbon compounds, for example, compounds containing carbon and hydrogen, and compounds containing carbon, hydrogen and oxygen can be cited. As carbon-containing substances, amorphous carbon with low crystallinity, highly crystalline graphite, etc. can be used. As amorphous carbon, carbon black, coal, coke, charcoal, activated carbon, etc. can be cited. As graphite, natural graphite, artificial graphite, graphitized mesophase carbon particles, etc. can be cited. Among them, from the aspects of low hardness and large buffering effect on the silicon phase whose volume changes due to charge and discharge, amorphous carbon is preferred. The amorphous carbon can be easily graphitizable carbon (soft carbon), or can be hardly graphitizable carbon (hard carbon). As carbon black, acetylene black, Ketjen black, etc. can be cited.

[0087] The particle crushing strength Ma of the first silicon-containing material can be, for example, 300 MPa < Ma < 1000 MPa, 300 MPa < Ma < 800 MPa. In addition, the particle crushing strength Mb of the second silicon-containing material can be, for example, 300 MPa < Mb < 1000 MPa, and can be 300 MPa < Ma < 800 MPa. The first silicon-containing material and the second silicon-containing material having the particle crushing strength within the above range are not easily crushed during the manufacturing process of the electrode plate, charge and discharge cycles, etc., which is beneficial to suppressing the reduction of the cycle characteristics.

[0088] The particle crushing strength can be obtained by the following method, for example. 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 in the captured image are prepared. While gradually increasing the load, the particles are compressed with an indenter. The load until the particles are damaged is taken as the particle crushing strength of the particles. The particle crushing 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 front-end diameter of 50 μm, setting the displacement speed to 5 μm / second, measuring the particle crushing strength of 10 particles, and calculating the average value.

[0089] The ratio Mb / Ma of the particle crushing strength Mb of the second silicon-containing material to the particle crushing strength Ma of the first silicon-containing material can satisfy 0.5 < Mb / Ma < 2. At this time, one of the first silicon-containing material and the second silicon-containing material is not extremely hard or extremely soft relative to the other. Therefore, for example, during compression in the manufacturing process of the electrode plate, phenomena such as particles with a large particle crushing strength crushing particles with a small particle crushing strength are not likely to occur, and the cycle characteristics are not easily reduced. The closer the particle crushing strengths of the first silicon-containing material and the second silicon-containing material are, the easier it is to exert the effect of suppressing the reduction of the cycle characteristics.

[0090] Next, an example of the manufacturing method of the first silicon-containing material will be described in detail. Here, the case where the first silicon phase is dispersed in the lithium silicate phase will be described.

[0091] Step (i)

[0092] Lithium silicate is produced using a raw material mixture containing Si and Li in a specified ratio. The raw material mixture may also contain the aforementioned alkali metal elements, Group II elements, and / or element M. The raw material mixture is dissolved, and the melt is passed through a metal roller and sheeted to produce lithium silicate. The sheeted silicate is then heat-treated in an atmospheric atmosphere at a temperature above its glass transition point and below its melting point to crystallize it. It should be noted that the sheeted silicate can also be used without crystallization. Alternatively, the raw material mixture can be calcined at a temperature below its melting point without dissolving, thereby producing silicate through a solid-state reaction.

[0093] Silicon (Si) raw materials can include silicon dioxide. Li (Li) raw materials can include, for example, lithium carbonate, lithium oxide, lithium hydroxide, and lithium hydride. These can be used individually or in combination of two or more. Raw materials for alkali metals, Group II elements, and element M can include oxides, hydroxides, carbonates, hydrides, nitrates, and sulfates of each element.

[0094] Process (ii)

[0095] Next, raw material silicon is mixed into lithium silicate to perform composite formation. For example, a first silicon-containing material, which is a composite particle of lithium silicate and a first silicon phase (hereinafter also referred to as silicate composite particle), can be produced through the following steps (a) to (c).

[0096] Process (a)

[0097] For example, silicon powder and lithium silicate powder can be mixed at a mass ratio of 20:80 to 95:5. Coarse silicon particles with an average particle size of several μm to tens of μm can be used as the raw material.

[0098] Process (b)

[0099] Next, a grinding device such as a ball mill is used to simultaneously refine and compound the mixture of raw silicon and lithium silicate. At this stage, an organic solvent can be added to the mixture for wet grinding. The organic solvent serves to prevent the material from adhering to the inner wall of the grinding container.

[0100] As organic solvents, alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicates, metal alkoxides, etc. can be used.

[0101] It should be noted that silicon and lithium silicate can be finely granulated separately and then mixed. Alternatively, silicon nanoparticles and amorphous lithium silicate nanoparticles can be prepared and mixed without using a pulverizing device. Known methods such as gas-phase methods (e.g., plasma methods) and liquid-phase methods (e.g., liquid-phase reduction methods) can be used to prepare the nanoparticles.

[0102] Process (c)

[0103] Next, the mixture is pressurized and sintered in an inert gas atmosphere (e.g., argon, nitrogen, etc.) at a temperature of 600°C to 1000°C. Sintering can be performed using a sintering apparatus capable of applying pressure under an inert atmosphere such as hot pressing. During sintering, the silicate softens and flows to fill the gaps between the first silicon phases. As a result, a dense, blocky sintered body is obtained, with the silicate phase as the sea and the first silicon phase as the island. If the obtained sintered body is pulverized, silicate composite particles can be obtained.

[0104] Process (iii)

[0105] Next, a conductive layer can be formed by covering at least a portion of the surface of the composite particles with a conductive material. Examples of methods for covering the surface of the composite particles with a conductive carbon material include: CVD methods using hydrocarbon gases such as acetylene or methane as raw materials; and methods that mix the composite particles with coal tar pitch, petroleum tar pitch, phenolic resin, etc., and then carbonize them by heating in an inert atmosphere (e.g., argon, nitrogen, etc.) at 700°C to 950°C. Alternatively, carbon black can be deposited on the surface of the composite particles.

[0106] Process (iv)

[0107] A process of cleaning composite particles with acid (including those with a conductive layer on the surface) can be performed. For example, by cleaning the composite particles with an acidic aqueous solution, trace amounts of alkaline components that may be generated during the composite formation of raw silicon and lithium silicate can be dissolved and removed. As the acidic aqueous solution, aqueous solutions of inorganic acids such as hydrochloric acid, hydrofluoric acid, sulfuric acid, nitric acid, phosphoric acid, and carbonic acid, and aqueous solutions of organic acids such as citric acid and acetic acid can be used.

[0108] Figure 1 The cross-section of silicate composite particles 20 covered with a conductive layer is schematically shown as an example of a first silicon-containing material.

[0109] The silicate composite particles (base particles) 23 have a lithium silicate phase 21 and a silicon phase 22 dispersed within the lithium silicate phase 21. The silicate composite particles (base particles) 23 have an island structure in which fine silicon phase 22 is dispersed in the matrix of lithium silicate phase 21. The surface of the silicate composite particles (base particles) 23 is covered by a conductive layer 26.

[0110] A silicon oxide phase (not shown) may be dispersed in the lithium silicate phase 21. The SiO2 content in the silicate composite particles (basic particles) 23, as determined by Si-NMR, is preferably 30% by mass or less, more preferably less than 7% by mass.

[0111] In addition to the above, silicate composite particles (base particles) 23 may also contain other components. For example, carbon materials, oxides such as ZrO2, carbides and other reinforcing materials may be included in a manner that is less than 10% by mass relative to the base particles 23.

[0112] Next, an example of the manufacturing method of the second silicon-containing material will be given.

[0113] (i) Method 1

[0114] Raw silicon and a carbon source are mixed and pulverized using a ball mill or similar equipment, simultaneously refining and compounding the mixture. Alternatively, an organic solvent can be added to the mixture for wet pulverization. In this process, the raw silicon is micronized to form a second silicon phase. This second silicon phase is dispersed within the carbon source matrix.

[0115] As a carbon source, it can be water-soluble resins such as carboxymethyl cellulose (CMC), hydroxyethyl cellulose, polyacrylate, polyacrylamide, polyvinyl alcohol, polyethylene oxide, and polyvinylpyrrolidone, cellulose, sugars such as sucrose, petroleum asphalt, coal tar, and tar, without particular limitation.

[0116] As organic solvents, alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicates, metal alkoxides, etc. can be used.

[0117] Next, the composite of the second silicon phase and the carbon source is heated to 700°C–1200°C in an inert gas atmosphere (such as argon or nitrogen) to carbonize the carbon source and generate amorphous carbon. Thus, a second silicon-containing material in which the second silicon phase is dispersed within a carbon phase containing amorphous carbon can be obtained.

[0118] (ii) Method 2

[0119] Raw silicon and carbon materials (hereinafter also referred to as the second carbon material) are mixed and pulverized and compounded using a pulverizing device such as a ball mill. Alternatively, an organic solvent can be added to the mixture for wet pulverization. In this process, the raw silicon is micronized to form the second silicon phase. The second silicon phase is dispersed within the matrix of the second carbon material.

[0120] By combining the raw material silicon with 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 can be obtained. Then, the second silicon-containing material can be heated to 700°C to 1200°C in an inert gas atmosphere.

[0121] As the second carbon material, amorphous carbon is preferred, and easily graphitized carbon (soft carbon), difficult-to-graphitize 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, when a composite of the second silicon and carbon material is obtained using a pulverizing device, the crystal structure of graphite almost disappears, forming a carbon phase of amorphous carbon.

[0122] Next, the secondary battery according to an embodiment of the present invention includes a positive electrode, a negative electrode, an electrolyte, and a separator sandwiched 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. The negative electrode, positive electrode, electrolyte, and separator included in the secondary battery according to an embodiment of the present invention will be described below.

[0123] [negative electrode]

[0124] The negative electrode, for example, comprises 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 coating a negative electrode slurry in which the negative electrode composite material is dispersed in a dispersion medium onto the surface of the negative electrode current collector and then drying it. The dried coating film can also be calendered as needed.

[0125] The negative electrode composite material may contain a negative electrode active material as an essential component, and may also contain binders, conductive agents, thickeners, etc. as optional components. The negative electrode active material includes a first carbon material and first and second silicon-containing materials.

[0126] As the negative current collector, a non-porous conductive substrate (such as metal foil) or a porous conductive substrate (such as a mesh, wire mesh, or perforated sheet) can be used. Examples of materials used as the negative current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0127] Examples of adhesives include fluoropolymers, polyolefin resins, polyamide resins, polyimide resins, vinyl resins, styrene-butadiene rubber (SBR), polyacrylic acid, and their derivatives. These can be used individually or in combination of two or more.

[0128] Examples of conductive agents include carbon black, conductive fibers, fluorocarbons, and organic conductive materials. These can be used individually or in combination of two or more.

[0129] Examples of thickeners include carboxymethyl cellulose (CMC) and polyvinyl alcohol. These can be used individually or in combination of two or more.

[0130] Examples of dispersion media include water, alcohols, ethers, N-methyl-2-pyrrolidone (NMP), or mixtures thereof.

[0131] [positive electrode]

[0132] The positive electrode, for example, includes a positive current collector and a positive electrode composite material layer formed on the surface of the positive current collector. The positive electrode composite material layer can be formed by coating a positive electrode slurry in which the positive electrode composite material is dispersed in a dispersion medium onto the surface of the positive current collector and then drying it. The dried coating can also be calendered as needed.

[0133] Positive electrode composite materials can contain positive electrode active materials as essential components, and binders, conductive agents, etc. as arbitrary components.

[0134] Lithium-based composite metal oxides can be used as positive electrode active materials. Examples of lithium-based composite metal oxides 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 O4, LiMePO4, Li2MePO4F. Here, M is at least one element 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 one transition element (e.g., at least one element 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. It should be noted that the value of 'a', representing the molar ratio of lithium, is the value immediately after the active material is prepared and will increase or decrease according to charge and discharge.

[0135] As a binder and conductive agent, the same type as exemplified for the negative electrode can be used. As a conductive agent, natural graphite, artificial graphite, and other graphite can be used.

[0136] The positive current collector can be a conductive substrate based on the negative current collector. Examples of materials that can be used for the positive current collector include stainless steel, aluminum, aluminum alloys, and titanium.

[0137] [Electrolytes]

[0138] An electrolyte (or electrolyte solution) comprises a solvent and a lithium salt dissolved in the solvent. The concentration of the lithium salt in the electrolyte is, for example, 0.5–2 mol / L. The electrolyte may also contain known additives.

[0139] The solvent can be an aqueous or non-aqueous solvent. Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, and cyclic carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). One non-aqueous solvent can be used alone, or two or more can be used in combination.

[0140] As lithium salts, lithium salts containing chloric acid (LiClO4, LiAlCl4, LiB) can be used, for example. 10 Cl 10 Lithium salts include those containing fluorine acids (LiPF6, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts containing fluorinated imides (LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), and lithium halides (LiCl, LiBr, LiI, etc.). A single lithium salt can be used, or two or more can be used in combination.

[0141] [Separator]

[0142] A separator is desired to be sandwiched between the positive and negative electrodes. The separator should have high ion permeability and possess moderate mechanical strength and insulation. Microporous membranes, woven fabrics, and nonwoven fabrics can be used as separators. Materials used for separators include, for example, polyolefins such as polypropylene and polyethylene.

[0143] As an example of a secondary battery structure, one can exemplify a structure in which an electrode assembly consisting of a positive and a negative electrode wound together with a separator and an electrolyte housed within a casing. Alternatively, instead of a wound electrode assembly, other electrode assemblies such as a stacked electrode assembly consisting of positive and negative electrodes separated by a separator can be used. Secondary batteries can take any shape, such as cylindrical, square, coin-shaped, button-shaped, or laminated.

[0144] Figure 2 This is a schematic perspective view showing a portion of a square secondary battery according to one embodiment of the present invention cut away.

[0145] The battery comprises: a square-shaped battery casing 4, an electrode assembly 1 and an electrolyte (not shown) housed within the battery casing 4, and a sealing plate 5 for sealing the opening of the battery casing 4. The electrode assembly 1 has an elongated negative electrode, an elongated positive electrode, and a separator sandwiched between them. The sealing plate 5 has an inlet plug blocked by a sealing plug 8 and a negative terminal 6 insulated from the sealing plate 5 by a gasket 7.

[0146] One end of the negative lead 3 is mounted on the negative current collector by welding or the like. One end of the positive lead 2 is mounted on the positive current collector by welding or the like. The other end of the negative lead 3 is electrically connected to the negative terminal 6. The other end of the positive lead 2 is electrically connected to the sealing plate 5.

[0147] The present invention will be specifically described below based on embodiments and comparative examples, but the present invention is not limited to the following embodiments.

[0148] <Example 1>

[0149] [Preparation of the first silicon-containing material]

[0150] Silica and lithium carbonate were mixed with an atomic ratio of Si / Li of 1.05, and the mixture was calcined in air at 950°C for 10 hours to obtain lithium silicate with the formula Li₂Si₂O₅ (z = 0.5). The obtained lithium silicate was then pulverized to an average particle size of 10 μm.

[0151] Lithium silicate (Li₂Si₂O₅) with an average particle size of 10 μm and raw silicon (3N, average particle size of 10 μm) were mixed at a mass ratio of 70:30. The mixture was filled into a 500 mL pan of a planetary ball mill (Fritsch Co., Ltd., P-5), and 24 SUS balls (20 mm in diameter) were placed in the pan. The pan was then covered, and the mixture was pulverized at 200 rpm for 50 hours in an inactive atmosphere.

[0152] Next, the powdered mixture was taken out in an inactive atmosphere and calcined at 800°C for 4 hours under pressure applied by a hot press in an inactive atmosphere to obtain a sintered body of the mixture (silicon-silicate composite).

[0153] The silicon-silicate composite was then pulverized and passed through a 40 μm sieve. The resulting silicate composite particles were mixed with coal tar pitch (manufactured by JFE Chemical Corporation, MCP250), and the mixture was calcined at 800 °C in an inactive atmosphere. A conductive layer was formed by covering the surface of the silicate composite particles with conductive carbon. The coverage of the conductive layer was 5% by mass relative to the total mass of the silicate composite particles and the conductive layer. Then, using a sieve, silicate composite particles with an average particle size of 10 μm and a conductive layer (first silicon-containing particles) were obtained.

[0154] Here, the volume-based particle size distribution of the first silicon-containing particle was measured using a laser diffraction particle size distribution measuring device (MT3300EXII manufactured by Microtrac Co., Ltd.), and the particle size at 50% of the cumulative volume was taken as the average particle size.

[0155] The crystallite size of the first silicon phase, calculated using the Scherer equation, was 15 nm based on the diffraction peaks attributable to the Si(111) plane from the silicate composite particles by XRD analysis.

[0156] The composition of the lithium silicate phase was analyzed using the above method (ICP-AES). The results showed that the Si / Li ratio was 1.0, and the content of Li2Si2O5 measured by Si-NMR was 70% by mass (the content of the first silicon phase was 30% by mass).

[0157] [Preparation of the second silicon-containing material]

[0158] The carbon source, coal tar pitch (manufactured by JFE Chemical Corporation, MCP250), was mixed with raw silicon (3N, average particle size 10 μm) at a mass ratio of 50:50. The mixture was filled into a 500 mL pan of a planetary ball mill (manufactured by Fritsch Co., Ltd., P-5), and 24 SUS balls (20 mm in diameter) were placed in the pan. The pan was then covered, and the mixture was pulverized at 200 rpm for 50 hours in an inactive atmosphere to obtain a composite of the second silicon phase and the carbon source.

[0159] Next, the composite of the second silicon phase and the carbon source was calcined in an inert gas atmosphere to carbonize the carbon source, resulting in a second silicon-containing material in which the second silicon phase is dispersed within a carbon phase containing amorphous carbon. Then, using an air jet mill, second silicon-containing particles with an average particle size of 10 μm were obtained.

[0160] Here, the volume-based particle size distribution of the second silicon-containing particle was measured using a laser diffraction particle size distribution measuring device (MT3300EXII manufactured by Microtrac Co., Ltd.), and the particle size at 50% of the cumulative volume was taken as the average particle size.

[0161] The crystallite size of the second silicon phase, calculated using the Scherer equation, was 15 nm based on the diffraction peaks of the second silicon-containing particle belonging to the Si(111) plane by XRD analysis.

[0162] <Particle breaking strength>

[0163] The particle breaking strength of either the first or second silicon-containing material was determined. Ten particles each with a diameter of 5 μm or larger and 20 μm or smaller were obtained from the captured images. While gradually increasing the load, the displacement velocity was set to 5 μm / s, and the particles were compressed using a flat indenter with a front diameter of 50 μm. The load at which the particles broke was determined. A Shimadzu MCT-211 miniature compression tester was used. The average particle breaking strength of the ten particles was calculated.

[0164] [First Carbon Material]

[0165] Prepare spherical graphite with an average particle size Dc of 20 μm. Here, the volumetric particle size distribution of graphite is measured using a laser diffraction particle size distribution measuring device (MT3300EXII manufactured by Microtrac Co., Ltd.), and the particle size at 50% of the cumulative volume is taken as the average particle size.

[0166] [Making the negative electrode]

[0167] First and second silicon-containing particles with conductive layers were mixed with graphite at a mass ratio of 3:3:94 to form the negative electrode active material. The negative electrode active material was then mixed with sodium carboxymethyl cellulose (CMC-Na) and styrene-butadiene rubber (SBR) at a mass ratio of 97.5:1:1.5. After adding water, the mixture was stirred using a mixer (manufactured by PRIMIX Corporation, TKHIVISMIX.) to prepare the negative electrode slurry.

[0168] Next, on the surface of the copper foil, at 1m... 2 The negative electrode composite material was coated with a negative electrode slurry in a manner that resulted in a mass of 190g. After the coating dried, it was calendered to obtain a coating with a density of 1.5g / cm³ on both sides of the copper foil. 3 The negative electrode of the negative electrode composite material layer.

[0169] [The production of the positive electrode]

[0170] Lithium-nickel composite oxide (LiNi) 0.8 Co 0.18 Al 0.02O2, 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 (PRIMIX Corporation, TKHIVIS MIX.) to prepare a positive electrode slurry. Next, the positive electrode slurry was coated onto the surface of aluminum foil. After the coating dried, it was calendered to produce a film with a density of 3.6 g / cm³ on both sides of the aluminum foil. 3 The positive electrode of the positive electrode composite material layer.

[0171] [Preparation of Electrolyte]

[0172] An electrolyte was prepared by dissolving lithium salt in a non-aqueous solvent. The non-aqueous solvent used was a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and methyl acetate (MA) in a volume ratio of 20:40:40. The concentration of LiPF6 in the electrolyte was 1.0 mol / L.

[0173] [Making a Second-hand Battery]

[0174] Tabs are installed on each electrode, and the positive and negative electrodes are wound into a spiral shape with the tabs at the outermost periphery and separated by a separator to create an electrode assembly. The electrode assembly is inserted into an aluminum laminated film casing, vacuum dried at 105°C for 2 hours, electrolyte is injected, and the opening of the casing is sealed to obtain battery A1.

[0175] <Examples 2-4 and Comparative Examples 1-3>

[0176] Using the negative electrode active materials with the compositions shown in Table 1, 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. In Table 1 (and Tables 2 and 3 described later), the first silicon-containing material is referred to as the first Si material, and the second silicon-containing material is referred to as the second Si material. It should be noted that in battery A4, the particle breakage strength Mb was controlled by changing the calcination temperature.

[0177] [evaluate]

[0178] The batteries prepared above were evaluated using the following methods.

[0179] For each battery after fabrication, charge it with a constant current of 0.3 It until the voltage reaches 4.2V, then charge it with a constant voltage of 4.2V until the current reaches 0.015 It. Then, discharge it with a constant current of 0.3 It until the voltage reaches 2.75V. The pause between charging and discharging is 10 minutes. Charging and discharging are performed at 25°C.

[0180] It should be noted that (1 / X)It represents the current, and (1 / X)It(A) = rated capacity (Ah) / X(h), where X represents the time used to charge or discharge the rated capacity. For example, 0.5It means that X = 2 and the current value is rated capacity (Ah) / 2(h).

[0181] The charge-discharge cycle was repeated under the conditions described above. The percentage of the discharge capacity in the 300th cycle relative to the discharge capacity in the 1st cycle was calculated as the capacity retention rate during the charge-discharge cycle. The evaluation results are presented together with information on particle damage strength in Table 1.

[0182] [Table 1]

[0183]

[0184] As shown in Table 1, 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 is limited to a small amount in order to maintain cycle characteristics. Even when the first silicon-containing material accounts for 9% by mass in the negative electrode active material, the cycle life is much lower than when the first silicon-containing material accounts for 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 accounts for 9% by mass (A2 and A3), the cycle characteristics are still improved compared to the case where the first silicon-containing material is used alone (B2). Moreover, battery A1, with a B / A ratio of 1.0, achieves the best cycle characteristics.

[0185] <Examples 5-7 and Comparative Examples 4 and 5>

[0186] Using the negative electrode active material with the composition shown in Table 2, batteries A5, A6, and A7 of Examples 5, 6, and 7, and batteries B4 and B5 of Comparative Examples 4 and 5, were prepared in the same manner as in Example 1, and were evaluated in the same way. Here, the particle breakage strength Ma was controlled by applying pressure through hot pressing, and Mb was controlled by the calcination temperature.

[0187] [Table 2]

[0188]

[0189] <Examples 8 and 9 and Comparative Examples 6 and 7>

[0190] Using the negative electrode active material with the composition shown in Table 3, batteries A8 and A9 of Examples 8 and 9, and batteries B6 and B7 of Comparative Examples 6 and 7, were prepared in the same manner as in Example 1, and were evaluated in the same way. Here, the particle breakage strength Ma was controlled by applying pressure through hot pressing, and Mb was controlled by the calcination temperature.

[0191] [Table 3]

[0192]

[0193] As can be understood from Tables 2 and 3, even with various changes to the composition of the negative electrode active material, good cycle characteristics can still be obtained when graphite is used in combination with the first and second silicon-containing materials.

[0194] Industrial availability

[0195] The secondary battery of the present invention is useful in the main power supply of mobile communication devices, portable electronic devices, etc.

[0196] Explanation of reference numerals in the attached figures

[0197] 1: Electrode assembly; 2: Positive electrode lead; 3: Negative electrode lead; 4: Battery casing; 5: Sealing plate; 6: Negative terminal; 7: Gasket; 8: Sealing plug; 20: Silicate composite particles covered 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. The negative electrode comprises a carbon material, a first silicon-containing material, and a second silicon-containing material. The first silicon-containing material comprises a silicate phase and a first silicon phase dispersed within the silicate phase. The second silicon-containing material comprises a carbon phase and a second silicon phase dispersed within the carbon phase. The ratio B / A of the mass B of the second silicon-containing material to the mass A of the first silicon-containing material satisfies 0.2 relative to the total 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, and the total mass A of the first silicon-containing material to the mass B of the second silicon-containing material is 5% by mass or more and 30% by mass or less.

2. The secondary battery according to claim 1, wherein, The silicate phase comprises at least one element selected from the group consisting of alkali metals and Group II elements.

3. The secondary battery according to claim 1 or 2, wherein, The silicate phase also contains element M.

4. The secondary battery according to claim 3, wherein, The element M is selected from at least one of 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 particle breaking strength Ma of the first silicon-containing material is 300 MPa. <Ma<1000MPa。 5. The secondary battery according to claim 3, wherein, The silicate phase includes the formula Li 2z SiO 2+z The lithium silicate shown in the formula is 0 <z<2。 6. The secondary battery according to claim 1 or 2, wherein, The particle breaking strength Mb of the second silicon-containing material is 300 MPa. <Mb<1000MPa。 7. The secondary battery according to claim 1 or 2, wherein, 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, satisfies 0.

5. <Mb / Ma<2。 8. The secondary battery according to claim 7, wherein, ​

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