Secondary battery

Abstract

This secondary battery comprises a positive electrode, a negative electrode, and an electrolyte, the negative electrode including a negative electrode active material, the negative electrode active material including a silicon-containing material and a graphite material, the single-particle fracture strength of the silicon-containing material being within a range of not less than 50 MPa but less than 350 MPa, and the ratio Ss / Sg of the single-particle fracture strength Ss of the silicon-containing material to the single-particle fracture strength Sg of the graphite material being within a range of not less than 2.5 but less than 60.

Description

secondary battery

[0001] The present disclosure relates to secondary batteries.

[0002] Silicon-containing materials, such as silicon (Si) and silicon oxides, are known to be able to store more lithium ions per unit volume than carbon materials, such as graphite, and therefore have been proposed for use in the negative electrodes of lithium-ion secondary batteries.

[0003] Patent Document 1 proposes "a non-aqueous electrolyte secondary battery comprising: a positive electrode plate having a positive electrode mixture layer containing a positive electrode active material capable of absorbing and releasing lithium ions; a negative electrode plate having a negative electrode mixture layer containing a negative electrode active material capable of absorbing and releasing lithium ions; a separator; and a non-aqueous electrolyte, wherein the negative electrode active material is a mixture of a graphite material and silicon oxide represented by SiOx (0.5≦x<1.6), and wherein the fracture strength ratio Ss / Sg, which is the ratio of the fracture strength Ss of a single particle of the silicon oxide to the fracture strength Sg of a single particle of the graphite material, is 60 to 90."

[0004] Japanese Patent Application Laid-Open No. 2015-185407

[0005] Patent Document 1 proposes using a soft graphite material to absorb the stress of SiOx expansion and contraction. However, the soft graphite is crushed during charging, causing much of the electrolyte to be pushed out of the negative electrode, making it difficult to retain sufficient electrolyte in the negative electrode. Discharging without sufficient electrolyte present near the negative electrode active material increases interface resistance and reduces load characteristics. During the production process, if the graphite material is soft, it is over-rolled, reducing the ionic conduction path in the negative electrode. As a result, it is difficult to achieve sufficient charge / discharge cycles.

[0006] Attempts have also been made to adjust the hardness of the graphite material so that the graphite material can absorb the stress of expansion and contraction of the silicon-containing material, thereby improving the cycle retention rate.

[0007] However, there is a limit to how much the cycle retention rate can be improved by adjusting the hardness of the graphite material. Silicon-containing materials with high hardness have large expansion and contraction coefficients, so as the content of the silicon-containing material in the negative electrode active material increases, it tends to become difficult to maintain a high cycle retention rate simply by adjusting the hardness of the graphite material. On the other hand, silicon-containing materials with low hardness tend to crack easily during the rolling process in electrode production and during charge and discharge, which reduces the cycle retention rate.

[0008] One aspect of the present disclosure relates to a secondary battery including a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode includes a negative electrode active material, which includes a silicon-containing material and a graphite material, wherein the silicon-containing material has a single particle fracture strength in the range of 50 MPa or more and less than 350 MPa, and wherein the ratio Ss / Sg of the single particle fracture strength Ss of the silicon-containing material to the single particle fracture strength Sg of the graphite material is in the range of 2.5 or more and less than 60.

[0009] According to the present disclosure, a secondary battery with a high capacity retention rate during charge / discharge cycles can be obtained.

[0010] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0011] 1 is a schematic perspective view of a secondary battery according to an embodiment of the present invention, with a portion cut away;

[0012] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits for specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of the materials may be selected and used alone, or two or more materials may be used in combination.

[0013] The present disclosure also encompasses combinations of two or more features arbitrarily selected from the appended claims, i.e., two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0014] In the following description, the terms "contain" or "comprise" encompass "contain (or include)," "consist essentially of," and "consist of."

[0015] The secondary battery includes at least non-aqueous electrolyte secondary batteries such as lithium ion batteries and lithium metal secondary batteries.

[0016] A secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode includes a negative electrode active material, and the negative electrode active material includes a silicon-containing material and a graphite material.

[0017] The single particle fracture strength Ss of the silicon-containing material and the single particle fracture strength Sg of the graphite material satisfy the following conditions.

[0018] (i) The single particle fracture strength Ss of the silicon-containing material is in the range of 50 MPa or more and less than 350 MPa. The single particle fracture strength Ss of the silicon-containing material may be in the range of 100 MPa or more and less than 350 MPa, or in the range of 100 MPa or more and 300 MPa or less.

[0019] (ii) The ratio Ss / Sg of the single particle fracture strength Ss of the silicon-containing material to the single particle fracture strength Sg of the graphite material is in the range of 2.5 or more and less than 60. The ratio Ss / Sg may be in the range of 8.8 or more and less than 60, or in the range of 8.8 or more and 35 or less, or in the range of 10 or more and 35 or less.

[0020] The present inventors have found that a high cycle retention rate can be maintained when the single particle fracture strength of the silicon-containing material and graphite material satisfies the above conditions (i) and (ii). This tendency is particularly pronounced when the content of the silicon-containing material in the negative electrode active material is high. The present invention is based on this finding.

[0021] Particles of silicon-containing materials with a single particle fracture strength Ss of less than 350 MPa have a low density and large voids within the particles. Therefore, the internal voids absorb the expansion and contraction of the silicon-containing particles during charging and discharging, resulting in small displacements during expansion and contraction and a high cycle retention rate. Furthermore, by setting the ratio Ss / Sg to 2.5 or more, the silicon-containing material has a sufficiently high hardness compared to the graphite material, which can prevent the silicon-containing material from cracking due to stress during the rolling process in electrode production or during charging and discharging. On the other hand, the higher the Ss / Sg ratio, the greater the difference in hardness between the silicon-containing material and the graphite material. As the soft graphite is crushed during charging, much of the electrolyte is more likely to be extruded from the negative electrode. By setting the ratio Ss / Sg to 60 or less, sufficient electrolyte can be retained in the negative electrode, preventing a decrease in the cycle retention rate.

[0022] The density of the particles of the silicon-containing material is, for example, 1.6 to 2.1 g / cm ３ and in the range of 1.6 to 1.9 g / cm ３ The density of the particles of the silicon-containing material can be measured by a pycnometer using helium gas.

[0023] The single particle fracture strength Sg of the graphite material is not particularly limited as long as it satisfies the above conditions (i) and (ii), but may be in the range of 10 MPa or more and 40 MPa or less.

[0024] The total proportion of the mass of the silicon-containing material and the mass of the graphite material relative to the mass of the entire negative electrode active material may be 60% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more, or may be 100% by mass or less.

[0025] The mass ratio of the silicon-containing material to the total mass of the graphite material and the silicon-containing material may be 3% by mass or more and 98% by mass or less. More preferably, the content of the silicon-containing material to the total mass of the silicon-containing material and the graphite material in the negative electrode active material may be 6% by mass or more and 95% by mass or less. In this range, the expansion and contraction of the entire negative electrode can be controlled within a more suitable range, and the advantage of high capacity due to the silicon-containing material can be maximized.

[0026] The content of the silicon-containing material relative to the total of the silicon-containing material and the graphite material in the negative electrode active material can be estimated by considering the ratio of the area of ​​the silicon-containing material to the area of ​​the graphite material in the cross section of the negative electrode as the volume ratio, and estimating the content of the silicon-containing material from the ratio of these areas and the specific gravities of the silicon-containing material and the graphite material. For example, the secondary battery is disassembled to remove the negative electrode, the negative electrode is washed with anhydrous ethyl methyl carbonate or dimethyl carbonate, the negative electrode is cut perpendicular to the surface of the negative electrode current collector, the cross section of the negative electrode mixture layer is polished by ion milling, and the backscattered electron image of the cross section is observed using a scanning electron microscope (SEM), and the area of ​​the silicon-containing material and the area of ​​the graphite material observed in the cross section are calculated. The silicon-containing material and the graphite material can be distinguished because the brightness of the backscattered electron image differs.

[0027] (Silicon-containing material) The silicon-containing material refers to a silicon alloy, a silicon compound, a composite material containing silicon, etc. The silicon-containing composite material may have a so-called sea-island structure, and the silicon phase may constitute the island portion. The silicon-containing composite material may be a composite particle containing an ion-conducting phase and a silicon phase (silicon particles in one aspect) dispersed within the ion-conducting phase. The ion-conducting phase is a phase that conducts ions. The ion-conducting phase may be at least one selected from the group consisting of a silicate phase, a carbon phase, and a silicon oxide phase. These materials are in particulate form and can be obtained by various methods.

[0028] Among these, composite materials comprising a carbon phase and a silicon phase dispersed within the carbon phase are flexible and highly conductive, allowing them to maintain a good conductive network within the negative electrode even when the graphite material cannot fully absorb stress. Specifically, even if voids form around the silicon-containing material or cracks occur in the silicon-containing material, parts of the silicon-containing material are less likely to become isolated, and contact between the silicon-containing material and its surroundings is more likely to be maintained. This makes it easier to suppress capacity loss during repeated charge / discharge cycles. Furthermore, the negative electrode is less likely to expand.

[0029] The hardness of a silicon-containing material depends primarily on the hardness of the ion-conducting phase. The greater the porosity and the lower the density of the ion-conducting phase, the lower the hardness of the silicon-containing material and the smaller the single-particle fracture strength Ss. Silicon-containing materials with a single-particle fracture strength Ss of less than 350 MPa have sufficiently small particle density and many fine voids (pores) within the particles. Therefore, the expansion and contraction of the silicon phase during charge and discharge is alleviated through the voids, making it easier to achieve a high cycle retention rate.

[0030] A composite material comprising a carbon phase and a silicon phase dispersed within the carbon phase can exist in the form of particles with an islands-in-a-sea structure. The silicon phase (islands) are dispersed in a matrix (sea) of carbon phase and are covered by the carbon phase. The islands-in-a-sea structure limits contact between the silicon phase and the electrolyte, thereby suppressing side reactions. Furthermore, stresses generated by the expansion and contraction of the silicon phase are absorbed by the carbon phase matrix.

[0031] The carbon phase, which is an ion-conducting phase, may be composed of, for example, amorphous carbon (amorphous carbon), rather than a material with a developed graphite-type crystal structure, such as graphite. Examples of amorphous carbon that constitutes the carbon phase include hard carbon, soft carbon, and other amorphous carbon. Amorphous carbon is a carbon material in which the average interplanar spacing d002 of the (002) plane measured by X-ray diffraction exceeds 0.34 nm.

[0032] The silicon phase dispersed in the carbon phase may have an average particle size of 1 nm or more, or 5 nm or more. The average particle size may be 1000 nm or less, 500 nm or less, 200 nm or less, 100 nm or less, or 50 nm or less. A fine silicon phase is preferable in that it reduces volume change during charge and discharge and improves the structural stability of the silicon-containing material.

[0033] The average particle size of the silicon phase can be measured by observing the cross section of particles of the Si-containing material using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). Specifically, it can be determined by averaging the maximum diameters of 100 randomly selected silicon phase particles.

[0034] The crystallite size of the silicon phase is preferably 30 nm or less. When the crystallite size is 30 nm or less, the volume change of the silicon-containing material due to the expansion and contraction of the silicon phase during charge and discharge can be further reduced. The crystallite size is more preferably 30 nm or less, and even more preferably 20 nm or less. When the crystallite size is 20 nm or less, the expansion and contraction of the silicon phase are uniformed, microcracks in the silicon phase are reduced, and the cycle characteristics can be further improved.

[0035] The crystallite size of the silicon phase is calculated by Scherrer's formula from the half-width of the diffraction peak attributable to the (111) plane of the silicon phase (simple element Si) in the X-ray diffraction pattern.

[0036] The content of the silicon phase in the silicon-containing material may be, for example, 30% by mass or more, 40% by mass or more, or 50% by mass or more, or 80% by mass or less, or 70% by mass or less. In such a range, a sufficiently high capacity of the negative electrode is achieved, and the cycle characteristics are also likely to be improved. Furthermore, by maintaining a considerable proportion of the carbon phase, the carbon phase is more likely to penetrate into voids that are subsequently generated due to charge and discharge, and for example, the electrical connection between the silicon-containing material and its surroundings is more likely to be maintained.

[0037] The average particle size D50 of the silicon-containing material may be 1 μm or more, or 5 μm or more, or 20 μm or less, 15 μm or less, or 10 μm or less. The average particle size D50 of the silicon-containing material may be the median diameter (diameter at 50% cumulative volume) in a volume-based particle size distribution measured with a laser diffraction / scattering particle size distribution analyzer.

[0038] (Graphite Material) A graphite material refers to a material in which a graphite-type crystal structure has been developed, and generally refers to a carbon material in which the average interplanar spacing d002 of the (002) plane measured by X-ray diffraction is 0.34 nm or less. Typical graphite materials include natural graphite, artificial graphite, and graphitized mesophase carbon particles. Graphite materials are particulate, and graphite materials with various average particle sizes and average circularities are commercially available.

[0039] The average particle size D50 of the graphite material may be 5 μm or more, or 10 μm or more, or 50 μm or less, or 30 μm or less. The average particle size D50 of the graphite material may be the median diameter (diameter at 50% cumulative volume) in a volume-based particle size distribution measured with a laser diffraction / scattering particle size distribution analyzer.

[0040] In one example of the negative electrode, the graphite material has an average particle size in the range of 10 to 30 μm, and the silicon-containing material has an average particle size in the range of 1 to 15 μm. This configuration results in particularly good charge-discharge cycle characteristics.

[0041] In one example of the negative electrode, the value of (average particle size of the silicon-containing material) / (average particle size of the graphite material) is less than 1, and may be 0.8 or less, 0.6 or less, or 0.5 or less. There is no particular lower limit to this value, but it may be 0.05 or more, 0.1 or more, or 0.2 or more. By setting this value to 0.5 or less, the charge-discharge cycle characteristics become particularly good.

[0042] The negative electrode active material may contain carbon materials such as hard carbon and soft carbon in addition to graphite materials. Hard carbon is a carbon material in which minute graphite crystals are arranged in random directions and further graphitization hardly progresses, and the average interplanar spacing d002 of the 002 plane is greater than 0.38 nm. Hard carbon is preferable in terms of low resistance and high capacity. However, graphite materials are most preferable in terms of high capacity and small irreversible capacity.

[0043] The mass ratio of carbon materials other than graphite material to the mass of graphite material contained in the negative electrode active material is preferably 30 mass % or less, more preferably 20 mass % or less, or even 10 mass % or less.

[0044] (Single particle fracture strength) Here, the single particle fracture strength Ss of the silicon-containing material is in the range of 50 to 350 MPa. The single particle fracture strength of the silicon-containing material may also be in the range of 100 to 350 MPa. A single particle fracture strength in this range can be said to be quite small for a silicon-containing material. A silicon-containing material having a single particle fracture strength in this range tends to have relatively small expansion and contraction during charge and discharge, and even if the expansion and contraction are large, it can deform itself to relieve stress to some extent, making it easy to obtain high charge and discharge cycle characteristics.

[0045] A silicon-containing material having a single particle fracture strength Ss of less than 350 MPa can maintain high charge-discharge cycle performance by combining it with a graphite material having a ratio Ss / Sg of 2.5 or more to the single particle fracture strength Sg of the graphite material (described later). However, if the ratio Ss / Sg exceeds 60, the difference in hardness between the silicon-containing material and the graphite material becomes large, making the silicon-containing material more susceptible to stress and cracking. Furthermore, excessive compression of the graphite during the negative electrode production process reduces the space available for retaining the electrolyte inside the negative electrode, resulting in a decrease in the ion conduction path of the negative electrode and a decrease in charge-discharge cycle performance under high loads. Therefore, by setting the ratio Ss / Sg to 2.5 or more and 60 or less, high charge-discharge cycle performance can be achieved. In addition, the phenomenon of the graphite material being crushed during charging, causing much of the electrolyte to be extruded from the negative electrode, and the phenomenon of the graphite material being over-rolled during the production process are suppressed. As a result, not only the charge-discharge cycle performance of the secondary battery but also its load characteristics are improved.

[0046] The method for controlling the single particle fracture strength Ss of the silicon-containing material is not particularly limited. For example, the silicon-containing material may be manufactured using a method for controlling the single particle fracture strength Ss to a desired value. Examples of such methods include selecting a material constituting the ion-conducting phase and applying pressure to the silicon-containing material while heating it before pulverization. The heating temperature may be, for example, 300°C to 800°C. For example, if the material used to form the ion-conducting phase contains a substance that is removed by heating, the greater the amount of substance removed by heating, the more voids there will be in the formed ion-conducting phase, resulting in a lower density and a lower single particle fracture strength Ss.

[0047] The single particle fracture strength Sg of the graphite material is not particularly limited as long as Ss and the ratio Ss / Sg satisfy the above-mentioned conditions, but may be, for example, in the range of 10 to 40 MPa. The single particle fracture strength Sg of the graphite material may be in the range of 12 to 20 MPa. If the single particle fracture strength of the graphite material is less than 10 MPa, the graphite material is rolled during the production process, reducing the ion conduction path of the negative electrode and making it more likely that the charge / discharge cycle performance under high load will deteriorate. If the single particle fracture strength of the graphite material exceeds 40 MPa, the residual stress during rolling will increase, causing the negative electrode to expand, significantly increasing the internal pressure of the electrode assembly and making it more likely that the non-aqueous electrolyte will be extruded from the electrode assembly.

[0048] The method for controlling the single particle fracture strength Sg of the graphite material is not particularly limited. For example, a method of applying pressure to the graphite material can be used. Such a method may be, for example, cold isostatic pressing. The number of times of pressing may be controlled to achieve a target single particle fracture strength. The single particle fracture strength may also be controlled by processing the particle shape of the graphite material.

[0049] The single particle breaking strength of graphite materials and silicon-containing materials can be measured using a commercially available measuring device (for example, a microcompression tester (MCT-W201) manufactured by Shimadzu Corporation) according to the following procedure.

[0050] (1) A graphite material or a silicon-containing material is spread on the lower pressure plate (SKS flat plate) of the measuring device.

[0051] (2) Select as a sample particles a size close to the average particle size D50 of the graphite material or the average particle size D50 of the silicon-containing material when viewed under an optical microscope.

[0052] (3) A flat diamond indenter having a diameter of 50 μm is used as the upper pressure tool, and only one particle selected as the sample is placed between the upper pressure tool and the lower pressure plate.

[0053] (4) The upper pressure probe is slowly lowered, and from the point where it comes into contact with the sample (the lowering speed changes), a load is applied at a constant acceleration (specifically, a displacement speed of 2.7 mN / sec).

[0054] (5) The relationship between the load and the deformation of the sample is measured, and the point at which the deformation of the sample changes suddenly (the inflection point of the load-deformation profile) is taken as the breaking point, and the breaking strength is calculated from the load and particle diameter at that time using the following formula: Breaking strength is calculated as the average value of five measurements.

[0055] St = 2.8P / πd ２ St: Breaking strength [MPa or N / mm ２ ] P: Load [N] d: Particle diameter [mm]

[0056] The graphite material sample desirably has a maximum diameter that is ±5% of the average particle size D50 of the graphite material. The silicon-containing material sample desirably has a maximum diameter that is ±5% of the average particle size D50 of the silicon-containing material. The sphericity of each sample desirably is 85% or more. The sphericity may be determined as the diameter of a circle having the same area as the TEM or SEM image.

[0057] The graphite material and silicon-containing material to be measured can be prepared by disassembling a secondary battery, removing the negative electrode, peeling off the negative electrode mixture from the negative electrode, washing the negative electrode mixture with anhydrous ethyl methyl carbonate or dimethyl carbonate, pulverizing the negative electrode mixture in a mortar, and separating the components by a method such as centrifugation. The pulverized negative electrode mixture sample is dried in a dry atmosphere for 1 hour, immersed in gently boiling 6 M hydrochloric acid for 10 minutes, and then washed with ion-exchanged water, filtered, and dried at 200 °C for 1 hour. Centrifugation or other methods can then be performed.

[0058] Next, two methods for producing silicon-containing materials will be described. (a) First Method: Raw silicon and a carbon source are mixed, and the mixture of raw silicon and carbon source is pulverized and composited using a pulverizing device such as a ball mill. An organic solvent may be added to the mixture. At this time, the raw silicon is pulverized to produce a silicon phase. The silicon phase is dispersed in the carbon source matrix.

[0059] Examples of carbon sources that can be used include, but are not limited to, water-soluble resins such as carboxymethyl cellulose (CMC), hydroxyethyl cellulose, polyacrylates, polyacrylamide, polyvinyl alcohol, polyethylene oxide, and polyvinylpyrrolidone; sugars such as cellulose and sucrose; petroleum pitch, coal pitch, and tar.

[0060] As the organic solvent, alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicate esters, metal alkoxides, etc. can be used.

[0061] Next, the composite of the silicon phase and the carbon source is heated to 700°C to 1200°C in an inert gas atmosphere (e.g., an argon, nitrogen, or other atmosphere) to carbonize the carbon source and generate amorphous carbon, thereby obtaining a silicon-containing material in which the silicon phase is dispersed in the carbon phase containing amorphous carbon.

[0062] (b) Second Method: Raw silicon and a carbon material are mixed, and the mixture of raw silicon and carbon material is pulverized and composited using a pulverizing device such as a ball mill. An organic solvent may be added to the mixture. At this time, the raw silicon is pulverized to form a silicon phase. The silicon phase is dispersed in the carbon material matrix.

[0063] By compositing the raw silicon and the carbon material as described above, a silicon-containing material in which silicon is dispersed in the carbon phase of amorphous carbon is obtained. The silicon-containing material may then be heated to 700°C to 1200°C in an inert gas atmosphere.

[0064] The carbon material is preferably amorphous carbon, and graphitizable carbon (soft carbon), non-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 carbon material, the crystalline structure of the graphite is almost completely lost when a composite of the second silicon and the carbon material is obtained using a pulverizer, and a carbon phase of amorphous carbon is formed.

[0065] (Example of Secondary Battery Configuration) An example of the configuration of a secondary battery according to the present disclosure will be described below. Except for using the above-described negative electrode active material as the negative electrode active material, various selections are possible for the other components. For example, known components may be used for the components other than the negative electrode active material. A secondary battery according to the present disclosure typically includes a positive electrode, a negative electrode, an electrolyte, and a separator disposed between the positive electrode and the negative electrode. These components will be described below.

[0066] [Negative Electrode] The negative electrode comprises, for example, a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector and containing a negative electrode active material. The negative electrode mixture layer can be formed by applying a negative electrode slurry, in which the negative electrode mixture is dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the applied layer. The dried coating film may be rolled as necessary.

[0067] The negative electrode mixture contains a negative electrode active material as an essential component, and may contain optional components such as a binder, a conductive agent, a thickener, etc. The negative electrode active material contains a carbon material and a silicon-containing material.

[0068] The negative electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0069] Examples of binders include fluororesin, polyolefin resin, polyamide resin, polyimide resin, vinyl resin, styrene-butadiene copolymer rubber (SBR), polyacrylic acid and its derivatives, etc. These may be used alone or in combination of two or more.

[0070] Examples of the conductive agent include carbon black, conductive fibers, carbon fluoride, organic conductive materials, etc. These may be used alone or in combination of two or more.

[0071] Examples of the thickener include carboxymethyl cellulose (CMC), polyvinyl alcohol, etc. These may be used alone or in combination of two or more.

[0072] Examples of the dispersion medium include water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), and a mixed solvent thereof.

[0073] [Positive Electrode] The positive electrode includes, for example, a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by applying a positive electrode slurry, in which the positive electrode mixture is dispersed in a dispersion medium, to the surface of the positive electrode current collector and drying the applied film. The dried coating may be rolled as necessary.

[0074] The positive electrode mixture contains a positive electrode active material as an essential component, and may contain a binder, a conductive agent, and the like as optional components.

[0075] As the positive electrode active material, a material that absorbs and releases lithium ions can be used. As the positive electrode active material, a known positive electrode active material used in non-aqueous electrolyte secondary batteries can be used. Examples of the positive electrode active material include lithium composite metal oxides. Examples of the lithium 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 O 4、 LiMePO 4、 Examples include 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 includes at least a transition element (e.g., 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 value a, which indicates the molar ratio of lithium, is the value immediately after preparation of the active material and increases or decreases with charge and discharge.

[0076] The binder and conductive agent may be the same as those exemplified for the negative electrode. Graphite such as natural graphite or artificial graphite may also be used as the conductive agent.

[0077] The positive electrode current collector may be a conductive substrate having the same shape as that described for the negative electrode current collector. Examples of the material for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0078] [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.

[0079] The solvent may be an aqueous solvent or a non-aqueous solvent. Examples of non-aqueous solvents that can be used 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 type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0080] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 Lithium salts such as fluorine-containing acids (LiPF, LiBF, LiSbF, LiAsF, LiCF, SO, LiCF, CO, etc.), lithium salts of fluorine-containing acid imides (LiN(CF, SO), LiN(CF) SO (CF, SO), LiN(CF, SO) (CF, SO), LiN(CF, SO), etc.), and lithium halides (LiCl, LiBr, LiI, etc.). One type of lithium salt may be used alone, or two or more types may be used in combination.

[0081] The electrolyte may contain, for example, 1% to 5% by weight of vinylene carbonate (VC), or 0.5% to 5% by weight of fluoroethylene carbonate (FEC).

[0082] The secondary battery of this embodiment may be a non-aqueous electrolyte secondary battery using a non-aqueous electrolyte, which can be obtained, for example, by dissolving a lithium salt in a non-aqueous solvent.

[0083] [Separator] It is desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator can be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator can be made of polyolefins such as polypropylene and polyethylene.

[0084] An example of a secondary battery includes an electrode group, an electrolyte, and an exterior housing that houses them. The electrode group may be a wound electrode group in which a positive electrode and a negative electrode are wound with a separator interposed therebetween. Alternatively, the electrode group may have another form. For example, the electrode group may be a stacked electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween. The form of the secondary battery is not particularly limited, and may be, for example, a cylindrical type, a prismatic type, a coin type, a button type, a laminate type, or the like.

[0085] 1 is a schematic perspective view of a partially cutaway prismatic secondary battery according to an embodiment of the present disclosure. Note that the battery shown in FIG. 1 is an example, and the secondary battery of the present disclosure is not limited to the battery shown in FIG.

[0086] The battery includes a bottomed prismatic 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 includes a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed therebetween. The sealing plate 5 has a liquid filling hole closed with a sealing plug 8 and a negative electrode terminal 6 insulated from the sealing plate 5 by a gasket 7.

[0087] One end of a negative electrode lead 3 is attached to the negative electrode current collector by welding or the like. One end of a 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 a negative electrode terminal 6. The other end of the positive electrode lead 2 is electrically connected to a sealing plate 5.

[0088] EXAMPLES The present disclosure will be specifically described below based on examples and comparative examples, but the present disclosure is not limited to the following examples. A plurality of secondary batteries were fabricated in the following procedure.

[0089] Examples 1 to 4, Comparative Examples 1 to 5 Preparation of Silicon-Containing Material Coal pitch as a carbon source and raw silicon (purity 99.9% or higher, average particle size 100 nm) were mixed and stirred in a ball mill while controlling the temperature below the softening temperature of the pitch. The stirring device was then changed to a bead mill, and the mixture was mixed while maintaining the temperature above the softening temperature of the pitch. The resulting liquid mixture was extruded through a slit to form a 4 mm thick sheet. The extruded sheet was heated to 800°C in a furnace under a nitrogen atmosphere and pressed at a predetermined pressure with a pair of rollers heated to 550°C. The pressed sheet was crushed in a jaw crusher and further crushed in a jet mill to an average particle size of 6 μm to obtain particles of a silicon-containing material (particles P1) having the desired single particle fracture strength Ss.

[0090] The single particle breaking strength Ss of particle P1 was adjusted to a target value by adjusting the ratio of fixed carbon contained in the coal pitch. The lower the fixed carbon content of the pitch, the more components volatilize when heated and carbonized, resulting in a larger amount of internal voids, a smaller density, and a smaller single particle breaking strength Ss.

[0091] The composition of the particles P1 was analyzed by ICP (inductively coupled plasma atomic emission spectroscopy), and the silicon content was found to be 50.1% by mass.

[0092] [Preparation of Graphite Material] Natural graphite was subjected to cold isostatic pressing a predetermined number of times to obtain a graphite material having the target single particle fracture strength Sg. The graphite material was pulverized in a jet mill to obtain particles (particles P2) having an average particle size of 20 μm.

[0093] [Fabrication of Negative Electrode] Particles P1 and P2 were mixed in a mass ratio of P1:P2 = 10:90 to obtain a negative electrode active material. The negative electrode active material, sodium carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of negative electrode active material:CMC:PAA:SBR = 97:1:1:1 to obtain a negative electrode mixture. Water was added to the negative electrode mixture, and the mixture was stirred using a mixer to prepare a negative electrode slurry. The negative electrode slurry was applied to the surface of copper foil, which served as a negative electrode current collector, to form a coating. The coating was dried and then rolled to obtain a negative electrode in which a negative electrode mixture layer was formed on both sides of the copper foil.

[0094] [Preparation of Positive Electrode] Lithium nickel composite oxide (LiNi 0.88 Co 0.09 Al 0.03 O2), acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of lithium nickel composite oxide:acetylene black:polyvinylidene fluoride = 100:1:0.9 to obtain a mixture. N-methyl-2-pyrrolidone (NMP) was added to the mixture, and the mixture was stirred using a mixer to prepare a positive electrode slurry. The positive electrode slurry was applied to the surface of aluminum foil, which served as a positive electrode current collector, to form a coating film. The coating film was dried and then rolled to obtain a positive electrode in which a positive electrode mixture layer was formed on both sides of the aluminum foil.

[0095] [Preparation of non-aqueous electrolyte] LiPF ６ The non-aqueous electrolyte was prepared by dissolving the above in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of EC:EMC=30:70. ６ The concentration of vinylene carbonate (VC) was 1.0 mol / L. Vinylene carbonate (VC) was dissolved in the non-aqueous electrolyte to a content of 2.0 mass %.

[0096] [Preparation of Secondary Battery] A tab was attached to each electrode, and the positive and negative electrodes were spirally wound with a separator interposed therebetween to prepare an electrode assembly. The tab was placed on the outermost periphery of the electrode assembly. The electrode assembly was inserted into an exterior case made of aluminum laminate film and vacuum dried at 105°C for 2 hours. Next, a nonaqueous electrolyte was injected into the exterior case, and the opening of the exterior case was sealed to obtain a battery.

[0097] [Evaluation] [Charge-Discharge Cycle Test] Each battery was subjected to constant current charging at a current of 0.5 It until the voltage reached 4.2 V, and then constant voltage charging at a constant voltage of 4.2 V until the current was reduced to 1 / 20 It. Next, constant current discharging was performed at a current of 0.5 It until the voltage reached 2.5 V. 200 charge-discharge cycles were performed with a rest period of 10 minutes between charge and discharge, and the capacity retention rate R, as shown in the following formula, was measured.

[0098] Capacity retention rate R (%) = (discharge capacity at 1st cycle / discharge capacity at 200th cycle) × 100

[0099] Note that (1 / X)It represents the current. (1 / X)It (A) = rated capacity (Ah) / X (h). X represents the time required to charge or discharge the electricity equivalent to the rated capacity. For example, 0.5It means that X = 2, and the current value is rated capacity (Ah) / 2 (h).

[0100] The results are shown in Table 1, with the batteries of the example being designated A1 to A4 and the batteries of the comparative example being designated B1 to B5.

[0101]

[0102] As shown in Table 1, in Examples 1 to 4, in which the single particle fracture strength Ss of the silicon-containing material is in the range of 50 MPa or more and less than 350 MPa, and the ratio Ss / Sg of the single particle fracture strength Ss of the silicon-containing material to the single particle fracture strength Sg of the graphite material is in the range of 2.5 or more and less than 60, the capacity retention rate R is 88% or more, and the charge-discharge cycle characteristics are excellent. On the other hand, in Comparative Examples 1 to 3, in which the single particle fracture strength Ss of the silicon-containing material exceeds 350 MPa, and Comparative Example 4, in which the ratio Ss / Sg is less than 2.5, the capacity retention rate R is low.

[0103] The ratio Ss / Sg is preferably 2.5 or more and less than 60 from the viewpoint of preventing the silicon-containing material from cracking due to stress during the rolling process in manufacturing the electrode or during charge and discharge, and from the viewpoint of preventing the graphite from being excessively compressed after the rolling process, causing the electrolyte to be extruded from the negative electrode.

[0104] <<Appendix>> The above embodiments disclose the following technologies. (Technology 1) A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode includes a negative electrode active material, and the negative electrode active material includes a silicon-containing material and a graphite material, wherein the silicon-containing material has a single particle fracture strength in the range of 50 MPa or more and less than 350 MPa, and wherein the ratio Ss / Sg of the single particle fracture strength Ss of the silicon-containing material to the single particle fracture strength Sg of the graphite material is in the range of 2.5 or more and less than 60. (Technology 2) The secondary battery according to Technology 1, wherein the silicon-containing material has a single particle fracture strength in the range of 100 MPa or more and less than 350 MPa. (Technology 3) The secondary battery according to Technology 1 or 2, wherein the graphite material has a single particle fracture strength in the range of 10 MPa or more and less than 40 MPa. (Technology 4) The secondary battery according to any one of Technologies 1 to 3, wherein the silicon-containing material is a composite material having a carbon phase and a silicon phase dispersed within the carbon phase. (Technology 5) The secondary battery according to any one of Technologies 1 to 4, wherein the mass ratio of the silicon-containing material to the total mass of the silicon-containing material and the graphite material in the negative electrode active material is 6 mass% or more and 95 mass% or less.

[0105] The present disclosure can be used in secondary batteries, and is useful, for example, as main power sources for mobile communication devices, portable electronic devices, and the like.

[0106] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0107] 1: electrode group, 2: positive electrode lead, 3: negative electrode lead, 4: battery case, 5: sealing plate, 6: negative electrode terminal, 7: gasket, 8: sealing plug

Claims

1. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode comprises a negative electrode active material, the negative electrode active material comprises a silicon-containing material and a graphite material, wherein the single particle fracture strength of the silicon-containing material is in the range of 50 MPa or more and less than 350 MPa, and the ratio Ss / Sg of the single particle fracture strength Ss of the silicon-containing material to the single particle fracture strength Sg of the graphite material is in the range of 2.5 or more and less than 60.

2. The secondary battery according to claim 1, wherein the silicon-containing material has a single particle fracture strength in the range of 100 MPa or more and less than 350 MPa.

3. The secondary battery according to claim 1, wherein the graphite material has a single particle fracture strength in the range of 10 MPa or more and 40 MPa or less.

4. The secondary battery according to any one of claims 1 to 3, wherein the silicon-containing material is a composite material having a carbon phase and a silicon phase dispersed within the carbon phase.

5. A secondary battery according to any one of claims 1 to 3, wherein the ratio of the mass of the silicon-containing material to the total mass of the silicon-containing material and the graphite material in the negative electrode active material is 6 mass % or more and 95 mass % or less.

Images