Non-aqueous electrolyte secondary battery

Abstract

According to the present invention, a negative electrode mixture for a non-aqueous electrolyte secondary battery comprises a negative electrode active material, wherein: the negative electrode active material includes a carbon material A containing no silicon and a silicon-containing material B; the discharge capacity density of the negative electrode active material is 750 Ah / g to 1,220 Ah / g; and the silicon-containing material B includes carbon composite particles having a carbon phase and a silicon phase dispersed within the carbon phase; and the ratio SB / SA of the specific surface area SB of the silicon-containing material B to the specific surface area SA of the carbon material A is 0.7 to 19.

Description

Nonaqueous electrolyte secondary battery CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This disclosure claims the benefit of priority to Japanese Patent Application No. 2024-129948, filed on August 6, 2024, in the Japan Patent Office, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a non-aqueous electrolyte secondary battery.

[0003] A nonaqueous electrolyte secondary battery, such as a lithium ion secondary battery, includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. A nonaqueous electrolyte solution is typically used as the nonaqueous electrolyte. The negative electrode includes a negative electrode mixture containing a negative electrode active material. The negative electrode active material is a material capable of electrochemically absorbing and desorbing lithium ions. Examples of such materials include carbonaceous materials and silicon-containing materials. Furthermore, a carbonaceous material that does not absorb or desorb lithium ions, such as carbon fiber or carbon nanotubes, may be added to the negative electrode mixture as a conductive agent.

[0004] Patent Document 1 proposes "a negative electrode for a secondary battery, comprising a negative electrode composite layer having a negative electrode active material containing graphite particles and a Si-containing material, the Si-containing material including a Si-containing material A having a carbon phase and silicon particles dispersed in the carbon phase, and when the negative electrode composite layer is divided into two equal parts in the thickness direction, the average value of the internal particle porosity of the graphite particles contained in the upper half region is 13% or less and is smaller than the average value of the internal particle porosity of the graphite particles contained in the lower half region."

[0005] International Publication No. 2022 / 070818

[0006] Silicon-containing materials undergo large volume changes due to the absorption and desorption of lithium ions. Carbon composite particles containing a carbon phase and a silicon phase dispersed within the carbon phase have a large discharge capacity and a large amount of lithium absorbed / desorbed per volume, resulting in particularly large volume changes. Therefore, with each charge / discharge cycle, the silicon-containing material particles tend to crack, revealing new surfaces. Side reactions with the electrolyte tend to occur on the new surfaces, reducing capacity with repeated charge / discharge cycles, resulting in a decrease in capacity retention and cycle performance.

[0007] Attempts have been made to improve cycle performance by controlling the voids in the electrode or the electrode material to absorb the expansion of the negative electrode due to volume changes in the silicon-containing material. However, the higher the internal porosity of the silicon-containing material, the lower the lithium ion conductivity within the silicon-containing material. As a result, the rapid charge / discharge performance of the secondary battery may be reduced. In high-capacity secondary batteries, the proportion of silicon-containing material in the negative electrode active material is high, which is likely to cause a problem of reduced rapid charge / discharge performance.

[0008] One aspect of the present disclosure relates to a negative electrode mixture for a non-aqueous electrolyte secondary battery, comprising: a silicon-free carbon material A and a silicon-containing material B; a discharge capacity density of the negative electrode active material is 750 Ah / g or more and 1220 Ah / g or less; the silicon-containing material B comprises carbon composite particles including a carbon phase and a silicon phase dispersed within the carbon phase; and a ratio SB / SA of the specific surface area of ​​the silicon-containing material B to the specific surface area SA of the carbon material A is 0.7 or more and 19 or less.

[0009] Another aspect of the present disclosure relates to a non-aqueous electrolyte secondary battery including a negative electrode containing the above-described negative electrode mixture, a separator, a positive electrode facing the negative electrode with the separator interposed therebetween, and a non-aqueous electrolyte.

[0010] In a non-aqueous electrolyte secondary battery using a negative electrode containing carbon composite particles including a silicon phase, both high cycle characteristics and high rapid charge / discharge performance can be achieved. 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 longitudinal cross-sectional view schematically illustrating a structure of a negative electrode (negative electrode mixture layer) used in a nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure in a discharged state;

[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 of the 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] A negative electrode mixture according to one embodiment of the present disclosure is a negative electrode mixture for a non-aqueous electrolyte secondary battery, and includes a negative electrode active material. The negative electrode active material includes a silicon-free carbon material A and a silicon-containing material B. The negative electrode active material has a discharge capacity density of 750 Ah / g or more and 1220 Ah / g or less. The silicon-containing material B includes carbon composite particles including a carbon phase and a silicon phase dispersed within the carbon phase. The ratio SB / SA of the specific surface area of ​​the silicon-containing material B to the specific surface area SA of the carbon material A is 0.7 or more and 19 or less.

[0015] Another embodiment of the present disclosure is a nonaqueous electrolyte secondary battery including a negative electrode containing the above-described negative electrode mixture for a nonaqueous electrolyte secondary battery, a separator, a positive electrode facing the negative electrode with the separator interposed therebetween, and a nonaqueous electrolyte.

[0016] The present disclosure is based on the inventor's novel finding that, in a non-aqueous electrolyte secondary battery in which a silicon-free carbon material A and a silicon-containing material B are combined and used as a negative electrode active material, it is possible to achieve both good cycle characteristics and good rapid charge / discharge characteristics by controlling the ratio between the specific surface areas of the carbon material A and the silicon-containing material B.

[0017] Hereinafter, the silicon-containing material may be referred to as an "Si-containing material," and the silicon phase may be referred to as an "Si phase" or a "silicon phase."

[0018] In non-aqueous electrolyte secondary batteries, carbonaceous materials such as graphite are generally used as the negative electrode active material. Silicon-containing materials have a larger theoretical capacity than carbonaceous materials, and the use of silicon-containing materials allows for a higher capacity than carbonaceous materials.

[0019] On the other hand, silicon-containing materials undergo large volume changes due to the absorption and desorption of lithium ions, which can easily cause cracks in the active material particles and the formation of new surfaces. The new surfaces are prone to side reactions with the non-aqueous electrolyte, resulting in a decrease in capacity. Therefore, even when silicon-containing materials are used, the capacity retention rate after repeated charge and discharge cycles can decrease, making it difficult to ensure a sufficient lifespan.

[0020] Among silicon-containing materials, carbon composite particles containing a carbon phase and a silicon phase dispersed within the carbon phase are ideal from the viewpoint of obtaining high capacity, since they have a particularly high active material utilization rate and can obtain a large discharge capacity. However, when repeated charge / discharge cycles are performed, cracks tend to occur not only in the silicon phase but also in the carbon phase constituting the matrix due to volume changes in the silicon phase. Therefore, when carbon composite particles containing a silicon phase are used, more new surfaces are formed than when carbonaceous materials not containing a silicon phase are used. Therefore, when repeated charge / discharge cycles are performed, the capacity retention rate decreases, and cycle characteristics tend to deteriorate.

[0021] The silicon-containing material B containing carbon composite particles has voids inside. The internal voids act as spaces to retain the non-aqueous electrolyte, supplying the non-aqueous electrolyte to the silicon phase dispersed within the carbon composite particles and providing space for expansion when the silicon phase expands upon charging. As a result, cracking of the carbon phase due to volumetric changes in the silicon phase can be suppressed, and the expansion and contraction of the carbon composite particles during charge and discharge can also be suppressed. This makes it possible to suppress deterioration of cycle characteristics while maintaining high capacity using the silicon-containing material B containing carbon composite particles.

[0022] The intra-particle porosity of the silicon-containing material B (carbon composite particles) is, for example, in the range of 6% to 20%. In this case, it is easy to maintain a high capacity retention rate over a long period of time while ensuring a high initial discharge capacity due to the carbon composite particles. The intra-particle porosity can be determined from a cross-sectional image of the silicon-containing material B using the method described below.

[0023] On the other hand, as the internal porosity of the carbon composite particles increases, and as the proportion of the silicon phase in the carbon composite particles increases to increase capacity, the proportion of the carbon phase in the carbon composite particles decreases, and the lithium ion conductivity inside the carbon composite particles decreases. As a result, it becomes difficult for a nonaqueous electrolyte secondary battery using carbon composite particles with a large internal porosity as the negative electrode active material to maintain excellent rapid charge-discharge characteristics.

[0024] In contrast, in the negative electrode mixture and nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure, the ratio SB / SA of the specific surface area SB of the silicon-containing material B to the specific surface area SA of the carbon material A is 0.7 or more and 19 or less. By controlling the ratio of the specific surface areas of the carbon material A and the silicon-containing material B in this manner, rapid charge / discharge characteristics are improved and can be maintained at a high level while suppressing a deterioration in cycle characteristics.

[0025] As the specific surface areas SA and SB are increased, the rapid charge-discharge characteristics improve, but side reactions with the non-aqueous electrolyte tend to occur more easily, leading to a deterioration in cycle characteristics. By controlling the ratio of the specific surface areas of the carbon material A and the silicon-containing material B within the above range, it is possible to maintain high rapid charge-discharge characteristics while suppressing a deterioration in cycle characteristics.

[0026] The specific surface area SA of the carbon material A is set to, for example, 2.9 m from the viewpoint of suppressing the deterioration of cycle characteristics and obtaining high rapid charge / discharge characteristics. ２ / g or more is sufficient, and ２ / g or more, 20m ２ / g or less.

[0027] The specific surface area SB of the silicon-containing material B is, for example, 2.0 m from the viewpoint of suppressing the deterioration of cycle characteristics and obtaining high rapid charge / discharge characteristics. ２ / g or more, 400m ２ / g or less.

[0028] When the specific surface area SA of the carbon material A is large, the carbon material A can maintain high rapid charge / discharge characteristics, and therefore high rapid charge / discharge characteristics can be realized without increasing the specific surface area SB of the silicon-containing material B. On the other hand, when the specific surface area SA of the carbon material A is relatively small, it is necessary to increase the specific surface area SB of the silicon-containing material B in order to maintain high rapid charge / discharge characteristics. In this regard, the specific surface area SA (m ２ / g) and the specific surface area SB (m ２ The product SA·SB with the saturation coefficient (S / g) is preferably 5.8 or more.

[0029] The negative electrode active material has a discharge capacity density of 750 Ah / g or more. The discharge capacity density of the negative electrode active material may be 750 Ah / g or more and 1220 Ah / g or less. According to an embodiment of the present disclosure, by using a negative electrode active material having a high discharge capacity density of 750 Ah / g or more, a nonaqueous electrolyte secondary battery can be realized that has a high capacity, suppresses a decrease in capacity retention rate during repeated charge and discharge, and also has excellent rapid charge and discharge performance.

[0030] The discharge capacity density can be controlled by the content ratio of the carbon material A and the silicon-containing material B in a negative electrode active material containing the carbon material A and the silicon-containing material B. In conventional secondary batteries, the content ratio of the silicon-containing material B in the entire negative electrode active material is sufficiently small (e.g., 10 mass% or less), and the discharge capacity density is also small. In this case, in order to realize a high-capacity secondary battery (e.g., 3.4 Ah or more in the case of an 18650-type cylindrical battery) using a negative electrode active material with a low discharge capacity density, it is necessary to increase the total amount of negative electrode active material, which increases the thickness of the negative electrode (negative electrode mixture layer). As a result, the current collection ability of the negative electrode is reduced, which makes it easier for rapid charge / discharge characteristics to deteriorate.

[0031] In contrast, when the content ratio of silicon-containing material B in the entire negative electrode active material is sufficiently high and the discharge capacity density is 750 Ah / g or more, the thickness of the negative electrode (negative electrode mixture layer) is prevented from increasing when achieving high capacity. As a result, the rapid charge / discharge characteristics are not deteriorated due to a decrease in the current collection ability of the negative electrode. In addition, by appropriately controlling the specific surface area SA of carbon material A and the specific surface area SB of silicon-containing material B, the deterioration of the rapid charge / discharge characteristics caused by a decrease in lithium ion conductivity inside the carbon composite particles is also suppressed, and remarkably high rapid charge / discharge characteristics can be realized.

[0032] The discharge capacity density of the negative electrode active material is determined by preparing a test electrode containing the negative electrode active material, using this electrode to prepare a test battery with metallic lithium as the counter electrode, and determining the discharge capacity of the negative electrode active material from the quantity of electricity when the test battery is charged and discharged under the following charge and discharge conditions:

[0033] [Discharge] The test battery is discharged at a constant current of 0.1 C until the test electrode (positive electrode) becomes +0.01 V or less relative to the metallic lithium counter electrode (negative electrode). [Charge] The battery is then charged at a constant current of 0.1 C, and the charge is terminated when the test electrode becomes +1.0 V or more relative to the metallic lithium counter electrode. The amount of charge electricity flowing from the start to the end of charge is taken as the discharge capacity of the negative electrode active material. The discharge capacity is divided by the total weight of the negative electrode active material to obtain the discharge capacity density. Charging and discharging are performed in an environment of 25°C.

[0034] A discharge capacity density of 750 Ah / g or more can be achieved in a negative electrode active material containing a carbon material A and a silicon-containing material B by increasing the ratio of the silicon-containing material B to the carbon material A. To obtain a high capacity, the ratio MB / MA of the mass MB of the silicon-containing material B to the mass MA of the carbon material A may be 0.4 or more and 0.7 or less.

[0035] In terms of improving rapid charge / discharge characteristics, it is effective to reduce the average particle size of the silicon-containing material B. A silicon-containing material B with a small average particle size has a small specific surface area SB and the lithium ions inside the particles need only travel a short distance during charge / discharge, improving rapid charge / discharge characteristics. When the average particle size of the silicon-containing material B is 0.5 μm or more and 7.0 μm or less, it is easy to maintain high rapid charge / discharge characteristics while suppressing deterioration of cycle characteristics.

[0036] Here, the average particle size of silicon-containing material B refers to the particle size (volume average particle size) (D50 particle size) at which the volume cumulative value is 50% in the particle size distribution measured by laser diffraction scattering. For example, an "LA-750" manufactured by Horiba, Ltd. can be used as a measuring device. The average particle size may be determined from a cross-sectional sample of the negative electrode formed in a non-aqueous electrolyte secondary battery after manufacture to obtain a backscattered electron image by FE-SEM. The negative electrode is removed from the non-aqueous electrolyte secondary battery in a discharged state, and the equivalent circle diameters of the cross sections of 10 or more particles of silicon-containing material B are determined, and the average value thereof is determined as the average particle size. Here, the equivalent circle diameter refers to the diameter of a circle having the same area as the area of ​​the particle observed in the cross section of the negative electrode.

[0037] The density of silicon-containing material B is 1.4 g / cm ３ Above, 2.1g / cm ３ or less. In this case, silicon-containing material B has an appropriate internal porosity, and by controlling the specific surface area ratio SB / SA within the above range, it is possible to easily maintain high rapid charge / discharge characteristics while suppressing deterioration of cycle characteristics. The particle density of the silicon-containing material can be measured with a pycnometer using helium gas.

[0038] The single particle fracture strength of the silicon-containing material B may be 100 MPa or more and 350 MPa or less. A single particle fracture strength in this range is 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, the silicon-containing material can deform itself to relieve stress to some extent, making it easy to obtain high charge and discharge cycle characteristics.

[0039] The hardness of silicon-containing material B depends mainly on the hardness of the carbon phase, which is the ion-conducting phase. The greater the porosity and the lower the density of the ion-conducting phase, the lower the hardness of silicon-containing material B and the smaller the single-particle fracture strength. Silicon-containing materials with a single-particle fracture strength of less than 350 MPa have sufficiently small particle density and many fine voids (pores) within the particles. Therefore, the internal voids absorb the expansion and contraction of the silicon phase that accompanies charging and discharging, so the volume change during expansion and contraction is small and a high cycle retention rate is easily achieved.

[0040] In addition, when the single particle fracture strength of silicon-containing material B is 100 MPa or more, silicon-containing material B has a sufficiently high hardness compared to carbon material A. This prevents the silicon-containing material from cracking due to stress during the rolling process in manufacturing the electrode or during charge and discharge. Furthermore, when the single particle fracture strength of silicon-containing material B is 350 MPa or less, the difference in hardness between silicon-containing material B and carbon material A is small, and the soft carbon material A is crushed during charge, preventing the non-aqueous electrolyte from being extruded from the negative electrode. As a result, a sufficient amount of electrolyte can be retained in the negative electrode during charge and discharge, preventing a decrease in cycle retention rate.

[0041] The single particle breaking strength 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.

[0042] (1) The silicon-containing material is spread on the lower pressure plate (SKS plate) of the measuring device.

[0043] (2) Particles having a size close to the average particle size D50 of the silicon-containing material as measured under an optical microscope are selected as samples.

[0044] (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.

[0045] (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).

[0046] (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.

[0047] St = 2.8P / πd ２ St: Breaking strength [MPa or N / mm ２ ] P: Load [N] d: Particle diameter [mm]

[0048] Here, the specific surface area of ​​the silicon-containing material B or the carbon composite particles is, in principle, a physical property that is evaluated using carbon composite particles isolated from the negative electrode mixture layer as a sample. However, the carbon composite particles are sturdy and can maintain almost the same physical properties as the raw carbon composite particles blended into the negative electrode mixture. The specific surface area of ​​the carbon composite particles reflects the internal and external voids of the carbon composite particles.

[0049] The specific surface area SB (and average pore diameter) of the silicon-containing material B or the powder of carbon composite particles prepared during the manufacture of a nonaqueous electrolyte secondary battery is determined by the following method.

[0050] The pore size distribution of the carbon composite particle powder includes a pore size distribution resulting from voids between the carbon composite particles and a pore distribution resulting from voids within the carbon composite particles, but these can be easily distinguished because they have different pore size ranges. In the pore volume distribution, the pore size X at which the pore volume in the pore size D range of 1 nm to X nm is equal to the pore volume in the pore size D range of X nm to 500 nm is determined, and this is taken as the average pore size of the carbon composite particles.

[0051] A sample of 0.20 g to 0.25 g of carbon composite particles is collected and placed in a measurement cell consisting of a glass tube for specific surface area measurement. The measurement cell is dried and degassed, and then the measurement is performed. The drying and degassing is performed for at least one hour at a pressure of 6.67 Pa and a temperature of 250°C ± 5°C. The mass of the sample in the measurement cell is then measured to the nearest 0.1 mg. The amount of nitrogen adsorbed by the sample at a temperature of -196°C is then measured using a specific surface area measuring device. From the results of the adsorption amount measurement, the specific surface area of ​​the carbon composite particles is determined using the BET multipoint method within a partial pressure (relative pressure) range of 0.03 to 1.0.

[0052] Furthermore, the median diameter at which the cumulative volume is 50% is calculated as the average pore diameter from the volumetric pore diameter distribution of the carbon composite particles determined by the BET multipoint method. The pore diameter distribution is a distribution in the range of 1 nm to 500 nm.

[0053] The specific surface area SA of the carbon material A can be determined in the same manner as above.

[0054] The internal particle porosity of the silicon-containing material or carbon composite particle is 6% or more and 20% or less, preferably 8% or more and 20% or less, and more preferably 8% or more and 12% or less. When the internal particle porosity is 6% or more, a sufficient amount of nonaqueous electrolyte (i.e., lithium ions) can be retained in the internal voids, and a large amount of lithium ions can be smoothly supplied to the silicon phase during charging, thereby maintaining high cycle performance. When the internal particle porosity is 20% or less, high capacity can be maintained, and even when the silicon phase expands and contracts during charging and discharging, contact between the silicon phase and the lithium ion conductive phase can be maintained, improving current collection. In addition, since the strength of the silicon-containing material itself is increased, as described above, the silicon-containing material is less likely to crack during charging and discharging, and the formation of new surfaces is suppressed. As a result, high cycle performance can be maintained over a long period of time, and a long-life nonaqueous electrolyte secondary battery can be realized.

[0055] The density of the silicon-containing material (carbon composite particles) is the true density, which does not take into account the volume occupied by voids between particles of the silicon-containing material. However, the volume occupied by the internal voids of the silicon-containing material is taken into account in the density calculation. In the carbon composite particles, the true densities of the carbon phase and the silicon phase (without taking into account the internal voids) are both 2.2 g / cm in a fully discharged state. ３ Therefore, the internal porosity of the carbon composite particles before forming the negative electrode of the secondary battery is calculated by multiplying the density obtained by a helium pycnometer by the true density (2.2 g / cm) of the carbon phase and the silicon phase. ３ ) can be evaluated by dividing it by the value.

[0056] The internal particle porosity of the negative electrode active material particles is determined by the following method for the negative electrode taken out of the secondary battery after production.

[0057] The internal particle porosity refers to the ratio of the area SC2 of the internal void G1 to the area SC1 of the negative electrode active material particle in the cross section of the negative electrode active material particle, and is calculated by (SC2 / SC1) × 100. The area SC1 of the negative electrode active material particle is the area of ​​the region surrounded by the outline of the negative electrode active material particle, and refers to the combined area of ​​the portion occupied by the negative electrode active material and the internal void G1 contained within the outline. When multiple internal voids G1 are present, the area SC2 of the internal void G1 refers to the total area of ​​the multiple internal voids G1. The internal particle porosity is calculated from a cross-sectional image of a layer containing the negative electrode active material of the negative electrode (negative electrode mixture layer) using the following method.

[0058] First, the discharged secondary battery is disassembled and the negative electrode is removed. Next, the negative electrode is washed with an organic solvent (such as anhydrous ethyl methyl carbonate or dimethyl carbonate), and then vacuum dried. After that, only the negative electrode mixture layer is peeled off to obtain a sample. Next, the cross section of the negative electrode mixture layer is exposed. For example, a method for exposing the cross section is to cut out a portion of the negative electrode and process it with an ion milling device (e.g., IM4000PLUS, manufactured by Hitachi High-Tech Corporation) to obtain a cross section of the negative electrode mixture layer.

[0059] Next, a backscattered electron image of the cross section of the exposed negative electrode mixture layer is taken using a scanning electron microscope. The magnification when taking the backscattered electron image is, for example, 3000 to 5000 times. Particles of the silicon-containing material are selected from the cross-sectional image, and the boundaries of the silicon-containing material particles are determined. Carbon materials such as graphite materials are sometimes used together with the silicon-containing material as the negative electrode active material, but the silicon-containing material and the carbon material can be distinguished because of the different brightnesses of the backscattered electron images. The ratio of the area of ​​the silicon-containing material to the area of ​​the carbon material in the cross section of the negative electrode is considered to be the volume ratio, and the content of the silicon-containing material can also be calculated from the ratio of these areas and the specific gravities of the silicon-containing material and the carbon material.

[0060] The obtained image is subjected to image processing, and binarization is performed by coloring the particle cross sections of the silicon-containing material in the cross-sectional image black and the voids present within the particles white. In the binarization process, the particles of the silicon-containing material (including internal voids) are displayed in black and the voids between the particles are displayed in white in the cross-sectional image.

[0061] Using the binarized cross-sectional image, particles of silicon-containing material with a maximum diameter of 5 μm or more and 50 μm or less are arbitrarily selected, and the area SC1 and the area SC2 of the internal voids are calculated. Note that, for minute voids with a maximum diameter of 3 μm or less, it may be difficult to distinguish between internal voids and external voids in image analysis. For this reason, voids with a maximum diameter of 3 μm or less are considered internal voids.

[0062] Using the particle area SC1 of the silicon-containing material and the area SC2 of the internal voids obtained above, the internal particle porosity (%) expressed by the above formula is calculated for 10 or more silicon-containing material particles, and the average value is taken as the internal particle porosity of the silicon-containing material. As image analysis software for image processing, for example, ImageJ manufactured by the National Institutes of Health may be used.

[0063] It is also possible to calculate the average pore diameter of the silicon-containing material from the shape of the internal voids G1. Specifically, the diameter (equivalent diameter) of a circle having an area equal to the area of ​​the internal voids G1 is determined. The equivalent diameter is determined for each of the multiple voids G1 contained within 10 or more particles of the silicon-containing material, and the average value is used as the average pore diameter of the silicon-containing material. The average pore diameter determined in this way generally coincides with the average pore diameter measured for the powder of the silicon-containing material before it is used to construct the secondary battery.

[0064] The method for controlling the intra-particle porosity, density, and / or single particle fracture strength of silicon-containing materials or carbon composite particles is not particularly limited. For example, the intra-particle porosity may be controlled to a target value by a manufacturing method for the silicon-containing material. Examples of such methods include selecting a material constituting the ion-conductive phase and applying pressure to the silicon-containing material before pulverization while heating it. The heating temperature may be, for example, 300°C to 800°C. For example, if the material used to form the ion-conductive 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-conductive phase, and the greater the intra-particle porosity, specific surface area, and / or average pore size will also tend to be. Furthermore, the single particle fracture strength will also decrease.

[0065] The carbon composite particles can be obtained, for example, by mixing a carbon source and raw silicon, stirring the mixture while crushing it with a stirrer such as a ball mill, and then firing the mixture in an inert atmosphere. Examples of carbon sources that can be used include pitch, tar, sugars, and water-soluble resins. Among these, pitch, carboxymethyl cellulose (CMC), polyvinylpyrrolidone, cellulose, and sucrose are preferred. When mixing the carbon source and raw silicon, the carbon source and raw silicon may be dispersed in a dispersion medium such as alcohol.

[0066] A pore-forming material may be mixed into the mixture of the carbon source and raw silicon. The carbon composite particles can be made porous by removing the pore-forming material from the fired product. The internal porosity of the carbon composite particles can be controlled by the amount of pore-forming material mixed. The pore-forming material is not particularly limited, but for example, a powder that is easily dissolved in an acidic aqueous solution that barely dissolves the silicon phase may be used. Examples of such powder pore-forming agents include oxides of transition metals (e.g., Cu, Fe, etc.). The median diameter in the volume-based particle size distribution of the pore-forming agent is appropriately selected depending on the pore size distribution of the desired carbon composite particles. The acid used in the acidic aqueous solution may be dilute sulfuric acid, for example.

[0067] The content of the silicon-containing material (carbon composite particles) in the entire negative electrode active material may be, for example, 20% by mass or more, 25% by mass or more, or 30% by mass or more in order to achieve a high capacity of 750 Ah / g or more. From the viewpoint of ensuring high cycle characteristics, the content of the silicon-containing material (carbon composite particles) in the negative electrode active material may be, for example, 50% by mass or less, or 40% by mass or less. These lower and upper limits can be combined arbitrarily.

[0068] 1 is a longitudinal cross-sectional view schematically illustrating the structure of a negative electrode (negative electrode mixture layer) containing carbon composite particles as a silicon-containing material in a discharged state, which can be used in a nonaqueous electrolyte secondary battery according to this embodiment. The carbon composite particle 30 is porous and is composed of a carbon phase 301 and a silicon phase 302. The silicon phase 302 is present in a dispersed state within the carbon phase 301. Intra-particle voids 303 exist near the silicon phase 302. Inter-particle voids 33 are formed between the carbon composite particle 30 and other particles. A portion of the inter-particle voids 33 is an external void 34 that is surrounded by the inner wall of the carbon composite particle 30 but is connected to the inter-particle voids 33.

[0069] In FIG. 1 , particles other than those shown as carbon composite particles 30 are not particularly limited, and may be carbon composite particles, particles of a carbon material such as graphite particles, or particles of a silicon-containing material other than carbon composite particles.

[0070] In the discharged state, the volume of the silicon phase (shown here in particulate form, and therefore also referred to as "silicon particles") is small, and many intra-particle voids 303 are present. On the other hand, in the charged state, the silicon particles expand, and the volume of the silicon phase increases. However, because the intra-particle voids 303 maintain sufficient space, expansion of the carbon composite particles 30 themselves is suppressed. Furthermore, when the silicon phase 302 contracts, the volume of the originally existing intra-particle voids 303 is restored, thereby maintaining contact between particles in the negative electrode mixture layer.

[0071] Furthermore, the carbon composite particles 30 can retain a considerable amount of non-aqueous electrolyte (i.e., lithium ions) in their own intra-particle voids 303. Therefore, they can receive lithium ions from the non-aqueous electrolyte retained in both their own intra-particle voids 303 and inter-particle voids 33. Therefore, the supply of lithium ions to the carbon composite particles 30, which require a large amount of lithium ions during charging, is less likely to be delayed.

[0072] An example of the configuration of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in more detail below. Except for the use of the silicon-containing material described above as the negative electrode active material and the nonaqueous electrolyte containing the S-containing cyclic compound component described above, various selections of other components are possible. For example, known components may be used as 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.

[0073] (Negative Electrode) The negative electrode includes a negative electrode mixture. The negative electrode may include a negative electrode mixture and a negative electrode current collector that holds the negative electrode mixture. The negative electrode typically includes a layered negative electrode mixture (hereinafter referred to as a negative electrode mixture layer). The negative electrode mixture includes at least a negative electrode active material. The negative electrode mixture may further include at least one selected from the group consisting of a binder and a thickener. The negative electrode mixture may further include a conductive agent.

[0074] (Negative Electrode Mixture) (Negative Electrode Active Material) The negative electrode active material contains a silicon-free carbon material and a Si-containing material. The Si-containing material may contain carbon composite particles.

[0075] (Si-containing material) The carbon composite particles contain a carbon phase and a Si phase dispersed within the carbon phase. Because the carbon phase has electronic conductivity, even if cracks occur in the carbon composite particles due to the expansion and contraction of the Si phase, the carbon composite particles are less likely to become isolated and contact points between the carbon composite particles and their surroundings are easily maintained. Therefore, deterioration of cycle performance can be easily suppressed.

[0076] The carbon phase may be composed of, for example, amorphous carbon (i.e., amorphous carbon) or crystalline carbon. Amorphous carbon may be, for example, hard carbon, soft carbon, or other. Amorphous carbon generally refers to a carbonaceous material in which the average interplanar spacing d002 of the (002) plane measured by X-ray diffraction exceeds 0.340 nm. Examples of crystalline carbon include carbon having a graphite-type crystal structure, such as graphite. Crystalline carbon such as graphite refers to a carbonaceous material in which d002 is 0.340 nm or less (e.g., 0.3354 nm or more and 0.340 nm or less).

[0077] The content of the Si phase in the carbon composite particles is, for example, 30% by mass to 80% by mass, or 40% by mass to 70% by mass. Within this range, a higher initial capacity can be obtained and deterioration in cycle characteristics can be easily reduced. Furthermore, by including a relatively large amount of carbon phase, even if cracks occur in the particles due to charge and discharge, the carbon phase can easily penetrate into the formed voids, thereby easily maintaining the conductive path in the negative electrode mixture.

[0078] Carbon composite particles can be obtained, for example, by pulverizing a mixture of a carbon source and raw silicon while stirring it in a ball mill or the like to form fine particles, and then heat-treating the mixture in an inert atmosphere. Examples of carbon sources that can be used include petroleum resins such as coal pitch, petroleum pitch, and tar, as well as sugars and water-soluble resins such as carboxymethyl cellulose (CMC), polyvinylpyrrolidone, cellulose, and sucrose. When mixing the carbon source and raw silicon, the carbon source and raw silicon may be dispersed in a dispersion medium such as alcohol. The milled mixture is dried and then heated in an inert gas atmosphere, for example, at a temperature of 600°C or higher and 1000°C or lower, to carbonize the carbon source and form a carbon phase.

[0079] The negative electrode active material may contain a Si-containing material other than the carbon composite particles, such as elemental silicon, a silicon alloy, or a silicon compound.

[0080] The Si-containing material may contain, in addition to the carbon composite particles, composite particles other than the carbon composite particles. Examples of such composite particles include composite particles in which a Si phase (fine Si phase) is dispersed within a lithium ion conductive phase (matrix). When the Si-containing material contains such composite particles, a higher capacity can be obtained and the effect of suppressing the deterioration of cycle characteristics can be enhanced.

[0081] The lithium ion conducting phase is SiO ２ It is preferable that the composite particles contain at least one selected from the group consisting of a silicate phase and a silicate phase. The lithium ion conductive phase may further contain a carbon phase. The lithium ion conductive phase may form an amorphous phase. However, this is not limited to this case, and for example, at least a portion of each of the silicate phase and the carbon phase may be a crystalline phase containing crystalline silicate or crystalline carbon as described for the carbon composite particles. Specific examples of composite particles include SiO ２ phase and SiO ２ Examples of the composite particles include composite particles containing a silicate phase and a Si phase dispersed within the silicate phase (silicate composite particles). However, the composite particles are not limited to these specific examples.

[0082] SiO ２ The phase is an amorphous phase containing 95% or more by mass of silicon dioxide. ２ Composite particles with dispersed Si phases are SiO ｘ where x is, for example, 0.5≦x<2, and may be 0.8≦x≦1.6. ｘ is, for example, obtained by heat treating silicon monoxide to produce SiO ２ The SiO phase and the fine Si phase are separated using a transmission electron microscope (TEM). ｘ When observing the particle cross section, SiO ２The Si phase dispersed within the composite particles can be seen. Such composite particles are sometimes referred to as silicon oxides in this specification. When the negative electrode active material contains silicon oxide, it is easy to ensure a higher initial discharge capacity.

[0083] The silicate phase preferably contains at least one of an alkali metal element (a Group 1 element other than hydrogen in the long periodic table) and a Group 2 element in the long periodic table. The alkali metal element includes lithium (Li), potassium (K), sodium (Na), etc. The Group 2 element includes magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), etc. The lithium silicate phase has the formula: Li ２ｙ SiO ２＋ｙ The silicate composite particles may have a composition expressed as (0<y<2), where y may be 1 / 2 or 1. The silicate composite particles in which the Si phase is dispersed within the silicate phase can be obtained, for example, by pulverizing a mixture of silicate and raw silicon while stirring it in a ball mill or the like to form fine particles, and then heat-treating the mixture in an inert atmosphere.

[0084] The Si-containing material may contain only carbon composite particles, or may contain a combination of carbon composite particles and at least one other Si-containing material. For example, the Si-containing material may contain, in addition to the carbon composite particles, at least one selected from the group consisting of silicon oxide and silicate composite particles.

[0085] The composition of the Si-containing material can be determined, for example, by obtaining a backscattered electron image of the cross section of the negative electrode mixture layer using a field emission scanning electron microscope (FE-SEM), observing the particles of the Si-containing material, and performing elemental analysis on the observed particles of the Si-containing material. For example, the battery is disassembled, the negative electrode is removed, washed with a non-aqueous solvent such as ethylene carbonate, dried, and then the cross section of the negative electrode mixture layer is processed using a cross section polisher (CP) to obtain a sample. A backscattered electron image of the cross section of the sample is taken using the FE-SEM. For elemental analysis, for example, an electron probe microanalyzer (EPMA) analysis or the like is used. Qualitative and quantitative analysis of elements may be performed using an Auger Electron Spectroscopy (AES) analyzer. The composition of the lithium ion conductive phase can also be determined by this analysis. The composition of the carbon phase can be confirmed based on d002 determined by X-ray diffraction.

[0086] The Si-containing material is usually a particulate material. The average particle diameter (D50) of the Si-containing material may be 0.5 μm or more and 7.0 μm or less. Within this range, excellent rapid charge / discharge characteristics and good battery performance are likely to be obtained.

[0087] The Si phase dispersed in the carbon phase is usually composed of multiple crystallites. The crystallite size of the Si phase is, for example, 500 nm or less, and may be 30 nm or less. The lower limit of the crystallite size of the Si phase is not particularly limited, but is, for example, 5 nm or more. The crystallite size is calculated by the Scherrer formula from the half-width of the diffraction peak assigned to the Si (111) plane in the X-ray diffraction (XRD) pattern of the Si phase.

[0088] The content of the Si phase contained in the composite particles can be measured, for example, by Si-NMR. Desirable measurement conditions for Si-NMR are shown below.

[0089] Measurement equipment: Varian solid-state nuclear magnetic resonance spectrometer (INOVA-400) Probe: Varian 7 mm CPMAS-2 MAS: 4.2 kHz MAS speed: 4 kHz Pulse: DD (45° pulse + signal acquisition time 1 H decoupled) Repetition time: 1200 sec Observation width: 100 kHz Observation center: near -100 ppm Signal acquisition time: 0.05 sec Number of accumulations: 560 Sample amount: 207.6 mg

[0090] From the viewpoint of improving electrical conductivity, at least a portion of the particle surface of the Si-containing material may be coated with a conductive layer. The conductive layer contains a conductive material such as conductive carbon. The amount of the conductive layer is, for example, 1 part by mass or more and 10 parts by mass or less per 100 parts by mass of the total of the particles of the Si-containing material and the conductive layer. The particles of the Si-containing material having a conductive layer on their surface can be obtained, for example, by mixing coal pitch or the like with the particles of the Si-containing material and heat treating the mixture in an inert atmosphere.

[0091] (Carbon material) Carbon materials expand and contract less during charging and discharging than Si-containing materials. In contrast, Si-containing materials expand and contract in volume during charging and discharging. Therefore, when the Si-containing material accounts for a large proportion of the negative electrode active material, poor contact between the negative electrode active material and the negative electrode current collector during charging and discharging is likely to occur. However, by using a Si-containing material in combination with a carbonaceous material, the contact state between the negative electrode active material particles and between the negative electrode mixture and the negative electrode current collector can be maintained better during repeated charging and discharging. Therefore, by using a Si-containing material in combination with a carbonaceous material that does not contain a Si phase, it is easy to obtain excellent cycle characteristics while imparting the high capacity of the Si phase to the negative electrode.

[0092] Examples of the carbon material include graphite, easily graphitizable carbon (soft carbon), and non-graphitizable carbon (hard carbon). One type of carbon material may be used alone, or two or more types may be used in combination.

[0093] Among these, graphite is preferred as the carbon material because of its excellent charge / discharge stability and low irreversible capacity. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles. The graphite particles may partially contain amorphous carbon, graphitizable carbon, or non-graphitizable carbon.

[0094] Graphite is a carbonaceous material with a developed graphite crystal structure. The average interplanar spacing d002 of the (002) plane of graphite measured by X-ray diffraction may be, for example, 0.340 nm or less, or 0.3354 nm or more and 0.340 nm or less. The crystallite size Lc(002) of graphite may be, for example, 5 nm or more, or 5 nm or more and 200 nm or less. The crystallite size Lc(002) is measured, for example, by the Scherrer method. When the average interplanar spacing d002 of the (002) plane of graphite and the crystallite size Lc(002) are within the above ranges, high capacity is easily obtained.

[0095] (Other Negative Electrode Active Materials) Examples of negative electrode active materials other than the Si-containing material and the carbon material include at least one selected from the group consisting of simple Sn, Sn alloys, and Sn compounds (such as Sn oxides).

[0096] The total amount of the Si-containing material and the carbon material (carbon material not containing an Si phase) in the negative electrode active material is preferably 90% by mass or more, and may be 95% by mass or more or 98% by mass or more. The total amount of the Si-containing material and the carbon material in the negative electrode active material is 100% by mass or less. The negative electrode active material may be composed only of the Si-containing material and the carbon material.

[0097] (Binder) As the binder, for example, a resin material is used. Examples of binders include fluororesins (e.g., polytetrafluoroethylene, polyvinylidene fluoride), polyolefin resins (e.g., polyethylene, polypropylene), polyamide resins (e.g., aramid resin), polyimide resins (e.g., polyimide, polyamideimide), acrylic resins (e.g., polyacrylic acid, polymethacrylic acid, acrylic acid-methacrylic acid copolymer, ethylene-acrylic acid copolymer, or salts thereof), vinyl resins (e.g., polyvinyl acetate), and rubber-like materials (e.g., styrene-butadiene copolymer rubber (SBR)). One type of binder may be used alone, or two or more types may be used in combination.

[0098] (Thickener) Examples of thickeners include cellulose derivatives such as cellulose ether. Examples of cellulose derivatives include CMC and its modified products, methyl cellulose, etc. Modified CMC also includes salts of CMC. Examples of salts include alkali metal salts (e.g., sodium salts), ammonium salts, etc. One type of thickener may be used alone, or two or more types may be used in combination.

[0099] (Conductive Agent) Examples of the conductive agent include conductive fibers and conductive particles. Examples of the conductive fibers include carbon fibers and metal fibers. Carbon fibers also include carbon nanotubes (CNT). Examples of the conductive particles include conductive carbon (carbon black, etc.) and metal powder. One type of conductive agent may be used alone, or two or more types may be used in combination.

[0100] The volume change of the Si-containing material is large due to expansion and contraction during charging and discharging. When the negative electrode mixture contains CNTs, even if cracks occur in the particles due to expansion and contraction of the Si-containing material, the CNTs suppress the disconnection of the conductive path, making it easier to obtain higher cycle characteristics. In particular, when the content of the Si-containing material in the negative electrode active material is high (for example, when it is 4 mass% or more), the effect of CNTs is significantly observed.

[0101] CNTs are carbonaceous materials with nanometer diameters that have a cylindrical structure in which a sheet of graphene, a six-membered ring network formed by carbon atoms, is rolled up. CNTs have excellent electrical conductivity. When the number of graphene layers that make up the cylindrical structure is one, they are called single-walled carbon nanotubes (SWCNTs). When the number of layers is multiple, they are called multi-walled carbon nanotubes (MWCNTs).

[0102] The CNT preferably includes SWCNT, which makes it easier to ensure higher cycle characteristics.

[0103] The proportion of SWCNTs in the CNTs is, for example, 50% or more, or may be 75% or more, or 90% or more. The proportion of SWCNTs in the CNTs is 100% or less. The proportion of SWCNTs in the CNTs is the ratio of the number of SWCNTs to the total number of CNTs.

[0104] The presence of CNTs in the negative electrode mixture can be confirmed, for example, by an image of a cross section of the negative electrode mixture layer taken with a scanning electron microscope (SEM).

[0105] The proportion of SWCNTs in the CNTs contained in the negative electrode mixture can be determined by the following method: An image of the cross section of the negative electrode mixture layer or the CNTs is obtained using an SEM. A number of CNTs (e.g., 50 to 200) are randomly selected and observed in the SEM image, the number of SWCNTs is determined, and the proportion of the number of SWCNTs to the total number of selected CNTs is calculated.

[0106] Quantitative analysis of CNTs is performed, for example, by combining Raman spectroscopy and thermogravimetric analysis.

[0107] From the viewpoint of reducing disconnection of the conductive path during charging and discharging, the average diameter of the CNTs may be, for example, 1 nm or more and 10 nm or less, or 1 nm or more and 5 nm or less.

[0108] From the viewpoint of reducing disconnection of the conductive path during charging and discharging, the average length of the CNTs may be, for example, 1 μm or more and 100 μm or less, or 5 μm or more and 20 μm or less.

[0109] The average length and average diameter of the CNTs can be determined from an image of the cross section of the negative electrode mixture layer or the CNTs using at least one of an SEM and a TEM. More specifically, in the captured image, a plurality of CNTs (e.g., 50 to 200) are arbitrarily selected, and their lengths and diameters are measured and averaged to determine the average length and average diameter. The length of the CNTs refers to the length of the CNTs when they are stretched linearly.

[0110] The CNT content in the negative electrode mixture is, for example, 0.005% by mass to 1% by mass, or 0.01% by mass to 1% by mass, or 0.01% by mass to 0.05% by mass. When the CNT content in the negative electrode mixture is in such a range, the conductivity of the negative electrode is improved and the capacity retention rate at the beginning of the charge-discharge cycle is significantly improved.

[0111] (Negative electrode current collector) The negative electrode current collector is selected depending on the type of nonaqueous electrolyte secondary battery. Examples of the negative electrode current collector include a sheet-shaped current collector. Metal foil or the like may also be used as the current collector. Alternatively, a porous current collector may also be used as the current collector. Examples of the porous current collector include a mesh, a punched sheet, and an expanded metal.

[0112] Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy.

[0113] The thickness of the negative electrode current collector is not particularly limited, and may be, for example, 1 μm or more and 50 μm or less, or 5 μm or more and 30 μm or less.

[0114] (Others) The negative electrode can be formed, for example, by applying a negative electrode slurry, in which the components of the negative electrode mixture are dispersed in a dispersion medium, to the surface of a negative electrode current collector and drying the applied film. The dried coating film may be rolled, if necessary.

[0115] The dispersion medium is not particularly limited, and examples thereof include water, alcohol (e.g., ethanol), ether (e.g., tetrahydrofuran), amide (e.g., dimethylformamide), N-methyl-2-pyrrolidone (NMP), and mixed solvents thereof.

[0116] (Positive Electrode) The positive electrode may include a positive electrode current collector and a positive electrode mixture layer held 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 layer. The dried coating film may be rolled as necessary. The positive electrode mixture contains a positive electrode active material as an essential component, and may contain a binder, a conductive agent, etc. as optional components. The dispersion medium can be selected, for example, from the dispersion media exemplified for the negative electrode.

[0117] As the positive electrode active material, for example, a composite oxide containing lithium and a transition metal is used. Examples of the transition metal include Ni, Co, and Mn. Examples of the composite oxide containing lithium and a transition metal include Li ａ CoO ２ , Li ａ NiO ２ , Li ａ MnO ２ , Li ａ Co ｂ１ Ni １－ｂ１ O ２ , Li ａ Co ｂ１ M １－ｂ１ O ｃ１ , Li ａ Ni １－ｂ１ M ｂ１ O ｃ１ , Li ａ Mn ２ O ４ , Li ａ Mn ２－ｂ１ M ｂ１ O ４ Here, a = 0 to 1.2, b1 = 0 to 0.9, and c1 = 2.0 to 2.3. 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. The value a, which indicates the molar ratio of lithium, increases or decreases with charge and discharge.

[0118] Among them, Li ａ Ni ｂ２ M １－ｂ２ O ２ (0<a≦1.2, 0.3≦b2≦1, and M is at least one selected from the group consisting of Mn, Co, and Al) is preferred. From the viewpoint of increasing capacity, it is more preferred that 0.8≦b2≦1 or 0.85≦b2≦1 is satisfied. From the viewpoint of stability of the crystal structure, Li ａ Ni ｂ２ Co ｃ２ Al ｄ O ２ (0<a≦1.2, 0.8≦b2<1, 0<c2<0.2 (or 0<c2≦0.18), 0<d≦0.1, b2+c2+d=1) is more preferable.

[0119] The binder may be a resin material exemplified for the negative electrode. The conductive agent may be selected from the conductive agents exemplified for the negative electrode. Graphite may also be used as the conductive agent.

[0120] The shape and thickness of the positive electrode current collector can be selected from the shapes and ranges described for the negative electrode current collector. Examples of materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloys, and titanium.

[0121] (Separator) It is usually 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. Examples of the separator include a microporous thin film, a woven fabric, and a nonwoven fabric. The separator may have a single-layer structure or a multilayer structure. A multilayer separator may be a laminate containing at least two layers selected from the group consisting of a microporous thin film, a woven fabric, and a nonwoven fabric. The separator is preferably made of polyolefin (e.g., polypropylene, polyethylene).

[0122] (Non-aqueous electrolyte) The non-aqueous electrolyte is usually used in a liquid state, but its fluidity may be limited by a gelling agent or the like. The non-aqueous electrolyte usually contains a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The non-aqueous electrolyte may further contain an additive.

[0123] (Non-aqueous Solvent) Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC). 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). Examples of chain carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate (MA), ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. The non-aqueous electrolyte may contain one type of non-aqueous solvent or a combination of two or more types.

[0124] (Lithium Salt) Examples of lithium salts include LiClO ４ , LiBF ４ , LiPF ６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０ , lower aliphatic lithium carboxylates, LiCl, LiBr, LiI, phosphates, borates, and imide salts. Examples of phosphates include lithium difluorophosphate (LiPO ２ F ２Examples of the borate salt include lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxalato)borate (LiDFFOB). Examples of the imide salt include lithium bisfluorosulfonylimide (LiN(FSO ２ ) ２ ), lithium bistrifluoromethanesulfonyl imide (LiN(CF ３ SO ２ ) ２ ), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), lithium bispentafluoroethanesulfonyl imide (LiN(C ２ F ５ SO ２ ) ２ The non-aqueous electrolyte may contain one type of lithium salt or a combination of two or more types of lithium salts.

[0125] The concentration of the lithium salt in the electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0126] (Additive) The non-aqueous electrolyte may contain an S-containing cyclic compound component as an additive. The S-containing cyclic compound component is a cyclic compound component containing an S element as a ring constituent element. The S-containing cyclic compound may, for example, contain an oxygen atom as a ring constituent element, or may contain an oxo group (═O) bonded to the ring as a substituent, or may contain both. The oxo group may be bonded to a carbon element constituting the ring, but is preferably bonded to an S element constituting the ring.

[0127] Such an S-containing cyclic compound may be, for example, at least one selected from the group consisting of sulfate esters, sulfite esters, and sulfonate esters. Among these, cyclic sulfite esters and cyclic sulfonate esters are preferred. The cyclic compound may also be a salt of these esters.

[0128] Examples of cyclic sulfates include alkylene sulfates and alkenylene sulfates. Specific examples of cyclic sulfates include ethylene sulfate, propylene sulfate, trimethylene sulfate, butylene sulfate, and vinylene sulfate. Examples of cyclic sulfites include at least one selected from the group consisting of alkylene sulfites and alkenylene sulfites. Specific examples of cyclic sulfites include ethylene sulfite, propylene sulfite, trimethylene sulfite, butylene sulfite, and vinylene sulfite. Examples of cyclic sulfonates include at least one selected from the group consisting of alkanesultones and alkenesultones. Specific examples of cyclic sulfonates include 1,3-propane sultone, 1,4-butane sultone, and 1,3-propene sultone. The S-containing cyclic compounds may be those in which one or more hydrogen atoms of the compounds exemplified above are substituted with a substituent. Examples of the substituent include an alkyl group, an alkenyl group, a hydroxyalkyl group, a hydroxy group, an alkoxy group, and a halogen atom. The number of carbon atoms in the substituent may be 1 to 4, or 1 to 3. Examples of the halogen atom include a chlorine atom and a fluorine atom.

[0129] The S-containing ring contained in the S-containing cyclic compound is usually 5- or 6-membered. The S-containing cyclic compound component preferably contains an S-containing cyclic compound having a carbon-carbon unsaturated bond. The carbon-carbon unsaturated bond may constitute a part of the S-containing ring, or may be contained in a substituent of the S-containing ring.

[0130] The S-containing cyclic compound forms a coating on the surface of the silicon-containing material, which is the negative electrode active material. This prevents side reactions between the silicon-containing material and the nonaqueous electrolyte when the silicon-containing material expands and contracts due to the absorption and desorption of lithium ions, causing cracks in the active material particles and forming new surfaces. As a result, the capacity retention rate and cycle performance are prevented from decreasing with repeated charge and discharge.

[0131] In a nonaqueous electrolyte secondary battery, the concentration of the S-containing cyclic compound component in the nonaqueous electrolyte is, for example, 2% by mass or less, and may be 1% by mass or less, in an initial nonaqueous electrolyte secondary battery. The initial nonaqueous electrolyte secondary battery is, for example, a nonaqueous electrolyte secondary battery after assembly and running-in charge / discharge (and aging as necessary). A commercially available nonaqueous electrolyte secondary battery may be used as the initial nonaqueous electrolyte secondary battery, and the nonaqueous electrolyte may be collected and subjected to analysis.

[0132] In non-aqueous electrolyte secondary batteries, the S-containing cyclic compound component is used to form a coating, and therefore the concentration of the S-containing cyclic compound component in the non-aqueous electrolyte changes during storage or during charge-discharge cycles. Therefore, it is sufficient that the S-containing cyclic compound component remains in the non-aqueous electrolyte sampled from an initial non-aqueous electrolyte secondary battery at a concentration equal to or greater than the detection limit. The content of the S-containing cyclic compound component in the electrolyte may be 0.01% by mass or more, 0.1% by mass or more, 0.25% by mass or more, or 0.5% by mass or more. The concentration of 1,3-propene sultone may be within the above range.

[0133] The non-aqueous electrolyte may contain additives other than the S-containing cyclic compound component. Examples of such additives include sulfur-containing compounds other than the S-containing cyclic compound component, phosphorus-containing compounds, nitrogen-containing compounds, vinyl ethylene carbonate, FEC, and aromatic compounds (cyclohexylbenzene, fluorobenzene, etc.). Examples of sulfur-containing compounds (S-containing compounds) include at least one selected from the group consisting of chain sulfate esters (ethyl sulfate, methyl sulfate, etc.), chain sulfite esters, and chain sulfonate esters. Examples of S-containing compounds include salts of these esters (ethyl sulfate, methyl sulfate, etc.). The non-aqueous electrolyte may contain one of these additives or a combination of two or more of them.

[0134] The nonaqueous electrolyte secondary battery preferably contains FEC, which tends to provide better cycle characteristics. FEC may be contained in a small amount (e.g., 0.1% by mass or more and 2% by mass or less) as an additive, or in a relatively large amount (e.g., more than 2% by mass) as a nonaqueous solvent in the nonaqueous electrolyte.

[0135] (Other) An example of the structure of a non-aqueous electrolyte secondary battery is a structure in which an electrode group in which a positive electrode and a negative electrode are wound with a separator interposed therebetween is housed in an exterior body together with a non-aqueous electrolyte. However, the structure of a non-aqueous electrolyte secondary battery is not limited to this structure. For example, the electrode group may be a laminate type in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween. The shape of the non-aqueous electrolyte secondary battery is also not limited, and may be, for example, a cylindrical type, a prismatic type, a coin type, a button type, or a laminate type.

[0136] FIG. 2 is a schematic perspective view, partially cut away, of a prismatic nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure. The nonaqueous electrolyte secondary battery includes a bottomed prismatic battery case 4, an electrode group 1, and an electrolyte (not shown) housed within 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 negative electrode current collector is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. That is, the positive electrode is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The periphery of the sealing plate 5 fits into the open end of the battery case 4, and the fitting is laser-welded. The sealing plate 5 has an electrolyte injection hole, which is closed with a seal plug 8 after injection.

[0137] <<Appendix>> The above embodiments disclose the following techniques. (Technology 1) A negative electrode mixture for a non-aqueous electrolyte secondary battery, comprising a negative electrode active material, the negative electrode active material comprising a silicon-free carbon material A and a silicon-containing material B, the discharge capacity density of the negative electrode active material being 750 Ah / g or more and 1220 Ah / g or less, the silicon-containing material B comprising carbon composite particles comprising a carbon phase and a silicon phase dispersed within the carbon phase, and a ratio SB / SA of the specific surface area of ​​the silicon-containing material B to the specific surface area SA of the carbon material A being 0.7 or more and 19 or less. (Technology 2) The negative electrode mixture according to Technology 1, wherein the average particle diameter D50 of the silicon-containing material B is 0.5 μm or more and 7.0 μm or less. (Technology 3) The negative electrode mixture according to Technology 1 or Technology 2, wherein the ratio MB / MA of the mass MB of the silicon-containing material B to the mass MA of the carbon material A is 0.4 or more and 0.7 or less. (Technology 4) The density of the silicon-containing material B is 1.4 g / cm ３ Above, 2.1g / cm ３ The negative electrode mixture according to any one of Techniques 1 to 3, wherein the silicon-containing material has a single particle fracture strength of 100 MPa or more and 350 MPa or less. (Technology 5) The negative electrode mixture according to any one of Techniques 1 to 4, wherein the silicon-containing material has a single particle fracture strength of 100 MPa or more and 350 MPa or less. (Technology 6) A non-aqueous electrolyte secondary battery comprising: a negative electrode containing the negative electrode mixture according to any one of Techniques 1 to 5; a separator; a positive electrode facing the negative electrode with the separator interposed therebetween; and a non-aqueous electrolyte.

[0138] EXAMPLES The present invention will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0139] Examples 1-7 and Comparative Examples 1-3: Non-aqueous electrolyte secondary batteries were fabricated and evaluated according to the following procedure. (1) Fabrication of Negative Electrode: Pitch, a carbon source, and coarse particles of raw silicon (purity 99.9% or higher, average particle size 100 nm) were mixed in a predetermined ratio. 5-30 parts by mass of iron oxide (FeO) per 100 parts by mass of the mixture was added as a pore-forming agent and further mixed. The mixture was then fired at 600-1000°C for 1-10 hours in an inert gas atmosphere (non-oxidizing atmosphere) such as nitrogen or argon to obtain a sintered mass. The resulting fired mass was pulverized using a ball mill to the desired average particle size (median size), washed with an acidic aqueous solution containing sulfuric acid, and then dried to obtain carbon composite particles (silicon-containing material B) containing a carbon phase and a Si phase dispersed within the carbon phase. The pitch used was a petroleum-derived organic material with a softening point of 200°C and a fixed carbon content of 85%. The average particle size (D50) of the carbon composite particles was adjusted to a target value by pulverization using a ball mill.

[0140] The composition of the carbon composite particles was analyzed by ICP (inductively coupled plasma atomic emission spectroscopy), and the silicon content was found to be 50.1 mass %.

[0141] A mixture of graphite particles (carbon material A) and carbon composite particles was used as the negative electrode active material. The graphite particles used had an average particle size (D50) of 25 μm. The specific surface area SA of the graphite particles (carbon material A) was 4 m ２ / g.

[0142] The specific surface area SB of the carbon composite particles, which are silicon-containing material B, was adjusted to a target value by adjusting the content of iron oxide (FeO) in the mixture to be fired according to the average particle size of the carbon composite particles. The lower the fixed carbon content of the pitch, the more components are volatilized when heated and carbonized, resulting in a larger amount of internal voids, a larger internal particle porosity, and a larger specific surface area and / or average pore diameter. Furthermore, the higher the content of iron oxide, the more iron oxide is removed when washed with an acidic aqueous solution, resulting in a larger internal particle porosity, a larger specific surface area and / or average pore diameter.

[0143] The negative electrode active material, binder, and conductive agent were mixed to form a negative electrode mixture, and an appropriate amount of water was added to the mixture to obtain a negative electrode slurry. The binders used were sodium polyacrylate (PAA-Na), a sodium salt of CMC (CMC-Na), and SBR. The conductive agent used was CNT (average diameter approximately 1.6 nm, average length approximately 5 μm) containing 90% or more SWCNT.

[0144] The CNT content in the negative electrode mixture (dry solid content) was 0.05 mass %, and the PAA-Na, CMC-Na, and SBR contents in the negative electrode mixture were each 1 mass % in terms of dry solid content.

[0145] Next, the negative electrode slurry was applied to the surface of the copper foil, the coating was dried, and then the copper foil was rolled to form a negative electrode mixture layer (thickness 80 μm, density 1.6 g / cm ) on both sides of the copper foil. ３ ) was formed to obtain a negative electrode.

[0146] (2) Preparation of the positive electrode: Lithium-containing composite oxide (LiNi ０．８ Co ０．１８ Al ０．０２ O ２ To 95 parts by mass of acetylene black, 2.5 parts by mass of polyvinylidene fluoride, and an appropriate amount of NMP were added and mixed to obtain a positive electrode slurry. Next, the positive electrode slurry was applied to the surface of an aluminum foil, the coating was dried, and then rolled to form a positive electrode mixture layer (thickness 95 μm, density 3.6 g / cm ) on both sides of the aluminum foil. ３ ) was formed to obtain a positive electrode.

[0147] (3) Preparation of non-aqueous electrolyte: A mixed solvent of EC, DMC, and MA (EC:DMC:MA = 20:60:20 (volume ratio)) was added to LiPF ６ The non-aqueous electrolyte was prepared by dissolving LiPF in the non-aqueous electrolyte and mixing with FEC. ６ The concentration of FEC in the non-aqueous electrolyte was 1 mass %.

[0148] (4) Fabrication of a Non-Aqueous Electrolyte Secondary Battery: An Al positive electrode lead was attached to the positive electrode obtained above, and a Ni negative electrode lead was attached to the negative electrode obtained above. The positive electrode and negative electrode were spirally wound with a polyethylene thin film (separator) interposed therebetween in an inert gas atmosphere to fabricate a wound electrode assembly. The electrode assembly was housed in a bag-shaped exterior body formed of a laminate sheet with an Al layer, and a predetermined amount of the electrolyte solution was injected. The exterior body was then sealed to fabricate a non-aqueous electrolyte secondary battery. When the electrode assembly was housed in the exterior body, portions of the positive electrode lead and the negative electrode lead were exposed to the outside from the exterior body.

[0149] A plurality of types of nonaqueous electrolyte secondary batteries with a rated capacity of 0.3 Ah were fabricated while changing the average particle size (D50), specific surface area, and / or the mixing ratio of graphite particles and carbon composite particles in the negative electrode active material of the carbon composite particles, which are silicon-containing material B, to obtain secondary batteries A1 to A7 and B1 to B3 according to Examples 1 to 7 and Comparative Examples 1 to 3, respectively.

[0150] For secondary batteries A1 to A5, and B3 according to Examples 1 to 5 and Comparative Example 3, graphite particles and carbon composite particles were mixed in a mass ratio of graphite particles:carbon composite particles of 1:0.4 (MB / MA=0.4), resulting in a negative electrode active material with a discharge capacity density of 750 Ah / g. For secondary batteries A6 and A7 according to Examples 6 and 7, graphite particles and carbon composite particles were mixed in a mass ratio of graphite particles:carbon composite particles of 1:0.7 (MB / MA=0.7), resulting in a negative electrode active material with a discharge capacity density of 1220 Ah / g. For secondary battery B1 according to Comparative Example 1, graphite particles and carbon composite particles were mixed in a mass ratio of graphite particles:carbon composite particles of 90:10 (MB / MA=0.11), resulting in a negative electrode active material with a discharge capacity density of 500 Ah / g. In secondary battery B2 according to Comparative Example 2, graphite particles and carbon composite particles were mixed in a mass ratio of graphite particles:carbon composite particles = 80:20 (MB / MA = 0.25) to obtain a negative electrode active material with a discharge capacity density of 600 Ah / g. However, in Examples 1 to 5 and Comparative Example 3, the total capacity (Ah) of the negative electrode was the same, and the amount of negative electrode active material loaded (the amount of negative electrode slurry applied per area of ​​the negative electrode) was changed so that the total capacity (Ah) of the negative electrode was the same.

[0151] Examples 8 to 11 In the preparation of a negative electrode, a petroleum-derived organic pitch with a softening point of 100°C and a fixed carbon content of 70% was used to obtain carbon composite particles. The average particle size (D50) and specific surface area of ​​the carbon composite particles were adjusted to target values. Except for this, multiple types of nonaqueous electrolyte secondary batteries were prepared in the same manner as in Example 1, with different average particle sizes (D50) and specific surface areas of carbon composite particles, which were silicon-containing material B, to obtain secondary batteries A8 to A11 according to Examples 8 to 11, respectively. The mixing ratio of graphite particles to carbon composite particles in the negative electrode active material was the same as in Example 1, and the discharge capacity of the negative electrode active material was 750 Ah / g.

[0152] <Evaluation> The following evaluations were performed using the obtained nonaqueous electrolyte secondary battery. (1) Initial capacity In a 25°C environment, the nonaqueous electrolyte secondary battery was subjected to constant current charging at a current of 0.1 C until the voltage reached 4.2 V. Thereafter, constant voltage charging was performed at a voltage of 4.2 V until the current reached 0.02 C or less. After a 10-minute rest, the nonaqueous electrolyte secondary battery was subjected to constant current discharging at a current of 0.1 C until the voltage reached 2.5 V. The discharge capacity (Ci) at this time was determined as the 0.1 C capacity.

[0153] (2) Cycle Characteristics: The cycle of charge, rest, and discharge used to calculate the discharge capacity Ci was counted as one cycle, and 100 cycles were repeated to calculate the discharge capacity (Cc) at the 100th cycle. The ratio (%) of the discharge capacity Cc to the initial discharge capacity Ci was calculated as the capacity retention rate X, which was used as an index of cycle characteristics. Capacity retention rate X (%) = (Cc / Ci) x 100

[0154] (3) Rapid Discharge Characteristics In a 25°C environment, the nonaqueous electrolyte secondary battery was subjected to constant current charging at a current of 0.1 C until the voltage reached 4.2 V. Thereafter, constant voltage charging was performed at a voltage of 4.2 V until the current reached 0.02 C or less. After a 10-minute rest, constant current discharging was performed at a current of 2 C until the voltage of the nonaqueous electrolyte secondary battery reached 2.5 V. The discharge capacity (Cx) at this time was calculated as the 2 C capacity. The ratio (%) of the 2 C capacity Cx to the 0.1 C capacity Ci was calculated as the rapid discharge performance Y, which was used as an index of rapid discharge performance. Rapid discharge performance Y (%) = (Cx / Ci) × 100

[0155] The results of the examples and comparative examples are shown in Table 1, together with the average particle diameter (D50) of the carbon composite particles and the ratio SB / SA of the specific surface area SB of the carbon composite particles to the specific surface area SA of the graphite particles. In Table 1, A1 to A11 correspond to examples 1 to 11, and B1 to B3 correspond to comparative examples 1 to 3. The specific surface area SB was measured using a mercury porosimeter for the powder of the carbon composite particles immediately after production.

[0156]

[0157] As shown in Table 1, in the batteries A1 to A11 in which the ratio SB / SA of the specific surface area of ​​the carbon composite particles (silicon-containing material B) to the specific surface area SA of the graphite particles (carbon material A) was set to 0.7 or more and 19 or less, a high cycle retention rate was obtained and the rapid discharge characteristics were improved even when the discharge capacity density of the negative electrode active material was set to a high capacity of 750 Ah / g or more and 1220 Ah / g or less.

[0158] The nonaqueous electrolyte secondary battery of the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, etc. However, these are merely examples, and the uses of the nonaqueous electrolyte secondary battery are not limited to these.

[0159] 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.

[0160] 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 30: Carbon composite particle 301: Carbon phase 302: Silicon phase 303: Intra-particle void 33: Inter-particle void 34: External void

Claims

1. A negative electrode mixture for a non-aqueous electrolyte secondary battery, comprising: a negative electrode active material comprising a silicon-free carbon material A and a silicon-containing material B; a discharge capacity density of the negative electrode active material being 750 Ah / g or more and 1220 Ah / g or less; the silicon-containing material B comprising carbon composite particles comprising a carbon phase and a silicon phase dispersed within the carbon phase; and a ratio SB / SA of the specific surface area of ​​the silicon-containing material B to the specific surface area SA of the carbon material A being 0.7 or more and 19 or less.

2. The negative electrode mixture according to claim 1, wherein the silicon-containing material B has an average particle diameter D50 of 0.5 μm or more and 7.0 μm or less.

3. The negative electrode mixture according to claim 1, wherein the ratio MB / MA of the mass MB of the silicon-containing material B to the mass MA of the carbon material A is 0.4 or more and 0.7 or less.

4. The density of the silicon-containing material B is 1.4 g / cm ３ Above, 2.1g / cm ３ The negative electrode mixture according to claim 1 , wherein:

5. The negative electrode mixture according to claim 1, wherein the silicon-containing material has a single particle fracture strength of 100 MPa or more and 350 MPa or less.

6. A non-aqueous electrolyte secondary battery comprising: a negative electrode containing the negative electrode mixture according to any one of claims 1 to 5; a separator; a positive electrode facing the negative electrode with the separator interposed therebetween; and a non-aqueous electrolyte.

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