Negative active material for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery

Abstract

A negative electrode active material for a nonaqueous electrolyte secondary battery has composite particles that include a lithium silicate phase, a silicon phase dispersed within the lithium silicate phase, and a crystalline phase of silicon dioxide dispersed within the lithium silicate phase. The crystalline phase of silicon dioxide includes β-cristobalite and quartz.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to improvements in a negative electrode active material for a nonaqueous electrolyte secondary battery. BACKGROUND

[0002] In recent years, nonaqueous electrolyte secondary batteries have high voltage and high energy density, and are therefore expected to be used as power sources for small-sized civilian applications, power storage devices, and electric vehicles. In the process of requiring high energy density of batteries, the use of a material containing silicon that alloyizes with lithium is expected as a negative electrode active material having a high theoretical capacity density.

[0003] However, the material containing silicon has a large irreversible capacity, and therefore has a problem of low initial charge-discharge efficiency. Therefore, various techniques have been proposed in which lithium equivalent to the irreversible capacity is introduced into the material containing silicon in advance. Specifically, a scheme using a composite particle containing a lithium silicate phase and silicon particles (fine silicon phase) dispersed in the lithium silicate phase has been proposed. The silicon particles contribute to charge-discharge reactions (reversible storage and release of lithium).

[0004] Patent Document 1 proposes a scheme in which at least one element Q selected from the group consisting of a rare earth element and an alkaline earth element is dispersed in the lithium silicate phase of the composite particle.

[0005] PRIOR ART DOCUMENTS

[0006] PATENT DOCUMENTS

[0007] Patent Document 1: International Publication No. 2018 / 179969 SUMMARY

[0008] In the composite particle, the degree of expansion and contraction of the silicon phase accompanying storage and release of lithium at the time of charge and discharge is large, and a large stress is generated in the silicate phase around the silicon phase at the time of charge and discharge, and a crack is easily generated in the silicate phase. As a result, a side reaction caused by contact of a newly exposed surface accompanying the crack of the silicate phase with the nonaqueous electrolyte is promoted, and the cycle characteristics are easily reduced. In Patent Document 1, the silicate phase is somewhat hardened by the element Q, but the suppression of the crack of the silicate phase is still insufficient.

[0009] Therefore, it is a problem to reduce the cycle characteristics of a secondary battery using a composite particle having a silicon phase as a negative electrode active material.

[0010] In view of the above, one aspect of the present disclosure relates to a negative electrode active material for a nonaqueous electrolyte secondary battery, which has a composite particle containing: a lithium silicate phase, a silicon phase dispersed in the lithium silicate phase, and a crystal phase of silicon dioxide dispersed in the lithium silicate phase, the crystal phase of silicon dioxide containing β-cristobalite and quartz.

[0011] Another aspect of this disclosure relates to a non-aqueous electrolyte secondary battery comprising: a positive electrode, a negative electrode, and an electrolyte, wherein the aforementioned negative electrode comprises the aforementioned negative electrode active material for secondary batteries.

[0012] The effects of the invention

[0013] According to this disclosure, the degradation of the cycle characteristics of non-aqueous electrolyte secondary batteries can be suppressed. Attached Figure Description

[0014] Figure 1 This is a diagram showing an example of the XRD pattern of composite particles.

[0015] Figure 2 A cross-sectional view of a negative electrode active material (composite particles) according to one embodiment of the present disclosure is shown for illustrative purposes.

[0016] Figure 3 A simplified perspective view of a non-aqueous electrolyte secondary battery according to one embodiment of this disclosure, with a portion cut off. Detailed Implementation

[0017] [Negative electrode active material for non-aqueous electrolyte secondary batteries]

[0018] The negative electrode active material for a non-aqueous electrolyte secondary battery according to embodiments of this disclosure comprises composite particles (hereinafter also referred to as composite particles), said composite particles comprising: a lithium silicate phase, a silicon phase dispersed within the lithium silicate phase, and a silicon dioxide crystalline phase dispersed within the lithium silicate phase. The silicon dioxide crystalline phase comprises β-cristobalite and quartz. β-cristobalite has a cubic crystal structure and, compared to quartz, has a lower Vickers hardness.

[0019] By dispersing silica within the silicate phase, the strength of the silicate phase is improved to some extent. Furthermore, as a crystalline phase of silica, β-cristobalite with relatively low hardness is mixed with quartz, thereby improving the softness of the silicate phase and mitigating the stress generated in the silicate phase surrounding the silicon phase during silicon phase expansion. As a result, the fracturing of the silicate phase is suppressed, and the reduction in cycling characteristics associated with silicate phase fracturing is sufficiently suppressed.

[0020] Furthermore, when both β-cristobalite and quartz are present as crystalline phases of silica, the silicate phase can follow either the expansion or contraction of the silicon phase, thus fully maintaining its function as a lithium-ion conducting phase during charge and discharge. Therefore, the silicon phase can significantly contribute to the charge and discharge reaction, resulting in high capacity and excellent cycle characteristics.

[0021] When the silica dispersed within the silicate phase is almost entirely composed of β-cristobalite, the hardness of the silicate phase decreases. Consequently, the silicate phase, which expands significantly during charging in tandem with the expansion of the silicon phase, becomes less able to follow the contraction of the silicon phase during discharging. This creates a gap between the silicon and silicate phases, isolating the silicon phase and reducing its ability to adequately contribute to the charge-discharge reaction, thus lowering cycle performance.

[0022] When the silica dispersed in the silicate phase is almost entirely composed of quartz, the flexibility of the silicate phase decreases, making it less likely for silicon to follow the expansion of the phase during charging. This increases the stress generated in the silicate phase, leading to cracking and consequently reducing cycle performance.

[0023] In the XRD pattern of the composite particles obtained by X-ray diffraction (XRD), the peak originating from β-cristobalite appears near 2θ = 21.6°, and the peak originating from quartz appears near 2θ = 26.3°. Cu Kα rays were used for the XRD measurements. It should be noted that in this specification, "near x°" means, for example, within the range of x ± 1°.

[0024] In the XRD pattern of the composite particles, the intensity I of the peak originating from β-cristobalite appearing near 2θ = 21.6° is... A The intensity I relative to the peak originating from quartz that appears near 2θ = 26.3° B Ratio: I A / I B Preferably 0.1 or higher, more preferably 0.3 or higher. A / I B When the strength ratio is within the above range, quartz and β-cristobalite exist in good equilibrium within the silicate phase, and the cycling characteristics are easily improved. A / I B The upper limit is, for example, 2.0.

[0025] Here, Figure 1 An example of the XRD pattern of the composite particles is shown. Figure 1 In the diagram, a1 shows the XRD pattern of the negative electrode active material (composite particles) of this embodiment, and b1 shows the XRD pattern of a conventional negative electrode active material (composite particles). It should be noted that composite particles a1 correspond to Example 1 (battery A1) described later, and composite particles b1 correspond to Comparative Example 1 (battery B1) described later.

[0026] Both a1 and b1 show peaks originating from the Si(111) plane of the silicon phase near 2θ = 28°. Both a1 and b1 show peaks originating from the Li2Si2O5 phase of the lithium silicate phase near 2θ = 24°.

[0027] In a1, a peak derived from quartz was observed around 2Θ = 26.3°, and a peak derived from β-cristobalite was observed around 2Θ = 21.6°. In a1, the peak intensity ratio of the above I A / I B was 0.3. On the other hand, in b1, a peak derived from quartz was observed around 2Θ = 26.3°, but a peak derived from β-cristobalite was not observed around 2Θ = 21.6°.

[0028] In the composite particle, a plurality of primary particles including the lithium silicate phase and the silicon phase are combined to constitute a secondary particle. If the secondary particle as a whole is observed, the composite particle has a structure in which the fine silicon phase is dispersed in the lithium silicate phase. By controlling the amount of the silicon phase dispersed in the lithium silicate phase, high capacity can be achieved. The expansion and contraction of the silicon phase can be moderated by the lithium silicate phase. Thus, it is possible to easily achieve both high capacity and improvement of cycle characteristics of the battery.

[0029] The composite particle has a structure in which the fine silicon dioxide phase is dispersed in the lithium silicate phase. One silicon dioxide phase can include both β-cristobalite and quartz, or β-cristobalite and quartz phases can be formed separately in the lithium silicate phase.

[0030] The average particle diameter of the composite particle (secondary particle) is, for example, 1 μm or more and 25 μm or less, or 4 μm or more and 15 μm or less. In the above particle diameter range, stress generated by volume change of the composite particle accompanying charge and discharge is easily moderated, and it becomes easy to obtain good cycle characteristics. The surface area of the composite particle also becomes a moderate size, and capacity reduction due to side reactions with the nonaqueous electrolyte is also suppressed. The average particle diameter of the composite particle refers to the particle diameter at which the volume cumulative value becomes 50% in the particle size distribution measured by the laser diffraction scattering method (volume average particle diameter). As a measuring device, for example, "LA-750" manufactured by HORIBA Ltd. can be used. In the case where the surface of the composite particle is covered with the conductive layer, the thickness of the conductive layer is as small as to substantially not affect the average particle diameter of the composite particle, and thus the average particle diameter of the composite particle having the conductive layer can also be regarded as the average particle diameter of the composite particle.

[0031] The composite particles can be extracted from the battery by the following method. First, the battery in a fully discharged state is disassembled to extract the negative electrode, and the negative electrode is washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove the nonaqueous electrolyte components. As described later, the negative electrode has a negative electrode current collector and a negative electrode mixture layer supported on the surface thereof. Therefore, the negative electrode mixture layer is peeled off from the copper foil, pulverized with a mortar, and a sample powder is obtained. Next, the sample powder is dried in a dry atmosphere for 1 hour, immersed in 6M hydrochloric acid that is slightly boiled for 10 minutes, and elements other than the composite particles are removed. Next, the sample powder is washed with ion-exchange water, filtered, and dried at 200°C for 1 hour. Thereafter, the conductive layer is removed by heating to 900°C in an oxygen atmosphere, and thus only the composite particles can be separated. Note that the fully discharged state refers to a state in which the depth of discharge (DOD) is 90% or more (state of charge (SOC) is 10% or less).

[0032] (lithium silicate phase)

[0033] The lithium silicate phase can be amorphous, and can also be highly crystalline. The lithium silicate phase is low in crystallinity but high in flexibility, and is easily broken by expansion and contraction of the silicon phase. In addition, in the case where the lithium silicate phase is high in crystallinity, the crystal of SiO2 contains β-cristobalite, and thus the flexibility of the lithium silicate phase is improved and the lithium silicate phase is less likely to be broken.

[0034] The lithium silicate is a silicate containing lithium (Li), silicon (Si), and oxygen (O). The atomic ratio of O to Si in the lithium silicate: O / Si is, for example, more than 2 and less than 4. In the case where the O / Si ratio is more than 2 and less than 4 (z in the formula described later is 0 < z < 2), it is advantageous in terms of stability of the silicate phase and lithium ion conductivity. The O / Si ratio is preferably more than 2 and less than 3. In addition, the atomic ratio of Li to Si in the lithium silicate: Li / Si is, for example, more than 0 and less than 4.

[0035] The composition of the lithium silicate can be represented by the formula: Li 2z SiO 2+z (0 < z < 2). From the viewpoints of stability, ease of production, lithium ion conductivity, and the like, z preferably satisfies the relationship of 0 < z < 1, and more preferably z = 1 / 2. The lithium silicate satisfying z = 1 / 2 can be represented by Li2Si2O5. The lithium silicate is desired to contain Li2Si2O5 as a main component, and ideally Li2Si2O5 is the main component of the entire silicate phase. Here, the "main component" refers to a component that accounts for 50% by mass or more of the entire lithium silicate or the entire silicate phase, and can account for 70% by mass or more.

[0036] The silicate phase can further contain an element M other than Li, Si, and O. The silicate phase improves the chemical stability, lithium ion conductivity of the silicate phase, or the side reaction caused by the contact of the silicate phase with the nonaqueous electrolyte by containing the element M.

[0037] As the element M, at least one selected from the group consisting of sodium (Na), potassium (K), magnesium (Mg), barium (Ba), zirconium (Zr), niobium (Nb), lanthanum (La), and the like, tantalum (Ta), vanadium (V), titanium (Ti), phosphorus (P), bismuth (Bi), zinc (Zn), tin (Sn), lead (Pb), antimony (Sb), cobalt (Co), fluorine (F), tungsten (W), aluminum (Al), and boron (B) can be used, for example. From the viewpoint of the resistance to the nonaqueous electrolyte and the structural stability of the silicate phase, the element M preferably contains at least one selected from the group consisting of Zr, Ti, P, Al, and B.

[0038] The lanthanoid improves the initial charge-discharge efficiency of the charge-discharge cycle. From the viewpoint of improving the lithium ion conductivity, the lanthanoid more preferably contains La. The ratio of La in the entire lanthanoid is preferably 90 atomic% or more and 100 atomic% or less.

[0039] The silicate phase can further contain trace amounts of elements such as iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), and the like.

[0040] The element M can form a compound. As the compound, depending on the kind of the element M, it can be a silicate of the element M, or it can be an oxide of the element M, for example.

[0041] In the silicate phase, the content of the element M is, for example, 1 mol% or more and 40 mol% or less with respect to the total amount of elements other than oxygen.

[0042] The contents of Li, Si, and the element M in the silicate phase can be determined, for example, by analyzing the cross section of the negative electrode mixture layer.

[0043] First, the battery in the fully discharged state is disassembled, and the negative electrode is taken out. The negative electrode is washed with anhydrous ethyl methyl carbonate or dimethyl carbonate, and the nonaqueous electrolyte component is removed and dried, and then the cross section of the negative electrode mixture layer is obtained using a cross section polisher (CP). Next, the cross section of the negative electrode mixture layer is observed using a scanning electron microscope (SEM).

[0044] Then, the content of each element can be calculated by any of the following methods. In addition, the composition of the silicate phase is calculated from the content of each element.

[0045] <EDX>

[0046] From the cross-sectional images of the reflection electron images of the negative electrode mixture layers, 10 composite particles of which the maximum diameter was 5 μm or more were randomly selected, and for each, element mapping analysis based on energy dispersive X-ray (EDX) was performed. The contained area of the element that was the object was calculated with an image analysis software. The observation magnification was desirably 2000 to 20000 times. The measured values of the contained area of the prescribed element contained in the 10 particles were averaged. From the obtained average value, the content of the element that was the object was calculated.

[0047] The measurement conditions of the cross-sectional SEM-EDX analysis are shown below.

[0048] <SEM-EDX measurement conditions>

[0049] Processing device: SM-09010 (Cross Section Polisher) manufactured by JEOL

[0050] Processing conditions: acceleration voltage 6 kV

[0051] Current value: 140 μA

[0052] Degree of vacuum: 1 x 10 -3 ~ 2 x 10 -3 Pa

[0053] Measurement device: electron microscope SU-70 manufactured by HITACHI

[0054] Acceleration voltage at the time of analysis: 10 kV

[0055] Field: free mode

[0056] Probe current mode: medium

[0057] Probe current range: high

[0058] Anode Ap.: 3

[0059] OBJ Ap.: 2

[0060] Analysis area: 1 μm square

[0061] Analysis software: EDAX Genesis

[0062] CPS: 20500

[0063] Lsec: 50

[0064] Time constant: 3.2

[0065] <AES>

[0066] From the cross-sectional images of the reflection electron images of the negative electrode mixture layer, 10 composite particles each having a maximum diameter of 5 μm or more were randomly selected, and qualitative and quantitative analyses of elements were performed using an Auger electron spectroscopy (AES) analysis device (for example, JAMP-9510F manufactured by JEOL Ltd.). The measurement conditions can be set to, for example, an acceleration voltage of 10 kV, a beam current of 10 nA, and an analysis area of 1 μm x 1 μm. The content of each of the specified elements contained in the 10 composite particles was averaged to calculate the content.

[0067] Note that the EDX analysis and the AES analysis were performed on a range inside 1 μm or more from the peripheral edge of the cross section of the composite particle.

[0068] <ICP>

[0069] The sample of the composite particle was completely dissolved in a heated acid solution (mixed acid of hydrofluoric acid, nitric acid, and sulfuric acid), and the carbon of the solution residue was filtered and removed. Thereafter, the obtained filtrate was analyzed by inductively coupled plasma emission spectrophotometry (ICP) to measure the spectral intensity of each element. Then, a standard curve was prepared using a commercially available standard solution of elements, and the content of each element contained in the composite particle was calculated.

[0070] In addition, the quantitative analysis of each element can be performed using an electron probe micro-analyzer (EPMA), laser ablation ICP mass spectrometry (LA-ICP-MS), X-ray photoelectron spectroscopy (XPS), or the like.

[0071] In addition, the content of B, Na, K, and Al contained in the composite particle can be quantitatively analyzed in accordance with JIS R3105 (1995) (Method of Analysis for Borosilicate Glass).

[0072] The content of carbon contained in the composite particle can be measured using a carbon / sulfur analysis device (for example, EMIA-520 manufactured by HORIBA, Ltd.). A sample was weighed on a magnetic plate, a combustion aid was added, and it was inserted into a combustion furnace (carrier gas: oxygen) heated to 1350°C, and the amount of carbon dioxide gas generated at the time of combustion was detected by infrared absorption. A standard curve was prepared using a carbon steel (carbon content: 0.49%) manufactured by Bureau of Analysed Samples, Ltd., and the carbon content of the sample was calculated (high-frequency induction heating furnace combustion-infrared absorption method).

[0073] The oxygen content contained in the composite particles can be measured with an oxygen / nitrogen / hydrogen analysis device (for example, EGMA-830 manufactured by Horiyama Manufacturing Co., Ltd.). The sample is put in a Ni capsule, and together with Sn pellets and Ni pellets that become flux, it is put into a carbon crucible heated at 5.75 kW of electric power, and carbon monoxide gas released is detected. A standard curve is made with a standard sample Y2O3, and the oxygen content of the sample is calculated (non-active gas fusion-non-dispersive infrared absorption method).

[0074] The silicate phase, the silicon phase, and the SiO2phase are present in the composite particles. The Si content obtained by the above method is the total of the amount of Si constituting the silicon phase and the amounts of Si in the silicate phase and the SiO2phase. On the other hand, the amount of Si constituting the silicon phase can be quantified separately using Si-NMR. The amount of Si in the SiO2phase can also be quantified separately using Si-NMR. Thus, by using Si-NMR, it is possible to distinguish and quantify the amount of Si constituting the silicon phase from the amounts of Si in the SiO2phase and the silicate phase. Note that, in the standard material required for quantification, a mixture containing the silicate phase, the silicon phase, and the SiO2phase at a prescribed ratio and having a known Si content can be used.

[0075] The content of the SiO2phase in the composite particles measured by Si-NMR is, for example, 50% by mass or less, and can be 10% by mass or more and 40% by mass or less.

[0076] The following shows ideal measurement conditions for Si-NMR.

[0077] <Si-NMR Measurement Conditions>

[0078] Measurement device: solid nuclear magnetic resonance spectrometer (INOVA-400) manufactured by Varian Inc.

[0079] Probe: Varian 7mm CPMAS-2

[0080] MAS: 4.2 kHz

[0081] MAS speed: 4 kHz

[0082] Pulse: DD (45° pulse + signal collection time 1H decoupling)

[0083] Repetition time: 1200 seconds to 3000 seconds

[0084] Observation amplitude: 100 kHz

[0085] Observation center: around -100 ppm

[0086] Signal collection time: 0.05 seconds

[0087] Cumulative number: 560

[0088] Sample amount: 207.6 mg

[0089] (silicon phase)

[0090] The silicon phase is a phase of silicon (Si) element, and repeats absorption and release of lithium ions with charge and discharge of the battery. The capacity is embodied by the Faraday reaction in which the silicon phase participates. The capacity of the silicon phase is large, and therefore, the degree of expansion and shrinkage accompanying charge and discharge is also large. However, the silicon phase is dispersed in the silicate phase, and therefore, the stress generated by the expansion and shrinkage of the silicon phase is mitigated.

[0091] The silicon phase can be composed of a plurality of crystallites. The crystallite size of the silicon phase is preferably 30 nm or less. In the case where the crystallite size of the silicon phase is 30 nm or less, the volume change amount generated by the expansion and shrinkage of the silicon phase accompanying charge and discharge can be reduced, and the cycle characteristics can be further improved. For example, the isolation of the silicon phase caused by the formation of pores around the silicon phase at the time of shrinkage of the silicon phase is suppressed, and the decrease in charge and discharge efficiency is suppressed. The lower limit value of the crystallite size of the silicon phase is not particularly limited, and is, for example, 1 nm or more.

[0092] The crystallite size of the silicon phase is more preferably 20 nm or less. In this case, the expansion and shrinkage of the silicon phase can be easily homogenized, the stress generated in the composite particle can be easily mitigated, and the cycle characteristics can be easily improved.

[0093] The crystallite size of the silicon phase is further preferably 10 nm or less, and particularly preferably 2 nm or more and 10 nm or less. In the case where the fine particles of silicon having an average particle diameter of 200 nm or less are used in the raw material silicon in the second step described later, the specific surface area of the composite compound obtained in the second step becomes large, and heat is easily transferred to the silicon phase and the SiO2 phase dispersed in the lithium silicate phase at the time of heating in the third step (or the fifth step) described later. As a result, a crystalline silicon phase having a crystallite size of 10 nm or less is formed, and both quartz and β-cristobalite are easily formed as the crystal phase of SiO2.

[0094] The crystallite size of the silicon phase is calculated from the full width at half maximum of the peak corresponding to the Si (111) plane of the silicon phase in the XRD pattern of the composite particle according to the Scherrer formula. The above XRD pattern is obtained by XRD measurement using Cu Kα rays.

[0095] The silicon phase of the composite particles contained in the battery before the initial charge is, for example, particulate. The average particle diameter of the particulate silicon phase is preferably 500 nm or less, more preferably 200 nm or less, further preferably 100 nm or less, and particularly preferably 50 nm or less. In the case where the microparticles of silicon having an average particle diameter of 200 nm or less are used in the raw material silicon in the second process described later, the average particle diameter of the silicon phase finally obtained can be 100 nm or less. After the initial charge, the average particle diameter of the silicon phase is preferably 400 nm or less, and more preferably 100 nm or less. By making the silicon phase fine, the volume change of the composite particles at the time of charge and discharge becomes small, and the structural stability of the composite particles is further improved. The average particle diameter of the silicon phase is measured using a cross-sectional image of the composite particles obtained by SEM. Specifically, the average particle diameter of the silicon phase is obtained by averaging the maximum diameters of 100 silicon phases arbitrarily selected.

[0096] From the viewpoint of high capacity, the content of the silicon phase in the composite particles is preferably 30% by mass or more, more preferably 35% by mass or more, and further preferably 55% by mass or more. In this case, the diffusivity of lithium ions is good, and excellent load characteristics are obtained. On the other hand, from the viewpoint of improving the cycle characteristics, the content of the silicon phase in the composite particles is preferably 95% by mass or less, more preferably 75% by mass or less, and further preferably 70% by mass or less. In this case, the surface of the silicon phase exposed without being covered by the silicate phase is reduced, and the side reaction of the non-aqueous electrolyte with the silicon phase is easily suppressed.

[0097] (Electrically conductive layer)

[0098] An electrically conductive layer containing an electrically conductive material can be formed on at least a part of the surface of the composite particles (secondary particles). Thereby, the electric conductivity of the composite particles can be dramatically improved. The thickness of the electrically conductive layer is preferably substantially thin to the extent that it does not affect the average particle diameter of the composite particles. If the diffusivity of lithium ions and the securing of the electric conductivity are taken into consideration, the thickness of the electrically conductive layer is preferably 1 to 200 nm, and more preferably 5 to 100 nm. The thickness of the electrically conductive layer can be measured by cross-sectional observation of the composite particles using SEM or TEM.

[0099] The electrically conductive material is preferably an electrically conductive carbon material. As the electrically conductive carbon material, amorphous carbon, graphite, easily graphitizable carbon (soft carbon), hardly graphitizable carbon (hard carbon), and the like can be used. Among them, in terms of easy formation of a thin electrically conductive layer that covers the surface of the composite particles, amorphous carbon is preferred. As the amorphous carbon, carbon black, a baked product of pitch, coke, activated carbon, and the like can be given. As the graphite, natural graphite, artificial graphite, graphitized mesocarbon microbeads, and the like can be given.

[0100] Here, Figure 2 A cross-sectional view of the negative electrode active material (composite particles) of an embodiment of the present disclosure is schematically shown.

[0101] The composite particle 20 has a base particle 23 composed of secondary particles aggregated from a plurality of primary particles 24. The base particle 23 (primary particle 24) has a lithium silicate phase 21, a silicon phase 22 and a crystal phase 28 of SiO2 dispersed in the lithium silicate phase 21. The base particle 23 has an island-in-matrix structure in which the fine silicon phase and the SiO2 phase are dispersed in the matrix of the lithium silicate phase 21.

[0102] At least a part of the surface of the base particle 23 can be covered with the conductive layer 26. The lithium silicate phase 21 can contain an element M. With repetition of charge and discharge, the granular silicon phases 22 adjacent to each other are connected to each other, and a network-like silicon phase can be formed.

[0103] [Method for manufacturing composite particle]

[0104] The composite particle can be manufactured, for example, by a manufacturing method including the following 1st to 4th processes.

[0105] (1st process) Process for obtaining a lithium silicate as a raw material (hereinafter, also referred to as a raw silicate). The raw silicate contains SiO2.

[0106] (2nd process) Process for mixing the raw silicate and raw silicon, and performing a pulverization treatment (complexing) of the mixture in a ball mill or the like.

[0107] (3rd process) Process for obtaining a sintered body by heating while compressing the complex (pulverized product).

[0108] (4th process) Process for pulverizing the sintered body to obtain a composite particle.

[0109] In the 1st process, a mixture of SiO2 and a Li compound is used as a raw material, and SiO2 which does not react with the Li compound during the production of the raw silicate remains in the raw silicate. In the case where the amount of SiO2 used with respect to the Li compound is large, SiO2 is likely to remain.

[0110] The SiO2 remaining in the raw silicate in the 2nd process becomes largely amorphous, but a crystal of fine SiO2 can be precipitated by heating in the 3rd process (or the 5th process described later) thereafter. That is, a crystal phase of fine SiO2 can be dispersed in the lithium silicate phase. The crystal phase of SiO2 is stable, and reacts with lithium ions also at the time of charging, and does not become a key to irreversible reaction. Since it is fine, it is not likely to become an obstacle to expansion and contraction of the silicon phase.

[0111] By appropriately adjusting the particle size (average particle diameter) of the raw silicon used in the second step and / or the heating temperature and the degree of compression in the third step, it is possible to cause β-cristobalite and quartz to coexist as SiO2 crystals and precipitate. In addition, it is possible to control the balance between β-cristobalite and quartz. Depending on the particle size of the raw silicon, it is possible to adjust the ease of heat transfer to the SiO2 phase dispersed in the lithium silicate phase at the time of heating in the third step. The raw silicon uses, for example, the fine particles of silicon described later. The heating temperature of the composite is, for example, 450°C or higher and 1000°C or lower. The pressure applied to the composite is, for example, 100 MPa or higher and 400 MPa or lower. The time for heating and compressing the composite is, for example, 1 hour or more and 10 hours or less.

[0112] The manufacturing method described above can further include a fifth step of forming a conductive layer on at least a part of the surface of the composite particles. By the heating treatment in the fifth step, it is possible to cause β-cristobalite and quartz to coexist, and it is possible to adjust the balance between β-cristobalite and quartz.

[0113] Next, each step of the manufacturing method of the composite particles will be described in detail.

[0114] (First Step)

[0115] The first step includes, for example, the following steps: Step 1a, mixing silicon dioxide, a lithium compound, and a compound containing an element M as necessary to obtain a mixture; and Step 1b, firing the mixture to obtain a raw silicate. The firing of Step 1b is performed, for example, in an oxidizing atmosphere. The firing temperature of Step 1b is preferably 400°C or higher and 1200°C or lower, more preferably 800°C or higher and 1100°C or lower.

[0116] As the lithium compound, lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, and the like can be given. The lithium compound can be used alone as one kind, or two or more kinds can be used in combination.

[0117] As the compound containing the element M, oxides, hydroxides, hydrides, halides, carbonates, oxalates, nitrates, sulfates, and the like of the element M can be used. The compound containing the element M can be used alone as one kind, or two or more kinds can be used in combination.

[0118] (Second Step)

[0119] The second step includes, for example, the following step: Step 2, pulverizing a mixture of the raw silicate and the raw silicon while imparting a shearing force to the mixture to obtain a pulverized product (composite) that is micronized. Here, for example, the raw silicate and the raw silicon can be mixed at a prescribed mass ratio (for example, 20:80 to 95:5), and a pulverizing device such as a ball mill can be used to micronize the mixture while stirring.

[0120] It is preferable to use fine particles of silicon as the raw material silicon. The average particle diameter of the fine particles of silicon is, for example, 500 nm or less, and can be 200 nm or less or 150 nm or less. The lower limit of the average particle diameter of the fine particles of silicon is, for example, 10 nm. In the case of using the above fine particles of silicon (particularly, fine particles having an average particle diameter of 200 nm or less), the specific surface area of the composite compound obtained in the second step becomes large, and heat is easily transferred to the SiO2 phase dispersed in the lithium silicate phase at the time of heating in the third step (or the fifth step). Thus, the crystallinity of the SiO2 phase is easily improved, and heating to a temperature at which β-cristobalite can be formed together with quartz is easily performed. In addition, in the case of using the above fine particles of silicon, the crystallite size of the silicon phase in the final obtained composite particles can be 10 nm or less (or 2 nm or more and 10 nm or less). Note that the average particle diameter of the raw material silicon refers to the particle diameter at which the volume cumulative value becomes 50% in the particle size distribution measured by laser diffraction scattering.

[0121] (Third Step)

[0122] Sintering of the composite compound is performed for the purpose of producing a dense composite particle and moderately reducing the surface area of the composite particle. In the third step, for example, the pulverized product (composite compound) is compressed using a hot press or the like while being heated, and a sintered body is obtained. Alternatively, a sintered body can be obtained by conveying and calendering the pulverized product shaped into a sheet between a pair of calender rolls after heating. The third step is performed, for example, in a non-reactive atmosphere (for example, an atmosphere of argon, nitrogen, or the like).

[0123] The heating temperature in the third step can be 450°C or higher and 1000°C or lower. In the case of the above temperature range, it is easy to disperse fine silicon particles and SiO2 particles in a silicate phase having low crystallinity. The raw material silicate is stable in the above temperature range and hardly reacts with silicon. The heating time is, for example, 1 hour or more and 10 hours or less.

[0124] (Fourth Step)

[0125] In the fourth step, the sintered body is pulverized so as to have a desired particle size distribution, and a composite particle (secondary particle) including a silicate phase, a silicon phase dispersed in the silicate phase, and a SiO2 phase is obtained. The composite particle is, for example, pulverized so as to have an average particle diameter of 1 to 25 μm.

[0126] (Fifth Step)

[0127] At least a part of the surface of the composite particle (secondary particle) can be covered with a conductive material to form a conductive layer. The conductive material is preferably electrochemically stable, and is preferably a conductive carbon material. As a method of covering the surface of the composite particle with a conductive carbon material, a CVD method using a hydrocarbon gas such as acetylene or methane as a raw material, a method of mixing coal pitch, petroleum pitch, phenol resin, or the like with the composite particle and heating to carbonize the same, or the like can be exemplified. Alternatively, carbon black can be attached to the surface of the composite particle. In the fifth step, for example, a mixture of the composite particle and the conductive carbon material is heated at 700°C or higher and 950°C or lower, for example, in a non-active atmosphere (for example, an atmosphere of argon, nitrogen, or the like), whereby a conductive layer can be formed on the surface of the composite particle.

[0128] [Non-aqueous electrolyte secondary battery]

[0129] The non-aqueous electrolyte secondary battery of the embodiment of the present disclosure has a positive electrode, a negative electrode, and a non-aqueous electrolyte, and the negative electrode contains the negative electrode active material for non-aqueous electrolyte secondary batteries described above.

[0130] Hereinafter, the non-aqueous electrolyte secondary battery will be described in detail.

[0131] [Negative electrode]

[0132] The negative electrode can have a negative electrode current collector and a negative electrode mixture layer supported on the surface of the negative electrode current collector. The negative electrode mixture layer can be formed by applying a negative electrode slurry in which a negative electrode mixture is dispersed in a dispersion medium to the surface of the negative electrode current collector and drying, whereby it can be formed. The coating film after drying can be calendered as necessary. The negative electrode mixture layer can be formed on one surface of the negative electrode current collector, or on both surfaces.

[0133] The negative electrode mixture contains a negative electrode active material as an essential component, and can contain a binder, a conductive agent, a thickening agent, or the like as an arbitrary component. The negative electrode active material contains at least the composite particle described above.

[0134] The negative electrode active material preferably further contains a carbon material that electrochemically occludes and releases lithium ions. The composite particle swells and shrinks in volume accompanying charge and discharge, and therefore, if the ratio of the composite particle in the negative electrode active material becomes large, the negative electrode active material is likely to be in poor contact with the negative electrode current collector accompanying charge and discharge. On the other hand, by using the composite particle in combination with the carbon material, it is possible to impart high capacity of the silicon particle to the negative electrode while achieving excellent cycle characteristics. From the viewpoint of high capacity and cycle characteristic improvement, the ratio of the carbon material in the total of the composite particle and the carbon material is preferably 98% by mass or less, more preferably 70% by mass or more and 98% by mass or less, and further preferably 75% by mass or more and 95% by mass or less.

[0135] As the carbon material, for example, graphite, easy graphitizable carbon (soft carbon), hard graphitizable carbon (hard carbon), or the like can be exemplified. Among them, graphite in which stability of charge and discharge is excellent and in which irreversible capacity is also small is preferable. Graphite refers to a material having a graphite-type crystal structure, and for example, includes natural graphite, artificial graphite, graphitized mesophase carbon particles, and the like. The carbon material can be used alone or in combination of two or more.

[0136] As the negative current collector, a non-porous conductive substrate (metal foil or the like), a porous conductive substrate (sieve body, mesh body, punched metal plate, or the like) is used. As the material of the negative current collector, for example, stainless steel, nickel, nickel alloy, copper, copper alloy, or the like can be exemplified. The thickness of the negative current collector is not particularly limited, and from the viewpoint of balance between strength of the negative electrode and light weight, 1 to 50 μm is preferable, and 5 to 20 μm is more desirable.

[0137] As the binder, for example, a resin material, such as polytetrafluoroethylene, polyvinylidene fluoride (PVDF), or the like fluorine resin; polyethylene, polypropylene, or the like polyolefin resin; aromatic polyamide resin, or the like polyamide resin; polyimide, polyamide-imide, or the like polyimide resin; polyacrylic acid, polymethyl acrylate, ethylene-acrylic acid copolymer, or the like acrylic resin; polyacrylonitrile, polyvinyl acetate, or the like vinyl resin; polyvinylpyrrolidone; polyethersulfone; styrene-butadiene copolymer rubber (SBR), or the like rubber-like material, or the like can be exemplified. The binder can be used alone or in combination of two or more.

[0138] As the conductive agent, for example, carbon such as acetylene black; conductive fibers such as carbon fiber, metal fiber, or the like; fluorinated hydrocarbon; metal powder such as aluminum or the like; conductive whisker such as zinc oxide, potassium titanate, or the like; conductive metal oxide such as titanium oxide; organic conductive material such as phenylene derivative, or the like can be exemplified. The conductive agent can be used alone or in combination of two or more.

[0139] As the thickening agent, for example, carboxymethyl cellulose (CMC) and a modified product thereof (also including a salt such as Na salt), cellulose derivative (cellulose ether or the like) such as methyl cellulose, saponified product of a polymer having a vinyl acetate unit such as polyvinyl alcohol, polyether (polyalkylene oxide such as polyethylene oxide, or the like), or the like can be exemplified. The thickening agent can be used alone or in combination of two or more.

[0140] As the dispersion medium, there is no particular limitation, and for example, water, alcohol such as ethanol, ether such as tetrahydrofuran, amide such as dimethylformamide, N-methyl-2-pyrrolidone (NMP), or a mixed solvent thereof, or the like can be exemplified.

[0141] [Positive electrode]

[0142] The positive electrode can have a positive electrode current collector and a positive electrode mixture layer supported 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 a positive electrode mixture is dispersed in a dispersion medium to the surface of the positive electrode current collector and drying it, and thus can be formed. The coated film after drying can be calendered as needed. The positive electrode mixture layer can be formed on one surface of the positive electrode current collector, or on both surfaces. The positive electrode mixture contains, as an essential component of the positive electrode active material, and can contain a binder, a conductive agent, and the like as arbitrary components. As the dispersion medium of the positive electrode slurry, NMP or the like is used.

[0143] As the positive electrode active material, for example, a lithium-containing composite oxide can be used. For example, Li a CoO2, Li a NiO2, Li a MnO2, Li a Co b Ni 1-b O2, Li a Co b Me 1-b O c , Li a Ni 1-b Me b O c , Li a Mn2O4, Li a Mn 2-b Me b O4, LiMePO4, Li2MePO4F (Me 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). Here, a = 0 to 1.2, b = 0 to 0.9, and c = 2.0 to 2.3. Note that the value of a, which represents the molar ratio of lithium, increases and decreases depending on the charge and discharge.

[0144] Among them, Li a Ni b Me 1-b O2 (Me is at least one selected from the group consisting of Mn, Co, and Al, 0 < a ≤ 1.2, 0.3 ≤ b < 1) is preferable. From the viewpoint of high capacity, it is more preferable to satisfy 0.85 ≤ b < 1. From the viewpoint of stability of crystal structure, it is further preferable to contain Co and Al as Me. a Ni b Co c Al d O2 (0 < a ≤ 1.2, 0.85 ≤ b < 1, 0 < c < 0.15, 0 < d ≤ 0.1, b + c + d = 1).

[0145] As the binder and the conductive agent, the same substances as those for the negative electrode example can be used. As the conductive agent, graphite such as natural graphite, artificial graphite, or the like can also be used.

[0146] The shape and the thickness of the positive electrode current collector can be selected from those of the negative electrode current collector, respectively. As the material of the positive electrode current collector, for example, stainless steel, aluminum, aluminum alloy, titanium, or the like can be exemplified.

[0147] [Non-aqueous electrolyte]

[0148] The non-aqueous electrolyte contains a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the non-aqueous electrolyte is, for example, preferably 0.5 mol / L or more and 2 mol / L or less. By making the lithium salt concentration in the above range, a non-aqueous electrolyte having excellent ion conductivity and having moderate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0149] As the non-aqueous solvent, for example, a cyclic carbonate, a chain carbonate, a cyclic carboxylic acid ester, a chain carboxylic acid ester, or the like is used. As the cyclic carbonate, propylene carbonate (PC), ethylene carbonate (EC), or the like can be exemplified. As the chain carbonate, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), or the like can be exemplified. As the cyclic carboxylic acid ester, γ-butyrolactone (GBL), γ-valerolactone (GVL), or the like can be exemplified. As the chain carboxylic acid ester, methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, or the like can be exemplified. The non-aqueous solvent can be used alone as one kind, or two or more kinds can be used in combination.

[0150] As the lithium salt, for example, LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, LiB 10 Cl 10Examples of borates include lower aliphatic carboxylic acids such as lithium, LiCl, LiBr, LiI, borates, and imide salts. Examples of borates include lithium bis(1,2-phenylene glycol 2-O,O')borate, lithium bis(2,3-naphthalene glycol 2-O,O')borate, lithium bis(2,2'-biphenylene glycol 2-O,O')borate, and lithium bis(5-fluoro-2-ol 1-benzenesulfonic acid 2-O,O')borate. Examples of lithium bis(fluorosulfonyl)imide (LiN(FSO2)2), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF3SO2)2), lithium trifluoromethanesulfonate nonafluorobutanesulfonylimide (LiN(CF3SO2)(C4F9SO2)), and lithium bis(pentafluoroethylsulfonyl)imino (LiN(C2F5SO2)2) are examples of lithium bis(fluoroethylsulfonyl)imino. LiPF6 is preferred. A single lithium salt can be used alone, or two or more can be used in combination.

[0151] [Separator]

[0152] It is typically desirable to sandwich a separator between the positive and negative electrodes. The separator should have high ion permeability, moderate mechanical strength, and insulation properties. Suitable materials for the separator include microporous membranes, woven fabrics, and non-woven fabrics. Materials used for the separator may include polyolefins such as polypropylene and polyethylene.

[0153] As an example of the structure of a non-aqueous electrolyte secondary battery, one can exemplify a structure in which an electrode assembly consisting of a positive and a negative electrode wound together with a separator and housed together with a non-aqueous electrolyte in a casing. However, this is not a limitation, and other forms of electrode assemblies can also be used. For example, a stacked electrode assembly consisting of a positive and a negative electrode layered together with a separator can be used. The shape of the non-aqueous electrolyte secondary battery is also not limited; for example, it can be cylindrical, square, coin-shaped, button-shaped, laminated, etc.

[0154] The following is a reference. Figure 3 The structure of a square non-aqueous electrolyte secondary battery, which is an example of a non-aqueous electrolyte secondary battery disclosed herein, will be described.

[0155] The battery comprises: a square-shaped battery casing 4, and an electrode assembly 1 and a non-aqueous electrolyte housed within the battery casing 4. The electrode assembly 1 has a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator sandwiched between them. The negative electrode current collector is electrically connected to a negative terminal 6 located on a sealing plate 5 via a negative electrode lead 3. The negative 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 of the sealing plate 5 via a positive electrode lead 2. That is, the positive electrode is electrically connected to the battery casing 4, which also has a positive terminal. The periphery of the sealing plate 5 is fitted into the open end of the battery casing 4, and the fitting is laser-welded. The sealing plate 5 has an injection hole for the non-aqueous electrolyte, which is blocked by a sealing plug 8 after electrolyte injection.

[0156] Hereinafter, the present disclosure will be specifically described based on Examples and Comparative Examples, but the present application is not limited to the following Examples.

[0157] <Example 1>

[0158] [Preparation of composite particles]

[0159] (First step)

[0160] Lithium oxide, silicon dioxide, aluminum oxide, and lanthanum oxide were mixed so as to have a molar ratio of Li2O:SiO2:Al2O3:La2O3=21:75:3:1, and the mixture was melted at 1500°C for 5 hours in an inactive atmosphere, and the molten liquid was formed into a flake shape by a metal roller to obtain a lithium silicate composite oxide containing Li, Si, Al, and La. The obtained lithium silicate composite oxide was pulverized so as to have an average particle diameter of 10 μm to obtain a raw silicate.

[0161] (Second step)

[0162] Thereafter, the raw silicate (average particle diameter 10 μm) and the raw silicon were mixed at a mass ratio of 40:60. A fine powder of silicon (3N, average particle diameter 100 nm) was used as the raw silicon. The mixture was filled into a pot (SUS, volume: 500 mL) of a planetary ball mill (P-5, manufactured by Fritsch Co.), 24 SUS balls (diameter 20 mm) were put in, the lid was closed, and the mixture was subjected to a pulverization treatment at 200 rpm for 25 hours in an inactive atmosphere.

[0163] (Third step)

[0164] Thereafter, the pulverized product (composite) was heated while being compressed by a hot press in an inactive atmosphere to obtain a sintered body. At this time, the heating temperature of the pulverized product was set to 600°C, the pressure applied to the pulverized product was set to 190 MPa, and the heating (compression) time was set to 4 hours.

[0165] (Fourth step)

[0166] Thereafter, the sintered body was pulverized through a 40-μm sieve to obtain composite particles.

[0167] (Fifth step)

[0168] The composite particles were mixed with coal tar pitch (manufactured by JFE Chemical Corporation, MCP250). The mixture was fired at 800°C for 5 hours in a non-reactive atmosphere to cover the surface of the composite particles with electrically conductive carbon and form an electrically conductive layer. The amount of the electrically conductive layer was set to 5% by mass relative to the total mass of the composite particles and the electrically conductive layer. Thereafter, a sieve was used to obtain composite particles al (secondary particles) having an average particle diameter of 5 μm with the electrically conductive layer.

[0169] XRD measurement was performed on the composite particles al. The XRD pattern of the composite particles al is shown in Figure 1 . In the XRD pattern, peaks derived from Si, Si02, and Li2Si205 were confirmed. For Si02, a peak derived from quartz was observed at 2θ = 26.3°, and a peak derived from β-cristobalite was observed at 2θ = 21.6°. The intensity I A of the peak derived from β-cristobalite was 0.3 relative to the intensity I B of the peak derived from quartz. The ratio: I A / I B was 0.3. In addition, the crystallite diameter of the silicon phase was calculated from the XRD pattern of the composite particles al according to the above-described method. The crystallite diameter of the silicon phase was 8 nm.

[0170] [Production of the negative electrode]

[0171] The composite particles were mixed with graphite at a mass ratio of 5:95 and used as the negative electrode active material. A negative electrode slurry was prepared by adding water to and stirring a negative electrode binder containing the negative electrode active material, a Na salt of CMC, and SBR at a mass ratio of 97.5:1:1.5. Next, the negative electrode slurry was applied to the surface of a copper foil, and the coated film was dried and calendered to produce a negative electrode having negative electrode binder layers formed on both surfaces of the copper foil with a density of 1.5 g / cm 3 . [Production of the positive electrode]

[0172] [Production of the positive electrode]

[0173] A positive electrode slurry was prepared by adding NMP to and stirring a positive electrode binder containing lithium cobaltate, acetylene, and PVDF at a mass ratio of 95:2.5:2.5. Next, the positive electrode slurry was applied to the surface of an aluminum foil, and the coated film was dried and calendered to produce a positive electrode having positive electrode binder layers formed on both surfaces of the aluminum foil with a density of 3.6 g / cm 3 . [Production of the non-aqueous electrolyte secondary battery]

[0174] [Production of the non-aqueous electrolyte secondary battery]

[0175] A non-aqueous electrolyte was prepared by dissolving LiPF6 at a concentration of 1.0 mol / L in a mixed solvent containing EC and DEC at a volume ratio of 3:7.

[0176] [Production of the non-aqueous electrolyte secondary battery]

[0177] The positive electrode and the negative electrode each having a tab were wound with a separator interposed therebetween to produce an electrode group in which the tabs were located at the outermost peripheral portions. The electrode group was inserted into an outer case made of an aluminum laminate film, vacuum-dried at 105°C for 2 hours, and then injected with a nonaqueous electrolyte. The opening of the outer case was sealed to obtain a battery Al of Example 1.

[0178] <Comparative Example 1>

[0179] In the second step, a coarse powder of silicon (3N, average particle diameter 10 μm) was used as the raw material silicon, and otherwise, the composite particles bl were produced according to the same method as in Example 1.

[0180] XRD measurement was performed on the composite particles bl. The XRD pattern of the composite particles bl is shown in Figure 1 In the XRD pattern, peaks derived from Si, SiO2, and Li2Si2O5 were confirmed. For SiO2, a peak derived from quartz was observed, but a peak derived from β-cristobalite was not observed.

[0181] Using the composite particles bl obtained in the above, a negative electrode was produced according to the same method as in Example 1 to obtain a nonaqueous electrolyte secondary battery Bl.

[0182] For each battery of the examples and comparative examples, the following evaluations were performed.

[0183] [Thickness change rate of the negative electrode after the initial charge]

[0184] At 25°C, constant current charge was performed at a current of 1 It (800 mA) until the voltage became 4.2 V, and then constant voltage charge was performed at a voltage of 4.2 V until the current became 1 / 20 It (40 mA).

[0185] The battery after the initial charge was disassembled, and the negative electrode was taken out, washed with ethyl methyl carbonate or dimethyl carbonate, dried, and the electrolyte was removed. Thereafter, the thickness of 10 points of the negative electrode was measured, and the average value thereof was taken as the thickness Tl of the negative electrode after the initial charge. In addition, the thickness of the negative electrode produced in the above (the negative electrode used when the battery was produced) was taken as the thickness TO of the negative electrode before the initial charge.

[0186] The ratio of the thickness Tl to the thickness TO of the negative electrode before the initial charge (Tl / TO x 100) was taken as the thickness change rate of the negative electrode after the initial charge.

[0187] [Thickness change rate of the negative electrode after the initial discharge]

[0188] The battery was prepared separately and charged at a constant current of 1 It (800 mA) at 25°C until the voltage became 4.2 V, and then charged at a constant voltage of 4.2 V until the current became 1 / 20 It (40 mA). After a pause of 10 minutes, the battery was discharged at a constant current of 1 It (800 mA) until the voltage became 2.75 V.

[0189] The battery after the initial discharge was disassembled, and the thickness T2 of the negative electrode after the initial discharge was measured according to the same method as described above. The ratio (T2 / T0 x 100) of the thickness T2 of the negative electrode after the initial discharge to the thickness T0 of the negative electrode before the initial charge was measured as the thickness change rate of the negative electrode after the initial discharge.

[0190] [Charge / discharge cycle test]

[0191] The charge and discharge were repeated under the following conditions.

[0192] <Charge>

[0193] The battery was charged at a constant current of 1 It (800 mA) at 25°C until the voltage became 4.2 V, and then charged at a constant voltage of 4.2 V until the current became 1 / 20 It (40 mA).

[0194] <Discharge>

[0195] The battery was discharged at a constant current of 1 It (800 mA) at 25°C until the voltage became 2.75 V.

[0196] The pause time between the charge and the discharge was set to 10 minutes. The ratio of the discharge capacity of the 300th cycle to the discharge capacity of the 1st cycle was measured as the capacity maintenance rate.

[0197] For the batteries Al and Bl, the evaluation results are shown in Table 1.

[0198] [Table 1]

[0199]

[0200] In the battery Al, quartz and β-cristobalite were contained as the crystal phase of silicon dioxide, and thus the stress generated in the silicate phase accompanying the expansion of the silicon phase was mitigated, and therefore, the thickness change rate of the negative electrode after the initial charge was smaller by 5% than that of the battery Bl. Thus, in the battery Al, the capacity maintenance rate was increased by 8% compared to the battery Bl, and the cycle characteristics were greatly improved.

[0201] Industrial applicability

[0202] The present disclosure can provide a nonaqueous electrolyte secondary battery having high capacity and good charge / discharge cycle characteristics. The nonaqueous electrolyte secondary battery of the present disclosure is useful as a main power source for mobile body communication devices, portable electronic devices, and the like.

[0203] Explanation of Reference Signs

[0204] 1 electrode group

[0205] 2 positive electrode lead

[0206] 3 negative electrode lead

[0207] 4 battery case

[0208] 5 sealing plate

[0209] 6 negative electrode terminal

[0210] 7 gasket

[0211] 8 sealing plug

[0212] 20 composite particle

[0213] 21 lithium silicate phase

[0214] 22 silicon phase

[0215] 23 base particle

[0216] 24 primary particle

[0217] 26 conductive layer

[0218] 28 crystal phase of SiO2

Claims

1. A negative electrode active material for a nonaqueous electrolyte secondary battery, comprising a composite particle, the composite particle comprising: a lithium silicate phase, a silicon phase dispersed in the lithium silicate phase, and a crystalline phase of silicon dioxide dispersed in the lithium silicate phase, the crystalline phase of silicon dioxide comprising β-cristobalite and quartz, the silicon phase having a crystallite size of less than 10 nm. In an X-ray diffraction spectrum of the composite particle measured by X-ray diffraction, a peak derived from the β-cristobalite appears near 2θ = 21.6°, a peak derived from the quartz appears near 2θ = 26.3°. the lithium silicate phase comprises Li2Si2O5. the lithium silicate phase comprises at least one element selected from the group consisting of sodium, potassium, magnesium, barium, zirconium, niobium, lanthanoid elements, tantalum, vanadium, titanium, phosphorus, bismuth, zinc, tin, lead, antimony, cobalt, fluorine, tungsten, aluminum, and boron.

2. The negative active material for nonaqueous electrolyte secondary batteries according to claim 1, wherein 6. A nonaqueous electrolyte secondary battery comprising: a positive electrode, a negative electrode, and a nonaqueous electrolyte, the negative electrode comprising the negative electrode active material for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 5. ​ 3. The negative active material for nonaqueous electrolyte secondary batteries according to claim 2, wherein The ratio of the intensity I of the peak derived from the β-cristobalite to the intensity I of the peak derived from the quartz in the X-ray diffraction pattern of the composite particle A The ratio of the intensity I of the peak derived from the β-cristobalite to the intensity I of the peak derived from the quartz in the X-ray diffraction pattern of the composite particle B A B is 0.1 or more.​​ 4. The negative electrode active material for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein ​ 5. The negative active material for nonaqueous electrolyte secondary batteries according to any one of claims 1 to 3, wherein, ​ ​ ​

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