Anode material for non-aqueous electrolyte secondary batteries and non-aqueous electrolyte secondary batteries

Abstract

This negative electrode material for non-aqueous electrolyte secondary battery comprises a composite particle and an electric conductive layer arranged on the surface of the composite particle, wherein the composite particle comprises a silicate phase and a silicon phase dispersed within the silicate phase, and the electric conductive layer contains a calcium component.

Description

Technical Field

[0001] The present disclosure mainly relates to improvements in the negative electrode of a non-aqueous electrolyte secondary battery.

Background Art

[0002] Non-aqueous electrolyte secondary batteries, particularly lithium-ion secondary batteries, are expected to be used as power sources for small consumer applications, power storage devices, and electric vehicles because they have a high voltage and high energy density. As the energy density of batteries is required to be increased, the use of materials containing silicon (silicon) that alloy with lithium as a negative electrode active material with a high theoretical capacity density is expected.

[0003] Patent Document 1 proposes a negative electrode active material including a lithium silicate phase represented by Li 2z SiO 2+z (0 < z < 2) and silicon particles dispersed in the lithium silicate phase.

[0004] Patent Document 2 proposes core-shell composite particles, where the core is a porous carbon-based matrix containing silicon particles, the silicon particles are encapsulated in the pores of the matrix, the pores containing the silicon particles have a diameter of ≧ 60 nm, and the shell is obtained by carbonization of one or more carbon precursors selected from the group consisting of tar, pitch, hard carbon, soft carbon, and hydrocarbons having 1 to 20 carbon atoms, resulting in a non-porous shell. The pores containing the silicon particles are obtained by first coating the silicon particles with one or more sacrificial materials, then coating the resulting product with one or more carbon precursors, and subsequently removing the sacrificial material-based coating again at a later time, and the carbon precursor-based coating is converted to a carbon-based matrix before or during the removal of the sacrificial material.

[0005] Patent Document 3 proposes a composite particle having a base material and a coating layer covering the base material, characterized in that the coating layer is composed of a plurality of coating particles and the stress relaxation rate of the coating particles is 5.0% or more. Methods for adjusting the stress relaxation rate of the coating particles include chemically or physically surface-treating the surface of the coating particles, or adding additives. By using lubricants such as higher fatty acids like stearic acid, metal soaps such as magnesium stearate, calcium stearate, aluminum stearate, zinc stearate, and calcium montana, or organic compound waxes such as paraffin wax, friction between particles can be reduced and the stress relaxation rate reduced. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2016 / 35290 brochure [Patent Document 2] Special Publication No. 2018-524246 [Patent Document 3] Japanese Patent Publication No. 2014-197503 [Overview of the project]

[0007] Among the patent documents 1 to 3, the negative electrode active material comprising a lithium silicate phase and silicon particles dispersed therein, as described in Patent Document 1, is promising. However, lithium silicate is easily dissolved in decomposition products (e.g., HF) produced by side reactions of the non-aqueous electrolyte component. When lithium silicate dissolves, the surface area of ​​the lithium silicate phase increases, accelerating the progress of side reactions. As a result, the capacity degradation associated with repeated charge-discharge cycles of non-aqueous electrolyte secondary batteries becomes greater.

[0008] In view of the above, one aspect of the present disclosure relates to a negative electrode material for a non-aqueous electrolyte secondary battery, which includes composite particles and a conductive layer disposed on the surface of the composite particles. The composite particles include a silicate phase and a silicon phase dispersed in the silicate phase, and the conductive layer contains a calcium component.

[0009] Another aspect of the present disclosure relates to a non-aqueous electrolyte secondary battery, which includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode includes the above-mentioned negative electrode material for a non-aqueous electrolyte secondary battery.

[0010] According to the present disclosure, deterioration of the capacity associated with repeated charge-discharge cycles of a non-aqueous electrolyte secondary battery is suppressed. Hereinafter, such deterioration of the capacity is also referred to as "decrease in charge-discharge cycle characteristics".

Brief Description of the Drawings

[0011] [Figure 1] FIG. 1 is a diagram schematically showing a cross-section of composite particles having a Ca-containing conductive layer according to an embodiment of the present disclosure. [Figure 2] FIG. 2 is a diagram schematically showing a cross-section of composite particles having a Ca-containing conductive layer after several charge-discharge cycles. [Figure 3] FIG. 3 is a schematic perspective view of a non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure, with a part cut away. [Figure 4A] FIG. 4A is a TEM photograph of a cross-section of composite particles having a Ca-containing conductive layer according to an embodiment of the present disclosure. [Figure 4B] FIG. 4B is a spectrum obtained when region 1 in the Ca-containing conductive layer of FIG. 4A and region 2 in the composite particles are analyzed by electron energy loss spectroscopy (TEM-EELS).

Embodiments for Carrying Out the Invention

[0012] [Negative Electrode Material for Non-Aqueous Electrolyte Secondary Battery] The negative electrode material for a non-aqueous electrolyte secondary battery according to the embodiment of this disclosure comprises composite particles and a conductive layer disposed on the surface of the composite particles. The composite particles comprise a silicate phase and a silicon phase dispersed within the silicate phase. The conductive layer contains a calcium (Ca) component. Hereinafter, the conductive layer containing the Ca component will also be referred to as the "Ca-containing conductive layer". Furthermore, a negative electrode material comprising composite particles and a conductive layer disposed on its surface, wherein the conductive layer contains a Ca component, will also be referred to as "composite particles having a Ca-containing conductive layer".

[0013] The silicon phase repeatedly expands and contracts during charging and discharging of non-aqueous electrolyte secondary batteries. However, because the silicon phase is dispersed within the silicate phase, the composite particles have excellent structural stability. Furthermore, the silicate phase has fewer reaction sites with lithium ions compared to silica (SiO2), resulting in a smaller irreversible capacity.

[0014] Because the Ca-containing conductive layer is located on the surface of the composite particles, decomposition products such as HF (hereinafter referred to as electrolyte decomposition products) generated by side reactions of the non-aqueous electrolyte component have more opportunities to come into contact with the Ca-containing conductive layer than with the silicate phase. Furthermore, the Ca component in the Ca-containing conductive layer is highly reactive with the electrolyte decomposition products and incorporates them into the Ca-containing conductive layer during the reaction. As a result, the electrolyte decomposition products are converted into stable substances such as CaF2. By incorporating and stabilizing the electrolyte decomposition products in the Ca-containing conductive layer, the opportunities for contact between the electrolyte decomposition products and the silicate phase are significantly reduced. Therefore, the increase in the surface area of ​​the lithium silicate phase due to the dissolution of lithium silicate is suppressed, and the deterioration of charge-discharge cycle characteristics due to the progress of side reactions is suppressed.

[0015] Furthermore, the state in which the Ca-containing conductive layer is arranged on the surface of the composite particles includes states in which the Ca-containing conductive layer is attached to or deposited on at least a part of the surface of the composite particles, or states in which the Ca-containing conductive layer covers at least a part of the surface of the composite particles in a film-like manner.

[0016] The average particle size of the composite particles is, for example, 1 μm or more and 25 μm or less, and may also be 4 μm or more and 15 μm or less. Within the above particle size range, stress due to volume changes of the composite particles during charging and discharging is easily relieved, making it easier to obtain good cycle characteristics. The surface area of ​​the composite particles also becomes of an appropriate size, and the decrease in capacity due to side reactions with non-aqueous electrolytes is suppressed. The average particle size of the composite particles refers to the particle size (volume-average particle size) at which the integrated volume value in the particle size distribution measured by laser diffraction scattering method becomes 50%. For the measuring device, for example, the "LA-750" manufactured by HORIBA, Ltd. can be used. Since the Ca-containing conductive layer does not substantially affect the average particle size of the composite particles, the average particle size of the composite particles and the average particle size of composite particles with a Ca-containing conductive layer can be considered equivalent.

[0017] The composite particles can be extracted from the battery by the following method. First, the battery is disassembled in a completely discharged state and the negative electrode is removed. The negative electrode is then washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove non-aqueous electrolyte components. As described later, the negative electrode comprises a negative electrode current collector and a negative electrode composite layer supported on its surface. The negative electrode composite layer is then peeled off the copper foil and crushed in a mortar to obtain a sample powder. Next, the sample powder is dried in a dry atmosphere for 1 hour and immersed in weakly boiled 6M hydrochloric acid for 10 minutes to remove elements other than the composite particles. Next, the sample powder is washed with deionized water, filtered, and dried at 200°C for 1 hour. After that, the composite particles can be isolated by heating to 900°C in an oxygen atmosphere to remove the Ca-containing conductive layer. A completely discharged state is defined as a state where the depth of discharge (DOD) is 90% or more (state of charge (SOC) is 10% or less).

[0018] <Silicate phase> The silicate phase is composed of a compound containing a metal element, silicon (Si), and oxygen (O). The metal element is not particularly limited, but including at least lithium facilitates the movement of lithium ions into and out of the silicate phase. In other words, it is preferable that the silicate phase contains at least lithium silicate.

[0019] Lithium silicate is a silicate containing lithium (Li), silicon (Si), and oxygen (O). The atomic ratio of O to Si in lithium silicate, O / Si, is, for example, greater than 2 and less than 4. When the O / Si ratio is greater than 2 and less than 4 (z is 0 < z < 2 in the formula described later), it is advantageous in terms of the stability of the silicate phase and lithium ion conductivity. Preferably, the O / Si ratio is greater than 2 and less than 3. Also, the atomic ratio of Li to Si in lithium silicate, Li / Si, is, for example, greater than 0 and less than 4.

[0020] The composition of lithium silicate can be represented by the formula: Li 2z SiO 2+z (0 < z < 2). From the viewpoints of stability, ease of preparation, lithium ion conductivity, etc., z preferably satisfies the relationship 0 < z < 1, and z = 1 / 2 is more preferable. Lithium silicate satisfying z = 1 / 2 can be represented by Li2Si2O5. It is desirable for lithium silicate to contain Li2Si2O5 as a main component, and it is desirable for Li2Si2O5 to be the main component of the entire silicate phase. Here, the "main component" refers to a component that occupies 50 mass% or more of the mass of the entire lithium silicate or the entire silicate phase, and may occupy 70 mass% or more of the component.

[0021] The silicate phase may further contain another element M in addition to Li, Si, and O. When the silicate phase contains the element M, the chemical stability and lithium ion conductivity of the silicate phase are improved, or side reactions due to the contact between the silicate phase and the non-aqueous electrolyte are suppressed.

[0022] As element M, for example, at least one selected from the group consisting of sodium (Na), potassium (K), magnesium (Mg), barium (Ba), zirconium (Zr), niobium (Nb), 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), boron (B), and rare earth elements may be used. From the viewpoint of resistance to non-aqueous electrolytes and structural stability of the silicate phase, it is preferable that element M includes at least one selected from the group consisting of Zr, Ti, P, Al, and B.

[0023] Rare earth elements can improve the initial charge-discharge efficiency of the charge-discharge cycle. The rare earth elements may be scandium (Sc), yttrium (Y), or lanthanide elements. Preferably, the rare earth elements include at least one selected from the group consisting of lanthanum (La), cerium (Ce), praseodymium (Pr), and neodymium (Nd). From the viewpoint of improving lithium-ion conductivity, it is more preferable that the rare earth elements include La. The proportion of La in the total rare earth elements is preferably 90 atomic% or more and 100 atomic% or less.

[0024] The silicate phase may also contain trace amounts of elements such as iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), and molybdenum (Mo).

[0025] Element M may form a compound. Depending on the type of element M, the compound may be, for example, a silicate of element M or an oxide of element M.

[0026] In the silicate phase, the content of element M is, for example, between 1 mol% and 40 mol% relative to the total amount of elements other than oxygen.

[0027] The content of Li, Si, and element M in the silicate phase can be measured, for example, by analyzing the cross-section of the negative electrode mixture layer.

[0028] First, a fully discharged battery is disassembled, the negative electrode is removed, and the negative electrode is washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove non-aqueous electrolyte components. After drying, a 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).

[0029] The content of each element can then be determined by one of the following methods. Furthermore, the composition of the silicate phase can be calculated from the content of each element.

[0030] <edx> From the cross-sectional image of the reflection electron image of the negative electrode mixture layer, 10 composite particles with a maximum particle diameter of 5 μm or more are randomly selected, and for each of them, elemental mapping analysis is performed by energy-dispersive X-ray (EDX). The area of the target element is calculated using image analysis software. An observation magnification of 2000 to 20000 times is desirable. The measured values of the area of the predetermined element contained in the 10 particles are averaged. The content of the target element is calculated from the obtained average value.

[0031] The measurement conditions for the desirable cross-sectional SEM-EDX analysis are shown below.

[0032] <SEM-EDX Measurement Conditions> Processing device: JEOL, SM-09010 (Cross Section Polisher) Processing conditions: acceleration voltage 6 kV Current value: 140 μA Vacuum degree: 1×10 -3 ~2×10 -3 Pa Measurement device: electron microscope SU-70 made by HITACHI Acceleration voltage during analysis: 10 kV Field: Free mode Probe current mode: Medium Probe current range: High Anode Ap.: 3 OBJ Ap.: 2 Analysis area: 1 μm square Analysis software: EDAX Genesis CPS: 20500 Lsec: 50 Time constant: 3.2 <aes> Ten composite particles with a maximum diameter of 5 μm or more are randomly selected from the cross-sectional image of the backscattered electron image of the negative electrode mixture layer, and qualitative and quantitative elemental analysis is performed on each particle using an Auger electron spectroscopy (AES) analyzer (e.g., JEOL Ltd., JAMP-9510F). Measurement conditions can be, for example, an acceleration voltage of 10 kV, a beam current of 10 nA, and an analysis area of ​​20 μmφ. The content of a predetermined element is calculated by averaging the content of each of the ten particles.

[0033] Note that EDX analysis and AES analysis are performed on the area more than 1 μm inward from the peripheral edge of the cross-section of the composite particles.

[0034] <icp> The composite particle sample is completely dissolved in a heated acid solution (a mixed acid of hydrofluoric acid, nitric acid, and sulfuric acid), and the carbon residue in the solution is filtered out. The resulting filtrate is then analyzed by inductively coupled plasma atomic emission spectroscopy (ICP) to measure the spectral intensity of each element. Subsequently, a calibration curve is created using commercially available elemental standard solutions, and the content of each element in the composite particles is calculated.

[0035] In addition, the quantitative analysis of each element can also be performed using electron microanalyzers (EPMA), laser ablation ICP mass spectrometry (LA-ICP-MS), X-ray photoelectron spectroscopy (XPS), etc.

[0036] Furthermore, the content of B, Na, K, and Al in the composite particles can be quantitatively analyzed in accordance with JIS R3105 (1995) (Analytical Method for Borosilicate Glass).

[0037] The ratio of the amount of Ca contained in the Ca-containing conductive layer to the mass of the composite particles may be quantitatively analyzed using composite particles having a Ca-containing conductive layer in accordance with JIS R3101 (1995) (Analytical Method for Soda-Lime Glass).

[0038] The carbon content in the composite particles may be measured using a carbon-sulfur analyzer (for example, the EMIA-520 manufactured by Horiba, Ltd.). The sample is weighed onto a magnetic board, a combustion aid is added, and it is inserted into a combustion furnace heated to 1350°C (carrier gas: oxygen). The amount of carbon dioxide gas generated during combustion is detected by infrared absorption. A calibration curve is prepared, for example, using carbon steel (carbon content 0.49%) manufactured by Bureau of Analysed Samples, Ltd., and the carbon content of the sample is calculated (high-frequency induction heating furnace combustion - infrared absorption method).

[0039] The oxygen content contained in the composite particles may be measured using an oxygen, nitrogen, and hydrogen analyzer (for example, the EGMA-830 type manufactured by Horiba, Ltd.). The sample is placed in a Ni capsule, and together with Sn pellets and Ni pellets serving as a flux, it is put into a carbon crucible heated at 5.75 kW, and the carbon monoxide gas released is detected. A calibration curve is created using the standard sample Y2O3 to calculate the oxygen content of the sample (inert gas fusion-non-dispersive infrared absorption method).

[0040] In the composite particles, there exist a silicate phase and a silicon phase. The Si content obtained by the above method is the total of the amount of Si constituting the silicon phase and the amount of Si in the silicate phase. On the other hand, the amount of Si constituting the silicon phase can be separately quantified using Si-NMR. Therefore, by using Si-NMR, the amount of Si constituting the silicon phase and the amount of Si in the silicate phase can be distinguished and quantified. For the standard substance required for quantification, a mixture containing a silicate phase and a silicon phase with known Si contents in a predetermined ratio may be used.

[0041] The following shows the desirable measurement conditions for Si-NMR.

[0042] <Si-NMR measurement conditions> Measuring device: A solid nuclear magnetic resonance spectrometer (INOVA-400) manufactured by Varian Probe: Varian 7mm CPMAS-2 MAS: 4.2 kHz MAS speed: 4 kHz Pulse: DD (45° pulse + signal acquisition time 1H decoupling) Repetition time: 1200 sec to 3000 sec Observation width: 100 kHz Observation center: Around -100 ppm Signal acquisition time: 0.05 sec Number of integrations: 560 Sample amount: 207.6 mg <Silicon phase> The silicon phase is a phase of elemental silicon (Si) that repeatedly absorbs and releases lithium ions during charging and discharging of the battery. The capacity is generated by the Faraday reaction involving the silicon phase. Because the silicon phase has a large capacity, it also expands and contracts significantly during charging and discharging. However, since the silicon phase is dispersed within the silicate phase, the stress caused by the expansion and contraction of the silicon phase is relieved.

[0043] The silicon phase may be composed of multiple crystallites. Preferably, the crystallite size of the silicon phase is 30 nm or less. When the crystallite size of the silicon phase is 30 nm or less, the volume change due to expansion and contraction of the silicon phase during charging and discharging can be reduced, further improving the cycle characteristics. For example, isolation of the silicon phase due to the formation of voids around the silicon phase during contraction is suppressed, thus suppressing a decrease in charge-discharge efficiency. The lower limit of the crystallite size of the silicon phase is not particularly limited, but is, for example, 1 nm or more. More preferably, the crystallite size of the silicon phase is 10 nm or more and 30 nm or less, and even more preferably 15 nm or more and 25 nm or less. When the crystallite size of the silicon phase is 10 nm or more, the surface area of ​​the silicon phase can be kept small, making it less likely for the silicon phase to deteriorate, leading to the generation of irreversible capacitance. When the crystallite size of the silicon particles is 30 nm or less, the expansion and contraction of the silicon phase can be made more uniform, stress on the composite particles can be easily relieved, and the cycle characteristics can be improved. The crystallite size of the silicon phase is calculated using Scherrer's formula from the full width at half maximum of the diffraction peaks attributed to the (111) plane of the silicon phase (elementary Si) in the X-ray diffraction pattern.

[0044] The silicon phase of the composite particles contained in the battery before the first charge is, for example, particulate. The average particle size of the particulate silicon phase is preferably 500 nm or less, more preferably 200 nm or less, and even more preferably 50 nm or less. After the first charge, the average particle size of the silicon phase is preferably 400 nm or less, more preferably 100 nm or less. By refining the silicon phase, the volume change of the composite particles during charge and discharge is reduced, and the structural stability of the composite particles is further improved. The average particle size of the silicon phase is measured using a cross-sectional image of the composite particles obtained by SEM. Specifically, the average particle size of the silicon phase is obtained by averaging the maximum diameters of any 100 silicon phases.

[0045] From the viewpoint of increasing the 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 even more preferably 55% by mass or more. In this case, the diffusibility of lithium ions is good, and excellent load characteristics can be 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 even more 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 between the non-aqueous electrolyte and the silicon phase is easily suppressed.

[0046] <Ca-containing conductive layer> The Ca-containing conductive layer is disposed on the surface of the composite particles. The Ca-containing conductive layer imparts conductivity to the composite particles. That is, the Ca-containing conductive layer constitutes a part of the conductive network in the negative electrode. Although the silicate phase has no conductivity, the composite particles having a conductive layer have conductivity, so the current collection property of the negative electrode is improved.

[0047] The Ca-containing conductive layer covers at least a portion of the surface of the composite particles. Therefore, the Ca-containing conductive layer also acts as a barrier layer to suppress side reactions between the non-aqueous electrolyte and the composite particles, and between the electrolyte decomposition products and the composite particles. Among these, electrolyte decomposition products (especially HF) tend to degrade the silicate phase. In contrast, the calcium component contained in the Ca-containing conductive layer reacts preferentially with the electrolyte decomposition products rather than with the silicate phase, thus significantly suppressing the degradation of the silicate phase.

[0048] From the viewpoint of ensuring capacity, the Ca-containing conductive layer is preferably thin enough to not substantially affect the average particle size of the composite particles. Considering the need to ensure conductivity and lithium ion diffusion, the thickness of the Ca-containing conductive layer is preferably 1 nm or more and 200 nm or less, and more preferably 5 nm or more and 100 nm or less. The thickness of the conductive layer can be measured by cross-sectional observation of the composite particles using SEM or TEM. Specifically, the thickness of the Ca-containing conductive layer is determined by averaging the thickness of the Ca-containing conductive layer of any 10 composite particles having conductive layers. However, the thickness of the Ca-containing conductive layer is determined by measuring the thickness at any 10 locations for each composite particle and averaging them.

[0049] The mass ratio of the Ca-containing conductive layer to the total of the composite particles and the Ca-containing conductive layer may be 0.5% by mass or more and 10% by mass or less, or 1% by mass or more and 5% by mass or less.

[0050] The Ca-containing conductive layer comprises a conductive material and Ca components dispersed within the conductive material. Carbonaceous materials are preferred as the conductive material. Examples of carbonaceous materials include amorphous carbon, graphite, easily graphitizable carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon). Among these, amorphous carbon is preferred because it easily forms a thin conductive layer covering the surface of the composite particles. Examples of amorphous carbon include carbon black, calcined pitch, coke, and activated carbon. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon.

[0051] The Ca component is desirable as its reactivity with electrolyte decomposition products increases. From this viewpoint, for example, calcium carbonate (CaCO3), calcium oxide (CaO), calcium carbide (CaC2), and calcium hydroxide (Ca(OH)2) are desirable, with CaCO3 and CaO being particularly desirable. One of these may be included in the Ca-containing conductive layer alone, or two or more may be included in the Ca-containing conductive layer in any combination.

[0052] Even in trace amounts, the Ca component in the Ca-containing conductive layer reacts with electrolyte decomposition products to suppress the degradation of the silicate phase. Therefore, the amount of Ca contained in the Ca-containing conductive layer is not particularly limited, as long as its function as a conductive layer is not significantly impaired. It is sufficient that the Ca component is detected in the Ca-containing conductive layer. The amount of Ca contained in the Ca-containing conductive layer may be, for example, 0.01% or more and 0.5% or less, 0.01% or more and 0.3% or less, or 0.02% or more and 0.1% or less, relative to the mass of the composite particles.

[0053] The particle size of the Ca component (e.g., CaCO3 or CaO) contained in the Ca-containing conductive layer is preferably such that it can be uniformly dispersed in the Ca-containing conductive layer, and is preferably limited to a size less than or equal to the thickness of the conductive material constituting the conductive layer. Specifically, the average particle size of the Ca component is preferably 200 nm or less, and more preferably 100 nm or less. The average particle size of the Ca component is measured using a cross-sectional image of the composite particles obtained by SEM or TEM. Specifically, the average particle size of the Ca component is determined by averaging the maximum diameters of any 10 Ca components.

[0054] The presence of a Ca component (e.g., CaCO3 or CaO) in a Ca-containing conductive layer can be confirmed, for example, by analyzing a cross-section of a composite particle having a Ca-containing conductive layer using electron energy loss spectroscopy (TEM-EELS). If the Ca-containing conductive layer contains, for example, CaCO3, the TEM-EELS loss spectrum will have peaks originating from CaCO3 at or near 349 eV and at or near 352 eV. Similarly, if the Ca-containing conductive layer contains, for example, CaO, the TEM-EELS loss spectrum will have peaks originating from CaO. On the other hand, if the TEM-EELS loss spectrum does not show peaks originating from CaCO3 or CaO when the inside of the composite particle is analyzed, it can be said that the Ca component is selectively contained in the Ca-containing conductive layer.

[0055] Figure 4A is a TEM image of a cross-section near the interface between the Ca-containing conductive layer and the composite particle of the Ca-containing conductive layer of the composite particle having a Ca-containing conductive layer according to Example 1, which will be described later. Figure 4B is the spectrum obtained when region 1 in the Ca-containing conductive layer and region 2 in the composite particle in Figure 4A were analyzed by TEM-EELS. The spectrum of region 1 has peaks originating from CaCO3 at or near 349 eV and at or near 352 eV, similar to the standard CaCO3 data. Therefore, it can be confirmed that CaCO3 is present as a Ca component in the Ca-containing conductive layer. In the spectrum of region 2, no peaks originating from CaCO3 are observed at or near 349 eV or at or near 352 eV.

[0056] [Method for manufacturing composite particles having a conductive layer] Composite particles having a conductive layer are obtained by forming a Ca-containing conductive layer on the composite particles. The composite particles are manufactured, for example, by a manufacturing method including the following steps 1 to 4. (Step 1) Step to obtain the raw material silicate. (Second step) After the first step, the raw material silicate and raw material silicon are compounded and the silicon phase is dispersed in the silicate phase to obtain a composite intermediate. (Third step) A step of heat-treating the composite intermediate to obtain a sintered body containing a silicate phase and a silicon phase dispersed within the silicate phase. (Fourth step) A step of crushing the sintered body to obtain composite particles containing a silicate phase and a silicon phase dispersed within the silicate phase.

[0057] [1st step] The first step includes, for example, step 1a of mixing silicon dioxide, a lithium compound, and optionally a compound containing element M to obtain a mixture, and step 1b of calcining the mixture to obtain a raw material silicate. The calcination in step 1b is carried out, for example, in an oxidizing atmosphere. The calcination temperature in step 1b is preferably 400°C or higher and 1200°C or lower, and more preferably 800°C or higher and 1100°C or lower.

[0058] Examples of lithium compounds include lithium carbonate, lithium oxide, lithium hydroxide, and lithium hydride. Lithium compounds may be used individually or in combination of two or more.

[0059] Compounds containing element M can include oxides, hydroxides, hydrides, halides, carbonates, oxalates, nitrates, sulfates, etc. of element M. Compounds containing element M may be used individually or in combination of two or more.

[0060] [Second process] The second step involves, for example, applying shear force to a mixture of raw silicate and raw silicon while grinding the mixture to obtain a composite intermediate in the form of fine particles. Here, for example, the raw silicate and raw silicon can be mixed in a predetermined mass ratio, and the mixture can be ground into fine particles while stirring using a grinding device such as a ball mill.

[0061] For the raw material silicon, coarse silicon particles with an average particle size of several micrometers to several tens of micrometers can be used. Preferably, the silicon particles obtained in the final product are controlled so that the crystallite size, calculated by Scherrer's formula from the full width at half maximum of the diffraction peaks attributed to the Si(111) plane of the X-ray diffraction pattern, is 10 nm or larger.

[0062] The second step is not limited to the above. For example, silicon nanoparticles and raw silicate nanoparticles may be synthesized without using a grinding device, and these may be mixed.

[0063] [3rd step] The third step includes, for example, a step of firing a composite intermediate while applying pressure to the finely atomized composite intermediate using a hot press or the like to obtain a sintered body. The firing of the composite intermediate is carried out, for example, in an inert atmosphere (for example, an atmosphere such as argon or nitrogen). The firing temperature is preferably 450°C or higher and 1000°C or lower. In this temperature range, it is easy to disperse fine silicon particles in the silicate phase with low crystallinity. The raw material silicate is stable in this temperature range and hardly reacts with silicon. The firing temperature is preferably 550°C or higher and 900°C or lower, and more preferably 650°C or higher and 850°C or lower. The firing time is, for example, 1 hour or more and 10 hours or less.

[0064] [4th step] The fourth step is to grind the sintered body to have a desired particle size distribution to obtain composite particles containing a silicate phase and a silicon phase dispersed within the silicate phase. The composite particles are ground to, for example, an average particle size of 1 to 25 μm.

[0065] [Process for forming a Ca-containing conductive layer] Next, an example of the process of forming a Ca-containing conductive layer on composite particles will be described. As raw materials for the conductive material that forms the conductive layer, for example, coal pitch or coal tar pitch, petroleum pitch, phenolic resin, etc., can be used. The raw materials for the conductive material and the composite particles are mixed, and the mixture is fired to carbonize the raw materials for the conductive material, thereby forming a conductive layer that covers at least a portion of the surface of the composite particles.

[0066] Here, a Ca-containing conductive layer is formed by incorporating a Ca component raw material into the conductive material raw material. As the Ca component raw material, a Ca-containing organic substance is preferable. Ca-containing organic substances have a high affinity for the conductive material raw material and readily dissolve or disperse uniformly within it. Among Ca-containing organic substances, calcium fatty acid is preferable. As the calcium fatty acid, a Ca salt of a higher fatty acid (e.g., calcium stearate) is preferable. Calcium fatty acid dissolves in conductive material raw materials such as pitch. As a result, the affinity between the conductive material raw material and the composite particles increases, and the conductive material raw material easily forms a thin coating on the surface of the composite particles.

[0067] Higher fatty acids are not particularly limited, but examples include linear alkyl carboxylic acids such as octyl acid, nonanoic acid, decanoic acid, undecanoic acid, dodecanoic acid (lauric acid), tridecanoic acid, tetradecanoic acid (myristic acid), pentadecanoic acid, hexadecanoic acid, heptadecanoic acid, octadecanoic acid (stearic acid), nonadecanoic acid, eicosanoic acid, heneicosanoic acid, and docosanoic acid; branched alkyl carboxylic acids such as 2-ethylhexanoic acid, isododecanoic acid, isotridecanoic acid, isotetradecanoic acid, isohexadecanoic acid, and isooctadecanoic acid; and linear alkenyl carboxylic acids such as octadecenoic acid, octadecadienoic acid, octadecatrienoic acid, and oleic acid. Among these, oleic acid, stearic acid, lauric acid, and myristic acid are preferred.

[0068] The firing of the mixture of conductive material raw materials and composite particles is carried out, for example, in an inert atmosphere (e.g., an atmosphere of argon, nitrogen, etc.). The firing temperature is preferably 450°C or higher and 1000°C or lower. Within this temperature range, it is easy to form a highly conductive Ca-containing conductive layer on the silicate phase with low crystallinity. Through firing, the Ca component raw materials are converted into Ca components such as calcium carbonate and calcium oxide. The firing temperature is preferably 550°C or higher and 900°C or lower, and more preferably 650°C or higher and 850°C or lower. The firing time is, for example, 1 hour or more and 10 hours or less.

[0069] Furthermore, the Ca-containing conductive layer may be formed on the composite particles by other methods. For example, a Ca-containing conductive layer may be formed by reacting a hydrocarbon gas with an organic substance containing Ca on the surface of the composite particles using a gas-phase method such as CVD. Acetylene, methane, etc., can be used as the hydrocarbon gas. Alternatively, carbon black may be mixed with the organic substance containing Ca and the composite particles to deposit a precursor for the Ca-containing conductive layer on the surface of the composite particles, and then the precursor for the Ca-containing conductive layer may be calcined together with the composite particles to form the Ca-containing conductive layer.

[0070] Hereinafter, an example of a negative electrode material for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure will be described with reference to Figure 1. Figure 1 is a schematic diagram showing a cross-section of a composite particle 11 having a Ca-containing conductive layer 15. The composite particle 11 comprises a silicate phase 12, a silicon (elemental Si) phase 13 dispersed within the silicate phase 12, and a compound 14 of an optional element M. At least a portion of the surface of the composite particle 11 is covered with the Ca-containing conductive layer 15.

[0071] The composite particles 11 have, for example, a sea-island structure, and in any cross-section, fine silicon phases 13 are scattered substantially uniformly within the matrix of silicate phase 12 without being concentrated in any particular region. The silicate phase 12 may also contain SiO2 in amounts equivalent to the native oxide film formed on the surface of the silicon phase.

[0072] Figure 2 is a schematic cross-sectional view showing the state of composite particles 11 having a Ca-containing conductive layer after assembling a battery using composite particles 11 as the negative electrode active material and performing several charge-discharge cycles. Through charging and discharging, the particulate silicon phases 13 that were adjacent in Figure 1 become connected to each other, and a network-like silicon phase 16 can be formed.

[0073] [Nonaqueous electrolyte secondary battery] A non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode includes the above-mentioned negative electrode material for a non-aqueous electrolyte secondary battery.

[0074] The following provides a detailed explanation of non-aqueous electrolyte secondary batteries.

[0075] [Negative electrode] The negative electrode may comprise 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 coating the surface of the negative electrode current collector with a negative electrode slurry, which is obtained by dispersing the negative electrode mixture in a dispersion medium, and drying it. The dried coating may be rolled if necessary. The negative electrode mixture layer may be formed on one surface of the negative electrode current collector or on both surfaces.

[0076] The negative electrode mixture contains a negative electrode active material as an essential component and may include binders, conductive agents, thickeners, etc., as optional components. The negative electrode active material used is the negative electrode material described above (composite particles having a conductive layer).

[0077] The negative electrode active material preferably further contains a carbon material that electrochemically intercepts and releases lithium ions. Since the volume of composite particles having a conductive layer expands and contracts with charging and discharging, if the proportion of composite particles in the negative electrode active material becomes large, poor contact between the negative electrode active material and the negative electrode current collector is likely to occur with charging and discharging. On the other hand, by using composite particles having a conductive layer and a carbon material in combination, it becomes possible to achieve excellent cycle characteristics while imparting the high capacity of silicon particles to the negative electrode. From the viewpoint of increasing capacity and improving cycle characteristics, the proportion of carbon material in the total of composite particles having a conductive layer 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 even more preferably 75% by mass or more and 95% by mass or less.

[0078] Examples of carbon materials include graphite, easily graphitizable carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon). Among these, graphite is preferred because it has excellent charge-discharge stability and low irreversible capacity. Graphite refers to materials having a graphite-type crystal structure, and includes, for example, natural graphite, artificial graphite, and graphitized mesophase carbon particles. Carbon materials may be used individually or in combination of two or more types.

[0079] As the negative electrode current collector, non-porous conductive substrates (such as metal foil) and porous conductive substrates (such as mesh, net, or perforated sheet) are used. Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys. The thickness of the negative electrode current collector is not particularly limited, but from the viewpoint of balancing the strength of the negative electrode with weight reduction, 1 to 50 μm is preferred, and 5 to 20 μm is more desirable.

[0080] Examples of binders include resin materials such as fluororesins like polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins like polyethylene and polypropylene; polyamide resins like aramid resins; polyimide resins like polyimide and polyamideimide; acrylic resins like polyacrylic acid, methyl polyacrylate, and ethylene-acrylic acid copolymers; vinyl resins like polyacrylonitrile and polyvinyl acetate; polyvinylpyrrolidone; polyethersulfone; and rubber-like materials like styrene-butadiene copolymer rubber (SBR). A single binder may be used alone, or two or more may be used in combination.

[0081] Examples of conductive agents include carbon compounds such as acetylene black; conductive fibers such as carbon fibers and metal fibers; carbon fluoride; metal powders such as aluminum; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and organic conductive materials such as phenylene derivatives. Conductive agents may be used individually or in combination of two or more.

[0082] Examples of thickening agents include carboxymethylcellulose (CMC) and its modified forms (including salts such as sodium salts), cellulose derivatives such as methylcellulose (cellulose ethers, etc.); saponified polymers having vinyl acetate units such as polyvinyl alcohol; and polyethers (polyalkylene oxides such as polyethylene oxide, etc.). A single thickening agent may be used alone, or two or more may be used in combination.

[0083] The dispersion medium is not particularly limited, and examples thereof include water, alcohols such as ethanol, ethers such as tetrahydrofuran, amides such as dimethylformamide, N-methyl-2-pyrrolidone (NMP), or a mixed solvent thereof.

[0084] [Positive electrode] The positive electrode may include 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 onto the surface of the positive electrode current collector and drying it. The dried coating film may be rolled if necessary. The positive electrode mixture layer may be formed on one surface of the positive electrode current collector or on both surfaces. 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. As the dispersion medium of the positive electrode slurry, NMP or the like is used.

[0085] 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 O 4、 LiMePO 4、 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, B). Here, a = 0 to 1.2, b = 0 to 0.9, c = 2.0 to 2.3. The a value indicating the molar ratio of lithium increases or decreases during charge and discharge.

[0086] Among them, Li a Ni b Me 1-b O2 (where Me is at least one selected from the group consisting of Mn, Co, and Al, 0 < a ≤ 1.2, and 0.3 ≤ b ≤ 1). A lithium nickel composite oxide represented by this is preferred. From the perspective of increasing the capacity, it is more preferred to satisfy 0.85 ≤ b < 1. From the perspective of the stability of the crystal structure, Li containing 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) is even more preferred.

[0087] As the binder and the conductive agent, the same ones as those exemplified for the negative electrode can be used. As the conductive agent, graphite such as natural graphite and artificial graphite may be used.

[0088] The shape and thickness of the positive electrode current collector can be selected respectively from the shape and range corresponding to the negative electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, titanium, etc.

[0089] [Non-aqueous electrolyte] 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 preferably, for example, 0.5 mol / L or more and 2 mol / L or less. By setting the lithium salt concentration within the above range, a non-aqueous electrolyte excellent in ionic conductivity and having appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0090] Examples of non-aqueous solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC) and ethylene carbonate (EC). Examples of linear carbonate esters 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 linear carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. The non-aqueous solvent may be used alone or in combination of two or more types.

[0091] Examples of lithium salts include LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, LiB 10 Cl 10 Examples include lithium lower aliphatic carboxylates, LiCl, LiBr, LiI, borates, and imide salts. Examples of borates include lithium bis(1,2-benzenediolate(2-)-O,O')borate, lithium bis(2,3-naphthalenediolate(2-)-O,O')borate, lithium bis(2,2'-biphenyldiolate(2-)-O,O')borate, and lithium bis(5-fluoro-2-oleate-1-benzenesulfonic acid-O,O')borate. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO2)2), lithium bistrifluoromethanesulfonate imide (LiN(CF3SO2)2), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF3SO2)(C4F9SO2)), and lithium bispentafluoroethanesulfonate imide (LiN(C2F5SO2)2). Among these, LiPF6 is preferred. The lithium salt may be used alone or in combination of two or more types.

[0092] [Separator] Generally, it is desirable to interpose a separator between the positive and negative electrodes. The separator should have high ion permeability and appropriate mechanical strength and insulating properties. As the separator, a microporous thin film, woven fabric, nonwoven fabric, etc., can be used. As the material of the separator, polyolefins such as polypropylene and polyethylene are preferred.

[0093] One 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 around each other with a separator, is housed together with a non-aqueous electrolyte in an outer casing. However, it is not limited to this, and other forms of electrode groups may be used. For example, a stacked electrode group in which the positive electrode and negative electrode are stacked with a separator in between may also be used. The form of the non-aqueous electrolyte secondary battery is also not limited, and may be cylindrical, prismatic, coin-type, button-type, laminate-type, etc.

[0094] Below, the structure of a rectangular non-aqueous electrolyte secondary battery, as an example of a non-aqueous electrolyte secondary battery related to this disclosure, will be described with reference to Figure 3.

[0095] The battery comprises a bottomed rectangular battery case 4, an electrode group 1 housed within the battery case 4, and a non-aqueous electrolyte. The electrode group 1 has a long, strip-shaped negative electrode, a long, strip-shaped positive electrode, and a separator interposed between them. 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 portion is laser-welded. The sealing plate 5 has an injection hole for the non-aqueous electrolyte, which is sealed by a seal 8 after injection.

[0096] The present disclosure will be described in detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0097] <Example 1> [Preparation of composite particles] [1st step] Silicon dioxide and Li2CO3 were mixed, and the mixture was calcined in air at 950°C for 10 hours to obtain silicate. The obtained silicate was pulverized to an average particle size of 10 μm.

[0098] [Second process] Silicate and raw silicon (3N, average particle size 10 μm) were mixed. In the mixture, the mass ratio of silicate to raw silicon was 40:60.

[0099] The mixture was filled into a 500mL pot (made of stainless steel) of a planetary ball mill (Fritsch, P-5), 24 stainless steel balls (20mm in diameter) were placed in the pot, the lid was closed, and the mixture was ground at 200rpm for 50 hours in an inert atmosphere.

[0100] [3rd step] Next, the powdered mixture was removed in an inert atmosphere, and then fired at 600°C for 4 hours under pressure from a hot press in an inert atmosphere to obtain a sintered body of the mixture.

[0101] [4th step] Next, the obtained sintered body was crushed and passed through a 40 μm mesh to obtain composite particles.

[0102] [Analysis of composite particles] The crystallite size of the silicon phase in the composite particles, as determined by the method described above, was 15 nm.

[0103] The composition of the main component of the silicate phase in the composite particles, as determined by the method described above, was Li2Si2O5.

[0104] [Process for forming a Ca-containing conductive layer] Next, 100 parts by mass of composite particles, 5 parts by mass of coal tar pitch, and 1 part by mass of calcium stearate were mixed, and then fired at 800°C in an argon atmosphere to form a Ca-containing conductive layer covering at least a portion of the surface of the composite particles. During firing, the coal tar pitch was converted into amorphous carbon.

[0105] [Analysis of Ca-containing conductive layer] The thickness of the Ca-containing conductive layer, determined by the method described above, was 10 nm.

[0106] The mass ratio of the Ca-containing conductive layer to the total mass of the composite particles and the Ca-containing conductive layer was determined from the mass difference of the composite particles before and after the formation of the Ca-containing conductive layer, and was found to be 3% by mass.

[0107] Analysis by TEM-EELS confirmed that the Ca component in the Ca-containing conductive layer was CaCO3. Furthermore, the ratio of Ca to the mass of composite particles in the Ca-containing conductive layer, as determined by the method described above, was 0.07% by mass.

[0108] [Fabrication of the negative electrode] A composite particle having a Ca-containing conductive layer and graphite was mixed in a mass ratio of 5:95 and used as the negative electrode active material. A negative electrode slurry was prepared by adding water to a negative electrode mixture containing the negative electrode active material, the Na salt of CMC, and SBR in a mass ratio of 97.5:1:1.5 and stirring. Next, 1 m was applied to the surface of the copper foil. 2 The negative electrode slurry is applied so that the mass of the negative electrode mixture per unit is 190g. After the coating is dried, it is rolled out to create a copper foil with a density of 1.5g / cm³ on both sides. 3 A negative electrode was fabricated with a negative electrode mixture layer formed thereon.

[0109] [Fabrication of the positive electrode] A positive electrode slurry was prepared by adding NMP to a positive electrode mixture containing lithium cobalt oxide, acetylene black, and PVDF in a mass ratio of 95:2.5:2.5 and stirring. Next, the positive electrode slurry was applied to the surface of aluminum foil, and after the coating film dried, it was rolled to coat both sides of the aluminum foil with a density of 3.6 g / cm³. 3 A positive electrode was fabricated with a positive electrode mixture layer formed thereon.

[0110] [Preparation of non-aqueous electrolytes] A non-aqueous electrolyte (electrolyte solution) was prepared by dissolving LiPF6 at a concentration of 1.0 mol / L in a mixed solvent containing EC and DEC in a volume ratio of 3:7.

[0111] [Fabrication of non-aqueous electrolyte secondary batteries] A positive electrode and a negative electrode, each fitted with a tab, were wound around a separator to create an electrode group with the tabs located on the outermost part. The electrode group was inserted into an aluminum laminate film casing, vacuum-dried at 105°C for 2 hours, then a non-aqueous electrolyte was injected, and the opening of the casing was sealed to obtain battery A1 of Example 1.

[0112] Example 2 Battery A2 of Example 2 was manufactured in the same manner as in Example 1, except that in the step of forming a Ca-containing conductive layer on the composite particles, the content of calcium stearate per 100 parts by mass of composite particles was changed to 5 parts by mass, and the ratio of Ca contained in the Ca-containing conductive layer to the mass of composite particles was set to 0.33% by mass.

[0113] Comparative Example 1 Battery B1 of Comparative Example 1 was prepared in the same manner as in Example 1, except that calcium stearate was not used in the process of forming a Ca-containing conductive layer on the composite particles.

[0114] Comparative Example 2 Battery B2 of Comparative Example 2 was manufactured in the same manner as in Example 1, except that calcium stearate was not used in the step of forming a Ca-containing conductive layer on the composite particles, and 0.5% by mass of CaO (0.36% by mass as Ca) was included in the negative electrode mixture relative to the composite particles in the preparation of the negative electrode.

[0115] Comparative Example 3 Battery B3 of Comparative Example 3 was prepared in the same manner as in Example 1, except that calcium stearate was not used in the step of forming a Ca-containing conductive layer on the composite particles, and in the second step of preparing the composite particles, 3% by mass of CaO was added to the silicate, and 0.92% by mass of Ca was dispersed in the composite particles.

[0116] Comparative Example 4 Battery B4 of Comparative Example 4 was prepared in the same manner as in Example 1, except that in the process of forming a conductive layer on the composite particles, magnesium stearate was used at a ratio of 1 part by mass per 100 parts by mass of composite particles instead of calcium stearate to form a Mg-containing conductive layer. The ratio of Mg contained in the conductive layer to the mass of the composite particles is estimated to be 0.05% by mass based on the amount of magnesium stearate used.

[0117] Comparative Example 5 Battery B5 of Comparative Example 5 was prepared in the same manner as in Example 1, except that in the process of forming a conductive layer on the composite particles, aluminum stearate was used at a ratio of 1 part by mass per 100 parts by mass of composite particles instead of calcium stearate to form an Al-containing conductive layer. The ratio of Al contained in the conductive layer to the mass of the composite particles is estimated to be 0.06% by mass based on the amount of aluminum stearate used.

[0118] [Cycle Test] <Charging> Constant current charging was performed with a current of 1 It (800 mA) until the voltage reached 4.2 V, and then constant voltage charging was performed with a constant voltage of 4.2 V until the current was reduced to 1 / 20 It (40 mA).

[0119] <Discharge> Constant current discharge was performed with a current of 1 It (800mA) until the voltage reached 2.75V.

[0120] The pause period between charging and discharging was set to 10 minutes.

[0121] The charging and discharging were performed in an environment of 25°C.

[0122] For each battery, the discharge capacity C1 for the first cycle was determined. Table 1 shows the C1 values ​​of each battery relative to the discharge capacity C1 value obtained for battery B1 in Comparative Example 1, which was set to 100.

[0123] Furthermore, for each battery, the discharge capacity C100 at 100 cycles was determined, and the capacity degradation rate R was calculated as the ratio expressed by (C1-C100) / C1. Table 1 shows the R values ​​of each battery relative to the R value obtained for battery B1 in Comparative Example 1, with the R value set to 100. The smaller the R value, the smaller the deterioration in charge-discharge cycle characteristics, and the better the battery.

[0124] [Table 1]

[0125] In batteries A1 and A2 of Examples 1 and 2, a lower capacity degradation rate was obtained compared to battery B1 of Comparative Example 1. This is thought to be because the Ca-containing conductive layer covering at least a portion of the surface of the composite particles acted as a barrier layer to suppress side reactions between electrolyte decomposition products and composite particles, thereby suppressing the degradation of the silicate phase. On the other hand, in Comparative Example 2, in which CaO was included in the negative electrode mixture, the capacity degradation rate actually increased. This is thought to be because the alkaline CaO promoted the degradation of components such as the binder. In Comparative Example 3, in which CaO was included in the silicate phase, the capacity degradation rate also increased. This is thought to be because the alkaline CaO attracted electrolyte decomposition products into the silicate phase, thereby promoting the degradation of the silicate phase. Furthermore, in batteries B4 and B5, in which components other than Ca were added to the conductive layer, the capacity degradation rate tended to increase. This indicates that the effect of the Ca component as a barrier layer to suppress side reactions between electrolyte decomposition products and composite particles is unique to the Ca component. [Industrial applicability]

[0126] The non-aqueous electrolyte secondary battery described herein is useful as a main power source for mobile communication devices, portable electronic devices, and the like. [Explanation of symbols]

[0127] 1 electrode group 2 Positive leads 3 Negative lead 4 Battery case 5 Sealing plate 6 Negative terminal 7 Gasket 8. Sealing 11 Composite particles 12 Silica Phase 13(16) Silicon phase 14 Compounds of element M 15 Ca-containing conductive layer< / icp> < / aes> < / edx>

Claims

1. The device comprises composite particles and a conductive layer disposed on the surface of the composite particles, The composite particle comprises a silicate phase and a silicon phase dispersed within the silicate phase. The conductive layer contains a calcium component, A negative electrode material for a non-aqueous electrolyte secondary battery, wherein the average particle size of the calcium component is 200 nm or less.

2. The negative electrode material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the silicate phase includes lithium silicate.

3. The lithium silicate is Li ２ Si ２ O ５ A negative electrode material for a non-aqueous electrolyte secondary battery according to claim 2, comprising as a main component.

4. The negative electrode material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the silicate phase further comprises at least one selected from the group consisting of sodium, potassium, magnesium, barium, zirconium, niobium, tantalum, vanadium, titanium, phosphorus, bismuth, zinc, tin, lead, antimony, cobalt, fluorine, tungsten, aluminum, boron, and rare earth elements.

5. The negative electrode material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the conductive layer is formed of amorphous carbon.

6. The anode material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, wherein the calcium component comprises at least one selected from the group consisting of calcium carbonate and calcium oxide.

7. The negative electrode material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 6, wherein the calcium content in the conductive layer is 0.01% or more and 0.5% or less relative to the mass of the composite particles.

8. The negative electrode material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 7, wherein the spectrum obtained when the conductive layer is analyzed by electron energy loss spectroscopy has a peak originating from calcium carbonate.

9. The negative electrode material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 8, wherein the spectrum obtained when the interior of the composite particles is analyzed by electron energy loss spectroscopy does not have a peak originating from calcium carbonate.

10. It comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode comprises the negative electrode material described in any one of claims 1 to 9, in a non-aqueous electrolyte secondary battery.

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