Composite particles for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

Abstract

Composite particles for a non-aqueous electrolyte secondary battery according to the present invention each include a lithium zirconate phase and silicon phases dispersed in the lithium zirconate phase. In the composite particles, the content ratio MZr of zirconium with respect to all elements other than oxygen is, for example, 14.6-54.6 mass%, and the content ratio MLi of lithium with respect to all elements other than oxygen is, for example, 0.9-10.4 mass%.

Description

Technical Field

[0001] The present disclosure mainly relates to composite particles that can be used as an electrode active material for 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] In Patent Document 1, a negative electrode active material (hereinafter also referred to as LSX) including a lithium silicate phase represented by Li 2z SiO 2+z (0 < z < 2) and silicon particles dispersed in the lithium silicate phase has been proposed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

[0005] Compared with SiO x in which silicon particles are dispersed in the SiO2 phase, LSX has a smaller irreversible capacity and an increased initial charge-discharge efficiency.

[0006] However, the lithium silicate phase in LSX may cause a side reaction with the non-aqueous electrolyte, and it is required to reduce the amount of gas generation during battery storage.

[0007] In view of the above, one aspect of the present disclosure relates to composite particles for non-aqueous electrolyte secondary batteries, which include a lithium zirconate phase and a silicon phase dispersed within the lithium zirconate phase.

[0008] Another aspect of the present disclosure relates to a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode contains the composite particles.

[0009] According to the present disclosure, the amount of gas generated during storage of a nonaqueous electrolyte secondary battery can be reduced. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 shows an example of an XRD pattern of a composite particle according to one embodiment of the present disclosure. [Figure 2] FIG. 2 is a cross-sectional view schematically illustrating a composite particle according to an embodiment of the present disclosure. [Figure 3] FIG. 3 is a schematic perspective view of a nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure, with a portion cut away. DETAILED DESCRIPTION OF THE INVENTION

[0011] [Non-aqueous electrolyte secondary battery negative electrode active material] The composite particles for a non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure include a lithium zirconate phase and a silicon phase dispersed within the lithium zirconate phase. The composite particles (hereinafter also referred to as LZX particles) can be used, for example, as a high-capacity negative electrode active material to be incorporated into the negative electrode of a non-aqueous electrolyte secondary battery.

[0012] The lithium zirconate phase in LZX particles has better acid and alkali resistance than the lithium silicate phase in LSX particles, which has a silicon phase dispersed within the lithium silicate phase. Therefore, LZX particles are less susceptible to side reactions with acid or alkali components contained in the non-aqueous electrolyte than LSX particles. As a result, gas generation associated with side reactions during battery storage is suppressed.

[0013] The LZX particles may be substantially free of lithium silicate and SiO 2. The total content of lithium silicate and SiO 2 in the LZX particles may be, for example, 3 mass % or less.

[0014] In the LZX particles, the zirconium content ratio MZr relative to all elements other than oxygen may be 14.6% by mass or more and 54.6% by mass or less, and the lithium content ratio MLi relative to all elements other than oxygen may be 0.9% by mass or more and 10.4% by mass or less. When the zirconium content ratio MZr and the lithium content ratio MLi are within the above ranges, a zirconate phase with excellent stability and ionic conductivity is easily obtained. Note that the stability mentioned above includes both chemical stability (acid resistance and alkali resistance) and thermal stability. The zirconium content ratio MZr is more preferably 29.0% by mass or more and 54.6% by mass or less.

[0015] Furthermore, from the viewpoint of easily obtaining a lithium zirconate phase excellent in stability and ionic conductivity, the ratio of the lithium content MLi to the zirconium content MZr (i.e., MLi / MZr) may be 4.7 or more and 23.2 or less, when the zirconium content MZr is 100. The lithium content MLi is more preferably 1.8 mass% or more and 9.7 mass% or less, provided that MLi / MZr satisfies the above numerical range.

[0016] From the viewpoint of achieving both high capacity and improved cycle characteristics, the silicon content MSi of the LZX particles relative to all elements other than oxygen is, for example, preferably 40% by mass or more and 90% by mass or less, and more preferably 42% by mass or more and 82% by mass or less. The silicon content MSi is the amount of Si that constitutes the silicon phase in the LZX particles.

[0017] In the XRD pattern of the composite particles obtained by X-ray diffraction (XRD) measurement, a peak derived from the lithium zirconate phase can be observed near 2θ=x°. x° is at least one selected from the group consisting of 18.6°, 26.5°, and 36.5°. Cu Kα rays are used as the X-rays for the XRD measurement. In this specification, "near x°" means, for example, within the range of x±1°.

[0018] A ZrO2 phase may be dispersed within the lithium zirconate phase. Highly crystalline, fine ZrO2 phases may be distributed in the form of islands within the zirconate phase matrix. Because the ZrO2 phase has high hardness, it is easy to suppress the expansion and cracking of the zirconate phase due to the expansion and contraction of the silicon phase. Furthermore, because the ZrO2 phase is highly stable, it is easy to suppress side reactions during battery storage, and the amount of gas generated is easy to reduce. In the X-ray diffraction pattern of the composite particles obtained by X-ray diffraction measurement, a peak derived from the ZrO2 phase may be observed around 2θ = 30.7°. The content of the ZrO2 phase in the LZX particles is, for example, 0% by mass or more and 10% by mass or less.

[0019] Here, FIG. 1 shows an example of an XRD pattern of a composite particle according to one embodiment of the present disclosure (Example 4 (LZX4) described below).

[0020] For LZX, a peak originating from the Si(111) plane of the silicon phase is observed near 2θ=28°, and a peak originating from the ZrO2 phase is observed near 2θ=30.7°.

[0021] Lithium zirconate phases Li6Zr2O7, Li2ZrO3 and Li are present around 2θ=18.6°, 2θ=26.5° and 2θ=36.5°. 5.52 Zr 2.62 Peaks derived from O8 are observed (peaks (i), (ii), and (iii) in Figure 1).

[0022] The composite particles have a sea-island structure in which fine silicon phases, which form islands, are dispersed within a lithium zirconate phase, which forms a sea. The lithium zirconate phase has good ionic conductivity, allowing the silicon phase to smoothly absorb and release lithium ions via the lithium zirconate phase. High capacity can be achieved by controlling the amount of silicon phase dispersed in the lithium zirconate phase. The lithium zirconate phase alleviates the expansion and contraction of the silicon phase. Therefore, it is easy to achieve both high battery capacity and improved cycle characteristics. From the perspective of alleviating the expansion and contraction of the silicon phase, the lithium zirconate phase may be amorphous.

[0023] In the composite particles, multiple primary particles containing a lithium zirconate phase and a silicon phase are bonded to form secondary particles. The average particle size of the composite particles (secondary particles) is, for example, 1 μm or more and 25 μm or less, and may be 4 μm or more and 15 μm or less. This particle size range facilitates the relaxation of stress caused by volumetric changes in the composite particles during charging and discharging, making it easier to achieve good cycle characteristics. The surface area of ​​the composite particles is also appropriately sized, further suppressing capacity loss due to side reactions with the non-aqueous electrolyte. The average particle size of the composite particles refers to the particle size (volume-average particle size) at which the volume-integrated value is 50% in the particle size distribution measured by laser diffraction scattering. For example, a measuring device such as the HORIBA LA-750 can be used. When the surface of the composite particles is covered with a conductive layer, the thickness of the conductive layer is small enough to not substantially affect the average particle size of the composite particles, and therefore the average particle size of the composite particles having the conductive layer can be considered the average particle size of the composite particles.

[0024] The composite particles can be extracted from a battery by the following method. First, a fully discharged battery is disassembled to remove the electrode (e.g., a negative electrode) containing the composite particles. The electrode is then washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove the nonaqueous electrolyte components. As described below, an electrode comprises a current collector and an electrode mixture layer supported on its surface. The electrode mixture layer is then peeled off from the current collector and crushed in a mortar to obtain a sample powder. The sample powder is then dried in a dry atmosphere for 1 hour and immersed in gently boiling 6 M hydrochloric acid for 10 minutes to remove elements other than those derived from the composite particles. Next, the sample powder is washed with ion-exchanged water, filtered, and dried at 200 °C for 1 hour. The conductive layer is then removed by heating to 900 °C in an oxygen atmosphere, allowing the composite particles to be isolated. Note that a 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).

[0025] (lithium zirconate phase) Lithium zirconate contains lithium (Li), zirconium (Zr), and oxygen (O). The atomic ratio of O to Zr in lithium zirconate (i.e., O / Zr) is, for example, 2.0 or more and 6.0 or less. When the O / Zr atomic ratio is within the above range, it is advantageous in terms of the stability and ionic conductivity of the lithium zirconate phase.

[0026] The lithium zirconate phases are Li6Zr2O7, Li2ZrO3 and Li 5.52 Zr 2.62 O8 as a main component. Here, the "main component" refers to a component that accounts for 50% by mass or more of the mass of the entire lithium zirconate phase, and may also account for 70% by mass or more.

[0027] The lithium zirconate phase may further contain another element M in addition to Li, Zr, and O. The element M may be, for example, at least one element selected from the group consisting of sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), boron (B), phosphorus (P), and lanthanum (La). When the lithium zirconate phase contains the element M, the stability and ionic conductivity of the lithium zirconate phase are further improved. Furthermore, side reactions due to contact between the lithium zirconate phase and the non-aqueous electrolyte are suppressed. From the viewpoints of resistance to the non-aqueous electrolyte and structural stability of the lithium zirconate phase, it is preferable that the element M contains at least one element selected from the group consisting of P and B.

[0028] The element M may form a compound. The compound may be, for example, an oxide of the element M or a zirconate of the element M depending on the type of the element M. In the lithium zirconate phase, the content of the element M is, for example, 0.3 mol % or more and 3 mol % or less with respect to all elements other than oxygen.

[0029] The lithium zirconate phase may further contain trace amounts of elements such as chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), and molybdenum (Mo).

[0030] The contents of Li, Zr, and element M in the lithium zirconate phase can be measured, for example, by analyzing a cross section of the electrode mixture layer.

[0031] First, a fully discharged battery was disassembled, the electrodes were removed, washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove the non-aqueous electrolyte, and then dried. After that, a cross-section of the electrode mixture layer was obtained using a cross-section polisher (CP), and the cross-section of the electrode mixture layer was then observed using a scanning electron microscope (SEM).

[0032] The content of each element can be determined by any of the following methods: The composition of the lithium zirconate phase is calculated from the content of each element.

[0033] <edx> From the cross-sectional image of the backscattered electron image of the 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 content 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 content areas of the predetermined element contained in the 10 particles are averaged. The content of the target element is calculated from the obtained average value.

[0034] The measurement conditions for the desirable cross-sectional SEM-EDX analysis are shown below.

[0035] <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> From the cross-sectional image of the backscattered electron image of the electrode mixture layer, ten composite particles with a maximum particle diameter of 5 μm or more are randomly selected, and each is subjected to qualitative and quantitative elemental analysis using an Auger electron spectroscopy (AES) analyzer (e.g., JAMP-9510F manufactured by JEOL Ltd.). Measurement conditions may 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 given element contained in the ten particles is calculated by averaging the content.

[0036] The EDX analysis and AES analysis are carried out within a range of 1 μm or more inside from the peripheral edge of the cross section of the composite particle.

[0037] <icp> A sample of composite particles is completely dissolved in a heated acid solution (a mixture of hydrofluoric acid, nitric acid, and sulfuric acid), and the residual carbon in the solution is filtered off. The filtrate is then analyzed by inductively coupled plasma optical emission spectroscopy (ICP) to measure the spectral intensity of each element. A calibration curve is then created using commercially available elemental standard solutions, and the content of each element in the composite particles is calculated.

[0038] Alternatively, the quantitative determination of each element can be performed using an electron microanalyzer (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), X-ray photoelectron spectroscopy (XPS), or the like.

[0039] The contents of B, Na, K and Al contained in the composite particles may be quantitatively analyzed in accordance with JIS R3105 (1995) (method of analysis of borosilicate glass).

[0040] The content of Ca contained in the composite particles may be quantitatively analyzed in accordance with JIS R3101 (1995) (method of analysis of soda-lime glass).

[0041] The carbon content of the composite particles can be measured using a carbon / sulfur analyzer (e.g., EMIA-520 manufactured by Horiba, Ltd.). The sample is weighed onto a magnetic board, a combustion improver is added, and the board is inserted into a combustion furnace (carrier gas: oxygen) heated to 1350°C. The amount of carbon dioxide gas generated during combustion is detected by infrared absorption. A calibration curve is created using, for example, carbon steel (carbon content: 0.49%) manufactured by the Bureau of Analyzed Samples, Ltd., and the carbon content of the sample is calculated (high-frequency induction heating furnace combustion - infrared absorption method).

[0042] The oxygen content contained in the composite particles may be measured using an oxygen-nitrogen-hydrogen analyzer (for example, the EGMA-830 type manufactured by Horiba, Ltd.). Put the sample in a Ni capsule, and together with the Sn pellets and Ni pellets that serve as a flux, put it into a carbon crucible heated at 5.75 kW, and detect the carbon monoxide gas released. The 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).

[0043] The amount of Si constituting the silicon phase in the composite particles can be quantified using Si-NMR.

[0044] The following shows the desirable Si-NMR measurement conditions.

[0045] <Si-NMR Measurement Conditions> Measuring device: Solid nuclear magnetic resonance spectrometer (INOVA-400) manufactured by Varian Probe: Varian 7mm CPMAS- The silicon phase is a phase of elemental silicon (Si), and it repeats the occlusion and release of lithium ions as the battery is charged and discharged. The capacity is manifested by the Faraday reaction involving the silicon phase. Since the silicon phase has a large capacity, the degree of expansion and contraction accompanying charge and discharge is also large. However, since the silicon phase is dispersed in the lithium zirconate phase, the stress due to the expansion and contraction of the silicon phase is relaxed.<0​​​​​​​​​​​​​​​​​​​​​​​The silicon phase can be composed of multiple crystallites. The crystallite size of the silicon phase is preferably 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 charge and discharge can be reduced, further improving cycle characteristics. For example, isolation of the silicon phase due to the formation of voids around the silicon phase during contraction is suppressed, and a decrease in charge and discharge efficiency is suppressed. The lower limit of the crystallite size of the silicon phase is not particularly limited, but is, for example, 1 nm or more.

[0047] The crystallite size of the silicon phase is more preferably 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 that the silicon phase will deteriorate, which leads to the generation of irreversible capacity. When the crystallite size of the silicon particles is 30 nm or less, it is easier to make the expansion and contraction of the silicon phase uniform, which makes it easier to alleviate stress generated in the composite particles, thereby improving cycle characteristics. The crystallite size of the silicon phase is calculated using the Scherrer equation from the half-width of the diffraction peak assigned to the (111) plane of the silicon phase (simple element Si) in the X-ray diffraction pattern.

[0048] 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 miniaturizing the silicon phase, the volume change of the composite particles during charge and discharge is reduced, further improving the structural stability of the composite particles. 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 determined by averaging the maximum diameters of 100 arbitrary silicon phases.

[0049] From the viewpoint of increasing 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, lithium ion diffusibility is good, and excellent load characteristics are obtained. On the other hand, from the viewpoint of improving 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 that is exposed without being covered with the silicate phase is reduced, and side reactions between the non-aqueous electrolyte and the silicon phase are likely to be suppressed.

[0050] (Conductive layer) A conductive layer containing a conductive material may be formed on at least a portion of the surface of the composite particle (secondary particle). This improves the conductivity of the composite particle. The thickness of the conductive layer is preferably thin enough that it does not substantially affect the average particle size of the composite particle. In consideration of ensuring conductivity and the diffusibility of lithium ions, the thickness of the conductive layer is preferably 1 to 200 nm, more preferably 5 to 100 nm. The thickness of the conductive layer can be measured by observing the cross section of the composite particle using SEM or TEM.

[0051] The conductive material is preferably a conductive carbon material. Examples of conductive carbon materials that can be used include amorphous carbon, graphite, easily graphitized carbon (soft carbon), and hardly graphitized carbon (hard carbon). Among these, amorphous carbon is preferred because it is easy to form a thin conductive layer that covers the surface of the composite particles. Examples of amorphous carbon include carbon black, burned pitch, coke, and activated carbon. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon.

[0052] [Method of manufacturing composite particles] The composite particles are produced, for example, by a production method including the following first to fourth steps. (First step) A step of obtaining lithium zirconate (hereinafter also referred to as raw zirconate) as a raw material. (Second step) A step of compositing raw zirconate and raw silicon to disperse a silicon phase within the zirconate phase, thereby obtaining a composite intermediate. (Third step) A step of subjecting the composite intermediate to heat treatment to obtain a sintered body containing a zirconate phase and a silicon phase dispersed within the zirconate phase. (Fourth step) A step of pulverizing the sintered body to obtain composite particles containing a zirconate phase and a silicon phase dispersed within the zirconate phase.

[0053] (1st step) The first step includes, for example, step 1a of mixing a zirconium compound, a lithium compound, and, if necessary, a compound containing element M to obtain a mixture, and step 1b of calcining the mixture to obtain a starting zirconate. 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, more preferably 800°C or higher and 1100°C or lower.

[0054] Examples of zirconium compounds include zirconium oxide (ZrO2), zirconium hydroxide, zirconium carbonate, etc. One type of zirconium compound may be used alone, or two or more types may be used in combination.

[0055] Examples of the lithium compound include lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, etc. The lithium compounds may be used alone or in combination of two or more.

[0056] The compound containing the element M may be an oxide, hydroxide, hydride, halide, carbonate, oxalate, nitrate, sulfate, or the like of the element M. The compound containing the element M may be used alone or in combination of two or more.

[0057] In the first step, zirconium compounds that did not react with the lithium compounds during the preparation of the raw zirconate may remain in the raw zirconate. When the amount of zirconium compounds used is large relative to the lithium compounds, the zirconium compounds are likely to remain. If the zirconium compounds remaining in the raw zirconate are ZrO2, a ZrO2 phase dispersed within the lithium zirconate phase may be formed in the final composite particles.

[0058] (2nd process) The second step includes, for example, a step of pulverizing a mixture of raw zirconate and raw silicon while applying shear force to the mixture to obtain a finely divided composite intermediate. Here, for example, the raw zirconate and raw silicon are mixed in a predetermined mass ratio, and the mixture is pulverized while being stirred using a pulverizer such as a ball mill.

[0059] The raw silicon may be coarse silicon particles with an average particle size of several μm to several tens of μm. The silicon particles finally obtained are preferably controlled so that the crystallite size calculated from the half-width of the diffraction peak attributed to the Si(111) plane in the X-ray diffraction pattern using the Scherrer equation is 10 nm or more.

[0060] The second step is not limited to the above. For example, silicon nanoparticles and raw zirconate nanoparticles may be synthesized and then mixed together without using a pulverizer.

[0061] (3rd step) The third step includes, for example, a step of sintering the microparticulated composite intermediate while applying pressure to the composite intermediate using a hot press or the like to obtain a sintered body. The composite intermediate is sintered, for example, in an inert atmosphere (e.g., an argon, nitrogen, or the like). The sintering temperature is preferably 450°C or higher and 1000°C or lower. Within this temperature range, fine silicon particles are easily dispersed within the zirconate phase, which has low crystallinity. The raw zirconate is stable within this temperature range and hardly reacts with silicon. The sintering temperature is preferably 550°C or higher and 900°C or lower, more preferably 650°C or higher and 850°C or lower. The sintering time is, for example, 1 hour or higher and 10 hours or lower.

[0062] (4th step) The fourth step is to pulverize the sintered body to have a desired particle size distribution, thereby obtaining composite particles containing a zirconate phase and a silicon phase dispersed within the zirconate phase. The composite particles are pulverized to have an average particle size of, for example, 1 to 25 μm.

[0063] (5th step) Furthermore, the method for producing composite particles may include a fifth step of coating at least a portion of the surface of the composite particles with a conductive material to form a conductive layer. The conductive material is preferably electrochemically stable, and a conductive carbon material is preferred. Examples of methods for coating the surface of the composite particles with a conductive carbon material include a CVD method using a hydrocarbon gas such as acetylene or methane as a raw material, and a method of mixing coal pitch, petroleum pitch, phenolic resin, etc. with the composite particles and heating to carbonize them. Carbon black may also be attached to the surface of the composite particles. In the fifth step, for example, a conductive layer may be formed on the surface of the composite particles by heating a mixture of the composite particles and the conductive carbon material in an inert atmosphere (e.g., an argon, nitrogen, etc.) at a temperature of 700°C or higher and 950°C or lower.

[0064] Here, FIG. 2 is a cross-sectional view schematically showing a negative electrode active material (composite particles) according to one embodiment of the present disclosure.

[0065] The composite particle 20 includes a base particle 23 composed of secondary particles formed by agglomeration of a plurality of primary particles 24. The base particle 23 (primary particle 24) includes a lithium zirconate phase 21 and a silicon phase 22 dispersed within the lithium zirconate phase 21. The base particle 23 has a sea-island structure in which fine silicon phases are dispersed within a matrix of the lithium zirconate phase 21.

[0066] Furthermore, fine ZrO2 phases 28 may be dispersed within the lithium zirconate phases 21. At least a portion of the surface of the base particles 23 may be coated with a conductive layer 26. The lithium zirconate phases 21 may contain element M. With repeated charge and discharge, adjacent particulate silicon phases 22 may be connected to each other to form a network-like silicon phase.

[0067] [Nonaqueous electrolyte secondary battery] A non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, and the negative electrode contains the composite particles.

[0068] The nonaqueous electrolyte secondary battery will be described in detail below.

[0069] [Negative electrode] The negative electrode may include 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 the negative electrode mixture is dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the applied layer. The dried coating may be rolled as necessary. The negative electrode mixture layer may be formed on one surface or both surfaces of the negative electrode current collector.

[0070] The negative electrode mixture contains a negative electrode active material as an essential component, and may contain optional components such as a binder, a conductive agent, a thickener, etc. The negative electrode active material contains at least the above composite particles.

[0071] The negative electrode active material preferably further contains a carbon material that electrochemically absorbs and releases lithium ions. Because the composite particles expand and contract in volume with charge and discharge, a high ratio of the composite particles to the negative electrode active material is likely to result in poor contact between the negative electrode active material and the negative electrode current collector during charge and discharge. On the other hand, the combined use of the composite particles and a carbon material makes it possible to achieve excellent cycle characteristics while imparting the high capacity of the silicon particles to the negative electrode. From the viewpoint of increasing capacity and improving cycle characteristics, the ratio of the carbon material to the total of the composite particles 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.

[0072] Examples of carbon materials include graphite, easily graphitizable carbon (soft carbon), and non-graphitizable carbon (hard carbon). Among these, graphite is preferred because it has excellent charge / discharge stability and low irreversible capacity. Graphite refers to a material having a graphite-type crystal structure, and includes, for example, natural graphite, artificial graphite, graphitized mesophase carbon particles, and the like. One type of carbon material may be used alone, or two or more types may be used in combination.

[0073] The negative electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of 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 is preferably 1 to 50 μm, and more preferably 5 to 20 μm, from the viewpoint of balancing the strength and weight of the negative electrode.

[0074] Examples of binders include resin materials, such as fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resin; polyimide and polyamideimide; acrylic resins such as polyacrylic acid, polymethyl acrylate, and ethylene-acrylic acid copolymer; vinyl resins such as polyacrylonitrile and polyvinyl acetate; polyvinylpyrrolidone; polyethersulfone; and rubber-like materials such as styrene-butadiene copolymer rubber (SBR). One type of binder may be used alone, or two or more types may be used in combination.

[0075] Examples of conductive agents include carbons such as acetylene black, conductive fibers such as carbon fiber and metal fiber, 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. One type of conductive agent may be used alone, or two or more types may be used in combination.

[0076] Examples of thickeners include carboxymethyl cellulose (CMC) and its modified products (including salts such as Na salt), cellulose derivatives such as methyl cellulose (cellulose ethers, etc.), saponified polymers having vinyl acetate units such as polyvinyl alcohol, polyethers (polyalkylene oxides such as polyethylene oxide, etc.), etc. One type of thickener may be used alone, or two or more types may be used in combination.

[0077] The dispersion medium is not particularly limited, but examples thereof include water, alcohols such as ethanol, ethers such as tetrahydrofuran, amides such as dimethylformamide, N-methyl-2-pyrrolidone (NMP), and mixed solvents thereof.

[0078] [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.

[0079] 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 Oc, 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, 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 by charge and discharge.

[0080] 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, where 0 < a ≤ 1.2 and 0.3 ≤ b ≤ 1) is preferred. From the perspective of increasing the capacity, it is more preferable 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 (where 0 < a ≤ 1.2, 0.85 ≤ b < 1, 0 < c < 0.15, 0 < d ≤ 0.1, and b + c + d = 1) is more preferable.

[0081] 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.

[0082] 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.

[0083] [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.

[0084] As the non-aqueous solvent, for example, cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, chain carboxylic acid esters, etc. are used. Examples of the cyclic carbonate include propylene carbonate (PC), ethylene carbonate (EC), etc. Examples of the chain carbonate include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), etc. Examples of the cyclic carboxylic acid ester include γ-butyrolactone (GBL), γ-valerolactone (GVL), etc. Examples of the chain carboxylic acid ester include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, etc. The non-aqueous solvent may be used alone or in combination of two or more.

[0085] Examples of lithium salts include LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, and LiB 10 Cl 10 Examples of the lithium salt include lithium carboxylates, lower aliphatic carboxylates, LiCl, LiBr, LiI, borates, imide salts, etc. Examples of the 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 bistrifluoromethanesulfonyl imide (LiN(CF3SO2)2), lithium trifluoromethanesulfonyl nonafluorobutanesulfonyl imide (LiN(CF3SO2)(C4F9SO2)), and lithium bispentafluoroethanesulfonyl imide (LiN(C2F5SO2)2). Among these, LiPF6 is preferred. The lithium salts may be used alone or in combination of two or more.

[0086] [Separator] It is usually desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator may be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin such as polypropylene or polyethylene.

[0087] An example of the structure of a nonaqueous electrolyte secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween is housed in an outer casing together with a nonaqueous electrolyte. However, this is not limited thereto, and other forms of electrode groups may also be applied. For example, a laminated electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween may also be used. The shape of the nonaqueous electrolyte secondary battery is also not limited, and may be, for example, a cylindrical type, a prismatic type, a coin type, a button type, a laminate type, or the like.

[0088] Hereinafter, the structure of a prismatic nonaqueous electrolyte secondary battery will be described as an example of the nonaqueous electrolyte secondary battery according to the present disclosure with reference to FIG.

[0089] The battery includes a bottomed, rectangular battery case 4, an electrode group 1, and a nonaqueous electrolyte housed within the battery case 4. The electrode group 1 includes a long, strip-shaped negative electrode, a long, strip-shaped positive electrode, and a separator interposed therebetween. The negative electrode current collector is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. In other words, 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 edge of the battery case 4, and the fitting is laser-welded. The sealing plate 5 has an injection hole for the nonaqueous electrolyte, which is closed with a seal 8 after injection.

[0090] The present disclosure will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0091] <Examples 1 to 6> [Preparation of composite particles (LZX particles)] (1st step) ZrO2 and Li2CO3 were mixed and fired in air at 950°C for 10 hours to obtain raw lithium zirconate (LZX particles) A1 to A6. In the mixture, the molar ratio of ZrO2 to Li2CO3 was set so that the mass ratio of MLi to MZr (MLi / MZr) when the zirconium content ratio MZr was 100 was the value shown in Table 1. The raw lithium zirconate was pulverized to an average particle size of 10 μm.

[0092] (2nd process) Next, raw lithium zirconate (average particle size 10 μm) and raw silicon (3N, average particle size 10 μm) were mixed. In the mixture, the mass ratio of raw lithium zirconate to raw silicon was adjusted so that the mass ratio of Zr to Si was the value shown in Table 1.

[0093] The mixture was placed in a pot (SUS, volume: 500 mL) of a planetary ball mill (Fritsch, P-5), 24 SUS balls (diameter 20 mm) were placed in the pot, the lid was closed, and the mixture was milled at 200 rpm for 50 hours in an inert atmosphere.

[0094] (3rd step) Next, the powder mixture was taken out in an inert atmosphere and sintered at 800°C for 4 hours while applying pressure using a hot press in an inert atmosphere to obtain a sintered body of the mixture.

[0095] (4th step) The obtained sintered body was pulverized and passed through a 40 μm mesh to obtain composite particles.

[0096] (5th step) The composite particles were mixed with coal pitch (MCP250, manufactured by JFE Chemical Corporation). The mixture was fired at 800°C for 5 hours in an inert atmosphere to form a conductive layer containing a conductive carbon material on the surface of the composite particles. The coating amount of the conductive layer was 5 mass% based on the total mass of the composite particles and the conductive layer. Then, composite particles with an average particle size of 5 μm and a conductive layer (LZX particles in which a silicon phase is dispersed within a lithium zirconate phase) were obtained using a sieve.

[0097] Figure 1 shows the XRD pattern of LZX4. In the XRD patterns obtained by XRD measurement of LZX particles 1 to 6 (hereinafter also referred to as LZX1 to 6) of Examples 1 to 6, peaks derived from the silicon phase, lithium zirconate phase, and ZrO2 phase were confirmed. The crystallite size of the silicon phase in the LZX particles determined by the method described above was 15 nm.

[0098] The Li content ratio MLi, Si content ratio MSi, and Zr content ratio MZr in the LZX particles were determined by the method described above. The values ​​of each content ratio are shown in Table 1.

[0099] [Preparation of negative electrode] The composite particles having a conductive layer and graphite were mixed in a mass ratio of 5:95 to prepare the negative electrode active material. A negative electrode mixture containing the negative electrode active material, a sodium salt of CMC, and SBR in a mass ratio of 97.5:1:1.5 was added with water and stirred to prepare a negative electrode slurry. The negative electrode slurry was then applied to the surface of copper foil, dried, and rolled to form a copper foil with a density of 1.5 g / cm on both sides. 3 A negative electrode having the negative electrode mixture layer formed thereon was fabricated.

[0100] [Preparation of positive electrode] A positive electrode mixture containing lithium cobalt oxide, acetylene black, and PVDF in a mass ratio of 95:2.5:2.5 was mixed with NMP and stirred to prepare a positive electrode slurry. The positive electrode slurry was then applied to the surface of an aluminum foil, the coating was dried, and the aluminum foil was rolled to form a positive electrode slurry having a density of 3.6 g / cm on both sides. 3 A positive electrode having the positive electrode mixture layer formed thereon was fabricated.

[0101] [Preparation of non-aqueous electrolyte] 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 in a volume ratio of 3:7.

[0102] [Fabrication of non-aqueous electrolyte secondary battery] The positive and negative electrodes, each with a tab attached, were wound with a separator interposed therebetween to produce an electrode assembly with the tabs located at the outermost periphery. The electrode assembly was inserted into an exterior case made of aluminum laminate film and vacuum dried at 105°C for 2 hours. After that, a nonaqueous electrolyte was poured into the exterior case, and the opening of the exterior case was sealed to obtain batteries A1 to A6 corresponding to LZX1 to LZX6 in Examples 1 to 6.

[0103] <Comparative Example 1> In the second step, lithium silicate was used instead of raw lithium zirconate, and lithium silicate (average particle size 10 μm) was mixed with raw silicon (3N, average particle size 10 μm) in a mass ratio of 50:50. Lithium silicate was obtained by mixing SiO2 and Li2CO3 in a molar ratio of SiO2:Li2CO3 = 70:30 and firing the mixture in air at 950 °C for 10 hours. The raw silicate was pulverized to an average particle size of 10 μm.

[0104] Except for the above, the same method as in Example 1 was used to obtain composite particles having a conductive layer (LSX particles in which a silicon phase is dispersed within a lithium silicate phase).

[0105] The XRD pattern of the LSX particles obtained by XRD measurement confirmed peaks attributable to lithium silicate phases (Li2Si2O5 and Li2SiO3) and silicon phases. The crystallite size of the silicon phase in the LSX particles was 15 nm.

[0106] A battery B1 was produced in the same manner as in Example 1, except that LSX particles having a conductive layer were used instead of the LZX particles having a conductive layer.

[0107] <Comparative Example 2> In the second step, ZrO2 was used instead of raw zirconate, and ZrO2 (average particle size 10 μm) and raw silicon (3N, average particle size 10 μm) were mixed so that the mass ratio of Zr to Si was the value shown in Table 1. In the subsequent step, an attempt was made to obtain composite particles, but it was not possible to disperse the silicon phase in zirconia, and composite particles could not be synthesized.

[0108] <Comparative Example 3> In the first step, an attempt was made to synthesize lithium zirconate (LZX7) by mixing ZrO2 and Li2CO3 so that the mass ratio of Li to Zr was the value shown in Table 1, but the production of lithium zirconate was insufficient, resulting in the formation of a multiphase mixture. In the subsequent step, an attempt was made to obtain composite particles using LZX particles 7, but granulation was difficult.

[0109] The batteries A1 to A6 and B1 obtained above were initially charged and discharged in the following manner, and then charged.

[0110] [Initial charge / discharge efficiency] <Charging> At 25°C, constant current charging was performed at a current of 1 It (800 mA) until the voltage reached 4.2 V, and then constant voltage charging was performed at a voltage of 4.2 V until the current reached 1 / 20 It (40 mA).

[0111] <Discharge> After a 10-minute rest, constant current discharge was carried out at 25°C at a current of 1 It (800 mA) until the voltage reached 2.75 V.

[0112] Each charged battery was stored at 80°C for 3 days, and the amount of gas generated in the battery was measured using the Archimedes method. Specifically, the increase in battery volume due to gas generation was measured. The amount of gas generated in each battery was expressed as a relative value, with the amount of gas generated in Battery B1 of Comparative Example 1 being set at 100. The evaluation results are shown in Table 1.

[0113] [Table 1]

[0114] In the batteries A1 to A6 using LZX particles, the amount of gas generated was smaller than in the battery B1 using LZX particles. In particular, the amount of gas generated was small in the batteries A1 to A4. [Industrial Applicability]

[0115] The nonaqueous electrolyte secondary battery according to the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, and the like. [Explanation of symbols]

[0116] 1 electrode group 2 positive leads 3 Negative lead 4 Battery case 5 Sealing plate 6 Negative terminal 7 Gasket 8. Seal 20 Composite particles 21 Lithium zirconate phase 22 Silicon phase 23 Mother particle 24 Primary particles 26 Conductive layer 28 ZrO2 phase< / icp> < / aes> < / edx>

Claims

1. A composite particle comprising a lithium zirconate phase and a silicon phase dispersed within the lithium zirconate phase, the composite particles are pulverized sintered bodies containing the lithium zirconate phase and the silicon phase dispersed within the lithium zirconate phase, (a) In the composite particles, The zirconium content ratio MZr to all elements other than oxygen is 14.6 mass% or more and 54.6 mass% or less, and The lithium content ratio MLi relative to all elements other than oxygen is 0.9 mass% or more and 10.4 mass% or less, or (b) In the composite particles, The zirconium content ratio MZr to all elements other than oxygen is 14.6 mass% or more and 54.6 mass% or less, and The content ratio MLi of lithium to all elements other than oxygen is 0.9 mass% or more and 10.4 mass% or less, The ratio of the lithium content ratio MLi to the zirconium content ratio MZr is 4.7 or more and 23.2 or less, when the zirconium content ratio MZr is 100; or (c) the lithium zirconate phase comprises at least one selected from the group consisting of Li6Zr2O7, Li2ZrO3, and Li5.52Zr2.62O8, or (d) In the X-ray diffraction pattern of the composite particle obtained by X-ray diffraction measurement, A peak derived from the lithium zirconate phase appears near 2θ=x°, The composite particles for a non-aqueous electrolyte secondary battery, wherein the x° is at least one selected from the group consisting of 18.6°, 26.5°, and 36.5°.

2. A composite particle comprising a lithium zirconate phase and a silicon phase dispersed within the lithium zirconate phase, A ZrO 2 phase is dispersed within the lithium zirconate phase, (a) In the composite particles, The zirconium content ratio MZr to all elements other than oxygen is 14.6 mass% or more and 54.6 mass% or less, and The lithium content ratio MLi relative to all elements other than oxygen is 0.9 mass% or more and 10.4 mass% or less, or (b) In the composite particles, The zirconium content ratio MZr to all elements other than oxygen is 14.6 mass% or more and 54.6 mass% or less, and The content ratio MLi of lithium to all elements other than oxygen is 0.9 mass% or more and 10.4 mass% or less, The ratio of the lithium content ratio MLi to the zirconium content ratio MZr is 4.7 or more and 23.2 or less, when the zirconium content ratio MZr is 100; or (c) the lithium zirconate phase comprises at least one selected from the group consisting of Li6Zr2O7, Li2ZrO3, and Li5.52Zr2.62O8, or (d) In the X-ray diffraction pattern of the composite particle obtained by X-ray diffraction measurement, A peak derived from the lithium zirconate phase appears near 2θ=x°, The composite particles for a non-aqueous electrolyte secondary battery, wherein the x° is at least one selected from the group consisting of 18.6°, 26.5°, and 36.5°.

3. ZrO in the lithium zirconate phase 2 The composite particle of claim 1 , wherein the phase is dispersed.

4. In the X-ray diffraction pattern of the composite particles obtained by X-ray diffraction measurement, the ZrO 2 The composite particle according to claim 2 or 3, wherein a peak derived from a phase appears.

5. 5. The composite particle according to claim 1, wherein the lithium zirconate phase contains at least one element selected from the group consisting of sodium, potassium, calcium, magnesium, boron, phosphorus, and lanthanum.

6. A positive electrode, a negative electrode, and a non-aqueous electrolyte, A non-aqueous electrolyte secondary battery, wherein the negative electrode comprises the composite particles according to any one of claims 1 to 5.

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