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

Abstract

This negative electrode active material for a non-aqueous electrolyte secondary battery comprises composite particles which include: lithium aluminate phases; and silicon phases dispersed in in the lithium aluminate phases.

Description

Technical Field

[0001] This disclosure mainly relates to the improvement of 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 increase, 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 SiOx 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 has low alkali resistance (stability against Li), and still may cause a side reaction with Li ions during the initial charging, and further improvement of the initial charge-discharge efficiency is required.

[0007] In view of the above, one aspect of the present disclosure relates to a negative electrode active material for a non-aqueous electrolyte secondary battery, which includes composite particles containing a lithium aluminate phase and a silicon phase dispersed in the lithium aluminate 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 above-described negative electrode active material for a non-aqueous electrolyte secondary battery.

[0009] According to the present disclosure, it is possible to suppress a decrease in the initial charge-discharge efficiency of a non-aqueous electrolyte secondary battery.

Brief Description of the Drawings

[0010] [Figure 1] FIG. 1 is a diagram showing an example of an XRD pattern of a negative electrode active material (composite particles) according to an embodiment of the present disclosure. [Figure 2] FIG. 2 is a cross-sectional view schematically showing a negative electrode active material (composite particles) according to an embodiment of the present disclosure. [Figure 3] FIG. 3 is a schematic perspective view of a part of a non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure, with a part cut away.

Modes for Carrying Out the Invention

[0011] [Negative Electrode Active Material for Non-Aqueous Electrolyte Secondary Battery] The negative electrode active material for a non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure includes composite particles (hereinafter also referred to as LAX particles) containing a lithium aluminate phase and a silicon phase dispersed in the lithium aluminate phase.

[0012] The lithium aluminate phase in the LAX particles is more alkali-resistant than the lithium silicate phase in LSX particles in which the silicon phase is dispersed in the lithium silicate phase. Therefore, in the LAX particles, the side reaction with Li ions during the initial charging is suppressed more than in the LSX particles, and the deterioration of the negative electrode active material due to the side reaction and the resulting decrease in the initial capacity are suppressed. That is, the decrease in the initial charge-discharge efficiency is highly suppressed.

[0013] The LAX particles may not substantially contain lithium silicate and SiO2. The LAX particles may contain lithium silicate and SiO2, but it is desirable that the amount is small. The total content of lithium silicate and SiO2 in the LAX particles may be, for example, 3% by mass or less.

[0014] In the LAX particles, the content ratio MAl of aluminum to the total of elements other than oxygen is 10% by mass or more and 47% by mass or less, and the content ratio MLi of lithium to the total of elements other than oxygen is preferably 0.7% by mass or more and 13.5% by mass or less. When the content ratio MAl of aluminum and the content ratio MLi of lithium are within the above ranges, an aluminate phase excellent in stability and ionic conductivity is easily obtained. Note that the above stability includes both chemical stability (alkali resistance) and thermal stability. The content ratio MAl of aluminum is more preferably 11.5% by mass or more and 45.5% by mass or less. The content ratio MLi of lithium is more preferably 1.0% by mass or more and 9.5% by mass or less.

[0015] Also, from the viewpoint of easily obtaining a lithium aluminate phase excellent in stability and ionic conductivity, the ratio of the content ratio MAl of aluminum to the content ratio MLi of lithium: MAl / MLi is preferably 2 or more and 20 or less. MAl / MLi is more preferably 4 or more and 12 or less.

[0016] From the viewpoint of achieving both high capacity and improved cycle characteristics, in the LAX particles, the content ratio MSi of silicon to the total of elements other than oxygen is preferably, for example, 40% by mass or more and 90% by mass or less, and more preferably 50.8% by mass or more and 85.5% by mass or less. The above content ratio MSi of silicon is the amount of Si constituting the silicon phase in the LAX particles.

[0017] In the XRD pattern of composite particles obtained by XRD measurement of composite particles, a peak originating from the lithium aluminate phase may be observed around 2θ=x°. x° is at least one selected from the group consisting of 19.4°, 22.3°, 31.9°, 34.3°, and 37.5°. Cu Kα rays are used for the XRD measurement. In this specification, "around x°" means, for example, within the range of x±1°.

[0018] The Al2O3 phase may be dispersed within the lithium aluminate phase. Fine, highly crystalline Al2O3 phases may be distributed in island-like formations within the matrix of the aluminate phase. In this case, expansion and cracking of the aluminate phase due to the expansion and contraction of the silicon phase are easily suppressed, and the cycle characteristics are easily improved. In this case, a peak originating from the Al2O3 phase may be observed around 2θ = 25.4° in the X-ray diffraction pattern of the composite particles obtained by X-ray diffraction measurement. The Al2O3 phase content in the LAX particles is, for example, 0% by mass or more and 10% by mass or less.

[0019] Here, Figure 1 shows an example of the XRD pattern of a negative electrode active material (composite particle) according to one embodiment of the present disclosure. The XRD pattern shown by the solid line in Figure 1 represents the XRD pattern of an Al-rich composite particle (LAX2) with a large MAl / MLi ratio. The XRD pattern shown by the dashed line in Figure 1 represents the XRD pattern of a Li-rich composite particle (LAX4) with a small MAl / MLi ratio. LAX2 corresponds to Example 2 (Battery A2) described later, and LAX4 corresponds to Example 4 (Battery A4) described later.

[0020] In both LAX2 and LAX4, a peak originating from the Si(111) plane of the silicon phase is observed around 2θ=28°. In the Al-rich LAX2, a peak originating from the Al2O3 phase is observed around 2θ=25.4°.

[0021] Peaks (i) to (v) in Figure 1 are those originating from the lithium aluminate phase. In LAX2, peaks originating from the lithium aluminate phase, Li2Al4O7 and LiAlO2, are observed around 2θ=19.4° and 2θ=31.9° (peaks (i) and (ii) in Figure 1). In addition, in LAX2, peaks originating from the lithium aluminate phase, Li2Al4O7 and LiAl5O8, are observed around 2θ=37.5° (peak (iii) in Figure 1).

[0022] In LAX4, peaks originating from the lithium aluminate phases LiAlO2 and Li5AlO4 are observed around 2θ=22.3° and 2θ=34.3° (peaks (iv) and (v) in Figure 1).

[0023] The composite particles have a sea-island structure in which fine silicon phases, representing islands, are dispersed within a lithium aluminate phase, which represents the sea. The lithium aluminate phase has good ionic conductivity, and the intercalation and release of lithium ions by the silicon phase through the lithium aluminate phase proceeds smoothly. High capacity can be achieved by controlling the amount of silicon phase dispersed in the lithium aluminate phase. The expansion and contraction of the silicon phase is mitigated by the lithium aluminate phase. Therefore, it is easy to achieve both high battery capacity and improved cycle characteristics. From the viewpoint of mitigating the expansion and contraction of the silicon phase, the lithium aluminate phase may be amorphous.

[0024] In composite particles, multiple primary particles containing lithium aluminate and silicon phases are bonded together 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 also be 4 μm or more and 15 μm or less. Within the above particle size range, stress due to volume changes in 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 capacity reduction due to side reactions with non-aqueous electrolytes is suppressed. The average particle size of composite particles refers to the particle size (volume-average particle size) at which the volume integrated 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. If the surface of the composite particles is covered with a conductive layer, the thickness of the conductive layer is substantially small enough not to affect the average particle size of the composite particles, so the average particle size of composite particles with a conductive layer may be considered as the average particle size of the composite particles.

[0025] 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 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).

[0026] (Lithium aluminate phase) Lithium aluminate contains lithium (Li), aluminum (Al), and oxygen (O). The atomic ratio of O to Al in lithium aluminate (O / Al) is, for example, between 1.6 and 4. Similarly, the atomic ratio of Li to Al in lithium aluminate (Li / Al) is, for example, between 1 / 5 and 5. When the O / Al and Li / Al atomic ratios are within these ranges, it is advantageous in terms of the stability and ionic conductivity of the lithium aluminate phase.

[0027] The composition of lithium aluminate is given by the formula: Li u AlO (3+u) / 2 It can be expressed as follows. From the viewpoint of ease of fabrication, stability and ionic conductivity, etc., u in the formula can be, for example, greater than 0 and 5 or less, or greater than 0 and 1 or less. For example, when u=1 / 5, it can be expressed as LiAl5O8, and when u=1 / 2, it can be expressed as Li2Al4O7. When u=1, it can be expressed as LiAlO2, and when u=5, it can be expressed as Li5AlO4. From the viewpoint of ease of fabrication, stability and ionic conductivity, etc., it is preferable that the lithium aluminate phase contains at least one selected from the group consisting of LiAl5O8, Li2Al4O7, LiAlO2 and Li5AlO4. Among these, it is more preferable that the lithium aluminate phase contains LiAlO2 as the main component. Here, "main component" refers to a component that accounts for 50% or more by mass of the total mass of the lithium aluminate phase, and may account for 70% or more by mass.

[0028] The lithium aluminate phase may contain, in addition to Li, Al, and O, another element M. Element M is at least one selected from the group consisting of sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), zirconium (Zr), iron (Fe), boron (B), phosphorus (P), and lanthanum (La). The inclusion of element M in the aluminate phase improves its stability and ionic conductivity. Furthermore, it suppresses side reactions caused by contact between the aluminate phase and the non-aqueous electrolyte. From the viewpoint of resistance to the non-aqueous electrolyte and structural stability of the aluminate phase, it is preferable that element M contains at least one selected from the group consisting of Zr, Fe, P, and B. La can further improve the initial charge-discharge efficiency.

[0029] Element M may form a compound. Depending on the type of element M, the compound may be, for example, an oxide of element M or an aluminate of element M. In the lithium aluminate phase, the content of element M is, for example, 0.3 mol% or more and 3 mol% or less relative to the total amount of elements other than oxygen.

[0030] The lithium aluminate phase may further contain trace amounts of elements such as chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), and molybdenum (Mo).

[0031] The content of Li, Al, and element M in the lithium aluminate phase can be measured, for example, by analyzing the cross-section of the negative electrode mixture layer.

[0032] 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).

[0033] The content of each element can then be determined by one of the following methods. Furthermore, the composition of the lithium aluminate phase can be calculated from the content of each element.

[0034] <edx> From the cross-sectional image of the reflected electron image of the negative electrode mixture layer, randomly select 10 composite particles with a maximum particle diameter of 5 μm or more, and perform elemental mapping analysis on each of them by energy-dispersive X-ray (EDX). Calculate the content area of the target element using image analysis software. The observation magnification is preferably 2000 to 20000 times. Average the measured values of the content area of the predetermined element contained in the 10 particles. The content of the target element is calculated from the obtained average value.

[0035] The following shows the measurement conditions for desirable cross-sectional SEM-EDX analysis.

[0036] <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 manufactured 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.

[0037] 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.

[0038] <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.

[0039] 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.

[0040] Furthermore, the content of B, Na, K, and Al in the composite particles may be quantitatively analyzed in accordance with JIS R3105 (1995) (Analytical Method for Borosilicate Glass).

[0041] The Ca content in the composite particles may be quantitatively analyzed in accordance with JIS R3101 (1995) (Analytical Method for Soda-Lime Glass).

[0042] 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).

[0043] 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 introduced into a carbon crucible heated at a power of 5.75 kW together with Sn pellets and Ni pellets serving as fluxes, 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).

[0044] The amount of Si constituting the silicon phase in the composite particles can be quantified using Si-NMR.

[0045] The following shows the desirable measurement conditions for Si-NMR.

[0046] <Si-NMR Measurement Conditions> Measuring device: 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) and repeatedly occludes and releases lithium ions with the charge and discharge of the battery. 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 within the lithium aluminate phase, the stress due to the expansion and contraction of the silicon phase is relaxed.

[0047] 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, thereby 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.

[0048] 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 for the silicon phase to deteriorate with 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, the stress generated in the composite particles can be easily relieved, and the cycle characteristics can be improved. The crystallite size of the silicon phase is calculated from the full width at half maximum of the diffraction peak attributed to the (111) plane of the silicon phase (pure Si) in the X-ray diffraction pattern using Scherrer's formula.

[0049] Before the initial charge, the silicon phase of the composite particles contained in the battery 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 initial charge, the average particle size of the silicon phase is preferably 400 nm or less, and more preferably 100 nm or less. By refining the silicon phase, the volume change of the composite particles during charging and discharging is reduced, and the structural stability of the composite particles is further improved. The average particle size of the silicon phase is measured using cross-sectional images of the composite particles obtained by SEM. Specifically, the average particle size of the silicon phase is determined by averaging the maximum diameters of any 100 silicon phases.

[0050] From the viewpoint of increasing capacity, the silicon phase content 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 diffusion is good, and excellent loading characteristics can be obtained. On the other hand, from the viewpoint of improving cycle characteristics, the silicon phase content 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 area of ​​the silicon phase that is exposed without being covered by the silicate phase is reduced, and side reactions between the non-aqueous electrolyte and the silicon phase are easily suppressed.

[0051] (Conductive layer) A conductive layer containing a conductive material may be formed on at least a portion of the surface of the composite particles (secondary particles). This improves the conductivity of the composite particles. The thickness of the conductive layer is preferably thin enough not to substantially affect the average particle size of the composite particles. Considering the assurance of conductivity and the diffusion of lithium ions, the thickness of the conductive layer is preferably 1 to 200 nm, and more preferably 5 to 100 nm. The thickness of the conductive layer can be measured by cross-sectional observation of the composite particles using SEM or TEM.

[0052] Conductive materials are preferably conductive carbon materials. Examples of conductive carbon 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.

[0053] [Method for manufacturing composite particles] The composite particles are manufactured, for example, by a manufacturing method that includes the following steps 1 through 4. (First step) A step to obtain lithium aluminate, which is the raw material (hereinafter also referred to as raw material aluminate). (Second step) A step to obtain a composite intermediate by compounding raw material aluminate and raw material silicon and dispersing the silicon phase in the silicate phase. (Third step) A step of heat-treating the composite intermediate to obtain a sintered body containing an aluminate phase and a silicon phase dispersed within the aluminate phase. (Fourth step) A step of crushing the sintered body to obtain composite particles containing an aluminate phase and a silicon phase dispersed within the aluminate phase.

[0054] (1st step) The first step includes, for example, step 1a of mixing an aluminum compound, a lithium compound, and optionally a compound containing element M to obtain a mixture, and step 1b of calcining the mixture to obtain raw material aluminate. 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.

[0055] Examples of aluminum compounds include aluminum oxide (Al2O3), aluminum hydroxide, and aluminum carbonate. Aluminum compounds may be used individually or in combination of two or more.

[0056] 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.

[0057] 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 types.

[0058] In the first step, aluminum compounds that did not react with the lithium compound during the production of the raw aluminate may remain in the raw aluminate. If the amount of aluminum compound used is large relative to the lithium compound, the aluminum compound is more likely to remain. If the aluminum compound remaining in the raw aluminate is Al2O3, an Al2O3 phase dispersed within the lithium aluminate phase may be formed in the final composite particles.

[0059] (2nd process) The second step involves, for example, applying shear force to a mixture of raw material aluminate and raw material silicon while crushing the mixture to obtain a composite intermediate in the form of fine particles. Here, for example, raw material aluminate and raw material silicon can be mixed in a predetermined mass ratio, and the mixture can be crushed into fine particles while stirring using a crushing device such as a ball mill.

[0060] 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 peak attributed to the Si(111) plane of the X-ray diffraction pattern, is 10 nm or larger.

[0061] The second step is not limited to the above. For example, silicon nanoparticles and nanoparticles of the raw material aluminate may be synthesized without using a grinding device, and these may be mixed.

[0062] (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 aluminate phase with low crystallinity. The raw material aluminate 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.

[0063] (4th step) The fourth step is to grind the sintered body to obtain composite particles containing an aluminate phase and a silicon phase dispersed within the aluminate phase, so that they have a desired particle size distribution. The composite particles are ground to, for example, an average particle size of 1 to 25 μm.

[0064] (5th step) Furthermore, the method for producing composite particles may include a fifth step of forming a conductive layer by coating at least a portion of the surface of the composite particles with a conductive material. The conductive material is preferably electrochemically stable, and a conductive carbon material is preferred. Examples of methods for coating the surface of composite particles with a conductive carbon material include the CVD method using hydrocarbon gases such as acetylene and methane as raw materials, and a method of mixing coal pitch, petroleum pitch, phenolic resin, etc. with the composite particles and heating to carbonize them. Alternatively, carbon black may 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 composite particles and a conductive carbon material in an inert atmosphere (for example, an atmosphere such as argon or nitrogen) at a temperature of 700°C to 950°C.

[0065] Here, Figure 2 is a schematic cross-sectional view showing a negative electrode active material (composite particles) according to one embodiment of the present disclosure.

[0066] The composite particle 20 comprises a mother particle 23 composed of secondary particles formed by the aggregation of multiple primary particles 24. The mother particle 23 (primary particles 24) comprises a lithium aluminate phase 21 and a silicon phase 22 dispersed within the lithium aluminate phase 21. The mother particle 23 has a sea-island structure in which fine silicon phase is dispersed within the matrix of the lithium aluminate phase 21.

[0067] Furthermore, fine Al2O3 phase 28 may be dispersed within the lithium aluminate phase 21. At least a portion of the surface of the parent particles 23 may be coated with a conductive layer 26. The lithium aluminate phase 21 may also contain element M. With repeated charging and discharging, adjacent particulate silicon phases 22 may connect with each other, forming a network-like silicon phase.

[0068] [Nonaqueous electrolyte secondary battery] A non-aqueous electrolyte secondary battery according to an embodiment of this disclosure comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode includes the above-mentioned negative electrode active material for a non-aqueous electrolyte secondary battery.

[0069] The following provides a detailed explanation of non-aqueous electrolyte secondary batteries.

[0070] [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.

[0071] The negative electrode mixture contains a negative electrode active material as an essential component and may contain binders, conductive agents, thickeners, etc., as optional components. The negative electrode active material includes at least the composite particles described above.

[0072] The negative electrode active material preferably further contains a carbon material that electrochemically intercepts and releases lithium ions. Since the volume of the composite particles 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 and 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 and 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] The dispersion medium is not particularly limited, but examples include water, alcohols such as ethanol, ethers such as tetrahydrofuran, amides such as dimethylformamide, N-methyl-2-pyrrolidone (NMP), or mixed solvents thereof.

[0079] [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 includes a positive electrode active material as an essential component and may include a binder, a conductive agent, etc. as optional components. As the dispersion medium of the positive electrode slurry, NMP or the like is used.

[0080] 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 during charge and discharge.

[0081] Among them, Li a Ni b Me 1-b O2 (Me is at least one selected from the group consisting of Mn, Co, and Al, 0 < a ≤ 1.2, 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.

[0082] 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.

[0083] The shape and thickness of the positive electrode current collector can be respectively selected from the shape and range corresponding to those of the negative electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, titanium, etc.

[0084] [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 ion conductivity and having appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0085] 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.

[0086] 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.

[0087] [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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] <Examples> 2~4, reference examples 1 、5~ 10> [Preparation of composite particles (LAX particles)] (1st step) Al2O3 and Li2CO3 were mixed, and the mixture was calcined in air at 950°C for 10 hours to obtain the raw aluminate. The molar ratio of Al2O3 to Li2CO3 in the mixture was as shown in Table 1. The raw aluminate was ground to an average particle size of 10 μm.

[0093] (2nd process) Next, raw material aluminate (average particle size 10 μm) and raw material silicon (3N, average particle size 10 μm) were mixed. In the mixture, the mass ratio of raw material aluminate to raw material silicon was as shown in Table 1.

[0094] 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 in an inert atmosphere at 200rpm for 50 hours.

[0095] (3rd step) Next, the powdered mixture was taken out in an inert atmosphere and fired at 800°C for 4 hours in an inert atmosphere while applying pressure using a hot press to obtain a sintered body of the mixture.

[0096] (4th step) The obtained sintered body was crushed and passed through a 40 μm mesh to obtain composite particles.

[0097] (5th step) Composite particles were mixed with coal pitch (JFE Chemical Co., Ltd., MCP250). The mixture was calcined in an inert atmosphere at 800°C for 5 hours to form a conductive layer containing a conductive carbon material on the surface of the composite particles. The amount of conductive layer covering was 5% by mass relative to the total mass of the composite particles and the conductive layer. Subsequently, composite particles with an average particle size of 5 μm and a conductive layer (LAX particles in which the silicon phase is dispersed within the lithium aluminate phase) were obtained using a sieve.

[0098] XRD measurements of the LAX particles revealed peaks originating from the silicon phase and the lithium aluminate phase in the XRD patterns. Peaks originating from the Al2O3 phase were also observed in particles LAX1-LAX3 and LAX6-LAX8. The crystallite size of the silicon phase in the LAX particles, determined by the method described above, was 15 nm.

[0099] Based on the composition of the lithium aluminate phase and the amount of Si constituting the silicon phase in the LAX particles, determined by the method described above, the Li content ratio MLi, Si content ratio MSi, and Al content ratio MAl in the LAX particles were determined. The values ​​for each content ratio are shown in Table 1.

[0100] [Fabrication of the negative electrode] A composite particle having a conductive layer and graphite were 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, the negative electrode slurry was applied to the surface of a copper foil, and after the coating film dried, it was rolled to create a surface on both sides of the copper foil with a density of 1.5 g / cm³. 3 A negative electrode was fabricated with a negative electrode mixture layer formed thereon.

[0101] [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.

[0102] [Preparation of non-aqueous electrolytes] 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.

[0103] [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 edge. The electrode group was inserted into an aluminum laminate film enclosure, vacuum-dried at 105°C for 2 hours, then a non-aqueous electrolyte was injected, and the opening of the enclosure was sealed to obtain a non-aqueous electrolyte secondary battery. (LAX in Table 1) 2 ~ 4 and A 2 ~A 4 Examples 2 ~ 4 This shows composite particles and batteries. In Table 1, LAX1, 5-10 and A1, A5-A10 represent the composite particles and batteries of Reference Examples 1, 5-10.

[0104] <Comparative Example 1> In the second step, silicate was used instead of aluminate, and silicate (average particle size 10 μm) and silicon (3N, average particle size 10 μm) were mixed in a mass ratio of 25:75. The silicate was obtained by mixing SiO2 and Li2CO3 in a molar ratio of SiO2:Li2CO3=70:30 and calcining the mixture in air at 950°C for 10 hours. The silicate was then ground to an average particle size of 10 μm.

[0105] Other than the above, reference Composite particles having a conductive layer (LSX particles in which a silicon phase is dispersed within a lithium silicate phase) were obtained using the same method as in Example 1.

[0106] XRD measurements of LSX particles revealed peaks originating from the lithium silicate phase (Li2Si2O5 and Li2SiO3) and the silicon phase in the XRD patterns. The crystallite size of the silicon phase in the LSX particles was 15 nm.

[0107] Aside from using LSX particles with a conductive layer instead of LAX1 particles with a conductive layer, reference Battery B1 was fabricated using the same method as in Example 1.

[0108] For each battery obtained above, the initial charge-discharge efficiency was determined using the following method.

[0109] [Initial charge / discharge efficiency] <Charging> At 25°C, constant current charging was performed with a current of 1 It (800mA) until the voltage reached 4.2V, and then constant voltage charging was performed with a voltage of 4.2V until the current was reduced to 1 / 20 It (40mA).

[0110] <Discharge> After a 10-minute rest period, constant current discharge was performed at 25°C with a current of 1 It (800 mA) until the voltage reached 2.75 V.

[0111] For each battery, the charge and discharge capacities were determined from the charge and discharge data described above, and the ratio of the discharge capacity to the charge capacity was calculated as the initial charge-discharge efficiency. Table 1 shows the initial charge-discharge efficiency values ​​for each battery as relative values, with the initial charge-discharge efficiency value obtained for battery B1 of Comparative Example 1 set to 100. The evaluation results are shown in Table 1.

[0112] [Table 1]

[0113] Batteries A1-A10, which used LAX particles, achieved higher initial charge-discharge efficiencies than battery B1, which used LSX particles. In particular, batteries A2-A4 and A7-A9 achieved even higher initial charge-discharge efficiencies. [Industrial applicability]

[0114] 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]

[0115] 1 electrode group 2 Positive leads 3 Negative lead 4 Battery case 5 Sealing plate 6 Negative terminal 7 Gasket 8. Sealing 20 Composite particles 21 Lithium aluminate phase 22 Silicon phase 23 Mother particle 24 Primary particles 26 Conductive layer 28 Al2O3 phase< / icp> < / aes> < / edx>

Claims

1. The composite particle comprises a lithium aluminate phase and a silicon phase dispersed within the lithium aluminate phase. The content of the lithium aluminate phase in the composite particles is 25% by mass or more and 45% by mass or less. The content of the silicon phase in the composite particles is 55% by mass or more and 75% by mass or less. In the aforementioned composite particle, The content ratio of aluminum element (ML) to the total amount of elements other than oxygen is 11.5% by mass or more and 45.5% by mass or less. The ratio of lithium element MLi to the total amount of elements other than oxygen is 1.0% by mass or more and 9.5% by mass or less. The ratio of the aluminum element content MAl to the lithium element content MLi (MLi) is 4 or more and 12 or less. However, this excludes cases where the content ratio of silicon element (MSi) to the total amount of elements other than oxygen is 57.5% by mass or less, as a negative electrode active material for non-aqueous electrolyte secondary batteries.

2. The lithium aluminate phase is LiAl ５ O ８ Li ２ Al ４ O ７ LiAlO ２ and Li ５ AlO ４ The negative electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, comprising at least one selected from the group consisting of the following.

3. In the X-ray diffraction pattern of the composite particles obtained by X-ray diffraction measurement, A peak originating from the lithium aluminate phase appears near 2θ = x°. The negative electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein x° is at least one selected from the group consisting of 19.4°, 22.3°, 31.9°, 34.3°, and 37.5°.

4. Al is dispersed in the lithium aluminate phase ２ O ３ The negative electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, in which a phase is dispersed

5. In the X-ray diffraction pattern of the composite particle obtained by X-ray diffraction measurement, the Al is located around 2θ = 25.4°. ２ O ３ The negative electrode active material for a non-aqueous electrolyte secondary battery according to claim 4, wherein a peak originating from the phase appears.

6. The lithium aluminate phase comprises at least one element selected from the group consisting of sodium, potassium, calcium, magnesium, zirconium, iron, boron, phosphorus, and lanthanum, as a negative electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 5.

7. It comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode comprises a negative electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 6, wherein the negative electrode is a non-aqueous electrolyte secondary battery.

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