Negative electrode active material for secondary battery and secondary battery

Abstract

This negative electrode active material for secondary batteries comprises silicate composite particles, each of which contains silicon particles and a silicate phase. The silicon particles are dispersed in a lithium silicate phase; and the silicon particles contain at least one first element that is selected from the group consisting of germanium and aluminum.

Description

[Technical field]

[0001] The present disclosure primarily relates to improvements in negative electrode active materials for secondary batteries. [Background technology]

[0002] In recent years, secondary batteries such as non-aqueous electrolyte secondary batteries have been expected to be used for small consumer applications, power storage devices, and power sources for electric vehicles because of their high voltage and high energy density. As there is a demand for higher energy density batteries, there are expectations for the use of materials containing silicon, which alloys with lithium, as negative electrode active materials with high theoretical capacity density.

[0003] However, silicon-containing materials have a problem in that their initial charge-discharge efficiency (particularly the ratio of the initial discharge capacity to the initial charge capacity) is low due to their large irreversible capacity. Therefore, various techniques have been proposed for introducing lithium equivalent to the irreversible capacity into silicon-containing materials in advance. Specifically, it has been proposed to use composite particles containing a lithium silicate phase and silicon particles dispersed in the lithium silicate phase (Patent Document 1). The silicon particles contribute to the charge-discharge reaction (reversible absorption and release of lithium).

[0004] The composite particles are produced, for example, by sintering a mixture of glassy lithium silicate powder and silicon particles in a high-temperature and high-pressure atmosphere. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2015-153520 A Summary of the Invention

[0006] It is known that the degree of expansion and contraction of the silicon particles accompanying the absorption and release of lithium during charging and discharging is large. Therefore, as the silicon particles expand and contract, a large stress is generated in the lithium silicate phase present around the silicon particles, which causes cracks and fractures in the composite particles. As a result, the binding force between the composite particles and the binder around them is weakened, and the fractured composite particles may lose the conductive path with the surrounding particles and become isolated. In addition, a side reaction between the electrolyte and the silicon particles is promoted. As a result, it causes a deterioration in the charge and discharge cycle characteristics. In particular, the larger the crystallite size of the silicon particles, the greater the expansion and contraction, and the easier it is for the charge and discharge cycle characteristics to deteriorate when used as a negative electrode active material.

[0007] In view of the above, one aspect of the present disclosure relates to a negative electrode active material for a secondary battery, comprising silicate composite particles including silicon particles and a silicate phase, the silicon particles being dispersed in the lithium silicate phase, and the silicon particles containing at least one first element selected from the group consisting of germanium and aluminum.

[0008] Another aspect of the present disclosure relates to a secondary battery including a positive electrode, a negative electrode, an electrolyte, and a separator interposed between the positive electrode and the negative electrode, the negative electrode including a current collector and a negative electrode active material layer, and the negative electrode active material layer including the above-described negative electrode active material for secondary batteries.

[0009] In the negative electrode active material in which silicon particles are dispersed in a silicate phase, the crystallite size of the silicon particles can be made small. Therefore, the negative electrode active material of the present disclosure can be used to obtain a secondary battery negative electrode and a secondary battery having excellent charge-discharge cycle characteristics. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view that illustrates a negative electrode active material for a secondary battery (LSX particles) according to one embodiment of the present disclosure. [Diagram 2] FIG. 2 is a schematic perspective view of a secondary battery according to an embodiment of the present disclosure with a portion cut away. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The negative electrode active material for a secondary battery according to an embodiment of the present disclosure includes a silicate composite particle including silicon particles and a silicate phase. The silicon particles are dispersed in the silicate phase within the silicate composite particle. The silicon particles contain at least one first element selected from the group consisting of germanium and aluminum.

[0012] The silicate composite particles have a sea-island structure in which silicon particles, which are islands, are dispersed within the silicate phase, which is the sea. By controlling the amount of silicon particles dispersed in the silicate phase, it is possible to increase the capacity, and since the silicon particles are dispersed within the silicate phase, the stress associated with the expansion and contraction of the silicon particles during charging and discharging is alleviated by the silicate phase, and the expansion and contraction of the LSX particles can be reduced. Therefore, cracks and breaks in the silicate composite particles can be reduced, and it is easy to achieve both high capacity and improved cycle characteristics of the battery.

[0013] Conventionally, composite particles are manufactured by mixing silicate powder and silicon particles, pulverizing the mixture in a ball mill or the like to form a composite, and sintering the pulverized mixture in a high-temperature and high-pressure atmosphere. However, in this case, the crystallite size of the silicon particles increases due to heat sintering. As a result, the silicon particles are more susceptible to deterioration due to expansion and contraction, and high charge-discharge cycle characteristics may not be maintained. On the other hand, if heat sintering is insufficient, the porosity of the sintered body increases, and the porosity inside the silicate composite particles tends to increase. As a result, the surface area that reacts with the electrolyte increases, and side reactions tend to increase. As a result, it may be difficult to maintain high charge-discharge cycle characteristics.

[0014] Therefore, in the negative electrode active material for secondary batteries of this embodiment, the silicon particles contain a first element. The first element is an element that can dissolve in silicon to form a solid solution, and for example, germanium and / or aluminum having an atomic radius close to that of silicon can be preferably used. At least a part of the first element can exist in the silicon particles in a state of forming a solid solution with silicon. This can retard the crystal growth of the silicon particles during heat sintering. As a result, the crystallite size of the silicon particles can be reduced, and high cycle characteristics can be maintained.

[0015] Furthermore, the inclusion of the first element lowers the melting point of the silicon particles, improving wettability at the interface between the lithium silicate phase and the silicon particles during heat sintering. As a result, the lithium silicate phase adheres closely to the silicon particles, reducing the porosity of the LSX particles and achieving even higher cycle characteristics.

[0016] Here, the porosity (%) of the LSX particles means the ratio of the area occupied by voids to the total area of ​​the base particle in the particle cross section, and can be determined by SEM observation of the particle cross section. The specific method for measuring the porosity is as follows. (1) Using an ion milling device manufactured by Hitachi High-Technologies Corporation (ex. IM4000), the cross section of the base particle is exposed. (2) The exposed cross-section of the particle is observed by SEM, the ratio of the void area to the total area of ​​the particle cross-section is measured, and the porosity (void area x 100 / total area of ​​particle cross-section) is calculated. The porosity is the average value for 10 particles.

[0017] For the sintered body before pulverization, the porosity may be determined by mercury intrusion porosimetry.

[0018] The first element may partly exist in the form of a solid solution with silicon in the silicon particles and the remaining part may precipitate in the silicon particles. In other words, the content ratio of the first element contained in the silicon particles may be below the solid solubility limit with silicon or may exceed the solid solubility limit with silicon. For example, when the first element is germanium and / or aluminum, the germanium and / or aluminum precipitated in the silicon particles have a lower elastic modulus than silicon and are easily compressed, and thus may have an effect of alleviating the expansion and contraction of silicon accompanying charge and discharge.

[0019] The first element may be contained in the silicon particles at a ratio of 0.1 atomic % or more and 20 atomic % or less, 0.1 atomic % or more and 10 atomic % or less, or 1 atomic % or more and 20 atomic % or less. The content ratio of the first element is determined by performing inductively coupled plasma atomic emission spectrometry (ICP-AES) on the silicate composite particles taken out from the battery by the method described below. The content ratio of the first element contained in the silicon particles is determined by performing energy dispersive X-ray spectroscopy (Energy Dispersive X-ray Spectroscopy) using a transmission electron microscope on the cross section of the silicate composite particles.

[0020] The silicate phase may be a lithium silicate phase. In this case, the silicate composite particles are composite particles in which silicon particles are dispersed in the lithium silicate phase (hereinafter also referred to as LSX particles). Alternatively, the silicate phase may be a silicate of an alkali metal other than Li (for example, sodium silicate). In the LSX particles, the lithium silicate phase contains, for example, an oxide phase represented by the formula: Li 2z SiO 2+z (0 < z < 1). From the viewpoints of stability, ease of production, lithium ion conductivity, etc., z = 1 / 2 is preferable.

[0021] The lithium silicate phase may contain elements other than Li, Si, and O (oxygen). The lithium silicate phase may contain, for example, at least one element selected from the group consisting of alkali metal elements (excluding Li) and Group II elements.

[0022] By including an alkali metal element other than Li in the lithium silicate phase, crystallization becomes difficult, the viscosity in the softened state is low, and the fluidity is high. Therefore, even in a heat treatment at a low temperature, the gaps between silicon particles are easily filled, and dense composite particles are easily produced. The alkali metal element may be Na and / or K, which are inexpensive. The atomic ratio of the alkali element X (e.g., K) other than Li contained in the lithium silicate phase to Li: X / Li may be, for example, 0.1 to 7.1, and may be 0.4 to 5, or 0.7 to 2.

[0023] The lithium silicate phase may contain a Group II element. In general, the silicate phase exhibits alkalinity, but the Group II element has the effect of suppressing the elution of alkali metals from the silicate phase. Therefore, the slurry viscosity is easily stabilized when preparing a slurry containing a negative electrode active material. Therefore, the need for a treatment (e.g., an acid treatment) for neutralizing the alkaline components of the LSX particles is also reduced.

[0024] As the Group II element, at least one selected from the group consisting of Be, Mg, Ca, Sr, Ba and Ra can be used. Among them, Ca is preferable because it can improve the Vickers hardness of the lithium silicate phase and further improve the cycle characteristics. The content of the Group II element is, for example, 20 mol % or less, or may be 15 mol % or less, or may be 10 mol % or less, based on the total amount of elements other than O contained in the lithium silicate phase.

[0025] The lithium silicate phase may contain an element M other than the alkali metal elements and the group II elements. The element M may be at least one selected from the group consisting of B, Al, Zr, Nb, Ta, La, V, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F and W. Specifically, for example, B has a low melting point and is advantageous for improving the fluidity during sintering. Ca has the effect of increasing the hardness of the lithium silicate phase, although it reduces the ion conductivity. Al, Zr, Nb, Ta and La can improve the hardness while maintaining the ion conductivity. The content of the element M may be, for example, 10 mol% or less, or 5 mol% or less, based on the total amount of elements other than O contained in the lithium silicate phase.

[0026] The contents of B, Na, K and Al contained in the lithium silicate phase are determined by quantitative analysis in accordance with JIS R3105 (1995) (analysis method for borosilicate glass), and the Ca content is determined by quantitative analysis in accordance with JIS R3101 (1995) (analysis method for soda-lime glass).

[0027] The other elements contained are determined by the following method. First, a sample of the lithium silicate phase or LSX particles containing it is completely dissolved in a heated acid solution (a mixed acid of hydrofluoric acid, nitric acid, and sulfuric acid), and the carbon remaining in the solution is filtered off. The filtrate is then analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES) to measure the spectral intensity of each element. Next, a calibration curve is created using commercially available standard solutions of the elements, and the content of each element contained in the lithium silicate phase is calculated.

[0028] In the silicate composite particles, there are a silicate phase and silicon particles, and these can be distinguished and quantified by using Si-NMR. The Si content obtained by ICP-AES as described above is the sum of the amount of Si constituting the silicon particles and the amount of Si in the silicate phase. On the other hand, the amount of Si constituting the silicon particles can be quantified separately by using Si-NMR. Therefore, the amount of Si in the silicate phase can be quantified by subtracting the amount of Si constituting the silicon particles from the Si content obtained by ICP-AES. For the standard substance required for quantification, a mixture containing a silicate phase and silicon particles with a known Si content in a predetermined ratio may be used.

[0029] The following shows the desirable measurement conditions for Si-NMR.

[0030] <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: Near -100 ppm Signal acquisition time: 0.05 sec Number of integrations: 560 Sample amount: 207.6 mg For increasing the capacity and improving the cycle characteristics, the content of silicon particles in the silicate composite particles may be, for example, 30% by mass or more and 80% by mass or less. By setting the content of silicon particles to 30% by mass or more, the proportion occupied by the silicate phase becomes smaller, and the initial charge-discharge efficiency is likely to be improved. By setting the content of silicon particles to 80% by mass or less, it becomes easier to reduce the degree of expansion and contraction of the LSX particles during charge and discharge. The content of silicon particles in the silicate composite particles is preferably 40% by mass or more, and more preferably 50% by mass or more.

[0031] The silicon particles dispersed in the silicate phase have a particulate phase of silicon (Si) alone, and are composed of single or multiple crystallites. The crystallite size of the silicon particles is preferably 30 nm or less. When the crystallite size of the silicon particles is 30 nm or less, the volume change due to the expansion and contraction of the silicon particles accompanying charging and discharging can be reduced, and the cycle characteristics can be further improved. For example, when the silicon particles contract, voids are formed around the silicon particles, reducing the contact points of the particles with the surroundings, suppressing the isolation of the particles, and suppressing the decrease in charge and discharge efficiency due to the isolation of the particles. The lower limit of the crystallite size of the silicon particles is not particularly limited, but is, for example, 5 nm.

[0032] The crystallite size of the silicon particles is more preferably 2 nm or more and 30 nm or less, and even more preferably 2 nm or more and 20 nm or less. When the crystallite size of the silicon particles is 20 nm or less, the expansion and contraction of the silicon particles can be made uniform, and the cycle characteristics can be improved by reducing fine cracks in the particles due to the expansion and contraction of the silicon particles during charging and discharging. The crystallite size of the silicon particles is calculated by the Scherrer formula from the half-width of the diffraction peak assigned to the Si(111) plane in the X-ray diffraction (XRD) pattern of the silicon particles.

[0033] At least a part of the surface of the silicate composite particle may be coated with a conductive material. Since the silicate phase has poor electronic conductivity, the conductivity of the composite particle tends to be low. However, by coating the surface of the base particle with a conductive material to form a conductive layer, the conductivity of the LSX particle can be dramatically increased. The conductive material is preferably a carbon material. The carbon material preferably contains at least one selected from the group consisting of carbon compounds and carbonaceous materials.

[0034] The thickness of the conductive layer is preferably thin enough not to substantially affect the average particle size of the silicate composite particles. In consideration of ensuring electrical conductivity and the diffusibility of ions (e.g., lithium ions) that contribute to charging and discharging, 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 silicate composite particles using a SEM or a TEM (transmission electron microscope).

[0035] Examples of carbon compounds include compounds containing carbon and hydrogen, and compounds containing carbon, hydrogen, and oxygen. Examples of carbonaceous materials include amorphous carbon with low crystallinity and graphite with high crystallinity. Examples of amorphous carbon include carbon black, coal, coke, charcoal, and activated carbon. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles. Among these, amorphous carbon is preferred because it has low hardness and a large buffering effect against silicon particles that change in volume during charging and discharging. The amorphous carbon may be either easily graphitized carbon (soft carbon) or difficult to graphitize carbon (hard carbon). Examples of carbon black include acetylene black and ketjen black.

[0036] The silicate composite particles can be taken out of the battery by the following method. First, the battery is disassembled to remove the negative electrode, which is washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove the electrolyte. Next, the negative electrode mixture is peeled off from 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 alkali metals such as Na and Li that may be contained in the binder, etc. Next, the sample powder is washed with ion-exchanged water, filtered, and dried at 200 °C for 1 hour. After that, it is heated to 900 °C in an oxygen atmosphere to remove the carbon components, and only the LSX particles can be isolated.

[0037] The cross-sectional observation of the silicate composite particles can be carried out, for example, by the following method. First, disassemble the battery, take out the negative electrode, and obtain the cross-section of the negative electrode mixture layer using a cross-section polisher (CP). Observe the cross-section of the negative electrode mixture layer using a scanning electron microscope (SEM). Randomly select 10 LSX particles with a maximum particle diameter of 5 μm or more from the cross-sectional image of the backscattered electron image of the negative electrode mixture layer, and perform elemental (e.g., carbon) mapping analysis on each of them using energy-dispersive X-ray (EDX). Calculate the contained area of the target element using image analysis software. The observation magnification is preferably 2000 to 20000 times. Average the measured values of the contained areas of a predetermined element for the 10 obtained particles.

[0038] In addition, during the charge and discharge process, a film is formed on the surface of the silicate composite particles due to the decomposition of the electrolyte, etc. Further, as will be described later, the silicate composite particles may further include a conductive layer that covers the surface of the composite particles. Therefore, the mapping analysis by EDX is performed on the range within 1 μm from the peripheral edge of the cross-section of the silicate composite particles so that the thin film and the conductive layer are not included in the measurement range. The distribution state of the carbon material inside the silicate composite particles can also be confirmed by the mapping analysis by EDX. Since it is difficult to distinguish from the decomposition products of the electrolyte at the end of the cycle, it is preferable to measure the sample before the cycle or at the initial stage of the cycle.

[0039] The measurement conditions for the desirable cross-sectional SEM-EDX analysis are shown below.

[0040] <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 In addition, the quantitative determination of each element in the silicate composite particles contained in the negative electrode active material layer in a discharged state can be performed not only by SEM-EDX analysis, but also by Auger electron spectroscopy (AES), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), X-ray photoelectron spectroscopy (XPS), etc.

[0041] Next, a method for producing silicate composite particles will be described in detail, taking the production of LSX particles having a lithium silicate phase as an example.

[0042] Process (i) The raw material for lithium silicate is a raw material mixture containing a Si-containing raw material and a Li raw material in a predetermined ratio. The raw material mixture may contain the above-mentioned alkali metal element, group II element, and / or element M. A mixture of the above raw materials in a predetermined amount is melted, and the melt is passed through a metal roll to form flakes to produce lithium silicate. The flaked silicate is then crystallized by heat treatment in an air atmosphere at a temperature above the glass transition point and below the melting point. The flaked silicate can also be used without being crystallized. It is also possible to produce silicate by a solid-phase reaction by firing a mixture of the above-mentioned raw materials in a predetermined amount at a temperature below the melting point without melting it.

[0043] Silicon oxide can be used as the Si raw material. For example, lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, etc. can be used as the Li raw material. These may be used alone or in combination of two or more. Raw materials for alkali metal elements, Group II elements, and element M include oxides, hydroxides, carbonates, hydrides, nitrates, sulfates, etc. of each element.

[0044] The Si raw material that has not reacted with the Li raw material may remain in the lithium silicate. The remaining Si raw material is dispersed in the lithium silicate as a silicon oxide phase.

[0045] Process (ii) Next, raw silicon is mixed with the lithium silicate to form a composite. The raw silicon used contains a predetermined amount of the first element. For example, the composite particles are produced through the following steps (a) to (c).

[0046] Process (a) First, raw silicon powder and lithium silicate powder are mixed in a mass ratio of, for example, 20:80 to 95:5. The raw silicon may be coarse silicon particles having an average particle size of several μm to several tens of μm. The coarse particles contain a predetermined amount of the first element.

[0047] Process (b) Next, the mixture of raw silicon and lithium silicate is pulverized and compounded while being finely divided using a pulverizing device such as a ball mill. At this time, an organic solvent may be added to the mixture and wet-pulverized. A predetermined amount of the organic solvent may be charged into the pulverizing vessel at once at the beginning of the pulverization, or a predetermined amount of the organic solvent may be charged into the pulverizing vessel intermittently in multiple batches during the pulverization process. The organic solvent serves to prevent the material to be pulverized from adhering to the inner wall of the pulverizing vessel.

[0048] As the organic solvent, alcohol, ether, fatty acid, alkane, cycloalkane, silicate ester, metal alkoxide, etc. can be used.

[0049] The raw silicon and lithium silicate may be separately pulverized and then mixed. Alternatively, silicon nanoparticles and amorphous lithium silicate nanoparticles may be produced and mixed without using a pulverizer. The nanoparticles may be produced by a known method such as a gas phase method (e.g., a plasma method) or a liquid phase method (e.g., a liquid phase reduction method).

[0050] Process (c) Next, the mixture is sintered by heating to 450°C to 1000°C while applying pressure in an inert gas atmosphere (such as an argon or nitrogen atmosphere). For sintering, a sintering device capable of applying pressure in an inert atmosphere, such as a hot press or a spark plasma sintering device, may be used. During sintering, the silicate softens and flows to fill the gaps between the silicon particles. As a result, a dense block-shaped sintered body can be obtained, with the silicate phase as the sea portion and the silicon particles as the islands.

[0051] The obtained sintered body is pulverized to obtain silicate composite particles. By appropriately selecting the pulverization conditions, silicate composite particles having a predetermined average particle size can be obtained. The average particle size of the silicate composite particles is, for example, 1 to 20 μm. The average particle size of the silicate composite particles means the particle size (volume average particle size) at which the volume integrated value is 50% in the volume particle size distribution measured by the laser diffraction scattering method.

[0052] Sintering may be performed by rolling the powder of the mixture previously molded into a sheet in a heated state. Rolling is usually performed by passing a sheet of the mixture through the gap between two rotating rolls. By passing the sheet of the mixture through the gap between the heated rolls, the mixture can be heated and pressurized at the same time. The sheet of the mixture may be heated before and / or after passing through the rolls. By performing a heat treatment while rolling the mixture molded into a sheet, the negative electrode active material for secondary batteries (LSX particles) can be produced with high productivity.

[0053] Process (iii) Next, at least a part of the surface of the composite particle may be coated with a conductive material to form a conductive layer. The conductive material is preferably electrochemically stable, and is preferably a conductive carbon material. Examples of a method for coating the surface of the composite particle 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 particle and heating to carbonize it. Carbon black may also be attached to the surface of the composite particle. The coating with the conductive carbon material can be performed, for example, by heating a mixture of the material from which the fine particles have been removed and the conductive carbon material in an inert atmosphere (for example, an atmosphere of argon, nitrogen, etc.) at 700°C to 950°C.

[0054] Process (iv) A step of washing the composite particles (including those having a conductive layer on the surface) with an acid may be carried out. For example, by washing the composite particles with an acidic aqueous solution, it is possible to dissolve and remove trace amounts of alkaline components present on the surface of the composite particles that may be generated when the raw silicon and lithium silicate are combined. As the acidic aqueous solution, an aqueous solution of an inorganic acid such as hydrochloric acid, hydrofluoric acid, sulfuric acid, nitric acid, phosphoric acid, or carbonic acid, or an aqueous solution of an organic acid such as citric acid or acetic acid can be used.

[0055] In the steps (ii)(c) and (iii), the heat treatment is performed at a relatively high temperature, and therefore the crystallite size of the silicon particles in the silicate composite particles may increase with heating. However, by including the first element in the silicon particles, the increase in the crystallite size of the silicon particles can be suppressed.

[0056] FIG. 1 shows a schematic cross section of an LSX particle 20 as an example of a negative electrode material.

[0057] The base particle 23 includes a lithium silicate phase 21 and silicon particles 22 dispersed within the lithium silicate phase 21. The base particle 23 has a sea-island structure in which fine silicon particles are dispersed within the matrix of the lithium silicate phase 21. The surface of the base particle 23 is covered with a conductive layer 26, and the LSX particle 20 is formed thereon.

[0058] The silicon particles 22 contain a first element other than silicon. The SiO 2 The content is, for example, preferably 30% by mass or less, and more preferably less than 7% by mass.

[0059] The base particle 23 may contain other components in addition to the lithium silicate phase 21, the silicon particles 22, the silicon oxide phase, and the carbon material. For example, from the viewpoint of improving the strength of the base particle 23, ZrO 2 The base particle 23 may contain a reinforcing material such as an oxide or carbide thereof up to 10% by weight.

[0060] The average particle size of the silicon particles 22 before the first charge is 1 nm or more and 1000 nm or less, and may be 500 nm or less, 200 nm or less, or 50 nm or less. By appropriately miniaturizing the silicon particles 22 in this way, the volume change during charging and discharging is reduced, and structural stability is improved. In addition, the expansion and contraction of the silicon particles is made uniform, and particle cracking is suppressed, thereby improving cycle characteristics. The average particle size of the silicon particles 22 is measured by observing the cross section of the negative electrode material using a SEM or TEM. Specifically, it is obtained by averaging the maximum diameters of 100 arbitrary silicon particles 22.

[0061] The secondary battery according to the embodiment of the present disclosure includes a positive electrode, a negative electrode, an electrolyte, and a separator interposed between the positive electrode and the negative electrode. The negative electrode includes a current collector and a negative electrode active material layer including the negative electrode active material for secondary batteries. The negative electrode, positive electrode, electrolyte, and separator included in the secondary battery according to the embodiment of the present disclosure will be described below.

[0062] [Negative electrode] The negative electrode comprises, for example, a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector and containing a negative electrode active material. 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 it. The coating film after drying 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.

[0063] The negative electrode mixture contains the above-mentioned LSX particle-containing negative electrode active material for secondary batteries as an essential component as a negative electrode active material, and may contain optional components such as a binder, a conductive agent, a thickener, etc. The silicon particles in the LSX particles can absorb many lithium ions, thereby contributing to a high capacity negative electrode.

[0064] The negative electrode active material may further contain other active material materials that electrochemically absorb and release lithium ions. For example, carbon-based active materials are preferable as other active material materials. Since the LSX particles expand and contract in volume with charging and discharging, when the ratio of the LSX particles in the negative electrode active material increases, poor contact between the negative electrode active material and the negative electrode current collector with charging and discharging is likely to occur. On the other hand, by using the LSX particles and the carbon-based active material in combination, it is possible to achieve excellent cycle characteristics while imparting the high capacity of the silicon particles to the negative electrode. The ratio of the LSX particles to the total of the LSX particles and the carbon-based active material is, for example, preferably 0.5 to 15 mass%, more preferably 1 to 5 mass%. This makes it easier to achieve both high capacity and improved cycle characteristics.

[0065] Examples of carbon-based active materials include graphite, graphitizable carbon (soft carbon), and non-graphitizable carbon (hard carbon). Among them, graphite is preferable because it has excellent charge / discharge stability and low irreversible capacity. Graphite means a material having a graphite-type crystal structure, and includes, for example, natural graphite, artificial graphite, and graphitized mesophase carbon particles. The carbon-based active materials may be used alone or in combination of two or more.

[0066] As the negative electrode current collector, a non-porous conductive substrate (metal foil, etc.) or a porous conductive substrate (mesh, net, punched sheet, etc.) is used. Examples of the material of the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, copper alloy, etc. The thickness of the negative electrode current collector is not particularly limited, but is preferably 1 to 50 μm, more preferably 5 to 20 μm, from the viewpoint of the balance between the strength and weight reduction of the negative electrode.

[0067] Examples of the binder include fluororesin, polyolefin resin, polyamide resin, polyimide resin, vinyl resin, styrene-butadiene copolymer rubber (SBR), polyacrylic acid and its derivatives. These may be used alone or in combination of two or more. Examples of the conductive agent include carbon black, conductive fiber, carbon fluoride, organic conductive material, etc. These may be used alone or in combination of two or more. Examples of the thickener include carboxymethyl cellulose (CMC), polyvinyl alcohol, etc. These may be used alone or in combination of two or more.

[0068] Examples of the dispersion medium include water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), and a mixture of these.

[0069] [Positive electrode] The positive electrode includes, for example, a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by applying a positive electrode slurry in which the positive electrode mixture is dispersed in a dispersion medium to the surface of the positive electrode current collector and drying it. The coating film after drying may be rolled as necessary. The positive electrode mixture layer may be formed on one surface of the positive electrode current collector or on both surfaces.

[0070] The positive electrode mixture contains a positive electrode active material as an essential component, and may contain a binder, a conductive agent, and the like as optional components.

[0071] The positive electrode active material may be a lithium composite metal oxide. For example, Lia Chief of Staff 2 , Li a NiO 2 , Li a MnO 2 , Li a Co b Ni 1-b O 2 , Li a Co b M 1-b O c , Li a Ni 1-b M b O c , Li a Mn 2 O 4 , Li a Mn 2-b M b O 4、 LiMePO 4、 Li 2 MePO 4 Examples of the metal element include F. Here, M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. Me includes at least a transition element (for example, includes at least one selected from the group consisting of Mn, Fe, Co, and Ni). Here, 0≦a≦1.2, 0≦b≦0.9, and 2.0≦c≦2.3. The value a, which indicates the molar ratio of lithium, is a value immediately after the active material is produced, and increases or decreases with charging and discharging.

[0072] The binder and conductive agent may be the same as those exemplified for the negative electrode. As the conductive agent, graphite such as natural graphite or artificial graphite may be used.

[0073] The shape and thickness of the positive electrode current collector can be 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, and titanium.

[0074] [Electrolytes] The electrolyte contains a solvent and a lithium salt dissolved in the solvent. The concentration of the lithium salt in the electrolyte is, for example, 0.5 to 2 mol / L. The electrolyte may contain known additives.

[0075] The solvent used may be an aqueous solvent or a non-aqueous solvent. Examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, and cyclic carboxylates. Examples of the cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Examples of the chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of the cyclic carboxylates include γ-butyrolactone (GBL) and γ-valerolactone (GVL). The non-aqueous solvents may be used alone or in combination of two or more.

[0076] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO 4 , LiAlCl 4 , LiB 10 Cl 10 etc.), lithium salts of fluorine-containing acids (LiPF 6 , LiBF 4 , LiSbF 6 , LiAsF 6 , LiCF 3 SO 3 , LiCF 3 CO 2 etc.), lithium salts of fluorine-containing acid imides (LiN(CF 3 SO 2 ) 2 , LiN(CF 3 SO 2 )(C 4 F 9 SO 2 ), LiN(C 2 F 5 SO 2 ) 2 Lithium salts that can be used include lithium halides (LiCl, LiBr, LiI, etc.), etc. The lithium salts may be used alone or in combination of two or more.

[0077] [Separator] Usually, it is desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and has appropriate mechanical strength and insulation properties. As the separator, a microporous thin film, a woven fabric, a nonwoven fabric, etc. can be used. As the material of the separator, for example, a polyolefin such as polypropylene or polyethylene can be used.

[0078] An example of the structure of the 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 and an electrolyte are housed in an exterior body. Alternatively, instead of a wound type electrode group, other types of electrode groups may be applied, such as a stacked type electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween. The secondary battery may be in any form, such as a cylindrical type, a square type, a coin type, a button type, a laminate type, or the like.

[0079] FIG. 2 is a schematic perspective view, with a portion cut away, of a prismatic secondary battery according to an embodiment of the present disclosure.

[0080] The battery includes a bottomed rectangular battery case 4, an electrode group 1 and an electrolyte housed in the battery case 4, and a sealing plate 5 that seals the opening of 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, positive electrode, and separator are wound around a flat winding core, and the electrode group 1 is formed by removing the winding core. The sealing plate 5 includes a liquid injection port closed with a plug 8, and a negative electrode terminal 6 insulated from the sealing plate 5 by a gasket 7.

[0081] One end of a negative electrode lead 3 is attached to the negative electrode current collector of the negative electrode by welding or the like. One end of a positive electrode lead 2 is attached to the positive electrode current collector of the positive electrode by welding or the like. The other end of the negative electrode lead 3 is electrically connected to a negative electrode terminal 6. The other end of the positive electrode lead 2 is electrically connected to a sealing plate 5. A resin frame is disposed on the upper part of the electrode group 1 to separate the electrode group 1 from the sealing plate 5 and to separate the negative electrode lead 3 from the battery case 4.

[0082] <Example> 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.

[0083] <Example 1> [Preparation of LSX particles] Lithium carbonate and silicon dioxide are mixed together to form Li 2 CO 3 :SiO 2 The mixture was melted in an inert gas atmosphere at 1500°C for 5 hours, the molten liquid was passed through a metal roller to form flakes, and the flakes were heat-treated at 750°C for 5 hours to obtain a lithium silicate composite oxide that exists as a mixed phase of amorphous and crystalline phases. The obtained lithium silicate composite oxide was pulverized to an average particle size of 10 μm.

[0084] Lithium silicate composite oxide with an average particle size of 10 μm was mixed with silicon germanium particles (average particle size 10 μm) in a mass ratio of 70:30. The silicon germanium particles used had a germanium content of 2 atomic % of the total particles. Silicon germanium was produced by arcing a mixture of silicon and germanium in an inert gas atmosphere and melting it with the heat. The mixture was filled into a pot (made of stainless steel, volume: 500 mL) of a planetary ball mill (manufactured by Fritsch, P-5), 24 stainless steel balls (diameter 20 mm) were placed in, the lid was closed, and the mixture was ground at 200 rpm for 25 hours in an inert atmosphere to form a composite.

[0085] The crushed material was then pressure molded into a sheet having a thickness of 4 to 5 mm at room temperature to obtain a sheet-like molded body. The molded body was heated to 800°C and rolled between rolling rolls to obtain a sintered body. The rolling was performed so that a pressure of 240 MPa was applied to the molded body.

[0086] The sintered body was then crushed and passed through a 40 μm mesh, and then 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 coat the surfaces of the LSX particles with conductive carbon to form a conductive layer. The amount of the conductive layer was 5 mass% based on the total mass of the LSX particles and the conductive layer. A sieve was then used to obtain LSX particles with an average particle size of 5 μm and a conductive layer.

[0087] XRD analysis was performed on the crushed composite before sintering, the sintered body, and the LSX particles after the conductive layer was formed, and the half-width of the peak derived from the Si (111) plane was measured. The crystallite size was calculated from the half-width of the peak using the Scherrer formula. In addition, the porosity of the sintered body was calculated by observing the cross section with a SEM.

[0088] [Preparation of negative electrode] LSX particles and graphite were mixed in a mass ratio of 5:95 and used as the negative electrode active material. The negative electrode active material, sodium carboxymethylcellulose (CMC-Na), styrene-butadiene rubber (SBR), and lithium polyacrylate were mixed in a mass ratio of 96.5:1:1.5:1, water was added, and the mixture was stirred using a mixer (TK Hivismix, manufactured by Primix Corporation) to prepare a negative electrode slurry. Next, 1 m of the mixture was applied to the surface of the copper foil. 2 The negative electrode slurry was applied so that the mass of the negative electrode mixture per sheet was 190 g, the coating was dried, and then rolled to form a copper foil with a density of 1.5 g / cm on both sides. 3 Thus, a negative electrode having a negative electrode mixture layer of the above formula was fabricated.

[0089] [Preparation of positive electrode] Lithium cobalt oxide, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 95:2.5:2.5, N-methyl-2-pyrrolidone (NMP) was added, and the mixture was stirred using a mixer (TK Hivismix, manufactured by Primix Corporation) 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 mixture was rolled to form a positive electrode slurry having a density of 3.6 g / cm on both sides of the aluminum foil. 3 A positive electrode having a positive electrode mixture layer of the above formula was fabricated.

[0090] [Preparation of electrolyte] LiPF was dissolved in a mixed solvent containing ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7. 6 was dissolved at a concentration of 1.0 mol / L to prepare a non-aqueous electrolyte solution.

[0091] [Preparation of secondary battery] A tab was attached to each electrode, and the positive and negative electrodes were spirally wound with a separator interposed therebetween so that the tabs were located at the outermost periphery to prepare an electrode group. The electrode group was inserted into an exterior body made of an aluminum laminate film and vacuum dried at 105°C for 2 hours. After that, a nonaqueous electrolyte was poured in and the opening of the exterior body was sealed to obtain a secondary battery A1.

[0092] <Comparative Example 1> In Example 1, lithium silicate composite oxide and silicon particles (3N, average particle size 10 μm) were mixed in a mass ratio of 70:30. The mixture was filled into a pot (made of SUS, volume: 500 mL) of a planetary ball mill (manufactured by Fritsch, P-5), 24 SUS balls (diameter 20 mm) were placed in the pot, the lid was closed, and the mixture was pulverized in an inert atmosphere at 200 rpm for 25 hours. The pulverized product was sintered in the same manner as in Example 1 to obtain a sintered body of the mixture.

[0093] The sintered body was then crushed and passed through a 40 μm mesh, and then 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 coat the surfaces of the LSX particles with conductive carbon to form a conductive layer. The amount of the conductive layer was 5 mass% based on the total mass of the LSX particles and the conductive layer. A sieve was then used to obtain LSX particles with an average particle size of 5 μm and a conductive layer.

[0094] Using the above LSX particles, a negative electrode was produced in the same manner as in Example 1, to obtain a secondary battery B1.

[0095] The following evaluations were carried out for each of the batteries of the Examples and Comparative Examples.

[0096] [Charge / discharge cycle test] Each battery was repeatedly charged and discharged under the following conditions.

[0097] <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).

[0098] <Discharge> At 25° C., constant current discharge was performed at a current of 1 It (800 mA) until the voltage reached 2.75 V.

[0099] The rest period between charging and discharging was 10 minutes. The ratio of the discharge capacity at the 500th cycle to the discharge capacity at the 1st cycle was defined as the cycle retention rate.

[0100] For each of batteries A1 and B1, the half-width (full-width at half-maximum) of the XRD peak derived from the Si(111) surface after the pulverized product, sintered body, and conductive layer formation of the LSX particles used as the negative electrode active material, as well as the crystallite size of the silicon particles calculated from the half-width, are shown in Table 1. The porosity of the sintered body was also measured, and the results are shown in Table 1.

[0101] As shown in Table 1, after the composite is formed, the half-width of the XRD peak increases and the crystallite size of the silicon particles increases as the sintering process and conductive layer formation process are performed. This is thought to be because each process involves heating, which promotes the crystal growth of silicon. However, as shown in Table 1, when silicon particles containing germanium are used, the increase in the half-width of the XRD peak and the increase in the crystallite size are suppressed compared to when silicon particles not containing germanium are used. In addition, the porosity of the LSX particles is also reduced.

[0102] [Table 1] [Industrial Applicability]

[0103] The present disclosure can provide a secondary battery having a high capacity and good charge / discharge cycle characteristics. The secondary battery of the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, and the like. [Explanation of symbols]

[0104] 1 electrode group 2 Positive Lead 3 Negative lead 4 Battery case 5 Sealing plate 6 Negative terminal 7 Gasket 8. Sealing 20 LSX particles 21 Lithium silicate phase 22 Silicon particles 23 Mother particle 26 Conductive Layer

Claims

1. The silicate composite particles include silicon particles and a silicate phase, The silicon particles are dispersed in the silicate phase, The silicon particles contain at least one first element selected from the group consisting of germanium and aluminum, the first element includes germanium, and at least a portion of the first element is dissolved in the silicon particles to form a solid solution; The first element is contained in the silicon particles at a ratio of 0.1 atomic % to 20 atomic %.

2. 2 . The negative electrode active material for a secondary battery according to claim 1 , wherein the silicon particles have an average particle size of 1 nm or more and 1000 nm or less.

3. 3. The negative electrode active material for a secondary battery according to claim 1, wherein the silicate phase contains Li and at least one element selected from the group consisting of alkali metal elements other than Li and Group II elements.

4. The silicate phase further comprises an element M, The element M is at least one selected from the group consisting of B, Al, Zr, Nb, Ta, La, V, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F and W. The negative electrode active material for secondary batteries according to any one of claims 1 to 3.

5. A battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator interposed between the positive electrode and the negative electrode; the negative electrode includes a current collector and a negative electrode active material layer, A secondary battery, wherein the negative electrode active material layer comprises the negative electrode active material for secondary batteries according to any one of claims 1 to 4.

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