Negative electrode active material for secondary batteries, and secondary battery

Abstract

The disclosed negative electrode active material for secondary batteries comprises silicate composite particles that contain a silicate phase and silicon particles dispersed in the silicate phase. The peak in the energy-loss near-edge structure (ELNES) for the oxygen K-edge for the silicate phase / silicon particle interface is shifted 2.0 eV to 3.0 eV to the lower energy side relative to the peak in the energy-loss near-edge structure for the oxygen K-edge for the silicate phase.　A secondary battery having excellent charge / discharge cycle characteristics can be realized by using the negative electrode active material according to the present invention.

Description

Negative electrode active material for secondary battery and secondary battery

[0001] The present invention mainly relates to an improvement in a negative electrode active material for a secondary battery.

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

[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 to pre-introduce lithium equivalent to the irreversible capacity into silicon-containing materials. Specifically, the use of composite particles containing a lithium silicate phase and silicon particles dispersed within the lithium silicate phase has been proposed (Patent Document 1). The silicon particles contribute to the charge-discharge reaction (reversible lithium absorption and desorption).

[0004] JP 2015-153520 A

[0005] However, it is known that the silicon particles of the composite particles expand and contract significantly during charge and discharge due to the absorption and release of lithium. This causes significant stress in the lithium silicate phase surrounding the silicon particles, resulting in cracks and fractures in the composite particles. This weakens the bonding strength between the composite particles and the surrounding binder, and fractured composite particles, in particular, lose their conductive paths to the surrounding particles and become isolated. This also promotes side reactions between the electrolyte and the silicon particles. This results in a deterioration of charge and discharge cycle performance.

[0006] In view of the above, one aspect of the present invention relates to a negative electrode active material for a secondary battery, which includes silicate composite particles including a silicate phase and silicon particles dispersed in the silicate phase, and in which the oxygen K-edge energy-loss near-edge structure (ELNES) peak at the interface between the silicate phase and the silicon particles is shifted to lower energy by 2.0 eV to 3.0 eV relative to the oxygen K-edge energy-loss near-edge structure peak in the silicate phase.

[0007] Another aspect of the present invention 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.

[0008] By using the negative electrode active material of the present invention, a secondary battery having excellent charge-discharge cycle characteristics can be realized. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0009] 1 is a cross-sectional view schematically illustrating a negative electrode active material for a secondary battery (silicate composite particles) according to one embodiment of the present invention. 2 is a schematic perspective view of a secondary battery according to one embodiment of the present invention, with a portion cut away.

[0010] Hereinafter, embodiments of the present disclosure will be described using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values, materials, etc. may be exemplified, but other numerical values, materials, etc. may be applied as long as the effects of the present disclosure are obtained. Note that components other than those characteristic of the present disclosure may be components of known secondary batteries. In this specification, when a "range of numerical value A to numerical value B" is mentioned, the range includes numerical value A and numerical value B. For example, when a "range of A to B mol %" is mentioned, it is synonymous with "A mol % or more and B mol % or less."

[0011] A negative electrode active material for a secondary battery according to an embodiment of the present disclosure includes silicate composite particles including a silicate phase and silicon particles dispersed in the silicate phase.

[0012] The silicate composite particles have a sea-island structure in which silicon particles, which form islands, are dispersed within a silicate phase, which forms a sea. High capacity can be achieved by controlling the amount of silicon particles dispersed in the silicate phase. Furthermore, because the silicon particles are dispersed within the silicate phase, the stress associated with the expansion and contraction of the silicon particles during charge and discharge is alleviated by the silicate phase, reducing the expansion and contraction of the silicate composite particles. This reduces cracking and fractures in the silicate composite particles, making it easy to achieve both high battery capacity and improved cycle performance.

[0013] The denser and more stable the interface between the silicate phase and the silicon particles, the more easily the silicate phase can alleviate stresses caused by the expansion and contraction of the silicon particles, and the more easily cracks and fractures in the silicate composite particles can be suppressed. In the silicate composite particles according to the embodiments of the present disclosure, by increasing the oxygen abundance ratio at the interface between the silicate phase and the silicon particles, numerous Si—O—Si bonds are formed between the silicate phase and the silicon particles, and a dense and stable interface is formed between the silicate phase and the silicon particles. This reduces cracks and fractures in the silicate composite particles and further improves cycle characteristics.

[0014] The interface state between the silicate phase and the silicon particles can be evaluated, for example, by observing a cross section of the negative electrode active material layer (negative electrode mixture layer) using a TEM, identifying the boundary between the silicate phase and the silicon particles based on the cross-sectional image, and performing electron energy loss spectroscopy (EELS) on the boundary position. For example, a fine structure appears near the absorption edge in the EELS spectrum associated with the excitation of K-shell electrons of oxygen (O) atoms. This fine structure reflects the bonding state of the oxygen atoms at the interface between the silicate phase and the silicon particles.

[0015] In the negative electrode active material for a secondary battery according to an embodiment of the present disclosure, the peak of the oxygen K-edge energy loss near edge structure (ELNES) at the interface between the silicate phase and the silicon particles is shifted to a lower energy side by 2.0 to 3.0 eV relative to the peak of the oxygen K-edge energy loss near edge structure in the silicate phase. In this case, sufficient Si—O—Si bonds are formed between the silicate phase and the silicon particles, forming a dense and stable interface and improving cycle characteristics.

[0016] The desirable conditions for measuring the oxygen K-edge energy loss absorption fine structure are shown below. <TEM-ELNES measurement conditions> Analysis equipment: JEOL JEM-F200 EELS detector Quantum ER (Gatan: attached to JEM-F200) Conditions: Acceleration voltage 200 kV Vacuum degree: 1×10 －６ ~8.0 x 10 －５ Pa dispersion: 0.050 eV / ch Probe diameter: 0.16 nm Probe current: 0.05 nA Camera length: 40 mm Pixel time: approximately 0.1 (s)

[0017] In the measurement, one silicate composite particle is selected from the cross-sectional image. From the cross-sectional image of the selected composite particle, a position at the interface between the silicate phase and the silicon particle and a position within the silicate phase are selected. At each position, oxygen K-edge ELNES measurement is performed to observe the ELNES peak in the range of 550 eV to 570 eV, which reflects the coordination environment of the O atoms, and the peak position (the position where the intensity is maximum) is determined. The shift amount of the peak position at the interface between the silicate phase and the silicon particle relative to the peak position in the silicate phase is determined. The peak position in the silicate phase may be determined by averaging measured values ​​at multiple positions (e.g., 10 arbitrarily selected positions) within the silicate phase. Similarly, the peak position at the interface between the silicate phase and the silicon particle may be determined by averaging measured values ​​at multiple positions (e.g., 10 arbitrarily selected positions) at the interface.

[0018] The porosity of the silicate composite particles is preferably less than 6%, preferably 3% or less, and more preferably 2% or less. In this case, the gaps between the silicate phase and the silicon particles are small, and the contact area between the silicate phase and the silicon particles is large. Furthermore, Si-O-Si bonds are formed in the contact areas between the silicate phase and the silicon particles, forming a dense and stable interface. This further improves cycle characteristics.

[0019] Here, "porosity" refers to 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. When using SEM observation, the specific method for measuring porosity is as follows. It is desirable to form the particle cross section on the particle in a state before charging and discharging, but when using particles that have been charged at least once, the particle is fully discharged before forming the cross section. (1) The cross section of the base particle is exposed using an ion milling device (e.g., IM4000) manufactured by Hitachi High-Tech Corporation. (2) The exposed particle cross section is observed using SEM, the ratio of the area of ​​voids to the total area of ​​the particle cross section is measured, and the porosity (area of ​​voids × 100 / total area of ​​particle cross section) is calculated. The porosity is the average value for 10 particles.

[0020] The silicate composite particles are usually produced by mixing silicate powder with silicon particles, pulverizing the mixture in a ball mill or the like to form a composite, and then sintering the pulverized mixture at high temperature.

[0021] The amount of peak shift can be controlled by the conditions of the process for obtaining the lithium silicate powder used in the composite. For example, by controlling the temperature of the solid-phase reaction when obtaining the silicate to 850° C. or higher and the melting point or lower, Li can be more uniformly dispersed. ２ O and SiO ２ and Al ２ O ３reacts with the silicon dioxide, and in the compounding step, a bond is easily formed at the silicon interface via O (oxygen). That is, under the above solid-state reaction conditions, the silicate powder is more strongly hardened, and the silicon pulverization force in the compounding step using a ball mill or the like is improved. As the silicon is finely pulverized and the lithium silicate and the silicon metal are more closely integrated, a bond is easily formed at the silicon interface via O (oxygen).

[0022] In addition, silicon oxide (SiO ２ In addition to increasing the amount of aluminum oxide (Al ), adding a specific amount of aluminum compound is also important in controlling the amount of peak shift at the interface between the silicate phase and silicon particles. By adding a specific amount of aluminum compound, the mixture after pulverization is less likely to harden even when sintered at high temperatures, and lithium silicate becomes more likely to flow during the sintering process. As a result, the porosity of the silicate composite particles is reduced, making it easier to control the amount of peak shift within the range of 2.0 eV to 3.0 eV. In the production of silicate powder, aluminum oxide (Al ) is used as the aluminum compound. ２ O ３ ), aluminum hydroxide, etc. may also be added.

[0023] In the silicate composite particles, the silicate phase is silicon oxide (SiO ２ ) in the range of 69 to 78 mol %, and lithium oxide (Li ２ The silicate phase of such a composition preferably contains aluminum oxide (Al ２ O ３ ) in the range of 2 to 6 mol %. That is, in a preferred example of a silicate phase, silicon oxide (SiO ２ ) content is in the range of 69 to 78 mol %, and lithium oxide (Li ２ O) content is in the range of 14 to 25 mol %, and aluminum oxide (Al ２ O ３ The content of the Li ions is in the range of 2 to 6 mol %. ２ O and SiO ２ In the content range, Al ２ O ３If the content is less than 2 mol % or more than 6 mol %, the reduction of voids by sintering becomes difficult to proceed. As a result, the ELNES peak shift decreases (sufficient Si-O-Si bonds are not formed between the silicate phase and the silicon particles), and the cycle characteristics also deteriorate. Note that in the silicate composite particles, the silicate phase is composed of silicon oxide (SiO ２ In the silicate composite particles, the silicate phase contains lithium oxide (Li ２ O) is more preferably contained in an amount of 21 mol % or less, and further preferably contained in an amount of 20 mol % or less. ２ O ３ The silicon oxide, lithium oxide, and aluminum oxide are preferably contained in an amount of 3 mol % or more, and more preferably 4 mol % or more. Silicon oxide, lithium oxide, and aluminum oxide are combined as a silicate and do not necessarily exist alone.

[0024] The silicate phase is Li ２ SiO ３ , Li ２ Si ２ O ５ , and SiO ２ The composite particles may have at least one crystalline phase selected from the group consisting of: The presence or absence of crystalline phases contained in the composite particles and their content ratios can be evaluated by X-ray diffraction (XRD). For example, in a diffraction pattern obtained by X-ray diffraction (XRD) using Cu-Kα radiation, a peak (peak position) derived from the Si(111) plane of the silicon particles may appear near 2θ = 28° (in the range of 27° to 30°). In this specification, "near 2θ = X°" means a range in which 2θ is (X - 0.5)° to (X + 0.5)°, unless otherwise specified. When a range referred to as "near 2θ = X°" is described, that description is followed.

[0025] In the diffraction pattern by X-ray diffraction (XRD) using Cu-Kα radiation, lithium silicate Li ２ SiO ３In addition, peaks due to lithium silicate (Li) may appear in the range of 2θ=23° to 25° (specifically, around 2θ=23.7°, 24.5°, and / or 24.8°). ２ Si ２ O ５ That is, the silicate phase has a peak derived from lithium silicate in the vicinity of at least one angle selected from the group consisting of 2θ=19.0°, 2θ=23.7°, 2θ=24.5°, and 2θ=24.8° in a diffraction pattern obtained by X-ray diffraction using Cu—Kα radiation.

[0026] In addition, in the diffraction pattern by X-ray diffraction (XRD) using Cu-Kα radiation, silicon oxide (SiO ２ That is, the silicate phase has a peak derived from silicon oxide in the vicinity of at least one angle selected from the group consisting of 2θ=20.4°, 2θ=26.0°, and 2θ=49.2° in a diffraction pattern obtained by X-ray diffraction using Cu—Kα radiation.

[0027] Silicate composite particles are produced by pulverizing and compounding silicon and silicate. In the silicate production process, if the silicon raw material is in excess relative to the lithium raw material, silicon oxide may be produced. Silicon oxide may precipitate as fine crystals in the silicate phase through a heating process after pulverization. Crystalline silicon oxide is stable and does not react with lithium ions during charging, thereby not being essential for irreversible reactions. Because it is so fine, it is unlikely to interfere with the expansion and contraction of silicon particles. The silicon oxide fine crystals may be dispersed within the silicate phase within the silicate composite particles. That is, the silicon oxide fine crystals and silicon particles may be dispersed within the silicate phase.

[0028] In the silicate composite particles, the silicate phase may be, for example, a compound having the formula: Li ２ｚ SiO ２＋ｚIt contains an oxide phase represented by (0<z<1). From the viewpoints of stability, ease of preparation, lithium ion conductivity, etc., z=1 / 2 is preferred.

[0029] The silicate phase may contain, in addition to Li, Si, and O (oxygen), another element M. When the silicate phase contains the element M, the chemical stability and lithium ion conductivity of the silicate phase are improved, or side reactions due to contact between the silicate phase and a non-aqueous electrolyte are suppressed. The silicate phase may contain, for example, at least one element M selected from the group consisting of Na, K, Ca, Mg, Zr, Fe, B, Al, P, and La.

[0030] By including an alkali metal element other than Li in the silicate phase, crystallization becomes difficult, the viscosity in the softened state is low, and the fluidity is high. Therefore, in the heat treatment step, it is easy to fill the gaps between silicon particles, and it is easy to produce dense composite particles. As the alkali metal element, Na and / or K are preferable because they are inexpensive.

[0031] The silicate phase may contain a Group II element such as Ca or Mg. Generally, the silicate phase exhibits alkalinity, and 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., acid treatment) to neutralize the alkaline component of the silicate composite particles is reduced. Among these, Ca is preferred because it can improve the Vickers hardness of the silicate phase and further improve the cycle characteristics.

[0032] As elements M other than alkali metal elements and Group II elements, for example, B has a low melting point and is advantageous for improving fluidity during sintering. Al, Zr, and La can improve hardness while maintaining ionic conductivity. Furthermore, rare earth elements such as La can improve charge / discharge efficiency at the initial stage of charge / discharge cycles. Furthermore, Zr, Ti, P, Al, and B have the effect of increasing resistance to non-aqueous electrolytes and structural stability of the silicate phase.

[0033] The silicate phase may further contain trace amounts of elements such as Cr, Ni, Mn, Cu, and Mo.

[0034] The element M may form a compound. The compound may be, for example, a silicate of the element M or an oxide of the element M depending on the type of the element M.

[0035] The contents of Li, Si, and element M in the silicate phase can be measured, for example, by analyzing a cross section of the negative electrode mixture layer.

[0036] First, a fully discharged battery is disassembled, the negative electrode is removed, the negative electrode is washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove the non-aqueous electrolyte component, and the negative electrode is dried. Then, 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).

[0037] The content of each element can be determined by any of the following methods: The composition of the silicate phase is calculated from the content of each element.

[0038] <EDX> From the cross-sectional image of the backscattered electron image of the negative electrode mixture layer, ten silicate composite particles with a maximum particle diameter of 5 μm or more are randomly selected, and elemental mapping analysis is performed on each particle using energy dispersive X-ray (EDX). The area percentage of the target element is calculated using image analysis software. The observation magnification is preferably 2000 to 20,000 times. The measured values ​​of the area percentage of a specific element contained in the ten particles are averaged. The content of the target element is calculated from the obtained average value.

[0039] Desirable measurement conditions for cross-sectional SEM-EDX analysis are shown below. <SEM-EDX measurement conditions> Processing equipment: SM-09010 (Cross Section Polisher) manufactured by JEOL Processing conditions: Acceleration voltage 6 kV Current value: 140 μA Vacuum degree: 1×10 -3 ~2 x 10 -3Pa Measuring device: Electron microscope HITACHI SU-70 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

[0040] <AES> In the measurement, 10 silicate composite particles having a maximum particle 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 each is subjected to qualitative and quantitative elemental analysis using an Auger electron spectroscopy (AES) analyzer (e.g., JAMP-9510F manufactured by JEOL Ltd.). The measurement conditions may be, for example, an acceleration voltage of 10 kV, a beam current of 10 nA, and an analysis area of ​​20 μmφ. The content of a predetermined element contained in the 10 particles is calculated by averaging the content.

[0041] During the charge / discharge process, a coating is formed on the surface of the silicate composite particles due to the decomposition of the electrolyte, etc. Furthermore, as described below, the silicate composite particles may further comprise a conductive layer covering the surface of the composite particles. Therefore, mapping analysis using EDX or AES is performed within a range of 1 μm inside from the peripheral edge of the cross section of the silicate composite particle so that the thin coating or conductive layer is not included in the measurement range. Mapping analysis also makes it possible to confirm the distribution of the carbon material inside the composite particles. Since it becomes difficult to distinguish from electrolyte decomposition products at the end of the cycle, it is preferable to measure samples taken before or at the beginning of the cycle.

[0042] <ICP> A sample of silicate composite particles 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 emission spectroscopy (ICP) 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 Si-containing particles is calculated.

[0043] The contents of B, Na, K and Al contained in the silicate phase can be quantitatively analyzed in accordance with JIS (Japanese Industrial Standard) R3105 (1995) (method of analysis of borosilicate glass).

[0044] The silicate composite particles contain a silicate phase and silicon particles, and these can be distinguished and quantified using Si-NMR. The Si content obtained by the above method is the sum of the amount of Si constituting the silicon particles and the amount of Si in the silicate phase (and the amount of Si constituting the silicon oxide). The amount of Si element contained in the silicate composite particles is distributed among the silicon particles, silicon oxide phase, and silicate phase using the results of quantitative analysis by Si-NMR. The standard substance required for quantification can be a mixture containing silicon particles, a silicon oxide phase, and a silicate phase in predetermined proportions with a known Si content.

[0045] Desirable Si-NMR measurement conditions are shown below. <Si-NMR measurement conditions> Measurement device: Varian solid-state nuclear magnetic resonance spectrometer (INOVA-400) Probe: Varian 7 mm CPMAS-2 MAS: 4.2 kHz MAS speed: 4 kHz Pulse: DD (45° pulse + signal acquisition time 1H decoupled) Repetition time: 1200 sec to 3000 sec Observation width: 100 kHz Observation center: near -100 ppm Signal acquisition time: 0.05 sec Number of accumulations: 560 Sample amount: 207.6 mg

[0046] To increase capacity and improve 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 of the silicate phase decreases, making it easier to improve initial charge / discharge efficiency. By setting the content of silicon particles to 80% by mass or less, it is easier to reduce the degree of expansion and contraction of the silicate composite particles during charge / 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.

[0047] The silicon particles dispersed within the silicate phase have a particulate phase of simple silicon (Si) and are composed of single or multiple crystallites. The crystallite size of the silicon particles is preferably 50 nm or less. When the crystallite size of the silicon particles is 50 nm or less, the volume change due to the expansion and contraction of the silicon particles during charging and discharging can be reduced, further improving cycle characteristics. For example, when the silicon particles contract, voids are formed around the silicon particles, reducing the contact points with the surrounding area of ​​the particles, thereby suppressing particle isolation, and suppressing a decrease in charge and discharge efficiency due to particle isolation. The lower limit of the crystallite size of the silicon particles is not particularly limited, but is, for example, 2 nm.

[0048] The crystallite size of the silicon particles is more preferably 5 nm or more and 30 nm or less, and even more preferably 5 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 microcracks in the particles due to the expansion and contraction of the silicon particles during charging and discharging can be reduced, thereby improving cycle characteristics. The crystallite size of the silicon particles is calculated using the Scherrer equation 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.

[0049] At least a portion of the surface of the silicate composite particles may be coated with a conductive material. Because the silicate phase has poor electronic conductivity, the conductivity of the silicate composite particles 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 silicate composite particles can be dramatically increased. A carbon material is preferred as the conductive material. The carbon material preferably contains at least one selected from the group consisting of carbon compounds and carbonaceous materials.

[0050] The thickness of the conductive layer is preferably thin enough not to substantially affect the average particle size of the silicate composite particles. Considering the need to ensure conductivity and the diffusibility of lithium ions, the thickness of the conductive layer is preferably 1 to 200 nm, more preferably 5 to 100 nm. The thickness of the conductive layer can be measured by observing the cross section of the silicate composite particles using a SEM or a TEM (transmission electron microscope).

[0051] Examples of carbon compounds include compounds containing carbon and hydrogen, and compounds containing carbon, hydrogen, and oxygen. Examples of carbonaceous materials that can be used 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 of its low hardness and its strong buffering effect against silicon particles, which 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.

[0052] The silicate composite particles can be removed from the battery by the following method. First, the battery is disassembled to remove the negative electrode, which is then 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 gently boiling 6 M 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. Then, the sample is heated to 900 °C in an oxygen atmosphere to remove the carbon component, allowing the silicate composite particles to be isolated.

[0053] Next, an example of a method for producing silicate composite particles will be described in detail. Step (i) (Step of Obtaining Lithium Silicate) A raw material mixture containing a Si-containing raw material and a Li raw material in a predetermined ratio is used as the raw material for lithium silicate. The above-mentioned element M may be contained in the raw material mixture. As the raw material containing element M, it is preferable to include an Al raw material (e.g., the aluminum compound described above) in the raw material compound. A mixture of predetermined amounts of the above raw materials is melted, and the melt is passed through a metal roll to form flakes, thereby producing 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. Note that the flaked silicate can also be used without crystallization. Alternatively, a predetermined amount of the mixture can be fired without melting, for example, at a temperature above 850°C and below the melting point, to produce silicate by a solid-phase reaction.

[0054] 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. Examples of raw materials for element M include oxides, hydroxides, carbonate compounds, hydrides, nitrates, sulfates, etc. of each element. 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 fine crystals of silicon oxide.

[0055] Step (ii) (Step of Obtaining Silicate Composite Particles) Next, lithium silicate is compounded with raw silicon to form composite particles. For example, the composite particles are produced through the following steps (a) to (c).

[0056] Step (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 powder may be coarse silicon particles having an average particle size of several μm to several tens of μm.

[0057] Step (b): Next, using a grinding device such as a ball mill, the mixture of raw silicon and lithium silicate is ground and compounded while being finely divided. At this time, an organic solvent may be added to the mixture and wet-ground. A predetermined amount of organic solvent may be added to the grinding vessel all at once at the beginning of grinding, or a predetermined amount of organic solvent may be added to the grinding vessel intermittently in multiple batches during the grinding process. The organic solvent serves to prevent the material to be ground from adhering to the inner wall of the grinding vessel.

[0058] As the organic solvent, alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicate esters, metal alkoxides, etc. can be used.

[0059] The raw silicon may be coarse silicon particles having an average particle size of several μm to several tens of μm. The silicon particles finally obtained are preferably controlled so that the crystallite size calculated from the half-width of the diffraction peak attributable to the Si(111) plane in the X-ray diffraction pattern using the Scherrer equation is 5 nm or more and 50 nm or less.

[0060] The raw silicon and lithium silicate may be separately microparticulated and then mixed. Alternatively, silicon nanoparticles and amorphous lithium silicate nanoparticles may be prepared without using a pulverizer and then mixed. The nanoparticles may be prepared by known methods such as a gas phase method (e.g., a plasma method) or a liquid phase method (e.g., a liquid phase reduction method).

[0061] Step (c) Next, the pulverized material is fired under pressure using a hot press or the like to obtain a sintered body. The firing is carried out, for example, in an inert atmosphere (e.g., an argon, nitrogen, or other atmosphere). The firing temperature is preferably 450°C or higher and 1000°C or lower. Within this temperature range, fine silicon particles are easily dispersed within the silicate phase with low crystallinity. During sintering, the lithium 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 forming a sea portion and the silicon particles forming islands. The firing temperature is preferably 550°C or higher and 900°C or lower, more preferably 650°C or higher and 850°C or lower. The firing time is, for example, 1 hour or higher and 10 hours or lower.

[0062] The resulting 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.

[0063] Step (iii) (Step of forming a conductive layer on the surface of silicate composite particles) Next, at least a portion of the surface of the silicate composite particles may be coated with a conductive material to form a conductive layer. The conductive material is preferably electrochemically stable, and a conductive carbon material is preferred. Examples of conductive carbon materials that can be used include coal pitch or coal tar pitch, petroleum pitch, and phenolic resin. The raw material for the conductive material is mixed with the composite particles, and the mixture is fired to carbonize the raw material for the conductive material, thereby forming a conductive layer that covers at least a portion of the surface of the composite particles.

[0064] The mixture of the raw conductive material and the composite particles is fired, for example, in an inert atmosphere (e.g., an argon, nitrogen, or other atmosphere). The firing temperature is preferably 450°C or higher and 1000°C or lower. Within this temperature range, a highly conductive layer is likely to be formed in the silicate phase with low crystallinity. 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 higher and 10 hours or lower.

[0065] The conductive layer may be formed on the composite particles by other methods. For example, a gas phase method such as CVD may be used to form the conductive layer by reacting a hydrocarbon gas on the surface of the Si-containing particles. Examples of the hydrocarbon gas include acetylene and methane. Alternatively, the conductive layer may be formed by mixing carbon black with the composite particles to deposit a precursor of the conductive layer on the surface of the composite particles, and then firing the precursor together with the composite particles.

[0066] Step (iv) A step of washing the composite particles (including those having a conductive layer on the surface) with an acid may be performed. For example, washing the composite particles with an acidic aqueous solution can 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. The acidic aqueous solution can be 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.

[0067] 1 shows a schematic cross section of a silicate composite particle 20 as an example of a negative electrode material. A base particle 23 includes a lithium silicate phase 21 and silicon particles 22 dispersed within the silicate phase 21. The base particle 23 has a sea-island structure in which fine silicon particles are dispersed within a matrix of the lithium silicate phase 21. The surface of the base particle 23 is coated with a conductive layer 26, thereby forming a silicate composite particle 20.

[0068] The base particles 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.

[0069] The average particle size of the silicon particles 22 before the first charge is 500 nm or less, preferably 200 nm or less, and more preferably 50 nm or less. By appropriately miniaturizing the silicon particles 22 in this manner, volume change during charge and discharge is reduced, improving structural stability. Furthermore, uniform expansion and contraction of the silicon particles is achieved, suppressing particle cracking and 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 determined by averaging the maximum diameters of 100 randomly selected silicon particles 22.

[0070] A secondary battery according to an embodiment of the present invention 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 containing the negative electrode active material for a secondary battery. Examples of the negative electrode, positive electrode, electrolyte, and separator included in the secondary battery according to an embodiment of the present invention will be described below.

[0071] [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 dried coating may be rolled as necessary. The negative electrode mixture layer may be formed on one surface or both surfaces of the negative electrode current collector.

[0072] The negative electrode mixture contains, as a negative electrode active material, the above-described negative electrode active material for a secondary battery containing silicate composite particles as an essential component, and may contain, as optional components, a binder, a conductive agent, a thickener, etc. The silicon particles in the silicate composite particles can occlude many lithium ions, thereby contributing to increasing the capacity of the negative electrode.

[0073] The negative electrode active material may further contain another active material that electrochemically absorbs and releases lithium ions. A preferred example of such another active material is a carbon-based active material. Because silicate composite particles expand and contract in volume with charge and discharge, a high silicate composite particle content in the negative electrode active material can easily cause poor contact between the negative electrode active material and the negative electrode current collector during charge and discharge. On the other hand, the combined use of silicate composite particles and a carbon-based active material makes it possible to achieve excellent cycle characteristics while still providing the negative electrode with the high capacity of the silicon particles. The proportion of silicate composite particles in the total of the silicate composite particles and the carbon-based active material is preferably 0.5 to 15 mass%, more preferably 1 to 5 mass%. This facilitates achieving both high capacity and improved cycle characteristics.

[0074] Examples of carbon-based active materials include graphite, easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon). Among these, graphite is preferred because of its excellent charge / discharge stability and low irreversible capacity. Graphite refers to a material having a graphite-type crystal structure, and includes, for example, natural graphite, artificial graphite, and graphitized mesophase carbon particles. One type of carbon-based active material may be used alone, or two or more types may be used in combination.

[0075] As the negative electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet) is used. Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys. The thickness of the negative electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm, from the viewpoint of balancing the strength and weight of the negative electrode.

[0076] Examples of binders 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 conductive agents include carbon black, conductive fiber, carbon fluoride, and organic conductive materials. These may be used alone or in combination of two or more. Examples of thickeners include carboxymethyl cellulose (CMC) and polyvinyl alcohol. These may be used alone or in combination of two or more.

[0077] Examples of the dispersion medium include water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), and a mixed solvent thereof.

[0078] [Positive Electrode] The positive electrode comprises, 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 a positive electrode mixture is dispersed in a dispersion medium, to the surface of the positive electrode current collector and drying it. The dried coating may be rolled as necessary. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode current collector.

[0079] 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.

[0080] As the positive electrode active material, a lithium composite metal oxide can be used. Examples of the lithium composite metal oxide include Li a CoO2, Li a NiO2, Li a MnO2, Li a Co b Ni 1-b O2, Li a Co b M 1-b O c , Li a Ni 1-b M b O c , Li a Mn2O4, Li a Mn 2-b M b O 4、 LiMePO 4、 Examples include Li2MePO4F. 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 (e.g., 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 the value immediately after preparation of the active material and increases or decreases with charge and discharge.

[0081] 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.

[0082] 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.

[0083] [Electrolyte] 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.

[0084] The solvent used may be an aqueous solvent or a non-aqueous solvent. Examples of non-aqueous solvents that may be used include cyclic carbonates, chain carbonates, and cyclic carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0085] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 Lithium salts such as fluorine-containing acids (LiPF, LiBF, LiSbF, LiAsF, LiCF, SO, LiCF, CO, etc.), lithium salts of fluorine-containing acid imides (LiN(CF, SO), LiN(CF) SO (CF, SO), LiN(CF, SO) (CF, SO), LiN(CF, SO), etc.), and lithium halides (LiCl, LiBr, LiI, etc.). One type of lithium salt may be used alone, or two or more types may be used in combination.

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

[0087] An example of the structure of a 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 outer casing. Alternatively, instead of a wound electrode group, other types of electrode groups may be used, such as a stacked electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween. The secondary battery may be in any shape, such as a cylindrical shape, a prismatic shape, a coin shape, a button shape, or a laminate shape.

[0088] FIG. 2 is a schematic perspective view of a partially cutaway prismatic secondary battery according to one embodiment of the present invention.

[0089] The battery includes a bottomed, rectangular battery case 4, an electrode group 1 and an electrolyte (not shown) 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, plate-shaped winding core, and the electrode group 1 is formed by removing the winding core. The sealing plate 5 includes a liquid inlet closed with a sealing plug 8 and a negative electrode terminal 6 insulated from the sealing plate 5 by a gasket 7.

[0090] 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 top of the electrode group 1, separating the electrode group 1 from the sealing plate 5 and separating the negative electrode lead 3 from the battery case 4.

[0091] The present invention will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0092] Examples 1 to 5 and Comparative Examples 1 to 3 [Preparation of Silicate Composite Particles] Lithium carbonate (Li ２ CO ３ ) and silicon dioxide (SiO ２) and a compound of element M were mixed in the molar ratios shown in Table 1, and the mixture was fired at 800°C for 10 hours in an inert gas atmosphere to obtain lithium silicate. The obtained lithium silicate was pulverized to an average particle size of 10 µm. In Table 1, the molar ratios of lithium and element M each represent the molar ratio when converted to oxide (excluding Fe).

[0093] Lithium silicate having an average particle size of 10 μm and raw silicon (3N, average particle size of 10 μm) were mixed in a mass ratio of 40:60.

[0094] The mixture was charged into a pot (SUS, volume: 500 mL) of a planetary ball mill (Fritsch, P-5), 24 SUS balls (diameter 20 mm) were placed inside, the lid was closed, and the mixture was milled at 200 rpm for 25 hours in an inert atmosphere.

[0095] Next, the powder mixture was taken out in an inert atmosphere and fired at 600°C for 4 hours under pressure applied by a hot press in an inert atmosphere to obtain a sintered body of the mixture. The obtained sintered body was pulverized and passed through a 40 µm mesh to obtain silicate composite particles.

[0096] 100 parts by mass of silicate composite particles and 5 parts by mass of coal tar pitch were mixed and then fired at 800°C in an argon atmosphere to form a conductive layer covering at least a portion of the surface of the composite particles. The firing converted the coal tar pitch to amorphous carbon. The thickness of the conductive layer was 10 nm. The mass ratio of the conductive layer to the total mass of the Si-containing particles and the conductive layer was 3 mass%. Then, silicate composite particles with an average particle size of 5 μm and a conductive layer were obtained using a sieve. XRD analysis of the silicate composite particles was performed to determine the Si, SiO ２ , Li ２ Si ２ O ５ , Li ２ SiO ３ The peaks due to

[0097] [Preparation of Negative Electrode] Silicate composite particles and graphite were mixed in a mass ratio of 5:95 and used as the negative electrode active material. Water was added to a negative electrode mixture containing the negative electrode active material, carboxymethyl cellulose sodium (CMC-Na), and styrene-butadiene rubber (SBR) in a mass ratio of 97.5:1:1.5, and the mixture was stirred to prepare a negative electrode slurry. Next, a 1 m thick layer was applied to the surface of a copper foil. ２ The negative electrode slurry was applied to both sides of the copper foil so that the mass of the negative electrode mixture per sheet was 190 g, and the coating was dried and then rolled to form a sheet with a density of 1.5 g / cm 3 A negative electrode having the negative electrode mixture layer formed thereon was fabricated.

[0098] [Preparation of positive electrode] LiNi 0.88 Co 0.09 Al 0.03 A positive electrode mixture containing O2, acetylene black, and polyvinylidene fluoride (PVDF) in a mass ratio of 95:2.5:2.5 was mixed with N-methyl-2-pyrrolidone (NMP) and stirred to prepare a positive electrode slurry. The positive electrode slurry was then applied to the surface of an aluminum foil, the coating was dried, and the foil was rolled to form a positive electrode slurry having a density of 3.6 g / cm3 on both sides of the aluminum foil. 3 A positive electrode having the positive electrode mixture layer formed thereon was fabricated.

[0099] [Preparation of Electrolyte Solution] A non-aqueous electrolyte solution was prepared by dissolving LiPF6 at a concentration of 1.0 mol / L in a mixed solvent containing ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7.

[0100] [Fabrication 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 positioned at the outermost periphery to fabricate an electrode assembly. The electrode assembly was inserted into an exterior case made of aluminum laminate film, and vacuum dried at 105°C for 2 hours. After that, a nonaqueous electrolyte solution was poured into the exterior case, and the opening of the exterior case was sealed to obtain a secondary battery.

[0101] In preparing silicate composite particles, when obtaining lithium silicate by calcination, the mixing ratio of the Li raw material, the Si raw material, and the oxide of element M was changed to obtain lithium silicates with different compositions. Using each lithium silicate, secondary batteries A1 to A5 and B1 to B3 were fabricated, each using silicate composite particles with different silicate phase compositions as the negative electrode active material. Table 1 shows the silicate phase compositions of secondary batteries A1 to A5 and B1 to B3 in terms of the mixing ratio (molar ratio) of the Li raw material, the Si raw material, and the compound of element M. In Table 1, A1 to A5 correspond to Examples 1 to 5, and B1 to B3 correspond to Comparative Examples 1 to 3.

[0102] The secondary batteries A1 to A5 and B1 to B3 were evaluated as follows.

[0103] (1) Measurement of Peak Shift in Oxygen K-Edge ELNES Measurement of the cross section of the negative electrode mixture layer was performed using the method described above. One position at the interface between the silicate phase and the silicon particles and one position within the silicate phase were selected in the cross section image, and the peak shift was calculated from the ELNES peak at the interface between the silicate phase and the silicon particles and the ELNES peak in the silicate phase.

[0104] (2) Measurement of Porosity of Silicate Composite Particles The porosity (%) of the silicate composite particles was determined by the method described above.

[0105] (3) Charge-Discharge Cycle Test Each battery was repeatedly charged and discharged under the following conditions: <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).

[0106] <Discharge> Constant current discharge was carried out at 25° C. with a current of 1 It (800 mA) until the voltage reached 2.75 V.

[0107] The rest period between charging and discharging was 10 minutes. １ The discharge capacity C at the 300th cycle was also determined for each battery. ３００ Calculate 100 x C ３００ / C１ The ratio expressed as a percentage was determined as the capacity retention rate R. The larger the R value, the better the charge-discharge cycle characteristics.

[0108]

[0109] The evaluation results for each of the secondary batteries A1 to A5 and B1 to B3 are shown in Table 1. Table 1 shows the evaluation results for the porosity, peak shift, and capacity retention rate R of the silicate composite particles used as the negative electrode active material of each battery. In Table 1, the capacity retention rate R is shown as a relative value, with the capacity retention rate of battery A1 set to 100. Table 1 also shows the composition of the silicate phase of the silicate composite particles used as the negative electrode active material of each battery in the form of a mixing ratio (molar ratio) of the Li raw material, Si raw material, and compound of element M used during synthesis.

[0110] The peak shift in Table 1 indicates how much the ELNES peak at the interface between the silicate phase and the silicon particles is shifted to the lower energy side relative to the ELNES peak in the silicate phase. Table 1 shows that secondary batteries A1 to A6, whose peak shift amounts were in the range of 2.0 eV to 3.0 eV, had higher capacity retention rates R than batteries B1 to B3.

[0111] The battery A1 of Example 1 was made of SiO ２ The amount of addition is the same as that of battery B2, but Al ２ O ３ By adding the above, the porosity was reduced and the capacity retention rate R was improved. In addition, the peak shift amount could be controlled within the range of 2.0 eV to 3.0 eV.

[0112] From the results of batteries A1 to A5, B1 and B2, SiO ２ As the amount of SiO increases, the capacity retention rate R tends to increase. ２ As the amount of SiO added increases, the amount of peak shift tends to increase. ２ In Battery B3, which contained a larger amount of SiO than Battery A6, the cycle characteristics were significantly reduced, and it was not possible to repeat charge / discharge cycles up to 300 cycles. The peak shift also decreased to less than 2.0 eV. This is because SiO ２When the amount of addition is excessive, the porosity is large and Li ２ SiO ３ and Li ２ Si ２ O ５ This is thought to be because a different lithium silicate was formed. ２ The amount added is in the range of 69 mol% to 78 mol%, and Li ２ In the batteries A1 to A5 in which the O addition amount was in the range of 14 mol % to 25 mol %, the peak shift amount was in the range of 2.0 eV to 3.0 eV, and in this case, the porosity was significantly reduced and the capacity retention rate was significantly improved.

[0113] The present invention can provide a nonaqueous electrolyte secondary battery having high capacity and good charge / discharge cycle characteristics. The nonaqueous electrolyte secondary battery of the present invention is useful as a main power source for mobile communication devices, portable electronic devices, and the like. Although the present invention has been described with reference to presently preferred embodiments, such disclosure should not be construed as limiting. Various modifications and alterations will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. Accordingly, the appended claims should be construed to include all modifications and alterations without departing from the true spirit and scope of the present invention.

[0114] 1: electrode group, 2: positive electrode lead, 3: negative electrode lead, 4: battery case, 5: sealing plate, 6: negative electrode terminal, 7: gasket, 8: sealing plug, 20: silicate composite particles, 21: lithium silicate phase, 22: silicon particles, 23: mother particles, 26: conductive layer

Claims

1. silicate composite particles including a silicate phase and silicon particles dispersed in the silicate phase; a peak of an oxygen K-edge energy-loss absorption fine structure at an interface between the silicate phase and the silicon particles is shifted to a lower energy side by 2.0 eV to 3.0 eV relative to a peak of the oxygen K-edge energy-loss absorption fine structure in the silicate phase.

2. The negative electrode active material for a secondary battery according to claim 1 , wherein the silicate composite particles have a porosity of less than 6%.

3. In the silicate composite particles, the silicate phase is silicon oxide (SiO ２ ) in the range of 69 to 78 mol %, and lithium oxide (Li ２ O) in the range of 14 to 25 mol %, and aluminum oxide (Al ２ O ３ 2. The negative electrode active material for a secondary battery according to claim 1, wherein the negative electrode active material for a secondary battery contains 2 to 6 mol % of the compound (II).

4. The silicate phase is Li ２ SiO ３ , Li ２ Si ２ O ５ , and SiO ２ 2. The negative electrode active material for a secondary battery according to claim 1, having at least one crystalline phase selected from the group consisting of:

5. The silicate phase has a diffraction pattern determined by an X-ray diffraction method using Cu-Kα radiation, having a peak derived from lithium silicate in the vicinity of at least one selected from the group consisting of 2θ = 19.0 °, 2θ = 23.7 °, 2θ = 24.5 °, and 2θ = 24.8 °; 5. The negative electrode active material for a secondary battery according to claim 4, having a peak derived from silicon oxide in the vicinity of at least one angle selected from the group consisting of 2θ = 20.4°, 2θ = 26.0°, and 2θ = 49.2°.

6. The silicate phase further comprises an element M, 2. The negative electrode active material for a secondary battery according to claim 1, wherein the element M is at least one selected from the group consisting of Na, K, Ca, Mg, Zr, Fe, B, Al, P, and La.

7. 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 6.