Negative electrode active material for secondary battery and secondary battery using the same

Abstract

This negative electrode active material for a secondary battery contains composite particles. The composite particles include a lithium silicate phase and silicon phases dispersed within the lithium silicate phase. The silicon phases have linear portions within the lithium silicate phase, and the relationship between the largest diameter D1 and the smallest diameter D2 of the linear portions is 3 ≤ D1 / D2. The composite particles contain metal Me dispersed within the lithium silicate phase. The metal Me is at least one selected from the group consisting of Fe, Pb, Zn, Sn, Cu, Ni, Cr, Zr and Ti.

Description

[Technical Field]

[0001] The present invention mainly relates to a negative electrode active material for a secondary battery. [Background technology]

[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, silicon-containing materials are expected to be used as negative electrode active materials with high theoretical capacity densities.

[0003] Patent Document 1 proposes a silicon-containing material as an active material for secondary batteries, which contains a continuous phase containing silicon having Si-Si bonds and having a three-dimensionally continuous foam skeleton, and a dispersed phase containing silicon having Si-O bonds and being encapsulated in regions partitioned by the continuous phase and in a dispersed state. Patent Document 2 proposes a negative electrode for non-aqueous electrolyte secondary batteries, in which a portion containing silicon at a high concentration in the negative electrode active material forms a three-dimensionally continuous silicon network. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-10890 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-191936 Summary of the Invention [Problem to be solved by the invention]

[0005] In secondary batteries that use a silicon-containing material as the negative electrode active material, improvements in initial charge / discharge efficiency and cycle characteristics are required. [Means for solving the problem]

[0006] In view of the above, one aspect of the present invention relates to a negative electrode active material for a secondary battery, comprising composite particles including a lithium silicate phase and a silicon phase dispersed in the lithium silicate phase, wherein the silicon phase has linear portions within the lithium silicate phase, and the linear portions have a maximum diameter D1 and a minimum diameter D2 that satisfy the relationship 3≦D1 / D2, and the composite particles contain metal Me dispersed in the lithium silicate phase, wherein the metal Me is at least one selected from the group consisting of Fe, Pb, Zn, Sn, Cu, Ni, Cr, Zr, and Ti.

[0007] Another aspect of the present invention relates to a secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode contains the above-described negative electrode active material for a secondary battery. [Effects of the Invention]

[0008] According to the present invention, the initial charge / discharge efficiency and cycle characteristics of a secondary battery can be improved. 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. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 2 is a diagram showing an example of the distribution state of a silicon phase obtained by STEM-EELS analysis of a negative electrode active material (composite particles) for a secondary battery according to one embodiment of the present invention. [Figure 2] FIG. 1 shows EELS spectra of a Si standard sample and a SiO 2 standard sample. [Figure 3] FIG. 2 is a cross-sectional view schematically showing a negative electrode active material (composite particles) included in a secondary battery before the first charge. [Figure 4] 1 is a schematic perspective view of a secondary battery according to an embodiment of the present invention, with a portion cut away; DETAILED DESCRIPTION OF THE INVENTION

[0010] [Negative electrode active material for secondary batteries] A negative electrode active material for a secondary battery according to an embodiment of the present invention includes composite particles comprising a lithium silicate phase and a silicon phase dispersed within the lithium silicate phase. The silicon phase has linear portions within the lithium silicate phase, and the maximum diameter D1 (nm) and minimum diameter D2 (nm) of the linear portions satisfy the relationship 3≦D1 / D2. The composite particles include a metal Me dispersed within the lithium silicate phase, where the metal Me is at least one selected from the group consisting of Fe, Pb, Zn, Sn, Cu, Ni, Cr, Zr, and Ti. The silicon phase may form a network structure within the lithium silicate phase, and the linear portions may be linear portions of the network structure. The following description will be given using an example in which the silicon phase forms a network structure, but the shape of the silicon phase is not particularly limited. Furthermore, the term "linear" does not necessarily mean strictly linear, but can also include branched, rod-shaped, and constricted shapes.

[0011] The above D1 / D2 refers to the ratio D1 / D2 of the maximum diameter D1 of the linear portion to the minimum diameter D2 of the linear portion when the secondary battery is in a fully discharged state (a state in which most of the lithium ions have been released from the silicon phase). The fully discharged state refers to a state in which the secondary battery is discharged to a depth of discharge (DOD) of 90% or more, for example, a state in which the secondary battery is discharged to the lowest voltage within a predetermined voltage range in the field of equipment in which the secondary battery is used.

[0012] The silicon phase dispersed within the silicate phase can form a network structure as it absorbs lithium ions during charging, thereby forming a conductive network of silicon phases inside the composite particles.

[0013] Conventionally, the network structure is composed of thin linear segments L2. While the thin linear segments L2 are advantageous in terms of alleviating stress caused by the expansion and contraction of the silicon phase during charge and discharge, the deterioration of the silicon phase during charge and discharge can block the electron conduction path provided by the linear segments L2, which can reduce the conductivity inside the composite particle.

[0014] In contrast, in the present invention, the ratio D1 / D2 of the maximum diameter D1 to the minimum diameter D2 of the linear portions constituting the network structure is set to 3 or more, and the thickness of the linear portions constituting the network structure is varied, allowing thick linear portions L1 and thin linear portions L2 to coexist. The thick linear portions L1 form thick electron conduction paths, maintaining a good conductive network during charge and discharge. The thick linear portions L1 ensure electron conduction paths between the thin linear portions L2. Therefore, a decrease in conductivity inside the composite particle caused by interruption of the electron conduction paths of the linear portions L2 due to deterioration of the silicon phase during charge and discharge is suppressed. D1 / D2 may be 4 or more, or 5 or more.

[0015] By dispersing the malleable metal Me within the silicate phase, the local stress concentration caused by the expansion and contraction of the thick linear portion L1 during charge and discharge is alleviated, particle cracking caused by the stress concentration is suppressed, and the deterioration of cycle characteristics caused by particle cracking is suppressed.

[0016] As described above, the decrease in electrical conductivity inside the composite particles during charge and discharge and the suppression of particle cracking can be suppressed, resulting in significant improvements in the initial charge and discharge efficiency and cycle characteristics. This effect is particularly achieved when the silicon phase with a D1 / D2 ratio of 3 or greater, in which the thickness of the linear portions constituting the network structure varies, and the metal Me are dispersed within the lithium silicate phase.

[0017] From the viewpoint of alleviating local stress concentration due to expansion and contraction of the thick linear portion L1 during charge and discharge, the maximum diameter D1 may be 60 nm or less or 40 nm or less, and from the viewpoint of maintaining a good conductive network, the maximum diameter D1 may be more than 20 nm or 21 nm or more.

[0018] From the viewpoint of facilitating the formation of a good conductive network, the minimum diameter D2 may be 1 nm or more, or may be 3 nm or more. From the viewpoint of alleviating stress caused by expansion and contraction of the silicon phase during charge and discharge, the minimum diameter D2 may be 15 nm or less, or may be 10 nm or less.

[0019] When the difference between D1 and D2 is more than 10 nm, the diameter of the thick linear portion L1 is, for example, in the range of (D2+10) nm or more and D1 nm or less. The diameter of the thin linear portion L2 is, for example, in the range of D2 nm or more and less than (D2+10) nm. Note that the diameter of the linear portion is determined by the coefficient b Si D1 refers to the minimum value of the diameter passing through the point where D is maximum. From the viewpoint of maintaining a good conductive network during charge and discharge and easily suppressing particle cracking, in the above case, in the linear portions constituting the network structure, the ratio of the number NL2 of thin linear portions L2 to the number NL1 of thick linear portions L1: NL2 / NL1 may be, for example, 5 or more and 20 or less. In the above case, the difference between D1 and D2 may be, for example, 15 nm or more, or 15 nm or more and 65 nm or less. In the above case, D2 may be, for example, 2 nm or more and 8 nm or less.

[0020] The fact that the silicon phase forms a network structure within the lithium silicate phase can be confirmed by elemental mapping using electron energy loss spectroscopy (EELS) using cross-sectional images of the composite particles taken with a scanning transmission electron microscope (STEM).

[0021] The above D1 / D2 can be determined by the following method. (1) Cross-sectional imaging of composite particles using a scanning transmission electron microscope (STEM) The battery (fully discharged state) is disassembled and the negative electrode is removed. The negative electrode (negative electrode composite layer) is processed into a thin film using a CP (cross-section polisher) method, FIB (focused ion beam) method, or the like to obtain a thin film sample (e.g., 30 nm thick) for STEM observation of the negative electrode cross section. The thin film sample is used to observe the cross section of the negative electrode (composite particle) using STEM. Observation using STEM is performed at high magnification (e.g., 20,000 to 1,000,000 times).

[0022] (2) Elemental analysis by electron energy loss spectroscopy (EELS) An elemental analysis by EELS is performed using a STEM image (dark-field image) of the cross section of the composite particle. The size (area) of the dark-field image is, for example, 0.02 μm2 ~2 μm 2 It is. Here, an example of silicon mapping of the composite particles by STEM-EELS after charge and discharge is shown in Fig. 1. In Fig. 1, the lighter the color, the higher the silicon concentration. The parts with a very high silicon concentration and a light color are distributed in a network pattern, corresponding almost to the distribution of the silicon phase.

[0023] The desirable STEM-EELS measurement conditions are shown below. <STEM-EELS Measurement Conditions> Measuring device: JEM-F200 (manufactured by JEOL Ltd.) EELS detector: Quantum ER (manufactured by Gatan) Acceleration voltage: 200 kV Vacuum degree: 1.0×10 -6 ~8.0×10 -5 Pa Dispersion: 0.050 eV / cH Spot size: 7 Camera length: 40 mm Pixel time: 0.1 second

[0024] (3) Creation of a map of the silicon phase A map of the silicon phase is obtained by separating the SiO2 component derived from the silicate phase and extracting the Si component derived from the silicon phase from the EELS analysis data of the cross-section of the composite particles. The silicon phase is almost a single Si phase, and the Si component can be regarded as a component derived from the silicon phase. The silicate phase is represented by Li2O·xSiO2, and the SiO2 component can be regarded as a component derived from the silicate phase.

[0025] Specifically, the measured value obtained by EELS analysis is regarded as the value obtained by adding the Si component and the SiO2 component, and fitting is performed based on the following formula to obtain the coefficient b Si and the coefficient b SiO2 to obtain. b Si +b SiO2 satisfies 1. Measured value = b Si ×S Si +b SiO2 ×SSiO2

[0026] In the formula, S Si and S SiO2 are the spectral values ​​of the Si and SiO2 standard samples, respectively. The spectra of the Si and SiO2 standard samples are based on the EELS spectral data (shown in Figure 2) of the Si and SiO2 standard samples stored in the database of the image analysis software (Gatan Microscopy Suite software, manufactured by Gatan).

[0027] Based on the above fitting, coefficient b Si The region where the value of σ is 0.14 or more is extracted as the silicon phase, and a map of the silicon phase is obtained. From the map of the silicon phase obtained above, it is confirmed that the silicon phase is distributed in a mesh-like pattern.

[0028] Coefficient b Si In regions where the value is large, the thickness of the silicon phase in the thin film sample is large, and the diameter of the linear portion of the network structure confirmed in the silicon phase map described below tends to be large.

[0029] (4) Measurement of the maximum diameter D1 of the linear parts that make up the network structure The coefficient b in the linear part of the network structure shown in the map of the silicon phase above Si The point P1 where is the largest (for example, 0.234) is found, and the smallest value of the diameter of the linear portion passing through point P1 is found and this is taken as the maximum diameter D1 (nm). If there are multiple points P1, the maximum diameter D1 is found for each point, and the largest value among them is selected.

[0030] (5) Measurement of the minimum diameter D2 of the linear part that constitutes the network structure The network structure shown in the above silicon phase map has a coefficient b Si Find the point P2 where the coefficient b is 0.150, find the minimum diameter of the linear portion passing through point P2, and set this as the minimum diameter D2 (nm). If there are multiple points P2, find the minimum diameter D2 for each point, and select the smallest value among them. SiIn the region where the ratio is less than 0.15, the silicon phase (portions other than the linear portions constituting the network structure) distributed in dots in the map may be included.

[0031] (6) In (1) above, any five points on the cross section of the composite particle are observed by STEM, D1 and D2 are determined for each point by the procedures in (2) to (5) above, and the average value of D1 / D2 is calculated.

[0032] From the viewpoint of alleviating stress caused by expansion and contraction of the silicon phase during charge and discharge, it is preferable that the majority of the linear portions constituting the network structure have a diameter D3 (nm) of 20 nm or less. In other words, the ratio N1 / N0 of the number of linear portions N1 having a diameter D3 of 20 nm or less to the total number N0 of linear portions constituting the network structure is preferably greater than 1 / 2. Note that the diameter D3 of the linear portions referred to here is the ratio of the coefficient b Si means the minimum value of the diameter passing through the point where is the maximum.

[0033] The above N1 / N0 can be calculated by the following method. 10 to 20 linear segments constituting the network structure shown in the map of the silicon phase obtained in (3) above are arbitrarily selected, and the coefficient b Si The point P3 where D3 is the largest is found, and the smallest diameter of the linear portion passing through point P3 is found, and this is taken as the diameter D3 (nm). The ratio of the number N1 of linear portions with a diameter D3 of 20 nm or less to the total number N0 of selected linear portions is found, i.e., N1 / N0. N1 / N0 is found for each of five arbitrary points on the cross section of the composite particle in the STEM image, and their average value is found.

[0034] When the difference between D1 and D2 exceeds 10 nm, the above NL2 / NL1 can be determined by the following method. 20 to 50 linear segments constituting the network structure shown in the map of the silicon phase obtained in (3) above are arbitrarily selected, and the coefficient b SiFind the point P3 where is largest, and find the smallest diameter of the linear portion that passes through point P3, and let this be the diameter D3 (nm). A linear portion where the diameter D3 is in the range of (D2+10) nm or more and D1 nm or less is called a linear portion L1. A linear portion where the diameter D3 is in the range of D2 nm or more and less than (D2+10) nm is called a linear portion L2. Find the number NL1 of linear portions L1 and the number NL2 of linear portions L2, and calculate NL2 / NL1.

[0035] Malleable metal Me is dispersed within the lithium silicate phase. This prevents cracking and collapse of the composite particles due to expansion and contraction of the composite particles during charge and discharge. The metal Me may be dispersed in the form of particles within the lithium silicate phase. The metal Me is at least one selected from the group consisting of Fe, Pb, Zn, Sn, Cu, Ni, Cr, Zr, and Ti. Among these, Fe, Pb, and Cu are preferred as the metal Me from the viewpoint of excellent malleability.

[0036] From the viewpoint of suppressing side reactions with the electrolyte, the content of metal Me contained in the composite particles may be 20% by mass or less, or 5% by mass or less, and from the viewpoint of suppressing particle cracking, the content of metal Me contained in the composite particles may be 0.1% by mass or more, or 0.5% by mass or more.

[0037] The content of metal Me (e.g., Fe) in the composite particles can be measured, for example, by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Specifically, a sample of the composite particles is completely dissolved in a heated acid solution (a mixed acid of hydrofluoric acid, nitric acid, and sulfuric acid), the carbon residue in the solution is filtered off, and the resulting filtrate is analyzed by ICP-AES to measure the spectral intensity of metal Me. Subsequently, a calibration curve is created using commercially available standard solutions of the elements, and the content of metal Me is calculated.

[0038] The composite particles include a lithium silicate phase, a silicon phase dispersed within the lithium silicate phase, and a metal Me dispersed within the lithium silicate phase. The lithium silicate phase results in composite particles with a small irreversible capacity and a high capacity. During charging, lithium ions are occluded in the silicon phase, and during discharging, lithium ions are released from the silicon phase. The silicon phase is dispersed within the matrix of the lithium silicate phase, thereby restricting the contact between the silicon phase and the electrolyte and suppressing side reactions. Also, the stress generated by the expansion and contraction of the silicon phase is alleviated by the matrix of the lithium silicate phase. Furthermore, the stress generated by the expansion and contraction of the silicon phase is also alleviated by the malleable metal Me, thereby reducing the stress applied to the lithium silicate phase.

[0039] In the composite particles, usually, a plurality of primary particles including a lithium silicate phase, a silicon phase, and a metal Me are bonded to form secondary particles. The average particle diameter (D50) of the composite particles (secondary particles) is, for example, 1 μm or more and 25 μm or less, and may also be 4 μm or more and 15 μm or less. In this specification, the average particle diameter (D50) means the particle diameter (volume average particle diameter) at which the volume integration value becomes 50% in the particle size distribution measured by the laser diffraction scattering method. For the measuring device, for example, "LA-750" manufactured by Horiba, Ltd. can be used.

[0040] The lithium silicate phase can have, for example, a composition represented by the formula: Li 2z SiO 2+z (0 < z < 2). The lithium silicate is lightweight and has excellent lithium ion conductivity. The lithium silicate phase may be an oxide phase containing Li, Si, and O, and may contain other elements. The atomic ratio of O to Si in the lithium silicate phase: O / Si is, for example, greater than 2 and less than 4. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase: Li / Si is, for example, greater than 0 and less than 4. It is preferable that z satisfies the relationship 0 < z < 1, and z = 1 / 2 is more preferable.

[0041] The lithium silicate phase may further contain another element M1 in addition to Li, Si, and O. The element M1 may be, for example, at least one element selected from the group consisting of K, Na, Mg, Ca, B, Al, Nb, Ta, La, Y, P, Bi, Sb, Co, Er, F, and W. Depending on the type of element M1, the ionic conductivity of the silicate phase may be improved. Furthermore, the resistance of the silicate phase to electrolytes may be improved. For example, from the viewpoint of improving the initial charge / discharge efficiency, the element M1 is preferably La.

[0042] The element M1 may form a compound. The compound may be, for example, an oxide of the element M1 or a silicate of the element M1 depending on the type of the element M1. In the lithium silicate phase, the content of the element M1 is, for example, 0.3 mol % or more and 3 mol % or less with respect to the total amount of elements other than oxygen.

[0043] The composition of the lithium silicate phase can be analyzed by the following method. The composition analysis is preferably performed using the composite particles or the negative electrode composite layer in a discharged state. Furthermore, to eliminate the influence of electrolyte decomposition products, it is preferable to analyze a sample of the composite particles in the battery before or at the beginning of a charge-discharge cycle.

[0044] The content of each element contained in the composite particles can be measured, for example, by ICP-AES. Specifically, a sample of the composite particles is completely dissolved in a heated acid solution, and the carbon remaining in the solution is filtered off. The resulting filtrate is then analyzed by ICP-AES to measure the spectral intensity of each element. Subsequently, a calibration curve is created using commercially available standard solutions of each element, and the content of each element is calculated.

[0045] When analyzing the composition of the lithium silicate phase, the composite particles may be taken out of the battery by, for example, the following method. Specifically, the battery (fully discharged state) is disassembled to take out the negative electrode, and the negative electrode is washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove the electrolyte. Next, the negative electrode composite layer is peeled off from the negative electrode current collector and pulverized in a mortar to obtain sample powder. Next, the sample powder is dried in a dry atmosphere for 1 hour and immersed in gently boiling 6M hydrochloric acid for 10 minutes to remove alkali metals such as Na and Li that may be contained in the binder and the like. Next, the sample powder is washed with ion-exchanged water, filtered, and dried at 200 ° C for 1 hour. Thereafter, only the composite particles can be isolated by heating in an oxygen atmosphere to remove the carbon component.

[0046] The lithium silicate phase and the silicon phase in the composite particles can be distinguished and quantified by using Si-NMR. The Si content obtained by ICP-AES as described above is the total of the amount of Si constituting the silicon phase and the amount of Si in the lithium silicate phase. On the other hand, the amount of Si constituting the silicon phase can be quantified separately by using Si-NMR. Therefore, the amount of Si in the lithium silicate phase can be quantified by subtracting the amount of Si constituting the silicon phase from the Si content obtained by ICP-AES. For the standard substances required for quantification, a mixture containing lithium silicate and silicon particles with a known Si content in a predetermined ratio may be used.

[0047] The following shows the desirable measurement conditions for Si-NMR. <Si-NMR measurement conditions> Measuring device: Solid nuclear magnetic resonance spectrometer (INOVA-400) manufactured by Varian Probe: Varian 7mm CPMAS-2 MAS: 4.2 kHz MAS speed: 4 kHz Pulse: DD (45° pulse + signal acquisition time 1H decoupling) Repetition time: 1200 sec to 3000 sec Observation width: 100 kHz Observation center: Around -100 ppm Signal acquisition time: 0.05 sec Number of integrations: 560 Sample amount: 207.6 mg

[0048] In addition, quantification of each element in the composite particles can also be performed by SEM-EDX analysis, Auger electron spectroscopy (AES), laser ablation ICP mass spectrometry (LA-ICP-MS), X-ray photoelectron spectroscopy (XPS), etc.

[0049] For example, quantification of each element in the composite particles by SEM-EDX analysis can also be performed by observing the cross-section of the composite particles in the cross-section of the negative electrode composite material layer. The cross-section observation can be performed by, for example, the following method. First, disassemble the battery, take out the negative electrode, and obtain the cross-section of the negative electrode composite material layer using a cross-section polisher (CP). Observe the cross-section of the negative electrode composite material layer using a scanning electron microscope (SEM). Randomly select 10 composite 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 composite material layer, and perform elemental mapping analysis by energy-dispersive X-ray (EDX) for each of them. Calculate the contained area of the target element using image analysis software. The observation magnification is preferably 2000 to 20000 times. Obtain the average of the measured values of the contained areas of a predetermined element for the 10 obtained particles.

[0050] Note that a film is formed on the surface of the composite particles due to decomposition of the electrolyte or the like during the charge and discharge process. In addition, the composite particles may have a conductive layer on their surface. Therefore, the mapping analysis by EDX is performed on a region 1 μm or more inside from the peripheral edge of the cross-section of the composite particles so that the film and the conductive layer are not included in the measurement range.

[0051] The following shows the measurement conditions for desirable cross-sectional SEM-EDX analysis. <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 equipment: HITACHI SU-70 electron microscope Accelerating 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

[0052] From the viewpoint of increasing capacity and improving cycle characteristics, the content of the silicon phase in the composite particles is desirably, for example, 30% by mass or more and 80% by mass or less. By making the content of the silicon phase 30% by mass or more, the proportion of the lithium silicate phase decreases, and the initial charge / discharge efficiency tends to improve. By making the content of the silicon phase 80% by mass or less, the degree of expansion and contraction of the composite particles during charge / discharge tends to be reduced. The content of the silicon phase in the composite particles is preferably 40% by mass or more, and more preferably 50% by mass or more.

[0053] In a secondary battery immediately after preparation of the composite particles or before the first charge, a silicon phase may be dispersed in the form of particles within the lithium silicate phase of the composite particles. From the viewpoint of forming a network structure with D1 / D2 of 3 or more, the particle size (maximum diameter) of the particulate silicon phase contained in the composite particles may be, for example, in the range of 20 nm or more and 200 nm or less. From the viewpoint of reducing stress caused by expansion and contraction of the silicon phase and suppressing particle cracking due to the stress, the average particle size of the particulate silicon phase contained in the composite particles may be 100 nm or less, 70 nm or less, or 50 nm or less. The average particle size of the silicon phase is determined by measuring the maximum diameter of any 100 silicon phases using SEM or TEM images of the cross section of the composite particle and averaging the measured diameters.

[0054] At least a portion of the surface of the composite particle may be coated with a conductive layer. By forming a conductive layer on the surface of the composite particle, the conductivity of the composite particle can be dramatically increased. The conductive material constituting the conductive layer is preferably a carbon material. The carbon material preferably contains at least one selected from the group consisting of carbon compounds and carbonaceous materials.

[0055] The thickness of the conductive layer is preferably thin enough not to substantially affect the average particle size of the composite particles. In consideration of ensuring conductivity and the diffusibility of lithium ions, the thickness of the conductive layer is preferably 1 to 200 nm, more preferably 5 to 100 nm. The thickness of the conductive layer can be measured by observing the cross section of the composite particles using a SEM or TEM.

[0056] 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 on 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.

[0057] Next, an example of a method for producing composite particles will be described in detail. Process (i) The raw material for lithium silicate is a raw material mixture containing a Si raw material and a Li raw material in a predetermined ratio. The raw material mixture may also contain the above-mentioned element M1. The raw material mixture is melted, and the molten liquid is passed through a metal roll to form flakes, 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. It is also possible to produce silicate by firing the raw material mixture at a temperature below the melting point without melting it, resulting in a solid-state reaction.

[0058] Silicon oxide can be used as the Si raw material. Li raw materials can be, for example, lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, etc. These can be used alone or in combination of two or more. Raw materials for element M1 include oxides, hydroxides, carbonates, hydrides, nitrates, sulfates, etc. of each element.

[0059] Process (ii) Next, raw material silicon is mixed with lithium silicate to form composite particles. For example, composite particles are produced by the following steps (a) to (c).

[0060] Process (a) A raw silicon powder and a lithium silicate powder are mixed in a mass ratio of, for example, 20:80 to 95:5 to obtain a mixed powder.

[0061] The raw silicon powder may be, for example, a silicon powder with a wide particle size range, or may be a silicon powder having two peaks in the volumetric particle size distribution. The difference between the particle size corresponding to one of the two peaks and the particle size corresponding to the other peak is, for example, 50 μm or more. The raw silicon powder may also be, for example, a mixture of coarse silicon powder (e.g., an average particle size (D50) of 80 μm or more and 300 μm or less) and fine silicon powder (e.g., an average particle size (D50) of 1 μm or more and 20 μm or less) in a predetermined mass ratio. The particle size distribution of the raw silicon powder can adjust D1 / D2 during formation of the network structure.

[0062] Furthermore, powder of metal Me (for example, Fe) is added to the mixed powder. The powder of metal Me may be added to the raw material mixture in step (i) instead of to the mixed powder in step (a).

[0063] Process (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 for wet grinding. The organic solvent serves to prevent the material to be ground from adhering to the inner wall of the grinding container. For example, in the case of a ball mill, D1 / D2 during the formation of the network structure can be adjusted by changing the grinding time, the rotation speed of the pot, the amount of balls filled, etc.

[0064] As the organic solvent, alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicate esters, metal alkoxides, etc. can be used.

[0065] The raw silicon and lithium silicate may be separately microparticulated and then mixed. Alternatively, silicon nanoparticles and amorphous lithium silicate nanoparticles may be prepared and mixed without using a pulverizer. 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).

[0066] Process (c) Next, the mixture is heated to 600-1000°C in an inert gas atmosphere (e.g., argon, nitrogen, etc.), pressurized, and sintered. A sintering device capable of applying pressure in an inert atmosphere, such as a hot press, can be used for sintering. 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 phase and metal Me as the island portion. The resulting sintered body can be crushed to obtain composite particles.

[0067] Process (iii) Subsequently, at least a portion of the surface of the composite particles may be coated with a conductive material to form a conductive layer. Examples of methods for coating the surfaces of composite particles with a conductive carbon material include a CVD method using hydrocarbon gases such as acetylene and methane as raw materials, and a method in which the composite particles are mixed with coal pitch, petroleum pitch, phenolic resin, or the like, and then heated at 700°C to 950°C in an inert atmosphere (e.g., an argon, nitrogen, or other atmosphere) to carbonize them. Alternatively, carbon black may be attached to the surfaces of the composite particles.

[0068] 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, washing the composite particles with an acidic aqueous solution can dissolve and remove trace amounts of alkaline components that may be generated when the raw silicon and lithium silicate are combined. The acidic aqueous solution may 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.

[0069] Here, FIG. 3 shows a schematic cross section of a composite particle 20 whose surface is coated with a conductive layer, as an example of the negative electrode active material included in a secondary battery before the initial charge.

[0070] The composite particle 20 includes a base particle 23 composed of a secondary particle formed by aggregating a plurality of primary particles 24. The base particle 23 (primary particle 24) includes a lithium silicate phase 21 and a silicon phase 22 dispersed within the lithium silicate phase 21. The base particle 23 has a structure in which the silicon phase 22 in the form of fine particles is dispersed in a matrix of the lithium silicate phase 21. Carbon (amorphous carbon) derived from the organic solvent used in step (b) may be present at the interface 25 of the primary particles 24.

[0071] Furthermore, fine metal Me particles 27 are dispersed within the lithium silicate phase 21. The average particle size of the metal Me particles 27 is, for example, 1 nm or more and 100 nm or less. The average particle size of the metal Me particles 27 is determined by measuring the maximum diameters of any 100 metal Me particles using an SEM image or TEM image of the cross section of the composite particle and averaging the measurements. The average particle size of the metal Me particles is, for example, 1 nm or more and 100 nm or less even after the first charge / discharge.

[0072] At least a portion of the surface of the base particle 23 can be coated with a conductive layer 26. With repeated charge and discharge, adjacent particulate silicon phases 22 can be connected to each other to form a silicon phase having a network structure. The lithium silicate phase 21 may further contain an element M1.

[0073] Next, a secondary battery according to an embodiment of the present invention includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode includes a negative electrode active material containing the composite particles. The negative electrode, positive electrode, and electrolyte included in the secondary battery according to an embodiment of the present invention will be described below.

[0074] [Negative electrode] The negative electrode includes, for example, a negative electrode current collector and a negative electrode composite layer formed on the surface of the negative electrode current collector and containing a negative electrode active material. The negative electrode composite layer can be formed by applying a negative electrode slurry, in which the negative electrode composite is dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the applied layer. The dried coating film may be rolled as necessary.

[0075] The negative electrode mixture contains, as an essential component, a negative electrode active material containing the above-described composite particles, and may contain, as optional components, a binder, a conductive agent, a thickener, etc. The silicon phase of the composite particles can occlude many lithium ions, thereby obtaining a high-capacity negative electrode.

[0076] 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. Since the composite particles expand and contract in volume with charge and discharge, a large proportion of the composite particles in the negative electrode active material can easily cause poor contact between the negative electrode active material and the negative electrode current collector with charge and discharge. On the other hand, the combined use of the composite particles and a carbon-based active material makes it possible to achieve excellent cycle characteristics while providing the negative electrode with a high capacity due to the silicon phase. The proportion of the composite particles in the total of the 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.

[0077] Examples of carbon-based active materials include graphite, easily graphitizable carbon (soft carbon), and non-graphitizable carbon (hard carbon). Among these, graphite is preferred because it has excellent charge / discharge stability and low irreversible capacity. Graphite refers to a material having a graphite-type crystal structure, and includes, for example, natural graphite, artificial graphite, 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.

[0078] The negative electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0079] Examples of binders include fluororesin, polyolefin resin, polyamide resin, polyimide resin, vinyl resin, styrene-butadiene copolymer rubber (SBR), polyacrylic acid and its derivatives, etc. These may be used alone or in combination of two or more.

[0080] Examples of conductive agents include carbon black, conductive fibers, carbon fluoride, organic conductive materials, etc. These may be used alone or in combination of two or more.

[0081] Examples of thickeners include carboxymethyl cellulose (CMC), polyvinyl alcohol, etc. These may be used alone or in combination of two or more.

[0082] Examples of the dispersion medium include water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), and a mixed solvent of these.

[0083] [Positive electrode] The positive electrode includes, for example, a positive electrode current collector and a positive electrode composite layer formed on the surface of the positive electrode current collector. The positive electrode composite layer can be formed by applying a positive electrode slurry, in which the positive electrode composite is dispersed in a dispersion medium, to the surface of the positive electrode current collector and drying the applied layer. The dried coating film may be rolled as necessary.

[0084] 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.

[0085] The positive electrode active material may be a lithium composite metal oxide. 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 bExamples of suitable lithium-ion batteries include Li2O4, LiMePO4, and Li2MePO4F. Here, M is at least one element selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. Me contains at least a transition element (for example, at least one element 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 varies with charge and discharge.

[0086] 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.

[0087] The positive electrode current collector may be a conductive substrate similar to that of the negative electrode current collector. Examples of materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloys, and titanium.

[0088] [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.

[0089] The solvent may be an aqueous solvent or a non-aqueous solvent. Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain 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). Examples of chain carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0090] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 etc.), lithium salts of fluorine-containing acids (LiPF6, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (LiCl, LiBr, LiI, etc.), etc. The lithium salts may be used alone or in combination of two or more.

[0091] [Separator] It is desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator may be made of a microporous thin film, a woven fabric, a nonwoven fabric, or the like. The separator may be made of a polyolefin such as polypropylene or polyethylene.

[0092] 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.

[0093] Hereinafter, the structure of a prismatic non-aqueous electrolyte secondary battery will be described as an example of the secondary battery according to the present invention with reference to FIG.

[0094] The battery includes a bottomed, rectangular battery case 4, and an electrode group 1 and a nonaqueous electrolyte (not shown) housed within the battery case 4. The electrode group 1 includes a long, strip-shaped negative electrode, a long, strip-shaped positive electrode, and a separator interposed therebetween. The negative electrode current collector is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. In other words, the positive electrode is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The periphery of the sealing plate 5 fits into the open edge of the battery case 4, and the fitting is laser-welded. The sealing plate 5 has an injection hole for the nonaqueous electrolyte, which is closed with a seal plug 8 after injection.

[0095] 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.

[0096] Example 1 [Synthesis of lithium silicate] Lithium carbonate and silicon dioxide were mixed in a molar ratio of Li2CO3:SiO2 = 34:66, and the mixture was fired in an air atmosphere at 750 °C for 5 hours to obtain lithium silicate (Li2Si2O5). Lithium silicate powder (average particle size (D50) 10 μm) was obtained by pulverization.

[0097] [Preparation of composite particles] In an inert atmosphere, Si powder and Li2SiO5 powder (average particle size (D50) 10 μm) were mixed in a mass ratio of 58:42, and then a predetermined amount of iron powder (average particle size (D50) 100 μm) was added. The mixture was then loaded into a pot (SUS, volume: 500 mL) of a planetary ball mill (Fritsch, P-5). The Si powder used was a mixture of coarse Si powder (average particle size (D50) 150 μm) and fine Si powder (average particle size (D50) 10 μm) in a mass ratio of 1:9.

[0098] Twenty-four SUS balls (20 mm diameter) were placed in a pot, the lid was closed, and the mixture was pulverized at 200 rpm for 50 hours. The powder was then removed from the pot and heat-treated at 800°C for 4 hours in an inert atmosphere to obtain composite particles A1 containing 1% by mass of iron. The iron content was measured by ICP-AES.

[0099] The composite particles A1 were pulverized and passed through a 40 μm mesh, then mixed with coal pitch (MCP250, manufactured by JFE Chemical Corporation) and heat-treated at 800°C in an inert atmosphere to coat the surfaces of the composite particles A1 with carbon and form a conductive layer. The amount of carbon coating was approximately 5% by mass based on the total of the composite particles A1 and the conductive layer. Then, composite particles A1 (average particle size (D50) 5 μm) with a conductive layer were obtained using a sieve.

[0100] [Preparation of negative electrode] A mixture containing composite particles A1 with a conductive layer and graphite in a mass ratio of 5:95 was mixed with carboxymethylcellulose sodium salt (CMC-Na) and styrene-butadiene rubber (SBR) in a mass ratio of 97.5:1.0:1.5, water was added, and the mixture was stirred using a mixer (TK Hibismix, manufactured by Primix) to prepare a negative electrode slurry. Next, the negative electrode slurry was applied to both sides of copper foil, the coating was dried, and then rolled to form a copper foil with a density of 1.6 g / cm. 3 A negative electrode A1 having the above negative electrode mixture layer formed thereon was obtained.

[0101] [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 Hibismix, manufactured by Primix Corporation) to prepare a positive electrode slurry. The positive electrode slurry was then applied to both sides 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 / cm. 3 A positive electrode having a positive electrode mixture layer of the above formula was obtained.

[0102] [Preparation of non-aqueous electrolyte] 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 ethyl methyl carbonate (EMC) in a volume ratio of 3:7.

[0103] [Secondary battery production] A tab was attached to each electrode, and the positive and negative electrodes A1 were spirally wound with the separator interposed therebetween so that the tabs were positioned at the outermost periphery to produce 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 A1.

[0104] <Example 2> In preparing the composite particles, a composite particle A2, a negative electrode A2, and a secondary battery A2 were obtained in the same manner as in Example 1, except that the Si powder was a mixture of coarse Si powder (average particle size (D50) 500 μm) and fine Si powder (average particle size (D50) 10 μm) in a mass ratio of 1:9.

[0105] Example 3 In preparing the composite particles, a composite particle A3, a negative electrode A3 and a secondary battery A3 were obtained in the same manner as in Example 1, except that the Si powder was a mixture of coarse Si powder (average particle size (D50) 150 μm) and fine Si powder (average particle size (D50) 10 μm) in a mass ratio of 8:2.

[0106] <Comparative Example 1> In preparing the composite particles, only fine Si powder (average particle size (D50) 10 μm) was used as the Si powder. Iron powder was not added to the mixture of Si powder and Li2Si2O5 powder. Other than the above, composite particles B1, negative electrode B1, and secondary battery B1 were obtained in the same manner as in Example 1.

[0107] <Comparative Example 2> In preparing the composite particles, only fine Si powder (average particle size (D50) 10 μm) was used as the Si powder. The time for the pulverization treatment using the ball mill was 40 hours. No iron powder was added to the mixture of Si powder and Li2Si2O5 powder. Other than the above, composite particles B2, anode B2, and secondary battery B2 were obtained in the same manner as in Example 1.

[0108] <Comparative Example 3> In preparing the composite particles, only fine Si powder (average particle size (D50) 10 μm) was used as the Si powder. The time for the pulverization treatment using the ball mill was 40 hours. Other than the above, composite particles B3, negative electrode B3, and secondary battery B3 were obtained in the same manner as in Example 1.

[0109] <Comparative Example 4> Composite particles B4, negative electrode B4, and secondary battery B4 were obtained in the same manner as in Example 1, except that in preparing the composite particles, iron powder was not added to the mixture of Si powder and Li2Si2O5 powder.

[0110] [Charge / discharge cycle test] A charge-discharge cycle test was carried out on each of the batteries of the Examples and Comparative Examples 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).

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

[0112] The rest period between charge and discharge was 10 minutes. The ratio (percentage) of the discharge capacity at the first cycle to the charge capacity at the first cycle was calculated as the initial charge / discharge efficiency (%). The ratio (percentage) of the discharge capacity at the 50th cycle to the discharge capacity at the first cycle was calculated as the capacity retention rate (%) at the 50th cycle.

[0113] The evaluation results are shown in Table 1. In Table 1, the initial charge-discharge efficiency and capacity retention rate are expressed as relative values, with the initial charge-discharge efficiency and capacity retention rate of Battery B1 of Comparative Example 1 set to 100.

[0114] [Analysis of composite particles immediately after their preparation] As a result of XRD analysis of each composite particle, peaks derived from Si and Li2Si2O5 were confirmed in the XRD pattern of each composite particle.

[0115] For each of the composite particles A1 to A3, the cross section of the particle was observed by SEM, and it was confirmed that the particulate silicon phase was dispersed uniformly within the lithium silicate phase, and that the Fe particles (metallic Me particles) were dispersed uniformly within the lithium silicate phase.

[0116] The average particle size of the silicon phase determined by the above-described method was 10 nm for Composite Particle A1, 15 nm for Composite Particle A2, and 30 nm for Composite Particle A3. The average particle size of the Fe particles determined by the above-described method was 15 nm for Composite Particle A1, 20 nm for Composite Particle A2, and 30 nm for Composite Particle A3.

[0117] The cross-sections of the composite particles B1 to B4 were observed by SEM, and it was confirmed that the particulate silicon phase was dispersed uniformly within the lithium silicate phase. In the composite particle B3, it was also confirmed that the Fe particles were dispersed uniformly within the lithium silicate phase.

[0118] [Analysis of composite particles after charge-discharge cycle test] After the charge-discharge cycle test, each battery (fully discharged state) of the examples and comparative examples was disassembled, the negative electrode was removed, and the silicon phase was confirmed to have formed a network structure by the method described above. Furthermore, the maximum diameter D1 of the linear portions constituting the network structure, the ratio of the maximum diameter D1 to the minimum diameter D2 of the linear portions constituting the network structure (D1 / D2), and the ratio N1 / N0 of the number of linear portions N1 with a diameter (the above-mentioned diameter D3) of 20 nm or less to the total number N0 of the linear portions constituting the network structure were determined. The analytical results are shown in Table 1.

[0119] [Table 1]

[0120] Batteries A1 to A3 exhibited higher initial charge-discharge efficiency and capacity retention than batteries B1 to B4. Among batteries A1 to A3, battery A1, which had D1 of 60 nm or less and N1 / N0 of more than 1 / 2, exhibited particularly high initial charge-discharge efficiency and capacity retention.

[0121] Battery B4 exhibited a higher initial charge-discharge efficiency than Battery B1, but its capacity retention rate was lower. In contrast, Battery A1 exhibited a higher initial charge-discharge efficiency and a higher capacity retention rate than Battery B1. In Battery A1, metal Me is dispersed within the lithium silicate phase, which is thought to have suppressed particle cracking during charge-discharge cycling in the composite particles containing the thick linear portions of the silicon phase network structure.

[0122] When D1 / D2 was 2.4, the improvements in the initial charge / discharge efficiency and capacity retention were 9 and 8, respectively, from Battery B2 to Battery B3. In contrast, when D1 / D2 was 5.2, the improvements in the initial charge / discharge efficiency and capacity retention were 18 and 33, respectively, from Battery B4 to Battery A1. Thus, the improvements in the initial charge / discharge efficiency and capacity retention were much greater from Battery B4 to Battery A1 than from Battery B2 to Battery B3. This shows that dispersing a silicon phase with a D1 / D2 ratio of 3 or more and metal Me within the silicate phase significantly improves the initial charge / discharge efficiency and capacity retention. [Industrial Applicability]

[0123] The negative electrode active material for secondary batteries according to the present invention is useful in secondary batteries that serve as the main power sources for mobile communication devices, portable electronic devices, and the like. While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention. [Explanation of symbols]

[0124] 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: composite particle, 21: lithium silicate phase, 22: silicon phase, 23: mother particle, 24: primary particle, 25: interface of primary particle, 26: conductive layer, 27: metal Me particle

Claims

1. Composite particles comprising a lithium silicate phase and a silicon phase dispersed within the lithium silicate phase, the silicon phase has a network structure in the lithium silicate phase, and the network structure has linear portions; The maximum diameter D of the linear portion １ and minimum diameter D ２ , 3≦D １ / D ２ and the maximum diameter D 1 satisfies the relationship 20 nm≦D 1 ≦60 nm; the minimum diameter D 2 satisfies the relationship 1 nm≦D 2 ≦15 nm, the composite particles comprise metal Me dispersed within the lithium silicate phase; The metal Me is at least one selected from the group consisting of Fe, Pb, Zn, Sn, Cu, Ni, Cr, Zr, and Ti.

2. The negative electrode active material for a secondary battery according to claim 1 , wherein a majority of the linear portions have a diameter of 20 nm or less.

3. The linear portion is a linear portion L １ and the linear portion L ２ Including, The linear portion L １ The diameter of (D ２ +10) nm or more, D １ nm or less, The linear portion L ２ The diameter of ２ nm or more, (D ２ +10) nm, The maximum diameter D １ and the minimum diameter D ２ The difference is more than 10 nm, The linear portion L １ Number of NL １ The linear portion L ２ Number of NL ２ Ratio: NL ２ / NL １ The negative electrode active material for a secondary battery according to claim 1 , wherein is 5 or more and 20 or less.

4. 4. The negative electrode active material for a secondary battery according to claim 1, wherein the metal Me particles have an average particle size of 1 nm or more and 100 nm or less.

5. A positive electrode, a negative electrode, and an electrolyte, A secondary battery, wherein the negative electrode comprises the negative electrode active material for secondary batteries according to any one of claims 1 to 4.

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