Negative electrode active material for secondary battery and secondary battery

Abstract

This negative-electrode active material for secondary batteries comprises a lithium silicate phase and a silicon phase dispersed in the lithium silicate phase, wherein the lithium silicate phase includes at least one element M selected from the group consisting of alkali metal elements (excluding lithium), group 2 elements, rare earth elements, zirconium (Zr), niobium (Nb), tantalum (Ta), vanadium (V), titanium (Ti), phosphorus (P), bismuth (Bi), zinc (Zn), tin (Sn), lead (Pb), antimony (Sb), cobalt (Co), fluorine (F), tungsten (W), aluminum (Al), and boron (B), and has a spot image in an electron beam diffraction image from a transmission electron microscope.

Description

Technical Field

[0001] The present disclosure relates to secondary batteries, and mainly relates to the improvement of the negative electrode of non-aqueous electrolyte secondary batteries.

Background Art

[0002] Non-aqueous electrolyte secondary batteries, particularly lithium-ion secondary batteries, are expected to be used as power sources for small consumer applications, power storage devices, and electric vehicles because they have high voltage and high energy density. As the energy density of batteries is required to be increased, the use of materials containing silicon (silicon) that alloy with lithium as a negative electrode active material with a high theoretical capacity density is expected.

[0003] In Patent Document 1, in a non-aqueous electrolyte secondary battery, it has been proposed to use a negative electrode active material including a lithium silicate phase represented by Li 2z SiO 2+z (0 < z < 2) and silicon particles dispersed in the lithium silicate phase.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

[0005] The negative electrode active material described in Patent Document 1 has a smaller irreversible capacity associated with charge and discharge compared to a composite (SiO x ) in which fine silicon is dispersed in the SiO2 phase, and is advantageous for improving the initial charge and discharge efficiency.

[0006] However, with the improvement of the performance of portable electronic devices and the like, an improvement in the cycle capacity retention rate is also required.

[0007] In view of the above, one aspect of the present disclosure relates to a negative electrode active material for a secondary battery, comprising a lithium silicate phase and a silicon phase dispersed within the lithium silicate phase, wherein the lithium silicate phase contains an alkali metal element (excluding lithium), a Group 2 element, a rare earth element, and at least one element M selected from the group consisting of zirconium (Zr), niobium (Nb), tantalum (Ta), vanadium (V), titanium (Ti), phosphorus (P), bismuth (Bi), zinc (Zn), tin (Sn), lead (Pb), antimony (Sb), cobalt (Co), fluorine (F), tungsten (W), aluminum (Al), and boron (B), and the negative electrode active material has a spot image in an electron beam diffraction image obtained by a transmission electron microscope.

[0008] Another aspect of the present disclosure relates to a secondary battery including a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode contains the above-described negative electrode active material for a secondary battery.

[0009] According to the present disclosure, the cycle capacity retention rate of a secondary battery can be increased. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram schematically illustrating a cross section of a negative electrode active material according to one embodiment of the present disclosure. [Figure 2] FIG. 2 is a diagram schematically showing a cross section of a negative electrode active material after undergoing several charge and discharge cycles. [Figure 3] FIG. 3 is a schematic perspective view of a secondary battery according to an embodiment of the present disclosure, with a portion cut away. [Figure 4A] FIG. 4A shows an example of an electron beam diffraction image of the negative electrode active material of this embodiment obtained using a transmission electron microscope. [Figure 4B] FIG. 4B shows an example of an electron beam diffraction image obtained with a transmission electron microscope for a conventional negative electrode active material. DETAILED DESCRIPTION OF THE INVENTION

[0011] [Negative electrode active material for secondary batteries] A negative electrode active material for a secondary battery according to an embodiment of the present disclosure (hereinafter also referred to simply as "negative electrode active material" or "composite particles") includes a lithium silicate phase and a silicon phase dispersed within the lithium silicate phase. The lithium silicate phase includes lithium (Li), silicon (Si), and oxygen (O). The lithium silicate phase further includes at least one element M selected from the group consisting of an alkali metal element (excluding lithium), a Group 2 element, a rare earth element, zirconium (Zr), niobium (Nb), tantalum (Ta), vanadium (V), titanium (Ti), phosphorus (P), bismuth (Bi), zinc (Zn), tin (Sn), lead (Pb), antimony (Sb), cobalt (Co), fluorine (F), tungsten (W), aluminum (Al), and boron (B). The negative electrode active material has a spot image in an electron beam diffraction image obtained by a transmission electron microscope.

[0012] It is known that negative electrode active materials containing a lithium silicate phase and silicon particles dispersed within the lithium silicate phase undergo significant expansion and contraction of the silicon particles due to the absorption and release of lithium during charging and discharging. As a result, the expansion and contraction of the silicon particles generates significant stress in the lithium silicate phase surrounding the silicon particles, causing cracks and fractures in the composite particles. This weakens the bonding strength between the composite particles and the surrounding binder, and the fractured composite particles may lose their conductive path to the surrounding particles and become isolated. Furthermore, side reactions between the electrolyte and the silicon particles are promoted. As a result, the cycle capacity retention rate may decrease.

[0013] However, according to the negative electrode active material of this embodiment, by incorporating element M into the lithium silicate phase, the precipitation of microcrystals in the low-crystalline or amorphous lithium silicate phase can be promoted during the manufacturing process, where heat is applied. This increases the strength of the lithium silicate phase and suppresses cracks and fractures associated with expansion and contraction during charge and discharge. Furthermore, the high crystallinity of the lithium silicate phase at the micro level reduces the number of sites within the lithium silicate phase that can react with lithium ions, thereby reducing irreversible capacity. Furthermore, the presence of microcrystals can suppress rapid expansion and contraction. In addition, by subjecting the composite particles to sufficient heat treatment, it is easy to reduce the porosity within the composite particles, and side reactions between the electrolyte and the silicon particles are also suppressed. These factors increase the initial charge and discharge efficiency and maintain high capacity even after multiple charge and discharge cycles.

[0014] When the temperature in the heating process is low, the coarsening of silicon crystals can be suppressed, resulting in a good cycle retention rate. However, the amorphous lithium silicate phase easily reacts with lithium due to the low crystalline precipitation of lithium silicate, resulting in a decrease in initial efficiency. On the other hand, when the heating temperature is high, the amount of crystalline lithium silicate increases, reducing reactivity with lithium and improving initial efficiency, but the silicon crystals coarsen, resulting in a decrease in cycle retention rate. By adding element M, it is possible to increase initial efficiency while suppressing coarsening of silicon crystals, making it possible to achieve both initial efficiency and cycle capacity retention rate.

[0015] In the negative electrode active material of this embodiment, the lithium silicate phase maintains a morphology in which fine crystals are dispersed within amorphous lithium silicate, forming a mixture of amorphous and crystalline phases. Therefore, when electron beam diffraction is performed on a single particle of the negative electrode active material immediately after production or on a negative electrode active material in a discharged state after several charge-discharge cycles in a battery state enclosed in an electrode plate using a transmission electron microscope (TEM), spots attributed to the lithium silicate phase and / or silicon phase appear in the electron beam diffraction image. The distance from the center of the spot corresponds to the interplanar spacing of the microcrystals of the lithium silicate phase and / or silicon phase. In contrast, when the crystallinity of the lithium silicate phase and / or silicon phase is low, a concentric pattern appears in the diffraction image, with the distance (radius) from the center corresponding to the interplanar spacing. Alternatively, if the lithium silicate phase is completely amorphous, no diffraction pattern attributed to the lithium silicate phase appears.

[0016] In electron diffraction images, whether the diffraction pattern is ring-shaped or spot-shaped depends on the relationship between the crystallite size of the sample and the beam width of the irradiated electron beam. When the crystallite size is equal to or larger than the beam width, the number of crystallites contributing to diffraction is limited, and large, discrete diffraction spots are observed. On the other hand, when the crystallite size is sufficiently small compared to the beam width, small diffraction spots from many crystallites overlap, resulting in a continuous, ring-shaped diffraction pattern.

[0017] 4A and 4B show examples of electron diffraction images of anode active materials obtained using a transmission electron microscope. FIG. 4A shows an electron diffraction image of the anode active material of this embodiment. FIG. 4B shows an electron diffraction image of a conventional anode active material. In FIG. 4A, spots attributed to the lithium silicate phase and the silicon phase are observed. In FIG. 4A, the spot at a position corresponding to a 2.78 Å interplanar spacing is believed to belong to the (130) or (200) plane of Li2SiO3. The spot near a position corresponding to a 2.50 Å interplanar spacing is believed to belong to the (002) plane of Li2SiO5. Furthermore, spots attributed to the (220) and (111) planes of Si are observed at positions corresponding to interplanar spacings of 1.93 Å and 3.14 Å, respectively. In FIG. 4B, a ring-shaped diffraction image attributed to the silicon phase is observed, but no diffraction image attributed to the lithium silicate phase is observed.

[0018] The microcrystals in the lithium silicate phase tend to change to amorphous due to physical and / or chemical effects during charge / discharge. Therefore, the spots change to ring shapes as the number of charge / discharge cycles increases. Therefore, the spots can be clearly identified by observing a negative electrode active material with a low number of charge / discharge cycles. It is sufficient to observe spot images in the diffraction image; even if spot images and ring-shaped diffraction images are mixed, there is no difference in battery characteristics compared to when spots alone are observed.

[0019] An example of desirable measurement conditions for electron diffraction using a transmission electron microscope is shown below.

[0020] Electron diffraction method: Selected area electron diffraction Selected area aperture: 200nmΦ Sample thickness: 60 to 80 nm Accelerating voltage: 200 kV The spot diameter of a spot image in an electron beam diffraction pattern is determined from the width (full width at half maximum) of region R, which has an intensity of 50% or more of the maximum intensity of the spot image. When the position of region R in the diffraction pattern is expressed in two-dimensional polar coordinates with the position where the non-diffracted light of the electron beam is incident as the origin, the width of region R in the deflection angle direction (circumferential direction) and the width of region R in the radial direction are determined. The difference ΔΦ in deflection angle Φ between both ends of region R in the deflection angle direction is determined. The difference Δr in the distance r between both ends of region R in the radial direction is determined, and Δr is converted to the difference Δ(2θ) in diffraction angle 2θ using the distance from the origin of the diffraction pattern to the sample. In the spot image, ΔΦ is, for example, 7° or less.

[0021] The average particle size of the negative electrode active material is preferably 8 μm or less, more preferably 6 μm or less, which tends to increase the crystallinity of the lithium silicate phase at the micro level, resulting in significant improvements in irreversible capacity and cycle characteristics.

[0022] The negative electrode active material may have an average particle size of 3 μm or more, which provides an appropriate surface area for the negative electrode active material and prevents a decrease in capacity due to a side reaction with the electrolyte.

[0023] The average particle size of the negative electrode active material refers to the particle size (volume average particle size) at which the volume cumulative value is 50% in the particle size distribution measured by a laser diffraction scattering method. For example, an "LA-750" manufactured by HORIBA Ltd. can be used as the measuring device.

[0024] The lithium silicate phase has the formula: Li a M b SiO xThe atomic ratio of Li element to Si contained in the lithium silicate phase: Li / Si is, for example, 0.3 or more and 2 or less (0.3≦a≦2). The atomic ratio of M element to Si contained in the lithium silicate phase: M / Si is, for example, 0.01 or more and 0.4 or less (0.01≦b≦0.4). The atomic ratio of oxygen to Si contained in the lithium silicate phase: O / Si is, for example, 1 or more and 3.5 or less (1≦x≦3.5). When the atomic ratios satisfy these conditions, microcrystals are easily precipitated in the lithium silicate phase, cracks and fractures caused by expansion and contraction during charge and discharge are suppressed, and the cycle capacity retention rate is improved.

[0025] The lithium silicate phase may contain at least one of an alkali metal element (excluding lithium) and an element of Group 2 of the long periodic table as the element M. The alkali metal element and / or the element of Group 2 may be used alone or in combination of two or more.

[0026] By including an alkali metal element other than Li in the lithium silicate phase, crystallization becomes difficult, the viscosity in the softened state is low, and fluidity is increased. Therefore, even in heat treatment at low temperatures, the gaps between silicon particles are easily filled, and dense composite particles are easily produced. The alkali metal element may be at least one of potassium (K) and sodium (Na), which are inexpensive. The atomic ratio of the alkali element X (e.g., K) other than Li contained in the lithium silicate phase to Li: X / Li may be, for example, 0.01 or more and 1 or less, or may be 0.01 or more and 0.8 or less, or may be 0.01 or more and 0.2 or less.

[0027] Furthermore, while silicate phases generally exhibit alkalinity, Group 2 elements have the effect of suppressing the elution of alkali metals from the silicate phase. This facilitates the stabilization of the slurry viscosity when preparing a slurry containing the negative electrode active material. This also reduces the need for treatment (e.g., acid treatment) to neutralize the alkaline components of the negative electrode active material particles.

[0028] The Group 2 element may be at least one selected from the group consisting of magnesium (Mg), calcium (Ca), and barium (Ba). Among these, Ca is preferred because it can improve the Vickers hardness of the lithium silicate phase and further improve cycle characteristics. The content of the Group 2 element is, for example, 20 mol% or less, or may be 15 mol% or less, or may be 10 mol% or less, based on the total amount of elements other than O (oxygen) contained in the lithium silicate phase.

[0029] The lithium silicate phase may contain a rare earth element RE as the element M. The rare earth element may be present in the form of a silicate of the rare earth element dispersed within the lithium silicate phase. La2Si2O7 is preferred as the silicate of the rare earth element RE. The silicate of the rare earth element RE may be contained in the negative electrode active material in a crystalline phase and may be present dispersed within the lithium silicate phase.

[0030] Since a crystalline phase containing a rare earth element RE has low reactivity with lithium ions, dispersing the crystalline phase in a lithium silicate phase matrix reduces the number of sites in the lithium silicate phase that can react with lithium ions, reduces the irreversible capacity, and improves the initial charge / discharge efficiency. To disperse the crystalline phase in the lithium silicate phase matrix, the crystalline phase may be generated in the silicate phase during the manufacturing process of the negative electrode active material. In this case, the number of sites that can react with lithium ions can be more efficiently reduced.

[0031] The rare earth element RE preferably includes at least one selected from the group consisting of lanthanum (La), cerium (Ce), praseodymium (Pr), and neodymium (Nd). From the viewpoint of improving lithium ion conductivity, it is more preferable that the rare earth element includes La. The proportion of La in the total rare earth elements is preferably 90 atomic % or more and 100 atomic % or less.

[0032] The higher the crystallinity of the rare earth element silicate, the more desirable it is. In this case, the reactivity of the crystalline phase with ions such as lithium ions is further reduced, and the lithium ion conductivity is improved, reducing resistance during discharge and improving initial charge / discharge efficiency. When the rare earth element silicate has high crystallinity, a diffraction peak is observed, for example, around a diffraction angle 2θ = 33° in the X-ray diffraction pattern of the composite material obtained by X-ray diffraction measurement using Cu-Kα radiation.

[0033] The crystalline phase RE may have a composition represented by the general formula (RE)2O3·ySiO2, for example. y is, for example, 1.0 to 2.0. The crystalline phase preferably contains a compound A represented by the general formula (RE)2Si2O7, because it has a high structural stability and is less likely to dissolve in the electrolyte. Among these, La2Si2O7 is more preferred, because it remains stable without structural change even during charge and discharge.

[0034] The content of the rare earth element in the negative electrode active material is preferably 0.2% by mass or more and 21% by mass or less, more preferably 2.4% by mass or more and 15% by mass or less, and even more preferably 5.5% by mass or more and 14% by mass or less, based on the total amount of elements other than oxygen. When the content of the rare earth element is 0.2% by mass or more based on the total amount of elements other than oxygen, reactivity with lithium ions decreases, making it easier to improve initial charge / discharge efficiency. When the content of the rare earth element is 21% by mass or less based on the total amount of elements other than oxygen, a certain amount of amorphous portion is secured within the lithium silicate phase, making it easier to improve lithium ion conductivity.

[0035] The lithium silicate phase may contain the following element E1 as the element M. Element E1 may include zirconium (Zr), niobium (Nb), tantalum (Ta), vanadium (V), titanium (Ti), phosphorus (P), bismuth (Bi), zinc (Zn), tin (Sn), lead (Pb), antimony (Sb), cobalt (Co), fluorine (F), tungsten (W), aluminum (Al), boron (B), etc. Element E1 may be used singly or in combination of two or more. When the lithium silicate phase contains element E1, the chemical stability and lithium ion conductivity of the composite particles are improved. Furthermore, side reactions due to contact between the silicate phase and the electrolyte are suppressed. From the viewpoints of electrolyte resistance and structural stability, element E1 is preferably at least one element selected from the group consisting of Zr, Ti, P, Al, and B. The element E1 may be present in the lithium silicate phase in the form of a compound of the element E1, which may be, for example, a silicate of the element E1 or an oxide of the element E1, depending on the type of the element E1.

[0036] The negative electrode active material may further contain trace amounts of other elements such as iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), and molybdenum (Mo). Unlike other elements, these elements may be contained in the negative electrode active material mainly in the form of metal fine particles rather than oxides. The surface layer of the metal may be in the form of an oxide.

[0037] When metal fine particles are dispersed in the negative electrode active material, the elastic metal reduces expansion and contraction, improving cycle performance. Examples of metal elements contained in the metal fine particles include aluminum (Al) in addition to the above elements. Among these, iron, chromium, nickel, and aluminum are preferred, with iron being the most preferred. The metal fine particles may also be alloys of these elements. The average particle diameter of the metal fine particles is preferably 2 nm to 100 nm. If the average particle diameter is less than 2 nm, they may easily form alloys with silicon, which may cause a decrease in capacity. On the other hand, if the average particle diameter exceeds 100 nm, they may easily become the starting point for cracks, which may result in a decrease in cycle retention. Furthermore, the number of metal fine particles dispersed in the negative electrode active material should be 10 to 1000 particles / μm.2 When the number of particles is 10 / μm, the cycle retention rate is likely to improve. 2 If the number is less than 1000 / μm, it is difficult to obtain the effect of mitigating expansion and contraction. 2 Above this value, silicon alloy formation becomes significant and causes a decrease in capacity.

[0038] The inclusion of element M increases the viscosity of lithium silicate at high temperatures during the sintering process (the process for obtaining a composite intermediate, described below). Therefore, it is necessary to improve sinterability by increasing the sintering temperature to reduce viscosity. As a result, dense composite particles can be formed, and the crystal growth of lithium silicate and silicon particles proceeds appropriately. On the other hand, without element M, increasing the heating temperature or prolonging the heating time significantly reduces the viscosity of lithium silicate, leading to significant contact between silicon particles and crystal growth, which tends to result in large silicon crystals. As a result, the expansion and contraction during charging and discharging may become delocalized, significantly reducing the cycle retention rate. However, when element M is included, even when the heating temperature is increased or the heating time is prolonged, the viscosity of the lithium silicate remains moderately softened, preventing the silicon crystals from coarsening, and achieving a good cycle retention rate.

[0039] The composition of the lithium silicate phase of the composite material can be analyzed, for example, by the following method.

[0040] The battery was disassembled, the negative electrode removed, washed with a non-aqueous solvent such as ethylene carbonate, and dried. The cross-section of the negative electrode mixture layer was then polished using a cross-section polisher (CP) to obtain a sample. A field-emission scanning electron microscope (FE-SEM) was used to obtain a backscattered electron image of the sample cross-section, and the cross-section of the composite particles was observed. An Auger electron spectroscopy (AES) analyzer (JEOL, JAMP-9510F) was used to qualitatively and quantitatively analyze the elements of the silicate phase of the observed composite particles (accelerating voltage 10 kV, beam current 10 nA, analysis area 20 μmφ). For example, the composition of the silicate phase was determined based on the obtained content of lithium (Li), silicon (Si), oxygen (O), and other elements. Quantification of each element in the composite particles in a discharged state can be performed using energy-dispersive X-ray spectroscopy (EDX), electron microanalyzer (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), X-ray photoelectron spectroscopy (XPS), and other techniques.

[0041] When observing or analyzing the cross-section of the above sample, a carbon sample stage can be used to fix the sample to prevent diffusion of Li. To prevent deterioration of the sample cross-section, a transfer vessel can be used to hold and transport the sample without exposing it to the atmosphere.

[0042] The content of the rare earth element in the negative electrode active material can be determined, for example, by the following method.

[0043] The battery was disassembled, the negative electrode removed, washed with a non-aqueous solvent such as ethylene carbonate, and dried. The negative electrode mixture layer was then cross-sectionally polished using a cross-section polisher (CP) to obtain a sample. A field-emission scanning electron microscope (FE-SEM) was used to obtain a backscattered electron image of the sample cross section, and the cross section of the composite particle was observed. An Auger electron spectroscopy (AES) analyzer (JEOL, JAMP-9510F) was used to perform qualitative and quantitative elemental analysis of a specific region in the center of the cross section of the observed composite particle (accelerating voltage 10 kV, beam current 10 nA, analysis area 20 μmφ). Based on the results of this analysis, the content of rare earth elements in the composite particle (the mass ratio of rare earth elements to the total mass of elements other than oxygen contained in the composite particle) was determined. Analysis was then performed on 10 observed composite particles, and the average rare earth element content was calculated.

[0044] The dispersion of a crystalline phase containing a rare earth element, silicon, and oxygen in the matrix of the lithium silicate phase can be confirmed by observing a cross-sectional image (backscattered electron image) of the composite material obtained using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). The equivalent circle diameter of the crystalline phase dispersed in the silicate phase is, for example, 10 nm or more and 1 μm or less. The equivalent circle diameter of the crystalline phase can be determined using a cross-sectional image (backscattered electron image) of the composite material obtained using SEM or TEM. Specifically, the area of ​​100 crystalline phases is converted to the diameter of an equivalent circle and then averaged.

[0045] The crystalline phase containing a rare earth element, silicon, and oxygen can be confirmed by X-ray diffraction measurement using Cu-Kα radiation.

[0046] The crystalline phase containing a rare earth element, silicon, and oxygen may also be confirmed by electron diffraction measurement using a field emission transmission electron microscope (JEOL Ltd., JEM2100F, accelerating voltage 200 kV, accelerating current 110 μA). The interplanar spacing and crystalline structure attributable to Compound A can be determined based on the diffraction point data (distance from the center point) obtained by electron diffraction measurement. The composition of the crystalline phase can be identified based on the interplanar spacing and crystalline structure obtained and the elements contained in the crystalline phase obtained by energy dispersive X-ray analysis (EDX).

[0047] Negative electrode active materials are produced by mixing lithium silicate particles and silicon particles and then compositing the mixture, for example, by grinding and sintering. Immediately after production, the negative electrode active material (composite particles) has a sea-island structure in which silicon particles (silicon phase) are dispersed within a lithium silicate phase (sea portion). However, after several charge-discharge cycles, adjacent silicon particles connect to each other through the silicon phase, forming a three-dimensional network. This is thought to be because, during charging, silicon particles located on the outermost surface of the negative electrode active material expand in volume due to the absorption of lithium ions, forming connections with adjacent silicon particles. Subsequently, even more adjacent silicon particles expand in volume due to the absorption of lithium ions, forming connections with other silicon particles, resulting in the entire silicon particles being alloyed into a network of lithium silicide. Subsequently, during discharge, the lithium ions are released, converting the regions between adjacent silicon particles into silicon phases. As a result, the silicon phases become connected to each other, improving initial efficiency and cycle retention.

[0048] The smaller the average particle size of the silicon particles dispersed in the lithium silicate phase, the greater the number of silicon particles per mass, and the shorter the distance between the silicon particles dispersed in the lithium silicate phase. Therefore, adjacent silicon particles are connected by charge and discharge, and the silicon phase is more likely to form a network. Because the distance between the silicon particles dispersed in the lithium silicate phase is sufficiently short and the silicon phase is more likely to form a network, the average particle size of the silicon particles is preferably 50 nm or less before the first charge.

[0049] Furthermore, because the silicon phase is dispersed within the silicate phase, expansion and contraction of the composite particles during charge and discharge are suppressed. From the viewpoint of suppressing cracks in the silicon phase, the average particle size of the silicon particles before the first charge is preferably 500 nm or less, more preferably 200 nm or less, and even more preferably 50 nm or less. After the first charge, the average particle size of the silicon particles is preferably 400 nm or less, more preferably 100 nm or less. By miniaturizing the silicon particles, volume change during charge and discharge is reduced, further improving the structural stability of the composite material.

[0050] The average particle size of silicon particles is measured using a cross-sectional image of the composite material obtained by SEM. Specifically, the average particle size of silicon particles is determined by averaging the maximum diameters of 100 randomly selected silicon particles and / or, in the case of a silicon phase formed in a network, the maximum diameters of the portions excluding the portions connecting adjacent silicon particles.

[0051] The content of the silicon phase in the negative electrode active material is preferably 50% by mass or more. This makes it easier for the silicon phase to form a network and for a high energy density to be obtained. Furthermore, the diffusibility of lithium ions is good, making it easier to obtain excellent load characteristics. On the other hand, from the viewpoint of improving cycle characteristics, the content of the silicon phase in the negative electrode active material is preferably 95% by mass or less, more preferably 75% by mass or less, and even more preferably 70% by mass or less. In this case, the surface of the silicon particles that is exposed and not covered with the lithium silicate phase is reduced, making it easier to suppress the reaction between the non-aqueous electrolyte and the silicon particles.

[0052] In addition, in a state in which the silicon phase is formed into a network shape by charge and discharge, the content of the silicon phase in the negative electrode active material may be slightly reduced from the content of silicon particles in the negative electrode active material before charge and discharge due to the influence of oxidation of the silicon particles, etc. The content of the silicon phase in the negative electrode active material may be, for example, 80% or more or 90% or more of the content of silicon particles in the negative electrode active material before charge and discharge.

[0053] The composite particles contain a lithium silicate phase and a silicon phase, and these can be distinguished and quantified by Si-NMR. The following are the preferred measurement conditions for Si-NMR.

[0054] Measurement equipment: Varian solid-state nuclear magnetic resonance spectrometer (INOVA-400) Probe: Varian 7mm CPMAS-2 MAS:4.2kHz MAS speed: 4kHz Pulse: DD (45° pulse + signal acquisition time 1H decoupled) Repeat time: 1200 seconds Observation width: 100kHz Observation center: Around -100 ppm Signal acquisition time: 0.05 sec Accumulation count: 560 Sample amount: 207.6 mg The standard substance required for quantification may be a mixture containing a silicate phase with a known Si content and silicon particles in a predetermined ratio.

[0055] At least a portion of the surface of the composite particle may be coated with a conductive layer. This enhances the conductivity of the composite material. The conductive layer is preferably thin enough that it does not substantially affect the average particle size of the composite particle. In consideration of ensuring conductivity and the diffusibility of lithium ions, the thickness of the conductive layer is preferably 1 to 200 nm, more preferably 5 to 100 nm. The thickness of the conductive layer can be measured by observing the cross section of the composite material using a SEM or TEM.

[0056] [Method for producing negative electrode active material] A method for producing a negative electrode active material includes, for example, a first step of obtaining lithium silicate, a second step of compounding the lithium silicate with raw silicon to obtain a composite intermediate in which silicon particles are dispersed within the lithium silicate phase, and a third step of heat-treating the composite intermediate to obtain composite particles in which the crystallinity of the lithium silicate phase and silicon particles is enhanced.

[0057] [1st step] The first step includes, for example, step 1a of mixing a raw material containing Si with a Li raw material to obtain a mixture, and step 1b of heating the mixture to obtain lithium silicate. Silicon oxide can be used as the Si raw material. Lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, etc. can be used as the Li raw material. These can be used alone or in combination of two or more. The raw material mixture can also contain a compound containing the above-mentioned element M. For example, the raw material mixture can also contain a compound of an alkali metal element other than lithium, a Group 2 element, and / or a rare earth element.

[0058] The firing in step 1b is carried out, for example, in an oxidizing atmosphere. The firing temperature in step 1b is preferably 400°C or higher and 1200°C or lower, and more preferably 800°C or higher and 1100°C or lower.

[0059] For example, a mixture of the above raw materials is melted, and the melt 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. The flaked silicate can also be used without crystallization. Alternatively, a predetermined amount of the mixture can be mixed and fired at a temperature below the melting point without melting it, to produce silicate through a solid-phase reaction.

[0060] When element M is an alkali metal element or a Group 2 element, examples of the compound containing element M include carbonates, oxides, hydroxides, and hydrides of element M. One compound containing element M may be used alone, or two or more compounds may be used in combination.

[0061] When the element M is a rare earth element, examples of compounds containing the element M include oxides, oxalates, nitrates, sulfates, halides, and carbonates of the rare earth element. For example, an example of a lanthanum compound is lanthanum oxide. The compounds containing rare earth elements may be used alone or in combination of two or more.

[0062] [Second process] In the second step, lithium silicate is mixed with raw silicon to form a composite. The raw silicon can be coarse silicon particles with an average particle size of several μm to several tens of μm. The silicon particles ultimately obtained are preferably controlled so that their crystallite size, calculated from the half-width of the diffraction peak attributed to the Si(111) plane in the X-ray diffraction pattern using the Scherrer equation, is 10 nm or more.

[0063] The second step includes, for example, step 2a of pulverizing the mixture of lithium silicate and raw silicon while applying shear force to the mixture to obtain a finely divided mixture, and step 2b of firing the finely divided mixture to obtain a composite intermediate.

[0064] In step 2a, for example, raw silicate and raw silicon are mixed in a predetermined mass ratio, and the mixture is stirred while being pulverized into fine particles using a grinding device such as a ball mill. 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 may be added intermittently in multiple batches during the grinding process. The organic solvent serves to prevent adhesion of the material to be ground to the inner wall of the grinding vessel. Examples of organic solvents that can be used include alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicate esters, and metal alkoxides.

[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 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).

[0066] In step 2b, the mixture may be fired while applying pressure using a hot press or the like to produce a sintered body (composite intermediate) of the mixture. The sintered body may then be pulverized into granules to form composite intermediate particles. By appropriately selecting the pulverization conditions, composite intermediate particles with an average particle size of 3 to 8 μm can be obtained.

[0067] The firing in step 2b is carried out, for example, in an inert atmosphere (e.g., an argon, nitrogen, or other atmosphere). The firing temperature in step 2b is preferably 450° C. or higher and 1000° C. or lower. Lithium silicate is stable within the above temperature range and hardly reacts with silicon, so any decrease in capacity is minor.

[0068] In the first or second step, a compound containing element E1 may be further added. Examples of compounds containing element E1 include oxides, oxalates, nitrates, sulfates, halides, and carbonates of element E1. Among these, oxides are preferred because they are stable and have good ionic conductivity. One compound containing element E1 may be used alone, or two or more compounds may be used in combination.

[0069] [3rd step] In the third step, the composite intermediate is subjected to a predetermined heat treatment. This improves the crystallinity of the lithium silicate phase and silicon particles. Furthermore, if the composite intermediate contains a rare earth element, the crystallinity of the silicate of the rare earth element dispersed within the silicate phase is also improved. Because the rare earth element forms ionic bonds by cutting the silicate framework, the heat treatment easily converts it into a stable, crystalline rare earth silicate.

[0070] The heat treatment temperature is preferably 550°C or higher and 900°C or lower, more preferably 650°C or higher and 850°C or lower. When the heat treatment temperature is 550°C or higher, a crystalline phase of silicate of rare earth elements is likely to be formed. When the heat treatment temperature is 900°C or lower, crystal precipitation in the lithium silicate phase is maintained in a state where battery performance is not significantly reduced. Furthermore, silicon particles dispersed in the silicate phase are likely to be maintained at a small size. The heat treatment time is, for example, 1 hour or higher and 10 hours or lower. The heat treatment is preferably carried out in an inert atmosphere.

[0071] [4th step] The method for producing a negative electrode active material may further include a fourth step of forming a conductive layer containing a conductive material on at least a portion of the surface of the composite particles. The conductive material is preferably electrochemically stable, and a carbon material is preferred. Examples of a method for forming a conductive layer on the surface of the composite material include mixing coal pitch, petroleum pitch, phenolic resin, or the like with the composite material particles and heating to carbonize them. The heating performed for the purpose of carbonization may also serve as the heat treatment in the third step. Alternatively, a conductive layer containing a carbon material may be formed on the surface of the composite particles by a CVD method using a hydrocarbon gas such as acetylene or methane as a raw material. Carbon black may also be attached to the surface of the composite particles.

[0072] [5th ​​step] The method for producing a negative electrode active material may further include a fifth step of washing the composite particles with an acid. For example, washing the lithium silicate-containing composite particles with an acidic aqueous solution can dissolve and remove trace amounts of components such as Li2SiO3 that may be generated when combining raw silicon and lithium silicate. 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.

[0073] An example of the negative electrode active material for a secondary battery according to an embodiment of the present disclosure will now be described with reference to Fig. 1. Fig. 1 is a diagram schematically illustrating a cross section of the negative electrode active material (composite particle 11).

[0074] The composite particle 11 is in the form of a particle and comprises a lithium silicate phase 12 and silicon (elemental Si) particles 13 dispersed within the lithium silicate phase 12. The composite particle 11 may also comprise a crystalline phase 14 of a silicate of a rare earth element dispersed within the lithium silicate phase 12. As shown in Fig. 1, at least a portion of the surface of the particulate composite material 11 may be coated with a conductive layer 15 containing a conductive material.

[0075] The composite particle 11 has, for example, an island-in-a-sea structure, and in any cross section, fine silicon particles 13 and crystalline phases 14 are scattered approximately uniformly in a matrix of a lithium silicate phase 12. Most of the crystalline phases 14 are larger in size than the silicon particles 13.

[0076] The silicate phase 12 may further contain element E2. The silicate phase 12 may also contain SiO2 at a level equivalent to a natural oxide film formed on the surface of silicon particles.

[0077] The composite particle 11 may contain other components in addition to the lithium silicate phase 12, the silicon particles 13, and the crystalline phase 14. For example, from the viewpoint of improving the strength of the composite particle 11, a reinforcing material such as an oxide such as ZrO or a carbide may be contained in an amount of less than 10 wt % relative to the composite particle.

[0078] 2 is a schematic cross-sectional view showing the state of composite particle 11 after several charge-discharge cycles of a battery assembled using composite particle 11 as the negative electrode active material. Through charge-discharge cycles, adjacent silicon particles 13 in FIG. 1 may connect with each other, forming a network-like silicon phase 16.

[0079] [Secondary battery] A secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, and an electrolyte, and the negative electrode contains the above-described negative electrode active material (composite particles).

[0080] The secondary battery will be described in detail below, taking a lithium ion secondary battery as an example.

[0081] [Negative electrode] The negative electrode may include a negative electrode current collector and a negative electrode mixture layer supported on the surface of the negative electrode current collector. The negative electrode mixture layer can be formed by applying a negative electrode slurry, in which the negative electrode mixture is dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the applied layer. The dried coating may be rolled as necessary. The negative electrode mixture layer may be formed on one surface or both surfaces of the negative electrode current collector.

[0082] The negative electrode mixture contains a negative electrode active material as an essential component, and may contain optional components such as a binder, a conductive agent, a thickener, etc. The negative electrode active material (composite particles) having the above-mentioned lithium silicate phase and silicon phase is used.

[0083] The negative electrode active material preferably further contains a carbon material that electrochemically absorbs and releases lithium ions. Because the composite particles expand and contract in volume with charge and discharge, a high ratio of the composite particles to the negative electrode active material is likely to result in poor contact between the negative electrode active material and the negative electrode current collector with charge and discharge. On the other hand, the combined use of the composite particles and a carbon material makes it possible to achieve excellent cycle characteristics while imparting the high capacity of the silicon particles to the negative electrode. From the viewpoint of increasing capacity and improving cycle characteristics, the ratio of the carbon material to the total of the silicon-containing material and the carbon material is preferably 98% by mass or less, more preferably 70% by mass or more and 98% by mass or less, and even more preferably 75% by mass or more and 95% by mass or less.

[0084] Examples of carbon materials include graphite, easily graphitizable carbon (soft carbon), and non-graphitizable carbon (hard carbon). Among these, graphite is preferred because it has excellent charge / discharge stability and low irreversible capacity. Graphite refers to a material having a graphite-type crystal structure, and includes, for example, natural graphite, artificial graphite, graphitized mesophase carbon particles, and the like. One type of carbon material may be used alone, or two or more types may be used in combination.

[0085] The negative electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys. The thickness of the negative electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm, from the viewpoint of balancing the strength and weight of the negative electrode.

[0086] Examples of binders include resin materials, such as fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resin; polyimide and polyamideimide; acrylic resins such as polyacrylic acid, polymethyl acrylate, and ethylene-acrylic acid copolymer; vinyl resins such as polyacrylonitrile and polyvinyl acetate; polyvinylpyrrolidone; polyethersulfone; and rubber-like materials such as styrene-butadiene copolymer rubber (SBR). One type of binder may be used alone, or two or more types may be used in combination.

[0087] Examples of conductive agents include carbons such as acetylene black, conductive fibers such as carbon fiber and metal fiber, carbon fluoride, metal powders such as aluminum, conductive whiskers such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, and organic conductive materials such as phenylene derivatives. One type of conductive agent may be used alone, or two or more types may be used in combination.

[0088] Examples of thickeners include carboxymethyl cellulose (CMC) and its modified products (including salts such as Na salt), cellulose derivatives such as methyl cellulose (cellulose ethers, etc.), saponified polymers having vinyl acetate units such as polyvinyl alcohol, polyethers (polyalkylene oxides such as polyethylene oxide, etc.), etc. One type of thickener may be used alone, or two or more types may be used in combination.

[0089] The dispersion medium is not particularly limited, but examples thereof include water, alcohols such as ethanol, ethers such as tetrahydrofuran, amides such as dimethylformamide, N-methyl-2-pyrrolidone (NMP), and mixed solvents thereof.

[0090] [Positive electrode] The positive electrode may include a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by applying a positive electrode slurry, in which the positive electrode mixture is dispersed in a dispersion medium, to the surface of the positive electrode current collector and drying the applied layer. 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. The positive electrode mixture contains a positive electrode active material as an essential component and may contain a binder, a conductive agent, etc. as optional components. NMP, etc., is used as the dispersion medium for the positive electrode slurry.

[0091] As the positive electrode active material, for example, a lithium-containing composite oxide can be used. a CoO2, Li a NiO2, Li a MnO2, Li a Co b Ni 1-b O2, Li a Co b Me 1-b O c , Li a Ni 1-b Me b O c , Li a Mn2O4, Li a Mn 2-b Me b O 4、 LiMePO 4、 Li2MePO4F (Me is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B). Here, a = 0 to 1.2, b = 0 to 0.9, and c = 2.0 to 2.3. The value a, which indicates the molar ratio of lithium, increases or decreases during charge and discharge.

[0092] Among them, Li a Ni b Me 1-bLiNi a a b b c c d dO2 (Me is at least one selected from the group consisting of Mn, Co, and Al, 0 < a ≤ 1.2, 0.3 ≤ b ≤ 1) is preferred. From the perspective of increasing the capacity, it is more preferable to satisfy 0.85 ≤ b ≤ 1. From the perspective of the stability of the crystal structure, Li a Ni b Co c Al d Ni a a b b c c d dO2 (0 < a ≤ 1.2, 0.85 ≤ b < 1, 0 < c < 0.15, 0 < d ≤ 0.1, b + c + d = 1) is even more preferred.

[0093] As the binder and the conductive agent, the same ones as those exemplified for the negative electrode can be used. As the conductive agent, graphite such as natural graphite and artificial graphite may be used.

[0094] The shape and thickness of the positive electrode current collector can be selected respectively from the shape and range according to the negative electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, titanium, etc.

[0095] [Electrolyte] The electrolyte contains a solvent and a lithium salt dissolved in the solvent. The concentration of the lithium salt in the electrolyte is preferably, for example, 0.5 mol / L or more and 2 mol / L or less. By setting the lithium salt concentration within the above range, an electrolyte excellent in ionic conductivity and having appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0096] 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.

[0097] Examples of lithium salts include LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, and LiB 10 Cl 10Examples of the lithium salt include lithium carboxylates, lower aliphatic carboxylates, LiCl, LiBr, LiI, borates, imide salts, etc. Examples of the borates include lithium bis(1,2-benzenediolate(2-)-O,O')borate, lithium bis(2,3-naphthalenediolate(2-)-O,O')borate, lithium bis(2,2'-biphenyldiolate(2-)-O,O')borate, and lithium bis(5-fluoro-2-oleate-1-benzenesulfonic acid-O,O')borate. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO2)2), lithium bistrifluoromethanesulfonyl imide (LiN(CF3SO2)2), lithium trifluoromethanesulfonate nonafluorobutanesulfonyl imide (LiN(CF3SO2)(C4F9SO2)), and lithium bispentafluoroethanesulfonyl imide (LiN(C2F5SO2)2). Among these, LiPF6 is preferred. LiPF6 easily forms a passive film on the surface of battery components such as the positive electrode current collector. The passive film can protect the components. One type of lithium salt may be used alone, or two or more types may be used in combination.

[0098] [Separator] It is usually desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator may be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin such as polypropylene or polyethylene.

[0099] An example of the structure of the secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and a non-aqueous 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 non-aqueous electrolyte 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.

[0100] Hereinafter, the structure of a prismatic secondary battery as an example of the secondary battery according to the present disclosure will be described with reference to Fig. 3. Fig. 3 is a schematic perspective view of a secondary battery according to an embodiment of the present disclosure, with a portion cut away.

[0101] The battery includes a bottomed prismatic battery case 4, an electrode group 1, and a non-aqueous electrolyte housed within the battery case 4. The electrode group 1 includes a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed between them to prevent direct contact. The electrode group 1 is formed by winding the negative electrode, positive electrode, and separator around a flat plate-shaped winding core and then removing the winding core.

[0102] 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. The other end of the negative electrode lead 3 is electrically connected to a negative electrode terminal 6 provided on the sealing plate 5 via a resin insulating plate. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. 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 positive electrode lead 2 is connected to the back surface of the sealing plate 5 via an insulating plate. In other words, the positive electrode lead 2 is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The insulating plate separates the electrode group 1 from the sealing plate 5 and also separates the negative electrode lead 3 from the battery case 4. The periphery of the sealing plate 5 fits into the open edge of the battery case 4, and the fitting portion is laser-welded. In this way, the opening of the battery case 4 is sealed with the sealing plate 5. The injection hole for the non-aqueous electrolyte provided in the sealing plate 5 is closed by a sealing plug 8 .

[0103] The present disclosure will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0104] <Examples 1 to 4 and Comparative Example 1> [Preparation of negative electrode active material (composite particles)] [1st step] Silicon dioxide and a compound containing element X were mixed, and the mixture was fired in air at 950° C. for 10 hours, thus obtaining a silicate containing element X.

[0105] The element X is selected from the group consisting of Li, Ca, B, Al, and La, and contains at least Li. The compounds containing element X were carbonates, hydroxides, or oxides of element X (Li2CO3, CaCO3, H3BO3, Al(OH)3, La2O3). The compounds containing element X were mixed in a mixing ratio that yielded the molar fractions shown in Table 1 when converted to oxides, and the mixture was fired to obtain lithium silicate. The obtained lithium silicate was pulverized to an average particle size of 10 μm.

[0106] [Second process] The silicate containing element X and raw material silicon (3N, average particle size 10 μm) were mixed in the mass ratio shown in Table 1.

[0107] The mixture was placed in a pot (SUS, volume: 500 mL) of a planetary ball mill (Fritsch, P-5), 24 SUS balls (diameter 20 mm) were placed in the pot, the lid was closed, and the mixture was milled at 200 rpm for 50 hours in an inert atmosphere.

[0108] Next, the powder mixture was taken out in an inert atmosphere and sintered for 4 hours under pressure using a hot press to obtain a sintered body (composite intermediate). The obtained composite intermediate was pulverized and classified to obtain composite intermediate particles (average particle size 7 μm).

[0109] [3rd step] The composite intermediate particles were then subjected to a heat treatment at 600° C. for 5 hours in an inert atmosphere.

[0110] [4th step] Next, coal pitch (MCP250, manufactured by JFE Chemical Corporation) and the heat-treated powder were mixed in a weight ratio of 5:100 using a stream mill, and the mixture was fired at 800°C for 5 hours in an inert atmosphere to coat the surfaces of the composite particles with conductive carbon to form a conductive layer, thereby obtaining negative electrode active materials (composite particles) a1 to a5 and b1.

[0111] For the negative electrode active material (composite particles) a4, X-ray diffraction measurement was carried out, and it was confirmed that a crystalline phase of La2Si2O7 was formed.

[0112] For the negative electrode active materials (composite particles) a3, a4, and b1, in the second step, iron fine powder (average particle size 1 μm) was mixed in a mixing ratio of 1 mass % relative to the total of silicon and raw silicon, and the mixture was fired to obtain a sintered body.

[0113] The sintering temperature in the second step was 650°C for negative electrode active material (composite particles) a1, 700°C for a2 to a4, and 600°C for b1.

[0114] Table 1 shows the content ratio of silicon particles and lithium silicate phase in the second step, the mixing ratio of each compound in the first step, and the composition of the lithium silicate phase (composition formula Li a M b SiO x The values ​​of a, b, and x when expressed as , as well as the presence or absence of iron particles are listed.

[0115] [Preparation of negative electrode] The composite particles and graphite were mixed in a mass ratio of 5:95 and used as the negative electrode active material. The negative electrode active material, carboxymethyl cellulose sodium (CMC-Na), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 97.5:1:1.5, water was added, and the mixture was stirred using a mixer (TK Hibismix, manufactured by Primix Corporation) to prepare a negative electrode slurry. Next, a 1 m thick film was applied to the surface of copper foil. 2 The negative electrode slurry was applied 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 copper foil with a density of 1.5 g / cm on both sides. 3 A negative electrode having the negative electrode mixture layer formed thereon was fabricated.

[0116] [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 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 / cm on both sides. 3 A positive electrode having the positive electrode mixture layer formed thereon was fabricated.

[0117] [Preparation of electrolyte] The electrolyte 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.

[0118] [Secondary battery production] A tab was attached to each electrode, and the positive and negative electrodes were spirally wound with the separator interposed between them so that the tabs were positioned at the outermost periphery to prepare 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 was poured into the exterior case, and the opening of the exterior case was sealed to obtain a battery.

[0119] In the production of the above negative electrodes, composite particles a1 to a4 and b1 were used to obtain batteries A1 to A4 according to Examples 1 to 4, respectively, and battery B1 according to Comparative Example 1. The following cycle test was carried out on each of the obtained batteries.

[0120] [Cycle test] <Charging> The battery was charged at a constant current of 1 It (800 mA) until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 1 / 20 It (40 mA).

[0121] <Discharge> A constant current discharge was carried out at a current of 1 It (800 mA) until the voltage reached 2.75 V.

[0122] The rest period between charging and discharging was 10 minutes, and charging and discharging were carried out at 25°C.

[0123] For each battery, the ratio of the first-cycle discharge capacity to the first-cycle charge capacity was calculated as the initial charge-discharge efficiency. Also, for each battery, the ratio of the first-cycle discharge capacity to the 100th-cycle discharge capacity was calculated as the cycle capacity retention.

[0124] The evaluation results are shown in Table 2. In Table 2, the initial charge-discharge efficiency and cycle capacity retention rate are each shown as relative values, with the evaluation result of Battery A1 set to 100.

[0125] In addition, the discharged battery after one cycle was disassembled, the negative electrode mixture layer was removed, and a particle of the negative electrode active material was selected and subjected to electron diffraction measurement to observe the presence and shape of diffraction images attributable to the silicon phase and / or the lithium silicate phase. The obtained electron diffraction images are also shown in Table 2.

[0126] <Comparative Example 2> A raw material consisting of 30 parts by mass of metallic silicon and 70 parts by mass of silicon dioxide was placed in a reactor, and the mixture was vaporized in an atmosphere of 1400°C and 100 Pa vacuum, and deposited on an adsorption plate. After the deposit was sufficiently cooled, it was removed from the reactor and pulverized in a ball mill to obtain a silicon compound (SiO ) with an average particle size of 5 μm. x Particles consisting of (x = 1.4) were prepared. Pyrolytic CVD using methane gas as the raw material was carried out at 1100°C for 5 hours to form a carbon coating on the particles, thereby preparing negative electrode active material (composite particles) b2. The carbon coating was controlled so that 5 parts by mass was coated per 100 parts by mass of silicon compound powder.

[0127] Using composite particles b2, battery B2 according to Comparative Example 2 was produced in the same manner as batteries A1 to A4 and B1, and evaluated in the same manner. Furthermore, the battery in a discharged state after one cycle was disassembled, and electron diffraction measurement of the negative electrode active material was performed to observe the presence and shape of a diffraction pattern. The evaluation results are shown in Table 2.

[0128] [Table 1]

[0129] [Table 2]

[0130] Batteries A1 to A4 of Examples 1 to 4, which used a negative electrode active material in which a silicon phase was dispersed in a lithium silicate phase and the lithium silicate phase contained element M, exhibited higher initial charge-discharge efficiency than battery B1, which used a negative electrode active material in which the silicon phase was dispersed in a lithium silicate phase but did not contain element M, and battery B2, which used a negative electrode active material in which the silicon phase was dispersed in a silicon oxide phase. Batteries A1 to A4 also maintained high cycle capacity retention rates. [Industrial Applicability]

[0131] The secondary battery according to the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, and the like. [Explanation of symbols]

[0132] 1 electrode group 2 positive leads 3 Negative lead 4 Battery case 5 Sealing plate 6 Negative terminal 7 Gasket 8. Seal 11 Composite particles 12 Lithium silicate phase 13 Silicon particles 14 Crystalline phases of rare earth silicates 15 Conductive layer 16 Silicon phase

Claims

1. A negative electrode active material comprising a lithium silicate phase and a silicon phase dispersed within the lithium silicate phase, The lithium silicate phase contains at least one element M selected from the group consisting of alkali metal elements (excluding lithium), Group 2 elements, rare earth elements, and zirconium (Zr), niobium (Nb), tantalum (Ta), vanadium (V), titanium (Ti), phosphorus (P), bismuth (Bi), zinc (Zn), tin (Sn), lead (Pb), antimony (Sb), cobalt (Co), fluorine (F), tungsten (W), aluminum (Al), and boron (B); a negative electrode active material for a secondary battery, the negative electrode active material having a spot image attributable to at least the lithium silicate phase out of the silicon phase and the lithium silicate phase, in an electron beam diffraction image obtained by a transmission electron microscope using a selected area electron beam diffraction method with a narrowed area of ​​200 nmΦ.

2. 2. The negative electrode active material for a secondary battery according to claim 1, wherein the element M includes at least one element selected from the group consisting of sodium (Na), potassium (K), magnesium (Mg), calcium (Ca), and barium (Ba).

3. 2. The negative electrode active material for a secondary battery according to claim 1, wherein the element M includes at least one selected from the group consisting of lanthanum (La), cerium (Ce), praseodymium (Pr), and neodymium (Nd).

4. 2. The negative electrode active material for a secondary battery according to claim 1, wherein the element M includes at least one of aluminum and boron.

5. The lithium silicate phase has the composition formula: Li ａ M ｂ SiO ｘ It has a composition represented by 5. The negative electrode active material for a secondary battery according to claim 1, wherein 0.3≦a≦2, 0.01≦b≦0.4, and 1≦x≦3.5 are satisfied.

6. The element M includes a rare earth element RE, 6. The negative electrode active material for a secondary battery according to claim 1, wherein a crystalline phase containing the rare earth element RE, silicon, and oxygen is dispersed within the lithium silicate phase.

7. The crystalline phase has the general formula: (RE) ２ Si ２ O ７ The negative electrode active material for a secondary battery according to claim 6 , comprising a compound represented by the formula:

8. The crystalline phase is La ２ Si ２ O ７ The negative electrode active material for a secondary battery according to claim 7 , comprising a compound represented by the formula:

9. 9. The negative electrode active material for a secondary battery according to claim 1, wherein the negative electrode active material contains iron (Fe).

10. 10. The negative electrode active material for a secondary battery according to claim 1, wherein the silicon phase is formed in a network shape.

11. The negative electrode active material for a secondary battery according to claim 10 , wherein the silicon phase is formed in a network shape by silicon particles dispersed in the lithium silicate phase and interconnecting with each other during charging and discharging.

12. A positive electrode, a negative electrode, and an electrolyte, A secondary battery, wherein the negative electrode comprises the negative electrode active material for a secondary battery according to any one of claims 1 to 11.

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