Negative electrode active material, its preparation method, and use

Abstract

The present invention provides a negative electrode active material and a method for preparing and using the same. The negative electrode active material includes negative electrode active material particles, the negative electrode active material particles having a silicon-based material core and a carbon film layer covering the surface of the silicon-based material core, the silicon-based material core including a silicon-oxygen compound, and a median diameter D of the negative electrode active material. 50 and the calculated particle diameter D c The ratio of D 50 / D c ≦15 and the calculated particle size D c = 7 × A / S, where A is the particle size distribution span value of the negative electrode active material, and S is the specific surface area of ​​the negative electrode active material. The negative electrode active material carbon film layer provided by the present invention has fewer defects, high interfacial stability in an electrochemical system, better cycle stability, high temperature properties, and less expansion and gas generation.

Description

[Technical Field]

[0001] The present application relates to the field of batteries, and more particularly to a negative electrode active material for secondary batteries, a method for preparing the same, an electrode, and a battery. [Background technology]

[0002] Currently, there are three main types of silicon anode active materials being developed: Type 1 is composites of silicon (including nanosilicon, porous silicon, amorphous silicon, etc.) and carbon materials; Type 2 is alloy materials composed of silicon with other metals (iron, manganese, nickel, chromium, cadmium, tin, copper, etc.) or nonmetallic components (carbon, nitrogen, phosphorus, boron, etc.); and Type 3 is composites of silicon oxide compounds and their carbon materials. However, due to a number of issues with the silicon itself, Type 1 and Type 2 materials result in poor cycle performance for the resulting electrode materials, which are prone to pulverization and easy to separate from the current collector.

[0003] Because silicon-oxygen compounds contain more inactive materials, their capacity is lower than that of silicon-only negative electrode active materials. However, at the same time, the presence of these inactive components effectively suppresses the expansion of silicon during cycling, which has an obvious advantage in terms of cycle stability.

[0004] Silicon-oxygen compounds also present unique challenges. During initial lithium intercalation, numerous side reactions with the electrolyte often result in the formation of a thick SEI film on the particle surface, and irreversible lithium deintercalation species, such as lithium silicate and lithium oxide, are formed within the particles, resulting in the irreversible loss of lithium ions within the battery. These two irreversible reactions result in low initial coulombic efficiency in lithium-ion batteries incorporating silicon-oxygen compound anodes, limiting the improvement of full-cell energy density. Silicon-oxygen compounds also have other challenges, such as low ionic and electronic conductivity and low coulombic efficiency during battery cycling.

[0005] On the other hand, numerous studies have shown that the surface of silicon-oxygen compound particles has a significant impact on electrochemical stability, including the stability of interfacial reactions between the silicon-oxygen particle surface and the electrolyte and susceptibility to transition metal precipitation. The above-mentioned interfacial stability significantly affects the cycle life of silicon-containing battery systems, charge-discharge and storage stability in high-temperature environments, and cell expansion. The presence of interfacial side reactions can lead to the generation of large amounts of gas after battery cells undergo multiple charge-discharge cycles or are exposed to high-temperature environments, potentially posing a safety risk. Therefore, the preparation of silicon-oxygen compound particles with stable surface properties has become a pressing issue in the industry.

[0006] The content of the background art is only art known to the applicant and does not represent prior art in the field. Summary of the Invention [Means for solving the problem]

[0007] In order to solve at least one of the above technical problems, the present invention provides a negative electrode active material comprising a negative electrode active material particle, the negative electrode active material particle comprising a silicon-based material core and a carbon film layer covering the surface of the silicon-based material core, the silicon-based material core comprising a silicon-oxygen compound, The median diameter D of the negative electrode active material 50 and the calculated particle diameter D c The ratio of D 50 / D c ≦15 and the calculated particle diameter D c = 7 × A / S, where A is the particle size distribution span value of the negative electrode active material, and S is the specific surface area of ​​the negative electrode active material.

[0008] In some embodiments of the present invention, the median diameter D 50 and the calculated particle diameter D c The ratio of D 50 / D c ≦10, preferably D 50 / D c ≦5, more preferably D 50 / Dc Meets ≦2.5.

[0009] In some embodiments of the present invention, the specific surface area S of the negative electrode active material is 0.1 to 15 m 2 / g, preferably 0.3 to 10m 2 / g, more preferably 0.3 to 6m 2 / g.

[0010] In some embodiments of the present invention, the median diameter D 50 is 0.2 to 20 μm, preferably 1 to 15 μm, more preferably 3 to 13 μm, The particle size distribution span value A of the negative electrode active material is 2.0 or less, preferably 1.5 or less, and more preferably 1.2 or less.

[0011] In some embodiments of the present invention, the silicon-based material core further comprises elemental lithium.

[0012] In some embodiments of the present invention, the content of lithium element in the negative electrode active material particles is 0.1 to 20 wt%, preferably 2 to 18 wt%, more preferably 4 to 15 wt%.

[0013] In some embodiments of the present invention, the content of silicon element in the negative electrode active material particles is 30 to 80 wt %, preferably 35 to 65 wt %, and more preferably 40 to 65 wt %.

[0014] In some embodiments of the present invention, the negative electrode active material particles further comprise silicon nanoparticles, and the median diameter of the silicon nanoparticles dispersed within the silicon-based material core is in the range of 0.1 to 35 nm, preferably 0.5 to 20 nm, and more preferably 1 to 15 nm.

[0015] In some embodiments of the present invention, the carbon film layer has a thickness of 0.001 to 5 μm, preferably 0.005 to 2 μm, and more preferably 0.01 to 1 μm.

[0016] In some embodiments of the present invention, the mass proportion of the carbon film layer is 0.01 to 20 wt %, preferably 0.1 to 15 wt %, more preferably 1 to 12 wt %, of the total mass of the negative electrode active material particles.

[0017] The present invention also provides an electrode containing the above-described negative electrode active material.

[0018] The present invention also provides a battery comprising the above electrode.

[0019] The present invention provides providing silicon-oxygen compound particles; a step of coating the surface of the silicon-oxygen compound particles with a carbon film layer; Also provided is a method for preparing the negative electrode active material, which includes:

[0020] In some embodiments of the invention, the method further comprises: The method further includes the step of doping the silicon-oxygen compound particles covering the carbon film layer with lithium. [Effects of the Invention]

[0021] The negative electrode active material carbon film layer provided by the present invention has fewer defects, high interfacial stability in electrochemical systems, better cycle stability, high-temperature properties, and less expansion and gas generation.

[0022] Additional aspects and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention. [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a flowchart of a preparation process provided by one embodiment of the present invention. [Figure 2] 1 is a flowchart of a preparation process provided by another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0024] In order to better understand the technical means of the present invention and the advantages of each aspect thereof, the specific embodiments of the present invention will be described in more detail below with reference to the drawings and examples. However, the specific embodiments and examples described below are for illustrative purposes only and are not intended to limit the present invention.

[0025] It should be noted that all similar substitutions and modifications are obvious to those skilled in the art, and such substitutions and modifications are deemed to be included in the present invention. Although the method and use of the present invention have been described through preferred embodiments, it is obvious that those skilled in the art can make modifications or appropriate changes and combinations to the method and use described herein to realize and use the technology of the present invention without departing from the content, spirit and scope of the present invention.

[0026] <Negative electrode active material> The present invention provides a negative electrode active material for a battery, comprising negative electrode active material particles. The negative electrode active material particles include a silicon-based material core and a carbon film layer covering the surface of the silicon-based material core. The silicon-based material core includes a silicon-oxygen compound. The silicon-based material core may be a silicon-oxygen compound particle or a lithium-intercalated silicon-oxygen compound particle, i.e., the silicon-based material core may include lithium element.

[0027] The median diameter D of the negative electrode active material provided by the present invention 50 and the calculated particle diameter D c The ratio of D 50 / D c ≦15 and the calculated particle diameter D c = 7 × A / S, where A is the particle size distribution span value of the negative electrode active material, and S is the specific surface area of ​​the negative electrode active material. 90 -D 10 ) / D 50 , where particle diameter D 10 is the particle size at which the cumulative distribution of particles is 10%, that is, the volume content of particles smaller than this particle size accounts for 10% of all particles, and the median diameter D 50is the particle size at which the cumulative distribution of particles is 50%, i.e., 50% of the particles are above this value and 50% of the particles are below this value, and the particle size D 90 is the particle size at which the cumulative distribution of particles is 90%, meaning that the volume content of particles smaller than this particle size accounts for 90% of all particles.

[0028] As a result of their research, the present inventors have found that the surface properties of negative electrode active material particles used in batteries, particularly the interfacial stability in electrochemical systems, are closely related to the structure of the carbon film layer coated on the surface.

[0029] Common structural defects in carbon film layers include pores within the carbon film layer, impurities, disrupted crystalline structures in the carbon layer, a high concentration of hydrocarbon / carbon-oxygen / carbon-sulfur bonds, surface wrinkles, and cracks. These structural defects can increase the specific surface area of ​​the anode active material, increase side reactions with the electrolyte, and cause stronger interactions with transition metals eluted from the cathode, potentially degrading the electrochemical properties of the anode active material, such as cycle stability, high-temperature stability, and expansion rate. The presence of interfacial side reactions can cause significant expansion and gas generation in battery cells after cycling or high-temperature environments, posing a safety risk.

[0030] The median diameter D of the negative electrode active material provided by the present invention 50 and the calculated particle diameter D c The ratio of D 50 / D c When the ratio satisfies ≦15, the carbon film layer has fewer defects, resulting in higher interfacial stability in the electrochemical system, better cycle stability, higher high-temperature properties, and less expansion and gas generation.

[0031] In some embodiments of the present invention, D 50 / D c ≦10, preferably D 50 / D c ≦5, more preferably D 50 / D c ≦2.5. The smaller this value is, the fewer defects there are in the carbon film layer on the surface of the negative electrode active material.

[0032] In some embodiments of the present invention, the specific surface area S of the negative electrode active material is 0.1 to 15 m 2 / g, preferably 0.3 to 10m 2 / g, more preferably 0.3 to 6m 2 / g.

[0033] In some embodiments of the present invention, the median diameter D of the negative electrode active material 50 is 0.2 to 20 μm, preferably 1 to 15 μm, and more preferably 3 to 13 μm.

[0034] In some embodiments of the present invention, the particle size distribution span value A of the negative electrode active material is 2.0 or less, preferably 1.5 or less, and more preferably 1.2 or less.

[0035] In some embodiments of the present invention, the content of silicon element in the negative electrode active material particles can be 30 to 80 wt%, preferably 35 to 65 wt%, more preferably 40 to 65 wt%, so that the material has a very high reversible capacity.

[0036] In some embodiments of the present invention, the content of lithium element in the silicon-based material core may be 0.1 to 20 wt%, preferably 2 to 18 wt%, more preferably 4 to 15 wt%.

[0037] In some embodiments of the present invention, the negative electrode active material particles also contain nanosilicon elements that can be uniformly dispersed within the silicon-based material core. The nanosilicon elements can exist within the silicon-based material core in the form of nanoparticles, and can have a median diameter in the range of 0.1 to 35 nm, preferably 0.5 to 20 nm, and more preferably 1 to 15 nm. Particles within this particle size range experience minimal particle expansion and are less likely to burst during repeated lithium ion insertion and extraction. This allows lithium ion secondary batteries using this material to experience minimal expansion during cycling and achieve stable cycling.

[0038] In some embodiments of the present invention, the thickness of the carbon film layer is 0.001 to 5 μm, preferably 0.005 to 2 μm, and more preferably 0.01 to 1 μm. The presence of the carbon film layer effectively improves the conductivity of the particles and reduces the contact resistance between particles in the negative electrode plate and between the negative electrode plate and the current collector, thereby improving the lithium insertion / extraction efficiency of the material, reducing polarization in the lithium ion battery, and promoting cycle stability.

[0039] In some embodiments of the present invention, the mass proportion of the carbon film layer may be 0.01 to 20 wt %, preferably 0.1 to 15 wt %, more preferably 1 to 12 wt %, of the total mass of the negative electrode active material particles.

[0040] The negative electrode active material for a battery provided herein has relatively stable surface properties, resulting in high interfacial stability with the electrolyte and minimal interaction with transition metals deposited from the positive electrode. Batteries prepared using this negative electrode active material have advantages such as excellent cycle stability, high-temperature characteristics, minimal expansion, and minimal gas generation.

[0041] <Method for preparing negative electrode active material> FIG. 1 is a flowchart showing the steps for preparing a negative electrode active material according to an exemplary embodiment of the present invention.

[0042] S101: Silicon-oxygen compound particles are prepared.

[0043] The specific preparation process can be carried out by the following steps: First, a mixture of metal silicon powder and silicon dioxide powder is heated in an inert gas atmosphere or under reduced pressure at a temperature between 900°C and 1600°C to generate silicon oxide gas. The molar ratio of metal silicon powder to silicon dioxide powder is set to a range of 0.5 to 1.5. The gas generated by the thermal reaction of the raw materials is deposited on an adsorption plate. When the temperature inside the reactor drops below 100°C, the deposit is removed and pulverized using equipment such as a ball mill or jet mill to obtain silicon-oxygen compound particles.

[0044] The silicon-oxygen compound particles of the present invention may be silicon oxide (silicon monoxide and / or silicon dioxide) materials. In an exemplary embodiment of the present invention, the stoichiometric ratio of silicon oxide in the silicon oxide compound particles may be 1:0.4 to 1:2, alternatively 1:0.6 to 1:1.5, or further alternatively 1:0.8 to 1:1.2. Of course, trace impurity elements other than silicon and oxygen may also be present.

[0045] S102: The surface of the silicon-oxygen compound particles is coated with a carbon film layer.

[0046] The structure of the carbon film layer on the particle surface is closely related to its preparation process, and carbon film layers obtained under different process parameters (e.g., type of carbon source, coating temperature, atmosphere, air pressure, heating rate, carbon content, magnetic field, electric field, etc.) and different furnace structural designs have significant differences in structure and also have significantly different degrees of defects.

[0047] According to an exemplary embodiment, the silicon-oxygen compound may be a silicon oxide compound that has not been disproportionated, or may be a silicon-oxygen compound that has been subjected to a disproportionation heat treatment. The temperature of the disproportionation heat treatment may be 600-1100°C, alternatively 700-1000°C, and more preferably 800-1000°C.

[0048] In the present invention, a carbon film layer can be directly obtained by chemical vapor deposition (CVD). The carbon source used in CVD is a hydrocarbon compound gas, whose decomposition temperature can be 600-1100°C, preferably 700-1000°C, and more preferably 800-1000°C. In the above-mentioned carbon film coating process, the selection of the carbon source, the carbonization temperature, the amount of carbon source added, the gas ratio, and the flow rate significantly affect the final carbon film structure. Furthermore, the structure of the carbon film can be significantly affected by controlling the pressure (vacuum level) and atmosphere ratio during the carbon film growth process, as well as the unique design of the air flow direction. Preferably, a mixture of a carbon source gas and an inert gas or a reducing gas (such as argon, nitrogen, helium, or hydrogen) is introduced at a certain ratio. The quality of the resulting carbon film is superior to that obtained by introducing only a pure carbon source gas. Furthermore, controlling the volume fraction of the carbon source gas in the mixed gas to 90% or less, preferably 75% or less, and more preferably 65% ​​or less, helps to obtain a carbon film layer with better coating quality and uniformity. In addition, by reducing the gas pressure inside the furnace during the carbon film growth process and maintaining a certain degree of negative pressure (vacuum), the uniformity of the carbon film layer can be significantly improved, resulting in a carbon film layer with greater smoothness and density.

[0049] Carbon film layers can also be obtained by first performing carbon reactive coating followed by heat treatment and carbonization in a non-oxidizing atmosphere. The carbon reactive coating method can use a mechanical melter, VC mixer, coating kettle, spray dryer, sand mill, or high-speed disperser. The solvent used during coating is one or more combinations of water, methanol, ethanol, isopropyl alcohol, n-butanol, ethylene glycol, ether, acetone, N-methylpyrrolidone, methylbutanone, tetrahydrofuran, benzene, toluene, xylene, N,N-dimethylformamide, N,N-dimethylacetamide, and chloroform. The carbon source can be one or more combinations of coal pitch, petroleum pitch, polyvinyl alcohol, epoxy resin, polyacrylonitrile, polymethyl methacrylate, glucose, sucrose, polyacrylic acid, and polyvinylpyrrolidone. The equipment used for heat treatment and carbonization can be a rotary kiln, ladle refining furnace, roller hearth kiln, pusher kiln, atmospheric box furnace, or tubular furnace. The heat treatment / carbonization temperature can be 600-1100°C, preferably 700-1000°C, and more preferably 800-1000°C, with a holding time of 0.5-24 hours. The non-oxidizing atmosphere can be provided by at least one gas selected from nitrogen, argon, hydrogen, and helium. In the carbon film coating process, the selection of the carbon source, the carbonization temperature, the amount of carbon source added, the heating rate, the design of the heating and temperature holding procedures, the strength of the mechanical stirring, and the selection of the coating equipment all have a significant impact on the final carbon film structure. Preferably, designing a heating procedure with multiple temperature holding steps helps to obtain a higher-quality carbon film coating structure. Selecting an appropriate intermediate temperature holding procedure ensures that the carbon source compound is fully melted, achieves appropriate fluidity, and performs a preliminary decoking reaction, thereby helping to obtain a more uniform and dense carbon coating layer. In addition, in the heat treatment and carbonization process, dynamic carbonization is preferred, in which the material is dynamically stirred during the temperature increase and temperature maintenance processes, as this promotes effective removal of volatile matter during the carbonization process of the carbon source, helping to obtain a carbon film with a denser structure and fewer defects.

[0050] 2 is a flowchart illustrating a process for preparing a negative electrode active material according to another exemplary embodiment of the present invention, including the following steps: S201: A step of preparing silicon-oxygen compound particles. S202: A step of coating the surface of the silicon-oxygen compound particles with a carbon film layer. S203: Doping the silicon-oxygen compound particles coated with the carbon film layer with lithium.

[0051] In the present invention, the doping (insertion of lithium element) of silicon-oxygen compound particles can be performed by electrochemical doping, liquid phase doping, thermal doping, etc. The doping atmosphere of lithium element is a non-oxidizing atmosphere, which is composed of at least one of nitrogen, argon, hydrogen, and helium.

[0052] The following methods are available for inserting lithium elements (lithium doping modification methods).

[0053] 1) Electrochemical method An electrochemical cell is prepared, containing four components: a tank, an anode electrode, a cathode electrode, and a power source. The anode electrode and cathode electrode are connected to both ends of the power source. At the same time, the anode electrode is connected to a lithium source, and the cathode electrode is connected to a container containing silicon-oxygen compound particles. The tank is filled with an organic solvent, and the lithium source (anode electrode) and the container containing silicon-oxygen compound particles (cathode electrode) are immersed in the organic solvent. After applying electricity, an electrochemical reaction occurs, causing lithium ions to be inserted into the silicon-oxygen compound structure, resulting in lithium-doped modified silicon-oxygen compound particles. Examples of the organic solvent include ethylene carbonate, propylene carbonate, butylene carbonate, fluorinated ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl acetate, propyl acetate, propionic acid ethyl ester, propyl propionate, and dimethyl sulfoxide. The organic solvent also contains an electrolyte lithium salt, such as lithium hexafluorophosphate (LiPF), lithium hexafluorophosphate (LiBF), or lithium perchlorate (LiClO). The lithium source (anode electrode) may be a lithium foil or a lithium compound such as lithium carbonate, lithium oxide, lithium hydroxide, lithium cobalt oxide, lithium iron phosphate, lithium manganate, lithium vanadium phosphate, or lithium nickelate.

[0054] 2) Liquid phase doping method Metallic lithium, an electron transfer catalyst, and silicon-oxygen compound particles are added to an ether-based solvent, and the mixture is stirred and heated under a non-oxidizing atmosphere. The isothermal reaction is maintained until the metallic lithium in the solution is completely consumed. Due to the action of the electron transfer catalyst, metallic lithium dissolves in the ether-based solvent and forms a lithium ion coordination compound with a low reduction potential, which reacts with the silicon-oxygen compound, allowing the lithium ion to enter the silicon-oxygen compound structure. Examples of the electron transfer catalyst include biphenyl and naphthalene. Examples of the ether-based solvent include methyl butyl ether, ethylene glycol butyl ether, tetrahydrofuran, and ethylene glycol dimethyl ether. The temperature of the isothermal reaction is 25 to 200°C. The non-oxidizing atmosphere is provided by at least one gas selected from nitrogen, argon, hydrogen, and helium.

[0055] 3) Thermal doping method Silicon-oxygen compound particles and a lithium-containing compound are uniformly mixed and then heat-treated in a non-oxidizing atmosphere. Examples of the lithium-containing compound include lithium hydroxide, lithium carbonate, lithium oxide, lithium peroxide, lithium hydride, lithium nitrate, lithium acetate, and lithium oxalate. The mixing method may be a high-speed disperser, a high-speed stirring mill, a ball mill, a conical mixer, a spiral mixer, a stirring mixer, or a VC mixer. The equipment used for the heat treatment may be a rotary furnace, a ladle refining furnace, a liner furnace, a roller hearth kiln, a pusher kiln, an atmospheric box furnace, or a tubular furnace. The heat treatment temperature is 400 to 850°C, preferably 550 to 800°C, the holding time is 1 to 12 hours, and the heating rate is greater than 0.1°C / min and less than or equal to 10°C / min. The non-oxidizing atmosphere is provided by at least one gas selected from the group consisting of nitrogen, argon, hydrogen, and helium.

[0056] The lithium intercalation process is performed after the carbon film layer is coated, which can suppress silicon grain growth in the silicon-oxygen compound during heat treatment. This allows nanoscale silicon particles to be uniformly dispersed and fixed in the lithium silicate compound or silicon-oxygen compound matrix, effectively suppressing the expansion of silicon nanoparticles and preventing the silicon particles from gradually coalescing into larger particles during charge and discharge. This reduces the battery's expansion and deformation during cycling, reduces electrical failure of the silicon material, and enables lithium-ion secondary batteries using this material to exhibit small cycling expansion and stable cycling. Furthermore, the carbon film layer coating process is performed before the lithium intercalation process, which helps to obtain a carbon film layer with better quality and more complete coverage.

[0057] At the same time, the lithium insertion process affects the structure of the carbon film layer on the surface of the silicon-oxygen compound particles to different degrees: if the diffusion process is gentle and controllable, the impact on the carbon film layer structure on the particle surface is minor; if the impact on the diffusion process is relatively severe, it may cause changes in the carbon film layer structure.

[0058] <Method for evaluating the characteristics of negative electrode active materials> 1. Material Measurement: The characteristics of the negative electrode active materials prepared in each example and comparative example were evaluated using the following equipment. The particle size distribution of the negative electrode active material was measured using a BetterSize 2000 laser particle size distribution analyzer (manufactured by Dandong Baite Co., Ltd.). The surface morphology of the negative electrode active material was observed using a scanning electron microscope (SEM) (manufactured by Hitachi, Ltd., SU8010). The specific surface area of ​​the negative electrode active material was measured using a specific surface area analyzer (manufactured by Quantum Chrome Instruments, NOVA 4200e). The specific surface area measurement requirements were as follows: The sample was weighed into a sample tube, and the specific surface area of ​​the sample was measured using nitrogen gas using a multipoint method within a relative pressure range of p / p0 = 0.05 to 0.3.

[0059] 2. Preparation of uniform slurry and electrode plate: 30 parts of the above negative electrode active material, 64 parts of artificial graphite, 2.5 parts of conductive additive, and 3.5 parts of binder are taken and made into an aqueous slurry, which is then applied, dried, and rolled to obtain a negative electrode plate containing the negative electrode active material of the present application.

[0060] 3. Full-cell evaluation: Negative plates obtained from the negative active materials prepared in each example and comparative example were cut, vacuum-baked, and rolled with a corresponding ternary positive plate and separator. Then, they were packed into aluminum-plastic shells of the corresponding size, filled with a certain amount of electrolyte, degassed, and sealed. After formation, lithium-ion batteries with a capacity of approximately 3.2 Ah were obtained. These full cells were subjected to a 60-day fully charged storage test at 60°C to test the thermal expansion coefficient, capacity retention, and capacity recovery rate of the system after high-temperature fully charged storage. The thermal expansion coefficient test was performed by storing the battery cells at 60°C for 60 days fully charged, then immediately removing the battery cells and testing their thickness (d), where d0 is the initial thickness of the battery cell at a half-charged state, and the thermal expansion coefficient = (d-d0) / d0. Next, after cooling the battery cell, a discharge test is performed in a battery testing system manufactured by NEWARE, and the discharge capacity is D1, the discharge capacity of the battery cell before high-temperature storage is D0, and the capacity retention rate = D1 / D0.The battery cell is then subjected to one charge / discharge cycle, and the discharge capacity is D2, and the capacity recovery rate = D2 / D0.

[0061] The present application will now be further described with reference to specific examples.

[0062] (Example 1-1) 1000 g of silicon-oxygen compound particles with a median diameter of 6 μm (silicon:oxygen atomic ratio 1:1) and low-temperature coal pitch powder were mixed uniformly in a coating kettle using a dry method. 2000 g of dimethylformamide was then added with stirring to uniformly disperse the mixed powder in the dimethylformamide. The coating kettle was then heated to 340°C and held at that temperature for 3 hours to obtain a silicon-oxygen compound material coated with coal pitch. The material was then statically heated to 680°C under a nitrogen gas atmosphere and held for 4 hours to carbonize the coal pitch. After cooling, the resulting material was sieved through a 500-mesh sieve to obtain a silicon-oxygen compound powder coated with a carbon film. The carbon content of the carbon-coated silicon-oxygen compound powder was measured and found to be 7 wt%.

[0063] The specific surface area of ​​the negative electrode active material obtained in the above process is S = 15 m 2 / g, median diameter of product D 50 The particle size distribution span value A is 1.52, and the particle diameter D calculated from the specific surface area is 10.64 μm. c = 0.71 μm, so the median diameter D 50 and the calculated particle diameter D c The ratio was 15.

[0064] 30 parts of the negative electrode active material, 64 parts of artificial graphite, 3.5 parts of conductive additive, and 2.5 parts of binder were mixed and coated in an aqueous slurry, which was then dried and rolled to obtain a silicon-containing negative electrode plate.

[0065] In this example, the results of high-temperature storage evaluation of a full cell containing the negative electrode active material were as follows: after being stored at 60°C for 60 days while fully charged, the thermal expansion coefficient of the battery cell was 22.7%, air bubbles were present in the battery cell, the capacity retention rate after high-temperature storage was 78.6%, and the capacity recovery rate was 86.3%.

[0066] (Example 1-2) The process was the same as in Example 1-1, except that the carbonization temperature of the coal pitch was adjusted to 780°C, but the carbon content was still 7 wt%.

[0067] The performance parameters are shown in Table 1.

[0068] (Examples 1-3) 1000 g of silicon-oxygen compound particles with a median diameter of 9 μm and medium-temperature coal pitch powder were mixed uniformly in a VC reactor using a dry method, then directly heated to 720°C and held at that temperature while stirring for 4 hours to carbonize the coal pitch in situ. After cooling, the resulting material was sieved through a 500-mesh sieve to obtain a silicon-oxygen compound powder coated with a carbon film. The carbon content of the carbon-coated silicon-oxygen compound powder was measured and found to be 7 wt%.

[0069] Table 1 shows the performance parameters and high-temperature storage characteristics of a battery cell using this negative electrode active material.

[0070] (Examples 1-4) 1000g of silicon-oxygen compound particles with a median diameter of 8μm and high-temperature coal pitch powder were mixed uniformly in a VC reactor using a dry process. The mixture was then heated to 450°C and stirred at a constant temperature for 3 hours to give the coal pitch sufficient fluidity and achieve a uniform coating. The mixture was then heated to 780°C and held for 5 hours for carbonization. Sufficient stirring was maintained during the carbonization process to promote effective removal of volatiles during the pitch carbonization process, resulting in a denser carbon film layer. After cooling, the resulting material was sieved through a 500-mesh sieve to obtain the carbon-coated silicon-oxygen compound powder. The carbon content of the carbon-coated silicon-oxygen compound powder was measured and found to be 6wt%.

[0071] Table 1 shows the performance parameters and high-temperature storage characteristics of a battery cell using this negative electrode active material.

[0072] (Examples 1-5) 1000g of silicon-oxygen compound particles with a median diameter of 7μm were weighed out and placed in a CVD furnace. Using methane as the carbon source, a coating reaction was carried out at 800℃ with a methane to argon flow ratio of 1:1, resulting in silicon-oxygen compound particles coated with a carbon film layer. The carbon content of the carbon-coated silicon-oxygen compound powder was measured and found to be 5wt%.

[0073] Table 1 shows the performance parameters and high-temperature storage characteristics of a battery cell using this negative electrode active material.

[0074] (Examples 1 to 6) 1000g of silicon-oxygen compound particles with a median diameter of 10μm were weighed out and placed in a CVD furnace. Using methane as the carbon source, a coating reaction was carried out at 800℃ with a methane to argon flow ratio of 2:1, resulting in silicon-oxygen compound particles coated with a carbon film layer. The carbon content of the carbon-coated silicon-oxygen compound powder was measured and found to be 7wt%.

[0075] Table 1 shows the performance parameters and high-temperature storage characteristics of a battery cell using this negative electrode active material.

[0076] (Examples 1-7) 1000g of silicon-oxygen compound particles with a median diameter of 6μm were weighed out and placed in a CVD furnace. Using methane as the carbon source, a low-pressure coating reaction was carried out at 800°C with a methane to argon flow ratio of 2:1. Although low-pressure CVD reactions have low production efficiency, they can significantly improve the uniformity of the carbon film layer, resulting in a smoother, denser, and less defective carbon film layer. The carbon content of the silicon-oxygen compound powder coated with the carbon film was measured and found to be 7wt%.

[0077] Table 1 shows the performance parameters and high-temperature storage characteristics of a battery cell using this negative electrode active material.

[0078] (Examples 1-8) 1000g of silicon-oxygen compound particles with a median diameter of 5μm were weighed out and placed in a CVD furnace. Using methane as the carbon source, a coating reaction was carried out at 800℃ with a methane to argon flow ratio of 0.6:1, yielding silicon-oxygen compound particles coated with a carbon film layer. The carbon content of the carbon-coated silicon-oxygen compound powder was measured and found to be 6wt%.

[0079] Table 1 shows the performance parameters and high-temperature storage characteristics of a battery cell using this negative electrode active material.

[0080] (Examples 1-9 to 1-17) As in Examples 1-1 to 1-8, silicon-oxygen compound particles were used as the raw material, and different carbon film-coated silicon-oxygen compound powders in Examples 1-9 to 1-17 were obtained using different carbon film coating processes, including adjustments of the carbon source selection, carbonization temperature, amount of carbon source added, air pressure (vacuum level), atmosphere ratio, air flow direction, as well as the temperature rise rate, temperature rise and temperature maintenance procedure design, mechanical stirring strength, and equipment selection.

[0081] Table 1 shows the performance parameters and high-temperature storage characteristics of a battery cell using this negative electrode active material.

[0082] (Comparative Example 1-1) 1000g of silicon-oxygen compound particles and sucrose were mixed uniformly in a coating kettle using a dry method, then heated to 200°C and stirred at that temperature for 3 hours, then further heated to 500°C and held there for 5 hours for carbonization. After cooling, the resulting material was sieved through a 500-mesh sieve to obtain a silicon-oxygen compound powder coated with a carbon film. The carbon content of the carbon-coated silicon-oxygen compound powder was measured and found to be 10wt%.

[0083] Table 1 shows the performance parameters and high-temperature storage characteristics of a battery cell using this negative electrode active material.

[0084] (Comparative Example 1-2) 1000g of silicon-oxygen compound particles and sucrose were mixed uniformly in a coating kettle using a dry method, then heated to 180°C and stirred at that temperature for 3 hours, then further heated to 400°C and held there for 8 hours for carbonization. After cooling, the resulting material was sieved through a 500-mesh sieve to obtain a silicon-oxygen compound powder coated with a carbon film. The carbon content of the carbon-coated silicon-oxygen compound powder was measured and found to be 8wt%.

[0085] Table 1 shows the performance parameters and high-temperature storage characteristics of a battery cell using this negative electrode active material.

[0086] [Table 1] JPEG0007804092000002.jpg80169

[0087] As can be seen from Table 1, the median diameter D 50 and the calculated particle diameter D c Ratio of D 50 / D c The smaller the value, the better the high temperature stability of the negative electrode active material, the smaller the expansion, and the lower the possibility of gas generation.

[0088] Furthermore, as can be seen from Table 1, the carbon film layers obtained under different process parameters (type of carbon source, coating temperature, atmosphere, air pressure, heating rate, carbon content, heating and temperature holding procedure, mechanical stirring, etc.) and different furnace structural designs have significant differences in structure and also have clearly different degrees of defects.

[0089] Example 2-1 Using the negative electrode active material of Example 1-16 as the raw material, the powder, metallic lithium ribbon, and biphenyl were placed in a sealable glass container, and then methyl butyl ether was added and the mixture was stirred under an argon gas atmosphere to react. After the reaction was completed and the mixture was dried, the resulting powder was placed in an argon atmosphere and subjected to heat treatment, heating it to 680°C at a rate of 2°C per minute, and then maintained at that temperature for 10 hours. After natural cooling, a lithium-doped silicon-oxygen compound powder was obtained.

[0090] Table 2 shows the performance parameters and high-temperature storage characteristics of a battery cell using this negative electrode active material.

[0091] (Example 2-2)

[0092] The product of Example 1-16 was used as the raw material, and the procedure was the same as Example 2-1, except that the heat treatment process after lithium doping was changed as follows: The temperature was raised to 720°C at a rate of 5°C per minute, and then the temperature was maintained for 3 hours. After natural cooling, a lithium-doped silicon-oxygen compound powder was obtained.

[0093] Table 2 shows the performance parameters and high-temperature storage characteristics of a battery cell using this negative electrode active material.

[0094] (Example 2-3) The product of Example 1-16 was used as the raw material, and the procedure was the same as Example 2-1, except that the heat treatment process after lithium doping was changed as follows: The temperature was raised to 720°C at a rate of 10°C per minute, and then the temperature was maintained for 6 hours. After natural cooling, a lithium-doped silicon-oxygen compound powder was obtained.

[0095] Table 2 shows the performance parameters and high-temperature storage characteristics of a battery cell using this negative electrode active material.

[0096] (Examples 2-4) The product of Example 1-16 was used as the raw material, and the procedure was the same as Example 2-1, except that the heat treatment process after lithium doping was changed as follows: The temperature was raised to 850°C at a rate of 10°C per minute, and then the temperature was maintained for 2 hours. After natural cooling, a lithium-doped silicon-oxygen compound powder was obtained.

[0097] Table 2 shows the performance parameters and high-temperature storage characteristics of a battery cell using this negative electrode active material.

[0098] (Examples 2-5) Using the product of Examples 1-10 as the raw material, the above particles were mixed with a lithium-containing compound (lithium oxide, lithium hydride, lithium hydroxide, lithium carbonate, etc.), and the mixed powder was placed in an argon atmosphere and subjected to heat treatment. The temperature was raised to 400°C at a rate of 2°C per minute and held for 3 hours, then further raised to 650°C and held for 10 hours, and after natural cooling, a lithium-containing silicon-oxygen compound powder coated with a carbon film was obtained.

[0099] Table 2 shows the performance parameters and high-temperature storage characteristics of a battery cell using this negative electrode active material.

[0100] (Examples 2-6) The product of Example 1-10 was used as the raw material, and the procedure was the same as Example 2-5, except that the heat treatment process after lithium doping was changed as follows: The temperature was directly raised to 700°C at a rate of 5°C per minute, and then the temperature was maintained for 6 hours. After natural cooling, a lithium-doped silicon-oxygen compound powder was obtained.

[0101] Table 2 shows the performance parameters and high-temperature storage characteristics of a battery cell using this negative electrode active material.

[0102] (Examples 2-7) The product of Examples 1-10 was used as the raw material, and the procedure was the same as in Examples 2-5, except that the heat treatment process after lithium doping was changed as follows: The temperature was directly raised to 820°C at a rate of 10°C per minute, and then the temperature was maintained for 2 hours. After natural cooling, a lithium-doped silicon-oxygen compound powder was obtained.

[0103] Table 2 shows the performance parameters and high-temperature storage characteristics of a battery cell using this negative electrode active material.

[0104] [Table 2]

[0105] Combining the data in Tables 1 and 2, the lithium intercalation process affects the structure of the carbon film layer on the surface of the silicon-oxygen compound particles to different degrees. If the diffusion process is gentle and controllable, the impact on the carbon film layer structure on the particle surface is minor. However, if the impact on the diffusion process is relatively severe, it will cause changes in the carbon film layer structure and reduce the interfacial stability of the material, resulting in poor high-temperature storage properties and increased swelling of batteries using this negative electrode active material.

[0106] The above examples are merely examples for clarifying the present invention and are not intended to limit the embodiments. Those skilled in the art can make various changes or modifications based on the above description. It is not necessary to list all the embodiments here. However, obvious changes or modifications derived therefrom still fall within the protection scope of the present invention.

Claims

1. A negative electrode active material comprising negative electrode active material particles, the negative electrode active material particles comprising a silicon-based material core and a carbon film layer covering a surface of the silicon-based material core, the silicon-based material core containing a silicon-oxygen compound and elemental lithium; The median diameter D of the negative electrode active material 50 The calculated particle diameter D c The ratio of D 50 / D c ≦2.5, and the calculated particle diameter D c = 7 × A / S, where A is the particle size distribution span value of the negative electrode active material, and S is the specific surface area of ​​the negative electrode active material. A negative electrode active material characterized by:

2. The specific surface area S of the negative electrode active material is 0.1 to 15 m 2 The negative electrode active material according to claim 1 , wherein the average molecular weight of the negative electrode active material is 1000 kJ / g.

3. The median diameter D of the negative electrode active material 50 is 0.2 to 20 μm, The particle size distribution span value A of the negative electrode active material is 2.0 or less. The negative electrode active material according to claim 1 .

4. 2. The negative electrode active material according to claim 1, wherein the content of lithium element in the negative electrode active material particles is 0.1 to 20 wt %.

5. 2. The negative electrode active material according to claim 1, wherein the content of silicon element in the negative electrode active material particles is 30 to 80 wt %.

6. 2. The negative electrode active material according to claim 1, wherein the negative electrode active material particles further comprise silicon nanoparticles, and the silicon nanoparticles dispersed within the silicon-based material core have a median diameter in the range of 0.1 to 35 nm.

7. 2. The negative electrode active material according to claim 1, wherein the carbon film layer has a thickness of 0.001 to 5 μm.

8. 2. The negative electrode active material according to claim 1, wherein the mass ratio of the carbon film layer is 0.01 to 20 wt % of the total mass of the negative electrode active material particles.

9. An electrode, comprising the negative electrode active material according to any one of claims 1 to 8.

10. A battery, characterized in that it comprises an electrode according to claim 9.

11. A method for preparing a negative electrode active material, comprising: providing silicon-oxygen compound particles; coating the surface of the silicon-oxygen compound particles with a carbon film layer; doping the silicon-oxygen compound particles covering the carbon film layer with lithium; the lithium doping step is carried out at a temperature of 550°C to 800°C with a heating rate of more than 0.1°C / min to 2°C / min or less; the negative electrode active material includes negative electrode active material particles, the negative electrode active material particles have a silicon-based material core and a carbon film layer covering the surface of the silicon-based material core; the silicon-based material core comprises a silicon-oxygen compound; The ratio of the median diameter D50 of the negative electrode active material to the calculated particle diameter Dc satisfies D50 / Dc≦2.5, and the calculated particle diameter Dc = 7×A / S, where A is the particle size distribution span value of the negative electrode active material and S is the specific surface area of ​​the negative electrode active material.

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