WO2023124603A1 - Negative electrode active material and preparation method therefor, electrode, and battery - Google Patents

Abstract

The present invention provides a negative electrode active material and a preparation method therefor, an electrode, and a battery. The negative electrode active material comprises negative electrode active material particles; the negative electrode active material particles comprise silicon-oxygen compound particles, a lithium element embedded into the silicon-oxygen compound particles, and a carbon film layer; the silicon-oxygen compound particles comprise inner cores and porous shell layers; and the surfaces of the porous shell layers are partially or completely covered with the carbon film layer. The negative electrode active material for the battery, provided by the present invention, has electrochemical characteristics of high efficiency, high energy density, and relatively small expansion when in use. The battery prepared from the negative electrode active material has advantages such as high energy density, excellent cycle performance, relatively small cycle expansion, and good high-temperature performance.

Description

Negative electrode active material, preparation method thereof, electrode, battery technical field

The present application relates to the field of batteries, in particular, to a negative electrode active material for batteries, a preparation method thereof, an electrode, and a battery.

Background technique

In recent years, with the continuous development of various portable electronic devices and electric vehicles, the demand for batteries with high energy density and long cycle life has become increasingly urgent. At present, the negative electrode active material of commercialized lithium-ion batteries is mainly graphite, but due to the low theoretical capacity (372mAh/g), the further improvement of battery energy density is limited. The elemental silicon negative electrode active material has a high capacity advantage (the lithium intercalation state at room temperature is Li ₁₅ Si ₄ , and the theoretical lithium storage capacity is about 3600mAh/g), which is about 10 times the theoretical capacity of the current commercial graphite negative electrode active material. The high-capacity advantage that other negative electrode active materials cannot match has become a research and development hotspot in academia and industry for many years, and it has gradually moved from laboratory research to commercial application.

At present, there are three main developments for silicon negative electrode active materials, one is simple silicon (including nano-silicon, porous silicon, amorphous silicon, etc.) and its composite material with carbon materials; the other is silicon and other metals (such as iron, manganese, Nickel, chromium, cadmium, tin, copper, etc.), non-metallic (carbon, nitrogen, phosphorus, boron, etc.) Among the above three structures, the theoretical capacity of the elemental silicon material is the highest, so the theoretical energy density is also the highest. However, the elemental silicon negative electrode active material has a serious volume effect in the process of intercalation and extraction of lithium, and the volume change rate is about 300%, which will cause the pulverization of the electrode material and the separation of the electrode material and the current collector. In addition, due to the continuous expansion and contraction of the silicon negative electrode active material during the charging and discharging process of the battery and continuous rupture, the resulting fresh interface is exposed to the electrolyte to form a new SEI film, thereby continuously consuming the electrolyte and reducing the cycle performance of the electrode material. . The above defects severely limit the commercial application of elemental silicon anodes.

Due to the presence of more inactive substances in silicon oxide compounds, its capacity is lower than that of simple silicon anode active materials; however, at the same time, due to the presence of these inactive components, the expansion of silicon during cycling is effectively suppressed by the inactive phase, so Its cycle stability has obvious advantages. But siloxanes also have their own specific problems. For example, a thicker SEI film will still be formed, lithium silicate and lithium oxide and other substances that cannot be reversibly delithiated will be generated inside the particles, low ion and electronic conductivity, and low Coulombic efficiency during battery cycling.

The content in the background section is only the technology known to the applicant, and does not represent the prior art in the field.

Contents of the invention

In order to solve one of the above technical problems, the present invention provides a negative electrode active material for batteries, which includes negative electrode active material particles;

The negative active material particles include silicon oxide particles, lithium elements embedded in the silicon oxide particles and a carbon film layer, the silicon oxide particles include an inner core and a porous shell, and the surface of the porous shell is partially or completely covered by the carbon film layer.

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

In some embodiments of the present invention, the relative specific surface area of the negative electrode active material is ≤5, preferably ≤3, more preferably ≤2, further preferably ≤1.5.

In some embodiments of the present invention, the relative tap density of the negative electrode active material particles is ≥0.8, preferably ≥0.85.

In some embodiments of the present invention, the content of lithium element in the silicon oxide compound particles is 0.1-20wt%, preferably 2-18wt%, more preferably 4-15wt%.

In some embodiments of the present invention, the silicon element content in the silicon oxide compound particles is 30-80wt%, preferably 35-65wt%, more preferably 40-65wt%.

In some embodiments of the present invention, the proportion of silicon in the porous shell is higher than the proportion of silicon in the inner core.

In some embodiments of the present invention, the proportion of lithium element in the porous shell layer is lower than the proportion of lithium element in the inner core.

In some embodiments of the present invention, the median diameter of the silicon oxide compound particles is 0.2-20 μm, preferably 1-15 μm, more preferably 3-13 μm.

In some embodiments of the present invention, the negative electrode active material particles also include elemental silicon nanoparticles, and the median diameter of the elemental silicon nanoparticles dispersed in the negative electrode active material particles is between 0.1-35nm, preferably 0.5-20nm, more preferably 1-15nm.

In some embodiments of the present invention, the thickness of the carbon film layer is 0.001-5 μm, preferably 0.005-2 μm, more preferably 0.01-1 μm.

In some embodiments of the present invention, the carbon film layer accounts for 0.01-20 wt%, preferably 0.1-15 wt%, more preferably 1-12 wt% in the negative electrode active material particles.

In some embodiments of the present invention, the coverage of the carbon film layer on the surface of the porous shell layer is ≥95%, preferably ≥98%.

In some embodiments of the present invention, a cladding layer is further included, the cladding layer includes organic compounds and/or metal oxygen compounds, and the surface of the carbon film layer is partially or completely covered by the cladding layer.

In some embodiments of the present invention, the metal oxygen-containing compound is a composite oxide of metal and phosphorus.

In some embodiments of the present invention, the metal includes one or more of lithium, titanium, magnesium, aluminum, zirconium, calcium, and zinc.

The present invention also provides an electrode comprising the above-mentioned negative electrode active material.

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

The present invention also provides a method for preparing negative electrode active materials for batteries, which comprises the following steps:

Coating a carbon film layer on the surface of silicon oxide particles;

Corrosion and pore forming are carried out on the silicon oxide compound particles coated with the carbon film layer, so that the silicon oxide compound particles coated with the carbon film layer form a structure including an inner core and a porous shell layer; wherein, the porous shell layer The surface is partially or completely covered by the carbon film layer;

Lithium doping is performed on the silicon oxide compound particles formed by etching holes.

In some embodiments of the present invention, the method also includes:

A coating layer comprising organic compounds and/or metal oxygen compounds is formed on the surface of the carbon film layer.

The present invention also provides a method for preparing negative electrode active materials for batteries, which comprises the following steps:

Coating a carbon film layer on the surface of silicon oxide particles;

Lithium doping is performed on the silicon oxide compound particles coated with the carbon film layer;

corroding the doped silicon-oxygen compound particles to form pores, so that the doped silicon-oxygen compound particles form a structure including a core and a porous shell; wherein, the surface of the porous shell is partially or completely covered by the covered by the carbon film layer.

In some embodiments of the present invention, the method also includes:

A coating layer comprising organic compounds and/or metal oxygen compounds is formed on the surface of the carbon film layer.

The negative electrode active material used in the battery provided by the invention has the electrochemical characteristics of high efficiency, high energy density and small expansion when used. The battery prepared by using the negative electrode active material has the advantages of high energy density, excellent cycle performance, small cycle expansion, good high temperature performance and the like.

Additional aspects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.

Description of drawings

Fig. 1 is a flow chart of the preparation of the negative electrode active material according to an exemplary embodiment of the present invention.

Fig. 2 is a flow chart of the preparation of the negative electrode active material according to another exemplary embodiment of the present invention.

Fig. 3(a) and Fig. 3(b) are SEM images of silica particles with porous shells prepared in an exemplary embodiment of the present invention.

Fig. 4 is an SEM image of silica particles with porous shells prepared in another exemplary embodiment of the present invention.

FIG. 5 is a cross-sectional SEM image of the silicon-oxygen particles with a porous shell shown in FIG. 4 , and the figure shows an enlarged cross-section of the porous shell.

Detailed ways

The specific implementation manners of the present invention will be described in more detail below with reference to the drawings and examples, so as to better understand the solution of the present invention and its advantages in various aspects. However, the specific embodiments and examples described below are for the purpose of illustration only, rather than limiting the present invention.

In particular, it should be pointed out that all similar replacements and modifications are obvious to those skilled in the art, and they are all considered to be included in the present invention. The method and application of the present invention have been described through preferred embodiments, and the relevant personnel can obviously make changes or appropriate changes and combinations to the method and application described herein without departing from the content, spirit and scope of the present invention to realize and Apply the technology of the present invention.

【Negative electrode active material】

The present invention provides a negative electrode active material for batteries, which has negative electrode active material particles. The negative electrode active material particles contain silicon-oxygen compound and lithium element, the silicon-oxygen compound exists in the form of particles, and the lithium element is embedded in the silicon-oxygen compound particle. The siloxane particles include a solid inner core and a porous shell, and the surface of the porous shell is partially or completely covered by a carbon film layer. That is, the negative electrode active material provided by the present invention includes an inner core, a porous middle layer (ie, a porous shell layer of silicon oxide particles) and a carbon film layer.

The porous shell layer as the middle layer can effectively accommodate part of the volume expansion of silicon during the lithium intercalation process, release the stress inside the particles, and reduce the damage of the particles. At the same time, in the cycle process, it can effectively inhibit the repeated damage and rapid thickening of the SEI film, reduce the cycle expansion and internal resistance increase of the battery after multiple charge and discharge, and improve the service life and stability of the battery.

The existence of the solid core can improve the mechanical strength of the negative electrode active material particles and the volume ratio of the effective active material. On the one hand, it can avoid the rupture of the negative electrode active material particles during the rolling process of the pole piece, and on the other hand, it can also increase the volume of the material. Specific capacity and volume specific energy.

In the present invention, the specific surface area of the negative electrode active material particles may be 0.1-15 m ² /g, preferably 0.3-10 m ² /g, more preferably 0.3-6 m ² /g. Within the range of the specific surface area, side reactions on the surface of the negative electrode active material particles are less and the stability is higher.

In the present invention, the relative specific surface area of the negative electrode active material may be ≤5, preferably ≤3, more preferably ≤2, even more preferably ≤1.5. In the present invention, the specific surface area of the negative electrode active material is defined as A, the specific surface area of the silicon-containing material without a porous shell but other structures consistent with the negative electrode active material is defined as B, and the relative specific surface area is A/B. The "silicon-containing material without a porous shell but other structures consistent with the negative electrode active material" mentioned here means that the silicon-containing material is the same as the negative electrode active material of the present invention except that it does not have a porous shell.

In the present invention, the relative tap density of the negative electrode active material particles is defined as D, and optionally, the value of D satisfies D≥0.8, preferably D≥0.85. In the present invention, the tap density of the negative electrode active material particles is defined as D1, and the tap density of the silicon-containing material that does not have a porous shell but other structures are consistent with the negative electrode active material is defined as D2, and the relative tap density D= D1/D2. Within the range of the above-mentioned relative tap density, the volume ratio of the porous shell and the solid core in the particle is relatively balanced, which can not only realize the function of the porous shell to accommodate the volume expansion of the silicon negative electrode and reduce SEI damage, but also avoid the mechanical strength of the particle. The decline and volume specific energy loss.

In the present invention, the porous shell layer may contain micropores and/or mesopores, wherein micropores are pores with a diameter of less than 2 nm, and mesopores are pores with a diameter of 2-50 nm. The porous shell may also contain macropores, which are pores with a diameter of 50 nm or more.

In the present invention, the lithium element content in the silicon oxide compound particles may be 0.1-20wt%, preferably 2-18wt%, more preferably 4-15wt%.

In the present invention, the silicon element content of the silicon oxide compound particles may be 30-80wt%, preferably 35-65wt%, more preferably 40-65wt%, so the negative electrode active material of the present invention has a high reversible capacity.

Further, in the negative electrode active material particle of the present invention, the proportion of silicon element in the porous shell layer may be greater than the proportion of silicon element in the solid inner core. This element distribution is beneficial to improve the first effect and reversible capacity of the negative electrode active material particles.

Further, in the negative electrode active material particle of the present invention, the proportion of lithium element in the porous shell layer may be lower than that of lithium element in the solid inner core.

In the present invention, the median diameter of the silicon oxide compound particles may be 0.2-20 μm, preferably 1-15 μm, more preferably 3-13 μm.

The negative electrode active material particles of the present invention may also contain elemental silicon nanoparticles, which can be uniformly dispersed in the negative electrode active material particles. Wherein, the median diameter of the elemental silicon nanoparticles may be between 0.1-35 nm, preferably 0.5-20 nm, more preferably 1-15 nm. When the particle undergoes a cycle of intercalation and extraction of lithium ions, the particle expands less and is not easily broken, so that the lithium ion secondary battery using the material has a small cycle expansion and stable cycle.

In the present invention, the thickness of the carbon film layer may be 0.001-5 μm, preferably 0.005-2 μm, more preferably 0.01-1 μm. The existence of the carbon film layer can effectively improve the conductivity of the particles, reduce the contact resistance between the particles in the negative electrode sheet, the negative electrode sheet and the current collector, thereby improving the lithium-deintercalation efficiency of the material, reducing the polarization of the lithium-ion battery and promoting its cyclic stability.

Further, the proportion of the carbon film layer in the negative electrode active material particles may be 0.01-20wt%, preferably 0.1-15wt%, more preferably 1-12wt%.

Further, the coverage of the carbon membrane layer on the surface of the porous shell layer is greater than 95%, preferably greater than 98%. The higher the coverage of the carbon film layer on the surface of the porous shell, the more effective it is to isolate the direct contact between the porous shell and the electrolyte, and greatly reduce the adverse effects of the large specific surface area of the porous shell itself. The high coverage of the carbon film layer can also reduce the specific surface area of the negative electrode active material particles containing the porous shell layer, reduce the side reaction of the material and the electrolyte, and improve its stability in the battery.

In the present invention, the surface of the negative electrode active material particles may also include a coating layer, and the coating layer completely or partially covers the outside of the carbon film layer. The cladding layer may include organic compounds and/or metal oxo compounds. The coating layer can further isolate the contact between the porous shell layer and the electrolyte, thereby further reducing the specific surface area of the negative electrode active material particles containing the porous shell layer, reducing the side reactions between the material and the electrolyte solution, and improving its stability in the battery.

The metal oxygen-containing compound may be a composite oxide of metal and phosphorus. The metal may include one or more of lithium, titanium, magnesium, aluminum, zirconium, calcium, and zinc.

The negative electrode active material used in the battery provided by the present application has the electrochemical characteristics of high efficiency, high energy density and small expansion. The battery prepared by using the negative electrode active material has the advantages of high energy density, excellent cycle performance, small cycle expansion, good high temperature performance and the like.

【Preparation method of negative electrode active material】

Fig. 1 is a flow chart of the preparation of the negative electrode active material according to an exemplary embodiment of the present invention.

S101: Prepare silicon oxide particles.

The concrete process of preparation can adopt following steps to carry out:

First, in an inert gas atmosphere or under reduced pressure, the mixture of metal silicon powder and silicon dioxide powder is heated in the temperature range of 900 ° C to 1600 ° C to generate silicon oxide gas, and the mixture of metal silicon powder and silicon dioxide powder The molar ratio is set in the range of 0.5-1.5. The gas generated by the heating reaction of the raw materials will be deposited on the adsorption plate. When the temperature in the reaction furnace is lowered to below 100°C, the sediment is taken out, crushed and pulverized using equipment such as a ball mill, jet mill, etc., to obtain silicon oxide compound particles.

Silicon oxide particles include silicon oxide (silicon monoxide and/or silicon dioxide) material. In an exemplary embodiment of the present invention, the silicon-oxygen stoichiometric ratio in the silicon-oxygen compound particles may be 1:0.4-1:2, optionally 1:0.6-1:1.5, more preferably 1 :0.8-1:1.2. Of course, there may be other trace impurity elements besides silicon and oxygen.

S102: Coating a carbon film layer on the surface of the silicon oxide compound particles.

According to an exemplary embodiment, the silicon oxide compound may be a silicon oxide compound that has not been disproportionated, or a silicon oxide compound that has undergone a disproportionation heat treatment. Wherein, the disproportionation heat treatment temperature may be 600-1100°C, optionally 700-1000°C, more preferably 800-1000°C.

In the present invention, the carbon film layer can be directly obtained by chemical vapor deposition (CVD). The carbon source used in CVD is hydrocarbon gas, and the decomposition temperature of hydrocarbon gas may be 600-1100°C, preferably 700-1000°C, more preferably 800-1000°C.

The carbon film layer can also be obtained by first carrying out carbon reaction coating and then carrying out heat treatment and carbonization in a non-oxidizing atmosphere. The carbon reaction coating method can use any one of mechanical fusion machine, VC mixer, coating kettle, spray drying, sand mill or high-speed disperser. The solvent used for coating is water, methanol, ethanol, isopropyl Alcohol, n-butanol, ethylene glycol, ether, acetone, N-methylpyrrolidone, methyl butanone, tetrahydrofuran, benzene, toluene, xylene, N,N-dimethylformamide, N,N-dimethylformamide One or more combinations of acetamide and chloroform. The carbon reactive source may be one or more of coal tar pitch, petroleum pitch, polyvinyl alcohol, epoxy resin, polyacrylonitrile, polymethylmethacrylate, glucose, sucrose, polyacrylic acid, and polyvinylpyrrolidone. The equipment used for heat treatment and carbonization can be any one of rotary furnace, ladle furnace, roller kiln, pusher kiln, atmosphere box furnace or tube furnace. The heat treatment carbonization temperature can be 600-1100°C, preferably 700-1000°C, more preferably 800-1000°C, and the holding time is 0.5-24 hours. The non-oxidizing atmosphere may be provided by at least one of the following gases: nitrogen, argon, hydrogen or helium.

S103: Corroding the silicon oxide compound particles coated with the carbon film layer to form pores, so that the silicon oxide compound particles coated with the carbon film layer form a structure including a solid inner core and a porous shell layer.

Wherein, the surface of the porous shell layer is partially or completely covered by the carbon film layer.

The corrosive pore-forming agent used may include various acids, alkalis or oxidants used in combination. Among them, the acids that may be used include nitric acid, sulfuric acid, hydrochloric acid, hydrofluoric acid, perchloric acid, chloric acid, etc., the alkalis that may be used include sodium hydroxide, potassium hydroxide, barium hydroxide, etc., and the oxidants that may be used in combination include hydrogen peroxide wait. By adjusting the ratio and concentration of the corrosion pore-forming agent used, as well as the temperature, stirring speed and time during the reaction, porous shells with different pore sizes and different thicknesses (or volume ratios) can be obtained.

The step of corroding the silicon oxide particles to form holes is placed after the step of coating the carbon film layer, which is beneficial to obtain a carbon film layer with better quality and more complete coating. The coating rate of the carbon film layer on the particle surface is preferably above 95%, more preferably above 98%. In this way, a relatively complete and continuous carbon film covering layer can be formed on the particle surface.

After corroding and forming pores on the silicon oxide compound particles coated with the carbon film layer, the carbon film layer remains and completely covers the surface of the porous shell layer, which plays a role of isolation and protection for the porous shell layer. If the above steps are reversed and the carbon film layer is coated after the hole is made, it is difficult to form a continuous carbon film coating layer due to the unevenness and unevenness caused by the porosity of the surface, thus resulting in the coverage and protection of the carbon film layer. The isolation effect is greatly reduced. In addition, after the silicon oxide compound particle coated with the carbon film layer is corroded and pore-forming, the particle can be coated with carbon twice to further increase the coverage of the carbon film layer on the surface and further optimize the carbon film layer on the surface. Isolation and protection of the porous shell.

S104: Perform lithium doping on the silicon oxide compound particles formed by etching holes.

In the present invention, the doping (intercalation of lithium element) of the silicon-oxygen compound particles can be done by means of electrochemical doping, liquid phase doping and thermal doping. The doping atmosphere of the lithium element is a non-oxidizing atmosphere, and the non-oxidizing atmosphere is composed of at least one of nitrogen, argon, hydrogen or helium.

The method of inserting lithium element (lithium doping modification method) can be:

1) Electrochemical method

An electrochemical cell is provided, which includes four components of a bath, an anode electrode, a cathode electrode and a power supply, and the anode electrode and the cathode electrode are respectively connected to two ends of the power supply. Simultaneously, the anode electrode is connected to a lithium source, and the cathode electrode is connected to a container containing silicon oxide particles. The bath is filled with an organic solvent, and the lithium source (anode electrode) and the container containing silicon oxide particles (cathode electrode) are immersed in the organic solvent. After the power is turned on, due to the occurrence of electrochemical reaction, lithium ions are intercalated into the silicon oxide compound structure to obtain lithium-doped modified silicon oxide compound particles. Above-mentioned organic solvent can adopt ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl acetate, propyl acetate, ethyl propionate Solvents such as ester, propyl propionate, dimethyl sulfoxide, etc. In addition, the organic solvent also contains electrolyte lithium salt, such as lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium perchlorate (LiClO ₄ ), etc. can be used. The above-mentioned lithium source (anode electrode) can be lithium foil, or lithium compounds, such as lithium carbonate, lithium oxide, lithium hydroxide, lithium cobaltate, lithium iron phosphate, lithium manganate, lithium vanadium phosphate, lithium nickelate, etc.

2) Liquid phase doping method

Add lithium metal, electron transfer catalyst, and silicon oxide compound particles into an ether-based solvent, continuously stir and heat in a non-oxidizing atmosphere to maintain a constant temperature reaction until the lithium metal in the solution completely disappears. Under the action of electron transfer catalyst, metal lithium can be dissolved in ether-based solvent and form a coordination compound of lithium ions, which has a lower reduction potential, so it can react with silicon-oxygen compounds, and lithium ions enter the structure of silicon-oxygen compounds middle. The electron transfer catalyst includes biphenyl, naphthalene and the like. The ether-based solvent includes methyl butyl ether, ethylene glycol butyl ether, tetrahydrofuran, ethylene glycol dimethyl ether, and the like. The constant temperature reaction temperature is 25-200°C. The non-oxidizing atmosphere is provided by at least one of the following gases: nitrogen, argon, hydrogen or helium.

3) Thermal doping method

The silicon oxide compound particles are uniformly mixed with the lithium-containing compound, and then heat-treated in a non-oxidizing atmosphere. The lithium-containing compound includes lithium hydroxide, lithium carbonate, lithium oxide, lithium peroxide, lithium hydride, lithium nitrate, lithium acetate, lithium oxalate and the like. The mixing method adopts any one of 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 is any one of a rotary furnace, a ladle furnace, a liner furnace, a roller kiln, a pusher kiln, an atmosphere box furnace or a tube furnace. The heat treatment temperature is 400-850°C, preferably 550-800°C; the holding time is 1-12 hours; the heating rate is greater than 0.1°C per minute and less than 10°C per minute. The non-oxidizing atmosphere is provided by at least one of the following gases: nitrogen, argon, hydrogen or helium.

The step of intercalating the lithium element is carried out after coating the carbon film layer, which can inhibit the growth of silicon crystal grains in the silicon oxide compound during heat treatment. Thus, the nano-scale elemental silicon particles are evenly dispersed and fixed in the lithium silicate compound or silicon oxide matrix, which can effectively inhibit the expansion of silicon nanoparticles and prevent the silicon particles from gradually merging into larger particles during charging and discharging. The size of the particles, thereby reducing the expansion deformation of the battery during the cycle and reducing the electrical failure of the silicon material, so that the cycle expansion of the lithium-ion secondary battery using this material is small and the cycle is stable. In addition, the step of coating the carbon film layer is carried out before intercalating the lithium element, which is beneficial to obtain a carbon film layer with better quality and more complete coating.

Fig. 2 shows the preparation process of the negative electrode active material according to another exemplary embodiment of the present invention. Include the following steps:

S201: Prepare silicon oxide particles.

S202: Coating a carbon film layer on the surface of the silicon oxide compound particles.

S203: Doping the silicon oxide compound particles coated with the carbon film layer with lithium.

S204: Etching the doped silicon-oxygen compound particles to form pores, so that the doped silicon-oxygen compound particles form a structure including a core and a porous shell.

The embodiment shown in FIG. 2 differs from the embodiment shown in FIG. 1 only in the order of etching holes. In the embodiment shown in Figure 1, the carbon film layer is coated and then corroded to form holes, and then lithium doping is carried out. In the embodiment shown in Figure 2, the carbon film layer is coated, followed by lithium doping, and then corroded to form holes .

According to an exemplary embodiment, after the above steps are completed, a coating layer of an organic compound and/or a metal oxygen-containing compound may also be coated on the surface of the negative electrode active material particle. In the process of forming the coating layer, the coating layer can be formed on the surface of the negative electrode active material particles by a solid phase mechanical mixing method, a liquid phase mixing method, a liquid phase in situ growth method or a gas phase method. Wherein, when coating by the liquid-phase in-situ growth method, water-soluble or alcohol-soluble reactants are prepared into a solution with a certain concentration, and then a coating layer is grown in-situ on the surface of the negative electrode active material particles by the solution method. The vapor phase method can be selected from atomic layer deposition (ALD), physical vapor deposition, chemical vapor deposition, evaporation and the like. The step of forming the cladding layer may include a heat treatment step, the temperature of the heat treatment is not higher than 850°C, the holding time is 0.1-12 hours, and the atmosphere can be vacuum or non-oxidizing atmosphere. Wherein the non-oxidizing atmosphere includes at least one of nitrogen, argon, hydrogen or helium. In addition, the heat treatment temperature must not be higher than the heat treatment temperature for intercalating lithium elements.

[Characterization method of negative electrode active material]:

1. Material testing: The following equipment was used to characterize the negative electrode active materials prepared in each example and comparative example: Dandong BetterSize 2000 laser particle size analyzer was used to test the particle size distribution of the negative electrode active materials. A Hitachi SU8010 scanning electron microscope (SEM) was used to observe the surface morphology of the negative electrode active materials. The NOVA 4200e specific surface area tester of Quantachrome Instruments was used to test the specific surface area of the negative electrode active material. The test requirements for the specific surface area are as follows: use a sample tube to weigh the sample, use nitrogen gas, and use the multi-point method to test the specific surface area of the sample within the range of relative pressure p/p0=0.05-0.3.

The tap density of the obtained negative electrode material was tested by a Dandong Baite BT-301 tap density tester. Among them, the test requirements for tap density are as follows: prepare a 25ml measuring cylinder, fix the measuring cylinder on the equipment base, align the origin on the sample stage with the origin on the equipment, and tighten the base; then add 10-20 grams of sample powder, the powder mass Record it as m; try to keep the powder surface in the graduated cylinder in a horizontal state, and insert a rubber stopper at the mouth of the graduated cylinder; then vibrate the sample 3000 times according to the vibration frequency of 200Hz; after the test, if the upper surface of the sample in the graduated cylinder is horizontal , directly read V; if the surface of the sample in the measuring cylinder is not horizontal, read the highest point V1 and the lowest point V2, and take the average value V of the two; then obtain the tap density ρ=m/V of the sample.

2. Homogenization and pole piece production: take 30 parts of the above-mentioned negative electrode active material, 64 parts of artificial graphite, 2.5 parts of conductive additives, and 3.5 parts of binder, perform homogenization coating in an aqueous system, then dry and roll, A negative electrode sheet containing the negative electrode active material of the present application was obtained.

3. Full battery evaluation: The negative electrode sheet of the negative electrode active material prepared in each example and comparative example was cut, vacuum baked, wound together with the paired lithium cobalt oxide positive electrode sheet and separator, and packed into a corresponding size A certain amount of electrolyte is injected into the aluminum-plastic case and sealed with degassing. After formation, a lithium-ion full battery of about 3.2Ah is obtained. The efficiency, capacity, energy and cycle stability of the full battery at 0.2C were tested with a battery tester from Shenzhen Newwell Electronics Co., Ltd. In addition, the full battery was also tested for 10 days of high-temperature storage at 60°C with full charge, and the cold expansion rate of the system after high-temperature storage was tested to evaluate the high-temperature storage stability of the system. Among them, the test method of the cold expansion rate is: after the cell is fully charged at 60°C for 10 days, the cell is taken out and cooled for 2 hours, and the thickness of the test cell is d, where the initial fully charged thickness of the cell is d0 , the cold state expansion rate=(d-d0)/d0.

The present application will be further described below in conjunction with specific embodiments.

Example 1-1

Weigh 1000 g of silicon oxide particles with a median diameter of 6 μm (atomic ratio of silicon to oxygen is 1:1), and place them in a CVD furnace. Using acetylene as a carbon source, the coating reaction was carried out at 950°C to obtain silicon oxide compound particles coated with a relatively complete carbon film layer, wherein the coverage rate of the carbon film layer reached 95%, and the thickness of the carbon film layer was 20nm.

Next, configure a hydrofluoric acid solution with a concentration of 2mol/L, add the above-mentioned silicon-oxygen particles coated with a carbon film layer, and continue to react for 24 hours at a stirring speed of 300r/min to obtain silicon-oxygen particles with a porous shell. Among them, the porous shell layer is mainly macropores with a pore diameter greater than 400nm, and the carbon film layer remains intact (as shown in Figure 3(a) and Figure 3(b)).

Next, the hot doping method is used to do lithium metal doping, specifically: take the above particles and mix lithium-containing compounds (such as lithium oxide, lithium hydride, lithium hydroxide, lithium carbonate, etc.), and place the mixed powder under an argon atmosphere. For heat treatment, the temperature was raised to 720° C. for 3 hours at a heating rate of 3° C. per minute, and then cooled naturally to obtain a negative electrode active material coated with a carbon film and a porous shell.

The relative tap density of the negative electrode active material obtained in the above steps is 0.8, and the relative specific surface area (relative BET) is 3.

Take 30 parts of the above-mentioned negative electrode active material, 64 parts of artificial graphite, 3.5 parts of conductive additives, and 2.5 parts of binder, perform homogenate coating in an aqueous system, then dry and roll to obtain a silicon-containing negative electrode sheet. The pole piece can withstand a compacted density of at least 1.55-1.6 g/cm ³ , under this compacted density, the negative electrode active material particles in this embodiment will not be damaged by the rolling process.

In this embodiment, the evaluation results of the full battery containing the negative electrode active material are: the first cycle coulombic efficiency (FCE) of the full battery is 85.3%, the volumetric energy density at 0.2C is 801.6Wh/L, and the full battery is cycled The retention rate after 400 cycles is 79%, and the cell expansion rate after 400 cycles is 14.5%. The full battery has also been tested for 10 days of high-temperature storage at 60°C, and the cold expansion rate of the high-temperature storage is 4.6%.

Example 1-2

The silicon oxide compound particles were coated with a carbon film layer using a process similar to that of Example 1-1, wherein the carbon film layer had a coverage rate of 96% and a thickness of 40 nm.

Next, configure a hydrofluoric acid solution with a concentration of 12mol/L, add the above-mentioned silicon-oxygen particles coated with a carbon film layer, and continue to react for 1 hour at a stirring speed of 500r/min to obtain silicon-oxygen particles with a porous shell. The carbon film remains intact, and the existence of macropores cannot be seen from the outer surface of the particles (as shown in Figure 4). Then observe the cross-section of the porous shell of the particle, as shown in Figure 5, it can be seen that the material mainly contains mesopores of 10-30nm; through nitrogen adsorption and desorption test analysis, it is found that the material also contains a small amount of micropores .

Next, the above silicon oxide powder, metal lithium ribbon and biphenyl were added into a sealable glass container, and then methyl butyl ether was added and reacted with stirring under an argon atmosphere. After the reaction is completed and dried, the obtained powder is placed in an argon atmosphere for heat treatment, and the temperature is raised to 680°C at a rate of 5°C per minute, and then kept for 2 hours, and then naturally cooled to obtain a lithium-doped negative electrode activity. Material.

The relative tap density of the negative electrode active material obtained in the above steps is 0.97, and the relative specific surface area is 2.

Adopt the same method as embodiment 1-1 to make the negative electrode pole piece, this pole piece can bear at least 1.7g/cm The compacted density, under this ^compacted density, the negative electrode active material particle in the present embodiment can not be by destroyed by the rolling process.

In this embodiment, the evaluation results of the negative electrode active material full battery are: the FCE of the full battery is 86.1%, the volumetric energy density at 0.2C is 805Wh/L, and the retention rate of the full battery after 400 cycles is 81.2%. The cell expansion rate after 400 cycles is 13.6%. The high-temperature storage cold expansion rate of the full battery is 3.8%.

Example 1-3

Silicon oxide particles coated with carbon film were obtained by a process similar to that of Example 1-2.

Next, the silicon oxide compound coated with the carbon film was doped with lithium using a process similar to that of Example 1-2.

Next, prepare a hydrofluoric acid solution with a concentration of 1 mol/L, add the above-mentioned silicon-oxygen particles, and continue to react for 2 hours at a stirring speed of 300 r/min to obtain silicon-oxygen particles with a porous shell. The performance results of this material are shown in Table 1.

Example 1-4

The processes of Examples 1-4 are similar to those of Examples 1-3, except that the corrosion pore-forming process is adjusted, and the corrosion pore-forming agent is replaced with 2 mol/L sodium hydroxide solution.

Examples 1-5 to 1-7

Using a process similar to that of Example 1-1, a carbon-coated silicon oxide compound with a porous shell is obtained. By adjusting the concentration and reaction time of the corrosion pore-forming agent, it is possible to obtain porous compounds with different relative tap densities and porosities. Shell material. Before the lithium doping step, the material is subjected to secondary carbon coating treatment. The specific process is: after the material and coal tar pitch powder are dry mixed evenly, heated and stirred so that the coal tar pitch is evenly coated on the surface of the material, and then heated To 900 ℃ to carbonize coal tar pitch. After the second carbon coating treatment, the coverage of the carbon film on the surface of the material is further improved, which effectively isolates the direct contact between the porous shell and the electrolyte, and can also reduce the specific surface area of the negative electrode active material particles containing the porous shell.

Next, the material was doped with lithium using a process similar to that of Example 1-2.

Examples 1-8

A process similar to that of Examples 1-4 is adopted, except that the coverage of the carbon film layer of the silicon oxide compound in the first step is reduced to 90%.

Examples 1-9 and 1-10

Using a process similar to that of Example 1-1, only by adjusting the amount of pore-forming corrosive agent added and the reaction time, the proportion of the porous shell and the proportion of pores in the material are further increased, and the relative tap densities of the obtained products are respectively decreased. are 0.75 and 0.7. Although materials with a higher pore ratio can effectively reduce the volume expansion after lithium intercalation, the final volume specific capacity and volume specific energy of the material decrease due to the low volume ratio of effective active materials.

Comparative example 1-1

Using a process similar to that of Example 1-1, but omitting the step of preparing a porous shell layer, a lithium-containing silicon oxide compound coated with a carbon film layer without a porous shell layer was obtained. The full battery FCE of this negative electrode active material is 87%. However, due to the large volume expansion of the material, the thickness expansion of the battery is significant when the battery is fully charged, resulting in a decrease in its volumetric energy density at 0.2C to 790.1Wh/L. At the same time, after repeated expansion and contraction of the battery during multiple cycles, the battery has obvious deformation and diaphragm bonding failure, resulting in a retention rate of only 66% of the full battery after 400 cycles, and the expansion rate of the cell is only 66%. Up to 24.1%. In addition, its cold expansion rate for high-temperature storage is 3.4%.

Comparative example 1-2

Using a process similar to that of Examples 1-3, but omitting the step of coating the carbon film layer in the first step, a lithium-containing silicon oxide compound with a porous surface layer without a carbon film layer was obtained. The relative specific surface area of the negative electrode active material is as high as 30, so the FCE of the full battery is only 80.2%, and the volumetric energy density at 0.2C is only 754Wh/L. After 400 cycles, the retention rate of the battery is only 50%, and the battery core undergoes obvious lithium precipitation and expansion deformation, and the expansion rate is as high as 31.9%. At the same time, the cold expansion rate of its high-temperature storage is as high as 17.8%.

Comparative example 1-3

A process similar to that of Examples 1-3 is adopted, but the degree of pore-forming corrosion is further increased, so that the material does not have a solid inner core, but only a surface carbon film layer and a porous inner core. The relative tap density of the material decreased to 0.5 while the relative specific surface area increased to 15. The same method as in Example 1-1 was used to make the negative electrode sheet, which can only withstand a compaction density of 0.8g/cm ³ , and a larger compaction density will cause the particles to be broken during the rolling process. Therefore, the FCE of the full battery using this material is only 81.6%, and its volumetric energy density at 0.2C is only 748.9Wh/L. The retention rate of the battery after 400 cycles is only 59.8%, and the battery cell undergoes obvious lithium precipitation and deformation, and the expansion rate is as high as 27.3%. At the same time, the cold expansion rate of its high-temperature storage is as high as 12.3%.

Table 1

Examples 2-1 to 2-7

Using a process similar to that of Example 1-1 to Example 1-4, negative electrode active materials with porous shells having different carbon film coverages and relative tap densities were obtained. Then, different types of coating layers were coated on the material to obtain different negative electrode active materials of Examples 2-1 to 2-7, and their performance parameters are shown in Table 2.

Embodiment 2-8 to 2-11

The surfaces of the negative electrode active materials obtained in Example 1-1 to Example 1-4 are respectively coated with different types of coating layers to obtain the different negative electrode active materials of Examples 2-8 to 2-11, and their performance parameters are as shown in the table 2.

Table 2

Apparently, the above-mentioned embodiments are only examples for clearly illustrating the present invention, rather than limiting the implementation. For those of ordinary skill in the art, other changes or changes in different forms can be made on the basis of the above description. It is not necessary and impossible to exhaustively list all the implementation manners here. And the obvious changes or variations derived therefrom are still within the protection scope of the present invention.

Claims (22)

1. A negative electrode active material for batteries, characterized in that it includes negative electrode active material particles;

   The negative active material particles include silicon oxide particles, lithium elements embedded in the silicon oxide particles and a carbon film layer, the silicon oxide particles include an inner core and a porous shell, and the surface of the porous shell is partially or completely covered by the carbon film layer.
2. The negative electrode active material according to claim 1, characterized in that, the specific surface area of the negative electrode active material is 0.1-15 m ² /g, preferably 0.3-10 m ² /g, more preferably 0.3-6 m ² /g.
3. The negative electrode active material according to claim 1, characterized in that, the relative specific surface area of the negative electrode active material is ≤5, preferably ≤3, more preferably ≤2, further preferably ≤1.5.
4. The negative electrode active material according to claim 1, characterized in that, the relative tap density of the negative electrode active material particles is ≥0.8, preferably ≥0.85.
5. The negative electrode active material according to claim 1, characterized in that the content of lithium element in the silicon oxide compound particles is 0.1-20wt%, preferably 2-18wt%, more preferably 4-15wt%.
6. The negative electrode active material according to claim 1, characterized in that the content of silicon element in the silicon oxide compound particles is 30-80wt%, preferably 35-65wt%, more preferably 40-65wt%.
7. The negative electrode active material according to claim 1, characterized in that the proportion of silicon in the porous shell is higher than the proportion of silicon in the inner core.
8. The negative electrode active material according to claim 1, wherein the proportion of lithium element in the porous shell layer is lower than the proportion of lithium element in the inner core.
9. The negative electrode active material according to claim 1, characterized in that, the median diameter of the silicon oxide compound particles is 0.2-20 μm, preferably 1-15 μm, more preferably 3-13 μm.
10. The negative electrode active material according to claim 1, wherein the negative electrode active material particles also include elemental silicon nanoparticles, and the median diameter of the elemental silicon nanoparticles dispersed in the negative electrode active material particles is 0.1 Between -35nm, preferably 0.5-20nm, more preferably 1-15nm.
11. The negative electrode active material according to claim 1, characterized in that, the thickness of the carbon film layer is 0.001-5 μm, preferably 0.005-2 μm, more preferably 0.01-1 μm.
12. The negative electrode active material according to claim 1, wherein the carbon film layer accounts for 0.01-20wt% of the negative electrode active material particles, preferably 0.1-15wt%, more preferably 1-12wt% .
13. The negative electrode active material according to claim 1, characterized in that, the coverage of the carbon film layer on the surface of the porous shell layer is ≥95%, preferably ≥98%.
14. The negative electrode active material according to claim 1, further comprising a coating layer, the coating layer comprising organic compounds and/or metal oxygen compounds, and the surface of the carbon film layer is partially or completely covered by the coating layer. Cladding covers.
15. The negative electrode active material according to claim 14, characterized in that the metal oxygen-containing compound is a composite oxide of metal and phosphorus.
16. The negative electrode active material according to claim 15, wherein the metal comprises one or more of lithium, titanium, magnesium, aluminum, zirconium, calcium, and zinc.
17. An electrode, characterized by comprising the negative electrode active material according to any one of claims 1-16.
18. A battery, characterized by comprising the electrode according to claim 17.
19. A method for preparing a negative electrode active material for a battery, comprising the following steps:

    Coating a carbon film layer on the surface of silicon oxide particles;

    Corrosion and pore forming are carried out on the silicon oxide compound particles coated with the carbon film layer, so that the silicon oxide compound particles coated with the carbon film layer form a structure including an inner core and a porous shell layer; wherein, the porous shell layer The surface is partially or completely covered by the carbon film layer;

    Lithium doping is performed on the silicon oxide compound particles formed by etching holes.
20. The method according to claim 19, further comprising:

    A coating layer comprising organic compounds and/or metal oxygen compounds is formed on the surface of the carbon film layer.
21. A method for preparing a negative electrode active material for a battery, comprising the following steps:

    Coating a carbon film layer on the surface of silicon oxide particles;

    Lithium doping is performed on the silicon oxide compound particles coated with the carbon film layer;

    corroding the doped silicon-oxygen compound particles to form pores, so that the doped silicon-oxygen compound particles form a structure including a core and a porous shell; wherein, the surface of the porous shell is partially or completely covered by the covered by the carbon film layer.
22. The method according to claim 21, further comprising:

    A coating layer comprising organic compounds and/or metal oxygen compounds is formed on the surface of the carbon film layer.

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