A negative electrode material and an electrochemical device

Abstract

The application provides a negative electrode material, which comprises a silicon-carbon material, the silicon-carbon material contains silicon elements, carbon elements, oxygen elements and metal elements, the mass content of the silicon elements is a, and 10%≤a≤90% based on the mass of the silicon-carbon material, the mass content of the metal elements is x, and 0.01%<x<0.5%, and the metal elements comprise at least one of barium, calcium, magnesium, iron, aluminum and manganese. The application also provides an electrochemical device.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of lithium ion batteries, in particular to a negative electrode material and an electrochemical device. BACKGROUND

[0002] Silicon as a negative electrode material of lithium ion batteries has a theoretical specific capacity of up to 4200 mAh / g, and is therefore considered to be the next generation of lithium ion negative electrode material most likely to replace traditional graphite negative electrode material and improve the energy density of lithium ion batteries. However, during the charging and discharging process, as lithium ions are inserted into the silicon lattice, the silicon-silicon bond gradually breaks and the lithium-silicon bond gradually forms, and the silicon particles will undergo a huge volume expansion (300-400%) during the lithium insertion process. This huge volume expansion is inevitable during the lithium insertion process of silicon particles, and the volume change of silicon particles during the cycle process will cause the fragmentation of silicon particles, and then the electrode sheet will be pulverized, an unstable SEI film will be formed and rapid capacity decay will occur. Considering the many shortcomings of silicon as a negative electrode material, a large number of studies in the field mainly focus on solving the performance of silicon, while ignoring the problems in practical application.

[0003] In recent years, it has been found in the industrialization and scale-up production of silicon-carbon materials that the silicon-carbon material slurry produces a lot of hydrogen gas, which affects the subsequent coating and cold pressing process, which leads to the actual capacity and the first coulombic efficiency of the full battery design not corresponding to the capacity and the first coulombic efficiency after coating, so that the capacity of the full battery is unstable. At present, in order to stabilize the processing of silicon-carbon materials in water-based slurry, a coating method can be used, and the coating method is mainly physical coating. This coating method cannot form a close contact with the surface of the silicon-carbon material, and the coating layer is easy to fall off during slurry stirring, so it cannot play a coating effect and cannot effectively solve the problem of slurry gas production. SUMMARY

[0004] In order to solve the above problems, the purpose of the present application is to provide a negative electrode material and an electrochemical device using the same.

[0005] In a first aspect, the present application provides a negative electrode material, the negative electrode material comprising a silicon-carbon material, the silicon-carbon material comprising silicon elements, carbon elements, oxygen elements and metal elements, the mass content of the silicon elements being a, and 10%≤a≤90%, the mass content of the metal elements being x, and 0.01%<x<0.5%, the metal elements comprising at least one of barium, calcium, magnesium, iron, aluminum and manganese. When the silicon-carbon material comprises metal elements, the gas production of the silicon-carbon material can be reduced, the processing performance of the slurry can be improved, and the electrochemical device can obtain better cycle performance and expansion performance.

[0006] By means of the technical solution according to the present application, by reacting the passivated silicon-carbon material precursor with the hydroxide of the metal, a uniform insoluble salt layer is coated on the silicon-carbon material in situ, the coating layer is insoluble in water, so that when the silicon-carbon material is processed in an aqueous slurry, the contact between the nanosilicon in the silicon-carbon material and water can be effectively avoided, thereby avoiding the problem of hydrogen gas generated during slurry processing, so that the subsequent coating, cold pressing and other processes will not be affected due to gas generation.

[0007] According to some embodiments of the present application, the silicon-carbon material is immersed in water in a closed space with an air atmosphere for 48 hours, and the volume content of hydrogen gas is y, and y < 5%, measured based on the total volume of the gas in the closed space. In this case, it is indicated that the gas generation of the silicon-carbon material is reduced, and the reduced gas generation can reduce the negative impact on the cycle performance and swelling rate of the battery.

[0008] According to some embodiments of the present application, the silicon-carbon material has a coating layer on the surface, and the thickness of the coating layer is z, and 50 nm < z < 1000 nm. Preferably, the thickness is obtained by CROSS-SECTION test, in which the silicon-carbon material is cut and the thickness of the coating layer is measured. By chemically reacting to form a dense and uniform coating layer on the surface of the silicon-carbon material with a thickness in this range, the coating layer is in close contact with the internal silicon-carbon material and will not fall off during slurry stirring, thereby effectively avoiding the problem of gas generation during slurry processing. In this chemical reaction, the nanosilicon exposed on the surface will participate in the reaction, and the nanosilicon on the surface will be consumed in the reaction, which will slightly reduce the capacity of the silicon-carbon material, but also avoid the side reaction of the surface silicon and the electrolyte. At the same time, the reaction of the surface nanosilicon also avoids the problem of lack of buffer space for the nanosilicon during swelling, so as to improve the cycle performance and swelling performance of the electrochemical device, but an excessively thick coating layer will reduce the capacity of the silicon-carbon material.

[0009] According to some embodiments of the present application, the coating layer is a silicate layer.

[0010] According to some embodiments of the present application, the silicon-carbon material is in a powder form, and specifically, the average particle size of the silicon-carbon material according to the present application is 5 μm to 15 μm. When in this range, the side reaction is less when the silicon-carbon material contacts with the electrolyte, and the active ions can be better embedded, and specifically, the capacity is higher.

[0011] According to some embodiments of the present application, the mass content of the metal element and the mass content of the silicon element satisfy: 0.0001 ≤ x / a ≤ 0.05. When the above relationship is satisfied, the advantages of high energy density of the silicon negative electrode can be achieved, and at the same time, good cycle performance of the electrochemical device and processing performance of the slurry can be obtained

[0012] According to some embodiments of the present application, the silicon-carbon material precursor is passivated by oxygen to obtain the silicon-carbon material according to the present application, thus the silicon-carbon material according to the present application contains oxygen element.

[0013] According to some embodiments of the present application, the mass content of oxygen element is b, and 1% < b < 10% based on the mass of the silicon-carbon material. When the oxygen content of the silicon-carbon material is within this range, active ions are consumed to form an inert phase on the surface of the silicon-carbon material, which can effectively improve the expansion performance and thus improve the cycle life of the electrochemical device.

[0014] In the second aspect of the present application, the present application provides a method for preparing a negative electrode material, characterized in that the method comprises at least the following steps:

[0015] (i) providing a porous carbon skeleton and depositing silane gas on the porous carbon skeleton to obtain a carbon-silicon material precursor;

[0016] (ii) passivating the carbon-silicon material obtained from step (i) by oxygen to obtain a passivated carbon-silicon material precursor;

[0017] (iii) placing the passivated carbon-silicon material precursor obtained from step (ii) in an alkaline solution to obtain a silicon-carbon material, wherein the alkaline solution is an aqueous solution of a hydroxide of a metal element, and the metal element includes at least one of barium, calcium, magnesium, iron, aluminum and manganese.

[0018] According to some embodiments of the present application, the porous carbon skeleton in step (i) has a pore volume > 0.3 cc / g, and more than 60% of the pore volume is the pore volume of micropores and mesopores.

[0019] According to some embodiments of the present application, in step (i), the porous carbon skeleton is placed in a deposition reactor (such as a fluidized bed reactor) and a silicon-containing gas is introduced at a temperature of 400-900°C, wherein the silicon-containing gas contains 1% to 100% by volume of silane gas and the rest is inert gas, and the silane deposition is carried out for 1-12h under this condition.

[0020] According to some embodiments of the present application, the inert gas can be one or a combination of nitrogen, argon and helium.

[0021] According to some embodiments of the present application, in step (ii), after the deposition reaction is completed, the temperature is reduced to 30-200°C, and an oxygen-containing gas is introduced, wherein the oxygen-containing gas contains 1% to 30% by volume of oxygen and the rest is inert gas, and the passivation reaction occurs for 1-12h.

[0022] According to some embodiments of the present application, in the step (iii), the passivated carbon-silicon material is dispersed in an alkaline solution with a concentration of 0.001 M-0.2 M, stirred for 1-12 h at a temperature of 20-80 ℃, and then filtered with deionized water until the pH of the filtrate is 6-8, and the filter cake is baked in an oven at 70 ℃ for about 12 h to obtain the negative electrode material.

[0023] According to some embodiments of the present application, the silicon-carbon material can be used to prepare an aqueous negative electrode slurry, which can be coated on a current collector after sufficient stirring and then standing for 48 h without abnormal morphology caused by gas production, and has excellent processing performance.

[0024] In a third aspect of the present application, the present application provides an electrochemical device, which comprises a positive electrode, a negative electrode and an electrolyte, wherein the negative electrode comprises the negative electrode material according to the present application, the electrolyte comprises lithium hexafluorophosphate, and the mass content of lithium hexafluorophosphate is c based on the mass of the electrolyte, and 5%≤c≤33%, so that the electrochemical device has excellent cycle performance and rate performance.

[0025] According to some embodiments of the present application, the mass content x of the metal element in the silicon-carbon material and the mass content c of lithium hexafluorophosphate in the electrochemical device satisfy: 0003≤x / c≤0.1. When the content of the metal element is small, more silicon in the silicon-carbon material is directly exposed to the electrolyte to react with lithium hexafluorophosphate in the electrolyte, causing loss of active silicon, and on the other hand, consumption of lithium hexafluorophosphate or weakening of its function in the electrolyte, affecting the cycle performance and swelling performance of the battery. When the mass percentage content of the metal element in the silicon-carbon material and the content of lithium hexafluorophosphate satisfy the above relationship, the metal element can reduce the side reaction of the silicon-carbon material and lithium hexafluorophosphate, and improve the cycle performance and swelling performance of the battery.

[0026] According to some embodiments of the present application, the electrochemical device is a lithium ion battery, which has a cycle retention rate of more than 80% and / or a swelling rate of less than 10% when fully charged at a 3C charge rate for 800 cycles at 25 ℃. The electrochemical device has excellent cycle performance and swelling performance.

[0027] The negative electrode material provided by the present application can effectively improve the processing performance of the negative electrode slurry, reduce gas production, and the electrochemical device using the negative electrode material has significantly improved cycle performance, rate performance and swelling performance. DETAILED DESCRIPTION

[0028] For the purpose of clarity, technical solutions and advantages of the present application will be described below in detail with embodiments. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. The embodiments described herein are illustrative in nature and serve to provide an overall understanding of the present application. The embodiments of the present application should not be construed as a limitation of the present application. Based on the technical solutions provided by the present application and the embodiments given, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0029] For the purpose of clarity, technical solutions and advantages of the present application will be described below in detail with embodiments. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. The embodiments described herein are illustrative in nature and serve to provide an overall understanding of the present application. The embodiments of the present application should not be construed as a limitation of the present application. Based on the technical solutions provided by the present application and the embodiments given, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0030] Unless otherwise defined, all terms used in the present application have the meanings commonly understood by those skilled in the art. Unless otherwise specified, the values of each parameter mentioned in the present application can be measured by various measurement methods commonly used in the art (for example, can be tested according to the methods given in the examples of the present application).

[0031] The list of items connected by the terms "at least one of", "at least one", "at least one of" or other similar terms can mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A can include a single component or multiple components. Item B can include a single component or multiple components. Item C can include a single component or multiple components.

[0032] I. Negative electrode

[0033] The negative electrode includes a current collector and a negative active material layer located on the surface of the current collector, the negative active material layer including the negative electrode material of the first aspect. In some embodiments, the current collector includes: a copper foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, a polymer substrate coated with a conductive metal, or any combination thereof.

[0034] In some embodiments, the negative active material layer further includes a binder, which includes, but is not limited to, polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene butadiene rubber, acrylated styrene butadiene rubber, epoxy resin, or nylon, etc.

[0035] In some embodiments, the negative active material layer further includes a conductive agent, which includes, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, and mixtures thereof. In some embodiments, the carbon-based materials are selected from natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fibers, or any combination thereof. In some embodiments, the metal-based materials are selected from metal powders, metal fibers, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymers are polyphenylene derivatives.

[0036] The negative electrode of the present application can be prepared by known methods in the art. Typically, the negative active material and optional conductive agent (e.g., carbon materials such as carbon black and metal particles), binder (e.g., SBR), other optional additives (e.g., PTC thermistor material), etc. are mixed together and dispersed in a solvent (e.g., deionized water), stirred uniformly, and then uniformly coated on a negative current collector, and dried to obtain a negative electrode containing a negative electrode film. A metal foil or a porous metal plate, etc. can be used as the negative current collector.

[0037] II. Positive Electrode

[0038] The materials, compositions, and methods of making the same that can be used in the positive electrode of the embodiments of the present application include any of the techniques disclosed in the prior art.

[0039] In some embodiments, the positive electrode includes a current collector and a positive active material layer on the current collector.

[0040] In some embodiments, the positive active material includes a positive electrode material capable of absorbing and releasing lithium or sodium. The positive electrode material that releases lithium includes, but is not limited to, lithium cobaltate, lithium nickel cobalt manganese acid, lithium nickel cobalt aluminate, lithium manganate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxyphosphate, lithium iron phosphate, lithium titanate, and lithium-rich manganese-based materials. The positive electrode material that releases sodium can be at least one of transition metal layered oxides, sodium polyanion compounds, Prussian blue, Prussian white, etc.

[0041] In some embodiments, the positive active material layer further includes a binder and, optionally, a conductive material. The binder improves the binding of the positive active material particles to each other and also improves the binding of the positive active material to the current collector.

[0042] In some embodiments, the binder includes, but is not limited to, polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymer, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene butadiene rubber, acrylated styrene butadiene rubber, epoxy resin, or nylon, etc.

[0043] In some embodiments, the conductive material includes, but is not limited to, carbon-based material, metal-based material, conductive polymer, and mixtures thereof. In some embodiments, the carbon-based material is selected from natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material is selected from metal powder, metal fiber, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer is polyphenylene derivative.

[0044] In some embodiments, the current collector can include, but is not limited to, aluminum.

[0045] The positive electrode can be prepared by a method known in the art. For example, the positive electrode can be obtained by mixing the active material, the conductive material, and the binder in a solvent to prepare an active material composition, and coating the active material composition on the current collector. In some embodiments, the solvent can include, but is not limited to, N-methyl pyrrolidone. The electrochemical device of the present application has higher energy density, cycle performance, and can meet the application requirements.

[0046] III. Separation Film

[0047] According to some embodiments of the present application, the material and shape of the separation film are not particularly limited, and can be any of the techniques disclosed in the prior art. In some embodiments, the separation film includes a polymer or inorganic material formed of a material stable to the electrolyte of the present application, etc.

[0048] For example, the separation film can include a base layer and a surface treatment layer. The base layer is a non-woven fabric, a film, or a composite film having a porous structure, and the material of the base layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric, a ceglard composite film, or a polypropylene-polyethylene-polypropylene porous composite film can be used.

[0049] The surface treatment layer is provided on at least one surface of the base layer, and can be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic material.

[0050] The inorganic layer includes inorganic particles and a binder, the inorganic particles are selected from at least one of alumina, silica, magnesia, titania, hafnia, tin oxide, ceria, nickel oxide, zinc oxide, calcium oxide, zirconia, yttria, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder is selected from at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinylalkoxide, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene.

[0051] The polymer layer includes a polymer, and the material of the polymer is selected from at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinylalkoxide, polyvinylidene fluoride, and poly(vinylidene fluoride-hexafluoropropylene).

[0052] IV. Electrolyte

[0053] According to some embodiments of the present application, the electrolyte of the present application can include similar components to the technology disclosed in the prior art, wherein the electrolyte according to the present application preferably includes lithium hexafluorophosphate, and the mass content of lithium hexafluorophosphate is c based on the mass of the electrolyte, and 5%≤c≤33%. In some embodiments, the lithium battery electrolyte includes at least one of fluoroether, fluoroethylene carbonate, or ether nitrile. In some embodiments, the electrolyte further includes a lithium salt, the lithium salt includes lithium bis(fluorosulfonyl)imide and lithium hexafluorophosphate, the concentration of the lithium salt is 1 mol / L to 2 mol / L, and the molar ratio of lithium bis(fluorosulfonyl)imide to lithium hexafluorophosphate is 0.05 to 4.

[0054] In some embodiments, the sodium battery electrolyte includes an organic solvent and a sodium salt, etc., wherein the organic solvent can be at least one of EC, PC, DMC, DEC, EMC, EA, FEC, VC, etc.; the sodium salt can be at least one of NaClO4, NaPF6, NaBF4, NaFSI, NaTFSI, etc.

[0055] In some embodiments, the electrolyte can further include a non-aqueous solvent. The non-aqueous solvent can be a carbonate compound, a carboxylic acid ester compound, an ether compound, other organic solvents, or a combination thereof.

[0056] The carbonate compound can be a chain carbonate compound, a cyclic carbonate compound, a fluoro-carbonate compound, or a combination thereof.

[0057] Examples of the chain carbonate compound are diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), and combinations thereof. Examples of the cyclic carbonate compound are ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinyl ethylene carbonate (VEC), or combinations thereof. Examples of the fluoro carbonate compound are fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methyl ethylene carbonate, 1-fluoro-1-methyl ethylene carbonate, 1,2-difluoro-1-methyl ethylene carbonate, 1,1,2-trifluoro-2-methyl ethylene carbonate, trifluoromethyl ethylene carbonate, or combinations thereof.

[0058] Examples of the carboxylic acid ester compound are methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decalactone, valerolactone, methylvaleronolactone, caprolactone, methyl formate, or combinations thereof.

[0059] Examples of the ether compound are dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, or combinations thereof.

[0060] Examples of the other organic solvent are dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidinone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, and phosphoric acid ester, or combinations thereof.

[0061] V. Electrochemical device

[0062] The present application provides an electrochemical device including a negative electrode, a positive electrode, an electrolyte, and a separator.

[0063] In some embodiments, the electrochemical device of the present application includes, but is not limited to, a primary battery, a secondary battery, a fuel cell, a solar cell, or a capacitor of all kinds.

[0064] In some embodiments, the electrochemical device is a sodium ion battery.

[0065] In some embodiments, the electrochemical device is a lithium secondary battery.

[0066] In some embodiments, the lithium secondary battery includes, but is not limited to, a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

[0067] VI. Test Methods

[0068] 1. Silicate Layer Thickness Test (CROSS-SECTION Test)

[0069] The cross-section polisher uses an ion source to ionize inert gas to generate inert ions. After acceleration and focusing, the high-speed inert ions will impact the atoms or molecules on the surface of the sample to achieve ion polishing. After CP cutting, the sample is placed on a sample stage for SEM test. The instrument model is IB-09010CP, the ion acceleration voltage is 2-6 kV, and the gas used is argon. The CROSS-SECTION test can be used to test the thickness of the etching / silicate layer at the cross-section of the silicon-carbon material.

[0070] 2. Particle Size Test

[0071] About 0.02 g of the silicon-carbon material sample is added to a clean beaker, about 20 ml of deionized water is added, and a few drops of 1% surfactant are added to make the powder completely dispersed in the water. Ultrasonic cleaning for 5 minutes in a 120W ultrasonic cleaner, and then the particle size distribution is tested using a MasterSizer2000 to calculate the average particle size of the silicon-carbon material.

[0072] 3. Gas Production Test

[0073] The silicon-carbon material sample needs to be stored in a sealed bag. After mixing the unopened sample, 0.5 g of silicon-carbon material powder and 4 g of water are mixed in a 20 ml vial (the gas in the bottle is air at this time), ensuring that the powder does not float and is completely in the water phase. After sealing and standing for 48 h (± 0.5 h), the bottle is shaken clockwise for 30 s before testing.

[0074] 1 mL of the upper gas in the vial is extracted using a 2.5 mL gas sampling needle for GC-TCD testing. The testing equipment is Aglient 7890A, the sampling port temperature is 150°C, the constant flow mode is used for splitting, the carrier gas is helium, the chromatographic column flow rate is 8 ml / min, the split ratio is 2:1, the detector temperature is 200°C, the reference flow is 30 mL / min, and the tail gas flow is 4 mL / min. The testing principle is that when the sample enters the chromatographic column, the components are repeatedly distributed between the two phases. Due to the different adsorption capacities of the stationary phase for each component, after a certain column length, each component is separated from each other and exits the chromatographic column in the order of flow into the detector for detection. The chromatographic peak flow curve of each component is drawn on the recorder. By testing the peak area, the mass fraction of each component can be determined by external standard quantification.

[0075] 4. Metal Element Content Test

[0076] Weigh 0.1 g of silicon-carbon material sample in a PTFE beaker, add 10 ml of HNO3 and 2 ml of HF, place it on a heating plate at 220°C, heat to near dryness, but not completely dry, then add 10 ml of nitric acid to fully dissolve the sample, but not dry, then cool the dissolved sample to room temperature, then filter three times with single layer filter paper, then dissolve to 100 ml, then bring the liquid sample into the physical chemistry system with carrier gas, and then into the plasma in aerosol form, which is fully evaporated, dissociated, atomized, ionized and excited, emitting characteristic spectral lines of atoms, which are used to qualitatively determine the content of alkali elements according to the wavelength of the spectral line, and quantitatively determine the content of alkali elements according to the fact that the intensity of the spectral line is proportional to the concentration.

[0077] 5. Silicon content test

[0078] Take 0.1000 g of the silicon-carbon material and put it into a nickel crucible, add 1.5 g of KOH, cover the crucible cover, and slowly heat the muffle furnace to 400°C, with a temperature rising program of heating from room temperature to 300°C in 2 h and heating from 300°C to 400°C in 2 h, then start the cooling digestion program, and take out the crucible after the program ends; After the crucible is cooled to about 100°C, take out the material and place it in a beaker, slowly add 30 ml of boiling water, leach for 1 h, use tweezers to clean the crucible, control the volume to 50 ml, filter into a 400 ml beaker, after filtering, add 20 ml of concentrated nitric acid to neutralize the solution, so that the solution is acidic, after the test solution is cooled to room temperature, add solid potassium chloride to saturation under constant stirring, then continue to add 2 g of potassium chloride, add 10 mL of potassium fluoride solution, a white precipitate appears after adding potassium fluoride, age for 15 min, filter with medium-speed quantitative filter paper, wash the beaker and precipitate with 8 mL of potassium chloride solution each time, a total of three times, take out the filter paper and put it back into the original beaker, add 20 mL of potassium chloride ethanol solution, 10 drops of phenolphthalein, neutralize the residual acid with sodium hydroxide standard solution, carefully stir the filter paper and scrub the cup wall until the solution is light red, use a glass rod to stir the paper pulp as much as possible, so that the reaction is complete, add 200 m of neutralized boiling water to the cup, after boiling, add 10 drops of phenolphthalein, neutralize to light red with sodium hydroxide standard solution, titrate to light red with sodium hydroxide standard solution as the end point, record the titration consumption volume; The silicon content is calculated according to the following formula:

[0079] ω Si = (V-V0) x c x 0.0072 / m x 100%,

[0080] Wherein: c is the concentration of sodium hydroxide standard solution, mol / L; V is the volume of sodium hydroxide standard solution consumed by titration, V0 is the volume of sodium hydroxide standard solution consumed by blank; 0.0072 is the molar mass of 1 / 4 Si; m is the mass of the sample.

[0081] 6. O element content test

[0082] Oxygen element content test: take an appropriate amount of sample, accurate to 0.001 mg, use Shanghai Microscale Organic Element Analyzer to test, model: American Platinum Gold Series II 2400, test method is JY / T 0580-2020, more than twice parallel determination is carried out continuously according to the sample determination procedure until the adjacent two results meet the requirements of the analysis error. After the sample is determined, the standard substance should be determined again. If the result deviates from the range of instrument operation procedure, the sample should be re-determined. The oxygen element mass percentage of the sample is calculated according to the following formula: Where e is the absolute mass of hydrogen element in the sample obtained from the calibration curve, unit: mg, m is the mass of the sample, unit: mg, k is the calibration factor of oxygen element on the same day.

[0083] 7. Full battery test

[0084] 7.1 Cycle performance test

[0085] The test temperature is 25℃ / 45℃, charged to 4.4V at 0.7C constant current, charged to 0.025C constant voltage, and discharged to 3.0V at 0.5C after standing for 5 minutes. The capacity obtained by this step is the initial capacity, and the cycle test is carried out at 0.7C charge / 0.5C discharge. The capacity decay curve is obtained by comparing the capacity of each step with the initial capacity. The number of cycles at 25℃ until the capacity retention rate is 90% is recorded as the room temperature cycle performance of the battery, and the number of cycles at 45℃ until the capacity retention rate is 80% is recorded as the high temperature cycle performance of the battery. The cycle performance of the material is compared by comparing the cycle number in the above two cases.

[0086] 7.2 Discharge rate test

[0087] At 25℃, discharge to 3.0V at 0.2C, stand for 5min, charge to 4.45V at 0.5C, constant voltage charge to 0.05C, then stand for 5min, adjust the discharge rate, discharge test at 0.2C, 0.5C, 1C, 1.5C, 2.0C respectively, get discharge capacity respectively, compare the capacity obtained at each rate with the capacity obtained at 0.2C, and compare the rate performance by comparing the ratio of 2C to 0.2C.

[0088] 7.3 Swelling rate test of battery full charge

[0089] Test the thickness of fresh battery at half charge (50% state of charge (SOC)) with a screw micrometer, cycle to 400 cycles, the battery is in full charge (100% SOC) state, then test the thickness of the battery at this time with a screw micrometer, and compare it with the thickness of the fresh battery at initial half charge (50% SOC), so as to obtain the swelling rate of the battery at full charge (100% SOC) at this time.

[0090] Example 1

[0091] Silicon-carbon material

[0092] The silicon-carbon material of Example 1 was prepared according to the following steps:

[0093] - providing a porous carbon skeleton (Celanese-YP-50F 6 micron model) 100 g, wherein the porous carbon skeleton has a pore volume of 0.8 cc / g, wherein the pore volume of micropores and mesopores is 87% of the total pore volume;

[0094] - placing the porous carbon skeleton in a deposition reactor, for example as a fluidized bed reactor, at a temperature of 700°C, introducing a silicon-containing gas into the reactor to carry out a silane deposition reaction for 8h, based on the volume of the silicon-containing gas, the silicon-containing gas contains 25% by volume of silane gas, the rest is nitrogen, thereby obtaining a silicon-carbon material;

[0095] - after the completion of the deposition reaction, reducing the temperature to the range of 45°C, and then introducing an oxygen-containing gas into the reactor to carry out a passivation reaction, wherein, based on the volume of the oxygen-containing gas, the oxygen-containing gas contains 25% by volume of oxygen, the rest is nitrogen, and the introduction time is 8h, to obtain a passivated silicon-carbon material;

[0096] - taking 20g of the prepared passivated silicon-carbon material and dispersing it in 200ml of a barium hydroxide solution with a concentration of 0.005M, stirring the dispersion at a temperature of 25°C for 4h to obtain a dispersion;

[0097] - after the completion of the above reaction, the dispersion is filtered with deionized water until the pH of the filtrate is 7, and the obtained filter cake is baked in a 70°C oven for 12h to obtain a silicon-carbon material.

[0098] Lithium ion button cell

[0099] The lithium ion button cell of Example 1 was prepared according to the following steps:

[0100] The silicon-carbon material of Example 1, conductive carbon black and polymer were added to deionized water in a mass ratio of 80:10:10, a slurry was formed by stirring, the slurry was coated on a copper foil to form a negative electrode active coating with a thickness of 100μm, vacuum dried at 85°C for 12h, cut into a round piece with a diameter of 1cm in a dry environment to obtain a negative electrode, a piece of lithium metal was used as the positive electrode, and a cegland composite membrane was used as the separator membrane. After adding the electrolyte, a lithium ion button cell was assembled.

[0101] Lithium ion full cell

[0102] The lithium ion button cell of Example 1 was prepared according to the following steps:

[0103] - Preparation of the positive electrode: LiCoO2, conductive carbon black, and polyvinylidene fluoride (PVDF) were added to N-methylpyrrolidone in a mass ratio of about 95%:2.5%:2.5%, and after being mixed thoroughly, a positive electrode slurry was obtained. The obtained positive electrode slurry was coated on a positive electrode current collector aluminum foil, and after drying and cold pressing, a positive electrode was obtained;

[0104] - Preparation of the negative electrode: graphite, the silicon-carbon material according to Example 1, conductive carbon black, and a binder polyacrylic acid (PAA) were mixed in a weight ratio of about 70%:15%:5%:10%, and an appropriate amount of water was added so that the solid content was about 55 mass% to 70 mass%, and the viscosity of the slurry was adjusted to about 4000-6000 Pa·s, thereby obtaining a negative electrode slurry. The obtained negative electrode slurry was coated on a negative electrode current collector copper foil, dried, and cold pressed to obtain a negative electrode;

[0105] - Preparation of the electrolyte: propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC) (weight ratio of 1:1:1) were mixed to form a solvent under a dry argon environment, LiPF6 was added to the solvent and mixed thoroughly, wherein the concentration of LiPF6 was about 1.15 mol / L, and then about 12.5 mass% of fluoroethylene carbonate (FEC) was added and mixed thoroughly to obtain an electrolyte;

[0106] - Preparation of the separator: a porous polymeric film of polyethylene (PE) was used as a separator, and the porosity was 35%.

[0107] - Assembly: the positive electrode, the separator, and the negative electrode were stacked in order, with the separator between the positive electrode and the negative electrode to serve as a separator. A bare cell was obtained by winding, and the bare cell was placed in a housing, and then electrolyte was injected and packaged. After processes such as formation, degassing, and edge cutting, a lithium ion full battery of Example 1 was obtained.

[0108] Examples 2-23

[0109] Examples 2-23 were prepared according to the steps of Reference Example 1, wherein the preparation parameters are shown in Table 1.

[0110] Comparative Examples 1-8

[0111] Comparative Examples 1-8 were prepared according to the steps of Reference Example 1, wherein the preparation parameters are shown in Table 1.

[0112] Table 1

[0113]

[0114]

[0115] ​The mass content x of metal elements (barium and calcium), the mass content a of silicon element, the thickness z of silicate layer, and the mass content b of oxygen element of the silicon-carbon materials obtained from Examples 1-14 and Comparative Examples 1-3 were determined;

[0116] The silicon-carbon material powders obtained from Examples 1-14 and Comparative Examples 1-3 were subjected to gas production test, and the volume content y of hydrogen gas produced by the silicon-carbon material after being immersed in water for 48 hours was determined;

[0117] After the negative electrode slurries obtained from Examples 1-14 and Comparative Examples 1-3 were allowed to stand for 48 hours, it was determined whether abnormality occurred in the processing performance of the slurry, and the processing performance was classified into the following three types according to the observed morphology and performance

[0118] - Good: no obvious abnormality was observed

[0119] - General: the slurry produced gas, and the coating could be broken off;

[0120] - Poor: the slurry produced gas, and the coating could not be broken off;

[0121] The lithium ion button cells obtained from Examples 1-14 and Comparative Examples 1-3 were subjected to battery performance test, and the negative electrode gram capacity of the lithium ion button cells was obtained.

[0122] The lithium ion full cells obtained from Examples 1-14 and Comparative Examples 1-3 were subjected to battery performance test, including cycle performance test, discharge rate test, and battery swelling rate test.

[0123] The results of the above tests are summarized in Tables 2, 3, and 4.

[0124] Table 2

[0125]

[0126] As shown in the test results of Examples 1-6 and Comparative Examples 1-3 in Table 2, when the content of metal elements (barium and calcium) in the silicon-carbon material is increased, the amount of gas produced is obviously reduced, so that the battery can obtain better cycle performance and performance, while Comparative Example 1 takes less silicate layer coating, the improvement in gas production is not obvious, and there is a problem of gas production in slurry processing, resulting in poor cycle performance of the battery. The negative electrode active material obtained in Comparative Example 3 has a too high thickness of silicate layer, although the processing performance is good, but a lot of active silicon is lost, resulting in a low gram capacity, which is not conducive to the advantages of high energy density of silicon negative electrode. At the same time, the thick silicate layer is not conducive to the transmission of lithium ions and electrons, affecting the cycle performance of the lithium ion battery.

[0127] Table 3

[0128]

[0129] The test results of Examples 2, 7-14 are shown in Table 3. As can be seen from Comparative Examples 2 and 7-11, when the silicon content in the silicon-carbon material is appropriate, the high energy density of the silicon negative electrode can be utilized, and good cycle performance of the lithium ion battery and processing performance of the slurry can be obtained under the same other conditions. As can be seen from Comparative Examples 2 and 12-14, when the silicon content is the same, the higher the metal element (barium) content, the thicker the silicate layer, which can more significantly improve gas production and lead to optimized battery cycle performance and reduced cycle expansion rate.

[0130] Table 4

[0131]

[0132] The comparison of Examples 2, 15-19 and Comparative Examples 4-5 shows that when the oxygen content of the silicon-carbon material is increased, the oxygen content on the surface of the nanosilicon is also increased, and these increased oxygen contents consume active lithium, thereby forming an inert phase, which can effectively improve the expansion performance and thus the cycle life. As shown in Example 19, when the oxygen content is increased to 10%, the higher oxygen content can affect the initial coulombic efficiency, and the general expansion performance and cycle performance are improved. However, when the oxygen content is further increased, for example, the oxygen content is increased to 12% and 15% in Comparative Examples 4 / 5, the initial coulombic efficiency is reduced, and in this case, the excessively low initial coulombic efficiency cannot fully utilize the high energy density of the silicon-carbon material.

[0133] Table 5

[0134]

[0135]

[0136] As can be seen from Table 5, under the same conditions, when the content of lithium hexafluorophosphate in the electrolyte is increased, lithium hexafluorophosphate can fully play the role of lithium salt additive, act as a bridge for lithium ions between the positive and negative electrodes, and thus effectively improve the lithium ion transport kinetics and rate performance. In addition, abundant lithium salt can ensure excellent cycle performance. As shown in Example 21, when the content of lithium hexafluorophosphate is 22% by mass, good cycle and rate performance can be obtained, while as shown in Example 22, when the content of lithium hexafluorophosphate is increased to 33%, the excessive electrolyte viscosity will lead to a decrease in lithium ion transport kinetics, and at the same time, increase the electrolyte cost. The excessively low and high content of lithium hexafluorophosphate in Comparative Examples 7 and 8 both lead to undesirable cycle performance. In addition, as can be seen from the comparison of Examples 2, 23-25 and Comparative Example 6, the increase in the mass percentage of metal elements in the silicon-carbon material is beneficial to solving the gas production problem and improving the cycle performance and expansion performance. Comparative Example 6 contains only 0.005% of metal elements, which means that more silicon is directly exposed to the electrolyte and reacts with lithium hexafluorophosphate in the electrolyte. Thus, on the one hand, the generated hydrofluoric acid will further consume the nano-silicon, causing the loss of active silicon, and on the other hand, the consumption of lithium hexafluorophosphate or the weakening of its function in the electrolyte will affect the cycle performance and expansion performance.

Claims

1. A negative electrode material, characterized by, The negative electrode material includes silicon-carbon material, which contains silicon, carbon, oxygen, and a metallic element. The mass content of silicon is 'a', and 10% ≤ a ≤ 90% based on the mass of the silicon-carbon material. The mass content of the metallic element is 'x', and 0.01% < x < 0.5%. The metallic element includes at least one selected from barium, calcium, magnesium, iron, aluminum, and manganese. The silicon-carbon material has a silicate layer on its surface, the thickness of which is z, and 50 nm < z < 100 nm. 1000nm.

2. The negative electrode material according to claim 1, characterized in that, The silicon-carbon material was immersed in water for 48 hours in a closed space with an air atmosphere. The volume content of hydrogen gas was measured as y based on the total volume of gas in the closed space, and y < 5%.

3. The negative electrode material of claim 1, wherein, The average particle size of the silicon-carbon material is 5 μm to 15 μm.

4. The negative electrode material according to any one of claims 1 to 3, characterized in that, The mass content of the metal element and the mass content of the silicon element satisfy the following condition: 0.0001≤x / a≤0.

05.

5. The negative electrode material according to any one of claims 1 to 3, characterized in that, Based on the mass of the silicon-carbon material, the mass content of oxygen is b, and 1% < b < 10%.

6. A method for preparing a negative electrode material, characterized in that, The method includes at least the following steps: (i) A porous carbon framework is provided, and silane gas is deposited on the porous carbon framework to obtain a silicon-carbon material precursor; (ii) Passivate the silicon carbide precursor obtained in step (i) with oxygen to obtain a passivated silicon carbide precursor; (iii) The passivated silicon carbide precursor obtained in step (ii) is placed in an alkaline solution to obtain silicon carbide material, wherein the alkaline solution is an aqueous solution of a hydroxide of a metal element, including at least one of barium, calcium, magnesium, iron, aluminum and manganese.

7. An electrochemical device, characterized in that, The electrochemical device includes a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode comprises a negative electrode material according to any one of claims 1 to 5, the electrolyte contains lithium hexafluorophosphate, and the mass content of lithium hexafluorophosphate is c based on the mass of the electrolyte, and 5% ≤ c ≤ 33%.

8. The electrochemical device according to claim 7, characterized in that, The mass content x of the metal element in the silicon-carbon material and the mass content c of lithium hexafluorophosphate in the electrochemical device satisfy the following condition: 0003 ≤ x / c ≤ 0.

1.

9. The electrochemical device according to claim 7 or 8, characterized in that, The electrochemical device is a lithium-ion battery, which has a cycle retention rate of more than 80% and / or a thickness expansion rate of less than 10% when fully charged after 800 cycles at 25°C and 3C charging rate.

Images