Negative electrode material, secondary battery, and electronic device

Abstract

A negative electrode material, a secondary battery, and an electronic device, in which the negative electrode material includes silicon carbon particles having a surface region, the proportion of the number of oxygen atoms in the surface region being A1% based on the sum of the number of oxygen atoms, carbon atoms, and silicon atoms in the surface region, 10 < = A1 < = 35; and the surface region refers to a region which is 0-200nm away from the surface of the silicon-carbon particle. By introducing oxygen atoms to the surfaces of the silicon-carbon particles, the side reaction of the silicon-carbon particles can be reduced, the kinetics can be improved, and the expansion and rate capability of the secondary battery can be improved.

Description

Technical Field

[0001] The present application belongs to the field of energy storage technology, and specifically relates to a negative electrode material, a secondary battery, and an electronic device. Background Art

[0002] Secondary batteries are devices that use the reversibility of chemical reactions to store and release electrical energy. The core principle is that after a chemical reaction is converted into electrical energy, the electrical energy can be used to repair the chemical system, and then the chemical reaction can be used again to convert it into electrical energy. In secondary batteries, the choice of negative electrode material is crucial to battery performance. Currently, common negative electrode materials include carbon materials (such as graphite, soft carbon, hard carbon) and silicon-based materials. These materials each have their own advantages and disadvantages. For example, graphite has the advantages of low cost and good cycle stability, but its gram capacity is low; silicon-based materials have extremely high theoretical gram capacity, but their volume expands violently during lithium insertion and extraction, resulting in poor cycle performance.

[0003] In the practical application of secondary batteries, poor kinetics and expansion are two technical issues that require special attention. Poor kinetics mainly refers to the slow reaction rate of the battery during the charging and discharging process, which will lead to increased internal resistance, capacity loss and shortened cycle life of the battery. The expansion problem mainly occurs during the lithium insertion and extraction process of the negative electrode material. For example, the volume expansion of silicon-based materials during charging and discharging is drastic, which may cause structural damage such as pole piece delamination and diaphragm puncture, and then cause safety hazards such as short circuit and thermal runaway. Therefore, it is necessary to improve the kinetics and expansion of secondary batteries to increase the reaction rate and cycle stability of the battery and ensure the safety and long life of the battery. Summary of the Invention

[0004] In view of this, the present application provides a negative electrode material, a secondary battery and an electronic device, which can reduce the side reactions of the silicon-carbon particles and improve the kinetics by introducing oxygen atoms into the surface of the silicon-carbon particles, thereby improving the expansion and rate performance of the secondary battery.

[0005] In the first aspect, the present application provides a negative electrode material, which includes silicon-carbon particles. The silicon-carbon particles have a surface area. Based on the sum of the atomic numbers of oxygen atoms, carbon atoms, and silicon atoms in the surface area, the atomic number of oxygen atoms in the surface area accounts for A1%, 10≤A1≤35; the surface area refers to the area from 0nm to 200nm away from the surface of the silicon-carbon particles. The present application introduces oxygen atoms into the surface area of ​​the silicon-carbon particles, which can form Si-O bonds in the surface area. By controlling the atomic number ratio of oxygen atoms in the surface area, the reactivity of silicon atoms in the surface area can be reduced, and the side reactions of the silicon-carbon particles with the electrolyte can be reduced. Moreover, since the atomic number ratio of silicon atoms in the surface area is reduced, the volume expansion of nano-silicon in the surface area is reduced, and the excessive expansion of nano-silicon in the surface area causes it to overflow from the silicon-carbon particles, exacerbating the side reactions with the electrolyte. In addition, the surface area can also act as a buffer layer to absorb and relieve the expansion stress inside the silicon-carbon particles, reduce the risk of pulverization and breakage of the silicon-carbon particles, further reduce side reactions, and improve the expansion problem of the secondary battery. In addition, the Si-O bonds in the surface area combine with lithium ions to form a layer containing Li x SiO y The solid electrolyte interface film (SEI film) composed of silicon carbon particles can promote the diffusion of lithium ions in silicon carbon particles, improve the dynamics of silicon carbon particles, and thus improve the rate performance of secondary batteries.

[0006] In some embodiments, 15≤A1≤30. When the atomic ratio of oxygen atoms in the surface region of the silicon-carbon particles is regulated to fall within this range, side reactions of the silicon-carbon particles can be further reduced, and the number of silicon atoms in the surface region can be indirectly regulated, which helps reduce the expansion of nano-silicon in the surface region and promotes the diffusion of lithium ions, thereby balancing the expansion stress and dynamics within the silicon-carbon particles, further improving the expansion and rate performance of the secondary battery.

[0007] In some embodiments, the silicon-carbon particles comprise porous carbon and nano-silicon located within the pores of the porous carbon; a carbon layer is provided on the surface of the silicon-carbon particles. The carbon layer on the surface of the silicon-carbon particles can cooperate with oxygen atoms in the surface region, further reducing the side reactions between the silicon-carbon particles and the electrolyte, and can also alleviate the expansion stress inside the silicon-carbon particles, as well as improve the conductivity of the silicon-carbon particles, thereby improving the expansion and rate performance of the secondary battery. In addition, the carbon layer can also prevent the silicon-carbon particles from coming into contact with the air, reducing the risk of violent reactions between the nano-silicon inside the silicon-carbon particles and the air; at the same time, it can also enhance the stability of the silicon-carbon particles in the aqueous slurry, thereby improving the problem of gas generation when the silicon-carbon particles come into contact with water.

[0008] In some embodiments, the atomic number percentage of silicon atoms in the surface region is B1%, based on the sum of the atomic numbers of oxygen atoms, carbon atoms, and silicon atoms in the surface region, with 25≤B1≤50. Regulating the atomic number percentage of silicon atoms in the surface region to conform to the above range helps reduce the expansion of nano-silicon in the surface region, optimizes the composition of the SEI film, and promotes the diffusion of lithium ions, thereby balancing the expansion stress and dynamics within the silicon-carbon particles, further improving the expansion and rate performance of the secondary battery.

[0009] In some embodiments, the silicon-carbon particles have an interior region, where the interior region refers to a region of the silicon-carbon particles excluding the surface region, and the interior region satisfies at least one of the following conditions:

[0010] (1) Based on the sum of the atomic numbers of oxygen atoms, carbon atoms, and silicon atoms in the inner region, the atomic number of oxygen atoms in the inner region accounts for A2%, 0<A2≤8;

[0011] (2) Based on the sum of the atomic numbers of oxygen atoms, carbon atoms, and silicon atoms in the inner region, the atomic number of silicon atoms in the inner region accounts for B2%, and B2>B1.

[0012] By controlling the atomic ratio of oxygen atoms or silicon atoms in the interior of the silicon-carbon particles to conform to the above relationship and coordinating them with oxygen atoms on the surface, this method can, on the one hand, alleviate the volume expansion of the silicon-carbon particles during lithium insertion, and on the other hand, optimize the ion conductivity of the SEI film, promote the diffusion of lithium ions, improve the expansion and kinetics of the material, and thus improve the expansion and rate performance of the secondary battery. Furthermore, it helps to increase the initial coulombic efficiency of the negative electrode material.

[0013] In some embodiments, the thickness of the carbon layer is T nm, where 200 ≤ T × A1 ≤ 1500. When the carbon layer thickness and the ratio of oxygen atoms in the surface area meet the above relationship, the interfacial stability of the silicon-carbon particles can be further improved, while promoting more uniform adhesion of the SEI to the surface of the silicon-carbon particles, further improving the expansion and rate performance of the secondary battery.

[0014] The present application provides a method for preparing the aforementioned negative electrode material, comprising the following steps:

[0015] Step S1, keeping the porous carbon at T1°C for t1 h in a silane gas atmosphere, where the volume percentage of silane gas in the silane gas atmosphere is 5-100%; then keeping it at T2°C for t2 h in a carbon dioxide atmosphere, where the volume percentage of carbon dioxide in the carbon dioxide atmosphere is 1-10%, and the volume flow rate of the carbon dioxide atmosphere is 40-100 L / min; then keeping it at T2°C for t2 h in an acetylene gas atmosphere for surface passivation to obtain passivated silicon carbon; 420≤T1≤600, 6≤t1≤16; 450≤T2≤650, 0.2≤t2≤2;

[0016] Step S2: heating the passivated silicon carbon to T3°C in an inert gas atmosphere, and then keeping it in an acetylene gas atmosphere for t3h to obtain silicon carbon particles with a carbon layer on the surface, wherein the volume proportion of acetylene in the acetylene gas atmosphere is 20-50%; 480≤T3≤700, 1≤t3≤5.

[0017] This application uses carbon dioxide gas to introduce oxygen atoms into the surface region of silicon-carbon particles. By controlling the temperature and duration of the carbon dioxide atmosphere, the atomic ratio of oxygen atoms in the surface region can be regulated, thereby improving the expansion and rate performance of the secondary battery. This, combined with the carbon layer on the surface of the silicon-carbon particles, can further improve the expansion and rate performance of the secondary battery.

[0018] On the other hand, the present application also provides a secondary battery, including a positive electrode, a negative electrode and an electrolyte; the negative electrode includes a negative electrode collector and a negative electrode material layer arranged on at least one surface of the negative electrode collector; the negative electrode material layer includes the aforementioned negative electrode material or the negative electrode material prepared by the aforementioned preparation method.

[0019] In some embodiments, the electrolyte includes fluoroethylene carbonate and lithium difluorophosphate; based on the mass of the electrolyte, the mass proportion of fluoroethylene carbonate is S1%, the mass proportion of lithium difluorophosphate is S2%, 5≤S1≤20, 0.05≤S2≤0.95. When the electrolyte is regulated to include the above-mentioned mass proportions of fluoroethylene carbonate (FEC) and lithium difluorophosphate (LiPO2F2), fluoroethylene carbonate can add LiF and other components to the SEI film, thereby improving the toughness and self-healing ability of the SEI film. However, the viscosity of FEC is relatively high, which will affect the migration rate of lithium ions. Lithium difluorophosphate can react with the Si-O bonds on the surface of the silicon-carbon particles to add Li to the SEI film. x SiO y components, increasing the diffusion rate of lithium ions in silicon-carbon particles, and synergistically with FEC to form a more stable SEI film, reducing side reactions, thereby further improving the expansion and rate performance of secondary batteries.

[0020] In some embodiments, the electrolyte further comprises ethyl fluoroacetate; based on the mass of the electrolyte, the mass fraction of ethyl fluoroacetate is S3%, 1≤S3 / S1≤5; preferably, 1.5≤S3 / S1≤4. By controlling the mass fraction of ethyl fluoroacetate (FEA) in the electrolyte to fall within the aforementioned range, the present application can promote the desolvation process of lithium ions, improve the ability to insert and extract lithium ions, and, together with FEC and lithium difluorophosphate, optimize the toughness and stability of the SEI film, buffer the volume expansion of nano-silicon, and reduce side reactions, thereby further improving the expansion and rate performance of the secondary battery.

[0021] The present application also provides an electronic device comprising the aforementioned secondary battery. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical solutions and advantages of this application more clear, the following further describes this application in detail with reference to the embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0023] The present application provides a negative electrode material, which includes silicon-carbon particles. The silicon-carbon particles have a surface area. Based on the sum of the atomic numbers of oxygen atoms, carbon atoms, and silicon atoms in the surface area, the atomic number of oxygen atoms in the surface area accounts for A1%, 10≤A1≤35; the surface area refers to the area from 0nm to 200nm away from the surface of the silicon-carbon particles. By controlling the atomic number ratio of oxygen atoms in the surface area, the atomic number ratio of silicon atoms in the surface area can be indirectly reduced, which can reduce the volume expansion of nano-silicon in the surface area and reduce the risk of particle pulverization and breakage. In addition, because oxygen atoms can form Si-O bonds with silicon atoms, the reactivity of silicon atoms in the surface area is reduced to a certain extent, thereby helping to reduce side reactions between silicon-carbon particles and electrolytes and improving the expansion and rate performance of secondary batteries.

[0024] In some embodiments, 10≤A1≤35; preferably, 15≤A1≤30. For example, the value of A1 is 10, 12, 15, 16, 21, 23, 25, 29, 32, 33, 35, or a range consisting of two of these values. When the atomic ratio of oxygen atoms in the surface region of the silicon-carbon particles is adjusted to fit within this range, the expansion and rate performance of the secondary battery can be further improved.

[0025] In some embodiments, the silicon-carbon particles comprise porous carbon and nano-silicon within the pores of the porous carbon; and a carbon layer is disposed on the surface of the silicon-carbon particles. The carbon layer on the surface of the silicon-carbon particles can cooperate with oxygen atoms in the surface region to further improve the expansion and rate performance of the secondary battery.

[0026] In some embodiments, the atomic number fraction of silicon atoms in the surface region is B1%, based on the sum of the atomic numbers of oxygen atoms, carbon atoms, and silicon atoms in the surface region, with 25≤B1≤50. For example, the value of B1 is 25, 26, 30, 31, 35, 38, 40, 42, 46, 49, 50, or a range consisting of any two of these values. Adjusting the atomic number fraction of silicon atoms in the surface region to conform to the above range helps further improve the expansion and rate performance of the secondary battery.

[0027] In some embodiments, the silicon-carbon particles have an interior region, which refers to the region of the silicon-carbon particle excluding the surface region. Based on the sum of the atomic numbers of oxygen atoms, carbon atoms, and silicon atoms in the interior region, the atomic number of oxygen atoms in the interior region is A2%, where 0 < A2 ≤ 8; for example, the value of A2 is 1, 2, 3, 4, 5, 6, 7, 8, or a range consisting of any two of these values. By controlling the atomic number of oxygen atoms in the interior region of the silicon-carbon particles to conform to the above relationship, the present application can further improve the expansion and rate performance of the secondary battery.

[0028] In some embodiments, based on the sum of the atomic numbers of oxygen atoms, carbon atoms, and silicon atoms in the interior region, the atomic number ratio of silicon atoms in the interior region is B2%, where B2>B1. When the atomic number ratio of silicon atoms in the interior region and the surface region of the silicon-carbon particle meet this relationship, the expansion and rate performance of the secondary battery can be further improved, and the initial coulombic efficiency can also be increased.

[0029] In some embodiments, the thickness of the carbon layer is T nm, and 200 ≤ T×A1 ≤ 1500. For example, the value of T×A1 is 200, 285, 412, 565, 756, 895, 1008, 1106, 1317, 1435, 1500, or a range consisting of any two of these values. When the carbon layer thickness and the ratio of oxygen atoms in the surface area meet the above relationship, the interfacial stability of the silicon-carbon particles is further improved, while promoting more uniform adhesion of the SEI to the surface of the silicon-carbon particles, further improving the expansion and rate performance of the secondary battery.

[0030] The present application provides a method for preparing the aforementioned negative electrode material, comprising the following steps:

[0031] Step S1, keeping the porous carbon at T1°C for t1 h in a silane gas atmosphere, wherein the volume proportion of silane gas in the silane gas atmosphere is 5-100%, preferably 5-40%; then keeping the porous carbon at T2°C for t2 h in a carbon dioxide atmosphere, wherein the volume proportion of carbon dioxide in the carbon dioxide atmosphere is 1-10%, and the volume flow rate of the carbon dioxide atmosphere is 40-100 L / min; then keeping the porous carbon in an acetylene gas atmosphere at T2°C for t2 h for surface passivation to obtain passivated silicon carbon; 420≤T1≤600, 6≤t1≤16; 450≤T2≤650, 0.2≤t2≤2;

[0032] Step S2: heating the passivated silicon carbon to T3°C in an inert gas atmosphere, and then keeping it in an acetylene gas atmosphere for t3h to obtain silicon carbon particles with a carbon layer on the surface, wherein the volume proportion of acetylene in the acetylene gas atmosphere is 20-50%; 480≤T3≤700, 1≤t3≤5.

[0033] The porous carbon is subjected to silane vapor deposition at T1°C, and carbon dioxide gas is introduced after the silane vapor deposition, which can oxidize the surface area of ​​the silicon-carbon particles, and silicon-carbon particles are obtained after oxidation. Among them, increasing the holding temperature or time in the carbon dioxide atmosphere, or increasing the concentration or flow rate of the carbon dioxide atmosphere, can increase the atomic number ratio of oxygen atoms in the surface area. Controlling the number ratio of oxygen atoms in the surface area of ​​the silicon-carbon particles to conform to the above relationship can improve the expansion and rate performance of the secondary battery. In addition, increasing the holding time and temperature in acetylene gas can increase the thickness of the carbon layer on the surface of the silicon-carbon particles. The silicon-carbon particles cooperate with the carbon layer on their surface to further improve the expansion and rate performance of the secondary battery.

[0034] In this application, when the volume percentage of the active gas in the silane gas atmosphere, carbon dioxide atmosphere, and acetylene gas atmosphere is not 100%, the corresponding atmosphere is a mixed gas atmosphere of the active gas and the inert gas. The inert gas can be selected from at least one of nitrogen, argon, or other gases known in the art that do not negatively affect the target effect.

[0035] In the present application, the temperature and time of the insulation treatment in the carbon dioxide atmosphere and the acetylene gas atmosphere are both selected from T2°C and t2 h, but the specific conditions may be the same or different; the volume proportion of acetylene gas in the acetylene gas atmosphere of step S1 and step S2 is both selected from the range of 20 to 50%, but the specific volume proportion may be the same or different.

[0036] In the present application, the degree of oxidation of the internal region can be controlled by adjusting the temperature T2 during treatment in a carbon dioxide atmosphere, as well as regulating the concentration and flow rate of the carbon dioxide atmosphere, so that the atomic number of oxygen atoms in the internal region meets the above range, thereby further improving the expansion and rate performance of the secondary battery. For example, when the temperature T2 during heat preservation treatment in a carbon dioxide atmosphere is increased to meet 551≤T2≤650, and the volume ratio of carbon dioxide in the carbon dioxide atmosphere is increased to 7-10%, and the volume flow rate of the carbon dioxide atmosphere is 80-100L / min, carbon dioxide tends to react in the surface area of ​​the silicon-carbon particles and is difficult to enter the internal area of ​​the silicon-carbon particles. Therefore, the degree of oxidation of the internal region can be reduced, thereby reducing the atomic number of oxygen atoms in the internal region to A2%.

[0037] On the other hand, the present application also provides a secondary battery, including a positive electrode, a negative electrode and an electrolyte.

[0038] secondary batteries

[0039] The secondary battery of the present application is not particularly limited. It is classified into various categories according to the type of electron transport material. For example, when the electron transport material is lithium (Li, including ions), the secondary battery is a lithium-ion battery; when the electron transport material is sodium (Na, including ions), the secondary battery is a sodium-ion battery.

[0040] According to one embodiment of the present application, a secondary battery may include a battery cell and an electrolyte. The battery cell may include a packaging material and an electrode assembly disposed within the packaging material, and the electrolyte may be filled in an internal space formed by the packaging material. Depending on the shape of the packaging material, the battery cell may be prismatic, cylindrical, or pouch-type.

[0041] The electrode assembly includes a positive electrode, a negative electrode, and a separator, as well as other components known in the art in a secondary battery, which are not limited in this application. The separator may be located between the positive electrode and the negative electrode.

[0042] The present application does not particularly limit the preparation method of the secondary battery, and for example, the following steps may be included: stacking the positive electrode, the separator and the negative electrode in order, and winding, folding and other operations as needed to obtain an electrode assembly, placing the electrode assembly in a packaging material, injecting the electrolyte into the packaging material and sealing it to obtain a secondary battery.

[0043] positive electrode

[0044] In this application, there are no special restrictions on the positive electrode, as long as the purpose of this application can be achieved. The positive electrode includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector; the above-mentioned "positive electrode material layer provided on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be located on one surface of the positive electrode current collector along its own thickness direction, or on two surfaces of the positive electrode current collector along its own thickness direction. It should be noted that the "surface" here can be the entire area of ​​the positive electrode current collector surface, or it can be a partial area of ​​the positive electrode current collector surface. This application has no special restrictions, as long as the purpose of this application can be achieved.

[0045] The present application does not particularly limit the type, size, and shape of the positive electrode current collector, as long as it does not cause chemical changes in the battery cell and has conductivity. For example, the positive electrode current collector can be made of materials such as stainless steel, aluminum, nickel, titanium, calcined carbon, or a surface treatment of aluminum or stainless steel with carbon, nickel, titanium, or silver. In the present application, the positive electrode current collector may also contain non-metallic elements, for example, non-metallic elements including at least one of fluorine, phosphorus, boron, chlorine, silicon, and sulfur.

[0046] The positive electrode current collector may have an appropriate thickness as needed. Although not particularly limited, the positive electrode current collector may have a thickness in the range of 1 μm to 500 μm, or may have a thickness in the range of 1 μm to 300 μm, or may have a thickness in the range of 1 μm to 100 μm, or may have a thickness in the range of 1 μm to 50 μm, or may have a thickness in the range of 1 μm to 20 μm.

[0047] Unless otherwise specifically stated, the terms thickness (or height), width and length used in this application refer to average values ​​and can be measured by a measuring instrument that can measure thickness (or height), width and length respectively and according to a method in the art.

[0048] The positive electrode current collector may have fine concavo-convex shapes on its surface, thereby further enhancing the adhesion to the positive electrode material layer. For example, the positive electrode current collector may be in the form of one or more selected from a film, sheet, foil, mesh, porous body, foam, and non-woven fabric.

[0049] In the present application, the positive electrode material layer includes a positive electrode active material. The present application has no particular restrictions on the type of positive electrode active material, as long as the purpose of the present application can be achieved. For example, the positive electrode active material may include lithium nickel cobalt manganese oxide (LiNi 0.90 Co 0.05 Mn 0.05 At least one of lithium manganese phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium iron manganese phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel lithium manganese oxide, spinel lithium nickel manganese oxide and lithium titanate. In the present application, the positive electrode active material may further contain non-metallic elements, for example, non-metallic elements include at least one of fluorine, phosphorus, boron, chlorine, silicon or sulfur. In the present application, there is no particular restriction on the thickness of the positive current collector and the positive electrode material layer, as long as the purpose of the present application can be achieved.

[0050] In some embodiments, the positive electrode material layer may further include a positive electrode binder. The present application does not particularly limit the type of positive electrode binder, as long as the purpose of the present application can be achieved. For example, the positive electrode binder may include but is not limited to polyvinylidene fluoride, polytetrafluoroethylene, polyolefins, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, modified polyvinylidene fluoride, modified styrene-butadiene rubber (SBR), polyolefin ester, polyolefin alcohol, polyacrylic acid or polyurethane. In some embodiments, the polyolefin binder includes at least one of polyethylene or polypropylene.

[0051] In some embodiments, the positive electrode material layer may further include a conductive agent. The present application does not particularly limit the type of conductive agent in the positive electrode material layer, as long as the purpose of the present application can be achieved. In some exemplary embodiments, the conductive agent includes a carbon-based material, such as graphite such as natural graphite or artificial graphite, carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, or carbon fiber; conductive polymers, such as polyphenylene derivatives; conductive metal oxides, such as zinc oxide, titanium oxide, etc.; conductive whiskers, such as potassium titanate, etc.; or a mixture formed by any combination of these substances.

[0052] In the present application, the positive electrode material layer can be formed by coating a positive electrode slurry on at least one side of a positive electrode current collector and drying it. Rolling can be performed after drying as needed. The positive electrode slurry contains the above-mentioned positive electrode active material, a positive electrode binder, and a conductive agent. Furthermore, the positive electrode slurry may also contain a solvent. The present application does not particularly limit the type of solvent, as long as it can achieve the purpose of the present application. For example, N-methylpyrrolidone can be used as the solvent.

[0053] The present application does not impose any particular restrictions on the mass ratios of the positive electrode active material, conductive agent, and positive electrode binder in the positive electrode material layer. Those skilled in the art can select these mass ratios according to actual needs, as long as the purpose of the present application can be achieved. These mass ratios can apply known mass ratios.

[0054] negative electrode

[0055] This application does not impose any particular restrictions on the negative electrode, as long as the purpose of this application can be achieved. For example, the negative electrode includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. In this application, the negative electrode material layer can be disposed on one surface in the thickness direction of the negative electrode current collector, or on two surfaces in the thickness direction of the negative electrode current collector. It should be noted that the "surface" here can be the entire area of ​​the negative electrode current collector or a partial area of ​​the negative electrode current collector. This application does not impose any particular restrictions, as long as the purpose of this application can be achieved.

[0056] The present application does not particularly limit the type, size, or shape of the negative electrode current collector, as long as it does not cause chemical changes in the battery cell and is conductive. For example, the negative electrode current collector may be made of stainless steel, copper, nickel, titanium, calcined carbon, or a material that has been surface-treated with carbon, nickel, titanium, or silver.

[0057] The negative electrode current collector may have an appropriate thickness as needed. Although not particularly limited, the negative electrode current collector may have a thickness in the range of 1 μm to 500 μm, or in the range of 1 μm to 300 μm, or in the range of 1 μm to 100 μm, or in the range of 1 μm to 50 μm, or in the range of 1 μm to 20 μm, or in the range of 5 μm to 10 μm.

[0058] The negative electrode current collector may have fine concavo-convex shapes on its surface, thereby further enhancing the adhesion to the negative electrode material layer. For example, the negative electrode current collector may be in the form of one or more selected from a film, a sheet, a foil, a mesh, a porous body, a foam, and a non-woven fabric.

[0059] In the present application, the negative electrode material layer includes the aforementioned negative electrode material or the negative electrode material prepared by the aforementioned preparation method. In some embodiments, the negative electrode material may also include other materials, such as but not limited to carbon materials such as graphite (artificial graphite, natural graphite or graphitized carbon fiber) or amorphous carbon; metals such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys or Al alloys that can be alloyed with lithium or alloys formed with lithium; SiO β (0<β≤2), metal oxides that can be doped or dedoped with lithium, such as SnO, SnO2, vanadium oxide, lithium vanadium oxide, or alloys thereof with lithium; or Si-C composites or Sn-C composites, etc., containing the metal and carbon materials; or lithium titanate with spinel structure TiO2-Li4Ti5O 12 , and any one of them or a mixture of two or more thereof can be used. Specifically, the carbon material can use low-crystalline carbon and high-crystalline carbon, etc. Representative low-crystalline carbons are soft carbon and hard carbon. Examples of high-crystalline carbon can be amorphous, plate-like, flaky, spherical or fibrous natural graphite or artificial graphite, primary graphite, pyrolytic carbon, mesophase pitch-based carbon fibers, carbon microspheres (mesophase carbon microspheres), mesophase pitch, and high-temperature calcined carbons such as petroleum and coal-based coke (coke derived from petroleum or coal tar pitch).

[0060] The negative electrode material layer in the present application also includes a negative electrode binder. The present application does not particularly limit the type of negative electrode binder, as long as the purpose of the present application can be achieved. For example, the negative electrode binder can include but is not limited to, and can be selected from polyvinylidene fluoride (PVDF), polyvinyl alcohol, styrene butadiene rubber (SBR), polyethylene oxide, carboxymethyl cellulose (CMC), hydroxypropyl cellulose, cellulose acetate, diacetyl cellulose, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, polymethyl acrylate, polymethyl methacrylate, polybutyl acrylate, polyacrylonitrile, polyvinyl pyrrolidone, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyarylate, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymer containing ethylene oxide, polyurethane, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, styrene butadiene rubber, acrylated styrene butadiene rubber, epoxy resin or nylon at least one.

[0061] The present application does not particularly limit the type of conductive agent in the negative electrode material layer, as long as it can achieve the purpose of the present application. In some exemplary embodiments, the conductive agent includes a carbon-based material, such as graphite such as natural graphite or artificial graphite, carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, or carbon fiber; a metal-based material, such as metal powder or metal fiber of copper, nickel, aluminum, silver, etc.; a conductive polymer, such as a polyphenylene derivative; a conductive metal oxide, such as zinc oxide, titanium oxide, etc.; a conductive whisker, such as potassium titanate, etc.; or a mixture formed by any combination of these substances.

[0062] The present application does not particularly limit the mass ratios of the negative electrode material, negative electrode conductive agent, and negative electrode binder in the negative electrode material layer. Those skilled in the art can select them according to actual needs, as long as the purpose of the present application can be achieved. These mass ratios can apply known mass ratios.

[0063] In the present application, the negative electrode material layer can be formed by coating a negative electrode slurry on at least one side of a negative electrode current collector and drying it. Rolling can be performed after drying as needed. The negative electrode slurry contains the aforementioned negative electrode material and a negative electrode binder, and may further contain a conductive agent as needed. Furthermore, the negative electrode slurry may also contain a solvent. The present application does not particularly limit the type of solvent, as long as it can achieve the objectives of the present application. For example, deionized water can be used as the solvent.

[0064] diaphragm

[0065] The present application does not particularly limit the separator, as long as it can achieve the purpose of the present application. For example, the separator material may include, but is not limited to, at least one of polyethylene (PE), polypropylene (PP)-based polyolefins (PO), polyesters (such as polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, and aramid; the separator type may include at least one of woven membranes, non-woven membranes, microporous membranes, composite membranes, rolled membranes, and spun membranes.

[0066] According to some embodiments of the present application, the separator may include a substrate layer and a surface treatment layer. The substrate layer may be a non-woven fabric, a film, or a composite film having a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Optionally, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric, or a polypropylene-polyethylene-polypropylene porous composite film may be used.

[0067] Optionally, a surface treatment layer is provided on at least one surface of the substrate layer, and the surface treatment layer can be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer with an inorganic substance. For example, the inorganic layer includes inorganic particles and a binder, and the present application has no particular restrictions on the inorganic particles, and can include, for example, at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The present application has no particular restrictions on the binder, and can include, for example, at least one of the above-mentioned positive electrode binder or negative electrode binder. The polymer layer contains a polymer, and the present application has no particular restrictions on the polymer, and can include, for example, at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinyl pyrrolidone, polyvinyl ether or polyvinylidene fluoride and poly(vinylidene fluoride-hexafluoropropylene). In the present application, the thickness of the separator is not particularly limited as long as the purpose of the present application can be achieved. For example, the thickness of the separator can be 5 μm to 500 μm.

[0068] electrolyte

[0069] In the present application, the electrolyte refers to a medium that causes the movement of electron transport substances to smoothly carry out the electrochemical reaction of the positive electrode and the negative electrode. The electrolyte can use commonly used organic liquid electrolytes, inorganic liquid electrolytes, gel-type polymer electrolytes, molten inorganic electrolytes, etc., but is not limited thereto. Solid electrolytes such as gel-type polymer electrolytes can also be used instead of the electrolyte. Batteries using solid electrolytes are usually called solid-state batteries or all-solid-state batteries. The liquid electrolyte (electrolyte) usually contains a non-aqueous solvent and a lithium salt.

[0070] In some embodiments, the electrolyte includes fluoroethylene carbonate and lithium difluorophosphate; based on the mass of the electrolyte, the mass proportion of fluoroethylene carbonate is S1%, the mass proportion of lithium difluorophosphate is S2%, 5≤S1≤20, and 0.05≤S2≤0.95. Exemplarily, the value of S1 is 5, 6, 8, 10, 11, 13, 14, 15, 18, 19, 20, or a value within a range consisting of any two of these values; the value of S2 is 0.05, 0.13, 0.24, 0.33, 0.41, 0.53, 0.64, 0.71, 0.75, 0.80, 0.93, 0.95, or a value within a range consisting of any two of these values. When the electrolyte is regulated to include fluoroethylene carbonate (FEC) and lithium difluorophosphate (LiPO2F2) in the above-mentioned mass proportions, the synergistic cooperation of the two can improve the ion diffusion capacity and stability of the SEI film, thereby further improving the expansion and rate performance of the secondary battery.

[0071] In some embodiments, the electrolyte further includes ethyl fluoroacetate; based on the mass of the electrolyte, the mass proportion of ethyl fluoroacetate is S3%, 1≤S3 / S1≤5; preferably, 1.5≤S3 / S1≤4. Exemplarily, the value of S3 / S1 is 1.5, 1.8, 2.0, 2.2, 2.4, 2.8, 3.1, 3.3, 3.6, 3.8, 4 or a value within the range of any two of these values. The present application controls the mass proportion of ethyl fluoroacetate (FEA) in the electrolyte to conform to the above range, thereby optimizing the toughness and stability of the SEI film together with FEC and lithium difluorophosphate, and improving the ion transport capacity, thereby further improving the expansion and rate performance of the secondary battery.

[0072] According to some embodiments of the present application, the lithium salt may also include, but is not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(oxalatoborate) {LiB(C2O4)2, LiBOB}, lithium difluorooxalatoborate {LiBF2(C2O4), LiDFOB}, LiNO3, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiC(SO2CF3)3, Li2SiF6, or lithium difluoroborate. The present application does not impose any restrictions on the content of the lithium salt in the electrolyte, as long as the purpose of the present application can be achieved.

[0073] The application has no particular restrictions on non-aqueous solvents. For example, non-aqueous solvents may include but are not limited to at least one of carbonate compounds, carboxylate compounds, ether compounds or other organic solvents. Above-mentioned carbonate compounds may include but are not limited to at least one of linear carbonate compounds, cyclic carbonate compounds or fluorocarbon compounds. Above-mentioned linear carbonate compounds may include but are not limited to at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate or ethyl methyl carbonate (EMC). Above-mentioned cyclic carbonate may include but are not limited to at least one of propylene carbonate (PC), butylene carbonate or vinyl ethylene carbonate. The fluorinated carbonate compound may include, but is not limited to, at least one of 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The carboxylate compound may include, but is not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanoic acid lactone, valerolactone, or caprolactone. The above-mentioned ether compound may include but is not limited to at least one of 1,3-dioxolane (DOL), ethylene glycol dimethyl ether (1,2-dimethoxyethane, DME), dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran or tetrahydrofuran.

[0074] electronic devices

[0075] The present application further provides an electronic device comprising the aforementioned secondary battery. Based on the aforementioned secondary battery, the electronic device of the present application can adapt to the conditions of rapid charge and discharge and achieve excellent service life and safety.

[0076] The electronic device of the present application is not particularly limited and can be any electronic device known in the art. For example, the electronic device can include, but is not limited to, a laptop computer, a pen-type computer, a mobile computer, an electronic book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a headset, a video recorder, an LCD television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0077] Measurement method

[0078] The present application can be measured using the following methods, and the physical properties in the following examples and comparative examples are measured using the following methods.

[0079] Atom number ratio test

[0080] Argon ion sputtering technology is used to etch silicon carbon particles from the surface, and the etching depth is controlled by controlling the etching time. XPS is used to test the ratio of the number of atoms in areas with different etching depths. The XPS testing equipment is the ESCLAB250Xi from Thermo Fisher Scientific, and Al is used as the target material as the excitation source. The etching depth is the distance from the surface of the silicon carbon particles. When the etching depth is 0nm, no etching is performed, and the ratio of the number of atoms on the surface of the silicon carbon particles is tested. When testing the surface area, the ratio of the number of atoms at three positions with an etching depth of 0nm, 100nm, and 200nm is tested respectively, and then the arithmetic average is calculated as the ratio of the number of atoms in the surface area. When testing the internal area, the ratio of the number of atoms at three positions with an etching depth of 250nm, 350nm, and 450nm is tested respectively, and then the arithmetic average is calculated as the ratio of the number of atoms in the internal area.

[0081] Carbon layer thickness test

[0082] Silicon-carbon particles with a carbon layer on their surfaces were sliced ​​and polished using a focused ion beam (FIB) to produce FIB samples. The thickness of the carbon layer on the FIB samples was then measured using a transmission electron microscope (TEM).

[0083] Cyclic expansion performance test

[0084] Place the lithium-ion full battery in a constant temperature box at 25°C ± 1°C for 30 minutes, charge it at a constant current of 0.5C to 4.35V, then charge it at a constant voltage of 4.35V to 0.025C, let it rest for 5 minutes, and then discharge it at 0.5C to 3.0V. This is one charge and discharge cycle. Record the initial thickness H0 of the lithium-ion full battery. Then, cycle it 600 times according to the above cycle process. Record the thickness H1 after the 600th cycle.

[0085] Cycle expansion ratio = H0 / H1×100%.

[0086] Rate performance test

[0087] The lithium-ion full battery was left standing at a test temperature of 25°C for 5 minutes, then constant-current charged to 4.35V at a current of 0.5C, and then charged to 0.025C at a constant voltage of 4.35V; left standing for 5 minutes, and then discharged to 3.0V at a constant current of 0.2C, and the 0.2C discharge capacity was recorded; then left standing for 5 minutes, the above charging process was repeated, and then the battery was discharged at a constant current of 2C, and the 2C discharge capacity was recorded.

[0088] Rate capacity retention rate = 2C discharge capacity / 0.2C discharge capacity × 100%.

[0089] The following uses lithium-ion batteries as an example to illustrate the solution of this application in conjunction with the following specific embodiments. Unless otherwise specified, the raw materials used in the following examples are all from common commercial products, and the devices or equipment used are all purchased from conventional market sales channels.

[0090] Example 1-1

[0091] Preparation of negative electrode materials:

[0092] Step S1, take porous carbon and place it in a fluidized bed equipment, introduce nitrogen to make it completely fluidized, and then gradually heat it to 500℃ at 5℃ / min; after the temperature in the fluidized bed stabilizes, introduce a silane gas atmosphere with a volume share of 30%, and keep it warm for 13 hours. Then adjust the temperature to 450℃ under nitrogen atmosphere, switch to carbon dioxide atmosphere and keep it warm for 0.2h, wherein the volume share of carbon dioxide in the carbon dioxide atmosphere is 5%, and the volume flow rate of the carbon dioxide atmosphere is 60L / min. Then switch to acetylene gas atmosphere with a volume share of 35%, keep it warm at 450℃ for 1h for surface passivation, and then cool it to room temperature;

[0093] Step S2: Transfer the cooled passivated silicon-carbon to a rotary kiln, introduce nitrogen, and heat the kiln to 550°C at a rate of 10°C / min. Then, introduce an acetylene gas atmosphere with a volume fraction of 35%. After maintaining the temperature for 1 hour, the acetylene gas is turned off and the temperature is lowered to obtain silicon-carbon particles having a carbon layer on the surface. The remaining components of the silane gas atmosphere, the carbon dioxide atmosphere, and the acetylene gas atmosphere are inert gases, all of which are nitrogen.

[0094] Preparation of negative electrode:

[0095] Silicon-carbon particles (20 wt%) with a carbon layer on their surface, artificial graphite (76 wt%), carbon nanotubes (0.8 wt%), lithium carboxymethyl cellulose (0.6 wt%), and polyacrylic acid (2.6 wt%) were mixed, then deionized water was added and stirred to form a negative electrode slurry. This negative electrode slurry was evenly coated on one surface of a copper foil, dried, and repeated on the other surface of the foil to obtain a negative electrode sheet coated on both sides with a negative electrode material layer. The coated copper foil was dried, pressurized, and cut into negative electrode sheets measuring 76.6 mm x 875 mm. The tabs were then welded to the negative electrode.

[0096] Preparation of positive electrode:

[0097] Lithium cobalt oxide (97.4 wt%), conductive carbon black (0.6 wt%), polyvinylidene fluoride (1.5 wt%), and carbon nanotubes (0.5 wt%) were mixed and then stirred with N-methylpyrrolidone to prepare a positive electrode slurry. The positive electrode slurry was evenly coated on one surface of aluminum foil. After drying, the coating process was repeated on the other surface of the foil to obtain a positive electrode sheet coated on both sides with a positive electrode material layer. The coated aluminum foil was dried, pressurized, and cut into positive electrode sheets with a size of 74 mm x 867 mm. The tabs were welded to obtain the positive electrode.

[0098] Preparation of diaphragm: A 12 μm thick polyethylene (PE) microporous membrane was selected as the diaphragm.

[0099] Preparation of electrolyte:

[0100] In an argon atmosphere glove box with a water content of less than 10 ppm, dimethyl carbonate, diethyl carbonate, and ethylene carbonate (mass ratio 1:1:1) were mixed to form a base solvent. Lithium hexafluorophosphate and fluoroethylene carbonate were then added to the base solvent and mixed thoroughly to form an electrolyte. Based on the mass of the electrolyte, lithium hexafluorophosphate accounted for 12.5% ​​by weight, fluoroethylene carbonate accounted for 14% by weight, and the remainder was the base solvent.

[0101] Battery production:

[0102] The positive electrode, separator, and negative electrode are stacked in order, with the separator placed between the positive and negative electrodes to act as an isolater. The electrode assembly is wound to obtain an electrode assembly, which is placed in an outer packaging aluminum-plastic film. After baking, the above-mentioned electrolyte is injected. After vacuum packaging, standing, formation, shaping, capacity testing and other processes, a lithium-ion battery is obtained.

[0103] Example 1-2 to Example 1-21, Comparative Example 1-1 to Comparative Example 1-5

[0104] The only difference from Example 1-1 is that the atomic number ratio of oxygen atoms and silicon atoms in the surface area and / or internal area, as well as the thickness of the carbon layer are regulated according to Table 1. The specific control parameters and performance test results are shown in Table 1. Among them, when the holding temperature of the porous carbon in the carbon dioxide atmosphere after silane vapor deposition is controlled to meet 450≤T2≤550, and the volume ratio of carbon dioxide in the carbon dioxide atmosphere is 1-6.9%, and the volume flow rate of the carbon dioxide atmosphere is 40-79L / min, oxygen atoms can be uniformly introduced into the surface area and internal area of ​​the silicon-carbon particles. By extending the holding time t2, the atomic number ratio of oxygen atoms in the surface area and the internal area can be increased. When 551≤T2≤650 is met, and the volume ratio of carbon dioxide in the carbon dioxide atmosphere is 7-10%, and the volume flow rate of the carbon dioxide atmosphere is 80-100L / min, it can promote more oxidation of oxygen atoms in the surface area and reduce the number of oxygen atoms in the internal area. In addition, the thickness of the carbon layer is increased by increasing the holding time t3 in step S2, and the thickness of the carbon layer is decreased by decreasing the holding time t3 in step S2.

[0105]

[0106]

[0107] As can be seen from Table 1, the present application improves the expansion and rate performance of the secondary battery by regulating the sum of the atomic numbers of oxygen atoms, carbon atoms and silicon atoms in the surface area, so that the atomic number of oxygen atoms in the surface area accounts for A1% and satisfies 10≤A1≤35; in particular, when 15≤A1≤30 is satisfied, the expansion and rate performance can be further improved.

[0108] When a carbon layer is provided on the surface of the silicon-carbon particles, it can cooperate with the surface oxygen atoms, which is beneficial to improving the expansion and rate performance of the secondary battery. In particular, when the sum of the atomic numbers of oxygen atoms, carbon atoms, and silicon atoms in the internal region is regulated, the atomic number ratio A2% of oxygen atoms in the internal region satisfies 0<A2≤8, for example, 0.5≤A2≤8, which can further improve the expansion and rate performance of the secondary battery. In particular, when the atomic number ratio of silicon atoms in the internal region is greater than the atomic number ratio of silicon atoms in the surface region, it is beneficial to further improve the expansion and rate performance of the secondary battery.

[0109] Example 2-1 to Example 2-11

[0110] The only difference compared to Examples 1-7 is that the mass ratios of fluoroethylene carbonate, lithium difluorophosphate, and ethyl fluoroacetate in the electrolyte were adjusted according to Table 2. The specific adjustment parameters and performance test results are shown in Table 2. During the adjustment process, the content of the base solvent was adaptively adjusted, wherein the mass ratio of dimethyl carbonate, diethyl carbonate, and ethylene carbonate remained unchanged.

[0111] Table 2

[0112]

[0113] As shown in Table 2, the inclusion of both fluoroethylene carbonate and lithium difluorophosphate in the controlled electrolyte, combined with their interaction, further improves the expansion and rate performance of the secondary battery compared to the embodiment containing only fluoroethylene carbonate. Furthermore, in the controlled electrolyte, the mass percentage S1 of fluoroethylene carbonate satisfies 5≤S1≤20, and the mass percentage S2 of lithium difluorophosphate satisfies 0.05≤S2≤0.95, further improving the expansion and rate performance of the secondary battery.

[0114] In particular, adding ethyl fluoroacetate to the electrolyte and adjusting its mass ratio S3 to meet 1≤S3 / S1≤5 can better improve the expansion and rate performance of the secondary battery. In particular, when meeting 1.5≤S3 / S1≤4, the expansion and rate performance of the secondary battery can be further improved.

[0115] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the principles of the present application should be included in the scope of protection of the present application.

Claims

1. A negative electrode material, characterized in that The negative electrode material includes silicon-carbon particles, the silicon-carbon particles having a surface area, and the atomic number of oxygen atoms in the surface area accounts for A1% based on the sum of the atomic numbers of oxygen atoms, carbon atoms, and silicon atoms in the surface area, with 10≤A1≤35; The surface region refers to a region ranging from 0 nm to 200 nm from the surface of the silicon-carbon particles.

2. The negative electrode material according to claim 1, characterized in that 15≤A1≤30。 3. The negative electrode material according to claim 1, characterized in that The silicon-carbon particles contain porous carbon and nano-silicon located in the pores of the porous carbon; and a carbon layer is provided on the surface of the silicon-carbon particles.

4. The negative electrode material according to any one of claims 1 to 3, characterized in that Based on the sum of the atomic numbers of oxygen atoms, carbon atoms, and silicon atoms in the surface region, the atomic number of silicon atoms in the surface region accounts for B1%, 25≤B1≤50.

5. The negative electrode material according to any one of claims 1 to 3, characterized in that The silicon-carbon particles have an internal region, which refers to a region of the silicon-carbon particles excluding the surface region, and the internal region satisfies at least one of the following conditions: (1) Based on the sum of the atomic numbers of oxygen atoms, carbon atoms, and silicon atoms in the inner region, the atomic number of oxygen atoms in the inner region accounts for A2%, 0<A2≤8; (2) Based on the sum of the atomic numbers of oxygen atoms, carbon atoms, and silicon atoms in the inner region, the atomic number of silicon atoms in the inner region accounts for B2%, and B2>B1.

6. The negative electrode material according to claim 3, characterized in that The thickness of the carbon layer is T nm, 200≤T×A1≤1500.

7. The method for preparing a negative electrode material according to any one of claims 1 to 6, characterized in that: The following steps are involved: Step S1, keeping the porous carbon at T1°C for t1 h in a silane gas atmosphere, wherein the volume proportion of the silane gas in the silane gas atmosphere is 5-100%; then keeping the porous carbon at T2°C for t2 h in a carbon dioxide atmosphere, wherein the volume proportion of the carbon dioxide in the carbon dioxide atmosphere is 1-10%, and the volume flow rate of the carbon dioxide atmosphere is 40-100 L / min; then keeping the porous carbon at T2°C for t2 h in an acetylene gas atmosphere to perform a surface passivation treatment to obtain passivated silicon carbon; 420≤T1≤600, 6≤t1≤16; 450≤T2≤650, 0.2≤t2≤2; Step S2: heating the passivated silicon carbon to T3°C in an inert gas atmosphere, and then keeping it in an acetylene gas atmosphere for t3h to obtain silicon carbon particles with a carbon layer on the surface, wherein the volume proportion of acetylene in the acetylene gas atmosphere is 20-50%; 480≤T3≤700, 1≤t3≤5.

8. A secondary battery, characterized in that: The invention comprises a positive electrode, a negative electrode and an electrolyte; the negative electrode comprises a negative electrode current collector and a negative electrode material layer provided on at least one surface of the negative electrode current collector; The negative electrode material layer includes the negative electrode material according to any one of claims 1 to 6 or the negative electrode material prepared by the preparation method according to claim 7.

9. The secondary battery according to claim 8, characterized in that The electrolyte includes fluoroethylene carbonate and lithium difluorophosphate; Based on the mass of the electrolyte, the mass proportion of the fluoroethylene carbonate is S1%, the mass proportion of the lithium difluorophosphate is S2%, 5≤S1≤20, 0.05≤S2≤0.

95.

10. The secondary battery according to claim 9, wherein The electrolyte further includes ethyl fluoroacetate; Based on the mass of the electrolyte, the mass proportion of the ethyl fluoroacetate is S3%, 1≤S3 / S1≤5; preferably, 1.5≤S3 / S1≤4.

11. An electronic device, characterized in that: A secondary battery comprising the secondary battery according to any one of claims 8 to 10.

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