Negative electrode material, negative electrode sheet and secondary battery

Abstract

The negative electrode material comprises a core body and a coating layer coating at least part of the surface of the core body, the core body comprises a base body and an active substance arranged on the base body, the protection strength of the coating layer is defined as gamma, gamma = 0.0129 T0-0.323, gamma is greater than or equal to 4 and less than or equal to 9, T0 is the initial temperature of mass increase of the negative electrode material in thermogravimetric analysis, T0 is the initial temperature of mass increase of the negative electrode material in thermogravimetric analysis, and T0 is the initial temperature of mass increase of the negative electrode material in thermogravimetric analysis. The thermogravimetric analysis comprises the step of determining the quality of the negative electrode material in an oxygen atmosphere; the negative electrode material further comprises an oxygen element, the atomic percent content of the oxygen element at any position, away from the interior of the negative electrode material by X, of the surface of the negative electrode material is defined as Px, X is larger than or equal to 60 nm and smaller than or equal to 160 nm, and Px is larger than or equal to 2 and smaller than or equal to 8.

Description

Technical Field

[0001] The present application relates to the field of electrochemical energy storage, and in particular to a negative electrode material, a negative electrode sheet and a secondary battery. Background Art

[0002] The negative electrode material may include a matrix and active substances distributed in the pores of the matrix or on the surface of the matrix. Taking silicon particles as an example, during the homogenization or battery circulation of the negative electrode material, some silicon particles will dissolve and come into contact with and react with water or electrolyte in the homogenization, thereby causing the undesirable phenomenon of gas production. In addition, the silicon particles will also expand abnormally during the battery circulation process, resulting in an undesirable volume effect. These problems limit the long-term use of the negative electrode material. Summary of the invention

[0003] In view of this, the present application provides a negative electrode material to solve at least one of the above technical problems.

[0004] To achieve the above object, the present application provides a negative electrode material, including a core body and a coating layer covering at least a portion of the surface of the core body, the core body including a substrate and an active material disposed on the substrate, and the protection strength of the coating layer is defined as γ, γ = 0.0129T 0 -0.323, where 4≤γ≤9, T 0 is the starting temperature of the mass increase of the negative electrode material in the thermogravimetric analysis. The thermogravimetric analysis includes measuring the mass of the negative electrode material under an oxygen atmosphere. The negative electrode material also includes oxygen element. The atomic percentage content of oxygen element at any point on the surface of the negative electrode material at a distance of X from the inside of the negative electrode material is defined as P. x , 60nm≤X≤160nm, 2≤P x ≤8.

[0005] In some possible implementations, along the direction from the surface of the negative electrode material to the inside of the negative electrode material, P x First lower then raise.

[0006] In some possible implementations, the specific surface area of ​​the substrate is 800 m 2 / g to 3000m 2 / g.

[0007] In some possible implementations, the substrate has pores, and the total pore volume of the substrate is 0.5 cm 3 / g to 2.0cm 3 / g.

[0008] In some possible implementations, the matrix has pores, and the porosity of the matrix is ​​40% to 80%.

[0009] In some possible implementations, the average particle size of the active material is 0.1 nm to 100 nm.

[0010] In some possible implementations, the coating layer has a thickness of 1 nm to 60 nm.

[0011] In some possible implementations, based on the mass of the negative electrode material, the mass of the coating layer accounts for less than or equal to 10%.

[0012] In some possible implementations, the median particle size D50 of the negative electrode material is 3 μm to 15 μm.

[0013] In some possible implementations, the specific surface area of ​​the negative electrode material is 1 m 2 / g to 10m 2 / g.

[0014] In some possible implementations, the matrix includes one of a carbon matrix and a non-carbon matrix; the carbon matrix includes one or more of hard carbon, soft carbon, graphite, carbon nanotubes, carbon fibers and graphene; the non-carbon matrix includes at least one of metal oxides, silicides, silicates, phosphates, titanates and aluminum borates; the metal oxides include one or more of aluminum oxide, zirconium oxide, germanium dioxide and manganese dioxide; the silicides include one or more of silicon carbide and silicon nitride; the silicates include one or more of cordierite, mullite and zeolite; the phosphates include one or more of aluminum phosphate, magnesium phosphate, calcium phosphate, titanium phosphate, chromium phosphate, cobalt phosphate, nickel phosphate, germanium phosphate, zirconium phosphate, niobium phosphate, molybdenum phosphate, tantalum phosphate, tungsten phosphate and lanthanum phosphate; the titanates include one or more of calcium titanate, iron titanate, lithium titanate and barium titanate.

[0015] In some possible implementations, the active material includes one or more of silicon-based materials, Li, Na, K, Sn, Ge, Fe, Mg, Ti, Zn, Al, P and Cu; the silicon-based material includes one or more of amorphous silicon, crystalline silicon, silicon oxide, silicon alloy and a composite of crystalline silicon and amorphous silicon.

[0016] In some possible implementations, the coating layer includes one or more of carbon materials, metal oxides, amorphous silicon, conductive polymers, fluorides, phosphates, and nitrides.

[0017] In some possible implementations, the matrix includes a carbon matrix, the active material includes silicon particles, the coating layer includes a carbon coating layer, and based on the mass of the negative electrode material, the mass proportion of silicon element in the negative electrode material is 20% to 65%.

[0018] In some possible implementations, based on the mass of the negative electrode material, the mass proportion of silicon element in the negative electrode material is 45% to 55%.

[0019] In some possible implementations, based on the mass of the negative electrode material, the mass proportion of carbon element in the negative electrode material is 30% to 60%.

[0020] In some possible implementations, based on the mass of the negative electrode material, the mass proportion of carbon element in the negative electrode material is 45% to 55%.

[0021] The present application also provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, wherein the negative electrode active material layer comprises the above-mentioned negative electrode material.

[0022] The present application also provides a secondary battery, comprising the above-mentioned negative electrode sheet.

[0023] The present application uses the protective strength γ of the coating layer to characterize the density, thickness and uniformity of the coating layer. When 4≤γ≤9, it means that the coating layer has good density, thickness and uniformity, and the active material in the coated core is not easy to be exposed, nor is it easy to seep through the coating layer and contact with external substances (such as slurry or electrolyte) to react, which is beneficial to reduce the gas generation of the negative electrode material and improve the cycle performance of the secondary battery. At the same time, when the oxygen percentage content P at a depth of X (60nm≤X≤160nm) from the surface of the negative electrode material x Satisfy 2≤P x ≤8, the oxidizing element mainly comes from the oxidation of the active material (such as silicon particles), and the corresponding oxide layer can provide protection for the active material (such as silicon particles), which is beneficial to reduce the gas generation value and expansion rate of the negative electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 (a) and (b) are schematic structural diagrams of a secondary battery provided in one embodiment of the present application during charging and discharging, respectively.

[0025] Main component symbols

[0026] Electrode assembly 100

[0027] Positive electrode 101

[0028] Negative electrode 102

[0029] Isolation film 103 DETAILED DESCRIPTION

[0030] The embodiments of the present application are described in detail below. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present application, and cannot be understood as limiting the present application; it should be noted that, unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by technicians in the technical field of the present application; in the absence of conflict, the embodiments of the present application and the features in the embodiments can be combined with each other; many specific details are elaborated in the following description to facilitate a full understanding of the present application, and the described embodiments are only part of the embodiments of the present application, not all of the embodiments.

[0031] An embodiment of the present application provides a secondary battery, comprising a housing, an electrode assembly and an electrolyte. The electrode assembly and the electrolyte are both located in the housing.

[0032] The outer shell can be a packaging bag encapsulated by an encapsulation film (such as an aluminum-plastic film), for example, a soft-pack battery. In other embodiments, it can also be a steel shell battery, an aluminum shell battery, etc.

[0033] See also Figure 1 The electrode assembly 100 includes a positive electrode sheet 101, a negative electrode sheet 102 and a separator 103. The separator 103 is disposed between the positive electrode sheet 101 and the negative electrode sheet 102. When an electrolyte (not shown) is provided, when charging, please refer to Figure 1 In (a), active ions (such as lithium ions) are released from the positive electrode material (such as the lithiated intercalation compound) of the positive electrode sheet 101, pass through the separator 103 through the electrolyte, reach the negative electrode sheet 102 and are embedded in the negative electrode material. Electrons correspondingly travel from the positive electrode sheet 101 through the external circuit to the negative electrode sheet 102. During discharge, please refer to Figure 1 In (b), active ions (such as lithium ions) are released from the negative electrode material of the negative electrode sheet 102, pass through the isolation membrane 103 through the electrolyte, reach the positive electrode sheet 101 and are embedded in the positive electrode material (such as the lithiated intercalation compound), and the electrons correspondingly reach the positive electrode sheet 101 from the negative electrode sheet 102 through the external circuit.

[0034] In some embodiments, the electrode assembly 100 may be a laminated structure, which is formed by alternately stacking the positive electrode sheet 101, the separator 103, and the negative electrode sheet 102. In other embodiments, the electrode assembly 100 may also be a wound structure, which is formed by stacking the positive electrode sheet 101, the separator 103, and the negative electrode sheet 102 in sequence and then winding them.

[0035] Positive electrode

[0036] The positive electrode sheet 101 includes a positive electrode current collector and a positive electrode material active layer provided on at least one surface of the positive electrode current collector. The positive electrode current collector may be made of aluminum foil or nickel foil, or may be any composite current collector disclosed in the prior art, such as but not limited to a current collector formed by combining the aforementioned conductive foil and a polymer substrate. The positive electrode material active layer includes a positive electrode active material, and the positive electrode active material includes a compound that can reversibly embed and deintercalate lithium ions (i.e., a lithiated intercalation compound). In some embodiments, the positive electrode active material may include a lithium transition metal composite oxide. The lithium transition metal composite oxide contains lithium and at least one element selected from cobalt, manganese and nickel. In some embodiments, the positive electrode active material may include but is not limited to lithium cobalt oxide (LiCoO 2 ), lithium nickel manganese cobalt ternary materials (NCM), lithium manganese oxide (LiMn 2 O 4 ), lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 O 4 ) or lithium iron phosphate (LiFePO 4 ) at least one of.

[0037] The positive electrode material active layer also includes a binder to bond the positive electrode active material particles to facilitate the formation of a film layer, and at the same time, it can also improve the bonding force between the positive electrode material active layer and the positive electrode current collector. In some embodiments, the binder may include but is not limited to at least one of 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, acrylic (ester) styrene-butadiene rubber, epoxy resin or nylon.

[0038] The positive electrode material active layer may also include a conductive material, including but not limited to a carbon-based material, a metal-based material, a conductive polymer, or any combination thereof. In some embodiments, the carbon-based material may include but is not limited to natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material may include but is not limited to metal powder or metal fiber, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0039] Negative plate

[0040] The negative electrode sheet 102 includes a negative electrode current collector and a negative electrode material active layer disposed on at least one surface of the negative electrode current collector. The negative electrode current collector may be at least one of copper foil, nickel foil, stainless steel foil, titanium foil or a carbon-based current collector, or may be any composite current collector disclosed in the prior art, such as but not limited to a current collector formed by combining the aforementioned conductive foil and a polymer substrate.

[0041] The negative electrode material active layer includes a negative electrode material, the negative electrode material includes a core body and a coating layer covering at least a portion of the surface of the core body, the core body includes a substrate and an active material disposed on the substrate, and the protection strength of the coating layer is defined as γ, γ = 0.0129T 0 -0.323, where 4≤γ≤9, T 0 is the starting temperature of the mass increase of the negative electrode material in the thermogravimetric analysis, and the thermogravimetric analysis includes measuring the mass of the negative electrode material under an oxygen atmosphere. For example, the protective strength γ of the coating layer can be 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9 or any value within the range formed by any two of the above values.

[0042] The present invention conducts thermogravimetric analysis (TG) on the negative electrode material under oxygen atmosphere to determine the temperature T at which the mass of the negative electrode material increases. 0 , and obtain the protective strength γ of the coating layer. The principles include: Generally, since the material forming the coating layer has pores at the microscopic level, oxygen molecules can pass through these pores, so that oxygen contacts the core body coated by the coating layer, and then due to the high reactivity of the active substances in the core body (such as silicon particles), oxygen tends to preferentially undergo oxidation reactions with the active substances (such as silicon particles) to generate oxides (such as silicon oxide), thereby causing the quality of the negative electrode material to increase. When the pores of the coating layer change, making it more difficult for oxygen molecules to pass through, increasing the temperature helps to make the thermal motion of the gas molecules more intense, the frequency of collisions between molecules increases, and the average kinetic energy of the molecules also increases, thereby accelerating the diffusion rate of the gas molecules in space, thereby enabling the oxygen molecules to pass through these pores again, pass through the coating layer, and cause the negative electrode material to oxidize again, resulting in an increase in mass. Therefore, in the above-mentioned TG test, the temperature T at which the mass of the negative electrode material increases is measured. 0 It can better reflect the protective strength of the coating layer on the core body, and the temperature T 0 The higher the value, the better the protection strength of the coating layer to the core. Therefore, this application defines the protection strength of the coating layer as γ. 0 =25℃,γ=0, when T 0 = 800℃, γ = 10, so T 0 and γ can be expressed by the formula γ = 0.0129T 0 -0.323 builds contact.

[0043] The above protection strength γ can be used to characterize the density, thickness and uniformity of the coating layer. When γ satisfies 4≤γ≤9, it means that the coating layer has good density, thickness and uniformity, and the active material in the coated core is not easy to be exposed, nor is it easy to seep through the coating layer and react with external substances (such as slurry or electrolyte), which is beneficial to reduce the gas production of the negative electrode material and improve the electrochemical performance of the secondary battery. When γ<4, the density, thickness or uniformity of the coating layer is poor. When coating the core body, part of the active material in the core body is easily exposed and directly contacted with external substances, or when there is active material shedding, it is easy to pass through the coating layer and contact with external substances, resulting in instability of the negative electrode material and serious reaction gas production, thereby seriously reducing the electrochemical performance of the secondary battery.

[0044] The negative electrode material also includes oxygen, and the atomic percentage content of oxygen at any point on the surface of the negative electrode material at a distance of X from the inside of the negative electrode material is defined as P. x , 60nm≤X≤160nm, 2≤P x ≤8. For example, the atomic percentage content of oxygen element P x It can be 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8 or any value within the range formed by any two of the above values.

[0045] The present application further proposes that the oxygen element at the depth X (60nm≤X≤160nm) mainly comes from the oxidation of active substances (such as silicon particles). Generally speaking, the outermost layer of the negative electrode material is the coating layer, followed by the active substance (such as silicon particles) region or the matrix, and the active substance is distributed in the matrix; because there are oxygen-containing groups on the matrix, the active substance will be partially oxidized when it is distributed to form an oxide layer. A certain amount of oxide layer can act as a buffer medium to reduce the expansion pressure during the cycle, and also reduce the direct contact between silicon particles and water or electrolyte, but too much oxide layer will cause the capacity and the first coulomb efficiency to decrease, so the oxygen content needs to be controlled. When the percentage content P of oxygen at the corresponding depth X x Satisfy 2≤P x ≤8, the corresponding oxide layer can provide protection for the active material (such as silicon particles), which is beneficial to reduce the gas generation value and expansion rate of the negative electrode material. x When the oxygen content is too high, there will be more oxide layers. During the charge and discharge cycle, taking the active material as silicon particles as an example, the silicon oxide in the oxide layer will form lithium silicate compounds. This part of lithium cannot participate in the reversible reaction, thus reducing the first coulomb efficiency and reversible capacity of the secondary battery. When the oxygen percentage content P xWhen it is too low, some active substances are difficult to protect, and are therefore more likely to seep out and produce side reactions with water or electrolyte, increasing gas production or causing structural instability of the negative electrode material.

[0046] At the same time, the present application also found that controlling the above oxygen percentage content P x Satisfy 2≤P x ≤8, the above-mentioned oxide layer is also beneficial to maintain or increase the protection strength γ of the above-mentioned coating layer to a certain extent, so as to make the protection strength γ of the coating layer meet a specific range. This may be because the appropriate oxygen content can promote better nucleation and growth of the coating layer material by affecting the cracking and surface reaction kinetics of the coating layer precursor material, thereby helping to improve the density, thickness or uniformity of the coating layer. However, too high an oxygen content will promote the oxidation reaction of the deposited coating layer material, resulting in excessive oxidation of the coating layer material after deposition, generating unexpected oxides or mixed phases, thereby changing the chemical composition and phase structure of the coating layer, which is not conducive to improving the density, thickness or uniformity of the coating layer. On the other hand, too low an oxygen content may cause the deposition rate of the coating layer material to slow down, which is not conducive to the formation of a continuous coating material deposition layer, nor is it conducive to improving the density, thickness or uniformity of the coating layer.

[0047] In some embodiments, along the direction from the surface of the negative electrode material to the center of the negative electrode material, P x First decrease and then increase. The oxygen atomic percentage corresponding to the surface of the negative electrode material will be higher, because the negative electrode material is exposed to the air and oxygen-containing groups are adsorbed on the surface; as the depth inside increases, the first thing detected is the coating layer, which comes from the cracking of the coating layer precursor material. The probability of introducing oxygen in this process is small, corresponding to the decrease in the percentage of oxygen atoms; as the detection depth increases further, the active material area is detected. During the process of the active material forming a distribution on the matrix, it will be oxidized by the oxygen-containing groups on the matrix to form an oxide layer, which increases the percentage of oxygen atoms detected. The oxide layer inside the negative electrode material at a certain depth is beneficial to act as a buffer layer for the matrix or active material, improve the stability of the matrix or active material, and thus further improve the cycle stability of the negative electrode material.

[0048] In some embodiments, the specific surface area of ​​the substrate is 800 m 2 / g to 3000m 2 / g. For example, the specific surface area of ​​the matrix can be 800m 2 / g、1000m 2 / g、1200m 2 / g、1400m 2 / g、1600m 2 / g、1800m 2 / g, 2000m 2 / g, 2300m2 / g, 2500m 2 / g、2800m 2 / g、3000m 2 / g or any value within the range formed by any two of the above values. Controlling the specific surface area of ​​the substrate within the above range is conducive to the uniform dispersion and stable attachment of the active substance on the substrate, thereby reducing the risk of the active substance falling off the substrate and improving the stability and durability of the active substance.

[0049] In some embodiments, the substrate has pores and the total pore volume of the substrate is 0.5 cm 3 / g to 2.0cm 3 / g. For example, the total pore volume of the matrix can be 0.5 cm 3 / g, 0.8cm 3 / g, 1cm 3 / g, 1.2cm 3 / g, 1.5cm 3 / g, 1.8cm 3 / g, 2cm 3 / g or any value within the range formed by any two of the above values. When the matrix has abundant pores, these pores can accommodate the active substances and reserve space for the volume expansion of the accommodated active substances.

[0050] In some embodiments, the substrate has pores, and the porosity of the substrate is 40% to 80%. For example, the porosity of the substrate can be 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or any value within the range formed by any two of the above values. Porosity refers to the percentage of the pore volume in the material to the total volume of the material in its natural state. Controlling the porosity of the substrate within the above range is conducive to the good deposition of active particles, thereby reducing the volume expansion effect.

[0051] In some embodiments, the average particle size of the active material is 0.1nm to 100nm. For example, the average particle size of the active material can be 0.1nm, 0.5nm, 1nm, 3nm, 5nm, 8nm, 10nm, 20nm, 30nm, 40nm, 50nm, 65nm, 80nm, 95nm, 100nm or any value within the range formed by any two of the above values. For example, the active material can be silicon particles. By setting the average particle size of the silicon particles within the above range, the mechanical stress of the silicon particles when the volume expands can be reduced, so that the secondary battery maintains a good battery capacity, and reduces irreversible capacity loss, and can also shorten the electron and ion transmission path. At the same time, the size of the silicon particles is reduced, and the gap between adjacent silicon particles is increased, which can reserve space for the volume expansion of the silicon particles.

[0052] In some embodiments, the coating layer has a thickness of 1 nm to 60 nm. For example, the coating layer may have a thickness of 1 nm, 3 nm, 5 nm, 8 nm, 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, or any value within the range formed by any two of the above values. Controlling the thickness of the coating layer within the above range is beneficial to maintaining the stability of the particle structure of the negative electrode material during the cycle, can reduce the exposed active material on the surface of the negative electrode material, reduce the exposed active material resulting in the generation of a large amount of SEI during the charge and discharge process, and improve the specific capacity and electrochemical performance of the negative electrode material.

[0053] In some embodiments, based on the mass of the negative electrode material, the mass proportion of the coating layer is less than or equal to 10%. For example, the mass proportion of the coating layer can be 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, or any value within the range formed by any two of the above values. The mass proportion of the coating layer in the negative electrode material is within the above range, which can make the electrode active ions have sufficient embedding amount, thereby making the battery have a higher charge and discharge capacity, and is conducive to reducing the risk of decreased conductivity.

[0054] In some embodiments, the volume median particle size D50 of the negative electrode material is 3 μm to 15 μm. For example, the volume median particle size D50 of the negative electrode material can be 3 μm, 4 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 15 μm, or any value within the range formed by any two of the above values. Controlling the volume median particle size D50 of the negative electrode material within the above range is beneficial to improving the cycle performance of the negative electrode material.

[0055] In some embodiments, the specific surface area of ​​the negative electrode material is 1 m 2 / g to 10m 2 / g. For example, the specific surface area of ​​the negative electrode material can be 1m 2 / g, 2m 2 / g, 3m 2 / g, 4m 2 / g, 5m 2 / g, 6m 2 / g, 7m 2 / g, 8m 2 / g, 9m 2 / g, 10m 2 / g or any value within the range formed by any two of the above values. Controlling the specific surface area of ​​the negative electrode material within the above range is beneficial to improving the capacity of the first discharge ratio and the first coulombic efficiency of the battery, reducing the risk of excessive lithium salt consumption by the SEI film, or electrical separation between particles caused by the volume effect, resulting in reduced reversible capacity and coulombic efficiency of the battery.

[0056] In some embodiments, the matrix includes a carbon matrix. In some embodiments, the carbon matrix includes one or more of hard carbon, soft carbon, graphite, carbon nanotubes, carbon fibers, and graphene. The carbon matrix can provide distribution sites for the active material and form a conductive network.

[0057] In other embodiments, the matrix includes a non-carbon matrix. The non-carbon matrix includes at least one of metal oxides, silicides, silicates, phosphates, titanates, and aluminum borates. In some embodiments, the metal oxide includes one or more of aluminum oxide, zirconium oxide, germanium dioxide, and manganese dioxide. In some embodiments, the silicide includes one or more of silicon carbide and silicon nitride. In some embodiments, the silicate includes one or more of cordierite, mullite, and zeolite. In some embodiments, the phosphate includes one or more of aluminum phosphate, magnesium phosphate, calcium phosphate, titanium phosphate, chromium phosphate, cobalt phosphate, nickel phosphate, germanium phosphate, zirconium phosphate, niobium phosphate, molybdenum phosphate, tantalum phosphate, tungsten phosphate, and lanthanum phosphate. In some embodiments, the titanate includes one or more of calcium titanate, iron titanate, lithium titanate, and barium titanate. The selection of the above materials for the non-carbon matrix is ​​conducive to optimizing the transmission path of lithium ions, improving the diffusion rate and embedding efficiency of lithium ions, and improving the electrochemical performance.

[0058] In some embodiments, the active material includes one or more of silicon-based materials, Li, Na, K, Sn, Ge, Fe, Mg, Ti, Zn, Al, P, and Cu.

[0059] In some embodiments, the silicon-based material includes one or more of amorphous silicon, crystalline silicon, silicon oxide, silicon alloy, and a composite of crystalline silicon and amorphous silicon. When the active material includes a silicon-based material, the silicon-based material is used as a component of the negative electrode active material, which can increase the specific capacity of the negative electrode material and thus increase the energy density of the secondary battery. Preferably, the silicon-based material includes amorphous silicon. When the active material further includes amorphous silicon, amorphous silicon expands isotropically during the lithium insertion process, which can reduce the collapse of pores, inhibit the rapid attenuation of the specific capacity of the negative electrode material, and is more conducive to improving the lithium insertion cycle performance of the negative electrode material.

[0060] In some embodiments, the coating layer includes one or more of a carbon material, a metal oxide, amorphous silicon, a conductive polymer, a fluoride, a phosphate, and a nitride. When the coating layer includes a carbon material, the carbon coating layer formed is beneficial to improving the stability and conductivity of the substrate. The carbon coating layer can also coat the active material exposed on the surface of the carbon substrate, reduce the continuous oxidation of the exposed active material during placement, and reduce the risk of reducing the specific capacity and first coulombic efficiency (ICE) of the negative electrode material. The carbon coating layer can also reduce the direct contact between the active material and the electrolyte, improve the stability of the SEI film, and thus improve the first coulombic efficiency of the negative electrode material.

[0061] In some embodiments, the carbon coating layer may be a single-layer carbon coating layer formed of the above single material, or a carbon coating layer formed of a combination of the above multiple materials, or a single-layer carbon coating layer, or a multiple-layer carbon coating layer, etc. The layer structure of the carbon coating layer may be selected according to actual needs. It is understandable that when the carbon coating layer is a multi-layer coating structure, its density is higher.

[0062] In some embodiments, the matrix includes a carbon matrix, the active material includes silicon particles, and the coating layer includes a carbon coating layer. Based on the mass of the negative electrode material, the mass proportion of silicon element in the negative electrode material is 20% to 65%. For example, the mass proportion of silicon element in the negative electrode material can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65% or any value within the range composed of any two of the above values. The negative electrode material obtained by compounding the carbon matrix (such as porous carbon) with silicon particles can effectively alleviate the expansion problem of silicon particles during the cycle, maintain the stability of the battery material, and improve the volume capacity of the material. Controlling the mass proportion of silicon element within the above range, the lithium battery can store a higher amount of electricity, that is, the first discharge specific capacity is higher. Further, based on the mass of the negative electrode material, the mass proportion of silicon element in the negative electrode material is 45% to 55%. For example, it can be 45%, 48%, 50%, 52%, 54%, 55% or any value within the range composed of any two of the above values.

[0063] In some embodiments, the matrix includes a carbon matrix, the active material includes silicon particles, and the coating layer includes a carbon coating layer. Based on the mass of the negative electrode material, the mass proportion of the carbon element of the negative electrode material is 30% to 60%. For example, the mass proportion of the carbon element of the negative electrode material can be 30%, 35%, 40%, 45%, 50%, 55%, 60% or any value within the range composed of any two of the above values. The carbon matrix is ​​composited with silicon particles to provide a conductive platform and buffer space for silicon particles, improving the problems of structural instability and poor conductivity of silicon particles during the cycle. The carbon element of the negative electrode material mainly comes from the carbon matrix and the carbon coating layer. Controlling the mass proportion of the carbon element within the above range can establish a sufficient carbon-based substrate, provide sufficient distribution sites for the active material, and facilitate the formation of an effective conductive network, thereby improving conductivity and cycle stability. Further, based on the mass of the negative electrode material, the mass proportion of the carbon element of the negative electrode material is 45% to 55%. For example, it may be 45%, 48%, 50%, 52%, 54%, 55%, or any value within the range formed by any two of the above values.

[0064] The negative electrode material active layer also includes a binder to bind the negative electrode active material particles to facilitate the formation of a film layer, and can also improve the binding force between the negative electrode material active layer and the negative electrode current collector. In some embodiments, the binder may include but is not limited to polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene butadiene rubber, acrylic (ester) styrene butadiene rubber, epoxy resin or nylon, etc.

[0065] The negative electrode material active layer may also include a conductive material, including but not limited to a carbon-based material, a metal-based material, a conductive polymer, or any combination thereof. In some embodiments, the carbon-based material may include but is not limited to natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material may include but is not limited to metal powder or metal fiber, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0066] Isolation film

[0067] The isolation membrane 103 includes a membrane layer with a porous structure, and its material includes but is not limited to at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide or aramid. For example, the isolation membrane 103 can be a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene non-woven fabric, a polyethylene non-woven fabric or a polypropylene-polyethylene-polypropylene porous composite membrane.

[0068] Electrolytes

[0069] The electrolyte has the function of conducting ions between the positive electrode and the negative electrode. The electrolyte can be in one or more of a gel state, a solid state, and a liquid state. In some embodiments, the electrolyte is an electrolyte. The electrolyte plays a role in conducting active ions between the positive electrode and the negative electrode. In some embodiments, the electrolyte includes a lithium salt and an organic solvent. The lithium salt can be selected from, but is not limited to, lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4 ) lithium hexafluoroarsenate (LiAsF 6 ), lithium perchlorate (LiClO 4 ), lithium tetraphenylborate (LiB(C 6 H 5 ) 4 ), lithium methanesulfonate (LiCH 3 SO 3), lithium bis(trifluoromethanesulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiCF 3 SO 3 ), lithium bis(trifluoromethylsulfonyl)imide (LiN(SO 2 CF 3 ) 2 、Tris(trifluoromethylsulfonyl)methyllithium (LiC(SO 2 CF 3 ) 3 ), lithium bis(oxalatoborate) (LiBOB) and lithium difluorophosphate (LiPO 2 F 2 ) in one or more. For example, the lithium salt is selected from LiPF 6 The organic solvent may be a carbonate compound, a carboxylate compound, an ether compound, a nitrile compound, other organic solvents or a combination thereof. Examples of carbonate compounds include, but are not limited to, diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), ethyl methyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinyl ethylene carbonate (VEC), 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-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, trifluoromethylethylene carbonate, or combinations thereof.

[0070] Another embodiment of the present application further provides a method for preparing a negative electrode material, comprising:

[0071] The first step: providing a substrate precursor and heat-treating the substrate precursor in a mixed gas atmosphere. The mixed gas includes an inert gas and hydrogen. The inert gas includes one or more of nitrogen, helium, neon, argon and krypton. The volume proportion of hydrogen in the mixed gas is less than 35%, thereby obtaining a first intermediate.

[0072] Taking the preparation of a carbon matrix as an example, the matrix precursor may be a carbon material, and the matrix precursor is heat-treated using a mixed gas including the inert gas and hydrogen, wherein the hydrogen has a reducing property and can combine with the oxygen atoms in the oxygen-containing groups to generate water (H 2O) or other compounds with low oxygen content. This reduction reaction helps to adjust the oxide layer on the surface of the material and improve the surface properties of the material, thereby reducing the oxygen content in the carbon matrix, and then regulating the content of the oxide layer around the active material, controlling the hydrogen in the mixed gas within the above range, which is conducive to obtaining P x Negative electrode material that meets the preset range.

[0073] In some embodiments, the temperature of the heat treatment is 400°C to 1000°C, and the time is 0.5h to 10h. For example, the temperature of the heat treatment can be 400°C, 500°C, 600°C, 700°C, 800°C, 900°C, 1000°C, or any value within the range formed by any two of the above values. The time of the heat treatment can be 0.5h, 1h, 2h, 3h, 4h, 6h, 8h, 10h, or any value within the range formed by any two of the above values.

[0074] Step 2: Mix the first intermediate with the active substance gas phase carrier, heat it, and make at least part of the active substance deposit on the first intermediate to obtain the second intermediate.

[0075] When the substrate precursor has pores, the corresponding first intermediate also has pores, and the active substance gas phase carrier can be partially deposited in the pores of the first intermediate under heating conditions, and partially attached to the surface of the first intermediate to obtain a second intermediate. At the same time, in order to control the deposition reaction speed and reduce the risk of excessive deposition, the reaction process can control the content of the active substance gas phase carrier to remain unchanged.

[0076] In some embodiments, "mixing the first intermediate with the active material gas phase carrier" also includes adding nitrogen and hydrogen for mixing. Adding nitrogen as an inert gas is helpful to reduce the risk of oxidation of the active material gas phase carrier under heating conditions, and adding hydrogen helps to make the active material gas phase carrier evenly distributed and reduce the agglomeration of the active material, thereby helping to improve the density and uniformity of the active material deposition. In order to control the deposition reaction speed and reduce the risk of excessive deposition, the hydrogen content can also be appropriately increased during the reaction process.

[0077] In some embodiments, the heating temperature is 300°C to 800°C, and the heating time is 2h to 15h. For example, the heating temperature can be 300°C, 400°C, 500°C, 600°C, 700°C, 800°C, or any value within the range formed by any two of the above values. The heating time can be 2h, 3h, 5h, 7h, 9h, 10h, 12h, 14h, 15h, or any value within the range formed by any two of the above values.

[0078] Step 3: coating the second intermediate, including one or more of carbon coating, oxide coating, metal salt coating and polymer coating; carbon coating includes introducing carbon material and carrier gas, the carrier gas includes nitrogen, argon and hydrogen, and the volume proportion of hydrogen in the carrier gas is greater than or equal to 2%.

[0079] When the second intermediate is carbon-coated, the above-mentioned carrier gas is also introduced when the carbon material is introduced, which can control the reaction rate of the carbon material and slow down the cracking rate of the carbon material, thereby helping to improve the density and uniformity of the coating layer. The nitrogen in the carrier gas helps to isolate oxygen; the above-mentioned carbon material will also produce hydrogen when cracked, and the hydrogen in the carrier gas is controlled within the above-mentioned range, so that the hydrogen can slow down the cracking rate of the carbon material gas, which is beneficial to improve the density and uniformity of the carbon coating layer; the argon in the carrier gas can enhance the surface reaction of the material, thereby promoting the efficient deposition of the carbon coating layer. The higher the density and the better the uniformity of the coating layer, the higher the protection strength of the coating layer for the second intermediate (i.e., the core body), the more effectively the reaction of silicon and water can be suppressed during the pulping process, and the reaction of silicon and electrolyte can also be suppressed during the battery cycle, thereby alleviating the gas production problem.

[0080] In some embodiments, the carbon material introduced includes one or more of gas phase carbon, liquid phase carbon and solid phase carbon. Among them, gas phase carbon may include one or more of methane, acetylene, ethylene, ethane, propane, propylene, benzene, toluene, xylene, ethanol, cyclohexane, methanol and acetone; liquid phase carbon may include one or more of n-hexane, toluene, benzene, xylene, methanol, ethanol, propanol, butanol, pentanol, acetone, butanone, 2-pentanone, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, amyl acetate and glycerol; solid phase carbon may include one or more of polyvinyl chloride, polyvinyl butyral, polyacrylonitrile, polyacrylic acid, polyethylene glycol, polypyrrole, polyaniline, sucrose, glucose, maltose, citric acid, asphalt, furfural resin, epoxy resin and phenolic resin.

[0081] The scheme of the present application will be explained below in conjunction with the embodiments. Those skilled in the art will understand that the following examples are only used to explain the present application and cannot be construed as limiting the present application. Unless otherwise stated, the reagents, software and instruments not specifically stated in the following embodiments are all conventional commercial products or open source.

[0082] Embodiment 1:

[0083] A negative electrode material based on a carbon matrix and silicon particles, the preparation method comprising:

[0084] S1, placing a porous carbon substrate (ie, substrate precursor) in a reaction chamber, heating it to 800° C. in a nitrogen atmosphere, and then heat-treating the porous carbon for 2 h with a mixed gas consisting of 90% nitrogen and 10% hydrogen to obtain a first intermediate.

[0085] S2, deliver silane, nitrogen and hydrogen to the heated chamber at a gas rate of 1L / min to 10L / min, with nitrogen providing an inert atmosphere and hydrogen assisting in the uniform distribution of silane in the reaction chamber. Control the silane concentration at 15%, raise the temperature to 520°C, and react for 12 hours to obtain a second intermediate.

[0086] S3, after the obtained second intermediate is cooled to room temperature, a carrier gas containing nitrogen, argon and hydrogen is continuously introduced, with nitrogen: argon: hydrogen = 14:5:1, and methane gas is introduced at a concentration of 15%. Heat treatment is performed at 700°C for 4 hours, and then the methane gas is turned off, nitrogen is introduced, and the temperature is lowered to room temperature.

[0087] S4, screening and grading the obtained samples to obtain the negative electrode material.

[0088] Embodiment 2:

[0089] The difference from Example 1 is that in S1, the mixed gas is adjusted to 85% nitrogen and 15% hydrogen, and the heat treatment time is adjusted to 1 hour;

[0090] In S3, "simultaneously introduce methane gas with a concentration of 15%" is adjusted to "simultaneously use a bubbling device to gasify xylene and adjust the concentration to 15%, and let it enter the reaction chamber together with the carrier gas". Accordingly, after the heat treatment and insulation is completed, "turn off the methane gas" is adjusted to "turn off the bubbling device".

[0091] Embodiment 3:

[0092] The difference from Example 1 is that in S1, the mixed gas is adjusted to 88% nitrogen and 12% hydrogen, and the heat treatment time is adjusted to 3h;

[0093] S3 is, after the obtained second intermediate is cooled to room temperature, the second intermediate and asphalt are mixed in a mass ratio of 50:15, and then the mixed material is placed in a high-temperature box furnace, the same carrier gas (nitrogen: argon: hydrogen = 14:5:1) is introduced, and heat treatment is carried out at 800°C for 2 hours.

[0094] Embodiment 4:

[0095] The difference from Example 1 is that in S3, the carrier gas is adjusted to nitrogen:argon:hydrogen=10:5:5.

[0096] Embodiment 5:

[0097] The difference from Example 1 is that in S1, the mixed gas is adjusted to 95% nitrogen and 5% hydrogen, and the heat treatment time is adjusted to 1 h;

[0098] In S3, “simultaneously introduce methane gas with a concentration of 15%, heat treat at 700°C, keep warm for 4 hours, and then turn off the methane gas” is adjusted to “simultaneously introduce propylene gas with a concentration of 10%, heat treat at 750°C, keep warm for 1 hour, and then turn off the propylene gas”;

[0099] A new step S3-1 is added after S3, and the material obtained in S3 is further coated with a metal salt, including: taking 14.2 g of isopropyl titanate and 1.98 g of lithium acetate and adding them to deionized water, and then adding 0.345 g of CTAB and stirring to obtain a mixed solution A; adding a chelating agent 2.89 g of citric acid to the prepared mixed solution A, adding 345 g of the sample obtained in S3, stirring for 1 hour to react to obtain a suspension B; then transferring the mixed solution to a beaker, stirring in a water bath at 60° C. for 12 hours to obtain a slurry C; spray drying the slurry C to obtain a metal salt Li 4 Ti 5 O 12 The material of the covering;

[0100] Finally, the negative electrode material is obtained through S4.

[0101] Embodiment 6:

[0102] The difference from Example 1 is that in S3, the carrier gas is adjusted to nitrogen:argon:hydrogen=25:3:1, and "simultaneously introducing methane gas with a concentration of 15%, heat treating at 700°C, keeping warm for 4 hours, and then turning off the methane gas" is adjusted to "simultaneously introducing propylene gas with a concentration of 10%, heat treating at 750°C, keeping warm for 1 hour, and then turning off the propylene gas".

[0103] Comparative Example 1:

[0104] The difference from Example 1 is that in S1, a porous carbon substrate (ie, substrate precursor) is placed in a reaction chamber to obtain a first intermediate, that is, the porous carbon is not subjected to heat treatment in S1.

[0105] Comparative Example 2:

[0106] The difference from Example 1 is that in S1, the mixed gas is adjusted to 60% nitrogen and 40% hydrogen.

[0107] Comparative Example 3:

[0108] The difference from Example 2 is that in S3, the carrier gas is adjusted to contain only nitrogen.

[0109] Comparative Example 4:

[0110] The difference from Example 3 is that S1 is to place a porous carbon substrate (i.e., substrate precursor) in a reaction chamber to obtain a first intermediate; i.e., the porous carbon is not subjected to heat treatment in S1;

[0111] In S3, the carrier gas is adjusted to nitrogen:argon:hydrogen=50:2:1.

[0112] Comparative Example 5:

[0113] The difference from Example 3 is that in S1, the mixed gas is adjusted to 60% nitrogen and 40% hydrogen;

[0114] In S3, the carrier gas is adjusted to nitrogen:argon:hydrogen=50:2:1.

[0115] The physical and electrochemical properties of the negative electrode materials obtained in the above examples 1-6 and comparative examples 1-5 were tested, and the testing methods were as follows:

[0116] 1. Test method for pore volume and proportion of carbon matrix and negative electrode material after removing silicon material:

[0117] The pore volume of a carbon matrix refers to the total volume of pores per unit mass of the carbon matrix, and the pore volume of a carbon matrix can be measured by gas adsorption measurement. Nitrogen adsorption is a technique that characterizes the porosity and pore size distribution of a material by condensing a gas in the pores of a solid. As the pressure increases, the gas first condenses in the pores with the smallest diameter, and the pressure increases until a saturation point is reached, at which all pores are filled with liquid. The nitrogen pressure is then gradually reduced to evaporate the liquid from the system. Analysis of the adsorption and desorption isotherms enables the determination of the pore volume and pore size distribution, as well as the proportion of the respective pore volumes of micropores, mesopores, and macropores in the total pore volume. Among them, the method of removing silicon material from the negative electrode material: adding 1M concentrated nitric acid to the negative electrode material and soaking it for 4h, and then dripping a 20% mass fraction HF acid solution into the negative electrode material drop by drop, yellow smoke will be generated, and the dripping is repeated many times until no yellow smoke is generated in the solution; finally, the residue is digested with 1M concentrated nitric acid, and then washed and dried to obtain the negative electrode material after the silicon material is removed. The present application may consider that the negative electrode material is a carbon matrix without silicon material.

[0118] 2. Test method for median particle size of negative electrode materials:

[0119] The particle size distribution range of the negative electrode material was tested by Malvern laser particle size analyzer (Mastersizer 3000), and the volume-based cumulative particle size distribution of the particle size distribution was measured by laser diffraction method. D10 represents the particle size corresponding to 10% of the cumulative particle size distribution percentage of the powder, D50 represents the particle size corresponding to 50% of the cumulative particle size distribution percentage, and D90 represents the particle size corresponding to 90% of the cumulative particle size distribution percentage, thereby obtaining the particle size distribution of the negative electrode material (D90-D10) / D50.

[0120] 3. Coating thickness:

[0121] The material was sectioned using a FIB-SEM device, 10 particles were randomly selected in the SEM, the coating thickness of each particle was measured three times, and the average coating thickness was obtained.

[0122] 4. Powder conductivity test:

[0123] The four-probe method is used to determine the volume resistivity of the sample. The MCP-PD51 powder conductivity meter produced by Mitsubishi Chemical of Japan can measure the conductivity of the powder at five pressure points of 4KN, 8KN, 12KN, 16KN and 20KN, and then the computer automatically calculates the resistivity of the powder.

[0124] 5. Test method for mass content of carbon: Using the G4 ICARUS HF infrared carbon-sulfur analyzer from Bruker, Germany, the sample is burned under high temperature and oxygen-rich conditions, and the carbon element it contains is oxidized into carbon dioxide. The generated gas enters the infrared detector with the carrier gas, and the carbon content can be calculated by quantitatively analyzing the changes in the carbon dioxide signal.

[0125] 6. Test method for mass content of silicon element: Use Nanyang Xinyu's SA2-9-17TP box-type atmosphere furnace to burn in an oxygen atmosphere, so that the silicon in the sample reacts into silicon dioxide, and the carbon is burned and converted into carbon dioxide and discharged, and the silicon content is calculated by weighing.

[0126] 7. Specific surface area test method: The specific surface area of ​​the powder sample was tested by gas adsorption method (nitrogen adsorption multi-point BET) at low temperature controlled by liquid nitrogen cooling using the American Micromeritics TriStar 3000 & 3020 specific surface area and pore size analyzer.

[0127] 8. Gas production test method:

[0128] a) Place 5 g of negative electrode material in a sealed container and add deionized water until the remaining volume above the container is 60 ml;

[0129] b) sealing the container, mixing well, and storing at room temperature for one day;

[0130] c) After 24 hours, shake the container again to disperse the precipitate into the liquid.

[0131] d) Open the lid of the container, use a hydrogen detector to detect the concentration of hydrogen, and convert it to cc / (kg·day).

[0132] 9. γ test of the protective strength of the coating layer: TG method is used to characterize the protective strength of the coating layer. Under oxygen atmosphere conditions, the test temperature range is 25℃ to 1200℃.

[0133] 10. Oxygen percentage P at different depths xTest: The negative electrode material was tested by X-ray photoelectron spectroscopy using the American Thermo Scientific K-Alpha equipment. The entire test process was carried out in a glove box filled with argon gas. Each time, a fine spectrum test was performed on the O element. The argon ion sputtering depth was set to 20nm each time, and the sputtering was performed 8 times. This application defines the sputtering depth as 20nm for the first time, 40nm for the second time, and so on. The specific test mode is: spectrum acquisition + etching 20nm, and sputtering 8 times in this way. The maximum value of the test results at different depths was selected as the test result, and the test result is the atomic percentage ratio.

[0134] 11. Electrochemical performance test:

[0135] a) The negative electrode material is respectively mixed with sodium carboxymethyl cellulose, styrene-butadiene rubber, conductive graphite (KS-6) and carbon black (SP) in a ratio of 92:2:2:2:2 to prepare a slurry, which is uniformly coated on a copper foil and dried to form a negative electrode sheet, which is assembled into a button cell in an argon atmosphere glove box, the separator used is a polypropylene microporous membrane, the electrolyte used is 1 mol / L lithium hexafluorophosphate (the solvent is a mixed slurry of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate), and the counter electrode used is a metal lithium sheet.

[0136] b) The button cell was charged and discharged on a battery program-controlled tester. The test conditions for the first discharge specific capacity and the first coulombic efficiency were as follows: the charge and discharge rate was 0.1C, and the voltage range was 0.01V to 1.5V; the average value C of the first charge capacity of the four button cells was calculated. (cha) , in milliampere hours (mAh) and the average value of the first discharge capacity C (dis) , in milliampere hours (mAh); calculate the first charge specific capacity Q of the sample (cha) 、First discharge specific capacity Q (dis) , the first Coulomb efficiency E, where Q (cha) =C (cha) / m,Q (dis) =C (dis) / m, E = Q (dis) / Q (cha) × 100%, where m is the mass of active substance in grams (g).

[0137] 12. Expansion rate test: The button cell was charged and discharged 50 times at a current density of 1C in the charge and discharge range of 0.01V to 1.5V, and then the electrode was observed with a scanning electron microscope. According to the corresponding scale, the average particle size of the silicon particles before lithium insertion was obtained to be D1; after the charge and discharge cycle, the button cell was disassembled, the electrode was cleaned with DEC, and the electrode was observed with a scanning electron microscope. According to the corresponding scale, the average particle size of the silicon particles after lithium insertion was obtained to be D2; considering the silicon particles as a spherical structure, the expansion rate is (D2-D1) / D1.

[0138] The above test results are shown in Table 1.

[0139] Table 1. Test results of physical properties and electrochemical properties of negative electrode materials of Examples 1-6 and Comparative Examples 1-5 of the present application

[0140]

[0141]

[0142] Please refer to Table 1 for the coating layer protection strength γ and oxygen percentage content P of the negative electrode materials of Examples 1-6 of the present application. x All satisfy 4≤γ≤9 and 2≤P x ≤8, and it has been determined that these negative electrode materials also have significantly lower gas generation values. When applied to button batteries, it has been determined that these negative electrode materials also have significantly lower expansion rates, and the button batteries all have significantly higher initial specific capacity and initial coulombic efficiency, indicating that when the coating layer protection strength γ meets the preset range, the coating layer has good density, thickness and uniformity, and the silicon particles in the coated core are not easy to be exposed, nor are they easy to seep through the coating layer to contact and react with external substances (such as slurry or electrolyte) to reduce the gas generation of the negative electrode material and improve the electrochemical performance of the secondary battery. When the percentage content of oxygen P x When the preset range is also met, the corresponding oxide layer can provide protection for the silicon particles, which is beneficial to reducing the gas generation value and expansion rate of the negative electrode material.

[0143] Compared with Example 1, since the porous carbon matrix was not heat-treated in Comparative Example 1, the percentage of oxygen in the matrix was higher, resulting in the P x If the concentration is too high, there will be more oxide layers around the silicon particles. During the charge and discharge cycle, the silicon oxide in the oxide layer will form lithium silicate compounds. This part of lithium cannot participate in the reversible reaction, resulting in a decrease in the first coulombic efficiency and reversible capacity of the resulting button battery.

[0144] Compared with Example 1, in Comparative Example 2, when the porous carbon matrix is ​​heat-treated, the content of hydrogen in the mixed gas is too high, resulting in a low percentage of oxygen in the matrix, resulting in a low content of P in the obtained negative electrode material. x If the temperature is too low, there will be less oxide layer around the silicon particles, and the protective effect on the silicon particles will be significantly reduced, resulting in serious gas production and serious expansion rate, affecting the electrochemical performance of the resulting button battery.

[0145] It can also be concluded from Example 1 and Comparative Examples 1-2 that the negative electrode materials obtained in Comparative Examples 1-2 contain not only P x The protective strength γ of the coating layer is not as good as that of Example 1, which indicates that Px Exceeding the preset range (too high or too low) will also have a certain adverse effect on the protective strength γ of the negative electrode material coating layer. x Within the preset range, negative electrode materials whose γ satisfies the preset range can be obtained more efficiently, and the undesirable phenomena of gas production and expansion of the negative electrode materials can be improved at the same time, which is beneficial to improving the electrochemical performance of the button battery.

[0146] Compared with Example 2, during carbon coating, the carrier gas in Comparative Example 3 contains only nitrogen and lacks hydrogen and argon. The carbon material used for carbon coating will produce hydrogen during high-temperature cracking. Due to the lack of hydrogen in the reaction environment, it is difficult to control the cracking rate of the carbon material, resulting in a decrease in the density and uniformity of the carbon coating layer. In addition, due to the lack of argon, the surface reaction activity of the material is also reduced, resulting in poor carbon coating effect, which leads to a low protection strength γ of the coating layer of the obtained negative electrode material, making it easier for silicon particles to be exposed and directly contact with external substances, or easier to pass through the coating layer and contact with external substances when falling off, resulting in instability of the negative electrode material and serious reaction gas production, thereby seriously reducing the electrochemical performance of the secondary battery.

[0147] Compared with Example 3, Comparative Example 4 did not perform heat treatment on the porous carbon substrate, while in Comparative Example 5, the content of hydrogen in the mixed gas for heat treatment of the porous carbon substrate was too high, resulting in the P x Too high and P of comparative example 5 x In addition, in Comparative Examples 4 and 5, the proportion of hydrogen in the carrier gas during the carbon coating process was also reduced. The insufficient hydrogen content made it difficult to control the cracking rate of the carbon material, resulting in a decrease in the density and uniformity of the carbon coating layer. The protective strength γ of the coating layer of the formed negative electrode material was also too low, which ultimately led to serious gas production and expansion of the material, seriously reducing the initial specific capacity and initial coulombic efficiency of the obtained button battery.

[0148] The above implementation modes are only used to illustrate the technical solutions of the present application and are not intended to limit the present application. Although the present application has been described in detail with reference to the above preferred implementation modes, a person skilled in the art should understand that the technical solutions of the present application may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present application.

Claims

1. A negative electrode material, comprising a core and a coating layer covering at least a portion of the surface of the core, wherein the core comprises a substrate and an active material disposed on the substrate, characterized in that: The protective strength of the coating layer is defined as γ, γ=0.0129T0-0.323, wherein 4≤γ≤9, T0 is the starting temperature of the mass increase of the negative electrode material in thermogravimetric analysis, and the thermogravimetric analysis includes measuring the mass of the negative electrode material under an oxygen atmosphere; The negative electrode material further includes an oxygen element, and the atomic percentage content of the oxygen element at any point on the surface of the negative electrode material at a distance X from the inside of the negative electrode material is defined as P. x , 60nm≤X≤160nm, 2≤P x ≤8.

2. The negative electrode material according to claim 1, characterized in that Along the surface of the negative electrode material pointing to the inside of the negative electrode material, the P x Lower first and then raise.

3. The negative electrode material according to claim 1, characterized in that The substrate satisfies at least one of the following conditions: (1) The specific surface area of ​​the substrate is 800 m 2 / g to 3000m 2 / g; (2) The substrate has pores, and the total pore volume of the substrate is 0.5 cm 3 / g to 2.0cm 3 / g; (3) The substrate has pores, and the porosity of the substrate is 40% to 80%.

4. The negative electrode material according to claim 1, characterized in that The negative electrode material satisfies at least one of the following conditions: (1) The average particle size of the active substance is 0.1 nm to 100 nm; (2) The thickness of the coating layer is 1 nm to 60 nm; (3) Based on the mass of the negative electrode material, the mass proportion of the coating layer is less than or equal to 10%; (4) The median particle size D50 of the negative electrode material is 3 μm to 15 μm; (5) The specific surface area of ​​the negative electrode material is 1 m 2 / g to 10m 2 / g.

5. The negative electrode material according to claim 1, characterized in that The matrix includes one of a carbon matrix and a non-carbon matrix; The carbon matrix includes one or more of hard carbon, soft carbon, graphite, carbon nanotubes, carbon fibers and graphene; The non-carbon matrix includes at least one of metal oxides, silicides, silicates, phosphates, titanates and aluminum borates; the metal oxides include one or more of aluminum oxide, zirconium oxide, germanium dioxide and manganese dioxide; the silicides include one or more of silicon carbide and silicon nitride; the silicates include one or more of cordierite, mullite and zeolite; the phosphates include one or more of aluminum phosphate, magnesium phosphate, calcium phosphate, titanium phosphate, chromium phosphate, cobalt phosphate, nickel phosphate, germanium phosphate, zirconium phosphate, niobium phosphate, molybdenum phosphate, tantalum phosphate, tungsten phosphate and lanthanum phosphate; the titanates include one or more of calcium titanate, iron titanate, lithium titanate and barium titanate.

6. The negative electrode material according to claim 1, characterized in that The active material includes one or more of silicon-based materials, Li, Na, K, Sn, Ge, Fe, Mg, Ti, Zn, Al, P and Cu; The silicon-based material includes one or more of amorphous silicon, crystalline silicon, silicon oxide, silicon alloy, and a composite of crystalline silicon and amorphous silicon.

7. The negative electrode material according to claim 1, characterized in that The coating layer includes one or more of carbon materials, metal oxides, amorphous silicon, conductive polymers, fluorides, phosphates and nitrides.

8. The negative electrode material according to claim 1, characterized in that The substrate includes a carbon substrate, the active material includes silicon particles, the coating layer includes a carbon coating layer, and the negative electrode material further satisfies at least one of the following conditions: (1) Based on the mass of the negative electrode material, the mass proportion of silicon element in the negative electrode material is 20% to 65%; (2) Based on the mass of the negative electrode material, the mass proportion of silicon element in the negative electrode material is 45% to 55%; (3) Based on the mass of the negative electrode material, the mass proportion of carbon element in the negative electrode material is 30% to 60%; (4) Based on the mass of the negative electrode material, the mass proportion of carbon element in the negative electrode material is 45% to 55%.

9. A negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, characterized in that: The negative electrode active material layer comprises the negative electrode material according to any one of claims 1 to 8.

10. A secondary battery, characterized in that: Comprising the negative electrode sheet as claimed in claim 9.

Images