Negative electrode material, negative electrode sheet, and secondary battery

Abstract

A negative electrode material, a negative electrode sheet, and a secondary battery. The negative electrode material includes a core and a carbon coating layer that coats at least part of the surface of the core, the core including a base and an active material. The 10-day gas generation amount A of the negative electrode material is 100 mL / kg or less, the 10-day gas generation amount A being measured by a drainage method. The residual carbon rate γ of the negative electrode material is 20% or less, the negative electrode material being stirred in a slurry stirring tank, the mass of the stirred negative electrode material being m1, the negative electrode material being immersed in a hydrofluoric acid solution for 1 h, and the mass after cleaning and drying being m2, the mass of the negative electrode material being m1, the negative electrode material being immersed in a hydrofluoric acid solution for 1 h, and the mass after cleaning and drying being m3. The secondary battery based on the above negative electrode material has a low electrode sheet expansion rate, good capacity, and cycle stability.

Description

TECHNICAL FIELD

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

[0002] Silicon negative electrode materials have a high theoretical energy density, and are therefore generally considered to have the potential to improve the performance of lithium ion batteries. Existing silicon materials are often combined with metals, oxides, organic polymers, carbon and other materials to obtain negative electrode materials with better conductivity and lower expansion effect, such as negative electrode materials based on amorphous silicon and carbon matrix, which are considered to have the advantages of high initial efficiency, high capacity, low expansion and good cycle stability. However, it has been found in practice that such materials have the problem that the surface coating layer is easily detached, thereby causing the expansion performance and capacity of the material to be less than expected, and the performance of the material to decay. SUMMARY

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

[0004] To achieve the above-mentioned purpose, the present application provides a negative electrode material, which comprises a core and a carbon coating layer coating at least part of the surface of the core, and the core comprises a matrix and an active material. The 10-day gas production A of the negative electrode material is less than or equal to 100 mL / kg. The residual carbon rate γ of the negative electrode material is defined as The residual carbon rate γ is less than or equal to 20%. The test method of the 10-day gas production A comprises: placing 50 g of the negative electrode material in a 300 mL slurry stirring tank, adding 50 g of sodium carboxymethyl cellulose with a mass fraction of 5% and 100 mL of pure water into the stirring tank, stirring at a frequency of 50 Hz for 1 h to obtain a slurry, placing the slurry in an aluminum plastic film, and measuring the 10-day gas production A of the slurry by the drainage method. The test method of m2 comprises: placing the negative electrode material in a slurry stirring tank and stirring at a frequency of 50 Hz for 1 h, taking out the stirred negative electrode material with a mass of m1, immersing it in a 20% hydrofluoric acid solution for 1 h, taking out the immersed negative electrode material, and measuring the mass m2 after cleaning and drying. The test method of m3 comprises: taking the negative electrode material with a mass of m1, immersing it in a 20% hydrofluoric acid solution for 1 h, taking out the immersed negative electrode material, and measuring the mass m3 after cleaning and drying.

[0005] In some possible implementations, the powder conductivity of the negative electrode material at 20 kN is 0.5 S / cm to 9.5 S / cm.

[0006] In some possible implementations, the ID / IG of the negative electrode material is 0.5 to 5.0.

[0007] In some possible implementations, the specific surface area of the negative electrode material is less than or equal to 5 m 2 / g.

[0008] In some possible implementations, the total pore volume of the negative electrode material is 0.001 cm 3 / g to 0.1 cm 3 / g.

[0009] In some possible implementations, the compaction density of the negative electrode material is 0.8 g / cm 3 to 1.2 g / cm 3 .

[0010] In some possible implementations, the negative electrode material includes micropores, mesopores and macropores, the volume percentage of the micropores is less than or equal to 5%, the volume percentage of the mesopores is 87% to 97%, and the volume percentage of the macropores is less than or equal to 13%, based on the total pore volume of the negative electrode material.

[0011] In some possible implementations, the particle size D10 of the negative electrode material is 1 μm to 5 μm, in some possible implementations, the particle size D50 of the negative electrode material is 6 μm to 16 μm, and in some possible implementations, the particle size D90 of the negative electrode material is 16 μm to 24 μm.

[0012] In some possible implementations, the thickness of the carbon coating layer is 10 nm to 1000 nm.

[0013] In some possible implementations, the active substance includes one or more of Si, Sn, Ge, Pb, Ag, Mg, Zn, Ga, In, Sb, Bi and alloy materials thereof.

[0014] In some possible implementations, the active substance includes a silicon material, the silicon material includes silicon particles, and the silicon particles include one of amorphous silicon and a composite of crystalline silicon and amorphous silicon.

[0015] In some possible implementations, the silicon material includes at least one of silicon oxide and a silicon alloy.

[0016] In some possible implementations, the silicon material includes silicon particles and a silicon oxide layer located on the surface of the silicon particles, and the silicon oxide layer includes silicon oxide.

[0017] In some possible implementations, the average particle size of the active substance is 0.1 nm to 500 nm.

[0018] In some possible implementations, the total pore volume of the substrate is 0.5 cm 3 / g to 2.0 cm 3 / g.

[0019] In some possible implementations, the substrate has a specific surface area of ​​600 m 2 / g to 3000m 2 / g.

[0020] In some possible implementations, the matrix includes a carbon matrix, and the carbon matrix includes one or more of amorphous carbon, graphitized carbon, mesocarbon microbeads, and carbon gel.

[0021] In some possible implementations, the matrix includes a non-carbon matrix, and the non-carbon matrix includes one or more of metal oxides, silicides, silicates, phosphates, titanates, and aluminum borates.

[0022] In some possible implementations, the matrix of the negative electrode material includes a carbon matrix, the active material of the negative electrode material includes a silicon material, and based on the mass of the negative electrode material, the mass proportion of the carbon element of the negative electrode material is 40% to 60%, or the mass proportion of the silicon element of the negative electrode material is 35% to 55%.

[0023] 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.

[0024] The present application also provides a secondary battery, comprising the above-mentioned negative electrode sheet.

[0025] In the present application, by providing a carbon coating layer on at least part of the surface of the core body, the carbon coating layer can protect the core body, thereby reducing the gas production A of the negative electrode material, and also helping to reduce the expansion of the negative electrode material during the cycle, thereby improving the cycle stability of the secondary battery based on this negative electrode material. At the same time, under the premise that the negative electrode material has a certain amount of carbon coating to reduce the gas production, the residual carbon rate γ of the negative electrode material is less than or equal to 20%, so the carbon coating layer is also more dense, and the arrangement between carbon atoms is closer, which is conducive to forming sufficient conductive channels and facilitating the transmission of electrons in the carbon coating layer, thereby maintaining good electrical conductivity. High-conductivity materials generally have better ion conductivity, which helps to reduce the internal resistance of the battery and improve the charge and discharge efficiency, thereby helping to improve the capacity and first coulombic efficiency of the resulting secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1A This is a schematic structural diagram of a secondary battery provided in one embodiment of the present application during charging.

[0027] Figure 1B This is a schematic structural diagram of a secondary battery provided in one embodiment of the present application during discharge.

[0028] Description of main component symbols

[0029] Electrode assembly 100

[0030] positive electrode sheet 101

[0031] negative electrode sheet 102

[0032] separator 103 DETAILED DESCRIPTION

[0033] Embodiments of the present application are described in detail below. The embodiments described below with reference to the drawings are exemplary and are only used to explain the present application and cannot be understood as a limitation of the present application; it should be noted that, unless otherwise defined, all technical and scientific terms used herein have the same meaning as understood by those skilled in the art in the technical field to which the present application belongs; the embodiments and the features in the embodiments of the present application can be combined with each other without conflict; in the following description, many specific details are set forth in order to provide a thorough understanding of the present application, and the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments.

[0034] The negative electrode material obtained by compounding silicon material and carbon material has excellent electrochemical performance, such as the advantages of high initial efficiency, high capacity, low expansion, and good cycle stability of amorphous silicon-carbon material. However, the coating layer on the surface of the amorphous silicon-carbon material is easy to fall off, resulting in performance degradation of the material, so how to prepare a low-expansion, high-capacity silicon-carbon composite material with a stable outer coating layer has become one of the development directions of negative electrode materials.

[0035] The present application researches and finds that improving the preparation process of the negative electrode material helps to achieve a coating layer with better compactness, thereby achieving the purpose of reducing the falling off of the coating layer on the surface of the negative electrode material.

[0036] Based on this, an embodiment of the present application provides a secondary battery, which comprises a shell, an electrode assembly, and an electrolyte. The electrode assembly and the electrolyte are both located in the shell.

[0037] The shell can be a packaging bag obtained by packaging with a packaging film (such as an aluminum plastic film), for example, a soft package battery. In other embodiments, it can also be a steel shell battery, an aluminum shell battery, etc.

[0038] Please refer to Figure 1A or Figure 1B The electrode assembly 100 comprises a positive electrode sheet 101, a negative electrode sheet 102, and a separator 103, and the separator 103 is arranged between the positive electrode sheet 101 and the negative electrode sheet 102. When the electrolyte (not shown in the figure) is arranged, during charging, please refer to Figure 1A Active ions (such as lithium ions) are deintercalated from the crystal lattice of the positive electrode material (such as a 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 inserted into the crystal lattice of the negative electrode material. During discharging, please refer to Figure 1BActive ions (e.g., lithium ions) are deintercalated from the crystal lattice of the negative active material of the negative electrode sheet 102, pass through the separator 103 via the electrolyte, reach the positive electrode sheet 101, and are intercalated into the crystal lattice of the positive active material (e.g., a lithiated intercalation compound), generating electrons that move from the negative electrode sheet 102 to the positive electrode sheet 101 via an external circuit. The reverse movement of the electrons forms an electric current that can be used by an electrical device.

[0039] In some embodiments, the electrode assembly 100 can be a stacked structure 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 can also be a wound structure formed by winding the positive electrode sheet 101, the separator 103, and the negative electrode sheet 102 after being alternately stacked.

[0040] Positive electrode sheet

[0041] The positive electrode sheet 101 includes a positive current collector and a positive active material active layer disposed on at least one surface of the positive current collector. The positive current collector can be an aluminum foil or a nickel foil, or any composite current collector known in the art, such as, but not limited to, a current collector formed by combining a conductive foil and a polymer substrate. The positive active material active layer includes a positive active material, which includes a compound that reversibly intercalates and deintercalates lithium ions (i.e., a lithiated intercalation compound). In some embodiments, the positive active material can 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 active material can include, but is not limited to, at least one of lithium cobaltate (LiCoO2), lithium nickel manganese cobalt ternary material (NCM), lithium manganate (LiMn2O4), lithium nickel manganate (LiNi 0.5 Mn 1.5 O4), or lithium iron phosphate (LiFePO4).

[0042] The positive active material active layer also includes a binder to bind the positive active material particles to facilitate the formation of a film layer and to improve the adhesion between the positive active material active layer and the positive current collector. In some embodiments, the binder can include, but is not limited to, at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, an ethylene oxide-containing polymer, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene butadiene rubber, acrylated styrene butadiene rubber, epoxy resin, or nylon.

[0043] The positive material active layer can 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 can 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 can include, but is not limited to, metal powder or metal fiber, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer can be a polyphenylene derivative.

[0044] Negative electrode sheet

[0045] The negative tab 102 includes a negative current collector and a negative material active layer disposed on at least one surface of the negative current collector. The negative current collector can use at least one of a copper foil, a nickel foil, a stainless steel foil, a titanium foil, or a carbon-based current collector, and can also 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.

[0046] The negative material active layer includes a negative material, which includes a core and a carbon coating layer coating at least part of the surface of the core, and the core includes a matrix and an active substance. The 10-day gas production A of the negative material is less than or equal to 100 mL / kg. For example, the 10-day gas production A of the negative material can be 100 mL / kg, 85 mL / kg, 70 mL / kg, 65 mL / kg, 30 mL / kg, 20 mL / kg, 5 mL / kg, 4 mL / kg, 3 mL / kg, or any value within the range formed by any two of the above values. The test method of the 10-day gas production A includes: placing 50 g of the negative material in a 300 mL slurry stirring tank, adding 50 g of sodium carboxymethyl cellulose with a mass fraction of 5% and 100 mL of pure water to the stirring tank, stirring at a frequency of 50 Hz for 1 h, obtaining a slurry, and placing the slurry in an aluminum plastic film. The 10-day gas production A of the slurry is measured by the drainage method.

[0047] The present application sets a carbon coating layer on at least part of the surface of the core, which can protect the core, reduce the gas production A of the negative material, and also help to reduce the expansion of the negative material during the cycle, thereby improving the cycle stability of the secondary battery based on the negative material.

[0048] Definition of the residual carbon rate of the negative material The residual carbon rate γ is less than or equal to 20%. For example, the residual carbon rate γ of the negative electrode material can be 20%, 16%, 15%, 12%, 9%, 8%, 5%, 4%, 3%, or any value within a range defined by any two of the above values. The test method of m2 includes: placing the negative electrode material in a slurry stirring tank for stirring, the stirring frequency is 50 Hz, the stirring time is 1 h, taking out the stirred negative electrode material with a mass of m1, placing it in a 20% hydrofluoric acid solution for 1 h, taking out the soaked negative electrode material, and cleaning and drying to obtain a mass of m2. The test method of m3 includes: taking the negative electrode material with a mass of m1, placing it in a 20% hydrofluoric acid solution for 1 h, taking out the soaked negative electrode material, and cleaning and drying to obtain a mass of m3.

[0049] The embodiments of the present application improve the coating process of the negative electrode material, adjust the compactness of the obtained negative electrode material coating layer, and thus achieve the purpose of reducing the above residual carbon rate. In the present application, the above residual carbon rate represents the proportion of the mass of the part of the core coated by the unstable carbon coating layer in the total mass of the negative electrode material. In the present application, the negative electrode material is stirred by an external force to change the coating effect of the carbon coating layer, so that part of the carbon coating layer loses the protective effect on the coated core. Then, the part of the core is dissolved or reacts in the acid solution, and thus the negative electrode material loses a part of the mass. In this process, the relatively stable carbon coating layer under the action of the external force still has a protective effect on the coated core, so as to reduce the dissolution or reaction of part of the core in the acid solution, and thus the part of the core is retained in the negative electrode material and participates in the remaining mass of the negative electrode material. Therefore, the part of the mass lost by the negative electrode material is used to represent the mass of the part of the core coated by the unstable carbon coating layer, and the calculation formula of the above residual carbon rate γ is constructed, and the coating compactness of the carbon coating layer of the negative electrode material is further represented.

[0050] The present application further finds that, under the premise that the negative electrode material has a certain carbon coating amount to reduce the gas production, in the negative electrode material with the residual carbon rate γ less than or equal to 20%, the protection effect of the carbon coating layer on the core is better, so that the gas production behavior of the negative electrode material is significantly reduced, and the swelling effect is also significantly reduced, and the cycle stability of the secondary battery based on the negative electrode material is obviously improved; and in the negative electrode material with the residual carbon rate γ less than or equal to 20%, the compactness of the carbon coating layer is also higher, and the arrangement between carbon atoms is tight enough, which is conducive to forming sufficient conductive channels, maintaining the movement of electrons in the carbon layer, thereby maintaining good electrical conductivity. The material with high electrical conductivity usually has better ion conduction capacity, which helps to reduce the internal resistance of the battery and improve the charge and discharge efficiency, thereby helping to improve the capacity and the first coulomb efficiency of the obtained secondary battery. When the residual carbon rate γ is greater than 20%, the protection effect of the carbon coating layer on the core in the negative electrode material is reduced, resulting in a dramatic gas production behavior of the negative electrode material, and the compactness of the carbon layer is reduced, resulting in a reduction in the efficiency of electron passing through the carbon layer, so that the swelling effect of the negative electrode material cannot be effectively alleviated, reducing the cycle stability of the obtained secondary battery, and the capacity and the first coulomb efficiency of the obtained secondary battery are also negatively affected.

[0051] The negative electrode material of the present application meets the above-mentioned 10-day gas production A and residual carbon rate γ falling within the preset range, and such setting can also avoid the phenomenon that the negative electrode material with very low coating amount also has a low residual carbon rate γ, but the gas production of the negative electrode material with very low coating amount is large, and does not have good performance.

[0052] In some embodiments, the residual carbon rate γ of the negative electrode material is less than or equal to 10%, and the 10-day gas production A is less than or equal to 50 mL / kg. For example, the residual carbon rate γ of the negative electrode material can be 9%, 8%, 5%, 4%, 3%, or any value within the range composed of any two of the above values; the 10-day gas production A can be 50 mL / kg, 30 mL / kg, 20 mL / kg, 5 mL / kg, 4 mL / kg, 3 mL / kg, or any value within the range composed of any two of the above values. The negative electrode material with the residual carbon rate γ less than or equal to 10% exhibits a greater degree of reduced gas production behavior, and the protection effect of the carbon coating layer on the core is more obvious, which can further reduce the contact of the core with external substances. The 10-day gas production A of the negative electrode material is controlled within the above range, which indicates that the negative electrode material itself has a certain coating amount, and on this basis, a coating layer with excellent compactness is formed through process improvement.

[0053] In some embodiments, the powder conductivity of the negative electrode material at 20 kN is 0.5 S / cm to 9.5 S / cm. For example, the powder conductivity can be 0.5 S / cm, 1.0 S / cm, 1.5 S / cm, 2.0 S / cm, 3.0 S / cm, 4.5 S / cm, 5.5 S / cm, 6.5 S / cm, 7.5 S / cm, 8.5 S / cm, 9.5 S / cm, or any value within a range between any two of the aforementioned values. The powder conductivity within the aforementioned range indicates that the negative electrode material has good electrical conductivity, and the electron has good passability in the carbon coating layer, thereby helping to improve the capacity and the first coulombic efficiency of the obtained secondary battery.

[0054] In some embodiments, the ID / IG of the negative electrode material is 0.5 to 5.0. The ID is the intensity of the D peak in the Raman spectrum of the negative electrode material, and the IG refers to the intensity of the G peak in the Raman spectrum of the negative electrode material. The ID / IG value can be used to characterize the degree of surface defects of the negative electrode material. In this application, when the ID / IG value is within the aforementioned range, it indicates that the graphitization degree of the carbon material in the carbon coating layer is higher, i.e., the arrangement of carbon atoms is more ordered, thereby forming more sp 2 hybrid carbon atoms in the in-plane vibration (G peak). This ordered structure is conducive to the transmission of electrons and improves the electrical conductivity of the material. Due to the rearrangement of carbon atoms during the carbonization process to form a more stable layered structure, the voids and defects are reduced, and therefore the carbon coating layer with a higher graphitization degree often has a more compact structure, i.e., higher compactness. Therefore, controlling the ID / IG within the aforementioned range is conducive to improving the compactness of the carbon coating, thereby helping to reduce the residual carbon rate γ.

[0055] In some embodiments, the specific surface area of the negative electrode material is less than or equal to 5 m 2 / g, specifically 0.5 m 2 / g, 1 m 2 / g, 1.5 m 2 / g, 2 m 2 / g, 2.5 m 2 / g, 3 m 2 / g, 3.5 m 2 / g, 4 m 2 / g, 4.5 m 2 / g, 5 m 2 / g, or any value within a range between any two of the aforementioned values. It can be understood that the specific surface area of the negative electrode material will affect the contact area between the negative electrode material and the electrolyte. When the specific surface area of the negative electrode material is within the aforementioned range, the amount of lithium ions consumed by the SEI film formed during the first charge-discharge process of the battery prepared from the negative electrode material can be reduced, and the irreversible capacity loss of the battery can be reduced.

[0056] In some embodiments, the particle size D10 of the negative electrode material is 1 μm to 5 μm. For example, the particle size D10 of the negative electrode material can be 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, or any value within a range between any two of the above values.

[0057] In some embodiments, the particle size D50 of the negative electrode material is 6 μm to 16 μm. For example, the particle size D50 of the negative electrode material can be 6 μm, 6.5 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 14 μm, 16 μm, or any value within a range between any two of the above values.

[0058] In some embodiments, the particle size D90 of the negative electrode material is 16 μm to 24 μm. For example, the particle size D90 of the negative electrode material can be 16 μm, 16.5 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 24 μm, or any value within a range between any two of the above values.

[0059] The volume-based cumulative particle size distribution D10 measured represents the particle size corresponding to 10% of the cumulative particle size distribution percentage, 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. When the particle size of the negative electrode material is within the above range, the diffusion path of lithium ions is short, the time for lithium ion intercalation and deintercalation can be ensured, the negative electrode material can achieve a state of fast and sufficient intercalation of lithium, and thus the charge and discharge performance of the battery can be ensured. In addition, when the particle size distribution of the negative electrode material is within the above range, the large particles with a large particle size and the small particles with a small particle size of the negative electrode material can cooperate with each other, the small particles fill the pores between the large particles, the tap density of the negative electrode material is improved, and thus the close degree between the particles of the positive electrode material is improved, the transport of active particles and the conduction of electrons are improved, the energy density of the obtained secondary battery is improved, the cycle life is prolonged, and the safety performance is improved.

[0060] In some embodiments, the carbon coating layer has a thickness of 10 nm to 1000 nm. For example, the carbon coating layer can have a thickness of 0.1 nm, 0.5 nm, 1 nm, 30 nm, 60 nm, 100 nm, 150 nm, 200 nm, 300 nm, 500 nm, 800 nm, 1000 nm, or any value within a range defined by any two of the above values. The carbon coating layer can coat the active material exposed on the surface of the core, reduce the direct contact between the active material and the electrolyte, thereby reducing the solubility of the negative electrode material and the amount of gas generated by the reaction between the dissolved active material (such as silicon particles) and the electrolyte, and reducing the risk of generating a large amount of SEI caused by the exposed active material during the charging and discharging process. Controlling the thickness of the carbon coating layer within the above range is beneficial to maintaining the stability of the particle structure of the negative electrode material during the cycle process and maintaining the good electrical conductivity of the negative electrode material, thereby helping to improve the specific capacity and cycle stability performance of the obtained secondary battery.

[0061] In some embodiments, the carbon coating layer can be a single-layer carbon coating layer formed of a single material as described above, a carbon coating layer formed of a combination of multiple materials as described above, a multi-layer carbon coating layer formed of a single material, a multi-layer carbon coating layer formed of multiple materials, or the like, and the layer structure of the carbon coating layer can be selected as needed. It can be understood that when the carbon coating layer has a multi-layer coating structure, it has a higher density.

[0062] In some embodiments, the active material includes one or more of Si, Sn, Ge, Pb, Ag, Mg, Zn, Ga, In, Sb, Bi, and alloy materials thereof.

[0063] In some embodiments, the active material includes a silicon material, the silicon material includes silicon particles, and the silicon particles include one of amorphous silicon, crystalline silicon, and a composite of crystalline silicon and amorphous silicon. When the active material includes a silicon material, the silicon material serves as a component of the negative active material, which can improve the specific capacity of the negative electrode material and further improve the energy density of the secondary battery. Preferably, the silicon material includes amorphous silicon. When the active material further includes amorphous silicon, the amorphous silicon isotropically expands during lithium intercalation, which can reduce the collapse of pores and thus help to reduce the expansion effect, and can inhibit the rapid decay of the specific capacity of the negative electrode material, which helps to improve the lithium intercalation cycle performance of the negative electrode material, thereby helping to improve the initial coulombic efficiency, capacity, and cycle stability performance of the obtained secondary battery.

[0064] In some embodiments, the silicon material includes at least one of silicon oxide and silicon alloy.

[0065] In some embodiments, the silicon material includes silicon particles and a silicon oxide layer on the surface of the silicon particles. The silicon oxide layer includes silicon oxide, and the general formula of the silicon oxide is SiOx, where 0.5≤x<2. Specifically, SiOx can be SiO0.5 SiO 0.7 SiO 0.9 SiO, SiO 1.2 SiO 1.5 SiO 1.8 SiO 1.9 etc. without limitation. In some embodiments, the mass percentage of oxygen atoms in the silicon material is 1% to 18%, based on 100% of the mass of the silicon material. Specifically, the mass percentage of oxygen atoms in the silicon material can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, or any value within a range between any two of the above values. Controlling the mass percentage of oxygen atoms in the silicon material within the above range is conducive to forming a stable silicon oxide layer on the surface of the silicon particles, which can reduce direct contact between the silicon particles and the electrolyte, thereby reducing side reactions between the silicon material and the electrolyte and improving the cycle stability of the negative electrode material. It can also ensure that the silicon material has stable activity and improve the specific capacity of the negative electrode material.

[0066] In some embodiments, the active substance includes a silicon material, and the mass content of silicon elements in the negative electrode material is 35% to 55%, specifically, it can be 35%, 38%, 40%, 43%, 45%, 48%, 50%, 52%, 55%, or any value within a range between any two of the above values. When the mass percentage of silicon elements is within this range, the secondary battery formed thereby can store a higher amount of electricity, i.e., has a higher initial specific discharge capacity.

[0067] In some embodiments, the average particle size of the active substance is 0.1 nm to 500 nm. For example, the average particle size of the active substance can be 0.1 nm, 0.5 nm, 1 nm, 5 nm, 10 nm, 30 nm, 50 nm, 100 nm, 150 nm, 200 nm, 300 nm, 400 nm, 500 nm, or any value within a range between any two of the above values. For example, the active substance can be a silicon material, i.e., a silicon particle. By setting the average particle size of the silicon particle within the above range, the mechanical stress of the silicon particle when it swells in volume can be reduced, the battery capacity of the secondary battery can be maintained, the irreversible capacity loss can be reduced, and the electron and ion transport paths can be shortened. At the same time, the size of the silicon particle is reduced, the gap between adjacent silicon particles is increased, and space can be reserved for the volume expansion of the silicon particle, which helps to reduce the risk of pulverization and fragmentation of the obtained negative electrode material due to the expansion effect.

[0068] In some embodiments, the morphology of the silicon particle includes at least one of a point shape, a spherical shape, an ellipsoidal shape, and a sheet shape.

[0069] In some embodiments, the purity of the silicon particles is greater than 99%. High purity silicon particles are more conducive to Li-Si alloying with lithium, improving the cycle performance of lithium ion batteries.

[0070] In some embodiments, the total pore volume of the substrate is 0.5 cm 3 / g to 2.0 cm 3 / g. For example, the total pore volume of the substrate can be 0.5 cm 3 / g, 0.8 cm 3 / g, 1 cm 3 / g, 1.2 cm 3 / g, 1.5 cm 3 / g, 1.8 cm 3 / g, 2.0 cm 3 / g, or any value within a range between any two of the above values. A total pore volume of the substrate within the above range indicates that the substrate has abundant pores that can accommodate active materials and provide space for volume expansion of the accommodated active materials, thereby effectively reducing the risk of pulverization and fragmentation of the resulting negative electrode material due to expansion effects.

[0071] In some embodiments, the total pore volume of the negative electrode material is 0.001 cm 3 / g to 0.1 cm 3 / g. For example, the total pore volume of the negative electrode material can be specifically 0.001 cm 3 / g, 0.002 cm 3 / g, 0.005 cm 3 / g, 0.008 cm 3 / g, 0.01 cm 3 / g, 0.02 cm 3 / g, 0.03 cm 3 / g, 0.04 cm 3 / g, 0.05 cm 3 / g, 0.06 cm 3 / g, 0.07 cm 3 / g, 0.08 cm 3 / g, 0.09 cm 3 / g, 0.1 cm 3 / g, or any value within a range between any two of the above values.

[0072] Compared with the negative electrode material after removing the active material such as silicon particles, i.e. the matrix, the pore volume of the negative electrode material containing the active material is significantly reduced, indicating that the density of the negative electrode material is increased, and the specific capacity of the negative electrode material can be effectively improved. The total pore volume of the negative electrode material filled with active material such as silicon particles and the negative electrode material removed from the active material such as silicon particles is controlled in the above range, which can not only improve the specific capacity of the negative electrode material, but also ensure that the negative electrode material can reserve a proper amount of pores for relieving the volume expansion caused by the deintercalation of lithium during the process of the active material such as silicon particles, which is beneficial to improve the cycle performance of the negative electrode material.

[0073] In some embodiments, taking the active material as silicon particles as an example, 150 mL of 20% mass fraction HF acid solution is added dropwise into 10 g of negative electrode material under stirring, SiF4 and H2 gas are generated, and heat is released. After no gas is generated, the supernatant acid solution is removed by centrifugation, 150 mL of 20% mass fraction HF acid solution is added into the negative electrode material again, and after stirring for 12 h, the supernatant acid solution is removed by centrifugation again. Then, the negative electrode material is washed with pure water until it is neutral and dried to obtain the negative electrode material after removing the silicon material, i.e. the matrix.

[0074] In some embodiments, the average pore diameter of the pores of the negative electrode material is 0.5 nm to 20 nm. Specifically, the average pore diameter of the pores of the negative electrode material can be 0.5 nm, 0.8 nm, 1.0 nm, 1.3 nm, 1.5 nm, 1.8 nm, 2.0 nm, 3.0 nm, 4.0 nm, 5.0 nm, 6.0 nm, 7.0 nm, 10 nm, 12 nm, 15 nm, 18 nm, 20 nm or any value within the range consisting of any two of the above values. Controlling the average pore diameter of the pores in the negative electrode material can improve the rate performance of the negative electrode material while buffering the volume expansion of the active material and improving the structural stability of the negative electrode material.

[0075] In some embodiments, the tap density of the negative electrode material is 0.8 g / cm 3 to 1.2 g / cm 3 . Specifically, it can be 0.8 g / cm 3 , 0.9 g / cm 3 , 1.0 g / cm 3 , 1.1 g / cm 3 , 1.2 g / cm 3 or any value within the range consisting of any two of the above values.

[0076] In some embodiments, the pores in the negative electrode material include micropores, wherein the volume percentage of the micropores in all pores is less than or equal to 5%. In some embodiments, the pores in the negative electrode material include mesopores, wherein the volume percentage of the mesopores in all pores is 87% to 97%. In some embodiments, the pores in the negative electrode material include macropores, wherein the volume percentage of the macropores in all pores is less than or equal to 13%.

[0077] It can be understood that, since the active substance such as silicon particles fills a large number of pores in the carbon material, especially micropores, the pores of the negative electrode material are mainly mesopores and macropores. It can be understood that, since the molecular size generated by the electrolyte is generally less than or equal to the pore size of the micropores, under the strong capillary adsorption capacity of the micropores, the adsorption capacity of the negative electrode material is positively proportional to the pore volume of the micropores to a large extent, for example, as the micropore volume increases, the adsorption capacity of the negative electrode material increases, and thus the side reaction of the negative electrode material with the electrolyte increases. Therefore, by controlling the volume percentage of the micropores, mesopores and macropores in the negative electrode material within the range, the uniformity of the distribution of the silicon material inside the negative electrode material can be improved. Specifically, the micropores are basically filled with active substances, and the remaining majority of the pores are mesopores, which can effectively alleviate the volume expansion of the active substance and reduce the local excessive expansion stress of the negative electrode material caused by the non-uniform volume change of the active substance during the cycle process, thereby preventing the negative electrode material from being broken and pulverized.

[0078] In some embodiments, the specific surface area of the substrate is 600 m 2 / g to 3000 m 2 / g. The substrate meeting the above specific surface area range has a relatively developed pore, and a large number of micropores, which is beneficial to the deposition of the active substance.

[0079] In some embodiments, the substrate includes a carbon substrate, and the carbon substrate includes one or more of amorphous carbon, graphitized carbon, mesocarbon microbeads and carbon gel. The carbon substrate selected from the above materials can provide distribution sites for the active substance and form a conductive network.

[0080] In some embodiments, the substrate comprises a carbon substrate, and the mass content of carbon element in the negative electrode material is 40% to 60%, specifically can be 40%, 42%, 43%, 45%, 48%, 50%, 52%, 55%, 57%, 59%, 60% or any value within the range between any two of the above values. The carbon element includes the carbon substrate and the carbon coating layer. When the mass content of the carbon element is within the range, sufficient carbon substrate can be established to provide sufficient distribution sites for the active material, which is conducive to the formation of an effective conductive network, and improves the electrical conductivity and cycle stability. In some embodiments, the substrate comprises a non-carbon substrate, and the non-carbon substrate comprises one or more of metal oxides, silicides, silicates, phosphates, titanates and aluminum borates. The non-carbon substrate adopts the above-mentioned materials, which can all play a supporting role. Compared with the existing conductive carbon substrate, the non-carbon substrate adopted by the present application has better strength and rigidity, so that the negative electrode material has a higher compaction density during the battery preparation process, which can improve the structural stability of the negative electrode material, reduce the particle breakage and pulverization of the negative electrode material, and is conducive to improving the cycle performance of the negative electrode material. At the same time, the non-carbon substrate has electronic insulation and good ionic conductivity, which can play a role similar to an artificial SEI film, slow down the generation of a subsequent natural SEI film, reduce the direct contact between the negative electrode material and the electrolyte, and reduce the occurrence of side reactions. In addition, the non-carbon substrate has a lower cost advantage compared with the carbon substrate. Due to the complexity of the activation and pore forming process, the energy consumption and environmental cost involved in the existing carbon substrate are relatively high. If a porous ceramic or other material naturally having porous pores is used, the pore forming process is omitted, so that the cost can be significantly reduced compared with the porous carbon substrate.

[0081] The negative electrode material active layer further comprises a binder for binding the negative electrode active material particles to facilitate the formation of a film layer, and can also improve the bonding force between the negative electrode material active layer and the negative electrode current collector. In some embodiments, the binder can include but is not limited to polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, butadiene rubber, acrylic butadiene rubber, epoxy resin or nylon, etc.

[0082] The negative electrode material active layer can further comprise a conductive material, which includes but is not limited to carbon-based materials, metal-based materials, conductive polymers or any combination thereof. In some embodiments, the carbon-based material can include but is not limited to natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fibers or any combination thereof. In some embodiments, the metal-based material can include but is not limited to metal powder or metal fiber, such as copper, nickel, aluminum or silver. In some embodiments, the conductive polymer can be a polyphenylene derivative.

[0083] Separator film

[0084] The separator 103 includes a membrane layer having a porous structure, and the material thereof includes, but is not limited to, at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, the separator 103 can be a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane, etc.

[0085] Electrolyte

[0086] The electrolyte has a role of conducting ions between the positive electrode sheet 101 and the negative electrode sheet 102. The state of the electrolyte can be one or more of a gel state, a solid state, and a liquid state. In some embodiments, the electrolyte employs an electrolytic solution. The electrolytic solution has a role of conducting active ions between the positive electrode sheet 101 and the negative electrode sheet 102. In some embodiments, the electrolytic solution includes a lithium salt and an organic solvent. The lithium salt can be selected from, but is not limited to, one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium tetraphenylborate (LiB(C6H5)4), lithium methanesulfonate (LiCH3SO3), lithium bisfluorosulfonylimide (LiFSI), lithium bis-trifluoromethanesulfonylimide (LiTFSI), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(trifluoromethylsulfonyl)imide (LiN(SO2CF3)2, lithium tris(trifluoromethylsulfonyl)methide (LiC(SO2CF3)3), lithium bis(oxalato)borate (LiBOB), and lithium difluorophosphate (LiPO2F2). For example, the lithium salt is selected as LiPF6 because it can give a high ionic conductivity and improve cycle characteristics. The organic solvent can be a carbonate compound, a carboxylic acid ester compound, an ether compound, a nitrile compound, other organic solvents, or a combination thereof. Examples of the carbonate compound include, but are not limited to, diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl 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-methyl ethylene carbonate, 1-fluoro-1-methyl ethylene carbonate, 1,2-difluoro-1-methyl ethylene carbonate, 1,1,2-trifluoro-2-methyl ethylene carbonate, trifluoromethyl ethylene carbonate, or a combination thereof.

[0087] Another embodiment of the present application also provides a method for preparing a negative electrode material, comprising:

[0088] The first step is to activate the substrate precursor material to obtain a porous substrate.

[0089] In some embodiments, the substrate precursor material comprises one or more of nutshell carbon, straw carbon, resin carbon, pitch carbon, and sugar. The porous carbon substrate can be obtained based on the above-mentioned materials.

[0090] In some embodiments, the activation treatment comprises one of steam activation and carbon dioxide activation, and the activation treatment comprises introducing steam or carbon dioxide gas under high-temperature conditions, and forming a porous material through subsequent treatment.

[0091] In some embodiments, before the activation treatment of the substrate precursor material, the method further comprises pre-carbonization treatment of the substrate precursor material, which comprises heat-treating the substrate precursor material at 300-500°C for about 3h and crushing the material. The pre-carbonization treatment is beneficial to making the material structure more stable, thereby improving the quality and performance of the subsequent activation product. Crushing the material after the pre-carbonization treatment is beneficial to expanding the contact area of the material in the subsequent activation treatment, improving the activation effect, and obtaining a porous material with good performance.

[0092] In some embodiments, the temperature of the activation treatment is 800-1100°C. In some embodiments, the pressure of the activation treatment is 0.01-10kpa. In some embodiments, the total gas flow of the activation treatment is 10-100L / min. In some embodiments, the activation treatment lasts for about 12h. In some embodiments, the subsequent treatment comprises acid washing, water washing, drying, crushing, and sieving.

[0093] The second step is to composite the porous substrate with an active material to obtain a core.

[0094] In some embodiments, the compositing of the porous substrate with the active material comprises gas phase deposition, which comprises mixing the porous substrate with a gas phase carrier of the active material under high-temperature conditions, making the gas phase carrier of the active material crack under high temperature, and depositing the active material component on the porous substrate.

[0095] In some embodiments, the gas phase deposition is carried out in a plasma-enhanced chemical vapor deposition device, and the reaction conditions of the gas phase deposition comprise a pressure of 1.0-10.0kpa, a microwave operating frequency of 2.5GHz, a power of 2600-3200W, a temperature of 400-700°C, and a total gas flow rate of 50-500Sccm. In some embodiments, the gas phase deposition is carried out in an inert gas atmosphere.

[0096] A third step: in a plasma enhanced chemical vapor deposition device, the core is mixed with a reaction gas including an inert gas and a gas-phase carbon source at a rotation speed frequency of 5 Hz to 40 Hz and a temperature of 550°C to 700°C, the concentration of the gas-phase carbon source in the reaction gas is 10% to 70%, and the gas-phase carbon source is cracked to form a carbon coating layer, thereby obtaining a coated core.

[0097] In the coating process described above, the rotation speed frequency of the device affects the deposition effect in the coating process. When the rotation speed of the reactor is too slow, the heat generated by the cracking of acetylene will cause the local temperature to be higher than the set value, resulting in a change in the crystal form of the material, although the density of the coating layer is improved, the performance of the negative electrode material is reduced. When the rotation speed of the reactor is too fast, the collision between particles is intensified, and the violent collision can damage the structure of the coating layer that has been formed, or make it difficult for the carbon source to crack and uniformly deposit on the surface of the particles, thereby affecting the growth of the coating layer and the density of the coating layer. Therefore, controlling the rotation speed frequency of the above-mentioned device within the above-mentioned range is beneficial to maintaining the crystal form of the material and promoting the growth and density of the coating layer, thereby reducing the carbon residue rate. It can be understood that controlling the reaction temperature within the above-mentioned range is also beneficial to maintaining the crystal form of the material, such as maintaining the silicon material as amorphous silicon, and improving the density of the coating layer, which is beneficial to reducing the carbon residue rate.

[0098] At the same time, the concentration of the gas-phase carbon source in the reaction gas also affects the deposition effect in the coating process. When the concentration of the gas-phase carbon source is too low, the coating amount is insufficient, and the performance improvement of the material is limited and requires a longer coating treatment time. When the concentration of the gas-phase carbon source is too high, the carbon source cracks violently, and the carbon source decomposes in a large amount in a short time. On the one hand, it can cause carbon source excess in some areas, forming an excessively thick coating layer, while in another part of the area, the coating may not be complete due to insufficient carbon source, forming a relatively thin coating layer or even a bare area. On the one hand, a large amount of gas and impurities can also be produced. If these gases and impurities cannot be effectively discharged during the formation of the coating layer, pores, cracks and other defects will be formed, thereby reducing the quality and density of the coating layer. On the other hand, since the carbon source needs to interact with the surface of the substrate (such as chemical bonding, adsorption, etc.) during the coating process to form a firm coating layer, this violent cracking can destroy this interaction, causing the bonding force between the coating layer and the substrate to decrease, affecting the stability and performance of the coating layer.

[0099] In some embodiments, in the plasma enhanced chemical vapor deposition device, the coating treatment conditions further include a pressure of 5.0 kPa to 20.0 kPa, a microwave operating frequency of 2.0 GHz to 2.5 GHz, a power of 2500 W to 3500 W, and a total gas flow rate of 50 Sccm to 500 Sccm.

[0100] In some embodiments, the gaseous carbon source includes one or more of methane, acetylene, propylene, benzene, ethanol, methanol, ethylene, propane, and butane.

[0101] Step 4: crush and sieve the coated core to obtain the negative electrode material.

[0102] The present invention will be explained below in conjunction with the embodiments. It will be understood by those skilled in the art that the following examples are only used to explain the present invention and are not to be construed as limiting the present invention. Unless otherwise indicated, the reagents, software, and instruments not specifically described in the following examples are all conventional commercially available products or open source.

[0103] Example 1:

[0104] A negative electrode material based on a carbon matrix and silicon particles, the preparation method comprising:

[0105] S1, place 10kg of fruit shell carbon in a kiln, heat it to 300℃ to 500℃, pre-carbonize it for 3h, and crush it mechanically. Place the crushed material in a reaction furnace, set the pressure to 0.01kpa to 10.0kpa, and introduce water vapor at 900℃ with a total air flow of 30L / min. The reaction time is 12h. After the reaction is completed, the reaction material is acid-washed, washed with water, dried, crushed, and sieved to obtain a porous carbon precursor.

[0106] S2, place 100g of porous carbon precursor in the reactor, set the pressure to 1.0kPa to 10.0kPa, the microwave operating frequency to 2.5GHz, and the power range to 3000W; when the reactor temperature reaches 500℃, introduce nitrogen and disilane, the concentration of disilane is 30%, the total gas flow rate is 300Sccm, and the holding time is 6h to obtain a core body.

[0107] S3, place the core body in the reactor, set the pressure to 5.0kPa to 20.0kPa, the microwave operating frequency to 2.0GHz, the power range to 3000W, and the rotation speed frequency to 15Hz; when the reactor temperature reaches 550℃, introduce nitrogen and acetylene, the acetylene concentration is 30%, the total gas flow rate is 200Sccm, and the holding time is 6h to obtain the coated core body.

[0108] S4, screening and grading the obtained coated core bodies to obtain negative electrode materials.

[0109] Example 2:

[0110] The difference from Example 1 is that in S3, the ultimate temperature of the reactor is adjusted to 600°C.

[0111] Example 3:

[0112] The difference from Example 1 is that in S3, the temperature adjustment of the reactor is 650℃.

[0113] Example 4:

[0114] The difference from Example 1 is that in S3, the temperature adjustment of the reactor is 700℃.

[0115] Example 5:

[0116] The difference from Example 3 is that in S3, the concentration of acetylene is adjusted to 10%, and the holding time is adjusted to 18h.

[0117] Example 6:

[0118] The difference from Example 3 is that in S3, the concentration of acetylene is adjusted to 50%, and the holding time is adjusted to 3.6h.

[0119] Example 7:

[0120] The difference from Example 3 is that in S3, the concentration of acetylene is adjusted to 70%, and the holding time is adjusted to 2.6h.

[0121] Example 8:

[0122] The difference from Example 3 is that in S3, the rotational frequency is adjusted to 5Hz.

[0123] Example 9:

[0124] The difference from Example 3 is that in S3, the rotational frequency is adjusted to 25Hz.

[0125] Example 10:

[0126] The difference from Example 3 is that in S3, the rotational frequency is adjusted to 40Hz.

[0127] Example 11:

[0128] The difference from Example 3 is that in S3, the microwave operating frequency is 2.5GHz, and the power range is 3500W.

[0129] Comparative Example 1:

[0130] The difference from Example 1 is that in S3, the temperature adjustment of the reactor is 480℃.

[0131] Comparative Example 2:

[0132] The difference from Example 7 is that in S3, the concentration of acetylene is adjusted to 90%, and the holding time is adjusted to 2h.

[0133] Comparative Example 3:

[0134] The difference from Example 10 is that in S3, the rotating speed frequency is adjusted to 50 Hz.

[0135] The negative electrode materials obtained in Examples 1-11 and Comparative Examples 1-3 were subjected to physical property and electrochemical property tests, and the test methods were as follows:

[0136] 1. Test of residual carbon rate γ of negative electrode material:

[0137] (1) The negative electrode material was placed in a slurry stirring tank for stirring, the stirring frequency was 50 Hz, and the stirring time was 1 h. The mass of the stirred negative electrode material was m1, which was placed in a 20% hydrofluoric acid solution for 1 h. The soaked negative electrode material was taken out, washed and dried, and the mass was m2.

[0138] (2) The negative electrode material with a mass of m1 was placed in a 20% hydrofluoric acid solution for 1 h. The soaked negative electrode material was taken out, washed and dried, and the mass was m3.

[0139] (3) The residual carbon rate of the negative electrode material was calculated as

[0140] 2. Test of gas production A of negative electrode material:

[0141] 50 g of negative electrode material was placed in a 300 mL slurry stirring tank, 50 g of 5% carboxymethyl cellulose sodium and 100 mL of pure water were added to the stirring tank, and stirring was performed, the stirring frequency was 50 Hz, and the stirring time was 1 h. The slurry was obtained, and the slurry was placed in an aluminum plastic film. The 10-day gas production A of the slurry was measured by the drainage method.

[0142] 3. Test of powder conductivity of negative electrode material:

[0143] 3 g of powder negative electrode material was placed in a 80°C drying oven for sufficient drying, and the dried powder negative electrode material was shaken and mixed. 2 g of the treated powder negative electrode material was pressed into a round sheet under a force of 20 kN for 30 s. The thickness of the sample was measured using a digital vernier caliper and recorded. The PRCD2110 powder resistivity of Yuan Energy Technology Co., Ltd. was selected, and the pressed round sheet sample was placed on the test table of the four-probe tester. The four probes of the four-probe tester were placed at different positions on the surface of the sample. According to the operation instructions of the four-probe tester, the test program was started. The tester automatically applied a certain current to the current probe, and measured the voltage response on the sample through the voltage probe. Then the resistivity or conductivity of the sample was calculated using Ohm's law and other physical formulas.

[0144] 4. Test of ID / IG of negative electrode material:

[0145] Take 100 mg of negative electrode material sample, the sample is in a dry, stable, non-volatile and non-toxic state. Before testing, calibrate the Raman spectrometer to ensure the accuracy of the test results. Place the sample on the sample stage of the Raman spectrometer. Set appropriate test parameters, such as laser wavelength 532 nm, etc. Start the Raman spectrometer Anton Paar Cora 5X00 series and perform spectral scanning. Record the Raman spectrum obtained by testing, find the positions of the D peak and the G peak in the spectrum, and measure their intensities. Calculate the ID / IG value, i.e. the ratio of the intensity of the D peak to the intensity of the G peak. According to the size of the ID / IG value, the disorder degree of the negative electrode material is judged.

[0146] 5. Particle size D10, D50, D90 test of negative electrode material:

[0147] Take 1 g of powder negative electrode material and dissolve it in 30 mL of pure water, ultrasonic oscillation for 2 min, pour the ultrasonic oscillation mixed solution into the measuring cylinder of Malvern 3000 test instrument, start the test instrument, and get the particle size distribution data.

[0148] 6. Active material average particle size and carbon coating thickness test of negative electrode material:

[0149] Select JEM-2100F test instrument of Japan Electronics Corporation, test the TEM image of the material under high magnification, count the particle size in the TEM image for data analysis, and obtain the average particle size. Count the outer carbon layer in the TEM image, there is an obvious boundary in the outer layer of the particle in the TEM image, and the outermost layer is the carbon coating layer. Count the thickness of the carbon coating layer. Properly process and prepare 10 mg of material, dissolve it in alcohol, titrate, dry, fix, and ultrathin section to ensure that the sample is suitable for TEM observation. Use a special sample clamp to load the processed sample onto the sample stage of the TEM, and adjust the position and angle of the sample to make it suitable for electron beam irradiation. Start the TEM system and vacuumize after initialization. Adjust the brightness, focus, magnification and other parameters of the electron gun. Calibrate the working voltage and current of the instrument to ensure that the equipment is in the best working state. Use the focusing device of the TEM system to adjust the focusing of the sample to obtain a clear TEM image.

[0150] 7. Pore volume and proportion and specific surface area test of negative electrode material and matrix:

[0151] Use the McBain specific surface test instrument ASAP 2425, take 0.5 g of material for degassing treatment, after degassing, place it in the test instrument, start the test instrument, and after the program ends, analyze the adsorption and desorption isotherms to determine the pore volume and pore size distribution, and the respective pore volumes of micropores, mesopores and macropores in the total pore volume, and the specific surface area.

[0152] 8. The mass content of carbon element test method: using Germany's G4 ICARUS HF infrared carbon and sulfur analyzer, the sample is burned in a high-temperature oxygen-rich state, the carbon element contained therein is oxidized to carbon dioxide, the generated gas enters the infrared detector with the carrier gas, and the content of the carbon element can be calculated by quantitatively counting the change of the carbon dioxide signal.

[0153] 9. The mass content of silicon element test method: using Nanyang Xiyu's SA2-9-17TP box atmosphere furnace, burning in an oxygen atmosphere to make the silicon in the sample react to become silicon dioxide, and the carbon burns to become carbon dioxide and is discharged, and the silicon content is calculated by weighing.

[0154] 10. Electrochemical related performance test:

[0155] (1) Assemble lithium ion button cell:

[0156] According to the mass ratio of polyacrylic acid to sodium carboxymethyl cellulose 1:1, a certain amount of pure water is added, and magnetic stirring is carried out for 6-12h to obtain the binder;

[0157] According to the mass ratio of silicon composite material, conductive agent and binder 70:15:15, silicon composite material and conductive agent are added to the binder, and magnetic stirring is carried out for 6-12h to obtain the slurry;

[0158] The slurry is uniformly coated on the copper foil, dried, sliced, and dried to obtain the silicon composite material electrode sheet;

[0159] Assemble the button cell, use the battery shell CR2032, the cathode is a lithium sheet, and 1M LiPF6 is the electrolyte.

[0160] (2) Capacity and initial coulombic efficiency test: place the button cell in the mold clamp of the high-precision battery test system, and perform static treatment on the button cell, the static treatment time is 12h, the discharge test conditions are discharge current 0.1C, rated capacity 1000mAh, and discharge cutoff voltage 5mV, the charge test conditions are discharge current 0.1C, rated capacity 1000mAh, and discharge cutoff voltage 1.5V, and the capacity and initial coulombic efficiency of the button cell are tested by using the high-precision battery test system.

[0161] (3) Cycle retention rate: the button cell tested for capacity and initial coulombic efficiency is subjected to 1 C charge-discharge cycle stability test, and the capacity ratio of the 50th week to the 1st week is the cycle retention rate.

[0162] (4) Swelling rate: the button cell tested for capacity and initial coulombic efficiency is subjected to 1 C charge-discharge cycle stability test, and the ratio of the 50th week to the original electrode thickness is the swelling rate.

[0163] The preparation conditions of the above-mentioned Examples 1-11 and Comparative Examples 1-3 are shown in Table 1, and the test results are shown in Table 2.

[0164] Table 1. Preparation conditions of the above-mentioned Examples 1-11 and Comparative Examples 1-3 in step S3

[0165] Example Rotation speed frequency (Hz) Temperature reached by the reactor (°C) Acetylene concentration (%) Time (h) Example 1 15 550 30 6 Example 2 15 600 30 6 Example 3 15 650 30 6 Example 4 15 700 30 6 Example 5 15 650 10 18 Example 6 15 650 50 3.6 Example 7 15 650 70 2.6 Example 8 5 650 30 6 Example 9 25 650 30 6 Example 10 40 650 30 6 Example 11 15 610 30 6 Comparative Example 1 15 480 30 6 Comparative Example 2 15 650 90 2 Comparative Example 3 50 650 30 6

[0166] Table 2. Test results of the above-mentioned Examples 1-11 and Comparative Examples 1-3

[0167]

[0168] Under the improved preparation process conditions, the residual carbon rates γ of the negative electrode materials of Examples 1-11 all meet the preset range, and the carbon coating layers of these negative electrode materials have better protection effects on the cores, so that the gas production of the slurries of these negative electrode materials is effectively reduced. In Example 11, the microwave frequency and power are increased, which is beneficial to reducing the cracking temperature of silane and acetylene, so that the sample prepared by adjusting the temperature to 610°C in Example 11 has basically the same performance as the sample prepared by cracking at a temperature of 650°C in Example 3. Therefore, the swelling effect of the negative electrode materials of Examples 1-11 is alleviated after being made into negative electrode sheets, and the secondary batteries prepared therefrom have better cycle stability. At the same time, the carbon coating layers of these negative electrode materials have good compactness, so that the electronic carbon layer maintains good passability, so that the secondary batteries prepared therefrom have higher capacity and first coulombic efficiency.

[0169] Compared with Example 1, the temperature of the coating treatment in Comparative Example 1 is lowered, and the low temperature leads to reduced cracking efficiency of the carbon source, affects the growth of the coating layer, affects the coating amount in the negative electrode material, and is also not conducive to the formation of a uniform and compact coating layer. The residual carbon rate of the negative electrode material does not meet the preset range, thereby leading to increased gas production of the obtained negative electrode material, higher swelling rate, and reduced cycle stability.

[0170] Compared with Example 7, the concentration of the carbon source in Comparative Example 2 is increased, and too much carbon source promotes the intensification of carbon source cracking and the phenomenon of concentrated cracking, which is not conducive to the formation of a uniform and compact coating layer, leading to a residual carbon rate that does not meet the preset range, thereby leading to increased gas production of the obtained negative electrode material, higher swelling rate, and reduced cycle stability.

[0171] Compared with Example 10, the rotation frequency in the coating treatment in Comparative Example 3 is increased, and the high rotation frequency leads to intensified collision between particles, which is also not conducive to the formation of a uniform and compact coating layer, leading to a residual carbon rate that does not meet the preset range, thereby leading to increased gas production of the obtained negative electrode material, higher swelling rate, and reduced cycle stability.

[0172] The above embodiments are only used to illustrate the technical solutions of the present application but not limit the present application, and although the present application is explained in detail with reference to the above preferred embodiments, it should be understood by those skilled in the art that the technical solutions of the present application can be modified or equivalent replaced 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 carbon coating covering at least a portion of the surface of the core, wherein the core comprises a matrix and an active material, characterized in that: The gas production A of the negative electrode material over 10 days is less than or equal to 100 mL / kg; Define the residual carbon rate of the negative electrode material The residual carbon rate γ is less than or equal to 20%; The test method for the 10-day gas production A comprises: placing 50 g of the negative electrode material in a 300 mL slurry stirring tank, adding 50 g of 5% sodium carboxymethyl cellulose and 100 mL of pure water to the stirring tank, stirring at a frequency of 50 Hz and for 1 hour to obtain a slurry, placing the slurry in an aluminum-plastic film, and measuring the 10-day gas production A of the slurry by a water displacement method; The test method of m2 includes: placing the negative electrode material in a slurry stirring tank and stirring at a frequency of 50 Hz for 1 hour, taking out a mass of m1 of the stirred negative electrode material and soaking it in a 20% hydrofluoric acid solution for 1 hour, taking out the soaked negative electrode material, washing and drying it, and measuring the mass of m2; The test method of m3 includes: taking the negative electrode material with a mass of m1 and placing it in a hydrofluoric acid solution with a mass fraction of 20% and soaking it for 1 hour, taking out the soaked negative electrode material, cleaning and drying it, and measuring the mass of m3.

2. The negative electrode material according to claim 1, wherein The powder conductivity of the negative electrode material at 20 kN is 0.5 S / cm to 9.5 S / cm.

3. The negative electrode material according to claim 1, wherein The ID / IG of the negative electrode material is 0.5 to 5.

0.

4. The negative electrode material according to claim 1, wherein The negative electrode material satisfies at least one of the following conditions: (1) The specific surface area of ​​the negative electrode material is less than or equal to 5m 2 / g; (2) The total pore volume of the negative electrode material is 0.001 cm 3 / g to 0.1cm 3 / g; (3) The compaction density of the negative electrode material is 0.8 g / cm 3 to 1.2g / cm 3 ; (4) The negative electrode material includes micropores, mesopores, and macropores. Based on the total pore volume of the negative electrode material, the volume proportion of the micropores is less than or equal to 5%, the volume proportion of the mesopores is 87% to 97%, and the volume proportion of the macropores is less than or equal to 13%; (5) The particle size D10 of the negative electrode material is 1 μm to 5 μm, the particle size D50 of the negative electrode material is 6 μm to 16 μm, and the particle size D90 of the negative electrode material is 16 μm to 24 μm.

5. The negative electrode material according to claim 1, wherein The carbon coating layer has a thickness of 10 nm to 1000 nm.

6. The negative electrode material according to claim 1, wherein The active substance satisfies at least one of the following conditions: (1) The active material includes one or more of Si, Sn, Ge, Pb, Ag, Mg, Zn, Ga, In, Sb, Bi and their alloys; (2) The active material includes a silicon material, the silicon material includes silicon particles, and the silicon particles include one of amorphous silicon, crystalline silicon, and a composite of crystalline silicon and amorphous silicon; (3) The active material includes a silicon material, and the silicon material includes at least one of silicon oxide and a silicon alloy; (4) The active material includes a silicon material, the silicon material includes silicon particles and a silicon oxide layer located on the surface of the silicon particles, and the silicon oxide layer includes silicon oxide; (5) The average particle size of the active substance is 0.1 nm to 500 nm.

7. The negative electrode material according to claim 1, wherein The substrate satisfies at least one of the following conditions: (1) The total pore volume of the matrix is ​​0.5 cm 3 / g to 2.0cm 3 / g; (2) The specific surface area of ​​the substrate is 600 m 2 / g to 3000m 2 / g; (3) The matrix includes a carbon matrix, and the carbon matrix includes one or more of amorphous carbon, graphitized carbon, mesocarbon microbeads, and carbon gel; (4) The matrix includes a non-carbon matrix, and the non-carbon matrix includes one or more of metal oxides, silicides, silicates, phosphates, titanates and aluminum borates.

8. The negative electrode material according to claim 1, wherein The matrix of the negative electrode material includes a carbon matrix, the active material of the negative electrode material includes a silicon material, 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 carbon element mass of the negative electrode material accounts for 40% to 60%; (2) Based on the mass of the negative electrode material, the mass of the silicon element in the negative electrode material accounts for 35% to 55%.

9. A negative electrode sheet comprising a negative electrode current collector and a negative electrode active material layer provided 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.

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