Negative electrode material, negative electrode sheet, and secondary battery

Abstract

A kind of negative material, negative pole piece and secondary battery, negative material contains nitrogen element, including carbon matrix and active substance, carbon matrix is equipped with hole, at least part active substance is arranged in the hole of carbon matrix, define the region of the depth of 200nm from the outer surface of negative material as first area, the region of the depth of 1000nm from the outer surface of negative material as second area, the atomic percentage content of nitrogen element in first area is α, the atomic percentage content of nitrogen element in second area is β, α<β;Define the deposition parameter of active substance as γ, γ≥0.86.The above-mentioned negative material, relatively most active substance is distributed in the hole of carbon matrix, reduces the gas generation behavior of negative material and is advantageous to the conductivity of carbon matrix to play, so that the secondary battery with the negative material has higher discharge specific capacity, first coulomb efficiency and better 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] With the rapid development and wide application of electric vehicles, the market puts forward higher requirements for the energy density and cycle life of secondary batteries (such as lithium ion batteries). The energy density of lithium ion batteries mainly depends on the selection of electrode materials. The traditional mainstream negative electrode material mainly includes graphite, but the specific capacity of the graphite negative electrode has approached the theoretical limit, and it is almost difficult to further improve. Silicon has a theoretical capacity of about 4200 mAh / g, has the advantages of high capacity, wide source, environmental friendliness, moderate lithium intercalation potential, etc., and is generally considered to be one of the next generation of high specific capacity candidate lithium ion battery negative electrode materials.

[0003] However, silicon expands significantly in volume during lithium extraction and intercalation, which easily leads to the breakage and pulverization of silicon particles, thereby causing the destruction of the electrode structure or the repeated growth of the solid electrolyte interface (SEI) film, etc., resulting in rapid decay of the electrochemical performance of the silicon negative electrode. SUMMARY

[0004] Therefore, the present application provides a negative electrode material to solve at least one of the above problems.

[0005] To achieve the above-mentioned purpose, the present application provides a negative electrode material, the negative electrode material contains nitrogen element, the negative electrode material includes carbon matrix and active material, the carbon matrix is provided with pores, at least part of the active material is arranged in the pores of the carbon matrix, a region with a depth of 200 nm from the outer surface of the negative electrode material is defined as a first region, a region with a depth of 1000 nm from the outer surface of the negative electrode material is defined as a second region, the atomic percentage of the nitrogen element in the first region is α, the atomic percentage of the nitrogen element in the second region is β, and α < β; the deposition parameter of the active material is defined as γ, γ≥0.86; wherein, ρ is the density of the active material; m1 is the mass of the negative electrode material, the mass percentage of the active material based on m1 is a1, and the specific pore volume of the negative electrode material is p1; m2 is the mass of the negative electrode material after removing the active material, the mass percentage of the active material based on m2 is a2, and the specific pore volume of the negative electrode material after removing the active material is p2.

[0006] In some possible implementation manners, 0.5≤α<β≤10.

[0007] In some possible implementation manners, the negative electrode material includes a core and a coating layer arranged on at least part of the surface of the core, the core includes the carbon matrix and the active material, and the material of the coating layer includes one or more of carbon material, metal oxide, amorphous silicon, conductive polymer, fluoride, phosphate and nitride.

[0008] In some possible implementations, the thickness of the coating layer is 5 nm to 200 nm.

[0009] In some possible implementations, the mass percentage of the coating layer in the negative electrode material is less than or equal to 10%.

[0010] In some possible implementations, the specific surface area of the carbon matrix is 800 m 2 / g to 3000 m 2 / g.

[0011] In some possible implementations, the total pore volume of the carbon matrix is 0.5 cm 3 / g to 2.0 cm 3 / g.

[0012] In some possible implementations, the porosity of the carbon matrix is 40% to 80%.

[0013] In some possible implementations, the carbon matrix comprises one or more of hard carbon, soft carbon, natural graphite, artificial graphite, carbon nanotube, carbon fiber, and graphene.

[0014] In some possible implementations, the active substance comprises one or more of Li, Na, K, Sn, Ge, Si, Fe, Mg, SiO, Ti, Zn, Al, P, and Cu.

[0015] In some possible implementations, the active substance comprises a silicon material, and the silicon material comprises one or more of amorphous silicon, crystalline silicon, and a composite of crystalline silicon and amorphous silicon.

[0016] In some possible implementations, the silicon material comprises one or more of silicon particles, silicon oxide, and silicon alloy.

[0017] In some possible implementations, the silicon material comprises silicon particles and a silicon oxide layer on the surface of the silicon particles, and the silicon oxide layer comprises silicon oxide.

[0018] In some possible implementations, the average particle size of the silicon material is 0.1 nm to 500 nm.

[0019] In some possible implementations, the active substance comprises a silicon material, the coating layer comprises a carbon material, and the mass percentage of carbon in the negative electrode material is 40% to 60% based on the mass of the negative electrode material.

[0020] In some possible implementations, the mass percentage of silicon in the negative electrode material is 30% to 65% based on the mass of the negative electrode material.

[0021] In some possible implementations, the specific surface area of the negative electrode material is 0.5 m 2 / g to 10m 2 / g.

[0022] In some possible implementations, the powder conductivity of the negative electrode material ranges from 0.5 S / cm to 3 S / cm.

[0023] In some possible implementations, the 24-hour gas production value of the negative electrode material is less than or equal to 4 cc / kg.

[0024] In some possible implementations, the median particle size of the anode material is 5 μm to 20 μm.

[0025] In some possible implementations, the particle size distribution (D90-D10) / D50 of the anode material is 0.9 to 5.

[0026] In some possible implementations, the compaction density of the negative electrode material under 1T pressure is 0.8 g / cm³. 3 Up to 1.3 g / cm 3 .

[0027] In some possible implementations, the tap density of the negative electrode material after 3000 vibrations is 0.5 g / cm³. 3 Up to 1.5g / cm 3 .

[0028] This application also provides a negative electrode sheet, including 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 includes the aforementioned negative electrode material.

[0029] This application also provides a secondary battery, including the aforementioned negative electrode.

[0030] In this application, the second region is closer to the interior of the carbon matrix than the first region. By setting the relationship between α and β, more nitrogen elements are distributed inside the carbon matrix rather than on the surface. This distribution promotes the deposition of more active materials within the pores of the carbon matrix, resulting in an active material deposition parameter γ greater than or equal to 0.86. This relatively reduces the proportion of active materials distributed on the surface of the carbon matrix, thereby reducing side reactions between the negative electrode material surface and the electrolyte, reducing gas generation behavior of the negative electrode material, and improving the conductivity of the carbon matrix. Therefore, secondary batteries using this negative electrode material can have higher discharge specific capacity, initial coulombic efficiency, and better cycle stability. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the structure of a secondary battery during charging, according to one embodiment of this application.

[0032] Figure 2 This is a schematic diagram of the structure of a secondary battery during discharge, provided in one embodiment of this application.

[0033] Figure 3 The morphology test image of the negative electrode material provided for Embodiment 1 of the present application.

[0034] Figure 4 The XRD test spectrum of the negative electrode material provided for Embodiment 1 of the present application.

[0035] Main element symbol explanation

[0036] Electrode assembly 100

[0037] Positive electrode sheet 101

[0038] Negative electrode sheet 102

[0039] Separator film 103. DETAILED DESCRIPTION

[0040] 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 of the present application and the features in the embodiments can be combined with each other without conflict; in the following description, many specific details are set forth in order to fully understand the present application, and the described embodiments are only part of the embodiments of the present application, rather than all the embodiments.

[0041] To solve the problem caused by the expansion of silicon particles in the silicon negative electrode, an innovation of silicon-carbon negative electrode material has appeared in the field of lithium ion batteries, which combines the high specific capacity of silicon and the stability of carbon, aiming to improve the energy density and cycle life of the battery. In order to further exert the advantages of silicon-carbon negative electrode material and improve the capacity of silicon-carbon negative electrode, the present application starts from the structural design and seeks a method to improve the loading amount of silicon. The present application finds that in the process of silicon and carbon compounding, a large amount of silicon is easily exposed on the surface of carbon material, which causes a huge volume effect of the silicon-carbon negative electrode material during lithium extraction and embedding, and affects the conductivity of the whole negative electrode material; the exposed silicon particles may also have obvious gas production behavior in the aqueous homogenate or electrolyte system, which brings safety hazards, and is also not conducive to the capacity of the silicon-carbon negative electrode material, resulting in a decrease in the first coulombic efficiency.

[0042] Based on this, the present application improves the preparation method of the silicon-carbon negative electrode material, reduces the content of silicon exposed on the surface of the carbon material on the basis of the same silicon deposition amount, so as to achieve the purpose of improving the capacity, the first coulombic efficiency and the cycle stability of the silicon-carbon negative electrode material.

[0043] Based on this, one embodiment of the present application provides a secondary battery, which includes a housing, an electrode assembly, and an electrolyte. The electrode assembly and the electrolyte are both located in the housing.

[0044] The housing 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.

[0045] Please refer to Figure 1 and Figure 2 The electrode assembly 100 includes a positive electrode sheet 101, a negative electrode sheet 102, and a separator 103 arranged between the positive electrode sheet 101 and the negative electrode sheet 102. When the electrolyte (not shown in the figure) is arranged, please refer to Figure 1 , 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 via the electrolyte, reach the negative electrode sheet 102, and are intercalated into the crystal lattice of the negative electrode material. When discharging, please refer to Figure 2 , active ions (such as lithium ions) are deintercalated from the crystal lattice of the negative electrode 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 electrode material (such as a lithiated intercalation compound), generating electrons from the negative electrode sheet 102 to the positive electrode sheet 101 via an external circuit, and the reverse movement of the electrons forms an electric current, which can be used by an electrical appliance.

[0046] In some embodiments, the electrode assembly 100 can be a stacked structure, which is formed by alternately stacking the positive electrode sheet 101, the separator 103, and the negative electrode sheet 102 in sequence. In other embodiments, the electrode assembly 100 can also be a wound structure, which is formed by winding the positive electrode sheet 101, the separator 103, and the negative electrode sheet 102 after being stacked in sequence.

[0047] The positive electrode sheet

[0048] The positive electrode sheet 101 includes a positive electrode current collector and a positive electrode material active layer arranged on at least one surface of the positive electrode current collector. The positive electrode current collector can use an aluminum foil or a nickel foil, etc., or any composite current collector disclosed in the prior art, such as but not limited to the aforementioned conductive foil and polymer substrate combined to form a current collector. The positive electrode material active layer includes a positive electrode active material, which includes a compound (i.e., a lithiated intercalation compound) that can reversibly intercalate and deintercalate lithium ions. In some embodiments, the positive electrode 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 electrode active material can include but is not limited to lithium cobaltate (LiCoO2), lithium nickel manganese cobalt ternary material (NCM), lithium manganate (LiMn2O4), lithium nickel manganate (LiNi0.5Mn1.5O4), lithium iron phosphate (LiFePO4), etc. 0.5 Mn 1.5O4) or at least one of lithium iron phosphate (LiFePO4).

[0049] The positive electrode material active layer further includes a binder to bind the positive electrode active material particles to facilitate the formation of a film layer, and to improve the binding force between the positive electrode material active layer and the positive electrode 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, polyethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene butadiene rubber, acrylated styrene butadiene rubber, epoxy resin, or nylon.

[0050] The positive electrode material active layer can further include a conductive material, which includes, but is 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.

[0051] The negative electrode sheet

[0052] 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 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 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.

[0053] The negative electrode material active layer includes a negative electrode material containing nitrogen elements, the negative electrode material including a carbon matrix and an active material, the carbon matrix being provided with pores, and at least part of the active material being disposed in the pores of the carbon matrix. A region with a depth of 200 nm from an outer surface of the negative electrode material is defined as a first region, and a region with a depth of 1000 nm from the outer surface of the negative electrode material is defined as a second region. The atomic percentage of nitrogen elements in the first region is α, and the atomic percentage of nitrogen elements in the second region is β, where α < β.

[0054] A deposition parameter of the active material is defined as γ, γ≥0.86; wherein ρ is the density of the active material; m1 is the mass of the negative electrode material, based on m1, the mass percentage of the active material is a1, and the specific pore volume of the negative electrode material is p1; m2 is the mass of the negative electrode material after removing the active material, based on m2, the mass percentage of the active material is a2, and the specific pore volume of the negative electrode material after removing the active material is p2.

[0055] The present application researches and finds that when the negative electrode material contains nitrogen element distribution, the nitrogen atom can cause the change of the local surface chemical activity of the carbon matrix, which can affect the contact between the carbon matrix and other substances. The present application further finds that when the negative electrode material is divided into the above-mentioned first region and the second region, the second region is closer to the inside of the carbon matrix than the first region. By setting the relationship between a and β, more nitrogen element is distributed in the inside of the carbon matrix rather than the surface layer. Taking the gas-phase deposition of active substances as an example, such distribution is conducive to promoting more gas-phase active substance sources (such as gas-phase silane) to crack and deposit in the pores of the carbon matrix, thereby facilitating the deposition of more active substances in the pores of the carbon matrix, so that the active substance deposition parameter γ is greater than or equal to 0.86, thereby relatively reducing the proportion of active substances distributed on the surface of the carbon matrix, further reducing the side reaction between the surface of the negative electrode material and the electrolyte, reducing the gas production behavior of the negative electrode material, and being conducive to the conductive performance of the carbon matrix. The secondary battery using the negative electrode material can have higher discharge specific capacity, first coulombic efficiency and better cycle stability.

[0056] Understandably, when a is greater than β, the nitrogen atom is distributed in the region of the carbon matrix relatively close to the surface layer, the content of nitrogen atoms distributed in the inside of the carbon matrix is relatively small, the role of nitrogen atoms in enhancing the adsorption capacity of the pores of the carbon matrix to active substances is weak, and the active substances tend to be deposited on the surface of the carbon matrix, which can lead to an increase in the side reaction between the negative electrode material and water or electrolyte, aggravate the gas production behavior of the negative electrode material, and be not conducive to the capacity of the secondary battery, resulting in a decrease in the first coulombic efficiency and cycle stability.

[0057] In the calculation formula of the above-mentioned γ, the polynomial m2×p2-m1×p1 represents the difference between the pore volume of the negative electrode material after removal of active substances and the pore volume of the provided negative electrode material, which is used to represent the volume of the active substances provided in the pores of the carbon matrix in the provided negative electrode material; the polynomial m1×a1-m2×a2 represents the difference between the mass of the active substances in the provided negative electrode material and the mass of the active substances in the negative electrode material after removal of the active substances, which is used to represent the total mass of the active substances removed by the acid solution in the provided negative electrode material (including all active substances provided in the pores of the carbon matrix and other positions of the carbon matrix). The ratio of the polynomial m1×a1-m2×a2 to the density ρ of the active substances corresponds to the representation of the total volume of the active substances in the provided negative electrode material. Therefore, the calculation formula of γ is used to represent the proportion of the active substances provided in the pores of the carbon matrix in the total active substances in the negative electrode material.

[0058] In some embodiments, the removing active material from the negative electrode material comprises mixing the negative electrode material with an acid solution. Mixing the negative electrode material with an acid solution of a sufficient concentration, stirring sufficiently, etching the negative electrode material with the acid solution, and cleaning and drying the etched negative electrode material can remove the active material in the pores of the carbon matrix and on the surface of the carbon matrix to a certain extent.

[0059] In some embodiments, the acid solution comprises one or more of hydrochloric acid, nitric acid, and hydrofluoric acid.

[0060] In some embodiments, taking silicon material as an example of the active material in the negative electrode material, the removing active material from the negative electrode material comprises using hydrochloric acid with a concentration of 70% and hydrofluoric acid with a concentration of 50% to configure an acid solution with a volume ratio of 2:1, placing the negative electrode material in the acid solution and stirring for at least 10 hours, and then cleaning and drying the negative electrode material. In the calculation formula of γ, ρ can be 2.34, the deposition parameter γ of the silicon material can be calculated, and the proportion of the silicon material deposited in the pores of the carbon matrix can be further characterized.

[0061] In the related art, taking silicon material as an active material as an example, when the content of the silicon material reaches a certain level, part of the silicon material will begin to adhere to the surface of the porous carbon, thereby significantly increasing the specific surface area of the negative electrode material, resulting in increased contact between the negative electrode material and the electrolyte during the charging and discharging process, increased side reactions, and difficulty in maintaining a stable SEI film. In addition, under the condition that the content of the silicon material in the negative electrode material is certain, a higher content of silicon material on the surface of the carbon matrix means a lower content of silicon material filled in the interior of the carbon matrix. In this way, a pore surplus may be generated in the interior of the particle, resulting in a problem of decreased strength of the negative electrode material particle. The present application measures the pore volume change and mass change of the negative electrode material before and after removing the active material, and then characterizes the proportion of the active material arranged in the pores of the carbon matrix in the negative electrode material, i.e., the deposition parameter γ. In the present application, when the active material deposition parameter γ is greater than or equal to 0.86, it indicates that most of the active material is distributed in the pores of the carbon matrix, which can reduce the side reactions between the negative electrode material and the electrolyte during the charging and discharging process, improve the cycle performance of the battery, and reduce the hydrolysis of the active material during the slurry preparation and battery use process, i.e., reduce the generation of gas, thereby improving the safety performance of the battery. In addition, under the condition that the content of the silicon material in the negative electrode material is certain, the increase in the content of the silicon material in the pores of the carbon matrix can also improve the particle strength of the negative electrode material. In other embodiments, when the deposition parameter γ is less than 0.86, the specific pore volume, specific surface area, micropore (pore with a pore size less than or equal to 2 nm) proportion, and gas production of the obtained negative electrode material significantly increase, which is not conducive to obtaining good electrochemical performance. This also indicates that the present application has important practical application value in obtaining a negative electrode material with relatively more active material arranged in the pores of the carbon matrix by setting the deposition parameter γ to be greater than or equal to 0.86.

[0062] The negative electrode material satisfying the above α, β, and γ requirements has excellent active material filling effect, with relatively most of the active material distributed in the pores of the carbon matrix, which reduces the gas production behavior of the negative electrode material and is conducive to the conductive performance of the carbon matrix, thereby enabling the secondary battery using the negative electrode material to have a higher discharge specific capacity, a first coulomb efficiency, and better cycle stability.

[0063] In some embodiments, 0.5≤a<β≤10. For example, a can be exemplified independently as 0.5, 1.0, 1.5, 1.8, 1.9, 2.2, 2.7, 2.9, 3, 4, 5, 6, 7, 8, 9, or any value within a range between any two of the above-mentioned values; or β can be exemplified independently as 1, 2, 3, 4, 4.7, 4.8, 5.2, 5.6, 7.8, 8.9, 9.5, 9.8, 10, or any value within a range between any two of the above-mentioned values; or a and β can be 1.5 and 5.6, 2.1 and 10, 1.0 and 4.8, 1.9 and 8.9, 0.5 and 4.7, 2.9 and 9.8, 2.7 and 9.5, 2.2 and 7.8, 1.8 and 5.2, or any other combination of values satisfying the above-mentioned relationship. It can be understood that controlling the size of a and β can regulate the content of nitrogen atoms in the carbon matrix, and when a and β satisfy the above-mentioned relationship and range, it is beneficial to form sufficient nitrogen doping in the carbon matrix, so as to better exert the effect of nitrogen atoms enhancing the adsorption capacity of the pores of the carbon matrix to active substances, promote relatively more active substances to be distributed in the pores of the carbon matrix, further reduce the side reactions between the surface of the negative electrode material and the electrolyte, further limit the gas generation behavior of the negative electrode material, improve the capacity of the secondary battery, and improve the first coulombic efficiency and cycle stability.

[0064] In some embodiments, the negative electrode material comprises a core and a coating layer arranged on at least part of the surface of the core, the core comprises a carbon matrix and an active substance, and the material of the coating layer comprises one or more of carbon material, metal oxide, amorphous silicon, conductive polymer, fluoride, phosphate, and nitride. The coating layer can reduce the solubility of the core part of the negative electrode material in the electrolyte, thereby reducing the amount of gas generated by the reaction of the dissolved active substance (such as silicon material) with the electrolyte. The carbon material can comprise one or more of graphene, soft carbon, and hard carbon, and the carbon coating layer on the outer layer of the negative electrode material has good electrical conductivity, which can improve the electrical conductivity of the negative electrode material.

[0065] In some embodiments, the coating layer can be a single-layer coating layer formed by a single material, or a coating layer formed by a combination of multiple materials, or a multi-layer coating layer formed by a single material, or a multi-layer coating layer formed by multiple materials, etc. The layer structure of the coating layer can be selected according to actual needs. When the coating layer is a multi-layer coating structure, the density is higher.

[0066] In some embodiments, the thickness of the coating layer is 5 nm to 200 nm. For example, the thickness of the coating layer can be 5 nm, 10 nm, 20 nm, 30 nm, 60 nm, 80 nm, 100 nm, 120 nm, 150 nm, 180 nm, 200 nm, or any value within a range between any two of the above-mentioned values. Controlling the thickness of the coating layer within the above-mentioned range can ensure the strength of the coating layer, without affecting the energy density, which is conducive to maintaining the stability of the particle structure of the negative electrode material in the cycle process, reducing the exposed active material on the surface of the negative electrode material, reducing the risk of generating a large amount of SEI in the charging and discharging process caused by the exposed active material, and at the same time maintaining a good interfacial resistance of the negative electrode material, thereby improving the specific capacity and electrochemical performance of the negative electrode material.

[0067] In some embodiments, the mass percentage of the coating layer in the negative electrode material is less than or equal to 10%. For example, the mass percentage of the coating layer can be 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, or any value within a range between any two of the above-mentioned values. Controlling the mass percentage of the coating layer in the negative electrode material within the above-mentioned range can enable the electrode active ions to have sufficient intercalation capacity, thereby enabling the battery to have a higher charging and discharging capacity, and being conducive to reducing the risk of conductivity reduction.

[0068] In some embodiments, the specific surface area of the carbon matrix is 800 m 2 / g to 3000 m 2 / g. For example, the specific surface area of the carbon matrix can be 800 m 2 / g, 1000 m 2 / g, 1200 m 2 / g, 1400 m 2 / g, 1600 m 2 / g, 1800 m 2 / g, 2000 m 2 / g, 2300 m 2 / g, 2500 m 2 / g, 2800 m 2 / g, 3000 m 2 / g, or any value within a range between any two of the above-mentioned values. Controlling the specific surface area of the carbon matrix within the above-mentioned range is conducive to uniform dispersion and stable adhesion of the active material on the carbon matrix, thereby reducing the risk of the active material falling off from the carbon matrix and improving the stability of the active material.

[0069] In some embodiments, the total pore volume of the carbon matrix is 0.5 cm 3 / g to 2.0 cm 3 / g. For example, the total pore volume of the carbon matrix can be 0.5 cm 3 / g, 0.8 cm3 / 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 defined by any two of the above values. The carbon matrix has abundant pores, which can accommodate active materials and reserve space for volume expansion of the accommodated active materials, thus improving the capacity and cycle stability of the battery prepared from the negative electrode material.

[0070] In some embodiments, the carbon matrix has a porosity of 40% to 80%. For example, the carbon matrix can have a porosity of 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or any value within a range defined by any two of the above values. The porosity refers to the percentage of the pore volume in the material to the total volume of the material in a natural state. Controlling the porosity of the carbon matrix within the above range is beneficial for good deposition of the active material, thus reducing the volume expansion effect of the negative electrode material.

[0071] In some embodiments, the carbon matrix comprises one or more of hard carbon, soft carbon, natural graphite, artificial graphite, carbon nanotube, carbon fiber, and graphene. The carbon matrix is selected from the above materials, which can all provide distribution sites for the active material and form a conductive network.

[0072] In some embodiments, the active material comprises one or more of Li, Na, K, Sn, Ge, Si, Fe, Mg, SiO, Ti, Zn, Al, P, and Cu. The combination of the carbon matrix with the above metal materials can induce lithium (or sodium, potassium) to be precipitated in the form of alloy with the metal materials in the pores, thus improving the lithium (or sodium, potassium) storage space of the carbon matrix. The combination of the carbon matrix with phosphorus can improve the electrical conductivity and is also beneficial for reducing the volume change of the negative electrode material during discharge / charge.

[0073] In some embodiments, the active material comprises a silicon material, which comprises one or more of amorphous silicon, crystalline silicon, and a composite of crystalline silicon and amorphous silicon. When the active material comprises the 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 thus the energy density of the secondary battery. Preferably, the silicon material comprises amorphous silicon. When the active material further comprises amorphous silicon, the amorphous silicon isotropically expands during lithium intercalation, which can reduce the collapse of pores in the material and inhibit the rapid decay of the specific capacity of the negative electrode material, thus being more beneficial for improving the lithium intercalation cycle performance of the negative electrode material.

[0074] In some embodiments, the silicon material includes one or more of silicon particles, silicon oxide, and silicon alloy. In some embodiments, the silicon material includes silicon particles and a silicon oxide layer on the surface of the silicon particles, and the silicon oxide layer includes silicon oxide.

[0075] In some embodiments, the average particle size of the silicon material is 0.1 nm to 500 nm. For example, the average particle size of the silicon material can be 0.1 nm, 0.5 nm, 1 nm, 5 nm, 10 nm, 20 nm, 30 nm, 50 nm, 80 nm, 100 nm, 200 nm, 300 nm, 400 nm, 450 nm, 500 nm, or any value within a range defined by any two of the above values. For example, the silicon 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 silicon particles swell in volume can be reduced, the battery capacity of the secondary battery can be maintained, the irreversible capacity loss can be reduced, the electron and ion transport paths can be shortened, and the size of the silicon particles is reduced, the gap between adjacent silicon particles is increased, and space for the volume expansion of the silicon particles can be reserved.

[0076] In some embodiments, the coating layer includes carbon material, and the mass percentage of carbon elements in the negative electrode material is 40% to 60% based on the mass of the negative electrode material. For example, the mass percentage of carbon elements can be 40%, 43%, 45%, 50%, 53%, 55%, 58%, 60%, or any value within a range defined by any two of the above values. Further preferably, the mass percentage of carbon elements can be 40% to 55%. The carbon material is derived from the carbon matrix and the coating layer, and the carbon material is compounded with the silicon material, which can provide an electrically conductive platform and a buffer space for the silicon material, and improve the structural instability and poor electrical conductivity of the negative electrode material during the cycle process.

[0077] In some embodiments, the active material includes silicon material, and the mass percentage of silicon elements in the negative electrode material is 30% to 65% based on the mass of the negative electrode material. For example, the mass percentage of silicon elements in the negative electrode material can be 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, or any value within a range defined by any two of the above values. Further preferably, the mass percentage of silicon elements can be 45% to 60%. The main element composition of the silicon material is silicon elements, and the negative electrode material compounded with the carbon matrix and the silicon material can effectively solve the expansion problem of the silicon material during the cycle process, maintain the stability of the battery material, and improve the volume capacity of the material.

[0078] In some embodiments, the specific surface area of the negative electrode material is 0.5 m 2 / g to 10 m 2 / g. For example, the specific surface area of the negative electrode material can be 0.5 m 2 / g, 1 m 2 / g, 2 m 2 / g, 3 m2 / 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 a range between any two of the above. When the specific surface area of the negative electrode material is large, the SEI film on the surface of the negative electrode material will consume excessive lithium salt, and the volume effect is easy to cause electrical detachment between particles, resulting in a decrease in the discharge specific capacity and coulombic efficiency of the battery. Therefore, the specific surface area of the negative electrode material of the present embodiment is small, which is beneficial to improve the first discharge specific capacity and the first coulombic efficiency of the battery. Further, the specific surface area of the negative electrode material can be preferably 0.5m 2 / g to 2.5m 2 / g. The specific surface area of the negative electrode material is further controlled within the above range, which is beneficial to further improve the first discharge specific capacity and the first coulombic efficiency of the battery.

[0079] In some embodiments, the powder conductivity of the negative electrode material is 0.5 S / cm to 3 S / cm. For example, the powder conductivity of the negative electrode material can be 0.5 S / cm, 1 S / cm, 1.5 S / cm, 2 S / cm, 2.5 S / cm, 3 S / cm or any value within a range between any two of the above. The powder conductivity of the negative electrode material reflects the conductivity performance of the negative electrode material, and good conductivity performance is beneficial to the capacity of the battery. Further, the powder conductivity of the negative electrode material can be preferably 1.5 S / cm to 3 S / cm. The powder conductivity of the negative electrode material is further controlled within the above range, which is beneficial to further improve the utilization of lithium in the lithium ion battery.

[0080] In some embodiments, the 24-hour gas production value of the negative electrode material is less than or equal to 4 cc / kg. For example, the 24-hour gas production value of the negative electrode material can be 0.5 cc / kg, 0.6 cc / kg, 0.8 cc / kg, 0.9 cc / kg, 1 cc / kg, 1.1 cc / kg, 1.2 cc / kg, 1.3 cc / kg, 2 cc / kg, 2.5 cc / kg, 3 cc / kg, 4 cc / kg or any value within a range between any two of the above. The low gas production value of the negative electrode material is beneficial to reduce the consumption of active material and improve the cycle stability of the battery. Further, the 24-hour gas production value of the negative electrode material can be preferably less than 2.7 cc / kg. The gas production value of the negative electrode material is further controlled within the above range, which is beneficial to further reduce the gas production behavior of the negative electrode material and improve the cycle stability.

[0081] In some embodiments, the median particle size D50 of the negative electrode material is 5-20 μm. For example, the median particle size D50 of the negative electrode material can be 5 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 17 μm, 20 μm, or any value within the range between any two of the above-mentioned values. Controlling the median particle size of the negative electrode material within the above-mentioned range is beneficial to the improvement of the cycle performance of the negative electrode material.

[0082] In some embodiments, the particle size distribution (D90-D10) / D50 of the negative electrode material is 0.9-5. For example, the particle size distribution (D90-D10) / D50 of the negative electrode material can be 0.9, 1, 1.2, 1.5, 2, 2.6, 3, 3.5, 4, 4.3, 4.6, 4.8, 5, or any value within the range between any two of the above-mentioned values. When the particle size distribution of the negative electrode material is within the above-mentioned range, the large particles with large particle sizes and the small particles with small particle sizes in the negative electrode material can cooperate with each other, and the small particles can fill the pores between the large particles, which can improve the tap density of the negative electrode material.

[0083] In the volume-based cumulative particle size distribution determined, 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.

[0084] In some embodiments, the tap density of the negative electrode material under a pressure of 1T is 0.8 g / cm 3 to 1.3 g / cm 3 . For example, the tap density 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 , 1.3 g / cm 3 , or any value within the range between any two of the above-mentioned values. Controlling the tap density within the above-mentioned range is helpful to reduce the diffusion path of lithium ions in the negative electrode material, thereby improving the rate performance of the battery.

[0085] In some embodiments, the tap density of the negative electrode material after 3000 vibrations is 0.5 g / cm 3 to 1.5 g / cm 3 . For example, the tap density can be 0.5 g / cm 3 , 0.7 g / cm 3 , 0.9 g / cm 3 , 1.1 g / cm 3 , 1.3 g / cm 31.5 g / cm3 3 or any value within a range between any two of the above values. Controlling the tap density within the above range is advantageous for the negative active material to form a proper tightness of internal structure, thereby improving the transport of lithium ions and the conduction of electrons, enhancing the energy density of the battery, prolonging the cycle life, and improving the safety performance.

[0086] The negative active material active layer further includes a binder to bind the negative active material particles to facilitate the formation of a film layer, and to improve the adhesion between the negative active material active layer and the negative 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, styrene butadiene rubber, acrylated styrene butadiene rubber, epoxy resin, or nylon, etc.

[0087] The negative active material active layer can further include 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 materials can include, but are 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 materials can include, but are not limited to, metal powders or metal fibers, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer can be a polyphenylene derivative.

[0088] Separator film

[0089] The separator film 103 includes a film 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 film 103 can be a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric, or a polypropylene-polyethylene-polypropylene porous composite film, etc.

[0090] Electrolyte

[0091] 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 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 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), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl 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.

[0092] Another embodiment of the present application also provides a method for preparing a negative electrode material, comprising:

[0093] The first step is to mix the carbon matrix, zinc salt, and organic active agent, which includes one or more of polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), and Pluronic F127. The mixture is heated in a reducing gas atmosphere to reduce the zinc ions in the zinc salt and attach them to the carbon matrix, obtaining a first precursor.

[0094] The above-mentioned organic active agent has amino and hydroxyl groups, for example, can be a non-ionic active agent or an anionic active agent, and can promote the reduction of the zinc salt and the attachment of the reduced elemental zinc to the surface and pores of the carbon matrix.

[0095] In some embodiments, the zinc salt comprises one or more of zinc chloride, zinc nitrate, zinc acetate, and zinc sulfate.

[0096] In some embodiments, the reducing gas comprises hydrogen. Understandably, the reducing gas atmosphere can further comprise an inert gas, for example, the reducing gas atmosphere can be hydrogen, a mixture of hydrogen and helium, a mixture of hydrogen and nitrogen, etc.

[0097] In some embodiments, the heating temperature is 600-800°C, and the heating time is 2-6h. For example, the heating temperature can be 600°C, 620°C, 650°C, 700°C, 750°C, 780°C, 800°C, or any value within a range defined by any two of the above values. The heating time can be 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, 6h, or any value within a range defined by any two of the above values.

[0098] Second step: mixing the first precursor with an acid solution and dissolving to obtain a second precursor.

[0099] The acid solution can act on the metallic zinc attached to the surface of the carbon matrix, reducing the metallic zinc on the surface of the carbon matrix, so as to obtain a carbon matrix precursor with metallic zinc attached to the pores, i.e., a second precursor.

[0100] In some embodiments, the acid solution comprises one or two of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid.

[0101] In some embodiments, the concentration of the acid solution is 0.5-2mol / L. For example, the concentration of the acid solution can be 0.5mol / L, 0.7mol / L, 1mol / L, 1.2mol / L, 1.5mol / L, 1.8mol / L, 2mol / L, or any value within a range defined by any two of the above values. In some embodiments, the dissolution time of the acid solution is 1-20h. For example, the dissolution time can be 1h, 3h, 5h, 8h, 10h, 12h, 15h, 17h, 19h, 20h, or any value within a range defined by any two of the above values. Controlling the concentration and dissolution time of the acid solution within the above ranges is conducive to the acid solution playing a role in removing the zinc on the surface of the carbon matrix, while being conducive to reducing the reaction between the acid solution and the zinc in the pores of the carbon matrix.

[0102] The third step is to react the second precursor with a mixture of ammonia gas and a protective gas under high temperature conditions, wherein the concentration of the ammonia gas is 1% to 50%, the reaction temperature is 400°C to 1000°C, the reaction time is 0.5h to 10h, and the reaction pressure is 1Pa to 100kPa, to obtain a third precursor.

[0103] For example, the concentration of the ammonia gas can be 1%, 5%, 10%, 15%, 20%, 30%, 40%, 45%, 50%, or any value within a range defined by any two of the above values. The reaction temperature can be 400°C, 600°C, 650°C, 700°C, 750°C, 800°C, 850°C, 900°C, 950°C, 1000°C, or any value within a range defined by any two of the above values. The reaction time can be 0.5h, 1h, 2h, 4h, 5h, 7h, 8h, 9h, 10h, or any value within a range defined by any two of the above values. The reaction pressure can be 1Pa, 50Pa, 500Pa, 1kPa, 10kPa, 20kPa, 40kPa, 60kPa, 80kPa, 100kPa, or any value within a range defined by any two of the above values.

[0104] The ammonia gas molecules are allowed to fully enter the pores of the carbon matrix under negative pressure conditions, which helps to dope nitrogen atoms in the pores of the carbon matrix, and the nitrogen doping can improve the surface chemical activity of the carbon material to enhance the adsorption capacity of the material for gas-phase silane molecules and improve the deposition effect. Controlling the concentration of the ammonia gas in the above process within the above range is conducive to doping sufficient nitrogen atoms in the negative electrode material to play the role of the above nitrogen doping, and is conducive to forming a nitrogen atom distribution in the negative electrode material that conforms to a preset trend. Controlling the reaction temperature and the reaction time in the above process within the above range is conducive to promoting the reaction of the ammonia gas and the second precursor, thereby facilitating the doping of sufficient nitrogen atoms in the negative electrode material and facilitating the formation of a nitrogen atom distribution that conforms to a preset trend.

[0105] In some embodiments, the protective gas includes at least one of nitrogen, helium, neon, argon, and krypton.

[0106] The fourth step is to mix the third precursor with an active material precursor to perform gas-phase deposition to obtain a negative electrode material.

[0107] In the gas-phase deposition process, the active material is deposited in the pores of the carbon matrix under the catalytic action of the metallic zinc, so that the active material in the prepared negative electrode material is mainly distributed in the pores of the carbon matrix, thereby reducing the reaction between the active material attached to or dissolved on the surface of the negative electrode material and the electrolyte, effectively reducing the gas production value of the negative electrode material and improving the powder conductivity of the negative electrode material, thereby improving the performance of the battery prepared from the negative electrode material.

[0108] In some embodiments, when the active material is a silicon material, the active material precursor comprises a silicon-containing gas, which comprises one or more of silane, disilane, trisilane, and tetrasilane.

[0109] In some embodiments, the concentration of the silicon-containing gas is 10% to 80%. For example, the concentration of the silicon-containing gas can be 10%, 15%, 20%, 30%, 50%, 70%, 80%, or any value within a range defined by any two of the aforementioned values. Further, the gas used to dilute the silicon-containing gas can be selected from one or both of inert gas and hydrogen.

[0110] In some embodiments, the temperature for vapor deposition is 300°C to 800°C. For example, the temperature for vapor deposition can be 300°C, 400°C, 450°C, 500°C, 520°C, 550°C, 600°C, 700°C, 800°C, or any value within a range defined by any two of the aforementioned values.

[0111] In some embodiments, the time for vapor deposition is 0.5h to 15h. For example, the time for vapor deposition can be 0.5h, 1h, 2h, 5h, 8h, 10h, 12h, 15h, or any value within a range defined by any two of the aforementioned values.

[0112] The process conditions for vapor deposition, such as the deposition temperature and the deposition time, affect the deposition of the silicon material in the pores of the carbon material. Controlling the temperature and the time for vapor deposition within the aforementioned ranges is advantageous for reducing the decomposition and deposition of the reaction gas before it enters the pores of the carbon material, and for promoting the decomposition and deposition of the reaction gas after it enters the pores.

[0113] In some embodiments, after the aforementioned vapor deposition, the obtained product (i.e., the inner core) is further subjected to a coating process. The coating includes at least one of carbon coating, oxide coating, metal salt coating, polymer coating, etc. Further, a combination of multiple coating methods is preferred, such as carbon coating followed by oxide coating, or carbon coating followed by metal salt coating, etc.

[0114] In some embodiments, when the coating comprises a carbon coating, the material of the coating layer comprises a carbon material, the carbon material is derived from at least one of a gas-phase carbon, a liquid-phase carbon, and a solid-phase carbon. The gas-phase carbon comprises at least one of methane, acetylene, ethylene, ethane, propane, propylene, gaseous benzene, gaseous toluene, gaseous xylene, gaseous ethanol, gaseous cyclohexane, gaseous methanol, and gaseous acetone; the liquid-phase carbon source comprises at least one 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. The solid-phase carbon source comprises at least one of polyvinyl chloride, polyvinyl butyral, polyacrylonitrile, polyacrylic acid, polyethylene glycol, polypyrrole, polyaniline, sucrose, glucose, maltose, citric acid, pitch, furfural resin, epoxy resin, and phenolic resin.

[0115] In some embodiments, the coating process comprises mixing the coating material with the core and performing a heat treatment.

[0116] The solution of the present application will be explained below in combination with embodiments. Those skilled in the art will understand that the following examples are only for explaining the present application and cannot be understood as a limitation of the present application. Unless otherwise stated, the reagents, software and instruments involved in the following examples which are not specifically stated are all conventional commercially available products or open source.

[0117] Embodiment 1:

[0118] A negative electrode material, the preparation method thereof comprises:

[0119] S1, weigh 2 kg of zinc chloride and 400 g of Pluronic F127, dissolve them in 20 L of water, then add 5 kg of activated carbon, stir for 24 h, filter, dry, and then place in a rotary furnace, calcine at 900 ℃ under an argon atmosphere for 4 h, to obtain a first precursor.

[0120] S2, place the first precursor into 20 L of 1 mol / L dilute sulfuric acid and stir for 4 h, then wash the sample with pure water, dry, and reserve, to obtain a second precursor.

[0121] S3, place the second precursor in a reaction cavity, heat to 800 ℃ under a nitrogen atmosphere, maintain the pressure in the cavity at 100 kPa, then use a mixed gas composed of 90% nitrogen and 10% ammonia to react with the second precursor for 2 h, to obtain a third precursor.

[0122] S4, adjust the temperature of the cavity to 520 ℃, introduce silane and a carrier gas into the cavity, the carrier gas is a mixed gas of nitrogen, argon, and hydrogen, control the concentration of the silane to be 15%, and react for 10 h, to obtain a core material.

[0123] S5, adjust the temperature of the cavity to 700℃, introduce methane and carrier gas into the cavity, the carrier gas is a mixture of nitrogen, argon and hydrogen, control the methane concentration to be 15%, and keep for 4h. Then close the methane gas, introduce nitrogen, and reduce the temperature to room temperature.

[0124] S6, screen and grade the obtained sample to obtain the negative electrode material.

[0125] Example 2:

[0126] The difference from Example 1 is that in S3, the composition of the mixed gas is adjusted to be 80% nitrogen and 20% ammonia.

[0127] Example 3:

[0128] The difference from Example 1 is that in S3, the reaction temperature is adjusted to be 600℃.

[0129] Example 4:

[0130] The difference from Example 1 is that in S3, the reaction temperature is adjusted to be 1000℃.

[0131] Example 5:

[0132] The difference from Example 1 is that in S3, the reaction time is adjusted to be 0.5h.

[0133] Example 6:

[0134] The difference from Example 1 is that in S3, the reaction time is adjusted to be 4h.

[0135] Example 7:

[0136] The difference from Example 1 is that in S3, the composition of the mixed gas is adjusted to be 80% nitrogen and 20% ammonia, the reaction temperature is adjusted to be 700℃, the reaction pressure is adjusted to be 50kPa, and the reaction time is adjusted to be 1h.

[0137] Example 8:

[0138] The difference from Example 1 is that in S3, the composition of the mixed gas is adjusted to be 50% nitrogen and 50% ammonia, the reaction pressure is adjusted to be 50kPa, and the reaction time is adjusted to be 0.5h.

[0139] Example 9:

[0140] The difference from Example 1 is that in S3, the composition of the mixed gas is adjusted to be 95% nitrogen and 5% ammonia, the reaction temperature is adjusted to be 700℃, and the reaction time is adjusted to be 10h.

[0141] Example 10:

[0142] The difference from Example 1 is that S5 is not performed.

[0143] Example 11:

[0144] The difference from Example 1 is that in S3, the treatment is carried out under normal pressure.

[0145] Example 12:

[0146] The difference from Example 1 is that in S3, the composition of the mixed gas is adjusted to 99% nitrogen and 1% ammonia.

[0147] Example 13:

[0148] The difference from Example 1 is that in S3, the reaction temperature is adjusted to 400°C.

[0149] Example 14:

[0150] The difference from Example 1 is that in S3, the reaction time is adjusted to 10 min.

[0151] Comparative Example 1:

[0152] The difference from Example 1 is that S3 is not carried out.

[0153] Comparative Example 2:

[0154] The difference from Example 1 is that in S3, the composition of the mixed gas is adjusted to 20% nitrogen and 80% ammonia.

[0155] Comparative Example 3:

[0156] The difference from Example 1 is that in S3, the reaction temperature is adjusted to 1800°C.

[0157] Comparative Example 4:

[0158] The difference from Example 1 is that in S3, the reaction time is adjusted to 20 h.

[0159] Example 1, a scanning electron microscope (Hitachi S4800) is used to test the negative electrode material obtained in Example 1, the voltage is 3 kV, the current size is 10 μA, the picture is focused and the astigmatism is eliminated, please refer to Figure 3 The negative electrode material prepared in the embodiments of the present application is uniformly distributed and has no obvious agglomeration phenomenon, which indicates that the use of the preparation method of the present application can realize the uniform distribution of the coating on the graphite material.

[0160] Example 1, an X-ray diffraction analyzer (Japan Science Ultima lv.) is used to test the negative electrode material obtained in Example 1, the X-ray source is copper (Cu) Kα line, the incident angle range is set to 10° to 90°, the step size is set to 0.02°, please refer to Figure 4The characteristic bread peak appears at 2θ = 28.4°, indicating that the silicon in the negative electrode material is mainly amorphous silicon.

[0161] The negative electrode materials obtained in Examples 1-14 and Comparative Examples 1-4 were subjected to the following physical property and electrochemical property tests:

[0162] 1. α, β test of the negative electrode material: The negative electrode material was subjected to X-ray photoelectron spectroscopy test using a Thermo Scientific K-Alpha device from the United States, and the whole test process was carried out in an argon-filled glove box. Four kinds of C, O, Si, and N were tested each time, and the argon ion sputtering depth was 50 nm each time. The specific test mode was as follows: sputtering 4 times to measure the nitrogen atom percentage at a depth of 200 nm, and sputtering 20 times to measure the nitrogen atom percentage at a depth of 1000 nm. Ten random samplings were tested at the same depth, and the average value was obtained to obtain α and β.

[0163] 2. Test of deposition parameter γ of silicon material in the negative electrode material: Take the negative electrode material with a mass of m1, use the ASAP2460 micropore specific surface area and pore size analyzer of American Micromeritics to measure the specific pore volume p1, use the SA2-9-17TP box-type atmosphere furnace of Nanyang Xiyu to burn in oxygen atmosphere, so that silicon and silicon monoxide in the sample react to form silicon dioxide, and carbon is burned to become carbon dioxide and discharged. Calculate the silicon content ratio a1. Use a solution prepared by mixing 70% HCl and 50% HF at a volume ratio of 2:1, and put the negative electrode material into the solution and stir for more than 10 hours. After cleaning and drying, the mass of the remaining material is m2, the specific pore volume is p2, and the silicon content ratio is a2. The density value of the silicon material is 2.34. The deposition parameter γ of the silicon material in the negative electrode material is calculated by the following formula:

[0164]

[0165] 3. Test of specific surface area, total pore volume, and porosity of the negative electrode material and carbon matrix:

[0166] a) Taking the active material as an example, in the state of stirring, 150 mL of 20% HF acid solution was added dropwise into 10 g of negative electrode material, which produced SiF4 and H2 gas and released heat. After the gas production stopped, the supernatant acid solution was removed by centrifugation. Then, 150 mL of 20% HF acid solution was added into the negative electrode material again, and the negative electrode material was stirred for 12 h and then centrifuged to remove the supernatant acid solution. Then, the negative electrode material was washed with pure water until it was neutral and dried to obtain the negative electrode material after removing the silicon material, i.e., the carbon matrix.

[0167] b) the specific surface area of the material is measured using the American Micromeritics TriStar 3000 specific surface area and pore size analyzer apparatus;

[0168] c) the pore volume of the material is measured by gas adsorption, while nitrogen adsorption is a technique for characterizing 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 condenses first in the pores with the smallest diameter, and the pressure increases until the saturation point is reached, at which point all the 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 makes it possible to determine the pore volume and the pore size distribution, as well as the respective pore volumes of micropores, mesopores and macropores in the total pore volume.

[0169] 4. Powder conductivity test of the negative electrode material: the conductivity under a 20 KN pressure point is tested using the MCP-PD51 powder resistance test system of Mitsubishi Chemical, Japan, and the volume resistivity of the sample is determined using the four-probe method. Using this instrument, the resistance of the powder can be measured, and then the conductivity and resistivity of the powder are automatically calculated by the computer.

[0170] 5. Gas production test of the negative electrode material:

[0171] a) 5 g of the negative electrode material is loaded into a sealed container, and deionized water is added to a remaining volume of 60 mL above the container;

[0172] b) the container is sealed and mixed evenly, and stored at room temperature for one day;

[0173] c) after 24 hours, the container is thoroughly shaken again to disperse the precipitate into the liquid.

[0174] The lid of the container is opened, the concentration of hydrogen gas is detected using a hydrogen gas detector, and converted to cc / (kg-day).

[0175] 6. Particle size test of the negative electrode material: D50 is measured using a laser particle size analyzer, which has a symmetric distribution of normal distribution. In the volume-based distribution, the cumulative 50% diameter is D50, and in turn, the cumulative 90% diameter is D90, and the cumulative 10% diameter is D10, so that the particle size distribution (D90-D10) / D50 of the material can also be obtained.

[0176] 7. Compacted density test of the negative electrode material: the specified mass m of the sample is placed in the mold using the American Micromeritics CARVER 4350.22 powder compacted density instrument, and a pressure of 1.0 T is applied, the pressure is maintained for 30 S, and then the pressure is removed to test the thickness, and the compacted density is calculated.

[0177] 8. Tap density test of negative electrode material: using Kuntai DAT-6-220 tap density instrument, a sample of a specified mass is placed in a measuring cylinder, and vibration is performed for a specified number of times (conventional test tap 3000 times), the volume of the measuring cylinder after tapping is read and the tap density is calculated.

[0178] 9. Carbon content test of negative electrode material: using G4 ICARUS HF infrared carbon and sulfur analyzer of Germany Bruck, 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.

[0179] 10. Electrochemical performance test:

[0180] The negative electrode materials prepared in the above examples and comparative examples are respectively mixed with carboxymethyl cellulose sodium, butadiene rubber, and 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 prepare a negative electrode sheet. The negative electrode sheet is assembled into a button cell in an argon atmosphere glove box, the used separator is a polypropylene microporous membrane, the used electrolyte is 1 mol / L lithium hexafluorophosphate (the solvent is a mixed slurry of ethylene carbonate, methyl ethyl carbonate and dimethyl carbonate), and the used counter electrode is a lithium metal sheet.

[0181] The negative electrode materials prepared in the above examples and comparative examples are prepared into batteries, and discharge specific capacity test is performed on a Blue Electric CT2001A battery test system. The ratio of the electric quantity discharged in 1 hour to the capacity of the battery is the discharge specific capacity.

[0182] The negative electrode materials prepared in the above examples and comparative examples are prepared into batteries, and first coulombic efficiency test is performed on a Blue Electric CT2001A battery test system. The charging and discharging current is 0.05C, and the first coulombic efficiency is measured. The measured value is the average value of 3 to 5 button cells of each material.

[0183] The preparation conditions of part of the above examples 1-14 and comparative examples 1-4 are shown in Table 1, and the above test results are shown in Table 2.

[0184] Table 1. Part of the preparation conditions of examples 1-14 and comparative examples 1-4 of the present application

[0185] Reaction temperature (°C) Reaction pressure (kPa) Ammonia content (%) Reaction time (h) Example 1 800 100 10 2 Example 2 800 100 20 2 Example 3 600 100 10 2 Example 4 1000 100 10 2 Example 5 800 100 10 0.5 Example 6 800 100 10 4 Example 7 700 50 20 1 Example 8 800 50 50 0.5 Example 9 700 100 5 10 Example 10 800 100 10 2 Example 11 800 / 10 2 Example 12 800 100 1 2 Example 13 400 100 10 2 Example 14 800 100 10 10 min Comparative Example 1 / / / / Comparative Example 2 800 100 80 2 Comparative Example 3 1800 100 10 2 Comparative Example 4 800 100 10 20

[0186] Table 2. Performance test results of examples 1-14 and comparative examples 1-4 of the present application

[0187]

[0188] Under the preparation process conditions of the present application, the negative electrode materials of Examples 1-14 contain nitrogen elements, also form a structure of a carbon matrix core with silicon material deposition and a cladding layer on the surface of the core, and the nitrogen elements have a preset content and distribution trend in the negative electrode material, so that the pores of the carbon matrix have enhanced adsorption capacity for the gaseous silicon source, thereby facilitating to increase the value of the deposition parameter γ to a certain extent, facilitating to make the silicon material deposition parameter γ meet the preset range, thereby relatively reducing the deposition of the silicon material on the surface of the carbon matrix, reducing the side reaction between the surface of the negative electrode material and the electrolyte, and effectively reducing the gas production behavior of the negative electrode material. The above negative electrode material has excellent silicon material filling effect, low gas production value, and the secondary battery using the same has relatively high discharge specific capacity, first coulombic efficiency and good cycle stability.

[0189] Among them, Examples 1-9 and Examples 11-14 also perform carbon cladding, so the comprehensive performance of the obtained negative electrode material is better. Examples 1-11 and 13 further meet the preset ranges of α and β, so the comprehensive performance of the obtained negative electrode material is better. In the carbon matrix nitrogen doping process of Examples 1 and 2, Examples 3 and 4, and Examples 5 and 6, the ammonia dosage, reaction temperature and reaction time are increased in turn within the appropriate range, and the values of α and β in the obtained negative electrode material are increased accordingly and α < β is maintained.

[0190] Compared with Example 1, Comparative Example 1 does not perform nitrogen doping treatment on the carbon matrix, and the detection amount of nitrogen atoms in the negative electrode material is low, which can be regarded as the background value of the nitrogen element in the negative electrode material. The nitrogen element in the negative electrode material is uniformly distributed, so α and β are basically equal, and the adsorption capacity of the carbon matrix of Comparative Example 1 for the gaseous silicon source is dependent, and the silicon material filling effect of the obtained negative electrode material is not ideal (γ is smaller than that of Example 1), the proportion of the silicon material distributed on the surface of the carbon matrix is relatively large, the gas production behavior is relatively severe, affects the play of the conductivity of the carbon matrix, also leads to the increase of the specific surface area of the negative electrode material, thereby leading to the discharge specific capacity and the first coulombic efficiency of the obtained secondary battery are not as good as those of Example 1.

[0191] Compared with Example 1, the excessive ammonia gas was used in the process of nitrogen doping of the carbon matrix of Comparative Example 2, which led to excessive nitrogen doping in the carbon matrix, was not conducive to the control of the relationship between α and β, and resulted in α>β. In the process of nitrogen doping of the carbon matrix of Comparative Example 3, a too high temperature was used, which may have led to a too fast cracking rate of ammonia gas, chaotic attachment of nitrogen atoms, and possible collapse of the structure of the carbon matrix, which was not conducive to the control of the relationship between α and β, and resulted in α>β. Meanwhile, the too high temperature also intensified the graphitization degree of the carbon matrix, which was not conducive to the deposition of silicon material. In the process of nitrogen doping of the carbon matrix of Comparative Example 4, a too long reaction time was used, which may have led to easy doping of the subsequently cracked nitrogen in the relatively surface layer of the carbon matrix, thereby resulting in α>β. In the above negative electrode materials, the proportion of the silicon material distributed on the surface of the carbon matrix was also relatively large, the gas production behavior was violent, the conductive performance of the carbon matrix was affected, and the specific surface area of the negative electrode material was increased, thereby limiting the discharge specific capacity and the initial coulombic efficiency of the obtained secondary battery.

[0192] In summary, the application provides an improved preparation method of a negative electrode material. Under the process conditions of the preparation method, the obtained negative electrode material has α, β and γ that meet the preset ranges, and α and β meet the preset relationship, so that the distribution of the active material in the negative electrode material has an excellent filling effect, the gas production behavior of the negative electrode material is reduced, and the conductive performance of the carbon matrix is facilitated, thereby making the secondary battery using the negative electrode material have a higher discharge specific capacity, an initial coulombic efficiency and a better cycle stability.

[0193] The above embodiments are only used to illustrate the technical solutions of the application and not to limit the application. Although the application has been described in detail with reference to the above preferred embodiments, those skilled in the art should understand that the technical solutions of the application can be modified or replaced by equivalents without departing from the spirit and scope of the application.

Claims

1. A negative electrode material, characterized by, The negative electrode material contains nitrogen element, the negative electrode material includes a carbon matrix and an active material, the carbon matrix is provided with pores, and at least part of the active material is arranged in the pores of the carbon matrix, a region with a depth of 200 nm from an outer surface of the negative electrode material is defined as a first region, a region with a depth of 1000 nm from the outer surface of the negative electrode material is defined as a second region, an atomic percentage of the nitrogen element in the first region is α, and an atomic percentage of the nitrogen element in the second region is β, a deposition parameter of the active material is defined as γ, wherein ρ is the density of the active material; a mass percentage of the active material in the negative electrode material is a1, and a specific pore volume of the negative electrode material is p1; the negative electrode material with a mass of m1 is placed in a sufficient amount of acid solution for stirring, and after cleaning and drying, the mass of the remaining material is m2, a mass percentage of the active material in the remaining material is a2, and a specific pore volume of the remaining material is p2.

2. The negative electrode material of claim 1, wherein, The negative electrode material includes an inner core and a coating layer arranged on at least part of a surface of the inner core, the inner core includes the carbon matrix and the active material, and the coating layer satisfies at least one of the following conditions: (1) the material of the coating layer includes one or more of carbon material, metal oxide, amorphous silicon, conductive polymer, fluoride, phosphate and nitride; (2) the thickness of the coating layer is 5 nm to 200 nm; (3) the mass percentage of the coating layer in the negative electrode material is less than or equal to 10%.

3. The negative electrode material of claim 1, wherein, The carbon matrix satisfies at least one of the following conditions: (1) the carbon matrix has a specific surface area of 800 m 2 / g to 3000 m 2 / g; (2) the total pore volume of the carbon matrix is 0.5 cm 3 / g to 2.0 cm 3 / g; (3) the porosity of the carbon matrix is 40% to 80%; (4) the carbon matrix includes one or more of hard carbon, soft carbon, natural graphite, artificial graphite, carbon nanotube, carbon fiber and graphene.

4. The negative electrode material of claim 1, wherein, The active material satisfies at least one of the following conditions: (1) the active material includes one or more of Li, Na, K, Sn, Ge, Si, Fe, Mg, SiO, Ti, Zn, Al, P and Cu; (2) the active material includes a silicon material, and the silicon material includes one or more 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 one or more of silicon particles, silicon oxide and silicon alloy; (4) the active material includes a silicon material, and the silicon material includes silicon particles and a silicon oxide layer arranged on a surface of the silicon particles, and the silicon oxide layer includes silicon oxide; (5) the active material includes a silicon material, and the average particle size of the silicon material is 0.1 nm to 500 nm.

5. The negative electrode material of claim 2, wherein the carbon-based material is selected from the group consisting of graphite, carbon black, and carbon nanotubes. The active material includes a silicon material, the coating layer includes a carbon material, and the negative electrode material satisfies at least one of the following conditions: (1) based on the mass of the negative electrode material, the mass percentage of carbon element in the negative electrode material is 40% to 60%; (2) based on the mass of the negative electrode material, the mass percentage of silicon element in the negative electrode material is 30% to 65%.

6. The negative electrode material of claim 1, wherein, The negative electrode material further satisfies at least one of the following conditions: (1) the specific surface area of the negative electrode material is 0.5 m 2 / g to 10 m 2 / g; (2) the powder conductivity of the negative electrode material is 0.5 S / cm to 3 S / cm; (3) the negative electrode material has a 24-hour gas generation value of 4 cc / kg or less.

7. The negative electrode material of claim 1, wherein, The negative electrode material also satisfies at least one of the following conditions: (1) the negative electrode material has a median particle diameter of 5 μm to 20 μm; (2) the negative electrode material has a particle size distribution (D90-D10) / D50 of 0.9 to 5; (3) the compaction density of the negative electrode material under 1 T pressure is 0.8 g / cm 3 to 1.3 g / cm 3 ; (4) the tap density of the negative electrode material after 3000 times of vibration is 0.5 g / cm 3 to 1.5 g / cm 3 .

8. 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 by The negative electrode active material layer comprises the negative electrode material according to any one of claims 1 to 7.

9. A secondary battery characterized by comprising: The negative electrode sheet according to claim 8.

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