Negative electrode material, negative electrode sheet and secondary battery

Abstract

The present application relates to the technical field of secondary batteries, and provides a negative electrode material, a negative electrode sheet and a secondary battery. The negative electrode material comprises a porous matrix and an active substance, wherein at least part of the active substance is located in pores of the porous matrix; and the porous matrix comprises a carbon material and a silicon-nitrogen material, with the mass content of the silicon-nitrogen material in the negative electrode material being 0.5-15%. A secondary battery assembled from the negative electrode material has excellent expansion resistance, electronic conduction performance and lithium-ion conduction performance.

Description

Negative electrode material, negative electrode sheet and secondary battery

[0001] The present application claims priority to the Chinese patent application No. 202510099669.9, filed on January 21, 2025, and entitled "Negative electrode material, negative electrode sheet and secondary battery", the content of which is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application relates to the technical field of secondary batteries, in particular, to a negative electrode material, a negative electrode sheet and a secondary battery. BACKGROUND

[0003] Lithium ion batteries are widely used in new energy vehicles and 3C electronic products due to their small size, light weight, environmental protection and other advantages. With the rapid development and wide application of new energy vehicles, higher requirements are put forward for the energy density and cycle life of lithium ion batteries. The negative electrode material is an important component of lithium ion batteries, and its performance directly affects the electrochemical performance of lithium ion batteries. At present, graphite negative electrode materials are more commonly used, but there is little room for improvement in their specific capacity. Silicon-based negative electrode materials have attracted widespread attention due to their higher theoretical specific capacity, but silicon-based negative electrode materials have volume expansion problems during charging and discharging, and the particles are easily broken and pulverized, leading to rapid decay and failure of electrochemical performance, which restricts the application of lithium ion batteries. At present, porous carbon materials and silicon-based materials are often used to composite negative electrode materials to inhibit expansion, but the particle strength of porous carbon materials is low, and the lithium ion and electron conduction capacity is limited, which makes it difficult to further improve the anti-expansion performance, lithium ion and electron conduction performance of the negative electrode material composed of porous carbon materials and silicon-based materials.

[0004] SUMMARY

[0005] The main purpose of the present application is to provide a negative electrode material, a negative electrode sheet and a secondary battery to solve the problems of volume expansion and insufficient lithium ion and electron conduction performance of the negative electrode material composed of porous carbon materials and silicon-based materials during charging and discharging in the prior art.

[0006] In order to achieve the above-mentioned purpose, according to the first aspect of the present application, a negative electrode material is provided, which comprises a porous matrix and an active material, at least part of the active material being located in the pores of the porous matrix; the porous matrix comprises a carbon material and a silicon-nitrogen material.

[0007] The mass content of the silicon-nitrogen material in the negative electrode material is 0.5% to 15%. According to the second aspect of the present application, a negative electrode sheet is provided, which comprises the negative electrode material provided in the first aspect.

[0008] According to the third aspect of the present application, a secondary battery is provided, which comprises the negative electrode sheet provided in the second aspect.

[0009] By introducing the silicon-nitrogen material with a specific mass content into the porous matrix, on the one hand, the silicon-nitrogen material has high mechanical strength, and appropriate addition can enhance the structural stability of the porous matrix, effectively improve the particle strength of the negative electrode material, and inhibit the volume expansion of the negative electrode material in the charging and discharging process; on the other hand, the silicon-nitrogen material can be in-situ converted into Li3N during lithiation, forming a lithium ion conductive network, Li3N can act as a lithium ion conductor, and the carbon material in the porous matrix can act as an electron conductor, the two characteristics are complementary, and together form a uniformly distributed electron / lithium ion transmission network, providing a fast and efficient channel for charge transmission, improving the charge conduction characteristics of the negative electrode material, and reducing the internal stress that may be generated during lithiation. In addition, Li3N as an inorganic substance has excellent mechanical strength, which can further improve the particle strength of the negative electrode material, so that the secondary battery assembled by the negative electrode material has optimal anti-expansion performance, electronic conduction performance and lithium ion conduction performance. BRIEF DESCRIPTION OF DRAWINGS

[0010] Fig. 1 is an SEM diagram of the negative electrode material in Example 1 of the present application;

[0011] Fig. 2 is an XRD diagram of the Si3N4 raw material, the negative electrode material in Example 1 and Example 2 of the present application;

[0012] Fig. 3 is a schematic diagram of the structure of the secondary battery provided by an embodiment of the present application during charging;

[0013] Fig. 4 is a schematic diagram of the structure of the secondary battery provided by an embodiment of the present application during discharging;

[0014] Fig. 5 is an N1s XPS spectrum of the negative electrode material of the present application.

[0015] BRIEF DESCRIPTION OF DRAWINGS: 100-electrode assembly; 101-positive electrode sheet; 102-negative electrode sheet; 103-separation film. DETAILED DESCRIPTION

[0016] In order to make the purpose, technical scheme and advantages of the embodiments of the present application more clear, the technical scheme in the embodiments of the present application will be described clearly and completely below. The specific conditions not noted in the embodiments are carried out according to the conventional conditions or the conditions suggested by the manufacturer. The reagents or instruments not noted by the manufacturer are all conventional products that can be obtained by market purchase.

[0017] As described in the background of the present application, the prior art porous carbon material and silicon-based material composite negative electrode material has the problems of volume expansion and insufficient lithium conductivity during the charge and discharge process. In order to solve the above problems, in a typical embodiment of the present application, a negative electrode material is provided, which includes a porous matrix and an active material, at least part of the active material is located in the pores of the porous matrix; the porous matrix includes a carbon material and a silicon-nitrogen material, and the mass content of the silicon-nitrogen material in the negative electrode material is 0.5% to 15%. The negative electrode material of the present application is a key component in a secondary battery, which is used to store lithium ions during the discharge process of a lithium ion battery and release lithium ions during the charging process.

[0018] The negative electrode material includes a porous matrix and an active material, and the active material is filled in the pores of the porous matrix. In the charge and discharge cycle of the secondary battery, the porous structure can absorb the volume change of the active material such as silicon material, which helps to improve the cycle stability of the negative electrode material, and this structure makes the contact area of the active material and the electrolyte larger, which helps to improve the charge and discharge efficiency of the secondary battery, thereby improving the electrochemical performance.

[0019] The porous matrix includes a carbon material and a silicon-nitrogen material, and the silicon-nitrogen material is located in the carbon material. Among them, the silicon-nitrogen material contains silicon and nitrogen elements, and the carbon material is an element or compound containing carbon elements. It can be understood that the material of the porous matrix includes a carbon-silicon-nitrogen composite material, and it can also be understood that the carbon material and the silicon-nitrogen material are combined with each other through chemical or physical methods, so that the carbon material and the silicon-nitrogen material together form a porous structure. Among them, the carbon material has good electrical conductivity and chemical stability, while the silicon-nitrogen material provides good mechanical stability and thermal stability and promotes the formation of a lithium ion conduction network, and the porous structure formed by this composite material helps to improve the specific surface area of the porous matrix.

[0020] In some embodiments, the negative electrode material includes a carbon material and an active material, and the active material is distributed in the carbon material. After the negative electrode material is etched by argon ions to a depth of 200 nm, X-ray photoelectron spectroscopy test is performed, and the N element high-resolution fine spectrum (N 1s XPS spectrum) is obtained, as shown in Figure 5. The data is analyzed by peak fitting method, and the results show that there is a characteristic peak of Si-N chemical bond at a binding energy of 398 eV ± 1 eV, indicating that the negative electrode material of the present application includes a silicon-nitrogen material. The negative electrode material with an initial mass of m1g is placed in a box-type atmosphere furnace and calcined at 900℃ in an oxygen atmosphere for 1 hour to obtain a sintered material. The sintered material is stirred with 500mL of 5% hydrofluoric acid solution for 1 hour, and the remaining material is washed with water, dried, and weighed as m2g. The mass content of the silicon-nitrogen material in the negative electrode material is X%, X% = m2 / m1x100%, 0.5≤X≤15, indicating that the mass content of the silicon-nitrogen material in the negative electrode material of the present application is 0.5% to 15%.

[0021] The active material is a material involved in the secondary battery chemical reaction in the negative electrode material, which is used to undergo redox reaction in the charging and discharging process of the secondary battery, thereby realizing energy storage and release. The active material is filled in the pores of the porous matrix, so that the specific surface area of the porous matrix can be maximized, and the energy density and charging and discharging efficiency of the secondary battery are improved.

[0022] In the negative electrode material of the present application, the mass content of the silicon-nitrogen material is 0.5% to 15%, and specifically can be 0.5%, 1%, 3%, 5%, 7%, 9%, 13%, 15% or a range consisting of any two of them.

[0023] The present application believes that when the mass content of the silicon-nitrogen material is 0.5% to 15%, the negative electrode material exhibits optimal anti-expansion performance, cycle stability and lithium ion conduction performance. By reasonably optimizing the mass content of the silicon-nitrogen material in the negative electrode material, the mechanical strength and lithium conductivity of the negative electrode material can be effectively improved, effectively solving the problems of volume expansion and insufficient lithium conductivity of the negative electrode material in the charging and discharging process, and reducing the negative impact on electronic conductivity and specific capacity, ensuring the comprehensive optimization of the performance of the negative electrode material.

[0024] According to the research of the present application, the secondary battery assembled by the above negative electrode material has excellent anti-expansion performance, electronic conduction performance and lithium ion conduction performance. This is because, on the one hand, the silicon-nitrogen material has high mechanical strength and can form a stable skeleton, and an appropriate amount of silicon-nitrogen material can enhance the structural stability of the porous matrix, effectively improve the particle strength of the negative electrode material, inhibit the volume expansion of the negative electrode material in the charging and discharging process, reduce the destruction of the electrode structure and the repeated growth of the SEI film, thereby improving the overall structural stability and cycle life of the negative electrode material; on the other hand, the silicon-nitrogen material can be in situ converted to Li3N during lithiation, forming a lithium ion conductive network, Li3N has good lithium ion conductivity, providing a path for the rapid transport of lithium ions, reducing the resistance of lithium ion transport, speeding up the charging speed, and improving the fast charging performance and lithium ion diffusion efficiency; at the same time, the carbon material in the porous matrix acts as an electronic conductor, which is complementary to the characteristics of the ion conductor Li3N, forming a uniformly distributed electronic / lithium ion transport network during lithiation, ensuring the rapid and efficient transport of electric charge, which not only improves the charge conduction characteristics of the material, but also reduces the internal stress that may be generated during lithiation, further improving the cycle stability and overall performance of the material, and Li3N as an inorganic substance has excellent mechanical strength, which can further improve the particle strength of the negative electrode material. Therefore, by limiting the mass content of the silicon-nitrogen material in the negative electrode material, the mechanical strength, lithium ion conductivity and electronic conductivity of the material can be optimized while maintaining high capacity, achieving the optimal balance between capacity, conductivity, lithium conductivity and volume expansion rate of the negative electrode material.

[0025] In some embodiments, the ratio of the pore volume of the micropores to the pore volume of the mesopores of the negative electrode material is (1-45):(55-99), and specifically can be 1:99, 5:95, 10:90, 20:80, 30:65, 40:50, 45:55, or a range consisting of any two of them. In this application, the proportion of the pore volume of the mesopores in the negative electrode material increases, and the proportion of the pore volume of the micropores decreases. 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 largely proportional to the pore volume of the micropores, that is, as the micropore volume increases, the adsorption capacity of the negative electrode material increases, and then the side reaction of the negative electrode material with the electrolyte will increase. Therefore, by controlling the proportion of the pore volume of the micropores of the negative electrode material, the active sites of the side reaction of the negative electrode material with the electrolyte can be reduced, and then the thickening of the solid electrolyte membrane caused by the continuous invasion of the electrolyte can be reduced, which is beneficial to improve the cycle performance of the negative electrode material. In addition, the mesopores with increased volume proportion can provide sufficient buffer space for the volume expansion of the active material, which is beneficial to improve the particle structure stability of the negative electrode material.

[0026] In some embodiments, the proportion of the pore volume of the mesopores in the negative electrode material is ≥80%, and specifically can be 80%, 82%, 85%, 87%, 90%, 93%, 95%, 99%, or a range consisting of any two of them. The mesopores are pore structures with a pore size of 2-50 nm.

[0027] In some embodiments, the proportion of the pore volume of the micropores in the negative electrode material is ≤10%, and specifically can be 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or a range consisting of any two of them, which is not limited herein. The micropores are pores with a pore size of less than 2 nm.

[0028] In some embodiments, the proportion of the pore volume of the macropores in the negative electrode material is ≤20%, and specifically can be 20%, 18%, 15%, 12%, 10%, 8%, 7%, 5%, 4%, 3%, 2%, 0, or a range consisting of any two of them, which is not limited herein. The macropores are pores with a pore size of greater than 50 nm.

[0029] It can be understood that by controlling the volume proportions of the micropores, mesopores, and macropores in the negative electrode material within the above ranges, the uniformity of the distribution of the active material inside the negative electrode material can be improved. Since most of the pores are mesopores, the volume expansion of the active material can be effectively alleviated, and the local excessive expansion stress caused by the non-uniform volume change of the active material during the cycle process can be reduced, which can prevent the negative electrode material from being broken and pulverized, and improve the cycle stability of the negative electrode material.

[0030] In some embodiments, the total pore volume of the negative electrode material is 0.001 cm3 / g~0.1 cm3 / g, specifically can be 0.001 cm3 / g, 0.002 cm3 / g, 0.003 cm3 / g, 0.004 cm3 / g, 0.005 cm3 / g, 0.006 cm3 / g, 0.007 cm3 / g, 0.008 cm3 / g, 0.009 cm3 / g, 0.01 cm3 / g, 0.02 cm3 / g, 0.03 cm3 / g, 0.04 cm3 / g, 0.05 cm3 / g, 0.06 cm3 / g, 0.07 cm3 / g, 0.08 cm3 / g, 0.09 cm3 / g, 0.1 cm3 / g or a range consisting of any two of them, which is not limited here. 3 / g~0.1 cm 3 / g, specifically can be 0.001 cm 3 / g, 0.005 cm 3 / g, 0.006 cm 3 / g, 0.007 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, 0.08 cm 3 / g, 0.1 cm 3 / g or a range consisting of any two of them, which is not limited here. After filling the active material, the remaining pores can reserve space for the volume expansion of the active material, alleviate the expansion effect of the negative electrode material, improve the cycle stability of the negative electrode material, and also can adsorb or accommodate a small amount of gas generated by the side reaction between the active material and the electrolyte, improve the gas production phenomenon of the negative electrode material.

[0031] In some embodiments, the average pore size of the pores of the negative electrode material is 0.4 nm~50 nm; specifically can be 0.4 nm, 0.5 nm, 1 nm, 1.5 nm, 2 nm, 3 nm, 5 nm, 10 nm, 20 nm, 30 nm, 40 nm, 50 nm or a range consisting of any two of them, and of course can also be other values within the above range, which is not limited here.

[0032] The carbon material in the porous matrix plays a supporting and conductive role, and the active material is filled in the pores of the porous matrix. Too much active material ratio of the negative electrode material may cause the volume expansion of the negative electrode material in the charging and discharging process to be unable to be fully buffered, affecting the cycle stability; and too much porous matrix ratio will reduce the active material ratio of the negative electrode material and reduce the theoretical capacity of the negative electrode material.

[0033] The carbon material is included in the porous matrix. In some embodiments, the mass content of carbon elements in the negative electrode material is 35% to 60%. Specifically, the mass content of carbon elements in the negative electrode material is 35%, 40%, 45%, 50%, 55%, 60% or any range formed by any two of them, and of course, other values in the above range are also not limited herein. By limiting the mass content of carbon elements in the negative electrode material to the above range, a balance is achieved in improving the capacity of the negative electrode material and improving the structural stability, thereby significantly improving the electrochemical performance and service life of the secondary battery material. In the specific implementation process of the present application, the mass ratio of the porous matrix and the active material can be determined according to the actual situation to achieve the optimization of performance.

[0034] The present application does not limit the specific mass ratio of carbon material and silicon-nitrogen material, as long as the mass content of silicon-nitrogen material in the negative electrode material is 0.5% to 15%.

[0035] The present application does not limit the specific type of silicon-nitrogen material, which can be a compound containing silicon and nitrogen elements in the art. For example, the silicon-nitrogen material can be Si3N4, which can be divided into crystal and amorphous according to different forms of Si3N4.

[0036] In some embodiments, the silicon-nitrogen material includes at least one of crystalline Si3N4 and amorphous Si3N4. Among them, the crystalline Si3N4 can be α-Si3N4 and / or β-Si3N4, etc. By adjusting the form of Si3N4 (such as crystal, amorphous), the performance of the negative electrode material can be further improved, such as improving the particle strength, lithium ion diffusion coefficient, electrical conductivity and reducing the expansion rate, thereby realizing a negative electrode material with high capacity, long life and fast charge and discharge.

[0037] The nano-sized silicon-nitrogen material can be more uniformly dispersed in the porous matrix and tightly contact with the carbon material in the porous matrix, forming a more uniform composite structure, which is beneficial to the stability and consistency of the electrochemical performance of the negative electrode material.

[0038] In some embodiments, the average particle size of the silicon-nitrogen material is ≤400 nm. By limiting the average particle size of the silicon-nitrogen material to ≤400 nm, the carbon material and the silicon-nitrogen material can be combined to form a porous matrix. In addition, due to its small size and high surface energy, nano-material has higher mechanical strength and toughness than bulk material. By limiting the average particle size of the silicon-nitrogen material to ≤400 nm, the silicon-nitrogen material nanoparticles in the negative electrode material can effectively enhance the mechanical properties of the negative electrode material as a whole, resist volume changes during charging and discharging, reduce particle breakage, and improve cycle life. The test method of the average particle size of the silicon-nitrogen material: the diameters of the silicon-nitrogen material particles in the transmission electron microscope image of the negative electrode material are mathematically counted and averaged.

[0039] The active material refers to an element or a compound used as a negative electrode material in a secondary battery.

[0040] In some embodiments, the active material includes at least one of Si, Sn, Ge, Pb, Ag, Mg, Zn, Ga, In, Sb, Bi, Al, and it can be understood that the active material can be an element or a compound containing any of the above elements, or an alloy containing any two or more of the above elements.

[0041] In some preferred embodiments, the active material can be an element or a compound containing silicon elements.

[0042] In some embodiments, the active material includes a silicon material, and the silicon material includes at least one of a silicon element, a silicon oxide, and a silicon alloy. Specifically, the silicon material as the active material can be in different forms, wherein the amorphous silicon is an amorphous silicon with no long-range order in atomic arrangement; the crystalline silicon is a crystalline form of silicon with regular atomic arrangement; the silicon oxide is a compound containing silicon elements and oxygen elements, for example, can be silicon dioxide; and the silicon alloy is an alloy formed by the silicon elements and other metal elements.

[0043] In some embodiments, the silicon material includes amorphous silicon, and when the active material further includes the amorphous silicon, the amorphous silicon isotropically expands during lithium intercalation, which can reduce the collapse of pores and inhibit the rapid decay of the specific capacity of the negative electrode material, and is more conducive to improving the lithium intercalation cycle performance of the negative electrode material.

[0044] In some embodiments, the silicon oxide includes silicon elements and oxygen elements, and the atomic ratio of the silicon elements to the oxygen elements is 0-2, and 0 is excluded. Specifically, the atomic ratio of the silicon elements to the oxygen elements can be 0.05, 0.11, 0.21, 0.26, 0.31, 0.41, 0.51, 0.59, 0.61, 0.69, 0.71, 0.74, 0.76, 0.79, 0.89, 0.99, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, and 2, and the like, which are not limited herein. Preferably, the atomic ratio of the silicon elements to the oxygen elements is 0-1, and 0 is excluded.

[0045] In some embodiments, the silicon oxide has a general chemical formula of SiO x wherein 0x<2, and x can be 0.05, 0.11, 0.21, 0.26, 0.31, 0.41, 0.51, 0.59, 0.61, 0.69, 0.71, 0.74, 0.76, 0.79, 0.89, 0.99, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, and 1.9, and the like, which are not limited herein. Preferably, 0

[0046] In order to realize the active material filled in the pores of the porous matrix, in some embodiments, the active material includes silicon material, and the average particle size of the silicon material is 0.1-100 nm. Alternatively, the average particle size of the silicon material can be specifically 0.1 nm, 1 nm, 2 nm, 5 nm, 10 nm, 20 nm, 25 nm, 30 nm, 40 nm, 45 nm, 50 nm, 80 nm, 100 nm or a range formed by any two of them, and of course, other values in the above range are also possible, which are not limited herein. The mechanical stress of the silicon material when swelling decreases with the decrease of the particle size, and the size reduction can shorten the electron and ion transmission path, at the same time, the size of the silicon material is reduced, the gap between adjacent silicon material particles is increased, and space for swelling is reserved. It can be understood that the average particle size of the silicon material in the above range can ensure the battery capacity of the lithium ion battery and reduce the irreversible capacity loss.

[0047] In some embodiments, the average particle size of the silicon material is 0.1-5 nm. The test method of the average particle size of the silicon material: the diameters of the silicon material particles in the transmission electron microscope image of the negative electrode material are mathematically counted, and the average value is obtained.

[0048] In some embodiments, the morphology of the silicon material includes at least one of point, spherical, ellipsoidal and flaky, and the morphology of the silicon material can be selected according to actual needs, which are not limited herein.

[0049] In some embodiments, the purity of the silicon material is greater than 99%, and it can be understood that the high-purity silicon material is beneficial to the Li-Si alloying with lithium and improves the cycle performance of the lithium ion battery.

[0050] In order to ensure that there is enough active material to participate in the electrochemical reaction, in some embodiments, the mass content of the silicon element in the negative electrode material is 20%-55%, for example, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55% or a range formed by any two of them. The silicon material has a high theoretical specific capacity, but the silicon material will swell and shrink during the charging and discharging process, which can cause the destruction of the structure of the secondary battery. By limiting the mass content of the silicon element in the above range, the energy density of the secondary battery can be improved while the cycle stability of the secondary battery is improved.

[0051] In some embodiments, the negative electrode material further includes a coating layer, and the coating layer is located on at least part of the surface of the porous matrix and / or the active material. The main function of the coating layer is a protective layer, which reduces the direct contact of the active material with the electrolyte, improves the chemical stability and corrosion resistance of the negative electrode material. On the other hand, the coating layer can also improve the structural stability of the negative electrode material, and reduce the destruction of the structure of the negative electrode material caused by the volume expansion and shrinkage of the active material during the charging and discharging process of the battery.

[0052] If the material of the coating layer is selected to have good electronic conductivity, the coating layer can not only play a protective role, but also effectively improve the electronic conductivity of the negative electrode material, reduce the internal resistance, and improve the charge-discharge efficiency and cycle performance of the secondary battery. In some embodiments, the material of the coating layer includes at least one of a carbon material, a metal oxide, a metal salt, and a polymer material. If the material of the coating layer includes a carbon material (such as graphitized carbon, carbon nanotubes, or graphene), the carbon material has good electronic conductivity, which can effectively improve the electronic conductivity of the negative electrode material, reduce the internal resistance, and improve the charge-discharge efficiency and cycle performance of the secondary battery. If the material of the coating layer includes a metal oxide, the metal oxide has high thermal stability, which can provide additional thermal protection and improve the safety performance of the secondary battery in a high-temperature environment. If the material of the coating layer includes a metal salt or a polymer material, the coating layer can provide a path for fast transport of lithium ions, improve the diffusion performance of lithium ions in the material, and improve the fast-charging performance of the secondary battery. In the specific implementation process of the present application, different coating layer materials can be selected according to actual needs.

[0053] The thickness of the suitable coating layer can ensure the effective transport of electrons and lithium ions. In some embodiments, the thickness of the coating layer is 10 nm to 200 nm, such as 10 nm, 50 nm, 100 nm, 150 nm, 200 nm, or a range formed by any two of the foregoing values. By limiting the thickness of the coating layer to the above suitable range, the present application can not only provide effective protection, but also ensure the uniformity of the coating layer, which is conducive to the uniform performance of the electrochemical reaction and improves the overall performance and consistency of the secondary battery.

[0054] In some embodiments, the particle strength of the negative electrode material is 50 MPa to 500 MPa, preferably 100 MPa to 300 MPa. The negative electrode material has a relatively high particle strength, which can effectively resist the volume shrinkage and expansion of the active material during the charge and discharge process, reduce the particle breakage and pulverization of the negative electrode material, help maintain the performance stability of the secondary battery during long-term charge and discharge cycles, and improve the cycle life of the secondary battery.

[0055] In some embodiments, the conductivity of the negative electrode material is 0.7 S / cm to 1.5 S / cm. The negative electrode material has a suitable conductivity, which not only allows for fast transport of electrons, but also promotes uniform charge distribution, improves the thermal stability of the negative electrode material, ensures the consistency of the electronic conductivity of the negative electrode material under different batches and different conditions, and improves the consistency and reliability of the secondary battery.

[0056] In some embodiments, the lithium ion diffusion coefficient of the negative electrode material is 0.7 x 10 -7 cm 2 / s to 0.9 x 10 -7 cm 2By limiting the lithium ion diffusion coefficient of the negative electrode material, it can be understood that the lithium ion can quickly and uniformly diffuse in the negative electrode material, which helps the uniform performance of the electrochemical reaction, thereby improving the overall electrochemical performance of the secondary battery.

[0057] In some embodiments, the median particle size D50 of the negative electrode material is 6-15 μm; for example, the median particle size D50 of the negative electrode material can be 6 μm, 8 μm, 10 μm, 12 μm, 15 μm or any value within the range between any two of the above values. Controlling the median particle size D50 of the negative electrode material within the above range is beneficial to the improvement of the cycle performance of the negative electrode material. The median particle size D50 represents the particle size corresponding to the cumulative particle size distribution percentage of 50%. The test method refers to GB / T 19077-2016. It can be conveniently measured by a laser particle size analyzer, such as the Mastersizer 3000 laser particle size analyzer of Malvern Instruments Ltd., UK, in which the cumulative 50% diameter is D50 in the volume-based distribution.

[0058] In some embodiments, the specific capacity of the negative electrode material is 1600-2200 mAh / g. The specific capacity is a measure of the amount of charge that can be stored by a unit mass of negative electrode material. By limiting the specific capacity of the negative electrode material to 1600-2200 mAh / g, it is shown that the negative electrode material of the present application has a higher specific capacity and can store more electrical energy, thereby significantly improving the energy density of the secondary battery. The limited specific capacity range helps to control the consistency of the composition and structure of the negative electrode material, ensures the performance consistency of the secondary battery in batch production, and is also beneficial to the processing and molding of the material, improving the production efficiency of the secondary battery.

[0059] The present application does not limit the specific preparation process of the negative electrode material, as long as the negative electrode material meets the above specific requirements. For example, in some embodiments, the preparation method of the negative electrode material of the present application can include the following steps:

[0060] S1, acid washing the biomass material, then carbonizing to obtain a precursor;

[0061] S2, mixing the precursor with a silicon-nitrogen material and performing activation treatment to obtain an activated product;

[0062] S3, heat pressing and crushing the activated product to obtain a porous matrix;

[0063] S4, filling the active substance into the pores of the porous matrix to obtain an inner core;

[0064] S5, coating the inner core with a coating layer to obtain the negative electrode material.

[0065] The present application can use biomass material as raw material, which refers to organic matter formed by photosynthesis, such as bamboo, wood, rice husk, etc. The most suitable biomass raw material can be selected according to the characteristics of the negative electrode material.

[0066] S1 includes acid washing and carbonization treatment. Specifically, the biomass material can be soaked with hydrochloric acid solution to achieve acid washing. The concentration of the hydrochloric acid solution is 5-7 M (mol / L), and the acid washing time is 6-12 h. Through acid washing, impurities and part of the soluble components on the surface of the biomass material are removed, improving the purity of the material and the degree of graphitization in the subsequent process. After acid washing, the material can be washed with water to remove residual acidic substances, and then dried at 70-120°C for 10-14 h to ensure that the material is dry. The dried biomass material is placed in a high-temperature carbonization furnace and carbonized under a first inert atmosphere. During the carbonization process, most of the volatile components in the biomass material are removed and converted into a carbon material with stable structure, i.e., a carbon precursor. After carbonization is completed, the material needs to be naturally cooled to room temperature to avoid temperature shock that can damage the material structure. The first inert atmosphere can be at least one of argon, nitrogen, and helium. The carbonization temperature is 800-1000°C, and the duration is 6-12 h.

[0067] S2 includes crushing, mixing, and activation. Specifically, the carbon precursor can be initially crushed by air flow to increase its surface area, which is beneficial for subsequent uniform mixing with the silicon-nitrogen material. Then, the initially crushed carbon precursor and the silicon-nitrogen material are mixed in a certain mass ratio in a vibration ball mill for 4-8 h to achieve sufficient mixing of the carbon precursor and the silicon-nitrogen material, obtaining a mixed material. The mixed material is activated, which can be done in a mixed atmosphere of water vapor and nitrogen, obtaining an activated product. The volume concentration of water vapor in the mixed gas is 6-10%, the activation temperature is 700-1000°C, and the activation time is 4-8 h. Water vapor activation further increases the porosity and specific surface area of the material, providing more active sites for subsequent filling and coating processes.

[0068] S3 includes hot pressing, crushing, and screening. Specifically, the activated product is placed in a hot press for hot pressing, which helps to compact the material particles and form small pores while maintaining the porous structure of the material. After hot pressing is completed, the material needs to be cooled to room temperature, and then the hot-pressed product is crushed to restore or adjust its particle size. Finally, through screening, particles that are too large or too small are removed, obtaining a porous matrix with uniform particle size distribution to meet the needs of subsequent filling and coating. The hot pressing temperature is 150-250°C, the hot pressing strength is 2-3 MPa, and the hot pressing time is 40-60 min.

[0069] In S4, the active material can be filled into the pores of the porous substrate by various methods, such as chemical vapor deposition (CVD). Specifically, the porous substrate is placed in a deposition device, an environment of a second inert atmosphere is ensured, the device is preheated to a suitable temperature for deposition, an active material precursor is introduced into the deposition device, and deposition is performed at a predetermined temperature. The deposition time is adjusted according to the required deposition amount and deposition rate, and is generally 1-10 hours. This process aims to uniformly fill the active material into the pores of the porous substrate to form an inner core; wherein the second inert atmosphere can be at least one of nitrogen, helium, and argon, and the flow rate of the second inert atmosphere is 50-1000 seem. The type of active material precursor can be selected according to actual needs, for example, the active material precursor can be a gas-phase silicon source, specifically, the active material precursor can be silane and / or disilane, and the active material formed is silicon; in the mixed gas of the second inert atmosphere and the to-be-deposited material, the volume concentration of the to-be-deposited material is 10-20%, the deposition temperature is 450-750°C, and the time is 1-10 hours.

[0070] In S5, the coating layer can be formed by various methods, such as chemical vapor deposition (CVD), atomic layer deposition (ALD), liquid phase coating, etc. Specifically, the inner core is mixed with a coating layer precursor material, and coating is performed under the condition of a third inert atmosphere; wherein the third inert atmosphere can be at least one of nitrogen, helium, and argon, and the flow rate of the third inert atmosphere is 50-1000 seem; the coating layer precursor material can be in a gas phase, a solid phase, or a liquid phase, for example, the coating layer precursor material can be at least one of a gas-phase carbon source, a solid-phase carbon source, and a liquid-phase carbon source, and the material of the coating layer formed is a carbon material. The coating temperature is 400-1000°C, and the time is 1-10 hours.

[0071] In a second aspect of the present application, a negative electrode sheet is provided, which comprises the negative electrode material provided in the first aspect.

[0072] The negative electrode sheet of the present application comprises a negative electrode current collector and a negative electrode material active layer provided on at least one surface of the negative electrode current collector. The negative electrode material active layer comprises the negative electrode material provided in the first aspect or the second aspect. 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 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. Due to the inclusion of the negative electrode material with excellent performance, the application of the negative electrode sheet to a secondary battery helps to improve the anti-swelling performance and electrochemical performance of the secondary battery.

[0073] The negative 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.

[0074] In particular, when preparing the negative electrode sheet, the negative material, the conductive agent, and the binder can be dispersed in an appropriate amount of water, and mixed by sufficient stirring to form a uniform negative electrode slurry. The negative electrode slurry is uniformly coated on the negative electrode current collector, and then subjected to drying, rolling, and slitting to obtain the negative electrode sheet. In one particular embodiment, the negative active layer includes 70% to 99% of the negative material, 0.5% to 15% of the conductive agent, and 0.5% to 15% of the binder by mass percentage.

[0075] The conductive agent can be selected from at least one of natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, and graphene. The binder can be selected from at least one of carboxymethyl cellulose, styrene butadiene rubber, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, an oxirane-containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyvinyl alcohol, and sodium polyacrylate.

[0076] In a third aspect, the present application provides a secondary battery including the negative electrode sheet provided in the second aspect.

[0077] Due to the inclusion of the negative electrode sheet with excellent performance, the secondary battery has excellent anti-swelling performance and electrochemical performance.

[0078] Specifically, the secondary battery includes a housing, an electrode assembly, and an electrolyte. The electrode assembly and the electrolyte are both located in the housing.

[0079] 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.

[0080] Referring to FIGS. 3 and 4, the electrode assembly 100 includes a positive electrode sheet 101, a negative electrode sheet 102, and a separator 103 disposed between the positive electrode sheet 101 and the negative electrode sheet 102. During charging, referring to FIG. 3, active ions (e.g., lithium ions) are deintercalated from a crystal lattice of a positive electrode material (e.g., a lithiated intercalation compound) of the positive electrode sheet 101, pass through the separator 103 via an electrolyte, reach the negative electrode sheet 102, and are intercalated into a crystal lattice of a negative electrode material. During discharging, referring to FIG. 4, active ions (e.g., 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 (e.g., the 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.

[0081] 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 sequence. 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 in sequence.

[0082] The positive electrode sheet 101 includes a positive electrode current collector and a positive electrode material active layer disposed on at least one surface of the positive electrode current collector. The positive electrode 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 electrode material active layer includes a positive electrode active material, which includes a compound that reversibly intercalates and deintercalates lithium ions (i.e., a lithiated intercalation compound). 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, 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).

[0083] The positive electrode material active layer also includes a binder to bind the positive electrode active material particles to facilitate the formation of a film layer and to improve the adhesion 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, an ethylene oxide-containing polymer, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene butadiene rubber, acrylated styrene butadiene rubber, an epoxy resin, or nylon.

[0084] The positive electrode 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.

[0085] 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.

[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 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), but is not limited thereto. 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-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, trifluoromethyl ethylene carbonate, or a combination thereof.

[0087] In the preparation of the secondary battery, the positive electrode sheet, the separator, and the negative electrode sheet are wound or laminated to obtain an electrode core, the electrode core is packaged into an aluminum-plastic film that is pre-stamped and molded, the secondary battery after packaging is dried of moisture, the electrolytic solution is injected into the dried secondary battery, and the secondary battery after being left, formed, and double-sealed is completed.

[0088] The application will be further described in conjunction with specific embodiments, which are not to be understood as limiting the scope of the application as claimed.

[0089] Example 1

[0090] The preparation method of the negative electrode material of the present embodiment includes the following steps:

[0091] S1, bamboo material is first pickled with 6M hydrochloric acid solution for 8h, after pickling, repeatedly immersed with water, then dried at 80℃ for 12h, after drying, carbonized at 850℃ under nitrogen atmosphere for 9h, naturally cooled to room temperature, to obtain carbon precursor;

[0092] S2, the carbon precursor is pulverized by airflow pulverization, and mixed with crystal Si3N4 in a vibration ball mill for 6h to obtain a Si3N4 / carbon precursor composite; wherein the added mass of crystal Si3N4 is 1% of the mass of the carbon precursor;

[0093] S3, the Si3N4 / carbon precursor composite is activated at 800℃ under water vapor and inert atmosphere for 6h to obtain an activated product; then the activated product is placed in a hot press, and hot pressed at a hot pressing temperature of 200℃ and a hot pressing strength of 2.5MPa for 50min, then the hot pressed product is pulverized and sieved to obtain a porous matrix; wherein the volume ratio of water vapor in the mixed gas composed of water vapor and inert atmosphere is 8%;

[0094] S4, the porous matrix is placed in a kiln, 800sccm of nitrogen and a gas phase silicon source are introduced, and deposited at 470℃ for 10h to obtain a core; wherein the volume ratio of the gas phase silicon source in the mixed gas composed of nitrogen and the gas phase silicon source is 15%;

[0095] S5, the kiln is repeatedly purged 5 times with 600sccm of nitrogen to remove residual silicon source, then the temperature of the kiln is increased to 700℃, 800sccm of nitrogen and propylene are introduced, and reacted for 8h to obtain the negative electrode material of the embodiment; wherein the volume ratio of propylene in the mixed gas composed of nitrogen and propylene is 15%.

[0096] Example 2

[0097] The difference from example 1 is that in S2, the added mass of crystal Si3N4 is 15% of the mass of the carbon precursor.

[0098] Example 3

[0099] The difference from example 1 is that in S2, the added mass of crystal Si3N4 is 20% of the mass of the carbon precursor.

[0100] Example 4

[0101] The difference from example 1 is that in S2, the added mass of crystal Si3N4 is 30% of the mass of the carbon precursor.

[0102] Example 5

[0103] The difference from example 1 is that the crystalline Si3N4 in S2 is replaced by amorphous Si3N4. Among them, the mass of amorphous Si3N4 is 15% of the mass of the carbon precursor.

[0104] Example 6

[0105] The difference from example 1 is that the gas-phase silicon source in S4 is replaced by a mixed gas of gas-phase silicon source and gas-phase aluminum source AlCl3 with a volume ratio of 3:1, so that the silicon-aluminum alloy fills the pores of the porous matrix, and the mass content of silicon element in the silicon-aluminum alloy is 75%.

[0106] Example 7

[0107] The difference from example 1 is that the reaction time of 8h in S5 is modified to 1h to protect the silicon in the core from side reactions in the subsequent oxide coating process, obtaining a core with a carbon coating layer; then 30g of NbCl5 (inorganic metal salt) is dissolved in 1000mL of anhydrous ethanol, and magnetic stirring is performed for 30min to obtain a metal salt solution; 70g of the core with a carbon coating layer is dispersed in 1000mL of anhydrous ethanol, and magnetic stirring is performed for 30min to obtain a suspension; the metal salt solution and the suspension are mixed, stirred at room temperature for 1h, and then subjected to hydrothermal reaction at 170℃ for 20h to perform Nb2O5 oxide coating. The reaction system is suction filtered and washed with anhydrous ethanol, and is dried in a vacuum oven at 80℃. The obtained sample is screened and graded to obtain the negative electrode material of the present example.

[0108] Example 8

[0109] The difference from example 1 is that the reaction time of 8h in S5 is modified to 1h to protect the silicon in the core from side reactions in the subsequent oxide coating process, obtaining a core with a carbon coating layer; then 14.2g of isopropyl titanate (organic metal salt) and 1.98g of lithium acetate (organic metal salt) are added to deionized water, and 0.345g of cetyltrimethylammonium bromide CTAB (quaternary ammonium salt) is added, and stirring is performed to obtain a first mixed solution; 2.89g of citric acid (chelating agent) is added to the first mixed solution, and 345g of the core with a carbon coating layer is added, and stirring is performed for 1h to obtain a second mixed solution; then the second mixed solution is stirred in a water bath at 60℃ for 12h to prepare a slurry; the slurry is subjected to spray drying to form a Li4Ti5O12 coating layer, and the obtained sample is screened and graded to obtain the negative electrode material of the present example. 12

[0110] Example 9

[0111] ​The difference from Example 1 is that the reaction time in S5 is changed from 8h to 1h to protect the silicon in the core from side reactions in the subsequent polymer coating process to obtain a core with a carbon coating layer; 15g of aniline monomer is dispersed in deionized water to form a stable emulsion, then 345g of the core with a carbon coating layer is added, and magnetic stirring is carried out at 45℃ for 1 hour, then 3g of ammonium persulfate is added as an initiator, and stirring is continued for 3 hours for polyaniline coating. The reaction system is suction filtered and washed with deionized water, and dried in a vacuum oven at 80℃. The obtained sample is sieved and graded to obtain the negative electrode material of the present example.

[0112] Example 10

[0113] The difference from Example 1 is that in S3, the activation time is 1 hour.

[0114] Example 11

[0115] The difference from Example 1 is that in S3, the activation time is 10 hours.

[0116] Example 12

[0117] The difference from Example 1 is that in S4, the deposition is carried out at 470℃ for 3h.

[0118] Example 13

[0119] The difference from Example 1 is that S5 is omitted, and the core of S4 is directly used as the negative electrode material.

[0120] Comparative Example 1

[0121] The difference from Example 1 is that in S2, Si3N4 is not added.

[0122] Comparative Example 2

[0123] The difference from Example 1 is that in S2, the mass of Si3N4 added is 35% of the mass of the carbon precursor.

[0124] Comparative Example 3

[0125] The difference from Example 1 is that in S2, the mass of Si3N4 added is 70% of the mass of the carbon precursor.

[0126] Comparative Example 4

[0127] The difference from Example 1 is that in S2, the mass of Si3N4 added is 0.4% of the mass of the carbon precursor.

[0128] Comparative Example 5

[0129] The difference from Example 1 is that in S4, the deposition is carried out at 470℃ for 12h.

[0130] The negative electrode materials obtained in Examples 1-13 and Comparative Examples 1-5 were subjected to physical property and electrochemical property tests, which were as follows:

[0131] 1. Pore volume, pore size and pore size ratio test:

[0132] The iPore620 pore size tester of Linkage Physical and Chemical Co., Ltd. and the Brunauer Emmett Teller (BET) pore size distribution test method were used. The pore size distribution data of the material were obtained by density functional theory (DFT) simulation analysis using the isothermal adsorption characteristic curve of nitrogen, and then the average pore size, total pore volume and micropore, mesopore and macropore pore volume ratio of the material were obtained.

[0133] 2. XPS test

[0134] The X-ray photoelectron spectroscopy test of the negative electrode material after argon ion etching for 200 nm depth was performed using the Thermo Scientific K-Alpha equipment of the United States. The N element high-resolution fine spectrum (N 1s XPS spectrum) was obtained. The data were analyzed by peak fitting method. The results showed that there was a characteristic peak of Si-N chemical bond at a binding energy of 398 eV ± 1 eV.

[0135] 3. XRD test

[0136] The X-ray diffractometer of Panalytical with model X'pert PRO was used to determine the phase of the powder;

[0137] The qualitative analysis scanning angle of XRD test was 10°-90°. The scanning speed was 5° / min. The scanning mode was step scanning with a speed of 0.013°.

[0138] 4. Particle size and particle size distribution test: the test method refers to GB / T 19077-2016. It can be conveniently measured by a laser particle size analyzer, such as the Mastersizer 3000 laser particle size analyzer of Malvern Instruments Ltd., UK. In the volume-based distribution, the cumulative 50% diameter is D50.

[0139] 5. Test of average particle size of the material: the diameter of the measured substance in the transmission electron microscope image of the negative electrode material was mathematically counted. Specifically, 20 measured substances were randomly selected to test their particle size and average value.

[0140] 6. Coating layer thickness: the cross-section of the material was processed by a focused ion beam scanning electron microscope (FIB-SEM) device. In the SEM, 10 particles were randomly selected, and the coating layer thickness was measured 3 times for each particle. The average coating layer thickness was measured.

[0141] 7. Test of electrical conductivity

[0142] The test is performed using a Mitsubishi Chemical Powder Conductivity Tester MCP-PD51 with a four-probe method. The powder sample is compressed using a pressure of 20 KN. The four probes are placed on the sample, a current is applied, and the voltage passing through the sample is tested. The resistance is calculated from the voltage and the current. Finally, the electrical conductivity of the powder is calculated from the resistance value.

[0143] 8. Test of particle strength

[0144] The test is performed using a Shimadzu DUH-211S Dynamic Ultra Micro Hardness Tester. Under the action of electromagnetic force, the corresponding force on the indenter gradually increases at a certain speed. When the particle reaches the breaking point, the particle breaks. At this time, the resistance to the indenter disappears, causing its displacement to rapidly decrease. The instrument records the displacement of the indenter in real time to determine the breaking point of the particle (rapid increase in displacement) and records the pressure on the particle at that time. The particle strength of the negative electrode material is obtained according to the relationship between the pressure F, the particle size D, and the particle strength Cx (Cx = 2480 x F / (πD 2 ).

[0145] 9. Test of the mass content of silicon elements in the negative electrode material and the mass content of silicon-nitrogen materials in the negative electrode material

[0146] (1) Place the negative electrode material with an initial mass of m1 g in a box-type atmosphere furnace (brand: Nanyang Xiyu, model: SA2-9-17TP) and burn it at 900°C for 1 hour in an oxygen atmosphere to obtain a sintered product. This causes silicon and silicon monoxide in the negative electrode material sample to react with oxygen to form silicon dioxide, and carbon to burn and release carbon dioxide.

[0147] (2) After crushing the sintered product obtained in (1), add it to 500 mL of 5% dilute hydrofluoric acid (in a dilute hydrofluoric acid solution, silicon dioxide dissolves quickly, and silicon-nitrogen materials hardly react). Gently stir the solution at room temperature for 1 hour with a polytetrafluoroethylene (PTFE) stirring rod. A small amount of gas bubbles (SiF4 gas) will be produced in the solution, and the silicon dioxide powder will gradually decrease until it disappears. The silicon-nitrogen material always exists in solid form. When the reaction solution no longer produces gas bubbles, filter it with a polytetrafluoroethylene funnel (or a filter with a polytetrafluoroethylene filter membrane). The filtrate contains fluorosilicic acid (or unreacted HF), and the filter residue is the preliminary purified silicon-nitrogen material.

[0148] (3) Wash the filter residue repeatedly with deionized water (3-5 times) to remove residual hydrofluoric acid and fluorosilicic acid (the impurities can be removed by detecting the pH value of the washing solution. When the pH value approaches 7, it indicates that the impurities have been washed out).

[0149] (5) The washed material is placed in an oven at 80-100°C for 2-3 hours to dry, and the dried silicon-nitrogen material is weighed as m2g, and the mass content of the silicon-nitrogen material is X% = m2 / m1 x 100%.

[0150] The mass content of the silicon element in the negative electrode material is Y% = (m1-m2) x 28.0855 / 60.084; the atomic weight of Si is 28.0855, and the molecular weight of SiO2 is 60.084.

[0151] 10. Test of the mass content of carbon element in the negative electrode material

[0152] A G4 ICARUS HF infrared carbon-sulfur analyzer of Bruker, Germany, is used, the sample is combusted in a high-temperature oxygen-rich state, the carbon element contained therein is oxidized into carbon dioxide, the generated gas enters an infrared detector with a carrier gas, and the content of the carbon element is calculated by quantitatively counting the change of the carbon dioxide signal; specifically, a certain amount of sample is weighed and loaded into a ceramic crucible, a fluxing agent is added, high-purity oxygen gas is passed into a high-frequency furnace to combust to generate carbon dioxide, and the mass content of the carbon element in the gas is determined by an infrared detector.

[0153] 11. Test of the specific surface area of the negative electrode material

[0154] A TriStar3000&3020 specific surface area and pore size analyzer of Micromeritics, USA, is used, the specific surface area of a powder sample is tested by a gas adsorption method (nitrogen adsorption multi-point BET) under low temperature control of liquid nitrogen cooling; specifically, a certain amount of sample is weighed and loaded into a specific bubble tube for specific surface area, nitrogen gas is used to purge at 300°C for a certain time in a degassing station, after degassing is completed, the sample is cooled to room temperature, the actual mass of the sample is weighed, the sample-loaded specific bubble tube is installed into the specific surface area and pore size analyzer, the mass of the sample is input, and the specific surface area of the sample is determined; the point range is 0.05g / cm 3 to 0.30g / cm 3 , and a point is taken every 0.05.

[0155] 12. Test of the lithium ion diffusion coefficient

[0156] The negative electrode material is configured into slurry with sodium carboxymethyl cellulose, styrene-butadiene rubber, and conductive graphite (KS-6) and carbon black (SP) according to a mass ratio of 92:2:2:2:2, to form negative electrode slurry; the negative electrode slurry is uniformly coated on a copper foil, dried to form 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 metal lithium sheet.

[0157] The negative electrode materials prepared in the above examples and comparative examples were prepared into button cells, and the lithium ion diffusion coefficients of the negative electrode materials were measured by using the potentiostatic intermittent titration technique (PITT). First, the button cell was discharged from the open circuit potential, and the potential was instantaneously reduced by 0.02 V, which was maintained for 15 minutes, and then the potential excitation was removed, and a relaxation time of 15 minutes was performed; this cycle was repeated in turn for each reduction of the potential by 0.02 V, until the lower limit of 0.01 V was reached, and after the discharge stage was completed, a charging stage was entered, in which the potential was increased by 0.02 V each time, and the rest of the test was similar to the discharge stage. By measuring the current change during the holding and relaxation stages at different potentials, the diffusion coefficient D of the negative electrode material was obtained according to the relationship between the diffusion coefficient and the current.

[0158] 13. Test of electrochemical performance

[0159] The negative electrode material was mixed with sodium carboxymethyl cellulose, butadiene rubber, and conductive graphite (KS-6) and carbon black (SP) according to a mass ratio of 92:2:2:2:2 to form a negative electrode slurry; the negative electrode slurry was uniformly coated on a copper foil, and was dried to form a negative electrode sheet; the negative electrode sheet was assembled into a button cell in an argon atmosphere glove box, the used separator was a polypropylene microporous membrane, the used electrolyte was 1 mol / L lithium hexafluorophosphate (the solvent was a mixed slurry of ethylene carbonate, methyl ethyl carbonate, and dimethyl carbonate), and the used counter electrode was a lithium metal sheet.

[0160] The negative electrode materials prepared in the above examples and comparative examples were prepared into button cells, and the discharge specific capacity and the expansion rate of the negative electrode material were tested on a Blue Electro CT2001A battery test system. Specifically, the initial thickness D0 of the negative electrode sheet was tested, and the battery including the negative electrode sheet was discharged at a discharge rate of 0.05 C to 0.005 V at 25°C, and was charged to 1.5 V at 0.05 C after being left to stand for 3 minutes, and this charging and discharging cycle was repeated 50 times, and then the battery was discharged to 0.005 V, and after the battery was taken out, it was left to stand at room temperature for 3 hours, and the negative electrode sheet was disassembled, and the thickness D1 of the negative electrode sheet at this time was tested. The expansion rate of the negative electrode material was calculated according to the negative electrode material expansion rate = (D1-D0) / D0 x 100%.

[0161] The above button cells were respectively tested for cycle performance on a blue CT2001A battery test system at room temperature (25℃±2℃). The test was performed according to the following charge-discharge system: (1) stand for 6 hours; (2) 0.1C constant current discharge to 0.01V, constant voltage discharge to a current of 0.05C; (3) stand for 30 minutes; (4) 0.1C constant current charge to 1.5V; (5) stand for 30 minutes; (6) 0.1C constant current discharge to 0.01V, constant voltage discharge to a current of 0.05C; (7) stand for 180 minutes at 25℃; (8) 1C constant current charge to 1.5V; (9) stand for 60 minutes; (10) 1C constant current discharge to 0.01V, constant voltage discharge to a current of 0.05C; (11) stand for 30 minutes; (12) repeat steps (8) to (11) for 100 times, stop the test. The capacity 100-week cycle capacity retention rate was calculated according to 100-week cycle capacity retention rate = discharged gram capacity after 100-week cycle / discharged gram capacity in the first week.

[0162] The above button cells were respectively tested for first coulombic efficiency on a blue CT2001A battery test system at room temperature (25℃±2℃), the current for charge-discharge was 0.05C, the charge-discharge interval was 0.01-5V, and the measured value was the average value of 3-5 button cells of each negative electrode material. The first coulombic efficiency = first charge specific capacity / first discharge specific capacity.

[0163] The test results are shown in Table 1 and Table 2.

[0164] Table 1

[0165] Table 2

[0166] Fig. 1 is an SEM image of the negative electrode material in Example 1 of the present application, Fig. 2 is an XRD image of Si3N4 raw material, the negative electrode material in Example 1 and Example 2 of the present application, and according to Fig. 2, it can be known that the negative electrode materials in Example 1 and Example 2 both contain Si3N4.

[0167] According to Tables 1 and 2, the negative electrode materials of Examples 1-12 all satisfy that the silicon-nitrogen material is located inside and on the surface of the carbon material, and the mass content of the silicon-nitrogen material in the negative electrode material is 0.5%-15%, while Comparative Example 1 does not add the silicon-nitrogen material, the mass content of Si3N4 in the negative electrode materials of Comparative Examples 2 and 3 exceeds 15%, and the mass content of Si3N4 in the negative electrode materials of Comparative Examples 4 and 5 is less than 0.5%. The negative electrode materials of Examples 1-12 have excellent conductivity and lithium-ion conductivity compared to Comparative Examples 2 and 3, the volume expansion rate of the negative electrode materials of Examples 1-12 is lower compared to Comparative Examples 1, 4 and 5, although the volume expansion rate of Comparative Examples 2 and 3 is improved, the conductivity and lithium-ion conductivity are poor, and although the conductivity and lithium-ion conductivity of Comparative Examples 1, 4 and 5 are improved, the volume expansion rate is high. Therefore, by making the negative electrode material satisfy that the silicon-nitrogen material is located inside and on the surface of the carbon material, and the mass content of the silicon-nitrogen material in the negative electrode material is 0.5%-15%, the volume expansion rate and the conductivity and lithium-ion conductivity of the negative electrode material can be improved.

[0168] Compared with Comparative Example 1, in Examples 1-12, as the mass content of Si3N4 in the negative electrode material increases, the particle strength of the negative electrode material gradually increases, indicating that introducing Si3N4 into the porous matrix is beneficial to improving the particle strength of the negative electrode material, and accordingly, the volume expansion rate gradually decreases, which is beneficial to ensuring the structural integrity of the negative electrode material during the cycle process. In Comparative Examples 2 and 3, the mass content of Si3N4 exceeds 15%, the lithium-ion diffusion coefficient, conductivity and specific capacity of the negative electrode material sharply decrease, especially in Comparative Example 3, the applicant believes that too much Si3N4 destroys the porous structure of the porous matrix and collapses the 3D conductive network, resulting in a decrease in the lithium-ion diffusion coefficient and conductivity of the negative electrode material, which is not conducive to improving the electrochemical performance of the secondary battery. Therefore, by introducing a specific mass content of silicon-nitrogen material into the porous matrix, the secondary battery assembled from the negative electrode material has optimal anti-expansion performance, electronic conduction performance and lithium-ion conduction performance.

[0169] Further, compared with Example 12, the negative electrode materials of Examples 1-9 further satisfy that the mass content of the silicon element in the negative electrode material is 20%-55% on the basis of satisfying that the mass content of the silicon-nitrogen material in the negative electrode material is 0.5%-15%, the mass content of the silicon element in the negative electrode material of Example 12 is less than 20%, and the specific capacity of the negative electrode materials of Examples 1-9 is significantly improved compared with Example 12.

[0170] The above descriptions are only the preferred embodiments of the present application, and are not intended to limit the present application. The present application can have various modifications and changes for those skilled in the art. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A negative electrode material, characterized by, The negative electrode material comprises a porous matrix and an active material, at least part of the active material is located in the pores of the porous matrix; the porous matrix comprises a carbon material and a silicon-nitrogen material; The mass content of the silicon-nitrogen material in the negative electrode material is 0.5% to 15%.

2. A negative electrode material, characterized in that, The negative electrode material comprises a carbon material and an active material, the active material is distributed in the carbon material; The negative electrode material is subjected to argon ion etching for 200 nm in depth, and then X-ray photoelectron spectroscopy is performed to obtain a high-resolution fine spectrum of N element (N 1s XPS spectrum), data analysis is performed by peak fitting, and the results show that there is a characteristic peak of Si-N chemical bond at a binding energy of 398 eV ± 1 eV; The negative electrode material with an initial mass of m1 g is placed in a box-type atmosphere furnace, and is calcined at 900 ℃ in an oxygen atmosphere for 1 hour to obtain a sintered material, the sintered material is stirred with 500 mL of a 5% hydrofluoric acid solution for 1 hour, the remaining material is washed with water, dried, and weighed as m2 g, the mass content of the silicon-nitrogen material in the negative electrode material is X%, X% = m2 / m1 × 100%, 0.5 ≤ X ≤ 15.

3. The negative electrode material according to claim 1 or 2, characterized in that, The mass content of the silicon-nitrogen material in the negative electrode material is 0.5%, 1%, 3%, 5%, 7%, 9%, 13%, 15%, or any value within a range formed by any two of the above values.

4. The negative electrode material according to claim 1 or 2, characterized in that, The mass content of carbon element in the negative electrode material is 35% to 60%.

5. The negative electrode material according to claim 1 or 2, characterized in that, The negative electrode material satisfies at least one of the following characteristics: (1) The silicon-nitrogen material comprises at least one of crystalline Si3N4 and amorphous Si3N4; (2) The average particle size of the silicon-nitrogen material is ≤400 nm.

6. The negative electrode material according to any one of claims 1 to 4, characterized in that, The active material comprises at least one of Si, Sn, Ge, Pb, Ag, Mg, Zn, Ga, In, Sb, Bi, and Al.

7. The negative electrode material according to claim 1 or 2, characterized in that, The active material satisfies at least one of the following characteristics: (1) The active material comprises a silicon material, and the silicon material comprises at least one of silicon element, silicon oxide, and silicon alloy; (2) The active material comprises a silicon material, and the mass content of silicon element in the negative electrode material is 20% to 55%; (3) The active material comprises a silicon material, and the average particle size of the silicon material is 0.1 nm to 5 nm.

8. The negative electrode material according to claim 1 or 2, characterized in that, The negative electrode material further comprises a coating layer, and the coating layer satisfies at least one of the following characteristics: (1) The coating layer is located on at least part of the surface of the porous matrix and / or the active material; (2) The material of the coating layer comprises at least one of a carbon material, a metal oxide, a metal salt, and a polymer material; (3) The thickness of the coating layer is 10 nm to 200 nm.

9. The negative electrode material according to claim 1 or 2, characterized in that, The particle strength of the negative electrode material is 100 MPa to 300 MPa.

10. The negative electrode material according to claim 1 or 2, characterized in that, The conductivity of the negative electrode material is 0.7 S / cm to 1.5 S / cm.

11. The negative electrode material according to claim 1 or 2, characterized in that, The lithium ion diffusion coefficient of the negative electrode material is 0.7 x 10 -7 ~ 0.9 x 10 -7 cm 2 / s.

12. The negative electrode material according to claim 1 or 2, characterized in that, The negative electrode material satisfies at least one of the following characteristics: (1) the total pore volume of the negative electrode material is 0.001 cm 3 / g to 0.1 cm 3 / g; (2) The average pore size of the pores of the negative electrode material is 0.4 nm to 50 nm; (3) The pore volume ratio of mesopores in the negative electrode material is ≥80%.

13. The negative electrode material according to claim 1 or 2, characterized in that, The median particle size D50 of the negative electrode material is 6 μm to 15 μm.

14. A negative electrode sheet characterized by comprising: The negative electrode sheet includes the negative electrode material according to any one of claims 1 to 13.

15. A secondary battery characterized by comprising: The secondary battery includes the negative electrode sheet according to claim 14.

Images