A negative electrode material, a method for preparing the same, and an electrochemical device

Abstract

The application provides a negative electrode material, a preparation method thereof, and an electrochemical device. The negative electrode material comprises: a core, wherein the core comprises silicon-based particles; a first layer comprising carbon, wherein the first layer is present outside the core; a second layer comprising a first conductive material and an organic lithium salt, wherein the second layer is present outside the first layer; and a third layer comprising a second conductive material and a polymer, wherein the third layer is present outside the second layer. The negative electrode material provided by the application can effectively alleviate the fragmentation and pulverization of the negative electrode material during the cycle process and slow down the growth of the SEI film, thereby reducing the volume expansion of the electrochemical device after the cycle, and improving the cycle performance of the electrochemical device.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of electrochemical energy storage, and in particular to a negative electrode material, a preparation method thereof, and an electrochemical device. BACKGROUND

[0002] Graphite is the most widely used negative electrode material, which has the advantages of high efficiency, stable charge and discharge platform, etc. However, the lower specific capacity hinders the further application of graphite. Compared with graphite, elemental silicon is considered as an ideal negative electrode material that can replace graphite due to its higher theoretical specific capacity and suitable working voltage.

[0003] However, the volume expansion of silicon after lithium intercalation is as high as 400%, and the repeated expansion and contraction of silicon-based negative electrode materials during the cycle process will lead to the destruction and growth of the SEI film and the fragmentation of silicon-based negative electrode particles, resulting in huge electrode expansion and rapid capacity decay, which affects the thickness and cycle performance of lithium ion batteries and restricts the large-scale application of silicon negative electrode materials in lithium ion batteries. SUMMARY

[0004] The purpose of the present application is to provide a negative electrode material, a preparation method thereof, and an electrochemical device to improve the cycle performance of the electrochemical device. The specific technical solutions are as follows:

[0005] The first aspect of the present application provides a negative electrode material, comprising: a core, the core comprising silicon-based particles; a first layer, the first layer existing outside the core; a second layer, the second layer existing outside the first layer; a third layer, the third layer existing outside the second layer; wherein the first layer comprises carbon; the second layer comprises a first conductive material and an organic lithium salt; and the third layer comprises a second conductive material and a polymer.

[0006] In an embodiment of the present application, the silicon-based particles comprise at least one of silicon-carbon particles or silicon-oxygen particles.

[0007] In an embodiment of the present application, the organic lithium salt comprises a polyacrylic lithium salt.

[0008] In an embodiment of the present application, the polymer comprises polyurethane.

[0009] In an embodiment of the present application, the first conductive material and the second conductive material are each independently selected from at least one of single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), or conductive carbon black.

[0010] In an embodiment of the present application, the polyacrylic lithium salt comprises at least one of lithium polyacrylate or lithium polyacrylate-acrylamide.

[0011] In an embodiment of the present application, the mass percentage of silicon element is 40% to 50% based on the mass of the negative electrode material.

[0012] In an embodiment of the present application, the negative electrode material satisfies at least one of the following: (1) the Dv50 of the negative electrode material is 6 μm to 10 μm; (2) the Dv90 of the negative electrode material is less than or equal to 30 μm; (3) the mass percentage of carbon in the first layer is 1% to 5% based on the mass of the negative electrode material; (4) the mass percentage of the organic lithium salt is 0.9% to 9% based on the mass of the negative electrode material; (5) the mass percentage of the polymer is 0.9% to 9% based on the mass of the negative electrode material; (6) the mass percentage of the first conductive material is 0.1% to 1% based on the mass of the negative electrode material; (7) the mass percentage of the second conductive material is 0.1% to 1% based on the mass of the negative electrode material.

[0013] In an embodiment of the present application, the mass ratio of the first conductive material to the organic lithium salt is 1:9 to 2:9, and the mass ratio of the second conductive material to the polymer is 1:9 to 2:9.

[0014] The second aspect of the present application provides a preparation method of the negative electrode material provided in the first aspect of the present application, which comprises: providing silicon-based particles; performing carbon coating on the silicon-based particles to obtain a first intermediate; dispersing the first intermediate in water to obtain a first dispersion, dispersing an organic lithium salt and a first conductive material in water to obtain a second dispersion, mixing the first dispersion and the second dispersion and stirring uniformly, and obtaining a second intermediate through spray drying; dispersing the second intermediate in water to obtain a third dispersion, dispersing a polymer and a second conductive material in water to obtain a fourth dispersion, mixing the third dispersion and the fourth dispersion and stirring uniformly, and obtaining the negative electrode material through spray drying. The coating can be partial coating or full coating.

[0015] In an embodiment of the present application, the carbon coating is placing the silicon-based particles in a fluidized bed, passing a carbon source gas at a temperature of 500°C to 650°C, and reacting for 2 h to 3 h to obtain the first intermediate, wherein the carbon source gas comprises at least one of acetylene, methane or propylene.

[0016] The third aspect of the present application provides an electrochemical device, which comprises a positive electrode sheet, a separator film, an electrolyte and a negative electrode sheet, the negative electrode comprises a negative electrode active material layer, and the negative electrode active material layer comprises the negative electrode material provided in the first aspect of the present application.

[0017] In an embodiment of the present application, the electrolyte comprises an additive, the additive comprises at least one of difluoroethylene carbonate or fluoroethylene carbonate, and the mass percentage of the additive is 0.5% to 10% based on the total mass of the electrolyte.

[0018] In one embodiment of the present application, the mass percentage of the additive is 2% to 7% based on the mass of the electrolyte.

[0019] The fourth aspect of the present application provides a use electric device comprising the electrochemical device provided by the third aspect of the present application.

[0020] The beneficial effects of the present application are as follows:

[0021] The silicon-based particles of the present application improve the cycle performance of the electrochemical device through three-layer coating. The first carbon coating layer can alleviate the problem of gas production caused by the direct contact of the silicon-based particles with water during stirring. The second layer can reduce the reaction between the electrolyte and the silicon-based particles, improve the composition of the SEI film, and improve the cycle performance of the electrochemical device. The third layer can reduce the specific surface area of the silicon-based particles, reduce the volume expansion of the silicon-based particles, improve the first cycle charge-discharge efficiency of the negative electrode material, and reduce the thickness expansion rate of the electrochemical device. The synergistic effect of the three coating layers can reduce the volume expansion of the silicon-based particles, while reducing the reaction between the electrolyte and the silicon-based particles, improving the composition of the SEI, and further improving the cycle performance of the electrochemical device and reducing the expansion of the electrochemical device. BRIEF DESCRIPTION OF DRAWINGS

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other embodiments can also be obtained by those skilled in the art based on these drawings.

[0023] Figure 1 Structure schematic diagram of the negative electrode material prepared for Example 1 of the present application;

[0024] Figure 2 Cycle performance comparison chart of lithium ion batteries assembled with the negative electrode materials in Example 1-1 and Comparative Examples 1 to 3 at 25℃;

[0025] Figure 3 Cycle performance comparison chart of lithium ion batteries assembled with the negative electrode materials in Example 1-1 and Comparative Examples 1 to 3 at 45℃;

[0026] Figure 4 Expansion rate comparison chart of lithium ion batteries assembled with the negative electrode materials in Example 1-1 and Comparative Examples 1 to 3 at 25℃;

[0027] Figure 5 Expansion rate comparison chart of lithium ion batteries assembled with the negative electrode materials in Example 1-1 and Comparative Examples 1 to 3 at 45℃. DETAILED DESCRIPTION

[0028] The technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are only some of the embodiments of the present application, but not all of the embodiments. All other embodiments obtained by those skilled in the art based on the present application shall fall within the scope of the present application.

[0029] It should be noted that in the specific embodiments of the present application, the present application is explained by taking lithium ion batteries as examples of electrochemical devices, but the electrochemical devices of the present application are not limited to lithium ion batteries.

[0030] The first aspect of the present application provides a negative electrode material, comprising: a core, the core comprising silicon-based particles; a first layer, the first layer existing outside the core; a second layer, the second layer existing outside the first layer; a third layer, the third layer existing outside the second layer; wherein the first layer comprises carbon; the second layer comprises a first conductive material and an organic lithium salt; and the third layer comprises a second conductive material and a polymer. In the present application, the first layer exists outside the entire core or part of the core, the second layer exists outside the entire first layer or part of the first layer, and the third layer exists outside the entire second layer or part of the second layer. Specifically, the structural schematic diagram of the negative electrode material is as shown in Figure 1 The core 11, the first layer 12, the second layer 13, and the third layer 14 are sequentially arranged from inside to outside. In an embodiment of the present application, a mutual solubility layer may be formed between the core and the first layer. In another embodiment of the present application, a mutual solubility layer may be formed between the first layer and the second layer. In another embodiment of the present application, a mutual solubility layer may be formed between the second layer and the third layer. The mutual solubility layer described above is not the first layer, the second layer, or the third layer described in the present application.

[0031] Without being limited to any theory, the inventors of the present application found that the introduction of the carbon-containing first layer can alleviate the problem of gas production caused by the direct contact of the silicon-based particles with water during stirring; the introduction of the second layer can reduce the reaction between the electrolyte and the silicon-based particles, improve the composition of the SEI film, and improve the cycle performance of the lithium ion battery; the introduction of the third layer can reduce the specific surface area of the silicon-based particles, reduce the volume expansion of the silicon-based particles, improve the first cycle charge-discharge efficiency of the negative electrode material, and reduce the thickness expansion rate of the lithium ion battery. The synergistic effect of the three coating layers can not only reduce the volume expansion of the silicon-based particles, but also reduce the reaction between the electrolyte and the silicon-based particles, improve the composition of the SEI, and further improve the cycle performance of the lithium ion battery and reduce the expansion rate of the lithium ion battery.

[0032] In an embodiment of the present application, the silicon-based particles comprise at least one of silicon-carbon particles or silicon-oxygen particles.

[0033] In an embodiment of the present application, the organic lithium salt comprises a polyacrylic lithium salt.

[0034] In an embodiment of the present application, the polymer comprises polyurethane.

[0035] In an embodiment of the present application, the first conductive material and the second conductive material are each independently selected from at least one of single-walled carbon nanotubes, multi-walled carbon nanotubes, or conductive black.

[0036] The silicon-based particles select the material in the above range, which can make the negative electrode have a higher capacity, so as to apply the negative electrode to the lithium ion battery, which can make the lithium ion battery have a higher energy density; the polymer selects polyurethane, which has good adhesion, can form a stable coating layer outside the inner core, and bond the second intermediate together to form the negative electrode material, which can reduce the volume expansion of the negative electrode, thereby reducing the thickness expansion rate of the lithium ion battery, and the specific surface area of the negative electrode material is also reduced; the first conductive material and the second conductive material select the material in the above range, which can make the negative electrode material have good conductivity.

[0037] In an embodiment of the present application, the polyacrylic lithium salt comprises at least one of lithium polyacrylate or lithium polyacrylate-acrylamide. The polyacrylic lithium salt selects the material in the above range, which can reduce the direct contact of the electrolyte with the active silicon, and also change the composition of the SEI, thereby improving the cycle performance of the lithium ion battery.

[0038] In an embodiment of the present application, the silicon-based particles satisfy at least one of the following: (1) the specific surface area of the silicon-based particles is 0.5m 2 / g to 10m 2 / g; (2) the particle size Dv50 of the silicon-based particles is 1.5μm to 2.5μm; (3) the particle size Dv90 of the silicon-based particles is less than or equal to 10μm. The particle classification technology in the present application is not particularly limited as long as it can achieve the purpose of the present application, and the particle classification technology can be any known classification means used in the art, such as fluid classification and cyclone classification. In the present application, Dv50 refers to the particle size reaching 50% of the volume accumulation from the small particle size in the particle size distribution of the material on the volume basis; Dv90 refers to the particle size reaching 90% of the volume accumulation from the small particle size in the particle size distribution of the material on the volume basis.

[0039] The specific surface area of the silicon-based particles can be 0.5m 2 / g, 1m 2 / g, 2m 2 / g, 3m 2 / g, 4m 2 / g, 5m 2 / g, 6m 2 / g, 7m 2 / g, 8m 2 / g, 9m2 / g, 10 m 2 / g or a range consisting of any two of the numerical values; the shape of the silicon-based particles can be at least one of spherical, spheroidal, flaky, or massive; without being limited to any theory, the inventors of the present application have found that when the specific surface area of the silicon-based particles is 0.5 m 2 / g to 10 m 2 / g, the reactivity of the silicon-based particles with water during stirring can be reduced, while a high degree of coating of the subsequent three layers on the outside of the silicon-based particles can be achieved.

[0040] The particle size Dv50 of the silicon-based particles can be 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, or a range consisting of any two of the numerical values. Without being limited to any theory, the inventors of the present application have found that when the particle size Dv50 of the silicon-based particles is 1.5 μm to 2.5 μm, the diffusion path of Li + can be shortened, the ionic conductivity of the negative electrode material can be improved, and the volume expansion of the silicon-based particles can be reduced.

[0041] The particle size Dv90 of the silicon-based particles can be 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, or a range consisting of any two of the numerical values. Without being limited to any theory, the inventors of the present application have found that when the particle size Dv90 of the silicon-based particles is less than or equal to 10 μm, the particle size of the negative electrode material during coating can be improved, Li + transport can be improved, and thus the cycle performance and expansion performance of the lithium ion battery can be improved.

[0042] In an embodiment of the present application, the mass percentage of silicon element is 40% to 50% based on the mass of the negative electrode material. For example, the mass percentage of silicon element can be 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, or a range consisting of any two of the numerical values. Without being limited to any theory, the inventors of the present application have found that when the mass percentage of silicon element is 40% to 50%, the negative electrode material can have a high reversible capacity and a first cycle charge-discharge efficiency, and when applied to a lithium ion battery, the lithium ion battery also has good cycle performance, thereby facilitating improvement of the energy density and cycle performance of the lithium ion battery.

[0043] In an embodiment of the present application, the particle size Dv50 of the negative electrode material is 6 μm to 10 μm. For example, the particle size Dv50 of the negative electrode material can be 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm, 10 μm, or a range between any two of them. Without being limited to any theory, the inventors of the present application have found that by controlling the particle size Dv50 of the negative electrode material to be 6 μm to 10 μm, the diffusion path of Li + + can be shortened, the ionic conductivity of Li + + can be improved, the first cycle charge-discharge efficiency of the negative electrode material can be improved; at the same time, the defects of increased electrolyte consumption and low material compaction density can be improved, and the risk of low ionic conductivity and poor rate performance caused by large particle size can be reduced.

[0044] In an embodiment of the present application, the particle size Dv90 of the negative electrode material is less than or equal to 30 μm. For example, the particle size Dv90 of the negative electrode material can be 12 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, 26 μm, 27 μm, 28 μm, 29 μm, 30 μm, or a range between any two of them. Without being limited to any theory, the inventors of the present application have found that when the particle size Dv90 of the negative electrode material is less than or equal to 30 μm, the dispersion uniformity of the slurry during the preparation of the negative electrode material can be improved, the transport of Li + + can be improved, and the cycle performance and swelling performance of the lithium ion battery can be improved.

[0045] In an embodiment of the present application, the mass percentage of carbon in the first layer is 1% to 5% based on the mass of the negative electrode material. For example, the mass percentage of carbon in the first layer can be 1%, 1.5%, 2%, 2.5%, 3%, 4%, 4.5%, 5%, or a range between any two of them. When the mass percentage of carbon in the first layer is within the above range, the contact between the active silicon in the silicon-based particles and air or solution can be effectively reduced, and the active silicon can be protected, so that the negative electrode material has a higher specific capacity and stability, thereby improving the cycle stability and capacity of the lithium ion battery.

[0046] In an embodiment of the present application, the mass percentage of organic lithium salt is 0.9% to 9% based on the mass of the negative electrode material. For example, the mass percentage of organic lithium salt can be 0.9%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or a range between any two of them. When the mass percentage of organic lithium salt is within the above range, the direct contact between the electrolyte and the active silicon can be reduced, and the composition of the SEI can be changed, thereby improving the cycle performance of the lithium ion battery.

[0047] In an embodiment of the present application, the mass percentage of the polymer is 0.9% to 9% based on the mass of the negative electrode material. For example, the mass percentage of the polymer can be 0.9%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or a range between any two of them. When the mass percentage of the polymer is within the above range, a stable coating layer can be formed by its good adhesion, and the second intermediate body is bonded together to form the negative electrode material, reducing the volume expansion of the negative electrode, thereby reducing the thickness expansion rate of the lithium ion battery. At the same time, the specific surface area of the negative electrode material is also reduced, and the first cycle charge-discharge efficiency of the negative electrode material is improved.

[0048] In an embodiment of the present application, the mass percentage of the first conductive material is 0.1% to 1% based on the mass of the negative electrode material. For example, the mass percentage of the first conductive material can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, or a range between any two of them. When the mass percentage of the first conductive material is within the above range, the negative electrode material can have good conductivity while introducing the second layer of organic lithium salt.

[0049] In an embodiment of the present application, the mass percentage of the second conductive material is 0.1% to 1% based on the mass of the negative electrode material. For example, the mass percentage of the second conductive material can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, or a range between any two of them. When the mass percentage of the second conductive material is within the above range, the negative electrode material can have good conductivity while introducing the third layer of polymer.

[0050] In an embodiment of the present application, the mass ratio of the first conductive material to the organic lithium salt is 1:9 to 2:9. For example, the mass ratio of the first conductive material to the organic lithium salt can be 1:9, 1.1:9, 1.2:9, 1.3:9, 1.4:9, 1.5:9, 1.6:9, 1.7:9, 1.8:9, 1.9:9, 2:9, or a range between any two of them. When the mass ratio of the first conductive material to the organic lithium salt is within the above range, the direct contact of the electrolyte with the active silicon can be reduced, the composition of the SEI can be changed, and at the same time the lithium ion battery has lower impedance, thereby improving the cycle performance of the lithium ion battery.

[0051] In an embodiment of the present application, the mass ratio of the second conductive material to the polymer is 1:9 to 2:9. For example, the mass ratio of the second conductive material to the polymer can be 1:9, 1.1:9, 1.2:9, 1.3:9, 1.4:9, 1.5:9, 1.6:9, 1.7:9, 1.8:9, 1.9:9, 2:9, or a range defined by any two of the above values. When the mass ratio of the polymer to the second conductive material is within the above range, the polymer is better adhered to the second intermediate body through its better adhesion, a stable coating layer is formed, and the second intermediate body is adhered together to form the negative electrode material, which reduces the volume expansion of the negative electrode while having good conductivity and first cycle charge-discharge efficiency, and reduces the thickness expansion rate of the lithium ion battery.

[0052] The second aspect of the present application provides a preparation method of the negative electrode material of the first aspect of the present application, which comprises: providing silicon-based particles; carbon-coating the silicon-based particles to obtain a first intermediate body; dispersing the first intermediate body in water to obtain a first dispersion, dispersing an organic lithium salt and a first conductive material in water to obtain a second dispersion, mixing and uniformly stirring the first dispersion and the second dispersion, and obtaining a second intermediate body by spray drying; dispersing the second intermediate body in water to obtain a third dispersion, dispersing a polymer and a second conductive material in water to obtain a fourth dispersion, mixing and uniformly stirring the third dispersion and the fourth dispersion, and obtaining the negative electrode material by spray drying. The above preparation method is simple in steps, easy to operate, and suitable for industrial production.

[0053] In an embodiment of the present application, the carbon coating is placing the silicon-based particles in a fluidized bed, passing a carbon source gas at a temperature of 500-650°C, and reacting for 2-3h to obtain the first intermediate body, wherein the carbon source gas comprises at least one of acetylene, methane, and propylene. For example, the coating temperature can be 500°C, 510°C, 520°C, 530°C, 540°C, 650°C, 560°C, 570°C, 580°C, 590°C, 600°C, 610°C, 620°C, 630°C, 640°C, 650°C, or a range defined by any two of the above values. For example, the coating time is 2h, 2.2h, 2.4h, 2.6h, 2.8h, 3h, or a range defined by any two of the above values. Without being limited to any theory, the present inventors have found that the carbon source gas is deposited on the outside of the target particles after cracking at high temperature, and within a suitable temperature range, the higher the deposition temperature, the faster the cracking rate and the higher the utilization rate of the carbon source gas, and the faster the growth rate of the coating layer; at the same deposition temperature, the length of the deposition time determines the thickness and integrity of the coating layer. By controlling the time and temperature of the carbon coating, the content of the carbon material in the carbon coating process can be controlled.

[0054] In an embodiment of the present application, the carbon source gas and the inert gas are mixed and then introduced into the fluidized bed, the inert gas includes at least one of nitrogen or argon; the volume percentage of the carbon source gas is 10% to 50% based on the volume of the mixed gas. For example, the volume percentage of the carbon source gas can be 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or a range between any two of them. Without being limited to any theory, the inventors of the present application have found that the provision of the inert gas is conducive to improving the uniformity of the distribution of carbon elements in the coating layer.

[0055] In an embodiment of the present application, the Dv50 of the first intermediate is 1.5 μm to 2.5 μm. For example, the Dv50 of the first intermediate can be 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, or a range between any two of them.

[0056] In an embodiment of the present application, the solid content of the first dispersion is 10 to 15%. For example, the solid content of the first dispersion can be 10%, 11%, 12%, 13%, 14%, 15%, or a range between any two of them.

[0057] In an embodiment of the present application, the ratio of the mass of the first intermediate to the sum of the mass of the mixture of the organic lithium salt and the first conductive material is 100:1 to 100:10. For example, the ratio of the mass of the first intermediate to the sum of the mass of the mixture of the organic lithium salt and the first conductive material can be 100:1, 100:2, 100:3, 100:4, 100:5, 100:6, 100:7, 100:8, 100:9, 100:10, or a range between any two of them; without being limited to any theory, the inventors of the present application have found that when the ratio of the mass of the first intermediate to the sum of the mass of the mixture of the organic lithium salt and the first conductive material is 100:1 to 100:10, the reaction of the electrolyte and the silicon-based particles can be effectively reduced, the composition of the SEI film can be improved, the cycle performance of the lithium ion battery can be improved, and the consumption of the silicon element in the negative electrode material in the reaction can be reduced, thereby making the negative electrode material have a higher reversible capacity.

[0058] In an embodiment of the present application, the solid content of the second dispersion is 20 to 30%. For example, the solid content of the second dispersion can be 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, or a range between any two of them.

[0059] In an embodiment of the present application, the solid content of the mixture of the first dispersion and the second dispersion is 10-15%. For example, the solid content of the mixture of the first dispersion and the second dispersion can be 10%, 11%, 12%, 13%, 14%, 15%, or a range defined by any two of the numbers.

[0060] In an embodiment of the present application, the inlet air temperature of the spray drying of the second intermediate is 150-170°C, and the outlet air temperature is 100-110°C. Without being limited by any theory, the present inventors have found that when the parameters of the spray drying are within the above ranges, the particle size distribution of the second intermediate after the spray drying can be controlled while taking into account the drying capacity of the machine and the production capacity, and meanwhile, the increase in particle size caused by the adhesion of particles can be effectively reduced.

[0061] In an embodiment of the present application, the Dv50 of the second intermediate is 1.6-2.7 μm. For example, the particle size Dv50 can be 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, 2.6 μm, 2.7 μm, or a range defined by any two of the numbers.

[0062] In an embodiment of the present application, the solid content of the third dispersion is 20-30%. For example, the solid content of the third dispersion can be 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, or a range defined by any two of the numbers.

[0063] In an embodiment of the present application, the mass ratio of the polymer and the second conductive material is 9:1-9:2. For example, the mass ratio of the polymer and the second conductive material can be 9:1, 9:1.1, 9:1.2, 9:1.3, 9:1.4, 9:1.5, 9:1.6, 9:1.7, 9:1.8, 9:1.9, 9:2, or a range defined by any two of the numbers. Without being limited by any theory, the present inventors have found that the introduction of the conductive material can alleviate the problem of the reduction in the electrical conductivity of the negative electrode material caused by the polymer coating.

[0064] In an embodiment of the present application, the ratio of the mass of the second intermediate to the sum of the mass of the polymer and the second conductive material is 100:1 to 100:10. For example, the ratio of the mass of the second intermediate to the sum of the mass of the polymer and the second conductive material can be 100:1, 100:2, 100:3, 100:4, 100:5, 100:6, 100:7, 100:8, 100:9, 100:10, or a range defined by any two of the aforementioned values. Without being limited to any theory, the inventors of the present application have found that when the ratio of the mass of the first intermediate to the sum of the mass of the organic lithium salt and the first conductive material is 100:1 to 100:10, the reaction of the electrolyte and the silicon-based particles can be effectively reduced, the composition of the SEI film can be improved, the cycle performance of the lithium ion battery can be improved, and the consumption of the silicon element content of the negative electrode material in the reaction can be reduced, thereby enabling the negative electrode material to have a higher reversible capacity.

[0065] In an embodiment of the present application, the solid content of the fourth dispersion is 18% to 22%. For example, the solid content of the fourth dispersion can be 18%, 18.5%, 19%, 19.5%, 20%, 20.5%, 21%, 21.5%, 22%, or a range defined by any two of the aforementioned values.

[0066] In an embodiment of the present application, the solid content of the mixture of the third dispersion and the fourth dispersion is 20% to 25%. For example, the solid content of the mixture of the third dispersion and the fourth dispersion can be 20%, 21%, 22%, 23%, 24%, 25%, or a range defined by any two of the aforementioned values.

[0067] In some embodiments, the inlet air temperature of the spray drying of the negative electrode material is 200°C to 230°C, and the outlet air temperature is 100°C to 110°C. Without being limited to any theory, the inventors of the present application have found that when the parameters of the spray drying are within the above-mentioned ranges, the particle size distribution of the negative electrode material after spray drying can be controlled while taking into account the drying capacity of the machine and the production capacity.

[0068] In an embodiment of the present application, the coating referred to in the present application can be partial coating or complete coating.

[0069] The third aspect of the present application provides an electrochemical device, which comprises a positive electrode, a separator, an electrolyte, and a negative electrode, wherein the negative electrode comprises a negative electrode active material layer, and the negative electrode active material layer comprises the negative electrode material provided in the first aspect of the present application.

[0070] In one embodiment of the present application, the electrolyte comprises an additive comprising at least one of difluoroethylene carbonate or fluoroethylene carbonate, and the mass percentage of the additive is 0.5% to 10%, preferably 2% to 7%, based on the total mass of the electrolyte. For example, the mass percentage of the additive can be 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range defined by any two of them. Without being limited to any theory, the inventors of the present application have found that difluoroethylene carbonate or fluoroethylene carbonate can improve the contact between the electrolyte and the negative electrode by interacting with the negative electrode material during the cycling of the lithium ion battery, thereby improving the cycling performance.

[0071] In the present application, the electrolyte further comprises a lithium salt, which is not particularly limited in the present application, and any lithium salt known in the art can be used as long as the purpose of the present application can be achieved. For example, the lithium salt can be selected from at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, and LiPO2F2. The mass percentage of the lithium salt can be 8% to 20%, based on the mass of the electrolyte, for example, the mass percentage of the lithium salt can be 8%, 10%, 12%, 14%, 16%, 18%, 20%, or a range defined by any two of them.

[0072] The base solvent is not particularly limited in the present application, as long as the object of the present application can be achieved, for example, the base solvent can include, but is not limited to, at least one of a carbonate compound, a carboxylic acid ester compound, an ether compound, or other organic solvents. The above-mentioned carbonate compound can include, but is not limited to, at least one of a chain carbonate compound, a cyclic carbonate compound, or a fluorinated carbonate compound. The above-mentioned chain carbonate compound can include, but is not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (MEC). The above-mentioned cyclic carbonate can include, but is not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), or vinyl ethylene carbonate (VEC). The fluorinated carbonate compound can include, but is not limited to, at least one of 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methyl ethylene carbonate, 1-fluoro-1-methyl ethylene carbonate, 1,2-difluoro-1-methyl ethylene carbonate, 1,1,2-trifluoro-2-methyl ethylene carbonate, or trifluoromethyl ethylene carbonate. The above-mentioned carboxylic acid ester compound can include, but is not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valerolactone, or caprolactone. The above-mentioned ether compound can include, but is not limited to, at least one of dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The above-mentioned other organic solvents can include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. The content of the base solvent in the electrolyte is not particularly limited in the present application, as long as the object of the present application can be achieved.

[0073] In the present application, the electrochemical device further includes a positive electrode sheet including a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector. The above-mentioned "positive electrode active material layer provided on at least one surface of the positive electrode current collector" means that the positive electrode active material layer can be provided on one surface of the positive electrode current collector in the thickness direction of the positive electrode current collector, or can be provided on both surfaces of the positive electrode current collector in the thickness direction of the positive electrode current collector. It should be noted that the "surface" here can be the entire layer of the surface of the positive electrode current collector, or can be part of the layer of the surface of the positive electrode current collector, which is not particularly limited in the present application, as long as the object of the present application can be achieved.

[0074] The positive electrode current collector is not particularly limited as long as the object of the present application can be achieved, and for example, an aluminum foil, an aluminum alloy foil, or a composite current collector (for example, an aluminum-carbon composite current collector), or the like can be included.

[0075] The positive electrode active material is not particularly limited as long as the object of the present application can be achieved, and for example, the positive electrode active material can include, but is not limited to, at least one of lithium cobaltate, lithium nickel-manganese cobaltate, lithium nickel-manganese aluminate, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel lithium manganate, spinel lithium nickel-manganese, and lithium titanate.

[0076] The positive electrode material layer can further include a conductive agent and a binder, and the kind of the conductive agent and the binder is not particularly limited as long as the object of the present application can be achieved. The mass ratio of the positive electrode active material, the conductive agent, and the binder in the positive electrode material layer is not particularly limited, and a person skilled in the art can select according to actual needs as long as the object of the present application can be achieved.

[0077] The binder is not particularly limited as long as the object of the present application can be achieved, and for example, the binder can include, but is not limited to, at least one of an adhesive polymer such as polyvinylidene fluoride, polytetrafluoroethylene, a polyolefin-based, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, modified polyvinylidene fluoride, modified SBR rubber, or polyurethane, wherein the polyolefin-based binder includes at least one of polyethylene, polypropylene, polyolefin ester, polyolefin alcohol, or polyacrylic acid.

[0078] The conductive agent is not particularly limited as long as the object of the present application can be achieved, and for example, the conductive agent can include, but is not limited to, a carbon-based material, a metal-based material, a conductive polymer, or a mixture thereof; wherein the carbon-based material includes natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, or carbon fiber; the metal-based material includes metal powder or metal fiber of copper, nickel, aluminum, silver, or the like; and the conductive polymer includes a polyphenylene derivative.

[0079] The thickness of the positive electrode current collector and the positive electrode material layer is not particularly limited as long as the object of the present application can be achieved. For example, the thickness of the positive electrode current collector is 5 μm to 20 μm, and the thickness of the single-sided positive electrode material layer is 30 μm to 120 μm.

[0080] Optionally, the positive electrode sheet can further include a conductive layer, and the conductive layer is located between the positive electrode current collector and the positive electrode material layer. The composition of the conductive layer is not particularly limited and can be a commonly used conductive layer in the art. The conductive layer includes a conductive agent and a binder. The conductive agent and the binder in the conductive layer are not particularly limited, and for example, can be at least one of the above-mentioned conductive agent and the above-mentioned binder.

[0081] In the present application, the electrochemical device further includes a separator film. The present application does not particularly limit the separator film as long as the purpose of the present application can be achieved. For example, the material of the separator film can include, but is not limited to, at least one of polyethylene (PE), polyolefin (PO) based on polypropylene (PP), polyester (for example, polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of the separator film can include at least one of a woven film, a nonwoven film, a microporous film, a composite film, a calendered film, or a spunlaced film.

[0082] In some embodiments of the present application, the separator film can include a base layer and a surface treatment layer. The base layer can be a nonwoven fabric, a film, or a composite film having a porous structure, and the material of the base layer can include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous film, a polyethylene porous film, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite film can be used.

[0083] Optionally, a surface treatment layer is provided on at least one surface of the base layer, and the surface treatment layer can be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic substance.

[0084] In some embodiments of the present application, the inorganic layer includes inorganic particles and a binder. The present application does not particularly limit the inorganic particles, and for example, the inorganic particles can include at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. The present application does not particularly limit the binder, and for example, the binder can be at least one of the above-described binders.

[0085] In some embodiments of the present application, the polymer layer includes a polymer, and the material of the polymer includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylic acid salt, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene).

[0086] In some embodiments of the present application, the inorganic layer can further include a thickening agent and a wetting agent, and the present application does not particularly limit the kind of the thickening agent and the wetting agent as long as the purpose of the present application can be achieved. For example, the thickening agent can include, but is not limited to, at least one of sodium carboxymethyl cellulose or lithium carboxymethyl cellulose; and the wetting agent can include, but is not limited to, at least one of dimethylsiloxane, sodium dodecyl sulfate, trialkyl phosphate, methyl decanoate, dodecyl acetate.

[0087] In the present application, the thickness of the separator film is not particularly limited as long as the object of the present application can be achieved, for example, the thickness of the separator film can be 4 μm to 30 μm.

[0088] In the present application, the electrochemical device further comprises a negative electrode tab, and the negative electrode tab comprises a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The above-mentioned "the negative electrode active material layer is disposed on at least one surface of the negative electrode current collector" means that the negative electrode active material layer can be disposed on one surface of the negative electrode current collector along the thickness direction of the negative electrode current collector, or can be disposed on two surfaces of the negative electrode current collector along the thickness direction of the negative electrode current collector. It should be noted that the "surface" here can be the entire layer of the surface of the negative electrode current collector, or can be part of the layer of the surface of the negative electrode current collector, and the present application is not particularly limited as long as the object of the present application can be achieved.

[0089] The present application does not particularly limit the negative electrode current collector as long as the object of the present application can be achieved, for example, it can comprise a copper foil, a copper alloy foil, a nickel foil, a stainless steel foil, a titanium foil, a foamed nickel, a foamed copper or a composite current collector, and exemplarily, the composite current collector can be a lithium-copper composite current collector, a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, etc.

[0090] In some embodiments of the present application, the negative electrode active material layer can further comprise a conductive agent and a binder, and the present application does not particularly limit the types of the conductive agent and the binder as long as the object of the present application can be achieved. For example, 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 polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, butadiene-styrene rubber, acrylated butadiene-styrene rubber, epoxy resin or nylon, etc. The conductive agent can include but is not limited to: carbon-based materials, metal-based materials, conductive polymers and mixtures thereof. Among them, the carbon-based materials are selected from carbon black, acetylene black, ketjen black, carbon fiber or any combination thereof; the metal-based materials are selected from metal powder, metal fiber, copper, nickel, aluminum or silver; and the conductive polymer is a polyphenylene derivative. The present application does not particularly limit the mass ratio of the negative electrode material, the conductive agent and the binder in the negative electrode active material layer, and the person skilled in the art can select according to the actual needs as long as the object of the present application can be achieved.

[0091] The present application does not particularly limit the thickness of the negative electrode active material layer as long as the object of the present application can be achieved, for example, the thickness of the single-sided negative electrode active material layer is 30 μm to 120 μm.

[0092] The present application does not particularly limit the thickness of the negative electrode current collector as long as the object of the present application can be achieved, for example, the thickness of the negative electrode current collector is 4 μm to 15 μm.

[0093] The electrochemical device also includes a case for accommodating the positive electrode sheet, the separator, the negative electrode sheet, and the electrolyte solution, and other components known in the art of electrochemical devices, which are not limited by the present application. The case is not particularly limited by the present application and can be a case known in the art as long as the purpose of the present application is achieved. For example, the case can be a hard case or a flexible case. The material of the hard case can be a metal, which is not particularly limited by the present application and can be a metal hard case known in the art as long as the purpose of the present application is achieved. The flexible case can be a metal plastic film such as an aluminum plastic film, a steel plastic film, or the like.

[0094] The preparation process of the electrochemical device of the present application is well known to those skilled in the art and is not particularly limited by the present application. For example, the preparation process of the electrochemical device can include, but is not limited to, the following steps: stacking the positive electrode sheet, the separator, and the negative electrode sheet in order, and performing operations such as winding, folding, or the like as needed to obtain an electrode assembly in a wound structure, placing the electrode assembly in the case, injecting the electrolyte solution into the case and sealing it to obtain the electrochemical device. Alternatively, the positive electrode sheet, the separator, and the negative electrode sheet can be stacked in order, and then the four corners of the entire stack structure can be fixed with a tape to obtain an electrode assembly in a stack structure, the electrode assembly can be placed in the case, the electrolyte solution can be injected into the case and sealed to obtain the electrochemical device. In addition, the overcurrent prevention element, the guide plate, or the like can be placed in the case as needed to prevent the pressure inside the electrochemical device from rising and overcharging and discharging.

[0095] The fourth aspect of the present application provides a power consuming device comprising the electrochemical device of the third aspect of the present application.

[0096] The power consuming device of the present application is not particularly limited and can be any power consuming device known in the art. In some embodiments, the power consuming device can include, but is not limited to, a notebook computer, a pen input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile machine, a portable copying machine, a portable printer, a headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable audio player, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting appliance, a toy, a game machine, a clock, a power tool, a flashlight, a camera, a household large storage battery, and a lithium ion capacitor, or the like.

[0097] Examples

[0098] Hereinafter, embodiments and comparative examples are presented to more specifically describe the embodiments of the present application. Various tests and evaluations were performed according to the following methods. In addition, unless otherwise specified, "parts" and "%" are on a mass basis.

[0099] Test method and equipment

[0100] Si element mass percentage content test

[0101] Preparation of digestion sample: take 0.1000 g of silicon-based particles or negative electrode material, put it into a nickel crucible, add 1.5 g of KOH, cover the crucible cover, and heat the muffle furnace to 400℃. The temperature rising program is to rise from room temperature to 300℃ in 2 h, and then rise from 300℃ to 400℃ in 2 h. Then start to cool down, and cool down naturally to 80℃. The digestion program ends, the crucible is taken out, and cooled to room temperature. The digested sample is taken out and placed in a polytetrafluoroethylene beaker.

[0102] Titration of sodium hydroxide standard solution of the digestion sample: add 30 mL of boiling water in a polytetrafluoroethylene beaker, leach for 1 h, then use tweezers to clean the crucible, control the volume to 50 mL, then filter the above solution into a 400 mL beaker. After filtration is completed, add 20 mL of concentrated nitric acid to the beaker at one time to neutralize the solution, so that the solution is acidic. After the solution is cooled to room temperature, add solid KCl to saturation with constant stirring, and add an excess of 2 g. Then add 10 mL of 200 g / L potassium fluoride solution, a white precipitate appears, age for 15 min, filter with medium-speed quantitative filter paper, and wash the beaker and precipitate with 8 mL of potassium chloride solution each time, a total of three times. Take out the filter paper and put it back into the original beaker, add 20 mL of potassium chloride ethanol solution, 10 drops of phenolphthalein, then neutralize the residual acid with sodium hydroxide standard solution, stir the filter paper and the scrubbing cup wall until the solution is light red. In this process, use a glass rod to stir the paper pulp, and react for 1 h. Add 200 mL of neutralized boiling water (boil and add 10 drops of phenolphthalein, neutralize with sodium hydroxide standard solution to light red) into the cup, titrate with sodium hydroxide standard solution to light red as the end point, and record the volume V of sodium hydroxide standard solution consumed.

[0103] Titration of sodium hydroxide standard solution of the blank sample: the steps are the same as those of the titration of sodium hydroxide standard solution of the digestion sample, except that the digested sample is not added. The blank sample is prepared, and the volume V0 of sodium hydroxide standard solution consumed by the titration of the blank sample is recorded.

[0104] The silicon content is calculated according to the following formula: ω Si = (V-V0) x c x 7.02 / m x 100%, where: c is the concentration of sodium hydroxide standard solution, in mol / L; V is the volume of sodium hydroxide standard solution consumed, in L; V0 is the volume of sodium hydroxide standard solution consumed by the blank sample, in L; 7.02 is the molar mass of 1 / 4 Si, in g / mol; and m is the mass of the sample, in g.

[0105] Specific surface area test

[0106] The specific surface area of the silicon-based particles of each example and the comparative example was tested by nitrogen adsorption method using a specific surface area analyzer (TriStar II 3020M, USA). The specific test was performed in accordance with the national standard GB / T 19587-2017 “Gas adsorption BET method for determining the specific surface area of solid substances”.

[0107] Test of mass percentage of carbon in the first layer

[0108] The negative electrode material was heated and combusted at high temperature under oxygen-rich conditions to oxidize carbon into carbon dioxide. The gas was treated and then entered the corresponding absorption cell to absorb the corresponding infrared radiation, which was then converted into a corresponding signal by a detector. The signal was sampled by a computer, linearly corrected, and converted into a value proportional to the concentration of carbon dioxide. The values of the entire analysis process were then added, and after the analysis was completed, the cumulative value was divided by the weight value in the computer, multiplied by the correction coefficient, and subtracted from the blank to obtain the mass percentage of carbon. A high-frequency infrared carbon and sulfur analyzer (Shanghai Deke HCS-140) was used for testing.

[0109] Test of resistivity of negative electrode material

[0110] The resistivity of the negative electrode material in each example and the comparative example was tested in a drying room using a powder resistivity meter.

[0111] Test of particle size

[0112] The Dv50 and Dv90 of the silicon-based particles and the negative electrode material were measured using a Mastersizer 3000 particle size tester produced by Malvern.

[0113] Preparation of coin-type half cells

[0114] (1) The negative electrode material, acetylene black, and sodium alginate prepared in each example or the comparative example were added to deionized water in a mass ratio of 80:10:10, and after being stirred thoroughly, a slurry with a solid content of 40% was formed. The slurry was coated on a copper foil with a thickness of 8 μm using a doctor blade to form a coating layer with a thickness of 100 μm. After drying in a vacuum drying oven at 85°C for 12 hours, the coating layer was cut into a circular sheet with a diameter of 1 cm using a punching machine in a dry environment to obtain a positive electrode.

[0115] Electrolyte and separator: In a dry argon environment, ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed uniformly in a volume ratio of 1:1 to obtain a mixed solution. 1 mol / L of LiPF6 and 5 vol% of fluoroethylene carbonate (FEC) were added to the mixed solution and stirred uniformly to obtain an electrolyte. A polyethylene film with a thickness of 12 μm was used as a separator.

[0116] (2) Assembly of coin-type half cells:

[0117] The above positive electrode sheet is assembled into a button-type half battery in a glove box with lithium metal sheet as the counter electrode, in the order of positive electrode, separator, lithium metal sheet, and adding the above electrolyte.

[0118] First cycle charge and discharge specific capacity test

[0119] For the button-type half battery, at 25°C in a normal pressure environment, the prepared button-type half battery is discharged at a rate of 0.1C to 0.01V, and then left for 5 minutes. The discharge specific capacity at this time is recorded, which is the first cycle discharge specific capacity. Then, the button-type half battery is charged at a rate of 0.1C to 1.5V, and then charged at a constant voltage until the current is 0.05C. After that, the button-type half battery is left for 5 minutes, which is one cycle of charge and discharge process. The charge capacity at this time is recorded, which is the first cycle charge specific capacity. The first cycle charge and discharge efficiency is (first cycle charge specific capacity / first cycle discharge specific capacity) x 100%.

[0120] First cycle full lithium intercalation expansion rate test

[0121] For the button-type half battery, before preparing the button-type half battery, the thickness H0 of the positive electrode sheet is measured with a micrometer. At 25°C in a normal pressure environment, the prepared button-type half battery is discharged at a rate of 0.1C to 0.01V, and then left for 30 minutes. Then, the button-type half battery is disassembled in a glove box, and the thickness H1 of the positive electrode sheet after full lithium intercalation is measured with a micrometer. The first cycle full lithium intercalation expansion rate is (H1-H0) / H0 x 100%.

[0122] Cycle performance test and expansion rate test

[0123] Before the test, the original lithium ion battery thickness is measured with a micrometer. At test temperatures of 25 and 45°C, the lithium ion battery is charged at a rate of 3.4C to 4.4V, and then charged at a constant voltage until the current is 0.025C. After that, the lithium ion battery is discharged at a rate of 0.5C to 3.0V. The capacity obtained in this step is the initial capacity of the lithium ion battery. The cycle test is carried out at a rate of 3.4C charge / 0.5C discharge. The capacity decay curve is obtained by comparing the capacity of each step with the initial capacity. The cycle number at which the capacity retention rate is 90% at 25°C is used to represent the room temperature cycle performance of the battery, and the cycle number at which the capacity retention rate is 80% at 45°C is used to represent the high temperature cycle performance of the battery. The cycle performance of the lithium ion battery is compared by comparing the cycle numbers in the above two cases. The above cycles at 25°C and 45°C are carried out to the 610th cycle to obtain Figure 2 and Figure 3 , i.e. the cycle performance comparison chart of the lithium ion battery at 25°C and the cycle performance comparison chart of the lithium ion battery at 45°C.

[0124] The thickness of the lithium ion battery cycled to 400 cycles at 25℃ and 45℃ and discharged to 3.0V is measured respectively. The relative difference of the thickness of the lithium ion battery cycled to 400 cycles at 25℃ and 3.0V and the original thickness of the lithium ion battery and the ratio of the original thickness of the lithium ion battery represent the room temperature cycle expansion rate of the lithium ion battery. The relative difference of the thickness of the lithium ion battery cycled to 400 cycles at 45℃ and 3.0V and the original thickness of the lithium ion battery and the ratio of the original thickness of the lithium ion battery represent the high temperature cycle expansion rate of the lithium ion battery. The above cycles at 25℃ and 45℃ are carried out to the 600th cycle to obtain Figure 4 and Figure 5 i.e. the expansion rate comparison chart of the lithium ion battery at 25℃ and the thickness expansion rate comparison chart of the lithium ion battery at 45℃.

[0125] At 25℃, when cycled to the 2nd, 4th, 50th, 100th, 150th, 200th, 300th, 400th, and 500th cycle, the lithium ion battery is subjected to small rate charging and discharging, and the process is as follows: the lithium ion battery is charged at 0.7C constant current to 4.53V, then charged at 4.53V constant voltage to 0.05C, and then discharged at 0.2C constant current to 3.0V after standing for 5 minutes, and then the next cycle is continued.

[0126] Example 1-1

[0127] <Preparation of negative electrode material>

[0128] (1) Take 200 kg of silicon-carbon particles and perform classification treatment by jet classification to obtain silicon-based particles with a particle size Dv50 of 2.0 μm and Dv90 of 7.4 μm.

[0129] (2) 20 kg of the classified silicon-based particles are transferred into a fluidized bed by high-pressure transmission, and after standing for about 30 min, the inlet valve and the exhaust valve of the fluidized bed are closed and vacuumized. When the cavity pressure reaches -101 kPa, the vacuumization is stopped and nitrogen is introduced at a rate of 100 L / min to positive pressure. The vacuumization and nitrogen introduction are repeated for more than 5 times, and then the oxygen content in the cavity is detected. When the oxygen content is reduced to below 10 ppm, the exhaust valve is opened, the stirring paddle of the fluidized bed is started, the rotating speed is 150 rpm, nitrogen is introduced at a rate of 200 L / min, and the temperature is raised to 550℃ at a rate of 5℃ / min. After 1 h of heat preservation, the fluidized bed inlet pipeline valve is switched, and acetylene / nitrogen mixed gas is introduced into the fluidized bed, the volume fraction of acetylene is 25%, the gas flow rate is 200 L / min, and the reaction time is 150 min. After the reaction is completed, nitrogen is introduced at a flow rate of 150 L / min, the stirring paddle speed is adjusted to 100 rpm for cooling, and the material is discharged after cooling to room temperature to obtain the first intermediate.

[0130] (3) Take 14 kg of deionized water, and add 2 kg of the first intermediate powder to the deionized water while stirring to obtain a first dispersion. After stirring for 2 h, add 0.4 kg of a second dispersion of lithium polyacrylate and single-walled carbon nanotubes (mass ratio of lithium polyacrylate to single-walled carbon nanotubes is 9:1, and the solvent is water) with a solid content of 25% to the first dispersion while stirring, and stir for 3 h. Finally, a slurry with a solid content of about 12.8% is obtained. The amount of lithium polyacrylate added is 4.5% based on the mass of the first intermediate, and the amount of single-walled carbon nanotubes added is 0.5%. Start the spray dryer, set the inlet temperature to 160°C and the outlet temperature to 105°C, and after the inlet and outlet temperatures reach the set range, first supply deionized water at a supply rate of 70 mL / min, and after stable operation for about 30 min, switch to supplying the slurry. The dried second intermediate powder is obtained at the outlet.

[0131] (4) Take 6 kg of deionized water, and add 2 kg of the second intermediate powder to the deionized water while stirring to obtain a third dispersion. After stirring for 2 h, add 0.5 kg of a fourth dispersion of polyurethane and single-walled carbon nanotubes (mass ratio of polyurethane to single-walled carbon nanotubes is 9:1, and the solvent is water) with a solid content of 20% to the third dispersion while stirring, and stir for 3 h. Finally, a slurry with a solid content of about 24.7% is obtained. The amount of polyurethane added is 4.5% based on the mass of the second intermediate, and the amount of single-walled carbon nanotubes added is 0.5%. Start the spray dryer, set the inlet temperature to 220°C and the outlet temperature to 105°C, and after the inlet and outlet temperatures reach the set range, first supply deionized water at a supply rate of 300 mL / min, and after stable operation for about 30 min, switch to supplying the slurry. The dried negative electrode material is obtained at the outlet.

[0132] wherein the mass percentage of lithium polyacrylate is 4.05%, the mass percentage of the first conductive material is 0.45%, the mass percentage of polyurethane is 4.14%, and the mass percentage of the second conductive material is 0.46%, based on the mass of the negative electrode material.

[0133] A schematic structural diagram of the negative electrode material prepared in Example 1-1 is shown in FIG. 1. Figure 1

[0134] Examples 1-2 to 1-11

[0135] Except that the mass percentage of carbon, the mass percentage of organic lithium salt, the mass percentage of the first conductive material, the mass ratio of organic lithium salt to the first conductive material, the mass percentage of polyurethane, the mass percentage of the second conductive material, and the mass ratio of polyurethane to the second conductive material in the first layer are adjusted according to Table 1 in the preparation of the negative electrode material, the rest is the same as Example 1-1. ​

[0136] Example 1-12

[0137] Except that the silicon-carbon particles are replaced with silicon-oxygen particles in the <Preparation of the negative electrode material>, and except that the silicon content of the silicon-oxygen particles is 49.4%, the specific surface area is 3.8 m 2 / g, the Dv50 is 2.1 μm, and the Dv90 is 7.5 μm, the rest is the same as Example 1-1.

[0138] Example 1-13

[0139] Except that the lithium polyacrylate is replaced with lithium polyacrylate-acrylamide in the <Preparation of the negative electrode material>, the rest is the same as Example 1-1.

[0140] Example 1-14

[0141] Except that the single-walled carbon nanotubes of the first conductive material are replaced with multi-walled carbon nanotubes in the <Preparation of the negative electrode material>, the rest is the same as Example 1-1.

[0142] Example 1-15

[0143] Except that the single-walled carbon nanotubes of the first conductive material are replaced with conductive carbon black in the <Preparation of the negative electrode material>, the rest is the same as Example 1-1.

[0144] Example 1-16

[0145] Except that the single-walled carbon nanotubes of the second conductive material are replaced with multi-walled carbon nanotubes in the <Preparation of the negative electrode material>, the rest is the same as Example 1-1.

[0146] Example 1-17

[0147] Except that the single-walled carbon nanotubes of the second conductive material are replaced with conductive carbon black in the <Preparation of the negative electrode material>, the rest is the same as Example 1-1.

[0148] Example 2-1

[0149] <Preparation of the positive electrode sheet>

[0150] The positive electrode active material LiCoO2, conductive carbon black, and polyvinylidene fluoride (PVDF) are mixed uniformly in a mass ratio of 95:2.5:2.5 in an N-methylpyrrolidone solvent system by fully stirring, to prepare a positive electrode slurry. The negative electrode slurry is uniformly coated on one surface of a negative electrode current collector copper foil with a thickness of 6 μm, and dried at 120°C, to obtain a negative electrode sheet with a single-sided coated negative electrode material layer, with a coating weight of the negative electrode material layer of 142 mg / 1540 mm 2Then, the above steps are repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of negative electrode material. After drying at 120℃, it is cold-pressed, then cut and welded with tabs to obtain a negative electrode sheet with a size of 78mm×875mm for use. The thickness of the single-sided negative electrode material layer is 54.5μm.

[0151] <Preparation of Negative Electrode Sheets>

[0152] Graphite, the negative electrode material prepared according to Examples 1-1, and the conductive agent (conductive carbon black, Super) were used. A negative electrode slurry with a solid content of 70 wt% was prepared by dissolving the positive electrode slurry and binder PAA in a ratio of 80:10:5:5 in deionized water. Deionized water was then added to adjust the viscosity of the slurry to 5000 Pa·s. The positive electrode slurry was then uniformly coated onto one surface of a 10 μm thick aluminum foil current collector and dried at 120 °C to obtain a positive electrode sheet with a single-sided coating of positive electrode material. The coating weight of the positive electrode material layer was 267.8 mg / 1540 mm². 2 Then, repeat the above steps on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode material. After drying at 120℃ and cold pressing, the sheet is cut and tabs are welded to obtain a positive electrode sheet with dimensions of 74mm × 867mm for later use. The thickness of the single-sided positive electrode material layer is 42μm. <Preparation of Electrolyte>

[0153] In a dry argon atmosphere, propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC) are mixed in a mass ratio of 1:1:1 to obtain a base solvent. Lithium salt LiPF6 is then added and mixed thoroughly to obtain an electrolyte. The electrolyte contains 12.5% ​​lithium salt LiPF6 based on the total mass of the electrolyte, with the remainder being the base solvent.

[0154] <Isolation membrane>

[0155] A polyethylene / polypropylene composite film with a thickness of 8μm was used as the separator.

[0156] <Preparation of Lithium-ion Batteries>

[0157] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The cells are then wound to obtain a bare cell. After welding the tabs, the bare cell is placed in an outer packaging foil aluminum-plastic film. The prepared electrolyte is injected into the dried bare cell. The lithium-ion battery is obtained through processes such as vacuum sealing, settling, formation, shaping, degassing, and capacity testing.

[0158] Examples 2-2 to 2-17

[0159] The rest is the same as Example 2-1 except that the negative electrode material in <Preparation of negative electrode sheet> is selected from the negative electrode materials prepared in Comparative Examples 1-1 to 1-3.

[0160] Comparative Example 1

[0161] The rest is the same as Example 1-1 except that the silicon-based particles have no coating layer in <Preparation of negative electrode material>, and the specific data are shown in Table 1.

[0162] Comparative Example 2

[0163] The rest is the same as Example 1-1 except that the silicon-based particles have no second layer of organic lithium salt coating in <Preparation of negative electrode material>, and the specific data are shown in Table 1.

[0164] Comparative Example 3

[0165] The rest is the same as Example 1-1 except that the silicon-based particles have no third layer of polymer coating in <Preparation of negative electrode material>, and the specific data are shown in Table 1.

[0166] Comparative Examples 4 to 6

[0167] The rest is the same as Example 2-1 except that the negative electrode material in <Preparation of negative electrode sheet> is selected from the negative electrode materials prepared in Comparative Examples 1-1 to 1-3.

[0168] Examples 3-1 to 3-15

[0169] The rest is the same as Example 2-1 except that difluoroethylene carbonate (DFEC) and fluoroethylene carbonate (FEC) are added to the electrolyte according to Table 3 in the preparation of electrolyte, and the mass percentage of DFEC and FEC is adjusted, the mass percentage of the base solvent is changed accordingly, the mass ratio of each component of the base solvent remains unchanged, and the mass percentage of lithium salt LiPF6 remains unchanged.

[0170] The preparation parameters and performance parameters of each example and comparative example are shown in Tables 1 to 3.

[0171]

[0172]

[0173] As can be seen from Examples 1-1 to 1-17, Examples 2-1 to 2-17 and Comparative Examples 1 to 6, when the negative electrode material has the three-layer coating structure of the present application, the negative electrode material has a lower resistivity, a higher silicon content, and has a higher first cycle charge-discharge efficiency and a lower first cycle full intercalation lithium expansion rate; the obtained lithium ion battery has a higher cycle number and a lower lithium ion battery expansion rate at room temperature and at high temperature, thereby indicating that the cycle performance of the lithium ion battery is improved and the expansion rate is reduced.

[0174] As can be seen from the description of the accompanying Figures 2 to 5 As can be seen from the description of the accompanying

[0175] As can be seen from Examples 1-1 to 1-17, Examples 2-1 to 2-17, when the silicon content of the negative electrode material is 40% to 50%, the Dv50 is 6 μm to 10 μm, the Dv90 is less than or equal to 30 μm, the mass percentage of carbon in the first layer is 1% to 5%, the mass percentage of organic lithium salt is 0.9% to 9%, the mass percentage of polyurethane is 0.9% to 9%, and / or the mass percentage of the second conductive material is 0.1% to 1%, the negative electrode material has a lower resistivity, a higher silicon content, and has a higher first cycle charge-discharge efficiency and a lower first cycle full intercalation lithium expansion rate, the obtained lithium ion battery has a higher cycle number and a lower lithium ion battery expansion rate at room temperature and at high temperature, thereby indicating that the cycle performance of the lithium ion battery is improved and the expansion rate is reduced.

[0176] Table 3

[0177]

[0178]

[0179] Note: " / " in Table 4 indicates that the corresponding preparation parameter, substance or performance parameter does not exist.

[0180] The mass percentage of the type of additive in the electrolyte usually affects the cycle performance of the lithium ion battery. As can be seen from Examples 1-1, Examples 3-1 to 3-15, when the additive is added to the electrolyte and the mass percentage is 0.5% to 10%, the lithium ion battery prepared using the negative electrode material provided by the present application has higher room temperature and high temperature cycle performance.

[0181] The above description is only the preferred embodiment of the present application, and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the principle of the present application shall be included in the scope of protection of the present application.

Claims

1. A negative electrode material, comprising: a core, the core comprising silicon-based particles; a first layer, the first layer being present outside the core; a second layer, the second layer being present outside the first layer; a third layer, the third layer being present outside the second layer; wherein the first layer comprising carbon; the second layer comprising a first conductive material and an organic lithium salt; the third layer comprising a second conductive material and a polymer.

2. The negative electrode material of claim 1, wherein, the silicon-based particles comprising at least one of silicon-carbon particles or silicon-oxygen particles; or the organic lithium salt comprising at least one of a polyacrylic lithium salt; or the polymer comprising polyurethane; or the first conductive material and the second conductive material are each independently selected from at least one of single-walled carbon nanotubes, multi-walled carbon nanotubes or conductive carbon black.

3. The negative electrode material of claim 2, wherein, the polyacrylic lithium salt comprising at least one of lithium polyacrylate or lithium polyacrylate-acrylamide.

4. The negative electrode material according to any one of claims 1 to 3, wherein, The mass percentage of silicon element is 40% to 50% based on the mass of the negative electrode material.

5. The negative electrode material according to any one of claims 1 to 3, wherein, The negative electrode material satisfies at least one of the following: (1) the Dv50 of the negative electrode material is 6 μm to 10 μm; (2) the Dv90 of the negative electrode material is less than or equal to 30 μm; (3) the mass percentage of carbon in the first layer is 1% to 5% based on the mass of the negative electrode material; (4) the mass percentage of the organic lithium salt is 0.9% to 9% based on the mass of the negative electrode material; (5) the mass percentage of the polymer is 0.9% to 9% based on the mass of the negative electrode material; (6) the mass percentage of the first conductive material is 0.1% to 1% based on the mass of the negative electrode material; (7) the mass percentage of the second conductive material is 0.1% to 1% based on the mass of the negative electrode material.

6. The negative electrode material of claim 5, wherein, The mass ratio of the first conductive material to the organic lithium salt is 1:9 to 2:9, and the mass ratio of the second conductive material to the polymer is 1:9 to 2:

9. 7.A method for preparing the negative electrode material according to any one of claims 1 to 6, comprising: providing silicon-based particles; carbon-coating the silicon-based particles to obtain a first intermediate; dispersing the first intermediate in water to obtain a first dispersion, dispersing an organic lithium salt and a first conductive material in water to obtain a second dispersion, mixing the first dispersion and the second dispersion and stirring uniformly, and obtaining a second intermediate by spray drying; dispersing the second intermediate in water to obtain a third dispersion, dispersing a polymer and a second conductive material in water to obtain a fourth dispersion, mixing the third dispersion and the fourth dispersion and stirring uniformly, and obtaining a negative electrode material by spray drying.

8. The production method according to claim 7, wherein The carbon-coating is placing the silicon-based particles in a fluidized bed, passing a carbon source gas at a temperature of 500 ℃ to 650 ℃, and reacting for 2 h to 3 h to obtain the first intermediate, wherein the carbon source gas comprises at least one of acetylene, methane or propylene. 9.An electrochemical device, comprising a positive electrode sheet, a separator film, an electrolyte and a negative electrode sheet, the negative electrode sheet comprising a negative electrode active material layer, the negative electrode active material layer comprising the negative electrode material according to any one of claims 1 to 6.

10. The electrochemical device of claim 9, wherein, The electrolyte contains an additive comprising at least one of difluoroethylene carbonate or fluoroethylene carbonate, the mass percentage of the additive being 0.5% to 10% based on the mass of the electrolyte.

11. The electrochemical device of claim 10, wherein, The mass percentage of the additive is 2% to 7% based on the mass of the electrolyte.

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