Negative electrode material, manufacturing method therefor and electrochemical apparatus

Abstract

Provided in the present application are a negative electrode material, a manufacturing method therefor, and an electrochemical apparatus. The negative electrode material comprises: an inner core, the inner core comprising silicon-based particles; a first layer comprising carbon, the first layer being outside the inner core; a second layer comprising an inorganic lithium salt, the second layer being outside the first layer; and a third layer comprising a conductive material and a polymer, the third layer being outside the second layer. The negative electrode material provided in the present application can effectively alleviate fragmentation and pulverization of silicon-carbon negative electrode materials during cycling and inhibit the growth of SEI films, thereby reducing volume expansion of electrochemical apparatuses after cycling, and improving the cycling performance of electrochemical apparatuses.

Description

A negative electrode material, a preparation method thereof, and an electrochemical device

[0001] The present application claims priority to the Chinese patent application No. 202410850565.2 filed on June 27, 2024, and entitled "A negative electrode material, a preparation method thereof, and an electrochemical device", the whole content of which is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application relates to the field of electrochemical energy storage, in particular to a negative electrode material, a preparation method thereof, and an electrochemical device. BACKGROUND

[0003] Graphite is the most widely used negative electrode material, which has the advantages of high efficiency and stable charge-discharge platform. However, the low 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 high theoretical specific capacity and suitable working voltage.

[0004] 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 can cause the destruction and growth of the solid electrolyte interface (SEI) film and the fragmentation of silicon-based particles, resulting in huge electrode expansion and rapid capacity decay, which affects the thickness and cycle performance of the electrochemical device and restricts the large-scale application of silicon negative electrode materials in electrochemical devices. SUMMARY

[0005] The present application aims to provide a negative electrode material and a preparation method thereof, and an electrochemical device using the negative electrode material, so as to improve the cycle performance of the electrochemical device.

[0006] 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 inorganic lithium salt; and the third layer comprises conductive material and polymer.

[0007] In an embodiment of the present application, the silicon-based particles include at least one of silicon-carbon particles or silicon-oxygen particles.

[0008] In an embodiment of the present application, the inorganic lithium salt comprises at least one of lithium fluoride and lithium carbonate.

[0009] In an embodiment of the present application, the polymer comprises polyurethane.

[0010] In an embodiment of the present application, the conductive material comprises at least one of single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (MWCNT), and conductive carbon black.

[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 Dv50 of the negative electrode material is 6 μm to 10 μm.

[0013] In an embodiment of the present application, the Dv90 of the negative electrode material is less than or equal to 30 μm.

[0014] 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.

[0015] In an embodiment of the present application, the mass percentage of inorganic lithium salt is 0.5% to 2% based on the mass of the negative electrode material.

[0016] In an embodiment of the present application, the mass percentage of polymer is 0.9% to 9% based on the mass of the negative electrode material.

[0017] In an embodiment of the present application, the mass percentage of conductive material is 0.1% to 1% based on the mass of the negative electrode material.

[0018] In an embodiment of the present application, the mass ratio of polymer to conductive material is 9:1 to 9:2 based on the mass of the negative electrode material.

[0019] A second aspect of the present application provides a preparation method of the negative electrode material of the first aspect of the present application, 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, adding an aqueous solution of a lithium source substance to the first dispersion to obtain a second dispersion, sufficiently dispersing and stirring, and obtaining a second intermediate by spray drying; dispersing the second intermediate in water to obtain a third dispersion, dispersing a polymer and a 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.

[0020] 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 ℃, and reacting for 2-3 h to obtain the first intermediate, wherein the carbon source gas comprises at least one of acetylene, methane, and propylene.

[0021] The third aspect of the present application provides an electrochemical device, comprising a positive electrode, a separator, an electrolyte and a negative electrode, the negative electrode comprises a negative electrode active material layer, and the negative electrode active material layer comprises the negative electrode material according to the first aspect of the present application.

[0022] In an embodiment of the present application, the electrolyte comprises an additive, the additive comprises at least one of lithium oxalate borate and lithium difluoro oxalate borate, and the mass percentage of the additive in the total mass of the electrolyte is 0.01% to 1%.

[0023] In an embodiment of the present application, the mass percentage of the additive in the total mass of the electrolyte is 0.1% to 0.6%.

[0024] The fourth aspect of the present application provides an electronic device comprising the electrochemical device according to the third aspect of the present application.

[0025] The present application has the following advantages:

[0026] 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; and a third layer, the third layer existing outside the second layer; wherein the first layer comprises carbon; the second layer comprises inorganic lithium salt; and the third layer comprises conductive material and polymer. At least part of the outside of the silicon-based particles as the core is coated and protected by amorphous carbon, effectively avoiding the problem of gas production caused by the direct contact of the silicon-based particles with water during stirring; the coating of the inorganic lithium salt can change the composition of the SEI film and reduce the thickness of the SEI, improve the lithium ion transference rate of the SEI, and improve the cycle performance of the electrochemical device; the coating of the polymer and the conductive material can inhibit the volume expansion of the silicon-carbon particles, reduce the thickness expansion rate of the electrochemical device, and at the same time, play the role of secondary granulation, reduce the specific surface area of the negative electrode material, and improve the first cycle charge-discharge efficiency. Through the synergistic effect of the three coating layers, the fragmentation and pulverization of the silicon-carbon negative electrode material during the cycle process can be effectively alleviated, and the growth of the SEI film can be inhibited, the volume expansion of the electrochemical device after the cycle can be reduced, and the cycle performance of the electrochemical device can be improved.

[0027] Of course, implementing any product or method of the present application does not necessarily require achieving all the advantages described above at the same time. BRIEF DESCRIPTION OF DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. 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 according to these drawings.

[0029] FIG. 1 is a structural schematic diagram of the negative electrode material prepared in Example 1-1.

[0030] FIG. 2 is a comparison diagram of the cycle performance of the lithium ion batteries of Example 2-1 and Comparative Examples 4 to 6 at 25°C.

[0031] FIG. 3 is a comparison diagram of the cycle performance of the lithium ion batteries of Example 2-1 and Comparative Examples 4 to 6 at 45°C.

[0032] FIG. 4 is a comparison diagram of the expansion rate of the lithium ion batteries of Example 2-1 and Comparative Examples 4 to 6 at 25°C.

[0033] FIG. 5 is a comparison diagram of the expansion rate of the lithium ion batteries of Example 2-1 and Comparative Examples 4 to 6 at 45°C. DETAILED DESCRIPTION

[0034] The technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on the present application belong to the scope of protection of the present application.

[0035] It should be noted that in the specific embodiments of the present application, the lithium ion battery is taken as an example of the electrochemical device to explain the present application, but the electrochemical device of the present application is not limited to the lithium ion battery.

[0036] 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 inorganic lithium salt; and the third layer comprises conductive material and polymer. Specifically, the structure of the negative electrode material is shown in FIG. 1, and from inside to outside, it is the core 11, the first layer 12, the second layer 13, and the third layer 14. The first layer can protect the active silicon on the silicon-based particles, and isolate the direct contact between water and active silicon during stirring, thereby preventing gas production. The second layer can inhibit the direct contact between the electrolyte and the active silicon, and can also change the composition of the SEI, reduce the thickness of the SEI, improve the lithium ion transference rate of the SEI, and improve the cycle performance of the electrochemical device. The third layer can inhibit the volume expansion of the silicon-carbon particles, reduce the thickness expansion rate of the electrochemical device, and also reduce the specific surface area of the negative electrode material through secondary granulation, thereby improving the first charge-discharge efficiency. The application of the negative electrode material with the above three coating layers can synergistically improve the energy density of the electrochemical device and optimize the cycle performance. In an embodiment of the present application, a mutual solubility layer can be formed between the core and the first layer. In another embodiment of the present application, a mutual solubility layer can be formed between the first layer and the second layer. In another embodiment of the present application, a mutual solubility layer can 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.

[0037] In an embodiment of the present application, the silicon-based particles include at least one of silicon-carbon particles or silicon-oxygen particles.

[0038] In an embodiment of the present application, the inorganic lithium salt comprises at least one of lithium fluoride or lithium carbonate.

[0039] In an embodiment of the present application, the polymer comprises polyurethane.

[0040] In an embodiment of the present application, the conductive material comprises at least one of single-walled carbon nanotubes, multi-walled carbon nanotubes, or conductive carbon black.

[0041] After the silicon-based particles are carbon-coated by chemical vapor deposition (CVD) of a carbon source gas, at least part of the outer side of the silicon-based particles is protected by an amorphous carbon layer, which can avoid the problem of gas production caused by direct contact of the silicon-based particles with water during stirring. Preferably, the entire outer side of the silicon-based particles is coated with amorphous carbon. The coating of inorganic lithium salt can improve the lithium ion migration rate of SEI and improve the cycle performance of the electrochemical device. The coating of polymer and conductive material can inhibit the volume expansion of the silicon-carbon particles, reduce the thickness expansion rate of the electrochemical device, alleviate the problem of reduced electrical conductivity of the negative electrode material, and at the same time, play a secondary granulation role, reduce the specific surface area of the negative electrode material, improve the first circle charge-discharge efficiency, and further improve the cycle performance of the electrochemical device.

[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 formed by any two of the above values. By adjusting the mass percentage of silicon element within the above range, the negative electrode material has a high reversible capacity and first circle efficiency, and is beneficial to improve the energy density and cycle performance of the electrochemical device.

[0043] In an embodiment of the present application, the negative electrode material satisfies at least one of the following conditions: (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 inorganic lithium salt is 0.5% to 2% based on the mass of the negative electrode material; (5) the mass percentage of polymer is 0.9% to 9% based on the mass of the negative electrode material; (6) the mass percentage of conductive material is 0.1% to 1% based on the mass of the negative electrode material. This is beneficial to further control the specific surface area of the negative electrode material, improve the electrical conductivity of the negative electrode material, improve the first circle charge-discharge efficiency, and further improve the cycle performance and reduce the expansion rate of the electrochemical device.

[0044] 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 formed by any two of the above values. By controlling the particle size Dv50 of the negative electrode material to be 6 μm to 10 μm, it is beneficial to shorten the diffusion path of Li + + ​The ion conductivity can be improved, the first circle efficiency of the negative electrode material can be improved, the defects of increased consumption of electrolyte and low material compaction density can be improved, and the risk of low ion conductivity caused by large particle size can be reduced.

[0045] 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. By adjusting the particle size Dv90 of the secondary particles to be 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 transmission of Li +

[0046] 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.

[0047] In an embodiment of the present application, the mass percentage of inorganic lithium salt is 0.5% to 2% based on the mass of the negative electrode material. For example, the mass percentage of inorganic lithium salt can be 0.5%, 1%, 1.5%, 2%, or a range between any two of them.

[0048] In an embodiment of the present application, the mass percentage of polymer is 0.9% to 9% based on the mass of the negative electrode material. For example, the mass percentage of polymer can be 0.9%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or a range between any two of them.

[0049] In an embodiment of the present application, the mass percentage of conductive material is 0.1% to 1% based on the mass of the negative electrode material. For example, the mass percentage of polymer 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.

[0050] ​In an embodiment of the present application, the mass ratio of the polymer to the conductive material is 9:1 to 9:2. For example, the mass ratio of the conductive material to the polymer 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 between any two of the values.

[0051] The second aspect of the present application provides a preparation method of the negative electrode material of the first aspect of the present application, 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, adding an aqueous solution containing a lithium source substance to the first dispersion to obtain a second dispersion, fully dispersing and stirring, and obtaining a second intermediate by spray drying; dispersing the second intermediate in water to obtain a third dispersion, dispersing a polymer and a 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 by spray drying. The preparation method is simple in steps, easy to operate, and suitable for industrial production.

[0052] Without being limited to any theory, the lithium source substance can be lithium fluoride. The inventors believe that, by successively adding aqueous solutions of ammonium fluoride and a lithium source compound to the first dispersion and fully stirring to generate lithium fluoride, the lithium fluoride can be more uniformly coated on the outside of the first intermediate. The lithium source compound comprises at least one of lithium acetate, lithium nitrate, and lithium sulfate.

[0053] Without being limited to any theory, the lithium source substance can be lithium carbonate. The inventors believe that, by first dissolving a lithium source compound in an aqueous solution, adding the aqueous solution to the first dispersion, and then passing carbon dioxide, the lithium carbonate can be more uniformly coated on the outside of the first intermediate. The lithium source compound comprises lithium hydroxide.

[0054] 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 0.5 m 2 / g to 10 m 2 / g; (2) the particle size Dv50 of the silicon-based particles is 1.5 μm to 2.5 μm; and (3) the particle size Dv90 of the silicon-based particles is less than or equal to 10 μm. The particle classification technology is not particularly limited in the present application, as long as the purpose of the present application can be achieved. The particle classification technology can be any known classification means in the art, such as fluidized classification and cyclone classification.

[0055] In an embodiment of the present application, the specific surface area of the silicon-based particles can be 0.5 m 2 / g, 1 m 2 / g, 2 m 2 / g, 3 m 2 / g, 4 m2 / g, 5m 2 / g, 6m 2 / g, 7m 2 / g, 8m 2 / g, 9m 2 / g, 10m 2 / g, or a range formed by any two of the above values; the shape of the silicon-based particles can be at least one of spherical, spheroidal, flaky, or massive. By regulating the specific surface area of the silicon-based particles within the above range, the reactivity of the silicon-based particles with water during stirring is inhibited, while a high degree of coverage of the outer side of the silicon-based particles can be achieved. If the specific surface area is too large (greater than 10 m 2 / g), the risk of exposure of active silicon increases.

[0056] In an embodiment of the present application, the particle size Dv50 of the silicon-based particles can be 1.5 pm, 1.6 pm, 1.7 pm, 1.8 pm, 1.9 pm, 2 pm, 2.1 pm, 2.2 pm, 2.3 pm, 2.4 pm, 2.5 pm, or a range formed by any two of the above values. By regulating the particle size Dv90 of the silicon-based particles within the above range, the diffusion path of Li + is shortened, the ionic conductivity of the negative electrode material is improved, and the volume expansion of the silicon-based particles is reduced.

[0057] In an embodiment of the present application, the particle size Dv90 of the silicon-based particles can be 3 pm, 4 pm, 5 pm, 6 pm, 7 pm, 8 pm, 9 pm, 10 pm, or a range formed by any two of the above values. By regulating the particle size Dv90 of the silicon-based particles within the above range, the particle size of the negative electrode material that undergoes the coating process is controlled, the transport of Li + is improved, and thus the cycle performance and expansion performance of the lithium ion battery are improved.

[0058] In an embodiment of the present application, after the carbon source gas and the inert gas are mixed and introduced into the fluidized bed, the inert gas includes at least one of nitrogen or argon; the volume percentage content of the carbon source gas is 10% to 50% based on the total volume of the mixed gas. For example, the volume percentage content of the carbon source gas can be 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or a range formed by any two of the above values. The provision of the inert gas is beneficial to improving the uniformity of the distribution of carbon elements in the first layer.

[0059] In an embodiment of the present application, the carbon coating is performed by placing the silicon-based particles in a fluidized bed, passing a carbon source gas at a temperature of 500-650°C for 2-3h to obtain a first intermediate, 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, 550°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 can be 2h, 2.2h, 2.4h, 2.6h, 2.8h, 3h, or a range defined by any two of the above values. By controlling the carbon coating time and temperature, the content of carbon material in the carbon coating process can be controlled. After the carbon source gas is cracked at high temperature, it is deposited on the outside of the silicon-based particles. Within a suitable temperature range, the higher the deposition temperature, the faster the cracking rate of the carbon source gas, the higher the utilization rate, and the faster the growth rate of the carbon-containing first layer. At the same deposition temperature, the longer the deposition, the thicker and more complete the carbon-containing first layer.

[0060] In an embodiment of the present application, the solid content of the first dispersion is 15-20%. For example, the solid content of the first dispersion can be 15%, 16%, 17%, 18%, 19%, 20%, or a range defined by any two of the above values. By adjusting the solid content of the first intermediate in deionized water within the above range, the particle size distribution of the second intermediate after spray drying can be controlled while taking into account the drying capacity and productivity of the machine, effectively reducing the risk of particle size increase caused by particle adhesion.

[0061] In an embodiment of the present application, the concentration of the lithium source compound aqueous solution is 1-10%. For example, the concentration of the lithium source compound aqueous solution can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range defined by any two of the above values.

[0062] In an embodiment of the present application, the mass ratio of the first intermediate to the inorganic lithium salt is 100:0.5-100:2. For example, the mass ratio can be 100:0.5, 100:0.6, 100:0.7, 100:0.8, 100:0.9, 100:1, 100:1.5, 100:2, or a range defined by any two of the above values. By adjusting the mass ratio of the first intermediate to the inorganic lithium salt within the above range, the composition of the SEI film can be changed and the thickness of the SEI can be reduced, the lithium ion mobility of the SEI can be improved, and the cycle performance of the electrochemical device can be improved, while the mass percentage of silicon in the final secondary particles is not significantly reduced, thereby ensuring that the secondary particles have sufficient reversible capacity.

[0063] In an embodiment of the present application, the solid content of the second dispersion is 8% to 20%, for example, the solid content of the second dispersion can be 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, or a range between any two of the aforementioned values. By regulating the solid content within the aforementioned range, the particle size distribution of the secondary particles after spray drying can be controlled while taking into account the drying capacity of the equipment and the production capacity.

[0064] In an embodiment of the present application, the stirring and dispersing time of the second dispersion is 3h to 6h, for example, the stirring and dispersing time can be 3h, 4h, 5h, 6h, or a range between any two of the aforementioned values. By regulating the stirring and dispersing time within the aforementioned range, sufficient stirring can ensure that the first intermediate and the lithium source substance in the aqueous solution are fully and uniformly contacted, thereby improving the uniformity of the second layer.

[0065] In an embodiment of the present application, the inlet air temperature of the spray drying of the second intermediate is 180°C to 220°C, and the outlet air temperature is 100°C to 110°C. By regulating the parameters of the spray drying within the aforementioned range, the particle size distribution of the second intermediate after spray drying can be controlled while taking into account the drying capacity of the equipment and the production capacity, thereby effectively reducing the risk of particle size increase caused by particle adhesion.

[0066] In an embodiment of the present application, the particle size Dv50 of the second intermediate particles is 1.7μm to 2.7μm; the particle size Dv90 of the second intermediate particles is less than or equal to 10.2μm. Compared with the first intermediate, the particle size of the second intermediate particles increases, which is due to the fact that spray drying can cause some particles to adhere to each other, thereby increasing the particle size.

[0067] In an embodiment of the present application, the particle size Dv50 of the second intermediate particles can be 1.7μm, 1.8μm, 1.9μm, 2μm, 2.1μm, 2.2μm, 2.3μm, 2.4μm, 2.5μm, 2.6μm, 2.7μm, or a range between any two of the aforementioned values.

[0068] In an embodiment of the present application, the particle size Dv90 of the second intermediate particles can be 3.2μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 10.2μm, or a range between any two of the aforementioned values.

[0069] In an embodiment of the present application, the solid content of the third dispersion is 20% to 30%. For example, the solid content can be 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, or a range defined by any two of the aforementioned values. By regulating the solid content of the second intermediate in deionized water within the aforementioned range, the particle size distribution of the secondary particles after spray drying can be controlled while taking into account the drying capacity of the machine and the production capacity.

[0070] In an embodiment of the present application, the mass ratio of the polymer and the conductive material is 9:1 to 9:2. For example, the mass ratio of the polymer and the 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 aforementioned values. The introduction of the conductive material can alleviate the problem of reduced electrical conductivity of the negative electrode material caused by the polymer.

[0071] In an embodiment of the present application, the solid content of the fourth dispersion is 18% to 22%. For example, the solid content can be 18%, 19%, 20%, 21%, 22%, or a range defined by any two of the aforementioned values. The addition of the conductive material can alleviate the problem of reduced electrical conductivity of the material powder caused by the polymer.

[0072] In an embodiment of the present application, the total mass of the mixture of the second intermediate, the polymer, and the conductive material is 100:1 to 100:10. For example, the total mass of the mixture of the second intermediate, the polymer, and the 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. By regulating the ratio within the aforementioned range, the polymer can play a role in secondary granulation, and at the same time, due to the strong coating of the polymer, the volume expansion of the silicon-carbon negative electrode particles is inhibited, and the thickness expansion rate of the electrochemical device is reduced.

[0073] 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 can be 20%, 21%, 22%, 23%, 24%, 25%, or a range defined by any two of the aforementioned values.

[0074] In an embodiment of the present application, the stirring and dispersing time of the negative material is 3h to 6h, for example, the stirring and dispersing time can be 3h, 4h, 5h, 6h or a range between any two of the above values. Sufficient stirring can make the mixture of the second intermediate, the polymer and the conductive material in the aqueous solution fully and uniformly contact, and improve the uniformity of the third layer. For example, the stirring and dispersing time can be 3h, 4h, 5h, 6h or a range between any two of the above values.

[0075] In an embodiment of the present application, the inlet air temperature of the spray drying of the negative material is 200-230℃, and the outlet air temperature is 100-110℃. For example, the inlet air temperature can be 200℃, 205℃, 210℃, 215℃, 220℃, 225℃, 230℃ or a range between any two of the above values; the outlet air temperature can be 100℃, 102℃, 105℃, 108℃, 110℃ or a range between any two of the above values. The parameters of the spray drying are within the above ranges, and the particle size distribution of the secondary particles after spray drying is controlled under the consideration of the drying capacity and production capacity of the machine.

[0076] In an embodiment of the present application, the coating referred to in the present application can be partial coating or complete coating.

[0077] The third aspect of the present application provides an electrochemical device, which comprises a positive electrode, a separator, an electrolyte and a negative electrode, the negative electrode comprises a negative electrode active material layer, and the negative electrode active material layer comprises the negative material of the first aspect of the present application.

[0078] In an embodiment of the present application, the electrolyte comprises an additive, the additive comprises at least one of lithium oxalate borate and lithium difluoro oxalate borate, and the mass percentage of the additive in the total mass of the electrolyte is 0.01% to 1%. For example, the mass percentage of the additive can be 0.01%, 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, 0.85%, 0.9%, 0.95%, 1% or a range between any two of the above values. The above additive can interact with the negative material during the cycle of the lithium ion battery, further improve the migration rate of lithium ions, and improve the cycle performance of the electrochemical device.

[0079] In an embodiment of the present application, the electrolyte includes an additive, the additive includes at least one of lithium oxalate borate, lithium difluoro oxalate borate, and the mass percentage of the additive is 0.1% to 0.6% based on the total mass of the electrolyte. For example, the mass percentage of the additive can be 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, or a range between any two of them.

[0080] In the electrochemical device of the present application, the electrolyte further includes a lithium salt and a non-aqueous solvent.

[0081] In some embodiments of the present application, the lithium salt is selected from one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium tetraphenylborate (LiB(C6H5)4), lithium methylsulfonate (LiCH3SO3), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis-trifluoromethanesulfonimide (LiN(SO2CF3)2), lithium hexafluorosilicate (LiSiF6), lithium bisoxalate borate (LiBOB), and lithium difluoroborate (LiF2OB). For example, the lithium salt can be LiPF6, because it has a high ionic conductivity and improves the cycle characteristics. 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 between any two of them.

[0082] The non-aqueous solvent can be a carbonate compound, a carboxylic acid ester compound, an ether compound, other organic solvents, or a combination thereof.

[0083] The carbonate compound described above can be a chain carbonate compound, a cyclic carbonate compound, a fluorinated carbonate compound, or a combination thereof.

[0084] Examples of the above-mentioned chain carbonate compounds are dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (EMC), and combinations thereof. Examples of the above-mentioned cyclic carbonate compounds are ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinyl ethylene carbonate (VEC), and combinations thereof. Examples of the above-mentioned fluorinated carbonate compounds are fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methyl ethylene carbonate, 1-fluoro-1-methyl ethylene carbonate, 1,2-difluoro-1-methyl ethylene carbonate, 1,1,2-trifluoro-2-methyl ethylene carbonate, trifluoromethyl ethylene carbonate, and combinations thereof.

[0085] Examples of the above-mentioned carboxylic acid ester compounds are methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, gamma-butyrolactone, decalactone, valerolactone, methylvaleronolactone, hexalactone, and combinations thereof.

[0086] Examples of the above-mentioned ether compounds are dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, and combinations thereof.

[0087] Examples of the above-mentioned other organic solvents are dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidinone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, and combinations thereof.

[0088] In the present application, the electrochemical device further includes a negative electrode tab including 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 "negative electrode active material layer 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 in the thickness direction thereof, or can be disposed on both surfaces of the negative electrode current collector in the thickness direction thereof. It is to be noted that the "surface" herein can be the entire layer of the surface of the negative electrode current collector, or can be a partial 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 negative current collector is not particularly limited in the present application, and can be, for example, a copper foil, a copper alloy foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, or a composite current collector. The composite current collector can be, for example, a lithium-copper composite current collector, a carbon-copper composite current collector, a nickel-copper composite current collector, or a titanium-copper composite current collector.

[0090] In some embodiments of the present application, the negative active material layer can further include a conductive agent and a binder. The type of the conductive agent and the binder is not particularly limited in the present application, and can be, for example, polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, an ethylene oxide-containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, butadiene-styrene rubber, acrylated butadiene-styrene rubber, an epoxy resin, or nylon. The conductive agent can include, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, and mixtures thereof. The carbon-based materials can be selected from natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fibers, or any combination thereof. The metal-based materials can be selected from metal powder, metal fibers, copper, nickel, aluminum, or silver. The conductive polymer can be a polyphenylene derivative. The mass ratio of the negative active material, the conductive agent, and the binder in the negative active material layer is not particularly limited in the present application, and can be selected by a person skilled in the art according to actual needs, as long as the purpose of the present application can be achieved.

[0091] The thickness of the negative active material layer is not particularly limited in the present application, and can be, for example, 30 μm to 120 μm.

[0092] The thickness of the negative current collector is not particularly limited in the present application, and can be, for example, 4 μm to 15 μm.

[0093] In the present application, the electrochemical device further includes a positive electrode tab including a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer disposed on at least one surface of the positive current collector means that the positive active material layer can be disposed on one surface of the positive current collector in the thickness direction of the positive current collector, or can be disposed on both surfaces of the positive current collector in the thickness direction of the positive current collector. It should be noted that the surface can be the entire layer of the surface of the positive current collector, or can be a partial layer of the surface of the positive current collector. The present application is not particularly limited, as long as the purpose of the present application can be achieved.

[0094] The positive electrode current collector is not particularly limited in the present application, as long as the object of the present application can be achieved, for example, it can include an aluminum foil, an aluminum alloy foil, or a composite current collector (for example, an aluminum-carbon composite current collector), and the like.

[0095] The positive electrode active material is not particularly limited in the present application, as long as the object of the present application can be achieved, 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 phosphate, and lithium titanate.

[0096] The positive electrode active 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 in the present application, 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 active material layer is not particularly limited in the present application, 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.

[0097] The binder is not particularly limited in the present application, as long as the object of the present application can be achieved, for example, the binder can include, but is not limited to, at least one of an adhesive polymer, for example, polyvinylidene fluoride, polytetrafluoroethylene, a polyolefin-based, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, modified polyvinylidene fluoride, or modified butadiene-styrene rubber, wherein the polyolefin-based binder includes at least one of polyethylene, polypropylene, polyolefin ester, polyolefin alcohol, or polyacrylic acid.

[0098] The conductive agent is not particularly limited in the present application, as long as the object of the present application can be achieved, 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, and the like; and the conductive polymer includes a polyphenylene derivative.

[0099] The thickness of the positive electrode current collector and the positive electrode active material layer is not particularly limited in the present application, 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 active material layer is 30 μm to 120 μm.

[0100] 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 active material layer. The composition of the conductive layer is not particularly limited, and it 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 in the present application, for example, they can be at least one of the above-mentioned conductive agent and the above-mentioned binder.

[0101] 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 spunlaid film.

[0102] 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.

[0103] 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.

[0104] 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 alumina, silica, magnesia, titania, hafnia, tin oxide, ceria, nickel oxide, zinc oxide, calcium oxide, zirconia, yttria, 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. 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).

[0105] 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, decanoic acid methyl ester, or dodecyl acetate.

[0106] In the present application, the thickness of the separator film is not particularly limited as long as the purpose of the present application can be achieved, and for example, the thickness of the separator film can be 4 μm to 20 μm.

[0107] 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.

[0108] 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 are stacked in order, and then the four corners of the entire stack structure are fixed with adhesive tape to obtain an electrode assembly in a stack structure, the electrode assembly is placed in the case, the electrolyte solution is injected into the case and sealed to obtain the electrochemical device. In addition, a current protection element, a 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 or discharging.

[0109] The fourth aspect of the present application provides an electronic device comprising the electrochemical device of the third aspect of the present application.

[0110] The electronic device of the present application is not particularly limited and can be any electric device known in the art. In some embodiments, the electric 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 head-mounted stereo 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 recorder, 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, an electric tool, a flashlight, a camera, a household large storage battery, and a lithium ion capacitor, or the like.

[0111] Examples

[0112] Hereinafter, embodiments and comparative examples are presented to more specifically explain 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.

[0113] Test method and apparatus

[0114] Si element mass percentage content test

[0115] Preparation of digestion sample: take 0.1000g of silicon-based particles or negative electrode material, put it into a nickel crucible, add 1.5g 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 2h, and then to 400℃ in 2h. Then start to cool down, naturally cool down to 80℃, end the digestion program, take out the crucible, cool down to room temperature, take out the digested sample and place it in a F4 beaker.

[0116] Titration of the digestion sample with sodium hydroxide standard solution: add 30mL of boiling water in the F4 beaker, leach for 1h, then use tweezers to clean the crucible, control the volume to 50mL, then filter the above solution into a 400mL beaker. After filtering, add 20mL of concentrated nitric acid to the beaker at one time to neutralize the solution, making the solution acidic. After the solution cools to room temperature, add solid KCl to saturation with constant stirring, and add 2g in excess. Then add 10mL of potassium fluoride solution, a white precipitate appears, age for 15min, filter with medium-speed quantitative filter paper, wash the beaker and precipitate with 8mL 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 20mL 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 the process, use a glass rod to stir the paper pulp, react for 1h. Add 200mL of boiled and neutralized water to the cup (boil and add 10 drops of phenolphthalein, neutralize with sodium hydroxide standard solution to light red), titrate with sodium hydroxide standard solution to light red as the endpoint, record the volume V of sodium hydroxide standard solution consumed.

[0117] Titration of the blank sample with sodium hydroxide standard solution: the steps are the same as those of the titration of the digestion sample with sodium hydroxide standard solution, except that the digested sample is not added, a blank sample is prepared, and the volume V0 of sodium hydroxide standard solution consumed by the blank sample is recorded.

[0118] The mass percentage content of silicon element 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; m is the mass of the sample, in g.

[0119] Specific surface area test

[0120] The specific surface area of the negative electrode material particles of each example and the comparative example was tested by nitrogen adsorption method using a specific surface area analyzer (TriStar II 3020M, USA Micromeritics). 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".

[0121] Carbon mass percentage test

[0122] The negative electrode material was heated and combusted at high temperature under oxygen-rich conditions by a high-frequency furnace to oxidize carbon into carbon dioxide. The gas was treated and then entered the corresponding absorption cell, absorbed the corresponding infrared radiation, and then converted into a corresponding signal by the detector. The signal was sampled by the computer, converted into a value proportional to the carbon dioxide concentration after linear correction, and then the values of the entire analysis process were added. After the analysis was completed, the cumulative value was divided by the weight value in the computer, multiplied by the correction coefficient, and then the blank was deducted to obtain the mass percentage of carbon. The high-frequency infrared carbon and sulfur analyzer (Shanghai Deke HCS-140) was used for testing.

[0123] Powder conductivity test

[0124] The conductivity of each synthesized negative electrode material was tested in a dry room using a powder resistivity meter. The reciprocal of the obtained conductivity is the resistance (Ω.cm) of the negative electrode material.

[0125] Granulometry test

[0126] 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.

[0127] The preparation process of the coin-type half-cell includes:

[0128] (1) Preparation process of the positive electrode sheet: the negative electrode material prepared in the examples or the comparative example was used as the active material, acetylene black was used as the conductive agent, and sodium alginate was used as the binder. The mass ratio of the active material, acetylene black, and sodium alginate was 90:5:5. The active material and acetylene black were thoroughly mixed and ground according to the proportion, and the sodium alginate aqueous solution was added and stirred for 4 h according to the proportion. Finally, the mixture slurry was uniformly coated on a copper foil and vacuum dried at 70°C, and then punched into a circular electrode sheet with a diameter of 10 mm.

[0129] Electrolyte and separator: In an argon glove box with water content <10 ppm, propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC) were mixed in a ratio of 1:1:1 to obtain a base solvent, and lithium salt LiPF6 was added to dissolve and mix uniformly to obtain an electrolyte; wherein the mass percentage of lithium salt LiPF6 based on the total mass of the electrolyte was 12.5%, and the balance was the base solvent. A PE / PP composite film with a thickness of 8 μm was used as a separator.

[0130] (2) Assembly of button-type half battery: In an argon glove box with water content <10 ppm, a lithium metal sheet was used as a negative electrode sheet, and the negative electrode sheet, the above-prepared separator, and the above-prepared positive electrode sheet were stacked in order, with the separator between the positive electrode sheet and the negative electrode sheet to act as a separator. After assembly, the above-prepared electrolyte was injected to obtain a button-type half battery.

[0131] The test method for the first cycle specific charge capacity and the first cycle lithium intercalation expansion rate includes:

[0132] At 25°C and under normal pressure, the above-prepared button-type half battery was discharged at a 0.1C rate to 0.01V, and then rested for 5 min. The discharge specific capacity at this time was recorded, which was the first cycle discharge specific capacity. Then, the battery was charged at a 0.1C rate to 1.5V, and then charged at a constant voltage until the current was 0.05C. After resting for 5 min, one cycle of charge and discharge was completed, and the charge capacity at this time was recorded, which was the first cycle charge specific capacity.

[0133] The first cycle charge-discharge efficiency = the first cycle charge specific capacity / the first cycle discharge specific capacity x 100%.

[0134] First cycle lithium intercalation expansion rate test

[0135] Before assembling the button-type half battery, the thickness H0 of the positive electrode sheet was measured with a micrometer. At 25°C and under normal pressure, the above-prepared button-type half battery was discharged at a 0.1C rate to 0.01V, and then rested for 30 min. The button-type half battery was disassembled in the glove box, and the thickness H1 of the positive electrode sheet after lithium intercalation was measured with a micrometer. The first cycle lithium intercalation expansion rate of the positive electrode sheet of the button-type half battery = (H1-H0) / H0 x 100%.

[0136] Lithium ion battery cycle performance test and expansion rate test

[0137] The lithium ion battery prepared in the example or the comparative example was charged at 3.4C to 4.4V in a constant current mode, charged in a constant voltage mode to 0.025C, and discharged at 0.5C to 3.0V after standing for 5 minutes at 25℃ / 45℃. The capacity obtained in this step was the initial capacity of the lithium ion battery. The capacity decay curve was obtained by comparing the capacity of each cycle with the initial capacity, and the cycle was repeated to the 600th cycle to obtain FIGS. 2 and 3.

[0138] The cycle number at which the capacity retention rate was 90% at 25℃ was recorded as the room temperature cycle performance of the battery, and the cycle number at which the capacity retention rate was 80% at 45℃ was recorded as the high temperature cycle performance of the battery. The cycle performance of the lithium ion battery was compared by comparing the cycle numbers in the above two cases.

[0139] The relative difference between the thickness of the lithium ion battery after being cycled at 25℃ to the 400th cycle and discharged to 3.0V and the original thickness was recorded as the room temperature cycle expansion rate, and the relative difference between the thickness of the lithium ion battery after being cycled at 45℃ to the 400th cycle and discharged to 3.0V and the original thickness was recorded as the high temperature cycle expansion rate. The above cycles were performed at 25℃ and 45℃ to the 500th cycle to obtain FIGS. 4 and 5, which are the thickness expansion rate comparison diagrams of the lithium ion battery at 25℃ and 45℃, respectively.

[0140] At 25℃, the lithium ion battery was subjected to small rate charging and discharging when cycled to the 2nd, 4th, 50th, 100th, 150th, 200th, 300th, 400th, and 500th cycles. The process was as follows: the lithium ion battery was charged at 0.7C to 4.53V in a constant current mode, then charged at 4.53V to 0.05C in a constant voltage mode, and discharged at 0.2C to 3.0V after standing for 5 minutes, and then the next cycle was continued after standing for 5 minutes.

[0141] Example 1-1

[0142] <Preparation of a negative electrode material>

[0143] (1) 200 kg of silicon-carbon particles were taken and subjected to classification treatment using a jet classification (or cyclone classification), to obtain silicon-carbon particles with a particle size Dv50 of 1.97 μm and a Dv90 of 7.4 μm.

[0144] (2) 20 kg of the classified silicon-carbon particles were transferred into the fluidized bed by high-pressure transmission. After standing for about 30 min, the gas inlet valve and the tail gas valve of the fluidized bed were closed and vacuum was applied. When the pressure in the chamber reached -101 kPa, the vacuum was turned off and nitrogen was introduced at a rate of 100 L / min until the pressure became positive. This process was repeated for more than 5 times, and then the oxygen content in the chamber was detected. When the oxygen content was reduced to less than 10 ppm, the tail gas valve was opened, the stirring paddle of the fluidized bed was started at a speed of 150 rpm, and nitrogen was introduced at a rate of 200 L / min. The temperature was raised to 550°C at a rate of 5°C / min, and the temperature was maintained for 1 h. After the temperature maintenance, the fluidized bed gas inlet pipeline valve was switched, and acetylene / nitrogen mixed gas was introduced into the fluidized bed. The volume fraction of acetylene was 25%, and the gas flow rate was 200 L / min. The reaction time was 150 min. After the reaction, nitrogen was introduced at a flow rate of 150 L / min, the stirring paddle speed was adjusted to 100 rpm, and the temperature was lowered to room temperature. The material was discharged to obtain the first intermediate.

[0145] (3) 10 kg of deionized water was taken, and 2 kg of the first intermediate powder was added to the deionized water while stirring to obtain a first dispersion. After sufficient stirring for 1 h, 508.8 g of a 10% lithium acetate aqueous solution was added to the first dispersion while stirring, and after sufficient stirring for 1 h, 285.6 g of a 10% ammonium fluoride aqueous solution was added while stirring. After sufficient stirring for 4 h, a second dispersion with a solid content of about 16.8% was finally obtained. The spray dryer was started, the inlet air temperature was set to 200°C, and the outlet air temperature was set to 105°C. After the inlet air temperature and the outlet air temperature reached the set range, deionized water was supplied at a supply rate of 300 mL / min. After stable operation for about 30 min, the supply of the slurry was switched. The second intermediate powder after drying was obtained at the outlet.

[0146] (4) 6 kg of deionized water was taken, and 2 kg of the second intermediate powder was added to the deionized water while stirring to obtain a third dispersion. After sufficient stirring for 2 h, 0.5 kg of a fourth dispersion with a solid content of 20% (the mass ratio of polyurethane to single-walled carbon nanotubes was 9:1, and the solvent was deionized water) was added to the third dispersion while stirring. After sufficient stirring for 3 h, a slurry with a solid content of about 24.7% was finally obtained. The spray dryer was started, the inlet air temperature was set to 220°C, and the outlet air temperature was set to 105°C. After the inlet air temperature and the outlet air temperature reached the set range, deionized water was supplied at a supply rate of 300 mL / min. After stable operation for about 30 min, the supply of the slurry was switched. The dried anode material particles were obtained at the outlet.

[0147] The silicon-based particles prepared in Example 1-1 were selected as silicon-carbon particles, the mass percentage of silicon element was 50.1%, the specific surface area was 7.2 m 2Dv50 of 1.97 pm, Dv90 of 7.4 pm; the mass percentage of carbon was 3%, the mass percentage of LiF was 0.94%, the mass percentage of polyurethane was 4.3%, and the mass percentage of SWCNT was 0.48% based on the mass of the negative electrode material, as shown in Table 1, Example 1-1. The structural schematic diagram of the negative electrode material is shown in FIG. 1.

[0148] Examples 1-2 to 1-16

[0149] Except that the negative electrode material adjusts each relevant parameter according to Table 1, the rest is the same as Example 1-1.

[0150] Example 1-17

[0151] Except that the silicon-based particles in the preparation of the negative electrode material are selected to be silicon-oxygen particles with a mass percentage of silicon element of 49.4%, a specific surface area of 3.8 m 2 / g, Dv50 of 2.1 pm, Dv90 of 7.5 pm, the rest is the same as Example 1-1.

[0152] Comparative Example 1

[0153] Except that the silicon-based particles in the preparation of the negative electrode material are selected to be silicon-oxygen particles with a mass percentage of silicon element of 49.4%, a specific surface area of 3.8 m

[0154] Comparative Example 2

[0155] Except that the silicon-based particles in the preparation of the negative electrode material are selected to be silicon-oxygen particles with a mass percentage of silicon element of 49.4%, a specific surface area of 3.8 m

[0156] Comparative Example 3

[0157] Except that the silicon-based particles in the preparation of the negative electrode material are selected to be silicon-oxygen particles with a mass percentage of silicon element of 49.4%, a specific surface area of 3.8 m

[0158] Example 2-1

[0159] Preparation of the negative electrode sheet

[0160] Graphite, the negative electrode material prepared according to Example 1-1, a conductive agent (conductive carbon black, Super PAA) were uniformly mixed in a solvent deionized water to prepare a negative electrode slurry with a solid content of 70wt%, deionized water was added, and the viscosity of the slurry was adjusted to 5000Pa·s to prepare a negative electrode slurry. The negative electrode slurry was uniformly coated on one surface of a negative electrode current collector copper foil with a thickness of 6 pm, and dried at 120°C to obtain a negative electrode sheet with a single-side coated negative electrode material layer, and the coating weight of the negative electrode material layer was 142mg / 1540mm2 Then the above steps are repeated on the other surface of the copper foil, i.e. a negative electrode sheet with double-side coated negative electrode material layer is obtained. After drying at 120℃ and cold pressing, the negative electrode sheet is cut and the tab is welded to obtain a negative electrode sheet with a size of 78mm x 875mm for use. The thickness of the single-side negative electrode active material layer is 54.5μm.

[0161] <Preparation of the positive electrode sheet>

[0162] 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 to prepare a positive electrode slurry. The positive electrode slurry is uniformly coated on one surface of the positive electrode current collector aluminum foil with a thickness of 10μm, and dried at 120℃ to obtain a positive electrode sheet with a single-side coated positive electrode material layer. The coating weight of the positive electrode material layer is 267.8mg / 1540mm 2 Then the above steps are repeated on the other surface of the aluminum foil, i.e. a positive electrode sheet with double-side coated positive electrode material layer is obtained. After drying at 120℃ and cold pressing, the positive electrode sheet is cut and the tab is welded to obtain a positive electrode sheet with a size of 74mm x 867mm for use. The thickness of the single-side positive electrode material layer is 42μm.

[0163] <Preparation of the electrolyte>

[0164] Under a dry argon environment, propylene carbonate (PC), ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed in a ratio of 1:1:1 to obtain a base solvent, and then lithium salt LiPF6 is added and mixed uniformly to dissolve and mix uniformly to obtain an electrolyte. The mass percentage of lithium salt LiPF6 based on the total mass of the electrolyte is 12.5%, and the balance is the base solvent.

[0165] <Separator>

[0166] An 8μm PE / PP composite film is used as the separator.

[0167] <Preparation of the lithium ion battery>

[0168] The positive electrode sheet, the separator and the negative electrode sheet are stacked in order with the separator between the positive electrode sheet and the negative electrode sheet to play a separating role, and then wound to obtain a bare cell. The bare cell is placed in an outer package, injected with electrolyte, packaged, and subjected to processes such as formation, degassing and edge cutting to obtain a lithium ion battery.

[0169] Examples 2-2 to 2-17

[0170] Example 2-1 was the same except that the negative electrode material in the <preparation of negative electrode sheet> was selected from the negative electrode material prepared in Comparative Example 1 according to Table 2.

[0171] Comparative Example 4

[0172] Example 2-1 was the same except that the negative electrode material in the <preparation of negative electrode sheet> was selected from the negative electrode material prepared in Comparative Example 1 according to Table 2.

[0173] Comparative Example 5

[0174] Example 2-1 was the same except that the negative electrode material in the <preparation of negative electrode sheet> was selected from the negative electrode material prepared in Comparative Example 1 according to Table 2.

[0175] Comparative Example 6

[0176] Example 2-1 was the same except that the negative electrode material in the <preparation of negative electrode sheet> was selected from the negative electrode material prepared in Comparative Example 1 according to Table 2.

[0177] Examples 3-1 to 3-13

[0178] Example 2-1 was the same except that lithium oxalate borate (LiBOB) and lithium difluoro oxalate borate (LiODFB) were added in the electrolyte according to Table 3 in the <preparation of electrolyte>, and the mass percentage of LiBOB and LiODFB was adjusted, the mass percentage of the base solvent was changed accordingly, and the mass percentage of lithium salt LiPF6 was unchanged.

[0179] The preparation parameters and performance parameters of each example and comparative example are shown in Tables 1 to 3.

[0180] Table 2

[0181] 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 resistance and a higher mass percentage of silicon element, and thus has a higher first cycle efficiency and a lower first cycle full intercalation lithium electrode expansion rate, and when the negative electrode battery of the present application is applied to a lithium ion battery, the lithium ion battery has good room temperature cycle performance, high temperature cycle performance, and a lower room temperature cycle expansion rate and high temperature cycle expansion rate. When the mass percentage of silicon element in the negative electrode material is 40% to 50%, the mass percentage of carbon in the first layer is 1% to 5%, the mass percentage of inorganic salt is 0.5% to 2%, the mass percentage of polymer is 0.9% to 9%, and / or the mass percentage of conductive material is 0.1% to 1%, the cycle performance and expansion performance of the lithium ion battery can be further improved.

[0182] FIG. 1 shows a structural schematic diagram of the negative electrode material prepared in Example 1-1. Example 2-1 applies a negative electrode containing the negative electrode material in Example 1-1 to a lithium ion battery. As can be seen from FIGS. 2 to 3, the lithium ion battery of Example 2-1 has good room temperature cycle performance and high temperature cycle performance; the negative electrode material in Comparative Example 5 does not contain a third layer, and the room temperature cycle performance and high temperature cycle performance of the lithium ion battery thereof are poor; and the room temperature cycle performance and high temperature cycle performance of the lithium ion batteries of Comparative Examples 4 and 6 are low because they do not contain a second layer. As can be seen from FIGS. 4 to 5, as the cycle number increases, the room temperature cycle expansion rate and high temperature cycle expansion rate of the lithium ion battery of Example 2-1 increase slowly, and the room temperature cycle expansion rate and high temperature cycle expansion rate of Comparative Examples 4 to 6 increase rapidly. Therefore, the negative electrode material of the present application has good expansion performance when applied to a lithium ion battery.

[0183] Table 3

[0184] The mass percentage and type of the additive in the electrolyte generally affect the expansion performance of the lithium ion battery. As can be seen from Examples 2-1, 3-1 to 3-13, when an additive is added to the electrolyte and the mass percentage thereof is 0.01% to 1%, the lithium ion battery prepared using the negative electrode material of the present application has a lower room temperature cycle expansion rate and high temperature cycle expansion rate.

[0185] It is to be noted that, in the present document, relational terms such as first and second and the like can be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Moreover, the terms "comprises", "comprising", or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but can include other elements not expressly listed or inherent to such process, method, article, or apparatus.

[0186] Various embodiments are described herein with reference to particular applications with a specific structure and / or specific components. Those skilled in the art will recognize that the embodiments described herein can be used in other applications with different structures and / or different components. In the description provided herein, numerous specific details are discussed, such as specific process parameters, materials, and equipment. However, embodiments described herein can be practiced without resorting to the details specifically mentioned. In other instances, well known structures and / or processes are not shown or described in detail in order to avoid obscuring aspects of the described embodiments.

[0187] The preferred embodiments of the present application have been disclosed herein and shown in the accompanying drawings. It is to be understood that the application is not limited to the preferred embodiments, and numerous modifications, equivalents and alternatives might be made thereto without departing from the spirit and scope of the application as disclosed in the specification and as defined in the appended claims.

Claims

1. A negative electrode material comprising: The core comprises silicon-based particles; The first layer exists outside the kernel; The second layer exists outside the first layer; The third layer exists outside the second layer; in, The first layer contains carbon; the second layer contains an inorganic lithium salt; and the third layer contains a conductive material and a polymer.

2. The negative electrode material according to claim 1, wherein, The silicon-based particles include at least one of silicon-carbon particles or silicon-oxygen particles; or the inorganic lithium salt includes at least one of lithium fluoride and lithium carbonate; or the polymer includes polyurethane; or the conductive material includes at least one of single-walled carbon nanotubes, multi-walled carbon nanotubes, and conductive carbon black.

3. The negative electrode material according to claim 1 or 2, wherein, Based on the mass of the negative electrode material, the silicon content is 40% to 50% by mass.

4. 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) Based on the mass of the negative electrode material, the mass percentage of carbon in the first layer is 1% to 5%; (4) Based on the mass of the negative electrode material, the mass percentage of the inorganic lithium salt is 0.5% to 2%; (5) Based on the mass of the negative electrode material, the polymer has a mass percentage content of 0.9% to 9%; (6) Based on the mass of the negative electrode material, the mass percentage of the conductive material is 0.1% to 1%.

5. The negative electrode material according to any one of claims 1 to 4, wherein, Based on the mass of the negative electrode material, the mass ratio of the polymer to the conductive material is 9:1 to 9:

2.

6. A method for preparing the negative electrode material according to any one of claims 1 to 5, comprising: Provides silicon-based particles; The silicon-based particles are carbon-coated to obtain a first intermediate. The first intermediate is dispersed in water to obtain a first dispersion. An aqueous solution containing a lithium source is added to the first dispersion to obtain a second dispersion. The dispersion is fully dispersed and stirred, and then spray-dried to obtain the second intermediate. The second intermediate is dispersed in water to obtain a third dispersion. The polymer and the conductive material are dispersed in water to obtain a fourth dispersion. The third dispersion and the fourth dispersion are mixed and stirred evenly, and the negative electrode material is obtained by spray drying.

7. The method for preparing the negative electrode material according to claim 6, wherein, The carbon coating process involves placing the silicon-based particles in a fluidized bed, introducing a carbon source gas at a temperature of 500°C to 650°C, and reacting for 2 to 3 hours to obtain the first intermediate. The carbon source gas includes at least one of acetylene, methane, and propylene.

8. An electrochemical device comprising a positive electrode, a separator, an electrolyte, and a negative electrode, wherein the negative electrode comprises a negative electrode active material layer comprising the negative electrode material according to any one of claims 1 to 5.

9. The electrochemical device according to claim 8, wherein, The electrolyte includes an additive, which includes at least one of lithium oxalate borate and lithium difluorooxalate borate, wherein the mass percentage of the additive is 0.01% to 1% based on the total mass of the electrolyte.

10. The electrochemical device according to claim 9, wherein, The additive has a mass percentage content of 0.1% to 0.6% based on the total mass of the electrolyte.

Images