A negative electrode material, a preparation method thereof, and a battery

Abstract

The present application relates to the field of battery technology, and specifically to a negative electrode material, a preparation method thereof, and a battery. The negative electrode material comprises silicon particles, a first hard carbon coating layer, and a second hard carbon coating layer; the first hard carbon coating layer is at least one of an eggshell-shaped matrix, a honeycomb-shaped matrix, and a sponge-shaped matrix, and the silicon particles are disposed in the cavities of the first hard carbon coating layer to form a first carbon-silicon composite; the second hard carbon coating layer is disposed on the surface of the first carbon-silicon composite to form a second carbon-silicon composite. The silicon@hard carbon I@hard carbon II double-layer hard carbon-coated carbon-silicon negative electrode material prepared by the present invention can effectively improve the problems of battery cell cycle failure and excessive expansion that exist in lithium-ion battery applications of silicon negative electrode materials.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a negative electrode material, a preparation method thereof, and a battery. Background Art

[0002] Silicon anode materials, due to their extremely high specific capacity, have become the most promising anode materials for lithium-ion batteries. However, their application is limited by the significant volume changes during charge and discharge, which can lead to silicon particle pulverization, electrode conductive structure failure, and repeated growth of the solid electrolyte interface (SEI). A common method for improving silicon materials is carbon coating to suppress expansion, but different coating carbon materials have varying effects on silicon expansion. Hard carbon materials, due to their extremely low expansion during charge and discharge, are ideal carbon sources for silicon coatings. Conventional silicon-coated carbon layers are simple single-layer coatings with limited expansion suppression.

[0003] The patent with publication number CN113659125A discloses a triple-core structure in which the surface of nano-silicon is coated with three layers of carbon shell. The surface of the nano-silicon particles is coated with a hard carbon layer, a graphene layer and a soft carbon layer in sequence. The carbon-silicon composite material prepared by this method has the characteristics of low specific surface area and high tap density. In terms of chemical performance indicators, it has the characteristics of high first charge and discharge coulomb efficiency, high gram capacity, stable cycle performance and excellent rate performance. However, the preparation process of the three-layer coating is complicated, and the physical properties of the carbon material of each coating layer vary greatly. The expansion level during the charge and discharge process is different, and there is a risk of separation or gaps between the coating layers during long cycles. Summary of the Invention

[0004] In light of this, the present invention provides a negative electrode material, a preparation method, and a battery. The carbon-silicon negative electrode material, a double-layer hard carbon-coated silicon@hard carbon I@hard carbon II anode material prepared by the present invention, can effectively alleviate the problems of cell cycle failure and excessive expansion that silicon negative electrode materials often encounter in lithium-ion battery applications.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0006] The present invention provides a negative electrode material, which includes silicon particles, a first hard carbon coating layer and a second hard carbon coating layer;

[0007] The first hard carbon coating layer is at least one of an eggshell-shaped matrix, a honeycomb-shaped matrix, and a sponge-shaped matrix, and silicon particles are disposed in cavities of the first hard carbon coating layer to form a first carbon-silicon composite;

[0008] The second hard carbon coating layer is arranged on the surface of the first carbon-silicon composite body to form a second carbon-silicon composite body.

[0009] The negative electrode material has a double-layer coating structure, the structure of silicon particles @ first hard carbon coating layer is a first carbon-silicon complex (silicon @ hard carbon I), and the structure of the first carbon-silicon complex @ second hard carbon coating layer is a second carbon-silicon complex, that is, the negative electrode material of the present invention (silicon @ hard carbon I @ hard carbon II).

[0010] In one embodiment of the present invention, silicon particles are first deposited within a honeycomb matrix formed by a first coating layer precursor. After low-temperature crosslinking and high-temperature carbonization, the honeycomb structure is dispersed into single or multiple spherical or ellipsoidal first carbon-silicon composites. The outer coating of this first carbon-silicon composite is a hard carbon layer formed by carbonizing starch, and the inner layer is composed of deposited nano-silicon particles. This coating is mostly complete, with a small portion being incomplete.

[0011] In one embodiment of the present invention, the first hard carbon coating layer comprises an eggshell-shaped substrate. The eggshell-shaped substrate is formed by dehydrating a first hard carbon precursor. The eggshell-shaped substrate has an eggshell-like shape and an inner cavity. The eggshell-shaped substrate may be a complete eggshell or an incomplete eggshell.

[0012] Preferably, the thickness of the first hard carbon coating layer is 200 to 500 nm, preferably 210 to 350 nm, for example, 210 nm, 230 nm, 250 nm, 290 nm, 300 nm, 320 nm, 340 nm or 350 nm.

[0013] Preferably, the median particle size Dv50 of the first carbon-silicon composite is 3.5 to 4.5 μm, for example, 3.5 μm, 3.8 μm, 4 μm, 4.3 μm, 4.4 μm or 4.5 μm.

[0014] Preferably, the specific surface area of the first carbon-silicon composite is 5 to 8 m 2 / g.

[0015] Preferably, the thickness of the second hard carbon coating layer is 700 to 1000 nm, preferably 800 to 950 nm, for example, 800 nm, 820 nm, 860 nm, 890 nm, 900 nm, 920 nm or 950 nm.

[0016] Preferably, the median particle size Dv50 of the second carbon-silicon composite is 4 to 6.2 μm, for example, 4 μm, 4.5 μm, 4.8 μm, 5 μm, 5.2 μm, 5.5 μm, 5.8 μm, 6 μm, or 6.2 μm.

[0017] Preferably, the specific surface area of the second carbon-silicon composite is 6 to 9 m 2 / g.

[0018] Preferably, the mass percentage of silicon particles in the negative electrode material is 40% to 60%, preferably 45% to 58%, for example 45%, 46%, 48%, 49%, 50%, 51%, 53%, 55%, 57%, or 58%.

[0019] Preferably, the silicon particles are nano-silicon particles.

[0020] Preferably, the particle size of the nano-silicon particles is 20 to 100 nm;

[0021] The present invention also provides a method for preparing the above-mentioned negative electrode material, comprising the following steps:

[0022] (1) dehydrating the first hard carbon precursor (C0) to obtain a first hard carbon coating layer (C1);

[0023] (2) introducing a silicon source gas and an inert gas into the first hard carbon coating layer, and depositing silicon in the first hard carbon coating layer by chemical vapor deposition to obtain a first carbon-silicon mixture (C2);

[0024] (3) crosslinking and carbonizing the first carbon-silicon mixture in an inert gas atmosphere to obtain a first carbon-silicon composite (C3);

[0025] (4) The second hard carbon precursor liquid phase is coated on the surface of the first carbon-silicon composite, and a second carbon-silicon composite (C4) is obtained by carbonization treatment.

[0026] Preferably, the first hard carbon precursor is a starch-based biomass hard carbon precursor.

[0027] Preferably, the starch-based biomass hard carbon precursor includes but is not limited to at least one of phosphate-modified starch, corn starch, mung bean starch, cassava starch, sweet potato starch, yams, potato starch, wheat starch, water chestnut starch, lotus root starch, and rice starch.

[0028] Preferably, the D50 value of the starch-based biomass hard carbon precursor is 0.5 to 5 μm.

[0029] Preferably, the second hard carbon precursor includes at least one of a resin-based hard carbon precursor, an organic polymer pyrolytic carbon-based hard carbon precursor, a carbon black-based hard carbon precursor, and a biomass carbon-based hard carbon precursor.

[0030] Preferably, the resin hard carbon precursor is selected from one or more of phenolic resin, epoxy resin, polyfurfuryl alcohol resin, furfural resin and furan resin.

[0031] Preferably, the organic polymer pyrolytic carbon hard carbon precursor is selected from one or more of aromatic compounds, polyfurfuryl alcohol, polyvinyl chloride, tetrafluoroethylene-perfluoroalkyl vinyl ether, polyvinylidene fluoride, polyacrylonitrile, and polyvinyl pyrrolidone; preferably, the aromatic compounds include naphthalene, anthracene, phenanthrene, benzene and their derivatives.

[0032] Preferably, the carbon black hard carbon precursor is selected from one or more of acetylene black, Ketjen black, and superconducting carbon black.

[0033] Preferably, the biomass carbon hard carbon precursor is selected from sugars and / or amino acids. Sugars include but are not limited to sucrose, glucose, maltose, etc., and amino acids include but are not limited to glycine, alanine, etc.

[0034] Preferably, in step (1), the dehydration treatment is as follows: under the protection of inert gas, the temperature is raised to 200-230°C at a rate of 0.5-2°C / min, and the temperature is kept constant for 2-5 hours.

[0035] Preferably, in step (2), the volume ratio of the silicon source gas to the inert gas is 1:(1-5); preferably 1:(1-3).

[0036] Preferably, in step (2), the flow rate of the mixed gas (silicon source gas and inert gas) is 0.5-5 L / min, preferably 1-2 L / min.

[0037] Preferably, the temperature of chemical vapor deposition is 400-600° C., and the time of chemical vapor deposition is 3-7 hours.

[0038] Preferably, in step (3), the cross-linking treatment is: under the protection of inert gas, heating to 200-230°C at a rate of 0.5-2°C / min, and treating at a constant temperature for 20-28h;

[0039] Preferably, in step (3), the carbonization treatment is: under the protection of inert gas, the temperature is raised to 900-1000°C at a rate of 1-3°C / min, and the temperature is kept constant for 1-3 hours.

[0040] Preferably, in step (4), the solvent for liquid coating is selected from one or more of water, ethanol, ethylene glycol, acetone, ether, and N-dimethylformamide.

[0041] Preferably, the second hard carbon precursor accounts for 20 wt% to 70 wt% in the coating liquid of the liquid phase coating.

[0042] Preferably, the solvent is removed by rotary evaporation.

[0043] Preferably, the temperature of the rotary evaporation is 80-90° C., and the rotation speed of the rotary evaporation is 25-35 r / min.

[0044] Preferably, in step (4), the carbonization treatment is: under the protection of inert gas, the temperature is raised to 900-1000°C at a rate of 1-2°C / min, and the temperature is kept constant for 1-3 hours.

[0045] In the embodiment provided by the present invention, in step (4), a pre-stabilization treatment step is further included between the liquid phase coating and the carbonization treatment.

[0046] Preferably, the pre-stabilization treatment is: under the protection of inert gas, heating to 250-350° C. at a rate of 4-6° C. / min and pre-stabilizing for 1-3 hours.

[0047] In the embodiment provided by the present invention, step (4) further includes a crushing step.

[0048] Preferably, the silicon source gas is selected from at least one of SiH4, Si2H6, SiClH3, SiCl2H2, SiCl3H, and SiClH3.

[0049] Preferably, the inert gas is selected from at least one of nitrogen, helium, argon, xenon and radon.

[0050] The present invention also provides a negative electrode active material, comprising the above-mentioned negative electrode material and an active material acceptable in the battery field.

[0051] In the embodiments provided by the present invention, the active material acceptable in the field of batteries includes a conductive agent and / or a binder.

[0052] In the embodiment provided by the present invention, the conductive agent is selected from one or more of conductive carbon black, acetylene black, Ketjen black, carbon fiber, graphene, single-walled carbon nanotubes, and multi-walled carbon nanotubes.

[0053] In the embodiments provided by the present invention, the binder is selected from one or more of carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyethylene, polyvinyl alcohol, polyvinyl chloride, polyvinyl fluoride, polyvinyl pyrrolidone, polytetrafluoroethylene, polypropylene, styrene-butadiene rubber, epoxy resin, butadiene rubber binder, and acrylonitrile binder.

[0054] In the embodiment provided by the present invention, the active materials acceptable in the battery field include a conductive agent and a binder, and the mass percentage of each component in the negative electrode active material is: 85%~98% negative electrode material, 0.5%~7% conductive agent, and 1.5%~8% binder.

[0055] The present invention also provides an electrode sheet, comprising the above-mentioned negative electrode material, the above-mentioned negative electrode active substance and / or the negative electrode material prepared by the above-mentioned preparation method.

[0056] Preferably, the electrode sheet further includes a current collector.

[0057] In the embodiment provided by the present invention, the current collector is selected from one or more of copper foil, carbon-coated copper foil, and perforated copper foil.

[0058] In the embodiment provided by the present invention, the above-mentioned electrode sheet can be obtained by the following method: mixing water and negative electrode active material to obtain negative electrode slurry; coating the negative electrode slurry on a current collector, and drying to obtain an electrode sheet.

[0059] In the embodiment provided by the present invention, the viscosity of the negative electrode slurry is 4000-6000 mPa.s, and the solid content is 35 wt%-45 wt%.

[0060] In the embodiment provided by the present invention, the drying conditions are: vacuum, temperature 80-90° C., and time 7-10 h.

[0061] The present invention also provides a battery comprising the above-mentioned electrode sheet.

[0062] Preferably, the battery is a lithium-ion battery.

[0063] Preferably, the battery is a wound cell or a laminated cell.

[0064] In a specific embodiment of the present invention, the battery further includes a positive electrode sheet.

[0065] In a specific embodiment of the present invention, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material coated on at least one side of the surface of the positive electrode current collector, and the positive electrode active material includes a positive electrode material.

[0066] In some embodiments of the present invention, the positive electrode current collector is selected from one or more of aluminum foil, carbon-coated aluminum foil, and perforated aluminum foil.

[0067] In some embodiments of the present invention, the positive electrode material is selected from one or more of lithium iron phosphate, lithium manganese phosphate, lithium vanadium phosphate, lithium iron silicate, lithium cobalt oxide, nickel-cobalt-manganese ternary material, nickel-manganese / cobalt-manganese / nickel-cobalt binary material, lithium manganese oxide and lithium-rich manganese-based material.

[0068] According to an embodiment of the present invention, the battery further comprises a separator. In some embodiments, the separator is selected from one or more of polyethylene or polypropylene.

[0069] According to an embodiment of the present invention, the battery further includes an electrolyte. In some embodiments, the electrolyte is a non-aqueous electrolyte comprising a non-aqueous organic solvent and a lithium salt. In some embodiments, the non-aqueous organic solvent is selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), ethylene carbonate, γ-butyrolactone, methylpropyl carbonate, and ethyl propionate. In some embodiments, the lithium salt is selected from one or more of LiPF6, LiBF4, LiSbF6, LiClO4, LiCF3SO3, LiAlO4, LiAlCl4, Li(CF3SO2)2N, LiBOB, and LiDFOB.

[0070] The present invention provides a negative electrode material, a preparation method thereof, and a battery. The negative electrode material comprises silicon particles, a first hard carbon coating layer, and a second hard carbon coating layer; the first hard carbon coating layer is at least one of an eggshell-shaped matrix, a honeycomb-shaped matrix, and a sponge-shaped matrix, and the silicon particles are disposed in the cavities of the first hard carbon coating layer to form a first carbon-silicon composite; the second hard carbon coating layer is disposed on the surface of the first carbon-silicon composite to form a second carbon-silicon composite. The negative electrode material is denoted as silicon@hard carbon I@hard carbon II. Compared with the prior art, the present invention has the following beneficial effects:

[0071] The present invention prepares a carbon-silicon negative electrode material with a double-layer coating structure of silicon @ hard carbon I @ hard carbon II, wherein small-sized nano-silicon particles are prepared by vapor deposition to reduce the occurrence of particle pulverization or cracking during the silicon deintercalation process. The porous structure treated with starch dehydration can allow the nano-silicon particles to be deposited into the pore structure for efficient coating. The first hard carbon coating layer prepared with a starch precursor effectively alleviates the internal stress caused by the expansion of the internal nano-silicon particles. The double-layer hard carbon coating more effectively suppresses expansion, while avoiding direct contact between silicon and electrolyte materials, thereby improving capacity retention. Moreover, the two coating layers are made of the same type of carbon material (hard carbon), and the expansion coefficients during the lithium deintercalation process are close, which makes the structural stability of the coating layer better and less likely to cause interlayer separation problems after cycling.

[0072] The first hard carbon coating layer in the silicon@hard carbon I@hard carbon II of the present invention is made of a starch-based biomass hard carbon precursor. The starch-based raw materials have the advantages of being environmentally friendly and low-cost. At the same time, the eggshell-shaped, honeycomb-shaped or sponge-shaped matrix formed after dehydration can provide the required macroporous structure for silicon deposition, thereby achieving more complete coating. BRIEF DESCRIPTION OF THE DRAWINGS

[0073] Figure 1 is a schematic cross-sectional view of a carbon-silicon negative electrode material;

[0074] Figure 2 Schematic diagram of the synthesis of the negative electrode material of the present invention;

[0075] Figure 3 This is the SEM cross-sectional view of the carbon-silicon negative electrode material;

[0076] Figure 4 4a and 4b are schematic diagrams of silicon particles deposited in an eggshell-shaped matrix; among them, 4a shows silicon particles arranged on the inner wall of the eggshell-shaped matrix, and 4b shows the inner cavity of the eggshell-shaped matrix filled with silicon particles; 1 shows the first hard carbon coating layer, 2 shows silicon particles, and 3 shows the cavity. DETAILED DESCRIPTION

[0077] The present invention discloses a negative electrode material, a method for preparing the same, and a battery. Those skilled in the art can refer to the contents herein and appropriately improve the process parameters to achieve the desired results. It should be noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in the present invention. The methods and applications of the present invention have been described through preferred embodiments. It is obvious that relevant personnel can modify or appropriately alter and combine the methods and applications described herein without departing from the content, spirit, and scope of the present invention to implement and apply the technology of the present invention.

[0078] To address the problems of existing silicon materials experiencing large volume changes during charge and discharge, which can lead to separation between the negative electrode current collector and the negative electrode active material, and repeated rupture and growth of the SEI film, the present invention provides a negative electrode material and a battery incorporating the same. The negative electrode material has a multilayer structure of silicon@hard carbon I@hard carbon II. Through the combined action of a double-layer hard carbon coating, the negative electrode material's volume changes during charge and discharge are effectively controlled, resulting in a battery with excellent cycling performance and coulombic efficiency.

[0079] The present invention provides a negative electrode material, which includes nano-silicon particles, a first hard carbon coating layer and a second hard carbon coating layer;

[0080] The first hard carbon coating layer is on the surface of the nano-silicon particles, and the second hard carbon coating layer is on the surface of the first hard carbon coating layer;

[0081] The negative electrode material has a double-layer coating structure, the structure of the silicon particles @ the first hard carbon coating layer is a first carbon-silicon composite, and the structure of the first carbon-silicon composite @ the second hard carbon coating layer is a second carbon-silicon composite, i.e., the final carbon-silicon negative electrode material;

[0082] In a specific embodiment of the present invention, the silicon nanoparticles are first deposited in a honeycomb matrix formed by a first cladding layer precursor.

[0083] In a specific embodiment of the present invention, the thickness of the first hard carbon coating layer is 200 nm to 500 nm, preferably 210 nm to 350 nm, for example, 210 nm, 230 nm, 250 nm, 290 nm, 300 nm, 320 nm, 340 nm or 350 nm.

[0084] In a specific embodiment of the present invention, the thickness of the second hard carbon coating layer is 700 nm to 1000 nm, preferably 800 nm to 950 nm, for example, 800 nm, 820 nm, 860 nm, 890 nm, 900 nm, 920 nm or 950 nm.

[0085] In a specific embodiment of the present invention, the median particle size Dv50 of the first carbon-silicon composite is 3.5 μm to 4.5 μm, for example, 3.5 μm, 3.8 μm, 4 μm, 4.3 μm, 4.4 μm or 4.5 μm.

[0086] The specific surface area of the first carbon-silicon composite is 5-8 m 2 / g.

[0087] In a specific embodiment of the present invention, the median particle size Dv50 of the second carbon-silicon composite, i.e., the carbon-silicon negative electrode material, is 4 μm to 6.2 μm, for example, 4 μm, 4.5 μm, 4.8 μm, 5 μm, 5.2 μm, 5.5 μm, 5.8 μm, 6 μm, or 6.2 μm.

[0088] The specific surface area of the first carbon-silicon composite is 6-9 m 2 / g.

[0089] In a specific embodiment of the present invention, the mass of the silicon material accounts for 40-60wt% of the total mass of the negative electrode material, preferably 45-58wt%, for example, 45 wt%, 46 wt%, 48 wt%, 49 wt%, 50 wt%, 51 wt%, 53 wt%, 55wt%, 57wt%, or 58 wt%.

[0090] In a specific embodiment of the present invention, the mass of the carbon material accounts for 40-60 wt% of the total mass of the negative electrode material, preferably 50-60 wt%, for example, 50 wt%, 51 wt%, 53 wt%, 54 wt%, 56 wt%, 58 wt%, or 60 wt%.

[0091] In a specific embodiment of the present invention, the mass ratio of the nano-silicon, the first hard carbon coating layer, and the second hard carbon coating layer is, for example, in the range of (40-60): (5-15): (35-55).

[0092] In a specific embodiment of the present invention, the schematic cross-sectional view of the negative electrode material of the present invention is as follows Figure 1 As shown, the carbon silicon negative electrode material is prepared by the following method, and the preparation process is as follows Figure 2 :

[0093] In step (1), a first hard carbon precursor (C0) of starch-based biomass is subjected to low-temperature dehydration treatment to prepare a porous honeycomb-shaped first hard carbon coating layer (C1).

[0094] Step (2) introduces silicon source gas and inert gas into the first hard carbon coating layer, and deposits nano-silicon in the first hard carbon coating layer by chemical deposition to obtain a first carbon-silicon mixture (C2).

[0095] In step (3), the first carbon-silicon composite is placed in an inert gas atmosphere and subjected to low-temperature crosslinking treatment and high-temperature carbonization treatment to obtain a core-shell silicon@hard carbon I carbon-silicon negative electrode material, namely the first carbon-silicon composite (C3).

[0096] Step (4): The second hard carbon precursor liquid phase is coated on the surface of the first carbon-silicon composite, and the second carbon-silicon composite (C4) of silicon@hard carbon I@hard carbon II is finally obtained by high-temperature pyrolysis. The SEM cross-sectional view is as follows: Figure 3 shown.

[0097] Wherein, in step (1), the starch-based biomass hard carbon precursor is a specially prepared starch with a D50 of 0.5-5 μm. The raw materials for producing the starch include one or more of phosphate-modified starch, corn starch, mung bean starch, cassava starch, sweet potato starch, red potato starch, potato starch, wheat starch, water chestnut starch, lotus root starch, and rice starch.

[0098] Among them, in step (1), low-temperature pretreatment refers to heating the temperature at a rate of 0.5-2°C / min to 200-230°C and treating the temperature under inert atmosphere for 2-5 hours, which is essentially a dehydration reaction.

[0099] Wherein, in step (2), the silicon source gas is selected from at least one of SiH4, Si2H6, SiClH3, SiCl2H2, SiCl3H, and SiClH3.

[0100] Therefore, the inert atmosphere in each step is selected from at least one of nitrogen, helium, argon, xenon and radon.

[0101] Wherein, in step (2), the volume ratio of the silicon source gas to the inert gas is 1: (1-5); for example, 1: (1-3).

[0102] Wherein, in step (2), the temperature of the chemical vapor deposition is 400-600° C., and the time of the chemical vapor deposition is 3 h-7 h.

[0103] Wherein, in step (3), the low-temperature treatment is a cross-linking treatment, and the reaction conditions are heating at a rate of 0.5-2°C / min to 200-230°C and constant temperature treatment for 20-28 hours under inert atmosphere. After the low-temperature treatment, the porous honeycomb structure is dispersed into a single particle or multiple particles of a spherical or ellipsoidal first carbon-silicon mixture. The first carbon-silicon mixture has an outer coating layer of a starch layer to be carbonized and an inner layer of deposited nano-silicon particles. Most of this coating is complete, and a small part is incomplete.

[0104] Wherein, in step (3), the high-temperature carbonization condition is to heat the material to 900-1000°C at a heating rate of 1-3°C / min and perform constant temperature treatment for 1-3 hours under the protection of an inert atmosphere.

[0105] Wherein, in step (4), the second hard carbon precursor includes resin, organic polymer pyrolytic carbon, carbon black, and biomass carbon precursor. The resin precursor is selected from one or more of phenolic resin, epoxy resin, polyfurfuryl alcohol resin, furfural resin, and furan resin; the organic polymer pyrolytic carbon precursor includes one or more of naphthalene, anthracene, phenanthrene, benzene carbon, polyfurfuryl alcohol pyrolytic carbon, polyvinyl chloride pyrolytic carbon, phenolic pyrolytic carbon, tetrafluoroethylene-perfluoroalkyl vinyl ether, polyvinylidene fluoride, polyacrylonitrile, and polyvinyl pyrrolidone; the carbon black precursor is selected from one or more of acetylene black, Ketjen black, and superconducting carbon black; the biomass carbon precursor is selected from sugars such as sucrose, glucose, and maltose, and amino acids such as glycine and alanine.

[0106] In step (4), the liquid coating solvent is selected from one or more of deionized water, ethanol, ethylene glycol, acetone, diethyl ether, and N-N-dimethylformamide, and the second hard carbon precursor accounts for 20-70 wt % of the coating liquid. The solvent is removed by rotary evaporation, with the rotary evaporation temperature being 80-90° C. and the rotary evaporation speed being 25-35 rpm.

[0107] Wherein, in step (4), the high-temperature pyrolysis conditions are heating to 900-1000°C at a heating rate of 1-2°C / min, and carbonizing at a constant temperature for 1-3 hours under the protection of an inert atmosphere.

[0108] The present invention also provides a negative electrode sheet, which comprises the above-mentioned negative electrode material.

[0109] According to an embodiment of the present invention, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer coated on at least one side of the negative electrode current collector, wherein the negative electrode active material layer includes the above-mentioned negative electrode material.

[0110] According to an embodiment of the present invention, the negative electrode active material layer further comprises a conductive agent. In some embodiments, the conductive agent is selected from one or more of conductive carbon black, acetylene black, Ketjen black, carbon fiber, graphene, single-walled carbon nanotubes, and multi-walled carbon nanotubes.

[0111] According to an embodiment of the present invention, the negative electrode active material layer further includes a binder. In some embodiments, the binder is selected from one or more of carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyethylene, polyvinyl alcohol, polyvinyl chloride, polyvinyl fluoride, polyvinyl pyrrolidone, polytetrafluoroethylene, polypropylene, styrene-butadiene rubber, epoxy resin, butadiene rubber binder, and acrylonitrile binder.

[0112] According to an embodiment of the present invention, the negative electrode current collector is selected from one or more of ordinary copper foil, carbon-coated copper foil, and perforated copper foil.

[0113] According to an embodiment of the present invention, the mass percentage of each component in the negative electrode active material layer is:

[0114] 85~98wt% negative electrode material, 0.5~7wt% conductive agent, 1.5~8wt% binder.

[0115] According to an embodiment of the present invention, the negative electrode sheet can be obtained by the following method:

[0116] The above-mentioned negative electrode materials, conductive agent and binder are mixed in deionized water to obtain a negative electrode slurry (viscosity of 4000-6000 mPa.s and solid content of 35-45wt%). The negative electrode slurry is coated on the negative electrode current collector, and then transferred to a vacuum oven at 80-90°C for drying for 7-10 hours. Finally, it is roll-pressed and slit to obtain the negative electrode sheet.

[0117] The present invention also provides a battery comprising the above-mentioned negative electrode sheet.

[0118] According to an embodiment of the present invention, the battery is a lithium-ion battery.

[0119] According to an embodiment of the present invention, the battery further includes a positive electrode sheet.

[0120] According to an embodiment of the present invention, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer coated on at least one surface of the positive electrode current collector, and the positive electrode active material layer includes a positive electrode material.

[0121] In some embodiments, the positive electrode current collector is selected from one or more of ordinary aluminum foil, carbon-coated aluminum foil, and perforated aluminum foil.

[0122] In some embodiments, the positive electrode material is selected from one or more of lithium iron phosphate, lithium manganese phosphate, lithium vanadium phosphate, lithium iron silicate, lithium cobalt oxide, nickel-cobalt-manganese ternary material, nickel-manganese / cobalt-manganese / nickel-cobalt binary material, lithium manganese oxide and lithium-rich manganese-based material.

[0123] According to an embodiment of the present invention, the battery further comprises a separator. In some embodiments, the separator is selected from one or more of polyethylene or polypropylene.

[0124] According to an embodiment of the present invention, the battery further includes an electrolyte. In some embodiments, the electrolyte is a non-aqueous electrolyte comprising a non-aqueous organic solvent and a lithium salt. In some embodiments, the non-aqueous organic solvent is selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), ethylene carbonate, γ-butyrolactone, methylpropyl carbonate, and ethyl propionate. In some embodiments, the lithium salt is selected from one or more of LiPF6, LiBF4, LiSbF6, LiClO4, LiCF3SO3, LiAlO4, LiAlCl4, Li(CF3SO2)2N, LiBOB, and LiDFOB.

[0125] Unless otherwise specified, the experimental methods used in the following examples are conventional methods; the reagents, materials, etc. used in the following examples are all commercially available unless otherwise specified.

[0126] In the description of the present invention, it should be noted that the terms "first", "second", etc. are only used for descriptive purposes and do not indicate or imply relative importance.

[0127] The present invention will be further described below in conjunction with the embodiments:

[0128] Example 1 Negative Electrode Material (Silicon@Hard Carbon I@Hard Carbon II) and Preparation Method Thereof

[0129] The structural diagram of the carbon-silicon negative electrode material of this embodiment is shown in FIG. Figure 1 , including silicon particles 21 , a first hard carbon coating layer 11 and a second hard carbon coating layer 12 . Figure 1 Only the honeycomb-shaped first hard carbon coating layer is shown, and the sponge-shaped and eggshell-shaped first hard carbon coating layers are not shown.

[0130] The first hard carbon coating layer is honeycomb-shaped and has a thickness of 218 nm. The raw material for preparing the first hard carbon coating layer is rice starch, and the D50 value of the rice starch is 3.9 μm.

[0131] Nano-silicon particles with a particle size of 20 to 100 nm are placed in the cavities of the first hard carbon coating layer to form the first carbon-silicon composite (silicon@hard carbon I). The median particle size Dv50 of the first carbon-silicon composite is 4.1 μm, and the specific surface area is 6.8 m 2 / g.

[0132] The second hard carbon coating layer is provided on the surface of the first carbon-silicon composite, forming a second carbon-silicon composite (silicon@hard carbon I@hard carbon II). The thickness of the second hard carbon coating layer is 865 nm. The raw material for its preparation is polyacrylonitrile. The median particle size Dv50 of the second carbon-silicon composite is 4.97 μm, and the specific surface area is 7.2 m 2 / g.

[0133] In the negative electrode material, the mass percentage of nano-silicon particles is 53.1%.

[0134] The negative electrode material has a double-layer coating structure, the structure of silicon particles @ first hard carbon coating layer is a first carbon-silicon complex, and the structure of first carbon-silicon complex @ second hard carbon coating layer is a second carbon-silicon complex, that is, the negative electrode material of the present invention.

[0135] The preparation method comprises the following steps (preparation process as shown in FIG. Figure 2 shown):

[0136] Step (1), placing 10g of rice starch in an inert gas atmosphere and heating it to 200°C at a heating rate of 1°C / min, and pre-stabilizing and dehydrating it for 2h to obtain a first hard carbon coating layer (C1).

[0137] In step (2), the first hard carbon coating layer is placed in an atmosphere of SiH4:Ar=1:1.5 (volume ratio) (gas flow rate is 1.4 L / min) and deposited at 500°C for 5 hours to prepare a first carbon-silicon mixture (C2).

[0138] Step (3), placing the first carbon-silicon mixture in an inert gas atmosphere and heating it to 230°C at a heating rate of 1°C / min for cross-linking treatment for 20 hours, then heating it to 1000°C at a heating rate of 2°C / min, maintaining it for 1 hour, and cooling it to room temperature to obtain the first carbon-silicon composite (C3) of "silicon@hard carbon I".

[0139] Step (4), the silicon @ hard carbon I material is mixed with 20g polyacrylonitrile and 50mL NN dimethylformamide by stirring for 30min, the solvent is removed by rotary evaporation, and then the mixture is placed in an inert atmosphere and heated to 300℃ at 5℃ / min for pre-stabilization for 2h to obtain the negative electrode material of the first carbon silicon complex coated with polyacrylonitrile, and then heated to 900℃ at 2℃ / min for carbonization for 3h, and then cooled naturally to room temperature. After air flow crushing and classification, the second carbon silicon complex (C4) of silicon @ hard carbon I @ hard carbon II can be obtained, and its SEM shows Figure 3 .

[0140] Example 2:

[0141] The specific preparation method of this embodiment refers to that of Example 1, with the only difference being that, in step (1), the starch used is phosphate-modified starch.

[0142] Example 3:

[0143] The specific preparation method of this embodiment refers to that of Example 1, with the only difference being that in step (1), the starch used is corn starch.

[0144] Example 4:

[0145] The specific preparation method of this embodiment refers to that of Example 1, with the only difference being that in step (1), the starch used is potato starch.

[0146] Example 5:

[0147] The specific preparation method of this embodiment refers to that of Example 1, except that, in step (4), the silicon@hard carbon I material is fully mixed with 50 mL of ethanol and 30 g of phenolic resin and placed at room temperature for 12 h, and then dried and crushed to obtain a phenolic resin-coated first carbon-silicon composite negative electrode material, and finally placed in an inert atmosphere and heated to 800°C at 3°C / min for carbonization for 3 h to obtain a silicon@hard carbon I@hard carbon II second carbon-silicon composite negative electrode material.

[0148] Example 6:

[0149] The specific preparation method of this embodiment refers to that of Example 1, except that, in step (4), the silicon@hard carbon I material is thoroughly mixed with 50 mL of ethanol and 38 g of polyvinyl pyrrolidone for 3 hours, and then placed at room temperature for 10 hours, and then dried and crushed to obtain a polyvinyl pyrrolidone-coated first carbon-silicon composite negative electrode material, and finally placed in an inert atmosphere and heated to 800°C at 3°C / min for carbonization for 3 hours to obtain a silicon@hard carbon I@hard carbon II second carbon-silicon composite negative electrode material.

[0150] Example 7:

[0151] The specific preparation method of this embodiment refers to that of Example 1, except that, in step (4), the silicon@hard carbon I material is fully mixed with 50 mL of ethanol and 65 g of glucose for 3 h, and then vacuum dried at 100°C for 12 h for caramel dehydration treatment, and then dried and crushed to obtain a negative electrode material of the first carbon-silicon complex coated with glucose, and finally placed in an inert atmosphere and heated to 900°C at 3°C / min for carbonization for 3 h to obtain a second carbon-silicon complex negative electrode material of silicon@hard carbon I@hard carbon II.

[0152] Comparative Example 1:

[0153] The specific preparation method of this comparative example refers to that of Example 1, except that ordinary starch with a D50 of 15-18 μm is used in step (1).

[0154] Comparative Example 2:

[0155] The specific preparation method of this comparative example refers to that of Example 1, except that there is no experimental treatment process of step (4), and the core-shell silicon@hard carbon I negative electrode material of step (3) is the final product.

[0156] Preparation of lithium-ion batteries

[0157] The carbon-silicon materials of silicon@hard carbon I@hard carbon II and silicon@hard carbon I prepared in Examples 1-7 and Comparative Examples 1-2 were used as negative electrode materials to assemble batteries. The specific production method is as follows:

[0158] (1) The negative electrode material prepared above, sodium carboxymethyl cellulose, styrene-butadiene rubber, Super P, and single-walled carbon nanotubes were mixed in a mass ratio of 92:2:5:0.95:0.05, and deionized water was added to obtain a negative electrode slurry under the action of a vacuum mixer. The negative electrode slurry was evenly coated on a copper foil with a thickness of 6 μm. The surface density of the negative electrode slurry coated on the surface of the negative electrode current collector was 4.5 mg / cm 2 The copper foil was transferred to a 90°C oven and dried for 12 hours, and then rolled and cut to obtain the negative electrode sheet.

[0159] (2) Lithium cobalt oxide (LCO), polyvinylidene fluoride (PVDF), acetylene black and carbon nanotubes (CNTs) were mixed in a mass ratio of 96:2:1.5:0.5, N-methylpyrrolidone was added, and the mixture was stirred in a vacuum mixer until a uniform positive electrode slurry was formed. The positive electrode slurry was evenly coated on an aluminum foil with a thickness of 9 μm. The surface density of the positive electrode slurry coated on the surface of the positive electrode current collector was 15.0 mg / cm 2 The coated aluminum foil was baked in an oven and then dried in a 120°C oven for 8 hours. The foil was then rolled and slit to obtain the desired positive electrode sheet. The positive electrode sheet was smaller than the negative electrode sheet, and the reversible capacity per unit area of the positive electrode sheet was 4% lower than that of the negative electrode sheet.

[0160] (3) The positive and negative electrode sheets are welded with their metal tabs.

[0161] (4) Use a polyethylene diaphragm with a thickness of 12 μm.

[0162] (5) The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in order, ensuring that the separator is between the positive and negative electrode sheets to play an isolating role, and then the bare cell is obtained by winding. The bare cell is placed in an aluminum-plastic film shell with a thickness of 0.086 mm. After packaging, electrolyte injection, secondary packaging, static, formation, shaping, and sorting, the desired lithium-ion battery is obtained.

[0163] Preparation of button half-cell

[0164] The carbon-silicon materials of silicon@hard carbon I@hard carbon II and silicon@hard carbon I prepared in Examples 1-7 and Comparative Examples 1-2 were used as negative electrode materials to assemble into button-type half-cells for testing. The specific preparation method is as follows:

[0165] (1) The negative electrode material prepared above, Super P, single-walled carbon nanotubes, sodium carboxymethyl cellulose, and styrene-butadiene rubber were mixed in a mass ratio of 92:0.95:0.05:2:5, deionized water was added, and the mixture was mixed evenly in a vacuum mixer to obtain a negative electrode slurry;

[0166] (2) The negative electrode slurry in step (1) was coated on copper foil, dried in an 80°C oven, and then transferred to a 100°C vacuum oven and dried for 12 hours to obtain a surface density of about 6.0 mg / cm 2 The negative electrode;

[0167] (3) In a dry environment, the negative electrode sheet in step (2) was heated to about 1.3 g / cm 3 The negative electrode discs with a diameter of 16 mm were formed by roller compaction and then punching.

[0168] (4) In a glove box, use the negative electrode disc in step (3) as the working electrode, a metal lithium sheet with a diameter of 18 mm as the counter electrode, and a polyethylene diaphragm with a thickness of 8 μm as the isolation membrane. Add electrolyte and assemble into a button half-cell.

[0169] Test Case

[0170] (1) Performance test of lithium-ion batteries

[0171] The performance of the lithium-ion battery prepared above was tested using a LAND test system at a test temperature of 25°C. Specifically:

[0172] Charge to 3.85V at a constant current of 0.7C, charge to 0.01C at a constant voltage, dissect the battery at this time, measure and calculate the thickness of the negative electrode coating, and use this as the initial coating thickness of the negative electrode. Charge to 4.45V at a constant current of 3C, charge to 0.05C at a constant voltage, let it stand for 10 minutes, discharge to 2.5V at 1C, let it stand for 10 minutes, and cycle this charge and discharge step. The highest discharge capacity in the first three weeks is the initial capacity of the battery, and the ratio of the capacity after 100 cycles to the initial capacity is the capacity retention rate of the battery. Dissect the battery that has been cycled for 100 weeks and has an SOC of 50%, measure and calculate the thickness of the negative electrode's cycle coating, and divide the difference between this thickness and the initial coating thickness by the initial coating thickness to obtain the half-electric expansion rate of the negative electrode coating.

[0173] (2) Performance test of button half-cell

[0174] The performance of the button half-cell prepared above was tested using a LAND test system at a test temperature of 25°C. Specifically:

[0175] Lithium was inserted to 0.005V at a current of 0.05C, and the mixture was allowed to stand for 10 minutes. Lithium was inserted to 0.005V at a current of 0.05mA, and the mixture was allowed to stand for 10 minutes. Lithium was then removed to 1.5V at a current of 0.1C to obtain the first lithium insertion and removal capacity. The first lithium removal capacity was divided by the mass of the negative electrode material in the above-mentioned negative electrode disc to obtain the gram capacity of the negative electrode material.

[0176] (3) Test results

[0177] The test results are shown in Table 1.

[0178] Table 1. Performance test results of negative electrode materials prepared in Examples 1-7 and Comparative Examples 1-2

[0179]

[0180] The results in Table 1 show that the hard carbon coating layer precursors of Examples 1-7 are different, resulting in differences in particle size, but all have high capacity retention rates, with a capacity retention rate of more than 92% after 100 cycles, proving the feasibility of the silicon@hard carbon I@hard carbon II double-layer coating structure material in inhibiting the expansion of silicon negative electrodes and improving the cycle performance of battery cells. In Comparative Example 1, the large-particle carbon-silicon negative electrode obtained by using an ordinary starch precursor as a coating layer has a lower capacity retention rate than that of small-particle starch, proving that small-particle carbon-silicon negative electrode materials are more effective in improving silicon negative electrode expansion. Comparative Example 2 does not use secondary carbon coating, and its carbon-silicon negative electrode material cycle expansion and capacity retention are both lower than those of the silicon@hard carbon I@hard carbon II double-layer coating structure material.

[0181] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A negative electrode material, characterized in that comprising silicon particles, a first hard carbon coating layer, and a second hard carbon coating layer; The first hard carbon coating layer is at least one of an eggshell-shaped matrix and a honeycomb-shaped matrix, and the silicon particles are disposed in the cavities of the first hard carbon coating layer to form a first carbon-silicon composite; The second hard carbon coating layer is disposed on the surface of the first carbon-silicon composite body to form a second carbon-silicon composite body; The thickness of the first hard carbon coating layer is 200nm to 500nm; the median particle size Dv50 of the first carbon-silicon composite is 3.5μm to 4.5μm; the specific surface area of the first carbon-silicon composite is 5m 2 / g ~ 8m 2 / g; The method for preparing the first carbon-silicon composite comprises the following steps: (1) dehydrating a first hard carbon precursor to obtain the first hard carbon coating layer, wherein the first hard carbon precursor is a starch-based biomass hard carbon precursor, and the particle size D50 of the starch-based biomass hard carbon precursor is 0.5 μm to 5 μm; (2) introducing a silicon source gas and an inert gas into the first hard carbon coating layer, and depositing silicon in the first hard carbon coating layer by a chemical vapor deposition method to obtain a first carbon-silicon mixture; (3) In an inert gas atmosphere, the first carbon-silicon mixture is cross-linked and carbonized to obtain a first carbon-silicon composite.

2. The negative electrode material according to claim 1, characterized in that The median particle size Dv50 of the second carbon-silicon composite is 4 μm to 6.2 μm; the specific surface area of the second carbon-silicon composite is 6 m 2 / g ~ 9m 2 / g.

3. The negative electrode material according to claim 1, characterized in that The thickness of the second hard carbon coating layer is 700-1000 nm.

4. The negative electrode material according to claim 1, characterized in that In the negative electrode material, the mass percentage of silicon particles is 40% to 60%.

5. The negative electrode material according to claim 1, characterized in that The silicon particles are nano-silicon particles; the particle size of the nano-silicon particles is 20 to 100 nm.

6. The method for preparing the negative electrode material according to any one of claims 1 to 5, characterized in that: The steps include: (1) dehydrating the first hard carbon precursor to obtain a first hard carbon coating layer; (2) introducing a silicon source gas and an inert gas into the first hard carbon coating layer, and depositing silicon in the first hard carbon coating layer by a chemical vapor deposition method to obtain a first carbon-silicon mixture; (3) crosslinking and carbonizing the first carbon-silicon mixture in an inert gas atmosphere to obtain a first carbon-silicon composite; (4) coating the surface of the first carbon-silicon composite with a second hard carbon precursor liquid phase, and carbonizing the surface to obtain a second carbon-silicon composite; The first hard carbon precursor is a starch-based biomass hard carbon precursor, and the particle size D50 value of the starch-based biomass hard carbon precursor is 0.5 μm to 5 μm.

7. The preparation method according to claim 6, characterized in that The starch-based biomass hard carbon precursor includes at least one of phosphate-modified starch, corn starch, mung bean starch, cassava starch, sweet potato starch, red potato starch, potato starch, wheat starch, water chestnut starch, lotus root starch, and rice starch.

8. The preparation method according to claim 6 or 7, characterized in that The second hard carbon precursor includes at least one of a resin-based hard carbon precursor, an organic polymer pyrolytic carbon-based hard carbon precursor, a carbon black-based hard carbon precursor, and a biomass carbon-based hard carbon precursor.

9. An electrode sheet, characterized in that: The negative electrode material comprises the negative electrode material according to any one of claims 1 to 5, and / or the negative electrode material prepared by the preparation method according to any one of claims 6 to 8.

10. A battery, characterized in that: Including the electrode sheet according to claim 9.

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