Negative active material, preparation method thereof, negative electrode sheet and lithium ion battery

Abstract

The application relates to the field of lithium ion batteries, and discloses a negative electrode active material, a preparation method thereof, a negative electrode sheet and a lithium ion battery. The application adopts a core-shell structure, uses hard carbon-coated graphite as an outer shell to wrap a negative electrode material as an inner core, can greatly reduce the expansion of silicon in the battery cycle process, improves the cycle performance and low-temperature performance of the battery, and has a relatively low internal resistance; meanwhile, when the hard carbon-coated graphite is used in combination with other negative electrode materials such as graphite, the conductivity of the prepared lithium ion battery can be greatly improved.

Description

Technical Field

[0001] This application relates to the field of lithium-ion batteries, and more particularly to a negative electrode active material and its preparation method, a negative electrode sheet, and a lithium-ion battery. Background Technology

[0002] Graphite, as a low-cost material with good electrical conductivity, is widely used in battery anode materials. Currently, the theoretical specific capacity of graphite anode materials used in the market is 372 mAh / g, while the theoretical specific capacity of silicon-carbon composite materials is approximately 4200 mAh / g, more than 10 times higher than that of graphite anodes. The high capacity of silicon-carbon anode materials can fully meet the energy requirements of pure electric vehicle batteries. However, the huge volume expansion effect generated during the charging and discharging process of silicon-based lithium-ion batteries makes their industrialization difficult.

[0003] Currently, most silicon anode materials are pre-lithiated / pre-magnesiated / carbon-coated, which improves the expansion effect during silicon cycling. However, compared with conventional artificial graphite, they are inferior in terms of cycling performance, low-temperature performance and internal resistance, and their performance still needs to be further improved. Summary of the Invention

[0004] In view of this, the purpose of this application is to provide a negative electrode active material and a method for preparing the same, such that the negative electrode active material can have high cycle performance, low temperature performance and low internal resistance;

[0005] Another objective of this application is to provide a negative electrode composite active material, a negative electrode sheet, and a lithium-ion battery based on the aforementioned negative electrode active material.

[0006] In order to solve the above-mentioned technical problems / achieve the above-mentioned objectives, or at least partially solve the above-mentioned technical problems / achieve the above-mentioned objectives, as a first aspect of this application, a negative active electrode material is provided, having a core-shell structure, the core-shell structure including a core, a first shell layer covering the surface of the core, and a second shell layer covering the surface of the first shell layer; the core contains a negative electrode material, the first shell layer includes graphite, and the second shell layer includes hard carbon.

[0007] Optionally, the first shell layer of graphite and the second shell layer of hard carbon account for 3-8% of the mass percentage of the negative electrode active material.

[0008] Optionally, the mass ratio of the second shell hard carbon to the first shell graphite is (12-20):100.

[0009] Optionally, the negative electrode material is a graphite negative electrode material and / or a silicon negative electrode material.

[0010] Optionally, the silicon anode material is silicon suboxide; further optionally, the silicon suboxide is pre-lithiated silicon suboxide, pre-magnesiated silicon suboxide, or carbon-coated silicon suboxide.

[0011] Optionally, the hard carbon is derived from bitumen or biomass.

[0012] To address the problems of large expansion during cycling and low cycle performance and low-temperature performance of existing silicon anode materials, this application develops a core-shell structured anode active material that significantly reduces the expansion of silicon in the middle during battery cycling, improves the battery's cycle performance and low-temperature performance, and has low internal resistance. This significantly improves the conductivity of lithium-ion batteries made with this anode active material. Therefore, the anode active material described in this application can be used in the preparation of anode composite active materials, anode sheets, or lithium-ion batteries. In these applications, any conventional materials and processes can be used with the anode active material provided in this application to prepare anode sheets and lithium-ion batteries.

[0013] As a second aspect of this application, a negative electrode composite active material is provided, comprising the negative electrode active material described in this application and graphite. Optionally, in the negative electrode composite active material, the mass ratio of the negative electrode active material to graphite is (0.1:99.9)-(20:80); optionally, in the negative electrode composite active material, the D of the graphite is... 50 The micrometer diameter is 7-8 μm, and the tap density is 0.7-0.8 g / cm³. 3 Its specific surface area is 2.0-2.2 m². 2 / g. As a third aspect of this application, a negative electrode sheet is provided, comprising a negative electrode active material or a negative electrode composite active material according to any of the foregoing embodiments of this application, as well as a conductive agent, a binder, and a current collector.

[0014] As a fourth aspect of this application, a lithium-ion battery is provided, including the aforementioned negative electrode, as well as a separator, an electrolyte, and a lithium source positive electrode.

[0015] As a fifth aspect of this application, a method for preparing the aforementioned negative electrode active material is provided, comprising:

[0016] After mixing graphite and hard carbon precursor, the mixture is subjected to high-temperature treatment to coat the graphite with the hard carbon precursor. Then, it is cured, carbonized, and cooled to obtain graphite coated with hard carbon.

[0017] The hard carbon-coated graphite and the negative electrode material are subjected to high-temperature heat treatment, and the negative electrode active material is obtained after cooling.

[0018] Optionally, the preparation method includes:

[0019] After mixing graphite and hard carbon precursor, the mixture is treated at 250-350℃ in an inert gas atmosphere to coat the graphite with the hard carbon precursor. Then, it is mixed with Tween, hydrogen peroxide and phosphoric acid and dried and cured. Then, it is pre-carbonized at 550-650℃ in an inert gas atmosphere, depolymerized after cooling to room temperature, and then carbonized at 400-1500℃ in an inert gas atmosphere. After cooling, hard carbon-coated graphite is obtained.

[0020] The hard carbon-coated graphite and the negative electrode material are subjected to heat treatment at 700℃-1100℃ in an inert gas atmosphere, and the negative electrode active material is obtained after cooling.

[0021] Compared with existing silicon anode materials, this application adopts a core-shell structure, using hard carbon-coated graphite as the outer shell to wrap the anode material as the core. This can significantly reduce the expansion of silicon in the middle during battery cycling, improve the cycle performance and low-temperature performance of the battery, and have lower internal resistance. At the same time, when used in combination with other anode materials such as graphite, the conductivity of the prepared lithium-ion battery can be greatly improved. Attached Figure Description

[0022] Figure 1 The diagram shown is a schematic diagram of the structure of the negative electrode active material of this application. Detailed Implementation

[0023] This application discloses a negative electrode active material, its preparation method, applications, as well as a negative electrode sheet and a lithium-ion battery. Those skilled in the art can refer to the content of this application and appropriately modify the process parameters to achieve the desired results. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this application. The products, processes, and applications described in this application have been described through preferred embodiments. Those skilled in the art can obviously make modifications or appropriate changes and combinations to the products, processes, and applications described herein without departing from the content, spirit, and scope of this application to realize and apply the technology of this application. Obviously, the described embodiments are only some, not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0024] It should be noted that, in this document, relational terms such as "first" and "second," "step 1" and "step 2," and "(1)" and "(2)" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element. Moreover, the embodiments and features described in this application can be combined with each other without conflict.

[0025] In a first aspect of this application, a core-shell structured silicon anode active material is provided. The core-shell structure includes a silicon anode material and / or a graphite anode material as the core, graphite as a first shell layer coated on the surface of the core, and hard carbon as a second shell layer coated on the surface of the first shell layer. A schematic diagram is shown below. Figure 1 The graphite anode material can be natural graphite, artificial graphite, or a mixture of both.

[0026] In some embodiments of this application, the first shell graphite and the second shell hard carbon account for 3-8% of the mass percentage of the negative electrode active material; in other embodiments of this application, the first shell graphite and the second shell hard carbon account for 3%, 4%, 5%, 6%, 7% or 8% of the mass percentage of the negative electrode active material.

[0027] In some embodiments of this application, the mass ratio of the second shell hard carbon to the first shell graphite is (12-20):100; in other embodiments of this application, the mass ratio of the second shell hard carbon to the first shell graphite is 12:100, 13:100, 14:100, 15:100, 16:100, 17:100, 18:100, 19:100 or 20:100.

[0028] In some embodiments of this application, the D of the negative electrode material 50 The thickness is 5-13 μm; in some other embodiments of this application, the D of the negative electrode material is... 50 It is 5-8μm.

[0029] In some embodiments of this application, the D of the hard carbon precursor 50The diameter is 4-10 μm; in some other embodiments of this application, the D of the hard carbon precursor is... 50 It is 4-7 μm.

[0030] In some embodiments of this application, the D of the first shell graphite 50 The diameter is 5-15 μm; in some other embodiments of this application, the D of the first shell graphite is... 50 It is 7-13μm.

[0031] In some embodiments of this application, the silicon anode material is silicon suboxide; in other embodiments of this application, the silicon suboxide is pre-lithiated silicon suboxide, pre-magnesiated silicon suboxide, or carbon-coated silicon suboxide, wherein the pre-lithiated silicon suboxide is an exemplary experimental material in the embodiments, which is doped with lithium source lithium powder or LiH by referring to conventional processes, and the mass ratio of lithium source lithium powder or LiH to silicon suboxide is usually (0.5-1):1.

[0032] In some embodiments of this application, the hard carbon is based on asphalt or biomass, such as petroleum asphalt, peanut shells, coconut shells, etc.; in other embodiments of this application, the petroleum asphalt is medium-temperature petroleum asphalt.

[0033] In some embodiments of this application, the graphite may be artificial graphite, such as coal-based artificial graphite, petroleum-based artificial graphite, or a mixture of both.

[0034] In a second aspect of this application, the negative electrode active material under any of the above-mentioned schemes is used in combination with other negative electrode materials. Differences in particle size, specific surface area, and tap density also affect its compatibility with other negative electrode materials. Based on this, this application has found that the performance of the above-mentioned negative electrode active material is optimal when mixed with graphite. Furthermore, different mass ratios of the above-mentioned negative electrode active material and artificial graphite result in significant differences in the cycle life and conductivity of the resulting lithium-ion battery. Therefore, in some embodiments of this application, the negative active electrode material further includes a negative electrode composite active material formed by mixing with graphite.

[0035] In some embodiments of the negative electrode composite active material of this application, the mass ratio of the negative electrode active material to graphite is (0.1:99.9)-(20:80). When the negative electrode composite active material accounts for 95% of the entire negative electrode slurry, this mass ratio can be converted to (0.095:94.905)-(19:76). In other embodiments of this application, the mass ratio of the negative electrode active material to graphite is (5:95)-(15:85). When the negative electrode composite active material accounts for 95% of the entire negative electrode slurry, this mass ratio can be converted to (4.75:90.25)- (14.25:80.75); In some other embodiments of this application, the mass ratio of the negative electrode active material to graphite is 8:92. When the negative electrode composite active material accounts for 95% of the entire negative electrode slurry, this mass ratio can be converted to 7.6:87.4; In some other embodiments of this application, when the negative electrode composite active material accounts for 95% of the entire negative electrode slurry, the mass ratio of the negative electrode active material to graphite is (5:90)-(10:85), for example, 10:85, 5:90, 6:89 or 9:86; It should be noted that all of the above mass ratios can be converted proportionally.

[0036] In the negative electrode composite active materials of certain embodiments of this application, the D of graphite as another composite negative electrode material 50 The micrometer diameter is 7-8 μm, and the tap density is 0.7-0.8 g / cm³. 3 Its specific surface area is 2.0-2.2 m². 2 / g.

[0037] In the negative electrode composite active material of certain embodiments of this application, the other negative electrode material graphite as composite may also be selected as artificial graphite, such as coal-based artificial graphite, petroleum-based artificial graphite or a mixture of both.

[0038] This application provides the application of the aforementioned negative electrode composite active material in the preparation of battery negative electrodes or lithium-ion batteries; the lithium battery prepared using the aforementioned negative electrode composite active material, compared with the experimental group using graphite or negative electrode active material alone, has a low-temperature discharge capacity retention rate of over 89% and a capacity retention rate of over 81% after 1000 cycles.

[0039] In a third aspect of this application, a negative electrode sheet is provided, comprising a negative electrode active material or a negative electrode composite active material according to any of the foregoing schemes of this application, as well as a conductive agent, a binder and a current collector, wherein the negative electrode active material or negative electrode composite active material, as well as the conductive agent and the binder, constitute a negative electrode slurry, and the negative electrode sheet is formed by coating the negative electrode slurry onto the surface of the current collector.

[0040] In some embodiments of this application, the mass ratio of the negative electrode active material or negative electrode composite active material, conductive agent, and binder in the negative electrode slurry is l:m:n, and l+m+n=100, where l is 95-99, m is 0.5-3, and n is 1-5, for example: 96:1.5:2.5, 95:1:4, etc.

[0041] In some embodiments of this application, the conductive agent is carbon nanotubes, which may be selected with a specific surface area of ​​200-300 m². 2 / g of carbon nanotubes; the binder is CMC, for example, CMC with a pH of 6.5-8; and / or SBR, for example, SBR with a pH of 7-9.

[0042] In some embodiments of this application, the current collector is a metal foil, such as aluminum foil, copper foil, etc.

[0043] In a fourth aspect of this application, a lithium-ion battery is provided, including the aforementioned negative electrode, as well as a separator, an electrolyte, and a lithium source positive electrode.

[0044] In some embodiments of this application, the lithium source material in the positive electrode is selected from at least one of lithium cobalt oxide, lithium manganese oxide, and lithium nickel cobalt manganese oxide; the separator is selected from polyethylene.

[0045] In a fifth aspect of this application, a method for preparing the aforementioned negative electrode active material is provided, comprising:

[0046] After mixing graphite and hard carbon precursor, the mixture is subjected to high-temperature treatment to coat the graphite with the hard carbon precursor. Then, it is cured, carbonized, and cooled to obtain graphite coated with hard carbon.

[0047] The hard carbon-coated graphite and the negative electrode material are subjected to high-temperature heat treatment, and the negative electrode active material is obtained after cooling.

[0048] In some embodiments of this application, the carbonization may further include pre-carbonization, carried out at 550-650°C in an inert gas atmosphere; the carbonization is carried out at 400-1500°C in an inert gas atmosphere; in other embodiments of this application, the carbonization temperature is 800-1300°C or 1000-1200°C.

[0049] In some embodiments of this application, the pre-carbonization or carbonization time is 1-10 hours; in other embodiments of this application, the time is 2-8 hours; in still other embodiments of this application, the time is 4-6 hours.

[0050] In some embodiments of this application, the temperature during the high-temperature heat treatment of the hard carbon-coated graphite and the negative electrode material is 700-1100℃, for example, 1000℃; and the treatment time is 4-12h, for example, 8h.

[0051] In some embodiments of this application, the preparation method includes:

[0052] After mixing graphite and hard carbon precursor, the mixture is treated at 250-350℃ in an inert gas atmosphere to coat the graphite with the hard carbon precursor. Then, it is mixed with Tween, hydrogen peroxide and phosphoric acid and dried and cured. Then, it is pre-carbonized at 550-650℃ in an inert gas atmosphere, depolymerized after cooling to room temperature, and then carbonized at 400-1500℃ in an inert gas atmosphere. After cooling, hard carbon-coated graphite is obtained.

[0053] The hard carbon-coated graphite and the negative electrode material are subjected to heat treatment at 700℃-1100℃ in an inert gas atmosphere, and the negative electrode active material is obtained after cooling.

[0054] In some embodiments of this application, the inert gas is nitrogen.

[0055] In some embodiments of this application, the desired temperature can be reached by heating at a rated heating rate of 2-5°C / min.

[0056] In other embodiments of this application, the preparation method includes:

[0057] Artificial graphite (D) 50 =13μm) and hard carbon precursors (medium-temperature petroleum asphalt powder or peanut shell or coconut shell, D 50 =6μm) were mixed in a mixer at a mass ratio of 100:15, and after discharge, the mixture was transferred to a coating kettle and treated at 300℃ for 4 hours under a nitrogen atmosphere at a rate of 2℃ / min to coat the surface of artificial graphite with hard carbon precursor. The hard carbon precursor coating was then mixed with Tween 20, hydrogen peroxide (30%), and phosphoric acid at a mass ratio of 100:3.5:14:2 to prepare a water slurry with a solid content of 50%. The slurry was dried in a 100℃ drying oven for 8 hours to carry out the curing reaction. The resulting surface hard carbon cured coating was loaded into a pit furnace and pre-carbonized at 600℃ for 1 hour under nitrogen protection at a rate of 2℃ / min. After cooling to a room temperature, the coating was depolymerized in a mixer for 30 minutes and then loaded into an atmosphere furnace and carbonized at 1150℃ for 4 hours under nitrogen protection at a rate of 2℃ / min. After cooling to a room temperature, the coating was discharged and screened to obtain hard carbon coated graphite.

[0058] Pre-lithiated silicon suboxide powder is placed in a rotary furnace and heated to 700℃-1100℃ at a set heating rate of 5℃ / min under nitrogen protection. Hard carbon-coated graphite is then introduced and kept at this temperature for 4-12 hours. After cooling, the silicon anode active material is obtained.

[0059] In the comparative experiments provided in this application, unless otherwise specified, all experimental conditions and materials are kept consistent to ensure comparability. Unless otherwise specified, all experimental materials and reagents used in the examples are commercially available.

[0060] Unless otherwise specified, "room temperature" as used in this application refers to the temperature measured in a typical laboratory using a Celsius thermometer, generally 20-30°C, preferably 25°C (Kelvin temperature is 298.15K).

[0061] The following provides a further description of the negative electrode active material, its preparation method, the negative electrode sheet, and the lithium-ion battery provided in this application.

[0062] First embodiment: Preparation of the silicon anode active material of this application (the core is a silicon anode material).

[0063] Artificial graphite (D) 50 =13μm) and medium-temperature petroleum asphalt powder (D 50 =6μm) were mixed in a mixer at a mass ratio of 100:15, and after discharge, the mixture was transferred to a coating kettle and treated at 300℃ for 4 hours under a nitrogen atmosphere at a rate of 2℃ / min, so that the asphalt melted and coated the surface of the artificial graphite. The obtained asphalt coating was mixed with Tween 20, hydrogen peroxide (30%), and phosphoric acid at a mass ratio of 100:3.5:14:2 to prepare a water slurry with a solid content of 50%, and dried in a 100℃ drying oven for 8 hours for curing reaction. The obtained surface asphalt cured coating was loaded into a pit furnace and pre-carbonized at 600℃ for 1 hour under nitrogen protection at a rate of 2℃ / min. After cooling to a greenhouse, it was depolymerized in a mixer for 30 minutes, and then loaded into an atmosphere furnace and carbonized at 1150℃ for 4 hours under nitrogen protection at a rate of 2℃ / min. After cooling to a greenhouse, it was discharged and screened to obtain hard carbon coated graphite.

[0064] Pre-lithiated silicon suboxide powder is placed in a rotary furnace and heated to 700℃-1100℃ at a set heating rate of 5℃ / min under nitrogen protection. Hard carbon-coated graphite is then introduced and kept at this temperature for 4-12 hours. After cooling, the silicon anode active material is obtained.

[0065] Table 1

[0066]

[0067] Second embodiment: Preparation of negative electrode sheet

[0068] Artificial graphite was used as another negative electrode active material, and silicon negative electrode active material prepared in the first embodiment was added to form a silicon negative electrode composite active material. The silicon negative electrode composite active material (based on the mass of negative electrode slurry), 1% conductive carbon black (based on the mass of negative electrode slurry), 2.5% SBR (based on the mass of negative electrode slurry), and 1.5% CMC (based on the mass of negative electrode slurry) were dissolved in deionized water solvent in a mixer under vacuum to mix and disperse, so as to prepare a uniform negative electrode slurry without bubbles. The slurry was uniformly coated on copper foil to prepare a negative electrode sheet. The differences between the experimental groups are shown in Table 2 below. Other parts not mentioned are consistent.

[0069] Table 2

[0070]

[0071]

[0072] Third Example: Preparation and Performance Testing of Lithium-ion Batteries

[0073] 1. Lithium-ion battery manufacturing

[0074] (1) Preparation of positive electrode sheet

[0075] Lithium nickel cobalt manganese oxide is used as the positive electrode active material, wherein the D of lithium nickel cobalt manganese oxide is... 50 Its diameter is 13 μm, and its specific surface area is 4.2 m². 2 / g. 96% of the positive electrode active material (based on the mass of the positive electrode slurry), 2% PVDF (based on the mass of the positive electrode), 1% conductive carbon black (based on the mass of the positive electrode slurry), and 1% carbon nanotubes (based on the mass of the positive electrode slurry) were dissolved in N-methylpyridinyl ketone. The mixture was then dispersed under vacuum in a mixer to prepare a uniform, bubble-free slurry, which was then uniformly coated onto aluminum foil to prepare the positive electrode sheet.

[0076] (2) Packaging and Formation

[0077] The negative electrode sheet, PVC separator and positive electrode sheet prepared in the second embodiment are stacked to form a battery cell. The battery cell has tabs on the same side. The tabs are welded to the current collector by an ultrasonic welding machine and then packaged with an aluminum-plastic film.

[0078] After the battery cell is baked, a non-aqueous electrolyte is injected into the cell. After being formed and tested, a 4Ah lithium-ion battery is prepared for battery performance testing.

[0079] 2. Performance Testing Methods

[0080] Battery cycle performance test methods:

[0081] (1) The experimental cell was charged to 4.2V at a constant current and constant voltage of 0.5C under the condition of 25℃±3℃, and the cut-off current was 0.02C;

[0082] (2) Let it sit for 30 minutes;

[0083] (3) Under the condition of 25℃±3, discharge at 1C constant current to 3.0V, and record the capacity D1 at this time;

[0084] (4) Let it sit for 30 minutes;

[0085] (5) Repeat (1)-(4) to record the capacity D2 when the capacity decays to 80%;

[0086] Capacity retention rate D = D2 / D1 * 100%

[0087] Battery low-temperature performance test methods:

[0088] (1) The experimental cell was charged to 4.2V at a constant current and constant voltage of 0.5C under the condition of 25℃±3℃, and the cut-off current was 0.02C;

[0089] (2) Let it sit for 30 minutes;

[0090] (3) Under the condition of 25℃±3, discharge at 1C constant current to 3.0V, and record the capacity D1 at this time;

[0091] (4) Let it sit for 30 minutes;

[0092] (5) Under the condition of 25℃±3, the battery cell is charged to 4.2V with constant current and constant voltage, and the cut-off current is 0.02C;

[0093] (6) Let it stand at -40℃±3 for 20 hours;

[0094] (7) Discharge at 5C to 3.0V and record the capacitance D2 at this time;

[0095] Capacity retention rate D = D2 / D1 * 100%.

[0096] 3. Experimental Results

[0097] See Table 3;

[0098] Table 3

[0099]

[0100] As shown in Table 3, experimental groups 9 and 10 are experimental groups that use artificial graphite alone and silicon anode active material of this application alone, respectively. Their low-temperature performance and cycle performance are poor. The low-temperature discharge capacity retention rate is 52% and 75%, respectively. The capacity retention rate decays to 80% before reaching 1000 cycles.

[0101] Among the other experimental groups, experimental groups 1-5, 7, and 11-12 showed better low-temperature performance and cycling performance. Considering low-temperature performance, internal resistance, and cycling performance, experimental groups 1, 4, and 5 were the best.

[0102] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A negative electrode active material, characterized by, The material has a core-shell structure, comprising a core, a first shell layer covering the surface of the core, and a second shell layer covering the surface of the first shell layer; the core contains a negative electrode material, which is a silicon negative electrode material; the first shell layer contains graphite; and the second shell layer contains hard carbon; the negative electrode active material is prepared according to the following method: After mixing graphite and hard carbon precursor, the mixture is treated at 250-350℃ in an inert gas atmosphere to coat the graphite with the hard carbon precursor. Then, it is mixed with Tween, hydrogen peroxide, and phosphoric acid, dried and cured, and then pre-carbonized at 550-650℃ in an inert gas atmosphere. After cooling to a room temperature, it is depolymerized and then carbonized at 400-1500℃ in an inert gas atmosphere. After cooling, hard carbon-coated graphite is obtained. The hard carbon is based on pitch or biomass as a precursor. The hard carbon-coated graphite and the negative electrode material are subjected to heat treatment at 700℃-1100℃ in an inert gas atmosphere, and the negative electrode active material is obtained after cooling.

2. The negative active material according to claim 1, characterized in that, The graphite and hard carbon account for 3-8% of the mass of the negative electrode active material.

3. The negative active material according to claim 1 or 2, characterized in that, The mass ratio of hard carbon to graphite is (12-20):

100.

4. The negative electrode active material according to claim 1, characterized in that, The silicon anode material is silicon suboxide.

5. The negative active material according to claim 4, characterized in that, The silicon suboxide is pre-lithiated silicon suboxide, pre-magnesiated silicon suboxide, or carbon-coated silicon suboxide.

6. A negative electrode composite active material, characterized by, It includes the negative electrode active material as described in any one of claims 1-5 and graphite; the mass ratio of the negative electrode active material to graphite is (5:90)-(10:85); D of the graphite is 7-8 μm, the tap density is 0.7-0.8 g / cm 50 3, the specific surface area is 2.1-2.2 m 3 2 / g. 2 2 / g.

7. A negative electrode sheet characterized by comprising: It includes the negative electrode composite active material as described in claim 6, as well as a conductive agent, a binder, and a current collector.

8. A lithium-ion battery, characterized by, It includes the negative electrode, separator, electrolyte, and lithium source positive electrode as described in claim 7.

9. The method for preparing the negative electrode active material according to claim 1, characterized in that, include: After mixing graphite and hard carbon precursor, the mixture is treated at 250-350℃ in an inert gas atmosphere to coat the graphite with the hard carbon precursor. Then, it is mixed with Tween, hydrogen peroxide and phosphoric acid and dried and cured. Then, it is pre-carbonized at 550-650℃ in an inert gas atmosphere, depolymerized after cooling to room temperature, and then carbonized at 400-1500℃ in an inert gas atmosphere. After cooling, hard carbon-coated graphite is obtained. The hard carbon-coated graphite and the negative electrode material are subjected to heat treatment at 700℃-1100℃ in an inert gas atmosphere, and the negative electrode active material is obtained after cooling.

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