A hard carbon-coated iron-based oxide composite material and its preparation method

Abstract

The present invention relates to the technical field of the preparation of lithium-ion battery materials, and provides a hard carbon-coated iron-based oxide composite material and a preparation method thereof. The preparation method of the hard carbon-coated iron-based oxide composite material includes: S1, mixing an iron source, a sodium source, ammonia water, a stabilizer and water for reaction to obtain an iron-based sodium salt compound; S2, mixing a hard carbon precursor with an acid solution, and reacting to obtain an oxidized hard carbon precursor; S3, performing a hydrothermal reaction on the oxidized hard carbon precursor, the iron-based sodium salt compound and graphene oxide to obtain a graphene oxide / polymer-coated iron-based sodium salt composite material; S4, reducing the graphene oxide / polymer-coated iron-based sodium salt composite material, performing gas surface treatment, and carbonizing to obtain a hard carbon-coated iron-based oxide composite material. Through the above technical solution, the problems in the prior art that when using iron oxide as the negative electrode material, there are large swelling and low ionic conductivity, resulting in poor rate performance and cycling performance of the lithium-ion battery are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion battery material preparation, and specifically, to a hard carbon-coated iron-based oxide composite material and a preparation method thereof. Background Art

[0002] At present, the commercialized anodes are mainly artificial graphite and natural graphite, but they have a relatively low specific capacity (350-360 mAh / g), which cannot meet the market demand for high energy density. Although silicon-carbon materials have high energy, their large expansion makes them unable to meet the widespread promotion in electric vehicles. Therefore, while studying carbon materials, people have gradually turned their attention to the research and development of other non-carbon anode materials with high specific capacity, especially various metal oxide anode materials.

[0003] Iron oxide is considered to be a promising anode material for lithium-ion batteries due to its high theoretical specific capacity (1007 mAh / g), rich material sources, low price, environmental friendliness, etc. Among them, iron oxides usually have a high theoretical specific capacity. For example, the specific capacity of Fe 3 O 4 is 924 mAh / g, and the specific capacity of Fe 2 O 3 is 1007 mAh / g, which is more than twice the specific capacity of graphite and has a relatively high electronic conductivity. However, when using iron oxide as the anode material, there are problems such as large expansion and low ionic conductivity, which are not conducive to improving the rate performance. Summary of the Invention

[0004] The present invention provides a hard carbon-coated iron-based oxide composite material and a preparation method thereof, which solve the problems of large expansion and low ionic conductivity when using iron oxide as the anode material in the related art, resulting in poor rate performance and cycle performance of lithium-ion batteries.

[0005] The technical solution of the present invention is as follows:

[0006] A preparation method of a hard carbon-coated iron-based oxide composite material includes the following steps:

[0007] S1. Mix an iron source, a sodium source, ammonia water, a stabilizer and water, and then react to obtain an iron-based sodium salt compound;

[0008] S2. Mix a hard carbon precursor with an acid solution, and react to obtain an oxidized hard carbon precursor;

[0009] S3. Mix the oxidized hard carbon precursor, the iron-based sodium salt compound, graphene oxide and water, and then carry out a hydrothermal reaction to obtain a graphene oxide / polymer-coated iron-based sodium salt composite material;

[0010] S4. After reducing the graphene oxide / polymer-coated iron-based sodium salt composite material, perform gas surface treatment and carbonization to obtain a hard carbon-coated iron-based oxide composite material.

[0011] As a further technical solution, it further includes at least one of the following technical features:

[0012] In S1, the mass ratio of the iron source, sodium source, ammonia water, stabilizer to water is 100:100 - 300:500 - 1000:1 - 10:1000;

[0013] In S1, the stabilizer includes one or two of sodium iminodisuccinate and ethylenediaminetetraacetic acid;

[0014] In S1, the reaction temperature is 50 - 100 °C, and the reaction time is 1 - 6 h.

[0015] As a further technical solution, in S1, the iron source is ferrous oxalate; the sodium source is sodium sulfate.

[0016] As a further technical solution, in S2, the addition amounts of the hard carbon precursor and the acid solution are calculated based on the concentration of the hard carbon precursor after mixing being 10 - 20 wt%.

[0017] As a further technical solution, in S2, the hard carbon precursor includes one or several of coconut shell, apricot shell, starch, phenolic resin, epoxy resin, and sucrose.

[0018] As a further technical solution, in S2, the acid solution includes one or several of sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid.

[0019] As a further technical solution, in S2, the mass concentration of the acid solution is 1 - 20%.

[0020] As a further technical solution, in S2, the reaction temperature is 100 - 200 °C, and the reaction time is 10 - 60 min.

[0021] As a further technical solution, in S3, the mass ratio of the oxidized hard carbon precursor, iron-based sodium salt compound, graphene oxide to water is 100:10 - 50:100 - 500:1000.

[0022] As a further technical solution, in S3, the hydrothermal reaction temperature is 100 - 200 °C, the time is 1 - 6 h, and the pressure is 1 - 3 MPa.

[0023] As a further technical solution, the reduction in S4 is carried out under a hydrogen and argon atmosphere with a volume ratio of 1 - 5:10 for the reduction reaction.

[0024] As a further technical solution, the reduction temperature in S4 is 200 - 300 °C, and the reduction time is 1 - 3 h.

[0025] As a further technical solution, gas surface treatment is carried out in S4, specifically: a mixed gas of sulfur-containing gas and argon with a volume ratio of 1 - 5:10 is introduced at a flow rate of 50 mL / min.

[0026] As a further technical solution, the sulfur-containing gas includes one or more of hydrogen sulfide, sulfur dioxide, and sulfur trioxide.

[0027] As a further technical solution, the carbonization temperature in S4 is 700 - 1100 °C, and the carbonization time is 1 - 6 h.

[0028] As a further technical solution, the hard carbon-coated iron-based oxide composite material prepared by the preparation method of the hard carbon-coated iron-based oxide composite material.

[0029] As a further technical solution, the core of the hard carbon-coated iron-based oxide composite material is an iron-based sodium salt compound, and the shell is a hard carbon composite material;

[0030] In the iron-based sodium salt compound, the molar ratio of Na + to Fe 2+ is 2:1.

[0031] As a further technical solution, the mass ratio of the core to the shell is 1 - 3:7 - 9.

[0032] The working principle and beneficial effects of the present invention are as follows:

[0033] 1. The present invention first prepares an iron-based sodium salt compound and adds it to the oxidized hard carbon precursor. Through hydrothermal reaction, gas surface treatment, and carbonization, a spherical hard carbon-coated iron-based oxide composite material is obtained. The present invention utilizes the specific capacity and electronic conductivity of the iron-based oxide itself to improve the power performance, and reduces the expansion through the hard carbon of the shell; at the same time, the iron-based sodium salt compound in the core can improve the ion transport rate during charge and discharge, that is, improve the electronic and ionic conductivity of the material, and improve the power performance and cycle performance of the composite material.

[0034] 2. The present invention improves the electronic conductivity of the material by doping graphene in the iron-based sodium salt compound material, and at the same time utilizes the high specific capacity and high lithium ion conductivity of the iron-based sodium salt compound itself to improve the rate performance. At the same time, a hard carbon is coated on the shell to reduce the expansion of the iron-based sodium salt compound during charge and discharge and improve the cycle performance. And through the sulfur-containing gas, the hard carbon material is modified to reduce defects, improve the first efficiency, and the sulfur and carbon will form a -C-S- chemical bond to improve the structural stability of the material. Description of the Drawings

[0035] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0036] Figure 1 It is a scanning electron microscope image of the hard carbon-coated iron-based oxide composite material obtained in Example 1 of the present invention. Specific embodiments

[0037] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts fall within the scope of protection of the present invention.

[0038] Example 1

[0039] S1. Add 14.4 g (0.1 mol) of ferrous oxalate and 14.2 g (0.1 mol) of sodium sulfate to 144 g of deionized water, and add 112 g of ammonia water (25 wt%) and 0.7 g of sodium iminodisuccinate. React at 80 °C for 3 h. After filtration, dry in vacuum at 80 °C for 24 h to obtain Na 2 Fe(C 2 O 4 )(SO 4 ) compound;

[0040] S2. Prepare a solution with a mass concentration of 10% sulfuric acid solution of 666 g and 100 g of coconut shell, with a coconut shell concentration of 13 wt%. Carry out hydrothermal reaction in a closed microwave reaction kettle at 150 °C for 30 min, wash to obtain an oxidized hard carbon precursor;

[0041] S3. Add 100 g of the oxidized hard carbon precursor to 1000 g of deionized water and mix evenly, then add 30 g of Na 2 Fe(C 2 O 4 )(SO 4 ) compound and mix evenly, then add 300 g of a 5% graphene oxide solution. Carry out hydrothermal reaction at 150 °C and 2 MPa for 3 h. After filtration, dry in vacuum at 80 °C for 24 h to obtain a graphene oxide / polymer-coated iron-based sodium salt composite material;

[0042] S4. Transfer the obtained graphene oxide / polymer-coated iron-based sodium salt composite material into a tubular furnace. After purging the air in the tube with argon, then introduce a mixed reducing gas of hydrogen and argon with a volume ratio of 3:10 at a flow rate of 50 mL / min. After reducing at 250 °C for 2 h, then introduce a mixed gas of sulfur dioxide and argon with a volume ratio of 3:10 at a flow rate of 50 mL / min, heat up to 900 °C and hold for 3 h, and then pulverize to obtain a hard carbon-coated iron-based oxide composite material.

[0043] Example 2

[0044] S1. Add 7.2 g (0.05 mol) of ferrous oxalate and 14.2 g (0.1 mol) of sodium sulfate to 72 g of deionized water, and add 36 g of ammonia water (25 wt%) and 0.072 g of tetrasodium iminodisuccinate. React at 50 °C for 6 h. After filtration, dry in vacuum at 80 °C for 24 h to obtain Na 2 Fe(C 2 O 4 )(SO 4 ) compound;

[0045] S2. Add 100 g of apricot shells to 900 g of hydrochloric acid solution with a mass concentration of 10% to prepare a solution with an apricot shell concentration of 10 wt%. Carry out hydrothermal reaction in a sealed microwave reactor at 100 °C for 60 min, wash to obtain an oxidized hard carbon precursor;

[0046] S3. Add 100 g of the oxidized hard carbon precursor to 1000 g of deionized water and mix evenly, then add 10 g of Na 2 Fe(C 2 O 4 )(SO 4 ) compound and mix evenly, then add 100 g of graphene oxide solution with a mass concentration of 5%. Carry out hydrothermal reaction at 100 °C and 3 MPa for 6 h. After filtration, dry in vacuum at 80 °C for 24 h to obtain a graphene oxide / polymer-coated iron-based sodium salt composite material;

[0047] S4. Transfer the obtained graphene oxide / polymer-coated iron-based sodium salt composite material into a tubular furnace. After purging the air in the tube with argon, then introduce a mixed reducing gas of hydrogen and argon with a volume ratio of 1:10 at a flow rate of 50 mL / min. After reducing at 200 °C for 3 h, then introduce a mixed gas of sulfur trioxide and argon with a volume ratio of 1:10 at a flow rate of 50 mL / min, heat up to 700 °C and hold for 6 h, and then pulverize to obtain a hard carbon-coated iron-based oxide composite material.

[0048] Example 3

[0049] S1. Add 14.4 g (0.1 mol) of ferrous oxalate and 42.6 g (0.3 mol) of sodium sulfate to 144 g of deionized water, and add 144 g of ammonia water (25 wt%) and 1.44 g of ethylenediaminetetraacetic acid. React at 100 °C for 1 h. After filtration, dry in vacuum at 80 °C for 24 h to obtain Na 2 Fe(C 2 O 4 )(SO 4 ) compound;

[0050] S2. Add 100 g of sucrose to 400 g of phosphoric acid solution with a mass concentration of 10% to prepare a solution with a sucrose concentration of 20 wt%. Carry out hydrothermal reaction in a sealed microwave reactor at 200 °C for 10 min, wash to obtain an oxidized hard carbon precursor;

[0051] S3. After adding 100 g of the oxidized hard carbon precursor to 1000 g of deionized water and mixing evenly, add 50 g of Na 2 Fe(C 2 O 4 )(SO 4 ) compound and mix evenly, then add 500 g of graphene oxide solution with a mass concentration of 5%. Carry out hydrothermal reaction at 200 °C and 1 MPa for 1 h. After filtration, dry in vacuum at 80 °C for 24 h to obtain a graphene oxide / polymer-coated iron-based sodium salt composite;

[0052] S4. Transfer the obtained graphene oxide / polymer-coated iron-based sodium salt composite to a tubular furnace. After purging the air in the tube with argon, then introduce a mixed reducing gas of hydrogen and argon with a volume ratio of 1:2 at a flow rate of 50 mL / min. Reduce at 300 °C for 1 h, then introduce a mixed gas of hydrogen sulfide and argon with a volume ratio of 1:2 at a flow rate of 50 mL / min, heat up to 1100 °C and hold for 1 h, and pulverize to obtain a hard carbon-coated iron-based oxide composite.

[0053] Comparative Example 1

[0054] S1. Add 100 g of coconut shell to 666 g of sulfuric acid solution with a mass concentration of 10% to prepare a solution with a coconut shell concentration of 13 wt%. Carry out hydrothermal reaction in a sealed microwave reactor at 150 °C for 30 min, wash to obtain an oxidized hard carbon precursor;

[0055] S2. After adding 100 g of the oxidized hard carbon precursor to 1000 g of deionized water and mixing evenly, add 300 g of graphene oxide solution with a mass concentration of 5%. Carry out hydrothermal reaction at 150 °C and 2 MPa for 3 h. After filtration, dry in vacuum at 80 °C for 24 h to obtain a graphene oxide / oxidized hard carbon precursor composite;

[0056] S3. Transfer the obtained graphene oxide / hard carbon oxide precursor composite material to a tubular furnace. After purging the air in the tube with argon, introduce a mixed reducing gas of hydrogen and argon with a volume ratio of 3:10 at a flow rate of 50 mL / min. After reducing at 250 °C for 2 h, then introduce a mixed gas of sulfur dioxide and argon with a volume ratio of 3:10 at a flow rate of 50 mL / min. Heat up to 900 °C and hold for 3 h, then pulverize to obtain a hard carbon-coated graphene oxide composite material.

[0057] Comparative Example 2

[0058] S1. Add 14.4 g (0.1 mol) of ferrous oxalate and 14.2 g (0.1 mol) of sodium sulfate to 144 g of deionized water, and add 112 g of ammonia water (25 wt%) and 0.7 g of sodium iminodisuccinate. React at 80 °C for 3 h. After filtration, dry in vacuum at 80 °C for 24 h to obtain Na 2 Fe(C 2 O 4 )(SO 4 ) compound;

[0059] S2. Add 100 g of coconut shell to 666 g of sulfuric acid solution with a mass concentration of 10% to prepare a solution with a coconut shell concentration of 13 wt%. Add the Na 2 Fe(C 2 O 4 )(SO 4 ) compound obtained in S1 and mix evenly. Then add 100 g of graphene oxide solution with a mass concentration of 5%. Carry out hydrothermal reaction at 150 °C and 2 MPa for 3 h. After filtration, dry in vacuum at 80 °C for 24 h to obtain a graphene oxide / polymer-coated iron-based sodium salt composite material;

[0060] S3. Transfer the obtained graphene oxide / polymer-coated iron-based sodium salt composite material to a tubular furnace. After purging the air in the tube with argon, introduce a mixed reducing gas of hydrogen and argon with a volume ratio of 3:10 at a flow rate of 50 mL / min. After reducing at 250 °C for 2 h, then introduce a mixed gas of sulfur dioxide and argon with a volume ratio of 3:10 at a flow rate of 50 mL / min. Heat up to 900 °C and hold for 3 h, then pulverize to obtain a hard carbon-coated iron-based oxide composite material.

[0061] Comparative Example 3

[0062] S1. Add 14.4 g (0.1 mol) of ferrous oxalate and 14.2 g (0.1 mol) of sodium sulfate to 144 g of deionized water, and add 112 g of ammonia water (25 wt%) and 0.7 g of sodium iminodisuccinate and stir evenly to obtain solution A;

[0063] S2. Add 10 g of phenolic resin to 100 g of deionized water to prepare Solution B;

[0064] S3. Add Solution B to Solution A, place it in a ball mill, ball mill at a rotation speed of 1200 r / min for 60 min, filter, vacuum dry at 80 °C for 24 h, then calcine at 800 °C for 3 h under an argon atmosphere, and ball mill to obtain amorphous carbon / Na 2 Fe(C 2 O 4 )(SO 4 ) composite material.

[0065] Performance test:

[0066] (1) SEM test: Conduct an SEM test on the hard carbon-coated iron-based oxide composite material obtained in Example 1. The test result is Figure 1 , from Figure 1 it can be seen that the hard carbon-coated iron-based oxide composite material provided in Example 1 of the present invention presents a granular structure, with a reasonable size distribution, and the particle size is between 2 and 10 μm.

[0067] (2) Physical and chemical property test: Refer to GB / T 24533-2019 "Graphite Anode Materials for Lithium-Ion Batteries" to test the particle size, tap density, specific surface area, ash content, specific capacity, and initial efficiency of the hard carbon-coated iron-based oxide composite materials obtained in Examples 1 to 3 and Comparative Examples 1 to 3. The test data are recorded in Table 1.

[0068] (3) Button cell test: Assemble the hard carbon-coated iron-based oxide composite materials obtained in Examples 1 to 3 and Comparative Examples 1 to 3 as the anode materials of lithium-ion batteries into button cells A1, A2, A3, B1, B2, and B3 respectively; the preparation method is as follows: Mix 90 g of the hard carbon-coated iron-based oxide composite materials obtained in Examples 1 to 3 and Comparative Examples 1 to 3 with 4 g of LA132 binder, 6 g of conductive agent SP, and 220 mL of secondary distilled water, stir to make a slurry, coat it on a copper foil, and obtain the anode electrode sheet after drying and rolling; the electrolyte is LiPF 6 / EC+DEC (volume ratio 1:1, concentration 1.3 mol / L); a metal lithium sheet is used as the counter electrode, and the separator uses a polyethylene, polypropylene, or polyisopropylene composite membrane. The simulation battery is assembled in a glove box filled with argon, and the electrochemical performance is tested on a Wuhan Blue Electric CT2001A battery tester. The charge and discharge voltage range is 0.005 to 2.0 V, the charge and discharge rate is 0.1 C, and at the same time, the rate (3C, 0.1C) and cycle performance (0.5C, 0.5C, 100 times) of the button cells are tested. The test data are recorded in Table 1.

[0069] Table 1 Various properties of the hard carbon-coated iron-based oxide composite material and button cell

[0070] Item Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Particle Size (D50, μm) 5.5 6.2 5.9 6.4 6.6 6.7 <![CDATA[Tap density (g / cm 3 )]]> 0.92 0.90 0.93 0.80 0.83 0.81 <![CDATA[Specific surface area (m 2 / g)]]> 5.5 5.1 6.0 4.8 4.1 3.8 Ash Content (%) 0.05 0.04 0.06 0.10 0.12 0.10 Initial Discharge Specific Capacity (mAh / g) 449 446 428 335 382 345 Initial Coulombic Efficiency (%) 86.1 85.5 84.1 80.4 81.8 85.3 Rate Performance (3C, 0.1C, %) 97.5 96.6 97.6 85.3 87.3 86.4 Cycling Performance (%) 97.8 96.7 97.3 93.3 94.0 93.6

[0071] As can be seen from Table 1, the hard carbon-coated iron-based oxide composite material obtained in Example 1 of the present invention has a high initial discharge specific capacity and initial Coulombic efficiency. The reason is that the doping of iron-based sodium salt compounds in the hard carbon material improves the specific capacity and ion diffusion rate of the material, thereby improving the rate performance and cycle performance. At the same time, the present invention modifies the hard carbon material by introducing a sulfur-containing gas to reduce defects and improve the initial Coulombic efficiency. In addition, the composite material prepared by the hydrothermal method in the present invention has the advantages of high tap density, high density, and low electronic impedance, thereby improving the rate performance.

[0072] (4) Soft-pack battery:

[0073] Using the hard carbon-coated iron-based oxide composite materials obtained in Examples 1 to 3 and Comparative Examples 1 to 3 as the negative electrode material, a ternary material (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 ) as the positive electrode, and an electrolyte of LiPF 6 / EC + DEC (volume ratio 1:1, concentration 1.3 mol / L), and celegard 2400 as the separator to prepare 1 Ah soft-pack batteries C1, C2, C3 and D1, D2, D3, that is, ternary soft-pack lithium batteries are obtained.

[0074] ① Rate performance test: The charge-discharge voltage range is 2.75 to 4.2 V, the temperature is 25 ± 3.0 °C, charge at 1.0C, 3.0C, 5.0C, 10.0C, and discharge at 1.0C to test the charging ratio and constant current ratio of the battery; the test results are recorded in Table 2.

[0075] Table 2 Rate performance of Examples 1 to 3 and Comparative Examples 1 to 3

[0076]

[0077] As can be seen from Table 2, using the hard carbon-coated iron-based oxide composite materials provided in Examples 1 to 3 of the present invention as the negative electrode material, the rate charging performance of the prepared soft-pack batteries is significantly better than that of Comparative Examples 1 to 3, that is, the charging time is shorter. The reason for the analysis is that during the battery charging process, the migration of lithium ions is required. The core of the material in the example is an iron-based sodium salt compound, which has the characteristics of high ionic conductivity, improves the diffusion rate of the material, improves the rate performance, and at the same time has a high specific surface area to improve the kinetic performance of the material and improve the rate performance.

[0078] ② Cycle performance test: charge and discharge currents are 2C and 2C, voltage range is 2.5 - 4.2V, and the number of cycles is 1000; the test results are recorded in Table 3.

[0079] Table 3 Cycle performance of Examples 1 - 3 and Comparative Examples 1 - 3

[0080]

[0081] As can be seen from Table 3, when the hard carbon-coated iron-based oxide composite materials provided in Examples 1 - 3 of the present invention are used as the negative electrode material, the cycle performance of the prepared lithium-ion batteries is significantly better than that of Comparative Examples 1 - 3 at all stages. The experimental results show that the hard carbon coating on the outer shell of the materials of the present invention reduces the expansion of iron-based sodium salt compounds during the charge and discharge process, thereby improving the cycle performance.

[0082] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of a hard carbon-coated iron-based oxide composite material, characterized in that, it includes the following steps: S1. After mixing an iron source, a sodium source, ammonia water, a stabilizer and water, carry out a reaction to obtain an iron-based sodium salt compound; S2. Mix a hard carbon precursor with an acid solution, and obtain an oxidized hard carbon precursor after the reaction; S3. After mixing the oxidized hard carbon precursor, the iron-based sodium salt compound, graphene oxide and water, carry out a hydrothermal reaction to obtain a graphene oxide / polymer-coated iron-based sodium salt composite material; S4. After reducing the graphene oxide / polymer-coated iron-based sodium salt composite material, carry out gas surface treatment and carbonization to obtain a hard carbon-coated iron-based oxide composite material; In S1, the stabilizer includes one or two of tetrasodium iminodisuccinate and ethylenediaminetetraacetic acid; In S2, the reaction temperature is 100~200°C, and the reaction time is 10~60 min; In S3, the hydrothermal reaction temperature is 100~200°C, the time is 1~6 h, and the pressure is 1~3 MPa; In S4, for the gas surface treatment, specifically: introduce a mixed gas of a sulfur-containing gas and argon with a volume ratio of 1~5:10 at a flow rate of 50 mL / min.

2. A preparation method of a hard carbon-coated iron-based oxide composite material according to claim 1, characterized in that, it further includes at least one of the following technical features: In S1, the mass ratio of the iron source, the sodium source, ammonia water, the stabilizer and water is 100:100~300:500~1000:1~10:1000; In S1, the reaction temperature is 50~100°C, and the reaction time is 1~6 h.

3. A preparation method of a hard carbon-coated iron-based oxide composite material according to claim 1, characterized in that, In S2, the addition amounts of the hard carbon precursor and the acid solution are based on the concentration of the hard carbon precursor after mixing being 10~20 wt%.

4. A preparation method of a hard carbon-coated iron-based oxide composite material according to claim 1, characterized in that, In S3, the mass ratio of the oxidized hard carbon precursor, the iron-based sodium salt compound, graphene oxide and water is 100:10~50:100~500:1000.

5. A preparation method of a hard carbon-coated iron-based oxide composite material according to claim 1, characterized in that, The reduction in S4 is carried out under a hydrogen and argon atmosphere with a volume ratio of 1~5:

10.

6. A hard carbon-coated iron-based oxide composite material prepared by the preparation method of a hard carbon-coated iron-based oxide composite material according to any one of claims 1~5.

7. A hard carbon-coated iron-based oxide composite material according to claim 6, characterized in that, the inner core is an iron-based sodium salt compound, and the outer shell is a hard carbon composite material; In the iron-based sodium salt compound, Na + and Fe 2+ have a molar ratio of 2:

1.

8. A hard carbon-coated iron-based oxide composite material according to claim 7, characterized in that, the mass ratio of the inner core to the outer shell is 1~3:7~9.

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