Hard carbon negative electrode material, preparation method thereof and secondary battery

Abstract

The invention provides a hard carbon negative electrode material, a preparation method thereof and a secondary battery. The preparation method comprises the following steps: performing surface functionalization treatment on a nitrogen-doped hard carbon material to obtain a half-step precursor material; mixing the half-step precursor material, a metal source, a first phosphorus source and a solvent, and performing hydrothermal reaction to obtain a precursor material; and finally calcining the precursor material and a second phosphorus source to obtain the hard carbon negative electrode material. According to the invention, composite modification is carried out by adopting technical means of element doping and surface coating, so that the capacity and the first coulombic efficiency of the hard carbon negative electrode material as well as the rapid charge-discharge capability and the long-term cycle stability are effectively improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of negative electrode materials, and in particular relates to a hard carbon negative electrode material, a preparation method thereof, and a secondary battery. Background Art

[0002] Sodium-ion batteries have become an important alternative to lithium-ion batteries due to their advantages such as abundant sodium resources and low cost. Hard carbon materials are ideal negative electrode materials due to their stable skeleton structure and excellent sodium storage performance. However, their low electrical conductivity and slow ion diffusion rate limit further improvement of rate performance. Traditional hard carbon negative electrode materials are mainly made by direct carbonization of biomass materials (such as coconut shells and wood) or resin materials. Their microstructure and pore distribution are difficult to precisely control, resulting in unstable electrochemical performance.

[0003] To address these issues, existing technologies have mostly addressed these challenges through coating modifications (e.g., carbon layers, metal oxide layers, etc.). While these coatings can partially improve performance, they still suffer from issues such as high interfacial impedance, insufficient ion migration efficiency, and poor rate performance. Furthermore, existing hard carbon anode materials suffer from low capacity (<300 mAh / g), low initial coulombic efficiency (40%-80%), and low rapid sodium insertion / extraction capabilities, limiting their large-scale application.

[0004] Therefore, how to provide a preparation method of hard carbon negative electrode materials with simple process and strong feasibility, which can effectively improve their electrochemical properties and enhance the application prospects of hard carbon negative electrode materials in sodium ion batteries, is a technical problem that technicians in this field urgently need to solve. Summary of the Invention

[0005] In view of the shortcomings of the existing technology and in order to solve the technical problems of low capacity and poor fast charging capability of traditional hard carbon negative electrode materials, the purpose of the present invention is to provide a hard carbon negative electrode material, a preparation method and a secondary battery thereof. By adopting element doping and surface coating technical means for composite modification, the capacity and first coulombic efficiency of the hard carbon negative electrode material, as well as the fast charging and discharging capability and long-term cycle stability are effectively improved.

[0006] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a method for preparing a hard carbon negative electrode material, the preparation method comprising the following steps:

[0008] S1. Performing surface functionalization treatment on the nitrogen-doped hard carbon material to obtain a half-step precursor material;

[0009] S2. mixing the half-step precursor material, the metal source, the first phosphorus source and the solvent, and obtaining a precursor material after a hydrothermal reaction;

[0010] S3. calcining the precursor material and the second phosphorus source to obtain the hard carbon negative electrode material.

[0011] The present invention provides a method for preparing a hard carbon negative electrode material with both high rate and high capacity. The preparation method is mainly divided into three stages: first, a nitrogen-doped hard carbon material with a surface rich in functional groups is prepared as the core; second, a metal phosphide coating layer is in situ grown on the surface of the core material by constructing chemical bonds; and finally, a metal phosphide / phosphorus composite coating layer is further formed on the surface of the core material through high-temperature phosphating treatment, thereby effectively improving the battery capacity at high rate, so that the secondary battery containing the hard carbon negative electrode material has high rate performance and cycle performance. The specific performance is as follows:

[0012] (1) On the one hand, the present invention introduces more defect sites and π-electron systems by doping the hard carbon negative electrode material with nitrogen, thereby improving the conductivity of the material and serving as adsorption sites to further improve the storage of metal ions. On the other hand, the present invention performs surface functionalization treatment on the nitrogen-doped hard carbon negative electrode material to provide it with more active groups (such as hydroxyl or carboxyl groups), which is beneficial for the subsequent coating treatment of the metal phosphide / phosphorus composite coating layer, thereby improving the bonding strength between the two and preventing peeling.

[0013] (2) The metal phosphide / phosphorus composite coating forms a stable interface with the hard carbon core material through CP and COM chemical bonds. This significantly improves the electron conduction efficiency of the coating while ensuring uniform distribution, reduces interfacial impedance, accelerates metal ion transport, and thus improves the rate performance of the secondary battery. Furthermore, the composite coating can also undergo an alloying reaction with metal ions, thereby providing a higher capacity, thereby overcoming technical issues such as low battery capacity volatilization at high rates.

[0014] Preferably, in step S1, the mass percentage of nitrogen element in the nitrogen-doped hard carbon material is 5wt% to 30wt%, for example, it can be 5wt%, 8wt%, 10wt%, 15wt%, 20wt%, 25wt% or 30wt%, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0015] Preferably, in step S1, the method for preparing the nitrogen-doped hard carbon material comprises the following steps:

[0016] The nitrogen-containing polymer precursor is sintered to obtain the nitrogen-doped hard carbon material.

[0017] Preferably, the nitrogen-containing polymer precursor comprises polyethyleneimine.

[0018] In the present invention, the polyethyleneimine can be a commercially available product, or can be prepared by the following method:

[0019] (1) Dissolve polyethylene glycol in 20 mL to 80 mL of solvent to obtain solution A, dissolve p-toluenesulfonyl chloride and pyridine in 20 mL to 80 mL of solvent to obtain solution B, slowly add solution B dropwise to solution A while maintaining 0°C, react at room temperature for 6 h to 24 h, transfer to ice water to precipitate a white solid, wash and vacuum dry to obtain an intermediate, wherein the mass ratio of polyethylene glycol, p-toluenesulfonyl chloride and pyridine is 1:(0.01 to 100):(0.01 to 100);

[0020] (2) The intermediate product, the cross-linking agent, and ethylenediamine are mixed in a mass ratio of 1:(0.01-100):(0.01-100) and dissolved in 50 mL of anhydrous solvent. The mixture is reacted at 80° C. to 120° C. for 12 h to 48 h. The ethylenediamine and anhydrous solvent are removed by rotary evaporation to obtain polyethyleneimine.

[0021] In the present invention, polyethylene glycol and p-toluenesulfonyl chloride react to form polyethylene glycol p-toluenesulfonate using pyridine as an acid absorbent. The p-toluenesulfonate group in the polyethylene glycol p-toluenesulfonate structure is a good leaving group, easily undergoing nucleophilic substitution. Therefore, when this group encounters ethylenediamine, the bifunctional nucleophilic reagent ethylenediamine attacks the p-toluenesulfonate group at the end of the polyethylene glycol chain, forming an amino-substituted product, which then reacts with a crosslinker to produce polyethyleneimine.

[0022] In the present invention, the solvent in step (1) includes at least one of water, ethanol, methanol or N,N-dimethylformamide.

[0023] In the present invention, the cross-linking agent in step (2) includes glutaraldehyde and / or epichlorohydrin.

[0024] In the present invention, the anhydrous solvent in step (2) includes anhydrous toluene and / or anhydrous tetrahydrofuran.

[0025] In the present invention, the cross-linked network of the precursor can be effectively adjusted by controlling the reaction conditions of polyethyleneimine, thereby better regulating the skeleton structure and porosity of the nitrogen-doped hard carbon material.

[0026] Preferably, the sintering treatment has a temperature of 500-1000℃, such as 500℃, 600℃, 700℃, 800℃, 900℃ or 1000℃, etc.; and a time of 1-12h, such as 1h, 5h, 8h, 10h or 12h, etc., not limited to the listed values, other values not listed within the range are also applicable.

[0027] Preferably, the sintering treatment has a temperature increasing rate of 1-5℃ / min, such as 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min or 5℃ / min, etc., not limited to the listed values, other values not listed within the range are also applicable.

[0028] In the present application, the sintering treatment is carried out under an inert atmosphere, which includes at least one of nitrogen, argon or helium.

[0029] Preferably, the surface functionalization treatment includes dispersing the nitrogen-doped hard carbon material and a mixed acid solution to obtain the semi-step precursor material.

[0030] Preferably, the mixed acid solution includes a nitric acid solution and a sulfuric acid solution.

[0031] Preferably, the volume ratio of the nitric acid solution and the sulfuric acid solution is 1:(2-4), such as 1:2, 1:2.5, 1:3, 1:3.5 or 1:4, etc., not limited to the listed values, other values not listed within the range are also applicable.

[0032] Preferably, the dispersing has a temperature of 25-100℃, such as 25℃, 50℃, 60℃, 80℃ or 100℃, etc.; and a time of 1-24h, such as 1h, 5h, 8h, 10h, 12h, 16h, 18h, 20h or 24h, etc., not limited to the listed values, other values not listed within the range are also applicable.

[0033] Preferably, in step S2, the mass ratio of the semi-step precursor material, the metal source and the first phosphorus source is 1:(0.01-100):(0.01-100), preferably 1:(0.2-10):(0.2-10), such as 1:0.01:100, 1:0.05:90, 1:0.1:80, 1:1:70, 1:1:1, 1:10:60, 1:20:50, 1:50:40, 1:80:30 or 1:100:0.01, etc., not limited to the listed values, other values not listed within the range are also applicable.

[0034] In the present invention, by regulating the mass ratio of the half-step precursor material, the metal source, and the first phosphorus source, the in-situ growth of the metal phosphide on the surface of the core is achieved to form a coating layer, significantly improving the electronic conduction efficiency of the material and reducing the interface impedance. If a relatively low-quality half-step precursor material, metal source, and first phosphorus source are used, the functional groups on the surface of the core will not react completely, generating phosphorus oxides during the subsequent phosphorus coating, increasing irreversible sodium storage, and the metal phosphide coating is less, reducing the material's conductivity and increasing the interface impedance, and the rate performance of the secondary battery will also deteriorate. If a relatively high-quality half-step precursor material, metal source, and first phosphorus source are used, the thickness of the metal phosphide coating layer will be too thick, and during the cycle process, the metal phosphide will undergo an alloying reaction, resulting in excessive volume expansion, which in turn destroys the material structure and causes the cycle performance of the secondary battery to deteriorate.

[0035] Preferably, in step S2, the metal source comprises any one or a combination of at least two of a niobium-containing compound, a tantalum-containing compound, or a cerium-containing compound. By optimizing the compounds of the aforementioned metal elements, the present invention further improves the capacity, rate capability, and ion diffusion performance of the hard carbon anode material.

[0036] Preferably, the niobium-containing compound includes any one of niobium pentachloride, niobium pentoxide, niobium nitrate, niobium oxalate, hydrated niobium carbonate, sodium niobate or niobium ethoxide, or a combination of at least two thereof.

[0037] Preferably, the tantalum-containing compound includes any one of tantalum chloride, sodium tantalate, tantalum nitrate, tantalum oxalate, tantalum carbonate or tantalum pentoxide, or a combination of at least two thereof.

[0038] Preferably, the cerium-containing compound includes any one of cerium dioxide, cerium sulfate, cerium chloride or ammonium cerium nitrate, or a combination of at least two thereof.

[0039] Preferably, in step S2, the first phosphorus source includes any one or a combination of at least two of phosphoric acid, phosphorous acid, phytic acid, sodium hypophosphite, diammonium hydrogen phosphate or triphenylphosphine.

[0040] Preferably, in step S2, the solvent includes any one of water, ethanol, methanol or N,N-dimethylformamide, or a combination of at least two thereof.

[0041] Preferably, in step S2, the temperature of the hydrothermal reaction is 50°C to 220°C, for example, it can be 50°C, 80°C, 100°C, 120°C, 150°C, 180°C, 200°C or 220°C; the time of the hydrothermal reaction is 1h to 24h, for example, it can be 1h, 5h, 8h, 10h, 12h, 16h, 18h, 20h or 24h, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0042] Preferably, in step S3, the mass ratio of the precursor material to the second phosphorus source is 1:(0.01~100), preferably 1:(0.2~10), for example, it can be 1:0.01, 1:0.05, 1:0.1, 1:1, 1:5, 1:10, 1:20, 1:50, 1:80 or 1:100, etc., and is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0043] In the present invention, by regulating the mass ratio of the precursor material and the second phosphorus source, a uniform coating of the metal phosphide / phosphorus composite coating layer is achieved, significantly improving the electronic conduction efficiency of the material and reducing the interface impedance. If a precursor material and a second phosphorus source of relatively low mass are used, the distribution of the metal phosphide / phosphorus composite coating layer will be uneven, reducing the electrical conductivity of the material and increasing the interface impedance, and the rate performance of the secondary battery will deteriorate accordingly; if a precursor material and a second phosphorus source of relatively high mass are used, the thickness of the metal phosphide / phosphorus composite coating layer will be too thick, and during the cycle process, the metal phosphide / phosphorus will undergo an alloying reaction, resulting in excessive volume expansion, thereby destroying the material structure and causing the cycle performance of the secondary battery to deteriorate.

[0044] Preferably, in step S3, the second phosphorus source includes any one of red phosphorus, black phosphorus, sodium hypophosphite, diammonium hydrogen phosphate or triphenylphosphine, or a combination of at least two thereof.

[0045] Preferably, in step S3, the calcination temperature is 500°C to 1000°C, for example, 500°C, 600°C, 700°C, 800°C, 900°C or 1000°C; the calcination time is 1h to 12h, for example, 1h, 5h, 8h, 10h or 12h, etc., and is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0046] Preferably, in step S3, the heating rate of the calcination treatment is 1°C / min to 5°C / min, for example, it can be 1°C / min, 2°C / min, 3°C / min, 4°C / min or 5°C / min, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0047] In the present invention, the calcination treatment is performed under an inert atmosphere, and the inert atmosphere includes at least one of nitrogen, argon or helium.

[0048] Preferably, in step S3, after the precursor material and the second phosphorus source are calcined, the following step is further included: stirring the material obtained after the calcination, the conductive polymer monomer, the fast ion conductor, the initiator and the acid solution to react to obtain the hard carbon negative electrode material.

[0049] Preferably, the mass ratio of the material obtained after the calcination treatment, the conductive polymer monomer, the fast ion conductor and the initiator is 1:(0.01~10):(0.01~10):(0.01~10), preferably 1:(0.2~5):(0.2~5):(0.5~10), for example, it can be 1:0.01:0.01:10, 1:0.05:9:9, 1:0.1:10:0.01, 1:1:1:1, 1:1:2:8, 1:2:3:7, 1:4:5:6, 1:6:8:10 or 1:8:7:10, etc., not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0050] Preferably, the conductive polymer monomer includes any one of aniline, pyrrole or 3,4-ethylenedioxythiophene, or a combination of at least two thereof.

[0051] Preferably, the fast ion conductor comprises Na3PS4, Na3SbS4 or Na3Zr2Si2PO 12 Any one or a combination of at least two of .

[0052] In the present invention, the initiator includes ammonium persulfate and / or ferric chloride.

[0053] Preferably, the concentration of the acid solution is 0.5 mol / L to 3 mol / L, for example, 0.5 mol / L, 1 mol / L, 2 mol / L or 3 mol / L, etc., and is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0054] In the present invention, the acid solution includes at least one of hydrochloric acid, sulfuric acid or nitric acid.

[0055] Preferably, the stirring reaction temperature is 0°C to 25°C, for example, it can be 0°C, 5°C, 10°C, 15°C, 20°C or 25°C; the stirring reaction time is 1h to 24h, for example, it can be 1h, 5h, 8h, 10h, 12h, 16h, 18h, 20h or 24h, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0056] In the present invention, by further providing a mixed coating layer of a conductive polymer and a fast ion conductor on the outermost surface, it can not only synergistically optimize the dual-channel transmission of electrons and ions, but also alleviate the volume expansion caused by the alloying reaction of the metal phosphide / phosphorus composite coating, thereby improving the structural stability of the material.

[0057] In a second aspect, the present application provides a hard carbon negative electrode material, which is prepared by the method for preparing a hard carbon negative electrode material according to the first aspect, and comprises a nitrogen-doped hard carbon material core and a metal phosphide / phosphorus composite coating layer coated on the surface of the nitrogen-doped hard carbon material core.

[0058] Preferably, the nitrogen-doped hard carbon material core has an interlayer spacing of 0.35 nm to 0.4 nm, for example, 0.35 nm, 0.36 nm, 0.37 nm, 0.38 nm, 0.39 nm or 0.4 nm, etc., and is not limited to the listed values, and other values not listed within the range are also applicable.

[0059] Preferably, the nitrogen-doped hard carbon material core has a pore size of 2 nm to 50 nm, for example, 2 nm, 5 nm, 10 nm, 20 nm, 30 nm, 40 nm or 50 nm, etc., and is not limited to the listed values, and other values not listed within the range are also applicable.

[0060] Preferably, the nitrogen-doped hard carbon material core has a mass percentage of nitrogen element of 5 wt% to 30 wt%, for example, 5 wt%, 8 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt% or 30 wt%, etc., and is not limited to the listed values, and other values not listed within the range are also applicable.

[0061] Preferably, the metal phosphide / phosphorus composite coating layer has a thickness of 5 nm to 200 nm, for example, 5 nm, 10 nm, 20 nm, 50 nm, 80 nm, 100 nm, 120 nm, 150 nm or 200 nm, etc., and is not limited to the listed values, and other values not listed within the range are also applicable.

[0062] Preferably, the metal phosphide / phosphorus composite coating layer has a mass percentage of phosphorus element of 0.5 wt% to 10 wt%, for example, 0.5 wt%, 1 wt%, 2 wt%, 5 wt%, 8 wt% or 10 wt%, etc., and is not limited to the listed values, and other values not listed within the range are also applicable.

[0063] Preferably, the hard carbon negative electrode material further comprises a mixed coating layer, which is arranged on the surface of the metal phosphide / phosphorus composite coating layer.

[0064] Preferably, the material of the mixed coating layer comprises a conductive polymer and a fast ion conductor.

[0065] Preferably, the mass percentage of the fast ion conductor in the mixed coating layer is 0.1wt% to 5wt%, for example, it can be 0.1wt%, 1wt%, 2wt%, 3wt% or 5wt%, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0066] Preferably, the thickness of the mixed coating layer is 5nm to 200nm, for example, it can be 5nm, 10nm, 20nm, 50nm, 80nm, 100nm, 120nm, 150nm or 200nm, etc., and is not limited to the listed values. Other unlisted values ​​within this numerical range are also applicable.

[0067] Preferably, the specific surface area of ​​the hard carbon negative electrode material is 5m 2 / g~300m 2 / g, for example, 5m 2 / g、10m 2 / g, 20m 2 / g, 50m 2 / g、100m 2 / g, 150m 2 / g, 200m 2 / g, 250m 2 / g or 300m 2 / g, etc., are not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0068] In a third aspect, the present invention provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte, wherein the active material of the negative electrode sheet comprises the hard carbon negative electrode material according to the second aspect.

[0069] The secondary battery of the present invention adopts the hard carbon negative electrode material according to the second aspect. Since the hard carbon negative electrode has the advantages of fast kinetics, high capacity and stable structure, it can realize a high energy density and high power density secondary battery.

[0070] In the present invention, the secondary battery exemplarily includes a sodium ion battery or a lithium ion battery.

[0071] In the present invention, the electrolyte includes a liquid electrolyte, a gel electrolyte or a solid electrolyte.

[0072] The numerical range described in the present invention includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0073] Compared with the prior art, the present invention has the following beneficial effects:

[0074] The present invention provides a method for preparing a hard carbon negative electrode material. The preparation method is mainly divided into three stages: first, a nitrogen-doped hard carbon material with a surface rich in functional groups is prepared as a core; second, a metal phosphide coating layer is in situ grown on the surface of the core material by constructing a chemical bond; finally, a metal phosphide / phosphorus composite coating layer is further formed on the surface of the core material through a high-temperature phosphating treatment, thereby effectively improving the battery capacity at high rate, so that the secondary battery containing the hard carbon negative electrode material has higher rate performance and cycle performance.

[0075] The hard carbon negative electrode material prepared by the present invention is applied to sodium ion batteries. -1 The capacity at current density is ≥350mAh / g, the first coulombic efficiency is >90%, and it has excellent cycle stability and high rate performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0076] Figure 1 This is a cycle performance diagram of the sodium ion button battery provided in Application Example 1. DETAILED DESCRIPTION

[0077] The technical solution of the present invention is further described below with reference to the accompanying drawings and specific embodiments. It should be understood by those skilled in the art that the embodiments are only for the purpose of helping to understand the present invention and should not be regarded as specific limitations of the present invention.

[0078] Example 1

[0079] This embodiment provides a hard carbon negative electrode material and a preparation method thereof. The hard carbon negative electrode material includes a nitrogen-doped hard carbon material core and a niobium phosphide / phosphorus composite coating layer and a mixed coating layer sequentially coated on the surface of the nitrogen-doped hard carbon material core from the inside to the outside.

[0080] Among them, the interlayer spacing of the nitrogen-doped hard carbon material core is 0.38nm, the pores of the nitrogen-doped hard carbon material core are 20nm, and the mass percentage of nitrogen element is 5wt%; the thickness of the niobium phosphide / phosphorus composite coating layer is 80nm, and the mass percentage of phosphorus element is 1wt%; the material of the mixed coating layer includes polyaniline and Na3PS4, the mass percentage of Na3PS4 is 0.1wt%, and the thickness is 50nm.

[0081] The preparation method comprises the following steps:

[0082] S1. Dissolve 1 g of polyethylene glycol in 50 mL of ethanol to obtain solution A. Dissolve 0.5 g of p-toluenesulfonyl chloride and 0.5 g of pyridine in 50 mL of ethanol to obtain solution B. Slowly add solution B dropwise to solution A at 0°C. After reacting at room temperature for 12 h, transfer to ice water to precipitate a white solid, wash, and vacuum dry to obtain the intermediate.

[0083] 1 g of the intermediate product, 0.1 g of glutaraldehyde, and 2 g of ethylenediamine were mixed and dissolved in 50 mL of anhydrous toluene, and the mixture was reacted at 100° C. for 12 h. The ethylenediamine and anhydrous solvent were removed by rotary evaporation to obtain polyethyleneimine;

[0084] The polyethyleneimine was transferred to a tube furnace, heated to 800°C at a rate of 2°C / min under an argon atmosphere and kept at that temperature for 6 h. After cooling to room temperature, the nitrogen-doped hard carbon material was obtained.

[0085] The nitrogen-doped hard carbon material was dispersed in mixed acid (prepared by mixing nitric acid solution and sulfuric acid solution in a volume ratio of 1:3), stirred at 70°C for 12 hours, and washed and dried to obtain a surface-functionalized half-step precursor material.

[0086] S2. 1 g of the surface-functionalized half-step precursor material, 0.3 g of sodium hypophosphite, and 0.1 g of niobium pentachloride were mixed and dispersed in 50 mL of ethanol. The mixture was then transferred to a reactor and hydrothermally reacted at 150 ° C for 12 h. The precursor material was washed and dried.

[0087] S3. Mix 1g of the above-mentioned precursor material and 0.5g of red phosphorus and transfer them to a tubular furnace. Heat the mixture to 500℃ at 2℃ / min under an argon atmosphere and keep it warm for 2h. Then disperse 1g of the calcined material, 0.1g of aniline, 2g of ammonium persulfate and 0.01g of Na3PS4 in 50mL of hydrochloric acid (concentration of 1mol / L) solution and react at 0℃ for 12h to obtain the hard carbon negative electrode material.

[0088] Example 2

[0089] This embodiment provides a hard carbon negative electrode material and a preparation method thereof. The hard carbon negative electrode material includes a nitrogen-doped hard carbon material core and a niobium phosphide / phosphorus composite coating layer and a mixed coating layer sequentially coated on the surface of the nitrogen-doped hard carbon material core from the inside to the outside.

[0090] Among them, the interlayer spacing of the nitrogen-doped hard carbon material core is 0.38nm, the pores of the nitrogen-doped hard carbon material core are 20nm, and the mass percentage of nitrogen element is 5wt%; the thickness of the niobium phosphide / phosphorus composite coating layer is 140nm, and the mass percentage of phosphorus element is 2wt%; the material of the mixed coating layer includes polyaniline and Na3PS4, the mass percentage of Na3PS4 is 0.1wt%, and the thickness is 50nm.

[0091] The difference between the preparation method and Example 1 is that:

[0092] S2. 1 g of the surface-functionalized half-step precursor material, 0.5 g of phosphoric acid, and 0.15 g of niobium pentoxide were mixed and dispersed in 50 mL of DMF. The mixture was then transferred to a reactor and hydrothermally reacted at 160°C for 14 h. The precursor material was washed and dried.

[0093] S3. 1 g of the above precursor material and 1 g of red phosphorus were mixed and transferred to a tube furnace. The temperature was raised to 550°C at 2°C / min under an argon atmosphere and kept at this temperature for 2 h. The rest of the preparation method was the same as in Example 1.

[0094] Example 3

[0095] This embodiment provides a hard carbon negative electrode material and a preparation method thereof. The hard carbon negative electrode material includes a nitrogen-doped hard carbon material core and a niobium phosphide / phosphorus composite coating layer and a mixed coating layer sequentially coated on the surface of the nitrogen-doped hard carbon material core from the inside to the outside.

[0096] Among them, the interlayer spacing of the nitrogen-doped hard carbon material core is 0.38nm, the pores of the nitrogen-doped hard carbon material core are 20nm, and the mass percentage of nitrogen element is 5wt%; the thickness of the niobium phosphide / phosphorus composite coating layer is 190nm, and the mass percentage of phosphorus element is 5wt%; the material of the mixed coating layer includes polyaniline and Na3PS4, the mass percentage of Na3PS4 is 0.1wt%, and the thickness is 50nm.

[0097] The difference between the preparation method and Example 1 is that:

[0098] S2. 2 g of the surface-functionalized half-step precursor material, 2 g of phytic acid, and 0.5 g of niobium oxalate were mixed and dispersed in 50 mL of water, and then transferred to a reactor for hydrothermal reaction at 200 ° C for 18 h, washed and dried to obtain a precursor material;

[0099] S3. 1 g of the above precursor material and 2 g of black phosphorus were mixed and transferred to a tube furnace. The temperature was raised to 600°C at 2°C / min under an argon atmosphere and kept at this temperature for 4 h. The rest of the preparation method was the same as in Example 1.

[0100] Example 4

[0101] This embodiment provides a hard carbon negative electrode material and a preparation method thereof. The hard carbon negative electrode material includes a nitrogen-doped hard carbon material core and a cerium phosphide / phosphorus composite coating layer and a mixed coating layer sequentially coated on the surface of the nitrogen-doped hard carbon material core from the inside to the outside.

[0102] Among them, the interlayer spacing of the nitrogen-doped hard carbon material core is 0.39nm, the pores of the nitrogen-doped hard carbon material core are 40nm, and the mass percentage of nitrogen element is 10wt%; the thickness of the cerium phosphide / phosphorus composite coating layer is 100nm, and the mass percentage of phosphorus element is 1wt%; the material of the mixed coating layer includes poly 3,4-ethylenedioxythiophene and Na3SbS4, the mass percentage of Na3SbS4 is 0.2wt%, and the thickness is 80nm.

[0103] The preparation method comprises the following steps:

[0104] S1. Dissolve 1 g of polyethylene glycol in 50 mL of ethanol to obtain solution A. Dissolve 0.6 g of p-toluenesulfonyl chloride and 0.3 g of pyridine in 50 mL of ethanol to obtain solution B. Slowly add solution B dropwise to solution A at 0°C. After reacting at room temperature for 18 h, transfer to ice water to precipitate a white solid, wash, and vacuum dry to obtain the intermediate.

[0105] 1 g of the intermediate product, 0.3 g of epichlorohydrin and 4 g of ethylenediamine were mixed and dissolved in 50 mL of anhydrous tetrahydrofuran, and reacted at 120° C. for 8 h. The ethylenediamine and anhydrous solvent were removed by rotary evaporation to obtain polyethyleneimine;

[0106] The polyethyleneimine was transferred to a tube furnace, heated to 1000°C at a rate of 2°C / min under an argon atmosphere, and kept at that temperature for 10 h. After cooling to room temperature, the nitrogen-doped hard carbon material was obtained.

[0107] The nitrogen-doped hard carbon material was dispersed in mixed acid (prepared by mixing nitric acid solution and sulfuric acid solution in a volume ratio of 1:3), stirred at 80°C for 12 hours, and washed and dried to obtain a surface-functionalized half-step precursor material.

[0108] S2. 1 g of the surface-functionalized half-step precursor material, 0.5 g of phosphorous acid, and 0.2 g of ceria were mixed and dispersed in 50 mL of ethanol, and then transferred to a reactor for hydrothermal reaction at 100 ° C for 24 h, washed, and dried to obtain a precursor material;

[0109] S3. Mix 1g of the above-mentioned precursor material and 0.5g of red phosphorus and transfer them to a tube furnace. Heat the mixture to 500℃ at 2℃ / min under an argon atmosphere and keep it warm for 2h. Then disperse 1g of the calcined material, 0.15g of 3,4-ethylenedioxythiophene, 5g of ferric chloride, and 0.02g of Na3SbS4 in 50mL of sulfuric acid (concentration of 1mol / L) solution, and react at 0℃ for 18h to obtain the hard carbon negative electrode material.

[0110] Example 5

[0111] The embodiment provides a hard carbon negative electrode material and a preparation method thereof.

[0112] The nitrogen-doped hard carbon material core has an interlayer spacing of 0.39 nm, a pore of 40 nm, and a mass percentage of nitrogen of 10 wt%; the cerium phosphide / phosphorus composite coating layer has a thickness of 100 nm and a mass percentage of phosphorus of 1 wt%; and the mixed coating layer is made of poly-3,4-ethylenedioxythiophene and Na3SbS4, and has a mass percentage of Na3SbS4 of 0.3 wt% and a thickness of 120 nm.

[0113] The preparation method is different from that of the embodiment 4 in that:

[0114] S3. 1g of the material obtained after the calcination treatment, 0.2g of 3,4-ethylenedioxythiophene, 5g of ammonium persulfate and 0.03g of Na3SbS4 are dispersed in 50mL of a hydrochloric acid solution (with a concentration of 1mol / L) to react for 20h at 0℃, and the rest is the same as in the preparation method of the embodiment 4.

[0115] Embodiment 6

[0116] The embodiment provides a hard carbon negative electrode material and a preparation method thereof.

[0117] The nitrogen-doped hard carbon material core has an interlayer spacing of 0.39 nm, a pore of 40 nm, and a mass percentage of nitrogen of 10 wt%; the cerium phosphide / phosphorus composite coating layer has a thickness of 100 nm and a mass percentage of phosphorus of 1 wt%; and the mixed coating layer is made of poly-3,4-ethylenedioxythiophene and Na3SbS4, and has a mass percentage of Na3SbS4 of 0.3 wt% and a thickness of 120 nm.

[0118] The preparation method is different from that of the embodiment 4 in that:

[0119] S3. 1g of the material obtained after the calcination treatment, 0.2g of 3,4-ethylenedioxythiophene, 5g of ammonium persulfate and 0.03g of Na3SbS4 are dispersed in 50mL of a hydrochloric acid solution (with a concentration of 1mol / L) to react for 20h at 0℃, and the rest is the same as in the preparation method of the embodiment 4.

[0120] Embodiment 7

[0121] This embodiment provides a hard carbon negative electrode material and a preparation method thereof. The hard carbon negative electrode material includes a nitrogen-doped hard carbon material core and a niobium phosphide / phosphorus composite coating layer and a mixed coating layer sequentially coated on the surface of the nitrogen-doped hard carbon material core from the inside to the outside.

[0122] Among them, the interlayer spacing of the nitrogen-doped hard carbon material core is 0.38nm, the pores of the nitrogen-doped hard carbon material core are 20nm, and the mass percentage of nitrogen element is 30wt%; the thickness of the niobium phosphide / phosphorus composite coating layer is 180nm, and the mass percentage of phosphorus element is 10wt%; the material of the mixed coating layer includes polyaniline and Na3PS4, the mass percentage of Na3PS4 is 5wt%, and the thickness is 80nm.

[0123] The preparation method comprises the following steps:

[0124] S1. Dissolve 1 g of polyethylene glycol in 50 mL of ethanol to obtain solution A. Dissolve 10 g of p-toluenesulfonyl chloride and 20 g of pyridine in 50 mL of ethanol to obtain solution B. Slowly add solution B dropwise to solution A at 0°C. After reacting at room temperature for 24 h, transfer to ice water to precipitate a white solid, wash, and vacuum dry to obtain the intermediate.

[0125] 1 g of the intermediate product, 3 g of glutaraldehyde, and 5 g of ethylenediamine were mixed and dissolved in 50 mL of anhydrous toluene, and the mixture was reacted at 120° C. for 18 h. The ethylenediamine and anhydrous solvent were removed by rotary evaporation to obtain polyethyleneimine;

[0126] The polyethyleneimine was transferred to a tube furnace, heated to 800°C at a rate of 2°C / min under an argon atmosphere and kept at that temperature for 10 h. After cooling to room temperature, the nitrogen-doped hard carbon material was obtained.

[0127] The nitrogen-doped hard carbon material was dispersed in mixed acid (prepared by mixing nitric acid solution and sulfuric acid solution in a volume ratio of 1:3), stirred at 70°C for 12 hours, and washed and dried to obtain a surface-functionalized half-step precursor material.

[0128] S2. 1 g of the surface-functionalized half-step precursor material, 10 g of sodium hypophosphite, and 10 g of niobium pentachloride were mixed and dispersed in 50 mL of ethanol, and then transferred to a reactor for hydrothermal reaction at 200 ° C for 16 h, washed and dried to obtain a precursor material;

[0129] S3. Mix 1g of the above-mentioned precursor material and 10g of red phosphorus and transfer them to a tubular furnace. Heat the mixture to 800℃ at 2℃ / min under an argon atmosphere and keep it warm for 6h. Then disperse 1g of the calcined material, 5g of aniline, 2g of ammonium persulfate and 10g of Na3PS4 in 50mL of hydrochloric acid (concentration of 1mol / L) solution and react at 0℃ for 20h to obtain the hard carbon negative electrode material.

[0130] Example 8

[0131] The difference between this embodiment and Example 1 is that in step S1, an adjustment is made to transfer an equal mass of urea into a tubular furnace, heat the temperature to 800°C at 2°C / min under an argon atmosphere and keep it warm for 6 hours, and then cool it to room temperature to obtain a nitrogen-doped hard carbon material, so that the interlayer spacing of the core of the nitrogen-doped hard carbon material is 0.39 nm, the pores of the core of the nitrogen-doped hard carbon material are 50 nm, the mass percentage of the nitrogen element is 5wt%, and the rest are the same as in Example 1.

[0132] Example 9

[0133] The difference between this embodiment and Example 1 is that in step S2, 1 g of the surface-functionalized half-step precursor material, 0.005 g of sodium hypophosphite, and 0.005 g of niobium pentachloride are mixed and dispersed in 50 mL of ethanol, so that the mass percentage of phosphorus in the niobium phosphide / phosphorus composite coating layer is 0.03 wt %. All other conditions are the same as in Example 1.

[0134] Example 10

[0135] The difference between this embodiment and embodiment 1 is that in step S2, niobium pentachloride is replaced by aluminum trichloride of equal mass, and the rest is the same as embodiment 1.

[0136] Example 11

[0137] The difference between this embodiment and embodiment 1 is that in step S3, 1 g of the above-mentioned precursor material and 0.005 g of red phosphorus are mixed and transferred to the tube furnace, so that the mass percentage of phosphorus element in the niobium phosphide / phosphorus composite coating layer is 0.6 wt %. Other aspects are the same as in embodiment 1.

[0138] Example 12

[0139] The difference between this embodiment and embodiment 1 is that step S3 is adjusted to include only the following steps: 1 g of the above-mentioned precursor material and 0.5 g of red phosphorus are mixed and transferred to a tube furnace, and the temperature is increased to 500°C at 2°C / min under an argon atmosphere and kept warm for 2 hours. The rest is the same as in embodiment 1.

[0140] Example 13

[0141] The difference between the present embodiment and embodiment 1 is that the material of the mixed coating layer in the hard carbon negative electrode material is adjusted to only include polyaniline, and step S3 is correspondingly adjusted to include the following steps: 1 g of the above precursor material and 0.5 g of red phosphorus are mixed and transferred into a tube furnace, and heated to 500℃ at 2℃ / min under an argon atmosphere for 2 h, and then 1 g of the material obtained after the calcination treatment, 0.1 g of aniline and 2 g of ammonium persulfate are dispersed in 50 mL of a hydrochloric acid solution (concentration of 1 mol / L), and reacted at 0℃ for 12 h, and the others are the same as in embodiment 1.

[0142] Comparative example 1

[0143] The difference between the present comparative example and embodiment 1 is that only a nitrogen-doped hard carbon material is provided, wherein the interlayer spacing of the core of the nitrogen-doped hard carbon material is 0.38 nm, the pore of the core of the nitrogen-doped hard carbon material is 20 nm, the mass percentage of nitrogen element is 5 wt%, and no coating treatment is performed, and the preparation method of the nitrogen-doped hard carbon material is the same as in embodiment 1.

[0144] Comparative example 2

[0145] The difference between the present comparative example and embodiment 1 is that the entire treatment in step S3 is not performed, i.e., the precursor material obtained in step S2 is used as the final hard carbon negative electrode material, and the others are the same as in embodiment 1.

[0146] Comparative example 3

[0147] The difference between the present comparative example and embodiment 1 is that the entire treatment in step S2 is not performed, i.e., the surface-functionalized semi-precursor material and red phosphorus are directly subjected to calcination treatment, and the others are the same as in embodiment 1.

[0148] Comparative example 4

[0149] The difference between the present comparative example and embodiment 1 is that the surface functionalization treatment in step S1 is not performed, i.e., the nitrogen-doped hard carbon material is directly subjected to subsequent treatment, and the others are the same as in embodiment 1.

[0150] Application examples 1 to 13 and comparative application examples 1 to 4

[0151] The hard carbon negative electrode materials prepared in the above embodiments and comparative examples are mixed and stirred uniformly with a binder polyacrylic acid, conductive carbon black and a dispersant sodium carboxymethyl cellulose, wherein the mass ratio of the hard carbon negative electrode material, the polyacrylic acid, the conductive carbon black and the sodium carboxymethyl cellulose is 95:1:2:2, the above mixture is mixed uniformly with water to obtain a negative electrode slurry, and then the negative electrode slurry is coated on a copper foil and dried to obtain a negative electrode sheet.

[0152] In an argon-filled glove box, the negative electrode sheets prepared above were assembled with a sodium metal sheet, a polypropylene separator, and an electrolyte to obtain sodium ion button batteries. The electrolyte used was a 1 mol / L NaCF3SO3 solution in diethylene glycol dimethyl ether.

[0153] Test conditions

[0154] The hard carbon negative electrode materials provided in Examples 1 to 13 and Comparative Examples 1 to 4 were subjected to conductivity tests under the following test conditions:

[0155] The electrical conductivity of the powder material was tested using a four-probe tester.

[0156] The sodium ion button batteries provided in Application Examples 1 to 13 and Comparative Application Examples 1 to 4 were subjected to performance tests. The test methods are as follows:

[0157] (1) At 200mAg -1 The capacity and capacity retention rate of the first and 100th charge and discharge tests were carried out under the same current density, and the electrode expansion rate of the sodium ion button battery at 100% SOC was tested at the same time.

[0158] (2) Test the capacity retention rate of sodium ion button batteries after 100 charge and discharge tests at different current densities.

[0159] The test results are shown in Tables 1-3:

[0160] Table 1

[0161]

[0162]

[0163] Table 2

[0164]

[0165]

[0166] Table 3

[0167]

[0168]

[0169] Combine Figure 1As can be seen from Tables 1-3, compared with Comparative Examples 1 to 4, the hard carbon negative electrode materials with both high rate and high capacity prepared in Examples 1 to 7 of the present invention have higher electrical conductivity, and the presence of the metal phosphide / phosphorus composite coating layer and the mixed coating layer can effectively reduce the interface resistance and improve the intrinsic conductivity of the material, thereby further improving the overall performance of the sodium ion battery. Specifically, the hard carbon negative electrode material prepared by the present invention has a high capacity and excellent rate performance, and still has a high capacity retention rate even at a current density of 3A / g, indicating that the material has excellent fast charging capability.

[0170] Comparison of Examples 1 to 3 shows that increasing the phosphorus content in the metal phosphide / phosphorus composite coating layer can effectively improve the capacity of the hard carbon negative electrode material. Comparison of Examples 4 to 6 shows that increasing the fast ion conductor content in the mixed coating layer can effectively improve the rate performance of the hard carbon negative electrode material.

[0171] Comparison of Example 1 and Examples 8 to 11 shows that the present invention further improves the electrochemical performance of the hard carbon negative electrode material by optimizing the types of raw material components and their content ranges.

[0172] Comparing Examples 1 and 12, it can be seen that in-situ coating with a conductive polymer not only facilitates sodium ion conduction but also preforms a stable interface layer during the initial charge and discharge process, thereby reducing side reactions between the electrolyte and the negative electrode material and improving the initial coulombic efficiency of the hard carbon negative electrode material. Furthermore, the conductive polymer alleviates the stress generated by volume expansion during the alloying process of the metal phosphide / phosphorus composite coating layer, thereby improving the structural stability of the negative electrode.

[0173] By comparing Example 1 and Comparative Examples 2 to 3, it can be seen that the doping of metal phosphide and phosphorus can effectively improve the rate performance and sodium ion diffusion coefficient of the hard carbon negative electrode material. This is because the metal phosphide has a strong desolvation ability and can accelerate the desolvation of sodium ions.

[0174] The applicant declares that the above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A method for preparing a hard carbon negative electrode material, characterized in that: The preparation method comprises the following steps: S1. Performing surface functionalization treatment on the nitrogen-doped hard carbon material to obtain a half-step precursor material; S2. mixing the half-step precursor material, the metal source, the first phosphorus source and the solvent, and obtaining a precursor material after a hydrothermal reaction; S3. calcining the precursor material and the second phosphorus source to obtain the hard carbon negative electrode material.

2. The preparation method according to claim 1, characterized in that In step S1, the mass percentage of nitrogen in the nitrogen-doped hard carbon material is 5 wt% to 30 wt%; Preferably, in step S1, the method for preparing the nitrogen-doped hard carbon material comprises the following steps: sintering the nitrogen-containing polymer precursor to obtain the nitrogen-doped hard carbon material; Preferably, the nitrogen-containing polymer precursor comprises polyethyleneimine; Preferably, the sintering temperature is 500° C. to 1000° C., and the sintering time is 1 hour to 12 hours; Preferably, the heating rate of the sintering treatment is 1°C / min to 5°C / min.

3. The preparation method according to claim 1 or 2, characterized in that The surface functionalization treatment process includes: dispersing the nitrogen-doped hard carbon material and the mixed acid solution to obtain the half-step precursor material; Preferably, the mixed acid solution comprises a nitric acid solution and a sulfuric acid solution; Preferably, the volume ratio of the nitric acid solution to the sulfuric acid solution is 1:(2-4); Preferably, the dispersion temperature is 25° C. to 100° C., and the dispersion time is 1 hour to 24 hours.

4. The preparation method according to any one of claims 1 to 3, characterized in that In step S2, the mass ratio of the half-step precursor material, the metal source and the first phosphorus source is 1:(0.01-100):(0.01-100), preferably 1:(0.2-10):(0.2-10); Preferably, in step S2, the metal source comprises any one or a combination of at least two of a niobium-containing compound, a tantalum-containing compound or a cerium-containing compound; Preferably, the niobium-containing compound comprises any one or a combination of at least two of niobium pentachloride, niobium pentoxide, niobium nitrate, niobium oxalate, hydrated niobium carbonate, sodium niobate or niobium ethoxide; Preferably, the tantalum-containing compound includes any one of tantalum chloride, sodium tantalate, tantalum nitrate, tantalum oxalate, tantalum carbonate or tantalum pentoxide, or a combination of at least two thereof; Preferably, the cerium-containing compound comprises any one or a combination of at least two of cerium dioxide, cerium sulfate, cerium chloride or ammonium cerium nitrate; Preferably, in step S2, the first phosphorus source comprises any one or a combination of at least two of phosphoric acid, phosphorous acid, phytic acid, sodium hypophosphite, diammonium hydrogen phosphate or triphenylphosphine; Preferably, in step S2, the solvent comprises any one of water, ethanol, methanol or N,N-dimethylformamide, or a combination of at least two thereof; Preferably, in step S2, the temperature of the hydrothermal reaction is 50° C. to 220° C., and the time of the hydrothermal reaction is 1 h to 24 h.

5. The preparation method according to any one of claims 1 to 4, characterized in that In step S3, the mass ratio of the precursor material to the second phosphorus source is 1:(0.01-100), preferably 1:(0.2-10); Preferably, in step S3, the second phosphorus source includes any one or a combination of at least two of red phosphorus, black phosphorus, sodium hypophosphite, diammonium hydrogen phosphate or triphenylphosphine; Preferably, in step S3, the calcination temperature is 500° C. to 1000° C., and the calcination time is 1 hour to 12 hours; Preferably, in step S3, the heating rate of the calcination treatment is 1°C / min to 5°C / min.

6. The preparation method according to any one of claims 1 to 5, characterized in that In step S3, after the precursor material and the second phosphorus source are calcined, the following steps are further included: The material obtained after the calcination treatment, a conductive polymer monomer, a fast ion conductor, an initiator and an acid solution are stirred and reacted to obtain the hard carbon negative electrode material.

7. The preparation method according to claim 6, characterized in that The mass ratio of the material obtained after the calcination treatment, the conductive polymer monomer, the fast ion conductor and the initiator is 1:(0.01-10):(0.01-10):(0.01-10), preferably 1:(0.2-5):(0.2-5):(0.5-10); Preferably, the conductive polymer monomer comprises any one or a combination of at least two of aniline, pyrrole or 3,4-ethylenedioxythiophene; Preferably, the fast ion conductor comprises Na3PS4, Na3SbS4 or Na3Zr2Si2PO 12 Any one or a combination of at least two of the following: Preferably, the concentration of the acid solution is 0.5 mol / L to 3 mol / L; Preferably, the stirring reaction temperature is 0°C to 25°C, and the stirring reaction time is 1 hour to 24 hours.

8. A hard carbon negative electrode material, characterized in that The hard carbon negative electrode material is prepared by the preparation method of the hard carbon negative electrode material according to any one of claims 1 to 7, and the hard carbon negative electrode material includes a nitrogen-doped hard carbon material core and a metal phosphide / phosphorus composite coating layer coated on the surface of the nitrogen-doped hard carbon material core.

9. The hard carbon negative electrode material according to claim 8, characterized in that The interlayer spacing of the nitrogen-doped hard carbon material core is 0.35nm to 0.4nm; Preferably, the pores of the nitrogen-doped hard carbon material core are 2 nm to 50 nm; Preferably, the mass percentage of nitrogen in the core of the nitrogen-doped hard carbon material is 5wt% to 30wt%; Preferably, the thickness of the metal phosphide / phosphorus composite coating layer is 5 nm to 200 nm; Preferably, the mass percentage of phosphorus element in the metal phosphide / phosphorus composite coating layer is 0.5wt% to 10wt%; Preferably, the hard carbon negative electrode material further comprises a mixed coating layer, and the mixed coating layer is arranged on the surface of the metal phosphide / phosphorus composite coating layer; Preferably, the material of the mixed coating layer comprises a conductive polymer and a fast ion conductor; Preferably, the mass percentage of the fast ion conductor in the mixed coating layer is 0.1wt% to 5wt%; Preferably, the thickness of the mixed coating layer is 5 nm to 200 nm.

10. A secondary battery, characterized in that: The secondary battery includes a positive electrode sheet, a negative electrode sheet, and an electrolyte, and the active material of the negative electrode sheet includes the hard carbon negative electrode material according to claim 8 or 9.

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