Negative electrode material, preparation method thereof and battery

Abstract

The invention relates to the field of batteries, in particular to a negative electrode material, a preparation method thereof and a battery. The negative electrode material comprises a hard carbon material and a coating layer coating at least part of the surface of the hard carbon material, the coating layer comprises a carbon material, the carbon material comprises a carbon material matrix and doping elements doped in the carbon material matrix, and the doping elements comprise P and N; the oxygen element content in the negative electrode material is 0. The preparation method of the negative electrode material comprises the following steps: calcining a hard carbon precursor in a reducing atmosphere to obtain a hard carbon material; and in an inert atmosphere, mixing the mixed solution of the hard carbon material, the phosphorus source and the nitrogen source with the metal hydride, and calcining to obtain the negative electrode material. According to the negative electrode material provided by the invention, the sodium storage capacity per gram of a hard carbon material is improved, and the high reversible capacity and the low potential capacity of the material are improved; and P and N double-atom doping also improves the reaction activity and the electronic conductivity of the negative electrode material, and the circulation and rate capability of the negative electrode material is remarkably improved.

Description

Technical Field

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

[0002] Due to increasingly serious environmental and energy crises, there is an urgent need to develop renewable and sustainable energy sources, such as wind and solar energy. To address these intermittent energy conversion issues, the development of large-scale energy storage systems is crucial. Sodium-ion batteries (Na-ion batteries) are considered the most promising alternative to Li-ion batteries for large-scale energy storage applications due to their low cost, naturally abundant sodium resources, and similar rocking-chair operating mechanism. Over the past decade, significant progress has been made in the practical application of Na-ion battery cathodes, including polyanionic compounds, layered oxides, and Prussian analogs. Regarding the anode, various materials, such as carbonaceous materials, alloy metals, and metal chalcogenides, have been extensively explored. However, due to their complex preparation processes and inherent electrochemical defects, most candidate materials exhibit serious limitations for large-scale applications. Carbonaceous materials are considered the most promising anode materials due to their low cost, ease of preparation, and good reproducibility. Unlike the successful application of graphite in conventional Li-ion batteries, the narrow carbon interlayer spacing (0.335 nm) and thermodynamic instability of graphite intercalation compounds hinder the intercalation of Na ions. Currently, graphite is generally considered inadequate as a direct anode material for sodium-ion batteries. Hard carbon, composed of interlaced graphite microcrystalline layers, is rich in micropores and defects. Its relatively large interlayer spacing allows it to store large amounts of sodium ions and exhibits a high reversible capacity. Furthermore, with the increasing demands for batteries, there is an urgent need for hard carbon anodes with high reversible capacity and an appropriate low-potential plateau capacity to meet the requirements of full batteries for practical applications. Summary of the Invention

[0003] In view of this, the present invention is committed to providing a negative electrode material and a preparation method thereof and a battery, wherein the negative electrode material has high reversible capacity and low potential platform capacity.

[0004] In order to solve the above technical problems, this application is implemented as follows:

[0005] The present invention provides a negative electrode material, comprising a hard carbon material and a coating layer coated on at least a portion of the surface of the hard carbon material;

[0006] The coating layer includes a carbon material, the carbon material includes a carbon material matrix and doping elements doped in the carbon material matrix, and the doping elements include P and N;

[0007] The oxygen content in the negative electrode material is 0.

[0008] In any embodiment, the hard carbon material further includes a doping element, and the doping element includes P and N.

[0009] In any embodiment, based on the mass of the negative electrode material, the doping amount of the P element in the negative electrode material is 0.5 wt % to 1 wt %.

[0010] In any embodiment, based on the mass of the negative electrode material, the doping amount of the N element in the negative electrode material is 0.5 wt % to 1 wt %.

[0011] In any embodiment, the coating layer has a thickness of 2 nm to 10 nm.

[0012] In any embodiment, the particle size of the hard carbon material is 4 μm to 8 μm.

[0013] In any embodiment, the hard carbon material comprises a spherical hard carbon material.

[0014] In any embodiment, based on the atomic content of the coating layer, the doping amount of the P element in the carbon material is 10 at % to 15 at %.

[0015] In any embodiment, based on the atomic content of the coating layer, the doping amount of the N element in the carbon material is 5 at % to 10 at %.

[0016] The second aspect of the present application provides a method for preparing a negative electrode material, comprising the following steps:

[0017] calcining the hard carbon precursor in a reducing atmosphere to obtain a hard carbon material;

[0018] Under a first inert atmosphere, mixing a mixed solution of a hard carbon material, a phosphorus source, and a nitrogen source with a metal hydride, and performing a second calcination to obtain a negative electrode material;

[0019] Wherein, the negative electrode material comprises a hard carbon material and a coating layer coated on at least a portion of the surface of the hard carbon material;

[0020] The coating layer includes a carbon material, the carbon material includes a carbon material matrix and doping elements doped in the carbon material matrix, and the doping elements include P and N;

[0021] The oxygen content in the negative electrode material is 0.

[0022] In any embodiment, the hard carbon precursor includes a resin and / or a biomass material.

[0023] In any embodiment, the resin includes at least one of a phenolic resin, an epoxy resin, a polyacrylonitrile, and a polyimide.

[0024] In any embodiment, the particle size of the phenolic resin is 5 μm to 10 μm.

[0025] In any embodiment, the reducing atmosphere comprises a mixed atmosphere of hydrogen and a second inert atmosphere.

[0026] In any embodiment, the volume percentage of hydrogen in the mixed atmosphere is 3% to 5%.

[0027] In any embodiment, the second inert atmosphere includes at least one of nitrogen, argon, helium, and neon.

[0028] In any embodiment, the flow rate of the reducing atmosphere is 50 mL / min to 100 mL / min.

[0029] In any embodiment, the temperature of the first calcination is 1000° C. to 1400° C., the time of the first calcination is 2 h to 5 h, and the heating rate of the first calcination is 2° C. / min to 5° C. / min.

[0030] In any embodiment, the first inert atmosphere includes at least one of nitrogen, argon, helium, and neon.

[0031] In any embodiment, the flow rate of the first inert atmosphere is 50 mL / min to 150 mL / min.

[0032] In any embodiment, the molar ratio of the phosphorus source to the nitrogen source in the mixed solution of the phosphorus source and the nitrogen source is 1:1 to 4:1.

[0033] In any embodiment, the ratio of the mixed solution of the phosphorus source and the nitrogen source to the hard carbon material is (2-20) mL:1 g.

[0034] In any embodiment, the mass ratio of the metal hydride to the hard carbon material is 0.5 to 2:1.

[0035] In any embodiment, the phosphorus source includes at least one of phosphorus trichloride, phenylphosphonium dichloride, and phosphorus tribromide.

[0036] In any embodiment, the nitrogen source comprises at least one of pyrrole, pyridine, and quinoline.

[0037] In any embodiment, the metal hydride includes at least one of calcium hydride, magnesium hydride, potassium hydride, and beryllium hydride.

[0038] In any embodiment, the temperature of the second calcination is 600° C. to 900° C., the time of the second calcination is 2 h to 5 h, and the heating rate of the second calcination is 2° C. / min to 5° C. / min.

[0039] In any embodiment, after the second calcination is completed, the method further includes: cooling and acid washing the product obtained after the second calcination to obtain a negative electrode material.

[0040] In any embodiment, the pickling reagent includes at least one of a hydrochloric acid solution, a sulfuric acid solution, and an oxalic acid solution.

[0041] In any embodiment, the concentration of the reagent is 1 mol / L to 3 mol / L.

[0042] In any embodiment, the pickling time is 1 hour to 4 hours.

[0043] A third aspect of the present application provides a battery, comprising a negative electrode sheet, wherein the negative electrode sheet comprises the above-mentioned negative electrode material, or the negative electrode material prepared by the above-mentioned method for preparing the negative electrode material.

[0044] Through the above technical solution, the beneficial technical effects of the present invention are:

[0045] The negative electrode material of the present application is a core-shell structure, in which the core is made of hard carbon material, and the oxygen content in the hard carbon material is 0, and the outer shell or shell layer includes a carbon material co-doped with phosphorus (P) and nitrogen (N). That is, the negative electrode material mainly includes a hard carbon material and a P and N co-doped carbon material coated on the surface of the hard carbon material, and the oxygen content in the negative electrode material is 0. By constructing an oxygen-free environment, the sodium storage capacity of the hard carbon material is improved, thereby improving the high reversible capacity and low potential capacity of the material. At the same time, P and N diatomic doping also improves the reactivity and electronic conductivity of the negative electrode material, significantly improving the cycle and rate performance of the negative electrode material.

[0046] Other features and advantages of the present invention will be described in detail in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. Together with the following specific embodiments, they are used to explain the present invention, but do not constitute a limitation of the present invention.

[0048] Figure 1 Shown is a diagram of a synthesis device for the negative electrode material of the present invention;

[0049] Figure 2 Shown is an EDS surface scan of the hard carbon material prepared in Example 1;

[0050] Figure 3 Shown is the EDS line scan of the hard carbon material prepared in Example 1;

[0051] Figure 4The EDS surface scan of the negative electrode material prepared in Example 1 is shown; wherein, Figure 4 (a) is the EDS surface scan of the N element in the negative electrode material, and (b) is the EDS surface scan of the P element in the negative electrode material;

[0052] Figure 5 Shown is a TEM image of the negative electrode material prepared in Example 1.

[0053] Legend:

[0054] 10-air inlet;

[0055] 20- stirrer;

[0056] 30-air outlet;

[0057] 40-Tube furnace. DETAILED DESCRIPTION

[0058] The present invention discloses a negative electrode material, a method for preparing the same, and a battery. Those skilled in the art can refer to the contents herein and appropriately improve the process parameters to achieve the desired results. It should be noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in the present invention. The methods and applications of the present invention have been described through preferred embodiments. It is obvious that relevant personnel can modify or appropriately alter and combine the methods and applications described herein without departing from the content, spirit, and scope of the present invention to implement and apply the technology of the present invention.

[0059] In the description of the present invention, a list of items connected by the term "at least one of" or other similar terms can mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A can include a single element or multiple elements. Item B can include a single element or multiple elements. Item C can include a single element or multiple elements.

[0060] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range or the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0061] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0062] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0063] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0064] In sodium-ion batteries, cathode materials such as layered oxides, polyanionic compounds, and Prussian blue have been well developed, but there is a lack of high-performance anode materials with high sodium storage capacity and low cost. Among the anode materials of sodium-ion batteries, "hard carbon", as a non-graphitizable carbon, has become the most suitable SIB anode microstructure due to its randomly distributed vortex structure with large interlayer spacing (over 0.37 nm) and closed nanopores. Generally, hard carbon is synthesized from thermosetting precursors (such as cellulose, peanut shells, sucrose, glucose, phenolic resins, starch, anthracite, etc.) by high-temperature carbonization. During the pyrolysis process, the carbon layer in the precursor easily forms vortex layers and disordered microstructures consisting of surface defects, nanopores or voids and graphitic domains. During the curing process, Na ions are first adsorbed in the inclined region (0.1-1V vs Na / Na + ) on the hard carbon defect sites, and then in the plateau region (0-0.1V vs Na / Na + ) forms quasi-metallic Na clusters. However, the hard carbon electrode exhibits a very low platform capacity, and the platform capacity plays a key role in determining the operating voltage and energy density of the fully charged battery. In the prior art, phosphorus (P) element is usually used to dope and modify the hard carbon material. However, since the phosphorus element inevitably forms POx functional groups on the surface of the material during the doping process, it hinders the further doping of the P element into the bulk phase, resulting in a low doping amount, which cannot effectively improve the problem of low sodium storage capacity of the hard carbon material. Therefore, there is an urgent need to develop a hard carbon negative electrode material for sodium ion batteries with high capacity and low potential platform capacity ratio.

[0065] In view of this, the present application proposes a negative electrode material, a preparation method thereof, and a battery, wherein the negative electrode material has high reversible capacity and low potential plateau capacity. The present application and optional embodiments are described in more detail below.

[0066] [Anode material]

[0067] In some embodiments, the present application provides a negative electrode material, the negative electrode material comprising a hard carbon material and a coating layer coated on at least a portion of the surface of the hard carbon material;

[0068] Wherein, the coating layer comprises a carbon material, the carbon material comprises a carbon material matrix and doping elements doped in the carbon material matrix, and the doping elements comprise P and N;

[0069] The oxygen content in the negative electrode material is 0.

[0070] It should be noted that, in this application, the term "coating" is not limited to direct coating but also includes indirect coating. For example, when a coating layer coats a hard carbon material, there may be no other structure between the coating layer and the outer surface of the hard carbon material, or there may be one or more layers of other structures between the coating layer and the outer surface of the hard carbon material. Preferably, there is no other structure between the coating layer and the outer surface of the hard carbon material.

[0071] The coating layer formed on or coated on at least a portion of the surface of the hard carbon material can protect or improve the hard carbon material, enhancing its structural stability, reversible capacity, and low-potential capacity, and optimizing the conductivity of the negative electrode material. Forming or coating the coating layer on at least a portion of the surface of the hard carbon material means that the coating layer can completely encapsulate the hard carbon material within the coating layer, or it can cover only a portion of the outer surface of the hard carbon material; that is, the coating layer can completely cover the hard carbon material or only a portion of the surface of the hard carbon material, preferably completely covering the hard carbon material.

[0072] The negative electrode material of the present application is a core-shell structure, in which the core is made of hard carbon material, and the oxygen content in the hard carbon material is 0, and the outer shell or shell layer includes a carbon material co-doped with P and N. That is, the negative electrode material mainly includes a hard carbon material and a P and N co-doped carbon material coated on the surface of the hard carbon material, and the oxygen content in the negative electrode material is 0. By constructing an oxygen-free environment, the sodium storage capacity of the hard carbon material is improved, thereby improving the high reversible capacity and low potential capacity of the material. At the same time, P and N diatomic doping also improves the reactivity and electronic conductivity of the negative electrode material, significantly improving the cycle and rate performance of the negative electrode material. In detail:

[0073] The present application dopes P and N elements in the coating layer of the hard carbon material by constructing an oxygen-free environment, effectively avoiding the generation of POx and NOx functional groups of the doped elements P and N on the surface of the material, increasing the doping amount of P and N in the material, providing more defect structures, and then providing more sodium ion storage active sites, thereby significantly improving the sodium storage capacity of the material; at the same time, the dual doping of P and N elements in the coating layer can also change the local bonding environment and electron distribution on the surface of the negative electrode material, significantly expand the interlayer spacing of the material, thereby improving the reactivity and electronic conductivity of the negative electrode material. In addition, the presence of the carbon material coating layer can further reduce the specific surface area of ​​the material, increase the number of closed pores in the material, and thus significantly improve the cycle and rate performance of the material. Thus, the negative electrode material can improve the cycle and rate performance of sodium ion batteries.

[0074] This application introduces doping elements P and N simultaneously into the carbon material of the coating layer, based on the hard carbon material coating the carbon material. This changes the local bonding environment and electron distribution on the surface of the hard carbon negative electrode material, providing a high density of active sites, promoting the rapid migration of sodium ions, and improving the structural stability of the material. Therefore, the overall electrochemical performance of the material can be improved.

[0075] Therefore, the negative electrode material provided by the present application can be a negative electrode material with a stable structure, high sodium storage capacity, and good electrical conductivity. The negative electrode material comprises a hard carbon material and a P and N co-doped carbon material coated on the surface of the hard carbon material, and the oxygen content in the negative electrode material is zero. This not only achieves a uniform carbon material coating on the surface of the hard carbon material, but also achieves high P and N doping levels in the coating layer, providing more sodium storage space, improving the material's reactivity and electrical conductivity, and significantly enhancing the negative electrode material's cycle and rate performance.

[0076] In this application, the term "hard carbon material" generally refers to an amorphous carbon material formed by high-temperature carbonization (usually at 800-1500°C) of a carbon-containing precursor (such as resin, polymer, biomass, etc.); wherein the oxygen content in the above-mentioned "hard carbon material" is 0, and it can be a carbonization product of a carbon-containing precursor under a reducing atmosphere. By constructing an oxygen-free environment, it can effectively avoid the doping elements P and N from generating POx and NOx functional groups on the surface of the material, providing conditions for subsequently increasing the doping amount of P and N in the material.

[0077] In this application, the term "carbon material" refers to a material primarily composed of carbon, without a consistent structure or properties. The oxygen content of the "carbon material" herein is zero. This material can be a carbon material produced by the pyrolysis of phosphorus-containing compounds and nitrogen-containing organic matter under an inert atmosphere, which is then uniformly deposited on the surface of a hard carbon material to form a coating. By creating an oxygen-free environment and simultaneously limiting the generation of POx and NOx functional groups on the material surface by doping elements P and N, a carbon coating is effectively prevented. This, in turn, improves the conductivity of the negative electrode material and enhances its cycling performance.

[0078] In some embodiments, the hard carbon material further includes doping elements, and the doping elements include P and N.

[0079] In this application, by introducing doping elements P and N into the hard carbon material, the synergistic effect of the two can enhance the conductivity of the hard carbon material, reduce the charge transfer impedance, and improve the overall rate performance and cycle life of the negative electrode material.

[0080] In some embodiments, the amount of P doped in the negative electrode material is 0.5 wt% to 1 wt% based on the mass of the negative electrode material. For example, the amount of P doped in the hard carbon material can be any one of 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, and 1 wt%, or a range therebetween, based on the mass of the negative electrode material.

[0081] In the present application, the calculation method of the doping amount of the P element in the negative electrode material is: ((total mass of the negative electrode material-mass of the carbon material matrix)×mass content of the P element in the negative electrode material) / total mass of the negative electrode material.

[0082] In some embodiments, the amount of nitrogen doped in the negative electrode material is 0.5 wt% to 1 wt% based on the mass of the negative electrode material. For example, the amount of nitrogen doped in the hard carbon material can be any one of 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, and 1 wt%, or a range therebetween, based on the mass of the negative electrode material.

[0083] In the present application, the calculation method of the doping amount of the N element in the negative electrode material is: ((total mass of the negative electrode material-mass of the carbon material matrix)×mass content of the N element in the negative electrode material) / total mass of the negative electrode material.

[0084] In this application, by controlling the doping amount of P and N elements in the negative electrode material within the above-mentioned mass range, the sodium storage capacity, cycle stability and rate performance of the material can be improved, and the structural stability can be maintained. If the doping amount of P and N is too much, the electron transmission path in the material will be blocked, hindering the diffusion of sodium ions and triggering side reactions, which will lead to a decrease in the conductivity of the material and a decrease in cycle stability. If the doping amount of P and N is too little, the effect of improving the conductivity of the material will not be achieved.

[0085] In some embodiments, the coating layer has a thickness of 2 nm to 10 nm. For example, the coating layer may have a thickness of 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, or 10 nm, or a range of any two thereof.

[0086] In the present application, the thickness of the coating layer is controlled within the above range. The coating layer can form a continuous and dense protective layer, which can not only achieve rapid diffusion of sodium ions, but also prevent the penetration of electrolyte, thereby improving the rate performance and cycle stability of the negative electrode material. If the thickness of the coating layer is too thick, the diffusion path of the sodium ions will be extended, the diffusion impedance will increase, and the rate performance and cycle stability of the negative electrode material will be affected. If the thickness of the coating layer is too thin, the effect of improving the rate performance and cycle stability of the material will not be achieved.

[0087] In some embodiments, the particle size of the hard carbon material is 4 μm to 8 μm. As an example, the particle size of the hard carbon material can be any one of 4 μm, 5 μm, 6 μm, 7 μm, and 8 μm, or a range between any two of them.

[0088] In some embodiments, the hard carbon material includes a spherical hard carbon material.

[0089] In the present application, the spherical hard carbon material is beneficial to improving the compaction density of the negative electrode material, which can further improve the overall energy density of the battery.

[0090] In some embodiments, the doping amount of the P element in the carbon material is 10 at % to 15 at %, based on the atomic content of the coating layer. As an example, the doping amount of the P element in the carbon material can be any one of 10 at %, 11 at %, 12 at %, 13 at %, 14 at %, and 15 at %, or a range between any two of these values, based on the atomic content of the coating layer.

[0091] In some embodiments, the doping amount of the nitrogen element in the carbon material is 5 at % to 10 at % based on the atomic content of the coating layer. As an example, the doping amount of the nitrogen element in the carbon material can be any one of 5 at %, 6 at %, 7 at %, 8 at %, 9 at %, and 10 at %, or a range between any two of the above, based on the atomic content of the coating layer.

[0092] In this application, the test method for the doping amount of P element and N element in carbon material is: using a scanning electron microscope and an EDS spectrometer for testing; specifically, under a scanning electron microscope voltage of 15KV and a magnification of 10K, an area is selected for EDS surface scanning to obtain the element ratio.

[0093] In this application, by creating an oxygen-free environment, the formation of POx and NOx functional groups on the material surface by the doping elements P and N is avoided, and the P and N doping levels are significantly increased, achieving a P doping level of 5at% to 10at% and a N doping level of 3at% to 6at% in the carbon material. This is beneficial for increasing the material's sodium storage capacity, and improving the reactivity and electronic conductivity of the negative electrode material. If the doping level of the doping elements P and N is too high, it will destroy the skeleton continuity of the coating layer carbon material, hinder electron transmission, and affect the conductivity of the negative electrode material.

[0094] Thus, the provided negative electrode material, such as a P and N co-doped carbon material coated with a hard carbon material, has an oxygen content of 0. By constructing an oxygen-free environment, the doping elements P and N are introduced on the basis of carbon coating, thereby achieving a high doping amount of P and N elements, which not only increases the sodium storage capacity of the material, but also increases the high reversible capacity and low potential capacity of the material; at the same time, the dual doping of P and N also changes the local bonding environment and electron distribution on the surface of the hard carbon negative electrode material, thereby improving the reaction activity and conductivity of the material, and at the same time forming a uniform carbon material coating layer, thereby improving the structural stability of the material.

[0095] [Method for preparing negative electrode material]

[0096] A second aspect of the present application provides a method for preparing a negative electrode material, the method comprising the following steps:

[0097] calcining the hard carbon precursor in a reducing atmosphere to obtain a hard carbon material;

[0098] Under a first inert atmosphere, mixing a mixed solution of a hard carbon material, a phosphorus source, and a nitrogen source with a metal hydride, and performing a second calcination to obtain a negative electrode material;

[0099] The negative electrode material includes a hard carbon material and a coating layer coated on at least a portion of the surface of the hard carbon material;

[0100] The coating layer includes a carbon material, the carbon material includes a carbon material matrix and doping elements doped in the carbon material matrix, and the doping elements include P and N;

[0101] The oxygen content in the negative electrode material is 0.

[0102] The present application provides a method for preparing a negative electrode material. A hard carbon material precursor is calcined in a reducing atmosphere, and then calcined with a phosphorus source, a nitrogen source, and a metal hydride in an inert atmosphere to obtain the negative electrode material. In this method, an oxygen-free environment is created to achieve coating of the hard carbon material with a P and N dual-doped carbon material. Specifically:

[0103] Oxygen is removed from the hard carbon material derived from the hard carbon precursor under a reducing atmosphere to produce a hard carbon material with zero oxygen content. A mixed solution of the hard carbon material, a phosphorus source, and a nitrogen source is then calcined with a metal hydride under an inert atmosphere. While avoiding the introduction of oxygen, the metal oxide decomposes during the calcination process to produce reducing hydrogen, enhancing the oxygen-free purity of the material surface and preventing the ingress of oxygen during the preparation process.

[0104] In the synthesis process of the negative electrode material of the present embodiment, an oxygen-free carbon microsphere substrate is obtained through a reducing atmosphere, thus constructing an oxygen-free synthesis environment. Simultaneously, a phosphorus source and a nitrogen source are introduced to achieve high-doping levels of N and P in the coating layer. The dual-atom doping can change the local bonding environment and electron distribution on the surface of the hard carbon negative electrode material, resulting in a significantly expanded interlayer spacing in the material structure, thereby improving the reactivity and electronic conductivity of the hard carbon negative electrode material. The presence of the coating further reduces the material's specific surface area, increases the number of closed pores in the material, and enhances the material's cycle and rate performance.

[0105] It should be understood that all the features and advantages described above for the “negative electrode material” are also applicable to the “method for preparing the negative electrode material” and will not be described in detail here.

[0106] In some specific embodiments, the method for preparing the phosphorus-containing composite negative electrode material comprises the following steps (a) to (b):

[0107] Step (a): preparing a hard carbon material.

[0108] In some embodiments, in step (a), the hard carbon precursor is first calcined under a reducing atmosphere to obtain a hard carbon material.

[0109] Optionally, in step (a), the hard carbon precursor includes resin and / or biomass material. For example, the hard carbon precursor may include resin, biomass material, or both.

[0110] Optionally, the resin includes at least one of phenolic resin, epoxy resin, polyacrylonitrile and polyimide.

[0111] Preferably, the resin comprises a phenolic resin.

[0112] Optionally, the particle size of the phenolic resin is 5 μm to 10 μm. As an example, the particle size of the phenolic resin can be any one of 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, and 10 μm, or a range between any two of them.

[0113] Optionally, in step (a), the reducing atmosphere includes a mixed atmosphere of hydrogen and a second inert atmosphere.

[0114] Optionally, the volume percentage of hydrogen in the mixed atmosphere is 3% to 5%. As an example, the volume percentage of hydrogen in the mixed atmosphere is any one of 3%, 3.5%, 4%, 4.5%, and 5%, or a range between any two of them.

[0115] In the present application, by controlling the hydrogen content in the reducing atmosphere within the above-mentioned volume percentage range, the oxygen element in the hard carbon material can be effectively removed, ensuring that the oxygen content in the hard carbon material is 0, and promoting the densification of the carbon skeleton. Normally, the oxygen in the hard carbon after high-temperature calcination exists in the form of functional groups (such as carboxyl, carbonyl, and ester functional groups), while in the present application, a reducing atmosphere is introduced during high-temperature calcination, and the hydrogen in the reducing atmosphere can react with the functional groups in the hard carbon to achieve the purpose of removing the oxygen element in the hard carbon material (specifically, in a high-temperature environment, the CO bond in the functional group breaks, the oxygen atoms escape, and the hydrogen undergoes oxidation-reduction with the oxygen-containing functional groups to generate spillable substances such as water, which are thereby removed).

[0116] Optionally, the second inert atmosphere includes at least one of nitrogen, argon, helium and neon.

[0117] Optionally, in step (a), the flow rate of the reducing atmosphere is 50 mL / min to 100 mL / min. As an example, the flow rate of the reducing atmosphere can be any one of 50 mL / min, 60 mL / min, 70 mL / min, 80 mL / min, 90 mL / min, and 100 mL / min, or a range therebetween.

[0118] Optionally, in step (a), the temperature of the first calcination is 1000° C. to 1400° C., the time of the first calcination is 2 h to 5 h, and the heating rate of the first calcination is 2° C. / min to 5° C. / min. As an example, the temperature of the first calcination can be any one of 1000° C., 1050° C., 1100° C., 1150° C., 1200° C., 1250° C., 1300° C., 1350° C., and 1400° C., or a range between any two thereof, the time of the first calcination can be any one of 2 h, 2.5 h, 3 h, 3.5 h, 4 h, 4.5 h, and 5 h, or a range between any two thereof, and the heating rate of the first calcination is any one of 2° C. / min, 2.5° C. / min, 3° C. / min, 3.5° C. / min, 4° C. / min, 4.5° C. / min, and 5° C. / min, or a range between any two thereof.

[0119] In this application, the first calcination process involves pyrolysis of a hard carbon precursor (e.g., resin, biomass, etc.) at high temperatures, eliminating oxygen and forming a hard carbon material. By controlling the first calcination conditions within the aforementioned range, a hard carbon material with excellent sodium storage performance and cyclic stability can be prepared.

[0120] As an example, step (a) includes:

[0121] A hard carbon precursor (such as a phenolic resin with a particle size of 5μm to 10μm) is calcined at 1000℃ to 1400℃ for 2h to 5h at a heating rate of 2℃ / min to 5℃ / min in a reducing atmosphere with a flow rate of 50mL / min to 100mL / min (such as a mixed atmosphere of hydrogen and nitrogen, with a volume percentage of hydrogen of 3% to 5%) to obtain a hard carbon material.

[0122] Step (b): coating and doping

[0123] In some embodiments, in step (b), a phosphorus source and a nitrogen source are mixed to obtain a mixed solution; a hard carbon material and a metal hydride are mixed to obtain a mixture; the mixed solution is mixed with the mixture by a first inert gas, and a second calcination is performed. After the second calcination is completed, the product is naturally cooled to room temperature, and then acid-washed to obtain a negative electrode material. For example, the device in step (b) is as follows Figure 1 As shown, a certain amount of phosphorus source and nitrogen source are weighed and placed in a three-necked flask, and dispersed at room temperature under a stirrer 20 to obtain a mixed solution; a first inert atmosphere is introduced into the mixed solution through the air inlet 10, and then the mixed solution is sent into a tubular furnace 40 through the air outlet 30; the hard carbon material and the metal hydride are mixed and placed in the tubular furnace 40 for a second calcination. After the second calcination is completed, it is naturally cooled to room temperature, and then the obtained product is pickled to obtain a negative electrode material.

[0124] Optionally, the first inert atmosphere includes at least one of nitrogen, argon, helium and neon.

[0125] Optionally, the flow rate of the first inert atmosphere is 50 mL / min to 150 mL / min. As an example, the flow rate of the first inert atmosphere can be any one of 50 mL / min, 60 mL / min, 80 mL / min, 100 mL / min, 120 mL / min, 140 mL / min, and 150 mL / min, or a range therebetween.

[0126] Optionally, the molar ratio of the phosphorus source to the nitrogen source in the mixed solution of the phosphorus source and the nitrogen source is 1:1 to 4:1. As an example, the molar ratio of the phosphorus source to the nitrogen source in the mixed solution of the phosphorus source and the nitrogen source can be any one of 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, and 4:1, or a range between any two of them.

[0127] In the present application, by controlling the amount of phosphorus source and nitrogen source within the above-mentioned molar ratio range, the sodium storage capacity of the material can be increased through synergistic doping effect and surface chemical regulation, and the reactivity and conductivity of the material can be enhanced, thereby obtaining a high-performance negative electrode material.

[0128] Optionally, the ratio of the mixed solution of the phosphorus source and the nitrogen source to the hard carbon material is (2-20) mL:1 g. As an example, the ratio of the mixed solution of the phosphorus source and the nitrogen source to the hard carbon material can be any one of 2:1 g, 5:1 g, 10 mL:1 g, 11 mL:1 g, 12 mL:1 g, 13 mL:1 g, 14 mL:1 g, 15 mL:1 g, 16 mL:1 g, 17 mL:1 g, 18 mL:1 g, 19 mL:1 g, and 20 mL:1 g, or a range between any two of the above.

[0129] In the present application, by controlling the usage ratio of the mixed solution of phosphorus source and nitrogen source and the hard carbon material within the above range, it can be ensured that the subsequent mixed solution forms a P and N co-doped carbon material coating layer of appropriate thickness on the surface of the hard carbon material, and it can also be ensured that the P and N doping amounts in the carbon material coating layer are also within a reasonable range.

[0130] Optionally, the mass ratio of the metal hydride to the hard carbon material is 0.5 to 2: 1. As an example, the mass ratio of the metal hydride to the hard carbon material can be any one of 0.5:1, 0.8:1, 1:1, 1.2:1, 1.4:1, 1.5:1, 1.6:1, 1.8:1, and 2:1, or a range therebetween.

[0131] Optionally, the phosphorus source includes at least one of phosphorus trichloride, phenylphosphonium dichloride and phosphorus tribromide.

[0132] Optionally, the nitrogen source includes at least one of pyrrole, pyridine and quinoline.

[0133] In the present application, the above-mentioned phosphorus source and nitrogen source are selected to control the entry of oxygen elements during the preparation process, which is beneficial to increase the doping amount of P and N and improve the electrochemical properties of the material.

[0134] Optionally, the metal hydride includes at least one of calcium hydride, magnesium hydride, potassium hydride and beryllium hydride.

[0135] Optionally, the temperature of the second calcination is 600° C. to 900° C., the time of the second calcination is 2 h to 5 h, and the heating rate of the second calcination is 2° C. / min to 5° C. / min. As an example, the temperature of the second calcination can be any one of 600° C., 650° C., 700° C., 750° C., 800° C., 850° C., and 900° C., or a range between any two of them, the time of the second calcination can be any one of 2 h, 2.5 h, 3 h, 3.5 h, 4 h, 4.5 h, and 5 h, or a range between any two of them, and the heating rate of the second calcination can be any one of 2° C. / min, 2.5° C. / min, 3° C. / min, 3.5° C. / min, 4° C. / min, 4.5° C. / min, and 5° C. / min, or a range between any two of them.

[0136] During this second calcination, the phosphorus and nitrogen sources decompose at high temperature to form a P- and N-codoped carbon coating. Simultaneously, the hydrogen released by the decomposition of the metal hydride reacts with oxygen-containing functional groups, further improving the material's oxygen-free purity and preventing oxygen intrusion during the preparation process. By precisely controlling calcination conditions (temperature, atmosphere, time) and element ratios, the doping efficiency and material integrity are balanced, ultimately optimizing the electrochemical performance of the hard carbon material.

[0137] Optionally, after the second calcination is completed, the product obtained after the second calcination is naturally cooled to room temperature and acid-washed to obtain a negative electrode material.

[0138] In this application, unless otherwise specified, room temperature refers to 10 to 30°C.

[0139] The pickling can be performed using conventional procedures known to those skilled in the art. For example, the pickling reagent includes at least one of a hydrochloric acid solution, a sulfuric acid solution, and an oxalic acid solution; the concentration of the reagent is 1 mol / L to 3 mol / L; and the pickling time is 1 to 4 hours. For example, the concentration of the reagent can be any one of 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, and 3 mol / L, or a range between any two of them; and the pickling time can be any one of 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, and 4 hours, or a range between any two of them.

[0140] As an example, step (b) includes:

[0141] In such Figure 1 In the synthesis apparatus shown in the figure, a phosphorus source (such as phenylphosphonium dichloride) and a nitrogen source (such as pyrrole) are placed in a three-necked flask at a molar ratio of 1:1 to 4:1, and stirred and mixed at room temperature under a stirrer 20 to obtain a mixed solution;

[0142] A first inert atmosphere (such as nitrogen) is introduced into the mixed solution through the gas inlet 10, and then the mixed solution is sent into the tube furnace 40 through the gas outlet 30;

[0143] The hard carbon material and the metal hydride (such as calcium hydride) are mixed in a mass ratio of 0.5 to 2:1 and then sent into a tubular furnace 40 using a corundum boat, wherein the ratio of the mixed solution of the phosphorus source and the nitrogen source to the hard carbon material is (10 to 20) mL: 1 g. Then, the mixture is calcined at 600 to 900 ° C for 2 h to 5 h at a heating rate of 2 ° C / min to 5 ° C / min. After the calcination, the obtained product is naturally cooled to room temperature, and then pickled with a reagent (such as a hydrochloric acid solution with a concentration of 1 mol / L to 3 mol / L) for 1 h to 4 h to obtain a negative electrode material.

[0144] A third aspect of the present application provides a battery, comprising a negative electrode sheet, wherein the negative electrode sheet comprises the above-mentioned negative electrode material, or the negative electrode material prepared by the above-mentioned method for preparing the negative electrode material.

[0145] Since the negative electrode sheet includes the negative electrode material provided in the embodiments of the present application, it can exhibit good electrochemical properties, such as a higher initial charge and discharge capacity, excellent initial coulombic efficiency, and the like.

[0146] In some embodiments, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. For example, the negative electrode current collector has two opposing surfaces in its thickness direction, and the negative electrode active material layer is disposed on the two opposing surfaces of the negative electrode current collector. It is understood that the negative electrode active material layer can also be laminated on either surface of the negative electrode current collector.

[0147] The present application does not specifically limit the material of the negative electrode current collector, as long as it can achieve the purpose of this application, and can be selected according to actual needs. As an example, the negative electrode current collector can be made of a metal material such as aluminum, copper, nickel, stainless steel, nickel-plated steel, or a foil with a surface coating layer made of a related foil.

[0148] In some embodiments, the negative electrode active material layer includes the hard carbon negative electrode material described above as provided herein. Furthermore, the negative electrode active material layer may optionally include a conductive agent. Furthermore, the negative electrode active material layer may optionally include a binder. The embodiments of this application do not particularly limit the types of conductive agents and binders in the negative electrode active material layer, as long as they can achieve the objectives of this application.

[0149] In some embodiments, the battery further comprises a positive electrode sheet, an electrolyte, and a separator. That is, the battery comprises a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator.

[0150] In the embodiments of the present application, in the battery, there is no limitation on the specific materials and structures of the positive electrode sheet, separator, and electrolyte. Components and structures that are well known in the art and can be used for secondary batteries can be selected as long as the purpose of the present application can be achieved.

[0151] The battery mentioned in the embodiments of this application may be a single physical module including one or more battery cells to provide higher voltage and capacity. For example, the battery mentioned in this application may be a battery cell, or the battery may include a battery module (or battery module) or a battery pack.

[0152] In some embodiments, a battery includes an electrode assembly and an electrolyte. The electrode assembly includes a positive electrode sheet, a negative electrode sheet, and a separator located between the positive electrode sheet and the negative electrode sheet. The electrode assembly can be a wound structure or a stacked structure, which is not limited in the present embodiment.

[0153] In some embodiments, the battery may be a battery module; when there are multiple battery cells, the multiple battery cells are arranged and fixed to form a battery module.

[0154] In some embodiments, the battery may be a battery pack, which may include a case and battery cells, wherein the battery cells or battery modules are housed in the case.

[0155] It should be understood that multiple battery cells can be assembled into a battery module or a battery pack. The number of battery cells contained in a battery module or a battery pack can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module or battery pack.

[0156] In some embodiments, the battery may be an energy storage device, including an energy storage container, an energy storage cabinet, and the like.

[0157] In the embodiments of the present application, the preparation method of the battery cell or battery is well known. In some embodiments, the positive electrode sheet, the separator, the negative electrode sheet and the electrolyte can be assembled to form a battery cell. As an example, the positive electrode sheet, the separator and the negative electrode sheet can be formed into an electrode assembly through a winding process and / or a lamination process, and the electrode assembly is placed in an outer package, dried and injected with electrolyte, and then vacuum packaged, allowed to stand, formed, and shaped to obtain a battery cell. Optionally, multiple battery cells can be further connected in series, in parallel or in a mixed connection to form a battery module. Optionally, multiple battery modules can also be connected in series, in parallel or in a mixed connection to form a battery pack. Optionally, in some embodiments, multiple battery cells can also directly form a battery pack.

[0158] The present invention is further described in detail below by way of examples. The raw materials used in the examples can all be obtained through commercial sources.

[0159] Example 1

[0160] The method for preparing the negative electrode material comprises the following steps:

[0161] (1) 100 g of phenolic resin microspheres with a particle size of 5 μm were placed in a porcelain boat and placed in a tube furnace. The mixture was heated at a rate of 2°C / min to a calcination temperature of 1300°C in a hydrogen-argon mixed atmosphere with a flow rate of 50 mL / min and a hydrogen content of 5%, and the mixture was kept at this temperature for 2 h to obtain a hard carbon material.

[0162] (2) 179 g of phenylphosphonium dichloride and 67 g of pyrrole were placed in a three-necked flask and stirred at room temperature for 1 h to obtain 205 mL of a mixed liquid; nitrogen was introduced into the bottom of the mixed solution through the air inlet, and the mixed solution was sent into a tubular furnace through a nitrogen flow at a nitrogen flow rate of 100 mL / min; 26 g of magnesium hydride and 50 g of hard carbon material were mixed evenly and placed in a corundum boat, and then the corundum boat was placed in the middle of the tubular furnace. Under a nitrogen atmosphere, the temperature was increased at a rate of 2 ° C / min to a calcination temperature of 800 ° C, and the temperature was kept for 2 h. After the temperature was reached, the material was naturally cooled, and then the material was pickled in 500 mL of 1 mol / L hydrochloric acid solution for 2 h to obtain a negative electrode material.

[0163] In the negative electrode material prepared in Example 1, the doping amount of the N element in the overall negative electrode material is 0.61wt%, and the doping amount of the P element is 0.92wt%, based on the total mass of the negative electrode material; in the coating layer of the negative electrode material, the doping amount of the N element is 6.7at%, and the doping amount of the P element is 14.2at%, based on the atomic content of the coating layer.

[0164] Example 2

[0165] The method for preparing the negative electrode material comprises the following steps:

[0166] (1) 100 g of phenolic resin microspheres with a particle size of 5 μm were placed in a porcelain boat and placed in a tube furnace. The mixture was heated at a rate of 2°C / min to a calcination temperature of 1300°C in a hydrogen-argon mixed atmosphere with a flow rate of 50 mL / min and a hydrogen content of 5%, and the mixture was kept at this temperature for 2 h to obtain a hard carbon material.

[0167] (2) 205 g of phosphorus trichloride and 67 g of pyrrole were placed in a three-necked flask and stirred at room temperature for 1 h to obtain 203 mL of a mixed liquid; nitrogen was introduced into the bottom of the mixed solution through the air inlet, and the mixed solution was sent into a tubular furnace through a nitrogen flow at a nitrogen flow rate of 100 mL / min; 42 g of calcium hydride and 50 g of hard carbon material were mixed evenly and placed in a corundum boat, which was then placed in the middle of the tubular furnace. Under a nitrogen atmosphere, the temperature was increased at a rate of 2 °C / min to a calcination temperature of 800 °C, and the temperature was kept for 2 h. After the temperature was reached, the mixture was naturally cooled, and then the material was pickled in 500 mL of 1 mol / L hydrochloric acid solution for 2 h to obtain a negative electrode material.

[0168] Example 3

[0169] The method for preparing the negative electrode material comprises the following steps:

[0170] (1) 100 g of phenolic resin microspheres with a particle size of 5 μm were placed in a porcelain boat and placed in a tube furnace. The mixture was heated at a rate of 2°C / min to a calcination temperature of 1400°C in a hydrogen-argon mixed atmosphere with a flow rate of 50 mL / min and a hydrogen content of 5%, and the mixture was kept at this temperature for 2 h to obtain a hard carbon material.

[0171] (2) 205 g of phosphorus trichloride and 67 g of pyrrole were placed in a three-necked flask and stirred at room temperature for 1 h to obtain 203 mL of a mixed liquid; argon gas was introduced into the bottom of the mixed solution through the air inlet, and the mixed solution was sent into a tubular furnace by a nitrogen flow with a nitrogen flow rate of 100 mL / min; 42 g of calcium hydride and 50 g of hard carbon material were mixed evenly and placed in a corundum boat, which was then placed in the middle of the tubular furnace. Under a nitrogen atmosphere, the temperature was increased at a rate of 2 °C / min to a calcination temperature of 700 °C, and the temperature was kept for 2 h. After the temperature was reached, the material was naturally cooled, and then the material was pickled in 500 mL of 1 mol / L hydrochloric acid solution for 2 h to obtain a negative electrode material.

[0172] Example 4

[0173] The method for preparing the negative electrode material comprises the following steps:

[0174] (1) 100 g of phenolic resin microspheres with a particle size of 5 μm were placed in a porcelain boat and placed in a tube furnace. The mixture was heated at a rate of 2°C / min to a calcination temperature of 1400°C in a hydrogen-argon mixed atmosphere with a flow rate of 50 mL / min and a hydrogen content of 5%, and the mixture was kept at this temperature for 2 h to obtain a hard carbon material.

[0175] (2) 205 g of phosphorus trichloride and 129 g of quinoline were placed in a three-necked flask and stirred at room temperature for 1 h to obtain 251 mL of a mixed liquid; nitrogen was introduced into the bottom of the mixed solution through the air inlet, and the mixed solution was sent into a tubular furnace through a nitrogen flow at a nitrogen flow rate of 100 mL / min; 11 g of beryllium hydride and 50 g of hard carbon material were mixed evenly and placed in a corundum boat, which was then placed in the middle of the tubular furnace. Under a nitrogen atmosphere, the temperature was increased at a rate of 2 °C / min to a calcination temperature of 800 °C, and the temperature was kept for 2 h. After the temperature was reached, the mixture was naturally cooled, and then the material was pickled in 500 mL of 1 mol / L hydrochloric acid solution for 2 h to obtain a negative electrode material.

[0176] Example 5

[0177] The preparation method of the negative electrode material is similar to that of Example 1, except that:

[0178] In step (1), the heating rate is 5°C / min, the calcination temperature is 1400°C, and the holding time is 5h.

[0179] Example 6

[0180] The preparation method of the negative electrode material is similar to that of Example 1, except that:

[0181] In step (2), the heating rate is 5°C / min, the calcination temperature is 1200°C, and the holding time is 2h.

[0182] Example 7

[0183] The preparation method of the negative electrode material is similar to that of Example 1, except that:

[0184] In step (2), the mass of phenylphosphonium dichloride is 208.5 g, the mass of pyrrole is 100.5 g, and the volume of the solution is 308 ml.

[0185] Comparative Example 1

[0186] The method for preparing the negative electrode material comprises the following steps:

[0187] 100 g of phenolic resin microspheres with a particle size of 5 μm were placed in a porcelain boat and placed in a tubular furnace. In a nitrogen atmosphere with a flow rate of 50 mL / min, the temperature was increased at a heating rate of 2°C / min to a calcination temperature of 1400°C and kept warm for 2 hours to obtain phenolic resin-based carbon spheres, which are the negative electrode materials.

[0188] Comparative Example 2

[0189] The method for preparing the negative electrode material comprises the following steps:

[0190] (1) 100 g of phenolic resin microspheres with a particle size of 5 μm were placed in a porcelain boat and placed in a tube furnace. The mixture was heated at a rate of 2°C / min to a calcination temperature of 1300°C in a hydrogen-argon mixed atmosphere with a flow rate of 50 mL / min and a hydrogen content of 5%, and the mixture was kept at this temperature for 2 h to obtain a hard carbon material.

[0191] (2) 50 g of hard carbon material, 20 g of sodium dihydrogen phosphate and 10 g of urea were placed in a porcelain boat in the middle of a tube furnace. Under a nitrogen atmosphere, the temperature was raised at a rate of 2 °C / min to a calcination temperature of 700 °C, and the temperature was kept for 2 h. After the temperature was raised, the mixture was naturally cooled to obtain phenolic resin-based carbon spheres - OPN, which were the negative electrode materials.

[0192] In the negative electrode material prepared in Comparative Example 2, based on the atomic content in the negative electrode material, the doping amount of the N element is 0.51 at %, the doping amount of the P element is 0.42 at %, and the doping amount of the O element is 4.6 at %.

[0193] Comparative Example 3

[0194] The method for preparing the negative electrode material comprises the following steps:

[0195] (1) 100 g of phenolic resin microspheres with a particle size of 5 μm were placed in a porcelain boat and placed in a tube furnace. The temperature was raised to 1400°C at a heating rate of 2°C / min in a nitrogen atmosphere with a flow rate of 50 mL / min and kept at this temperature for 2 h to obtain a hard carbon material.

[0196] (2) 50 g of hard carbon material, 20 g of sodium dihydrogen phosphate, and 10 g of urea were placed in a porcelain boat in the middle of a tube furnace. Under a nitrogen atmosphere, the temperature was increased at a rate of 2 °C / min to a calcination temperature of 700 °C, and the temperature was kept at this temperature for 2 h. After the temperature was reached, the mixture was naturally cooled to obtain an intermediate material.

[0197] (3) 30 g of the intermediate material and 5 g of asphalt were mixed and stirred for 1 h. The mixture was then placed in a porcelain boat in the middle of a tube furnace. Under a nitrogen atmosphere, the temperature was raised at a rate of 2 °C / min to a calcination temperature of 700 °C. The mixture was kept at this temperature for 2 h and then naturally cooled to obtain phenolic resin carbon balls@ONP-coated, which were the negative electrode materials.

[0198] Battery preparation and performance testing

[0199] 1. Preparation of sodium ion batteries

[0200] (1) The negative electrode materials, conductive agent carbon black, and binder carboxymethyl cellulose prepared in the above examples and comparative examples were mixed and added to N-methylpyrrolidone (NMP) solvent to obtain a slurry, where the mass ratio of the negative electrode material, conductive agent, and binder was 8:1:1; the obtained slurry was evenly applied on aluminum foil and dried in a vacuum drying oven at 100°C for 12 hours to obtain a negative electrode sheet;

[0201] (2) providing a metal sodium sheet as the positive electrode;

[0202] (3) providing a glass fiber diaphragm as an isolation membrane;

[0203] (4) providing an electrolyte, which is a 1 mol / L NaPF6 diethylene glycol dimethyl ether solution;

[0204] (5) Assemble the negative electrode sheet, the separator, and the positive electrode sheet in this order and soak them in electrolyte solution to form a CR2032 button battery in an argon glove box.

[0205] 2. Battery performance test

[0206] (1) First discharge capacity test: Using the Land battery test system, at 25°C, discharge the battery to 0V at a constant current density of 0.1C. The discharge capacity at this time is recorded as the first discharge capacity. The test results are shown in Table 1.

[0207] (2) First charge capacity test: Using the Land battery test system, the battery was charged to 2V at a current density of 0.1C at 25°C. The charge capacity at this time was recorded as the first charge capacity. The test results are shown in Table 1.

[0208] (3) First coulombic efficiency (first efficiency) test: at 25°C, discharge to 0V at a constant current density of 0.1C, and record the discharge capacity at this time as the first discharge capacity; charge to 2V at a constant current density of 0.1C, and record the charge capacity at this time as the first charge capacity; first coulombic efficiency = first charge capacity / first discharge capacity * 100%; the test results are shown in Table 1;

[0209] (4) Capacity retention (cycling performance) test: At 25°C, discharge the battery to 0V at a constant current of 1C, then charge it to 2V at a constant current of 1C. This constitutes a complete charge-discharge cycle, and record "1 cycle." Repeat this process until the number of cycles reaches 500. End the test and record the charge capacity at the 500th cycle. Therefore, the capacity retention at the 500th cycle = charge capacity at the 500th cycle / charge capacity at the 1st cycle * 100%. The test results are shown in Table 1.

[0210] Table 1

[0211]

[0212]

[0213] As shown in Table 1, the button-type half-cells of the high-performance sodium ion battery anode composite material prepared from the negative electrode material in the embodiment have a high initial charge and discharge capacity (over 420 mAh / g), and the first storage efficiency exceeds 90%, while the first discharge capacity of Comparative Example 1 is only 332.5 mAh / g, the first charge capacity is only 281.3 mAh / g, and the first coulomb efficiency is 84.6%. This shows that the carbon coating layer effectively reduces the specific surface area of ​​the material, reduces the defect concentration, and is conducive to improving the first-cycle coulomb efficiency of the material. At the same time, the diatomic doping expands the carbon layer spacing of the carbon material, provides more active sites, and improves the gram capacity. Therefore, the negative electrode material prepared by the present invention has high reversible capacity and low potential capacity.

[0214] The button-type half-cells of the new sodium-ion battery anode composite material made from the negative electrode material in the embodiment all have a high cycle retention rate (greater than 90%) and excellent cycle stability, while the capacity retention rate of Comparative Example 1 is only 80.1%. This shows that the high doping amount of N and P in the coating layer is achieved. The double atom doping can change the local bonding environment and electron distribution on the surface of the hard carbon negative electrode material. The material structure has a significantly expanded interlayer spacing, thereby improving the reactivity and electronic conductivity of the hard carbon negative electrode material. The presence of the coating layer further reduces the specific surface area of ​​the material, increases the number of closed pores in the material, and improves the cycle and rate performance of the material. In Comparative Example 2, element doping is carried out in an aerobic environment to increase the active adsorption sites, thereby achieving the purpose of increasing the capacity of the material, but the performance of the negative electrode material is lower than that of the embodiment; Comparative Example 3 performs a coating process on the basis of Comparative Example 2. The coating process effectively improves the initial efficiency of the material, but the effect is lower than that of the embodiment. This shows that it is impossible to achieve a large amount of doping elements in an aerobic environment, and thus it is impossible to significantly improve the performance of the material.

[0215] 3. Performance Test of Negative Electrode Materials The specific surface areas of the negative electrode materials in the examples and comparative examples were tested using a nitrogen adsorption and desorption method. The test results are shown in Table 2.

[0216] Table 2

[0217]

[0218]

[0219] As shown in Table 2, the negative electrode materials prepared in the examples all have relatively small specific surface areas (less than 2 m 2 / g), while the specific surface areas of the negative electrode materials prepared in the comparative examples are all greater than 6m 2 / g, even more than 10m 2 / g. Comparative Example 2, which was not coated, had the largest specific surface area, which was caused by the open pores generated during the calcination process. Comparative Example 3 could effectively reduce the specific surface area after being coated with asphalt, but the coating effect was not uniform, and some pores were still not coated. In Example 3, the carbon source was uniformly transported by airflow, achieving uniform coating of the material, resulting in a greater reduction in specific surface area.

[0220] The EDS surface scan of the hard carbon material prepared in Example 1 is as follows: Figure 2 shown.

[0221] The EDS line scan of the hard carbon material prepared in Example 1 is as follows: Figure 3 As shown. Figure 2 、 Figure 3 It can be seen that the hard carbon material contains only carbon element, indicating that the hard carbon material does not contain oxygen element.

[0222] The EDS surface scan of the negative electrode material prepared in Example 1 is as follows: Figure 4 As shown. Figure 4 It can be seen that the P and N elements in the negative electrode material are successfully doped and evenly distributed.

[0223] The TEM image of the negative electrode material prepared in Example 1 is as follows: Figure 5 As shown. Figure 5 It can be seen that the carbon material is successfully coated in the negative electrode material, and the coating layer thickness is 4.6 nm.

[0224] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A negative electrode material, characterized in that The negative electrode material includes a hard carbon material and a coating layer coated on at least a portion of the surface of the hard carbon material; The coating layer includes a carbon material, the carbon material includes a carbon material matrix and doping elements doped in the carbon material matrix, and the doping elements include P and N; The oxygen content in the negative electrode material is 0.

2. The negative electrode material according to claim 1, characterized in that The hard carbon material further includes doping elements, and the doping elements include P and N; Based on the mass of the negative electrode material, the doping amount of the P element in the negative electrode material is 0.5wt% to 1wt%; Based on the mass of the negative electrode material, the doping amount of the N element in the negative electrode material is 0.5wt% to 1wt%.

3. The negative electrode material according to claim 1 or 2, characterized in that The thickness of the coating layer is 2nm to 10nm; and / or, the particle size of the hard carbon material is 4 μm to 8 μm; and / or, the hard carbon material comprises a spherical hard carbon material; and / or, based on the atomic content of the coating layer, the doping amount of the P element in the carbon material is 10 at % to 15 at %; And / or, based on the atomic content of the coating layer, the doping amount of the N element in the carbon material is 5 at % to 10 at %.

4. A method for preparing a negative electrode material, characterized in that: The following steps are involved: calcining the hard carbon precursor in a reducing atmosphere to obtain a hard carbon material; Under a first inert atmosphere, mixing a mixed solution of a hard carbon material, a phosphorus source, and a nitrogen source with a metal hydride, and performing a second calcination to obtain a negative electrode material; Wherein, the negative electrode material comprises a hard carbon material and a coating layer coated on at least a portion of the surface of the hard carbon material; The coating layer includes a carbon material, the carbon material includes a carbon material matrix and doping elements doped in the carbon material matrix, and the doping elements include P and N; The oxygen content in the negative electrode material is 0.

5. The method for preparing the negative electrode material according to claim 4, wherein: The hard carbon precursor includes a resin and / or a biomass material; Preferably, the resin includes at least one of phenolic resin, epoxy resin, polyacrylonitrile and polyimide; Preferably, the particle size of the phenolic resin is 5 μm to 10 μm.

6. The method for preparing the negative electrode material according to claim 4, wherein: The reducing atmosphere comprises a mixed atmosphere of hydrogen and a second inert atmosphere; Preferably, the volume percentage of hydrogen in the mixed atmosphere is 3% to 5%; Preferably, the second inert atmosphere comprises at least one of nitrogen, argon, helium and neon; Preferably, the flow rate of the reducing atmosphere is 50 mL / min to 100 mL / min.

7. The method for preparing the negative electrode material according to claim 4, wherein: The temperature of the first calcination is 1000° C. to 1400° C., the time of the first calcination is 2 h to 5 h, and the heating rate of the first calcination is 2° C. / min to 5° C. / min.

8. The method for preparing the negative electrode material according to claim 4, wherein: The first inert atmosphere comprises at least one of nitrogen, argon, helium and neon; Preferably, the flow rate of the first inert atmosphere is 50 mL / min to 150 mL / min; and / or, the molar ratio of the phosphorus source to the nitrogen source in the mixed solution of the phosphorus source and the nitrogen source is 1:1 to 4:1; and / or, the ratio of the mixed solution of the phosphorus source and the nitrogen source to the hard carbon material is (2-20) mL: 1 g; and / or, the mass ratio of the metal hydride to the hard carbon material is 0.5 to 2:1; Preferably, the phosphorus source includes at least one of phosphorus trichloride, phenylphosphonium dichloride and phosphorus tribromide; Preferably, the nitrogen source comprises at least one of pyrrole, pyridine and quinoline; Preferably, the metal hydride includes at least one of calcium hydride, magnesium hydride, potassium hydride and beryllium hydride.

9. The method for preparing a negative electrode material according to any one of claims 4 to 8, characterized in that: The temperature of the second calcination is 600° C. to 900° C., the time of the second calcination is 2 h to 5 h, and the heating rate of the second calcination is 2° C. / min to 5° C. / min; And / or, after the second calcination is completed, the method further comprises: cooling and acid washing the product obtained after the second calcination to obtain a negative electrode material; Preferably, the pickling reagent includes at least one of hydrochloric acid solution, sulfuric acid solution and oxalic acid solution; Preferably, the concentration of the reagent is 1 mol / L to 3 mol / L; Preferably, the pickling time is 1 hour to 4 hours.

10. A battery comprising a negative electrode sheet, characterized in that: The negative electrode sheet comprises the negative electrode material according to any one of claims 1 to 3, or the negative electrode material prepared by the method for preparing the negative electrode material according to any one of claims 4 to 9.

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