Metal nitride and metal oxide co-coated anode material and preparation method thereof, anode and battery

Abstract

The application relates to the field of lithium ion batteries, and discloses a negative electrode material coated with a metal nitride and a metal oxide, a preparation method of the negative electrode material, a negative electrode and a battery, wherein the negative electrode material comprises graphite and a coating layer, the coating layer is coated on the surface of the graphite, and the coating layer comprises a mixed layer of a metal nitride and a metal oxide. The negative electrode material provided by the application exhibits small polarization, high specific capacity and excellent cycle stability under a large-current charging condition.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and in particular to a negative electrode material co-coated with a metal nitride and a metal oxide, a preparation method thereof, a negative electrode and a battery. Background Art

[0002] As a representative of green travel, electric vehicles are experiencing a growing industry scale. However, issues such as short driving range and slow charging speeds are hindering the further development of the electric vehicle industry. The fast-charging capabilities of electric vehicles are closely related to the physical and chemical properties of the positive and negative electrode materials in lithium-ion batteries. Graphite negative electrode materials have a lithium storage capacity of approximately 360 mAh / g, but they experience severe polarization under fast charging conditions, resulting in a lower utilization of their lithium storage capacity. Furthermore, under high-current charging conditions, lithium dendrites are easily formed on the surface of the graphite electrode, causing battery short circuits and potentially triggering safety issues such as fires and explosions.

[0003] The transport process of lithium ions embedded in graphite electrodes mainly includes the following four steps: transport of solvated lithium ions in the electrolyte, desolvation of solvated lithium ions on the electrode surface, transport of lithium ions in the solid electrolyte interface film (SEI film) on the graphite surface, and transport of lithium ions in the graphite bulk phase. Among them, lithium ion desolvation and transport in the SEI film are considered to be the rate-limiting steps. Based on this, the fast charging capability of graphite negative electrode materials can be improved through structural design, such as designing expanded layer, porous or nano-sized graphite negative electrode materials, but the above methods are at the expense of the energy density of graphite negative electrode materials.

[0004] How to maintain high capacity under high current charging conditions and inhibit the formation of lithium dendrites is of great research significance. Summary of the Invention

[0005] The present invention aims to overcome the problems of capacity loss and poor cycling stability in prior art negative electrode materials under high-current charging conditions. The present invention provides a negative electrode material, a preparation method thereof, a negative electrode, and a battery. The negative electrode material provided by the present invention exhibits minimal polarization, high specific capacity, and excellent cycling stability under high-current charging conditions.

[0006] To achieve the above objectives, the present invention provides a negative electrode material in a first aspect, comprising graphite and a coating layer, wherein the coating layer is coated on the surface of the graphite, and the coating layer comprises a mixture layer of metal nitride and metal oxide.

[0007] A second aspect of the present invention provides a method for preparing a negative electrode material, wherein the preparation method comprises: performing a first reaction on dopamine hydrochloride and graphite in a first solvent to obtain a material I having a surface coated with a polydopamine layer, performing a second reaction on the material I and a metal salt in a second solvent to obtain a material II having a surface coated with a metal salt and a polydopamine layer, performing a first heat treatment on the material II in an ammonia atmosphere to obtain a material III having a surface coated with a metal nitride, and performing a second heat treatment on the material III in an oxygen atmosphere to obtain a negative electrode material having a surface coated with a mixture layer including a metal nitride and a metal oxide;

[0008] The temperature of the first heat treatment is 500-1000° C., the temperature of the second heat treatment is 100-500° C., the time is 0.1-1 h, and the volume proportion of ammonia in the ammonia atmosphere is greater than 50%.

[0009] A third aspect of the present invention provides a negative electrode for a lithium-ion battery, wherein the negative electrode comprises the negative electrode material described in the first aspect of the present invention or the negative electrode material prepared by the preparation method described in the second aspect of the present invention.

[0010] A fourth aspect of the present invention provides a battery, wherein the battery comprises the negative electrode described in the third aspect of the present invention.

[0011] Through the above technical solution, the present invention prepares a negative electrode material co-coated with metal nitride and metal oxide, and the metal nitride and metal oxide are in situ converted into an inorganic-rich SEI layer containing metal elements and Li3N and Li2O during the lithiation process, thereby improving the fast charging capability and suppressing the formation of lithium dendrites under high current charging conditions.

[0012] The negative electrode material prepared by the present invention still has high capacity and excellent cycle stability under high current charge and discharge conditions, with a specific capacity of more than 220 mAh / g at a current density of 4C and a capacity retention rate of more than 90% after 300 cycles. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is a SEM image of the negative electrode material prepared in Example 2;

[0014] Figure 2 is the EDS spectrum of the negative electrode material prepared in Example 2;

[0015] Figure 3 is the XRD pattern of graphite and the negative electrode material prepared in Example 5;

[0016] Figure 4 dQ / dV curves corresponding to the first cycle of charge and discharge process of graphite and the negative electrode material prepared in Example 7;

[0017] Figure 5 This is a capacity cycle diagram of the negative electrode material prepared in Example 1 at a current density of 4C. DETAILED DESCRIPTION

[0018] 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 and 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.

[0019] The terms "first" and "second" are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly specify the number of the technical features indicated. Therefore, features specified as "first" or "second" may explicitly or implicitly include one or more of such features. In the description of this application, unless otherwise specified, "plurality" means two or more.

[0020] In the description of this application, the interface refers to the solid electrolyte interface (SEI) layer, that is, a thin and dense passivation film formed by the reduction and decomposition of solvent molecules and lithium salts in the electrolyte on the surface of the negative electrode material during the charging and discharging process of the lithium-ion battery.

[0021] A first aspect of the present invention provides a negative electrode material, comprising graphite and a coating layer, wherein the coating layer is coated on the surface of the graphite, and the coating layer comprises a mixture layer of metal nitride and metal oxide.

[0022] During the lithiation process, the interface between the metal nitride and metal oxide in the coating layer is in situ transformed into an inorganic-rich SEI layer containing metal elements as well as Li3N and Li2O. The lithium ion conductivity of Li3N is higher than that of traditional SEI components, which can accelerate the transmission speed of lithium ions, increase the desolvation rate, achieve rapid transmission of lithium ions at the interface, and improve fast charging capabilities. The metal element layer has higher electronic conductivity, which can reduce interfacial impedance and weaken polarization, thereby increasing the lithium nucleation overpotential and inhibiting the formation of lithium dendrites under high current conditions. Constructing a mixture layer of metal nitride and metal oxide can avoid oxidative decomposition of nitride, which is beneficial to improving the stability of the negative electrode material.

[0023] In some embodiments, preferably, the metal nitride comprises Fe4N, and the metal oxide comprises Fe2O3. In the coating layer, the metal nitride mainly exists in the form of Fe4N, in addition to which there are small amounts of Fe3N, Fe2N, and Fe6N2. The metal oxide mainly exists in the form of Fe2O3, in addition to small amounts of FeO and Fe3O4.

[0024] In some embodiments, preferably, the coating layer has a thickness of 10-100 nm. The coating layer thickness can be any value between any two of 10 nm, 30 nm, 50 nm, 70 nm, and 100 nm. The coating layer thickness within this range is beneficial for increasing the desolvation rate of the negative electrode material and the transmission rate in the SEI film, while effectively suppressing the formation of lithium dendrites under high current conditions.

[0025] A second aspect of the present invention provides a method for preparing a negative electrode material, wherein the preparation method comprises: performing a first reaction on dopamine hydrochloride and graphite in a first solvent to obtain a material I having a surface coated with a polydopamine layer, performing a second reaction on the material I and a metal salt in a second solvent to obtain a material II having a surface coated with a metal salt and a polydopamine layer, performing a first heat treatment on the material II in an ammonia atmosphere to obtain a material III having a surface coated with a metal nitride, and performing a second heat treatment on the material III in an oxygen atmosphere to obtain a negative electrode material having a surface coated with a mixture layer including a metal nitride and a metal oxide;

[0026] The temperature of the first heat treatment is 500-1000° C., the temperature of the second heat treatment is 100-500° C., the time is 0.1-1 h, and the volume proportion of ammonia in the ammonia atmosphere is greater than 50%.

[0027] Material I includes an inner layer of graphite and a polydopamine layer coated on the surface of the graphite, material II includes an inner layer of graphite and a polydopamine layer and a metal salt layer coated on the surface of the graphite, material III includes an inner layer of graphite and a metal nitride layer coated on the surface of the graphite, and the negative electrode material includes an inner layer of graphite and a mixture layer of metal nitride and metal oxide coated on the surface of the graphite.

[0028] In some embodiments, the metal salt preferably includes one or more of ferric oxalate, ferric sulfate, ferric nitrate, and ferric chloride. The metal salt containing the iron element is converted into iron nitride and iron oxide after a two-step heat treatment. The iron nitride and iron oxide at the interface are converted into elemental iron, lithium nitride, and lithium oxide during the lithiation process. This inorganic-rich fast ion conductive layer helps accelerate interfacial ion transport, thereby improving the rate performance of the negative electrode material.

[0029] In some embodiments, the volume percentage of ammonia in the ammonia atmosphere is preferably greater than 90%. When the volume percentage of ammonia is greater than 50%, it is favorable for the nitridation reaction to form metal nitrides. The volume percentage of ammonia can be greater than 50%, 60%, 70%, 80%, 90%, 95%, 99%, etc.

[0030] In some embodiments, preferably, the first step of heat treatment is performed for 1-4 hours at a heating rate of 1-5°C / min.

[0031] In some embodiments, preferably, the second step heat treatment is performed for 0.1-0.6 h, and the heating rate is 1-5° C. / min.

[0032] During the first heat treatment step, the metal salt layer coating the graphite surface transforms into a metal nitride layer, while polydopamine undergoes thermal decomposition and transforms into an amorphous carbon layer. During the second heat treatment step, a portion of the metal nitride transforms into a metal oxide, forming a mixture layer of metal nitride and metal oxide. The first heat treatment time should not be too short, otherwise the metal salt cannot be completely reduced, resulting in incomplete formation of the nitride layer. The second heat treatment time should not be too long to prevent the complete conversion of the metal nitride to metal oxide. A heating rate within the range of 1-5°C / min for both heat treatments is conducive to the complete reaction.

[0033] The temperature of the first step heat treatment can be any value between any two of 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, 800°C, 850°C, 900°C, and 1000°C; the time can be any value between any two of 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, and 4h; and the heating rate can be any value between any two of 1°C / min, 2°C / min, 3°C / min, 4°C / min, and 5°C / min. The temperature of the second step heat treatment can be any value between any two numbers of 100℃, 120℃, 150℃, 180℃, 200℃, 300℃, 400℃, and 500℃; the time can be any value between any two numbers of 0.1h, 0.2h, 0.3h, 0.4h, 0.6h, 0.8h, and 1h; the heating rate can be any value between any two numbers of 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, and 5℃ / min.

[0034] In some embodiments, preferably, the mass ratio of dopamine hydrochloride to graphite is 0.01-0.3. By constructing a polydopamine layer on the graphite surface, the adsorption capacity between graphite and metal salt can be significantly improved, which helps to construct a uniformly coated metal salt layer. The mass ratio of dopamine hydrochloride to graphite can be any value between any two numbers of 0.01, 0.05, 0.1, 0.15, 0.2, 0.25, and 0.3. The mass ratio of dopamine hydrochloride to graphite is conducive to forming a uniform polydopamine layer on the graphite surface within this range, thereby forming a uniform metal salt layer.

[0035] In some embodiments, the mass ratio of the metal salt to the graphite is preferably 0.01-0.5. The mass ratio of the metal salt to the graphite can be any value between any two of 0.01, 0.1, 0.2, 0.3, 0.4, and 0.5. The mass ratio of the metal salt to the graphite within this range is conducive to forming a layer of a mixture of metal nitrides and metal oxides uniformly coated on the surface of the graphite. When the mass ratio of the metal salt to the graphite is too high, the coating layer formed is too thick, which is not conducive to the transmission of lithium ions and instead leads to a decrease in the rate performance of the negative electrode material.

[0036] In some embodiments, preferably, the regulator for regulating the pH value of the first reaction is ammonia water and / or Tris solution.

[0037] In some embodiments, preferably, the pH value of the first reaction is 8 to 9. A pH value of 8 to 9, especially around 8.5, is conducive to the polymerization of dopamine molecules to form polydopamine.

[0038] The pH value of the first reaction can be any value between any two of 8, 8.2, 8.4, 8.5, 8.7, and 9.

[0039] In some embodiments, preferably, the first reaction process includes: first stirring the dopamine hydrochloride, graphite and the first solvent.

[0040] In some embodiments, preferably, the first stirring time is 12-36 hours.

[0041] The dopamine hydrochloride and graphite are uniformly dispersed by stirring to form a uniformly coated polydopamine layer. The first stirring time can be any value between any two of 12h, 16h, 20h, 24h, 28h, 32h, and 36h.

[0042] In some embodiments, preferably, the second reaction process includes: mixing the material I, the metal salt and the second solvent and heating until the second solvent is evaporated to dryness, thereby forming the material II having the metal salt and polydopamine layer coated on the surface.

[0043] In some embodiments, preferably, the mixing process comprises: subjecting the material I, the metal salt, and the second solvent to ultrasonic treatment, and then performing the second reaction under a second stirring condition.

[0044] Material I and the metal salt are uniformly dispersed in the second solvent by ultrasound and second stirring, and the second solvent is gradually evaporated by heating. During the evaporation process, the metal salt gradually coats the surface of Material II to form a uniform metal salt layer.

[0045] The first solvent may be deionized water, and the second solvent may be deionized water.

[0046] A third aspect of the present invention provides a negative electrode for a lithium-ion battery, wherein the negative electrode comprises the negative electrode material described in the first aspect of the present invention or the negative electrode material prepared by the preparation method described in the second aspect of the present invention.

[0047] A fourth aspect of the present invention provides a battery, wherein the battery comprises the negative electrode described in the third aspect of the present invention.

[0048] According to a particularly preferred embodiment of the present invention, dopamine hydrochloride and graphite are first stirred in a first solvent to obtain material I having a surface-coated polydopamine layer, wherein the mass ratio of dopamine hydrochloride to graphite is 0.2-0.3, the pH of the first stirring is 8.4-8.6, the pH adjuster is ammonia water, and the first stirring time is 20-25h;

[0049] Ultrasonic treatment is performed on the material I, the metal salt, and the second solvent, and then heated under a second stirring condition until the second solvent is evaporated to dryness, to obtain the material II having a surface coated with a metal salt and a polydopamine layer, wherein the metal salt is ferric oxalate, and the mass ratio of the metal salt to the graphite is 0.18-0.22;

[0050] The material II is subjected to a first heat treatment in an ammonia atmosphere to obtain a material III whose surface is coated with a metal nitride, and the material III is subjected to a second heat treatment in an oxygen atmosphere to obtain a negative electrode material whose surface is coated with a mixture layer including a metal nitride and a metal oxide, wherein the volume proportion of ammonia in the ammonia atmosphere is greater than 95%, the temperature of the first heat treatment is 550-650°C, the time is 2-4h, and the heating rate is 3-5°C / min, and the temperature of the second heat treatment is 120-180°C, the time is 0.3-0.6h, and the heating rate is 3-5°C / min.

[0051] The negative electrode material prepared by this scheme maintains a high specific capacity of not less than 300 mAh / g at a current density of 4C, and the capacity retention rate after 300 cycles at a current density of 4C is not less than 98%, preferably 100%, and the capacity retention rate after 500 cycles exceeds 90%.

[0052] The present invention is described in detail below through examples. In the following examples, scanning electron microscopy (SEM) images of the negative electrode materials were obtained using a scanning electron microscope, EDS spectra were obtained using a scanning electron microscope, and X-ray diffraction (XRD) patterns were obtained using an X-ray diffractometer. The dQ / dV curve corresponding to the first cycle of charge and discharge and the capacity cycling diagram at a current density of 4C were obtained by performing charge and discharge tests on assembled batteries using a battery tester.

[0053] Example 1

[0054] 0.25 g of dopamine hydrochloride and 1 g of natural graphite were dissolved in 100 mL of water and uniformly dispersed by the first stirring at a speed of 400 rpm. Ammonia water was added to adjust the pH value to 8.5. After stirring for 24 h, the mixture was washed with deionized water and ethanol alternately by filtration until the pH was neutral. The powder was collected and vacuum dried at 60 ° C for 12 h to obtain material I.

[0055] All the above materials I and 0.2 g of ferric oxalate were dissolved in 20 mL of deionized water, ultrasonicated for 30 min, heated under a second stirring state until the deionized water was evaporated, the second stirring speed was 400 rpm, the heating temperature was 80°C, and then dried in an 80°C oven for 4 h to obtain material II.

[0056] Material II was subjected to a two-step heat treatment. The first step of heat treatment was carried out in an ammonia atmosphere, in which the volume proportion of ammonia was 99%. The temperature was raised to 800°C at a heating rate of 5°C / min and kept warm for 3 hours, and then cooled. Then, the second step of heat treatment was carried out in an oxygen atmosphere, in which the temperature was raised to 150°C at a heating rate of 5°C / min and kept warm for 0.5 hours, and cooled to obtain a negative electrode material co-coated with metal nitride and metal oxide, in which the metal nitride was mainly Fe4N and the metal oxide was mainly Fe2O3.

[0057] Example 2

[0058] The method of Example 1 was followed, except that: the amount of dopamine hydrochloride added was 0.15 g, that is, the mass ratio of dopamine hydrochloride to graphite was 0.15; ferric nitrate was used instead of ferric oxalate, and the amount of ferric nitrate added was 0.5 g, that is, the mass ratio of metal salt to graphite was 0.5; and the temperature of the first heat treatment was 800°C.

[0059] The negative electrode materials of Examples 1-16 and Comparative Examples 1-6 have similar SEM images and EDS spectra. Example 2 is a representative example, and its SEM image is as follows: Figure 1 As shown in the figure, the surface of the negative electrode material is smooth, without obvious particle aggregation, and the particle size is about 10μm. Figure 2 As shown, it shows that the surface of the negative electrode material contains Fe, C, O, and N elements and they are evenly distributed.

[0060] Example 3

[0061] The method of Example 2 was followed, except that the amount of dopamine hydrochloride added was 0.3 g, that is, the mass ratio of dopamine hydrochloride to graphite was 0.3.

[0062] Example 4

[0063] The method of Example 2 was followed, except that the amount of dopamine hydrochloride added was 0.05 g, that is, the mass ratio of dopamine hydrochloride to graphite was 0.05.

[0064] Example 5

[0065] The method of Example 2 was followed, except that the amount of iron nitrate added was 0.2 g, i.e. the mass ratio of metal salt to graphite was 0.2.

[0066] The negative electrode materials of Examples 1-16 had similar XRD patterns, of which Example 5 is representative. The XRD patterns of graphite and the negative electrode material prepared in Example 5 are shown in Figure 1. In addition to the diffraction peaks of the graphite itself, there were diffraction peaks of iron oxide (Fe2O3) and iron nitride (Fe4N), proving that a mixed coating layer of iron oxide and iron nitride was formed on the surface of the graphite, while the graphite itself did not have such diffraction peaks. Figure 3

[0067] Example 6

[0068] The method of Example 2 was followed, except that the amount of iron nitrate added was 0.05 g, i.e. the mass ratio of metal salt to graphite was 0.05.

[0069] Example 7

[0070] The method of Example 5 was followed, except that the temperature of the first heat treatment was 1000°C.

[0071] Example 8

[0072] The method of Example 5 was followed, except that the temperature of the first heat treatment was 600°C.

[0073] Example 9

[0074] The method of Example 5 was followed, except that the time of the first heat treatment was 1 h and the heating rate was 1°C / min.

[0075] Example 10

[0076] The method of Example 5 was followed, except that the temperature of the second heat treatment was 300°C and the heating rate was 3°C / min.

[0077] Example 11

[0078] The method of Example 5 was followed, except that the temperature of the second heat treatment was 500°C, the time was 0.1 h and the heating rate was 1°C / min.

[0079] Example 12

[0080] The method of Example 5 was followed, except that the time of the second heat treatment was 1 h.

[0081] Example 13 ​

[0082] The method of Example 2 was followed, except that Tris solution was used instead of ammonia water, and the first stirring time was 12 h.

[0083] Example 14

[0084] The method of Example 2 was followed, except that the first stirring time was 36 h.

[0085] Example 15

[0086] The method of Example 2 was followed, except that the amount of dopamine hydrochloride added was 0.5 g, that is, the mass ratio of dopamine hydrochloride to graphite was 0.5.

[0087] Example 16

[0088] The method of Example 2 was followed, except that the amount of ferric nitrate added was 0.7 g, that is, the mass ratio of metal salt to graphite was 0.7.

[0089] Comparative Example 1

[0090] The method of Example 5 was followed, except that the temperature of the first heat treatment was 200° C., the time was 4 h, and the heating rate was 3° C. / min.

[0091] Comparative Example 2

[0092] The method of Comparative Example 1 was followed, except that the temperature of the first heat treatment step was 80°C.

[0093] Comparative Example 3

[0094] The method of Example 5 was followed, except that the temperature of the second heat treatment was 50°C.

[0095] Comparative Example 4

[0096] The method of Example 11 was followed, except that the temperature of the second heat treatment was 650°C.

[0097] Comparative Example 5

[0098] The method of Example 5 was followed, except that the second heat treatment time was 2 h.

[0099] Comparative Example 6

[0100] The method of Example 1 was followed, except that the volume proportion of ammonia in the ammonia atmosphere was 30%.

[0101] Test Example 1

[0102] The negative electrode materials prepared in Examples 1-16 and Comparative Examples 1-6 were prepared into battery negative electrodes and further assembled into button batteries for electrochemical testing. The LB315 electrolyte used was 1 mol / L LiPF6 dissolved in a mixed solution of ethylene carbonate and diethyl carbonate, wherein the mass ratio of ethylene carbonate to diethyl carbonate was 7:3.

[0103] The first cycle charge and discharge test was conducted at a temperature of 25°C and a voltage window of 0.01-1.5V at a current density of 0.1C (1C = 300mAh / g), and the dQ / dV curves corresponding to the first cycle charge and discharge process of the graphite and negative electrode materials were obtained. Examples 1-16 have similar dQ / dV curves corresponding to the first cycle charge and discharge process. Taking Example 7 as an example, its dQ / dV curve is as follows: Figure 4 As shown in the figure. The positive region represents the charging process, with a reduction reaction occurring at the negative electrode, and the corresponding peak is the reduction peak. The negative region represents the discharging process, with an oxidation reaction occurring at the negative electrode, and the corresponding peak is the oxidation peak. As can be seen from the figure, compared with unmodified graphite, the negative electrode material co-coated with iron nitride and iron oxide has a higher reduction potential and a lower oxidation potential, indicating that the negative electrode material has weaker polarization, which is conducive to the performance of rate performance and inhibits the formation of lithium dendrites.

[0104] The capacity cycle diagram of the negative electrode material prepared in Example 1 at a current density of 4C is shown in FIG. Figure 5 As shown, the iron nitride and iron oxide co-coated anode material has a specific capacity of 300 mAh / g at a current density of 4C, while the graphite itself can only exert a specific capacity of about 100 mAh / g. In addition, the capacity retention rate of the anode material after 300 cycles at a current density of 4C is 100%.

[0105] The electrochemical performance of Examples 1-16 and Comparative Examples 1-6 was tested at a temperature of 25° C., a voltage window of 0.01-1.5 V, and a current density of 4 C. The results are shown in Table 1.

[0106] Table 1 Specific capacity and capacity retention of negative electrode materials at 4C current density

[0107] Example Specific capacity before cycle test / mAh / g Capacity retention rate after 300 cycles / % Example 1 300 100 Example 2 320 95 Example 3 280 98 Example 4 340 93 Example 5 300 95 Example 6 290 90 Example 7 250 95 Example 8 220 90 Example 9 240 95 Example 10 220 90 Example 11 250 95 Example 12 250 95 Example 13 300 95 Example 14 300 95 Example 15 280 95 Example 16 280 95 Comparative Example 1 150 90 Comparative Example 2 100 80 Comparative Example 3 100 80 Comparative Example 4 180 90 Comparative Example 5 150 80 Comparative Example 6 100 80

[0108] The results in Table 1 show that the negative electrode material prepared by the present invention has higher specific capacity and capacity retention than unmodified graphite under high current charging conditions. Compared with Example 2, the capacity retention of the negative electrode materials prepared in Examples 3, 4, and 15 shows a trend of first increasing and then decreasing, and the specific capacity of the negative electrode materials prepared in Examples 5, 6, and 16 all shows a trend of first increasing and then decreasing. This is because when the mass ratio of dopamine hydrochloride to graphite is too large, or when the mass ratio of metal salt to graphite is too large, a thick coating layer will be formed on the surface of the negative electrode material, hindering the effective transmission of lithium ions, resulting in a decrease in the specific capacity or cycle stability of the material.

[0109] Compared with Example 5, the specific capacity and capacity retention rate of the negative electrode materials prepared in Comparative Examples 1 and 2 are reduced. This is because the temperature of the first step heat treatment is too low, and the metal salt layer cannot be effectively converted into a metal nitride layer. Compared with Example 5, the specific capacity and capacity retention rate of the negative electrode material prepared in Comparative Example 3 are reduced. This is because the temperature of the second step heat treatment is too low, and the metal nitride layer cannot be effectively converted into a metal oxide layer. The specific capacity and capacity retention rate of the negative electrode material prepared in Comparative Example 4 are slightly lower than those in Example 11. This is because the temperature of the second step heat treatment in Comparative Example 4 is higher, resulting in more nitride layers being converted into oxide layers, reducing the interfacial lithium ion transmission capacity, reducing the interface stability, and reducing the specific capacity and capacity retention rate of the negative electrode material. The specific capacity of the negative electrode material prepared in Comparative Example 5 is significantly reduced compared with Example 5. This is because the second step heat treatment time is extended in Comparative Example 5, resulting in the proportion of the nitride layer on the surface of the negative electrode material converted into the oxide layer exceeding the acceptable range, reducing the interfacial ion transmission, and having an adverse effect on the performance of the material, indicating that the time of the second step heat treatment needs to be strictly controlled. The specific capacity and capacity retention rate of the negative electrode material prepared in Comparative Example 6 are much lower than those in Example 1. This is because in Comparative Example 6, when the first step of heat treatment is performed, the volume proportion of ammonia in the ammonia atmosphere is 30%, indicating that the volume proportion of ammonia in the ammonia atmosphere is greater than 50% in the present invention.

[0110] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as the contents disclosed by the present invention and fall within the scope of protection of the present invention.

Claims

1. A negative electrode material, characterized in that The invention comprises graphite and a coating layer, wherein the coating layer is coated on the surface of the graphite, and the coating layer comprises a mixture layer of metal nitride and metal oxide, wherein the metal nitride comprises Fe4N, and the metal oxide comprises Fe2O3; The preparation method of the negative electrode material comprises: performing a first reaction on dopamine hydrochloride and graphite in a first solvent to obtain a material I having a surface coated with a polydopamine layer, performing a second reaction on the material I and a metal salt in a second solvent to obtain a material II having a surface coated with a metal salt and a polydopamine layer, performing a first heat treatment on the material II in an ammonia atmosphere to obtain a material III having a surface coated with a metal nitride, and performing a second heat treatment on the material III in an oxygen atmosphere to obtain a negative electrode material having a surface coated with a mixture layer including a metal nitride and a metal oxide; The temperature of the first heat treatment is 500-1000° C., the temperature of the second heat treatment is 100-500° C., the time is 0.1-1 h, and the volume proportion of ammonia in the ammonia atmosphere is greater than 50%.

2. The negative electrode material according to claim 1, wherein The thickness of the coating layer is 10-100 nm.

3. A method for preparing the negative electrode material according to claim 1 or 2, characterized in that: The preparation method comprises: performing a first reaction on dopamine hydrochloride and graphite in a first solvent to obtain a material I having a surface coated with a polydopamine layer; performing a second reaction on the material I and a metal salt in a second solvent to obtain a material II having a surface coated with a metal salt and a polydopamine layer; performing a first heat treatment on the material II in an ammonia atmosphere to obtain a material III having a surface coated with a metal nitride; and performing a second heat treatment on the material III in an oxygen atmosphere to obtain a negative electrode material having a surface coated with a mixture layer including a metal nitride and a metal oxide; The temperature of the first heat treatment is 500-1000° C., the temperature of the second heat treatment is 100-500° C., the time is 0.1-1 h, and the volume proportion of ammonia in the ammonia atmosphere is greater than 50%.

4. The preparation method according to claim 3, wherein The metal salt includes one or more of ferric oxalate, ferric sulfate, ferric nitrate and ferric chloride; And / or, the volume proportion of ammonia in the ammonia atmosphere is greater than 90%; And / or, the first step heat treatment time is 1-4 h, and the heating rate is 1-5°C / min; And / or, the second step heat treatment time is 0.1-0.6 h, and the heating rate is 1-5°C / min; And / or, the mass ratio of dopamine hydrochloride to graphite is 0.01-0.3; And / or, the mass ratio of the metal salt to the graphite is 0.01-0.

5.

5. The preparation method according to claim 4, wherein The pH value of the first reaction is 8-9.

6. The preparation method according to claim 5, wherein The regulator for regulating the pH value of the first reaction is ammonia water and / or Tris solution.

7. The preparation method according to claim 5, wherein The process of the first reaction includes: first stirring the dopamine hydrochloride, graphite and a first solvent.

8. The preparation method according to claim 7, wherein The first stirring time is 12-36 h.

9. The preparation method according to claim 4, wherein The second reaction process includes: mixing the material I, the metal salt and the second solvent, and heating until the second solvent is evaporated to dryness, thereby forming the material II with the metal salt and polydopamine layer coated on the surface.

10. A negative electrode of a lithium ion battery, characterized in that: The negative electrode comprises the negative electrode material according to claim 1 or 2 or the negative electrode material prepared by the preparation method according to any one of claims 3 to 9.

11. A battery, characterized in that: The battery comprises the negative electrode according to claim 10.

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