A composite lithium supplement and its preparation method

Abstract

The present invention relates to a composite lithium supplement agent, which is lithium borate doped with a transition metal coated with carbon, and / or lithium thioborate doped with a transition metal coated with carbon. The composite lithium supplement agent of the present invention has a high theoretical decomposition capacity and has ion transport performance, which is beneficial to improving the battery rate performance; and by regulating the doping elements and ratios, as well as carbon coating, a composite lithium supplement agent with two decomposition mechanisms is prepared, significantly improving the electronic conductivity of the material and effectively reducing the decomposition potential; and according to its different decomposition reactions, the charging voltage can be regulated to achieve on-demand lithium supplementation. The composite lithium supplement agent prepared by the present invention has good air stability and moisture resistance, and is compatible with the existing battery preparation process. Moreover, the material has good chemical stability and high safety, and is suitable for industrial-scale preparation.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly relates to a composite lithium supplement agent and a preparation method thereof. Background Art

[0002] With the continuous advancement of transportation electrification, electric vehicles have entered a golden era of development. However, limited by the current development level of power battery technology, the cruising range of new energy vehicles is relatively short and the battery life is limited. Therefore, improving the energy density and extending the service life of power batteries have become key scientific issues that urgently need to be solved. On the one hand, researchers are committed to developing new material systems, such as high-nickel cathodes, silicon-based anodes, etc. On the other hand, in-depth research on the failure mechanism of lithium-ion batteries is carried out to propose corresponding solutions. Currently, well-known domestic enterprises have all laid out high-energy density battery systems based on high-nickel ternary cathodes matched with silicon oxide or nano-silicon anodes, striving to break through the energy density of 350 Wh / kg. However, during the charge and discharge process, the silicon-based anode undergoes severe volume expansion, resulting in particle pulverization and continuous damage to the solid electrolyte interphase film, causing a large amount of irreversible lithium loss, and further making the battery exhibit a low first-cycle Coulombic efficiency. At the same time, due to the consumption of active lithium on the anode side, the cathode is in a lithium-deficient state for a long time and irreversible phase transformation occurs, all of which result in the battery energy density being lower than the theoretical energy density of the material. Therefore, developing a lithium supplement agent to improve the first-cycle Coulombic efficiency of the battery and compensate for lithium loss during long-term cycling is a key technology to improve the battery energy density.

[0003] Existing lithium supplement agents include lithium-rich materials, decomposable lithium salts, and some binary lithium-containing compounds. Although a large number of studies have verified the lithium supplement effect of these materials, no mature lithium supplement product that can be applied on a large scale has been found yet.

[0004] CN110867584B discloses a ternary lithium-rich material Li5MO4. Although its theoretical specific capacity is relatively high, its air stability is poor, and short-term exposure to air will have an adverse impact on the electrochemical performance of the material. And since the decomposition products of such materials contain transition metal oxides, it will not only reduce the energy density of the battery system, but may also continuously catalyze the side reactions of the electrolyte, resulting in battery failure. CN112951620B discloses a lithium nitride modified with a coating layer as a lithium supplement agent, which improves the water resistance of lithium nitride to a certain extent and improves its compatibility with the battery preparation process. However, this method still does not solve the problems of strong intrinsic reducibility and poor chemical stability of lithium nitride, and this method uses organic solvents to treat lithium nitride, which may bring potential environmental pollution.

[0005] At present, although extensive research has been carried out in the academic community on cathode lithium supplements, anode prelithiation, and some new lithium supplement methods, there is still no lithium supplement method that can meet the actual requirements of current battery production. There is still a lack of a lithium supplement with a high theoretical specific capacity, good air stability, and simple preparation process in the market. Therefore, the development of new composite lithium supplements is crucial for improving the first-cycle Coulombic efficiency of batteries, effectively increasing the energy density of batteries, and extending the battery life. Summary of the Invention

[0006] To solve the above technical problems, the present invention provides a new composite lithium supplement and its preparation method and application. The composite lithium supplement prepared by the present invention through carbon coating and transition metal doping has two lithium deintercalation mechanisms, can be applied to long-term lithium supplementation in lithium-ion batteries, effectively improves the electronic conductivity of the material, reduces its decomposition potential, and realizes efficient lithium supplementation for lithium-ion batteries.

[0007] The present invention solves the above technical problems through the following technical solutions:

[0008] A composite lithium supplement is lithium borate doped with transition metals coated with carbon, and / or lithium thioborate doped with transition metals coated with carbon.

[0009] In the composite lithium supplement, the molar ratio of Li to B is 1-5:1. Lithium borate / lithium thioborate has various forms, such as lithium borate being Li2B4O7, LiBO2, Li4B2O5, Li3BO3, Li5BO4, and lithium thioborate being Li2B2S5, Li3BS3, etc. Theoretically, the higher the lithium content in the lithium supplement, the higher the lithium supplementation efficiency. However, if the lithium content is too high, the air stability of the material will deteriorate, and lithium carbonate is likely to form on the surface. On the other hand, the increase in lithium content will lead to an increase in residual alkali on the surface of the material, which is likely to react with the binder PVDF during the slurry preparation process, resulting in gelation. Therefore, considering the comprehensive aspects of the lithium supplementation effect of lithium batteries, raw material and production costs, and the industrial applicability of the manufacturing process, lithium borate / lithium thioborate is preferably in the form of Li3BO3 / Li3BS3.

[0010] Furthermore, the thickness of the carbon coating layer is 1-10 nm, preferably 2-5 nm.

[0011] Furthermore, the chemical formula of the transition metal-doped lithium borate is expressed as Li x M a B y O z and the chemical formula of the transition metal-doped lithium thioborate is expressed as Li x M a B y S z, where x ranges from 2 to 5, y ranges from 1 to 4, z ranges from 2 to 7, a ranges from 0.01 to 1, preferably from 0.1 to 0.5, and the values of x, y, z, and a satisfy the charge conservation of the compound. The transition metal is selected from at least one of Al, Ti, Mn, Fe, Co, Ni, and Cu.

[0012] In a preferred technical solution of the present invention, the transition metal-doped lithium borate is Li 3-2x M x BO3, and the transition metal-doped lithium thioborate is Li 3-2x M x BO3; the transition metal M is selected from at least one of Al, Ti, Mn, Fe, Co, Ni, and Cu; x is from 0.01 to 1, preferably from 0.05 to 0.1.

[0013] Lithium borate and / or lithium thioborate is used as a lithium supplement agent, and its lithium deintercalation is an oxygen anion redox reaction mechanism. During the formation stage, the oxygen anion redox reaction corresponding to the lithium deintercalation contributes most of the irreversible capacity loss, and the inevitable oxygen release can be removed by methods such as reserving an air bag. After doping with an appropriate amount of transition metal element M, in addition to the oxygen anion redox reaction, there is also a transition metal cation redox mechanism in the lithium deintercalation mechanism. The lithium deintercalation is accompanied by a change in the valence state of the transition metal, with a small volume expansion change and no gas generation. This feature can be used to achieve on-demand lithium supplementation during the battery cycle. When the battery has cycled a certain number of times, for the capacity attenuation caused by the loss of active lithium, by adjusting the charging voltage range, the lithium ions in the lithium supplement agent can be effectively released to restore the capacity of the electrode material. Through the dual lithium deintercalation mechanism of the lithium supplement additive of the present invention, the first-cycle Coulombic efficiency of the battery can be effectively improved. At the same time, during the battery cycle, on-demand lithium supplementation is achieved by regulating the voltage, improving the cycle stability of the battery.

[0014] Taking the above-mentioned transition metal-doped lithium borate Li 3-2x M x BO3 as an example, during the operation of the battery, the decomposition process of the lithium supplement additive includes (i) the transition metal cation valence change mechanism and (ii) the oxygen anion valence change mechanism. There are two different decomposition potentials during the charging process.

[0015] i)

[0016] ii) xLiMBO3 → xLi + + xMBO3++ xe -

[0017] Taking the above-mentioned transition metal-doped lithium thioborate Li 3-2x M xTaking BS3 as an example, during the operation of the battery, the decomposition process of the lithium supplement additive includes: (i) the variable valence mechanism of transition metal cations, and (ii) the variable valence mechanism of oxygen anions.

[0018] iii)

[0019] iv)xLiMBS3→xLi + +xMBS3++xe -

[0020] Among them, the decomposition potentials corresponding to reactions i) and iii) are 4.0 - 4.5 V (Vs, Li + / Li), and the decomposition potentials corresponding to reactions ii) and iv) are in the range of 4.5 - 4.7 V (Vs, Li + / Li).

[0021] The second object of the present invention is to provide a preparation method of the above composite lithium supplement agent, including the following steps:

[0022] (S1) A lithium source, a lithium source, a boron source, and optionally, a sulfur source and a transition metal source are mixed evenly in a solvent, the solvent is evaporated to dryness, mechanically ground, and calcined in an inert atmosphere to obtain transition metal-doped lithium borate and / or transition metal-doped lithium thiosulfate;

[0023] (S2) The material obtained in step (S1) is crushed and then mixed evenly with a carbon source, and calcined in an inert atmosphere to obtain carbon-coated transition metal-doped lithium borate and / or transition metal-doped lithium thiosulfate, that is, the composite lithium supplement agent of the present invention.

[0024] Furthermore, in step (S1), the lithium source includes at least one of lithium carbonate (Li2CO3), lithium hydroxide monohydrate (LiOH·H2O), lithium nitrate (LiNO3), lithium acetate (CH3COOLi), and lithium sulfide (Li2S); the transition metal source is a salt of a transition metal, such as oxalate, nitrate, halide salt, etc. of transition metal M. The dosages of the lithium source, boric acid and / or thiosulfuric acid, and the transition metal source satisfy Li 3-2x M x BO3 and / or Li 3-2x M x BS3 is sufficient.

[0025] Furthermore, in steps (S1) and (S2), the inert atmosphere is one of nitrogen and argon or a mixture of the two in any proportion; the calcination temperature in step (S1) is 600 - 800 °C, and the calcination time is 6 - 10 h; the calcination temperature in step (S2) is 500 - 700 °C, and the calcination time is 2 - 4 h. Step (S2) is for the carbonization of organic substances. If the temperature is too high, the lithium supplement agent will melt and lose its original carbon-coated morphology.

[0026] Further, in step (S2), the carbon source is an organic carbon source and / or an inorganic carbon source; the organic carbon source includes at least one of organic acids and polysaccharides, the organic acids include at least one of citric acid, oxalic acid, and malic acid, and the polysaccharides include at least one of glucose, sucrose, and fructose; the inorganic carbon source includes at least one of graphite oxide, single-layer graphene oxide, and carbon nanotubes (such as hydroxyl multi-walled carbon nanotubes, fluorinated carbon nanotubes, single-walled carbon nanotubes, etc.). The addition amount of the carbon source is 5%-10% of the mass of the material obtained in step (S1). If the addition amount of the carbon source is too small, the conductivity of the lithium supplement agent is poor, the decomposition voltage increases, and during the conventional charge-discharge voltage cycle process, the lithium supplement effect cannot be effectively exerted, resulting in a decrease in battery capacity; if the amount of the carbon source used is too large, the content of the active material of the electrode sheet is reduced, and the volumetric energy density of the battery is decreased.

[0027] Further, the carbon source is a compound carbon source composed of an organic carbon source and a graphite-based carbon source in a mass ratio of 90-95:5-10. Using organic components such as citric acid, graphite carbon such as graphite oxide and the lithium supplement agent can be connected, and the material with the best coating uniformity can be obtained in this way. In the composite system of the organic carbon source and the inorganic carbon source, by introducing a graphite-based carbon source with a higher degree of graphitization, the overall electronic conductivity can be effectively improved, and the decomposition voltage of the lithium supplement agent can be reduced. Due to the lack of strong interaction between the graphite-based carbon source and the lithium supplement material, it is difficult for a single graphite-based carbon source to form a uniform coating layer on the surface of the lithium supplement agent. In the composite lithium supplement system, organic substances such as citric acid have reducibility, can interact with the oxygen-containing functional groups on the surface of graphite oxide, and at the same time form an acid-base interaction with the residual alkali on the surface of the lithium supplement material, establishing a strong connection between the carbon layer and the lithium supplement agent. During the high-temperature calcination process, the carbon material is uniformly attached to the surface of the lithium supplement agent, and a uniform carbon coating layer is obtained after calcination.

[0028] Further, in step (S2), there is no particular limitation on the crushing method. In a specific embodiment of the present invention, it is a high-energy ball milling method, the ball-to-material ratio is 5:1 to 30:1, the ball milling speed is 300 rpm - 700 rpm, and the ball milling time is 2 h - 20 h.

[0029] The third object of the present invention is to provide a lithium-ion battery, including a positive electrode, a negative electrode, a separator, and an electrolyte; the positive electrode contains the above-mentioned composite lithium supplement agent.

[0030] Further, in the positive electrode, the composite lithium supplement agent accounts for 5-15 wt% of the positive electrode active material, preferably 10-15 wt%, such as 12.5 wt%. The positive electrode active material is well-known in the art and includes, but is not limited to, lithium iron phosphate, lithium cobaltate, CNM ternary positive electrode materials, etc.

[0031] Furthermore, the positive electrode containing the composite lithium supplement can be obtained by directly coating a slurry containing the composite lithium supplement on the positive electrode sheet (the slurry includes the composite lithium supplement, a binder, and an organic solvent, and the ratio of the composite lithium supplement to the binder is 80 - 95:5 - 20, and the solid content of the slurry is 50 - 70%); alternatively, the positive electrode material, the composite lithium supplement, a conductive agent, and a binder can be formulated into a slurry to prepare a working positive electrode sheet.

[0032] The binder, conductive agent, and organic solvent are well-known in the art. For example, the binder is selected from at least one of polyvinylidene fluoride, polyacrylonitrile, and polyethylene glycol, the conductive agent is selected from at least one of Super P, Ketjen black, carbon nanotubes, and graphene, and the organic solvent is selected from at least one of N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), and N,N-dimethylformamide (DMF).

[0033] During the first charge-discharge process of a lithium-ion battery, the formation of a stable solid electrolyte interface (SEI) at the interface between the positive and negative electrodes will irreversibly consume active lithium, resulting in a relatively low first-cycle Coulombic efficiency. For a silicon-based negative electrode with a higher specific capacity, the SEI will continuously reconstruct during cycling, further exacerbating the consumption of active lithium. The present invention has developed a novel composite lithium supplement that has different decomposition mechanisms at different voltages, and by regulating the potential, on-demand lithium supplementation can be achieved. This can not only improve the first-cycle Coulombic efficiency of a high-energy battery system but also extend the battery cycle life.

[0034] Compared with existing inventions, the advantages of the present invention are as follows:

[0035] First, the present invention has prepared a composite lithium supplement based on lithium borate and lithium thioborate, which has a high theoretical decomposition capacity and ionic transport properties, facilitating the improvement of the battery rate performance; and by regulating the doping elements and ratios, as well as carbon coating, a composite lithium supplement with two decomposition mechanisms is prepared. This significantly improves the electronic conductivity of the material and effectively reduces its decomposition potential; and according to its different decomposition reactions, regulating the charging voltage can achieve on-demand lithium supplementation.

[0036] Second, the composite lithium supplement prepared by the present invention has good air stability and moisture resistance, and is compatible with existing battery manufacturing processes. Moreover, the material has good chemical stability and high safety, making it suitable for large-scale industrial production.

[0037] Third, the preparation process of the present invention is simple and has low requirements for equipment, facilitating industrialization and large-scale promotion. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a process flow chart of the composite lithium supplement of the present invention;

[0039] Figure 2SEM photograph of the composite lithium supplement agent in Example 1;

[0040] Figure 3 TEM photograph of the composite lithium supplement agent in Example 1;

[0041] Figure 4 First charge-discharge curve of the full battery assembled with the composite lithium supplement agent in Example 1 of the present invention;

[0042] Figure 5 EDS of the composite lithium supplement agent in Example 4. Detailed implementation manners

[0043] The present invention will be further described below in conjunction with specific embodiments, but is not limited to specific embodiments. Unless otherwise specified, all ratios in the embodiments of the present invention are mass ratios.

[0044] The assembled battery in the following embodiments is a button battery (CR2032), the positive electrode is lithium nickel cobalt manganese oxide (NCM622), and the negative electrode is a silicon / graphite composite material. The separator model is Celgard 2500, and the electrolyte is 1M lithium hexafluorophosphate (LiPF6) dissolved in a mixed solvent of equal volumes of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). The above-assembled battery is subjected to charge-discharge tests on a LAND charge-discharge tester. The charge-discharge rate of the first cycle is 0.02C (the rate is calculated according to the nominal specific capacity of the used positive electrode material, such as 180 mAh / g for NCM622), and the cut-off voltage is 2.5 - 4.5V. The charge-discharge rates of the second and third cycles are 0.1C, and the charge-discharge rate during subsequent cycling is 0.5C, and the cut-off voltage is 2.5 - 4.3V.

[0045] Unless otherwise specified, the experimental methods in the following embodiments are all conventional methods; unless otherwise specified, the reagents and materials can all be obtained from commercial sources.

[0046] Example 1

[0047] (1) Disperse LiOH (0.1 mol), H3BO3, and MnC2O4·2H2O in 100 ml of water according to a molar ratio of 2.9:1:0.05, heat to 50 °C, and stir for 2 h until completely dissolved. Evaporate the solvent at 100 °C, dry the obtained mixture, mechanically grind for 15 min, and calcine at 600 °C for 10 h in an argon atmosphere to obtain lithium manganese borate (Li 2.9 Mn 0.05 BO3);

[0048] (2) Use high-energy ball milling to process the L 2.9 Mn0.05 The BO3 material is mixed with citric acid, Li 2.9 Mn 0.05 The mass ratio of BO3 to citric acid is 100:10, and high-energy ball milling is carried out. The conditions of high-energy ball milling are: the ball-to-material ratio (the ratio of grinding balls to the mixture) is 20:1, the rotational speed of the ball mill is 600 rpm, and the ball milling time is 10 h; the ball-milled mixture is calcined in an argon atmosphere for 5 h at a calcination temperature of 600 °C to obtain a composite lithium supplement agent.. Figure 2 It is the SEM photograph of the composite lithium supplement agent obtained in Example 1. It can be seen that the particle size of the material after ball milling is 1-5 μm. Figure 3 It is the TEM photograph of the composite lithium supplement agent obtained in Example 1.

[0049] (3) The composite lithium supplement agent is used as a cathode material additive, and a cathode electrode sheet is prepared by mixing with ternary cathode material NCM622, conductive agent Super P, and binder PVDF according to a mass ratio of 10:80:10:10, and a coin cell is assembled with a silicon / graphite composite anode for electrochemical testing. Figure 4 It is the first charge-discharge curve of the lithium-ion battery assembled according to the above method with the cathode electrode sheet obtained in Example 1.

[0050] Example 2

[0051] (1) Li2S, B, S and MnC2O4·2H2O are mixed according to a molar ratio of 1.45:1:0.1:0.05 and then mechanically ground for 45 min. In an argon atmosphere, the mixture is encapsulated in a quartz tube and then calcined at 600 °C for 10 h to obtain manganese-doped lithium thioborate (Li 2.9 Mn 0.05 BS3, abbreviated as M 0.05 -LBS).

[0052] (2) M 0.05 -LBS and citric acid are mixed by high-energy ball milling, and M 0.5 -LBS is crushed to 1-5 μm. In an argon atmosphere, the mass ratio of M 0.05 -LBS to citric acid is 100:10, the ball-to-material ratio (the ratio of grinding balls to the mixture) is 20:1, the rotational speed of the ball mill is 600 rpm, and the ball milling time is 10 h.

[0053] (3) The ball-milled mixture is calcined in an argon atmosphere for 2 h at a calcination temperature of 600 °C to obtain a composite lithium supplement agent

[0054] (4) The composite lithium supplement agent is used as a cathode material additive, and a cathode electrode sheet is prepared by mixing with NCM622, conductive agent Super P, and binder PVDF according to a mass ratio of 10:80:10:10, and a coin cell is assembled with a silicon / graphite composite anode for electrochemical testing.

[0055] Example 3

[0056] The rest is the same as in Example 3, except that in step 1), the molar ratio of LiOH, H3BO3 and MnC2O4·2H2O is 2.8:1:0.1, and the resulting product is Li 2.8 Mn 0.1 BO3.

[0057] Example 4

[0058] The rest is the same as in Example 1, except that in step 1), the doping element is FeC2O4·2H2O, and the molar ratio of LiOH, H3BO3 and FeC2O4·2H2O is 2.9:1:0.05, and the resulting product is Li 2.9 Fe 0.05 BO3. Figure 5 is the EDS spectrum of the resulting product.

[0059] Example 5

[0060] The rest is the same as in Example 1, except that in step 2), the mass ratio of citric acid to lithium borate doped with manganese is 5:100.

[0061] Example 6

[0062] The rest is the same as in Example 1, except that in step 2), the mass ratio of citric acid to lithium borate doped with manganese is 15:100.

[0063] Example 7

[0064] The rest is the same as in Example 1, except that in step 2), the mass ratio of citric acid to lithium borate doped with manganese is 3:100.

[0065] Example 8

[0066] The rest is the same as in Example 1, except that in step 2), the carbon source added is a mixture of citric acid and graphite oxide in a mass ratio of 95:5, and the mass ratio of Li 2.9 Mn 0.05 BO3 to the carbon source is 100:10.

[0067] Example 9

[0068] The rest is the same as in Example 1, except that in step 2), the carbon source added is a mixture of citric acid and graphite oxide in a mass ratio of 90:10, and the mass ratio of Li 2.9 Mn 0.05 BO3 to the carbon source is 100:10.

[0069] Example 10

[0070] The rest is the same as in Example 1, except that the carbon source added in step 2) is graphite oxide.

[0071] Comparative Example 1

[0072] The positive electrode sheet was prepared from NCM622, conductive agent Super P, and binder PVDF according to a mass ratio of 80:10:10, and a coin cell was assembled with a silicon / graphite composite negative electrode for electrochemical testing.

[0073] In the examples included in the present invention, the lithium supplementing effects of different composite lithium supplementing agents in the battery are shown in Table 1.

[0074] Table 1 Electrochemical performance of lithium-ion batteries in different examples

[0075]

[0076] As can be seen from the above examples, different composite lithium supplementing agents all have good lithium supplementing effects. Compared with the original lithium borate material, element doping and carbon coating significantly improve the lithium supplementing effect. Optimizing the doping elements and doping amounts can improve the conductivity of the material and release more active lithium at a lower voltage. From the comparison between Example 1 and Examples 8 and 9, it can be seen that the conductivity of the material is closely related to the quality of the carbon coating layer. Compared with the amorphous carbon formed after carbonization of the organic carbon source, adding a small amount of graphite oxide has a more obvious effect on improving the conductivity of the composite lithium supplementing agent, and the electrochemical performance is also more excellent. On the other hand, Example 10 shows that adding graphite oxide alone is difficult to achieve good results because there is a lack of interaction between graphite oxide and the lithium supplementing agent, making it difficult to uniformly coat the surface of the lithium supplementing material. Adding a composite carbon source of an organic carbon source and graphite oxide can improve the electronic conductivity of the composite lithium supplementing agent, and further reduce the decomposition voltage of the lithium supplementing agent, enabling the lithium supplementing agent to be completely decomposed in the conventional positive electrode working voltage range, thereby improving the first-cycle Coulombic efficiency and long-cycle stability.

[0077] In summary, the preparation method of the composite lithium supplementing agent in the present invention is simple. By doping and carbon coating, the decomposition voltage of the lithium supplementing agent is reduced, which is compatible with the existing conventional battery systems and has a good lithium supplementing effect. Moreover, the composite lithium supplementing agent prepared by the present invention has good air stability, is compatible with the battery preparation process, and has the potential for large-scale application.

Claims

1. A preparation method of a composite lithium supplement, characterized in that, Comprising the following steps: (S1) adding a lithium source, a boron source, a transition metal source, and optionally, a sulfur source, mixing them uniformly in a solvent, evaporating the solvent, mechanically grinding, and calcining under an inert atmosphere to obtain transition metal-doped lithium borate and / or transition metal-doped lithium thioborate; the chemical formula of transition metal-doped lithium borate is Li x M a B y O z , the chemical formula of transition metal-doped lithium thioborate is Li x M a B y S z , wherein x is between 2-5, y is between 1-4, z is between 2-7, a is between 0.01-1, and the values ​​of x, y, z, and a satisfy the charge conservation of the compound; the transition metal is selected from at least one of Al, Ti, Mn, Fe, Co, Ni, and Cu; the lithium source is selected from at least one of lithium carbonate, lithium hydroxide monohydrate, lithium nitrate, lithium acetate, and lithium sulfide; the boron source is boric acid or boron element; (S2) After crushing the material obtained in step (S1), it is mixed evenly with a carbon source and calcined in an inert atmosphere to obtain a product, lithium borate doped with a transition metal and / or lithium thioborate doped with a transition metal, with a carbon coating; the carbon source is a compound carbon source composed of an organic carbon source and a graphite-based carbon source in a mass ratio of 90-95:5-10; the organic carbon source is citric acid; the graphite-based carbon source is graphene oxide; the addition amount of the carbon source is 5%-10% of the mass of the material obtained in step (S1), and the thickness of the carbon coating layer is 1-10 nm.

2. The preparation method according to claim 1, characterized in that, The thickness of the carbon coating layer is 2-5 nm.

3. The preparation method according to claim 1, characterized in that, a ranges from 0.1 to 0.

5.

4. The preparation method according to claim 1, characterized in that, The transition metal-doped lithium borate is Li 3-2x M x BO3, and the transition metal-doped lithium thioborate is Li 3-2x M x BO3; x is 0.01 - 1.

5. The preparation method according to claim 4, characterized in that, x is 0.05-0.

1.

6. The preparation method according to claim 1, characterized in that, In step (S1), the transition metal source is a salt of a transition metal; the sulfur source is elemental sulfur.

7. The preparation method according to claim 6, characterized in that, The salt of the transition metal is an oxalate, nitrate, or halide salt of transition metal M.

8. The preparation method according to claim 1, characterized in that, In steps (S1) and (S2), the inert atmosphere is one of nitrogen and argon or a mixture of the two in any proportion; the calcination temperature in step (S1) is 600-800 °C, and the calcination time is 6-10 h; the calcination temperature in step (S2) is 500-700 °C, and the calcination time is 2-4 h.

9. A lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte; the positive electrode contains the composite lithium supplement prepared by the preparation method according to any one of claims 1-8.

10. The lithium-ion battery according to claim 9, characterized in that, The composite lithium supplement accounts for 5-15 wt% of the cathode active material.

11. The lithium-ion battery according to claim 9, characterized in that, The composite lithium supplement accounts for 10-15 wt% of the cathode active material.

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