CN115588734A - Composite lithium supplement agent and preparation method thereof - Google Patents

Abstract

The invention relates to a composite lithium replenishing agent, which is carbon-coated transition metal-doped lithium borate, and/or carbon-coated transition metal-doped lithium thioborate. The composite lithium replenishing agent of the present invention has a high theoretical decomposition capacity, and has ion transport performance, which is beneficial to improve the battery rate performance; and by adjusting the doping element and ratio, and carbon coating, a composite with two decomposition mechanisms is prepared. The lithium supplementation agent significantly improves the electronic conductivity of the material and effectively reduces the decomposition potential; and according to its different decomposition reactions, adjusting the charging voltage can realize lithium supplementation on demand. The composite lithium supplement prepared by the 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 large-scale industrial preparation.

Description

A kind of composite lithium replenishing agent and preparation method thereof

technical field

The invention relates to the technical field of lithium ion batteries, in particular to a composite lithium replenishing agent and a preparation method thereof.

Background technique

With the continuous advancement of transportation electrification, electric vehicles have ushered in a golden age 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 prolonging the service life of power batteries has become a key scientific problem that needs to be solved urgently. 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 study of the failure mechanism of lithium-ion batteries and corresponding solutions are proposed. At present, well-known domestic companies have deployed high-energy-density battery systems based on high-nickel ternary positive electrodes matched with silicon oxide or nano-silicon negative electrodes, and strive to break through the energy density of 350Wh/kg. However, silicon-based anodes undergo severe volume expansion during charge and discharge, resulting in particle pulverization and continuous destruction of the solid electrical cut-off film, resulting in a large irreversible loss of lithium, which in turn causes the battery to exhibit a low initial cycle Coulombic efficiency. At the same time, due to the consumption of active lithium on the negative electrode side, the positive electrode is in a lithium-deficient state for a long time and an irreversible phase transition occurs, all of which cause the energy density of the battery to be lower than the theoretical energy density of the material. Therefore, the development of a lithium supplement to improve the first-cycle Coulombic efficiency of the battery and compensate for the lithium loss during the long-cycle process is a key technology for improving the energy density of the battery.

Existing lithium supplements include lithium-rich materials, decomposable lithium salts, and some binary lithium-containing compounds. Although a large number of studies have verified the lithium supplementation effect of these materials, there is still no one that can be applied on a large scale. Mature lithium supplement products.

CN110867584B discloses a ternary lithium-rich material Li ₅ MO ₄ , although its theoretical specific capacity is high, but its air stability is poor, and a short exposure in air will have a bad influence on the electrochemical performance of the material. And because the decomposition products of such materials contain transition metal oxides, it will not only reduce the energy density of the battery system, but also may continue to catalyze the side reaction of the electrolyte, resulting in battery failure. CN112951620B discloses a coating modified lithium nitride as a lithium supplement, 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 reducibility and poor chemical stability of lithium nitride, and this method uses an organic solvent to treat lithium nitride, which may cause potential environmental pollution.

At present, although the academic community has conducted extensive research on positive electrode lithium supplementation agents, negative electrode pre-lithiation and some new lithium supplementation methods, there is still no lithium supplementation method that can meet the actual requirements of current battery production, and there is still a lack of a lithium supplementation method in the market. Lithium supplementation agent with high theoretical specific capacity, good air stability and simple preparation process. Therefore, the development of new composite lithium supplements is very important to improve the first-cycle Coulombic efficiency of batteries, effectively improve battery energy density and prolong battery life.

Contents of the invention

In order to solve the above technical problems, the present invention provides a novel composite lithium supplement and its preparation method and application. The compound lithium replenishing agent prepared by carbon coating and transition metal doping in the present invention has two delithiation mechanisms, can be applied to long-term lithium supplementation in lithium ion batteries, effectively improves the electronic conductivity of the material, and reduces its Decompose the potential to realize efficient lithium replenishment for lithium-ion batteries.

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

A composite lithium replenishing agent is carbon-coated transition metal-doped lithium borate, and/or carbon-coated transition metal-doped lithium thioborate.

In the composite lithium supplement, the molar ratio of Li and B is 1-5:1. Lithium borate/lithium thioborate exists in many forms, such as lithium borate is Li ₂ B ₄ O ₇ , LiBO ₂ , Li ₄ B ₂ O ₅ , Li ₃ BO ₃ , Li ₅ BO ₄ and lithium thioborate is Li ₂ B ₂ S ₅ , Li ₃ BS ₃ and so on. Theoretically, the higher the lithium content in the lithium supplement agent, the higher the lithium supplement efficiency. However, if the lithium content is too high, the air stability of the material will be deteriorated, and lithium carbonate will be easily formed on the surface. On the other hand, an increase in the lithium content will lead to an increase in the residual alkali on the surface of the material, and it is easy to react with the binder PVDF during the beating preparation process, resulting in gelation. Therefore, considering the practical solution to the effect of lithium supplementation of lithium batteries, the cost of raw materials and production, and the industrial applicability of the manufacturing process, lithium borate/lithium thioborate is preferably in the form of Li ₃ BO ₃ /Li ₃ BS ₃ .

Further, the thickness of the carbon coating layer is 1-10 nm, preferably 2-5 nm.

Further, the chemical formula of transition metal-doped lithium borate is expressed as Li _x M _a B _y O _z , and the chemical formula of transition metal-doped lithium borate is expressed as Li _x M _a B _y S _z , wherein x is between 2-5, y Between 1-4, z between 2-7, a between 0.01-1, preferably 01-0.5, the values of x, y, z, 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.

In a preferred technical solution of the present invention, the transition metal-doped lithium borate is Li _3-2x M _x BO ₃ , the transition metal-doped lithium thioborate is Li _3-2x M _x BO ₃ ; the transition metal M is selected from At least one of Al, Ti, Mn, Fe, Co, Ni, Cu; x is 0.01-1, preferably 0.05-0.1.

Lithium borate and/or lithium thioborate are used as lithium replenishing agents, and the delithiation is an oxyanion redox reaction mechanism. In the formation stage, the oxidation-reduction reaction of oxyanions corresponding to the extraction of lithium contributes most of the irreversible capacity loss, and the unavoidable oxygen release can be removed by reserving air pockets and other methods. However, after doping an appropriate amount of transition metal element M, the mechanism of delithiation is not only the oxidation-reduction reaction of oxyanions, but also the oxidation-reduction mechanism of transition metal cations. The removal of lithium is accompanied by the change of valence state of the transition metal, the change of volume expansion is small, and no gas is generated. This feature can be used to achieve on-demand lithium replenishment during battery cycling. When the battery is cycled for a certain number of cycles, in view of the capacity decay caused by the loss of active lithium, by adjusting the charging voltage range, the lithium ions in the lithium supplement can be effectively released and the capacity of the electrode material can be restored. The present invention can effectively improve the coulombic efficiency of the first cycle of the battery through the double delithiation mechanism of the lithium supplement additive, and at the same time, realize the on-demand lithium supplement by regulating the voltage during the battery cycle, and improve the cycle stability of the battery.

Taking the above-mentioned transition metal-doped lithium borate as Li _3-2x M _x BO ₃ as an example, during battery operation, the decomposition process of lithium-supplementing additives includes (i) transition metal cation change mechanism, and (ii) oxygen anion change valence mechanism. Corresponding to two different decomposition potentials during charging.

i)

ii)xLiMBO ₃ →xLi ⁺ +xMBO ₃ ++xe ^-

Taking the above-mentioned transition metal-doped lithium thioborate as Li _3-2x M _x BS ₃ as an example, during the operation of the battery, the decomposition process of lithium-supplementing additives includes: (i) transition metal cation valence change mechanism, and (ii) Oxygen anion change mechanism.

iii)

iv)xLiMBS ₃ →xLi ⁺ +xMBS ₃ ++xe ^-

The decomposition potentials corresponding to reactions i) and iii) are 4.0-4.5V (Vs, Li ⁺ /Li), and the decomposition potentials corresponding to reactions ii) and iv) are 4.5-4.7V (Vs, Li ⁺ /Li).

The second object of the present invention is to provide a preparation method for the above-mentioned composite lithium supplement, comprising the following steps:

(S1) Lithium source, lithium source, boron source, optionally, sulfur source is also added, transition metal source is mixed uniformly in the solvent, the solvent is evaporated, mechanically ground, and calcined under an inert atmosphere to obtain transition metal-doped boric acid Lithium and/or transition metal doped lithium thioborate;

(S2) crushing the material obtained in step (S1) and mixing it with a carbon source evenly, and calcining it under an inert atmosphere to obtain carbon-coated carbon-coated transition metal-doped lithium borate and/or transition metal-doped sulfur Lithium borate, that is, the composite lithium supplement of the present invention.

Further, in step (S1), the lithium source includes lithium carbonate (Li ₂ CO ₃ ), lithium hydroxide monohydrate (LiOH·H ₂ O), lithium nitrate (LiNO ₃ ), lithium acetate (CH ₃ COOLi) , at least one of lithium sulfide (Li ₂ S); the transition metal source is a transition metal salt, such as transition metal M oxalate, nitrate, halide and the like. The amount of lithium source, boric acid and/or thioboric acid, and transition metal source should meet Li _3-2x M _x BO ₃ and/or Li _3-2x M _x BS ₃ .

Further, in steps (S1), (S2), the inert atmosphere is nitrogen, one of argon or a mixture of both in any proportion; the calcination temperature of step (S1) is 600-800°C, and the calcination time is 6-10 hours; the calcination temperature in step (S2) is 500-700° C., and the calcination time is 2-4 hours. Step (S2) is for carbonization of organic matter. If the temperature is too high, the lithium supplement will be melted and the original carbon-coated morphology will be lost.

Further, in step (S2), the carbon source is an organic carbon source and/or an inorganic carbon source; the organic carbon source includes organic acids, at least one of polysaccharides, and the organic acids include citric acid, oxalic acid, At least one of malic acid, the polysaccharide includes at least one of glucose, sucrose, and fructose; the inorganic carbon source includes graphite oxide, monolithic graphene oxide, carbon nanotubes (hydroxyl multi-walled carbon nanotubes, At least one of fluorinated carbon nanotubes, single-walled carbon nanotubes, etc.). The amount of carbon source added is 5%-10% of the mass of the material obtained in step (S1). If the amount of carbon source added is too small, the conductivity of the lithium supplement agent is not good, and the decomposition voltage increases. The content of the active material of the pole piece is reduced, and the volumetric energy density of the battery is reduced.

Further, the carbon source is a composite carbon source in which the mass ratio of organic carbon source and graphitic carbon source is 90-95:5-10. Using organic components such as citric acid, graphitized carbon such as graphite oxide, and lithium supplementation agent can be connected, so that the uniform coating effect of the material is the best. The composite system of organic carbon source and inorganic carbon source can effectively improve the overall electronic conductivity and reduce the decomposition voltage of the lithium supplement by introducing a graphite carbon source with a high degree of graphitization. Due to the lack of strong interaction between the graphite-based carbon source and the lithium-supplementing material, it is difficult for a single graphite-based carbon source to form a uniform coating layer on the surface of the lithium-supplementing agent. The organic matter in the composite lithium supplement system, such as citric acid, is reductive and 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, and establish a bond between the carbon layer and the lithium supplement agent. Strong connection. During the high-temperature calcination process, the carbon material is uniformly attached to the surface of the lithium supplementing agent, and a uniform carbon coating layer is obtained after calcination.

Further, in step (S2), the crushing method is not particularly limited. In a specific embodiment of the present invention, it is a high-energy ball milling method, the ball-to-material ratio is 5:1-30:1, and the ball milling speed is 300rpm-700rpm , The ball milling time is 2h-20h.

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

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

Further, the positive electrode containing the composite lithium supplementing agent can be directly coated with the slurry containing the composite lithium supplementing agent on the positive electrode sheet (the slurry includes a composite lithium supplementing agent, a binder and an organic solvent, and the composite lithium supplementing agent The ratio of the binder to the binder is 80-95:5-20, and the solid content of the slurry is 50-70%); the positive electrode material, the composite lithium replenishing agent, the conductive agent and the binder can also be formulated into a slurry to prepare Working positive plate.

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, and the conductive agent is selected from Super P, Ketjen Black, conductive carbon tubes, and graphene. At least one, the organic solvent is selected from at least one of N-methylpyrrolidone (NMP), dimethylsulfoxide (DMSO), and N,N-dimethylformamide (DMF).

During the first charging and discharging process of lithium-ion batteries, the positive and negative electrodes react with the interface to form a stable solid electric cut-off film (SEI), which will irreversibly consume active lithium, resulting in low first-cycle Coulombic efficiency. For silicon-based anodes with higher specific capacity, the SEI will continue to restructure during cycling, further aggravating the consumption of active lithium. The present invention develops a novel composite lithium supplementation agent, which has different decomposition mechanisms under different voltages, and lithium supplementation on demand can be realized by regulating the potential. This can not only improve the first cycle Coulombic efficiency of the high specific energy battery system, but also prolong the battery cycle life.

Compared with prior inventions, the present invention has the advantages of:

1. The present invention has prepared a composite lithium supplement agent based on lithium borate and lithium thioborate, which has high theoretical decomposition capacity and ion transport performance, which is conducive to improving the battery rate performance; and by regulating the doping element and ratio, and carbon Coating, the composite lithium supplement agent with two decomposition mechanisms is prepared. The electronic conductivity of the material is significantly improved, and its decomposition potential is effectively reduced; and according to its different decomposition reactions, adjusting the charging voltage can realize lithium supplementation on demand.

2. The composite lithium supplement 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 large-scale industrial preparation.

3. The preparation process of the present invention is simple to operate, has low requirements on equipment, is conducive to realizing industrialization, and is suitable for large-scale promotion.

Description of drawings

Fig. 1 is a process flow diagram of the composite lithium replenishing agent of the present invention;

Fig. 2 is the scanning electron micrograph (SEM) of composite lithium replenishing agent in embodiment 1;

Fig. 3 is the projection electron micrograph (TEM) of composite lithium supplementing agent in embodiment 1;

Fig. 4 is the first cycle charge and discharge curve of the full battery assembled with the addition of a composite lithium supplement in Example 1 of the present invention;

Fig. 5 is the X-ray energy spectrum analysis (EDS) of composite lithium supplement agent in embodiment 4.

detailed description

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

The battery assembled in the following examples is a button battery (CR2032), the positive pole is nickel cobalt lithium manganese oxide (NCM622), and the negative pole is a silicon/graphite composite material. The diaphragm model is Celgard 2500, and the electrolyte is 1M lithium hexafluorophosphate (LiPF ₆ ) dissolved in an equal volume of a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC). The battery assembled above was charged and discharged on the LAND charge and discharge tester. The charge and discharge rate of the first cycle was 0.02C (the rate is calculated according to the nominal specific capacity of the positive electrode material used, such as NCM622 is 180mAh/g), and the cut-off The voltage is 2.5-4.5V. The charge and discharge rate of the second and third cycles is 0.1C, the charge and discharge rate of the subsequent cycles is 0.5C, and the cut-off voltage is 2.5-4.3V.

The experimental methods described in the following examples, unless otherwise specified, are conventional methods; the reagents and materials, unless otherwise specified, can be obtained from commercial sources.

Example 1

(1) Disperse LiOH (0.1mol), H ₃ BO ₃ and MnC ₂ O ₄ ·2H ₂ O in 100ml of water at a molar ratio of 2.9:1:0.05, heat to 50°C and stir for 2 hours until completely dissolved. Evaporate the solvent at 100°C, dry the obtained mixture, grind it mechanically for 15min, and calcinate at 600°C for 10h under an argon atmosphere to obtain manganese-doped lithium borate (Li _2.9 Mn _0.05 BO ₃ );

(2) Mix the L _2.9 Mn _0.05 BO material obtained in step (2 ₎ with citric acid by using high-energy ball milling, the mass ratio of Li _2.9 Mn _0.05 BO ₃ and citric acid is 100:10, and carry out high-energy ball milling. The high-energy ball milling conditions are: The ball-to-material ratio (the ratio of grinding balls to the mixture) is 20:1, the speed of the ball mill is 600rpm, and the ball milling time is 10h; the ball-milled mixture is calcined for 5h in an argon atmosphere at a calcination temperature of 600°C to obtain a composite lithium supplement . . Figure 2 is the SEM photo 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. Fig. 3 is the TEM photo of the composite lithium supplement agent obtained in Example 1.

(3) The composite lithium supplement is used as the positive electrode material additive, and the positive electrode sheet is prepared with the ternary positive electrode material NCM622, the conductive agent Super P, and the binder PVDF according to the mass ratio of 10:80:10:10, matching the silicon/graphite composite negative electrode Assemble the coin cells for electrochemical testing. Fig. 4 is the first cycle charge and discharge curve of the lithium-ion battery obtained by assembling the positive pole piece obtained in Example 1 according to the above method.

Example 2

(1) Mix Li ₂ S, B, S and MnC ₂ O ₄ ·2H ₂ O according to the molar ratio of 1.45:1:0.1:0.05, then mechanically grind for 45 minutes, and seal the mixture in a quartz tube under an argon atmosphere, Calcined at 600° C. for 10 h to obtain manganese-doped lithium thioborate (Li _2.9 Mn _0.05 BS ₃ , M _0.05 -LBS for short).

(2) M _0.05 -LBS and citric acid were mixed by high-energy ball milling, and the M _0.5 -LBS was crushed to 1-5 μm. Under an argon atmosphere, the mass ratio of M _0.05 -LBS to citric acid was 100:10, the ball-to-material ratio (the ratio of grinding balls to the mixture) was 20:1, the rotational speed of the ball mill was 600 rpm, and the ball milling time was 10 h.

(3) Calcining the ball-milled mixture for 2 hours under an argon atmosphere at a calcination temperature of 600°C to obtain a composite lithium supplement

(4) The composite lithium supplement is used as the positive electrode material additive, and the positive electrode sheet is prepared with NCM622, the conductive agent Super P, and the binder PVDF according to the mass ratio of 10:80:10:10, and the button battery is assembled with the silicon/graphite composite negative electrode Perform electrochemical tests.

Example 3

The rest is the same as Example 3, except that the molar ratio of LiOH, H ₃ BO ₃ and MnC ₂ O ₄ ·2H ₂ O in step 1) is 2.8:1:0.1, and the obtained product is Li _2.8 Mn _0.1 BO ₃ .

Example 4

The rest is the same as in Example 1, except that the doping element in step 1) is FeC ₂ O ₄ ·2H ₂ O, LiOH, H ₃ BO ₃ and FeC ₂ O ₄ ·2H ₂ O in a molar ratio of 2.9:1 : 0.05, the obtained product is Li _2.9 Fe _0.05 BO ₃ . Figure 5 is the EDS spectrum of the product obtained.

Example 5

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

Example 6

The rest are the same as in Example 1, except that in step 2), the mass ratio of citric acid and manganese-doped lithium borate is 15:100.

Example 7

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

Example 8

The rest is the same as in Example 1, except that the carbon source added in step 2) is a mixture of citric acid and graphite oxide according to the mass ratio of 95:5, and the mass ratio of Li _2.9 Mn _0.05 BO ₃ to the carbon source is 100:10.

Example 9

The rest is the same as in Example 1, except that the carbon source added in step 2) is a mixture of citric acid and graphite oxide according to the mass of 90:10, and the mass ratio of Li _2.9 Mn _0.05 BO ₃ to the carbon source is 100:10.

Example 10

All the other are the same as Example 1, except that the carbon source added in step 2) is graphite oxide.

Comparative example 1

NCM622, conductive conductive agent Super P, and binder PVDF were prepared according to the mass ratio of 80:10:10. The positive electrode sheet was matched with the silicon/graphite composite negative electrode to assemble the button cell for electrochemical testing.

In the examples included in the present invention, the lithium supplementation effects of different composite lithium supplementation agents in batteries are shown in Table 1.

Table 1 Electrochemical properties of lithium-ion batteries of different embodiments

According to the above examples, it can be seen that different composite lithium supplementation agents have better lithium supplementation effects. Compared with the original lithium borate material, element doping and carbon coating significantly improve the lithium supplementation effect. Optimizing the doping elements and doping amount can improve the conductivity of the material and release more active lithium at a lower voltage. From the comparison of 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. Compared with the amorphous carbon formed after carbonization of the organic carbon source, adding a small amount of graphite oxide has a significant impact on the carbon coating. The conductivity of the composite lithium supplementation agent is more obvious, and the electrochemical performance is also more excellent. On the other hand, Example 10 shows that it is difficult to achieve a better effect by adding graphite oxide alone, because graphite oxide and the lithium supplementing agent lack interaction, and it is difficult to uniformly coat the surface of the lithium supplementing material. Adding an organic carbon source and a composite carbon source of graphite oxide can improve the electronic conductivity of the composite lithium supplement agent, thereby reducing the decomposition voltage of the lithium supplement agent, so that the lithium supplement agent can be completely decomposed in the normal positive electrode working voltage range, and the first Cyclical Coulombic efficiency and long-term cycle stability.

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

Claims (10)

1. A composite lithium supplement is a carbon-coated transition metal doped lithium borate and/or a carbon-coated transition metal doped lithium thioborate.

2. The composite lithium supplement agent as claimed in claim 1, wherein the molar ratio of Li to B in the composite lithium supplement agent is 1-5:1; preferably, the lithium borate comprises Li ₂ B ₄ O ₇ ，LiBO ₂ ，Li ₄ B ₂ O ₅ ，Li ₃ BO ₃ ，Li ₅ BO ₄ Of (a), the lithium thioborate includes Li ₂ B ₂ S ₅ ，Li ₃ BS ₃ At least one of (1).

3. The composite lithium supplementing agent according to claim 1, wherein the carbon coating has a thickness of 1 to 10nm, preferably 2 to 5nm.

4. The composite lithium supplement agent according to claim 1, wherein the transition metal doped lithium borate is represented by the chemical formula Li _x M _a B _y O _z Transition metal doped lithium thioborate formula expressed as Li _x M _a B _y S _z Wherein x is between 2 and 5, y is between 1 and 4, z is between 2 and 7, a is between 0.01 and 1, preferably 01 to 0.5, and the values of x, y, z and a satisfy the conservation of charge of the compound; the transition metal is selected from at least one of Al, ti, mn, fe, co, ni and Cu;

further, the transition metal-doped lithium borate is Li _3-2x M _x BO ₃ The transition metal doped lithium thioborate is Li _{3- 2x} M _x BO ₃ (ii) a x is 0.01 to 1, preferably 0.05 to 0.1.

5. The preparation method of the composite lithium supplementing agent of any one of claims 1 to 4, which is characterized by comprising the following steps:

(S1) adding a lithium source, a boron source and a transition metal source, optionally adding a sulfur source, uniformly mixing in a solvent, evaporating the solvent to dryness, mechanically grinding, and calcining in an inert atmosphere to obtain transition metal doped lithium borate and/or transition metal doped lithium thioborate;

and (S2) crushing the material obtained in the step (S1), uniformly mixing the crushed material with a carbon source, and calcining the mixture in an inert atmosphere to obtain carbon-coated transition metal-doped lithium borate and/or transition metal-doped lithium thioborate.

6. The production method according to claim 5, wherein, in step (S1), the lithium source includes lithium carbonate (Li) ₂ CO ₃ ) Lithium hydroxide monohydrate (LiOH. H) ₂ O), lithium nitrate (LiNO) ₃ ) Lithium acetate (CH) ₃ COOLi), lithium sulfide (Li) ₂ S) at least one of; the transition metal source is a salt of a transition metal, such as an oxalate, a nitrate, a halide salt of a transition metal M; the boron source is boric acid or a boron simple substance, and the sulfur source is a sulfur simple substance.

7. The method according to claim 5, wherein in steps (S1) and (S2), the inert atmosphere is one or a mixture of nitrogen and argon in any proportion; the calcining temperature of the step (S1) is 600-800 ℃, and the calcining time is 6-10h; the calcining temperature of the step (S2) is 500-700 ℃, and the calcining time is 2-4h.

8. The production method according to claim 5, wherein in the step (S2), the carbon source is an organic carbon source and/or an inorganic carbon source; the organic carbon source comprises at least one of organic acid and polysaccharide, the organic acid comprises at least one of citric acid, oxalic acid and malic acid, and the polysaccharide comprises at least one of glucose, sucrose and fructose; the inorganic carbon source comprises at least one of graphite oxide, single-layer graphene oxide and carbon nanotubes (hydroxyl multi-walled carbon nanotubes, fluorinated carbon nanotubes, single-walled carbon nanotubes and the like); the adding amount of the carbon source is 5-10% of the mass of the material obtained in the step (S1).

9. The preparation method according to claim 8, wherein the carbon source is an organic carbon source and a graphite-based carbon source in a mass ratio of 90-95:5-10 of compound carbon source.

10. A lithium ion battery comprises a positive electrode, a negative electrode, a diaphragm and an electrolyte; the positive electrode contains the composite lithium supplement agent according to any one of claims 1 to 4; preferably, the composite lithium supplement agent accounts for 5-15wt% of the positive electrode active material, and preferably 10-15wt%.

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