Positive electrode lithium supplement composite additive and its preparation method and application

Abstract

The present application belongs to the field of battery material technology, and in particular relates to a positive electrode lithium supplement composite additive and its preparation method, as well as a positive electrode material, a positive electrode sheet, and a secondary battery. Among them, the positive electrode lithium supplement composite additive includes a lithium supplement material core and a carbon coating layer grown on the outer surface of the lithium supplement material core; wherein, the lithium supplement material core includes a lithium supplement additive and a transition metal element doped in the structure of the lithium supplement additive and / or between the structures. The positive electrode lithium supplement composite additive of the present application improves the degree and quality of coating of the carbon coating layer on the lithium supplement material core, improves the bonding stability of the two, and thus improves the environmental stability of the positive electrode lithium supplement composite additive. In addition, the transition metal elements doped in the structure of the lithium supplement additive and / or between the structures can further improve the structural stability of the lithium supplement additive through the influence of electrostatic forces between elements and the like.

Description

Technical Field

[0001] The present application belongs to the field of battery material technology, and in particular relates to a positive electrode lithium supplement composite additive and its preparation method and application. Background Art

[0002] With the rapid development of energy storage technology, the use of portable digital devices and vehicle-mounted power supplies is increasing. People have higher and higher requirements for the energy density of batteries. It is imperative to develop secondary batteries with large capacity, long life and high safety. During the first charge and discharge process of lithium-ion batteries, a SEI film will form at the interface of the negative electrode material. Studies have shown that the components of SEI are mainly lithium salt materials such as LiF, Li2CO3, R-COOLi, and R-CH2OLi. The formation of SEI is an irreversible process. The Li used to form SEI + During the discharge process, it can no longer be embedded in the positive electrode material, resulting in a loss of battery capacity.

[0003] The study found that the formation of SEI film consumes part of the Li in the positive electrode material. + , which in turn leads to irreversible capacity loss of the electrode material. Therefore, in order to further improve the energy density of lithium-ion batteries, this capacity loss can be compensated by pre-replenishing lithium. There are two main types of pre-replenishing lithium technologies. One is the negative electrode material lithium replenishment technology, which has high requirements for the operating environment. The lithium replenisher is generally metal lithium foil and inert lithium powder, which are too active and cannot be stored stably for a long time, thereby increasing the difficulty of operation and production risks; the other is the positive electrode material lithium replenishment technology, which requires relatively safer and easier operation.

[0004] At present, although the commonly used positive electrode lithium supplement composite additives have advantages such as high theoretical specific capacity, moderate operating voltage, less impact on existing production processes, and better safety, the existing lithium supplement additives all have problems such as poor structural stability and high residual alkalinity value, and are prone to gelation during battery homogenization. In addition, a large amount of gas is easily generated during the charging and discharging process, and a series of side reactions are triggered due to the unstable structure of the lithium supplement additive. In order to address the problem of poor structural stability of positive electrode lithium supplement composite additives, the stability of lithium supplement additives is generally improved by element substitution doping, carbon coating, etc. Among them, although element substitution doping can optimize the crystal framework structure of lithium supplement additives, it has a poor effect on improving the environmental stability of lithium supplement additives; and the existing carbon coating has poor stability in combination with lithium supplement additives. Therefore, the effect of improving the stability of lithium supplement additives needs to be further improved. Summary of the Invention

[0005] The purpose of this application is to provide a positive electrode lithium supplement composite additive and its preparation method, as well as a positive electrode material, a positive electrode sheet, and a secondary battery, aiming to solve to a certain extent the problems of poor environmental stability of existing positive electrode lithium supplementation and poor carbon coating layer bonding and coating effect.

[0006] To achieve the above application objectives, the technical solutions adopted in this application are as follows:

[0007] In a first aspect, the present application provides a positive electrode lithium-supplementing composite additive, which comprises a lithium-supplementing material core and a carbon coating layer grown on the outer surface of the lithium-supplementing material core; wherein the lithium-supplementing material core comprises a lithium-supplementing additive and a transition metal element doped within and / or between the structures of the lithium-supplementing additive.

[0008] In a second aspect, the present application provides a method for preparing a positive electrode lithium supplement composite additive, comprising the following steps:

[0009] obtaining a lithium supplement additive, and mixing the lithium supplement additive with a transition metal precursor to prepare a composite lithium supplement material doped with a transition metal element;

[0010] A carbon coating layer is generated on the surface of the composite lithium-replenishing material to obtain a positive electrode lithium-replenishing composite additive.

[0011] In a third aspect of the present application, a positive electrode material is provided, comprising a positive electrode active material and the positive electrode lithium replenishing additive described above or the positive electrode lithium replenishing additive prepared by the method described above.

[0012] In a fourth aspect, the present application provides a positive electrode sheet, which contains the above-mentioned positive electrode lithium-replenishing composite additive, or contains the positive electrode lithium-replenishing composite additive prepared by the above-mentioned method.

[0013] In a fifth aspect, the present application provides a secondary battery comprising the above-mentioned positive electrode sheet.

[0014] The positive electrode lithium supplement composite additive provided in the first aspect of the present application includes a lithium supplement material core, and the transition metal element is doped within the crystal structure framework and / or between the crystal structures of the lithium supplement additive, rather than replacing the framework sites in the crystal structure of the lithium supplement additive. This doping form enables the transition metal element doped in the lithium supplement material core to maintain catalytic activity, and the doped transition metal element has a strong affinity with the carbon source, so that the carbon source uses the doped transition metal element as the catalytic active center, and the carbon source continuously precipitates and grows into a carbon coating layer on the outer surface of the lithium supplement material core. The carbon coating layer directly catalytically grows on the surface of the lithium supplement material core, compared to the carbon coating layer formed by deposition, effectively improves the degree and quality of the carbon coating layer on the lithium supplement material core, improves the combination stability of the two, and thus improves the environmental stability of the positive electrode lithium supplement composite additive. In addition, the transition metal element doped in the structure and / or between the structures of the lithium supplement additive can further improve the structural stability of the lithium supplement additive through the influence of electrostatic forces between elements.

[0015] The second aspect of the present application provides a method for preparing a positive electrode lithium-replenishing composite additive, which comprises mixing a lithium-replenishing additive with a transition metal precursor so that the transition metal precursor is fully dispersed and doped in the lithium-replenishing additive, and then preparing a composite lithium-replenishing material doped with transition metal elements; and then preparing a carbon coating layer on the surface of the composite lithium-replenishing material. Since the composite lithium-replenishing material is doped with transition metal elements, it has high catalytic activity and can adsorb and react with carbon atoms to form carbides, so that the carbon source grows on the surface of the composite lithium-replenishing material to form a carbon coating layer, thereby improving the bonding stability and tightness of the carbon coating layer with the core lithium-replenishing material. The preparation method of the positive electrode lithium-replenishing composite additive of the present application has a simple process. The prepared positive electrode lithium-replenishing composite additive has a lithium-replenishing material doped with transition metal elements in the gap as the core, and carbon grown on the outer surface of the core of the lithium-replenishing material as the coating layer, which improves the structural stability and environmental stability of the positive electrode lithium-replenishing composite additive, thereby improving the lithium-replenishing effect of the positive electrode lithium-replenishing composite additive.

[0016] The positive electrode material provided in the third aspect of the present application includes a positive electrode active material and the above-mentioned positive electrode lithium-replenishing composite additive. The additive has a lithium-replenishing material doped with transition metal elements in the gaps as a core, and carbon grown on the outer surface of the core of the lithium-replenishing material as a coating layer. It not only has a good lithium-replenishing capacity, but also has good structural stability and environmental stability. It can effectively resist the influence of external environmental factors such as moisture and carbon dioxide, thereby ensuring the lithium-replenishing effect of the additive.

[0017] The positive electrode sheet provided in the fourth aspect of the present application, because it contains the above-mentioned positive electrode material, which includes a positive electrode active material and a positive electrode lithium replenishing composite additive, not only has a good lithium replenishing capacity, but also has good structural stability and environmental stability, and can effectively resist the influence of external environmental factors such as moisture and carbon dioxide, thereby ensuring the lithium replenishing effect of the additive. The positive electrode sheet has a good lithium replenishing effect and high safety, and can effectively improve the electrochemical properties of the positive electrode sheet, such as capacity retention, cycle life, and safety.

[0018] The secondary battery provided in the fifth aspect of the present application, because it includes the aforementioned positive electrode sheet, which is added with the aforementioned positive electrode lithium-replenishing composite additive, can effectively compensate for the active lithium ions consumed by the formation of the SEI film during the battery's initial charge, effectively maintaining the active lithium in the battery system and improving the battery's capacity retention rate. As a result, the secondary battery provided by the present application has high energy density and good capacity retention rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments or descriptions of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0020] Figure 1 It is a schematic flow chart of the preparation method of the positive electrode lithium supplement composite additive provided in the embodiment of the present application. DETAILED DESCRIPTION

[0021] In order to make the technical problems, technical solutions and beneficial effects to be solved by this application more clearly understood, the present application is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0022] In this application, the term "and / or" describes the relationship between associated objects, indicating that three possible relationships exist. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. A and B can be singular or plural. The character " / " generally indicates that the associated objects are in an "or" relationship.

[0023] In this application, "at least one" means one or more, and "plurality" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single items or plural items. For example, "at least one of a, b or c", or "at least one of a, b and c" can all mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, c can be single or multiple.

[0024] It should be understood that in the various embodiments of the present application, the size of the serial numbers of the above-mentioned processes does not mean the order of execution. Some or all of the steps can be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0025] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application. The singular forms "a", "an" and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms unless the context clearly indicates otherwise.

[0026] The weights of the relevant components mentioned in the examples of this application may not only refer to the specific content of each component, but also represent the weight ratio between the components. Therefore, as long as the content of the relevant components is proportionally enlarged or reduced according to the examples of this application, it is within the scope disclosed in the examples of this application. Specifically, the mass in the examples of this application may be μg, mg, g, kg, etc., which are mass units known in the chemical industry.

[0027] The terms "first" and "second" are used solely for descriptive purposes to distinguish objects, such as substances, from one another and should not be understood to indicate or imply relative importance or to implicitly specify the quantity of the technical features being referred to. For example, without departing from the scope of the embodiments of this application, a first XX may also be referred to as a second XX, and similarly, a second XX may also be referred to as a first XX. Thus, features defined as "first" or "second" may explicitly or implicitly include one or more of such features.

[0028] In a first aspect of an embodiment of the present application, a positive electrode lithium-supplementing composite additive is provided, which includes a lithium-supplementing material core and a carbon coating layer grown on the outer surface of the lithium-supplementing material core; wherein the lithium-supplementing material core includes a lithium-supplementing additive and a transition metal element doped within and / or between the structures of the lithium-supplementing additive.

[0029] The positive electrode lithium supplement composite additive provided in the first aspect of the embodiment of the present application includes a lithium supplement material core, which includes a lithium supplement additive and a transition metal element doped within the structure and / or between the structures of the lithium supplement additive. Specifically, the transition metal element is doped within the crystal structure framework of the lithium supplement additive and / or between the crystal structures, rather than replacing the framework sites in the crystal structure of the lithium supplement additive. This doping form enables the transition metal element doped in the lithium supplement material core to maintain catalytic activity, and the doped transition metal element has a strong affinity with the carbon source, so that the carbon source uses the doped transition metal element as the catalytic active center, and the carbon source continuously precipitates and grows into a carbon coating layer on the outer surface of the lithium supplement material core. The carbon coating layer directly catalytically grows on the surface of the lithium supplement material core, compared to the carbon coating layer formed by deposition, effectively improves the degree and quality of the carbon coating layer on the lithium supplement material core, improves the bonding stability of the two, and thus improves the environmental stability of the positive electrode lithium supplement composite additive. In addition, the transition metal element doped within the structure and / or between the structures of the lithium supplement additive can further improve the structural stability of the lithium supplement additive through the influence of electrostatic forces between elements.

[0030] In some embodiments, the transition metal element is selected from at least one of Ti, Mn, Mo, W, and Ce; these transition metal elements not only have a strong affinity for carbon atoms, but also have high catalytic activity. During the carbon coating process, they cannot maintain the corresponding metal form, but react with carbon to form a new carbide, which can not only improve the bonding stability and tightness between the growing carbon coating layer and the lithium-supplementing material core, but also this carbide can further serve as a catalyst in the growth process of the carbon coating layer, better promote the growth and formation of the carbon coating layer, and improve the degree and quality of the carbon layer coating on the outer surface of the lithium-supplementing material core.

[0031] In some embodiments, the transition metal element is doped in the crystal structure and / or between the crystal structures of the lithium supplement additive in the form of a transition metal element and / or a transition metal oxide. The transition metal element doped in the crystal structure and / or between the crystal structures of the lithium supplement additive in the embodiment of the present application can be doped in the form of a transition metal element or in the form of a transition metal oxide. In actual application, a reasonable selection can be made based on the catalytic effect of the transition metal element. If the transition metal element has better catalytic activity in the form of a single substance, it is doped in the form of a transition metal element; if the transition metal element has better catalytic activity in the form of an oxide, it is doped in the form of a transition metal oxide.

[0032] In some embodiments, in the core of the lithium-supplementing material, the molar ratio of the lithium-supplementing additive to the doped transition metal element is 1:(0.001-0.1). This doping molar ratio of the transition metal element not only ensures the improvement of the coating stability and coating effect of the transition metal element on the carbon coating layer, but also improves the structural stability of the lithium-supplementing material. If the doping molar ratio of the transition metal element is too high, the carbon coating layer formed will be too thick, which will hinder the ion transport in the core of the lithium-supplementing material and reduce the lithium-supplementing capacity of the positive electrode lithium-supplementing composite additive, thereby reducing the lithium-supplementing effect. In some specific embodiments, in the core of the lithium-supplementing material, the molar ratio of the lithium-supplementing additive to the doped transition metal element includes but is not limited to 1:(0.001-0.09), 1:(0.01-0.08), 1:(0.03-0.07), and 1:(0.04-0.06).

[0033] In some embodiments, the mass percentage of the carbon coating layer in the positive electrode lithium supplement composite additive is 2.0-5.5%. The positive electrode lithium supplement composite additive of the present application embodiment effectively increases the coating amount of the carbon coating layer by doping the transition metal element in the core of the lithium supplement material, and improves the coating effect and coating stability. In some specific embodiments, the mass percentage of the carbon coating layer in the positive electrode lithium supplement composite additive includes but is not limited to 2.0%-5.5%, 2.8%-5.0%, 4.0%-5.0%, 3.3%-4.8%, 4.5%-4.8%, etc.

[0034] In some embodiments, the chemical formula of the lithium supplement additive is Li x M y O z , where 0 < x ≤ 5, 0 < y ≤ 3, 0 < z ≤ 4, and M is selected from at least one of Fe, Co, Ni, Mn, and Cu. This lithium-supplementing material can provide abundant lithium, effectively releasing lithium during the initial charge cycle and replenishing the irreversible lithium ions consumed by the formation of the SEI film at the negative electrode, thereby increasing the lithium content of the lithium cathode material in the electrode, thereby improving the electrode's capacity and cycle performance.

[0035] In some embodiments, the positive electrode lithium-replenishing composite additive further includes an encapsulation layer coated on the outer surface of the carbon coating layer, the encapsulation layer comprising at least one of an isolation encapsulation layer, an ion conductor encapsulation layer, and an electronic conductor encapsulation layer. These encapsulation layers can effectively improve the electronic and ion conductivity of the lithium-replenishing material in the core, enhancing lithium release during charging. They can also serve to isolate moisture, improving the stability of the positive electrode lithium-replenishing composite additive and achieving a stable lithium-replenishing effect. Furthermore, the stability, uniformity of dispersion, and good processing performance of the positive electrode lithium-replenishing composite additive in the electrode active slurry and active layer can be further improved. Specifically, the isolation encapsulation layer further protects the positive electrode lithium-replenishing composite additive, preventing the core of the lithium-replenishing material from contact with water and carbon dioxide in the environment; the electronic conductor encapsulation layer can enhance the electronic conductivity of the coating layer, thereby enhancing the electronic conductivity of the lithium-replenishing composite additive and reducing the internal impedance of the electrode; and the ion conductor encapsulation layer can enhance the ionic conductivity of the positive electrode lithium-replenishing composite additive, thereby enhancing the ionic conductivity of the lithium-replenishing composite additive and facilitating the outward transport of lithium ions from the core of the lithium-replenishing material.

[0036] In some embodiments, the encapsulation layer can be a separate isolation encapsulation layer that fully encapsulates the lithium-supplementing material core and the carbon coating layer to play a protective role and further improve the stability of the lithium-supplementing material core. It can also be a composite laminated structure of an isolation encapsulation layer and an electronic conductor encapsulation layer. The preferred structure is that the isolation encapsulation layer is coated on the outer surface of the carbon coating layer, and the electronic conductor encapsulation layer is coated on the outer surface of the isolation encapsulation layer. It can also be a composite laminated structure of an isolation encapsulation layer and an ion conductor encapsulation layer. The preferred structure is that the isolation encapsulation layer is coated on the outer surface of the carbon coating layer, and the ion conductor encapsulation layer is coated on the outer surface of the isolation encapsulation layer. It can also be a composite laminated structure of an isolation encapsulation layer, an electronic conductor encapsulation layer, and an ion conductor encapsulation layer. The preferred structure is that the isolation encapsulation layer is coated on the outer surface of the carbon coating layer, the ion conductor encapsulation layer is coated on the outer surface of the isolation encapsulation layer, and the electronic conductor encapsulation layer is coated on the outer surface of the ion conductor encapsulation layer; or, the isolation encapsulation layer is coated on the outer surface of the carbon coating layer, the electronic conductor encapsulation layer is coated on the outer surface of the isolation encapsulation layer, and the ion conductor encapsulation layer is coated on the outer surface of the electronic conductor encapsulation layer.

[0037] In some embodiments, the material of the isolation packaging layer includes at least one of ceramic, polymer, or carbon material. In some specific embodiments, the ceramic includes at least one of Al2O3, SiO2, boehmite, Si3N4, SiC, and BN. In some specific embodiments, the polymer includes [C6H7O6Na] n Organic polymers with the structure [C6H7O2(OH)2OCH2COONa] n Organic polymers with the structure [C3H4O2] n Organic polymers with the structure [C3H3O2Ma ] n Organic polymers with the structure [C3H3N] n An organic polymer with the structure -[CH2-CF2] n -structured organic polymers, organic polymers containing -[NHCO]-structures, organic polymers containing an imide ring -[CO-N-CO]-structure on the main chain, and one or more of polyvinyl pyrrolidone, wherein M a It is an alkali metal element. Specifically, the polymer includes one or more of polyvinylidene fluoride, sodium alginate, sodium carboxymethyl cellulose, polyacrylic acid, polyacrylic acid salt, polyacrylonitrile, polyamide, polyimide, polyvinyl pyrrolidone, polyethylene oxide (PEO), polypyrrole (PPy), polytetrafluoroethylene (PTFE), and polyurethane (PU). Further, the polymer includes one or more of sodium carboxymethyl cellulose and polyacrylic acid. Sodium carboxymethyl cellulose and polyacrylic acid are two-dimensional surface-type polymers with good adhesion, which can effectively coat the core of the lithium-rich material, thereby avoiding contact between the core of the lithium-rich material and the air and improving the stability of the lithium-supplementing additive. In some embodiments, the molecular weight of the polymer is greater than or equal to 100,000. The molecular weight of the polymer can be, but is not limited to, 100,000, 150,000, 200,000, 300,000, 500,000 or 1 million. The larger the molecular weight of the polymer, the higher the density and structural strength of the polymer layer, and the more conducive to achieving protection of the core of the lithium-rich material. In some specific embodiments, the carbon material includes at least one of graphene, carbon nanotubes, amorphous carbon, graphite, and carbon black.

[0038] In some embodiments, the thickness of the isolation and encapsulation layer is 5-200 nm, more preferably 5-50 nm. By adjusting the material and thickness of the isolation and encapsulation layer, the present invention can further improve the barrier between water and carbon dioxide and the lithium source core in the lithium supplement material core, thereby improving the stability of the lithium source core.

[0039] In some embodiments, the material of the electronic conductor encapsulation layer includes at least one of a carbon material, a conductive polymer, or a conductive oxide. In some specific embodiments, the carbon material includes at least one of mesoporous carbon, carbon nanotubes, graphite, carbon black, graphene, etc. The conductive polymer may be, but is not limited to, the conductive polymer included in the isolation encapsulation layer described above. The conductive oxide includes at least one of In2O3, ZnO, and SnO2.

[0040] In some embodiments, the thickness of the electronic conductor encapsulation layer is 5-200 nm, more preferably 5-50 nm. By adjusting the thickness of the electronic conductor encapsulation layer, the electronic conductivity of the positive electrode lithium supplement composite additive can be further improved.

[0041] In some embodiments, the material of the ion conductor encapsulation layer includes at least one of a perovskite-type, a NASICON-type, a garnet-type, or a polymer-type solid electrolyte. In some specific embodiments, the perovskite-type includes Li 3x La 2 / 3-x TiO3(LLTO), specifically Li 0.5 La 0.5 TiO3、Li 0.33 La 0.57 TiO3、Li 0.29 La 0.57 TiO3、Li 0.33 Ba 0.25 La 0.39 TiO3、(Li 0.33 La 0.56 ) 1.005 Ti 0.99 Al 0.01 O3、Li 0.5 La 0.5 Ti 0.95 Zr 0.05 At least one of O3, etc.; NASICON type includes but is not limited to Li 1.4 Al 0.4 Ti 1.6 (PO4)3(LATP); garnet type including but not limited to Li7La3Zr2O 12 (LLZO), Li 6.4 La3Zr 1.4 Ta 0.6 O 12 , Li 6.5 La3Zr 1.5 Ta 0.5 O 12 At least one of the following: the polymer solid electrolyte includes at least one of PEO / PPO / PVDF etc. which dissolves lithium salt.

[0042] In some embodiments, the thickness of the ion conductor encapsulation layer is 5-200 nm, more preferably 5-50 nm. The present invention can further improve the ionic conductivity of the positive electrode lithium supplement composite additive by adjusting the thickness and material of the ion conductor encapsulation layer.

[0043] Based on the structure and performance of the positive electrode lithium-replenishing composite additives described in the examples of the present application, the positive electrode lithium-replenishing composite additives described in the examples of the present application have excellent storage and processability, as well as stable electrochemical performance. Testing has shown that the capacity decay rate of a positive electrode sheet directly prepared from the positive electrode lithium-replenishing composite additives of the present application, such as a positive electrode sheet prepared from the positive electrode lithium-replenishing composite additives, a binder, and a conductive agent, after 24 hours of storage at an ambient humidity of 25% relative to the capacity decay rate after 0 hours of storage is no greater than 30%, further no greater than 25%, and even further no greater than 17.3%. Ideally, the lithium-replenishing additives described in the examples of the present application are stored in a dry, oxygen-free environment, such as a vacuum environment, to maximize the electrochemical performance of the lithium-replenishing additives described in the examples of the present application.

[0044] The positive electrode lithium supplement composite additive of the embodiment of the present application can be prepared by the following embodiment method.

[0045] As attached Figure 1 As shown, the second aspect of the embodiment of the present application provides a method for preparing a positive electrode lithium supplement composite additive, comprising the following steps:

[0046] S10 obtains a lithium supplement additive, and mixes the lithium supplement additive with a transition metal precursor to prepare a composite lithium supplement material doped with a transition metal element;

[0047] S20. Generate a carbon coating layer on the surface of the composite lithium replenishing material to obtain a positive electrode lithium replenishing composite additive.

[0048] The second aspect of the embodiment of the present application provides a method for preparing a positive electrode lithium supplement composite additive, which comprises mixing a lithium supplement additive with a transition metal precursor so that the transition metal precursor is fully dispersed and doped in the lithium supplement additive, and then preparing a composite lithium supplement material doped with transition metal elements; and then preparing a carbon coating layer on the surface of the composite lithium supplement material. Since the composite lithium supplement material is doped with transition metal elements, it has high catalytic activity and can adsorb and react with carbon atoms to form carbides, so that the carbon source grows on the surface of the composite lithium supplement material to form a carbon coating layer, thereby improving the bonding stability and tightness of the carbon coating layer with the core lithium supplement material. The method for preparing a positive electrode lithium supplement composite additive in the embodiment of the present application has a simple process. The prepared positive electrode lithium supplement composite additive has a lithium supplement material doped with transition metal elements in the gap as the core, and carbon grown on the outer surface of the core of the lithium supplement material as the coating layer, which improves the structural stability and environmental stability of the positive electrode lithium supplement composite additive, thereby improving the lithium supplement effect of the positive electrode lithium supplement composite additive.

[0049] The present embodiment does not specifically limit the type of lithium-supplementing additive in step S10. Any lithium-supplementing additive that meets the application requirements can be used. The preparation method of the lithium-supplementing additive includes, but is not limited to, mixing a lithium source and a metal source, and then calcining the mixture in an inert atmosphere to obtain the lithium-supplementing additive.

[0050] In some embodiments, the steps of preparing a composite lithium-supplementing material doped with transition metal elements include: mixing and grinding the lithium-supplementing additive with a transition metal precursor, and then performing a reduction or oxidation treatment to obtain a composite lithium-supplementing material doped with transition metal elements. In the embodiment of the present application, the lithium-supplementing additive and the transition metal precursor are mixed and ground so that the transition metal precursor is fully mixed and doped with the lithium-supplementing additive and evenly distributed in the lithium-supplementing additive; then a reduction treatment is performed to reduce the transition metal precursor to a transition metal element, or an oxidation treatment is performed to convert the transition metal precursor into a transition metal oxide, thereby obtaining a composite lithium-supplementing material doped with transition metal elements. In the actual preparation process, reduction treatment or oxidation treatment can be selected according to the catalytic activity of the transition metal element; that is, if the transition metal has a higher catalytic activity in the form of a element, a reduction treatment is performed; if the transition metal has a higher catalytic activity in the form of an oxide, an oxidation treatment is performed.

[0051] In other embodiments, a transition metal precursor may be mixed with a precursor of a lithium-supplementing material. While the precursor of the lithium-supplementing material reacts to generate a lithium-supplementing additive material, the transition metal precursor may be converted into a transition metal oxide and doped into the structure of or between the structures of the lithium-supplementing additive material, thereby achieving more uniform doping of the transition metal element.

[0052] In some embodiments, the transition metal precursor includes at least one of a transition metal nitrate, a transition metal sulfate, a transition metal carbonate, a transition metal hydroxide, and a transition metal ammonium salt. In some embodiments, the transition metal element in the transition metal precursor includes at least one of titanium, manganese, molybdenum, tungsten, cerium, etc. In some specific embodiments, the transition metal precursor includes but is not limited to Ce(NO3)3, (NH4) 10 W 12 O 41 , (NH4)2MoO4, etc. These transition metal precursors are easily reduced to transition metal elements or oxidized to transition metal oxides, and other elements are volatilized and removed in gaseous form and are not easy to remain.

[0053] In some embodiments, in step S20, forming a carbon coating on the surface of the composite lithium-replenishing material includes mixing the composite lithium-replenishing material with a carbon material and then calcining the mixture. Under the high-temperature calcination conditions, the carbon material, with the transition metal element doped in the composite lithium-replenishing material as a catalytic center, adsorbs the carbon material to form a transition metal carbide. The carbide can further serve as a catalyst for the formation of the carbon coating, further promoting the growth of the carbon coating and improving the bonding stability and tightness between the carbon coating and the composite lithium-replenishing material. The resulting positive electrode lithium-replenishing composite additive with the carbon coating grown on its surface has better structural stability and environmental stability, thereby improving its lithium-replenishing effect.

[0054] In some embodiments, the calcination treatment conditions include: keeping warm in an inert atmosphere and / or reducing atmosphere at a temperature of 400-600°C for 1-3 hours; the calcination conditions are conducive to the growth of the carbon coating layer. If the temperature is too low or the holding time is too short, the growth rate and effect of the carbon coating layer will be reduced; if the temperature is too high or the holding time is too long, the performance of the composite lithium supplement material will be affected and the lithium supplement effect will be reduced.

[0055] In some embodiments, the carbon material is selected from at least one of polyaniline, polyethylene, and benzene. These organic carbon materials easily react with transition metal elements to form carbides during high-temperature calcination, so that the carbon material is transformed and grown into a carbon coating layer on the surface of the composite lithium supplement material.

[0056] In some embodiments, after forming a carbon coating on the surface of the composite lithium-supplementing material, the method further includes forming at least one of an isolation encapsulation layer, an ion conductor encapsulation layer, and an electronic conductor encapsulation layer on the surface of the carbon coating. Methods for forming the isolation encapsulation layer, the ion conductor encapsulation layer, and the electronic conductor encapsulation layer include, but are not limited to, chemical deposition, magnetron sputtering, or atomic layer deposition.

[0057] In some specific embodiments, when the material of the packaging layer is a ceramic layer, the step of preparing the ceramic packaging layer can be, but not limited to, magnetron sputtering, in which a ceramic target material is sputtered onto the surface of the carbon coating layer to deposit a ceramic packaging layer, wherein the conditions of magnetron sputtering are adjusted according to the specific properties of the target material.

[0058] In some specific embodiments, when the encapsulation layer is a polymer layer, the step of forming the polymer isolation encapsulation layer may include dispersing the carbon-coated composite lithium-replenishing material in a solution containing the polymer, followed by vacuum drying to form a dense polymer encapsulation layer on the surface of the carbon coating layer. The solvent of the solution is a solvent capable of uniformly dispersing or dissolving the polymer, such as one or more of N-methylpyrrolidone, methanol, ethanol, isopropanol, acetone, tetrahydrofuran, and diethyl ether.

[0059] In some specific embodiments, when the material of the encapsulation layer is a carbon material layer, the method for forming a carbon material isolation encapsulation layer includes the following steps: dispersing a carbon-coated composite lithium supplement material in a solution containing a carbon source, performing a carbonization treatment after drying, and forming a dense carbon encapsulation layer on the carbon-coated surface. Among them, the carbon source can be, but is not limited to, PEO, and can also be other carbon sources. As long as it can form a coated carbon source layer on the surface of the lithium supplement material, it is suitable for the present invention. Specifically, the carbon-coated lithium supplement material is evenly mixed with PEO, PEO reaches its melting point at 300°C, and is evenly coated on the surface of the carbon-coated lithium supplement material. The coated material is sintered in an inert atmosphere at 600 degrees Celsius for 6 hours. After sintering, a dense carbon layer is formed.

[0060] Thirdly, the embodiments of the present application further provide a positive electrode material. The positive electrode material of the embodiments of the present application comprises a positive electrode active material and the positive electrode lithium replenishing additive of the embodiments of the present application. Thus, the positive electrode material of the embodiments of the present application has excellent lithium replenishing performance and good processing properties, can improve the quality of the positive electrode active material layer, thereby improving the quality of the positive electrode active material layer and imparting electrochemical performance to the corresponding positive electrode sheet.

[0061] In the embodiment, the content of the positive electrode lithium supplement additive in the positive electrode material of the embodiment of the present application can be controlled. The mass content of the positive electrode material in the positive electrode active layer of the embodiment of the present application can be 0.1 to 10%.

[0062] In some embodiments, the mass percentage of the positive electrode lithium replenishing composite additive in the positive electrode material is 0.1-10%. This ratio can just compensate for the loss of active lithium in the battery during the first charge process. Since most of the lithium provided by the positive electrode lithium replenishing composite additive cannot be circulated during the operation of the battery, if the amount of the positive electrode lithium replenishing composite additive added to the positive electrode sheet is too high, the excessive lithium will cause lithium ions to precipitate on the negative electrode surface during the operation of the battery, forming lithium dendrites; if the amount of the positive electrode lithium replenishing composite additive added to the positive electrode sheet is too low, the active lithium lost in the positive electrode material cannot be fully replenished, which is not conducive to improving the energy density and capacity retention rate of the battery. In some specific embodiments, the mass percentage of the positive electrode lithium replenishing composite additive in the positive electrode material includes but is not limited to 0.1-1%, 1-2%, 2-5%, 5-8%, 8-10%, etc.

[0063] In other embodiments, the positive electrode active material contained in the positive electrode material of the embodiment of the present application can be a phosphate positive electrode active material, a ternary positive electrode active material or a lithium transition metal oxide. In specific embodiments, it includes one or more of lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxyphosphate, lithium fluorovanadium phosphate, lithium titanate, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide.

[0064] A fourth aspect of the embodiments of the present application provides a positive electrode sheet, which contains the above-mentioned positive electrode lithium-replenishing composite additive, or contains the positive electrode lithium-replenishing composite additive prepared by the above-mentioned method.

[0065] The positive electrode sheet provided in the fourth aspect of the present application comprises the aforementioned positive electrode lithium-replenishing composite additive, which comprises a lithium-replenishing material interstitially doped with a transition metal element and a carbon coating formed on the outer surface of the lithium-replenishing material core. The additive not only has a good lithium-replenishing capacity but also exhibits excellent structural and environmental stability, effectively resisting the effects of environmental factors such as moisture and carbon dioxide, thereby ensuring the additive's lithium-replenishing effect. Consequently, the positive electrode sheet exhibits excellent lithium-replenishing effects and high safety, effectively improving the electrochemical properties of the positive electrode sheet, such as capacity retention, cycle life, and safety.

[0066] In some embodiments, the positive electrode sheet includes a current collector and an active material layer laminated together, and the weight percentage of the positive electrode lithium-replenishing composite additive in the active material layer is 0.1-10%. This ratio can just compensate for the loss of active lithium during the battery's initial charge. Because most of the lithium provided by the positive electrode lithium-replenishing composite additive cannot circulate during battery operation, if the amount of the positive electrode lithium-replenishing composite additive added to the positive electrode sheet is too high, the excess lithium will cause lithium ions to precipitate on the negative electrode surface during battery operation, forming lithium dendrites. If the amount of the positive electrode lithium-replenishing composite additive added to the positive electrode sheet is too low, the active lithium lost in the positive electrode material cannot be fully replenished, which is not conducive to improving the battery's energy density and capacity retention. In some specific embodiments, the weight percentage of the positive electrode lithium-replenishing composite additive in the active material layer of the positive electrode sheet includes, but is not limited to, 0.1-1%, 1-2%, 2-5%, 5-8%, 8-10%, etc.

[0067] In some embodiments, the positive electrode active material in the positive electrode sheet includes but is not limited to at least one of lithium iron phosphate, lithium cobalt oxide, lithium manganese iron phosphate, lithium manganese oxide, lithium nickel cobalt manganese oxide, and lithium nickel manganese oxide. These positive electrode materials have high specific capacity, which is beneficial to improving the energy density of the battery.

[0068] In some embodiments, the positive electrode current collector includes but is not limited to any one of copper foil and aluminum foil.

[0069] In some embodiments, the positive electrode active layer also includes components such as a conductive agent and a binder. The embodiments of the present application do not specifically limit these materials, and appropriate materials can be selected according to actual application requirements.

[0070] In some embodiments, the binder content in the positive electrode active layer is 2 wt% to 4 wt%. In specific embodiments, the binder content can be 2 wt%, 3 wt%, 4 wt%, and other typical but non-limiting amounts. In specific embodiments, the binder includes one or more of polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethylcellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives.

[0071] In some embodiments, the conductive agent comprises 3 wt% to 5 wt% of the positive electrode active layer. In specific embodiments, the conductive agent comprises 3 wt%, 4 wt%, 5 wt%, or other typical but non-limiting amounts. In specific embodiments, the conductive agent comprises one or more of graphite, carbon black, acetylene black, graphene, carbon fiber, C60, and carbon nanotubes.

[0072] In some embodiments, the preparation process of the positive electrode sheet is: mixing the positive electrode active material, the positive electrode lithium supplement composite additive, the conductive agent and the binder to obtain an electrode slurry, coating the electrode slurry on the current collector, and preparing the positive electrode sheet through steps such as drying, rolling, and die cutting.

[0073] A fifth aspect of an embodiment of the present application provides a secondary battery, which includes the above-mentioned positive electrode sheet.

[0074] The secondary battery provided in the fifth aspect of the present application comprises the aforementioned positive electrode sheet, which is supplemented with the aforementioned positive electrode lithium-replenishing composite additive. This effectively compensates for the active lithium ions consumed by the formation of the SEI film during the initial charge of the battery, effectively maintaining the active lithium in the battery system and improving the battery's capacity retention. Consequently, the secondary battery provided in the present application has a high energy density and a good capacity retention rate.

[0075] The secondary battery in the embodiment of the present application may be a lithium ion battery or a lithium metal battery.

[0076] The negative electrode sheet, electrolyte, separator, etc. of the secondary battery in the embodiment of the present application are not specifically limited and can be applied to any battery system.

[0077] In order to make the above implementation details and operations of this application clearly understood by those skilled in the art, as well as to demonstrate the significant improvement in performance of the positive electrode lithium supplement composite additive and its preparation method, positive electrode sheet, and secondary battery in the embodiments of this application, the above technical solution is illustrated by multiple embodiments below.

[0078] Example 1

[0079] A positive electrode lithium supplement composite additive, the preparation of which comprises the following steps:

[0080] 1. Li2CO3 and Fe(OH)3 were mechanically crushed in a molar ratio of 2.5:1, stirred evenly, sintered at 850°C for 12h under an argon atmosphere, crushed, and sieved to obtain Li5FeO4 lithium supplement additive.

[0081] 2. The above-mentioned Li5FeO4 lithium supplement additive and Ce(NO3)3 were mixed evenly in a molar ratio of 1:0.02, sintered at 550°C for 3h under an argon atmosphere, crushed, and sieved to obtain a Li5FeO4·0.01Ce2O3 composite lithium supplement additive.

[0082] 3. First, dissolve polyaniline in NMP, mix it evenly with the composite lithium supplement additive, place it in a reactor, react at 200°C for 2 hours, and then vacuum dry it to obtain C&Li5FeO4·0.01Ce2O3 coated with a carbon layer.

[0083] Example 2

[0084] A positive electrode lithium supplement composite additive, the preparation of which comprises the following steps:

[0085] 1. LiOH and CS2 were mechanically crushed and stirred in a molar ratio of 4:1, sintered at 800 °C for 10 h under a nitrogen atmosphere, crushed, and sieved to obtain Li2S lithium supplement additive.

[0086] 2. The above-mentioned Li2S lithium supplement additive and (NH4) 10 W 12 O 41 After being uniformly mixed in a molar ratio of 1:0.005, the mixture was sintered at 550°C for 3 h under an argon atmosphere, crushed, and sieved to obtain a Li2S·0.06WO3 composite lithium supplement additive.

[0087] 3. First, polyethylene and the composite lithium supplement additive are mixed evenly, placed in a nitrogen atmosphere muffle furnace, and sintered at 500°C for 2 hours to obtain C&Li2S·0.06WO3 coated with a carbon layer.

[0088] Example 3

[0089] A positive electrode lithium supplement composite additive, the preparation of which comprises the following steps:

[0090] 1. Li2O and Ni(OH)2 were mechanically crushed in a molar ratio of 1:1, stirred evenly, sintered at 900°C for 8h under a nitrogen atmosphere, crushed, and sieved to obtain Li2NiO2 lithium supplement additive.

[0091] 2. The above-mentioned Li2NiO2 lithium supplement additive and (NH4)2MoO4 were mixed uniformly in a molar ratio of 1:0.005, sintered at 450°C for 2h in a nitrogen and hydrogen atmosphere, crushed, and sieved to obtain a Li2NiO2·0.005Mo composite lithium supplement additive.

[0092] 3. First, benzene and the composite lithium supplement additive are evenly mixed, placed in a reactor, reacted at 300°C for 2 hours, and then vacuum dried to obtain C&Li2NiO2·0.005Mo coated with a carbon layer.

[0093] Example 4

[0094] A positive electrode lithium supplement composite additive, the preparation of which differs from that of Example 1 in that: in step 2, the Li5FeO4 lithium supplement additive and Ce(NO3)3 are mixed in a ratio of 1:0.1 and then reacted.

[0095] Example 5

[0096] A positive electrode lithium supplement composite additive, the preparation of which differs from that of Example 1 in that: in step 2, the Li5FeO4 lithium supplement additive and Ce(NO3)3 are mixed in a ratio of 1:0.12 and then reacted.

[0097] Comparative Example 1

[0098] A positive electrode lithium supplement composite additive, the preparation of which comprises the following steps:

[0099] 1. Li2CO3 and Fe(OH)3 were mechanically crushed in a molar ratio of 2.5:1, stirred evenly, sintered at 850°C for 12h under an argon atmosphere, crushed, and sieved to obtain Li5FeO4 lithium supplement additive.

[0100] 2. First, dissolve polyaniline in NMP, mix it evenly with Li5FeO4 lithium supplement additive, place it in a reactor, react at 200℃ for 2h, and then vacuum dry to obtain C&Li5FeO4 coated with a carbon layer.

[0101] Comparative Example 2

[0102] A positive electrode lithium supplement composite additive, the preparation of which comprises the following steps:

[0103] 1. LiOH and CS2 were mechanically crushed and stirred in a molar ratio of 4:1, sintered at 800 °C for 10 h under a nitrogen atmosphere, crushed, and sieved to obtain Li2S lithium supplement additive.

[0104] 2. First, polyethylene and Li2S lithium supplement additive are mixed evenly, placed in a nitrogen atmosphere muffle furnace, and sintered at 500℃ for 2h to obtain C&Li2S·0.06WO3 coated with a carbon layer.

[0105] Comparative Example 3

[0106] A positive electrode lithium supplement composite additive, the preparation of which comprises the following steps:

[0107] 1. Li2O and Ni(OH)2 were mechanically crushed in a molar ratio of 1:1, stirred evenly, sintered at 900°C for 8h under a nitrogen atmosphere, crushed, and sieved to obtain Li2NiO2 lithium supplement additive.

[0108] 2. The above-mentioned Li2NiO2 lithium supplement additive and Al(NO3)3 were mixed evenly in a ratio of 1:2, sintered at 750°C for 5h under a nitrogen atmosphere, crushed, and sieved to obtain a Li2NiO2·Al2O3 composite lithium supplement additive.

[0109] 3. First, benzene and the composite lithium supplement additive are mixed evenly, placed in a reactor, reacted at 300°C for 2 hours, and then vacuum dried to obtain C&Li2NiO2·Al2O3 coated with a carbon layer.

[0110] In order to verify the progress of the examples of the present application, the gram capacity and carbon content of the positive electrode lithium supplementation composite materials prepared in Examples 1 to 5 and Comparative Examples 1 to 3 were tested respectively. The test results are shown in Table 1 below:

[0111] Table 1

[0112]

[0113]

[0114] As can be seen from the test results in Table 1 above, the positive electrode lithium supplement composite additives prepared in Examples 1 to 5 of the present application not only maintain a high gram capacity but also increase the coating amount of the carbon coating layer. The gram capacity of Example 3 after being placed at a humidity of about 25% for 24 hours shows a certain degree of attenuation, and the gram capacity attenuation rate relative to 0 hours is no more than 17.3%. The attenuation rates of the other examples of the present application are all no more than 16%. The gram capacity of the composite lithium supplement additive at 25% humidity is much higher than that of conventional lithium supplement additives, indicating that the carbon coating layer has a good bonding stability with the lithium supplement material core, and doping can effectively improve the environmental stability of the additive. However, in Comparative Examples 1 and 2, no transition metal elements are doped, the carbon coating effect is low, and the gram capacity of the composite additive is reduced, especially when placed at 25% humidity for 24 hours. The non-transition metal Al doped in Comparative Example 2 also does not improve the coating effect of the carbon material, and at the same time reduces the gram capacity of the composite additive. (Note: Attenuation rate = (1-gram capacity after 24 hours of placement / gram capacity after 0 hours of placement) * 100%).

[0115] Furthermore, in order to verify the lithium replenishing effect of the positive electrode lithium replenishing composite additive in the battery, the mixture of the positive electrode lithium replenishing additive and lithium iron phosphate provided in the above Examples 1 to 5 and Comparative Examples 1 to 3 was mixed with SP:PVDF respectively according to the ratio of (lithium iron phosphate + positive electrode lithium replenishing composite additive): SP:PVDF = 95:2:3, and the mixing method was ball milling, and the ball milling time was 60min; the rotation speed was set to 30HZ: after the homogenization-coating-drying-cutting operations, positive electrode sheets were prepared respectively. Then, button-type lithium-ion batteries were assembled in an inert atmosphere glove box in the assembly order of lithium metal sheet-diaphragm-electrolyte-positive electrode sheet. The lithium-ion batteries prepared in Examples 1 to 5 and Comparative Examples 1 to 2 were charged to 4.3V at 0.05C, and constant voltage at 4.3V until the current was less than 0.01C; their first charge gram capacity and cycle capacity retention rate were tested, and the test results are shown in Table 2 below:

[0116] Table 2

[0117]

[0118] The above test results show that compared to the lithium-supplementing composite additives prepared in Comparative Examples 1 and 2 without transition metal doping, and the non-transition metal aluminum doping in Comparative Example 3, the positive-electrode lithium-supplementing composite materials prepared in Examples 1 to 5 of the present application exhibited higher initial charge capacity, cycle capacity retention, and cycle stability when added to lithium iron phosphate, indicating that the positive-electrode materials incorporating the positive-electrode lithium-supplementing composite additives prepared in the present application have higher charge capacity and stability. Furthermore, due to the excessively high amount of transition metal doped in Example 5, the carbon coating layer was too thick, which to some extent reduced the efficiency of lithium ion migration and transport, resulting in a lower lithium-supplementing effect than in Examples 1 to 4, which had a doping molar ratio of 0.001 to 0.1.

[0119] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present application should be included in the scope of protection of the present application.

Claims

1. A positive electrode lithium supplement composite additive, characterized in that: The positive electrode lithium supplement composite additive comprises a lithium supplement material core and a carbon coating layer grown on the outer surface of the lithium supplement material core; wherein the lithium supplement material core comprises a lithium supplement additive and a transition metal element doped in the structure and / or between the structures of the lithium supplement additive; the transition metal element is doped in the crystal structure and / or between the crystal structures of the lithium supplement additive in the form of a transition metal element and / or a transition metal oxide; the chemical formula of the lithium supplement additive is Li x M y O z , wherein 0<x≤5, 0<y≤3, 0<z≤4, M is selected from at least one of Fe, Co, Ni, Mn, and Cu; the transition metal element is selected from at least one of Ti, Mn, Mo, W, and Ce; the molar ratio of the lithium supplement additive to the doped transition metal element is 1:(0.001~0.1); the transition metal element reacts with carbon to form carbide, which catalyzes the growth and formation of the carbon coating layer.

2. The positive electrode lithium supplement composite additive according to claim 1, characterized in that In the positive electrode lithium supplement composite additive, the mass percentage of the carbon coating layer is 2.0-5.5%.

3. The positive electrode lithium supplement composite additive according to claim 2, characterized in that The positive electrode lithium supplement composite additive further includes an encapsulation layer coated on the outer surface of the carbon coating layer.

4. The positive electrode lithium supplement composite additive according to claim 3, characterized in that The packaging layer includes at least one of an isolation packaging layer, an ion conductor packaging layer, and an electronic conductor packaging layer.

5. The positive electrode lithium supplement composite additive according to any one of claims 1 to 4, characterized in that: The capacity attenuation rate of the positive electrode sheet prepared by the positive electrode lithium supplement composite additive, the conductive agent and the binder after being stored for 24 hours at an ambient humidity of 25% relative to that of the positive electrode sheet stored for 0 hours is not greater than 25%.

6. A method for preparing the positive electrode lithium supplement composite additive according to any one of claims 1 to 5, characterized in that: The following steps are involved: Obtaining a lithium supplement additive, and mixing the lithium supplement additive with a transition metal precursor to prepare a composite lithium supplement material doped with a transition metal element; the transition metal element in the transition metal precursor includes at least one of titanium, manganese, molybdenum, tungsten, and cerium; A carbon coating layer is generated on the surface of the composite lithium-replenishing material to obtain a positive electrode lithium-replenishing composite additive.

7. The method for preparing the positive electrode lithium supplement composite additive according to claim 6, wherein: The step of preparing the composite lithium-supplementing material doped with transition metal elements comprises: mixing and grinding the lithium-supplementing additive with the transition metal precursor, and then performing reduction or oxidation treatment to obtain the composite lithium-supplementing material doped with transition metal elements; And / or, the step of forming a carbon coating layer on the surface of the composite lithium supplement material comprises: mixing the composite lithium supplement material with a carbon material, and then calcining the mixture to obtain the positive electrode lithium supplement composite additive having a carbon coating layer grown on the surface; And / or, after the carbon coating layer is formed on the surface of the composite lithium supplementing material, the method further comprises the step of preparing at least one of an isolation encapsulation layer, an ion conductor encapsulation layer, and an electronic conductor encapsulation layer on the surface of the carbon coating layer.

8. The method for preparing the positive electrode lithium supplement composite additive according to claim 7, wherein: The transition metal precursor includes at least one of a transition metal nitrate, a transition metal sulfate, a transition metal carbonate, a transition metal hydroxide, and a transition metal ammonium salt; And / or, the calcination treatment conditions include: keeping the temperature at 400-600° C. in an inert atmosphere and / or a reducing atmosphere for 1-3 hours; And / or, the carbon material is selected from at least one of polyaniline, polyethylene, and benzene.

9. A positive electrode material, characterized in that The positive electrode lithium-replenishing composite additive comprises the positive electrode lithium-replenishing composite additive according to any one of claims 1 to 5, or the positive electrode lithium-replenishing composite additive prepared by the method according to any one of claims 6 to 8.

10. The positive electrode material according to claim 9, wherein The mass percentage of the positive electrode lithium supplement composite additive in the positive electrode material is 0.1-10%.

11. A positive electrode sheet, characterized in that: The positive electrode sheet contains the positive electrode material as claimed in claim 10.

12. A secondary battery, characterized in that: The secondary battery includes the positive electrode sheet according to claim 11.

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