Lithium supplementing material, preparation method thereof, positive electrode sheet and secondary battery

Abstract

The application discloses a lithium supplementing material, a preparation method thereof, a positive pole piece and a secondary battery. The lithium supplementing material comprises a core and a first carbon framework, and the first carbon framework forms a cage structure. The core comprises lithium-rich compound particles, and the core is accommodated in the first carbon framework of the cage structure. The lithium supplementing material provided by the application has high lithium supplementing specific capacity and excellent electronic conductivity.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, in particular to a lithium supplementing material, a preparation method thereof, a positive electrode sheet and a secondary battery. BACKGROUND

[0002] Lithium ion batteries are one of the best energy storage and power battery technologies in terms of comprehensive performance. However, during the initial charging process, lithium ions are transferred from the positive electrode to the negative electrode for storage, accompanied by the formation of a solid-state electrolyte (SEI) on the surface of the negative electrode. This process irreversibly consumes a portion of the active lithium and reduces the capacity and energy density of the battery. Pre-lithiation can introduce additional active lithium into the battery system and is very promising in terms of compensating for initial lithium loss and increasing the energy density of lithium ion batteries.

[0003] To date, various pre-lithiation methods have been explored, including electrochemical pre-lithiation, chemical pre-lithiation and the use of electrode pre-lithiation additives. Among them, lithium supplementing materials exhibit higher open-circuit voltage and better stability in atmospheric environment. In addition, some lithium supplementing materials generally have better compatibility with NMP (the most widely used solvent in the preparation of cathode slurry), making them more practical in the battery industry.

[0004] Among lithium supplementing materials, most binary lithium supplementing materials such as Li3N, Li2S, LiF, Li2O, Li2O2, Li3P, LiI, LiBr, etc. generally have a capacity greater than ternary lithium supplementing materials such as Li2NO2, Li2CoO2, etc. However, binary lithium supplementing materials often have problems such as poor ionic conductivity, poor electronic conductivity, instability in air, reaction with moisture, decomposition to produce gas, etc. SUMMARY

[0005] The purpose of the present application is to provide a lithium supplementing material, a preparation method thereof, a positive electrode sheet and a secondary battery.

[0006] The present application provides the following technical solutions:

[0007] In a first aspect, the present application provides a lithium supplementing material, comprising a core and a first carbon framework, wherein the core comprises lithium-rich compound particles; and the first carbon framework forms a cage structure, and the core is accommodated in the first carbon framework of the cage structure. The lithium supplementing material provided by the present application can enhance the ability of the lithium supplementing material to transport lithium ions outward by coating the core with the first carbon framework having a cage structure. On the one hand, the first carbon framework has strong conductivity, which can enhance the ability of the lithium supplementing material to transport lithium ions outward. On the other hand, the cage structure of the first carbon framework has a large specific surface area, which can further improve the lithium ion transport efficiency of the lithium supplementing material. In addition, the nano-space confinement effect of the precursor molecular cage of the first carbon framework on the precursor of the lithium-rich compound can realize the nano-structurization of the core and the lithium-rich compound particles contained therein. The nano-sized core and the lithium-rich compound particles contained therein have the structural advantages of smaller polarization and lower voltage platform during charging, so that the lithium supplementing material can have a lower activation barrier and can significantly improve the deintercalation efficiency of lithium ions, thereby realizing a higher specific capacity of the lithium supplementing material.

[0008] In a possible implementation, the first carbon framework has a plurality of holes, and part of the lithium-rich compound particles are embedded in the holes. Specifically, the first carbon framework has a cage structure, which is not limited in shape and can include a cube, a polyhedron, a sphere, etc. It can be understood that the cage structure is a structure formed by a plurality of rod-shaped or rope-shaped solids in a cross-linked manner, so the cage structure is a hollow structure, i.e., has holes, and the shape of the holes is not limited and can include a square, a polygon, a circle or an ellipse.

[0009] In a possible implementation, the lithium supplementing material comprises non-lithium compound particles having a first metal element and metal particles having a second metal element, wherein the first metal element and the second metal element are different, the first metal element comprises one or more of Zn, Fe, Cu, Ni and Co, and the second metal element comprises one or more of Co, Mo, Mn, Pb, Pt and Ru.

[0010] In a possible implementation, the mass ratio of the lithium-rich compound particles, the non-lithium compound particles and the metal particles is 100:(5-20):(2-8). By controlling the mass ratio of the lithium-rich compound particles, the non-lithium compound particles and the metal particles within the above range, it can be ensured that the lithium supplementing material has a suitable lithium ion supply amount, and the core has a suitable conductivity. Because none of the particles other than the lithium-rich compound particles can provide lithium ions, when the proportion of the lithium-rich compound particles is low, the lithium supplementing performance of the lithium supplementing material will be reduced.

[0011] In a possible implementation, the lithium-rich compound particles, the non-lithium compound particles and / or the metal particles are embedded in the network pores of the first carbon framework. Specifically, the formation of the lithium-rich compound particles, the non-lithium compound particles and the metal particles all need the participation of carbon, so after the organic matter in the metal-organic framework forms a carbon skeleton, the lithium ions and the coordinated metal ions need to react with the carbon on the carbon skeleton to form an etching of the first carbon framework, so that the first carbon framework can form a pore structure again.

[0012] In a possible implementation, the particle size D50 of the inner core is 200 nm to 300 nm. The particle size of the inner core in the implementation of the present application can be controlled to be sub-micron level by the confinement effect of the molecular cage. By greatly reducing the particle size, the sub-micron inner core can shorten the diffusion length and increase the active specific surface area, which is beneficial to the insertion and extraction of lithium ions, thereby increasing the capacity of the lithium supplement material. When the particle size of the inner core is less than the above range, the difficulty of preparing the molecular cage increases, thereby increasing the preparation cost of the lithium supplement material, which is not conducive to industrial production. When the particle size of the inner core is greater than the above range, the specific surface area of the inner core is reduced, which reduces the efficiency of the insertion and extraction of lithium ions, thereby resulting in poor lithium supplement effect of the lithium supplement material.

[0013] In a possible implementation, the particle size D50 of the lithium-rich compound particles is 3 nm to 8 nm. Specifically, the lithium-rich compound particles are primary particles in the inner core, and have a smaller particle size than the inner core. The nanoscale lithium-rich compound particles can be formed in the internal space of the molecular cage, which further increases the specific surface area of the inner core, so that the lithium ion transmission efficiency between the lithium-rich compound particles in the inner core is higher.

[0014] In a possible implementation, the particle size D50 of the non-lithium compound particles is 5 nm to 10 nm.

[0015] In a possible implementation, the particle size D50 of the metal particles is 10 nm to 100 nm.

[0016] In a possible implementation, the first carbon framework includes carbon network units, and the carbon network units are polygons, including one or more of pentagons, hexagons, octagons and dodecagons.

[0017] In a possible implementation, the lithium supplement material further includes a second carbon framework, and the second carbon framework is coated on the outer layer of the first carbon framework.

[0018] In a possible implementation, the first carbon framework is a metal-organic framework carbonized layer, and the second carbon framework is a covalent organic framework carbonized layer.

[0019] In a possible implementation, the thickness A1 of the first carbon framework is 10 nm to 50 nm. Understandably, the thickness of the first carbon framework ensures both the specific capacity of the lithium supplement material and the electronic conductive environment. When the thickness of the first carbon framework is less than the above range, the first carbon framework does not completely coat the core, which is not conducive to building a good electronic conductive environment; when the thickness of the first carbon framework is greater than the above range, since the first carbon framework does not contribute lithium ions, the overall gram capacity of the lithium supplement material is reduced.

[0020] In a possible implementation, the density of the first carbon framework is 0.10 g / cm 3 -0.20 g / cm 3 .

[0021] In a possible implementation, the thickness A2 of the second carbon framework is 2 nm to 10 nm. Understandably, the thickness of the second carbon framework ensures both the specific capacity of the lithium supplement material and the electronic conductive environment. When the thickness of the second carbon framework is less than the above range, the second carbon framework does not completely coat the core, which is not conducive to building a good electronic conductive environment; when the thickness of the second carbon framework is greater than the above range, since the second carbon framework does not contribute lithium ions, the overall gram capacity of the lithium supplement material is reduced.

[0022] In a possible implementation, the density of the second carbon framework is 0.40 g / cm 3 -0.60 g / cm 3 .

[0023] In a possible implementation, the mass ratio of the core, the first carbon framework, and the second carbon framework is 100:(9-15):(1-5). By controlling the mass ratio of the core and the carbon layer within a suitable range, not only can the thickness of the first carbon framework and the second carbon framework be adjusted so that the carbon layer provides sufficient conductive environment, but also can ensure that all lithium ions and other metal ions in the core are fully reacted to obtain the required product. When the proportion of the core is less than the above range, the proportion of the first carbon framework or the second carbon framework is too large, and since the carbon layer does not contribute lithium ions, the overall gram capacity of the lithium supplement material is reduced; when the proportion of the core is greater than the above range, the proportion of the first carbon framework or the second carbon framework is too small, which may lead to incomplete coating, so that part of the lithium ions are not reacted to form lithium-rich compounds or part of the lithium-rich compounds are exposed to the external environment.

[0024] In a possible implementation, thicknesses of the first carbon framework and the second carbon framework satisfy a relationship: 1≤A1 / A2≤50. By controlling the thickness ratio of the first carbon framework and the second carbon framework within the above range, the thickness of the second carbon framework is controlled, so as to avoid the phenomenon of the second carbon being too thick on the basis of ensuring that the second carbon framework achieves complete coating. When less than the range of the above relationship, it indicates that the second carbon framework is too thick, thereby reducing the overall gram capacity of the lithium supplement material; when greater than the range of the above relationship, it indicates that the second carbon framework is too thin, and the situation of incomplete coating or firm coating is prone to occur.

[0025] In a possible implementation, densities of the first carbon framework and the second carbon framework satisfy a relationship: 2≤ρ2 / ρ1≤6. It can be understood that, when the above relationship is satisfied, it indicates that the density of the second carbon framework is greater than the density of the first carbon framework, because the second carbon framework should be a dense framework coated on the surface layer of the first carbon framework, and is thinner than the first carbon framework.

[0026] In a possible implementation, the chemical formula of the lithium-rich compound particle includes Li x M y , wherein M is one or more elements in S, P, L, Br, 1≤x≤3, and 1≤y≤2.

[0027] In a second aspect, the application further provides a preparation method of a lithium supplement material, including: uniformly mixing a first metal precursor, a second metal precursor, a lithium-rich compound precursor, and a first organic ligand in a solvent, and obtaining a precursor mixture after washing and drying; and obtaining the lithium supplement material after sintering the precursor mixture; the lithium supplement material includes an inner core and a first carbon framework, the first carbon framework forms a cage structure, and the inner core is accommodated in the first carbon framework of the cage structure.

[0028] In a third aspect, the application further provides a positive electrode tab, including a current collector and an active material layer arranged on the current collector, the active material layer including a positive electrode material and the lithium supplement material in any one of the embodiments of the first aspect, or the active material layer including a positive electrode material and the lithium supplement material obtained by the preparation method of the lithium supplement material in any one of the embodiments of the second aspect.

[0029] In a fourth aspect, the application further provides a secondary battery including the positive electrode tab in the third aspect, or the battery including the lithium supplement material in any one of the embodiments of the first aspect, or the battery including the lithium supplement material obtained by the preparation method of the lithium supplement material in any one of the embodiments of the second aspect. BRIEF DESCRIPTION OF DRAWINGS

[0030] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the accompanying drawings needed to be used in the description of the embodiments or the prior art will be briefly introduced. Obviously, the accompanying drawings in the following description only only some embodiments of the present application, and all other embodiments obtained by those of ordinary skill in the art based on the accompanying drawings without creative labor should be within the protection scope of the present application.

[0031] Figure 1 The schematic diagram of the appearance of the lithium supplement material in an embodiment;

[0032] Figure 2 The schematic diagram of the cross section of the first carbon frame and the core in an embodiment;

[0033] Figure 3 The schematic diagram of the cross section of the first carbon frame, the second carbon frame and the core in an embodiment;

[0034] Figure 4 The SEM diagram of the appearance structure of the lithium supplement material in an embodiment;

[0035] Figure 5 The TEM diagram of the lithium supplement material in an embodiment;

[0036] Figure 6 The TEM diagram of the partial position structure of the lithium supplement material in an embodiment; Figure 5

[0037] Figure 7 The flow chart of the preparation method of the lithium supplement material in an embodiment;

[0038] Figure 8 The flow chart of the preparation method of the lithium supplement material including the second frame in an embodiment. DETAILED DESCRIPTION

[0039] The technical solutions in the embodiments of the present application will be described clearly and completely below with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor should be within the protection scope of the present application.

[0040] It should be noted that when a component is referred to as being "fixed" to another component, it can be directly on the other component or there can be an intervening component. When a component is referred to as being "connected" to another component, it can be directly connected to the other component or there can be an intervening component.

[0041] ​Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description of the application herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety.

[0042] Some embodiments of the present application are described in detail below with reference to the accompanying drawings. The following examples and features in the examples can be combined with each other in the case of no conflict.

[0043] In a first aspect, the present application provides a lithium supplement material 100, please refer to Figures 1 to 3 , comprising a core 10 and a first carbon framework 20, the core 10 comprises lithium-rich compound particles 101; the first carbon framework 20 forms a cage structure, and the core 10 is accommodated in the first carbon framework 20 of the cage structure.

[0044] Specifically, the core can be a secondary particle composed of a plurality of primary particles with smaller sizes. Among them, the core includes lithium-rich compound particles (with nanoscale size) with smaller particle size, and the lithium-rich compound particles are lithium source cores for lithium supplement. Preferably, the lithium-rich compound is a binary lithium-rich compound. For example, the lithium-rich compound particles can be lithium sulfide, and the precursor thereof can be lithium sulfate. The lithium supplement material with lithium sulfide particles constituting the core is added to the electrode, so that it acts as a "sacrifice" during the first charging process, and releases all the lithium ions contained at one time as much as possible, to supplement the irreversible lithium ions consumed by the formation of SEI film in the negative electrode, thereby maintaining the abundance of lithium ions in the battery system, improving the first efficiency of the battery and the overall electrochemical performance.

[0045] Compared with other lithium supplement materials, the lithium supplement material made of binary lithium-rich compound has weaker binding force between lithium ions and anions, smaller polarization of binary lithium-rich compound, and lower voltage platform required during charging, so as to effectively reduce the activation barrier of binary lithium-rich compound, thereby significantly improving the deintercalation efficiency of lithium ions, so that the lithium supplement material can have higher specific capacity of lithium supplement. Moreover, the binary lithium-rich compound will not produce oxygen during the process of lithium supplement, which can reduce the risk of swelling phenomenon of lithium battery.

[0046] Optionally, the first carbon framework can be obtained by carbonization of a ZIF material (a porous crystalline material, organic imidazolate cross-linked to transition metal, forming a polyhedral framework). Based on the characteristics of the precursor, the first carbon framework is in the shape of a molecular cage with a cage structure, and the core is located inside the molecular cage, so the first carbon framework can have abundant pores. The reason for the formation of pores in the first carbon framework can be that, on the one hand, based on the porous structure of the precursor, the first carbon framework after carbonization can still retain abundant pore structure; on the other hand, during the preparation of the lithium supplement material, the lithium source reacts with the first carbon framework, so that when the lithium source is reduced to a lithium-rich compound, it can simultaneously etch the surface of the first carbon framework to form pores.

[0047] Optionally, at least part of the core is embedded in the network pores of the first carbon framework. Based on the abundant network pore structure of the first carbon framework, at least part of the core can be embedded in the pores. It can be understood that, based on the above reasons for the formation of pores, part of the core can be embedded in the pores originally present on the first carbon framework, and part of the core can be embedded in the pores after the lithium source in the core etches the surface of the first carbon framework.

[0048] Optionally, part of the core embedded in the first carbon framework can penetrate the first carbon framework, and can not penetrate the first carbon framework. The advantage of penetrating the first carbon framework is that not only the first carbon framework enhances the conductivity of the lithium supplement material, but also the first carbon framework reduces the contact between the core and the external environment.

[0049] For example, the formation process of the core and the first carbon framework can be that cobalt ions (Co 2+ ) and zinc ions (Zn 2+ ) are coordinated with 2-aminoimidazole to form a molecular cage (metal-organic framework). Then lithium sulfate is added, which can be accommodated in the metal-organic framework. Under high temperature reaction, the imidazole skeleton is converted into a carbon skeleton to form the first carbon framework, part of the zinc ions are converted into zinc sulfide nanoparticles, the cobalt ions are converted into cobalt nanoparticles, and the lithium sulfate reacts with the carbon skeleton, so that the carbon inside the first carbon framework is consumed by reaction, and the lithium sulfide generated by the reaction of lithium sulfate can be embedded therein. The specific reaction is as follows:

[0050] Li2SO4+2C=Li2S+2CO2

[0051] Li2SO4+4C=Li2S+4CO

[0052] Zn 2+ +SO4 2- +2C=ZnS+2CO2

[0053] Co 2+ +SO4 2-+ 2C = CoS + 2CO2

[0054] 2CoS + C = 2Co + CS2

[0055] Preferably, the advantages of using zinc ions and cobalt ions are that the zinc ions are partially converted into zinc sulfide nanoparticles and the cobalt ions are converted into cobalt nanoparticles, which can fix the intermediate products of material decomposition and prevent them from undergoing irreversible reactions with the carbonate electrolyte, while also bringing about catalysis and gas production inhibition.

[0056] The lithium supplementing material provided in the application has a first carbon framework with a cage structure coated outside the core. On the one hand, the first carbon framework with strong conductivity can enhance the ability of the lithium supplementing material to transport lithium ions outward, and the first carbon framework with a cage structure also has a larger specific surface area, which can further improve the lithium ion transport efficiency of the lithium supplementing material. On the other hand, the nano-space confinement effect of the precursor molecular cage of the first carbon framework on the precursor of the lithium-rich compound can realize the nano-structurization of the core and the lithium-rich compound particles contained therein. By utilizing the structural advantages of the nanoscale core and the lithium-rich compound particles contained therein, the polarization of the lithium-rich compound particles is smaller, and the voltage platform during charging is lower, so that the lithium supplementing material can have a lower activation barrier, which can significantly improve the deintercalation efficiency of lithium ions, thereby realizing a higher specific capacity of the lithium supplementing material.

[0057] In a possible implementation, the first carbon framework has a plurality of holes, and part of the lithium-rich compound particles are embedded in the holes. Specifically, refer to Figure 4 The first carbon framework has a cage structure and is not limited in shape, which can include a cube, a polyhedron, a sphere, etc. It can be understood that the cage structure should be a structure formed by a plurality of rod-shaped or rope-shaped solids in a cross-linking manner, so the cage structure is a hollow structure, i.e., has holes, and the shape of the holes is not limited, which can include a square, a polygon, a circle or an ellipse.

[0058] In a possible implementation, refer to Figure 3 The core 10 further includes non-lithium compound particles 102 having a first metal element and metal particles 103 having a second metal element, wherein the first metal element and the second metal element are different, the first metal element includes one or more of Zn, Fe, Cu, Ni and Co, and the second metal element includes one or more of Co, Mo, Mn, Pb, Pt and Ru.

[0059] Specifically, based on the formation process of the core and the first carbon framework provided in the above embodiments, the coordinated metal ions in the metal-organic framework can react with carbon at high temperature to form compounds or metal elements. Among them, the formed compounds are non-lithium compound particles, and the metal elements are metal particles. It can be understood that, since the coordinated metal in the ZIF material can have diversity, the formed non-lithium compound particles and metal particles are not specifically limited. Therefore, the primary particles in the core can also include the non-lithium compound particles and metal particles described above.

[0060] Optionally, in order to ensure that other by-products are avoided in the formation process of the lithium-rich compound, other metal ions such as Mn 2+ , Mo 2+ , Ni + , Fe 2+ , Cu + ions can also be added in the reaction. These metal ions can form non-lithium compound particles or metal particles as the coordinated metal ions described above. The metal ions described above can consume excess anions in the lithium-rich compound, such as sulfur ions, so as to avoid the formation of by-products such as lithium polysulfide in the core. For example, the core contains lithium sulfide (Li2S) as the lithium-rich compound particle. Through the consumption of sulfur ions by the metal ions described above, the generation of by-product lithium polysulfide can be reduced, thereby ensuring the lithium supplement effect of lithium sulfide.

[0061] Therefore, through these coordinated metal ions, not only can the intermediate products of material decomposition be fixed, but also irreversible reactions with carbonate electrolyte can be prevented, while catalysis and gas inhibition effects can be brought.

[0062] In one possible embodiment, the mass ratio of the lithium-rich compound particles, the non-lithium compound particles, and the metal particles is 100:(5-20):(2-8). Specifically, the mass ratio of the lithium-rich compound particles, the non-lithium compound particles, and the metal particles can be, but is not limited to, 100:20:2, 100:18:3, 100:16:4, 100:14:5, 100:12:6, 100:10:6, 100:8:7, and 100:5:8. By controlling the mass ratio of the lithium-rich compound particles, the non-lithium compound particles, and the metal particles within the above range, it can be ensured that the lithium supplement material has a suitable lithium ion supply amount, and the core has a suitable conductivity. Because the other particles except the lithium-rich compound particles do not provide lithium ions, when the proportion of the lithium-rich compound particles is low, the lithium supplement performance of the lithium supplement material will be reduced.

[0063] In a possible implementation, the non-lithium compound particles and / or metal particles are embedded in the network pores of the first carbon framework. Specifically, according to the reaction equation provided in the above implementation, the formation of the lithium-rich compound particles, the non-lithium compound particles and the metal particles all need the participation of carbon, so after the organic matter in the metal-organic framework forms the carbon skeleton, the lithium ions and the coordinated metal ions need to react with the carbon on the carbon skeleton to form an etching of the first carbon framework, so that the first carbon framework can form a pore structure again.

[0064] Optionally, the lithium-rich compound particles, the non-lithium compound particles and the metal particles embedded in the first carbon framework can penetrate the first carbon framework, or can not penetrate the first carbon framework.

[0065] In a possible implementation, the particle size D50 of the core is 200 nm to 300 nm. Specifically, the particle size of the core can be, but is not limited to, 200 nm, 210 nm, 220 nm, 230 nm, 250 nm, 270 nm or 300 nm. The particle size of the existing commercial binary lithium-rich compound (lithium sulfide) powder is usually 10 μm to 30 μm. Due to the insulating property, the internal volume of most particles hinders the conversion of sulfur, thereby showing a high potential barrier and a low capacity. The particle size of the core in the implementation of the present application can be controlled to be submicron level by the confinement effect of the molecular cage. By greatly reducing the particle size to submicron level, the diffusion length can be shortened and the active specific surface area can be enlarged, which is beneficial to the embedding and extraction of lithium ions, thereby increasing the capacity of the lithium supplement material. When the particle size of the core is less than the above range, the difficulty of preparing the molecular cage is increased, thereby increasing the preparation cost of the lithium supplement material, which is not conducive to industrial production. When the particle size of the core is greater than the above range, the specific surface area of the core is reduced, which reduces the embedding and extraction efficiency of lithium ions, thereby resulting in poor lithium supplement effect of the lithium supplement material.

[0066] In a possible implementation, the particle size D50 of the lithium-rich compound particles is 3 nm to 8 nm. Specifically, the particle size of the lithium-rich compound particles can be, but is not limited to, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm or 8 nm. Specifically, the lithium-rich compound particles are primary particles in the core, and have a smaller particle size than the core. The nanoscale lithium-rich compound particles can be formed in the internal space of the molecular cage, which further enlarges the specific surface area of the core, so that the lithium ion transmission efficiency between the lithium-rich compound particles in the core is higher.

[0067] In a possible implementation, the particle size D50 of the non-lithium compound particles is 5 nm to 10 nm. Specifically, the particle size of the non-lithium compound particles can be, but is not limited to, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm or 10 nm.

[0068] In a possible implementation, the particle size D50 of the metal particles is 10 nm to 100 nm. Specifically, the particle size of the metal particles can be, but is not limited to, 10 nm, 20 nm, 30 nm, 50 nm, 70 nm, or 100 nm.

[0069] Controlling the particle size of the above-mentioned particles in a suitable range can ensure that the core has a suitable submicron size, and can avoid occupying the space of the lithium-rich compound particles, resulting in a lower content of the lithium-rich compound particles.

[0070] In a possible implementation, referring to Figure 4 , the first carbon framework includes carbon net units, and the carbon net units are polygons, including one or more of pentagons, hexagons, octagons, and dodecagons. Specifically, since the precursor of the first carbon framework is an organic framework formed by cross-linking of organic imidazole esters, after high-temperature sintering, when non-carbon atoms are removed, the carbon atoms can combine with each other to form polygons such as pentagons, hexagons, octagons, or dodecagons. Therefore, the lithium supplementing material can also form a polyhedral shape.

[0071] In a possible implementation, referring to Figure 3 , Figure 5 and Figure 6 , the lithium supplementing material 100 further includes a second carbon framework 30, and the second carbon framework 30 is coated on the outer surface of the first carbon framework 20. Specifically, the second carbon framework can be formed by carbonization of a covalent organic framework formed by polycondensation of trimesamyl formyl and p-phenylenediamine. It can be understood that the precursor of the second carbon framework is an organic framework, and then a stable ultrathin carbon layer (the second carbon framework) is formed at high temperature. The density of the second carbon framework should be greater than the density of the first carbon framework, and the thickness of the second carbon framework should be less than the thickness of the first carbon framework. As can be seen from Figure 6 , there is a clear boundary between the first carbon framework and the core, which also indicates that the core is located in the first carbon framework; and the second carbon framework is located on the outer surface of the first carbon framework, forming a structure that is thinner than the first carbon framework. The second carbon framework can inhibit the reaction of the core with water, and ultimately effectively improve the comprehensive performance of the lithium supplementing agent.

[0072] It can be understood that since the core 10 is embedded in the hole of the first carbon framework 20, at least part of the core 10 is exposed to the external environment, and the core 10 exposed to the external environment is prone to react with water and carbon dioxide in the environment, thereby reducing the electrochemical performance of the lithium supplementing material 100. Therefore, the second carbon framework 30 can be arranged outside the first carbon framework 20, and the second carbon framework 30 can coat the first carbon framework 20 while covering the exposed part of the core 10, so that the core 10 is isolated from the external environment.

[0073] Of course, in other embodiments, the second carbon framework can also directly coat the outer surface of the core through the first carbon framework. By setting the second carbon framework, not only can the electronic and ionic conduction performance in the core be effectively improved, the lithium release during charging can be improved; but also can play a certain role in isolating moisture, improve the stability of the lithium supplement material, and achieve stable lithium supplement effect. In addition, it can also ensure the stability, uniformity and good processing performance of the lithium supplement material in the electrode active paste and active layer.

[0074] In a possible implementation, the first carbon framework is a metal-organic framework carbonized layer, and the second carbon framework is a covalent organic framework carbonized layer. It can be understood that the first carbon framework is a combination of the above-mentioned metal and organic ligand, so it is a metal-organic framework carbonized layer. And the second carbon framework is a combination of different organic ligands in the above, so it is a covalent organic framework carbonized layer.

[0075] In a possible implementation, the thickness A1 of the first carbon framework is 10nm-50nm. Specifically, the thickness A1 of the first carbon framework can be but not limited to 10nm, 15nm, 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, 50nm. It can be understood that the thickness of the first carbon framework ensures the specific capacity and electronic conductive environment of the lithium supplement material at the same time. When the thickness of the first carbon framework is less than the above range, the first carbon framework is not completely coated on the core, which is not conducive to building a good electronic conductive environment; when the thickness of the first carbon framework is greater than the above range, since the first carbon framework does not contribute lithium ions, the overall gram capacity of the lithium supplement material will be reduced.

[0076] In a possible implementation, the density of the first carbon framework is 0.10g / cm 3 -0.20g / cm 3 .

[0077] In a possible implementation, the thickness A2 of the second carbon framework is 2nm-10nm. Specifically, the thickness A2 of the second carbon framework can be but not limited to 2nm, 3nm, 4nm, 5nm, 6nm, 7nm, 8nm, 9nm, 10nm. It can be understood that the thickness of the second carbon framework ensures the specific capacity and electronic conductive environment of the lithium supplement material at the same time. When the thickness of the second carbon framework is less than the above range, the second carbon framework is not completely coated on the core, which is not conducive to building a good electronic conductive environment; when the thickness of the second carbon framework is greater than the above range, since the second carbon framework does not contribute lithium ions, the overall gram capacity of the lithium supplement material will be reduced.

[0078] In a possible implementation, the density of the second carbon framework is 0.40g / cm 3 -0.60g / cm 3 .

[0079] In a possible implementation, the mass ratio of the core, the first carbon framework and the second carbon framework is 100:(9-15):(1-5). Specifically, the mass ratio of the core, the first carbon framework and the second carbon framework can be, but is not limited to, 100:9:1, 100:12:1, 100:15:1, 100:9:5, 100:12:5, 100:15:5. By controlling the mass ratio of the core and the carbon layer within a suitable range, the thickness of the first carbon framework and the second carbon framework can be adjusted, so that the carbon layer provides sufficient conductive environment; and it can also be ensured that all lithium ions and other metal ions in the core are fully reacted to obtain the required product. When the proportion of the core is less than the above range, the proportion of the first carbon framework or the second carbon framework is too large, and since the carbon layer does not contribute lithium ions, the overall gram capacity of the lithium supplement material is reduced; when the proportion of the core is greater than the above range, the proportion of the first carbon framework or the second carbon framework is too small, which can lead to incomplete coating, so that part of the lithium ions are not reacted to form lithium-rich compounds or part of the lithium-rich compounds are exposed to the external environment.

[0080] In a possible implementation, the thickness of the first carbon framework and the second carbon framework satisfies the relationship: 1≤A1 / A2≤50. Specifically, A1 / A2 can be, but is not limited to, 1, 2, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50. By controlling the thickness ratio of the first carbon framework and the second carbon framework within the above range, the thickness of the second carbon framework can be controlled, so as to avoid the phenomenon of too thick second carbon on the basis of ensuring that the second carbon framework achieves complete coating. When less than the above relationship range, it means that the second carbon framework is too thick, thereby reducing the overall gram capacity of the lithium supplement material; when greater than the above relationship range, it means that the second carbon framework is too thin, which is prone to incomplete coating or insecure coating.

[0081] In a possible implementation, the density of the first carbon framework ρ1 and the density of the second carbon framework ρ2 satisfy the relationship: 2≤ρ2 / ρ1≤6. It can be understood that when the above relationship is satisfied, it means that the density of the second carbon framework is greater than the density of the first carbon framework, because the second carbon framework should be a dense framework coated on the surface layer of the first carbon framework, and is thinner than the first carbon framework.

[0082] In a possible implementation, the lithium-rich compound particles can be ternary compounds and can also be binary compounds. The chemical formula of the binary compound includes Li x M y , wherein M is one or more elements of S, P, L, Br, 1≤x≤3, 1≤y≤2. Specifically, the lithium-rich compound particles can be one or more of lithium sulfide (Li2S), lithium phosphide (Li3P), lithium iodide (LiI), and lithium bromide (LiBr).

[0083] In a second aspect, the present application also provides a preparation method of the lithium supplementing material, please refer to Figure 7 , which is specifically used for the preparation of the lithium supplementing material in the first aspect. The preparation method comprises the following steps:

[0084] Step S10, the first metal precursor, the second metal precursor, the lithium-rich compound precursor and the first organic ligand are added into a solvent and uniformly mixed, and after washing and drying, a precursor mixture is obtained;

[0085] Step S20, the precursor mixture is sintered to obtain the lithium supplementing material; the lithium supplementing material comprises an inner core and a first carbon framework, the first carbon framework forms a cage structure, and the inner core is accommodated in the first carbon framework of the cage structure.

[0086] Optionally, the first metal precursor can be a zinc salt, including but not limited to zinc nitrate, zinc acetate, zinc sulfate, etc. The second metal precursor can be a cobalt salt, including but not limited to cobalt nitrate, cobalt acetate, cobalt sulfate, etc. The lithium-rich compound precursor can be a lithium salt, including but not limited to lithium sulfate, lithium phosphate, lithium iodide, lithium bromide, etc. The first organic ligand can be an organic imidazole ester, including but not limited to 2-aminoimidazole, etc.

[0087] In a possible implementation, in the above step S10, the first metal precursor and the second metal precursor can be first dissolved into a solvent. Then the lithium-rich compound precursor is dissolved into the above solvent, and after ultrasonic stirring, a homogeneous solution is obtained.

[0088] On the basis of the above implementation, the first organic ligand is added into the homogeneous solution. After ultrasonic stirring and normal temperature stirring, the precursor mixture is obtained after washing and drying. The first metal precursor, the second metal precursor and the first organic ligand can react to form a molecular cage (metal-organic framework), and the molecular cage is wrapped around the outer periphery of the lithium-rich compound precursor.

[0089] In a possible implementation, in the above step S20, during the sintering process, the organic framework in the molecular cage is carbonized to form a carbon skeleton; the metal ions participating in coordination and the lithium-rich compound precursor react with the carbon skeleton and the carbonized second carbon source to form lithium-rich compound particles, non-lithium compound particles and metal particles. Finally, the lithium-rich compound particles, the non-lithium compound particles and the metal particles jointly constitute the inner core, and the unreacted carbon skeleton forms the first carbon framework.

[0090] In a possible implementation, in the above step S10, the molar ratio of the first metal precursor, the second metal precursor, the lithium-rich compound precursor and the first organic ligand is (0.1-0.5) :(0.1-0.5) :(0.5-4) :(0.3-1.5).

[0091] In a possible implementation, in the step S10, other metal salts can also be added, including but not limited to manganese salts, molybdenum salts, nickel salts, ferrous salts, and copper salts; and the ratio of the other metal salts to the above-mentioned implementation is (0.02-0.06). The purpose of adding the other metal salts is to consume the excess anions in the lithium-rich compounds and avoid the generation of other by-products.

[0092] In a possible implementation, in the step S20, the sintering temperature of the precursor mixture can be 600-800°C, and the sintering time can be 5-10h.

[0093] In a possible implementation, in the step S20, the sintering environment of the precursor mixture can be an atmosphere formed by any one of nitrogen, argon, and nitrogen-argon mixed gas.

[0094] In a possible implementation, please refer to Figure 8 A second carbon framework can also be made on the outer surface of the first carbon framework, and the specific steps are as follows:

[0095] In step S11, the first metal precursor, the second metal precursor, the lithium-rich compound precursor, and the first organic ligand are added to a solvent and uniformly mixed to obtain a material with a metal-organic framework coating;

[0096] In step S21, the second organic ligand and the third organic ligand are added to the above-mentioned solution, and after sufficient reaction, a precursor mixture with a covalent organic framework is obtained,

[0097] In step S31, the precursor mixture is sintered to obtain a lithium supplement material; the lithium supplement material includes a core, a first carbon framework, and a second carbon framework, the first carbon framework forms a cage structure, the core is accommodated in the first carbon framework of the cage structure, and the second carbon framework forms a coating on the outer surface of the first carbon framework.

[0098] Specifically, the first carbon framework is formed after the metal-organic framework in step S11 is carbonized, and the second carbon framework is formed after the covalent organic framework in step S21 is carbonized.

[0099] The second organic ligand and the third organic ligand are the materials provided in the first aspect, which are not described herein.

[0100] In a third aspect, the application further provides a positive electrode sheet, which comprises a current collector and an active material layer arranged on the current collector, the active material layer comprising a positive electrode material and the lithium supplementing material of the first aspect, or the active material layer comprising the lithium supplementing material obtained by the preparation method of the second aspect. The positive electrode sheet provided by the application contains the lithium supplementing material described above, and the lithium supplementing material can provide active lithium ions to compensate for the consumption of active lithium ions due to the formation of SEI film during the first charging of the battery, thereby effectively maintaining the gram capacity of the positive electrode sheet and improving the capacity retention rate of the positive electrode sheet. At the same time, the lithium supplementing material can release lithium polysulfide to eliminate lithium precipitation, not only maintaining the performance of the positive electrode sheet, but also improving the service life of the positive electrode sheet.

[0101] In a possible implementation, the positive electrode sheet comprises a positive electrode current collector, and the positive electrode current collector has a positive electrode active layer, which comprises positive electrode material, conductive agent, binder and the like. The application does not make specific limitations on these materials, and suitable materials can be selected according to actual application requirements. The positive electrode current collector includes, but is not limited to, any one of copper foil and aluminum foil. The positive electrode active material can be phosphate positive electrode active material and ternary positive electrode active material, and in specific embodiments, it includes one or more of lithium cobaltate, lithium manganate, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxyphosphate, fluorinated lithium vanadium phosphate, lithium titanate, lithium nickel cobalt manganate, and lithium nickel cobalt aluminum. The conductive agent includes one or more of graphite, carbon black, acetylene black, graphene, carbon fiber, C60 and carbon nanotube, and the content of the conductive agent in the positive electrode active layer is 3wt%-5wt%. The types of the binder include one or more of polyvinylidene chloride, soluble polytetrafluoroethylene, butadiene rubber, hydroxypropyl methyl cellulose, methyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan and chitosan derivatives, and the content of the binder in the positive electrode active layer is 2wt%-4wt%.

[0102] In a possible implementation, the content of the lithium supplementing material in the positive electrode material can be controlled to be 1%-6% of the mass of the positive electrode active material. This ratio can exactly compensate for the loss of active lithium during the first charging of the battery. If the amount of the lithium supplementing material added in the positive electrode sheet is too low, the lost active lithium in the positive electrode material cannot be completely supplemented, which is not conducive to improving the energy density and capacity retention rate of the battery. If the amount of the lithium supplementing material added in the positive electrode material is too high, the lithium precipitation of the negative electrode may become serious, and the cost may increase. In some specific embodiments, the mass percentage of the lithium supplementing material in the positive electrode material can be 1%, 2%, 4%, 6% and the like.

[0103] In a fourth aspect, the application further provides a secondary battery comprising the positive electrode plate described above. The positive electrode plate added with the lithium supplementing material can effectively compensate for the active lithium ions consumed due to the formation of SEI film during the first charging of the battery, effectively maintain the gram capacity of the positive electrode plate, and improve the capacity retention rate of the positive electrode plate.

[0104] The technical solutions of the application are described in detail below through specific examples.

[0105] Example 1

[0106] The present embodiment provides a lithium supplementing material and a preparation method thereof. The lithium supplementing material comprises a core, a first carbon framework and a second carbon framework. The core comprises a lithium-rich compound Li2S, ZnS and Co element.

[0107] The preparation method of the lithium supplementing material comprises the following steps:

[0108] (1) 1.5 g of Zn(AC)2·4H2O and 1.5 g of Co(AC)2·4H2O are dissolved in 50 mL of deionized water. 15 g of Li2SO4·H2O is dissolved in 50 mL of deionized water, and ultrasonic stirring is performed for 10 min to form a homogeneous solution.

[0109] (2) 12 g of 2-aminoimidazole is added, ultrasonic stirring is performed for 1 min, stirring is performed at room temperature for 12 h, and standing is performed for 2 h. The above materials are collected by centrifugation and washed with anhydrous ethanol for 3 times, and freeze-drying is performed for 24 h. The powder is collected and dried at 80℃ for 2 h.

[0110] (3) The dried powder is added to a 0.5M p-formaldehyde aqueous solution and stirred at room temperature for 3 h to produce a cross-linking reaction with the amino group on the 2-aminoimidazole. Then, a 0.75M p-phenylenediamine acetone solution is added and reacted at 60℃ for 5 h to form a covalent organic framework by cross-linking. After filtration, it is dried for standby use.

[0111] (4) The sample powder is placed in a tube furnace and heat-treated at 700℃ for 2 h under the protection of an inert atmosphere, wherein the temperature rising rate in the heating stage is 2℃ / min. Finally, the lithium supplementing material is obtained.

[0112] In the lithium supplementing material, the mass ratio of the lithium-rich compound particles Li2S, ZnS and metal element (Co) is 100:10:5, the particle size D50 of the core is 200 nm, the particle size D50 of the Li2S particles is 5 nm, the particle size D50 of the ZnS is 10 nm, the particle size D50 of the Co element is 10 nm, the thickness of the first carbon framework is 30 nm, and the thickness of the second carbon framework is 5 nm.

[0113] Example 2

[0114] The embodiment provides a lithium supplementing material and a preparation method thereof. The lithium supplementing material comprises a core, a first carbon framework and a second carbon framework. The core comprises a lithium-rich compound Li2S, ZnS and Co element.

[0115] The preparation method of the lithium supplementing material comprises the following steps:

[0116] (1) 4.5 g of Zn(AC)2·4H2O and 0.3 g of Co(AC)2·4H2O are dissolved in 50 mL of deionized water. 15 g of Li2SO4·H2O is dissolved in 50 mL of deionized water, and ultrasonic stirring is performed for 10 min to form a homogeneous solution.

[0117] (2) 12 g of 2-aminoimidazole is added, ultrasonic stirring is performed for 1 min, stirring is performed at room temperature for 12 h, and standing is performed for 2 h. The above materials are collected by centrifugation and washed with anhydrous ethanol for 3 times, and freeze-drying is performed for 24 h. The powder is collected and dried at 80℃ for 2 h.

[0118] (3) The dry powder is added to a 0.5M aqueous solution of p-formaldehyde and stirred at room temperature for 3 h to generate a cross-linking reaction with the amino group on the 2-aminoimidazole. Then, a 0.75M solution of p-phenylenediamine acetone is added and reacted at 60℃ for 5 h to form a covalent organic framework by cross-linking. After filtration, drying is performed for standby use.

[0119] (4) The sample powder is placed in a tube furnace and heat-treated at 700℃ for 2 h under the protection of an inert atmosphere, wherein the temperature rising rate in the temperature rising stage is 2℃ / min. Finally, the lithium supplementing material is obtained.

[0120] In the lithium supplementing material, the mass ratio of the lithium-rich compound particles Li2S, ZnS and metal element (Co) is 100:20:2, the particle size D50 of the core is 200 nm, the particle size D50 of the Li2S particles is 5 nm, the particle size D50 of the ZnS is 10 nm, the particle size D50 of the Co element is 10 nm, the thickness of the first carbon framework is 30 nm, and the thickness of the second carbon framework is 5 nm.

[0121] Embodiment 3

[0122] The embodiment provides a lithium supplementing material and a preparation method thereof. The lithium supplementing material comprises a core, a first carbon framework and a second carbon framework. The core comprises a lithium-rich compound Li2S, ZnS and Co element.

[0123] The preparation method of the lithium supplementing material comprises the following steps:

[0124] (1) 4.5 g of Zn(AC)2·4H2O and 0.3 g of Co(AC)2·4H2O are dissolved in 50 mL of deionized water. 15 g of Li2SO4·H2O is dissolved in 50 mL of deionized water, and ultrasonic stirring is performed for 10 min to form a homogeneous solution.

[0125] (2) Add 12 g of 2-aminoimidazole, ultrasonic stirring for 1 min, stirring at room temperature for 12 h, standing for 2 h, centrifugal collection of the above materials and washing with anhydrous ethanol for 3 times, freeze-drying for 24 h, collecting the powder and drying at 80℃ for 2 h.

[0126] (3) Add the dry powder into 0.5M p-phenylenedimethylol water solution, stirring at room temperature for 3 h to make cross-linking reaction with the amino group on 2-aminoimidazole, and then add 0.75M p-phenylenediamine acetone solution, reacting at 60℃ for 5 h to form covalent organic framework by cross-linking. After filtration, dry for standby.

[0127] (4) Put the sample powder into a tube furnace, heat treat at 700℃ for 2 h under the protection of inert atmosphere, and the temperature rising rate is 2℃ / min in the temperature rising stage, finally obtain the lithium supplement material.

[0128] In the lithium supplement material, the mass ratio of lithium-rich compound particles Li2S, ZnS and metal element (Co) is 100:5:8, the particle size D50 of the inner core is 200 nm, the particle size D50 of the Li2S particle is 5 nm, the particle size D50 of the ZnS is 10 nm, the particle size D50 of the Co element is 10 nm, the thickness of the first carbon framework is 30 nm, and the thickness of the second carbon framework is 5 nm.

[0129] Example 4

[0130] The embodiment provides a lithium supplement material and a preparation method thereof. The lithium supplement material comprises an inner core, a first carbon framework and a second carbon framework, and the inner core comprises lithium-rich compounds Li2S, ZnS and Co element.

[0131] The preparation method of the lithium supplement material comprises the following steps:

[0132] (1) Dissolve 1.5 g of Zn(AC)2·4H2O and 1.5 g of Co(AC)2·4H2O in 50 mL of deionized water. Dissolve 15 g of Li2SO4·H2O in 50 mL of deionized water, ultrasonic stirring for 10 min to form a homogeneous solution.

[0133] (2) Add 12 g of 2-aminoimidazole, ultrasonic stirring for 1 min, stirring at room temperature for 12 h, standing for 2 h, centrifugal collection of the above materials and washing with anhydrous ethanol for 3 times, freeze-drying for 24 h, collecting the powder and drying at 80℃ for 2 h.

[0134] (3) Put the sample powder into a tube furnace, heat treat at 700℃ for 2 h under the protection of inert atmosphere, and the temperature rising rate is 2℃ / min in the temperature rising stage, finally obtain the lithium supplement material.

[0135] In the lithium supplementing material, the mass ratio of lithium-rich compound particles Li2S, ZnS, metal element (Co) is 100:10:5, the particle size D50 of the inner core is 200 nm, the particle size D50 of the Li2S particle is 5 nm, the particle size D50 of the ZnS is 10 nm, the particle size D50 of the Co element is 10 nm, the thickness of the first carbon framework is 30 nm, and the thickness of the second carbon framework is 0 nm.

[0136] Example 5

[0137] The embodiment provides a lithium supplementing material and a preparation method thereof. The lithium supplementing material comprises an inner core, a first carbon framework and a second carbon framework, and the inner core comprises lithium-rich compounds Li2S, ZnS and Co element.

[0138] The preparation method of the lithium supplementing material comprises the following steps:

[0139] (1) 1.5 g of Zn(AC)2·4H2O and 1.5 g of Co(AC)2·4H2O are dissolved in 50 mL of deionized water. 15 g of Li2SO4·H2O is dissolved in 50 mL of deionized water, and ultrasonic stirring is performed for 10 min to form a homogeneous solution.

[0140] (2) 0.1 g of manganese nitrate, 0.1 g of nickel nitrate and 12 g of 2-aminoimidazole are added, ultrasonic stirring is performed for 1 min, stirring is performed at room temperature for 12 h, and standing is performed for 2 h. The above materials are collected by centrifugation and washed with anhydrous ethanol for 3 times, and frozen drying is performed for 24 h. The powder is collected and dried at 80℃ for 2 h.

[0141] (3) The dried powder is added to a 0.5M aqueous solution of p-formaldehyde and stirred at room temperature for 3 h to produce a cross-linking reaction with the amino group on the 2-aminoimidazole. Then, a 0.75M solution of p-phenylenediamine acetone is added and reacted at 60℃ for 5 h to form a covalent organic framework by cross-linking. After filtration, it is dried for standby use.

[0142] (4) The sample powder is placed in a tube furnace and heat treated at 700℃ for 2 h under the protection of an inert atmosphere, wherein the temperature rising rate in the temperature rising stage is 2℃ / min. Finally, the lithium supplementing material is obtained.

[0143] In the lithium supplementing material, the mass ratio of lithium-rich compound particles Li2S, ZnS, metal element (Co, Ni, Mn) is 100:10:8, the particle size D50 of the inner core is 200 nm, the particle size D50 of the Li2S particle is 5 nm, the particle size D50 of the ZnS is 10 nm, the average particle size D50 of the Co, Ni and Mn elements is 10 nm, the thickness of the first carbon framework is 30 nm, and the thickness of the second carbon framework is 5 nm.

[0144] Example 6

[0145] The embodiment provides a lithium supplementing material and a preparation method thereof. The lithium supplementing material comprises a core, a first carbon framework and a second carbon framework. The core comprises a lithium-rich compound Li3P, ZnS and Co element.

[0146] The preparation method of the lithium supplementing material comprises the following steps:

[0147] (1) 1.5 g of Zn(AC)2·4H2O and 1.5 g of Co(AC)2·4H2O are dissolved in 50 mL of deionized water. 18 g of Li3PO4·4H2O is dissolved in 50 mL of deionized water, and ultrasonic stirring is performed for 10 min to form a homogeneous solution.

[0148] (2) 12 g of 2-aminoimidazole is added, ultrasonic stirring is performed for 1 min, stirring is performed at room temperature for 12 h, and standing is performed for 2 h. The above materials are collected by centrifugation and washed with anhydrous ethanol for 3 times, and freeze-drying is performed for 24 h. The powder is collected and dried at 80℃ for 2 h.

[0149] (3) The dried powder is added to a 0.5M aqueous solution of p-formaldehyde and stirred at room temperature for 3 h to generate a cross-linking reaction with the amino group on the 2-aminoimidazole. Then, a 0.75M solution of p-phenylenediamine acetone is added and reacted at 60℃ for 5 h to form a covalent organic framework by cross-linking. After filtration, drying is performed for standby use.

[0150] (4) The sample powder is placed in a tube furnace and heat-treated at 700℃ for 2 h under the protection of an inert atmosphere, wherein the temperature rising rate in the temperature rising stage is 2℃ / min. Finally, the lithium supplementing material is obtained.

[0151] In the lithium supplementing material, the mass ratio of the lithium-rich compound particles Li3P, Zn3P2 and metal element (Co) is 100:10:5, the particle size D50 of the core is 200 nm, the particle size D50 of the Li3P particles is 5 nm, the particle size D50 of the Zn3P2 is 10 nm, the particle size D50 of the Co element is 10 nm, the thickness of the first carbon framework is 30 nm, and the thickness of the second carbon framework is 5 nm.

[0152] Comparative Example 1

[0153] The lithium supplementing material provided in the comparative example is different from the lithium supplementing material in the embodiment 1 only in that the lithium supplementing material comprises a core and a carbon layer, and the core comprises lithium-rich compound particles Li2S and the carbon layer is an amorphous carbon layer. The preparation method of the lithium supplementing material comprises the following steps:

[0154] (1) 15 g of Li2SO4·H2O is dissolved in 50 mL of deionized water, and 48 g of glucose is dissolved in 50 mL of deionized water. Ultrasonic stirring is performed for 10 min to form a homogeneous solution.

[0155] (2) After freeze-drying for 24 h, the sample was broken and sieved, and then was placed in a tube furnace for heat treatment at 700℃ for 2 h under the protection of inert atmosphere, wherein the temperature rising rate was 2℃ / min in the temperature rising stage, and finally the lithium supplement material was obtained.

[0156] The lithium supplement material provided by the above-mentioned examples 1 to 6 and the lithium supplement material provided by comparative example 1 were respectively assembled into lithium sulfide positive electrode, mutual matching positive electrode and lithium ion battery according to the following method:

[0157] Positive electrode: the lithium supplement material was mixed with polyvinylidene fluoride and SP-Li at a mass ratio of 80:8:12, and then was stirred and ball-milled to obtain positive electrode slurry, the positive electrode slurry was coated on the surface of aluminum foil, vacuum dried at 110℃ overnight, and then was rolled to obtain positive electrode sheet;

[0158] Negative electrode: graphite was mixed with carboxymethyl cellulose (CMC), SBR and SP at a mass ratio of 95.8:1.2:2:1, and then was stirred and ball-milled to obtain negative electrode slurry, the negative electrode slurry was coated on the surface of copper foil, vacuum dried at 110℃ overnight, and then was rolled to obtain negative electrode sheet;

[0159] Electrolyte: ethylene carbonate and methyl ethyl carbonate were mixed at a volume ratio of 3:7, and then LiPF6 was added to form electrolyte, and the concentration of LiPF6 was 1 mol / L;

[0160] Separator: polypropylene microporous separator;

[0161] Lithium ion battery assembly: the lithium ion full battery was assembled in the glove box under inert atmosphere according to the assembly order of graphite negative electrode sheet-separator-electrolyte-positive electrode sheet.

[0162] The electrochemical performance of each lithium ion battery assembled in the above lithium ion battery examples was tested, and the test conditions were as follows:

[0163] Constant current and constant voltage charging, first circle charge and discharge voltage 2.5-4.3V, current 0.1C, cutoff current 0.01C. The test results are shown in the following table 1:

[0164] Table 1

[0165]

[0166] As can be seen from the test results of Example 1 and Comparative Example 1 in Table 1, the ordinary carbon-coated lithium supplement agent in Comparative Example 1 has a large resistance and by-products reacting with the electrolyte, which makes it difficult to play a capacity advantage in the carbonate electrolyte, so the charge gram capacity and resistivity of the secondary battery in Comparative Example 1 are extremely poor. The lithium supplement material coated by the first carbon framework and the second carbon framework (Example 1) has excellent performance, so the performance of the secondary battery assembled therefrom is relatively excellent. Therefore, it is also illustrated that the ultra-fine nanoscale lithium-rich core prepared by the metal-organic carbon framework confinement greatly reduces the diffusion resistance, on the other hand, the compound nanoparticles and metal nanoparticles converted during the high-temperature sintering process can fix the intermediate products of material decomposition, prevent irreversible reaction with the carbonate electrolyte, and at the same time bring catalysis and gas inhibition effect, the outer cross-linked stable covalent carbon framework with uniform thickness and density plays a certain role in isolating water molecules, and the 0.1C charge capacity is fully played.

[0167] As can be seen from the test results of Examples 1-3 in Table 1, by changing the mass ratio of the lithium-rich compound particles, non-lithium compound particles, and metal particles in the core, the performance of the prepared lithium supplement material is obviously affected. Among them, the proportion of non-lithium compound particles in Example 2 is relatively high, which leads to a decrease in the proportion of lithium-rich compound particles, and the lithium ion content is affected, so the charge gram capacity is lower than that of Example 1. In Example 2, the content of metal particles is slightly increased, so that the metal particles can fix the intermediate products of material decomposition, reduce the reaction of lithium-rich compounds and intermediate products, and thus improve the efficiency of lithium ion supply. However, by controlling the proportion of lithium-rich compound particles, non-lithium compound particles, and metal particles within the range provided in the present application, lithium supplement materials with good performance can be obtained.

[0168] As can be seen from the test results of Examples 1 and 4 in Table 1, the presence or absence of the second carbon framework also has a key influence on the performance of the lithium supplement material. The charge gram capacity and resistivity of the secondary battery assembled by the lithium supplement material without the second carbon framework are poor. This is because part of the lithium-rich compound is exposed to the external environment through the pores of the first framework, so that the reaction of the lithium-rich compound with the external water vapor is intensified.

[0169] As can be seen from the test results of Examples 1 and 5 in Table 1, on the basis of Example 1, by adding a variety of catalytically active nano-metal particles, the capacity of the secondary battery is improved to a certain extent. This is because the metal particles can fix the intermediate products of material decomposition, reduce the reaction of lithium-rich compounds and intermediate products, and thus improve the efficiency of lithium ion supply.

[0170] As can be seen from the test results of Examples 1 and 6 in Table 1, when the core is replaced with lithium phosphide, the charging capacity at 0.1C reaches as high as 893 mAh / g. This indicates that the structure provided in this application can be applied to a variety of lithium-rich compound materials and can effectively improve the physicochemical properties of lithium-rich compound materials.

[0171] In summary, lithium-rich cores confined within metal-organic carbon frameworks, coupled with cross-linked coating of covalent organic frameworks, and transformed into multilayered carbon framework-coated ultrafine nanoscale lithium-rich cores through high-temperature sintering, exhibit excellent electrochemical performance. These lithium-rich cores provide an average "donor" lithium-ion specific capacity of 700 mAh / g in existing carbonate electrolyte systems. The desired lithium "donor" effect is observed when more catalytically active metals are incorporated. Due to their high "donor" lithium-ion capacity, potential compatibility with cathode materials, and battery manufacturing processes, the synthesized lithium-rich cores can be used in existing lithium-ion batteries (LIBs) and next-generation rechargeable lithium-ion battery systems with significant initial lithium loss.

[0172] In the description of the embodiments of this application, it should be noted that the orientation or positional relationship of the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and other indicators are based on the orientation or positional relationship of the drawings, and are only for the convenience of describing this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0173] The above-disclosed embodiments are merely preferred embodiments of this application and should not be construed as limiting the scope of this application. Those skilled in the art will understand that all or part of the processes for implementing the above embodiments and equivalent variations made in accordance with the claims of this application are still within the scope of this application.

Claims

1. A lithium supplementing material, characterized by, The lithium supplement material comprises: a first carbon framework forming a cage structure; a core comprising lithium-rich compound particles, the core being accommodated in the first carbon framework of the cage structure; the lithium supplement material further comprises a second carbon framework, the second carbon framework being coated on an outer surface layer of the first carbon framework, the first carbon framework being a metal-organic framework carbonized layer, and the second carbon framework being a covalent organic framework carbonized layer. 2.The lithium supplementing material of claim 1, wherein, The first carbon framework has a plurality of pores, and part of the lithium-rich compound particles are embedded in the pores. 3.The lithium supplementing material of claim 1, wherein, The core further comprises non-lithium compound particles having a first metal element and metal particles having a second metal element, wherein the first metal element and the second metal element are different, the first metal element comprises one or more of Zn, Fe, Cu, Ni, and Co, and the second metal element comprises one or more of Co, Mo, Mn, Pb, Pt, and Ru.

4. The lithium supplement material according to claim 3, wherein a mass ratio of the lithium-rich compound particles, the non-lithium compound particles, and the metal particles is 100: (5-20): (2-8); and / or the non-lithium compound particles and / or the metal particles are embedded in the pores of the first carbon framework.

5. The lithium supplement material according to claim 3, wherein a particle size D50 of the core is 200 nm-300 nm; and / or a particle size D50 of the lithium-rich compound particles is 3 nm-8 nm; and / or a particle size D50 of the non-lithium compound particles is 5 nm-10 nm; and / or a particle size D50 of the metal particles is 10 nm-100 nm.

6. The lithium supplement material according to claim 1, wherein The thickness A1 of the first carbon framework is 10 nm to 50 nm; the density of the first carbon framework is 0.10 g / cm 3 -0.20 g / cm 3 ; And / or, the thickness A2 of the second carbon framework is 2nm-10nm; the density of the second carbon framework is 0.40g / cm 3 -0.60g / cm 3 ; and / or a mass ratio of the core, the first carbon framework, and the second carbon framework is 100: (9-15): (1-5); and / or a thickness of the first carbon framework and the second carbon framework satisfies a relationship 1≤A1 / A2≤50; and / or a density p1 of the first carbon framework and a density p2 of the second carbon framework satisfy a relationship 2≤p2 / p1≤6.

7. A method for preparing a lithium supplement material, characterized in that, The method comprises: adding a first metal precursor, a second metal precursor, a lithium-rich compound precursor, and a first organic ligand into a solvent to uniformly mix, to obtain a material with a metal-organic framework coating; adding a second organic ligand and a third organic ligand into a solution of the material with the metal-organic framework coating, and after sufficient reaction, obtaining a precursor mixture with a covalent organic framework; sintering the precursor mixture to obtain the lithium supplement material; the lithium supplement material comprises a core, a first carbon framework, and a second carbon framework, the first carbon framework forms a cage structure, the core is accommodated in the first carbon framework of the cage structure, the second carbon framework is coated on an outer surface layer of the first carbon framework, the first carbon framework is a metal-organic framework carbonized layer, and the second carbon framework is a covalent organic framework carbonized layer.

8. A positive electrode sheet characterized by comprising: The positive electrode tab comprises a current collector and an active material layer disposed on the current collector, the active material layer comprising a positive electrode material and the lithium supplementing material as claimed in any one of claims 1-6, or the active material layer comprising a positive electrode material and the lithium supplementing material obtained by the preparation method as claimed in claim 7.

9. A secondary battery characterized by comprising: The battery comprises the positive electrode tab as claimed in claim 8, or the battery comprises the lithium supplementing material as claimed in any one of claims 1-6, or the battery comprises the lithium supplementing material obtained by the preparation method as claimed in claim 7.

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