Positive electrode lithium supplement additive, preparation method and application thereof

Abstract

The present application discloses a cathode lithium supplement additive, a preparation method thereof, and an application. The cathode lithium supplement additive of the present application includes a core body and a hermetic encapsulation layer coated on the core body. The core body includes a lithium-rich lithium supplement material, and the material of the hermetic encapsulation layer includes an inorganic aluminum compound. The cathode lithium supplement additive of the present application contains a hermetic encapsulation layer, has high density, high lithium ion insertion / extraction and lithium ion conductivity, low residual alkali content, and excellent lithium supplement effect, lithium supplement stability and processing performance. In addition, the preparation method of the cathode lithium supplement additive can ensure the stable structure and electrochemical performance of the prepared cathode lithium supplement additive, and has high efficiency and low production cost.

Description

Technical Field

[0001] This application belongs to the field of secondary batteries, and specifically relates to a cathode lithium supplement additive, its preparation method and application. Background Art

[0002] The oil energy crisis problems in the 1960s and 1970s forced people to search for new alternative energy sources. With the increasing awareness of environmental protection and energy crisis among people, lithium-ion batteries are considered to be one of the most promising energy sources due to their advantages such as high working voltage, high energy density, relatively low self-discharge level, no memory effect, no heavy metal element pollution such as lead and cadmium, and extremely long cycle life.

[0003] During the first charging process of lithium-ion batteries, a solid electrolyte interface (SEI) film is usually formed on the surface of the negative electrode, and this process will consume a large amount of Li + , which means that part of the Li + removed from the cathode material is irreversibly consumed, corresponding to a decrease in the reversible specific capacity of the battery cell. Negative electrode materials, especially silicon-based negative electrode materials, will further consume Li + , resulting in lithium loss of the cathode material and reducing the first Coulombic efficiency and battery capacity of the battery. For example, in a lithium-ion battery system using a graphite negative electrode, about 10% of the lithium source is consumed during the first charging. When using high specific capacity negative electrode materials, such as alloy-based (silicon, tin, etc.), oxide-based (silicon oxide, tin oxide) and amorphous carbon negative electrodes, the consumption of the cathode lithium source will be further aggravated.

[0004] To improve the low Coulombic efficiency problem caused by irreversible loss of the negative electrode, in addition to pre-lithiation of the negative electrode material and electrode sheet, lithium supplementation of the cathode can also meet the requirements of high energy density. For example, the currently publicly reported lithium-rich iron-based material has a theoretical capacity as high as 867 mAh / g, the working voltage window is the same as that of conventional lithium-ion batteries, and it basically does not participate in the electrochemical process in the later stage. It is a cathode lithium supplement additive with broad prospects. In another publicly reported cathode lithium supplement material Li5FeO4, it is prepared by the sol-gel method. This material used as a cathode lithium supplement material for lithium-ion batteries has the characteristics of large charging capacity and small discharge capacity. However, this material is extremely demanding on the environmental adaptability, and there is a large amount of residual alkali on the surface layer, which is not easy to process. In another publicly reported carbon-coated lithium ferrite material, the carbon coating isolates the external environment, alleviates the contact between lithium ferrite and water in the air, thereby improving the material stability; nevertheless, the coating layer is always difficult to completely isolate the contact with water in the air, resulting in material deterioration and failure. Moreover, after carbon coating, although the electronic conductivity of the carbon-coated lithium ferrite material can be improved, the path of ion insertion and extraction is increased and the ionic conductivity is affected, resulting in the lithium supplementation effect of the carbon-coated lithium ferrite material. At the same time, there is still residual alkali in the coating layer of the carbon-coated lithium ferrite material or at the interface between the coating layer and the lithium ferrite core, resulting in its difficult processing. Summary of the Invention

[0005] The purpose of the present application is to overcome the above deficiencies of the prior art, and provide a cathode lithium supplement additive and a preparation method thereof, so as to solve the technical problems that the ionic conductivity of the cathode lithium supplement additive with the existing coating structure is reduced or the processability is further unsatisfactory.

[0006] Another purpose of the present application is to provide a cathode sheet and a secondary battery containing the electrode sheet, so as to solve the technical problems that the initial Coulomb efficiency and battery capacity of the existing secondary battery are unsatisfactory.

[0007] In order to achieve the above application purpose, in the first aspect of the present application, a cathode lithium supplement additive is provided. The cathode lithium supplement additive of the present application includes a core body and a sealing encapsulation layer coated on the core body. The core body includes a lithium-rich lithium supplement material, and the material of the sealing encapsulation layer includes an inorganic aluminum compound.

[0008] Further, the inorganic aluminum compound is distributed on the inner surface of the sealing encapsulation layer close to the core body.

[0009] Or further, in the direction from the inner surface to the outer surface of the sealing encapsulation layer close to the core body, the content of the inorganic aluminum compound decreases in a gradient.

[0010] Further, the inorganic aluminum compound includes an inorganic aluminum compound generated by heat treatment.

[0011] Further, the inorganic aluminum compound includes at least one of Al2O3, LiAlO2, and Li5AlO4.

[0012] Furthermore, the inorganic aluminum compound includes Li5AlO4, LiAlO2, and Al2O3, and Li5AlO4, LiAlO2, and Al2O3 respectively form a Li5AlO4 coating layer, a LiAlO2 coating layer, and an Al2O3 coating layer. In the direction from the inside to the surface of the core body, the Li5AlO4 coating layer, the LiAlO2 coating layer, and the Al2O3 coating layer are sequentially coated to form a transition aluminum-containing coating layer, and the Li5AlO4 coating layer is coated on the surface of the core body.

[0013] Further, the sealing encapsulation layer includes an ion conductor encapsulation layer, and the ion conductor encapsulation layer coats the surface of the core body. The material of the ion conductor encapsulation layer includes an inorganic aluminum compound.

[0014] Further, the mass content of the inorganic aluminum compound in the sealing encapsulation layer is 0.1-5.0%.

[0015] Further, the lithium-rich lithium supplement material is a lithium supplement material with an anti-fluorite structure.

[0016] Further, the lithium-rich lithium supplement material is L x M y Nz O q ; wherein, L in the molecular formula is Li or / and a mixed alkali metal element of Li and at least one of K and Na not exceeding 30%; M includes at least one of Fe, Co, Mn, Ni, Si, and Al; N includes at least one of Fe, Co, Mn, Ni, Si, Al or other equivalent or hetero-valent metal elements, O is oxygen element, x is 4-6, y is 0.7-1.0, z is 0.01-0.3, and q is 4-5.

[0017] Further, the particle size of the core is 0.2 μm - 20 μm.

[0018] Still further, the ionic conductor encapsulation layer includes a first ionic conductor encapsulation layer and a second ionic conductor encapsulation layer. The first ionic conductor encapsulation layer coats the surface of the core, and the material of the first ionic conductor encapsulation layer is an inorganic aluminum compound. The second ionic conductor encapsulation layer coats the surface of the first ionic conductor encapsulation layer facing away from the core.

[0019] Still further, the total molar ratio of L to M and N in the molecular formula is 4-7:1.

[0020] Still further, when M in the molecular formula is Fe, N is Al.

[0021] Still further, the hermetic encapsulation layer further includes an electronic conductor encapsulation layer, and the electronic conductor encapsulation layer coats the surface of the ionic conductor encapsulation layer facing away from the core.

[0022] Still further, the material of the electronic conductor encapsulation layer includes at least one of carbon materials, conductive oxides, and conductive organic compounds.

[0023] Still further, the thickness of the electronic conductor encapsulation layer is 50 - 200 nm.

[0024] Still further, the material of the electronic conductor encapsulation layer is a carbon material, and the content range of the carbon material in the positive electrode lithium supplement additive is 2 wt% - 10 wt%.

[0025] Further, the specific surface area of the positive electrode lithium supplement additive is 0.2 - 5.0 m 2 / g.

[0026] Further, the loose bulk density of the positive electrode lithium supplement additive is 0.35 - 0.80 g / mL, and its tapped density is 0.50 - 1.20 g / mL.

[0027] Further, the resistivity of the positive electrode lithium supplement additive at 25 °C is lower than 5 Ω·cm.

[0028] Further, the material of the electronic conductor encapsulation layer includes at least one of carbon materials, conductive oxides, and conductive organic compounds.

[0029] Further, the capacity decay rate of the positive electrode sheet prepared from the positive electrode lithium supplement additive, binder and conductive agent after being stored at an environmental humidity of 25% for 20 hours relative to the capacity after being stored for 0.5 hours is not more than 30%; and / or

[0030] Further, the capacity decay rate of the positive electrode sheet prepared from the positive electrode lithium supplement additive, binder and conductive agent after being stored at an environmental humidity of 10% for 20 hours relative to the capacity after being stored for 0.5 hours is not more than 20%.

[0031] In the second aspect of the present application, a preparation method of the positive electrode lithium supplement additive of the present application is provided. The preparation method of the positive electrode lithium supplement additive of the present application includes the following steps:

[0032] Form a coating film layer on the surface of the lithium-rich lithium supplement material with the material containing an aluminum source, and then perform a first heat treatment in an oxygen-containing environment to form a hermetic encapsulation layer on the surface of the lithium-rich lithium supplement material;

[0033] Or

[0034] Perform a second heat treatment on the lithium-rich lithium supplement material in an aluminum-containing container and in an oxygen-containing environment to form a hermetic encapsulation layer on the surface of the lithium-rich lithium supplement material.

[0035] Further, the method for forming a coating film layer on the surface of the lithium-rich lithium supplement material with the material containing an aluminum source includes the following steps:

[0036] Prepare a solution of the aluminum source, disperse the lithium-rich lithium supplement material in the solution, and then initiate an aluminum precipitation reaction to deposit an aluminum compound on the surface of the lithium-rich lithium supplement material to form a coating film layer containing an aluminum compound.

[0037] Further, the aluminum source includes at least one of aluminum isopropoxide, aluminum organic acid salt, aluminum trioxide, aluminum nitrate, aluminum chloride, aluminum sulfate, and aluminum silicate.

[0038] Further, the temperature of the first heat treatment is 700 - 1000 °C, and the time is 20 - 48 hours.

[0039] Further, the temperature of the second heat treatment is 500 - 1000 °C, and the time is 20 - 48 hours.

[0040] In the third aspect of the present application, a positive electrode material is provided. The positive electrode material of the present application includes a positive electrode active material and the positive electrode lithium supplement additive of the present application or a positive electrode lithium supplement additive prepared by the preparation method of the positive electrode lithium supplement additive of the present application.

[0041] In a fourth aspect of the present application, a positive electrode sheet is provided. The positive electrode sheet of the present application includes a positive electrode current collector and a positive electrode active layer combined on the surface of the positive electrode current collector, and the positive electrode active layer is doped with the positive electrode lithium supplement additive of the present application or the positive electrode lithium supplement additive prepared by the preparation method of the positive electrode lithium supplement additive of the present application.

[0042] In a fifth aspect of the present application, a secondary battery is provided. The present application includes a positive electrode sheet and a negative electrode sheet, and the positive electrode sheet is the positive electrode sheet of the present application.

[0043] Compared with the prior art, the present application has the following technical effects:

[0044] The positive electrode lithium supplement additive of the present application coats the core containing the lithium-rich material with a sealing layer. In this way, the sealing layer can effectively coat the lithium-rich material contained in the core, isolate the core from the outside world, and avoid the contact between the core and moisture and carbon dioxide in the outside world, ensuring the stability of the core and thus ensuring the lithium supplement effect and stability of the positive electrode lithium supplement additive. On the one hand, the inorganic aluminum compound contained in the sealing layer enhances the densification performance of the sealing layer and the isolation effect between the core and the outside world; on the other hand, it can act as an ionic conductivity additive to improve the insertion and extraction of lithium ions and the lithium ion conductivity, thereby improving the lithium supplement effect of the positive electrode lithium supplement additive; thirdly, the inorganic aluminum compound contained in the sealing layer can reduce the residual alkali content in the core and the sealing layer, endowing the positive electrode lithium supplement additive of the present application with excellent processing performance. Since the core of the positive electrode lithium supplement additive of the present application contains a lithium-rich material, it endows the positive electrode lithium supplement additive of the present application with the ability to provide abundant lithium, so that it can act as a "sacrificial agent" during the first cycle charging process and release all lithium ions at one time as much as possible, improving the first efficiency and overall electrochemical performance of the battery.

[0045] The preparation method of the positive electrode lithium supplement additive of the present application can effectively prepare the positive electrode lithium supplement additive with a core-shell structure, and can make the sealing layer contain an inorganic aluminum compound to effectively coat the core containing the lithium-rich material. At the same time, the prepared positive electrode lithium supplement additive has a low residual alkali content, thus ensuring excellent lithium supplement effect, stable lithium supplement performance and good processing performance of the prepared positive electrode lithium supplement additive. In addition, the preparation method of the positive electrode lithium supplement additive can ensure the stability of the structure and electrochemical performance of the prepared positive electrode lithium supplement additive, with high efficiency and cost savings in production.

[0046] Due to the inclusion of the positive electrode lithium supplement additive of the present application in the positive electrode material and the positive electrode sheet of the present application, the components contained in the positive electrode active layer of the positive electrode sheet of the present application are uniformly dispersed, the film layer has high quality, and excellent electrochemical performance is imparted to the positive electrode sheet of the present application. Moreover, during the charge and discharge process, the contained positive electrode lithium supplement additive can serve as a lithium source and act as a "sacrificial agent" during the first cycle charging process to supplement the irreversible lithium ions consumed by the negative electrode to form the SEI film, thereby maintaining an abundant supply of lithium ions in the battery system and improving the first efficiency and overall electrochemical performance of the battery.

[0047] Due to the inclusion of the electrode sheet of the present application in the secondary battery of the present application, the lithium-ion battery of the present application has excellent initial Coulomb efficiency, battery capacity, and cycle performance, a long service life, and stable electrochemical performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0049] Figure 1 It is a schematic structural diagram of the positive electrode lithium supplement additive in the embodiment of the present application;

[0050] Figure 2 It is a schematic structural diagram of the ion conductor encapsulation layer included in the positive electrode lithium supplement additive in the embodiment of the present application;

[0051] Figure 3 For Figure 2 It is a schematic structural diagram of the ion conductor encapsulation layer included in the positive electrode lithium supplement additive in the embodiment of the present application, showing the first ion conductor encapsulation layer;

[0052] Figure 4 For Figure 3 It is a schematic structural diagram of the ion conductor encapsulation layer included in the positive electrode lithium supplement additive in the embodiment of the present application, showing the second ion conductor encapsulation layer;

[0053] Figure 5 It is a schematic structural diagram of the positive electrode lithium supplement additive in the embodiment of the present application, which simultaneously includes an ion conductor encapsulation layer and an electron conductor encapsulation layer 22;

[0054] Figure 6 It is a scanning electron microscope photograph of the positive electrode lithium supplement additive provided in Embodiment 1 of the present application;

[0055] Figure 7 It is an X-ray diffraction pattern of the positive electrode lithium supplement additive provided in Embodiment 1 of the present application;

[0056] Figure 8 TEM image of the cathode lithium supplement additive provided in Embodiment 1 of the present application;

[0057] Figure 9 Initial charge specific capacity curve of the battery assembled with the cathode sheet containing the cathode lithium supplement additive provided in Embodiment 1. Detailed implementation manners

[0058] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application more clear and understandable, the present application will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0059] In the present application, the term "and / or" describes the association relationship of associated objects and indicates that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. Where A and B can be singular or plural. The character " / " generally represents an "or" relationship between the associated objects before and after.

[0060] In the present application, "at least one" means one or more, and "a plurality" means two or more. "At least one (item)" or similar expressions thereof refer to any combination of these items, including any combination of single item (item) or plural items (items). For example, "at least one (item) of a, b, or c", or, "at least one (item) of a, b, and c" can both represent: a, b, c, a - b (i.e., a and b), a - c, b - c, or a - b - c, where a, b, and c can be single or plural respectively.

[0061] It should be understood that in various embodiments of the present application, the sequence numbers of the above processes do not mean the order of execution. Some or all 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 to the implementation process of the embodiments of the present application.

[0062] 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", "the" and "said" used in the embodiments of the present application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0063] In the description of the embodiments of the present application, the weight of the relevant components mentioned not only can refer to the specific content of each component, but also can represent the proportional relationship of the weights between the components. Therefore, as long as the content of the relevant components in the description of the embodiments of the present application is scaled up or down proportionally, it is within the scope disclosed in the description of the embodiments of the present application. Specifically, the mass described in the description of the embodiments of the present application can be mass units well-known in the chemical industry such as μg, mg, g, kg, etc.

[0064] The terms "first" and "second" are only used for descriptive purposes to distinguish objects such as substances from each other, and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. For example, without departing from the scope of the embodiments of the present application, the first XX can also be referred to as the second XX, and similarly, the second XX can also be referred to as the first XX. Thus, the features defined with "first" and "second" can explicitly or implicitly include one or more of such features.

[0065] In a first aspect, the embodiments of the present application provide a cathode lithium supplement additive. The cathode lithium supplement additive in the embodiments of the present application includes a core body and a sealing encapsulation layer coated on the core body, that is, the cathode lithium supplement additive in the embodiments of the present application has a core-shell structure. As in the embodiments, the structure of the cathode lithium supplement additive in the embodiments of the present application is as Figures 1 to 5 shown, including a core body 10 and a sealing encapsulation layer 20 coated on the core body 10.

[0066] Among them, the core body 10 includes a lithium-rich lithium supplement material, that is, in the cathode lithium supplement additive in the embodiments of the present application, the core body 10 is a lithium source for lithium supplementation. Since the core body 10 is rich in lithium, it is ensured that the cathode lithium supplement additive in the embodiments of the present application can provide abundant lithium. As an additive added to the electrode, it acts as a "sacrificial agent" during the first cycle charging process, and releases all the lithium ions contained in the cathode lithium supplement additive as much as possible at one time to supplement the irreversible lithium ions consumed by the formation of the SEI film on the negative electrode.

[0067] At the same time, the lithium-rich lithium supplement material contained in the core body 10 can be a conventional lithium supplement material or a newly developed lithium supplement material. Based on the function of the cathode lithium supplement additive in the embodiments of the present application, in the embodiments, the lithium-rich lithium supplement material is an anti-fluorite structure lithium supplement material. The anti-fluorite structure lithium supplement material endows the lithium-rich lithium supplement material with a unidirectional capacity characteristic, which can effectively de-lithiate during the first charging process and prevent lithium ions from re-embedding into the lithium supplement material during discharging, thereby ensuring the lithium supplementation effect of the lithium supplement additive of the present application.

[0068] The lithium-rich material contained in the core body 10 can be a ternary lithium supplement material or a binary lithium supplement material. As in the embodiments, the lithium-rich lithium supplement material is L x M y N z Oq ; wherein, L in the molecular formula is Li or / and a mixed alkali metal element of Li and at least one of K and Na not exceeding 30%; M includes at least one of Fe, Co, Mn, Al, Ni, and Si; N includes at least one of Fe, Co, Mn, Al, Ni, Si or other equivalent or heterovalent metal elements, O is oxygen element, x is 4 - 6, y is 0.7 - 1.0, z is 0.01 - 0.3, and q is 4 - 5. In a further embodiment, the molecular formula L x M y N z O q the total molar ratio of L, M, and N in is 4 - 7:1, specifically, typical but non-limiting molar ratios such as 4:1, 5:1, 6:1, 7:1, etc. In a specific embodiment, the L x M y N z O q The lithium-rich lithium-supplementing material shown is an aluminum-doped iron-based lithium-supplementing material, that is, M is Fe and N is Al at the same time. At this time, L x M y N z O q The lithium-rich lithium-supplementing material shown can be Li5Fe 0.98 Al 0.02 O4. These lithium-rich materials are rich in lithium and can release lithium ions during the first-cycle charging process to play an effective lithium-supplementing role. When the lithium-rich material has an antifluorite structure, it can also improve the unidirectional capacity characteristics of the lithium-rich material, thereby ensuring the lithium-supplementing effect of the lithium-supplementing additive in this application. When the lithium-rich material contains aluminum element doping, Al atoms exist in the form of substituting the iron atom lattice, and the Al atoms existing in this form can broaden the lithium-ion transmission channels and improve the lithium-ion extraction rate.

[0069] In the embodiment, the core body 10 can be at least one of primary particles and secondary particles, specifically, at least one of primary particles and secondary particles formed by the lithium-rich material contained in the core body 10. In the embodiment, the particle size of the core body 10 can be 0.2 μm - 20 μm. For example, when the core body 10 is a primary particle, the particle size of the primary particle, that is, the particle size distribution of the core body 10, is 0.2 μm - 2 μm; when the core body 10 is a secondary particle, the particle size of the secondary particle, that is, the particle size distribution of the core body 10, is 0.4 μm - 20 μm. Among them, the secondary particle refers to an agglomerated particle formed by the aggregation of more than one primary particle. By controlling the particle size morphology and particle size of the core body 10, on the basis of its ability to provide abundant lithium ions, the processability of the cathode lithium-supplementing additive in the preparation of lithium battery slurry is also improved. Among them, the smaller primary particle size can also extract more lithium.

[0070] In addition, although the lithium-rich material contained in the core body 10 in each of the above embodiments is rich in lithium, it is unstable in the presence of water and carbon dioxide, and it is prone to react with water and carbon dioxide, resulting in a reduction in the lithium supplementing effect of the cathode lithium supplementing additive in the embodiments of the present application. At the same time, the lithium-rich material generally also contains residual alkali formed due to processing, and these residual alkalis will further lead to a reduction in its processing performance. For example, it will cause a sharp increase in the viscosity of the slurry containing the above-mentioned lithium-rich material, and quickly gel and lose fluidity, so that subsequent processing cannot be carried out. Therefore, on the basis of the core body 10 in each of the above embodiments, the sealing encapsulation layer 20 contained in the cathode lithium supplementing additive in each of the above embodiments is coated on the core body 10 to form a complete coating layer, such as Figure 1 as shown. In this way, the sealing encapsulation layer 20 can effectively encapsulate the lithium-rich material contained in the core body 10, isolate the core body 10 from the outside world, avoid the contact between the core body 10 and moisture and carbon dioxide in the outside world, and ensure the stability of the core body 10, thereby ensuring the lithium supplementing effect and the stability of lithium supplementation of the cathode lithium supplementing additive.

[0071] Among them, the material of the sealing encapsulation layer 20 includes an inorganic aluminum compound. In this way, an inorganic aluminum compound is added to the sealing encapsulation layer 20, and the presence of this inorganic aluminum compound enhances the densification performance of the sealing encapsulation layer 20 and enhances the isolation effect between the core body 10 and the outside world. Moreover, this inorganic aluminum compound can also act as an ionic conductivity additive to improve the insertion and extraction of lithium ions and the lithium ion conductivity, thereby improving the lithium supplementing effect of the cathode lithium supplementing additive. At the same time, the presence of the inorganic aluminum compound also makes the content of residual alkali in the core body 10 and the sealing encapsulation layer 20 low, endowing the cathode lithium supplementing additive in the embodiments of the present application with excellent processing performance.

[0072] The distribution of the added inorganic aluminum compound in the sealing encapsulation layer 20 can be the following several situations:

[0073] In the embodiment, the inorganic aluminum compound is distributed on the inner surface of the sealing encapsulation layer 20 close to the core body 10.

[0074] In other embodiments, in the direction from the inner surface to the outer surface of the sealing encapsulation layer 20 close to the core body 10, the content of the inorganic aluminum compound decreases in a gradient.

[0075] By controlling the distribution of the inorganic aluminum compound in the sealing encapsulation layer 20, the inorganic aluminum compound plays the role described above in the sealing encapsulation layer 20, thereby improving the densification, ionic conductivity of the sealing encapsulation layer 20 and reducing the content of residual alkali, and thus improving the lithium supplementing stability, lithium supplementing effect and processing performance of the cathode lithium supplementing additive in the embodiments of the present application.

[0076] In the embodiments, the above inorganic aluminum compound includes an inorganic aluminum compound generated by heat treatment. For example, in the embodiments, the above inorganic aluminum compound includes at least one of Al2O3, LiAlO2, and Li5AlO4. These inorganic aluminum compounds have high ionic conductivity and strong binding force with the lithium-rich lithium-supplementing material of the core body 10, thereby enhancing the binding strength between the hermetic encapsulation layer 20 and the core body 10. At the same time, these inorganic aluminum compounds can form a dense film layer, improving the density of the hermetic encapsulation layer 20 and the lithium-supplementing stability and effect of the cathode lithium-supplementing additive.

[0077] In the embodiments, when the inorganic aluminum compound includes Li5AlO4, it can form a Li5AlO4 coating layer to coat the core body 10, or it can be further distributed and doped into the core body 10. For example, it diffuses from the Li5AlO4 coating layer to the core body 10 in a diffusion manner to form a transition layer containing Li5AlO4 on the surface layer of the core body 10. Of course, the Li5AlO4 can also be molecules in the core body 10, such as diffusing into the core body 10 to form a transition layer containing Li5AlO4 on the surface layer of the core body 10.

[0078] In the embodiments, when the inorganic aluminum compound includes Al2O3 and LiAlO2, the Al2O3 and LiAlO2 can form a mixture coating layer with mutual doping, or a composite layer structure of a LiAlO2 coating layer and an Al2O3 coating layer. Of course, the inorganic aluminum compound can also be any one of Al2O3 and LiAlO2, and any one of them forms a corresponding coating layer.

[0079] In a further embodiment, the above inorganic aluminum compound simultaneously includes Al2O3, LiAlO2, and Li5AlO4. Moreover, the Li5AlO4, LiAlO2, and Al2O3 respectively form a Li5AlO4 coating layer, a LiAlO2 coating layer, and an Al2O3 coating layer. And from the inside to the surface direction of the core body 10, the Li5AlO4 coating layer, the LiAlO2 coating layer, and the Al2O3 coating layer are sequentially coated to form a transition aluminum-containing coating layer that coats the core body 10. That is, the Li5AlO4 coating layer coats the surface of the core body 10, the LiAlO2 coating layer coats the outer surface of the Li5AlO4 coating layer, and the Al2O3 coating layer coats the outer surface of the LiAlO2 coating layer. At this time, the hermetic sealing layer 20 includes this transition aluminum-containing coating layer. Among them, Li5AlO4 can also be doped in the core body 10. As described above, it can be diffused from the Li5AlO4 coating layer into the core body 10 to form a transition layer containing Li5AlO4 in the surface layer of the core body 10. The outer surface refers to the surface of the corresponding coating layer that is away from the core body 10. When the Li5AlO4 coating layer, the LiAlO2 coating layer, and the Al2O3 coating layer form a transition aluminum-containing coating layer, the interfaces of the three layers can have a clear cross-section, or there can be a transition layer formed by the materials of the adjacent two layers between the interfaces of the adjacent two layers. Whether there is a transition layer at the interface between the adjacent two layers is specifically determined according to the method of forming the Li5AlO4 coating layer, the LiAlO2 coating layer, and the Al2O3 coating layer. When including the formation of a transition aluminum-containing coating layer by the Li5AlO4 coating layer, the LiAlO2 coating layer, and the Al2O3 coating layer, the Li5AlO4 coating layer directly coats the surface of the core body 10. The Li5AlO4 contained in the Li5AlO4 coating layer is rich in lithium, and its crystal form is similar to the crystal form of the lithium-rich material contained in the core body 10. It can be considered that aluminum atoms partially replace the metal atoms contained in the lithium-rich lithium-supplementing material in the core body 10 in terms of crystal form. Therefore, the Li5AlO4 coating layer can effectively broaden the transmission channels of lithium ions and improve the ionic conductivity of the Li5AlO4 coating layer. Moreover, the LiAlO2 contained in the LiAlO2 coating layer also has good ionic conductivity characteristics, which is also conducive to the conduction of lithium ions. And the LiAlO2 coating layer and the Al2O3 coating layer form a composite dense composite layer, which also has good denseness and can effectively isolate the core body 10 from adverse factors in the environment such as water vapor and dioxide, and can also effectively isolate the direct contact between the electrolyte and the core body 10 and the Li5AlO4 coating layer, thereby effectively avoiding or reducing the negative reaction between the electrolyte and the core body 10 and the Li5AlO4 coating layer, and thus improving the electrochemical performance such as the lithium-supplementing performance and effect of the positive lithium-supplementing additive of the present application.

[0080] Based on the distribution of the above inorganic aluminum compound in the hermetic sealing layer 20. In the embodiment, as Figure 2As shown, the hermetic encapsulation layer 20 includes an ionic conductor encapsulation layer 21, and the ionic conductor encapsulation layer 21 coats the surface of the core body 10. The material of the ionic conductor encapsulation layer 21 includes the above-mentioned inorganic aluminum compound. Specifically, the ionic conductor encapsulation layer 21 can be the coating layer formed by the above-mentioned inorganic aluminum compound. Further, it can be the transition aluminum-containing coating layer formed by sequentially coating the above-mentioned Li5AlO4 coating layer, LiAlO2 coating layer, and Al2O3 coating layer.

[0081] In the above embodiments, the mass content of the inorganic aluminum compound contained in the hermetic encapsulation layer 20 in the hermetic encapsulation layer 20 can be controlled to be 0.1%-5.0%. By controlling and adjusting the content of the inorganic aluminum compound in the hermetic encapsulation layer 20, the above-mentioned functions of the inorganic aluminum compound are improved, the compactness and ionic conductivity of the hermetic encapsulation layer 20 are increased, and the residual alkali content is reduced, thereby improving the lithium supplement stability, effect, and processing performance of the positive electrode lithium supplement additive.

[0082] Based on Figure 2 the structure of the positive electrode lithium supplement additive shown, in the embodiment, as Figure 3 shown, the ionic conductor encapsulation layer 21 includes a first ionic conductor encapsulation layer 211. The first ionic conductor encapsulation layer 211 coats the surface of the core body 10, and the material of the first ionic conductor encapsulation layer 211 is an inorganic aluminum compound. The inorganic aluminum compound is formed into the first ionic conductor encapsulation layer 211 to coat the core body 10, so that the first ionic conductor encapsulation layer 211 has high compactness and ionic conductivity, effectively reduces the thickness of the hermetic encapsulation layer 20, thereby shortening the lithium ion migration path, and improving the lithium supplement stability and effect of the positive electrode lithium supplement additive. Moreover, it can effectively reduce the residual alkali content in the core body 10 and the first ionic conductor encapsulation layer 211, and further improve the processing performance of the positive electrode lithium supplement additive in the embodiment of the present application.

[0083] In the embodiment, the thickness of the first ionic conductor encapsulation layer 211 can be controlled to be 10-50 nm. The thickness of the first ionic conductor encapsulation layer 211 is low, and it can independently form the hermetic encapsulation layer 20.

[0084] Based on Figure 2 and Figure 3 the structure of the positive electrode lithium supplement additive shown, as Figure 4As shown, the ionic conductor encapsulation layer 21 may further include a second ionic conductor encapsulation layer 212, and the second ionic conductor encapsulation layer 212 is coated on the surface of the first ionic conductor encapsulation layer 211 facing away from the core body 10, that is, the second ionic conductor encapsulation layer 212 coats the first ionic conductor encapsulation layer 211, and forms an ionic conductor encapsulation layer 21 with a composite structure with the first ionic conductor encapsulation layer 211. Adding the second ionic conductor encapsulation layer 212 can form a composite ionic conductor encapsulation layer structure with the first ionic conductor encapsulation layer 211 to enhance the ionic conductivity and compactness of the ionic conductor encapsulation layer 21, thereby enhancing the ionic conductivity and lithium supplementation stability of the positive electrode lithium supplementation additive, which is beneficial to the outward transport of lithium ions in the core body. At the same time, after the core body 10 releases all lithium ions as a "sacrificial body", the ionic conductor encapsulation layer 21 can be reused for the second time to play an auxiliary role in enhancing ion transport inside the electrode. Based on the function of the ionic conductor encapsulation layer 21, when the functional encapsulation 20 only contains the ionic conductor encapsulation layer 21, the ionic conductor encapsulation layer 21 should be a dense structure and fully coated. At this time, the first ionic conductor encapsulation layer 211 and the second ionic conductor encapsulation layer 212 can each be partially coated or fully coated, but the composite ionic conductor encapsulation layer structure formed by the two should be fully coated and dense. In addition, the thickness of the second ionic conductor encapsulation layer 212 can be adjusted according to actual needs, such as controlling the thickness of the second ionic conductor encapsulation layer 212 according to the requirement that the composite ionic conductor encapsulation layer structure of the first ionic conductor encapsulation layer 211 and the second ionic conductor encapsulation layer 212 is densely coated.

[0085] In the embodiment, the material of the second ionic conductor encapsulation layer 212 may include at least one of perovskite type, NASICON type, and garnet type. In a specific embodiment, the perovskite type includes Li 3x La 2 / 3-x TiO3 (LLTO), specifically such as 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 O3, etc., and the NASICON type is such as but not limited to Li 1.4 Al0.4 Ti 1.6 (PO4)3 (LATP), the garnet type includes 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 them. By selecting the material of the second ion conductor encapsulation layer 212, the ionic conductivity of the ion conductor encapsulation layer 21 can be further improved. Of course, the thickness of the above-mentioned first ion conductor encapsulation layer 211 and the second ion conductor encapsulation layer 212 can also be adjusted to adjust the thickness of the ion conductor encapsulation layer 21, so as to optimize the densification of the hermetic encapsulation layer 20 and adjust the ion transport path.

[0086] In a further embodiment, on the basis of Figures 2 to 4 , the hermetic encapsulation layer 20 further includes an electronic conductor encapsulation layer 22. As shown in Figure 5 , the electronic conductor encapsulation layer 22 is coated on the surface of the ion conductor encapsulation layer 21 facing away from the core body 10, that is, the electronic conductor encapsulation layer 22 is coated on the ion conductor encapsulation layer 21. The added electronic conductor encapsulation layer 22 in the above-mentioned hermetic encapsulation layer 20 can enhance the electronic conductivity of the hermetic encapsulation layer 20, thereby enhancing the electronic conductivity of the cathode lithium supplement additive, which is beneficial to reducing the impedance inside the electrode; at the same time, during and after the release process of the core body 10 as a "sacrificial material", the electronic conductor encapsulation layer 22 can also be reused and play an auxiliary role as a conductive agent inside the electrode. Moreover, the electronic conductor encapsulation layer 22 can also play a synergistic role in densification with the ion conductor encapsulation layer 21, improving the densification of the hermetic encapsulation layer 20, thereby improving the lithium supplement stability and lithium supplement effect of the cathode lithium supplement additive. Based on the function of the electronic conductor encapsulation layer 22, it can be fully coated or partially coated.

[0087] In the embodiments, the thickness of the electronic conductor encapsulation layer 22 is 50 - 200 nm. In some other embodiments, the material of the electronic conductor encapsulation layer 22 includes at least one of carbon materials, conductive oxides, and conductive organic compounds. In a specific embodiment, when the material of the electronic conductor encapsulation layer 22 is a carbon material, the carbon material includes at least one of amorphous carbon, carbon nanotubes, graphite, carbon black, graphene, etc. For example, when the material of the electronic conductor encapsulation layer 22 is a carbon material, and the content range of the carbon material in the cathode lithium supplement additive is 2 wt% - 10 wt%, specifically, it can be typical but non-limiting contents such as 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, etc. In some other specific embodiments, when the material of the electronic conductor encapsulation layer 22 is a conductive oxide, the conductive oxide can include at least one of In2O3, ZnO, and SnO2. The conductive organic compound can be a conductive polymer, etc. By adjusting the content and material of the electronic conductor encapsulation layer 22, its electronic conductivity can be further improved. For example, it is detected that the resistivity of the cathode lithium supplement additive at 25 °C is lower than 5 Ω·cm in the presence of the electronic conductor encapsulation layer 22.

[0088] In addition, due to the high density and high surface quality of the sealing encapsulation layer 20 of the cathode lithium supplement additive in the above embodiments, it is measured that the specific surface area of the cathode lithium supplement additive is 0.2 - 5.0 m 2 / g. The loose bulk density of the cathode lithium supplement additive is 0.35 - 0.80 g / mL, and further can be 0.50 - 0.80 g / mL; its tapped density is 0.50 - 1.20 g / mL, and further can be 0.80 - 0.80 g / mL.

[0089] Based on the structure and performance of the lithium supplement additive in the embodiments of the present application, as the lithium supplement additive in the embodiments of the present application has a sealing encapsulation layer 20 containing an inorganic aluminum compound, therefore, it has excellent storage properties, processability, and stable electrochemical performance. For example, it is detected that for the cathode sheet directly prepared from the lithium supplement additive in the embodiments of the present application, such as the cathode sheet prepared from the cathode lithium supplement additive, binder, and conductive agent, the capacity decay rate of the capacity after storing for 20 hours at an environmental humidity of 25% relative to the capacity after storing for 0.5 hours is not more than 30%, and further not more than 14%. The capacity decay rate of the capacity after storing for 20 hours at an environmental humidity of 10% relative to the capacity after storing for 0.5 hours is not more than 20%, and further not more than 2.4%. This demonstrates that the lithium supplement additive in the embodiments of the present application has excellent storage properties, high lithium supplement effect, and lithium supplement stability. Ideally, the lithium supplement additive in the above embodiments of the present application is stored in a favorable environment such as a vacuum environment with dryness and no oxygen to maximize the electrochemical performance of the lithium supplement additive in the present application.

[0090] Second aspect, embodiments of the present application further provide a preparation method of the above-mentioned cathode lithium supplement additive. In the embodiment, the preparation method of the cathode lithium supplement additive of the present application embodiment includes the following steps:

[0091] S01: Form a coating film layer on the surface of the lithium-rich lithium supplement material with the material containing an aluminum source;

[0092] S02: Subject the lithium-rich lithium supplement material with the coating film layer formed in step S01 to a first heat treatment in an oxygen-containing environment to form a hermetic encapsulation layer on the surface of the lithium-rich lithium supplement material.

[0093] Among them, the lithium-rich lithium supplement material in step S01 is the lithium-rich lithium supplement material in the core body 10 for forming the above-mentioned cathode lithium supplement additive, and the hermetic encapsulation layer is also the hermetic encapsulation layer 20 in the above-mentioned cathode lithium supplement additive. Then the coating film layer is the precursor layer of the hermetic encapsulation layer. Therefore, the materials, structures, thicknesses, etc. of the lithium-rich lithium supplement material and the hermetic encapsulation layer in step S01 are the same as those of the core body 10 and the hermetic encapsulation layer 20 of the above-mentioned cathode lithium supplement additive. To save space, the lithium-rich lithium supplement material and the hermetic encapsulation layer in step S01 will not be elaborated here.

[0094] In addition, the lithium-rich lithium supplement material can be prepared according to the preparation methods of each lithium-rich lithium supplement material.

[0095] In the embodiment, the method of forming a coating film layer on the surface of the lithium-rich lithium supplement material with the material containing an aluminum source in step S01 includes the following steps:

[0096] Prepare a solution of the aluminum source, disperse the lithium-rich lithium supplement material in the solution, and then initiate an aluminum precipitation reaction to deposit an aluminum compound on the surface of the lithium-rich lithium supplement material to form a coating film layer containing the aluminum compound.

[0097] The coating film layer is in-situ formed on the surface of the lithium-rich lithium supplement material by the solution in-situ deposition method to improve the integrity and compactness of the coating film layer. Then the aluminum source can be the precursor material of the material forming the coating film layer. In the embodiment, the aluminum source can be at least one of aluminum isopropoxide, aluminum organic acid root salt, aluminum trioxide, aluminum nitrate, aluminum chloride, aluminum sulfate, and aluminum silicate. Among them, the aluminum organic acid root salt can be at least one of aluminum acetate, aluminum citrate, and aluminum isopropoxide. When the aluminum source is these inorganic aluminum oxides, its particle size can be in the nanometer range. These aluminum sources can effectively decompose or react to generate aluminum-containing precipitates and be in-situ combined on the surface of the lithium-rich lithium supplement material. At the same time, the solvent of the solution should ensure the stability of the lithium-rich lithium supplement material, and the type of solvent can be selected according to the solubility of the aluminum source. In addition, the content or thickness of the coating film layer can be controlled by controlling the mixing ratio of the aluminum source and the lithium-rich lithium supplement material.

[0098] In a specific embodiment, the solvent of the solution can be, but is not limited to, tetrahydrofuran. At this time, the method for forming a coating film layer of a material containing an aluminum source on the surface of the lithium-rich lithium-supplementing material in step S01 includes the following steps:

[0099] After mixing aluminum isopropoxide in tetrahydrofuran with the lithium-rich lithium-supplementing material, a thermal decomposition reaction is carried out to deposit aluminum oxide precipitate on the surface of the lithium-rich lithium-supplementing material, forming an aluminum oxide coating film layer containing an aluminum compound.

[0100] Of course, in step S01, in addition to forming a coating film layer of a material containing an aluminum source on the surface of the lithium-rich lithium-supplementing material by the above solution precipitation method, other methods can also be used to form a coating film layer on the surface of the lithium-rich lithium-supplementing material.

[0101] In step S02, during the first heat treatment of the composite material containing the coating layer in step S01, the coating film layer undergoes the first heat treatment under the action of heat to form a dense coating layer. At the same time, under the heat treatment conditions, the aluminum element in the coating film layer can also or may further react with the surface of the lithium-rich lithium-supplementing material to generate aluminum-containing compounds, such as reacting with lithium to generate LiAlO2, Li5AlO4, etc. Therefore, the generated dense encapsulation layer. Therefore, in the embodiment, the first heat treatment can be a sintering treatment, such as the temperature can be 700 - 1000 °C, specifically it can be typical but non-limiting temperatures such as 700 °C, 800 °C, 900 °C, 1000 °C, etc. The first heat treatment time should ensure that the aluminum source fully reacts to generate the target inorganic aluminum compound, such as it can be 20 - 48 hours. By controlling and optimizing the conditions of the first heat treatment, the coating film layer can be fully heat-treated, improving the density of the formed dense encapsulation layer and reducing the residual alkali content of the prepared cathode lithium-supplementing additive. In addition, by controlling the time of the first heat treatment, the type of inorganic aluminum compound in the dense encapsulation layer can be controlled or the structure of the dense encapsulation layer forming the inorganic aluminum compound can be indirectly controlled. For example, by controlling the heat treatment, such as the sintering treatment time, while the aluminum source generates inorganic aluminum, the inorganic aluminum coating the surface of the core further reacts with the lithium-rich lithium-supplementing material contained in the core. Specifically, aluminum atoms partially replace the metal atoms contained in the lithium-rich lithium-supplementing material, thereby generating a Li5AlO4 coating layer. On the surface of this Li5AlO4 coating layer, there is a LiAlO2 coating layer, and on the surface of this LiAlO2 coating layer, there is an Al2O3 coating layer, thus forming the above-mentioned Li5AlO4 coating layer, LiAlO2 coating layer, and Al2O3 coating layer that are sequentially coated to form a transition aluminum-containing coating layer. Of course, the time of the first heat treatment can also be further controlled so that the thickness of the Li5AlO4 coating layer thickens in the direction away from the core, and at the same time, the thicknesses of the LiAlO2 coating layer and the Al2O3 coating layer decrease correspondingly, or all further react to generate a Li5AlO4 coating layer.

[0102] Meanwhile, the embodiment of the present application also provides another preparation method of the above-mentioned cathode lithium supplement additive. In the embodiment, the preparation method of the cathode lithium supplement additive of the embodiment of the present application includes the following steps:

[0103] S03: Subject the lithium-rich lithium supplement material to a second heat treatment in an aluminum-containing container and in an oxygen-containing environment to form a hermetic encapsulation layer on the surface of the lithium-rich lithium supplement material.

[0104] Among them, the lithium-rich lithium supplement material in step S03 is the same as the lithium-rich lithium supplement material in the above step S01. During the second heat treatment, aluminum in the aluminum-containing container reacts with oxygen in the environment to form a deposited inorganic aluminum compound on the surface of the lithium-rich lithium supplement material, forming a hermetic encapsulation layer. Therefore, the function of the second heat treatment is the same as that of the first heat treatment. In the embodiment, the second heat treatment can also be a sintering treatment. For example, the temperature can be 500-1000 °C, specifically 500 °C, 600 °C, 700 °C, 800 °C, 900 °C, 1000 °C and other typical but non-limiting temperatures. The second heat treatment time should ensure that the aluminum source fully reacts to generate the target inorganic aluminum compound, such as 20-48 hours. Then, during the second heat treatment, the type of inorganic aluminum generated by the second heat treatment and the change of the inorganic aluminum-containing layer structure can also be realized by controlling the heat treatment, such as the sintering temperature and time, just like the types of Li5AlO4, LiAlO2, Al2O3 or the Li5AlO4 coating layer, LiAlO2 coating layer and Al2O3 coating layer generated in the first heat treatment are sequentially coated to form a transition aluminum-containing coating layer structure.

[0105] In a further embodiment, after the above step S02 or step S03, it further includes coating other functional layers on the surface of the hermetic encapsulation layer, such as forming the second ion conductor encapsulation layer 212 contained in the above-mentioned cathode lithium supplement additive or further coating the electronic conductor encapsulation layer 22 contained in the above-mentioned cathode lithium supplement additive. Specifically, the method of forming other functional layers such as the second ion conductor encapsulation layer 212 and / or the electronic conductor encapsulation layer 22 on the surface of the hermetic encapsulation layer can be selected according to the film layer structure and materials to form the corresponding or suitable method.

[0106] Therefore, the above preparation method of the cathode lithium supplement additive can effectively prepare the cathode lithium supplement additive of the embodiment of the present application with a core-shell structure, and can ensure that the hermetic encapsulation layer contains an inorganic aluminum compound, while making the residual alkali content of the prepared cathode lithium supplement additive low, so as to ensure excellent lithium supplement effect, stable lithium supplement performance and good processing performance of the prepared cathode lithium supplement additive. In addition, the preparation method of the cathode lithium supplement additive can ensure the stable structure and electrochemical performance of the prepared cathode lithium supplement additive, and has high efficiency and saves production costs.

[0107] In a third aspect, an embodiment of the present application further provides a positive electrode material. The positive electrode material of the embodiment of the present application includes a positive electrode active material and the positive electrode lithium supplement additive in the above-mentioned embodiment of the present application. In this way, the positive electrode material of the embodiment of the present application has excellent lithium supplement performance and good processing performance, can improve the quality of the positive electrode active material layer, and thus improve the quality of the positive electrode active material layer, endowing the corresponding positive electrode sheet with electrochemical performance.

[0108] In the embodiment, the content of the positive electrode lithium supplement additive in the above-mentioned embodiment of the present application in the positive electrode material of the embodiment of the present application can control the mass content of the positive electrode material of the embodiment of the present application in the positive electrode active layer to be 1.5%-5.5%, and further can be 2.0%.

[0109] In other embodiments, the positive electrode active material contained in the positive electrode material of the embodiment of the present application may be a phosphate positive electrode active material and a ternary positive electrode active material. 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, lithium fluorovanadate phosphate, lithium titanate, lithium nickel cobalt manganate, and lithium nickel cobalt aluminate.

[0110] In a fourth aspect, an embodiment of the present application further provides a positive electrode sheet. The positive electrode sheet of the embodiment of the present application includes a positive electrode current collector and a positive electrode active layer bonded to the surface of the positive electrode current collector, and the positive electrode active layer contains the positive electrode lithium supplement additive in the above-mentioned embodiment of the present application. Since the positive electrode sheet of the embodiment of the present application contains the above-mentioned positive electrode lithium supplement additive of the embodiment of the present application, during the charge and discharge process, the positive electrode lithium supplement additive contained in the positive electrode sheet plays the above-mentioned role, can be consumed first as a "sacrificial agent" during the first cycle charging process as a lithium source to supplement the irreversible lithium ions consumed by the negative electrode to form the SEI film, thereby maintaining the abundance of lithium ions in the battery system and improving the first efficiency and overall electrochemical performance of the battery. Moreover, the quality of the positive electrode sheet is stable and the yield is high.

[0111] In one embodiment, the mass content of the positive electrode lithium supplement additive in the above-mentioned embodiment of the present application contained in the positive electrode active layer can be 1.5%-5.5%, and further can be 2.0%. In addition to this positive electrode lithium supplement additive, the positive electrode active layer further includes a positive electrode active material, a binder, and a conductive agent. Among them, the binder can be a commonly used electrode binder, such as including one or more of polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methyl cellulose, methyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives. In the embodiment of the present application, the conductive agent can be a commonly used conductive agent, such as including one or more of graphite, carbon black, acetylene black, graphene, carbon fiber, C60, and carbon nanotubes. The positive electrode active material may include one or more of lithium cobaltate, lithium manganate, lithium iron phosphate, lithium vanadium phosphate, lithium vanadium oxyphosphate, lithium fluorovanadate phosphate, lithium titanate, lithium nickel cobalt manganate, and lithium nickel cobalt aluminate.

[0112] In the embodiment, the preparation process of the positive electrode sheet may be as follows: mixing a positive electrode active material, a positive electrode lithium supplement additive, a conductive agent, and a binder to obtain an electrode paste, coating the positive electrode paste on a positive electrode current collector, and preparing the positive electrode sheet through steps such as drying, rolling, and die-cutting.

[0113] In a fifth aspect, the embodiments of the present application further provide a secondary battery. The secondary battery in the embodiments of the present application includes necessary components such as a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, and of course, also includes other necessary or auxiliary components. Among them, the positive electrode sheet is the positive electrode sheet in the embodiments of the present application, that is, the positive electrode active layer contained in the positive electrode sheet contains the positive electrode lithium supplement additive in the above embodiments of the present application.

[0114] Since the secondary battery in the embodiments of the present application contains the positive electrode lithium supplement additive in the above embodiments of the present application, based on the excellent lithium supplement performance of the positive electrode lithium supplement additive in the above embodiments of the present application, or further having ionic conductivity and / or electronic conductivity, the secondary battery in the embodiments of the present application is given excellent initial Coulomb efficiency, battery capacity, and cycle performance, with a long lifespan and stable electrochemical performance.

[0115] The following uses multiple specific embodiments to illustrate the positive electrode lithium supplement additive, its preparation method, and application in the embodiments of the present application, etc.

[0116] 1. Embodiments of the positive electrode lithium supplement additive and its preparation method:

[0117] Embodiment 1

[0118] This embodiment provides a positive electrode lithium supplement additive and its preparation method. The positive electrode lithium supplement additive includes a Li5FeO4 core, a hydrophobic alumina-containing layer coating the core, and a carbon layer coating the outer surface of the hydrophobic alumina-containing layer. That is, the hydrophobic alumina-containing layer and the carbon layer form a composite hermetic packaging layer.

[0119] The preparation method of the positive electrode lithium supplement additive in this embodiment includes the following steps:

[0120] S1. Synthesis method of Li5FeO4:

[0121] Synthesize Li5FeO4 using iron oxide and lithium carbonate as the iron source and lithium source. Ball-mill and mix the raw materials according to the molar ratio of Fe to Li of 1:7. After mixing is completed, perform sintering. The sintering procedure is as follows: 1. Raise the temperature from room temperature to 900 °C at a heating rate of 5 K / min; 2. Keep the temperature at 850 °C for 24 hours; 3. Cool down to room temperature. Argon protection is passed through during the entire sintering process to obtain the Li5FeO4 material;

[0122] S2. Preparation method of the dense hydrophobic layer containing inorganic aluminum:

[0123] Li5FeO4 powder with a molar ratio of 0.98:0.02 and aluminum isopropoxide are uniformly dispersed in a tetrahydrofuran solution, and the reaction is carried out in a high-temperature oven at 200 °C for 20 hours. After the reaction vessel is cooled to room temperature, the reaction liner is taken out, and the hydrothermal reaction product is filtered out and then sintered at 1000 °C for 20 hours in an oxygen atmosphere to in-situ coat a dense hydrophobic layer containing inorganic aluminum on the surface of the Li5FeO4 material; among them, the dense hydrophobic layer containing inorganic aluminum is a transition aluminum-containing coating layer formed by sequentially coating a Li5AlO4 coating layer, a LiAlO2 coating layer, and an Al2O3 coating layer, and the Li5AlO4 coating layer is coated on Li5FeO4;

[0124] S3. Form a carbon coating on the surface of the dense hydrophobic layer containing inorganic aluminum:

[0125] Put this material in a blender, add PEO accounting for 3%-10% of the mass of the composite material with a dense hydrophobic layer containing inorganic aluminum in step S2, and disperse it evenly. Then, keep it at 500 °C for 24 hours in an argon or nitrogen atmosphere. Under high-temperature conditions, let PEO crack to generate carbon, which is uniformly coated on the surface of the composite material with a dense hydrophobic layer containing inorganic aluminum in step S2 to form a dense conductive carbon coating layer, and finally obtain the end product C@inorganic aluminum@Li5FeO4 (or denoted as: C-Li5Fe 0.98 Al 0.02 O4, @ means coating).

[0126] It is measured that the average particle size of the core body Li5FeO4 is 5 μm, the thickness of the dense hydrophobic layer containing inorganic aluminum is 20 nm nm, the thickness of the carbon coating layer is 80 nm, the specific surface area is 0.82 m 2 / g, and the measured resistivity is lower than 5 Ω·cm at 25 °C.

[0127] The ratio characteristics of the intensity of the 2theta standard peaks of the XRD of C@Al2O3@Li5FeO4 in this example are shown in Table 1 below: (see the xrd diagram):

[0128] Table 1

[0129] Intensity ratio of the standard peak positions of 2theta in XRD Range 16.62° / 23.58° 0.23-0.42 21.55° / 23.58° 0.76-0.96 33.80° / 23.58° 0.89-1.24 36.65° / 23.58° 0.33-0.43 43.70° / 23.58° 0.37-0.39 46.18° / 23.58° 0.38-0.42 56.55° / 23.58° 0.37-0.39

[0130] Example 2

[0131] This example provides a cathode lithium supplement additive and its preparation method. This example provides a cathode lithium supplement additive and its preparation method. The cathode lithium supplement additive includes a Li5FeO4 core body, a hydrophobic layer containing inorganic aluminum coating the core body, and a carbon layer coating the outer surface of the hydrophobic layer containing alumina, that is, the hydrophobic layer containing inorganic aluminum and the carbon layer constitute a composite dense packaging layer.

[0132] The preparation method of the cathode lithium supplement additive in this example includes the following steps:

[0133] S1. Synthesis method of Li5FeO4 material:

[0134] Mix ferric oxide and lithium carbonate by ball milling at a molar ratio of Fe:Li of 1:6. Select agate beads as the ball milling beads, an agate pot as the ball milling pot, a ball milling speed of 50 HZ, and a ball milling time of 24 h; put the mixed material into a tube furnace for sintering, and set the sintering program to heat up to 1000 °C at 10 °C / min and keep it warm in an inert atmosphere such as nitrogen or argon at 1000 °C for 48 h. After the reaction is completed, a high-purity Li5FeO4 material is obtained;

[0135] S2. Synthesis method of the inorganic aluminum-containing dense hydrophobic layer:

[0136] Take 99.00 g of the synthesized Li5FeO4 material and 1.00 g of nano-aluminum oxide material and put them into a beaker, add 500 mL of absolute ethanol, and stir on a magnetic stirrer. The stirring time is 24 h, and the stirring speed is selected as 600 rpm; after stirring is completed, carry out suction filtration. The filter residue obtained by suction filtration is kept warm in an oxygen atmosphere at 1000 °C for 48 h, and after the reaction is completed, a high-purity Li5FeO4 material coated with an inorganic aluminum-containing hydrophobic layer is obtained; among them, the inorganic aluminum dense hydrophobic layer is a transition aluminum-containing coating layer formed by sequentially coating a Li5AlO4 coating layer, a LiAlO2 coating layer, and an Al2O3 coating layer, and the Li5AlO4 coating layer is coated on Li5FeO4;

[0137] S3. Form a carbon coating on the surface of the inorganic aluminum-containing dense hydrophobic layer:

[0138] Put this material into a mixer, add PVDF accounting for 3%-10% of the mass of the inorganic aluminum-containing dense hydrophobic layer composite material in step S2, and disperse it evenly. Then keep it warm at 500 °C for 24 hours under an argon atmosphere. Let PVDF crack to generate carbon at high temperature, and evenly coat the surface of the inorganic aluminum-containing dense hydrophobic layer composite material to form a dense conductive carbon coating layer, and finally obtain the end product C@inorganic aluminum@Li5FeO4.

[0139] Example 3

[0140] This example provides a cathode lithium supplement additive and its preparation method. This example provides a cathode lithium supplement additive and its preparation method. The cathode lithium supplement additive includes a Li5FeO4 core, an inorganic aluminum-containing hydrophobic layer coating the core, and a carbon layer coating the outer surface of the aluminum oxide-containing hydrophobic layer. That is, the inorganic aluminum-containing hydrophobic layer and the carbon layer form a composite hermetic packaging layer.

[0141] The preparation method of the cathode lithium supplement additive in this example includes the following steps:

[0142] S1. Synthesis method of Li5FeO4 material:

[0143] Add ferric nitrate, lithium carbonate and lithium hydroxide into an aqueous solution in a molar ratio of 1:3:3, and mix them with stirring at 600 rpm for 24 h; after mixing, evaporate the water, and finally collect the solid powder. The powder is put into a tube furnace for sintering. The sintering program is set to heat up to 1000 °C at a rate of 10 °C / min, and keep it warm in an inert atmosphere such as nitrogen or argon at 1000 °C for 20 h; after the reaction is completed, a high-purity Li5FeO4 material is obtained;

[0144] S2. Synthesis method of inorganic aluminum-containing dense hydrophobic layer:

[0145] Take 95.00 g of the synthesized Li5FeO4 material and 10 g of aluminum citrate material and put them into a beaker, add 500 mL of absolute ethanol, and stir on a magnetic stirrer. The stirring time is 36 h, keep the temperature at 50 °C during stirring, and select a stirring speed of 600 rpm; after stirring, perform suction filtration. The filter residue obtained by suction filtration is kept warm in an oxygen atmosphere at 1000 °C for 24 h, and after the reaction is completed, a high-purity Li5FeO4 material coated with an inorganic aluminum hydrophobic layer is obtained; among them, the inorganic aluminum dense hydrophobic layer is a transition aluminum-containing coating layer formed by sequentially coating the Li5AlO4 coating layer, the LiAlO2 coating layer and the Al2O3 coating layer, and the Li5AlO4 coating layer is coated on Li5FeO4;

[0146] S3. Formation of carbon coating on the surface of the inorganic aluminum-containing dense hydrophobic layer:

[0147] Put this material into a mixer, add few-layer graphene accounting for 3%-10% of the mass of the inorganic aluminum-containing dense hydrophobic layer composite material in step S2, and perform ball milling and mixing. Adjust the ball milling speed to 50 Hz, set the ball milling time to 48 h, and after the ball milling is completed, take out the material and keep it warm at 1000 °C in an argon atmosphere for 30 hours. Under high-temperature conditions, graphene is uniformly coated on the surface of the inorganic aluminum-containing dense hydrophobic layer composite material to form a dense conductive carbon coating layer.

[0148] Comparative Example 1

[0149] This comparative example provides a cathode lithium supplement additive and its preparation method. Compared with Example 2, the cathode lithium supplement additive in this comparative example does not contain an in-situ coated inorganic aluminum-containing dense hydrophobic layer and carbon layer on the surface of the Li5FeO4 material core.

[0150] Comparative Example 2

[0151] This comparative example provides a cathode lithium supplement additive and its preparation method. Compared with Example 2, the cathode lithium supplement additive in this comparative example does not have an in-situ coated dense hydrophobic inorganic aluminum layer on the surface of the Li5FeO4 material core, but retains a carbon coating layer. That is, the cathode lithium supplement additive in this comparative example is C@Li5FeO4.

[0152] S1. Synthesis of Li5FeO4:

[0153] Iron oxide and lithium carbonate are ball-milled and mixed at a molar ratio of Fe:Li of 1:6. The ball-milling beads are selected as agate beads, the ball-milling tank is selected as an agate tank, the ball-milling speed is selected as 50 HZ, and the ball-milling time is selected as 24 h. The mixed material is put into a tubular furnace for sintering. The sintering program is set to heat up to 1000 °C at a rate of 10 °C / min and keep it warm in an inert atmosphere such as nitrogen or argon at 1000 °C for 48 h. High-purity Li5FeO4 material is obtained after the reaction is completed.

[0154] S3. Formation of carbon coating on the surface of Li5FeO4:

[0155] The Li5FeO4 material is ground and mixed with PEO at a mass ratio of 3%-10%, and after being evenly dispersed, it is kept warm at 1000 °C in an argon atmosphere for 48 hours. Under high-temperature conditions, PEO pyrolyzes and uniformly coats the surface of the Li5FeO4 material, forming a dense conductive carbon coating layer.

[0156] 2. Lithium-ion battery examples:

[0157] The cathode lithium supplement additives provided in Examples 1 to 3 above and the cathode lithium supplement additive provided in the comparative example are assembled into a positive electrode and a lithium-ion battery respectively according to the following method:

[0158] Positive electrode: Under the same conditions, NMP: Super P-Li: PVDF: cathode lithium supplement additive are mixed at a mass ratio of 25:8:1:1. The mixing method is ball-milling, the ball-milling time is 60 min, the rotation speed is set to 30 HZ, and after homogenization - coating - drying - slicing operations, a positive electrode sheet is prepared. The positive electrode sheet is baked in a vacuum oven at 100 °C to remove trace water. Among them, the cathode lithium supplement additives are the cathode lithium supplement additives provided in Examples 1 to 3 above and the cathode lithium supplement additive provided in the comparative example respectively, and the storage conditions are the same;

[0159] In addition, the positive electrodes containing the cathode lithium supplement additive provided in Example 1 are stored at a relative humidity of 25% (25 °C) for 0.5 h, 1 h, 2 h, 5 h, 10 h, and 20 h respectively;

[0160] Negative electrode: The lithium metal sheet is a lithium metal sheet with a diameter of 16 mm produced by Tianjin Zhongneng Lithium Industry;

[0161] Electrolyte: The electrolyte is a 1mol / L LiPF6 solution, and the solvent is composed of EC (ethylene carbonate) and DEC (diethyl carbonate) in a volume ratio of 1:1;

[0162] Separator: A PE separator produced by Shanghai Enjie;

[0163] Battery case: (including the negative electrode case, stainless steel gasket and positive electrode case) with a model of CR2032

[0164] Lithium-ion battery assembly: Assembled into a button lithium-ion battery in the order of negative electrode case - stainless steel gasket - lithium metal sheet - separator - electrolyte - positive electrode sheet - positive electrode case in an inert atmosphere glove box.

[0165] 3. Related performance tests

[0166] 1. Related tests on the positive electrode lithium supplement additive

[0167] 1.1 Electron microscopy analysis (SEM) of the positive electrode lithium supplement additive:

[0168] The positive electrode lithium supplement additives provided in Examples 1 to 3 and the comparative example above were respectively subjected to scanning electron microscopy analysis. Among them, the scanning electron microscope photos of the positive electrode lithium supplement additive provided in Example 1 are respectively as Figure 6 shown. The SEM photos of the positive electrode lithium supplement additives provided in other examples are similar to Figure 6 . It can be seen from the SEM photos that the particle size distribution of the positive electrode lithium supplement additive prepared in the examples of the present application is relatively uniform, the surface is dense, and the morphology, particle size and other properties of the prepared positive electrode lithium supplement additive are stable. In addition, the resistivity of the positive electrode lithium supplement additive prepared in Example 1 is less than 5 Ω·cm, while the resistivity of the positive electrode lithium supplement additive in Comparative Example 1 is greater than 200 Ω·cm.

[0169] 1.2 X-ray diffraction (XRD) characterization of the positive electrode lithium supplement additive:

[0170] The positive electrode lithium supplement additives provided in Examples 1 to 3 and the comparative example above were respectively subjected to XRD analysis. Among them, the positive electrode lithium supplement additive in Example 1 is as Figure 7 shown. It can be seen from Figure 7 that there is an obvious small peak at the position of 2θ of 19.8°. This peak is the peak of aluminum-doped Li5AlO4, the small peak at 28.4° is the LiAlO2 peak, and there is a small peak at 43.4° which is the characteristic peak of Al2O3. It can be determined that there are at least three forms of existence of aluminum elements. The XRD patterns of the positive electrode lithium supplement additives provided in other examples are similar to Figure 7 , and all have the characteristic peaks of Li5AlO4, LiAlO2 and Al2O3.

[0171] 1.3 Transmission Electron Microscopy (TEM) Characterization of the Positive Electrode Lithium Supplement Additive:

[0172] The positive electrode lithium supplement additives provided in Examples 1 to 3 and the Comparative Example above were respectively subjected to TEM analysis. Among them, the TEM image of the positive electrode lithium supplement additive in Example 1 is as Figure 8 shown. It can be seen from Figure 8 that the positive electrode lithium supplement additive in Example 1 has an aluminum characteristic peak, and the TEM photos of the positive electrode lithium supplement additives provided in other examples are similar to Figure 8 . It can be seen from the TEM image that the material is coated with a continuous and dense inorganic layer. This dense and continuous inorganic layer can effectively isolate the core material from contact with water and carbon dioxide in the air, reduce the overall residual alkali amount of the material, and improve the processability of the material for making the positive electrode slurry. When the material is prepared into a battery, it can also effectively separate the core material from the electrolyte and reduce side reactions.

[0173] 2. Examples of Lithium-Ion Batteries:

[0174] The electrochemical performances of the lithium-ion batteries assembled in the above lithium-ion battery examples were tested under the following conditions:

[0175] Charge at 0.05C to 4.3V and hold at 4.3V until the current is less than 0.01C; test its initial charge specific capacity, and the test instrument is the battery test cabinet model LANHE-CT3001A produced by Wuhan Blue Electronic Co., Ltd.

[0176] The test results of the initial charge specific capacity are as follows:

[0177] In the batteries assembled with the positive electrode sheets containing the positive electrode lithium supplement additives of each example prepared and stored under the same conditions, the charge specific capacity can reach more than 650 mAh / g at 4.3V. Among them, the curve of the initial charge specific capacity of the battery containing the positive electrode lithium supplement additive provided in Example 1 is as Figure 9 shown. The initial charge specific capacities of the batteries with the positive electrode lithium supplement additives provided in other examples are close to those of the battery with the positive electrode lithium supplement additive provided in Example 1. However, the highest value of the initial charge specific capacity of the battery containing the positive electrode lithium supplement additive provided in Comparative Example 1 is significantly lower than 650 mAh / g. Therefore, due to the presence of inorganic aluminum compounds in the hermetic encapsulation layer of the positive electrode lithium supplement additive in the examples of the present application, the dense performance of the hermetic encapsulation layer is significantly improved, the isolation between the core body and the environment is enhanced, and the content of residual alkali is reduced. As a result, the positive electrode lithium supplement additive in the examples of the present application has high lithium supplement performance and processing stability, and significantly improves the electrochemical performance of the battery.

[0178] Among the batteries assembled with the positive electrode sheets provided in Example 1 containing the positive electrode lithium supplement additive after storage for different times and at different humidities, the initial charge specific capacities of each battery at an environmental humidity of 25% are shown in Table 2 below, the initial charge specific capacities of each battery at an environmental humidity of 20% are shown in Table 3 below, and the initial charge specific capacities of each battery at an environmental humidity of 10% are shown in Table 4 below. The initial charge specific capacities in the batteries assembled with the positive electrode sheets provided in Comparative Example 1 and Comparative Example 2 containing the positive electrode lithium supplement additive after storage for 0.5 h are shown in Table 2 respectively.

[0179] As can be seen from Table 2, the positive electrode sheet (after drying) prepared with the positive electrode lithium supplement additive in Example 1 of the present application can still maintain a relatively high capacity after being placed for 2 hours under the condition of a humidity not higher than 25% (25 °C). However, when the positive electrode sheets containing the positive electrode lithium supplement additives in Comparative Example 1 were placed for 0.5 h with reference to the positive electrode sheets containing the positive electrode lithium supplement additives in Example 1, a sharp attenuation of the capacity was shown. For example, the specific capacity of the battery containing the positive electrode lithium supplement additive in Comparative Example 1 was only 235 mAh / g, and the specific capacity of the battery containing the positive electrode lithium supplement additive in Comparative Example 2 was 477.2 mAh / g.

[0180] Comparing Table 2 to Table 4, it can be seen that although the specific capacities of the batteries assembled with the positive electrode sheets prepared with the positive electrode lithium supplement additive in Example 1 of the present application vary at different humidities, the differences are almost negligible. Further testing on Examples 2 to 3 shows that the changes in the specific capacities of the batteries assembled with the positive electrode sheets containing the positive electrode lithium supplement additives in Examples 2 to 3 at different humidities are not significant. This shows that when the positive electrode lithium supplement additive in the examples of the present application is stored at a humidity below 25%, the capacity of the positive electrode sheet basically does not attenuate significantly; when the storage environmental humidity is less than 20%, the capacity of the positive electrode lithium supplement additive in the examples of the present application can basically be made not to attenuate, or the attenuation is very small, having excellent storage performance and stable electrochemical performance. Further detection shows that after the positive electrode lithium supplement additive provided in Example 1 is placed in air with water and oxygen for a sufficient time, its structure will change, generating products such as LiFeO2, LiOH, and Li2CO3.

[0181] Table 2

[0182]

[0183] Table 3

[0184]

[0185] Table 4

[0186]

[0187] The above embodiments merely represent several implementation manners of the present application. The description thereof is relatively specific and detailed, but it should not be construed as a limitation to the patent scope of the present application. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several modifications and improvements can still be made, and these all fall within the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the appended claims.

Claims

1. A cathode lithium supplement additive, comprising a core body and a hermetic encapsulation layer coated on the core body, characterized in that: The core includes a lithium-rich lithium-supplementing material, and the material of the hermetic encapsulation layer includes an inorganic aluminum compound; the inorganic aluminum compound includes Li5AlO4, LiAlO2, and Al2O3, and the Li5AlO4, LiAlO2, and Al2O3 respectively form a Li5AlO4 coating layer, a LiAlO2 coating layer, and an Al2O3 coating layer. From the inside to the surface direction of the core, the Li5AlO4 coating layer, the LiAlO2 coating layer, and the Al2O3 coating layer are sequentially coated to form a transition aluminum-containing coating layer, and the Li5AlO4 coating layer is coated on the surface of the core.

2. The cathode lithium supplement additive according to claim 1, wherein: The inorganic aluminum compound is distributed on the inner surface of the hermetic encapsulation layer close to the core; or in the direction from the inner surface to the outer surface of the hermetic encapsulation layer close to the core, the content of the inorganic aluminum compound decreases in a gradient. Or / and The inorganic aluminum compound includes an inorganic aluminum compound generated by heat treatment.

3. The cathode lithium supplement additive according to any one of claims 1-2, characterized in that: The hermetic encapsulation layer includes an ion conductor encapsulation layer, and the ion conductor encapsulation layer coats the surface of the core. The material of the ion conductor encapsulation layer includes the inorganic aluminum compound; or / and The mass content of the inorganic aluminum compound in the hermetic encapsulation layer is 0.1-5.0%; or / and The lithium-rich lithium-supplementing material is an anti-fluorite structure lithium-supplementing material; or / and The lithium-rich lithium supplement material is L x M y N z O q ; wherein, L in the molecular formula is Li or / and a mixed alkali metal element of Li and at least one of K and Na not exceeding 30%; M includes at least one of Fe, Co, Mn, Ni, Si, and Al; N includes at least one of Fe, Co, Mn, Ni, Si, Al or other equivalent or hetero-valent metal elements, O is oxygen element, x is 4-6, y is 0.7-1.0, z is 0.01-0.3, q is 4-5; or / and The particle size of the core is 0.2μm-20μm.

4. The cathode lithium supplement additive according to claim 3, wherein: The ion conductor encapsulation layer includes a first ion conductor encapsulation layer and a second ion conductor encapsulation layer. The first ion conductor encapsulation layer coats the surface of the core, and the material of the first ion conductor encapsulation layer is the inorganic aluminum compound; The second ion conductor encapsulation layer is coated on the surface of the first ion conductor encapsulation layer facing away from the core.

5. The cathode lithium supplement additive according to claim 3, wherein: The total molar ratio of L to M and N in the molecular formula is 4-7:1; and / or When M in the molecular formula is Fe, N is Al.

6. The cathode lithium supplement additive according to claim 3, characterized in that: The hermetic encapsulation layer further includes an electronic conductor encapsulation layer, and the electronic conductor encapsulation layer is coated on the surface of the ion conductor encapsulation layer facing away from the core.

7. The cathode lithium supplement additive according to claim 6, wherein: The material of the electronic conductor encapsulation layer includes at least one of a carbon material, a conductive oxide, and a conductive organic compound; and / or The thickness of the electronic conductor encapsulation layer is 50-200nm; and / or The material of the electronic conductor encapsulation layer is a carbon material, and the content range of the carbon material in the positive electrode lithium-supplementing additive is 2wt%-10wt%.

8. The cathode lithium supplement additive according to any one of claims 1-2, 4-7, characterized in that: The specific surface area of the positive electrode lithium supplement additive is 0.2 - 5.0 m 2 / g; and / or The loose bulk density of the positive electrode lithium-supplementing additive is 0.35-0.80g / mL, and its tapped density is 0.50-1.20g / mL; and / or The resistivity of the positive electrode lithium-supplementing additive at 25°C is less than 5Ω·cm; and / or The material of the electronic conductor encapsulation layer includes at least one of a carbon material, a conductive oxide, and a conductive organic compound.

9. The cathode lithium supplement additive according to any one of claims 1-2, 4-7, characterized in that The capacity decay rate of the positive electrode sheet prepared from the positive electrode lithium-supplementing additive, a binder, and a conductive agent after being stored at an ambient humidity of 25% for 20 hours relative to the capacity after being stored for 0.5 hours is not more than 30%; and / or The capacity decay rate of the positive electrode sheet prepared from the positive electrode lithium supplement additive, binder and conductive agent after being stored at an environmental humidity of 10% for 20 hours relative to the capacity after being stored for 0.5 hours is not more than 20%.

10. The preparation method of the cathode lithium supplement additive according to any one of claims 1-9, characterized in that: It includes the following steps: Form a coating film layer on the surface of the lithium-rich lithium supplement material with the material containing an aluminum source, and then perform a first heat treatment in an oxygen-containing environment to form a hermetic encapsulation layer on the surface of the lithium-rich lithium supplement material; Or Perform a second heat treatment on the lithium-rich lithium supplement material in an aluminum-containing container and in an oxygen-containing environment to form a hermetic encapsulation layer on the surface of the lithium-rich lithium supplement material.

11. The preparation method according to claim 10, wherein, The method for forming the coating film layer by forming the material containing an aluminum source on the surface of the lithium-rich lithium supplement material includes the following steps: Prepare a solution of the aluminum source, disperse the lithium-rich lithium supplement material in the solution, and then initiate an aluminum precipitation reaction to deposit an aluminum compound on the surface of the lithium-rich lithium supplement material to form a coating film layer containing the aluminum compound.

12. The preparation method according to claim 10 or 11, characterized in that, The aluminum source includes at least one of aluminum isopropoxide, aluminum organic acid salt, aluminum oxide, aluminum nitrate, aluminum chloride, aluminum sulfate, and aluminum silicate; and / or The temperature of the first heat treatment is 700 - 1000 °C, and the time is 20 - 48 hours; and / or The temperature of the second heat treatment is 500 - 1000 °C, and the time is 20 - 48 hours.

13. A cathode material, characterized in that: It includes a positive electrode active material and the positive electrode lithium supplement additive according to any one of claims 1 - 9 or the positive electrode lithium supplement additive prepared by the preparation method according to any one of claims 10 - 12.

14. A positive electrode sheet, comprising a positive electrode current collector and a positive electrode active layer bonded to the surface of the positive electrode current collector, characterized in that: The positive electrode active layer contains the positive electrode lithium supplement additive according to any one of claims 1 - 9 or the positive electrode lithium supplement additive prepared by the preparation method according to any one of claims 10 - 12.

15. A secondary battery, comprising a positive electrode sheet and a negative electrode sheet, characterized in that: The positive electrode sheet is the positive electrode sheet according to claim 14.

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