Lithium supplement agent, preparation method thereof, positive plate and secondary battery

Abstract

The invention relates to the technical field of batteries, particularly provides a lithium supplement agent and a preparation method thereof, a positive plate and a secondary battery, and aims to solve the problem that an existing lithium oxalate lithium supplement agent cannot adapt to a battery cell system due to overhigh decomposition voltage. In order to achieve the purpose, the lithium supplementing agent comprises lithium oxalate, a conductive agent and an auxiliary lithium supplementing agent, and the auxiliary lithium supplementing agent generates a substance capable of catalyzing lithium removal of the lithium oxalate in the formation process of the lithium ion battery adopting the lithium supplementing agent and does not generate a substance which reacts with an electrolyte of the lithium ion battery. The auxiliary lithium supplementing material in the lithium supplementing agent generates a substance capable of catalyzing lithium removal of lithium oxalate in the battery formation process, so that the lithium oxalate can be decomposed at lower voltage to release more capacity, the decomposition voltage of the lithium oxalate is effectively reduced, meanwhile, the auxiliary lithium supplementing material is decomposed and does not generate a product which reacts with an electrolyte, and the lithium supplementing efficiency is improved. The lithium oxalate lithium supplement agent can be suitable for a battery cell system, and the subsequent battery cell performance is prevented from being influenced.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and particularly provides a lithium supplement agent, a preparation method thereof, a positive electrode sheet, and a secondary battery. Background Art

[0002] During the formation process of lithium-ion batteries, an SEI film is formed on the negative electrode, which consumes a large amount of active lithium, directly resulting in the loss of irreversible capacity.

[0003] Supplying active lithium is an effective improvement measure, which can improve the energy density, cycling and storage performance of the battery. Currently, commercial lithium supplementation is mainly carried out on the positive electrode. Among them, iron-based and nickel-based lithium supplement agents are the most popular in research and use, but there are problems such as high price.

[0004] The sacrificial lithium supplement agent has gradually come into people's sight. It has a high theoretical specific capacity and no residue after the capacity is exerted. Only Li + and gas are left. The generated gas CO2 has a protective effect on the battery cell system and will not react with the electrolyte. Moreover, after the gas is exhausted during formation, "zero residue" lithium supplementation can be achieved. A typical example is lithium oxalate material, and the theoretical decomposition voltage is 3.0V. However, such lithium supplement materials have high insulation and poor conductivity. It is very difficult to remove the first lithium, and the actual decomposition voltage is relatively high, reaching above 4.7V, which is not suitable for the battery cell system.

[0005] In order to solve the problem of the relatively high decomposition voltage of the lithium oxalate lithium supplement material, in the prior art, a catalyst is usually added to lithium oxalate to reduce the decomposition voltage. For example, a metal-based catalyst (such as a metal oxide) is added to lithium oxalate. Although the metal-based catalyst can reduce the decomposition voltage of the lithium oxalate lithium supplement material, the metal-based catalyst will remain in the battery system and cannot be discharged. With the cycling and storage of the battery, it may have a catalytic side effect on the battery electrolyte later, accelerating the attenuation of the battery and even causing safety problems.

[0006] Therefore, it is crucial to develop a sacrificial lithium supplement agent with a low decomposition voltage and no negative effects on the products. Summary of the Invention

[0007] The present invention aims to solve at least to some extent the above technical problems, that is, to solve at least to some extent the problem that the existing lithium oxalate lithium supplement agent has too high a decomposition voltage and cannot adapt to the battery cell system.

[0008] In a first aspect, the present invention provides a lithium supplement agent for supplementing lithium to a lithium-ion battery. The lithium supplement agent includes lithium oxalate, a conductive agent, and an auxiliary lithium supplement agent. The auxiliary lithium supplement material generates a substance capable of catalyzing the de-lithiation of lithium oxalate during the formation of a lithium-ion battery using the lithium supplement agent and does not generate a substance that reacts with the electrolyte of the lithium-ion battery.

[0009] In the preferred technical solution of the above lithium supplement agent, the auxiliary lithium supplement material includes one or any combination of lithium transition metal oxides and lithium oxides.

[0010] In the preferred technical solution of the above lithium supplement agent, the auxiliary lithium supplement material includes one or any combination of Li2NiO2, Li5FeO4, Li2O, and LiCoO2.

[0011] In the preferred technical solution of the above lithium supplement agent, the mass ratio of lithium oxalate to the auxiliary lithium supplement material is 40:(0.4 - 2.5), and preferably the mass ratio of lithium oxalate to the auxiliary lithium supplement material is 40:(0.4 - 2.0).

[0012] In the preferred technical solution of the above lithium supplement agent, the conductive agent includes one or any combination of carbon nanotubes or carbon fibers.

[0013] In the preferred technical solution of the above lithium supplement agent, the mass ratio of lithium oxalate to the conductive agent is 40:(0.5 - 3.0), and preferably the mass ratio of lithium oxalate to the conductive agent is 40:(0.5 - 2.5).

[0014] In the preferred technical solution of the above lithium supplement agent, the particle size D50 of the lithium supplement agent is 2 μm - 8 μm.

[0015] In a second aspect, the present invention also provides a preparation method of a lithium supplement agent. The preparation method includes the following steps: S1: Provide a mixed solution mainly composed of a conductive agent, lithium oxalate, and a solvent to form a lithium oxalate solution mixed with the conductive agent; S2: Recrystallize the lithium oxalate solution mixed with the conductive agent to obtain a crystalline substance; S3: Mix and granulate the crystalline substance obtained in step S2 with an auxiliary lithium supplement material to obtain the lithium supplement agent.

[0016] In the preferred technical solution of the above preparation method of the lithium supplement agent, the step of "providing a mixed solution mainly composed of a conductive agent, lithium oxalate, and a solvent" specifically includes: S11: Dissolve lithium oxalate crystals in a solvent to obtain a lithium oxalate solution; S12: Mix the conductive agent into the lithium oxalate solution to obtain the mixed solution.

[0017] In the preferred technical solution of the above preparation method of the lithium supplement agent, the step of "dissolving lithium oxalate crystals in a solvent to obtain a lithium oxalate solution" specifically includes: Dissolve lithium oxalate crystals in deionized water under the condition of 5 - 20 °C to obtain the lithium oxalate solution.

[0018] In the preferred technical solution of the above lithium supplement preparation method, the step of "recrystallizing the lithium oxalate solution mixed with the conductive agent" specifically includes: recrystallizing the lithium oxalate solution mixed with the conductive agent under the condition of 30-45 °C.

[0019] In the preferred technical solution of the above lithium supplement preparation method, the step of "mixing and granulating the crystallized product with the auxiliary lithium supplement material" specifically includes: mixing and granulating the crystallized product with the auxiliary lithium supplement material under the condition that the environmental humidity is less than 10%.

[0020] In the third aspect, the present invention also provides a positive electrode sheet, which includes a current collector and a positive electrode active material disposed on the surface of the current collector. Among them, the positive electrode active material contains any one of the lithium supplement agents described in the first aspect or a lithium supplement agent prepared by using the preparation method of any one of the lithium supplement agents described in the second aspect.

[0021] In the fourth aspect, the present invention also provides a secondary battery, which includes the positive electrode sheet described in the third aspect.

[0022] In the case of adopting the above preferred technical solution, by adding an auxiliary lithium supplement material to the lithium oxalate lithium supplement agent, on the one hand, the auxiliary lithium supplement material generates a substance that can catalyze the de-lithiation of lithium oxalate during the battery formation process, reducing the decomposition voltage of lithium oxalate, so that lithium oxalate is applicable to the cell system; on the other hand, the auxiliary lithium supplement material does not generate substances that react with the electrolyte during the formation process of the lithium-ion battery, avoiding affecting the subsequent cell performance. In addition, this application can reduce the decomposition voltage of lithium oxalate to below 4.2V, and the preparation cost of the lithium supplement agent is relatively low, without residual catalyst, and thus there is no side effect on the cell system. After lithium oxalate plays its role, it basically does not damage the cell, and there is no risk of performance attenuation caused by structural collapse.

[0023] Furthermore, by reasonably regulating the mass ratio of lithium oxalate to the auxiliary lithium supplement material, on the one hand, it can avoid the content of the auxiliary lithium supplement material in the lithium supplement agent being too low to effectively reduce the decomposition voltage of lithium oxalate, and thus unable to effectively improve the lithium supplement effect of the lithium supplement agent; on the other hand, it can avoid the content of the auxiliary lithium supplement material in the lithium supplement agent being too high, resulting in excessive oxygen free radicals generated by the decomposition of the auxiliary lithium supplement material, which affects the battery system.

[0024] Still further, compared with setting the conductive agent in the form of a solid conductive agent, setting the conductive agent as a hollow conductive agent with a hollow structure inside can effectively coat lithium oxalate, improve conductivity, and the structure of the conductive agent is stable after lithium oxalate plays its role and will not collapse, and the cell structure will not be damaged.

[0025] Furthermore, when preparing the lithium supplement agent, by crystallizing the mixed solution of lithium oxalate solution and conductive agent, the lithium oxalate crystals can be recrystallized, thereby better optimizing the crystallization of lithium oxalate crystals, shortening the extraction path of Li ions in the crystal lattice, increasing the ion diffusion rate, and more effectively reducing the decomposition voltage of lithium oxalate. At the same time, by first mixing the conductive agent with the lithium oxalate solution and then crystallizing the mixed solution to obtain the crystallization product, the coating effect of the conductive agent can be improved, thereby increasing the conductivity of the lithium supplement agent, which helps to improve the ability of the lithium supplement agent to extract the first lithium ion and reduce the decomposition voltage. In addition, recrystallization can further improve the purity of lithium oxalate, which helps to improve the cycle performance and safety of the battery.

[0026] Furthermore, by dissolving lithium oxalate crystals in a solvent at 5-20°C, the dissolution rate of lithium oxalate can be increased, and the preparation efficiency of the lithium supplement agent can be improved. By crystallizing the mixed solution at 30-45°C, on the one hand, it can avoid the crystallization precipitation rate being too slow due to too low temperature, which affects the crystallization efficiency. On the other hand, it can also avoid the crystallization precipitation being too fast due to too high temperature, resulting in too large crystal size, which affects the coating effect of the conductive agent, and thus the crystallization precipitation rate and crystal size are both within a better range. At the same time, by mixing and granulating under the condition that the environmental humidity is less than 10%, the auxiliary lithium supplement material can be made more stable, avoiding the environmental humidity being too high and affecting the stability of the auxiliary lithium supplement material. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The preferred embodiments of the present invention will be described below with reference to the drawings, in which:

[0028] Figure 1 is a flowchart of the preparation method of the lithium supplement agent of the present invention;

[0029] Figure 2 is a flowchart of an embodiment of the preparation method of the lithium supplement agent of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] The preferred embodiments of the present invention will be described below with reference to the drawings. Those skilled in the art should understand that these embodiments are only used to explain the technical principle of the present invention and are not intended to limit the protection scope of the present invention.

[0031] Unless otherwise specified, the terms used in this application have the well-known meanings commonly understood by those skilled in the art. Unless otherwise specified, the numerical values of the various parameters mentioned in this application can be measured by various commonly used measurement methods in the art (for example, they can be tested according to the methods given in the embodiments of this application).

[0032] Unless otherwise specified, the test methods in the following examples are all conventional methods; unless otherwise specified, the raw materials, reagent materials, etc. used in the following examples are all commercially available products.

[0033] The list of items connected by the term "at least one of" or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may include a single component or multiple components. Item B may include a single component or multiple components. Item C may include a single component or multiple components.

[0034] In a first aspect, the present invention provides a lithium supplement agent for supplementing lithium to a lithium-ion battery.

[0035] Specifically, the lithium supplement agent of the present invention includes lithium oxalate, a conductive agent, and an auxiliary lithium supplement material. Among them, the auxiliary lithium supplement material generates a substance capable of catalyzing the de-lithiation of lithium oxalate during the formation of the lithium-ion battery using the lithium supplement agent and does not generate a substance that reacts with the electrolyte of the lithium-ion battery.

[0036] By adding an auxiliary lithium supplement material to the lithium oxalate lithium supplement agent, on the one hand, the auxiliary lithium supplement material generates a substance capable of catalyzing the de-lithiation of lithium oxalate during the formation of the battery, so that lithium oxalate can decompose and release more capacity at a lower voltage; on the other hand, the auxiliary lithium supplement material does not generate a substance that reacts with the electrolyte of the lithium-ion battery, avoiding the influence of its products on the performance of the subsequent battery cells.

[0037] It should be noted that in practical applications, the present invention does not impose any limitation on the particle size of lithium oxalate. For example, the particle size of lithium oxalate can be set to 50μm, 55μm, 60μm, 65μm, 70μm, 75μm, 80μm, or a range composed of any two of these values. In some embodiments, the particle size of lithium oxalate is 50μm - 80μm.

[0038] It should also be noted that in practical applications, the present invention does not make any particular limitation on the specific type of the auxiliary lithium supplement material, as long as it can generate a substance capable of catalyzing the de-lithiation of lithium oxalate during the formation of the lithium-ion battery and does not generate a substance that reacts with the electrolyte of the lithium-ion battery. For example, the auxiliary lithium supplement material can be set as a transition metal oxide of lithium, or the auxiliary lithium supplement material can also be set as an oxide of lithium, etc. Such adjustments and changes to the specific type of the auxiliary lithium supplement material do not deviate from the principle and scope of the present invention and should all be included in the protection scope of the present invention.

[0039] Preferably, the auxiliary lithium supplementing material includes one or any combination of transition metal oxides of lithium and oxides of lithium.

[0040] It should be noted that the present invention does not make any limitation on the specific type of the auxiliary lithium supplementing material. For example, the auxiliary lithium supplementing material may include one or any combination of Li2NiO2, Li5FeO4, Li2O, and LiCoO2. Or, the auxiliary lithium supplementing material may also be set to any other possible transition metal oxides of lithium or oxides of lithium. Such adjustments and changes to the specific type of the auxiliary lithium supplementing material do not deviate from the principle and scope of the present invention and should be included within the protection scope of the present invention.

[0041] Preferably, the auxiliary lithium supplementing material may include one or any combination of Li2NiO2, Li5FeO4, Li2O, and LiCoO2.

[0042] Exemplarily, the auxiliary lithium supplementing material is Li5FeO4. Li5FeO4 decomposes to generate oxygen free radicals and LiFeO2. Among them, the oxygen free radicals can catalyze the de-lithiation of lithium oxalate, and LiFeO2 is an inert substance that does not react with the electrolyte of the lithium ion battery and will not affect the performance of the subsequent battery cell.

[0043] Exemplarily, the auxiliary lithium supplementing material is Li2O. During the formation process of the lithium ion battery, the oxygen generated by Li2O can catalyze the de-lithiation of lithium oxalate, and the generated lithium ions can participate in lithium intercalation and de-lithiation. It will not only generate substances that have harmful side reactions with the electrolyte of the lithium ion battery, but also further improve the lithium supplementing effect.

[0044] It should be noted that in practical applications, those skilled in the art can set the auxiliary lithium supplementing material to only any one of Li2NiO2, Li5FeO4, Li2O, and LiCoO2. Or, the auxiliary lithium supplementing material can also be set to include any two of Li2NiO2, Li5FeO4, Li2O, and LiCoO2. Or, the auxiliary lithium supplementing material can also be set to include any three of Li2NiO2, Li5FeO4, Li2O, and LiCoO2, and so on. Such adjustments and changes to the specific composition of the auxiliary lithium supplementing material do not deviate from the principle and scope of the present invention and should be included within the protection scope of the present invention.

[0045] Exemplarily, the auxiliary lithium supplementing material only includes any one of Li2NiO2, Li5FeO4, Li2O, and LiCoO2.

[0046] Next, the auxiliary lithium supplementing material of the present invention will be introduced by taking Li5FeO4 as an example.

[0047] Specifically, the auxiliary lithium supplementing material is Li5FeO4. The oxygen free radicals generated by its decomposition can participate in the formation of the negative electrode SEI film. At the same time, the highly active oxygen provides energy for the decomposition of lithium oxalate, enabling it to decompose and release more capacity at a lower voltage. Moreover, the decomposition product of Li5FeO4 is LiFeO2, which is an inert substance and will not affect the subsequent performance of the battery cell.

[0048] It should be noted that in practical applications, the present invention does not make any particular limitation on the mass ratio of lithium oxalate to the auxiliary lithium supplementing material. For example, the mass ratio of lithium oxalate to the auxiliary lithium supplementing material can be set to 40:0.4, 40:0.5, 40:0.6, 40:0.7, 40:0.8, 40:0.9, 40:1.0, 40:1.1, 40:1.2, 40:1.3, 40:1.4, 40:1.5, 40:1.6, 40:1.7, 40:1.8, 40:1.9, 40:2.0, 40:2.5, or the range composed of any two of these values.

[0049] Preferably, the mass ratio of lithium oxalate to the auxiliary lithium supplementing material is 40:(0.4 - 2.5).

[0050] More preferably, the mass ratio of lithium oxalate to the auxiliary lithium supplementing material is 40:(0.4 - 2.0).

[0051] By reasonably regulating the mass ratio of lithium oxalate to the auxiliary lithium supplementing material, on the one hand, it can avoid the content of the auxiliary lithium supplementing material in the lithium supplementing agent being too low, resulting in the inability to effectively reduce the decomposition voltage of lithium oxalate, and thus the inability to effectively improve the lithium supplementing effect of the lithium supplementing agent; on the other hand, it can avoid the content of the auxiliary lithium supplementing material in the lithium supplementing agent being too high, resulting in the decomposition of the auxiliary lithium supplementing material generating excessive oxygen free radicals and affecting the battery system.

[0052] It should be noted that in practical applications, the present invention does not make any limitation on the specific type of the conductive agent, as long as it can improve the conductivity of the lithium supplementing agent. For example, the conductive agent can be set as carbon nanotubes, or it can also be set as carbon fibers, or it can further be set as a combination of carbon nanotubes and carbon fibers, etc. Such adjustments and changes to the specific type of the conductive agent do not deviate from the principle and scope of the present invention and should all be included in the protection scope of the present invention.

[0053] Preferably, the conductive agent includes one or any combination of carbon nanotubes or carbon fibers.

[0054] More preferably, the conductive agent includes carbon nanotubes.

[0055] By setting the conductive agent to include carbon nanotubes, since the carbon nanotubes have a hollow structure inside, compared with the form of setting the conductive agent as a solid conductive agent, it can effectively coat lithium oxalate, improve conductivity, and the structure of the conductive agent after lithium oxalate plays is stable and will not collapse, and the cell structure of the lithium-ion battery will not be damaged, avoiding affecting the performance of the cell.

[0056] It should be noted that in practical applications, the present invention does not make any special limitations on the mass ratio of lithium oxalate to the conductive agent. For example, the mass ratio of lithium oxalate to the conductive agent can be set to 40:0.5, 40:0.6, 40:0.7, 40:0.8, 40:0.9, 40:1.0, 40:1.1, 40:1.2, 40:1.3, 40:1.4, 40:1.5, 40:2.5, 40:3.0 or the range composed of any two of these values.

[0057] Preferably, the mass ratio of lithium oxalate to the conductive agent is 40:(0.5 - 3.0).

[0058] More preferably, the mass ratio of lithium oxalate to the conductive agent is 40:(0.5 - 2.5).

[0059] By reasonably regulating the mass ratio of lithium oxalate to the conductive agent, on the one hand, it can enable the conductive agent to effectively coat lithium oxalate, and then effectively improve the conductive performance of lithium oxalate and improve the ability of the lithium supplement agent to release the first lithium ion; on the other hand, it can avoid the blockage of the internal lithium ion transmission channels caused by excessive addition of the conductive agent, and thus avoid the hindrance of lithium ion transmission.

[0060] It should be noted that in practical applications, the present invention does not make any limitations on the particle size of the lithium supplement agent. For example, the D50 of the lithium supplement agent can be set to 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm or the range composed of any two of these values.

[0061] In some embodiments, the D50 of the lithium supplement agent is 2μm - 8μm.

[0062] In a second aspect, the present invention also provides a preparation method of the lithium supplement agent as introduced in any one of the above.

[0063] Specifically, as Figure 1 shown, the preparation method of the lithium supplement agent of the present invention includes the following steps:

[0064] S1: Provide a mixed solution mainly composed of a conductive agent, lithium oxalate and a solvent to form a lithium oxalate solution mixed with the conductive agent;

[0065] S2: Recrystallize the lithium oxalate solution mixed with the conductive agent to obtain a crystalline substance;

[0066] S3: Mix and granulate the crystallization product obtained in step S2 with an auxiliary lithium supplement material to obtain a lithium supplement agent.

[0067] With such a setting, when preparing the lithium supplement agent, by recrystallizing the mixed solution of lithium oxalate solution and the conductive agent, on the one hand, it can better optimize the crystallization of lithium oxalate crystals, shorten the extraction path of Li ions in the crystal lattice, increase the ion diffusion rate, and more effectively reduce the decomposition voltage of lithium oxalate; on the other hand, it can improve the coating effect of the conductive agent and thus improve the conductivity of the lithium supplement agent, which helps to improve the ability of the lithium supplement agent to extract the first lithium ion, reduce the decomposition voltage. In addition, recrystallizing the mixed solution can further improve the purity of lithium oxalate, which helps to improve the cycle performance and safety of the battery.

[0068] It should be noted that in practical applications, those skilled in the art can first dissolve lithium oxalate crystals in a solvent to obtain a lithium oxalate solution, then mix the conductive agent into the lithium oxalate solution to obtain a mixed solution. Or, they can directly mix the conductive agent and lithium oxalate crystals into a solvent to obtain a mixed solution. Or, they can also obtain a mixed solution mainly composed of a conductive agent, lithium oxalate, and a solvent through any other possible means, etc. Such adjustments and changes to the specific preparation method of the lithium oxalate solution mixed with the conductive agent do not deviate from the principle and scope of the present invention and should all be included in the protection scope of the present invention.

[0069] Preferably, as Figure 2 shown, the step of "providing a mixed solution mainly composed of a conductive agent, lithium oxalate, and a solvent" specifically includes:

[0070] S11: Dissolve lithium oxalate crystals in a solvent to obtain a lithium oxalate solution;

[0071] S12: Mix the conductive agent into the lithium oxalate solution to obtain the mixed solution.

[0072] Preferably, in step S11, the step of "dissolving lithium oxalate crystals in a solvent to obtain a lithium oxalate solution" specifically includes:

[0073] Under the condition of 5 - 20°C, dissolve lithium oxalate crystals in deionized water to obtain a lithium oxalate solution.

[0074] It should be noted that the present invention does not make any limitation on the specific temperature for dissolving lithium oxalate crystals in a solvent. For example, the dissolution temperature of lithium oxalate crystals can be set to 5°C, 6°C, 7°C, 8°C, 9°C, 10°C, 11°C, 12°C, 13°C, 14°C, 15°C, 16°C, 17°C, 18°C, 19°C, 20°C or the range composed of any two of these values.

[0075] Through multiple experiments and verifications, the inventors found that by dissolving lithium oxalate crystals in a solvent under the condition of 5-20°C, the dissolution rate of lithium oxalate can be increased, and the preparation efficiency of the lithium supplement agent can be improved.

[0076] It should be noted that in practical applications, the solvent is not limited to deionized water. For example, the solvent can also be set as an aqueous solution, or it can be set as any other possible type as long as it can dissolve lithium oxalate. Such adjustments and changes to the specific types of solvents do not deviate from the principles and scope of the present invention and should be included within the protection scope of the present invention.

[0077] Of course, preferably, the solvent is deionized water.

[0078] It should be noted that the higher the purity of lithium oxalate crystals, the higher the cycle performance and safety of the lithium-ion battery.

[0079] Preferably, the purity of lithium oxalate crystals is greater than or equal to 99.9%.

[0080] It should also be noted that in practical applications, the present invention does not make any special limitations on the particle size of lithium oxalate. For example, the particle size of lithium oxalate can be set to 50μm, 55μm, 60μm, 65μm, 70μm, 75μm, 80μm, or the range composed of any two of these values.

[0081] Preferably, the particle size of lithium oxalate is 50μm - 80μm.

[0082] It should be noted that the present invention does not make any special limitations on the specific types of conductive agents. For example, the conductive agent can be set as carbon nanotubes, or it can be set as carbon fibers, or it can be set as a mixture of carbon nanotubes and carbon fibers. Such adjustments and changes to the specific types of conductive agents do not deviate from the principles and scope of the present invention and should be included within the protection scope of the present invention.

[0083] In some embodiments, the conductive agent is at least one of carbon nanotubes and carbon fibers.

[0084] It should be noted that in practical applications, the present invention does not make any limitations on the specific length of the conductive agent. For example, the length of the conductive agent can be set to 1μm, 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm, 50μm, or the range composed of any two of these values.

[0085] In some embodiments, the length of the conductive agent is 1μm - 50μm.

[0086] Preferably, the step of "mixing the conductive agent into the lithium oxalate solution obtained in step S11 to form a mixed solution" specifically includes:

[0087] The conductive agent is mixed into the lithium oxalate solution obtained in step S11 and ultrasonic vibration is performed to form a mixed solution.

[0088] Through such a setting, the conductive agent and the lithium oxalate solution can be mixed more evenly, which helps to improve the coating effect of the conductive agent on the lithium oxalate.

[0089] It should be noted that, in practical applications, the conductive agent is not limited to being added to the lithium oxalate solution for ultrasonic vibration to form a mixed solution, as long as the conductive agent and the lithium oxalate solution can be stirred evenly to obtain a mixed solution, for example, the conductive agent can also be added to the lithium oxalate solution for sufficient stirring to form a mixed solution, etc. Such flexible adjustment and change does not deviate from the principle and scope of the present invention, and should be included in the protection scope of the present invention. Of course, preferably, the conductive agent is added to the lithium oxalate solution for ultrasonic vibration to form a mixed solution.

[0090] It should also be noted that the present invention does not impose any restrictions on the crystallization temperature conditions for recrystallizing the lithium oxalate solution mixed with a conductive agent, as long as the mixed solution can be recrystallized. For example, the crystallization temperature of the mixed solution can be set to 30°C, 31°C, 32°C, 33°C, 34°C, 35°C, 36°C, 37°C, 38°C, 39°C, 40°C, 41°C, 42°C, 43°C, 44°C, 45°C or a range consisting of any two of these values.

[0091] Preferably, in step S2, the step of "recrystallizing the lithium oxalate solution mixed with the conductive agent" specifically includes:

[0092] The lithium oxalate solution mixed with the conductive agent is recrystallized at 30-45°C.

[0093] Through such a setting, the mixed solution is recrystallized at 30-45°C. On the one hand, it can avoid the crystallization rate being too slow due to too low temperature, which affects the crystallization efficiency. On the other hand, it can also avoid the crystallization being too fast due to too high temperature, which leads to excessively large crystal size and affects the coating effect of the conductive agent. The inventors have verified through multiple experiments that crystallizing the mixed solution at 30-45°C can make the crystallization rate and crystal size within a better range.

[0094] It should be noted that in practical applications, the present invention does not impose any restrictions on the environmental humidity for mixing the crystalline substance and the auxiliary lithium supplement material. For example, the environmental humidity can be set to 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range composed of any two of these values.

[0095] In some embodiments, the crystalline substance and the auxiliary lithium supplement material are mixed and granulated under the condition that the environmental humidity is less than 10%.

[0096] Through such a setting, under the condition that the environmental humidity is less than 10%, the auxiliary lithium supplement material is relatively stable, avoiding the influence of excessive environmental humidity on the stability of the auxiliary lithium supplement material.

[0097] It should be noted that the present invention does not impose any restrictions on the equipment used for mixing and granulating the crystalline substance and the auxiliary lithium supplement material. For example, granulation can be carried out by a ball mill, or alternatively, granulation can be carried out by a granulator, etc. Such adjustments and changes to the equipment used for granulation do not deviate from the principle and scope of the present invention and should all be included within the protection scope of the present invention.

[0098] Preferably, the crystalline substance and the auxiliary lithium supplement material are granulated by a ball mill.

[0099] It should be noted that the present invention does not impose any restrictions on the rotation speed of the ball mill during granulation. For example, the rotation speed of the ball mill during granulation can be set to 400 rmp, 500 rmp, 600 rmp, 700 rmp, 800 rmp, 900 rmp, 1000 rmp, 1100 rmp, 1200 rmp, or a range composed of any two of these values.

[0100] Preferably, the rotation speed of the ball mill during granulation is 400 rmp - 1200 rmp.

[0101] In a third aspect, the present invention further provides a positive electrode sheet, wherein the positive electrode sheet includes a current collector and a positive electrode active material provided on the surface of the current collector, and the positive electrode active material contains any one of the lithium supplement agents introduced in the first aspect or a lithium supplement agent prepared by using any one of the preparation methods of the lithium supplement agents introduced in the second aspect.

[0102] In a fourth aspect, the present invention further provides a secondary battery, and the secondary battery includes the positive electrode sheet introduced in the third aspect.

[0103] It should be noted that although the present invention introduces the lithium supplement agent and the preparation method of the lithium supplement agent by taking Li5FeO4 as an example of the auxiliary lithium supplement material, this is not restrictive. The lithium supplement agent and the preparation method of the lithium supplement agent of the present invention are also applicable to other types of auxiliary lithium supplement materials, etc. Such flexible adjustments and changes do not deviate from the principle and scope of the present invention and should all be included in the protection scope of the present invention.

[0104] The following will introduce in detail the preparation method of the lithium supplement agent of the present invention in combination with the following embodiments.

[0105] Example 1:

[0106] The preparation method of the lithium supplement agent in this example includes the following steps:

[0107] S11: Dissolve 40 g of lithium oxalate crystals in 1 L of deionized water, stir until dissolved to obtain a lithium oxalate solution;

[0108] S12: Add 187.5 g of a conductive agent dispersion with a mass fraction of 0.8% to the lithium oxalate solution, disperse it, and perform ultrasonic vibration treatment to form a mixed solution;

[0109] S2: Evaporate the mixed solution in step S12 at 35 °C, while performing ultrasonic vibration. After completion, dry it for 12 h to obtain a crystalline substance;

[0110] S3: Under an environmental humidity of less than 10%, weigh 1.2 g of Li5FeO4 and mix it with the crystalline substance obtained in step S3. Then put it into a ball mill for ball milling. The rotation speed of the ball mill is 800 rmp, and the ball milling time is 4 h - 6 h to obtain a lithium supplement agent, where the particle size D50 of the lithium supplement agent is 2 μm - 8 μm.

[0111] In this example, the conductive agent is carbon nanotubes, the mass ratio of lithium oxalate to the conductive agent is 40:1.0, the auxiliary lithium supplement material is Li5FeO4, and the mass ratio of lithium oxalate to the auxiliary lithium supplement material is 40:1.2.

[0112] Example 2:

[0113] The preparation method of the lithium supplement agent in Example 2 is the same as that in Example 1. The types of the conductive agent and the auxiliary lithium supplement material in Example 2 are the same as those in Example 1, and the mass ratio of lithium oxalate to the conductive agent is the same as that in Example 1. The difference lies in the different mass ratio of lithium oxalate to the auxiliary lithium supplement material.

[0114] In this example, the mass ratio of lithium oxalate to the auxiliary lithium supplement material is 40:0.4.

[0115] Example 3:

[0116] The preparation method of the lithium supplement agent in Example 3 is the same as that in Example 1. The types of the conductive agent and the auxiliary lithium supplement material in Example 3 are the same as those in Example 1, and the mass ratio of lithium oxalate to the conductive agent is the same as that in Example 1. The difference lies in that the mass ratio of lithium oxalate to the auxiliary lithium supplement material is different.

[0117] In this example, the mass ratio of lithium oxalate to the auxiliary lithium supplement material is 40:2.0.

[0118] Example 4:

[0119] The preparation method of the lithium supplement agent in Example 4 is the same as that in Example 1. The types of the conductive agent and the auxiliary lithium supplement material in Example 4 are the same as those in Example 1, and the mass ratio of lithium oxalate to the auxiliary lithium supplement material is the same as that in Example 1. The difference lies in that the mass ratio of lithium oxalate to the conductive agent is different.

[0120] In this example, the mass ratio of lithium oxalate to the conductive agent is 40:0.5.

[0121] Example 5:

[0122] The preparation method of the lithium supplement agent in Example 5 is the same as that in Example 1. The types of the conductive agent and the auxiliary lithium supplement material in Example 5 are the same as those in Example 1, and the mass ratio of lithium oxalate to the auxiliary lithium supplement material is the same as that in Example 1. The difference lies in that the mass ratio of lithium oxalate to the conductive agent is different.

[0123] In this example, the mass ratio of lithium oxalate to the conductive agent is 40:1.5.

[0124] Example 6:

[0125] The preparation method of the lithium supplement agent in Example 6 is the same as that in Example 1. The types of the conductive agent and the auxiliary lithium supplement material in Example 6 are the same as those in Example 1, and the mass ratio of lithium oxalate to the auxiliary lithium supplement material is the same as that in Example 1. The difference lies in that the mass ratio of lithium oxalate to the conductive agent is different.

[0126] In this example, the mass ratio of lithium oxalate to the conductive agent is 40:2.5.

[0127] Example 7:

[0128] The preparation method of the lithium supplement agent in Example 7 is the same as that in Example 1. The types of the conductive agent and the auxiliary lithium supplement material in Example 7 are the same as those in Example 1, and the mass ratio of lithium oxalate to the conductive agent is the same as that in Example 1. The difference lies in that the mass ratio of lithium oxalate to the auxiliary lithium supplement agent in this example is different.

[0129] In this example, the mass ratio of lithium oxalate to the auxiliary lithium supplement material is 40:2.5.

[0130] Example 8:

[0131] The preparation method of the lithium supplement agent in Example 8 is the same as that in Example 1. The types of the conductive agent and the auxiliary lithium supplement material in Example 8 are the same as those in Example 1. The mass ratio of lithium oxalate to the auxiliary lithium supplement material in Example 8 is the same as that in Example 1. The difference lies in that the mass ratio of lithium oxalate to the conductive agent in this example is different.

[0132] In this example, the mass ratio of lithium oxalate to the conductive agent is 40:3.

[0133] Example 9:

[0134] The preparation method of the lithium supplement agent in Example 9 is the same as that in Example 1. The type of the conductive agent in Example 9 is the same as that in Example 1. The mass ratio of lithium oxalate to the conductive agent in Example 9 is the same as that in Example 1. The mass ratio of lithium oxalate to the auxiliary lithium supplement material is also the same. The difference lies in that the type of the auxiliary lithium supplement material is different.

[0135] In this example, the auxiliary lithium supplement material is Li2NiO2.

[0136] Example 10:

[0137] The preparation method of the lithium supplement agent in Example 10 is the same as that in Example 1. The types of the conductive agent in Example 10 are the same as those in Example 1. The mass ratio of lithium oxalate to the conductive agent in Example 10 is the same as that in Example 1. The difference lies in that the type of the auxiliary lithium supplement material is different and the mass ratio of lithium oxalate to the auxiliary lithium supplement material is different.

[0138] In this example, the auxiliary lithium supplement material is Li2NiO2, and the mass ratio of lithium oxalate to the auxiliary lithium supplement material is 40:0.4.

[0139] Example 11:

[0140] The preparation method of the lithium supplement agent in Example 11 is the same as that in Example 1. The types of the conductive agent in Example 11 are the same as those in Example 1. The mass ratio of lithium oxalate to the conductive agent in Example 11 is the same as that in Example 1. The difference lies in that the type of the auxiliary lithium supplement material is different and the mass ratio of lithium oxalate to the auxiliary lithium supplement material is different.

[0141] In this example, the auxiliary lithium supplement material is Li2NiO2, and the mass ratio of lithium oxalate to the auxiliary lithium supplement material is 40:0.8.

[0142] Example 12:

[0143] The preparation method of the lithium supplement agent in Example 12 is the same as that in Example 1. The types of conductive agents in Example 12 are the same as those in Example 1, and the mass ratio of lithium oxalate to the conductive agent is the same as that in Example 1. The difference lies in the different types of auxiliary lithium supplement materials and the different mass ratios of lithium oxalate to the auxiliary lithium supplement materials.

[0144] In this example, the auxiliary lithium supplement material is Li2NiO2, and the mass ratio of lithium oxalate to the auxiliary lithium supplement material is 40:1.6.

[0145] Example 13:

[0146] The preparation method of the lithium supplement agent in Example 13 is the same as that in Example 1. The types of conductive agents in Example 13 are the same as those in Example 1, and the mass ratio of lithium oxalate to the conductive agent is the same as that in Example 1. The difference lies in the different types of auxiliary lithium supplement materials and the different mass ratios of lithium oxalate to the auxiliary lithium supplement materials.

[0147] In this example, the auxiliary lithium supplement material is Li2NiO2, and the mass ratio of lithium oxalate to the auxiliary lithium supplement material is 40:2.0.

[0148] Example 14:

[0149] The preparation method of the lithium supplement agent in Example 14 is the same as that in Example 1. The types of conductive agents in Example 14 are the same as those in Example 1, and the mass ratio of lithium oxalate to the auxiliary lithium supplement material is the same as that in Example 1. The difference lies in the different types of auxiliary lithium supplement materials and the different mass ratios of lithium oxalate to the conductive agent.

[0150] In this example, the auxiliary lithium supplement material is Li2NiO2, and the mass ratio of lithium oxalate to the conductive agent is 40:0.5.

[0151] Example 15:

[0152] The preparation method of the lithium supplement agent in Example 15 is the same as that in Example 1. The types of conductive agents in Example 15 are the same as those in Example 1, and the mass ratio of lithium oxalate to the auxiliary lithium supplement material is the same as that in Example 1. The difference lies in the different types of auxiliary lithium supplement materials and the different mass ratios of lithium oxalate to the conductive agent.

[0153] In this example, the auxiliary lithium supplement material is Li2NiO2, and the mass ratio of lithium oxalate to the conductive agent is 40:1.5.

[0154] Comparative Example 1:

[0155] The preparation method of the lithium supplement agent in Comparative Example 1 is the same as that in Example 1. The type of conductive agent in Comparative Example 1 is the same as that in Example 1, and the mass ratio of lithium oxalate to the conductive agent is also the same as that in Example 1. The difference is that in step S3 of this comparative example, no auxiliary lithium supplement material is added, and the crystallized product is directly ball-milled to obtain the lithium supplement agent.

[0156] Comparative Example 2:

[0157] The preparation method of the lithium supplement agent in Comparative Example 2 is the same as that in Example 1. The type of auxiliary lithium supplement material in Comparative Example 2 is the same as that in Example 1, and the mass ratio of lithium oxalate to the auxiliary lithium supplement material in Comparative Example 2 is also the same as that in Example 1. The difference is that in step S2 of this comparative example, no conductive agent is added, and the lithium oxalate solution is directly dispersed and subjected to ultrasonic vibration treatment to form a mixed solution.

[0158] Comparative Example 3:

[0159] The preparation method of the lithium supplement agent in Comparative Example 3 is the same as that in Example 1. The mass ratio of lithium oxalate to the auxiliary lithium supplement material in Comparative Example 3 is also the same as that in Example 1. The difference is that the type of the auxiliary lithium supplement material in this comparative example is different, and in step S2 of this comparative example, no conductive agent is added, and the lithium oxalate solution is directly dispersed and subjected to ultrasonic vibration treatment to form a mixed solution.

[0160] In this comparative example, the auxiliary lithium supplement material is Li2NiO2.

[0161] Testing method:

[0162] (1) The lithium supplement agents prepared in Examples 1 to 15 and Comparative Examples 1 to 3 are made into lithium-ion batteries.

[0163] Specifically, the lithium supplement agents, cathode active material (lithium iron phosphate), SP, and PVDF prepared in Examples 1 to 15 and Comparative Examples 1 to 3 are used as the cathode material to prepare a cathode sheet, and graphite is used as the anode material to make an anode sheet. The cathode sheet, separator, anode sheet, and separator are wound to form an electric core, and then an electrolyte composed of lithium hexafluorophosphate (LiPF6), ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) is injected into the electric core and sealed to obtain a lithium-ion battery.

[0164] (2) The obtained lithium-ion batteries are formed and tested

[0165] The obtained lithium-ion batteries in (1) are formed and tested. First, they are charged with a constant current of 0.05C to 3.0V, then charged with a constant current of 0.2C to 4.0V, and finally charged with a constant current of 0.1C to 4.7V. The first decomposition voltage of the lithium supplement agent is recorded, and the test results are shown in the following table.

[0166] Table 1: Test data of examples and comparative examples

[0167] It can be seen from the data in Table 1 that:

[0168] 1. Comparing the data of Examples 1 to 15 with that of Comparative Example 1, the initial decomposition voltage of the lithium supplement in Examples 1 to 15 is significantly lower than that of Comparative Example 1. Thus, it can be seen that adding an auxiliary lithium supplement material to the lithium oxalate lithium supplement can significantly reduce the initial decomposition voltage of lithium oxalate. Further, when the mass ratio of lithium oxalate to the auxiliary lithium supplement material in the lithium supplement is 40:(0.4 - 2.5), the initial decomposition voltage of lithium oxalate can be significantly reduced.

[0169] 2. Comparing the data of Examples 1 to 3 with that of Example 7, and comparing the data of Examples 9 to 13, as the mass ratio of lithium oxalate to the auxiliary lithium supplement material in the lithium supplement gradually decreases, that is, as the content of the auxiliary lithium supplement material in the lithium supplement increases, the initial decomposition voltage of lithium oxalate gradually decreases, and the lithium supplement effect is significantly improved. However, when the mass ratio of lithium oxalate to the auxiliary lithium supplement material further decreases, that is, when the content of the auxiliary lithium supplement material in the lithium supplement further increases, the initial decomposition voltage of lithium oxalate will gradually increase. That is to say, as the content of the auxiliary lithium supplement material in the lithium supplement increases, the initial decomposition voltage of lithium oxalate gradually decreases, indicating that the auxiliary lithium supplement material can generate substances that catalyze the de-lithiation of lithium oxalate, enabling lithium oxalate to decompose and release more capacity at a lower voltage, thereby gradually reducing the initial decomposition voltage of lithium oxalate. However, when the content of the auxiliary lithium supplement material in the lithium supplement further increases, it will cause the auxiliary lithium supplement material to decompose and generate excessive highly reactive oxygen, and too much oxygen will affect the battery system, resulting in a gradual increase in its initial decomposition voltage.

[0170] Therefore, it is necessary to reasonably control the mass ratio between lithium oxalate and the auxiliary lithium supplement material in the lithium supplement. When the mass ratio of lithium oxalate to the auxiliary lithium supplement material in the lithium supplement is 40:(0.4 - 2.0), the initial decomposition voltage of lithium oxalate is relatively low, and the lithium supplement effect of the lithium supplement is better. Further, when the mass ratio of lithium oxalate to the auxiliary lithium supplement material in the lithium supplement is 40:(0.4 - 1.2), the initial decomposition voltage of lithium oxalate is even lower, and the lithium supplement effect of the lithium supplement is better. Even further, when the mass ratio of lithium oxalate to the auxiliary lithium supplement material in the lithium supplement is 40:1.2, the initial decomposition voltage of lithium oxalate is the lowest, and the lithium supplement effect of the lithium supplement is the best.

[0171] 3. Compare the data of Example 1, Examples 4 to 6, Example 8, and Comparative Example 2. The initial decomposition voltage of lithium oxalate in Example 1, Examples 4 to 6 is significantly lower than that of lithium oxalate in Comparative Example 2. Compare the data of Example 9, Examples 14 to 15 with that of Comparative Example 3. The initial decomposition voltage of lithium oxalate in Example 9, Examples 14 to 15 is significantly lower than that of lithium oxalate in Comparative Example 3. This shows that adding a conductive agent to the lithium supplement agent can effectively improve the conductivity of the lithium supplement agent and effectively reduce the initial decomposition voltage of lithium oxalate. Further, when the mass ratio of lithium oxalate to the conductive agent in the lithium supplement agent is 40:(0.5 - 3.0), the initial decomposition voltage of lithium oxalate can be effectively reduced.

[0172] 4. Compare the data of Example 1, Examples 4 to 6 with that of Example 8, and compare the data of Example 9, Examples 14 and 15. As the mass ratio of lithium oxalate to the conductive agent in the lithium supplement agent decreases, that is, as the content of the conductive agent in the lithium supplement agent gradually increases, the initial decomposition voltage of lithium oxalate gradually decreases. However, when the mass ratio of lithium oxalate to the conductive agent in the lithium supplement agent further decreases, that is, when the content of the conductive agent in the lithium supplement agent further increases, the initial decomposition voltage of lithium oxalate gradually increases again. This is because too much conductive agent will block the lithium ion transmission channels, resulting in hindered lithium ion transmission, thereby increasing the decomposition voltage.

[0173] Therefore, it is necessary to reasonably control the mass ratio between lithium oxalate and the conductive agent in the lithium supplement agent. When the mass ratio of lithium oxalate to the conductive agent is 40:(0.5 - 2.5), the initial decomposition voltage of lithium oxalate is relatively low, and the lithium supplement effect of the lithium supplement agent is better. Further, when the mass ratio of lithium oxalate to the conductive agent in the lithium supplement agent is 40:(0.5 - 1.5), the initial decomposition voltage of lithium oxalate is significantly reduced. Furthermore, when the mass ratio of lithium oxalate to the conductive agent in the lithium supplement agent is 40:1, the initial decomposition voltage of lithium oxalate is the lowest, and the lithium supplement effect of the lithium supplement agent is the best.

[0174] So far, the technical solution of the present invention has been described in conjunction with the preferred embodiments shown in the drawings. However, it is easy for those skilled in the art to understand that the protection scope of the present invention is obviously not limited to these specific embodiments. Without departing from the principle of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will all fall within the protection scope of the present invention.

Claims

1. A lithium supplement agent for supplementing lithium to a lithium-ion battery, characterized in that, The lithium supplement agent includes lithium oxalate, a conductive agent, and an auxiliary lithium supplement material. Among them, the auxiliary lithium supplement material generates a substance capable of catalyzing the de-lithiation of lithium oxalate during the formation of a lithium-ion battery using the lithium supplement agent, and does not generate a substance that reacts with the electrolyte of the lithium-ion battery.

2. The lithium supplement agent according to claim 1, wherein The auxiliary lithium supplement material includes one or any combination of transition metal oxides of lithium and oxides of lithium.

3. The lithium supplement agent according to claim 2, wherein The auxiliary lithium supplement material includes one or any combination of Li2NiO2, Li5FeO4, Li2O, and LiCoO2.

4. The lithium supplement agent according to claim 1, characterized in that, The mass ratio of the lithium oxalate to the auxiliary lithium supplementing material is 40:(0.4 - 2.5). ， Preferably, the mass ratio of the lithium oxalate to the auxiliary lithium supplementing material is 40:(0.4 - 2.0).

5. The lithium supplement agent according to claim 1, characterized in that, The conductive agent includes one or any combination of carbon nanotubes or carbon fibers.

6. The lithium supplement according to claim 1, wherein The mass ratio of the lithium oxalate to the conductive agent is 40:(0.5 - 3.0), and preferably the mass ratio of the lithium oxalate to the conductive agent is 40:(0.5 - 2.5).

7. The lithium supplement agent according to any one of claims 1 to 6, characterized in that, The particle size D50 of the lithium supplement agent is 2 μm - 8 μm.

8. A method for preparing a lithium supplement agent according to any one of claims 1 to 7, characterized in that, The preparation method includes the following steps: S1: Provide a mixed solution mainly composed of a conductive agent, lithium oxalate, and a solvent to form a lithium oxalate solution mixed with the conductive agent; S2: Recrystallize the lithium oxalate solution mixed with the conductive agent to obtain a crystal; S3: Mix and granulate the crystal obtained in step S2 with the auxiliary lithium supplement material to obtain the lithium supplement agent.

9. The preparation method of the lithium supplement agent according to claim 8, wherein, The step of "providing a mixed solution mainly composed of a conductive agent, lithium oxalate, and a solvent" specifically includes: S11: Dissolve lithium oxalate crystals in a solvent to obtain a lithium oxalate solution; S12: Mix the conductive agent into the lithium oxalate solution to obtain the mixed solution.

10. The preparation method of the lithium supplement agent according to claim 9, wherein The step of "dissolving lithium oxalate crystals in a solvent to obtain a lithium oxalate solution" specifically includes: Under the condition of 5 - 20 °C, dissolve lithium oxalate crystals in deionized water to obtain the lithium oxalate solution.

11. The preparation method of the lithium supplement agent according to claim 8, characterized in that, The step of "recrystallizing the lithium oxalate solution mixed with the conductive agent" specifically includes: Recrystallize the lithium oxalate solution mixed with the conductive agent under the condition of 30 - 45 °C.

12. The preparation method of the lithium supplement agent according to claim 8, wherein The step of "mixing and granulating the crystal with the auxiliary lithium supplement material" specifically includes: The crystal and the auxiliary lithium supplement material are mixed and granulated under the condition that the environmental humidity is less than 10%.

13. A positive electrode sheet, characterized in that, The positive electrode sheet includes a current collector and a positive electrode active material provided on the surface of the current collector. Among them, the positive electrode active material contains the lithium supplement agent according to any one of claims 1 to 7 or the lithium supplement agent prepared by the preparation method of the lithium supplement agent according to any one of claims 8 - 12.

14. A secondary battery, characterized in that, Including the positive electrode sheet according to claim 13.

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