A positive electrode lithium replenishing agent and its preparation method, positive electrode sheet and lithium battery

Abstract

This invention relates to the field of battery technology, and more particularly to a positive electrode lithium replenisher, its preparation method, a positive electrode sheet, and a lithium battery. The aforementioned positive electrode lithium replenisher includes a core and a shell, with the shell covering the surface of the core to isolate the core from the side reaction environment. The core is a lithium-rich material, specifically at least one of Li5FeO4, Li5ReO6, and Li6CoO4. The shell is cobalt boride. This core-shell structured positive electrode lithium replenisher with cobalt boride as the shell effectively coats the core material, improves the air stability of the positive electrode lithium replenisher, reduces residual alkali, and thus enhances the battery's energy density and cycle performance.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a positive electrode lithium replenishing agent and its preparation method, a positive electrode sheet, and a lithium battery. Background Technology

[0002] During the first charge of a lithium battery, some Li₂ will be released from the positive electrode. + Instead of embedding into the negative electrode, it participates in the formation of the SEI film on the negative electrode surface. The above process is irreversible and consumes some of the active lithium, thereby reducing the energy density and cycle performance of the battery. Summary of the Invention

[0003] To address the aforementioned technical problems, this application discloses a positive electrode lithium replenishing agent and its preparation method, a positive electrode sheet, and a lithium battery, in order to solve the problems of poor air stability and large residual alkali in current positive electrode lithium replenishing agents.

[0004] In a first aspect, the present invention provides a positive electrode lithium replenishing agent, the positive electrode lithium replenishing agent comprising a core and a shell, wherein the shell covers the surface of the core and separates the core from the side reaction environment, the core being a lithium-rich material, the lithium-rich material being at least one of Li5FeO4, Li5ReO6, and Li6CoO4, and the shell being cobalt boride.

[0005] Preferably, the lithium-rich material is Li6CoO4.

[0006] Furthermore, the thickness of the outer shell is 1nm to 20nm.

[0007] Furthermore, the pH of the positive electrode lithium replenishing agent after being placed in air for a specified time is less than or equal to 9.5, and the specified time is 0h to 48h.

[0008] Preferably, the pH value of the positive electrode lithium replenishing agent after being placed in air for a specified time is less than or equal to 9.25, and the specified time is 18h to 24h.

[0009] Secondly, the present invention provides a method for preparing a positive electrode lithium supplement as described in the first aspect, comprising the following steps:

[0010] Mixing: The lithium-rich material is mixed and stirred with a cobalt-containing organic solution and a boron-containing organic solution to form a solid-liquid mixture;

[0011] In-situ deposition: The solid-liquid mixture is heated, causing the cobalt-containing compound in the cobalt-containing organic solution and the boron-containing compound in the boron-containing organic solution to undergo an in-situ deposition reaction on the surface of the lithium-rich material to generate cobalt boride. The cobalt boride is deposited and coats the surface of the lithium-rich material to form the positive electrode lithium replenishing agent.

[0012] The cobalt-containing compound is one or more of cobalt chloride, cobalt sulfate, cobalt iodide, and cobalt acetate, and the boron-containing compound is one or more of potassium borohydride, lithium borohydride, sodium thioborohydride, trisec-butyllithium borohydride, and nickel borohydride.

[0013] Furthermore, in the in-situ deposition step, the in-situ deposition reaction is carried out by heating at 40℃~78℃ for 0.5h~10h.

[0014] Further, the mixing step is as follows: the lithium-rich material is first mixed with one of the cobalt-containing organic solution or the boron-containing organic solution, and then the other of the cobalt-containing organic solution or the boron-containing organic solution is added dropwise to form the solid-liquid mixture.

[0015] Furthermore, in the mixing step, the dropping rate is 5 mL / min to 20 mL / min.

[0016] Furthermore, in the mixing step, the lithium-rich material accounts for 1% to 70% of the mass percentage of the solid-liquid mixture.

[0017] Further, in the mixing step, the cobalt content in the cobalt-containing organic solution is 0.001 mol / L to 0.1 mol / L, and / or the boron content in the boron-containing organic solution is 0.001 mol / L to 0.1 mol / L.

[0018] Furthermore, in the mixing step, the organic solution is an anhydrous organic solution, which is one or more of anhydrous ethanol, anhydrous methanol, tetrahydrofuran, n-butanol, or toluene.

[0019] Furthermore, the method for preparing the positive electrode lithium replenishing agent further includes, after the in-situ deposition step, performing:

[0020] Centrifugal washing: The solid-liquid mixture after the in-situ deposition reaction is centrifuged and washed to obtain the precipitate;

[0021] Drying: The precipitate is dried in a vacuum or inert atmosphere.

[0022] Thirdly, this application provides a positive electrode sheet, the positive electrode sheet comprising the positive electrode lithium supplement agent as described in the first aspect, or the positive electrode sheet comprising the positive electrode lithium supplement agent prepared by the preparation method described in the second aspect.

[0023] Fourthly, this application provides a lithium battery, the lithium battery including the positive electrode as described in the third aspect.

[0024] Compared with the prior art, this application has at least the following beneficial effects:

[0025] This application provides a core-shell structured positive electrode lithium supplement with cobalt boride as the outer shell, which can effectively coat the core material, form a dense and high-strength outer shell, reduce the occurrence of side reactions, avoid the positive electrode lithium supplement from reacting with water vapor and carbon dioxide, improve the air stability of the positive electrode lithium supplement, reduce residual alkali, and thus improve the battery energy density and cycle performance.

[0026] Because cobalt boride contains no oxygen, it can form stable chemical bonds with oxygen in Li5FeO4, Li5ReO6, and Li6CoO4, resulting in better coating performance than traditional oxygen-containing materials such as zirconium oxide and alumina. Especially when lithium cobalt oxide is used as the core, the presence of cobalt in cobalt boride reduces stress at the interface between the outer shell and the core, leading to a denser coating. Therefore, using the above-mentioned positive electrode lithium supplement as an additive in the positive electrode sheet can effectively improve the battery's energy density and cycle performance. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 A process flow diagram of the preparation method of the positive electrode lithium replenishing agent provided in this application;

[0029] Figure 2 This is a schematic diagram of the structure of the positive electrode lithium replenishing agent provided in Embodiment 1 of this application;

[0030] Figure 3 This is a SEM image of the positive electrode lithium replenishment agent provided in Example 1 of this application;

[0031] Figure 4 SEM image of the positive electrode lithium replenishment agent provided in Comparative Example 1 of this application;

[0032] Figure 5 SEM image of the positive electrode lithium replenishment agent provided in Comparative Example 2 of this application;

[0033] Figure 6 The image shows the SEM image of the positive electrode lithium replenishment agent provided in Comparative Example 3 of this application.

[0034] Explanation of the attached diagram labels: 1. Kernel; 2. Shell. Detailed Implementation

[0035] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0036] During the initial charge and discharge of a lithium-ion battery, some active lithium ions migrate from the positive electrode to the negative electrode surface, forming an SEI film, resulting in irreversible loss of the active lithium material. Currently, widely used graphite electrode active materials experience irreversible capacity losses exceeding 6%, while silicon-based electrode active materials suffer losses as high as 10%–20%. This irreversible loss leads to a decrease in the battery's energy density and cycle performance. To compensate for the reduced active lithium ions during this process, lithium replenishment materials can be used to compensate for the consumed active lithium ions, thereby improving the cycle performance and energy density of the lithium-ion battery.

[0037] While lithium-rich materials can provide active lithium ions, highly reactive lithium-rich materials, such as lithium cobalt oxide, are very sensitive to water and carbon dioxide in the air, easily undergoing side reactions to generate lithium hydroxide and lithium carbonate. This leads to increased residual alkali. The generated lithium hydroxide and lithium carbonate can also react with binders in the positive electrode, such as polyvinylidene fluoride, resulting in poor adhesion of the positive electrode and consequently reduced battery stability and performance. Coating lithium-rich materials with alumina or zirconium oxide powder often fails to completely cover the entire lithium-rich material particle, instead creating island-like distribution on the surface. This means that part of the lithium-rich material particle is coated with the powder, while another part is exposed to the air. This structural form cannot effectively isolate the lithium-rich material from the environment where side reactions occur, such as water or carbon dioxide in the air, thus failing to effectively improve the problems of poor stability and high residual alkali in air. Residual alkali refers to the lithium hydroxide (LiOH) and lithium carbonate (Li2CO3) remaining on the material surface.

[0038] Based on this, in order to improve the air stability of the positive electrode lithium replenishing agent, reduce residual alkali, and effectively coat lithium-rich materials, this application proposes a new positive electrode lithium replenishing agent to solve the above problems and improve the battery energy density and cycle performance.

[0039] In a first aspect, embodiments of this application provide a positive electrode lithium replenishing agent, the positive electrode lithium replenishing agent comprising a core and a shell, wherein the shell covers the surface of the core and separates the core from the side reaction environment, the core being a lithium-rich material, the lithium-rich material being at least one of Li5FeO4, Li5ReO6, and Li6CoO4, and the shell being cobalt boride.

[0040] It should be noted that the above-mentioned side reaction environment refers to the environment in which lithium-rich materials can directly contact substances such as water and carbon dioxide to produce lithium hydroxide and lithium carbonate. For example, it can be an air environment containing water vapor and carbon dioxide.

[0041] This application uses cobalt boride as the outer shell and at least one lithium-rich material selected from Li5FeO4, Li5ReO6, and Li6CoO4 as the core. Cobalt boride, containing only boron and cobalt, can combine with the oxygen element in the aforementioned lithium-rich materials to form stable chemical bonds. This allows the cobalt boride shell to have a stronger bond with the lithium-rich material core through chemical bonding, ensuring that the shell can completely cover the surface of the core. This results in a superior and more complete coating effect, effectively separating the core from water vapor, carbon dioxide, and other pollutants in the air, significantly reducing side reactions and gas generation problems associated with these lithium-rich materials. It should be noted that when using powders such as alumina or zirconium oxide to coat lithium-rich materials, the interaction between these powders and the lithium-rich materials is only a weak intermolecular van der Waals force, and therefore does not have a strong coating capacity.

[0042] Meanwhile, when cobalt boride is used as the outer shell, its excellent electronic and ionic conductivity ensures that the rate at which active lithium ions in the core pass through the cobalt boride shell is not affected, and it also has no impact on the conductivity of the positive electrode, thus fully leveraging the lithium replenishment effect of lithium-rich materials. Therefore, using the above-mentioned positive electrode lithium replenishing agent as an additive to the positive electrode can effectively improve the energy density and cycle performance of the battery.

[0043] In a preferred embodiment, the lithium-rich material is Li6CoO4.

[0044] When lithium cobalt oxide is used as the core material, the presence of cobalt in cobalt boride reduces the stress at the interface between the shell and the core, improving the material compatibility between them. This results in a denser coating and better encapsulation of the core by the shell. Consequently, it also prevents cobalt ions from dissolving into the electrolyte during cycling, which could catalyze the electrolyte and reduce battery cycle performance.

[0045] Optionally, the thickness of the outer shell is 1 nm to 20 nm.

[0046] The shell thickness in this application embodiment can be any value within the above range, such as 1nm, 2.5nm, 3.1nm, 4.2nm, 5.7nm, 7.3nm, 8.9nm, 10.1nm, 11.3nm, 12.5nm, 13.6nm, 14.7nm, 15.1nm, 16.8nm, 17.6nm, 18.4nm, 19.5nm, or 20nm.

[0047] Furthermore, the pH of the positive electrode lithium replenishing agent after being placed in air for a specified time is less than or equal to 9.5, and the specified time is 0h to 48h.

[0048] Preferably, the pH value of the positive electrode lithium replenishing agent after being placed in air for a specified time is less than or equal to 9, and the specified time is 18h to 24h.

[0049] The positive electrode lithium replenisher provided in this application, when placed in air for less than 48 hours, has a measured pH value of less than or equal to 9.5. This indicates that the lithium-rich material does not undergo significant side reactions to generate a large amount of residual alkali when placed in air. This demonstrates that the cobalt boride shell used in the positive electrode lithium replenisher provides a good coating effect, effectively isolating the core lithium-rich material from external water and carbon dioxide reactions. This improves the stability of the lithium-rich material in air and extends the shelf life of the positive electrode lithium replenisher, thus meeting the requirements of large-scale industrial production storage. Residual alkali refers to the lithium hydroxide and lithium carbonate generated after the lithium-rich material comes into contact with water and carbon dioxide.

[0050] When the specified placement time is within 18h to 24h, the measured pH value is less than or equal to 9.25. That is, within a shorter placement time, the positive electrode lithium replenishment agent provided by this application has fewer side reactions with external carbon dioxide and water, which can better meet the stability requirements in the actual preparation process and ensure higher production efficiency.

[0051] Secondly, the present invention provides a method for preparing a positive electrode lithium supplement as described in the first aspect, comprising the following steps:

[0052] Mixing: The lithium-rich material is mixed and stirred with a cobalt-containing organic solution and a boron-containing organic solution to form a solid-liquid mixture;

[0053] In-situ deposition: The solid-liquid mixture is heated, causing the cobalt-containing compound in the cobalt-containing organic solution and the boron-containing compound in the boron-containing organic solution to undergo an in-situ deposition reaction on the surface of the lithium-rich material to generate cobalt boride. The cobalt boride is deposited and coats the surface of the lithium-rich material to form the positive electrode lithium replenishing agent.

[0054] The cobalt-containing compound is one or more of cobalt chloride, cobalt sulfate, cobalt iodide, and cobalt acetate, and the boron-containing compound is one or more of potassium borohydride, lithium borohydride, sodium thioborohydride, trisec-butyllithium borohydride, and nickel borohydride.

[0055] The applicant's research found that the positive electrode lithium replenishing agent provided in the embodiments of this application, prepared by the above-mentioned in-situ deposition method, can effectively coat lithium-rich materials.

[0056] Combination Figure 1As shown, through a mixing step, lithium-rich materials and raw materials required for preparing cobalt boride are initially mixed, so that the lithium-rich materials are distributed in the solid-liquid mixture. Then, the solid-liquid mixture is heated, so that the cobalt-containing organic solution and the boron-containing organic solution undergo an in-situ deposition reaction on the surface of the lithium-rich materials under heating conditions, so that the generated cobalt boride can fully coat the outer surface of the lithium-rich materials. In the above reaction process, the introduction of water and carbon dioxide can be avoided when using organic solutions as the reaction system. Moreover, since the in-situ deposition method can control the thermodynamic and kinetic conditions in the above reaction system, compared with the difficulty of control in traditional grinding and mixing coating methods, the in-situ deposition coating method provided by this application can achieve stable and controllable coating. Since there is not only a chemical reaction in the cobalt boride formation process during the deposition process, but also chemical bonding between cobalt boride and oxygen in the lithium-rich materials, the bonding strength between the shell and the core material is also better.

[0057] In summary, the above-mentioned positive electrode lithium replenishing agent prepared by the in-situ deposition method provided in this application can coat the surface of lithium-rich materials with a dense and high-strength shell, effectively isolating the lithium-rich materials from contact with water and carbon dioxide, reducing the occurrence of side reactions, and thus achieving the lithium replenishing effect during the first charge and discharge process of the battery, thereby improving the energy density and cycle performance of the battery.

[0058] Optionally, in the in-situ deposition step, the in-situ deposition reaction is carried out by heating at 40℃ to 78℃ for 0.5h to 10h.

[0059] In the preparation method provided in this application, the heating reaction conditions in the in-situ deposition step can be any value within the above range. For example, the heating temperature can be 40℃, 41℃, 43℃, 48℃, 52℃, 55℃, 63℃, 68℃, 73℃, 77℃ or 78℃, and the heating time can be 0.5h, 1.2h, 1.5h, 2.3h, 3.4h, 4.1h, 4.7h, 5.2h, 5.5h, 6.8h, 7.9h, 8.3h, 9.1h or 10h.

[0060] By controlling the reaction temperature and reaction time in this step, the thermodynamic and kinetic conditions of the reaction process can be controlled, meeting the requirements for precise and controllable reaction process in batch preparation. For example, when the reaction temperature is increased, the in-situ deposition reaction rate increases, and the reaction time can be controlled to regulate the thickness of the final positive electrode lithium replenishment shell.

[0061] Optionally, the mixing step is as follows: first, the lithium-rich material is mixed with one of the cobalt-containing organic solution or the boron-containing organic solution, and then the other of the cobalt-containing organic solution or the boron-containing organic solution is added dropwise to form the solid-liquid mixture.

[0062] Furthermore, in the mixing step, the dropping rate is 5 mL / min to 20 mL / min.

[0063] In the above mixing steps, the two solutions must be added separately. When adding one solution, the dropping rate of the other solution needs to be controlled; the dropping rate should not be too fast or too slow. If the solution is added too quickly, the ion concentration in some areas will be too high, resulting in new nucleation and preventing deposition on the substrate surface, thus affecting the coating effect and the purity of the material.

[0064] Preferably, in the mixing step, the lithium-rich material accounts for 10% to 50% of the mass percentage of the solid-liquid mixture.

[0065] Preferably, the solid-liquid ratio is controlled within the above-mentioned range during the mixing step. The mass percentage of lithium-rich material in the solid-liquid mixture should not be too high, otherwise the resulting solid-liquid mixture will have a high viscosity, making it more difficult to operate during stirring and difficult to mix evenly, thus increasing the difficulty of the process. It should also not be too low, otherwise the content of lithium-rich material in the solid-liquid mixture will be too low, resulting in a significant reduction in production efficiency and an increase in production costs.

[0066] Optionally, in the mixing step, the cobalt content in the cobalt-containing organic solution is 0.001 mol / L to 0.1 mol / L, and / or the boron content in the boron-containing organic solution is 0.001 mol / L to 0.1 mol / L.

[0067] Because a controllable in-situ deposition method is used, the preparation method provided in this application requires lower concentrations of cobalt and boron in the cobalt-containing organic solution and the boron-containing organic solution, reducing the difficulty of the process. During preparation, when the concentrations of the raw materials are low, the reaction rate can be controlled by adjusting other process parameters such as reaction temperature and reaction time to achieve a good coating effect. The cobalt content in the cobalt-containing organic solution can be any value within the range of 0.001 mol / L to 0.1 mol / L, such as 0.001 mol / L, 0.005 mol / L, 0.017 mol / L, 0.023 mol / L, 0.028 mol / L, 0.036 mol / L, 0.041 mol / L, 0.050 mol / L, 0.057 mol / L, 0.062 mol / L, 0.074 mol / L, 0.081 mol / L, 0.094 mol / L, or 0.1 mol / L. L; The boron content in the boron-containing organic solution can be any value within the range of 0.001 mol / L to 0.1 mol / L, such as 0.001 mol / L, 0.005 mol / L, 0.017 mol / L, 0.023 mol / L, 0.028 mol / L, 0.036 mol / L, 0.041 mol / L, 0.050 mol / L, 0.057 mol / L, 0.062 mol / L, 0.074 mol / L, 0.081 mol / L, 0.094 mol / L, or 0.1 mol / L.

[0068] Optionally, in the mixing step, the organic solution is an anhydrous organic solution, which is one or more of anhydrous ethanol, anhydrous methanol, tetrahydrofuran, n-butanol, or toluene.

[0069] Since lithium-rich materials are sensitive to water, this application uses anhydrous organic solution as a solvent to control the absence of water in the reaction system and performs liquid-phase coating in the organic solution. This avoids side reactions between the lithium-rich materials and water during the reaction process, which would otherwise result in poor lithium replenishment effect of the final positive electrode lithium replenishing agent. This effectively improves the air stability of lithium-rich cobalt oxide and overcomes the problem of high residual alkali.

[0070] Optionally, the method for preparing the positive electrode lithium replenishing agent further includes, after the in-situ deposition step, performing:

[0071] Centrifugal washing: The solid-liquid mixture after the in-situ deposition reaction is centrifuged and washed to obtain the precipitate;

[0072] Drying: The precipitate is dried in a vacuum or inert atmosphere.

[0073] To further improve the final product quality of the positive electrode lithium replenishment agent prepared by the aforementioned in-situ deposition method, this application embodiment also provides the above-mentioned means for improvement. After the in-situ deposition is completed, the solid-liquid mixture after the reaction can be centrifuged and cleaned. During cleaning, anhydrous organic solutions such as anhydrous ethanol and anhydrous methanol can be used to wash away the residual solution and precipitate particles on the surface of the lithium-rich material coated with cobalt boride. Then, the cleaned precipitate is dried in a vacuum or inert atmosphere to further remove the residual organic solution during cleaning. The inert atmosphere refers to an atmosphere containing one or more inert gases such as nitrogen and argon. The drying temperature is preferably 100°C.

[0074] Thirdly, this application provides a positive electrode sheet, the positive electrode sheet comprising the positive electrode lithium supplement agent as described in the first aspect, or the positive electrode sheet comprising the positive electrode lithium supplement agent prepared by the preparation method described in the second aspect.

[0075] Fourthly, this application provides a lithium battery, which includes a positive electrode as described in the third aspect. The lithium battery also includes an electrolyte, a negative electrode, and a separator, with the separator disposed between the positive and negative electrodes. The positive electrode, negative electrode, and separator are assembled to form a battery cell. The electrolyte is used to inject liquid into the battery cell and wet the positive and negative electrodes.

[0076] The solution of this application will be further described below with reference to specific embodiments and experimental data:

[0077] Example 1

[0078] Combination Figure 2 As shown, Figure 2 This is a schematic diagram of the positive electrode lithium replenishment agent provided in this embodiment. See also... Figure 2 This embodiment provides a positive electrode lithium replenishing agent, which has a core-shell structure, including a core 1 and an outer shell 2, with the outer shell 2 covering the surface of the core 1. The core is Li6CoO4, and the outer shell is cobalt boride.

[0079] The preparation process of this positive electrode lithium supplement includes the following steps:

[0080] Mixing: Weigh 50g of Li6CoO4 powder and add it to 100mL of anhydrous ethanol solution containing 0.08mol / L cobalt nitrate. Then add 30mL of anhydrous ethanol solution containing 0.05mol / L sodium borohydride at a dropping rate of 8mL / min to 10mL / min to form a solid-liquid mixture.

[0081] In-situ deposition: The above solid-liquid mixture was heated in a water bath at 50°C for 2 hours to carry out an in-situ deposition reaction, so that cobalt nitrate and sodium borohydride reacted on the surface of Li6CoO4 powder to generate cobalt borohydride.

[0082] Centrifugal washing: The product processed by the in-situ deposition step is centrifuged and then the separated precipitate is washed three times with anhydrous ethanol solution.

[0083] Drying: The precipitate was placed in a vacuum oven at 100°C and dried for 12 hours to obtain the positive electrode lithium replenishment agent.

[0084] Example 2

[0085] The positive electrode lithium supplement provided in this embodiment uses the same core and shell materials as in Example 1, the difference being the solid content. The solid content in Example 1 is 28%, and the solid content in Example 2 is 20%. The preparation process of the positive electrode lithium supplement provided in this embodiment includes the following steps:

[0086] Mixing: Weigh 40g of Li6CoO4 powder and add it to 130mL of anhydrous ethanol solution containing 0.08mol / L cobalt nitrate. Then add 30mL of anhydrous ethanol solution containing 0.05mol / L sodium borohydride at a dropping rate of 8mL / min to 10mL / min to form a solid-liquid mixture.

[0087] In-situ deposition: The above solid-liquid mixture was heated in a water bath at 50°C for 2 hours to carry out an in-situ deposition reaction, so that cobalt nitrate and sodium borohydride reacted on the surface of Li6CoO4 powder to generate cobalt borohydride.

[0088] Centrifugal washing: The product processed by the in-situ deposition step is centrifuged and then the separated precipitate is washed three times with anhydrous ethanol solution.

[0089] Drying: The precipitate was placed in a vacuum oven at 100℃ and dried for 12 hours to obtain the positive electrode lithium replenishment sample;

[0090] Example 3

[0091] The positive electrode lithium replenishing agent provided in this embodiment uses the same core and shell materials as in Example 1, the difference being the concentration of the sodium borohydride solution. The preparation process of the positive electrode lithium replenishing agent provided in this embodiment includes the following steps:

[0092] Mixing: Weigh 50g of Li6CoO4 powder and add it to 100mL of anhydrous ethanol solution containing 0.08mol / L cobalt nitrate. Then add 30mL of anhydrous ethanol solution containing 0.1mol / L sodium borohydride at a dropping rate of 8mL / min to 10mL / min to form a solid-liquid mixture.

[0093] In-situ deposition: The above solid-liquid mixture was heated in a water bath at 40°C for 2 hours to carry out an in-situ deposition reaction, so that cobalt nitrate and sodium borohydride reacted on the surface of Li6CoO4 powder to generate cobalt borohydride.

[0094] Centrifugal washing: The product processed by the in-situ deposition step is centrifuged and then the separated precipitate is washed three times with anhydrous ethanol solution.

[0095] Drying: The precipitate was placed in a vacuum oven at 100℃ and dried for 12 hours to obtain the positive electrode lithium replenishment sample;

[0096] Example 4

[0097] The positive electrode lithium replenishing agent provided in this embodiment uses the same core and shell materials as in Example 1, the difference being the different cobalt nitrate concentration. The preparation process of the positive electrode lithium replenishing agent provided in this embodiment includes the following steps:

[0098] Mixing: Weigh 50g of Li6CoO4 powder and add it to 100mL of anhydrous ethanol solution containing 0.15mol / L cobalt nitrate. Then add 30mL of anhydrous ethanol solution containing 0.05mol / L sodium borohydride at a dropping rate of 8mL / min to 10mL / min to form a solid-liquid mixture.

[0099] In-situ deposition: The above solid-liquid mixture was heated in a water bath at 50°C for 2 hours to carry out an in-situ deposition reaction, so that cobalt nitrate and sodium borohydride reacted on the surface of Li6CoO4 powder to generate cobalt borohydride.

[0100] Centrifugal washing: The product processed by the in-situ deposition step is centrifuged and then the separated precipitate is washed three times with anhydrous ethanol solution.

[0101] Drying: The precipitate was placed in a vacuum oven at 100°C and dried for 10 hours to obtain the positive electrode lithium replenishment sample;

[0102] Example 5

[0103] The positive electrode lithium replenishing agent provided in this embodiment uses the same core and shell materials as in Example 1, the difference being the holding temperature and time of the water bath heating. The preparation process of the positive electrode lithium replenishing agent provided in this embodiment includes the following steps:

[0104] Mixing: Weigh 50g of Li6CoO4 powder and add it to 100mL of anhydrous ethanol solution containing 0.08mol / L cobalt nitrate. Then add 30mL of anhydrous ethanol solution containing 0.05mol / L sodium borohydride at a dropping rate of 8mL / min to 10mL / min to form a solid-liquid mixture.

[0105] In-situ deposition: The above solid-liquid mixture was heated in a water bath at 70°C for 3 hours to carry out an in-situ deposition reaction, so that cobalt nitrate and sodium borohydride reacted on the surface of Li6CoO4 powder to generate cobalt borohydride.

[0106] Centrifugal washing: The product processed by the in-situ deposition step is centrifuged and then the separated precipitate is washed three times with anhydrous ethanol solution.

[0107] Drying: The precipitate was placed in a vacuum oven at 100℃ and dried for 8 hours to obtain the positive electrode lithium replenishment sample;

[0108] Example 6

[0109] The core material of the positive electrode lithium supplement provided in this embodiment is Li5FeO4; the preparation process of the positive electrode lithium supplement provided in this embodiment includes the following steps:

[0110] Mixing: Weigh 50g of Li5FeO4 powder and add it to 100mL of anhydrous ethanol solution containing 0.08mol / L cobalt nitrate. Then add 30mL of anhydrous ethanol solution containing 0.05mol / L sodium borohydride at a dropping rate of 8mL / min to 10mL / min to form a solid-liquid mixture.

[0111] In-situ deposition: The above solid-liquid mixture was heated in a water bath at 50°C for 2 hours to carry out an in-situ deposition reaction, so that cobalt nitrate and sodium borohydride reacted on the surface of Li5FeO4 powder to generate cobalt borohydride.

[0112] Centrifugal washing: The product processed by the in-situ deposition step is centrifuged and then the separated precipitate is washed three times with anhydrous ethanol solution.

[0113] Drying: The precipitate was placed in a vacuum oven at 100°C and dried for 12 hours to obtain the positive electrode lithium replenishment agent.

[0114] Example 7

[0115] The core material of the positive electrode lithium supplement provided in this embodiment is Li5ReO6. The preparation process of the positive electrode lithium supplement provided in this embodiment includes the following steps:

[0116] Mixing: Weigh 50g of Li5ReO6 powder and add it to 100mL of anhydrous ethanol solution containing 0.08mol / L cobalt nitrate. Then add 30mL of anhydrous ethanol solution containing 0.05mol / L sodium borohydride at a dropping rate of 8mL / min to 10mL / min to form a solid-liquid mixture.

[0117] In-situ deposition: The above solid-liquid mixture was heated in a water bath at 50°C for 2 hours to carry out an in-situ deposition reaction, so that cobalt nitrate and sodium borohydride reacted on the surface of Li5ReO6 powder to generate cobalt borohydride.

[0118] Centrifugal washing: The product processed by the in-situ deposition step is centrifuged and then the separated precipitate is washed three times with anhydrous ethanol solution.

[0119] Drying: The precipitate was placed in a vacuum oven at 100°C and dried for 12 hours to obtain the positive electrode lithium replenishment agent.

[0120] Example 8

[0121] The positive electrode lithium replenishing agent provided in this embodiment uses the same core and shell materials as in Example 1. The difference is that the dropping rate of the anhydrous ethanol solution of cobalt nitrate in this embodiment is 15 mL / min to 20 mL / min. The preparation process of this positive electrode lithium replenishing agent includes the following steps:

[0122] Mixing: Weigh 50g of Li6CoO4 powder and add it to 100mL of anhydrous ethanol solution containing 0.08mol / L cobalt nitrate. Then add 30mL of anhydrous ethanol solution containing 0.05mol / L sodium borohydride at a dropping rate of 15mL / min to 20mL / min to form a solid-liquid mixture.

[0123] In-situ deposition: The above solid-liquid mixture was heated in a water bath at 50°C for 2 hours to carry out an in-situ deposition reaction, so that cobalt nitrate and sodium borohydride reacted on the surface of Li6CoO4 powder to generate cobalt borohydride.

[0124] Centrifugal washing: The product processed by the in-situ deposition step is centrifuged and then the separated precipitate is washed three times with anhydrous ethanol solution.

[0125] Drying: The precipitate was placed in a vacuum oven at 100°C and dried for 12 hours to obtain the positive electrode lithium replenishment agent.

[0126] Comparative Example 1

[0127] This comparative example is uncoated Li6CoO4.

[0128] Comparative Example 2

[0129] This comparative example provides a positive electrode lithium supplement agent, which is made by coating Li6CoO4 with nano-alumina. The preparation process of the positive electrode lithium supplement agent provided in this comparative example includes the following steps:

[0130] Step 1: Weigh 10g of lithium cobalt oxide powder and 0.1g of nano lithium oxide powder, and mix them using a high-energy ball mill to obtain the coated precursor.

[0131] Step 2: The precursor is sintered at 500℃ under nitrogen protection for 8 hours to obtain the positive electrode lithium replenishment sample.

[0132] Comparative Example 3

[0133] The only difference between this comparative example and Comparative Example 2 is that the coating material used is nano-zirconia.

[0134] The performance tests of the positive electrode sheets in the embodiments and comparative examples are described below:

[0135] 1) Preparation of button cells

[0136] ① The positive electrode lithium replenishing agent samples provided in Examples 1 to 3, along with conductive carbon black and PVDF, were dispersed in NMP. Aluminum foil was used as the positive electrode current collector, coated onto the positive electrode current collector, and dried to obtain the positive electrode sheet.

[0137] ② Using lithium metal as the negative electrode, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1. Then, fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent at a ratio of 1 mol / L to prepare the electrolyte. The positive electrode, negative electrode, polyethylene separator, and other components are assembled together and then injected into the electrolyte to form a coin cell.

[0138] 2) First charge specific capacity test

[0139] At 25°C, the coin cells prepared in Examples 1 to 3 were charged to 4.5V at a rate of 0.066C, followed by constant voltage charging with a cutoff current of 0.01C to obtain the initial charge specific capacity of the positive electrode lithium replenishment. After being placed in air for 24 hours, the charge specific capacity was measured again.

[0140] 3) Macroscopic color changes

[0141] The initial color of all the positive electrode lithium replenishment samples was cobalt blue. After being left in the air for 24 hours, the color change of the positive electrode lithium replenishment samples provided in Examples 1 to 3 was observed.

[0142] 4) Scanning electron microscopy characterizes the microstructure.

[0143] The microstructure of the samples was observed using a Zeiss electron microscope to further examine the differences between the samples before and after coating.

[0144] See Figures 3 to 6 Therefore, Figure 4 The image shows the SEM image of the positive electrode lithium replenishment agent in Comparative Example 1. The image in Comparative Example 1 shows the microstructure of Li6CoO4 without surface coating. Figure 5 and Figure 6 The images show the microstructures of Li6CoO4 surfaces coated with nano-alumina and nano-zirconia, respectively, in Comparative Examples 2 and 3. The coating method was ball milling and mixing. The coating materials were distributed in an island-like pattern on the Li6CoO4 surface, exposing more of the core surface. Figure 4 The image shows the microstructure of Li6CoO4 surface coated with cobalt boride in Example 1. The coating method was in-situ deposition. The cobalt boride coating on Li6CoO4 was significantly higher, and the coating effect was better, with no exposed core surface.

[0145] Table 1 Comparison of changes in charging specific capacity, color change, and pH value.

[0146]

[0147]

[0148] The test results are explained in detail below:

[0149] I. Comparing the experimental results of Example 1 and Comparative Examples 1 to 3, it can be seen that compared with traditional alumina and zirconium oxide-coated Li6CoO4 materials, especially uncoated Li6CoO4 materials, the novel positive electrode lithium replenisher provided in this application, which uses cobalt boride as the outer shell to coat the Li6CoO4 core, has a higher specific capacity during the first charge and discharge, reaching 803.7 mAh / g, and still has a battery specific capacity of 795.9 mAh / g after being placed in air for 24 hours. The lithium replenisher performance is excellent. The capacity decay is less than 3%, while the Li6CoO4 cathode lithium replenishment agent prepared by the traditional coating method decays to 647.9 mAh / g and 702.3 mAh / g after being placed in air for 24 hours. Moreover, from the comparison of macroscopic color observation results and pH test results, it can be seen that after being placed in air for 24 hours, the traditional alumina and zirconium oxide coated Li6CoO4 materials show obvious color changes and the pH reaches above 11.69, while the cathode lithium replenishment agent provided in this application does not show obvious color changes and the pH is 8.52.

[0150] In summary, the novel cathode lithium replenishing agent provided in this application, which uses cobalt boride as the outer shell to coat the Li6CoO4 core, can achieve excellent coating effect and effectively separate Li6CoO4 from the side reaction environment. That is, in the cathode lithium replenishing agent provided in this application, the cobalt boride shell can effectively coat the surface of the lithium-rich material in the core, improve the air stability of the lithium-rich material, reduce residual alkali, and thus improve the battery energy density and cycle performance.

[0151] II. Comparing the experimental results of Examples 1 to 5 and Example 8, it is evident that by adjusting the solid content, cobalt nitrate concentration, and sodium borohydride concentration during the reaction process, and controlling reaction parameters such as the holding temperature and time during water bath heating, and the solution droplet acceleration rate, the mass ratio of the cobalt boride shell in the final prepared positive electrode lithium supplement will change accordingly. The actually prepared positive electrode lithium supplement sample still exhibits good air stability, and the battery's energy density and cycle performance are also good. Therefore, while achieving excellent coating effects, controllable deposition of the cobalt boride shell can be achieved by adjusting parameters such as the reactant concentration of raw materials, the heating temperature and heating time in the in-situ deposition step, thereby improving production efficiency, reducing production costs, and possessing potential advantages of higher efficiency and greater environmental friendliness.

[0152] III. Comparison of the experimental results of Examples 1, 6, and 7 shows that when the core material is Li5FeO4 or Li5ReO6, the positive electrode lithium replenishing agent provided in this application can effectively coat the core material, exhibiting good air stability. The pH value of the sample after 24 hours is less than 9.25, and the final coin cells also have high specific capacity. Furthermore, when the core material is Li6CoO4 as in Example 1, the air stability of the prepared positive electrode lithium replenishing agent and the final measured battery specific capacity are both superior. That is, when the core material of the positive electrode lithium replenishing agent provided in this application is Li6CoO4, the outer shell can achieve a better coating effect on the core, thereby improving the air stability of the positive electrode lithium replenishing agent.

[0153] The foregoing has provided a detailed description of a positive electrode lithium replenishing agent and its preparation method, as well as the positive electrode sheet and lithium battery disclosed in the embodiments of this application. Specific examples have been used to illustrate the principles and implementation methods of this application. The descriptions of the above embodiments are only for the purpose of helping to understand the technical solutions and core inventive points of the embodiments of this application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this invention. Therefore, the content of this specification should not be construed as a limitation of this invention.

Claims

1. A positive electrode lithium replenishing agent, characterized in that, The positive electrode lithium replenishment agent includes a core and a shell, and the shell covers the surface of the core to isolate the core from the side reaction environment. The core is a lithium-rich material, which is at least one of Li5FeO4, Li5ReO6, and Li6CoO4. The shell is cobalt boride, which is chemically bonded to oxygen in the lithium-rich material.

2. The positive electrode lithium replenishing agent according to claim 1, characterized in that, The lithium-rich material is Li6CoO4.

3. The positive electrode lithium replenishing agent according to claim 1, characterized in that, The thickness of the outer shell is 1 nm to 20 nm.

4. The positive electrode lithium replenishing agent according to claim 1, characterized in that, The positive electrode lithium replenishing agent has a pH value of less than or equal to 9.5 after being placed in air for a specified time, wherein the specified time is 0 h to 48 h.

5. The positive electrode lithium replenishing agent according to claim 4, characterized in that, The positive electrode lithium replenishing agent has a pH value of less than or equal to 9.25 after being placed in air for a specified time, wherein the specified time is 18 h to 24 h.

6. A method for preparing a positive electrode lithium replenishing agent as described in any one of claims 1 to 5, characterized in that, Includes the following steps: Mixing: The lithium-rich material is mixed and stirred with a cobalt-containing organic solution and a boron-containing organic solution to form a solid-liquid mixture; In-situ deposition: The solid-liquid mixture is heated, causing the cobalt-containing compound in the cobalt-containing organic solution and the boron-containing compound in the boron-containing organic solution to undergo an in-situ deposition reaction on the surface of the lithium-rich material to generate cobalt boride. The cobalt boride is deposited and coats the surface of the lithium-rich material to form the positive electrode lithium replenishing agent. The cobalt-containing compound is one or more of cobalt chloride, cobalt sulfate, cobalt iodide, and cobalt acetate, and the boron-containing compound is one or more of potassium borohydride, lithium borohydride, sodium thioborohydride, trisec-butyllithium borohydride, and nickel borohydride.

7. The method for preparing the positive electrode lithium supplement agent according to claim 6, characterized in that, In the in-situ deposition step, the in-situ deposition reaction is carried out by heating at 40℃~78℃ for 0.5 h~10 h.

8. The method for preparing the positive electrode lithium replenishing agent according to claim 6, characterized in that, The mixing step is as follows: first, the lithium-rich material is mixed with one of the cobalt-containing organic solution or the boron-containing organic solution, and then the other of the cobalt-containing organic solution or the boron-containing organic solution is added dropwise to form the solid-liquid mixture.

9. The method for preparing the positive electrode lithium replenishing agent according to claim 8, characterized in that, During the mixing step, the dropping rate is 5 mL / min to 20 mL / min.

10. The method for preparing the positive electrode lithium replenishing agent according to claim 6, characterized in that, In the mixing step, the lithium-rich material accounts for 10% to 50% of the mass percentage of the solid-liquid mixture.

11. The method for preparing the positive electrode lithium replenishing agent according to claim 6, characterized in that, In the mixing step, the cobalt content in the cobalt-containing organic solution is 0.001 mol / L to 0.1 mol / L, and / or the boron content in the boron-containing organic solution is 0.001 mol / L to 0.1 mol / L.

12. The method for preparing the positive electrode lithium replenishing agent according to any one of claims 6 to 11, characterized in that, In the mixing step, the organic solution is an anhydrous organic solution, which is one or more of anhydrous ethanol, anhydrous methanol, tetrahydrofuran, n-butanol, or toluene.

13. The method for preparing the positive electrode lithium replenishing agent according to claim 6, characterized in that, The method for preparing the positive electrode lithium replenishing agent further includes, after the in-situ deposition step, performing: Centrifugal washing: The solid-liquid mixture after the in-situ deposition reaction is centrifuged and washed to obtain the precipitate; Drying: The precipitate is dried in a vacuum or inert atmosphere.

14. A positive electrode plate, characterized in that, The positive electrode sheet includes the positive electrode lithium replenishing agent as described in any one of claims 1 to 5, or the positive electrode sheet includes the positive electrode lithium replenishing agent prepared by the preparation method as described in any one of claims 6 to 13.

15. A lithium battery, characterized in that, The lithium battery includes the positive electrode as described in claim 14.

Images