Lithium-replenishing agent and preparation method therefor, positive electrode sheet and secondary battery

Abstract

The present application belongs to the technical field of energy storage, and provides a lithium-replenishing agent. The lithium-replenishing agent comprises: a lithium-rich lithium ferrite material and a coating layer that coats the surface of the lithium-rich lithium ferrite material, wherein the material of the coating layer comprises lithium pyrophosphate. The present application is beneficial for solving the technical problems of the poor air stability, relatively poor safety, etc., of lithium-rich positive electrode materials in the related art, and thus can effectively replenish lithium.

Description

Lithium replenishing agent and its preparation method, positive electrode sheet, secondary battery Technical Field

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

[0002] Lithium-ion batteries are widely used in energy storage fields such as batteries for new energy vehicles. During the first charge of a lithium-ion battery, the organic electrolyte will decompose and reduce on the surface of the negative electrode, such as graphite, forming a solid electrolyte interphase (SEI) film. This permanently consumes a large amount of lithium from the positive electrode, resulting in a low initial coulombic efficiency (ICE) for the first cycle, which reduces the capacity and energy density of the lithium battery.

[0003] To address the low coulombic efficiency of lithium-ion batteries, lithium replenishment is necessary to compensate for irreversible lithium loss. Currently, the main methods for lithium replenishment are negative electrode lithium replenishment and positive electrode lithium replenishment. Compared to negative electrode lithium replenishment, which is more difficult and requires greater investment, positive electrode lithium replenishment has become the primary approach because high-capacity materials can be added during the positive electrode slurry preparation process. An ideal positive electrode lithium replenishing agent needs to meet several conditions, such as good air stability, high specific energy and volumetric energy density, a lithium delithiation potential and lithium insertion potential between those of the positive electrode material, and compatibility with currently used manufacturing processes and battery systems.

[0004] Currently, common cathode lithium supplements mainly include lithium-rich cathode materials, such as lithium-rich lithium iron phosphate and lithium-rich lithium nickel phosphate. However, these lithium-rich cathode materials suffer from poor air stability, easy decomposition, and relatively poor safety. Summary of the Invention

[0005] In view of the technical problems existing in the background art, this application provides a lithium replenishing agent and its preparation method, a positive electrode containing the lithium replenishing agent and a secondary battery containing the positive electrode, aiming to solve the technical problems of poor air stability and relatively poor safety of lithium-rich positive electrode materials in the related art.

[0006] In a first aspect, embodiments of this application provide a lithium replenishing agent, which includes: lithium-rich lithium iron phosphate material, and a coating layer covering the surface of the lithium-rich lithium iron phosphate material; wherein the material of the coating layer includes: lithium pyrophosphate.

[0007] In the technical solution of this application embodiment, by coating the surface of lithium-rich lithium iron phosphate material with a coating layer of lithium pyrophosphate, on the one hand, the lithium pyrophosphate is an ion conductor, providing a partial capacity while providing a channel for lithium ion insertion / extraction, facilitating the movement of lithium ions during the battery charging-discharging process, and improving lithium ion transport and conduction performance. On the other hand, the lithium pyrophosphate protects the lithium-rich lithium iron phosphate material, reducing the contact between the lithium-rich lithium iron phosphate material and air, thereby improving the air stability of the lithium-rich lithium iron phosphate material, achieving the effect of improving the air stability and safety stability of the lithium replenishing agent, and enhancing the lithium replenishing effect of the lithium replenishing agent.

[0008] In some embodiments, the lithium iron phosphate material includes titanium.

[0009] In the above embodiments, by doping titanium into lithium iron ferrite material, the collapse and phase transition of the crystal structure of lithium iron ferrite material are suppressed during charge-discharge, thereby improving the material stability of the lithium replenishing agent. Simultaneously, titanium doping can also regulate the lattice electric field, suppress cation mixing, and thus optimize the ion diffusion channel, making lithium ion diffusion more orderly and accelerating the lithium ion diffusion rate, thereby improving the capacity and lithium replenishment efficiency of the lithium replenishing agent. Furthermore, titanium doping helps to improve the electronic conductivity of lithium iron ferrite material, thereby enhancing the conductivity of the lithium replenishing agent. In addition, titanium doping helps to enhance the chemical bond energy of lithium iron ferrite material, reducing structural damage and performance degradation of the lithium replenishing agent at high temperatures, thus improving the safety and reliability of the battery in high-temperature environments.

[0010] In some embodiments, the thickness of the coating layer is 10 nm to 50 nm, and / or the coating coverage is greater than or equal to 94%.

[0011] In the above embodiments, controlling the thickness of the coating layer within the aforementioned range facilitates the effective coating of lithium pyrophosphate onto lithium iron ferrite materials, reducing the contact between the lithium iron ferrite materials and air. By controlling the coating layer coverage rate to be greater than or equal to 94%, the integrity of the coating of lithium pyrophosphate onto lithium iron ferrite materials can be improved, further reducing the contact between the lithium iron ferrite materials and air.

[0012] Secondly, embodiments of this application provide a method for preparing a lithium supplement, comprising:

[0013] We provide lithium iron ferrite materials;

[0014] A coating layer is formed on the surface of a lithium-rich lithium iron phosphate material, the material of which includes lithium pyrophosphate.

[0015] In the technical solution of this application embodiment, a lithium replenishing agent can be prepared by forming a coating layer on the surface of a lithium-rich lithium iron phosphate material. This preparation method is simple and easy to implement. The lithium replenishing agent prepared in this way has a coating layer of lithium pyrophosphate on the surface of the lithium-rich lithium iron phosphate material. On the one hand, the lithium pyrophosphate is an ion conductor, providing some capacity while providing a channel for lithium ion insertion and extraction, facilitating the movement of lithium ions during the battery charging and discharging process, and improving lithium ion transport and conduction performance. On the other hand, the lithium pyrophosphate protects the lithium-rich lithium iron phosphate material, reducing the contact between the lithium-rich lithium iron phosphate material and air, thereby improving the air stability of the lithium-rich lithium iron phosphate material. This achieves the effect of improving the air stability and safety stability of the lithium replenishing agent, and enhancing the lithium replenishing effect of the lithium replenishing agent.

[0016] In some embodiments, the step of providing lithium-rich lithium iron phosphate material includes:

[0017] Provides first lithium salt and ferrous salt;

[0018] The first lithium salt and the ferrous salt were mixed and calcined under a protective gas atmosphere to obtain the lithium-rich lithium iron phosphate material.

[0019] In the above embodiments, after the first lithium salt and ferrous salt are mixed, they are calcined under the protection of a protective gas, which facilitates the more effective fusion of lithium and iron elements and reduces the generation of impurities, thereby improving the product performance of the resulting lithium supplement.

[0020] In some embodiments, the first lithium salt includes lithium nitrate, the ferrous salt includes ferrous oxalate, the ferrous oxalate includes titanium, and the molar ratio of lithium in the first lithium salt to the total amount of iron and titanium in the ferrous salt is (5.0-5.3):1.

[0021] In the above embodiments, when lithium nitrate and ferrous oxalate are used as raw materials and calcined under a protective gas atmosphere, compared with the related technologies that use a mixture of lithium salts or lithium oxides and trivalent iron oxides to obtain lithium-rich lithium ferrite, firstly, it eliminates the need for large quantities of high-cost lithium oxides, resulting in lower costs; secondly, the calcination temperature is lower and the calcination time is shorter when lithium nitrate and ferrous oxalate react, thus saving energy; simultaneously, the redox reaction between lithium nitrate and ferrous oxalate reduces nitrogen oxides to nitrogen gas, which helps reduce the generation of nitrogen oxides, thus reducing environmental pollution and corresponding treatment costs; furthermore, the lithium-rich lithium ferrite material prepared by calcining lithium nitrate and ferrous oxalate under a protective gas atmosphere exhibits less agglomeration, making subsequent pulverization easier and facilitating the preparation of lithium-rich lithium ferrite materials with larger specific surface areas, which is beneficial for improving the electrochemical performance of lithium-rich lithium ferrite materials and enhancing the lithium supplementation effect of lithium supplementation agents. In addition, the presence of titanium in the ferrous salt facilitates the introduction of titanium into lithium iron ferrite materials, which is beneficial to improving the electrochemical performance and thermal stability of lithium iron ferrite materials and further enhancing the lithium replenishment effect.

[0022] In some embodiments, calcination includes a first stage and a second stage performed sequentially.

[0023] In the first stage, the calcination temperature is 200℃~400℃, and the calcination holding time is 3h~5h;

[0024] In the second stage, the calcination temperature is 600℃~700℃, and the calcination holding time is 4h~6h.

[0025] In the above embodiments, by setting two calcination stages, it is beneficial to allow the redox reaction of the first lithium salt and ferrous salt to proceed fully in the first stage, while also allowing the generated water vapor, carbon dioxide, nitrogen oxides, etc. to be discharged as soon as possible, reducing the reaction of these gases with lithium; in the second stage, by further increasing the reaction temperature, it is beneficial to allow the first lithium salt and ferrous salt to react more fully, thereby obtaining lithium iron ferrite material with improved purity.

[0026] In some embodiments, a coating layer is formed on the surface of a lithium-rich lithium iron phosphate material, including:

[0027] The second lithium salt, pyrophosphate source, and first solvent are mixed to obtain the first reactant.

[0028] The lithium iron ferrite material is mixed with the first reactant and then subjected to a first impurity removal treatment to obtain the lithium replenishing agent.

[0029] In the above embodiments, by mixing the lithium-rich lithium iron phosphate material with the first reactant, it is beneficial to deposit lithium pyrophosphate on the surface of the lithium-rich lithium iron phosphate material while the second lithium salt and pyrophosphate source react to obtain lithium pyrophosphate, thereby forming a coating layer on the surface of the lithium-rich lithium iron phosphate material. After the first impurity removal treatment, a lithium replenishing agent is obtained.

[0030] In some embodiments, the first impurity removal process includes evaporating a first solvent, solid-liquid separation, drying, sieving, and iron removal.

[0031] In the above embodiments, evaporating the first solvent helps to evaporate and remove the volatile gases generated after mixing the lithium iron phosphate material with the first reactant, thereby obtaining lithium iron phosphate coated with high purity. At the same time, by evaporating and removing the first solvent, lithium pyrophosphate gradually precipitates and coats the surface of the lithium iron phosphate material, thereby forming a coating layer. Then, through solid-liquid separation, drying, sieving and iron removal, a lithium supplement agent with high purity is obtained.

[0032] In some embodiments, the method for preparing the lithium supplement satisfies at least one of the following conditions:

[0033] (1) The molar ratio of lithium element in the lithium iron phosphate material, the second lithium salt, and the pyrophosphate ion in the pyrophosphate source is 1:(0.1~0.15):(0.025~0.04);

[0034] (2) The first solvent includes: ethanol;

[0035] (3) The second lithium salt includes at least one of lithium acetate, lithium chloride, lithium nitrate and lithium citrate;

[0036] (4) The pyrophosphate source includes at least one of pyrophosphate, ammonium pyrophosphate and ammonium hydrogen pyrophosphate;

[0037] (5) The temperature for evaporating the first solvent is 80℃~120℃.

[0038] In the above embodiments, by controlling the molar ratio of lithium element in the lithium-rich lithium iron ferrite material and the second lithium salt to pyrophosphate ions in the pyrophosphate source to 1:(0.1-0.15):(0.025-0.04), it is beneficial to form an appropriate amount of lithium pyrophosphate as a coating layer on the surface of the lithium-rich lithium iron ferrite material, making the coating more complete and effectively reducing the contact between the lithium-rich lithium iron ferrite material and air, thereby effectively improving the capacity and stability of the lithium supplement agent during use. By using ethanol as the first solvent, its low boiling point facilitates evaporation. By using at least one of the above lithium salts as the second lithium salt, the azeotropic principle of the acid corresponding to the anion in the second lithium salt can be utilized during the reaction between the second lithium salt and the pyrophosphate source to achieve the decomposition and / or volatilization of the anion of the second lithium salt, as well as the precipitation and crystallization of lithium pyrophosphate, and facilitate heterogeneous nucleation on the surface of the lithium-rich lithium iron ferrite material. During the heterogeneous nucleation of lithium pyrophosphate on the surface of lithium-rich lithium iron phosphate, lithium ions and pyrophosphate ions slowly combine as the first solvent evaporates, maintaining a low supersaturation of lithium pyrophosphate. This allows lithium pyrophosphate to slowly crystallize and deposit on the surface of the lithium-rich lithium iron phosphate material, resulting in a denser and more complete coating. Using at least one of the aforementioned pyrophosphate and pyrophosphate salts as the pyrophosphate source, the products generated by the reaction of the second lithium salt and the pyrophosphate source, other than lithium pyrophosphate, are easily decomposed and volatilized, facilitating the precipitation and crystallization of lithium pyrophosphate on the surface of the lithium-rich lithium iron phosphate material while reducing the introduction of impurities. By controlling the evaporation temperature to 80℃~120℃, azeotropic reactions between the first solvent and the anions of the second lithium salt are facilitated.

[0039] Thirdly, embodiments of this application provide a positive electrode sheet, including a current collector and a positive electrode material disposed on at least one side of the current collector along its thickness direction, the positive electrode material including the lithium supplement agent as described in the first aspect.

[0040] In this embodiment, the positive electrode contains the aforementioned lithium replenishing agent, thus enabling effective lithium replenishment and improving the battery's safety, stability, and air stability during the lithium replenishment process.

[0041] Fourthly, embodiments of this application provide a secondary battery, including: a positive electrode, a negative electrode, and a separator; wherein the positive electrode is the positive electrode as described in the third aspect.

[0042] In this embodiment, the secondary battery includes the aforementioned positive electrode sheet, which contains the aforementioned lithium replenishing agent. Therefore, the secondary battery has high safety stability and air stability during the lithium replenishment process.

[0043] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, specific embodiments of this application are given below. Attached Figure Description

[0044] To more clearly illustrate the technical solutions of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0045] Figure 1 is a schematic flowchart of a method for preparing a lithium supplement provided in an embodiment of this application. Detailed Implementation

[0046] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0047] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0048] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0049] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0050] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0051] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0052] Currently used cathode lithium supplementation materials, such as lithium-rich nickel oxide (Li2NiO2) and lithium-rich iron oxide (Li5FeO4), suffer from poor air stability and low safety, which limits their application in lithium supplementation materials.

[0053] To address the technical problems of poor stability and relatively poor safety stability of lithium-rich cathode materials in air, this application provides a lithium replenishing agent and its preparation method, a cathode electrode, and a secondary battery. The method involves coating the surface of a lithium-rich lithium iron phosphate material with a lithium pyrophosphate layer. This lithium pyrophosphate is an ion conductor, providing some capacity while also offering lithium-ion insertion / extraction channels, facilitating lithium-ion movement during battery charging and discharging, and improving lithium-ion transport and conduction performance. Furthermore, the lithium pyrophosphate protects the lithium-rich lithium iron phosphate material, reducing its contact with air and thus improving its air stability. This improves the air stability and safety stability of the lithium replenishing agent, enhancing its lithium replenishment effect. Consequently, the electrochemical performance (such as battery capacity and cycle performance), air stability, and safety stability of the cathode electrode and secondary battery are also improved.

[0054] In a first aspect, embodiments of this application provide a lithium replenishing agent, comprising: lithium iron ferrite rich material, and a coating layer coated on the surface of the lithium iron ferrite rich material;

[0055] The coating material includes lithium pyrophosphate.

[0056] In some embodiments, the lithium iron phosphate material includes titanium.

[0057] In the above embodiments, by doping titanium into lithium iron ferrite material, the collapse and phase transition of the crystal structure of lithium iron ferrite material are suppressed during charge-discharge, thereby improving the material stability of the lithium replenishing agent. Simultaneously, titanium doping can also regulate the lattice electric field, suppress cation mixing, and thus optimize the ion diffusion channel, making lithium ion diffusion more orderly and accelerating the lithium ion diffusion rate, thereby improving the capacity and lithium replenishment efficiency of the lithium replenishing agent. Furthermore, titanium doping helps to improve the electronic conductivity of lithium iron ferrite material, thereby enhancing the conductivity of the lithium replenishing agent. In addition, titanium doping helps to enhance the chemical bond energy of lithium iron ferrite material, reducing structural damage and performance degradation of the lithium replenishing agent at high temperatures, thus improving the safety and reliability of the battery in high-temperature environments.

[0058] In some embodiments, the chemical formula of the above-mentioned lithium iron phosphate material is: Li x Fe y Ti (1-y) O4, where x is 5.05 to 5.2 and y is 0.978 to 0.995. For example, x can be 5.05, 5.1, or 5.2, etc., and y can be 0.978, 0.980, 0.982, 0.985, 0.988, 0.990, or 0.995, etc.

[0059] In the above embodiments, the lithium iron ferrite material is doped with Ti element, and the molar ratio of Ti element to Fe element is (0.005~0.022):(0.995~0.978). This is beneficial to improve the stability of the lithium supplement agent, as well as the capacity and lithium supplementation efficiency of the lithium supplement agent. As the molar ratio of Ti element to Fe element increases or decreases, it is not conducive to further improving the comprehensive performance of the lithium iron ferrite material, such as electrochemical performance and thermal stability.

[0060] In some embodiments, the chemical formula of the above-mentioned lithium supplement is: Li x Fe y Ti (1-y) O4@Li m (P2O7) n Where x is 5.05–5.2, y is 0.978–0.995, m is 0.1–0.15, and n is 0.025–0.04. For example, x can be 5.05, 5.1, or 5.2, y can be 0.978, 0.980, 0.982, 0.985, 0.988, 0.990, or 0.995, m can be 0.1, 0.11, 0.12, 0.13, 0.14, or 0.15, and n can be 0.025, 0.026, 0.028, 0.03, 0.032, 0.034, 0.035, 0.037, 0.038, 0.039, or 0.04.

[0061] In some embodiments, the thickness of the coating layer is 10 nm to 50 nm. For example, the thickness of the coating layer can be 10 nm, 20 nm, 30 nm, 40 nm, or 50 nm, etc.

[0062] In the above embodiments, by controlling the thickness of the coating layer within the above range, it is beneficial for lithium pyrophosphate to effectively coat lithium iron phosphate materials and reduce the contact between lithium iron phosphate materials and air.

[0063] In some embodiments, the thickness of the coating layer is 19 nm to 48 nm.

[0064] In the above embodiments, further controlling the thickness of the coating layer is beneficial for lithium pyrophosphate to more effectively coat lithium iron phosphate materials, further reducing the contact between lithium iron phosphate materials and air, and maintaining good lithium ion transport and conduction performance.

[0065] In some embodiments, the coating coverage is greater than or equal to 94%. The coating coverage refers to the percentage of the area covered by the coating on the surface of the lithium-rich lithium iron ferrite material relative to the total surface area of ​​the lithium-rich lithium iron ferrite material. For example, the coating coverage may be 94%, 95%, 96%, 97%, 98%, or 99%, etc.

[0066] In the above embodiments, by controlling the coating rate of the coating layer to be greater than or equal to 94%, it is beneficial to improve the coating integrity of lithium pyrophosphate on lithium iron phosphate material and further reduce the contact between lithium iron phosphate material and air.

[0067] In some embodiments, the coverage of the coating layer is 94.1% to 99.1%.

[0068] In the above embodiments, by controlling the coating rate of the coating layer within the range of 94.1% to 99.1%, the integrity of the coating of lithium pyrophosphate on lithium iron phosphate material is improved, while maintaining good lithium ion transport and conduction performance.

[0069] In some embodiments, the compaction density of the lithium supplement is 2.89 g / mL to 3.14 g / mL, preferably 2.89 g / mL to 3 g / mL.

[0070] For example, the compaction density of the lithium supplement can be 2.89 g / mL, 2.90 g / mL, 2.91 g / mL, 2.92 g / mL, 2.93 g / mL, 2.94 g / mL, 2.95 g / mL, 2.96 g / mL, 2.97 g / mL, 2.99 g / mL, 3.0 g / mL, 3.05 g / mL, 3.08 g / mL, 3.1 g / mL, 3.11 g / mL, 3.12 g / mL, 3.13 g / mL, or 3.14 g / mL, etc.

[0071] In the above embodiments, by controlling the compaction density of the lithium replenishing agent, it is beneficial to maintain a high energy density of the lithium replenishing agent, thereby improving the lithium replenishment effect.

[0072] In some embodiments, the tap density of the lithium supplement is 1.68 g / mL to 1.89 g / mL, preferably 1.68 g / mL to 1.74 g / mL.

[0073] For example, the tap density of the lithium supplement is 1.68 g / mL, 1.70 g / mL, 1.72 g / mL, 1.75 g / mL, 1.76 g / mL, 1.78 g / mL, 1.79 g / mL, 1.80 g / mL, 1.82 g / mL, 1.85 g / mL, 1.86 g / mL, 1.88 g / mL, or 1.89 g / mL, etc.

[0074] In some embodiments, the specific surface area of ​​the lithium supplement is 5.68 m². 2 / g~8.49m 2 / g, preferably 7.27m 2 / g~7.45m 2 / g, which helps improve the lithium replenishment effect.

[0075] For example, the specific surface area of ​​this lithium supplement is 5.68 m². 2 / g, 5.70m 2 / g, 5.72m 2 / g, 5.75m 2 / g, 5.78m 2 / g, 5.79m 2 / g, 5.82m 2 / g, 5.85m 2 / g, 5.89m 2 / g, 5.92m 2 / g, 5.94m 2 / g, 5.98m 2 / g, 6.0m 2 / g, 6.05m 2 / g, 6.08m 2 / g, 6.12m 2 / g, 6.15m 2 / g, 6.18m 2 / g、6.19m 2 / g、6.21m 2 / g, 6.25m 2 / g, 6.28m 2 / g, 6.29m 2 / g, 6.32m 2 / g, 6.35m 2 / g, 6.37m 2 / g, 6.39m 2 / g, 6.45m 2 / g, 6.48m 2 / g, 6.52m 2 / g, 6.59m 2 / g, 6.68m 2 / g, 6.75m 2 / g、6.79m 2 / g、6.85m 2 / g、6.89m 2 / g、6.94m 2 / g、6.98m 2 / g、7.01m 2 / g、7.08m 2 / g、7.12m 2 / g、7.15m 2 / g、7.19m 2 / g、7.25m 2 / g、7.28m 2 / g、7.31m 2 / g、7.35m 2 / g、7.37m 2 / g、7.39m 2 / g、7.45m 2 / g、7.48m 2 / g、7.51m 2 / g、7.56m 2 / g、7.58m 2 / g、7.61m 2 / g、7.64m 2 / g、7.69m 2 / g、7.72m 2 / g、7.76m 2 / g、7.79m 2 / g、7.83m 2 / g、7.85m 2 / g、7.89m 2 / g、7.92m 2 / g、7.94m 2 / g、7.96m 2 / g、7.99m 2 / g、8.01m 2 / g、8.05m 2 / g、8.08m 2 / g、8.1m 2 / g、8.13m 2 / g、8.15m 2 / g、8.16m 2 / g、8.19m 2 / g、8.25m 2 / g、8.29m 2 / g、8.34m 2 / g、8.35m 2 / g、8.37m 2 / g、8.40m2 / g, 8.45m 2 / g or 8.49m 2 / g etc.

[0076] In some embodiments, the pH value of the lithium supplement is 9.01 to 10.35, optionally 9.01 to 9.89.

[0077] In the above embodiments, the pH value of the lithium replenishing agent is closer to neutral, which reduces the residual alkali on the surface of the lithium replenishing agent, which is conducive to making full use of the capacity of the lithium replenishing agent and improving the safety and stability of the lithium replenishing agent during lithium replenishment.

[0078] For example, the pH value of the lithium supplement can be 9.01, 9.05, 9.17, 9.15, 9.16, 9.19, 9.25, 9.28, 9.34, 9.36, 9.38, 9.40, 9.43, 9.46, 9.48, 9.5, 9.52, 9.55, 9.57, 9.59, 9.62, 9.64, or 9. 68, 9.7, 9.73, 9.78, 9.8, 9.85, 9.88, 9.9, 9.94, 9.97, 10.0, 10.05, 10.09, 10.12, 10.15, 10.16, 10.18, 10.2, 10.23, 10.25, 10.28, 10.3, 10.34, or 10.35, etc.

[0079] In some embodiments, the lithium supplement has a D10 particle size of 0.32 μm to 0.79 μm, a D50 particle size of 1.67 μm to 5.78 μm, and a D90 particle size of 9.01 μm to 14.67 μm.

[0080] For example, the D10 particle size of this lithium supplement can be 0.32 μm, 0.35 μm, 0.38 μm, 0.4 μm, 0.42 μm, 0.45 μm, 0.48 μm, 0.47 μm, 0.5 μm, 0.52 μm, 0.55 μm, 0.56 μm, or 0.57 μm, etc., and the D50 particle size of this lithium supplement can be 1.67 μm, 1.69 μm, 1.71 μm, 1.75 μm, 1.78 μm, 1.8 μm, 1.82 μm, 1.85 μm, 1.86 μm, 1.89 μm, 1.91 μm, 1.94 μm, 1.96 μm, 1.98 μm, 2.00 μm, 2.02 μm, 2.05 μm, 2.06 μm, 2.08 μm, etc. m, 2.1μm, 2.12μm, 2.15μm, 2.17μm, 2.19μm, 2.20μm, 2.23μm, 2.25μm, 2.28μm m, 2.30μm, 2.34μm, 2.38μm, 2.39μm, 2.45μm, 2.47μm, 2.52μm, 2.58μm, 2.62μm m, 2.65μm, 2.67μm, 2.73μm, 2.78μm, 2.79μm, 2.81μm, 2.85μm, 2.87μm, 2.9μm m, 2.91μm, 2.94μm, 2.96μm, 2.98μm, 3.0μm, 3.04μm, 3.08μm, 3.09μm, 3.12μm , 3.15μm, 3.18μm, 3.20μm, 3.25μm, 3.28μm, 3.30μm, 3.34μm, 3.38μm, 3.39μm m, 3.45μm, 3.47μm, 3.52μm, 3.58μm, 3.62μm, 3.65μm, 3.67μm, 3.73μm, 3.78μm m, 3.79μm, 3.81μm, 3.85μm, 3.87μm, 3.9μm, 3.91μm, 3.94μm, 3.96μm, 3.98μm m, 4.0μm, 4.04μm, 4.08μm, 4.09μm, 4.12μm, 4.15μm, 4.18μm, 4.20μm, 4.25μm , 4.28μm, 4.30μm, 4.34μm, 4.38μm, 4.39μm, 4.45μm, 4.47μm, 4.52μm, 4.58μm m, 4.62μm, 4.65μm, 4.67μm, 4.73μm, 4.78μm, 4.79μm, 4.81μm, 4.85μm, 4.87μm m, 4.9μm, 4.91μm, 4.94μm, 4.96μm, 4.98μm, 5.0μm, 5.04μm, 5.08μm, 5.09μm, 5.12μm, 5.15μm, 5.18μm, 5.20μm, 5.25μm, 5.28μm, 5.3μm, 5.38μm, 5.4μm, 5.42μm, 5.45μm, 5.48μm, 5.49μm, 5.55μm, 5.56μm, 5.59μm, 5.62μm, 5.65μm, 5.67μm, 5.68μm, 5.7μm, 5.72μm, 5.75μm, or 5.78μm, etc.

[0081] In the above embodiments, the lithium replenishing agent has a relatively concentrated particle size distribution, which can improve the uniformity of distribution in the positive electrode when applied to the positive electrode sheet.

[0082] In some embodiments, the magnetic foreign matter content of the lithium replenishing agent is 0.11 ppm to 0.13 ppm.

[0083] In some embodiments, the free lithium content of the lithium supplement is 321 ppm to 599 ppm.

[0084] In some embodiments, under conditions of 25°C and humidity less than or equal to 10%, the initial charge specific capacity of the lithium replenisher at a 0.1C rate is 701.3 mAh / g to 760 mAh / g, preferably 714 mAh / g to 760 mAh / g; under conditions of 25°C and humidity less than or equal to 10%, the initial discharge specific capacity of the lithium replenisher at a 0.1C rate is 4.6 mAh / g to 16.7 mAh / g.

[0085] For example, under conditions of 25°C and humidity less than or equal to 10%, the initial charge specific capacity of this lithium replenisher at a 0.1C rate is 701.3 mAh / g, 703 mAh / g, 705 mAh / g, 710 mAh / g, 715 mAh / g, 718 mAh / g, 720 mAh / g, 723 mAh / g, 728 mAh / g, 731 mAh / g, 735 mAh / g, 739 mAh / g, 740 mAh / g, 745 mAh / g, 749 mAh / g, 750 mAh / g, and 752 mA. The specific capacities of the lithium supplement at 0.1C rate are 756 mAh / g, 758 mAh / g, 759 mAh / g, or 759.9 mAh / g, etc.; under conditions of 25℃ and humidity less than or equal to 10%, the initial discharge specific capacities are 4.6 mAh / g, 4.9 mAh / g, 5.2 mAh / g, 5.5 mAh / g, 5.8 mAh / g, 5.9 mAh / g, 6.1 mAh / g, 6.2 mAh / g, 6.5 mAh / g, 6.8 mAh / g, 6.9 mAh / g, 7.2 mAh / g, and 7.5 mAh / g, respectively. / g, 7.8mAh / g, 7.9mAh / g, 8.1mAh / g, 8.2mAh / g, 8.5mAh / g, 8.8mAh / g, 9.1mAh / g, 9.2mAh / g, 9.5mAh / g, 9.8mAh / g, 9.9mAh / g, 10.2mAh / g, 10.5mAh / g, 10.8mAh / g, 10.9mAh / g, 11.1mAh / g, 11.2mAh / g, 11.5mAh / g, 11.8mAh / g, 12.1mAh / g, 12.5mAh / g mAh / g, 12.6mAh / g, 12.9mAh / g, 13.2mAh / g, 13.5mAh / g, 13.8mAh / g, 13.9mAh / g, 14.1mAh / g, 14.2mAh / g, 14.5mAh / g, 14 .8mAh / g, 14.9mAh / g, 15.2mAh / g, 15.5mAh / g, 15.8mAh / g, 15.9mAh / g, 16.1mAh / g, 16.2mAh / g, 16.5mAh / g or 16.7mAh / g, etc.

[0086] In some embodiments, under conditions of 25°C and humidity less than or equal to 10%, the initial charge specific capacity of the lithium replenisher at a 1C rate is 662 mAh / g to 728 mAh / g, preferably 689 mAh / g to 728 mAh / g; under conditions of 25°C and humidity less than or equal to 10%, the initial charge specific capacity of the lithium replenisher at a 1C rate is 2.1 mAh / g to 6.9 mAh / g.

[0087] For example, under conditions of 25°C and humidity less than or equal to 10%, the initial charge specific capacity of this lithium replenisher at a 1C rate is 672.4 mAh / g, 675 mAh / g, 678 mAh / g, 679 mAh / g, 680 mAh / g, 685 mAh / g, 687 mAh / g, 689 mAh / g, 691 mAh / g, 693 mAh / g, 695 mAh / g, 699 mAh / g, 700 mAh / g, 701 mAh / g, 701.3 mAh / g, 703 mAh / g, 705 mAh / g, 710 mAh / g, 715 mAh / g, 718 mAh / g, 720 mAh / g, 723 mAh / g, 725 mAh / g, 726 mAh / g, 727 mAh / g, or 727.5 mAh / g, etc. Under conditions of 25℃ and humidity less than or equal to 10%, the initial discharge specific capacity of the lithium replenishment agent at a 0.1C rate is 2.1mAh / g, 2.3mAh / g, 2.5mAh / g, 2.6mAh / g, 2.8mAh / g, 2.9mAh / g, 3.0mAh / g, 3.2mAh / g, 3.4mAh / g, 3.7mAh / g, 3.8mAh / g, 4.0mAh / g, 4.1mAh / g, 4.3mAh / g, 4.5mAh / g, 4.6mAh / g, 4.8mAh / g, 4.9mAh / g, 5.2mAh / g, 5.5mAh / g, 5.8mAh / g, 5.9mAh / g, 6.1mAh / g, 6.2mAh / g, 6.5mAh / g, 6.8mAh / g, or 6.9mAh / g, etc.

[0088] In some embodiments, after being placed at 25℃±1℃ and humidity less than or equal to 85%±5% for 4 hours, the lithium replenisher has an initial charge specific capacity of 643mAh / g to 750mAh / g at a 0.1C rate, preferably 671mAh / g to 750mAh / g, for example, 651.5mAh / g, 671.5mAh / g, 674.3mAh / g, 683.2mAh / g, 701.4mAh / g, 749.5mAh / g, etc.; after being placed at 25℃±1℃ and humidity less than or equal to 85%±5% for 4 hours, the lithium replenisher has an initial discharge specific capacity of 2.4mAh / g to 11.2mAh / g at a 0.1C rate, for example, 3.1mAh / g, 3.2mAh / g, 4.8mAh / g, 4.9mAh / g, 8.1mAh / g, 10.1mAh / g, etc.

[0089] In some embodiments, after being placed at 25℃±1℃ and humidity less than or equal to 85%±5% for 4 hours, the lithium replenisher has an initial charge specific capacity of 601mAh / g to 720mAh / g at a 1C rate, preferably 643mAh / g to 720mAh / g, for example, 635.3mAh / g, 687.5mAh / g, 698.1mAh / g, 701.3mAh / g, etc.; after being placed at 25℃±1℃ and humidity less than or equal to 85%±5% for 4 hours, the lithium replenisher has an initial discharge specific capacity of 1.4mAh / g to 6.2mAh / g at a 1C rate, for example, 1.6mAh / g, 2.1mAh / g, 4.2mAh / g, 6.1mAh / g, etc.

[0090] In some embodiments, after being placed at 25℃±1℃ and humidity less than or equal to 85%±5% for 12 hours, the lithium replenisher has an initial charge specific capacity of 587mAh / g to 749mAh / g at a 0.1C rate, preferably 641mAh / g to 749mAh / g, for example, 601.5mAh / g, 633.8mAh / g, 646.4mAh / g, 700.2mAh / g, 707.8mAh / g, 748.1mAh / g, etc.; after being placed at 25℃±1℃ and humidity less than or equal to 85%±5% for 12 hours, the lithium replenisher has an initial discharge specific capacity of 2.6mAh / g to 8.6mAh / g at a 0.1C rate, for example, 2.7mAh / g, 3.2mAh / g, 3.6mAh / g, 5.3mAh / g, 7.3mAh / g, 8.1mAh / g, etc.

[0091] In some embodiments, after being placed at 25℃±1℃ and humidity less than or equal to 85%±5% for 12 hours, the lithium replenisher has an initial charge specific capacity of 497mAh / g to 718mAh / g at a 1C rate, preferably 617mAh / g to 718mAh / g, for example, 511.3mAh / g, 604.8mAh / g, 614.6mAh / g, 683.7mAh / g, etc.; after being placed at 25℃±1℃ and humidity less than or equal to 85%±5% for 12 hours, the lithium replenisher has an initial discharge specific capacity of 1.9mAh / g to 5.9mAh / g at a 1C rate, for example, 2.1mAh / g, 2.4mAh / g, 4.2mAh / g, 5.1mAh / g, 5.9mAh / g, etc.

[0092] In some embodiments, the first charge specific capacity and first discharge specific capacity of the lithium replenishing agent are obtained by preparing a positive electrode sheet having the lithium replenishing agent, assembling the positive electrode sheet into a button cell, and testing it.

[0093] In some embodiments, the positive electrode sheet includes a positive electrode active material, a binder, and a conductive agent. The positive electrode active material is a lithium supplement agent as described above. The binder is preferably PVDF (polyvinylidene fluoride), but other adhesive materials suitable for the positive electrode sheet can also be used. The conductive agent is preferably a carbon conductive material, such as conductive carbon black (SP). The above-mentioned positive electrode active material, binder, and conductive agent are mixed in a suitable mass ratio, and an aluminum sheet is used as the current collector. The mass ratio of the positive electrode active material, binder, and conductive agent to prepare the positive electrode sheet can be 78:8:14.

[0094] Secondly, embodiments of this application provide a method for preparing a lithium supplement, as shown in Figure 1, including:

[0095] S11, Provide lithium iron ferrite materials;

[0096] S12. A coating layer is formed on the surface of a lithium-rich lithium iron phosphate material; the material of the coating layer includes lithium pyrophosphate.

[0097] In the method for preparing the lithium replenishing agent provided in this application embodiment, a coating layer is formed on the surface of a lithium-rich lithium iron phosphate material. The coating layer material includes lithium pyrophosphate, thus obtaining the lithium replenishing agent. This preparation method is simple and easy to implement. The lithium replenishing agent prepared in this way coats the surface of the lithium-rich lithium iron phosphate material with a coating layer containing lithium pyrophosphate. On the one hand, the lithium pyrophosphate is an ion conductor, providing some capacity while providing a channel for lithium ion insertion / extraction, facilitating the movement of lithium ions during the battery charging-discharging process, and improving lithium ion transport and conduction performance. On the other hand, the lithium pyrophosphate protects the lithium-rich lithium iron phosphate material, reducing the contact between the lithium-rich lithium iron phosphate material and air, thereby improving the air stability of the lithium-rich lithium iron phosphate material. This achieves the effect of improving the air stability and safety stability of the lithium replenishing agent, and enhancing the lithium replenishing effect of the lithium replenishing agent.

[0098] The aforementioned lithium iron ferrite materials can be obtained commercially or produced in-house.

[0099] In some embodiments, the lithium supplement as described above is obtained by the lithium supplement preparation method provided in this application.

[0100] In some embodiments, step S11, providing the lithium iron phosphate material, may include:

[0101] S111, providing a first lithium salt and a ferrous salt;

[0102] S112. The first lithium salt and the ferrous salt are mixed and calcined under a protective gas to obtain lithium iron ferrite material.

[0103] In the above embodiments, after the first lithium salt and ferrous salt are mixed, they are calcined under the protection of a protective gas, which facilitates the more effective fusion of lithium and iron elements and reduces the generation of impurities, thereby improving the product performance of the resulting lithium supplement.

[0104] In some embodiments, the first lithium salt includes lithium nitrate, the ferrous salt is ferrous oxalate, and the ferrous oxalate includes titanium.

[0105] In the above embodiments, when lithium nitrate and ferrous oxalate are used as raw materials and calcined under a protective gas atmosphere, compared with the related technologies that use a mixture of lithium salts or lithium oxides and trivalent iron oxides to obtain lithium-rich lithium ferrite, firstly, it eliminates the need for large quantities of high-cost lithium oxides, resulting in lower costs; secondly, the calcination temperature is lower and the calcination time is shorter when lithium nitrate and ferrous oxalate react, thus saving energy; simultaneously, the redox reaction between lithium nitrate and ferrous oxalate reduces nitrogen oxides to nitrogen gas, which helps reduce the generation of nitrogen oxides, thus reducing environmental pollution and corresponding treatment costs; furthermore, the lithium-rich lithium ferrite material prepared by calcining lithium nitrate and ferrous oxalate under a protective gas atmosphere exhibits less agglomeration, making subsequent pulverization easier and facilitating the preparation of lithium-rich lithium ferrite materials with larger specific surface areas, which is beneficial for improving the electrochemical performance of lithium-rich lithium ferrite materials and enhancing the lithium supplementation effect of lithium supplementation agents. In addition, the presence of titanium in the ferrous salt facilitates the introduction of titanium into lithium iron ferrite materials, which is beneficial to improving the electrochemical performance and thermal stability of lithium iron ferrite materials and further enhancing the lithium replenishment effect.

[0106] The reaction equation for the redox reaction between lithium nitrate and ferrous oxalate is shown in equation (1) below: LiNO3 + FeC2O4 → Li5FeO4 + NO2 + N2 + CO2 (Equation (1))

[0107] In some embodiments, the ferrous oxalate may be doped with Ti element at a mass percentage of 0.5% to 4%, preferably with Ti element at a mass percentage of 1% to 3%. For example, in the ferrous oxalate, the mass percentage of Ti element is 1%, 2% or 3%.

[0108] In some embodiments, the particle size of ferrous oxalate can be 5 μm to 10 μm, and the particle size of lithium nitrate can be 5 μm to 10 μm. For example, the particle size of the ferrous oxalate can be 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, or 10 μm, etc., and the particle size of the lithium nitrate can be 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, or 10 μm, etc.

[0109] In the above embodiments, by controlling the particle size of ferrous oxalate to be 5μm to 10μm and the particle size of lithium nitrate to be 5μm to 10μm, it is beneficial to ensure that lithium nitrate and ferrous oxalate are mixed more effectively and react fully at a lower calcination temperature, thereby reducing the generation of nitrogen oxides and reducing the agglomeration of lithium-rich lithium iron ferrite materials. This results in lithium-rich lithium iron ferrite materials with good dispersion performance and large specific surface area, thereby improving the lithium replenishment effect of the lithium replenishing agent.

[0110] In some embodiments, to improve the reaction conversion rate and reduce raw material residue, the molar ratio of lithium in the first lithium salt to the total molar ratio of iron and titanium in the ferrous salt is (5.0–5.3):1. Preferably, the molar ratio of lithium in the first lithium salt to the total molar ratio of iron and titanium in the ferrous salt is (5.05–5.2):1. For example, the molar ratio of lithium in the first lithium salt to the total molar ratio of iron and titanium in the ferrous salt can be 5.0:1, 5.05:1, 5.1:1, 5.2:1, or 5.3:1, etc.

[0111] In some embodiments, the first lithium salt and ferrous salt can be mixed using an inclined mixer or a twin-screw mixer for a mixing time of 2 to 4 hours. For example, a mixing time of 2, 3, or 4 hours can achieve effective mixing.

[0112] In S112, during the calcination process, a protective gas is introduced. On the one hand, the first lithium salt and the ferrous salt can undergo a redox reaction under the protection of the protective gas, thereby improving the conversion rate of the redox reaction between the first lithium salt and the ferrous salt, reducing the generation of impurities and improving the purity of the obtained lithium iron ferrite material. On the other hand, the protective gas can also carry away the waste gas (such as nitrogen oxides) generated during the calcination process in a timely manner, further improving the yield of the obtained lithium iron ferrite material.

[0113] During the calcination process, the flow rate of the protective gas can be 0.1 m / s to 0.3 m / s. Simultaneously, an induced draft fan can be used to circulate the protective gas within the reactor and carry away the waste gas. For example, the flow rate of the protective gas can be 0.1 m / s, 0.2 m / s, or 0.3 m / s.

[0114] In some embodiments, the protective gas may include nitrogen, argon, and / or helium, etc.

[0115] In some embodiments, during S112, the process of mixing the first lithium salt and the ferrous salt and calcining them under a protective gas atmosphere to obtain lithium-rich lithium iron ferrite material further includes: using a hydrogen peroxide solution with a mass fraction of 0.1% to 0.3% to absorb the waste gas carried out by the protective gas, thereby obtaining a dilute nitric acid solution. This dilute nitric acid solution can be used to further prepare lithium nitrate, and the prepared lithium nitrate can be recycled as the first lithium salt in the preparation of lithium-rich lithium iron ferrite material, realizing the recycling of waste gas, which helps to reduce process costs and environmental pollution.

[0116] In some embodiments, when a dilute nitric acid solution is obtained by absorbing waste gas with hydrogen peroxide solution, lithium carbonate can be added to the dilute nitric acid solution to dissolve it, and then lithium nitrate crystals can be obtained by concentration and crystallization to achieve the preparation of lithium nitrate.

[0117] In some embodiments, the calcination described above may include: a first stage and a second stage performed sequentially;

[0118] In the first stage, the calcination temperature is 200℃~400℃, and the calcination holding time is 3h~5h;

[0119] In the second stage, the calcination temperature is 600℃~700℃, and the calcination holding time is 4h~6h.

[0120] For example, in the first stage, the calcination temperature can be 200℃, 220℃, 230℃, 240℃, 250℃, 260℃, 270℃, 280℃, 290℃, 300℃, 310℃, 320℃, 330℃, 340℃, 350℃, 360℃, 380℃, 390℃ or 400℃, and the calcination holding time can be 4h, 5h or 6h.

[0121] In the second stage, the calcination temperature can be 600℃, 620℃, 630℃, 640℃, 650℃, 660℃, 670℃, 680℃, 690℃ or 700℃, and the calcination holding time can be 4h, 5h or 6h.

[0122] In the above embodiments, by setting two calcination stages, it is beneficial to allow the redox reaction of the first lithium salt and ferrous salt to proceed fully in the first stage, while also allowing the generated water vapor, carbon dioxide, nitrogen oxides, etc. to be discharged as soon as possible, reducing the reaction of these gases with lithium; in the second stage, by further increasing the reaction temperature, it is beneficial to allow the first lithium salt and ferrous salt to react more fully, thereby obtaining lithium iron ferrite material with improved purity.

[0123] In some embodiments, in the first stage, the heating rate of calcination can be 100℃ / h to 200℃ / h, and in the second stage, the heating rate of calcination can be 50℃ / h to 100℃ / h. For example, in the first stage, the heating rate of calcination can be 100℃ / h, 120℃ / h, 130℃ / h, 150℃ / h, 160℃ / h, 180℃ / h, or 200℃ / h, etc., and in the second stage, the heating rate of calcination can be 50℃ / h, 60℃ / h, 70℃ / h, 80℃ / h, 90℃ / h, or 100℃ / h, etc.

[0124] In some embodiments, after calcination, the material can be cooled to a temperature less than or equal to 80°C before being discharged.

[0125] In some embodiments, in order to further improve the lithium replenishment performance of the lithium replenishing agent and the coating effect of lithium pyrophosphate on the surface of lithium-rich lithium iron ferrite material, the particle size of the lithium-rich lithium iron ferrite material can be 1 μm to 5 μm.

[0126] In some embodiments, after the calcination is completed, the material is cooled to a temperature less than or equal to 80°C to obtain a cooled material. After the cooled material is pulverized, a lithium-rich lithium iron phosphate material is obtained.

[0127] In some embodiments, after discharge and before forming a coating layer on the surface of the lithium-rich lithium iron ferrite material, the lithium-rich lithium iron ferrite material can be conveyed to a sealed reactor via negative pressure conveying. Specifically, to reduce the contact between the lithium-rich lithium iron ferrite material and air, it can be conveyed to a pulverizer for pulverization via negative pressure conveying. After pulverization, lithium-rich lithium iron ferrite material with a particle size of 1μm to 5μm is obtained. To further reduce the contact between the lithium-rich lithium iron ferrite material and air, a protective gas can be introduced into the pulverizer to protect the lithium-rich lithium iron ferrite material during the pulverization process. The protective gas can, for example, be nitrogen with a dew point ≤ -75°C. To reduce contamination of the lithium-rich lithium iron ferrite material by the pulverizer, the parts in contact with the lithium-rich lithium iron ferrite material during pulverization can be made of ceramic material.

[0128] Similar to the above-described pulverization process, to further reduce the contact between the lithium-rich lithium iron ferrite material and air before it reacts with the first lithium salt and pyrophosphate source in the sealed reactor, a protective gas can be introduced into the sealed reactor during the negative pressure transport process to protect the lithium-rich lithium iron ferrite material transported to the sealed reactor. This protective gas can, for example, be nitrogen with a dew point ≤ -75°C.

[0129] In some embodiments, the specific implementation of forming a coating layer on the surface of lithium-rich lithium iron phosphate material is not limited, and all preparation methods that can coat lithium pyrophosphate on the surface of lithium-rich lithium iron phosphate material are within the protection scope of this application.

[0130] In some embodiments, forming a coating layer on the surface of a lithium iron phosphate material may include:

[0131] The second lithium salt, pyrophosphate source, and first solvent are mixed to obtain the first reactant.

[0132] The lithium iron ferrite material is mixed with the first reactant and then subjected to a first impurity removal treatment to obtain the lithium replenishing agent.

[0133] In the above embodiments, by mixing lithium-rich lithium iron phosphate material with a second lithium salt and a pyrophosphate source, the second lithium salt and the pyrophosphate source react to obtain lithium pyrophosphate, while lithium pyrophosphate is deposited on the surface of the lithium-rich lithium iron phosphate material to form a coating layer on the surface of the lithium-rich lithium iron phosphate material. After a first impurity removal treatment, a lithium replenishing agent is obtained.

[0134] In some embodiments, the aforementioned lithium iron phosphate material, the second lithium salt, and the pyrophosphate source can be mixed in a sealed reactor, and the second lithium salt and the pyrophosphate source can undergo a metathesis reaction under a protective gas atmosphere. For example, before the reaction, nitrogen gas with a dew point ≤ -75°C can be introduced into the sealed reactor to replace the air inside, resulting in a humidity ≤ 5% within the reactor. Then, by heating the sealed reactor, the second lithium salt and the pyrophosphate source can react at a preset temperature.

[0135] During this process, the mixed solution of lithium-rich lithium iron phosphate material, second lithium salt, and pyrophosphate source can be stirred and heated while stirring. This facilitates the full reaction of the second lithium salt and pyrophosphate source to obtain lithium pyrophosphate, thereby achieving the deposition of lithium pyrophosphate on the surface of lithium-rich lithium iron phosphate material. The stirring speed can be 150 r / min to 300 r / min.

[0136] In some embodiments, the second lithium salt comprises at least one of lithium acetate, lithium chloride, lithium nitrate, and lithium citrate.

[0137] In the above embodiments, at least one of lithium acetate, lithium chloride, lithium nitrate, and lithium citrate is used as the second lithium salt. During the reaction between the second lithium salt and the pyrophosphate source, the azeotropic principle of the acid corresponding to the anion in the second lithium salt is utilized to achieve the decomposition and / or volatilization of the anion of the second lithium salt, as well as the precipitation and crystallization of lithium pyrophosphate. This facilitates heterogeneous nucleation on the surface of lithium-rich lithium ferrite materials. During the heterogeneous nucleation of lithium pyrophosphate on the surface of lithium-rich lithium ferrite, as the first solvent evaporates, lithium ions and pyrophosphate ions slowly combine, maintaining a low supersaturation of lithium pyrophosphate. This allows lithium pyrophosphate to slowly crystallize and deposit on the surface of the lithium-rich lithium ferrite material, resulting in a denser and more complete coating.

[0138] In some embodiments, the pyrophosphate source includes at least one of pyrophosphate, ammonium pyrophosphate, and ammonium hydrogen pyrophosphate.

[0139] In the above embodiments, at least one of pyrophosphate, ammonium pyrophosphate, and ammonium hydrogen pyrophosphate is used as the pyrophosphate source. The products generated by the reaction of the second lithium salt and the pyrophosphate source, excluding lithium pyrophosphate, are easily decomposed and volatilized, which facilitates the precipitation and crystallization of lithium pyrophosphate on the surface of lithium-rich lithium iron phosphate material while reducing the introduction of impurities.

[0140] For example, the second lithium salt and the pyrophosphate source can undergo a metathesis reaction to generate lithium pyrophosphate and volatile gases. The volatile gases can be removed by the first impurity removal treatment, thereby obtaining a lithium supplement.

[0141] In some embodiments, the first solvent includes ethanol.

[0142] In the above embodiments, ethanol is used as the first solvent because it has a low boiling point, which facilitates the evaporation of the first solvent.

[0143] In some embodiments, the initial amount of the first solvent added is 5 to 15 times the mass of the second lithium salt.

[0144] In the above embodiments, by controlling the initial addition amount of the first solvent to be 5 to 15 times the mass of the second lithium salt, it is easier to control the precipitation rate of lithium pyrophosphate and make the lithium pyrophosphate coating more complete.

[0145] The lithium iron phosphate material can be in powder form. The second lithium salt and pyrophosphate can be dissolved in the first solvent. After being heated to a preset temperature under stirring, the second lithium salt and pyrophosphate react. The solvent is then evaporated by stirring, thereby causing lithium pyrophosphate to be deposited on the surface of the lithium iron phosphate material.

[0146] In some embodiments, the first impurity removal process includes evaporating a first solvent, solid-liquid separation, drying, sieving, and iron removal.

[0147] In the above embodiments, by evaporating the first solvent, the volatile gases generated after mixing the lithium iron ferrite material with the first reactant can be removed, thereby obtaining lithium iron ferrite coated with lithium pyrophosphate; at the same time, by evaporating and removing the first solvent, lithium pyrophosphate is gradually precipitated and coated on the surface of the lithium iron ferrite material, and then a lithium supplement with high purity is obtained through solid-liquid separation, drying, sieving and iron removal treatment.

[0148] In some embodiments, the temperature at which the first solvent is evaporated can be 80°C to 120°C. For example, the temperature at which the first solvent is evaporated can be 80°C, 90°C, 100°C, 110°C, or 120°C.

[0149] In the above embodiments, by controlling the evaporation temperature, it is convenient for the first solvent and the anion of the second lithium salt to undergo azeotropic reaction.

[0150] In some embodiments, the volume of the first solvent after evaporation is reduced to 1 / 4 to 1 / 3 of its initial volume. For example, the volume of the first solvent after evaporation is reduced to 1 / 4, 3 / 10, or 1 / 3 of its initial volume.

[0151] By controlling the volume of the first solvent after evaporation to be reduced to 1 / 4 to 1 / 3 of the initial volume, an appropriate amount of lithium pyrophosphate precipitate can be controlled and coated on the surface of lithium-rich lithium iron phosphate material, thereby effectively controlling the thickness of the coating layer.

[0152] In some embodiments, to facilitate the evaporation of the first solvent and volatile gas, an exhaust pipe can be connected to the sealed reactor to evaporate the solvent and volatile gas and recover them through condensation.

[0153] In some embodiments, the molar ratio of the lithium element in the lithium-rich lithium iron ferrite material and the second lithium salt to the pyrophosphate ion in the pyrophosphate source is 1:(0.1 to 0.15):(0.025 to 0.04). For example, the molar ratio of the lithium element in the lithium-rich lithium iron ferrite material and the second lithium salt to the pyrophosphate ion in the pyrophosphate source can be 1:0.1:0.025, 1:0.1:0.03, 1:0.1:0.035, 1:0.1:0.04, 1:0.12:0.025, 1:0.12:0.03, 1:0.12:0.035, 1:0.12:0.04, 1:0.15:0.025, 1:0.15:0.03, 1:0.15:0.035, or 1:0.15:0.04, etc.

[0154] In the above embodiments, by controlling the molar ratio of lithium element in the lithium-rich lithium iron ferrite material and the lithium element in the second lithium salt to the pyrophosphate ion in the pyrophosphate source to be 1:(0.1~0.15):(0.025~0.04), it is beneficial to form an appropriate amount of lithium pyrophosphate as a coating layer on the surface of the lithium-rich lithium iron ferrite material, so that the coating is more complete, effectively reducing the contact between the lithium-rich lithium iron ferrite material and air, thereby effectively improving the capacity and stability of the lithium replenishing agent during use.

[0155] In some embodiments, the coating is completed when the volume of the first solvent evaporates to 1 / 4 to 1 / 3 of the initial volume. The solvent can be removed by centrifugation and drying, and then the lithium supplement can be obtained by sieving and iron removal.

[0156] In some embodiments, the drying process can be carried out under a protective gas atmosphere. For example, the centrifuged material can be placed in a nitrogen oven for drying. The dew point of the nitrogen gas introduced into the nitrogen oven can be ≤-75°C, the drying temperature can be 100°C to 150°C, and the drying time can be 3 hours to 6 hours. After drying, the lithium supplement is obtained by sieving, iron removal, and packaging.

[0157] In some embodiments, during the centrifugation, drying, sieving, iron removal, and packaging processes described above, the centrifuge used for centrifugation, the nitrogen oven used for drying, the vibrating screen used for sieving, the iron remover used for iron removal, and the packaging machine used for packaging can all be housed in a constant temperature and humidity chamber, which can improve the quality stability of the finished product. For example, the humidity in this constant temperature and humidity chamber can be controlled to ≤5%, and the temperature can be 20℃~30℃.

[0158] Thirdly, embodiments of this application provide a positive electrode sheet, including a current collector and a positive electrode material disposed on at least one side of the current collector along its thickness direction, the positive electrode material including the lithium supplement agent as described in the first aspect.

[0159] In the positive electrode provided in the embodiments of this application, since the positive electrode material contained in the positive electrode includes the lithium replenishing agent as described in the first aspect, the positive electrode can effectively replenish lithium and improve the safety stability and air stability of the battery during the lithium replenishment process, while maintaining the battery with high lithium-ion transport and conduction performance.

[0160] Fourthly, embodiments of this application provide a secondary battery, including a positive electrode, a negative electrode, and a separator; wherein the positive electrode can be the positive electrode described in the third aspect.

[0161] In the secondary battery provided in the embodiments of this application, since the secondary battery includes a positive electrode as described in the third aspect, the secondary battery has high safety stability and air stability during lithium replenishment, and has high lithium-ion transport and conduction performance.

[0162] Fifthly, embodiments of this application provide an electrical device comprising: a plurality of batteries connected in series and / or in parallel, wherein at least one battery is a secondary battery as described in the fourth aspect.

[0163] In the electrical device provided in the embodiments of this application, since the battery included in the electrical device is a secondary battery as described in the fourth aspect, the electrical device has good safety stability and power stability.

[0164] The electrical devices provided in this application embodiment can be, but are not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc. Among them, electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc., and spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc.

[0165] The following are some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0166] Example 1

[0167] The preparation method of the lithium supplement provided in Example 1 is as follows:

[0168] Step 1) Lithium nitrate and ferrous oxalate are mixed and calcined at high temperature under nitrogen protection to obtain lithium-rich lithium iron nitrate. The ferrous oxalate is doped with 2% Ti by mass, and the particle size of the ferrous oxalate is 8.2 μm. The total molar ratio of lithium in lithium nitrate to the total molar ratio of iron and titanium in ferrous oxalate is 5.1:1, and the particle size of lithium nitrate is 8.5 μm. A slant mixer is used for mixing, and the mixing time is 3 hours. During the calcination process, in the first stage, the heating rate is 150℃ / h, and the temperature is increased to... The temperature is 300℃, and it is held at this temperature for 4 hours. The second stage involves further heating to 650℃ at a rate of 80℃ / h, with a sintering time of 5 hours. Nitrogen gas is introduced to maintain a gas velocity of 0.2 m / s within the furnace. Simultaneously, the exhaust gas is drawn out using an induced draft fan and absorbed by a 0.2% hydrogen peroxide solution to obtain a dilute nitric acid solution. This solution is then added back to dissolve lithium carbonate, concentrated, and crystallized to prepare lithium nitrate. The sintered material is cooled to 80℃ before being discharged.

[0169] Step 2) The lithium iron ferrite prepared in Step 1) is conveyed to a pulverizer under negative pressure for pulverization. During the negative pressure conveying process, nitrogen gas with a dew point ≤ -75℃ is used for gas replenishment. During the pulverization process, nitrogen gas with a dew point ≤ -75℃ is introduced into the pulverizer for protection. All parts of the pulverizer that come into contact with the lithium iron ferrite are made of ceramic material. Pulverization is stopped when the particle size of the lithium iron ferrite reaches 2.7μm. Then, anhydrous ethanol is added to a sealed reactor, followed by lithium acetate and pyrophosphate. The mixture is stirred and slurried to completely dissolve the lithium acetate and pyrophosphate. The pulverized material is then conveyed to the sealed reactor. At the same time, nitrogen gas with a dew point ≤ -75℃ is introduced to replace the air in the sealed reactor, so that the humidity in the sealed reactor is ≤ 5%. Then, the temperature is increased under stirring at a stirring speed of 200 r / min. An exhaust pipe is installed to evaporate and condense the subsequently evaporated ethanol and acetic acid vapors. The molar ratio of lithium iron ferrite, lithium acetate, and pyrophosphate added was 1:0.12:0.04, and the amount of anhydrous ethanol added was 10 times the mass of lithium acetate. The reaction was heated to 100℃ with stirring, and continued until the ethanol volume evaporated to 1 / 4 of its original volume, at which point the reaction was terminated. The material in the sealed reactor was piped to a centrifuge for centrifugation, then washed with alcohol. The washed material was then dried in a nitrogen oven, followed by sieving, iron removal, and packaging to obtain lithium iron ferrite coated with lithium pyrophosphate. The mother liquor generated from centrifugation was returned to be reused after adding anhydrous ethanol, lithium acetate, and pyrophosphate. During drying in the nitrogen oven, the dew point of the nitrogen gas was ≤-75℃, the drying temperature was 130℃, and the drying time was 5 hours. The centrifuge, oven, vibrating screen, iron remover, and packaging machine were all housed in a constant temperature and humidity chamber, with humidity controlled to ≤5% and temperature at 20℃.

[0170] Example 2

[0171] The preparation method of the lithium supplement provided in Example 2 is basically the same as that of the lithium supplement provided in Example 1, except that:

[0172] In step 1), ferrous oxalate is doped with 1% Ti by mass, and the particle size of ferrous oxalate is 5 μm. The total molar ratio of lithium to iron and titanium in lithium nitrate is 5.05:1, and the particle size of lithium nitrate is 10 μm. A slant mixer is used for mixing, and the mixing time is 4 h. During the calcination process, the first stage is characterized by a heating rate of 100 °C / h, heating to 200 °C, and holding at this temperature for 3 h. The second stage involves further heating to 600 °C at a heating rate of 50 °C / h, with a sintering time of 4 h. Nitrogen gas is introduced to make the gas flow rate in the furnace 0.1 m / s. At the same time, the waste gas is drawn out by an induced draft fan and absorbed by a 0.1% hydrogen peroxide solution to obtain a dilute nitric acid solution. This solution is then added back to dissolve lithium carbonate, and the solution is concentrated and crystallized to prepare lithium nitrate.

[0173] In step 2), lithium iron ferrite is pulverized to a particle size of 1.0 μm and then pulverization is stopped. During the stirring and slurrying process of lithium iron ferrite, lithium acetate, and pyrophosphate, the stirring speed is 150 r / min. The molar ratio of lithium iron ferrite, lithium acetate, and pyrophosphate is 1:0.1:0.025, and the amount of anhydrous ethanol added is 5 times the mass of lithium acetate. The mixture is heated to 80℃ under stirring, and the reaction is stopped when the ethanol volume evaporates to 1 / 3 of its original volume. When drying the lithium iron ferrite coated with lithium pyrophosphate in a nitrogen oven, the dew point of the nitrogen gas is ≤-75℃, the drying temperature is 100℃, and the drying time is 6 hours. The centrifuge, oven, vibrating screen, iron remover, and packaging machine are all placed in a constant temperature and humidity room, with humidity controlled within ≤5% and temperature at 30℃.

[0174] Example 3

[0175] The preparation method of the lithium supplement provided in Example 3 is basically the same as that of the lithium supplement provided in Example 1, except that:

[0176] In step 1), ferrous oxalate is doped with 3% Ti by mass, and the particle size of ferrous oxalate is 10 μm. The total molar ratio of lithium to iron and titanium in lithium nitrate is 5.2:1, and the particle size of lithium nitrate is 5 μm. A slant mixer is used for mixing, and the mixing time is 2 hours. During the calcination process, the first stage is characterized by a heating rate of 200℃ / h, heating to 400℃, and holding at this temperature for 5 hours. The second stage involves further heating to 700℃ at a heating rate of 100℃ / h, with a sintering time of 6 hours. Nitrogen gas is introduced to achieve a gas flow rate of 0.3 m / s in the furnace. Simultaneously, the waste gas is drawn out by an induced draft fan and absorbed using a 0.3% hydrogen peroxide solution to obtain a dilute nitric acid solution. This solution is then added back to dissolve lithium carbonate, and the solution is concentrated and crystallized to prepare lithium nitrate.

[0177] In step 2), lithium iron ferrite is pulverized to a particle size of 5 μm, and pulverization is stopped. During the stirring and slurrying process of lithium iron ferrite, lithium acetate, and pyrophosphate, the stirring speed is 300 r / min. The molar ratio of lithium iron ferrite, lithium acetate, and pyrophosphate is 1:0.15:0.03, and the amount of anhydrous ethanol added is 15 times the mass of lithium acetate. The mixture is heated to 120℃ under stirring, and the reaction is stopped when the ethanol volume evaporates to 1 / 4 of its original volume. When drying the lithium iron ferrite coated with lithium pyrophosphate in a nitrogen oven, the dew point of the nitrogen gas is ≤-75℃, the drying temperature is 150℃, and the drying time is 3 hours. The centrifuge, oven, vibrating screen, iron remover, and packaging machine are all placed in a constant temperature and humidity room, with humidity controlled within ≤5% and temperature at 25℃.

[0178] Example 4

[0179] The preparation method of the lithium supplement provided in Example 4 is basically the same as that of the lithium supplement provided in Example 1, except that:

[0180] In step 1), ferrous oxalate is doped with 0.5% Ti by mass.

[0181] Example 5

[0182] The preparation method of the lithium supplement provided in Example 5 is basically the same as that of the lithium supplement provided in Example 1, except that:

[0183] In step 1), ferrous oxalate is doped with 4% Ti by mass.

[0184] Example 6

[0185] The preparation method of the lithium supplement provided in Example 6 is basically the same as that of the lithium supplement provided in Example 1, except that:

[0186] In step 1), ferrous oxalate is not doped with Ti.

[0187] Example 7

[0188] The preparation method of the lithium supplement provided in Example 7 is basically the same as that of the lithium supplement provided in Example 1, except that:

[0189] In step 1), the total molar ratio of lithium in lithium nitrate to iron and titanium in ferrous oxalate is 5.0:1.

[0190] Example 8

[0191] The preparation method of the lithium supplement provided in Example 8 is basically the same as that of the lithium supplement provided in Example 1, except that:

[0192] In step 1), the total molar ratio of lithium in lithium nitrate to iron and titanium in ferrous oxalate is 5.3:1.

[0193] Example 9

[0194] The preparation method of the lithium supplement provided in Example 9 is basically the same as that of the lithium supplement provided in Example 1, except that:

[0195] In step 2), the molar ratio of lithium iron ferrite, lithium acetate, and pyrophosphate is 1:0.20:0.04.

[0196] Example 10

[0197] The preparation method of the lithium supplement provided in Example 10 is basically the same as that of the lithium supplement provided in Example 1, except that:

[0198] In step 2), the molar ratio of lithium ferrite, lithium acetate, and pyrophosphate is 1:0.10:0.02.

[0199] Comparative Example 1

[0200] The preparation method of the lithium replenishing agent provided in Comparative Example 1 is basically the same as that of the lithium replenishing agent provided in Example 1, except that:

[0201] The lithium iron ferrite prepared in step 1) was directly pulverized to a particle size of 2.7 μm and used as a lithium supplement.

[0202] Test methods and test results

[0203] 1. Testing of the chemical composition and physicochemical properties of lithium supplements

[0204] The lithium supplements provided in Examples 1-12 and Comparative Example 1 were tested for chemical composition and physicochemical properties. The specific test results are shown in Table 1 below.

[0205] Table 1

[0206] In Table 1 above, the Fe content in the lithium supplement was determined by potentiometric titration, the phosphorus content by gravimetric analysis of quinoline phosphomolybdate, and the contents of lithium, nickel, cobalt, chromium, and titanium were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). The free lithium content was determined by potentiometric titration. The moisture content in the lithium supplement was determined by the KF method. Magnetic foreign matter in the lithium supplement could be collected using a magnet, dissolved in aqua regia, and then measured using atomic absorption spectrometry.

[0207] Table 2

[0208] In Table 2, the tap density of the lithium supplement is measured using a tap density meter with 5000 vibrations. The compacted density of the lithium supplement is measured using a compacted density meter with a test pressure of 3T and a pressing time of 30s. The pH value of the lithium supplement was determined by acid-base potentiometric titration and measured with a pH meter; the specific surface area of ​​the lithium supplement was determined by the BET specific surface area determination method, such as the nitrogen adsorption multi-point BET method; the D10, D50, and D90 particle sizes in the particle size distribution of the lithium supplement could be determined by a laser particle size analyzer, specifically the Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK; where D10 particle size represents the particle size corresponding to a cumulative volume distribution percentage of 10% for the lithium supplement, D50 particle size represents the particle size corresponding to a cumulative volume distribution percentage of 50% for the lithium supplement, and D90 particle size represents the particle size corresponding to a cumulative volume distribution percentage of 90% for the lithium supplement.

[0209] In Table 2, the thickness of the coating layer was measured using transmission electron microscopy. The coating ratio was defined as the percentage of the coating layer's coverage area on the particle surface (i.e., the lithium iron ferrite core of Examples 1-10 above) to the total particle surface area. The coating amount was measured using diffuse reflectance infrared Fourier transform spectroscopy. The coating ratio could be calculated based on the coating amount and the cross-sectional area of ​​the coating layer, using the following formula: Where n represents the coating ratio, M represents the coating amount, q represents the molecular weight of the coating material, and NA is a constant (6.023 × 10⁻⁶). 23 ), a0 represents the cross-sectional area of ​​the coating layer, S w This represents the specific surface area of ​​the particles.

[0210] As shown in Tables 1 and 2, the lithium replenishing agents provided in Examples 1 to 10 of this application all have a lithium pyrophosphate coating layer on the surface of the lithium iron phosphate material. Compared with the lithium replenishing agent provided in Comparative Example 1, the lithium replenishing agents provided in Examples 1 to 10 have significantly lower free lithium content, magnetic material content, and residual alkali content, and significantly increased specific surface area due to the presence of the coating layer. This is beneficial to improving the capacity and lithium replenishment effect of the lithium replenishing agent when applied to secondary batteries. At the same time, the calcined materials obtained by the preparation methods of the lithium replenishing agents provided in Examples 1 to 10 have weak agglomeration and are therefore easy to crush. The particle size of the final lithium replenishing agent is smaller and more concentrated than that of the lithium replenishing agent provided in Comparative Example 1, which is also beneficial to improving the capacity and lithium replenishment effect of the lithium replenishing agent when applied to secondary batteries.

[0211] Furthermore, comparing Examples 1-5 with Example 6, it is evident that doping ferrous oxalate with Ti helps reduce the free lithium content, thereby improving the capacity and lithium replenishment effect of the lithium replenishing agent when applied to secondary batteries. Comparing Examples 1-5, Example 7, and Example 8, it is evident that controlling the total molar ratio of lithium in lithium nitrate to the total molar ratio of iron and titanium in ferrous oxalate within the range of (5.05-5.2):1 helps reduce the free lithium content and residual alkali content, while also making the pH value of the lithium replenishing agent more suitable, thus improving the stability and lithium replenishment effect when applied to secondary batteries. Comparing Examples 1-5, Example 9, and Example 10, it is evident that controlling the molar ratio of lithium ferrite, lithium acetate, and pyrophosphate within the range of 1:(0.1-0.15):(0.025-0.04) helps reduce the free lithium content and residual alkali content, makes the pH value of the lithium replenishing agent more suitable, and improves the stability and lithium replenishment effect when applied to secondary batteries.

[0212] The lithium replenishing agents provided in Examples 1-10 have high compaction density, tap density, and specific surface area, as well as small and uniformly distributed particle size, which is beneficial for improving the energy density of the lithium replenishing agent. The lithium replenishing agents provided in Examples 1-10 have a large coating layer thickness and coating ratio, which protects the inner lithium-rich lithium iron phosphate material, reduces the contact between the lithium-rich lithium iron phosphate material and air, and is beneficial for improving the stability of the lithium replenishing agent. At the same time, the lithium pyrophosphate as a coating layer is beneficial for improving lithium-ion conduction efficiency and maintaining a high capacity of the lithium replenishing agent.

[0213] 2. Electrical performance tests of lithium supplements

[0214] The lithium supplements provided in Examples 1-10 and Comparative Example 1 were used as positive electrode active materials, and positive electrode sheets were prepared by mixing them with SP and PVDF in proportions of 78%, 14%, and 8%, respectively. The areal density of the positive electrode sheets was 3.5 mg / cm³. 2 .

[0215] A button cell is assembled from a positive electrode, a separator, a negative electrode, and an electrolyte. The separator is made of glass fiber, the negative electrode is made of lithium metal, the electrolyte is made of LiPF6, and the solvent is a mixture of EC (ethylene carbonate) and DEC (diethyl carbonate). The concentration of LiPF6 is 1 mol / L.

[0216] (1) The prepared button battery was left to stand for 10 hours at room temperature (25℃) and humidity ≤10%. Then, the button battery was activated by charge-discharge. The button battery prepared above was tested at room temperature (25℃) with a nominal specific capacity of 700mAh / g and a voltage range of 2.00V to 4.35V. The specific capacity of the first charge and the specific capacity of the first discharge at 0.1C rate and the specific capacity of the first charge and the first discharge at 1C rate were tested. The test results of the specific capacity of the first charge and the specific capacity of the first discharge are shown in Table 3 below.

[0217] Table 3

[0218] As shown in Table 3, the button batteries with the lithium replenishing agents provided in Examples 1 to 10 have a large initial charge capacity at 0.1C and 1C rates. For example, the button batteries with the lithium replenishing agents provided in Examples 1 to 10 have an initial charge specific capacity of more than 700 mAh / g at 0.1C rate and more than 660 mAh / g at 1C rate.

[0219] Comparing the results of the button batteries corresponding to the lithium replenishing agents provided in Examples 1-5 and Example 6 in Table 3, it can be seen that: doping with Ti element is beneficial to improving the initial charge specific capacity. When the doping ratio of Ti element in ferrous oxalate is 1% to 3%, it is beneficial to obtain a higher initial charge specific capacity. Comparing the results of the button batteries corresponding to the lithium replenishing agents provided in Examples 1-3 and Examples 7-8 in Table 3, it can be seen that when the total molar ratio of lithium nitrate to iron and titanium in ferrous oxalate is (5.05:1 to 5.2):1, it is beneficial to obtain a higher initial charge specific capacity. Comparing the results of the button batteries corresponding to the lithium replenishing agents provided in Examples 1-3 and Examples 9-10 in Table 3, it can be seen that when the molar ratio of lithium-rich lithium iron phosphate, lithium acetate and pyrophosphate is in the range of 1:(0.1 to 0.15):(0.025 to 0.04), it is beneficial to obtain a higher initial charge specific capacity.

[0220] The method for conducting charge-discharge cycle tests on the button batteries prepared above at room temperature (25°C) within a voltage range of 2.00V to 4.35V is as follows:

[0221] (2) The button batteries prepared above were placed in an environment with a humidity of 85±5% and a temperature of 25±1℃ for 4h and 12h, and then their electrical performance was tested. The specific test method was the same as the test method above for standing for 10 hours under the conditions of room temperature (25℃) and humidity ≤10%. The results of the first charge-discharge capacity test are shown in Table 4 below:

[0222] Table 4

[0223] As shown in Table 4, the button batteries corresponding to the lithium replenishing agents provided in Examples 1-10 still exhibit high initial charge specific capacity under high humidity conditions, and the change in initial charge specific capacity measured after prolonged exposure to high humidity is minimal. This indicates a high initial charge specific capacity retention rate under high humidity conditions, demonstrating the good stability of the lithium replenishing agents provided in Examples 1-10. In contrast, Comparative Example 1, lacking coating, allowed lithium-rich lithium iron phosphate to directly contact carbon dioxide and water vapor in the air, resulting in hydrolysis and decomposition of the lithium iron phosphate, leading to a sharp capacity decay.

[0224] Comparing the results of the button batteries corresponding to Examples 1-5 and Example 6 in Table 4, it can be seen that Ti doping is beneficial to the stability of the lithium replenishing agent under high humidity conditions; that is, Ti doping is beneficial to improving the stability of the lithium replenishing agent. Comparing the results of the button batteries corresponding to Examples 1-5 and Examples 7-8 in Table 4, it can be seen that when the molar ratio of lithium nitrate to the total amount of iron and titanium in ferrous oxalate is in the range of (5.05-5.2):1, it is more beneficial to the stability of the lithium replenishing agent under high humidity conditions. Comparing the results of the button batteries corresponding to Examples 1-3 and Examples 9-10 in Table 4, it can be seen that when the molar ratio of lithium ferrite, lithium acetate, and pyrophosphate is in the range of 1:(0.1-0.15):(0.025-0.04), it is more beneficial to maintaining the stability of the lithium replenishing agent.

[0225] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A lithium supplement agent, wherein, include: Lithium iron oxide rich in lithium, and a coating layer covering the surface of the lithium iron oxide rich in lithium; The material of the coating layer includes lithium pyrophosphate.

2. The lithium supplement agent according to claim 1, wherein, The lithium iron ferrite material contains titanium.

3. The lithium supplement agent according to claim 1 or 2, wherein, The thickness of the coating layer is 10 nm to 50 nm; and / or, the coating coverage of the coating layer is greater than or equal to 94%.

4. A method for preparing a lithium supplement, wherein, include: We provide lithium iron ferrite materials; A coating layer is formed on the surface of the lithium iron ferrite material; The material of the coating layer includes lithium pyrophosphate.

5. The method for preparing the lithium supplement according to claim 4, wherein, The steps for providing lithium-rich lithium iron phosphate materials include: Provides first lithium salt and ferrous salt; The first lithium salt and the ferrous salt are mixed and calcined under a protective gas atmosphere to obtain the lithium-rich lithium iron ore material.

6. The method for preparing the lithium supplement according to claim 5, wherein, The first lithium salt comprises lithium nitrate, and the ferrous salt comprises ferrous oxalate; the ferrous oxalate comprises titanium, and the molar ratio of lithium in the first lithium salt to the total amount of iron and titanium in the ferrous salt is (5.0–5.3):1; and / or The calcination includes a first stage and a second stage performed sequentially; in the first stage, the calcination temperature is 200℃~400℃ and the calcination holding time is 3h~5h; in the second stage, the calcination temperature is 600℃~700℃ and the calcination holding time is 4h~6h.

7. The method for preparing the lithium supplement according to any one of claims 4 to 6, wherein, A coating layer is formed on the surface of the lithium iron phosphate material, comprising: The second lithium salt, pyrophosphate source, and first solvent are mixed to obtain the first reactant. The lithium iron phosphate material is mixed with the first reactant and subjected to a first impurity removal treatment to obtain the lithium replenishing agent.

8. The method for preparing the lithium supplement according to claim 7, wherein, The first impurity removal process includes evaporation of the first solvent, solid-liquid separation, drying, sieving, and iron removal; and / or, The method for preparing the lithium supplement meets at least one of the following conditions: (1) The molar ratio of the lithium-rich lithium iron oxide material, the lithium element in the second lithium salt, and the pyrophosphate ion of the pyrophosphate source is 1:(0.1~0.15):(0.025~0.04); (2) The first solvent includes: ethanol; (3) The second lithium salt includes at least one of lithium acetate, lithium chloride, lithium nitrate and lithium citrate; (4) The pyrophosphate source includes at least one of pyrophosphate, ammonium pyrophosphate and ammonium hydrogen pyrophosphate; (5) The temperature for evaporating the first solvent is 80℃~120℃.

9. A positive electrode plate, wherein, It includes a current collector and a positive electrode material disposed on at least one side of the current collector along its thickness direction, the positive electrode material including the lithium supplement as described in any one of claims 1 to 3.

10. A secondary battery, wherein, include: Positive electrode, negative electrode, and separator; The positive electrode sheet is the positive electrode sheet as described in claim 9.

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