Lithium-rich lithium iron phosphate lithium supplement and preparation method thereof, positive electrode sheet and secondary battery

Abstract

The application discloses a lithium-rich lithium iron phosphate lithium supplement, a preparation method thereof, a positive pole piece and a secondary battery, and relates to the technical field of secondary batteries. The lithium-rich lithium iron phosphate lithium supplement comprises a core and a carbon coating layer located on at least part of the surface of the core. The core comprises aluminum-doped lithium-rich lithium iron phosphate, the mass fraction of aluminum in the lithium-rich lithium iron phosphate lithium supplement is 1.57% to 5.5%, and the mass fraction of elemental iron in the lithium-rich lithium iron phosphate lithium supplement is less than or equal to 950 ppm. The lithium-rich lithium iron phosphate lithium supplement has good specific capacity and cycle stability.

Description

Technical Field

[0001] This application relates to the field of secondary battery technology, and more specifically, to a lithium-rich lithium iron phosphate lithium supplement agent and its preparation method, a positive electrode sheet, and a secondary battery. Background Technology

[0002] Improving the energy density of current lithium-ion battery cathode materials faces severe challenges, especially the loss of active lithium due to the irreversible formation of the solid electrolyte interphase (SEI) film during the first cycle of graphite anodes, which severely restricts the full-cell capacity. To compensate for this irreversible lithium loss, "pre-lithiation" technology has been widely recognized as a key strategy to overcome the energy density bottleneck. Traditional lithium replenishing agents such as Li₂NiO₂ and Li₅VO₄ can partially compensate for lithium loss, but they still suffer from insufficient capacity, gas production, or poor industrial compatibility. Lithium-rich lithium iron ore oxide (Li₅FeO₄), with its high theoretical specific capacity of 700 mAh / g and moderate delithiation potential (~4.0V vs. Li₂), offers a solution. + Lithium ions (Li₂ / Li) are considered ideal candidates for next-generation high-capacity lithium replenishment agents; however, their poor stability in air severely limits their storage life. Summary of the Invention

[0003] In view of the technical problems existing in the background art, this application provides a lithium-rich lithium iron phosphate supplement and its preparation method, positive electrode sheet and secondary battery, aiming to solve the technical problems of poor stability and short storage life of lithium-rich lithium iron phosphate supplement in air.

[0004] In a first aspect, embodiments of this application provide a lithium-rich lithium iron ferrite supplement, which includes a core and a carbon coating layer located on at least a portion of the surface of the core. The core includes aluminum-doped lithium iron ferrite, and the mass fraction of aluminum in the lithium-rich lithium iron ferrite supplement is 1.57% to 5.5%, and the mass fraction of elemental iron in the lithium-rich lithium iron ferrite supplement is ≤950ppm.

[0005] In the technical solution of this application embodiment, by introducing aluminum into the core for doping and controlling the mass fraction of aluminum in the lithium iron ferrite supplement within the aforementioned range, it is beneficial for aluminum to enter the lattice of lithium iron ferrite, forming intrinsic modification at the atomic scale, inducing lattice contraction, enhancing lithium-ion diffusion capability, and improving the capacity of the lithium iron ferrite supplement. Simultaneously, controlling the mass fraction of aluminum within the aforementioned range also helps form an appropriate amount of Al-O bonds, which helps improve the lattice energy of aluminum-doped lithium iron ferrite, enhancing the thermodynamic stability of the lithium iron ferrite supplement, thus helping to improve the stability of the lithium iron ferrite supplement in air and extend its storage life. Controlling the mass fraction of elemental iron in the lithium iron ferrite supplement within the aforementioned range helps to improve battery safety and reduce irreversible capacity loss during battery cycling.

[0006] In some embodiments, the mass fraction of the second phase in the lithium iron phosphate supplement is ≤5%.

[0007] In this embodiment, the second phase refers to the components in the core of the lithium-rich lithium iron ferrite supplement, excluding the aluminum-doped lithium-rich lithium iron ferrite solid solution, such as aluminum oxide, lithium-rich lithium aluminate (Li5AlO4), lithium aluminate (LiAlO2), etc. The second phase is generally an intrinsic phase at the micron or nanometer scale, usually located at the grain boundaries or particle surface of lithium-rich lithium iron ferrite, and is an inert component. Controlling the mass fraction of the second phase within the above range is beneficial to improving the lithium-ion transport performance, thereby helping to increase the capacity of the lithium-rich lithium iron ferrite supplement.

[0008] In some embodiments, the mass fraction of carbon in the lithium iron phosphate supplement is 1.0% to 3.5%.

[0009] In this embodiment, controlling the mass fraction of carbon elements within the above-mentioned range helps to form a carbon coating layer of suitable thickness. On the one hand, this helps to fully coat the core, reduce the probability of the core coming into contact with air, and improve the stability of the lithium iron ferrite supplement in air. On the other hand, it helps to balance the specific capacity of the lithium iron ferrite supplement, allowing it to fully exert its lithium supplementation performance.

[0010] In some embodiments, the carbon coating in the lithium iron ferrite supplement has a coating rate of ≥96%.

[0011] In this embodiment, by adjusting the coating rate of the carbon coating layer, it is helpful to isolate the aluminum-doped lithium iron ferrite in the core from the outside air, reduce the probability of the aluminum-doped lithium iron ferrite in the core decomposing to produce strong alkaline substances such as lithium oxide, lithium carbonate, and lithium hydroxide, and help improve the processing performance, first charge specific capacity, and storage stability of lithium iron ferrite supplement.

[0012] In some embodiments, the D50 particle size of the lithium iron phosphate supplement meets the following requirements: 2μm≤D50≤5.5μm.

[0013] In this embodiment, by controlling the D50 particle size of the lithium iron ferrite supplement, it is helpful for the lithium iron ferrite supplement to be more fully activated during the first charge and discharge process (battery formation stage), resulting in a higher first charge and discharge specific capacity. It also helps the lithium iron ferrite supplement to completely release oxygen during the first charge and discharge process, reducing the gas generation rate during subsequent battery cycles, thereby helping to improve the cycle stability of the battery.

[0014] In some embodiments, the molar ratio of lithium to iron in the lithium-rich lithium iron ferrite supplement is (6.0~8.03):1.

[0015] In this embodiment, by adjusting the molar ratio of lithium to iron in the lithium-rich lithium iron ferrite supplement, it is helpful to improve the capacity of the lithium-rich lithium iron ferrite supplement and enhance its overall electrochemical performance.

[0016] In some embodiments, aluminum-doped lithium iron ferrite also contains zirconium, and the mass fraction of zirconium in the lithium iron ferrite supplement is 0.2% to 2.5%.

[0017] In this embodiment, zirconium is further introduced into aluminum-doped lithium iron ferrite and the mass fraction of zirconium is controlled within the above-mentioned range. The zirconium element works synergistically with the aluminum element to regulate the degree of distortion of the crystal structure, so that aluminum-doped lithium iron ferrite has a suitable lattice energy, which helps to reduce the migration energy barrier of lithium ions, promotes the redox of oxygen anions, thereby reducing the interfacial impedance. While improving the storage stability of lithium iron ferrite supplement, the capacity of lithium iron ferrite supplement is significantly improved.

[0018] In some embodiments, after being placed in an environment with a temperature of 20°C to 30°C and a relative humidity of 20% to 30% for 24 hours, the first charge specific capacity decay rate under 0.05C conditions is less than 7%, and after being placed for 48 hours, the first charge specific capacity decay rate under 0.05C conditions is less than 15%.

[0019] In this embodiment, the lithium-rich lithium iron ferrite supplement exhibits good stability in air, indicating that aluminum-doped lithium-rich lithium iron ferrite effectively suppresses the probability of side reactions occurring in humid environments, thus solving the technical bottleneck of rapid capacity decay caused by the poor air stability of traditional lithium-rich lithium iron ferrite supplements.

[0020] Secondly, embodiments of this application provide a method for preparing a lithium-rich lithium iron phosphate supplement, comprising the following steps:

[0021] A mixture of lithium source, iron source and doping source is obtained; the doping source includes aluminum source.

[0022] The mixture is subjected to calcination treatment, which includes: subjecting the mixture to a first heat treatment to obtain a first intermediate product; the temperature of the first heat treatment is 100℃~300℃, and the time of the first heat treatment is 4h~8h; raising the temperature to 600℃~800℃, subjecting the first intermediate product to a second heat treatment to obtain a second intermediate product; the time of the second heat treatment is 24h~48h.

[0023] The second intermediate product was subjected to carbon coating treatment to obtain lithium-rich lithium iron ferrite supplement.

[0024] The lithium-rich lithium iron ferrite supplement includes a core and a carbon coating layer located on at least part of the surface of the core. The core includes aluminum-doped lithium iron ferrite, and the mass fraction of aluminum in the lithium-rich lithium iron ferrite supplement is 1.57% to 5.5%.

[0025] In the technical solution of this application embodiment, by pre-mixing the lithium source, iron source, and dopant source, it is helpful to obtain a mixture with uniform distribution of each element, thereby helping to shorten the migration and diffusion path of ions during sintering and forming a uniform aluminum-doped lithium iron ferrite solid solution. By performing a step-by-step calcination process, firstly, a first heat treatment is performed at a low temperature, which helps to accelerate the decomposition of the aluminum source, promote the volatilization of the aluminum source decomposition products, and avoid side reactions between the aluminum source decomposition products and the iron and lithium sources; then, a second heat treatment is performed at a high temperature, which helps the dopant source enter the lattice of lithium iron ferrite to form a solid solution and reduce the formation rate of the second phase; thereby helping to improve the stability of lithium iron ferrite supplement in air and the charge and discharge specific capacity.

[0026] In some embodiments, the doping source further includes a zirconium source, and the mass fraction of zirconium in the lithium iron phosphate supplement is 0.2% to 2.5%.

[0027] In this embodiment, by simultaneously introducing aluminum and zirconium sources for co-doping into the lithium-rich lithium iron ferrite supplement, zirconium (0.2%–2.5% by mass) synergistically interacts with aluminum (1.57%–5.5%) in the crystal structure, significantly reducing the migration energy barrier of lithium ions in the crystal lattice, thereby contributing to an increase in the specific capacity of the lithium-rich lithium iron ferrite supplement. Furthermore, the introduction of zirconium optimizes lattice distortion, reduces the probability of side reactions between the lithium-rich lithium iron ferrite supplement and moisture and titanium dioxide in the air, and significantly improves the air stability of the lithium-rich lithium iron ferrite supplement.

[0028] In some embodiments, the lithium source includes at least one of lithium oxide, lithium hydroxide, lithium carbonate, and lithium oxalate.

[0029] In this embodiment, lithium oxide can directly participate in the solid-phase reaction without decomposition, which is beneficial for the rapid synthesis of high-purity aluminum-doped lithium iron ferrite; aluminum hydroxide, lithium carbonate and lithium oxalate have good ion diffusion performance, which also helps the lithium source to fully react with the iron source and doping source to form aluminum-doped lithium iron ferrite with uniform elemental distribution; thus helping to improve the air stability of lithium iron ferrite supplementation agent.

[0030] In some embodiments, the iron source includes at least one of ferric oxide, iron tetroxide, ferric hydroxide, and ferric hydroxide.

[0031] In this embodiment, ferric oxide can directly participate in the solid-phase reaction to generate lithium iron ferrite rich in lithium; iron tetroxide helps to lower the sintering temperature and promote the solid-phase reaction; iron hydroxide and iron hydroxyl oxide can be dehydrated at low temperature to form high-valence oxides, which helps to improve the purity of the phase, reduce the content of the second phase, and thus help to suppress the occurrence of side reactions and improve the stability of lithium iron ferrite supplement in air.

[0032] In some embodiments, the aluminum source includes at least one of aluminum oxide, γ-Al(OH)3, α-Al(OH)3, AlO(OH), aluminum isopropoxide, aluminum nitrate, aluminum acetate, and aluminum oxalate.

[0033] In this embodiment, each aluminum source has good reactivity and readily reacts with iron and lithium sources to form aluminum-doped lithium iron ferrite solid solutions, thereby helping to improve the stability of lithium iron ferrite supplements in air.

[0034] In some embodiments, the doping source includes a zirconium source, which includes at least one of tetragonal zirconium oxide, monoclinic zirconium oxide, zirconium hydroxide, and zirconium isopropoxide.

[0035] In this embodiment, the zirconium source can promote the entry of zirconium ions into the lithium iron ferrite lattice to replace Fe. 3+ The site forms Zr-O bonds, increasing the lattice energy and thus helping to improve the thermodynamic stability of lithium iron ferrite supplements.

[0036] In some implementations, the average particle size of the aluminum source is <5 μm.

[0037] In this embodiment, controlling the average particle size of the aluminum source within the above-mentioned range helps to improve processing performance and facilitates the uniform coating of lithium and iron sources on its surface, which in turn helps ion migration and refusion during the calcination process, allowing the solid-phase reaction to proceed more fully and improving the reaction conversion rate.

[0038] In some embodiments, the doping source includes a zirconium source with an average particle size of <5 μm.

[0039] In this embodiment, controlling the average particle size of the zirconium source within the above-mentioned range helps to improve the efficiency of the mixing process, allowing the lithium and iron sources to be uniformly coated on its surface. This not only helps to promote the solid-phase reaction but also helps to improve the purity of the lithium iron ferrite supplement and enhance the overall performance of the lithium iron ferrite supplement.

[0040] In some embodiments, when the aluminum source is alumina, the BET specific surface area of ​​the aluminum source is ≥100 m². 2 / g; When the zirconium source is at least one of tetragonal zirconium oxide and monoclinic zirconium oxide, the BET specific surface area of ​​the zirconium source is ≥15m². 2 / g.

[0041] In this embodiment, controlling the BET specific surface area of ​​the aluminum source and zirconium source within the above-mentioned range helps to reduce the reaction energy barrier of the aluminum source and zirconium source, so as to promote the entry of aluminum ions and zirconium ions into the lattice of lithium iron ferrite at a lower temperature, reduce the content of the second phase, and thus help to reduce the probability of side reactions between the lithium iron ferrite supplement and moisture and carbon dioxide in the air, and improve the stability of the lithium iron ferrite supplement in the air.

[0042] In some embodiments, the mixing speed is 400 r / min to 600 r / min, and the mixing time is 0.5 h to 1.5 h; further, the mixing is carried out in a protective atmosphere or an environment with humidity below 20%, the protective atmosphere including at least one of nitrogen, argon, helium, neon and krypton.

[0043] In this embodiment, controlling the mixing conditions within the above-mentioned range helps to improve the uniformity of the lithium source, iron source and dopant source after mixing, making the distribution of lithium, iron and dopant elements in the mixture obtained by the mixing treatment more uniform. This helps to shorten the migration and diffusion path of ions during the subsequent sintering process, form a uniform solid solution, and reduce the content of second phases such as alumina and iron oxide. This is beneficial to improving the stability of lithium iron ferrite supplement in air and the charge and discharge specific capacity.

[0044] In some embodiments, the lithium source fed into the mixing equipment has a water content of less than 1000 ppm (refer to GB / T 45330), a magnetic foreign matter content of less than 10 ppm (refer to GB / T 41704—2022), and a purity of ≥95%; the iron source fed into the mixing equipment has a water content of less than 5000 ppm, a magnetic foreign matter content of less than 100 ppm, and a purity of ≥95%. The average particle size of the iron source is <300 nm.

[0045] In this embodiment, further adjusting the performance parameters of the lithium source and iron source helps to suppress the agglomeration of raw materials such as the lithium source during the mixing process, thereby improving the uniformity of the mixture. It also helps to reduce the content of magnetic foreign matter in the lithium-rich lithium iron ferrite supplement, thus improving the purity of the supplement. Furthermore, controlling the average particle size of the iron source within the aforementioned range helps to ensure the full solid-phase reaction between the lithium source and the iron source, improving the reaction conversion rate and the purity of the lithium-rich lithium iron ferrite supplement.

[0046] In some embodiments, the aluminum source is selected from at least one of aluminum citrate, aluminum acetate, aluminum oxalate and aluminum hydroxide; the temperature of the first heat preservation treatment is 200℃~300℃ and the time of the first heat preservation treatment is 4h~8h; the temperature of the second heat preservation treatment is 600℃~750℃ and the time of the second heat preservation treatment is 24h~48h.

[0047] In this embodiment, during the first heat preservation process, the aluminum source can be fully decomposed, allowing the decomposition products water and carbon dioxide to be fully released. After the water and carbon dioxide are released, the second heat preservation process helps to suppress the formation of the second phase, improve the purity of the lithium iron ferrite supplement, and improve the electrochemical performance and air stability of the lithium iron ferrite supplement.

[0048] In some embodiments, the aluminum source is alumina, the temperature of the first heat preservation treatment is 100℃~200℃, and the time of the first heat preservation treatment is 4h~8h; the temperature of the second heat preservation treatment is 650℃~800℃, and the time of the second heat preservation treatment is 24h~40h.

[0049] In this embodiment, during the first heat preservation process, the moisture in the aluminum source can be fully evaporated, which helps to reduce the probability of side reactions occurring during the second heat preservation process. Under the above conditions, the second heat preservation process helps aluminum ions in the aluminum source enter the lithium iron ferrite lattice to form a solid solution, reducing the formation rate of the second phase, thereby helping to improve the stability and electrochemical performance of the lithium iron ferrite supplement in air.

[0050] In some embodiments, the calcination process is carried out in a nitrogen or inert atmosphere; specifically, the inert atmosphere includes at least one of helium, argon and krypton.

[0051] In this embodiment, controlling the calcination process under the above conditions helps to suppress the probability of second phase formation, improve the purity of lithium iron ferrite supplement, and thus help to improve the stability of lithium iron ferrite supplement in air.

[0052] In some embodiments, after the second heat preservation treatment is completed, the product obtained from the second heat preservation treatment is pulverized to obtain a second intermediate product; the dew point temperature of the pulverization environment is ≤-40℃; the D100 of the second intermediate product is ≤35μm, the D50 of the second intermediate product is ≤15μm, and the D10 of the second intermediate product is ≥0.1μm; further, the D100 of the second intermediate product is ≤30μm, the 2μm of the second intermediate product is ≤D50 is ≤5μm, and the D10 of the second intermediate product is ≥0.1μm.

[0053] In this embodiment, pulverizing under the above conditions helps reduce the probability of the product obtained from the second heat preservation treatment reacting with water. Furthermore, controlling the particle size of the second intermediate product within the above range helps to fully activate the lithium-rich lithium iron phosphate supplement during the first charge-discharge process, reducing its gas generation rate when subsequently compounded with other cathode materials, and improving battery life.

[0054] In some embodiments, the carbon coating process includes:

[0055] Phenolic resin is dissolved in an organic solvent to obtain a carbon-coated solution;

[0056] The carbon-coated liquid is mixed with the second intermediate product and dried to obtain the carbon-coated intermediate.

[0057] The carbon-coated intermediate was sintered and pulverized to obtain lithium-rich lithium iron ferrite supplement.

[0058] By dissolving phenolic resin in an organic solvent to form a carbon coating liquid, and then mixing it with a second intermediate product, the uniformity of carbon coating can be improved, forming a continuous and complete carbon coating layer. This helps to block the probability of aluminum-doped lithium iron ferrite coming into contact with moisture and carbon dioxide in the air, thereby improving the stability of lithium iron ferrite supplement in the air.

[0059] In some embodiments, the organic solvent includes anhydrous ethanol, acetone, etc.

[0060] In this embodiment, by selecting the organic solvent, it is helpful for the phenolic resin to dissolve fully, which in turn facilitates the uniform coating of the second intermediate product.

[0061] In some embodiments, the concentration of phenolic resin in the carbon coating solution is 0.03 g / mL to 0.07 g / mL.

[0062] In this embodiment, controlling the concentration of phenolic resin in the carbon coating solution within the above-mentioned range facilitates subsequent adjustment of the mass ratio of the carbon coating solution to the second intermediate product within a suitable range, which helps to improve processing performance and enhance the uniformity of the carbon coating intermediate.

[0063] In some embodiments, the second intermediate product is added to a mixer, and at least one of nitrogen, argon, and helium is introduced for gas replacement to control the oxygen content in the mixer to be below 50 ppm; then stirring is started at a speed of 800 r / min to 1200 r / min; at the same time, using at least one of nitrogen, argon, and helium as a pressure source, the carbon coating liquid is sprayed into the mixer through an atomizing nozzle, and the mass ratio of the carbon coating liquid to the second intermediate product is controlled to be (3.3 to 8.2): 100.

[0064] In this embodiment, by regulating the mixing conditions of the carbon coating liquid and the second intermediate product, it is helpful for the carbon coating liquid and the second intermediate product to be fully mixed and form a uniform contact, thereby increasing the coating rate of the carbon coating layer. This helps to reduce the probability of aluminum-doped lithium iron ferrite coming into contact with air and reduce the generation rate of strongly alkaline substances, which in turn helps to improve the first charge specific capacity of the lithium iron ferrite supplement and improve the homogenization performance of the lithium iron ferrite supplement.

[0065] In some embodiments, after all the carbon coating liquid is sprayed into the mixer, the stirring speed is adjusted to 600 r / min to 800 r / min, and the temperature inside the mixer is adjusted to 80℃ to 100℃. The mixture is stirred and dried for 2 h to 4 h to obtain a carbon-coated intermediate. Further, at least one of nitrogen, argon, and helium is introduced during the drying process for gas replacement.

[0066] In this embodiment, by regulating the mixing conditions, it is helpful to quickly remove organic solvents and regulate the uniform coating and solidification rate of phenolic resin on the surface of aluminum-doped lithium iron ferrite, thereby improving the coating effect. Controlling the atmospheric conditions during the drying process helps to reduce the probability of decomposition of aluminum-doped lithium iron ferrite upon contact with air, which in turn helps to improve the purity of lithium iron ferrite supplement and improve the stability of lithium iron ferrite supplement in air.

[0067] In some embodiments, phenolic resins include linear thermoplastic phenolic resins (Novolac type), liquid methyl phenolic resins (Resol type), etc.

[0068] In this embodiment, the phenolic resin has good solubility in organic solvents, which helps to prepare a stable and uniform carbon coating liquid. This avoids the phenolic resin precipitating out and clogging the equipment or accumulating after spraying during the subsequent atomization coating process, preventing the formation of a uniform carbon coating intermediate with the second intermediate product.

[0069] In some embodiments, the sintering process is carried out in an inert atmosphere with an oxygen content ≤20ppm. The sintering process includes a first sintering process and a second sintering process. The first sintering process is carried out at 90℃~110℃ for 2h~4h. After the first sintering process, the temperature is increased to 600℃~680℃ at a rate of 2℃ / min~5℃ / min for the second sintering process, and the time of the second sintering process is 4h~8h.

[0070] In this embodiment, controlling the sintering conditions within the aforementioned range helps the phenolic resin to undergo a stable pyrolysis reaction. During this process, the gas is released slowly, which helps suppress the formation of structural defects and obtain a dense and uniform carbon coating layer. On the other hand, controlling the heating rate within the aforementioned range facilitates free radical recombination, deoxygenation condensation, and the closure and stacking of six-membered rings, thereby helping to improve the graphitization degree and conductivity of the carbon coating layer, reduce the powder resistivity of the lithium iron ferrite supplement, and thus improve the charging specific capacity and structural stability of the lithium iron ferrite supplement.

[0071] Thirdly, embodiments of this application provide a positive electrode sheet, including a current collector and a positive electrode active material, a conductive agent, a binder, and a lithium replenishing agent located on one or both sides of the current collector; the lithium replenishing agent is the lithium-rich lithium iron ferrite replenishing agent in the above embodiments of this application.

[0072] In this embodiment, the positive electrode contains the aforementioned lithium-rich lithium iron phosphate supplement, thus exhibiting good discharge specific capacity and cycle stability.

[0073] 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 in the above embodiments of this application.

[0074] In this embodiment, the secondary battery includes the aforementioned positive electrode sheet, thus possessing the advantages of good energy density and lifespan.

[0075] 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, the following are specific embodiments of this application. Attached Figure Description

[0076] 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.

[0077] Figure 1 The images show the XRD patterns of lithium-rich lithium iron ferrite supplements in Examples 1 and 4 of this application. Detailed Implementation

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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).

[0084] In the description of the embodiments of this application, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0085] In the description of the embodiments of this application, unless otherwise expressly specified and limited, technical terms such as "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. For those skilled in the art, the specific meaning of the above terms in the embodiments of this application can be understood according to the specific circumstances.

[0086] Lithium iron oxide rich in lithium has a high theoretical specific capacity and is suitable as a lithium replenishing agent; however, it has poor stability in air and is prone to react with moisture and carbon dioxide in the air. When used in positive electrode sheets, it will result in more residual alkali on the surface of the positive electrode sheet, which is prone to side reactions during battery cycling and accelerates the capacity decay of the battery.

[0087] To address the technical problems of poor stability and short storage life of lithium iron ferrite supplements in air, this application provides a lithium iron ferrite supplement, its preparation method, a positive electrode, and a secondary battery. The lithium iron ferrite supplement is designed with a core-shell structure, and aluminum-doped lithium iron ferrite is selected as the core. On the one hand, the shell layer effectively isolates the aluminum-doped lithium iron ferrite in the core from contact with air. On the other hand, aluminum doping helps improve the thermodynamic stability of the lithium iron ferrite supplement, thereby improving its stability in air and storage life. Consequently, the performance of the positive electrode and the secondary battery is also enhanced.

[0088] In a first aspect, embodiments of this application provide a lithium-rich lithium iron ferrite supplement, comprising a core and a carbon coating layer located on at least a portion of the surface of the core. The core comprises aluminum-doped lithium iron ferrite, the general chemical formula of which is Li5Fe. 1-x M x O4, wherein x > 0, M includes Al, and the mass fraction of aluminum in the lithium iron ferrite supplement is 1.57%~5.5%, and the mass fraction of elemental iron in the lithium iron ferrite supplement is ≤950ppm. Specifically, but not limitingly, the mass fraction of aluminum in the lithium iron ferrite supplement can be 1.57%, 2%, 2.5%, 3%, 3.5%, 4%, 5%, 5.5%, or any value within the range of any two of the above values. More preferably, the mass fraction of aluminum in the lithium iron ferrite supplement is 2.0%~3.2%. Specifically, but not limitingly, the mass fraction of elemental iron in the lithium iron ferrite supplement can be 950ppm, 900ppm, 850ppm, 800ppm, 750ppm, 700ppm, 650ppm, 600ppm, 550ppm, 500ppm, or any value within the range of any two of the above values.

[0089] In the technical solution of this application embodiment, by introducing aluminum into the core for doping and controlling the mass fraction of aluminum in the lithium iron ferrite supplement within the aforementioned range, it is beneficial for aluminum to enter the lattice of lithium iron ferrite, forming intrinsic modification at the atomic scale, inducing lattice contraction, enhancing lithium-ion diffusion capability, and improving the capacity of the lithium iron ferrite supplement. Simultaneously, controlling the mass fraction of aluminum within the aforementioned range also helps form an appropriate amount of Al-O bonds, which helps improve the lattice energy of aluminum-doped lithium iron ferrite, enhancing the thermodynamic stability of the lithium iron ferrite supplement, thus helping to improve the stability of the lithium iron ferrite supplement in air and extend its storage life. Controlling the mass fraction of elemental iron in the lithium iron ferrite supplement within the aforementioned range helps to improve battery safety, suppress the occurrence of side reactions, and improve initial coulombic efficiency and cycle stability.

[0090] Furthermore, in some embodiments, the mass fraction of the second phase in the lithium iron ferrite supplement is ≤5%. Specifically, but not limitingly, the mass fraction of the second phase in the lithium iron ferrite supplement can be 1%, 2%, 3%, 4%, 5%, or any value within the range of any two of the above values.

[0091] In the technical solution of this application embodiment, the second phase refers to the impurity phase in the core of the lithium iron ferrite supplement, excluding the aluminum-doped lithium iron ferrite solid solution, such as aluminum oxide, lithium aluminate (Li5AlO4), lithium aluminate (LiAlO2), etc.; the second phase is generally an intrinsic phase at the micron or nanometer scale, usually located at the grain boundaries or particle surface of lithium iron ferrite, and is an inert component. Controlling the mass fraction of the second phase within the above range is beneficial to improving the lithium ion transport performance, thereby helping to increase the capacity of the lithium iron ferrite supplement.

[0092] Further, in some embodiments, the mass fraction of carbon in the lithium iron ferrite supplement is 1.0% to 3.5%. Specifically, but not limitingly, the mass fraction of carbon in the lithium iron ferrite supplement is 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, or any value within the range of any two of the above values. More preferably, the mass fraction of carbon in the lithium iron ferrite supplement is 2.0% to 3.5%.

[0093] In the technical solution of this application embodiment, controlling the mass fraction of carbon elements within the above range helps to form a carbon coating layer of suitable thickness. On the one hand, it helps to fully coat the core, reduce the probability of the core contacting the air, and improve the stability of lithium iron ferrite supplement in air; on the other hand, it helps to balance the specific capacity of lithium iron ferrite supplement, so that it can give full play to its lithium supplementation performance.

[0094] Furthermore, in some embodiments, the carbon coating coverage of the lithium iron ferrite supplement is ≥96%. Specifically, but not limitingly, the carbon coating coverage can be 96%, 96.5%, 97%, 97.5%, 98%, 98.5%, 99%, or any value within the range of any two of the above values.

[0095] In the technical solution of this application embodiment, by adjusting the coating rate of the carbon coating layer, it is helpful to isolate the aluminum-doped lithium iron ferrite in the core from the outside air, reduce the probability of the aluminum-doped lithium iron ferrite in the core decomposing to produce strong alkaline substances such as lithium oxide, lithium carbonate, and lithium hydroxide, and help improve the processing performance, first charge specific capacity and storage stability of lithium iron ferrite supplement.

[0096] Furthermore, in some embodiments, the D50 particle size of the lithium-rich lithium iron ferrite supplement meets the following condition: 2μm ≤ D50 ≤ 5.5μm. Specifically, but not limitingly, the D50 particle size of the lithium-rich lithium iron ferrite supplement can be any value within the range of 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, 5.5μm, or any combination of the above values.

[0097] In the technical solution of this application embodiment, by controlling the D50 particle size of the lithium iron ferrite supplement, it is helpful for the lithium iron ferrite supplement to be more fully activated in the first charge and discharge process (battery formation stage), and to have a higher first charge and discharge specific capacity. It also helps the lithium iron ferrite supplement to completely release oxygen in the first charge and discharge process, and to discharge the gas in the battery formation stage; thereby reducing the gas generation rate in the subsequent battery cycle process.

[0098] Furthermore, in some embodiments, the molar ratio of lithium to iron in the lithium-rich lithium iron ferrite supplement is (6.0~8.03):1.

[0099] In the technical solution of this application embodiment, by adjusting the molar ratio of lithium to iron in the lithium-rich lithium iron ferrite supplement, it is helpful to improve the capacity of the lithium-rich lithium iron ferrite supplement and enhance its comprehensive electrochemical performance.

[0100] Furthermore, in some embodiments, the aluminum-doped lithium iron ferrite also contains zirconium (M further includes Zr), and the mass fraction of zirconium in the lithium iron ferrite lithium supplement is 0.2% to 2.5%. Specifically, but not limitingly, the mass fraction of zirconium in the lithium iron ferrite lithium supplement is 0.2%, 0.4%, 0.6%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, or any value within the range of any two of the above values. More preferably, the mass fraction of zirconium in the lithium iron ferrite lithium supplement is 1.0% to 2.0%.

[0101] In the technical solution of this application embodiment, zirconium element is further introduced into aluminum-doped lithium iron ferrite and the mass fraction of zirconium element is controlled within the above-mentioned range. Zirconium element works with aluminum element to control the degree of distortion of crystal structure, so that aluminum-doped lithium iron ferrite has a suitable crystal energy, which helps to reduce the migration energy barrier of lithium ions, promote the redox of oxygen anions, thereby reducing the interface impedance. While improving the storage stability of lithium iron ferrite supplement, the capacity of lithium iron ferrite supplement is significantly improved.

[0102] Furthermore, in some embodiments, after being placed in an environment with a temperature of 20°C to 30°C and a relative humidity of 20% to 30% for 24 hours, the first charge specific capacity decay rate under 0.05C conditions is less than 7%, and after being placed for 48 hours, the first charge specific capacity decay rate under 0.05C conditions is less than 15%.

[0103] In the technical solution of this application embodiment, the lithium iron ferrite supplement has good stability in air, indicating that aluminum doping of lithium iron ferrite effectively suppresses the probability of side reactions in the lithium iron ferrite supplement in a humid environment, and solves the technical bottleneck of traditional lithium iron ferrite supplements causing rapid capacity decay and poor battery cycle stability due to poor air stability.

[0104] Secondly, embodiments of this application provide a method for preparing a lithium-rich lithium iron phosphate supplement, comprising the following steps:

[0105] A mixture of lithium source, iron source and doping source is obtained; the doping source includes aluminum source.

[0106] The mixture is subjected to calcination treatment, which includes: subjecting the mixture to a first heat treatment to obtain a first intermediate product; the temperature of the first heat treatment is 100℃~300℃, specifically but not limitingly, the temperature of the first heat treatment can be 100℃, 150℃, 200℃, 250℃, 300℃ or any value within the range of any two of the above values; the time of the first heat treatment is 4h~8h, specifically but not limitingly, the time of the first heat treatment can be 4h, 5h, 6h, 7h, 8h or any value within the range of any two of the above values; The temperature is raised to 600℃~800℃, and the first intermediate product is subjected to a second heat treatment to obtain a second intermediate product; the second heat treatment time is 24h~48h; specifically, but not limitingly, the temperature of the second heat treatment can be 600℃, 650℃, 700℃, 750℃, 800℃ or any value within the range of any two of the above values; specifically, but not limitingly, the time of the second heat treatment can be 24h, 28h, 30h, 32h, 36h, 40h, 42h, 46h, 48h or any value within the range of any two of the above values.

[0107] The second intermediate product was subjected to carbon coating treatment to obtain lithium-rich lithium iron ferrite supplement.

[0108] The lithium-rich lithium iron ferrite supplement comprises a core and a carbon coating layer located on at least a portion of the surface of the core. The core comprises aluminum-doped lithium iron ferrite, and the mass fraction of aluminum in the lithium-rich lithium iron ferrite supplement is 1.57% to 5.5%. Specifically, but not limitingly, the mass fraction of aluminum in the lithium-rich lithium iron ferrite supplement can be 1.57%, 2%, 2.5%, 3%, 3.5%, 4%, 5%, 5.5%, or any value within the range of any two of the above values.

[0109] The process involves pre-mixing the lithium source, iron source, and dopant source to obtain a uniformly distributed mixture of elements. This helps shorten the migration and diffusion paths of ions during sintering, resulting in a homogeneous aluminum-doped lithium iron ferrite solid solution. The step-by-step calcination process, starting with a low-temperature holding treatment, accelerates the decomposition of the aluminum source, promotes the volatilization of its decomposition products, and prevents side reactions between the aluminum decomposition products and the iron and lithium sources. A second high-temperature holding treatment then facilitates the entry of the dopant source into the lithium iron ferrite crystal lattice to form a solid solution, reducing the formation rate of the second phase. This, in turn, improves the stability of the lithium iron ferrite supplement in air and its charge / discharge specific capacity.

[0110] Furthermore, in some embodiments, the doping source also includes a zirconium source, and the mass fraction of zirconium in the lithium iron ferrite supplement is 0.2% to 2.5%. Specifically, but not limitingly, the mass fraction of zirconium in the lithium iron ferrite supplement is 0.2%, 0.4%, 0.6%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, or any value within the range of any two of the above values.

[0111] In the technical solution of this application embodiment, by simultaneously introducing aluminum and zirconium sources for co-doping into the lithium-rich lithium iron ferrite supplement, zirconium, at a mass fraction of 0.2%~2.5%, synergistically acts with aluminum (1.57%~5.5%) on the crystal structure of aluminum-doped lithium iron ferrite, significantly reducing the migration energy barrier of lithium ions in the crystal lattice, thereby helping to improve the specific capacity of the lithium-rich lithium iron ferrite supplement. Furthermore, the introduction of zirconium optimizes lattice distortion, reduces the probability of side reactions between the lithium-rich lithium iron ferrite supplement and moisture and titanium dioxide in the air, and significantly improves the air stability of the lithium-rich lithium iron ferrite supplement.

[0112] Furthermore, in some embodiments, the lithium source includes at least one of lithium oxide, lithium hydroxide, lithium carbonate, and lithium oxalate.

[0113] In the technical solution of this application embodiment, lithium oxide can directly participate in the solid-phase reaction without decomposition, which is beneficial for the rapid synthesis of high-purity aluminum-doped lithium iron ferrite; aluminum hydroxide, lithium carbonate and lithium oxalate have good ion diffusion performance, which also helps the lithium source to fully react with the iron source and doping source to form aluminum-doped lithium iron ferrite with uniform element distribution; thus helping to improve the air stability of lithium iron ferrite supplementing agent.

[0114] Furthermore, in some embodiments, the iron source includes at least one of ferric oxide, iron(II) oxide, ferric hydroxide, and ferric hydroxide.

[0115] In the technical solution of this application embodiment, ferric oxide can directly participate in the solid-phase reaction to generate lithium iron ferrite rich in lithium; iron tetroxide helps to reduce the sintering temperature and promote the solid-phase reaction; iron hydroxide and iron hydroxyl oxide can be dehydrated at low temperature to form high-valence oxides, which helps to improve the phase purity, reduce the content of the second phase, and thus help to suppress the occurrence of side reactions and improve the stability of lithium iron ferrite supplement in air.

[0116] Furthermore, in some embodiments, the aluminum source includes at least one selected from aluminum oxide, γ-Al(OH)3, α-Al(OH)3, AlO(OH), aluminum isopropoxide, aluminum nitrate, aluminum acetate, basic aluminum acetate, and aluminum oxalate.

[0117] In the technical solutions of this application embodiment, each of the above-mentioned aluminum sources has good reactivity and is easy to react with iron and lithium sources to generate aluminum-doped lithium iron ferrite solid solutions, thereby helping to improve the stability of lithium iron ferrite supplements in air.

[0118] Furthermore, in some embodiments, the doping source includes a zirconium source, which includes at least one of tetragonal zirconium oxide, monoclinic zirconium oxide, zirconium hydroxide, and zirconium isopropoxide.

[0119] In the technical solution of this application embodiment, the zirconium source can promote the entry of zirconium ions into the lithium iron ferrite lattice to replace Fe. 3+ The site forms Zr-O bonds, increasing the lattice energy and thus helping to improve the thermodynamic and air stability of lithium iron ferrite supplements.

[0120] Furthermore, in some embodiments, the average particle size of the aluminum source is <5 μm. Specifically, but not limitingly, the average particle size of the aluminum source can be 1 μm, 2 μm, 3 μm, 4 μm, or any value within the range of any two of the above values, and is not specifically limited herein.

[0121] In the technical solution of this application embodiment, controlling the average particle size of the aluminum source within the above-mentioned range helps to improve processing performance and facilitates the uniform coating of lithium and iron sources on its surface, which helps ion migration and refusion during the calcination process, allowing the solid-phase reaction to proceed more fully and improving the reaction conversion rate.

[0122] Furthermore, in some embodiments, the doping source includes a zirconium source with an average particle size of <5 μm. Specifically, but not limitingly, the average particle size of the zirconium source can be 1 μm, 2 μm, 3 μm, 4 μm, or any value within the range of any two of the above values, and is not specifically limited herein.

[0123] In the technical solution of this application embodiment, controlling the average particle size of the zirconium source within the above-mentioned range helps to improve the efficiency of the mixing process, so that the lithium source and iron source are uniformly coated on its surface, which not only helps to promote the solid-phase reaction, but also helps to improve the purity of the lithium iron ferrite supplement and improve the overall performance of the lithium iron ferrite supplement.

[0124] Furthermore, in some embodiments, the average particle size of the lithium source is <300 nm. Specifically, but not limitingly, the average particle size of the lithium source can be 50 nm, 80 nm, 100 nm, 150 nm, 200 nm, 250 nm, or any value within the range of any two of the above values.

[0125] In the technical solution of this application embodiment, controlling the average particle size of the lithium source within the above-mentioned range helps to improve the diffusion performance of lithium ions during the calcination process, which is conducive to the full progress of the solid phase reaction, helps to improve the purity of lithium iron ferrite supplementary agent, and reduces the content of impurity phase.

[0126] Furthermore, in some embodiments, the average particle size of the iron source is <300 nm. Specifically, but not limitingly, the average particle size of the iron source can be 50 nm, 80 nm, 100 nm, 150 nm, 200 nm, 250 nm, or any value within the range of any two of the above values.

[0127] In the technical solution of this application embodiment, controlling the average particle size of the iron source within the above-mentioned range helps to shorten the diffusion path of iron ions, making the solid-phase reaction faster and more complete.

[0128] Furthermore, in some embodiments, when the aluminum source is alumina, the BET specific surface area of ​​the aluminum source is ≥100m². 2 / g, specifically, but not limitingly, the BET specific surface area of ​​the aluminum source can be 100m². 2 / g, 150m 2 / g、200m 2 / g、250m 2 / g、300m 2 / g or any value within the range of any two of the above values; when the zirconium source is at least one of tetragonal zirconium oxide and monoclinic zirconium oxide, the BET specific surface area of ​​the zirconium source is ≥15m². 2 / g, specifically, but not limitingly, the BET specific surface area of ​​the zirconium source can be 15m². 2 / g、20m 2 / g、25m 2 / g、30m 2 / g、35m 2 / g or any value within the range of any two of the above values.

[0129] In the technical solution of this application embodiment, controlling the BET specific surface area of ​​the aluminum source and zirconium source within the above-mentioned range helps to reduce the reaction energy barrier of the aluminum source and zirconium source, so as to promote the entry of aluminum ions and zirconium ions into the lattice of lithium iron ferrite at a lower temperature, reduce the content of the second phase, and thus help to reduce the probability of side reactions between the lithium iron ferrite supplement and moisture and carbon dioxide in the air, and improve the stability of the lithium iron ferrite supplement in the air.

[0130] Further, in some embodiments, the mixing speed is 400 r / min to 600 r / min. Specifically, but not limitingly, the mixing speed can be 400 r / min, 450 r / min, 500 r / min, 550 r / min, 600 r / min, or any value within the range of any two of the above values. The mixing time is 0.5 h to 1.5 h. Specifically, but not limitingly, the mixing time can be 0.5 h, 0.8 h, 1 h, 1.2 h, 1.5 h, or any value within the range of any two of the above values. Further, the mixing process is carried out in a protective atmosphere or an environment with humidity below 20%. The protective atmosphere includes at least one of nitrogen, argon, helium, neon, and krypton.

[0131] In the technical solution of this application embodiment, controlling the mixing conditions within the above range helps to improve the uniformity of the lithium source, iron source and dopant source after mixing, making the distribution of lithium, iron and dopant elements in the mixture obtained by mixing more uniform, which helps to shorten the migration and diffusion path of ions during subsequent sintering, form a uniform solid solution, and reduce the content of second phases such as alumina and iron oxide; thereby improving the stability of lithium iron ferrite supplement in air and the charge and discharge specific capacity.

[0132] Furthermore, in some embodiments, the lithium source fed into the mixing equipment has a water content of less than 1000 ppm (refer to GB / T 45330), a magnetic foreign matter content of less than 10 ppm (refer to GB / T 41704—2022), and a purity of ≥95%; the iron source fed into the mixing equipment has a water content of less than 5000 ppm, a magnetic foreign matter content of less than 100 ppm, and a purity of ≥95%.

[0133] In the technical solution of this application embodiment, by further adjusting the performance parameters of the lithium source and iron source, it is helpful to suppress the agglomeration of raw materials such as lithium source during the mixing process and improve the uniformity of the mixture. It also helps to reduce the content of magnetic foreign matter in the lithium-rich lithium iron ferrite supplement, thereby improving the purity of the lithium-rich lithium iron ferrite supplement. Furthermore, controlling the average particle size of the iron source within the aforementioned range helps the solid-phase reaction between the lithium source and iron source to proceed fully, improving the reaction conversion rate and the purity of the lithium-rich lithium iron ferrite supplement.

[0134] Further, in some embodiments, the aluminum source is selected from at least one of aluminum citrate, aluminum acetate, aluminum oxalate, and aluminum hydroxide; the temperature of the first heat treatment is 200°C to 300°C; specifically, but not limitingly, the temperature of the first heat treatment can be 200°C, 220°C, 240°C, 260°C, 280°C, 300°C, or any value within the range of any two of the above values; the time of the first heat treatment is 4 hours to 8 hours; specifically, but not limitingly, the time of the first heat treatment can be 4 hours, 4.5 hours, 5 hours, 6 hours, 7 hours, 8 hours, or... The temperature of the second heat preservation treatment is 600℃~750℃. Specifically, but not limitingly, the temperature of the second heat preservation treatment can be 600℃, 650℃, 700℃, 750℃ or any value within the range of any two values ​​above; the time of the second heat preservation treatment is 24h~48h. Specifically, but not limitingly, the time of the second heat preservation treatment can be 24h, 28h, 30h, 32h, 36h, 40h, 42h, 48h or any value within the range of any two values ​​above.

[0135] In the technical solution of this application embodiment, during the first heat preservation process, the aluminum source can be fully decomposed, so that the decomposition products of the aluminum source, water and carbon dioxide, are fully released; after the water and carbon dioxide are released, the second heat preservation process is carried out, which helps to suppress the formation of the second phase, improve the purity of lithium iron ferrite supplement, and improve the electrochemical performance and air stability of lithium iron ferrite supplement.

[0136] Furthermore, in some embodiments, the aluminum source is alumina, and the temperature of the first heat treatment is 100℃~200℃. Specifically, but not limitingly, the temperature of the first heat treatment can be any value within the range of 100℃, 120℃, 140℃, 160℃, 180℃, 200℃, or any two of the above values. The duration of the first heat treatment is 4h~8h. Specifically, but not limitingly, the duration of the first heat treatment can be 4h, 4.5h, 5h, 6h, 7h, 8h, or any two of the above values. The temperature of the second heat treatment is 650℃~800℃, specifically but not limitingly, the temperature of the second heat treatment can be 650℃, 700℃, 750℃, 800℃ or any value within the range of any two of the above values; the time of the second heat treatment is 24h~40h, specifically but not limitingly, the time of the second heat treatment can be 24h, 28h, 30h, 32h, 36h, 40h or any value within the range of any two of the above values.

[0137] In the technical solution of this application embodiment, during the first heat preservation process, the moisture in the aluminum source can be fully evaporated, which helps to reduce the probability of side reactions occurring during the second heat preservation process. Under the above conditions, the second heat preservation process helps aluminum ions in the aluminum source enter the lithium iron ferrite lattice to form a solid solution, reducing the generation rate of the second phase, thereby helping to improve the stability and electrochemical performance of the lithium iron ferrite supplement in air.

[0138] Furthermore, in some embodiments, the calcination process is carried out in a nitrogen or inert atmosphere; specifically, the inert atmosphere includes at least one of helium, argon, and krypton.

[0139] In the technical solution of this application embodiment, controlling the calcination treatment under the above conditions helps to suppress the probability of second phase formation, improve the purity of lithium iron ferrite supplement, and thus help to improve the stability of lithium iron ferrite supplement in air.

[0140] Furthermore, in some embodiments, after the second heat preservation treatment is completed, the product obtained from the second heat preservation treatment is pulverized to obtain a second intermediate product; the dew point temperature of the pulverization environment is ≤-40℃; the D100 of the second intermediate product is ≤35μm, the D50 of the second intermediate product is ≤15μm, and the D10 of the second intermediate product is ≥0.1μm; further, the D100 of the second intermediate product is ≤30μm, the 2μm of the second intermediate product is ≤D50 is ≤5μm, and the D10 of the second intermediate product is ≥0.1μm.

[0141] In the technical solution of this application embodiment, pulverizing under the above conditions helps reduce the probability of the product obtained from the second heat preservation treatment reacting with water. Furthermore, controlling the particle size of the second intermediate product within the above range helps to fully activate the lithium-rich lithium iron phosphate supplement during the first charge-discharge process, reducing its gas generation rate when subsequently compounded with other cathode materials, and improving battery life.

[0142] Furthermore, in some embodiments, the carbon coating treatment method includes:

[0143] Phenolic resin is dissolved in an organic solvent to obtain a carbon-coated solution;

[0144] The carbon-coated liquid is mixed with the second intermediate product and dried to obtain the carbon-coated intermediate.

[0145] The carbon-coated intermediate was sintered and pulverized to obtain lithium-rich lithium iron ferrite supplement.

[0146] In the technical solution of this application embodiment, by dissolving phenolic resin in an organic solvent to form a carbon coating liquid, and then mixing it with a second intermediate product, it helps to improve the uniformity of carbon coating and form a continuous and complete carbon coating layer. This helps to block the probability of aluminum-doped lithium iron ferrite coming into contact with moisture and carbon dioxide in the air, thereby improving the stability of lithium iron ferrite supplement in the air.

[0147] Furthermore, in some embodiments, the organic solvent includes anhydrous ethanol, acetone, etc.

[0148] In the technical solution of this application embodiment, by selecting the organic solvent, it is helpful for the phenolic resin to be fully dissolved, which in turn facilitates the uniform coating of the second intermediate product.

[0149] Furthermore, in some embodiments, the concentration of phenolic resin in the carbon coating solution is 0.03 g / mL to 0.07 g / mL. Specifically, but not limitingly, the concentration of phenolic resin in the carbon coating solution can be 0.03 g / mL, 0.04 g / mL, 0.05 g / mL, 0.06 g / mL, 0.07 g / mL, or any value within the range of any two of the above values.

[0150] In the technical solution of this application embodiment, controlling the concentration of phenolic resin in the carbon coating liquid within the above-mentioned range facilitates subsequent adjustment of the mass ratio of carbon coating liquid to the second intermediate product within a suitable range, which helps to improve processing performance and enhance the uniformity of the carbon coating intermediate.

[0151] Furthermore, in some embodiments, the second intermediate product is added to a mixer, and at least one gas selected from nitrogen, argon, and helium is introduced for gas replacement to control the oxygen content in the mixer to be below 50 ppm; then stirring is started at a speed of 800 r / min to 1200 r / min. Specifically, but not limitingly, the stirring speed can be 800 r / min, 900 r / min, 1000 r / min, 1100 r / min, 1200 r / min, or any two of the above. The value can be any value within the range of the numerical composition; at the same time, using at least one of nitrogen, argon, and helium as a pressure source, the carbon coating liquid is sprayed into the mixer through an atomizing nozzle, and the mass ratio of the carbon coating liquid to the second intermediate product is controlled to be (3.3~8.2):100. Specifically, but not limitingly, the mass ratio of the carbon coating liquid to the second intermediate product can be 3.3:100, 4:100, 5:100, 6:100, 7:100, 8.2:100 or any value within the range of any two of the above ratios.

[0152] In the technical solution of this application embodiment, by controlling the mixing conditions of the carbon coating liquid and the second intermediate product, it is helpful for the carbon coating liquid and the second intermediate product to be fully mixed, form a uniform contact, improve the coating rate of the carbon coating layer, and thus help reduce the probability of aluminum-doped lithium iron ferrite coming into contact with air and reduce the generation rate of strong alkaline substances. This is beneficial to improve the first charge specific capacity of the lithium iron ferrite supplement and improve the homogenization performance of the lithium iron ferrite supplement.

[0153] Further, in some embodiments, after all the carbon coating liquid is sprayed into the mixer, the stirring speed is adjusted to 600 r / min to 800 r / min, and the temperature inside the mixer is adjusted to 80℃ to 100℃. The mixture is then stirred and dried for 2 h to 4 h to obtain the carbon-coated intermediate. Further, during the drying process, at least one of nitrogen, argon, and helium is introduced for gas replacement. Specifically, but not limitingly, the stirring speed can be 600 r / min, 6500 r / min, 700 r / min, 800 r / min, or any value within the range of any two of the above values; the temperature inside the mixer can be 80℃, 85℃, 90℃, 95℃, 100℃, or any value within the range of any two of the above values; the stirring and drying time can be 2 h, 3 h, 4 h, or any value within the range of any two of the above values.

[0154] In the technical solution of this application embodiment, by regulating the mixing conditions, on the one hand, it helps to quickly remove organic solvents, and on the other hand, it helps to regulate the uniform coating and solidification rate of phenolic resin on the surface of aluminum-doped lithium iron ferrite, so as to improve the coating effect; controlling the atmospheric conditions of the drying process helps to reduce the probability of aluminum-doped lithium iron ferrite decomposing upon contact with air, thereby helping to improve the purity of lithium iron ferrite supplement and improve the stability of lithium iron ferrite supplement in air.

[0155] Furthermore, in some embodiments, the phenolic resin includes linear thermoplastic phenolic resin (Novolac type), liquid methyl phenolic resin (Resol type), etc.

[0156] In the technical solution of this application embodiment, the phenolic resin has good solubility in organic solvents, which helps to prepare a stable and uniform carbon coating liquid and avoids the phenolic resin precipitating and clogging the equipment or accumulating after spraying during the subsequent atomization coating process, so as to prevent the formation of a uniform carbon coating intermediate with the second intermediate product.

[0157] Furthermore, in some embodiments, the sintering process is carried out in an inert atmosphere with an oxygen content ≤20ppm. The sintering process includes a first sintering process and a second sintering process. The first sintering process is carried out at 90℃~110℃ for 2h~4h. After the first sintering process, the temperature is increased to 600℃~680℃ at a rate of 2℃ / min~5℃ / min for the second sintering process, which lasts for 4h~8h. Specifically, but not limitingly, the temperature of the first sintering treatment can be 90℃, 95℃, 100℃, 105℃, 110℃, or any value within the range of any two of the above values; the time of the first sintering treatment can be 2h, 2.5h, 3h, 3.5h, 4h, or any value within the range of any two of the above values; the temperature of the second sintering treatment can be 600℃, 620℃, 640℃, 660℃, 680℃, or any value within the range of any two of the above values; the time of the second sintering treatment can be 4h, 5h, 6h, 7h, 8h, or any value within the range of any two of the above values.

[0158] In the technical solution of this application embodiment, controlling the sintering conditions within the above-mentioned range helps the phenolic resin to undergo a stable pyrolysis reaction. During this process, the gas is released slowly, which helps to suppress the formation of structural defects and obtain a dense and uniform carbon coating layer. On the other hand, controlling the heating rate within the above-mentioned range helps to promote free radical recombination, deoxygenation condensation, and the closure and stacking of six-membered rings, thereby helping to improve the graphitization degree and conductivity of the carbon coating layer, reduce the powder resistivity of the lithium iron ferrite supplement, and thus improve the charging specific capacity and structural stability of the lithium iron ferrite supplement.

[0159] Thirdly, embodiments of this application provide a positive electrode sheet, including a current collector and a positive electrode active material, a conductive agent, a binder, and a lithium replenishing agent located on one or both sides of the current collector; the lithium replenishing agent is the lithium-rich lithium iron ferrite replenishing agent in the above embodiments of this application.

[0160] 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 in the above embodiments of this application.

[0161] 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.

[0162] I. Preparation Method

[0163] α-Al(OH)3: average particle size 3 μm, BET specific surface area 2 m² 2 / g.

[0164] Monoclinic zirconium oxide: average particle size 3 μm, BET specific surface area 20 m² 2 / g.

[0165] Basic aluminum acetate: Al(OH)(C2H3O2)2, with an average particle size of 3μm and a BET specific surface area of ​​20m². 2 / g.

[0166] Alumina: average particle size 3μm, BET specific surface area 152m² 2 / g.

[0167] Example 1

[0168] One embodiment of the lithium-rich lithium iron ferrite supplement agent of this application is described below, and the preparation method of the lithium-rich lithium iron ferrite supplement agent described in this embodiment is as follows:

[0169] S1, weigh 821.78g lithium oxide, 677.72g ferric oxide, 134.51g α-Al(OH)3, and 31.25g monoclinic zirconium oxide, with a lithium to iron molar ratio of 6.64. The materials are fed into a high-speed mixer for mixing to obtain a mixture. The mixing time is 1.5 hours, the mixing speed is 500 r / min, and the equipment is kept completely sealed during the mixing process. The entire mixing process is carried out under nitrogen protection. The lithium oxide has a water content of 800 ppm, a magnetic impurity content of 6.35 ppm, a purity of 96.35%, and an average particle size of 200 nm. The ferric oxide has a water content of 3500 ppm, a magnetic impurity content of 80 ppm, and a purity of 97.85%. The average particle size of the ferric oxide is 200 nm.

[0170] S2, the mixture is placed in a sintering furnace for calcination under argon atmosphere. First, a first holding treatment is performed at 240℃ for 6 hours; then, the temperature is increased to 650℃ at a rate of 3℃ / min for a second holding treatment for 38 hours. After the second holding treatment, the product is pulverized to obtain a second intermediate product. The dew point temperature of the pulverization environment is ≤-40℃. The D100 particle size of the second intermediate product is 27.52μm, the D50 particle size is 4.96μm, and the D10 particle size is 0.362μm.

[0171] S3, 98.91g of linear thermoplastic phenolic resin (Novolac type) was dissolved in anhydrous ethanol, and the insoluble matter was removed by filtration to obtain a homogeneous and stable carbon-coated solution; the concentration of linear thermoplastic phenolic resin in the carbon-coated solution was 0.05g / mL; the second intermediate product was added to a high-speed mixer, and nitrogen gas was introduced for gas replacement to control the oxygen content in the high-speed mixer to be below 50ppm. The stirring was started at a speed of 1000r / min, and the carbon-coated solution was sprayed into the high-speed mixer through an atomizing nozzle using nitrogen as a pressure source. The mass ratio of carbon-coated solution to the second intermediate product was 6.4:100; after all the carbon-coated solution was sprayed in, the stirring speed was adjusted to 700r / min, and the temperature inside the high-speed mixer was adjusted to 90℃ through heat transfer oil. The mixture was stirred and dried for 3h to obtain the carbon-coated intermediate; nitrogen gas was continuously introduced for replacement throughout the stirring and drying process to remove organic solvents;

[0172] S4. The carbon-coated intermediate is placed in a vacuum sintering furnace and sintered in an inert atmosphere (nitrogen with a purity greater than 99.99%). First, it is held at 100℃ for 3 hours, and then the temperature is increased to 660℃ at 4℃ / min and held for 6 hours. The oxygen content in the furnace is ≤20ppm throughout the sintering process. The sintered material is then dispersed by airflow pulverization to obtain the lithium-rich lithium iron ferrite supplement.

[0173] Example 2

[0174] One embodiment of the lithium-rich lithium iron ferrite supplement agent of this application differs from that of Example 1 in the following ways:

[0175] In S1, the mass of ferric oxide is 744.84 g, the mass of α-Al(OH)3 is 70.38 g, and the molar ratio of lithium to iron is 6.01.

[0176] Example 3

[0177] One embodiment of the lithium-rich lithium iron ferrite supplement agent of this application differs from that of Example 1 in the following ways:

[0178] In S1, the mass of ferric oxide is 560.84 g, the mass of α-Al(OH)3 is 246.18 g, and the molar ratio of lithium to iron is 8.03.

[0179] Example 4

[0180] One embodiment of the lithium-rich lithium iron ferrite supplement agent of this application differs from that of Example 1 in the following ways:

[0181] In S1, the mass of monoclinic zirconium oxide is 4.18 g.

[0182] Example 5

[0183] One embodiment of the lithium-rich lithium iron ferrite supplement agent of this application differs from that of Example 1 in the following ways:

[0184] In S1, the mass of monoclinic zirconium oxide is 52.24 g.

[0185] Example 6

[0186] One embodiment of the lithium-rich lithium iron ferrite supplement agent of this application differs from that of Example 1 in the following ways:

[0187] In S1, basic aluminum acetate is used to replace α-Al(OH)3, and the mass of basic aluminum acetate is 272.98g;

[0188] In S2, the temperature of the first heat preservation treatment is 260℃; after the first heat preservation treatment is completed, the temperature is increased to 650℃ at a rate of 4℃ / min for the second heat preservation treatment, and the time of the second heat preservation treatment is 36h.

[0189] Example 7

[0190] One embodiment of the lithium-rich lithium iron ferrite supplement agent of this application differs from that of Example 1 in the following ways:

[0191] In S3, the mass of linear thermoplastic phenolic resin (Novolac type) is 51g, the concentration of linear thermoplastic phenolic resin in the carbon coating liquid is 0.03g / mL, and the mass ratio of carbon coating liquid to the second intermediate product is 3.3:100.

[0192] Example 8

[0193] One embodiment of the lithium-rich lithium iron ferrite supplement agent of this application differs from that of Example 1 in the following ways:

[0194] In S3, the mass of linear thermoplastic phenolic resin (Novolac type) is 126.73 g, the concentration of linear thermoplastic phenolic resin in the carbon coating liquid is 0.07 g / mL, and the mass ratio of carbon coating liquid to the second intermediate product is 8.2:100.

[0195] Example 9

[0196] One embodiment of the lithium-rich lithium iron ferrite supplement agent of this application differs from that of Example 1 in the following ways:

[0197] In S1, no zirconium source was added for zirconium doping. The mixture consisted of 821.78g lithium oxide, 610.60g ferric oxide, and 198.64g α-Al(OH)3, with a lithium to iron molar ratio of 7.37.

[0198] Example 10

[0199] One embodiment of the lithium-rich lithium iron ferrite supplement agent of this application differs from that of Example 1 in the following ways:

[0200] In S1, the mass of monoclinic zirconium oxide is 2.09 g.

[0201] Example 11

[0202] One embodiment of the lithium-rich lithium iron ferrite supplement agent of this application is described below, and the preparation method of the lithium-rich lithium iron ferrite supplement agent described in this embodiment is as follows:

[0203] S1, weigh 821.78g lithium oxide, 677.72g ferric oxide, 134.51g α-Al(OH)3, and 31.25g monoclinic zirconium oxide, with a lithium to iron molar ratio of 6.64. The materials are fed into a high-speed mixer for mixing to obtain a mixture. The mixing time is 1.5 hours, the mixing speed is 400 r / min, and the equipment is kept completely sealed during the mixing process, which is carried out under nitrogen protection. The lithium oxide has a water content of 800 ppm, a magnetic impurity content of 6.35 ppm, a purity of 96.35%, and an average particle size of 200 nm. The ferric oxide has a water content of 3500 ppm, a magnetic impurity content of 80 ppm, and a purity of 97.85%. The average particle size of the ferric oxide is 200 nm.

[0204] S2, the mixture is placed in a sintering furnace for calcination under argon atmosphere. First, a first holding treatment is performed at 300℃ for 8 hours; then, the temperature is increased to 700℃ at a rate of 3℃ / min for a second holding treatment for 30 hours. After the second holding treatment, the product is pulverized to obtain a second intermediate product. The dew point temperature of the pulverization environment is ≤-40℃. The D100 particle size of the second intermediate product is 27.31μm, the D50 particle size is 4.74μm, and the D10 particle size is 0.357μm.

[0205] S3, 98.91g of linear thermoplastic phenolic resin (Novolac type) was dissolved in anhydrous ethanol, and the insoluble matter was removed by filtration to obtain a homogeneous and stable carbon-coated solution; the concentration of linear thermoplastic phenolic resin in the carbon-coated solution was 0.05g / mL; the second intermediate product was added to a high-speed mixer, and nitrogen gas was introduced for gas replacement to control the oxygen content in the high-speed mixer to be below 50ppm. The stirring was started at a speed of 800r / min, and the carbon-coated solution was sprayed into the high-speed mixer through an atomizing nozzle using nitrogen as a pressure source. The mass ratio of carbon-coated solution to the second intermediate product was 6.4:100; after all the carbon-coated solution was sprayed in, the stirring speed was adjusted to 700r / min, and the temperature inside the high-speed mixer was adjusted to 80℃ through heat transfer oil. The mixture was stirred and dried for 4h to obtain the carbon-coated intermediate; nitrogen gas was continuously introduced for replacement throughout the stirring and drying process to remove organic solvents.

[0206] S4. The carbon-coated intermediate is put into a vacuum sintering furnace and sintered in an inert atmosphere (nitrogen with a purity greater than 99.99%). First, it is held at 90°C for 4 hours, and then the temperature is increased to 600°C at 2°C / min and held for 8 hours. The oxygen content in the furnace is ≤20ppm throughout the sintering process. The sintered material is then dispersed by airflow pulverization to obtain the lithium-rich lithium iron ferrite supplement.

[0207] Example 12

[0208] One embodiment of the lithium-rich lithium iron ferrite supplement agent of this application is described below, and the preparation method of the lithium-rich lithium iron ferrite supplement agent described in this embodiment is as follows:

[0209] S1, weigh 821.78g lithium oxide, 677.72g ferric oxide, 134.51g α-Al(OH)3, and 31.25g monoclinic zirconium oxide, with a lithium to iron molar ratio of 6.64. The materials are fed into a high-speed mixer for mixing to obtain a mixture. The mixing time is 0.5h, the mixing speed is 600r / min, and the equipment is kept completely sealed during the mixing process, which is carried out under nitrogen protection. The lithium oxide has a water content of 800ppm, a magnetic impurity content of 6.35ppm, a purity of 96.35%, and an average particle size of 200nm. The ferric oxide has a water content of 3500ppm, a magnetic impurity content of 80ppm, and a purity of 97.85%. The average particle size of the ferric oxide is 200nm.

[0210] S2, the mixture is placed in a sintering furnace for calcination under argon atmosphere. First, a first holding treatment is performed at 200℃ for 4 hours; then, the temperature is increased to 750℃ at a rate of 3℃ / min for a second holding treatment for 24 hours. After the second holding treatment, the product is pulverized to obtain a second intermediate product. The dew point temperature of the pulverization environment is ≤-40℃. The D100 particle size of the second intermediate product is 26.24μm, the D50 particle size is 4.85μm, and the D10 particle size is 0.371μm.

[0211] S3, 98.91g of linear thermoplastic phenolic resin (Novolac type) was dissolved in anhydrous ethanol, and the insoluble matter was removed by filtration to obtain a homogeneous and stable carbon-coated solution; the concentration of linear thermoplastic phenolic resin in the carbon-coated solution was 0.05g / mL; the second intermediate product was added to a high-speed mixer, and nitrogen gas was introduced for gas replacement to control the oxygen content in the high-speed mixer to be below 50ppm. The stirring was started at a speed of 1200r / min, and the carbon-coated solution was sprayed into the high-speed mixer through an atomizing nozzle using nitrogen as a pressure source. The mass ratio of carbon-coated solution to the second intermediate product was 6.4:100; after all the carbon-coated solution was sprayed in, the stirring speed was adjusted to 700r / min, and the temperature inside the high-speed mixer was adjusted to 100℃ through heat transfer oil. The mixture was stirred and dried for 2h to obtain the carbon-coated intermediate; nitrogen gas was continuously introduced for replacement throughout the stirring and drying process to remove organic solvents.

[0212] S4. The carbon-coated intermediate is put into a vacuum sintering furnace and sintered in an inert atmosphere (nitrogen with a purity greater than 99.99%). First, it is held at 110℃ for 2 hours, and then the temperature is increased to 680℃ at 5℃ / min and held for 4 hours. The oxygen content in the furnace is ≤20ppm throughout the sintering process. The sintered material is then dispersed by airflow pulverization to obtain the lithium-rich lithium iron ferrite supplement.

[0213] Example 13

[0214] One embodiment of the lithium-rich lithium iron ferrite supplement agent of this application differs from that of Example 1 in the following ways:

[0215] In S1, aluminum oxide replaces α-Al(OH)3, with an aluminum oxide mass of 87.91 g, ferric oxide mass of 744.84 g, and a lithium to iron molar ratio of 6.01.

[0216] In S2, the first insulation treatment is carried out at 140°C.

[0217] Comparative Example 1

[0218] This application presents a comparative example of a lithium-rich lithium iron ferrite supplement. The difference between the preparation method of the lithium-rich lithium iron ferrite supplement described in this comparative example and that of Example 1 is as follows:

[0219] In S1, the mass of ferric oxide is 802.13 g, the mass of α-Al(OH)3 is 15.64 g, and the molar ratio of lithium to iron is 5.61.

[0220] Comparative Example 2

[0221] This application presents a comparative example of a lithium-rich lithium iron ferrite supplement. The difference between the preparation method of the lithium-rich lithium iron ferrite supplement described in this comparative example and that of Example 1 is as follows:

[0222] In S1, the mass of ferric oxide is 491.1 g, the mass of α-Al(OH)3 is 312.81 g, and the molar ratio of lithium to iron is 9.17.

[0223] Comparative Example 3

[0224] This application presents a comparative example of a lithium-rich lithium iron ferrite supplement. The difference between the preparation method of the lithium-rich lithium iron ferrite supplement described in this comparative example and that of Example 6 is as follows:

[0225] In S2, the calcination treatment method is as follows: the temperature is increased from room temperature (25℃) to 650℃ at a heating rate of 4℃ / min, and held for 36 hours.

[0226] Comparative Example 4

[0227] This application presents a comparative example of a lithium-rich lithium iron ferrite supplement. The difference between the preparation method of the lithium-rich lithium iron ferrite supplement described in this comparative example and that of Example 1 is as follows:

[0228] In S1, without the addition of α-Al(OH)3 and monoclinic zirconium oxide, 821.78 g of lithium oxide and 818.5 g of ferric oxide were mixed to obtain a mixture; wherein the molar ratio of lithium to iron was 5.5.

[0229] Comparative Example 5

[0230] This application presents a comparative example of a lithium-rich lithium iron ferrite supplement. The difference between the preparation method of the lithium-rich lithium iron ferrite supplement described in this comparative example and that of Example 1 is as follows:

[0231] In S1, without adding an aluminum source, 821.78g of lithium oxide, 818.5g of ferric oxide and 92.57g of monoclinic zirconium oxide were mixed to obtain a mixture; wherein the molar ratio of lithium to iron was 5.5.

[0232] II. Testing Methods

[0233] 1. Mass fraction of aluminum and zirconium in lithium iron ferrite supplement: Mass fraction of metal elements was determined by ICP test.

[0234] 2. Mass fraction of carbon in lithium iron ferrite supplement: The mass fraction of carbon was tested using an infrared carbon-sulfur analyzer.

[0235] 3. Carbon coating rate in lithium iron ferrite supplement: The morphology and size of the material are observed by scanning electron microscopy (SEM). The area S1 of the primary particle outline and the area S2 of the carbon coating layer shown in the primary particle outline are obtained. The percentage of S2 to S1 is calculated. Data from fifty random particles are tested and the average value is taken.

[0236] 4. Mass fraction of the second phase in lithium-rich lithium iron ferrite supplement: The types of phases are determined by XRD, and the proportion of the second phase in the lithium-rich lithium iron ferrite supplement is calculated. The proportion of the second phase in the lithium-rich lithium iron ferrite supplement is determined by the ratio of the highest diffraction peak of the non-lithium iron ferrite phase to the highest diffraction peak of the lithium iron ferrite phase. The sum of these ratios for all non-lithium iron ferrite phases is taken as the proportion of the second phase in the lithium-rich lithium iron ferrite supplement.

[0237] 5. Mass fraction of elemental iron in lithium iron ferrite supplement: ICP test was performed according to GB / T 37167-2018.

[0238] 6. Types of phases in lithium iron ferrite supplements: The types of phases are determined by XRD.

[0239] 7. D50 particle size of lithium iron phosphate supplement: The D50 particle size (median volume diameter) was measured by laser diffraction using a laser particle size analyzer.

[0240] 8. Residual alkali content in lithium iron phosphate supplement: The test shall be conducted in accordance with the method specified in GB / T 41704—2022 "Test Methods for Cathode Materials of Lithium-ion Batteries: Determination of Magnetic Foreign Matter Content and Residual Alkali Content".

[0241] 9. Initial charge specific capacity of lithium iron phosphate supplement: The test was conducted in accordance with GB / T 45327-2025 at 25℃ and 5% relative humidity. The battery was charged to 4.2V at a current density of 0.05C and then charged to 0.01C at a constant voltage to obtain the initial charge specific capacity C1.

[0242] 10. Air stability of lithium-rich lithium iron ferrite supplement: The lithium-rich lithium iron ferrite supplement was placed in an environment of 25°C and 25% relative humidity. After 24 hours and 48 hours of storage, the initial charge specific capacity C2 and C3 of the lithium-rich lithium iron ferrite supplement were tested using the method described in test method 9 above. The initial charge specific capacity decay rate R of the lithium-rich lithium iron ferrite supplement after 24 hours and 48 hours of storage was calculated. 24h and R 28h R 24h =(C1-C2) / C1, R 48h =(C1-C3) / C1.

[0243] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0244] Tables 1 and 2 show the performance test results of lithium-rich lithium iron ferrite supplements in the examples and comparative examples.

[0245] Table 1

[0246]

[0247] Table 2

[0248]

[0249] Figure 1 The XRD patterns of lithium-rich lithium iron ferrite supplements in Example 1 and Comparative Example 4 are shown. As can be seen from the figures, all diffraction peaks observed after aluminum doping shifted towards the higher 2θ angle, indicating a reduction in unit cell volume. In addition, no other impurity peaks appeared, indicating that the lithium-rich lithium iron ferrite supplements in this application have high purity and low second phase content.

[0250] As shown in Tables 1 and 2, the mass fraction of aluminum in the lithium iron phosphate supplement in the embodiments of this application is 1.57% to 5.5%, and the content of elemental iron is ≤950ppm; the above-mentioned supplement has good electrochemical performance, with a residual alkali content of less than 0.5%, and an initial charge specific capacity of more than 790mAh / g. 24h <8%, R48h <15%, exhibiting good storage stability and lithium replenishment performance.

[0251] In Comparative Example 1, the aluminum content in the lithium-rich lithium iron phosphate supplement was less than 1.57%, resulting in significantly lower storage stability compared to the examples. In Comparative Example 2, the aluminum content in the lithium-rich lithium iron phosphate supplement was too high, leading to a significantly lower initial charge specific capacity and poorer lithium replenishment effect compared to the examples. In Comparative Example 3, the elemental iron content in the lithium-rich lithium iron phosphate supplement was high, resulting in significantly lower stability compared to the examples. In Comparative Examples 4 and 5, the lithium-rich lithium iron phosphate supplements contained no aluminum, resulting in extremely poor storage stability.

[0252] Furthermore, comparing the performance test results of Example 9 with those of Examples 1, 6, and 8, it can be seen that the lithium iron ferrite supplement doped with zirconium exhibits better specific capacity and storage stability. Comparing the performance test results of Example 10 with those of Examples 1, 6, and 8, it can be seen that the lithium iron ferrite supplement doped with zirconium, especially when the mass fraction of zirconium in the supplement is 0.2% to 2.5%, exhibits better specific capacity and storage stability.

[0253] In addition, comparing the performance test results of Examples 2-3 with Examples 1, 6, and 8, it can be seen that when the mass fraction of aluminum in the lithium-rich lithium iron ferrite supplement is 2.0%~3.2%, the lithium-rich lithium iron ferrite supplement exhibits better lithium replenishment performance and storage stability. Furthermore, comparing the performance test results of Examples 4-5 with Examples 1, 6, and 8, it can be seen that when zirconium is doped in the lithium-rich lithium iron ferrite supplement, and the mass fraction of zirconium is 1%~2%, the lithium-rich lithium iron ferrite supplement exhibits both good lithium replenishment performance and storage stability. Comparing the performance test results of Example 7 with Examples 1 and 8, it can be seen that when the mass fraction of carbon in the lithium-rich lithium iron ferrite supplement is 2.0%~3.5%, its storage stability is significantly higher.

[0254] 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-rich lithium iron phosphate lithium supplement, characterized in that, The lithium-rich lithium iron ferrite supplement includes a core and a carbon coating layer located on at least a portion of the surface of the core. The core comprises aluminum-doped lithium iron ferrite, and the mass fraction of aluminum in the lithium-rich lithium iron ferrite supplement is 1.57% to 5.5%, and the mass fraction of elemental iron in the lithium-rich lithium iron ferrite supplement is ≤950ppm.

2. The lithium-rich lithium iron phosphate supplement agent according to claim 1, characterized in that, At least one of the following conditions must be met: (1) The mass fraction of the second phase in the lithium-rich lithium iron phosphate supplement is ≤5%; (2) The mass fraction of carbon in the lithium iron ferrite supplement is 1.0%~3.5%; (3) The carbon coating rate of the lithium iron ferrite supplement is ≥96%; (4) The D50 particle size of the lithium iron ferrite supplement meets the following requirements: 2μm≤D50≤5.5μm; (5) The molar ratio of lithium to iron in the lithium-rich lithium iron oxide supplement is (6.0~8.03):

1.

3. The lithium-rich lithium iron ferrite supplement agent according to claim 1 or 2, characterized in that, The aluminum-doped lithium iron ferrite also contains zirconium, and the mass fraction of zirconium in the lithium iron ferrite supplement is 0.2% to 2.5%.

4. The lithium-rich lithium iron phosphate supplement agent according to claim 1 or 2, characterized in that, The lithium-rich lithium iron ferrite supplement exhibits a first-charge capacity decay rate of less than 7% after being placed in an environment with a temperature of 20℃~30℃ and a relative humidity of 20%~30% for 24 hours, and a first-charge capacity decay rate of less than 15% after being placed in an environment with a relative humidity of 0.05C for 48 hours.

5. A method for preparing a lithium-rich lithium iron ferrite lithium supplement, characterized in that, Includes the following steps: A lithium source, an iron source, and a doping source are mixed to obtain a mixture; the doping source includes an aluminum source. The mixture is subjected to calcination treatment, the calcination treatment comprising: subjecting the mixture to a first heat treatment to obtain a first intermediate product; the temperature of the first heat treatment is 100℃~300℃, and the time of the first heat treatment is 4h~8h; raising the temperature to 600℃~800℃, subjecting the first intermediate product to a second heat treatment to obtain a second intermediate product; the time of the second heat treatment is 24h~48h. The second intermediate product is subjected to carbon coating treatment to obtain the lithium-rich lithium iron ferrite supplement. The lithium-rich lithium iron ferrite supplement includes a core and a carbon coating layer located on at least a portion of the surface of the core. The core comprises aluminum-doped lithium iron ferrite, and the mass fraction of aluminum in the lithium-rich lithium iron ferrite supplement is 1.57% to 5.5%.

6. The method for preparing lithium-rich lithium iron ferrite supplementary agent according to claim 5, characterized in that, The doping source also includes a zirconium source, and the mass fraction of zirconium in the lithium iron ferrite supplement is 0.2% to 2.5%.

7. The method for preparing lithium-rich lithium iron ferrite supplementary agent according to claim 5 or 6, characterized in that, At least one of the following conditions must be met: (1) The lithium source includes at least one of lithium oxide, lithium hydroxide, lithium carbonate and lithium oxalate; (2) The iron source includes at least one of ferric oxide, iron(II) oxide, ferric hydroxide and ferric hydroxide; (3) The aluminum source includes at least one of aluminum oxide, γ-Al(OH)3, α-Al(OH)3, AlO(OH), aluminum isopropoxide, aluminum nitrate, aluminum acetate and aluminum oxalate; (4) The doping source includes a zirconium source, which includes at least one of tetragonal zirconium oxide, monoclinic zirconium oxide, zirconium hydroxide and zirconium isopropoxide; (5) The average particle size of the aluminum source is <5 μm; (6) The doping source includes a zirconium source, and the average particle size of the zirconium source is <5 μm.

8. The method for preparing lithium-rich lithium iron ferrite supplementary agent according to claim 5 or 6, characterized in that, At least one of the following conditions must be met: (1) The mixing speed is 400 r / min to 600 r / min, and the mixing time is 0.5 h to 1.5 h; (2) The aluminum source is selected from at least one of aluminum citrate, aluminum acetate, aluminum oxalate and aluminum hydroxide. The temperature of the first heat preservation treatment is 200℃~300℃ and the time of the first heat preservation treatment is 4h~8h. The temperature of the second heat preservation treatment is 600℃~750℃ and the time of the second heat preservation treatment is 24h~48h. (3) The aluminum source is alumina, the temperature of the first heat preservation treatment is 100℃~200℃, and the time of the first heat preservation treatment is 4h~8h; the temperature of the second heat preservation treatment is 650℃~800℃, and the time of the second heat preservation treatment is 24h~40h; (4) The calcination treatment is carried out in a nitrogen or inert atmosphere; (5) After the second heat preservation treatment is completed, the product obtained by the second heat preservation treatment is pulverized to obtain the second intermediate product; the dew point temperature of the pulverization treatment environment is ≤-40℃; the D100 of the second intermediate product is ≤35μm, the D50 of the second intermediate product is ≤15μm, and the D10 of the second intermediate product is ≥0.1μm. (6) The carbon coating treatment method includes: Phenolic resin is dissolved in an organic solvent to obtain a carbon-coated solution; The carbon-coated liquid is mixed with the second intermediate product and dried to obtain a carbon-coated intermediate. The carbon-coated intermediate is sintered and pulverized to obtain the lithium-rich lithium iron ferrite supplement.

9. A positive electrode sheet, characterized in that, It includes a current collector and a positive electrode active material located on one or both sides of the current collector, a conductive agent, a binder, and a lithium supplement agent; the lithium supplement agent includes the lithium-rich lithium iron ferrite supplement agent according to any one of claims 1 to 4, or the lithium-rich lithium iron ferrite supplement agent prepared by the preparation method of the lithium-rich lithium iron ferrite supplement agent according to any one of claims 5 to 8.

10. A secondary battery, characterized in that, It includes a positive electrode, a negative electrode, and a separator, wherein the positive electrode is the positive electrode as described in claim 9.

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