Lithium manganese iron phosphate material, preparation method therefor, positive electrode sheet, and lithium-ion battery

Abstract

The present application relates to the field of lithium-ion batteries, and provides a lithium manganese iron phosphate material, a preparation method therefor, a positive electrode sheet, and a lithium-ion battery, wherein the lithium manganese iron phosphate material has a molecular formula of LixMnyFe1-y(PO4)z, where 0.7≤x≤1.1, 0.15≤y<1, 0.9≤z≤1.05, a manganese dissolution of 25-33 ppm, a powder resistivity of 12-28 Ω·cm, and a magnetic substance amount of 0.05-0.12 ppm. The lithium manganese iron phosphate material provided by the present application has a low powder resistivity, a small amount of magnetic substance, and a significantly reduced manganese dissolution, and when being used as a positive electrode active material for a lithium-ion battery, the corresponding lithium-ion battery can exhibit a higher charge-discharge rate and efficiency, thereby improving the electrical performance and safety stability of the lithium-ion battery.

Description

Lithium manganese iron phosphate materials, their preparation methods, positive electrode sheets and lithium-ion batteries Technical Field

[0001] This application relates to the field of lithium-ion battery technology, specifically to a lithium manganese iron phosphate material, its preparation method, positive electrode sheet, and lithium-ion battery. Background Technology

[0002] A lithium-ion battery is a rechargeable battery that uses lithium ions to store and release energy by moving between the positive and negative electrodes. Due to its high energy density, long lifespan, and low self-discharge rate, lithium-ion batteries are widely used in the field of new energy vehicles.

[0003] Currently, there are many types of cathode materials commonly used in lithium-ion batteries, mainly including lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, ternary materials, and lithium iron phosphate. Among them, phosphate-based lithium intercalation materials with an olivine-type crystal structure, such as LiMPO4 (M: Mn, Fe, Co, Ni), have become promising cathode materials for lithium-ion batteries. Lithium iron phosphate (LiFePO4) has a very stable charge-discharge platform, stable structure during charge-discharge processes, and is non-toxic, non-polluting, and can be used in high-temperature environments. Its raw materials are widely available, and it exhibits excellent electrochemical performance, making it the most widely used type of cathode material in lithium-ion batteries.

[0004] However, with the development of lithium iron phosphate, its performance improvement has encountered a bottleneck. Based on this, lithium manganese iron phosphate (LFP) cathode materials, formed by introducing manganese, have higher voltage plateaus and energy densities. However, the mainstream process for LFP is still the solid-state process, commonly using iron phosphate and manganese oxides as precursors. During preparation, the mixing and solid-state fusion between the two are poor, resulting in severe manganese and iron segregation. The resulting LFP has extremely poor conductivity, and its electrical performance is also poor in manganese-rich regions. Furthermore, during the precipitation reaction, the K+ of the intermediate product, ferrous manganese phosphate, is significantly reduced. sp The ferrous ions are relatively large, requiring a higher pH environment for precipitation. At higher pH values, the reaction system is prone to localized overconcentration, causing some ferrous ions to precipitate as hydroxides, resulting in impurities in the ferrous manganese phosphate phase and further affecting the phase purity of the final lithium manganese iron phosphate material. Furthermore, the aforementioned localized overconcentration during the preparation process also affects the nucleation and crystallization process of lithium manganese iron phosphate and its intermediates, consequently affecting the particle size distribution and size uniformity of the lithium manganese iron phosphate product, ultimately leading to poor electrochemical performance in lithium-ion battery electrodes.

[0005] In view of this, how to provide a lithium iron phosphate material with low manganese segregation, so that it can exhibit higher electrochemical performance when used as a positive electrode active material for lithium-ion batteries, is one of the important technical problems that need to be solved in this field.

[0006] Summary of the Invention

[0007] In view of the technical problems existing in the background art, this application provides a lithium manganese iron phosphate material, its preparation method, positive electrode sheet and lithium-ion battery, aiming to solve the technical problems of severe manganese dissolution and poor electrochemical performance of the active material, namely lithium manganese iron phosphate material, in the positive electrode sheet of lithium-ion batteries in the prior art.

[0008] In a first aspect, embodiments of this application provide a lithium manganese iron phosphate material, the core of which has the molecular formula Li. x Mn y Fe 1-y (PO4) z Among them, 0.7≤x≤1.1, 0.15≤y<1, 0.9≤z≤1.05, the manganese leaching amount of lithium manganese iron phosphate material is 25ppm~33ppm, the powder resistivity is 12Ω·cm~28Ω·cm, and the amount of magnetic material is 0.05ppm~0.12ppm.

[0009] The lithium manganese iron phosphate powder provided in this application has low resistivity, low magnetic content, and significantly reduced manganese dissolution. When used as a positive electrode active material for lithium-ion batteries, it enables the corresponding lithium-ion batteries to exhibit higher charging and discharging rates and efficiencies, thereby improving the electrical performance and safety stability of lithium-ion batteries. Furthermore, the core of this application is essentially a core-shell structure of lithium manganese iron phosphate, with manganese located on the outside of the core. Because the lithium migration distance on the outside is shorter and it can be better wetted by the electrolyte, the capacity of manganese can be better utilized. Correspondingly, the internal lithium iron phosphate has good conductivity, so it does not affect the overall capacity of the material, thus resulting in particularly excellent electrical performance of the obtained lithium manganese iron phosphate material.

[0010] In the preferred embodiment of this application, the lithium manganese iron phosphate material includes a core and a carbon layer disposed on the surface of the core; the D50 particle size of the lithium manganese iron phosphate material is 1.35 μm to 1.9 μm; and the mass fraction of the carbon layer is 1% to 3% based on 100% of the weight of the lithium manganese iron phosphate material.

[0011] The lithium manganese iron phosphate material provided in this application has a more uniform particle size, and the combination of the core phosphate and the outer carbon is better, thus exhibiting better electrochemical performance in lithium-ion batteries, resulting in higher electrical performance and better safety and stability of the lithium-ion batteries.

[0012] In some preferred embodiments, the core of the lithium manganese iron phosphate material has the molecular formula Li. x Mn y Fe 1-y (PO4) z In, 1≤(1-y) / y≤5.

[0013] The lithium manganese iron phosphate material provided in this application has a suitable ratio of manganese and iron elements, resulting in more coordinated electrochemical performance and thus enabling the corresponding lithium-ion battery to have better performance.

[0014] Secondly, embodiments of this application provide a method for preparing lithium manganese iron phosphate material. The method includes: mixing an iron source with a phosphoric acid solution to perform a first reaction, obtaining a ferrous dihydrogen phosphate solution; adding a manganese salt and a first alkali source to the ferrous dihydrogen phosphate solution, performing a second reaction, obtaining a ferrous manganese phosphate slurry; adding a lithium salt and a second alkali source to the ferrous manganese phosphate slurry, performing a third reaction; obtaining a first product after filtration and washing; obtaining a second product after slurrying and drying of the first product; and obtaining the lithium manganese iron phosphate material after calcination of the second product; wherein the first and second alkali sources are urea.

[0015] Compared to traditional solid-state methods, the above preparation method can achieve a core-shell structure of lithium manganese iron phosphate. Manganese is located on the outside of the core, where lithium migration distance is shorter and it can be better wetted by the electrolyte, thus better utilizing manganese capacity. The internal lithium iron phosphate, due to its good conductivity, does not affect its capacity utilization, resulting in excellent electrochemical performance of the lithium manganese iron phosphate material. Specifically, a ferrous dihydrogen phosphate solution is first obtained through a first reaction; in the second reaction, the slow decomposition of urea is used to gradually increase the pH of the reaction system, causing ferrous and manganese ions to precipitate successively in the form of phosphates and reducing the formation of ferrous hydroxide impurities, resulting in a more uniform mixture of manganese and iron; the third reaction further utilizes urea decomposition to further slowly increase the pH, causing lithium ions in the added lithium salt to precipitate further in the form of phosphates. The synergistic effect of each reaction step results in a more uniform mixture of lithium, manganese, iron, and phosphorus, yielding a lithium manganese iron phosphate material with better electrochemical performance.

[0016] In the preferred embodiment of this application, the molar ratio of the first alkali source to the ferrous dihydrogen phosphate solution is (2.2-2.5):1. This allows the urea, as the first alkali source, to better promote a gradual increase in the pH value of the reaction system when it decomposes to obtain ammonia and reacts with hydrogen ions in dihydrogen phosphate, thereby obtaining lithium manganese iron phosphate material with higher purity and better electrochemical performance. Preferably, the molar ratio of the second alkali source to the first alkali source is (0.3-0.6):1. This allows for better coordination in the amount of urea added to the reaction system twice, thereby better controlling the change in the pH value of the system and obtaining lithium manganese iron phosphate material with superior electrochemical performance.

[0017] In some preferred embodiments, the molar ratio of manganese in the manganese salt to the molar ratio of iron in the ferrous dihydrogen phosphate solution is (0.2-1):1, and the ratio of the sum of the molar amounts of manganese in the manganese salt and iron in the ferrous dihydrogen phosphate solution to the molar amount of lithium in the lithium salt is 1:(1.08-1.1), to form the aforementioned core-shell structure, resulting in a more stable lithium manganese iron phosphate material with better cycle performance in lithium-ion batteries. Preferably, the iron source is iron sheet and / or iron powder, the manganese salt is selected from one or more of manganese sulfate, manganese chloride, and manganese acetate, and the lithium salt is selected from one or more of lithium sulfate, lithium chloride, and lithium acetate. The use of the aforementioned divalent manganese and iron salts can effectively save costs, reduce the introduction of impurities and the generation of waste gas during subsequent calcination, and ultimately significantly improve the purity of the material, enabling it to exhibit more stable long-cycle characteristics in lithium-ion batteries.

[0018] In some preferred embodiments, the temperature of the first reaction is 50°C to 70°C, and the pH value of the ferrous dihydrogen phosphate solution is 2.0 to 2.5, thereby obtaining a ferrous dihydrogen phosphate solution with higher purity and improving its dispersibility to facilitate subsequent processes; preferably, the concentration of the phosphoric acid solution is 3 mol / L to 6 mol / L, so as to better dissolve the iron source and obtain a ferrous dihydrogen phosphate solution with better dispersibility, thereby promoting the improvement of particle size uniformity of the intermediate and final products.

[0019] In some preferred embodiments, the temperature of the second reaction is 95°C to 105°C, and the endpoint pH of the second reaction is 4.5 to 5.5, so as to obtain a more stable intermediate product with ferrous phosphate as the core and manganese phosphate as the shell. Preferably, the second reaction further includes continuing the reaction for 30 min to 60 min after reaching the endpoint pH of the second reaction, so as to make the co-precipitation reaction more complete, significantly improve the mixing uniformity of each element in the structure of the obtained intermediate product, as well as the particle size uniformity of the intermediate product itself, stabilize its chemical structure, and thus improve its electrochemical performance, especially reducing the dissolution of manganese and making its chemical performance more stable.

[0020] In some preferred embodiments, the second reaction is preferably carried out under conditions where the oxygen content is less than 100 ppm, thereby reducing the generation of oxygen-containing compound impurities during the reaction, improving the purity of the lithium manganese iron phosphate material, and ultimately making the lithium-ion battery prepared when it is used as a positive electrode active material have superior electrical performance.

[0021] In some preferred embodiments, the temperature of the third reaction is 100℃~110℃, and the endpoint pH value of the third reaction is 7.1~7.5, so as to form a more uniform lithium phosphate on the surface of ferrous manganese phosphate, thereby more effectively inhibiting the dissolution of manganese in subsequent applications and improving its electrochemical performance, especially stability. Preferably, the third reaction also includes continuing the reaction for 30min~60min after reaching the endpoint pH value of the third reaction. Preferably, the third reaction is carried out under conditions where the oxygen content is less than 50ppm, thereby further reducing impurities and improving the purity of the obtained lithium manganese iron phosphate material.

[0022] In some preferred embodiments, the preparation method of lithium manganese iron phosphate material further includes: slurrying a first product to obtain a first product slurry, adding a carbon source to the first product slurry, and spray drying to obtain a second product. This application further utilizes a carbon source to coat the surface of the obtained lithium manganese iron phosphate material to further improve its structural stability and conductivity.

[0023] In some preferred embodiments, the solid content of the first product slurry is 30%–40%, the particle size of the second product is 5 μm–8 μm, and the water mass fraction in the second product is less than or equal to 1%, thereby making the carbon layer coating more uniform and controlling the particle size uniformity of the resulting material, resulting in more stable electrochemical performance. Preferably, the carbon source is a water-soluble carbon source, more preferably one or more of water-soluble starch, sucrose, glucose, or polyethylene glycol. These carbon sources can cooperate with the above-mentioned lithium manganese iron phosphate core and form a more uniform and stable carbon layer on its surface, thereby effectively improving its electrochemical performance.

[0024] In some preferred embodiments, calcination is carried out under conditions of oxygen content less than 5 ppm and humidity less than 3%, with a holding temperature of 700℃ to 750℃ and a holding time of 8h to 15h. Calcination under these conditions results in a lower content of magnetic materials and superior electrochemical performance in the obtained lithium manganese iron phosphate product.

[0025] Thirdly, embodiments of this application provide a positive electrode sheet comprising the aforementioned lithium manganese iron phosphate material, or lithium manganese iron phosphate material prepared by the aforementioned method for preparing lithium manganese iron phosphate material. This positive electrode sheet contains the aforementioned lithium manganese iron phosphate material with low manganese leaching, low powder resistivity, and low magnetic properties, thus exhibiting high electrochemical performance.

[0026] Fourthly, embodiments of this application provide a lithium-ion battery, including the positive electrode sheet as described above. Because the lithium manganese iron phosphate material obtained in this application possesses both excellent electrochemical performance and structural stability, when it is used as a positive electrode sheet component in a lithium-ion battery, the resulting lithium-ion battery also exhibits comprehensively improved electrochemical performance, especially in terms of initial charge capacity, initial discharge capacity, and initial discharge efficiency.

[0027] 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

[0028] To more clearly illustrate the technical solution 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.

[0029] Figure 1 is a SEM image of the lithium iron phosphate cathode material obtained in Example 1 of this application;

[0030] Figure 2 is the XRD pattern of the lithium iron phosphate cathode material obtained in Example 1 of this application;

[0031] Figure 3 is an EDS result diagram of the elemental composition of the lithium manganese iron phosphate cathode material obtained in Example 1 of this application;

[0032] Figure 4 is a SEM image of the lithium iron phosphate cathode material obtained in Example 2 of this application;

[0033] Figure 5 shows the XRD patterns of the lithium iron phosphate cathode materials obtained in Examples 2 and 3 of this application;

[0034] Figure 6 is a SEM image of the lithium iron phosphate cathode material obtained in Example 3 of this application;

[0035] Figure 7 is a process flow diagram of the preparation method of lithium manganese iron phosphate material provided in the embodiments of this application. Detailed Implementation

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] As described in the background art, the active material in the positive electrode of lithium-ion batteries, namely lithium manganese iron phosphate, suffers from severe manganese leaching and poor electrochemical performance.

[0042] To address the aforementioned technical problems, in a first aspect, embodiments of this application provide a lithium manganese iron phosphate material, the core of which has the molecular formula Li. x Mn y Fe 1-y (PO4) z Among them, 0.7≤x≤1.1, 0.15≤y<1, 0.9≤z≤1.05; the manganese leaching amount of lithium manganese iron phosphate material is 25ppm~33ppm, the powder resistivity is 12Ω·cm~28Ω·cm, and the amount of magnetic material is 0.05ppm~0.12ppm.

[0043] This lithium manganese iron phosphate powder has low resistivity. When used as a positive electrode active material in lithium-ion batteries, it enables the corresponding lithium-ion batteries to exhibit higher charge and discharge rates and efficiencies, thereby improving the overall performance of lithium-ion batteries. Simultaneously, the resulting lithium manganese iron phosphate material contains a lower amount of magnetic material, which is beneficial to the safety performance of lithium manganese iron phosphate, resulting in better safety and lower self-discharge in the final battery cell. Specifically, the calcination process of lithium manganese iron phosphate generates reducing gases, which reduce manganese and iron elements in the material. When reduced, the electron configuration of iron and manganese changes. The number of unpaired electrons in iron increases, enhancing magnetism; while the number of unpaired electrons in manganese is relatively lower, its magnetism is still enhanced. When the content of these two magnetic materials increases, they become more susceptible to erosion by the electrolyte, leading to metal deposition under electrochemical action. This causes black spots on the separator, affecting the cycle performance, safety performance, and self-discharge of the battery cell.

[0044] The lithium manganese iron phosphate material provided in this application has a manganese leaching amount of 25ppm to 33ppm, which is significantly reduced. This is beneficial for maintaining the structural stability of the cathode material, thereby improving the cycle life of the battery. It also reduces the phase transition of the cathode material during charge and discharge, thus improving the battery's capacity retention rate. This means that the battery experiences less capacity loss after prolonged use and can maintain a high energy density. In particular, manganese leaching can damage the crystal structure of the material, affecting the normal insertion and extraction of lithium ions during charge and discharge. Therefore, reducing manganese leaching helps the cathode material maintain its structural integrity, thereby improving its thermal stability. Thermal stability is crucial for battery safety, especially at high temperatures. Batteries with good thermal stability are less prone to thermal runaway and have better safety performance. Simultaneously, manganese leaching can cause manganese to deposit on the separator under electrochemical action, making it easier for manganese to puncture the separator and cause micro-short circuits in the cell.

[0045] In the preferred embodiment of this application, the lithium manganese iron phosphate material includes a core and a carbon layer disposed on the surface of the core; the D50 of the lithium manganese iron phosphate material is 1.35 μm to 1.9 μm; and the mass fraction of the carbon layer is 1% to 3% based on 100% of the weight of the lithium manganese iron phosphate material. The lithium manganese iron phosphate material provided by this application has a more uniform particle size, and the combination of the core phosphate and the outer carbon layer is better, thereby exhibiting better electrochemical performance in lithium-ion batteries, resulting in higher electrical performance and better safety and stability of the obtained lithium-ion batteries.

[0046] In some preferred embodiments, the molecular formula of lithium manganese iron phosphate material is Li x Mn y Fe 1-y (PO4) zIn this application, 1 ≤ (1-y) / y ≤ 5. The lithium manganese iron phosphate material provided by this application has a suitable ratio of elements, resulting in more coordinated electrochemical performance and thus enabling the corresponding lithium-ion battery to exhibit better performance.

[0047] In particular, the phosphate core provided in this application is essentially a core-shell structure of lithium manganese iron phosphate, with manganese located on the outside of the core. Because the lithium on the outside has a shorter migration distance and can be better wetted by the electrolyte, the capacity of manganese can be better utilized. Correspondingly, the lithium iron phosphate inside has good conductivity, so it does not affect the overall capacity of the material, thus making the electrical performance of the resulting lithium manganese iron phosphate material particularly excellent.

[0048] Secondly, embodiments of this application provide a method for preparing lithium manganese iron phosphate material. The method includes: mixing an iron source with a phosphoric acid solution to perform a first reaction, obtaining a ferrous dihydrogen phosphate solution; adding a manganese salt and a first alkali source to the ferrous dihydrogen phosphate solution, performing a second reaction, obtaining a ferrous manganese phosphate slurry; adding a lithium salt and a second alkali source to the ferrous manganese phosphate slurry, performing a third reaction; obtaining a first product after filtration and washing; obtaining a second product after slurrying and drying of the first product; and obtaining the lithium manganese iron phosphate material after calcination of the second product; wherein the first and second alkali sources are urea.

[0049] The preparation method provided in this application first prepares ferrous manganese phosphate using a homogeneous precipitation method. Specifically, ferrous dihydrogen phosphate is prepared by adding phosphoric acid to iron. Then, ferrous salt and manganese salt are mixed sequentially, with urea added as a supplement. During the reaction, urea decomposes to produce ammonia, which reacts with hydrogen ions from dihydrogen phosphate, causing the pH to gradually increase, preventing localized overconcentration, and ultimately yielding a purer phase and more uniform particle size of ferrous manganese phosphate. In this case, ferrous phosphate forms the core, and manganese phosphate grows on the surface of the ferrous phosphate. Then, lithium salt and urea are mixed to form uniform lithium phosphate on the surface of the ferrous manganese phosphate. The reaction process utilizes the different pH characteristics of manganese, ferrous, and lithium phosphate precipitation. Initially, at a lower pH, the slow decomposition of urea gradually raises the pH, causing iron and manganese to precipitate as phosphates. Then, lithium salt is added, followed by more urea and continued heating to further raise the pH, allowing lithium to precipitate as phosphates as well. Due to homogeneous precipitation and the rate of urea decomposition, heterogeneous nucleation preferentially leads to the precipitation of the precipitate, forming a homogeneous co-precipitate of lithium, manganese, iron, and phosphorus. This is followed by slurrying, drying, and calcination to obtain lithium manganese iron phosphate material with low manganese leaching, low powder resistivity, and low magnetic material content. Its electrochemical performance is superior, significantly improving battery performance when applied to lithium-ion batteries.

[0050] Compared to traditional solid-state methods, the above preparation method can achieve a core-shell structure of lithium manganese iron phosphate. The outer manganese layer, due to the shorter migration distance of lithium and better wetting by the electrolyte, can better utilize its capacity. The inner lithium iron phosphate layer, with its good conductivity, does not affect its capacity utilization, resulting in excellent electrical performance. Simultaneously, the resulting material exhibits uniform elemental distribution, particularly uniform manganese and iron element arrangement, avoiding manganese segregation. This leads to higher capacity, as manganese segregation can cause lithium manganese phosphate aggregation. Since lithium manganese phosphate has poor ionic conductivity, lithium cannot be extracted, resulting in reduced charging and discharging capacity. The preparation method provided in this application, with its uniform mixing of manganese and iron, avoids this situation, resulting in a higher capacity lithium-ion battery with superior cycle performance.

[0051] Furthermore, the method provided in this application eliminates the need for mixing and grinding processes, enabling the uniform mixing of multiple elements such as manganese, iron, phosphorus, and lithium. Specifically, the preparation method provided in this application successfully achieves uniform mixing of elements in lithium manganese iron phosphate material through two sequential co-precipitation steps, namely the second reaction and the third reaction, resulting in a cathode material with a more complete and stable crystal structure. Conventional solid-state methods require processes such as mixing, slurry formation, grinding, and nano-sizing, which result in a long process flow and poor mixing effect. In comparison, the preparation method of lithium manganese iron phosphate material provided in this application has a simpler process and better mixing effect. Meanwhile, the preparation method of lithium manganese iron phosphate material provided in this application eliminates oxidation, drying, grinding and other processes compared to traditional processes. According to actual calculations, the cost of these process steps is about 4,000 yuan. Then, adding the cost reduction of oxidant and carbon reduction, it can be roughly estimated that the cost reduction per ton of product (lithium manganese iron phosphate) compared to the traditional solid-phase method is 4,300 yuan / ton. Based on a production line with an annual output of 10,000 tons, the cost savings reach 43 million yuan, which is of great economic value.

[0052] The cathode material obtained by calcination in this application is then crushed, sieved, and packaged to finally obtain a cathode material that can be used as a product in practical applications.

[0053] In the preferred embodiment of this application, the molar ratio of the first alkali source to the ferrous dihydrogen phosphate solution is (2.2-2.5):1. This allows the urea, acting as the first alkali source, to better promote a gradual increase in pH during the reaction of ammonia with hydrogen ions in the dihydrogen phosphate ion, resulting in a lithium manganese iron phosphate material with higher purity and better electrochemical performance. Through extensive experimentation, the inventors further optimized the molar ratio of the second alkali source to the first alkali source to be (0.3-0.6):1. This allows for better coordination of the amounts of urea added to the reaction system in both steps, thereby better controlling the pH changes and obtaining a lithium manganese iron phosphate material with even superior electrochemical performance.

[0054] In some preferred embodiments, to obtain a higher purity ferrous dihydrogen phosphate solution and improve its dispersibility for subsequent processes, the preferred temperature for the first reaction is 50°C to 70°C, and the pH value of the ferrous dihydrogen phosphate solution is 2.0 to 2.5. Simultaneously, the preferred concentration of the phosphoric acid solution is 3 mol / L to 6 mol / L to facilitate better dissolution of the iron source, resulting in a better-dispersed ferrous dihydrogen phosphate solution, thereby promoting improved particle size uniformity of the intermediate and final products.

[0055] In some preferred embodiments, the temperature of the second reaction is 95°C to 105°C, and the endpoint pH of the second reaction is 4.5 to 5.5, in order to obtain a more stable intermediate product with ferrous phosphate as the core and manganese phosphate as the shell. Furthermore, through extensive experimentation, the inventors have optimized the second reaction by continuing the reaction for 30 to 60 minutes after reaching the endpoint pH, in order to significantly improve the particle size uniformity of the obtained intermediate product, stabilize its chemical structure, and thus improve its electrochemical performance, especially by reducing manganese dissolution and making its chemical properties more stable.

[0056] It should be further clarified that, in this application, the endpoint pH value means that the reaction is considered essentially complete once the reaction system reaches a specific pH value. As for reactions that continue for a period of time after reaching their endpoint pH value, these reactions are still carried out at the same temperature and stirring speed.

[0057] In some preferred embodiments, to ensure a more complete reaction, the stirring speed of the second reaction is preferably 60 r / min to 120 r / min, thereby obtaining intermediate products with higher particle size uniformity and effectively optimizing the performance of the final lithium manganese iron phosphate material. Preferably, the second reaction is carried out under conditions where the oxygen content is less than 100 ppm, thereby reducing the generation of oxygen-containing compound impurities during the reaction, improving the purity of the lithium manganese iron phosphate material, and ultimately making the lithium-ion battery prepared using it as a positive electrode active material have superior electrical performance.

[0058] In some preferred embodiments, the temperature of the third reaction is 100°C to 110°C, and the final pH value of the third reaction is 7.1 to 7.5. The inventors optimized these experimental conditions through extensive experimentation to facilitate the formation of a more uniform lithium phosphate layer on the surface of ferrous manganese phosphate, thereby more effectively suppressing the dissolution of manganese during subsequent applications and improving its electrochemical performance, especially its stability. To ensure a more complete lithium phosphate formation reaction, the third reaction preferably includes continuing the reaction for 30 to 60 minutes after reaching the final pH value. More preferably, the third reaction is carried out under conditions where the oxygen content is less than 50 ppm, thereby further reducing impurities and improving the purity of the resulting lithium manganese iron phosphate material.

[0059] In some preferred embodiments, to better form the core-shell structure and obtain a more stable lithium manganese iron phosphate material with better cycle performance in lithium-ion batteries, the inventors optimized the addition ratio of each element in the core and shell through extensive experiments. Specifically, the preferred ratio of the molar amount of manganese in the manganese salt to the molar amount of iron in the ferrous dihydrogen phosphate solution is (0.2-1):1, and the ratio of the sum of the molar amounts of manganese in the manganese salt and iron in the ferrous dihydrogen phosphate solution to the molar amount of lithium in the lithium salt is 1:(1.08-1.1). Preferably, the iron source is iron sheet and / or iron powder, the manganese salt is selected from one or more of manganese sulfate, manganese chloride, and manganese acetate, and the lithium salt is selected from one or more of lithium sulfate, lithium chloride, and lithium acetate. Through extensive experimentation, the inventors have selected divalent manganese salts and ferrous salts as the preferred materials. Unlike traditional processes, this eliminates the need to add oxidants during the preparation of iron phosphate. Furthermore, the final calcination process does not require additional carbon to reduce the high-valence metal ions. This effectively saves costs, reduces the introduction of impurities and the generation of waste gas during subsequent calcination, and ultimately significantly improves the purity of the material, enabling it to exhibit more stable long-cycle characteristics in lithium-ion batteries.

[0060] In some preferred embodiments, the preparation method of lithium manganese iron phosphate material further includes: slurrying a first product to obtain a first product slurry, adding a carbon source to the first product slurry, and drying to obtain a second product. This application further utilizes a carbon source to coat the surface of the obtained lithium manganese iron phosphate material to further improve its structural stability and conductivity.

[0061] Furthermore, spray drying is preferred.

[0062] In some preferred embodiments, to achieve a more uniform carbon layer coating and improve the particle size uniformity of the resulting material, thereby exhibiting more stable electrochemical performance, the solid content of the first product slurry is preferably 30%–40%, the particle size of the second product is 5 μm–8 μm, and the mass fraction of water in the second product is less than or equal to 1%. Here, the solid content refers to the mass fraction of insoluble solid particles in the first product slurry. Through extensive experimentation, the inventors have further preferred a water-soluble carbon source, more preferably one or more of water-soluble starch, sucrose, glucose, or polyethylene glycol, so that the carbon source can cooperate with the aforementioned lithium manganese iron phosphate core and form a more uniform and stable carbon layer on its surface, thereby effectively improving its electrochemical performance.

[0063] In some preferred embodiments, calcination is carried out under conditions of oxygen content less than 5 ppm, CO content less than 50 ppm, hydrogen content less than 100 ppm, and humidity less than 3%, with a heating rate of 300℃ / h to 400℃ / h, a holding temperature of 700℃ to 750℃, and a holding time of 8h to 15h. The inventors optimized the calcination conditions through extensive experiments to obtain the above-mentioned scheme. Under these conditions, calcination reduces the generation of additional waste gases (such as those from lithium salt decomposition and redox reactions) beyond the crystal water and carbon source decomposition of the material itself. This results in a more stable atmosphere inside the furnace (because the main components of the waste gas are H2O and CO2, which react with incandescent carbon at high temperatures to produce gases such as H2 and CO, thus consuming carbon and resulting in an incomplete carbon coating layer. Furthermore, the strong reducing properties of the produced H2 and CO gases may reduce iron to its elemental state, thereby generating magnetic materials and causing safety risks such as micro-short circuits and dendrite formation in the battery). This results in a lower content of magnetic materials and superior electrochemical performance in the resulting lithium manganese iron phosphate product.

[0064] Thirdly, embodiments of this application provide a positive electrode sheet, which includes the above-mentioned lithium manganese iron phosphate material, or lithium manganese iron phosphate material prepared by the above-mentioned method for preparing lithium manganese iron phosphate material.

[0065] In this embodiment, the positive electrode sheet contains lithium manganese iron phosphate material with low manganese leaching, low powder resistivity, and low magnetic properties, thus exhibiting high electrochemical performance.

[0066] Fourthly, embodiments of this application provide a lithium-ion battery, including the positive electrode sheet as described above.

[0067] In this embodiment, since the lithium manganese iron phosphate material obtained in this application has both good electrochemical performance and structural stability, when it is used as a positive electrode component in a lithium-ion battery, the resulting lithium-ion battery also has comprehensively improved electrochemical performance, especially the first charge-discharge capacity and the first discharge efficiency.

[0068] I. Preparation Method

[0069] Example 1

[0070] Preparation of a lithium manganese iron phosphate material:

[0071] 1. Electrolytic iron powder was added to a 4.5 mol / L phosphoric acid solution, and the reaction was carried out until the final pH was 2.3. The reaction temperature was 60℃. After filtration, a ferrous dihydrogen phosphate solution was obtained.

[0072] 2. Add manganese acetate to the ferrous dihydrogen phosphate solution, with the molar ratio of iron in the ferrous dihydrogen phosphate solution to manganese in the added manganese acetate being 1:1. Then add urea, with the molar amount of urea being 2.4 times the molar amount of ferrous dihydrogen phosphate. Stir the reaction in a sealed reactor under nitrogen protection. After purging nitrogen to reduce the oxygen content in the sealed reactor to below 100 ppm, raise the temperature and react at a stirring speed of 100 r / min and a temperature of 100℃. Control the endpoint pH to 4.9. Continue the reaction under these conditions for 50 min to obtain ferrous manganese phosphate slurry.

[0073] 3. Continue to add urea and lithium chloride to the ferrous manganese phosphate slurry. The molar ratio of lithium in the added lithium salt to the sum of the molar ratios of the added ferrous and manganese salts is 1.09:1, and the molar ratio of the added urea to the urea added in step 2 is 0.45:1. Continue to maintain the reaction at 105℃. During the reaction, nitrogen gas is continuously introduced to keep the oxygen content in the sealed reactor below 50ppm. Control the endpoint pH to 7.3. Continue to react under these conditions for 50min, then filter and wash with pure water until the conductivity of the washing water is ≤200μS / cm, then stop washing to obtain the first product.

[0074] 4. The first product is added to pure water and slurried. After slurrying, the solid content of the slurry is 35wt%. Then, water-soluble starch is added as a carbon source (which is converted into a carbon layer during subsequent calcination). After stirring and dissolving, the mixture is spray-dried. The particle size of the sprayed material is 5.6μm and the moisture content is controlled at ≤1wt%. The spray-dried material is obtained, which is the second product.

[0075] 5. The spray-dried material is placed in an atmosphere furnace for calcination under a nitrogen atmosphere. The heating rate is 350℃ / h, reaching a temperature of 725℃, and held at this temperature for 12 hours. During the calcination process, nitrogen is continuously introduced into the furnace, while an external induced draft fan extracts the exhaust gas. The pressure inside the furnace is 40Pa higher than the external atmospheric pressure. The oxygen content in the atmosphere furnace is maintained below 5ppm, CO content below 50ppm, hydrogen content below 100ppm, and humidity below 3%. The material is then discharged after the temperature is reduced to ≤100℃. The calcined material is then pulverized and sieved through a 150-mesh ultrasonic vibrating screen. Iron is removed using an electromagnetic iron separator. The packaging process is performed in a constant temperature and humidity room under vacuum, with a temperature of 20℃-25℃ and humidity ≤10%. The resulting carbon layer has a mass fraction of 1.85%, and the molecular formula of the core is Li. 1.05 Mn 0.50 Fe 0.50 (PO4) 1.04 Lithium iron manganese phosphate material.

[0076] The process flow of the above preparation method is shown in Figure 7. The SEM results of the obtained lithium manganese iron phosphate cathode material are shown in Figure 1, and the XRD results are shown in Figure 2.

[0077] During SEM testing, the obtained lithium manganese iron phosphate was cross-sectioned, and the EDS results of its elemental composition are shown in Figure 3. In Figure 3, elements that were not displayed due to overlapping labels are marked with arrows.

[0078] Analysis of the above figure shows that the elemental distribution is uniform, especially the uniform arrangement of manganese and iron, avoiding manganese segregation. This results in a higher capacity of the material because the segregation of manganese alone would cause the aggregation of lithium manganese phosphate. Lithium manganese phosphate has very poor ionic conductivity, so lithium cannot be extracted, resulting in low charge-discharge capacity. In this embodiment, because the manganese and iron are uniformly mixed, this situation is avoided, resulting in higher capacity and better subsequent cycle performance.

[0079] Example 2

[0080] Preparation of a lithium manganese iron phosphate material:

[0081] 1. Electrolytic iron powder was added to a 3 mol / L phosphoric acid solution, and the reaction was carried out until the pH reached 2. The reaction temperature was 50℃. After filtration, a ferrous dihydrogen phosphate solution was obtained.

[0082] 2. Add manganese acetate to the ferrous dihydrogen phosphate solution, with the molar ratio of iron in the ferrous dihydrogen phosphate solution to manganese in the added manganese acetate being 1:0.8. Then add urea, with the molar amount of urea being 2.5 times the molar amount of ferrous dihydrogen phosphate. Stir the reaction in a sealed reactor under nitrogen protection. After purging nitrogen to reduce the oxygen content in the sealed reactor to below 100 ppm, raise the temperature and react at a stirring speed of 120 r / min and a temperature of 95℃. Control the endpoint pH to 5.5. Continue the reaction for 30 min under these conditions to obtain ferrous manganese phosphate slurry.

[0083] 3. Continue to add urea and lithium chloride to the ferrous manganese phosphate slurry. The ratio of the molar amount of lithium in the added lithium salt to the sum of the molar amounts of the added ferrous and manganese salts is 1.08:1. The molar amount of added urea is 0.6:1 to the molar amount of urea added in step (2). Continue to maintain the reaction at 100℃. During the reaction, nitrogen gas is continuously introduced to keep the oxygen content in the sealed reactor below 50ppm. The final pH is controlled at 7.5. Continue to react under these conditions for 30min. Then filter and wash with pure water. Stop washing when the conductivity of the washing water is ≤200μS / cm to obtain the first product.

[0084] 4. The first product is added to pure water and slurried. After slurrying, the solid content of the slurry is 30wt%. Then, a mixture of glucose and water-soluble starch in a mass ratio of 1:1 is added as a carbon source (the carbon source is converted into a carbon layer in the subsequent calcination process). After stirring and dissolving, the mixture is spray-dried. The particle size of the sprayed material is 3μm and the moisture content is controlled at ≤1wt%. The spray-dried material is obtained, which is the second product.

[0085] 5. The spray-dried material is placed in an atmosphere furnace for calcination under a nitrogen atmosphere. The heating rate is 300℃ / h, reaching 700℃, and held at this temperature for 15 hours. During calcination, nitrogen is continuously introduced into the furnace, while an external induced draft fan extracts waste gas. The pressure inside the furnace is 40Pa higher than the external atmospheric pressure. The oxygen content, CO content, and hydrogen content in the atmosphere furnace are maintained below 5ppm, 50ppm, 100ppm, and humidity below 3%. The material is then discharged after the temperature is reduced to ≤100℃. The calcined material is then pulverized and sieved through a 150-mesh ultrasonic vibrating screen. Iron is removed using an electromagnetic iron separator. The packaging process involves vacuum packaging in a constant temperature and humidity room at 20℃-25℃ and humidity ≤10%. The resulting carbon layer has a mass fraction of 2.96wt%, and the core molecular formula is Li. 1.02 Mn 0.45 Fe 0.55 (PO4) 0.98 Lithium iron manganese phosphate material.

[0086] The SEM results of the obtained lithium manganese iron phosphate cathode material are shown in Figure 4, and the XRD results are shown in Figure 5.

[0087] Example 3

[0088] Preparation of a lithium manganese iron phosphate material:

[0089] 1. Electrolytic iron powder was added to a 6 mol / L phosphoric acid solution, and the reaction was carried out until the pH reached 2.5. The reaction temperature was 50℃. After filtration, a ferrous dihydrogen phosphate solution was obtained.

[0090] 2. Add manganese acetate to the ferrous dihydrogen phosphate solution, with the molar ratio of iron in the ferrous dihydrogen phosphate solution to manganese in the added manganese acetate being 1:0.2. Then add urea, with the molar amount of urea being 2.2 times the molar amount of ferrous dihydrogen phosphate. Stir the reaction in a sealed reactor under nitrogen protection. After purging nitrogen to reduce the oxygen content in the sealed reactor to below 100 ppm, raise the temperature and react at a stirring speed of 60 r / min and a temperature of 105℃. Control the endpoint pH to 4.5. Continue the reaction under these conditions for 60 min to obtain ferrous manganese phosphate slurry.

[0091] 3. Continue to add urea and lithium chloride to the ferrous manganese phosphate slurry. The ratio of the molar amount of lithium in the added lithium salt to the sum of the molar amounts of the added ferrous and manganese salts is 1.1:1. The molar amount of added urea is 0.3:1 compared with the molar amount of urea added in step (2). Continue to maintain the reaction at 110°C. Nitrogen gas is continuously introduced during the reaction process to keep the oxygen content in the sealed reactor below 50ppm. The final pH is controlled at 7.1. Continue to react under these conditions for 60min. Then filter and wash with pure water. Stop washing when the conductivity of the washing water is ≤200μS / cm to obtain the first product.

[0092] 4. The first product is added to pure water and slurried. After slurrying, the solid content of the slurry is 30wt%. Then, sucrose is added as a carbon source (which is converted into a carbon layer in the subsequent calcination process). After stirring and dissolving, it is spray-dried. The particle size of the sprayed material is 8μm and the moisture content is controlled at ≤1wt% to obtain the spray-dried material, which is the second product.

[0093] 5. The spray-dried material is placed in an atmosphere furnace for calcination under a nitrogen atmosphere. The heating rate is 400℃ / h, reaching a temperature of 750℃, and held at this temperature for 8 hours. During the calcination process, nitrogen is continuously introduced into the furnace, while an external induced draft fan extracts the exhaust gas. The pressure inside the furnace is 40Pa higher than the external atmospheric pressure. The oxygen content in the atmosphere furnace is maintained below 5ppm, CO content below 50ppm, hydrogen content below 100ppm, and humidity below 3%. The material is then discharged after the temperature is reduced to ≤100℃. The calcined material is then crushed and sieved through a 150-mesh ultrasonic vibrating screen. Iron is removed using an electromagnetic iron separator. The packaging process is carried out in a constant temperature and humidity room under vacuum, with a temperature of 20℃-25℃ and humidity ≤10%. The resulting carbon layer has a mass fraction of 1.06wt%, and the core molecular formula is Li. 0.79 Mn 0.18 Fe 0.82 (PO4) 1.03 Lithium iron manganese phosphate material.

[0094] The SEM results of the obtained lithium manganese iron phosphate cathode material are shown in Figure 6, and the XRD results are shown in Figure 5.

[0095] Example 4

[0096] Preparation of a lithium manganese iron phosphate material:

[0097] The difference between this embodiment and Embodiment 1 lies only in step 1, specifically:

[0098] 1. Electrolytic iron powder was added to a 2 mol / L phosphoric acid solution, and the reaction was carried out until the pH reached 3.0. The reaction temperature was 80℃. After filtration, a ferrous dihydrogen phosphate solution was obtained.

[0099] The remaining steps are consistent with those in Example 1.

[0100] Example 5

[0101] Preparation of a lithium manganese iron phosphate material:

[0102] The difference between this embodiment and Embodiment 1 lies only in step 1, specifically:

[0103] 1. Electrolytic iron powder was added to a 7 mol / L phosphoric acid solution, and the reaction was carried out until the pH reached 1.5. The reaction temperature was 40℃. After filtration, a ferrous dihydrogen phosphate solution was obtained.

[0104] The remaining steps are consistent with those in Example 1.

[0105] Example 6

[0106] Preparation of a lithium manganese iron phosphate material:

[0107] The difference between this embodiment and Embodiment 1 lies only in step 2, specifically:

[0108] 2. Add manganese acetate to the ferrous dihydrogen phosphate solution, with the molar ratio of iron in the ferrous dihydrogen phosphate solution to manganese in the added manganese acetate being 1:0.1. Then add urea, with the molar amount of urea being twice the molar amount of ferrous dihydrogen phosphate. Stir the reaction in a sealed reactor under nitrogen protection. After purging nitrogen to reduce the oxygen content in the sealed reactor to below 100 ppm, raise the temperature and react at a stirring speed of 150 r / min and a temperature of 110℃. Control the endpoint pH to 4.0. Continue the reaction under these conditions for 70 min to obtain ferrous manganese phosphate slurry.

[0109] The remaining steps are consistent with those in Example 1.

[0110] Example 7

[0111] Preparation of a lithium manganese iron phosphate material:

[0112] The difference between this embodiment and Embodiment 1 lies only in step 2, specifically:

[0113] 2. Add manganese acetate to the ferrous dihydrogen phosphate solution, with the molar ratio of iron in the ferrous dihydrogen phosphate solution to manganese in the added manganese acetate being 1:1.2. Then add urea, with the molar amount of urea being 3 times the molar amount of ferrous dihydrogen phosphate. Stir the reaction in a sealed reactor under nitrogen protection. After purging nitrogen to reduce the oxygen content in the sealed reactor to below 100 ppm, raise the temperature and react at a stirring speed of 50 r / min and a temperature of 90℃. Control the endpoint pH to 6.0 and continue the reaction for 20 min under these conditions to obtain ferrous manganese phosphate slurry.

[0114] The remaining steps are consistent with those in Example 1.

[0115] Example 8

[0116] Preparation of a lithium manganese iron phosphate material:

[0117] The difference between this embodiment and Embodiment 1 lies only in step 3, specifically:

[0118] 3. Continue to add urea and lithium chloride to the ferrous manganese phosphate slurry. The ratio of the molar amount of lithium in the added lithium salt to the sum of the molar amounts of the added ferrous and manganese salts is 1.0:1. The molar amount of urea added is 0.2:1 compared to the molar amount of urea added in step (2). Continue to maintain the reaction at 120℃. During the reaction, nitrogen gas is continuously introduced to keep the oxygen content in the sealed reactor below 50ppm. The final pH is controlled at 7.0. Continue to react under these conditions for 70min. Then filter and wash with pure water. Stop washing when the conductivity of the washing water is ≤200μS / cm to obtain the first product.

[0119] The remaining steps are consistent with those in Example 1.

[0120] Example 9

[0121] Preparation of a lithium manganese iron phosphate material:

[0122] The difference between this embodiment and Embodiment 1 lies only in step 3, specifically:

[0123] 3. Continue to add urea and lithium chloride to the ferrous manganese phosphate slurry. The ratio of the molar amount of lithium in the added lithium salt to the sum of the molar amounts of the added ferrous and manganese salts is 1.2:1. The molar amount of added urea is 0.8:1 compared to the molar amount of urea added in step (2). Continue to maintain the reaction at 90°C. During the reaction, continue to introduce nitrogen gas to keep the oxygen content in the sealed reactor below 50ppm. Control the endpoint pH to 8.0. Continue to react under these conditions for 20 minutes. Then filter and wash with pure water. Stop washing when the conductivity of the washing water is ≤200μS / cm to obtain the first product.

[0124] The remaining steps are consistent with those in Example 1.

[0125] Example 10

[0126] Preparation of a lithium manganese iron phosphate material:

[0127] The difference between this embodiment and Embodiment 1 lies only in step 5, specifically:

[0128] 5. Place the spray-dried material into an atmosphere furnace for calcination. The calcination process is carried out under nitrogen atmosphere protection. The heating rate is 250℃ / h, and the temperature is raised to 650℃. Hold the temperature at this temperature for 18h, and then keep the conditions consistent with those in Example 1.

[0129] The remaining steps are consistent with those in Example 1.

[0130] Example 11

[0131] Preparation of a lithium manganese iron phosphate material:

[0132] The difference between this embodiment and Embodiment 1 lies only in step 5, specifically:

[0133] 5. Place the spray-dried material into an atmosphere furnace for calcination. The calcination process is carried out under nitrogen atmosphere protection. The heating rate is 450℃ / h, and the temperature is raised to 800℃. Hold the temperature at this temperature for 6 hours, and then keep the conditions the same as in Example 1.

[0134] The remaining steps are consistent with those in Example 1.

[0135] Comparative Example 1

[0136] Preparation of a lithium manganese iron phosphate material:

[0137] 1. Ferrous dihydrogen phosphate, manganese acetate, lithium chloride, and water-soluble starch are mixed to obtain a solid material, wherein the molar ratio of iron, manganese, and lithium in the solid material is 0.5:0.5:1.09.

[0138] 2. The solid material is placed in an atmosphere furnace for calcination under nitrogen atmosphere protection. The heating rate is 350℃ / h, reaching a temperature of 725℃, and held at this temperature for 12 hours. During the calcination process, nitrogen is continuously introduced into the furnace, while an external induced draft fan extracts the exhaust gas. The pressure inside the furnace is 40Pa higher than the external atmospheric pressure. The oxygen content in the atmosphere furnace is maintained below 5ppm, CO content below 50ppm, hydrogen content below 100ppm, and humidity below 3%. The material is then discharged after the temperature is reduced to ≤100℃. The calcined material is then crushed and sieved through a 150-mesh ultrasonic vibrating screen. Iron is removed using an electromagnetic iron separator. The packaging process is carried out in a constant temperature and humidity room under vacuum, with a temperature of 20℃-25℃ and humidity ≤10%, resulting in lithium manganese iron phosphate material with a carbon layer mass fraction of 1.85%.

[0139] II. Testing Methods

[0140] Performance testing of lithium manganese iron phosphate materials

[0141] 1. Element content: obtained by ICP-OES testing.

[0142] 2. Manganese leaching amount: Add 1g of the test sample to 100mL of deionized water, stir and dissolve at 25℃ for 30min, then filter. The manganese content in the filtrate is measured by ICP-OES, which is the manganese leaching amount.

[0143] 3. Free lithium: Measured by automatic potentiometric titration according to SJT 11794-2021.

[0144] 4. Magnetic Material Content: Weigh 100g of the material and pour it into a capped plastic bottle. Add 1000g of pure water, then add an 8000GS magnetic magnet (protected with PTFE). Tighten the cap, place the bottle horizontally, and rotate it at 10-20 rpm for 30-45 minutes. Remove the magnetic magnet, add another 1000g of pure water, place the bottle back into the plastic bottle, tighten the cap, and rotate it horizontally at 10-20 rpm for 5-10 minutes. Remove the magnetic magnet, dissolve it in aqua regia, and measure the solution using ICP. Make up the volume and measure the nickel, chromium, copper, zinc, and iron content. Calculate the total mass of nickel, chromium, copper, zinc, and iron, and divide it by the weight of the material to obtain the magnetic foreign matter content of the material.

[0145] 5. Moisture content: Tested using the Karl Fischer (KF) method.

[0146] 6. D50: Particle size analysis by laser diffraction method according to GB / T 19077.

[0147] 7. BET: According to GB / T 19587, the specific surface area of ​​solid substances is determined by the gas adsorption BET method.

[0148] 8. Compacted density: Powder compacted density is determined according to GB / T 24533.

[0149] 9. Tap density: Powder tap density is determined according to GB / T 5162-2021.

[0150] 10. Powder resistivity: The four-probe method was used to test the resistivity at a pressure of 10 MPa.

[0151] Half-cell performance testing: Half-cell samples were prepared using the cathode materials obtained from each example and comparative example, and their first charge-discharge capacity and first discharge efficiency at 0.2C were tested.

[0152] The results of the above tests are shown in Tables 1 to 4.

[0153] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0154] Table 1

[0155] Table 2

[0156] Table 3

[0157] Table 4

[0158] The results above show that the embodiments described in this application have achieved the preparation of lithium manganese iron phosphate materials with low manganese leaching, low magnetic material mass, and low powder resistivity. Specifically:

[0159] A comparison of the various embodiments with the comparative examples shows that the preparation method used in this application has superior performance compared to the conventional lithium manganese iron phosphate material prepared by solid-state method. After being used as a positive electrode material to prepare a lithium-ion battery, the corresponding lithium-ion battery exhibits higher initial charge-discharge specific capacity and initial efficiency.

[0160] Comparing Examples 1 to 3 with Examples 4 and 5, it can be seen that in the preparation process of ferrous dihydrogen phosphate solution, the preferred temperature of the first reaction is 50℃ to 70℃, the preferred pH value of the ferrous dihydrogen phosphate solution is 2.0 to 2.5, and the preferred concentration of the phosphoric acid solution is 3mol / L to 6mol / L. This can obtain a ferrous dihydrogen phosphate solution with better dispersibility, thereby promoting the improvement of the particle size uniformity of the intermediate and final products, and thus obtaining lithium iron manganese phosphate cathode material with better electrochemical performance.

[0161] Comparing Examples 1 to 3 with Examples 6 and 7, it can be seen that in the preparation process of ferrous manganese phosphate slurry, the preferred molar ratio of the first alkali source to ferrous dihydrogen phosphate in the ferrous dihydrogen phosphate solution is (2.2-2.5):1, and the preferred molar ratio of manganese in the manganese salt to iron in the ferrous dihydrogen phosphate solution is (0.2-1):1. Simultaneously, the preferred temperature for the second reaction is 95℃-105℃, the preferred stirring speed is 60r / min-120r / min, the preferred endpoint pH value for the second reaction is 4.5-5.5, and after reaching the endpoint pH value of the second reaction, the reaction is continued for 30-60 minutes. This yields a more stable intermediate product with ferrous phosphate as the core and manganese phosphate as the shell, thereby improving the electrochemical performance of the final lithium iron manganese phosphate cathode material.

[0162] Comparing Examples 1 to 3 with Examples 8 and 9, it can be seen that in the process of preparing the first product from ferrous manganese phosphate slurry, the preferred ratio of the sum of the molar amounts of manganese in the manganese salt and iron in the ferrous dihydrogen phosphate solution to the molar amounts of lithium in the lithium salt is 1:(1.08~1.1), and the molar ratio of the second alkali source to the first alkali source is (0.3~0.6):1. Simultaneously, the preferred temperature for the third reaction is 100℃~110℃, the endpoint pH of the third reaction is 7.1~7.5, and continuing the reaction for 30min~60min after reaching the endpoint pH of the third reaction allows for better coordination of the amount of urea added to the reaction system in both reactions, thereby better controlling the pH changes in the system and obtaining a lithium manganese iron phosphate material with superior electrochemical performance.

[0163] Comparing Examples 1 to 3 with Examples 10 and 11, it can be seen that in the final calcination process to obtain lithium manganese iron phosphate material, the preferred calcination heating rate is 300℃ / h to 400℃ / h, the holding temperature is 700℃ to 750℃, and the holding time is 8h to 15h. This can more effectively suppress the generation of gases such as H2 and CO, thereby better protecting the integrity of the carbon coating layer and reducing the generation of magnetic materials. As a result, the obtained lithium manganese iron phosphate product has a lower content of magnetic materials and better electrochemical performance.

[0164] 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 iron manganese phosphate material, characterized in that, The molecular formula of the inner core of the lithium manganese iron phosphate material is Li x Mn y Fe 1-y (PO4) z wherein 0.7≤x≤1.1, 0.15≤y<1, 0.9≤z≤1.05; the manganese dissolution amount of the lithium manganese iron phosphate material is 25ppm-33ppm, the powder resistivity is 12Ω·cm-28Ω·cm, and the amount of magnetic substance is 0.05ppm-0.12ppm. 2.The lithium iron manganese phosphate material of claim 1, characterized in that, The lithium manganese iron phosphate material further comprises a carbon layer arranged on the surface of the inner core; The D50 particle size of the lithium manganese iron phosphate material is 1.35 μm-1.9 μm; The mass fraction of the carbon layer is 1%-3% based on 100% of the weight of the lithium manganese iron phosphate material. 3.The lithium iron manganese phosphate material of claim 1 or 2, characterized in that, The molecular formula of the inner core of the lithium iron manganese phosphate material is Li x Mn y Fe 1-y (PO4) z 1≤(1-y) / y≤5.

4. A method for preparing a lithium iron manganese phosphate material, characterized by, The preparation method of the lithium manganese iron phosphate material comprises: mixing an iron source with a phosphoric acid solution to perform a first reaction to obtain a ferrous phosphate solution; adding a manganese salt and a first alkali source into the ferrous phosphate solution to perform a second reaction to obtain a manganese ferrous phosphate slurry; adding a lithium salt and a second alkali source into the manganese ferrous phosphate slurry to perform a third reaction, and after filtration and washing, a first product is obtained, the first product is slurried and dried to obtain a second product, and the second product is calcined to obtain the lithium manganese iron phosphate material; the first alkali source and the second alkali source are urea.

5. The method of claim 4, wherein the lithium iron manganese phosphate material is prepared by the steps of: mixing a lithium source, a manganese source, and an iron source; and heating the mixture to a temperature of 600 to 800 °C for 1 to 10 hours. The molar ratio of the first alkali source to the ferrous phosphate in the ferrous phosphate solution is (2.2-2.5):1; Preferably, the molar ratio of the second alkali source to the first alkali source is (0.3-0.6):1; Preferably, the molar ratio of manganese in the manganese salt to iron in the ferrous phosphate solution is (0.2-1):1, and the ratio of the sum of the molar amount of manganese in the manganese salt and the molar amount of iron in the ferrous phosphate solution to the molar amount of lithium in the lithium salt is 1:(1.08-1.1); More preferably, the iron source is iron sheet and / or iron powder, the manganese salt is selected from one or more of manganese sulfate, manganese chloride and manganese acetate, and the lithium salt is selected from one or more of lithium sulfate, lithium chloride and lithium acetate.

6. The method of producing a lithium iron manganese phosphate material according to claim 4 or 5, characterized in that, The temperature of the first reaction is 50-70°C, and the pH value of the ferrous phosphate solution is 2.0-2.5; Preferably, the concentration of the phosphoric acid solution is 3-6 mol / L.

7. The method of producing a lithium iron manganese phosphate material according to any one of claims 4 to 6, characterized in that, The temperature of the second reaction is 95-105°C, and the end-point pH value of the second reaction is 4.5-5.5; Preferably, the second reaction further comprises continuing to react for 30-60 min after the end-point pH value of the second reaction is reached. Preferably, the second reaction is performed under the condition that the oxygen content is less than 100 ppm.

8. The method of producing a lithium iron manganese phosphate material according to any one of claims 4 to 7, characterized in that, The temperature of the third reaction is 100-110°C, and the end-point pH value of the third reaction is 7.1-7.5; Preferably, the third reaction further comprises continuing to react for 30-60 min after the end-point pH value of third reaction is reached. Preferably, the third reaction is performed under the condition that the oxygen content is less than 50 ppm.

9. The method of producing a lithium iron manganese phosphate material according to any one of claims 4 to 8, characterized in that, The preparation method of the lithium manganese iron phosphate material further comprises: after the first product is slurried, a first product slurry is obtained, a carbon source is added into the first product slurry, and after spray drying, the second product is obtained; Preferably, the solid content of the first product slurry is 30%-40%, the particle size of the second product is 5-8 μm, and the mass fraction of water in the second product is less than or equal to 1%; Preferably, the carbon source is a water-soluble carbon source, more preferably one or more of water-soluble starch, sucrose, glucose or polyethylene glycol.

10. The method of producing a lithium iron manganese phosphate material according to any one of claims 4 to 9, characterized in that, The calcination treatment is performed under conditions of an oxygen content of less than 5 ppm and a humidity of less than 3%, and the holding temperature of the calcination treatment is 700-750°C, and the holding time of the calcination treatment is 8-15h.

11. A positive electrode sheet characterized by comprising: The positive electrode plate comprises the lithium iron manganese phosphate material of any one of claims 1 to 3, or is prepared by the preparation method of the lithium iron manganese phosphate material of any one of claims 4 to 10.

12. A lithium-ion battery, characterized by, The positive electrode plate as claimed in claim 11 is included.

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