Lithium manganese iron phosphate positive electrode material, and preparation method therefor and use thereof

Abstract

The present application provides a lithium manganese iron phosphate positive electrode material, and a preparation method therefor and a use thereof. The lithium manganese iron phosphate positive electrode material comprises a doped manganese iron phosphate core and a coating layer coated on the surface of the doped manganese iron phosphate core, the coating layer being a lithium-containing carbon layer; and the doped manganese iron phosphate core comprises doping ions, and in particles of the doped manganese iron phosphate core, from inside to outside, the content of manganese ions and the content of iron ions gradually decrease and the content of the doping ions gradually increases. The lithium manganese iron phosphate positive electrode material having the described structure can effectively improve the poor phase structure stability and poor conductivity in lithium manganese iron phosphate positive electrode materials, and mitigate the series of problems caused by the Jahn-Teller effect and manganese dissolution that occur in the material itself, thereby improving the conductivity of the lithium manganese iron phosphate positive electrode material, and the comprehensive performance in various aspects such as gram capacity and cycle stability when the lithium manganese iron phosphate positive electrode material is used in batteries.

Description

Lithium manganese iron phosphate cathode material, its preparation method and application

[0001] This application is based on and claims priority to Chinese application CN application number 202411843248.4 filed on December 13, 2024, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0002] This application relates to the field of lithium batteries, and more specifically, to a lithium manganese iron phosphate cathode material, its preparation method, and its application. Background Technology

[0003] Lithium iron phosphate (LFP) cathode materials not only possess advantages such as high energy density and long cycle life, but also high safety and no memory effect. In recent years, LFP cathode materials have been applied in the fields of power batteries and energy storage. However, due to their low voltage platform (3.4V relative to Li / Li...),... + The low packing density and low rate performance of lithium iron phosphate cathode materials have limited their wider application.

[0004] Lithium manganese iron phosphate (LiFePO4) is a cathode material obtained by incorporating manganese into lithium iron phosphate. It exhibits a higher discharge platform (3.5–4.1 V relative to Li / Li). + This allows the overall theoretical energy density of the material to be increased by approximately 21% compared to lithium iron phosphate. However, lithium iron manganese phosphate exhibits a rapid energy decay at a plateau of 4.1V during battery cycling. The main reasons for this are: ① the material's inherently unstable phase structure and irregular microstructure; ② the Jahn-Teller effect inherent in lithium iron manganese phosphate; ③ manganese dissolution within the lithium iron manganese phosphate material; and ④ the poor lithium-ion conductivity of lithium iron manganese phosphate. There are many methods for preparing lithium iron manganese phosphate, some of which are listed below:

[0005] Chinese patent application CN102249208A discloses a hydrothermal method for preparing lithium iron phosphate (LiMn) battery cathode materials. This method first utilizes a hydrothermal synthesis reaction to prepare LiMn. x Fe 1-x PO4: Lithium hydroxide aqueous solution, ferrous sulfate aqueous solution, and phosphoric acid are stirred and mixed. Under sealed conditions, the temperature is raised to 150–180°C over 0.5–2.0 hours, and the reaction is carried out at a pressure of 0.48–1.0 MPa for 0.5–4 hours. The mixture is then cooled to below 80°C and filtered. The wet filter cake is then mixed with a soluble carbon source organic matter and spray-dried or flash-dried. Finally, LiMn... x Fe 1-xThe PO4 carbon source composite powder was calcined at 600–750°C for 4–6 hours under inert gas conditions and then cooled to below 150°C to obtain carbon-coated lithium manganese iron phosphate lithium-ion battery cathode material.

[0006] Chinese patent application CN103000898A discloses a method for preparing carbon-composite lithium manganese iron phosphate material. This method ensures uniform mixing of raw materials at the molecular level, facilitating the acquisition of a homogeneous precursor. The method mainly comprises two parts: First, a manganese source and a phosphorus source are dissolved to form an aqueous solution, and anhydrous ethanol is added dropwise. Once a precipitate forms, the solution is stirred, filtered, washed, and dried to obtain the precursor. Second, the precursor is ball-milled with a lithium source and a ferrous salt, dried to form a mixture, and then calcined and cooled to obtain the carbon-composite lithium manganese iron phosphate material.

[0007] Chinese patent application CN102969506A discloses a modified lithium iron manganese phosphate cathode material and its preparation method. The specific preparation process disclosed in this invention is as follows: (1) Phosphorus source, iron source and manganese source are placed in a reaction vessel and stirred at a speed of 800-1500 r / min and heated to 60-100℃ for 5-18 h. After the reaction is completed, the product is dried and pulverized; (2) Pre-calcination: Under nitrogen protection, the temperature is raised to 500-700℃ at a speed of 2-10℃ / min and held for 5-10 h, and then cooled with the furnace; (3) Mixing: The pre-calcined product is mixed with a certain amount of lithium source and a carbon source is added. Anhydrous ethanol is used as a dispersant. The product is ball-milled at 800-2000 r / min for 2-10 h. After ball milling, the product is dried and pulverized; (4) High-temperature sintering: Under nitrogen protection, the temperature is raised to 600-900℃ at a speed of 2-10℃ / min and held for 5-20 h, and then cooled with the furnace. Finally, lithium iron manganese phosphate cathode material is obtained.

[0008] Chinese patent application CN102738465A discloses a solid-state method for preparing lithium manganese iron phosphate (LiMn) materials. The method involves placing a lithium source, a trivalent iron source, manganese dioxide, a phosphorus source, and a carbon source into a ball mill jar, adding a dispersant and a complexing agent, mixing the mixture, and then ball milling it. Following drying and sintering, LiMn is obtained. x Fe 1-x PO4 cathode material.

[0009] Chinese patent application CN115321507A discloses a method for preparing lithium manganese iron phosphate material via coprecipitation. This method involves adding a mixed solution of manganese source, iron source, phosphoric acid, and perchloric acid to an alkaline solution to perform a coprecipitation reaction. The coprecipitated product is then washed and dried to obtain the lithium manganese iron phosphate cathode material. However, this preparation method uses cyanide as both the manganese and iron sources, which is environmentally harmful and difficult to scale up for production and application.

[0010] The lithium iron manganese phosphate cathode materials prepared by the above-mentioned methods all have certain defects. In order to improve the series of problems caused by defects such as poor phase structure stability, irregular microstructure, poor conductivity, Jahn-Teller effect and manganese dissolution in the lithium iron manganese phosphate cathode materials, this application is hereby proposed. Summary of the Invention

[0011] The main objective of this application is to provide a lithium iron manganese phosphate cathode material, its preparation method, and its application, in order to solve the problems of poor cycle performance and severe capacity decay caused by lithium iron manganese phosphate cathode materials in the battery process due to poor phase structure stability, poor lithium ion conductivity, and factors such as the Jahn-Teller effect and manganese dissolution that occur in the material itself.

[0012] To address the aforementioned issues, this application provides a lithium iron phosphate (LFP) cathode material. The LFP cathode material comprises a doped manganese iron phosphate core and a coating layer on the surface of the doped manganese iron phosphate core, the coating layer being a lithium-containing carbon layer. The doped manganese iron phosphate core contains dopant ions, and from the inside out, the content gradient of manganese and iron ions decreases while the content gradient of dopant ions increases. This LFP cathode material can effectively improve the manganese dissolution phenomenon existing in LFP materials during use, and can further improve the cycling performance and severe capacity decay problems of LFP cathode materials. It can also further improve the conductivity of LFP cathode materials and enhance the overall performance of the material.

[0013] Furthermore, the chemical formula of this lithium manganese iron phosphate cathode material is: LiFe 1-x-y Mn x M y PO4 / C, wherein 0.1≤x≤0.9, 0.002≤y≤0.02, and M is one or more of nickel, cobalt, magnesium, titanium, copper, vanadium, zirconium, and chromium; preferably, LiFe 1-x-y Mn x M yIn the PO4 / C mixture, 0.5 ≤ x ≤ 0.7, 0.01 ≤ y ≤ 0.02; preferably, the particle size of the lithium manganese iron phosphate cathode material is 0.5–10 μm; preferably, the carbon coating content of the coating layer is 1%–3%; preferably, M is cobalt and / or magnesium. Controlling the component content of the lithium manganese iron phosphate cathode material within the above ranges has a better effect on improving the overall performance of the material.

[0014] According to another aspect of this application, a method for preparing the above-mentioned lithium manganese iron phosphate cathode material is also provided. This method includes the following steps: Step S1, dissolving a manganese source and an iron source in water to obtain a manganese-iron mixed solution; dissolving a soluble compound of the dopant ion in water to obtain a dopant ion solution; Step S2, placing an aqueous complexing agent solution in a reaction vessel and adding phosphoric acid to adjust the pH; Step S3, adding the manganese-iron mixed solution and the dopant ion solution to the reaction vessel in a co-precipitation crystallization reaction to obtain a crystalline precipitate; during the co-precipitation process, gradually increasing the flow rate of the dopant ion solution; Step S4, filtering, washing, and drying the crystalline precipitate to obtain a gradient-doped manganese iron phosphate precursor; Step S5, mixing the manganese iron precursor with a lithium source, a carbon source, and water to obtain a mixed slurry; and subjecting the mixed slurry to grinding, spray drying, sintering, and pulverizing to obtain the lithium manganese iron phosphate cathode material. Using the above preparation method, the obtained lithium manganese iron phosphate cathode material exhibits better performance.

[0015] Further, in step S1, the concentration of the manganese-iron mixed solution is 0.8–1.2 mol / L; preferably, the concentration of the doped ion solution is 0.3–0.6 mol / L. Controlling the concentrations of the manganese-iron mixed solution and the doped ion solution within the above ranges is beneficial to improving the overall performance of the lithium manganese iron phosphate cathode material.

[0016] Further, the co-current dropwise addition operation is performed under stirring, with the manganese-iron mixed solution added at a rate of 0.5–5 L / h; preferably, the initial addition rate of the doped ion solution is 0.005–0.5 L / h, and when the crystal particle size reaches 3–5 μm, the addition rate of the doped ion solution is controlled at 1–5 L / h; preferably, the stirring speed is 100–1500 r / min. Controlling the co-current dropwise addition operation conditions within the above range can improve the performance of the obtained lithium manganese iron phosphate cathode material. Preferably, when the crystal particle size reaches 3–5 μm, the process further includes: continuously adding a surfactant to the co-precipitation crystallization reaction system until the co-precipitation crystallization reaction is completed; preferably, the surfactant is vinylpyrrolidone; preferably, the surfactant is added at a rate of 0.2–0.5 L / h; preferably, the surfactant is added in the form of an aqueous solution with a concentration of 0.1–1 mol / L. Adding a surfactant to the co-precipitation crystallization reaction system can more effectively promote the formation of a gradient-doped structure of the manganese iron phosphate precursor.

[0017] Further, in step S3, feeding is stopped when the particle size of the particles to be crystallized is 5–100 μm, and aging is carried out; preferably, the particle size of the crystallized particles is 5–30 μm; preferably, the aging time is 4–8 h. Controlling the crystallization conditions within the above range can result in smaller and more uniform particle size of the obtained gradient-doped manganese iron phosphate precursor, and better performance of the lithium manganese iron phosphate cathode material.

[0018] Further, the doping ion compound is one or more of nickel sulfate, nickel nitrate, cobalt nitrate, cobalt sulfate, magnesium nitrate, magnesium sulfate, magnesium acetate, tetrabutyl titanate, ammonium fluorotitanate, copper sulfate, ammonium polyvanadate, ammonium metavanadate, zirconium nitrate, or chromium sulfate; preferably, the iron source is one or more of ferrous sulfate, ferrous oxalate, ferrous acetate, and ferrous nitrate; preferably, the manganese source is one or more of manganese sulfate, manganese oxalate, manganese acetate, and manganese nitrate; preferably, the lithium source is one or more of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate; preferably, the complexing agent is one or more of ammonia, ammonium oxalate, citric acid, malic acid, and EDTA-2Na; preferably, the carbon source is one or more of polyethylene glycol, glucose, chitosan, PVP-K30, and CNT. Choosing the above compounds yields better results when preparing lithium manganese iron phosphate cathode materials.

[0019] Further, in step S2, when adding the complexing agent, a protective gas is first introduced into the reactor to adjust the pH to 3-6; preferably, the concentration of the complexing agent is 1.0-3.0 mol / L; preferably, the amount of complexing agent added is 1-2 times the total molar amount of metal ions in the manganese-iron mixed solution and the doped ion solution; preferably, the concentration of phosphoric acid is 1.0-2.0 mol / L; preferably, the temperature of the co-precipitation crystallization reaction is 40-80℃; preferably, the washing solvent in step S4 is one or more of deionized water, ethanol, and acetone; preferably, the drying temperature is 100-500℃, and the drying time is 5-10 h. These conditions allow the obtained iron-manganese precursor to have high purity.

[0020] Furthermore, the solid content of the mixed slurry is 30% to 40%. These conditions can improve the mixing effect and are beneficial to improving the overall performance of lithium manganese iron phosphate cathode materials.

[0021] According to another aspect of this application, a lithium-ion battery is also provided, wherein the positive electrode of the lithium-ion battery comprises the lithium manganese iron phosphate positive electrode material prepared above. This lithium-ion battery has the advantages of high discharge specific capacity and high capacity retention, and it can effectively mitigate manganese deposition during cycling.

[0022] This application provides a lithium iron phosphate (LFP) cathode material, comprising a doped LFP core and a coating layer on the surface of the LFP core, the coating layer being a lithium-containing carbon layer. The LFP core contains dopant ions, and from the inside out, the content gradient of manganese and iron ions decreases while the content gradient of dopant ions increases. The LFP cathode material with the above structure can effectively improve the poor phase structure stability, poor conductivity, and a series of problems caused by the Jahn-Teller effect and manganese dissolution inherent in LFP cathode materials, thereby improving the conductivity, specific capacity, cycle stability, and other comprehensive performance aspects of the LFP cathode material in applications such as batteries. Attached Figure Description

[0023] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0024] Figure 1 illustrates Mn according to Embodiment 1 of this application. 0.6 Fe 0.38 Mg 0.02 SEM image of the HPO4 precursor. Detailed Implementation

[0025] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0026] As described in the background section, lithium manganese iron phosphate (LFP) cathode materials have a higher discharge platform than lithium iron phosphate (LFP) cathode materials, and their theoretical energy density is about 21% higher. However, due to factors such as the low phase structure stability of the material itself, irregular microstructure, the Jahn-Teller effect and manganese dissolution phenomenon inherent in LFP materials, and poor lithium-ion conductivity, LFP cathode materials exhibit rapid capacity decay at a 4.1V voltage platform during battery cycling. These factors, in turn, lead to poor cycle performance and severe capacity decay when LFP cathode materials are applied in batteries.

[0027] To address the aforementioned issues, this application provides a lithium iron manganese phosphate cathode material comprising a doped manganese iron phosphate core and a coating layer covering the surface of the doped manganese iron phosphate core, wherein the coating layer is a lithium-containing carbon layer; the doped manganese iron phosphate core includes dopant ions, and from the inside out, the content gradient of manganese ions and iron ions in the particles of the doped manganese iron phosphate core decreases while the content gradient of dopant ions increases.

[0028] The lithium iron phosphate (LFP) cathode material described in this application comprises two parts: a doped manganese iron phosphate core and a coating layer covering the surface of the doped manganese iron phosphate core. The coating layer is a lithium-containing carbon layer. LFP cathode material is a cathode material obtained by further doping manganese into lithium iron phosphate. In conventional LFP cathode materials, manganese is often uniformly present within the material. This leads to manganese dissolution, decreased material structural stability, and severe battery capacity decay during practical applications due to the direct, large-area contact between the LFP cathode material and the electrolyte. This application uses the doped manganese iron phosphate core as the cathode material and further coats it with a lithium-containing carbon coating layer. The presence of this coating layer effectively improves the manganese dissolution phenomenon during use and further improves the cycle life and capacity decay issues of the LFP cathode material. Furthermore, the coating layer of the lithium manganese iron phosphate cathode material described in this application is a lithium-containing carbon layer. Using lithium and carbon together as the coating layer of the doped manganese iron phosphate core can further improve the conductivity of the lithium manganese iron phosphate cathode material and improve the overall performance of the material.

[0029] In addition, in the doped manganese iron phosphate core of the lithium manganese iron phosphate cathode material described in this application, the content gradient of manganese ions and iron ions decreases from the inside out, while the content gradient of dopant ions increases. The doped manganese iron phosphate core of the lithium manganese iron phosphate cathode material described in this application adopts the aforementioned relatively special structure, in which the content gradient of manganese ions and iron ions decreases from the inside out, while the content gradient of dopant ions increases from the inside out. Firstly, this gradient change in elemental content allows dopant ions to better embed into the manganese iron phosphate core to form a solid solution. While not changing the electron transport performance of the manganese iron phosphate core, it further enhances the conductivity of the dopant ions in the lithium manganese iron phosphate cathode material, effectively improving the conductivity and cycle stability of the lithium manganese iron phosphate cathode material. Furthermore, the existence of dopant ions and the manganese iron phosphate core in the form of a solid solution enables the material itself to have a more stable phase structure. Secondly, the dopant ions exist in the doped manganese iron phosphate core in a unique gradient manner, which can improve the interfacial conditions of the material and thus mitigate the Jahn-Teller effect. This significantly improves the material's cycle performance and stability, and addresses issues such as severe capacity decay in batteries. Further addition of dopant ions to lithium iron phosphate cathode materials can also broaden the lithium-ion transport channels and improve the overall conductivity of the material.

[0030] In summary, the lithium manganese iron phosphate cathode material provided in this application can effectively improve the problems of poor phase structure stability, poor conductivity, manganese dissolution, and a series of problems caused by the Jahn-Teller effect inherent in lithium manganese iron phosphate cathode materials. Furthermore, it further improves the problems of poor cycle stability and severe capacity decay of lithium manganese iron phosphate cathode materials when applied to batteries.

[0031] In a preferred embodiment, the chemical formula of the lithium manganese iron phosphate cathode material is: LiFe 1-x-y Mn x M y PO4 / C, where 0.1≤x≤0.9, 0.002≤y≤0.02, and M is one or more of nickel, cobalt, magnesium, titanium, copper, vanadium, zirconium, and chromium. The content and relative content of manganese, iron, lithium, and doped metals in lithium manganese iron phosphate cathode materials affect the overall performance of the material. According to the stoichiometric ratio of the components in the above chemical formula, the resulting lithium manganese iron phosphate cathode material exhibits better overall performance during use. Preferably, LiFe... 1-x-y Mn x M yIn PO4 / C, 0.5 ≤ x ≤ 0.7, 0.01 ≤ y ≤ 0.02, and the content ratio of each component within the above range results in better overall performance. Preferably, the carbon coating amount of the coating layer is 1% to 3%. Carbon and lithium, as the main components in the coating layer, allow carbon, lithium, and the various components in the doped manganese iron phosphate core to work together, further improving the conductivity of the lithium manganese iron phosphate cathode material and enhancing its overall performance. Controlling the carbon coating amount within the above range results in better material performance. Preferably, M is cobalt and / or magnesium. These two doped metal ions can improve the performance of the resulting lithium manganese iron phosphate cathode material. Preferably, the particle size of the lithium manganese iron phosphate cathode material is 0.5 μm to 10 μm.

[0032] According to another aspect of this application, a method for preparing the above-mentioned lithium manganese iron phosphate cathode material is also provided. The preparation method includes the following steps: Step S1, dissolving a manganese source and an iron source in water to obtain a manganese iron mixed solution; dissolving a soluble compound of dopant ions in water to obtain a dopant ion solution; Step S2, placing an aqueous complexing agent solution in a reaction vessel and adding phosphoric acid to adjust the pH; Step S3, adding the manganese iron mixed solution and the dopant ion solution to the reaction vessel in a co-precipitation crystallization reaction in a parallel-flowing manner to obtain a crystalline precipitate; during the parallel-flowing addition process, the flow rate of the dopant ion solution is gradually increased; Step S4, filtering, washing, and drying the crystalline precipitate to obtain a gradient-doped manganese iron phosphate precursor; Step S5, mixing the manganese iron precursor with a lithium source, a carbon source, and water to obtain a mixed slurry, and subjecting the mixed slurry to grinding, spray drying, sintering, and pulverizing to obtain the lithium manganese iron phosphate cathode material.

[0033] Specifically, the preparation method first dissolves manganese and iron sources in water to obtain a manganese-iron mixed solution; then, a soluble compound of the dopant ion is dissolved in water to obtain a dopant ion solution; then, an aqueous solution of the complexing agent is placed in a reaction vessel, and the pH is adjusted with phosphoric acid; then, the prepared manganese-iron mixed solution and the dopant ion solution are added to the reaction vessel in a co-current dropwise manner, so that the manganese-iron mixed solution and the dopant ion solution undergo a co-precipitation crystallization reaction to obtain a crystalline precipitate, and the flow rate of the dopant ion solution is gradually increased when it is added; then, the obtained crystalline precipitate is filtered, washed, and dried to obtain an iron-manganese precursor; finally, the iron-manganese precursor is mixed with a lithium source, a carbon source, and water to obtain a mixed slurry, and the mixed slurry is ground, spray-dried, sintered, and pulverized to obtain a lithium manganese iron phosphate cathode material.

[0034] In this preparation process, a manganese-iron mixed solution and a doped ion solution are first subjected to a co-precipitation crystallization reaction under the action of a complexing agent and phosphoric acid. By controlling the addition of the manganese-iron mixed solution and the doped ion solution to the reaction vessel in a parallel-flow dropping manner, and gradually increasing the flow rate of the doped ion solution during the parallel-flow dropping process, a crystalline precipitate with a decreasing gradient of manganese and iron ion content and an increasing gradient of doped ion content is obtained from the inside out. Further processing yields a gradient-doped manganese iron phosphate precursor. During the co-precipitation crystallization reaction, the manganese-iron mixed solution and the doped ion solution affect the growth rate of the crystal facets of the precipitate due to the different types of metal ions and the changes in the metal ion content in the system, thus resulting in a gradient structure in the formed crystalline precipitate. Subsequently, the gradient-doped manganese iron phosphate precursor is mixed with a lithium source, a carbon source, and water, and subjected to a series of treatments to obtain lithium iron manganese phosphate cathode material. The above preparation method can make the prepared lithium iron manganese phosphate cathode material have a doped iron manganese phosphate core and a coating layer structure on the surface of the doped iron manganese phosphate core. Furthermore, from the inside to the outside of the particles of the doped iron manganese phosphate core, the content gradient of manganese ions and iron ions decreases while the content gradient of doped ions increases.

[0035] In fact, during the actual preparation of the above-mentioned lithium manganese iron phosphate cathode material, due to the possibility of lithium loss, an excess of lithium is often added during preparation to obtain the material with the corresponding molecular formula. It is best to control the amount of lithium added to be 1.02 to 1.08 times the sum of the molar amounts of Fe, Mn and Mg.

[0036] In a preferred embodiment, in step S1, the concentration of the manganese-iron mixed solution is 0.8–1.2 mol / L; preferably, the concentration of the doping ion solution is 0.3–0.6 mol / L. The concentrations of the manganese-iron mixed solution and the doping ion solution affect the co-precipitation crystallization reaction. Controlling the concentrations of the manganese-iron mixed solution and the doping ion solution within the above-mentioned ranges can effectively control the crystal size of the generated iron-manganese precursor to a smaller range, which is beneficial to improving the overall performance of the lithium manganese iron phosphate cathode material.

[0037] In a preferred embodiment, a co-current dropwise addition operation is performed under stirring, with the manganese-iron mixed solution added at a rate of 0.5–5 L / h. Preferably, the initial addition rate of the dopant ion solution is 0.005–0.5 L / h, and when the crystallization particle size reaches 3–5 μm, the addition rate of the dopant ion solution is controlled to 1–5 L / h. During the co-precipitation crystallization reaction of the manganese-iron mixed solution and the dopant ion solution under the action of a complexing agent and phosphoric acid, by controlling the addition rate of the dopant ion solution within a certain range, the structure of the doped manganese-iron phosphate core particles in the obtained lithium manganese-iron phosphate cathode material can be such that, from the inside out, the content gradient of manganese and iron ions decreases, while the content gradient of dopant ions increases. Controlling the addition rates of the manganese-iron mixed solution and the dopant ion solution, as well as the variation of the addition rates, within the above range, allows the obtained lithium manganese-iron phosphate cathode material to possess superior overall performance. Preferably, when the crystallization particle size reaches 3–5 μm, the method further includes: continuously adding a surfactant to the coprecipitation crystallization reaction system until the coprecipitation crystallization reaction is completed; preferably, the surfactant is vinylpyrrolidone; preferably, the surfactant is added at a rate of 0.2–0.5 L / h; preferably, the surfactant is added in the form of an aqueous solution with a concentration of 0.1–1 mol / L. Adding a surfactant to the coprecipitation crystallization reaction system allows for further control of the crystal growth rate of the crystalline precipitate by utilizing the difference in adsorption rates of the surfactant on the crystal surface, which is more conducive to the formation of a gradient structure in the crystalline precipitate, resulting in a better gradient-doped structure generated from the iron-manganese phosphate precursor. Preferably, the stirring speed during addition is controlled at 100–1500 r / min.

[0038] In a preferred embodiment, in step S3, feeding is stopped when the particle size of the particles to be crystallized is 5–100 μm, and aging is carried out. The amount of manganese-iron mixed solution and doped ion solution added during the co-precipitation crystallization reaction affects the particle size of the crystalline precipitate. By controlling the amount of reactants added in the manner described above, this application can obtain a good iron-manganese phosphate precursor, thereby resulting in better performance of the obtained lithium iron manganese phosphate cathode material. Preferably, the particle size of the crystallized particles is 5–30 μm. Feeding is stopped in step S3 when the particle size of the particles to be crystallized is 5–30 μm. Controlling the amount of reactants added in the manner described above can result in a smaller and more uniform particle size of the obtained iron-manganese phosphate precursor, corresponding to better performance of the obtained lithium iron manganese phosphate cathode material. Preferably, the aging time is 4–8 hours. Controlling the aging time within the above range allows the co-precipitation crystallization reaction to proceed more fully.

[0039] For example, but not limitingly, the doping ion compound is one or more of nickel sulfate, nickel nitrate, cobalt nitrate, cobalt sulfate, magnesium nitrate, magnesium acetate, tetrabutyl titanate, ammonium fluorotitanate, copper sulfate, ammonium polyvanadate, ammonium metavanadate, zirconium nitrate, or chromium sulfate; preferably, the iron source is one or more of ferrous sulfate, ferrous oxalate, ferrous acetate, and ferrous nitrate; preferably, the manganese source is one or more of manganese sulfate, manganese oxalate, manganese acetate, and manganese nitrate; preferably, the lithium source is one or more of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate; preferably, the complexing agent is one or more of ammonia, ammonium oxalate, citric acid, malic acid, and EDTA-2Na (disodium ethylenediaminetetraacetate); preferably, the carbon source is at least one of polyethylene glycol (PEG), glucose, chitosan, PVP-K30 (polyvinylpyrrolidone-K30), and CNT (carbon nanotubes). Theoretically, any compound that can achieve the scheme of this application is acceptable, but the above-mentioned compounds are preferred for better results.

[0040] In a preferred embodiment, when adding the complexing agent in step S2, a protective gas is first introduced into the reactor to adjust the pH to 3-6; preferably, the concentration of the complexing agent is 1.0-3.0 mol / L; preferably, the amount of complexing agent added is 1-2 times the total molar amount of metal ions in the manganese-iron mixed solution and the doped ion solution; preferably, the concentration of phosphoric acid is 1.0-2.0 mol / L. Controlling the reaction conditions within the above range during the co-precipitation crystallization reaction can result in a lithium manganese iron phosphate cathode material with better cycle stability. Preferably, the temperature of the co-precipitation crystallization reaction is 40-80°C, which is more conducive to the co-precipitation crystallization reaction. Preferably, the washing solvent in step S4 is one or more of deionized water, ethanol, and acetone; preferably, the drying temperature is 100-500°C, and the drying time is 5-10 h. The above conditions can result in a high purity of the iron-manganese precursor. Preferably, the protective gas is at least one of nitrogen or argon.

[0041] In a preferred embodiment, the solid content of the mixed slurry is 30% to 40%. Controlling the solid content of the mixed slurry and the conditions for grinding, spray drying, and sintering during the preparation of lithium manganese iron phosphate cathode materials can result in lithium manganese iron phosphate cathode materials with better overall performance.

[0042] According to another aspect of this application, a lithium-ion battery is also provided, wherein the positive electrode of the lithium-ion battery comprises lithium manganese iron phosphate positive electrode material prepared by the above-described preparation method.

[0043] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0044] Example 1

[0045] The molecular formula of a lithium manganese iron phosphate cathode material is: LiFe 0.38 Mn 0.6 Mg 0.02 The PO4 / C cathode material has a carbon coating of 2% and a particle size of 0.5 μm ≤ D50 ≤ 2 μm. The iron source for preparing this lithium manganese iron phosphate cathode material is ferrous nitrate, the manganese source is manganese nitrate, the soluble compound for doping ions is magnesium sulfate, the lithium source is lithium carbonate, and the carbon source is a 1:1 mass mixture of glucose and PEG. During preparation, the molar ratio of Fe, Mn, and Mg in the manganese source, iron source, and soluble compound for doping ions is 0.38:0.6:0.02, and the molar amount of Li in the lithium source is 1.02 times the sum of the molar amounts of Fe, Mn, and Mg.

[0046] The preparation method of the above-mentioned lithium manganese iron phosphate cathode material is as follows:

[0047] (1) Dissolve the manganese source and the iron source in water to obtain a 1 mol / L manganese-iron mixed solution; dissolve the soluble compound of the doped ion in water to obtain a 0.5 mol / L doped ion solution; place water in a reaction vessel, add citric acid to make its concentration 1 mol / L, wherein the molar amount of citric acid is twice the total molar amount of metal ions in the manganese-iron mixed solution and the doped ion solution, and at the same time turn on the stirring of the reaction vessel and introduce nitrogen gas to make the reaction vessel be in the protective gas range, then add 2 mol / L phosphoric acid to adjust the pH to 3, and heat the reaction vessel to 55°C.

[0048] (2) Under the condition of stirring speed of 1500 r / min in the reactor, the above manganese-iron mixed solution and the above doped ion solution were added to the reactor in a co-precipitation crystallization reaction. The addition rate of the manganese-iron mixed solution was controlled at 5 L / h, and the initial addition rate of the doped ion solution was controlled at 0.2 L / h. When the crystal particle size reached 3-5 μm, the addition rate of the doped ion was controlled at 3 L / h. During this process, 1 mol / L of vinylpyrrolidone aqueous solution was added at an addition rate of 0.2 L / h. When the crystal particle size in the reactor was 8 ≤ D50 ≤ 10 μm, the addition was stopped, and aging continued for 6 h. The aged precipitate was filtered, washed with water, and dried at 300℃ for 10 h to obtain the gradient Mg-doped iron-manganese phosphate precursor Fe. 0.38 Mn 0.6 Mg 0.02 HPO4 was used to prepare the precursor, and the SEM image is shown in Figure 1. As can be seen from the figure, the gradient-doped iron manganese phosphate precursor prepared by the method of this application exhibits a spherical aggregate morphology and high uniformity. In addition, no obvious fragments were found. The above structural features can further improve the overall conductivity of the material.

[0049] (3) The above-mentioned gradient Mg-doped iron-manganese phosphate precursor, lithium source, carbon source and water are mixed to obtain a mixed slurry, wherein the solid content of the mixed slurry is 35%. The mixed slurry is subjected to grinding, spray drying, sintering and air jet pulverization to obtain lithium iron-manganese phosphate cathode material LiFe with a carbon coating of 2%. 0.38 Mn 0.6 Mg 0.02 PO4 / C.

[0050] Example 2

[0051] The molecular formula of a lithium manganese iron phosphate cathode material is: LiFe 0.39 Mn 0.6 Co 0.01 The PO4 / C cathode material has a carbon coating of 2% and a particle size of 0.5 μm ≤ D50 ≤ 2 μm. The iron source for preparing this lithium manganese iron phosphate cathode material is ferrous sulfate, the manganese source is manganese sulfate, the soluble compound for doping ions is cobalt sulfate, the lithium source is lithium carbonate, and the carbon source is a 1:1 mass mixture of glucose and PEG. During preparation, the molar ratio of Fe, Mn, and Co in the manganese source, iron source, and soluble compound for doping ions is 0.39:0.6:0.01, and the molar amount of Li in the lithium source is 1.08 times the sum of the molar amounts of Fe, Mn, and Co.

[0052] The preparation method of the above-mentioned lithium manganese iron phosphate cathode material is as follows:

[0053] (1) Dissolve the manganese source and the iron source in water to obtain a 1 mol / L manganese-iron mixed solution; dissolve the soluble compound of the doped ion in water to obtain a 0.5 mol / L doped ion solution; place water in a reactor and add ammonia water to make the concentration 1 mol / L, wherein the molar amount of ammonia water is twice the total molar amount of metal ions in the manganese-iron mixed solution and the doped ion solution. At the same time, turn on the stirring of the reactor and introduce argon gas to make the reactor in the protective gas range. Then add 2 mol / L phosphoric acid to adjust the pH to 6 and heat the reactor to 55°C.

[0054] (2) Under the condition of stirring speed of 1500 r / min in the reactor, the above manganese-iron mixed solution and the above doped ion solution were added to the reactor in a co-current dropwise manner to carry out co-precipitation crystallization reaction. The addition rate of the manganese-iron mixed solution was controlled at 5 L / h, and the initial addition rate of the doped ion solution was controlled at 0.2 L / h. When the crystal particle size reached 3-5 μm, the addition rate of the doped ion was controlled at 3 L / h. During this process, 0.1 mol / L of vinylpyrrolidone aqueous solution was added at a rate of 0.5 L / h. When the crystal particle size in the reactor was 8 ≤ D50 ≤ 10 μm, the dropwise addition was stopped, and aging continued for 6 h. The aged precipitate was filtered, washed with ethanol, and dried at 300 °C for 10 h to obtain the graded Co-doped iron-manganese phosphate precursor Fe. 0.39 Mn 0.6 Co 0.01 HPO4.

[0055] (3) The above-mentioned graded Co-doped iron-manganese phosphate precursor, lithium source, carbon source and water are mixed to obtain a mixed slurry, wherein the solid content of the mixed slurry is 35%. The mixed slurry is subjected to grinding, spray drying, sintering and air jet pulverization to obtain lithium iron-manganese phosphate cathode material LiFe with a carbon coating of 2%. 0.39 Mn 0.6 Co 0.01 PO4 / C.

[0056] Example 3

[0057] The molecular formula of a lithium manganese iron phosphate cathode material is: LiFe 0.49 Mn 0.5 Co 0.01 The PO4 / C cathode material has a carbon coating of 2% and a particle size of 0.5 μm ≤ D50 ≤ 2 μm. The ferrous sulfate used in its preparation is manganese sulfate, the manganese source is cobalt nitrate, the lithium source is lithium carbonate, and the carbon source is a 1:1 mass mixture of glucose and PEG. During preparation, the molar ratio of Fe, Mn, and Co in the manganese source, iron source, and soluble dopant compound is 0.49:0.5:0.01, and the molar amount of Li in the lithium source is 1.02 times the sum of the molar amounts of Fe, Mn, and Co.

[0058] The preparation method of the above-mentioned lithium manganese iron phosphate cathode material is as follows:

[0059] (1) Dissolve the manganese source and the iron source in water to obtain a 1 mol / L manganese-iron mixed solution; dissolve the soluble compound of the doped ion in water to obtain a 0.5 mol / L doped ion solution; place water in a reactor and add ammonia water to make the concentration 1 mol / L, wherein the molar amount of ammonia water is twice the total molar amount of metal ions in the manganese-iron mixed solution and the doped ion solution. At the same time, turn on the stirring of the reactor and introduce argon gas to make the reactor in the protective gas range. Then add 2 mol / L phosphoric acid to adjust the pH to 6 and heat the reactor to 55°C.

[0060] (2) Under the condition of stirring speed of 1500 r / min in the reactor, the above manganese-iron mixed solution and the above doped ion solution were added to the reactor in a co-current dropwise manner to carry out co-precipitation crystallization reaction. The addition rate of the manganese-iron mixed solution was controlled at 5 L / h, and the initial addition rate of the doped ion solution was controlled at 0.2 L / h. When the crystal particle size reached 3-5 μm, the addition rate of the doped ion was controlled at 3 L / h. During this process, 0.1 mol / L of vinylpyrrolidone aqueous solution was added at a rate of 0.5 L / h. When the crystal particle size in the reactor was 8 ≤ D50 ≤ 10 μm, the dropwise addition was stopped, and aging continued for 6 h. The aged precipitate was filtered, washed with ethanol, and dried at 300 °C for 10 h to obtain the graded Co-doped iron-manganese phosphate precursor Fe. 0.49 Mn 0.5 Co 0.01 HPO4.

[0061] (3) The above-mentioned graded Co-doped iron-manganese phosphate precursor, lithium source, carbon source and water are mixed to obtain a mixed slurry, wherein the solid content of the mixed slurry is 35%. The mixed slurry is subjected to grinding, spray drying, sintering and air jet pulverization to obtain lithium iron-manganese phosphate cathode material LiFe with a carbon coating of 2%. 0.49 Mn 0.5 Co 0.01 PO4 / C.

[0062] Example 4

[0063] The molecular formula of a lithium manganese iron phosphate cathode material is: LiFe 0.28 Mn 0.7 Mg 0.02The PO4 / C cathode material has a carbon coating of 2% and a particle size of 0.5 μm ≤ D50 ≤ 2 μm. The ferrous sulfate used in its preparation is manganese sulfate, the manganese source is magnesium acetate, the soluble compound for doping is magnesium acetate, the lithium source is lithium carbonate, and the carbon source is a 1:1 mass mixture of glucose and PEG. During preparation, the molar ratio of Fe, Mn, and Mg in the manganese source, iron source, and soluble compound for doping is 0.28:0.7:0.02, and the molar amount of Li in the lithium source is 1.02 times the sum of the molar amounts of Fe, Mn, and Mg.

[0064] The preparation method of the above-mentioned lithium manganese iron phosphate cathode material is as follows:

[0065] (1) Dissolve the manganese source and the iron source in water to obtain a 1 mol / L manganese-iron mixed solution; dissolve the soluble compound of the doped ion in water to obtain a 0.5 mol / L doped ion solution; place water in a reactor and add ammonia water to make the concentration 1 mol / L, wherein the molar amount of ammonia water is 1.5 times the total molar amount of metal ions in the manganese-iron mixed solution and the doped ion solution. At the same time, turn on the stirring of the reactor and introduce argon gas to make the reactor in the protective gas range. Then add 2 mol / L phosphoric acid to adjust the pH to 6 and heat the reactor to 55°C.

[0066] (2) Under the condition of stirring speed of 1500 r / min in the reactor, the above manganese-iron mixed solution and the above doped ion solution were added to the reactor in a co-precipitation crystallization reaction. The addition rate of the manganese-iron mixed solution was controlled at 5 L / h, and the initial addition rate of the doped ion solution was controlled at 0.2 L / h. When the crystal particle size reached 3-5 μm, the addition rate of the doped ion was controlled at 3 L / h. During this process, 0.1 mol / L of vinylpyrrolidone aqueous solution was added at a rate of 0.5 L / h. When the crystal particle size in the reactor was 8 ≤ D50 ≤ 10 μm, the addition was stopped, and aging continued for 6 h. The aged precipitate was filtered, washed with ethanol, and dried at 300 °C for 10 h to obtain the gradient Mg-doped iron-manganese phosphate precursor Fe. 0.49 Mn 0.5 Mg 0.01 HPO4.

[0067] (3) The above-mentioned gradient Mg-doped iron-manganese phosphate precursor, lithium source, carbon source and water are mixed to obtain a mixed slurry, wherein the solid content of the mixed slurry is 35%. The mixed slurry is subjected to grinding, spray drying, sintering and air jet pulverization to obtain lithium iron-manganese phosphate cathode material LiFe with a carbon coating of 2%. 0.28 Mn 0.7 Mg 0.02 PO4 / C.

[0068] Example 5

[0069] The molecular formula of a lithium manganese iron phosphate cathode material is: LiFe 0.898 Mn 0.1 Co 0.002 The PO4 / C cathode material has a carbon coating of 2% and a particle size of 0.5 μm ≤ D50 ≤ 2 μm. The ferrous sulfate used in its preparation is manganese sulfate, the manganese source is cobalt nitrate, the lithium source is lithium carbonate, and the carbon source is glucose. During preparation, the molar ratio of Fe, Mn, and Co in the manganese source, iron source, and soluble compound of the dopant ion is 0.898:0.1:0.002, and the molar amount of Li in the lithium source is 1.02 times the sum of the molar amounts of Fe, Mn, and Co.

[0070] The preparation method of the above-mentioned lithium manganese iron phosphate cathode material is as follows:

[0071] (1) Dissolve the manganese source and the iron source in water to obtain a 1 mol / L manganese-iron mixed solution; dissolve the soluble compound of the doped ion in water to obtain a 0.5 mol / L doped ion solution; place water in a reaction vessel, add ammonia water to make the concentration 1 mol / L, wherein the molar amount of ammonia water is 1 times the total molar amount of metal ions in the manganese-iron mixed solution and the doped ion solution, and at the same time turn on the stirring of the reaction vessel and introduce argon gas to make the reaction vessel in the protective gas range, then add 2 mol / L phosphoric acid to adjust the pH to 6, and heat the reaction vessel to 55℃.

[0072] (2) Under the condition of stirring speed of 1500 r / min in the reactor, the above manganese-iron mixed solution and the above doped ion solution were added to the reactor in a co-current dropwise manner to carry out co-precipitation crystallization reaction. The addition rate of the manganese-iron mixed solution was controlled at 5 L / h, and the initial addition rate of the doped ion solution was controlled at 0.2 L / h. When the crystal particle size reached 3-5 μm, the addition rate of the doped ion was controlled at 3 L / h. During this process, 0.1 mol / L of vinylpyrrolidone aqueous solution was added at a rate of 0.5 L / h. When the crystal particle size in the reactor was 8 ≤ D50 ≤ 10 μm, the dropwise addition was stopped, and aging continued for 6 h. The aged precipitate was filtered, washed with ethanol, and dried at 300 °C for 10 h to obtain the graded Co-doped iron-manganese phosphate precursor Fe. 0.898 Mn 0.1 Co 0.002 HPO4.

[0073] (3) The above-mentioned graded Co-doped iron-manganese phosphate precursor, lithium source, carbon source and water are mixed to obtain a mixed slurry, wherein the solid content of the mixed slurry is 35%. The mixed slurry is subjected to grinding, spray drying, sintering and air jet pulverization to obtain lithium iron-manganese phosphate cathode material LiFe with a carbon coating of 2%. 0.898 Mn 0.1Co 0.002 PO4 / C.

[0074] Example 6

[0075] The molecular formula of a lithium manganese iron phosphate cathode material is: LiFe 0.08 Mn 0.9 Mg 0.02 The PO4 / C cathode material has a carbon coating of 2% and a particle size of 0.5 μm ≤ D50 ≤ 2 μm. The ferrous sulfate used in its preparation is manganese sulfate, the manganese source is magnesium sulfate, the soluble compound for doping is magnesium nitrate, the lithium source is lithium carbonate, and the carbon source is glucose. During preparation, the molar ratio of Fe, Mn, and Mg in the manganese source, iron source, and soluble compound for doping is 0.08:0.9:0.02, and the molar amount of Li in the lithium source is 1.02 times the sum of the molar amounts of Fe, Mn, and Mg.

[0076] The preparation method of the above-mentioned lithium manganese iron phosphate cathode material is as follows:

[0077] (1) Dissolve the manganese source and the iron source in water to obtain a 1 mol / L manganese-iron mixed solution; dissolve the soluble compound of the doped ion in water to obtain a 0.5 mol / L doped ion solution; place water in a reactor and add ammonia water to make the concentration 1 mol / L, wherein the molar amount of ammonia water is twice the total molar amount of metal ions in the manganese-iron mixed solution and the doped ion solution. At the same time, turn on the stirring of the reactor and introduce argon gas to make the reactor in the protective gas range. Then add 2 mol / L phosphoric acid to adjust the pH to 6 and heat the reactor to 55°C.

[0078] (2) Under the condition of stirring speed of 1500 r / min in the reactor, the above manganese-iron mixed solution and the above doped ion solution were added to the reactor in a co-precipitation crystallization reaction. The addition rate of the manganese-iron mixed solution was controlled at 5 L / h, and the initial addition rate of the doped ion solution was controlled at 0.2 L / h. When the crystal particle size reached 3-5 μm, the addition rate of the doped ion was controlled at 3 L / h. During this process, 0.1 mol / L of vinylpyrrolidone aqueous solution was added at a rate of 0.5 L / h. When the crystal particle size in the reactor was 8 ≤ D50 ≤ 10 μm, the addition was stopped, and aging continued for 6 h. The aged precipitate was filtered, washed with ethanol, and dried at 300 °C for 10 h to obtain the gradient Mg-doped iron-manganese phosphate precursor Fe. 0.08 Mn 0.9 Mg 0.02 HPO4.

[0079] (3) The above-mentioned gradient Mg-doped iron-manganese phosphate precursor, lithium source, carbon source and water are mixed to obtain a mixed slurry, wherein the solid content of the mixed slurry is 35%. The mixed slurry is subjected to grinding, spray drying, sintering and air jet pulverization to obtain lithium iron-manganese phosphate cathode material LiFe with a carbon coating of 2%. 0.88 Mn 0.1 Mg 0.02 PO4 / C.

[0080] Example 7

[0081] The difference between Example 7 and Example 2 is that the rates of the parallel droplet addition process are different.

[0082] The molecular formula of a lithium manganese iron phosphate cathode material is: LiFe 0.39 Mn 0.6 Co 0.01 The PO4 / C cathode material has a carbon coating of 2% and a particle size of 0.5 μm ≤ D50 ≤ 2 μm. The iron source for preparing this lithium manganese iron phosphate cathode material is ferrous sulfate, the manganese source is manganese sulfate, the soluble compound for doping ions is cobalt sulfate, the lithium source is lithium carbonate, and the carbon source is a 1:1 mass mixture of glucose and PEG. During preparation, the molar ratio of Fe, Mn, and Co in the manganese source, iron source, and soluble compound for doping ions is 0.39:0.6:0.01, and the molar amount of Li in the lithium source is 1.02 times the sum of the molar amounts of Fe, Mn, and Co.

[0083] The preparation method of the above-mentioned lithium manganese iron phosphate cathode material is as follows:

[0084] (1) Dissolve the manganese source and the iron source in water to obtain a 1 mol / L manganese-iron mixed solution; dissolve the soluble compound of the doped ion in water to obtain a 0.5 mol / L doped ion solution; place water in a reactor and add ammonia water to make the concentration 1 mol / L, wherein the molar amount of ammonia water is twice the total molar amount of metal ions in the manganese-iron mixed solution and the doped ion solution. At the same time, turn on the stirring of the reactor and introduce argon gas to make the reactor in the protective gas range. Then add 2 mol / L phosphoric acid to adjust the pH to 6 and heat the reactor to 55°C.

[0085] (2) Under the condition of stirring speed of 1500 r / min in the reactor, the above manganese-iron mixed solution and the above doped ion solution were added to the reactor in a co-current dropwise manner to carry out co-precipitation crystallization reaction. The addition rate of the manganese-iron mixed solution was controlled at 0.5 L / h, and the initial addition rate of the doped ion solution was controlled at 0.005 L / h. When the crystal particle size reached 3-5 μm, the addition rate of the doped ion was controlled at 1 L / h. During this process, 0.1 mol / L of vinylpyrrolidone aqueous solution was added at an addition rate of 0.5 L / h. When the crystal particle size in the reactor was 8 ≤ D50 ≤ 10 μm, the dropwise addition was stopped, and aging continued for 6 h. The aged precipitate was filtered, washed with ethanol, and dried at 300 °C for 10 h to obtain the graded Co-doped iron-manganese phosphate precursor Fe. 0.39 Mn 0.6 Co 0.01 HPO4.

[0086] (3) The above-mentioned graded Co-doped iron-manganese phosphate precursor, lithium source, carbon source and water are mixed to obtain a mixed slurry, wherein the solid content of the mixed slurry is 35%. The mixed slurry is subjected to grinding, spray drying, sintering and air jet pulverization to obtain lithium iron-manganese phosphate cathode material LiFe with a carbon coating of 2%. 0.39 Mn 0.6 Co 0.01 PO4 / C.

[0087] Example 8

[0088] The difference between Example 8 and Example 2 is that the rates of the parallel droplet addition process are different.

[0089] The molecular formula of a lithium manganese iron phosphate cathode material is: LiFe 0.39 Mn 0.6 Co 0.01 The PO4 / C cathode material has a carbon coating of 2% and a particle size of 0.5 μm ≤ D50 ≤ 2 μm. The iron source for preparing this lithium manganese iron phosphate cathode material is ferrous sulfate, the manganese source is manganese sulfate, the soluble compound for doping ions is cobalt sulfate, the lithium source is lithium carbonate, and the carbon source is a 1:1 mass mixture of glucose and PEG. During preparation, the molar ratio of Fe, Mn, and Co in the manganese source, iron source, and soluble compound for doping ions is 0.39:0.6:0.01, and the molar amount of Li in the lithium source is 1.02 times the sum of the molar amounts of Fe, Mn, and Co.

[0090] The preparation method of the above-mentioned lithium manganese iron phosphate cathode material is as follows:

[0091] (1) Dissolve the manganese source and the iron source in water to obtain a 1 mol / L manganese-iron mixed solution; dissolve the soluble compound of the doped ion in water to obtain a 0.5 mol / L doped ion solution; place water in a reactor and add ammonia water to make the concentration 1 mol / L, wherein the molar amount of ammonia water is twice the total molar amount of metal ions in the manganese-iron mixed solution and the doped ion solution. At the same time, turn on the stirring of the reactor and introduce argon gas to make the reactor in the protective gas range. Then add 2 mol / L phosphoric acid to adjust the pH to 6 and heat the reactor to 55°C.

[0092] (2) Under the condition of stirring speed of 1500 r / min in the reactor, the above manganese-iron mixed solution and the above doped ion solution were added to the reactor in a co-current dropwise manner to carry out co-precipitation crystallization reaction. The addition rate of the manganese-iron mixed solution was controlled at 5 L / h, and the initial addition rate of the doped ion solution was controlled at 0.5 L / h. When the crystal particle size reached 3-5 μm, the addition rate of the doped ion was controlled at 5 L / h, and 0.1 mol / L of vinylpyrrolidone aqueous solution was added at a rate of 0.5 L / h during this process. When the crystal particle size in the reactor was 8 ≤ D50 ≤ 10 μm, the dropwise addition was stopped, and aging continued for 6 h. The aged precipitate was filtered, washed with ethanol, and dried at 300 °C for 10 h to obtain the graded Co-doped iron-manganese phosphate precursor Fe. 0.39 Mn 0.6 Co 0.01 HPO4.

[0093] (3) The above-mentioned graded Co-doped iron-manganese phosphate precursor, lithium source, carbon source and water are mixed to obtain a mixed slurry, wherein the solid content of the mixed slurry is 35%. The mixed slurry is subjected to grinding, spray drying, sintering and air jet pulverization to obtain lithium iron-manganese phosphate cathode material LiFe with a carbon coating of 2%. 0.39 Mn 0.6 Co 0.01 PO4 / C.

[0094] Example 9

[0095] The difference between Example 9 and Example 2 is that the feeding was stopped when the particle size of the crystallized particles was 30-100 μm.

[0096] The molecular formula of a lithium manganese iron phosphate cathode material is: LiFe 0.39 Mn 0.6 Co 0.01The PO4 / C cathode material has a carbon coating of 2% and a particle size of 0.5 μm ≤ D50 ≤ 2 μm. The iron source for preparing this lithium manganese iron phosphate cathode material is ferrous sulfate, the manganese source is manganese sulfate, the soluble compound for doping ions is cobalt sulfate, the lithium source is lithium carbonate, and the carbon source is a 1:1 mass mixture of glucose and PEG. During preparation, the molar ratio of Fe, Mn, and Co in the manganese source, iron source, and soluble compound for doping ions is 0.39:0.6:0.01, and the molar amount of Li in the lithium source is 1.02 times the sum of the molar amounts of Fe, Mn, and Co.

[0097] The preparation method of the above-mentioned lithium manganese iron phosphate cathode material is as follows:

[0098] (1) Dissolve the manganese source and the iron source in water to obtain a 1 mol / L manganese-iron mixed solution; dissolve the soluble compound of the doped ion in water to obtain a 0.5 mol / L doped ion solution; place water in a reactor and add ammonia water to make the concentration 1 mol / L, wherein the molar amount of ammonia water is twice the total molar amount of metal ions in the manganese-iron mixed solution and the doped ion solution. At the same time, turn on the stirring of the reactor and introduce argon gas to make the reactor in the protective gas range. Then add 2 mol / L phosphoric acid to adjust the pH to 6 and heat the reactor to 55°C.

[0099] (2) Under the condition of stirring speed of 1500 r / min in the reactor, the above manganese-iron mixed solution and the above doped ion solution were added to the reactor in a co-current dropwise manner to carry out co-precipitation crystallization reaction. The addition rate of the manganese-iron mixed solution was controlled at 5 L / h, and the initial addition rate of the doped ion solution was controlled at 0.2 L / h. When the crystal particle size reached 3-5 μm, the addition rate of the doped ion was controlled at 3 L / h. During this process, 0.1 mol / L of vinylpyrrolidone aqueous solution was added at a rate of 0.5 L / h. When the crystal particle size in the reactor was 30 ≤ D50 ≤ 100 μm, the dropwise addition was stopped, and aging continued for 6 h. The aged precipitate was filtered, washed with ethanol, and dried at 300 °C for 10 h to obtain the graded Co-doped iron-manganese phosphate precursor Fe. 0.39 Mn 0.6 Co 0.01 HPO4.

[0100] (3) The above-mentioned graded Co-doped iron-manganese phosphate precursor, lithium source, carbon source and water are mixed to obtain a mixed slurry, wherein the solid content of the mixed slurry is 35%. The mixed slurry is subjected to grinding, spray drying, sintering and air jet pulverization to obtain lithium iron-manganese phosphate cathode material LiFe with a carbon coating of 2%. 0.39 Mn 0.6 Co 0.01 PO4 / C.

[0101] Example 10

[0102] The molecular formula of a lithium manganese iron phosphate cathode material is: LiFe 0.39 Mn 0.6 V 0.01 The PO4 / C cathode material has a carbon coating of 1% and a particle size of 0.5 μm ≤ D50 ≤ 2 μm. The iron source for preparing this lithium manganese iron phosphate cathode material is ferrous sulfate, the manganese source is manganese sulfate, the soluble compound for doping ions is a mixture of ammonium polyvanadate and ammonium metavanadate in a 1:1 molar ratio, the lithium source is lithium carbonate, and the carbon source is PEG. During preparation, the molar ratio of Fe, Mn, and V in the manganese source, iron source, and soluble compound for doping ions is 0.39:0.6:0.01, and the molar amount of Li in the lithium source is 1.02 times the sum of the molar amounts of Fe, Mn, and V.

[0103] The preparation method of the above-mentioned lithium manganese iron phosphate cathode material is as follows:

[0104] (1) Dissolve the manganese source and the iron source in water to obtain a 0.8 mol / L manganese-iron mixed solution; dissolve the soluble compound of the doped ion in water to obtain a 0.6 mol / L doped ion solution; place water in a reactor, add ammonia water to make the concentration 3 mol / L, wherein the molar amount of ammonia water is 1 times the total molar amount of metal ions in the manganese-iron mixed solution and the doped ion solution, and at the same time turn on the stirring of the reactor and introduce nitrogen gas to make the reactor in the protective gas range, then add 1 mol / L phosphoric acid to adjust the pH to 3, and heat the reactor to 40°C.

[0105] (2) With the stirring speed of the reactor at 100 r / min, the above-mentioned manganese-iron mixed solution and the above-mentioned doped ion solution were added to the reactor in a co-precipitation crystallization reaction. The addition rate of the manganese-iron mixed solution was controlled at 5 L / h, and the initial addition rate of the doped ion solution was controlled at 0.2 L / h. When the crystal particle size reached 3-5 μm, the addition rate of the doped ion was controlled at 3 L / h. During this process, 0.1 mol / L of vinylpyrrolidone aqueous solution was added at a rate of 0.5 L / h. The addition was stopped when the crystal particle size in the reactor was 8 ≤ D50 ≤ 10 μm, and aging continued for 4 h. The aged precipitate was filtered, washed with acetone, and dried at 100 °C for 10 h to obtain the gradient V-doped iron-manganese phosphate precursor Fe. 0.39 Mn 0.6 V 0.01 HPO4.

[0106] (3) The gradient V-doped iron-manganese phosphate precursor, lithium source, carbon source, and water are mixed to obtain a mixed slurry with a solid content of 30%. The mixed slurry is then subjected to grinding, spray drying, sintering, and air jet milling to obtain a lithium iron-manganese phosphate cathode material LiFe with a carbon coating of 1%.0.39 Mn 0.6 V 0.01 PO4 / C.

[0107] Example 11

[0108] The molecular formula of a lithium manganese iron phosphate cathode material is: LiFe 0.39 Mn 0.6 Zr 0.01 The PO4 / C cathode material has a carbon coating of 3% and a particle size of 0.5 μm ≤ D50 ≤ 2 μm. The iron source for preparing this lithium manganese iron phosphate cathode material is ferrous sulfate, the manganese source is manganese sulfate, the soluble compound for doping ions is zirconium nitrate, the lithium source is lithium carbonate, and the carbon source is glucose. During preparation, the molar ratio of Fe, Mn, and Zr in the manganese source, iron source, and soluble compound for doping ions is 0.39:0.6:0.01, and the molar amount of Li in the lithium source is 1.08 times the sum of the molar amounts of Fe, Mn, and Zr.

[0109] The preparation method of the above-mentioned lithium manganese iron phosphate cathode material is as follows:

[0110] (1) Dissolve the manganese source and the iron source in water to obtain a 1.2 mol / L manganese-iron mixed solution; dissolve the soluble compound of the doped ion in water to obtain a 0.3 mol / L doped ion solution; place water in a reactor and add ammonia water to make the concentration 1 mol / L, wherein the molar amount of ammonia water is 1 times the total molar amount of metal ions in the manganese-iron mixed solution and the doped ion solution. At the same time, turn on the stirring of the reactor and introduce argon gas to make the reactor in the protective gas range. Then add 2 mol / L phosphoric acid to adjust the pH to 6 and heat the reactor to 80°C.

[0111] (2) With the stirring speed of the reactor at 1500 r / min, the above-mentioned manganese-iron mixed solution and the above-mentioned doped ion solution were added to the reactor in a co-precipitation crystallization reaction. The addition rate of the manganese-iron mixed solution was controlled at 5 L / h, and the initial addition rate of the doped ion solution was controlled at 0.2 L / h. When the crystal particle size reached 3-5 μm, the addition rate of the doped ion was controlled at 3 L / h. During this process, 0.1 mol / L of vinylpyrrolidone aqueous solution was added at a rate of 0.5 L / h. The addition was stopped when the crystal particle size in the reactor was 8 ≤ D50 ≤ 10 μm, and aging continued for 8 h. The aged precipitate was filtered, washed with water, and dried at 500 °C for 5 h to obtain the gradient Zr-doped iron-manganese phosphate precursor Fe. 0.39 Mn 0.6 Zr 0.01 HPO4.

[0112] (3) The above-mentioned gradient Zr-doped iron-manganese phosphate precursor, lithium source, carbon source and water are mixed to obtain a mixed slurry, wherein the solid content of the mixed slurry is 40%. The mixed slurry is subjected to grinding, spray drying, sintering and air jet pulverization to obtain lithium iron-manganese phosphate cathode material LiFe with a carbon coating of 3%. 0.39 Mn 0.6 Zr 0.01 PO4 / C.

[0113] Example 12

[0114] The molecular formula of a lithium manganese iron phosphate cathode material is: LiFe 0.39 Mn 0.6 Co 0.01 The PO4 / C cathode material has a carbon coating of 2% and a particle size of 0.5 μm ≤ D50 ≤ 2 μm. The iron source for preparing this lithium manganese iron phosphate cathode material is ferrous sulfate, the manganese source is manganese sulfate, the soluble compound for doping ions is cobalt sulfate, the lithium source is lithium carbonate, and the carbon source is a 1:1 mass mixture of glucose and PEG. During preparation, the molar ratio of Fe, Mn, and Co in the manganese source, iron source, and soluble compound for doping ions is 0.39:0.6:0.01, and the molar amount of Li in the lithium source is 1.02 times the sum of the molar amounts of Fe, Mn, and Co.

[0115] The preparation method of the above-mentioned lithium manganese iron phosphate cathode material is as follows:

[0116] (1) Dissolve the manganese source and the iron source in water to obtain a 1.5 mol / L manganese-iron mixed solution; dissolve the soluble compound of the dopant ion in water to obtain a 1 mol / L dopant ion solution; place water in a reactor and add ammonia water to make the concentration 1 mol / L, wherein the molar amount of ammonia water is twice the total molar amount of metal ions in the manganese-iron mixed solution and the dopant ion solution. At the same time, turn on the stirring of the reactor and introduce argon gas to make the reactor in the protective gas range. Then add 2 mol / L phosphoric acid to adjust the pH to 7 and heat the reactor to 55°C.

[0117] (2) Under the condition of stirring speed of 1500 r / min in the reactor, the above manganese-iron mixed solution and the above doped ion solution were added to the reactor in a co-current dropwise manner to carry out co-precipitation crystallization reaction. The addition rate of the manganese-iron mixed solution was controlled at 5 L / h, and the initial addition rate of the doped ion solution was controlled at 0.2 L / h. When the crystal particle size reached 3-5 μm, the addition rate of the doped ion was controlled at 3 L / h. During this process, 0.1 mol / L of vinylpyrrolidone aqueous solution was added at a rate of 0.5 L / h. When the crystal particle size in the reactor was 100 ≤ D50 ≤ 150 μm, the dropwise addition was stopped, and aging continued for 2 h. The aged precipitate was filtered, washed with ethanol, and dried at 100 °C for 5 h to obtain the graded Co-doped iron-manganese phosphate precursor Fe. 0.39 Mn 0.6 Co 0.01 HPO4.

[0118] (3) The above-mentioned graded Co-doped iron-manganese phosphate precursor, lithium source, carbon source and water are mixed to obtain a mixed slurry, wherein the solid content of the mixed slurry is 35%. The mixed slurry is subjected to grinding, spray drying, sintering and air jet pulverization to obtain lithium iron-manganese phosphate cathode material LiFe with a carbon coating of 2%. 0.39 Mn 0.6 Co 0.01 PO4 / C.

[0119] Comparative Example 1

[0120] The molecular formula of a lithium manganese iron phosphate cathode material is: LiFe 0.4 Mn 0.6 The PO4 / C cathode material has a carbon coating of 2% and a particle size of 0.5 μm ≤ D50 ≤ 2 μm. The iron source for preparing this lithium manganese iron phosphate cathode material is ferrous nitrate, the manganese source is manganese nitrate, the lithium source is lithium carbonate, and the carbon source is glucose. During preparation, the molar ratio of Fe to Mn in the manganese and iron sources is 0.4:0.6, and the molar amount of Li in the lithium source is 1.02 times the sum of the molar amounts of Fe and Mn.

[0121] The preparation method of the above-mentioned lithium manganese iron phosphate cathode material is as follows:

[0122] (1) Dissolve manganese and iron sources in water to obtain a 1 mol / L manganese-iron mixed solution; place water in a reaction vessel, add citric acid to make its concentration 1 mol / L, wherein the molar amount of citric acid is twice the total molar amount of metal ions in the manganese-iron mixed solution, at the same time turn on the stirring of the reaction vessel and introduce nitrogen gas to make the reaction vessel in the protective gas range, then add 2 mol / L phosphoric acid to adjust the pH to 3, and heat the reaction vessel to 55℃.

[0123] (2) With the stirring speed of the reactor at 1500 r / min, the above manganese-iron mixed solution was added to the reactor to carry out a co-precipitation crystallization reaction. The reaction was stopped when the crystal particle size in the reactor was 8 ≤ D50 ≤ 10 μm, and aging continued for 6 h. The aged precipitate was filtered, washed with water, and dried at 60 °C for 10 h to obtain the manganese-iron phosphate precursor Fe. 0.4 Mn 0.6 HPO4.

[0124] (3) The above-mentioned iron-manganese phosphate precursor, lithium source, carbon source and water are mixed to obtain a mixed slurry, wherein the solid content of the mixed slurry is 35%. The mixed slurry is subjected to grinding, spray drying, sintering and air jet pulverization to obtain lithium iron phosphate cathode material LiFe with a carbon coating of 2%. 0.4 Mn 0.6 PO4 / C.

[0125] Comparative Example 2

[0126] The molecular formula of a lithium manganese iron phosphate cathode material is: LiFe 0.40 Mn 0.6 The PO4 / C cathode material has a carbon coating of 2% and a particle size of 0.5 μm ≤ D50 ≤ 2 μm. The iron source for preparing this lithium manganese iron phosphate cathode material is ferrous nitrate, the manganese source is manganese nitrate, the lithium source is lithium carbonate, and the carbon source is a 1:1 mass mixture of glucose and PEG. During preparation, the molar ratio of Fe, Mn, and Mg in the manganese and iron sources is 0.40:0.6, and the molar amount of Li in the lithium source is 1.02 times the sum of the molar amounts of Fe and Mn.

[0127] The preparation method of the above-mentioned lithium manganese iron phosphate cathode material is as follows:

[0128] (1) Dissolve manganese and iron sources in water to obtain a 1 mol / L manganese-iron mixed solution; place water in a reaction vessel, add citric acid to make its concentration 1 mol / L, wherein the molar amount of citric acid is twice the total molar amount of metal ions in the manganese-iron mixed solution, and at the same time turn on the stirring of the reaction vessel and introduce nitrogen gas to make the reaction vessel in the protective gas range, then add 2 mol / L phosphoric acid to adjust the pH to 3, and heat the reaction vessel to 55℃.

[0129] (2) With the stirring speed of the reactor at 1500 r / min, the above-mentioned manganese-iron mixed solution was added dropwise to the reactor for co-precipitation crystallization. The addition rate of the manganese-iron mixed solution was controlled at 5 L / h, and the addition rate of 0.1 mol / L vinylpyrrolidone was controlled at 0.5 L / h. Dropwise addition was stopped when the crystal particle size in the reactor was 8 ≤ D50 ≤ 10 μm, and aging continued for 6 h. The aged precipitate was filtered, washed with water, and dried at 300 °C for 10 h to obtain the graded iron-manganese phosphate precursor Fe.0.4 Mn 0.6 HPO4.

[0130] (3) The above-mentioned gradient manganese iron phosphate precursor, lithium source, carbon source and water are mixed to obtain a mixed slurry, wherein the solid content of the mixed slurry is 35%. The mixed slurry is subjected to grinding, spray drying, sintering and air jet pulverization to obtain a lithium iron phosphate cathode material LiFe with a carbon coating of 2%. 0.4 Mn 0.6 PO4 / C.

[0131] Comparative Example 3

[0132] The molecular formula of a lithium manganese iron phosphate cathode material is: LiFe 0.39 Mn 0.6 Co 0.01 The PO4 / C cathode material has a carbon coating of 2% and a particle size of 0.5 μm ≤ D50 ≤ 2 μm. The iron source for preparing this lithium manganese iron phosphate cathode material is ferrous sulfate, the manganese source is manganese sulfate, the soluble compound for doping ions is cobalt sulfate, the lithium source is lithium carbonate, and the carbon source is a 1:1 mass mixture of glucose and PEG. During preparation, the molar ratio of Fe, Mn, and Co in the manganese source, iron source, and soluble compound for doping ions is 0.39:0.6:0.01, and the molar amount of Li in the lithium source is 1.02 times the sum of the molar amounts of Fe, Mn, and Co.

[0133] The preparation method of the above-mentioned lithium manganese iron phosphate cathode material is as follows:

[0134] (1) Dissolve the manganese source and the iron source in water to obtain a 1 mol / L manganese-iron mixed solution; dissolve the soluble compound of the doped ion in water to obtain a 0.5 mol / L doped ion solution; place water in a reactor and add ammonia water to make the concentration 1 mol / L, wherein the molar amount of ammonia water is twice the total molar amount of metal ions in the manganese-iron mixed solution and the doped ion solution. At the same time, turn on the stirring of the reactor and introduce argon gas to make the reactor in the protective gas range. Then add 2 mol / L phosphoric acid to adjust the pH to 6 and heat the reactor to 55°C.

[0135] (2) With the stirring speed of the reactor at 1500 r / min, the above manganese-iron mixed solution and the above doped ion solution were mixed in the specified molecular formula element ratio and added to the reactor; the reaction was stopped when the crystallization particle size in the reactor was 8 ≤ D50 ≤ 10 μm, and aging was continued for 6 h. The aged precipitate was filtered, washed with ethanol, and dried at 300 °C for 10 h to obtain the Co-doped iron-manganese phosphate precursor Fe. 0.39 Mn 0.6 Co 0.01 HPO4.

[0136] (3) The Co-doped manganese iron phosphate precursor, lithium source, carbon source, and water were mixed to obtain a mixed slurry with a solid content of 35%. The mixed slurry was then subjected to grinding, spray drying, sintering, and air jet milling to obtain a lithium iron phosphate cathode material (LiFe) with a carbon coating of 2%. 0.39 Mn 0.6 Co 0.01 PO4 / C.

[0137] The lithium manganese iron phosphate cathode materials prepared in Examples 1 to 12 and Comparative Examples 1 to 3 were subjected to relevant performance tests, and were then used in lithium-ion batteries for further performance testing. The preparation methods of the battery cathode, the lithium-ion battery, and the relevant testing methods are described below:

[0138] Explanation of the preparation of the battery positive electrode and the preparation of lithium-ion batteries:

[0139] (1) Preparation of slurry: 23.75g of positive electrode material, 0.5g of superconducting carbon black (SP) and 0.75g of binder polyvinylidene fluoride (PVDF) were added to a 500mL agate ball mill jar, and 16g of N-methylpyrrolidone (NMP) was added as solvent. The slurry was prepared by ball milling at 360r / min for 4h.

[0140] (2) Preparation of the positive electrode sheet: Adjust the scale of the doctor blade of the coating machine, and evenly coat the prepared slurry onto the aluminum foil. Place the coated electrode sheet in a vacuum drying oven and bake at 130℃ for 3 hours. Place the coated aluminum foil flat in the middle of the roller and roll the electrode sheet. Place the rolled electrode sheet with the front side tightly against the perforated area and punch the sheet in sequence. The compaction density of the electrode sheet is controlled between 2.0 and 2.4 g / cm³. 3 The diameter is 14mm and the thickness is 0.05~0.10mm; the punched electrode is placed in a vacuum drying oven at 130℃ and baked for 3h to obtain the positive electrode.

[0141] (3) Assembly of button cells: In the glove box, the negative electrode shell, spring, steel sheet, lithium sheet, separator, positive electrode sheet and positive electrode shell are assembled in sequence. During the process, 10 μL of 1 mol / L LiPF6 electrolyte is injected. The electrolyte uses a 1:1 volume mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) as solvent. Then, the button cells are sealed with a sealing machine to obtain button cells.

[0142] Explanation of the testing method:

[0143] Powder resistivity test: The powder resistivity was tested using the four-probe method. 0.4g of lithium manganese iron phosphate cathode material powder was taken and its resistivity was tested under a pressure of 30MPa for 10s. The results are shown in Table 1.

[0144] Test method for manganese leaching after 500 cycles: After 500 cycles, the negative electrode sheet was dried, and 5g of powder was scraped off with a blade and dissolved in 200mL of 0.008mol / L hydrochloric acid solution. The solution was stirred at 880rpm for 30min, and then placed in a 25℃ constant temperature water bath for 2h. After filtration, 1ml of the filtrate was placed in a 50ml volumetric flask, 2mL of GR concentrated nitric acid was added, and the solution was diluted to the mark for ICP detection. The manganese content in the negative electrode sheet can reflect the specific manganese leaching of the positive electrode sheet prepared in this application.

[0145] The lithium manganese iron phosphate cathode materials prepared in Examples 1 to 13 and Comparative Examples 1 to 3 were subjected to powder resistivity testing according to the above method. The lithium manganese iron phosphate cathode materials prepared in the above examples and comparative examples were respectively prepared into corresponding coin cells according to the above method for preparing coin cells. The 0.2C discharge capacity, 1C discharge capacity, and capacity retention rate after 500 cycles of 1C charge and discharge were tested respectively. The amount of Mn dissolved after 500 cycles was also tested.

[0146] Table 1 shows the performance test results of lithium manganese iron phosphate cathode material and the relevant performance test results of lithium-ion batteries when it is used in lithium-ion batteries.

[0147] Table 1

[0148] From the above description, it can be seen that the embodiments of this application achieve the following technical effects: The gradient-doped lithium manganese iron phosphate cathode material prepared in the embodiments of this application has a lower resistivity, and exhibits better specific capacity performance when applied to batteries. In addition, the lithium manganese iron phosphate cathode material prepared in the embodiments of this application has less manganese dissolution when applied to batteries, indicating that the material structure proposed in this application can effectively improve defects such as the Jahn-Teller effect and manganese dissolution, thereby making the battery exhibit better conductivity and cycle stability. By controlling the preparation process parameters within the preferred range, the obtained lithium manganese iron phosphate cathode material exhibits better overall performance. In contrast, the lithium manganese iron phosphate cathode materials prepared in Comparative Examples 1 to 3 are either undoped with metal ions or are not the gradient structure proposed in this application, and their overall performance is far different from that of the structure proposed in this application.

[0149] In summary, the gradient-doped lithium manganese iron phosphate cathode material proposed in this application can effectively improve the poor phase structure stability, poor lithium-ion conductivity, and problems such as the Jahn-Teller effect and manganese dissolution that occur in lithium manganese iron phosphate cathode materials. This improves the overall performance of lithium manganese iron phosphate cathode materials in applications, including conductivity and cycle stability.

[0150] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A lithium manganese iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material includes a doped manganese iron phosphate core and a coating layer covering the surface of the doped manganese iron phosphate core. The coating layer is a lithium-containing carbon layer. The doped manganese iron phosphate core contains dopant ions, and the content gradient of manganese ions and iron ions decreases from the inside to the outside of the particles of the doped manganese iron phosphate core, while the content gradient of dopant ions increases.

2. The lithium iron phosphate cathode material according to claim 1, characterized in that, The chemical formula of the lithium manganese iron phosphate cathode material is: LiFe 1-x-y Mn x M y PO4 / C, where 0.1≤x≤0.9, 0.002≤y≤0.02, and M is one or more of nickel, cobalt, magnesium, titanium, copper, vanadium, zirconium, and chromium; Preferably, the LiFe 1-x-y Mn x M y In PO4 / C, 0.5 ≤ x ≤ 0.7, 0.01 ≤ y ≤ 0.02; Preferably, the particle size of the lithium manganese iron phosphate cathode material is 0.5–10 μm; Preferably, the carbon coating content of the coating layer is 1% to 3%; Preferably, M is cobalt and / or magnesium.

3. A method for preparing the lithium manganese iron phosphate cathode material according to claim 1 or 2, characterized in that, The preparation method includes the following steps: Step S1: Dissolve the manganese source and the iron source in water to obtain a manganese-iron mixed solution; dissolve the soluble compound of the doped ion in water to obtain a doped ion solution; Step S2: Place the complexing agent aqueous solution in a reaction vessel and add phosphoric acid to adjust the pH; Step S3: The manganese-iron mixed solution and the doped ion solution are added to the reaction vessel in a co-precipitation crystallization reaction to obtain a crystalline precipitate; during the co-precipitation process, the flow rate of the doped ion solution is gradually increased. Step S4: The crystalline precipitate is filtered, washed and dried to obtain a gradient-doped manganese iron phosphate precursor; Step S5: The iron-manganese precursor, lithium source, carbon source and water are mixed to obtain a mixed slurry. The mixed slurry is then subjected to grinding, spray drying, sintering and pulverizing to obtain the lithium iron manganese phosphate cathode material.

4. The method for preparing lithium manganese iron phosphate cathode material according to claim 3, characterized in that, In step S1, the concentration of the manganese-iron mixed solution is 0.8–1.2 mol / L; Preferably, the concentration of the doped ion solution is 0.3 to 0.6 mol / L.

5. The method for preparing lithium manganese iron phosphate cathode material according to claim 4, characterized in that, The co-current dropwise addition operation is carried out under stirring, and the addition rate of the manganese-iron mixed solution is 0.5-5 L / h. Preferably, the initial addition rate of the doped ion solution is 0.005–0.5 L / h, and when the crystal particle size reaches 3–5 μm, the addition rate of the doped ion solution is controlled to be 1–5 L / h. Preferably, when the crystallization particle size reaches 3-5 μm, the method further includes: continuously adding a surfactant to the coprecipitation crystallization reaction system until the coprecipitation crystallization reaction is completed; Preferably, the surfactant is vinylpyrrolidone; Preferably, the surfactant is added at a rate of 0.2–0.5 L / h; Preferably, the surfactant is added in the form of an aqueous solution with a concentration of 0.1 to 1 mol / L; Preferably, the stirring speed is 100-1500 r / min.

6. The method for preparing the lithium manganese iron phosphate cathode material according to any one of claims 3 to 5, characterized in that, In step S3, feeding is stopped when the particle size of the crystallized particles is 5-100 μm, and aging is carried out. Preferably, the particle size of the crystalline particles is 5–30 μm; Preferably, the aging time is 4 to 8 hours.

7. The method for preparing the lithium manganese iron phosphate cathode material according to any one of claims 3 to 5, characterized in that, The doped ion compound is one or more of nickel sulfate, nickel nitrate, cobalt nitrate, cobalt sulfate, magnesium nitrate, magnesium sulfate, magnesium acetate, tetrabutyl titanate, ammonium fluorotitanate, copper sulfate, ammonium polyvanadate, ammonium metavanadate, zirconium nitrate, or chromium sulfate. Preferably, the iron source is one or more selected from ferrous sulfate, ferrous oxalate, ferrous acetate, and ferrous nitrate; Preferably, the manganese source is one or more selected from manganese sulfate, manganese oxalate, manganese acetate, and manganese nitrate; Preferably, the lithium source is one or more of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate; Preferably, the complexing agent is one or more of ammonia, ammonium oxalate, citric acid, malic acid, and EDTA-2Na; Preferably, the carbon source is one or more of polyethylene glycol, glucose, chitosan, PVP-K30, and CNT.

8. The method for preparing the lithium manganese iron phosphate cathode material according to any one of claims 3 to 5, characterized in that, When adding the complexing agent in step S2, a protective gas is first introduced into the reactor to adjust the pH to 3-6. Preferably, the concentration of the complexing agent is 1.0–3.0 mol / L; Preferably, the amount of complexing agent added is 1 to 2 times the total molar amount of metal ions in the manganese-iron mixed solution and the doped ion solution; Preferably, the concentration of phosphoric acid is 1.0–2.0 mol / L; Preferably, the temperature of the co-precipitation crystallization reaction is 40–80°C; Preferably, the solvent used for washing in step S4 is one or more of deionized water, ethanol, and acetone; Preferably, the drying temperature is 100–500°C, and the drying time is 5–10 hours.

9. The method for preparing the lithium manganese iron phosphate cathode material according to any one of claims 3 to 5, characterized in that, The solid content of the mixed slurry is 30% to 40%.

10. A lithium-ion battery, characterized in that, The positive electrode of the lithium-ion battery comprises lithium manganese iron phosphate positive electrode material prepared by the preparation method of lithium manganese iron phosphate positive electrode material according to any one of claims 3 to 9.

Images