Lithium manganese iron phosphate positive electrode material, preparation method thereof and electrochemical device

Abstract

The invention discloses a lithium manganese iron phosphate positive electrode material, a preparation method thereof and an electrochemical device, and the preparation method comprises the following steps: mixing a manganese source, an iron source, a phosphorus source, a lithium source and a first carbon source, and carrying out primary grinding to obtain first mixed slurry; performing primary sintering on the first mixed slurry to obtain a base material; mixing the base material with water, and carrying out pressure filtration to obtain a solid material; mixing the solid material and a second carbon source, and performing secondary grinding to obtain second mixed slurry; and performing secondary sintering on the second mixed slurry to form a coating layer on the surface of the base material, the coating layer comprising a carbon layer, thereby obtaining the lithium manganese iron phosphate positive electrode material. The pH and specific surface area of the lithium manganese iron phosphate positive electrode material are effectively reduced by means of water washing and pressure filtration, and the processing performance of the lithium manganese iron phosphate positive electrode material in preparation of a rear-end electrochemical device is improved.

Description

Technical Field

[0001] The present application relates to the field of new energy technologies, and particularly to lithium iron manganese phosphate cathode materials, methods for preparing the same, and electrochemical devices. Background Art

[0002] As an emerging cathode material for lithium batteries, lithium iron manganese phosphate (LMFP) combines the stability of lithium iron phosphate (LFP) and the high-voltage advantage of lithium manganese phosphate (LMP). It can be used alone as a cathode material for batteries or can be graded with ternary (NCM) cathode materials to improve the energy density of the cathode material and reduce costs.

[0003] However, existing lithium iron manganese phosphate cathode materials have problems such as high pH value and high specific surface area. Due to the high pH value and high specific surface area, the force of mutual adsorption on the particle surface is relatively strong, resulting in particle aggregation and poor dispersibility. This makes it difficult to increase the solid content when stretching the slurry, and the viscosity of the slurry is relatively large, restricting the processing performance of the lithium iron manganese phosphate cathode material. As a result, the electrodes prepared widely exhibit a rough surface. Summary of the Invention

[0004] In view of this, to solve at least one of the above technical problems, the present application provides a method for preparing a lithium iron manganese phosphate cathode material.

[0005] In addition, an embodiment of the present application further provides a lithium iron manganese phosphate cathode material prepared by the method for preparing the above lithium manganese oxide cathode material, and an electrochemical device applying the lithium manganese oxide cathode material.

[0006] In a first aspect, an embodiment of the present application provides a method for preparing a lithium iron manganese phosphate cathode material, the preparation method comprising the following steps: Mixing a manganese source, an iron source, a phosphorus source, a lithium source, and a first carbon source and performing primary grinding to obtain a first mixed slurry; performing primary sintering on the first mixed slurry to obtain a matrix material; mixing the matrix material with water and performing pressure filtration to obtain a solid material; mixing the solid material and a second carbon source and performing secondary grinding to obtain a second mixed slurry; and performing secondary sintering on the second mixed slurry to form a coating layer on the surface of the matrix material, the coating layer comprising a carbon layer, thereby obtaining the lithium iron manganese phosphate cathode material.

[0007] In some possible embodiments, in the step of mixing the matrix material with water, the water contains a coating material, and the coating material contains a metal element; During the secondary sintering process, a metal coating layer is further formed on the surface of the matrix material, the metal coating layer is located between the matrix material and the carbon layer, and the metal coating layer and the carbon layer constitute the coating layer.

[0008] In some possible embodiments, the metal element includes at least one of iron, titanium, vanadium, and zirconium.

[0009] In some possible embodiments, the mass percentage of the coating material in the matrix material is 0.05% - 0.5%.

[0010] In some possible embodiments, the pressure of the pressure filtration is 0.1 MPa - 0.3 MPa.

[0011] In some possible embodiments, in the step of mixing the matrix material with water, the mass ratio of water to the matrix material is (0.5 - 2):1; and / or After the step of pressure filtration, the moisture content of the solid material is 3% - 6%.

[0012] In some possible embodiments, the conditions for the first sintering include: the sintering temperature is 500 - 750 °C, and the heat preservation time is 180 min - 900 min; and / or The conditions for the second sintering include: the sintering temperature is 650 - 850 °C, and the heat preservation time is 180 min - 900 min.

[0013] In a second aspect, an embodiment of the present application further provides a lithium iron manganese phosphate cathode material. The lithium manganese oxide cathode material is prepared by the preparation method of the lithium iron manganese phosphate cathode material described above. The lithium iron manganese phosphate cathode material includes a matrix material and a coating layer on the surface of the matrix material. The matrix material is lithium iron manganese phosphate, and the coating layer includes a carbon layer.

[0014] In some possible embodiments, the coating layer further includes a metal coating layer, and the metal coating layer is located between the matrix material and the carbon layer.

[0015] In a third aspect, an embodiment of the present application further provides an electrochemical device. The electrochemical device includes a positive electrode plate, and the positive electrode plate includes a positive electrode material, and the positive electrode material is the lithium iron manganese phosphate cathode material described above.

[0016] Compared with the prior art, the lithium iron manganese phosphate cathode material and its preparation method provided by the embodiments of the present application can remove the residual alkali on the surface of the lithium iron manganese phosphate matrix material through a water washing method of mixing with water and pressurized filtration, thereby effectively reducing the pH value of the lithium iron manganese phosphate cathode material. At the same time, small particle impurities remaining on the surface of the matrix material are also removed during the water washing and pressurized filtration processes. These residual substances adhering to the surface of the matrix material particles will increase the specific surface area of the lithium iron manganese phosphate cathode material. Removing the remaining small particle impurities can obtain a purer lithium iron manganese phosphate phase and is also beneficial to reducing the specific surface area of the lithium iron manganese phosphate cathode material. In addition, a carbon coating layer is formed on the surface of the matrix material, which is beneficial to forming a conductive network and improving the conductivity of the lithium iron manganese phosphate cathode material.

[0017] The present application can effectively reduce the pH value and specific surface area of the lithium iron manganese phosphate cathode material only through two sinterings in combination with one water washing. During the preparation process of the electrode sheet, the lower pH value and specific surface area are beneficial to reducing the viscosity of the slurry of the lithium iron manganese phosphate cathode material, improving the fluidity and stability of the slurry, realizing the increase of the solid content of the slurry, thereby improving the processing performance of the lithium iron manganese phosphate cathode material prepared into the electrode sheet and improving the quality of the electrode sheet. Brief Description of the Drawings

[0018] Figure 1 is a process flow chart of a preparation method of a lithium iron manganese phosphate cathode material provided by an embodiment of the present application.

[0019] Figure 2 is a schematic structural diagram of a lithium iron manganese phosphate cathode material provided by an embodiment of the present application.

[0020] Figure 3 is a schematic structural diagram of a lithium iron manganese phosphate cathode material provided by another embodiment of the present application. Detailed Description of the Embodiments

[0021] The embodiments of the present application will be described in detail below. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present application and should not be construed as a limitation of the present application; it should be noted that unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present application belongs; without conflict, the embodiments of the present application and the features in the embodiments can be combined with each other; many specific details are set forth in the following description to fully understand the present application, and the described embodiments are only some embodiments of the present application, not all embodiments.

[0022] Please refer to Figure 1 As shown, the embodiments of the present application provide a preparation method of a lithium iron manganese phosphate cathode material, which specifically includes the following steps: Step S1, mix a manganese source, an iron source, a phosphorus source, a lithium source, and a first carbon source and perform primary grinding to obtain a first mixed slurry.

[0023] Specifically, mixing the manganese source, the iron source, the phosphorus source, the lithium source, and the first carbon source and then performing primary grinding can fully mix the above raw materials, make the raw materials evenly dispersed, and obtain a first mixed slurry. The method of primary grinding can include at least one of sand grinding, high-energy ball milling, vibration milling, blender grinding, and soybean milk machine grinding. It can be understood that grinding includes but is not limited to the above methods, and any method that can achieve grinding of materials is acceptable.

[0024] In some embodiments, the manganese source, the iron source, the phosphorus source, the lithium source, and the first carbon source can be dissolved in a solvent and mixed evenly. The solvent can be water, which can make the raw materials fully dispersed in the solvent, increase the contact probability between the raw material particles and the medium during grinding, and thus improve the grinding effect.

[0025] In some embodiments, grinding can be carried out in a step-by-step manner. For example, in two steps, in the first step, the above raw materials are ground to a particle size D50 of 0.8 μm to 1.2 μm (the diameter of the zirconia beads added is 0.6 mm), and in the second step, the grinding product of the first step is continuously ground to a particle size D50 of 0.30 μm to 0.45 μm (the diameter of the zirconia beads added is 0.3 mm), thereby obtaining a first mixed slurry. By step-by-step grinding, the particle size can be better controlled.

[0026] The average particle size D50 of the particles in the first mixed slurry is 0.30 μm to 0.45 μm, which is beneficial to reducing particle agglomeration and improving the stability and uniformity of the first mixed slurry. The average particle size D50 of the particles in the first mixed slurry can exemplarily be 0.3 μm, 0.35 μm, 0.4 μm, 0.45 μm, or any value within the range composed of any two of the above values.

[0027] In some embodiments, the molar ratio of Mn:Fe:P:Li in the manganese source, the iron source, the phosphorus source, and the lithium source can be mixed as (0.2 - 0.8):(0.2 - 0.8):(1.02 - 1.05):(1 - 1.05). Optimizing the ratio of each component in the first mixed slurry can reduce the residues of the lithium source and the iron source, which is beneficial to reducing the residual alkali on the surface of the matrix material.

[0028] In some embodiments, a doping element can also be added to the first mixed slurry. Among them, the doping element can include at least one of Mg, Ti, V, Nb, W, Mo, Er, Co, Ni, Zr, Sr, and Al, etc. The above doping elements can play a role in improving the electrochemical performance of the material.

[0029] In some embodiments, the mass ratio of the total mass of the iron source and the manganese source to the mass of the first carbon source can be (10 - 20):1.

[0030] In some embodiments, the manganese source includes at least one of manganese dioxide, manganese tetroxide, manganese carbonate, etc.

[0031] In some embodiments, the iron source may include at least one of ferrous oxalate, iron phosphate, iron(III) oxide, etc.

[0032] In some embodiments, the phosphorus source may include at least one of phosphoric acid, lithium dihydrogen phosphate, ammonium dihydrogen phosphate, etc.

[0033] In some embodiments, the lithium source includes at least one of lithium dihydrogen phosphate, lithium hydroxide, lithium carbonate, lithium phosphate, lithium nitrate, etc.

[0034] In some embodiments, the first carbon source may include at least one of glucose, sucrose, starch, polyethylene glycol, etc. The first carbon source is beneficial to inhibiting the oxidation of the carbon source and the manganese source to high valence states, and reducing the capacity loss of the lithium iron manganese phosphate cathode material.

[0035] Step S2, subject the first mixed slurry to a first sintering to obtain a matrix material.

[0036] Specifically, subject the first mixed slurry to a first sintering under an inert atmosphere (which can be nitrogen) and certain sintering conditions to obtain a matrix material of lithium iron manganese phosphate. The chemical general formula of the matrix material is LiMn x Fe 1-x PO 4 , where 0 < x < 1, and x can exemplarily be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or any value within the range composed of any two of the above numerical values.

[0037] In some embodiments, the conditions of the first sintering may include: the sintering temperature is 500~750 °C, and the heat preservation time is 180 min~900 min. Through appropriate sintering temperature and sintering time, it promotes the uniform dispersion of manganese elements and iron elements in the lithium iron phosphate matrix material, and helps the formation of the crystal structure of the lithium iron phosphate matrix material.

[0038] In some embodiments, the heating rate of the first sintering can be 1.5 °C / min~5 °C / min, and the heating time can be 150 min~500 min. Under the above heating rate and heating time, it is beneficial to reduce the thermal stress and further improve the crystal quality of the matrix material.

[0039] In some embodiments, before the step of subjecting the first mixed slurry to a first sintering, the preparation method may further include: drying the first mixed slurry by spray drying, which is beneficial to removing the solvent and effectively reducing the caking phenomenon of the first mixed slurry during the first sintering process.

[0040] Further, the inlet air temperature for spray drying can be 200°C to 250°C, and the outlet air temperature can be 75°C to 120°C. Setting the ranges of the inlet air temperature and the outlet air temperature helps to increase the evaporation rate of the solvent and the drying degree during the spray drying process.

[0041] Step S3: Mix the matrix material with water and perform pressure filtration to obtain solid materials.

[0042] Specifically, water is added to the matrix material and stirred at a speed of 50 rpm to 200 rpm in a magnetic stirrer. After being fully mixed evenly, pressure filtration is carried out. By means of dissolution and filtration, the residual alkali on the surface of the matrix material and other small particle impurities adsorbed on the surface of the matrix material are removed. The filtrate is removed to obtain the washed solid materials. Among them, pressure filtration applies an external pressure to push the liquid through the filter medium (such as filter membranes, filter cloths, etc.), and the solid particles are retained on the filter medium, which can improve the water washing efficiency and is suitable for scenarios where a large amount of slurry needs to be processed. It can be understood that any method capable of realizing solid-liquid separation and removing surface residual alkali and small particle impurities is acceptable.

[0043] In some embodiments, the mass ratio of water to the matrix material can be (0.5 to 2):1, which helps to fully dissolve and remove the residual alkali on the surface of the matrix material and the small particle impurities adsorbed on the surface, and is beneficial to the efficient progress of pressure filtration. Exemplarily, the mass ratio of water to the matrix material can be 0.5:1, 0.7:1, 0.8:1, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.5:1, 1.8:1, 2:1 or any value within the range formed by any two of the above values.

[0044] In some embodiments, the water content of the solid materials obtained after pressure filtration is 3% to 6%, indicating that most of the water in the solid materials has been effectively removed, facilitating the subsequent secondary sintering reaction and improving the quality of the lithium iron phosphate manganese cathode material.

[0045] In some embodiments, the pressure for pressure filtration can be 0.1 MPa to 0.3 MPa, which can accelerate the filtration speed and improve the removal effect of surface residual alkali and impurities. Exemplarily, the pressure for pressure filtration can be 0.1 MPa, 0.15 MPa, 0.2 MPa, 0.25 MPa, 0.3 MPa or any value within the range formed by any two of the above values.

[0046] Exemplarily, the pressure filtration can be mechanical filtration. By mechanically applying pressure (such as hydraulic pressure, air pressure) to push the liquid through the filter medium, the matrix material can be washed in large quantities and with high efficiency, removing the residual alkali and impurities on the surface of the matrix material.

[0047] The residual alkali on the surface of the matrix material may include unreacted raw materials or reaction by-products during the synthesis process. Among them, the unreacted raw materials may include lithium sources, such as alkaline lithium sources like lithium carbonate and lithium hydroxide, and the reaction by-products may include iron hydroxide and ferrous hydroxide formed by the hydrolysis of iron sources. These alkaline substances remaining on the surface of the matrix material will cause the surface pH to rise. During the preparation of the electrode, too high a pH of the lithium iron phosphate manganese cathode material will cause a defluorination reaction between the lithium iron phosphate manganese cathode material and the binder (polyvinylidene fluoride PVDF), resulting in the gelation of the positive electrode slurry, a significant increase in the viscosity of the positive electrode slurry, poor fluidity, difficulty in achieving uniform coating, and difficulty in increasing the solid content, thus affecting the electrochemical performance and processability of the lithium iron phosphate manganese cathode material; and in an alkaline environment, the electrolyte salt lithium hexafluorophosphate will decompose to produce HF, reducing the battery cycle.

[0048] Small particle impurities may include other unreacted raw materials and reaction by-products. The attachment of these residual substances on the surface of the matrix material particles will increase the specific surface area of the lithium iron phosphate manganese cathode material. Removing the residual small particle impurities can improve the purity of the matrix material and is beneficial to reducing the specific surface area of the lithium iron phosphate manganese cathode material. The lithium iron phosphate manganese cathode material with a high specific surface area will also have a relatively high surface energy, strong intermolecular forces between particles, and is prone to particle agglomeration; and there are more small particles in the cathode material with a high specific surface area, which is prone to metal dissolution, affecting the long-term performance of the battery. The lithium iron phosphate manganese cathode material with a smaller specific surface area has a reduced adsorption force on the particle surface, is not prone to particle agglomeration and excessive adsorption of the solvent of the positive electrode slurry (such as N-methylpyrrolidone), making the lithium iron phosphate manganese cathode material easier to handle and disperse during the production process, helping to improve the processability of the lithium iron phosphate manganese cathode material, thereby reducing the viscosity of the slurry, increasing the fluidity and solid content of the slurry. A smaller specific surface area also helps to reduce unnecessary transmission paths of electrons and ions on the surface of the lithium iron phosphate manganese cathode material and improve conductivity.

[0049] The reduction of the pH and specific surface area of the lithium iron phosphate manganese cathode material synergistically reduces the viscosity of the slurry during the preparation of the electrode sheet, improves the stability and fluidity of the slurry, is beneficial to the increase of the solid content of the lithium iron phosphate manganese cathode material slurry and the uniform progress of coating, and reduces the corrosion of the lithium iron phosphate manganese cathode material on electrode materials such as solvents, coatings, and binders, which is beneficial to preparing a smooth and uniform electrode, and jointly improves the electrochemical performance and processability of the lithium iron phosphate manganese cathode material in the preparation of the subsequent electrochemical device (such as a battery).

[0050] In step S2, coating materials containing metal elements can also be added to water. Pressurized filtration is carried out using the water containing the coating materials, so that the coating materials can be in-situ coated on the surface of the matrix material during the water washing and pressure filtration process. After the pressurized filtration process ends, the coating materials still remain on the surface of the matrix material. After subsequent sintering, a metal coating layer can be formed on the surface of the matrix material, which can improve the capacity of the lithium iron phosphate manganese cathode material. The surface water washing and in-situ coating of the matrix material are carried out simultaneously, optimizing the preparation process and improving the production efficiency. Moreover, due to the removal of the residual alkali and impurities on the surface of the matrix material, the coating materials can better combine with the matrix material, improving the uniformity and firmness of the coating.

[0051] In some embodiments, the coating materials can include at least one of metal element oxides and salts. Among them, the metal elements can include at least one of iron, titanium, vanadium, and zirconium, and the salt can be phosphate. The coating materials can effectively improve the capacity of the lithium iron phosphate manganese cathode material.

[0052] In some embodiments, the mass percentage of the coating materials in the matrix material can be 0.05% - 0.5%, which can effectively control the coating amount, effectively improve the capacity of the lithium iron phosphate manganese cathode material, and at the same time, the internal impedance of the lithium iron phosphate manganese cathode material is not affected.

[0053] Step S4: Mix the solid materials and the second carbon source and perform secondary grinding to obtain a second mixed slurry.

[0054] Specifically, after mixing the solid materials and the second carbon source and performing secondary grinding, the solid materials and the second carbon source can be fully dispersed and homogenized to obtain a second mixed slurry. The method of secondary grinding can include at least one of sand grinding, high-energy ball milling, vibration milling, blender grinding, and soybean milk machine grinding, etc. It can be understood that grinding includes but is not limited to the above methods, and any method that can achieve grinding of materials can be used.

[0055] In some embodiments, the solid materials and the second carbon source can be dispersed in a solvent and mixed evenly. The solvent can be water, which can make the solid materials and the second carbon source fully dispersed in the solvent, increasing the contact probability between the raw material particles (solid materials and the second carbon source) and the medium during the grinding process, and thus improving the grinding effect.

[0056] The average particle size D50 of the particles in the second mixed slurry is 0.25μm - 0.40μm, which is beneficial to reducing particle agglomeration, improving the stability and uniformity of the second mixed slurry, and enabling the lithium iron phosphate manganese cathode material to have both high charge-discharge capacity and good processing performance. The average particle size D50 of the particles in the second mixed slurry can be exemplarily 0.25μm, 0.3μm, 0.35μm, 0.4μm, or any value within the range composed of any two of the above values.

[0057] In some embodiments, the mass ratio of the solid material to the second carbon source may be (15 to 25):1, which is beneficial for the second carbon source to moderately coat the surface of the solid material.

[0058] In some embodiments, the second carbon source may include at least one of glucose, sucrose, starch, polyethylene glycol, etc.

[0059] Step S5: Perform secondary sintering on the second mixed slurry to form a coating layer on the surface of the matrix material. The coating layer includes a carbon layer, thereby obtaining the lithium iron phosphate manganese cathode material.

[0060] Specifically, reference can be made to Figure 2 , perform secondary sintering on the second mixed slurry under an inert atmosphere (which can be nitrogen) and certain sintering conditions to form a coating layer on the surface of the matrix material 10, thereby obtaining the lithium iron phosphate manganese cathode material 100, wherein the coating layer includes a carbon layer 20.

[0061] In some embodiments, the conditions for secondary sintering may include: the sintering temperature is 650 to 850 °C, and the heat preservation time is 180 min to 900 min. By optimizing the conditions for secondary sintering, the microstructure and coating effect of the lithium iron phosphate manganese cathode material can be improved, the defects and porosity of the particles can be reduced, a uniform and dense coating layer can be formed, and thus the specific surface area of the lithium iron phosphate manganese cathode material can be further reduced.

[0062] In some embodiments, the heating rate for secondary sintering may be 1.5 °C / min to 5 °C / min, and the heating time may be 150 min to 500 min. Under the above heating rate and heating time, it is beneficial to reduce the thermal stress and further improve the uniformity and density of the coating layer.

[0063] In some embodiments, before the step of performing secondary sintering on the second mixed slurry, the preparation method may further include: drying the second mixed slurry by spray drying, which is beneficial for removing the solvent and effectively reducing the caking phenomenon of the second mixed slurry during secondary sintering.

[0064] Furthermore, the inlet air temperature for spray drying may be 200 °C to 250 °C, and the outlet air temperature may be 75 °C to 120 °C. Setting the range of the inlet air temperature and the outlet air temperature helps to increase the evaporation rate of the solvent and the degree of drying during spray drying.

[0065] In another embodiment, reference can be made to Figure 3, in step S3, when the coating material is added to water, a coating material containing metal elements is in-situ coated on the surface of the matrix material 10. Therefore, after secondary sintering, a composite coating layer is formed, that is, a metal coating layer 30 and a carbon layer 20 are sequentially formed on the surface of the matrix material 10, forming a double coating layer structure of the matrix material 10, the metal coating layer 30, and the carbon layer 20 arranged in sequence from the inside to the outside. The preparation method provided by this embodiment improves the capacity of the lithium iron phosphate manganese cathode material 200 while coating the metal elements inside the carbon layer, reducing the exposure of metal elements, and lowering the pH and specific surface area of the lithium iron phosphate manganese cathode material 200.

[0066] Compared with the prior art, the preparation method of the lithium iron phosphate manganese cathode material provided by the embodiment of the present application has the following beneficial effects: 1. By adopting a water washing method of mixing the matrix material with water and performing pressure filtration, the residual alkali content and other small particle impurities on the surface of the matrix material are reduced, and the pH and specific surface area of the lithium iron phosphate manganese cathode material are lowered, thereby effectively improving the processing performance and safety of the lithium iron phosphate manganese cathode material in the preparation of backend electrochemical devices (such as batteries). Compared with the prior art method of using three grindings and three sinterings to reduce the pH and specific surface area, the process flow is shortened and the production cost is reduced.

[0067] 2. The pressure for pressure filtration can be 0.1 MPa to 0.3 MPa, which can push the liquid through the filter medium, and wash the matrix material in large quantities and with high efficiency, effectively removing the residual alkali and impurities on the surface of the matrix material.

[0068] 3. Reducing the specific surface area of the lithium iron phosphate manganese cathode material, reducing the unnecessary transmission paths of electrons and ions on the surface of the lithium iron phosphate manganese cathode material, is beneficial to improving the conductivity of the lithium iron phosphate manganese cathode material. Further combined with the formed carbon layer, it can synergistically improve the conductivity of the lithium iron phosphate manganese cathode material.

[0069] 4. In the step of filtering after mixing the matrix material with water, adding a coating material to the water can simultaneously in-situ coat metal elements on the surface of the matrix material to form a metal coating layer. Without adding additional coating operations, a coating layer can be formed on the surface of the matrix material during the water washing and filtration process, improving the capacity of the lithium iron phosphate manganese cathode material, which is simple and efficient; and this metal coating layer is located inside the carbon layer, reducing the exposure of metal elements and lowering the pH and specific surface area of the lithium iron phosphate manganese cathode material.

[0070] In addition, the embodiment of the present application also provides a lithium iron phosphate manganese cathode material prepared by the foregoing preparation method of the lithium iron phosphate manganese cathode material. Please refer to again Figure 2, the lithium iron manganese phosphate cathode material 100 includes a matrix material 10 and a coating layer located on the surface of the matrix material 10. Among them, the matrix material 10 is lithium iron manganese phosphate, and the coating layer includes a carbon layer 20. Due to the water washing treatment by pressure filtration, the pH value and specific surface area of the lithium iron manganese phosphate cathode material 100 are both relatively low.

[0071] Among them, the lithium iron manganese phosphate cathode material has a relatively low pH value, and the particles of the lithium iron manganese phosphate cathode material... The relatively low pH value also indicates that there are fewer basic substances or residual alkali metals (such as hydroxides) on the surface of the lithium iron manganese phosphate cathode material, which is beneficial to reducing the increase in the viscosity of the cathode slurry caused by adverse side reactions (such as defluorination reaction with PVDF or decomposition of electrolyte salt lithium hexafluorophosphate in an alkaline environment), and improving the fluidity and stability of the lithium iron manganese phosphate cathode material. During the battery preparation process, the relatively low pH value reduces the corrosion of materials such as solvents, coatings, and binders due to the presence of strong alkalis, thereby improving the processing performance of the battery, reducing the damage to the electrolyte and electrodes, and the gas generation during the cycling process, and enhancing the stability of the battery manufacturing process.

[0072] The reduction in the specific surface area of the lithium iron manganese phosphate cathode material makes the lithium iron manganese phosphate cathode material not agglomerate to form large particles during the coating process, which is beneficial to reducing the slurry viscosity and improving the stability and fluidity of the slurry when the lithium iron manganese phosphate cathode material slurry is stretched, thereby facilitating the increase in the solid content of the slurry. During the preparation process of the electrode sheet, a smooth and uniform electrode can be prepared, improving the processing performance of the lithium iron manganese phosphate cathode material in the preparation of the subsequent electrochemical device (such as a battery). Moreover, the lithium iron manganese phosphate cathode material has a relatively low specific surface area, indicating that there are fewer exposed active sites on the lithium iron manganese phosphate cathode material, which can reduce the chance of adverse side reactions (such as reaction with the electrolyte to form an ineffective solid electrolyte interface film (SEI)) occurring in the battery, thereby enhancing the stability of the lithium iron manganese phosphate cathode material. Secondly, the relatively low specific surface area also helps to reduce the risk of thermal runaway during the use of the battery. In the case of overcharging or high-temperature environment, the smaller surface area can reduce the excessive reaction area, thereby improving safety and reducing the risk of thermal runaway or fire. In addition, it can also improve the electronic conductivity and electrochemical performance of the lithium iron manganese phosphate cathode material.

[0073] The increase in the solid content of the lithium iron manganese phosphate cathode material slurry means that there is more positive active material (lithium iron manganese phosphate) in the slurry, which is beneficial to increasing the capacity of the lithium iron manganese phosphate cathode material in the subsequent electrochemical device (such as a battery).

[0074] In addition, the coating layer includes a carbon layer, and the carbon layer can synergistically improve the conductivity of the lithium iron manganese phosphate cathode material with the low specific surface area.

[0075] Please refer to again Figure 3, in another embodiment, the coating layer of the lithium iron manganese phosphate cathode material 200 further includes a metal coating layer 30, and the metal coating layer 30 is located between the matrix material 10 and the carbon layer 20. A double coating layer structure of matrix material 10 - metal coating layer 30 - carbon layer 20 is formed. This structure can improve the capacity of the lithium iron manganese phosphate cathode material 200 while coating metal elements inside the carbon layer, reducing the exposure of metal elements, and further reducing the pH and specific surface area of the lithium iron manganese phosphate cathode material 200.

[0076] Compared with the prior art, the lithium iron manganese phosphate cathode material provided by the embodiment of the present application has a lower pH value and specific surface area, which can improve the processing performance and safety of the lithium iron manganese phosphate cathode material in the preparation of subsequent electrochemical devices (such as batteries). The capacity of the lithium iron manganese phosphate cathode material can also be improved by adding a metal coating layer.

[0077] The embodiment of the present application provides an electrochemical device (such as a battery), which includes a positive electrode plate, and the positive electrode plate includes a positive electrode material, and the positive electrode material is the aforementioned lithium iron manganese phosphate cathode material.

[0078] Compared with the prior art, the electrochemical device provided by the embodiment of the present application uses the aforementioned lithium iron manganese phosphate cathode material with good processability, and prepares a positive electrode plate with a high solid content and smooth and uniform surface, improving the capacity of the electrochemical device and reducing the resistivity of the electrochemical device.

[0079] The aforementioned lithium iron manganese phosphate cathode material, its preparation method and the electrochemical device are further described below through specific embodiments.

[0080] Example 1 Step 1: Mix 17.32 kg of lithium carbonate, 66.89 kg of iron phosphate, 8.18 kg of glucose, 50 kg of manganese tetraoxide, 69.53 kg of lithium dihydrogen phosphate, 1.81 kg of magnesium carbonate, 0.5 kg of niobium pentoxide with 320 kg of water evenly and conduct primary grinding. The primary grinding includes introducing the above materials into a sand mill for rough grinding (zircon beads with a diameter of 0.6 mm) until the particle size D50 = 1.3 μm, and then conducting fine grinding (zircon beads with a diameter of 0.3 mm) until the particle size D50 = 0.35 μm to obtain a first mixed slurry.

[0081] Step 2: Conduct spray drying on the first mixed slurry. The inlet air temperature for spray drying is 220 °C and the outlet air temperature is 90 °C. Then conduct primary sintering under the protection of an inert atmosphere, raise the temperature to 660 °C at a rate of 2.5 °C / min, sinter for 7.5 h, and finally cool down to obtain the matrix material.

[0082] Step 3: Add 100 kg of matrix material and 120 kg of water into a filter press, stir and mix at a speed of 300 rpm, and perform pressure filtration to obtain solid materials. Among them, the pressure of pressure filtration is 0.2 MPa, and the moisture content of the solid materials is 3% - 6%.

[0083] Step 4: Mix 40 kg of solid materials, 1.2 kg of glucose, 2.4 kg of polyethylene glycol (molecular weight 6000) and 60 kg of water evenly and perform secondary grinding. The secondary grinding includes introducing the above materials into a sand mill for grinding (zircon beads with a diameter of 0.3 mm), grinding until the particle size D50 = 0.3 μm, and obtaining a second mixed slurry.

[0084] Step 5: Perform spray drying on the second mixed slurry. The inlet air temperature of spray drying is 220 °C, and the outlet air temperature is 90 °C. Then, perform secondary sintering under the protection of an inert atmosphere, raise the temperature to 730 °C at a speed of 2.5 °C / min, sinter for 7.5 h, finally cool down, and perform air flow crushing and iron removal by sieving with a crushing strength of 120 to obtain a lithium iron phosphate cathode material with a carbon layer.

[0085] Example 2 The difference from Example 1 is that in Step 3, 0.1 kg of TiO 2 is pre-dissolved in water as a coating material. Other steps are basically the same as those in Example 1, please refer to Example 1.

[0086] Example 3 The difference from Example 1 is that in Step 3, 0.1 kg of Nb 2 O 5 is pre-dissolved in water as a coating material. Other steps are basically the same as those in Example 1, please refer to Example 1.

[0087] Example 4 The difference from Example 1 is that in Step 4, 0.1 kg of TiO 2 is added as a coating material and ground together. Other steps are basically the same as those in Example 1, please refer to Example 1.

[0088] Comparative Example 1 The difference from Example 1 is that Step 2 is not carried out, that is, the matrix material is not washed and pressure-filtered. Other steps are basically the same as those in Example 1, please refer to Example 1.

[0089] Comparative Example 2 The difference from Comparative Example 1 is that when performing secondary grinding in Step 4, 0.1 kg of TiO 2 is added as a coating material and ground together. Other steps are basically the same as those in Comparative Example 1, please refer to Comparative Example 1.

[0090] The following tests were conducted on the lithium iron manganese phosphate cathode materials obtained in Examples 1-4 and Comparative Examples 1-2.

[0091] (I) pH test: According to GB / T 24533-2019, the pH value at which the lithium iron manganese phosphate cathode material is stable in water was tested.

[0092] (II) Specific surface area test: The nitrogen adsorption-desorption method was used. At the liquid nitrogen temperature, the equilibrium adsorption amount of nitrogen on the object surface is related to its specific surface area and other characteristics. Combining the law of the change of the adsorption amount with the relative pressure during the adsorption process, the specific surface area test can be carried out.

[0093] (III) Carbon content test: The carbon content of the LMFP cathode material was tested by infrared analysis. Using a carbon-sulfur analyzer, the sample was burned under high-temperature and oxygen-rich conditions. The carbon element contained in it was oxidized to carbon dioxide and entered the infrared detector with the carrier gas. The content of the carbon element was quantitatively calculated by statistically analyzing the change in the infrared absorption wavelength intensity of the carbon dioxide signal.

[0094] (IV) Surface residual alkali (Li 2 CO 3 ) test: A potentiometric titrator was used to conduct the test according to the method specified in the standard drafted by the National Nonferrous Metals Standardization Technical Committee, "Determination of Magnetic Foreign Matter Content and Residual Alkali Content in Cathode Materials for Lithium-Ion Batteries".

[0095] (V) Powder tap density test: A tap density tester was used to conduct the test according to the method specified in the standard drafted by the National Nonferrous Metals Standardization Technical Committee, "Determination of the Tap Density of Lithium-Ion Battery Cathode Material Powder".

[0096] (VI) Resistivity test: A conductivity tester was used to conduct the test according to the method specified in the standard drafted by the National Nonferrous Metals Standardization Technical Committee, "Determination of the Resistivity of Lithium-Ion Battery Cathode Material Powder".

[0097] (VII) Electrical performance test: The SP:PVDF:lithium iron manganese phosphate cathode material was mixed evenly in a mass ratio of 5:3:2 to make a positive electrode plate. Using metallic lithium as the negative electrode material, a CR2025 button cell was made and tested within the voltage range of 2.5~4.5V.

[0098] The test steps for the first charge-discharge efficiency are as follows: Under normal temperature conditions of 25°C, the lithium-ion battery is charged at a constant current of 0.1C (C represents the current rate, 1C = 2A) until 4.5V, and then charged at a constant voltage until the current drops to 0.05C, at which point the charging stops, and the first charging capacity is recorded. Then, it is discharged at 0.1C to the cut-off voltage of 2.5V, and the first discharge capacity is recorded. The 0.1C gram capacity of the positive electrode material is calculated based on the first discharge capacity, and the first charge-discharge efficiency of the battery, i.e., the first efficiency, is calculated according to the following formula: First charge-discharge efficiency = (First discharge capacity / First charging capacity) * 100%.

[0099] Then, repeat the above charge-discharge process once at a current of 1C, record the 2nd charging capacity and the 2nd discharge capacity, and calculate the 1C gram capacity of the positive electrode material based on the 2nd discharge capacity.

[0100] The relevant test results of Examples 1-4 and Comparative Examples 1-2 are shown in Table 1.

[0101] The above results show that: Combined with the data in Table 1, comparing Example 1 and Comparative Example 1, it can be seen that the pH value of the lithium iron phosphate manganese cathode material after water washing in Example 1 decreased significantly from 9.9 in Comparative Example 1 to 8.95, and the specific surface area also decreased from 14.98 m 2 / g in Comparative Example 1 to 12.40 m 2 / g, and the surface residual alkali (Li 2 CO 3 ) decreased significantly from 2589 ppm to 205 ppm, and the rate performance of the battery prepared was also improved.

[0102] Comparing Example 1 and Example 2, it is found that after adding the coating material TiO 2 to the water during pressure filtration in Example 2, its physical and chemical indexes (such as pH, specific surface area, carbon content, Li 2 CO 3 content, tap density, and powder resistivity) hardly changed, but the gram capacity of the battery increased significantly. For example, the 0.1C gram capacity increased from 154.22 mAh / g in Example 1 to 158.19 mAh / g, indicating that the coating material remained on the surface of the matrix material after water washing, improving the electrochemical performance of the lithium iron phosphate manganese cathode material.

[0103] Comparing Example 2 with Example 4, the capacity difference between the two is not significant, but the specific surface area and pH value of the lithium iron manganese phosphate cathode material in Example 2 are lower. This indicates that compared with adding the coating material in the secondary grinding stage, adding the coating material in the water washing and pressure filtration stage can better coat the metal elements on the surface of the matrix material to form a metal coating layer, and the second carbon source added in the subsequent secondary grinding stage can be coated on the surface of the metal coating layer to form a structure of matrix material - metal coating layer - carbon layer, enabling the lithium iron manganese phosphate cathode material to have both low pH, low specific surface area, and high capacity.

[0104] Comparing Example 4 with Comparative Example 2, the capacity difference between the two is not significant, but the specific surface area and pH value of the lithium iron manganese phosphate cathode material in Example 4 are lower, and the surface residual alkali (Li 2 CO 3 ) in Comparative Example 2 is as high as 2435 ppm, while the surface residual alkali (Li 2 CO 3 ) in Example 4 is only 309 ppm. This shows that even by adding metal coating auxiliary materials during secondary grinding to improve the capacity of the lithium iron manganese phosphate cathode material, due to the water washing of the metal matrix by pressure filtration, the specific surface area and pH value of the lithium iron manganese phosphate cathode material can still be significantly reduced.

[0105] It can be seen from this that in Examples 1 - 4 of the present application, by adopting the water washing method of mixing the matrix material with water and performing pressure filtration during the preparation process, the content of residual alkali and other small particle impurities on the surface of the matrix material is reduced, effectively reducing the pH and specific surface area of the lithium iron manganese phosphate cathode material, thereby effectively improving the processing performance and safety of the lithium iron manganese phosphate cathode material in the preparation of subsequent electrochemical devices (such as batteries). In addition, by adding a metal coating layer in Examples 2 - 4, the capacity of the prepared battery is effectively increased, and the pH value and specific surface area still remain at a low level.

[0106] It can be understood that the above embodiments are merely exemplary embodiments adopted to illustrate the principle of the present invention, but the present invention is not limited thereto. For those of ordinary skill in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also regarded as the protection scope of the present invention.

Claims

1. A method for preparing a lithium manganese iron phosphate positive electrode material, characterized in that: The preparation method comprises: Mixing a manganese source, an iron source, a phosphorus source, a lithium source and a first carbon source and grinding them once to obtain a first mixed slurry; sintering the first mixed slurry once to obtain a matrix material; The base material is mixed with water and filtered under pressure to obtain a solid material; mixing the solid material and the second carbon source and performing secondary grinding to obtain a second mixed slurry; and The second mixed slurry is subjected to secondary sintering to form a coating layer on the surface of the base material, wherein the coating layer includes a carbon layer, thereby obtaining the lithium manganese iron phosphate positive electrode material.

2. The method for preparing the lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: In the step of mixing the base material with water, the water contains a coating material, and the coating material contains a metal element; During the secondary sintering process, a metal coating layer is formed on the surface of the base material. The metal coating layer is located between the base material and the carbon layer. The metal coating layer and the carbon layer constitute the coating layer.

3. The method for preparing the lithium iron manganese phosphate positive electrode material according to claim 2, characterized in that: The metal element includes at least one of iron, titanium, vanadium and zirconium.

4. The method for preparing the lithium iron manganese phosphate positive electrode material according to claim 2, characterized in that: The mass percentage of the coating material to the base material is 0.05% to 0.5%.

5. The method for preparing the lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: The pressure of the pressurized filtration is 0.1MPa~0.3MPa.

6. The method for preparing the lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: In the step of mixing the base material with water, the mass ratio of water to the base material is (0.5-2):1; and / or After the pressure filtration step, the moisture content of the solid material is 3% to 6%.

7. The method for preparing lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: The conditions of the primary sintering include: a sintering temperature of 500-750° C. and a holding time of 180 min-900 min; and / or The conditions of the secondary sintering include: a sintering temperature of 650-850° C. and a heat preservation time of 180 min-900 min.

8. A lithium manganese iron phosphate positive electrode material, characterized in that: Prepared by the preparation method of the lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 7, the lithium iron manganese phosphate positive electrode material comprises a base material and a coating layer located on the surface of the base material, the base material is lithium iron manganese phosphate, and the coating layer comprises a carbon layer.

9. The lithium iron manganese phosphate positive electrode material according to claim 8, characterized in that: The coating layer further includes a metal coating layer, and the metal coating layer is located between the base material and the carbon layer.

10. An electrochemical device, characterized in that: The electrochemical device comprises a positive electrode sheet, the positive electrode sheet comprises a positive electrode material, and the positive electrode material is the lithium iron manganese phosphate positive electrode material as described in any one of claims 8 to 9.

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