Preparation method of doped lithium manganese iron phosphate positive electrode material

Abstract

The application discloses a preparation method of a doped lithium manganese iron phosphate positive electrode material. The lithium manganese iron phosphate is prepared through adding a phosphorus source, a lithium source, an iron source, a manganese source, a carbon source and a dopant, and through processes of ball milling, sand milling, spray drying and high-temperature calcining, and the low conductivity problem of the lithium manganese iron phosphate is effectively solved by using the composite carbon source and the metal dopant. The polymer in the carbon source acts as a surfactant, and the problem of uneven element distribution in the solid-phase synthesis is solved. In addition, the multi-metal doping technology is adopted, the lattice parameter is increased through ion occupation, and the problems of low ion conductivity in the lithium manganese iron phosphate are effectively solved. The doped lithium manganese iron phosphate material prepared through the preparation method has the advantages of uniform manganese iron element distribution, thin and dense carbon coating and the like, the ion conductivity is effectively improved by the dopant, and the doped lithium manganese iron phosphate material exhibits high first circle charge-discharge efficiency and rate performance.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, and in particular to a method for preparing a doped lithium manganese iron phosphate cathode material. Background Technology

[0002] With the advancement of dual-carbon policies, the implementation of new national standards, and the rapid decline in lithium battery costs, the penetration rate of lithium batteries has increased rapidly. Currently, lithium-ion batteries are widely used in various devices, such as portable electronic products and electric / hybrid vehicles. However, developing cathode materials that can guarantee high energy density, long cycle life, and low cost remains a crucial research hotspot. In terms of cathode materials, olivine-structured LiFePO4 has been extensively studied due to its advantages such as high theoretical capacity, high safety, long cycle life, and low cost. However, LiFePO4 has a relatively low redox potential (3.4V). vs Li + The olivine-type LiMnPO4 exhibits a lower energy density than LiFePO4 (4.1 V), thus the cathode shows a lower redox potential. vs Li + LiMnPO4 has a higher energy density than LiFePO4. However, previous studies have reported that LiMnPO4 exhibits low ionic and electronic conductivity, resulting in poor conductivity and rate performance when used as a cathode. Furthermore, electrochemical Mn... 3+ During the formation reaction or material synthesis process, lithium iron phosphate (LiFePO4) is easily distorted into an unstable octahedron, reducing its symmetry (Jahn-Teller effect), leading to structural instability and slow interfacial dynamics in the electrode. To improve the energy density of LiFePO4 cathode materials, Mn is introduced into the olivine structure for doping, increasing the voltage plateau and thus the energy density. However, LiFePO4 suffers from problems such as a dual voltage plateau, low electronic conductivity, slow lithium-ion diffusion rate, and severe capacity loss at high-rate charge and discharge rates. Specifically, this manifests as insufficient cycle life and insufficient compaction density, resulting in limited capacity improvement, poor rate performance, and limited applications for standalone use. Currently, modification of lithium iron phosphate can be used to enhance the performance of the binary transition metal phosphate LiMn. 1−x Fe x Electrochemical properties of PO4, such as coating and doping. Summary of the Invention

[0003] This invention addresses the shortcomings of lithium manganese iron phosphate (LMP) cathode materials, such as poor conductivity and poor cycle performance, by providing a method for preparing doped LMP cathode materials. Through doping and coating, high-performance LMP cathode materials are prepared. Multiple methods, including bulk doping, particle structure control, and surface coating, are used synergistically to effectively improve the lithium-ion conductivity and rate performance of LMP. Doped LMP is prepared using a solid-state method and applied to the preparation of high-performance lithium-ion battery cathodes. The use of metal dopants to obtain LMP improves the electrode conductivity, resulting in higher first-cycle efficiency and rate performance. The preparation process of this invention is simple, and the reaction process is easily controlled.

[0004] The technical solution of this invention is as follows:

[0005] A method for preparing lithium manganese iron phosphate cathode material, the specific synthesis steps are as follows:

[0006] S1, according to the designed stoichiometric ratio of manganese iron lithium phosphate, weigh out the phosphorus source, lithium source, iron source, manganese source, carbon source, metal dopant and ultrapure water in sequence;

[0007] S2, put the mixed slurry from step S1 into a vertical stirred ball mill, and adjust the speed for ball milling;

[0008] S3, transfer the slurry after ball milling in step S2 to a horizontal sand mill with a rotation speed of 1800~2800 r / min, and measure the particle size at intervals.

[0009] S4. The slurry from step S3 is spray-dried, and the feed rate is used to control the feed temperature and the discharge temperature to obtain a dried brown powder precursor of lithium manganese iron phosphate.

[0010] S5. Weigh the brown powder from step S4 and place it in a graphite sagger. Place it in a tube furnace and calcine it at high temperature in an inert gas atmosphere to obtain black solid powder - lithium manganese iron phosphate.

[0011] S6, black lithium manganese iron phosphate is crushed, sieved and demagnetized to obtain lithium manganese iron phosphate cathode material.

[0012] The molar ratio of iron source to manganese source in S1 is (3~5):(5~7), the molar ratio of phosphorus source to lithium source is 1:(1.01~1.05), and the carbon source accounts for 2.1%~4.2% of the total mass of lithium manganese iron phosphate.

[0013] The phosphorus source in S1 includes one or more of phosphoric acid (85 acid), diammonium hydrogen phosphate, diammonium dihydrogen phosphate, lithium phosphate, and lithium dihydrogen phosphate.

[0014] The lithium source in S1 includes one or more of lithium carbonate, lithium phosphate, lithium dihydrogen phosphate, or lithium acetate.

[0015] The iron source in S1 includes one of iron phosphate, iron nitrate, iron oxide, or iron(II,III) oxide.

[0016] The manganese source in S1 includes one of manganese oxide, manganese dioxide, or manganese tetroxide.

[0017] The carbon source in S1 includes one or more of sucrose, β-cyclodextrin, glucose, polyvinylpyrrolidone, or polyethylene glycol.

[0018] The nitrogen source in S1 includes one of imidazole, urea, dimethylimidazolium, or thiourea.

[0019] The metal dopant in S1 includes one or more of titanium dioxide, tetrabutyl titanate, isopropyl titanate, potassium titanium oxalate, vanadium pentoxide, or ammonium metavanadate (the doping amount is 500~3000 ppm of the total mass of lithium manganese iron phosphate).

[0020] The particle size D of the ball mill slurry output in S2 50 Controlled within ≤500 nm.

[0021] The particle size D of the grinding slurry output in S3 50 Controlled within ≤220 nm.

[0022] In step S4, the inlet air temperature is controlled at 150 ℃, the outlet air temperature at 70~80 ℃, and the feed rate at 1100~1300 mL / h;

[0023] The precursor in S5 is calcined in a tube furnace under a nitrogen or argon atmosphere, with the heating and cooling rates controlled at 1.5~2.5℃ / min. The pre-calcination temperature is 280~320℃ and the holding time is 1.5~2.5 h. The sintering temperature is 625~675℃ and the holding time is 6~12 h.

[0024] The particle size D of the lithium manganese iron phosphate pulverized in S6 50 The size should be controlled within 1~4 μm.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] (1) Compared with the prior art, the present invention is prepared by solid-state method, which is simple, low-cost and easy to scale up for production. The combination of ball milling and sand milling improves the grinding efficiency, makes the particle size more nanoscale, which is conducive to shortening the primary particle size and accelerating the lithium ion diffusion rate. In addition, the element distribution is more uniform, which greatly improves the consistency of lithium manganese iron phosphate products and enhances the cycle stability of the products.

[0027] (2) This invention uses a composite carbon source. Polyethylene glycol enhances the stability of the slurry and prevents sedimentation during long-term spraying. Polyvinylpyrrolidone forms a uniform carbon coating on the surface of lithium manganese iron phosphate, enhancing electrochemical cycle stability and effectively solving the problem of uneven element distribution in solid-state products. At the same time, the carbon coating improves the conductivity of the lithium manganese iron phosphate surface. In addition, multi-metal doping is used to increase the lattice parameters through ion occupancy, causing the lithium manganese iron phosphate lattice to expand, which is beneficial to the insertion and extraction of lithium ions during charging and discharging. This effectively solves the problem of poor internal conductivity of lithium manganese iron phosphate and improves rate performance. Attached Figure Description

[0028] Figure 1 This is a particle size distribution diagram of lithium manganese iron phosphate prepared by sand milling in Embodiment 1 of the present invention;

[0029] Figure 2 This is a scanning electron microscope (SEM) image of lithium manganese iron phosphate prepared in Embodiment 1 of the present invention;

[0030] Figure 3 This is the X-ray diffraction (XRD) pattern of lithium manganese iron phosphate prepared in Embodiment 1 of the present invention;

[0031] Figure 4 This is a rate performance diagram of lithium manganese iron phosphate prepared in Embodiment 1 of the present invention;

[0032] Figure 5 The graph shows the first charge-discharge performance of lithium manganese iron phosphate prepared according to Embodiments 2, 3 and 4 of this invention at a rate of 0.1C. Detailed Implementation

[0033] The technical solution of this product will be described in detail below through specific embodiments.

[0034] Example 1

[0035] First, weigh 138.03 g of ammonium dihydrogen phosphate and pour it into 400 mL of ultrapure water. Then, slowly add 73.64 g of lithium carbonate, paying attention to the generation of bubbles during the reaction. Measure the pH at the endpoint where no more bubbles are generated. After the reaction stops, weigh 120.65 g of ferric phosphate, 91.61 g of manganese tetroxide, 44.61 g of polyethylene glycol, 13.73 g of sucrose, and 1.48 g of polyvinylpyrrolidone according to the stoichiometric ratio and pour them into 290 g of deionized water. Stir in a vertical sand mill at 400 r / min until evenly dispersed. Then, add 0.15 g of imidazole, 0.43 g of titanium dioxide, and 0.27 g of vanadium pentoxide. Ball mill to control the particle size D. 50After the particle size reaches ≤500 nm, the material is discharged and transferred to a horizontal sand mill. The ball mill slurry is washed with 476 g of deionized water in the sand mill. The speed of the horizontal sand mill is controlled at 2500 r / min and the agitator speed is 100 r / min. The particle size is measured at regular intervals, and the discharge particle size D is recorded. 50 The wavelength was 198 nm. The resulting brown slurry was granulated and dried by spray drying. The lithium manganese iron phosphate precursor was then placed in a tube furnace under a nitrogen atmosphere and kept at 650 °C for 12 h. After natural cooling, it was crushed and sieved to obtain titanium nitride and vanadium-doped lithium manganese iron phosphate cathode material.

[0036] Example 2

[0037] First, weigh 138.03 g of ammonium dihydrogen phosphate and pour it into 400 mL of ultrapure water. Then, slowly add 73.64 g of lithium carbonate, paying attention to the generation of bubbles during the reaction. Measure the pH at the endpoint where no more bubbles are generated. After the reaction stops, weigh 120.65 g of ferric phosphate, 91.61 g of manganese tetroxide, 1.37 g of polyvinylpyrrolidone, and 23.71 g of glucose according to the stoichiometric ratio and pour them into 274 g of deionized water. Stir the mixture in a vertical sand mill at 400 r / min until it is evenly dispersed. Then, add 0.16 g of urea, 1.12 g of tetrabutyl titanate, and 0.27 g of vanadium pentoxide. Ball mill the mixture to control the particle size D. 50 After the particle size reaches ≤500 nm, the material is discharged and transferred to a horizontal sand mill. The ball mill slurry is washed with 476 g of deionized water in the sand mill. The speed of the horizontal sand mill is controlled at 2000 r / min and the agitator speed is 100 r / min. The particle size is measured at regular intervals, and the discharge particle size D is recorded. 50 The wavelength was 212 nm. The resulting brown slurry was granulated and dried by spray drying. The lithium manganese iron phosphate precursor was then placed in a tube furnace under a nitrogen atmosphere and kept at 635 °C for 6 h. After natural cooling, it was crushed and sieved to obtain titanium nitride and vanadium-doped lithium manganese iron phosphate cathode material.

[0038] Example 3

[0039] First, weigh 136.74 g of ammonium dihydrogen phosphate and pour it into 400 mL of ultrapure water. Then, slowly add 64.27 g of lithium carbonate, paying attention to the generation of bubbles during the reaction. Measure the pH at the endpoint where no more bubbles are generated. After the reaction stops, weigh 78.50 g of ferric phosphate, 91.12 g of manganese tetroxide, 1.78 g of polyvinylpyrrolidone, and 20.57 g of sucrose according to the stoichiometric ratio and pour them into 306 g of deionized water. Stir in a vertical sand mill at 400 r / min until evenly dispersed. Then, add 0.17 g of dimethylimidazole, 0.829 g of isopropyl titanate, and 0.18 g of ammonium metavanadate. After ball milling, control the particle size D. 50After the particle size reaches ≤500 nm, the material is discharged and transferred to a horizontal sand mill. The ball mill slurry is washed with 476 g of deionized water in the sand mill. The speed of the horizontal sand mill is controlled at 2600 r / min and the agitator speed is 20 r / min. The particle size is measured at regular intervals, and the discharge particle size D is recorded. 50 The nm value was 210 nm. The obtained brown slurry was granulated and dried by spray drying. The lithium manganese iron phosphate precursor was then placed in a tube furnace under a nitrogen atmosphere and pre-calcined at 350 °C for 2 h, followed by a maximum temperature of 650 °C for 8 h. After natural cooling, it was pulverized and sieved to obtain titanium nitride and vanadium-doped lithium manganese iron phosphate cathode material.

[0040] Example 4

[0041] First, weigh 123.63 g of ammonium dihydrogen phosphate and pour it into 400 mL of ultrapure water. Then, slowly add 54.23 g of lithium carbonate, paying attention to the generation of bubbles during the reaction. Measure the pH at the endpoint where no more bubbles are generated. After the reaction stops, weigh 110.4 g of ferric phosphate, 54.29 g of manganese tetroxide, 5.01 g of polyethylene glycol, and 23.05 g of sucrose according to the stoichiometric ratio and pour them into 246 g of deionized water. Stir in a vertical sand mill at 400 r / min until evenly dispersed. Then, add 0.21 g of thiourea, 0.847 g of potassium titanium oxalate, and 0.29 g of ammonium metavanadate. Ball mill to control the particle size D. 50 After the particle size reaches ≤500 nm, the material is discharged and transferred to a horizontal sand mill. The ball mill slurry is washed with 476 g of deionized water in the sand mill. The speed of the horizontal sand mill is controlled at 2000 r / min and the agitator speed is 30 r / min. The particle size is measured at regular intervals, and the discharge particle size D is recorded. 50 The wavelength was 205 nm. The resulting brown slurry was granulated and dried by spray drying. The lithium manganese iron phosphate precursor was then placed in a tube furnace under a nitrogen atmosphere and pre-calcined at 350 °C for 2 h, followed by a maximum temperature of 650 °C for 10 h. After natural cooling, it was pulverized and sieved to obtain titanium nitride and vanadium-doped lithium manganese iron phosphate cathode material.

[0042] Comparative Example 1

[0043] First, weigh 138.03 g of ammonium dihydrogen phosphate and pour it into 400 mL of ultrapure water. Then, slowly add 73.64 g of lithium carbonate, paying attention to the generation of bubbles during the reaction. Measure the pH at the endpoint where no more bubbles are generated. After the reaction stops, weigh 120.65 g of ferric phosphate, 91.61 g of manganese tetroxide, 44.61 g of polyethylene glycol, 13.73 g of sucrose, and 1.48 g of polyvinylpyrrolidone according to the stoichiometric ratio and pour them into 290 g of deionized water. Stir in a vertical sand mill at 400 r / min until evenly dispersed. Then, add 0.08 g of imidazole, 0.22 g of titanium dioxide, and 0.14 g of vanadium pentoxide. Ball mill to control the particle size D. 50 After the particle size reaches ≤500nm, the material is discharged and transferred to a horizontal sand mill. The ball mill slurry is washed with 476 g of deionized water in the sand mill. The speed of the horizontal sand mill is controlled at 2500 r / min and the agitator speed is 100 r / min. The particle size is measured at regular intervals, and the discharge particle size D is recorded. 50 The wavelength was 198 nm. The resulting brown slurry was granulated and dried by spray drying. The lithium manganese iron phosphate precursor was then placed in a tube furnace under a nitrogen atmosphere and kept at 650 °C for 12 h. After natural cooling, it was crushed and sieved to obtain titanium nitride and vanadium-doped lithium manganese iron phosphate cathode material.

[0044] The difference between this comparative example and Example 1 is that this comparative example reduces the doping content of titanium nitride and vanadium.

[0045] Comparative Example 2

[0046] First, weigh 138.03 g of ammonium dihydrogen phosphate and pour it into 400 mL of ultrapure water. Then, slowly add 73.64 g of lithium carbonate, paying attention to the generation of bubbles during the reaction. Measure the pH at the endpoint where no more bubbles are generated. After the reaction stops, weigh 120.65 g of ferric phosphate, 91.61 g of manganese tetroxide, 44.61 g of polyethylene glycol, 13.73 g of sucrose, and 1.48 g of polyvinylpyrrolidone according to the stoichiometric ratio and pour them into 290 g of deionized water. Stir in a vertical sand mill at 400 r / min until evenly dispersed. Then, add 0.21 g of imidazole, 0.58 g of titanium dioxide, and 0.38 g of vanadium pentoxide. Ball mill to control the particle size D. 50 After the particle size reaches ≤500nm, the material is discharged and transferred to a horizontal sand mill. The ball mill slurry is washed with 476 g of deionized water in the sand mill. The speed of the horizontal sand mill is controlled at 2500 r / min and the agitator speed is 100 r / min. The particle size is measured at regular intervals, and the discharge particle size D is recorded. 50The wavelength was 198 nm. The resulting brown slurry was granulated and dried by spray drying. The lithium manganese iron phosphate precursor was then placed in a tube furnace under a nitrogen atmosphere and kept at 650 °C for 12 h. After natural cooling, it was crushed and sieved to obtain titanium nitride and vanadium-doped lithium manganese iron phosphate cathode material.

[0047] The difference between this comparative example and Example 1 is that this comparative example increases the content of titanium nitride and vanadium doping.

[0048] Comparative Example 3

[0049] First, weigh 138.03 g of ammonium dihydrogen phosphate and pour it into 400 mL of ultrapure water. Then, slowly add 73.64 g of lithium carbonate, paying attention to the generation of bubbles during the reaction. Measure the pH at the endpoint where no more bubbles are generated. After the reaction stops, weigh 120.65 g of ferric phosphate, 91.61 g of manganese tetroxide, 44.61 g of polyethylene glycol, 13.73 g of sucrose, and 1.48 g of polyvinylpyrrolidone according to the stoichiometric ratio and pour them into 290 g of deionized water. Stir in a vertical sand mill at 400 r / min until evenly dispersed. Then, add 0.15 g of imidazole and 0.43 g of titanium dioxide, and ball mill to control the particle size D. 50 After the particle size reaches ≤500 nm, the material is discharged and transferred to a horizontal sand mill. The ball mill slurry is washed with 476 g of deionized water in the sand mill. The speed of the horizontal sand mill is controlled at 2500 r / min and the agitator speed is 100 r / min. The particle size is measured at regular intervals, and the discharge particle size D is recorded. 50 The nm value was 198 nm. The resulting brown slurry was granulated and dried by spray drying. The lithium manganese iron phosphate precursor was then placed in a tube furnace under a nitrogen atmosphere and kept at 650 °C for 12 h. After natural cooling, it was pulverized and sieved to obtain titanium nitride lithium manganese iron phosphate cathode material.

[0050] The difference between this comparative example and Example 1 is that this comparative example changed the doping material to prepare lithium manganese iron phosphate doped only with titanium nitride.

[0051] Comparative Example 4

[0052] First, weigh 138.03 g of ammonium dihydrogen phosphate and pour it into 400 mL of ultrapure water. Then, slowly add 73.64 g of lithium carbonate, paying attention to the generation of bubbles during the reaction. Measure the pH at the endpoint where no more bubbles are generated. After the reaction stops, weigh 120.65 g of ferric phosphate, 91.61 g of manganese tetroxide, 44.61 g of polyethylene glycol, 13.73 g of sucrose, and 1.48 g of polyvinylpyrrolidone according to the stoichiometric ratio and pour them into 290 g of deionized water. Stir in a vertical sand mill at 400 r / min until evenly dispersed. Then, add 0.43 g of titanium dioxide and 0.27 g of vanadium pentoxide, and ball mill to control the particle size D.50 After the particle size reaches ≤500 nm, the material is discharged and transferred to a horizontal sand mill. The ball mill slurry is washed with 476 g of deionized water in the sand mill. The speed of the horizontal sand mill is controlled at 2500 r / min and the agitator speed is 100 r / min. The particle size is measured at regular intervals, and the discharge particle size D is recorded. 50 The wavelength was 198 nm. The resulting brown slurry was granulated and dried by spray drying. The lithium manganese iron phosphate precursor was then placed in a tube furnace under a nitrogen atmosphere and kept at 650 °C for 12 h. After natural cooling, it was crushed and sieved to obtain titanium and vanadium doped lithium manganese iron phosphate cathode material.

[0053] The difference between this comparative example and Example 1 is that the dopants in this comparative example are changed to titanium and vanadium.

[0054] Comparative Example 5

[0055] First, weigh 138.03 g of ammonium dihydrogen phosphate and pour it into 400 mL of ultrapure water. Then, slowly add 73.64 g of lithium carbonate, paying attention to the generation of bubbles during the reaction. Measure the pH at the endpoint where no more bubbles are generated. After the reaction stops, weigh 120.65 g of ferric phosphate, 91.61 g of manganese tetroxide, 44.61 g of polyethylene glycol, 13.73 g of sucrose, and 1.48 g of polyvinylpyrrolidone according to the stoichiometric ratio and pour them into 290 g of deionized water. Stir in a vertical sand mill at 400 r / min until evenly dispersed. Then, ball mill to control the particle size D. 50 After the particle size reaches ≤500 nm, the material is discharged and transferred to a horizontal sand mill. The ball mill slurry is washed with 476 g of deionized water in the sand mill. The speed of the horizontal sand mill is controlled at 2500 r / min and the agitator speed is 100 r / min. The particle size is measured at regular intervals, and the discharge particle size D is recorded. 50 The nm value was 198 nm. The resulting brown slurry was granulated and dried by spray drying. The lithium manganese iron phosphate precursor was then placed in a tube furnace under a nitrogen atmosphere and kept at 650 °C for 12 h. After natural cooling, it was pulverized and sieved to obtain the lithium manganese iron phosphate cathode material.

[0056] The difference between this comparative example and Example 1 is that this comparative example does not contain any metal dopants.

[0057] Comparative Example 6

[0058] First, weigh 138.03 g of ammonium dihydrogen phosphate and pour it into 400 mL of ultrapure water. Then, slowly add 73.64 g of lithium carbonate, paying attention to the generation of bubbles during the reaction. Measure the pH at the endpoint where no more bubbles are generated. After the reaction stops, weigh 120.65 g of ferric phosphate, 91.61 g of manganese tetroxide, and 53.24 g of sucrose according to the stoichiometric ratio and pour them into 276 g of deionized water. Stir the mixture in a vertical sand mill at 400 r / min until it is evenly dispersed. Then, add 0.15 g of imidazole, 0.43 g of titanium dioxide, and 0.27 g of vanadium pentoxide. Ball mill the mixture to control the particle size D. 50 After the particle size reaches ≤500 nm, the material is discharged and transferred to a horizontal sand mill. The ball mill slurry is washed with 476g of deionized water in the sand mill. The speed of the horizontal sand mill is controlled at 2500 r / min and the agitator speed is 100 r / min. The particle size is measured at regular intervals, and the discharge particle size D is recorded. 50 The nm value was 198 nm. The resulting brown slurry was granulated and dried by spray drying. The lithium manganese iron phosphate precursor was then placed in a tube furnace under a nitrogen atmosphere and kept at 650 °C for 12 h. After natural cooling, it was pulverized and sieved to obtain the lithium manganese iron phosphate cathode material.

[0059] The difference between this comparative example and Example 1 is that no surfactant was added in this comparative example, and the amount of carbon added was kept consistent.

[0060] Comparative Example 7

[0061] First, weigh 138.03 g of ammonium dihydrogen phosphate and pour it into 400 mL of ultrapure water. Then, slowly add 71.54 g of lithium carbonate, paying attention to the generation of bubbles during the reaction. Measure the pH at the endpoint where no more bubbles are generated. After the reaction stops, weigh 120.65 g of ferric phosphate, 91.61 g of manganese tetroxide, 44.61 g of polyethylene glycol, 13.73 g of sucrose, and 1.48 g of polyvinylpyrrolidone according to the stoichiometric ratio and pour them into 290 g of deionized water. Stir in a vertical sand mill at 400 r / min until evenly dispersed. Then, add 0.15 g of imidazole, 0.43 g of titanium dioxide, and 0.27 g of vanadium pentoxide. Ball mill to control the particle size D. 50 After the particle size reaches ≤500nm, the material is discharged and transferred to a horizontal sand mill. The ball mill slurry is washed with 476 g of deionized water in the sand mill. The speed of the horizontal sand mill is controlled at 2500 r / min and the agitator speed is 100 r / min. The particle size is measured at regular intervals, and the discharge particle size D is recorded. 50 The wavelength was 198 nm. The resulting brown slurry was granulated and dried by spray drying. The lithium manganese iron phosphate precursor was then placed in a tube furnace under a nitrogen atmosphere and kept at 650 °C for 12 h. After natural cooling, it was pulverized and sieved to obtain the lithium manganese iron phosphate cathode material.

[0062] The difference between this comparative example and Example 1 is that this comparative example reduces the ratio of phosphorus source to lithium source, controlling it at 1:1.01.

[0063] Comparative Example 8

[0064] First, weigh 138.03 g of ammonium dihydrogen phosphate, 108.73 g of manganese sulfate, and 72.92 g of ferrous sulfate, and dissolve them in 346 g of water. After complete dissolution, add 32.63 g of ammonia water and stir continuously. A precipitate slowly forms, which is then filtered and washed to obtain the ferrous manganese ammonium phosphate precursor. Weigh 73.64 g of lithium carbonate, 44.61 g of polyethylene glycol, 13.73 g of sucrose, 1.48 g of polyvinylpyrrolidone, and the manganese ammonium phosphate precursor, and pour them into 590 g of deionized water. Stir in a vertical sand mill at 400 r / min until uniformly dispersed. Then, ball mill the mixture to control the particle size D. 50 After the particle size reaches ≤500 nm, the material is discharged and transferred to a horizontal sand mill. The ball mill slurry is washed with 476 g of deionized water in the sand mill. The speed of the horizontal sand mill is controlled at 2500 r / min and the agitator speed is 100 r / min. The particle size is measured at regular intervals, and the discharge particle size D is recorded. 50 The nm value was 198 nm. The resulting brown slurry was granulated and dried by spray drying. The lithium manganese iron phosphate precursor was then placed in a tube furnace under a nitrogen atmosphere and kept at 650 °C for 12 h. After natural cooling, it was pulverized and sieved to obtain the lithium manganese iron phosphate cathode material.

[0065] The difference between this comparative example and Example 1 is that this comparative example uses a liquid phase method and does not add any dopants to prepare the manganese iron ammonium phosphate precursor.

[0066] Electrochemical performance tests were conducted on coin cell half-cells assembled with lithium manganese iron phosphate cathode material in a glove box. During half-cell assembly, a lithium metal sheet was used as the counter electrode, and a lithium-ion battery electrolyte solution with a volume ratio of 1 mol / L LiPF6 / EC:DEC:DMC = 1:1:1 (ethylene carbonate EC, diethyl carbonate DEC, dimethyl carbonate DMC) was selected. Using N-methylpyrrolidone (NMP) as the solvent, the active material, acetylene black, and polyvinylidene fluoride (PVDF) were mixed uniformly at a mass ratio of 90:5:5. The slurry was then uniformly coated onto aluminum foil using a 150 μm thick doctor blade. The foil was placed in a vacuum oven and dried at 80 °C for 12 hours. After cooling to room temperature, the foil was compacted using a roller press and then cut into φ12 mm circular electrode sheets using a sheet press. Finally, the mass of the electrode sheets was accurately weighed and recorded, and the mass of the active material on each electrode sheet was calculated using the percentage of the active material's mass. The cells were assembled into half-cells in a glove box and allowed to stand for 10 hours before electrochemical performance testing.

[0067] The above-described Implementation Example 1 is the best specific implementation of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

[0068] like Figure 1 As shown, the particle size distribution of the lithium manganese iron phosphate slurry prepared in Example 1 was measured by a Malvern 3000 particle size analyzer, where D... 10 =124 nm, D 50 =198 nm, D 90 =312 nm, D 99 =393 nm, and the overall distribution is normally distributed.

[0069] like Figure 2 As shown in Figures a and b, these are SEM images of lithium manganese iron phosphate prepared in Example 1. It can be observed that the final sample particles are spherical and evenly distributed.

[0070] like Figure 3 As shown, the XRD pattern of the titanium- and vanadium-doped lithium manganese iron phosphate prepared in Example 1 was measured using a Bruker D8 ADVANCE X powder diffractometer (Germany). Furthermore, LiMn... 0.6 Fe 0.4 Compared to LiMnPO4, the diffraction peaks of the PO4 / C sample show a slight shift towards higher diffraction angles within the 27–37° range. Bragg's formula 2dsinθ = nλ indicates that a smaller lattice parameter d corresponds to a larger diffraction angle. Therefore, this phenomenon is related to lattice shrinkage, suggesting that Fe... 2+ It was successfully incorporated into the LiMnPO4 lattice because Fe 2+ The ionic crystal radius (0.092 Å) is smaller than that of Mn. 2+ (0.097 Å), by Figure 1 It can be seen that the titanium and vanadium-doped lithium manganese iron phosphate cathode material produced has no other impurity phases generated.

[0071] like Figure 4 The figure shows the electrochemical charge-discharge curves of the lithium manganese iron phosphate prepared in Example 1 at rates of 0.1 C, 0.33 C, and 0.5 C. When used as the positive electrode of a lithium-ion battery, the lithium manganese iron phosphate prepared in Example 1 exhibits discharge specific capacities of 158.85, 153.43, and 150.91 mAh / g at different charge-discharge rates of 0.1 C, 0.33 C, and 0.5 C, respectively, with an initial charge-discharge efficiency of 96.1%.

[0072] Figure 5The figures show the charge-discharge curves of lithium manganese iron phosphate prepared in Examples 2, 3, and 4 at a 0.1 C rate. The discharge specific capacities are 157.28, 156.51, and 157.45 mAh / g, respectively, and the first-cycle charge-discharge efficiencies are 95.8%, 95.6%, and 95.7%, respectively. Compared with Example 1, Comparative Examples 1-8 controlled multiple influencing factors such as carbon source type, surfactant, metal dopant, and manganese-iron ratio, which have a crucial impact on electrochemical performance.

[0073] The results of all implementation cases and comparison cases are shown in Table 1 below.

[0074] Table 1

[0075]

[0076] As shown in Table 1, by comparing the electrochemical performance of the implementation case and the comparative case, the solid-state method of the present invention, with the addition of a certain amount of titanium nitride and vanadium dopants, can effectively increase the first charge-discharge efficiency and rate performance of the lithium manganese iron phosphate cathode material.

Claims

1. A method for preparing a doped lithium manganese iron phosphate cathode material, characterized in that, Includes the following steps: S1. According to the designed stoichiometric ratio of manganese iron lithium phosphate, the phosphorus source, lithium source, iron source, manganese source, carbon source, nitrogen source, metal dopant and ultrapure water are weighed in sequence. The carbon source is a composite carbon source including polyvinylpyrrolidone and polyethylene glycol. Nitrogen source: one of imidazole, urea, dimethylimidazolium or thiourea; metal dopant is a combination of titanium dioxide, tetrabutyl titanate, isopropyl titanate, potassium titanium oxalate and vanadium pentoxide or ammonium metavanadate; doping method is titanium-vanadium co-doping; doping amount is 500~3000 ppm of the total mass of lithium manganese iron phosphate. S2, put the mixed slurry from step S1 into a vertical stirred ball mill, and adjust the speed for ball milling; S3, transfer the ball-milled slurry from step S2 to a horizontal sand mill at a speed of 1800~2800 r / min, and measure the particle size at regular intervals. The particle size D of the milled slurry output is... 50 Controlled within ≤220 nm; S4. The slurry from step S3 is spray-dried, and the feed rate is used to control the feed temperature and the discharge temperature to obtain a dried brown powder precursor of lithium manganese iron phosphate. S5, weigh the brown powder from step S4 and put it into a graphite sagger, place it in a tube furnace and calcine it at high temperature in an inert gas atmosphere to obtain black solid powder - lithium manganese iron phosphate. S6, black lithium manganese iron phosphate is crushed, sieved and demagnetized to obtain lithium manganese iron phosphate cathode material.

2. The method for preparing a doped lithium manganese iron phosphate cathode material according to claim 1, characterized in that: The molar ratio of iron source to manganese source in S1 is (3~5):(5~7), the molar ratio of phosphorus source to lithium source is 1:(1.01~1.05), and the carbon source accounts for 2.1%-4.2% of the total mass of lithium manganese iron phosphate.

3. The method for preparing a doped lithium manganese iron phosphate cathode material according to claim 1, characterized in that: The phosphorus source in S1 includes one or more of phosphoric acid, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, lithium phosphate, or lithium dihydrogen phosphate; the lithium source includes one or more of lithium carbonate, lithium phosphate, lithium dihydrogen phosphate, or lithium acetate; the iron source includes one of iron phosphate, iron nitrate, iron oxide, or iron(II,III) oxide; and the manganese source includes one of manganese dioxide or manganese(II,III) oxide.

4. The method for preparing a doped lithium manganese iron phosphate cathode material according to claim 1, characterized in that: The particle size D of the ball mill slurry output in S2 50 Controlled within ≤500 nm.

5. A method for preparing a doped lithium manganese iron phosphate cathode material according to claim 1, characterized in that: In S4, the inlet air temperature is controlled at 140~160℃, the outlet air temperature at 70~80℃, and the feed rate at 1100~1300 mL / h.

6. A method for preparing a doped lithium manganese iron phosphate cathode material according to claim 1, characterized in that: The precursor in S5 is calcined in a tube furnace under a nitrogen or argon atmosphere, with the heating and cooling rates controlled at 1.5~2.5℃ / min, the pre-calcination temperature at 280~320℃, the holding time at 1.5~2.5 h, the sintering temperature at 625~675℃, and the holding time at 6~12 h.

7. A method for preparing a doped lithium manganese iron phosphate cathode material according to claim 1, characterized in that: The particle size D of the lithium manganese iron phosphate pulverized in S6 50 The size should be controlled within 1~4 μm.

Images