Lithium manganese iron phosphate positive electrode material and preparation method thereof

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a lithium manganese iron phosphate positive electrode material and a preparation method thereof. The preparation method comprises the following steps: mixing a lithium source, a manganese source, an iron source, a carbon source, a magnesium source, a niobium source and water to obtain slurry; and carrying out spray drying and sintering on the slurry to obtain the lithium manganese iron phosphate positive electrode material. According to the invention, through magnesium-niobium co-doping, the obtained lithium manganese iron phosphate primary particle has good uniformity, and the particle size of the primary particle is 200-400 nm. According to the invention, the crystal structure of the lithium manganese iron phosphate is improved, the structural deformation caused by the Jahn-Teller effect is reduced, the problems of low coulombic efficiency and poor high-temperature cycling stability of the lithium manganese iron phosphate at normal temperature are solved, and meanwhile, the problem of too fast attenuation of the lithium manganese iron phosphate under high-temperature cycling is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a lithium iron manganese phosphate positive electrode material and a preparation method thereof. Background Art

[0002] Lithium iron phosphate cathode material is highly favored in electric vehicles, energy storage systems and other applications. It is particularly suitable for electric buses, logistics vehicles and some electric tools that require high safety and long cycle life. x Fe 1-x Lithium iron phosphate (LiFePO4) is a new type of phosphate-based lithium-ion battery cathode material formed by doping a certain proportion of manganese (Mn) into lithium iron phosphate (LiFePO4). This material combines the advantages of lithium iron phosphate and lithium manganese oxide (LiMn2O4) crystals and has attracted widespread attention from researchers.

[0003] Compared with traditional nickel cobalt manganese oxide, the raw materials used in lithium iron manganese phosphate batteries are relatively cheap and abundant in resources, and the manganese element is also widely recognized in environmentally friendly materials, which provides strong support for its application in the field of new energy. However, the presence of manganese makes the deintercalation and movement of lithium ions more difficult, thereby reducing the conductivity of lithium iron manganese phosphate, which affects the coulombic efficiency of the battery to a certain extent. Prior art CN111477862A discloses a carbon-coated lithium iron manganese phosphate lithium ion battery positive electrode material and its preparation method, using biological rapeseed pollen as a template, and preparing porous lithium iron manganese phosphate by high-pressure hot solvent method and high-temperature thermal cracking method, so that lithium iron manganese phosphate forms a rich pore structure, which is conducive to the diffusion and transmission of lithium ions and improves the diffusion rate of lithium ions. However, this method has the problem of poor cycle performance under high temperature conditions. Summary of the invention

[0004] Therefore, the technical problem to be solved by the present invention is to overcome the defects of low coulombic efficiency and / or poor high temperature cycle performance of lithium manganese iron phosphate batteries in the prior art, thereby providing a lithium manganese iron phosphate positive electrode material and a preparation method thereof.

[0005] To this end, the present invention provides the following technical solutions:

[0006] In a first aspect, the present invention provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising the following preparation steps:

[0007] Mixing a lithium source, a manganese source, an iron source, a carbon source, a magnesium source, a niobium source, and water to obtain a slurry;

[0008] The slurry is spray-dried and sintered to obtain the lithium manganese iron phosphate positive electrode material.

[0009] In an optional embodiment, the mass ratio of the lithium source, manganese source, iron source, carbon source and magnesium source is (8000-9000): (4500-5500): (6000-7000): (1550-1650): (50-150).

[0010] In an optional embodiment, the mass ratio of the lithium source to the niobium source is (8000-9000): (170-200).

[0011] In an optional embodiment, the mass ratio of the lithium source to water is (8000-9000): (40000-41000).

[0012] In an optional embodiment, the lithium source includes: at least one of lithium carbonate, lithium dihydrogen phosphate, lithium phosphate, lithium hydroxide, dilithium hydrogen phosphate, and lithium oxalate;

[0013] In an optional embodiment, the manganese source includes: at least one of trimanganese tetroxide, manganese dioxide, manganese sulfate, manganese carbonate, and manganese dihydrogen phosphate;

[0014] In an optional embodiment, the iron source includes at least one of ferric phosphate, ferric nitrate, ferric sulfate, and ferrous oxalate.

[0015] In an optional embodiment, the carbon source includes at least one of starch, glucose, sucrose, melamine, citric acid, β-cyclodextrin, fructose, and polyethylene glycol;

[0016] In an optional embodiment, the magnesium source includes at least one of magnesium oxide, magnesium hydroxide, magnesium sulfate, and magnesium acetate;

[0017] In an optional embodiment, the niobium source includes at least one of niobium oxide, niobium hydroxide, niobium sulfate and niobium nitrate.

[0018] Furthermore, the mixing process includes premixing and grinding, the stirring speed of the premixing is 500-800r / min, and the time is 0.5-2h; the grinding includes coarse grinding and fine grinding, the stirring speed of the coarse grinding is 1000-2000r / min, and the time of coarse grinding is 20-40min (coarse grinding to a particle size of about 3μm), and fine grinding is performed. After fine grinding, the particle size of the slurry is less than 300nm, and the mass solid content of the slurry is 35%-40%.

[0019] Furthermore, the inlet air temperature of the spray drying is 160-180° C., the outlet air temperature is 85-110° C., and the air pressure is 0.2-0.6 MPa.

[0020] Preferably, during spray drying, a peristaltic pump is used to pump the slurry into the spray drying device, and the rotation speed of the peristaltic pump is 35-80 rpm.

[0021] In an optional embodiment, the sintering is performed under a protective atmosphere. Preferably, the protective atmosphere includes nitrogen and / or argon.

[0022] In an optional embodiment, the sintering includes a heating process and a heat preservation process;

[0023] The heating rate of the heating process is 2-5°C / min, the temperature of the heat preservation process is 650-800°C, and the time is 8-12h.

[0024] Further, the sintered material is crushed to obtain lithium manganese iron phosphate positive electrode material;

[0025] The pulverization is performed by a jet mill, and the D50 of the lithium manganese iron phosphate positive electrode material after pulverization is 200-400nm, wherein the D50 is tested by a particle size analyzer.

[0026] Furthermore, the classification frequency of the air flow mill is 80-250 Hz, the air pressure is 0.2-0.7 MPa, and the fan frequency is 25-40 Hz.

[0027] Preferably, a motor is used to convey the sintered material to the air flow mill, and the motor frequency is 5 to 10 Hz.

[0028] In a second aspect, the present invention provides a lithium manganese iron phosphate positive electrode material prepared by the method for preparing the lithium manganese iron phosphate positive electrode material.

[0029] The technical solution of the present invention has the following advantages:

[0030] The invention provides a method for preparing a lithium iron manganese phosphate positive electrode material, comprising the following preparation steps: mixing a lithium source, a manganese source, an iron source, a carbon source, a magnesium source, a niobium source and water to obtain a slurry; spray-drying the slurry, and sintering to obtain the lithium iron manganese phosphate positive electrode material.

[0031] The addition of magnesium into the crystal structure can inhibit the growth of primary particles, maintain a smaller particle size, have a shorter ion diffusion particle diameter, and increase the diffusion rate of lithium ions. In addition, magnesium doping can reduce structural defects, improve the insertion / extraction efficiency of lithium ions, and thus increase the coulombic efficiency of the battery.

[0032] After doping with niobium, the Jahn-Teller effect can be suppressed, structural deformation can be reduced, and the stability of the material during the cycle process can be maintained, thereby improving the cycle performance and capacity retention rate of the lithium manganese iron phosphate positive electrode material, solving the problem of Mn dissolution at high temperatures, and improving the high-temperature cycle stability of the lithium manganese iron phosphate positive electrode material.

[0033] The present invention co-doped magnesium and niobium to obtain lithium manganese iron phosphate primary particles with good uniformity, and the particle size of the primary particles is between 200 and 400 nm. The crystal structure of lithium manganese iron phosphate is improved, the structural deformation caused by the Jahn-Teller effect is reduced, and the problems of low coulomb efficiency and poor high-temperature cycle stability of lithium manganese iron phosphate at room temperature are solved, and the problem of excessive attenuation of lithium manganese iron phosphate under high-temperature cycle is solved. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0035] Figure 1 is a SEM image of the lithium manganese iron phosphate positive electrode material prepared in Example 1 of the present invention;

[0036] Figure 2 is the XRD diagram of the lithium manganese iron phosphate positive electrode material prepared in Example 1 of the present invention;

[0037] Figure 3 This is a 0.1C specific capacity test graph of a battery prepared with the lithium manganese iron phosphate positive electrode material prepared in Example 1 of the present invention;

[0038] Figure 4 It is a test chart of the cycle capacity retention rate at 45° C. of batteries prepared with the lithium manganese iron phosphate positive electrode materials obtained in Example 1 of the present invention and Comparative Example 1. DETAILED DESCRIPTION

[0039] The following examples are provided for a better understanding of the present invention, but are not intended to limit the best mode of implementation, nor to limit the content and protection scope of the present invention. Any product identical or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with other prior arts shall fall within the protection scope of the present invention.

[0040] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in the field can be used. If no manufacturer is specified for the reagents or instruments used, they are all conventional reagent products that can be obtained commercially.

[0041] The information of the manufacturers, batch numbers, etc. of the raw materials used in the examples and comparative examples are shown in Table 1.

[0042] Table 1 Raw material information

[0043] Iron phosphate Guizhou Zhongwei KZFP104-2310294-27 Manganese tetraoxide Sinosteel Tianyuan MYC3-ZG6230216233 Lithium carbonate Hunan Jinkai Magnesium Oxide Hebei Jinghuang Niobium oxide Hebei Jinghuang glucose Zhucheng Dongxiao Polyethylene glycol Shanghai Yipinhui Mw: 2000 Lithium dihydrogen phosphate Shanghai Xingyan

[0044] Example 1

[0045] This embodiment provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising the following preparation steps:

[0046] Weigh 1206.10g of glucose, 399.89g of polyethylene glycol, 5000g of manganese tetraoxide, 6595.36g of iron phosphate, 1588.90g of lithium carbonate, 6927.62g of lithium dihydrogen phosphate, 58.09g of magnesium oxide, and 183.25g of niobium oxide, mix, add 40456.91g of water, and premix at a speed of 650r / min for 1.25h; after the premixing, grind in a coarse sand mill at 1500r / min for 30min, and then grind in a fine sand mill to obtain a slurry with a particle size of less than 300nm.

[0047] A centrifugal spray dryer is selected to spray dry the slurry, and the spray drying inlet air temperature is 175°C, the outlet air temperature is 95°C, and the air pressure is 0.5Mpa. A peristaltic pump is used to pump the slurry into the centrifugal spray dryer, and the rotation speed of the peristaltic pump is 35rpm.

[0048] The spray-dried material was placed in a furnace and sintered in a nitrogen atmosphere. The temperature was raised to 700°C at a heating rate of 2°C / min and kept at this temperature for 10 h.

[0049] The sintered material is crushed by an air flow mill with a grading frequency of 220 Hz, an air pressure of 0.2 MPa, and a fan frequency of 35 Hz. The sintered material is conveyed to the air flow mill by a motor with a motor frequency of 5 Hz to obtain a lithium iron manganese phosphate positive electrode material.

[0050] The lithium manganese iron phosphate positive electrode material obtained in this example was subjected to SEM and XRD tests, as shown in FIG. Figure 1 , Figure 2 shown.

[0051] from Figure 1 It can be seen that the primary particles of the lithium manganese iron phosphate positive electrode material obtained by the present invention have good uniformity, and the particle size range is between 200 and 400 nm.

[0052] from Figure 2 It can be seen that the lithium manganese iron phosphate positive electrode material is an olivine structure.

[0053] Example 2

[0054] This embodiment provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising the following preparation steps:

[0055] Weigh 1211.95g of glucose, 400.75g of polyethylene glycol, 5000g of manganese tetraoxide, 6534.26g of iron phosphate, 1640.17g of lithium carbonate, 7044.05g of lithium dihydrogen phosphate, 116.19g of magnesium oxide, and 185.64g of niobium oxide, mix them, add 40543.61g of water, and premix them at a speed of 600r / min for 1.5h. After the premixing, grind them in a coarse sand mill at 1200r / min for 35min, and then grind them in a fine sand mill to obtain a slurry with a particle size of less than 300nm.

[0056] A centrifugal spray dryer is selected to spray dry the slurry, and the spray drying inlet air temperature is 170°C, the outlet air temperature is 90°C, and the air pressure is 0.4Mpa. A peristaltic pump is used to pump the slurry into the centrifugal spray dryer, and the rotation speed of the peristaltic pump is 40rpm.

[0057] The spray-dried material was placed in a furnace and sintered in an argon atmosphere. The temperature was raised to 690°C at a heating rate of 3°C / min and kept at this temperature for 10.5 h.

[0058] The sintered material is crushed by an air flow mill with a grading frequency of 200 Hz, an air pressure of 0.25 MPa, and a fan frequency of 30 Hz. The sintered material is conveyed to the air flow mill by a motor with a motor frequency of 6 Hz to obtain a lithium manganese iron phosphate positive electrode material.

[0059] Example 3

[0060] This embodiment provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising the following preparation steps:

[0061] Weigh 1208.22g of glucose, 400.60g of polyethylene glycol, 5000g of manganese tetraoxide, 6595.36g of iron phosphate, 1691.11g of lithium carbonate, 6927.62g of lithium dihydrogen phosphate, 58.09g of magnesium oxide, and 183.25g of niobium oxide, and add 40528.27g of water to perform premixing at a speed of 500r / min for 2h; after the premixing, grind in a coarse sand mill at 1000r / min for 40min, and then grind in a fine sand mill to obtain a slurry with a particle size of less than 300nm;

[0062] A centrifugal spray dryer is selected to spray dry the slurry, and the spray drying inlet air temperature is 160°C, the outlet air temperature is 85°C, and the air pressure is 0.25Mpa. A peristaltic pump is used to pump the slurry into the centrifugal spray dryer, and the rotation speed of the peristaltic pump is 45rpm.

[0063] The spray-dried material was placed in a furnace for sintering, and the temperature was raised to 660°C at a heating rate of 4°C / min under a nitrogen atmosphere and kept at this temperature for 11 hours.

[0064] The sintered material is crushed by an air flow mill with a grading frequency of 170 Hz, an air pressure of 0.4 MPa, and a fan frequency of 28 Hz. The sintered material is conveyed to the air flow mill by a motor with a motor frequency of 7 Hz to obtain a lithium manganese iron phosphate positive electrode material.

[0065] Example 4

[0066] This embodiment provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising the following preparation steps:

[0067] Weigh 1214.05 g of glucose, 401.45 g of polyethylene glycol, 5000 g of manganese tetraoxide, 6534.26 g of iron phosphate, 1675.44 g of lithium carbonate, 7044.05 g of lithium dihydrogen phosphate, 116.19 g of magnesium oxide, and 185.64 g of niobium oxide, and add 40614.32 g of water to perform premixing at a speed of 750 r / min for 1 h. After the premixing, grind in a coarse sand mill at 1800 r / min for 25 min, and then grind in a fine sand mill to obtain a slurry with a particle size of less than 300 nm.

[0068] A centrifugal spray dryer was selected to spray dry the slurry, with an inlet air temperature of 177°C, an outlet air temperature of 100°C, and an air pressure of 0.55 MPa. A peristaltic pump was used to pump the slurry into the centrifugal spray dryer, and the rotation speed of the peristaltic pump was 35 rpm.

[0069] The spray-dried material was placed in a furnace for sintering, and the temperature was raised to 720°C at a heating rate of 4.5°C / min in a nitrogen atmosphere and kept at this temperature for 9 hours.

[0070] The sintered material is crushed by an air flow mill with a grading frequency of 225 Hz, an air pressure of 0.5 MPa, and a fan frequency of 36 Hz. The sintered material is conveyed to the air flow mill by a motor with a motor frequency of 8 Hz to obtain a lithium manganese iron phosphate positive electrode material.

[0071] Example 5

[0072] This embodiment provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising the following preparation steps:

[0073] Weigh 1210.88g of glucose, 401.48g of polyethylene glycol, 5000g of manganese tetraoxide, 6595.36g of iron phosphate, 1719.53g of lithium carbonate, 6927.62g of lithium dihydrogen phosphate, 58.09g of magnesium oxide, and 185.64g of niobium oxide, and add 40617.49g of water for premixing at a speed of 800r / min for 0.5h. After the premixing, grind in a coarse sand mill at 2000r / min for 20min, and then grind in a fine sand mill to obtain a slurry with a particle size of less than 300nm.

[0074] A centrifugal spray dryer is selected to spray dry the slurry, and the spray drying inlet air temperature is 180°C, the outlet air temperature is 110°C, and the air pressure is 0.6Mpa. A peristaltic pump is used to pump the slurry into the centrifugal spray dryer, and the rotation speed of the peristaltic pump is 50rpm.

[0075] The spray-dried material was placed in a furnace for sintering, and the temperature was raised to 800°C at a heating rate of 5°C / min under a nitrogen atmosphere and kept at this temperature for 8h.

[0076] The sintered material is crushed by an air flow mill with a grading frequency of 250 Hz, an air pressure of 0.65 MPa, and a fan frequency of 40 Hz. The sintered material is conveyed to the air flow mill by a motor with a motor frequency of 9 Hz to obtain a lithium manganese iron phosphate positive electrode material.

[0077] Example 6

[0078] This embodiment provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising the following preparation steps:

[0079] Weigh 1216.68g of glucose, 402.32g of polyethylene glycol, 5000g of manganese tetraoxide, 6534.26g of iron phosphate, 1719.53g of lithium carbonate, 7044.05g of lithium dihydrogen phosphate, 116.19g of magnesium oxide, and 185.64g of niobium oxide, and add 40702.70g of water to perform premixing. The premixing speed is 650r / min, and the time is 1.25h. After the premixing is completed, the mixture is sand-milled at 1500r / min for 30min in a coarse sand mill, and then sand-milled in a fine sand mill to obtain a slurry with a particle size of less than 300nm.

[0080] A centrifugal spray dryer is selected to spray dry the slurry, and the spray drying inlet air temperature is 175°C, the outlet air temperature is 95°C, and the air pressure is 0.5Mpa. A peristaltic pump is used to pump the slurry into the centrifugal spray dryer, and the rotation speed of the peristaltic pump is 35rpm.

[0081] The spray-dried material was placed in a furnace for sintering, and the temperature was raised to 700°C at a heating rate of 2°C / min in an argon atmosphere and kept at this temperature for 10 h.

[0082] The sintered material is passed through an air flow mill with a classification frequency of 220Hz, an air pressure of 0.2Mpa, and a fan frequency of 35Hz; wherein, a motor is used to convey the sintered material to the air flow mill, and the motor frequency is 5Hz, and the material is crushed to obtain lithium manganese iron phosphate positive electrode material.

[0083] Comparative Example 1

[0084] This comparative example provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising the following preparation steps:

[0085] Weigh 1199.83g of glucose, 398.89g of polyethylene glycol, 5000g of manganese tetraoxide, 6650.79g of ferric phosphate, 1669.42g of lithium carbonate, and 6811.19g of lithium dihydrogen phosphate, mix, add 40355.95g of water, and premix, the premixing speed is 650r / min, and the time is 1.25h; after the premixing is completed, grind in a coarse sand mill at 1500r / min for 30min, and then grind in a fine sand mill to obtain a slurry with a particle size of less than 300nm;

[0086] A centrifugal spray dryer is selected to spray dry the slurry, and the spray drying inlet air temperature is 175°C, the outlet air temperature is 95°C, and the air pressure is 0.5Mpa. A peristaltic pump is used to pump the slurry into the centrifugal spray dryer, and the rotation speed of the peristaltic pump is 35rpm.

[0087] The spray-dried material was placed in a furnace for sintering, and the temperature was raised to 700°C at a heating rate of 2°C / min under a nitrogen atmosphere and kept at this temperature for 10 h.

[0088] The sintered material is crushed by an air flow mill with a grading frequency of 220 Hz, an air pressure of 0.2 MPa, and a fan frequency of 35 Hz. The sintered material is conveyed to the air flow mill by a motor with a motor frequency of 5 Hz to obtain a lithium iron manganese phosphate positive electrode material.

[0089] Comparative Example 2

[0090] This comparative example provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising the following preparation steps:

[0091] Weigh 1216.68g of glucose, 402.32g of polyethylene glycol, 5000g of manganese tetraoxide, 6821.32g of iron phosphate, 1712.23g of lithium carbonate, 6985.84g of lithium dihydrogen phosphate, and 183.25g of niobium oxide, mix, add 40702.70g of water, and premix at a speed of 650r / min for 1.25h; after the premixing, grind in a coarse sand mill at 1500r / min for 30min, and then grind in a fine sand mill to obtain a slurry with a particle size of less than 300nm;

[0092] A centrifugal spray dryer is selected to spray dry the slurry, and the spray drying inlet air temperature is 175°C, the outlet air temperature is 95°C, and the air pressure is 0.5Mpa. A peristaltic pump is used to pump the slurry into the centrifugal spray dryer, and the rotation speed of the peristaltic pump is 35rpm.

[0093] The spray-dried material was placed in a furnace for sintering, and the temperature was raised to 700°C at a heating rate of 2°C / min under a nitrogen atmosphere and kept at this temperature for 10 h.

[0094] The sintered material is crushed by an air flow mill with a grading frequency of 220 Hz, an air pressure of 0.2 MPa, and a fan frequency of 35 Hz. The sintered material is conveyed to the air flow mill by a motor with a motor frequency of 5 Hz to obtain a lithium iron manganese phosphate positive electrode material.

[0095] Comparative Example 3

[0096] This comparative example provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising the following preparation steps:

[0097] Weigh 1205.63g of glucose, 399.76g of polyethylene glycol, 5000g of manganese tetraoxide, 6592.59g of iron phosphate, 1654.81g of lithium carbonate, 6924.71g of lithium dihydrogen phosphate, and 58.09g of magnesium oxide, mix, add 40443.96g of water, and premix, the speed of premixing is 650r / min, and the time is 1.25h; after the premixing is completed, grind in a coarse sand mill at 1500r / min for 30min, and then put it into a fine sand mill to obtain a slurry with a particle size of less than 300nm;

[0098] A centrifugal spray dryer is selected to spray dry the slurry, and the spray drying inlet air temperature is 175°C, the outlet air temperature is 95°C, and the air pressure is 0.5Mpa. A peristaltic pump is used to pump the slurry into the centrifugal spray dryer, and the rotation speed of the peristaltic pump is 35rpm.

[0099] The spray-dried material was placed in a furnace for sintering, and the temperature was raised to 700°C at a heating rate of 2°C / min under a nitrogen atmosphere and kept at this temperature for 10 h.

[0100] The sintered material is crushed by an air flow mill with a grading frequency of 220 Hz, an air pressure of 0.2 MPa, and a fan frequency of 35 Hz. The sintered material is conveyed to the air flow mill by a motor with a motor frequency of 5 Hz to obtain a lithium iron manganese phosphate positive electrode material.

[0101] Test Case

[0102] The specific surface areas of the lithium iron manganese phosphate positive electrode materials prepared in the above-mentioned embodiments and comparative examples were measured by using a specific surface analyzer.

[0103] The positive electrode active materials, acetylene black and polyvinylidene fluoride PVDF prepared in the above examples 1-6 and comparative examples 1-3 were mixed in NMP at a mass ratio of 92:3:5, and the obtained positive electrode slurry was dried on aluminum foil and punched into a positive electrode sheet with a diameter of 12 mm at a pressure of 200 MPa. The sheet was then placed in a vacuum drying oven and dried at 120°C for 12 hours. After drying, in a glove box under an Ar protective atmosphere, a 2032 button battery shell was used, a Li metal disc was used as the negative electrode, the electrolyte was a LiPF6 electrolyte, and the solvent was EC:DC:DMC (1:1:1), and the battery was assembled together.

[0104] 0.1C discharge capacity at 25°C: The assembled battery was charged at 0.1C at 25°C to a cut-off voltage of 4.25V, then charged to 4.3V at a constant voltage of 0.05C, and then discharged to 2.5V at a rate of 0.1C. Figure 3 As shown, it can be seen that the discharge specific capacity is 153.81 mAh / g.

[0105] 1C discharge capacity at 25℃: The assembled battery is charged at 0.1C at 25℃ to a cut-off voltage of 4.25V, then charged to 4.3V at a constant voltage of 0.05C, and then discharged to 2.5V at a rate of 1C.

[0106] 45℃ cycle capacity retention rate 50th: The assembled battery was cycled for 50 cycles at 45℃, with a current density of 1C in the voltage range of 2.5~4.3V, and the cycle capacity retention rate was calculated.

[0107] The cycle capacity retention rate after 50 cycles at 45° C. = the discharge capacity at the 50th cycle / the discharge capacity at the 1st cycle.

[0108] The above test results are shown in Table 2.

[0109] Among them, the cycle capacity of Example 1 and Comparative Example 1 is as follows Figure 4 As shown, it can be seen that the high temperature cycling performance of the lithium manganese iron phosphate positive electrode material is improved after the addition of magnesium and niobium elements.

[0110] Table 2 Performance test results

[0111]

[0112]

[0113] From the comparison between the embodiment and the comparative example, it can be seen that after doping with niobium and magnesium elements, the 45°C cycle capacity retention rate, 25°C discharge specific capacity and coulomb efficiency of the technical solution are significantly improved. The problem of low coulomb efficiency at room temperature and poor high-temperature cycle stability of lithium manganese iron phosphate positive electrode materials is solved.

[0114] At the same time, it can be seen from the examples that the technical solutions obtained by Examples 1 to 6 have a maximum discharge capacity of 145.92 mAh / g under the conditions of 25° C., 2.5-4.3 V, and 1 C.

[0115] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection of the invention.

Claims

1. A method for preparing a lithium manganese iron phosphate positive electrode material, characterized in that: The method comprises the following preparation steps: Mixing a lithium source, a manganese source, an iron source, a carbon source, a magnesium source, a niobium source, and water to obtain a slurry; The slurry is spray-dried and sintered to obtain 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: The mass ratio of the lithium source, manganese source, iron source, carbon source and magnesium source is (8000-9000): (4500-5500): (6000-7000): (1550-1650): (50-150); Optionally, the mass ratio of the lithium source to the niobium source is (8000-9000): (170-200); Optionally, the mass ratio of the lithium source to water is (8000-9000): (40000-41000).

3. The method for preparing the lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: At least one of the following conditions is met: (1) The lithium source includes at least one of lithium carbonate, lithium dihydrogen phosphate, lithium phosphate, lithium hydroxide, dilithium hydrogen phosphate, and lithium oxalate; (2) the manganese source comprises at least one of manganese tetraoxide, manganese dioxide, manganese sulfate, manganese carbonate, and manganese dihydrogen phosphate; (3) The iron source includes at least one of ferric phosphate, ferric nitrate, ferric sulfate, and ferrous oxalate.

4. The method for preparing the lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: At least one of the following conditions is met: (1) The carbon source comprises at least one of starch, glucose, sucrose, melamine, citric acid, β-cyclodextrin, fructose, and polyethylene glycol; (2) The magnesium source includes at least one of magnesium oxide, magnesium hydroxide, magnesium sulfate, and magnesium acetate; (3) The niobium source includes at least one of niobium oxide, niobium hydroxide, niobium sulfate and niobium nitrate.

5. The method for preparing the lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 4, characterized in that: The mixing process includes grinding, and the particle size of the slurry after grinding is less than 300nm.

6. The method for preparing the lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 4, characterized in that: The spray drying has an inlet air temperature of 160-180°C, an outlet air temperature of 85-110°C, and an air pressure of 0.2-0.6Mpa; Preferably, during spray drying, a peristaltic pump is used to pump the slurry into the spray drying device, and the rotation speed of the peristaltic pump is 35-80 rpm.

7. The method for preparing the lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 4, characterized in that: The sintering satisfies at least one of the following conditions: (1) The sintering is carried out under a protective atmosphere; Preferably, the protective atmosphere comprises nitrogen and / or argon; (2) The sintering includes a heating process and a heat preservation process; Preferably, the heating rate of the heating process is 2-5°C / min, the temperature of the insulation process is 650-800°C, and the time is 8-12h.

8. The method for preparing the lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 4, characterized in that: The sintered material is crushed to obtain a lithium manganese iron phosphate positive electrode material; Preferably, the pulverization is performed by a jet mill, and the particle size D50 of the lithium manganese iron phosphate positive electrode material after pulverization is 200-400 nm.

9. The method for preparing lithium iron manganese phosphate positive electrode material according to claim 8, characterized in that: The classification frequency of the air flow mill is 80-250 Hz, the air pressure is 0.2-0.7 MPa, and the fan frequency is 25-40 Hz; Preferably, a motor is used to convey the sintered material to the air flow mill, and the motor frequency is 5 to 10 Hz.

10. The lithium iron manganese phosphate positive electrode material prepared by the method for preparing the lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 9.

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