Modified lithium iron manganese phosphate positive electrode material, preparation method thereof and lithium ion battery

Abstract

The invention provides a modified lithium iron manganese phosphate positive electrode material, a preparation method thereof and a lithium ion battery, and relates to the technical field of lithium ion battery materials. The positive electrode material comprises a phosphorus source, an iron source, a manganese source, a lithium source and a magnesium source. Mixing a titanium source solution, carrying out hydrothermal reaction, centrifugally washing and drying to synthesize the lithium manganese iron phosphate, and coating polypyrrole and a glucose carbon layer to improve the electronic conductivity of the lithium manganese iron phosphate. The magnesium-titanium co-doped stable crystal structure is introduced, the lithium ion diffusion path is shortened, the dissolution of the manganese element is inhibited, the structural change in the charging and discharging process is reduced, the side reaction with the electrolyte is inhibited by coating the cobalt boride, and the electrochemical performance of the lithium manganese iron phosphate is improved. The primary nano-particles improve ionic conductivity, and the secondary nano-particles are nano-particles formed by assembling the primary nano-particles and have the advantages of being good in rate capability, high in compaction density and high in tap density.

Description

Technical Field

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

[0002] Lithium iron manganese phosphate cathode materials have advantages such as cheap raw materials, excellent cycle stability and high thermal safety. At the same time, divalent manganese ions replace part of divalent iron ions, which increases the working voltage and thus improves the energy density. Compared with ternary materials, other commercial cathodes for lithium-ion batteries without nickel and cobalt can reduce cost and environmental issues. However, lithium iron manganese phosphate cathode materials still have poor electronic and ionic conductivity, as well as Mn dissolution and crystal structure destruction caused by the Jan-Taylor effect, resulting in poor cycle performance. Summary of the invention

[0003] The present invention improves the electronic conductivity and ionic conductivity of lithium manganese iron phosphate by co-doping with anions and cations and coating with a composite carbon source. The wet synthesis route is adopted, which has the characteristics of low energy consumption and uniform particle size distribution compared with solid phase reaction.

[0004] One of the purposes of the present invention is to provide a modified lithium manganese iron phosphate positive electrode material.

[0005] The second object of the present invention is to provide a method for preparing the modified lithium manganese iron phosphate positive electrode material.

[0006] A third object of the present invention is to provide a lithium-ion battery comprising the modified lithium manganese iron phosphate positive electrode material.

[0007] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are particularly adopted:

[0008] In a first aspect, the present invention provides a modified lithium manganese iron phosphate positive electrode material, comprising doped lithium manganese iron phosphate, and a coating layer;

[0009] The chemical formula of doped lithium manganese iron phosphate is Li a Fe b Mn c Mg d Ti e PO4, where 0.9≤a≤1.6, 0.1≤b≤0.9, 0.1≤c≤0.9, 0.01≤d≤0.4, 0.01≤e≤0.4, b+c+d+e=1, a:(b+c+d+e)=(0.8~1.3):1;

[0010] The coating includes carbon and cobalt boride.

[0011] The chemical formula of doped lithium manganese iron phosphate is Li a Feb Mn c Mg d Ti e PO4, a range is 0.9-1.6, for example 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, etc., and for example 1.03, 1.04, 1.05 or 1.06, b range is 0.1-0.9, for example 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, etc., and for example 0.37, 0.39, 0.40 or 0.42, c range is 0.1-0.9, for example 0.1, 0.2, 0.3, 0.4, 0.5, 0 .6, 0.7, 0.8, 0.9, etc., and 0.58, 0.59 or 0.62, d ranges from 0.01-0.4, for example, 0.01, 0.02, 0.03, 0.04, 0.05, 0.1, 0.2, 0.3, 0.4, etc., e ranges from 0.01-0.4, for example, 0.01, 0.02, 0.03, 0.04, 0.05, 0.1, 0.2, 0.3, 0.4, etc., and b+c+d+e=1, a:(b+c+d+e)=(0.8~1.3):1.

[0012] The coating includes carbon and cobalt boride.

[0013] In some embodiments, based on the total mass of the modified lithium manganese iron phosphate positive electrode material as 100%, the content of cobalt boride in the coating layer is 0.1-1%, for example, 0.1%, 0.2%, 0.5%, 1%, etc.

[0014] In some embodiments, based on the total mass of the modified lithium manganese iron phosphate positive electrode material as 100%, the mass content of carbon in the coating layer is 1% to 15%, for example, 1.0%, 2.0%, 3.0%, 4.0%, 5.0%, 10.0%, 15.0%, etc.

[0015] In a second aspect, the present invention provides a method for preparing a modified lithium manganese iron phosphate positive electrode material, comprising the following steps:

[0016] (1) dispersing a manganese source, an iron source, a magnesium source and a titanium source in a mixed solution of deionized water and ethylene glycol, stirring the solution uniformly, and recording the result as solution A; dispersing a lithium source and a phosphorus source in a mixed solution of water and ethylene glycol, stirring the solution uniformly, and recording the result as solution B; slowly adding solution B to solution A, stirring the solution uniformly, adjusting the pH to 6 to 8, transferring the mixed solution to a hydrothermal reactor, performing a hydrothermal reaction, and obtaining a solid product after centrifugal washing;

[0017] (2) The solid product is mixed evenly with cobalt boride, polypyrrole and glucose in deionized water and then ball-milled. After the slurry particle size reaches the standard, it is spray-dried. After the solid powder is collected, it is sintered in an inert atmosphere and sieved to remove magnetism to obtain a modified lithium manganese iron phosphate positive electrode material.

[0018] Step (1):

[0019] In step (1), the manganese source includes but is not limited to manganese dioxide, manganese carbonate, manganese tetraoxide, manganese sulfate or ferromanganese phosphate, etc., preferably manganese sulfate; the iron source includes but is not limited to ferrous sulfate, ferric oxide, ferric phosphate, etc., preferably ferrous sulfate; the magnesium source includes but is not limited to magnesium nitrate, magnesium chloride, magnesium acetate, magnesium sulfate, etc., preferably magnesium sulfate; the titanium source includes but is not limited to titanium chloride, titanium sulfate, tetrabutyl titanate, etc., preferably titanium sulfate. The lithium source includes but is not limited to lithium phosphate, lithium hydroxide, lithium metaphosphate, lithium monohydrogen phosphate, lithium dihydrogen phosphate or lithium acetate, etc., preferably lithium hydroxide; the phosphorus source includes but is not limited to phosphate, phosphoric acid or ammonium dihydrogen phosphate, etc., preferably phosphoric acid.

[0020] In some embodiments, the molar ratio of the metal elements of the manganese source, iron source, magnesium source, titanium source, lithium source, and phosphorus source satisfies the general formula Li a Fe b Mn c Mg d Ti e PO4, where 0.9≤a≤1.6, 0.1≤b≤0.9, 0.1≤c≤0.9, 0.01≤d≤0.4, 0.01≤e≤0.4, b+c+d+e=1, a:(b+c+d+e)=(0.8~1.3):1.

[0021] In some embodiments, the volume ratio of deionized water to ethylene glycol in the mixed solution of deionized water and ethylene glycol is 1:3-6.

[0022] In some embodiments, the reagent for adjusting pH is lithium hydroxide solution, sodium hydroxide solution, ammonia solution, phosphoric acid solution, hydrochloric acid solution, sulfuric acid solution.

[0023] In some embodiments, the temperature of the hydrothermal reaction is 160-230° C., more preferably 180-210° C., and the reaction time is 4-12 h, more preferably 5-8 h.

[0024] In some embodiments, the solvent for centrifugal washing is methanol, ethanol, deionized water, etc., preferably ethanol; the centrifugal speed is 5000-9000 rpm, preferably 6000-7000 rpm.

[0025] Step (2):

[0026] In some embodiments, the preparation method of cobalt boride is to grind the cobalt source, the solvent tin powder, and the boron powder uniformly, then sinter them, and then wash them.

[0027] The cobalt source includes but is not limited to cobalt nitrate, cobalt chloride, cobalt sulfate, cobalt acetate, etc., preferably cobalt acetate.

[0028] Preferably, the molar ratio of the cobalt source, the tin powder and the boron powder is 1:(0.5-2):(1-6).

[0029] Preferably, the sintering atmosphere in the preparation of cobalt boride is a mixture of a reducing atmosphere and an inert atmosphere, preferably a mixture of hydrogen and argon, a mixture of hydrogen and nitrogen, a mixture of ammonia and argon, a mixture of ammonia and nitrogen, etc., preferably a mixture of hydrogen and argon.

[0030] Preferably, in the preparation of cobalt boride, the sintering temperature is 500-900°C, more preferably 600-850°C; the holding time is 5-14h, more preferably 6-12h; the heating rate is 1-20°C / min, more preferably 1-10°C / min.

[0031] Preferably, the solution used for washing in the preparation of cobalt boride includes but is not limited to hydrochloric acid solution, oxalic acid solution, phosphoric acid solution, organic acid solution, etc., preferably oxalic acid solution.

[0032] In some embodiments, the mass percentage of cobalt boride in the solid product is 0.2% to 1.2%, and the mass percentage of the sum of polypyrrole and glucose in the solid product is 6.5% to 12.5%.

[0033] In some embodiments, the diameter of the ball milling medium is 0.1 mm-1 mm, preferably 0.1 mm-0.3 mm; the material of the ball milling medium is zirconia, agate, yttrium-stabilized zirconia, etc., preferably yttrium-stabilized zirconia; the filling rate of the ball milling medium is 50-90%, preferably 70-85%; the particle size D50 of the slurry after ball milling is 0.01-1 μm, preferably 0.1-0.5 μm.

[0034] In some embodiments, the spray drying temperature is 100-200°C, preferably 170-210°C; the inert atmosphere is nitrogen or argon, preferably nitrogen; the sintering temperature is 400-800°C, preferably 500-700°C; the sintering time is 7-13h, preferably 8-10h.

[0035] The present invention introduces magnesium-titanium co-doping to stabilize the crystal structure, shorten the lithium ion diffusion path, inhibit the dissolution of manganese elements and reduce the structural changes during the charge and discharge process, and coats cobalt boride and mixed carbon to inhibit side reactions with electrolytes, thereby improving the electrochemical performance of lithium manganese iron phosphate. The primary nanoparticles improve ion conductivity, and the secondary particles are nanoparticles assembled from the primary particles, which have the advantages of good rate performance, high compaction density, and high tap density.

[0036] Preferably, the carbon content of the finished product is about 1.5% to 2.8%.

[0037] In a third aspect, the present invention further provides a lithium-ion battery comprising the above-mentioned modified lithium manganese iron phosphate positive electrode material.

[0038] Beneficial Effects

[0039] 1. The present invention adopts hydrothermal reaction to synthesize lithium iron manganese phosphate material. Compared with the traditional solid phase process, the prepared lithium iron manganese phosphate particles are more uniform in size and suitable for application scenarios with high requirements for cycle performance; the use of ethylene glycol as a solvent for the hydrothermal reaction can not only make the particles more evenly dispersed, but also the two hydroxyl groups in ethylene glycol can be adsorbed on the crystal surface, inducing particle growth to expose more (010) crystal planes, thereby increasing the diffusion rate of lithium ions.

[0040] 2. In the hydrothermal reaction synthesis, the present invention introduces magnesium and titanium doping, wherein the introduction of magnesium can optimize the crystal structure of the LMFP material, especially by increasing the octahedral LiO6 gap, thereby improving the diffusion rate of lithium ions in the material; the doping of titanium may help stabilize the structure of the LMFP material, while refining the grains to improve the conductivity.

[0041] 3. The present invention coats cobalt boride on the surface of the primary nano-sized lithium manganese iron phosphate through the ball milling process, introduces anion and cation coating modification, and improves the rate performance and high-temperature cycle performance of lithium manganese iron phosphate. The synthesis of secondary large-particle manganese iron phosphate increases the tap density of the material and improves its processing performance in the homogenization process, which is more conducive to the mass application of lithium manganese iron.

[0042] 4. The present invention uses a mixed carbon source of polypyrrole and glucose for coating. The introduction of nitrogen doping and the chemical bonds formed between carbon atoms can enhance the structural stability of the material. The mixed carbon source of polypyrrole and glucose can form a tighter and continuous conductive network, thereby further improving the conductive properties of the material, while also improving the mechanical strength and compaction density of the material.

[0043] The present invention has been described in detail above, but the above embodiments are only illustrative in nature and are not intended to limit the present invention. In addition, this article is not limited by any theory described in the above prior art or invention content or the following examples. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 The figure is a schematic diagram of the process of preparing the lithium manganese iron phosphate material of the present invention;

[0045] Figure 2 is the SEM image of Example 1;

[0046] Figure 3 is the SEM image of Example 2;

[0047] Figure 4 is the SEM image of Example 3; DETAILED DESCRIPTION

[0048] The present invention is further described below in conjunction with examples. It should be noted that the following examples are provided for illustrative purposes only and do not constitute a limitation on the scope of protection claimed for the present invention.

[0049] Unless otherwise specified, the raw materials, reagents, methods, etc. used in the examples are conventional raw materials, reagents, methods in the art.

[0050] Example 1

[0051] Synthesis of cobalt boride: Weigh 4.98g of cobalt acetate, 3.56g of tin powder, and 1.08g of boron powder, grind them evenly, and sinter them in a mixed atmosphere of H2 and argon with a mass fraction of 5%, with a heating rate of 3℃ / min, a sintering temperature of 780℃, and a holding time of 8h; the sintered powder is crushed, pickled, and dried to obtain cobalt boride.

[0052] Weigh the raw materials MnSO4·4H2O 15.33g, FeSO4·7H2O 12.16g, magnesium sulfate 0.602g, titanium sulfate 1.80g, disperse them in 32ml deionized water and 128ml ethylene glycol mixed solution, stir evenly and record it as solution A; weigh 5.45g lithium hydroxide and disperse it in 40ml ethylene glycol and 20ml deionized mixed solution, pour 5ml of 0.025mol / ml phosphoric acid solution and record it as solution B; slowly pour solution B into solution A, adjust pH to 6-7 with ammonia water, stir evenly and transfer to the lining of the hydrothermal reactor, the temperature of the hydrothermal reaction is 180℃, and the reaction time is 6h. After the hydrothermal reaction is completed, wash it three times with ethanol centrifugation, the centrifugal speed is 8000rpm, and dry it.

[0053] 15 g of the product after the hydrothermal reaction, 0.09 g of cobalt boride, 0.9 g of glucose, and 0.45 g of polypyrrole were weighed and dispersed in a mixed solution of 20 ml of ethanol and 20 ml of deionized water. The ball milling medium used a 0.1 mm zirconium ball with a filling rate of 85%. The ball milling speed was 700 rpm / min. The slurry was ball milled to a particle size D50 < 0.25 μm. After the particle size reached the standard, spray drying was performed at a spray drying temperature of 190° C. After spraying, sintering was performed in a nitrogen atmosphere at a sintering temperature of 670° C. The holding time was 8 hours. Finally, the slurry was sieved and demagnetized to prepare a slurry with a molecular formula of Li 1.04 Mn 0.55 Fe 0.35 Ti 0.06 Mg 0.04 PO4 lithium manganese iron phosphate.

[0054] Example 2

[0055] Synthesis of cobalt boride: Weigh 4.98g of cobalt acetate, 3.56g of tin powder, and 1.08g of boron powder, grind them evenly, and sinter them in a mixed atmosphere of H2 and argon with a mass fraction of 5%, with a heating rate of 3℃ / min, a sintering temperature of 780℃, and a holding time of 8h; the sintered powder is crushed, pickled, and dried to obtain cobalt boride.

[0056] Weigh the raw materials MnSO4·4H2O 15.33g, FeSO4·7H2O 12.16g, magnesium sulfate 0.752g, titanium sulfate 1.50g, disperse them in 32ml deionized water and 128ml ethylene glycol mixed solution, stir evenly and record it as solution A; weigh 5.45g lithium hydroxide and disperse it in 40ml ethylene glycol and 20ml deionized mixed solution, slowly pour 5ml of 0.025mol / ml phosphoric acid solution and record it as solution B; slowly introduce solution B into solution A, adjust pH to 6-7 with ammonia water, stir evenly and transfer to the lining of the hydrothermal reactor, the temperature of the hydrothermal reaction is 180℃, and the reaction time is 6h. After the hydrothermal reaction is completed, wash it three times with ethanol centrifugation, the centrifugal speed is 8000rpm, and dry it.

[0057] 15 g of the product after the hydrothermal reaction, 0.09 g of cobalt boride, 0.9 g of glucose, and 0.45 g of polypyrrole were weighed and dispersed in a mixed solution of 20 ml of ethanol and 20 ml of deionized water. The ball milling medium used a 0.1 mm zirconium ball with a filling rate of 85%. The ball milling speed was 700 rpm / min. The slurry was ball milled to a particle size D50 < 0.25 μm. After the particle size reached the standard, spray drying was performed at a spray drying temperature of 190° C. After spraying, sintering was performed in a nitrogen atmosphere at a sintering temperature of 670° C. The holding time was 8 hours. Finally, the slurry was sieved and demagnetized to prepare a slurry with a molecular formula of Li 1.04 Mn 0.55 Fe 0.35 Ti 0.05Mg 0.05 PO4 lithium manganese iron phosphate.

[0058] Example 3

[0059] Synthesis of cobalt boride: Weigh 4.98g of cobalt acetate, 3.56g of tin powder, and 1.08g of boron powder, grind them evenly, and sinter them in a mixed atmosphere of H2 and argon with a mass fraction of 5%, with a heating rate of 3℃ / min, a sintering temperature of 780℃, and a holding time of 8h; the sintered powder is crushed, pickled, and dried to obtain cobalt boride.

[0060] Weigh the raw materials MnSO4·4H2O 15.33g, FeSO4·7H2O 12.16g, magnesium sulfate 0.902g, titanium sulfate 1.20g, disperse them in 32ml deionized water and 128ml ethylene glycol mixed solution, stir evenly and record it as solution A; weigh 5.45g lithium hydroxide and disperse it in 40ml ethylene glycol and 20ml deionized mixed solution, slowly pour 5ml of 0.025mol / ml phosphoric acid solution and record it as solution B; slowly introduce solution B into solution A, adjust pH to 6-7 with ammonia water, stir evenly and transfer to the lining of the hydrothermal reactor, the temperature of the hydrothermal reaction is 180℃, and the reaction time is 6h. After the hydrothermal reaction is completed, wash it three times with ethanol centrifugation, the centrifugal speed is 8000rpm, and dry it.

[0061] 15 g of the product after the hydrothermal reaction, 0.09 g of cobalt boride, 0.9 g of glucose, and 0.45 g of polypyrrole were weighed and dispersed in a mixed solution of 20 ml of ethanol and 20 ml of deionized water. The ball milling medium used a 0.1 mm zirconium ball with a filling rate of 85%. The ball milling speed was 700 rpm / min. The slurry was ball milled to a particle size D50 < 0.25 μm. After the particle size reached the standard, spray drying was performed at a spray drying temperature of 190° C. After spraying, sintering was performed in a nitrogen atmosphere at a sintering temperature of 670° C. The holding time was 8 hours. Finally, the slurry was sieved and demagnetized to prepare a slurry with a molecular formula of Li 1.04 Mn 0.55 Fe 0.35 Ti 0.04 Mg 0.06 PO4 lithium manganese iron phosphate.

[0062] Comparative Example 1

[0063] Synthesis of cobalt boride: Weigh 4.98g of cobalt acetate, 3.56g of tin powder, and 1.08g of boron powder, grind them evenly, and sinter them in a mixed atmosphere of H2 and argon with a mass fraction of 5%, with a heating rate of 3℃ / min, a sintering temperature of 780℃, and a holding time of 8h; the sintered powder is crushed, pickled, and dried to obtain cobalt boride.

[0064] Weigh the raw materials MnSO4·4H2O 15.33g, FeSO4·7H2O 12.16g, and titanium sulfate 1.80g, disperse them in 32ml of deionized water and 128ml of ethylene glycol mixed solution, stir evenly and record it as solution A; weigh 5.45g of lithium hydroxide, disperse it in 40ml of ethylene glycol and 20ml of deionized mixed solution, slowly pour 5ml of 0.025mol / ml phosphoric acid solution and record it as solution B; slowly introduce solution B into solution A, adjust the pH to 6-7 with ammonia water, stir evenly and transfer it to the lining of the hydrothermal reactor, the temperature of the hydrothermal reaction is 180℃, and the reaction time is 6h. After the hydrothermal reaction is completed, wash it three times with ethanol centrifugation, the centrifugal speed is 8000rpm, and dry it.

[0065] 15 g of the product after the hydrothermal reaction, 0.09 g of cobalt boride, 0.9 g of glucose, and 0.45 g of polypyrrole were weighed and dispersed in a mixed solution of 20 ml of ethanol and 20 ml of deionized water. The ball milling medium used a 0.1 mm zirconium ball with a filling rate of 85%. The ball milling speed was 700 rpm / min. The slurry was ball milled to a particle size D50 < 0.25 μm. After the particle size reached the standard, spray drying was performed at a spray drying temperature of 190° C. After spraying, sintering was performed in a nitrogen atmosphere at a sintering temperature of 670° C. The holding time was 8 hours. Finally, the slurry was sieved and demagnetized to prepare a slurry with a molecular formula of Li 1.04 Mn 0.55 Fe 0.35 Ti 0.06 PO4 lithium manganese iron phosphate.

[0066] Comparative Example 2

[0067] Synthesis of cobalt boride: Weigh 4.98g of cobalt acetate, 3.56g of tin powder, and 1.08g of boron powder, grind them evenly, and sinter them in a mixed atmosphere of H2 and argon with a mass fraction of 5%, with a heating rate of 3℃ / min, a sintering temperature of 780℃, and a holding time of 8h; the sintered powder is crushed, pickled, and dried to obtain cobalt boride.

[0068] Weigh the raw materials MnSO4·4H2O 15.33g, FeSO4·7H2O 12.16g, and magnesium sulfate 0.902g, disperse them in 32ml of deionized water and 128ml of ethylene glycol mixed solution, stir evenly and record it as solution A; weigh 5.45g of lithium hydroxide, disperse it in 40ml of ethylene glycol and 20ml of deionized mixed solution, slowly pour 5ml of 0.025mol / ml phosphoric acid solution and record it as solution B; slowly introduce solution B into solution A, adjust the pH to 6-7 with ammonia water, stir evenly and transfer it to the lining of the hydrothermal reactor, the temperature of the hydrothermal reaction is 180℃, and the reaction time is 6h. After the hydrothermal reaction is completed, wash it three times with ethanol centrifugation, the centrifugal speed is 8000rpm, and dry it.

[0069] 15 g of the product after the hydrothermal reaction, 0.09 g of cobalt boride, 0.9 g of glucose, and 0.45 g of polypyrrole were weighed and dispersed in a mixed solution of 20 ml of ethanol and 20 ml of deionized water. The ball milling medium used a 0.1 mm zirconium ball with a filling rate of 85%. The ball milling speed was 700 rpm / min. The slurry was ball milled to a particle size D50 < 0.25 μm. After the particle size reached the standard, spray drying was performed at a spray drying temperature of 190° C. After spraying, sintering was performed in a nitrogen atmosphere at a sintering temperature of 670° C. The holding time was 8 hours. Finally, the slurry was sieved and demagnetized to prepare a slurry with a molecular formula of Li 1.04 Mn 0.55 Fe 0.35 Mg 0.06 PO4 lithium manganese iron phosphate.

[0070] Comparative Example 3

[0071] Synthesis of cobalt boride: Weigh 4.98g of cobalt acetate, 3.56g of tin powder, and 1.08g of boron powder, grind them evenly, and sinter them in a mixed atmosphere of H2 and argon with a mass fraction of 5%, with a heating rate of 3℃ / min, a sintering temperature of 780℃, and a holding time of 8h; the sintered powder is crushed, pickled, and dried to obtain cobalt boride.

[0072] Weigh the raw materials MnSO4·4H2O 15.33g, FeSO4·7H2O 12.16g, magnesium sulfate 0.602g, titanium sulfate 1.80g, disperse them in 32ml deionized water and 128ml ethylene glycol mixed solution, stir evenly and record it as solution A; weigh 5.45g lithium hydroxide and disperse it in 40ml ethylene glycol and 20ml deionized mixed solution, slowly pour 5ml of 0.025mol / ml phosphoric acid solution and record it as solution B; slowly introduce solution B into solution A, adjust pH to 6-7 with ammonia water, stir evenly and transfer to the lining of the hydrothermal reactor, the temperature of the hydrothermal reaction is 180℃, and the reaction time is 6h. After the hydrothermal reaction is completed, centrifuge and wash three times with ethanol, the centrifugal speed is 8000rpm, and dry.

[0073] 15 g of the product after the hydrothermal reaction, 0.09 g of cobalt boride, and 1.5 g of glucose were weighed and dispersed in a mixed solution of 20 ml of ethanol and 20 ml of deionized water. The ball milling medium used a 0.1 mm zirconium ball with a filling rate of 85%. The ball milling speed was 700 rpm / min. The slurry was milled to a particle size D50 < 0.25 μm. After the particle size reached the standard, spray drying was performed at a spray drying temperature of 190° C. After spraying, sintering was performed in a nitrogen atmosphere at a sintering temperature of 670° C. The holding time was 8 hours. Finally, the slurry was sieved and demagnetized to prepare a slurry with a molecular formula of Li 1.04 Mn 0.55 Fe 0.35 Ti 0.06 Mg 0.04PO4 lithium manganese iron phosphate.

[0074] Comparative Example 4

[0075] Synthesis of cobalt boride: Weigh 4.98g of cobalt acetate, 3.56g of tin powder, and 1.08g of boron powder, grind them evenly, and sinter them in a mixed atmosphere of H2 and argon with a mass fraction of 5%, with a heating rate of 3℃ / min, a sintering temperature of 780℃, and a holding time of 8h; the sintered powder is crushed, pickled, and dried to obtain cobalt boride.

[0076] Weigh the raw materials MnSO4·4H2O 15.33g, FeSO4·7H2O 12.16g, magnesium sulfate 0.602g, titanium sulfate 1.80g, disperse them in 32ml deionized water and 128ml ethylene glycol mixed solution, stir evenly and record it as solution A; weigh 5.45g lithium hydroxide and disperse it in 40ml ethylene glycol and 20ml deionized mixed solution, slowly pour 5ml of 0.025mol / ml phosphoric acid solution and record it as solution B; slowly introduce solution B into solution A, adjust pH to 6-7 with ammonia water, stir evenly and transfer to the lining of the hydrothermal reactor, the temperature of the hydrothermal reaction is 180℃, and the reaction time is 6h. After the hydrothermal reaction is completed, centrifuge and wash three times with ethanol, the centrifugal speed is 8000rpm, and dry.

[0077] 15 g of the product after the hydrothermal reaction, 0.9 g of glucose, and 0.45 g of polypyrrole were weighed and dispersed in a mixed solution of 20 ml of ethanol and 20 ml of deionized water. The ball milling medium used a 0.1 mm zirconium ball with a filling rate of 85%. The ball milling speed was 700 rpm / min, and the slurry was ball milled to a particle size D50 < 0.25 μm. After the particle size reached the standard, spray drying was performed at a spray drying temperature of 190° C. After spraying, sintering was performed in a nitrogen atmosphere at a sintering temperature of 670° C. The holding time was 8 hours, and finally sieving and removing magnetism to prepare a slurry with a molecular formula of Li 1.04 Mn 0.55 Fe 0.35 Ti 0.06 Mg 0.04 PO4 lithium manganese iron phosphate.

[0078] The test data and compaction density statistics of the half-cells of the embodiments and comparative examples are shown in the following table:

[0079]

[0080]

[0081] Compared with Example 1, Comparative Example 1 does not introduce magnesium doping, Comparative Example 2 does not introduce titanium doping, Comparative Example 3 uses glucose as the carbon source, and Comparative Example 4 does not have cobalt boride coating compared with Example 1. The SEM images of Examples 1, 2, and 3 are all typical secondary spherical particles formed by the accumulation of primary nanoparticles. The test results also confirm that the secondary spherical large-particle lithium manganese iron phosphate positive electrode material prepared by coating with cobalt boride and composite carbon source effectively improves the gram-specific capacity and compaction density of lithium manganese iron phosphate.

[0082] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical solutions described in the above embodiments may be modified, or some or all of the technical features thereof may be replaced by equivalents, without departing from the spirit and essence of the claims of the present invention; and these modifications or replacements are still within the scope defined by the claims of the present invention.

Claims

1. A modified lithium manganese iron phosphate positive electrode material, characterized in that: including doped lithium manganese iron phosphate, and a coating layer; The chemical formula of doped lithium manganese iron phosphate is Li a Fe b Mn c Mg d Ti e PO4, where 0.9≤a≤1.6, 0.1≤b≤0.9, 0.1≤c≤0.9, 0.01≤d≤0.4, 0.01≤e≤0.4, b+c+d+e=1, a:(b+c+d+e)=(0.8~1.3):1; The coating includes carbon and cobalt boride.

2. The modified lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: Based on the total mass of the modified lithium manganese iron phosphate positive electrode material being 100%, the content of cobalt boride in the coating layer is 0.1-1%.

3. The modified lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: Taking the total mass of the modified lithium manganese iron phosphate positive electrode material as 100%, the mass content of carbon in the coating layer is 1% to 15%.

4. A method for preparing the modified lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 3, characterized in that: The steps include: (1) dispersing a manganese source, an iron source, a magnesium source and a titanium source in a mixed solution of deionized water and ethylene glycol, stirring the solution uniformly, and recording the result as solution A; dispersing a lithium source and a phosphorus source in a mixed solution of water and ethylene glycol, stirring the solution uniformly, and recording the result as solution B; slowly adding solution B to solution A, stirring the solution uniformly, adjusting the pH to 6-8, transferring the mixed solution to a hydrothermal reactor, performing a hydrothermal reaction, and obtaining a solid product after centrifugal washing; (2) The solid product is mixed evenly with cobalt boride, polypyrrole and glucose in deionized water and then ball-milled. After the slurry particle size reaches the standard, it is spray-dried. After the solid powder is collected, it is sintered in an inert atmosphere and sieved to remove magnetism to obtain a modified lithium manganese iron phosphate positive electrode material.

5. The preparation method according to claim 4, characterized in that: In step (1), the manganese source is one or more selected from manganese dioxide, manganese carbonate, manganese tetraoxide, manganese sulfate or ferromanganese phosphate; the iron source is one or more selected from ferrous sulfate, ferric oxide, and ferric phosphate; the magnesium source is one or more selected from magnesium nitrate, magnesium chloride, magnesium acetate, and magnesium sulfate; the titanium source is one or more selected from titanium chloride, titanium sulfate, and tetrabutyl titanate; the lithium source is one or more selected from lithium phosphate, lithium hydroxide, lithium metaphosphate, lithium monohydrogen phosphate, lithium dihydrogen phosphate, or lithium acetate; and the phosphorus source is one or more selected from phosphate, phosphoric acid, or ammonium dihydrogen phosphate.

6. The preparation method according to claim 4, characterized in that: In step (1), the temperature of the hydrothermal reaction is 160 to 230° C., and the reaction time is 4 to 12 hours.

7. The preparation method according to claim 4, characterized in that: In step (2), the preparation method of cobalt boride is to grind the cobalt source, tin powder and boron powder evenly, sinter them, and then wash them.

8. The preparation method according to claim 4, characterized in that: In step (2), the mass percentage of cobalt boride in the solid product is 0.2% to 1.2%, and the mass percentage of the sum of polypyrrole and glucose in the solid product is 6.5% to 12.5%.

9. The preparation method according to claim 4, characterized in that: In step (2), the diameter of the ball milling medium is 0.1 mm to 1 mm; the material of the ball milling medium is zirconia, agate, or yttrium-stabilized zirconia; the filling rate of the ball milling medium is 50 to 90%; and the particle size D50 of the slurry after ball milling is 0.01 to 1 μm; The spray drying temperature is 100-200°C; the sintering temperature is 400-800°C; and the sintering time is 7-13h.

10. A lithium ion battery, characterized in that: The invention comprises the modified lithium iron manganese phosphate positive electrode material as described in any one of claims 1 to 3 or the modified lithium iron manganese phosphate positive electrode material prepared by the preparation method as described in any one of claims 4 to 9.

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