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

Abstract

The application provides a modified lithium manganese iron phosphate positive electrode material and a preparation method and a lithium ion battery thereof, and relates to the technical field of lithium ion battery materials. The positive electrode material is prepared by mixing a phosphorus source, an iron source, a manganese source, a lithium source, a magnesium source and a titanium source, and then performing hydrothermal reaction, centrifugal washing and drying to synthesize lithium manganese iron phosphate. The electronic conductivity of the lithium manganese iron phosphate is improved by coating polypyrrole and a glucose carbon layer. The application introduces magnesium and 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 in the charging and discharging process. The coating of cobalt boride inhibits the side reaction with the electrolyte and improves the electrochemical performance of the lithium manganese iron phosphate. The primary nanoparticles improve the ion conductivity, and the secondary nanoparticles are nanoparticles assembled by the primary nanoparticles, which have the advantages of good rate performance, high tap density and high vibration density.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of lithium ion battery materials, and particularly relates to a modified lithium manganese iron phosphate positive electrode material, a preparation method thereof and a lithium ion battery. BACKGROUND

[0002] The lithium manganese iron phosphate positive electrode material has the advantages of cheap raw materials, excellent cycle stability, high thermal safety, and the like, and the replacement of part of the divalent iron ions by divalent manganese ions improves the working voltage and further improves the energy density. Compared with ternary materials, other commercial cathodes of lithium ion batteries without nickel and cobalt can alleviate the cost and environmental problems. However, the lithium manganese iron phosphate positive electrode material still has poor cycle performance due to poor electronic and ionic conductivity and Mn dissolution and crystal structure destruction caused by the introduction of the Jiang Taylor effect. SUMMARY

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

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

[0005] The second purpose of the present application is to provide a preparation method of the modified lithium manganese iron phosphate positive electrode material.

[0006] The third purpose of the present application 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 purposes of the present application, the following technical solutions are adopted:

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

[0009] The chemical general formula of the doped lithium manganese iron phosphate is Li a Fe b Mn c Mg d Ti e PO4, wherein 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 layer comprises carbon and cobalt boride.

[0011] The chemical general formula of the doped lithium manganese iron phosphate is Li a Feb Mn c Mg d Ti e PO4, a ranges from 0.9 to 1.6, such as 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, etc., further such as 1.03, 1.04, 1.05 or 1.06, b ranges from 0.1 to 0.9, such as 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, etc., further such as 0.37, 0.39, 0.40 or 0.42, c ranges from 0.1 to 0.9, such as 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, etc., further such as 0.58, 0.59 or 0.62, d ranges from 0.01 to 0.4, such as 0.01, 0.02, 0.03, 0.04, 0.05, 0.1, 0.2, 0.3, 0.4, etc., e ranges from 0.01 to 0.4, such as 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 layer comprises carbon and cobalt boride.

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

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

[0015] In a second aspect, the present application provides a preparation method of a modified lithium iron manganese 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 uniformly, and recording as solution A; dispersing a lithium source and a phosphorus source in a mixed solution of water and ethylene glycol, stirring uniformly, and recording as solution B; slowly adding solution B into solution A while stirring, stirring uniformly, adjusting pH to 6-8, and then transferring the mixed solution into a hydrothermal reaction kettle to perform hydrothermal reaction, and then centrifuging and washing to obtain a solid product;

[0017] (2) The solid product is mixed with cobalt boride, polypyrrole and glucose in deionized water, and then ball-milled. After the particle size of the slurry reaches the standard, it is spray dried. After collecting the solid powder, it is sintered in an inert atmosphere, and then sieved to remove the 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, trimanganese tetroxide, manganese sulfate or manganese iron phosphate, and is preferably manganese sulfate; the iron source includes but is not limited to ferrous sulfate, iron oxide, iron phosphate, and is preferably ferrous sulfate; the magnesium source includes but is not limited to magnesium nitrate, magnesium chloride, magnesium acetate, magnesium sulfate, and is preferably magnesium sulfate; the titanium source includes but is not limited to titanium chloride, titanium sulfate, tetrabutyl titanate, and is preferably titanium sulfate. The lithium source includes but is not limited to lithium phosphate, lithium hydroxide, lithium metaphosphate, monohydrogen lithium phosphate, dihydrogen lithium phosphate or lithium acetate, and is preferably lithium hydroxide; the phosphorus source includes but is not limited to phosphate, phosphoric acid or ammonium dihydrogen phosphate, and is preferably phosphoric acid.

[0020] In some embodiments, the molar ratio of metal elements of the manganese source, the iron source, the magnesium source, the titanium source, the lithium source and the phosphorus source satisfies the general formula Li a Fe b Mn c Mg d Ti e PO4, wherein 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 or sulfuric acid solution.

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

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

[0025] Step (2):

[0026] In some embodiments, the cobalt boride is prepared by sintering a cobalt source, a tin powder, and a boron powder after being uniformly ground, and then washing to obtain the cobalt boride.

[0027] The cobalt source includes, but is not limited to, cobalt nitrate, cobalt chloride, cobalt sulfate, cobalt acetate, and the like, and is 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 the cobalt boride is a mixture of a reducing atmosphere and an inert atmosphere, and is 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, or the like, and is preferably a mixture of hydrogen and argon.

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

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

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

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

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

[0035] The application introduces magnesium-titanium co-doping to stabilize the crystal structure, shortens the lithium ion diffusion path, inhibits the dissolution of manganese elements, reduces the structural changes in the charging and discharging process, coats cobalt boride and mixed carbon to inhibit the side reaction with the electrolyte, and improves the electrochemical performance of the lithium manganese iron phosphate.

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

[0037] In a third aspect, the application also provides a lithium ion battery comprising the modified lithium manganese iron phosphate cathode material.

[0038] Advantages

[0039] 1. The application uses hydrothermal reaction to synthesize lithium manganese iron phosphate material, which has more uniform particle size compared to traditional solid-phase method process, and is suitable for application scenarios with high cycle performance requirements; the hydrothermal reaction selects ethylene glycol as the solvent, which can make the particles more uniformly dispersed, and the two hydroxyl groups in the ethylene glycol can be adsorbed on the crystal surface to induce the growth of particles to expose more (010) crystal surfaces, thereby improving the diffusion rate of lithium ions.

[0040] 2. In the hydrothermal reaction synthesis, magnesium and titanium elements are doped, 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 elements can help stabilize the structure of the LMFP material, while refining the grains to improve the electrical conductivity.

[0041] 3. The application coats cobalt boride on the surface of the nanometerized lithium manganese iron phosphate primary particles through a ball milling process, which introduces anions and cations for modification at one time, thereby improving the rate performance and high-temperature cycle performance of the lithium manganese iron phosphate.

[0042] 4. The application uses mixed carbon sources of polypyrrole and glucose for coating, and the chemical bond formed between the nitrogen element doping and the carbon atoms can enhance the structural stability of the material, and the mixed carbon source of polypyrrole and glucose can form a more compact and continuous conductive network, thereby further improving the electrical conductivity of the material, while also improving the mechanical strength and compaction density of the material.

[0043] The application has been described in detail in the foregoing, but the above-described embodiments are merely illustrative in nature and are not intended to limit the application. Furthermore, the present text is not limited by any theory described in the foregoing prior art or summary of the application or in the following examples. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 The flow chart of the preparation method of the lithium manganese iron phosphate material of the present application is shown in the figure.

[0045] Figure 2 The SEM image of Example 1 is shown in the figure.

[0046] Figure 3 The SEM image of Example 2 is shown in the figure.

[0047] Figure 4 The SEM image of Example 3 is shown in the figure. DETAILED DESCRIPTION

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

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

[0050] Example 1

[0051] Synthesis of cobalt boride: weigh cobalt acetate 4.98 g, tin powder 3.56 g, and boron powder 1.08 g, grind them uniformly, and then sinter under a mixed atmosphere of 5% H2 and argon, with a heating rate of 3°C / min, a sintering temperature of 780°C, and a holding time of 8 h; after sintering, the powder is crushed, washed with acid, and dried to obtain cobalt boride.

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

[0053] The hydrothermal reaction product 15 g, cobalt boride 0.09 g, glucose 0.9 g, polypyrrole 0.45 g were weighed and dispersed in a mixed solution of 20 ml of ethanol and 20 ml of deionized water, the medium for ball milling was 0.1 mm zirconium balls, the filling rate was 85%, the ball milling speed was 700 rpm / min, the slurry particle size D50 was less than 0.25 μm after ball milling; after the particle size reached the standard, spray drying was carried out, the temperature for spray drying was 190°C, after the spray was completed, sintering was carried out in a nitrogen atmosphere, the sintering temperature was 670°C, the holding time was 8h, and finally sieving was carried out to remove the magnetic particles to prepare lithium manganese iron phosphate with a molecular formula of Li 1.04 Mn 0.55 Fe 0.35 Ti 0.06 Mg 0.04 PO4.

[0054] Example 2

[0055] Synthesis of cobalt boride: cobalt acetate 4.98 g, tin powder 3.56 g, and boron powder 1.08 g were weighed and ground uniformly, and then sintered in a mixed gas atmosphere of 5% H2 and argon, the heating rate was 3°C / min, the sintering temperature was 780°C, and the holding time was 8h; after sintering, the powder was broken, acid washed, and dried to obtain cobalt boride.

[0056] MnSO4·4H2O 15.33 g, FeSO4·7H2O 12.16 g, magnesium sulfate 0.752 g, and titanium sulfate 1.50 g were weighed and dispersed in a mixed solution of 32 ml of deionized water and 128 ml of ethylene glycol, and stirred uniformly to obtain solution A; lithium hydroxide 5.45 g was weighed and dispersed in a mixed solution of 40 ml of ethylene glycol and 20 ml of deionized water, and slowly poured into 5 ml of a 0.025 mol / ml concentration of phosphoric acid solution to obtain solution B; solution B was slowly introduced into solution A, and the pH was adjusted to 6-7 with ammonia water, and then stirred uniformly and transferred into the inner liner of a hydrothermal reaction kettle, the hydrothermal reaction temperature was 180°C, and the reaction time was 6h. After the hydrothermal reaction was completed, the product was washed with ethanol by centrifugation three times at a speed of 8000 rpm, and dried.

[0057] The hydrothermal reaction product 15 g, cobalt boride 0.09 g, glucose 0.9 g, polypyrrole 0.45 g were weighed and dispersed in a mixed solution of 20 ml of ethanol and 20 ml of deionized water, the medium for ball milling was 0.1 mm zirconium balls, the filling rate was 85%, the ball milling speed was 700 rpm / min, the slurry particle size D50 was less than 0.25 μm after ball milling; after the particle size reached the standard, spray drying was carried out, the temperature for spray drying was 190°C, after the spray was completed, sintering was carried out in a nitrogen atmosphere, the sintering temperature was 670°C, the holding time was 8h, and finally sieving was carried out to remove the magnetic particles to prepare lithium manganese iron phosphate with a molecular formula of Li 1.04 Mn 0.55 Fe 0.35 Ti 0.05Mg 0.05 Manganese iron lithium phosphate of PO4.

[0058] Example 3

[0059] Synthesis of cobalt boride: weigh cobalt acetate 4.98g, tin powder 3.56g, boron powder 1.08g, grind uniformly, sinter under the mixed atmosphere of 5% H2 and argon, heating rate 3℃ / min, sintering temperature 780℃, holding time 8h; after sintering, the powder is crushed, pickled, dried to obtain cobalt boride.

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

[0061] Weigh the hydrothermal reaction product 15g, cobalt boride 0.09g, glucose 0.9g, and polypyrrole 0.45g, disperse in 20ml ethanol and 20ml deionized water mixed solution, the ball milling medium uses 0.1mm zirconium ball, the filling rate is 85%, the ball milling speed is 700rpm / min, ball mill the slurry to a particle size D50<0.25μm; after the particle size reaches the standard, spray drying is carried out, the spray drying temperature is 190℃, after the spray drying is completed, sintering is carried out in a nitrogen atmosphere, the sintering temperature is 670℃, the holding time is 8h, and finally sieve to remove the magnet to prepare the molecular formula Li 1.04 Mn 0.55 Fe 0.35 Ti 0.04 Mg 0.06 Manganese iron lithium phosphate of PO4.

[0062] Comparative Example 1

[0063] Synthesis of cobalt boride: weigh cobalt acetate 4.98g, tin powder 3.56g, boron powder 1.08g, grind uniformly, sinter under the mixed atmosphere of 5% H2 and argon, heating rate 3℃ / min, sintering temperature 780℃, holding time 8h; after sintering, the powder is crushed, pickled, dried to obtain cobalt boride.

[0064] MnSO4·4H2O 15.33g, FeSO4·7H2O 12.16g, titanium sulfate 1.80g were weighed and dispersed in 32ml deionized water and 128ml ethylene glycol mixed solution, stirred uniformly, recorded as solution A; lithium hydroxide 5.45g was weighed and dispersed in 40ml ethylene glycol and mixed with 20ml deionized solution, slowly poured into 5ml 0.025mol / ml concentration phosphoric acid solution, recorded as solution B; solution B was slowly introduced into solution A, the pH was adjusted to 6-7 with ammonia water, and then transferred to the inner liner of the hydrothermal reaction kettle, the hydrothermal reaction temperature was 180℃, and the reaction time was 6h. After the hydrothermal reaction was completed, ethanol was used for centrifugal washing three times, the centrifugal speed was 8000rpm, and drying.

[0065] The hydrothermal reaction product 15g, cobalt boride 0.09g, glucose 0.9g, and polypyrrole 0.45g were weighed and dispersed in 20ml ethanol and 20ml deionized water mixed solution, the medium for ball milling was 0.1mm zirconium ball, the filling rate was 85%, the ball milling speed was 700rpm / min, the slurry particle size D50 was less than 0.25μm after ball milling; after the particle size reached the standard, spray drying was carried out, the spray drying temperature was 190℃, and after the spray drying was completed, sintering was carried out in a nitrogen atmosphere, the sintering temperature was 670℃, the holding time was 8h, and finally sieving was carried out to remove the magnetic substance to prepare lithium manganese iron phosphate with the molecular formula of Li 1.04 Mn 0.55 Fe 0.35 Ti 0.06 PO4.

[0066] Comparative example 2

[0067] Synthesis of cobalt boride: cobalt acetate 4.98g, tin powder 3.56g, and boron powder 1.08g were ground uniformly, and then sintered in a mixed gas atmosphere of 5% mass fraction H2 and argon, the heating rate was 3℃ / min, the sintering temperature was 780℃, and the holding time was 8h; after sintering, the powder was crushed, acid washed, and dried to obtain cobalt boride.

[0068] MnSO4·4H2O 15.33g, FeSO4·7H2O 12.16g, magnesium sulfate 0.902g were weighed and dispersed in 32ml deionized water and 128ml ethylene glycol mixed solution, stirred uniformly, recorded as solution A; lithium hydroxide 5.45g was weighed and dispersed in 40ml ethylene glycol and mixed with 20ml deionized solution, slowly poured into 5ml 0.025mol / ml concentration phosphoric acid solution, recorded as solution B; solution B was slowly introduced into solution A, the pH was adjusted to 6-7 with ammonia water, and then transferred to the inner liner of the hydrothermal reaction kettle, the hydrothermal reaction temperature was 180℃, and the reaction time was 6h. After the hydrothermal reaction was completed, ethanol was used for centrifugal washing three times, the centrifugal speed was 8000rpm, and drying.

[0069] The water-reaction product 15 g, cobalt boride 0.09 g, glucose 0.9 g, and polypyrrole 0.45 g were weighed and dispersed in a mixed solution of 20 ml of ethanol and 20 ml of deionized water, the medium for ball milling was 0.1 mm zirconium balls, the filling rate was 85%, and the ball milling speed was 700 rpm / min, the slurry was ball milled until the particle size D50 was less than 0.25 μm, after the particle size reached the standard, spray drying was performed, the spray drying temperature was 190°C, after the spray was completed, sintering was performed in a nitrogen atmosphere, the sintering temperature was 670°C, the holding time was 8 h, and finally, sieving was performed to remove the magnetic particles to prepare lithium manganese iron phosphate with a molecular formula of Li 1.04 Mn 0.55 Fe 0.35 Mg 0.06 PO4.

[0070] Comparative Example 3

[0071] Synthesis of cobalt boride: cobalt acetate 4.98 g, tin powder 3.56 g, and boron powder 1.08 g were weighed and ground uniformly, and then sintering was performed in a mixed gas atmosphere of 5% H2 and argon, the heating rate was 3°C / min, the sintering temperature was 780°C, and the holding time was 8 h, after sintering, the powder was crushed, acid washed, and dried to obtain cobalt boride.

[0072] MnSO4·4H2O 15.33 g, FeSO4·7H2O 12.16 g, magnesium sulfate 0.602 g, and titanium sulfate 1.80 g were weighed and dispersed in a mixed solution of 32 ml of deionized water and 128 ml of ethylene glycol, and the mixture was stirred uniformly to obtain solution A, lithium hydroxide 5.45 g was weighed and dispersed in a mixed solution of 40 ml of ethylene glycol and 20 ml of deionized water, 5 ml of 0.025 mol / ml phosphoric acid solution was slowly poured into the mixture to obtain solution B, solution B was slowly introduced into solution A, the pH was adjusted to 6-7 with ammonia water, and the mixture was stirred uniformly and then transferred into the inner liner of a hydrothermal reaction kettle, the hydrothermal reaction temperature was 180°C, and the reaction time was 6 h, after the hydrothermal reaction was completed, the product was washed with ethanol by centrifugation three times at a speed of 8000 rpm, and then dried.

[0073] The water-reaction product 15 g, cobalt boride 0.09 g, glucose 0.9 g, and polypyrrole 0.45 g were weighed and dispersed in a mixed solution of 20 ml of ethanol and 20 ml of deionized water, the medium for ball milling was 0.1 mm zirconium balls, the filling rate was 85%, and the ball milling speed was 700 rpm / min, the slurry was ball milled until the particle size D50 was less than 0.25 μm, after the particle size reached the standard, spray drying was performed, the spray drying temperature was 190°C, after the spray was completed, sintering was performed in a nitrogen atmosphere, the sintering temperature was 670°C, the holding time was 8 h, and finally, sieving was performed to remove the magnetic particles to prepare lithium manganese iron phosphate with a molecular formula of Li 1.04 Mn 0.55 Fe 0.35 Ti 0.06 Mg 0.04Lithium manganese iron phosphate of PO4.

[0074] Comparative Example 4

[0075] Synthesis of cobalt boride: weigh cobalt acetate 4.98 g, tin powder 3.56 g, and boron powder 1.08 g, grind uniformly, and then sinter under a mixed atmosphere of 5% H2and argon, with a heating rate of 3°C / min, a sintering temperature of 780°C, and a holding time of 8 h; after sintering, the powder is crushed, pickled, and dried to obtain cobalt boride.

[0076] Weigh raw materials MnSO4·4H2O 15.33 g, FeSO4·7H2O 12.16 g, magnesium sulfate 0.602 g, and titanium sulfate 1.80 g, and disperse them in a mixed solution of 32 ml of deionized water and 128 ml of ethylene glycol, to obtain solution A; weigh lithium hydroxide 5.45 g, disperse it in a mixed solution of 40 ml of ethylene glycol and 20 ml of deionized water, slowly pour into 5 ml of a 0.025 mol / ml concentration of phosphoric acid solution, and label as solution B; slowly introduce solution B into solution A, adjust the pH to 6-7 with ammonia water, stir uniformly, and then transfer into the inner liner of a hydrothermal reaction kettle; the hydrothermal reaction temperature is 180°C, and the reaction time is 6 h. After the hydrothermal reaction is completed, the product is washed by centrifugation with ethanol three times at a centrifugal speed of 8000 rpm, and then dried.

[0077] Weigh the hydrothermally reacted product 15 g, glucose 0.9 g, and polypyrrole 0.45 g, disperse them in a mixed solution of 20 ml of ethanol and 20 ml of deionized water, use 0.1 mm zirconium balls as the ball milling medium, the filling rate is 85%, the ball milling speed is 700 rpm / min, ball mill until the slurry particle size D50 is less than 0.25 μm; after the particle size meets the requirements, spray drying is performed at a temperature of 190°C, and then sintering is performed in a nitrogen atmosphere, the sintering temperature is 670°C, the holding time is 8 h, and finally sieving is performed to remove the magnetic material to obtain lithium manganese iron phosphate of PO4. 1.04 Mn 0.55 Fe 0.35 Ti 0.06 Mg 0.04 Lithium manganese iron phosphate of PO4.

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

[0079]

[0080]

[0081] Comparative Example 1 does not introduce magnesium doping compared to Example 1, Comparative Example 2 does not introduce titanium doping compared to Example 1, Comparative Example 3 is single glucose as carbon source compared to Example 1, and Comparative Example 4 does not have cobalt boride coating compared to Example 1. The SEM images of Example 1, Example 2, and Example 3 are all typical secondary spherical particles formed by accumulation of primary nanoparticles, and the test results also confirm that the secondary spherical particles of lithium manganese iron phosphate cathode material prepared by using cobalt boride and composite carbon source coating effectively improve the specific capacity and compaction density of lithium manganese iron phosphate.

[0082] The above examples are only used to illustrate the technical solutions of the present application, but not to limit it. Although the present application has been described in detail with reference to the foregoing examples, it should be understood by those skilled in the art that the technical solutions recorded in the foregoing examples can be modified or some or all of the technical features can be replaced equivalently without departing from the spirit and essence defined in the claims of the present application.

Claims

1. A modified lithium iron manganese phosphate cathode material, characterized in that, The modified manganese iron lithium phosphate cathode material comprises doped manganese iron lithium phosphate and a coating layer. The chemical general formula of the doped lithium manganese iron phosphate is Li a Fe b Mn c Mg d Ti e PO4, wherein 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 layer comprises carbon and cobalt boride. The preparation method of the modified manganese iron lithium phosphate cathode material comprises the following steps: (1) dispersing manganese source, iron source, magnesium source and titanium source in a mixed solution of deionized water and ethylene glycol, stirring uniformly, and recording as solution A; dispersing lithium source and phosphorus source in a mixed solution of water and ethylene glycol, stirring uniformly, and recording as solution B; slowly adding solution B into solution A while stirring, stirring uniformly, adjusting pH to 6-8, and then transferring the mixed solution into a hydrothermal reaction kettle to perform hydrothermal reaction, and then performing centrifugal washing to obtain a solid product; (2) mixing the solid product with cobalt boride, polypyrrole and glucose in deionized water, performing ball milling, performing spray drying after the particle size of the slurry reaches the standard, collecting the solid powder, and then performing sintering in an inert atmosphere, and then performing magnetic screening to obtain the modified manganese iron lithium phosphate cathode material; In step (2), the preparation method of cobalt boride is to grind cobalt source, tin powder and boron powder uniformly and then perform sintering and washing to prepare.

2. The modified lithium iron manganese phosphate cathode material of claim 1, wherein, The content of cobalt boride in the coating layer is 0.1-1% based on the total mass of the modified manganese iron lithium phosphate cathode material.

3. The modified lithium iron manganese phosphate cathode material of claim 1, wherein, The mass content of carbon in the coating layer is 1-15% based on the total mass of the modified manganese iron lithium phosphate cathode material.

4. The modified lithium iron manganese phosphate cathode material of claim 1, wherein, In step (1), the manganese source is one or more selected from manganese dioxide, manganese carbonate, trimanganese tetroxide, manganese sulfate or manganese iron phosphate; the iron source is one or more selected from ferrous sulfate, iron oxide and iron 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, monobasic lithium phosphate, dibasic lithium phosphate and lithium acetate; and the phosphorus source is one or more selected from phosphate, phosphoric acid and ammonium dihydrogen phosphate.

5. The modified lithium iron manganese phosphate cathode material of claim 1, wherein, In step (1), the temperature of the hydrothermal reaction is 160-230°C, and the reaction time is 4-12h.

6. The modified lithium iron manganese phosphate cathode material of claim 1, wherein, In step (2), the mass percentage of cobalt boride in the solid product is 0.2-1.2%, and the mass percentage of the sum of polypyrrole and glucose in the solid product is 6.5-12.5%.

7. The modified lithium iron manganese phosphate cathode material of claim 1, wherein, In step (2), the diameter of the ball milling medium is 0.1-1mm; the material of the ball milling medium is zirconia, agate or yttrium stabilized zirconia; the filling rate of the ball milling medium is 50-90%; and the particle size D50 of the slurry after ball milling is 0.01-1μm. The temperature of the spray drying is 100-200°C; the sintering temperature is 400-800°C; and the sintering time is 7-13h.

8. A lithium-ion battery, characterized by The modified manganese iron lithium phosphate cathode material comprises the modified manganese iron lithium phosphate cathode material according to any one of claims 1-7.

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