Lithium manganese iron phosphate composite material and preparation method thereof

Abstract

The invention discloses a lithium manganese iron phosphate composite material and a preparation method thereof, and belongs to the technical field of lithium ion battery positive electrode materials. Comprising the following steps: S1, dissolving MnCl2. 4H2O and FeCl3. 6H2O in ethoxyacetic acid to prepare a first solution; adding 1, 3, 5-benzene tricarbonic acid into absolute ethyl alcohol to prepare a second solution; slowly adding the first solution into the second solution, then adding NaOH, carrying out hydrothermal reaction, cooling, centrifugally washing and drying to obtain a ferromanganese precursor; s2, SeOx (at) rGO-EDA is prepared; and S3, uniformly mixing the ferromanganese precursor, SeOx (at) rGO-EDA, lithium carbonate and ammonium dihydrogen phosphate, heating and calcining in nitrogen, and cooling to obtain the lithium iron manganese phosphate composite material. The strong oxygen affinity of SeOx can be combined with active oxygen on the surface of LiFePO4, and the chemical adsorption effect of SeOx can reduce the dissolution of manganese ions and improve the cycling stability of the composite material.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion battery positive electrode materials, and particularly relates to a lithium manganese iron phosphate composite material and a preparation method thereof. Background Art

[0002] As a positive electrode material for lithium-ion batteries, lithium iron phosphate (LiFePO4) has become an important choice in the field of power batteries and energy storage since it was first reported in 1997 due to the stability and high safety of its olivine structure. In its crystal structure, FeO6 octahedrons and PO4 tetrahedrons are connected by common vertices, and lithium ions diffuse in one dimension along the direction, giving the material excellent thermal stability and cycle life, and a high theoretical specific capacity. However, its extremely low intrinsic electronic conductivity and slow lithium ion diffusion rate result in insufficient high-rate performance. At the same time, the low tap density limits the volume energy density, making it difficult to meet the needs of high-power and compact applications. To address the above problems, research has focused on multi-scale modification technologies: in terms of conductivity improvement, carbon coating and bulk doping can improve conductivity; tap density optimization improves density through particle grading and morphology control.

[0003] Patent application publication number CN114678526A discloses a method for preparing a carbon-coated lithium iron phosphate composite material, comprising the following steps: pulverizing biomass and pre-treating it to obtain biomass powder; adding the biomass powder to an aqueous iron salt solution and stirring it, then adding an aqueous ammonia solution dropwise while stirring until no more precipitation is produced, filtering and washing the precipitate; carbonizing and activating the precipitate under a protective atmosphere to obtain a nanoscale iron oxide / carbon composite material; and mixing the nanostructured carbon with a lithium source and a phosphorus source according to the stoichiometric ratio of lithium iron phosphate, and calcining the composite material at high temperature under a protective atmosphere. However, biomass carbon generally has a low degree of crystallinity and may have insufficient electronic conductivity compared to a graphitized carbon coating, making it difficult to significantly improve the high-rate performance of lithium iron phosphate.

[0004] Therefore, preparing a positive electrode material that can improve the actual specific capacity and cycle stability is an urgent problem to be solved. Summary of the Invention

[0005] The object of the present invention is to provide a lithium manganese iron phosphate composite material and a preparation method thereof, so as to improve the actual specific capacity and cycle stability of the positive electrode material.

[0006] The purpose of the present invention can be achieved through the following technical solutions: A lithium manganese iron phosphate composite material, comprising lithium manganese iron phosphate and SeO modified on the surface of lithium manganese iron phosphate x @rGO-EDA; the lithium manganese iron phosphate comprises a manganese / iron-1,3,5-benzenetricarboxylic acid organic framework, a phosphorus source and a lithium source; the rGO is coated with SeO in situ generated by a magnesium thermal reduction reaction xThe adjacent rGO sheets were covalently cross-linked by EDA.

[0007] Furthermore, the preparation method of the lithium manganese iron phosphate composite material comprises the following steps: S1. Dissolving MnCl2·4H2O and FeCl3·6H2O in ethoxyacetic acid to prepare a first solution; adding 1,3,5-benzenetricarboxylic acid to anhydrous ethanol to prepare a second solution; slowly adding the first solution to the second solution, then adding NaOH, performing a hydrothermal reaction, cooling, centrifuging, washing, and drying to obtain a manganese iron precursor; S2. Graphene oxide is dissolved in deionized water and dispersed evenly to obtain a third solution; SeO2 is added to a 50% ethanol solution and dispersed evenly to obtain a fourth solution; the third solution and the fourth solution are mixed evenly, EDA (ethylenediamine) solution is added dropwise, subjected to hydrothermal reaction, washed and freeze-dried to obtain SeO2@GO-EDA; S3, grind and mix SeO2@GO-EDA and magnesium, heat and keep warm under argon environment, cool and immerse in hydrochloric acid solution; filter, wash, freeze-dry and obtain SeO x @rGO-EDA, 0<X≤2; S4, the manganese iron precursor, SeO x After @rGO-EDA, lithium carbonate and ammonium dihydrogen phosphate are evenly mixed, the temperature is increased and calcined under nitrogen, and then cooled to obtain a lithium manganese iron phosphate composite material.

[0008] Furthermore, the usage ratio of the MnCl2·4H2O, FeCl3·6H2O and ethoxyacetic acid is (1-2.5) g: (0.25-1.25) g: (40-100) mL.

[0009] Furthermore, the usage ratio of the 1,3,5-benzenetricarboxylic acid, anhydrous ethanol and NaOH is (0.5-2) g: (10-40) mL: (0.1-0.3) g.

[0010] Furthermore, the hydrothermal reaction is carried out at 160-180° C. for 16-48 hours.

[0011] Furthermore, the washing is performed by alternating washing with deionized water and ethanol for 3 times; and the drying is performed at 60-80° C. for 12-24 hours.

[0012] Furthermore, the usage ratio of the graphene oxide, deionized water, SeO2, 50% ethanol solution and EDA is (30-50) mg: (20-40) mL: (400-600) mg: (10-30) mL: (100-400) μL.

[0013] Furthermore, the mass ratio of SeO2@GO-EDA and magnesium is (0.8-1): (0.8-1).

[0014] Furthermore, the heating and heat preservation is to raise the temperature to 650-700°C at a rate of 1-5°C / min and keep the temperature for 3-4h.

[0015] Furthermore, the concentration of the hydrochloric acid solution is 1 mol / L, and the immersion time is 18-24 hours.

[0016] Furthermore, the ferromanganese precursor, SeO x The mass ratio of @rGO-EDA, lithium carbonate and ammonium dihydrogen phosphate is (30-45):(6-9):(3.5-5):(10-15).

[0017] Furthermore, the temperature-raising calcination is to raise the temperature to 700-750° C. at a rate of 1-5° C. / min and calcine for 8-10 hours.

[0018] Beneficial effects of the present invention: (1) The present invention provides a method for preparing a lithium manganese iron phosphate composite material, wherein the surface of lithium manganese iron phosphate (LMFP) particles is modified with SeO x @rGO-EDA functional unit. SeO x Selenium (Se 4+ / Se 6+ ) has strong electronegativity and forms Se-OP or Se-OM (M=Mn / Fe) covalent bonds with the oxygen atoms exposed on the surface of LMFP particles to achieve chemical modification. The residual carboxyl and hydroxyl groups on the edge of rGO and the Mn on the surface of LMFP 2+ / Fe 2+ Bonding occurs, enhancing the interface bonding strength.

[0019] (2) The present invention adopts a hydrothermal method to synthesize a manganese / iron-1,3,5-benzenetricarboxylic acid organic framework (manganese iron precursor). The high stability of the organic framework not only improves the stability of the composite material, but its high specific surface area and regular pores can provide a rapid diffusion path for lithium ions, thereby improving electronic conductivity; the introduction of manganese ions can increase the additional capacity of the battery; in addition, the chelation effect of the MOF (metal organic framework) framework on iron ions can reduce the dissolution of iron during the cycle and extend the cycle life.

[0020] (3) SeO used in the present invention x @rGO-EDA, rGO (reduced graphene oxide) coated with SeO in situ generated by Mg-thermal reduction reaction xWhen \(0 < x\leq2\), a core-shell structure is formed; GO is reduced to rGO with stronger conductivity at high temperature; in addition, adjacent rGO sheets are covalently cross-linked by ethylenediamine (EDA) molecules to construct a three-dimensional continuous conductive network. This structure significantly improves the electron conductivity, structural stability and electrochemical activity of the composite material.

[0021] (4) The graphene oxide used in the present invention, the synergistic effect of its oxygen-containing functional groups and the amino groups of EDA can regulate the electrolyte decomposition path, form a dense and stable SEI film, and improve the stability of the lithium iron phosphate material; graphene oxide is used as a surface coating layer on SeO2, synergistically inhibiting oxygen release and structural degradation. In addition, due to the strong oxygen affinity of SeO x can preferentially bind to the surface active oxygen of LiFePO4, and its chemisorption effect can inhibit the disproportionation reaction of manganese ions, reduce the dissolution of manganese ions, and improve the cycle stability of the composite material. Specific embodiments

[0022] The specific embodiments of the present invention will be described in detail below, but it should be understood that the protection scope of the present invention is not limited by the specific embodiments.

[0023] Example 1 This example provides a lithium manganese iron phosphate composite material, which is prepared by the following steps: S1. Dissolve 1.5 g of manganese chloride tetrahydrate (MnCl2·4H2O) and 0.5 g of ferric chloride hexahydrate (FeCl3·6H2O) in 60 mL of ethoxyacetic acid to prepare a first solution; dissolve 1 g of 1,3,5-benzenetricarboxylic acid in 20 mL of absolute ethanol to prepare a second solution; slowly add the first solution to the second solution under magnetic stirring, then add 0.2 g of NaOH, transfer it to a stainless steel autoclave, and place it in an oven at 180°C for 48 h. After the sample is naturally cooled in the autoclave, it is centrifuged and collected, washed alternately with deionized water and ethanol 3 times, and dried in an oven at 80°C for 24 h to obtain a manganese-iron precursor; S2. Dissolve 40 mg of graphene oxide in 20 mL of deionized water, and obtain a third solution through stirring and ultrasonic dispersion; add 500 mg of SeO2 to 10 mL of 50% ethanol solution, and obtain a fourth solution through stirring and ultrasonic dispersion; mix the third solution and the fourth solution, stir, ultrasonic disperse, then add 200 μL of EDA solution, stir for 10 min and then put it into a reactor, place the reaction kettle in an oven, carry out hydrothermal reaction at 180°C for 12 h. After the reaction is completed, wash it alternately with deionized water and ethanol 3 times, and freeze-dry for 24 h to obtain SeO2@GO-EDA; S3. Grind and mix 300 mg of SeO2@GO-EDA and 300 mg of magnesium, place them in a graphite crucible, and heat them in a tube furnace under an argon atmosphere. The temperature is gradually increased at a rate of 5°C / min until it reaches 680°C and kept warm for 3 hours. The reaction product is then cooled to room temperature and immersed in a 1 mol / L hydrochloric acid solution for 24 hours. The product is separated by vacuum filtration technology, washed with deionized water several times until the pH is 7, and finally freeze-dried for 12 hours to obtain SeO2@GO-EDA. x @rGO-EDA (0<X≤2); S4, 35 parts by weight of ferromanganese precursor, 8 parts by weight of SeO x After @rGO-EDA, 4 parts by weight of lithium carbonate, and 12 parts by weight of ammonium dihydrogen phosphate were fully mixed, the temperature was raised to 730°C at a rate of 5°C / min under high-purity nitrogen and calcined for 8 hours. After cooling, a lithium manganese iron phosphate composite material was obtained.

[0024] Example 2 Compared with Example 1, this embodiment differs in that the ferromanganese precursor is increased and the dosage ratios of the other components in S3 are adjusted. The specific implementation steps are as follows: S1. Dissolve 1.5 g of manganese chloride tetrahydrate (MnCl2·4H2O) and 0.5 g of ferric chloride hexahydrate (FeCl3·6H2O) in 60 mL of ethoxyacetic acid to prepare a first solution; add 1 g of 1,3,5-benzenetricarboxylic acid to 20 mL of anhydrous ethanol to prepare a second solution; slowly add the first solution to the second solution under magnetic stirring, then add 0.2 g of NaOH, transfer to a stainless steel autoclave, and place in an oven at 180°C for 48 hours. After the sample is naturally cooled in the autoclave, it is collected by centrifugation, washed alternately with deionized water and ethanol three times, and dried in an oven at 80°C for 24 hours to obtain a manganese iron precursor; S2. Dissolve 40 mg of graphene oxide in 20 mL of deionized water and obtain a third solution by stirring and ultrasonic dispersion. Add 500 mg of SeO2 to 10 mL of 50% ethanol solution and obtain a fourth solution by stirring and ultrasonic dispersion. Mix the third and fourth solutions, stir, and ultrasonically disperse them. Then, add 200 μL of EDA solution dropwise, stir for 10 min, and place in a reactor. Place the reactor in an oven and hydrothermally react at 180°C for 12 h. After the reaction, wash the mixture alternately with deionized water and ethanol three times and freeze-dry for 24 h to obtain SeO2@GO-EDA. S3. Grind and mix 300 mg of SeO2@GO-EDA and 300 mg of magnesium, place them in a graphite crucible, and heat them in a tube furnace under an argon atmosphere. The temperature is gradually increased at a rate of 5°C / min until it reaches 680°C and kept warm for 3 hours. Then, the reaction product is cooled to room temperature and immersed in a 1 mol / L hydrochloric acid solution for 24 hours. The product is separated by vacuum filtration technology, washed with deionized water several times until the pH is 7, and finally freeze-dried for 12 hours to obtain SeO2@GO-EDA. x @rGO-EDA (0<X≤2); S4, 45 parts by weight of ferromanganese precursor, 6 parts by weight of SeO x After @rGO-EDA, 3.5 parts by weight of lithium carbonate, and 10 parts by weight of ammonium dihydrogen phosphate were fully mixed, the temperature was raised to 720°C at a rate of 5°C / min under high-purity nitrogen and calcined for 8 hours. After cooling, a lithium manganese iron phosphate composite material was obtained.

[0025] The remaining raw materials and preparation process remain the same as in Example 1.

[0026] Example 3 Compared with Example 1, this embodiment differs in that the ferromanganese precursor is reduced and the dosage ratios of the other components in S3 are adjusted. The specific implementation steps are as follows: S1. Dissolve 1.5 g of manganese chloride tetrahydrate (MnCl2·4H2O) and 0.5 g of ferric chloride hexahydrate (FeCl3·6H2O) in 60 mL of ethoxyacetic acid to prepare a first solution; add 1 g of 1,3,5-benzenetricarboxylic acid to 20 mL of anhydrous ethanol to prepare a second solution; slowly add the first solution to the second solution under magnetic stirring, then add 0.2 g of NaOH, transfer to a stainless steel autoclave, and place in an oven at 180°C for 48 hours. After the sample is naturally cooled in the autoclave, it is collected by centrifugation, washed alternately with deionized water and ethanol three times, and dried in an oven at 80°C for 24 hours to obtain a manganese iron precursor; S2. Dissolve 40 mg of graphene oxide in 20 mL of deionized water and obtain a third solution by stirring and ultrasonic dispersion. Add 500 mg of SeO2 to 10 mL of 50% ethanol solution and obtain a fourth solution by stirring and ultrasonic dispersion. Mix the third and fourth solutions, stir, and ultrasonically disperse them. Then, add 200 μL of EDA solution dropwise, stir for 10 min, and place in a reactor. Place the reactor in an oven and hydrothermally react at 180°C for 12 h. After the reaction, wash the mixture alternately with deionized water and ethanol three times and freeze-dry for 24 h to obtain SeO2@GO-EDA. S3. Grind and mix 300 mg of SeO2@GO-EDA and 300 mg of magnesium, place them in a graphite crucible, and heat them in a tube furnace under an argon atmosphere. The temperature is gradually increased at a rate of 5°C / min until it reaches 660°C and kept warm for 3 hours. The reaction product is then cooled to room temperature and immersed in a 1 mol / L hydrochloric acid solution for 24 hours. The product is separated by vacuum filtration technology, washed with deionized water several times until the pH is 7, and finally freeze-dried for 12 hours to obtain SeO2@GO-EDA. x @rGO-EDA (0<X≤2); S4, 30 parts by weight of ferromanganese precursor, 9 parts by weight of SeO x After @rGO-EDA, 5 parts by weight of lithium carbonate, and 15 parts by weight of ammonium dihydrogen phosphate were fully mixed, the temperature was raised to 710°C at a rate of 5°C / min under high-purity nitrogen and calcined for 8 hours. After cooling, a lithium manganese iron phosphate composite material was obtained.

[0027] The remaining raw materials and preparation process remain the same as in Example 1.

[0028] Example 4 Compared with Example 1, this embodiment differs in that the mass ratio of SeO2@GO-EDA and magnesium is adjusted. The specific implementation steps are as follows: S1. Dissolve 1.5 g of manganese chloride tetrahydrate (MnCl2·4H2O) and 0.5 g of ferric chloride hexahydrate (FeCl3·6H2O) in 60 mL of ethoxyacetic acid to prepare a first solution; add 1 g of 1,3,5-benzenetricarboxylic acid to 20 mL of anhydrous ethanol to prepare a second solution; slowly add the first solution to the second solution under magnetic stirring, then add 0.2 g of NaOH, transfer to a stainless steel autoclave, and place in an oven at 180°C for 48 hours. After the sample is naturally cooled in the autoclave, it is collected by centrifugation, washed alternately with deionized water and ethanol three times, and dried in an oven at 80°C for 24 hours to obtain a manganese iron precursor; S2. Dissolve 40 mg of graphene oxide in 20 mL of deionized water and obtain a third solution by stirring and ultrasonic dispersion. Add 500 mg of SeO2 to 10 mL of 50% ethanol solution and obtain a fourth solution by stirring and ultrasonic dispersion. Mix the third and fourth solutions, stir, and ultrasonically disperse them. Then, add 200 μL of EDA solution dropwise, stir for 10 min, and place in a reactor. Place the reactor in an oven and hydrothermally react at 180°C for 12 h. After the reaction, wash the mixture alternately with deionized water and ethanol three times and freeze-dry for 24 h to obtain SeO2@GO-EDA. S3. Grind and mix 300 mg of SeO2@GO-EDA and 240 mg of magnesium, place them in a graphite crucible, and heat them in a tube furnace under an argon atmosphere. The temperature is gradually increased at a rate of 5°C / min until it reaches 690°C and kept warm for 3 hours. The reaction product is then cooled to room temperature and immersed in a 1 mol / L hydrochloric acid solution for 24 hours. The product is separated by vacuum filtration technology, washed with deionized water several times until the pH is 7, and finally freeze-dried for 12 hours to obtain SeO2@GO-EDA. x @rGO-EDA (0<X≤2); S4, 35 parts by weight of ferromanganese precursor, 8 parts by weight of SeO x After @rGO-EDA, 4 parts by weight of lithium carbonate, and 12 parts by weight of ammonium dihydrogen phosphate were fully mixed, the temperature was raised to 740°C at a rate of 5°C / min under high-purity nitrogen and calcined for 8 hours. After cooling, a lithium manganese iron phosphate composite material was obtained.

[0029] The remaining raw materials and preparation process remain the same as in Example 1.

[0030] Example 5 Compared with Example 1, this embodiment differs in that the amount of EDA is increased and the amounts of other components in S2 are adjusted. The specific implementation steps are as follows: S1. Dissolve 1.5 g of manganese chloride tetrahydrate (MnCl2·4H2O) and 0.5 g of ferric chloride hexahydrate (FeCl3·6H2O) in 60 mL of ethoxyacetic acid to prepare a first solution; add 1 g of 1,3,5-benzenetricarboxylic acid to 20 mL of anhydrous ethanol to prepare a second solution; slowly add the first solution to the second solution under magnetic stirring, then add 0.2 g of NaOH, transfer to a stainless steel autoclave, and place in an oven at 180°C for 48 hours. After the sample is naturally cooled in the autoclave, it is collected by centrifugation, washed alternately with deionized water and ethanol three times, and dried in an oven at 80°C for 24 hours to obtain a manganese iron precursor; S2. Dissolve 35 mg of graphene oxide in 20 mL of deionized water and obtain a third solution by stirring and ultrasonic dispersion. Add 400 mg of SeO2 to 10 mL of 50% ethanol solution and obtain a fourth solution by stirring and ultrasonic dispersion. Mix the third and fourth solutions, stir, and ultrasonically disperse them. Then, add 400 μL of EDA solution dropwise, stir for 10 min, and place in a reactor. Place the reactor in an oven and hydrothermally react at 180°C for 12 h. After the reaction, wash the mixture alternately with deionized water and ethanol three times and freeze-dry for 24 h to obtain SeO2@GO-EDA. S3. Grind and mix 300 mg of SeO2@GO-EDA and 300 mg of magnesium, place them in a graphite crucible, and heat them in a tube furnace under an argon atmosphere. The temperature is gradually increased at a rate of 5°C / min until it reaches 700°C and kept warm for 3 hours. The reaction product is then cooled to room temperature and immersed in a 1 mol / L hydrochloric acid solution for 24 hours. The product is separated by vacuum filtration technology, washed with deionized water several times until the pH is 7, and finally freeze-dried for 12 hours to obtain SeO2@GO-EDA. x @rGO-EDA (0<X≤2); S4, 35 parts by weight of ferromanganese precursor, 8 parts by weight of SeO x After @rGO-EDA, 4 parts by weight of lithium carbonate, and 12 parts by weight of ammonium dihydrogen phosphate were fully mixed, the temperature was raised to 750°C at a rate of 5°C / min under high-purity nitrogen and calcined for 8 hours. After cooling, a lithium manganese iron phosphate composite material was obtained.

[0031] The remaining raw materials and preparation process remain the same as in Example 1.

[0032] Example 6 Compared with Example 1, this embodiment differs in that the amount of EDA is reduced and the amounts of other components in S2 are adjusted. The specific implementation steps are as follows: S1. Dissolve 1.5 g of manganese chloride tetrahydrate (MnCl2·4H2O) and 0.5 g of ferric chloride hexahydrate (FeCl3·6H2O) in 60 mL of ethoxyacetic acid to prepare a first solution; add 1 g of 1,3,5-benzenetricarboxylic acid to 20 mL of anhydrous ethanol to prepare a second solution; slowly add the first solution to the second solution under magnetic stirring, then add 0.2 g of NaOH, transfer to a stainless steel autoclave, and place in an oven at 180°C for 48 hours. After the sample is naturally cooled in the autoclave, it is collected by centrifugation, washed alternately with deionized water and ethanol three times, and dried in an oven at 80°C for 24 hours to obtain a manganese iron precursor; S2. Dissolve 50 mg of graphene oxide in 40 mL of deionized water and obtain a third solution by stirring and ultrasonic dispersion. Add 600 mg of SeO2 to 20 mL of 50% ethanol solution and obtain a fourth solution by stirring and ultrasonic dispersion. Mix the third and fourth solutions, stir, and ultrasonically disperse them. Then, add 100 μL of EDA solution dropwise, stir for 10 min, and place in a reactor. Place the reactor in an oven and hydrothermally react at 180°C for 12 h. After the reaction, wash the mixture alternately with deionized water and ethanol three times and freeze-dry for 24 h to obtain SeO2@GO-EDA. S3. Grind and mix 300 mg of SeO2@GO-EDA and 300 mg of magnesium, place them in a graphite crucible, and heat them in a tube furnace under an argon atmosphere. The temperature is gradually increased at a rate of 5°C / min until it reaches 650°C and kept warm for 3 hours. The reaction product is then cooled to room temperature and immersed in a 1 mol / L hydrochloric acid solution for 24 hours. The product is separated by vacuum filtration technology, washed with deionized water several times until the pH is 7, and finally freeze-dried for 12 hours to obtain SeO2@GO-EDA. x @rGO-EDA (0<X≤2); S4, 35 parts by weight of ferromanganese precursor, 8 parts by weight of SeO x After @rGO-EDA, 4 parts by weight of lithium carbonate, and 12 parts by weight of ammonium dihydrogen phosphate were fully mixed, the temperature was raised to 700°C at a rate of 5°C / min under high-purity nitrogen and calcined for 8 hours. After cooling, a lithium manganese iron phosphate composite material was obtained.

[0033] The remaining raw materials and preparation process remain the same as in Example 1.

[0034] Example 7 Compared with Example 1, this embodiment differs in that the dosage of each component in S1 is adjusted. The specific implementation steps are as follows: S1. Dissolve 2 g of manganese chloride tetrahydrate (MnCl2·4H2O) and 1 g of ferric chloride hexahydrate (FeCl3·6H2O) in 80 mL of ethoxyacetic acid to prepare a first solution; add 2 g of 1,3,5-benzenetricarboxylic acid to 30 mL of anhydrous ethanol to prepare a second solution; slowly add the first solution to the second solution under magnetic stirring, then add 0.2 g of NaOH, transfer to a stainless steel autoclave, and place in an oven at 180°C for 48 hours. After the sample is naturally cooled in the autoclave, it is collected by centrifugation, washed alternately with deionized water and ethanol three times, and dried in an oven at 80°C for 24 hours to obtain a manganese iron precursor; S2. Dissolve 40 mg of graphene oxide in 20 mL of deionized water and obtain a third solution by stirring and ultrasonic dispersion. Add 500 mg of SeO2 to 10 mL of 50% ethanol solution and obtain a fourth solution by stirring and ultrasonic dispersion. Mix the third and fourth solutions, stir, and ultrasonically disperse them. Then, add 200 μL of EDA solution dropwise, stir for 10 min, and place in a reactor. Place the reactor in an oven and hydrothermally react at 180°C for 12 h. After the reaction, wash the mixture alternately with deionized water and ethanol three times and freeze-dry for 24 h to obtain SeO2@GO-EDA. S3. Grind and mix 300 mg of SeO2@GO-EDA and 300 mg of magnesium, place them in a graphite crucible, and heat them in a tube furnace under an argon atmosphere. The temperature is gradually increased at a rate of 5°C / min until it reaches 660°C and kept warm for 3 hours. The reaction product is then cooled to room temperature and immersed in a 1 mol / L hydrochloric acid solution for 24 hours. The product is separated by vacuum filtration technology, washed with deionized water several times until the pH is 7, and finally freeze-dried for 12 hours to obtain SeO2@GO-EDA. x @rGO-EDA (0<X≤2); S4, 35 parts by weight of ferromanganese precursor, 8 parts by weight of SeO x After @rGO-EDA, 4 parts by weight of lithium carbonate, and 12 parts by weight of ammonium dihydrogen phosphate were fully mixed, the temperature was raised to 720°C at a rate of 5°C / min under high-purity nitrogen and calcined for 8 hours. After cooling, a lithium manganese iron phosphate composite material was obtained.

[0035] The remaining raw materials and preparation process remain the same as in Example 1.

[0036] Comparative Example 1 Compared with Example 1, this comparative example is different in that EDA is not added. The specific implementation steps are as follows: S1. Dissolve 1.5 g of manganese chloride tetrahydrate (MnCl2·4H2O) and 0.5 g of ferric chloride hexahydrate (FeCl3·6H2O) in 60 mL of ethoxyacetic acid to prepare a first solution; add 1 g of 1,3,5-benzenetricarboxylic acid to 20 mL of anhydrous ethanol to prepare a second solution; slowly add the first solution to the second solution under magnetic stirring, then add 0.2 g of NaOH, transfer to a stainless steel autoclave, and place in an oven at 180°C for 48 hours. After the sample is naturally cooled in the autoclave, it is collected by centrifugation, washed alternately with deionized water and ethanol three times, and dried in an oven at 80°C for 24 hours to obtain a manganese iron precursor; S2. Dissolve 40 mg of graphene oxide in 20 mL of deionized water and obtain a third solution by stirring and ultrasonic dispersion. Add 500 mg of SeO2 to 10 mL of 50% ethanol solution and obtain a fourth solution by stirring and ultrasonic dispersion. Mix the third and fourth solutions, stir, and ultrasonically disperse them. After stirring for 10 min, place them in a reactor, place the reactor in an oven, and hydrothermally react at 180°C for 12 h. After the reaction, wash them alternately with deionized water and ethanol three times, and freeze-dry them for 24 h to obtain SeO2@GO. S3. Grind and mix 300 mg of SeO2@GO and 300 mg of magnesium, place them in a graphite crucible, and heat them in a tube furnace under an argon atmosphere. The temperature is gradually increased at a rate of 5°C / min until it reaches 680°C and kept warm for 3 hours. The reaction product is then cooled to room temperature and immersed in a 1 mol / L hydrochloric acid solution for 24 hours. The product is separated by vacuum filtration technology, washed with deionized water several times until the pH is 7, and finally freeze-dried for 12 hours to obtain SeO2@GO. x @rGO(0<X≤2); S4, 35 parts by weight of ferromanganese precursor, 4 parts by weight of SeO x After @rGO, 4 parts by weight of lithium carbonate and 12 parts by weight of ammonium dihydrogen phosphate were fully mixed, the temperature was raised to 730°C at a rate of 5°C / min under high-purity nitrogen and calcined for 8 hours. After cooling, a lithium manganese iron phosphate composite material was obtained.

[0037] The remaining raw materials and preparation process remain the same as in Example 1.

[0038] Comparative Example 2 Compared with Example 1, this comparative example differs in that no SeO2 is added. The specific implementation steps are as follows: S1. Dissolve 1.5 g of manganese chloride tetrahydrate (MnCl2·4H2O) and 0.5 g of ferric chloride hexahydrate (FeCl3·6H2O) in 60 mL of ethoxyacetic acid to prepare a first solution; add 1 g of 1,3,5-benzenetricarboxylic acid to 20 mL of anhydrous ethanol to prepare a second solution; slowly add the first solution to the second solution under magnetic stirring, then add 0.2 g of NaOH, transfer to a stainless steel autoclave, and place in an oven at 180°C for 48 hours. After the sample is naturally cooled in the autoclave, it is collected by centrifugation, washed alternately with deionized water and ethanol three times, and dried in an oven at 80°C for 24 hours to obtain a manganese iron precursor; S2. Dissolve 40 mg of graphene oxide in 20 mL of deionized water and obtain a third solution by stirring and ultrasonic dispersion. Then, add 200 μL of EDA solution dropwise, stir for 10 min, and place in a reactor. Place the reactor in an oven and perform hydrothermal reaction at 180°C for 12 h. After the reaction, wash with deionized water and ethanol alternately three times and freeze-dry for 24 h to obtain GO-EDA. S3. Grind and mix 300 mg of GO-EDA and 300 mg of magnesium, place them in a graphite crucible, and heat them in a tube furnace under an argon atmosphere. The temperature is gradually increased at a rate of 5°C / min until it reaches 680°C and kept at this temperature for 3 hours. The reaction product is then cooled to room temperature and immersed in a 1 mol / L hydrochloric acid solution for 24 hours. The resulting product is separated by vacuum filtration, washed with deionized water several times until the pH reaches 7, and finally freeze-dried for 12 hours to obtain rGO-EDA. S4. After fully mixing 35 parts by weight of manganese iron precursor, 6 parts by weight of rGO-EDA, 4 parts by weight of lithium carbonate, and 12 parts by weight of ammonium dihydrogen phosphate, the mixture was heated to 730°C at a rate of 5°C / min under high-purity nitrogen and calcined for 8 hours. After cooling, a manganese iron lithium phosphate composite material was obtained.

[0039] The remaining raw materials and preparation process remain the same as in Example 1.

[0040] Comparative Example 3 Compared with Example 1, this comparative example differs in that S2 only adds graphene oxide, and the specific implementation steps are as follows: S1. Dissolve 1.5 g of manganese chloride tetrahydrate (MnCl2·4H2O) and 0.5 g of ferric chloride hexahydrate (FeCl3·6H2O) in 60 mL of ethoxyacetic acid to prepare a first solution; add 1 g of 1,3,5-benzenetricarboxylic acid to 20 mL of anhydrous ethanol to prepare a second solution; slowly add the first solution to the second solution under magnetic stirring, then add 0.2 g of NaOH, transfer to a stainless steel autoclave, and place in an oven at 180°C for 48 hours. After the sample is naturally cooled in the autoclave, it is collected by centrifugation, washed alternately with deionized water and ethanol three times, and dried in an oven at 80°C for 24 hours to obtain a manganese iron precursor; S2. After fully mixing 35 parts by weight of manganese iron precursor, 3 parts by weight of graphene oxide, 4 parts by weight of lithium carbonate, and 12 parts by weight of ammonium dihydrogen phosphate, the mixture was heated to 750°C at a rate of 5°C / min under high-purity nitrogen and calcined for 8 hours. After cooling, a manganese iron lithium phosphate composite material was obtained.

[0041] The remaining raw materials and preparation process remain the same as in Example 1.

[0042] Comparative Example 4 Compared with Example 1, this comparative example differs in that no magnesium thermal reaction is performed. The specific implementation steps are as follows: S1. Dissolve 1.5 g of manganese chloride tetrahydrate (MnCl2·4H2O) and 0.5 g of ferric chloride hexahydrate (FeCl3·6H2O) in 60 mL of ethoxyacetic acid to prepare a first solution; add 1 g of 1,3,5-benzenetricarboxylic acid to 20 mL of anhydrous ethanol to prepare a second solution; slowly add the first solution to the second solution under magnetic stirring, then add 0.2 g of NaOH, transfer to a stainless steel autoclave, and place in an oven at 180°C for 48 hours. After the sample is naturally cooled in the autoclave, it is collected by centrifugation, washed alternately with deionized water and ethanol three times, and dried in an oven at 80°C for 24 hours to obtain a manganese iron precursor; S2. Dissolve 40 mg of graphene oxide in 20 mL of deionized water and obtain a third solution by stirring and ultrasonic dispersion. Add 500 mg of SeO2 to 10 mL of 50% ethanol solution and obtain a fourth solution by stirring and ultrasonic dispersion. Mix the third and fourth solutions, stir, and ultrasonically disperse them. Then, add 200 μL of EDA solution dropwise, stir for 10 min, and place in a reactor. Place the reactor in an oven and hydrothermally react at 180°C for 12 h. After the reaction, wash the mixture alternately with deionized water and ethanol three times and freeze-dry for 24 h to obtain SeO2@GO-EDA. S3. After thoroughly mixing 35 parts by weight of manganese iron precursor, 8 parts by weight of SeO2@GO-EDA, 4 parts by weight of lithium carbonate, and 12 parts by weight of ammonium dihydrogen phosphate, the mixture was heated to 750°C at a rate of 5°C / min under high-purity nitrogen and calcined for 8 hours. After cooling, a manganese iron lithium phosphate composite material was obtained.

[0043] The remaining raw materials and preparation process remain the same as in Example 1.

[0044] Comparative Example 5 Compared with Example 1, this comparative example differs in that no manganese / iron-1,3,5-benzenetricarboxylic acid organic skeleton is formed. The specific implementation steps are as follows: S1. Dissolve 1.5 g of manganese chloride tetrahydrate (MnCl2·4H2O) and 0.5 g of ferric chloride hexahydrate (FeCl3·6H2O) in 60 mL of ethoxyacetic acid to prepare a first solution; add 0.2 g of NaOH, transfer to a stainless steel autoclave, and place in an oven at 180°C for 48 hours. After the sample is naturally cooled in the autoclave, it is collected by centrifugation, washed alternately with deionized water and ethanol three times, and dried in an oven at 80°C for 24 hours to obtain a manganese iron precursor; S2. Dissolve 40 mg of graphene oxide in 20 mL of deionized water and obtain a third solution by stirring and ultrasonic dispersion. Add 500 mg of SeO2 to 10 mL of 50% ethanol solution and obtain a fourth solution by stirring and ultrasonic dispersion. Mix the third and fourth solutions, stir, and ultrasonically disperse them. Then, add 200 μL of EDA solution dropwise, stir for 10 min, and place in a reactor. Place the reactor in an oven and hydrothermally react at 180°C for 12 h. After the reaction, wash the mixture alternately with deionized water and ethanol three times and freeze-dry for 24 h to obtain SeO2@GO-EDA. S3. Grind and mix 300 mg of SeO2@GO-EDA and 300 mg of magnesium, place them in a graphite crucible, and heat them in a tube furnace under an argon atmosphere. The temperature is gradually increased at a rate of 5°C / min until it reaches 690°C and kept warm for 3 hours. The reaction product is then cooled to room temperature and immersed in a 1 mol / L hydrochloric acid solution for 24 hours. The product is separated by vacuum filtration technology, washed with deionized water several times until the pH is 7, and finally freeze-dried for 12 hours to obtain SeO2@GO-EDA. x @rGO-EDA (0<X≤2); S4, 35 parts by weight of ferromanganese precursor, 8 parts by weight of SeO x After @rGO-EDA, 4 parts by weight of lithium carbonate, and 12 parts by weight of ammonium dihydrogen phosphate were fully mixed, the temperature was raised to 740°C at a rate of 5°C / min under high-purity nitrogen and calcined for 8 hours. After cooling, a lithium manganese iron phosphate composite material was obtained.

[0045] The remaining raw materials and preparation process remain the same as in Example 1.

[0046] Performance Testing Lithium iron phosphate, acetylene black, and polytetrafluoroethylene were added to N-methylpyrrolidone in a mass ratio of 80:12:8, ground and mixed thoroughly, and the resulting slurry was coated on an aluminum foil surface and dried at 100°C for 24 hours to obtain a positive electrode sheet. A metallic lithium sheet was used as the negative electrode, a polypropylene microporous membrane was used as the separator, and an organic solution containing 1 mol / L LiPF6 was used as the electrolyte. The solvent used for the organic solution was a mixed solution of dimethyl carbonate and ethylene carbonate. The battery was assembled in a glove box. The prepared battery was subjected to capacity testing and cycle testing according to SJ / T 11797-2022; the test results are shown in Table 1: Table 1

[0047] As can be seen from Table 1, the only difference between Examples 2-7 and Example 1 is the adjustment of the component ratio within a reasonable range. From the results, the prepared lithium manganese iron phosphate composite materials can improve the specific capacity and cycle performance of the battery; compared with Example 1, Example 2 increases the amount of manganese iron precursor, which means that the iron content is increased, which destroys the balance of iron, lithium and phosphorus atoms, resulting in a downward trend in battery performance compared with Example 1; compared with Example 1, Example 5 increases the amount of EDA, which will hinder the transmission path of lithium ions and reduce the specific capacity of the battery; compared with Example 1, Example 7 increases the molar ratio of iron and manganese in the manganese iron precursor, resulting in an unstable structure of the manganese iron precursor, which will cause a decline in subsequent performance test results.

[0048] Comparative Examples 1-3 are compared with Example 1 in that the SeO xAdjustment of raw material components during the preparation of @rGO-EDA improves stability because EDA can form a dense and stable SEI film with graphene oxide; SeO x It not only improves electronic conductivity, but also reduces manganese ion dissolution and improves battery cycle stability; thus, the battery performance of Comparative Examples 1-3 decreases compared to Example 1. Compared with Example 1, in Comparative Example 4, after the magnesium thermal reaction is not performed, GO cannot form rGO with stronger conductivity, and SeO2 cannot further improve the conductivity, resulting in a decrease in the specific capacity of the battery. Compared with Example 1, Comparative Example 5 adopts a solution that does not form a manganese / iron-1,3,5-benzenetricarboxylic acid organic skeleton. At this time, the stability of the lithium iron phosphate material decreases, and the manganese and iron ions will accelerate the dissolution, and the battery performance is further reduced.

[0049] The above disclosures are only a few specific embodiments of the present invention. However, the embodiments of the present invention are not limited thereto. Any changes that can be conceived by those skilled in the art should fall within the scope of protection of the present invention.

Claims

1. A lithium manganese iron phosphate composite material, characterized in that: Including lithium manganese iron phosphate and lithium manganese iron phosphate surface modified SeO x @rGO-EDA; The lithium manganese iron phosphate comprises a manganese / iron-1,3,5-benzenetricarboxylic acid organic framework, a phosphorus source and a lithium source; The rGO is coated with SeO generated in situ via a magnesium thermal reduction reaction. x The surface forms a core-shell structure, and the adjacent rGO sheets are covalently cross-linked by EDA to form SeO x @rGO-EDA, 0<X≤2.

2. A method for preparing a lithium manganese iron phosphate composite material, characterized in that: The method for preparing the lithium manganese iron phosphate composite material according to claim 1 comprises the following steps: S1. Dissolving MnCl2·4H2O and FeCl3·6H2O in ethoxyacetic acid to prepare a first solution; adding 1,3,5-benzenetricarboxylic acid to anhydrous ethanol to prepare a second solution; slowly adding the first solution to the second solution, then adding NaOH, performing a hydrothermal reaction, cooling, centrifuging, washing, and drying to obtain a manganese iron precursor; S2. Graphene oxide is dissolved in deionized water and dispersed evenly to obtain a third solution; SeO2 is added to a 50% ethanol solution and dispersed evenly to obtain a fourth solution; the third solution and the fourth solution are mixed evenly, EDA solution is added dropwise, hydrothermally reacted, washed and freeze-dried to obtain SeO2@GO-EDA; S3, grind and mix SeO2@GO-EDA and magnesium, heat and keep warm under argon environment, cool and immerse in hydrochloric acid solution; filter, wash, freeze-dry to obtain SeO x @rGO-EDA, 0<X≤2; S4, the manganese iron precursor, SeO x After @rGO-EDA, lithium carbonate and ammonium dihydrogen phosphate are evenly mixed, the temperature is increased and calcined under nitrogen, and then cooled to obtain a lithium manganese iron phosphate composite material.

3. The method for preparing the lithium iron manganese phosphate composite material according to claim 2, wherein: The usage ratio of the MnCl2·4H2O, FeCl3·6H2O and ethoxyacetic acid is (1-2.5) g: (0.25-1.25) g: (40-100) mL.

4. The method for preparing the lithium iron manganese phosphate composite material according to claim 2, wherein: The usage ratio of the 1,3,5-benzenetricarboxylic acid, anhydrous ethanol and NaOH is (0.5-2) g: (10-40) mL: (0.1-0.3) g.

5. The method for preparing the lithium iron manganese phosphate composite material according to claim 2, wherein: The hydrothermal reaction is carried out at 160-180° C. for 16-48 hours; the washing is carried out by alternating washing with deionized water and ethanol for 3 times; and the drying is carried out at 60-80° C. for 12-24 hours.

6. The method for preparing the lithium iron manganese phosphate composite material according to claim 2, characterized in that: The usage ratio of the graphene oxide, deionized water, SeO2, 50% ethanol solution and EDA is (30-50) mg: (20-40) mL: (400-600) mg: (10-30) mL: (100-400) μL.

7. The method for preparing the lithium iron manganese phosphate composite material according to claim 2, characterized in that: The mass ratio of SeO2@GO-EDA and magnesium is (0.8-1): (0.8-1).

8. The method for preparing the lithium iron manganese phosphate composite material according to claim 2, wherein: The heating and heat preservation process is to increase the temperature to 650-700° C. at a rate of 1-5° C. / min and keep the temperature for 3-4 hours; the temperature rising and calcining process is to increase the temperature to 700-750° C. at a rate of 1-5° C. / min and calcine for 8-10 hours.

9. The method for preparing the lithium iron manganese phosphate composite material according to claim 2, characterized in that: The concentration of the hydrochloric acid solution is 1 mol / L, and the immersion time is 18-24 hours.

10. The method for preparing the lithium iron manganese phosphate composite material according to claim 2, characterized in that: The ferromanganese precursor, SeO x The mass ratio of @rGO-EDA, lithium carbonate and ammonium dihydrogen phosphate is (30-45):(6-9):(3.5-5):(10-15).

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