A lithium iron manganese phosphate composite cathode material and a preparation method thereof

Abstract

The application discloses a lithium manganese iron phosphate composite positive electrode material and a preparation method thereof. The lithium manganese iron phosphate composite positive electrode material comprises carbon particles containing lithium manganese iron phosphate, and the surface of the lithium manganese iron phosphate particles is coated with polymer A and carbonized surfactant B; wherein the chemical formula of the lithium manganese iron phosphate particles is LiFe x Mn 1‑x PO4, and the value of x is 0.2-0.5; the polymer A is a polysiloxane compound; and the surfactant B is at least one selected from sodium dodecyl sulfate, sorbitan oleate polyoxyethylene ether, methyl stearate polyoxyethylene ether and alkylphenol polyoxyethylene ether. The application can increase the conductivity by mixing the reaction system with micro-nano bubble water to hydrothermally synthesize small-particle-diameter products and adding the surfactant B, and can inhibit the dissolution of manganese ions by adding the polymer A; the positive electrode material prepared by the method has excellent conductivity and good cycle performance.

Description

Technical Field

[0001] This invention belongs to the field of battery technology and relates to a lithium manganese iron phosphate composite cathode material and its preparation method. Background Technology

[0002] In recent decades, high-energy-density batteries for electric vehicles have developed rapidly, and several high-reversible-capacity cathodes, such as NCM-811 and NCA, have been adopted in commercial equipment. However, almost all well-known electric vehicles have experienced safety incidents, some even escalating into fires, partly attributed to the poor stability of the cathode. Therefore, attention has gradually shifted to the safer olivine cathode.

[0003] Besides the well-known lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4) is another important material in the olive phosphate family. (The last sentence appears to be incomplete and possibly contains errors.) + Compared to Li, it also has a redox plateau at 4.1V, which is about 700mV higher than LiFePO4. Furthermore, due to its stable PO structure, it exhibits similar thermal stability, making it one of the most promising cathode materials, boasting advantages such as high energy density, good safety, and low raw material cost. However, LiMnPO4 has extremely poor electronic conductivity. The band gap for electron transitions in LiFePO4 is 0.3eV, exhibiting semiconductor characteristics, while the band gap for transitions in LiMnPO4 is 2eV, classifying it as an insulator.

[0004] Because LiMnPO4 and LiFePO4 can together form a solid solution of lithium manganese iron phosphate (LiFe... x Mn 1-x Mn-doped LiFePO4 exhibits a much higher operating voltage than LiFePO4 and a much higher conductivity than LiMnPO4. Therefore, doping LiFePO4 with Mn and LiMnPO4 with Fe are effective strategies for improving their electrochemical performance. However, LiFe... x Mn 1-x While the conductivity of PO4 is improved compared to LiMnPO4, the improvement is limited, making it difficult to fully realize the electrochemical performance of the material. Furthermore, researchers primarily utilize solid-state reactions and sol-gel methods to synthesize nanomaterials, which, due to the difficulty in obtaining small particles, typically result in low reversible capacity.

[0005] Another drawback restricting the development of lithium manganese iron phosphate is the nucleophilic fluoride F in the electrolyte. -Attacking electrophilic manganese atoms causes manganese to dissolve into the electrolyte. The dissolved manganese ions migrate towards the anode and are likely to deposit on its surface. This severely interferes with the formation of the solid electrolyte interphase (SEI) layer, leading to continuous decomposition and regeneration of the SEI film on the graphite surface, and sustained consumption of active lithium. Therefore, to maintain the system's performance, it is necessary to minimize manganese dissolution. Summary of the Invention

[0006] The purpose of this invention is to provide a lithium manganese iron phosphate composite cathode material and its preparation method. This invention involves hydrothermal synthesis of small-particle products by mixing the reaction system with micro / nano bubble water, adding surfactant B to increase conductivity, and adding polymer A to inhibit the dissolution of manganese ions. The cathode material prepared by this method exhibits excellent conductivity and good cycle performance.

[0007] The cathode material prepared by the method of this invention has excellent conductivity and good cycle performance, and is easy to operate.

[0008] The present invention provides a lithium manganese iron phosphate composite cathode material, comprising carbon particles containing lithium manganese iron phosphate, wherein the surface of the lithium manganese iron phosphate particles is coated with polymer A and carbonized surfactant B;

[0009] The chemical formula of the lithium manganese iron phosphate particles is LiFe. x Mn 1-x PO4, the value of x is 0.2 to 0.5;

[0010] The polymer A is a polysiloxane compound;

[0011] The surfactant B is selected from at least one of sodium dodecyl sulfate, sorbitan oleate polyoxyethylene ether (T-80), methyl stearate polyoxyethylene ether, and alkylphenol polyoxyethylene ether.

[0012] In the aforementioned lithium manganese iron phosphate composite cathode material, the polysiloxane compound is selected from at least one of polydimethylsiloxane, polydiethylsiloxane, and polymethylvinylsiloxane.

[0013] This invention also provides a method for preparing the above-mentioned lithium manganese iron phosphate composite cathode material, comprising the following steps:

[0014] 1) Mix lithium source, phosphorus source, iron source, manganese source and deionized water according to the molar ratio of lithium, phosphorus, iron and manganese elements 3:1:x:1-x, where the value of x is 0.2 to 0.5, to obtain a mixture;

[0015] 2) A lithium source is slowly added dropwise to the mixture to obtain a gray suspension;

[0016] 3) Adjust the pH of the suspension by adding an acidic solution and stir to obtain the reactants;

[0017] 4) After mixing and stirring the reactants with micro-nano bubble water containing the surfactant B, the mixture is transferred to a reaction vessel with a polytetrafluoroethylene liner, so that the surfactant B in the micro-nano bubble water comes into full contact with the reactants and adheres to the surface of the reactant particles.

[0018] 5) The system in step 4) is subjected to a carbonization reaction, and the gray precipitate is separated to obtain the hydrothermal product;

[0019] 6) The hydrothermal products are sintered to obtain the composite material LiFe. x Mn 1-x PO4 / C;

[0020] 7) The composite material LiFe x Mn 1-x PO4 / C is wet ball-milled with polymer A to obtain a polymer-coated composite material;

[0021] 8) Dry the polymer-coated composite material to obtain the lithium manganese iron phosphate composite cathode material.

[0022] In the above preparation method, the lithium source is selected from at least one of LiOH·H2O, Li2CO3 and CH3COOLi;

[0023] The phosphorus source is selected from at least one of phytic acid, NH4H2PO4 and phosphoric acid;

[0024] The iron source is selected from at least one of FeSO4·7H2O, Fe3(PO4)2 and Fe2O3;

[0025] The manganese source is at least one of MnSO4, Mn2O3 and manganese acetate.

[0026] In the above preparation method, the acidic solution is selected from at least one of hydrochloric acid, sulfuric acid, and phytic acid;

[0027] The pH value of the suspension is adjusted to 6.5-7.5, specifically 6.5, 7.0, or 7.5, and the stirring time is 30-60 minutes, specifically 30 minutes, 40 minutes, or 50 minutes.

[0028] In the above preparation method, the micro-nano bubble water is prepared by a micro-nano bubble generating device.

[0029] In the above preparation method, the average particle size of the bubbles in the micro-nano bubble water is 200 nm to 4 μm;

[0030] The micro-nano bubble water has a bubble content of 80% to 90%, uses H2 and Ar as gas sources, and has a hydrogen volume percentage of 2% to 4% (specifically 2%, 2.5%, 3%, 3.5% or 2% to 3.5%). The solvent is a mixed solution of deionized water, ethylene glycol and surfactant B with a mass ratio of 100:15 to 30:5 to 15.

[0031] In the above preparation method, the volume ratio of the reactant to the micro / nano bubble water containing the surfactant B can be 1:1 to 1.5, specifically 1:1, 1:1.25, 1:1 to 1.25 or 1:1.5.

[0032] In the above preparation method, in step 4), the stirring time can be 1 to 2 hours, specifically 1 hour, 1.5 hours or 2 hours.

[0033] In the above preparation method, in step 5), the carbonization reaction temperature is 150-200℃ and the time is 2-6h; specifically, the hydrothermal reaction temperature can be 150℃, 160℃, 170℃, 180℃, 185℃, 195℃ or 200℃, and the reaction time can be 2h, 2.5h, 3h, 4h or 2-4h; the carbonization reaction is to carbonize the surfactant B at high temperature, so that the carbonized surfactant B is coated on the surface of the lithium manganese iron phosphate particles, which can form a conductive carbon network on the particle surface.

[0034] In the above preparation method, step 5) further includes the steps of naturally cooling the system after the carbonization reaction to room temperature, filtering and washing to obtain the gray precipitate, and drying it;

[0035] The drying conditions are as follows: vacuum drying at 60-100℃ for 10-20 hours; specifically, vacuum drying at 60℃ for 10 hours, vacuum drying at 70℃ for 12 hours, vacuum drying at 80℃ for 14 hours, vacuum drying at 85℃ for 16 hours, vacuum drying at 90℃ for 17 hours, vacuum drying at 95℃ for 18 hours, vacuum drying at 90℃ for 19 hours, and vacuum drying at 100℃ for 20 hours.

[0036] In the above preparation method, step 6) involves the following sintering process: In a tube furnace, N2 is used as the protective atmosphere, and the temperature is increased to 700–770°C at a rate of 3–5°C / min, then sintered at 700–770°C for 6–12 hours; specifically, the temperature can be increased to 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 760°C, or 770°C at rates of 3°C / min, 4°C / min, 3–4°C / min, or 5°C / min, and then sintered at 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 760°C, or 770°C for 6 hours, 8 hours, 10 hours, or 12 hours.

[0037] In step 7), the composite material LiFe x Mn 1-x The mass percentage of polymer A added in PO4 / C is 0.1% to 2.5%, specifically 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 1.0%, 1.5%, 2.0%, or 0.4% to 2.0%.

[0038] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0039] 1. This invention utilizes micro / nano bubble water: nano to micrometer in size (200nm to 4μm), with a stable bubble fluid sourced from H2 and Ar. This results in a smaller particle size in the final product, with an average particle size controlled within the range of 80nm to 200nm. The nanobubbles, possessing sufficient surface energy, effectively prevent the aggregation of nanoparticles and homogenize the size of the microparticles, which is beneficial for their electrochemical properties and improves conductivity. The stable bubble fluid sourced from H2 and Ar also helps prevent the oxidation of divalent ions during the reaction process.

[0040] 2. Surfactant B has a significant impact on the evolution of bubble surface properties and size. Surfactant B promotes the stable existence of bubbles and allows for sufficient contact with reactants when attached to the bubble surface. After high-temperature carbonization, the surfactant coated on the particle surface can form a conductive carbon network on the particle surface, improving conductivity, and can also prevent oxidation during sintering.

[0041] 3. Polymer A coats the material surface, preventing manganese leaching; siloxane polymers can bind to fluoride ions, preventing them from reacting with manganese ions; nucleophilic fluoride F in the electrolyte. - Attacking electrophilic manganese atoms causes manganese to dissolve into the electrolyte. The dissolved manganese ions migrate towards the anode and are likely to deposit on its surface. This severely interferes with the formation of the solid electrolyte interphase (SEI) layer, which is crucial for the stable cycling of the anode material. Therefore, to maintain the system's performance, it is necessary to minimize manganese dissolution. Silicon (in siloxane groups) is effective against F... - It exhibits high binding affinity, preferentially binding to fluoride ions and preventing their reaction with manganese ions. Furthermore, due to the abundance of siloxane functional groups on the polymer backbone, only a small amount of polymer A needs to be embedded to achieve this effect, thus having minimal impact on ion kinetics diffusion.

[0042] One method to reduce manganese dissolution is to add additives to the electrolyte, but this reduces the electrolyte's conductivity. These additives are often highly electrophilic and easily decompose. This invention achieves a more desirable effect by coating with a very small amount of polymer A. Attached Figure Description

[0043] Figure 1 This is a flowchart illustrating the preparation process of the present invention. Detailed Implementation

[0044] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0045] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0046] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.

[0047] This invention provides a lithium iron phosphate composite cathode material;

[0048] This includes the hydrothermal synthesis of lithium manganese iron phosphate (LiFe) by mixing lithium, phosphorus, iron, and manganese sources in a molar ratio of lithium, phosphorus, iron, and manganese of 3:1:x:1-x, where x is 0.2–0.5, and by mixing polymer A and surfactant B with micro / nano bubble water (NBW). x Mn 1-x PO4).

[0049] NBW is prepared by a micro-nano bubble generator. The average particle size of the generated bubbles is between 200 nm and 4 μm, and the bubble content is 80% to 90%. H2 (V(H2)% = 2% to 4%) and Ar are used as gas sources, and a mixed solution of deionized water, ethylene glycol and surfactant B with a mass ratio of 100:15 to 30:5 to 15 is used as solvent.

[0050] The present invention provides a method for preparing a lithium manganese iron phosphate composite cathode material, comprising the following steps:

[0051] 1) The molar ratio of lithium, phosphorus, iron and manganese is 3:1:x:1-x, where the value of x is 0.2 to 0.5. The lithium source, phosphorus source, iron source, manganese source and a certain amount of deionized water are mixed.

[0052] 2) Then, lithium source is slowly added dropwise to the mixture to obtain a gray suspension;

[0053] 3) Add an acidic solution to adjust the pH of the slurry to 6.5–7.5, and stir for 30–60 minutes;

[0054] 4) Mix with micro-nano bubble water at a volume ratio of 1:1 to 1.5 and continue stirring for 1 to 2 hours. Then transfer to the PTFE reactor liner. The surfactant B in the micro-nano bubble water will come into full contact with the reactants and adhere to the particle surface.

[0055] 5) After reacting at 150-200℃ for 2-6 hours, the mixture is naturally cooled to room temperature. After filtration and washing, a gray precipitate is obtained and then vacuum dried at 60-100℃ for 10-20 hours.

[0056] 6) The dried hydrothermal product was sintered in a tube furnace under a protective atmosphere of N2 at a heating rate of 3–5 °C / min at 700–770 °C for 6–12 h to obtain the LiFe composite material. x Mn 1-x PO4 / C;

[0057] 7) The sintered LiFe composite material x Mn 1-x PO4 / C was wet-ball-milled with a trace amount of polymer A to ensure that the polymer fully coats the LiFe. x Mn 1-x A protective layer is formed on the PO4 / C surface;

[0058] 8) Finally, the finished product is dried to obtain a polymer-coated composite material;

[0059] Furthermore, the lithium source is at least one of LiOH·H2O, Li2CO3, and CH3COOLi; the phosphorus source is at least one of phytic acid, NH4H2PO4, and phosphoric acid; the iron source is at least one of FeSO4·7H2O, Fe3(PO4)2, and Fe2O3; and the manganese source is at least one of MnSO4, Mn2O3, and manganese acetate.

[0060] Furthermore, the acidic solution is at least one of hydrochloric acid, sulfuric acid, and phytic acid.

[0061] Further, polymer A is a polysiloxane, such as at least one of polydimethylsiloxane, polydiethylsiloxane, and polymethylvinylsiloxane, and the amount of polymer A added is the LiFe composite material after sintering. x Mn 1- x The PO4 / C content is 0.1% to 2.5% by mass.

[0062] Further, the surfactant B is at least one of sodium dodecyl sulfate, sorbitan oleate polyoxyethylene ether (T-80), methyl stearate polyoxyethylene ether, and alkylphenol polyoxyethylene ether.

[0063] Furthermore, the average particle size of the bubbles in the micro-nano bubble water is 200 nm to 4 μm;

[0064] The micro-nano bubble water has a bubble content of 80% to 90%, uses H2 and Ar as gas sources, has a hydrogen volume percentage of 2% to 4%, and uses a mixed solution of deionized water, ethylene glycol and surfactant B with a mass ratio of 100:15 to 30:5 to 15 as solvent.

[0065] Furthermore, the volume ratio of the reactant to the micro / nano bubble water containing the surfactant B is 1:1 to 1.5.

[0066] The following examples illustrate the above-mentioned lithium manganese iron phosphate composite cathode material for lithium batteries and its preparation method.

[0067] Example 1

[0068] LiOH·H2O, phytic acid, FeSO4·7H2O, and MnSO4 were weighed according to an elemental molar ratio of Li:P:Fe:Mn of 3:1:0.2:0.8. First, FeSO4·7H2O, MnSO4, phytic acid solution, and a certain amount of deionized water were mixed. After the solids dissolved and the mixture was homogeneous, LiOH·H2O was slowly added dropwise to the above mixed solution. After a few minutes, a gray suspension was obtained. After the LiOH·H2O reaction was complete, the pH was adjusted to 6.5 with an acidic solution and stirred for 30 minutes. After stirring, it was mixed with micro / nano bubble water at a volume ratio of 1:1 and stirred for another 1 hour. The micro / nano bubble water was prepared using a micro / nano bubble generator, using H2 (V(H2)% = 2%) and Ar as gas sources, and a solution of deionized water, ethylene glycol, and sodium dodecyl sulfate in a mass ratio of 100:15:5 as the solvent. The average particle size of the generated bubbles was between 200 nm and 4 μm, and the bubble content was 80%–90%. The above reaction system was then transferred to a reactor for hydrothermal reaction at 150°C for 2 hours. After the reaction, it was allowed to cool naturally to room temperature. After filtration and washing, a gray precipitate was obtained and dried under vacuum at 60°C for 10 hours. The dried hydrothermal product was then sintered in a tube furnace at 700°C for 6 hours under a protective atmosphere of N2 and a heating rate of 3°C / min to obtain the composite material LiFe. x Mn 1-x PO4 / C. The sintered composite material was wet-milled with polydimethylsiloxane (0.4% by mass of the sintered composite material) to ensure good coating of the polymer on LiFe. x Mn 1-x The PO4 / C surface is dried to obtain a polymer-coated composite material.

[0069] Example 2

[0070] Li₂CO₃, phytic acid, FeSO₄·7H₂O, and MnSO₄ were weighed according to an elemental molar ratio of Li:P:Fe:Mn of 3:1:0.2:0.8. First, FeSO₄·7H₂O, MnSO₄, phytic acid solution, and a certain amount of deionized water were mixed. After the solids dissolved and the mixture was homogeneous, Li₂CO₃ was slowly added dropwise to the mixture. After a few minutes, a gray suspension was obtained. After the Li₂CO₃ reacted completely, the pH was adjusted to 6.5 with an acidic solution and stirred for 30 minutes. After stirring, it was mixed with micro / nano bubble water at a volume ratio of 1:1 and stirred for another 1 hour. The micro / nano bubble water was prepared using a micro / nano bubble generator, using H₂ (V(H₂)% = 2.5%) and Ar as gas sources, and a mixed solution of water:ethylene glycol:T80 at a mass ratio of 100:18:7 as the solvent. The generated bubbles had an average particle size between 200 nm and 4 μm, and a bubble content of 80%-90%. The above reaction system was then transferred to a reactor for hydrothermal reaction at 160℃ for 2 hours. After the reaction, it was allowed to cool naturally to room temperature. After filtration and washing, a gray precipitate was obtained and dried under vacuum at 70℃ for 12 hours. The dried hydrothermal product was then sintered in a tube furnace at 710℃ for 6 hours under a protective atmosphere of N2 and a heating rate of 3℃ / min to obtain the composite material LiFe. x Mn 1-x PO4 / C. The sintered composite material was wet-ball-milled with polydiethylsiloxane (0.5% by mass of the sintered composite material) to ensure good coating of the polymer on LiFe. x Mn 1-x The PO4 / C surface is dried to obtain a polymer-coated composite material.

[0071] Example 3

[0072] LiOH·H2O, NH4H2PO4, Fe3(PO4)2, and MnSO4 were weighed according to the elemental molar ratio of Li:P:Fe:Mn 3:1:0.25:0.75. First, Fe3(PO4)2, MnSO4, NH4H2PO4 and a certain amount of deionized water were mixed. After the solids were dissolved and mixed evenly, LiOH·H2O was slowly added dropwise to the above mixed solution. After a few minutes, a gray suspension was obtained. After the LiOH·H2O reaction was complete, the pH was adjusted to 6.5 with an acidic solution and stirred for 30 minutes. After stirring, it was mixed with micro-nano bubble water at a volume ratio of 1:1 and stirred for another 1 hour. The micro-nano bubble water was prepared using a micro-nano bubble generator with H2 (V(H2)% = 2.5%) and Ar as gas sources, and a mixed solution of water, ethylene glycol, and methyl stearate polyoxyethylene ether at a mass ratio of 100:20:10 as the solvent. The average particle size of the generated bubbles was between 200 nm and 4 μm, and the bubble content was 80%-90%. The reaction system was then transferred to a reactor for hydrothermal reaction at 170 °C for 2.5 h. After the reaction, the mixture was allowed to cool naturally to room temperature. After filtration and washing, a gray precipitate was obtained and vacuum-dried at 80 °C for 14 h. The dried hydrothermal product was then sintered in a tube furnace at 720 °C for 8 h under a protective atmosphere of N2, with a heating rate of 3 °C / min, to obtain the LiFe composite material. x Mn 1-x PO4 / C. The sintered composite material was wet-ball-milled with polymethylvinylsiloxane (0.6% by mass of the sintered composite material) to better coat the LiFe. x Mn 1-x The PO4 / C surface is dried to obtain a polymer-coated composite material.

[0073] Example 4

[0074] LiOH·H2O, NH4H2PO4, Fe2O3, and MnSO4 were weighed according to the molar ratio of Li:P:Fe:Mn of 3:1:0.3:0.7. Fe2O3, MnSO4, NH4H2PO4, and a certain amount of deionized water were mixed first. After the solids were dissolved and mixed evenly, LiOH·H2O was slowly added dropwise to the above mixed solution. After a few minutes, a gray suspension was obtained. After the LiOH·H2O reaction was complete, the pH was adjusted to 7 with an acidic solution and stirred for 40 minutes. After stirring, it was mixed with micro-nano bubble water at a volume ratio of 1:1.25 and stirred for another 1.5 hours. The micro-nano bubble water was prepared using a micro-nano bubble generator, with H2 (V(H2)% = 3%) and Ar as gas sources, and a mixed solution of water:ethylene glycol:alkylphenol polyoxyethylene ether at a mass ratio of 100:25:10 as the solvent. The average particle size of the generated bubbles was between 200 nm and 4 μm, and the bubble content was 80%-90%. The above reaction system was then transferred to a reactor for hydrothermal reaction at 180℃ for 2 hours. After the reaction, it was naturally cooled to room temperature, filtered, and washed to obtain a gray precipitate, which was then vacuum dried at 85℃ for 16 hours. The dried hydrothermal product was then sintered in a tube furnace at 730℃ for 8 hours under a protective atmosphere of N2, with a heating rate of 4℃ / min, to obtain the LiFe composite material. x Mn 1- x PO4 / C. The sintered composite material was wet-ball-milled with polydimethylsiloxane (0.7% by mass of the sintered composite material) to better coat the LiFe. x Mn 1-x PO4 / C surface. Finally, the finished product is dried to obtain a polymer-coated composite material.

[0075] Example 5

[0076] Li₂CO₃, phytic acid, Fe₂O₃, and Mn₂O₃ were weighed according to an elemental molar ratio of Li:P:Fe:Mn of 3:1:0.35:0.65. First, Fe₂O₃, Mn₂O₃, phytic acid solution, and a certain amount of deionized water were mixed. After the solids dissolved and the mixture was homogeneous, Li₂CO₃ was slowly added dropwise to the mixture. After a few minutes, a gray suspension was obtained. After the Li₂CO₃ had completely reacted, the pH was adjusted to 7 with an acidic solution and stirred for 50 minutes. After stirring, it was mixed with micro / nano bubble water at a volume ratio of 1:1.25 and stirred for another 1.5 hours. The micro / nano bubble water was prepared using a micro / nano bubble generator, with H₂ (V(H₂)% = 3%) and Ar as gas sources, and a mixed solution of water:ethylene glycol:alkylphenol polyoxyethylene ether at a mass ratio of 100:20:12 as the solvent. The average particle size of the generated bubbles was between 200 nm and 4 μm, and the bubble content was 80%-90%. The above reaction system was then transferred to a reactor for hydrothermal reaction at 185℃ for 3 hours. After the reaction, the mixture was allowed to cool naturally to room temperature. After filtration and washing, a gray precipitate was obtained and vacuum-dried at 90℃ for 17 hours. The dried hydrothermal product was then sintered in a tube furnace at 740℃ for 10 hours under a protective atmosphere of N2 and a heating rate of 4℃ / min to obtain the composite material LiFe. x Mn 1-x PO4 / C. The sintered composite material was wet-milled with polydiethylsiloxane (0.8% by mass of the sintered composite material) to ensure good coating of the polymer on LiFe. x Mn 1-x The PO4 / C surface is dried to obtain a polymer-coated composite material.

[0077] Example 6

[0078] CH3COOLi, NH4H2PO4, FeSO4·7H2O, and (CH3COO)2Mn were weighed according to the elemental molar ratio of Li:P:Fe:Mn 3:1:0.4:0.6. First, FeSO4·7H2O, (CH3COO)2Mn, NH4H2PO4, and a certain amount of deionized water were mixed. After the solids were dissolved and mixed evenly, CH3COOLi was slowly added dropwise to the above mixed solution. After a few minutes, a gray suspension was obtained. After CH3COOLi reacted completely, the pH was adjusted to about 7.5 with an acidic solution and stirred for 50 minutes. After stirring, it was mixed with micro-nano bubble water at a volume ratio of 1:1.25 and stirred for another 1.5 hours. The micro-nano bubble water was prepared using a micro-nano bubble generator, with H2 (V(H2)% = 3.5%) and Ar as gas sources, and a mixed solution of water:ethylene glycol:sodium dodecyl sulfate in a mass ratio of 100:25:12 as the solvent. The average particle size of the generated bubbles was between 200 nm and 4 μm, and the bubble content was 80%-90%. The reaction system was then transferred to a reactor for hydrothermal reaction at 190 °C for 3.5 h. After the reaction, the mixture was allowed to cool naturally to room temperature. After filtration and washing, a gray precipitate was obtained and vacuum-dried at 95 °C for 18 h. The dried hydrothermal product was then sintered in a tube furnace at 750 °C for 10 h under a protective atmosphere of N2, with a heating rate of 4 °C / min, to obtain the LiFe composite material. x Mn 1-x PO4 / C. The sintered composite material was wet-milled with polymethylvinylsiloxane (1.0% by mass of the sintered composite material) to better coat the LiFe. x Mn 1-x The PO4 / C surface is dried to obtain a polymer-coated composite material.

[0079] Example 7

[0080] CH3COOLi, phosphoric acid, Fe2O3, and (CH3COO)2Mn were weighed according to an elemental molar ratio of Li:P:Fe:Mn of 3:1:0.45:0.55. First, Fe2O3, (CH3COO)2Mn, phosphoric acid solution, and a certain amount of deionized water were mixed. After the solids dissolved and mixed evenly, CH3COOLi was slowly added dropwise to the above mixed solution. After a few minutes, a gray suspension was obtained. After the CH3COOLi reaction was complete, the pH was adjusted to 7.5 with an acidic solution and stirred for 60 minutes. After stirring, it was mixed with micro / nano bubble water at a volume ratio of 1:1.5 and stirred for another 2 hours. The micro / nano bubble water was prepared using a micro / nano bubble generator, using H2 (V(H2)% = 3.5%) and Ar as gas sources, and a mixed solution of water:ethylene glycol:T80 at a mass ratio of 100:25:15 as the solvent. The average particle size of the generated bubbles was between 200 nm and 4 μm, and the bubble content was 80%-90%. The above reaction system was then transferred to a reactor for hydrothermal reaction at 195℃ for 4 hours. After the reaction, the mixture was allowed to cool naturally to room temperature. After filtration and washing, a gray precipitate was obtained and vacuum-dried at 90℃ for 19 hours. The dried hydrothermal product was then sintered in a tube furnace at 760℃ for 12 hours under a protective atmosphere of N2 and a heating rate of 5℃ / min to obtain the composite material LiFe. x Mn 1- x PO4 / C. The sintered composite material was wet-milled with polydiethylsiloxane (1.5% by mass of the sintered composite material) to better coat the LiFe. x Mn 1-x The PO4 / C surface is dried to obtain a polymer-coated composite material.

[0081] Example 8

[0082] CH3COOLi, phosphoric acid, FeSO4·7H2O, and MnSO4 were weighed according to an elemental molar ratio of Li:P:Fe:Mn of 3:1:0.5:0.5. First, FeSO4·7H2O, MnSO4, phosphoric acid solution, and a certain amount of deionized water were mixed. After the solids dissolved and the mixture was homogeneous, CH3COOLi was slowly added dropwise to the above mixed solution. After a few minutes, a gray suspension was obtained. After the CH3COOLi reaction was complete, the pH was adjusted to approximately 7.5 with an acidic solution and stirred for 60 minutes. After stirring, it was mixed with micro / nano bubble water at a volume ratio of 1:1.5 and stirred for another 2 hours. The micro / nano bubble water was prepared using a micro / nano bubble generator, using H2 (V(H2)% = 4%) and Ar as gas sources, and a mixed solution of water:ethylene glycol:alkylphenol polyoxyethylene ether at a mass ratio of 100:30:15 as the solvent. The average particle size of the generated bubbles was between 200 nm and 4 μm, and the bubble content was 80%-90%. The above reaction system was then transferred to a reactor for hydrothermal reaction at 200℃ for 4 hours. After the reaction, it was allowed to cool naturally to room temperature. After filtration and washing, a gray precipitate was obtained and dried under vacuum at 100℃ for 20 hours. The dried hydrothermal product was then sintered in a tube furnace at 770℃ for 12 hours under a protective atmosphere of N2 and a heating rate of 5℃ / min to obtain the composite material LiFe. x Mn 1-x PO4 / C. The sintered composite material was wet-milled with polymethylvinylsiloxane (2.0% by mass of the sintered composite material) to better coat the LiFe. x Mn 1-x The PO4 / C surface is dried to obtain a polymer-coated composite material.

[0083] Comparative Example 1

[0084] The preparation method is the same as in Example 1 of this invention, except that no micro / nano bubble water mixing is used to obtain the LiFe composite material. x Mn 1-x PO4 / C.

[0085] Comparative Example 2

[0086] The preparation method is the same as in Example 1 of this invention, except that no polymer A is added, resulting in the LiFe composite material. x Mn 1-x PO4 / C.

[0087] A battery was fabricated using artificial graphite as the negative electrode material, LiPF6 / EC+DEC (volume ratio 1:1) as the electrolyte, and Celgard 2400 membrane as the separator. The lithium-ion diffusion coefficient and cycle performance at 1C were tested, and the data are shown in Table 1.

[0088] Table 1

[0089]

[0090] As shown in Table 1 above, the lithium-ion battery using the lithium manganese iron phosphate composite cathode material provided in the embodiments of the present invention as the cathode has a higher lithium-ion diffusion coefficient and a higher cycle capacity retention rate, which are significantly higher than the relevant performance of the lithium-ion batteries in Comparative Examples 1 and 2. Therefore, the cathode material provided in the above embodiments can significantly improve the battery's electrical performance and cycle performance.

[0091] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A lithium iron phosphate composite cathode material, characterized in that, The invention includes carbon particles containing lithium manganese iron phosphate, wherein the surface of the lithium manganese iron phosphate particles is coated with polymer A and carbonized surfactant B; The chemical formula of the lithium manganese iron phosphate particles is LiFe. x Mn 1-x PO4, the value of x is 0.2~0.5; The polymer A is a polysiloxane compound; the polysiloxane compound is selected from at least one of polydimethylsiloxane, polydiethylsiloxane, and polymethylvinylsiloxane; The surfactant B is selected from at least one of sodium dodecyl sulfate, sorbitan oleate polyoxyethylene ether, methyl stearate polyoxyethylene ether, and alkylphenol polyoxyethylene ether. The preparation method of the lithium manganese iron phosphate composite cathode material includes the following steps: 1) Mix the phosphorus source, iron source, manganese source and deionized water according to the molar ratio of lithium, phosphorus, iron and manganese as 3:1:x:1-x, where the value of x is 0.2~0.5, to obtain a mixture; 2) A lithium source is slowly added dropwise to the mixture to obtain a gray suspension; 3) Adjust the pH of the suspension to 6.5-7.5 with an acidic solution and stir to obtain the reactants; 4) After mixing and stirring the reactants with micro-nano bubble water containing the surfactant B, the mixture is transferred to a reaction vessel with a polytetrafluoroethylene liner, so that the surfactant B in the micro-nano bubble water comes into full contact with the reactants and adheres to the surface of the reactant particles. 5) The system in step 4) is subjected to a carbonization reaction, and the gray precipitate is separated to obtain the hydrothermal product; 6) The hydrothermal products are sintered to obtain the composite material LiFe. x Mn 1-x PO4 / C; 7) The composite material LiFe x Mn 1-x PO4 / C is wet ball-milled with polymer A to obtain a polymer-coated composite material; 8) Dry the polymer-coated composite material to obtain the lithium manganese iron phosphate composite cathode material.

2. The preparation method of the lithium manganese iron phosphate composite cathode material according to claim 1, characterized in that, Includes the following steps: 1) Mix the phosphorus source, iron source, manganese source and deionized water according to the molar ratio of lithium, phosphorus, iron and manganese as 3:1:x:1-x, where the value of x is 0.2~0.5, to obtain a mixture; 2) A lithium source is slowly added dropwise to the mixture to obtain a gray suspension; 3) Adjust the pH of the suspension to 6.5-7.5 with an acidic solution and stir to obtain the reactants; 4) After mixing and stirring the reactants with micro-nano bubble water containing the surfactant B, the mixture is transferred to a reaction vessel with a polytetrafluoroethylene liner, so that the surfactant B in the micro-nano bubble water comes into full contact with the reactants and adheres to the surface of the reactant particles. 5) The system in step 4) is subjected to a carbonization reaction, and the gray precipitate is separated to obtain the hydrothermal product; 6) The hydrothermal products are sintered to obtain the composite material LiFe. x Mn 1-x PO4 / C; 7) The composite material LiFe x Mn 1-x PO4 / C is wet ball-milled with polymer A to obtain a polymer-coated composite material; 8) Dry the polymer-coated composite material to obtain the lithium manganese iron phosphate composite cathode material.

3. The preparation method according to claim 2, characterized in that, The lithium source is selected from at least one of LiOH·H2O, Li2CO3 and CH3COOLi; The phosphorus source is selected from at least one of phytic acid, NH4H2PO4 and phosphoric acid; The iron source is selected from at least one of FeSO4·7H2O, Fe3(PO4)2 and Fe2O3; The manganese source is at least one of MnSO4, Mn2O3 and manganese acetate.

4. The preparation method according to claim 2 or 3, characterized in that, The acidic solution is selected from at least one of hydrochloric acid, sulfuric acid, and phytic acid; The stirring time is 30-60 minutes.

5. The preparation method according to claim 2 or 3, characterized in that, The micro-nano bubble water is prepared using a micro-nano bubble generator.

6. The preparation method according to claim 5, characterized in that, The average particle size of the bubbles in the micro-nano bubble water is 200nm~4μm; The micro-nano bubble water has a bubble content of 80%~90%, uses H2 and Ar as gas sources, has a hydrogen volume percentage of 2%~4%, and uses a mixed solution of deionized water, ethylene glycol and surfactant B with a mass ratio of 100:15~30:5~15 as solvent.

7. The preparation method according to claim 2 or 6, characterized in that, The volume ratio of the reactants to the micro / nano bubble water containing the surfactant B is 1:1 to 1.

5.

8. The preparation method according to claim 2 or 3, characterized in that, In step 4), the stirring time is 1-2 hours; In step 5), the carbonization reaction is carried out at a temperature of 150~200℃ for 2~6 hours. Step 5) also includes the steps of naturally cooling the system after the carbonization reaction to room temperature, filtering and washing to obtain the gray precipitate, and drying it; The drying conditions are as follows: vacuum drying at 60~100℃ for 10~20h.

9. The preparation method according to claim 2 or 3, characterized in that, In step 6), the sintering process is as follows: N2 is used as a protective atmosphere in a tube furnace, and the temperature is raised to 700-770°C at a heating rate of 3-5°C / min, and sintered at 700-770°C for 6-12 hours. In step 7), the composite material LiFe x Mn 1-x The mass percentage of polymer A added in PO4 / C is 0.1~2.5%.

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