Graphene-coated lithium iron manganese phosphate and preparation method and application thereof

Abstract

This invention provides a graphene-coated lithium manganese iron phosphate, its preparation method, and its application. The preparation method includes the following steps: mixing a precursor, a lithium source, and a phosphorus source to obtain a mixture; the precursor is a co-precipitation product of iron and manganese, and includes doped metal ions; calcining the mixture, and after the heat preservation stage during calcination, introducing carbon source gas to continue heat preservation, thereby obtaining the graphene-coated lithium manganese iron phosphate. The preparation method utilizes metal active sites combined with chemical vapor deposition to obtain uniformly graphene-coated lithium manganese iron phosphate, significantly improving the material's conductivity, cycle performance, and rate performance, reducing battery internal resistance and charge / discharge polarization, and the overall process is simple, easy to operate, and can effectively reduce reaction temperature and cost.

Description

Technical Field

[0001] This invention belongs to the field of battery technology and relates to a graphene-coated lithium manganese iron phosphate, its preparation method and application. Background Technology

[0002] Lithium iron manganese phosphate (LMP) suffers from low electrical conductivity. The electron transition energy in LMP is much higher than in lithium iron phosphate (LFP). While LFP can be considered a semiconductor, LMP is essentially an insulator. Furthermore, the small particle size of LMP makes its processing significantly more difficult than that of LFP. Therefore, conductive substances need to be added to the material, such as carbon coating the surface of LMP. Current technology typically involves mixing LMP and a carbon source through ball milling, followed by calcination to obtain carbon-coated LMP material.

[0003] For example, CN 114628660A discloses a hydrothermal synthesis method for lithium iron manganese phosphate nanoparticles, including the following steps: preparing a solution of lithium, iron, manganese and phosphorus source raw materials; mixing the raw material solutions to obtain a mixed slurry; conveying the mixed slurry to a preheating unit for preheating; then sending the heated mixed slurry into a reactor; carrying out a hydrothermal synthesis reaction in the reactor; filtering, washing and drying the product to obtain lithium iron manganese phosphate powder; mixing the lithium iron manganese phosphate powder with a carbon source and calcining it at 700°C for 4 hours under inert gas protection to obtain carbon-coated lithium iron manganese phosphate; however, the above method cannot uniformly coat the carbon source on the material surface, and the lithium iron manganese material is easily broken during ball milling.

[0004] Based on the above research, there is a need to provide a method for preparing graphene-coated lithium manganese iron phosphate. The preparation method can grow a uniform graphene coating layer on the material surface, significantly reduce the internal resistance and charge / discharge polarization of the battery, improve high-temperature performance and cycle performance, and at the same time improve rate performance. Summary of the Invention

[0005] The purpose of this invention is to provide a graphene-coated lithium manganese iron phosphate, its preparation method, and its application. The preparation method utilizes metal active sites combined with chemical vapor deposition to obtain graphene-coated lithium manganese iron phosphate, which significantly improves the conductivity, cycle performance, and rate performance of the material, reduces the battery internal resistance and charge / discharge polarization, and the overall process is simple, easy to operate, and can effectively reduce the reaction temperature and cost.

[0006] To achieve this objective, the present invention employs the following technical solution:

[0007] In a first aspect, the present invention provides a method for preparing graphene-coated lithium manganese iron phosphate, the method comprising the following steps:

[0008] (1) Mix the precursor, lithium source and phosphorus source to obtain a mixture;

[0009] The precursor is a co-precipitation product of iron and manganese, and includes doped metal ions.

[0010] (2) The mixture described in step (1) is calcined. After the heat preservation stage during the calcination process is completed, carbon source gas is introduced to continue heat preservation, and the graphene-coated lithium manganese iron phosphate is obtained.

[0011] This invention employs a manganese-iron precursor doped with metal ions for graphene coating. The graphene coating is carried out during calcination, achieving uniform coating while simultaneously preparing the material. The metal-ion-doped precursor serves as the raw material for calcination, and the coating process during calcination allows the doped metal ions to act as active sites for carbon source attachment, resulting in the uniform growth of graphene on the material surface. The conductive carbon and lithium iron phosphate form a fast conductive network, enabling rapid electron migration between active materials during charging and discharging, reducing battery internal resistance and charge / discharge polarization. Simultaneously, the coating reduces the contact surface area between the active material and the electrolyte, thus preventing side reactions and improving high-temperature and cycle performance. It also effectively inhibits the aggregation and growth of modified material particles, maintaining the nanostructure of the particles and effectively reducing Li-. + The diffusion distance within the active particles gives the material superior rate performance.

[0012] Preferably, step (2) further includes a step of introducing a protective gas to continue the heat preservation after introducing the carbon source gas.

[0013] In this invention, a carbon source gas is introduced during the calcination process to achieve graphene deposition and coating. Then, a protective gas is introduced to further improve the uniformity of the coating, enhance the bonding strength between the graphene coating layer and the core, and further ensure that the carbon layer on the surface is a graphene coating layer.

[0014] Preferably, after the protective gas is introduced and the temperature is maintained, the material is further cooled to room temperature under the protective gas environment to obtain the graphene-coated lithium manganese iron phosphate.

[0015] Preferably, the time for continuing to maintain the temperature by introducing protective gas is 1-3 hours, for example, it can be 1 hour, 2 hours or 3 hours, but it is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0016] Preferably, the protective gas includes any one or a combination of at least two of nitrogen, argon, helium or krypton, with typical but not limited combinations including a combination of nitrogen and argon, or a combination of helium and krypton.

[0017] Preferably, the calcination in step (2) is carried out in a protective gas.

[0018] Preferably, the calcination in step (2) includes pre-calcination, primary sintering, and secondary sintering performed sequentially.

[0019] Preferably, after the heat preservation stage of the first sintering is completed and / or after the heat preservation stage of the second sintering is completed, carbon source gas is introduced to continue heat preservation. More preferably, carbon source gas is introduced to continue heat preservation after both the heat preservation stage of the first sintering and the heat preservation stage of the second sintering.

[0020] This invention ensures uniform graphene coating by controlling the timing of introducing the carbon source gas and combining it with staged sintering. Preferably, after the first sintering and holding stage, the carbon source gas is turned on and kept at the first sintering temperature. Then, the carbon source gas is turned off, and the temperature is kept at the first sintering temperature again in a protective gas. Then, a high-temperature second sintering is performed. After the second sintering and holding stage, the carbon source gas is turned on and kept at the second sintering temperature again in a protective gas. This achieves graphene coating during the calcination process. Therefore, this invention achieves uniform graphene coating during the high-temperature calcination of the precursor.

[0021] Preferably, after the holding stage of the first sintering is completed, the temperature at which the carbon source gas is introduced to continue holding is the same as the temperature of the first sintering.

[0022] Preferably, after the heat preservation stage of the secondary sintering is completed, the temperature at which the carbon source gas is introduced for continued heat preservation is the same as the temperature of the secondary sintering.

[0023] Preferably, the preheating temperature is 200-400℃, for example, 200℃, 300℃ or 400℃, and the time is 2-4h, for example, 2h, 3h or 4h, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0024] Preferably, the temperature of the first sintering is 750-900℃, for example, 750℃, 800℃, 850℃ or 900℃, and the time is 8-24h, for example, 8h, 10h, 15h, 20h or 24h, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0025] Preferably, the temperature of the secondary sintering is 900-1200℃, for example, 900℃, 1000℃, 1100℃ or 1200℃, and the time is 1-3h, for example, 1h, 2h or 3h, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0026] Preferably, the time for introducing carbon source gas in step (2) is 5-50 min, for example, it can be 5 min, 10 min, 20 min, 30 min, 40 min or 50 min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0027] Preferably, the flow rate of the carbon source gas introduced in step (2) is 1-30 sccm, for example, it can be 1 sccm, 10 sccm, 20 sccm or 30 sccm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0028] Preferably, the carbon source gas in step (2) includes methane and / or acetylene.

[0029] Preferably, the molar ratio of the precursor, lithium source and phosphorus source in step (1) is 1:(1.01-1.1):1, for example, it can be 1:1.01:1, 1:1.05:1, 1:1.07 or 1:1.1:1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0030] Preferably, the valence state of the doped metal ions in step (1) is higher than that of manganese and iron in lithium manganese iron phosphate.

[0031] Preferably, the doped metal ions in step (1) include Zr. 4+ Ti 4+ V 5+ or Nb 5+ Any one or at least two of the above, preferably Zr 4+ .

[0032] This invention involves the incorporation of Li + Cations with similar ionic radii and higher valence states, such as Zr 4+ This can generate more vacancies at Li sites, which is more beneficial for inner Li layers. + The migration of these molecules further enhances the electrochemical performance of the doped lithium manganese iron phosphate.

[0033] Preferably, the precursor in step (1) is prepared by the following method:

[0034] A mixed iron source, manganese source, and complexing agent are used to adjust the pH of the system. Then, a dopant source is added to carry out a co-precipitation reaction to obtain the precursor.

[0035] Preferably, an antioxidant is also added to the system.

[0036] Preferably, the antioxidant includes citric acid.

[0037] Preferably, the complexing agent comprises ammonium oxalate.

[0038] Preferably, the doping source comprises an oxide doped with metal ions.

[0039] Preferably, the pH of the system is adjusted to 2-3, for example, 2, 2.5 or 3, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0040] Preferably, the pH of the system is adjusted using ammonia and / or sulfuric acid.

[0041] Preferably, the temperature of the coprecipitation reaction is 40-60℃, for example, 40℃, 50℃ or 60℃, and the time is 4-6h, for example, 4h, 5h or 6h, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0042] As a preferred embodiment of the preparation method of the present invention, the preparation method includes the following steps:

[0043] (1) Mix iron source, manganese source, complexing agent and antioxidant, then adjust the pH of the system to 2-3 with ammonia and / or sulfuric acid, then add dopant source, and carry out co-precipitation reaction at 40-60℃ for 4-6h to obtain precursor. Mix precursor, lithium source and phosphorus source in a molar ratio of 1:(1.01-1.1):1 to obtain mixture;

[0044] The precursor is a co-precipitation product of iron and manganese, and includes doped metal ions.

[0045] (2) In a protective gas, the mixture described in step (1) is pre-fired at 200-400℃ for 2-4 hours, sintered once at 750-900℃ for 8-24 hours, and then carbon source gas is introduced at a flow rate of 1-30 sccm and kept at the temperature for 5-50 minutes. Then, it is kept at the temperature under the protective gas for 1-3 hours, and then sintered again at 900-1200℃ for 1-3 hours. Then, carbon source gas is introduced at a flow rate of 1-30 sccm and kept at the temperature for 5-50 minutes. Then, it is kept at the temperature under the protective gas for 1-3 hours to obtain the graphene-coated lithium manganese iron phosphate.

[0046] In a second aspect, the present invention provides a graphene-coated lithium manganese iron phosphate, wherein the graphene-coated lithium manganese iron phosphate is prepared by the preparation method described in the first aspect.

[0047] Thirdly, the present invention provides a lithium-ion battery comprising graphene-coated lithium manganese iron phosphate as described in the second aspect.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] This invention utilizes a manganese-iron precursor doped with metal ions, which serve as active sites for carbon attachment. Combined with chemical vapor deposition coating during calcination, this generates uniformly coated graphene on the material surface, forming a fast-conducting network with the lithium manganese-iron lithium. This allows electrons to migrate rapidly between the active materials during charging and discharging, reducing the battery's internal resistance and charge / discharge polarization. Simultaneously, the coating reduces the contact surface area between the active materials and the electrolyte, preventing side reactions and improving high-temperature and cycle performance. Furthermore, it effectively inhibits the aggregation and growth of the modified material particles, maintaining their nanostructure and effectively reducing Li-. + The diffusion distance within the active particles gives the material superior rate performance. Detailed Implementation

[0050] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0051] Example 1

[0052] This embodiment provides a method for preparing graphene-coated lithium manganese iron phosphate, the method comprising the following steps:

[0053] (1) FeSO4·7H2O and MnSO4·2H2O were mixed according to a molar ratio of n(Mn):n(Fe) = 4:6 to prepare a salt solution. Citric acid was added to prevent oxidation. Then, the solution was pumped into a reaction vessel with (NH4)2·C2O4 and stirred. The pH of the system was adjusted to 2.5 using ammonia and sulfuric acid. Zirconia was then added and the mixture was stirred at 50°C for 5 hours to carry out a co-precipitation reaction to obtain the precursor. The precursor, Li2CO3 and NH4H2PO4 were mixed in a molar ratio of 1:1.03:1 to obtain a mixture.

[0054] (2) The mixture described in step (1) is pre-calcined at 300°C for 3 hours in a nitrogen atmosphere, and then sintered at 800°C for 10 hours. The carbon source gas switch is turned on, and carbon source gas methane is introduced at a flow rate of 15 sccm. The temperature is maintained at 800°C for 40 minutes, and then the carbon source gas switch is turned off. The temperature is then maintained at 800°C for 2 hours in a nitrogen atmosphere, and then sintered again at 1000°C for 2 hours. The carbon source gas switch is turned on, and carbon source gas methane is introduced at a flow rate of 15 sccm. The temperature is then maintained at 1000°C for 30 minutes, and then the carbon source gas switch is turned off. The temperature is then maintained at 1000°C for 2 hours in a nitrogen atmosphere, and then cooled to room temperature under nitrogen protection to obtain the graphene-coated lithium manganese iron phosphate material.

[0055] Example 2

[0056] This embodiment provides a method for preparing graphene-coated lithium manganese iron phosphate, the method comprising the following steps:

[0057] (1) FeSO4·7H2O and MnSO4·2H2O were mixed according to a molar ratio of n(Mn):n(Fe) = 4:6 to prepare a salt solution. Citric acid was added to prevent oxidation. Then, the solution was pumped into a reaction vessel with (NH4)2·C2O4 and stirred. The pH of the system was adjusted to 2 using ammonia and sulfuric acid. Zirconia was then added and the mixture was stirred at 40°C for 6 hours to carry out a co-precipitation reaction to obtain the precursor. The precursor, Li2CO3 and NH4H2PO4 were mixed in a molar ratio of 1:1.01:1 to obtain a mixture.

[0058] (2) The mixture described in step (1) is pre-calcined at 200°C for 4 hours in an argon atmosphere, and then sintered at 750°C for 24 hours. The carbon source gas switch is turned on, and carbon source gas methane is introduced at a flow rate of 1 sccm. The temperature is maintained at 750°C for 50 minutes, and then the carbon source gas switch is turned off. The temperature is then maintained at 750°C for 1 hour in an argon atmosphere, and then sintered again at 900°C for 3 hours. The carbon source gas switch is turned on, and carbon source gas methane is introduced at a flow rate of 1 sccm. The temperature is then maintained at 900°C for 50 minutes, and then the carbon source gas switch is turned off. The temperature is then maintained at 900°C for 1 hour in an argon atmosphere, and then cooled to room temperature under argon protection to obtain the graphene-coated lithium manganese iron phosphate material.

[0059] Example 3

[0060] This embodiment provides a method for preparing graphene-coated lithium manganese iron phosphate, the method comprising the following steps:

[0061] (1) FeSO4·7H2O and MnSO4·2H2O were mixed according to a molar ratio of n(Mn):n(Fe) = 4:6 to prepare a salt solution. Citric acid was added to prevent oxidation. Then, the solution was pumped into a reaction vessel with (NH4)2·C2O4 and stirred. The pH of the system was adjusted to 3 using ammonia and sulfuric acid. Zirconia was then added, and the mixture was stirred at 60°C for 4 hours to carry out a co-precipitation reaction to obtain the precursor. The precursor, Li2CO3 and NH4H2PO4 were mixed in a molar ratio of 1:1.1:1 to obtain a mixture.

[0062] (2) The mixture described in step (1) is pre-calcined at 400°C for 2 hours in a nitrogen atmosphere, and then sintered at 900°C for 8 hours. The carbon source gas switch is turned on, and acetylene is introduced at a flow rate of 30 sccm. The mixture is then kept at 900°C for 5 minutes and then the carbon source gas switch is turned off. The mixture is then kept at 900°C for 3 hours in a nitrogen atmosphere, and then sintered again at 1200°C for 1 hour. The carbon source gas acetylene is introduced at a flow rate of 30 sccm, and the mixture is then kept at 1200°C for 20 minutes and then the carbon source gas switch is turned off. The mixture is then kept at 1200°C for 3 hours in a nitrogen atmosphere, and then cooled to room temperature under nitrogen protection to obtain the graphene-coated lithium manganese iron phosphate material.

[0063] Example 4

[0064] This embodiment provides a method for preparing graphene-coated lithium manganese iron phosphate. Except for step (2), which is prepared by the following method, the preparation method is the same as that in Example 1.

[0065] Step (2) is prepared by the following method: the mixture described in step (1) is pre-calcined at 300°C for 3 hours in a nitrogen atmosphere, sintered once at 800°C for 10 hours, and then sintered again at 1000°C for 2 hours. Then, the carbon source gas switch is turned on, and methane is introduced at 15 sccm. The temperature is then maintained at 1000°C for 30 minutes, and then the carbon source gas switch is turned off. The temperature is then maintained at 1000°C for 2 hours in a nitrogen atmosphere, and then cooled to room temperature under nitrogen protection to obtain the graphene-coated lithium manganese iron phosphate material.

[0066] Example 5

[0067] This embodiment provides a method for preparing graphene-coated lithium manganese iron phosphate. Except for step (2), which is prepared by the following method, the preparation method is the same as that in Example 1.

[0068] Step (2) is prepared by the following method: the mixture described in step (1) is pre-calcined at 300°C for 3 hours in a nitrogen atmosphere, sintered once at 800°C for 10 hours, the carbon source gas switch is turned on, and carbon source gas methane is introduced at 15 sccm. The mixture is then kept at 800°C for 40 minutes and the carbon source gas switch is turned off. The mixture is then kept at 800°C for 2 hours in a nitrogen atmosphere and sintered again at 1000°C for 2 hours to obtain the graphene-coated lithium manganese iron phosphate material.

[0069] Example 6

[0070] (2) The mixture described in step (1) is pre-calcined at 300°C for 3 hours in a nitrogen atmosphere. The carbon source gas switch is turned on, and methane is introduced at 15 sccm. The mixture is then kept at 300°C for 40 minutes and then the carbon source gas switch is turned off. The mixture is then kept at 300°C for 2 hours in a nitrogen atmosphere, sintered at 800°C for 10 hours, and then sintered again at 1000°C for 2 hours. Finally, the mixture is cooled to room temperature under nitrogen protection to obtain the graphene-coated lithium manganese iron phosphate material.

[0071] Example 7

[0072] This embodiment provides a method for preparing graphene-coated lithium manganese iron phosphate. Except for step (2), after introducing carbon source gas, the secondary sintering is carried out directly instead of being held at 800°C for 2 hours under nitrogen.

[0073] Example 8

[0074] This embodiment provides a method for preparing graphene-coated lithium manganese iron phosphate. Except for step (2), after introducing the carbon source gas, before cooling down to room temperature, the preparation method is the same as in embodiment 1, except that the carbon source gas is introduced and the carbon source gas is introduced and the carbon source gas is introduced and the carbon source gas is introduced and cooled down to room temperature, the carbon source gas is cooled down to room temperature directly.

[0075] Example 9

[0076] This embodiment provides a method for preparing graphene-coated lithium manganese iron phosphate. The preparation method is the same as in Example 1, except that the zirconium oxide in step (1) is replaced with titanium oxide.

[0077] Example 10

[0078] This embodiment provides a method for preparing graphene-coated lithium manganese iron phosphate. The preparation method is the same as in Example 1, except that the zirconium oxide in step (1) is replaced by vanadium pentoxide.

[0079] Comparative Example 1

[0080] This comparative example provides a method for preparing graphene-coated lithium manganese iron phosphate. The preparation method is the same as in Example 1 except that zirconium oxide is not added in step (1).

[0081] Comparative Example 2

[0082] This comparative example provides a method for preparing graphene-coated lithium manganese iron phosphate, the method comprising the following steps:

[0083] (1) FeSO4·7H2O and MnSO4·2H2O were mixed according to a molar ratio of n(Mn):n(Fe) = 4:6 to prepare a salt solution. Citric acid was added to prevent oxidation. Then, the solution was pumped into a reaction vessel with (NH4)2·C2O4 and stirred. The pH of the system was adjusted to 2.5 using ammonia and sulfuric acid. Zirconia was then added and the mixture was stirred at 50°C for 5 hours to carry out a co-precipitation reaction to obtain the precursor. The precursor, Li2CO3 and NH4H2PO4 were mixed in a molar ratio of 1:1.03:1 to obtain a mixture.

[0084] (2) The mixture described in step (1) is pre-fired at 300°C for 3 hours in a nitrogen atmosphere, sintered once at 700°C for 10 hours, and then sintered again at 1000°C for 2 hours. Then it is cooled to room temperature under nitrogen protection to obtain lithium manganese iron phosphate material.

[0085] (3) Mix lithium manganese iron phosphate material and graphene, and then calcine at 1000℃ for 40 min in a nitrogen atmosphere to obtain the graphene-coated lithium manganese iron phosphate material.

[0086] The graphene-coated lithium manganese iron phosphate obtained in the above examples and comparative examples was used to prepare a positive electrode, which was then used with a lithium metal negative electrode, LiPF6 (EC:DMC:EMC = 1:1:1) electrolyte and Celgard 2400 separator to prepare a 2032 coin cell. The obtained coin cells were tested for cycle performance at 1C with a cutoff voltage range of 2.5 to 4.5V in a 25°C constant temperature chamber, and the rate performance was tested by cycling 5 times each at 0.1C, 0.2C, 0.5C, 1C, 3C and 5C. The capacity retention rate after 200 cycles and the capacity retention rate at 3C / 1C are shown in Table 1.

[0087] Table 1

[0088]

[0089]

[0090] As can be seen from the table above:

[0091] (1) By using a precursor doped with metal ions and depositing graphene during calcination, this invention can improve the uniformity of graphene deposition and the bonding strength with the core, thereby improving the electrochemical performance of the material. As shown in Examples 1 and 4-6, this invention preferably introduces carbon source gas to deposit graphene after both the first and second sintering. Through the two-step deposition process, in which pre-deposition is performed at a lower temperature during the first sintering, followed by the second sintering and the second deposition, the bonding strength between the graphene coating layer and the core can be improved, as well as the uniformity of deposition, thereby improving the cycle performance and rate performance. As shown in Examples 1 and 7-8, after the graphene deposition, the heat preservation step with protective gas can further promote the uniform deposition of graphene and improve the stability of the coating, thereby further improving the electrochemical performance.

[0092] (2) As can be seen from Examples 1 and 9-10, the present invention uses zirconium ions as doped metal ions. Its higher valence state can dop more vacancies, which is more conducive to the migration of lithium ions than other doped ions. As can be seen from Examples 1 and Comparative Example 1, when the precursor does not contain doped ions, it cannot provide deposition sites during the deposition process and cannot provide more vacancies. Therefore, the electrochemical performance of the obtained material is greatly reduced. As can be seen from Examples 1 and Comparative Example 2, the conventional coating method of mixed calcination cannot uniformly coat graphene. However, the present invention can achieve coating through calcination, which can not only improve the uniformity of coating, but also simplify the process steps and reduce costs.

[0093] In summary, this invention provides a graphene-coated lithium manganese iron phosphate, its preparation method, and its application. The preparation method utilizes metal active sites combined with chemical vapor deposition to obtain graphene-coated lithium manganese iron phosphate, which significantly improves the material's conductivity, cycle performance, and rate performance, reduces battery internal resistance and charge / discharge polarization, and the overall process is simple, easy to operate, and can effectively reduce reaction temperature and cost.

[0094] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing graphene-coated lithium manganese iron phosphate, characterized in that, The preparation method includes the following steps: (1) Mix the precursor, lithium source and phosphorus source to obtain a mixture; The precursor is a co-precipitation product of iron and manganese, and includes doped metal ions. (2) The mixture described in step (1) is calcined. After the heat preservation stage during the calcination process is completed, carbon source gas is introduced to continue heat preservation, and the graphene-coated lithium manganese iron phosphate is obtained. The metal ion doping in step (1) comprises any one or a combination of at least two of Zr 4+ , Ti 4+ , V 5+ or Nb 5+ . The calcination in step (2) includes pre-calcination, primary sintering, and secondary sintering performed sequentially; After the holding stage of the first sintering and after the holding stage of the second sintering, carbon source gas is introduced to continue the holding process. Step (2) after introducing the carbon source gas also includes the step of introducing a protective gas to continue the heat preservation.

2. The preparation method according to claim 1, characterized in that, The time for continuing to maintain the temperature by introducing protective gas is 1-3 hours.

3. The preparation method according to claim 1, characterized in that, The protective gas includes any one or a combination of at least two of nitrogen, argon, helium, or krypton.

4. The preparation method according to claim 1, characterized in that, The calcination in step (2) is carried out in a protective gas.

5. The preparation method according to claim 1, characterized in that, After the holding stage of the first sintering is completed, the carbon source gas is introduced to continue holding at the same temperature as the first sintering.

6. The preparation method according to claim 1, characterized in that, After the holding stage of the secondary sintering is completed, the temperature at which carbon source gas is introduced to continue holding is the same as the temperature of the secondary sintering.

7. The preparation method according to claim 1, characterized in that, The pre-firing temperature is 200-400℃, and the time is 2-4 hours.

8. The preparation method according to claim 1, characterized in that, The temperature of the first sintering is 750-900℃, and the time is 8-24h.

9. The preparation method according to claim 1, characterized in that, The secondary sintering temperature is 900-1200℃, and the time is 1-3 hours.

10. The preparation method according to claim 1, characterized in that, The time for introducing the carbon source gas in step (2) is 5-50 minutes.

11. The preparation method according to claim 1, characterized in that, The flow rate of the carbon source gas introduced in step (2) is 1-30 sccm.

12. The preparation method according to claim 1, characterized in that, The carbon source gas in step (2) includes methane and / or acetylene.

13. The preparation method according to claim 1, characterized in that, The molar ratio of the precursor, lithium source and phosphorus source in step (1) is 1:(1.01-1.1):

1.

14. The preparation method according to claim 1, characterized in that, The doped metal ion in step (1) is Zr 4+ .

15. The preparation method according to claim 1, characterized in that, The precursor in step (1) is prepared by the following method: A mixed iron source, manganese source, and complexing agent are used to adjust the pH of the system. Then, a dopant source is added to carry out a co-precipitation reaction to obtain the precursor.

16. The preparation method according to claim 15, characterized in that, Antioxidants were also added to the system.

17. The preparation method according to claim 16, characterized in that, The antioxidants include citric acid.

18. The preparation method according to claim 15, characterized in that, The complexing agent includes ammonium oxalate.

19. The preparation method according to claim 15, characterized in that, The pH of the system was adjusted to 2-3.

20. The preparation method according to claim 15, characterized in that, The pH of the system is adjusted using ammonia and / or sulfuric acid.

21. The preparation method according to claim 15, characterized in that, The coprecipitation reaction is carried out at a temperature of 40-60℃ for 4-6 hours.

22. The preparation method according to claim 1, characterized in that, The preparation method includes the following steps: (1) Mix iron source, manganese source, complexing agent and antioxidant, then adjust the pH of the system to 2-3 with ammonia and / or sulfuric acid, then add dopant source, and carry out co-precipitation reaction at 40-60℃ for 4-6h to obtain precursor. Mix precursor, lithium source and phosphorus source in a molar ratio of 1:(1.01-1.1):1 to obtain mixture; The precursor is a co-precipitation product of iron and manganese, and includes doped metal ions. (2) In a protective gas, the mixture described in step (1) is pre-fired at 200-400℃ for 2-4 hours, sintered once at 750-900℃ for 8-24 hours, and then carbon source gas is introduced at a flow rate of 1-30 sccm and kept at the temperature for 5-50 minutes. Then, it is kept at the temperature under the protective gas for 1-3 hours, and then sintered again at 900-1200℃ for 1-3 hours. Then, carbon source gas is introduced at a flow rate of 1-30 sccm and kept at the temperature for 5-50 minutes. Then, it is kept at the temperature under the protective gas for 1-3 hours to obtain the graphene-coated lithium manganese iron phosphate.

23. A graphene-coated lithium manganese iron phosphate, characterized in that, The graphene-coated lithium manganese iron phosphate is prepared by the preparation method described in any one of claims 1-22.

24. A lithium-ion battery, characterized in that, The lithium-ion battery includes the graphene-coated lithium manganese iron phosphate as described in claim 23.

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