A lithium iron manganese phosphate positive electrode material modified by composite carbon doping with vegetable oil and a preparation method and application thereof

Abstract

The application provides a lithium manganese iron phosphate positive electrode material modified by composite carbon doping with vegetable oil and a preparation method and application thereof. The preparation method comprises the following steps: 1) placing a lithium source, a manganese source, an iron source, a phosphorus source and a solid carbon source in a ball mill tank, dry grinding to obtain a mixture; adding vegetable oil and an ethanol solvent, wet grinding to obtain a slurry, and reserving; 2) drying the slurry obtained in the step 1) to obtain a dried substance, and reserving; 3) grinding the dried substance obtained in the step 2), sieving, and then high-temperature sintering; 4) cooling to room temperature with a furnace, grinding, sieving, and then obtaining the lithium manganese iron phosphate positive electrode material. The composite carbon sources sucrose, polyvinyl alcohol and vegetable oil are used to participate in carbon doping modification, the uniform coating of the carbon source is realized, and the discharge specific capacity, cycle performance and voltage platform of the lithium manganese iron phosphate are effectively improved. The method has simple operation process, and the carbon source is low in price, so that the production cost of the lithium manganese iron phosphate is reduced, and the conductivity of the lithium manganese iron phosphate positive electrode material is improved.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery cathode material technology, specifically relating to a carbon-modified manganese iron phosphate cathode material using vegetable oil, its preparation method, and its application. Background Technology

[0002] Lithium manganese iron phosphate (LFP) is a lithium-ion battery cathode material developed based on lithium iron phosphate. Its theoretical capacity is the same as lithium iron phosphate (170 mAh / g), but its Li... + The electrode potential of lithium manganese iron phosphate (LFP) is 4.1V, while that of lithium iron phosphate is only 3.4V. The high potential of LFP gives it the potential advantage of high energy density, but its conductivity is a problem that urgently needs to be addressed.

[0003] There are two existing methods to improve the conductivity of lithium manganese iron phosphate (LFP): one is to reduce the size of primary particles to improve the crystallinity and elemental composition uniformity of the material; the other is to coat or dope it with carbon materials that have good conductivity to reduce the material's conductivity. Commonly used carbon materials include glucose, sucrose, citric acid, graphene, and carbon nanotubes. To improve the uniform introduction of carbon, spray drying and sand milling are often required, which greatly increases the synthesis difficulty and production cost of LFP. While carbon nanotubes and graphene can avoid complex processes, their high cost as carbon sources also increases the production cost of LFP.

[0004] Therefore, it is necessary to explore a carbon doping method that is inexpensive and simple to process, so as to improve the conductivity of lithium manganese iron phosphate while reducing its production cost. Summary of the Invention

[0005] To address the problems of high production cost and poor electrochemical performance of current carbon-doped modified lithium manganese iron phosphate cathode materials, the present invention aims to provide a carbon-doped lithium manganese iron phosphate cathode material using vegetable oil composite, its preparation method, and its application.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] This invention provides a method for preparing a carbon-doped lithium manganese iron phosphate cathode material using vegetable oil, comprising the following steps:

[0008] Step 1): Place the lithium source, manganese source, iron source, phosphorus source and solid carbon source in a ball mill jar, dry mill to obtain a mixture; then add vegetable oil and ethanol solvent to the mixture to make the solid content of the ball mill 15-60%, wet mill to obtain a slurry for later use.

[0009] The volume of vegetable oil added is 6-18% of the volume of ethanol solvent; the solid carbon source is polyvinyl alcohol and / or sucrose.

[0010] Step 2): Dry the slurry obtained in Step 1) to obtain a dried product for later use;

[0011] Step 3): Grind the dried material obtained in Step 2), sieve it, and then sinter it at high temperature;

[0012] Step 4): After cooling to room temperature in the furnace, grind the product and sieve it to obtain the final product.

[0013] Preferably, in step 1), according to the chemical formula Li 1-1.05 Fe 0.3-0.7 Mn 0.3-0.7 PO4&C 0.5 Weigh out each raw material, namely the mixed raw material lithium source, manganese source, iron source, phosphorus source and solid carbon source, and the molar ratio of lithium element, iron element, manganese element, phosphorus element and carbon element is 1-1.05:0.3-0.7:0.3-0.7:1:0.5;

[0014] The solid carbon source is polyvinyl alcohol and sucrose; the molar ratio of carbon in polyvinyl alcohol to carbon in sucrose is (0.02-0.03):(0.07-0.13).

[0015] Preferably, the solid content of the ball mill is 40-60%; the volume of the added vegetable oil is 6-12% of the volume of the ethanol solvent.

[0016] Preferably, in step 1), during dry grinding, the ball milling time is 3-5 hours and the ball milling speed is 300-500 r / min; during wet grinding, the ball milling time is 3-5 hours and the ball milling speed is 300-500 r / min.

[0017] Preferably, the lithium source is any one or a mixture of lithium dihydrogen phosphate, lithium carbonate, and lithium hydroxide monohydrate;

[0018] The manganese source is any one or a mixture of manganese carbonate, manganese dihydrogen phosphate, manganese dioxide, and manganese tetroxide;

[0019] The iron source is ferrous oxalate dihydrate and / or ferric phosphate;

[0020] The phosphorus source is any one or a mixture of phosphoric acid, ammonium dihydrogen phosphate, manganese dihydrogen phosphate, and lithium dihydrogen phosphate.

[0021] Preferably, in step 1), the total volume of the slurry does not exceed 1 / 3 of the volume of the ball mill jar; the grinding beads are zirconia beads, and the mass of the zirconia beads is 5-7 times the total mass of the raw materials in the ball mill jar.

[0022] Preferably, step 2) drying is vacuum drying or room temperature stirring drying; vacuum drying is carried out in a vacuum drying oven at a drying temperature of 80°C for a drying time of ≤24h; room temperature stirring drying is carried out at a stirring speed of 400-600r / min.

[0023] Preferably, the specific method of step 3) is as follows: after grinding, pass through a 100-mesh sieve and place in a tube furnace. Under an argon atmosphere with a gas flow rate of 200 mL / min, perform high-temperature sintering. At a heating rate of 5 °C / min, first heat to 300-500 °C and hold for 4-6 hours, then heat to 600-700 °C and hold for 7-9 hours.

[0024] Step 4) Grind the material and then pass it through a 200-mesh sieve.

[0025] This invention provides a lithium manganese iron phosphate cathode material, which is prepared by the aforementioned preparation method.

[0026] This invention provides an application of the lithium manganese iron phosphate cathode material in lithium-ion batteries.

[0027] Beneficial effects:

[0028] This invention involves dry grinding lithium, manganese, iron, phosphorus, and solid carbon sources in a ball mill jar, followed by the addition of vegetable oil and solvent for wet grinding. The resulting slurry is dried in a vacuum drying oven, ground, sieved, and then placed in a tube furnace for high-temperature sintering under an argon atmosphere. After furnace cooling, it is ground and sieved again to obtain carbon-modified lithium manganese iron phosphate cathode material. This method is simple to operate and uses inexpensive carbon sources, reducing the production cost of lithium manganese iron phosphate while improving the conductivity of the lithium manganese iron phosphate cathode material.

[0029] The method of this invention uses a composite carbon source: sucrose, polyvinyl alcohol, and vegetable oil. Polyvinyl alcohol and sucrose are solid carbon sources; while participating in carbon doping modification, their addition during dry milling reduces the particle size of lithium, manganese, iron, and phosphorus sources, simultaneously reducing the particle size of the solid carbon source. Vegetable oil, as an excellent liquid carbon source, participates in carbon doping modification, achieving uniform coating of the carbon source and effectively improving the conductivity of lithium manganese iron phosphate, thereby increasing its discharge specific capacity and cycle performance. Simultaneously, the effective introduction of carbon can increase the median charge-discharge voltage in the electrochemical reaction of lithium manganese phosphate. Vegetable oil also acts as a dispersant and grinding aid; its addition during wet milling improves mixing uniformity and assists in ball milling to reduce the particle size of the raw materials. Attached Figure Description

[0030] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. Wherein:

[0031] Figure 1 The graph shows the rate performance of button batteries prepared from the cathode materials obtained in Examples 1-3 and Comparative Example 1 of this invention.

[0032] Figure 2 The graph shows the cycle performance of button batteries prepared from the cathode materials obtained in Examples 1-3 and Comparative Example 1 of this invention.

[0033] Figure 3 This is a scanning electron microscope (SEM) image of the cathode material obtained in Comparative Example 1 of the present invention.

[0034] Figure 4 This is a SEM image of the cathode material obtained in Example 1 of the present invention.

[0035] Figure 5 This is a SEM image of the cathode material obtained in Example 2 of the present invention.

[0036] Figure 6 This is a SEM image of the cathode material obtained in Example 3 of the present invention. Detailed Implementation

[0037] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention are within the scope of protection of the present invention.

[0038] The present invention will now be described in detail with reference to embodiments. It should be noted that, unless otherwise specified, the embodiments and features described in the embodiments of the present invention can be combined with each other.

[0039] This invention addresses the existing problems by providing a method for preparing a carbon-modified manganese iron phosphate cathode material using vegetable oil, comprising the following steps:

[0040] Step 1): Place the lithium source, manganese source, iron source, phosphorus source and solid carbon source in a ball mill jar and dry mill to obtain a mixture; then add vegetable oil and ethanol solvent to the mixture to make the solid content of the ball mill 15-60% (e.g. 15%, 30%, 40%, 50% or 60%), and wet mill to obtain a slurry for later use.

[0041] The volume of the added vegetable oil is 6-18% of the volume of the ethanol solvent (e.g., 6%, 8%, 10%, 12%, 14%, 16% or 18%); the solid carbon source is polyvinyl alcohol and / or sucrose.

[0042] Step 2): Dry the slurry obtained in Step 1) to obtain a dried product for later use;

[0043] Step 3): Grind the dried material obtained in Step 2), sieve it, and then sinter it at high temperature;

[0044] Step 4): After cooling to room temperature in the furnace, the material is ground and sieved to obtain carbon-modified lithium manganese iron phosphate cathode material.

[0045] In a preferred embodiment of the present invention, step 1) is carried out according to the chemical formula Li 1-1.05 Fe 0.3-0.7 Mn 0.3-0.7 PO4&C 0.5 Weigh out each raw material, namely the mixed raw material lithium source, manganese source, iron source, phosphorus source, and solid carbon source, and find that the molar ratio of lithium, iron, manganese, phosphorus, and carbon is 1-1.05:0.3-0.7:0.3-0.7:1:0.5 (for example, 1:0.5:0.5:1:0.5, 1:0.3:0.3:1:0.5, or 1.05:0.7:0.7:1:0.5).

[0046] In a preferred embodiment of the present invention, the solid carbon source is polyvinyl alcohol and sucrose; the molar ratio of carbon in polyvinyl alcohol and sucrose is (0.02-0.03):(0.07-0.13) (for example, 0.02:0.13, 0.028:0.082 or 0.03:0.07).

[0047] In a preferred embodiment of the present invention, the solid content of the ball mill is 40-60% (e.g., 40%, 50%, or 60%); the volume of the added vegetable oil is 6-12% (e.g., 6%, 8%, 10%, or 12%) of the volume of the ethanol solvent.

[0048] In a preferred embodiment of the present invention, during dry grinding, the ball milling time is 3-5 hours (e.g., 3 hours, 4 hours, or 5 hours), and the ball milling speed is 300-500 r / min (e.g., 300 r / min, 400 r / min, or 500 r / min); during wet grinding, the ball milling time is 3-5 hours (e.g., 3 hours, 4 hours, or 5 hours), and the ball milling speed is 300-500 r / min (e.g., 300 r / min, 400 r / min, or 500 r / min).

[0049] In a preferred embodiment of the present invention, the lithium source is any one or a mixture of lithium dihydrogen phosphate, lithium carbonate, and lithium hydroxide monohydrate;

[0050] The manganese source is any one or a mixture of manganese carbonate, manganese dihydrogen phosphate, manganese dioxide, and manganese tetroxide;

[0051] The iron source is ferrous oxalate dihydrate and / or ferric phosphate;

[0052] The phosphorus source is any one or a mixture of phosphoric acid, ammonium dihydrogen phosphate, manganese dihydrogen phosphate, and lithium dihydrogen phosphate.

[0053] In a preferred embodiment of the present invention, the total volume of the slurry does not exceed 1 / 3 of the volume of the ball mill jar; the grinding beads are zirconia beads, and the mass of the zirconia beads is 5-7 times (e.g., 5 times, 6 times or 7 times) the total mass of the raw materials in the ball mill jar.

[0054] In a preferred embodiment of the present invention, step 2) drying is vacuum drying or room temperature stirring drying; vacuum drying is carried out in a vacuum drying oven at a drying temperature of 80°C for a drying time of ≤24h; room temperature stirring drying is carried out at a stirring speed of 400-600r / min (e.g., 400r / min, 500r / min or 600r / min).

[0055] In a preferred embodiment of the present invention, the specific method of step 3) is as follows: after grinding, the material is passed through a 100-mesh sieve and placed in a tube furnace. Under an argon atmosphere, high-temperature sintering is carried out. The temperature is raised to 300-500℃ (e.g., 300℃, 400℃ or 500℃) at a heating rate of 5℃ / min and held for 4-6 hours (e.g., 4 hours, 5 hours or 6 hours). Then, the temperature is raised to 600-700℃ (e.g., 600℃, 650℃ or 700℃) and held for 7-9 hours (e.g., 7 hours, 8 hours or 9 hours).

[0056] In a preferred embodiment of the present invention, the flow rate of the argon atmosphere is 200 mL / min; the temperature is first raised to 400°C and held for 5 hours, and then raised to 650°C and held for 8 hours.

[0057] In a preferred embodiment of the present invention, after grinding in step 4), the material is passed through a 200-mesh sieve.

[0058] This invention provides a lithium manganese iron phosphate cathode material, which is prepared by the aforementioned preparation method.

[0059] This invention provides an application of the lithium manganese iron phosphate cathode material in lithium-ion batteries.

[0060] The following detailed embodiments illustrate the present invention's carbon-modified manganese iron lithium cathode material using vegetable oil composites, its preparation method, and its applications.

[0061] Example 1

[0062] This embodiment provides a method for preparing a carbon-modified manganese iron phosphate cathode material using vegetable oil composites. The specific steps are as follows:

[0063] Step 1): Place the lithium source, manganese source, iron source, phosphorus source, and solid carbon source in a ball mill jar and dry mill for 4 hours at a speed of 400 r / min to obtain a mixture;

[0064] Among them, the lithium and phosphorus sources are lithium dihydrogen phosphate, the manganese source is manganese carbonate, and the iron source is ferrous oxalate dihydrate; the solid carbon sources are polyvinyl alcohol and sucrose; according to the chemical formula LiFe 0.5 Mn 0.5 PO4&C 0.5 The molar ratio of lithium, iron, manganese, phosphorus, carbon in polyvinyl alcohol, and carbon in sucrose is 1:0.5:0.5:1:0.028:0.082. Weigh out each raw material.

[0065] Then, add liquid carbon source (vegetable oil) and ethanol solvent to the mixture to make the solid content of ball milling 40% (mass percentage). The volume of vegetable oil added is 6% of the volume of ethanol solvent. Then wet mill for 4 hours at a speed of 400 r / min to obtain slurry for later use.

[0066] The total volume of the slurry is 1 / 3 of the volume of the ball mill jar; the grinding beads are zirconia beads, and the mass of the zirconia beads is 6 times the total mass of the raw material in the ball mill jar;

[0067] Step 2): Place the slurry obtained in Step 1) in a vacuum drying oven at 80°C for 24 hours to obtain the dried product for later use.

[0068] Step 3): Grind the dried material obtained in Step 2), pass it through a 100-mesh sieve, and place it in a tube furnace. Under an argon atmosphere (gas flow rate of 200 mL / min), perform high-temperature sintering. The temperature is increased to 400℃ at a heating rate of 5℃ / min and held for 5 hours, and then increased to 650℃ and held for 8 hours.

[0069] Step 4): After cooling to room temperature in the furnace, grind the material and pass it through a 200-mesh sieve to obtain carbon-modified lithium manganese iron phosphate cathode material.

[0070] Example 2

[0071] This embodiment provides a method for preparing a carbon-modified manganese iron lithium cathode material using vegetable oil. The only difference from Embodiment 1 is that in step 1), the volume of vegetable oil added is 12% of the volume of ethanol solvent. Other parameters and steps are the same as in Embodiment 1.

[0072] Example 3

[0073] This embodiment provides a method for preparing a carbon-modified manganese iron lithium cathode material using vegetable oil. The only difference from Embodiment 1 is that in step 1), the volume of vegetable oil added is 18% of the volume of ethanol solvent. Other parameters and steps are the same as in Embodiment 1.

[0074] Example 4

[0075] This embodiment provides a method for preparing a carbon-modified lithium manganese iron phosphate cathode material using vegetable oil composite. The only difference from Embodiment 1 is that in step 1), after adding liquid carbon source (vegetable oil) and ethanol solvent, the solid content of the ball mill is 15% (mass percentage). Other parameters and steps are the same as in Embodiment 1.

[0076] Example 5

[0077] This embodiment provides a method for preparing a carbon-modified lithium manganese iron phosphate cathode material using vegetable oil composite. The only difference from Embodiment 1 is that in step 1), after adding liquid carbon source (vegetable oil) and ethanol solvent, the solid content of the ball mill is 60% (mass percentage). Other parameters and steps are the same as in Embodiment 1.

[0078] Compare with Example 1

[0079] The only difference between this comparative example and Example 1 is that no vegetable oil was added, only ethanol solvent was added. That is, in step 1), the volume of vegetable oil added was 0% of the volume of ethanol solvent. Other parameters and steps are the same as in Example 1.

[0080] Compare with Example 2

[0081] The only difference between this comparative example and Example 1 is that the vegetable oil is replaced with glycerin, i.e., in step 1), glycerin and ethanol solvent are added to the mixture to make the solid content of the ball mill 40% (mass percentage), wherein the volume of glycerin added is 6% of the volume of ethanol solvent. Other parameters and steps are the same as in Example 1.

[0082] Compare with Example 3

[0083] The only difference between this comparative example and Example 1 is that the vegetable oil is replaced with mineral oil, i.e., in step 1), mineral oil and ethanol solvent are added to the mixture to make the solid content of the ball mill 40% (mass percentage), wherein the volume of the added mineral oil is 6% of the volume of the ethanol solvent. Other parameters and steps are the same as in Example 1.

[0084] Application Example 1

[0085] The positive electrode materials obtained in Examples 1-5 and Comparative Examples 1-3 were dried with acetylene black (AB) and binder (PVDF) in a drying oven at 60°C for 2 hours, respectively. They were then mixed uniformly at a mass ratio of 8:1:1, and an appropriate amount of N-methylpyrrolidone (NMP) was added and stirred thoroughly to form a uniform paste. This paste was then uniformly coated onto aluminum foil and finally dried in a vacuum drying oven at 90°C for 12 hours to obtain the positive electrode sheet. The positive electrode sheet, lithium sheet, separator (Celgard 2325), and electrolyte (EC / DMC / EMC / VC of 1 mol / L LiPF6) were assembled into a CR2016 type button cell in an argon-filled hand-packing box and sealed using a sealing machine.

[0086] Rate and cycle tests were conducted: After the button cells were left at room temperature for 12 hours, charge-discharge tests were performed using the Shenzhen Xinwei Battery Testing System (CT4008W). The room temperature was 25℃, and the charge / discharge cutoff voltage was 2.0–4.5V. First, activation was achieved by cycling for 5 cycles at a low rate of 0.1C, and the discharge specific capacity at different rates was measured. Then, cycle performance tests were conducted at 1C (50 cycles) to obtain the capacity retention rate of the material. The results are shown in Table 1. Figure 1 , Figure 2 As shown.

[0087] Table 1: Electrochemical performance test results of Comparative Examples 1-3 and Examples 1-5

[0088]

[0089] Figure 1 , Figure 2 The electrochemical performance test results for Comparative Example 1 and Examples 1-3 are shown in Table 1. The performance and capacity retention at each rate are also shown in Table 1. Comparison of Comparative Example 1 and Examples 1-3 reveals that the discharge specific capacity at different rates increases with increasing vegetable oil content. However, at 1C rate, long-cycle operation is optimal when the vegetable oil content relative to ethanol is 12% by volume. When this content is further increased to 18%, the capacity retention decreases. This is because excessive carbon content makes interfacial accumulation more likely during cycling, and the uneven distribution of carbon leads to a gradual increase in irreversible phase transitions, thus exacerbating capacity decay.

[0090] By comparing Examples 1, 4, and 5, it can be found that as the solid content increases, the discharge specific capacity at different rates all show an increasing trend. However, the capacity retention rate during long-term cycling at 1C rate is basically the same. This is because, during the vacuum drying stage, a large amount of solvent evaporates, which also causes some loss of the liquid carbon source. Therefore, when the solid content decreases, the vacuum drying stage will lose more liquid carbon source, which will affect the effective utilization of lithium manganese phosphate and reduce the initial energy (first discharge specific capacity).

[0091] By comparing Example 1, Comparative Example 2, and Comparative Example 3, it can be found that after changing the type of liquid carbon source, the discharge specific capacity at different rates all showed a decreasing trend, and the capacity retention rate during long cycles at 1C rate also decreased. This is because mineral oil has more impurities and poor viscosity; while glycerol has a low carbon content and is easily lost during sintering. After changing the liquid carbon source, the reduction in the amount of carbon source introduced and the insufficient distribution of carbon source caused a decrease in both material capacity and cycle performance.

[0092] Meanwhile, the effective introduction of carbon improves the electrochemical reaction performance of lithium manganese phosphate relative to Li. + The electrode potential of / Li was measured. In Example 1, the median charging voltage was 4.0634V and the median discharging voltage was 3.5470V; in Control Example 1, the median charging voltage was 4.0258V and the median discharging voltage was 3.5408V. It is evident that the further introduction of a liquid carbon source significantly improves the median voltage of lithium manganese iron phosphate, overcoming the low potential limitation of lithium iron phosphate and enabling the fabrication of high-energy-density batteries.

[0093] Application Example 2

[0094] SEM and EDS tests were performed on the samples of Control Example 1 and Examples 1-3, and the results are as follows: Figure 3-6 As shown in Table 2.

[0095] Table 2: Surface scan carbon atom percentage parameters in EDS tests of Comparative Example 1 and Examples 1-3

[0096] sample C atoms / % Compare with Example 1 27.01 Example 1 32.46 Example 2 35.93 Example 3 39.53

[0097] Table 2 shows the carbon content test results of surface scan EDS for Comparative Example 1 and Examples 1-3. It can be observed that the carbon atom content gradually increases with the increase in the amount of vegetable oil introduced. Figure 3-6 The images are scanning electron microscope (SEM) images of Comparative Example 1 and Examples 1-3. Lithium manganese iron phosphate exhibits a polyhedral morphology, with those completely coated with carbon appearing as spherical shapes. As can be seen from the SEM images, when the amount of vegetable oil added is 18%, it is difficult to find lithium manganese iron phosphate coated with carbon in the SEM image. This is because the high carbon content makes the material more prone to agglomeration and also makes it easier for carbon to migrate and accumulate at the interface during the electrochemical reaction, resulting in a decrease in cycle performance.

[0098] From Comparative Example 1 ( Figure 3 ) and Example 1 ( Figure 4 The comparison of SEM images shows that the introduction of liquid carbon source results in a more uniform distribution of carbon source, with more carbon source existing in the form of coated lithium manganese iron phosphate. This helps to reduce contact with the electrolyte during the electrochemical reaction and reduce the dissolution of manganese. Therefore, Example 1 has a higher capacity retention rate.

[0099] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a lithium iron manganese phosphate cathode material modified by composite carbon doping with vegetable oil, characterized in that, The method comprises the following steps: Step 1): a lithium source, a manganese source, an iron source, a phosphorus source and a solid carbon source are placed in a ball mill jar, and after dry grinding, a mixture is obtained; then plant oil and an ethanol solvent are added to the mixture, so that the solid content of the ball milling is 15-60%, and after wet grinding, a slurry is obtained for standby; The volume of the added plant oil is 6-12% of the volume of the ethanol solvent; the solid carbon source is polyvinyl alcohol and / or sucrose; Step 2): the slurry obtained in step 1) is dried to obtain a dried material for standby; Step 3): the dried material obtained in step 2) is ground, sieved, and then high-temperature sintered; Step 4): after cooling to room temperature in the furnace, grinding and sieving are performed, and then the lithium manganese iron phosphate positive electrode material is obtained. The specific method of step 3) is: after grinding, the material is sieved through a 100-mesh sieve and placed in a tube furnace, and high-temperature sintering is performed under an argon atmosphere with a gas flow rate of 200 mL / min, and the temperature is raised at a rate of 5 ℃ / min, first raised to 300-500 ℃ and kept for 4-6 h, and then raised to 600-700 ℃ and kept for 7-9 h.

2. The production method according to claim 1, wherein Step 1), according to the chemical formula Li 1-1.05 Fe 0.3- 0.7 Mn 0.3-0.7 PO 4 &C 0.5 , the amount of substance of lithium, iron, manganese, phosphorus and carbon elements in the mixed raw materials, lithium source, manganese source, iron source, phosphorus source and solid carbon source, is 1-1.05:0.3-0.7:0.3-0.7:1:0.

5. The solid carbon source is polyvinyl alcohol and sucrose; the molar ratio of carbon elements in polyvinyl alcohol to carbon elements in sucrose is (0.02-0.03):(0.07-0.13).

3. The production method according to claim 1, wherein The solid content of the ball milling is 40-60%; the volume of the added plant oil is 6-12% of the volume of the ethanol solvent.

4. The production method according to claim 1, wherein In step 1), during dry grinding, the ball milling time is 3-5 h, and the ball milling speed is 300-500 r / min; during wet grinding, the ball milling time is 3-5 h, and the ball milling speed is 300-500 r / min.

5. The production method according to claim 1, wherein The lithium source is a mixture of any one or more of lithium dihydrogen phosphate, lithium carbonate, and lithium hydroxide monohydrate; The manganese source is a mixture of any one or more of manganese carbonate, manganese dihydrogen phosphate, manganese dioxide, and trimanganese tetraoxide; The iron source is ferrous oxalate dihydrate and / or iron phosphate; The phosphorus source is a mixture of any one or more of phosphoric acid, ammonium dihydrogen phosphate, manganese dihydrogen phosphate, and lithium dihydrogen phosphate.

6. The production method according to claim 1, wherein In step 1), the total volume of the slurry does not exceed 1 / 3 of the volume of the ball mill jar; the ball milling beads are zirconia beads, and the mass of the zirconia beads is 5-7 times the total mass of the raw materials in the ball mill jar.

7. The production method according to claim 1, wherein In step 2), the drying is vacuum drying or room temperature stirring drying; the vacuum drying is performed in a vacuum drying oven at a drying temperature of 80 ℃ for ≤24 h; the stirring speed of the room temperature stirring drying is 400-600 r / min.

8. The production method according to claim 1, wherein After grinding in step 4), the material is sieved through a 200-mesh sieve.

9. A lithium iron manganese phosphate cathode material, characterized in that, The lithium manganese iron phosphate positive electrode material is prepared by the preparation method of any one of claims 1-8.

10. The application of the lithium manganese iron phosphate positive electrode material in a lithium ion battery according to claim 9.

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