A high-capacity coated lithium manganese iron phosphate cathode material

Abstract

The present invention belongs to the field of lithium battery technology and provides a high-capacity coated lithium iron manganese phosphate positive electrode material. The lithium iron manganese phosphate positive electrode material comprises lithium iron manganese phosphate particles and a hectorite / reduced graphene oxide composite material coated on the surface of the lithium iron manganese phosphate particles. The hectorite / reduced graphene oxide composite material is coated on the surface of the lithium iron manganese phosphate particles. The intercalation structure of the hectorite and reduced graphene oxide forms a uniformly coated conductive network, improving the conductivity of the coating layer. The hectorite not only improves the dispersibility of graphene, but also acts as a lithium supplement, replenishing lithium lost during the charge and discharge process, thereby increasing the battery capacity and cycle life. The lithium iron manganese phosphate positive electrode material prepared by the present invention has high capacity and excellent cycle performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and in particular relates to a high-capacity coated lithium manganese iron phosphate positive electrode material. Background Art

[0002] Lithium iron phosphate crystals are olivine-type, and the basic structural unit is composed of LiO6 octahedron, FeO6 octahedron and PO4 tetrahedron, among which FeO6 octahedron and PO4 tetrahedron are cross-linked to form a polyanion framework structure. + Transmission along a single b-axis. Lithium manganese iron phosphate is based on the structure of lithium iron phosphate and Mn element is added to achieve the combination of manganese and iron, taking advantage of the strengths and making up for the weaknesses. 2+ With Mn 2+ The ionic radii of the two materials are very similar. Lithium manganese iron phosphate relies on the synergistic effect between LiFePO4 and LiMnPO4 to form a stable and uniform solid solution, thus combining the stable electrochemical performance of LiFePO4 with the high potential of LiMnPO4. The Mn element can increase the discharge voltage to 4.1V, significantly improving the energy density of the positive electrode material.

[0003] Lithium manganese iron phosphate (LMFP) boasts approximately 15% higher energy density than lithium iron phosphate (LFP), offering advantages such as high energy density, improved safety, and lower cost. However, its application is still limited by its poor electrical conductivity. Currently, improvements to its electrical conductivity are primarily sought through modification methods, with mainstream approaches including surface coating, ion doping, and micromorphology control. Surface coatings include carbon coating, graphene coating, and metal oxide coating.

[0004] Due to its ultra-high electrical conductivity, graphene coating can effectively increase the material's electronic conductivity, thereby improving the battery's charge-discharge performance. However, graphene's flaky structure makes it prone to agglomeration when mixed with other materials, making it difficult to disperse evenly. This can affect the electrochemical performance of the resulting cathode material. Therefore, improving the electrochemical performance of graphene-coated cathode materials is a key issue that needs to be addressed. Summary of the Invention

[0005] The purpose of the present invention is to provide a high-capacity coated lithium manganese iron phosphate positive electrode material to improve the electrochemical performance of the graphene-coated lithium manganese iron phosphate positive electrode material.

[0006] The purpose of the present invention can be achieved through the following technical solutions:

[0007] The present invention provides a high-capacity coated lithium iron manganese phosphate positive electrode material, wherein the lithium iron manganese phosphate positive electrode material comprises lithium iron manganese phosphate particles and a hectorite / reduced graphene oxide composite material coated on the surface of the lithium iron manganese phosphate particles;

[0008] The chemical formula of the lithium manganese iron phosphate particles is LiFe x Mn y PO4, where 0.2≤x≤0.5, 0.5≤y≤0.8, x+y=1.

[0009] Furthermore, the preparation steps of the lithium manganese iron phosphate positive electrode material are as follows:

[0010] Step 1: mixing ferrous salt, manganese salt, reducing agent and water to form a first mixed solution;

[0011] Step 2: Under inert atmosphere, add the first mixed solution and ammonium oxalate solution into a reactor, heat and stir to react, maintain the reaction pH at 3-4, filter, wash and dry the reaction product to obtain a ferrous manganese oxalate precursor;

[0012] Step 3: Mix the ferrous manganese oxalate precursor, lithium salt and phosphate, use anhydrous ethanol as a medium, ball-mill under an inert atmosphere, dry the ball-milled slurry, and calcine under an inert atmosphere to obtain lithium iron manganese phosphate particles;

[0013] Step 4: mixing the hectorite aqueous dispersion and the graphene oxide dispersion at a mass ratio of hectorite to graphene oxide of 1:1-2, adding hydrazine hydrate to the mixed solution, and heating the reaction to reduce the graphene oxide to obtain a hectorite / reduced graphene oxide dispersion;

[0014] Step 5: Add lithium manganese iron phosphate particles to the hectorite / reduced graphene oxide dispersion, stir and mix, dry, and grind under an inert atmosphere to obtain a hectorite / reduced graphene oxide-coated lithium manganese iron phosphate positive electrode material.

[0015] The reaction equation in the above step 2 is:

[0016] xF 2+ +yMn 2+ +C2O4 2- →Fe x Mn y C2O4 (x+y=1);

[0017] The reaction equation in the above step 3 is:

[0018] Fe x Mn y C2O4+Li + +PO4 3- →LiFe x Mn y PO4+CO2.

[0019] Furthermore, the ferrous salt and manganese salt are prepared in a manganese to iron molar ratio of (1-4): 1. A lower Mn content cannot significantly increase the platform voltage, while a higher Mn content results in a low discharge capacity and rapid decay due to the John-Teller effect of the Mn substance.

[0020] Furthermore, the ferrous salt is at least one of ferrous sulfate and ferrous chloride.

[0021] Furthermore, the manganese salt is at least one of manganese sulfate and manganese chloride.

[0022] Furthermore, the reducing agent is at least one of citric acid and ascorbic acid.

[0023] Furthermore, the total concentration of ferrous ions and manganese ions in the first mixed solution is 0.5-1.5 mol / L, and the concentration of the reducing agent is 0.001-0.01 mol / L.

[0024] Furthermore, the concentration of the ammonium oxalate solution is 0.4-0.8 mol / L, and the ingredients are prepared according to a molar ratio of oxalate ions to metal ions of 1.05-1.1:1.

[0025] Furthermore, in step 2, the reaction temperature during heating and stirring is 50-60°C, the reaction time is 4-6 hours, and the reaction pH is controlled by dropwise addition of sulfuric acid and ammonia water. Both sulfuric acid and ammonia water are low-concentration solutions, with a sulfuric acid concentration of 1 mol / L and an ammonia water solution containing 2 wt% ammonia. Too high a concentration will cause a sharp increase in temperature during the dropwise addition, which is not conducive to controlling the reaction temperature.

[0026] Furthermore, the ferrous manganese oxalate precursor, lithium salt and phosphate are prepared according to a molar ratio of n(Fe+Mn):n(Li):n(PO4)=1:(1.05-1.1):1.

[0027] Furthermore, the lithium salt is at least one of lithium carbonate, lithium chloride and lithium hydroxide.

[0028] Furthermore, the phosphate is at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate and phosphoric acid.

[0029] Furthermore, in step three, the ball milling speed is 400-600 r / min, the ball milling time is 6-10 h, the calcination temperature is 400-800° C., and the calcination time is 10-16 h.

[0030] Furthermore, in step 4, the concentration of the hectorite aqueous dispersion is 4-5 mg / mL, the concentration of the graphene oxide dispersion is 0.5-1 mg / mL, the molar concentration of hydrazine hydrate is 85%, and the ingredients are prepared according to the mass ratio of graphene oxide to hydrazine hydrate of 10:(7-10). The heating reaction temperature is 85-95°C and the reaction time is 1-2h.

[0031] When hectorite is added to water, it forms a gel dispersion. The layered structure of hectorite makes it easy to dissociate in aqueous solution, forming a "house of cards" structure. When the graphene oxide dispersion is added to the mixture, an intercalation structure is formed due to electrostatic interaction and steric hindrance effect. After hydrazine hydrate reduction, the hectorite is evenly distributed on the surface of the reduced graphene oxide, achieving uniform dispersion of the reduced graphene oxide.

[0032] Furthermore, in step five, the mass ratio of the lithium manganese iron phosphate particles to the hectorite is 100:(0.1-0.5), the grinding speed is 400-600 r / min, and the grinding time is 2-4 h.

[0033] Beneficial effects of the present invention:

[0034] (1) The present invention coats the surface of lithium manganese iron phosphate particles prepared by a co-precipitation method with a hectorite / reduced graphene oxide composite material, and utilizes the intercalation structure of hectorite and reduced graphene oxide to form a uniformly coated conductive network, thereby improving the conductivity of the coating layer. In addition, the hectorite contains a high content of lithium, which can replenish the lithium lost during the charge and discharge process, thereby improving the battery capacity and cycle life.

[0035] (2) The present invention utilizes a sol-gel method to coat the surface of lithium iron manganese phosphate particles with a hectorite / reduced graphene oxide composite material. The hectorite aqueous dispersion is in a gel state, which can prevent the agglomeration of graphene oxide and achieve uniform dispersion of graphene oxide in the coating layer. When lithium iron manganese phosphate particles are added to form a coating layer, no further calcination is required. After drying and grinding, the lithium iron manganese phosphate positive electrode material can be obtained, thereby reducing energy consumption. DETAILED DESCRIPTION

[0036] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0037] Example 1

[0038] Preparation of lithium manganese iron phosphate positive electrode material:

[0039] Step 1: Weigh ferrous sulfate, manganese sulfate and citric acid, add them into water and mix to form a first mixed solution, wherein ferrous sulfate and manganese sulfate are weighed according to a manganese-iron molar ratio of 1:1, and citric acid is weighed at a concentration of 0.001 mol / L, and the total concentration of ferrous ions and manganese ions in the first mixed solution is controlled to be 1 mol / L.

[0040] Step 2: Under inert atmosphere protection, the first mixed solution and 0.5 mol / L ammonium oxalate solution are added to a reactor at a molar ratio of oxalate ion to metal ion of 1.05:1, and the mixture is stirred at 50° C. for 5 hours. During the reaction, 1 mol / L sulfuric acid or 2 wt% ammonia water is added dropwise to maintain the reaction pH at 3-4. The reaction product is precipitated and filtered, washed with anhydrous ethanol and deionized water, and then vacuum dried at 90° C. for 12 hours to obtain a ferrous manganese oxalate precursor;

[0041] Step 3: Mix the ferrous manganese oxalate precursor, lithium carbonate and ammonium dihydrogen phosphate in a molar ratio of n (Fe + Mn): n (Li): n (PO4) = 1: 1.05: 1, use anhydrous ethanol as a medium, and ball mill at 500r / min for 6h under an inert atmosphere. Dry the milled slurry at 80℃ for 6h, calcine at 400℃ for 2h under an inert atmosphere, and then heat it to 800℃ and calcine it for 10h to obtain lithium iron manganese phosphate particles. The chemical formula of lithium iron manganese phosphate particles is LiFe 0.5 Mn 0.5 PO4;

[0042] Step 4: 5 mg / mL of hectorite aqueous dispersion and 1 mg / mL of graphene oxide dispersion were mixed at a mass ratio of hectorite to graphene oxide of 1:1, hydrazine hydrate with a molar concentration of 85% was added to the mixed solution at a mass ratio of graphene oxide to hydrazine hydrate of 10:7, and the graphene oxide was reduced after heating at 85°C for 2 hours to obtain a hectorite / reduced graphene oxide dispersion;

[0043] Step 5. Add the lithium iron manganese phosphate particles to the lithium iron manganese phosphate / reduced graphene oxide dispersion at a mass ratio of m (lithium iron manganese phosphate particles): m (hectorite) = 100:0.1, stir and mix, and then dry. Grind at 400 r / min for 4 h under an inert atmosphere, and pass through a 300 mesh sieve to obtain the lithium iron manganese phosphate coated hectorite / reduced graphene oxide positive electrode material.

[0044] Example 2

[0045] The difference from Example 1 is that the mass ratio of lithium manganese iron phosphate particles m (lithium manganese iron phosphate particles): m (heterolithite) is changed to 100:0.3, and steps 1, 2, 3 and 4 are the same as those in Example 1.

[0046] Step 5. Add the lithium iron manganese phosphate particles to the lithium iron manganese phosphate / reduced graphene oxide dispersion at a mass ratio of m (lithium iron manganese phosphate particles): m (hectorite) = 100:0.3, stir and mix, and then dry. Grind at 400 r / min for 4 h under an inert atmosphere, and pass through a 300-mesh sieve to obtain the lithium iron manganese phosphate-coated hectorite / reduced graphene oxide positive electrode material.

[0047] Example 3

[0048] The difference from Example 1 is that the mass ratio of lithium manganese iron phosphate particles m (lithium manganese iron phosphate particles): m (heterolithite) is changed to 100:0.5, and steps 1, 2, 3 and 4 are the same as those in Example 1.

[0049] Step 5. Add the lithium iron manganese phosphate particles to the lithium iron manganese phosphate / reduced graphene oxide dispersion at a mass ratio of m (lithium iron manganese phosphate particles): m (hectorite) = 100:0.5, stir and mix, and then dry. Grind at 400 r / min for 4 h under an inert atmosphere, and pass through a 300 mesh sieve to obtain the lithium iron manganese phosphate coated hectorite / reduced graphene oxide positive electrode material.

[0050] Example 4

[0051] The difference from Example 2 is that the mass ratio of hectorite and graphene oxide is changed to 1:1.5, and steps 1, 2, 3 and 5 are the same as those in Example 2.

[0052] Step 4: 5 mg / mL of hectorite aqueous dispersion and 1 mg / mL of graphene oxide dispersion were mixed at a mass ratio of hectorite to graphene oxide of 1:1.5, and hydrazine hydrate with a molar concentration of 85% was added to the mixed solution at a mass ratio of graphene oxide to hydrazine hydrate of 10:7. The reaction was heated at 85°C for 2 hours to reduce the graphene oxide to obtain a hectorite / reduced graphene oxide dispersion.

[0053] Example 5

[0054] The difference from Example 2 is that the mass ratio of hectorite and graphene oxide is changed to 1:2, and steps 1, 2, 3 and 5 are the same as those in Example 2.

[0055] Step 4: 5 mg / mL of hectorite aqueous dispersion and 1 mg / mL of graphene oxide dispersion were mixed at a mass ratio of hectorite to graphene oxide of 1:2, and hydrazine hydrate with a molar concentration of 85% was added to the mixed solution at a mass ratio of graphene oxide to hydrazine hydrate of 10:7. The mixture was heated at 85°C for 2 hours to reduce the graphene oxide to obtain a hectorite / reduced graphene oxide dispersion.

[0056] Example 6

[0057] The difference from Example 4 is that the manganese-iron molar ratio is changed to 3:2, and steps 2, 4 and 5 are the same as those in Example 4.

[0058] Step 1, weighing ferrous sulfate, manganese sulfate and citric acid, adding them to water and mixing to form a first mixed solution, wherein the ferrous sulfate and manganese sulfate are weighed according to a manganese-iron molar ratio of 3:2, and the citric acid is weighed at a concentration of 0.001 mol / L, and the total concentration of ferrous ions and manganese ions in the first mixed solution is controlled to be 1 mol / L;

[0059] Step 3: Mix the ferrous manganese oxalate precursor, lithium carbonate and ammonium dihydrogen phosphate in a molar ratio of n (Fe + Mn): n (Li): n (PO4) = 1: 1.05: 1, use anhydrous ethanol as a medium, and ball mill at 500r / min for 6h under an inert atmosphere. Dry the milled slurry at 80℃ for 6h, calcine at 400℃ for 2h under an inert atmosphere, and then heat it to 800℃ and calcine it for 10h to obtain lithium iron manganese phosphate particles. The chemical formula of lithium iron manganese phosphate particles is LiFe 0.4 Mn 0.6 PO4.

[0060] Example 7

[0061] The difference from Example 4 is that the manganese-iron molar ratio is changed to 4:1, and steps 2, 4 and 5 are the same as those in Example 4.

[0062] Step 1, weighing ferrous sulfate, manganese sulfate and citric acid, adding them to water and mixing to form a first mixed solution, wherein the ferrous sulfate and manganese sulfate are weighed according to a manganese-iron molar ratio of 4:1, and the citric acid is weighed at a concentration of 0.001 mol / L, and the total concentration of ferrous ions and manganese ions in the first mixed solution is controlled to be 1 mol / L;

[0063] Step 3: Mix the ferrous manganese oxalate precursor, lithium carbonate and ammonium dihydrogen phosphate in a molar ratio of n (Fe + Mn): n (Li): n (PO4) = 1: 1.05: 1, use anhydrous ethanol as a medium, and ball mill at 500r / min for 6h under an inert atmosphere. Dry the milled slurry at 80℃ for 6h, calcine at 400℃ for 2h under an inert atmosphere, and then heat it to 800℃ and calcine it for 10h to obtain lithium iron manganese phosphate particles. The chemical formula of lithium iron manganese phosphate particles is LiFe 0.2 Mn 0.8 PO4.

[0064] Comparative Example 1

[0065] Preparation of conventional lithium manganese iron phosphate positive electrode material:

[0066] Step 1: Weigh ferrous sulfate, manganese sulfate and citric acid, add them into water and mix to form a first mixed solution, wherein ferrous sulfate and manganese sulfate are weighed according to a manganese-iron molar ratio of 1:1, and citric acid is weighed at a concentration of 0.001 mol / L, and the total concentration of ferrous ions and manganese ions in the first mixed solution is controlled to be 1 mol / L.

[0067] Step 2: Under inert atmosphere protection, the first mixed solution and 0.5 mol / L ammonium oxalate solution are added to a reactor at a molar ratio of oxalate ion to metal ion of 1.05:1, and the mixture is stirred at 50° C. for 5 hours. During the reaction, 1 mol / L sulfuric acid or 2 wt% ammonia water is added dropwise to maintain the reaction pH at 3-4. The reaction product is precipitated and filtered, washed with anhydrous ethanol and deionized water, and then vacuum dried at 90° C. for 12 hours to obtain a ferrous manganese oxalate precursor;

[0068] Step 3. Mix the ferrous manganese oxalate precursor, lithium carbonate and ammonium dihydrogen phosphate in a molar ratio of n(Fe+Mn):n(Li):n(PO4)=1:1.05:1, use anhydrous ethanol as a medium, and ball mill at 500r / min under an inert atmosphere for 6h. Dry the milled slurry at 80℃ for 6h, calcine at 400℃ for 2h under an inert atmosphere, then heat to 800℃ and calcine for 10h. Grind and pass through a 300-mesh sieve to obtain a lithium manganese iron phosphate positive electrode material.

[0069] Comparative Example 2

[0070] Preparation of conventional carbon-coated lithium manganese iron phosphate cathode material:

[0071] Preparation of lithium manganese iron phosphate positive electrode material:

[0072] Step 1: Weigh ferrous sulfate, manganese sulfate and citric acid, add them into water and mix to form a first mixed solution, wherein ferrous sulfate and manganese sulfate are weighed according to a manganese-iron molar ratio of 1:1, and citric acid is weighed at a concentration of 0.001 mol / L, and the total concentration of ferrous ions and manganese ions in the first mixed solution is controlled to be 1 mol / L.

[0073] Step 2: Under inert atmosphere protection, the first mixed solution and 0.5 mol / L ammonium oxalate solution are added to a reactor at a molar ratio of oxalate ion to metal ion of 1.05:1, and the mixture is stirred at 50° C. for 5 hours. During the reaction, 1 mol / L sulfuric acid or 2 wt% ammonia water is added dropwise to maintain the reaction pH at 3-4. The reaction product is precipitated and filtered, washed with anhydrous ethanol and deionized water, and then vacuum dried at 90° C. for 12 hours to obtain a ferrous manganese oxalate precursor;

[0074] Step 3. Mix the ferrous manganese oxalate precursor, lithium carbonate and ammonium dihydrogen phosphate in a molar ratio of n(Fe+Mn):n(Li):n(PO4)=1:1.05:1 to obtain a mixture, add glucose at a mass fraction of 10%, use anhydrous ethanol as a medium, and ball mill at 500r / min for 6h under an inert atmosphere. Dry the ball-milled slurry at 80℃ for 6h, calcine at 400℃ for 2h under an inert atmosphere, then heat to 800℃ and calcine for 10h. Grind and pass through a 300-mesh sieve to obtain a carbon-coated lithium manganese iron phosphate positive electrode material.

[0075] Comparative Example 3

[0076] The only difference from Example 1 is that hectorite is not added during the preparation.

[0077] Preparation of lithium manganese iron phosphate positive electrode material:

[0078] Step 1: Weigh ferrous sulfate, manganese sulfate and citric acid, add them into water and mix to form a first mixed solution, wherein ferrous sulfate and manganese sulfate are weighed according to a manganese-iron molar ratio of 1:1, and citric acid is weighed at a concentration of 0.001 mol / L, and the total concentration of ferrous ions and manganese ions in the first mixed solution is controlled to be 1 mol / L.

[0079] Step 2: Under inert atmosphere protection, the first mixed solution and 0.5 mol / L ammonium oxalate solution are added to a reactor at a molar ratio of oxalate ion to metal ion of 1.05:1, and the mixture is stirred at 50° C. for 5 hours. During the reaction, 1 mol / L sulfuric acid or 2 wt% ammonia water is added dropwise to maintain the reaction pH at 3-4. The reaction product is precipitated and filtered, washed with anhydrous ethanol and deionized water, and then vacuum dried at 90° C. for 12 hours to obtain a ferrous manganese oxalate precursor;

[0080] Step 3: Mix the ferrous manganese oxalate precursor, lithium carbonate and ammonium dihydrogen phosphate in a molar ratio of n (Fe + Mn): n (Li): n (PO4) = 1: 1.05: 1, use anhydrous ethanol as a medium, and ball mill at 500r / min for 6h under an inert atmosphere. Dry the milled slurry at 80℃ for 6h, calcine at 400℃ for 2h under an inert atmosphere, and then heat it to 800℃ and calcine it for 10h to obtain lithium iron manganese phosphate particles. The chemical formula of lithium iron manganese phosphate particles is LiFe 0.5 Mn 0.5 PO4;

[0081] Step 4: preparing a 1 mg / mL graphene oxide dispersion, adding hydrazine hydrate with a molar concentration of 85% according to a mass ratio of graphene oxide to hydrazine hydrate of 10:7, heating the reaction at 85° C. for 2 h to reduce the graphene oxide, and obtaining a reduced graphene oxide dispersion;

[0082] Step 5. Add the lithium iron manganese phosphate particles to the reduced graphene oxide dispersion at a mass ratio of m (lithium iron manganese phosphate particles): m (graphene oxide) = 100:0.1, stir and mix, and then dry. Grind at 400 r / min for 4 hours under an inert atmosphere, and pass through a 300-mesh sieve to obtain a reduced graphene oxide-coated lithium iron manganese phosphate positive electrode material.

[0083] The performance tests of the positive electrode materials prepared in Examples 1 to 7 and Comparative Examples 1 to 3 were performed. The tests were performed using button batteries. The positive electrode materials used were the lithium manganese iron phosphate positive electrode materials prepared in Examples 1-7 and Comparative Examples 1-3. The binder used polytetrafluoroethylene, the conductive agent used conductive carbon black, the solvent used N-methylpyrrolidone (NMP), the mass ratio of the positive electrode material, the binder, and the conductive agent was 8:1:1, the negative electrode used a lithium sheet, the separator used a Celgard 2500 composite membrane, and the electrolyte used 1 mol / L LiPF6 (lithium hexafluorophosphate). The button battery assembly was completed in an argon atmosphere glove box. The test reference was T / CIAPS0029-2023 "Lithium manganese iron phosphate positive electrode material for lithium ion batteries". The results are shown in Table 1:

[0084] Table 1

[0085]

[0086] As shown in Table 1, the ratio of coating material to lithium iron manganese phosphate particles was adjusted in Examples 1-3. In Example 2, the ratio of m(lithium iron manganese phosphate particles):m(hectorite) was 100:0.3, resulting in a positive electrode material with a 0.1C initial discharge capacity of 153 mAh / g. However, in Example 3, the excessive coating material reduced the tap density of the positive electrode material, resulting in inferior electrochemical performance compared to Example 2. Based on Example 2, Examples 4 and 5 increased the proportion of graphene oxide in the coating material, further improving the conductivity of the positive electrode material and facilitating electron transfer. However, in Example 5, the excessive graphene oxide partially agglomerated due to limited dispersion, resulting in uneven coating. In Examples 6 and 7, increasing the manganese to iron ratio and the Mn content increased the operating voltage of the lithium iron manganese phosphate, thereby improving the battery's energy density. However, excessive Mn can trigger the Jahn-Teller effect, affecting cycling performance. Combining the results of Comparative Example 1, Comparative Example 2 and the embodiment, it can be seen that the lithium manganese iron phosphate positive electrode material prepared in the embodiment of the present invention has high capacity and excellent cycle performance. Combining Comparative Example 3 and Example 1, it can be seen that, in comparison, the specific capacity of the positive electrode materials coated with hectorite and graphene is higher than that of the graphene-coated positive electrode material, and hectorite has an improvement effect on the electrochemical performance of the graphene-coated positive electrode material.

[0087] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus.

[0088] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A high-capacity coated lithium manganese iron phosphate positive electrode material, characterized in that: The lithium iron manganese phosphate positive electrode material includes lithium iron manganese phosphate particles and a hectorite / reduced graphene oxide composite material coated on the surface of the lithium iron manganese phosphate particles; The chemical formula of the lithium manganese iron phosphate particles is LiFe x Mn y PO4, where 0.2≤x≤0.5, 0.5≤y≤0.8, x+y=1.

2. A high-capacity coated lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The steps for preparing the lithium manganese iron phosphate positive electrode material are as follows: Step 1: mixing ferrous salt, manganese salt, reducing agent and water to form a first mixed solution; Step 2: Under inert atmosphere, add the first mixed solution and ammonium oxalate solution into a reactor, heat and stir to react, maintain the reaction pH at 3-4, filter, wash and dry the reaction product to obtain a ferrous manganese oxalate precursor; Step 3: Mix the ferrous manganese oxalate precursor, lithium salt and phosphate, use anhydrous ethanol as a medium, ball-mill under an inert atmosphere, dry the ball-milled slurry, and calcine under an inert atmosphere to obtain lithium iron manganese phosphate particles; Step 4: mixing the hectorite aqueous dispersion and the graphene oxide dispersion at a mass ratio of hectorite to graphene oxide of 1:1-2, adding hydrazine hydrate to the mixed solution, and heating the reaction to reduce the graphene oxide to obtain a hectorite / reduced graphene oxide dispersion; Step 5: Add lithium manganese iron phosphate particles to the hectorite / reduced graphene oxide dispersion, stir and mix, dry, and grind under an inert atmosphere to obtain a hectorite / reduced graphene oxide-coated lithium manganese iron phosphate positive electrode material.

3. A high-capacity coated lithium manganese iron phosphate positive electrode material according to claim 2, characterized in that: The ferrous salt and the manganese salt are prepared according to a molar ratio of manganese to iron of (1-4):

1.

4. The high-capacity coated lithium manganese iron phosphate positive electrode material according to claim 2, characterized in that: The total concentration of ferrous ions and manganese ions in the first mixed solution is 0.5-1.5 mol / L, and the concentration of the reducing agent is 0.001-0.01 mol / L.

5. The high-capacity coated lithium manganese iron phosphate positive electrode material according to claim 2, characterized in that: The concentration of the ammonium oxalate solution is 0.4-0.8 mol / L, and the ingredients are prepared according to a molar ratio of oxalate ions to metal ions of 1.05-1.1:

1.

6. The high-capacity coated lithium manganese iron phosphate positive electrode material according to claim 2, characterized in that: In step 2, the reaction temperature of the heating and stirring is 50-60° C., the reaction time is 4-6 hours, and the reaction pH is maintained by dropwise adding sulfuric acid and ammonia water to control the reaction pH.

7. The high-capacity coated lithium manganese iron phosphate positive electrode material according to claim 2, characterized in that: The ferrous manganese oxalate precursor, lithium salt and phosphate are prepared according to a molar ratio of n(Fe+Mn):n(Li):n(PO4)=1:(1.05-1.1):

1.

8. The high-capacity coated lithium manganese iron phosphate positive electrode material according to claim 2, characterized in that: In step 3, the ball milling speed is 400-600 r / min, the ball milling time is 6-10 h, the calcination temperature is 400-800° C., and the calcination time is 10-16 h.

9. The high-capacity coated lithium manganese iron phosphate positive electrode material according to claim 2, characterized in that: In step 4, the concentration of the hectorite aqueous dispersion is 4-5 mg / mL, the concentration of the graphene oxide dispersion is 0.5-1 mg / mL, the molar concentration of hydrazine hydrate is 85%, and the ingredients are prepared according to the mass ratio of graphene oxide to hydrazine hydrate of 10: (7-10). The heating reaction temperature is 85-95 ° C, and the reaction time is 1-2 h.

10. The high-capacity coated lithium manganese iron phosphate positive electrode material according to claim 2, characterized in that: In step 5, the mass ratio of the lithium manganese iron phosphate particles to the hectorite is 100:(0.1-0.5), the grinding speed is 400-600 r / min, and the grinding time is 2-4 h.

Images