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

Abstract

The application discloses a lithium iron manganese phosphate composite material and a preparation method thereof, and belongs to the technical field of lithium ion batteries. The lithium iron manganese phosphate composite material is synthesized, wherein the lithium iron manganese phosphate composite material comprises an organic acid manganese iron precursor with carbon nanotubes embedded therein, which is synthesized by a coprecipitation reaction method, and the lithium iron manganese phosphate composite material with carbon nanotubes embedded therein and an outer carbon layer is synthesized by high-temperature calcination. The lithium iron manganese phosphate composite material synthesized in this way guarantees a high compaction density, and the carbon nanotubes inside the particles effectively improve the transmission efficiency of electrons / ions in the particle interior; in the high-temperature calcination process, the particle crystallinity becomes high, and the carbon layer formed on the particle surface can effectively improve the conductivity of electrons / ions between the particles. When the lithium iron manganese phosphate composite material is used as a positive electrode material, the good conductivity can ensure that the battery has good electrochemical performance, and the high compaction density can improve the volume energy density of the battery.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to a lithium manganese iron phosphate composite material and its preparation method. Background Technology

[0002] In recent years, LiFe y Mn 1-y LiFePO4, a material in the same group as LiFePO4 and LiMnPO4, combines the superior structural stability and high safety of LiFePO4 with the high operating voltage and high specific energy of LiMnPO4, making it an ideal cathode material for next-generation lithium-ion batteries. It is widely used in commercial fields such as electric vehicles, power tools, and energy storage. However, LiFe... y Mn 1-y PO4 materials have drawbacks such as low electronic / ionic conductivity and short lifetime due to Mn precipitation during cycling, which greatly limit their application in advanced energy storage fields with high energy density, high power, and fast response.

[0003] Currently, regarding LiFe y Mn 1-y The main solutions to the aforementioned drawbacks of PO4 are: (1) particle nanostructuring, (2) designing special structures / morphologies, and (3) constructing electron / ion conductive networks. To improve the performance of LiFe... y Mn 1-y The performance of PO4 is usually achieved by using one or more processing methods in combination.

[0004] The invention with authorization number CN115321507B discloses a method for preparing ferromanganese phosphate by coprecipitation and its application. A ferricyanide solution, a manganese salt solution, and a mixed solution of phosphoric acid and perchloric acid are prepared separately. The ferricyanide solution, manganese salt solution, mixed solution and alkaline solution are added to the bottom liquid in a parallel flow to react and the precipitate is ferromanganese phosphate. Then, it is mixed with lithium hydroxide and glucose and calcined to obtain manganese-assisted carbon-coated lithium manganese iron phosphate particles.

[0005] Patent application CN116374981A discloses a preparation method and application of in-situ growth of lithium manganese iron phosphate on the surface of conductive carbon microspheres to form a three-dimensional conductive network. The preparation method includes: first, preparing conductive carbon microspheres and mixing them with lithium manganese iron phosphate precursor solution, and then carrying out a hydrothermal reaction to obtain lithium manganese iron phosphate precursor; and then sintering the lithium manganese iron phosphate precursor at high temperature to obtain lithium manganese iron phosphate cathode material.

[0006] The invention disclosed in patent CN111740104B provides a method for preparing lithium iron manganese phosphate / carbon nanotube composite cathode material. Compared with traditional methods, this invention utilizes an iron-based catalyst to induce in-situ growth of well-dispersed carbon nanotubes, which are then used as raw materials to prepare lithium iron phosphate / carbon nanotube composite cathode material. Potassium permanganate is added to accelerate the oxidation of iron. This material exhibits good structural and thermal stability, high electrical conductivity, small particle size, and uniform distribution, effectively improving the cycle performance and rate performance of lithium iron manganese phosphate material, and contributing to the further industrial application of lithium iron manganese phosphate material.

[0007] The above-mentioned synthesis processes each have their own advantages and disadvantages. Among them, the co-precipitation method is simple, produces larger particle sizes, and has a higher compaction density, but the large particles suffer from poor conductivity. The in-situ growth of nanoparticles can effectively improve the conductivity of the material, but the compaction density is low, resulting in a low volumetric energy density. Although carbon material composites can effectively improve the conductivity between lithium manganese iron phosphate particles, they cannot improve the electron / ion transport efficiency within the particles. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention provides a lithium manganese iron phosphate composite material and its preparation method. A carbon nanotube-embedded organic manganese iron precursor is synthesized by co-precipitation. Subsequently, the precursor is calcined at high temperature with a lithium source, a phosphorus source, and a carbon source to finally synthesize a lithium manganese iron phosphate composite material with embedded carbon nanotubes and an outer carbon coating.

[0009] The specific technical solution of the present invention is as follows:

[0010] A method for preparing a lithium manganese iron phosphate composite material, wherein the lithium manganese iron phosphate composite material comprises lithium manganese iron phosphate material with embedded carbon nanotubes and a surface coated with a carbon layer, and the lithium manganese iron phosphate material is composed of LiFe. y Mn 1-y PO4, where 0.2 ≤ y ≤ 0.4,

[0011] The carbon nanotubes constitute 0.5-1.5 wt% of the lithium manganese iron phosphate composite material by mass; the carbon layer constitutes 0.5-1.5 wt% of the lithium manganese iron phosphate composite material by mass.

[0012] The preparation method includes the following steps:

[0013] (1) Add manganese source and iron source to an aqueous solution of organic acid to obtain an organic acid manganese iron mixed solution;

[0014] (2) Add the carbon nanotubes modified with aqua regia solution to the organic acid aqueous solution and mix them evenly to obtain solution A;

[0015] (3) Add the organic acid manganese iron mixed solution obtained in step (1) to the solution A obtained in step (2), mix evenly to obtain solution B, use ammonia water to adjust the pH value of solution B to 2-5, react to obtain reactants, dry the reactants to obtain organic acid manganese iron with embedded carbon nanotubes.

[0016] (4) The organic manganese iron, lithium source, phosphorus source and carbon source of embedded carbon nanotubes obtained in step (3) are added to the dispersant in sequence and ground thoroughly to obtain a mixed slurry.

[0017] (5) The mixed slurry obtained in step (4) is dried and then calcined under an inert gas. After calcination, the lithium manganese iron phosphate composite material is obtained.

[0018] Current methods for improving conductivity primarily involve coating the carbon source onto the outside of LFMPs. This invention, however, constructs point channels within secondary particles. Aqua regia is used to acid-treat the carbon nanotubes, allowing hydroxyl and carboxyl functional groups to be attached to their surface. This improves the dispersion of the carbon nanotubes in organic acid solutions. By controlling the acid treatment conditions, the aspect ratio, specific surface area, and pore volume of the carbon nanotubes can be adjusted. Simultaneously, the number of carboxyl and hydroxyl groups on the carbon nanotube surface increases, resulting in a higher concentration of hydroxyl and carboxyl functional groups. The organic acid, acting as a precipitant and solvent, effectively dissolves manganese and iron salts, increasing the concentration of manganese and iron ions in the solution. These functional groups adsorb manganese and iron ions, inducing the uniform distribution of organic acid manganese-iron primary particles on the carbon nanotube surface, which is beneficial for the atomic-level distribution of manganese and iron.

[0019] Primary particles are continuously stirred and aggregated to grow, eventually forming secondary particles with micron-sized structures. These secondary particles contain embedded carbon nanotubes. After high-temperature calcination, the bonds between the primary lithium manganese iron phosphate particles and the carbon nanotubes, as well as between the primary particles themselves, become stronger. The presence of carbon nanotubes effectively improves electron / ion transport within the material. Furthermore, the good toughness and elastic modulus of carbon nanotubes enhance the structural stability of the secondary lithium manganese iron phosphate particles during battery charge-discharge cycles, thus improving the battery's electrochemical performance and cycle stability. Mixing and grinding the organic manganese iron phosphate precursor with embedded carbon nanotubes with phosphorus, lithium, and carbon sources improves the dispersion of each component and facilitates the formation of a uniform coating layer on the material surface after high-temperature calcination. This coating layer effectively improves the conductivity between materials and slows down manganese ion precipitation. The synthesized lithium manganese iron phosphate material has a micron-sized structure and exhibits high compaction density and volumetric energy density.

[0020] Specifically, the organic acid in the organic acid aqueous solution is at least one of ethylenediaminetetraacetic acid, polyacrylic acid, tartaric acid, citric acid, and oxalic acid.

[0021] The manganese source is at least one selected from manganese oxalate, manganese carbonate, manganese sulfate, manganese nitrate, and manganese acetate.

[0022] The iron source is at least one of ferrous oxalate, ferrous sulfate, and ferrous chloride.

[0023] The molar ratio of manganese in the manganese source to iron in the iron source is 1.5 to 4.

[0024] The concentration of the organic acid manganese iron mixed solution is 0.1-1 mol / L.

[0025] Specifically, in step (2), the treatment conditions for carbon nanotubes modified by aqua regia solution are: heating in a water bath at 80-95℃ for 0.5-1.5h.

[0026] The acid-treated carbon nanotubes have numerous hydroxyl and carboxyl functional groups on their surface. These functional groups can induce the primary particles to be uniformly distributed on both sides of the carbon nanotubes, which is beneficial for the uniform distribution of manganese and iron. The primary particles aggregate to form secondary particles with micron-sized structures, and the synthesized secondary particles contain embedded carbon nanotubes.

[0027] Even if carbon nanotubes are acid-treated with aqua regia, functional groups such as carboxyl and hydroxyl groups can be attached to the surface of the carbon nanotubes, which can improve the dispersion of carbon nanotubes in organic acid solutions. After the functional groups dissociate in the solution, their surfaces become electronegative and can adsorb manganese and iron ions in the organic acid solution. Thus, during co-precipitation, the metal phase will preferentially form on the outside of the carbon nanotubes, inducing the growth and aggregation of primary organic acid manganese iron particles on the surface of the carbon nanotubes. Through continuous stirring and aging, the carbon nanotubes are "locked" in the organic acid manganese iron precursor, and finally, secondary spherical organic acid manganese iron particles with embedded carbon nanotubes are formed. Point channels are constructed inside the secondary particles.

[0028] In step (3), the reaction temperature is 55-75℃ and the reaction time is 4-6h;

[0029] The drying temperature is 75–95°C, and the drying time is 8–12 hours.

[0030] In step (4), the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium phosphate, and lithium dihydrogen phosphate.

[0031] The phosphorus source is at least one selected from phosphoric acid, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, and lithium dihydrogen phosphate.

[0032] The carbon source is at least one selected from anhydrous glucose, sucrose, fructose, phenolic resin, polyethylene glycol, and polyvinyl alcohol.

[0033] The dispersant is an aqueous solution of polyethylene glycol.

[0034] Polyethylene glycol (PEG) affects the uniformity and dispersibility of lithium manganese iron phosphate cathode materials embedded with carbon nanotubes. PEG exhibits good dispersibility due to its high polarity, allowing it to strongly interact with solutes, dispersing them in the solvent and forming a stable dispersion system. This effectively improves the rheological properties of the suspension, enhances its stability, reduces its viscosity, and improves its flowability, thus achieving the purpose of dispersion. Furthermore, PEG possesses good anti-crystallization capabilities, preventing solute crystallization and thus contributing to the dispersion objective.

[0035] The lithium source, the organic manganese iron with embedded carbon nanotubes, and the phosphorus source are present in a molar ratio of nLi∶n(Mn+Fe)∶nPO4. 3- Add the following ratios: 1 to 1.02: 1.1 to 1.5: 1.

[0036] The grinding conditions are: grinding speed of 350-450 rpm / min and grinding time of 5-8 h.

[0037] In step (5), the calcination conditions are to maintain the temperature at 600-800℃ for 3-10 hours. The sintering atmosphere is nitrogen.

[0038] The present invention also provides a lithium manganese iron phosphate composite material prepared by the preparation method described above.

[0039] The present invention also provides the application of the aforementioned lithium manganese iron phosphate composite material in the preparation of lithium-ion batteries.

[0040] The present invention also provides a lithium-ion battery comprising the aforementioned lithium manganese iron phosphate composite material.

[0041] The beneficial effects of this invention are:

[0042] This invention synthesizes a lithium manganese iron phosphate composite material, which includes synthesizing an organic manganese iron precursor with embedded carbon nanotubes via a co-precipitation reaction, followed by high-temperature calcination to synthesize the lithium manganese iron phosphate composite material with embedded carbon nanotubes and an outer carbon coating. This synthesized lithium manganese iron phosphate composite material ensures high compaction density, while the carbon nanotubes within the particles effectively improve the electron / ion transport efficiency within the particles. During high-temperature calcination, the crystallinity of the particles increases, and the carbon layer formed on the particle surface effectively improves the conductivity of electrons / ions between particles. When used as a cathode material, the good conductivity of the lithium manganese iron phosphate composite material ensures good electrochemical performance of the battery, and the high compaction density improves the battery's volumetric energy density. Attached Figure Description

[0043] Figure 1 This is a scanning electron microscope (SEM) image of the manganese iron oxalate material with embedded carbon nanotubes provided in Example 1 of the present invention.

[0044] Figure 2 This is a feature diagram of the manganese iron oxalate material with embedded carbon nanotubes provided in Embodiment 1 of the present invention;

[0045] Figure 3 The image shown is a scanning electron microscope (SEM) image of the lithium manganese iron phosphate composite material prepared in Example 1 of this invention; the magnification is 5000x.

[0046] Figure 4 The image shown is a scanning electron microscope (SEM) image of the lithium manganese iron phosphate composite material prepared in Example 1 of this invention; the magnification is 10,000 times.

[0047] Figure 5 The image shown is a scanning electron microscope (SEM) image of the lithium manganese iron phosphate composite material prepared in Example 1 of this invention; the magnification is 50,000 times.

[0048] Figure 6 The image shown is a scanning electron microscope (SEM) image of the lithium manganese iron phosphate composite material prepared in Example 1 of this invention; the magnification is 100,000 times.

[0049] Figure 7 This is a characteristic diagram of the lithium manganese iron phosphate composite material prepared in Example 1 of the present invention;

[0050] Figure 8 The graph shows the electrochemical performance of the battery assembled in the range of 2.0V-4.5V provided in Embodiment 1 of the present invention at the first charge and discharge performance at 0.1C, 0.2C, 0.5C, 1C, and 2C. Detailed Implementation

[0051] Example 1

[0052] Step 1: Add manganese oxalate and ferrous oxalate to an oxalic acid aqueous solution at a molar ratio of Mn:Fe = 1.5 to prepare a 1 mol / L mixed solution; prepare a solution of concentrated nitric acid and concentrated hydrochloric acid at a volume ratio of 1:3.

[0053] Step 2: Place the beaker containing the prepared aqua regia solution in an 80°C water bath and heat with stirring. Then, add carbon nanotubes (0.5% of the lithium manganese iron phosphate composite material) to the 80°C aqua regia solution and heat and stir for 0.5 hours. After drying the modified carbon nanotubes at 75°C for 12 hours, add them to an organic acid aqueous solution to form solution A.

[0054] Step 3: Add the mixed solution from Step 1 to Solution A, and simultaneously add ammonia solution to Solution A to adjust the pH value to 2. Control the reaction temperature at 55℃ and the reaction time at 4-6 hours. After the reaction is complete, a precipitate is obtained.

[0055] Step 4: After filtering the precipitate and washing it multiple times with deionized water and ethanol, place it in a drying oven at 75°C for 12 hours to obtain organic manganese iron powder with embedded carbon nanotubes.

[0056] Step 5: Mix the organic manganese iron powder with embedded carbon nanotubes with lithium carbonate and lithium dihydrogen phosphate in a molar ratio of nLi∶n(Mn+Fe)∶nPO4. 3- =1.01∶1.3∶1 and 0.5wt% anhydrous glucose of lithium manganese iron phosphate composite material were added sequentially to an aqueous solution of polyethylene glycol and ground thoroughly at 350 rpm for 10 h.

[0057] Step 6: After grinding, the slurry is dried at 75°C for 12 hours. Then, the dried powder is placed in a box furnace and calcined at room temperature to 600°C for 10 hours under a nitrogen atmosphere. The resulting lithium manganese iron phosphate composite material is obtained after calcination.

[0058] The sintered lithium manganese iron phosphate composite material was ground, and the ground lithium manganese iron phosphate composite material, SP, and PVDF were weighed and mixed in a mass ratio of 8:1:1 and ground evenly.

[0059] Add 1 wt% NMP dropwise to the homogenized mixture while grinding to homogenize it.

[0060] The ground slurry is evenly coated onto aluminum foil to prepare the electrode sheet;

[0061] The coated electrode sheet is dried in a vacuum environment at 70°C for 12 hours to obtain the positive electrode of a lithium-ion battery.

[0062] SEM image of the organic manganese iron powder with embedded carbon nanotubes prepared in step 4 is shown below. Figure 1 As shown in the figure, almost no individual free carbon nanotubes are visible. The surface of the carbon nanotubes is completely covered with secondary manganese and iron particles. This indicates that while treating the original carbon nanotubes with aqua regia and mixed acid removes impurities, the strong oxidizing properties of the mixed acid easily erode the unstable five-membered rings and seven-membered rings of the carbon nanotubes, as well as the defects and two ends of the carbon nanotubes. This forms functional groups such as hydroxyl and carboxyl groups on the carbon surface, which act as ligands to adsorb and capture manganese and iron ions. The carbon nanotubes that have captured manganese and iron ions can be monodispersed in the organic acid manganese iron solution. Subsequently, manganese and iron ions on different carbon nanotubes or at different positions on the same carbon nanotube combine, nucleate, merge and grow to form particles, and uniformly embed the carbon nanotubes within them.

[0063] The lithium manganese iron phosphate composite material prepared in step 6 is shown below. Figures 3-6Scanning electron microscopy images were used to observe the morphology and microstructure of the lithium manganese iron phosphate material prepared in Example 1. The prepared sample has a spherical structure with an average particle size of 100 nm to 150 nm. The carbon coating on the material surface is very uniform and complete, with a carbon layer thickness of about 1.5 to 3 nm.

[0064] Phase analysis was performed on the lithium manganese iron phosphate material prepared in Example 1, and the results are as follows: Figure 7 To date, the diffraction peaks of the prepared sample are similar to those of LiMn. 0.6 Fe 0.4 The standard spectrum of PO4 corresponds to the olivine structure of the orthorhombic crystal system; no impurity peaks appear in the spectrum, indicating that the sample prepared in this example is pure phase lithium manganese iron phosphate; no carbon diffraction peaks were detected, indicating that carbon exists in the form of amorphous carbon on the surface of the coating material.

[0065] Example 2

[0066] Step 1: Add manganese carbonate and ferrous chloride to a polyacrylic acid aqueous solution at a molar ratio of Mn:Fe = 2.33 to prepare a 0.5 mol / L mixed solution; prepare a solution of concentrated nitric acid and concentrated hydrochloric acid at a volume ratio of 1:3.

[0067] Step 2: Place the prepared aqua regia solution in a beaker and heat it in a 90℃ water bath with stirring. Then, add carbon nanotubes (1 wt% of lithium manganese iron phosphate composite material) to the 90℃ aqua regia and heat and stir for 1 hour. After drying the modified carbon nanotubes at 85℃ for 10.5 hours, add them to an organic acid aqueous solution to form solution A.

[0068] Step 3: Add the mixed solution from Step 1 to Solution A, and simultaneously add ammonia solution to Solution A to adjust the pH value to 3. Control the reaction temperature at 65℃ and the reaction time at 5h. After the reaction is complete, a precipitate is obtained.

[0069] Step 4: After filtering the precipitate and washing it multiple times with deionized water and ethanol, place it in an 85℃ drying oven and dry for 10 hours to obtain organic manganese iron powder with embedded carbon nanotubes.

[0070] Step 5: Mix the organic manganese iron powder with embedded carbon nanotubes with lithium hydroxide and ammonium dihydrogen phosphate in a molar ratio of nLi∶n(Mn+Fe)∶nPO4. 3- =1.02∶1.5∶1 and 1wt% polyvinyl alcohol of lithium manganese iron phosphate composite material were added sequentially to an aqueous solution of polyethylene glycol and ground thoroughly at 400 rpm for 8 hours.

[0071] Step 6: Place the ground slurry at 85℃ and dry for 10 hours. Then place the dried powder in a box furnace and calcine it from room temperature to 700℃, followed by holding at that temperature for 6 hours. The sintering atmosphere is nitrogen. After calcination, lithium manganese iron phosphate composite material is obtained.

[0072] The sintered lithium manganese iron phosphate composite material was ground, and the ground lithium manganese iron phosphate composite material, SP, and PVDF were weighed and mixed in a mass ratio of 8:1:1 and ground evenly.

[0073] Add 3 wt% NMP dropwise to the homogenized mixture while grinding to homogenize it.

[0074] The ground slurry is evenly coated onto aluminum foil to prepare the electrode sheet;

[0075] The coated electrode sheet is dried in a vacuum environment at 85°C for 10 hours to obtain the positive electrode of a lithium-ion battery.

[0076] Example 3

[0077] Step 1: Add manganese acetate and ferrous sulfate in a molar ratio of Mn:Fe = 4 to a citric acid aqueous solution to prepare a 0.1 mol / L mixed solution; prepare a solution of concentrated nitric acid and concentrated hydrochloric acid in a volume ratio of 1:3.

[0078] Step 2: Place the beaker containing the prepared aqua regia solution in a 95°C water bath and heat with stirring. Then, add carbon nanotubes (1.5% of the lithium manganese iron phosphate composite material) to the 95°C aqua regia solution and heat and stir for 1.5 hours. After drying the modified carbon nanotubes at 95°C for 12 hours, add them to an organic acid aqueous solution to form solution A.

[0079] Step 3: Add the mixed solution from Step 1 to Solution A, and simultaneously add ammonia solution to Solution A to adjust the pH value to 5. Control the reaction temperature at 75℃ and the reaction time at 4 hours. After the reaction is complete, a precipitate is obtained.

[0080] Step 4: After filtering the precipitate and washing it multiple times with deionized water and ethanol, place it in a 95℃ drying oven and dry for 8 hours to obtain organic manganese iron powder with embedded carbon nanotubes.

[0081] Step 5: Mix the organic manganese iron powder with embedded carbon nanotubes with lithium phosphate and lithium dihydrogen phosphate in a molar ratio of nLi∶n(Mn+Fe)∶nPO4. 3- =1.0∶1.1∶1 and 1.5wt% sucrose of lithium manganese iron phosphate composite material were added sequentially to an aqueous solution of polyethylene glycol and ground thoroughly at 450 rpm for 8 hours.

[0082] Step 6: After grinding, the slurry is dried at 95℃ for 8 hours. Then, the dried powder is placed in a box furnace and calcined at room temperature to 800℃ for 3 hours under a nitrogen atmosphere. The lithium manganese iron phosphate composite material is obtained after calcination.

[0083] The sintered lithium manganese iron phosphate composite material was ground, and the ground lithium manganese iron phosphate composite material, SP, and PVDF were weighed and mixed in a mass ratio of 8:1:1 and ground evenly.

[0084] Add 5 wt% NMP dropwise to the homogenized mixture while grinding to homogenize it.

[0085] The ground slurry is evenly coated onto aluminum foil to prepare the electrode sheet;

[0086] The coated electrode sheet is dried in a vacuum environment at 90°C for 8 hours to obtain the positive electrode of a lithium-ion battery.

[0087] Comparative Example 1 (no carbon nanotubes added, outer carbon coating)

[0088] Step 1: Add manganese oxalate and ferrous oxalate to an oxalic acid aqueous solution at a molar ratio of Mn:Fe = 1.5 to prepare a 1 mol / L mixed solution;

[0089] Step 2: Add ammonia solution to the mixed solution in Step 1 to adjust the pH value to 2, control the reaction temperature at 55℃, and the reaction time at 4h. After the reaction is completed, a precipitate is obtained.

[0090] Step 3: After filtering the precipitate and washing it multiple times with deionized water and ethanol, place it in a drying oven at 75°C for 12 hours to obtain organic acid manganese iron powder.

[0091] Step 4: The organic manganese iron powder obtained in Step 3 is mixed with lithium carbonate and lithium dihydrogen phosphate in a molar ratio of nLi∶n(Mn+Fe)∶nPO4. 3- =1.01∶1.3∶1 and 0.5wt% anhydrous glucose of lithium manganese iron phosphate composite material were added sequentially to an aqueous solution of polyethylene glycol and ground thoroughly at 350 rpm for 10 h.

[0092] Step 5: After grinding, the slurry is dried at 75°C for 12 hours. Then, the dried powder is placed in a box furnace and calcined at room temperature to 600°C for 10 hours under a nitrogen atmosphere. The resulting lithium manganese iron phosphate composite material is obtained after calcination.

[0093] The sintered lithium manganese iron phosphate composite material was ground, and the ground lithium manganese iron phosphate composite material, SP, and PVDF were weighed and mixed in a mass ratio of 8:1:1 and ground evenly.

[0094] Add 1 wt% NMP dropwise to the homogenized mixture while grinding to homogenize it.

[0095] The ground slurry is evenly coated onto aluminum foil to prepare the electrode sheet;

[0096] The coated electrode sheet is dried in a vacuum environment at 70°C for 12 hours to obtain the positive electrode of a lithium-ion battery.

[0097] Comparative Example 2 (addition of carbon nanotubes without aqua regia treatment, with an outer carbon coating)

[0098] Step 1: Add manganese oxalate and ferrous oxalate to an aqueous solution of ethylenediaminetetraacetic acid at a molar ratio of Mn:Fe = 1.5 to prepare a 1 mol / L mixed solution.

[0099] Step 2: Add ammonia solution to the mixed solution in Step 1 to adjust the pH value to 2, control the reaction temperature at 55℃, and the reaction time at 6h. After the reaction is completed, a precipitate is obtained.

[0100] Step 3: After filtering the precipitate and washing it multiple times with deionized water and ethanol, place it in a drying oven at 75°C for 12 hours to obtain organic acid manganese iron powder.

[0101] Step 4: Mix the organic acid manganese iron powder with lithium carbonate and lithium dihydrogen phosphate in a molar ratio of nLi∶n(Mn+Fe)∶nPO4. 3- =1.01∶1.3∶1 and 0.5wt% anhydrous glucose of lithium manganese iron phosphate composite material were added sequentially to an aqueous solution of polyethylene glycol and ground thoroughly at 350 rpm for 10 h.

[0102] Step 5: After grinding, the slurry is dried at 75°C for 12 hours. Then, the dried powder is placed in a box furnace and calcined at room temperature to 600°C for 10 hours under a nitrogen atmosphere. The resulting lithium manganese iron phosphate composite material is obtained after calcination.

[0103] The sintered lithium manganese iron phosphate composite material was ground, and the ground lithium manganese iron phosphate composite material, SP, and PVDF were weighed and mixed in a mass ratio of 8:1:1 and ground evenly.

[0104] Add 1 wt% NMP dropwise to the homogenized mixture while grinding to homogenize it.

[0105] The ground slurry is evenly coated onto aluminum foil to prepare the electrode sheet;

[0106] The coated electrode sheet is dried in a vacuum environment at 70°C for 12 hours to obtain the positive electrode of a lithium-ion battery.

[0107] Comparative Example 3 (carbon nanotubes added, outer layer not coated with carbon)

[0108] Step 1: Add manganese oxalate and ferrous oxalate to an oxalic acid aqueous solution at a molar ratio of Mn:Fe = 1.5 to prepare a 1 mol / L mixed solution;

[0109] Step 2: After drying carbon nanotubes at 75°C for 12 hours at a dosage of 0.5% of the lithium manganese iron phosphate composite material, add them to an organic acid aqueous solution to form solution A.

[0110] Step 3: Add the mixed solution from Step 1 to Solution A, and simultaneously add ammonia solution to Solution A to adjust the pH value to 2. Control the reaction temperature at 55℃ and the reaction time at 4 hours. After the reaction is complete, a precipitate is obtained.

[0111] Step 4: After filtering the precipitate and washing it multiple times with deionized water and ethanol, place it in a drying oven at 75°C for 12 hours to obtain organic manganese iron powder with embedded carbon nanotubes.

[0112] Step 5: Mix the organic manganese iron powder with embedded carbon nanotubes with lithium carbonate and lithium dihydrogen phosphate in a molar ratio of nLi∶n(Mn+Fe)∶nPO4. 3- The solutions were added sequentially in a ratio of 1.01:1.3:1 to an aqueous solution of polyethylene glycol and then thoroughly ground at 350 rpm for 10 hours.

[0113] Step 6: After grinding, the slurry is dried at 75°C for 12 hours. Then, the dried powder is placed in a box furnace and calcined at room temperature to 600°C for 10 hours under a nitrogen atmosphere. The resulting lithium manganese iron phosphate composite material is obtained after calcination.

[0114] The sintered lithium manganese iron phosphate composite material was ground, and the ground lithium manganese iron phosphate composite material, SP, and PVDF were weighed and mixed in a mass ratio of 8:1:1 and ground evenly.

[0115] Add 1 wt% NMP dropwise to the homogenized mixture while grinding to homogenize it.

[0116] The ground slurry is evenly coated onto aluminum foil to prepare the electrode sheet;

[0117] The coated electrode sheet is dried in a vacuum environment at 70°C for 12 hours to obtain the positive electrode of a lithium-ion battery.

[0118] Test Example 1

[0119] The electrode sheets prepared according to Examples 1-3 and Comparative Examples 1-3 used lithium metal sheets as the negative electrode; the separator was a Celgard 2400 polypropylene porous membrane; the electrolyte was a solution of EC, DMC, and EMC in a mass ratio of 1:1:1, with LiPF6 as the solute and a LiPF6 concentration of 1.0 mol / L; 2023 coin cells were assembled in a glove box. Charge-discharge cycle performance tests were conducted on the batteries. Continuous charge-discharge was performed at current densities of 0.1C, 0.2C, 0.5C, 1C, and 2C within the cutoff voltage range of 2.0–4.5V. The electrochemical performance results are shown in Table 1, and the discharge capacity at different rates is also presented.

[0120] As shown in Table 1, the comparison of Examples 1 to 3 indicates that the organic acid manganese iron ratio of 1.5 has the best discharge performance.

[0121] By adding aqua regia-treated carbon nanotubes during the preparation of the lithium manganese iron phosphate precursor, and using organic acids as precipitants and solvents, the concentration of manganese and iron ions in the solution is effectively increased, forming an organic acid-manganese iron mixture. During the co-precipitation of the two, the carbon nanotubes induce the formation of primary organic acid-manganese iron particles on the surface of the carbon nanotubes. With continuous stirring and aging, secondary organic acid-manganese iron particles with embedded carbon nanotubes are finally formed. The precursor, lithium source, and carbon source are thoroughly ground and then coated with a sintered outer layer. The discharge capacity of Example 1 at different rates (0.1C, 0.2C, 0.5C, 1C, 2C) is significantly improved compared to the comparative example. Figure 8 The discharge specific capacities were 152.7 mAh / g, 147.5 mAh / g, 144.1 mAh / g, 139.6 mAh / g and 134.2 mAh / g, respectively.

[0122] By comparing Examples 1-3 with Comparative Examples 1-3, it can be concluded that the organic manganese iron phosphate precursor with embedded carbon nanotubes treated with aqua regia can significantly improve the battery performance of lithium manganese iron phosphate cathode material.

[0123] Table 1. Discharge capacity (mAh / g) of the examples and comparative examples at different discharge rates.

[0124]

[0125] The data above shows that the aspect ratio, specific surface area, pore volume, and number of surface functional groups of carbon nanotubes embedded with aqua regia can be controlled by adjusting the acid treatment conditions. Manganese and iron ions on the carbon nanotube surface can achieve a molecular-level even distribution. After high-temperature calcination, the connection between the primary lithium manganese iron phosphate particles and the carbon nanotubes, as well as between the primary lithium manganese iron phosphate particles themselves, becomes stronger. The presence of carbon nanotubes effectively improves the transport of electron ions within the material, effectively enhancing the internal conductivity of the lithium manganese iron phosphate material. Simultaneously, due to the good toughness and elasticity of the carbon nanotube structure itself, it effectively improves the structural stability of the secondary lithium manganese iron phosphate particles during battery charge-discharge cycles, effectively reducing the volume resistivity of the lithium manganese iron phosphate material and improving the internal electronic conductivity and Li-Li content. + A high transmission rate is beneficial for improving the electrochemical performance and cycle stability of batteries.

Claims

1. A method for preparing a lithium manganese iron phosphate composite material, characterized in that, The lithium manganese iron phosphate composite material consists of lithium manganese iron phosphate material with embedded carbon nanotubes and a surface coated with a carbon layer. The lithium manganese iron phosphate material is composed of LiFe. y Mn 1-y PO4, where 0.2 ≤ y ≤ 0.4, The carbon nanotubes constitute 0.5-1.5 wt% of the lithium manganese iron phosphate composite material; the carbon layer constitutes 0.5-1.5 wt% of the lithium manganese iron phosphate composite material. The preparation method includes the following steps: (1) Add manganese source and iron source to an aqueous solution of organic acid to obtain an organic acid manganese iron mixed solution; the organic acid in the aqueous solution of organic acid is at least one of ethylenediaminetetraacetic acid, polyacrylic acid, tartaric acid, citric acid and oxalic acid; (2) Add the carbon nanotubes modified by aqua regia solution to the organic acid aqueous solution and mix them evenly to obtain solution A; the treatment conditions for the carbon nanotubes modified by aqua regia solution are: heating in a water bath at 80-95℃ for 0.5-1.5h; (3) Add the organic acid manganese iron mixed solution obtained in step (1) to the solution A obtained in step (2), mix evenly to obtain solution B, use ammonia water to adjust the pH value of solution B to 2~5, react to obtain reactants, dry the reactants to obtain organic acid manganese iron with embedded carbon nanotubes; (4) The organic manganese iron, lithium source, phosphorus source and carbon source of embedded carbon nanotubes obtained in step (3) are added to the dispersant in sequence and ground thoroughly to obtain a mixed slurry; (5) The mixed slurry obtained in step (4) is dried and then calcined under an inert gas. After calcination, the lithium manganese iron phosphate composite material is obtained.

2. The preparation method of the lithium manganese iron phosphate composite material as described in claim 1, characterized in that, The manganese source is at least one selected from manganese oxalate, manganese carbonate, manganese sulfate, manganese nitrate, and manganese acetate. The iron source is at least one of ferrous oxalate, ferrous sulfate, and ferrous chloride. The molar ratio of manganese in the manganese source to iron in the iron source is 1.5 to 4. The concentration of the organic acid manganese iron mixed solution is 0.1-1 mol / L.

3. The preparation method of the lithium manganese iron phosphate composite material as described in claim 1, characterized in that, In step (3), the reaction temperature is 55-75℃ and the reaction time is 4-6h; The drying temperature is 75~95℃, and the drying time is 8~12h.

4. The preparation method of the lithium manganese iron phosphate composite material as described in claim 1, characterized in that, In step (4), the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium phosphate, and lithium dihydrogen phosphate. The phosphorus source is at least one selected from phosphoric acid, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, and lithium dihydrogen phosphate. The carbon source is at least one selected from anhydrous glucose, sucrose, fructose, phenolic resin, polyethylene glycol, and polyvinyl alcohol. The lithium source, the organic manganese iron with embedded carbon nanotubes, and the phosphorus source are in the molar ratio nLi∶n(Mn+Fe)∶nPO4 3- Add values ​​of 1~1.02∶1.1~1.5∶1.

5. The preparation method of the lithium manganese iron phosphate composite material as described in claim 1, characterized in that, In step (5), the calcination conditions are to keep the temperature at 600~800℃ for 3-10 hours.

6. The lithium manganese iron phosphate composite material prepared by any one of the preparation methods described in claims 1 to 5.

7. The application of the lithium manganese iron phosphate composite material as described in claim 6 in the preparation of lithium-ion batteries.

8. A lithium-ion battery, characterized in that, Including the lithium manganese iron phosphate composite material as described in claim 6.

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