A lithium iron manganese phosphate material and preparation method thereof

Abstract

The present invention discloses a lithium iron manganese phosphate material and a preparation method thereof, belonging to the field of lithium battery technology. First, dopamine-doped lithium chloride is polymerized on the surface of lithium iron manganese phosphate particles to form a coating layer of polydopamine-doped lithium chloride, then titanium dioxide nanotubes are used to form a deposition layer, and finally the deposited titanium dioxide nanotubes are used as a growth matrix to achieve secondary coating of polydopamine, and the lithium iron manganese phosphate material is formed after carbonization. Lithium chloride and polydopamine form a chelate to promote cross-linking, inhibit the volume expansion and rupture of the particles, and utilize the Li-N-C structure to build a lithium ion migration channel to achieve rapid transmission of lithium ions, inhibit lithium dendrite growth, improve rate performance and extend the cycle life of the battery. The hydrogen bond anchoring of polydopamine phenolic hydroxyl groups and the hydroxyl groups on the surface of titanium dioxide nanotubes synergistically inhibits the dissolution and side reactions of manganese ions, forming a complete coating on the surface of the lithium iron manganese phosphate particles, and improving the conductive properties of the lithium iron manganese phosphate material.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and in particular relates to a lithium manganese iron phosphate material and a preparation method thereof. Background Art

[0002] Lithium iron manganese phosphate (LiMnFePO4) is a lithium-ion battery cathode material with advantages such as high energy density, high stability, high safety, and long cycle life. Demand for LiMnFePO4 in downstream industries has been increasing year by year, further spurring the search for improved performance. While the introduction of manganese improves some of the performance of existing lithium-ion battery cathode materials, it also introduces new issues such as low conductivity and manganese dissolution. Therefore, it is necessary to modify LiMnFePO4 based on mature industrial production to optimize battery performance and promote further development of the industry.

[0003] In this context, the surface coating technology of lithium manganese iron phosphate materials has been gradually advanced. The main coating material is a polysaccharide organic carbon source, and the coating methods include sol-gel method and mixed grinding. However, the carbon coating in the existing technology often has certain limitations in conductivity due to the low degree of graphitization, and the conductivity of the positive electrode material will affect the rate performance of the battery; in addition, although carbon coating can play a role in physical protection, inhibiting the dissolution of manganese and improving the cycle stability of the positive electrode material, during the charge and discharge cycle, the carbon coating layer is prone to cracking or falling off, which will also affect the cycle performance of the battery. Summary of the Invention

[0004] The present invention provides a lithium manganese iron phosphate material and a preparation method thereof, which can solve the problem of poor cycle performance of the lithium manganese iron phosphate material in the prior art.

[0005] The purpose of the present invention can be achieved through the following technical solutions:

[0006] The present invention provides a method for preparing a lithium manganese iron phosphate material, comprising the following steps:

[0007] Step 1: Prepare a Tris-HCl buffer solution, add lithium manganese iron phosphate particles, and ultrasonically disperse for 2-4 hours to obtain a dispersion. Add dopamine hydrochloride and lithium chloride to the dispersion, stir at room temperature for 12-24 hours, and filter and wash to obtain primary particles. The particles are lithium manganese iron phosphate coated with polydopamine doped with lithium. Under alkaline conditions, dopamine hydrochloride will first automatically oxidize to generate dopamine quinone, and then further react and polymerize to generate polydopamine. Alkaline conditions will consume the hydrogen ions generated during the polymerization process and promote the reaction to proceed in the forward direction. After adding lithium chloride, lithium ions will combine with the hydroxyl and amino groups in the dopamine structure to form Li-O and Li-N bonds. Dopamine and lithium ions form a chelate, which further promotes the polymerization reaction of dopamine and makes the formed polydopamine cross-linked structure tighter.

[0008] Step 2: Titanium dioxide nanotubes are dispersed in a Tris-HCl buffer solution to form a suspension. Primary particles are added to the suspension, stirred evenly, and then allowed to stand for 1-2 hours to allow the titanium dioxide nanotubes to be completely deposited. After filtering, the excess titanium dioxide nanotubes are removed by washing with water to obtain secondary particles. Titanium dioxide nanotubes are deposited on the surface of the secondary particles. The phenolic hydroxyl groups in polydopamine serve as anchor points for connecting titanium dioxide, which will adsorb the titanium dioxide nanotubes and deposit them on the surface of the particles.

[0009] Step 3: Disperse the secondary particles in a Tris-HCl buffer solution, add dopamine hydrochloride, stir at room temperature under open conditions for 12-24 hours, filter, wash, and dry, and calcine at 700-750°C under an inert atmosphere for 1-2 hours to obtain the lithium manganese iron phosphate material. After titanium dioxide is deposited on the surface of the particles in Step 2, polydopamine will grow around it as a matrix, and the titanium dioxide and polydopamine will be tightly bonded to form a uniform and complete surface structure.

[0010] During the calcination process under an inert atmosphere, the polydopamine in the coating layer will gradually carbonize. Compared with other carbon source materials, the polydopamine formed by cross-linking polymerization has an orderly structure and contains nitrogen elements, which can achieve a high degree of graphitization. After carbonization, a Li-NC structure will be formed, which will replenish ions while building lithium ion channels, thereby promoting the rapid transmission of lithium ions.

[0011] Furthermore, the pH of the Tris-HCl buffer solution is 8-9.

[0012] Furthermore, the D50 of the lithium manganese iron phosphate particles is 0.5-1.5 μm.

[0013] Furthermore, in step 1, the mass fraction of the lithium manganese iron phosphate particles in the Tris-HCl buffer solution is 10-20 g / L.

[0014] Furthermore, in step 1, the dopamine hydrochloride is added to the dispersion at a concentration of 0.5-2 g / L.

[0015] Furthermore, in step 1, the mass of the lithium chloride is 6-10% of the mass of dopamine hydrochloride.

[0016] Furthermore, the diameter of the titanium dioxide nanotubes is 5-10 nm;

[0017] The concentration of the titanium dioxide nanotubes in the suspension is 0.1-0.5 g / L.

[0018] Titanium dioxide nanotubes with smaller diameters can be better adsorbed and deposited on the surface of the particles, and their high specific surface area can provide a larger growth platform for the polydopamine coating in step three, forming a complete coating layer. The staggered coating of titanium dioxide nanotubes and polydopamine can effectively inhibit the dissolution of manganese, inhibit side reactions, and improve cycle stability.

[0019] Furthermore, in step 2, the mass ratio of the primary particles to the titanium dioxide nanotubes in the suspension is 20-40:1.

[0020] Furthermore, in step three, the mass fraction of the secondary particles in the Tris-HCl buffer solution is 10-20 g / L, and the mass-to-volume ratio of dopamine hydrochloride to the Tris-HCl buffer solution is 0.5-2 g / L.

[0021] Furthermore, the gas of the inert atmosphere is one of argon and nitrogen.

[0022] The present invention also provides a lithium manganese iron phosphate material, which is prepared using the above-mentioned preparation method.

[0023] Beneficial effects of the present invention:

[0024] (1) The present invention conducts dopamine self-polymerization on the surface of lithium manganese iron phosphate particles to form a polydopamine coating, and dopes lithium chloride to form a chelate to promote cross-linking. The tightly cross-linked polydopamine layer can alleviate the volume expansion during the ion insertion and extraction process, avoid the rupture of the particles, and use the Li-NC structure to build a lithium ion migration channel to achieve rapid lithium ion transmission, inhibit lithium dendrite growth, improve rate performance and extend the cycle life of the battery.

[0025] (2) In the preparation of the present invention, a nano-titanium dioxide nanotube layer is deposited, and the mechanical properties of the coating layer are enhanced by utilizing the high hardness and nanotube structure. The layer is then combined with the polydopamine carbonized layer to block direct contact of the electrolyte with lithium iron manganese phosphate, thereby inhibiting the dissolution of manganese ions and side reactions. Polydopamine utilizes phenolic hydroxyl groups and hydroxyl groups on the surface of the titanium dioxide nanotubes to anchor and combine through hydrogen bonds, thereby achieving uniform deposition of the nanotubes on the surface of the polydopamine.

[0026] (3) The present invention uses titanium dioxide nanotubes as a growth matrix, and polydopamine extends on its surface through π-π stacking and hydrogen bonding to form a complete covering layer, achieving complete coating of lithium manganese iron phosphate particles. Polydopamine can be carbonized into a highly graphitized nitrogen-doped carbon layer, and its ordered structure can reduce electron migration resistance, giving the lithium manganese iron phosphate material excellent conductive properties.

[0027] (4) The preparation process of the present invention does not require special equipment. The energy consumption of operations other than conventional carbonization and calcination is low, and no additional energy consumption is added. The operation is simple and easy to scale up. DETAILED DESCRIPTION

[0028] 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.

[0029] Example 1

[0030] Preparation of lithium manganese iron phosphate material:

[0031] Step 1: Add 0.8 g of tris (hydroxymethyl)aminomethane to a beaker, then add 14.7 mL of 0.1 mol / L hydrochloric acid, add ultrapure water and stir, and when the pH is 8.5, stop adding ultrapure water to obtain a Tris-HCl buffer solution.

[0032] Step 2: Purchase lithium manganese iron phosphate particles (LiMn 0.5 Fe 0.5 PO4) was added to a Tris-HCl buffer solution at a mass fraction of 10 g / L and ultrasonically dispersed for 2 hours to form a dispersion. Dopamine hydrochloride and lithium chloride were added to the dispersion at a concentration of 0.5 g / L dopamine hydrochloride and a mass of lithium chloride equal to 6% of the dopamine hydrochloride. The mixture was stirred at room temperature for 12 hours under open conditions. The mixture was filtered and washed three times with ultrapure water to remove excess unpolymerized dopamine hydrochloride, resulting in primary particles with a black appearance.

[0033] Step 3: Commercially purchased titanium dioxide nanotubes with a diameter of 5-10 nm were added to a Tris-HCl buffer solution at a concentration of 0.1 g / L, and ultrasonically dispersed for 20 minutes to form a suspension. Primary particles were added and stirred for 5 minutes, and then allowed to stand for 1 hour. The mass ratio of primary particles to titanium dioxide nanotubes in the suspension was 40:1. After filtering, the undeposited titanium dioxide nanotubes were washed three times with ultrapure water to obtain secondary particles.

[0034] Step 4: Add the secondary particles to a Tris-HCl buffer solution at a mass fraction of 10 g / L, ultrasonically disperse for 10 minutes, add dopamine hydrochloride at a concentration of 0.5 g / L, stir at room temperature for 12 hours, filter, wash three times with ultrapure water, dry at 50°C, and calcine at 700°C under an argon atmosphere for 2 hours to obtain lithium manganese iron phosphate material.

[0035] Example 2

[0036] The only difference from Example 1 is that the mass of lithium chloride is adjusted from 6% of dopamine hydrochloride to 8%.

[0037] Preparation of lithium manganese iron phosphate material:

[0038] Step 1: Add 0.8 g of tris (hydroxymethyl)aminomethane to a beaker, then add 14.7 mL of 0.1 mol / L hydrochloric acid, add ultrapure water and stir, and when the pH is 8.5, stop adding ultrapure water to obtain a Tris-HCl buffer solution.

[0039] Step 2: Purchase lithium manganese iron phosphate particles (LiMn 0.5 Fe 0.5 PO4) was added to a Tris-HCl buffer solution at a mass fraction of 10 g / L and ultrasonically dispersed for 2 hours to form a dispersion. Dopamine hydrochloride and lithium chloride were added to the dispersion at a concentration of 0.5 g / L dopamine hydrochloride and a mass of 8% of the dopamine hydrochloride. The dispersion was stirred at room temperature for 12 hours under open conditions. The mixture was filtered and washed three times with ultrapure water to remove excess unpolymerized dopamine hydrochloride, resulting in primary particles with a black appearance.

[0040] Step 3: Commercially purchased titanium dioxide nanotubes with a diameter of 5-10 nm were added to a Tris-HCl buffer solution at a concentration of 0.1 g / L, and ultrasonically dispersed for 20 minutes to form a suspension. Primary particles were added and stirred for 5 minutes, and then allowed to stand for 1 hour. The mass ratio of primary particles to titanium dioxide nanotubes in the suspension was 40:1. After filtering, the undeposited titanium dioxide nanotubes were washed three times with ultrapure water to obtain secondary particles.

[0041] Step 4: Add the secondary particles to a Tris-HCl buffer solution at a mass fraction of 10 g / L, ultrasonically disperse for 10 minutes, add dopamine hydrochloride at a concentration of 0.5 g / L, stir at room temperature for 12 hours, filter, wash three times with ultrapure water, dry at 50°C, and calcine at 700°C under an argon atmosphere for 2 hours to obtain lithium manganese iron phosphate material.

[0042] Example 3

[0043] The only difference from Example 1 is that the mass of lithium chloride is adjusted from 6% of dopamine hydrochloride to 10%.

[0044] Preparation of lithium manganese iron phosphate material:

[0045] Step 1: Add 0.8 g of tris (hydroxymethyl)aminomethane to a beaker, then add 14.7 mL of 0.1 mol / L hydrochloric acid, add ultrapure water and stir, and when the pH is 8.5, stop adding ultrapure water to obtain a Tris-HCl buffer solution.

[0046] Step 2: Purchase lithium manganese iron phosphate particles (LiMn 0.5 Fe 0.5PO4) was added to a Tris-HCl buffer solution at a mass fraction of 10 g / L and ultrasonically dispersed for 2 hours to form a dispersion. Dopamine hydrochloride and lithium chloride were added to the dispersion at a concentration of 0.5 g / L dopamine hydrochloride and a mass of lithium chloride equal to 10% of the dopamine hydrochloride. The dispersion was stirred at room temperature for 12 hours under open conditions. The mixture was filtered and washed three times with ultrapure water to remove excess unpolymerized dopamine hydrochloride, resulting in primary particles with a black appearance.

[0047] Step 3: Commercially purchased titanium dioxide nanotubes with a diameter of 5-10 nm were added to a Tris-HCl buffer solution at a concentration of 0.1 g / L, and ultrasonically dispersed for 20 minutes to form a suspension. Primary particles were added and stirred for 5 minutes, and then allowed to stand for 1 hour. The mass ratio of primary particles to titanium dioxide nanotubes in the suspension was 40:1. After filtering, the undeposited titanium dioxide nanotubes were washed three times with ultrapure water to obtain secondary particles.

[0048] Step 4: Add the secondary particles to a Tris-HCl buffer solution at a mass fraction of 10 g / L, ultrasonically disperse for 10 minutes, add dopamine hydrochloride at a concentration of 0.5 g / L, stir at room temperature for 12 hours, filter, wash three times with ultrapure water, dry at 50°C, and calcine at 700°C under an argon atmosphere for 2 hours to obtain lithium manganese iron phosphate material.

[0049] Example 4

[0050] The only difference from Example 2 is that in step 2, the concentration of dopamine hydrochloride is adjusted from 0.5 g / L to 1.5 g / L, and the stirring time at room temperature is adjusted from 12 h to 18 h.

[0051] Preparation of lithium manganese iron phosphate material:

[0052] Step 1: Add 0.8 g of tris (hydroxymethyl)aminomethane to a beaker, then add 14.7 mL of 0.1 mol / L hydrochloric acid, add ultrapure water and stir, and when the pH is 8.5, stop adding ultrapure water to obtain a Tris-HCl buffer solution.

[0053] Step 2: Purchase lithium manganese iron phosphate particles (LiMn 0.5 Fe 0.5 PO4) was added to a Tris-HCl buffer solution at a mass fraction of 10 g / L and ultrasonically dispersed for 2 hours to form a dispersion. Dopamine hydrochloride and lithium chloride were added to the dispersion at a concentration of 1.5 g / L dopamine hydrochloride and a mass of lithium chloride equal to 8% of the dopamine hydrochloride. The mixture was stirred at room temperature for 18 hours under open conditions. The mixture was filtered and washed three times with ultrapure water to remove excess unpolymerized dopamine hydrochloride, resulting in primary particles with a black appearance.

[0054] Step 3: Commercially purchased titanium dioxide nanotubes with a diameter of 5-10 nm were added to a Tris-HCl buffer solution at a concentration of 0.1 g / L, and ultrasonically dispersed for 20 minutes to form a suspension. Primary particles were added and stirred for 5 minutes, and then allowed to stand for 1 hour. The mass ratio of primary particles to titanium dioxide nanotubes in the suspension was 40:1. After filtering, the undeposited titanium dioxide nanotubes were washed three times with ultrapure water to obtain secondary particles.

[0055] Step 4: Add the secondary particles to a Tris-HCl buffer solution at a mass fraction of 10 g / L, ultrasonically disperse for 10 minutes, add dopamine hydrochloride at a concentration of 0.5 g / L, stir at room temperature for 12 hours, filter, wash three times with ultrapure water, dry at 50°C, and calcine at 700°C under an argon atmosphere for 2 hours to obtain lithium manganese iron phosphate material.

[0056] Example 5

[0057] The only difference from Example 2 is that in step 2, the concentration of dopamine hydrochloride is adjusted from 0.5 g / L to 2 g / L, and the stirring time at room temperature is adjusted from 12 h to 24 h.

[0058] Preparation of lithium manganese iron phosphate material:

[0059] Step 1: Add 0.8 g of tris (hydroxymethyl)aminomethane to a beaker, then add 14.7 mL of 0.1 mol / L hydrochloric acid, add ultrapure water and stir, and when the pH is 8.5, stop adding ultrapure water to obtain a Tris-HCl buffer solution.

[0060] Step 2: Purchase lithium manganese iron phosphate particles (LiMn 0.5 Fe 0.5 PO4) was added to a Tris-HCl buffer solution at a mass fraction of 10 g / L and ultrasonically dispersed for 2 hours to form a dispersion. Dopamine hydrochloride and lithium chloride were added to the dispersion at a concentration of 2 g / L dopamine hydrochloride and a mass of lithium chloride equal to 8% of the dopamine hydrochloride. The dispersion was stirred at room temperature for 24 hours under open conditions. The particles were filtered and washed three times with ultrapure water to remove excess unpolymerized dopamine hydrochloride, resulting in primary particles with a black appearance.

[0061] Step 3: Commercially purchased titanium dioxide nanotubes with a diameter of 5-10 nm were added to a Tris-HCl buffer solution at a concentration of 0.1 g / L, and ultrasonically dispersed for 20 minutes to form a suspension. Primary particles were added and stirred for 5 minutes, and then allowed to stand for 1 hour. The mass ratio of primary particles to titanium dioxide nanotubes in the suspension was 40:1. After filtering, the undeposited titanium dioxide nanotubes were washed three times with ultrapure water to obtain secondary particles.

[0062] Step 4: Add the secondary particles to a Tris-HCl buffer solution at a mass fraction of 10 g / L, ultrasonically disperse for 10 minutes, add dopamine hydrochloride at a concentration of 0.5 g / L, stir at room temperature for 12 hours, filter, wash three times with ultrapure water, dry at 50°C, and calcine at 700°C under an argon atmosphere for 2 hours to obtain lithium manganese iron phosphate material.

[0063] Example 6

[0064] The only difference from Example 4 is that the mass ratio of primary particles to titanium dioxide nanotubes in the suspension in step 3 is adjusted from 40:1 to 30:1.

[0065] Preparation of lithium manganese iron phosphate material:

[0066] Step 1: Add 0.8 g of tris (hydroxymethyl)aminomethane to a beaker, then add 14.7 mL of 0.1 mol / L hydrochloric acid, add ultrapure water and stir, and when the pH is 8.5, stop adding ultrapure water to obtain a Tris-HCl buffer solution.

[0067] Step 2: Purchase lithium manganese iron phosphate particles (LiMn 0.5 Fe 0.5 PO4) was added to a Tris-HCl buffer solution at a mass fraction of 10 g / L and ultrasonically dispersed for 2 hours to form a dispersion. Dopamine hydrochloride and lithium chloride were added to the dispersion at a concentration of 1.5 g / L dopamine hydrochloride and a mass of lithium chloride equal to 8% of the dopamine hydrochloride. The mixture was stirred at room temperature for 18 hours under open conditions. The mixture was filtered and washed three times with ultrapure water to remove excess unpolymerized dopamine hydrochloride, resulting in primary particles with a black appearance.

[0068] Step 3: Commercially purchased titanium dioxide nanotubes with a diameter of 5-10 nm were added to a Tris-HCl buffer solution at a concentration of 0.1 g / L, and ultrasonically dispersed for 20 minutes to form a suspension. Primary particles were added and stirred for 5 minutes, and then allowed to stand for 1 hour. The mass ratio of primary particles to titanium dioxide nanotubes in the suspension was 30:1. After filtering, the undeposited titanium dioxide nanotubes were removed and washed three times with ultrapure water to obtain secondary particles.

[0069] Step 4: Add the secondary particles to a Tris-HCl buffer solution at a mass fraction of 10 g / L, ultrasonically disperse for 10 minutes, add dopamine hydrochloride at a concentration of 0.5 g / L, stir at room temperature for 12 hours, filter, wash three times with ultrapure water, dry at 50°C, and calcine at 700°C under an argon atmosphere for 2 hours to obtain lithium manganese iron phosphate material.

[0070] Example 7

[0071] The only difference from Example 4 is that the mass ratio of primary particles to titanium dioxide nanotubes in the suspension in step 3 is adjusted from 40:1 to 20:1.

[0072] Preparation of lithium manganese iron phosphate material:

[0073] Step 1: Add 0.8 g of tris (hydroxymethyl)aminomethane to a beaker, then add 14.7 mL of 0.1 mol / L hydrochloric acid, add ultrapure water and stir, and when the pH is 8.5, stop adding ultrapure water to obtain a Tris-HCl buffer solution.

[0074] Step 2: Purchase lithium manganese iron phosphate particles (LiMn 0.5 Fe 0.5 PO4) was added to a Tris-HCl buffer solution at a mass fraction of 10 g / L and ultrasonically dispersed for 2 hours to form a dispersion. Dopamine hydrochloride and lithium chloride were added to the dispersion at a concentration of 1.5 g / L dopamine hydrochloride and a mass of lithium chloride equal to 8% of the dopamine hydrochloride. The mixture was stirred at room temperature for 18 hours under open conditions. The mixture was filtered and washed three times with ultrapure water to remove excess unpolymerized dopamine hydrochloride, resulting in primary particles with a black appearance.

[0075] Step 3: Commercially purchased titanium dioxide nanotubes with a diameter of 5-10 nm were added to a Tris-HCl buffer solution at a concentration of 0.1 g / L, and ultrasonically dispersed for 20 minutes to form a suspension. Primary particles were added and stirred for 5 minutes, and then allowed to stand for 1 hour. The mass ratio of primary particles to titanium dioxide nanotubes in the suspension was 20:1. After filtering, the undeposited titanium dioxide nanotubes were washed three times with ultrapure water to obtain secondary particles.

[0076] Step 4: Add the secondary particles to a Tris-HCl buffer solution at a mass fraction of 10 g / L, ultrasonically disperse for 10 minutes, add dopamine hydrochloride at a concentration of 0.5 g / L, stir at room temperature for 12 hours, filter, wash three times with ultrapure water, dry at 50°C, and calcine at 700°C under an argon atmosphere for 2 hours to obtain lithium manganese iron phosphate material.

[0077] Comparative Example 1

[0078] The only difference from Example 1 is that lithium chloride is not added in step 2 of this comparative example.

[0079] Preparation of lithium manganese iron phosphate material:

[0080] Step 1: Add 0.8 g of tris (hydroxymethyl)aminomethane to a beaker, then add 14.7 mL of 0.1 mol / L hydrochloric acid, add ultrapure water and stir, and when the pH is 8.5, stop adding ultrapure water to obtain a Tris-HCl buffer solution.

[0081] Step 2: Purchase lithium manganese iron phosphate particles (LiMn 0.5 Fe 0.5 PO4) was added to a Tris-HCl buffer solution at a mass fraction of 10 g / L and ultrasonically dispersed for 2 hours to form a dispersion. Dopamine hydrochloride was added to the dispersion at a concentration of 0.5 g / L. The mixture was stirred at room temperature for 12 hours under open conditions. The mixture was filtered and washed three times with ultrapure water to remove excess unpolymerized dopamine hydrochloride, resulting in primary particles with a black appearance.

[0082] Step 3: Commercially purchased titanium dioxide nanotubes with a diameter of 5-10 nm were added to a Tris-HCl buffer solution at a concentration of 0.1 g / L, and ultrasonically dispersed for 20 minutes to form a suspension. Primary particles were added and stirred for 5 minutes, and then allowed to stand for 1 hour. The mass ratio of primary particles to titanium dioxide nanotubes in the suspension was 40:1. After filtering, the undeposited titanium dioxide nanotubes were washed three times with ultrapure water to obtain secondary particles.

[0083] Step 4: Add the secondary particles to a Tris-HCl buffer solution at a mass fraction of 10 g / L, ultrasonically disperse for 10 minutes, add dopamine hydrochloride at a concentration of 0.5 g / L, stir at room temperature for 12 hours, filter, wash three times with ultrapure water, dry at 50°C, and calcine at 700°C under an argon atmosphere for 2 hours to obtain lithium manganese iron phosphate material.

[0084] Comparative Example 2

[0085] The only difference from Example 1 is that titanium dioxide nanotubes are not deposited in this comparative example.

[0086] Preparation of lithium manganese iron phosphate material:

[0087] Step 1: Add 0.8 g of tris (hydroxymethyl)aminomethane to a beaker, then add 14.7 mL of 0.1 mol / L hydrochloric acid, add ultrapure water and stir, and when the pH is 8.5, stop adding ultrapure water to obtain a Tris-HCl buffer solution.

[0088] Step 2: Purchase lithium manganese iron phosphate particles (LiMn 0.5 Fe 0.5 PO4) was added to a Tris-HCl buffer solution at a mass fraction of 10 g / L and ultrasonically dispersed for 2 hours to form a dispersion. Dopamine hydrochloride and lithium chloride were added to the dispersion at a concentration of 0.5 g / L dopamine hydrochloride and a mass of lithium chloride equal to 6% of the dopamine hydrochloride. The mixture was stirred at room temperature for 12 hours under open conditions. The mixture was filtered and washed three times with ultrapure water to remove excess unpolymerized dopamine hydrochloride, resulting in primary particles with a black appearance.

[0089] Step 3: The primary particles were added to a Tris-HCl buffer solution at a mass fraction of 10 g / L, ultrasonically dispersed for 10 min, dopamine hydrochloride was added at a concentration of 0.5 g / L, stirred at room temperature for 12 h, filtered, washed three times with ultrapure water, dried at 50°C, and calcined at 700°C under an argon atmosphere for 2 h to obtain lithium manganese iron phosphate material.

[0090] Comparative Example 3

[0091] In this comparative example, lithium iron manganese phosphate material is prepared by directly mixing lithium iron manganese phosphate particles with glucose, grinding them, and then calcining them at high temperature.

[0092] Preparation of lithium manganese iron phosphate material:

[0093] Step 1: Commercially available lithium manganese iron phosphate particles (LiMn 0.5 Fe 0.5 PO4) is mixed with carbon source glucose, the mass of glucose is 10% of the total mass of lithium manganese iron phosphate particles and glucose, pure water is added according to the solid content of 50%, the mixture is stirred and ground, and the particle size after grinding is 300-400nm;

[0094] Step 2: spray-dry the ground lithium manganese iron phosphate particles and calcine them at 700° C. for 2 h in a nitrogen atmosphere to obtain lithium manganese iron phosphate material.

[0095] The performance of batteries assembled with the lithium manganese iron phosphate materials prepared in Examples 1 to 7 and Comparative Examples 1 to 3 was tested with reference to the T / CIAPS0029-2023 "Lithium manganese iron phosphate positive electrode material for lithium ion batteries" standard. The results are shown in Table 1:

[0096]

[0097] As can be seen from Table 1, with the increase of the amount of lithium chloride added in the coating layer, its capacity and rate performance are significantly improved, and the cross-linking of polydopamine can be promoted. The coating layer structure is more compact and uniform, which can improve the conductivity of the material. However, after adding too much lithium chloride, the stability of the Li-NC structure is insufficient, resulting in a decrease in its cycle stability. Example 4 and Example 5 increase the proportion of polydopamine on the basis of Example 2. When the carbon content increases compared to the coating layer, the conductivity of the material is improved, but the excessive coating layer ratio will reduce the capacity of the material. Under the conditions of Example 4, its comprehensive performance is the best. In Example 6 and Example 7, with the increase of titanium dioxide nanotubes, the effect of suppressing side reactions is significantly improved, the cycle performance of the battery is improved, and the capacity retention rate can reach up to 99.1% under 1C cycle 500 weeks. Compared with Example 1, when lithium chloride is not added to the coating layer material, the specific capacity of the lithium manganese iron phosphate material is low, and the conductive performance is also reduced. When titanium dioxide nanotubes are not added for deposition, the conductivity and cycle performance of Comparative Example 2 are obviously inferior to those of Example 1. Comparative Example 3 is a conventional carbon coating technology in the prior art. Judging from the performance test results of Comparative Example 3, the performance indicators of the embodiment materials of the present invention are all better than those of Comparative Example 3, and they have excellent electrochemical properties.

[0098] 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.

[0099] 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 method for preparing lithium manganese iron phosphate material, characterized in that: The following steps are involved: Step 1: lithium manganese iron phosphate particles are dispersed in a Tris-HCl buffer solution to form a dispersion, dopamine hydrochloride and lithium chloride are added, the mixture is stirred at room temperature for 12-24 hours, and filtered and washed to obtain primary particles; Step 2: Titanium dioxide nanotubes are dispersed in a Tris-HCl buffer solution to form a suspension, primary particles are added, stirred evenly, and then allowed to stand for 1-2 hours, and filtered and washed to obtain secondary particles; Step 3: The secondary particles are dispersed in a Tris-HCl buffer solution, dopamine hydrochloride is added, and the mixture is stirred at room temperature for 12-24 hours. The mixture is filtered, washed, and dried. The mixture is calcined at 700-750° C. for 1-2 hours under an inert atmosphere to obtain a lithium manganese iron phosphate material. The pH of the Tris-HCl buffer solution is 8-9; The concentration of the titanium dioxide nanotubes in the suspension is 0.1-0.5 g / L.

2. The method for preparing a lithium iron manganese phosphate material according to claim 1, wherein: The D50 of the lithium manganese iron phosphate particles is 0.5-1.5 μm.

3. The method for preparing a lithium manganese iron phosphate material according to claim 1, wherein: In step 1, the mass fraction of the lithium manganese iron phosphate particles in the Tris-HCl buffer solution is 10-20 g / L.

4. The method for preparing a lithium iron manganese phosphate material according to claim 1, wherein: In step 1, the dopamine hydrochloride is added to the dispersion at a concentration of 0.5-2 g / L.

5. The method for preparing a lithium iron manganese phosphate material according to claim 1, wherein: In step 1, the mass of the lithium chloride is 6-10% of the mass of dopamine hydrochloride.

6. The method for preparing a lithium iron manganese phosphate material according to claim 1, characterized in that: The diameter of the titanium dioxide nanotube is 5-10 nm.

7. The method for preparing a lithium iron manganese phosphate material according to claim 1, characterized in that: In step 2, the mass ratio of the primary particles to the titanium dioxide nanotubes in the suspension is 20-40:

1.

8. The method for preparing a lithium iron manganese phosphate material according to claim 1, wherein: In step 3, the mass fraction of the secondary particles in the Tris-HCl buffer solution is 10-20 g / L, and the mass-to-volume ratio of dopamine hydrochloride to the Tris-HCl buffer solution is 0.5-2 g / L.

9. A lithium manganese iron phosphate material, characterized in that: The lithium iron manganese phosphate material is prepared by the preparation method of the lithium iron manganese phosphate material according to any one of claims 1 to 8.

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