Manganese iron phosphate precursor, manganese iron lithium phosphate positive electrode material, and preparation method and application thereof

Abstract

This invention relates to the field of lithium-ion batteries, specifically to a manganese iron phosphate precursor, a lithium manganese iron phosphate cathode material, its preparation method, and its application. The manganese iron phosphate precursor includes manganese, iron, and carbon. From the core to the surface, the content of manganese decreases, the content of iron increases, and the content of carbon decreases. The content of the three elements exhibits a gradient distribution and synergistic effect. Batteries containing lithium manganese iron phosphate cathode materials prepared from this precursor can achieve both high discharge capacity and discharge plateau voltage, while reducing the amount of manganese dissolved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion batteries, specifically to a manganese iron phosphate precursor, lithium manganese iron phosphate cathode material, its preparation method, and its application. Background Technology

[0002] Lithium manganese iron phosphate (LiMn) x Fe 1-x PO4 (abbreviated as LMFP) is a cathode material for lithium-ion batteries. With its advantages such as high safety performance, long cycle life, high voltage and wide availability of raw materials, it is gradually becoming an advanced alternative to lithium iron phosphate and will become an important choice for the future new energy vehicle and energy storage market.

[0003] However, lithium manganese iron phosphate also suffers from problems such as poor electronic conductivity and manganese leaching. This is because in the nonlinear MnO6 octahedron, the high-spin state of Mn... 3+ It has a very large magnetic moment in doubly degenerate eg orbits (containing dx) 2 -y 2 and dz 2 The orbital contains only one electron, resulting in an asymmetric electron distribution. Simultaneously, dx 2 -y 2 and dz 2 The electrons in the orbital exhibit varying degrees of shielding effect on the Mn nucleus in different directions, while the intramolecular structure aims to stabilize Mn. 3+ During migration, the longitudinal Mn-O bonds gradually elongate while the horizontal Mn-O bonds shorten, causing the linear MnO2 arrangement to elongate along the axial direction, resulting in Jahn-Teller distortion. Jahn-Teller distortion brings about drastic structural changes to the material, accelerating structural damage and causing material deactivation. To avoid Jahn-Teller distortion, the material can be modified to improve its performance. Modification processes include carbon coating, particle size reduction, ion doping, and optimization of manganese content.

[0004] However, the current process has the following problems: the introduction of manganese significantly reduces the conductivity of the material, and manganese may leach out. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is to overcome the defects of existing batteries that cannot simultaneously achieve high discharge capacity and discharge plateau voltage, and the defect that manganese elements in the battery will dissolve, thereby providing a precursor of manganese iron phosphate, lithium manganese iron phosphate cathode material, preparation method and application.

[0006] Therefore, the present invention provides the following technical solution:

[0007] The first aspect of this invention protects a manganese iron phosphate precursor, comprising manganese, iron and carbon elements; wherein, from the core to the surface, the content of manganese decreases, the content of iron increases, and the content of carbon decreases.

[0008] The manganese iron phosphate precursor provided by this invention has a decreasing manganese content, an increasing iron content, and a decreasing carbon content from the core to the surface. Batteries containing lithium manganese iron phosphate cathode materials obtained from the manganese iron phosphate precursor can achieve both high discharge specific capacity and discharge plateau voltage. The gradient distribution of carbon content enhances electronic conductivity, promotes lithium ion migration and diffusion, and improves battery conductivity.

[0009] According to the present invention, in the core, the molar ratio of manganese to iron is (8-9):(1-2).

[0010] According to the present invention, the carbon content is 5-8 wt%, based on the mass of the manganese iron phosphate precursor at the core.

[0011] According to the present invention, at the midpoint between the core and the surface, the molar ratio of manganese to iron is (6-7.5):(2.2-4).

[0012] According to the present invention, the carbon content is 2-4 wt%, based on the mass of the manganese iron phosphate precursor at the midpoint between the core and the surface.

[0013] According to the present invention, at the surface, the molar ratio of manganese to iron is (3-5):(5-7).

[0014] According to the present invention, the carbon content is 1-2 wt%, based on the mass of the manganese iron phosphate precursor at the surface.

[0015] In this invention, the average particle size of the manganese iron phosphate precursor is 0-1.8 μm at the core, the average particle size of the manganese iron phosphate precursor is 1.8-2.5 μm (excluding 1.8 μm) at the middle between the core and the surface, and the average particle size of the manganese iron phosphate precursor is 2.5-5 μm (excluding 2.5 μm) at the surface.

[0016] In this invention, the ICP method is used to determine the content of different metal elements in different parts of the manganese iron phosphate precursor. Specifically, 1g of sample is added to 20mL of hydrochloric acid solution (37wt% hydrochloric acid:pure water = 1:1) for digestion to obtain a liquid sample. The liquid sample is then drawn into a plasma torch by an nebulizer. The sample is excited, and the characteristic spectra of the emitted elements are reflected by a mirror and focused onto the entrance slit of the spectrometer. When the light enters the spectrometer, it strikes the grating, and the diffracted light, according to the analytical wavelength, illuminates the photocathode of the photomultiplier tube through the exit slit. The light corresponding to each sample is converted into electrical energy, and the computer directly converts the signal intensity into the corresponding sample concentration and records the reading.

[0017] In this invention, a carbon-sulfur analyzer is used to determine the carbon content. Specifically, the sample is oxidized by oxygen at high temperature in a combustion furnace, introduced into a carbon detection cell to determine the carbon content, and the reading is read on a computer.

[0018] According to the present invention, the average particle size of the manganese iron phosphate precursor is 3-5 μm.

[0019] A second aspect of this invention protects a method for preparing a manganese iron phosphate precursor, comprising the following steps:

[0020] Step 1: Mix the first manganese source, the first iron source, the first phosphorus source and the first carbon source to obtain the base liquid;

[0021] Step 2: Add manganese source solution, iron source solution, carbon source solution and complexing agent to the bottom liquid at the initial flow rate and carry out the reaction. When the average particle size reaches 1.5-2 μm, reduce the flow rate of manganese source solution and carbon source solution to 2 / 3-9 / 10 of their initial flow rates, respectively; increase the flow rate of iron source solution to 2-4 times its initial flow rate.

[0022] When the reaction reaches an average particle size of 2.5-2.8 μm, reduce the flow rates of the manganese source solution and the carbon source solution to 1 / 3-3 / 4 of their initial flow rates, respectively; and increase the flow rate of the iron source solution to 3-7 times its initial flow rate.

[0023] Step 3: The material obtained after the reaction in Step 2 is aged and separated to obtain the manganese iron phosphate precursor.

[0024] In this invention, a stepwise preparation method is adopted. First, a base liquid is obtained. Then, by controlling the flow rate in multiple stages, the content of manganese in the prepared manganese iron phosphate precursor decreases from the core to the surface, the content of iron increases, and the content of carbon decreases, presenting a gradient distribution. Finally, the battery containing the cathode material prepared from this precursor has excellent conductivity and the amount of manganese dissolved is reduced.

[0025] In this invention, step 3 involves separation in a conventional manner in the art. Typically, without limitation, separation includes filtration, washing, and drying. The conditions for washing and drying are not limited, as long as a pure and dry manganese iron phosphate precursor can be obtained.

[0026] According to the present invention, in step 1, the molar ratio of the first manganese source, the first iron source, the first phosphorus source and the first carbon source is (5-10):(0.5-2):(5-12):1, preferably (8-9):(1-1.5):(9-10):1.

[0027] According to the present invention, in step 1, a dispersant is added before mixing.

[0028] According to the present invention, the mass ratio of the dispersant to the first iron source is 8-12:1.

[0029] According to the present invention, the dispersant comprises polyethylene glycol and / or polyacrylamide.

[0030] According to the present invention, in step 2, the molar ratio of the complexing agent to the first iron source is 0.3-0.6:1.

[0031] According to the present invention, in step 2, the complexing agent includes ferrous phosphate and / or oxalate dihydrate.

[0032] According to the present invention, in step 2, the manganese source solution contains a second manganese source and water, wherein the concentration of the second manganese source is 1.8-2.2 mol / L.

[0033] According to the present invention, in step 2, the iron source solution contains a second iron source and water, wherein the concentration of the second iron source is 0.7-1.3 mol / L.

[0034] According to the present invention, in step 2, the carbon source solution contains a second carbon source and water, wherein the concentration of the second carbon source is 0.3-0.5 g / L.

[0035] According to the present invention, the first manganese source and the second manganese source are each independently selected from at least one of manganese nitrate, manganese sulfate and manganese carbonate.

[0036] According to the present invention, the first iron source and the second iron source are each independently selected from at least one of ferric nitrate, manganese sulfate and manganese carbonate.

[0037] According to the present invention, the first carbon source and the second carbon source are each independently selected from at least one of glucose, fructose and ribose.

[0038] In this invention, the mixing conditions described in step 1 are conventional conditions in the art and need not be limited, as long as they can be mixed evenly.

[0039] According to the present invention, in step 2, the initial flow rate is 0.005-0.1 L / s.

[0040] According to the present invention, the reaction conditions include a reaction temperature of 50-80°C.

[0041] A third aspect of this invention protects a manganese iron phosphate precursor prepared by the aforementioned preparation method.

[0042] The fourth aspect of this invention protects a lithium manganese iron phosphate cathode material, wherein the lithium manganese iron phosphate cathode material is obtained by sintering a lithium source and the aforementioned manganese iron phosphate precursor.

[0043] According to the present invention, the molar ratio of the lithium source to the manganese iron phosphate precursor is 0.5-1.2:1.

[0044] In this invention, the lithium source used in the preparation process includes at least one of lithium carbonate, lithium sulfate, and lithium nitrate.

[0045] In this invention, the ferromanganese phosphate precursor and the lithium source are mixed before sintering. The mixing method of the ferromanganese phosphate precursor and the lithium source is a conventional mixing method in the art. Typically, without limitation, the mixing method includes the following steps: mixing the ferromanganese phosphate precursor with the lithium source and water, sand milling, and then spray drying. The amount of water used is at a conventional level in the art. Typically, without limitation, the ratio of the lithium source to the water is (250-300):(0.5-0.7), and the unit of the ratio is g:mol.

[0046] In this invention, the sintering conditions are conventional sintering conditions in the art. Typically, without limitation, under N2 protection, the temperature is increased to 800-900°C at a rate of 1-2°C / min and held for 8-10 hours to obtain lithium manganese iron phosphate cathode material.

[0047] The fifth aspect of this invention protects the application of the aforementioned lithium manganese iron phosphate cathode material in lithium-ion batteries.

[0048] In this invention, the composition and preparation method of the lithium-ion battery are conventional in the field. Typically, and non-limitingly, the composition and preparation method of the positive electrode sheet includes: mixing the positive electrode material with conductive agent acetylene black and binder PVDF in a mass ratio of 90:5:5, adding 3-5g of 1-methyl-2-pyrrolidone, ball milling for 1 hour to form a slurry, uniformly coating it onto an aluminum sheet, drying, and pressing it to form the positive electrode sheet. The composition and preparation method of the negative electrode sheet includes: using a lithium metal sheet as the negative electrode; assembling it into a 2032 coin cell; using a Blue Electric testing system for electrical performance testing; a charge / discharge voltage of 2.0-4.2V; the first cycle at 0.1C / 0.1C; and then 200 cycles at 1C / 1C.

[0049] The technical solution of this invention has the following advantages:

[0050] 1. The present invention provides a manganese iron phosphate precursor, wherein the content of manganese decreases from the core to the surface, the content of iron increases, and the content of carbon decreases; the content of the three elements exhibits a gradient distribution and works synergistically. The battery containing the lithium manganese iron phosphate cathode material obtained from this precursor can achieve both high discharge specific capacity and discharge plateau voltage, and the amount of manganese dissolved is reduced; furthermore, the gradient distribution of carbon content enhances electronic conductivity, promotes the migration and diffusion of lithium ions, and improves the battery conductivity.

[0051] 2. This invention employs a stepwise preparation method, first preparing a base liquid containing manganese, iron, phosphorus, and carbon; then, by controlling multi-stage flow rates, manganese, iron, and carbon are added again. The two steps work together to ensure that the content of manganese, iron, and carbon in the prepared manganese iron phosphate precursor exhibits a gradient distribution, with the content of manganese decreasing from the core to the surface, the content of iron increasing, and the content of carbon decreasing. Batteries containing lithium manganese iron phosphate cathode materials prepared from this precursor can achieve both high discharge specific capacity and discharge plateau voltage, while reducing the amount of manganese dissolved. Detailed Implementation

[0052] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0053] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0054] The content of different metal elements in different parts of the manganese iron phosphate precursor was determined by ICP. The specific method was as follows: 1g of sample was added to 20mL of hydrochloric acid solution (37wt% hydrochloric acid:pure water = 1:1) for digestion to obtain a liquid sample. The liquid sample was then drawn into a plasma torch by an nebulizer. The sample was excited, and the characteristic spectra of the emitted elements were reflected by a mirror and then focused onto the entrance slit of the spectrometer. When the light entered the spectrometer, it struck the grating, and the diffracted light, according to the analytical wavelength, irradiated the photocathode of the photomultiplier tube through the exit slit. The light was converted into electrical energy corresponding to each sample, and the computer directly converted the signal intensity into the corresponding sample concentration and recorded the reading.

[0055] The carbon content was determined using a carbon-sulfur analyzer. The specific method involved oxidizing the sample with oxygen at high temperature in a combustion furnace, introducing it into a carbon detection cell to measure the carbon content, and then reading the values ​​on a computer.

[0056] Polyethylene glycol (weight average molecular weight of 5000 g / mol).

[0057] Example 1

[0058] This embodiment provides a lithium iron phosphate cathode material, the specific preparation steps and operating parameters of which are as follows:

[0059] Add 45 mol of manganese nitrate, 5 mol of ferric nitrate, 50 mol of ammonium dihydrogen phosphate, and 5 mol of glucose, with a molar ratio of manganese nitrate, ferric nitrate, ammonium dihydrogen phosphate, and glucose of 9:1:10:1; then add 12.5 kg of polyethylene glycol as a dispersant (polyethylene glycol to ferric nitrate mass ratio of 10:1), and mix thoroughly to obtain the base solution; add 2 mol of ferrous phosphate as a complexing agent (ferrous phosphate to ferric nitrate molar ratio of 0.4:1), and then add a 2 mol / L manganese nitrate solution, a 1 mol / L ferric nitrate solution, and a 0.3 g / L glucose solution at an initial flow rate of 0. The flow rates were pumped separately into the bottom liquid at 0.005 L / s. The reaction was carried out under a N2 atmosphere at 75 °C. When the average particle size reached 1.8 μm, the flow rates of the manganese nitrate solution and glucose solution were reduced to 9 / 10 of their initial flow rates, while the flow rate of the ferric nitrate solution was increased to twice its initial flow rate. When the average particle size reached 2.5 μm, the flow rates of the manganese nitrate solution and glucose solution were reduced to 3 / 4 of their initial flow rates, while the flow rate of the ferric nitrate solution was increased to three times its initial flow rate. The reaction was stopped when the average particle size reached 3 μm. After aging, filtration, washing, and drying, the manganese ferric phosphate precursor A1 was obtained.

[0060] In the core, the molar ratio of manganese to iron is 9:10, and based on the mass of the manganese iron phosphate precursor in the core, the carbon content is 6 wt%.

[0061] At the midpoint between the core and the surface, the molar ratio of manganese to iron is 7.5:2.5, and based on the mass of the manganese iron phosphate precursor at the midpoint between the core and the surface, the carbon content is 3 wt%.

[0062] At the surface, the molar ratio of manganese to iron is 5:5, and based on the mass of the manganese iron phosphate precursor at the surface, the carbon content is 2 wt%.

[0063] Lithium carbonate and manganese iron phosphate precursor A1 were mixed evenly in 250g of water at a molar ratio of 0.5:1, then sand-milled, spray-dried, and heated to 900℃ at a rate of 2℃ / min under N2 atmosphere and held for 10h. After cooling, lithium iron phosphate cathode material B1 was obtained.

[0064] Example 2

[0065] This embodiment provides a lithium iron phosphate cathode material, the specific preparation steps and operating parameters of which are as follows:

[0066] Add 45 mol of manganese nitrate, 5 mol of ferric nitrate, 50 mol of ammonium dihydrogen phosphate, and 5 mol of glucose, with a molar ratio of manganese nitrate, ferric nitrate, ammonium dihydrogen phosphate, and glucose of 9:1:10:1; then add 12.5 kg of polyethylene glycol as a dispersant (polyethylene glycol to ferric nitrate mass ratio of 10:1), and mix thoroughly to obtain the base solution; add 2 mol of oxalic acid dihydrate as a complexing agent (oxalic acid dihydrate to ferric nitrate molar ratio of 0.4:1), and then add 2 mol / L manganese nitrate solution, 1 mol / L ferric nitrate solution, and 0.3 g / L glucose solution at the initial flow rate. The flow rates were pumped into the bottom liquid at 0.005 L / s, and the reaction was carried out under a N2 atmosphere at 75 °C. When the average particle size reached 1.8 μm, the flow rates of the manganese nitrate solution and glucose solution were reduced to 3 / 4 of their initial flow rates, while the flow rate of the ferric nitrate solution was increased to 3 times its initial flow rate. When the average particle size reached 2.5 μm, the flow rates of the manganese nitrate solution and glucose solution were reduced to 1 / 2 of their initial flow rates, while the flow rate of the ferric nitrate solution was increased to 5 times its initial flow rate, until the reaction was stopped when the average particle size reached 3 μm. After aging, filtration, washing, and drying, manganese ferric phosphate precursor A2 was obtained.

[0067] At the core, the molar ratio of manganese to iron is 9:10, and based on the mass of the manganese iron phosphate precursor at the core, the carbon content is 5 wt%.

[0068] At the midpoint between the core and the surface, the molar ratio of manganese to iron is 7:3, and based on the mass of the manganese iron phosphate precursor at the midpoint between the core and the surface, the carbon content is 4 wt%.

[0069] At the surface, the molar ratio of manganese to iron is 5:5, and based on the mass of the manganese iron phosphate precursor at the surface, the carbon content is 3 wt%.

[0070] Lithium carbonate and manganese iron phosphate precursor A2 were mixed evenly in 250g of water at a molar ratio of 0.5:1, then sand-milled, spray-dried, and heated to 850℃ at a rate of 2℃ / min under N2 atmosphere and held for 10h. After cooling, lithium iron phosphate cathode material B2 was obtained.

[0071] Example 3

[0072] This embodiment provides a lithium iron phosphate cathode material, the specific preparation steps and operating parameters of which are as follows:

[0073] Add 45 mol of manganese nitrate, 5 mol of ferric nitrate, 50 mol of ammonium dihydrogen phosphate, and 5 mol of glucose, with a molar ratio of manganese nitrate, ferric nitrate, ammonium dihydrogen phosphate, and glucose of 9:1:10:1; then add 12.5 kg of polyethylene glycol as a dispersant (polyethylene glycol to ferric nitrate mass ratio of 10:1), mix thoroughly to obtain a base solution, and add 2 mol of ferrous phosphate as a complexing agent (ferrous phosphate to ferric nitrate molar ratio of 0.4:1). Dissolve 2 mol / L manganese nitrate solution, 1 mol / L ferric nitrate solution, and 0.3 g / L glucose solution at an initial flow rate of... The solution was pumped into the base liquid at a rate of 0.05 L / s. The reaction was carried out under a N2 atmosphere at 75 °C. When the average particle size reached 1.8 μm, the flow rates of the manganese nitrate solution and glucose solution were reduced to 2 / 3 of their initial flow rates, while the flow rate of the ferric nitrate solution was increased to 4 times its initial flow rate. When the average particle size reached 2.5 μm, the flow rates of the manganese nitrate solution and glucose solution were reduced to 1 / 3 of their initial flow rates, while the flow rate of the ferric nitrate solution was increased to 7 times its initial flow rate, until the average particle size reached 3 μm. The reaction was then stopped after aging, filtration, washing, and drying to obtain the manganese ferric phosphate precursor A3.

[0074] At the core, the molar ratio of manganese to iron is 9:10, and based on the mass of the manganese iron phosphate precursor at the core, the carbon content is 5 wt%.

[0075] At the midpoint between the core and the surface, the molar ratio of manganese to iron is 6:4, and based on the mass of the manganese iron phosphate precursor at the midpoint between the core and the surface, the carbon content is 4 wt%.

[0076] At the surface, the molar ratio of manganese to iron is 3:7, and based on the mass of the manganese iron phosphate precursor at the surface, the carbon content is 2 wt%.

[0077] Lithium carbonate and manganese iron phosphate precursor A3 were mixed evenly in 250g of water at a molar ratio of 0.5:1, then sand-milled, spray-dried, and heated to 800℃ at a rate of 2℃ / min under N2 atmosphere and held for 10h. After cooling, lithium iron phosphate cathode material B3 was obtained.

[0078] Example 4

[0079] This embodiment provides a lithium iron phosphate cathode material, the specific preparation steps and operating parameters of which are as follows:

[0080] Following the method of Example 1, except that the molar ratio of manganese nitrate, ferric nitrate, ammonium dihydrogen phosphate and glucose is 5:2:6:1 instead of 9:1:10:1, manganese iron phosphate precursor A4 is obtained.

[0081] At the core, the molar ratio of manganese to iron is 8:2, and based on the mass of the manganese iron phosphate precursor at the core, the carbon content is 5 wt%.

[0082] At the midpoint between the core and the surface, the molar ratio of manganese to iron is 6:4, and based on the mass of the manganese iron phosphate precursor at the midpoint between the core and the surface, the carbon content is 3 wt%.

[0083] At the surface, the molar ratio of manganese to iron is 4.8:5.2, and based on the mass of the manganese iron phosphate precursor at the surface, the carbon content is 2 wt%.

[0084] Lithium carbonate and manganese iron phosphate precursor A4 were mixed evenly in 250g of water at a molar ratio of 0.5:1, then sand-milled, spray-dried, and heated to 800℃ at a rate of 2℃ / min under N2 atmosphere and held for 10h. After cooling, lithium iron phosphate cathode material B4 was obtained.

[0085] Example 5

[0086] This embodiment provides a lithium iron phosphate cathode material, the specific preparation steps and operating parameters of which are as follows:

[0087] Following the method of Example 1, except that "initial flow rate is 0.1 L / s" is used instead of "initial flow rate is 0.005 L / s" to obtain manganese iron phosphate precursor A5;

[0088] At the core, the molar ratio of manganese to iron is 9:10, and based on the mass of the manganese iron phosphate precursor at the core, the carbon content is 8 wt%.

[0089] At the midpoint between the core and the surface, the molar ratio of manganese to iron is 7.8:2.2, and based on the mass of the manganese iron phosphate precursor at the midpoint between the core and the surface, the carbon content is 3.5 wt%.

[0090] At the surface, the molar ratio of manganese to iron is 5:5, and based on the mass of the manganese iron phosphate precursor at the surface, the carbon content is 2 wt%.

[0091] Lithium carbonate and manganese iron phosphate precursor A5 were mixed evenly in 250g of water at a molar ratio of 0.5:1, then sand-milled, spray-dried, and heated to 800℃ at a rate of 2℃ / min under N2 atmosphere and held for 10h. After cooling, lithium iron phosphate cathode material B5 was obtained.

[0092] Example 6

[0093] This embodiment provides a lithium iron phosphate cathode material, the specific preparation steps and operating parameters of which are as follows:

[0094] Following the method of Example 1, except that "0.2 g / L glucose solution" was used instead of "0.3 g / L glucose solution" to obtain manganese iron phosphate precursor A6;

[0095] In the core, the molar ratio of manganese to iron is 8.5:1.5, and based on the mass of the manganese iron phosphate precursor in the core, the carbon content is 8 wt%.

[0096] At the midpoint between the core and the surface, the molar ratio of manganese to iron is 7.5:2.5, and based on the mass of the manganese iron phosphate precursor at the midpoint between the core and the surface, the carbon content is 4 wt%.

[0097] At the surface, the molar ratio of manganese to iron is 4.5:5.5, and based on the mass of the manganese iron phosphate precursor at the surface, the carbon content is 2 wt%.

[0098] Lithium carbonate and manganese iron phosphate precursor A6 were mixed evenly in 250g of water at a molar ratio of 0.5:1, then sand-milled, spray-dried, and heated to 800℃ at a rate of 2℃ / min under N2 atmosphere and held for 10h. After cooling, lithium iron phosphate cathode material B6 was obtained.

[0099] Comparative Example 1

[0100] This comparative example provides a lithium iron phosphate cathode material, the specific preparation steps and operating parameters of which are as follows:

[0101] 45 mol of manganese nitrate, 5 mol of ferric nitrate, 50 mol of ammonium dihydrogen phosphate, and 5 mol of glucose were added, with a molar ratio of manganese nitrate, ferric nitrate, ammonium dihydrogen phosphate, and glucose of 9:1:10:1. 12.5 kg of polyethylene glycol was then added as a dispersant, and the mixture was thoroughly mixed to obtain a base solution. 2 mol of ferrous phosphate was added as a complexing agent. A 2 mol / L manganese nitrate solution, a 1 mol / L ferric nitrate solution, and a 0.3 g / L glucose solution were pumped into the base solution at an initial flow rate of 0.005 L / s. The reaction was carried out under a nitrogen atmosphere at 75 °C until the average particle size reached 3 μm. After aging, filtration, washing, and drying, the manganese ferric phosphate precursor DA1 was obtained.

[0102] In the core, the molar ratio of manganese to iron is 9:10, and based on the mass of the manganese iron phosphate precursor in the core, the carbon content is 6 wt%.

[0103] At the midpoint between the core and the surface, the molar ratio of manganese to iron is 9:10, and based on the mass of the manganese iron phosphate precursor at the midpoint between the core and the surface, the carbon content is 6 wt%.

[0104] At the surface, the molar ratio of manganese to iron is 9:10, and based on the mass of the manganese iron phosphate precursor at the surface, the carbon content is 6 wt%.

[0105] Lithium carbonate and manganese iron phosphate precursor DA1 were mixed evenly in 250g of water at a molar ratio of 0.5:1, then sand-milled, spray-dried, and heated to 850℃ at a rate of 2℃ / min under N2 atmosphere and held for 10h. After cooling, lithium iron phosphate cathode material DB1 was obtained.

[0106] Comparative Example 2

[0107] This comparative example provides a lithium iron phosphate cathode material, the specific preparation steps and operating parameters of which are as follows:

[0108] Following the method of Example 1, except that "adding 45 mol of manganese nitrate, 5 mol of ferric nitrate, and 50 mol of ammonium dihydrogen phosphate" was used instead of "adding 45 mol of manganese nitrate, 5 mol of ferric nitrate, 50 mol of ammonium dihydrogen phosphate and 5 mol of glucose" was used to obtain the manganese iron phosphate precursor DA2.

[0109] At the core, the molar ratio of manganese to iron is 9.5:0.5, and based on the mass of the manganese iron phosphate precursor at the core, the carbon content is 0.

[0110] At the midpoint between the core and the surface, the molar ratio of manganese to iron is 8.5:1.5, and based on the mass of the manganese iron phosphate precursor at the midpoint between the core and the surface, the carbon content is 0.

[0111] At the surface, the molar ratio of manganese to iron is 7:3, and based on the mass of the manganese iron phosphate precursor at the surface, the carbon content is 0.

[0112] Lithium carbonate and manganese iron phosphate precursor DA2 were mixed evenly in 250g of water at a molar ratio of 0.5:1, then sand-milled, spray-dried, and heated to 850℃ at a rate of 2℃ / min under N2 atmosphere and held for 10h. After cooling, lithium iron phosphate cathode material DB2 was obtained.

[0113] Comparative Example 3

[0114] 100g of manganese trioxide, trioxide, and lithium carbonate (manganese trioxide, trioxide, and lithium carbonate in a metal molar ratio of 6:4:10.3) were weighed out, 2g of glucose was weighed out, and 250g of deionized water were mixed. The mixture was then milled and spray-dried to obtain lithium iron phosphate cathode material precursor DA3. Under N2 atmosphere, the temperature was increased to 800℃ at a rate of 2℃ / min and held for 10h. After cooling, lithium iron phosphate cathode material DB3 was obtained.

[0115] The cathode materials obtained in the examples and comparative examples were assembled into batteries.

[0116] The prepared positive electrode material was mixed evenly with conductive agent acetylene black and binder PVDF at a mass ratio of 90:5:5. 4g of 1-methyl-2-pyrrolidone was added, and the mixture was ball-milled for 1 hour to form a slurry. This slurry was then evenly coated onto an aluminum sheet. The amount coated onto the current collector was measured, and the mixture was dried and pressed into a positive electrode sheet. A 2032 coin cell was assembled using a lithium metal sheet as the negative electrode. Electrical performance was tested using a Blue Electric testing system with a charge / discharge voltage of 2.0-4.2V. The first cycle was performed at 0.1C / 0.1C, followed by 200 cycles at 1C / 1C.

[0117] Test method for the discharge plateau voltage of batteries made of lithium iron phosphate cathode material: Data is collected at voltage intervals, and then the voltage curve is differentiated. The plateau voltage is determined by the peak value of dQ / dV.

[0118] The method for testing the manganese leaching amount in batteries made from lithium iron phosphate cathode materials is as follows: ICP method is used. Specifically, 1g of sample is added to 20mL of hydrochloric acid solution (37wt% hydrochloric acid:pure water = 1:1) for digestion to obtain a liquid sample. The liquid sample is then drawn into a plasma torch by an nebulizer. The sample is excited, and the characteristic spectrum of the emitted element is reflected by a mirror and focused onto the entrance slit of the spectrometer. When the light enters the spectrometer, it strikes the grating, and the diffracted light, according to the analytical wavelength, illuminates the photocathode of the photomultiplier tube through the exit slit. The light is converted into electrical energy for each sample, and the computer directly converts the signal intensity into the corresponding sample concentration and records the reading.

[0119] The specific test results are shown in Table 1.

[0120] Table 1

[0121] Discharge capacity (mAh / g) Manganese leaching amount (wt%) Discharge plateau voltage (V) Example 1 130.1 0.0032 3.84 Example 2 134.3 0.0048 3.8 Example 3 138.5 0.0083 3.65 Example 4 142 0.0099 3.6 Example 5 126.1 0.0123 3.87 Example 6 136.3 0.0078 3.77 Comparative Example 1 88.9 0.0158 3.95 Comparative Example 2 117.7 0.0224 3.9 Comparative Example 3 140.3 0.0185 3.63

[0122] The present invention provides a manganese iron phosphate precursor, wherein the content of manganese decreases from the core to the surface, the content of iron increases, and the content of carbon decreases; the contents of the three elements exhibit a gradient distribution and synergistic effect. The battery containing the lithium manganese iron phosphate cathode material obtained from this precursor can achieve both high discharge capacity and discharge plateau voltage, and the amount of manganese dissolved is reduced.

[0123] Comparing Example 1 with Comparative Example 2, it can be seen that the discharge plateau voltage of Comparative Example 2 still maintains a high value, but the discharge specific capacity is greatly reduced. This proves that the gradient distribution of carbon content enhances electronic conductivity, promotes the migration and diffusion of lithium ions, improves the battery conductivity, and reduces the amount of manganese dissolved.

[0124] Comparing Example 1 and Example 6, it can be seen that the discharge specific capacity of Example 6 remains at a high level, but the discharge plateau voltage decreases. This proves that a specific carbon source concentration can improve the electrical performance of the cathode material, achieving both a high discharge specific capacity and a high discharge plateau voltage.

[0125] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A manganese iron phosphate precursor, characterized in that, It includes manganese, iron, and carbon elements; among them, from the core to the surface, the content of manganese decreases, the content of iron increases, and the content of carbon decreases. At the core, the molar ratio of manganese to iron is (8-9):(1-2); Based on the mass of the manganese iron phosphate precursor at the core, the carbon content is 5-8 wt%; At the midpoint between the core and the surface, the molar ratio of manganese to iron is (6-7.5):(2.2-4); Based on the mass of the manganese iron phosphate precursor located between the core and the surface, the carbon content is 2-4 wt%. At the surface, the molar ratio of manganese to iron is (3-5):(5-7); The carbon content is 1-2 wt%, based on the mass of the manganese iron phosphate precursor on the surface.

2. The manganese iron phosphate precursor according to claim 1, characterized in that, The average particle size of the manganese iron phosphate precursor is 3-5 μm.

3. A method for preparing the manganese iron phosphate precursor according to claim 1 or 2, characterized in that, Includes the following steps: Step 1: Mix the first manganese source, the first iron source, the first phosphorus source and the first carbon source to obtain the base liquid; Step 2: Add manganese source solution, iron source solution, carbon source solution and complexing agent to the bottom liquid at the initial flow rate and carry out the reaction. When the average particle size reaches 1.5-2 μm, reduce the flow rate of manganese source solution and carbon source solution to 2 / 3-9 / 10 of their initial flow rates, respectively; increase the flow rate of iron source solution to 2-4 times its initial flow rate. When the reaction reaches an average particle size of 2.5-2.8 μm, reduce the flow rates of the manganese source solution and the carbon source solution to 1 / 3-3 / 4 of their initial flow rates, respectively; and increase the flow rate of the iron source solution to 3-7 times its initial flow rate. Step 3: The material obtained after the reaction in Step 2 is aged and separated to obtain the manganese iron phosphate precursor.

4. The preparation method according to claim 3, characterized in that, In step 1, the molar ratio of the first manganese source, the first iron source, the first phosphorus source, and the first carbon source is (5-10):(0.5-2):(5-12):

1.

5. The preparation method according to claim 4, characterized in that, The molar ratio of the first manganese source, the first iron source, the first phosphorus source, and the first carbon source is (8-9):(1-1.5):(9-10):

1.

6. The preparation method according to claim 3, characterized in that, In step 1, a dispersant is added before mixing.

7. The preparation method according to claim 6, characterized in that, The mass ratio of the dispersant to the first iron source is 8-12:1; And / or, the dispersant includes polyethylene glycol and / or polyacrylamide.

8. The preparation method according to claim 3, characterized in that, In step 2, the molar ratio of the complexing agent to the first iron source is 0.3-0.6:1; And / or, in step 2, the complexing agent includes ferrous phosphate and / or oxalate dihydrate; And / or, in step 2, the manganese source solution contains a second manganese source and water, wherein the concentration of the second manganese source is 1.8-2.2 mol / L; And / or, in step 2, the iron source solution contains a second iron source and water, wherein the concentration of the second iron source is 0.7-1.3 mol / L; And / or, in step 2, the carbon source solution contains a second carbon source and water, wherein the concentration of the second carbon source is 0.3-0.5 g / L; And / or, the first manganese source and the second manganese source are each independently selected from at least one of manganese nitrate, manganese sulfate and manganese carbonate; And / or, the first iron source and the second iron source are each independently selected from ferric nitrate; And / or, the first carbon source and the second carbon source are each independently selected from at least one of glucose and fructose.

9. The preparation method according to claim 3, characterized in that, In step 2, the initial flow rate is 0.005-0.1 L / s; And / or, the reaction conditions include: a reaction temperature of 50-80°C.

10. A lithium manganese iron phosphate cathode material, characterized in that, The lithium manganese iron phosphate cathode material is prepared by sintering a lithium source and a precursor of manganese iron phosphate as described in claim 1 or 2, or a precursor of manganese iron phosphate prepared by any one of claims 3-9.

11. The lithium iron phosphate cathode material according to claim 10, characterized in that, The molar ratio of the lithium source to the manganese iron phosphate precursor is 0.5-1.2:

1.

12. The application of the lithium manganese iron phosphate cathode material according to claim 10 or 11 in a lithium-ion battery.