Lithium manganese iron phosphate modified material, and positive electrode and lithium ion battery using same

Abstract

A lithium manganese iron phosphate modified material, and a positive electrode and a lithium ion battery using same. The lithium manganese iron phosphate modified material comprises a nickel-cobalt-manganese material, and, successively coating the outside of the nickel-cobalt-manganese material, a first carbon coating layer, a lithium manganese iron phosphate coating layer and a second carbon coating layer; the lithium manganese iron phosphate coating layer comprises a lithium manganese iron phosphate material, the chemical formula of the lithium manganese iron phosphate material being LiMn(1-a)FeaPO4, where 0.3≤a≤0.7; and the chemical formula of the nickel-cobalt-manganese material is LiNixCoyMn(1-x-y), where 0.5≤x≤0.8, and 0<x+y<1.

Description

A lithium manganese iron phosphate modified material and a positive electrode and lithium ion battery using the same

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on August 28, 2023, with application number 202311086956.3. The entire contents of the above application are incorporated by reference into this application. Technical Field

[0002] The present application relates to the technical field of lithium-ion batteries, and in particular to a lithium manganese iron phosphate modified material and a positive electrode and a lithium-ion battery using the same. Background Art

[0003] New energy electric vehicles have developed rapidly in recent years, and the research and development of power batteries has also been in full swing. However, current energy density-oriented subsidy policies have focused R&D efforts on the development of high-energy-density battery materials. Furthermore, the "Notice on Soliciting Enterprise Opinions on the 'Letter on Temporarily Suspending the Use of Ternary Material Power Lithium-ion Batteries in Commercial Vehicles'" issued by relevant authorities fully reflects public concerns about the safety of ternary materials, known for their high specific capacity. Meanwhile, while materials such as lithium iron phosphate (LFP) have a stable structure and batteries using them offer good safety, their low specific energy content results in a short driving range for passenger vehicles using LFP batteries, making them inadequate for travel needs.

[0004] Like lithium iron phosphate (LFP), lithium iron manganese phosphate (LFP) also has an olivine-type structure. The phosphate tetrahedrons in LFP are very stable, providing structural support during the charge and discharge process, resulting in LFP's excellent thermodynamic and kinetic stability. Comparing the performance of conventional commercial power battery cathode materials reveals that LFP has similar safety characteristics. However, its voltage plateau (4.1V) is 0.7V higher than that of LFP. Given the same battery design, LFP can increase battery energy density by 20%. Compared to lithium manganese oxide (LMO), LFP has higher energy density and superior high-temperature cycle life, despite having similar voltage plateaus. Compared to ternary materials, LFP offers greater safety and a lower material cost, despite similar energy densities.

[0005] In order to fully utilize the low-cost advantage of lithium manganese iron phosphate materials and improve their power performance and cycle performance, the relevant method mainly modifies the lithium manganese iron phosphate materials by doping them with other metal ions, thereby improving their electrochemical activity. The combination of lithium manganese iron phosphate and other positive electrode materials can complement each other in performance and produce a synergistic effect. Technical issues

[0006] When lithium manganese iron phosphate and nickel cobalt manganese materials are mixed during the slurry preparation process, the slurry viscosity changes too quickly and is unevenly dispersed, making coating difficult. To address this, a double-layer or even multi-layer coating method can be used, but this increases manufacturing costs and makes it difficult to control the coating surface density and coating uniformity. Technical Solutions

[0007] In the first aspect, the present application provides a lithium iron manganese phosphate modified material, comprising a nickel-cobalt-manganese material, a first carbon coating layer, a lithium iron manganese phosphate coating layer, and a second carbon coating layer; in the lithium iron manganese phosphate modified material, the nickel-cobalt-manganese material is sequentially coated with a first carbon coating layer, a lithium iron manganese phosphate coating layer, and a second carbon coating layer; the lithium iron manganese phosphate coating layer comprises a lithium iron manganese phosphate material, and the chemical formula of the lithium iron manganese phosphate material is LiMn (1-a) Fe a PO4, where 0.3≤a≤0.7; the chemical formula of nickel-cobalt-manganese material is LiNi x Co y Mn (1-x-y) , where 0.5≤x≤0.8, 0 <x+y<1。

[0008] In a second aspect, the present application provides a positive electrode, comprising a current collector and a positive electrode active coating disposed on the surface of the current collector, wherein the positive electrode active coating comprises the lithium manganese iron phosphate modified material provided in the present application.

[0009] In a third aspect, the present application provides a lithium-ion battery, comprising the positive electrode provided by the present application. Beneficial effects

[0010] The beneficial effects of the lithium iron manganese phosphate modified material provided by the present application are: through the separate coating of the first carbon coating layer and the second carbon coating layer, the viscosity stability of the lithium iron manganese phosphate modified material can be enhanced, and the high-temperature stability and cycle performance of the lithium iron manganese phosphate modified material can be improved; wherein, the coating of the first carbon coating layer can protect the nickel-cobalt-manganese material, and avoid structural damage to the lithium iron manganese phosphate modified material due to volume expansion of the nickel-cobalt-manganese material during the charge and discharge process; and the coating of the second carbon coating layer can protect the lithium iron manganese phosphate material to improve the manganese dissolution of the lithium iron manganese phosphate material at high temperature, and provide a second layer of protection for the nickel-cobalt-manganese material to avoid excessive attenuation of the nickel-cobalt-manganese material during the cycle; therefore, the lithium iron manganese phosphate modified material provided by the present application has good viscosity stability, high-temperature stability and cycle performance. Modes for Carrying Out the Invention

[0011] Example 1

[0012] Treatment group 1A

[0013] 1. Preparation of LiMnFePO4 Modified Materials

[0014] S1. 0.08 mol of nickel nitrate, 0.01 mol of manganese nitrate, 0.01 mol of cobalt nitrate, 0.15 mol of lithium carbonate, 0.15 mol of urea and 0.02 mol of ammonium fluoride were mixed and stirred for 25 minutes, and then the mixture was transferred to a hydrothermal reactor and hydrothermally reacted at 160°C for 420 minutes to obtain a nickel-cobalt-manganese material. The nickel-cobalt-manganese material was then centrifuged, dried and ground into a powder with a particle size of 1000 nm. The ground nickel-cobalt-manganese material was annealed at 650°C in an air atmosphere for 3 hours, and then the obtained nickel-cobalt-manganese material was ultrasonically infiltrated with a 0.01 mol / L glucose solution for 10 minutes, and then dried and the obtained solid was annealed at 800°C in a nitrogen atmosphere for 5 hours to obtain a nickel-cobalt-manganese composite material;

[0015] S2. 0.25 mol of a phosphorus source, 0.15 mol of ferrous sulfate, 0.1 mol of manganese sulfate, 0.25 mol of lithium hydroxide, 0.02 mol of ammonium fluoride and 0.1 mol of a nickel-cobalt-manganese composite material were mixed, and 80 mL of ethanol was added. After nitrogen protection, the mixture was sealed and stirred for 20 minutes to dissolve and disperse. The resulting suspension was transferred to a hydrothermal reactor, and the temperature was raised to 115 ° C for 300 minutes. After the hydrothermal reaction was completed, the resulting suspension was centrifuged to obtain a solid precursor of lithium iron manganese phosphate.

[0016] S3. The lithium iron manganese phosphate precursor is dried and ground into a powder with a particle size of 1500 nm. The obtained lithium iron manganese phosphate precursor powder is then ultrasonically immersed in a 0.1 mol / L glucose solution for 30 minutes, and then dried. Subsequently, the mixture is annealed in an inert atmosphere at 750°C for 480 minutes to obtain a lithium iron manganese phosphate modified material.

[0017] 2. Preparation of lithium-ion batteries

[0018] (1) Preparation of positive electrode sheet: The lithium manganese iron phosphate modified material, SP, CNT and PVDF were mixed and stirred in a mass ratio of 96:1.8:1.5:0.7, and NMP was used as a solvent to obtain a positive electrode slurry. The obtained positive electrode slurry was evenly coated on a 12 μm carbon-coated aluminum foil and dried at 90 °C to obtain a dry positive electrode sheet.

[0019] (2) Preparation of negative electrode sheet: artificial graphite, SP, CMC and SBR were mixed and stirred in a mass ratio of 95.5:2:0.5:2, and deionized water was used as a solvent to obtain a negative electrode slurry. The negative electrode slurry was then coated on an 8μm copper foil and the electrode was vacuum dried at 100°C.

[0020] (3) Preparation of lithium-ion battery electrolyte: The electrolyte is prepared in a glove box filled with argon, wherein the water content in the glove box is less than 10 ppm and the oxygen content is less than 1 ppm. The preparation of the electrolyte includes the following steps: ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) are mixed with 1.2 mol / L lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LFSI) in a volume ratio of 30:40:30, wherein the mass ratio of LiPF6:LFSI is 1.1:0.1, and then 0.5 wt.% of additives vinylene carbonate (VC), 1.5 wt.% of fluoroethylene carbonate (FEC), 0.5% of vinyl sulfate (DTD) and 0.5% of lithium difluorophosphate (LiPO2F2) are added, mixed evenly and set aside.

[0021] (4) Lithium-ion battery assembly: The vacuum-dried positive electrode sheet, negative electrode sheet and separator are punched out and then assembled into CR2032 button batteries in an argon-filled glove box.

[0022] Treatment group 2A

[0023] This treatment group prepared the lithium iron manganese phosphate modified material with reference to the formula and method provided by the treatment group 1A of Example 1. The difference between this treatment group and the treatment group 1A of Example 1 is that when preparing the lithium iron manganese phosphate modified material, this treatment group replaced the 0.01 mol / L glucose solution in S1 and S3 with 0.01 mol / L starch solution. Except for this difference, the operating steps for preparing the lithium iron manganese phosphate modified material and lithium-ion battery in this treatment group are strictly consistent with those of the treatment group 1A of Example 1.

[0024] Comparative group 1A

[0025] This comparative group prepared the lithium iron manganese phosphate modified material using the formula and method provided by treatment group 1A of Example 1. The difference between this comparative group and treatment group 1A of Example 1 is that when preparing the lithium iron manganese phosphate modified material, this comparative group did not perform carbon coating on the nickel-cobalt-manganese material in S1. Apart from this difference, the operating steps for preparing the lithium iron manganese phosphate modified material and lithium-ion battery in this comparative group were strictly consistent with those of treatment group 1A of Example 1. Specifically, the steps for preparing the lithium iron manganese phosphate modified material in this comparative group were as follows:

[0026] S1. 0.08 mol of nickel nitrate, 0.01 mol of manganese nitrate, 0.01 mol of cobalt nitrate, 0.15 mol of lithium carbonate, 0.15 mol of urea and 0.02 mol of ammonium fluoride were mixed and stirred for 25 minutes, and then the mixture was transferred to a hydrothermal reactor and hydrothermally reacted at 160 ° C for 420 minutes. The solid was centrifuged and dried and ground. The ground nickel-cobalt-manganese material was annealed at 650 ° C in an air atmosphere for 3 hours, and then sufficient nitrogen was introduced and annealed at 800 ° C in a nitrogen atmosphere for 5 hours to obtain a nickel-cobalt-manganese material;

[0027] S2 and S3 were strictly consistent with treatment group 1A in Example 1.

[0028] Comparative group 2A

[0029] This comparative group prepared the lithium iron manganese phosphate modified material using the formula and method provided by treatment group 1A of Example 1. The difference between this comparative group and treatment group 1A of Example 1 is that when preparing the lithium iron manganese phosphate modified material, this comparative group did not perform carbon coating on the lithium iron manganese phosphate precursor in S3. Apart from this difference, the operating steps for preparing the lithium iron manganese phosphate modified material and lithium-ion battery in this comparative group were strictly consistent with those of treatment group 1A of Example 1. Specifically, the steps for preparing the lithium iron manganese phosphate modified material in this comparative group were as follows:

[0030] S1 and S2 were strictly consistent with treatment group 1A in Example 1;

[0031] S3. Dry and grind the lithium iron manganese phosphate precursor into powder, and then anneal the obtained lithium iron manganese phosphate precursor in an inert atmosphere at 750° C. for 480 minutes to obtain a lithium iron manganese phosphate modified material.

[0032] Comparative group 3A

[0033] This comparative group prepares lithium iron manganese phosphate modified materials and lithium ion batteries using the formula and method provided by treatment group 1A of reference example 1. The difference between this comparative group and treatment group 1A of example 1 is that when preparing the lithium iron manganese phosphate modified material, this comparative group prepares lithium iron manganese phosphate material and nickel cobalt manganese material respectively by hydrothermal reaction, and carbon-coates the lithium iron manganese phosphate material and nickel cobalt manganese material respectively by high-temperature reaction to obtain lithium iron manganese phosphate composite material and nickel cobalt manganese composite material. Finally, the two are physically mixed during the slurry preparation process. Except for this difference, the operating steps for preparing lithium ion batteries in this comparative group are strictly consistent with those of treatment group 1A of example 1. Specifically, the steps for preparing the lithium iron manganese phosphate modified material in this comparative group are as follows:

[0034] S1. Strictly consistent with treatment group 1A of Example 1;

[0035] S2. 0.15 mol of ferrous sulfate, 0.1 mol of manganese sulfate, 0.25 mol of lithium hydroxide and 0.02 mol of ammonium fluoride were mixed, and 80 mL of ethanol was added. After nitrogen protection, the mixture was sealed, stirred and dissolved for 20 minutes; the obtained suspension was transferred to a hydrothermal kettle, the temperature was raised to 115°C and the hydrothermal reaction was carried out for 300 minutes. After the hydrothermal reaction was completed, the suspension was centrifuged, the obtained solid was dried and ground into powder, and the obtained solid was ultrasonically immersed in a 0.1 mol / L glucose solution for 30 minutes, and then dried. The solid was then placed in an inert atmosphere at 750°C for annealing for 480 minutes to obtain a lithium manganese iron phosphate composite material.

[0036] Preparation of positive electrode slurry: lithium manganese iron phosphate composite material, nickel cobalt manganese composite material, SP, CNT and PVDF were mixed and stirred in a mass ratio of 76.8:19.2:1.8:1.5:0.7, and NMP was used as a solvent to obtain a positive electrode slurry.

[0037] Control group 4A

[0038] This comparative group prepared lithium iron manganese phosphate modified materials and lithium-ion batteries using the formula and method provided by treatment group 1A of Example 1. The difference between this comparative group and treatment group 1A of Example 1 is that when preparing the lithium iron manganese phosphate modified material, this comparative group prepared the lithium iron manganese phosphate material and the nickel cobalt manganese material separately through hydrothermal reaction and high-temperature reaction, and finally physically mixed the two during the slurry preparation process. Apart from this difference, the operating steps for preparing the lithium iron manganese phosphate modified material and lithium-ion battery in this comparative group are strictly consistent with those of treatment group 1A of Example 1. Specifically, the steps for preparing the lithium iron manganese phosphate modified material in this comparative group are as follows:

[0039] S1. 0.08 mol of nickel nitrate, 0.01 mol of manganese nitrate, 0.01 mol of cobalt nitrate, 0.15 mol of lithium carbonate, 0.15 mol of urea and 0.02 mol of ammonium fluoride were mixed and stirred for 25 minutes, and then the mixture was transferred to a hydrothermal reactor and hydrothermally reacted at 160 ° C for 420 minutes. After centrifugation and drying, the solid was ground and the ground nickel-cobalt-manganese material was annealed at 650 ° C in an air atmosphere for 3 hours, followed by introduction of sufficient nitrogen and annealing at 800 ° C in a nitrogen atmosphere for 5 hours to obtain a nickel-cobalt-manganese material;

[0040] S2. 0.15 mol of ferrous sulfate, 0.1 mol of manganese sulfate, 0.25 mol of lithium hydroxide and 0.02 mol of ammonium fluoride were mixed, and 80 mL of ethanol was added. After nitrogen protection, the mixture was sealed, stirred and dissolved for 20 minutes; the obtained suspension was transferred to a hydrothermal kettle, the temperature was raised to 115°C and the hydrothermal reaction was carried out for 300 minutes. After the hydrothermal reaction was completed, the obtained suspension was centrifuged, the obtained solid was dried and ground into powder, and the obtained solid was placed in an inert atmosphere at 750°C for annealing for 480 minutes to obtain lithium manganese iron phosphate material.

[0041] Preparation of positive electrode slurry: lithium manganese iron phosphate material, nickel cobalt manganese material, SP, CNT and PVDF were mixed and stirred in a mass ratio of 76.8:19.2:1.8:1.5:0.7, and NMP was used as a solvent to obtain a positive electrode slurry.

[0042] Test Example 1

[0043] 1. Test subjects

[0044] Positive electrode slurries and lithium-ion batteries prepared in each treatment group and control group of Example 1.

[0045] 2. Test Method

[0046] (1) Cathode slurry viscosity test: Use a viscometer to monitor the viscosity of the cathode slurry for 12 hours.

[0047] (2) High-temperature storage performance: At room temperature, the lithium-ion battery is charged at a constant current of 0.5C (nominal capacity) to a voltage of 4.25V, and then charged at a constant voltage of 4.25V to a current ≤0.05C. The initial volume and initial discharge capacity of the battery cell are measured. The battery cell is then stored in a 60°C constant temperature box for 30 days. After that, the final volume of the battery cell is tested, and the battery is discharged at 0.5C to 2.5V to measure the battery's retention capacity and recovery capacity. Among them, the high-temperature battery capacity retention rate is calculated according to formula ①, and the high-temperature battery capacity loss rate is calculated according to formula ②.

[0048]

[0049]

[0050] (3) Cycle performance test: The prepared lithium-ion battery was placed at 25°C for 1 hour and then subjected to a charge and discharge cycle performance test at a rate of 3C. The charge and discharge voltage range was 2.5V~4.25V. The cycle capacity retention rate, DCIR growth rate, and charge constant current ratio were measured.

[0051] 3. Test results and analysis

[0052] The test results of this test example are shown in Table 1, in which this test example mainly explores the effect of carbon coating on the lithium iron manganese phosphate modified material and the lithium ion battery using it. The lithium iron manganese phosphate modified material prepared in comparison group 1A did not perform multi-layer coating on the nickel cobalt manganese material, resulting in a sharp drop in the cycle capacity of the prepared lithium ion battery during the charge and discharge process. In comparison group 2A, the prepared lithium iron manganese phosphate modified material did not undergo carbon coating on the lithium iron manganese phosphate precursor, resulting in the lithium iron manganese phosphate material being exposed, which in turn led to a significant decrease in the high-temperature storage performance of the lithium ion battery. This is because the coating of the lithium iron manganese phosphate with carbon material can effectively improve the dissolution of manganese in the lithium iron manganese phosphate material. In comparison group 3A, the lithium iron manganese phosphate composite material and the nickel cobalt manganese composite material were prepared respectively by hydrothermal reaction and high-temperature treatment, and the lithium iron manganese phosphate composite material and the nickel cobalt manganese composite material were physically mixed when preparing the positive electrode slurry. It can be found that even if the lithium iron manganese phosphate composite material and the nickel cobalt manganese composite material are coated with carbon material, the viscosity of the positive electrode slurry still changes significantly. Furthermore, in Comparative Group 4A, a lithium iron manganese phosphate material and a nickel-cobalt-manganese material without carbon coating were prepared separately through hydrothermal reaction and high-temperature treatment. The lithium iron manganese phosphate material and the nickel-cobalt-manganese material were then physically mixed when preparing the positive electrode slurry. The test results show that the change in the positive electrode slurry is greater than that prepared in Comparative Group 3A. Therefore, the comparison of Groups 3A to 4A demonstrates that the lithium iron manganese phosphate modified material provided in this application can improve the abnormal viscosity of the slurry after mixing lithium iron manganese phosphate and nickel-cobalt in the related art.

[0053] In treatment groups 1A~2A, the effects of different carbon sources on the prepared lithium iron manganese phosphate modified materials were compared. Among them, the test results show that the viscosity stability, high temperature storage stability and cycle performance of the positive electrode slurry measured in treatment group 1A are better than those of the lithium iron manganese phosphate modified material and lithium-ion battery prepared in treatment group 2A. Therefore, this test example proves that the lithium iron manganese phosphate modified material provided by this application can not only effectively improve the viscosity stability of the positive electrode slurry and improve the high temperature stability of the lithium iron manganese phosphate modified material, but also effectively slow down the attenuation rate of the nickel, cobalt and manganese material in the lithium iron manganese phosphate modified material.

[0054] Table 1. Test results of test case 1

[0055]

[0056] Example 2

[0057] Treatment group 1B

[0058] This treatment group prepared lithium manganese iron phosphate modified materials and lithium ion batteries according to the formula and method provided by treatment group 1A in Example 1.

[0059] Treatment group 2B

[0060] This treatment group prepared the lithium iron manganese phosphate modified material with reference to the formula and method provided by treatment group 1B of Example 2. The difference between this treatment group and treatment group 1B of Example 2 is that when preparing the lithium iron manganese phosphate modified material, this treatment group does not anneal the nickel-cobalt-manganese material in S1. Instead, the nickel-cobalt-manganese material is directly mixed with the carbon source and then subjected to the first high-temperature treatment. Apart from this difference, the operating steps for preparing the lithium iron manganese phosphate modified material and lithium-ion battery in this treatment group are strictly consistent with those of treatment group 1B of Example 2. Specifically, the steps for preparing the lithium iron manganese phosphate modified material in this treatment group are as follows:

[0061] S1. 0.08 mol of nickel nitrate, 0.01 mol of manganese nitrate, 0.01 mol of cobalt nitrate, 0.15 mol of lithium carbonate, 0.15 mol of urea and 0.02 mol of ammonium fluoride were mixed and stirred for 25 minutes, and then the mixture was transferred to a hydrothermal reactor and hydrothermally reacted at 160°C for 420 minutes to obtain a nickel-cobalt-manganese material. The nickel-cobalt-manganese material was then centrifuged, dried and ground into powder; the ground nickel-cobalt-manganese material was ultrasonically infiltrated with 0.01 mol / L glucose solution for 10 minutes, and then dried and the resulting solid was annealed at 800°C in a nitrogen atmosphere for 5 hours to obtain a nickel-cobalt-manganese composite material;

[0062] S2 and S3 were strictly consistent with treatment group 1B in Example 2.

[0063] Treatment group 3B

[0064] This treatment group prepared the lithium iron manganese phosphate modified material with reference to the formula and method provided by treatment group 1B of Example 2. The difference between this treatment group and treatment group 1B of Example 2 is that when preparing the lithium iron manganese phosphate modified material, this treatment group did not grind the nickel-cobalt-manganese material in S1. Apart from this difference, the operating steps for preparing the lithium iron manganese phosphate modified material and lithium-ion battery in this treatment group are strictly consistent with those of treatment group 1B of Example 2. Specifically, the steps for preparing the lithium iron manganese phosphate modified material in this treatment group are as follows:

[0065] S1. 0.08 mol of nickel nitrate, 0.01 mol of manganese nitrate, 0.01 mol of cobalt nitrate, 0.15 mol of lithium carbonate, 0.15 mol of urea and 0.02 mol of ammonium fluoride were mixed and stirred for 25 minutes, and then the mixture was transferred to a hydrothermal reactor and hydrothermally reacted at 160°C for 420 minutes to obtain a nickel-cobalt-manganese material. The nickel-cobalt-manganese material was then centrifuged and dried, and then annealed at 650°C in an air atmosphere for 3 hours. The nickel-cobalt-manganese material was then ultrasonically infiltrated with a 0.01 mol / L glucose solution for 10 minutes, and then dried and the resulting solid was annealed at 800°C in a nitrogen atmosphere for 5 hours to obtain a nickel-cobalt-manganese composite material;

[0066] S2 and S3 were strictly consistent with treatment group 1B in Example 2.

[0067] Treatment group 4B

[0068] This treatment group prepared the lithium iron manganese phosphate modified material with reference to the formula and method provided by treatment group 1B of Example 2. The difference between this treatment group and treatment group 1B of Example 2 is that when preparing the lithium iron manganese phosphate modified material, the hydrothermal reaction in S2 is carried out under an air atmosphere. Apart from this difference, the operating steps for preparing the lithium iron manganese phosphate modified material and lithium-ion battery in this treatment group are strictly consistent with those of treatment group 1B of Example 2. Specifically, the steps for preparing the lithium iron manganese phosphate modified material in this treatment group are as follows:

[0069] S1. Strictly consistent with treatment group 1B of Example 2;

[0070] S2. 0.15 mol of ferrous sulfate, 0.1 mol of manganese sulfate, 0.25 mol of lithium hydroxide, 0.02 mol of ammonium fluoride and 0.1 mol of nickel-cobalt-manganese composite material were mixed, and 80 mL of ethanol was added, and the mixture was sealed and stirred for 20 minutes to dissolve and disperse; the resulting suspension was transferred to a hydrothermal reactor, and the temperature was raised to 115 ° C for 300 minutes of hydrothermal reaction. After the hydrothermal reaction was completed, the resulting suspension was centrifuged to obtain a solid precursor of lithium manganese iron phosphate;

[0071] S3. Strictly consistent with treatment group 1B of Example 2.

[0072] Test Example 2

[0073] 1. Test subjects

[0074] The positive electrode slurry and lithium-ion battery prepared in each treatment group of Example 2.

[0075] 2. Test Method

[0076] The experiment of this test example was carried out according to the test method of Test Example 1.

[0077] 3. Test results and analysis

[0078] The test results of this test example are shown in Table 2, wherein this test example mainly explores the influence of the method of preparing lithium manganese iron phosphate on the lithium manganese iron phosphate modified material and the lithium ion battery using the same.

[0079] In treatment group 2B, the nickel-cobalt-manganese composite material was not annealed in S1 during the preparation of the lithium iron manganese phosphate modified material. Instead, the nickel-cobalt-manganese composite material was directly mixed with a carbon source and then subjected to the first high-temperature treatment. This resulted in a decrease in the structural stability of the nickel-cobalt-manganese composite material in the lithium iron manganese phosphate modified material of treatment group 2B, which in turn caused a decrease in the cycle capacity retention rate of the corresponding lithium-ion battery.

[0080] In treatment group 3B, since the nickel-cobalt-manganese material is not ground in S1 when preparing the lithium iron manganese phosphate modified material, the particle size of the nickel-cobalt-manganese material in the lithium iron manganese phosphate modified material prepared by treatment group 3B is uneven, which in turn increases the DCIR growth rate of the corresponding lithium-ion battery and reduces the charging constant current ratio.

[0081] In treatment group 4B, when preparing the lithium iron manganese phosphate modified material, the hydrothermal reaction in S2 was carried out under an air atmosphere, resulting in a decrease in the stability of the synergistic effect between the lithium iron manganese phosphate material and the nickel-cobalt-manganese composite material in the lithium iron manganese phosphate modified material prepared by treatment group 4B, thereby causing a decrease in the viscosity stability of the corresponding positive electrode slurry and a decrease in the high-temperature storage performance of the lithium-ion battery.

[0082] Table 2. Test results of test case 2

[0083]

[0084] Example 3

[0085] Based on the test results of Test Example 1, the carbon coating method used in the preparation of the lithium iron manganese phosphate modified material will affect the resulting lithium iron manganese phosphate modified material. The lithium iron manganese phosphate modified material prepared by Treatment Group 1A of Example 1 is the best, and the corresponding lithium battery has the best overall performance. Therefore, in this example, Treatment Group 1C is set up according to Treatment Group 1A of Example 1. In addition, Treatment Groups 1C to 3C of Example 3 use the surfactant material used in S2 in the preparation of the lithium iron manganese phosphate modified material as a variable. The variables of Treatment Groups 1C to 3C of Example 3 are shown in Table 3. Furthermore, Treatment Groups 1C to 3C of Example 3 prepare lithium iron manganese phosphate modified materials and lithium-ion batteries with reference to Treatment Group 1A of Example 1.

[0086] Table 3. Variables of silicon-carbon composites prepared by treatment groups 1C to 3C

[0087]

[0088] Test Example 3

[0089] 1. Test subjects

[0090] Positive electrode slurries and lithium-ion batteries prepared in each treatment group of Example 3.

[0091] 2. Test Method

[0092] The experiment of this test example was carried out according to the test method of Test Example 1.

[0093] 3. Test results and analysis

[0094] The test results of this test example are shown in Table 4, wherein this test example mainly explores the influence of the surfactant material on the lithium iron manganese phosphate modified material and the lithium ion battery using it when preparing the lithium iron phosphate precursor. Through the testing of the lithium iron manganese phosphate modified material and lithium ion battery corresponding to the treatment groups 1C~3C, it can be found that the surfactant will affect the growth of the lithium iron manganese phosphate material on the surface of the nickel cobalt manganese composite material, thereby affecting the stability of the lithium iron manganese phosphate modified material prepared by the treatment groups 1C~3C. In particular, in treatment group 3C, when no surfactant was introduced and only deionized water was used instead, the lithium iron manganese phosphate material grew unevenly on the surface of the nickel cobalt manganese composite material, thereby causing the positive electrode slurry viscosity stability and high temperature storage performance of the lithium iron manganese phosphate modified material prepared by treatment group 3C to decrease.

[0095] Table 4. Test results of test case 3

[0096] .

Claims

1. A lithium iron manganese phosphate modified material, comprising a nickel cobalt manganese material, a first carbon coating layer, a lithium iron manganese phosphate coating layer and a second carbon coating layer; In the lithium iron manganese phosphate modified material, the outside of the nickel-cobalt-manganese material is sequentially coated with the first carbon coating layer, the lithium iron manganese phosphate coating layer and the second carbon coating layer; The lithium iron manganese phosphate coating layer includes a lithium iron manganese phosphate material, and the chemical formula of the lithium iron manganese phosphate material is LiMn (1-a) Fe a PO4, where 0.3≤a≤0.7； The chemical formula of the nickel-cobalt-manganese material is LiNi x Co y Mn (1-x-y) , where 0.5≤x≤0.8, 0 <x+y<1。 2. The lithium iron manganese phosphate modified material according to claim 1, wherein: The particle size of the nickel-cobalt-manganese material is 1000nm~1500nm.

3. The lithium iron manganese phosphate modified material according to claim 2, wherein: The particle size of the lithium manganese iron phosphate modified material is 1100nm~1900nm.

4. The lithium iron manganese phosphate modified material according to claim 1, wherein: Calculated by mass percentage, the carbon content of the lithium manganese iron phosphate modified material is 0.5%~3%.

5. The lithium iron manganese phosphate modified material according to any one of claims 1 to 4, wherein: The method for preparing the lithium manganese iron phosphate modified material comprises the following steps: S1. mixing the carbon source for providing the first carbon coating layer with the nickel-cobalt-manganese material and performing a first high-temperature treatment to coat the surface of the nickel-cobalt-manganese material with the first carbon coating layer to obtain a nickel-cobalt-manganese composite material; S2. mixing a phosphorus source, an iron source, a manganese source, a lithium source, a surfactant, and the nickel-cobalt-manganese composite material under an inert gas atmosphere, and forming the lithium iron manganese phosphate coating layer on the surface of the nickel-cobalt-manganese composite material by a hydrothermal reaction, thereby obtaining a lithium iron manganese phosphate precursor; S3. Mix the lithium iron manganese phosphate precursor and the carbon source for forming the second carbon coating layer, and perform a second high-temperature treatment to form the second carbon coating layer on the surface of the lithium iron manganese phosphate coating layer, thereby preparing the lithium iron manganese phosphate modified material.

6. The lithium iron manganese phosphate modified material according to claim 5, wherein: In S1, the first high temperature treatment is an annealing treatment at 750° C. to 850° C. for 4 to 6 hours in an inert gas atmosphere.

7. The lithium iron manganese phosphate modified material according to claim 5, wherein: In S2, the surfactant is ammonium fluoride.

8. The lithium iron manganese phosphate modified material according to claim 5, wherein: In S3, the second high temperature treatment is an annealing treatment at 700° C. to 800° C. for 7 to 9 hours in an inert gas atmosphere.

9. A positive electrode, comprising a current collector and a positive electrode active coating disposed on the surface of the current collector, wherein the positive electrode active coating comprises the lithium manganese iron phosphate modified material according to any one of claims 1 to 8.

10. A lithium ion battery comprising the positive electrode according to claim 9.

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