Magnesium-manganese co-doped lithium iron phosphate as well as preparation method and application thereof

Abstract

The invention provides magnesium-manganese co-doped lithium iron phosphate as well as a preparation method and application thereof, and solves the problem that an existing LFP material is relatively low in specific discharge capacity under a 5C-10C high-rate condition. The preparation method comprises the following steps: firstly, adding iron phosphate dihydrate into water, uniformly mixing, adding a phosphoric acid solution of manganese phosphate and magnesium phosphate, and carrying out aging doping reaction to obtain slurry after aging reaction; pulping and washing the slurry subjected to the aging reaction, drying and calcining to obtain an anhydrous iron phosphate material doped with magnesium and manganese; finally, mixing the magnesium-manganese-doped anhydrous iron phosphate material, lithium carbonate and glucose, performing ball milling, and performing spray drying to obtain a precursor; and under the condition of filling rice husk charcoal to isolate oxygen, performing two-stage calcination on the precursor to prepare the magnesium-manganese co-doped lithium iron phosphate. The magnesium-manganese co-doped lithium iron phosphate is used as the positive electrode material, so that the specific discharge capacity under a high-rate condition is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the field of material chemistry, and in particular to magnesium-manganese co-doped lithium iron phosphate and a preparation method and application thereof. Background Art

[0002] The theoretical specific capacity of lithium iron phosphate LiFePO4 (LFP) cathode material is 170mAh / g, the discharge platform voltage is 3.4V, and it has a stable olivine structure. It has the advantages of high safety performance, low cost, and long cycle life. It is currently the lithium-ion battery cathode material with the highest market share and is widely used in various portable electronic devices, electric vehicles, energy storage systems and other fields. The common methods for synthesizing LFP are carbon thermal reduction, hydrothermal method, and sol-gel method. Among them, carbon thermal reduction is the main industrial-scale production method. It is generally prepared by mixing, ball milling, drying and calcining raw materials such as iron phosphate FePO4, lithium carbonate Li2CO3 and glucose. However, because the tetrahedral structure of PO4 affects the Li + The deintercalation and electron diffusion of LFP materials lead to the electronic conductivity of LFP materials (about 10 -9 s / cm) and the lithium ion diffusion coefficient (~10 -14 cm 2 / s) is low, resulting in the discharge capacity under high rate conditions such as 2C ~ 10C, especially under high rate conditions of 5C-10C, the discharge capacity is only about 60% of that under low rate conditions. The macroscopic manifestation is that the endurance of lithium iron phosphate power battery vehicles is seriously attenuated when moving at high speed. This performance defect seriously affects the further promotion and application of lithium iron phosphate positive electrode materials.

[0003] Chinese patent CN117486182A discloses a method for preparing modified lithium iron manganese phosphate, which specifically comprises: subjecting a mixture of a silver source, a manganese source, an iron source, a phosphorus source and a lithium source solution to a hydrothermal reaction to obtain an intermediate; then mixing the intermediate with a carbon source and subjecting the intermediate to a ball milling reaction and heat treatment to obtain the intermediate. This patent replaces part of the Li position with Ag, thereby increasing the ion diffusion channel in the lithium iron manganese phosphate lattice and improving the ion migration performance; at the same time, Ag doping also causes a volume change, increases the electron transmission rate, and further increases the conductivity of the obtained modified lithium iron manganese phosphate product, while ensuring that it has a high charge and discharge capacity. Under the condition of improving the rate performance. However, when this material is applied to lithium iron phosphate power battery vehicles, the discharge specific capacity under high rate conditions needs to be further improved. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides a method for preparing magnesium-manganese co-doped lithium iron phosphate. The magnesium-manganese co-doped lithium iron phosphate as a positive electrode material can improve the electronic conductivity and lithium ion diffusion coefficient, and significantly improve the discharge specific capacity under high rate conditions.

[0005] The technical solution of the present invention is as follows:

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

[0007] (1) Aging and doping: adding ferric phosphate dihydrate into water and stirring, raising the temperature to 50°C ± 5°C, adding phosphoric acid solution of manganese phosphate and magnesium phosphate, and continuing to raise the temperature to control at 95°C ± 2°C for aging reaction for 2 to 3 hours until the color of the slurry completely turns white, thereby obtaining a slurry after aging reaction;

[0008] (2) Calcination: After the aging reaction, the slurry is beaten and washed, and then dried and calcined at 520° C. to 580° C. for 4 to 5 hours to obtain anhydrous iron phosphate material doped with magnesium and manganese;

[0009] (3) Preparation of lithium iron phosphate: Anhydrous iron phosphate material doped with magnesium and manganese, lithium carbonate and glucose are mixed and ball-milled to obtain a mixed slurry; the mixed slurry is spray-dried to obtain a precursor; and then the precursor is pre-calcined at 450°C to 550°C for 1 to 2 hours and then calcined at 680°C to 780°C for 6 to 10 hours under the condition of filling biochar to isolate oxygen to obtain magnesium and manganese co-doped lithium iron phosphate.

[0010] In step (3), when the precursor is calcined under the condition of filling biochar to isolate oxygen, the biochar does not directly contact the precursor. Specifically, it can be achieved as follows: the precursor is placed in a small crucible, and the small crucible is then placed upside down in a large crucible, and the middle of the large and small crucibles is filled with biochar to isolate oxygen and form a reducing atmosphere during calcination, and the biochar does not contact the precursor, and the filled precursor is placed in a muffle furnace for calcination.

[0011] In the present invention, biochar refers to a carbon-rich solid material produced by high-temperature pyrolysis of biomass materials under oxygen-deficient or oxygen-limited conditions, such as rice husk charcoal, wood charcoal, bamboo charcoal, and the like.

[0012] In one embodiment, in step (1), the preparation method of ferric phosphate dihydrate is as follows: after mixing the ferrous solution with the phosphate source, controlling the temperature at 40°C to 70°C, and continuously adding hydrogen peroxide until Fe 2+ Completely oxidize, then stabilize the pH between 2.1 and 2.2, continue stirring the reaction for 30 to 60 minutes, filter under reduced pressure, and then pulp and wash to obtain ferric phosphate dihydrate; wherein the phosphate source is at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and phosphoric acid; and the molar ratio of ferrous ions to phosphate ions is 0.96 to 1:1.

[0013] The ferrous solution was pretreated before use, specifically: after heating to 50°C, 10% sodium hydroxide solution was added, the pH value was controlled at 4.2±0.1, stirred for 60 minutes, and the precipitate was removed by filtration to obtain a clear solution.

[0014] The inventor changed the addition of manganese phosphate and magnesium phosphate during aging to the addition in the oxidation synthesis step of preparing iron phosphate to prepare a doped iron phosphate precursor, but the experimental effect was extremely poor and the results could not be controlled at all. There are two main reasons. First, manganese and magnesium will also generate phosphate or hydroxide precipitation. The precipitation process will affect the formation of iron phosphate, making it difficult to accurately control the iron-phosphorus ratio in iron phosphate within the range of 0.96 to 0.99, thereby affecting the subsequent material structure and performance. Second, most of the manganese and magnesium elements will be adsorbed by iron phosphate in the form of impurities such as manganese hydroxide or magnesium hydroxide. Due to the obvious differences in solubility, precipitation rate, particle size and other properties of impurities such as manganese hydroxide or magnesium hydroxide and iron phosphate, the distribution of manganese and manganese elements in iron phosphate is also uneven, resulting in the inability to prepare a uniformly doped iron phosphate precursor during synthesis.

[0015] In a specific embodiment, in step (1), the manganese phosphate is at least one of manganese phosphate, manganese monohydrogen phosphate, and manganese dihydrogen phosphate; the magnesium phosphate is at least one of magnesium phosphate, magnesium monohydrogen phosphate, and magnesium dihydrogen phosphate; in the phosphoric acid solution of the manganese phosphate and the magnesium phosphate, the molar ratio of phosphoric acid to the total amount of the manganese phosphate and the magnesium phosphate is 1:1-6 calculated as phosphate radical; and the molar ratio of the manganese phosphate to the magnesium phosphate is 1:1-3 calculated as metal ions.

[0016] In a specific embodiment, in the anhydrous ferric phosphate material doped with magnesium and manganese obtained in step (2), the molar ratio of iron to phosphorus is 0.96-0.99, and the contents of magnesium and manganese are 1000-30000 ppm and 1000-20000 ppm, respectively.

[0017] By adopting the aging doping method of the present invention, the total doping amount is below 1000ppm, which is very different from ordinary iron phosphate. There is a slight effect between 100 and 5000pm, but it is not obvious. The total doping amount between 8000 and 15000pm has a good overall effect. While improving the high-rate performance, the cycle performance can also be slightly improved. After the total doping amount exceeds 18000pm, the rate performance does not change much from 8000 to 15000pm, but the cycle performance decreases significantly. The main reason is that the doping amount is too large, which destroys the original stable structure and causes the cycle performance to decrease.

[0018] In a specific embodiment, in step (3), the amount of anhydrous iron phosphate material doped with magnesium and manganese and lithium carbonate is 1:1.01-1.05 in terms of the molar ratio of iron to lithium.

[0019] In one embodiment, in step (3), the amount of glucose added is 3-6% of the total solid mass; preferably, the amount of glucose added is 4-5% of the total solid mass; more preferably, the amount of glucose added is 4% of the total solid mass.

[0020] In a specific embodiment, in step (3), the ball-to-material ratio during ball milling is 13-16:1, and the solid-liquid ratio is 1:1-1.2.

[0021] In one specific embodiment, in step (3), the air inlet temperature during spray drying is 180° C. to 220° C., and the peristaltic pump speed is 20 to 30 rpm.

[0022] The present invention also provides magnesium-manganese co-doped lithium iron phosphate prepared by the above preparation method.

[0023] The present invention also provides the use of magnesium-manganese co-doped lithium iron phosphate as a positive electrode material in a lithium ion battery.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] (1) The present invention uses a phosphoric acid-magnesium phosphate-manganese phosphate mixed solution as an aging agent. On the one hand, it eliminates the residual iron hydroxide impurities in the iron phosphate and realizes the precise control of the iron-phosphorus ratio in the iron phosphate raw material; on the other hand, during the aging reaction, the magnesium and manganese ions slowly and stably partially replace the iron ions in the iron phosphate in an in-situ doping manner. Compared with other doping methods, it has the advantages of controllable doping amount and extremely uniform and stable distribution, which lays a solid foundation for the subsequent improvement of the high-rate performance of lithium iron phosphate.

[0026] (2) The conventional calcination method of lithium iron phosphate positive electrode material is to mix about 10% glucose under nitrogen protective atmosphere. The present invention is to mix about 4-5% glucose and use biochar to isolate oxygen from the outside (biochar is not in contact with the calcined material) for calcination in a reducing atmosphere. Compared with the conventional protective atmosphere calcination method, less glucose is used and the reduction is more thorough. At the same time, because less glucose is used, the tap density and compaction density are greater, and the energy density of the battery is also higher. The in-situ doping of magnesium and manganese in the iron phosphate precursor can greatly improve the discharge specific capacity performance of the lithium iron phosphate material under high-rate conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a SEM image of the lithium iron phosphate prepared in Example 1;

[0028] Figure 2 This is a SEM image of the lithium iron phosphate prepared in Example 2;

[0029] Figure 3This is a SEM image of the lithium iron phosphate prepared in Comparative Example 1;

[0030] Figure 4 This is the equivalent circuit model diagram. DETAILED DESCRIPTION

[0031] The present invention is further described below with specific examples, but the embodiments of the present invention are not limited thereto.

[0032] Example 1

[0033] A method for preparing a magnesium-manganese co-doped high-performance lithium iron phosphate positive electrode material is provided, comprising the following steps:

[0034] (1) Raw material preparation: weigh about 1 kg of ferrous sulfate produced as a byproduct of titanium sulfate as a raw material, add about 2 L of deionized water and heat to about 50° C. to dissolve, add a small amount of 10% sodium hydroxide solution to remove impurities and separate, and the ferrous ion concentration is measured to be 1.68 mol / L, and let stand for use; measure 100 ml of 15% phosphoric acid solution, add 10 g each of magnesium dihydrogen phosphate and manganese dihydrogen phosphate, heat to dissolve until clear and set aside.

[0035] (2) Oxidation synthesis reaction: 1L of ferrous iron solution was placed in a reactor, 1.68 mol of 85% phosphoric acid was pumped in, and 27.5% hydrogen peroxide solution was slowly pumped in while heating until the ferrous iron was completely oxidized. Then, 10% sodium hydroxide solution was slowly pumped in, and the temperature was controlled not to exceed 65°C and the pH value was controlled to be 2.1-2.2. After stable stirring for half an hour, the mixture was filtered under reduced pressure to obtain a yellow ferric phosphate dihydrate filter cake.

[0036] (3) Aging and in-situ doping reaction: After the ferric phosphate dihydrate filter cake is slurried and filtered once, the filter cake is placed in an aging reactor, and about 2 L of deionized water is added. After the temperature is raised to 50°C, about 100 ml of a phosphate-magnesium-manganese mixed solution is added. The mixture is heated to 95°C±2°C and stirred for aging reaction for 2 hours until the suspension turns completely white.

[0037] (4) Washing, drying and calcining of iron phosphate: The white slurry obtained from the aging reaction was slurried, washed and filtered three times in succession to ensure that there was no residual sulfate ion, and then placed in a flash dryer for drying. After drying, it was placed in a box furnace and calcined at a constant temperature of 550° C. for 4 hours to obtain anhydrous iron phosphate. The iron-phosphorus molar ratio was found to be 0.97, and the magnesium and manganese contents were 5600 ppm and 4900 ppm, respectively.

[0038] (5) Preparation of lithium iron phosphate: First, according to the lithium iron molar ratio of 1.02, weigh the homemade magnesium-manganese doped FePO4, lithium carbonate (Li2CO3) and glucose (accounting for 5% of the solid material) and put them into a ball mill. Under the process conditions of ball-to-material ratio of 15:1, solid-liquid ratio of 1:1, rotation speed of 600rpm, and ball milling for 3h, a mixed slurry is obtained. Then, the dry air inlet temperature is set to 200℃, the peristaltic pump speed is set to 24rpm, and the slurry is spray dried to obtain a precursor. Finally, the precursor is placed in a small crucible, and the small crucible is placed upside down in a large crucible. Rice husk charcoal is filled in the middle of the large and small crucibles to isolate oxygen and form a reducing atmosphere during calcination. The filled precursor is placed in a muffle furnace, pre-calcined at 500℃ for 1h, and then calcined at 750℃ for 8h to obtain the LFP product.

[0039] Example 2

[0040] A method for preparing a magnesium-manganese co-doped high-performance lithium iron phosphate positive electrode material is provided, comprising the following steps:

[0041] (1) Raw material preparation: weigh about 2 kg of ferrous sulfate as raw material, add about 5 L of deionized water and heat to about 50° C. to dissolve, add a small amount of 10% sodium hydroxide solution to remove impurities and separate, and the ferrous ion concentration is measured to be 1.39 mol / L, and let stand for use; measure 100 ml of 15% phosphoric acid solution, add 15 g of magnesium dihydrogen phosphate and 12 g of manganese dihydrogen phosphate, heat to dissolve until clear and set aside.

[0042] (2) Oxidation synthesis reaction: 1L of ferrous iron solution was placed in a reactor, 1.39 mol of 85% phosphoric acid was pumped in, and 27.5% hydrogen peroxide solution was slowly pumped in while heating until the ferrous iron was completely oxidized. Then, 10% sodium hydroxide solution was slowly pumped in, and the temperature was controlled not to exceed 65°C and the pH value was controlled to be 2.1-2.2. After stable stirring for half an hour, the mixture was filtered under reduced pressure to obtain a yellow ferric phosphate dihydrate filter cake.

[0043] (3) Aging and in-situ doping reaction: After the ferric phosphate dihydrate filter cake is slurried and filtered once, the filter cake is placed in an aging reactor, and about 2 L of deionized water is added. After the temperature is raised to 50°C, about 90 ml of a phosphate-magnesium-manganese mixed solution is added. The mixture is heated to 95°C±2°C and stirred for aging reaction for 2 hours until the suspension turns completely white.

[0044] (4) Washing, drying and calcining of iron phosphate: The white slurry obtained from the aging reaction was slurried, washed and filtered three times in succession to ensure that there was no residual sulfate ion, and then placed in a flash dryer for drying. After drying, it was placed in a box furnace and calcined at a constant temperature of 550° C. for 4 hours to obtain anhydrous iron phosphate. The iron-phosphorus molar ratio was found to be 0.99, and the magnesium and manganese contents were 7200 ppm and 5500 ppm, respectively.

[0045] (5) Preparation of lithium iron phosphate: First, according to the lithium-iron molar ratio of 1.02, weigh the homemade magnesium-manganese doped FePO4, lithium carbonate (Li2CO3) and glucose (accounting for 4% of the solid material) and put them into a ball mill. Under the process conditions of ball-to-material ratio of 15:1, solid-liquid ratio of 1:1, rotation speed of 600rpm, and ball milling for 3h, a mixed slurry is obtained. Then, the dry air inlet temperature is set to 200℃, the peristaltic pump speed is set to 24rpm, and the slurry is spray dried to obtain a precursor. Finally, the precursor is placed in a small crucible, and the small crucible is placed upside down in a large crucible. Rice husk charcoal is filled in the middle of the large and small crucibles to isolate oxygen and form a reducing atmosphere during calcination. The filled precursor is placed in a muffle furnace, pre-calcined at 500℃ for 1h, and then calcined at 750℃ for 8h to obtain the LFP product.

[0046] Comparative Example 1

[0047] A method for preparing a lithium iron phosphate positive electrode material is provided, comprising the following steps:

[0048] (1) Raw material preparation: weigh about 1 kg of ferrous sulfate as raw material, add about 2 L of deionized water and heat to about 50° C. to dissolve, add a small amount of 10% sodium hydroxide solution to remove impurities and separate, and the ferrous ion concentration is measured to be 1.73 mol / L, and let stand for use;

[0049] (2) Oxidation synthesis reaction: 1L of ferrous iron solution was placed in a reactor, 1.73 mol of 85% phosphoric acid was pumped in, and 27.5% hydrogen peroxide solution was slowly pumped in while heating until the ferrous iron was completely oxidized. Then, 10% sodium hydroxide solution was slowly pumped in, and the temperature was controlled not to exceed 65°C and the pH value was controlled to be 2.1-2.2. After stable stirring for half an hour, the mixture was filtered under reduced pressure to obtain a yellow ferric phosphate dihydrate filter cake.

[0050] (3) Aging reaction: After the ferric phosphate dihydrate filter cake is slurried and filtered once, the filter cake is placed in an aging reaction kettle, and about 2 L of deionized water is added. After the temperature is raised to 50° C., about 0.18 mol of 85% phosphoric acid is added. The mixture is heated to 95° C.±2° C. and stirred for aging reaction for 2 hours until the suspension completely turns white.

[0051] (4) Washing, drying and calcining of iron phosphate: The white slurry obtained from the aging reaction is slurried, washed and filtered three times in succession to ensure that there is no residual sulfate ion, and then it is dried in a flash dryer. After drying, it is placed in a box furnace and calcined at a constant temperature of 550° C. for 4 hours to obtain anhydrous iron phosphate. The iron-phosphorus molar ratio is found to be 0.98.

[0052] (5) Preparation of lithium iron phosphate: First, FePO4, lithium carbonate (Li2CO3) and glucose (accounting for 12% of the solid material) were weighed according to a lithium-iron molar ratio of 1.02 and placed in a ball mill. A mixed slurry was obtained under the process conditions of a ball-to-material ratio of 15:1, a solid-liquid ratio of 1:1, a rotation speed of 600 rpm, and ball milling for 3 hours. Then, the dry air inlet temperature was set to 200°C, the peristaltic pump speed was set to 24 rpm, and the slurry was spray-dried to obtain a precursor. Finally, the precursor was placed in a tubular furnace, filled with nitrogen as a protective gas, pre-calcined at 500°C for 1 hour, and then calcined at 750°C for 8 hours to obtain the LFP product.

[0053] Comparative Example 2

[0054] A method for preparing a lithium iron phosphate positive electrode material is provided, comprising the following steps:

[0055] Steps (1) to (4) are performed in the same manner as in Comparative Example 1. Conventional FePO4 prepared in step (4) is used in step (5) to prepare lithium iron phosphate, as follows:

[0056] (5) Preparation of lithium iron phosphate: First, according to the lithium iron molar ratio of 1.02, weigh the homemade conventional FePO4, lithium carbonate (Li2CO3) and glucose (accounting for 5% of the solid material) and put them into a ball mill. Under the process conditions of ball-to-material ratio of 15:1, solid-liquid ratio of 1:1, rotation speed of 600rpm, and ball milling for 3h, a mixed slurry is obtained. Then, the dry air inlet temperature is set to 200℃, the peristaltic pump speed is set to 24rpm, and the slurry is spray dried to obtain a precursor. Finally, the precursor is placed in a small crucible, and the small crucible is placed upside down in a large crucible. Rice husk charcoal is filled in the middle of the large and small crucibles to isolate oxygen and form a reducing atmosphere during calcination. The filled precursor is placed in a muffle furnace, pre-calcined at 500℃ for 1h, and then calcined at 750℃ for 8h to obtain the LFP product.

[0057] Comparative Example 3

[0058] A method for preparing a lithium iron phosphate positive electrode material is provided, comprising the following steps:

[0059] Steps (1) to (4) are performed in the same manner as in Example 1. The self-made FePO4 doped with magnesium and manganese prepared in step (4) is used in step (5) to prepare lithium iron phosphate, as follows:

[0060] (5) Preparation of lithium iron phosphate: First, according to the lithium-iron molar ratio of 1.02, weigh the homemade magnesium-manganese doped FePO4, lithium carbonate (Li2CO3) and glucose (accounting for 12% of the solid material) and put them into a ball mill. Under the process conditions of ball-to-material ratio of 15:1, solid-liquid ratio of 1:1, rotation speed of 600rpm, and ball milling for 3h, a mixed slurry is obtained. Then, the dry air inlet temperature is set to 200℃, the peristaltic pump speed is set to 24rpm, and the slurry is spray-dried to obtain a precursor. Finally, the precursor is placed in a tubular furnace, filled with nitrogen as a protective gas, pre-calcined at 500℃ for 1h, and then calcined at 750℃ for 8h to obtain the LFP product.

[0061] Comparative Example 4

[0062] (1) Raw material preparation: weigh about 1 kg of ferrous sulfate produced as a by-product of titanium sulfate as a raw material, add about 2 L of deionized water and heat to about 50° C. to dissolve, add a small amount of 10% sodium hydroxide solution to remove impurities and separate, and the ferrous ion concentration is measured to be 1.60 mol / L, and let stand for use; measure 100 ml of 15% phosphoric acid solution, add 12 g each of magnesium dihydrogen phosphate and manganese dihydrogen phosphate, heat to dissolve until clear and set aside.

[0063] (2) Oxidation synthesis reaction: 1L of ferrous iron solution was placed in a reactor, 1.55 mol of 85% phosphoric acid was pumped in, and 27.5% hydrogen peroxide solution was slowly pumped in while heating until the ferrous iron was completely oxidized, and then 100 ml of the phosphoric acid solution of manganese dihydrogen phosphate and magnesium dihydrogen phosphate prepared in step (1) was slowly pumped in. After reacting for 30 minutes, 10% sodium hydroxide solution was slowly pumped in, and the temperature was controlled not to exceed 65°C and the pH value was 2.1-2.2. After stable stirring for half an hour, the yellow ferric phosphate dihydrate filter cake was obtained by filtration under reduced pressure.

[0064] (3) Aging reaction: After the ferric phosphate dihydrate filter cake is slurried and filtered once, the filter cake is placed in an aging reaction kettle, and about 2 L of deionized water is added. After the temperature is raised to 50° C., about 10 ml of 85% phosphoric acid solution is added, and the temperature is continued to be heated to 95° C. ± 2° C. and stirred for aging reaction for 2 hours until the suspension completely turns white.

[0065] (4) Washing, drying and calcining of iron phosphate: The white slurry obtained from the aging reaction was slurried, washed and filtered three times in succession to ensure that there was no residual sulfate ion, and then placed in a flash dryer for drying. After drying, it was placed in a box furnace and calcined at a constant temperature of 550° C. for 4 hours to obtain anhydrous iron phosphate. The iron-phosphorus molar ratio was found to be 1.00, and the magnesium and manganese contents were 4500 ppm and 3900 ppm, respectively.

[0066] (5) Preparation of lithium iron phosphate: First, according to the lithium iron molar ratio of 1.02, weigh the homemade magnesium-manganese doped FePO4, lithium carbonate (Li2CO3) and glucose (accounting for 5% of the solid material) and put them into a ball mill. Under the process conditions of ball-to-material ratio of 15:1, solid-liquid ratio of 1:1, rotation speed of 600rpm, and ball milling for 3h, a mixed slurry is obtained. Then, the dry air inlet temperature is set to 200℃, the peristaltic pump speed is set to 24rpm, and the slurry is spray dried to obtain a precursor. Finally, the precursor is placed in a small crucible, and the small crucible is placed upside down in a large crucible. Rice husk charcoal is filled in the middle of the large and small crucibles to isolate oxygen and form a reducing atmosphere during calcination. The filled precursor is placed in a muffle furnace, pre-calcined at 500℃ for 1h, and then calcined at 750℃ for 8h to obtain the LFP product.

[0067] The performance test results of the embodiments and comparative examples are shown in Table 1:

[0068] Table 1. Performance test results of examples and comparative examples

[0069]

[0070]

[0071] Note 1: The test method for rate discharge capacity is based on GB / T42161-2022-Lithium iron phosphate electrochemical performance test first discharge specific capacity and first charge and discharge efficiency test method.

[0072] Note 2: The test method for cycle performance is based on GB / T 42260-2022-Lithium iron phosphate electrochemical performance test cycle life test method.

[0073] Note 3: The test methods for tap density and compaction density are based on T / SNLCY002-2022-Lithium iron phosphate performance test method.

[0074] The results of the AC impedance test using Shanghai Chenhua CHI760E electrochemical workstation show that the average lithium ion diffusion coefficient of magnesium-manganese co-doped lithium iron phosphate (Examples 1 and 2 is 3.6×10 -13 cm 2 s -1 ) is higher than that of ordinary lithium iron phosphate (the lithium ion expansion factor of comparative example 1 is 1.3×10 -13 cm 2 s -1 )'s lithium ion diffusion coefficient increased by about 1 to 2 times.

[0075] The test method is to calculate the lithium ion diffusion coefficient of the positive electrode material through electrochemical impedance spectroscopy (EIS) simulation, and use Z-view3.1 software to fit it to the equivalent circuit model (such as Figure 4 ), where Formula 1 is the lithium ion migration coefficient DLi+ =R 2 T 2 / (2A 2 n 4 F 4 σ 2 C 2 ), where R is the gas constant, T is the absolute temperature, A is the electrode surface area, n is the number of transferred electrons per mole of substance participating in the electrode reaction, C is the concentration of lithium in the electrode, F is the Faraday constant, and σ is the Warburg coefficient (obtained by fitting); Obtain the Warburg coefficient (σ): Formula 2 is Zre=Rs+Rct+σω -1 / 2 , Rs represents the total resistance of the electrolyte, separator and current collector, Rct represents the charge transfer resistance, CPE represents the constant phase element, and Zw represents the + Warburg impedance related to the diffusion process. The lithium ion diffusion coefficient of the positive electrode material is obtained by calculation.

[0076] The above disclosure is only the preferred embodiment of the present invention, which certainly cannot be used to limit the scope of the present invention. Therefore, equivalent changes made according to the claims of the present invention still fall within the scope covered by the present invention.

Claims

1. A method for preparing magnesium-manganese co-doped lithium iron phosphate, characterized in that: The steps include: (1) Aging and doping: adding ferric phosphate dihydrate into water and stirring, raising the temperature to 50°C ± 5°C, adding phosphoric acid solution of manganese phosphate and magnesium phosphate, and continuing to raise the temperature to control at 95°C ± 2°C for aging reaction for 2 to 3 hours until the color of the slurry completely turns white, thereby obtaining a slurry after aging reaction; (2) Calcination: After the aging reaction, the slurry is beaten and washed, and then dried and calcined at 520° C. to 580° C. for 4 to 5 hours to obtain anhydrous iron phosphate material doped with magnesium and manganese; (3) Preparation of lithium iron phosphate: Anhydrous iron phosphate material doped with magnesium and manganese, lithium carbonate and glucose are mixed and ball-milled to obtain a mixed slurry; the mixed slurry is spray-dried to obtain a precursor; and then the precursor is pre-calcined at 450°C to 550°C for 1 to 2 hours and then calcined at 680°C to 780°C for 6 to 10 hours under the condition of filling biochar to isolate oxygen to obtain magnesium and manganese co-doped lithium iron phosphate.

2. The method for preparing magnesium-manganese co-doped lithium iron phosphate according to claim 1, characterized in that: In step (1), the preparation method of ferric phosphate dihydrate is as follows: after mixing the ferrous solution with the phosphate source, controlling the temperature at 40°C to 70°C, and continuously adding hydrogen peroxide until Fe 2+ Completely oxidize, then stabilize the pH between 2.1 and 2.2, continue stirring the reaction for 30 to 60 minutes, filter under reduced pressure, and then pulp and wash to obtain ferric phosphate dihydrate; wherein the phosphate source is at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and phosphoric acid; and the molar ratio of ferrous ions to phosphate ions is 0.96 to 1:

1.

3. The method for preparing magnesium-manganese co-doped lithium iron phosphate according to claim 1, characterized in that: In step (1), the manganese phosphate is at least one of manganese phosphate, manganese monohydrogen phosphate, and manganese dihydrogen phosphate; the magnesium phosphate is at least one of magnesium phosphate, magnesium monohydrogen phosphate, and magnesium dihydrogen phosphate; in the phosphoric acid solution of the manganese phosphate and the magnesium phosphate, the molar ratio of phosphoric acid to the total amount of the manganese phosphate and the magnesium phosphate is 1:1-6 calculated as phosphate radical; and the molar ratio of the manganese phosphate to the magnesium phosphate is 1:1-3 calculated as metal ions.

4. The method for preparing magnesium-manganese co-doped lithium iron phosphate according to claim 1, characterized in that: In the anhydrous ferric phosphate material doped with magnesium and manganese obtained in step (2), the molar ratio of iron to phosphorus is 0.96-0.99, and the contents of magnesium and manganese are 1000-30000 ppm and 1000-20000 ppm respectively.

5. The method for preparing magnesium-manganese co-doped lithium iron phosphate according to claim 1, characterized in that: In step (3), the amount of anhydrous iron phosphate material doped with magnesium and manganese and lithium carbonate is 1:1.01 to 1.05 in terms of the molar ratio of iron to lithium.

6. The method for preparing magnesium-manganese co-doped lithium iron phosphate according to claim 1, characterized in that: In step (3), the amount of glucose added is 3-6% of the total solid mass; preferably, the amount of glucose added is 4-5% of the total solid mass; more preferably, the amount of glucose added is 4% of the total solid mass.

7. The method for preparing magnesium-manganese co-doped lithium iron phosphate according to claim 1, characterized in that: In step (3), the ball-to-material ratio during ball milling is 13-16:1, and the solid-liquid ratio is 1:1-1.

2.

8. The method for preparing magnesium-manganese co-doped lithium iron phosphate according to claim 1, characterized in that: In step (3), the air inlet temperature during spray drying is 180°C to 220°C, and the peristaltic pump speed is 20 to 30 rpm.

9. Magnesium-manganese co-doped lithium iron phosphate prepared by the method for preparing magnesium-manganese co-doped lithium iron phosphate according to any one of claims 1 to 8.

10. Use of the magnesium-manganese co-doped lithium iron phosphate according to claim 9 as a positive electrode material in a lithium ion battery.

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