Lithium manganese iron phosphate positive electrode material, and preparation method therefor and use thereof

Abstract

Provided in the present disclosure are a lithium manganese iron phosphate positive electrode material, and a preparation method therefor and the use thereof. The preparation method comprises the following steps: (1) mixing an iron source, a manganese source and a solvent, freezing the mixture to obtain a frozen solution A, freezing a phosphorus source solution to obtain a frozen solution B, crushing the frozen solution A and the frozen solution B at a low temperature, and pressing and freezing same to obtain a frozen solution C; (2) placing the frozen solution C in a reflux device, mixing a complexing agent and hydrogen peroxide to obtain a mixed solution, controlling the reflux of the mixed solution between the reflux device and a reaction device, and controlling the concentration and pH of the complexing agent in the system until the frozen solution C is completely melted to obtain a turbid liquid; and (3) subjecting the turbid liquid to a solid-liquid separation treatment, then mixing same with a lithium source, and sintering the mixture to obtain a lithium manganese iron phosphate positive electrode material. In the present disclosure, the raw material solutions are prepared into frozen solutions in advance, and the dissolution of the frozen solutions is controlled by using a reflux method, such that a coprecipitation reaction is slowly carried out, thereby preparing a lithium manganese iron phosphate positive electrode material having a small particle size and uniform element distribution.

Description

A lithium manganese iron phosphate positive electrode material and its preparation method and application Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries and relates to a lithium iron manganese phosphate positive electrode material and a preparation method and application thereof. Background Art

[0002] With the increasing popularity of electric vehicles and various electronic products, the requirements for electric vehicle range and safety are becoming increasingly stringent. In response to this trend, lithium manganese phosphate, with its olivine crystal structure, has attracted attention and is considered one of the most promising cathode materials for lithium-ion power batteries. However, while lithium manganese phosphate cathode materials offer many advantages, their low ionic and electronic conductivity has become a major focus for many researchers. Replacing manganese ions with metal ions can significantly improve this problem, leading to the emergence of lithium iron manganese phosphate cathode materials.

[0003] CN104466161A discloses a solid phase synthesis method of lithium iron manganese phosphate, wherein the precursor materials of iron source, lithium source, phosphorus source, carbon source and manganese source are mixed in proportion, a dispersant is added, ball milling is performed, drying is performed, and primary sintering and secondary sintering are performed under nitrogen protective atmosphere respectively to finally obtain LiMn x Fe 1-x PO4 material. However, when using a single high-temperature solid-phase method to prepare lithium manganese iron phosphate positive electrode material, Mn and Fe elements are difficult to be evenly distributed in the main body of the material, which will cause serious Mn 3+ The Jahn-Teller effect affects the cycle and rate performance of the battery.

[0004] JP2023507212A discloses a co-precipitation method for preparing a lithium iron manganese phosphate precursor. The method involves mixing a mixed solution of manganese iron salts with phosphoric acid and oxalic acid solutions to prepare the lithium iron manganese phosphate precursor. However, the precipitation coefficients of manganese iron vary significantly, and the co-precipitation method can also result in uneven mixing of manganese iron in the precursor.

[0005] The lithium manganese iron phosphate prepared by the method described in the above scheme has the problem of uneven element distribution, which leads to a significant decrease in its performance and affects its practical application.

[0006] Summary of the Invention

[0007] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0008] The purpose of the present disclosure is to provide a lithium manganese iron phosphate positive electrode material and its preparation method and application. The present disclosure prepares a raw material solution into a frozen solution in advance, and uses a reflux method to control the dissolution of the frozen solution so that the coprecipitation reaction proceeds slowly, thereby preparing a lithium manganese iron phosphate positive electrode material with small particle size and uniform element distribution.

[0009] To achieve this disclosure purpose, the present disclosure adopts the following technical solutions:

[0010] In a first aspect, the present disclosure provides a method for preparing a lithium manganese iron phosphate positive electrode material, the preparation method comprising the following steps:

[0011] (1) An iron source, a manganese source, and a solvent are mixed and frozen to obtain a frozen liquid A, a phosphorus source solution is frozen to obtain a frozen liquid B, and the frozen liquids A and B are subjected to low-temperature crushing, followed by pressing and freezing to obtain a frozen liquid C;

[0012] (2) placing the frozen liquid C in a reflux device, mixing the complexing agent and hydrogen peroxide to obtain a mixed solution, controlling the mixed solution to reflux between the reflux device and the reaction device, and controlling the complexing agent concentration and pH in the system until the frozen liquid C is completely melted to obtain a suspension;

[0013] (3) performing solid-liquid separation on the suspension, mixing the obtained solid product with a lithium source, and sintering to obtain the lithium manganese iron phosphate positive electrode material.

[0014] In the method disclosed herein, by preparing a frozen solution A containing a uniform mixture of iron and manganese, the frozen solution can slowly react with the phosphorus source during the melting process, causing the reaction to proceed slowly and evenly. As a result, the manganese and iron in the prepared lithium manganese phosphate are evenly distributed, exhibiting good electrochemical performance. By using a reflux device, on the one hand, the manganese and iron can be slowly co-precipitated, resulting in a more uniform distribution of manganese and iron, and on the other hand, the growth process can be slowed, resulting in the preparation of a lithium manganese phosphate cathode material with a smaller particle size.

[0015] In one embodiment, the iron source in step (1) comprises a ferrous salt.

[0016] In one embodiment, the iron source includes any one of ferrous chloride, ferrous sulfate, or ferrous nitrate, or a combination of at least two thereof.

[0017] The present invention uses ferrous salt as the iron source for the reaction and adds hydrogen peroxide, so that during the reflux reaction, the divalent iron first reacts with the hydrogen peroxide to form trivalent iron, thereby slowing down the sedimentation rate of the trivalent iron and making the iron and manganese precipitation more uniform.

[0018] In one embodiment, the manganese source comprises a divalent manganese salt.

[0019] In one embodiment, the manganese source includes any one of manganese chloride, manganese nitrate, or manganese sulfate, or a combination of at least two thereof.

[0020] In one embodiment, the concentration of the iron source in the freezing liquid A is 0.1 to 2 mol / L, for example, 0.1 mol / L, 0.5 mol / L, 1 mol / L, 1.5 mol / L or 2 mol / L.

[0021] In one embodiment, the concentration of the manganese source in the freezing solution A is 0.1 to 2 mol / L, for example, 0.1 mol / L, 0.5 mol / L, 1 mol / L, 1.5 mol / L or 2 mol / L.

[0022] In one embodiment, based on the total molar amount of the manganese source and the iron source as 100%, the molar proportion of the manganese source is 10-40%, for example, 10%, 20%, 25%, 30% or 40%.

[0023] In one embodiment, the temperature of the freezing treatment in step (1) is -30 to -40°C, for example: -30°C, -32°C, -35°C, -38°C or -40°C.

[0024] In one embodiment, the freezing treatment time is 6 to 12 hours, for example: 6 hours, 7 hours, 8 hours, 10 hours or 12 hours.

[0025] In one embodiment, the solute of the phosphorus source solution in step (1) includes any one of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate or ammonium phosphate, or a combination of at least two thereof.

[0026] In one embodiment, the concentration of the phosphorus source solution is 1 to 2 mol / L, for example, 1 mol / L, 1.2 mol / L, 1.5 mol / L, 1.8 mol / L or 2 mol / L.

[0027] In one embodiment, the temperature of the low temperature crushing treatment is <-10°C.

[0028] In one embodiment, the pressing pressure is 0.1-10 MPa, for example, 0.1 MPa, 0.5 MPa, 1 MPa, 5 MPa or 10 MPa.

[0029] In one embodiment, the total concentration of iron and manganese in the freezing liquid C is 0.2 to 2 mol / L, for example, 0.2 mol / L, 0.5 mol / L, 1 mol / L, 1.5 mol / L or 2 mol / L.

[0030] In one embodiment, the molar ratio of the total molar amount of iron and manganese in the freezing liquid C to the phosphorus in the phosphorus source is 1:(1.1-2), for example: 1:1.1, 1:1.3, 1:1.5, 1:1.8 or 1:2.

[0031] In one embodiment, the complexing agent in step (2) includes citric acid and / or tartaric acid.

[0032] In one embodiment, the molar ratio of the hydrogen peroxide to the iron source is (1-3):1, for example: 1:1, 1.5:1, 2:1, 2.5:1 or 3:1.

[0033] In one embodiment, the concentration of the complexing agent in the mixed solution is 0.05 to 0.15 mol / L, for example, 0.05 mol / L, 0.08 mol / L, 0.1 mol / L, 0.12 mol / L or 0.15 mol / L.

[0034] In one embodiment, the concentration of hydrogen peroxide in the mixed solution is 1 to 2 mol / L, for example, 1 mol / L, 1.2 mol / L, 1.5 mol / L, 1.8 mol / L or 2 mol / L.

[0035] In one embodiment, the volume ratio of the mixed solution to the freezing solution C is (0.8-1.2):1, for example: 0.8:1, 0.9:1, 1:1, 1.1:1 or 1.2:1, etc.

[0036] In one embodiment, the reflux rate of the mixed solution is 100-200 mL / min, for example, 100 mL / min, 120 mL / min, 150 mL / min, 180 mL / min or 200 mL / min.

[0037] In one embodiment, the method of controlling the concentration of the complexing agent in the system in step (2) includes adding the complexing agent during the reflux process.

[0038] In one embodiment, the pH is 3 to 6, for example, 3, 3.5, 4, 5 or 6.

[0039] In one embodiment, the method of controlling the pH in the system includes adding ammonia water.

[0040] In one embodiment, the concentration of the ammonia water is 2 to 10 mol / L, for example, 2 mol / L, 4 mol / L, 6 mol / L, 8 mol / L or 10 mol / L.

[0041] In one embodiment, the reaction temperature in the system is 40-80°C, for example, 40°C, 50°C, 60°C, 70°C or 80°C.

[0042] In one embodiment, the frozen liquid C is completely melted and then aged.

[0043] In one embodiment, the aging treatment time is 1 to 5 hours, for example, 1 hour, 2 hours, 3 hours, 4 hours or 5 hours.

[0044] In one embodiment, the lithium source in step (3) includes lithium hydroxide and / or lithium carbonate.

[0045] In one embodiment, the total molar ratio of the lithium element in the lithium source to the manganese and iron elements in the solid product is (1-1.1):1, for example: 1:1, 1.02:1, 1.05:1, 1.08:1 or 1.1:1, etc.

[0046] In one embodiment, a carbon source is added during the mixing process.

[0047] In one embodiment, the carbon source comprises any one or a combination of at least two of glucose, sucrose, fructose, lactose, citric acid, soluble starch, benzoic acid or ascorbic acid.

[0048] In one embodiment, the sintering temperature is 600-900°C, for example, 600°C, 650°C, 700°C, 800°C or 900°C.

[0049] In one embodiment, the sintering time is 6 to 20 hours, for example, 6 hours, 8 hours, 10 hours, 15 hours or 20 hours.

[0050] In a second aspect, the present disclosure provides a lithium iron manganese phosphate positive electrode material, which is prepared by the method described in the first aspect.

[0051] In a third aspect, the present disclosure provides a positive electrode plate, which comprises the lithium manganese iron phosphate positive electrode material as described in the second aspect.

[0052] In a fourth aspect, the present disclosure provides a lithium-ion battery, wherein the lithium-ion battery comprises the positive electrode sheet as described in the third aspect.

[0053] Compared with the prior art, the present disclosure has the following beneficial effects:

[0054] (1) The present invention prepares a frozen solution of a uniform mixture of manganese and iron elements in advance, and the melting of ice allows the manganese and iron co-precipitation reaction to proceed slowly, resulting in a more uniform distribution of the manganese and iron components.

[0055] (2) The present disclosure can control the growth rate of the material in the system by using a reflux device, thereby producing a lithium manganese iron phosphate positive electrode material with more uniform particle size and smaller particle size.

[0056] (3) The present invention uses divalent iron as the iron source and adds hydrogen peroxide, which can make the divalent iron react with hydrogen peroxide to generate trivalent iron during the reflux reaction, thereby slowing down the sedimentation rate of the trivalent iron and making the iron and manganese precipitation more uniform.

[0057] (4) The 0.1C discharge capacity of the battery made of the lithium manganese iron phosphate positive electrode material prepared by the method disclosed in the present invention can reach more than 159.68mAh / g, the 0.5C / 0.1C capacity retention rate can reach more than 97.64%, and the 100-cycle capacity retention rate can reach more than 98.57%.

[0058] Still other aspects will become apparent upon reading and understanding the accompanying drawings and detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] The accompanying drawings are used to provide a further understanding of the technical solution of this article and constitute a part of the specification. Together with the embodiments of this application, they are used to explain the technical solution of this article and do not constitute a limitation on the technical solution of this article.

[0060] FIG1 is a schematic diagram of an apparatus used in a method for preparing a lithium manganese iron phosphate positive electrode material according to an embodiment of the present disclosure, wherein: 1 is a freezing liquid C; 2 is a reflux device; 3 is a pump; 4 is a reaction device; 5 is a stirring rod; and 6 is a mixed solution.

[0061] FIG2 is an SEM image of the ferromanganese phosphate precursor prepared in Example 1 of the present disclosure. DETAILED DESCRIPTION

[0062] The technical solution of the present disclosure is further described below through specific implementation methods. Those skilled in the art should understand that the embodiments are only used to help understand the present disclosure and should not be regarded as specific limitations of the present disclosure.

[0063] Example 1

[0064] This embodiment provides a lithium manganese iron phosphate positive electrode material. The schematic diagram of the device used in the preparation method of the lithium manganese iron phosphate positive electrode material is shown in Figure 1, wherein 1 is a freezing liquid C, 2 is a reflux device, 3 is a pump, 4 is a reaction device, 5 is a stirring rod, and 6 is a mixed solution. The preparation method is as follows:

[0065] (1) 0.7 mol / L ferrous nitrate and 0.3 mol / L manganese nitrate solution were mixed in a volume ratio of 1:1 and frozen in a spherical mold at -35°C for 8 h to obtain frozen liquid A, 1.5 mol / L ammonium dihydrogen phosphate solution was prepared and frozen at -35°C for 8 h to obtain frozen liquid B, frozen liquid A and frozen liquid B were crushed below -12°C, and then the shaved (particle size <2 mm) frozen liquid A and frozen liquid B were mixed evenly and pressed into a circular mold at a pressure of 2 MPa, and then frozen at -40°C for 10 h to obtain frozen liquid C (Fe+Mn:P=1:1.5);

[0066] (2) placing the frozen liquid C in a reflux device, mixing hydrogen peroxide and citric acid to obtain a mixed solution, wherein the concentration of hydrogen peroxide in the mixed solution is 1.5 mol / L, and the concentration of citric acid is 0.1 mol / L, controlling the mixed solution to reflux between the reflux device and the reaction device (the volume ratio of the mixed solution to the frozen liquid C is 1:1), and adding a complexing agent during the reflux process, maintaining the concentration of the complexing agent in the reaction solution at 0.1 mol / L, controlling the reflux flow rate to 150 mL / min, controlling the temperature in the reactor to 60°C, adjusting the pH value of the reaction solution to 5 with 3 mol / L ammonia water, reacting until the frozen mixed solution C is completely melted, and then keeping the temperature for 3 hours to obtain a suspension of manganese iron phosphate precursor;

[0067] (3) The obtained suspension was transferred to a solid-liquid separator for solid-liquid separation, and the solid product was washed twice with deionized water. The washed product was vacuum dried at 80°C for 10 hours to obtain a manganese iron phosphate precursor, which was mixed according to a molar ratio of Li: (Fe + Mn) = 1.03: 1. The lithium source was lithium hydroxide, and glucose was weighed to theoretically synthesize 5% of the mass of lithium manganese iron phosphate. The above materials were wet ball milled and dried, and then ball milled and placed in a tube furnace. Under a nitrogen atmosphere, the temperature was raised to 800°C at a rate of 3°C / min and kept warm for calcination for 12 hours to obtain the lithium manganese iron phosphate positive electrode material.

[0068] The SEM image of the ferromanganese phosphate precursor obtained in step (2) is shown in FIG2 . Four locations were randomly selected and the actual molar content ratio of metal Fe and Mn at each location was tested by EDS. The values ​​are as follows:

[0069] EDS1: 2.343, EDS2: 2.321, EDS3: 2.335, EDS4: 2.329. As can be seen from FIG2 , in the ferromanganese phosphate precursor prepared by the method disclosed in the present invention, the distribution uniformity of manganese and iron elements is good.

[0070] Example 2

[0071] This embodiment provides a lithium manganese iron phosphate positive electrode material. The schematic diagram of the device used in the preparation method of the lithium manganese iron phosphate positive electrode material is shown in Figure 1. The preparation method is as follows:

[0072] (1) 0.6 mol / L ferrous chloride and 0.4 mol / L manganese chloride solution were mixed in a volume ratio of 1:1 and frozen in a spherical mold at -30°C for 12 h to obtain frozen liquid A, 1 mol / L phosphoric acid solution was prepared and frozen at -30°C for 12 h to obtain frozen liquid B, frozen liquid A and frozen liquid B were crushed below -15°C, and then the shaved (particle size <2 mm) frozen liquid A and frozen liquid B were mixed evenly and pressed into a circular mold at a pressure of 0.1 MPa, and then frozen at -30°C for 12 h to obtain frozen liquid C (Fe+Mn:P=1:1.1);

[0073] (2) placing the frozen liquid C in a reflux device, mixing hydrogen peroxide and citric acid to obtain a mixed solution, wherein the concentration of hydrogen peroxide in the mixed solution is 1 mol / L, and the concentration of tartaric acid is 0.05 mol / L, controlling the mixed solution to reflux between the reflux device and the reaction device (the volume ratio of the mixed solution to the frozen liquid C is 1:1), and adding a complexing agent during the reflux process, maintaining the concentration of the complexing agent in the reaction solution at 0.05 mol / L, controlling the reflux flow rate to 100 mL / min, controlling the temperature in the reactor to 40°C, adjusting the pH value of the reaction solution to 3 with 2 mol / L ammonia water, reacting until the frozen mixed solution C is completely melted, and then keeping the temperature for 1 hour to obtain a suspension of manganese iron phosphate precursor;

[0074] (3) The obtained suspension was transferred to a solid-liquid separator for solid-liquid separation, and the solid product was washed twice with deionized water. The washed product was vacuum dried at 80°C for 10 hours to obtain a manganese iron phosphate precursor, which was mixed according to a molar ratio of Li: (Fe + Mn) = 1:1. The lithium source was lithium hydroxide, and sucrose was weighed to 5% of the theoretical mass of the synthesized lithium manganese iron phosphate. The above materials were wet ball milled and dried, and then ball milled and placed in a tube furnace. Under a nitrogen atmosphere, the temperature was raised to 600°C at a rate of 3°C / min and kept warm for calcination for 20 hours to obtain the lithium manganese iron phosphate positive electrode material.

[0075] Example 3

[0076] This embodiment provides a lithium manganese iron phosphate positive electrode material. The schematic diagram of the device used in the preparation method of the lithium manganese iron phosphate positive electrode material is shown in Figure 1. The preparation method is as follows:

[0077] (1) 0.9 mol / L ferrous sulfate and 0.1 mol / L manganese sulfate solution were mixed in a volume ratio of 1:1 and frozen in a spherical mold at -40°C for 8 h to obtain frozen liquid A, 2 mol / L ammonium phosphate solution was prepared and frozen at -40°C for 8 h to obtain frozen liquid B, frozen liquid A and frozen liquid B were crushed below -20°C, and then the shaved (particle size <2 mm) frozen liquid A and frozen liquid B were mixed evenly and pressed into a circular mold at a pressure of 10 MPa, and then frozen at -40°C for 8 h to obtain frozen liquid C (Fe+Mn:P=1:2);

[0078] (2) placing the frozen liquid C in a reflux device, mixing hydrogen peroxide and citric acid to obtain a mixed solution, wherein the concentration of hydrogen peroxide in the mixed solution is 2 mol / L, and the concentration of tartaric acid is 0.15 mol / L, controlling the mixed solution to reflux between the reflux device and the reaction device (the volume ratio of the mixed solution to the frozen liquid C is 1:1), and adding a complexing agent during the reflux process, maintaining the concentration of the complexing agent in the reaction solution at 0.15 mol / L, controlling the reflux flow rate to 200 mL / min, controlling the temperature in the reactor to 80°C, adjusting the pH value of the reaction solution to 6 using 10 mol / L ammonia water, reacting until the frozen mixed solution C is completely melted, and then keeping the temperature for 5 hours to obtain a suspension of manganese iron phosphate precursor;

[0079] (3) The obtained suspension was transferred to a solid-liquid separator for solid-liquid separation, and the solid product was washed twice with deionized water. The washed product was vacuum dried at 80°C for 10 hours to obtain a manganese iron phosphate precursor, which was mixed according to a molar ratio of Li: (Fe + Mn) = 1.1: 1. The lithium source was lithium hydroxide, and citric acid was weighed to theoretically synthesize 5% of the mass of lithium manganese iron phosphate. The above materials were wet ball milled and dried, and then ball milled and placed in a tube furnace. Under a nitrogen atmosphere, the temperature was raised to 900°C at a rate of 3°C / min and calcined for 6 hours to obtain the lithium manganese iron phosphate positive electrode material.

[0080] Example 4

[0081] The only difference between this embodiment and embodiment 1 is that the temperature in the reactor is 30° C., and other conditions and parameters are exactly the same as those in embodiment 1.

[0082] Example 5

[0083] The only difference between this embodiment and embodiment 1 is that the temperature in the reactor is 90° C., and other conditions and parameters are exactly the same as those in embodiment 1.

[0084] Comparative Example 1

[0085] This comparative example provides a lithium iron manganese phosphate positive electrode material, and the preparation method of the lithium iron manganese phosphate positive electrode material is as follows:

[0086] (1) 0.7 mol / L ferrous sulfate and 0.3 mol / L manganous sulfate solution were mixed in a volume ratio of 1:1 to prepare a mixed solution A; (2) 1.5 mol / L ammonium dihydrogen phosphate solution was prepared, and citric acid was added thereto so that the citric acid concentration in the phosphorus source mixed solution was 0.1 mol / L;

[0087] (2) Mixing the mixed solution A with the phosphorus source mixed solution, stirring at a speed of 60 r / min, controlling the solution temperature in the reactor wall to 60°C, adjusting the pH value of the reaction solution to 6 with 3 mol / L ammonia water, keeping the temperature and aging for 3 h to obtain a suspension, transferring the obtained suspension to a solid-liquid separator for solid-liquid separation, washing the solid product twice with deionized water, and vacuum drying the washed product at 80°C for 10 h;

[0088] (3) The mixture was mixed in a molar ratio of Li:(Fe+Mn)=1.03:1, with lithium hydroxide as the lithium source, and glucose (5% by mass of the theoretically synthesized lithium manganese iron phosphate) was weighed. The above materials were wet ball-milled and dried, and then ball-milled again and placed in a tube furnace. In a nitrogen atmosphere, the temperature was raised to 800°C at a rate of 3°C / min and calcined for 12 hours to obtain a carbon-coated lithium manganese iron phosphate positive electrode material.

[0089] Performance testing:

[0090] The lithium manganese iron phosphate prepared in the embodiment and the comparative example was assembled into a button cell in the ratio of lithium manganese iron phosphate: conductive carbon black: PVDF = 8:1:1. A Clgard 2300 type separator was used, and a metal lithium sheet was used as the counter electrode. Charge and discharge tests were performed. The test results are as follows, as shown in Table 1:

[0091] Table 1

[0092] As can be seen from Table 1, from Examples 1-3, the 0.1C discharge capacity of the battery made of the lithium manganese iron phosphate positive electrode material prepared by the method described in the present disclosure can reach more than 159.68 mAh / g, the 0.5C / 0.1C capacity retention rate can reach more than 97.64%, and the 100-cycle capacity retention rate can reach more than 98.57%.

[0093] By comparing Example 1 with Examples 4-5, it can be seen that the temperature in the reaction apparatus described in the present disclosure affects the performance of the obtained lithium manganese iron phosphate positive electrode material. When the reaction temperature is controlled at 40-80°C, the obtained lithium manganese iron phosphate has better performance. If the reaction temperature is too low, the synthesized lithium manganese phosphate has a larger particle size and poor crystallinity. If the reaction temperature is too high, the particle size of the manganese iron phosphate is smaller, but the contact between the manganese iron phosphate particles is increased, and agglomeration is likely to occur.

[0094] From the comparison between Example 1 and Comparative Example 1, it can be seen that the present disclosure can prepare a frozen solution A in which iron and manganese are uniformly mixed, so that the frozen solution can slowly mix and react with the phosphorus source during the melting process, so that the reaction proceeds slowly and evenly. Therefore, the manganese and iron in the prepared lithium manganese iron phosphate are uniformly distributed and have good electrochemical properties.

Claims

1. A method for preparing a lithium manganese iron phosphate positive electrode material, the preparation method comprising the following steps: (1) mixing an iron source, a manganese source and a solvent, freezing the mixture to obtain a frozen liquid A, freezing the phosphorus source solution to obtain a frozen liquid B, subjecting the frozen liquid A and the frozen liquid B to low temperature crushing, and then pressing and freezing the mixture to obtain a frozen liquid C; (2) placing the frozen liquid C in a reflux device, mixing the complexing agent and hydrogen peroxide to obtain a mixed solution, controlling the mixed solution to reflux between the reflux device and the reaction device, and controlling the complexing agent concentration and pH in the system until the frozen liquid C is completely melted to obtain a suspension; (3) performing solid-liquid separation on the suspension, mixing the obtained solid product with a lithium source, and sintering to obtain the lithium manganese iron phosphate positive electrode material.

2. The preparation method according to claim 1, wherein The iron source in step (1) comprises ferrous salt; Optionally, the iron source includes any one of ferrous chloride, ferrous sulfate or ferrous nitrate, or a combination of at least two of them.

3. The preparation method according to claim 1, wherein The manganese source includes a divalent manganese salt; Optionally, the manganese source includes any one of manganese chloride, manganese nitrate or manganese sulfate, or a combination of at least two of them.

4. The preparation method according to claim 1, wherein The concentration of the iron source in the freezing liquid A is 0.1-2 mol / L; Optionally, the concentration of the manganese source in the freezing liquid A is 0.1 to 2 mol / L; Optionally, based on the total molar amount of the manganese source and the iron source as 100%, the molar proportion of the manganese source is 10-40%.

5. The preparation method according to any one of claims 1 to 4, wherein: The temperature of the freezing treatment in step (1) is -30 to -40°C; Optionally, the freezing treatment time is 6 to 12 hours.

6. The preparation method according to any one of claims 1 to 5, wherein: The solute of the phosphorus source solution in step (1) includes any one of phosphoric acid, diammonium phosphate, diammonium hydrogen phosphate or ammonium phosphate, or a combination of at least two thereof; Optionally, the concentration of the phosphorus source solution is 1 to 2 mol / L; Optionally, the temperature of the low-temperature crushing treatment is <-10°C; Optionally, the pressing pressure is 0.1 to 10 MPa; Optionally, the total concentration of iron and manganese in the freezing liquid C is 0.2-2 mol / L; Optionally, the molar ratio of the total molar amount of the iron element and the manganese element in the refrigerant C to the phosphorus element in the phosphorus source is 1:(1.1-2).

7. The preparation method according to any one of claims 1 to 6, wherein: The complexing agent in step (2) includes citric acid and / or tartaric acid; Optionally, the molar ratio of the hydrogen peroxide to the iron source is (1-3):1; Optionally, the concentration of the complexing agent in the mixed solution is 0.05 to 0.15 mol / L; Optionally, the concentration of hydrogen peroxide in the mixed solution is 1 to 2 mol / L; Optionally, the volume ratio of the mixed solution to the freezing solution C is (0.8-1.2):1; Optionally, the reflux rate of the mixed solution is 100-200 mL / min.

8. The preparation method according to any one of claims 1 to 7, wherein: The method of controlling the concentration of the complexing agent in the system in step (2) includes adding the complexing agent during the reflux process; Optionally, the pH is 3 to 6; Optionally, the method of controlling the pH in the system includes adding ammonia water; Optionally, the concentration of the ammonia water is 2 to 10 mol / L; Optionally, the reaction temperature in the system is 40 to 80°C; Optionally, the frozen liquid C is subjected to aging treatment after being completely melted; Optionally, the aging treatment time is 1 to 5 hours.

9. The preparation method according to any one of claims 1 to 8, wherein: The lithium source in step (3) includes lithium hydroxide and / or lithium carbonate; Optionally, the total molar ratio of the lithium element in the lithium source to the manganese and iron elements in the solid product is (1-1.1):1; Optionally, a carbon source is added during the mixing process; Optionally, the carbon source includes any one or a combination of at least two of glucose, sucrose, fructose, lactose, citric acid, soluble starch, benzoic acid or ascorbic acid; Optionally, the sintering temperature is 600-900°C; Optionally, the sintering time is 6 to 20 hours.

10. A lithium iron manganese phosphate positive electrode material, wherein the lithium iron manganese phosphate positive electrode material is prepared by the method according to any one of claims 1 to 9.

11. A positive electrode plate, comprising the lithium iron manganese phosphate positive electrode material according to claim 10. 12 . A lithium ion battery, comprising the positive electrode sheet according to claim 11 .

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