Manganese iron phosphate precursor, preparation method thereof and application thereof

Abstract

The application discloses a manganese iron phosphate precursor and a preparation method and application thereof, and relates to the technical field of lithium ion batteries. The preparation method comprises the following steps: adding a phosphorus source, an organic manganese source and an organic iron source into an organic solvent to obtain a mixed solution; dissolving a doping element into water, adding starch, and heating and gelatinizing to form a sol; under a heating condition, the mixed solution is added into the sol and uniformly mixed to obtain a mixed system, and the mixed system is adjusted to be alkaline and then reacted; after the reaction is completed, drying and calcination are performed to obtain the manganese iron phosphate precursor. By adopting the starch sol as the medium, the branched chain molecules after starch gelatinization can be combined with metal ions, the metal ions can be uniformly dispersed in the sol, the segregation phenomenon of metal elements can be reduced, the accurate regulation of the Mn / Fe ratio and the doping element content is beneficial, the uniformity of element distribution is improved, and the lithium manganese iron phosphate material prepared by using the manganese iron phosphate precursor as the raw material has higher energy density and better electrochemical performance.

Description

Technical Field

[0001] The present invention belongs to the field of preparation of lithium manganese iron phosphate materials, and particularly relates to a manganese iron phosphate precursor and a preparation method and application thereof. Background Art

[0002] Lithium manganese iron phosphate (LMFP) material can provide higher energy density than lithium iron phosphate due to its higher discharge platform and has the same safety performance as lithium iron phosphate, thus it can become an upgraded substitute for lithium iron phosphate. However, manganese iron phosphate material also has relatively obvious disadvantages. On the one hand, it has low electronic conductivity and lithium ion diffusion coefficient; on the other hand, it has unsatisfactory cycle performance. This is due to the Jahn-Teller effect in the material during the charge and discharge process. The collapse of the structure causes the capacity to decay during the charge and discharge process. At this stage, morphology control, ion doping and surface coating are effective ways to optimize the conductivity and cycle performance of LMFP materials. By optimizing the synthesis method of the positive electrode material, the particle size, morphology and crystal orientation of the synthesized material can be effectively controlled. This is closely related to the electrochemical reaction kinetics and can significantly affect the electrochemical performance of the LMFP material.

[0003] Furthermore, research has shown that the Mn / Fe ratio determines the energy density and electrochemical performance of LMFP materials to a certain extent. When Mn / Fe = 6:4, the material exhibits excellent cycling stability, with an average voltage of around 3.65V and a high energy density. Therefore, synthesizing LMFP materials with a precise Mn / Fe ratio is crucial for improving their electrical properties.

[0004] The solid-phase method is a commonly used method for synthesizing LMFP materials. This method is simple in synthesis process and low in cost, and is widely popular in the industry. However, the raw materials are not easy to disperse evenly during the mixing process. LMFP materials contain two transition metal ions, which easily form metal ion segregation during the synthesis process, resulting in uneven grain growth. The manganese and iron ratio is difficult to accurately synthesize, and the content of doping elements cannot be accurately controlled, making it difficult to fully utilize the discharge capacity. Summary of the Invention

[0005] In order to solve the above technical problems, the purpose of the present invention is to provide a manganese iron phosphate precursor and its preparation method and application. The preparation method uses starch-based gel as a liquid medium to obtain a manganese iron phosphate precursor with a more accurate Mn / Fe ratio and doping element content, as well as a more uniform element distribution. The manganese iron phosphate lithium material prepared using this manganese iron phosphate precursor as raw material has good electrochemical properties.

[0006] The present invention provides a method for preparing a ferromanganese phosphate precursor, comprising the following steps:

[0007] S1. A phosphorus source, an organic manganese source and an organic iron source are added to an organic solvent to obtain a mixed solution;

[0008] S2. The doping element is dissolved in water, starch is added, and heated to form a sol;

[0009] S3. Under heating conditions, the mixed solution of step S1 and the sol of step S2 are uniformly mixed to obtain a mixed system, and the mixed system is adjusted to be alkaline after the reaction;

[0010] S4. After the reaction is completed, drying and calcining are performed to obtain a manganese iron phosphate precursor.

[0011] The present invention uses starch sol as a medium, and the branched molecules after starch gelatinization can bind to Mn 2+ 、Fe 2+ , so that Mn 2+ 、Fe 2+ Can be dispersed evenly in the sol to reduce Mn 2+ 、Fe 2+ The segregation phenomenon is beneficial to make the Mn / Fe ratio more accurate and improve the uniformity of the distribution of Mn elements and Fe elements. The doping elements can also be evenly dispersed in the sol, reducing the segregation phenomenon of the doping elements, which is beneficial to the precise regulation of the doping element content and improving the uniformity of the distribution of the doping elements. Furthermore, the manganese source and the iron source of the present invention are respectively an organic manganese source and an organic iron source, and form a mixed solution in an organic solvent. The organic solvent can dissolve the organic manganese source and the organic iron source well, and can also be well soluble in the doping elements. The distribution of the doping elements is more uniform, which is more conducive to the subsequent accurate and uniform doping of the doping elements into the manganese iron phosphate precursor. The mixed system after the metal salt solution and the sol are mixed in step S3 of the present invention is alkaline. Under alkaline conditions, the stability of the sol can be increased, and the Mn content can be further improved. 2+ 、Fe 2+ The uniformity of the doping elements in the mixed system is improved, thereby reducing the segregation of metal elements. Furthermore, when the ferromanganese phosphate precursor prepared by the preparation method of the present invention is used as a raw material to prepare lithium iron manganese phosphate, the prepared lithium iron manganese phosphate easily forms crystals of uniform size, thereby more conducive to the full utilization of its capacity. The steps in the preparation method of the present invention work synergistically, achieving a lithium iron manganese phosphate material prepared using the ferromanganese phosphate precursor as a raw material with higher energy density and better electrochemical performance.

[0012] Preferably, in S1, the amount of phosphorus in the phosphorus source is a, the amount of iron in the organic iron source is b, and the amount of manganese in the organic manganese source is c, and a / (b+c)=0.95~1.02. The value of a / (b+c) can be, for example, 0.95, 0.98, 1, or 1.02, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0013] Preferably, in said S1:

[0014] The organic iron source includes ferric acetate;

[0015] And / or, the organic manganese source includes at least one of manganese acetate and manganese oxalate;

[0016] and / or, the phosphorus source is phosphoric acid;

[0017] And / or, the organic solvent includes at least one of ethanol, propanol, and methanol.

[0018] In this solution, the organic iron source includes ferric acetate, which is more soluble in organic solvents, making its distribution more uniform; and / or, the organic manganese source includes at least one of manganese acetate and manganese oxalate, which is more soluble in organic solvents, making its distribution more uniform; and / or, the organic solvent includes at least one of ethanol, propanol, and methanol. The use of organic alcohol solvents can better dissolve the organic iron source and the organic manganese source, making their distribution uniform, and the subsequent doping elements can also be fully compatible with the organic alcohol solvent. Compared with water, starch gelatinized into a sol is more soluble in an organic alcohol solvent, and the sol is more uniform.

[0019] Preferably, in S2, the doping element includes at least one of Zr, Mg, Gr, V, and Ti.

[0020] Preferably, in S2, the starch is at least one of sweet potato starch, corn starch, and modified starch.

[0021] Preferably, in S2, the mass of the starch is 1.0wt%-5wt% of the mass of the water, for example, it can be 1.0wt%, 2.0wt%, 3.0wt%, 4.0wt%, 5.0wt%, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0022] The mass of the starch in this solution is 1.0wt%-5wt% of the mass of the water. Within this range, upon heating, the starch gelatinizes (breaks hydrogen bonds), and the resulting sol has better structural stability. This provides a stable sol environment as a medium, allowing metal ions to bind to the C chain. The metal ions are more evenly distributed in this medium, which can improve metal ion segregation. At the same time, the viscosity of the mixed system obtained by mixing the sol and the mixed solution is moderate, further facilitating the reaction to obtain a ferromanganese phosphate precursor with a precise manganese-iron ratio and doping element content.

[0023] Preferably, in S2, the heating is heating to 70°C to 90°C, for example, 70°C, 75°C, 80°C, 85°C, 90°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0024] Preferably, in S3, the alkalinity is pH=9-11, for example, 9, 10, 11, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0025] Preferably, in S4, the drying is specifically: heating to 100°C~200°C at a rate of 2~5°C / min (i.e., the drying temperature is 100°C~200°C) for drying, and the heating rate can be, for example, 2°C / min, 3°C / min, 4°C / min, 5°C / min, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable; the drying temperature can be, for example, 100°C / min, 120°C / min, 140°C / min, 160°C / min, 180°C / min, 200°C / min, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0026] The specific drying conditions of this scheme are: heating to 100℃~200℃ at a rate of 2~5℃ / min for drying. Under this drying condition, low-temperature drying of the organic solvent is conducive to the next step of calcination to obtain a manganese iron phosphate precursor with nano-sized particles.

[0027] Preferably, in S4, the calcination includes a first stage and a second stage, the calcination temperature of the first stage is 350-450°C, and the calcination temperature of the second stage is 500-700°C. The calcination temperature of the first stage can be, for example, 350°C, 380°C, 400°C, 420°C, 450°C, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable; the calcination temperature of the second stage can be, for example, 500°C, 550°C, 600°C, 650°C, 700°C, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0028] The specific calcination conditions of this scheme are as follows: it includes a first stage and a second stage, the calcination temperature of the first stage is 350-450°C, and the calcination temperature of the second stage is 500-700°C. Under the calcination conditions, the organic matter is burned out at medium temperature and the high-temperature phase is processed to synthesize the element-doped manganese iron phosphate precursor. The precursor has a nano-scale particle size, and the manganese iron phosphate lithium material synthesized using the precursor as a raw material has a porous structure. The small particle size and porous structure are conducive to improving the Li ion transmission rate in the manganese iron phosphate lithium positive electrode material and improving the kinetic reaction.

[0029] Preferably, in S4, the drying time is 12 to 20 hours, for example, it can be 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, or 20 hours, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0030] Preferably, in S4, the calcination time of the first stage is 6 to 10 hours, for example, 6 hours, 7 hours, 8 hours, 9 hours, or 10 hours, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0031] Preferably, in S4, the calcination time of the second stage is 10 to 18 hours, for example, it can be 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, or 18 hours, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0032] The present invention provides a ferromanganese phosphate precursor, including the ferromanganese phosphate precursor material prepared by the preparation method.

[0033] The present invention provides the use of the ferromanganese phosphate precursor in the preparation of lithium ferromanganese phosphate material. DETAILED DESCRIPTION

[0034] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention and the drawings in the embodiments. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0035] Example 1

[0036] This embodiment provides a Mg-doped ferromanganese phosphate precursor having a Mn / Fe molar ratio of 6:4, and a specific preparation method thereof includes:

[0037] (1) 381 mmol of phosphoric acid, 152.4 mmol of ferric acetate, and 228.6 mmol of manganese acetate were added to 300 mL of ethanol and stirred uniformly to form a mixed solution, wherein the molar ratio of manganese acetate to ferric acetate was 6:4, and the ratio of the total amount of manganese and iron elements to the amount of phosphate was 1:1.

[0038] (2) 0.76 mmol of Mg(NO3)2 was dissolved in 300 mL of deionized water, and 9 g of corn starch was added. The mixture was stirred at a constant speed in a stirred reactor until a uniform suspension was formed. The mixture was heated to 90°C, and the suspension swelled and gelatinized to form a sol.

[0039] (3) Add the mixed solution obtained in step (1) to the sol obtained in step (2) and stir and mix them uniformly at 90°C.

[0040] (4) The pH in the reactor was adjusted to 9, and stirring was continued at 90° C. for 3 h to obtain a gel.

[0041] (5) The stirred gel was placed in a forced air drying oven, heated to 150°C at a rate of 2°C / min, and dried for 12 h to dry the solvent and obtain a powder.

[0042] (6) The dried powder was ground and placed in a box-type muffle furnace. The temperature was increased at a rate of 2°C / min and calcined at 350°C for 6 h to burn out all organic matter. The temperature was then increased at a rate of 2°C / min and calcined at 600°C for 10 h. Finally, the powder was cooled to room temperature, washed thoroughly with deionized water, and dried to obtain a Mg-doped ferromanganese phosphate precursor material.

[0043] Example 2

[0044] This embodiment provides a V-doped ferromanganese phosphate precursor having a Mn / Fe molar ratio of 6:4. The specific preparation method thereof includes:

[0045] (1) 381 mmol of phosphoric acid, 152.4 mmol of ferric acetate, and 228.6 mmol of manganese acetate were added to 300 mL of methanol and stirred uniformly to form a mixed solution, wherein the molar ratio of manganese acetate to ferric acetate was 6:4, and the ratio of the total amount of manganese and iron elements to the amount of phosphate was 1:1.

[0046] (2) 0.76 mmol of NH4VO3 was dissolved in 300 mL of deionized water, and 3 g of sweet potato starch was added. The mixture was stirred at a constant speed in a stirred reactor until a uniform suspension was formed. The mixture was heated to 70 °C, and the suspension swelled and gelatinized to form a sol.

[0047] (3) Add the mixed solution obtained in step (1) to the sol obtained in step (2) and stir and mix them evenly at 70°C.

[0048] (4) The pH in the reactor was adjusted to 10, and stirring was continued at 70°C for 4 h.

[0049] (5) The stirred gel was placed in a forced air drying oven, heated to 100°C at a rate of 2°C / min, and dried for 20 h to dry the solvent and obtain a powder.

[0050] (6) The dried powder was ground and placed in a box-type muffle furnace. The temperature was increased at a rate of 5°C / min and calcined at 400°C for 6 h to burn out all organic matter. The temperature was then increased at a rate of 5°C / min and calcined at 700°C for 10 h. The powder was finally cooled to room temperature, washed thoroughly with deionized water, and dried to obtain a V-doped ferromanganese phosphate precursor material.

[0051] Example 3

[0052] This embodiment provides a Zr-doped ferromanganese phosphate precursor having a Mn / Fe molar ratio of 6:4. The specific preparation method thereof includes:

[0053] (1) 381 mmol of phosphoric acid, 152.4 mmol of ferric acetate, and 228.6 mmol of manganese acetate were added to 300 mL of propanol and stirred uniformly to form a mixed solution, wherein the molar ratio of manganese acetate to ferric acetate was 6:4, and the ratio of the total amount of manganese and iron elements to the amount of phosphate was 1:1.

[0054] (2) 0.76 mmol of Zr(NO3)4 was dissolved in 300 mL of deionized water, and 15 g of modified starch (carboxymethyl starch, purchased from Changzhou Shuangcheng Chemical Co., Ltd.) was added. The mixture was stirred at a constant speed in a stirred reactor until a uniform suspension was formed. The mixture was heated to 80°C, and the suspension swelled and gelatinized to form a sol.

[0055] (3) Add the mixed solution obtained in step (1) to the sol obtained in step (2) and stir and mix them evenly at 80°C.

[0056] (4) The pH in the reactor was adjusted to 11, and stirring was continued at 80°C for 5 h.

[0057] (5) The stirred gel was placed in a forced air drying oven, heated to 200°C at a rate of 5°C / min, and dried for 10 h to dry the solvent and obtain a powder.

[0058] (6) After the dried powder is ground, it is placed in a box muffle furnace and heated at a rate of 10°C / min, calcined at 450°C for 6h to burn off the organic matter. Then continue to heat, heated at a rate of 10°C / min, calcined at 500°C for 18h, and finally cooled to room temperature, washed with deionized water, and finally dried to obtain a Zr-doped manganese iron phosphate precursor material.

[0059] Example 4

[0060] This example provides a Mg-doped manganese iron phosphate precursor with a Mn / Fe molar ratio of 6:4, and the specific preparation method comprises:

[0061] In step (4) of this example, the pH in the reactor is adjusted to 8. The rest is the same as in Example 1.

[0062] Example 5

[0063] This example provides a Mg-doped manganese iron phosphate precursor with a Mn / Fe molar ratio of 6:4, and the specific preparation method comprises:

[0064] In step (4) of this example, the pH in the reactor is adjusted to 12. The rest is the same as in Example 1.

[0065] Example 6

[0066] This example provides a Mg-doped manganese iron phosphate precursor with a Mn / Fe molar ratio of 6:4, and the specific preparation method comprises:

[0067] In step (2) of this example, 2.5g of corn starch is added, and the rest is the same as in Example 1.

[0068] Example 7

[0069] This example provides a Mg-doped manganese iron phosphate precursor with a Mn / Fe molar ratio of 6:4, and the specific preparation method comprises:

[0070] In step (2) of this example, 17g of corn starch is added, and the rest is the same as in Example 1.

[0071] Comparative Example 1

[0072] This comparative example provides a manganese iron phosphate precursor, and the specific preparation method comprises:

[0073] (1) 381mmol of phosphoric acid, 152.4mmol of iron acetate, and 228.6mmol of manganese acetate are added to 300mL of ethanol and stirred uniformly to obtain a mixed solution, wherein the molar ratio of manganese acetate to iron acetate is 6:4, and the total amount of substance of manganese and iron elements to the amount of substance of phosphate is 1:1.

[0074] (2) Dissolve 0.76 mmol of Mg(NO3)2 in 300 mL of deionized water to obtain a doped metal salt solution.

[0075] (3) Add the mixed solution obtained in step (1) to the doped metal salt solution obtained in step (2) and stir at 90° C. to mix uniformly.

[0076] (4) The pH in the reactor was adjusted to 9, and stirring was continued at 90°C for 3 h.

[0077] (5) After stirring, place the mixture in a forced air drying oven, heat it to 150°C at a rate of 2°C / min, and dry it for 12 h to dry the solvent and obtain a powder.

[0078] (6) The dried powder was ground and placed in a box-type muffle furnace. The temperature was increased at a rate of 2°C / min and calcined at 350°C for 6 h to burn out all organic matter. The temperature was then increased at a rate of 2°C / min and calcined at 600°C for 10 h. Finally, the powder was cooled to room temperature, washed thoroughly with deionized water, and dried to obtain a Mg-doped ferromanganese phosphate precursor material.

[0079] Comparative Example 2

[0080] This comparative example provides a manganese ferrophosphate precursor, the specific preparation method of which includes:

[0081] Step (1) of this comparative example is as follows: 381 mmol of phosphoric acid, 76.2 mmol of ferric sulfate, and 228.6 mmol of manganese sulfate are added to 300 mL of deionized water and stirred uniformly to obtain a mixed solution, wherein the molar ratio of manganese sulfate to ferric sulfate is 6:2, and the ratio of the total amount of manganese and iron to the amount of phosphate is 1:1. The remaining steps are the same as in Example 1.

[0082] Comparative Example 3

[0083] This comparative example provides a manganese ferrophosphate precursor, the specific preparation method of which includes:

[0084] Step (2) of this comparative example is as follows: 9 g of corn starch is added to 300 mL of deionized water (without doping elements), stirred at a constant speed in a stirred reactor until a uniform suspension is formed, and heated to 90° C., and the suspension is swollen and gelatinized to form a sol. The rest of the steps are the same as in Example 1.

[0085] Comparative Example 4

[0086] In step (2) of this comparative example, 9 g of corn starch was added to 300 mL of deionized water (without doping elements), stirred at a constant speed in a stirred reactor until a uniform suspension was formed, and heated to 90° C. to allow the suspension to swell and gelatinize to form a sol. The remaining steps were the same as those in comparative example 2.

[0087] Comparative Example 5

[0088] The carbon-coated lithium manganese iron phosphate cathode material is directly prepared (without first preparing the Mg-doped lithium manganese iron phosphate precursor material), and the preparation method is as follows:

[0089] (1) 381 mmol of phosphoric acid, 152.4 mmol of ferric acetate, 228.6 mmol of manganese acetate, and 194.3 mmol of lithium carbonate were added to 300 mL of ethanol and stirred to form a mixed solution, wherein the molar ratio of manganese acetate to ferric acetate was 6:4, and the ratio of the total amount of manganese and iron to the amount of phosphate was 1:1.

[0090] (2) Dissolve 0.76 mmol of Mg(NO3)2 in 300 mL of deionized water, add 9 g of corn starch, stir uniformly in a stirred reactor until a uniform suspension is formed, and heat to 90°C until a sol is formed.

[0091] (3) Add the mixed solution obtained in step (1) to the sol obtained in step (2) and stir and mix them uniformly at 90°C.

[0092] (4) The pH in the reactor was adjusted to 9, and stirring was continued at 90° C. for 3 h to obtain a gel.

[0093] (5) The stirred gel was placed in a forced air drying oven, heated to 150°C at a rate of 2°C / min, and dried for 12 h to dry the solvent and obtain a powder.

[0094] (6) The dried powder was ground and placed in a box-type muffle furnace. The temperature was increased at a rate of 2°C / min and calcined at 350°C for 6 hours to burn out the organic matter to obtain a lithium manganese iron phosphate precursor. The lithium manganese iron phosphate precursor was mixed with 41970 mg of glucose and placed in a nitrogen atmosphere furnace. The temperature was then continued to be increased at a rate of 2°C / min and calcined at 600°C for 10 hours. Finally, it was cooled to room temperature, washed thoroughly with deionized water, and finally dried to obtain a carbon-coated lithium manganese iron phosphate material doped with Mg.

[0095] Performance Testing

[0096] The ferromanganese phosphate precursors prepared in Examples 1 to 7 and Comparative Examples 1 to 2 were ball-milled with lithium carbonate and glucose in a ratio of ferromanganese phosphate precursor: lithium carbonate: glucose = 1:0.5:0.6 (molar ratio), dried, and sintered at 350°C for 6 hours under the protection of a nitrogen atmosphere. After sintering, they were ball-milled again and sintered at 600°C in a N2 atmosphere for 10 hours to obtain a carbon-coated lithium manganese iron phosphate positive electrode material.

[0097] The carbon-coated lithium iron manganese phosphate positive electrode materials obtained in Examples 1 to 7 and Comparative Examples 1 to 3 were mixed into a positive electrode according to the ratio of carbon-coated LiFeMnPO4: PVDF (polyvinylidene fluoride): conductive carbon black = 9:0.5:0.5 (mass ratio), and lithium metal was used as the negative electrode to assemble into a lithium-ion button battery.

[0098] 2. Test items

[0099] The degree of element segregation was tested using ICP (inductively coupled plasma optical emission spectrometry).

[0100] ICP test method: Take 10 samples (0.2 g each) of the carbon-coated lithium manganese iron phosphate prepared in Example 1 and Comparative Examples 1 to 5, respectively, and digest each carbon-coated lithium manganese iron phosphate sample with 15 mL of 37 wt% concentrated hydrochloric acid (digestion at 210°C for 40 min). Remove and cool slightly, then filter, transfer the filtrate to a 250 mL volumetric flask, dilute to the scale with ultrapure water, and shake well to obtain the test solution.

[0101] Then, the content of the main elements in the solution to be tested was measured using an inductively coupled plasma emission spectrometer, and the average molar ratio of Mn:Fe (x=n / 10, n represents the sum of the molar ratios of Mn:Fe of 10 samples, and x represents the average molar ratio) and its variance (variance D1=((x1-x))) of the carbon-coated lithium manganese iron phosphate of Example 1 and Comparative Examples 1 to 5 were calculated. 2 +(x2-x) 2 +(x3-x) 2 +…+(x 10 -x) 2 ) / 10,x1,x2,x3,…,x 10 represents the Mn:Fe molar ratio of each of the 10 samples) and the doping transition element content (y = m / 10, m represents the sum of the mass fractions of the doping elements in the 10 samples, y represents the average mass fraction) and its variance (D2 = ((y1-y) 2 +(y²-y) 2 +(y3-y) 2 +…+(y 10 -y) 2 ) / 10,y1,y2,y3,…,y 10 represents the mass fraction of doping elements in 10 samples respectively), and the results are shown in Table 1 below.

[0102] Table 1

[0103]

[0104] As shown in Table 1, the ICP test results show that the molar ratio of Mn to Fe and the content of doping elements in Examples 1 to 3 are closer to the raw material input ratio, and the uniformity of element distribution is high, indicating that metal ions do not segregate during the synthesis process, the growth of grains is uniform, the manganese iron examples are accurately synthesized, the doping element content can also be precisely controlled, and the element uniformity can also be improved. Compared with Example 1, the pH in the reactor is adjusted to 8 and 12 in step (4) of Example 4 and Example 5, respectively, which are smaller and larger, respectively. The molar ratio of Mn to Fe and the content of doping elements deviate slightly from the raw material input ratio, and the uniformity of doping element distribution is slightly lower than that of Example 1, indicating that metal ion segregation is reduced during the synthesis process. Compared with Example 1, the amount of corn starch added in step (2) of Example 6 and Example 7 was 2.5g and 17g, respectively, that is, the mass of starch was 0.83% and 5.67% of the mass of water, respectively. The starch content was relatively low and relatively high, respectively. The molar ratio of Mn and Fe elements and the content of doping elements slightly deviated from the raw material input ratio, and the uniformity of doping element distribution was slightly lower than that of Example 1, indicating that the low starch content had little effect on metal ion segregation. The high starch content resulted in a viscosity increase in the mixed system obtained by mixing the sol and the mixed solution, which also had little effect on metal ion segregation. Compared with Example 1, Comparative Example 1 did not add starch to form the sol, the molar ratio of Mn and Fe elements seriously deviated from the raw material input ratio, and the uniformity of doping element distribution was significantly lower than that of Example 1. Compared with Example 1, step (1) of Comparative Example 2 uses an inorganic iron source, an inorganic manganese source, and an inorganic solvent, water, that is, Comparative Example 2 uses an inorganic system. Compared with the organic system of Example 1, the molar ratio of Mn and Fe elements and the content of doping elements in the inorganic system of Comparative Example 2 deviate from the raw material input ratio, and the uniformity of the distribution of doping elements is also worse than that of Example 1. Compared with Example 1, Comparative Example 3 is not doped with Mg elements; compared with Comparative Example 2, Comparative Example 4 is not doped with Mg elements. Comparative Examples 3 and 4 are basically the same in terms of the molar ratio of Mn and Fe elements and the uniformity of their element distribution, indicating that when preparing a manganese iron phosphate precursor without doping elements, the effect is the same whether in an organic system or an inorganic system. Compared with Example 1, Comparative Example 5 directly prepares a carbon-coated manganese iron phosphate lithium positive electrode material without first preparing the corresponding manganese iron phosphate precursor. Compared with Example 1, the molar ratio of Mn and Fe elements and the content of doping elements in Comparative Example 5 deviate from the raw material input ratio, and the uniformity of the distribution of doping elements is also worse than that of Example 1.

[0105] Electrical performance test:

[0106] Lithium-ion button cell capacity test conditions: 25°C, charge to 4.25V at 0.1C, cut-off current 0.05C, then discharge to 2.0V at 0.1C, and record the first discharge capacity in grams.

[0107] Lithium-ion button cell cycle test conditions: 25°C, charge to 4.25V at 0.5C, cut-off current 0.05C, then discharge to 2.0V at 0.5C, repeat this cycle, and record capacity retention.

[0108] Li + Diffusion coefficient test method:

[0109] Initial electrochemical impedance spectroscopy (EIS) tests were performed using a Reference 600 electrochemical workstation manufactured by GAMRY Instruments, Inc., USA. Lithium metal was used as the counter and reference electrodes, and the active material electrode was used as the working electrode. The tests were conducted at 28°C, with a scan frequency of 100 kHz to 0.01 Hz and a scan amplitude of 5 mV. The EIS of the lithium iron phosphate cathode material was used to investigate its electrochemical behavior, including charge transfer impedance and lithium ion diffusion rate. Li . D Li The following formula can be used for calculation:

[0110] D Li =R 2 T 2 / 2A 2 n 4 F 4 C 2 σ 2

[0111] Where R is the molar gas constant, 8.314 Pa·m 3 ·mol -1 ·K -1

[0112] T--absolute temperature (K)

[0113] A - surface area of ​​positive electrode material (m 2 )

[0114] n--the number of electrons transferred per mole of active material

[0115] F--Faraday constant (96485.3383±0.0083C / mol)

[0116] C--lithium ion concentration (mol / L)

[0117] σ--Warburg impedance coefficient

[0118] The relationship between the real part of electrochemical impedance and σ is:

[0119] Z Re =R Ω +R ct +R w=R Ω +Rct+σ·ω -1 / 2

[0120] where R Ω is the ohmic impedance, Rct is the charge transfer impedance, R w The Warburg impedance coefficient σ is numerically equal to the slope of the real part of the electrochemical impedance with respect to the inverse square root of the diagonal frequency.

[0121] The carbon-coated lithium manganese iron phosphate prepared by using the manganese iron phosphate precursors of Examples 1 to 7 and Comparative Examples 1 to 2 as raw materials and the carbon-coated lithium manganese iron phosphate prepared in Comparative Example 3 were used to prepare positive electrodes and then assembled into lithium ion button batteries. The first discharge gram capacity and the number of cycles when the capacity decayed to 80% of the lithium ion button batteries were tested. The results are shown in Table 2 below. At the same time, the Li + Diffusion coefficient, the results are shown in Table 2 below.

[0122] Table 2

[0123]

[0124]

[0125] As shown in Table 2, the molar ratio of Mn and Fe elements in the ferromanganese phosphate precursors prepared in Examples 1 to 3, and the content of doping elements are closer to the raw material input ratio, and the uniformity of element distribution is high, indicating that metal ions do not undergo segregation during the synthesis process, the growth of grains is uniform, the ferromanganese phosphate is precisely synthesized, and the content of doping elements can also be precisely controlled, while also improving the uniformity of elements. The lithium ion button cells assembled from the carbon-coated lithium manganese phosphate materials prepared using the ferromanganese phosphate precursors of Examples 1 to 3 as raw materials have excellent first discharge capacity and cycle performance, and have a larger porosity and a higher lithium ion diffusion rate. Compared with Example 1, the pH in the reactor is adjusted to 8 and 12 in step (4) of Example 4 and Example 5, respectively, which are smaller and larger, respectively. The molar ratio of Mn and Fe elements and the content of doping elements slightly deviate from the raw material input ratio, and the uniformity of doping element distribution is slightly lower than that of Example 1, indicating that metal ion segregation is reduced during the preparation process, and the first discharge capacity, cycle number and lithium ion diffusion rate are slightly reduced compared to Example 1. Compared with Example 1, the amount of corn starch added in step (2) of Example 6 and Example 7 is 2.5g and 17g, respectively, that is, the mass of starch is 0.83% and 5.67% of the mass of water, respectively. The starch content is relatively low and relatively high, the molar ratio of Mn element to Fe element and the content of doping element slightly deviate from the raw material input ratio, and the uniformity of doping element distribution is slightly lower than that of Example 1, indicating that the starch content is low and the improvement on metal ion segregation is small; the starch content is high, the viscosity of the mixed system obtained by mixing the sol and the mixed solution is small, and the improvement on metal ion segregation is also small. The first discharge capacity, cycle number and lithium ion diffusion rate are slightly reduced compared with Example 1. Compared with Example 1, Comparative Example 1 does not add starch to form a sol, the molar ratio of Mn element to Fe element seriously deviates from the raw material input ratio, the uniformity of doping element distribution is also significantly lower than that of Example 1, and the first discharge capacity, cycle number and lithium ion diffusion rate are significantly reduced compared with Example 1. Compared with Example 1, step (1) of Comparative Example 2 uses an inorganic iron source, an inorganic manganese source, and an inorganic solvent, water, that is, Comparative Example 2 uses an inorganic system. Compared with the organic system of Example 1, the molar ratio of Mn and Fe elements and the content of doping elements in the inorganic system of Comparative Example 2 deviate from the raw material input ratio, and the uniformity of the doping element distribution is also worse than that of Example 1. The initial discharge capacity, the number of cycles, and the lithium ion diffusion rate are all reduced compared with Example 1. Compared with Example 1, Comparative Example 3 is not doped with Mg elements; compared with Comparative Example 2, Comparative Example 4 is not doped with Mg elements. Comparative Examples 3 and 4 are basically the same in the molar ratio of Mn and Fe elements and the uniformity of their element distribution, indicating that when preparing a manganese phosphate iron precursor without doping elements, the effect is the same whether in an organic system or an inorganic system. Comparative Examples 3 and 4 are also basically the same in the initial discharge capacity, the number of cycles, and the lithium ion diffusion rate, and are all worse than Example 1.Compared with Example 1, the Comparative Example 5 is directly prepared into the carbon-coated lithium manganese iron phosphate positive electrode material, and the corresponding manganese iron phosphate precursor is not prepared first. Compared with Example 1, the molar ratio of Mn element and Fe element, the content of the doping element of the Comparative Example 5 deviates from the raw material input ratio, the uniformity of the distribution of the doping element is also worse than that of Example 1, and the first discharge capacity, the cycle number and the lithium ion diffusion speed are all smaller than those of Example 1.

[0126] In summary, it is illustrated that the carbon-coated lithium manganese iron phosphate positive electrode material prepared by taking the manganese iron phosphate precursor of the embodiment of the present application as the raw material has the more uniform distribution of Mn element, Fe element and doping element, does not generate other non-stoichiometric ratio of impurities, has the higher purity, is not affected by the impurities in the cycle process, and has the more stable structure.

[0127] The above examples are only used to illustrate the technical solutions of the present application but not to limit the protection scope of the present application. Although the present application is described in detail with reference to the above examples, it should be understood by those skilled in the art that the technical solutions of the present application can be modified or replaced equivalently, and these modifications or replacements are all within the protection scope of the present application.

Claims

1. A method for preparing a ferromanganese phosphate precursor, characterized in that: The steps include: S1. A phosphorus source, an organic manganese source and an organic iron source are added to an organic solvent to obtain a mixed solution; S2. The doping element is dissolved in water, starch is added, and heated to form a sol; S3. Under heating conditions, the mixed solution of step S1 and the sol of step S2 are uniformly mixed to obtain a mixed system, and the mixed system is adjusted to be alkaline after the reaction; S4. After the reaction is completed, drying and calcining to obtain a manganese iron phosphate precursor; In S2, the mass of the starch is 1.0 wt% to 5 wt% of the mass of the water.

2. The method for preparing the ferromanganese phosphate precursor according to claim 1, wherein: In S1, the amount of phosphorus in the phosphorus source is a, the amount of iron in the organic iron source is b, and the amount of manganese in the organic manganese source is c, and a / (b+c)=0.95~1.

02.

3. The method for preparing the ferromanganese phosphate precursor according to claim 1, wherein: In said S1: The organic iron source includes ferric acetate; And / or, the organic manganese source includes at least one of manganese acetate and manganese oxalate; and / or, the phosphorus source is phosphoric acid; And / or, the organic solvent includes at least one of ethanol, propanol, and methanol.

4. The method for preparing the ferromanganese phosphate precursor according to claim 1, wherein: In S2, the doping element includes at least one of Zr, Mg, Gr, V, and Ti.

5. The method for preparing the ferromanganese phosphate precursor according to claim 1, wherein: In the step S2, the heating is performed to a temperature of 70°C to 90°C.

6. The method for preparing the ferromanganese phosphate precursor according to claim 1, wherein: In the S3, the alkalinity is pH=9-11.

7. The method for preparing the ferromanganese phosphate precursor according to claim 1, wherein: In the step S4 , the calcination includes a first stage and a second stage. The calcination temperature of the first stage is 350-450° C., and the calcination temperature of the second stage is 500-700° C.

8. A ferromanganese phosphate precursor, characterized in that: The ferromanganese phosphate precursor is prepared by the preparation method according to any one of claims 1 to 7.

9. Use of the ferromanganese phosphate precursor according to claim 8 in the preparation of lithium ferromanganese phosphate material.

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