Mixed slurry for preparing positive electrode material and preparation method for mixed slurry

Abstract

The present invention relates to the technical field of lithium-ion batteries, and in particular to a mixed slurry for preparing a positive electrode material and a preparation method for the mixed slurry. The preparation method comprises the following steps: dispersing, for N times, a first slurry comprising a manganese iron phosphate precursor and a lithium source in an organic solvent to obtain a second slurry, and performing heat treatment until the pH value of the second slurry tends to be stable, so as to remove gas in the first slurry to obtain a mixed slurry for preparing a positive electrode material, wherein N is a positive integer greater than or equal to 2, the boiling point of the organic solvent is greater than or equal to 160°C, and the temperature of the heat treatment is 100-200°C. The preparation method can greatly reduce the specific surface area of a positive electrode material without affecting the electrochemical performance thereof, is simple to operate, and is suitable for mass production.

Description

Mixed slurry for preparing positive electrode material and preparation method thereof

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on August 3, 2023, with application number 202310968713.6 and application name “A mixed slurry for preparing positive electrode materials and its preparation method”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the technical field of lithium-ion batteries, and in particular to a mixed slurry for preparing positive electrode materials and a preparation method thereof. Background Art

[0003] Lithium-ion batteries, with their high energy density and excellent cycle performance, are rapidly replacing lead-acid batteries and are widely used in laptop computers, electric bicycles, new energy electric vehicles, and other fields. As a key component of lithium-ion batteries, the cathode material largely determines their performance.

[0004] Among them, lithium iron manganese phosphate (LiFePO4) materials have great potential for application in power lithium-ion batteries. They combine the advantages of LiFePO4's high cycle stability with its high voltage and energy density. However, existing LiFePO4 materials have low electronic conductivity. To improve the electrochemical performance of LiFePO4 materials, researchers typically employ single measures such as particle size reduction, conductive material coating, bulk ion doping, material structure design, and morphology control, or utilize a combination of these measures to improve the performance of LiFePO4 materials.

[0005] A Chinese invention patent application, publication number CN114649517A, discloses a method for synthesizing a nanoscale carbon-composite lithium manganese iron phosphate cathode material by preparing nanoscale precursors, manganese oxalate and ferrous oxalate. This method facilitates lithium ion diffusion, improves rate capability, and thus enhances electrochemical performance. However, the large specific surface area of ​​the nanoscale material makes homogenization and coating difficult during battery cell production, increasing cell manufacturing costs.

[0006] In summary, the particle size of existing lithium iron manganese phosphate materials is generally controlled at the nanometer level, and their specific surface area is large. Once exposed to humid air, the moisture easily contacts the hydrophilic Li phase on the material surface and undergoes a lithiation reaction, resulting in an increase in the water content of the lithium iron manganese phosphate cathode material, which in turn affects the battery's capacity retention rate. Moreover, during the slurry preparation process of the battery, the excessively large specific surface area of ​​the cathode material can easily lead to agglomeration. It can also cause the cathode slurry to crack and fall off the aluminum foil during drying, making the preparation of the cathode electrode sheet difficult. Therefore, it is of great significance to prepare a lithium iron manganese phosphate material with a low specific surface area and good electrochemical performance.

[0007] Summary of the Invention

[0008] The present application discloses a mixed slurry for preparing a cathode material and a preparation method thereof, so as to obtain a lithium iron manganese phosphate material with a low specific surface area and good electrochemical performance.

[0009] To achieve the above object, the present application provides the following technical solutions:

[0010] In a first aspect, the present application provides a preparation method of a mixed slurry for preparing a cathode material. The preparation method includes the following steps: dispersing a first slurry including a lithium iron manganese phosphate precursor and a lithium source in an organic solvent in N times to obtain a second slurry, and performing heat treatment until the pH value of the second slurry tends to be stable to remove the gas in the first slurry, so as to obtain a mixed slurry for preparing a cathode material; N is a positive integer greater than or equal to 2; the boiling point of the organic solvent is greater than or equal to 160°C, and the temperature of the heat treatment is 100°C to 200°C.

[0011] Further, the preparation method of the lithium iron manganese phosphate precursor includes the following steps: mixing raw materials including soluble phosphate, iron source, manganese source and ammonia source and reacting to obtain a precursor slurry containing the lithium iron manganese phosphate precursor, and performing pressure filtration, washing and drying on the precursor slurry to obtain the lithium iron manganese phosphate precursor, wherein the pH value of the precursor slurry is less than or equal to 12.

[0012] Further, the stable pH value of the second slurry is 7 to 10.

[0013] Further, the lithium iron manganese phosphate precursor includes at least one of NH4Fe x Mn 1-x PO4·H2O, Fe x Mn 1- x HPO4·yH2O; wherein, 0 < x < 0.5, and y is the water content.

[0014] Further, the lithium source includes at least one of lithium hydroxide, lithium oxide, lithium carbonate, lithium dihydrogen phosphate, lithium hydrogen phosphate or lithium phosphate; the molar ratio of the lithium source to the lithium iron manganese phosphate precursor is 1:1 to 1.06:1.

[0015] Further, the organic solvent includes at least one of ethylene glycol, dimethyl sulfoxide, N-methylpyrrolidone, and formamide.

[0016] Further, the median particle size of the particles in the first slurry is 0.1 μm to 1.0 μm, and the solid content of the first slurry is 10% to 50%.

[0017] Further, the time of the heat treatment is 4h to 10h.

[0018] Furthermore, the first slurry also includes a coating agent and / or a dopant.

[0019] In a second aspect, the present application provides a mixed slurry for preparing a positive electrode material, the mixed slurry comprising a phosphate precursor and a lithium source, and the pH value of the mixed slurry is 7-10.

[0020] Furthermore, the pH of the mixed slurry is 7-8.

[0021] Furthermore, the viscosity of the mixed slurry is 500 mPa·S to 10000 mPa·S, and the solid content of the mixed slurry is 10% to 40%.

[0022] Furthermore, the weight loss temperature of the mixed slurry at 1.6% to 2.0% measured by thermogravimetric analysis is 110° C. to 130° C.

[0023] Furthermore, the weight loss temperature of the mixed slurry at 9% to 11% measured by thermogravimetric analysis is 140° C. to 180° C.

[0024] The technical solution of this application has the following beneficial effects:

[0025] The present application provides a method for preparing a mixed slurry, wherein a first slurry including a ferromanganese phosphate precursor and a lithium source is dispersed in an organic solvent to obtain a second slurry, and the gas generated by the thermal decomposition of the ferromanganese phosphate precursor and the lithium source is removed by heat treatment. When the pH value of the second slurry tends to be stable, it indicates that the gas is effectively removed, thereby significantly reducing the specific surface area of ​​the positive electrode material without affecting its electrochemical performance. Moreover, the method in the present application is simple to operate and suitable for mass production. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG1 is a flow chart of a method for preparing a mixed slurry in an embodiment of the present application. DETAILED DESCRIPTION

[0027] To make the objectives, technical solutions, and advantages of this application more clear, this application will be further described in detail below with reference to the accompanying drawings. It should be understood that the embodiments described herein are only some of the embodiments of this application, and not all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of this application without inventive effort are intended to fall within the scope of protection of this application.

[0028] The application scenarios described in the embodiments of this application are intended to more clearly illustrate the technical solutions of the embodiments of this application and do not constitute a limitation on the technical solutions provided by the embodiments of this application. Persons skilled in the art will appreciate that, as new application scenarios emerge, the technical solutions provided by the embodiments of this application are equally applicable to similar technical problems. In the description of this application, unless otherwise specified, "multiple" means two or more.

[0029] Currently, lithium manganese iron phosphate cathode materials have a relatively large specific surface area. This specific surface area is primarily influenced by the inner layer of lithium manganese iron phosphate and the coating material. Furthermore, gas removal during the precursor sintering process creates pores on the cathode material surface, which can also increase the specific surface area. Therefore, it is crucial to develop a mixed slurry that can reduce the specific surface area of ​​the cathode material without compromising its electrochemical performance.

[0030] In view of this, an embodiment of the present application provides a method for preparing a mixed slurry for preparing a positive electrode material. FIG1 is a flow chart of the method for preparing the mixed slurry in the embodiment of the present application. Referring to FIG1 , the preparation method includes the following steps:

[0031] A first slurry including a manganese iron phosphate precursor and a lithium source is dispersed in an organic solvent N times to obtain a second slurry, and heat-treated until the pH value of the second slurry tends to be stable to remove the gas in the first slurry, thereby obtaining a mixed slurry for preparing a positive electrode material; N is a positive integer greater than or equal to 2; the boiling point of the organic solvent is greater than or equal to 160°C, and the temperature of the heating treatment is 100°C to 200°C.

[0032] To improve the dispersibility of the first slurry, the first slurry is dispersed in an organic solvent N times to obtain a second slurry, and the second slurry is continuously heated. N is a positive integer greater than or equal to 2, preferably 2, 3, 4, 5, or 6. This improves the dispersibility of the first slurry in the organic solvent and allows for more uniform heating of the first slurry without complicating the process.

[0033] In order to further improve the dispersibility of the first slurry, the second slurry may be stirred, and the specific stirring speed is set as needed.

[0034] Optionally, the first slurry is added to the organic solvent in two steps. First, a portion of the first slurry is added to the organic solvent at a first rotational speed. After a period of reaction, the remaining first slurry is added to the organic solvent at a second rotational speed until the gas components are discharged. Both the first and second rotational speeds are 500 to 2000 rpm.

[0035] In one embodiment of the present application, the boiling point of the organic solvent is greater than or equal to 160°C, and the temperature of the heat treatment is 100°C to 200°C. Because the boiling point of the organic solvent is relatively high, the corresponding groups in the first slurry can be decomposed into gases and discharged under the premise of reducing its volatilization, thereby effectively reducing the generation of gases during the subsequent sintering of the mixed slurry to prepare the positive electrode material, which leads to pores on the surface of the positive electrode material and an increase in specific surface area.

[0036] It is understood that the ferromanganese phosphate precursor and the lithium source are mechanically mixed to obtain a first slurry of a predetermined particle size. The present application does not limit the method of mechanical mixing, and the mechanical mixing method includes at least one of ball milling or sand milling to ensure that the median particle size of the particles in the first slurry is 0.1 μm to 1.0 μm. The small and similar particle sizes of the particles in the first slurry help improve the dispersibility of the subsequent first slurry in the second slurry.

[0037] The rotation speed of the ball mill or sand mill is 1200 rpm to 2800 rpm, and the temperature is room temperature to 40° C. Controlling the temperature to not exceed 40° C. can effectively avoid side reactions between the ferromanganese phosphate precursor and the lithium source.

[0038] In one embodiment of the present application, the solid content of the first slurry is 10% to 50%.

[0039] In one embodiment of the present application, the first slurry further includes a dopant and / or a coating agent. When the dopant is a metal material, specifically, it can be selected from magnesium, titanium or aluminum. The concentration of the dopant in the first solvent is 1000 to 10000 ppm. The coating agent can be a carbon material, specifically, it can be selected from at least one of sucrose, glucose, citric acid, polyvinyl alcohol, polyethylene glycol, carbon nanotubes, carbon black, acetylene black, graphene, etc. The mass ratio of the coating agent to the finally prepared lithium manganese iron phosphate positive electrode material is 1 to 5%.

[0040] Among them, the volatile substances in the ferromanganese phosphate precursor and the lithium source decompose and generate gas when heated, causing the pH value of the second slurry to change rapidly until it stabilizes. Therefore, the degree of gas removal in the second slurry can be determined based on the pH value of the second slurry. Specifically, during the heating process, the pH value of the second slurry can be closely monitored to control the removal effect of the corresponding substances that can generate gas. For example, it can be monitored in real time or at intervals.

[0041] In one embodiment of the present application, the stable pH value of the second slurry is 7 to 10, preferably 7 to 9. Within the above range, the occurrence of side reactions between the ferromanganese phosphate precursor and the lithium source is reduced.

[0042] In one embodiment of the present application, the boiling point of the organic solvent is greater than or equal to 160 °C. Preferably, the boiling point of the organic solvent is greater than or equal to 180 °C, which is beneficial to the removal of gas in the second slurry and reduces the volatilization of the organic solvent.

[0043] It can be understood that the type of the organic solvent is not limited in the present application. Specifically, the organic solvent can be a solvent with a high boiling point and good dispersibility, such as at least one selected from ethylene glycol, dimethyl sulfoxide, N-methylpyrrolidone or formamide.

[0044] In one embodiment of the present application, the temperature of the heat treatment is 100 °C to 200 °C, which is specifically set according to the type of the organic solvent and the length of the heating time. The purpose is to fully remove the gas in the second slurry and at the same time reduce the volatilization of the organic solvent.

[0045] In one embodiment of the present application, the time of the heat treatment is 4 h to 10 h, and the duration of the heat treatment is specifically determined by fully removing the gas in the second slurry.

[0046] In one embodiment of the present application, the lithium source includes at least one of lithium hydroxide, lithium oxide, lithium carbonate, lithium dihydrogen phosphate, lithium hydrogen phosphate or lithium phosphate; the molar ratio of the lithium source to the manganese iron phosphate precursor is 1:1 to 1.06:1.

[0047] In one embodiment of the present application, the manganese iron phosphate precursor includes, but is not limited to, NH4Fe x Mn<9000007> x PO4·H2O, Fe x Mn 1-x HPO4·yH2O; wherein, 0 < x < 0.5, and y is the water content.

[0048] In one embodiment of the present application, the preparation method of the manganese iron phosphate precursor includes the following steps: mixing raw materials including soluble phosphate, iron source, manganese source and ammonia source and reacting to obtain a precursor slurry containing the manganese iron phosphate precursor, and performing pressure filtration, washing and drying on the precursor slurry to obtain the manganese iron phosphate precursor, wherein the pH value of the precursor slurry is less than or equal to 12 so that the pH value of the second slurry is within a preset range.

[0049] In order to more clearly illustrate the preparation method of the precursor, the preparation method of the precursor is described below by taking the ammonium manganese iron phosphate precursor and the iron manganese hydrogen phosphate precursor as examples respectively, as follows:

[0050] Among them, the general formula of the ammonium manganese iron phosphate precursor is NH4Fe x Mn 1-x PO4·H2O, wherein, 0 < x < 0.5, and its preparation method includes the following steps:

[0051] Under the condition that the pH value is 5 - 7 and in a protective atmosphere, soluble phosphate, ammonia, an iron source and a manganese source are mixed according to a molar ratio of 1.0 - 1.5:1.0:x:1 - x (0 < x < 0.5), and kept at 30°C - 80°C for 1 h - 8 h for heat preservation reaction. After sufficient required precipitation is formed in the reaction system, pressure filtration, washing and drying are carried out to obtain an ammonium iron manganese phosphate precursor.

[0052] Among them, the molar concentration of the soluble phosphate is 1.0 mol / L - 1.5 mol / L, and the ammonia is ammonia water with a concentration of 4.5 mol / L - 5.0 mol / L; among them, the above reaction is carried out at a stirring speed of 200 rpm - 1000 rpm; among them, the drying temperature is 80 - 200°C, and the drying time is 2 - 12 hours.

[0053] To avoid the Fe 2+ ions introduced by the iron source during the above reaction process from being oxidized to generate Fe 3+ ions, the above reaction is carried out under the condition of the presence of an antioxidant. The antioxidant can be ascorbic acid.

[0054] In an embodiment of the present application, the protective atmosphere for the above reaction is at least one of argon, nitrogen and carbon dioxide, so as to ensure that any reactant and / or intermediate product does not undergo an oxidation reaction during the coprecipitation reaction process.

[0055] Among them, the general formula of the ammonium iron manganese phosphate precursor is Fe x Mn 1-x HPO4·yH2O, where 0 < x < 0.5 and y is the water content. Its preparation method includes the following steps:

[0056] In a protective atmosphere, ammonium bicarbonate, an iron source and a manganese source are added to a reaction kettle according to a molar ratio of 1:x:1 - x (0 < x < 0.5), and an antioxidant is added. A reaction slurry is obtained by reacting at room temperature for 2 h - 6 h. Then, soluble phosphate is added according to a molar ratio of 1:1 of the soluble phosphate to the ammonium bicarbonate under the protective atmosphere, and then the temperature is raised to 90°C for reaction. After the reaction is completed, the precipitate formed by the reaction is centrifugally washed with deionized water, and finally dried at 70°C - 130°C for 4 h - 15 h to obtain the ammonium iron manganese phosphate precursor.

[0057] It is understood that the types of soluble phosphate, iron source, manganese source and ammonia source are selected specifically according to the type of manganese ferrophosphate precursor. Specifically, the soluble phosphate (i.e., phosphorus source) is selected from at least one of phosphoric acid, ammonium dihydrogen phosphate, ammonium dihydrogen phosphate, manganese dihydrogen phosphate, calcium dihydrogen phosphate, calcium phosphate, ammonium phosphate, and ammonium pyrophosphate. The iron source is selected from at least one of iron powder, ferrous sulfate, ferric oxide, ferric acetate, ferric phosphate, ferrous phosphate, ferrous monohydrogen phosphate, ferric dihydrogen phosphate, ferric nitrate, and ferric chloride. The manganese source is selected from at least one of manganese monoxide, manganese dioxide, manganese tetraoxide, manganese sulfate, manganese carbonate, and manganese hydroxide.

[0058] Based on the same inventive concept, the present application provides a mixed slurry for preparing a positive electrode material, wherein the mixed slurry includes a phosphate precursor and a lithium source, and the pH of the mixed slurry is 7-10.

[0059] In a specific embodiment of the present application, the pH value of the mixed slurry is 7-8, and the pH value of the mixed slurry is close to neutral, indicating that the removal effect of related substances that can generate gas and affect the pH value of the solution is good, thereby effectively reducing the generation of gas during the subsequent sintering process of the mixed slurry and reducing the specific surface area of ​​the positive electrode material.

[0060] The mixed slurry can be prepared by the preparation method in various possible embodiments of the present application, by heating and exhausting the slurry including the ferromanganese phosphate precursor and the lithium source so that the pH value of the mixed slurry is 7-8.

[0061] In one embodiment of the present application, the viscosity of the mixed slurry is 500mPa·S to 10000mPa·S, for example 500mPa·S, 1000mPa·S, 1500mPa·S, 2000mPa·S, 2500mPa·S, 3500mPa·S, 4500mPa·S, 5500mPa·S, 7000mPa·S, 8500mPa·S, 10000mPa·S or a range between any two of them.

[0062] The solid content of the mixed slurry is 10% to 40%, for example 10%, 20%, 25%, 30%, 40% or any range between two thereof.

[0063] In one embodiment of the present application, the 1.6% to 2.0% weight loss temperature of the mixed slurry measured by thermogravimetric analysis is 110° C. to 130° C. Within the above temperature range, evaporation of free water in the mixed slurry causes the weight of the mixed slurry to decrease by 1.6% to 2.0%.

[0064] In one embodiment of the present application, the temperature at which the mixed slurry loses 9% to 11% of its weight as measured by thermogravimetric analysis is 140° C. to 180° C. Within the above temperature range, the decomposition of gas-generating substances in the mixed slurry and the evaporation of the gas result in a weight reduction of the mixed slurry of 9% to 11%.

[0065] Therefore, after heating treatment, the related substances that can generate gas in the mixed slurry in the embodiment of the present application can be decomposed and the corresponding gas can be discharged, thereby avoiding the generation of gas in the subsequent sintering process of the mixed slurry, resulting in pores on the surface of the positive electrode material and an increase in the specific surface area.

[0066] The mixed slurry and its preparation method in this application will be further described in detail below with reference to specific examples and comparative examples.

[0067] Example 1

[0068] This embodiment is a method for preparing a mixed slurry, which includes the following steps:

[0069] S1: First, prepare a ferromanganese solution: ferrous sulfate and manganese sulfate are mixed in a molar ratio of 0.4:0.6 to prepare 1 L of a 1.0 mol / L ferromanganese solution, wherein the ferromanganese solution also includes ascorbic acid, and the ascorbic acid is 0.04 wt% of the mass of the manganese source;

[0070] Then, under nitrogen atmosphere, 1L of 1.2mol / L ammonium dihydrogen phosphate solution, 1L of 1.0mol / L manganese iron solution and ammonia water were reacted at 60℃ and 500rpm stirring speed for 3h, and the pH value of the above solution was regulated to 6 by controlling the flow rate of ammonia water. The generated precipitate was filtered and washed by centrifugation with deionized water, and dried at 110℃ for 5h to obtain nano-scale manganese iron ammonium phosphate precursor NH4Fe 0.4 Mn 0.6 PO4·H2O.

[0071] S2: NH4Fe 0.4 Mn 0.6 PO4·H2O, lithium carbonate, glucose, and polyethylene glycol are mixed and then ball-milled and sand-milled to obtain a first slurry, wherein the molar ratio of lithium carbonate to ammonium dihydrogen phosphate is 1.02:1, the sum of the masses of glucose and polyethylene glycol is 2.1 wt% of the mass of the finally prepared lithium manganese iron phosphate, the ball milling speed is 2400 rpm, the sand milling speed is 1200 rpm, and the temperature is ≤25°C. The median particle size D50 of the particles in the obtained first slurry is 0.18 μm, and the solid content is 20%.

[0072] S3: The first slurry was dispersed in ethylene glycol twice to obtain a second slurry, and heated at 160°C for 5 hours. The initial pH value of the second slurry was 9. When the pH value of the second slurry stabilized at 7.8, the heating was stopped to obtain a mixed slurry for preparing a positive electrode material.

[0073] Thermogravimetric analysis of the mixed slurry in Example 1 showed that the weight of the mixed slurry decreased by 1.7% at 120°C, mainly due to the evaporation of free water in the mixed slurry; the weight decreased by 10% at 160°C, mainly due to the decomposition of ammonium and carbonate ions in the mixed slurry to produce gases such as ammonia and carbon dioxide, which were removed from the mixed slurry under heating conditions.

[0074] Examples 2-12 and Comparative Examples 1-2

[0075] Examples 2-12 and Comparative Examples 1-2 are all methods for preparing a mixed slurry. The specific steps can be referred to Example 1, and the differences are listed in Table 1.

[0076] Example 13

[0077] This embodiment is a method for preparing a mixed slurry, which includes the following steps:

[0078] S1: Under nitrogen atmosphere, ammonium bicarbonate, ferrous sulfate, and manganese sulfate were added to a reactor in a molar ratio of 1:0.4:0.6 and reacted at room temperature for 4 hours to obtain a reaction slurry (0.04 wt% of ascorbic acid as a manganese source was also added to the reactor), and 1 mol of ammonium dihydrogen phosphate was added. The pH value of the solution was adjusted to 10 by controlling the flow rate of ammonia water. The temperature was raised to 50°C until the reaction was completed. The precipitate generated by the reaction was washed with deionized water by centrifugation and finally dried at 90°C for 8 hours to obtain a manganese ferrohydrogen phosphate precursor Fe 0.4 Mn 0.6 HPO4·H2O.

[0079] S2: Fe 0.4 Mn 0.6 HPO4·H2O, lithium carbonate, glucose, and polyethylene glycol are mixed and then ball-milled and sand-milled to obtain a first slurry, wherein the molar ratio of lithium carbonate to ammonium dihydrogen phosphate is 1.02:1, the sum of the masses of glucose and polyethylene glycol is 2.1 wt % of the mass of the finally prepared lithium manganese iron phosphate, the ball milling speed is 2400 rpm, the sand milling speed is 1200 rpm, the temperature is ≤25°C, and the median particle size D50 of the particles in the obtained first slurry is 0.18 μm, and the solid content is 20%.

[0080] S3: The first slurry was dispersed in ethylene glycol twice to obtain a second slurry, and heated at 160°C for 5 hours. The initial pH value of the second slurry was 9. When the pH value of the second slurry stabilized at 7.8, the heating was stopped to obtain a mixed slurry for preparing a positive electrode material.

[0081] Comparative Example 3

[0082] Comparative Example 3 is a method for preparing a mixed slurry. The specific steps can refer to Example 13, except that the heating and exhaust process is missing, as specifically listed in Table 1.

[0083] Table 1

[0084] The actual weight loss in Table 1 refers to the weight loss percentage measured by thermogravimetric analysis under heating treatment at a certain temperature.

[0085] The pH value, viscosity and solid content of the mixed slurries in Examples 1-13 and Comparative Examples 1-3 were measured. The specific test results are shown in Table 2 below.

[0086] The mixed slurries in Examples 1-13 and Comparative Examples 1-3 were spray-dried and sintered to prepare positive electrode materials, which were then prepared into secondary batteries. The BET and discharge specific capacities were tested, and the test results are shown in Table 2 below.

[0087] Table 2

[0088] Referring to the data in Table 1 and Table 2, the higher the solid content of the first slurry in Examples 1-3, the higher the solid content of the prepared mixed slurry and the greater the viscosity.

[0089] The temperature of mechanical mixing during the preparation of the first slurry in Example 4 is higher than that in Example 1. The solid content of the mixed slurry in Example 4 is 26%, which is slightly higher than the 25% solid content of the mixed slurry in Example 1. This indicates that the lower the temperature of mechanical mixing, the better the grinding effect of the precursor, lithium source and carbon source, the more fully the three are mixed, the lower the solid content of the mixed slurry, the smaller the specific surface area of ​​the positive electrode material, and the higher the discharge specific capacity.

[0090] Referring to the data of Examples 1, 5 and 6, it can be seen that when the temperature of the heating treatment is high, the evaporation of the organic solvent will be accelerated, which is not conducive to the removal of gas in the second slurry, resulting in an increase in the pH value, viscosity and solid content of the mixed slurry, a significant increase in the specific surface area of ​​the positive electrode material, and a decrease in the discharge specific capacity; when the temperature of the heating treatment is low, the decomposition rate of the gas-generating substance in the first slurry will slow down, resulting in an increase in the pH value of the mixed slurry, a decrease in viscosity, a decrease in solid content, a significant increase in the specific surface area of ​​the positive electrode material, and a decrease in the discharge specific capacity.

[0091] Referring to the data of Examples 1, 7 and 8, it can be seen that the longer the heating treatment time, the more thorough the gas removal in the first slurry, and the higher the viscosity and solid content of the mixed slurry. However, as time goes by, the volatilization of the organic solvent increases, and the pH value may increase slightly, thereby affecting the electrochemical properties of the positive electrode material.

[0092] Referring to the data of Examples 1, 9 and 10, it can be seen that when the organic solvents are different, the boiling point and dispersibility thereof will affect the gas removal effect, and therefore, the viscosity, solid content and pH of the mixed slurry will be affected.

[0093] Referring to the data of Examples 1, 11 and 12, it can be seen that the higher the initial pH value of the second slurry, the higher the pH value of the mixed slurry. It can be seen that when preparing the first slurry, the selection of raw materials will directly affect the pH value of the mixed slurry.

[0094] With reference to the data of Example 1 and Example 13, it can be seen that the slurry preparation method in this application is applicable to different types of precursors.

[0095] Referring to the data of Example 1 and Comparative Example 2, it can be seen that after the heating exhaust temperature is increased, the actual weight loss is significantly reduced, the gas discharge is not sufficient, and the carbon coating quality is reduced, resulting in capacity loss.

[0096] In Comparative Example 2, the heating temperature of the heating exhaust treatment was too high, resulting in too rapid volatilization of the organic solvent, uneven carbon distribution, and incomplete gas removal. The solid content and viscosity of the mixed slurry were too high, the specific surface area of ​​the positive electrode material increased significantly, and the discharge specific capacity decreased.

[0097] Comparative Examples 1 and 3 lack the heating and exhaust process compared to Example 1 and Example 13, respectively, that is, there is no need to prepare the first slurry, and the gas-generating substances in the raw materials cannot be removed in advance, resulting in a significant increase in the specific surface area of ​​the positive electrode material and a decrease in the discharge specific capacity.

[0098] The above are only specific embodiments of the present application, but the scope of protection of this application is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in this application should be included in the scope of protection of this application. Therefore, the scope of protection of this application should be based on the scope of protection of the claims.

Claims

1. A method for preparing a mixed slurry for preparing a positive electrode material, characterized in that: It includes the following steps: Disperse the first slurry including the manganese iron phosphate precursor and the lithium source in an organic solvent in N times to obtain the second slurry, and heat-treat until the pH value of the second slurry tends to be stable to remove the gas in the first slurry, thereby obtaining the mixed slurry for preparing the cathode material, where N is a positive integer greater than or equal to 2; the boiling point of the organic solvent is greater than or equal to 160 °C, and the temperature of the heat treatment is 100 °C to 200 °C.

2. The preparation method according to claim 1, characterized in that: The preparation method of the manganese iron phosphate precursor includes the following steps: Mix the raw materials including soluble phosphate, iron source, manganese source and ammonia source and react to obtain the precursor slurry containing the manganese iron phosphate precursor, and the precursor slurry is subjected to pressure filtration, washing and drying to obtain the manganese iron phosphate precursor. Wherein, the pH value of the precursor slurry is less than or equal to 12.

3. The preparation method according to claim 1, characterized in that: The stable pH value of the second slurry is 7 to 10.

4. The preparation method according to any one of claims 1 to 3, characterized in that The ferromanganese phosphate precursor includes NH4Fe x Mn 1-x PO4·H2O、Fe x Mn 1-x At least one of HPO4·yH2O; Wherein, 0 < x < 0.5, and y is the water content.

5. The preparation method according to any one of claims 1 to 4, characterized in that: The lithium source includes at least one of lithium hydroxide, lithium oxide, lithium carbonate, lithium dihydrogen phosphate, lithium hydrogen phosphate or lithium phosphate; the molar ratio of the lithium source to the manganese iron phosphate precursor is 1:1 to 1.06:

1.

6. The preparation method according to any one of claims 1 to 5, characterized in that: The organic solvent includes at least one of ethylene glycol, dimethyl sulfoxide, N-methylpyrrolidone, and formamide.

7. The preparation method according to any one of claims 1 to 6, characterized in that: The median particle size of the particles in the first slurry is 0.1 μm to 1.0 μm, and the solid content of the first slurry is 10% to 50%.

8. The preparation method according to any one of claims 1 to 7, characterized in that: The first slurry further includes a coating agent and / or a doping agent.

9. A mixed slurry for preparing a positive electrode material, characterized in that: The mixed slurry includes a phosphate precursor and a lithium source, and the pH value of the mixed slurry is 7 to 10.

10. The mixed slurry according to claim 9, characterized in that: The pH of the mixed slurry is 7 - 8.

11. The mixed slurry according to claim 10, characterized in that: The viscosity of the mixed slurry is 500 mPa·S to 10,000 mPa·S, and the solid content of the mixed slurry is 10% to 40%.

12. The mixed slurry according to claim 10, characterized in that: The weight loss temperature of 1.6% to 2.0% of the mixed slurry measured by thermogravimetric analysis is 110 °C to 130 °C.

13. The mixed slurry according to claim 10, characterized in that: The weight loss temperature of 9% to 11% of the mixed slurry measured by thermogravimetric analysis is 140 °C to 180 °C.

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