Method for preparing lithium manganese iron phosphate material through differential carbon coating

Abstract

The invention discloses a method for preparing a high-performance lithium manganese iron phosphate material through differential carbon coating, which comprises the following steps: controlling the target particle size D50 range of a lithium manganese iron phosphate precursor to be 350-600nm, and then carrying out carbon coating, spray drying and air jet pulverization to obtain first dry powder; controlling the target particle size D50 range of the lithium manganese iron phosphate precursor to be 50-150nm, and then carrying out carbon coating, spray drying and air jet pulverization to obtain second dry powder; the first dry powder and the second dry powder are highly mixed, sintered and cooled, and a sintered material is obtained; and carrying out secondary jet milling on the sintered material to obtain the high-performance lithium iron manganese phosphate powder. According to the invention, large and small particles are subjected to differential carbon coating, so that the coating of a carbon layer on the surface of the small particles is more complete, and the probability that a lithium manganese iron phosphate body is in direct contact with an electrolyte is reduced, so that the electrolyte consumption caused by dissolution of trivalent manganese and side reaction is reduced, and the storage and cycle performance of the lithium manganese iron phosphate can be effectively optimized.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion battery positive electrode material preparation, and in particular to a method for preparing high-performance lithium manganese iron phosphate material by differentiated carbon coating. Background Art

[0002] Lithium-ion battery cathode material preparation technology is developing toward high performance, low cost, environmental friendliness, and high safety. Currently, large-scale commercialized lithium-ion battery cathode materials primarily include ternary and lithium iron phosphate cathodes. Ternary materials offer advantages in energy density and low-temperature environments, but their raw materials are expensive and their safety is relatively poor. Consequently, their share in automotive power batteries is gradually decreasing. By comparison, lithium iron phosphate, while inexpensive and offering excellent cycle stability and safety, suffers from low energy density and poor low-temperature performance, limiting its application scenarios.

[0003] The lithium iron manganese phosphate material involved in the present invention is a positive electrode material that can make up for the shortcomings of ternary lithium and lithium iron phosphate. The manganese platform voltage of lithium iron manganese phosphate is high, which can achieve a higher energy density, and the manganese platform capacity retention rate is better than that of lithium iron phosphate in a low temperature environment; at the same time, the raw materials of lithium iron manganese phosphate are widely available, and its olivine structure has good stability, which can meet the requirements of environmental friendliness and safety. At present, lithium iron manganese phosphate positive electrode materials have begun to be widely used in digital consumer batteries and two-wheeled vehicle batteries. However, due to the low compaction density and poor cycle stability of lithium iron manganese phosphate, its use in the fields of energy storage and large power batteries is still limited.

[0004] To address the low compaction density of lithium manganese iron phosphate, existing technologies use a method of grading large and small particles to increase compaction density. Large particles provide higher compaction density, while small particles enter the gaps between large particles, providing higher discharge capacity without affecting compaction density. For example, companies such as Defang Nano and Nantong Ruixiang use lithium manganese iron phosphate with a high manganese to iron ratio for small particles and lithium manganese iron phosphate with a low manganese to iron ratio for large particles, achieving high compaction performance.

[0005] However, existing technologies have the following problems: the presence of small particles increases electrolyte side reactions and the dissolution of trivalent manganese from the lithium manganese iron phosphate itself, seriously affecting the cycling stability of the discharge capacity. Specifically, the presence of small particles leads to a high specific surface area, resulting in a high side reaction interface, which in turn causes the dissolution of trivalent manganese, thereby worsening the cycling performance of the lithium manganese iron phosphate. Existing technologies do not use differentiated carbon coatings for large and small particles, making it difficult to ensure the long-term cycling performance of small particles, and failing to simultaneously optimize long-term cycling performance.

[0006] The compaction density of lithium manganese iron phosphate materials on the market is generally not high, or the cycle performance is poor. Therefore, the development and production of high-density, long-cycle lithium manganese iron phosphate positive electrodes is still a major challenge for the industry. Summary of the Invention

[0007] Based on the problems existing in the background technology, the present invention provides a method for preparing high-performance lithium iron manganese phosphate material by differentiated carbon coating, which adopts the large and small particle grading process to improve the powder compaction density of the lithium iron manganese phosphate material, and at the same time, differentiated carbon coating is performed on large particles and small particles, retaining large particles (350-600nm) that provide high compaction performance, and optimizing the carbon coating of small particles (50-150nm), so that the surface side reactions of small particles with high reactivity are reduced during the circulation process, thereby reducing the consumption of electrolyte and the dissolution of trivalent manganese; at the same time, the high compaction and long cycle performance of the lithium iron manganese phosphate material are achieved.

[0008] The present invention is implemented through the following technical solutions:

[0009] A method for preparing high-performance lithium manganese iron phosphate material by differentiated carbon coating comprises the following steps:

[0010] S1. Preparation of a first dry powder: An iron source, a manganese source, a phosphorus source, and a lithium source are placed in deionized water and stirred to form a mixed slurry. The mixed slurry is ground using a sand mill to a target particle size D50 of 350-600 nm. A first component carbon source is added, dissolved, and mixed, and spray-dried to obtain a first spray slurry. The first spray slurry is subjected to a first airflow milling to control the target particle size to obtain a first dry powder;

[0011] S2. Preparation of a second dry powder: An iron source, a manganese source, a phosphorus source, and a lithium source are placed in deionized water and stirred to form a mixed slurry. The mixed slurry is ground using a sand mill to a target particle size D50 of 50-150 nm. A second component, a carbon source, is added, dissolved, and mixed, and spray-dried to obtain a second spray slurry. The second spray slurry is subjected to a first airflow milling to control the target particle size to obtain a second dry powder;

[0012] S3. The first dry powder and the second dry powder are mixed in a high mixer and sintered in an inert atmosphere sintering furnace, and the sintered material is obtained after cooling;

[0013] S4. The sintered material is subjected to a second air flow crushing to control the target particle size to obtain high-density long-circulation lithium manganese iron phosphate powder.

[0014] Furthermore, the manganese source in steps S1 and S2 is one or more of manganese tetraoxide, manganese trioxide, manganese dioxide, manganese carbonate, manganese phosphate, ferromanganese phosphate, ferromanganese oxalate, ferromanganese oxide and manganese acetate;

[0015] The iron source in steps S1 and S2 is one or more of ferric oxide, ferromanganese oxide, ferrous oxalate, ferric phosphate and ferromanganese phosphate;

[0016] The phosphorus source in steps S1 and S2 is one or more of phosphoric acid, ammonium dihydrogen phosphate, ferric phosphate, lithium dihydrogen phosphate and ferromanganese phosphate;

[0017] The lithium source in steps S1 and S2 is one or more of lithium carbonate, lithium hydroxide monohydrate, and lithium dihydrogen phosphate;

[0018] The carbon source in steps S1 and S2 is one or more of glucose, sucrose, soluble starch, polyethylene glycol, polyvinyl pyrrolidone, polyvinyl alcohol, and phenolic resin.

[0019] Furthermore, the molar ratio of manganese element to iron element in the mixed slurry in step S1 and step S2 is x:(1-x), 0.2<x<0.8.

[0020] Furthermore, the amount of the first component carbon source used in step S1 corresponds to a target carbon content of 0.8-1.2 wt % in the first dry powder sintering material.

[0021] Furthermore, the amount of the second component carbon source used in step S2 corresponds to a target carbon content of 1.8-2.2 wt % in the second dry powder sintering material.

[0022] Furthermore, in step S1 and step S2, the target particle size D50 of the first spray slurry and the second spray slurry subjected to the first air flow pulverization is in the range of 0.5-0.8 μm.

[0023] Furthermore, in step S3, the mass ratio of the first dry powder to the second dry powder is w:(1-w), 0.2<w<0.8.

[0024] Furthermore, in step S3, the inert atmosphere is nitrogen or argon.

[0025] Furthermore, in step S4, the target particle size D50 of the sintered material subjected to the second air flow milling is in the range of 0.8-1.2 μm.

[0026] Beneficial effects of the present invention:

[0027] 1. The differentiated carbon coating of large and small particles used in the present invention provides a solution to the problems of many side reactions and poor circulation of small particles. The high carbon content process of small particles, combined with more high-molecular-weight carbon source components, can make the carbon layer coating on the surface of small particles more complete, reducing the chance of direct contact between the lithium manganese iron phosphate body and the electrolyte, thereby reducing the dissolution of trivalent manganese and the electrolyte consumption caused by side reactions, and effectively optimizing its storage and circulation performance.

[0028] 2. The process of the present invention does not add any additional equipment or steps with significant high energy consumption to the existing widely used process, and is therefore easy to promote and use on a large scale. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The accompanying drawings are used to further explain the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

[0030] Figure 1 This is a process flow chart for preparing lithium manganese iron phosphate by differentiated carbon coating according to the present invention;

[0031] Figure 2 This is an SEM image of the differentiated carbon-coated lithium manganese iron phosphate positive electrode material prepared in Example 2 of the present invention. DETAILED DESCRIPTION

[0032] The technical solution of the present invention is further described in detail below with reference to specific embodiments, but the protection scope of the present invention is not limited to the following embodiments.

[0033] Example 1

[0034] A method for preparing high-performance lithium manganese iron phosphate material by differentiated carbon coating comprises the following steps:

[0035] S1. Preparation of the first dry powder: 2000g of ferromanganese phosphate and 502g of lithium carbonate were placed in 6000g of deionized water, stirred thoroughly, and then ground using a sand mill to a particle size of D50 = 505nm. Subsequently, 160g of glucose was added and stirred thoroughly to dissolve to obtain a slurry to be sprayed; the slurry was spray-dried to a moisture content of less than 1.5%; the spray material was then pulverized using a jet mill to a D50 = 0.63μm and used as the first dry powder for standby use;

[0036] S2. Preparation of a second dry powder: 2000 g of ferromanganese phosphate and 502 g of lithium carbonate were placed in 6000 g of deionized water, stirred thoroughly, and ground using a sand mill to a particle size of D50 = 123 nm. 160 g of glucose and 160 g of polyethylene glycol were then added and stirred to dissolve to obtain a slurry to be sprayed; the slurry was spray-dried to a moisture content of less than 1.5%; the spray material was then pulverized with a jet mill to a D50 = 0.61 μm and prepared as a second dry powder.

[0037] S3. 300 g of the first dry powder and 700 g of the second dry powder were added to a high-speed mixer and mixed at high speed for 5 minutes. Then, 800 g of the mixed material was added to a bowl and added to a box furnace. Nitrogen was introduced at a rate of 10 L / min throughout the sintering process. The sintering temperature was increased from room temperature at a heating rate of 5 ° C / min to 400 ° C and held for 5 hours. Then, the temperature was increased at a heating rate of 5 ° C / min to 750 ° C and held for 8 hours, and then naturally cooled to below 80 ° C.

[0038] S4. The sintered material after cooling is subjected to a second air flow pulverization, and the pulverized particle size D50 is 1.1 μm to obtain lithium manganese iron phosphate positive electrode powder.

[0039] The positive electrode material tested had a compaction density of 2.40 g / cc at 225 mPa and a carbon content of 1.61%. The tested 0.1C gram capacity was 153 mAh / g, the 1C gram capacity was 138 mAh / g, and the capacity retention rate was 73% after 500 charge and discharge cycles at 1C.

[0040] Example 2

[0041] A method for preparing high-performance lithium manganese iron phosphate material by differentiated carbon coating comprises the following steps:

[0042] S1. Preparation of the first dry powder: 2000g of ferromanganese phosphate and 502g of lithium carbonate were placed in 6000g of deionized water, stirred thoroughly, and then ground using a sand mill to a particle size of D50 = 495nm. Subsequently, 160g of glucose was added and stirred thoroughly to dissolve to obtain a slurry to be sprayed; the slurry was spray-dried to a moisture content of less than 1.5%; the spray material was then pulverized using a jet mill to a D50 = 0.65μm and used as the first dry powder for standby use;

[0043] S2. Preparation of a second dry powder: 2000 g of ferromanganese phosphate and 502 g of lithium carbonate were placed in 6000 g of deionized water, stirred thoroughly, and ground using a sand mill to a particle size of D50 = 118 nm. 180 g of glucose and 180 g of polyethylene glycol were then added and stirred to dissolve to obtain a slurry to be sprayed; the slurry was spray-dried to a moisture content of less than 1.5%; the spray material was then pulverized with a jet mill to a D50 = 0.62 μm and prepared as a second dry powder.

[0044] S3. 300 g of the first dry powder and 700 g of the second dry powder were added to a high-speed mixer and mixed at high speed for 5 minutes. Then, 800 g of the mixed material was added to a bowl and added to a box furnace. Nitrogen was introduced at a rate of 10 L / min throughout the sintering process. The sintering temperature was increased from room temperature at a heating rate of 5 ° C / min to 400 ° C and held for 5 hours. Then, the temperature was increased at a heating rate of 5 ° C / min to 750 ° C and held for 8 hours, and then naturally cooled to below 80 ° C.

[0045] S4. The sintered material after cooling was subjected to a second air flow pulverization, and the pulverization particle size D50 was 1.0±0.2μm to obtain lithium manganese iron phosphate positive electrode powder, whose SEM morphology is as follows Figure 2 shown.

[0046] The positive electrode material tested had a compaction density of 2.37 g / cc at 225 mPa and a carbon content of 1.81%. The tested 0.1C gram capacity was 154 mAh / g, the 1C gram capacity was 141 mAh / g, and the capacity retention rate was 85% after 500 charge and discharge cycles at 1C.

[0047] Example 3

[0048] A method for preparing high-performance lithium manganese iron phosphate material by differentiated carbon coating comprises the following steps:

[0049] S1. Preparation of the first dry powder: 2000g of ferromanganese phosphate and 502g of lithium carbonate were placed in 6000g of deionized water, stirred thoroughly, and then ground using a sand mill to a particle size of D50 = 498nm. Subsequently, 160g of glucose was added and stirred thoroughly to dissolve to obtain a slurry to be sprayed; the slurry was spray-dried to a moisture content of less than 1.5%; the spray material was then pulverized using a jet mill to a D50 = 0.61μm and used as the first dry powder for standby use;

[0050] S2. Preparation of a second dry powder: 2000g of ferromanganese phosphate and 502g of lithium carbonate were placed in 6000g of deionized water, stirred thoroughly, and ground using a sand mill to a particle size of D50 = 119nm. Subsequently, 120g of glucose and 120g of polyvinylpyrrolidone were added and stirred thoroughly to dissolve to obtain a slurry to be sprayed; the slurry was spray-dried to a moisture content of less than 1.5%; the spray material was then pulverized with a jet mill to D50 = 0.59μm and used as a second dry powder for standby use;

[0051] S3. 300 g of the first dry powder and 700 g of the second dry powder were added to a high-speed mixer and mixed at high speed for 5 minutes. Then, 800 g of the mixed material was added to a bowl and added to a box furnace. Nitrogen was introduced at a rate of 10 L / min throughout the sintering process. The sintering temperature was increased from room temperature at a heating rate of 5 ° C / min to 400 ° C and held for 5 hours. Then, the temperature was increased at a heating rate of 5 ° C / min to 750 ° C and held for 8 hours, and then naturally cooled to below 80 ° C.

[0052] S4. The cooled sintered material is subjected to a second air flow pulverization, and the pulverized particle size D50 is 1.0±0.2 μm to obtain lithium manganese iron phosphate positive electrode powder.

[0053] The positive electrode material tested had a compaction density of 2.32 g / cc at 225 mPa and a carbon content of 1.79%. The tested 0.1C gram capacity was 153 mAh / g, the 1C gram capacity was 140 mAh / g, and the capacity retention rate after 500 charge and discharge cycles at 1C was 82%.

[0054] Example 4

[0055] A method for preparing high-performance lithium manganese iron phosphate material by differentiated carbon coating comprises the following steps:

[0056] S1. Preparation of the first dry powder: 2000g of ferromanganese phosphate and 502g of lithium carbonate were placed in 6000g of deionized water, stirred thoroughly, and then ground using a sand mill to a particle size of D50 = 499nm. Subsequently, 160g of glucose was added and stirred thoroughly to dissolve to obtain a slurry to be sprayed; the slurry was spray-dried to a moisture content of less than 1.5%; the spray material was then pulverized using a jet mill to a D50 = 0.57μm and used as the first dry powder for standby use;

[0057] S2. Preparation of a second dry powder: 2000 g of ferromanganese phosphate and 502 g of lithium carbonate were placed in 6000 g of deionized water, stirred thoroughly, and ground using a sand mill to a particle size of D50 = 116 nm. 250 g of glucose was then added and stirred to dissolve to obtain a slurry to be sprayed; the slurry was spray-dried to a moisture content of less than 1.5%; the spray material was then pulverized with a jet mill to a D50 = 0.59 μm and prepared as a second dry powder;

[0058] S3. 300 g of the first dry powder and 700 g of the second dry powder were added to a high-speed mixer and mixed at high speed for 5 minutes. Then, 800 g of the mixed material was added to a bowl and added to a box furnace. Nitrogen was introduced at a rate of 10 L / min throughout the sintering process. The sintering temperature was increased from room temperature at a heating rate of 5 ° C / min to 400 ° C and held for 5 hours. Then, the temperature was increased at a heating rate of 5 ° C / min to 750 ° C and held for 8 hours, and then naturally cooled to below 80 ° C.

[0059] S4. The cooled sintered material is subjected to a second air flow pulverization, and the pulverized particle size D50 is 1.0±0.2 μm to obtain lithium manganese iron phosphate positive electrode powder.

[0060] The positive electrode material tested had a compaction density of 2.36 g / cc at 225 mPa and a carbon content of 1.82%. The tested 0.1C gram capacity was 152 mAh / g, the 1C gram capacity was 139 mAh / g, and the capacity retention rate after 500 charge and discharge cycles at 1C was 72%.

[0061] Comparative Example 1

[0062] A method for preparing lithium manganese iron phosphate material by carbon coating, comprising the following steps:

[0063] S1. Preparation of the first dry powder: 2000g of ferromanganese phosphate and 502g of lithium carbonate were placed in 6000g of deionized water, stirred thoroughly, and then ground using a sand mill to a particle size of D50 = 500±10nm. Subsequently, 220g of glucose was added and stirred thoroughly to dissolve to obtain a slurry to be sprayed; the slurry was spray-dried to a moisture content of less than 1.5%; the spray material was then pulverized using a jet mill to a D50 = 0.6±0.1μm and used as the first dry powder for standby use;

[0064] S2. Preparation of the second dry powder: 2000g of ferromanganese phosphate and 502g of lithium carbonate were placed in 6000g of deionized water, stirred thoroughly, and ground using a sand mill to a particle size of D50 = 120 ± 10nm, followed by adding 220g of glucose and stirring thoroughly to dissolve to obtain a slurry to be sprayed; the slurry was spray-dried to a moisture content of less than 1.5%; the spray material was then pulverized with a jet mill to D50 = 0.6 ± 0.1μm and used as a second dry powder for standby use;

[0065] S3. 300 g of the first dry powder and 700 g of the second dry powder were added to a high-speed mixer and mixed at high speed for 5 minutes. Then, 800 g of the mixed material was added to a bowl and added to a box furnace. Nitrogen was introduced at a rate of 10 L / min throughout the sintering process. The sintering temperature was increased from room temperature at a heating rate of 5 ° C / min to 400 ° C and held for 5 hours. Then, the temperature was increased at a heating rate of 5 ° C / min to 750 ° C and held for 8 hours, and then naturally cooled to below 80 ° C.

[0066] S4. The cooled sintered material is subjected to a second air flow pulverization, and the pulverized particle size D50 is 1.0±0.2 μm to obtain lithium manganese iron phosphate positive electrode powder.

[0067] The positive electrode material tested had a compaction density of 2.35 g / cc at 225 mPa and a carbon content of 1.83%. The tested 0.1C gram capacity was 153 mAh / g, the 1C gram capacity was 138 mAh / g, and the capacity retention rate after 500 charge and discharge cycles at 1C was 63%.

[0068] The data for the above examples and comparative examples are summarized in Table 1. Based on the data in this table, it can be concluded that coating small particles with a high carbon content, polymeric carbon source, can significantly improve the 1C cycle capacity retention while maintaining the original carbon content and compaction density. Therefore, the differentiated carbon coating of the present invention has significant benefits for the performance of lithium manganese iron phosphate prepared with large and small particle gradations.

[0069] Table 1 Summary of experimental data of embodiments and comparative examples

[0070]

[0071] Finally, it should be noted that the above-described embodiments merely represent several implementation methods of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made by a person skilled in the art without departing from the spirit of the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention should be based on the appended claims.

Claims

1. A method for preparing high-performance lithium manganese iron phosphate material by differentiated carbon coating, characterized in that: The following steps are involved: S1. Preparation of a first dry powder: An iron source, a manganese source, a phosphorus source, and a lithium source are placed in deionized water and stirred to form a mixed slurry. The mixed slurry is ground using a sand mill to a target particle size D50 of 350-600 nm. A first component carbon source is added, dissolved, and mixed, and spray-dried to obtain a first spray slurry. The first spray slurry is subjected to a first airflow milling to control the target particle size to obtain a first dry powder; S2. Preparation of a second dry powder: An iron source, a manganese source, a phosphorus source, and a lithium source are placed in deionized water and stirred to form a mixed slurry. The mixed slurry is ground using a sand mill to a target particle size D50 of 50-150 nm. A second component, a carbon source, is added, dissolved, and mixed, and spray-dried to obtain a second spray slurry. The second spray slurry is subjected to a first airflow milling to control the target particle size to obtain a second dry powder; S3. The first dry powder and the second dry powder are mixed in a high mixer and sintered in an inert atmosphere sintering furnace, and the sintered material is obtained after cooling; S4. The sintered material is subjected to a second air flow crushing to control the target particle size to obtain high-density long-circulation lithium manganese iron phosphate powder.

2. The method according to claim 1, characterized in that The manganese source in steps S1 and S2 is one or more of manganese tetraoxide, manganese trioxide, manganese dioxide, manganese carbonate, manganese phosphate, ferromanganese phosphate, ferromanganese oxalate, ferromanganese oxide and manganese acetate; The iron source in steps S1 and S2 is one or more of ferric oxide, ferromanganese oxide, ferrous oxalate, ferric phosphate and ferromanganese phosphate; The phosphorus source in steps S1 and S2 is one or more of phosphoric acid, ammonium dihydrogen phosphate, ferric phosphate, lithium dihydrogen phosphate and ferromanganese phosphate; The lithium source in steps S1 and S2 is one or more of lithium carbonate, lithium hydroxide monohydrate, and lithium dihydrogen phosphate; The carbon source in steps S1 and S2 is one or more of glucose, sucrose, soluble starch, polyethylene glycol, polyvinyl pyrrolidone, polyvinyl alcohol, and phenolic resin.

3. The method according to claim 1, characterized in that The molar ratio of manganese element to iron element in the mixed slurry in step S1 and step S2 is x:(1-x), 0.2<x<0.

8.

4. The method according to claim 1, wherein The amount of the first component carbon source used in step S1 corresponds to a target carbon content of 0.8-1.2 wt % in the first dry powder sintering material.

5. The method according to claim 1, wherein The amount of the second component carbon source used in step S2 corresponds to a target carbon content of 1.8-2.2 wt % in the second dry powder sintering material.

6. The method according to claim 1, wherein The target particle size D50 of the first spray slurry and the second spray slurry subjected to the first air flow pulverization in step S1 and step S2 is in the range of 0.5-0.8 μm.

7. The method according to claim 1, characterized in that In step S3 , the mass ratio of the first dry powder to the second dry powder is w:(1−w), where 0.2<w<0.

8.

8. The method according to claim 1, characterized in that In step S3, the inert atmosphere is nitrogen or argon.

9. The method according to claim 1, characterized in that The target particle size D50 of the sintered material subjected to the second air flow milling in step S4 is in the range of 0.8-1.2 μm.

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