A double-coated positive electrode material and a preparation method thereof

Abstract

The application relates to a double-coated positive electrode material and a preparation method thereof, and the preparation steps are as follows: a soluble ferrous salt solution is heated and stirred, a phosphorus source is added, heating and stirring are continuously carried out, a lithium source, an additive and water are added, a first mixed material is obtained after ball milling, sand milling and spraying; the first mixed material is calcined under an inert gas atmosphere and then crushed to obtain a first material; the first material is dispersed in a citric acid solution and continuously stirred, a soluble iron salt solution and a lithium carbonate solution are added, continuous stirring is carried out, and finally a carbon source is added, a second mixed material is obtained after sand milling and spraying; the second mixed material is calcined to obtain a double-coated lithium iron phosphate composite material. Through coating lithium ferrite on the surface of lithium iron phosphate and then carrying out carbon coating treatment, the double-coated lithium iron phosphate positive electrode material is formed, and the cycle performance, processing performance and safety performance of the material can be effectively improved.

Description

Technical Field

[0001] This invention relates to the field of lithium battery materials technology, specifically to a double-coated cathode material and its preparation method. Background Technology

[0002] Lithium iron phosphate (LFP) batteries have become a research hotspot in the new energy field due to their low cost and long cycle life. In recent years, with the development of battery technologies such as blade batteries, CTC (cell-to-cell) batteries, and CTP (cell-to-pump) batteries, LFP batteries have also experienced rapid development.

[0003] During the first week of charge and discharge in a lithium-ion battery, the electrode material reacts with the electrolyte at the interface to form a solid electrolyte interphase (SEI) film, which consists of various organic and inorganic lithium salts. The SEI film plays a crucial role in the material's performance. It is insoluble in organic solvents, allowing it to exist stably in organic electrolyte solutions. Solvent molecules cannot pass through this passivation film, effectively preventing co-intercalation and avoiding damage to the electrode material caused by solvent molecule co-intercalation. This ensures the structural stability of the material, thereby improving the cell's cycle performance and lifespan. On the other hand, the formation of the SEI film consumes lithium ions, leading to a decrease in the first week's charge and discharge efficiency and reducing the cell's capacity.

[0004] To address this issue, adding an additional lithium source can effectively improve the first-cycle efficiency of the battery cell, thereby increasing its energy density. For example, CN109950514A discloses a method for preparing lithium iron phosphate coated with lithium ferrite, which uses lithium ferrite to coat lithium iron phosphate. However, coating lithium iron phosphate with lithium ferrite alone leads to severe water absorption by the material, significantly impacting its processing performance. Summary of the Invention

[0005] This invention provides a double-coated cathode material and its preparation method. The method involves coating lithium iron phosphate (LiFePO4) with lithium ferrite (Li5FeO4) to form a lithium ferrite layer, followed by carbon coating on the lithium ferrite layer to create a double-coated lithium iron phosphate cathode material. During the lithium ferrite coating process, soluble iron in the lithium iron phosphate is leached out through acid leaching, serving as an iron source and precipitating into lithium ferrite. This prevents subsequent negative electrode precipitation, effectively improving the material's cycle performance and safety. Simultaneously, the lithium ferrite coating effectively enhances the material's first-cycle efficiency and cycle performance. Furthermore, the carbon coating technology effectively reduces the material's water absorption, thereby improving the processing performance of the prepared cathode material and resulting in a lower internal resistance and better cycle performance in the composite material.

[0006] The present invention provides the following solution to the above-mentioned technical problems: a double-coated cathode material comprising a lithium iron phosphate substrate, wherein the surface of the substrate is sequentially coated with a lithium iron phosphate layer and a carbon coating layer from the inside out.

[0007] Preferably, based on the overall cathode material, the lithium iron phosphate layer contains 1-5 wt%, more preferably 2-4 wt%. The lithium iron phosphate content affects cycle performance and reversible capacity; a high coating content leads to lower composite material capacity, while a low coating content leads to poor cycle performance. The carbon coating layer contains 0.5-3 wt%, more preferably 1-2 wt%. The carbon content affects capacity and cycle performance; a high carbon content leads to lower composite material capacity, while a low carbon content leads to excessive internal resistance and poor cycle performance. The molar ratio of lithium, iron, and phosphorus in the lithium iron phosphate aggregate is (1-1.1):(0.9-1):1.

[0008] The preparation method of the double-coated cathode material described above includes the following steps:

[0009] S1. Dissolve soluble ferrous salt in water, heat to 60°C and stir to obtain a first solution; add phosphorus source to the first solution, maintain pH at 1-4, heat to 80-95°C and stir to obtain a first precursor; add lithium source, additive and water to the first precursor, and obtain a first mixture after ball milling, sand milling and spraying.

[0010] S2. The first mixture is calcined in an inert gas atmosphere and then pulverized by airflow to obtain the first material; the inert gas atmosphere may be a nitrogen atmosphere;

[0011] S3. Disperse the first material in citric acid solution and stir continuously. Then add soluble ferrous salt and lithium carbonate solution and stir continuously. Finally add carbon source. After sand milling and spraying, the second mixture is obtained.

[0012] S4. The second mixture is calcined in an inert gas atmosphere to obtain a double-coated cathode material; the inert gas atmosphere may be a nitrogen atmosphere.

[0013] Preferably, in S1, the soluble ferrous salt is any one or a combination of ferrous sulfate, ferrous nitrate, and ferrous chloride; the phosphorus source is any one or a combination of phosphoric acid, monohydrogen phosphate, and dihydrogen phosphate; and the lithium source is any one or a combination of lithium carbonate, lithium hydroxide, lithium acetate, and lithium nitrate.

[0014] Preferably, in S1, the particle size of the first mixture is 100-500 nm, the amount of additive added is 1000-3000 ppm of the mass percentage of the mixture, and the additive is a soluble sulfate or nitrate of rare earth elements such as titanium, magnesium, aluminum, zirconium, etc.

[0015] Preferably, in step S2, the calcination temperature is 650-750℃ and the calcination time is 4-8h.

[0016] Preferably, in step S2, the particle size of the first material is 500-2500 nm.

[0017] Preferably, in step S3, the concentration of citric acid solution is 0.1-0.2 mol / L; the concentration of lithium carbonate solution is 1-2 mol / L; the mass ratio of soluble ferrous salt to the first material is 1-5%:1; and the molar ratio of soluble ferric salt, citric acid and lithium carbonate is 1:1-1.2:5-5.5.

[0018] Preferably, in S3, the carbon source is organic carbon and inorganic carbon, and any one or any combination of glucose, white sugar, sucrose, polyethylene glycol, graphite, and graphene; the soluble ferrous salt is any one or any combination of ferrous sulfate, ferrous nitrate, and ferrous chloride.

[0019] Preferably, in step S4, the calcination temperature is 650-800℃ and the calcination time is 2-6h.

[0020] The beneficial effects of this invention are as follows:

[0021] 1. This invention uses lithium iron phosphate as a lithium replenishing agent to coat lithium iron phosphate, which significantly improves the first-cycle efficiency and cycle performance of lithium iron phosphate composite materials; by adding acid treatment during the coating process, the iron dissolution of lithium iron phosphate composite materials is significantly reduced, thereby improving the safety performance of the battery cell.

[0022] 2. This invention further coats the lithium iron phosphate composite material with a carbon source, forming a double-coated structure. This reduces the material's water absorption capacity, improves its processing performance, and simultaneously lowers its internal resistance and enhances its cycling performance. Furthermore, multiple coatings effectively suppress volume changes during cycling, improving the material's structural stability. Detailed Implementation

[0023] The principles and features of the present invention are described below with reference to embodiments. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0024] Example 1

[0025] S1. A certain amount of ferrous sulfate is dissolved in water, heated to 60°C and stirred to obtain a first solution; monoammonium phosphate and phosphoric acid are added to the first solution, and the pH value is maintained between 2 and 2. After the materials are completely added, the temperature is raised to 90°C and stirred for 6 hours. The mixture is filtered and washed to obtain a first precursor; a certain amount of lithium carbonate is added to the first precursor, and the mixture is ball-milled, sand-milled, and spray-dried to obtain a first mixture; wherein the molar ratio between lithium in lithium carbonate, iron in ferrous sulfate, monoammonium phosphate, and phosphorus in phosphoric acid is 1.05:1:1; the particle size of the first mixture is 200 nm.

[0026] S2. The first mixture is loaded into a bowl and calcined in a roller kiln under a nitrogen atmosphere. The first calcination temperature is set to 700℃ and the calcination time is 6h. Then, the mixture is pulverized by airflow to obtain the first material with a particle size of 1200nm.

[0027] S3. The first material is dispersed in a 0.1 mol / L citric acid solution and stirred for 3 hours. Then, a ferrous nitrate solution is added, followed by a slow addition of a 1 mol / L lithium carbonate solution. The mixture is stirred for 3 hours, and finally, glucose is added. The mixture is then milled and sprayed to obtain the second mixture. The mass ratio of ferrous nitrate to the first material is 3%:1, and the molar ratio of ferrous nitrate, citric acid, and lithium carbonate is 1:1.1:5.3.

[0028] S4. The second mixture is placed in a bowl and calcined in a roller kiln under a nitrogen atmosphere. The second calcination temperature is set to 750°C and the calcination time is 4 hours to obtain a double-coated cathode material. The carbon content of the double-coated cathode material is 1 wt%, and the lithium ferrite content is 3 wt%.

[0029] Example 2

[0030] The other steps are the same as in Example 1, except that in S3, the first material is dispersed in a 0.1 mol / L citric acid solution and stirred for 1 hour.

[0031] Example 3

[0032] The other steps are the same as in Example 1, except that in S3, the first material is dispersed in a 0.1 mol / L citric acid solution and stirred for 5 hours.

[0033] Example 4

[0034] The other steps are the same as in Example 1, except that in S4, the content of lithium ferrite in the double-coated cathode material is 1 wt%.

[0035] Example 5

[0036] The other steps are the same as in Example 1, except that in S4, the content of lithium ferrite in the double-coated cathode material is 5 wt%.

[0037] Example 6

[0038] The other steps are the same as in Example 1, except that in S4, the carbon content of the double-coated cathode material is 0.5 wt%.

[0039] Example 7

[0040] The other steps are the same as in Example 1, except that in S4, the carbon content of the double-coated cathode material is 3 wt%.

[0041] Example 8

[0042] The other steps are the same as in Example 1, except that in S2, the calcination temperature of the first mixture is 650°C.

[0043] Example 9

[0044] The other steps are the same as in Example 1, except that in S2, the calcination temperature of the first mixture is 750°C.

[0045] Example 10

[0046] The other steps are the same as in Example 1, except that in S4, the calcination temperature of the second mixture is 650°C.

[0047] Example 11

[0048] The other steps are the same as in Example 1, except that in S4, the calcination temperature of the second mixture is 850°C.

[0049] Comparative Example 1

[0050] The other steps are the same as in Example 1, except that in S3, ferrous nitrate solution and lithium carbonate solution are not added, and in S4, the lithium ferrite content of the positive electrode material is 0.

[0051] Comparative Example 2,

[0052] The other steps are the same as in Example 1, except that glucose is not added in S3 and the carbon content of the positive electrode material is 0 in S4.

[0053] The parameter variables of each embodiment and comparative example are summarized in Table 1. The soluble iron content and electrochemical performance of the double-coated cathode materials prepared in each embodiment and comparative example are tested, and the results are summarized in Table 2.

[0054] As shown by the comparison of Examples 1-3, the citric acid soaking time affects the iron leaching content in the final product, which in turn affects the cycle performance. Too short a soaking time will result in too high a soluble iron content. When the soaking time is 5 hours, the soluble iron content is lower than that of 3 hours, but the reduction is not significant. Considering all factors, a soaking time of 3 hours is optimal.

[0055] As shown by the comparison of Examples 1, 4, 5 and Comparative Example 1, the amount of lithium ferrite coating affects the cycle performance and reversible capacity. A high coating amount will result in a lower capacity of the composite material, while a low coating amount will result in a poor cycle performance of the composite material. The effect is particularly obvious in Comparative Example 1. Without lithium ferrite coating, the first-cycle efficiency and 200-cycle efficiency of the material are poor.

[0056] As shown by the comparison of Examples 1, 6, 7 and Comparative Example 2, carbon content affects capacity and cycle performance. High carbon content leads to low capacity of composite materials, while low carbon content leads to excessive internal resistance and poor cycle performance. The effect is particularly obvious in Comparative Example 1. Without carbon coating, the initial release specific capacity and 200-cycle efficiency of the material are poor.

[0057] As shown by the comparison of Examples 1 and 8-11, both excessively high and low calcination temperatures will affect the crystallinity and particle size of the material, ultimately resulting in poor capacity and cycle performance.

[0058] Considering both preparation cost and performance, Example 1 is the optimal example.

[0059] Table 1. Parameter variables among different embodiments

[0060]

[0061] Table 2 Characterization test results for each embodiment

[0062]

[0063]

[0064] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Those skilled in the art can readily implement the present invention as described above. However, any modifications, alterations, or variations made by those skilled in the art without departing from the scope of the present invention, based on the disclosed technical content, are equivalent embodiments of the present invention. Furthermore, any modifications, alterations, or variations made to the above embodiments based on the essential technology of the present invention are still within the protection scope of the present invention.

Claims

1. A double-coated cathode material, comprising a lithium iron phosphate matrix, characterized in that, The substrate surface is coated with a lithium ferrite layer and a carbon coating layer from the inside out. Based on the overall cathode material, the content of the lithium ferrite layer is 3 wt% and the content of the carbon coating layer is 1 wt%. The double-coated cathode material is prepared through the following steps: S1. Dissolve soluble ferrous salt in water, heat and stir to obtain a first solution; add phosphorus source to the first solution, maintain pH at 1-4, heat and stir to obtain a first precursor; add lithium source, additive and water to the first precursor, and obtain a first mixture after ball milling, sand milling and spraying. S2. The first mixture is calcined in an inert gas atmosphere and then pulverized by airflow to obtain the first material, which is the lithium iron phosphate matrix. S3. Disperse the first material in citric acid solution and stir continuously for 3 hours. Then add soluble ferrous salt and lithium carbonate solution and stir continuously. Finally add carbon source and obtain the second mixture after sand milling and spraying. S4. The second mixture is calcined in an inert gas atmosphere to obtain a double-coated cathode material.

2. The method for preparing a double-coated cathode material according to claim 1, characterized in that, Includes the following steps: S1. Dissolve the soluble ferrous salt in water, heat and stir to obtain a first solution; add a phosphorus source to the first solution, maintain the pH at 1-4, heat and stir to obtain a first precursor; add a lithium source, additives and water to the first precursor, and obtain a first mixture after ball milling, sand milling and spraying; wherein the molar ratio between lithium in the lithium source, iron in the soluble ferrous salt and phosphorus in the phosphorus source is 1.05:1:1; S2. The first mixture is calcined in an inert gas atmosphere and then pulverized by airflow to obtain the first material; S3. The first material is dispersed in a 0.1 mol / L citric acid solution and stirred continuously for 3 hours. Then, a soluble ferrous salt and a 1 mol / L lithium carbonate solution are added and stirred continuously. Finally, a carbon source is added. After sand milling and spraying, a second mixture is obtained. The mass ratio of soluble ferrous salt to the first material is 3%:

1. The molar ratio of soluble ferrous salt, citric acid and lithium carbonate is 1:1.1:5.

3. S4. The second mixture is calcined in an inert gas atmosphere to obtain a double-coated cathode material.

3. The method for preparing a double-coated cathode material according to claim 2, characterized in that, In S1, the soluble ferrous salt is any one or any combination of ferrous sulfate, ferrous nitrate, and ferrous chloride; the phosphorus source is any one or any combination of phosphoric acid, monohydrogen phosphate, and dihydrogen phosphate; and the lithium source is any one or any combination of lithium carbonate, lithium hydroxide, lithium acetate, and lithium nitrate.

4. The method for preparing a double-coated cathode material according to claim 3, characterized in that, In S1, the particle size of the first mixture is 100-500 nm, and the amount of additive added is 1000-3000 ppm by mass percentage of the mixture.

5. The method for preparing a double-coated cathode material according to claim 3, characterized in that, In S2, the calcination temperature is 650-750℃ and the calcination time is 4-8h.

6. The method for preparing a double-coated cathode material according to claim 3, characterized in that, In S2, the particle size of the first material is 500-2500 nm.

7. The method for preparing a double-coated cathode material according to claim 3, characterized in that, In S3, the carbon source is organic carbon and inorganic carbon, and any one or any combination of glucose, white sugar, sucrose, polyethylene glycol, graphite, and graphene; the soluble ferrous salt is any one or any combination of ferrous sulfate, ferrous nitrate, and ferrous chloride.

8. The method for preparing a double-coated cathode material according to claim 3, characterized in that, In S4, the calcination temperature is 650-800℃ and the calcination time is 2-6h.

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