Preparation method of interface stable medium AlF3 coated lithium iron phosphate material

Abstract

The invention provides a preparation method of an interface stable medium AlF3 coated lithium iron phosphate material. The preparation method comprises the following steps: firstly, respectively preparing a lithium iron phosphate matrix and an AlF3 solution, then uniformly mixing the lithium iron phosphate matrix and the AlF3 solution, atomizing the mixture through a pneumatic system, spraying the atomized mixture onto a high-temperature glass substrate to realize uniform thin layer coating of AlF3 on the surface of the lithium iron phosphate material, and finally preparing the AlF3-coated lithium iron phosphate material through high-temperature sintering. The material has sphere-like morphology, wide particle size distribution, good processability and high energy density, and the lithium ion battery prepared from the material has excellent cycle performance and stability.

Description

Technical Field

[0001] The present invention relates to the field of preparation of lithium ion battery positive electrode materials, and in particular to a method for preparing an interface stabilizing medium AlF3 coated lithium iron phosphate material. Background Art

[0002] With the rapid development of the new energy electric vehicle industry and the increasingly hot energy storage market, lithium iron phosphate (LFP) has become one of the most promising cathode materials for next-generation lithium-ion batteries due to its low cost, long life, and excellent stability. However, LFP has a hexagonal close-packed structure, with FeO6 octahedra cross-linked by PO4 tetrahedra, resulting in the absence of a continuous network of edge-sharing FeO6 octahedra, which makes it very poorly conductive. Furthermore, the PO4 tetrahedra located between the FeO6 octahedra block the lithium ion diffusion channels, restricting their movement to a one-dimensional channel. This results in a low lithium ion diffusion rate and poor electrochemical performance of LFP materials.

[0003] During the charge and discharge process of lithium-ion batteries, the electrode material and the electrolyte solution react at the solid-liquid interface, forming a passivation layer covering the electrode material surface, known as the solid electrolyte interface (SEI) film. The SEI film determines the performance of most batteries. A stable SEI film can improve the interfacial properties of the electrode material and enhance the battery's long-term cycling performance. However, in most cases, the SEI film fails to densely cover the electrode surface, or the SEI film is not an electronic insulator. This allows the solvent or lithium salt to continue to gain or lose electrons from the electrode, causing redox reactions. This further depletes the lithium source in the lithium-ion battery's positive electrode, increases the battery's internal resistance, and ultimately affects the battery's cycling performance and charge-discharge efficiency, and may even generate gas, reducing battery safety. To stabilize the electrode surface, surface modification is often performed on the electrode material or the electrode surface, or additives that can form a more stable SEI film are added to the electrolyte. AlF3, a hexagonal ionic crystal, is chemically very stable, insoluble in water and most organic solvents, and poorly soluble in acidic or alkaline solutions. Introducing it as an interfacial stabilizer into the coating of lithium iron phosphate materials can effectively improve the performance of lithium iron phosphate batteries. Furthermore, the AlF3 coating can limit the abnormal growth of lithium iron phosphate particles during high-temperature sintering, resulting in a lithium iron phosphate material with a more spherical morphology and particle size distribution. Summary of the Invention

[0004] The present invention provides a method for preparing an AlF3-coated lithium iron phosphate material with an interface stabilizing medium. The method involves mixing a lithium iron phosphate matrix with an AlF3 solution, atomizing the mixture through a pneumatic system to uniformly adhere the AlF3 to the surface of lithium iron phosphate particles, and sintering the mixture under an inert atmosphere to produce a uniformly AlF3-coated lithium iron phosphate material. The lithium-ion battery produced using this material exhibits excellent charge-discharge capacity and cycle performance.

[0005] To achieve the above object, the present invention provides the following technical solutions: A method for preparing an interface stabilizing medium AlF3-coated lithium iron phosphate material comprises the following steps: (1) Preparation of lithium iron phosphate matrix: iron source, phosphorus source, lithium source, carbon source, doping source and deionized water are mixed, and the lithium iron phosphate matrix is ​​obtained by stirring, grinding, spray drying, high temperature sintering, crushing and screening; (2) AlF3 pre-reaction: preheat the fluosilicic acid solution at a certain temperature, add excess aluminum hydroxide powder and stir thoroughly, raise the temperature, react for a certain time, and filter to obtain a filtrate; (3) Preparation of AlF3-coated lithium iron phosphate material: dilute the filtrate in step (2), uniformly mix it with the lithium iron phosphate matrix in step (1), and then atomize it through a pneumatic system and spray it onto a high-temperature glass substrate to achieve a thin layer of uniform AlF3 coating. Collect the resulting solid particles, sinter them at high temperature, crush and sieve them to obtain the AlF3-coated lithium iron phosphate material.

[0006] As a preferred embodiment, in step (1), the iron source includes ferrous sulfate, ferrous oxalate, ferric nitrate, and ferric phosphate; the phosphorus source includes phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; the lithium source includes lithium carbonate, lithium nitrate, lithium phosphate, and lithium hydroxide; the carbon source includes glucose, sucrose, PVDF, PEG, graphene, and carbon nanotubes; the doping source includes compounds containing titanium, vanadium, magnesium, niobium, and the like; and the molar ratio of the iron source, phosphorus source, lithium source, carbon source, and doping source is Fe:P:Li:C:M=1:(1-1.1):(1.02-1.05):(0.3-0.6):(0.005-0.2).

[0007] As a preferred embodiment, the concentration of the fluorosilicic acid solution in step (2) is 5%-25%, the aluminum hydroxide powder feeding coefficient is a molar ratio of F:Al=2:1, the preheating temperature is 60-80°C; the reaction temperature is 90-100°C, and the reaction time is 5-20 min.

[0008] As a preferred embodiment, in step (3), the filtrate is diluted to 0.5-3 wt% of AlF3, preferably 2 wt%, and the lithium iron phosphate matrix and the filtrate are mixed in a mass ratio of 1:(1-3); in step (3), the temperature of the high-temperature glass substrate is 180-300°C.

[0009] As a preferred embodiment, the sintering atmosphere in step (1) and step (3) is one of nitrogen, argon and helium; the sintering temperature in step (1) is 700-800°C, and the holding time is 8-12 h; the sintering temperature in step (3) is 400-600°C, and the holding time is 4-8 h.

[0010] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention adopts a pneumatic device to atomize the solid-liquid mixture of lithium iron phosphate matrix and AlF3 filtrate to form lithium iron phosphate particles wrapped with AlF3 liquid film, and then rolls on a high-temperature glass substrate to remove moisture to form a uniform AlF3 coating thin layer, and then performs high-temperature sintering to prepare lithium iron phosphate material uniformly coated with AlF3. The particles of this material are spherical, with a wide particle size distribution, and have excellent processing performance and energy density.

[0011] (2) When the battery prepared by coating lithium iron phosphate material with interface stabilizing medium AlF3 undergoes charge and discharge reaction, AlF3 can promote the dissociation of lithium salts and improve the conductivity and ion transfer efficiency of the electrolyte; on the other hand, it can promote the formation of an interface layer rich in lithium fluoride and make it densely wrapped on the surface of the lithium iron phosphate material, effectively inhibiting the subsequent reduction of lithium salts and the occurrence of side reactions, stabilizing the battery phase interface, and improving the cycle performance and stability of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 This is the SEM image of the AlF3-coated lithium iron phosphate material described in Example 2.

[0013] Figure 2 This is an SEM image of the lithium iron phosphate material described in Comparative Example 1.

[0014] Figure 3 This is an SEM image of the electrode sheet made of the material described in Example 2 after 200 cycles. DETAILED DESCRIPTION

[0015] The endpoints of the ranges and any values ​​disclosed in the present invention are not limited to the exact ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be considered as specifically disclosed in the present invention.

[0016] In order to better understand the present invention, the content of the present invention is further explained below in conjunction with embodiments and related drawings, but the content of the present invention is not limited to the following embodiments.

[0017] Example 1 (1) Iron phosphate, lithium carbonate, glucose, and titanium dioxide were mixed in a molar ratio of Fe:P:Li:C:Ti=1:1:1.02:0.3:0.005, and after wet grinding and spray drying, they were sintered at high temperature under nitrogen atmosphere protection. The sintering temperature was set at 700°C and the holding time was 12 h. After crushing and screening, a lithium iron phosphate matrix was obtained.

[0018] (2) Preheat a 5% fluorosilicic acid solution at 60 °C, then add excess aluminum hydroxide powder according to the molar ratio F:Al=2:1 and stir thoroughly. Heat to 95 °C, react for 5 min, and filter to obtain a filtrate.

[0019] (3) The filtrate in step (2) was diluted to 2 wt% AlF3, and the lithium iron phosphate matrix and the filtrate were uniformly mixed in a mass ratio of 1:1. The mixture was atomized by a pneumatic system and sprayed onto a high-temperature glass substrate. The temperature of the glass substrate was set to 180 °C. The solid particles on the glass substrate were then collected and sintered under nitrogen atmosphere. The sintering temperature was set to 400 °C and the holding time was 8 h. After crushing and screening, the AlF3-coated lithium iron phosphate material was obtained.

[0020] Example 2 (1) Iron phosphate, lithium carbonate, glucose, and titanium dioxide were mixed in a molar ratio of Fe:P:Li:C:Ti=1:1.05:1.03:0.42:0.013, and after wet grinding and spray drying, they were sintered at high temperature under nitrogen atmosphere protection. The sintering temperature was set at 750 °C and the holding time was 10 h. After crushing and screening, a lithium iron phosphate matrix was obtained.

[0021] (2) Preheat a 15% fluorosilicic acid solution at 70 °C, then add excess aluminum hydroxide powder according to the molar ratio F:Al=2:1 and stir thoroughly. Heat to 95 °C, react for 12 min, and filter to obtain a filtrate.

[0022] (3) The filtrate in step (2) was diluted to 2 wt% AlF3, and the lithium iron phosphate matrix and the filtrate were uniformly mixed in a mass ratio of 1:2. The mixture was atomized by a pneumatic system and sprayed onto a high-temperature glass substrate. The temperature of the glass substrate was set to 240 °C. The solid particles on the glass substrate were then collected and sintered under nitrogen atmosphere. The sintering temperature was set to 500 °C and the holding time was 6 h. After crushing and screening, the AlF3-coated lithium iron phosphate material was obtained.

[0023] Example 3 (1) Iron phosphate, lithium carbonate, glucose, and titanium dioxide were mixed in a molar ratio of Fe:P:Li:C:Ti=1:1.1:1.05:0.5:0.02. After wet grinding and spray drying, they were sintered at high temperature under nitrogen atmosphere protection. The sintering temperature was set at 800°C and the holding time was 8 h. After crushing and screening, a lithium iron phosphate matrix was obtained.

[0024] (2) Preheat a 25% fluorosilicic acid solution at 80 °C, then add excess aluminum hydroxide powder according to the molar ratio F:Al=2:1 and stir thoroughly. Heat to 95 °C, react for 20 min, and filter to obtain a filtrate.

[0025] (3) The filtrate in step (2) was diluted to 2 wt% AlF3, and the lithium iron phosphate matrix and the filtrate were uniformly mixed in a mass ratio of 1:3. The mixture was atomized by a pneumatic system and sprayed onto a high-temperature glass substrate. The temperature of the glass substrate was set to 300 °C. The solid particles on the glass substrate were then collected and sintered under nitrogen atmosphere. The sintering temperature was set to 600 °C and the holding time was 4 h. After crushing and screening, the AlF3-coated lithium iron phosphate material was obtained.

[0026] Example 4 (1) Ferrous oxalate, lithium dihydrogen phosphate, sucrose, and ammonium metavanadate were mixed in a molar ratio of Fe:P:Li:C:Ti=1:1.05:1.03:0.42:0.013, wet ground, spray dried, and then sintered at high temperature under nitrogen atmosphere at 750 °C for 10 h. The lithium iron phosphate matrix was obtained after crushing and sieving.

[0027] (2) Preheat a 15% fluorosilicic acid solution at 70 °C, then add excess aluminum hydroxide powder according to the molar ratio F:Al=2:1 and stir thoroughly. Heat to 95 °C, react for 12 min, and filter to obtain a filtrate.

[0028] (3) The filtrate in step (2) was diluted to 2 wt% AlF3, and the lithium iron phosphate matrix and the filtrate were uniformly mixed in a mass ratio of 1:2. The mixture was atomized by a pneumatic system and sprayed onto a high-temperature glass substrate. The temperature of the glass substrate was set to 240 °C. The solid particles on the glass substrate were then collected and sintered under nitrogen atmosphere. The sintering temperature was set to 500 °C and the holding time was 6 h. After crushing and screening, the AlF3-coated lithium iron phosphate material was obtained.

[0029] Comparative Example 1 Compared with Example 2, the difference of this comparative example is that the lithium iron phosphate substrate is not coated with AlF3, and other conditions are the same as those of Example 2.

[0030] (1) Iron phosphate, lithium carbonate, glucose, and titanium dioxide were mixed in a molar ratio of Fe:P:Li:C:Ti=1:1.05:1.03:0.42:0.013, and after wet grinding and spray drying, they were sintered at high temperature under nitrogen atmosphere protection. The sintering temperature was set at 750 °C and the holding time was 10 h. After crushing and screening, a lithium iron phosphate matrix was obtained.

[0031] (2) The lithium iron phosphate matrix and deionized water were uniformly mixed in a mass ratio of 1:2, and atomized and sprayed onto a high-temperature glass substrate through a pneumatic system. The temperature of the glass substrate was set to 240 °C. The solid particles on the glass substrate were then collected and sintered under nitrogen atmosphere. The sintering temperature was set to 500 °C and the holding time was 6 h. The lithium iron phosphate material was obtained after crushing and screening.

[0032] The above embodiments are merely examples for illustrative purposes only and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications may be made based on the above descriptions. It is not necessary and impossible to enumerate all implementation methods here. Therefore, any obvious variations or modifications derived therefrom are still within the scope of protection of the present invention.

[0033] Performance testing: The AlF3-coated lithium iron phosphate material, PVDF, and Super-P prepared in the Examples and Comparative Examples were homogenized at a ratio of 90:5:5, with a solids content of 30%. The mixture was then coated, dried, and punched to produce circular electrode sheets. Finally, the circular electrode sheets, separator, and lithium sheet were assembled into button cells in a glove box for testing. Charge and discharge tests were conducted on the button cells within a voltage range of 2.0 to 3.6 V.

[0034] Test results:

[0035] from Figure 1 It can be seen from the figure that the primary particles of the AlF3-coated lithium iron phosphate material prepared in Example 2 are tightly packed, the particles are spherical, the particle size distribution is wide, the processing performance is excellent, and the energy density is high; Figure 2 It can be seen that the lithium iron phosphate particles prepared in Comparative Example 1 are too large, have a narrow particle size distribution, have obvious gaps between particles, and have poor electrochemical performance; Figure 3It can be seen that even after long-term cycling, the electrode surface is generally dense and smooth, and no porous structure or lithium dendrites are produced. The formation of a fluorinated "interface fusion layer" (the marked area in the yellow dotted circle) during the electrochemical cycle is conducive to the uniformity of the lithium ion flow and avoids the tearing of the interface layer. It can be seen from the examples and comparative example data that the interface stabilizing medium AlF3 coated lithium iron phosphate material prepared using the solution of the present invention has good processing performance and high energy density. The prepared battery has a high discharge capacity at 0.1C, 0.5C, 1C and 3C, and the battery has excellent cycle performance and stability.

Claims

1. A method for preparing an interface stabilizing medium AlF3 coated lithium iron phosphate material, characterized in that: The following steps are involved: (1) AlF3 pre-reaction: preheat the hydrosilicic acid solution at a certain temperature, add excess aluminum hydroxide powder and stir thoroughly, increase the reaction and filter to obtain a filtrate; (2) Preparation of AlF3-coated lithium iron phosphate material: dilute the filtrate in step (1) and mix it evenly with the lithium iron phosphate matrix. Then, spray the mixture onto a high-temperature glass substrate through a pneumatic atomization system to achieve a thin layer of AlF3 uniform coating. The resulting solid particles are collected, sintered at high temperature, crushed and sieved to obtain the AlF3-coated lithium iron phosphate material.

2. The method for preparing the interface stabilizing medium AlF3 coated lithium iron phosphate material according to claim 1, characterized in that: In step (1), the mass concentration of the fluorosilicic acid solution is 5%-25%, the feeding coefficient of aluminum hydroxide powder is a molar ratio of F:Al=2:1, and the preheating temperature is 60-80°C.

3. The method for preparing the interface stabilizing medium AlF3 coated lithium iron phosphate material according to claim 1, characterized in that: In step (1), the reaction temperature is 90-100 °C and the reaction time is 5-20 min.

4. The method for preparing the interface stabilizing medium AlF3 coated lithium iron phosphate material according to claim 1, characterized in that: In the step (2), the filtrate is diluted to 0.5-3 wt% of AlF3, and the lithium iron phosphate matrix and the filtrate are mixed in a mass ratio of 1:(1-3).

5. The method for preparing the interface stabilizing medium AlF3 coated lithium iron phosphate material according to claim 4, characterized in that: The preparation method of the lithium iron phosphate matrix is ​​to mix an iron source, a phosphorus source, a lithium source, a carbon source, a doping source and deionized water, and prepare the mixture through stirring, grinding, spray drying, high-temperature sintering, crushing and screening.

6. The method for preparing the interface stabilizing medium AlF3 coated lithium iron phosphate material according to claim 5, characterized in that: The iron source includes any one of ferrous sulfate, ferrous oxalate, ferric nitrate or ferric phosphate; The phosphorus source includes any one of phosphoric acid, ammonium dihydrogen phosphate or diammonium hydrogen phosphate; The lithium source includes any one of lithium carbonate, lithium nitrate, lithium phosphate or lithium hydroxide; The carbon source includes any one of glucose, sucrose, PVDF, PEG, graphene or carbon nanotubes; The doping source includes any one of titanium, vanadium, magnesium or niobium compounds; The molar ratio of iron source, phosphorus source, lithium source, carbon source and doping source is Fe:P:Li:C:M=1:(1-1.1):(1.02-1.05):(0.3-0.6):(0.005-0.2).

7. The method for preparing the interface stabilizing medium AlF3 coated lithium iron phosphate material according to claim 5, characterized in that: The atmosphere during the high-temperature sintering process is one of nitrogen, argon or helium; the sintering temperature is 700-800 °C, and the holding time is 8-12 h.

8. The method for preparing the interface stabilizing medium AlF3 coated lithium iron phosphate material according to claim 1, characterized in that: In step (2), the temperature of the high-temperature glass substrate is 180-300°C.

9. The method for preparing the interface stabilizing medium AlF3 coated lithium iron phosphate material according to claim 1, characterized in that: The sintering temperature in step (2) is 400-600 °C, and the holding time is 4-8 h.

10. A battery positive electrode material, characterized in that: The AlF3-coated lithium iron phosphate material is prepared by the method according to any one of claims 1 to 9.

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