High-entropy fluoride electrode material and preparation method and application thereof

Abstract

The invention belongs to the technical field of lithium ion batteries, and particularly relates to a high-entropy fluoride electrode material and a preparation method and application thereof. The preparation method of the high-entropy fluoride electrode material comprises the following steps: dispersing a surfactant in a solvent, adding a soluble iron salt, a soluble nickel salt, a soluble manganese salt, a soluble copper salt and a soluble chromium salt, uniformly mixing, and adding a fluorine source to obtain a precursor solution; the precursor solution is placed at the temperature of 120 DEG C to 140 DEG C for a reaction, uniform FeF3, NiF2, MnF2, CuF2 and CrF3 are formed, and a high-entropy fluoride precursor is obtained; and annealing the high-entropy fluoride precursor in a protective atmosphere to obtain the high-entropy fluoride electrode material. The high-entropy fluoride electrode material prepared by the invention shows excellent reversible capacity and excellent cycling stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and particularly relates to a high-entropy fluoride electrode material, a preparation method thereof, and an application thereof. Background Art

[0002] As a storage device integrating high energy density and high voltage, rechargeable lithium-ion batteries are widely used in various electronic devices and electric vehicles; the full English name of rechargeable lithium-ion batteries is Rechargeable lithium-ion battery, abbreviated as LIBs. Currently, the cathode materials used in LIBs limit their voltage platforms and specific capacities. These cathode materials include LiFePO4, LiFePO4, and LiNi x Co y Mn 2-x-y O2. Due to the inherent defects of the insertion-type storage mechanism, these cathode materials cannot meet the high-capacity and high-energy requirements of LIBs. Therefore, developing cathode materials with high voltage platforms and high capacities has become a key factor in the research and development of high-performance LIBs.

[0003] Due to the occurrence of multi-electron redox reactions, the theoretical capacity of conversion-type cathode materials is much higher than that of insertion-type cathode materials. Among them, transition metal fluorides are regarded as a preferred system to break through the limits of insertion-type materials due to their multi-electron transfer characteristics and high redox potentials. However, transition metal fluorides still have undeniable disadvantages, making it impossible to directly replace the currently widely used insertion-type cathode materials. This is mainly reflected in large voltage hysteresis, poor electron / ion conductivity, and excessive volume change of metal fluorides during cycling, resulting in poor stability.

[0004] Based on the above problems, the prior art has introduced high-entropy alloys into the field of lithium-ion battery electrode materials to improve the performance of lithium-ion battery electrode materials. Document 1: Y. Wei, R. Yao, X. Liu, W. Chen, J. Qian, Y. Yin, D. Li, Y. Chen, Advanced Science. 2023, 10, 2300271. Document 1 synthesized a lithium-active Zn-Ge-Cu-Si-P quaternary solid solution in an Ar atmosphere using high-energy mechanical ball milling. Document 2: Y. Cui, P. A. Sukkurji, K. Wang, R. Azmi, A. M. Nunn, H. Hahn, B. Breitung, Y.-Y. Ting, P. M. Kowalski, P. Kaghazchi, Journal of Energy Chemistry. 2022, 72, 342. Document 2 synthesized high-entropy fluorides with 4 to 7 elements using mechanical ball milling respectively.

[0005] However, in the above-mentioned existing technologies, the high-energy-consuming ball milling method is used to synthesize high-entropy fluorides or high-entropy oxides as electrode materials. Since it is difficult for metal fluorides or metal oxides to be evenly distributed during the ball milling process, the ion diffusion rate of the electrode material is low and the specific surface area is small during the charge and discharge process, and significant excessive volume changes are caused during the cycling process. Summary of the Invention

[0006] In order to solve the technical problems in the existing technology that it is difficult for metal fluorides or metal oxides to be evenly distributed during the ball milling process, resulting in a low ion diffusion rate and a small specific surface area of the electrode material during the charge and discharge process, and significant excessive volume changes during the cycling process, the present invention provides a high-entropy fluoride electrode material, a preparation method thereof, and an application.

[0007] The present invention introduces a surfactant, combines a cationic surfactant and a non-ionic surfactant, and uses it as a dispersion solvent for soluble metal salts; dissolves the soluble metal salts and a fluorine source in the dispersion solvent to improve the dispersion and stability of the soluble metal salts in the solution, prevent the aggregation of metal ions in the soluble metal salts, and obtain a uniformly mixed precursor solution. The precursor solution is placed in a closed container for reaction, and by controlling the reaction temperature, the uniform growth of nanocrystals is promoted to obtain five different metal fluoride phases, and a high-entropy fluoride precursor is obtained. The problem that it is difficult for metal fluorides or metal oxides to be evenly distributed during the ball milling process is solved.

[0008] The present invention further combines the annealing method to remove the crystal water in the high-entropy fluoride precursor to obtain a high-entropy fluoride electrode material with high strength, high mixing type and high stability. The high-entropy fluoride electrode material prepared by the present invention exhibits excellent reversible capacity and excellent cycle stability.

[0009] The first object of the present invention is to provide a preparation method of a high-entropy fluoride electrode material, including the following steps:

[0010] Disperse the surfactant in a solvent, add soluble iron salt, soluble nickel salt, soluble manganese salt, soluble copper salt and soluble chromium salt, and after mixing evenly, add a fluorine source to obtain a precursor solution.

[0011] Place the precursor solution at 120 °C to 140 °C for reaction to form uniform FeF3, NiF2, MnF2, CuF2 and CrF3, and obtain a high-entropy fluoride precursor; anneal the high-entropy fluoride precursor under a protective atmosphere to obtain a high-entropy fluoride electrode material.

[0012] It should be noted that the atomic-level uniform dispersion of multi-metal ions lays the foundation for the subsequent hydrothermal coprecipitation reaction, directly affecting the component uniformity and electrochemical active site density of the final high-entropy fluoride, and thus affecting its electrochemical performance. In order to uniformly disperse the five soluble metal salts, the present invention introduces a surfactant as a dispersion solvent for the soluble metal salts; first, the surfactant is mixed uniformly with the solvent to form micelles, and then the five soluble metal salts are added. The micelles are used to improve the dispersibility and stability of the soluble metal salts, prevent the aggregation of metal ions, and obtain a uniformly mixed precursor solution.

[0013] A single surfactant is only effective for specific types of liquids and cannot comprehensively reduce the surface tension of the overall precursor solution, resulting in poor wetting performance and difficulty in achieving uniform mixing of the precursor solution. Cationic surfactants can reduce the surface tension of the solution, enabling solute particles to be better dispersed in the solution and preventing their aggregation and precipitation. Non-ionic surfactants can further improve the dispersibility of solute particles. The present invention combines cationic surfactants and non-ionic surfactants to prevent the aggregation of metal ions in the soluble metal salts by improving the dispersibility and stability of the soluble metal salts, and obtains a uniformly mixed precursor solution.

[0014] Preferably, the surfactant is a mixed system of a cationic surfactant and a non-ionic surfactant.

[0015] Preferably, the volume ratio of the non-ionic surfactant to the solvent is 1:1; the concentration of the cationic surfactant in the precursor solution is 0.1 M.

[0016] Preferably, the cationic surfactant is cetyltrimethylammonium bromide or sodium hexadecylsulfonate.

[0017] Preferably, the non-ionic surfactant is Tween80.

[0018] The specific method for preparing the precursor solution is as follows:

[0019] Disperse the surfactant in the solvent, and after uniform dispersion, add soluble iron salt, soluble nickel salt, soluble manganese salt, soluble copper salt, and soluble chromium salt to obtain a uniformly mixed five-metal salt solution; mix the five-metal salt solution and the fluorine source to obtain a precursor solution.

[0020] In the process of preparing the precursor solution, the surfactant needs to be mixed evenly first and then the metal salt is added. This is because the non-ionic surfactant in the surfactant is a viscous liquid and the cationic surfactant is a powder. Premature addition of the metal salt may cause the metal salt particles to directly adhere to the surfactant, forming particles that are difficult to distribute evenly. Moreover, after the five-element metal salt solution is mixed evenly, the fluorine source is added drop by drop to avoid directly adding hydrofluoric acid to the unmixed five-element metal salt solution, which may lead to incomplete reaction between the five-element metal salt solution and hydrofluoric acid, poor dispersion, or the occurrence of other side reactions.

[0021] Preferably, the molar ratio of iron ions in the soluble iron salt to fluoride ions in the fluorine source is 0.35:2 - 3. This is because an excessive fluorine source may generate other by-products, thereby reducing the purity and yield of the target product; while a small amount of fluorine source will reduce the utilization rate of raw materials and the target product with the expected output cannot be obtained.

[0022] Preferably, the fluorine source is hydrofluoric acid, sodium fluoride or potassium fluoride.

[0023] High-entropy materials usually require five or more main elements and the atomic ratio is between 5% and 35%; among all fluorides, since NiF2 and CuF2 have the highest voltage and the theoretical capacity can reach the average value; CrF2 and FeF3 have extremely high theoretical capacity and the theoretical voltage reaches the average value, and at the same time have good compatibility; MnF2 has excellent electrochemical cycle test performance. The present invention combines NiF2, CuF2, CrF2, FeF3 and MnF2 to form lattice distortion through the high-entropy effect, inhibit single-phase separation, and promote the formation of a high-entropy structure.

[0024] Preferably, Mn in the soluble manganese salt 2+ , Cu in the soluble copper salt 2+ , Cr in the soluble chromium salt 3+ , Ni in the soluble nickel salt 3+ and Fe in the soluble iron salt 3+ have a molar ratio of y:y:y:x:0.35; where, 0.1625 < x < 0.35, x > y, and x + 3y + 0.35 = 1.

[0025] Preferably, the soluble iron salt is Fe(CH3COO)3, Fe(NO)3·9H2O or FeCl3·6H2O.

[0026] Preferably, the soluble nickel salt is Ni(CH3COO)2·4H2O, Ni(NO3)2·6H2O or NiCl2·6H2O.

[0027] Preferably, the soluble manganese salt is (CH3COO)2Mn·4H2O, Mn(NO3)2·4H2O or MnCl2·4H2O.

[0028] Preferably, the soluble copper salt is Cu(CH3COO)2·H2O, Cu(NO3)2·3H2O or CuCl2·2H2O.

[0029] Preferably, the soluble chromium salt is Cr(CH3COO)3, Cr(NO3)3·9H2O or CrCl3·6H2O.

[0030] It should be noted that in the present invention, the penta-metal fluoride solution is subjected to a hydrothermal co-precipitation reaction, and the reaction temperature and time are controlled so that Fe 3+ , Ni 2+ , Mn 2+ , Cu 2+ and Cr 3+ in the penta-metal fluoride solution respectively react with F- in the fluorine source to form uniform precipitation compounds of FeF3, NiF2, MnF2, CuF2 and CrF3, obtaining five different metal fluoride phases and getting a high-entropy fluoride precursor.

[0031] Preferably, the temperature of the reaction is 120 °C to 140 °C, and the reaction time is 4 h to 6 h. High temperature can promote a more complete precipitation reaction and the uniform growth of nanocrystals; however, too high a temperature may lead to too large particles or agglomeration, resulting in uneven particle size.

[0032] In the present invention, the high-entropy fluoride precursor is annealed under a protective atmosphere because the annealing process can effectively reduce the defect density and lattice distortion inside the crystal; and promote the further crystallization and structural stability of the high-entropy fluoride. This change is beneficial to the formation of ion conduction channels and inhibits the volume expansion during the reaction process. At the same time, the crystallization water in the high-entropy fluoride precursor is further removed by annealing. If the annealing temperature is too low, the crystallization water may not be removed completely, and the residual moisture will affect the electrochemical performance; if the temperature is too high, the particles may sinter, reducing the specific surface area and affecting the lithium ion diffusion. Preferably, the annealing temperature is 230 °C to 400 °C, and the time is 3 h.

[0033] The second object of the present invention is to provide a high-entropy fluoride electrode material prepared by the above preparation method.

[0034] Preferably, the chemical formula of the high-entropy fluoride electrode material is (Fe 0.35 Ni x Mn y Cu y Cr y )F u; wherein, x > y, 2 ≤ u ≤ 3, 0.1625 < x < 0.35. This ratio is based on the most stable FeF3 as the main component, and NiF2 with second-best performance as the auxiliary component.

[0035] The third object of the present invention is to provide the application of the above high-entropy fluoride electrode material as a cathode material for lithium-ion batteries.

[0036] Preferably, the preparation method of the cathode material for lithium-ion batteries is as follows:

[0037] Mix the high-entropy fluoride electrode material, acetylene black conductive agent and polytetrafluoroethylene binder, and then add N-methylpyrrolidone to obtain a slurry; wherein, the mass ratio of the high-entropy fluoride electrode material, acetylene black conductive agent and polyvinylidene fluoride is 8:1:1, and the mass ratio of N-methylpyrrolidone to the powder slurry is 2:1.

[0038] Uniformly coat the slurry on the aluminum foil, and after curing, perform pressing to obtain the cathode.

[0039] Preferably, the curing temperature is 60°C to 100°C, and the curing time is 20h to 24h.

[0040] Compared with the prior art, the present invention has the following technical effects:

[0041] The present invention introduces a surfactant, combines a cationic surfactant and a non-ionic surfactant as a dispersion solvent for soluble metal salts; dissolves the soluble metal salts and a fluorine source in the dispersion solvent, and uses the surfactant to reduce the surface tension of the precursor solution, improve the dispersion and stability of the soluble metal salts in the precursor solution, prevent the aggregation of metal ions, and obtain a uniformly mixed precursor solution. Place the precursor solution in a closed container for reaction, and by controlling the reaction temperature, promote the uniform growth of nanocrystals, form uniform precipitation compounds of FeF3, NiF2, MnF2, CuF2 and CrF3, obtain five different metal fluoride phases, and obtain a high-entropy fluoride precursor. Solve the problem that metal fluorides or metal oxides are difficult to be evenly distributed during the ball milling process in the prior art.

[0042] The present invention anneals the high-entropy fluoride precursor under a protective atmosphere. Through the annealing treatment, the defect density and lattice distortion inside the crystal are effectively reduced; promote the further crystallization and structural stability of the high-entropy fluoride, facilitate the formation of ion conduction channels, and inhibit the volume expansion during the reaction process to obtain a high-entropy fluoride electrode material. At the same time, through annealing, the crystal water in the high-entropy fluoride precursor is further removed to avoid the adverse effects of crystal water on the high-entropy fluoride electrode material.

[0043] By simplifying the synthesis method of high-entropy fluoride, the present invention reduces the energy consumption during the synthesis process and enables it to be mass-produced. The high-entropy fluoride electrode material synthesized by the present invention is used as a cathode material in a lithium-ion battery, and at 0.2Ag -1 it has a high reversible capacity of 289.8 mAh g -1 , showing excellent reversible capacity and excellent cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 FIG. 1 is a thermogravimetric curve graph of the high-entropy fluoride electrode materials prepared in Comparative Example 1, Example 1, and Example 2.

[0045] Figure 2 FIG. 2 is an XRD graph of the high-entropy fluoride electrode materials prepared in Comparative Example 1, Example 1, and Example 2.

[0046] Figure 3 FIG. 3 is an XRD comparison graph of HEF-400 prepared in Example 1 and the Joint Committee on Powder Diffraction Standards (JCPDS) card of metal fluoride.

[0047] Figure 4 FIG. 4 is a transmission electron microscope (TEM) image of HEF-400 prepared in Example 1.

[0048] Figure 5 FIG. 5 is a TEM image of HEF-400 prepared in Example 1 at different magnifications; where, a is the TEM image at a magnification of 20 nm, and b is the TEM image at a magnification of 500 nm.

[0049] Figure 6 FIG. 6 is an elemental distribution map of HEF-400 prepared in Example 1; where, a is the F elemental distribution map, b is the Fe elemental distribution map, c is the Mn elemental distribution map, d is the Cr elemental distribution map, e is the Cu elemental distribution map, and f is the Ni elemental distribution map in d.

[0050] Figure 7 FIG. 7 is a cyclic voltammetry (CV) curve graph of the HEF-400 lithium battery for the first four cycles.

[0051] Figure 8 FIG. 8 is a discharge capacity graph of the HEF-400 lithium battery at different current densities.

[0052] Figure 9 FIG. 9 is a Nyquist plot of the HEF-400 lithium battery before and after cycling; where, the inset is an equivalent circuit diagram for fitting the EIS spectrum before cycling.

[0053] Figure 10 FIG. 10 is a cycle stability test graph of the HEF-400 lithium battery at 0.2Ag -1 .

[0054] Figure 11 Capacitance contribution diagram of HEF-400 lithium battery at different scanning rates.

[0055] Figure 12 Photographs of the precursor solution prepared in Example 1 at different stirring reaction times; among them, a is the initial photograph, b is the photograph after stirring reaction for 3 min, and c is the photograph after stirring reaction for 30 min.

[0056] Figure 13 Photographs of the precursor solution prepared in Comparative Example 2 at different reaction times; among them, a is the initial photograph, b is the photograph after stirring reaction for 3 min, and c is the photograph after stirring reaction for 30 min.

[0057] Figure 14 Photographs of the precursor solution prepared in Comparative Example 3 at different reaction times; among them, a is the initial photograph, b is the photograph after stirring reaction for 3 min, and c is the photograph after stirring reaction for 30 min.

[0058] Figure 15 Photographs of the precursor solution prepared in Comparative Example 4 at different reaction times; among them, a is the initial photograph, b is the photograph after stirring reaction for 3 min, and c is the photograph after stirring reaction for 30 min.

[0059] Detailed embodiments

[0060] In order to enable those skilled in the art to better understand and implement the technical solutions of the present invention, the present invention will be further described below with reference to specific embodiments.

[0061] In the description of the present invention, unless otherwise specified, the reagents used are commercially available, and the methods used are conventional techniques in the art.

[0062] Example 1

[0063] This example provides a preparation method of a high-entropy fluoride electrode material, including the following steps:

[0064] S1. Dissolve 1.822 g of cetyltrimethylammonium bromide in 25 mL of ethanol and 25 mL of Tween 80 to obtain a mixed solution; dissolve Fe(NO)3·9H2O, Ni(CH3COO)2·4H2O, (CH3COO)2Mn·4H2O, Cu(CH3COO)2·H2O, and Cr(CH3COO)3 in the mixed solution in a molar ratio of 0.35:0.20:0.15:0.15:0.15. Under stirring conditions, add 15 mL of 40 wt% HF dropwise, and continue stirring to obtain a dark green precursor solution.

[0065] S2. Transfer the precursor solution into a polytetrafluoroethylene liner, then place it into a reaction kettle and seal it. Heat it in an oven at 120 °C for 6 h. After the reaction is completed, cool it to room temperature, wash it with ethanol 5 times, and then place it in an oven. The drying temperature is 120 °C and the drying time is 3 h to obtain a green powder of high-entropy fluoride.

[0066] S3. Grind the high-entropy fluoride, and then anneal it in a tube furnace at 400 °C under a nitrogen atmosphere to obtain a high-entropy fluoride electrode material, denoted as HEF-400.

[0067] Example 2

[0068] This example provides a preparation method of a high-entropy fluoride electrode material, including the following steps:

[0069] S1. Dissolve 1.822 g of cetyltrimethylammonium bromide in 25 mL of ethanol and 25 mL of Tween 80 to obtain a mixed solution. Dissolve Fe(NO)3·9H2O, Ni(CH3COO)2·4H2O, (CH3COO)2Mn·4H2O, Cu(CH3COO)2·H2O, and Cr(CH3COO)3 in the mixed solution at a molar ratio of 0.35:0.20:0.15:0.15:0.15. Under stirring conditions, add 15 mL of 40 wt% HF dropwise, and continue stirring to obtain a dark green precursor solution.

[0070] S2. Transfer the precursor solution into a polytetrafluoroethylene liner, then place it into a reaction kettle and seal it. Heat it in an oven at 120 °C for 6 h. After the reaction is completed, cool it to room temperature, wash it with ethanol 5 times, and then place it in an oven. The drying temperature is 120 °C and the drying time is 3 h to obtain a green powder of high-entropy fluoride.

[0071] S3. Grind the high-entropy fluoride, and then anneal it in a tube furnace at 230 °C under a nitrogen atmosphere to obtain a high-entropy fluoride electrode material, denoted as HEF-230.

[0072] The difference between this example and Example 1 is:

[0073] The annealing temperature in this example is 230 °C.

[0074] Comparative Example 1

[0075] This comparative example provides a preparation method of a high-entropy fluoride electrode material, including the following steps:

[0076] S1. Dissolve 1.822 g of cetyltrimethylammonium bromide in 25 mL of ethanol and 25 mL of Tween 80 to obtain a mixed solution.

[0077] S2. Dissolve Fe(NO)3·9H2O, Ni(CH3COO)2·4H2O, (CH3COO)2Mn·4H2O, Cu(CH3COO)2·H2O, and Cr(CH3COO)3 in a mixed solution at a molar ratio of 0.35:0.20:0.15:0.15:0.15. Under stirring conditions, add dropwise 15 mL of 40 wt% HF, and continue stirring to obtain a dark green precursor solution.

[0078] S3. Transfer the precursor solution into a polytetrafluoroethylene inner liner, then place it into a reaction kettle and seal it. Heat it in an oven at 120 °C for 6 h. After the reaction is completed, cool it to room temperature, wash it with ethanol 5 times, and then place it in an oven. The drying temperature is 120 °C and the drying time is 3 h to obtain a green powder of high-entropy fluoride electrode material, denoted as HEF-25.

[0079] The difference between this comparative example and Example 1 is:

[0080] Annealing treatment was not carried out in this comparative example.

[0081] Comparative Example 2

[0082] This comparative example provides a preparation method of a high-entropy fluoride electrode material, including the following steps:

[0083] S1. Dissolve 1.822 g of cetyltrimethylammonium bromide in 50 mL of ethanol to obtain a mixed solution. Dissolve Fe(NO)3·9H2O, Ni(CH3COO)2·4H2O, (CH3COO)2Mn·4H2O, Cu(CH3COO)2·H2O, and Cr(CH3COO)3 in the mixed solution at a molar ratio of 0.35:0.20:0.15:0.15:0.15. Under stirring conditions, add dropwise 15 mL of 40 wt% HF, and continue stirring to obtain a dark green precursor solution.

[0084] S2. Transfer the precursor solution into a polytetrafluoroethylene inner liner, then place it into a reaction kettle and seal it. Heat it in an oven at 120 °C for 6 h. After the reaction is completed, cool it to room temperature, wash it with ethanol 5 times, and then place it in an oven. The drying temperature is 120 °C and the drying time is 3 h to obtain a high-entropy fluoride in the form of green powder.

[0085] S3. Grind the high-entropy fluoride, and then anneal it in a tube furnace at 400 °C under a nitrogen atmosphere to obtain a high-entropy fluoride electrode material.

[0086] The difference between this comparative example and Example 1 is:

[0087] Tween 80 was not added to the precursor solution in this comparative example.

[0088] Comparative Example 3

[0089] This comparative example provides a method for preparing a high-entropy fluoride electrode material, including the following steps:

[0090] S1. Mix 25 mL of ethanol and 25 mL of Tween 80 to obtain a mixed solution; dissolve Fe(NO)3·9H2O, Ni(CH3COO)2·4H2O, (CH3COO)2Mn·4H2O, Cu(CH3COO)2·H2O, and Cr(CH3COO)3 in the mixed solution at a molar ratio of 0.35:0.20:0.15:0.15:0.15. Under stirring conditions, add 15 mL of 40 wt% HF dropwise, and continue stirring to obtain a dark green precursor solution.

[0091] S2. Transfer the precursor solution to a polytetrafluoroethylene inner lining, then place it in a reaction kettle and seal it; heat it in an oven at 120 °C for 6 h; after the reaction is completed, cool it to room temperature, wash it 5 times with ethanol, and then place it in an oven. The drying temperature is 120 °C and the drying time is 3 h to obtain a green powder of high-entropy fluoride.

[0092] S3. Grind the high-entropy fluoride, and then anneal it in a tube furnace at 400 °C under a nitrogen atmosphere to obtain a high-entropy fluoride electrode material.

[0093] The difference between this comparative example and Example 1 is:

[0094] The precursor solution in this comparative example does not add cetyltrimethylammonium bromide.

[0095] Comparative Example 4

[0096] This comparative example provides a method for preparing a high-entropy fluoride electrode material, including the following steps:

[0097] S1. Dissolve Fe(NO)3·9H2O, Ni(CH3COO)2·4H2O, (CH3COO)2Mn·4H2O, Cu(CH3COO)2·H2O, and Cr(CH3COO)3 in 50 mL of ethanol at a molar ratio of 0.35:0.20:0.15:0.15:0.15. Under stirring conditions, add 15 mL of 40 wt% HF dropwise, and continue stirring to obtain a dark green precursor solution.

[0098] S2. Transfer the precursor solution to a polytetrafluoroethylene inner lining, then place it in a reaction kettle and seal it; heat it in an oven at 120 °C for 6 h; after the reaction is completed, cool it to room temperature, wash it 5 times with ethanol, and then place it in an oven. The drying temperature is 120 °C and the drying time is 3 h to obtain a green powder of high-entropy fluoride.

[0099] S3. Grind the high-entropy fluoride, and then anneal it in a nitrogen atmosphere in a tube furnace at 400 °C to obtain a high-entropy fluoride electrode material.

[0100] The difference between this comparative example and Example 1 is as follows:

[0101] In this comparative example, cetyltrimethylammonium bromide and Tween 80 were not added to the precursor solution.

[0102] Applied Example 1

[0103] This applied example provides a method for preparing a cathode electrode material for a lithium-ion battery, including the following steps:

[0104] Mix the HEF-400, acetylene black, and polytetrafluoroethylene binder prepared in Example 1 in a glass at a mass ratio of 8:1:1, and then add 200 μL of N-methylpyrrolidone to the glass and grind evenly for 6 h to obtain a slurry.

[0105] Coat the slurry evenly on the aluminum foil, dry it under vacuum conditions at 60 °C for 20 h, and then press it into a disc with a diameter of 6 mm to obtain a cathode electrode sheet.

[0106] Applied Comparative Example 1

[0107] This applied comparative example provides a method for preparing a cathode electrode material for a lithium-ion battery, including the following steps:

[0108] Mix the HEF-400, acetylene black, and polytetrafluoroethylene binder prepared in Example 1 in a glass at a mass ratio of 8:1:1, and then add 200 μL of N-methyl-2-pyrrolidone to the glass and grind evenly for 6 h to obtain a slurry.

[0109] Coat the slurry evenly on the aluminum foil, dry it under vacuum conditions at 60 °C for 20 h, and then press it into a disc with a diameter of 6 mm to obtain a cathode electrode sheet.

[0110] Experimental tests:

[0111] 1. Thermogravimetric analysis.

[0112] From Figure 1It can be seen that before 100 °C, the mass of HEF-25, HEF-230, and HEF-400 decreases slowly. This is because at lower temperatures, only the evaporation of adsorbed water occurs. After 100 °C, due to the evaporation of a large amount of crystal water content, the mass of HEF-25 and HEF-230 decreases sharply. Between 230 °C and 400 °C, the mass of HEF-230 decreases by 12%, while the mass of HEF-400 only decreases by 3%, indicating that HEF-230 still contains a considerable amount of crystal water. Between 400 °C and 494 °C, HEF-400, HEF-230, and HEF-25 show mass losses of 4%, 4%, and 5% respectively, which indicates that thermal decomposition of FeF3 begins above 400 °C. Therefore, the preferred annealing temperature of the present invention is not higher than 400 °C.

[0113] 2. X-ray diffraction analysis.

[0114] As can be seen from Figure 2 it that in the range of 10° to 25°, XRD peaks are observed for HEF-25 and HEF-230, while HEF-400 has no corresponding diffraction peaks. Combining with Figure 1 the thermogravimetric results in, the diffraction peaks of HEF-25 and HEF-230 in the range of 10° to 25° are attributed to hydroxyfluorides generated by crystal water. In the range of 25° to 60°, the diffraction peak intensities of HEF-25, HEF-230, and HEF-400 gradually increase, and the diffraction peak shapes become sharper because the annealing process effectively reduces the defect density and lattice distortion inside the crystal. As the annealing treatment progresses, the grain size increases and its distribution becomes more uniform. This change promotes the formation of channels beneficial to ion conduction and inhibits the volume expansion during the reaction process.

[0115] As can be seen from Figure 3It can be seen that the diffraction peaks of HEF-400 match well with the crystal planes of each metal fluoride. Among them, the crystal planes corresponding to FeF3 belong to the hexagonal crystal system (JCPDS 33-0647), including (012), (110), (113), (211), and (122) crystal planes; the crystal planes corresponding to MnF2 belong to the orthorhombic crystal system (JCPDS 17-0864) and are composed of (111), (102), and (130) crystal planes; the crystal planes corresponding to CrF3 also belong to the hexagonal crystal system (JCPDS 73-9443), specifically (012) and (104) crystal planes; for CuF2, only the (200) crystal plane is observed in its cubic crystal system (JCPDS 06-0343). For NiF2, the (110), (111), (210), (211), and (220) crystal planes are observed in the tetragonal crystal system (JCPDS 24-0792). This indicates that five different metal fluoride phases of FeF3, MnF2, CrF3, CuF2, and NiF2 are formed in the present invention.

[0116] 3. Surface morphology analysis.

[0117] It can be seen from Figure 4 , Figure 5 and Figure 6 that the interplanar spacings of HEF-400 are 0.260 nm, 0.328 nm, 0.236 nm, 0.270 nm, and 0.342 nm respectively, corresponding to the (110) crystal plane of FeF3, the (110) crystal plane of NiF2, the (102) crystal plane of MnF2, the (200) crystal plane of CuF2, and the (012) crystal plane of CrF3. This shows that HEF-400 contains five different metal fluoride phases rather than complex intermetallic compounds. In addition, as shown in d-i of Figure 4 , the element mappings in the mapping diagram show that all elements are evenly distributed without obvious accumulation or segregation, thus meeting the definition requirements of high-entropy materials.

[0118] 4. Electrochemical performance test.

[0119] (1) Battery assembly:

[0120] In the present invention, a lithium metal foil is used as the counter electrode and reference electrode, 4 mol / L LiClO4 is used as the electrolyte, and the solvent is a mixed solvent of ethylene carbonate and propylene carbonate with a volume ratio of 1:1. The positive electrode material prepared in Application Example 1 is used as the working electrode. In a glove box filled with argon, a CR2025 button battery is assembled to obtain the HEF-400 lithium battery; among them, O2 and H2O in the glove box < 1 ppm.

[0121] Electrochemical tests were carried out on the HEF-400 lithium battery. When performing cyclic voltammetry scanning, the range of the cyclic voltammetry characteristic curve was set to 1.5 V to 4.5 V, and the scanning rate was 0.2 mVs -1 ~2 mVs -1 。When performing constant current charge and discharge tests, the voltage range was set to 1.5 V to 4.5 V, and the current intensity was 20 μA to 640 μA. When performing impedance tests, the frequency range was set to 100 kHz to 10 mHz, and the voltage amplitude was 10 mV. When using constant current charge and discharge for cyclic stability tests, the voltage range was set to 1.5 V to 4.5 V, and the current density was set to 0.2 Ag -1 ,and the number of cycles was 494 times.

[0122] (2) Determination of electrochemical performance:

[0123] From Figure 7 it can be seen that during the first cycle discharge, a small peak appeared at ~2.46 V for the HEF-400 lithium battery, but this small peak disappeared in subsequent cycles, which can be attributed to electrolyte decomposition and the formation of the solid electrolyte interface film. In addition, it was observed that two reduction peaks also appeared at ~1.77 V and ~2.99 V for the HEF-400 lithium battery, corresponding to the lithiation of the HEF-400 lithium battery, and M 2+ 、M 3+ was reduced to M 0 。During the first charging process, the HEF-400 lithium battery had a relatively wide oxidation peak at ~2.9 V respectively, corresponding to M 0 being oxidized to M 2+ 、M 3+ 。The above lithiation / delithiation reactions indicate that the HEF-400 lithium battery has a high degree of redox reversibility during the charge / discharge process.

[0124] As Figure 8 shown, the HEF-400 lithium battery has excellent rate performance and can achieve deep charge and discharge at high current density.

[0125] As Figure 9 shown, the impedance data indicate that the inherent resistance of the HEF-400 lithium battery was ~13.1 Ω before cycling and ~16.9 Ω after cycling. The lower resistance helps the transport of electrons and ions, improves the utilization rate of metal fluoride, and the resistance change is small before and after cycling, indicating the stable structure of the HEF-400 lithium battery.

[0126] The cyclic stability of the HEF-400 lithium battery was further verified by long-term charge and discharge. As Figure 10 shown, when cycling at a current density of 0.2 Ag -1 , its initial discharge capacity was 162.6 mAh g-1 When the cycle reaches 65 cycles, the discharge capacity drops to 113.3 mAh g -1 . However, after that, the discharge capacity gradually increases. After 446 cycles are completed, the discharge capacity can reach 216.1 mAh g -1 . In addition, its average Coulomb efficiency can be maintained at 97.5% after 300 cycles.

[0127] As Figure 11 shown, by studying the capacitance contribution of the HEF-400 lithium battery, the utilization rate of metal fluoride in the HEF-400 lithium battery is further calculated to be ~68.3%.

[0128] From Figure 13 , Figure 14 and Figure 15 it can be seen that as the stirring time increases, after stirring for 60 min, the metal salt particles in the precursor solution are still not completely dissolved and dispersed; from Figure 12 it can be seen that after stirring for 60 min, under the action of the cationic surfactant and the non-ionic surfactant, the surface tension of the precursor solution is significantly reduced, promoting the dissolution of the metal salt in the precursor solution and obtaining a uniformly dispersed precursor solution.

[0129] This shows that the present invention introduces a surfactant after combining a cationic surfactant and a non-ionic surfactant as a dispersion solvent for soluble metal salts; by using the surfactant to reduce the surface tension of the precursor solution, the dispersibility and stability of the soluble metal salt in the precursor solution are further improved.

[0130] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention is also intended to include these modifications and variations. The above-described embodiments are only preferred embodiments given to fully illustrate the present invention, and the scope of its protection is not limited thereto.

Claims

1. A preparation method of a high-entropy fluoride electrode material, characterized in that, It includes the following steps: Disperse the surfactant in the solvent, add soluble iron salt, soluble nickel salt, soluble manganese salt, soluble copper salt and soluble chromium salt. After mixing evenly, add the fluorine source to obtain the precursor solution; Place the precursor solution at 120 °C to 140 °C for reaction to form uniform FeF3, NiF2, MnF2, CuF2 and CrF3, and obtain the high-entropy fluoride precursor; anneal the high-entropy fluoride precursor under a protective atmosphere to obtain the high-entropy fluoride electrode material; The surfactant is a mixed system of cationic surfactant and non-ionic surfactant.

2. The preparation method of the high-entropy fluoride electrode material according to claim 1, characterized in that The volume ratio of the non-ionic surfactant to the solvent is 1:1; The concentration of the cationic surfactant in the precursor solution is 0.1 M.

3. The preparation method of the high-entropy fluoride electrode material according to claim 1, wherein, The cationic surfactant is cetyltrimethylammonium bromide or sodium hexadecylsulfonate; The non-ionic surfactant is Tween80.

4. The preparation method of the high-entropy fluoride electrode material according to claim 1, characterized in that, Fe in soluble iron salt 3+ The molar ratio of Fe in 3+ to F- in the fluorine source is 0.35:2 to 3; Mn in soluble manganese salt 2+ 、Cu in soluble copper salt 2+ 、Cr in soluble chromium salt 3+ 、Ni in soluble nickel salt 3+ and Fe in soluble iron salt 3+ have a molar ratio of y:y:y:x:0.35; Wherein, 0.1625 < x < 0.35, x > y, x + 3y + 0.35 = 1.

5. The preparation method of the high-entropy fluoride electrode material according to claim 1, wherein The reaction time for preparing the high-entropy fluoride precursor is 4 h to 6 h.

6. The preparation method of the high-entropy fluoride electrode material according to claim 1, wherein The annealing temperature is 230 °C to 400 °C.

7. The preparation method of the high-entropy fluoride electrode material according to claim 1, wherein, The soluble iron salt is Fe(CH3COO)3, Fe(NO)3·9H2O or FeCl3·6H2O; The soluble nickel salt is Ni(CH3COO)2·4H2O, Ni(NO3)2·6H2O or NiCl2·6H2O; The soluble manganese salt is (CH3COO)2Mn·4H2O, Mn(NO3)2·4H2O or MnCl2·4H2O; The soluble copper salt is Cu(CH3COO)2·H2O, Cu(NO3)2·3H2O or CuCl2·2H2O; The soluble chromium salt is Cr(CH3COO)3, Cr(NO3)3·9H2O or CrCl3·6H2O.

8. The preparation method of the high-entropy fluoride electrode material according to claim 1, wherein The fluorine source is hydrofluoric acid, sodium fluoride or potassium fluoride.

9. A high-entropy fluoride electrode material, characterized in that, The high-entropy fluoride electrode material is prepared by the preparation method of the high-entropy fluoride electrode material according to any one of claims 1 to 8.

10. Application of the high-entropy fluoride electrode material according to claim 9 as a positive electrode material for lithium-ion batteries.

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