Metal fluoride heterojunction particle, preparation method thereof and application of metal fluoride heterojunction particle in lithium ion battery

Abstract

The invention relates to a metal fluoride heterojunction particle, a preparation method thereof and application of the metal fluoride heterojunction particle in a lithium ion battery. The preparation method of the metal fluoride heterojunction particles comprises the following steps: (1) adding a metal chlorine salt, an auxiliary metal chlorine salt and a fluorinating agent into a deep eutectic solvent, and then reacting at 50-70 DEG C for 6-12 hours to obtain a precipitate; and (2) washing, centrifuging, drying and carrying out heat treatment on the obtained precipitate to obtain the metal fluoride heterojunction particles.

Description

Technical Field

[0001] The present invention relates to a metal fluoride heterojunction particle, a preparation method thereof, and an application thereof in a lithium-ion battery, belonging to the technical field of new energy. Background Art

[0002] The rapid development of the energy storage market has promoted the continuous growth of the performance requirements of secondary batteries. However, the limited specific capacity and energy density of currently commercialized lithium-ion batteries make it impossible to meet the requirements of future large-scale and long-endurance energy storage. In addition, traditional cobalt-containing cathode materials have problems such as insufficient reserve abundance, high price cost, and chemical pollution. Therefore, it is of great significance to develop new cathode materials with high specific capacity, high energy density, low price, and environmental friendliness.

[0003] Due to the limitation of the single-electron transfer reaction mechanism, the theoretical specific capacity and energy density of traditional lithium-ion battery cathode materials (such as lithium cobaltate, lithium iron phosphate, and nickel-cobalt-based ternary cathode materials, etc.) are usually lower than 275 mAh / g and 750 Wh / kg. In contrast, new conversion reaction-type cathode materials have the potential for higher specific energy through multi-electron transfer reactions. For example, the iron trifluoride cathode based on the three-electron transfer reaction has a theoretical specific capacity of 712 mAh / g and a thermodynamic potential of ~2.73 V, which can provide a high energy density of 1947 Wh / kg. Combining its characteristics of abundant raw material reserves and environmental friendliness, it is thus regarded as one of the potential cathode candidates for the next-generation energy storage batteries.

[0004] However, the low intrinsic electronic conductivity of the iron fluoride cathode and the large degree of phase change in the conversion reaction result in poor cycle stability. To address this problem, most literature usually adopts cathode modification strategies such as particle nanosizing and conductive framework modification. Although the ion / electron transport on the particle surface has been improved to a certain extent, there are still great challenges in the conversion reaction kinetics and reversibility of the intrinsic structure of iron fluoride. In addition, the current synthesis of metal fluorides often uses methods such as high-pressure solvothermal and high-temperature heat treatment involving corrosive gas fluorine sources such as HF and NF3. These fluorination methods are highly dangerous. Another type of fluorination method using fluorinated ionic liquids can improve the safety of the preparation process, but the ionic liquids are expensive and not suitable for large-scale industrial production. Summary of the Invention

[0005] Aiming at the deficiencies of the above-mentioned prior art, the present invention provides a metal fluoride heterojunction particle, a preparation method thereof, and an application thereof in a lithium-ion battery.

[0006] In the first aspect, the present invention provides a preparation method of a metal fluoride heterojunction particle, comprising: (1) Add a metal chloride, an auxiliary metal chloride, and a fluorinating agent to a deep eutectic solvent, and then react at 50-70 °C for 6-12 hours to obtain a precipitate; (2) Wash, centrifuge, dry, and heat-treat the obtained precipitate to obtain the metal fluoride heterojunction particles.

[0007] Preferably, the deep eutectic solvent comprises: a mixture of choline chloride and a polyol; the polyol is selected from at least one of ethylene glycol, polyethylene glycol, glycerol, and tetraethylene glycol; the molar ratio of choline chloride to ethylene glycol is 1:(1-4).

[0008] Preferably, the metal chloride is selected from at least one of ferrous chloride tetrahydrate, ferric chloride hexahydrate, cobalt chloride hexahydrate, nickel chloride hexahydrate, and copper chloride dihydrate; the mass ratio of the metal chloride to the deep eutectic solvent is 1:(10-50).

[0009] Preferably, the auxiliary metal chloride is selected from at least one of cobalt chloride hexahydrate and nickel chloride hexahydrate, and the composition of the auxiliary metal chloride is different from that of the metal chloride; The molar ratio of the metal chloride to the auxiliary metal chloride is (8-1):1.

[0010] Preferably, the fluorinating agent is selected from at least one of ammonium hydrogen fluoride and ammonium fluoride; the molar ratio of the fluorinating agent to the metal chloride is (2-10):1. The temperature of the heat treatment is 250-350 °C, the time is 2-5 hours, and the atmosphere is an inert atmosphere or a nitrogen atmosphere. Preferably, first heat up to 125-150 °C and keep warm for 3-5 hours, and then heat up to 250-350 °C and keep warm for 2-5 hours.

[0011] In the present invention, a metal fluoride heterojunction is prepared based on a deep eutectic solvent and an auxiliary metal chloride. Specifically, choline chloride and a polyol can serve as a hydrogen bond acceptor and a hydrogen bond donor respectively, and form a transparent and homogeneous solution through hydrogen bond interaction. Among them, choline chloride and the polyol are both safe and non-toxic, and the deep eutectic solvent formed by the two not only has the characteristics similar to ionic liquids but also has a low price. In addition, metal cobalt ions or nickel ions in the auxiliary metal chloride can serve as an oxidation auxiliary agent to in-situ oxidize some Fe 2+ ions in the solution to Fe 3+ , and finally form an FeF2-FeF3 fluoride heterojunction.

[0012] In the second aspect, the present invention provides a method for preparing a cathode material for a lithium-ion battery, comprising: (1) Add a metal chloride, an auxiliary metal chloride, conductive carbon black, and a fluorinating agent to a deep eutectic solvent, and then react at a temperature of 50-70 °C for 6-12 hours to obtain a precipitate; (2) Wash, centrifuge, dry, and heat-treat the obtained precipitate to obtain the positive electrode material for the lithium-ion battery.

[0013] In the present invention, an auxiliary metal chloride is first introduced into the deep eutectic solvent as an oxidation auxiliary agent, and the phase ratios of the metal fluoride heterojunction are regulated by changing the ratio of metal ions. The tight hetero-contact between multiple phases of the metal fluoride can promote and stabilize the interfacial charge transfer, and improve the reversibility and kinetics of the electrochemical reaction of the metal fluoride. In addition, the metal ions (such as Co 2+ ) used as the oxidation auxiliary agent in the preparation process do not participate in the fluoride precipitation and can be recycled through recovery.

[0014] Preferably, the deep eutectic solvent includes a mixture of choline chloride and polyol; the polyol is selected from at least one of ethylene glycol, polyethylene glycol, glycerol, and tetraethylene glycol; the molar ratio of choline chloride to ethylene glycol is 1:(1-4).

[0015] Preferably, the metal chloride is selected from at least one of ferrous chloride tetrahydrate, ferric chloride hexahydrate, cobalt chloride hexahydrate, nickel chloride hexahydrate, and copper chloride dihydrate; the mass ratio of the metal chloride to the deep eutectic solvent is 1:(10-50).

[0016] Preferably, the auxiliary metal chloride is selected from at least one of cobalt chloride hexahydrate and nickel chloride hexahydrate, and the composition of the auxiliary metal chloride is different from that of the metal chloride; The molar ratio of the metal chloride to the auxiliary metal chloride is (8-1):1.

[0017] Preferably, the fluorinating agent is selected from at least one of ammonium hydrogen fluoride and ammonium fluoride; the molar ratio of the fluorinating agent to the metal chloride is (2-10):1.

[0018] Preferably, the conductive carbon black is selected from at least one of Ketjen black, Super-P, acetylene black, carbon nanotubes, and graphene; the mass ratio of the conductive carbon black to the metal chloride is (0.02-0.1):1. The temperature of the heat treatment is 250-350 °C, the time is 2-5 hours, and the atmosphere is an inert atmosphere or a nitrogen atmosphere. Preferably, first heat up to 125-150 °C and keep warm for 3-5 hours, and then heat up to 250-350 °C and keep warm for 2-5 hours.

[0019] In the third aspect, the present invention provides metal fluoride heterojunction particles prepared according to the above preparation method. The metal fluoride heterojunction particles exhibit a porous spherical morphology with tightly connected structures, and there is tight contact between the metal fluoride heterostructures; preferably, the metal fluoride heterojunction particles are composed of tightly connected porous spherical particles with a particle size of 100-200 nm.

[0020] Preferably, the metal fluoride heterojunction particles are composed of multi-phase fluorides, preferably biphasic iron fluoride of hexagonal tungsten bronze structure FeF3 and rutile structure FeF2, and more preferably the molar ratio of FeF3 with hexagonal tungsten bronze structure to FeF2 with rutile structure in the biphasic iron fluoride is (1-9):1, preferably (1-4):1.

[0021] Fourthly, the present invention provides an application of the metal fluoride heterojunction particles in the preparation of a cathode material for a lithium-ion battery.

[0022] Fifthly, the present invention provides an application of the metal fluoride heterojunction particles in a lithium-ion battery.

[0023] Sixthly, the present invention provides a cathode material for a lithium-ion battery prepared according to the above preparation method, comprising: metal fluoride heterojunction particles and conductive carbon black loaded on the metal fluoride heterojunction particles; preferably, the mass percentage content of the conductive carbon black is 10-30%. The conductive carbon black is loaded on the metal fluoride heterojunction particles as a modified conductive network.

[0014] Preferably, the metal fluoride heterojunction particles are composed of multi-phase fluorides, preferably biphasic iron fluoride of hexagonal tungsten bronze structure FeF3 and rutile structure FeF2, and more preferably the molar ratio of FeF3 with hexagonal tungsten bronze structure to FeF2 with rutile structure in the biphasic iron fluoride is (1-9):1, preferably (1-4):1.

[0025] Seventhly, the present invention provides an application of the above cathode material for a lithium-ion battery in a lithium-ion battery.

[0026] The beneficial effects of the present invention: (1) Based on the deep eutectic solvent and auxiliary metal chloride provided by the present invention, the metal fluoride heterojunction can be efficiently and controllably synthesized by the "one-pot method", and the preparation process is convenient; the cheap and safe deep eutectic solvent method is adopted to avoid the use of highly corrosive NF3 / HF gas and expensive ionic liquids in the conventional fluorination method; (2) For the metal fluoride heterojunction prepared by the deep eutectic solvent and auxiliary metal chloride of the present invention, the tight heterocontact between the metal fluoride phases can promote and stabilize the interfacial charge transfer and improve the reversibility and kinetics of the metal fluoride electrochemical reaction; (3) For the FeF3-FeF2 heterojunction / conductive carbon black composite cathode material for a lithium-ion battery prepared by the deep eutectic solvent and auxiliary metal chloride of the present invention, it can not only promote and stabilize the interfacial charge transfer and topological transformation reaction, but also combine the characteristics of the narrower band gap and more stable transformation reaction of FeF2 and the higher theoretical capacity and thermodynamic potential of FeF3, realizing the synergistic improvement of the battery specific capacity and cycle stability. Brief Description of the Drawings

[0027] Figure 1 XRD diffraction pattern of the synthesized FeF3-FeF2 heterojunction / KB composite material in Example 1 of the present invention and the corresponding Rietveld refinement results; Figure 2 SEM images of the synthesized FeF3-FeF2 heterojunction / KB composite material in Example 1 of the present invention, where the scale bar of (a) is 1 μm and the scale bar of (b) is 300 nm; Figure 3 TEM image of the synthesized FeF3-FeF2 heterojunction / KB composite material in Example 1 of the present invention; Figure 4 Cyclic voltammetry curve of the lithium battery assembled with the FeF3-FeF2 heterojunction / KB as the positive electrode material in Example 2 of the present invention; Figure 5 Specific capacity and coulombic efficiency cycle diagram of the lithium battery assembled with the FeF3-FeF2 heterojunction / KB as the positive electrode material in Example 2 of the present invention; Figure 6 Comparison diagram of the long-term cycling performance of lithium batteries assembled with FeF3-FeF2 heterojunction / KB, pure-phase FeF3, and FeF2 as the positive electrode materials in Example 2 of the present invention; Figure 7 Specific capacity and galvanostatic charge-discharge curves of the lithium battery assembled with the FeF3-FeF2 heterojunction / KB as the positive electrode material in Example 2 of the present invention at different current densities; Figure 8 Long-term cycling performance of the lithium battery assembled with the FeF3-FeF2 heterojunction / KB as the positive electrode material in Example 2 of the present invention under high loading of the positive electrode active material. Detailed Description of the Invention

[0028] The present invention will be further described below through the following embodiments. It should be understood that the following embodiments are only used to illustrate the present invention and do not limit the present invention.

[0029] The present invention discloses a metal fluoride heterojunction prepared by a deep eutectic solvent and a lithium-ion battery positive electrode material prepared from the metal fluoride heterojunction.

[0030] In the present invention, the deep eutectic solvent includes: choline chloride and one or more polyols. The polyols include, but are not limited to, ethylene glycol, polyethylene glycol, glycerol, tetraethylene glycol, etc.

[0031] In the present invention, a method for preparing metal fluoride heterojunctions by means of deep eutectic solvents includes: adding metal chlorides, auxiliary metal chlorides, and fluorinating agents into the deep eutectic solvent, reacting at a temperature of 50 - 70 °C for 6 - 12 hours, and after washing, centrifuging, drying, and heat-treating the precipitate, metal fluoride heterojunction particles are obtained.

[0032] In the above method for preparing metal fluoride heterojunctions, the metal chlorides include but are not limited to ferrous chloride tetrahydrate, ferric chloride hexahydrate, cobalt chloride hexahydrate, nickel chloride hexahydrate, copper chloride dihydrate; the auxiliary metal chlorides include but are not limited to cobalt chloride hexahydrate, nickel chloride hexahydrate or a mixture of the two; the fluorinating agents include but are not limited to ammonium bifluoride, ammonium fluoride or a mixture of the two.

[0033] For the above method for preparing metal fluoride heterojunctions, especially the metal fluoride heterojunction prepared based on ferrous chloride tetrahydrate and auxiliary metal chlorides is a biphasic iron fluoride of hexagonal tungsten bronze structure FeF3 and rutile structure FeF2.

[0034] Preparation of metal fluoride heterojunction / conductive carbon black composite materials: Loading conductive carbon black onto the above metal fluoride heterojunction particles can obtain metal fluoride heterojunction / conductive carbon black composite materials, which can be used as the cathode material for lithium-ion batteries. In the present invention, the metal fluoride heterojunction used as the active material for the cathode material of lithium-ion batteries is preferably a biphasic iron fluoride of hexagonal tungsten bronze structure FeF3 and rutile structure FeF2. The conductive carbon black includes but is not limited to Ketjen black, Super-P, acetylene black, carbon nanotubes, graphene, etc. In the present invention, Ketjen black is selected as the conductive carbon black.

[0035] The present invention also provides a method for preparing the above FeF3 - FeF2 heterojunction / conductive carbon black composite material, including: mixing choline chloride and polyol to obtain a deep eutectic solvent, then adding ferrous chloride tetrahydrate, auxiliary metal chlorides, fluorinating agents, and conductive carbon black, reacting at a temperature of 50 - 70 °C for 6 - 12 hours, and after washing, centrifuging, drying, and heat-treating the precipitate, the FeF3 - FeF2 heterojunction / conductive carbon black composite material is obtained.

[0036] In the process of preparing the above-mentioned FeF3-FeF2 heterojunction / conductive carbon black composite material, the auxiliary metal chloride includes but is not limited to cobalt chloride hexahydrate, nickel chloride hexahydrate, etc.; the fluorinating agent includes but is not limited to ammonium bifluoride, ammonium fluoride, etc. In the process of preparing the above-mentioned FeF3-FeF2 heterojunction / conductive carbon black composite material, the specific synthesis steps are as follows: Step (1), choline chloride and polyol (such as ethylene glycol) are mixed in a certain molar ratio (such as 1:3), and stirred at a heating state (such as 60 °C) for 4-6 hours to form a clear and transparent deep eutectic solvent. Step (2), take a certain amount of the above-mentioned deep eutectic solvent (such as 40-60 mL), add a certain amount of ferrous chloride tetrahydrate (such as: 5-10 mmol) and cobalt chloride hexahydrate (such as the molar ratio of ferrous chloride tetrahydrate to cobalt chloride hexahydrate is 1:1-8:1) and a certain mass of conductive carbon black (such as: 60-150 mg), and continue to stir under the above heating state until a uniform suspension is formed. Step (3), continue to add a certain amount of fluorinating agent (such as 2-3.5 g), and continue to stir for 6-12 hours for a sufficient fluorination reaction. Step (4), separate the mixture after the above fluorination reaction by centrifugation, recover the supernatant, and collect it after alternately washing the precipitate with ethanol and acetone for multiple times. Step (5), vacuum-dry the collected precipitate at 80 °C for 24 hours. Step (6), place the dried powder in a quartz crucible, transfer it to a tube furnace for heat treatment, in a nitrogen atmosphere, heat it to 150 °C at a heating rate of 3 °C per minute and hold for 5 hours, then continue to heat it to 300 °C at a heating rate of 5 °C per minute and hold for 2 hours, and then cool it naturally to obtain the FeF3-FeF2 heterojunction / conductive carbon black composite material.

[0037] In the FeF3-FeF2 heterojunction / conductive carbon black composite material prepared by the present invention, the FeF3-FeF2 heterojunction is composed of closely connected porous spherical particles with a particle size of 100-200 nm, and the mass fraction of conductive carbon black is 10-30%.

[0038] Preparation of FeF3-FeF2 heterojunction / conductive carbon black lithium-ion battery cathode material: Using the FeF3-FeF2 heterojunction / conductive carbon black composite material as the positive electrode active material, and mixing it with a binder (such as polyvinylidene fluoride PVDF, polytetrafluoroethylene PTFE, etc.) and a conductive agent (such as Super-P, acetylene black, etc.) in a certain mass ratio (such as 8:1:1) to prepare the FeF3-FeF2 heterojunction / conductive carbon black lithium-ion battery cathode material.

[0039] Based on the deep eutectic solvent and auxiliary metal chloride provided by the present invention, metal fluoride heterojunctions can be efficiently and controllably synthesized by the "one-pot method". The preparation process is convenient, and the use of highly corrosive NF3 / HF gas and expensive ionic liquids in conventional fluorination methods is avoided.

[0040] For the metal fluoride heterojunction prepared by the deep eutectic solvent and auxiliary metal chloride of the present invention, the tight hetero-contact between the metal fluoride phases can promote and stabilize the interfacial charge transfer, and improve the reversibility and kinetics of the metal fluoride electrochemical reaction.

[0041] The FeF3-FeF2 heterojunction / conductive carbon black composite cathode material for lithium-ion batteries prepared by the deep eutectic solvent and auxiliary metal chloride of the present invention can not only promote and stabilize the interfacial charge transfer and topological transformation reaction, but also combine the characteristics of the narrower band gap and more stable transformation reaction of FeF2 with the higher theoretical capacity and thermodynamic potential of FeF3, realizing the synergistic improvement of the specific capacity and cycle stability of the battery.

[0042] The following further exemplify embodiments to illustrate the present invention in detail. It should also be understood that the following embodiments are only used to further illustrate the present invention and cannot be construed as limiting the protection scope of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the above content of the present invention fall within the protection scope of the present invention. The specific process parameters and the like in the following examples are only an example within a suitable range, that is, those skilled in the art can make selections within a suitable range through the description herein, rather than being limited to the specific values in the following examples.

[0043] Example 1: Preparation of FeF3-FeF2 heterojunction / KB composite material by deep eutectic solvent method: (1) Weigh 25 g of choline chloride and measure 30 mL of ethylene glycol. After mixing the two, heat and stir at 60 °C until a clear and transparent deep eutectic solvent is formed; (2) Add 1.99 g of ferrous chloride tetrahydrate, 1.19 g of cobalt chloride hexahydrate, and 150 mg of Ketjenblack (KB). Maintain vigorous stirring under heating conditions until the metal chlorides are completely dissolved and a homogeneous suspension is formed; (3) Then add 3.42 g of ammonium bifluoride and continue stirring for 12 hours to ensure sufficient fluorination reaction; after waiting for the mixture to cool to room temperature, centrifuge at a speed of 6000 revolutions per minute, and alternately wash the precipitate with ethanol and acetone until the supernatant becomes colorless. Recover the supernatant and collect the precipitate; (4) Place the precipitate in an 80 °C oven and dry it under vacuum for 24 hours; (5) Then transfer the dried powder to a tube furnace for heat treatment. Under a nitrogen atmosphere, heat it to 150 °C at a heating rate of 3 °C per minute and hold for 5 hours. Subsequently, continue to heat it to 300 °C at a heating rate of 5 °C per minute and hold for 2 hours. After natural cooling, the FeF3-FeF2 heterojunction / KB composite material can be obtained. The content of Ketjen black in the obtained FeF3-FeF2 heterojunction / KB composite material is 30%.

[0044] The XRD diffraction pattern and the corresponding Rietveld refinement results of the FeF3-FeF2 heterojunction / KB composite material obtained in Example 1 are as shown in the appendix Figure 1 As shown, in this iron fluoride heterojunction, the mass fractions of hexagonal tungsten bronze structure FeF3 and rutile structure FeF2 are 79% and 21% respectively, and the corresponding molar fractions are 76% and 24% respectively, indicating that more than three-quarters of Fe 2+ is oxidized to Fe 3+ . In addition, no cobalt fluoride phase is matched in the XRD diffraction pattern, indicating that cobalt ions only act as an oxidation assistant and do not participate in the precipitation of fluorides. The SEM image is as shown in the appendix Figure 2 As shown, the FeF3-FeF2 heterojunction particles exhibit a porous spherical morphology with closely connected particles. The particle size is mostly between 100-200 nm and is embedded in the KB carbon particles. The TEM image is as shown in the appendix Figure 3 As shown, the contact boundary between the FeF3 and FeF2 phases and the characteristic crystal planes such as (002), (020), (110) belonging to the hexagonal tungsten bronze structure FeF3 and the (110) characteristic crystal plane belonging to the rutile structure FeF2 can be distinguished, which further verifies the successful preparation of the FeF3-FeF2 heterojunction.

[0045] Example 2: Preparation, battery assembly and testing of the iron fluoride cathode material electrode: 1) Electrode preparation: The FeF3-FeF2 heterojunction / KB, conductive agent Super-P, and binder polyvinylidene fluoride (PVDF) prepared in Example 1 are uniformly mixed by grinding in a mass ratio of 8:1:1, and an appropriate amount of N-methylpyrrolidone (NMP) is added dropwise to make a uniform slurry. Then it is coated on a clean aluminum foil current collector, and after natural drying, it is transferred to a vacuum oven at 80 °C and dried for 12 hours. The aluminum foil loaded with iron oxyfluoride after drying is cut into circular pieces with a diameter of 8 mm, which is used as the cathode material electrode.

[0046] 2) Preparation of electrolyte: In a glove box under an argon atmosphere with both the water value and oxygen value less than 0.1 ppm, 574.2 mg of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was weighed and added to a mixed solvent containing 1 mL of 1,3-dioxolane (DOL) and 1 mL of dimethoxyethane (DME) (the corresponding concentration of the solute was 1.0 mol / L). It was continuously stirred at room temperature for 24 hours until the white powder was completely dissolved to obtain the electrolyte (LiTFSI / DOL-DME).

[0047] 3) Assembly of the battery: In a glove box under an argon atmosphere with both the water value and oxygen value less than 0.1 ppm, a CR2025 coin-type battery was assembled for electrochemical performance testing. Among them, the positive electrode was the above-prepared FeF3-FeF2 heterojunction / KB positive electrode sheet, the negative electrode was a lithium metal sheet, the electrolyte was the above-prepared electrolyte, and the separator was Celgard 2400.

[0048] 4) Cyclic voltammetry test of the battery: To analyze the electrochemical reaction potential of the iron fluoride heterojunction positive electrode, a cyclic voltammetry test was performed on a battery assembled with FeF3-FeF2 heterojunction / KB as the positive electrode material and LiTFSI / DOL-DME as the electrolyte. The scan rate was set at 0.2 mV / s, and the voltage range was 1.2 - 4.0 V. The results are as follows Figure 4 shown. The reduction peak prominent near 2.75 V in the first-cycle curve verified that FeF3 dominated in the iron fluoride heterojunction. The appearance of a second reduction peak near 1.8 V during the continued lithiation reaction indicated a conversion reaction. In the subsequent several cycles, the reduction peaks corresponding to the iron fluoride insertion and conversion reactions were respectively shifted to higher voltage positions of 3.0 V and 2.0 V.

[0049] 5) Charge-discharge test of the battery: To test the electrochemical performance of the iron fluoride heterojunction positive electrode synthesized in Example 1, a battery assembled with FeF3-FeF2 heterojunction / KB as the positive electrode material and LiTFSI / DOL-DME as the electrolyte was placed on a BlueTEC CT2001A for constant-current charge-discharge testing. The current density was between 100 - 1000 mA / g, and the charge-discharge voltage range was 1.2 - 4.0 V. The obtained electrochemical performance is as follows Figures 5 to 8 shown. As shown Figure 5 in the figure, at a current density of 100 mA / g, the discharge specific capacity of the FeF3-FeF2 heterojunction / KB positive electrode was as high as 520 mAh / g within 10 cycles. Even after 200 cycles, it could still remain at 305 mAh / g. As shown Figure 6As shown, compared with the pure-phase hexagonal tungsten bronze structure FeF3 cathode and rutile structure FeF2 cathode prepared by the deep eutectic solvent method provided by the present invention, the FeF3-FeF2 heterojunction / KB cathode exhibits excellent comprehensive performance of high specific capacity and stable reversible cycling, indicating that the iron fluoride heterojunction cathode combines the performance characteristics of high specific capacity of FeF3 and stable cycling of FeF2. In addition, as shown in the appendix Figure 7 As shown, the FeF3-FeF2 heterojunction / KB cathode also exhibits high-rate charge-discharge capabilities. Even at a high current density of 1000 mA / g, the discharge specific capacity can still reach as high as 328 mAh / g.

[0050] As shown in the appendix Figure 8 When the loading of the cathode active material is increased to 5 mg / cm 2 , the FeF3-FeF2 heterojunction / KB cathode can still achieve stable cycling for more than 150 cycles. The above electrochemical performance test results demonstrate the significant improvement effect of the heterojunction structure on the reversibility and kinetics of the conversion reaction of the iron fluoride cathode.

[0051] Example 3: Preparation of FeF3-FeF2 Heterojunction Particles by Deep Eutectic Solvent Method (1) Weigh 25 g of choline chloride and measure 30 mL of ethylene glycol. After mixing the two, heat and stir at 60 °C until a clear and transparent deep eutectic solvent is formed; (2) Add 1.99 g of ferrous chloride tetrahydrate and 1.19 g of cobalt chloride hexahydrate, and maintain vigorous stirring under heating conditions until the metal chlorides are completely dissolved and a homogeneous suspension is formed; (3) Then add 3.42 g of ammonium bifluoride and continue stirring for 12 hours to ensure sufficient fluorination reaction; after waiting for the mixture to cool to room temperature, perform centrifugal separation at a speed of 6000 revolutions per minute, and wash the precipitate alternately with ethanol and acetone until the supernatant becomes colorless. Recover the supernatant and collect the precipitate; (4) Place the precipitate in an 80 °C oven and dry it under vacuum for 24 hours; (5) Then transfer the dried powder to a tube furnace for heat treatment. Under a nitrogen atmosphere, heat it to 150 °C at a heating rate of 3 °C per minute and hold for 5 hours, and then continue to heat it to 300 °C at a heating rate of 5 °C per minute and hold for 2 hours. After natural cooling, FeF3-FeF2 heterojunction particles can be obtained. In these iron fluoride heterojunction particles, the mass fractions of hexagonal tungsten bronze structure FeF3 and rutile structure FeF2 are 90% and 10% respectively, and the corresponding molar fractions are 88% and 12% respectively, indicating that 88% of Fe 2+ is oxidized to Fe 3+ .

[0052] Example 4 Using the FeF3-FeF2 heterojunction particles prepared in Example 3, the preparation, battery assembly and testing of the iron fluoride cathode material electrode were carried out, referring to Example 2. The discharge specific capacity of the obtained battery after 100 cycles at a current density of 100 mA / g has decayed to 261 mAh / g, which is significantly lower than the discharge specific capacity value (404 mAh / g) of the FeF3-FeF2 heterojunction / KB cathode in Example 2.

[0053] The preparation process of the FeF3 / KB particles in Comparative Example 1 includes: (1) Weigh 25 g of choline chloride and measure 30 mL of ethylene glycol. After mixing the two, heat and stir at 60 °C until a clear and transparent deep eutectic solvent is formed; (2) Add 2.70 g of ferric chloride hexahydrate and maintain vigorous stirring under heating conditions until the metal chloride is completely dissolved and a homogeneous suspension is formed; (3) Then add 3.42 g of ammonium bifluoride and continue stirring for 12 hours to ensure a sufficient fluorination reaction; after waiting for the mixture to cool to room temperature, centrifuge at a speed of 6000 revolutions per minute, and alternately wash the precipitate with ethanol and acetone until the supernatant becomes colorless. Recover the supernatant and collect the precipitate; (4) Place the precipitate in an 80 °C oven and dry it under vacuum for 24 hours; (5) Then transfer the dried powder to a tube furnace for heat treatment. Under a nitrogen atmosphere, heat it to 150 °C at a heating rate of 3 °C per minute and hold for 5 hours, and then continue to heat it to 300 °C at a heating rate of 5 °C per minute and hold for 2 hours. After natural cooling, the FeF3 / KB particles can be obtained.

[0054] Using the FeF3 / KB particles prepared in this Comparative Example 1, the preparation, battery assembly and testing of the iron fluoride cathode material electrode were carried out, referring to Example 2.

[0055] The preparation process of the FeF2 / KB particles in Comparative Example 2 includes: (1) Weigh 25 g of choline chloride and measure 30 mL of ethylene glycol. After mixing the two, heat and stir at 60 °C until a clear and transparent deep eutectic solvent is formed; (2) Add 1.99 g of ferrous chloride tetrahydrate and maintain vigorous stirring under heating conditions until the metal chloride is completely dissolved and a homogeneous suspension is formed; (3) Then add 2.28 g of ammonium bifluoride and continue stirring for 12 hours to ensure a sufficient fluorination reaction; after waiting for the mixture to cool to room temperature, centrifuge at a speed of 6000 revolutions per minute, and alternately wash the precipitate with ethanol and acetone until the supernatant becomes colorless. Recover the supernatant and collect the precipitate; (4) Place the precipitate in an oven at 80 °C for vacuum drying for 24 hours; (5) Then transfer the dried powder to a tube furnace for heat treatment. Under a nitrogen atmosphere, heat it to 150 °C at a heating rate of 3 °C per minute and hold for 5 hours. Subsequently, continue to heat it to 300 °C at a heating rate of 5 °C per minute and hold for 2 hours. After natural cooling, FeF2 / KB particles can be obtained.

[0056] Use the FeF2 / KB particles prepared in Comparative Example 2 to prepare, assemble and test the electrodes of the iron fluoride cathode material. Refer to Example 2.

[0057] Finally, it is necessary to state here that the above embodiments are only used to further illustrate the technical solutions of the present invention and should not be construed as limiting the protection scope of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the above content of the present invention fall within the protection scope of the present invention.

Claims

1. A method for preparing metal fluoride heterojunction particles, characterized in that, Comprising: (1) Adding a metal chloride salt, an auxiliary metal chloride salt, and a fluorinating agent to a deep eutectic solvent, and then reacting at 50 - 70 °C for 6 - 12 hours to obtain a precipitate; (2) Washing, centrifuging, drying, and heat-treating the obtained precipitate to obtain the metal fluoride heterojunction particles.

2. A method for preparing a cathode material of a lithium-ion battery, characterized in that, Comprising: (1) Adding a metal chloride salt, an auxiliary metal chloride salt, conductive carbon black, and a fluorinating agent to a deep eutectic solvent, and then reacting at a temperature of 50 - 70 °C for 6 - 12 hours to obtain a precipitate; (2) Washing, centrifuging, drying, and heat-treating the obtained precipitate to obtain the positive electrode material for a lithium-ion battery.

3. The preparation method according to claim 1 or 2, characterized in that, The deep eutectic solvent comprises: a mixture of choline chloride and a polyol; the polyol is selected from at least one of ethylene glycol, polyethylene glycol, glycerol, and tetraethylene glycol; the molar ratio of choline chloride to ethylene glycol is 1:(1 - 4).

4. The preparation method according to claim 1 or 2, characterized in that, The metal chloride salt is selected from at least one of ferrous chloride tetrahydrate, ferric chloride hexahydrate, cobalt chloride hexahydrate, nickel chloride hexahydrate, and copper chloride dihydrate; the mass ratio of the metal chloride salt to the deep eutectic solvent is 1:(10 - 50).

5. The preparation method according to claim 1 or 2, characterized in that, The auxiliary metal chloride salt is selected from at least one of cobalt chloride hexahydrate and nickel chloride hexahydrate, and the composition of the auxiliary metal chloride salt is different from that of the metal chloride salt; The molar ratio of the metal chloride salt to the auxiliary metal chloride salt is (8 - 1):

1.

6. The preparation method according to claim 1 or 2, characterized in that, The fluorinating agent is selected from at least one of ammonium hydrogen fluoride and ammonium fluoride; the molar ratio of the fluorinating agent to the metal chloride salt is (2 - 10):1; The temperature of the heat treatment is 250 - 350 °C, the time is 2 - 5 hours, and the atmosphere is an inert atmosphere or a nitrogen atmosphere; preferably, first heating to 125 - 150 °C and holding for 3 - 5 hours, and then heating to 250 - 350 °C and holding for 2 - 5 hours.

7. The preparation method according to claim 2, characterized in that, The conductive carbon black is selected from at least one of Ketjen black, Super-P, acetylene black, carbon nanotubes, and graphene; The mass ratio of the conductive carbon black to the metal chloride salt is (0.02 - 0.1):

1.

8. A metal fluoride heterojunction particle prepared by the preparation method according to claim 1, characterized in that, The metal fluoride heterojunction particles exhibit a porous spherical morphology with closely connected particles, and there is close contact between the metal fluoride heterostructures; Preferably, the metal fluoride heterojunction particles are composed of closely connected porous spherical particles with a particle size of 100 - 200 nm.

9. The metal fluoride heterojunction particles according to claim 8, wherein the metal fluoride heterojunction particles are composed of multiple-phase fluorides, preferably biphasic iron fluoride of hexagonal tungsten bronze structure FeF3 and rutile structure FeF2, and more preferably the molar ratio of hexagonal tungsten bronze structure FeF3 to rutile structure FeF2 in the biphasic iron fluoride is (1 - 9):1, preferably (1 - 4):

1.

10. Use of the metal fluoride heterojunction particles according to claim 8 in the preparation of a positive electrode material for a lithium-ion battery.

11. Use of the metal fluoride heterojunction particles according to claim 8 in a lithium-ion battery.

12. A lithium-ion battery cathode material prepared by the preparation method according to claim 2, characterized in that, Comprising: Metal fluoride heterojunction particles, and conductive carbon black loaded on the metal fluoride heterojunction particles; preferably, the mass percentage content of the conductive carbon black is 10 - 30%.

13. The lithium-ion battery cathode material according to claim 12, wherein the metal fluoride heterojunction particles are composed of multi-phase fluorides, preferably biphasic iron fluoride of hexagonal tungsten bronze structure FeF3 and rutile structure FeF2, and more preferably the molar ratio of hexagonal tungsten bronze structure FeF3 to rutile structure FeF2 in the biphasic iron fluoride is (1-9):1, preferably (1-4):

1.

14. An application of the lithium-ion battery cathode material according to claim 12 in a lithium-ion battery.

Images