Iron oxyfluoride positive electrode material and preparation method and application thereof

Abstract

The present application relates to a kind of ferrum oxyfluoride positive electrode material and its preparation method and application.The active chemical composition of the ferrum oxyfluoride positive electrode material includes FeO x F 2‑x ,0< x ≤1, and FeO x F 2‑x It is rutile structure;Preferably, the active composition of the ferrum oxyfluoride positive electrode material includes main phase FeO x F 2‑x And second phase FeF3· n H2O,0< x ≤1, n =0.33 or 0, wherein main phase FeO x F 2‑x Molar content of fluoride is 65-90%.

Description

TECHNICAL FIELD

[0001] The present application relates to a ferric oxyfluoride positive electrode material and a preparation method and application thereof, in particular to a high specific capacity and high energy density ferric oxyfluoride positive electrode material based on a conversion reaction and a preparation method and application thereof, and belongs to the technical field of new energy. BACKGROUND

[0002] Secondary batteries, as a kind of "green" energy storage technology, have played an important role in the market of wearable and portable electronic devices. However, with the increasing demand for energy storage, the current commercial lithium-ion batteries are still difficult to meet the performance requirements of long-lasting and large-scale energy storage devices such as electric vehicles and smart grids. For the secondary battery system, the development of high specific capacity and high energy density positive electrode materials is of great significance to break through the bottleneck of battery energy storage.

[0003] The positive electrode materials based on the intercalation reaction mechanism (including lithium cobaltate, lithium iron phosphate and nickel-cobalt-manganese-based ternary positive electrode materials, etc.) have a theoretical specific capacity and energy density of less than 275 mAh / g and 750 Wh / kg due to the limitation of single electron transfer. The positive electrode materials based on the conversion reaction mechanism can have a theoretical specific capacity and energy density close to or exceeding 1000 mAh / g and 2000 Wh / kg through multi-electron transfer. Among them, oxygen (O2) and sulfur (S) positive electrodes are of great concern due to their extremely high theoretical specific capacity. However, for such molecular-type positive electrodes, the problem of reaction confinement hinders their further development. The inefficient reaction confinement can cause loss of active materials and trigger adverse side reactions related to electrolyte or metal negative electrode. In addition, the loose configuration of molecular-type positive electrodes also reduces the performance of the battery in terms of volume quantization.

[0004] Metal fluorides with compact structure as another type of conversion positive electrode material can promote the increase of reaction potential due to the strongest electronegativity of fluorine element, and in combination with the large specific capacity obtained by conversion reaction, the fluorine positive electrode can provide high energy density (especially volume energy density) that can compete with O2 or S positive electrode. For example, the economic and environmentally friendly iron trifluoride (FeF3) can provide an extremely high energy density of 1947 Wh / kg based on the theoretical specific capacity of 712 mAh / g and the thermodynamic potential of ~2.73 V generated by the three-electron conversion reaction, and thus is considered as one of the most potential positive electrode candidates.

[0005] However, the low intrinsic electronic conductivity of iron fluoride cathode and the sluggish kinetics involved in the repeated cleavage / reconstruction of metal-fluorine bond during its multiphase conversion reaction, easily lead to rapid decay of reversible capacity, increased charge-discharge polarization and low energy efficiency (<60%). The conventional strategies of particle nanocrystallization and conductive network modification, although to some extent optimize the ion and electron transport pathways, do not essentially solve the problem of sluggish kinetics of iron-based fluoride conversion reaction, and reduce the overall energy storage efficiency of the battery.

[0006] For lithium-driven fluoride conversion reaction system, the heterogeneous deposition and coverage of passivation phase lithium fluoride on the electrode surface will hinder the chemical reaction between the internal active fluoride and lithium ions, and thus cause serious voltage polarization and low capacity release. Therefore, the reasonable spatial distribution of reactant phase and the activation of passivation phase lithium fluoride are the key factors to realize the efficient activation of conversion-type fluoride. SUMMARY

[0007] In view of the above problems, the purpose of the present application is to provide an iron-oxygen-fluoride cathode material with high specific capacity and high energy density based on conversion reaction and its preparation method and application.

[0008] In the first aspect, the present application provides an iron-oxygen-fluoride cathode material, the active chemical composition of which comprises FeO x F 2-x , 0 x F 2-x is a rutile structure.

[0009] In the present application, the iron fluoride trihydrate FeF3·3H2O is subjected to thermal phase transition and oxygen doping by heat treatment method, and the iron-oxygen-fluoride FeO x F 2-x with rutile structure is synthesized. The introduction of lattice oxygen not only can serve as an internal electron transport hub to improve the intrinsic electronic conductivity of fluoride, but also can regulate the phase conversion path, introduce a stable rock salt structure secondary parent phase in a limited voltage range and reduce the precipitation of passivation phase lithium fluoride. In addition, the intrinsic structure self-doping is beneficial to the uniform distribution of oxygen elements in the mixed anion structure, and thus effectively reduces the lithium ion diffusion barrier of anisotropic channels in the rutile structure framework. Therefore, the intrinsic lattice oxygen doping can potentially improve the conversion reaction reversibility of fluoride.

[0010] Preferably, the active composition of the iron-oxygen-fluoride cathode material comprises a main phase FeO x F 2-x and a second phase FeF3·nH2O, 0 x F 2-xThe molar content of fluoride is 65-90%. The composition of the iron oxyfluoride positive electrode material includes only the main phase FeO x F 2-x and the second phase FeF3·nH2O. FeF3·nH2O is a second phase existing in addition to the main phase FeO x F 2-x when the iron oxyfluoride is prepared by the method of the present application. According to the phase transition rule under given preparation conditions, the second phase can be FeF3·0.33H2O (n=0.33) or FeF3 (n=0). The electrochemical performance mainly depends on the main phase FeO x F 2-x , and the performance of the second phase has relatively small influence. The FeF3·nH2O is a conversion phase of hydrated iron fluoride that does not undergo oxygen doping during heat treatment. When n=0.33, FeF3·0.33H2O is hexagonal tungsten bronze structure iron fluoride; when n=0, FeF3 is rhombohedron structure iron fluoride.

[0011] Preferably, the active chemical composition of the iron oxyfluoride positive electrode material can be FeO 0.32 F 1.68 / FeF3·0.33H2O (wherein the molar content of the main phase FeO 0.32 F 1.68 is 68.55%) or FeO 0.77 F 1.23 / FeF3 (wherein the molar content of the main phase FeO 0.77 F 1.23 is 85.51%).

[0012] Preferably, the iron oxyfluoride positive electrode material further comprises conductive carbon black as an additional conductive modification network, and the content of the conductive carbon black is 10-25 wt% of the total mass of the conductive carbon black and the iron oxyfluoride positive electrode material. In terms of only the cycle / rate performance of the iron oxyfluoride positive electrode, the higher the proportion of the conductive carbon black, the better; but too high a proportion of non-active substances will reduce the actual energy density of the battery, but a certain proportion of conductive carbon black is still needed to compensate for the low electrical conductivity of the iron oxyfluoride, and therefore, 10-25 wt% is a relatively reasonable proportion range. If the comprehensive performance is considered, 25% is preferred. At this time, the iron oxyfluoride positive electrode material is essentially a composite material of FeO x F 2-x / FeF3·nH2O and conductive carbon black.

[0013] Preferably, the particle size of the iron oxyfluoride positive electrode material is 50-200 nm.

[0014] In the second aspect, the application further provides a preparation method of the ferric oxyfluoride cathode material, which comprises the following steps: placing the ferric fluoride trihydrate / conductive carbon black composite precursor in an inert protective atmosphere, and keeping the temperature at 250-350 DEG C for 2-5 hours to obtain the ferric oxyfluoride cathode material.

[0015] The application directly synthesizes the rutile-type ferric oxyfluoride from ferric fluoride trihydrate by using a heat treatment method for the first time, and the oxygen self-doping process is beneficial to the uniform distribution of chemical components and the optimization of intrinsic electron and ion transmission paths. In addition, the process flow does not need other auxiliary reagents, and the operation is simple and convenient, and is expected to realize mass production.

[0016] Preferably, the ferric fluoride trihydrate is alpha-crystal ferric fluoride trihydrate or / and beta-crystal ferric fluoride trihydrate; and the inert protective atmosphere is nitrogen or / and argon atmosphere. In the application, the water molecules in the intrinsic structure of the ferric fluoride trihydrate precursor can be used as an internal oxygen source, and through continuous heat-induced hydroxylation / dehydroxylation, more homogeneous and controllable oxygen self-doping can be realized.

[0017] Preferably, the ferric fluoride trihydrate / conductive carbon black composite precursor is synthesized by using an ion liquid-based dissolution-precipitation method, which comprises the following steps: the conductive carbon black is added into an ion liquid containing a tetrafluoroborate salt in advance and uniformly mixed, then a hydrated iron salt is added, stirring is carried out at 0-10 DEG C for 6-12 hours, and then washing, centrifugation and drying are carried out to obtain the ferric fluoride trihydrate / conductive carbon black composite precursor; preferably, the mass ratio of the conductive carbon black to the hydrated iron salt is 1:(15-40). The conductive carbon black is in-situ wrapped on the surface of the ferric fluoride trihydrate, which not only provides a reduction environment to promote the phase transition of the ferric fluoride trihydrate to the ferric oxyfluoride during the subsequent heat treatment, but also can be used as a surface conductive modification network of the fluoride to further improve the electron transmission of the cathode. In the application, the ion liquid is used as a fluorine source, which has excellent thermal stability (the decomposition temperature is above 400 DEG C) and is more environmentally friendly and safe, and in addition, can be used as a fluoride surface modification layer to inhibit the combination and growth of fluoride particles during the synthesis process. In the application, the conductive carbon black is an inorganic carbon source, which is a high-purity conductive carbon material itself and remains stable in the complete synthesis route, so that the carbon content in the ferric oxyfluoride product is more easily controlled. In the application, the conductive carbon black as a carbon source and the ion liquid surface modification layer as a fluorine source can both provide a reduction environment during the heat treatment process. In the application, the ion liquid-based dissolution-precipitation fluorination method is carried out at a temperature close to room temperature (0-10 DEG C), and the reaction process is more controllable and easy to mass production.

[0018] Preferably, the hydrated iron salt is at least one of ferric nitrate nonahydrate or ferric chloride hexahydrate.

[0019] In order to further realize the efficient reversible conversion reaction of the iron oxyfluoride positive electrode, the present invention also provides a functional ether electrolyte that can promote the dissociation of the passivation phase lithium fluoride and construct a solid-liquid fluorine transmission path, including a lithium salt (for example, at least one of lithium bis(trifluoromethanesulfonyl imide) (LiTFSI), lithium bis(fluorosulfonyl imide) (LiFSI), lithium hexafluorophosphate (LiPF6) and lithium difluorooxalatoborate (LiDFOB)) and an additive (for example, at least one of tris(pentafluorophenyl)borane (TPFPB) and lithium fluoride (LiF)).

[0020] Preferably, the solute of the functional ether electrolyte includes lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), tris(pentafluorophenyl)borane (TPFPB) and lithium fluoride (LiF).

[0021] Preferably, the concentration of the lithium salt in the functional ether electrolyte is 0.5-1 mol / L; the concentrations of the tris(pentafluorophenyl)borane and lithium fluoride additives are 0.01-0.05 mol / L and 0.01-0.5 mol / L, respectively; preferably, the concentration ratio of the tris(pentafluorophenyl)borane to the lithium salt is (0.01-0.05):1. Preferably, when the lithium salt is selected from lithium bis(trifluoromethanesulfonyl imide) and lithium fluoride, the concentration of the lithium bis(trifluoromethanesulfonyl imide is 0.5-1.0 mol / L, the concentration of tris(pentafluorophenyl)borane is 0.01-0.05 mol / L, and the concentration of the lithium fluoride is 0.01-0.5 mol / L. For example, when preparing the functional ether electrolyte, the added concentrations of lithium bis(trifluoromethanesulfonyl)imide, tris(pentafluorophenyl)borane and lithium fluoride are 0.5-1.0 mol / L, 0.01-0.05 mol / L and 0.01-0.5 mol / L, respectively, preferably 1.0 mol / L, 0.05 mol / L and 0.5 mol / L, respectively. In the present invention, lithium fluoride is an additive used in combination with tris(pentafluorophenyl)borane. Although the concentration of lithium fluoride added when preparing the electrolyte is high (0.5 M), lithium fluoride is almost insoluble in pure ether solvents. The present invention promotes the dissolution of lithium fluoride by means of a fluorophilic acceptor, tris(pentafluorophenyl)borane. Even so, the final solubility of lithium fluoride is only about 0.05 M (the undissolved lithium fluoride will be filtered after the electrolyte is stirred).

[0022] Preferably, the solvent of the functional ether electrolyte is at least one of ethylene glycol dimethyl ether (DME), diethylene glycol dimethyl ether (DGM), triethylene glycol dimethyl ether (TGM), and tetraethylene glycol dimethyl ether (TEGDME), preferably ethylene glycol dimethyl ether.

[0023] The application discloses a functional ether electrolyte capable of optimizing the fluorine transmission mechanism of a conversion-type iron oxyfluoride positive electrode, by adding tris(pentafluorophenyl)borane and lithium fluoride.

[0024] In a third aspect, the application further provides a battery, comprising: a positive electrode containing an iron oxyfluoride positive electrode material, or / and a functional ether electrolyte, and a negative electrode.

[0025] Preferably, the concentration of lithium salt in the functional ether electrolyte is 0.5-1 mol / L, and the concentration of tris(pentafluorophenyl)borane is 0.01-0.05 mol / L.

[0026] Preferably, the concentration ratio of tris(pentafluorophenyl)borane to lithium salt is (0.01-0.05):1.

[0027] Preferably, when the lithium salt is selected from lithium bis(trifluoromethanesulfonyl)imide and lithium fluoride, the concentration of lithium bis(trifluoromethanesulfonyl)imide is 0.5-1.0 mol / L, the concentration of tris(pentafluorophenyl)borane is 0.01-0.05 mol / L, and the concentration of lithium fluoride is 0.01-0.5 mol / L.

[0028] The application combines fluorine-based structure regulation and electrolyte component design to improve the conversion reaction of iron oxyfluoride positive electrode materials, and has the following positive and beneficial effects:

[0029] (1) In the application, rutile-type iron oxyfluoride is synthesized by a low-temperature heat treatment method for the first time, which is economic, environmentally friendly, more convenient to operate and stronger in controllability. The introduction of oxygen components into the fluorine-based structure framework can not only improve the intrinsic conductivity of the fluorides, but also regulate the phase conversion path, introduce a stable rock salt structure secondary parent phase in the conversion reaction process, and promote the reversible reconstruction of the fluorine-based structure framework.

[0030] (2) The synthetic process is not only simple and convenient, but also the intrinsic structure of oxygen self-doping can promote the uniform distribution of lattice oxygen compared with the external oxygen source penetration, which is beneficial to the electron and ion transmission of the fluorine-based structure.

[0031] (3) In the present application, when the functional ether-based electrolyte containing tris(pentafluorophenyl)borane is used for the conversion-type iron-oxygen-fluoride positive electrode, the dissociation of the passivated lithium fluoride phase can be promoted and a convenient solid-liquid fluorine transmission path can be constructed between the discharge product lithium fluoride and the iron-based phase, so as to reduce the overpotential of the conversion reaction in the charging process and promote the reversible reconstruction of the iron-oxygen-fluoride structure. In addition, in this electrolyte, a conformal and fluorinated enhanced positive electrode / electrolyte interface layer is derived in situ on the surface of the active grain, which effectively inhibits the dissolution of the active material and the related adverse interfacial reaction, and adjusts the structural stress generated in the lithiation / delithiation process, further improving the electrochemical stability and mechanical stability of the positive electrode.

[0032] (4) In the present application, thanks to the fine regulation of the fluorine-based structure and the reasonable design of the electrolyte composition, the iron-oxygen-fluoride positive electrode can perform convenient fluorine ion transmission and lithium ion deintercalation during charging and discharging, the two-stage intercalation / conversion lithiation platform can be maintained for a long time, and high energy efficiency is achieved to realize large reversible specific capacity and high positive electrode energy density. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 XRD pattern of the FeF3·3H2O / KB composite precursor synthesized in Example 1;

[0034] Figure 2 XRD pattern and Rietveld refinement results of the iron-oxygen-fluoride positive electrode material synthesized in Example 1 and Example 2, wherein a corresponds to FeO 0.32 F 1.68 / FeF3·0.33H2O / KB composite, b corresponds to FeO 0.77 F 1.23 / FeF3 / KB composite;

[0035] Figure 3 SEM images of the iron-oxygen-fluoride positive electrode material synthesized in Example 1 and Example 2, wherein a corresponds to FeO 0.32 F 1.68 / FeF3·0.33H2O / KB composite (scale 200 nm), b corresponds to FeO 0.77 F 1.23 / FeF3 / KB composite (scale 200 nm);

[0036] Figure 4 Photoelectron spectrograms of the two iron-oxygen-fluoride positive electrode materials synthesized in Example 1 and Example 2, wherein a is the Fe 2p sub-spectrum, and b is the O 1s sub-spectrum;

[0037] Figure 5 Li metal battery with FeO 0.32 F 1.68 Cyclic voltammograms of Li metal battery with FeO

[0038] Figure 6 Li metal battery with FeO 0.32 F 1.68 Galvanostatic charge-discharge curves of Li metal battery with FeO

[0039] Figure 7 Long cycle performance comparison of Li metal battery with FeO 0.32 F 1.68 F in LiTFSI-LiF-TPFPB / DME and LiTFSI / DOL-DME electrolyte systems, respectively;

[0040] Figure 8 Rate performance comparison of Li metal battery with FeO 0.32 F 1.68 F in LiTFSI-LiF-TPFPB / DME and LiTFSI / DOL-DME electrolyte systems, respectively;

[0041] Figure 9 Li metal battery with FeO 0.77 F 1.23 Galvanostatic charge-discharge curves of Li metal battery with FeO

[0042] Figure 10 Long cycle performance comparison of Li metal battery with FeO 0.77 F 1.23 F in LiTFSI-LiF-TPFPB / DME and LiTFSI / DOL-DME electrolyte systems, respectively;

[0043] Figure 11 Rate performance comparison of Li metal battery with FeO 0.77 F 1.23Rate performance comparison of Li metal batteries with / FeF3 / KB as cathode material in LiTFSI-LiF-TPFPB / DME and LiTFSI / DOL-DME electrolyte system respectively;

[0044] Figure 12 FeO 0.32 F 1.68 FeO 0.77 F 1.23 Energy density and energy efficiency comparison of Li metal batteries with / FeF3 / KB as cathode material in LiTFSI-LiF-TPFPB / DME electrolyte system;

[0045] Figure 13 FeO 0.32 F 1.68 FeO 0.77 F 1.23 Energy density vs. power density of Li metal batteries with / FeF3 / KB as cathode material in LiTFSI-LiF-TPFPB / DME electrolyte system;

[0046] Figure 14 FeO 0.32 F 1.68 First cycle constant current charge-discharge curves comparison of / FeF3·0.33H2O / KB cathode material in functional ether electrolyte containing different concentration of TPFPB / LiF additive;

[0047] Figure 15 FeO 0.32 F 1.68 Long cycle performance comparison of / FeF3·0.33H2O / KB cathode material in functional ether electrolyte containing different concentration of TPFPB / LiF additive. DETAILED DESCRIPTION

[0048] The present application is further illustrated by the following examples, which should not be construed as limiting the present application.

[0049] In the present application, a heat treatment method at a lower temperature is adopted to make iron fluoride trihydrate FeF3·3H2O undergo thermal induced phase transition and oxygen doping, and synthesize iron oxyfluoride (FeO x F 2-x / FeF3·nH2O, 0 x F 2-xThe host phase accounts for 65-90% of the molar content of the fluoride phase. The lattice oxygen acts as an internal electron transport hub to enhance the intrinsic electronic conductivity of the fluorine-based structure, and introduces a stable rock salt structure secondary parent phase in the conversion reaction process, avoiding the complete decomposition of the fluorine-based structure in a limited voltage range (1.2-4.0V), and thus significantly improving the conversion reaction reversibility. The iron oxyfluoride prepared by the low-temperature heat treatment synthesis method of the present application not only has a convenient process flow, but also avoids the use of additional solvents and harsh high-pressure conditions in wet chemical synthesis methods, as well as the high energy consumption of high-temperature (1000℃) high-pressure (65kbars) solid-phase fluorination reaction.

[0050] In an optional example, the iron oxyfluoride positive electrode material further comprises conductive carbon black. In essence, the iron oxyfluoride positive electrode material is an iron oxyfluoride / conductive carbon black composite material, and the conductive carbon black accounts for 10-25wt% of the composite positive electrode material.

[0051] In an optional example, the precursor for preparing the iron oxyfluoride positive electrode material is a trihydrate iron fluoride / conductive carbon black composite material, which can be synthesized based on an ionic liquid-based dissolution-precipitation fluorination method. The ionic liquid-based fluorination method avoids the use of unsafe gas fluorine sources (such as HF and F2). In addition, a small amount of ionic liquid left on the surface of the fluoride not only can provide a reduction environment together with the conductive carbon black to promote the thermal phase transition of trihydrate iron fluoride to iron oxyfluoride, but also can act as a surface toughness modification layer to adjust the structural stress generated during the lithiation / delithiation process of the fluoride.

[0052] Preparation of iron oxyfluoride / conductive carbon black composite material. Specifically, a trihydrate iron fluoride / conductive carbon black composite precursor is first synthesized, and then a phase transition is induced by heat treatment to obtain an iron oxyfluoride / conductive carbon black composite material. The following exemplary describes the preparation method of the iron oxyfluoride / conductive carbon black composite material provided by the present application.

[0053] A certain amount of conductive carbon black is added to an ionic liquid containing tetrafluoroborate salt, and the conductive carbon black is uniformly dispersed in the ionic liquid by stirring or ultrasonic treatment (e.g., stirring for 12 hours). A certain amount of hydrated iron salt is then added, and the mixture is stirred at 0-10°C for 6-12 hours to allow for complete fluorination and precipitation. The product is then washed repeatedly and centrifuged (e.g., washed 3-5 times with anhydrous acetone) and dried (e.g., vacuum dried at 80°C for 12 hours) to obtain a precursor of a fluorinated iron trihydrate / conductive carbon black composite. The π electrons of the carbon surface form strong interactions with the cationic groups (e.g., imidazole groups) of the ionic liquid. These interactions not only effectively disperse the conductive carbon black, but also promote the close adhesion of the ionic liquid-directed precipitated fluorinated iron to the conductive carbon black, enhancing the electrical contact between them. The ionic liquid not only acts as an adhesive between the fluorinated iron and the conductive carbon black, but also as an interfacial layer between the fluorinated iron particles, preventing the coarsening of the crystalline grains and effectively controlling the nanotopography of the fluorinated iron. The hydrated iron salt can be at least one of ferric nitrate nonahydrate or ferric chloride hexahydrate. The ionic liquid can be a low-temperature molten salt composed of a tetrafluoroborate anion and an organic cationic group, such as an imidazolium salt ion, a pyridinium salt ion, a quaternary ammonium salt ion, a quaternary phosphonium salt ion, etc.

[0054] The dried precursor of the fluorinated iron trihydrate / conductive carbon black composite is ground and placed in a ceramic crucible, which is then transferred to a tube furnace. After the introduction of an inert protective atmosphere (e.g., nitrogen) and the removal of air, the mixture is heat-treated at 300°C for 2-5 hours, followed by natural cooling, to obtain a ferrum oxyfluoride / conductive carbon black composite material. The powder product is ground to a fine and uniform state for physical property characterization and electrode sheet fabrication.

[0055] The application provides a functional ether electrolyte capable of improving the reversibility of conversion reaction of iron-oxygen-fluoride positive electrode. In optional examples, the functional ether electrolyte contains lithium bis-trifluoromethanesulfonimide, tris(pentafluorophenyl)borane, lithium fluoride and other solutes. The tris(pentafluorophenyl)borane in the functional ether electrolyte is added as a boron-based acceptor additive, which can promote the dissociation of lithium fluoride in the discharge product of iron-oxygen-fluoride positive electrode material, form a solvated fluoride ion intermediate at the phase interface between lithium fluoride and iron-based product phase, and further build a convenient solid-liquid fluoride transport path between lithium fluoride and iron-based product phase, so as to improve the reversibility of conversion reaction of iron-oxygen-fluoride positive electrode material. The lithium fluoride added during electrolyte preparation can pre-fluorinate the tris(pentafluorophenyl)borane to improve the electrochemical stability and weaken the tendency of decomposition of the tris(pentafluorophenyl)borane during charge-discharge cycle. The following exemplary preparation method of the functional ether electrolyte provided by the application is described. The key factor affecting the electrochemical performance of conversion-type iron-based fluoride is the phase conversion path and reaction kinetics. Based on this, the application develops a simple and effective thermal-induced oxygen self-doping method to synthesize iron-oxygen-fluoride to optimize the phase conversion path, and designs a functional ether electrolyte to promote the dissociation activation of LiF to upgrade the reaction kinetics in view of the slow reaction kinetics caused by the passivation property of conversion phase LiF, so as to significantly improve the cycle stability and rate performance of iron-oxygen-fluoride positive electrode.

[0056] In an inert atmosphere glove box, a certain volume of solvent is measured, and the solvent can be at least one of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether, preferably ethylene glycol dimethyl ether. According to the volume of the solvent, lithium bis-trifluoromethanesulfonimide, tris(pentafluorophenyl)borane and lithium fluoride are weighed and added into the solvent according to their respective concentrations, and then stirred vigorously for 12-24 hours, so that the lithium trifluoromethanesulfonimide is fully dissolved, and the tris(pentafluorophenyl)borane is fully dissolved and pre-fluorinated. Finally, the undissolved lithium fluoride powder is filtered to obtain a clear and transparent electrolyte. The addition concentrations of lithium bis-trifluoromethanesulfonimide, tris(pentafluorophenyl)borane and lithium fluoride are 0.5-1.0 mol / L, 0.01-0.05 mol / L and 0.01-0.5 mol / L, respectively, and the preferred concentrations are 1.0 mol / L, 0.05 mol / L and 0.5 mol / L, respectively. Preferably, the molar ratio of the concentrations of lithium bis-trifluoromethanesulfonimide, tris(pentafluorophenyl)borane and lithium fluoride is 1:(0.01-0.05):(0.01-0.5).

[0057] The application discloses a conversion type secondary battery which is assembled by an iron oxyfluoride positive electrode, a functional ether electrolyte containing tris(pentafluorophenyl)borane and a metal lithium negative electrode, and has a reversible specific capacity of about 500 mAh / g, an energy efficiency close to 80%, and an energy density of up to 1100 Wh / kg and 700 Wh / kg under power densities of 220 and 4300 W / kg, respectively. The iron oxyfluoride positive electrode used in the application is simple and safe to synthesize, and the functional ether electrolyte used in the application is suitable for the iron oxyfluoride positive electrode and promotes reversible conversion reconstruction of the iron oxyfluoride positive electrode. The conversion type lithium metal battery assembled by the iron oxyfluoride positive electrode and the functional ether electrolyte has a large specific capacity, a high energy efficiency, high energy density and high power density, and is expected to become a next-generation high-energy-density energy storage device.

[0058] The application will be further illustrated in detail by the following examples. It should be understood that the following examples are only used for further illustrating the application, and cannot be understood as limiting the protection scope of the application. Some non-essential improvements and adjustments made by those skilled in the art according to the above content of the application are within the protection scope of the application. The specific process parameters in the following examples are only one example in the suitable range, that is, those skilled in the art can select in the suitable range according to the description herein, and are not limited to the specific values in the following examples.

[0059] Example 1

[0060] 1) Preparation of FeF3·3H2O / KB precursor by ion liquid-based dissolution-precipitation method:

[0061] 10 mL of 1-butyl-3-methyl imidazole tetrafluoroborate (BmimBF4) ion liquid was measured and poured into a reaction bottle, and then 40 mg of conductive carbon black (KB) was weighed and added into the ion liquid, and the mixture was stirred at room temperature for 12 hours to obtain a black and uniform suspension. Then the reaction bottle was placed in an ice-water mixture and stirred for 30 minutes, and the temperature of the solution in the bottle was maintained at 0-10 DEG C. Then 1.0 g of Fe(NO3)3·9H2O was slowly added into the solution in the bottle under stirring, and the stirring was continued for 12 hours. During the stirring, ice cubes were added into the ice-water mixture from time to time to ensure that the temperature of the reaction environment was between 0-10 DEG C. After the reaction, the solution was washed with anhydrous acetone for 5 times and centrifuged, and then was placed in a vacuum oven at 80 DEG C for drying for 12 hours to obtain the FeF3·3H2O / KB precursor. The XRD diffraction pattern thereof is shown in FIG. 1, which shows characteristic diffraction peaks belonging to the alpha crystal type iron fluoride trihydrate. Figure 1

[0062] 2) Preparation of iron oxyfluoride FeO 0.32 F 1.68 / FeF3·0.33H2O / KB powder: ​

[0063] The FeF3·3H2O / KB precursor prepared in 1) was ground and placed in a ceramic crucible, then transferred to a tube furnace. High-purity nitrogen was introduced into the tube furnace to remove the air in the furnace tube. After ventilation for 2 hours, it was heated to 300°C at a heating rate of 5°C / min and kept at this temperature for 2 hours, and then naturally cooled. The ventilation state was maintained throughout the entire process. After cooling to room temperature, the iron oxyfluoride FeO was obtained. 0.32 F 1.68 / FeF3·0.33H2O / KB. The XRD diffraction pattern and the corresponding Rietveld refinement results are shown in the attached Figure 2 As shown in a, in this iron oxyfluoride, the rutile structure of FeO 0.32 F 1.68 The main phase (molar content is 68.55%), and a small amount of FeF3·0.33H2O with hexagonal tungsten bronze structure without oxygen doping is the second phase. Figure 3 As shown in Figure a, most of the fluoride particles have a square block morphology, with a particle size of approximately 50 to 100 nm, and are surrounded by spherical conductive carbon black particles. Figure 4 As shown, it is proved that lattice oxygen exists in the rutile iron oxyfluoride in the bonding state of O-Fe-F.

[0064] 3) FeO 0.32 F 1.68 Preparation of / FeF3·0.33H2O / KB electrode:

[0065] : The iron oxyfluoride FeO prepared in 2) 0.32 F 1.68 The FeF3·0.33H2O / KB, the conductive agent Super-P, and the binder polyvinylidene fluoride (PVDF) were uniformly mixed by grinding in a mass ratio of 8:1:1. An appropriate amount of N-methylpyrrolidone (NMP) was added dropwise to form a uniform slurry. The slurry was then coated onto a clean aluminum foil current collector. After air drying, the slurry was transferred to an 80°C vacuum oven and dried for 12 hours. The dried aluminum foil loaded with iron oxyfluoride was cut into 8mm diameter discs, which served as the positive electrode material.

[0066] 4) Preparation of functional ether and conventional ether electrolytes:

[0067] In an argon atmosphere glove box with water value and oxygen value less than 0.1 ppm, 574.2 mg of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 25.9 mg of lithium fluoride (LiF) and 51.2 mg of tris(pentafluorophenyl)borane (TPFPB) were weighed into 2 mL of ethylene glycol dimethyl ether (DME) solvent (the concentrations of the three solutes were 1.0 mol / L, 0.5 mol / L and 0.05 mol / L, respectively), and after continuous stirring at room temperature for 24 hours, the undissolved LiF powder was filtered to obtain a clear functional ether electrolyte containing tris(pentafluorophenyl)borane (LiTFSI-LiF-TPFPB / DME).

[0068] In an argon atmosphere glove box with water value and oxygen value less than 0.1 ppm, 574.2 mg of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was weighed into a mixed solvent containing 1 mL of 1,3-dioxolane (DOL) and 1 mL of ethylene glycol dimethyl ether (DME) (the concentration of the solute was 1.0 mol / L), and after continuous stirring at room temperature for 24 hours, the white powder was completely dissolved to obtain a conventional ether electrolyte (LiTFSI / DOL-DME).

[0069] 5) Based on FeO 0.32 F 1.68 Battery assembly and test of / FeF3·0.33H2O / KB positive electrode:

[0070] In an argon atmosphere glove box with water value and oxygen value less than 0.1 ppm, CR2025 button cells were assembled for electrochemical performance tests. The positive electrode was the above-mentioned iron-oxygen-fluoride electrode sheet, the negative electrode was a lithium metal sheet, the electrolyte was one of the electrolytes prepared in 4), and the separator was Celgard2400.

[0071] The cyclic voltammetry test of the battery was performed using an electrochemical workstation VersaSTAT3, with a scan rate of 0.1 mV / s and a voltage range of 1.2-4.0 V, and the results are shown in FIG. 3. Figure 5 Compared with the conventional electrolyte system, when the iron-oxygen-fluoride positive electrode was cycled in the LiTFSI-LiF-TPFPB / DME electrolyte, the oxidation peak shifted to a lower voltage, and the corresponding conversion reaction overpotential was reduced, which proved that the construction of a convenient solid-liquid fluorine transport path between the discharge product lithium fluoride and the iron-based phase could promote the structural reconstruction of the fluorine-based host during the charging process.

[0072] The constant current charge and discharge test of the battery was performed using a blue light CT2001A, with a current density of 100-2000 mA / g and a voltage range of 1.2-4.0 V, and the electrochemical performance of the batteries is shown in FIG. 4. Figures 6-8 Figure 6 ​and 7 The constant current charge-discharge curve and cycle stability diagram of the iron oxyfluoride cathode material at a current density of 100 mA / g are shown respectively. In the ether electrolyte system containing tris(pentafluorophenyl)borane, FeO 0.32 F 1.68 The / FeF3·0.33H2O / KB positive electrode shows a capacity recovery process during cycling, which is likely due to the construction of the solid-liquid fluorine transmission path, which reactivates the inactivated lithium fluoride accumulated in the previous cycles. This also contributes to the elevation and extension of the lithiation conversion platform from the 10th cycle to the 30th cycle. After 100 cycles, the discharge specific capacity can still be maintained at 472mAh / g, and its cycle stability is significantly improved compared to the performance in conventional ether electrolyte systems. As shown in the attached figure Figure 8 In the functional ether electrolyte system, FeO 0.32 F 1.68 / FeF3·0.33H2O / KB positive electrode exhibits more excellent rate performance, with discharge specific capacities of 470, 432, 389, 334 and 271 mAh / g at current densities of 200, 300, 500, 1000 and 2000 mA / g, respectively.

[0073] Example 2

[0074] 1) Iron oxyfluoride FeO 0.77 F 1.23 Preparation of / FeF3 / KB material:

[0075] The FeF3·3H2O / KB precursor prepared in Example 1 was ground and placed in a ceramic crucible, then transferred to a tube furnace. High-purity nitrogen was introduced into the tube furnace to remove the air in the furnace tube. After ventilation for 2 hours, it was heated to 300°C at a heating rate of 5°C / min and kept warm for 5 hours, and then naturally cooled. The ventilation state was maintained throughout the entire process. After cooling to room temperature, the iron oxyfluoride positive electrode material FeO 0.77 F 1.23 / FeF3 / KB. The XRD diffraction pattern and the corresponding Rietveld refinement results are shown in the attached Figure 2 As shown in b, after the heat treatment at 300 ° C is extended to 5 hours, the main phase in the iron oxyfluoride is rutile structure FeO with higher oxygen content. 0.77 F 1.23 , and the molar content of the main phase increased to 85.51%, and the secondary phase was transformed into a rhombohedral FeF3. Figure 3 As shown in Figure b, the fluoride particles present a more regular octahedral configuration, and the particle size also grows to between 100 and 200 nm. Figure 4 As shown in Figure 3, it is proved that the extension of heat treatment time leads to a deeper oxygen doping degree of iron oxyfluoride.

[0076] 2) FeO 0.77 F 1.23 Preparation of FeO

[0077] The iron oxyfluoride FeO 0.77 F 1.23 / FeF3 / KB was uniformly mixed with conductive agent Super-P and binder polyvinylidene fluoride (PVDF) by grinding at a mass ratio of 8:1:1, and a proper amount of N-methyl pyrrolidone (NMP) was added to make a uniform slurry. The slurry was then coated on a clean aluminum foil current collector, and after natural air drying, the aluminum foil was transferred to a vacuum oven at 80°C for drying for 12 hours. The dried aluminum foil loaded with iron oxyfluoride was cut into a round sheet with a diameter of 8 mm, which was used as the positive electrode material.

[0078] 3) FeO 0.77 F 1.23 Battery assembly and test of FeO

[0079] CR2025 button cells were assembled in an argon glovebox with water and oxygen values less than 0.1 ppm for electrochemical performance tests. The positive electrode was the iron oxyfluoride electrode sheet prepared in 2), the negative electrode was a lithium metal sheet, the electrolyte was one of the electrolytes prepared in Example 1, and the separator was Celgard 2400. The batteries were tested by constant current charge-discharge using a blue electric CT2001A, and the current density was between 100 and 2000 mA / g, and the charge-discharge voltage range was 1.2-4.0 V. The electrochemical performance of the batteries is shown in FIGS. Figures 9-11 Figure 9 and 10 are the constant current charge-discharge curves and cycle stability diagrams of the iron oxyfluoride positive electrode material at a current density of 100 mA / g. In the ether electrolyte system containing tris(pentafluorophenyl)borane, the FeO 0.77 F 1.23 / FeF3 / KB positive electrode exhibited a raised and prolonged two-stage lithiation platform from the 10th to the 50th cycle, and the discharge specific capacity could still be maintained at 484 mAh / g after 100 cycles. As shown in FIG. Figure 11 , the FeO 0.77 F 1.23 / FeF3 / KB positive electrode also has excellent high-rate performance, and the discharge specific capacity can still be maintained at 355 and 320 mAh / g at a large current density of 1000 and 2000 mA / g. Under the same test conditions, the electrochemical performance of the iron oxyfluoride positive electrode in the functional ether electrolyte system is better than that in the conventional ether electrolyte system.

[0080] ​Figure 1 Figure 12 Energy density cycling performance comparison of iron oxyfluoride cathode materials with different oxygen content in functional ether electrolyte system containing tris(pentafluorophenyl)borane. The energy density of FeO 0.32 F 1.68 The energy density of FeO 0.77 F 1.23 The energy density of FeO 0.32 F 1.68 The energy density of FeO 0.77 F 1.23 The energy density of FeO 0.32 Figure 2

[0081] Figure 3 Figure 13 Figure 4 Figure 5

[0082] Example 3

[0083] 1) Preparation of functional ether electrolyte:

[0084] Referring to the preparation method of functional ether electrolyte in Example 1, set the concentration of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as 1 moL / L, the concentration of tris(pentafluorophenyl)borane (TPFPB) as 0.05 moL / L, and the concentration of lithium fluoride (LiF) as 0, 0.05 moL / L, and 0.5 moL / L respectively, using ethylene glycol dimethyl ether (DME) as the solvent. After continuous vigorous stirring at room temperature for 24 hours, filter the undissolved LiF powder to obtain a series of clear functional ether electrolytes containing tris(pentafluorophenyl)borane, which are respectively recorded as 0.05M TPFPB, 0.05M TPFPB-0.05M LiF, and 0.05M TPFPB-0.5M LiF.

[0085] 2) Cell assembly and testing based on a series of functional ether electrolytes:

[0086] In an argon atmosphere glove box with water and oxygen values ​​less than 0.1 ppm, CR2025 button cells were assembled for electrochemical performance testing. The positive electrode was the FeO prepared in Example 1. 0.32 F 1.68 The electrodes were amorphous metal lithium sheets with a TPFPB / FeF3·0.33H2O / KB as the cathode. The electrolyte was one of the functional ether electrolytes prepared above containing different concentrations of TPFPB / LiF additives. The separator was Celgard 2400. The batteries were subjected to constant current charge and discharge tests using a Blue Power CT2001A at a current density of 100 mA / g and a charge and discharge voltage range of 1.2 to 4.0 V. The electrochemical performance of these batteries is shown in the attached figure. Figures 14-15 As shown. Figure 14 and 15 FeO 0.32 F 1.68 The first cycle constant current charge-discharge curves and cycle stability diagram of the / FeF3·0.33H2O / KB cathode material in functional ether electrolytes containing different concentrations of TPFPB / LiF additives. 0.32 F 1.68 The / FeF3·0.33H2O / KB cathode material exhibited similar lithiation / delithiation behavior in a range of functional ether electrolytes, but in cells with low concentrations (0 and 0.05M) of LiF additive, an additional tailing platform appeared below the discharge voltage of 1.5V. This platform is likely caused by the reductive decomposition of the strongly Lewis-acidic TPFPB without sufficient pre-fluorination by LiF. As the cycle progressed, although the additional electrolyte decomposition platform brought about an abnormal short-term increase in capacity, the battery then experienced rapid decay, accompanied by large fluctuations in Coulombic efficiency. In contrast, the cell using 0.05M TPFPB-0.5M LiF electrolyte showed excellent cycling stability and a stable Coulombic efficiency close to 100%. Therefore, the rational combination of TPFPB and LiF in the ether electrolyte system can effectively improve the electrochemical performance of the fluoride cathode and suppress the electrolyte decomposition problem. Referring to Example 3, the present invention sets different concentrations of tris(pentafluorophenyl)borane (TPFPB) and lithium fluoride (LiF) additives to optimize the electrochemical performance improvement effect of the functional ether electrolyte on the iron oxyfluoride positive electrode. Considering that the large molecular weight of TPFPB may increase the viscosity of the electrolyte to a certain extent and reduce the ion mobility, its concentration should not be too high and is set to 0.05M. In addition, the pre-fluorination effect of the LiF additive on TPFPB can inhibit the tendency of TPFPB to be reduced at low potentials due to its strong Lewis acidity.

[0087] The excellent electrochemical performance is attributed to the fine-tuning of fluorine-based structure and the rational design of electrolyte composition. Deepening the oxygen doping in fluorine-based structure, although reduces the initial discharge specific capacity of iron oxyfluoride cathode at 1.2 V cutoff voltage, the lattice oxygen not only can act as built-in electron conduction hub of fluorine-based structure, but also can regulate the phase transformation path to reduce the precipitation of passivation phase LiF and enhance the stability of oxygen-rich rock salt structure secondary parent phase. The higher crystallinity and more regular fluorine / oxygen structure arrangement obtained by prolonging the heat treatment time also contribute to the lithium ion diffusion and mechanical stability maintenance of the bulk lattice. These factors explain why the FeO 0.77 F 1.23 / FeF3 / KB cathode exhibits more excellent cycle stability and rate capability. In addition, the functional ether electrolyte containing tris(pentafluorophenyl)borane can provide a convenient solid-liquid fluorine transport path to assist the iron oxyfluoride cathode to carry out more durable and efficient conversion reaction.

[0088] Finally, it is necessary to point out that: the above examples are only used to further detail the technical solutions of the present application, and cannot be understood as a limitation on the protection scope of the present application. Some non-essential improvements and adjustments made by those skilled in the art based on the above content of the present application all belong to the protection scope of the present application.

Claims

1. A method for preparing an iron oxyfluoride positive electrode material, characterized in that: The active components of the iron oxyfluoride positive electrode material include the main phase FeO x F 2-x and the second phase FeF3· n H2O, 0< x ≤1, n =0.33 or 0, where FeO x F 2-x It has a rutile structure, and the fluoride in the composition of the iron oxyfluoride positive electrode material only includes the main phase FeO x F 2-x and the second phase FeF3·nH2O, the main phase FeO x F 2-x The molar content of fluoride is 65-90%; The preparation method of the iron oxyfluoride positive electrode material comprises the following steps: placing a ferric fluoride trihydrate / conductive carbon black composite precursor in an inert protective atmosphere and keeping the temperature at 250-350° C. for 2-5 hours to obtain the iron oxyfluoride positive electrode material; wherein, A ferric fluoride trihydrate / conductive carbon black composite precursor is synthesized by a dissolution-precipitation method based on an ionic liquid, comprising: pre-adding conductive carbon black to an ionic liquid containing tetrafluoroborate and uniformly mixing the mixture; then adding a hydrated iron salt; stirring the mixture at 0-10°C for 6-12 hours; and then washing, centrifuging, and drying the mixture to obtain the ferric fluoride trihydrate / conductive carbon black composite precursor; the mass ratio of the conductive carbon black to the hydrated iron salt is 1:(15-40).

2. The method for preparing the iron oxyfluoride positive electrode material according to claim 1, wherein: The active chemical composition of the iron oxyfluoride positive electrode material is FeO 0.32 F 1.68 / FeF3·0.33H2O or FeO 0.77 F 1.23 / FeF3.

3. The method for preparing the iron oxyfluoride cathode material according to claim 1, wherein: The iron oxyfluoride positive electrode material further comprises conductive carbon black, and the content of the conductive carbon black accounts for 10 to 25 wt % of the total mass of the conductive carbon black and the iron oxyfluoride positive electrode material.

4. The method for preparing the iron oxyfluoride cathode material according to claim 1, wherein: The ferric fluoride trihydrate is α-crystalline ferric fluoride trihydrate and / or β-crystalline ferric fluoride trihydrate; the inert protective atmosphere is nitrogen and / or argon atmosphere.

5. The method for preparing the iron oxyfluoride positive electrode material according to claim 1, wherein: The hydrated iron salt is at least one of ferric nitrate nonahydrate or ferric chloride hexahydrate.

6. An iron oxyfluoride cathode material prepared by the preparation method according to any one of claims 1 to 5, characterized in that: The size of the nanoparticles of the iron oxyfluoride cathode material is 50 to 200 nm; the iron oxyfluoride cathode material is nanoparticles with regular square or octahedral shapes.

7. A battery, characterized in that: include: A positive electrode comprising the iron oxyfluoride positive electrode material of claim 6, a functional ether electrolyte, and a negative electrode, wherein the solute in the functional ether electrolyte comprises a lithium salt and an additive; the lithium salt is selected from at least one of lithium bis(trifluoromethanesulfonyl imide), lithium bis(fluorosulfonyl imide), lithium hexafluorophosphate, and lithium difluorooxalatoborate; and the additive is selected from at least one of tris(pentafluorophenyl)borane and lithium fluoride; The solvent in the functional ether electrolyte is selected from at least one of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether.

8. The battery according to claim 7, characterized in that The solvent in the functional ether electrolyte is ethylene glycol dimethyl ether.

9. The battery according to claim 7, wherein The concentration of the lithium salt in the functional ether electrolyte is 0.5-1 mol / L; the concentrations of the tris(pentafluorophenyl)borane and lithium fluoride additive are 0.01-0.05 mol / L and 0.01-0.5 mol / L, respectively.

10. The battery according to claim 9, characterized in that The concentration ratio of tris(pentafluorophenyl)borane to lithium salt is (0.01-0.05):1.

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