Copper fluoride / polymer double-coated chromium oxide positive electrode material as well as preparation method and application thereof

Abstract

The invention discloses a copper fluoride / polymer double-coated chromium oxide positive electrode material as well as a preparation method and application thereof, and belongs to the technical field of lithium ion batteries. The specific scheme is as follows: the positive electrode material comprises an inner core, an inner coating layer coating the inner core, and an outer coating layer coating the inner coating layer, the inner core is chromium oxide, the inner coating layer is a copper fluoride coating layer, and the outer coating layer is a mixed coating layer of a polymer and a high-conductivity material. A copper fluoride coating layer with a high-voltage window and high discharge capacity is introduced on the surface of the chromium oxide by adopting a chemical precipitation method, and a mixed coating layer of a conductive material and a polymer is constructed on the outer surface of the material, so that the self-discharge phenomenon between the chromium oxide and an electrolyte is isolated by the double coating layers, the discharge platform of the material is improved, and the service life of the material is prolonged. The polymer layer constructed through in-situ polymerization enables the material to have excellent ion and electron transmission characteristics, and the specific discharge capacity, the rate capability and the storage performance of the chromium oxide are effectively improved.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion batteries, and specifically relates to a copper fluoride / polymer double-coated chromium oxide positive electrode material and a preparation method and application thereof. Background Art

[0002] With the increasing demand for energy storage batteries in civilian portable electronic devices and the defense field, the development of new battery systems with high energy efficiency and strong safety has become the current focus. As the main component of the battery, the positive electrode largely determines the energy density and power characteristics of the battery. At present, commercial batteries are mainly based on lithium oxides (such as LiFePO 4 、LiCoO 2 , NCM, NCA, etc.) as the positive electrode, and graphite, silicon oxide or a mixture of the two as the negative electrode, its energy density is difficult to be higher than 400Wh / Kg. The lithium-free positive electrode (metal oxide, metal phosphide and metal fluoride) based on the conversion reaction has the advantages of high specific energy, low price and easy synthesis. Compared with the lithium-containing positive electrode, it has great potential research value in many fields. As a lithium-free positive electrode material, the chromium oxide positive electrode (mainly Cr 8 O 21 ) has the advantages of extremely high theoretical specific capacity (642mAh / g), actual energy density (1210Wh / Kg) and low raw material cost, and is a very promising type of positive electrode material.

[0003] However, Cr 8 O 21 The actual capacity of the material is far lower than the theoretical capacity, and the rate performance is also relatively poor. In addition, the material is generally prepared by high-temperature solid-phase calcination and contains trace amounts of CrO 3 Difficult to remove, CrO 3 The strong oxidizing properties of chromium oxide make it easy to react with the electrolyte, releasing a large amount of heat and generating by-products, which affects the safety and storage performance of the battery, and also limits the application of chromium oxide positive electrode.

[0004] The invention patent with the patent publication number CN112968176 A prepared a positive electrode material composited with chromium oxide and carbon nanotubes, which improved the material's initial discharge specific capacity and rate performance. However, it is difficult to obtain a uniform and dense coating layer on the material surface by the high-temperature solid phase method, and the chromium oxide still contacts the electrolyte, causing side reactions, thus affecting the safety and storage performance of the battery.

[0005] The invention patent application with patent publication number CN116759569A prepared a chromium oxide positive electrode material composited with boron and other metal elements, which improved the conductivity, high temperature performance and rate performance of the material. However, the metal source is an organic salt, and the doping process adopts a high temperature solid phase method, resulting in poor material consistency. At the same time, the first cycle discharge capacity of 351.7mAh / g is not very high, which is difficult to meet the current performance requirements for primary batteries.

[0006] The invention patent CN112194182A prepared chromium oxide containing lithiated sulfide polyacrylonitrile, which improved the material's initial coulombic efficiency, reversibility and conductivity. However, the material will lose some capacity after lithium storage in lithiated sulfide polyacrylonitrile, resulting in its initial discharge capacity of only 326.5mAh / g, and there is a lot of room for performance optimization.

[0007] In summary, the current conventional modification mainly revolves around sintered Cr 8 O 21 It is difficult to form a uniform and stable coating layer without doping or coating, which can simultaneously improve the ionic and electronic conductivity of the chromium oxide material. Therefore, the improvement effect of the discharge capacity, rate performance and reserve performance of the modified material is difficult to achieve the expected effect. Summary of the invention

[0008] In order to solve the problems of low discharge specific capacity, poor rate performance and storage performance of chromium oxide positive electrode materials, the present invention provides a copper fluoride / polymer double-coated chromium oxide positive electrode material and a preparation method and application thereof.

[0009] In order to achieve the above object, the present invention adopts the following technical solutions:

[0010] A copper fluoride / polymer double-coated chromium oxide positive electrode material, the positive electrode material comprises an inner core, an inner coating layer coated on the inner core, and an outer coating layer coated on the inner coating layer; the inner core is chromium oxide, the inner coating layer is a copper fluoride coating layer, and the outer coating layer is a mixed coating layer of polymer and conductive material.

[0011] Furthermore, the chromium oxide is Cr 8 O 21 .

[0012] Furthermore, the thickness of the inner coating layer and the outer coating layer is 3-10 nm.

[0013] A method for preparing the copper fluoride / polymer double-coated chromium oxide positive electrode material comprises the following steps:

[0014] S1: CrO 3 Cr was prepared by high temperature calcination in oxygen atmosphere. 8 O21 ;

[0015] S2: Cr 8 O 21 、Cu(NO 3 ) 2 ·3H 2 O and dispersant were added to solvent I, and Cr 8 O 21 Evenly dispersed, making Cu(NO 3 ) 2 ·3H 2 O dissolves to obtain solution Ⅰ;

[0016] S3: slowly dropping the ammonium fluoride solution into solution I, stirring vigorously during the dropping process, stirring after the dropping is completed to allow the material to react fully, then separating, washing, and drying the sample, and then placing the sample in a tube furnace and calcining it under an inert atmosphere to obtain a chromium oxide material coated with copper fluoride;

[0017] S4: dissolving the lithium salt in a sulfone monomer solvent, adding an initiator, and obtaining a precursor solution;

[0018] S5: dissolving the precursor solution, the conductive material and the chromium oxide coated with copper fluoride in a sufficient amount of acetonitrile, heating and stirring to polymerize and solidify the sulfone monomer, and vacuum drying to obtain a chromium oxide positive electrode material with a copper fluoride / polymer double coating layer.

[0019] Furthermore, in S1, the high temperature calcination temperature is 270-330°C, the time is 6-24 hours, and the heating rate is 1-10°C / min. 3 After high temperature calcination in an oxygen atmosphere, the CrO 3 The reaction is more complete, and the pulverizing method adopts ball milling, the ball milling speed is 350-500rpm, and the ball milling time is 2-8 hours.

[0020] Furthermore, in S2, the dispersant includes a combination of one or more of oleic acid, linoleic acid, oleyl alcohol, alkylphenol polyoxyethylene ether, polyoxyethylene oleyl alcohol ether, and polyvinyl pyrrolidone, and the solvent I includes anhydrous ethanol.

[0021] Further, in S2, the Cr in the solution I 8 O 21 The dispersion concentration is 1-50g / L, Cu(NO 3 ) 2 ·3H 2 The concentration of O is 0.01-0.03 mol / L.

[0022] Furthermore, in S3, the concentration of the ammonium fluoride solution is 0.04-0.12 mol / L (twice the theoretical ratio of the Cu source); the precipitate is separated by centrifugation, and after obtaining the precipitate, it needs to be centrifuged and washed three times with ethanol to remove residual unreacted raw materials, and then vacuum dried after centrifugation to completely dry the precipitate; the temperature of the calcination process is 200-300°C, the calcination time is 5-10h, and the heating rate is 1-10°C / min.

[0023] Further, in S4, the sulfone monomer includes one or a combination of arylene ether sulfone, methyl vinyl sulfone, aryl ether sulfone, sulfone ether, 3-acryloyl cyclopentane sulfone, tetramethylene sulfone, sulfonated ether ketone sulfone, and phenyl sulfone; the lithium salt includes LiPF 6 , LiBF 4 , LiTFSI, LiFSI, LiDFOB, LiBOB, or a combination thereof; the initiator comprises azobisisobutyronitrile; the concentration of the lithium salt in the precursor solution is 0.1-4 mol / L; the mass fraction of the initiator in the precursor solution is 1wt.%.

[0024] Furthermore, in S5, the mass ratio of the chromium oxide coated with copper fluoride, the precursor solution and the conductive material is 100:5-20:0.1-1; the conductive material includes one of carbon nanotubes, carbon black, silver powder and graphene, the heating and stirring temperature is 60°C, and the time is 12h; the vacuum drying temperature is 60°C, and the time is 12-24h.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) The present invention forms CuF on the surface of chromium oxide by chemical precipitation method 2 The uniform and dense coating layer of CuF 2 High voltage platform (3.55V, vs Li / Li + ) and high discharge specific capacity (528mAh / g), which significantly improved the discharge platform and first-cycle discharge specific capacity of chromium oxide.

[0027] (2) The present invention further coats a mixed coating layer of polymer and conductive material on the surface of the chromium oxide material coated with copper fluoride, and the uniform dispersion of the conductive material improves the chromium oxide and CuF 2 The presence of the polymer layer creates a good transmission path for lithium ion insertion and extraction in the active material, which can greatly improve the kinetics and electrochemical properties of the electrode material.

[0028] (3) The inner coating layer and the outer coating layer of the present invention isolate the contact between the chromium oxide and the electrolyte, inhibit the self-discharge reaction between the highly oxidizing chromium oxide material and the electrolyte, and improve the storage performance of the material.

[0029] (4) Chemical precipitation method for coating CuF of the present invention 2 And the method of polymer coating has universality and can be extended to other lithium-ion battery systems for modifying electrode materials. Description of the Drawings

[0030] Figure 1 It is a schematic diagram of the chromium oxide cathode material with a copper fluoride / polymer double coating layer of the present invention;

[0031] Figure 2 For Comparative Example 1, the unmodified chromium oxide cathode Cr 8 O 21 Electrochemical performance diagram;

[0032] Figure 3 It is the room temperature impedance diagram of the polysulfone polymer electrolyte obtained after polymerization of the precursor solution prepared in Example 1;

[0033] Figure 4 For CuF in Example 1 2 And the electrochemical performance diagram of the polymer-coated chromium oxide cathode;

[0034] Figure 5 It is the impedance comparison diagram between the modified chromium oxide cathode in Example 1 and the unmodified chromium oxide cathode in Comparative Example 1;

[0035] Figure 6 It is the discharge curve of the battery with the modified chromium oxide cathode in Example 1 and the battery with the unmodified original chromium oxide cathode in Comparative Example 1 after storage at 55°C for 7 days. Detailed Embodiments

[0036] Next, the technical solutions in the present invention will be clearly and completely described in conjunction with the drawings and embodiments. Obviously, the described embodiments are only a part of the embodiments of the invention, rather than all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0037] Example 1

[0038] A preparation method of a copper fluoride / polymer double-coated chromium oxide cathode material, comprising the following steps:

[0039] (1) 4 g of chromium trioxide powder was placed in a tube furnace, high-purity oxygen was introduced into the tube furnace, and the tube furnace program was set to increase the temperature to 270°C at 5°C / min, and calcined at high temperature for 12 hours. The product was then cooled to room temperature and then taken out. The product was ground, and the unreacted chromium trioxide was removed by water washing. After vacuum drying at 60°C for 24 hours, the product was calcined again according to the above steps, ground again, sieved and washed with water to obtain a high-valent chromium oxide active material (Cr 8 O 21 ).

[0040] (2) Take 2g of Cr 8 O 21 , 1mmol Cu(NO 3 ) 2 ·3H 2 O was added to a beaker containing 40 ml of anhydrous ethanol, and oleic acid was added as a dispersant. Ultrasonication was performed for 30 min to uniformly disperse the chromium oxide in the anhydrous ethanol and promote the Cu(NO 3 ) 2 ·3H 2 O particles are completely dissolved to obtain solution I; 4 mmol of ammonium fluoride is dissolved in 40 ml of deionized water to obtain an ammonium fluoride solution, and then the ammonium fluoride solution is slowly dripped into solution I, with vigorous stirring during the process. After the dripping is complete, stir for another 5 hours to allow the material to fully react. Then the solution and the precipitate are transferred to a centrifuge tube, the centrifuge speed is set to 5000 rpm, the centrifugation time is 10 minutes, the precipitate is centrifuged out, and then ethanol is added to the precipitate, and the centrifugation is washed three times under the same parameters to remove the unreacted copper source and fluorine source, and the washed precipitate is dried at 60°C in a vacuum oven for 24 hours. Finally, the dried powder is ground into fine powder in an argon-filled glove box, and then placed in an argon-filled tubular furnace and heated to 250°C at a heating rate of 5°C / min for calcination for 8 hours to obtain the coated CuF 2 of chromium oxide materials.

[0041] (3) LiPF 6 The LiPF was dissolved in an arylene ether sulfone polymerization monomer solvent, azobisisobutyronitrile was added as an initiator, and magnetic stirring was performed for 15 minutes to obtain a precursor solution. 6 The concentration of azobisisobutyronitrile in the precursor solution is 1 mol / L, and the mass fraction of azobisisobutyronitrile in the precursor solution is 1 wt%. 2 The chromium oxide and a trace amount of carbon black and the precursor solution were mixed and dissolved in a sufficient amount of acetonitrile, heated at 60°C with magnetic stirring for 12 hours to polymerize the arylene ether sulfone polymerization monomer, wherein the CuF coated 2The mass ratio of chromium oxide, precursor solution and carbon black is 100:5:0.1. The polymerized lithium storage material is dried in a vacuum oven at 60°C for 12 hours, and the acetonitrile solvent is removed to obtain a chromium oxide positive electrode material with a copper fluoride / polymer double coating layer.

[0042] (4) The chromium oxide positive electrode material with copper fluoride / polymer double coating layer was dissolved in NMP with Super P and PVDF at a mass ratio of 8:1:1 and stirred for 12 hours. The slurry was coated on aluminum foil with a coating machine and placed in a vacuum oven at 60°C for 12 hours to obtain a modified chromium oxide electrode sheet. The above electrode sheet was used as the positive electrode, and 1M LiPF 6 +EC / DMC (volume ratio of 3:7) +FEC (mass fraction of 5%) was used as the electrolyte, lithium metal negative electrode and Celgard-2400 commercial polypropylene separator were used to assemble CR2025 button batteries, and the electrochemical performance was tested. The voltage test range was: 1.5V~4.5V.

[0043] Comparative Example 1

[0044] Cr 8 O 21 As the positive electrode material, Super P and PVDF were dissolved in NMP at a mass ratio of 8:1:1 and stirred for 12 hours. The slurry was coated on aluminum foil with a coating machine and placed in a vacuum oven at 60°C for 12 hours to obtain an unmodified chromium oxide electrode sheet. The above electrode sheet was used as the positive electrode, 1M LiPF 6 +EC / DMC (volume ratio of 3:7) +FEC (mass fraction of 5%) was used as the electrolyte, lithium metal negative electrode and Celgard-2400 commercial polypropylene separator were used to assemble CR2025 button batteries, and the electrochemical performance was tested. The voltage test range was: 1.5V~4.5V.

[0045] Figure 1 Schematic diagram of the structure of the chromium oxide positive electrode material with copper fluoride / polymer double coating layer in Example 1. Figure 2 This is the first cycle discharge curve of the unmodified chromium oxide positive electrode in Comparative Example 1, and its first cycle discharge capacity at a current density of 30 mA / g is 372 mAh / g. Figure 3 The room temperature impedance spectrum of the polysulfone polymer electrolyte obtained after polymerization of the precursor solution prepared in Example 1. The room temperature ionic conductivity is calculated to be 3.5×10 -4 S / cm, demonstrating that the polysulfone polymer coating can promote the rapid transport of lithium ions. Figure 4 is the first cycle discharge capacity of the modified chromium oxide positive electrode in Example 1. Its first cycle discharge capacity at a current density of 30 mA / g is 442 mAh / g, which is significantly improved compared with the unmodified chromium oxide positive electrode in Comparative Example 1. Figure 5The impedance comparison of the modified chromium oxide positive electrode in Example 1 and the unmodified chromium oxide positive electrode in Comparative Example 1 is shown in FIG. 1 . The impedances of Comparative Example 1 and Example 1 are 168Ω and 97Ω, respectively. Figure 6 The discharge curves of the battery with modified chromium oxide positive electrode and the battery with unmodified chromium oxide positive electrode in Example 1 after storage at 55°C for 7 days, the storage capacity of Comparative Example 1 and Example 1 are 314 mAh / g and 420 mAh / g, respectively, and the capacity loss is 15.6% and 5%, respectively. It can be seen that the double coating strategy not only improves the discharge capacity of chromium oxide, but also reduces the occurrence of self-reaction at the electrode / electrolyte interface, greatly improving the storage performance of the material.

[0046] Example 2

[0047] A method for preparing a copper fluoride / polymer double-coated chromium oxide positive electrode material comprises the following steps:

[0048] (1) Same as step (1) of Example 1.

[0049] (2) Take 2g of Cr 8 O 21 , 0.4 mmol Cu(NO 3 ) 2 ·3H 2 O was added to a beaker containing 40 ml of anhydrous ethanol, and oleic acid was added as a dispersant. Ultrasonication was performed for 30 min to uniformly disperse the chromium oxide in the anhydrous ethanol and promote the Cu(NO 3 ) 2 ·3H 2 O particles were completely dissolved to obtain solution I; 1.6 mmol of ammonium fluoride was dissolved in 40 ml of deionized water to obtain an ammonium fluoride solution, and then the ammonium fluoride solution was slowly dripped into solution I, with vigorous stirring during the process. After the dripping was complete, it was stirred for another 5 hours to allow the material to fully react. Then the solution and the precipitate were transferred to a centrifuge tube, and the centrifuge speed was set to 5000 rpm and the centrifugation time was 10 minutes. The precipitate was centrifuged out, and then ethanol was added to the precipitate. The precipitate was centrifuged and washed three times under the same parameters to remove the unreacted copper source and fluorine source. The washed precipitate was dried at 60°C in a vacuum oven for 24 hours. Finally, the dried powder was ground into fine powder in an argon-filled glove box, and then placed in an argon-filled tubular furnace and heated to 270°C at a heating rate of 3°C / min and calcined for 6 hours to obtain a coated CuF 2 of chromium oxide materials.

[0050] (3) Same as step (3) of Example 1.

[0051] (4) Same as step (4) in Example 1.

[0052] Example 3

[0053] A method for preparing a copper fluoride / polymer double-coated chromium oxide positive electrode material comprises the following steps:

[0054] (1) Same as step (1) of Example 1.

[0055] (2) Same as step (2) in Example 1.

[0056] (3) 1 mol / L LiPF 6 Dissolve in methyl vinyl sulfone polymerization monomer solvent, add 1wt% azobisisobutyronitrile as initiator, and stir magnetically for 15min to obtain a precursor solution. 2 The chromium oxide and a trace amount of carbon black and the precursor solution were mixed and dissolved in a sufficient amount of acetonitrile, heated at 60°C with magnetic stirring for 12 h to polymerize the methyl vinyl sulfone monomer, wherein the coated CuF 2 The mass ratio of chromium oxide, precursor solution and carbon black is 100:10:1. The polymerized lithium storage material is dried in a vacuum oven at 60°C for 12 hours to remove the acetonitrile solvent, thereby obtaining a chromium oxide positive electrode material having a copper fluoride / polymer double coating layer.

[0057] (4) Same as step (4) in Example 1.

[0058] Example 4

[0059] A method for preparing a copper fluoride / polymer double-coated chromium oxide positive electrode material comprises the following steps:

[0060] (1) Same as step (1) of Example 1.

[0061] (2) Same as step (2) in Example 1.

[0062] (3) 1 mol / L LiFSI was dissolved in methyl vinyl sulfone polymerization monomer solvent, 1 wt% azobisisobutyronitrile was added as initiator, and magnetic stirring was performed for 15 min to obtain a precursor solution. 2 The chromium oxide, trace amount of graphene and precursor solution were mixed and dissolved in sufficient acetonitrile, heated at 60℃ and magnetically stirred for 12h to polymerize the methyl vinyl sulfone monomer, wherein the CuF coated 2 The mass ratio of chromium oxide, precursor solution and carbon black is 100:10:1. The polymerized lithium storage material is dried in a vacuum oven at 60°C for 12 hours to remove the acetonitrile solvent, thereby obtaining a chromium oxide positive electrode material having a copper fluoride / polymer double coating layer.

[0063] (4) Same as step (4) in Example 1.

[0064] The present invention introduces a copper fluoride coating layer with a high voltage window and a high discharge capacity on the surface of the chromium oxide by a chemical precipitation method, and constructs a mixed coating layer of a conductive material and a polymer on the outer surface of the material. The double coating layer isolates the self-discharge phenomenon between the chromium oxide and the electrolyte, improves the discharge platform of the material, and the polymer layer constructed by in-situ polymerization enables the material to have excellent ion and electron transmission characteristics, effectively improving the discharge specific capacity, rate performance and storage performance of the chromium oxide.

[0065] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A copper fluoride / polymer double-coated chromium oxide positive electrode material, characterized in that: The positive electrode material comprises a core, an inner coating layer coated on the core, and an outer coating layer coated on the inner coating layer; the core is chromium oxide, the inner coating layer is a copper fluoride coating layer, and the outer coating layer is a mixed coating layer of polymer and conductive material.

2. The copper fluoride / polymer double-coated chromium oxide positive electrode material according to claim 1, characterized in that: The chromium oxide is Cr8O 21 .

3. The copper fluoride / polymer double-coated chromium oxide positive electrode material according to claim 1, characterized in that: The thickness of the inner coating layer and the outer coating layer is 3-10 nm.

4. A method for preparing the copper fluoride / polymer double-coated chromium oxide positive electrode material according to any one of claims 1 to 3, characterized in that: The following steps are involved: S1: CrO3 is calcined at high temperature in an oxygen atmosphere to prepare Cr8O 21 ; S2: Cr8O 21 , Cu(NO3)2·3H2O and dispersant are added into solvent I, dispersed and dissolved uniformly to obtain solution I; S3: adding the ammonium fluoride solution dropwise to the solution I, stirring vigorously, stirring after the addition is complete to allow the material to react fully, then separating, washing, and drying the sample, and then calcining in an inert atmosphere to obtain a chromium oxide material coated with copper fluoride; S4: dissolving the lithium salt in a sulfone monomer solvent, adding an initiator, and obtaining a precursor solution; S5: dissolving the precursor solution, the conductive material and the chromium oxide coated with copper fluoride in acetonitrile, heating and stirring to polymerize and solidify the sulfone monomer, and vacuum drying to obtain a chromium oxide positive electrode material with a copper fluoride / polymer double coating layer.

5. The preparation method according to claim 4, characterized in that: In S1, the high temperature calcination temperature is 270-330°C, the time is 6-24 hours, and the heating rate is 1-10°C / min.

6. The preparation method according to claim 4, characterized in that: In S2, the dispersant includes one or more of oleic acid, linoleic acid, oleyl alcohol, alkylphenol polyoxyethylene ether, polyoxyethylene oleyl alcohol ether, and polyvinyl pyrrolidone, and the solvent I includes anhydrous ethanol.

7. The preparation method according to claim 4, characterized in that: In S2, the Cr8O 21 The dispersion concentration is 1-50g / L, and the concentration of Cu(NO3)2·3H2O is 0.01-0.03mol / L.

8. The preparation method according to claim 4, characterized in that: In S3, the concentration of ammonium fluoride solution is 0.04-0.12mol / L; the temperature of the calcination process is 200-300℃, the calcination time is 5-10h, and the heating rate is 1-10℃ / min.

9. The preparation method according to claim 4, characterized in that: In S4, the sulfone monomer includes one or more of arylene ether sulfone, methyl vinyl sulfone, aromatic ether ether sulfone, sulfone ether, 3-acryloyl cyclobutane sulfone, tetramethylene sulfone, sulfonated ether ketone sulfone, and phenyl sulfone; the lithium salt includes one or more of LiPF6, LiBF4, LiTFSI, LiFSI, LiDFOB, LiBOB, etc.; the initiator includes azobisisobutyronitrile; the concentration of the lithium salt in the precursor solution is 0.1-4 mol / L; the mass fraction of the initiator in the precursor solution is 1wt.%.

10. The preparation method according to claim 4, characterized in that: In S5, the mass ratio of the chromium oxide coated with copper fluoride, the precursor solution and the conductive material is 100:5-20:0.1-1; the conductive material includes one of carbon nanotubes, carbon black, silver powder and graphene.

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