Iron vanadate@MIL-88A egg yolk and eggshell spindle composite material and its preparation and application

Abstract

The present invention discloses an iron vanadate @ MIL-88A egg yolk and eggshell spindle composite material, its preparation, and its application in preparing a negative electrode for a lithium-ion battery. The composite material comprises a hollow spindle-shaped amorphous iron vanadate eggshell, a solid spindle-shaped MIL-88A egg yolk is encapsulated inside the iron vanadate eggshell, and a gap is left between the iron vanadate eggshell and the MIL-88A egg yolk. Preparation method: First, a solid spindle-shaped MIL-88A is prepared, and then the MIL-88A is partially etched by ammonium metavanadate to obtain the iron vanadate @ MIL-88A egg yolk and eggshell spindle composite material. The material provided by the present invention has the characteristics of unique structure and simple synthesis process, and significantly improves the specific capacity and cycle performance of iron vanadate.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and in particular to an iron vanadate@MIL-88A egg yolk and eggshell spindle composite material, its preparation method, and its application in preparing a negative electrode of a lithium-ion battery. Background Art

[0002] Lithium-ion batteries offer advantages such as high energy density, minimal memory effect, environmental friendliness, and a long cycle life, significantly reducing the energy market's reliance on fossil fuels. However, the low theoretical capacity of graphite, the anode material used in commercial lithium-ion batteries, significantly limits their application in high-power applications such as electric vehicles. Therefore, finding new anode materials that can replace graphite is crucial.

[0003] Due to the synergistic effect between the two metal elements, bimetallic oxides can show better electrochemical performance than single metal oxides, which has attracted a lot of attention. -1 FeVO4's theoretical capacity is considered a promising new negative electrode material to replace graphite. However, during cycling, FeVO4 experiences significant volume expansion and contraction, which can cause structural pulverization, leading to rapid capacity decay and poor cycle performance. This presents a major technical challenge that urgently needs to be addressed in FeVO4's application as a negative electrode in lithium-ion batteries.

[0004] In order to improve the performance of FeVO4 lithium-ion batteries, people have conducted various explorations. The invention patent with publication number CN105958068A discloses a nanorod-shaped lithium-ion battery negative electrode material, iron vanadate, the invention patent with publication number CN112768679A discloses a one-dimensional pea-shaped iron vanadate nanowire material, and the invention patent with publication number CN111410235A discloses an ultra-thin iron vanadate nanosheet material. Overall, there is still little exploration of FeVO4 in the field of lithium-ion batteries, and there are not many related academic papers and invention patents. This has led to a lack of understanding of the structure of FeVO4 materials and innovative designs, and the synthesis methods are relatively simple, so the progress in this field is extremely limited. Summary of the Invention

[0005] In response to the technical problems existing in this field, the present invention provides an iron vanadate (FeVO4)@MIL-88A egg yolk and eggshell spindle composite material, which has the characteristics of unique structure and simple synthesis process, and significantly improves the specific capacity and cycle performance of iron vanadate.

[0006] A ferrous vanadate@MIL-88A egg yolk and eggshell spindle composite material, comprising a hollow spindle-shaped amorphous ferrous vanadate eggshell, a solid spindle-shaped MIL-88A egg yolk encapsulated in the ferrous vanadate eggshell, and a gap between the ferrous vanadate eggshell and the MIL-88A egg yolk.

[0007] Preferably, the iron vanadate eggshell is obtained by etching spindle-shaped MIL-88A with ammonium metavanadate.

[0008] Preferably, the middle part of the iron vanadate eggshell is in the shape of a hexagonal prism, the distance between the opposite side edges of the hexagonal prism is 100-1000 nm, and the side edge length of the hexagonal prism is 50-900 nm; the two end parts of the iron vanadate eggshell are in the shape of a hexagonal pyramid, and the height of the hexagonal pyramid is 50-500 nm; the thickness of the iron vanadate eggshell is 5-100 nm.

[0009] Preferably, the spindle-shaped MIL-88A yolk has an axial length of 100-1300 nm and a diameter of 50-800 nm at the thickest point in the middle.

[0010] The present invention also provides a method for preparing the iron vanadate@MIL-88A egg yolk and eggshell spindle composite material, comprising the steps of:

[0011] (1) Dissolve ferric chloride and fumaric acid in dimethylformamide, heat to 90-110°C, react for 0.5-4 hours with continuous stirring, centrifuge, wash, and dry to obtain spindle-shaped MIL-88A;

[0012] (2) The spindle-shaped MIL-88A prepared in step (1) is dispersed in ethanol, heated to 88-92°C, maintained for 3-10 min, and then an aqueous solution of NH4VO3 is added. The temperature is maintained for 0.5-4 h, centrifuged, washed, and dried to obtain the iron vanadate@MIL-88A egg yolk and eggshell spindle composite material; the ratio of the amount of the MIL-88A, the ethanol, the NH4VO3, and the water is 35-45 mg:25-35 mL:0.1-0.7 mmol:10 mL.

[0013] The preparation method of the present invention first synthesizes a solid spindle-shaped MIL-88A, then partially etches the solid spindle-shaped MIL-88A with ammonium metavanadate to obtain an iron vanadate@MIL-88A egg yolk and eggshell spindle composite material. This method can improve the specific capacity and cycle performance of iron vanadate.

[0014] Preferably, in step (1), the ratio of the amount of the ferric chloride, the fumaric acid and the dimethylformamide is 0.05-0.5 mol:0.05-0.5 mol:10 mL.

[0015] Preferably, the reaction environment of step (2) is an oil bath heated to 88-92° C. under reflux conditions.

[0016] The present invention also provides the use of the iron vanadate@MIL-88A egg yolk and eggshell spindle composite material in preparing a negative electrode of a lithium ion battery.

[0017] In one preferred embodiment, the iron vanadate@MIL-88A egg yolk and eggshell spindle composite material of the present invention is used to prepare a lithium-ion battery negative electrode: the iron vanadate@MIL-88A egg yolk and eggshell spindle composite material, acetylene black conductive agent, and PVDF binder are weighed at a mass ratio of 8:1:1. The PVDF is dissolved in an appropriate amount of N-methylpyrrolidone and stirred until completely dissolved. The evenly ground active material and acetylene black are then added to the solution, and stirring is continued to ensure that the slurry is uniformly mixed. The slurry is then evenly coated on a circular copper foil (diameter 12 mm), dried in a vacuum oven at 100°C, and finally flattened on a tablet press at a pressure of 10 MPa to produce an electrode sheet.

[0018] In a glove box filled with high-purity argon, the prepared electrode sheet, lithium sheet, and separator were assembled into a CR2025 button-type lithium-ion battery. The electrolyte was an EC / DMC solution containing 1 mol / L LiPF6. The charge-discharge and cycling performance of the lithium-ion battery was tested using a Xinwei battery testing system.

[0019] The present invention can obtain an iron vanadate@MIL-88A egg yolk and eggshell spindle composite material with high specific capacity and excellent cycle performance.

[0020] Compared with the prior art, the present invention has the following significant technical effects:

[0021] 1) The preparation method of the present invention partially etches solid MIL-88A with ammonium metavanadate, and based on the Kirkendall effect, a hollow spindle-shaped amorphous iron vanadate eggshell is synthesized, and the iron vanadate eggshell also encapsulates residual MIL-88A, namely the MIL-88A egg yolk. The temperature of the reaction, i.e., step (2) in the preparation method, is very critical. When the temperature is too high, the reaction is violent and the synthesized iron vanadate eggshell is very easy to break; when the temperature is too low, the reaction is too slow or even does not react. The suitable temperature range of the reaction is very narrow and needs to be strictly controlled. For this reason, in a preferred embodiment, an oil bath heating with high temperature control accuracy and a matching reflux device are designed for the reaction. In addition, in the reaction, the amount of ammonium metavanadate used and the etching time are also critical. Excessive use of ammonium metavanadate and excessive reaction time will cause the MIL-88A to dissolve completely. At this time, the synthesized material is a hollow iron vanadate spindle, and the egg yolk and eggshell structure cannot be obtained.

[0022] 2) In the iron vanadate@MIL-88A egg yolk and eggshell spindle composite material provided by the present invention, the hollow spindle-shaped amorphous iron vanadate eggshell produces a synergistic effect through the three elements of amorphous, hollow and spindle shape, achieving good cycle performance. The amorphous state makes the hollow spindle shell of the iron vanadate dense, improving its own structural strength, while making the stress change direction of the iron vanadate non-oriented and concentrated, but uniformly distributed in all directions, eliminating the strong destructive effect of stress concentration distribution on the material structure; hollowing makes the structure have strong self-regulation and self-adaptation ability to volume changes, which can effectively reduce the stress acting on the spindle, thereby improving the stability of the spindle structure; the spindle is composed of hexagonal prisms and hexagonal pyramids, both of which have good stability, play a significant role in resisting the volume change of iron vanadate and improving cycle stability, and are superior to conventional structures such as hollow spheres and hollow cubes. In particular, the hollow spindle of the present invention is short and thick in the middle, with elongated ends. This distinguishes it from slender spindles because hexagonal pyramids are more stable than hexagonal prisms. This structure maximizes the advantages of hexagonal pyramids and minimizes the disadvantages of hexagonal prisms. In contrast, slender spindles are more prone to breakage and experience faster performance degradation. These three essential elements work together to achieve the superior cycling performance of iron vanadate.

[0023] 3) The combination of the hollow, spindle-shaped amorphous iron vanadate eggshell and the spindle-shaped MIL-88A yolk produces an unexpected composite effect. While the reversible capacity of MIL-88A is low, far below that of iron vanadate, the reversible capacity of the iron vanadate@MIL-88A composite material is significantly higher than both iron vanadate and MIL-88A, resulting in a 1+1>2 composite effect. This composite effect is closely related to the yolk-shell structure of iron vanadate and MIL-88A. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a scanning electron microscope (SEM) photograph of MIL-88A prepared in Example 1;

[0025] Figure 2 This is a SEM photo of the iron vanadate@MIL-88A egg yolk and eggshell spindle prepared in Example 1;

[0026] Figure 3 This is a TEM image of the iron vanadate@MIL-88A egg yolk and eggshell spindle prepared in Example 1;

[0027] Figure 4 This is the full XPS spectrum of the iron vanadate@MIL-88A egg yolk and eggshell spindle prepared in Example 1;

[0028] Figure 5The iron vanadate@MIL-88A egg yolk and eggshell spindles and the iron vanadate hollow spindles prepared in Example 1 were compared with MIL-88A at a current density of 1 A g -1 cycle performance. DETAILED DESCRIPTION

[0029] The present invention will be further described below in conjunction with the accompanying drawings and specific examples. It should be understood that these examples are intended to illustrate the present invention and are not intended to limit the scope of the invention. The operating methods in the following examples where no specific conditions are specified are generally performed under conventional conditions or as recommended by the manufacturer.

[0030] Example 1

[0031] Preparation of iron vanadate@MIL-88A yolk and eggshell spindle composites:

[0032] (1) Dissolve 0.1 mol of ferric chloride and 0.1 mol of fumaric acid in 10 mL of dimethylformamide, heat to 100°C, stir for 2 h, centrifuge and separate the product, wash with ethanol several times, and dry at 70°C to obtain spindle-shaped MIL-88A;

[0033] (2) 40 mg of MIL-88A prepared in step (1) was dispersed in 30 mL of ethanol and heated to 90°C in an oil bath under condensation reflux conditions for 5 min; 10 mL of a 40 mM NH4VO3 aqueous solution was then added and the mixture was kept warm at 90°C for 2 h; the product was centrifuged, washed several times with ethanol, and dried at 70°C to obtain an iron vanadate@MIL-88A egg yolk and eggshell spindle composite material.

[0034] Figure 1 This is an SEM image of the spindle-shaped MIL-88A. The interior of MIL-88A is solid and shaped like a spindle. The middle part is hexagonal. The distance between the opposite side faces of the hexagonal prism is about 200nm, and the side length of the hexagonal prism is about 250nm; the two end parts are hexagonal pyramids with a height of about 120nm.

[0035] Figure 2 This is an SEM image of the iron vanadate@MIL-88A egg yolk and eggshell spindle, showing the spindle morphology intact. The ammonium metavanadate etching reaction causes the product to change in size, transforming the originally slender MIL-88A into a thicker, longer iron vanadate. The central portion of the iron vanadate is hexagonal, with the distance between the opposing side faces increasing to 350-400 nm and the side length of the hexagonal prism also increasing to approximately 300 nm. The ends of the iron vanadate are hexagonal pyramids, each with a height of approximately 150 nm.

[0036] Figure 3This is a TEM image of the yolk-shell spindle of an iron vanadate @MIL-88A egg. The interior shows a distinct yolk-shell structure. The outer layer is a FeVO4 shell, approximately 25nm thick. In the center lies a solid, spindle-shaped core, approximately 155nm in diameter at its thickest point and 420nm in axial length.

[0037] Figure 4 This is the full XPS spectrum of the iron vanadate@MIL-88A yolk and eggshell spindle. The characteristic peaks of Fe 2p, V 2p, and O1s can be seen, indicating that iron vanadate was successfully synthesized after ammonium metavanadate etching of MIL-88A.

[0038] The iron vanadate@MIL-88A egg yolk and eggshell spindle composite material of the present invention was used to prepare a lithium-ion battery negative electrode: the iron vanadate@MIL-88A egg yolk and eggshell spindle composite material, acetylene black conductive agent, and PVDF binder were weighed at a mass ratio of 8:1:1. The PVDF was dissolved in an appropriate amount of N-methylpyrrolidone and stirred until completely dissolved. The ground active material and acetylene black were then added to the solution, and stirring was continued to ensure that the slurry was uniformly mixed. The slurry was then evenly coated on a circular copper foil (12 mm diameter), dried in a vacuum oven at 100°C, and finally flattened on a tablet press at a pressure of 10 MPa to produce an electrode sheet.

[0039] In a glove box filled with high-purity argon, the prepared electrode sheet, lithium sheet, and separator were assembled into a CR2025 button-type lithium-ion battery. The electrolyte was an EC / DMC solution containing 1 mol / L LiPF6. The charge-discharge and cycling performance of the lithium-ion battery was tested using a Xinwei battery testing system.

[0040] Preparation of iron vanadate hollow spindles and MIL-88A. The difference between the preparation of the iron vanadate hollow spindles and the preparation of the iron vanadate@MIL-88A egg yolk and eggshell spindle composite material prepared above is only that in step (2), the amount of NH4VO3 used is 0.8mmol, and the amount of the aqueous solution of NH4VO3 used is still 10mL. At this time, ammonium metavanadate completely etches the solid MIL-88A. The difference between the preparation of MIL-88A and the preparation of the iron vanadate@MIL-88A egg yolk and eggshell spindle composite material prepared above is only that the preparation of MIL-88A only includes step (1). The iron vanadate hollow spindles and the corresponding lithium ion battery negative electrode of MIL-88A are prepared according to the above-mentioned method for making a lithium ion battery negative electrode.

[0041] Figure 5 The iron vanadate@MIL-88A egg yolk and eggshell spindles prepared in Example 1 and the iron vanadate hollow spindles, MIL-88A at a current density of 1A g -1The discharge capacity of iron vanadate@MIL-88A increased from 1343.5 mAh g to -1 Down to 915.5mAh g -1 , and then began to rise again, reaching 1136.8 mAh g at the 87th cycle. -1 , and then maintained a relatively stable floating state. After the 150th cycle, the discharge capacity began to slowly decrease, but was basically stable after the 270th cycle. The discharge capacity at the 320th cycle was 960.8 mAh g -1 In contrast, the discharge capacity of FeVO4 hollow spindles increased from 758.6 mAh g to -1 Down to 722.3mAh g -1 , and then began to rise gradually, reaching 865.3 mAh g at the 100th cycle. -1 After 270 cycles, the discharge capacity began to slowly decrease, and at the 320th cycle the discharge capacity was 717.6 mAh g -1 The discharge capacity of the MIL-88A spindle increased from 989.5 mAh g in the first 30 cycles. -1 Down to 148.1mAh g -1 The specific capacity of the subsequent cycles increased slightly, and the discharge specific capacity was 216.7 mAh g at the 320th cycle. -1 The discharge capacity of the iron vanadate@MIL-88A yolk and eggshell spindle is much higher than that of iron vanadate and MIL-88A, and a composite effect of 1+1>2 is produced.

[0042] The cycling performance of the iron vanadate@MIL-88A egg yolk and eggshell spindle composite material is better than that of a nanorod-shaped lithium-ion battery negative electrode material iron vanadate reported in the invention patent publication number CN105958068A at a current density of 0.1C (about 0.125A g -1 ) The discharge capacity after 50 cycles is about 866.9 mAh g -1 , which is superior to a FeVO4@TiO2 nanocomposite at a current density of 0.2 A g -1 The discharge capacity after 100 cycles is about 597 mAh g -1 (Controllable synthesis of FeVO4@TiO2nanostructures as anode for lithium ion battery,J Nanopart Res 19(2017)243).

[0043] Example 2

[0044] (1) Dissolve 0.1 mol of ferric chloride and 0.1 mol of fumaric acid in 10 mL of dimethylformamide, heat to 100°C, stir for 2 h, centrifuge and separate the product, wash with ethanol several times, and dry at 70°C to obtain spindle-shaped MIL-88A;

[0045] (2) 40 mg of MIL-88A prepared in step (1) was dispersed in 30 mL of ethanol and heated to 90°C in an oil bath under condensation reflux conditions for 5 min; 10 mL of a 55 mM NH4VO3 aqueous solution was then added and the mixture was kept warm at 90°C for 2 h; the product was separated by centrifugation, washed several times with ethanol, and dried at 70°C to obtain an iron vanadate@MIL-88A egg yolk and eggshell spindle composite material.

[0046] The structure of the resulting iron vanadate@MIL-88A egg yolk and eggshell spindle composite was similar to that of Example 1, with the main difference being that the thickness of the iron vanadate eggshell was increased to 28 nm. The MIL-88A egg yolk retained a solid spindle-shaped core, with a diameter of approximately 113 nm at its thickest point and an axial length of approximately 330 nm.

[0047] Example 3

[0048] (1) Dissolve 0.05 mol of ferric chloride and 0.05 mol of fumaric acid in 10 mL of dimethylformamide, heat to 100°C, stir for 2 h, centrifuge and separate the product, wash with ethanol several times, and dry at 70°C to obtain spindle-shaped MIL-88A;

[0049] The subsequent process is the same as that in Example 1.

[0050] The structure of the resulting iron vanadate@MIL-88A egg yolk and eggshell spindle composite material is similar to that of Example 1, with the main differences being that the central portion of the hollow, spindle-shaped amorphous iron vanadate eggshell is hexagonal, with the distance between opposing lateral faces of the hexagonal prisms approximately 160-200 nm and a lateral edge length of approximately 140 nm. The cones at both ends are approximately 72 nm high, and the shell thickness is approximately 20 nm. The MIL-88A egg yolk retains a spindle-shaped solid core, with a diameter of approximately 60 nm at its thickest point and an axial length of approximately 185 nm.

[0051] In addition, it should be understood that after reading the above description of the present invention, those skilled in the art may make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the claims attached to this application.

Claims

1. An iron vanadate@MIL-88A egg yolk and eggshell spindle composite material, characterized in that: The iron vanadate@MIL-88A egg yolk and eggshell spindle composite material includes a hollow spindle-shaped amorphous iron vanadate eggshell, a solid spindle-shaped MIL-88A egg yolk is encapsulated in the iron vanadate eggshell, and a gap is left between the iron vanadate eggshell and the MIL-88A egg yolk.

2. The iron vanadate@MIL-88A egg yolk and eggshell spindle composite material according to claim 1, characterized in that The iron vanadate eggshell is obtained by etching the spindle-shaped MIL-88A with ammonium metavanadate.

3. The iron vanadate@MIL-88A egg yolk and eggshell spindle composite material according to claim 1, characterized in that: The middle part of the iron vanadate eggshell is in the shape of a hexagonal prism, the distance between the opposite side edges of the hexagonal prism is 100-1000nm, and the side edge length of the hexagonal prism is 50-900nm; the two end parts of the iron vanadate eggshell are in the shape of a hexagonal pyramid, and the height of the hexagonal pyramid is 50-500nm; the thickness of the iron vanadate eggshell is 5-100nm.

4. The iron vanadate@MIL-88A egg yolk and eggshell spindle composite material according to claim 1, characterized in that The spindle-shaped MIL-88A yolk has an axial length of 100-1300 nm and a diameter of 50-800 nm at the thickest part in the middle.

5. The method for preparing the iron vanadate@MIL-88A egg yolk and eggshell spindle composite material according to any one of claims 1 to 4, characterized in that: Including steps: (1) Dissolve ferric chloride and fumaric acid in dimethylformamide, heat to 90-110°C, react for 0.5-4 hours with continuous stirring, centrifuge, wash, and dry to obtain spindle-shaped MIL-88A; (2) The spindle-shaped MIL-88A prepared in step (1) is dispersed in ethanol, heated to 88-92°C, maintained for 3-10 min, and then an aqueous solution of NH4VO3 is added. The temperature is maintained for 0.5-4 h, centrifuged, washed, and dried to obtain the iron vanadate@MIL-88A egg yolk and eggshell spindle composite material; the ratio of the amount of the MIL-88A, the ethanol, the NH4VO3, and the water is 35-45 mg:25-35 mL:0.1-0.7 mmol:10 mL.

6. The preparation method according to claim 5, characterized in that In step (1), the ratio of the amount of the ferric chloride, the fumaric acid and the dimethylformamide is 0.05-0.5 mol:0.05-0.5 mol:10 mL.

7. The preparation method according to claim 5, characterized in that The reaction environment of step (2) is to heat the oil bath to 88-92° C. under reflux conditions.

8. Use of the iron vanadate@MIL-88A egg yolk and eggshell spindle composite material according to any one of claims 1 to 4 in preparing a negative electrode for a lithium ion battery.

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