CN110668414B - Vanadium phosphate/graphene foam composite nano material with porous network structure and preparation method thereof - Google Patents

Vanadium phosphate/graphene foam composite nano material with porous network structure and preparation method thereof Download PDF

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CN110668414B
CN110668414B CN201911041516.XA CN201911041516A CN110668414B CN 110668414 B CN110668414 B CN 110668414B CN 201911041516 A CN201911041516 A CN 201911041516A CN 110668414 B CN110668414 B CN 110668414B
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graphene foam
vanadium phosphate
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vanadium
foam composite
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CN110668414A (en
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申泽骧
蒋海凤
来琳斐
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Sino Singapore International Joint Research Institute
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    • C01B25/00—Phosphorus; Compounds thereof
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    • C01B25/26—Phosphates
    • C01B25/45—Phosphates containing plural metal, or metal and ammonium
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    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
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    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
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    • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
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Abstract

The invention discloses a vanadium phosphate nano material with a porous network structure and a preparation method thereof, which is used for a lithium ion battery cathode material, and the preparation method comprises the following steps: firstly, growing self-supporting graphene foam by adopting a chemical vapor deposition method, and loading Vanadium Phosphate (VPO) on a graphene foam substrate by using a hydrothermal method4) Nano material to raise its conductivity and ionic conductivity. The vanadium phosphate nano material with the porous network structure can be used as a negative electrode material of a lithium ion battery, and has a good lithium ion energy storage behavior of 0.2A g‑1The charging capacity can reach 424.4mAh g under the current density of‑1(ii) a The capacity maintenance rate of the battery can still reach more than 50.3 percent after 3000 circles of charge-discharge cycles. The preparation method of the vanadium phosphate nano material is simple, avoids the complicated process of preparing the electrode plate by the traditional process, greatly reduces the cost problem of battery assembly, and can realize green large-scale production.

Description

Vanadium phosphate/graphene foam composite nano material with porous network structure and preparation method thereof
Technical Field
The invention relates to the technical field of energy storage and conversion, in particular to a vanadium phosphate/graphene foam composite nano material with a porous network structure applied to a lithium ion battery cathode material and a preparation method thereof.
Background
Since the commercialization of lithium ion batteries, which are a new type of electrochemical energy source, by sony corporation of japan in 1992, lithium ion batteries have been rapidly developed, and have gradually replaced nickel-chromium batteries and nickel-hydrogen batteries, which have become the third generation of small-sized energy storage batteries. The novel energy-saving control system has the advantages of high working voltage, large capacity, long cycle life, safety and the like, and is widely applied to various fields including smart power grids, mobile equipment, new energy electric vehicles and the like. Although lithium ion batteries have been used in human life (electronic devices), some large-scale devices (all-electric vehicles) have made higher requirements on energy density and power density of lithium ion batteries, and therefore researchers continue to explore and research the design of advanced electrode materials and the optimization of battery assembly processes, trying to maximize the electrochemical performance of lithium ion batteries, reducing the cost to a lower level and increasing the safety factor, and developing novel electrode materials with high energy density and long cycle performance has become the main direction of current lithium ion battery research.
For the electrode material of the lithium ion battery, the anode material is various, such as lithium iron phosphate, lithium cobaltate, nickel cobalt manganese ternary material, etc., and the anode material is widely applied to the lithium ion battery through the performance test and the marketization inspection of the system. However, among various cathode materials, the cathode material is relatively single, and the research is not as much as the cathode material, but the cathode material still plays a crucial role in improving the electrochemical performance of the whole battery, and the current commercialized cathode materials mainly use carbon materials, mainly including graphite, acetylene black, graphene and other materials. The reason is that the carbon cathode material has the advantages of wide source, large specific surface area, good conductivity, high mechanical strength, stable chemistry and thermodynamics and the like. However, the graphite is used as the negative electrode material of the lithium ion battery, and the capacity is only 372 mAh g-1The requirements for high energy density and power density of the lithium ion battery cannot be met, so that the development of a high-energy-density lithium ion battery cathode material is very important.
Vanadium Phosphate (VPO)4) Is one of the most promising lithium ion negative electrode materials and has higher specific capacity (550 mAhg)-1). The vanadium phosphate material of the crystal has an orthogonal structure and is composed of VO6Octahedron and PO4Tetrahedrally composed of, VO6Octahedron and PO4The tetrahedron is formed by connecting oxygen atoms at corners, has a three-dimensional network structure, and simultaneously, the distance between metal vanadium atoms is short, which can help the diffusion and the transfer of ions. However VPO4Having a PO4The tetrahedral structure has poor electrical conductivity, which restricts the development of vanadium phosphate.
In the research process of the invention, it is found that the introduction of the carbon material with better conductivity into the main material of vanadium phosphate is the most effective method for improving the poor conductivity of vanadium phosphate. Therefore, on the basis of these researches, it is urgently needed to provide a vanadium phosphate nano material with a porous network structure and a preparation method thereof.
Disclosure of Invention
The invention aims to solve the problem of VPO in the prior art4The problem of poor conductivity of the electrode material is that the vanadium phosphate/graphene foam composite nano material with a porous network structure is high in purity, good in crystallinity and excellent in conductivity and the preparation method thereof are provided.
The purpose of the invention can be achieved by adopting the following technical scheme:
a porous vanadium phosphate/graphene foam composite nanomaterial with a net structure is applied to a lithium ion battery cathode material, the net vanadium phosphate/graphene foam composite nanomaterial grows on self-supporting Graphene Foam (GF), the self-supporting graphene foam structure is prepared by a Chemical Vapor Deposition (CVD) method on the basis of a foam nickel framework, and then the vanadium phosphate/graphene foam composite nanomaterial is prepared on a graphene foam substrate by a hydrothermal method, wherein the preparation method comprises the following steps:
s1, adopting a low-valence vanadium source, a phosphorus source and a carbon source according to the weight ratio of 2: 4: 5 in deionized water, and mechanically mixing and stirring;
s2, adjusting the pH value of the solution obtained in the step S1 to 7 by ammonia water;
s3, transferring the mixture obtained in the step S2 to a reaction kettle for hydrothermal reaction;
s4, washing the sample obtained in the step S3 with water, and drying in an oven;
s5, mixing the self-supporting vanadium phosphate/graphene foam (VPO) obtained in the step S44and/GF) precursor is transferred into argon atmosphere to be calcined, and finally the vanadium phosphate/graphene foam composite nano material is obtained.
Further, the synthesized composite nano-material is a self-supporting structure, and the vanadium phosphate/graphene foam composite nano-material is a porous net loose structure.
Further, the vanadium source is vanadium pentoxide, the phosphorus source is ammonium dihydrogen phosphate, and the carbon source is citric acid.
Further, the temperature and time of the mechanical stirring in the step S1 were 80 ℃ and 2 hours, respectively.
Furthermore, the temperature of hydrothermal synthesis in the step S3 is 200 ℃, the time is 20 hours, and the reaction process is safe.
Further, the Graphene Foam (GF) is grown by adopting a Chemical Vapor Deposition (CVD) method, the number of deposited graphene layers is 8-12, and FeCl is used for growing3the/HCl solution removed the nickel substrate and the structure remained unchanged, resulting in a light, thin, flexible, self-supporting substrate.
Further, 2X 5 cm was first set2And placing the graphene foam with the size in a solution of a reaction kettle, growing a vanadium phosphate/graphene foam composite nano material on a substrate by a hydrothermal method, washing and drying the self-supporting structure, and calcining the self-supporting structure in a tubular furnace at 725 ℃ for 4 hours in an argon atmosphere to finally obtain the self-supporting nano structure.
The self-supporting composite electrode material is prepared, the electrode material can be cut into a proper size and then assembled into a battery, the complex process of assembling the battery in the traditional process is reduced (the active material, the conductive agent and the binding agent are uniformly mixed according to a certain proportion (the time of 1 day), the slurry is coated on a current collector by scraping, and the slurry is dried and cut into a proper size (the time of 1 day)), the cycle and the cost of battery preparation are greatly reduced, and the self-supporting composite electrode material has a commercial application prospect.
The vanadium phosphate active material is loaded on the graphene foam with good conductivity, so that the conductivity of the vanadium phosphate nano material is improved, the self-supporting flexible material is further prepared, the self-supporting nano material can be directly used as an electrode plate to assemble a battery, the use of a non-conductive binder is reduced, and the conductivity and the ion diffusion rate of the electrode material are increased. Meanwhile, when the vanadium phosphate/graphene foam composite nano material is synthesized, a carbon material is mixed, so that the problem of poor conductivity of the active material is further solved, the capacity of the active material is improved, and the lithium ion battery with high energy density and power density is prepared.
Compared with the prior art, the invention has the following advantages and effects:
(1) the vanadium source, the phosphorus source and the carbon source used in the method have low prices and low production cost.
(2) The method adopts a hydrothermal method to synthesize the material, has simple equipment and simple and convenient operation process, and provides guarantee for large-scale production of the electrode material.
(3) The invention also provides a self-supporting vanadium phosphate/graphene foam composite nano material of the lithium ion negative electrode material with high capacity and better cyclicity, and the conductivity of the material and the ionic conductivity are further improved.
(4) The preparation process of the traditional electrode material is simply described as follows, firstly, grinding and uniformly mixing an active material, a conductive agent and a binder according to a certain proportion, and dissolving the mixture by using an organic solvent (NMP) to prepare a honey-like slurry; uniformly scraping the slurry on a current collector by using a scraper; drying the active material on the current collector in an oven; and finally, cutting the electrode into electrode plates with proper sizes so as to assemble the battery, wherein the time spent in the process is generally 2-3 days. The process is complicated, the preparation process time is long, and the preparation cost is high. The method directly grows the active material on the graphene foam substrate with good conductivity, and the composite electrode material can be directly used for assembling the battery, so that the preparation period and cost of the electrode are greatly shortened, and the method has a great commercial application prospect.
Drawings
FIG. 1 is a self-supporting vanadium phosphate/graphene foam (VPO) prepared according to the present invention4@ GF) electron photographs of the nanomaterials;
FIG. 2 is a self-supporting vanadium phosphate/graphene foam (VPO) prepared according to the present invention4@ GF) powder X-ray diffraction patterns of nanomaterials and Graphene Foam (GF);
fig. 3 is a scanning electron microscope photograph of a prepared material of the present invention, in which,
FIG. 3 (a) is: scanning electron microscope pictures of Graphene (GF),
FIG. 3 (b) is: vanadium phosphate/graphene foam (VPO)4@ GF) scanning electron microscope pictures of nanomaterials;
FIG. 4 is a self-supporting vanadium phosphate/graphene foam (VPO) prepared according to the present invention4@ GF) projection electron microscope picture of nanomaterials, wherein,
FIG. 4 (a) is: the invention prepares a projection electron microscope picture with the sample magnification of 6 ten thousand times,
FIG. 4 (b) is: the invention prepares the projection electron microscope picture with the sample magnification of 15 ten thousand times
FIG. 5 is a self-supporting vanadium phosphate/graphene foam (VPO) prepared according to the present invention4@ GF) electrochemical characterization plots of composite nanomaterials and Graphene Foam (GF), where,
FIG. 5 (a) is: VPO4@ GF specific capacity values at different current densities,
FIG. 5 (b) is: the charge-discharge specific capacity values of the pure GF substrate under different current densities,
FIG. 5 (c) is: VPO4@ GF at 0.1A g-1A charge-discharge curve diagram under the current density,
FIG. 5 (d) is: VPO4@ GF at 0.2A g-1A cycle performance plot and a coulombic efficiency plot at current density;
FIG. 6 is a self-supporting vanadium phosphate/graphene foam (VPO) prepared according to the present invention4@ GF) composite nanomaterial on cyclic voltammograms at different scan rates, wherein,
fig. 6 (a) is: the scanning speed is from 0.2 to 1.0 mV/s of cyclic voltammogram,
FIG. 6 (b) is: scanning a cyclic voltammogram with the speed of 2-10 mV/s;
FIG. 7 is an analysis of VPO4The energy storage mechanism diagram of @ GF nano material, wherein,
FIG. 7 (a) is: the relationship between the peak currents of the cathode and the anode and the scanning speed when the scanning speed is 0.2-1.0 mV/s,
FIG. 7 (b) is: the relationship between the peak currents of the cathode and the anode and the scanning speed when the scanning speed is 2-10 mV/s,
FIG. 7 (c) is: when the scanning speed is 0.2 mV/time, the proportion of the current controlled by diffusion and the current controlled by capacitance,
FIG. 7 (d) is: under different scanning speeds, the proportion relation diagram of the current controlled by diffusion and the current controlled by capacitance;
FIG. 8 is a diagram of: VPO4The @ GF nano material has capacity surplus rate after 3000 cycles.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Example one
The embodiment discloses a vanadium phosphate nano material with a porous network structure and high purity, good crystallinity and excellent conductivity and a preparation method thereof4) Network nano-material in Vanadium Phosphate (VPO)4) The citric acid is added into the precursor solution to carry out carbon mixing treatment, thereby further improving the conductivity of the material.
The embodiment provides a vanadium phosphate/graphene foam composite nano material with a porous network structure for a lithium ion battery cathode material and a preparation method thereof, and the preparation method comprises the following steps:
1) adopting a low-valence vanadium source, a phosphorus source and a carbon source according to the weight ratio of 2: 4: 5 in deionized water, and mechanically mixing and stirring for 2 hours at the temperature of 80 ℃;
2) dropwise adding ammonia water into the solution prepared in the step 1) to enable the pH value of the solution to be 7;
3) transferring the mixture obtained in the step 2) into a reaction kettle, and carrying out hydrothermal reaction at 200 ℃ for 20 hours;
4) washing the sample obtained in the step 3) with water, and drying in a vacuum oven at 80 ℃;
5) subjecting the obtained self-supporting vanadium phosphate/graphene foam (VPO) in step 4)4and/GF) precursor is placed in an argon atmosphere for calcination treatment, so that the crystallinity of the material is improved, and the self-supporting vanadium phosphate/graphene foam composite nano material is finally obtained.
The vanadium phosphate/graphene foam self-supporting composite nano material for the lithium ion battery cathode material obtained by the preparation method is loose and porous, and is favorable for diffusion and transfer of lithium ions, and meanwhile, the contact area of the active material and the lithium ions is increased, so that VPO is synthesized4The graphene foam substrate and the carbon source are introduced in the process of the material, so that the conductivity of the material is further increased.
In the preparation method of the vanadium phosphate/graphene composite nanomaterial for the negative electrode material of the lithium ion battery, the vanadium source is vanadium pentoxide, the phosphorus source is ammonium dihydrogen phosphate or ammonium monohydrogen phosphate, and the carbon source is citric acid.
The self-supporting vanadium phosphate/graphene composite nano material prepared by the invention is used as a negative electrode material of a lithium ion battery, and the self-supporting material can be directly used as an electrode plate assembled battery, so that the influence of a binding agent (PVDF) on blocking lithium ion diffusion is reduced, and the performance of the material is improved.
The invention discovers that the vanadium phosphate lithium ion negative electrode material synthesized by a hydrothermal method shows the best electrochemical performance after being calcined at 725 ℃, and the capacity of the vanadium phosphate lithium ion negative electrode material is 0.2A g-1Can reach 424.4mAh g under the current density-1(ii) a Through 300 cycles (0.2A g)-1) The capacity maintenance rate can reach 61.5%; the capability of the material for long-term circulation under large current density is continuously researched, and the experimental result shows that the capacity of the material is improved after 3000 circles of charge-discharge cyclesThe amount maintenance rate can reach 50.3 percent.
The invention also analyzes the energy storage mechanism of the self-supporting vanadium phosphate/graphene composite nano material, the electrode material generally consists of two parts, namely a diffusion control process and a capacitance control process, and the capacitance control capacity of the electrode material accounts for 47% under the sweep rate of 0.2 mV/s.
Example two
The process of synthesizing the self-supporting vanadium phosphate/graphene foam composite nano material mainly comprises the following steps:
1) 0.364 g of vanadium pentoxide, 0.460 g of ammonium dihydrogen phosphate and 0.96 g of citric acid are respectively weighed and dissolved in 30 mL of DI water, and stirred for 2 hours at the temperature of 80 ℃;
2) after the mixed solution in the step 1) is stirred, adjusting the pH value of the solution to 7 by using concentrated ammonia water, then transferring the reaction solution into a 50mL reaction kettle, and reacting for 20 hours in a drying oven at 200 ℃;
3) after the reaction in the step 2) is finished, cooling to room temperature, and collecting the self-supporting vanadium phosphate/graphene composite nano material;
4) washing the sample obtained in the step 3) with deionized water for three times, and drying the self-supporting electrode material in an oven at 80 ℃;
5) calcining the sample dried in the step 4) in a tubular furnace, placing the calcined sample in an argon atmosphere, pre-calcining for 4 hours at 725 ℃, and obtaining the VPO with the porous network structure at the last, wherein the temperature rise speed is 5 ℃/min4@ GF composite electrode material.
As shown in FIG. 1, FIG. 1 is a self-supporting vanadium phosphate/graphene foam (VPO) prepared according to the present invention4@ GF) electrophotographic image of composite nanomaterial, VPO can be seen from FIG. 14The @ GF composite nano material has a unique porous structure and is completely different from a powdery nano material in appearance.
As shown in FIG. 2, FIG. 2 is a self-supporting vanadium phosphate/graphene foam (VPO) prepared according to the present invention4@ GF) powder X-ray diffraction patterns of composite nanomaterials and Graphene Foam (GF). From FIG. 2, one can see VPO4@ GF samplesIn addition to the X-ray diffraction peaks of graphene, other peaks are perfect and standard VPO4(PDF # 75-1621) diffraction peak pair, which shows that the sample prepared in this example has better purity.
As shown in FIG. 3, FIG. 3 is a diagram of the preparation of self-supporting vanadium phosphate/graphene foam (VPO) according to the present invention4@ GF) composite nanomaterial and Graphene Foam (GF). As can be seen from fig. 3 (a) and 3 (b), the structure of the graphene foam is still maintained after the nickel substrate is removed, and no structural collapse occurs; the vanadium phosphate with the nano-network structure uniformly grows on the surface of the graphene foam, does not agglomerate, and is a loose and porous structure.
FIG. 4 is a self-supporting vanadium phosphate/graphene foam (VPO) prepared according to the present invention4@ GF) projection electron microscope pictures of composite nanomaterials. As can be seen from FIGS. 4 (a) and 4 (b), in VPO4The nano structure has a structure with a plurality of small holes, and the holes are different in size, but the existence of the holes increases the contact between the active material and the electrolyte, and provides a premise for exerting high performance.
FIG. 5 is a self-supporting vanadium phosphate/graphene foam (VPO) prepared according to the present invention4@ GF) electrochemical characterization of composite nanomaterials and Graphene Foam (GF), fig. 5 (a) being VPO4@ GF specific capacity values at different current densities, FIG. 5 (b) are charge-discharge specific capacity values of pure GF base at different current densities, and FIG. 5 (c) is VPO4@ GF at 0.1A g-1FIG. 5 (d) is a VPO curve of charge and discharge at current density4@ GF at 0.2A g-1A plot of cycling performance at current density and a plot of coulombic efficiency. As can be seen from fig. 5 (a) and 5 (b), the specific capacity of the graphene foam substrate is very small and almost negligible. FIG. 5 (c) shows VPO4The coulombic efficiency of the first turn of @ GF can reach 74.5 percent, and the value of the coulombic efficiency is higher than that of a negative electrode material of a common lithium ion battery. FIG. 5 (d) illustrates VPO after 300 cycles have elapsed4The residual capacity rate of @ GF is still 61.5%, and the coulombic efficiency can reach 100%. Loose porous VPO4Active material, such asThe unique structure is beneficial to full contact of the electrolyte and the active material, and is beneficial to diffusion of lithium ions in the reaction of inserting/extracting the lithium ions, thereby further promoting the performance of the electrode material.
FIG. 6 is a self-supporting vanadium phosphate/graphene foam (VPO) prepared according to the present invention4@ GF) Cyclic voltammogram of the composite nanomaterial at different current densities, as can be seen from FIGS. 6 (a) and 6 (b), VPO4The @ GF sample showed a pair of redox peaks, but the peak currents were not large, which was required for VPO4The storage mechanism of @ GF was analyzed next.
FIG. 7 is an analysis of VPO4The @ GF electrode material energy storage mechanism diagram is shown in fig. 7 (a) and 7 (b) which are graphs showing the relationship between current and sweep rate, respectively, and fig. 7 (c) and 7 (d) which are graphs showing the analysis calculation of the ratio of capacity of diffusion control and capacity of capacitance diffusion at different sweep rates, respectively. As can be seen from fig. 7 (c) and 7 (d), the capacity of diffusion control occupies almost half of the ratio, which corresponds to a weaker redox peak; meanwhile, as the sweep rate increases, the capacity of the capacitance control and the capacity of the diffusion control both increase, but the capacity of the capacitance control increases by a large extent, and thus the ratio thereof increases.
FIG. 8 is a self-supporting vanadium phosphate/graphene foam (VPO) prepared according to the present invention4@ GF) composite nano material is 2A g-1Ability to cycle for long periods at large current densities, after 3000 cycles, VPO4The residual capacity of @ GF is still 50.3%, i.e. VPO after 3000 charge-discharge cycles4The @ GF capacity remains more than half.
The above embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents thereof, and all such changes, modifications, substitutions, combinations, and simplifications are intended to be included in the scope of the present invention.

Claims (5)

1. A porous vanadium phosphate/graphene foam composite nanomaterial with a net structure is used as a lithium ion battery cathode material, and is characterized in that the net vanadium phosphate/graphene foam composite nanomaterial grows on a self-supporting graphene foam substrate, the self-supporting graphene foam substrate grows by a chemical vapor deposition method, the number of deposited graphene layers is 8-12, and then a nickel substrate is removed by using a ferric trichloride/hydrochloric acid solution; then preparing a vanadium phosphate/graphene foam composite nano material on a graphene foam substrate by using a hydrothermal method, wherein the preparation method of the vanadium phosphate/graphene foam composite nano material is as follows:
s1, adopting a low-valence vanadium source, a phosphorus source and a carbon source according to the weight ratio of 2: 4: 5 in deionized water, and mechanically mixing and stirring;
s2, adjusting the pH value of the solution obtained in the step S1 to 7 by ammonia water;
s3, transferring the mixture obtained in the step S2 to a reaction kettle, adding a graphene foam substrate, and carrying out hydrothermal reaction to generate a vanadium phosphate nano material on the substrate to obtain a graphene foam sample;
s4, washing the graphene foam sample obtained in the step S3 with water, and drying in an oven to obtain a self-supporting vanadium phosphate/graphene foam precursor;
and S5, transferring the self-supporting vanadium phosphate/graphene foam precursor obtained in the step S4 into an argon atmosphere, and calcining to obtain the vanadium phosphate/graphene foam composite nano material.
2. The porous reticulated vanadium phosphate/graphene foam composite nanomaterial according to claim 1, wherein the vanadium phosphate/graphene foam composite nanomaterial is a porous reticulated porous structure.
3. The porous reticulated vanadium phosphate/graphene foam composite nanomaterial according to claim 1, wherein the vanadium source is vanadium pentoxide, the phosphorus source is ammonium dihydrogen phosphate, and the carbon source is citric acid.
4. The porous reticulated vanadium phosphate/graphene foam composite nanomaterial according to claim 1, wherein the temperature and time of the mechanical stirring in the step S1 are 80 ℃ and 2 hours, respectively.
5. The porous reticulated vanadium phosphate/graphene foam composite nanomaterial according to claim 1, wherein the hydrothermal synthesis temperature in step S3 is 200 ℃ and the hydrothermal synthesis time is 20 hours.
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