A method for preparing hemispherical array Li3VO4 / C lithium ion battery negative electrode material

Abstract

The present invention relates to a preparation method of a hemispherical array Li3VO4 / C anode material for lithium-ion batteries. Polyvinyl alcohol, lithium nitrate, ammonium metavanadate, and oxalic acid are added to an appropriate amount of deionized water and stirred evenly to obtain a green solution A. To construct a microreaction environment, an appropriate amount of ethylene glycol is directly measured and filled into syringes for double-needle mixed electrospray, and the mixture is collected on a rotating receiver covered with aluminum foil with a negative voltage of 2-5 kV, dried, and calcined to obtain a hemispherical array Li3VO4 / C composite material. For the first time, the present invention uses a simple double-needle electrospray to synthesize a hemispherical Li3VO4 / C composite material as an anode material for lithium-ion batteries. For the first time, ethylene glycol is used to construct a microemulsion reaction environment, and the combination of microemulsion and electrospray is used to solve the problem of difficult morphology control in electrospray. The prepared hemispherical Li3VO4 / C composite material shows excellent electrochemical performance as an anode material for lithium-ion batteries.

Description

Technical Field

[0001] The invention relates to a novel negative electrode material for lithium ion batteries, in particular to a method for preparing a hemispherical array Li3VO4 / C composite material as a negative electrode material for lithium ion batteries by electrostatic spraying, and belongs to the field of electrochemical energy. Background Art

[0002] Affected by global warming and the continuous consumption of non-renewable fossil energy, the global energy consumption structure is transforming towards low-carbonization, and the development of renewable energy has become the mainstream direction of global energy transformation. Lithium-ion batteries are widely used in communication facilities, power tools, and energy storage due to their advantages such as low environmental load, superior performance compared to other batteries, and reusability. Correspondingly, people have put forward higher and higher requirements for the performance of lithium-ion batteries. High energy density, high stability, and low-cost lithium-ion battery materials are the focus of current research.

[0003] There are many problems with traditional lithium-ion battery materials, such as high irreversible capacity, voltage hysteresis, and unclear discharge platform of carbon nanotubes. Silicon-based negative electrode materials are ideal negative electrode materials for lithium-ion batteries, but the volume expansion of silicon electrodes can reach 300%, which seriously affects the cycle performance of silicon negative electrode materials. Therefore, we selected a new type of lithium-ion battery negative electrode material: Li3VO4, which has the advantages of higher volumetric capacity and lower voltage platform. However, the electronic conductivity and ionic conductivity of Li3VO4 negative electrode materials are relatively low, which may lead to greater polarization during charging / discharging, poor electrochemical reaction dynamics, and poor battery cycle performance at high rates. Therefore, it is necessary to enhance the conductivity of Li3VO4 materials to improve overall performance.

[0004] Based on the above background, this patent has developed a method for preparing hemispherical Li3VO4 / C composite materials based on electrostatic spraying. The main body of the prepared material is hemispherical carbon interconnected, and a large number of ultra-small nanosheets grow on the surface of the carbon shell. The overall structure of the prepared Li3VO4 / C composite material has good mechanical properties and strong bearing capacity. It can effectively inhibit the structural collapse caused by volume expansion during charging and discharging. The local ultra-small nanosheets can provide an efficient way for the electrolyte to penetrate, and the uniform carbon composite can significantly improve the conductivity of the material. The prepared hemispherical Li3VO4 / C composite material exhibits excellent electrochemical properties as a negative electrode for lithium-ion batteries and has great potential application value. Summary of the invention

[0005] The present invention proposes a method for preparing a hemispherical array Li3VO4 / C lithium-ion battery negative electrode material. The prepared hemispherical Li3VO4 / C composite material is synthesized by using a simple double-needle electrostatic spray, and for the first time, ethylene glycol is used to construct a microemulsion reaction environment. The combination of microemulsion and electrostatic spray solves the problem of difficult electrostatic spray morphology control. The interconnection between the hemispheres forms an interconnected path for lithium ion diffusion, and the hemispherical structure is conducive to the contact and penetration of the electrolyte. The material has excellent comprehensive electrochemical properties as a negative electrode for lithium-ion batteries and has potential application value in lithium-ion batteries.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A method for preparing a hemispherical array Li3VO4 / C lithium ion battery negative electrode material, the method comprising the following steps:

[0008] (1) Add polyvinyl alcohol, lithium nitrate, ammonium metavanadate and oxalic acid into an appropriate amount of deionized water and stir evenly to obtain a mixed solution;

[0009] (2) transferring the mixed solution and ethylene glycol of step (1) into electrostatic spray syringes respectively, and performing electrostatic spraying respectively to obtain a precursor composite atomized film;

[0010] (3) The product obtained by steam crosslinking the precursor composite atomized film obtained in step (2) is placed in a nitrogen environment, heated to 200-300° C. and pre-fired for 2-5 hours, and then heated to 500-800° C. and calcined for 3-6 hours to obtain a hemispherical array Li3VO4 / C composite material.

[0011] Preferably, the molar ratio of lithium nitrate, oxalic acid and ammonium metavanadate in the mixed solution in step (1) is (3-4): (5-6): (1-1.3), and the mass fraction of polyvinyl alcohol is 4-6%.

[0012] The content of ethylene glycol in step (2) is higher than 99.5%.

[0013] Preferably, the spraying conditions in step (2) are: voltage 18-20 kV, time 6-8 hours, temperature 40-60° C., humidity 20-30%, and the distance between the spinning needle and the drum is 20-30 cm.

[0014] Preferably, step (2) is double-needle mixed electrostatic spraying, and the distance between the double needles is 7-8 cm.

[0015] Preferably, in step (3), the temperature of steam cross-linking is 60-80° C., and the steam cross-linking reaction is carried out for 10-12 hours.

[0016] Preferably, in step (3), the pre-sintering temperature is 200-300°C, the heating rate is 5-10°C / min, the pre-sintering time is 2-5 hours, and then the pre-sintering time is 500-800°C for 3-6 hours in a nitrogen atmosphere at a heating rate of 3-12°C / min.

[0017] The present invention forms charged droplets by atomizing a polymer solution in a strong electric field, and obtains hemispherical Li3VO4 / C electrode material by using double-needle mixed electrostatic spray for the first time. A large number of ultra-small nanosheet layers are embedded in the hemispherical carbon matrix, which significantly enhances the diffusion of lithium ions.

[0018] The principle is as follows: (1) The microemulsion of a mixed system of polyols and water is an important microreactor for preparing size-controlled nanomaterials. Ethylene glycol can form a hydrogen bond network different from water, and has high cohesive energy and low dielectric constant, and has good conductivity; (2) The water-based polymer polyvinyl alcohol (PVA) has good adhesion, and under vigorous stirring, a precursor solution containing lithium source, vanadium source, PVA, oxalic acid and water molecules is formed; (3) Due to the atomization effect of electrostatic spray, solution A forms droplets, and ethylene glycol is coated on the droplets that have not completely evaporated under the action of electrostatics, and is finally collected on a rotating aluminum foil to form an atomized film; (4) Under the action of electrostatics, the atomized film undergoes particle self-assembly, and at the same time, it grows neatly in the microreactor; (5) PVA molecules and oxalic acid molecules cross-link during heating, accompanied by the release of H2O, and water vapor escapes from the precursor particles to generate pores on the top of the particles. In this process, due to the tension in the precursor particles, hollow hemispheres are gradually formed. At the same time, various adjacent hollow hemispheres are neatly arranged to form a hollow hemisphere array; (6) The interconnection between the hemispheres forms an interconnected pathway for the diffusion of lithium ions. At the same time, the hemisphere structure is conducive to the contact and penetration of the electrolyte, so that the material has excellent comprehensive electrochemical properties as a negative electrode for lithium-ion batteries.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] (1) The new microemulsion method combined with double-needle electrostatic spraying is simple and easy to operate, low in production cost, and environmentally friendly;

[0021] (2) The synthesis process is simple, does not require high temperature and high pressure environment, the morphology is controllable, and the performance is excellent.

[0022] (3) The prepared composite material has an interconnected structure consisting of hemispheres connected to each other, which has the characteristics of strong structural bearing capacity and stable performance. It can withstand the volume expansion caused by charging and discharging. The spherical structure is conducive to the contact and penetration of the electrolyte, and has high capacity and excellent cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1Optical photographs of the sample precursor prepared after spraying in Example 1: (a) after spraying, (b) after drying, and (c) after sintering.

[0024] Figure 2 XRD pattern of the sample prepared in Example 1.

[0025] Figure 3 SEM image of the sample prepared in Example 1.

[0026] Figure 4 The first three cycles of charge and discharge curves and cycle performance diagram of the sample prepared in Example 1: (a) the first three cycles of charge and discharge curves, (b) cycle performance diagram.

[0027] Figure 5 Optical photograph of the sample prepared in Example 2 after drying.

[0028] Figure 6 SEM image of the sample prepared in Example 2.

[0029] Figure 7 The first three cycles of charge and discharge curves and cycle performance diagram of the sample prepared in Example 2: (a) the first three cycles of charge and discharge curves, (b) cycle performance diagram.

[0030] Figure 8 Optical photographs of the sample precursor prepared after spraying in Example 3: (a) after spraying, (b) after drying.

[0031] Fig. 9 SEM image of the sample prepared in Example 3.

[0032] Fig.10 The first three cycles of charge and discharge curves and cycle performance diagram of the sample prepared in Example 3: (a) the first three cycles of charge and discharge curves, (b) cycle performance diagram.

[0033] Fig.11 Optical photographs of the sample precursor prepared after spraying in Example 4: (a) after spraying, (b) after drying, and (c) after sintering.

[0034] Fig.12 The first three cycles of charge and discharge curves and cycle performance diagram of the sample prepared in Example 4: (a) the first three cycles of charge and discharge curves, (b) the cycle performance diagram. DETAILED DESCRIPTION

[0035] Example 1

[0036] 0.8 g polyvinyl alcohol, 7.5 mmol lithium nitrate, 2.5 mmol ammonium metavanadate, and 12.5 mmol oxalic acid were accurately weighed according to chemical stoichiometry and added to 15 mL deionized water and stirred for 48 hours to obtain a uniform green solution A; 3 mL ethylene glycol was directly measured as solution B. Solution B was loaded into a syringe and placed under solution A for double-needle mixed electrostatic spraying. The distance between the double needles was 8 cm, the voltage was 18 kV, and the temperature was 40°C. The spraying was carried out for 8 hours to obtain an atomized film of the two materials interwoven ( Figure 1 a), after spraying, the precursor material was quickly transferred to an 80℃ forced air drying oven for steam crosslinking for 12 hours. It can be seen that the precursor material is yellow and viscous ( Figure 1 b), and then pre-sintered at 250 °C for 3 hours in a nitrogen environment and calcined at 600 °C for 5 hours to obtain the Li3VO4 / C composite material ( Figure 1 c), XRD analysis of the carbonized composite material shows that the diffraction peaks correspond well to those of Li3VO4 (PDF#38-1247) ( Figure 2 The prepared Li3VO4 / C composite material has a hemispherical array structure with a hemispherical size of about 50 μm. Some damaged microspheres indicate that it is a hollow structure ( Figure 3 a, b). The formation of this structure comes from: 1) The material undergoes a cross-linking reaction during the heating process, accompanied by the release of H2O, forming gaps inside the material, and further H2O molecules escape from the top of the particles, resulting in the formation of a hollow structure. 2) The tension of PVA in the precursor enhances the overall structural stability of the hollow spheres and causes the hollow spheres to shrink to form a hemispherical array structure. ( Figure 3 a,b). The surface of the hollow sphere is composed of a large number of nanosheets ( Figure 3 c), which comes from the microemulsion reaction of Li3VO4 precursor in water@ethylene glycol environment and subsequent solid phase reaction during heating. In the Li3VO4 / C hemispherical array structure, local ultra-small nanosheets can provide efficient penetration pathways for electrolyte, and the uniform composite of carbon can significantly improve the conductivity of the material.

[0037] The material was made into a battery as follows: the prepared sample was mixed with acetylene black and polyvinylidene fluoride in a weight ratio of 8:1:1, and a slurry was prepared using N-methylpyrrolidone as a solvent. The slurry was coated on a copper foil with a thickness of 10 μm, dried at 60°C for 2 h, cut into discs with a diameter of 14 mm, and vacuum dried at 120°C for 10 h. A metal lithium sheet was used as the counter electrode, a Celgard membrane was used as the diaphragm, and a solution of EC+DMC+DEC (volume ratio of 1:1:1) containing LiPF6 (1 mmol / L) was used as the electrolyte. The CR2025 battery was assembled in an argon-protected glove box. After the battery was assembled, it was left to stand for 8 hours, and then a constant current charge and discharge test was performed using a CT3001 battery test system. The test voltage was 3~0.01 V and the current density was 200 mA g -1 . Figure 4 The first three cycles of charge and discharge curves and cycle performance of the prepared hemispherical array Li3VO4 / C lithium-ion battery. The first charge and discharge specific capacities are 725 and 1009.9 mAh g -1 ( Figure 4 a), there are obvious charging and discharging platforms. After 100 cycles, the charging and discharging capacities are 574 and 576.4 mAh g, respectively. -1 ( Figure 4 b) exhibits excellent electrochemical performance.

[0038] Example 2

[0039] In this example, only the ethylene glycol in Example 1 was replaced with DMF for spraying, and the surface was yellow rough granules after drying ( Figure 5 ), and then the material was obtained under the same sintering conditions. The prepared sample morphology showed an irregular porous structure ( Figure 6 The morphological difference between this embodiment and embodiment 1 is mainly due to the difference in microemulsion environment. The material obtained in embodiment 2 was made into a battery according to the method of embodiment 1. Figure 7 As shown in Figure 2, the first charge and discharge specific capacities are 524.3 and 849.9 mAh g -1 ( Figure 7 a), there are obvious charging and discharging platforms. After 100 cycles, the charging and discharging capacities are 298.3 and 300.4 mAh g, respectively. -1 ( Figure 7 b) Poor electrochemical performance.

[0040] Example 3

[0041] In this embodiment, only solution A in embodiment 1 is used for spraying, and the precursor material has a gray rough surface ( Figure 8 a), cross-linking reaction occurred during the drying process and yellow particles precipitated ( Figure 8b), and then the material was obtained under the same sintering conditions. The prepared sample had a honeycomb structure ( Fig. 9 The difference in morphology between this example and Example 1 indicates the key role of ethylene glycol in the formation of the Li3VO4 / C hemispherical array structure. The material obtained in Example 3 was made into a battery according to the method of Example 1. Fig.10 As shown in Figure 2, the first charge and discharge specific capacities are 516.2 and 767.9 mAh g -1 ( Fig.10 a), there are obvious charging and discharging platforms. After 100 cycles, the charging and discharging capacities are 466 and 469.4 mAh g, respectively. -1 ( Fig.10 b) Poor electrochemical performance.

[0042] Example 4

[0043] In this example, only solution A and solution B of Example 1 were uniformly mixed and sprayed with a single needle. After the solution was sprayed, a green smooth surface was formed ( Fig.11 a), Particle aggregation after drying ( Fig.11 b), and made it into a battery according to the method of Example 1. Fig.10 As shown in Figure 2, the first charge and discharge specific capacities are 480.1 and 735.6 mAh g -1 ( Fig.12 a), there are obvious charging and discharging platforms. After 100 cycles, the charging and discharging capacities are 442.3 and 444.9 mAh g, respectively. -1 ( Fig.12 b) Poor electrochemical performance.

[0044] Example 5

[0045] In this embodiment, the ethylene glycol content in Embodiment 1 is only increased to 5 mL. During the spraying process, due to excessive solution, it cannot be completely volatilized under high pressure, and the product collected on the aluminum foil cannot be separated from the aluminum foil.

[0046] Example 6

[0047] This embodiment is based on the embodiment 1 without adding oxalic acid. The obtained solution A is not uniform and has too strong viscosity. Therefore, electrostatic spraying cannot be performed, and battery products cannot be realized.

[0048] Example 7

[0049] In this example, only the distance between the two needles in Example 1 was changed to 5 cm. Since water and ethylene glycol are miscible in any ratio and the distance between the needles is too close, the water in solution A is combined with ethylene glycol and miscible without volatilization, resulting in the product collected on the aluminum foil being unable to be separated from the aluminum foil.

[0050] Example 8

[0051] In this embodiment, only the distance between the two needles in embodiment 1 is changed to 10 cm. Since the distance is too far, the mist droplet A and ethylene glycol cannot be evenly combined, and it is difficult to collect the spinning product on the aluminum foil, so the battery product cannot be realized.

Claims

1. A method for preparing a hemispherical array Li3VO4 / C lithium ion battery negative electrode material, characterized in that: The method comprises the following steps: (1) Add polyvinyl alcohol, lithium nitrate, ammonium metavanadate and oxalic acid into an appropriate amount of deionized water and stir evenly to obtain a mixed solution; (2) The mixed solution and ethylene glycol of step (1) are transferred to electrostatic spray syringes respectively, and electrostatic spraying is performed respectively to obtain a precursor composite atomized film, wherein the content of ethylene glycol is higher than 99.5%. The electrostatic spraying conditions are: double-needle mixed electrostatic spraying is performed at a temperature of 40-60°C, a humidity of 20-30%, a voltage of 18-20 kV, and the distance between the double needles is 7-8 cm, and the electrospinning is performed for 6-8 hours; (3) The precursor composite atomized film obtained in step (2) is placed in a nitrogen environment after steam cross-linking to obtain a product, and calcined to obtain a hemispherical array Li3VO4 / C composite material.

2. The method for preparing the hemispherical array Li3VO4 / C composite material according to claim 1, characterized in that: The molar ratio of lithium nitrate, oxalic acid and ammonium metavanadate in the mixed solution of step (1) is (3-4): (5-6): (1-1.3), and the mass fraction of polyvinyl alcohol is 4-6%.

3. The method for preparing the hemispherical array Li3VO4 / C composite material according to claim 1, characterized in that: In step (3), the precursor composite atomized film is steam cross-linked at 60-80° C. for 10-12 hours.

4. The method for preparing the hemispherical array Li3VO4 / C composite material according to claim 1, characterized in that: In step (4), the calcination is carried out at a pre-calcination temperature of 200-300°C, a heating rate of 3-10°C / min, pre-calcination for 2-5 hours, and then calcination at 500-800°C for 3-6 hours at a heating rate of 3-12°C / min.

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