A dual-carbon-layer modified Na 3.5 Mn 0.5 V 1.5 (PO4)3 cathode material preparation and application

Abstract

The present invention discloses the preparation and application of a dual-carbon-layer modified Na 3.5 Mn 0.5 V 1.5 (PO4)3 sodium-ion battery cathode material. A precursor is obtained by the sol-gel method, and then heat-treated under inert gas conditions to finally obtain an amorphous carbon and carbon nanotube dual-carbon-layer modified Na 3.5 Mn 0.5 V 1.5 (PO4)3@C@CNTs composite material. This preparation method avoids problems such as large particle size and insufficient contact of reactants brought about by the traditional solid-phase method. Citric acid acts as a reducing agent in the solution to reduce V 5+ to V 3+ , which decomposes into amorphous carbon at high temperature and in-situ coats the surface of sample particles. Carbon nanotubes (CNTs) are used as a conductive network to coat the electrode material, effectively improving the electron conduction between sample particles and significantly enhancing the electrochemical properties such as the reversible capacity and cycle stability of the material. The preparation process of the present invention is simple and the raw materials are easily available. When this material is used as the cathode of a sodium-ion battery, it exhibits a high specific capacity and a long cycle life.

Description

Technical Field

[0001] The present invention belongs to the technical field of cathode materials and electrochemical devices for sodium-ion batteries, and particularly relates to the preparation and application of a dual-carbon-layer modified Na 3.5 Mn 0.5 V 1.5 (PO4)3 cathode material. This material is synthesized by the sol-gel method and exhibits excellent electrochemical performance when applied as a cathode material for sodium-ion batteries. Background Art

[0002] Lithium-ion batteries dominate the field of portable electronic products due to their advantages such as long cycle life and high energy density, and are also used in new energy vehicles. However, the scarcity of lithium resources, high cost, and certain safety hazards have affected the development of lithium-ion batteries in large-scale energy storage devices. Therefore, it is necessary to develop a new energy storage system with rich resources and low cost. Sodium, which is in the same main group as lithium, has rich resources and low cost, and has similar physical and chemical properties to lithium. Therefore, sodium-ion batteries are very likely to replace lithium-ion batteries in the future.

[0003] NASICON-type (sodium superionic conductor) Na3V2(PO4)3 is considered an attractive SIB electrode material due to its easy preparation, large capacity, and high potential. However, V is a trace element with a high price and toxicity, which limits its practical application. Doping sodium vanadate phosphate with low-cost Mn element can not only increase the working voltage and capacity of the electrode, but also reduce the cost. For example, the invention patent CN106946238A discloses a sodium manganese vanadate phosphate electrode material, its preparation method and application, and uses the solid-phase method to synthesize Na 3.5 Mn 0.5 V 1.5 (PO4)3 electrode material and carbon-coated Na 3.5 Mn 0.5 V 1.5 (PO4)3 electrode material. Carbon coating improves the electronic conductivity of the material and to a certain extent increases the capacity of the material. However, the materials prepared by the solid-phase method have a large particle size, insufficient contact between reactants, and a large diffusion distance of Na + in the solid phase, which restricts the high-rate output of Na 3.5 Mn 0.5 V 1.5 (PO4)3 material. Carbon coating can only improve the electronic conductivity of a single particle, and the conductivity between particles cannot be effectively improved. Therefore, it is necessary to propose a new strategy to improve the electrochemical performance by reducing the particle size and improving the conduction of electrons between multiple particles. Summary of the Invention

[0004] The present invention addresses the problems existing in the prior art and provides a preparation and application of a dual-carbon-layer modified Na 3.5 Mn 0.5 V 1.5 (PO4)3 cathode material. By using the sol-gel method to prepare nanosized particles, the diffusion distance of sodium ions is shortened. The Na 3.5 Mn 0.5 V 1.5 (PO4)3 material is modified by carbon coating and carbon nanotubes in a synergistic manner to accelerate the electron transfer speed between particles. The method of this patent has the advantages of simple operation, high efficiency, wide application range, etc., which is conducive to the large-scale application of the Na 3.5 Mn 0.5 V 1.5 (PO4)3 cathode material.

[0005] To achieve the above object, the specific steps for preparing the dual-carbon-layer modified Na 3.5 Mn 0.5 V 1.5 (PO4)3 cathode material by the sol-gel method are as follows:

[0006] (1) Add vanadium pentoxide and citric acid to deionized water and stir magnetically until completely dissolved to obtain a dark green solution. Subsequently, weigh sodium acetate, manganese acetate tetrahydrate, and ammonium dihydrogen phosphate according to the stoichiometric ratio and add them to the solution. Finally, ultrasonically disperse the carbon nanotubes and add them to the mixed solution. The mixed solution is placed in an oil bath at 80 °C and stirred and dried to evaporate the water to obtain a gel precursor;

[0007] (2) Place the product obtained in step (1) in a vacuum drying oven and dry it at 120 °C for 12 hours, and then grind it to obtain a powder precursor;

[0008] (3) Press the precursor powder sample obtained in step (2) into tablets with a Ф15mm mold under a pressure of 2 - 4 MPa, and then place it in a tube furnace and pre-burn it at 400 °C for 4 h in an Ar atmosphere;

[0009] (4) Take out the sample sintered in step (3), grind it, and sinter it at 700 °C for 6 h to obtain a high-purity Na 3.5 Mn 0.5 V 1.5 (PO4)3@C@CNTs sample.

[0010] The preparation conditions involved in the present invention are simple and easy for large-scale industrial production. In-situ dual-carbon coating of the material improves the conductivity of the material, thereby showing better electrochemical performance. Brief Description of the Drawings

[0011] Figure 1 is the dual-carbon-layer modified Na of the present invention3.5 Mn 0.5 V 1.5 XRD refined data graph of the Na

[0012] Figure 2 This is the Na modified by the double carbon layer of the present invention 3.5 Mn 0.5 V 1.5 SEM image of the Na

[0013] Figure 3 This is the Na modified by the double carbon layer of the present invention 3.5 Mn 0.5 V 1.5 Cyclic voltammogram of the Na

[0014] Figure 4 This is the Na modified by the double carbon layer of the present invention 3.5 Mn 0.5 V 1.5 Cycling performance graph of the Na at 1 C rate

[0015] Figure 5 This is the Na modified by the double carbon layer of the present invention 3.5 Mn 0.5 V 1.5 Rate performance graph of the Na

[0016] Figure 6 This is the Na modified by the double carbon layer of the present invention 3.5 Mn 0.5 V 1.5 Cycling performance graph of the Na at 10 C rate Detailed implementation manners

[0017] To make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described below in conjunction with the accompanying drawings and specific implementation manners. However, the protection scope of the present invention is not limited to the following embodiments

[0018] Example 1:

[0019] (1) In this example, 5 mmol of the target product Na 3.5 Mn 0.5 V 1.5(PO4)3@C@CNTs material, weigh 0.6889 g of vanadium pentoxide and 1.8996 g of citric acid, add 20 mL of deionized water, and stir magnetically to completely dissolve to obtain a dark green solution. Subsequently, weigh 1.4935 g of sodium acetate, 0.6189 g of manganese acetate tetrahydrate, and 1.7429 g of ammonium dihydrogen phosphate and add them to the solution. Finally, ultrasonically disperse 0.13 g of carbon nanotubes and add them to the mixed solution. The mixed solution is placed in an oil bath at 80 °C and stirred and dried to evaporate the water to obtain a dry gel;

[0020] (2) Place the product obtained in step (1) in a vacuum drying oven and dry it at 120 °C for 12 hours, and grind it for 1 h to obtain a powder sample;

[0021] (3) Press the precursor powder sample obtained in step (2) into tablets with a Ф15mm mold under a pressure of 2 - 4 MPa, then place it in a tube furnace and pre-burn it at 400 °C in an Ar atmosphere for 4 h;

[0022] (4) Take out the sample sintered in step (3), grind it, and sinter it at 700 °C for 6 h to obtain a high-purity Na 3.5 Mn 0.5 V 1.5 (PO4)3@C@CNTs sample.

[0023] Figure 1 It shows that the double-carbon layer modified Na 3.5 Mn 0.5 V 1.5 (PO4)3 cathode material is successfully synthesized. It can be seen from the spectrum that the phase of the synthesized material at 700 °C is pure and there are no impurity peaks.

[0024] The material Na 3.5 Mn 0.5 V 1.5When (PO4)3@C@CNTs is used as the cathode material for sodium-ion batteries, the electrode is prepared according to the mass ratio of active material:superconductive carbon black:PVDF (binder) of 7:2:1. First, PVDF is placed in a 5-ml glass beaker, and an appropriate amount of N-methylpyrrolidone solution (NMP) is added. After magnetic stirring until it becomes clear, the active electrode material and superconductive carbon black are mixed and ground in a mortar for 60 minutes. Then, it is taken out and placed in a glass beaker and stirred for 8 hours. Finally, it is evenly coated on the current collector aluminum foil and dried in a vacuum drying oven at 100 °C for 12 hours. After taking it out and slicing, the diameter of the electrode sheet is 12 mm. Then, the electrode sheet is compacted using a tablet press to obtain the working electrode. A sodium metal sheet is used as the counter electrode and reference electrode, and glass fiber (GF / D) is used as the separator. The electrolyte is an organic solution of NaClO4 dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 1:1). The assembly of the CR2032 type button battery is carried out in a glove box filled with argon, and it is required that the oxygen content and water content in the glove box are both lower than 0.1 ppm. The charge and discharge capacity is calculated based on the mass of the active material.

[0025] Figure 2 is double-carbon layer modified Na 3.5 Mn 0.5 V 1.5 SEM image of the (PO4)3 cathode material. It can be seen from the figure that the particles are roughly spherical, with a diameter between 100 and 300 nm. Multiple ends of carbon nanotubes can be seen on the surface of the particles, indicating that the other end of the carbon nanotubes has been successfully embedded into the particles.

[0026] Figure 3 is double-carbon layer modified Na 3.5 Mn 0.5 V 1.5 CV curves of the (PO4)3 cathode material in the first three cycles at a scanning rate of 0.2 mV / s and a voltage range of 2.5 - 4.2 V. Except for the first cycle, the curves of the second and third cycles almost overlap, indicating that the electrode material has good cycle reversibility. Compared with the published patent CN106946238A, there is a pair of redox peaks at about 3.9 V for the electrode material of the present invention, which corresponds to the V 4+ / V 5+ redox reaction. This is because the added carbon nanotubes have been successfully embedded into the sample particles, effectively improving the electronic conductivity of the material, thus activating the V 4+ / V 5+ redox reaction.

[0027] Figure 4 is double-carbon layer modified Na 3.5 Mn 0.5 V 1.5(PO4)3 cathode material's cycling performance graph at 1 C rate. After 200 cycles, the discharge specific capacity of the material is as high as 100.4 mAh / g, and the capacity retention rate is 86.3%, indicating that the material has good cycling stability performance.

[0028] Figure 5 is double-carbon-layer modified Na 3.5 Mn 0.5 V 1.5 (PO4)3 cathode material's performance graphs at different rates. At current densities of 0.2 C, 0.5 C, 1 C, 2 C, 5 C, and 10 C, the discharge specific capacities reach 120 mAh / g, 114.1 mAh / g, 110.8 mAh / g, 106.6 mAh / g, 101.8 mAh / g, and 96.2 mAh / g respectively. When the current returns to 0.2 C, the discharge specific capacity is 112.9 mAh / g. This shows that the material still has good rate reversibility after cycling at high current densities and then returning to low current densities. Compared with the capacity of the Na 3.5 Mn 0.5 V 1.5 (PO4)3@C material in the high-rate 10 C in the published patent CN106946238A, the reversible capacity is increased by nearly 30 mAh / g.

[0029] Figure 6 is double-carbon-layer modified Na 3.5 Mn 0.5 V 1.5 (PO4)3 cathode material's long cycling performance graph at 10 C high rate. The initial discharge specific capacity is 101.2 mAh / g, and the capacity retention rate after 8000 cycles is still 76%, indicating that the material has excellent high-rate performance and long cycling life.

Claims

1. A preparation method of a dual-carbon-layer modified Na 3.5 Mn 0.5 V 1.5 (PO4)3 cathode material, characterized in that The specific steps are as follows: (1) Add vanadium pentoxide and citric acid to deionized water and stir magnetically to completely dissolve them to obtain a dark green solution; subsequently, weigh sodium acetate, manganese acetate tetrahydrate, and ammonium dihydrogen phosphate according to the stoichiometric ratio and add them to the solution; then, ultrasonically disperse the carbon nanotubes and add them to the mixed solution; finally, place the mixed solution in an oil bath at 80 °C and stir and dry it to evaporate the water to obtain a sol-gel; (2) Place the product obtained in step (1) in a vacuum drying oven and dry it at 120 °C for 12 hours, and then grind it to obtain a powder sample; (3) Press the precursor powder sample obtained in step (2) into tablets with a Ф10 mold under a pressure of 2-4 MPa, and then place it in a tube furnace and pre-burn it at 400 °C for 4 h under an Ar atmosphere; (4) Take out the sample sintered in step (3), after grinding, sinter at 700 °C for 6 h to obtain the double-carbon layer modified Na 3.5 Mn 0.5 V 1.5 (PO4)3 sample.

2. The preparation method according to claim 1, characterized in that: The double-carbon-layer modified Na 3.5 Mn 0.5 V 1.5 (PO4)3 cathode material is applied to the cathode material of sodium-ion batteries.

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