Recently, the research team led by Li Xianfeng, Zhang Huamin, and associate researcher Zheng Qiong of the Energy Storage Technology Research Department of Dalian Institute of Chemical Physics, Chinese Academy of Sciences has made new progress in the research of polyanion cathode materials for sodium ion batteries. ACS Energy Letters.
Sodium ion battery has the advantages of rich resources, low cost, high cost performance, etc. It has a good application prospect in the field of electric bicycles, low-speed electric vehicles, distributed energy storage, and large-scale energy storage. The working principle of sodium ion batteries and lithium ion batteries is similar, the positive electrode material determines the energy density of sodium ion batteries. Polyanion compounds have the advantages of higher voltage, higher theoretical specific capacity, and stable structure, and have become one of the preferred materials for the cathode materials of sodium ion batteries. However, the low anionic conductivity of polyanionic compounds limits the specific capacity and rate performance of batteries. Therefore, further improving its rate performance, optimizing the performance of the whole battery, and further reducing the cost of material preparation, and achieving large-scale preparation of materials are difficult problems to be solved. In response to this, the team has carried out a series of studies in recent years on the structural element control of polyanion cathode materials for sodium ion batteries, sodium deintercalation mechanism, carbon composite preparation, construction of full batteries and soft pack batteries, etc. Sodium vanadium fluorophosphate Na3V2 (PO4) 2F3 (Nano Energy, 2018; J. Mater. Chem. A, 2017), sodium vanadium fluorophosphate NaVPO4F (J. Mater. Chem. A, 2018), sodium vanadium phosphate Na3V2 (PO4) 3 (J. Mater. Chem. A, 2018; J. Mater. Chem. A, 2016) and other efficient synthesis and application of vanadium-based polyanionic compounds.
Sodium vanadium trifluorophosphate has a three-dimensional network structure formed by the intermittent connection of [V2O8F3] double octahedron and [PO4] tetrahedron, which is beneficial to the rapid insertion and extraction of Na +. The theoretical energy density is 500Wh / kg, which is equivalent to the energy density of LiFePO4 in lithium ion batteries (550Wh / kg), and it has attracted much attention in recent years. On the basis of previous research work, the research team proposed a low-temperature solvothermal-ball mill preparation method to realize the green economic synthesis of high conductivity carbon-coated sodium vanadium fluorophosphate (Na3V2 (PO4) 2F3). The study found that the solvent type and pH value in the low-temperature solvothermal process played a key role in Na3V2 (PO4) 2F3 morphology and product purity. In the acidic environment of ethanol and water blending solvent, the crystal has a very high surface energy, and Na3V2 (PO4) 2F3 with high purity and high yield can be obtained. After a short time (1h) ball milling with Ketjen Black (KB), the surface of Na3V2 (PO4) 2F3 is evenly coated with a layer of highly graphitized KB, which effectively improves its ion diffusion and electron conduction capabilities. The sodium ion battery assembled from Na3V2 (PO4) 2F3 has a high specific capacity of 138mAh / g under a current of 0.5C, and its capacity can still maintain 122mAh / g under a large current of 40C. This low-temperature solvent thermal-ball milling method will provide a new strategy for the practicalization of low-cost, high-performance sodium ion battery technology.
The above-mentioned research work was supported by the strategic pilot science and technology project of the Chinese Academy of Sciences.
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