Mg-Doped Na₃V₂(PO₄)₂F₃/C Cathodes for Sodium Ion Batteries

Lithium-ion batteries (LIBs) are competent energy storage devices that have been used for electronic devices. However, for large-scale applications, there are concerns about the cost and lithium reserves. The market requires batteries which have high energy density, high safety, low cost, and environmental benignity [1-2]. Recently, the sodium-ion batteries (SIBs) have received much attention because of their similar electrochemical properties with LIBs. Na3V2(PO4)2F3 has been developed as a cathode material for SIBs due to high voltage, high thermo-stability, and high capacity [3]. Na3V2(PO4)2F3/C (NVPF/C) with a NASICON structure has a Na+ intercalation potential of 3.76 V. However, the NVPF/C has a low electronic conductivity which hinders the capacities and rate capability. Carbon coating and particle size reduction can improve the electrochemical performance. In addition, this work use Mg2+ doping attempting to increase the bulk conductivity of NVPF. Na3V2-xMgx(PO4)2F3/C cathodes are successfully synthesized using a sol-gel method with citric acid as the reducing agent and carbon source. X-ray diffraction (XRD) analysis confirms that Mg has been doped into NVPF/C. The XRD peak shifts indicate a lattice contraction due to the smaller atomic size of Mg as compared to V. The Mg-doped NVPF/C cathodes show promising properties for SIB applications. Reference: [1] M. Arumugam and Y. Ya. Advance Energy Mater., 2018, (8), 1-11 [2] W. Feixiang, Z. Chenglong, C. Shuangqiang, L.Yaxiang, H. Yanglong, H. Yong-sheng, M. Joachim and Y. Yan, Material Today. doi.org/10.1016/j.mattod.2018.03.004, 2017 [3] Z. Liu, Y.-Y. Hu, M. T. Dunstan, H. Huo, X. Hao, H. Zou, G. Zhong, Y. Yang and C. P. Grey. Chem. Mater., 2014, (26), 2513–2521.