Improving Reversibility of Anionic Redox Reaction of NaVO3 By Metal Doping for Sodium-Ion Batteries

Abstract

Traditionally, positive electrode materials in lithium/sodium-ion batteries (LIBs/NIBs) undergo charge and discharge through redox reaction of the transition metal (TM) within the structure. As a result, the number of electron transfer is limited by the available valence states of the TM. Recently, there is an increased interest in solid-state anionic redox reaction (ARR) in positive electrode materials that compensates the charge transfer by the oxygen atoms. ARR can potentially increase the capacity of the active material with the additional charge-transfer site. It can also open the door to more transition-metal compounds that are originally thought to be inactive. Our previous work has shown that NaVO3 (NVO) can undergo anionic redox reaction while the oxidation state of V remains 5+ between 1.5 and 4.9 V vs. Na/Na+.[1] It can give a capacity of more than 100 mAh g-1, though its capacity fades with cycling. Here, we aim at improving the stability and capacity of the material by incorporating other transition metals into the structure to utilize both the anionic and cationic redox reactions. As a start, we synthesized a single phase Na1.3Co0.1V0.9O3 (NCVO) with Co2+ doping by a typical solid-state method. Both NVO and NCVO materials were first discharged to 1.2 V to investigate the amount of V5+/4+ redox reaction that is available in the material and then charged to 4.7 V. Figure 1a shows the first and second charge-discharge curve of the two materials. Interestingly, the incorporation of Co into the structure reduces the activity of V5+/4+ during initial discharge, with a smaller capacity of 38 mAh g-1 for NCVO as opposed to 94 mAh g-1 for NVO to 1.2 V. Though, the amount of charge and discharge capacities during 2nd cycle are the same, suggesting changes in reaction mechanism of the material with Co2+ addition. The available capacity of about 180 mAh g-1 is higher than that can be obtained from Co and V redox reactions alone, so part of the capacity comes from oxygen redox reaction. Figure 1b shows the cycle performance of NCVO and it gives good stability. In addition, NCVO can delivers better rate performance with a discharge capacity of 130 mAh g-1 at 50 mA g-1. Further works with techniques such as ex-situ XRD, XPS and GITT to characterize the material and understand the reaction mechanism are underway. More results on the effect of the metal dopant will be presented at the meeting. [1] J. Zhang, B. Su, A. Kitajou, M. Fujita, Y. Cui, M. Oda, W. Zhou, P. H.-L. Sit, D. Y. W. Yu, J. Power Sources 400, 377 (2018). Figure 1. (a) Initial discharge and second charge-discharge curves of NVO and NCVO electrodes at 10 mA g−1 between 1.2 and 4.7 V vs. Na/Na+; (b) Cycle performance of NCVO between 1.2 and 4.7 V. Figure 1