(Invited) Layered Sodium Transition Metal Oxides As Cathode Materials for Na-Ion Batteries

Abstract

Thanks to their excellent electrochemical performance, Li-ion batteries have been successfully utilized as a power source in electric vehicles and large-scale energy storage systems. However, there are some concerns that the current technology of Li-ion batteries is not able to meet the demands for power sources in future energy storage applications. For example, lithium resources are expensive and too geographically constrained, leading to a substantial increase in the cost of Li-ion batteries. In this connection, new rechargeable battery systems such as sodium ion, sodium metal, magnesium, metal-air, and metal-sulfur batteries are being considered as alternatives to replace Li-ion batteries. In particular, Na-ion batteries have attracted rapidly increasing attention owing to the low cost of Na-ion batteries. 1-3 Because of their low cost, the main potential application of Na-ion batteries is considered to be large-scale energy storage systems. However, it should be stressed that it is not easy to reduce the cost of Na-ion batteries relative to that of Li-ion batteries, in spite of the inexpensive Na precursors. The current Na-ion batteries have slightly lower energy densities than Li-ion batteries. This implies that the cost per energy ($/Wh) of Na-ion batteries cannot be easily reduced because the energy density limitations offset the cost savings due to the use of Na. Therefore, it is necessary to develop high capacity electrode materials to improve the energy density of Na-ion batteries. In this presentation, we will focus on our recent works involving the development of high capacity layered-structured sodium transition metal oxide cathode materials to improve the energy density of Na-ion batteries. We have examined various layered Na 1-x MO 2 (M = Mn, Fe, Ni, and their solid solution analogues) materials, along with understanding the role of dopants in the reaction mechanisms of those electrode materials. We also discuss the failure mechanisms of those cathode materials and suggest a strategy for improving electrochemical performance. [1] Y. Kim, K.-H. Ha, S. M. Oh, K. T. Lee, Chem. Eur. J., 20 (2014) 11980. [2] S. Y. Hong, Y. Kim, Y. Park, A. Choi, N.-S. Choi, K. T. Lee, Energy Environ. Sci., 6, (2013) 2067. [3] M.-S. Kwon, S. G. Lim, Y. Park, S.-M. Lee, K. Y. Chung, T. J. Shin, K. T. Lee, ACS Appl. Mater. Interfaces, 9 (2017) 14758.