Ionic Model Approach to Battery Voltage of NaMO2

Abstract

NaMO2 (M = Ti, V, Cr, Mn, Fe, Co, and Ni) with a layered rock-salt structure exhibits high battery voltage (V) and capacity (Q), and is considered to be a promising cathode material for sodium ion secondary batteries (SIBs). Among the battery parameters, V is directly related to the electronic structure of the compounds. Here, we apply the ionic model proposed by Torrance et al. [Physica C 182, 351 (1991) ]t o NaMO2 and estimated V using structural parameters. The ionic model reproduces the global feature of the M-dependence of V. This analysis suggests that an electron is removed from the O 2p state in the charge process in the late transition metal compounds (M = Mn–Ni).