(Invited) Oxide- and Polyanion- based Cathode Materials for Li-ion and Na-ion Batteries

Abstract

Na- ion layered oxide as cathodes have attracted both the scientific community and industry. They offer high gravimetric energy density, tunability of properties and ease of synthesis. This leads to both interesting science and low cost applied materials.Layered oxides are generally classified by the nomenclature introduced by Delmas et. al.1 which depends on the alkali environment and stack of layers in the unit cell. The same team also used layered oxides for the first-time as potential positive electrode for reversible sodium storage.2 In this talk we present our contributions on (i) O3, (ii) P3 and (iii) modified P3 type layered oxides as cathode materials for Na-ion batteries.O3-type layered sodium transition metal oxides (NaxMO2, M = transition metal ions) are one of the most attractive cathode materials considering their capacity. However, the use of O3 phases is limited due to their low redox voltage and several associated phase changes which are unfavorable for long cycling. 3-5 In this presentation, we show the effect of Cu, Ni and Ti doping in the O3 structure and relate the observed local environments to their storage performances. Cu2+ Jahn-Teller distortion leads to irreversibility and dictates the amount of Cu2+ that can be doped in the layer. Our results show that 10-12% of Cu-doping is optimal. In order to further examine the effect of local structures, we doped Ni2+ and achieved excellent structural reversibility which favors high-rate performance and cycling stability. 6 Na-sufficient Fe and Mn based P3 layered oxide is able to accommodate higher than 0.67 moles of Na-ions. 7,8 We present the effects of voltage limits on the storage performances and highlight associated phase changes during electrochemical cycling. XANES data confirm the charge compensation mechanism during electrochemical cycling. Full cell data further elucidate the sufficiency of Na-ions in the P3 structure. 8 Lastly, the storage performances of P3-Na0.9Fe0.5Mn0.5O2 are compared with those for modified P3 phase, P3 (major)/ O3 (minor) layered oxide. The phase changes during cycling and comprehension of charge compensation mechanism from XANES of this modified P3 structure are reported.