A Study of Cu Doping Effects in P2-Na0.75Mn0.6Fe0.2(CuxNi0.2-x)O2 Layered Cathodes for Sodium-Ion Batteries

Layered sodium metal oxide cathodes have much broader choice of transition metal elements than the Li counterparts. A reversible Cu2+/3+ redox couple has recently been introduced in such Na cathodes. To study the role of Cu, here P2-type layered Na0.75Mn0.6Fe0.2(CuxNi0.2-x)O-2 compounds have been designed, synthesized and investigated. It shows the high initial capacity of 206 mAh/g and good capacity retention. Reversible oxygen redox activity is observed in our experiments. Our DFT calculations suggest that Cu can stabilize the oxygen redox by modifying the electronic structure together with Fe at high voltages. In addition, the strong P ' 2 transition observed at low voltages is induced by Jahn-Teller active Cu2+ with two competing effects of enhanced Na ion diffusivity and reduced electronic conductivity.