First-principles insights on anion redox activity in NaxFe1/8Ni1/8Mn3/4O2: Toward efficient high-energy cathodes for Na-ion batteries

In the search for high-energy cathode materials for Na-ion batteries (NIBs), Fe-doped layered transition metal oxides have been recently proposed as promising systems that can ensure improved reversible capacity at high working voltage. Exploiting the anionic redox chemistry in this class of materials represents a great advance for the energy storage community, but uncontrolled oxidation process can lead to the formation of unbound molecular oxygen, with detrimental effects on overall capacity and stability upon cycling. The higher TM-O covalency provided by Fe doping seems to prevent oxygen loss and ensure full capacity recovery. Understanding anionic processes and the underlying mechanism with atomistic details can reinforce the experimental efforts and help to outline rational design strategies for novel high-performing NIB cathodes. To this end, we present a state-of-the-art first-principles study on the P2-type NaxTMO2 (TM = Fe, Ni, and Mn-NFNMO) oxide. We compare structural and electronic features of stoichiometric (NaxFe0.125Ni0.125Mn0.75O2) and Mn-deficient (NaxFe0.125Ni0.125Mn0.68O2) NFNMO to identify and discuss the contribution of each element sublattice on charge compensation processes. Although Mn deficiency is predicted to increase the cathode working voltage, we find the charge compensation being mostly exerted by the Ni and Fe sublattices. Oxygen redox is unfold via the formation of superoxide species at low Na loads with a preferential breaking of more labile Ni-O bonds and binding to Fe atoms. Our calculations predict no release of molecular O-2 upon desodiation, thus highlighting the key role of Fe dopant that provides a good TM-O bond strength, preventing oxygen loss while still enabling anionic redox reactions at high voltages with extra reversible capacity.