Moisture exposed layered oxide electrodes as Na-ion battery cathodes

Mn-rich layered oxides of P2 Na2/3Mn0.8Fe0.1Ti0.1O2 have been shown to exhibit a remarkably stable electrochemical performance even after exposure to moisture for extended periods of time. Here, a detailed investigation of the electrochemical performance of pristine, protonated, and hydrated electrodes is reported. Neutron powder diffraction and Na-23 NMR are employed in order to correlate the overall electrochemical performance of each electrode with that of the as-synthesized crystal structure. The effects of proton and water (or OH) moieties on the Na+ layers are discussed based on the electrochemical performance of each phase. The complete structural evolution of the protonated and pristine P2 Na2/3Mn0.8Fe0.1Ti0.1O2 electrodes during charge/discharge is determined via in situ synchrotron X-ray diffraction. The protonated phase at the potential cut-offs (1.5-4.2 and 2-4 V) and the applied currents used shows a predominantly solid-solution reaction with little evidence of a secondary phase while the pristine phase shows the formation of secondary phases and typically better electrochemical capacities. Therefore, the formation of the secondary phase, in part, enhances capacity in this system. Thus moisture exposure (and subsequent treatment) of generally P2 electrodes can lead to significantly different structural evolution during charge/discharge reactions and hence observed capacities.