Study on the Reversible Electrode Reaction of Na1–xNi0.5Mn0.5O2 for a Rechargeable Sodium-Ion Battery

Layered NaNi0.5Mn0.5O2 (space group: R3m), having an O3-type (α-NaFeO2 type) structure according to the Delmas’ notation, is prepared by a solid-state method. The electrochemical reactivity of NaNi0.5Mn0.5O2 is examined in an aprotic sodium cell at room temperature. The NaNi0.5Mn0.5O2 electrodes can deliver ca. 105–125 mAh g–1 at rates of 240–4.8 mA g–1 in the voltage range of 2.2–3.8 V and show 75% of the initial reversible capacity after 50 charge/discharge cycling tests. In the voltage range of 2.2–4.5 V, a higher reversible capacity of 185 mAh g–1 is achieved; however, its reversibility is insufficient because of the significant expansion of interslab space by charging to 4.5 V versus sodium. The reversbility is improved by adding fluoroethylene carbonate into the electrolyte solution. The structural transition mechanism of Na1–xNi0.5Mn0.5O2 is also examined by an ex situ X-ray diffraction method combined with X-ray absorption spectroscopy (XAS). The staking sequence of the [Ni0.5Mn0.5]O2 slabs chang...