Electrochemical Properties of Monoclinic NaNiO₂

Monoclinic NaNiO[subscript 2] is re-investigated as a positive electrode material for sodium ion batteries. We observe reversible Na extraction and insertion of about 120 mAh/g when NaNiO[subscript 2] is cycled between 1.25 V and 3.75 V, though discharge only proceeds to Na0.91NiO[subscript 2]. The voltage profile shows large and pronounced steps, which are fully reversible. Charging to 4.5 V leads to higher charge and discharge capacity, though with poor coulombic efficiency and rapid capacity fade. X-ray diffraction of charged samples indicates that high voltage charging leads to the formation of an inactive phase.