Factors Affecting Electrochemical Properties of O3-Type Layered Oxides for Sodium-Ion Batteries

Abstract

In recent years, rechargeable sodium-ion batteries made from earth abundant elements have been attracting much attention as energy storage devices free from material resource constraints. Among various positive electrode materials for sodium-ion batteries, layered transition metal oxides are promising materials due to their large reversible capacity. Sodium-containing layered metal oxides are divided into two major groups: O3-type and P2-type. Compared to P2-type oxides, O3-type oxides show larger theoretical capacities based on Na ion contents in the host layered structures. In this study, a series of O3-type layered oxides, NaNiO 2 , NaMnO 2 , and NaNi 0.5 Mn 0.5 O 2 , were synthesized, and the impacts of nickel and manganese ions on the electrochemical properties and phase transition behavior of these materials were carefully examined. Since both Ni 3+ and Mn 3+ ions are Jahn-Teller active ions, NaNiO 2 and NaMnO 2 have a distorted O'3-type layered structure. In contrast, NaNi 0.5 Mn 0.5 O 2 containing Jahn-Teller inactive Ni 2+ and Mn 4+ ions crystalizes into an O3-type layered structure without distortion. The cyclability of NaNiO 2 and NaNi 0.5 Mn 0.5 O 2 is significantly enhanced by limiting the upper cut-off voltage to 3.8 V, whereas the electrode reversibility of NaMnO 2 is less affected by the upper cut-off voltage. In-situ X-ray diffraction measurements reveal that a pronounced shrinkage of the interlayer distance associated with O3/P3 phase transition after charging above 3.8 V is observed for NaNiO 2 and NaNi 0.5 Mn 0.5 O 2 , but not for NaMnO 2 . Therefore, the precise control of state of charge is effective in achieving higher electrode reversibility of nickel-containing layered oxides. Based on these results, the effects of nickel and manganese ions on the electrochemical properties and phase transition behavior of O3-type layered oxide materials will be discussed in detail. Moreover, the impacts of electrolyte solutions and material synthesis conditions will also be presented.