Comprehensive Electrochemical Study of the Role of Transition Metals in O3-Type Na[Ni x Co y Mn z ]O2 (x=1/3, 0.5, 0.6, and 0.8) Cathodes for Sodium-Ion Batteries

Abstract

Sodium ion batteries (SIBs) have attracted significant interest due to the abundant natural resources of Na and their similar chemistry to Li ions.5 O3-type layered structured transition metal oxides appear to be attractive based on the success of using layered LiMO2 cathodes in LIBs. Recently, our group introduced micro-sphere NaMO2 (M=Ni, Co, and Mn) cathodes with high energy density and high thermal stability.1-3 However, the relationships between transition metal composition and the electrochemical and related structural properties in O3-type layered NaMO2compounds have not been systematically studied. In this study, a comprehensive study of Na[Ni x Co y Mn z ]O2 (x=1/3, 0.5, 0.6, and 0.8) cathodes is carried out to determine the optimal composition as the electrochemical, structural, and thermal properties in O3-type layered cathodes are strongly dependent on the transition metal composition. Here, the role of each transition metal in [Ni x Co y Mn z ]O2 cathodes is identified via electrochemical property characterization, structural analysis, and thermal stability testing. Briefly, an increase of the Ni fraction resulted in an increasingly higher capacity but is accompanied by progressively poor capacity retention. On the other hand, the Co metal played an important role in stabilizing the structure, while the Mn content contributed to enhancing the capacity retention and thermal stability. The present study highlights the importance of appropriately balancing the transition metal composition in a layered Na[Ni x Co y Mn z ]O2 cathode. Furthermore, this work provides a design guideline for developing an ideal Na[Ni x Co y Mn z ]O2 cathode with both high capacity and optimal cycle retention in addition to thermal stability.