Investigation on Improved Electrochemical Performance of Layered Sodium Manganese Oxide By Partial Transformation of P2-Phase to O3-Phase for Sodium Ion Battery

Abstract

Layered structure sodium manganese oxides are the most popular cathode materials for sodium ion battery due to their superior electrochemical performances with low cost. There are various structures of sodium manganese oxides based on the transition metal stacking arrangement and the coordination of Na ions in the structure. Among various structures of the layered structure materials, sodium manganese oxide has two representative structures which are P2- and O3- phases. The letter denoted as P and O indicates the trigonal prismatic and octahedral coordination of Na ions in the structure, respectively. And the number of transition metal layer in unit cells is written after the letter. Therefore, P2- and O-3 type phases have trigonal prismatic and octahedral sites for Na ions occupying, and two and three different MnO2 layers per unit cell, respectively. Generally, P2- and O3-type materials exhibit different intrinsic electrochemical behaviour due to their different Na ion occupation sites and Na ion concentrations. The P2-type materials show low initial charge capacity due to large unoccupied Na ion site and low reversible capacity. The low initial charge capacity cause severe problem when it is adopted as a cathode for full-cell system. O3-type materials provide higher initial capacity and reversible capacity than P2-type materials. However, O3-type materials exhibit lower Na ions diffusivity because Na ions should diffuse through face-shared interstitial tetrahedral sites in the structure. Furthermore, they show fast degradation when it is charged above 4.0V due to structural instability. In this study, we provide the enhanced electrochemical performances of sodium manganese oxide by partial transformation from P2-type to O3-type phase. The P2- and O3- mixed phase suggest the novel ways to overcome the disadvantages of each phase, by adopting the advantages of opponent phase. The mixed phase for layered structure cathode materials present the new ways for the development of cathode materials with improved properties.