Revealing High Na-Content P2-Type Layered Oxides as Advanced Sodium-Ion Cathodes

Abstract

Layered Na-based oxides with the general composition of Na_<i>x</i>TMO₂ (TM: transition metal) have attracted significant attention for their high compositional diversity that provides tunable electrochemical performance for electrodes in sodium-ion batteries. The various compositions bring forward complex structural chemistry that is decisive for the layered stacking structure, Na-ion conductivity, and the redox activity, potentially promising new avenues in functional material properties. In this work, we have explored the maximum Na content in P2-type layered oxides and discovered that the high-content Na in the host enhances the structural stability; moreover, it promotes the oxidation of low-valent cations to their high oxidation states (in this case Ni²⁺). This can be rationalized by the increased hybridization of the O(2<i>p</i>)-TM(3<i>d</i>-<i>e</i>_g*) states, affecting both the local TM environment as well as the interactions between the NaO₂ and TMO₂ layers. These properties are highly beneficial for the Na storage capabilities as required for cathode materials in sodium-ion batteries. It leads to excellent Na-ion mobility, a large storage capacity (>100 mAh g^-1 between 2.0-4.0 V), yet preventing the detrimental sliding of the TMO₂ layers (P2-O2 structural transition), as reflected by the ultralong cycle life (3000 (dis)charge cycles demonstrated). These findings expand the horizons of high Na-content P2-type materials, providing new insights of the electronic and structural chemistry for advanced cathode materials.