Thermodynamically stable low-Na O3 cathode materials driven by intrinsically high ionic potential discrepancy

Abstract

The thermodynamically stable window for an O3-type layered sodium cathode material is largely determined by its Na stoichiometry; a spontaneous transition to the P-type structure occurs when it is relatively low. With such limitation, the capacity and stability of O3-structured materials become restricted and a potentially promising class of O3-type materials that garner the structural stability of P2-type materials is underexplored. This work discovers that a large ionic potential discrepancy within the transition metal layer acts as a driving force that pushes the Na-ions from prismatic coordination to octahedral coordination. Utilizing this strategy, we have explored a class of off-stoichiometric O3-type materials with exceptionally low Na-stoichiometry (generally forming P2-type structures with higher thermodynamic stability) yet having the structural parameter features of P-type materials. These materials demonstrate rapid O3–P3 phase transition while maintaining a stable solid solution reaction at high voltages, resulting in an impressive P-phase range of 81.4%, thus showing superior performance compared with conventional O3-type materials. This principle provides a great extension to the existing family of layered cathode materials for sodium-ion batteries.