First-Principles Insights on Solid-State Phase Transitions in P2-Na _x MnO₂-Based High Energy Cathode during Na-Ion Battery Operations

Manganese-based layered oxides hold great promise as cathode materials for sodium-ion batteries (NIBs), offering environmental benefits and abundant resource availability. While promising NaxTMO2 cathodes can be fabricated by finely tuning the transition metal (TM) composition to achieve high energy density and good reversible capacity, practical applications of these materials are still hindered by severe phase transitions that affect intrinsic structural stability during battery operation. Understanding and controlling these transformations are of outmost importance to encourage their suitable application in highly efficient NIB devices. Using state-of-the-art density functional theory (DFT), we focus on the prototypical P2 -> P2 ' and P2 -> OP4 transitions in the NaxMnO2 system at high and low operating voltages, respectively. By unveiling structural and electronic feature variations at different states of charge (e.g., x(Na) ranging from 0.72 to 0.34, thus modeling the desodiation/sodiation cycling), we provide an atomistic perspective on the gliding-driven mechanism and the key factors mainly responsible for these undesired phase transitions. Cooperative Jahn-Teller effects (CJTE) associated with Mn3+ JT-active centers, together with changes in Na+ orderings, are highlighted as the main causes driving the phase transitions after Na insertion/extraction. Whether the Mn3+-based long-range interactions enhance stabilization throughout the crystalline lattice or intralayer Na+ motion leads to major distortions, the retention of the P2 structure strictly relies on a balance between electrostatic (Na+-Na+ intralayer and O2--O2- interlayer repulsions) and covalent (TM-O bond strengths) contributions. These theoretical insights can help to advance the future design principles towards more efficient and structurally stable layered oxides as advanced NIB cathodes.