Origin of Reversible Interlayer-Disorder-Induced Phase Transitions in Layered Sodium Manganese Oxide Cathodes

Reversible interlayer-disorder-induced phase transitions in sodium birnessite (Na x MnO 2 ·yH 2 O) enable layered oxide cathodes to approach their theoretical performance limits, yet their microscopic origin remains poorly understood. Conventional first-principles calculations predict a thermodynamically unstable spinel-like phase and an endothermic layered-to-spinel transition with a large kinetic barrier (>1.5 eV) throughout the operating potential window, contradicting experimental observations. Here, we resolve this discrepancy using a recently developed charge-dependent computational framework that explicitly incorporates potential-driven charge effects. We demonstrate that these effects fundamentally regulate interlayer species–MnO 2 interactions and qualitatively reshape both the thermodynamic and kinetic landscapes of the layered-to-spinel transition. Although the spinel-like phase is metastable at low desodiation and low potentials, it becomes thermodynamically favored at high desodiation under elevated potentials, accompanied by a substantial reduction of the transition barrier to 0.19 eV, rationalizing the experimentally observed reversibility. This behavior originates from distinct charge-dependent chemical reactivities and charge-induced structural responses of the layered and spinel phases. Our findings resolve a longstanding puzzle in sodium birnessite phase transitions and provide fundamental insights into potential-driven charge effects in layered oxide cathodes for next-generation alkali-metal-ion batteries.