Elevated-Temperature Behavior of Mn-Rich Layered Oxides: Stacking-Dependent Redox Processes and the Role of CeO ₂ Surface Engineering

Sodium-ion batteries require oxygen-redox cathodes that remain structurally and chemically stable under realistic electrochemical and elevated-temperature conditions. Here, we investigate the sodium-storage behavior of the layered oxide Na 2/3 Mg 1/3 Mn 2/3 O 2, which adopts both P2- and P3-type stackings and exhibits Mn/O redox chemistry, enabling a morphology-independent evaluation of stacking- and redox-dependent behavior at high temperature. Diffusion analysis shows that in Mn-redox regimes, Na + transport is strongly structure dependent, whereas oxygen-redox regimes exhibit enhanced diffusion but higher, stacking-independent activation energies. Ex-situ XRD and PDF reveal reversible layer transformations and crystallite fragmentation upon deep desodiation, while EPR and XPS link Mn 3+ disproportionation, Mn 2+ surface migration, and NaF-rich interphase formation to interfacial degradation processes that intensify at elevated temperatures. CeO 2 surface modification preserves the bulk structure, suppresses Mn 2+ release, and enables deeper desodiation, increasing capacity by 30−50 mAh g −1 (230−250 mAh g −1 total). However, CeO 2 also accelerates the formation of a more resistive cerium-containing interphase at high temperature, slightly reducing long-term stability. Overall, CeO 2 functions as an oxygen-redox stabilizer that enhances accessible capacity while reshaping interfacial chemistry. These insights establish design rules for balancing bulk redox stabilization with controlled interphase evolution in next-generation, thermally robust sodium-ion cathodes.