A Novel Quantification Method for High Voltage Structural Evolution in Sodium and Lithium Layered Oxides

Abstract High‐voltage cycling of layered alkali transition metal oxides in Li‐ and Na‐ion cells increases energy density but greatly diminishes their lifetime. One of the most common Na‐ion cathode materials, O3‐type Na[Ni _1/3 Fe _1/3 Mn _1/3 ]O ₂ (NFM111), exhibits a well understood reversible octahedral to prismatic (O3↔P3) phase transition below 4.0 V, but its structural changes at higher voltage remain unclear. In this study, the structural evolution of NFM111 upon high voltage cycling is investigated by in‐situ X‐ray diffraction (XRD). It is found that the P3 phase that persists upon charging to 4 V transforms into a new octahedral/prismatic (OP) hybrid structure after just one charge to 4.3 V, with no recovery of the P3 phase in further cycles. A novel XRD‐analysis method, centered on the stacking factor “ z ”, is developed to quantify the stacking fractions in the OP hybrid structure and track their evolution during cycling, revealing structural hysteresis in the OP phase. This method is extended to the O3/O1 stacking, providing deeper understanding of high‐voltage phase transitions in Li _x NiO ₂ and Li _x CoO ₂ (x < 0.25) by revisiting previously reported in‐situ XRD data. Together, it is demonstrated that the “z‐factor” method is widely applicable to probing the high‐voltage phase transitions in both Li and Na layered oxides.