Elucidating Structural Dynamics to Stabilize High‐Voltage Cycling in O3‐Type Layered Cathodes for Sodium‐Ion Batteries

The development of high-energy-density sodium-ion batteries (SIBs) is limited by the structural instability of O3-type layered oxide cathodes at high voltages. In this study, we present a comprehensive mechanistic analysis of the structural evolution in Na[Ni_0.5Mn_0.5]O₂ (NM55) and its derivatives, utilizing operando high-resolution X-ray diffraction to elucidate the complex phase transition sequence and how structural changes impact electrochemical performance. We identify a cascade of transformations-O3-O'3-P'3-P3-P'3'-P3'-O3'-O3'-marked by abrupt lattice distortions, sodium/vacancy ordering, and significant c-axis contraction at high states of charge. These structural dynamics directly contribute to capacity fading, mechanical degradation, and limited cycle life. Building on these insights, we explore systematic doping strategies with Fe, Ti, and Ca. Fe substitution suppresses monoclinic distortions and alters high-voltage transitions, while co-doping with Ti and Ca further stabilizes the lattice, inhibits undesirable phase evolution, and preserves interface integrity. The optimized quaternary composition, Na_0.96Ca_0.02[Ni_0.4Fe_0.2Mn_0.375Ti_0.025]O₂, achieves a stable capacity retention of 70.18% after 600 cycles at 1C. This work establishes a vital link between specific phase transition mechanisms in O3-type cathodes and their electrochemical durability, providing crucial guidance for the rational design of next-generation SIB cathode materials.