Phase‐Transition Engineering Enables Low‐Strain Cathodes for 192 Wh kg ⁻ ¹ Sodium‐Ion Batteries

Phase transitions in sodium layered transition metal (TM) oxides often induce microstrain and TM ion migration, leading to structural degradation and poor cycling stability. However, a rational design approach for optimizing phase transitions is still lacking. Here we introduce phase transition potential (Φ phase ) as a rational descriptor to predict and control phase evolution in these cathodes. A lower Φ phase enables smoother Na + migration and slower slab sliding during Na extraction, thereby facilitating a continuous phase transformation rather than abrupt phase changes. Guided by this concept, we design a calcium‐substituted layered oxide, Na 0.96 Ca 0.02 Ni 0.33 Fe 0.33 Mn 0.33 O 2 (NCNFMO), which delivers a specific capacity of 140 mAh g −1 at 0.1 C and retains 84.2% of its initial capacity after 500 cycles at 1 C, compared to only 26.4% retention for NaNi 0.33 Fe 0.33 Mn 0.33 O 2 . Moreover, the NCNFMO||Al@C full cell maintains a high‐capacity retention of 82.5% after 100 cycles, and 6.1 Ah full cell demonstrates an energy density of 192 Wh kg −1 entire cell . These findings offer fundamental insights into phase behavior‐induced microstrain and a promising path toward high‐energy, long‐life sodium‐ion batteries.