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_0.96Ca_0.02Ni_0.33Fe_0.33Mn_0.33O₂ (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_0.33Fe_0.33Mn_0.33O₂. 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.