Quasi-Zero-Strain High-Voltage P2-Type Na2/3Li1/6Ni1/3Mn1/2O2 Cathodes Enabled by Capacity-Strain Balance

Deep sodium extraction/insertion in Mn-based P2-type layered cathodes usually induces undesired lattice strain, transition-metal layer gliding, and irreversible phase transitions, which limit their high-voltage cycling stability. Here, Li incorporation is employed to construct P2–Na2/3Li1/6Ni1/3Mn1/2O2 and regulate the capacity-strain balance for high-voltage sodium-ion batteries. By introducing electrochemically inactive Li while maintaining the Ni content, the local transition-metal-layer environment is modified, enabling stable Na+ storage within the P2 structure. The obtained cathode delivers a discharge capacity of 107 mAh g–1 and an active-material-level specific energy of 363 Wh kg–1 at 0.1 C, while maintaining 95.23% capacity retention after 100 cycles at 1.0 C within 2.0–4.5 V. More importantly, XRD results reveal a reversible reaction pathway characterized by predominantly solid-solution-like structural evolution with suppressed P2–O2/OP4 phase transition during sodium extraction/insertion, giving a maximum volume change of 1.37%. These results suggest that Li incorporation effectively balances the reversible capacity output and structural strain. The relatively low volume change per unit capacity (0.0128% (mAh g–1)−1) further highlights the advantage of this capacity-strain balance strategy.