A Rational Biphasic Tailoring Strategy Enabling High‐Performance Layered Cathodes for Sodium‐Ion Batteries

Layered oxide cathodes usually exhibit high compositional diversity, thus providing controllable electrochemical performance for Na-ion batteries. These abundant components lead to complicated structural chemistry, closely affecting the stacking preference, phase transition and Na$^+$ kinetics. With this perspective, we explore the thermodynamically stable phase diagram of various P2/O3 composites based on a rational biphasic tailoring strategy. Then a specific P2/O3 composite is investigated and compared with its monophasic counterparts. A highly reversible structural evolution of P2/O3–P2/O3/P3–P2/P3–P2/Z/O3′–Z/O3′ based on the Ni$^{2+}$/Ni$^{3.5+}$, Fe$^{3+}$/Fe$^{4+}$ and Mn$^{3.8+}$/Mn$^{4+}$ redox couples upon sequential Na extraction/insertion is revealed. The reduced structural strain at the phase boundary alleviates the phase transition and decreases the lattice mismatch during cycling, endowing the biphasic electrode a large reversible capacity of 144 mAh g$^{−1}$ with the energy density approaching 514 Wh kg$^{−1}$