Interlocking biphasic chemistry for high-voltage P2/O3 sodium layered oxide cathode

Biphasic hybridization of layered cathode materials for sodium-ion batteries (SIBs) is crucial to enhance storage performances. The synergistic effect of biphases is generally considered to underlie the enhancement, yet the in-depth mechanism underneath remains unclear, in particular at high-voltages (> 4.2 V, vs Na⁺/Na). Herein, a unique high-voltage-stable P2/O3 composite layered oxide-Na_0.85Ni_0.34Mn_0.33Ti_0.33O₂ with a specific proportion (P2:O3 = 24.8%:75.2%) is reported via tuning the amounts of Ti substitution in Na_0.85Ni_0.34Mn_0.66-xTi_xO₂. Operando X-ray diffraction reveals that the biphasic Na_0.85Ni_0.34Mn_0.33Ti_0.33O₂ cathode successfully restrains the formation of O2 phase and undergoes completely reversible structural evolution of P2/O3–P2/P3–OP4/OP2 upon being charged to a high voltage of 4.4 V. Moreover, the interlocking effect between the phase boundaries is revealed to effectively mitigate the severe structural strain and large lattice volume change, thus further raising its structural stability under the high-voltage regions. Consequently, the obtained biphasic P2/O3-Na_0.85Ni_0.34Mn_0.33Ti_0.33O₂ cathode exhibits an excellent capacity retention of 80.6% at 1C after 200 cycles. More importantly, the full cell fabricated with hard carbon anode achieves a high energy density of 294.6 Wh kg⁻¹. These results highlight the excellent electrochemical performance of biphasic cathode materials and also grasp new insight into designing composite structure materials for SIBs.