Nonmetal Substitution in Interstitial Site of O3‐NaNi _0.5 Mn _0.5 O ₂ Induces the Generation of a Nearly Zero Strain P2&O3 Biphasic Structure as Ultrastable Sodium‐Ion Cathode

Abstract Co‐free O3‐type NaNi _0.5 Mn _0.5 O ₂ cathode material for sodium‐ion batteries has shown great promise due to its high theoretical capacity and plentiful Na reservoir. However, the rapid capacity recession caused by harmful phase transition and large volume strain severely restricts their practical application. Herein, the obstacle is well addressed by constructing a P2&O3 biphasic structure via a customized boron‐doping strategy. The light‐weight boron doping in the interstitial position reduces the energy gap of the formation energy of P2 and O3 structure, which induces the formation of P2&O3 biphase in high Na state. In addition, the biphasic structure exhibits near zero volume strain due to the lattice interlocking effect of P2&O3, as identified by in situ X‐ray diffraction measurement. As a result, it presents a remarkable cyclability with a capacity retention of 85.2% over 1000 cycles at a high rate of 5 C. More importantly, a pouch‐type full‐cell device can exhibit a long cycling life with 70.8% capacity retention over 150 cycles at 0.1 C. This work can offer a new inspiration for designing advanced high sodium electrode materials via light element doping for future energy storage devices.