Phase Transition Regulation Mechanisms in P2‐Type Oxide Cathodes

Layered sodium transition metal oxides (Na_xTMO₂) are one of the promising cathodes for sodium ion batteries due to the high energy density and advantages of large-scale manufacturing. Recently, P2-type oxides cathodes, as an important family of Na_xTMO₂, has been paid much attention because the oxygen redox chemistry can further boost the energy density of the cathodes through the special configurations of "Na-O-Li" or "Na-O-Mg". However, these materials suffer the phase transition due to highly depleting Na⁺ in the structure, when the oxygen redox chemistry is triggered. Past of the decade has witnessed the effective strategies to cope with the phase transition issues through elements doping, but the universal rule behind the elements doping strategies is still a mystery, which is essential to provide guidance to design the advanced cathodes. In this perspective, we present a discussion on phase transitions during charge/discharge process in P2-type oxides, drawing from the research within our group and recent publications. Phase transition process in P2-type cathodes is analyzed through the lens of structural geometry evolution toward intrinsic physical/chemical properties. Governing principles and regulation mechanisms of phase transition are elucidated as well, which can give contributions to the rational design of high-performance P2-type oxide cathodes.