Strain-Buffering Layered/Rocksalt Heterostructure Constructed via Oxygen Redox Activation for Durable O3-Type Layered Transition Metal Oxides.

Harnessing anionic oxygen redox is pivotal for boosting the capacity of O3-type layered transition metal oxides for sodium ion batteries (SIBs), yet it often triggers severe lattice distortion and irreversible oxygen release. Herein, we propose an in situ electrochemical surface reconstruction strategy to stabilize the O3-type NaNi1/3Fe1/3Mn1/3O2 (NFM) cathode. By strictly controlling the initial anionic redox reactions at an ultra-high potential of 4.6 V, a robust heterostructure comprising an ordered layered core and a disordered rocksalt shell is constructed. This unique surface architecture serves as an isotropic strain buffer that effectively mitigates particle cracking and provides dense surface passivation to suppress interfacial side reactions. Furthermore, the induced rocksalt phase stabilizes the surface TM-O coordination, thereby preventing lattice oxygen loss during subsequent cycling. Consequently, when cycled within 2.0-4.0 V, the electrochemically pretreated cathode (DRS(4.6)@NFM) exhibits significantly reduced lattice distortion and superior electrochemical durability (capacity retention ratio of 90.77% after 300 cycles) with negligible voltage decay (retaining 98.56% of the initial voltage). Even when cycled up to 4.2 V, a capacity retention ratio of 87.4% after 300 cycles is still maintained. This work elucidates the interplay between electrochemical pretreatment and structural evolution, providing a decisive foundation for designing high-voltage and durable sodium-ion cathodes.