Constructing high-capacity and ultra-stable biphasic layered cathodes via reductive coupling mechanism for sodium-ion...

Stimulated anionic redox reactions are commonly utilized to design layered oxide cathodes for high-energy sodium-ion batteries (SIBs). Unfortunately, excessive oxygen redox often causes irreversible lattice oxygen loss and cation migration, leading to rapid capacity and voltage fading, as well as slow reaction kinetics. Here, we design and synthesize a P2/O3 biphasic cathode material, Na_0.8Ni_0.3Mn_0.5Cu_0.1Ti_0.1O₂ (NNMCT), which can activate the redox of oxygen at significantly lower voltage while leveraging the high stability of the P2 phase and the high capacity of the O3 phase. Furthermore, we elucidate the reductive coupling mechanism (RCM) of the uncommon electron transfer from oxygen to Ni/Mn ions within the material, which enhances the reversibility of the anion redox reaction, and induces the formation of strong covalent Ni/Mn-(O-O) bonds that effectively inhibit excessive oxygen oxidation and avoid rapid capacity decay. As a result, NNMCT cathode materials exhibit excellent reversible capacity (134.0 mAh g^-1 at 10 mA g^-1) and superior long-cycle stability (85.2% capacity retention after 500 cycles at 100 mA g^-1). The intrinsic functional mechanism of RCM is fully elucidated through a series of in situ/ex situ characterizations as well as theoretical calculations. These findings underscore the advantages of biphasic/multiphasic cathodes for high-energy SIBs.