P2/O3 biphasic Fe/Mn-based layered oxide cathode with ultrahigh capacity and great cyclability for sodium ion batteries

As the representative layered oxide cathode for sodium ion batteries (SIBs) featuring the low cost, P2-type Na-Fe-Mn oxide (NFMO) delivers a high capacity but a limited cycling stability, while O3-type NFMO shows extended cycling lifespan but a lower capacity. Considering the complementarity of two phases in electrochemistry, we successfully designed and fabricated a Fe/Mn-based layered oxide Na0.67Li0.11Fe0.36Mn0.36Ti0.17O2 with a unique P2/O3 biphasic architecture through high-proportion Li/Ti co-substitution. High-proportion Li substitution in transition metal layers triggers the reversible O redox below 4.2 V due to the formation of the special O bonding environment, delivering a highest capacity of 235 mA h g1 ever reported among all Fe- and Mn-based layered oxide cathodes. Moreover, the unique intersected complex way at the phase boundary significantly suppressed the P2→OP4 phase transition and decreased the lattice mismatch between two phases at high potentials, greatly enhancing the cycling stability. This novel phase complex strategy benefits the design of promising cathode materials with high capacity and long lifespan for SIBs and beyond.