A novel P2/O3 composite cathode toward synergistic electrochemical optimization for sodium ion batteries

The P2-type and O3-type are the most common types of layered oxides. The P2-type is possessed with better air stability but lower capacity while the O3-type possesses higher capacity but poorer kinetic performance. Considering the complementarity of two phases, a series of P2/O3–Na0.80Li0.13Ni0.20FexMn0.67-xO2 (x = 0.05, 0.10, 0.15) are designed and successfully synthesized by Fe doping in this work. The consolidation of P2 and O3 structures in this layered oxide is clearly characterized by the X-ray diffraction (XRD) refinement. Surprisingly, the two-phase collaboration is well demonstrated in the superior electrochemical properties. When x = 0.15, Fe0.15-LNM reveals a high reversible capacity with 180.95 mAh·g−1 and when x = 0.10, Fe0.10-LNM presents a good capacity retention with 88.60% after 100 cycles. Here it is the O3–P3 reversible phase transition and the redox of Fe that mainly provide the capacity during sodium ion de-embedding. Of note, the residual sodium salt on the surface is greatly reduced in the two-phase, which cuts down the occurrence of side reactions and improves the efficiency. This novel composite phase strategy facilitates the design of promising cathode materials with high capacity and long lifetime for sodium-ion batteries (SIBs) and other alkali metal batteries.