Lithium-Doping Stabilized High-Performance P2-Na_0.66Li_0.18Fe_0.12Mn_0.7O₂ Cathode for Sodium Ion Batteries

While sodium-ion batteries (SIBs) hold great promise for large-scale electric energy storage and low speed electric vehicles, the poor capacity retention of the cathode is one of the bottlenecks in the development of SIBs. Following a strategy of using lithium doping in the transition-metal layer to stabilize the desodiated structure, we have designed and successfully synthesized a novel layered oxide cathode P2-Na_0.66Li_0.18Fe_0.12Mn_0.7O₂, which demonstrated a high  capacity of 190 mAh g^-1 and a remarkably high capacity retention of ∼87% after 80 cycles within a wide voltage range of 1.5-4.5 V. The outstanding stability is attributed to the reversible migration of lithium during cycling and the elimination of the detrimental P2-O2 phase transition, revealed by ex situ and in situ X-ray diffraction and solid-state nuclear magnetic resonance spectroscopy.