K+-doped P2-Na0.67Fe0.5Mn0.5O2 cathode for highly enhanced rate performance sodium-ion battery

Fe/Mn-based layered oxides are considered as competitive cathode materials for sodium-ion batteries (SIBs) owing to their satisfactory theoretical specific capacity, economy, and environmental friendliness. However, the inferior cycling performance and poor rate capability caused by structural evolution hinder their practical application. Herein, a novel K+-doped P2-Na0.67Fe0.5Mn0.5O2 cathode for SIBs is successfully achieved to address these issues. Due to the doping of 0.05 K+, the cathode displays good cycling stability at 1 C, with a capacity retention of 70% after 100 cycles, while the pristine one only yields 49% capacity retention. Even at a high rate of 3 C, it still maintains 72% capacity retention. The characterizations and calculations demonstrate that interplanar distance is enlarged by introducing large-sized K+ in the edge-sharing Na+ sites. Galvanostatic intermittent titration technique (GITT) results reveal that the Na+ diffusion coefficient is well enhanced by K+ doping. In-situ XRD investigation suggests that the K+ in the Na layer can act as a pillar to stabilize the host layered structure. The theoretical calculation demonstrates that the conductivity and the interaction between transition metal (TM) ions and oxygen of K+-doped samples are well improved, thus suppressing the structural evolution.