Suppressing Cation Migration and Reducing Particle Cracks in a Layered Fe-Based Cathode for Advanced Sodium-Ion Batteries.

Sodium-ion batteries have huge potential in large-scale energy storage applications. Layered Fe-based oxides are one of the desirable cathode materials due to abundance in the earth crust and high activity in electrochemical processes. However, Fe-ion migration to Na layers is one of the major hurdles leading to irreversible structural degradation. Herein, it is revealed that distinct Fe-ion migration in cycling NaFeO2 (NFO) should be mainly responsible for the strong local lattice strain and resulting particle cracks, all of which results in the deterioration of electrochemical performance. More importantly, a strategy of Ru doping could effectively suppress the Fe-ion migration and then reduce the local lattice strain and the particle cracks, finally to greatly enhance the sodium storage performance. Atomic-scale characterization shows that NFO electrode after cycling presents the intense lattice strain locally, accompanied by the remarkable particle cracks. Whereas, Ru-doped NFO electrode maintains the well-ordered layered structure by inhibiting the Fe-O distortion, so as to eliminate the resulting side effect. As a result, Ru-doped NFO could greatly improve the comprehensive electrochemical performance by delivering a reversible capacity of 120 mA h g-1 , about 80% capacity retention after 100 cycles. The findings provide new insights for designing high-performance electrodes for sodium-ion batteries.