Suppressing phase transition and improving electrochemical performances of O3-NaNi1/3Mn1/3Fe1/3O2 through ionic conductive Na2SiO3 coating

Layered oxide O3-NaNi1/3Mn1/3Fe1/3O2 is one of the most promising cathode materials with high open-air stability, low cost, and environmental friendliness, but it experiences a series of complicated phase transitions during the charge-discharge process that lead to inferior electrochemical properties. In this work, we find that phase transitions of O3-NaNi1/3Mn1/3Fe1/3O2 can be effectively suppressed through in situ coating of Na-ion conductor Na2SiO3. The Na2SiO3 coating layer can reduce polarization and suppress voltage drop, and also has three-dimensional pathways for Na-ion diffusion to ensure larger Na+ diffusion coefficient. Therefore, the electrochemical performances of layered oxide NaMO2 (M = Ni1/3Mn1/3Fe1/3) can be greatly improved. Specifically, the discharge capacity of 5 mol% Na2SiO3 coated electrodes increase by 28% compared to the pristine after 50 cycles. Na2SiO3 coated NaNi1/3Mn1/3Fe1/3O2 also shows outstanding rate capability (58 mAh g−1 at a high rate of 5 C, compared to only 12 mAh g−1 for the pristine). These results suggest this to be an effective and reliable surface-modification strategy to reinforce the electrochemical properties of layered oxide materials for sodium ion batteries.