Exceptionally increased reversible capacity of O3-type NaCrO₂ cathode by preventing irreversible phase transition

Although O3-type NaCrO₂ has various merits as a promising cathode material for Na-ion batteries, only ∼0.5 mol Na⁺ in O3-type NaCrO₂ can be used because of irreversible phase transition by Cr migration to the Na layers. Thus, it is important to increase the Na⁺ content that can be reversibly de/intercalated by O3-type NaCrO₂. Through combined studies using first-principles calculation and experiments, we demonstrate that the presence of Sb⁵⁺ in the NaCrO₂ structure can suppress Cr migration even after charging to 4.1 V (vs. Na⁺/Na) and enables an increase in the Na content that can be reversibly de/intercalated. During charge/discharge at C/20 (1C = 175 mA g⁻¹), O3-type Na_0.72Cr_0.86Sb_0.14O₂ delivers a specific capacity of ∼175 mAh g⁻¹ corresponding to ∼0.72 mol Na⁺ de/intercalation, representing highly enhanced electrochemical performance compared with that of O3-type NaCrO₂, which exhibits poor coulombic efficiency of only ∼37% under the same conditions.