K doping stabilizes three-dimensional K_0.2Na_1.3Mn_0.5O_2-δ as high-performance cathode for sodium-ion batteries

It is a difficult challenge to simultaneously employ the cationic and anionic redox chemistry in cathode materials for sodium-ion batteries with high energy. Even though layered oxides (classified as two-dimensional oxides) demonstrate excellent promise in the high discharge capacity, their poor oxygen transformation via redox reactions is limited by crystal instability. Therefore, a doping strategy was conceived to tackle this issue and increase redox efficiency. K doping was applied to transform the two-dimensional Na_1.3Mn_0.7O₂ (NMO) to three-dimensional K_0.2Na_1.3Mn_0.5O₂ (KNMO), preventing the irreversible phase shift and preserving the crystal structure’s stability while cycling. With this modification treatment, KNMO features manganese and oxygen reactive sites, delivering a promising energy density of 190 mAh·g⁻¹ at 5 mA·g⁻¹ in the 2.0–4.5 V voltage range (vs. 71.4 mAh·g⁻¹ for the pristine NMO). Moreover, it displays improved capacity retention of more than 83.5% after 50 cycles at 50 mA·g⁻¹. The results demonstrated that doped intercalation oxides were promising for redox oxygen transformation in sodium-ion batteries.