Unique Impacts of Scandium Doping on Electrode Performance of P'2- and P2-type Na_2/3MnO₂

Layered sodium manganese oxides are one of the promising candidates as cathode materials for high-capacity Na-ion batteries free from rare elements. Among the polytypes of layered sodium manganese oxides, the P'2-type Na_2/3MnO₂ electrode exhibits the highest reversible capacity ≈220 mAh g^-1 based on the Mn^3+/4+ redox couple; however, the cycle life remained a significant challenge. Because of Jahn-Teller (JT) active Mn³⁺ ions ([Ar]4s²3d⁴), the layered structure is cooperatively distorted and experiences complex structural changes during Mn^3+/4+ redox accompanied by Na extraction/insertion. The impact of scandium(III) dopant in Na_2/3[Mn_{1- x}Sc_x]O₂ on the cooperative lattice-distortion and electrode performance is systematically investigated, and prepartion for distortion-free hexagonal P2-type Na_2/3[(Mn_{1- x}Sc_x)_0.93□_0.07]O₂ (□ = vacancy) allows the contribution of the vacancies on the Mn site to be checked. Furthermore, a comparative study of the case of other trivalent metals of yttrium(III) and aluminum(III) as dopants is demonstrated. Thus, it is concluded that a unique synergetic effect of the Sc doping and JT distortion provides significant improvement of redox activity of P'2-Na_2/3[Mn_{1- x}Sc_x]O₂, demonstrating stable cycling for more than 300 cycles in a Na-ion cell. The findings highlight the critical role of the coexistence of the Sc dopant and honeycomb ordering of Mn^IV(Mn^III _{1- x}Sc^III _x) in [Mn_{1- x}Sc_x]O₂ slabs of P'2-Na_2/3[Mn_{1- x}Sc_x]O₂ on the stable redox for long-life Na-ion batteries.