Unique Impacts of Scandium Doping on Electrode Performance of P’2‐ and P2‐type Na _2/3 MnO ₂

Abstract

Abstract 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 2/3 MnO 2  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 Mn 3+  ions ([Ar] 4s 2 3d 4 ), 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 ]O 2  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 ]O 2  (□ = 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 ]O 2 , 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 ]O 2  slabs of P’2‐Na 2/3 [Mn 1‐ x Sc x ]O 2 on the stable redox for long‐life Na‐ion batteries.