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 Na2/3MnO2 electrode exhibits the highest reversible capacity ≈220 mAh g-1 based on the Mn3+/4+ redox couple; however, the cycle life remained a significant challenge. Because of Jahn-Teller (JT) active Mn3+ ions (Ar4s23d4), the layered structure is cooperatively distorted and experiences complex structural changes during Mn3+/4+ redox accompanied by Na extraction/insertion. The impact of scandium(III) dopant in Na2/3Mn1- xScxO2 on the cooperative lattice-distortion and electrode performance is systematically investigated, and prepartion for distortion-free hexagonal P2-type Na2/3(Mn1- xScx)0.93□0.07O2 (□ = 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-Na2/3Mn1- xScxO2, 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 MnIV(MnIII 1- xScIII x) in Mn1- xScxO2 slabs of P'2-Na2/3Mn1- xScxO2 on the stable redox for long-life Na-ion batteries.
Moriya et al. (2025) studied this question.