(Invited) Understanding Role of Li Substitution at Alkali-Site of NaX[Fe0.5Mn0.5]O2 As Cathode for Sodium Ion Batteries

Abstract

Sodium ion batteries(SIB) have gained significant attention as next generation of rechargeable batteries based on natural abundance and low cost. Transition metal oxides with layered structure is well known electrode material that have been successfully commercialized in LIB system and have been reported to be one of the promising electrode material for SIBs. Extensive studies have been reported about substitution of various foreign element in the layered structure. Substitution of foreign element could be achieved by substitution at either transition metal site or alkali site. Many studies focused on the composition and substitution at the transition metal site of the layered structure. Here, this study focused on the substitution at the alkali site of layered structure as cathode materials for SIBs. Substitution at alkali site could not only affect the properties of layered structure, but also interfere with intercalation of sodium. In this study, Li was substituted on alkali site of NaXFe0.5Mn0.5O2 as cathode material for SIBs. Although Li have been reported to be substituted at transition metal site in previous literature, Li substitution at alkali site was achieved by varying the ratio between Fe and Mn. Rietveld refinement synchrotron XRD and neutron diffraction patterns, XANES analysis, and ICP analysis were conducted in order to confirm the substitution of Li at alkali site. When Li was substituted at alkali site, it showed improved cycle life and rate capability in spite of decreased interlayer spacing. In situ XRD analysis and DFT calculation was conducted in order to understand the role of Li in structure stability and Na intercalation behavior. The result showed that Li substitution at alkali site stabilized layered structure during charge/discharge process and that Li lowered the energy barrier of Na hopping, leading to improved cycle life and rate capability.