(Invited) Layered Structure Cathodes; Prejudice, Inverse Concept and New Possibility

Abstract

Phase transformation of layered structure into spinel structure has been detrimental for most of layered oxide cathodes. Even if a lot of efforts have been made to relieve this highly irreversible phase transformation, there have been few successful results. However, we firstly observed the possibility to make this irreversible phase transformation extremely reversible by utilizing Na-birnessite (NaxMnO2•yH2O; Na-bir) or Li-birnessite (Li-bir) as basic structural units, which have distinctive layered structure containing crystal water. Herein, the crystal water in the structure contributes to generating metastable spinel-like phase, which is the key factor for making this unusual reversibility happen. The reversible structural rearrangement between layered and spinel-like phases during electrochemical reaction could activate new cation sites and enhance ion diffusion with higher structural stability. This unprecedented reversible phase transformation between spinel and layered structure was also observed for the Na-bir or Li-bir with some organic moelcules in place of crystal water finally setting a significant precedent for organic-inorganic hybrid electrodes for alkali ion batteries. All of materials were carefully analyzed via combined ex situ soft and hard X-ray absorption spectroscopy (XAS) analysis with in situ X-ray diffraction (XRD). Fundamental mechanism on this reversible phase transformation was theoretically elucidated and confirmed by kinetic investigation using first-principle calculation. These results provide deep insight into novel class of intercalating materials which can deal with highly reversible framework changes, and thus it can break up some typical prejudices which we have about the layered cathodes for Li or Na secondary batteries.