Irreversible phase transformations of layered oxide cathodes during charging have been detrimental for most of them. Even if a lot of efforts have been made to relieve this highly irreversible phase transformation, there have been just a few successful results, which definitely limit the amount of extracted alkali ions and therey the available capacitie of the layered oxides. So, this presentation will suggest two strategies to get over the limitation of previous researches.  

As an inverse conceptual strategy, we first observed the possibility to make this irreversible phase transformation extremely reversible by utilizing crystal water as a pillar. Although we found a few cyrstal structures working with this reversible phase transition, the works using Na-birnessite (Na_xMnO₂•yH₂O; Na-bir) or Li-birnessite (Li-bir) as basic structural units will be highlighted here. 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 maximized through modulating the steric coordination or amount of cyrstal water in the lattice resultantly optimizing the electrochemical performances of the birnessite layered oxides.

Pseudo Jahn-Teller effect (Pseudo JTE) will be also stressed out as a fundamental reason behind lattice distortion of layered oxide cathodes. Even though Jahn-Teller effect (JTE) has been regarded as one of the most important determinators of how much stress layered cathode materials undergo during charge and discharge, there have been many reports that traces of superstructure exist in pristine layered materials and irreversible phase transitions occur even after eliminating the JTE. A careful consideration of the energy of cationic distortion using a Taylor expansion indicated that second-order JTE (Pseudo JTE) is more widespread than the aforementioned JTE because of the various bonding states that occur between bonding and anti-bonding molecular orbitals in transition metal octahedra. As a model case, some of layered oxide cathodes including P2-type Mn-rich cathode (Na_3/4MnO₂) will be dealt with in this presentation

The two new insights 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.