Thermally Activated P2-O3 Mixed Layered Cathodes toward Synergistic Electrochemical Enhancement for Na Ion Batteries

Abstract

Layer-structured oxide cathodes have a lot of phases, which can be varied depending on Na ion contents and finally determine their electrochemical properties. Therefore, the off-stoichiometry of layer-structured oxides with the Na ions may differentiate not only their capacities but also the cyclic stabilities, kinetics, and so on, highlighting the importance of Na ion content. However, Na2CO3 tends to be irreversibly formed on surface by making use of the Na ions lost from the lattice. Thereby, the O3 phase with stoichiometric Na content changes into the off-stoichiometric P2 phase bringing about significant disadvantages. To address this issue, a thermal activation process is suggested to simultaneously decompose Na2CO3 into electrochemically active Na ions and modulate the off-stoichiometric P2 phase into the stoichiometric O3 phase. This study indicates that minimizing the loss of Na ions and maintaining the lattice framework with higher contents of Na ions during the synthesis of Na-incorporating layered cathodes should be a key toward attaining electrochemical superiority.