Reversible layered-disordered transitions enabled by entropic stabilization

High configurational entropy was shown to impart emergent physical and structural functionalities to various oxides, with the notable exception of transition metal layered lithium oxides which were observed to possess poor electrochemical performance associated with secondary impurity phases. Through a combined thermal analysis using operando X-ray diffraction and calorimetric techniques with scanning transmission electron microscopy (STEM) measurements, we reveal the crystallization pathway of compositionally complex lithiated oxides and show how entropystabilization can be manipulated to enable enhanced electrochemical performance associated with reversible structural transitions in layered oxides. Screening various high-entropy and mediumentropy layered oxides and studying their thermal behavior, we show how precise control of synthetic conditions yields co-existing layered and disordered rock salt (DRX) nano-domains and the formation of antisite defects, both enabling enhanced electrochemical performance with reversible layered to spinel and layered to DRX structural transitions upon Li (de)intercalation. By comparing oxides with similar configurational entropy but different enthalpy of mixing, we show that large configurational entropy, as calculated assuming an ideal mixing, cannot be used as sole criterion for designing enhanced intercalation materials, with transformation energies and non-ideal enthalpy and entropy of mixing offsetting the stabilization effect of configurational entropy. The fundamental insights gained in this work regarding the thermal and electrochemical behavior of high entropy layered materials constitute a crucial step towards the choice of synthetic conditions to control the complex interplay between thermodynamics and kinetics in stabilizing compositionally complex layered oxides.