Discovering a First-Order Phase Transition in the Li-CeO₂ System

An in-depth understanding of (de)lithiation induced phase transition in electrode materials is crucial to grasp their structure-property relationships and provide guidance to the design of more desirable electrodes. By operando synchrotron XRD (SXRD) measurement and Density Functional Theory (DFT) based calculations, we discover a reversible first-order phase transition for the first time during (de)lithiation of CeO₂ nanoparticles. The Li_xCeO₂ compound phase is identified to possess the same fluorite crystal structure with FM3M space group as that of the pristine CeO₂ nanoparticles. The SXRD determined lattice constant of the Li_xCeO₂ compound phase is 0.551 nm, larger than that of 0.541 nm of the pristine CeO₂ phase. The DFT calculations further reveal that the Li induced redistribution of electrons causes the increase in the Ce-O covalent bonding, the shuffling of Ce and O atoms, and the jump expansion of lattice constant, thereby resulting in the first-order phase transition. Discovering the new phase transition throws light upon the reaction between lithium and CeO₂, and provides opportunities to the further investigation of properties and potential applications of Li_xCeO₂.