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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 LixCeO2 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 LixCeO2 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 LixCeO2.