In Situ Neutron Diffraction Studies of LiCe(WO4)2 Polymorphs: Phase Transition and Structure–Property Correlation

Abstract

Polymorphs of LiCe(WO4)(2) alpha-LiCe(WO4)(2) and beta-LiCe(WO4)(2)] were successfully synthesized by a citric acid-assisted sol-gel method for the first time. Phase purity and crystallinity were confirmed by powder X-ray diffraction and further characterized by X-ray photoelectron spectroscopy, field emission scanning electron microscopy, Raman, and thermogravimetric-differential thermal analysis measurements. Investigation of the effect of calcination temperature and time indicated the existence of an irreversible structural phase transition from low-temperature (LT) beta-phase to high-temperature (HT) alpha-phase, which is systematically followed by in situ HT powder neutron diffraction (NPD) studies. Rietveld refinements using NPD data revealed that alpha-LiCe(WO4)(2) crystallizes in tetragonal Scheelite-type structure (I4(1)/a), while beta-LiCe(WO4)(2) crystallizes in triclinic alpha-LiPr-(WO4)(2)-type structure (P (1) over bar). Furthermore, beta-LiCe(WO4)(2) undergoes a reconstructive phase transition where WO6 of the beta-phase rearranges to WO4 at HT in alpha-phase and is supported by Raman measurements. Anodic redox activities of these polymorphs were determined by cyclic voltammetry and galvanostatic charge-discharge measurements. Interestingly, beta-phase has shown promising results compared with that of alpha-phase, which is attributed to the easy Li-ion diffusion in between the parallel layers of WO6 and also to the greater structural stability of the beta-phase. Thus, our initial understanding on the structure-property of these LiCe(WO4)(2) polymorphs provides insights into the design of new insertion anode electrodes for Li-ion batteries.