Optical and electrochemical properties of Li+ intercalated Zr–Ce oxide and Hf–Ce oxide films

Sputter deposited Zr–Ce oxide and Hf–Ce oxide films were investigated with regard to structure, optical absorption, and electrochemical properties. X-ray diffractometry and Rutherford backscattering spectrometry showed that polycrystalline Zr–Ce oxide and Hf–Ce oxide films had cubic crystal structures for 40–100 mol % ${\mathrm{CeO}}_{\mathrm{2}}$ and 50–100 mol % ${\mathrm{CeO}}_{\mathrm{2}},$ respectively. Cyclic voltammetry was performed in an electrolyte of propylene carbonate containing ${\mathrm{LiClO}}_{\mathrm{4}}.$ The charge capacity was $\text{∼60\hspace{0.17em}}{\mathrm{mC/cm}}^{\mathrm{2}}\mathrm{\mu m}$ for a Zr–Ce oxide film with a Ce/Zr atom ratio of $\text{∼13}$ as well as for a Hf–Ce oxide film with a Ce/Hf atom ratio of $\text{∼2.}$ A decrease of the charge capacity was noted after $\text{∼1000}$ voltammetric cycles, with the mixed oxide films being far more stable than ${\mathrm{CeO}}_{\mathrm{2}}.$ In situ optical transmittance measurements showed that both Zr–Ce and Hf–Ce oxide films remained essentially transparent during ${\mathrm{Li}}^{\mathrm{+}}$ intercalation. Chronopotentiometry measurements were used to elucidate effects of the electronic structure during ${\mathrm{Li}}^{\mathrm{+}}$ intercalation.