The role of in situ generated nano-sized metal particles on the coulombic efficiency of MGeO₃ (M = Cu, Fe, and Co) electrodes

To improve the coulombic efficiency of GeO₂ electrode, a Cu-containing ternary metal oxide (CuGeO₃) was prepared and the electrochemical behavior of Cu component was studied. The GeO₂ electrode shows a low coulombic efficiency in the first cycle (43%), which is mainly caused by a poor Ge oxidation kinetics (Ge + 2Li₂O → GeO₂ + 2Li⁺ + 2e^-). The X-ray absorption spectroscopy (XAS) data illustrate that the Cu component in CuGeO₃ is converted to nano-sized metallic Cu in the earlier stage of lithiation but idles thereafter. In contrast, the Ge component in CuGeO₃ behaves like the GeO₂ electrode. It is converted to nano-sized Ge by a conversion reaction and further lithiated by alloying reaction. The de-lithiation proceeds in the reverse order. The CuGeO₃ electrode shows a much improved coulombic efficiency (74%) in the first cycle, which is indebted to a facilitated Ge oxidation with a much reduced electrode polarization. This feature has been explained by the favorable roles provided by the in situ generated nano-sized metallic Cu particles that make such an intimate contact with the nano-sized Ge and Li₂O that they can catalyze Li₂O decomposition and provide an electronic conductive network for Ge oxidation. A similar favorable effect was observed with the other ternary oxides (FeGeO₃ and CoGeO₃), wherein the formation of nano-sized metallic Fe and Co can be assumed.