Kinetics of Lithium-Ion Transfer at the Interface between Li0.35La0.55TiO3 and Binary Electrolytes

The kinetics of lithium-ion transfer at a Li0.35La0.55TiO3 (LLT)/liquid electrolyte interface were investigated by four-probe ac impedance spectroscopy. In Nyquist plots for a system consisting of Li/electrolyte/LLT/electrolyte/Li, two semicircles were observed at high- and midfrequency regions and were attributed to grain-boundary resistance in LLT and interfacial resistance between LLT and the liquid electrolyte. The activation energies of interfacial lithium-ion transfer were evaluated from the temperature-dependence of interfacial conductivities at the LLT/liquid electrolyte. When binary electrolytes consisting of LiClO4 dissolved in mixtures of ethylene carbonate (EC) and dimethyl carbonate (DMC) of given volume ratios were used, the activation energies were around 50 kJ mol−1. However, an electrolyte without EC (LiClO4/DMC) gave an activation energy of 32 kJ mol−1. Since a theoretical calculation clarified that the solvation ability of EC is higher than that of DMC, the activation energies reflected energies for the desolvation of lithium-ion from solvent during interfacial lithium-ion transfer. Another important result is that the activation energies of interfacial lithium-ion transfer were almost the same (ca. 50 kJ mol−1) in EC:DMC (1:1 by vol) and EC:DMC (1:9 by vol). Furthermore, Raman spectra of EC:DMC binary electrolytes show that there is a dramatic difference in the solvation numbers of EC molecules per lithium-ion between EC:DMC (1:1 by vol) and EC:DMC (1:9 by vol). These results indicate that the desolvation of lithium-ion from the last solvent molecule is the rate-determining step in interfacial lithium-ion transfer.