Electrochemical intercalation of lithium into LiLaNb2O7 perovskite

The layered perovskite compound LiLaNb2O7 is a lithium-deficient material. This product is obtained by ion exchange in molten salt from KLaNb2O7. Electrochemical intercalation of lithium ions is possible and leads to the formation of Li1+xLaNb2O7. The intercalation is investigated in LiClO4(M)-PC electrolyte using galvanostatic discharge, the galvanostatic intermittent titration technique, and impedance spectroscopy. A maximum lithium uptake of x = 1 was found without any structural change. The discharge curve clearly shows the existence of interactions between the host material and the intercalated lithium ions as intercalation proceeds. The energy of these interactions remains small for x < 0.5 and increases drastically to 1 eV for x > 0.5. A detailed study of the discharge curve shows that superlattice orderings of Li ions appear at x = 0.25 and 0.5 leading to a small but sudden change of the electrode potential. Complex impedance spectroscopy, performed during the discharge, shows that for x < 0.5 the charge-transfer reaction occurs with the adsorption of intermediate species. This adsorption reaction disappears for x > 0.5. An equivalent electrical model which includes charge-transfer, adsorption, constant-phase elements, and diffusional impedance is proposed. The fit between the experimental data and this model is good and leads to the determination of the kinetics parameters as a function of x. The mechanism of the intercalation is discussed on the basis of the crystallographic structure of the layered perovskite.