Electrochemical lithiation of Si modified TiO₂ nanotube arrays, investigated in ionic liquid electrolyte

TiO₂ nanotube layers, synthesized by means of Ti foil anodization, have been modified by Si direct current (DC) magnetron sputtering, carried out at different power and deposition time. The performed SEM imaging suggests that two complementary factors (deposition power and time) play an important role for the surface morphology of the silicon structure deposited on TiO₂ nanotubes template. The variation of these parameters allows obtaining a specific "macaroni" like morphology, favorable for Si stabilization during its multiple electrochemical cycling in Li-ion electrolyte. The nanostructured Ti/TiO₂-Si layers are electrochemically tested for Li-ion insertion/extraction in 1-butyl-1-methylpyrrolidinium bis (trifluoromethyl) sulfonylimide ([BMP][TFSI]) containing 1 M Li[TFSI]. The voltammetric measurements show stable initial cycling, involving equal amounts of Si for the three selected deposition conditions. The potential ranges of Si and TiO₂ lithiation-delithiation processes are well resolved, which makes a separate estimation of Si and TiO₂ capacities from voltammetric data possible. Both factors, Si deposition parameters and substrate type, influence the long term galvanostatic cycling of the samples. It is found that the "macaroni" like structure deposited at 50 W/454 s on amorphous TiO₂ template exhibits a superior constant current cycling. The good electrochemical performance of this sample type is attributed to the specific morphology of the Si deposit and structural stability of the amorphous TiO₂ nanotubes.