Li and Na-ion diffusion and intercalation characteristics in vertically aligned TiS<sub>2</sub> nanowall network grown using atomic layer deposition

We present here the study of diffusion and intercalation mechanisms of Li-ion and Na-ion in titanium disulfide (TiS2) films grown by atomic layer deposition (ALD). The layered TiS2 has been explored here due to the interesting differences between the intercalation mechanisms of Li+ and Na+. The ALD grown TiS2 films further facilitate the study as this method provides compact and dense films with no polymer binder and carbon additives. The diffusion and intercalation processes are observed to depend on the ionic size and character of the solid electrolyte interphase. The increased capacity obtained for the present ALD synthesized samples is attributed to the enhanced anchoring ability of the TiS2 films, which comprises of an extended nanowall network. The charge transfer resistance (Rct) obtained from impedance data correlate well with the lithiation steps observed in the galvanostatic discharge-charge studies. At potentials where lithiation takes place, Rct value is observed to drop. This direct correlation is however, not observed between the Rct and sodiation potential. The diffusion coefficients, calculated using GITT and impedance methods, are observed to be independent of the type of Li-salts. However, the variation of diffusion coefficients with the lithiation/sodiation voltages are different. This is attributed to the combined differences in the ionic radii and phase formation. Intercalation-de-intercalation in to TiS2 coated with alumina is also studied here. Coating with alumina results in a stable SEI. However, coating leads to higher Rct, lower DLi+ and poor capacity retention as a function of cycle number.