Electrochemical study of the Li-ion storage process in MWCNT@TiO₂–SiO₂ composites

Tuning the Li-ion storage mechanism from battery-like to pseudocapacitive is a current strategy to improve the rate capability of intercalation materials. A widespread methodology is decreasing the particle size of the active material, offering larger number of active sites available to storage Li-ions on the surface of the materials. Core@shell composites were obtained by hydrolyzing TTIP and TEOS, at different ratios, over previously dispersed home-made MWCNTs in Isopropanol, in order to obtain a TiO₂ shell modified with 1 mol% and 9 mol% of SiO₂. This led to a detriment in the anatase crystalline size (TEM and XRD) and an increment in the specific surface area (BET) of the composite, but kept constant the TiO₂ shell thickness formed around the MWCNTs. A change in the Li-ion storage process from mostly insertion (at SiO₂ 1 mol%) to entirely pseudocapacitive (at SiO₂ 9 mol%), was observed. This allowed a better capacity retention at high cycling rates, when the material was tested between 3 and 1 V vs. Li/Li⁺. Nonetheless, when the potential windows during cycling was increased from 3 to 0.5 V, the specific capacity of the composite modified with 9 mol% of SiO₂, vanished at high cycling rates. The thoroughly EIS characterization in the whole potential window (from 3 to 0.5 V) of the tested half cells, evidenced the enlargement of charge transfer resistance; which was associated to highly reactive –OH groups (FTIR and TGA) present in the composite, promoted by the addition of SiO₂ in the shell.