Anatase TiO2 nanotubes as negative electrodes for Lithium-ion and Sodium-ion microbatteries

The aim of this thesis is to gain a deeper understanding of the anatase TiO2 nanotubes as negative electrodes in lithium-ion and sodium-ion microbatteries by researching their fundamental intercalation processes. The nano-structuring of the anatase TiO2 electrodes allows to improve their electrochemical behaviour, and thus the batteries capacities. To this end, a DFT study of the lithium and sodium insertions into the anatase bulk, slab and nanotubes has been carried out. The study shows that the anatase TiO2 exhibits a higher reactivity towards sodium than towards lithium, resulting in two different insertion mechanisms: Li ions are intercalated via a two-phases equilibrium, while Na ions insertion occurs spontaneously without phase transformation. This high reactivity towards sodium may induce Na trapping, partly explaining the irreversible capacity losses observed during the first cycle. Lithium ions are intercalated spontaneously with higher capacities into the surface than into the bulk and can explain the capacities exceeding the theoretical bulk value reported for the nanostructured electrodes. The lithium intercalation induces a gradual structural reconstruction, forming a rock salt LiTiO2 phase, but its inward growth is limited to a few nanometres. The sodium ions are favourably adsorbed above all the oxygen atoms of the anatase surface and spontaneously inserted into the surface. However, the surface reconstruction is restricted by the electronic repulsion of the Na ions. This limited stability of the sodiated surfaces can be a possible explanation of their transformation into amorphous structure. The most favourable intercalation sites of the nanotubes are located in their external wall, stabilizing the nt-TiO2 structure. Li and Na ions are also favourably adsorbed on-top the outermost oxygen atoms, explaining why more external wall exposure exhibit better capacities. Reactivity with the internal wall is spontaneous for Li ions, while the Na ions require low potentials and are associated with rupturing Ti-O bonds in agreement with the formation of amorphous nt-TiO2 below 0.5 V vs Na/Na+.