Long-Cycle-Life Na-Ion Anodes Based on Amorphous Titania Nanotubes-Interfaces and Diffusion

Amorphous self-assembled titania nanotube layers are fabricated by anodization in ethylene glycol based baths. The nanotubes having diameters between 70-130 nm and lengths between 4.5-17 μm are assembled in Na-ion test cells. Their sodium insertion properties and electrochemical behavior with respect to sodium insertion is studied by galvanostatic cycling with potential limitation and cyclic voltammetry. It is found that these materials are very resilient to cycling, some being able to withstand more than 300 cycles without significant loss of capacity. The mechanism of electrochemical storage of Na⁺ in the investigated titania nanotubes is found to present significant particularities and differences from a classical insertion reaction. It appears that the interfacial region between titania and the liquid electrolyte is hosting the majority of Na⁺ ions and that this interfacial layer has a pseudocapacitive behavior. Also, for the first time, the chemical diffusion coefficients of Na⁺ into the amorphous titania nanotubes is determined at various electrode potentials. The low values of diffusion coefficients, ranging between 4 × 10^-20 to 1 × 10^-21 cm²/s, support the interfacial Na⁺ storage mechanism.