High cyclability of ionic liquid-produced TiO2 nanotube arrays as an anode material for lithium-ion batteries

Abstract

TiO2 nanotubes (NTs) are considered as a potential SEI-free anode material for Li-ion batteries to offer enhanced safety. Organic solutions, dominatingly ethylene glycol (EG)-based, have widely been used for synthesizing TiO2 NTs via anodization because of their ability to generate long tubes and well-aligned structures. However, it has been revealed that the EG-produced NTs are composited with carbonaceous decomposition products of EG, release of which during the tube crystallization process inevitably causes nano-scale porosity and cracks. These microstructural defects significantly deteriorate the NTs' charge transport efficiency and mechanical strength/toughness. Here we report using ionic liquids (ILs) to anodize titanium to grow low-defect TiO2 NTs by reducing the electrolyte decomposition rate (less IR drop due to higher electrical conductivity) as well as the chance of the decomposition products mixing into the TiO2 matrix (organic cations repelled away). Promising electrochemical results have been achieved when using the IL-produced TiO2 NTs as an anode for Li-ion batteries. The ILNTs demonstrated excellent capacity retention without microstructural damage for nearly 1200 cycles of charge–discharge, while the NTs grown in a conventional EG solution totally pulverized in cycling, resulting in significant capacity fade.