Atomic Layer Deposition of Amorphous TiO₂ on Carbon Nanotube Networks and Their Superior Li and Na Ion Storage Properties

Titanium dioxide is a promising anode material for lithium and sodium-ion batteries (LIBs, SIBs). Current research of TiO₂ is mainly focused on the crystalline phase (anatase or rutile) with a battery-like diffusion mechanism. In the present work, it is aimed to demonstrate the advantages of amorphous titanium dioxide over crystalline ones for both Li and Na ion batteries. Amorphous TiO₂ thin layers of different thicknesses are deposited on carbon nanotube (CNT) network/carbon fiber paper (CFP) substrates, forming the double-walled TiO₂@CNT nanotubes. For comparison, the as-obtained amorphous TiO₂@CNT/CFP samples are annealed to achieve anatase TiO₂. It is shown that, as an anode of LIBs, the amorphous TiO₂ shows better performance than the anatase one in terms of specific capacity, rate capacity, as well as cycling performance. The difference in the electrochemical property stems from their different charge-storage mechanisms; different from the diffusion-limited process in crystalline TiO₂, the amorphous TiO₂ stores charges via a fast surface-controlled capacitive process similar to the orthorhombic Nb₂O₅. Moreover, the amorphous TiO₂@CNT/CFP electrodes are also confirmed to be suitable for SIBs.