Ordered Ti-Fe-O nanotubes as additive-free anodes for lithium ion batteries

Abstract

Manufacturing of binder-free mixed oxide electrodes with an unique 1D tubular morphology, high lithium storage ability and long durability represents a real challenge to assemble efficient lithium ion batteries for safe operation. Herein, the successful fabrication of mixed Ti-Fe-O nanotubes (NTs) by a single potentiostatic anodization step of a TiFe12 alloy at room temperature is reported. With the help of suitable characterization tools, the mixed oxide tubes in the different preparation stages were investigated for crystallinity and phase formation including an analysis on the structure defects which influence the material´s performance. Their morphology and element distribution were evaluated in the bulk and on the surface as well as oxidation state changes of the transition metal elements and the solid-electrolyte interphase formation were investigated up to the operando mode. The high anodization voltage along with the substrate composition induced the formation of a thin membrane layer covering the top of Ti-Fe-O NTs. The electrochemical performance of NTs as potential anodes in Li-ion batteries was evaluated vs. Li/Li+ without any binder or conductive additives. The effect of the annealing temperature on crystallinity and Li-ion storage ability of NTs was investigated as well. The Ti-Fe-O NTs electrodes demonstrated an initial discharge capacity of 291 mAh g−1 at a current rate of 0.15C (1C = 335 mA g−1). The annealed crystalline NTs showed a higher reversible capacity of 155 mAh g−1 than the amorphous nanotubes after direct fabrication at room temperature (98 mAh g−1) after 50 charging/discharging cycles with a Coulombic efficiency close to 100% without a provable decomposition of the tubular structure but slight visible changes in their morphology. The noticed increase of the capacity of the Ti-Fe-O NT arrays treated at 600 °C is attributed to the enhanced ionic conductivity originated from short pathways for the Li+ ion transport across the grain boundaries of the crystalline domains. In contrast, the presence of point defects, atomic displacements and planes sliding in the amorphous matrixes surrounding the crystalline entities hinder Li+ ion transport through scattered diffusion pathways, and hence a lower lithium storage is demonstrated.