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Ning Song1 Donald Thomas1 Charles Hall1 Yu Jiang1 James Hook1 Alison Lennon1

1, UNSW, Sydney, New South Wales, Australia

Nanostructured titanium oxide (TiO2-x) has attracted attention for use in both battery and supercapacitor electrodes in recent years due to its ease of fabrication, high cycling capability with fast charging/discharging, safety and low toxicity. Although initial studies used crystalline TiO2-x where lithium ions (Li+) intercalate in planes between octahedral crystal elements, more recent studies have focussed on amorphous nanostructured materials due to their increased charge storage capacity. This increased charge storage has been attributed to increased Van de Waals attraction at the roughened anodic oxide surface, with oxygen vacancies providing electrically positive sites that can trap electrons at the convex surfaces [1].
In this paper, we report the use of NMR measurements of diffusion and relaxation to probe the surface chemical environmental of self-supported amorphous TiO2-x nanotubes in 1 M LiPF6 in 1:1 EC:EMC. Of particular interest is the ability to use these measurements to obtain estimates of the electrolyte-accessible metal oxide surface area, surface to volume ratio and surface wettability as a function of different anodisation conditions. TiO2-x nanotubes were first formed by anodising titanium mesh or foam in an electrolyte of fluoride-containing glycerol. Then relaxation measurements (T1 and T2) were performed in 1 M LiPF6 in 1:1 EC:EMC for each species: Li+ (measured using 7Li NMR) and the electrolyte anion, PF6- (measured using both the 19F and the 31P NMR) in both the neat electrolyte and in anodised mesh soaked in electrolyte. T1 and T2 variations thus are being explored for different electrode charge states as well as different anodisation conditions.
Diffusion of the Li+ and PF6- species in the electrolyte by pulsed field gradient stimulated echo (PFGSTE), is also being investigated both in the absence and presence of a mesh TiO2-x nanotubes electrode. The Li+ diffusion coefficient in the bulk electrolyte is consistent with previous measurements in this electrolyte [2] thus confirming the accuracy of our PFGSTE measurement system. Diffusion coefficients, measured in the presence of self-supported TiO2-x nanotube electrodes at different states of charge, will be used to provide dynamic information about the exchange of ions between in-and ex-pore environments, and in combination with the relaxation measurements, to provide information on how both the electrolyte anion and Li+ interact with the surface.
[1] M. Fukahara et al. Sci. Rep., 6, 35870,2016.
[2] C. Capiglia et al., J. Power Sources, 81-82, 859,1999.

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