Understanding the Relationship of Nanostructure to Performance in Additive Free TiO2 Anodes for Lithium-Ion Batteries

TiO₂ has recently emerged as a potential material for use as anodes for lithium ion batteries owing to its low cost, abundance and high chemical stability. There has been considerable interest in nanostructured thin films for this application since they offer higher surface area and thus better performance compared to bulk films. The performance of the anodes has previously been correlated to the particle size in these nanostructures. However, morphology and crystal orientation of the nanostructure have not yet been co-related to the performance of the battery. Also, current fabrication methods consist of multiple steps involving the use of carbon as conducting material and binders for adhering the active material to the current collector substrate. These, in addition to increasing the cost of fabrication, also pose a safety concern for high rate applications.

In this study, we present the fabrication of TiO₂ nanostructures using a single step gas phase method directly on the current collector. Aerosol Chemical Vapor Deposition (ACVD) process has been successfully used for single step synthesis of nanostructured TiO₂ with different morphologies by controlling the aerosol dynamics in the process. One dimensional (columnar) single crystal TiO₂ nanostructures and granular structures were synthesized on stainless steel substrates (current collector) by the ACVD process. The electrochemical performance of these nanostructures was tested in a half-cell assembly against Li/Li⁺.

Single crystalline columnar structures exhibited enhanced specific capacity compared to the polycrystalline granular structures for the same structure height (Figure 1). The difference in performance of the columnar structures and the granular structures widens with increasing charging rates indicating the sluggish diffusion kinetics in the granular structures. This reduced performance of the granular structures was attributed to the presence of grain boundaries and the different crystallographic orientation in the structures.  Further the performance of the columnar and granular nanostructures depended on the height of the structures. Mass transfer characteristics of the different morphologies were studied using electrochemical impedance spectroscopy (EIS) and Galvanic Intermittent Titration Technique (GITT). Transport modelling of lithium in the nanostructure will be used to understand the correlation between the nanostructure and performance of the anodes.