Matt Charlton: High-Resolution Analysis of Electrochemical Charge Transfer Processes in Complex Materials Systems

Matthew Charlton is a fourth year PhD student under the supervision of Prof. Keith Stevenson. The main focus of his research is in the development of highly ordered model material interfaces to facilitate high-resolution analysis of electrochemical charge transfer processes in complex materials systems.

The development of advanced materials for electrochemical energy conversion and storage is an increasingly important area of research. In the technologies currently being investigated, such as next-generation lithium ion batteries, photovoltaics, and supercapacitors, an understanding of the chemical and electrochemical processes at the nanoscale will be paramount to their success. Specifically, the complex charge transfer processes at material interfaces is of particular interest. One alternative method to study these processes, rather than trying to probe complex and often structurally convoluted high performance materials, is through the use of model systems that can be precisely controlled geometrically and chemically.

Atomic layer deposition (ALD) utilizes self-limiting vapor-surface chemical reactions to deposit thin films of material with precise control over geometry, surface quality, structure, and chemistry. As a result, Matthew has chosen to evaluate ALD as a potential synthesis route for these model interfacial systems. Using ALD-grown films of TiO2 and V2O5, deposited onto smooth thin films of conductive carbon, as two representative electrode materials, he is combining electrochemical methods and spatially resolved surface analytical techniques to probe the lithium ion coupled electron transfer processes in these materials. The pristine nature of the interfaces in these systems has enabled a deeper understanding of the specific mechanisms that occur during lithiation. For example, by varying the thickness of the active layer, it can be possible to determine length-scale dependence on the transition in insertion mechanism from intercalative to pseudocapacitive.

Additionally, recent spatially resolved spectroscopy results suggest that in the titania system, the fluorine counter-ion may play a more significant role in the charge transfer process than previously understood. Figure 1 below shows the ion depth profiles attained from time-of-flight secondary ion mass spectroscopy of these titania thin film electrodes after lithium insertion. It shows that the F- ion has penetrated significantly into the oxide electrode, and the sharp transitions between the electrode layers can be used as depth markers to estimate the positions of F- and other ions found throughout the sample. More significantly, the F- trace after delithiation does not indicate the same penetration.

Figure 1. TOF SIMS spectra from TiO2/C layered electrode after lithiation. Fluorine (shown in red) is found to penetrate through the TiO2 layer.

Figure 1. TOF SIMS spectra from TiO2/C layered electrode after lithiation. Fluorine (shown in red) is found to penetrate through the TiO2 layer.