Lithium ion quantification using mercury amalgams as in situ electrochemical probes in nonaqueous media

We report on the quantitative, spatially resolved study of ionic processes for energy materials in nonaqueous environments by in situ electrochemical means at the micro- and nanoscale. Mercury-capped platinum ultramicroelectrodes (Hg/Pt UMEs) were tested as probes for alkali ions in propylene carbonate (PC) in an oxygen- and water-free environment. Anodic stripping voltammetry (ASV) performed at Hg/Pt UMEs displayed a linear response to Li(+) concentration extending from 20 μM to at least 5 mM. The sensitivities of these probes for ionic lithium are 1.93 and -23.2 pA μM(-1) by the steady-state amalgamation current and the peak stripping current, respectively. These values showed excellent agreement with simulated results as well as to those obtained experimentally for Cd(2+) in H2O. We further explored the interfacial imaging of lithium ion flux at an electrified interface. Scanning electrochemical microscopy (SECM) using Hg/Pt UMEs showed that the steady-state amalgamation of ionic lithium could be used to reliably position a probe close to a substrate. Investigations on a selectively insulated gold electrode in an organic solvent system showcased the response of Hg/Pt UMEs to lithium uptake by an electroactive material. Additionally, lithium stripping voltammetry at Hg deposits on a 120 nm carbon nanoelectrode demonstrated the possibility of implementing the introduced imaging strategy at the nanoscale. This work opens a way to directly correlate material defects and reactive heterogeneity in energy materials with unprecedented spatial and temporal resolution.