In our work, we investigate the suitability of different carbons as quasi-reference electrodes (QREs) for lithium-salt containing electrolytes. As a reference point, we directly use the lithium intercalation/de-intercalation reaction of nanoparticulate Li4Ti5O12. Low surface area carbon-based QREs are impaired by high rates of potential shift (140 mV after 5 d). All high surface area carbon-based QREs show very low, uniform, and reproducible rates of potential shift (40-50 mV after 5 d). Only a negligible influence of carbon pore size distribution, electrolyte solvent, and binder on the reference electrode stability was observed. A distinct influence of the electrolyte anion on the quasi-reference electrode stability was found (Figure 1). The stability of pristine activated carbon decreases in the following order LiTFSI>LiClO4>LiPF6>LiBF4.
We also studied the influence of carbon modification on the QRE stability. Activated carbon was de-functionalized (thermal annealing in vacuum or hydrogen) and functionalized (HNO3 treated) in order to unveil coherences of reference electrode stability with surface functional groups. The potential shift of activated carbon is drastically suppressed if the carbon surface is saturated by oxygen and nitrogen functional groups (Figure 2). After 15 days, the potential of heavily functionalized activated carbon is only marginally altered by 10 mV. Therefore functionalized activated carbon is a well suited quasi-reference electrode for the electrochemical characterization of Li-salt containing electrolytes which are not stable versus lithium metal.
1. P. W. Ruch, D. Cericola, M. Hahn, R. Kötz and A. Wokaun, Journal of Electroanalytical Chemistry, 636(1-2), 128–131 (2009).