Metallic lithium is the most frequently employed reference electrode in lithium ion battery research, but its high reactivity limits its utilization mainly to conventional carbonate based electrolytes. Novel concepts, like hybrid battery/supercapacitor devices, are often based on electrolytes with high ionic conductivity (e.g., acetonitrile). The utilization of these electrolytes brings up its own challenges as fundamental electrochemical characterizations are drastically aggravated by the high reactivity towards lithium metal. An alternative to Li/Li⁺ reference electrode for TEA-BF₄ containing organic electrolytes is PTFE bound activated carbon as was shown in 2009.¹ However, the latter approach yielded a pronounced drift of the reference electrode potential over time when a lithium-salt was employed. This effect is only poorly understood so far and a stable reference electrode for such systems is in high demand. Based on these issues, we provide a comprehensive study to establish (modified) carbon as a stable quasi-reference electrode for advanced lithium-salt containing electrolytes.²

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.

References

1. P. W. Ruch, D. Cericola, M. Hahn, R. Kötz and A. Wokaun, Journal of Electroanalytical Chemistry, 636(1-2), 128–131 (2009).

2. M. Widmaier, B. Krüner, N. Jäckel, M. Aslan, S. Fleischmann, C. Engel and V. Presser, J. Electrochem. Soc., 163(14), A2956-A2964 (2016).