Investigation of the electrochemical windows of aprotic alkali metal (Li,Na,K) salt solutions

This work is a comparative study of the electrochemical windows and the basic processes on gold electrodes in LIClO4, NaClO4, and KClO4 solutions in propylene carbonate (PC). The analytical tools included cyclic voltammetry, electrochemical quartz crystal microbalance, surface-sensitive Fourier transform infrared spectroscopy (ex situ, external reflectance mode), and X-ray photoelectron spectroscopy. The apparent electrochemical windows of these systems are anodically limited at potentials above 1.3 V (vs. Ag pseudoreference electrode corresponding to 4.3 vs. Li/Li+) due to solvent oxidation. The apparent cathodic side is Limited due to the reversible bulk active metal deposition occurring at approximately -3 and <-2.7 V vs. Ag pseudoreference electrode for Li and Na, respectively. In the case of the potassium salt solution, the electrochemical window is limited by a pronounced cathodic process below -2 V (vs. Ag reference electrode), which is attributed to irreversible reduction of solution species. Irreversible potassium deposition occurs at potentials below -2.5 V. This process cannot be separated from the reduction processes of the solution starting below -2 V. The study revealed that irreversible trace O-2, trace H2O, and PC reduction form passivating surface films on these electrodes. These films act as a solid electrolyte interphase, i.e., they allow transport of the alkali metal ions through them. The study also found that the major constituent in the surface films is the PC reduction product CH3CH(OCO2M)CH2OCO2M, In general, the surface films formed on the noble metal electrodes in the Li and K salt solutions are more stable than those formed in the Na salt solutions, because the sodium oxides, hydroxide, and carbonates thus formed are more soluble in PC than the corresponding Li and K compounds. <(c)> 2001 The Electrochemical Society. All rights reserved.