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 bas ic processes on gold electrodes in LIClO4, NaClO4, and KClO4 solutions in p ropylene carbonate (PC). The analytical tools included cyclic voltammetry, electrochemical quartz crystal microbalance, surface-sensitive Fourier tran sform 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 pseudorefe rence electrode corresponding to 4.3 vs. Li/Li+) due to solvent oxidation. The apparent cathodic side is Limited due to the reversible bulk active met al deposition occurring at approximately -3 and <-2.7 V vs. Ag pseudorefere nce electrode for Li and Na, respectively. In the case of the potassium sal t solution, the electrochemical window is limited by a pronounced cathodic process below -2 V (vs. Ag reference electrode), which is attributed to irr eversible reduction of solution species. Irreversible potassium deposition occurs at potentials below -2.5 V. This process cannot be separated from th e reduction processes of the solution starting below -2 V. The study reveal ed that irreversible trace O-2, trace H2O, and PC reduction form passivatin g surface films on these electrodes. These films act as a solid electrolyte interphase, i.e., they allow transport of the alkali metal ions through th em. The study also found that the major constituent in the surface films is the PC reduction product CH3CH(OCO2M)CH2OCO2M, In general, the surface fil ms 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 t he corresponding Li and K compounds. <(c)> 2001 The Electrochemical Society . All rights reserved.