Stability of sodium couple in organic and inorganic molten salt electrolytes investigated with electrochemical quartz crystal microbalance

An electrochemical quartz crystal microbalance (EQCM) was used to examine t he sodium plating-stripping reactions in two room-temperature molten salts: (i) buffered-neutral 1-methyl-3-propylimidazolium chloride and (ii) methan esulfonyl-aluminum chloride. In cyclic voltammetry scans on melt (i), sligh tly more mass was plated on the electrode than required by the charge passe d for sodium reduction. The excess mass deposition suggests coreduction of either electrolytes or impurities and formation of protective layers; this results in irreversible deposition and less than 100% coulombic efficiency. In chronoamperometry scans, the mass deposition rate was lower than the eq uivalent charge passed for pure sodium. This indicates possible reaction be tween sodium metal and the electrolytes or impurities at the electrode surf ace after depletion of sodium ions. The inorganic molten salt melts, (ii), were made of aluminum chloride, methanesulfonyl chloride, and sodium chlori de. The amount of sodium chloride added to melts was critical to the stabil ity of the sodium deposited. The highest coulombic efficiency for the sodiu m couple in these melts was 97%, and the self-discharge rate was 3 muA/cm(2 ). The EQCM results showed reduction of the electrolyte or reaction with so dium metal forming irreversible products. (C) 2001 The Electrochemical Soci ety. All rights reserved.