A MICROELECTRODE STUDY OF LITHIUM ELECTROKINETICS IN POLY(ETHYLENE GLYCOL DIMETHYL ETHER) AND 1,2-DIMETHOXYETHANE

A systematic characterization of the electrokinetics of the Li/Li+ couple was carried out in poly(ethylene glycol dimethyl ether) (PEGM) (MW = 400) and in its small molecule analog, 1,2-dimethoxyethane (DME), using LiAsF6 as the electrolyte salt. Microelectrodes (nickel disks of 25 or 50 mu m diam) were used for the investigations because of their many advantages over conventional macroelectrodes. In the kinetic studies, lithium was freshly deposited and then immediately stripped to minimize interference from lithium surface films. The exchange current densities, i(0), for the Li/Li+ couple are an order of magnitude smaller in PEGM than in DME between 296 and 338 K. A typical value of i(0) is 0.72 mA/cm(2) in PEGM and 9.8 mA/cm(2) DME, for salt concentrations of 0.3 M at 313 K. The transfer coefficient was close to 0.5 in both solvents at all temperatures. The exchange current density in PEGM is strongly temperature dependent, with a high activation energy of 66 kJ/mol. The concentration dependence of i(0) in PEGM is unusual, with a linear relationship between i(0) and C-salt(1/2) from 0.02 to 0.6 M after which the i(0) drops off sharply. The results can be interpreted microscopically in light of recent advances concerning the influence of solvent dynamics on electron transfer. The experiments also provide insight into the factors affecting lithium electrode kinetics in polymer electrolytes.