The application of microelectrodes to the study of lithium battery systems

Copper, nickel, carbon, aluminium, gold and platinum microdisc electrodes (radii 4-40 μm) have been used to study the deposition and dissolution of lithium metal in both organic (tetrahydrofuran) and inorganic (thionyl chloride) solvents each containing LiAsF_6 and LiAlCl_4 respectively. It has been shown that high quality, reproducible data, free of effects due to iR drop may be obtained even in these low dielectric constant solvents and at the high current densities of interest in lithium battery chemistry. Cyclic voltammetric and potential step techniques have been used to study the kinetics of the Li/Li^+ couple at freshly deposited lithium surfaces. It is suggested that a freshly deposited lithium surface, maintained at negative overpotentials is free from any surface films as a result of cathodic protection. (i) In LiAsF_6/THF (0.8 mol dm^-3) the data shows the couple to be rapid and the exchange current density was estimated to be 4.0 mA cm^-2 at room temperature. Temperature studied over the range 254K to 313K showed the energy of activation for the couple to be 56 kJ mol^-1 indicating that the Li^+ ions are strongly solvated by the THF. The mechanism and kinetics of the nucleation and growth of lithium metal on a nickel substrate was probed. It is suggested that lithium deposition occurs via the instantaneous nucleation of hemispherical centres which grow three dimensionally under kinetic control. (ii) In LiAlCl_4/SOCl_2 (1.7 mol dm^-3) the couple is again very rapid. At room temperature the exchange current density was estimated to be 50 mA cm^-2 and temperature studies over the range 200K to 343K led to a value for the energy of activation of 25 kJ mol^-1. It has been demonstrated that a lithium surface allowed to stand on open circuit is quickly covered by a film which inhibits dissolution and further deposition of lithium. Studies of such a filmed lithium surface led to a model to explain the behaviour of the Li/Li^+ couple in this electrolyte. The electrochemical reduction of SOCl_2 has been shown to depend on the electrode material and to be catalysed by various additives to carbon cathodes. The influence of construction and composition of the carbon cathode on the discharge behaviour of Li/SOCl_2 cells has been investigated. (D80014)