(632a) Electrochemical Kinetics in Solid Electrolytes for Lithium Metal Batteries

Solid electrolytes will enable safer, longer-lasting, next-generation batteries, but new techniques are required to determine electrochemical reaction kinetics. An electrochemical alternative to the rotating disk electrode has been developed to investigate oxidative electrolyte degradation and lithium plating/stripping kinetics in cells with solid polymer electrolyte. The solid polymer electrolyte used in this work consists of lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI) dissolved in polystyrene-b-poly(ethylene oxide) block copolymers. The electrochemical technique is pulsed voltammetry in which current is measured during short pulses to different overpotentials. Since convective mixing is not possible in solid electrolytes (for example by rotating the electrode), the pulses are separated by long periods of rest that allow concentration gradients to dissipate via diffusion. Current is measured immediately after capacitive charging but before any significant mass transfer resistance has developed, thus yielding measurements of pure charge transfer current. Butler-Volmer analysis of current versus overpotential is used to find exchange current density and apparent transfer coefficients for some key reactions in solid polymer electrolytes. The activation energy for charge transfer was also determined. This new technique was validated in a liquid electrolyte, with a ferrocyanide/ferricyanide redox couple, and also compared to results based on electrochemical impedance spectroscopy.