Electrochemical Kinetics in Solid Battery Electrolytes

Abstract

Solid electrolytes are of interest for improved battery safety and compatibility with advanced electrodes (such as lithium metal). They could potentially enable flexible, efficient batteries with the use of single-ion conducting polymer or composite electrolytes. However, the rate capability of solid-state batteries is problematic, as is long-term stability with certain electrode chemistries. In order to better understand these limitations, we have developed a pulsed voltammetry method to study electrode reaction kinetics in cells with solid polymer electrolytes. Traditionally, a rotating disk electrode has been used to study reaction kinetics in liquid electrolytes. This method minimizes mass transfer limitations by rotating the working electrode so that reaction kinetics can be accurately measured. Since electrode rotation is not possible in solid electrolytes, we use short, constant-voltage pulses and collect a current measurement. Then the cell is left at open circuit for an extended period of time to dissipate any concentration gradients that developed during the measurement. After sufficient relaxation, another pulse to a different voltage is made. Measurement at early time minimizes mass-transfer contribution to the measurement. We have validated this technique against rotating disk electrode measurements in a liquid electrolyte. It has also been applied to study oxidative degradation of polymer electrolyte and lithium and plating kinetics. The latter study is of particular interest due to the fact that a connection between reaction kinetics and lithium dendrite morphology has been observed in some systems. 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 kinetic measurements from pulsed voltammetry will be compared to electrochemical impedance spectroscopy measurements and lithium surface morphology analysis. Interesting effect of salt concentration will also be presented.