Poly(ethylene oxide)-Based Electrolyte for Solid-State-Lithium-Batteries with High Voltage Electrodes: Re-Evaluation of Common Beliefs

Abstract

Poly(ethylene oxide) (=PEO)-based solid polymer electrolytes (SPEs) are believed to be oxidatively instable, thus suitable only with “low potential” electrodes. Indeed, they reveal a sudden failure in Li metal cells when high potential positive electrodes, e.g. LiNi0.6Mn0.2Co0.2O2 (NMC622) are used. This is electrochemically detectable in an arbitrary, time – and voltage independent, “voltage noise” during charge.A relation with the believed SPE oxidation was evaluated, for validity reasons on different electrodes including different active materials in both, potentiodynamic and galvanostatic experiments. The results indicate an exponential current increase and a potential plateau at 4.6 V vs. Li|Li+, respectively, demonstrating that the oxidation onset of the SPE is above the used working potential of NMC622, which is only charged up to 4.3 V vs. Li|Li+.Surprisingly, a simple increase in SPE membrane thickness or a simple exchange of Li metal negative electrode with graphite electrode revealed an operation free of “voltage noise”. These experiments indicate that the Li | SPE interface, and in particular, Li dendrite formation and penetration through the SPE membrane is the significant source and counterintuitively not the literature believed SPE│NMC622 interface.(1) The concluded oxidative stability of the cheap and abundant PEO and the precisely diagnosed failure source led to modified and more systematic improvement strategies for a successful high voltage application of solid-state Li batteries, even at 40 °C.(2) G. Homann, L. Stolz, J. Nair, I. C. Laskovic, M. Winter and J. Kasnatscheew, Sci Rep, 10, 4390 (2020). G. Homann, L. Stolz, M. Winter and J. Kasnatscheew, iScience, [accepted] (2020). Figure 1