Electrochemical Characterization of PEO/LiTFSI Electrolytes Near the Solubility Limit

Abstract

Polymer electrolytes are commonly studied as potential solid replacements for conventional liquid electrolytes to enable the design of rechargeable batteries with a lithium metal anode. Poly(ethylene oxide) (PEO)/lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) has been studied extensively and serves as the foundation for advanced systems. We present the first full electrochemical characterization of this system at high salt concentrations above r = 0.30, where r is the molar ratio of lithium ions to ethylene oxide in the electrolyte. Above r = 0.28, we observe two coexisting phases: an amorphous phase with r = 0.28 and a crystalline phase with r = 0.50. We measure the limiting current, the maximum current that can be applied to an electrolyte before the time-dependent potential diverges exponentially, and find that at r = 0.50, the limiting current is essentially zero. This indicates that r = 0.50 is the solubility limit for PEO/LiTFSI. Based on this solubility limit, we use concentrated solution theory to predict the limiting current below r = 0.50. The theory and experiments agree qualitatively but not quantitatively because the theory was developed for single-phase systems.