Determining the Absolute Anodic Stability Threshold of Polymer Electrolytes: A Capacity-Based Electrochemical Method

Inherently safe, reversible, energy-dense solid-state lithium metal batteries require solid electrolytes that are electrochemically compatible at the high potentials of the cathode. In the scientific literature, there are significant discrepancies in the reported anodic stabilities of solid-state electrolytes, especially polymeric materials. Addressing the limitations of popular characterization approaches, such as linear sweep voltammetry, this study introduces a capacity-based electrochemical method, termed the reversibility test, as a reliable alternative in determining the anodic stability of polymer electrolytes. The reversibility test integrates the anodic and cathodic currents during a cyclic voltammetry measurement to identify irreversibility (ρ) through the capacity ratios. The capacity ratio is unchanged when the electrolyte is electrochemically stable but deviates at potentials exceeding the electrolyte’s stability threshold. Using this reversibility test, the anodic stabilities of poly(ethylene oxide) (PEO) and hydrogenated nitrile butadiene rubber (HNBR), both blended with lithium bis(trifluoromethanesulfonyl) imide (LiTFSI), were determined to be 3.6 and 3.7 V vs Li/Li+, respectively. The stability threshold of PEO is validated by size exclusion chromatography. At charge potentials exceeding 3.6 V, the M̅W of PEO monotonically decreases, indicating further oxidation. Thus, the reversibility test is established as a sensitive and versatile characterization method for determining the oxidative stability of polymer electrolytes.