Degradation of Styrene-Poly(ethylene oxide)-Based Block Copolymer Electrolytes at the Na and K Negative Electrode Studied by Microcalorimetry and Impedance Spectroscopy

The electrode-electrolyte interface of alkali metal electrodes and solid polymer electrolytes (SPE) is challenging to access because solid electrolytes are difficult to remove without damaging the interphase region. Herein, the two non-invasive techniques isothermal microcalorimetry (IMC) and electrochemical impedance spectroscopy (EIS) are combined to explored degradation processes of reactive sodium and potassium metal electrodes in contact with SPEs. Comparison of the parasitic heat flows and interfacial resistances at different current densities with a liquid electrolyte (LE) system showed marked differences in aging behaviour. The data also suggest that the electrochemically active surface area of alkali metal electrodes increase with cycling, leading to larger parasitic heat flows and indicating morphological changes. SPE-based cells exhibit similar levels of parasitic heat flow at different current densities, which is in stark contrast to the LE cell where a strong correlation between the two is evident. The ambiguity of EIS spectra is challenging due to the overlapping time constants of the underlying electrode processes. However, equivalent circuit modelling can be used to follow trends in resistance evolution, for example to track the rapidly increasing cell impedance in K/K symmetric cells during a 48 h equilibration interval prior to cycling, which abruptly disappeared once cycling begins.