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

Abstract

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.

Moving Down Group 1: Analytical Challenges and Current Trends for Solid Polymer Electrolytes in Post‐Li Battery Applications

“…This makes an accurate determination of the required parameter set from electrochemical impedance spectroscopy (EIS) more challenging. [13,23,44] An alternative approach are solid-state NMR (ssNMR) techniques, e. g. pulse-field gradient NMR (PFG-NMR) to measure (selfÀ )diffusion coefficients and electrophoretic NMR (eNMR) to measure cation migration in an electric field. Both techniques encounter challenges when moving from the NMRactive 7 Li nucleus to the heavier Group 1 elements 23 Na and 39 K, as the nuclei exhibit too short relaxation times.…”

“…The reactivity of alkali metals generally increases down the Group 1 elements, as demonstrated by the observation of parasitic heat flow in symmetric cells by isothermal calorimetry. [44] Polyethers typically display high cathodic stability towards alkali metals, [47] contrary to polycarbonates (e. g. PEC and PPC) which have been reported to depolymerize. [48] A major driver for this process might not be the alkali metal itself, but the fact that surface groups, like oxides and hydroxides, [49] initiate the degradation.…”

“…The pool of available techniques is comparatively limited. Although various electrochemical techniques can correlate the onset of degradation with potential (voltametric techniques [40,56] ) and parasitic currents, heat flow (microcalorimetry [44] ), gas evolution (online electrochemical mass spectrometry, OEMS [56] ) or resistance changes (e. g. electrochemical impedance spectroscopy, EIS [44] ). Zaghib and coworkers took a more visual approach, using in-situ scanning electron microscopy (SEM) experiments to investigate the surface layer evolution, dendrite growth and associated changes in electrolyte and electrode layer thickness.…”

“…A) Illustration of different cationic species formed in a SPE and a selection of commonly used analytical techniques to measure transference numbers. B) EIS spectra of Na/Na and K/K symmetric cells with a PEO-based block copolymer solid electrolyte (BPE) recorded at 55 °C after three consecutive cycling sequences comprising of 10 cycles at 10, 20 and 50 μA cm À 2 , respectively (Reprinted with permission [44]. Copyright 2024, Xing et al).…”