Reversible Ca-Ion Plating and Stripping from Poly(ethylene oxide)-Based Solid Polymer Electrolyte Enabled by Surface Functionalization

The native oxide layer of Ca-metal electrodes impedes Ca-ion transport properties across the electrolyte–electrode interphase. Bis(trifluorosulfonyl)imide-salts (TFSI-) were reported to inhibit any ion transport due to facile degradation at the reactive interface. Poly(ethylene oxide)-based solid polymer electrolytes (SPEs) frequently use Ca(TFSI)₂ for its comparatively high ionic conductivity and hence have not achieved reversible plating/stripping from Ca-electrodes yet. To overcome this roadblock, a Ca-electrode surface treatment with Bi-salt was introduced, enabling operation of Ca/Ca symmetrical cells from a PEO-Ca(TFSI)₂ SPE for the first time. The functionalization greatly reduced interfacial resistances thus allowing reversible Ca plating and stripping from the Ca-SPE.