Structure and Conductivity of Crosslinked Ca-POE Polymer Electrolytes for Secondary Batteries

Poly(oxyethylene) (POE) is frequently used as suitable component to prepare solid polymer electrolytes (SPEs), due to its: (i) ability to coordinate and dissociate doping salts; (ii) good mechanical properties; and (iii) high chemical and electrochemical stability. With the aim to obtain calcium secondary batteries, here we describe the preparation and studies of crosslinked Ca-polycondensate (NPCY) electrolytes with formula NPCY/(CaTf₂)_x based on fragments of POE chains and CaTf₂. The molecular weight of POE precursors is Y=400 and 1000 g ⋅ mol⁻¹. The effect of POE molecular weight on the thermal, structural, and electrical properties of NPCY/(CaTf₂)_x is investigated revealing that in mesoscale this materials show: (i) two different nanodomains with polyether chains both “free” (not coordinating the cation) and involved in 4–4 coordination cages of Ca²⁺ metal ions; (ii) f_α-fast, f_α-cross and f_α-slow relaxation modes of polyether chains, detected by broadband electrical spectroscopy, which are coupled with the long-range charge migration pathways of SPEs; (iii) that triflate (Tf⁻) anions, which act as plasticizers, modulate the inter-chain migration processes of Ca²⁺ between polyether coordination sites. Finally, the conductivity values of NPCY/(CaTf₂)_x, which is up to 10⁻⁴ S ⋅ cm⁻¹ at 80 °C, classify NPCY/(CaTf₂)_x as promising SPEs for the development of calcium secondary batteries.