Design of a Boron-Containing PTHF-Based Solid Polymer Electrolyte for Sodium-Ion Conduction with High Na⁺Mobility and Salt Dissociation

Solid polymer electrolytes based on semi-interpenetrating polymer networks (semi-IPNs) were developed for sodium-ion conduction using a boron-centered moiety with polytetrahydrofuran (PTHF) chains. The resulting solid polymer electrolytes (SPEs) combined the anion-trapping properties of the boron with the weakly coordinating PTHF to achieve enhanced salt dissociation and transference number. The structure of the polymer was confirmed by 1H NMR, 13C NMR, and 11B NMR, and the cross-linking reaction with hexamethylene diisocyanate (HMDI) was confirmed by Fourier transform infrared (FTIR) spectroscopy. The mechanical and thermal stabilities of the samples were evaluated, and FTIR spectroscopic analysis showed the efficiency of the boron centers in complexing with ClO4-. Dielectric studies also indicated the predominance of free Na+ in samples. The highest room-temperature ionic conductivity was delivered by boron-containing electrolytes with O/Na: 5, 7.54 × 10-5 S cm-1, and O/Na: 10, 1.13 × 10-5 S cm-1. Linear sweep voltammetry (LSV) revealed suitable electrochemical stability against Na metal, and the measured transference number was 0.88, confirming that electrolytes benefit from the looser coordination of Na+ ions with PTHF chains combined with the anion-trapping performance of boron moieties, indicating a potential direction in polymer electrolyte design.