Grain boundaries and the influence of the ionophilic–ionophobic balance on 7Li and 19F NMR and conductivity in low-dimensional polymer electrolytes with lithium tetrafluoroborate

Abstract

7Li and 19F NMR linewidths and impedance spectra are reported for low-dimensional CmOn ( I):LiBF 4 mixtures. Data for the ionophilic polymer C18O5 is compared with that for the ionophobic C18O1 and the block copolymer C16O1O5(21%) (21 mol% of C16O5). In C18O5:LiBF 4 (1:1) narrow 7Li linewidths, which were observed in the liquid crystal phase above the side chain melting temperature (∼50 °C), persist in the crystal down to ca. 0 °C and broaden below −20 °C. However, in C18O1:LiBF 4 (1:0.6) narrow 7Li linewidths were also observed down to −20 °C suggesting highly mobile neutral aggregates of salt since this system is non-conductive. In the copolymer C16O1O5(21%):LiBF 4 (1:0.7) the linewidths were even narrower down to −70 °C with weak temperature dependence. In all systems 19F linewidths were significantly broader than 7Li linewidths. The complex plane plots obtained by impedance spectroscopy exhibit characteristic minima identified with ‘grain boundary’ resistance and, following heat treatment, minima with weak temperature dependence identified with ‘internal crystal’ resistance, R i, and conductivities, σ i ≥ 10 −4 S cm −1. Four-component mixtures of copolymers CmO1O5 and CmO1O4 with LiBF 4 and ‘salt-bridge’ poly(tetramethylene oxide)–dodecamethylene copolymers gave conductivities of ca. 4 × 10 −4 S cm −1 at 20 °C with weak temperature dependence. A novel carrier-hopping mechanism of lithium transport decoupled from side chain melting in the crystalline state is postulated. ▪