Transport mechanism of lithium ions in non-coordinating P(VdF-HFP) copolymer matrix

Polymer films based on poly(vinylidene difluoride-co-hexafluoropropylene) (P(VdF-HFP)) with different amounts of bis(trifluoromethane)sulfonimide lithium salt (LiTfSI) were prepared from acetone solution in a doctor blade casting machine under controlled and reproducible drying conditions. The modification of the copolymer-based layers show a significant enhancement of conductivity over several orders of magnitude for increasing LiTfSI content and a constantly low electronic conductivity. The addition of salt results in a structural change of the crystalline areas in the semi-crystalline copolymer matrix from α- to γ-phase of P(VdF), which has been studied using Raman spectroscopy and X-Ray diffraction. Lithium ions are coordinated by oxygen atoms of TfSI− as verified by Raman spectroscopy and molecular dynamics simulations. Based on the experimental data and simulation results, we propose a transport mechanism for the lithium ions through salt channels in the amorphous regions of the non-coordinating copolymer matrix via hopping between stabilized positions.