Coordination and interactions in a Li-salt doped ionic liquid

We report on the coordination and interactions in the LiTFSI doped ionic liquid PyR14TFSI over a large concentration range, 0.01≤x≤0.4, using Raman spectroscopy. We find that the concentration dependence of the average number of TFSI anions coordinating to one Li-ion (NTFSI/Li) can be divided into three regimes. For low concentrations, x≤0.05, we find that a large number TFSI anions coordinate each Li-ion, N>2. The number decreases with increasing salt concentration and the interaction between the Li-ion and the TFSI anions is rather weak in this concentration range. At intermediate concentrations, 0.1≤x≤0.2, the number of TFSI anions coordinating each Li-ion (NTFSI/Li) is ~2 pointing towards the formation of [Li(TFSI)2]− ionic clusters. At higher concentrations, x>0.2, NTFSI/Li decreases further indicating the transition to more complex structures with Li-ions bridging TFSI anions. We also show that the evolution of the microscopic structure as a function of Li-salt concentration is mirrored in the behaviour of macroscopic properties such as the ionic conductivity and the glass transition temperature, which also show a crossover in the same concentration range.