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 (N-TFSI/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 (N-TFSI/Li) is similar to 2 pointing towards the formation of [Li(TFSI)(2)](-) ionic clusters. At higher concentrations, x> 0.2, N-TFSI/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