Ion correlations in quaternary ionic liquids electrolytes

Lithium-ion batteries are currently the most popular and widely used energy storage devices, almost omnipresent within modern society in portable devices, electrical vehicles, energy storage stations, and so on. The demand for more efficient, more durable, and more sustainable batteries is rapidly growing. The electrolyte is a key element to improve the performance of lithium-ion batteries. In this work, we focus on quaternary ionic liquid electrolyte (ILE), which uses a four-component ionic liquid as the solvent. Quaternary ILE has found wide applications in energy storage systems, but the ion transport in the electrolyte has not been fully characterized to provide the best strategy for performance optimisation. In this work, we systematically analyse the ion transport in the quaternary ILE and uncover how the correlations between various ions affect the conductivity of the electrolyte. We have found that lithium ions are transported in charge clusters, leading to a negative effective transference number of lithium ions. Furthermore, we identify the stable cluster conformations in ILE by cluster analysis and quantum chemical computing. This work highlights the necessity of considering ion correlations in multi-component electrolyte systems.