Discrepant lithium transference numbers due to heterogeneous speciation

Abstract

Understanding lithium secondary battery electrolytes to the point where they can be designed and tailored is of extraordinary importance. This is especially true for lithium transference and lithium mobility, both of which are to be maximized in the design of a high-performance battery. However, how these properties relate to the species present in the bulk electrolyte remains poorly understood. Here we show how different species in [Li(G1)₃][PO₂F₂] contribute to experimentally observable properties. We find unprecedented heterogeneity in the form of well-separated, negatively charged oligomeric aggregates co-existing with free solvent molecules. Our approach also allows us to estimate the electrophoretic mobilities of different species such as [Li(G1)₂]⁺, aggregates formed by communal solvation, or free glyme. Importantly, the widely used Bruce–Vincent method fails in this case, giving unreasonably high lithium transference numbers. Thus, we present a framework to rationalise the discrepancy in lithium transference and mobility obtained from different methods, which provides a new level of detail in the understanding of battery electrolytes.