Ion Correlations and Transport in Concentrated Electrolyte Solutions for Battery Applications

Highly concentrated electrolyte solutions for battery applications are characterized by a low flammability, a high thermal and electrochemical stability and by the formation of a stable solid electrolyte interphase (SEI) in contact to electrode materials. The classification of such concentrated electrolyte solutions in terms of the classical scheme "strong" or "weak" has been controversially discussed in the literature. Here, a comprehensive theoretical framework is presented for a more general classification, which is based on a comparison of charge and mass transport. By combining the Onsager transport formalism with linear response theory, correlated movements of the ions in equilibrium are related to transport properties in a lithium-ion battery cell. The relevance of the classification approach is substantiated by showing that (i) it is straightforward to integrate highly concentrated electrolytes into a general classification and that (ii) both fast charge transport and fast mass transport are indispensable for achieving fast Li
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transport in battery cells.