Electrolyte development for high-performance Li-ion cells: Additives, solvents, and agreement with a generalized molecular model

Abstract

The of electrolyte is one of the most important components of a Li-ion cell. In addition to facilitating the flow of ionic current between electrodes during charge and discharge, the electrolyte plays other important roles. The components of the electrolyte are responsible for forming the passivating solid electrolyte interphase (SEI) layers which enable long-term cycling of a Li-ion cell. Further, under abusive conditions such as extremely fast charge and discharge currents, the electrolyte must be able to sustain large ionic currents with little concentration polarization. As such, the main areas of research focus for the electrolyte of a Li-ion cell can be broken down into two main categories: (1) interfacial properties, which are primarily dictated by low weight percent electrolyte additives; and (2) bulk transport properties, which are dominated by choices of solvent and salt blends. In recent years, much advancement has been made in both areas of electrolyte development. New combinations of low wt. % additives have been developed to extend the cycle and calendar life of Li-ion cells to new highs. Low viscosity cosolvents such as methyl acetate (MA) or ethyl acetate (EA) have been shown to improve the rate capability of Li[Ni, Mn, Co]O2 (NMC)/graphite cells1-5 following the pioneering work of Smart et al.6 with minor compromises in cell lifetime. This paper will highlight some recent work on additive development, noting that subtle differences in cycling conditions can “make or break” a given additive combination. Additionally, work on mapping the transport properties for a wide variety of electrolyte systems of interest will be shown. Finally, the Advanced Electrolyte Model (AEM), a theoretical model for calculating the properties of electrolytes, will be presented with an emphasis on the AEM’s agreement to experiment and ongoing efforts to validate the model in a wide array of electrolyte systems.