The success of numerous “beyond Li-ion” technologies relies crucially on the availability of a safe, cycleable, and Coulombically reversible lithium anode. Achieving this aim will require improvements in interfacial stability at the solid electrolyte interphase (SEI), which can support homogeneous Li transport and the suppression of dendrites. As suggested in literature, a more homogeneous SEI can be achieved either by tailoring the composition through electrolyte and additive design; or by introducing non-native materials to the Li surface in ex situ processing, referred to as “Artificial SEI” design. Numerous recent studies have suggested that fluoride-rich materials, in particular, impart promising long-term cycling behavior in modified Li electrodes. However, there remains a lack of fundamental understanding regarding the precise chemical, ionic and mechanical roles of the newly imparted artificial interphases; and how promising preliminary laboratory results may translate to practical steps forward under realistic rates and depth-of-discharge during cycling. This talk examines the role of LiF in the SEI in a series of model systems in which the composition of LiF is intentionally controlled and enriched compared to the native SEI. Artificial interphases are introduced by the controlled reactivity of fresh Li metal with fluoride-containing chemicals under a range of temperature and pressure conditions. Notably, the artificial interphases are grown in the absence of liquid solvents, which allows for atomically pure LiF layers to be formed without confounding chemical effects of solvent co-reduction, as occurs when an SEI is formed in situ during battery assembly. We first examine how the composition, morphology, electronic/ionic properties, and mechanical strength of the resulting films can be tuned by varying the reaction conditions. By performing stepwise characterization at intervals of half-cycles, we next investigate step-by-step the process of Li stripping and plating from films when the fluoride-enriched SEI is paired with different electrolyte solvents. Finally, we relate these observations to long-term cycling performance. Such an approach enables examination of the intrinsic function of LiF, and a platform to explore whether a performance limit exists – how much LiF is enough, and can there be “too much of a good thing”?

Probing the intrinsic role of lithium fluoride in artificial solid electrolyte interphases

2019 Spring National Meeting

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