(349b) Advancing Quantitative Understanding of Functionality in the Lithium SEI in Liquid Electrolytes

Li metal anodes offer significantly higher capacities than graphite and are therefore central to strategies to develop advanced rechargeable battery chemistries that meet range and performance targets for electric vehicles. Although closer than ever, lithium (Li) anodes still cannot meet the >99.9% Coulombic efficiency (CE) consistently needed for >1,000 cycle life. This shortfall arises from uncontrolled reactivity at the solid electrolyte interphase (SEI) and its resulting properties, leading to inhomogeneous plating and stripping, continuous electrolyte consumption and loss of active Li inventory. Despite much recent progress in electrolyte development, the lack of quantitative understanding of functionality from the perspective of the SEI itself still hinders attempts to rationally design an improved interface and bridge the remaining gap in CE.

To help inform such efforts, our work is developing techniques to gain insights into SEI phases and reveal interplays between their chemistry, structure and function. First, we developed approaches to isolate and synthesize SEI-relevant ionic phases, including lithium oxide (Li₂O), lithium fluoride (LiF) and others, at representative nanometer-scale thicknesses directly on Li metal. These interfaces are interrogated via targeted electrochemical and spectroscopy techniques to reveal their transport properties, Li⁺ exchange kinetics and chemical reactivity, providing insight into how such phases bolster, or hinder, transport in the SEI. Recently, quantification efforts have been extended towards characterization of the native SEI. Using a combination of voltammetry and electrochemical impedance spectroscopy-based methods, we report on self-consistent Li⁺ exchange rates across a comprehensive range of electrolytes and show that Li CE and rate capability are tightly correlated with the rate of SEI Li⁺ exchange. The magnitudes of such exchange rates delineate critical electrolyte-dependent cycling rates beyond which deposition is interface-limited and CE plummets. For many electrolytes, this transition occurs around typical (~1 mA/cm²) imposed cycling currents, elucidating the importance of cycling protocol and rate on Li reversibility. Moreover, we examine the highly electrolyte-dependent changes in Li⁺ exchange upon Cu formation and cycling, which exhibit unexpected dynamics. These results are collectively helping to increase quantitative understanding of the SEI, which guides electrolyte and additive selection as well as design of cycling protocols to maximize CE.