Correlations Between Functional and Compositional Parameters and Coulombic Efficiency in the Lithium Anode SEI

The lithium (Li) metal anode offers significantly higher capacity than graphite and is therefore central to strategies to develop advanced rechargeable battery chemistries that meet range and performance targets for electric vehicles. Although closer than ever, Li anodes still cannot meet the &gt;99.9% Coulombic efficiency (CE) consistently needed for &gt;1,000 cycle life.¹ This shortfall arises from uncontrolled reactivity at the solid electrolyte interphase (SEI) and resulting SEI properties, leading to inhomogeneous plating and stripping, continuous electrolyte consumption and loss of active Li inventory. Despite progress in electrolyte development, the lack of precise understanding of the SEI still hinders attempts to rationally design an improved interface towards bridging remaining gaps in CE.<br/><br/>To inform such efforts, our work is advancing quantitative techniques to yield insights into SEI phases and the hidden interplays between their chemistry, properties, and function. We developed a methodology to probe the Li⁺ exchange rate between electrolyte and the Li anode, as modulated by the SEI, in a range of native Li SEI in diverse electrolytes and examined the robustness of the methodology by both electrochemical impedance and Tafel analysis.² Across numerous electrolytes, we find a strong correlation between Li⁺ exchange rate and CE, which indicates that phases that promote facile Li⁺ exchange can serve as a materials chemistry design descriptor for high-performance SEI. Second, we further examined composition-performance relationships in the SEI using a new parallelized titration protocol that enables quantification of lithium oxide (Li₂O) in cycled Li electrodes for the first time, alongside numerous other SEI phases.³ We reveal that Li₂O is the leading detectable phase underlying high performance of Li metal anodes, beyond that of lithium fluoride (LiF), opening new design trajectories for future electrolytes and potentially reducing the reliance on costly, unsustainable yet pervasive fluorinated electrolytes. <br/> <br/>1. G. M. Hobold, J. Lopez, R. Guo, <i>et al.</i>, <i>Nature Energy</i> 2021, 6, 951-960.<br/>2. G. M. Hobold, K.-H. Kim and B. M. Gallant, <i>Energy & Environmental Science</i> 2023, 16, 2247-2261. <br/>3. G. M. Hobold, C. Wang., K. Steinberg, Y. Li and B. M. Gallant, <i>Nature Energy</i> 2024, 9, 580-591.