Dissolution of the Solid Electrolyte Interphase and its Effects on Lithium Metal Anode Cyclability

Despite their potential to provide higher energy density rechargeable batteries, Lithium metal anodes (LMA) suffer from low cyclability. The cycle life of LMA is largely tied to the stability and physicochemical properties of the solid electrolyte interphase (SEI), a nanoscale layer that mitigates the thermodynamic instability of Li metal towards the electrolyte. Using cryogenic-TEM, recent studies by Boyle <i>et. al.</i> and Huang <i>et. al. </i>suggest that dissolution of SEI components can introduce nanoscopic interphasial heterogeneities in LMAs.^1,2 Dissolution compromises the efficacy of the passivation layer since additional electrolyte and Li must be consumed to repair the partially dissolved SEI, resulting in a thicker and more heterogeneous layer.^3,4 Despite its significance, fundamental understanding of SEI dissolution using electrolytes optimized for LMAs remain lacking. More importantly, it is difficult to gauge on how much SEI dissolution can affect cyclability without a quantitative and systematic study of the process.<br/><br/>In this presentation, we will report the utilization of <i>in-operando</i> electrochemical quartz crystal microbalance (EQCM) technique to systematically quantify and compare SEI solubility using ether-based electrolytes for LMAs. Correlating SEI solubility results with performance metrics obtained from coulometric experiments allowed us to establish a correlation between solubility, passivity, and cyclability. Building upon this, we illustrate that cycle life can be further extended for one of the best electrolytes by shifting the SEI solubility equilibria. This suggests that the cycle life of a state-of-the-art electrolyte can be extended by decreasing SEI dissolution, illustrating that solubility is another knob that can be tuned to effectively improve cyclability.<br/><br/>Together with EQCM, X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonance (NMR) spectroscopy results, we reveal that solubility is dependent on not just the SEI’s composition, but also the properties of the electrolyte. We show that comparing SEI solubilities solely based on its composition becomes less reliable for high-performance electrolytes. Our analyses indicate that the differences in the amount of anion-derived component in their SEIs become small enough that the physical and chemical property of the electrolytes start to play a more significant role in dissolution. This crucial piece of information could aid in future design of more passivating and stable SEIs by not just tuning the SEI composition, but also tuning the physical and chemical property of the electrolyte to minimize SEI dissolution even more effectively.<br/><br/><b>References</b><br/>1. D. T. Boyle, W. Huang, H. Wang, Y. Li, H. Chen, Z. Yu, W. Zhang, Z. Bao, Y. Cui, <i>Nat. Energy</i>. <b>6</b>, 487–494 (2021).<br/>2. W. Huang, H. Wang, D. T. Boyle, Y. Li, Y. Cui, <i>ACS Energy Lett.</i> <b>5</b>, 1128–1135 (2020).<br/>3. M. Broussely, Ph. Biensan, F. Bonhomme, Ph. Blanchard, S. Herreyre, K. Nechev, R. J. Staniewicz, <i>Sel. Pap. Press. 12th Int. Meet. Lithium Batter.</i> <b>146</b>, 90–96 (2005).<br/>4. S. K. Heiskanen, J. Kim, B. L. Lucht, <i>Joule</i>. <b>3</b>, 2322–2333 (2019).