Towards a Fully-Quantitative Picture of SEI Chemical Composition and Its Native Li+ Exchange in Liquid Electrolytes

Abstract

Energy density requirements for next-generation batteries make the lithium (Li) metal anode a key candidate to replace graphite in lithium-ion batteries due to Li’s improved capacity (3,860 mAh/g vs. 372 mAh/g). However, Li anodes display lower Coulombic efficiency (CE, <99.9%) than graphite (>99.95%),1 resulting in faster loss of reversible Li over cycling. This shortfall derives from parasitic Li-electrolyte reactions that lead to accumulation of inactive Li0 and electrolyte-derived byproducts, which result in the formation of a native solid electrolyte interphase (SEI) on Li. In addition to contributing to capacity loss, the SEI also has the important role of regulating Li+ exchange between Li and the electrolyte, which may affect Li plating morphology and reversibility.2 Despite significant advances, the framework on which the understanding of SEI composition and function was built has largely been qualitative, often due to experimental limitations and/or lack of self-consistent techniques.To bridge this gap, my thesis work focuses on advancing experimental techniques and methodologies to precisely quantify the chemical composition and rates of Li+ exchange in native SEIs. Using a combination of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), self-consistent rates of Li+ exchange are reported across a wide range of electrolytes.3 Notably, we found that native SEIs formed in high CE electrolytes displayed consistently higher rates of Li+exchange. The improved transport in high-CE electrolytes was found to emerge after an SEI formation cycle, and amplified over cycling, indicating that fast Li+ exchange is beneficial to Li reversibility.In order to identify which SEI phases grant high-CE electrolytes their beneficial functional properties, we also developed and expanded upon a series of titration experiments based on GC, ICP-AES and NMR that can track an extended array of key SEI phases: semicarbonates (ROCO2Li), lithium carbide (Li2C2), olefins (RLi), LiF, P-containing phases, and total Li loss.4 In the 1 M LiPF6 EC/DEC electrolyte, we demonstrate chemical resolution up to 71% of Li loss and 33% of SEI loss. Among the quantifiable SEI phases, ROCO2Li was consistently the major phase, but its proportions were invariant with CE. Instead, Li2C2, a minor phase, exhibited clear inverse correlation with CE. We also demonstrate further quantitative chemical resolution can be achieved with online GC, which can differentiate between the different types of semicarbonate (i.e., which “R” in ROCO2Li) and alkoxide phases that form on Li.5 Altogether, these results add further nuance to the current understanding of SEI composition and their function, which we hope will allow more quantitative electrolyte design. Hobold, G. M.; Lopez, J.; Guo, R.; Minafra, N.; Banerjee, A.; Shirley Meng, Y.; Shao-Horn, Y.; Gallant, B. M., Moving Beyond 99.9% Coulombic Efficiency for Lithium Anodes in Liquid Electrolytes. Nat. Energy 2021, 6 (10), 951-960. Zheng, J.; Kim, M. S.; Tu, Z.; Choudhury, S.; Tang, T.; Archer, L. A., Regulating Electrodeposition Morphology of Lithium: Towards Commercially Relevant Secondary Li Metal Batteries. Chem. Soc. Rev. 2020, 49 (9), 2701-2750. Hobold, G. M.; Kim, K.-H.; Gallant, B. M., Beneficial Vs. Inhibiting Passivation by the Native Lithium Solid Electrolyte Interphase Revealed by Electrochemical Li+ Exchange. 2022. Hobold, G. M.; Gallant, B. M., Quantifying Capacity Loss Mechanisms of Li Metal Anodes Beyond Inactive Li0. ACS Energy Lett. 2022, 7 (10), 3458-3466. Hobold, G. M.; Khurram, A.; Gallant, B. M., Operando Gas Monitoring of Solid Electrolyte Interphase Reactions on Lithium. Chem. Mater. 2020, 32 (6), 2341-2352.