Interplay of Transport and Reversibility of Lithium Electrodeposition through the Solid Electrolyte Interphase

Abstract

The ability to utilize lithium (Li) metal as a reversible anode remains a central goal to achieve targets for high-energy Li-ion batteries for electric vehicles. However, the Coulombic efficiency (CE) of Li plating and stripping, although reaching record values in recent years (99.6-99.9% over a portion of cycling), still falls below what is needed (>99.95% over 1,000 cycles).1-3 These issues arise from complex challenges at the Li solid electrolyte interphase (SEI), which causes Li to deposit inhomogeneously and suffer from parasitic electrolyte reactivity upon electroplating, leading to loss of active Li, insufficient reversibility, and poor cycle life.4 Following decades of research, understanding of the SEI properties, function and performance remains largely qualitative, due in part to the challenge of obtaining quantitative experimental data beyond that of CE. Collectively, this challenge hinders attempts to understand which phases, or combinations of phases, are desirable in an improved SEI, and thus select solvent/salt/additive combinations to rationally improve reversibility.To address this gap, we have been conducting studies into model interfaces generated on Li metal to elucidate phase-specific transport properties and how these properties relate to the uniformity/roughness of Li plating and the resulting CE upon charge. We began our efforts by examining two of the most-common ionic SEI phases, LiF and Li2O, grown as single-component SEI on Li. I will first discuss experimental findings related to ion transport in these distinct SEI phases as obtained from electrochemical impedance spectroscopy,5 where the chemical potential imposed by Li yields different properties than bulk analogues of these materials would predict. We measure roughly two-fold more-facile ionic conductivity in Li2O than LiF, which suggests LiF may be a transport-limiting phase within the SEI. The model interfaces are also yielding new insights into chemical instabilities and the dynamic chemical evolution of individual SEI phases in contact with particular carbonate electrolytes, especially in the presence of LiPF6 salts, that underpin increased Li+-transfer resistance and lead to higher required plating overpotentials and SEI breakdown.6 In this context, I discuss our recent efforts to understand Li+-ion exchange at the electrolyte/native SEI interface by quantification of electrochemical ion exchange currents, where a correlation with CE is observed. Collectively, these findings suggest strategies to control and improve Li+ transport in the SEI through future electrolyte and additive design, which can better protect Li during electrodeposition and bolster retention of electrochemically-active Li upon cycling. References Suo, L. M.; Xue, W. J.; Gobet, M.; Greenbaum, S. G.; Wang, C.; Chen, Y. M.; Yang, W. L.; Li, Y. X.; Li, J., Fluorine-donating electrolytes enable highly reversible 5-V-class Li metal batteries. P Natl Acad Sci USA 2018, 115 (6), 1156-1161. Xiao, J.; Li, Q. Y.; Bi, Y. J.; Cai, M.; Dunn, B.; Glossmann, T.; Liu, J.; Osaka, T.; Sugiura, R.; Wu, B. B.; Yang, J. H.; Zhang, J. G.; Whittingham, M. S., Understanding and applying coulombic efficiency in lithium metal batteries. Nat Energy 2020, 5 (8), 561-568. Yang, Y. Y. C.; Davies, D. M.; Yin, Y. J.; Borodin, O.; Lee, J. Z.; Fang, C. C.; Olguin, M.; Zhang, Y. H.; Sablina, E. S.; Wang, X. F.; Rustomji, C. S.; Meng, Y. S., High-Efficiency Lithium-Metal Anode Enabled by Liquefied Gas Electrolytes. Joule 2019, 3 (8), 1986-2000. Zhang, J. G.; Xu, W.; Xiao, J.; Cao, X.; Liu, J., Lithium Metal Anodes with Nonaqueous Electrolytes. Chem Rev 2020, 120 (24), 13312-13348. Guo, R.; Gallant, B. M., Li2O Solid Electrolyte Interphase: Probing Transport Properties at the Chemical Potential of Lithium. Chem Mater 2020, 32 (13), 5525-5533. Guo, R.; Wang, D.; Zuin, L.; Gallant, B. M., Reactivity and evolution of ionic phases in the lithium solid-electrolyte interphase. ACS Energy Letters 2021, 6.