Stable Lithium Metal Batteries Enabled by Dilute Cationic Additives

Conventional Li-ion electrolytes consist of carbonate solvents and Li-ion salts that were made for carbonaceous negative electrodes. When the electrolytes are used with metallic lithium anodes, carbonate electrolytes lead to low charge-discharge cycle stability due to adverse reactive species at the anode surface. 1-4 This reactive species are called the solid electrolyte interface (SEI). 1 SEI instabilities/inhomogeneities result in its continuous thickening and formation of electronically non-active dead lithium. 2 SEI composition and structure are affected by both solvent and electrolyte chemistry in bulk solution and, more importantly, at the electrode-electrolyte interface. 4 Current research studies suggest that X-rich (where X is a halogen) SEI yields superior performance compared to halogen-free SEI. 1,5 Electrolytes that have large volume fractions of halogenated species have statistically higher probability to be reduced at the electrode surface and yield X-rich SEI layers. 6 Therefore, the use of halogenated solvent and/or high concentrated (> 1 M) lithium salt with halogenated anions has been actively explored for LiMBs. However, the high cost of Li salts and high viscosity of the electrolyte at high salt concentration make the concentrated electrolytes unrealistic for commercial battery applications. In search of a cost-effective solution for high performance LiMBs, halogenated electrolyte additives could serve as an ideal approach since only a small amount of the additive would be potentially required to induce similar X-rich interface that can be seen in the case of the halogenated ether electrolyte. In this meeting abstract, we present a new concept that leverages favorable electrostatic interactions with the electrolyte additive to drive the formation of robust SEI layers even at low additive content. Specifically, custom-designed additives can be electrostatically attracted to the negatively charged electrode, creating a high population of halogenated species at the anode surface even at a dilute Li salt concentration. Effective SEI formation with a low-concentration additive circumvents the challenges associated with the current state-of-the-art approach of using halogenated species at high concentration (i.e., unfavorably high solution viscosity and high cost). Reference 1 Louli, A. J. et al. Diagnosing and correcting anode-free cell failure via electrolyte and morphological analysis. Nature Energy 5 , 693-702, (2020). 2 Chen, K.-H. et al. Dead lithium: mass transport effects on voltage, capacity, and failure of lithium metal anodes. Journal of Materials Chemistry A 5 , 11671-11681, (2017). 3 Lin, D. et al. Three-dimensional stable lithium metal anode with nanoscale lithium islands embedded in ionically conductive solid matrix. Proc Natl Acad Sci U S A 114 , 4613-4618, (2017). 4 Lin, D., Liu, Y. & Cui, Y. Reviving the lithium metal anode for high-energy batteries. Nat Nanotechnol 12 , 194-206, (2017). 5 Yu, Z. et al. Molecular design for electrolyte solvents enabling energy-dense and long-cycling lithium metal batteries. Nature Energy 5 , 526-533, (2020). 6 Yamada, Y., Wang, J., Ko, S., Watanabe, E. & Yamada, A. Advances and issues in developing salt-concentrated battery electrolytes. Nature Energy 4 , 269-280, (2019).