Analysis of Lithium Metal-Electrolyte Interfacial Phenomena Through Nonlinear Electrochemical Impedance Spectroscopy

Lithium metal batteries (LMB) are promising for next-generation energy storage due to their high theoretical energy densities, but they have not yet achieved the Coulombic efficiency (CE) of over 99.9% necessary for a cycle life of 1000+ cycles that would enable commercialization.¹ One promising method to achieve such a high CE is to engineer an electrolyte that forms a protective solid-electrolyte interphase (SEI) layer that enables uniform lithium deposition and dissolution, while remaining mechanically and chemically stable. Although recent advances in interfacial characterization have yielded valuable insights regarding the effect of the electrolyte on SEI structure/composition and deposited lithium morphology, many of the interfacial kinetic and transport properties remain unclear. Given that kinetics and transport phenomena control the lithium deposition process, and thus LMB cycle life, it is critical to gain a deeper understanding to guide future interfacial engineering efforts.

Electrochemical impedance spectroscopy (EIS) is a popular technique to estimate interfacial kinetics and transport properties, yet the degenerate nature of linear equivalent circuit models (ECM) has yielded several different interpretations of EIS data.^2–4 While EIS can offer helpful qualitative insights, the lack of consistency results in poor confidence in the quantitative analysis. This is circumvented in nonlinear electrochemical impedance spectroscopy (NLEIS), a novel in-situ characterization method that can break the degeneracy present in linear EIS analyses.⁵ With identical instrumentation as its linear counterpart, NLEIS uses a moderate AC current amplitude to elicit a weakly nonlinear voltage response from a system. In particular, the second harmonic voltage response is formed by the differences between battery electrodes, which makes it a powerful diagnostic signal for detecting subtle changes in electrode kinetics, transport, and thermodynamics. Complementary analysis of the first and second harmonic Nyquist plots reduces the degeneracy of ECM solutions, which enables consistent and physically meaningful interpretations of EIS data.

Here, we utilize NLEIS to characterize the interfaces created by a conventional carbonate-based electrolyte (CE ~92%) and a local high concentration electrolyte (CE ~99.5%) during cycling in lithium-copper and lithium symmetric cells. From a qualitative analysis of the first and second harmonic Nyquist plots, we propose an ECM reflecting the relevant lithium metal-electrolyte interfacial phenomena. We then quantify electrochemical kinetic and transport parameters using an open-source python package, nleis.py, to perform simultaneous model fitting.⁵ This work is the first of its kind in evaluating LMBs using higher harmonics from impedance spectroscopy and paves the way for consistent quantification of LMB kinetic and transport properties.

1. G. M. Hobold et al., Nat. Energy, 6, 951–960 (2021).
2. Y. Han et al., Adv. Funct. Mater., 29, 1904629 (2019).
3. D. T. Boyle et al., J. Am. Chem. Soc., 144, 20717–20725 (2022).
4. S. Drvarič Talian, J. Bobnar, A. R. Sinigoj, I. Humar, and M. Gaberšček, J. Phys. Chem. C, 123, 27997–28007 (2019).
5. Y. Ji and D. T. Schwartz, J. Electrochem. Soc., 170, 123511 (2023).