The electrification of the transportation sector and the rising deployment of portable devices vehicles have accelerated the development of high-energy-dense batteries and revived interest in lithium metal as a high-capacity anode material alternative to the commonly used graphite. In the presence of well-known organic carbonate-based electrolyte formulations in lithium metal batteries (LMBs), high surface area Li (HSAL) deposits are formed during galvanostatic cycling, precipitating rapid active material degradation and causing an early cell failure.1 Consequently, researchers have shifted their focus towards novel electrolyte formulations aimed at facilitating a dense Li deposition and prolonging the cycle life of LMBs. A promising approach is the application of localized high-concentration electrolytes (LHCEs) due to their unique solvation structure, causing the formation of an effective anion-derived Solid Electrolyte Interphase (SEI) and a dense Li deposition morphology.2–4 Although heavily investigated for decades, the formation and decisive properties of the SEI and their role in facilitating homogeneous Li deposits remain largely a mystery due to the ultrathin, nanostructured nature of the interphase layer.
In our study, we investigated the nanoscale structural and chemical information of the electrolyte-induced SEI by utilizing nano-FTIR5. The unique combination of atomic force microscopy (AFM) and IR spectroscopy allows a simultaneous investigation of the interphase’s mechanical properties, heterogeneity and chemical composition on a nanoscale resolution. Combining these essential SEI properties makes it possible to comprehensively compare the derived interphases of apparently good-performing LHCEs and poor-performing standard electrolytes based on organic carbonate solvent electrolytes and systematically work out their essential differences. With the knowledge gained, novel electrolyte compositions based on the set baseline LHCE have been developed to tune specific performance parameters and methodically investigate their impact on relevant battery properties.

References:
1. Horstmann, B. et al. Strategies towards enabling lithium metal in batteries: interphases and electrodes. Energy Environ. Sci. 14, 5289–5314 (2021).
2. Ren, X. et al. Enabling High-Voltage Lithium-Metal Batteries under Practical Conditions. Joule 3, 1662–1676 (2019).
3. Ren, F. et al. Solvent–Diluent Interaction-Mediated Solvation Structure of Localized High-Concentration Electrolytes. ACS Appl. Mater. Interfaces 14, 4211–4219 (2022).
4. Angarita-Gomez, S. & Balbuena, P. B. Ion mobility and solvation complexes at liquid–solid interfaces in dilute, high concentration, and localized high concentration electrolytes. Mater. Adv. 3, 6352–6363 (2022).
5. Larson, J. M., Dopilka, A. & Kostecki, R. Infrared nanoimaging and nanospectroscopy of electrochemical energy storage materials and interfaces. Curr. Opin. Electrochem. 47, 101548 (2024).