Reactivity and Evolution of Ionic Solid-Electrolyte-Interphases in Battery Electrolytes

The instability of the solid electrolyte interphase (SEI) on the lithium (Li) metal anode is a major challenge towards improving the Coulombic efficiency (CE) and cycle life of Li batteries. In classical SEI models with mosaic and layered structures, ionic phases (e.g., Li 2 O and LiF) are enriched closest to the Li|SEI interface, while the outer SEI are highly dependent on electrolytes and typically assigned to less-reduced species, such as semi-carbonates and organic Li salts. 1, 2 The formation of SEI on Li is often interpreted as a consequence of reactivity between Li metal and electrolytes; however, there remains a lack of understanding about the interplay between electrolytes and specific SEI phases once they are formed. Some recent computational studies have shed light on the chemical reactivity between thin (~1 nm) single-phase inorganic SEI (Li 2 O, LiOH, and Li 2 CO 3 ) on Li metal and DME electrolytes containing LiTFSI or LiFSI salts, using ab initio molecular dynamics (AIMD) calculations; 3, 4 experimental insights are still much-needed. Herein, we study two ionic SEI phases, Li 2 O (Fig. 1a-b) and LiF (Fig. 1c-d), that are nearly ubiquitously found across many native SEIs, and investigate their stability at the SEI|electrolyte interface. We find, by using a combination of electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and non-destructive X-ray absorption near-edge spectroscopy (XANES), that the ionic SEI|electrolyte interfaces can undergo significant chemical evolution as a function of electrolyte. As shown in Fig. 1a, distinctive lower-frequency semicircles emerged in the Nyquist plots of EIS when 1 M LiTFSI EC/DEC and 1 M LiPF 6 EC/DEC were used, indicating significant changes at the interface between the ionic Li 2 O SEI and carbonate-based electrolytes, especially in the presence of LiPF 6 salt. The surface atomic concentration of F also increased significantly from 4 at.% in 1 M LiTFSI DOL/DME to 44 at.% in 1 M LiPF 6 EC/DEC, suggesting that a F-rich Li 2 O|electrolyte interface leads to an additional charge transfer barrier. Non-destructive Li K-edge XANES of the Li 2 O SEI soaked in 1 M LiPF 6 EC/DEC electrolyte displayed a dominant LiF peak at 62.3 eV with an evident blue shift compared with the standard LiF peak at 62.0 eV (Fig. 1b), 5 which could be attributed to the solvation of F-rich species at the interface by organic phases from carbonates decomposition, forming an “organic/F-rich” outer SEI layer. Similar organic/F-rich outer SEI was also observed on the LiF SEI soaked in 1 M LiPF 6 EC/DEC electrolyte, as evidenced by EIS (Fig. 1c), XPS (Fig. 1d) and XANES results. The organic/F-rich outer SEI induced by reactivity between ionic SEI phases and carbonate-based electrolytes can significantly exacerbate the subsequent plating overpotentials. Our experimental results suggest that electrolyte solvent and salt selection is of great importance to optimize transport in ionic-rich Li interfaces, which can have important implications ultimately for SEI stability and thus CE. Figure 1. (a) Nyquist plot of EIS spectra of Li|Li 2 O SEI in the three different electrolytes, where the thickness L and conductivity σ of the Li 2 O SEI were acquired by fitting the higher-frequency semi-circle (or the sole semi-circle in the case of DOL/DME). 6 (b) Li K-edge XANES fluorescence yield (FLY) of Li|Li 2 O interface after soaking in EC/DEC solvent or 1 M LiPF 6 EC/DEC electrolyte for 20 hours. (c) Nyquist plot of EIS spectra of Li|LiF SEI in the three different electrolytes. (d) C1s and F1s XPS of the surface layer on Li|LiF SEI after soaking in each electrolyte. References: 1. E. Peled, D. Golodnitsky, and G. Ardel, J. Electrochem. Soc., 144 (8), L208-L210 (1997). 2. A. Schechter, D. Aurbach, and H. Cohen Langmuir, 15 (9), 3334-3342 (1999). 3. E. P. Kamphaus, S. Angarita-Gomez, X. P. Qin, M. H. Shao, M. Engelhard, K. T. Mueller, V. Murugesan, and P. B. Balbuena, ACS Appl. Mater. Interfaces, 11 (34), 31467-31476 (2019). 4. E. P. Kamphaus, S. A. Gomez, X. Qin, M. Shao, and P. B. Balbuena, ChemPhysChem, 21 (12), 1310-1317 (2020). 5. D. Wang and L. Zuin, J. Power Sources, 337 , 100-109 (2017). 6. R. Guo and B. M. Gallant, Chem. Mater., 32 (13), 5525-5533 (2020). Figure 1

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