Electrolyte Solvation Structure at Solid–Liquid Interface Probed by Nanogap Surface-Enhanced Raman Spectroscopy

Abstract

Understanding the fundamental factors that drive ion solvation structure and transport is key to design high-performance, stable battery electrolytes. Reversible ion solvation and desolvation are critical to the interfacial charge-transfer process across the solid-liquid interface as well as the resulting stability of the solid electrolyte interphase. Herein, we report the study of Li salt solvation structure in aprotic solution in the immediate vicinity (∼20 nm) of the solid electrode-liquid interface using surface-enhanced Raman spectroscopy (SERS) from a gold nanoparticle (Au NP) monolayer. The plasmonic coupling between Au NPs produces strong electromagnetic field enhancement in the gap region, leading to a 5 orders of magnitude increase in Raman intensity for electrolyte components and their mixtures namely, lithium hexafluorophosphate, fluoroethylene carbonate, ethylene carbonate, and diethyl carbonate. Further, we estimate and compare the lithium-ion solvation number derived from SERS, standard Raman spectroscopy, and Fourier transform infrared spectroscopy experiments to monitor and ascertain the changes in the solvation shell diameter in the confined nanogap region where there is maximum enhancement of the electric field. Our findings provide a multimodal spectroscopic approach to gain fundamental insights into the molecular structure of the electrolyte at the solid-liquid interface.

Understanding the Role of Fluorination on the Interaction of Electrolytic Carbonates with Li+ through an Electronic Structure Approach

“…Similar to the IR spectra, the carbonyl group stretching mode doublet in the Raman spectra shifted towards higher frequency (see Figure ) because of fluorine substituent. This shifting is consistent with a recent experimental report, where Nanda et.al found similar changes in the Raman spectra for FEC in comparison to EC, detailed in section 3.2.3.…”

“…For example, shifts to 1690 cm −1 from its original value of 1825 cm −1 due to Li + coordination in the case of DEC. is found to shift towards the higher frequency region for isolated carbonates, and their Li + ‐carbonate complexes due to fluorine substituent. For example, In the case of FEC and Li + FEC, shifts 20 cm −1 and 18 cm −1 , respectively as compared to EC and Li + EC, which is in good agreement with previous report, where a shift of 28 cm −1 is found due to fluorination of isolated EC …”

“…The IR spectra analysis shows that each vibrational mode is shifted towards the higher frequency region in the spectra due to fluorine substituent. The most affected vibrational mode is associated with the stretching of C=O bond as listed in the Table 6, which are in good agreement with previous reports ,. The highest shifting in the C=O vibrational mode is found in the case of Li + [EMC] complex which shows a ∼70.4 cm −1 shift towards the higher frequency region.…”

Lithium Salt Dissociation in Diblock Copolymer Electrolyte Using Fourier Transform Infrared Spectroscopy

“…Although its specialty is for identification of compounds, detection of functional groups, and qualitative analysis of chemical interactions, quantitative analysis is also possible. This requires precise and careful control of samples and experimental conditions along with appropriate analysis and proper assumptions (Kakihana et al, 1990;Fieldson and Barbari, 1993;Ferry et al, 1995;Sammon et al, 1998;Elabd et al, 2003;Philippe et al, 2004;Hallinan and Elabd, 2007;Oparaji et al, 2016;Yang et al, 2018;Kim and Hallinan, 2020). In this section, the dissociation of LiTFSI in SEO is investigated via detailed analysis of the fingerprint region.…”

Direct Operando Observation of Double Layer Charging and Early SEI Formation in Li-Ion Battery Electrolytes

“…The SEI is known to be a protective layer forming in situ on the negative electrode primarily in the 1 st cycle of the Li-ion cell and thereafter stabilizes its operation until the end of life. However, despite more than two decades of intense research activity, the formation mechanism, composition, and morphology of the SEI remain debated and not completely understood [1][2][3] . Numerous advanced micro-/spectroscopic characterization techniques have been developed and applied to study the SEI 4 .…”