Probing Electrolyte Solvents at Solid/Liquid Interface Using Gap-Mode Surface-Enhanced Raman Spectroscopy

Abstract

Understanding the aprotic solution structures at the immediate vicinity of solid/liquid interface (SLI) is critically important for next generation lithium ion battery development. Yet, it is still challenging to investigate the carbonate chemical profiles close to the diffuse layer (about 10 nm) of the electrical double layer at SLI due to the lack of a ultrahigh surface sensitive tool. In this work, we demonstrate the structures of commonly used carbonate solvents (ethylene carbonate (EC) and diethyl carbonate (DEC)) and a carbonate additive (fluoroethylene carbonate (FEC)) in a commercial Li-ion battery electrolyte can be determined at ∼17 nm above the electrode surface. This is only enabled by a nanogap surface-enhanced Raman spectroscopy (SERS) technique based on a monolayer gold nanoparticle (Au NP) ensemble. The SERS enhancement factor (EF) of those carbonates was found to depend on the molecular polarizability, with the maximum EF at ∼10 5 found for EC and FEC. Despite their alike chemical structures, this monolayer Au NP SERS substrate is fully capable of discrimiating the different Raman finger prints of EC and FEC. Compared to EC, several vibration modes in FEC, such as C-C skeletal deformation, ring breathing band and C=O stretching band, shift to higher frequencies because of the displacement of a hydrogen atom by a much heavier fluorine atom in a methylene bridge. This counterintuitive observation against the commonly used "ball and spring" model in vibrational spectroscopy is mostly due to the increased bond strength in the FEC ring versus that of EC. A second order empirical polynomial best describes the correlation between the SERS band integration of EC or DEC molar concentration. Our findings open up new opportunities for in-depth understanding of the electrolyte molecular vibrational behaviors at direct solid/liquid interface and developing advanced electrolytes for next generation lithium-ion batteries.

Ultrathin and Non‐Flammable Dual‐Salt Polymer Electrolyte for High‐Energy‐Density Lithium‐Metal Battery

“…Specifically, the peaks at 729 and 2245 cm -1 were ascribed to OCO ring breathing of FEC and CN stretching vibration of ADN, respectively. [46][47][48] The characteristic peaks of ADN and FEC mentioned above were shifted to higher frequencies in DSPE-2M, indicating the strengthening Li + -FEC and Li + -ADN coordination. [46][47][48] Previous studies have shown that nitriles are incompatible with LMAs.…”

Intrinsic chemical reactivity of solid-electrolyte interphase components in silicon–lithium alloy anode batteries probed by FTIR spectroscopy

“…Many techniques have been been developed to probe the nature of the electrochemicallygenerated SEI, with recent progress characterizing the ratio and spatial location of organic and inorganic species via neutron reflectivity (NR), 3 X-ray photoelectron spectroscopy (XPS), [4][5][6] scanning spreading resistance microscopy (SSRM) and time-of-flight secondary ion mass spectrometry (ToF-SIMS), 7,8 , surface-and tip-enhanced Raman spectroscopy (SERS and TERS), 9,10 and classical molecular dynamics/quantum chemical calculations. 11 These studies are coming to a consensus that the initial SEI formation under electrochemical bias involves primarily organic species from electrolyte reduction, which under further cycling becomes thinned at the expense of inorganic phases such as lithium silicates (Li x SiO y ), lithium oxide (Li 2 O), lithium peroxide (Li 2 O 2 ), lithium hydroxide (LiOH), and lithium carbonate (Li 2 CO 3 ).…”

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

“…However, turning to operando SERS shown in Figure 1 b, we clearly observe a potential-dependent growth of the EC+Li + peaks during negative polarization of the Au, which can be explained by the charging of the electric double layer and accumulation of Li + and solvation of EC at the SERS substrate surface. The intensities of the respective peaks can be extracted by fitting, and Figure 2 a shows that the EC+Li + peaks clearly increase in intensity, whereas the EC peaks remain constant until ∼2.3 V. On the basis of the EC+Li + to EC peak area ratio, a Li + concentration at the Au surface can be estimated (according to the relationship derived by Yang et al 16 ) and was found to increase from the expected 1 M at the OCP up to 1.5 M at 2.3 V vs Li + /Li (see SI section S3 for details). However, all of the peaks associated with the electrolyte decrease in intensity thereafter because of SEI formation.…”