(Invited) Solid-Electrolyte Interface and Interphase Depicted By Plasmon-Enhanced Raman Spectroscopy
Authors: Nanda Jagjit , Yang Guang , Veith Gabriel M. , Hallinan Daniel T. , Sokolov Alexei P. 
Published: February 2020

The growing need for energy storage systems requires batteries with even higher power and energy density with extended life and enhanced safety. This clearly calls for innovations in high energy density electrode materials as well as in design of robust and efficient charge transfer interfaces without electrolyte degradation or chemical side reactions. Charge transport across the solid electrode-liquid electrolyte interface (SLI) is believed to be one of the charge/discharge rate limiting steps. In lithium-ion batteries, the thermodynamic instability of the electrolyte at the SLI leads to formation of a passivation layer, commonly referred to as the solid-electrolyte interphase (SEI). Although the composition, morphology, and structure of SEI in lithium-ion batteries have been extensively studied, probing its evolution in situ at nanoscale is still challenging to a large extent. Plasmon-enhanced Raman spectroscopy (PERS) is promising to solve this challenge. PERS is the resonant oscillation of conduction electrons at the interface between negative and positive permittivity molecules stimulated by incident light. It includes surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS). The prerequisite of employing PERS to probe the surface molecules is to have a highly ordered plasmonic nanostructured SPR substrate, which is extremely challenging.
Large area (cm2) monolayers of gold nanoparticles (Au NPs) have long-range ordered nanogap arrays. The local electromagnetic field is extremely intense within the nanogap (<10 nm) region due to the coupling effect among adjacent nanoparticles, which allows for probing the molecules at immediate vicinity of SLI (distance from the solid surface is of 17 nm). 1 The Au NP monolayers exhibit high SERS sensitivity even down to single-molecule level (enhancement factor > 107). This allows for probing a broad range of trace electrolyte components, such as lithium hexafluorophosphate (LiPF6), fluoroethylene carbonate (FEC), ethylene carbonate (EC) and diethyl carbonate (DEC), etc. The investigation of a commercial lithium-ion battery electrolyte (LiPF6 in EC+DEC binary solvents) using SERS allows for the determination of the solvent coordination numbers, which ranges from 2 to 5, in sharp contrast to those calculated from bulk liquid electrolytes by standard confocal Raman and infra-red (IR) spectroscopy from 3 to 6. 2

SERS is promising to probe molecular fingerprints in nanoscale through SLI plane. However, it lacks the nanoscale resolution in sample plane. This makes it extremely hard for getting spatial heterogeneity of the SEI, which can be only on the order of 10 nm. Tip Enhanced Raman Spectroscopy (TERS) combines SERS with atomic force microscopy (AFM), capable of providing the chemical vibrational information and topography of the sample in the nanometer spatial resolution simultaneously. TERS analysis on cycled amorphous (a-Si) indicates that the nanometer scale SEI “islands” are unevenly distributed on the Si anode surface. Even for the same SEI “island”, the composition is different from point to point with inter-point distance smaller than 10 nm. The local chemical information studied by TERS is intrinsically different than that collected from the standard confocal Raman and IR spectroscopy.
Acknowledgement
This work is supported by the U.S. Department of Energy's Vehicle Technologies Office under the Silicon Electrolyte Interface Stabilization (SEISta) Consortium directed by Anthony Burrell and managed by Brian Cunningham