Probing Silicon Anodes in Lithium-Ion Batteries Using In Situ FTIR Spectroscopy

We have been exploring the intrinsic chemical and electrochemical reactivities of silicon nanoparticles (Si NPs), the electrode binder and finished 3D electrodes using both ex situ and in situ attenuated total reflectance-infrared Fourier transform (ATR-FTIR) spectroscopies. We will first detail ex situ FTIR results that probe the solid reaction products from the chemical reaction of aprotic battery electrolyte and three purported components of the Si-based anode solid-electrolyte interphase (SEI): SiO₂ nanoparticles (NPs), lithium silicate (Li_xSiO_y) powders and Si NPs. We use FTIR and classical molecular dynamics/density functional perturbation theory to assess the solid products remaining with these model materials after exposure to electrolyte. These species represent the initial stages of SEI growth and predict they likely drive subsequent chemical and electrochemical reactions by controlling molecular interactions at the Si active material interface.<br/>Additionally, we will present studies on poly(acrylic acid) (PAA), a commonly used binder for fabricating Si anodes, that show the evolution of PAA occurs via a cross-linking reaction during electrode curing. We will relate its chemical change to the final electrode properties and performance as well as local electrolyte structure in the 3D electrode. These studies are made possible using two types of in situ ATR-FTIR spectroscopy: thermal ATR-FTIR to probe the PAA cross-linking reaction, and ATR-FTIR spectroelectrochemistry of three-dimensional composite electrodes, a unique technique that probes the solvation dynamics of lithium ions at the silicon anode interface under electrochemical polarization. These latter studies reveal that PAA acts as an interfacial material that conducts lithium-ions, limits solvent molecule access to the Si surface, and stabilizes the electrode against parasitic lithium inventory loss at high state of charge for an extended period.