Probing Interphase Reactions on Lithium with Operando Gas Chromatograph
Authors: Hobold Gustavo M. , Gallant Betar M. 
Published: May 2020

Li battery chemistries exploit reactions that occur under very reducing conditions (~ 0 V vs. Li/Li+) at negative electrodes, which are held below the electrochemical stability window of known carbonate-based electrolyte systems (0.5-1 V vs. Li/Li+),1 resulting in solvent decomposition at the surface and leading to the formation of a solid electrolyte interphase (SEI). When formed on Li, however, this interphase is unable to fully protect the electrode, resulting in an SEI composition that changes over time.2 Due to this dynamic nature, its chemistry is challenging to characterize using conventional ex-situ surface analysis (e.g., Fourier transform infrared spectroscopy, FTIR; X-ray photoelectron spectroscopy, XPS).3, 4 Consequently, identifying the chemical mechanisms through which candidate systems for improving Li SEI stability operate – such as the use of highly fluorinated and/or concentrated electrolytes5 – remains difficult. Fortunately, interphase reactions are known to release gases that are easily detected by gas chromatography (GC) and mass spectrometry (MS),6 using which we are able to construct a time-resolved picture of the SEI chemistry. Here we show that these experiments can provide valuable insight into the dynamic chemical reaction pathways on Li, yielding an updated picture of factors that contribute to a good SEI.

Our experimental approach consists of a custom-designed electrochemical cell coupled to a GC instrument, which, by sampling the cell headspace periodically, enables operando quantification of CO2, CO, H2 and C1-2 products evolved at Li electrodes. By comparing gas evolution between rest and polarization regimes, we identified a significant increase in the rate of formation of all observed gases when the electrodes are polarized. Using electrodes that we identified to be gas-inert, such as LiFePO4, we identified that the deposition reaction, i.e., Li+ reduction, accounts for nearly all of the gas-releasing interphase reactions, implying that the SEI is only formed electrochemically when Li is plated. We next explored the nature of these reactions by quantifying how the evolution rate of each gas scales with current density, which enabled the distinction between gases that were formed in chemically-limiting reactions and gases that were formed from electrochemically-limiting steps. Because our experiments are able to quantify the abundance of each gas product, we also identified and quantified the branching of specific interphase reactions that occur on Li with conventional carbonate-based electrolyte systems. Then, by rationally tuning the electrolyte (i.e., changing the salt species and/or introducing fluorinated solvents), we discovered and quantified how each component drives specific interphase reactions, and how the ensuing SEI chemistry affects the Faradaic efficiency of the Li electrode over time. In particular, we observed that pathways that promote decarbonylation and/or decarboxylation (i.e., release of CO/CO2) of the solvent upon reduction correlate with higher Faradaic efficiencies. By varying salt concentration, we further explored how interphase formation pathways are affected by the solvation structure of the electrolyte in the contact-ion and solvent-separated regimes. Thus, our experiments provide a mechanistic picture of how organic solvent-derived products affect SEI chemistry and stability, which can be as important as known ionic phases like LiF.5

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