In situ infrared nanospectroscopy of the local processes at the Li/polymer electrolyte interface

Abstract

Solid-state batteries possess the potential to significantly impact energy storage industries by enabling diverse benefits, such as increased safety and energy density. However, challenges persist with physicochemical properties and processes at electrode/electrolyte interfaces. Thus, there is great need to characterize such interfaces in situ, and unveil scientific understanding that catalyzes engineering solutions. To address this, we conduct multiscale in situ microscopies (optical, atomic force, and infrared near-field) and Fourier transform infrared spectroscopies (near-field nanospectroscopy and attenuated total reflection) of intact and electrochemically operational graphene/solid polymer electrolyte interfaces. We find nanoscale structural and chemical heterogeneities intrinsic to the solid polymer electrolyte initiate a cascade of additional interfacial nanoscale heterogeneities during Li plating and stripping; including Li-ion conductivity, electrolyte decomposition, and interphase formation. Moreover, our methodology to nondestructively characterize buried interfaces and interphases in their native environment with nanoscale resolution is readily adaptable to a number of other electrochemical systems and battery chemistries.

Nano-FTIR Spectroscopy of the Solid Electrolyte Interphase Layer on a Thin-Film Silicon Li-Ion Anode

“…Previous results have suggested that the SEI is heterogenous on the nanoscale; ,− therefore, we aimed to use the high-resolution capabilities of the nano-FTIR to evaluate this assessment. We probed the film over a 200 nm line scan in 10 steps, close to the limit of the technique’s resolution …”

“…The topography of the surfaces is very smooth, and the resulting spectra do not vary significantly in terms of peak ratios as a function of position. For the non-washed electrode (Figure 5a), the spectra were taken at a different location from Spot 1 (in Figure 3a sensitivity to molecular orientation, 19 we speculate that these variations originate from different conformations of the LiEDC and EC molecules on the surface/subsurface region. Furthermore, the nano-FTIR spectrum from each location has similar IR absorption peak characteristics as the bulk ATR-FTIR measurement (Figure 4), which indicates that the chemical composition, structure, and distribution of the organic compounds in the SEI of the non-washed a-Si electrode are fairly homogenous.…”

“…To do so, an s-SNOM-like setup can be adopted, along with the utilization of a broadband IR light source (such as an ultrabroadband synchrotron or broadband laser and an asymmetric Michaelson interferometric detection scheme where the tip-sample system is incorporated into one of the arms of the interferometer) . In this way, local high-resolution nano-FTIR spectroscopy can be performed, which closely matches standard far-field FTIR references. , …”

“… 22 In this way, local high-resolution nano-FTIR spectroscopy can be performed, which closely matches standard far-field FTIR references. 19 , 23 …”

“… 14 Recently, this technique was used to investigate in situ Li metal plating and the associated chemical changes at the interface with a solid polymer electrolyte, demonstrating the effect of local heterogeneity during the Li plating process. 19 Ex situ tip-enhanced Raman spectroscopy, a related scanning probe technique, was applied to show local heterogeneity in the SEI on thin film Si anodes after cycling. 17 These studies, in addition to recent cryo-TEM work, 24 suggest that the variations in nanoscale heterogeneity of the SEI layer could be crucial for the functional structure and stability of the SEI.…”