Improved Operando Raman Cell Configuration for Commercially-Sourced Electrodes in Alkali-Ion Batteries

Abstract

Operando Raman spectroscopy is a well-established technique for monitoring chemical changes in active materials during electrochemical cycling of alkali-ion cells. To date, however, its application to the study of commercial electrodes under realistic operating conditions has been severely limited by cell design constraints. We present here an improved configuration for performing operando Raman spectroscopy on coated metal foil electrodes used in standard laboratory cell testing. Electrochemical modeling predicts much improved lithiation homogeneity compared to a previously used configuration; this observation is validated experimentally for a commercially-sourced graphite electrode. The new configuration delivers improved electrochemical performance at higher specific currents than was previously possible, ensuring that Raman measurements at a single location are representative of the entire electrode. Finally, the broad applicability of the configuration is demonstrated through a study of hard carbon sodium-ion negative electrodes over 50 cycles. These results provide a new configuration for performing reliable, validated operando Raman spectroscopy on commercial battery electrodes, as well as establishing a general methodological framework for the validation of operando spectroscopic techniques to ensure that their performance is relevant to the practical systems to which they are applied.

Lithium Insertion into Graphitic Carbon Observed via Operando Kerr-Gated Raman Spectroscopy Enables High State of Charge Diagnostics

“…Conversely, as described above, CW Raman microscopy enables accurate focusing and analysis of individual graphite flakes, probing a geometric area of <5 μm 2 , in which the inhomogeneity of the studied area is expected to be minimized. Such inhomogeneity in graphite electrodes during lithium intercalation has previously been demonstrated using operando/in situ CW Raman microscopy to compare spectra of different regions of the working electrode under the same cell voltage conditions. , …”

“…As the intercalation proceeds toward high depths of lithiation toward stages 2 and 1, the interior mode is lost (since no non-adjacent graphene layers remain), and this has been confirmed by CW Raman microscopy experiments. 12 , 14 , 27 At this point, where the cell voltage is <60 mV vs Li + /Li, the remaining G-band-related features in the Kerr-gated Raman spectra red-shift continuously until the lithiation step is completed. These final transitions through stage 2 and stage 1 of the intercalation process, as will be explored in greater depth later, have been difficult to reliably follow using conventional Raman spectroscopic techniques, and the efficacy of the Kerr-gated Raman method in removing the emission baseline plays a key role in enabling this analysis.…”

“…This may alleviate, or help to account for, local inhomogeneities in the states of lithiation across the electrode structure that otherwise require careful experimental and equipment design to overcome. 27 …”

“…Such inhomogeneity in graphite electrodes during lithium intercalation has previously been demonstrated using operando/in situ CW Raman microscopy to compare spectra of different regions of the working electrode under the same cell voltage conditions. 13 , 27 …”

“…Conversely, the Kerr-gated Raman spectra are collected by rastering the laser spot across the electrode surface, which ensures the measured signals are averages of the studied surface. This may alleviate, or help to account for, local inhomogeneities in the states of lithiation across the electrode structure that otherwise require careful experimental and equipment design to overcome …”