Electric Potential Gradient at the Buried Interface between Lithium-Ion Battery Electrodes and the SEI Observed Using Photoelectron Spectroscopy

Abstract

The buried interface between the bulk electrode material and the solid electrolyte interphase (SEI) in cycled Li-ion battery anodes is suggested to incorporate an electric potential gradient. This suggestion is based on photoelectron spectroscopy (PES) results from different anode materials that all show relative binding energy shifts between the components of the SEI and the active anode. Implications of this electric potential gradient on binding energy reference points in PES as well as on charge-transfer kinetics in Li-ion batteries are discussed. Specifically, we show that the separation of surface layer and bulk material spectral contributions (depth profiling) is crucial for consistent data interpretation. We conclude that previous interpretations of lithiation as cause for changes in PES spectra may need to be revised.

Binding Energy Referencing for XPS in Alkali Metal-Based Battery Materials Research (II): Application to Complex Composite Electrodes

“…(ii) Many of the investigated surfaces are covered with mixtures of several species, which makes their identification really complex. (iii) As also discussed in previous works [19,21], the absolute values of the BE shift from the "alkaline effect" obviously depend on the near-surface Li-concentration and -distribution, which varies in our examples from case to case. In addition, the exact determination of the shifts is not really beneficial.…”

“…These shifts were not caused simply by surface charging and depend on the surface concentration of the alkaline (Li, Na) elements in their elemental form. Similar results were found by Maibach et al [21] at HAXPES investigations on differently charged (lithiated) graphite anodes. They observed shifts of the SEI components depending on the charging state and Li concentration, respectively.…”

Different Shades of Li4Ti5O12 Composites: The Impact of the Binder on Interface Layer Formation

“…The general changes for CMC−Na are similar to those for PVdF‐HFP, with an increase of C−H bonds at OCV conditions and an increase of −C−O− and decrease of −C−C when cycling the electrode. The −C−C− peak position varies after calibration against the −C−H peak for the five spectra recorded, which has been observed previously, and was proposed to originate from an electric field gradient at the interface between electrode and SEI, as described in the work of Maibach et al …”

“…The general changes for CMCÀNa are similar to those for PVdF-HFP, with an increase of CÀH bonds at OCV conditions and an increase of ÀCÀOÀ and decrease of ÀCÀC when cycling the electrode. The ÀCÀCÀ peak position varies after calibration against the ÀCÀH peak for the five spectra recorded, which has been observed previously [51,52] and was proposed to originate from an electric field gradient at the interface between electrode and SEI, as described in the work of Maibach et al [53] For this binder, it is not straightforward to distinguish between carbonates formed as part of the SEI and the oxygenrich carbons in CMC, although the carboxymethyl moiety seem to appear at slightly lower binding energies than inorganic carbonates (which are expected at around 291 eV). In pristine LTO : CMCÀNa electrodes, the carbons bound to two oxygens (ÀOÀCÀOÀ; at 287.8 eV), [35] which should originate primarily from the glycosidic bond, is most prominent.…”

Interface identification of the solid electrolyte interphase on graphite

“…Nevertheless, small imbalance potentials of -0.81 and 0.78 eV of the top and bottom edges (taken at final points on the potential curve tails in Figure 1b and whose subtraction defines the edge potential difference) induce an electric field opposite to the up-direction of the c-axis. The presence of such electric gradient was recently assessed by means of photoelectron spectroscopic examination of the buried interfaces in cycled LIBs [42].…”