Operando Optical Microscopy on a Graphite Electrode in Photonics

Fengyuan 2026-04-17 viewed

Ultrathin Graphite's combination of conductivity, transparency and low density makes it ideal for nanoscale optical devices. However, there is a trade-off between the suppression of interband optical transitions due to electron-longitudinal phonon interaction and the high intraband Drude conductivity near the interband edge, which limit the transmission for photon energies around 2eV. We show that the presence of lithium can dramatically mitigate these limitations by significantly reducing the intraband Drude conductivity and increasing the transmission for a wide range of photon energies. The strong suppression of interband phonon - phonon interactions in LixC6 as well as its enhanced valence band jumping is responsible for this.

We have used operando microscopy to analyze the interaction of Li with graphite on one electrode in order to better understand this fascinating effect. It is possible to observe the dynamics in real time of phase separations, stripping and plating reactions that are not at equilibrium. In particular, the technique allows us to determine the phase-separation kinetics of graphite/silicon/graphite materials and the optoelectronic properties they exhibit.

Operando Optical Microscopy Depicts Depth heterogeneity of Lithiation & Declithiation

Figure 1 presents the results. 2. At a low state of charge (SOC = 0%), the graphite particles displayed an original color of dark gray. After being charged at 2 mA cm-2 for 4 h, the phase separation can be observed from the lateral surface of the electrode (LSE). Changes from dark gray (LSE) to yellow (LSE) clearly indicate the onset of first lithiation. Color signal intensities in micrographs can track the variation of yellow intensity along electrode depth. The micrographs can be used to determine the volume of graphite lithiated in comparison with unlithiated.

We also observe the influence of the ablation rate on the lithiation behavior and the appearance of the yellow color. We determine that the ablation rate has a logarithmic dependence on the pulse peak fluence, as expected from the logarithmic ablation model for metals.

The non-equilibrium phase seperation reaction also contributes significantly to the overall lithiation. The main reason for this is the formation of layers with lithium intercalated between the middle layer and the side layer. This increases the interlayer distance. As the pulse peak fluence increases, the intensity of the color signals will also increase.

Furthermore, we have compared the real-time response of both graphite and silicon/graphite electrodes by analyzing their frame-by-frame changes based on the yellow color intensity and the volume change induced by Si expansion. In fact, the graphite real-time response is superior than that from silicon/graphite. This can be attributed to Si's lower expansion coefficient. These findings illuminate the unorthodox modification of ultrathin-graphite optoelectronic characteristics and show that these are affected significantly by intercalation, non-equilibrium, phase separation and relaxation reactions as well as plating/stripping in large electrolyte density gradients.