Operando Photonic Stopband Monitoring of Lithium-Ion Battery Electrodes
Authors: Lonergan Alex , Gulzar Umair , Zhang Yan , O'Dwyer Colm 
Published: July 2022

Research in lithium-ion batteries is often focused on optimising the electrode performance of either the anode1 or cathode2. One common research strategy is to explore alternative electrode material candidates for use as both the anode3 and cathode4. Another approach involves optimising the performance of existing electrode materials through structured electrode architectures with nano-sized features. Incorporating nanostructure into electrode design has reported advantages of shorter ion diffusion lengths and improved rate capability during cycling5. A common and simple technique for introducing nanostructure to electrodes is the use of a photonic crystal template, particularly inverse opal photonic crystals. The porous geometry, highly interconnected material and nano-sized features of the pore walls are all believed to contribute to improved electrochemical performance in inverse opal electrodes6
7.
Photonic crystal materials are more than just a structural template and are renowned for their ability to tune the wavelengths of light propagating in the structure8
9. The repeating dielectric material comprising the structure reflects specific wavelengths, known as the photonic bandgap or stopband, depending on the size of the repeating lattice and the refractive index contrast between the composite materials. Tuning the photonic stopband of various inverse opal materials has been extensively studied10
11. Inverse opal battery electrodes have yet to exploit the optical potential of the photonic stopband.
Here, we showcase a fundamentally new operando analysis technique for lithium-ion battery electrodes adopting a photonic crystal structure. Visible spectroscopy is used to monitor the presence and position of the photonic stopband of the electrode material during battery cycling, see Figure 1. Capitalizing on the sensitivity of the photonic stopband to lattice size and refractive index contrast, shifts or changes in the optical spectrum can be correlated to the electrode environment and material performance. Several optical effects are observed throughout the cycling process and linked back to the battery performance of the anode. A TiO2 inverse opal anode is reported on here, yet the technique is versatile and should be applicable to a wide range of electrode materials possessing a photonic crystal structure.

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Figure 1 (a) Schematic diagram of simultaneous electrochemical and optical characterisation techniques for photonic crystal electrodes. (b) Galvanostatic charge/discharge data for a TiO2 inverse opal electrode. (c) Optical spectrum for a TiO2 inverse opal submerged in LiPF6 electrolyte. (d) SEM image showing the ordered structure of a pristine TiO2 inverse opal. (e) Operando optical spectra obtained at 0.5 V intervals during discharge of the TiO2 inverse opal electrode.

Figure 1