  Innovative new materials are consistently emerging as electrode candidates
from lithium-ion battery research, promising high energy densities and
high-rate capabilities. Understanding potential structural changes, morphology
evolution, degradation mechanisms and side reactions during lithiation is
important for designing, optimising and assessing aspiring electrode materials.
In-situ and operando analysis techniques provide a means to investigate these
material properties under realistic operating conditions. Here, we demonstrate
an operando spectroscopic method using photonic crystal-structured electrodes
that uses the optical transmission spectrum to monitor changes to the state of
charge or discharge during lithiation and the change to electrode structure, in
real-time. Photonic crystals possess a signature optical response, with a
photonic bandgap (or stopband) presenting as a structural colour reflection
from the material. We leverage the presence of this photonic stopband,
alongside its intricate relationship to the electrode structure and material
phase, to correlate electrode lithiation with changes to the optical spectrum
during operation. In this work, we explore the optical and electrochemical
behaviour of a TiO2 anode in a lithium-ion battery, structured as an inverse
opal photonic crystal. In principle, the operando technique demonstrated here
is versatile and applicable to a wide range of electrochemical electrode
material candidates when structured with ordered porosity akin to a photonic
crystal structure.
