Modelling the Through-Hole Coin Cell Configuration for <i>Operando</i> Optical Spectroscopy of Battery Electrodes

Operando optical methods are widely used for measuring the properties of battery electrodes under charge/discharge conditions and provide valuable insights into various phenomena. However, the need to gain optical access to the electrode of interest often requires a compromise, such that the electrochemical characteristics of the operando cell deviate from those in a conventional cell. One approach towards minimally invasive optical spectroscopy is the “through-hole” coin cell configuration in which the bottom electrode is optically accessed via a hole punched through the top electrode and separator, but the impact of this hole on the validity of the measurement has been given little consideration. Here we performed modelling of graphite and lithium nickel manganese cobalt oxide (LiNi 0.6 Mn 0.2 Co 0.2 O 2 ; NMC622) electrodes in a Li-ion half-cell arrangement, to explore this through-hole configuration. We show that the presence of a 0.5 mm radius hole results in a non-uniform Li + ion distribution in the electrode phase, but this can be reduced by decreasing the current, decreasing the hole size or increasing the separator thickness. The Li + non-uniformity varies non-linearly with the average degree of electrode lithiation and is more extreme for a graphite electrode compared to NMC622 due to their contrasting voltage-charge profiles.