Three-Electrode All-Solid-State Battery Cycling

Why include a reference electrode in battery testing?

A reference electrode makes it possible to distinguish between cathode and anode processes that otherwise overlap in a two-electrode measurement. In this application note, the separate potentials of the NMC cathode and the graphite anode could be monitored during cycling, and impedance spectra could be recorded simultaneously for both electrodes, as a function of temperature.

Three-electrode setups for solid-state electrolytes

Implementing a reference electrode in a solid-state electrolyte system poses several challenges. In addition to finding a suitable redox couple that can provide a known reference potential, it needs to be chemically stable with the electrolyte of choice, in order to avoid long-term potential drift as well as high surface resistance. Complicating the matter further, the geometry is very restricted in the thin cells that constitue solid-state batteries. A non-symmetric reference electrode can lead to measurement artifacts, especially in electrochemical impedance spectroscopy.

The CompreCell-3e can accommodate various wire or ring type reference electrodes, depending on the type of electrolyte sample used. In this application note we used a gold-plated tungsten wire that was lithiated in situ to obtain a stable lithium reference potential. This is based on a recent publication that employed a similar reference wire. Precise and active pressure and temperature control was of course achieved with the CompreDrive.

All-solid-state battery insights in three-electrode mode

Including the lithium reference electrode in this NMC | LPSCl | Graphite all-solid-state cell, it was possible to monitor both the NMC and graphite potentials during cycling, and also to enforce individual potential cutoff limits for both electrodes. An issue with the graphite electrode was easily identified due to the deconvuluted charge/discharge curves, something that would have been challenging in two-electrode mode.

Going one step further, electrochemical impedance spectroscopy can be recorded for both electrodes simultaneously (see figure to the right), enabling advanced characterization of the individual reactions and how they vary with temperature, state-of-charge, pressure, etc.