Solid-State Battery Characterization with OrigaFlex + CompreDrive

A true reference electrode for solid-state batteries

Integrating a reference electrode in a solid-state electrolyte battery cell is difficult, not least because those cells are limited to thin layers that have to be pressed in order to keep the internal resistance sufficiently low. Nevertheless, given the benefits of a three-electrode setup, such as separating the electrochemical contributions of the anode and cathode, reference electrodes are becoming more prevalent in the solid-state electrolyte research literature. Lithium metal is used as the reference electrode material in the vast majority of those cases.

True reference electrodes require a mixed-valence redox couple, i.e. both the reduced and oxidized form of an electrochemically active compound, such as lithium metal (Li⁰) and lithum ion (Li⁺). Both forms should have constant activity (concentration) in order for the potential to be stable. Therefore, a lithium metal electrode is only a true reference electrode if the lithium ion activity at the lithium/electrolyte interface is constant. Since lithium metal is highly reductive and reacts with most solid electrolytes, this is rarely the case. Hence, a lithium metal electrode can technically be considered a pseudo-reference electrode. The lithium metal potential is also dependent on the applied pressure as well as the solid electrolyte composition.

In a recent publication, we described an easy way to fabricate and employ reference electrodes based on a thin wire coated with Li₄Ti₅O₁₂ / Li₇Ti₅O₁₂ (i.e. partially reduced LTO). Since this is a mixed-valence redox couple with known and constant activities, this can be considered a true reference electrode. Hence, the potential was stable for several months, as LTO is much less reducing than lithium. This potential is also independent of the applied pressure, enabling the determination of the reaction volume of both anode and cathode individually. In this application note, a partially reduced LTO reference electrode was used for its stable potential during prolonged measurement times.

Cell breathing

Another element of solid-state battery research that is gaining more interest is dilatation experiments, revealing the volume changes of a cell during cycling (so called cell breathing and swelling). For example, lithium metal takes up a lot of space as it is being plated on a lithium metal anode while charging a battery. Most cathodes, on the other hand, increase much less in volume when taking up lithium, and therefore the cell overall typically expands upon charge and shrinks again during discharge. This cell breathing can be a useful diagnostic tool when characterizing the electrochemical processes occurring in the solid-state battery cell. The recently released LVDT Distance Add-on for the cylindrical CompreCells allows such dilatation measurements, as demonstrated in this application note.

Instrument synchronization

Finally, communication and synchronization between the potentiostat and the temperature/pressure controller (such as the CompreDrive) is crucial for correlating data collected by the different instruments, as well as for automation of measurement procedures. Since temperature and pressure usually affect the electrochemical properties of the active electrode materials significantly, this can help in gaining a deeper understanding of the system under test. In this application note, an OrigaFlex potentiostat was used, and its digital I/O capabilities leveraged to automate collection of impedance spectra at different temperatures as controlled by the CompreDrive. To ensure synchronization between the potential/current data recorded by the potentiostat and pressure/temperature/dilatation data recorded by the CompreDrive, the synchronization feature in Edelweiss was used. This works with various data sets that have already been recorded on different time bases (i.e. without synchronization between the instruments).