In–Li Counter Electrodes in Solid-State Batteries – A Comparative Approach on Kinetics, Microstructure, and Chemomechanics
A recent publication by Alt et al. in Advanced Energy Materials describes an investigation by the Janek research group into the usage of InLi as a counter electrode in solid-state battery (SSB) research. They studied the impact of the preparation method on the potential stability, rate capabilities, and microstructure of the electrode. Furthermore, they used the CompreDrive to quantify the cell height changes as an effect of lithium insertion and extraction during cycling.
Counter electrodes for solid-state battery materials development
High-capacity cathode and anode active materials play a vital role in achieving competitive power and energy densities in SSBs. To properly characterize the electrode materials, the influence of the employed counter electrode must be eliminated or at least minimized. The optimal solution for this challenge is the implementation of reference electrodes, such as those utilized in the three-electrode CompreCell system. However, two-electrode measurements are typically more straightforward to execute and interpret, and the counter electrode might still influence the results, for example if lithium dendrites are formed. To address these issues, it is essential to develop electrochemically stable counter electrodes that demonstrate high rate capability and low overpotential, characterized by consistent half-cell potential, favorable kinetics, and reversibility. The In–Li alloy system offers a variety of advantageous features, with the two-phase eutectic In/(InLi)x maintaining a stable potential of 0.62 V vs Li across a wide stoichiometric range, while supporting rapid lithium diffusion, making it useful as a counter electrode in SSB research.
Height changes (top) occurring during cycling (bottom) for two InLi//NCM SSB cells. InLi was prepared either via stacking of foil (left) or as an electrolyte composite (right).
Quantifying cell height changes with the CompreDrive
The CompreDrive actively regulates the pressure applied to a solid-electrolyte cell stack using a servo motor. The position information from the motor can be used to quantify height changes of the electrodes during cycling. This was used in a recent study to distinguish grain boundary and bulk conductivity in solid electrolytes. In the featured publication, this feature was instead used to assess the swelling of the InLi alloy during cycling, depending on the electrode preparation method. The authors prepared In/Li | LPSCl | NCM cells using the CompreCell system, applied 75 MPa in the CompreDrive, and monitored the height changes induced by repeated cycling. The InLi alloy was prepared either by stacking of In and Li foil, or by preparing a powder composite with solid electrolyte. They found height changes around 4.4 µm for both preparation methods, which is larger than the theoretical value expected from crystallographical data (2.6 µm), but similar to previous experimental data. This indicates that the electrode is less dense than pure InLi, irrespective of the preparation method. The chemomechanical reversibility was high when exposing these cells to extended cycling, i.e. little irreversible swelling was observed.
The full research article is available online. Follow the link to read the full text of this important contribution to the SSB research field!