Researchers from Philipps-Universität Marburg, with colleagues from Karlsruhe Institute of Technology and Toyota Motor Europe, have investigated the role of void space on ion tranport in a composite cathode for solid-state Li-ion batteries. Their study, published in the Journal of Power Sources, reports a significant effect of residual voids in the composite electrode on the ion transport tortuosity.

Based on their findings, the researchers caution that careful attention needs to be paid to the actual amount of void space formed during the preparation of composite electrodes as key component of all-solid-state batteries.

All-solid-state lithium batteries (ASSLIBs) are promising as next-generation energy source in electric vehicles. … However, a high power density of ASSLIBs can be achieved only if fast ion transport in their composite electrodes is realized. These composite electrodes consist of active material particles, SE particles, and (if necessary) conductive additives, such as carbon black. Unlike conventional lithium-ion batteries, in which a liquid electrolyte penetrates the entire void space of porous electrodes and wets all the active material particles, the composite electrodes of ASSLIBs are prepared by blending SE particles with active material particles. For cathode active materials like LiCoO₂ (LCO), this turns out to be a challenging task.

Since a reproducible, homogeneous structure of ASSLIB electrodes is not easily achieved, a detailed understanding of the influence of microstructural properties on battery performance becomes of major interest. One key parameter with a strong impact on battery performance is the tortuosity characterizing ion transport in the composite electrodes. This parameter reflects the reduction of ion transport in a composite electrode compared to the transport in an ideal composite system, where ions migrate along straight, uniform pathways. While ion transport tortuosity has been investigated for a number of electrodes and separators in liquid-electrolyte batteries, data available for ion transport in composite electrodes of ASSLIBs remain scarce.

—Hlushkou

In their study, the researchers took two different approaches to determine the ion transport tortuosity for a typical ASSLIB cathode (LCO active material particles and a sulfide-based SE).

1. Determination of the stationary Li⁺ current across the composite electrode by an impedance spectroscopic measurement on a symmetrical cell Li metal | solid electrolyte | composite electrode | solid electrolyte | Li metal.

2. Combining the three-dimensional (3D) physical reconstruction of the electrode microstructure by focused ion-beam scanning electron microscopy (FIB-SEM) with 3D numerical simulations of ion transport in the reconstructed electrode.

They found that the presence of the voids significantly changed the morphology of the solid electrolyte phase compared to a void-free cathode. The voids not only reduced the volume fraction of the phase available for ion transport, but also transformed the geometry of the solid electrolyte phase into a far more tortuous one through the generation of a large number of fine, highly tortuous paths hindering ion transport.

This problem associated with void space does not exist in conventional Li-ion batteries using liquid electrolytes, although entrapped air may give rise to similar consequences. The liquid electrolyte ideally saturates the complete void space in the electrodes. In contrast, close attention should be paid to the actual amount of void space formed during the preparation of composite electrodes for all-solid-state batteries. It is not unlikely that the void space problem becomes worse for composite electrodes with smaller volume fractions of the solid electrolyte than realized in the present study. Lower volume fractions are targeted to achieve batteries with high energy densities.

—Hlushkou et al.

Toyota Motor Corporation provided financial support and Rockwood Lithium provided lithium powder for the study.

Resources

• Dzmitry Hlushkou, Arved E. Reising, Nico Kaiser, Stefan Spannenberger, Sabine Schlabach, Yuki Kato, Bernhard Roling, Ulrich Tallarek (2018) “The influence of void space on ion transport in a composite cathode for all-solid-state batteries,” Journal of Power Sources, Volume 396, Pages 363-370 doi: 10.1016/j.jpowsour.2018.06.041