Solid State Electrolytes
Authors: Strand Dee , Caldwell Marissa 
Published: February 2020

Solid state batteries offer significant advantages over today’s cells containing liquid electrolytes. In large format cells required for automotive applications, replacement of volatile, flammable organic solvents with a solid phase electrolyte eliminates safety concerns. Other benefits include lower costs due to elimination of the costly separator and the potential to improve energy density by enabling safe lithium metal anodes. Finally, a solid state cell offers opportunities for novel cell formats, shapes, and packaging. While significant improvements in ionic conductivity of materials have been demonstrated, few examples of solid state batteries with practical electrode thicknesses are reported. A primary challenge in an all solid state battery is the difficulty of rapidly moving charge across solid/solid interfaces within the electrodes and across the electrode/electrolyte interface. Demonstrated progress in improving the ionic conductivity of solid electrolytes does not necessarily result in improved cell performance due to these interfacial impedances within the cathode and/or between the electrodes and the electrolyte.

Research in solid state batteries primarily centers on maximizing bulk ionic conductivity of the solid electrolyte or adding coating/interface layers to reduce impedance. However, all solid cells can be enabled through formulation optimization using known materials by reducing the dominant interfacial impedances. In this presentation, we present results showing the marked improvement in all solid cell performance through formulation optimization and the novel use of solid electrolyte additives. In liquid electrolyte cells, significant optimization of electrode composition and processing conditions are required to achieve desired porosity, connectivity, etc. for optimal performance. Similar logic is required to optimize ionic conductivity, electronic conductivity, and electrode density in solid state composite electrodes. Electrode additives are shown to reduce interfacial impedance both within the electrode and across the electrode/electrolyte interfaces.

Composite electrolytes based on ceramic particles embedded in an ionically conductive polymer can also benefit from formulation optimization. The inclusion of the ceramic particles into a polymer membrane can 1) improve ionic conductivity by altering the total crystallinity of the polymer and 2) improve the mechanical properties of the membrane. However, use of a composite membrane provides additional interfaces between the particles and the polymer, and can also alter the interfacial impedance between the solid electrolyte and the electrodes. Again, formulation with additives can affect both ionic conductivity and interfacial impedance.

A final aspect in formulation designs for solid state batteries are interactions between components. Similar to observations in liquid electrolyte cell where, for example, components in the cathode can have negative effects on other cell components, interactions can also be observed in solid state batteries. Thus, substantial opportunities will be shown for improvement in solid state cell performance by proper formulation of the cathode and electrolyte without the need for new ionic conductor material development.