Li Lu (National University of Singapore) said at ICREN Barcelona April 2018, "Solid state electrolytes have attracted tremendous attentions in recent years due largely to their commercial applications for highly safe energy storage and large energy storage devices. Although solid state electrolytes been studied for a few decades, breakthrough in ionic conductivity has been noted only recently. With applications of batteries in large format, safety issues become an extremely important in addition to challenges of high energy density. Replacement of highly flammable organic liquid electrolyte by solid and stable electrolyte leads to increased safety. Most solid electrolytes possess a wide operation potential range so that some cathodes materials that cannot be used in organic electrolyte can now be considered. Studies also note that use of solid electrolyte can significantly expand battery operation temperature range. Therefore, solid state battery is the future energy storage device. Solid electrolyte that is a key and also an essential component in the batteries can be categorized into following types: oxide, glassy, sulfate, and polymer and its composites. Different types of solid electrolytes show different advantages in different aspects. Based on safety concern, oxide-based electrolytes such as garnet-structured, nasicon-structured and lisicon-structured materials have demonstrated pretty good stability in ambient condition with reasonably high ionic conductivity of about 10-4 ~ 10-3 S/cm. Some of them can be potentially used in all-solid-state batteries and Li-air batteries." His presentation reported on recent development of solid state electrolyte for Li-air batteries and for all-solid-state batteries.
He compared options as below and concluded that interface engineering is key.
At the materials level issues are:
Ionic conductivity - Still low
Stability in ambient - reactive with gases and moisture