Jan Allen, US Army Research Laboratory
Synthesis and Characterization of Fast Li-ion Conducting Solid State Electrolytes
Written by Aashutosh Mistry
Solid electrolytes represent a key research area in lithium batteries for a safer future. Many different solid electrolytes have been invented in recent years with the primary aim of matching ionic conductivity to their liquid counterparts. Most of the electrolytes are poor ion conductors in their pristine form, and elemental doping appears to be an effective strategy to advance these materials. Toward that end, Jan Allen and his collaborators have studied various different solid electrolytes and their subsequent substitutions to assess the relative usefulness. They identified that a reduction of titanium Ti4+ is an issue for perovskite (e.g., LLTO lithium lanthanum titanium oxide) and NaSICON (e.g., LATP lithium aluminum titanium phosphate) type electrolytes. On the contrary, zirconium-based solid electrolytes, specifically LZP (lithium zirconium phosphate) and LLZO (lithium lanthanum zirconium oxide) are more stable. Both these materials are amenable to substitution via other cations (calcium, yttrium, aluminum, tantalum, gallium, etc.) and provide increased conductivity. For example, consider substitution to LLZO structure via Ta, Al, and Ga. Ta replaces Zr sites and changes the unit cell, Ga has an octahedral site preference, and Al prefers tetrahedral sites (which competes with Li). Ta-doped LLZO has higher conduction than Ga-doped and Al-doped is inferior given the competition of Al with Li distribution. Such experiments provide valuable design rules for choosing solid electrolytes and appropriate substitutions.