A team of Berkeley Lab researchers has invented a solid electrolyte for lithium-ion batteries that can reduce battery weight and improve safety, as liquid electrolytes are the most flammable component of lithium-ion cells.
The researchers modified a metal-organic framework (MOF), through which ions can flow; specifically, they modified a magnesium-based MOF that forms honeycomb-like hexagonal tubes 13 Å wide. These tubular structures have the ability to convey ions, but their natural resistance is too high to serve as a practical solid electrolyte. This limitation was overcome by a process that added a lithium isopropoxide to the MOF to increase its ionic conductivity to 10-4 S/cm, which is within the performance range necessary for implementation in lithium-ion batteries today. The rigid structure of this new solid-state MOF allows it to serve as both electrolyte and separator, which keeps anode and cathode layers physically apart.
The Berkeley Lab technology may enable the design of lithium-ion batteries with anodes made of lithium rather than lithium-intercalated graphite. Lithium anodes have a theoretical charge capacity much higher than those of graphite, but during charging cycles form destructive structures called dendrites that penetrate the separators, shorting the battery. The solid structure of the MOF electrolyte inhibits dendrite growth and enables lightweight, higher capacity lithium anodes, a valuable to future battery design.