In the pursuit of a rechargeable battery that can power electric vehicles (EVs) for hundreds of miles on a single charge, scientists have endeavored to replace the graphite anodes currently used in EV batteries with lithium metal anodes. But while lithium metal extends an EV's driving range by 30 to 50%, it also shortens the battery's useful life due to lithium dendrites — tiny, treelike defects that form on the lithium anode over the course of many charge and discharge cycles. Dendrites also short-circuit the cells in the battery if they make contact with the cathode.
For decades, researchers assumed that hard, solid electrolytes, such as those made from ceramics, would work best to prevent dendrites from working their way through the cell. But the problem with that approach is that it didn't stop dendrites from forming or nucleating in the first place, like tiny cracks in a car windshield that eventually spread.
Researchers have developed a new class of soft, solid electrolytes — made from both polymers and ceramics — that suppress dendrites in that early nucleation stage, before they can propagate and cause the battery to fail.
Solid-state energy storage technologies such as solid-state lithium metal batteries that use a solid electrode and a solid electrolyte can provide high energy density combined with excellent safety but the technology must overcome diverse materials and processing challenges. The new dendrite-suppressing technology could enable battery manufacturers to produce safer lithium metal batteries with both high energy density and a long cycle life. Lithium metal batteries manufactured with the new electrolyte could also be used to power electric aircraft.
Key to the design of the soft, solid electrolytes was the use of soft polymers of intrinsic microporosity (PIMs) whose pores were filled with nanosized ceramic particles. Because the electrolyte remains a flexible, soft, solid material, battery manufacturers will be able to manufacture rolls of lithium foils with the electrolyte as a laminate between the anode and the battery separator. The lithium-electrode sub-assemblies (LESAs) are attractive drop-in replacements for the conventional graphite anode, allowing battery manufacturers to use their existing assembly lines.
To demonstrate the dendrite-suppressing features of the new PIM composite electrolyte, the researchers created 3D images of the interface between lithium metal and the electrolyte to visualize lithium plating and stripping for up to 16 hours at high current. Continuously smooth growth of lithium was observed when the new PIM composite electrolyte was present, while in its absence, the interface showed telltale signs of the early stages of dendritic growth. These and other data confirmed predictions from a new physical model for electrodeposition of lithium metal.