Battery breakthrough for electric transport

Researchers at Berkeley Lab and Carnegie Mellon University have designed a new class of solid electrolytes that could facilitate the wider electrification of transportation, including electric aircraft and long-range electric cars.

Battery endeavours and the dendrite dilemma

To design a rechargeable battery that can power electric vehicles (EVs) for hundreds of kilometres on a single charge, scientists have focused on replacing the graphite anodes currently used in EV batteries with lithium metal anodes.

Lithium metal extends an EV’s driving range by 30–50%, but lithium dendrites — tiny tree-like defects that form on the lithium anode over the course of many charge and discharge cycles — shorten the battery’s life. 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 this approach doesn’t stop dendrites from forming or ‘nucleating’ in the first place.

A soft approach to dendrite suppression

Researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), in collaboration with Carnegie Mellon University, have developed a new class of soft, solid electrolytes — made from both polymers and ceramics — that suppresses dendrites in the early nucleation stage, before they can propagate and cause the battery to fail. The findings are reported in the journal Nature Materials.

“Our dendrite-suppressing technology has exciting implications for the battery industry,” said co-author Brett Helms, a staff scientist in Berkeley Lab’s Molecular Foundry.

“With it, battery manufacturers can produce safer lithium metal batteries with both high energy density and a long cycle life.”

Helms added that lithium metal batteries manufactured with the new electrolyte could also be used to power electric aircraft.

He explained that key to the design of the soft, solid-electrolytes was the use of soft polymers of intrinsic microporosity, or 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. These lithium-electrode subassemblies, or 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 Helms team used X-rays at Berkeley Lab’s Advanced Light Source to create 3D images of the interface between lithium metal and the electrolyte, and to visualise 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, which takes into account both chemical and mechanical characteristics of the solid electrolytes.

“In 2017, when the conventional wisdom was that you need a hard electrolyte, we proposed that a new dendrite suppression mechanism is possible with a soft solid electrolyte,” said co-author Venkat Viswanathan, associate professor of mechanical engineering and faculty fellow at Scott Institute for Energy Innovation at Carnegie Mellon University.

“It is amazing to find a material realisation of this approach with PIM composites.”

24M Technologies has integrated these materials into larger format batteries for EVs and eVTOL (electric vertical take-off and landing) aircraft.

“While there are unique power requirements for EVs and eVTOLs, the PIM composite solid electrolyte technology appears to be versatile and enabling at high power,” Helms said.

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