Kevlar fibers fortify lithium

With the capacity to store up to five times the energy of today's lithium-ion solutions, researchers have a keen interest in lithium-sulfur batteries, and a team at the University of Michigan has taken a step toward realizing their real-world potential. The breakthrough hinges on a naturally inspired membrane that overcomes stability issues and offers the battery a "nearly perfect" design, enabling it to last for a thousand-plus cycles.

“There are a number of reports claiming several hundred cycles for lithium-sulfur batteries, but it is achieved at the expense of other parameters – capacity, charging rate, resilience and safety," said leader of the research team, Nicholas Kotov. "The challenge nowadays is to make a battery that increases the cycling rate from the former 10 cycles to hundreds of cycles and satisfies multiple other requirements including cost."

In taking up that challenge, Kotov and his colleagues turned to aramid nanofibers, which are nanoscale versions of Kevlar fibers, and fashioned them into carefully engineered networks that mimic the structure of cell membranes. This material was infused with an electrolyte gel and prevents a common cause of battery failure, which is the formation of needle-like growths called dendrites that grow on one of the electrodes.

But the benefits of the novel membrane go further than that. As a lithium-sulfur battery is cycled, small particles of lithium and sulfur known as lithium polysulfides flow to the lithium and compromise the device's capacity. The team addressed this by integrating tiny, bio-inspired channels into its artificial membrane and adding an electrical charge, which repels the particles while allowing the positively charged lithium ions to flow freely.