Ion-conducting membranes (ICMs) are used to separate the cathode and anode in solid-ion batteries. Although ICMs are as important as the electrodes in batteries for when it comes to improving charge capacity and discharge rates, much less attention is paid to these structures.

ICMs are usually made from microporous polymer sheets impregnated with solutions of lithium salts in alkylcarbonates. These gel or liquid phases serve as conducting media and allow ions to be transported between electrodes while preventing electrons from passing between them. Many liquid electrolytes used with microporous ICMs are flammable and can leak out from batteries. When the battery shorts because of dendrites that grow from the anode to cathode and pierce the ICM, the batteries can catch fire.

A good ICM material

Finding a good ICM material is no easy task: not only does it need to suppress dendrite growth, it should also conduct ions well, and be flexible and tough. Most separator materials (such as gel and liquid electrolytes, or composite gel electrolytes with organic fillers) do not tick all these boxes.

This is where aramid nanofibres could come into their own, says a team of researchers led by Nicholas Kotov. The nanostructures form a tight mesh (filled with a solid electrolyte) with a pore size that is about the same size as that of the growing tip of dendrites, if not a little smaller. This allows the nanofibres to “block” the holes made by the dendrites as they grow and pierce the ICM.

Suppressing dendrite growth

The fibres are also as strong, stiff and tough as the Kevlar™ parent material from which they are made and are thus able to withstand the pressure of the growing metal dendrites. “In fact, we showed that these nanofibres not only stop lithium dendrites (which are relatively soft) from growing, they also stop hard copper dendrites in their tracks too,” says Kotov.

The researchers made their nanofibres by slowly dissolving Kevlar nanofibres in dimethylsulphoxide (DMSO) – a common industrial solvent. Kevlar™ is the registered trademark for a para-aramid synthetic fibre, first isolated in the 1960s by DuPont and widely used today in bulletproof vests, bicycle tyres and racing sails. The material has a high tensile strength-to-weight ratio, making it stronger than steel in this respect. “Varying the amount of water in the DMSO allows us to make nanofibres with different diameters (ranging from around 5 to 10 nm),” explains Kotov.

Better batteries

“Pairing the aramid nanofibre-based ICM with a high charge capacity cathode and anode (for example, a lithium metal anode), will allow us to make thinner batteries with greater charge capacities and higher discharge rates,” he tells nanotechweb.org. “The aramid nanofibres are also stable to higher temperatures than conventional ICM materials, which is an added advantage for such applications.”

The team says that it is now busy working on optimizing its separator as well as looking at how to scale up its production. “We are doing this part of the work with a Detroit and Ann Arbor-based company, Elegus Technologies, headed by the University of Michigan graduate John Hennessy and which employs several UM students,” says Kotov.

The research is detailed in Nature Communications doi:10.1038/ncomms7152.