Lithium metal anodes promise to double the energy storage capacity compared to the carbon-based anodes used in today’s lithium ion batteries. But uncontrolled growth of dendrites on lithium metal anodes during charging has limited their use. Dendrites are spikey depositions of lithium metal that takes place during charging. Sharp dendrites can grow large enough to pierce battery separation layers, shorting the battery and causing fires.
|The formation of spikey dendrites during charging of lithium metal batteries has limited their appllcations (Image source: Arizona State University)|
Battery researchers around the world are examining electrolyte chemistry, battery charging protocols, increases in battery internal pressure, and coatings on the surface of the lithium metal, all in an effort to inhibit dendrite growth and make lithium metal batteries safer. One group at Arizona State University, under Hanqing Jiang, professor in ASU’s School for Engineering of Matter, Transport and Energy, has published a paper in Nature Energy that describes the effects of stress on the growth of lithium dendrites.
|When lithium metal is deposited onto a rigid surface (the orange surface above), compressive stresses are formed, which cannot be relaxed and dendrites form (Image source: Arizona State University)|
The ASU researchers deposited a layer of lithium metal onto a soft substrate of polydimethylsiloxane (PDMS), more commonly known as the silicon material from which contact lenses and silly putty are made. In a press release published by ASU, Jiang said, “We already know that tiny tin needles or whiskers can protrude out of tin surfaces under stress, so by analogy we looked at the possibility of stress as a factor in lithium dendrite growth.”
|When lithium metal is deposited onto a flexible silicone substrate (in gray above), wrinkles form whcih reduce residual stresses and decrease dendrite growth (Image source: Arizona State University)|
When the lithium metal was deposited on the silicone substrate, the stresses created by the accumulation of the metal were relieved by the formation of wrinkles in the silicon substrate. The elimination of the residual stresses had a large effect on the dendrites. “There were remarkable reductions in dendrite growth,” said Jiang. The research team discovered that the reduction in dendrite growth was directly related to the reduction in stress caused by the deformation and wrinkling of the silicon substrate.
|Plating of the lithium metal onto the silicone (PDMS) substarate causes it to wrinkle in 2 dimensions, reducing the lithium metal residual stress and dendrite formation (Image source: Arizona State University)|
Jiang’s team also came up with a way to increase the life of lithium metal batteries while maintaining their high energy density—by giving the silicone a three dimensional form, almost like a sponge. “The PDMS, which serves as a porous, sponge-like layer, relieves the stress and effectively inhibits dendrite growth,” said Jiang.
The ASU research also has implications for other types of batteries. “Almost all metals used as battery anodes tend to develop dendrites. For example, these findings have implications for zinc, sodium and aluminum batteries as well,” said Jiang.
The low cost of flexible silicone materials and their potential to improve the performance of lithium metal anodes by reducing stress and thus dendrite formation is just one more step in the development of safe, high energy density lithium metal batteries.
Senior Editor Kevin Clemens has been writing about energy, automotive, and transportation topics for more than 30 years. He has set several world land speed records on electric motorcycles that he built in his workshop.