A new biologically inspired battery membrane enables a battery with five times the capacity of the industry standard lithium ion design to run more than 1000 cycles required to power electric vehicles. A team from the University of Michigan showed that the aramid nanofiber network recovered from Kevlar fibers can enable lithium sulfur batteries to overcome the fatal weakness of their cycle life - the number of times it can be charged and discharged.
Nicholas Kotov, distinguished professor of chemical science and engineering at Owen Langmuir University, who led the study, said: "many reports claim that lithium sulfur batteries have hundreds of cycles, but this is achieved at the expense of other parameters - capacity, charging rate, resilience and safety." Today's challenge is to make a battery that increases the cycle rate from the previous 10 cycles to hundreds of cycles and meets a variety of other requirements, including cost. The bionic engineering of these batteries integrates two scales - molecular and nano scale. For the first time, we integrated the ion selectivity of cell membrane and the toughness of cartilage. Our integrated systems approach enables us to address the overall challenges of lithium sulfur batteries.
Previously, his team had relied on the aramid nanofiber network injected with electrolyte gel to prevent one of the main reasons for short cycle life: the branches from one electrode to another electrode pierced the membrane. The toughness of aramid fiber prevents the emergence of dendrites.
But lithium sulfur batteries have another problem: small molecules of lithium and sulfur form and flow to lithium, adhere to themselves and reduce the capacity of the battery. The membrane needs particles that allow lithium ions to flow from lithium to sulfur and return - and block lithium and sulfur, called lithium polysulfide. This ability is called ion selectivity.
Ahmed EmrE, a postdoctoral researcher in chemical engineering and co-author of the paper in nature communication, said: "Inspired by the biological ion channel, we designed a highway for lithium ion, in which lithium polysulfide cannot pass through the toll station,"
Lithium ion and lithium polysulfide are similar in size, so it is not enough to make small channels to block lithium polysulfide. Mimicking the pores of biofilms, researchers at the University of Massachusetts added charges to the pores of battery membranes.
They do this by using lithium polysulfide itself. They stick to aramid nanofibers, and their negative charges repel the lithium polysulfide ions that continue to form on the sulfur electrode. However, positively charged lithium ions can pass freely.
"Achieving a record level of multiple parameters for multiple material properties is what car batteries need now. It's a bit like Olympic gymnastics - you have to be perfect in all aspects, including the sustainability of its production," Kotov said
As a battery, Kotov said the design was "almost perfect" and its capacity and efficiency were close to theoretical limits. It can also deal with the extreme temperatures in car life, from the heat of charging under full day light to the cold in winter. However, the real-world cycle life may be shortened by fast charging, more like 1000 cycles, he said. This is considered a ten-year life span.
In addition to higher capacity, lithium sulfur batteries have sustainability advantages over other lithium-ion batteries. Sulfur is much more abundant than cobalt in lithium ion electrode. In addition, the aramid fiber of the battery membrane can be recycled from the old bulletproof back core.
