The University of Michigan has developed a new lithium-sulfur battery that can cycle 1,000 times

According to foreign media reports, the research team at the University of Michigan found that the aramid nanofiber network recovered from Kevlar fiber can solve the problem of short cycle life of lithium-sulfur batteries, providing about 1,000 actual cycles.

(Image source: University of Michigan)

This battery diagram shows how lithium ions return to the lithium electrode, while lithium polysulfide cannot pass through the electrode diaphragm. In addition, the spike-like dendrites that grow from the lithium electrode do not pierce the film and reach the sulfur electrode, thus shorting the battery.

Nicholas Kotov, a professor of chemical sciences and engineering who led the study, said: "Many reports claim that lithium-sulfur batteries can achieve hundreds of cycles, but affect other performance parameters such as capacity, charge rate, elasticity and safety. The challenge, therefore, is to produce batteries that can greatly increase the number of cycles (from 10 to 100) while also meeting requirements such as cost.

The biomimetic engineering of these batteries integrates two scales: the molecular scale and the nanoscale. For the first time, we have integrated the ion selectivity and cartilage toughness of battery separators. Our integrated system approach addresses the overall challenges of lithium-sulfur batteries. ”

One of the main reasons for the short cycle life of the battery is the dendrite puncture of the diaphragm by the electrode growth. Previously, Kotov's team relied on a network of aramid nanofibers injected with electrolyte gels to solve this problem, because the toughness of the aramid fibers prevents dendrites. However, lithium-sulfur batteries have other problems, that is, small molecules of lithium and sulfur form and flow to lithium, thus attaching themselves and reducing the battery capacity. This diaphragm needs to allow lithium ions to flow from lithium to sulfur and return, as well as block lithium and lithium polysulfide. This ability is called ion selectivity.

Ahmet Emre, a chemical engineering postdoctoral fellow, co-first author of the paper, said: "Inspired by biological ion channels, we designed high-speed channels through which lithium ions can pass quickly, while lithium polysulfide cannot. ”

Lithium ions and lithium polysulfide are similar in size, so it is not enough to block lithium polysulfide by making only small channels. Researchers at the University of Michigan also mimic pores in biofilms, adding charges to the pores of battery membranes. The method they used was to adhere the lithium polysulfide itself to the aramid nanofibers, so that the negative charge repels the continuous formation of lithium polysulfide ions on the sulfur electrode. However, positively charged lithium ions can pass freely.

Kotov said the battery design is "nearly perfect," with capacity and efficiency approaching theoretical limits. The battery can also cope with the extreme temperatures in car life, from the charging heat full of sunshine to the cold of winter. However, after fast charging, the cycle life of the battery in the real world may become shorter, about 1000 times, that is, ten years.

In addition to higher capacity, lithium-sulfur batteries have a sustainability advantage over other lithium-ion batteries. Sulfur is more abundant than lithium ion electrode cobalt. In addition, the aramid fibers of battery separators can be recycled from old bulletproof vests.