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<\/p>\nJune 22, 2020<\/p>\n
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A new lithium-based electrolyte invented by Stanford University scientists could pave the way for the next generation of battery-powered electric vehicles.<\/p>\n
In a study<\/a> published June 22 in Nature Energy<\/em>, Stanford researchers demonstrate how their novel electrolyte design boosts the performance of lithium metal batteries, a promising technology for powering electric vehicles, laptops and other devices.<\/p>\n \u201cMost electric cars run on lithium-ion batteries, which are rapidly approaching their theoretical limit on energy density,\u201d said study co-author Yi Cui<\/a>, professor of materials science and engineering and of photon science at the SLAC National Accelerator Laboratory<\/a>.\u00a0\u201cOur study focused on lithium metal batteries, which are lighter than lithium-ion batteries and can potentially deliver more energy per unit weight and volume.\u201d<\/p>\n Lithium-ion batteries, used in everything from smartphones to electric cars, have two electrodes \u2013 a positively charged cathode containing lithium and a negatively charged anode usually made of graphite. An electrolyte solution allows lithium ions to shuttle back and forth between the anode and the cathode when the battery is used and when it recharges.<\/p>\n PhD candidates and lead authors Hansen Wang, left, and Zhiao Yu, right, testing an experimental cell in their laboratory. (Image credit: Hongxia Wang.)<\/span><\/p>\n<\/div>\n A lithium metal battery can hold about twice as much electricity per kilogram as today\u2019s conventional lithium-ion battery. Lithium metal batteries do this by replacing the graphite anode with lithium metal, which can store significantly more energy.<\/p>\n \u201cLithium metal batteries are very promising for electric vehicles, where weight and volume are a big concern,\u201d said study co-author Zhenan Bao<\/a>, the K.K. Lee Professor in the School of Engineering<\/a>. \u201cBut during operation, the lithium metal anode reacts with the liquid electrolyte. This causes the growth of lithium microstructures called dendrites on the surface of the anode, which can cause the battery to catch fire and fail.\u201d<\/p>\n Researchers have spent decades trying to address the dendrite problem.<\/p>\n \u201cThe electrolyte has been the Achilles\u2019 heel of lithium metal batteries,\u201d said co-lead author Zhiao Yu, a graduate student in chemistry. \u201cIn our study, we use organic chemistry to rationally design and create new, stable electrolytes for these batteries.\u201d<\/p>\n For the study, Yu and his colleagues explored whether they could address the stability issues with a common, commercially available liquid electrolyte.<\/p>\n \u201cWe hypothesized that adding fluorine atoms onto the electrolyte molecule would make the liquid more stable,\u201d Yu said. \u201cFluorine is a widely used element in electrolytes for lithium batteries. We used its ability to attract electrons to create a new molecule that allows the lithium metal anode to function well in the electrolyte.\u201d<\/p>\n The result was a novel synthetic compound, abbreviated FDMB, that can be readily produced in bulk.<\/p>\n \u201cElectrolyte designs are getting very exotic,\u201d Bao said. \u201cSome have shown good promise but are very expensive to produce. The FDMB molecule that Zhiao came up with is easy to make in large quantity and quite cheap.\u201d<\/p>\n The Stanford team tested the new electrolyte in a lithium metal battery.<\/p>\n The results were dramatic. The experimental battery retained 90 percent of its initial charge after 420 cycles of charging and discharging. In laboratories, typical lithium metal batteries stop working after about 30 cycles.<\/p>\n The researchers also measured how efficiently lithium ions are transferred between the anode and the cathode during charging and discharging, a property known as \u201ccoulombic efficiency.\u201d<\/p>\n \u201cIf you charge 1,000 lithium ions, how many do you get back after you discharge?\u201d Cui said. \u201cIdeally you want 1,000 out of 1,000 for a coulombic efficiency of 100 percent. To be commercially viable, a battery cell needs a coulombic efficiency of at least 99.9 percent. In our study we got 99.52 percent in the half cells and 99.98 percent in the full cells; an incredible performance.\u201d<\/p>\n For potential use in consumer electronics, the Stanford team also tested the FDMB electrolyte in anode-free lithium metal pouch cells \u2013 commercially available batteries with cathodes that supply lithium to the anode.<\/p>\n \u201cThe idea is to only use lithium on the cathode side to reduce weight,\u201d said co-lead author Hansen Wang, a graduate student in materials science and engineering. \u201cThe anode-free battery ran 100 cycles before its capacity dropped to 80 percent \u2013 not as good as an equivalent lithium-ion battery, which can go for 500 to 1,000 cycles, but still one of the best performing anode-free cells.\u201d<\/p>\n \u201cThese results show promise for a wide range of devices,\u201d Bao added. \u201cLightweight, anode-free batteries will be an attractive feature for drones and many other consumer electronics.\u201d<\/p>\nLithium-ion vs. lithium metal<\/h2>\n
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<\/a><\/p>\nNew electrolyte<\/h2>\n
\u2018Incredible performance\u2019<\/h2>\n
Anode-free battery<\/h2>\n
Battery500<\/h2>\n