X-ray batteries: Extra charges

Electric boost

 
Electric revelations made possible using X-rays

US researchers have used X-ray diffraction and transmission X-ray microscopy to take a closer look at how lithium-sulfur batteries operate. Their observations might facilitate improvements in these short-lived devices and give a much-needed boost to electric vehicles.

Current electric cars, including the likes of the Tesla Model S and the Nissan Leaf use rechargeable lithium-ion batteries, the same class of power supply as you might find in a mobile phone or laptop computer. These batteries are expensive and can account for more than half the cost of an electric vehicle. A promising and potentially less pricey technology is the related lithium-sulfur battery. Li-S batteries have three major benefits when compared to Li-ion batteries. First, they can, in theory at least, store up to five times as much energy as the more familiar battery type, which means longer journeys would be possible on a single charge at a much lower cost or much smaller batteries could be used to get the same distance. Secondly, sulfur is lighter and cheaper per charge unit available than the current materials (such as calcium oxide and manganese oxide) used for cathodes so the batteries weigh less - a 10 centimetre cube of Li-S material would weigh about the same as one litre of water - which means less dead weight in a vehicle. Sulfur is also "earth abundant" and so easy for manufacturers to obtain.

Unfortunately, despite all the advantages of lithium-sulfur technology over the more conventional battery class, there is a major drawback: these batteries fail after a few dozen cycles of charging and discharging, whereas lithium-ion batteries continue to function even after several thousand power cycles.

"The cycle life of lithium-sulfur batteries is very short," explains post-doctoral researcher Johanna Nelson of the SLAC National Accelerator Laboratory at Stanford University, California, USA. "Typically, after a few tens of cycles the battery will die, so it isn't viable for electric vehicles, which require many thousands of cycles over a ten- or twenty-year lifetime." The common format for a lithium-sulfur battery comprises two electrodes - a lithium metal anode and a sulfur-carbon cathode. This setup is embedded in the electrolyte. However, it is thought that unwanted chemical reactions ultimately deplete the cathode of sulfur leading to battery failure after several power cycles.

Nelson and her colleagues have now raised doubts as to whether or not earlier experiments that sought to validate this hypothesis regarding sulfur depletion stand up to closer scrutiny. They have used high-power X-ray microscopy at the Stanford Synchrotron Radiation Lightsource (SSRL, a part of SLAC) to observe the processes taking place in a working battery and discovered, surprisingly, that that the sulfur particles in the cathode seem to remain intact during the process of battery discharge. Writing in the Journal of the American Chemical Society, the team suggests that lithium-sulfur batteries must surely fail via some alternative mechanism and their work could provide important clues as to how that failure might be precluded to extend the life of such batteries to make them viable in electric vehicles.

"Based on previous experiments, we expected sulfur particles to completely disappear from the cathode when the battery discharges," adds Nelson. "Instead, we saw only negligible changes in the size of the particles, the exact opposite of what earlier studies found." X-ray microscopy allowed the team to obtain snapshots on the nanoscale of individual sulfur particles before, during and after discharge. This, the team says, is the first real-time imaging of a lithium-sulfur battery in operation.

"The standard way to do high-resolution imaging is with electron microscopes after the battery has partially discharged," Nelson said. "But electrons don't penetrate metal and plastic very well. With SLAC's X-ray microscope, we can actually see changes that are happening while the battery is running." When functioning normally, a Li-S battery generates a current as lithium ions in the anode react with sulfur particles at the cathode this discharges the battery but leads to production of a lithium polysulfide by-product. If some of the polysulfide leaks into the electrolyte and permanently bonds with the lithium metal anode then it cannot be recycled when the battery is recharged and so is lost forever hence the battery fails after several operations. "You don't want to lose active sulfur material every time the battery discharges. You want a battery that can be cycled multiple times," Nelson adds.

It's the dilithium crystals...

Previous experiments had suggested that dilithium sulfide crystals form during the discharge phase, which leads to a thin insulating film that blocks conduction of electrons and lithium ions causing battery failure. Electron microscopy studies inadvertently led researchers to the conclusion that much of the sulfur is chemically transformed into Li2S-polysulfide sheets that prevented the battery from operating. But, Nelson and her colleagues, suggest that the methodology of those earlier studies was flawed. "Typically, they would cycle the battery, disassemble it, wash away the electrolyte and then analyze it with X-ray diffraction or an electron microscope. But when you do that, you also wash away all of the polysulfides that are loosely trapped on the cathode. So when you image the cathode, you don't see any sulfur species at all," Nelson explains.

By instead using X-ray imaging, the Stanford-SLAC team could see that every particle retained its basic shape and size throughout the discharge cycle. "We expected the sulfur to completely disappear and form polysulfides in the electrolyte," Nelson adds. "Instead we found that, for the most part, the particles stayed where they were and lost very little mass. They did form polysulfides, but most of those were trapped near the carbon-sulfur cathode. We didn't have to disassemble the battery or even stop it, because we could image the sulfur content while the device was operating."

There was another surprise from the X-ray data. "Based on previous experiments, we expected that crystalline dilithium sulfide would form at the end of the discharge cycle," Nelson says. "But we did a very deep discharge and never saw any of this material in its crystalline state." The work suggests that the "polysulfide problem" may not be the insurmountable barrier earlier researchers first thought and so might allow Li-S technology to be developed more rapidly to take advantage of its many benefits.

"The next step is to use what we understand from the X-ray studies to design nanoscale materials to overcome the issues related to sulfur cathodes and to realize long cycle life, team leader Yi Cui told SpectroscopyNOW.