Out of all the different types of battery chemistries and architectures that have been proposed, perhaps the most interesting one in the short term is solid-state electrolytes. The lithium-ion batteries we use in our consumer goods does the job well, but in many applications, higher density is required and also higher levels of safety, especially in use cases that might involve the human body. The one flaw with Li-ion technology is that it can combust if mishandled, or even in some cases spontaneously. The electrolyte used in Li-ion batteries contains both the fuel and the oxygen necessary to sustain a fire. Li-ion fires can be cause by a short circuit, discharging too quickly, overcharging, defects, bad design or damage. When one cell catches fire, others next to it can also overheat and catch fire. These fires can not be treated like normal fires. There have been several occasions where the fires have gotten out of control, including one occasion last month on a cargo ship, the Felicity Ace, which was carrying a cargo of luxury vehicles at the time.
Solid-state batteries are much safer, and can be manufactured without a protective shell that is often used for Li-ion batteries when they have to be used in applications where a danger may occur, such as in implants. They also provide a higher energy density. However, there have been some challenges making solid-state batteries suitable for large scale usage, such as their tendency to be brittle. Researchers from the University of Waterloo, Canada, who are members of the Joint Center for Energy Storage Research (JCESR), headquartered at the U.S. Department of Energy’s (DOE) Argonne National Laboratory, have now developed a new solid electrolyte that has several important advantages.
The new electrolyte is composed of lithium, scandium, indium and chlorine, and it is capable of conducting lithium ions well, but electrons poorly. This combination is necessary for a solid-state battery that functions without significantly losing capacity for over a hundred cycles at over 4 volts and thousands of cycles at intermediate voltage. The chloride in the electrolyte is key to keeping it stable above 4 volts, making it suitable for use with typical cathode materials that form the mainstay of today’s lithium-ion cells.
Current solid-state electrolytes heavily use sulphides, which can oxidize and degrade above 2.5 volts. They require an insulating coating around the cathode material that operates above 4 volts, which impairs the ability of electrons and lithium ions to move from the electrolyte and into the cathode. “With sulphide electrolytes, you have a kind of conundrum — you want to electronically isolate the electrolyte from the cathode so it doesn’t oxidize, but you still require electronic conductivity in the cathode material,” Linda Nazar, a Distinguished Research Professor of Chemistry at UWaterloo and a long-time member of JCESR.
Half of the indium is swapped out for scandium to provide lower electronic and higher ionic conductivity. “Chloride electrolytes oxidize only at high voltages, and some are chemically compatible with the best cathodes we have,” Nazar said. “There’s been a few of them reported recently, but we designed one with distinct advantages.” One chemical key to the ionic conductivity lay in the material’s criss-crossing 3D structure called a spinel. The researchers had to load the spinel with as many charge carrying ions as possible, but also to leave sites open for the ions to move through. The ideal scenario would be having half the sites in the spinel structure be lithium occupied while the other half remained open, but that is hard to design.
Nazar said that it is not yet clear why the electronic conductivity is lower than many previously reported chloride electrolytes, but it helps establish a clean interface between the cathode material and solid electrolyte, a fact that is largely responsible for the stable performance even with high amounts of active material in the cathode.
A paper based on the research, “High areal capacity, long cycle life 4 V ceramic all-solid-state Li-ion batteries enabled by chloride solid electrolytes,” appeared in the January 3 online edition of Nature Energy. Other authors of the paper include Nazar’s graduate student, Laidong Zhou, a JCESR member who was responsible for the majority of the work, and Se Young Kim, Chun Yuen Kwok and Abdeljalil Assoud, all of UWaterloo. Additional authors included Tong-Tong Zuo and Professor Juergen Janek of Justus Liebig University, Germany and Qiang Zhang of the DOE’s Oak Ridge National Laboratory. The research was funded by the DOE’s Office of Science, Office of Basic Energy Sciences with some support from Canada’s National Sciences and Engineering Research Council.
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