Inside a lithium-ion battery, you might not want to keep everything neat and tidy; a little bit of disorder may improve its performance, according to new research.
Engineers meticulously arrange materials in rigid patterns in typical lithium-ion cathodes and anodes, the idea being that a more structured arrangement yields a more efficient battery. In the pursuit of greater energy densities, better performance and longevity, designers are seeking ways to structure battery components more rigorously and at smaller scales.
Letting material layers blend or lose their shape often corresponds to weaker battery performance, a situation that arises as these devices age. The lithium ions have a harder time moving through the cathode and thereby deliver less energy.
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"This understanding and experimental evidence has generally led battery scientists to overlook disordered materials," explained Jinhyuk Lee, a research assistant in materials science and engineering at the Massachusetts Institute of Technology.
However, he found that in some circumstances, disruption could have performance benefits. Lee and his collaborators published their findings last week in the journal Science.
"In this paper, we found a particular composition that when it becomes disordered, it can still have a good performance," said co-author Dong Su, a staff scientist in the electron microscopy group at Brookhaven National Laboratory.
Researchers used a blend of lithium, chromium and molybdenum for the cathode, initially alternating between lithium and transition metal oxide layers. After charging and discharging the battery a few times, they found the performance remained steady.
Path to higher energy density
The researchers then opened up the batteries, looking at them after different numbers of cycles using electron microscopes and X-ray diffraction. They found that the layers degraded and randomly mixed, and after 10 cycles, the layers were almost gone.
Nonetheless, the disordered cathode yielded an energy density of 660 watt-hours per kilogram at 2.5 volts, a result rarely observed even in perfectly structured devices.
What made this battery design work so well with a disordered structure compared to previous designs was the presence of excess lithium. Conventional lithium-ion batteries use equal amounts of lithium and a given transition metal. With the new formula, the extra lithium finds its own path through the cathode, creating faster channels for the ion and delivering a more consistent performance.
The disordered material also has other advantages over structured cathodes. According to Lee, the new cathode is highly stable in its volume, barely expanding or contacting as it cycles through charges and discharges. Despite the randomness, lithium ends up diffusing through the cathode more uniformly, as well.
This has implications for designing battery components, as what was once a menacing phenomenon can now become a useful trait over the life of a cell. "It's not until you get to 500 or 1,000 cycles or more that it's a useful battery material," said Eric Stach, the group leader for the electron microscopy group at Brookhaven, who was not involved in this research. "The bigger kind of outlook on this is now you have some reason to think that you would even want to do that."
"Our biggest accomplishment here is we first proposed a lithium diffusion mechanism" for disordered batteries, Lee said, adding that the findings open up a new class of potential cathode materials for lithium-ion batteries.
Researchers said they will now try running the cells for more cycles and investigate other materials that still perform well with disordered structures. "We want to answer the practical questions," Su said. "That's what we're driving for: to make these things useful."
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500
