Silicon Nanowire Batteries, Take Two: The “Core Shell” Approach
Since the late 1800s, the primary impediment to the adoption of electric vehicles has been battery technology. And while the technology has advanced by leaps and bounds in the last decade or two (compare your cell phone with one from the early 90s), with a threefold improvement in energy density and more than an order of magnitude improvement in power density, it still lags behind gasoline.
Some have argued that current technology is sufficient — that the ability to drive 1 1/2 hours to 3 hours nonstop is good enough for the overwhelming majority of trips, and that paired with a range extender, rapid chargers, or battery swapping, you have a viable means of replacing the gasoline car. However, there still is a great deal of pressure to get electric vehicle range up to that of gasoline.
For those who remember the original research, Dr. Cui’s team discovered that by using crystalline silicon nanowires in place of the conventional graphite anode, the anode could hold ten times more lithium than it normally could. Silicon also offers the advantage of having almost no side-reactions with the electrolyte, which are what limit shelf life in li-ion batteries. For his research, Dr. Cui received a Global Research Partnership (GRP) grant from King Abdullah University of Science and Technology (KAUST).
Silicon’s ability to absorb huge amounts of lithium has long been known, but it’s always had a fundamental problem: it absorbs so much that it swells, cracks, and pulverizes itself, becoming useless in short order. While the nanowires proved more resistant, they still went down to 8x capacity after just the first charge cycle.
While the original technology had promise, and research continues to be ongoing, the cycle life of the nanowires has led to research into alternate nanowire chemistries. One that was recently published was that of “core shell” nanowires, wherein there’s a crystalline nanowire core to conduct the electrons and an amorphous surface, which has better stability. This comes at the cost of only a 3x improvement in energy density over graphite. At this point in the research, they’re already up to 90% capacity retention after 100 cycles.
While this may not sound like much, factor in the fact that the rate of capacity loss drops off dramatically over time and the fact that increased capacity greatly offsets capacity loss. The larger the capacity, the fewer cycles the pack needs to perform to go the same distance, and at the same time, the less of the cell’s maximum capacity is drawn in a given amount of time to provide the needed amount of power. The net result is that for a gain this large, you don’t need a very long cycle life. The same applies to price; if it costs the same to produce but yields three times the energy density, the cost per watt-hour is 1/3rd as great.
The paper also mentions that this is with a very fast cycle of about seven minutes, which obviously invites comparisons to AltairNano’s “Nano-titanate” cells. However, that’s about where the comparison ends, for while AltairNano’s cells achieve 70Wh/kg energy density, and normal graphite cells achieve up to 180Wh/kg, the core shell anodes achieve about three times better than those of even graphite cells.
Note that to achieve such a density gain in total, you also need to advance the energy density for the cathode. Technologies for this include Argonne Laboratories’ composite Li2MnO3/LiMO2 or LiM2O4 cathodes, nanocomposite metal fluoride cathodes, various cathodes from Actacell, one from GM, and a LiMn2O4 nanorod cathode. Other competitors on the anode side include graphite-encased tin nanoparticles (tin is nearly as good of a lithium absorber as silicon), LVO, silicon monoxide with silicon nanoparticles, carbon nanotubes with silicon nanoparticles, carbon nanowires coated with silicon carbide, GM’s MgH metal hydride anode, and a porous silicon nanostructure anode.
With so many techs promising 2 to 10-fold increases in density for their respective battery component, it seems ever unlikely that lithium-ion battery technology will be stagnating any time soon. Quite to the contrary, the pace seems to be picking up. When it comes to range, gasoline may soon get a run for its money.
Image credit: Yi Cui