A team from the Stanford School of Engineering has made a breakthrough in lithium-ion battery design that is expected to result in more stable batteries for electric vehicles (EVs) and significantly reduce their cost. By taking inspiration from the structure of honeycomb, the team has resolved the issue of dendrite formation on the batteries, a mossy-looking leak of lithium ions that reduces efficiency and can pose a fire hazard. The result is a lithium-anode battery that could give a range of up to 300 miles and reduce the cost of an EV with that kind of range down to around $25,000.
Here’s the problem with current designs of EV lithium-ion batteries: the lithium is found in the battery’s electrolyte and expands during charging, making its surface uneven after repeated recharging. Lithium ions that are attracted to the battery’s anode can then escape through the gaps created by the uneven surface and “leak” out, creating a fibrous mass known as dendrite. Eventually the tiny dendrite fibers spread between the cathode and anode, which can cause the battery to short circuit and even catch on fire. Ideally, the battery design would also have a lithium anode, which would be much more efficient than the silicon or graphite currently used. Unfortunately, that would only compound the dendrite problem, as well as producing a lot of heat and causing the anode to poach lithium from the electrolyte.
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The Stanford team have resolved the issues of a lithium anode by covering it in a honeycomb of carbon nanospheres. It takes 5,000 layers of the carbon to make a 20-nanometer thick layer, but that is all that is necessary to keep the lithium’s surface smooth as it expands and contracts during charging and discharging. Tests have shown the battery performs at 99 percent efficiency over 150 usage cycles.
While the new design is a significant breakthrough, it’s not market ready yet. A commercial EV lithium-ion battery needs to achieve 99.9 percent efficiency over the long term, which is why we are still using graphite and silicon anodes. Previous test models of lithium anodes have started off with a bang – achieving 99.6 percent efficiency – but have dwindled down to 50 percent efficiency fairly quickly. That the new carbon honeycomb covering seems to have stabilized the efficiency is the key detail of the current breakthrough. It now remains for the team to experiment with the electrolyte composition to see if they can raise efficiency to marketable levels.
Via Clean Technica
Photos by Tesla Motors; and Kevin Krejci via Flickr
