jeudi 21 novembre 2013

Nanowires and Viruses Combine to Create High-Capacity Batteries

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While lithium-ion (Li-ion) batteries have managed to elevate hybrid vehicles into a significant segment of the automotive market, all-electric vehicles using that same battery technology have languished as a niche product. Li-ion batteries justdon’t have the long charge life or short recharging capabilities that would make all-electric vehicles a good fit for most people’s driving habits. (It also doesn’t help the marketing image of automakers when theseall-electric vehicles burst into flames.)
Marketing considerations aside, the recent demise of a few high-profile nanotech companies that tried to make Li-ion batteries for all-electric vehicles may indicate fundamental problems with the current approach. Maybe we need a different battery technology to make all-electric vehicles mainstream.
This realization already began to sink in with the research communities a few years back, and I started noticing more research into Lithium-air batteries.  Lithium-air batteries use the oxidation of lithium at the anode and the reduction of oxygen at the cathode to create a current. This battery design has promised up to 10 times as much energy as a lithium-ion battery at the same weight, giving them an energy density equal to that of gasoline.
Greater storage capacity certainly helps solve one EV limitation, but if a battery technology is going to displace fossil fuels, it also needs to recharge as quickly as—or on par with—filling up a car at gas station. Earlier this year, researchers at Massachusetts Institute of Technology (MIT) and Sandia National Laboratory peered into the lithium oxidation process of Li-air batteries and saw a possible way to speed up battery recharging.
Now researchers—again from MIT—have made progress on both fronts. They took nanowires made of manganese oxide (which is often used as the material for the cathode of Li-air batteries) and coated them with man-made viruses. These viruses create a larger surface area that improves the reaction between the lithium and oxygen, resulting in a greater storage capacity and a faster rate of recharging. A video describing the technology can be seen below.
The combination of man-made viruses and nanostructures has all the earmarks of Angela Belcher’s work.  And indeed, Belcher and four others are the authors of the research, which appears in the journal Nature Communications ("Biologically enhanced cathode design for improved capacity and cycle life for lithium-oxygen batteries").
This is not the first time that Belcher has applied her virus-based work to the problem of improving batteries. A few years back, she proposed something similar for use in Li-ion batteries. It would seem that Li-ion batteries have lost some favor even in this line of research.
Of course, this is just preliminary research and only addresses the material used in the cathode. However, the potential commercial success of the technology will really come down to its energy density in comparison to fossil fuels and the speed of recharging.
The MIT researchers offer some insight into these figures by estimating that their virus-enabled Li-air battery will have a two to three times better storage capacity than Li-ion batteries. This still may be somewhat short of the energy density needed to compete with fossil fuels.
The researchers don’t provide the exact storage capacity they used for calculating that their improvements. But an average Li-ion battery today has an energy density around 200 Wh/kg. Doubling or even tripling of that (based on doubling or tripling the storage capacity which can vary from the energy density) would still only bring it to around 400-600 Wh/kg, far short of the 1000 Wh/kg needed to be competitive with fossil fuels.
This is important research for an application with strong market pull, but it is still just the initial steps for finding a way to replace the internal combustion engine. Before years are spent on realizing its full potential, we should be sure that we aren't chasing after a technology that even at its best just won't be good enough.
Image, Video: MIT

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