A research team from the Stanford School of Engineering has just figured out how to stabilize the lithium in a lithium-ion battery, and that could help bring the typical EV down to the level of mainstream affordability. The team is looking at a price point of $25,000 for an EV battery range of 300 miles, which would be competitive with a 40 mpg gasmobile.
The dream of extending EV battery range usually comes with a high price tag (here and here, for example), so the idea that longer range could actually bring down costs is of particular interest, especially considering that former Energy Secretary Steven Chu is a member of that Standford research team.
Honeycomb (cropped) by prilfish.
Dendrites Are Attacking The Ship, Captain!
We love the word dendrite because it sounds like one of those evil alien creatures from Star Trek’s cheap season (oh wait, they were all cheap seasons), but in real life dendrites really are evil, at least for Li-ion batteries.
Our sister site Gas2.org took note of Chu’s interest in solving the dendrite problem back in 2008, before he embarked on a detour as Energy Secretary.
Now that Chu is back at his former position with Stanford Engineering, he has joined with research team leader Yi Cui and lead author Guangyuan Zheng in a paper published online yesterday in Nature Nanotechnology, titled “Interconnected hollow carbon nanospheres for stable lithium metal anodes,” which zeroes in on the dendrite problem.
Dendrites refers to those hairy mossy fibers that can grow out of your Li-ion battery over time. They are associated with decreased efficiency as well as safety risks. Our friends over at Lawrence Berkeley Laboratory offer a good rundown (break added for clarity):
Over the course of several battery charge/discharge cycles, particularly when the battery is cycled at a fast rate, microscopic fibers of lithium, called “dendrites,” sprout from the surface of the lithium electrode and spread like kudzu across the electrolyte until they reach the other electrode.
An electrical current passing through these dendrites can short-circuit the battery, causing it to rapidly overheat and in some instances catch fire.
The kudzu reference is to that weedy vine that has been swallowing up the southeastern US, btw.
Three Problems For Li-ion EV Battery Range
For those of you new to the subject, a Li-ion battery has two electrodes, an anode and a cathode. As the battery is discharging, electrons move from the anode to the cathode through a solution (or a solid) called an electrolyte.
The lithium in a Li-ion battery is in the electrolyte, not the anode or the cathode. A typical Li-ion battery has an anode made of graphite or silicon. Lithium would be a far more efficient choice, but that’s where the dendrite problem comes in.
Lithium expands during charging, to a degree far more extreme than silicon or graphite. That leaves an uneven surface. Lithium ions that are attracted to the anode from the electrolyte escape through pits and cracks. The result is a mossy growth, aka dendrites.
Dendrites aren’t the only problem, though. As described by Stanford, extending Li-ion EV battery range also depends on improving management of the chemical reaction between a lithium anode and the electrolyte, in order to prevent the anode from “using up” the electrolyte.
A third, related issue involves managing the heat that results from the interaction between a lithium anode and an electrolyte.
The Honeycomb Solution
Standford’s solution is a “honeycomb” of interlocking carbon nanospheres layered on to the lithium anode. At 20 nanometers thick you’d need 5,000 layers of this nano-comb to equal the width of a human hair, but it seems to have gotten the job done.
The honeycomb structure is flexible enough to stabilize the anode surface as it expands during charging, and also while it contracts during discharge.
According to Stanford, the results so far look promising. In tests the new lithium anode reached 99 percent efficiency over the course of 150 charge/discharge cycles.
The figure of 99 percent is significant because a marketable battery needs to be 99.9 percent efficient over numerous charging cycles (that’s Coulombic efficiency for those of you keeping score at home). Earlier attempts at lithium anodes have petered out pretty quickly, attaining a top mark of 99.6 percent at the beginning and dropping down to 50 percent efficiency after just 100 cycles.
In batteryspeak that figure of 99.6 percent is significantly lower than the desired 99.9 percent. By the same token Standford’s achievement of 99 percent over 150 cycles is pretty impressive and it comes a lot closer to commercial viability, but it does fall short of the desired 99.9 percent mark.
The next steps for Standford include tinkering around with the electrolytes, perhaps trying out some new ones.
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