Stanford postdoctoral researcher Chao Wang holds a container of self-healing polymer that can be applied to silicon electrodes to keep them from cracking and falling apart during battery operation. (Brad Plummer/SLAC)
Using a novel flexible polymer, researchers have successfully created a battery electrode that is self-healing; a breakthrough which could have major implications for the next generation of electric cars.
This prototype lithium ion battery, made in a Stanford lab, contains a silicon electrode protected with a coating of self-healing polymer. The cables and clips in the background are part of an apparatus for testing the performance of batteries during multiple charge-discharge cycles. (Brad Plummer/SLAC)
Currently, one of the most widely used materials for battery electrodes is silicon, due to its ability to take in a large quantity of lithium ions as the battery is charging and release all of these whilst the battery is at work.
Unfortunately, silicon is a relatively brittle material with a tendency to crack as it expands and contracts, thus degrading the battery’s performance quickly.
Now, in a scientific first, the scientists from Stanford University and the Department of Energy’s (DOE) SLAC National Accelerator Laboratory have coated a silicon electrode with a self-healing polymer to create a longer lasting lithium ion battery.
Left: An electron micrograph shows cracks left in a self-healing polymer coating due to swelling of its silicon electrode during charging. Right: Five hours later, the smaller cracks have healed. (C. Wang et al, Nature Chemistry)
The polymer itself was created via purposely weakening some of the chemical bonds in the polymer chain, whilst nanoparticles of carbon were also added to the polymer to increase its conductivity. So although this polymer material breaks apart quite easily, it is drawn back together via chemical attraction in a similar way to DNA. This means that the coating helps to bind the silicon electrode together for longer, healing small cracks that appear in the electrode.
Stanford Professor Zhenan Bao summarizes the findings below:
"We found that silicon electrodes lasted 10 times longer when coated with the self-healing polymer, which repaired any cracks within just a few hours,".
From left: Stanford Professor Zhenan Bao; postdoctoral researcher Chao Wang, holding a container of self-healing polymer; and Stanford/SLAC Associate Professor Yi Cui. (Brad Plummer/SLAC)
This new technique could help herald in a new age of electric vehicles with the ability to drive for hundreds of miles without the need for charging. It could also be extremely beneficial for smartphone users, as the new technology would lead to much longer lives for phone batteries. Further applications may also be found in the solar cell and semiconductor industries.
The electrodes did not lose any significant energy capacity over 100 charge-discharge cycles, which Yi Cui, an associate professor at SLAC and Stanford, believes is a positive sign.
"That's still quite a way from the goal of about 500 cycles for cell phones and 3,000 cycles for an electric vehicle, but the promise is there, and from all our data it looks like it's working."
Stanford postdoctoral researcher Chao Wang holds a solid piece of the stretchy, self-healing polymer used to coat and protect silicon battery electrodes. (Brad Plummer/SLAC)
Original source: DOE/SLAC National Accelerator Laboratory
Citation: C. Wang et al., Nature Chemistry, 17 October 2013 (10.1038/nchem.1802)