Battery capacity remains a big issue in devices ranging from phones to electric vehicles – and one of the biggest constraints is the materials used to make electrodes.
A paper published at Nature Nanotechnology (abstract here), offers a promising lead for improvements as boffins say they've hit on a way to replace today's anode materials with lithium.
Today's Li-ion batteries store charge by accumulating the positively-charged ions onto an anode made from materials like graphite or silicon. The researchers say if the anode could be made of lithium, it would make batteries lighter while giving them greater energy density.
As noted in the paper, lithium has a specific capacity of 3,860 mAh g-1, which tops the available metals, and has a low anode potential, but its physical properties have prevented it from being used as in anodes.
“The lithium anode forms dendritic and mossy metal deposits, leading to serious safety concerns and low Coulombic efficiency during charge/discharge cycles,” they write.
The dendrites cut a battery's efficiency and shorten its life, so the work, led by Stanford professor Yi Cui, has focussed on preventing their growth.
To do that, the researchers coated a lithium anode with a layer of hollow carbon nanospheres, to prevent the growth of the dendrites.
As the Stanford release – here at Science Codex – explains, the nanosphere layer is 20 nanometres thick and forms a honeycomb-like protective surface over the unstable lithium anode.
The carbon is chemically stable, so it doesn't react with the electrolyte, and it's flexible enough to expand and contract during the battery's charge cycle.
By providing a protective layer, contact between the anode and electrolyte (which causes heat and sometimes battery fires) is minimised, improving battery safety.
So far, the researchers say the Coulombic efficiency of the battery – the ratio of input charge to output – is over 99 per cent at 150 cycles, but their ultimate target is 99.9 per cent.
"Of all the materials that one might use in an anode, lithium has the greatest potential. Some call it the Holy Grail," comments professor Cui. ®