US researchers have taken a big step toward accomplishing what battery designers have been trying to do for decades - design a pure lithium anode that would be a huge boost to battery efficiency.
All batteries have three basic components: an electrolyte to provide electrons, an anode to discharge those electrons, and a cathode to receive them.
"Of all the materials that one might use in an anode, lithium has the greatest potential. Some call it the Holy Grail," said Yi Cui, a professor of Material Science and Engineering at Stanford University and leader of the research team.
"It is very lightweight and it has the highest energy density. You get more power per volume and weight, leading to lighter, smaller batteries with more power," Cui said.
"Lithium has major challenges that have made its use in anodes difficult. Many engineers had given up the search, but we found a way to protect the lithium from the problems that have plagued it for so long," said Guangyuan Zheng, a doctoral candidate in Cui's lab and first author of the study.
Lithium ions expand as they gather on the anode during charging. All anode materials, including graphite and silicon, expand somewhat during charging, but not like lithium.
Researchers said that lithium's expansion during charging is "virtually infinite" relative to the other materials.
Its expansion is also uneven, causing pits and cracks to form in the outer surface, like paint on the exterior of a balloon that is being inflated.
The resulting fissures on the surface of the anode allow the precious lithium ions to escape, forming hair-like or mossy growths, called dendrites. Dendrites, in turn, short circuit the battery and shorten its life.
Preventing this buildup is the first challenge of using lithium for the battery's anode.
The second engineering challenge is that a lithium anode is highly chemically reactive with the electrolyte. It uses up the electrolyte and reduces battery life.
An additional problem is that the anode and electrolyte produce heat when they come into contact.
To solve these problems the Stanford researchers built a protective layer of interconnected carbon domes on top of their lithium anode. This layer is what the team has called nanospheres.
The Stanford team's nanosphere layer resembles a honeycomb: it creates a flexible, uniform and non-reactive film that protects the unstable lithium from the drawbacks that have made it such a challenge.
The nanosphere layer is made of amorphous carbon, which is chemically stable, yet strong and flexible so as to move freely up and down with the lithium as it expands and contracts during the battery's normal charge-discharge cycle.
The study is published in the journal Nature Nanotechnology.
