Designing a Pure Lithium Anode
- Tuesday, 05 August 2014
The race is on to design smaller, cheaper, and more efficient rechargeable batteries to meet power storage needs. Now, a team of researchers at Stanford University report that they have taken a big step toward designing a pure lithium anode, which, they say, would greatly advance current lithium ion batteries.
All batteries have three basic components: an electrolyte to provide electrons, an anode to discharge those electrons, and a cathode to receive them. With lithium ion batteries, the lithium is in the electrolyte, but not in the anode. An anode of pure lithium could boost battery efficiency greatly.
Lithium is very lightweight and has the highest energy density, but has major challenges that have made its use in anodes difficult. The researchers say that they have found a way to protect the lithium from the problems that have plagued it for so long.
The research team includes Steven Chu, the former U.S. Secretary of Energy and Nobel laureate who recently resumed his professorship at Stanford. "In practical terms, if we can triple the energy density and simultaneously decrease the cost four-fold, that would be very exciting," Chu said.
However, they face several challenges. During charging, positively charged lithium ions in the electrolyte are attracted to the negatively charged anode and the lithium accumulates on the anode. But, lithium ions exponentially expand as they gather on the anode during charging relative to 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. These fissures on the surface of the anode allow precious lithium ions to escape, forming mossy growths called dendrites, which short-circuit the battery and shorten its life.
Their second engineering challenge involves finding a way to deal with the fact that lithium anodes are 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. Lithium batteries, including those in use today, can overheat to the point of fire, or even explosion.
To solve these problems the Stanford researchers built a protective layer of interconnected carbon domes, called nanospheres, on top of their lithium anode, which creates a flexible, uniform and non-reactive film that protects the unstable lithium from the drawbacks that have made it such a challenge.