Stanford University researchers have designed a pure lithium anode, the so-called “holy grail” of battery design.
“Of all the materials that one might use in an anode, lithium has the greatest potential. Some call it the Holy Grail,” explained study leader Yi Cui, a professor of Material Science and Engineering. “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.”
For the longest time, achieving this particular battery design seemed all but impossible.
According to Steven Chu, the former U.S. Secretary of Energy who now teaches at Stanford, a pure lithium anode would allow for longer-lasting cell phones and electric cars.
Researchers would like to utilize lithium for the anode, but so far they have not been able to accomplish this due to the fact that lithium ions expand as they collect on the anode during charging. Lithium’s expansion is also uneven, causing cracks to develop in the outer surface. These cracks allow lithium ions to escape, creating dendrites. These dendrites short circuit the battery and shorten its life.
This is the first of two problems complicating the development of a pure lithium anode. The second problem is that a lithium anode is highly chemically reactive with the electrolyte. Besides these two primary problems, lithium batteries can also overheat and cause a fire or even explosion.
To overcome these issues the researchers constructed a protective layer (they call nanospheres) of interconnected carbon domes on top of their lithium anode. The layer forms a flexible, uniform and non-reactive film that protects the unstable lithium from the issues that have made it so difficult to use.
“The ideal protective layer for a lithium metal anode needs to be chemically stable to protect against the chemical reactions with the electrolyte and mechanically strong to withstand the expansion of the lithium during charge,” Cui remarked.
According to the researchers, the new lithium metal anode achieves 99 percent efficiency even at 150 cycles. Typically, to be commercially viable, a battery must have coulombic efficiency of 99.9 percent or more. Earlier versions of unprotected lithium metal anodes achieved 96 percent efficiency for a shorter period of time.
The study’s findings are described in greater detail in the journal Nature Nanotechnology.