Stanford Team Achieves 'Holy Grail' of Battery
Design
Engineers use carbon nanospheres to protect lithium from
the reactive and expansive problems that have restricted its use as an anode
Engineers across the globe have been racing to design smaller, cheaper and
more efficient rechargeable batteries to meet the power storage needs of
everything from handheld gadgets to electric cars.
In a paper published today in the journal Nature Nanotechnology,
researchers at Stanford University report that they have taken a big step
toward accomplishing what battery designers have been trying to do for decades
– design a pure lithium anode.
All batteries have three basic components: an electrolyte to provide
electrons, an anode to discharge those electrons, and a cathode to receive
them.
Today, we say we have lithium batteries, but that is only partly true. What
we have are lithium ion batteries. The lithium is in the electrolyte, but not
in the anode. An anode of pure lithium would be a huge boost to battery
efficiency.
"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 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."
But engineers have long tried and failed to reach this Holy Grail.
"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
paper.
In addition to Zheng, the research team includes Steven Chu, the former U.S.
"In practical terms, if we can improve the capacity of batteries to,
say, four times today's, that would be exciting. You might be able to have cell
phone with double or triple the battery life or an electric car with a range of
300 miles that cost only $25,000—competitive with an internal combustion engine
getting 40 mpg," Chu said.
The engineering challenge
In the paper, the authors explain how they are overcoming the problems
posed by lithium.
Most lithium ion batteries, like those you might find in your smart phone
or hybrid car, work similarly. The key components include an anode, the
negative pole from which electrons flow out and into a power-hungry device, and
the cathode, where the electrons re-enter the battery once they have traveled
through the circuit. Separating them is an electrolyte, a solid or liquid
loaded with positively charged lithium ions that travel between the anode and
cathode.
During charging, the positively charged lithium ions in the electrolyte are
attracted to the negatively charged anode and the lithium accumulates on the
anode. Today, the anode in a lithium ion battery is actually made of graphite
or silicon.
Engineers would like to use lithium for the anode, but so far they have
been unable to do so. That's because the 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 say 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. Lithium batteries, including those in use today, can
overheat to the point of fire, or even explosion, and are, therefore, a serious
safety concern. The recent battery fires in Tesla cars and on Boeing's
Dreamliner are prominent examples of the challenges of lithium ion batteries.
Building the nanospheres
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 carbon nanosphere wall is
just 20 nanometers thick. It would take some 5,000 layers stacked one atop
another to equal the width of single human hair.
"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 said.
The Stanford nanosphere layer is just that. It 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.
Ideal within reach
In technical terms, the nanospheres improve the coulombic efficiency of the
battery—a ratio of the amount of lithium that can be extracted from the anode
when the battery is in use compared to the amount put in during charging. A
single round of this give-and-take process is called a cycle.
Generally, to be commercially viable, a battery must have a coulombic
efficiency of 99.9 percent or more, ideally over as many cycles as possible.
Previous anodes of unprotected lithium metal achieved approximately 96 percent
efficiency, which dropped to less than 50 percent in just 100 cycles—not nearly
good enough. The Stanford team's new lithium metal anode achieves 99 percent
efficiency even at 150 cycles.
"The difference between 99 percent and 96 percent, in battery terms,
is huge. So, while we're not quite to that 99.9 percent threshold, where we
need to be, we're close and this is a significant improvement over any previous
design," Cui said. "With some additional engineering and new
electrolytes, we believe we can realize a practical and stable lithium metal
anode that could power the next generation of rechargeable batteries."
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