A new type of rechargeable alkali metal-chlorine battery developed at Stanford holds six times more electricity than the commercially available rechargeable lithium-ion batteries commonly used today.
By Andrew Myers for Stanford News
Service
August 25, 2021 -- An international team
of researchers led by Stanford University has developed rechargeable batteries
that can store up to six times more charge than ones that are currently
commercially available.
The advance, detailed in a new paper
published Aug. 25 in the journal Nature, could accelerate the use of
rechargeable batteries and puts battery researchers one step closer toward
achieving two top stated goals of their field: creating a high-performance
rechargeable battery that could enable cellphones to be charged only once a week
instead of daily and electric vehicles that can travel six times farther
without a recharge.
The new so-called alkali metal-chlorine
batteries, developed by a team of researchers led by Stanford chemistry
Professor Hongjie Dai and doctoral candidate Guanzhou Zhu, relies on the
back-and-forth chemical conversion of sodium chloride (Na/Cl2) or
lithium chloride (Li/Cl2) to chlorine.
When electrons travel from one side of a
rechargeable battery to the other, recharging reverts the chemistry back to its
original state to await another use. Non-rechargeable batteries have no such
luck. Once drained, their chemistry cannot be restored.
“A rechargeable battery is a bit like a
rocking chair. It tips in one direction, but then rocks back when you add
electricity,” Dai explained. “What we have here is a high-rocking rocking
chair.”
Serendipitous Discovery
The reason no one had yet created a
high-performance rechargeable sodium-chlorine or lithium-chlorine battery is
that chlorine is too reactive and challenging to convert back to a chloride
with high efficiency. In the few cases where others were able to achieve a
certain degree of rechargeability, the battery performance proved poor.
In fact, Dai and Zhu did not set out to
create a rechargeable sodium and lithium-chlorine battery at all, but merely to
improve their existing battery technologies using thionyl chloride. This
chemical is one of the main ingredients of lithium-thionyl chloride batteries,
which are a popular type of single-use battery first invented in the 1970s.
But in one of their early experiments
involving chlorine and sodium chloride, the Stanford researchers noticed that
the conversion of one chemical to another had somehow stabilized, resulting in
some rechargeability. “I didn’t think it was possible,” Dai said. “It took us
about at least a year to really realize what was going on.”
Over the next several years, the team
elucidated the reversible chemistries and sought ways to make it more efficient
by experimenting with many different materials for the battery’s positive
electrode. The big breakthrough came when they formed the electrode using an
advanced porous carbon material from collaborators Professor Yuan-Yao Li and
his student Hung-Chun Tai from the National Chung Cheng University of Taiwan.
The carbon material has a nanosphere structure filled with many ultra-tiny
pores. In practice, these hollow spheres act like a sponge, sopping up copious
amounts of otherwise touchy chlorine molecules and storing them for later
conversion to salt inside the micropores.
“The chlorine molecule is being trapped
and protected in the tiny pores of the carbon nanospheres when the battery is
charged,” Zhu explained. “Then, when the battery needs to be drained or
discharged, we can discharge the battery and convert chlorine to make NaCl –
table salt – and repeat this process over many cycles. We can cycle up to 200
times currently and there’s still room for improvement.”
The result is a step toward the brass
ring of battery design – high energy density. The researchers have so far
achieved 1,200 milliamp hours per gram of positive electrode material, while
the capacity of commercial lithium-ion battery today is up to 200 milliamp
hours per gram. “Ours has at least six times higher capacity,” Zhu said.
The researchers envision their batteries
one day being used in situations where frequent recharging is not practical or
desirable, such as in satellites or remote sensors. Many otherwise usable
satellites are now floating in orbit, obsolete due to their dead batteries.
Future satellites equipped with long-lived rechargeable batteries could be
fitted with solar chargers, extending their usefulness many times over.
For now though, the working prototype
they’ve developed might still be suitable for use in small everyday electronics
like hearing aids or remote controls. For consumer electronics or electrical
vehicles, much more work remains to engineer the battery structure, increase
the energy density, scale up the batteries and increase the number of cycles.
Hongjie Dai is the J. G. Jackson and C.
J. Wood Professor in Chemistry in the School of Humanities and Sciences.
Additional researchers at Stanford are Xin Tian, Jiachen Li, Hao Sun, Peng
Liang, Michael Angell and Yongtao Meng. Additional co-authors are from National
Chung Cheng University, National Synchrotron Radiation Research Center,
National Central University, National Taiwan University of Science &
Technology – all in Taiwan; as well as Shandong University of Science &
Technology in China. This research was supported by Stanford’s Bits & Watts
Initiative and employed tools at the Stanford Nano Shared Facilities, which is
supported by the National Science Foundation.
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