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Stanford researchers make rechargeable metal-chlorine battery six times better than lithium-ion

Mon, 09/13/2021 - 11:25 -- Paul Crompton

An international team of researchers led by Stanford University has developed rechargeable alkali metal-chlorine cell that can store up to six times more charge than commercially available secondary lithium-ion batteries.

The new alkali metal-chlorine cell relies on the back-and-forth chemical conversion of sodium chloride (Na/Cl2) or lithium chloride (Li/Cl2) to chlorine.

The cell was developed by a team of researchers led by Stanford chemistry professor Hongjie Dai and doctoral candidate Guanzhou Zhu.

The technology was explained in a paper published in the journal Nature.

The cell can cycle up to 200 times and 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, stated Zhu.

Previous work on a high-performing secondary sodium-chlorine or lithium-chlorine battery was hindered because chlorine is too reactive and challenging to convert back to a chloride with high efficiency. 

In the few cases where a degree of rechargeability was enabled, the battery performance proved poor.

Dai and Zhu set out to improve their existing sodium and lithium-chlorine battery technologies using thionyl chloride— one of the main ingredients of lithium-thionyl chloride primary batteries first invented in the 1970s.

Technology breakthrough 

In one of the scientists’ early experiments involving chlorine and sodium chloride, they noticed the conversion of one chemical to another had stabilised, resulting in some rechargeability. 

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 that act like a sponge, sopping up chlorine molecules and storing them for later conversion to salt inside the micropores.

Zhu said: “The chlorine molecule is being trapped and protected in the tiny pores of the carbon nanospheres when the battery is charged.

“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 working prototype is suitable for small electronics like hearing aids or remote controls but further work to increase the energy density, scale up the size and increase cyclability is needed before before it is ready for larger applications such as consumer electronics or electrical vehicles, 

Image: An LED light is powered by a prototype rechargeable battery using the sodium-chlorine chemistry developed recently by Stanford researchers. (Image credit: Guanzhou Zhu)

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