Stanford University scientists experimenting with a decades-old, single-use battery architecture have developed a new version that is not only rechargeable, but offers around six times the capacity of today's lithium-ion solutions. The breakthrough hinges on the stabilization of volatile chlorine reactions within the device, and could one day provide the basis for high-performance batteries that power smartphones for a week at a time.
The new battery is described as an alkali metal-chlorine battery, and is based on chemistry that first emerged in the 1970s called lithium-thionyl chloride. These batteries are highly regarded for their high energy density, but rely on highly reactive chlorine that makes them unsuitable for anything other than a single use.
In a regular rechargeable battery, the electrons travel from one side to the other during discharging and then are reverted to their original form as the battery is recharged. In this case, however, the sodium chloride or lithium chloride is converted to chlorine, which is too reactive to be converted back to chloride with any great efficiency.
The authors of this new study may well have come up with a solution to this problem. The team was experimenting with sodium chloride and chlorine to try and improve this battery's performance, but found that the chemical had actually stabilized, which enabled the battery some degree of rechargeability. Subsequent investigations led the team to develop a new electrode material made of porous carbon that acts like a sponge, soaking up the erratic chlorine molecules and safely storing them to be converted back into sodium chloride.
“The chlorine molecule is being trapped and protected in the tiny pores of the carbon nanospheres when the battery is charged,” says Guanzhou Zhu. “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.”
A well maintained lithium-ion battery, for context, can be good for 500-1000 cycles.
Through their experiments, the team also demonstrated a very high energy density for the prototype battery, clocking 1,200 mAh per gram of the electrode material, around six times more than is offered by today's lithium-ion battery technology.
“A rechargeable battery is a bit like a rocking chair. It tips in one direction, but then rocks back when you add electricity,” says study author Hongjie Dai. “What we have here is a high-rocking rocking chair.”
The team imagines the battery finding use in hearing aids or remote controls, or being used to power devices that only require infrequent recharging like satellites or remote sensors that could be topped up with solar. For use in smartphones and electric vehicles, the scientists will need to scale up the battery and engineer a suitable structure, while also increasing the number of times it can be safely cycled.
The research was published in the journal Nature.
Source: Stanford
