Boffins interrogate sodium ion battery stability mystery

The mechanism still isn't completely understood, but research could help resolve SIB lifespan problems

The world is inching closer to replacing lithium in batteries with sodium – and Cornell University researchers think they've found a way to help address stability problems in the nascent energy storage technology. 

By shooting a highly synchronized X-ray beam at a sodium ion battery in the process of charging, researchers said they were able to spot transient crystal defects that only occur when the battery was in use and which vanish when at rest, the team said in its paper. 

"If we looked at the battery before and after the first charge-discharge cycle, we would see no defects. But during the operation, we see how the defects form and self-heal, leaving detectable 'scars' behind," said Andrej Singer, lead researcher and faculty fellow at Cornell's Department of Material Science and Engineering.

Sodium ion batteries have been at the cutting edge of battery research in recent years, but they degrade quickly when charged and discharged and such short lifespans make them far less practical as a replacement for long-lasting lithium ion batteries. 

While this discovery could help future researchers better understand how the cathodes in sodium ion batteries function, the team said they still don't quite understand the mechanism behind their formation. They were, however, able to use metallurgical modeling to predict the movement of the transient defects across the cathode. 

Armed with that knowledge, the Cornell/University of California, San Diego/University of Chicago/Argonne National Laboratory team now plans to study how the defects interact with the process of ionic diffusion as the battery charges and discharges. Singer said the defects suggest that particle shape plays a role in that interaction process, something he and his team want to explore further.

"For centuries, blacksmiths used defect engineering in metals to create stronger and more durable materials without even realizing it," Singer said, adding that he hopes using a defect-engineering approach to sodium ion cathodes could help solve the stability problem.

Sodium ion batteries are having a moment

Lots of different industries are including sustainability in their future product roadmaps, and batteries are no different. Lithium is a far rarer and much more expensive material than sodium, which is plentiful around the world, chemically similar to lithium, and relatively inexpensive. 

Chinese battery makers BYD and CATL have both been actively working on sodium ion batteries that avoid degradation issues and it was recently reported that both firms plan to field mixed Lithium-Sodium ion batteries in Chinese-made electric vehicles later this year. 

The US has already become a laggard in other forms of battery technology, and by all accounts China is ready to corner the market on sodium ion batteries once they reach a production-ready state. 

According to The New York Times, 16 out of 20 sodium battery factories currently planned or under construction are located in China; two years from now 95 percent of the world's sodium battery manufacturing capacity will belong to China.

While it is possible to refine sodium for batteries from sources like sea salt and other forms of sodium chloride, much of it comes from sources like soda ash, or sodium carbonate, the largest source of sodium for industrial applications. Wyoming, for example, hosts one of the largest ever discovered sources of soda ash that's been widely used in the US and could serve as a source of sodium for batteries.

Natural soda ash is far more sustainable to refine than lithium, but in China, where most sodium ion batteries are likely to be made in the next few years, synthetic soda ash made through the Solvay process dominates.

The Solvay process used to make China's synthetic sodium carbonate is chemically intensive and requires a lot of heat, typically coal, meaning China's sodium ion batteries likely won't be the sustainable solution the world needs. ®