Scientists want sodium-ion batteries to compete with Li-ion batteries

A researcher working on sodium-ion battery development. (Image courtesy of Delft University of Technology).

Scientists from the Chinese Academy of Sciences and Delft University of Technology are working to speed up research into sodium-ion batteries so that they can compete with the Li-ion batteries found in our smartphones, laptops and electric cars. 

In a paper published in the journal Science, the researchers present evidence of achieving something that not even the best supercomputers have been able to: developing a method to predict the atomic structure of sodium-ion batteries.

As the name says, sodium-ion batteries are based on sodium, which is found in kitchen salt, among other things. In theory, Na-ion batteries do not perform as well as Li-ion batteries, meaning, their energy density is 20% to 30% lower than that of Li-ion batteries. 

The study states that Na-ion batteries provide more opportunities in the use of raw materials to build up better and cheaper positive electrodes

The researchers say that even though this reduced performance doesn’t make them competitive when it comes to mobile phones or electric cars, they can be a good alternative for situations in which weight is slightly less important.

In their paper, the scientists mention maritime applications, vehicles that can be charged frequently and power walls at home or battery parks that store wind and solar energy as elements that could benefit from these batteries.

The document also states that Na-ion batteries provide more opportunities in the use of raw materials to build up better and cheaper positive electrodes, in which cobalt  -commonly used in Li-ion batteries – would not be needed. 

But getting to the precise recipe for the cathode has been a challenge. 

“Depending on the precise cocktail of elements you will end up with subtle differences in the atomic structure of the positive electrode, which have a major impact on the battery’s performance,” Marnix Wagemaker, co-author of the study, said in a media statement.

“With just a handful of elements, there are so many structural possibilities that even the fastest supercomputer can’t predict how the different combinations will turn out. As a result, the development of new materials is slow.”

This is why Wagemaker and his team decided to better understand the sandwich-like structure of the cathode and work on predicting the ideal formula for it.

“At first it seemed like the size of the ions determined the atomic structure,” Wagemaker said. “But it soon became clear that that wasn’t the only factor. The distribution of the electrical charge of the ions plays a pivotal role.”

According to the expert, this was a crucial insight because the ratio between the size of an ion and its charge, the so-called ‘ionic potential,’ is known to have predictive value. 

Thus, the researchers developed a simple formula based on the ionic potential and were able to predict which structure they were going to get at which ratio of a selection of raw materials. “The formula guides us through the enormous number of possibilities to the electrode materials that can deliver the best performance,” Wagemaker said.

The scientists also tested their formula by designing new materials and attempted to make a cathode with the highest possible energy density, and another one that charges very quickly.

“In both cases, we succeeded. In terms of energy density we were right at the upper limit of what is possible. I like the fact that such a simple formula, based on a very old idea from geology, can make predictions on the atomic scale with such accuracy,” Wagemaker said.

For the team, the next step is to also look at other types of structures, both in electrodes and electrolytes for various types of batteries.