A team of scientists at the U.S. Department of Energy's Brookhaven National Laboratory has found the cause of degraded performance in operating sodium-ion battery, helping them gain an insight that could help advance battery design to address energy storage needs in the future.

“We discovered that the loss in battery capacity is largely the result of sodium ions entering and leaving iron sulfide—the battery electrode material we studied—during the first charge/discharge cycle,” explained Brookhaven physicist Jun Wang, who led the research published online in Advanced Energy Materials. “The electrochemical reactions involved cause irreversible changes in the microstructure and chemical composition of iron sulfide, which has a high theoretical energy density. By identifying the underlying mechanism limiting its performance, we seek to improve its real energy density.”

Most portable electronics today are powered by rechargeable lithium-ion batteries. But lithium is expensive and limited in supply, so scientists have been looking for alternatives. Sodium has recently emerged as a prime candidate because it is less expensive, more abundant and has similar chemical properties.

Unfortunately, sodium-ion batteries, like their lithium counterparts, undergo changes during charge and discharge cycles that degrade their performance. While lithium-ion batteries have been extensively studied, little is known about the degradation mechanisms in sodium-ion batteries.

Wang’s team set out to change that. Using a full-field transmission x-ray microscope (TXM), the scientists imaged what happened as sodium ions were inserted into (sodiation) and extracted from (desodiation) an iron sulfide electrode over 10 cycles.

This study represents the first time that researchers have captured the structural and chemical evolution of a sodium–metal sulfide battery during its electrochemical reactions.

“Our full-field hard x-ray transmission microscope was critical because it provided nanoscale spatial resolution and a large field of view. Other microscopes typically provide one or the other but not both,” said Wang.

The TXM images reveal significant fractures and cracks in the battery material after the first cycle. These microstructural defects, which originate at the surface of the iron sulfide particles and then proceed inward toward their core, are the result of the particles expanding in volume upon initial sodiation during the discharge process. Although these expanded particles subsequently shrink during the first desodiation (charging) process, they are unable to reverse back to their original pristine condition—a phenomenon called irreversibility.

The scientists also tracked and mapped the corresponding chemical changes in real time using TXM in combination with a spectroscopy technique called x-ray absorption near edge structure, in which x-rays are fine-tuned to the energy at which there is a sharp decrease in the amount of x-rays that a chemical element absorbs. Because this energy is unique to each element, the resulting absorption spectra can be used to identify chemical composition.

The team’s spectra show that the iron sulfide particles undergo a chemical transformation following the same surface-to-core mechanism as seen with the microstructural defects. In the early stage of sodiation, only the surface of the particles reacts with the sodium ions and converts to pure iron; as more sodium ions are inserted, this conversion reaction spreads to the core. By the end of the first discharge, nearly all the iron sulfide particles are converted to iron. During desodiation, most areas of the particles transform back to their original iron sulfide phase except for a few regions in the core, where some sodium ions remain “trapped.”

To quantify the diffusion of sodium ions, the team measured the changes in voltage of the battery material during cycling. From these voltage measurements, they were able to calculate the rate at which the sodium ions were moving in and out of the iron sulfide particles.

They found that in the beginning of the first discharge, sodium ions diffuse very slowly. But at some voltage, the diffusivity increases significantly. The opposite occurs during the first charge: sodium ions diffuse quickly at first, and then at a certain voltage, the diffusivity suddenly drops. These results are consistent with the structural and chemical changes observed through TXM.

“It appears that on the one hand, the cracks and fractures created by volume expansion of the iron sulfide particles during discharge destroy the particles’ structure,” said Wang. “But on the other hand, these defects provide a path for sodium ions to get to the particles’ core. When the volume shrinks during charging, some of these paths are blocked, restricting the movement of sodium ions and trapping some in the core.”

After this volume expansion and shrinkage in the first cycle, the battery material seems to achieve microstructural and chemical “equilibrium.” Using the same TXM techniques, the team found that microstructure and chemical composition of the particles show robust reversibility as early as the second cycle and continuing through the tenth cycle. In other words, the battery material does not undergo significant subsequent changes in volume and is readily converted back to its original chemical form. They further confirmed their findings by performing real-time x-ray nanotomography to create 3D images of the battery material and measuring the percent volume change.

Now that the scientists know why the structural and chemical irreversibility occurs, they can start working on ways to improve battery capacity after the first cycle. For example, one possible solution to the problem of sodium ion mobility may be decreasing the size of the iron sulfide particles so that a one-phase reaction occurs, making it much easier for sodium to react. Wang’s team also plans to work with collaborators on modelling and simulations that will help inform the design of battery materials.

This work was supported by DOE’s Laboratory Directed Research and Development programme.