A new study investigating the chemical fundamentals, performance and microstructural defects of sodium–metal sulfide batteries has shown how the material changes during the charge/discharge process, insight that could lead to innovative battery design and optimization of materials microstructure for future energy storage needs.

Although most portable electronics are now powered by rechargeable lithium–ion batteries, the technology is constrained by the high cost and limited availability of lithium, leading to much research into alternatives. Sodium is seen as a potential candidate due to its cheapness, availability and similar chemical properties, but sodium–ion batteries go through changes in their charge and discharge cycles, degrading their performance.

The research, published in Advanced Energy Materials [Wang et al. Adv. Energy Mater. (2017) DOI: 10.1002/aenm.201602706], used full-field transmission x-ray microscopy (TXM) to ensure nanoscale spatial resolution and a large field of view to image the insertion of sodium ions into, and extracted from, an iron sulfide electrode over 10 cycles, the first time that the structural and chemical evolution of sodium–metal sulfide batteries have been captured during their electrochemical reactions.

"The cracks and fractures created by volume expansion of the iron sulfide particles during discharge destroy the particles' structure"Jun Wang

The team, from US DoE’s Brookhaven National Laboratory, found the loss in battery capacity was due largely to sodium ions entering and leaving iron sulfide, the electrode material used, from substantial cracks originating at the surface of the iron sulfide particles during the first charge/discharge cycle. The electrochemical reactions resulted in irreversible changes in the microstructure and chemical composition of the electrode; as iron sulphide has a high theoretical energy density, it is hoped that showing the underlying mechanism limiting performance will help to improve its real energy density.

They mapped the corresponding chemical changes using TXM combined with x-ray absorption near edge structure, where 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. As such energy is specific to each element, the absorption spectra can identify chemical composition, showing that the iron sulfide particles experience a chemical transformation following the same surface-to-core mechanism as found in the microstructural defects.

As team leader Jun Wang said, “It appears that … the cracks and fractures created by volume expansion of the iron sulfide particles during discharge destroy the particles' structure… On the other hand, these defects provide a path for sodium ions to get to the particles' core”. As volume shrinks during charging, some paths are blocked, which restricts the movement of sodium ions, trapping some in the core.

The researchers will now look for ways to improve battery capacity after the first cycle, and the results have inspired them to look at nanoengineering approaches to decrease interfacial resistance and ion diffusion barriers to enhance cycle reversibility of conversion-based battery materials.