Densifying argyrodite could prevent dendrite formation in all-solid-state batteries

All-solid-state batteries are emerging energy storage solutions in which flammable liquid electrolytes are substituted by solid materials that conduct lithium ions. In addition to being safer than lithium-ion batteries (LIBs) and other batteries based on liquid electrolytes, all-solid-state batteries could exhibit greater energy densities, longer lifespans and shorter charging times.

Despite their potential, most all-solid-state batteries introduced to date do not perform as well as expected. One main reason for this is the formation of so-called lithium dendrites, needle-like metal structures that form when the lithium inside the batteries is unevenly deposited during charging.

These structures can pierce solid electrolytes, which can adversely impact the performance of batteries and potentially elicit dangerous reactions. Identifying strategies to prevent the formation of dendrites in solid electrolytes, while also achieving high energy densities and overall battery performance is thus of key importance to enable the commercialization and widespread deployment of all-solid-state batteries.

Researchers at the University of Oxford and other institutes recently showed that densified argyrodite (Li₆PS₅Cl), a ceramic solid electrolyte material, could help to enhance the performance of all-solid-state batteries, while also preventing the formation of lithium dendrites. Their paper, published in Nature Energy, could open new possibilities for the fabrication of safer, better performing and fast-charging batteries based on solid electrolytes.

“Avoiding lithium dendrites at the lithium/ceramic electrolyte interface and, as a result, avoiding cell short circuit when plating at practical current densities remains a significant challenge for all-solid-state batteries,” wrote Dominic L. R. Melvin, Marco Siniscalchi and their colleagues in their paper. “Typically, values are limited to around 1 mA cm⁻², even, for example, for garnets with a relative density of >99%. It is not obvious that simply densifying ceramic electrolytes will deliver high plating currents.”

As part of their study, the researchers densified a Li₆PS₅Cl solid electrolyte, bringing its relative density from 83% up to 99%. This essentially means that they increased the density of the material in relation to its theoretical maximum density (i.e., how dense it would be if it had no defects, pores, etc.). They then used imaging and modeling tools to study the electrolyte’s microstructure, particularly focusing on the formation of lithium dendrites.

“We show that plating currents of 9 mA cm⁻² can be achieved without dendrite formation, by densifying argyrodite, Li₆PS₅Cl, to 99%,” wrote Melvin, Siniscalchi and their colleagues. “Changes in the microstructure of Li₆PS₅Cl on densification from 83 to 99% were determined by focused ion beam-scanning electron microscopy tomography and used to calculate their effect on the critical current density (CCD).”

Notably, the researchers found that the densification of argyrodite improved the material’s CCD, which is the maximum current at which lithium can be plated in the electrolyte without prompting the growth of dendrites. They then used modeling techniques to investigate how specific changes in the size of pores or cracks on the electrolyte affected its CCD.

“Modeling shows that not all changes in microstructure with densification act to increase CCD,” wrote the authors. “Whereas smaller pores and shorter cracks increase CCD, lower pore population and narrower cracks act to decrease CCD. Calculations show that the former changes dominate over the latter, predicating an overall increase in CCD, as observed experimentally.”

Overall, this recent study highlights the promise of densified argyrodite as an electrolyte for all-solid-state batteries, suggesting that it could suppress dendrite growth. Future works could integrate the electrolyte in battery cells, to further assess and validate its potential for real-world applications.

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