Ferric chloride (FeCl₃) could help solve one of the biggest safety challenges facing solid-state lithium-metal batteries. Researchers found that adding the widely used industrial chemical to polyethylene oxide-based electrolytes promotes the growth of lithium as spherical particles rather than needle-like dendrites. Surprisingly, the additive also softens the electrolyte, challenging the long-held view that strong electrolytes are essential for suppressing dendrite growth during battery cycling.
Lithium-metal batteries with solid-state electrolytes can store more energy than conventional lithium-ion batteries because they use lithium metal rather than graphite as the anode, potentially allowing batteries to be smaller and lighter. Replacing flammable liquid electrolytes with solid materials could also improve battery safety by eliminating the risk of leaks.
Among solid electrolytes, polyethylene oxide (PEO)-based materials stand out for their flexibility, low cost and compatibility with industrial manufacturing.
However, as lithium-metal batteries charge and discharge, lithium can deposit as spiky growths known as dendrites. Like tree roots pushing through concrete, dendrites can pierce the solid electrolyte, causing short circuits and thermal runaway. To prevent this, researchers have typically sought to increase the electrolyte’s stiffness.
‘The mainstream view in this field is that softer electrolytes are ineffective and cannot prevent lithium dendrite growth,’ says Ruo Zhao from Shenzhen University in China, who led the study. ‘This conventional wisdom has severely limited the scope of exploration.’
Instead of strengthening the electrolyte, Zhao’s team focused on controlling lithium deposition. Previous studies had explored lithophilic metal ions and fluorine-containing salts, explains Zhao, so out of curiosity the researchers investigated the effects of FeCl₃.
Incorporating the salt into the PEO matrix formed Fe–O/Cl centres. The researchers found the electronegative chloride anions direct lithium ions towards nucleation centres, while the redox-active iron cations act as an electron reservoir to encourage lithium reduction at the nucleation site. This synergy promotes the growth of lithium as spherical particles rather than dendritic needles. The team observed uniform spherical lithium deposits across a range of operating conditions, a result Zhao describes as the most compelling evidence for the mechanism. Symmetric lithium cells using the additive also cycled stably for more than 3000 hours.
Whitney Loo, who develops sustainable polymer-based battery electrolytes at the University of Wisconsin-Madison in US, says the study addresses a long-standing challenge in electrolyte design known as the strength–dendrite paradox. ‘Typically, in polymer-based electrolytes increasing the strength decreases the lithium-ion transport capabilities,’ Loo explains. Loo describes the experimental evidence presented by Zhao’s team as ‘convincing’ and ‘very thorough’.
The team plans to investigate how the composition and function of the solid electrolyte interface influences lithium nucleation and to test a range of metal salts to extend the approach. They are confident the strategy can be scaled up and integrated into existing battery manufacturing processes. If successful, it could provide a route to high-performance polymer electrolytes without relying on increasingly rigid materials.
