Lithium metal is considered the ultimate anode candidate for next-generation rechargeable batteries because it offers far higher energy density than graphite. Yet widespread adoption remains constrained by a recurring failure mode: during charging, lithium can grow as needle-like dendrites. These structures degrade cycle life, increase impedance, and can raise serious safety concerns, especially under high-rate or long-duration operation.
A team at Tohoku University’s Institute for Materials Research (IMR) has now pinpointed a design lever that goes beyond simply adding more salt to the electrolyte. Instead of treating electrolyte formulation as a one-way “concentration increase,” the researchers report that there is an optimal lithium-salt window that promotes both uniform plating and interfacial endurance.
The work focuses on electrolytes made from ethylene carbonate and propylene carbonate with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Using a suite of complementary techniques—pulsed-field gradient nuclear magnetic resonance (PFG-NMR), electrochemical testing, electron microscopy, impedance spectroscopy, and nanoindentation—the team connected ion transport behavior to the mechanical properties of the solid electrolyte interphase (SEI).
Their results indicate that electrolytes containing 1–2 molar (M) LiTFSI deliver the best performance. At these concentrations, lithium ions and the negatively charged TFSI anions move together at nearly matched rates. This “cooperative” transport creates a steadier supply of charge carriers to the electrode surface, reducing the likelihood of localized current spikes that trigger uneven deposition.
Equally important, the cooperative motion appears to strengthen the SEI. The researchers found that a more mechanically stable SEI can better resist deformation under electrochemical stress, limiting the growth of porous or filamentary lithium morphologies.
In contrast, dilute electrolytes form weaker interphases that allow voids and porous deposits to emerge. Highly concentrated electrolytes, on the other hand, hinder key transport processes: reduced ion mobility and impeded electron transfer contribute to non-uniform lithium growth.
“Our results show that achieving stable lithium metal deposition is not simply a matter of increasing the salt concentration,” said Hongyi Li of Tohoku University’s IMR. “Instead, the key is creating a balance where lithium ions and anions move cooperatively while maintaining a mechanically robust interfacial layer.”
These findings introduce a practical electrolyte design principle: tune salt concentration to achieve correlated ion-pair diffusion while simultaneously optimizing SEI mechanical stability. The approach may help accelerate the development of safer, longer-lasting lithium metal batteries for electric vehicles, portable electronics, and large-scale renewable energy storage.
Keywords
Lithium metal batteries; LiTFSI; electrolyte concentration; ion-pair diffusion; SEI stability; dendrite suppression; electrochemical interphases
