Fine-tuning electrolytes for lithium metal batteries

Lithium metal has long been considered suitable for next-generation rechargeable batteries because it can store more energy than the graphite anodes used in today’s batteries. However, lithium metal batteries have not been brought into widespread use because lithium tends to grow needle-like structures called dendrites during charging, reducing battery life and creating potential safety hazards.

Now, researchers from Tohoku University have identified a key factor that could overcome this challenge; rather than increasing the amount of lithium salt in the electrolyte, the researchers found that there is an optimal concentration that allows lithium to deposit evenly while forming a stronger protective layer on the battery’s surface.

The research findings have been published in ACS Electrochemistry.

Electrolytes carry lithium ions between a battery’s electrodes during charging and discharging. Scientists have often focused on highly concentrated electrolytes because they can suppress dendrite formation; however, it is unclear why some electrolytes perform better than others.

To investigate this, the researchers examined electrolytes containing different concentrations of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved in a mixture of ethylene carbonate and propylene carbonate. By combining multiple analytical techniques, they linked ion transport in the electrolyte with the mechanical properties of the protective solid electrolyte interphase (SEI).

The researchers found that electrolytes containing 1–2 molar (M) LiTFSI produced the best results. At these concentrations, lithium ions and negatively charged TFSI ions moved together through the electrolyte at nearly the same rate. This cooperative transport created a more uniform flow of ions to the electrode surface, allowing lithium to deposit as smooth, dense layers instead of uneven, dendritic structures.

This balanced ion transport also produced a harder, more mechanically stable SEI layer. In comparison, dilute electrolytes formed weaker protective layers that allowed porous lithium deposits to develop, while highly concentrated electrolytes reduced ion mobility and hindered electron transport, ultimately leading to non-uniform lithium growth.

Hong Li, an assistant professor at Tohoku University, said the research shows that achieving stable lithium metal deposition is not simply a matter of increasing the salt concentration. “Instead, the key is creating a balance where lithium ions and anions move cooperatively while maintaining a mechanically robust interfacial layer. This provides a new design principle for developing practical lithium metal batteries,” Li said.

The findings offer a new way of designing electrolytes by optimising both ion transport and the stability of the interface between the electrolyte and the electrode. Rather than relying solely on highly concentrated electrolytes, researchers can target this intermediate concentration regime to improve battery performance, safety and lifespan.

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