Ion Networks in Water-based Li-ion Battery Electrolytes

Area of Science:

• Electrochemistry
• Materials Science
• Physical Chemistry

Background:

• Water-in-salt electrolytes (WiSEs) are advanced electrolytes for lithium-ion batteries (LIBs), offering enhanced safety and nonflammability due to high salt concentrations.
• Unlike conventional electrolytes, WiSEs feature unique solvation structures and ion transport mechanisms dominated by ion networks and aggregates.
• The reduced availability of free water molecules shifts the solvation environment, promoting stronger ion-anion interactions and complex aggregate formation.

Purpose of the Study:

• To elucidate the critical role of ion networks in the performance of WiSEs.
• To investigate the unconventional ion transport mechanisms in WiSEs.
• To highlight the potential of WiSEs for next-generation energy storage technologies.

Main Methods:

• Utilized advanced spectroscopic techniques, including infrared pump-probe (IR-PP) and two-dimensional IR (2D-IR) spectroscopy.
• Employed molecular dynamics (MD) simulations to analyze ion behavior and solvation structures.
• Correlated spectroscopic findings with simulation data to understand ion transport pathways.

Main Results:

• Demonstrated that ion networks in WiSEs are central to electrolyte performance, governing transport properties and stability.
• Showcased that Li+ ions are transported along pathways within ion networks, a mechanism termed structural diffusion, rather than through bulk water.
• Identified that bulk-like water molecules form transient hydrogen-bond networks acting as conduits for Li+ ions, while anion-bound water is less mobile.

Conclusions:

• The formation of extensive, 3D ion networks, particularly stabilized by chaotropic anions like TFSI-, is crucial for Li+ ion mobility and electrochemical stability in WiSEs.
• WiSEs exhibit high ionic conductivity due to the decoupling of viscosity and ionic mobility, driven by structural diffusion within ion networks.
• Understanding and controlling ion aggregates in WiSEs is key to developing safer, high-performance electrolytes for LIBs and other aqueous energy storage systems.