Ion Networks in Water-based Li-ion Battery Electrolytes

Abstract

Water-in-salt electrolytes (WiSEs) are promising electrolytes for next-generation lithium-ion batteries (LIBs), offering critical advantages like nonflammability and improved safety. These electrolytes have extremely high salt concentrations and exhibit unique solvation structures and transport mechanisms dominated by the formation of ion networks and aggregates. These ion networks are central to the performance of WiSEs, govern the transport properties and stability of the electrolyte, deviating from conventional dilute aqueous or organic electrolytes.

The availability of free water molecules is significantly reduced in WiSEs, leading to a shift in the solvation environment. Lithium ions (Li⁺) typically travel with their solvation shells in dilute solutions and form stronger interactions with anions, resulting in the formation of complex ion aggregates. Despite the high viscosity of WiSEs, they exhibit surprisingly high ionic conductivity attributed to the decoupling of viscosity and ionic mobility. Instead of moving through free water, Li⁺ ions are transported along the pathways formed by the ion networks, minimizing direct solvent interaction and enhancing mobility.

Advanced spectroscopic techniques, such as infrared IR pump-probe (IR-PP) and two-dimensional IR (2D-IR) spectroscopy, and molecular dynamics (MD) simulations have illuminated the critical role of these ion networks in facilitating transport. These studies have shown that even at extreme salt concentrations, some water molecules retain properties similar to bulk water, essential for fast ion movement. In WiSEs, bulk-like water molecules form transient hydrogen-bond networks that serve as conduits for Li⁺ ions, while anion-bound water molecules play a less active role in transport due to their slower dynamics.

As the salt concentration increases, the structure of WiSEs becomes more dominated by 3D ion networks. MD simulations reveal that these networks, stabilized by chaotropic anions such as bis(trifluoromethanesulfonyl)imide (TFSI^–), disrupt the hydrogen-bonding network of water and provide a stable, interconnected structure that supports the movement of Li⁺ ions. The formation of these extensive ion networks is critical for maintaining ionic mobility and the electrochemical stability of the electrolyte.

The shift from traditional vehicular transport mechanisms to structural diffusion is a hallmark of WiSEs. Li⁺ ions no longer move with their solvation shells but hop between coordination sites within the ion network. This structural diffusion mechanism enables high ionic mobility despite the reduced presence of water and the increased viscosity of the solution. In conclusion, the formation of ion networks and aggregates in WiSEs not only stabilizes the electrolyte but also drives an unconventional ion transport mechanism. By understanding and controlling these aggregates, WiSEs offer a pathway toward safer, high-performance electrolytes for LIBs and other aqueous energy storage technologies.