Liquid Structure in Water-in-Salt Electrolytes

Lithium ion (Li⁺) batteries were first commercialized about three decades ago and have been widely used since then, especially in mobile electronics. However, fire and explosive accidents in recent years have raised safety concerns regarding the flammable organic-based electrolyte solutions usually used in these batteries. Organic electrolytes also have cost and environmental concerns and complicate battery recycling. Aqueous electrolytes offer a safer, more environmentally benign alternative. Unfortunately, the narrow electrochemical stability window of water (1.23 V) limits energy density of a battery and hence their appeal for commercial applications. The recent discovery of the water-in-salt (WIS) electrolyte concept provides an exciting and promising new route for the design of aqueous electrolyte batteries. However, fundamental knowledge of the solvation structure of these electrolytes, which is a prerequisite for the understanding ionic transport and electrochemical properties, remains limited.

In this work, two WIS systems were studied using combined X-ray scattering, X-ray absorption, FTIR experiments and classical molecular dynamics (MD) simulations. One WIS is a 20 molal (m) LiTFSI aqueous solution and the other consists of 1 m Zn(TFSI)₂ and 20 m LiTFSI. Classical MD simulations are validated against the experiments. Both experiments and simulations demonstrate that in these highly concentrated WIS electrolytes, the water network is disrupted and the majority of water molecules exist in the form of isolated monomers, clusters or small aggregates with chain-like configurations. On the other hand, TFSI^- anions are connected to each other and form a network. This description is fundamentally different from those proposed in earlier studies, which suggested that a water-rich domain and a TFSI^--rich domain coexist in the liquids. In addition, for the WIS doped with 1 m Zn(TFSI)₂, we found that Zn²⁺ ions are solvated by six waters in their first solvation shell and TFSI^- anions are completely excluded although TFSI^- concentration is as high as 22 m. This solvation structure is also fundamentally different from the picture that Zn²⁺ ions are completely coordinated by three TFSI^- anions proposed in a previous study. The new pictures of the liquid structures revealed in the current study suggest that new mechanisms are required to understand the transport and electrochemical properties of these WIS electrolytes.