Self-Crosslinking Poly(Ethylene Glycol) Diglycidyl Ether in Water-in-Salt Electrolytes for Minimal Hydrogen Evolution Reactions and Extended LiTFSI Solubility

Abstract

Water-in-salt electrolytes - WISEs - are prevailing thanks to their compelling extended voltage window due to the reduced free water molecules at the electrode interface. However, as has been reported elsewhere, free-water content still can be reduced further. In our previous work, an unstable phenomenon of solid electrolyte interphase (SEI) and salt precipitation/dissolution issue were revealed. Herein, we propose a novel approach in order to alleviate those issues using poly(ethylene glycol) diglycidyl ether (PDE) as an additive. Indeed, upon mixing LiTFSI, water and PDE at high concentrations, we observed a ring-opening reaction of PDE that was confirmed via Raman spectroscopy, FTIR and ionic conductivity measurements. These crosslinked networks could also increase the solubility limits of LiTFSI in water, which was identified by adding more LiTFSI or LiOTf. Differential scanning calorimetry (DSC) measurement demonstrated that these crosslinked electrolytes effectively suppress the crystallization of water molecules with the WISE. Linear sweep voltammetry (LSV) measurements revealed that these novel crosslinked electrolytes considerably reduce free water content which effectively drives the HER to more negative potentials. More significantly, the SEI formed with these novel electrolytes remains present and stable on the electrode surface after a resting period of 1 h. Our work herein offers a new approach to tackling SEI instability and precipitation/dissolution issues.

Thermodynamic, Kinetic, and Transport Contributions to Hydrogen Evolution Activity and Electrolyte-Stability Windows for Water-in-Salt Electrolytes

“…This electrolyte has a reported electrochemical stability window of ∼3 V, resulting in a significant increase in the aqueous battery energy density. Diverse electrolyte salts have now been used in WiSE systems, − including nitrates, acetates, water-in-bisalt electrolytes (LiTFSI + LiOTf), hydrate-melt electrolytes (LiTFSI + LiBETI), and molecular crowding electrolytes (2 m LiTFSI in 94% PEG) …”