No 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.

Aqueous batteries are regaining interest, thanks to the extended working stability voltage window in a highly concentrated electrolyte, namely the water-in-salt electrolyte. A solid-electrolyte interphase (SEI) forms on the negative electrode to prevent water access to the electrode surface. However, we further reported that the formed SEI layer was not uniform on the surface of the glassy carbon electrode. The SEI after passivation will also show degradation during the remaining time of opencircuit voltage (OCV); hence, it calls for a more stable passivation layer to cover the electrode surface. Here, a surface modification was successfully achieved via artificial diazonium grafting using monomers, such as poly(ethylene glycol), α-methoxy, ω-allyloxy (PEG), and allyl glycidyl cyclocarbonate (AGC), on glassy carbon. Physical and electrochemical measurements indicated that the hydrophobic layer composed of PEG or AGC species was well grafted on the electrode surface. The grafted hydrophobic coatings could protect the electrode surface from the water molecules in the bulk electrolyte and then suppress the free water decomposition (from LSV) but still migrating lithium ions. Furthermore, multiple cycles of CV with one-hour resting OCV identified the good stability of the hydrophobic grafting layer, which is a highlight compared with our precious work. These findings relying on the diazonium grafting design may offer a new strategy to construct a stable artificial SEI layer that can well protect the electrode surface from the free water molecule.