Molecular Crowding Electrolytes for High-Voltage Aqueous Batteries

Abstract

Replacing flammable nonaqueous electrolytes with non-flammable aqueous electrolytes is critical to achieve safe, low-cost and eco-friendly Li-ion batteries. However, the voltage window of an aqueous Li-ion battery is limited by the thermodynamic stability window of water (1.23 V), which limits the energy density of the aqueous Li-ion batteries. Recently developed LiTFSI-based “water-in-salt” electrolytes with highly-concentrated salts (>21 m) have expanded the electrochemical stability window of water to over 3.0 V by decreasing the amount of free water molecules and creating artificial solid-electrode interfaces, however, raise concerns of cost and toxicity.Molecular crowding is a common phenomenon in living cells where water activity is substantially reduced as a consequence of the changes in the water hydrogen-bonding structuring in the presence of molecular crowding agents. Herein, we introduce a molecular crowding electrolyte using water-miscible polymer poly(ethylene glycol) (PEG) as the crowding agent to decrease water activity, thereby allowing a high operational cell voltage (3.2 V) with a low salt concentration (2 m). Aqueous Li4Ti5O12/LiMn2O4 full cells with stable specific energies between 75-110 Wh kg-1 were demonstrated over 300 cycles. Extensive electrolyte characterizations including nuclear magnetic resonance spectroscopy (NMR), Fourier-transform infrared spectroscopy (FTIR), online electrochemical mass spectroscopy (OEMS) and density functional theory (DFT)-molecular dynamic (MD) simulation were used to confirm the HER suppression effect of the molecular crowding aqueous electrolyte.The molecular crowding electrolyte enables numerous negative electrode materials that can not be used in traditional aqueous battery systems. This work provides a new platform for designing aqueous electrolytes with high stability for low-cost and sustainable energy storage.