Molecular crowding electrolytes for high-voltage aqueous batteries

Developing low-cost and eco-friendly aqueous electrolytes with a wide voltage window is critical to achieve safe, high-energy and sustainable Li-ion batteries. Emerging approaches using highly concentrated salts (21-55 m (mol kg^-1)) create artificial solid-electrode interfaces and improve water stability; however, these approaches raise concerns about cost and toxicity. Molecular crowding is a common phenomenon in living cells where water activity is substantially suppressed by molecular crowding agents through altering the hydrogen-bonding structure. Here we demonstrate a 'molecular crowding' electrolyte using the water-miscible polymer poly(ethylene glycol) as the crowding agent to decrease water activity, thereby achieving a wide electrolyte operation window (3.2 V) with low salt concentration (2 m). Aqueous Li₄Ti₅O₁₂/LiMn₂O₄ full cells with stable specific energies between 75 and 110 W h kg^-1 were demonstrated over 300 cycles. Online electrochemical mass spectroscopy revealed that common side reactions in aqueous Li-ion batteries (hydrogen/oxygen evolution reactions) are virtually eliminated. This work provides a path for designing high-voltage aqueous electrolytes for low-cost and sustainable energy storage.