Suppressing Hydrogen Evolution in Aqueous Lithium-Ion Batteries with Double-Site Hydrogen Bonding

The recent concept of "molecular crowding agents" offering hydrogen bond (H-bond) accepting sites for free water molecules has alleviated parasitic hydrogen evolution in aqueous electrolytes. However, their cathodic limits are still not low enough to be compatible with the energy-dense Li4Ti5O12 anode (1.55 V vs Li+/Li). Inspired by nature's choice of a peptide unit featuring an amide group in forming extensive H-bond networks with water, herein, we select caprolactam, an imide analogous to the amide group in a peptide, to reduce water activity via regulating the H-bond. The introduced caprolactam containing both an H-bond acceptor and donor effectively confines water molecules in a double-site anchoring configuration with strengthened H-bonding interactions and interrupts the original H-bonding among water molecules. This unique solution structure delays the onset potential of hydrogen evolution to 1.3 V vs Li+/Li, which enables the cycling of a Li4Ti5O12/LiMn2O4 full cell with an average Coulombic efficiency of 99.7% and 78% capacity retention after 350 cycles.