Dynamic Modulation of Ions Solvation Sheath by Butyramide\nas Molecular Additives in Aqueous Batteries

The high activity of water in aqueous battery electrolytes\ncan\ntrigger side reactions, limiting their large-scale application. Additives\nthat form contact pairs (CPs) with cations by coordinating with them\ncan effectively reduce water’s activity. However, due to the\ncomplex interactions between ions, additives, and solvent molecules\nand the fact that current strategies for additive screening primarily\nrely on static physical parameters, the dynamic mechanisms that govern\nthe modulation of ion solvation sheaths are still poorly understood.\nIn this study, we introduce butyramide (BUT) as a molecular additive\nand employ molecular simulations to demonstrate its regulatory effect\non the hydration sheath of Ca<sup>2+</sup>, which is more pronounced\nthan that for Na<sup>+</sup>. The dynamic process by which BUT replaces\nwater molecules in the tight hydration sheath of Ca<sup>2+</sup> is\nelucidated by forming a stable [BUT-Ca<sup>2+</sup>(H<sub>2</sub>O)<sub>7</sub>] complex that suppresses water molecule activity. At a 2\nM concentration, the free energy barrier for the transition from contact\npair (CP) to solvent-shared pair (SP) for Ca<sup>2+</sup> is 11.7\nkJ/mol higher than that for Na<sup>+</sup> at 8.5 kJ/mol, consistent\nwith the cationic Hofmeister series. Furthermore, the stability and\ndynamic fluctuations among solvent-separated pair (SSP), SP, and CP\nstates are attributed to the balance between electrostatic attractive\npotential energy and hydration repulsive potential energy, supported\nby quantum chemical calculations of the ion desolvation process. Using\nBUT as an additive presents a promising strategy to enhance battery\nperformance by modulating the solvation environment of metal ions,\naddressing the growing demand for safer and more sustainable energy\nstorage solutions.