Breaking Zn²⁺ Local Hydration Shells via Dual-Track Anion Chemistry

Functional anion additives improve aqueous Zn-ion batteries by regulating water reactivity and Zn²⁺ solvation, yet their molecular mechanisms in Zn²⁺ desolvation remain unclear. Here, we combine gas-phase negative-ion photoelectron spectroscopy and well-tempered metadynamics to establish a cross-phase descriptor-to-mechanism framework for three widely applied anions: bis(oxalato)borate, BOB^-; difluoro(oxalate)borate, DFOB^-; and bis(fluorosulfonyl)imide, FSI^-. Anion stepwise hydration quantifies intrinsic anion-water binding energies of 7.0, 10.0, and 9.2 kcal/mol for BOB^-, DFOB^-, and FSI^-, respectively, and reveals distinct binding-site accessibility and conformational adaptability. Metadynamics simulations show DFOB^- and FSI^- directly coordinate Zn²⁺ to form [Zn(H₂O)₅-anion]⁺ contact ion pairs, whereas all three anions disrupt intershell water-water hydrogen bonding, converting first-shell waters from donor-donor-acceptor units to donor-donor environments. Rigid DFOB^- follows a constrained interchange-dissociation pathway, while flexible FSI^- enables adaptive desolvation through transient trans-to-cis switching. These findings reveal dual-track anion chemistry as a molecular basis for regulating the first step of Zn²⁺ desolvation.