Breaking Zn ²⁺ Local Hydration Shells via Dual-Track Anion Chemistry

Functional anion additives improve aqueous Zn-ion batteries by regulating water reactivity and Zn 2+ solvation, yet their molecular mechanisms in Zn 2+ 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 Zn 2+ to form [Zn(H 2 O) 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 Zn 2+ desolvation.