Zn2 + Coordination and Ligand-Exchange Dynamics in a ZnCl2-Based Deep Eutectic Electrolyte from Machine-Learning Molecular Dynamics

The Zn2+ coordination shell in ZnCl2-based battery electrolytes is far more dynamic than its time-averaged structure suggests. Using a machine-learning force field (MACE) fine-tuned with density functional theory data, we simulate nanosecond-scale molecular dynamics of a ZnCl2:urea:H2O deep eutectic electrolyte across 300-400 K, a time regime inaccessible to ab initio methods and poorly captured by classical force fields. The Zn2+ first coordination shell is tetrahedral and compositionally mixed with chloride ions, urea, and water contributing comparable shares. This overall occupancy remains essentially constant with temperature, masking markedly asymmetric underlying kinetics. Residence-time analysis reveals that chloride is essentially nonexchanging on the nanosecond time scale, while water and urea exchange continuously, with residence times an order of magnitude shorter and activation energies of 12.5 and 16.2 kJ/mol, respectively. Potential of mean force and speciation analyses identify ZnCl2 as the dominant solute species and show that ligand substitution proceeds predominantly through an associative pathway. Self-diffusion coefficients and activation energies for all four constituent species further establish a mobility hierarchy consistent with this coordination picture. Together, these results provide a molecular-level rationale for the transport behavior of ZnCl2-based electrolytes and a general framework for understanding ion speciation and lability in coordinating eutectic battery electrolytes.