Resolving the Zn²⁺ Solvation Structure and Transport Properties of ZnCl₂ Electrolytes Using Neural Network Potential from Salt-in-Water to Water-in-Salt

Aqueous zinc-ion batteries (ZIBs) are of great interest for next-generation energy storage applications due to their low cost, intrinsic safety, and environmental friendliness. ZnCl₂ solutions are a promising electrolyte for aqueous zinc ion batteries. We have performed a combined computational and experimental study of the structural and dynamic properties of aqueous ZnCl₂ electrolytes with concentrations ranging from salt-in-water (SIW) to water-in-salt (WIS). By developing a neural network potential (NNP) model, we perform molecular dynamics (MD) simulations with <i>ab initio</i> accuracy but at much larger length and time scales. The calculated NNP structural and dynamic properties of ZnCl₂ electrolytes are in good agreement with pair distribution function (PDF) experiments and conductivity measurements.<br/>The MD trajectories provide a comprehensive picture of the Zn²⁺ solvation shell structure, with a range of Zn(H₂O)_xCl_y first solvation shell species and their populations. The NNP solvation structures are validated by the structure factors of ZnCl₂ solutions obtained from the PDF experiment. Multiple co-existing solvation motifs of Zn(H₂O)_xCl_y were determined through this analysis. Dynamic properties dictated by the multi-stage charge carrier characteristics are deciphered and will be described, including differences in Zn²⁺ and Cl^- diffusion within the SIW and WIS regimes.<br/>Through microscopic insights of the structural and dynamic properties of ZnCl₂ electrolytes, we aim to reveal the underlying mechanism of the ion transport process and inform further studies on ion transport, desolvation energy, and electrode-electrolyte interface structures.