Critical Concentration Effect in NaCl–NaI Molten Salt: A DPMD Study of Structural and Transport Properties

Molten salt electrodeposition is a clean and efficient technique for gold recovery from e-waste, yet its low current efficiency restricts industrial application. By integration of density measurements with deep potential molecular dynamics (DPMD), this work reveals the concentration-dependent regulatory mechanism of molten salt electrochemical performance. The results showed that at NaI concentrations ≤30 mol %, the I^- was primarily dispersed as isolated species. The structure was dominated by a Cl^- network, evidenced by a characteristic I^--Cl^- bond length of 2.65 Å and an average Cl^- coordination number of 4.9 around I^-. At 20 mol % NaI, a slight 3.3% enhancement in Cl^- diffusivity was observed. The overall electrochemical performance deteriorated with increasing NaI concentration, with 30 mol % identified as a critical threshold. This threshold was demonstrated by the severe changes observed at 40 mol % NaI, which included a 62% increase in viscosity and a 39% reduction in conductivity. These significant property changes are consistent with a mechanism of hindered ion transport. This mechanistic insight provides a theoretical basis for concentration control thresholds in mixed-anion molten salts with significant ionic size disparities (e.g., Cl^-/Br^- and F^-/I^-). The concentration control strategy offers guidance for electrolyte engineering in metal electrorefining, solar thermal fluids, and nuclear fuel reprocessing.