Investigation of concentration-dependent solvation structure evolution and glass transition in MgCl2 electrolytes: Implications for aqueous magnesium ion battery performance

Abstract

The high safety of aqueous magnesium ion batteries (AMIBs) contrasts with their limited electrochemical performance. To overcome electrolyte-induced parasitic reactions, it is essential to understand the dynamic evolution of concentration-dependent metal ion solvation structures (MISSs). This study systematically reveals the solvation structure evolution of MgCl₂ aqueous solutions across a full concentration range (0–30 M) and its impact on electrochemical properties using molecular dynamics simulations and density functional theory calculations. Results indicate that six characteristic solvation configurations exist, exhibiting a dynamic, concentration-dependent inter-evolution defined as the solvation structure evolutionary processes (SSEP). The four-phase glass transition mechanism in solvation structure evolution is revealed by analyzing the percentage of each type of solvation structure in different concentrations. The study shows that conductivity is directly related to the dynamic transitions of dominant solvation structures, with a shift in the Mg²⁺ coordination mode—from octahedral through pentahedral intermediates to tetrahedral—revealing a concentration-dependent ion transport mechanism. At low concentrations, free-state stochastic diffusion predominates, reaching a maximum conductivity before transitioning to relay transport within a restricted network at high concentrations. Key contributions include: a general strategy for electrolyte design based on the solvation structure evolution process, which quantitatively correlates structural occupancy with migration properties, and the “Concentration Window” regulation model that balances high conductivity with reduced side reactions. These findings clarify the structural origins of anomalous conductivity in highly concentrated electrolytes and establish a mapping between microstructural evolution and macroscopic performance, providing a theoretical basis for engineering high-security electrolytes of AMIBs.