Structure and Transport of Solvent Ligated Octahedral Mg-Ion in an Aqueous Battery Electrolyte

The advancement of battery technologies has gone "beyond lithium" batteries to involve other ions. Magnesium ion batteries are now in the limelight. Mg-ion cells containing aqueous electrolytes are a relatively newer topic that has not obtained much attention; therefore, in this work, we model 4.0 m aqueous magnesium bistrifluorosulfonimide, Mg(TFSI)2, an appropriate electrolyte for Mg-ion batteries based on an experimental formulation. We wish to gain insights into the structure and dynamics of this electrolyte at the molecular level at two different temperatures (303.15 and 343.15 K), each with and without a charge scaling factor that controls the dynamics of the ions. We performed structural analysis by calculating radial distribution functions (RDFs) and combined distribution functions (CDFs). We find that the Mg2+ ion appears to be held in a solvation shell that is composed of oxygen atoms of TFSI– ions and water with an octahedral arrangement. We also identify that the cis and trans configurations of the TFSI– anion participate in coordination with Mg2+ through two and one oxygen atoms, respectively. The solvation shells do not change structurally with the change in temperature. However, the composition of the solvation shells around the Mg2+ tends to change when the charge scaling factor is changed. With scaled charges, Mg2+ is primarily surrounded by only water molecules; otherwise, we observe both water and anions. The combined distribution function plotted concerning Mg–N distances and the Mg–N–S angle report occurrences of both cis and trans-TFSI–. CDFs plotted between Mg–O distances and Mg–O–H angles between Mg2+ and H2O show only minimal differences with respect to the variation of temperature and charge scaling factor. We also observe enhanced diffusivity with an increase in temperature. With the knowledge of the diffusive regime of the simulation, we calculate the ionic conductivity of the systems that are within the acceptable range according to commercial standards. The hydrogen-bond autocorrelation functions showed fast hydrogen-bond dynamics, despite a well-structured solvation matrix. The correlation between conductivity and ion-cage lifetime provides the authenticity of the correlated ionic dynamics. This electrolyte system is observed to provide good ion transport at raised temperatures while maintaining a solvation environment containing both water and TFSI– ions.