IntroductionAqueous electrolytes have potential applications in electrochemical energy storage devices due to their intrinsic non-flammable properties. Though its electrochemical stability window is narrow and normally it is hard to form effective solid electrolyte interphase, some researchers conduct in-depth research and propose several related solutions. Water-in-salt (WiS) electrolytes, as one of super-concentrated aqueous electrolytes, become one of the most promising systems. Some studies indicate that they can maintain a good electrochemical performance in a wide temperature range. However, its microscopic mechanism is still unclear. Therefore, in this paper, the microscopic structure and dynamic properties of LiTFSI-based aqueous electrolytes were investigated via molecular dynamics simulations.MethodsAll-atom classical molecular dynamics simulations were performed by using a software package named LAMMPS. The calculated system was composed of Li cation, TFSI anion and water molecules. The 20 mol/kg salt concentration system was selected as a main research object, and the component ratio was LiTFS:H₂O of 128:355. In addition, a system with a salt concentration of 3.5 mol/kg (LiTFSI:H₂O of 32:507) was also used as a comparison for simultaneous study. The selection principle of the two systems was to be consistent with that of the previous work. The force field parameters also followed the previous selection, where the simulation results were verified via the experimental data. Each system was simulated at different temperatures (i.e., 253, 273, 298, 323 K, and 343 K). For each salt concentration system, a relatively large simulation box was established and randomly put the corresponding number of particles into the box. Afterwards, the equilibrium system at each temperature was obtained through the initial structure optimization, and the heating or cooling procedure under the NPT ensemble. Finally, the production runs of 100–200 ns was carried out under the NVT ensemble, and the results of this step were used for post-analysis. In the whole production runs, the simulation timestep was 1 fs, and the data output interval was 50 fs for stress tensors, 1 ps for unwrapped coordinates, and 10 ps for trajectory-based analysis.Results and discussionsThe RDF calculations indicate that in the super-concentrated system (20 mol/kg), it is hardly affected by temperature, and at a lower concentration (3.5 mol/kg), the peak value is slightly affected by temperature. This indicates that the microscopic solvation structure between Li and water molecules in the high concentration system is more thermodynamically stable. To further explain this phenomenon, we calculate the potential of mean force (PMF). The results show that the solvation binding energy and activation energy of Li–Ow in the high concentration system both are almost temperature-independent, while the low concentration system shows a certain temperature dependence for the activation energy differences of 1–2 k_BT in the temperature range.Furthermore, the connection matrix analysis (where the red region represents a closer contact distance between the two atoms and a higher binding strength) show that this part is mainly from Li–Ow, Li–O, Hw–O, Hw–O four pairs. The first two pairs represent O atom from the first solvation shell of Li, and the latter two groups represent the hydrogen bonds. In addition, the yellow region represents that although the contact distance between atoms is relatively far (i.e., still within 3 Å), the binding strength is still high, such as Li–Hw, Li–S, Hw–S, Ow–O, Ow–Ow, O–O pairs. The connection matrix also gives important information that the Li–Li pair is formed at a distance of about 3 Å, which is an interesting phenomenon.The properties of Li–Li dimers in high concentration systems are investigated. Through the RDF, the first peak appears at 3 Å, and the peak gradually increases as the temperature decreases, implying a more intense binding or a longer binding time. From the auto-correlation function (ACF) and the lifetime of the dimer existence, the ACF decreases rapidly with the increase of temperature, and the lifetime decreases significantly from 8500 ps to 160 ps, which is consistent with the Brownian motion and can also be confirmed via the analysis of dynamic properties.Also, the relationship among the viscosity, diffusion coefficient, ionic conductivity and their temperature dependence of the two studied systems is analyzed. In general, the dynamic properties are obviously affected by temperature, both in high concentration system and low concentration system. In the analysis of the ionic conductivity, the Haven ratio is calculated to characterize the effect of collective diffusion in the system. The results show that the collective diffusion behavior in the high concentration system significantly affects the ionic conductivity, and is independent of temperature. At a low concentration, the effect of collective diffusion on ionic conductivity changes with temperature, and the effect gradually disappears at high temperatures.ConclusionsIn this work, molecular dynamics simulations were used to investigate the microscopic structure and dynamic properties of a typical aqueous electrolyte at two concentrations and their performance in a wide temperature range. The thermal stability of the micro-solvation structure of the high concentration system was analyzed via the RDF and PMF, providing a microscopic explanation for the excellent performance of super-concentrated aqueous electrolyte at extreme temperatures. The analysis of the connection matrix could further clarify the microscopic picture of the studied system. The analysis of the properties of Li–Li dimers could be related to the structure and dynamic behavior. The study of dynamic properties such as viscosity, diffusivity and conductivity showed the intrinsic coupling between them and their relationship with structural properties. In particular, the coupling relationship between collective dynamics and conductivity in two different concentration electrolyte systems was proposed and compared.