Computational simulation assists in the design of solid electrolytes of polymers with high salt concentrations

On July 28, 2022, Dr. Fangfang Chen, Dr. Xiaoen Wang, Professor Maria Forsyth, Senior Research Fellow, Institute for Advanced Materials, Deakin University, Australia, and Professor Michel Armand of CIC EnergiGUNE in Spain published an online article titled “Cationic Polymer-in-salt electrolytes for fast metal” in the journal Nature Materials ion conduction and solid-state battery applications” new research.

Through molecular dynamics simulation, the study reveals that new high-salt concentration ionic liquid polymers can achieve rapid conduction of metal sodium and potassium ions, providing new ideas for the future development of different types of high-energy density solid batteries. This work demonstrates how to efficiently and rationally design the optimal structure of the electrolyte through theoretical calculations, and explores the transport mechanism of metal ions in such polymer solid electrolytes. The predicted material system was finally successfully verified experimentally.

The first and corresponding authors of the paper are Dr. Fangfang Chen, and the co-corresponding authors include Dr. Xiaoen Wang and Professor Maria Forsyth.

Future high-energy density battery technologies will require the use of a new generation of electrode materials, such as metallic lithium, sodium or potassium. At the same time, the development of new electrolyte materials to match them is essential for the safe, stable and long-term use of high-energy batteries. In recent years, solid-state batteries have been a hot topic in battery research and development. The development of solid-state batteries based on polymers is considered to be an important way to solve the safety hazards of batteries. At present, the concerned polymer electrolyte is generally unable to obtain high metal ion conductivity and ion migration number at the same time. The target ion mobility number characterizes the contribution of metal ion conduction to the total conductivity of the electrolyte. High ion mobility is an important indicator of supporting stable battery charge and discharge, and its size is highly correlated with the diffusion speed and concentration of metal ions. In conventional polymer electrolytes this indicator cannot be significantly improved due to the use of low concentrations of salt and faster anion movement. For example, the number of ion migration reported by the PEO system is about 0.2, which adversely affects the interface and battery performance. At present, although the ion migration of polymer single-ion electrolytes based on polymeric anions is high (up to 1), its conductivity is generally poor.

Cationic polymers based on ionic liquids are a novel electrolyte, and Dr. Fangfang Chen and Xiaoen Wang reported some unique properties of such electrolytes in their research into the application of this polymer in lithium batteries (Joule, 2019, 3, 2687-2702). In this work, Dr. Fangfang Chen used classical molecular dynamics methods to study the conduction of metal ions other than lithium ions in highly concentrated ionic liquid polymers, including sodium, potassium and magnesium metal ions. First, the work proposes a method for designing and predicting the optimized structure of ionic liquid polymers based on calculations. This is mainly determined by calculating the proportion of the three coordination structures of the anions at different salt concentrations. The salt concentration of the co-coordination structure of “polycation-anion-metal ion”, given the highest percentage, can be used as a starting point for experimental studies of materials.

Figure 1: (Fig 1c in the original figure) The ratio of FSI anions in three coordination structures in different systems.

Then, in the study of several metal ion systems with the same anion concentration, it was found that the self-diffusion coefficient of the metal ions was approximately linearly correlated with the binding energy of the alkali metal ion pair at 353 K (80 °C). That is, the lower the binding energy of metal ions and anions, the faster the diffusion. This suggests that the low binding energy of metal ion and anion pairs is a key to obtaining high metal ion motion.

Secondly, the chemical environment of the fastest and slowest moving sodium and anion groups was studied in the Na21 system. The results showed that (1) the speed and slowness of Na ions and anions were highly correlated; (2) Anions and metal ions that are close to the polymer or have more polycation coordination numbers move slowly, and vice versa. (3) Metal ions move faster in areas where the structure of lysate salts is enriched.

Fig. 2: (Fig2a, 2b in the original figure) (a) The relationship between the ion self-diffusion coefficient and the binding energy of the ion pair; (b) The chemical environment of fast (red) slow (blue) sodium ions.

Next, by tracking and analyzing the coordination structure of metal ions, it is found that metal ions diffuse through structural mechanisms in ionic liquid polymers with high salt concentrations, which is different from the diffusion mechanism of low-salt systems. Finally, the work experimentally verified the computational prediction system, which confirmed that the K12 system obtained the highest conductivity at the theoretical predicted temperature of 80 °C and above, while the Na12 system had the overall high conductivity at different temperatures. The Na12 system also carried out a preliminary battery cycle performance analysis, and obtained promising results. The work points to the enormous potential of highly concentrated ionic liquid polymers in the development of multi-type solid-state batteries.

Figure 3: Experimental study analysis and symmetric battery performance analysis of theoretical prediction system.

(Source: Science Network)

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