Research

The rapid growth in the global economy during the recent decades has resulted in an exponential rise in fossil fuel consumption. As a result of this, two major problems have come into light. The first is the accelerating depletion/exhaustion of existing fossil fuel reserves, and the second is climate change resulting from increased greenhouse gases. The need to develop and expand sustainable and clean energy resources and the related technologies is recognized as an urgent priority worldwide. Most renewable clean energy sources are highly dependent on the time of day and the local weather conditions. Therefore, in order to effectively collect energy from intermittent energy sources, it is necessary to develop efficient energy conversion and energy storage devices. In this regard, batteries and other electrochemical devices like supercapacitors are recognized as highly important.

In general, electrochemical devices consist of the cathode, the anode, the separator, and the electrolyte. Compared to the solid materials that make up the cathode and the anode, the components or the composition of the electrolyte have been less studied; even though the electrolyte is an essential part of an energy storage device. The electrolyte is placed between electrodes in the device and serves as the media which facilitates ion transport. Ideally, an electrolyte should have the following characteristics (1) a high ionic conductivity; (2) a wide electrochemical stability window; (3) a high chemical stability; (4) a wide operating temperature range; (5) low volatility and non-flammable; (6) environmentally friendly; and (7) have a low cost. In the case of lithium-ion batteries, where the electrolyte is not compatible with the cathode material at a high potential (> 4.2 V vs Li/Li⁺) and the anode material at a low potential (< 0 V vs Li/Li⁺) at the same time, the solid electrolyte interphase (SEI) on the anode, and/or the cathode electrolyte interphase (CEI) on the cathode are necessary for the smooth operation of the device. Such passivation layers do the job of preventing side-reactions between electrodes and electrolyte. Unfortunately, it is very challenging to find an electrolyte that meets all these requirements, as each electrolyte has its own set of advantages and disadvantages. Without a doubt, the performance of energy storage systems can be significantly improved by advancements in better performing electrolytes.

An electrolyte solution (liquid phase) typically comprised of solvents, salts, and additives. To enhance the property of electrolytic systems, the right components in the optimum molecular ratios should be carefully selected. These days, the performance of batteries with the electrolyte prepared at a non-conventional concentration, such as high a (> 3 M) or low (< 0.5 M) concentration, can dramatically improve the performance of the battery without the need of changing the components. In the electrolyte, the ion species can exist as free form, contact ion pairs (CIPs), or aggregates (AGGs); all of which possess different characteristics such as the nature of molecular interactions, molecular orbital energies, and transport properties. In other words, electrolytes with different solvation structures, modulated by the concentration, should be classified as different electrolyte systems.

However, so far, improvements in electrolyte performance have been conducted based on trial-and-error or experience-based methods, which is time-consuming. To develop the electrolyte system efficiently, a computer-aided design can be adapted. The candidate molecules can be screened by determining key properties via theoretical calculations: by quantum calculations, the HOMO level and the LUMO levels, the oxidation and reduction potentials, the interaction energies between molecules in the electrolyte, can be estimated; while molecular dynamics simulations can predict the solvation structure, ionic conductivity, and shear viscosity. Then the selected molecule or electrolyte system can be further evaluated by the experimental measurements. In addition, to increase the accuracy of predicting electrolyte properties better computational models can be developed.

Although prolonged scientific efforts have been devoted to developing electrolytes for different electrochemical systems, the types of chemical species used so far are limited. This is because it is quite challenging to find novel molecules that satisfy all the requirements mentioned previously. Furthermore, researchers have been more dedicated to applying known molecules to electrolyte systems and have devoted less time to finding novel molecules. In spite of the difficulties, it is quite meaningful to explore new chemical species. New molecules can be applied to improve the performance of a given electrochemical system. Furthermore, other research groups might further evaluate the chemical species for other various types of electrochemical systems, bringing breakthroughs in various devices. By using the aforementioned strategy, new chemical species can be proposed to the field of material science.