Electrolytes for Magnesium-Ion Batteries: Next Generation Energy Storage Solutions for Powering Electric Vehicles
Akhila Das, Anjumole P. Thomas, Neethu T.M. Balakrishnan, Jishnu N.S., Jabeen Fatima M. Jou-Hyeon Ahn, Prasanth Raghavan
Introduction
Rechargeable batteries are still in their infancy meeting the requirements of energy storage applications such as electric vehicles, portable energy storage devices, etc. [1, 2]. A need for sustainable and economically viable energy storage devices is crucial. Currently, lithium-ion batteries rule the electrochemical world, but they are handicapped by their risk of hazards and cost [3]. Magnesium-ion batteries are an alternative system for lithium-ion batteries and are superior in terms of safety. Redox
FIGURE 8.1 Schematic illustration of the structure and working principles of an Mg-ion battery (charging cycle).
potential of magnesium (Mg) is -2.38 V [4], which is greater than lithium (-3.04 V), paving the way for an easy charge-discharge process. The charging process of batteries leads to the intercalation of ions (Li7Mg2+) in the cathodes; the volume of occupancy for two lithium (Li+) ions is higher than that of a single Mg2+ ion [4, 5]. Therefore, because of the bivalency of Mg ions, more charge can be stored in a minimum volume, which is an added advantage for miniaturization of energy storage devices. During lithiation process, volume expansion occurs, and, as a result, structural deformation takes place in the process of delithation at the cathode, decreasing the electric contact and cyclability [6-8]. Alkali metals in anodes are not safe because they are reactive and have the potential for dendrite formation. Mg metals are compatible with anodes and are safe; a schematic diagram of Mg-ion battery is shown in Figure 8.1. Research on Mg-ion batteries has impelled its importance in many industries and academic institutions. Advanced Project Research Agency, Pellion Technologies, and Toyota Research Institute in North America initiated the development of Mg-ion batteries. Pellion Technologies focused mainly on the core issue related to Mg-ion deposition/dissolution and found an alternative method to tackle problems [8,9]. At the same time, Toyota Research Institute’s research focused mainly on developing high-energy, high-density Mg-sulfur (Mg-S) batteries.
Mg-ion Battery Chemistry
The basic chemistry behind the Mg-ion battery mechanism is similar to that of the lithium-ion battery. Mg-ion batteries consist of (i) anodes made of Mg metal/metal alloy; (b) electrolytes, which can be organic liquid electrolytes, non-nucleophilic electrolytes, polymer electrolytes, etc.; (d) cathode materials capable of reacting with Mg ions; and (d) separators, such as microglass fiber, glass fiber, polypropylene non- woven membranes, etc. [10, 11]. Cathodes are layered materials capable of intercalating Mg ions, whereas anodes are materials that release Mg cations. During the charging process, electrons are ejected from the cathode through the external circuit, whereas Mg ions will pass to anodes through the electrolytes. During the discharging process, the reverse process takes place in which electrons move from anodes to cathodes through the external circuit. The deintercalation of Mg ions occurs during this time. The basic mechanism behind charging and discharging is given in Equations 8.1 and 8.2:
Further research on Mg-ion batteries is favored because Mg metals are safer than lithium metals and it does not cause dendrite formation. However, when Mg metals come in contact with conventional organic solvents, a thin passivation layer develops on the surface of the Mg metal that is impermeable to the Mg-ion movement that imparts the electrochemical performance of the battery. Thus, the type of electrolytes chosen has a major role in the development of a sustainable battery. Therefore, researchers are focusing mainly on the development of suitable electrolytes that could avoid the problems experienced with conventional electrolytes.
Electrolytes for Mg-ion Batteries
Electrolytes are the core part leading the development of Mg-ion battery. The type of electrolyte chosen has a major role in determining the electrochemical stability and Mg metal compatibility of the battery. Salt-solvent interactions, chemical reactions at electrode-electrolyte interfaces, migration mechanisms of Mg2+, etc. are the major concerns when choosing electrolytes [5, 12, 13]. The major classification of electrolytes in Mg-ion batteries are given in the following sections.
