Roles of Additives in the Trihexyl(tetradecyl) Phosphonium Chloride Ionic Liquid Electrolyte for Primary Mg-Air Cells

Abstract

As reported previously, water saturated trihexyl(tetradecyl)phosphonium chloride ([P 6,6,6,14 ][Cl]) ionic liquid (IL) is a promising electrolyte for magnesium-air batteries. The added water plays an important role in enabling high rate and high efficiency Mg dissolution while stabilizing the Mg interphase. In this work, the role of the water was investigated by replacement with other additives such as toluene and tetrahydrofuran to specifically target the assumed roles of water, namely: (i) enhancement of transport properties; (ii) complexation and stabilization of the Mg anode; (iii) provision of active protons for the cathodic reaction. Discharge tests show that ethylene glycol supports comparable performance to that provided by water. Examination of the viscosity and conductivity of different [P 6,6,6,14 ][Cl]/additive mixtures indicates that a simple consideration of solution characteristics cannot explain the observed trends. Rather, other factors, such as the presence of active protons and/or oxygen-donor groups, are also key features for the development of IL electrolytes for practical magnesium-air cells. Finally, the presence of ethylene glycol in the electrolyte results in a complex gel on the Mg interface, similar to that found in the presence of water. This may also play a role in enabling stable discharge of the Mg anode.Metal-air batteries are attracting increasing levels of interest because their high theoretical energy densities can satisfy a growing number of high energy applications. A metal-air battery is composed of a metallic anode and a porous cathode, separated by an electrolyte. 1 The utilization of an air cathode means that the cathodic reactant (oxygen) does not contribute to the mass of the cell (at least in the fully charged state), which leads to inherently high values of specific energy. 1 Historically, metal-air cells and batteries have been viewed as primary (single-discharge) devices, due to the inherent irreversibility of the oxygen reduction reaction in traditional electrolytes. The ORR in aqueous systems leads to the formation of OH − which, in the presence of Mg 2+ from the anodic dissolution of Mg during discharge, likely forms Mg hydroxide, a compound that cannot be converted back to oxygen by electrochemical reduction (i.e., 'recharging'). By comparison, in the case of reversible metal-air systems, such as Lithium-air for instance, the ORR reaction is conducted in non-aqueous media in which the formation of the peroxide can be reversible. 1-3 In recent years, though, the development of greater levels of understanding of electrochemical processes in non-aqueous media has led to a more widespread consideration of the reversible operation of both the oxygen (air) cathode, and a range of reactive metal anodes. 1 To date only the zinc-air system has been commercialized in the form of primary cells and batteries. 2 So-called 'rechargeable' Znair, which attracted some attention during the 1990s, was actually a primary system that was mechanically recharged by replacing spent zinc...

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