Rechargeable Magnesium/Oxygen Batteries: Reaction Mechanisms and Their Dependence on Electrolyte Composition.

Abstract

Electrochemical energy storage devices that are robust, energy-dense, and cheap will accelerate the commercialization of electric vehicles. Magnesium/Oxygen (Mg/O2) batteries are a promising system with the potential for very high energy densities. The goal of this dissertation is to explore candidate magnesium electrolytes for use in Mg/O2 batteries, and to assess the reaction mechanisms and performance of Mg/O2 cells that employ these electrolytes. We first consider electrolytes based on ionic liquids (ILs), which are attractive because they are nonflammable and nonvolatile. The absence of voltammetric signatures of Mg plating from ILs with Tf2N– and BF4– suggests that strong Mg/anion Coulombic attraction inhibits electrodeposition. Cosolvent additions to Mg(Tf2N)2/PP13-Tf2N were explored but did not result in enhanced plating/stripping activity. The results highlight the need for IL solvents or cosolvent systems that promote Mg2+ dissociation. We next describe a room-temperature, non-aqueous, reversible Mg/O2 cell using a modified Grignard electrolyte. Electrochemical, microscopic, and spectroscopic analyses reveal characteristics that distinguish Mg/O2 chemistry from its alkali-metal/O2 analogues. The open-circuit voltage is 2.0 V, lower than the ~2.9 V expected for direct electrochemical formation of MgOx. The low voltage and two-phase product are consistent with a multi-step discharge reaction in which a superoxide-ion (O2–) intermediate forms at ~2 V vs. Mg/Mg2+. Bypassing the multi-step mechanism in favor of direct electrochemical MgOx formation would raise the discharge potential and the energy density. The performance of the preceding modified-Grignard-based cell is then compared with one based on an all-inorganic magnesium aluminum chloride complex (MACC) electrolyte. Mg/O2 cells using the MACC electrolyte exhibit higher discharge capacities than those based on the modified Grignard electrolyte. However, rechargeability is limited. The discharge product is found to be an inhomogeneous mixture of MgCl2 and Mg(ClO4)2. Electrochemical impedance spectroscopy measurements are used to identify that film formation on the Mg anode is main source of impedance in Mg/O2 cells. This research shows that the performance of Mg-based batteries is highly sensitive to the electrolyte. For future work, it would be useful to develop electrolytes that limit SEI film formation on the Mg anode and could result in a MgO2 discharge product to enhance rechargeability.