Electrical Energy Storage By a Mg/Cus Accumulator: An Alternative to Lithium Batteries
Authors: Westphal Anne , Neumann Johannes , Thiele Philipp , Ludwig Ralf 
Published: February 2020

Rechargeable magnesium batteries are considered to be very promising candidates for next generation non-lithium batteries.[1-5] They use divalent Mg2+ cations instead of monovalent Li+ leading to high theoretical capacities. In addition, magnesium (used as anode) has the lowest standard electrode potential compared to other multivalent metals like aluminum or zinc which will be reflected in high cell voltages. The higher safety, environmental compatibility of the electrode materials, and the low costs of magnesium due to its high abundance compared to lithium are further advantages. Numerous research activities focus on the investigation of new cathode materials.[6-9] In addition, a high electrochemical stability of the electrolyte, especially towards the magnesium anode, is absolutely essential. To develop new electrolytes with improved physical and electrochemical properties therefore is another challenging task in this field.[10-11]

In our presentation a different concept of a magnesium battery will be shown. The general idea is to use a CuS cathode which dissolves upon discharge when Cu2+ is reduced to Cu. At the anode Mg is oxidized to Mg2+ forming MgS as a solid discharge product. Here, in contrast to already known magnesium batteries, the divalent S2- anions will be used for the charge transfer instead of the Mg2+ cations. In consequence, a suitable sulfide containing electrolyte is required. It should be noted that, provided that the proposed electrode reactions are fully reversible, the suggested Mg/CuS accumulator can theoretically also be set up in the “discharged” state, i.e. MgS vs. Cu instead of Mg vs. CuS.
Besides the processing of the Mg, Cu, MgS, and CuS electrodes the preparation, simulation and electrochemical testing of potential electrolyte candidates is the main focus of this project. Alkali metal sulfides solved in mixtures of organic solvents are chosen as reference electrolytes to be compared with new electrolytes based on ionic liquids. The electrochemical properties are characterized by cyclic voltammetry and impedance spectroscopy. The electrochemical stability of the system and the reversibility of the proposed electrode reactions have to be analyzed by half-cell cycling and full-cell charge/discharge tests. At this, Raman spectroscopy will be a valuable analytical tool to investigate the individual electrode reactions, i.e. the formation and dissolution of MgS and CuS. Experiments are accompanied by MD simulations and physical properties like diffusion coefficients, ion conductivity, etc. can be understood at a molecular level.

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