How solution chemistry affects the electrochemical behavior of cathodes for Mg batteries, a classical electroanalytical study

Mo₆S₈ Chevrel phase (CP) is still the most effective magnesium ions insertion material that was examined in cathodes for rechargeable Mg battery model systems. Its synthesis includes CuMo₆S₈ as an intermediate which undergoes Cu ions leaching. Thereby, CP materials may contain small amounts of copper. Cu_xMo₆S₈ undergoes complex magnesium insertion processes that include reversible displacement of copper. We report herein on a study of Cu_0.09Mo₆S₈ electrodes in two electrolyte solutions which are important for secondary Mg batteries: C₆H₆MgCl/AlCl₃/THF (all phenyl complex solutions, APC) and MgTFSI₂/MgCl₂/DME (Mg imide/chloride/DME solutions, ICD). There systems are ideal playgrounds for studying the nature of complex magnesium insertion processes into inorganic hosts. The analysis reported herein shows how the electroanalytical behavior of CP electrodes is affected by the electrolyte solution identity. For instance, the main Mg ions insertion process from the APC solutions occurs at 160 mV lower than from the ICD ones; the heterogeneous rate constant for charge transfer across the interface are significantly higher in APC solutions; in turn, the conversion processes related to the Cu residue in the CP electrodes are kinetically and thermodynamically favorable in the ICD solutions. As expected, the magnesium solid-state diffusion coefficient was found to be solutions independent. The study demonstrates the sensitivity of electroanalytical tools such as slow scanning rate voltammetry for reflecting fine structural changes during intercalation processes and can be generalized to other systems.