New insights into Self‐discharge and Heat Generation in Magnesium Batteries

Mo$_6$S$_8$ in the Chevrel Phase (CP) and 14-polyanthraquinone (14PAQ) cathode materials-based coin cells were assembled against Mg-foil as an anode by using 0.3 M magnesium tetrakis (hexafluoroisopropyloxy) borate Mg[B(hfip)$_4$]$_2$/dimethoxyethane (DME), 0.5 M Mg[B(hfip)$_4$]$_2$/DME and 0.5 M Mg[B(hfip)$_4$]$_2$/tetraglyme (G4) electrolytes. The heat generation of those three variants was compared using a sensitive MS80 Tian-Calvet calorimeter. The Chevrel Phase Mo$_6$S$_8$ was found to generate less heat than the organic 14PAQ. However, its specific capacity was also comparatively lower than for the organic cathode material. It is equally important for battery kinetics to have a well-designed electrolyte, therefore different solvents with the same electrolyte salt were utilized. Noticeable differences were observed and in tetraglyme solvent stable cycling and fewer self-discharge phenomena were detected. However, the activation process needs more cycles to achieve the required capacity in the case of the Chevrel Phase. The generated heat during cycling indicated the high resistances, swelling/contraction in organic cathodes leading to higher heat generation, and poor capacity retention. ... mehrTo overcome self-discharging in Mg batteries, side reactions/dissolution of cathode materials, electrolyte saturation and the formed interfaces on the anode side must be considered.