The well-known all phenyl complex (APC) electrolyte for magnesium batteries is studied for the first time at high temperature using tetraglyme as a solvent. Combined with a molybdenum current collector, this enables the examination of positive electrode materials for Mg batteries at temperatures as high as 180 °C and up to 2 V vs. Mg, allowing discovery of the auspicious properties of CuS as a conversion cathode.