Chevrel compounds are regarded as positive-electrode materials of a magnesium rechargeable battery (MRB), but their redox potential is about 1.2 V \vsMg. In this work, we show logically and experimentally a new aspect of the Chevrel compounds that their redox potential can be as high as about 2-3 V \vsMg. A crucial basis on this aspect is that Cu cations can be extracted from \CMS\ around 1.2 V \vsMg\ in the conventional electrolyte (Grignard-reagent/tetrahydrofuran) while the Cu electrodeposition can occur at about 1.7 V \vsMg\ in the same electrolyte, which means that the chemical potential of Cu in the Chevrel compounds is higher than that in pure Cu metal. This thermodynamic contradiction inevitably compels us to consider a certain interaction with the solvent other than the simple deintercalation from the compound, which is discussed through the paper. With the use of large molecule solvent or ionic liquids, we have observed an intriguing relaxation phenomena, where the cations move to find more stable sites, which directly indicates that the Chevrel compounds have several sites for cations.