In this work, experimental and computational methods based on density functional theory (DFT) were used to study the spontaneous degradation (i.e. in the absence of any external electromotive force) of pure metallic magnesium (Mg) and magnesium stannide (Mg₂Sn) in all-phenyl complex (APC) electrolyte, a commonly used Mg-ion battery electrolyte. Our results reveal that Mg atoms can be stripped from metallic Mg and from Mg₂Sn through galvanic replacement reactions involving anions present in APC such as Ph₄Al^ˉ, Ph₂AlCl₂^ˉ, PhAlCl₃^ˉ and AlCl₄^ˉ, resulting in Mg dissolution and metallic aluminum deposition. Such a spontaneous degradation phenomenon is susceptible to impact the performance of Mg-ion battery cells in which metallic Mg or magnesium alloys are used as anodes. In particular, we have found that Mg-ion battery half-cells assembled using Mg₂Sn and metallic Mg as the working and counter electrodes, respectively, and APC as the electrolyte, exhibit Coulombic efficiencies higher than 100% at low C-rates. Such an unusual Coulombic efficiency is attributed to unbalance charge associated with the difference in Mg atoms dissolution rate between the Mg₂Sn working electrode and the Mg metal counter electrode, when these electrodes undergo spontaneous degradation in APC.