Conventional electrolytes made by mixing simple Mg²⁺ salts and aprotic solvents, analogous to those in Li-ion batteries, are incompatible with Mg anodes because Mg metal readily reacts with such electrolytes, producing a passivation layer which blocks Mg²⁺ cation transport. Here, we report that, through tuning a conventional electrolyte—Mg(TFSI)₂ (TFSI^- is N(SO₂CF₃)₂^-) with an Mg(BH₄)₂ additive—highly reversible Mg plating/stripping with high coulombic efficiency close to 100% is achievable, by decoupling the interaction between Mg²⁺ and TFSI^- and enhanced reductive stability of free TFSI^-. A critical adsorption step between the Mg⁰ atoms and active Mg cation clusters involving BH₄^- anions is demonstrated on evolving electrified interface based upon analysis of distribution of relaxation times (DRT) from operando electrochemical impedance spectroscopy (EIS), operando electrochemical X-ray absorption spectroscopy (XAS), nuclear magnetic resonance (NMR), and density functional theory (DFT) calculations. This study suggests a new approach for developing advanced electrolytes and provides an in-operando analysis kit of probing electrified interfaces with adsorption of active Mg cation clusters for rechargeable Mg batteries.