A method for forming a solvo-ionic liquid suitable for use as an electrolyte in an electrochemical cell is provided. The solvo-ionic liquid, a mixture including a multidentate ethereal solvent and magnesium borohydride, can be a liquid, a gel or a solid at room temperature and generally has high thermal stability including virtually no volatility at a typical cell operating temperature. An electrochemical cell having a solvo-ionic liquid as electrolyte is also disclosed. The electrochemical cell will typically be a rechargeable magnesium battery, having an anode suitable to accommodate magnesium oxidation during battery discharge.