Understanding Mg-ion speciation is of utmost importance to the rational design of electrolytes for rechargeable magnesium batteries. Herein, we report an effort to better understand Mg-ion speciation in ionic liquid (IL) electrolytes through the design of alkoxy-functionalized cations with different alkoxy substituent. In contrast with previous studies focusing on the coordination sphere of Mg2+, the comparison of Raman spectroscopy, electrochemical and DFT calculation results of various IL-based electrolytes suggests that the coordination sphere of transient Mg+ plays a key role in Mg reversible deposition/dissolution process. Finally, a prototype Mg/V2O5 cell using the noncorrosive IL-based electrolyte is demonstrated for the first time, exhibiting a remarkable initial discharge capacity of 140 mAh g−1 and reversible capacity of 100 mAh g−1.