Studying δ-MnO2/reduced graphene oxide composite cathode in a low-temperature and high-voltage-tolerant hybrid electrolyte for aqueous Mg-ion batteries

Abstract Optimization of the aqueous electrolyte concentration is a significant issue in the development of high-performance aqueous rechargeable magnesium ion batteries (MIBs). In this study, a novel magnesium ion-based hybrid electrolyte composed of 2 M magnesium sulfate (MgSO 4 )/2 M acetate (MgOAc) was designed, and its corresponding physiochemical properties were systemically investigated by simply tuning their molar ratios. Additionally, a δ -MnO 2 /reduced graphene oxide (rGO) composite cathode material was successfully synthesized and delivered a high specific capacity and excellent rate capability in the optimized hybrid electrolyte. The as-fabricated device based on the δ -MnO 2 /rGO composite cathode exhibited a high operating voltage of up to 2 V and delivered a maximum energy density of 29.8 Wh kg −1 at the power density of 823 W kg −1 . More importantly, the device showed impressive discharge capacity and excellent cycling stability even at the low temperature of −20 °C. In view of the outstanding electrochemical properties of the δ -MnO 2 /rGO composite cathode in an optimized hybrid electrolyte of MgSO 4 /MgOAc, it could be regarded as a novel prototype for low-cost aqueous MIBs.