A Se-Doped S@CMK3 Composite as a High-Performance Cathode for Magnesium–Sulfur Batteries with Mg2+/Li+ Hybrid Electrolytes

A magnesium–sulfur (Mg–S) battery system is considered a high-performance candidate for next-generation secondary batteries, because of its prominent advantages such as high safety, low cost, and high energy density. However, the shuttle effect caused by the dissolution of polysulfides leads to irreversible loss of capacity. Herein, we prepare a S0.96Se0.04@CMK3 composite (with a high S + Se content of 82.6 wt %) and employ the composite material as a cathode for Mg batteries with easily prepared hybrid electrolytes. The electronic conductivity of the cathode material increases by 18 orders of magnitude after Se doping, from 5 × 10–28 to 6.5 × 10–10 S m–1. Compared with S@CMK3, doped Se in S0.96Se0.04@CMK3 plays an important role in maintaining capacity stability by lowering polarization, facilitating electron transport, and improving reaction kinetics. The S0.96Se0.04@CMK3 cathode delivers a stable and reversible capacity of 492.9 mAh g–1 after 110 cycles, which is almost twice that of the S@CMK cathode without Se doping, and better rate performance. This Se-doping strategy proposes an effective method to exploit high-performance cathodes for Mg–S batteries.