  Searching for efficient and high-performance cathode materials for
rechargeable magnesium ion batteries (RMBs) is urgent for exploring sustainable
energy technologies. However, the majority of cathode materials for RMBs
usually suffer from low rate capacity and inferior cycle performance caused by
structure collapse. Herein, ternary CuCo$_2$S$_4$/Cu$_7$S$_4$ composites with
nano-sized are first synthesized by a facile solvothermal method in this work
and the magnesium ion storage behavior is discussed when applied in the cathode
for RMBs. Electrochemical results demonstrate that the nanosphere-like
CuCo$_2$S$_4$/Cu$_7$S$_4$ composites exhibit a high initial discharge capacity
of 256 mAh/g at 10 mA/g and 123 mAh/g at 300 mA/g at room temperature.
Furthermore, an outstanding long-term cyclic stability with a reversible
capacity of 106 mA/g after 300 cycles and the coulombic efficiency of about 99%
are achieved at 300 mA/g. Such remarkable electrochemical performance is owing
to the nanosphere-like structure, ternary composition and pseudocapacitive
storage behavior of CuCo$_2$S$_4$/Cu$_7$S$_4$. The results of this work
indicate that the CuCo$_2$S$_4$/Cu$_7$S$_4$ nanocomposites synthesized by a
facile and efficient method are promising cathodes for RMBs in energy
storage/conversion application.
