Hybrid Mg2+/Li+ batteries featuring dendrite-free Mg anode and Li-storage cathode are promising safe energy storage systems. However, improving the energy density remains a grand challenge due to the quite limited capacities of traditional Li-intercalation cathodes. To circumvent this limitation, a new type of conversion cathode for hybrid Mg2+/Li+ batteries, microflower-like Cu9S5, is reported herein. The Cu9S5 cathode is compatible with the dual-salt Mg2+/Li+ electrolyte, and delivers a high reversible capacity of 300 mAh g−1 at 50 mA g−1 via electrochemical conversion reaction, with 94% capacity retention over 100 cycles in hybrid Mg2+/Li+ batteries. The electrode also shows an outstanding rate capability providing 155 mAh g−1 at 1000 mA g−1 and a superior long-term cycleability over 1000 cycles. This excellent performance is attributed to the hierarchical architecture of Cu9S5 which facilitates the reversible conversion reactions and stabilizes the electrode structure during discharge/charge process. This work provides new insight for the nanostructure design of stable conversion electrodes, and opens a new avenue for building safe and high-performance hybrid Mg2+/Li+ batteries for practical applications.