To realize high-rate and long-term performances of an aqueous rechargeable battery, the most effective approach is to build electrode materials with more reaction active sites and stable structures. Transition metal sulfides have become up-and-coming electrodes due to their high conductivity. Herein, we demonstrated the in situ construction of core-shell Co9S8@C materials with controlled carbon content and thickness. Nanorod-like cobalt-organic chelates were used as the precursors. The cobalt in cobalt-organic chelates reacted with sublimed sulfur to generate the Co9S8 core in situ; meanwhile, the organic chelates were converted into carbon shells, which coated the Co9S8 core and connected with each other to maintain the whole rod shape. Moreover, tunable thickness and content of the carbon shell in the Co9S8@C composite could be achieved by regulating the composition of the reaction solvent. In addition, when 20 mL of dimethylcarbinol was used, the obtained Co9S8@C composite (H1) exhibited the most excellent electrochemical performances, in particular outstanding cycling stability. When assembled with a treated iron powder (TIP) electrode, the Co9S8@C//TIP aqueous rechargeable battery delivered 220.7 mA h g-1 discharge capacity at 1 A g-1, which decreased to 152.8 mA h g-1 even when the current density was increased by a factor of ten (10 A g-1), indicating surprising high-rate performance. Also, after 5000 cycles at 10 A g-1, 74.8% of capacity retention was obtained, further illustrating its excellent long-term cycling stability. Suitable electrode materials with a tunable carbon content have direct impact on the overall performance of an aqueous rechargeable battery, which will guide us for obtaining high-rate and long-term aqueous batteries.