Crystallinity Engineering of Coal-Derived Ball-Milled Carbon for High-Performance MnO₂ Cathodes in Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries (AZIBs) are attractive as a safe and affordable energy storage device, but enhancing the conductivity and ion transport in the electrodes is difficult. In this work, coal-derived ball-milled carbon (CBMC) with varying crystallinity was prepared as a conductive additive for MnO₂ and compared to commercial Super P. CBMC 1500 has the best performance, with a high specific capacity of 274 mAh g⁻¹ at 25 mA g⁻¹ and excellent cycle stability (89% retention after 900 cycles), in contrast to Super P (22% retention). It also demonstrates better rate performance and smaller charge transfer resistance than CBMC 850 and CBMC 2700. This improved performance is due to its intermediate structure, which offers a balance between electrical conductivity and ion diffusivity, ensuring efficient charge transfer and stable electrochemical processes. This structure was also found to have the best balance between electronic conductivity and Zn²⁺ binding energy, according to density functional theory (DFT) calculations. This study shows the coal-derived CBMC as a low-cost and high-performance alternative to traditional conductive additives for AZIBs.