Morphology-dependent synergy in high-performance MnO₂/activated carbon electrodes via facile dry-mixing

The pursuit of high-performance supercapacitors (SCs) often revolves around developing novel nanostructures through complex chemical synthesis, which poses challenges for scalable production. Herein, we demonstrate that exceptional electrochemical properties can be achieved by a simple dry-mixing of activated carbons (ACs) and manganese dioxide (MnO₂). A series of ACs with distinct morphologies were prepared and composited with petal-like MnO₂ clusters. We systematically investigated the profound impact of AC morphology on the synergistic effects with MnO₂. The optimal hybrid electrode, comprising spherical carbon (ACS-2.5) and 25 wt% MnO₂, delivered a superior gravimetric capacitance of 430.44 F g^-1, a remarkable volumetric capacitance of 357.27 F cm^-3 and an outstanding capacitance retention of 93.41 ± 0.23% after 5000 cycles. Furthermore, this electrode achieved an exceptional energy density of 14.96 Wh kg^-1 at a power density of 61 W kg^-1. Notably, the synergy was found to be highly morphology-dependent: a universal optimal MnO₂ ratio of ∼25 wt% was identified for lamellar ACs, whereas the ideal ratio for spherical ACs increased with their diameter. Electrochemical impedance spectroscopy (EIS) and detailed mechanistic analysis revealed that MnO₂ acts as a multifunctional agent, serving as an interparticle conductive bridge and intraparticle spacer, thereby drastically reducing charge transfer resistance and enhancing ion diffusion. Critically, the incorporation of MnO₂ simultaneously increased the electrode compaction density, which is crucial for volumetric energy storage performance. This work provides fundamental insight into the structure-property relationship in hybrid electrodes and establishes a simple, scalable, and effective paradigm for designing high-performance energy storage devices by leveraging the synergy between conventional materials.