Role of Mesoporosity in Hard Carbon Anodes for High-Energy and Stable Potassium-Ion Storage

Herein, NaCl-templated mesoporous hard carbons (NMCs) have been designed and engineered with tunable surface properties as anode materials for potassium-ion batteries (KIBs) and hybrid capacitors (KICs). By utilizing "water-in-oil" emulsions, the size of NaCl templates is precisely modified, leading to smaller particles that enable the formation of primary carbon structures with reduced particle size and secondary structures with 3D interconnected mesoporosity. These features significantly enhance electrode density, reduce particle-to-particle resistance, and improve electrolyte wettability. The resulting NMC delivers high gravimetric and volumetric capacities of 323.5 mAh g^-1 and 142.3 mAh cm^-3, respectively, with outstanding cycling stability, retaining 292.7 mAh g^-1 after 620 cycles. Full-cell evaluations demonstrate the potential of NMC-based anodes paired with KIB (Prussian blue cathode) and KIC (activated carbon-cathode), achieving a capacity of 109.4 mAh g^-1 and capacitance of 254.6 F g^-1, respectively. As a KIB, the cell delivers a high average voltage of 3.4 V, resulting in a specific energy of 256 Wh kg^-1, while KIC reaches 141.4 Wh kg^-1 and high specific power of 58.6 kW kg^-1. These results underscore the importance of morphological features and porosity in enhancing the electrochemical performance of materials, providing insights for future energy storage solutions.