Monodisperse Hierarchical N-Doped Carbon Microspheres with Uniform Pores as a Cathode Host for Advanced K–Se Batteries

K–Se batteries offer high energy density and cost-effectiveness, making them promising candidates for energy storage systems. However, their practical applications are hindered by Se aggregation, sluggish ion diffusion, and significant volumetric expansion. To address these challenges, monodisperse hierarchical N-doped carbon microspheres (NCHS) with uniformly sized pores were synthesized as cathode hosts. The flower-like microstructure, formed by the assembly of two-dimensional building blocks, mitigated Se aggregation and facilitated uniform distribution within the pores, enhancing Se utilization. Nitrogen doping, introduced during synthesis, strengthened chemical bonding between selenium and the carbon host, suppressed side reactions, and accelerated reaction kinetics. These synergistic effects enabled efficient ion transport, improved electrolyte accessibility, and enhanced redox reactions. Additionally, the uniform particle and pore sizes of NCHS effectively mitigated volumetric expansion and surface accumulation, ensuring long-term cycling stability and superior electrochemical performance. Se-loaded NCHS (Se@NCHS) exhibited a high discharge capacity of 199.4 mA h g−1 at 0.5 C after 500 cycles with 70.4% capacity retention and achieved 188 mA h g−1 at 3.0 C, outperforming conventional carbon hosts such as Super P. This study highlights the significance of structural and chemical modifications in optimizing cathode materials and offers valuable insights for developing high-performance energy storage systems.