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This work reports on the synthesis of hard carbon spheres (HCS) and the impact of the pyrolysis temperature (1500 to 1900 °C) on the properties of HC and its relationship with the electrochemical performance in potassium-ion batteries (KIBs). Comparison with commercial graphite performance is provided as well. Spherical morphology, disordered structure, and low surface area were obtained for the HCSs. Most properties (interlayer space, active surface area, and oxygen-based functional groups) were found to decrease with increasing pyrolysis temperature, except for the helium density and closed porosity, which increase. However, graphite presents a flake-like morphology with a larger particle size, a higher helium density, an ordered structure with a smaller interlayer distance, and no closed pores. Electrochemical tests in a half-cell vs K + /K showed that HCSs perform better than graphite with higher initial Coulombic efficiency (ICE) and better specific capacities. The HCSs pyrolyzed at 1500 and 1700 °C exhibit the best initial Coulombic efficiency, ICEs of 54 and 62%, and specific capacities of 254 and 247 mAh g −1 (C/20, 11.5 mA g −1 ) , respectively. The ICE is affected by multiple surface and bulk parameters but also by electrolyte formulation (67% for 0.8 M KFSI vs 62% for 0.8 M KPF 6 ). The capacity is governed by diffusive phenomena, and a larger interlayer graphitic spacing and defects favor a better insertion of K ions. Closed pores did not lead to an improvement in capacity. Furthermore, HCSs exhibit significantly better capacity retention (97%) than graphite (84%), especially when cycled at high current rates (up to 10C depotassiation rate).