We fabricated sulfur and nitrogen codoped cyanoethyl cellulose‐derived carbons (SNCCs) with state‐of‐the‐art electrochemical performance for potassium ion battery (PIB) and potassium ion capacitor (PIC) anodes. At 0.2, 0.5, 1, 2, 5, and 10 A g⁻¹, the SNCC shows reversible capacities of 369, 328, 249, 208, 150, and 121 mA h g⁻¹, respectively. Due to a high packing density of 1.01 g cm⁻³, the volumetric capacities are also uniquely favorable, being 373, 331, 251, 210, 151, and 122 mA h cm⁻³ at these currents, respectively. SNCC also shows promising initial Coulombic efficiency of 69.0% and extended cycling stability with 99.8% capacity retention after 1000 cycles. As proof of principle, an SNCC‐based PIC is fabricated and tested, achieving 94.3 Wh kg⁻¹ at 237.5 W kg⁻¹ and sustaining over 6000 cycles at 30 A g⁻¹ with 84.5% retention. The internal structure of S and N codoped SNCC is based on highly dilated and defective graphene sheets arranged into nanometer‐scale walls. Using a baseline S‐free carbon for comparison (termed NCC), the role of S doping and the resultant dilated structure was elucidated. According to galvanostatic intermittent titration technique and electrochemical impedance spectroscopy analyses, as well as COMSOL simulations, this structure promotes rapid solid‐state diffusion of potassium ions and a solid electrolyte interphase that is stable during cycling. X‐ray diffraction was used to probe the ion storage mechanisms in SNCC, establishing the role of reversible potassium intercalation and the presence of KC₃₆, KC_24, and KC₈ phases at low voltages.