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It is a major challenge to achieve fast charging and high reversible capacity in potassium ion storing carbons. Here, we synthesized sulfur-rich graphene nanoboxes (SGNs) by one-step chemical vapor deposition to deliver exceptional rate and cyclability performance as potassium ion battery and potassium ion capacitor (PIC) anodes. The SGN electrode exhibits a record reversible capacity of 516 mAh g^–1 at 0.05 A g^–1, record fast charge capacity of 223 mA h g^–1 at 1 A g^–1, and exceptional stability with 89% capacity retention after 1000 cycles. Additionally, the SGN-based PIC displays highly favorable Ragone chart characteristics: 112 Wh kg^–1at 505 W kg^–1 and 28 Wh kg^–1 at 14618 W kg^–1 with 92% capacity retention after 6000 cycles. X-ray photoelectron spectroscopy analysis illustrates a charge storage sequence based primarily on reversible ion binding at the structural–chemical defects in the carbon and …