Na-ion batteries (NIBs) have received much attention, especially for large-scale energy storage, due to its cost and sustainability advantage over Li-ion batteries (LIBs).  Rapid progress has been made on NIB cathodes, including layered metal oxides and polyanionic compounds.  For the anode side, despite great advances being made on various candidates, futuristic commercialization of NIBs most likely relies on high-performing carbon anodes considering safety, scalability, and cost.  Unfortunately, graphite that works very well for LIBs does not function for NIBs due to the unfavorable thermodynamics between graphite and Na.  Fortunately, nongraphitic carbons as anodes can provide meaningful capacities for NIBs.  Among them, hard carbon also known as nongraphitizable carbon has become the leading candidate since Stevens and Dahn first reported a reversible capacity of ~300 mAh/g on pyrolyzed glucose.  However, the capacity of hard carbon has not been improved much after that.  Herein, we dope heteroatoms, phosphorus to improve the capacity of hard carbon.  The capacity of phosphorus-doped hard carbon (P-HC) was increased to a record high value of 359 mAh/g from 280 mAh/g when up-doped.  In order to learn the mechanism of heteroatom doping, the specific surface area of un-doped hard carbon (HC) and P-HC were controlled to be low, i.e., less than 10 m²/g, for comparison.  Supported by different characterization techniques, we proposed that the doping-induced defects play an important role for the high capacity of P-doped hard carbon.