Nitrogen-Doping-Induced Defects of a Carbon Coating Layer Facilitate Na-Storage in Electrode Materials

A nitrogen‐doped, carbon‐coated Na3V2(PO4)3 cathode material is synthesized and the formation of doping type of nitrogen‐doped in carbon coating layer is systemically investigated. Three different carbon‐nitrogen species: pyridinic N, pyrrolic N, and quaternary N are identified. The most important finding is that different carbon‐nitrogen species in the carbon layer have different impacts on the improvement of the electrochemical properties of Na3V2(PO4)3. Pyridinic N and pyrrolic N significantly increase the electronic conductivity and create numerous extrinsic defects and active sites. Quaternary N only increases the electronic conductivity without creating extrinsic defects. Therefore, it is unexpectedly demonstrated that the Na3V2(PO4)3/C+N, in which with minimize content of quaternary N or exist most extrinsic defects, exhibits the best electrochemical performance, particularly the rate performance and cycling stability. For example, when the discharging rate increased from 0.2 C to 5 C, its capacity of 101.9 mAh g−1 decays to 84.3 mAh g−1 and an amazing capacity retention of 83% is achieved. Moreover, even at higher current density of 5 C, an excellent capacity retention of 93% is maintained even after 100 cycles. Nitrogen‐doped carbon coating, which is a simple and effective approach, is used to improve the electrochemical performance of a Na3V2(PO4)3 cathode material in sodium‐ion batteries and demonstrates obvious and significant modification effects. Moreover, the different impacts on the improvement of electrochemical performance by various carbon‐nitrogen species are analyzed and verified in detail.