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

Abstract

A nitrogen‐doped, carbon‐coated Na 3 V 2(PO 4)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 Na 3 V 2(PO 4)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 Na 3 V 2(PO 4)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.