Formation of N-doped molybdenum carbide confined in hierarchical and hollow carbon nitride microspheres with enhanced sodium storage properties

Although Na-ion batteries have attracted considerable attention as a competitive alternative to Li-ion batteries, the development of anode materials with satisfactory performances is still in its infancy, which leads to the increasing interest in the nanoscale design of the structure and chemistry of electrode materials. In this study, the confinement effect and template role provided by metal-organic supramolecular frameworks are exploited to fabricate hollow carbon nitride hierarchical microspheres with ultrafine N-doped MoC (MoCxNy) uniformly dispersed within the network. In the special nanoarchitecture, the carbon nitride matrix with the hollow structure can buffer the volume change of MoCxNy nanoparticles, inhibit their aggregation and improve the conductivities during sodiation/desodiation. Moreover, the pyrolysis of melamine ligands under confinement induces the incorporation of N-dopant into the lattice of molybdenum carbides, which further promotes the electrochemical activity of molybdenum carbides. Benefiting from these combined effects, the as-fabricated hybrids show remarkable cycle stability with a specific capacity of 410 mA h g(-1) after 200 cycles at 0.16 A g(-1) current. High specific discharge capacities are maintained at high current density, e.g. 370 mA h g(-1), 310 mA h g(-1), 240 mA h g(-1) and 190 mA h g(-1) at 0.33 A g(-1), 0.5 A g(-1), 0.67 A g(-1) and 0.83 A g(-1), respectively.