In-situ carbon encapsulation of ultrafine VN in yolk-shell nanospheres for highly reversible sodium storage

Abstract Transition metal nitrides as high-capacity anode materials for sodium-ion batteries (SIBs) have drawn increasing attention regarding their excellent electrical conductivity and efficiently multi-electron conversion reactions, but designing and developing metal nitrides-based electrodes with superior sodium storage properties remains highly challenging. Here, vanadium-polymer frameworks with yolk-shell architectures are designed via a facile self-polymerization route. In a subsequent calcining process, ultrafine VN (quantum dots) are in-situ created and steadily encapsulated in yolk-shell N-doped carbon nanospheres (VN QDs/N-C). The VN in quantum-dot levels could provide short-range ion-diffusion paths and reduce Na-intercalation stresses, and the open yolk-shell carbon nanospheres with robust shells, strong space-confinements and sufficient interior voids can establish stable conductive carbon matrixes for mitigating volume expansions of VN QDs and facilitating transfer kinetics of ions/electrons. Accordingly, the VN QDs/N-C demonstrates a high reversible capacity of 486.8 mAh g−1 at 0.05 A g−1, accompanied by highly reversible sodium storage properties (225.7 mAh g−1 reversible capacity with 105.2% capacity retention at 30.0 A g−1 after 30000 cycles). Benefiting from these architectural advantages, the products of metal nitride-based QDs in-situ encapsulated in carbon materials have enormous potentials in sustainable energy storage fields.