NbS₂ Nanosheets with M/Se (M = Fe, Co, Ni) Codopants for Li⁺ and Na⁺ Storage

Transition metal (M = Fe, Co, Ni) and Se codoped two-dimensional uniform NbS₂ (M_xNb_1-xS_2-ySe_y) nanosheets were synthesized via a facile oil-phase synthetic process. The morphology of M_xNb_1-xS_2-ySe_y can be adjusted by tuning the amount of metal and Se introduced into NbS₂. Among them, the optimized Fe_0.3Nb_0.7S_1.6Se_0.4 nanosheets, with lateral sizes of 1-2 μm and approximately 5 nm thick, achieve the best Li-ion and Na-ion storage properties. For example, the Fe_0.3Nb_0.7S_1.6Se_0.4 nanosheets depict excellent rate capabilities with fifth-cycle specific capacities of 461.3 mAh g^-1 at 10 A g^-1 for Li storage and 136 mAh g^-1 at 5 A g^-1 for Na storage. More significantly, ultralong cyclic stabilities were achieved with reversible specific capacities of 444 mAh g^-1 at 5 A g^-1 during the 3000th cycle for Li storage and 250 mAh g^-1 at 1 A g^-1 during the 750th cycle for Na storage. Post-treatment high-resolution transmission electron microscopy was studied to prove that the reversible Li-ion storage in NbS₂ was based on a conversion reaction mechanism.