Multiphase nano Co9S8/CoS encapsulated in N-doped carbon for high capacity sodium-ion battery anode

As anode materials for sodium-ion batteries (SIBs), transition metal sulfides (TMSs) have been widely explored due to their low cost, high theoretical capacity and rich redox reactions. However, poor cycling performance and capacity retention limit the application of materials. In this work, multiphase nano Co9S8/CoS encapsulated in N-doped carbon for high capacity SIBs anode is prepared via a facile two-step strategy: precipitation and heat treatment. When applied as anodes for SIBs, mass capacity of 389.2 mAh g−1 at 0.1 A g−1 after 60 cycles is obtained, and delivered ~300 mAh g−1 from 2nd to 300th cycle without depravation under 1.0 A g−1. In addition, according to the calculation of Na+ diffusion kinetics, it is evident that carbon encapsulation not only helps prevent electrolyte erosion but also enhances the DNa+ of the anode material. The high efficient strategy can be applied to synthesis of other multiphase analogues. Based on theoretical investigations, calculations of band gaps and rebuild energies for other TMSs (i.e., Cu7S4/Cu2S, FeS/Fe21.333O32/Fe3O4, MnS2/MnS, Ni3S2/Ni3C and cubic-ZnS/hexagonal-ZnS), as well as analysis of cyclic data, it can be concluded that Co9S8/CoS@NC material exhibits the highest conductivity and cycle stability.