Iron-based Composites Are Used As Anode Materials For Alkali Metal Ion Batteries

Iron sulfides and iron nitrides are very suitable anode materials for alkali metal ion batteries because of their relatively low price and high initial theoretical capacity.However,the huge volume expansion of the iron sulfides and iron nitrides leads to the poor cycling stability of the batteries.Recent studies have shown that adjusting the morphology and structure at the nanoscale and introducing carbon coating at the same time will improve the electrochemical performance of the materials.Nanostructured active materials can speed up the reaction kinetics and carbon coating will enhance the conductivity and provide a buffer layer to mitigate the huge volume changes during charging and discharging process.Based on the above results,we use different methods to improve the electrochemical performance of iron sulfides and iron nitrides.The details are as follows:First,we exploit a mild etching approach coupled with further sulfidation to prepare uniform yolk-shell Fe₇S₈@C nanoboxes.The yolk-shell Fe₇S₈@C nanoboxes can be served as general materials for the reversible and rapid storage of alkali metal ions?Li⁺,Na⁺,or K⁺?.For LIBs and SIBs tests,the discharge capacity is 765.5 and491.6 m Ah g^-1 after 200 cycles,respectively.Furthermore,the composite also exhibits satisfactory K⁺ion storage.It retains a high capacity of 110.3 m Ah g^-1 after 50 cycles in 0.8 M KPF₆ as the electrolyte.It can also retain a stable capacity of 97.8 m Ah g^-1even after 100 cycles in the 3 M KFSI electrolyte.The remarkable electrochemical behavior of the composite is ascribed to several factors,including the high theoretical specific capacity of Fe₇S₈,nanosized Fe₇S₈ active particles,the yolk-shell structure that buffers the large volume expansion,and the carbon shell that enhances both the conductivity and structural integrity.Second,we successfully prepare the double-coated Fe₂N@Ti N@C microboxes through simple hydrothermal coating and nitriding methods.The composite exhibits superior Li⁺ion storage(with a discharge capacity of 334.8 m Ah g^-1 after 500 cycles).The double-coated unique core-shell nano-hollow structure is favorable for Li⁺ion storage.Firstly,the synergistic effect of Fe₂N core and Ti N shell improves the rate capability and prolongs the cycle life of LIBs.Secondly,the outer carbon layer can effectively reduce the strain,buffer the volume expansion,enhance the conductivity of the electrode and promote the diffusion of lithium ions and electrons during charging and discharging process.Finally,the shape and size of the microbox structure are conducive to rapid and stable ion transport.During the cycling process,the active material has good compatibility with the electrolyte,which can provide more active sites and reduce the resistance to ion migration.Last,we synthesize the necklace-like FeS₂@CNFs microboxes by electrostatic spinning and further sulfidation reaction.The composite delivers excellent K⁺ion storage.At the current density of 100 m A g^-1,the specific capacity is 255.7 m Ah g^-1after 100 cycles.At a high current density of 500 m A g^-1,the specific capacity is 180.2m Ah g^-1 after 100 cycles.The internal voids generated by the necklace-like structure are beneficial to the volume expansion of the electrode material during the cycles,and effectively prevents the aggregation of FeS₂ nanoparticles.Carbon nanofibers improve the conductivity of the material and speed up the charge transfer during the cycles.