Carbon Encapsulated Transitional Metal Compounds Prepared For Energy Storage

Transitional metal compounds(TMCs)as anodes for lithium/sodium ion battery can provide high capacities which are expected to replace commercial graphite anodes and apply to electronic equipments and electric vehicles.However,some drawbacks of TMCs such as poor electron transfer and pulverization in the cycle process cause a poor cycling stability and rate performance,seriously hampering the application of TMCs in energy storage.In order to improve the electrochemical performance,there are mainly two kinds of schemes.One is designing elaborate structures,such as anisotropy of multidimensional metal compound nanostructures,to improve the contact with the electrolyte and current collector.The other is to design carbon coated TMCs which is not only improve the electrical conductivity by the carbon shell,but also to buffer the volume change.In fact,the above two options are limited.If combining the two methods,designing anisotropy multidimensional carbon coated metal compound nanostructures will be of great significance for promoting the actual energy storage applications.However,how to design the structure is a difficulty.This work presents a new concept of designing anisotropic multidimensional carbon coated metal compound nanostructures.Through the organic metal pyrolysis,the carbon coated nanocrystallines conduct Oriented Attachment(OA)growth,achieving the construct of three different multidimensional carbon coated structures.And they are branched carbon encapsulated MnS(MnS@C)nanochains,carbon coated Fe3O4(Fe3O4@C)nanosheets and carbon encapsulated iron selenium composites(Fe3Se4&Fe7Seg@C)nanosheets,respectively.By investigating the intermediate morphology,structure,dislocation as well as the growth rule between the nanocrystalline and carbon shell of three samples at different stages,we explain the mechanism of carbon coated nanocrystalline via the OA growth and demonstate a multidimensional carbon coated nanocrystalline building model.First,the organic metal is decomposed into metal clusters and small organic molecules.Then,the metal clusters react quickly with O,S and Se element to form metal compound primary nanocrystals under heating.Simultaneously,the small organic molecules are deposited on the surface of the metal compound primary nanocrystals to form amorphous carbon coatings.Due to the lower catalytic activity of metal compound,the initial metal compound nanoparticles cannot be encapsulated fully by carbon coatings,so the bare parts of metal compound nanoparticles tend to attach with each other to reduce the surface energy.With metal compound nanoparticles gradually attaching together,anisotropy multidimensional carbon coated metal compound nanostructures will be formed finally.All of the three products have exhibited good electrochemical performance.As anodes for lithium-ion batteries(LIBs),MnS@C nanochains demonstrated high specific capacity and long cycle life.At 500 mA g-1,the reversible specific capacity is ca.318 mA h g-1 at the initial cycle and is maintained at ca.200 mA h g-1 after 800 cycles.And the Fe3O4@C nanosheets and Fe3Se4&Fe7Se8@C nanomaterials also demonstrated perfect cycling stability and rate-performance when used as anode materials for LIBs and sodium-ion batteries(SIBs).As anodes for LIBs,the reversible specific capacity of Fe3O4@C nanosheets is ca.600 mA h g-1 at 500 mA g-1,and it is stably maintained after 100 cycles.At the same current density,the Fe3Se4&Fe7Se8@C's reversible specific capacity is ca.900 mA h g-1,and it is up to 1156 mA h g-1' after 100 cycles.As anodes for SIBs,the two samples' reversible specific capacity is respectively 100 and 380 mA h g-1 at 500 mA g-1,and they both demonstrate good cycling performance.