Preparation Of Transition-metal Selenides/Carbon Composites Derived From MOFs And The Investigation Of The Electrochemical Performance Towards Li And Na Ion Storage

Rechargeable lithium-ion batteries（LIBs）are prevailing in daily life for powering the portable electronics,electric vehicles（EVs）and smart grids,etc,owing to their high energy density,long lifespan and environmental friendliness.However,LIBs still suffer from the challenge in realizing widespread practical application due to the limited reserves of lithium resources.In recent years,sodium-ion batteries（SIBs）,emerging as a viable alternative for LIBs,have been attracting significant interests in the field of large-scale energy storage,because sodium is of high natural abundance and low costs.Unfortunately,as the ionic radius of Na is far larger than that of Li（0.102 vs 0.076 nm）,the electrode host materials that are good for LIBs are not suitable for SIBs.Yet,it is generally believed that the electrode materials for LIBs and SIBs are relevant to each other and it is highly desired to design electrode host materials that may be used for both LIBs and SIBs.Composite materials based on transition-metal selenides（TMSs）encapsulated in carbonaceous materials,especially those derived from metal-organic frameworks（MOFs）,have demonstrated great promise as dual-role anode materials for both LIBs and SIBs,primarily due to the high electric conductivity and theoretical capacity.In this paper,we developed a facile MOF-engaged approach to prepare TMSs/C composites as anode materials for both LIBs and SIBs,and further investigated their electrochemical properties and reaction mechanisms.The main contents are summarized as follows:（1）A composite material based on CoSe₂ nanoparticles encapsulated in N-doped carbon framework that was intertwined with carbon nanotubes（CoSe₂@N-CF/CNTs）was synthesized successfully by a facile MOF-derived selenidation strategy from the typical cobalt-based zeolitic imidazolate framework（ZIF-67）.As anode materials for LIBs,CoSe₂@N-CF/CNTs composites deliver a reversible capacity of 428 mAh g^-1 even after 500 cycles at a current density of 1.0 A g^-1.The charge/discharge mechanisms of CoSe₂ are characterized using ex-situ X-ray diffraction and Raman analysis,from which the lithiation products of CoSe₂ are found to be Li_xCoSe₂ and Li₂Se,which are further converted to CoSe₂ upon delithiation.Additionally,the CoSe₂@N-CF/CNTs composites also demonstrate excellent electrochemical performance as anode materials for SIBs,with high specific capacities of 606 and 501 mAh g^-1 at 0.1 and1.0 A g^-1 in the 100th cycle.More strikingly,the Na-storage capacities of 275 and 194 mAh g^-1were still obtained even at ultrahigh current rates of 30 and 50 A g^-1.The electrochemical reaction kinetics of the as-prepared electrodes is further studied by pseudocapacitance and galvanostatic intermittent titration technique（GITT）measurements.（2）Starting with Ni-MOF,the NiSe-C porous spheres were prepared successfully by a facile one-step carbonization/selenidation treatment procedure.Potentially such porous structure may not only significantly increase electrolyte/electrode contact and thus facilitate ions diffusion and electrons transfer,as well as electrolyte infiltration,but also effectively alleviate the large volume changes during repeated charge/discharge process.And as a consequence,the resultant NiSe-C composite can deliver excellent electrochemical properties.As anode materials for Li-storage,the as-synthesized NiSe-C porous spheres deliver a high discharge capacity of 571 mAh g^-1 after 40 cycles at a current density of 0.2 A g^-1.Besides,a stabilized reversible capacity of 387 mAh g^-1 can be remained at 0.5 A g^-1 in the 50th cycle when applied as an anode for SIBs.Furthermore,electrochemical impedance spectroscopy（EIS）measurements were conducted to study the electrochemical behavior of the NiSe-C composite.The interfacial charge transfer resistance(R_ct)retained a very low and stable level during cycling,which was beneficial for fast charge transfer and ions diffusion,thus resulting in improved rate capability and cycle stability.