Metal-organic Framework And Its Derivatives For Lithium-ion Storage

Metal-organic frameworks（MOFs）and their derived metal sulfides are the most widely investigated in the field of energy storage due to their high surface area,tunable pore size and controllable structure.However,some drawbacks such as low conductivity and poor cycling stability limited their practical applications.In fact,the design of new materials with micro/nano structures and hybridization engineering have been proved to be effective ways for improving electrochemical performance.In this dissertation,MOFs with different metals,organic ligands,structures,sizes,dimensions,and morphologies and their derived metal sulfides had been controllably synthesized through simple and pollution-free routes.Meanwhile,nanosizing,morphology controlling and hybridization were used to improve the electronic conductivity and cycling stability.Furthermore,the relationships between the inherent characteristics of materials and electrochemical performance have also been investigated in detail.（1）A simple method was developed to fabricate hierarchical Co-based metal-organic framework（H-Co-MOF）microflowers in presence of action of octylamine for the first time.It was found that these microflowers are composed of nanosheets,consisting of small nanoflakes.Such a unique hierarchical structure can expose more active sites and reduced the diffusion distance of Li⁺.Consequently,Co-MOF microflowers electrode delivered a high capacity of 828 mA h g^-1 after 700 cycles at 2 Ag^-1.（2）A simple route was developed to fabricate Co-MOF microsphere precursor,which was then calcined with sulfur to obtain CoS₂/nitrogen-doped carbon hollow spheres（CoS₂/NC）composite.It was found that CoS₂ nanoparticles are homogenously distributed into the N-doped carbon layer.Based on density functional theory calculation,the N-doped carbon layer and hollow sphere structure not only buffered the volume change and obstructed the aggregation,but also improved the conductivity.When acted as an anode for Li-ion batteries,CoS₂/NC electrode delivered outstanding electrochemical performance.At 2 Ag^-1,a large capacity of 721 mA h g^-1 after 1000cycles can be maintained.（3）A simple method was applied to fabricate the FeCoS₂@nitrogen-doped carbon-graphene（FeCoS₂@NC-G）composite via calcination of FeCo-MOF/GO precursor and sulfur.New composite of the FeCoS₂ nanoparticles were uniformly embedded into nitrogen-doped carbon nanorod,and then further wrapped by graphene.The unique double-carbon coated nanostructure could provide sufficient space to buffer the volume expansion of FeCoS₂nanoparticles,obstructed the aggregation,shorten the Li⁺diffusion distance and enhanced the electronic conductivity.As a result,it showed a large capacity of 736.2 mA h g^-1 after 1500 cycles at 5 Ag^-1.Even at 20 Ag^-1,it still retained a high capacity of 547.1 mA h g^-1.（4）A facile and novel one-step route was realized to prepare Co₉S₈/N,S co-doped carbon（Co₉S₈/NSC）hybrid composites for the first time,in which a sulfonate-based Co-MOF was used as a precursor in the absence of additional sulfur source.Specifically,sulfonic acid groups which belonged to Co-MOF precursor underwent in situ sulfurization reaction with cobalt cations to form Co₉S₈ during the calcination process,while the organic ligand was simultaneously converted into NSC matrix.Due to the synergistic effect of Co₉S₈ nanoparticles and NSC matrix,it not only facilitated the diffusion of Li⁺and improved the electronic conductivity,but also prevented the aggregation and the volume change.Accordingly,Co₉S₈/NSC composite demonstrated excellent lithium-ion storage properties,such as a superior capacity of 1179 mA h g^-1at 0.1 Ag^-1 can be achieved.（5）Ageneral route was firstly developed to prepare M_xS_y/N,S co-doped carbon（M_xS_y/NSC）by calcination of sulfonate-based M-MOF precursor.The organic ligand1,5-naphthalenedisulfonic acid in sulfonate-based M-MOF precursor was used as a sulfur source,and metal ions were sulfurized in situ to form M_xS_y nanoparticles,resulted in the formation of M_xS_y/NSC(M_xS_y=Fe₇S₈,Co₉S₈,Ni₉S₈,Cu_1.96S,Mn S,Zn S)composites.Benefiting from the synergistic effect of M_xS_y nanoparticles and NSC matrix,the composites exhibited an excellent electrochemical performance.Taking Fe₇S₈/NSC anode material as an example,a high capacity of 645 mA h g^-1 at 5 Ag^-1can be retained,as well as an outstanding cycling stability over 1500 cycles can be achieved.