Research On Preparation And Performance Of MOFs-based Lithium-ion Battery Anode Materials

The increasing demand for clean energy has promoted the large-scale application of lithium-ion batteries(LIBs)in modern society.With the rapid development of electric vehicles,the development of LIBs with higher power,energy and safety has become a research hotspot.As the most promising anode materials for the next-generation high-performance LIBs,Sn-based materials and transition metal oxide materials have attracted extensive research interest due to their low cost,high theoretical capacity,and environmental friendliness.But at the same time,they also have some disadvantages,such as severe volume expansion and poor conductivity,resulting in short cycle life and poor rate performance,which hinder their practical application.Metal-organic frameworks(MOFs)are crystalline porous materials with periodic networks composed of metal ions or ion clusters and organic ligands through coordination bonds.They have the characteristics of large specific surface area,high porosity,rich structure and easy functionalization.In recent years,MOFs have been used as ideal precursors to prepare various functional materials with diverse compositions and structures,which have shown great potential in energy storage and conversion field.Besides,MOFs-based materials have the advantages of simple preparation,rich variety,and easy adjustment of morphology and components.As a result,MOFs derived metal or metal oxide materials have become the most promising anode materials for high-performance lithium ion batteries.In this dissertation,to overcome the shortcomings of Sn-based materials and transition metal oxides as LIBs anodes,high-performance anode materials derived from MOFs are designed and synthesized from the perspective of micro-/nano-structure control and component adjustment,which provide a new strategy and approach for the development of high-performance LIBs anode materials.The main research contents are as follows:(1)The FeSn2/FeSn/Fe-rGO composites with Sn-Fe nano-alloy supported on reduced graphene oxide(rGO)sheets are synthesized by a simple MOFs precursor method.The FeSn2/FeSn/Fe-rGO composites show excellent lithium storage performance with high reversible capacity(1274 mAh g-1 after 200 cycles at 0.2 A g-1),exceptional rate performance(665 mAh g-1 at 5.0 A g-1)and excellent high-current cycling stability(cycled 1200 times at 2.0 A g-1,capacity decay per cycle is only 0.025%).In addition,the LiFePO4|FeSn2/FeSn/Fe-rGO full batteries showed enhanced lithium storage performance(capacity retention rate is as high as 87.1%after 100 cycles at 0.5C).The excellent electrochemical performance can be attributed to its unique three-stage buffer structure:Fe atoms in FeSn2 and FeSn act as buffer framework,metallic iron acts as an elastic buffer for the structural maintenance of cubic nanoparticles,and rGO serves as a two-dimensional elastic matrix to reduce the volume expansion and structure collapse of the electrode during the lithiation process,which synergistically improve the lithium storage performance.(2)The Sn-Co-based metal composite(Sn-Co/rGO)with double buffer structure are prepared by thermally decomposing Sn-Co-based MOFs precursor(Sn3[Co(CN)6]4/rGO)in a reducing atmosphere.When used as anode for LIBs,Sn-Co/rGO exhibits excellent lithium storage performance:high reversible capacity(1055 mAh g-1 after 250 cycles at 0.2 A g-1),good rate performance(320 mAh g-1 at 5.0 A g-1)and excellent long cycle life(720 mAh g-1 after 600 cycles at 1.0 A g-1).The inactive Co in Sn-Co/rGO acts as an elastic buffer that buffers the volume expansion of the Sn-Co alloy nanoparticles,while the N-doped rGO elastic matrix can not only inhibit the agglomeration of Sn-Co nanoparticles but also enhance the conductivity of the material,and jointly enhance the material's lithium storage performance.(3)The SnO2-x-Fe2O3/rGO composites with oxygen vacancies are synthesized by calcinating the Sn3[Fe(CN)6]4/rGO precursor in air.The oxygen vacancies can enhance the conductivity of the material and provide more Li+active sites;Fe2O3 can promote the reversible conversion reaction of SnO2-x and effectively prevent the agglomeration of Sn grains during the lithiation;rGO as an elastic two-dimensional matrix can effectively alleviate the volume expansion and enhance the conductivity of the material.As a result,the SnO2-x-Fe2O3/rGO composites show excellent lithium storage performance with reversible capacity of 1035 mAh g-1 after 200 cycles at 0.2 A g-1,rate capacity of 476 mAh g-1 at 3.0 A g-1,and discharge capacity of 621 mAh g-1 after 400 cycles at 1.0 A g-1.In addition,LiFePO4?SnO2-x-Fe2O3/rGO full cells have a decent capacity retention of 81.4%after 100 cycles at 0.5 C.This study effectively solves the disadvantages of large volume expansion and low electrical conductivity of the Sn-based oxide electrodes,and provides a useful exploration for the practical application of such materials.(4)Using Co[Fe(CN)5NO]@GO as the precursor,rGO-coated Co-Fe mixed oxides(Co3O4-CoFe2O4@rGO)are prepared by a facile two-step calcination method.The hierarchical porous Co3O4-CoFe2O4 nanocubes can shorten the diffusion distance of lithium ions and improve the rate performance;The tightly wrapped flexible rGO layer can effectively buffer the volume expansion and enhance the conductivity of Co3O4-CoFe2O4.As a result,the Co3O4-oFe2O4@rGO composites show excellent lithium storage performance.The reversible capacity reaches up to 1393 mAh g-1 after 300 cycles at 0.2 A g-1,and the rate capacity is as high as 420 mAh g-1 at 4.0 A g-1.This study provides a new route for the preparation of metal oxide based anodes with high capacity,high conductivity and high structural stability.(5)Through simply calcination the W-based metal-organic frameworks(Fe2W(CN)8·xH2O),a novel Fe2WO6 hierarchical porous octahedra with oxygen vacancies are successfully constructed.Oxygen vacancies can enhance the conductivity of the material,provide more Li+storage sites,while the hierarchical porous structure effectively buffers the volume expansion and promotes the diffusion of the electrolyte.Besides,by introducing the heavy element W,the density of Fe2WO6 is as high as 5.3 g cm-3.Thus,an ultrahigh volumetric capacity of 8745 mAh cm-3(0.2 A g-1)could be achieved.Furthermore,the mechanism of the significant increment in capacity is also investigated.The new findings indicate that the phase transition and structural rearrangement of the Fe2WO6 electrode,along with the enhanced reversible formation and decomposition of the polymer/gel-like film,collectively result in the capacity evolution during cycling.Remarkably,the introduction of oxygen vacancies and high density make hierarchical porous Fe2WO6 a promising anode for high volumetric lithium storage.