Synthesis And Application Of Carbon Coated Tin-based Anode Materials For Lithium-ion Batteries

Nowadays,as the most widely used rechargeable batteries,lithium ion batteries(LIBs)have made significant progress all over the world.But in some special areas such as the electric vehicles that call for energy storage system with both high power and high energy densities,conventional LIBs cannot meet the requirements.As for the anode materials,common used graphite is limited by the theoretical capacity of 372 mA h g-1 and can hardly meet the increasing demands.Tin-based anode materials are regarded as the potential substitutions because of their high specific capacities(Sn 991 mA h g-1,SnO2 781 mA h g-1,SnS 781 mA h g-1),low discharge plateau,abundant sources and so on.However,there exist some drawbacks in tin-based anode materials such as the low electrical conductivity and the volume change during charge-discharge processes that may lead to cracking or pulverization of the electrode.To reduce the capacity decrease caused by the volume effect,the most common used strategy is using nanosized tin-based materials and carbon coating.Nanosized particles have low volume effect and short Li+ diffusion length and carbon can act as a buffer layer to accommodate the volume change of tin-based material and increase the electrical conductivity as well.This thesis focuses on the design of carbon encapsulated tin-based anode composites mainly using the metal-organic frameworks(MOFs)as the precursor.The metals in MOFs and the added tin source formed tin-based nano particles and the organic ligands formed carbon layer,thus the micro-/nano-scale combined configurations were obtained.The synthesized materials includes carbon-coated Sn,SnO2,Zn2SnO4,Mn2SnO4,Sn-Co alloy,Sn02/Ti02 and flowerlike SnS.By investigating the morphology,structure and the lithium ion storage property of the materials,we try to explore the synthesis strategies,find the relationships between the morphology/structure and the superior performance,and reveal the lithium storage mechanism of tin-based materials.(1)Using the metal organic framework MOF-5 as carbon/metal sources and template,carbon coated Zn2SnO4 was synthesized by adding tin source to carbonized MOF-5 and following heat treatment.Sn and SnO2 nanoparticles were controlled to grow inside the porous carbons from MOF-5 under different gas conditions.All the materials show the structure of nanoparticles coated by porous carbon.And carbon coated SnO2 exhibits the best lithium storage performance:a reversible capacity of 676 mA h g-1after 100 cycles at current density of 100 mA g-1.This chapter provides a strategy to fabricate carbon coated tin-based materials by using MOFs template.(2)A flake-like carbon coated Mn2SnO4 nanomaterial was synthesized using flake Mn-MOFs as precursor.The unique 2D morphology is favorable for lithium storage because it provides more electrode/electrolyte contacts and reduces diffusion length of Li+.The prepared material give a high specific capacity of 986 mA h g-1 with 90.1%capacity retention after 100 cycles at 100 mA g-1 and a capacity of 428 mA h g-1 even at a high current of 2 A g-1.Moreover,the performance of carbon coated Mn2SnO4 was compared with carbon coated MnO/SnO2 which has the same element composition with Mn2SnO4.The results show that both the higher capacity and capacity retention of Mn2SnO4 can be ascribed to the production of uniformly dispersed MnO/SnO2 at atom scale,which had a synergistic effect during charge-discharge processes.(3)To investigate the effect of inactive metal on the stability of tin based materials,N-doped carbon coated Sn-Co nanoalloys were synthesized using Co-MOFs(ZIF-67)as precursor.The materials show a micro-/nano-scale combined configuration of monodispersed Sn-Co nanoalloys with size of?10 nm in microsized N-doped carbon polyhedron with diameter of 2 ?m.Benefiting from the N-doped carbon layer and inactive Co,the material exhibits excellent cycling stability:a capacity of 818 mA h g-1,and 86.5%capacity retention after 100 charge-discharge cycles at 100 mA g-1.Even at high current density of 2 A g-1,the capacity decreases slightly after 500 cycles.(4)Tin source was introduced to Ti-MOFs(MIL-125)precursor.After carbonization,the composite of SnO2/TiO2@C was synthesized which consists of SnO2 and Ti02 nano particles with size bellow 10 nm uniformly dispersed in cake-like carbon matrix with diameter of 500 nm and thickness of 200 nm.In this carbon composite,SnO2 nanoparticles contrite the most capacity.TiO2,as a low-strain material,plays critical role in separating SnO2 particles from each other and maintaining the structure integrity.As a result,the reversible capacity of the material remains above 1000 mA h g-1 at the current density of 100 mA g-1 and the cake-like morphology maintains well after 100 cycles.Moreover,nanosized particles,especially the Ti02,make the charge-discharge behavior a mixture of diffusion-limited and capacitive process,which gives the composite a superior rate performance.(5)Flowerlike SnS2 nanoparticles were synthesized through hydrothermal method.After that,tannic acid and diethlyenetriamine were co-deposited on the surface of flowerlike SnS2,realizing the uniform coating under simple condition.After carbonization,flower-like SnS nanoparticles with a?10 nm thickness of carbon layer were obtained and the N content in carbon coating was 9.9 at%.The abundant mesoporous structure formed by stacked nanoflake has advantages in improving the Li+transfer and volume buffering.The uniform thin carbon layer has the effect on stabilizing the structure and increasing the diffusivity coeffcient of Li+.Moreover,abundant pyridinic N and pyrrolic N in carbon layer increase the capacity of lithium ion storage.Thus the material exhibits a capacity of 1090 mA h g-1 after 100 cycles at 100 mA g-1,and a capacity retention of 91.4%.At high current density of 2 A g-1,the reversible capacity was as high as 735 mA h g-1.