Design And Synthesis Of Transition Metal Compounds And Their Applications In Lithium Ion Batteries Batteries

As a new type of energy storage equipment,the development of lithium-ion battery has attracted much attention,becoming a hotspot for national researchers.In order to meet the needs of social development,lithium-ion battery should have a high capacity,long cycle life,and excellent rate performance.However,the commercial lithium ion battery anode material – graphite has a low specific capacity of 370 mA g-1,which is far away from meeting the social needs.Therefore,developing an anode material with high capacity,good cycle stability and excellent performance becomes more and more important.So far,researchers have developed a variety of anode materials,such as various carbon materials,alloys,transition metal sulfides and transition metal oxides.Among these negative electrode materials,transition metal sulfides and transition metal oxides are favored by researchers because of their high theoretical specific capacity.MoS2,as a typical transition metal sulfide,has a grapheme-like two-dimensional structure,and its unique structure makes it present excellent electrochemical performance.But MoS2 has a poor conductivity,and can easy reunite in the actual application.These shortcomings limit the application of MoS2.In addition,the transition metal oxide(Co3O4)has a high theoretical specific capacity,almost three times of the graphite,but the high volume expansion during the charge and discharge process,causing the electrode material crushing and resulting capacity decline rapidly.In addition,the poor conductivity ofCo3O4 is also reasons forpoor performance.In view of the above problems,the effective solutions mainly include synthesis of nanomaterials and synthesis of composite materials,so as to improve the electrochemical properties of materials.The main studies include:(1)MoS2/N-C composites with micro-nanostructures were synthesized by one-step hydrothermal treatment.MoS2 nanodots were embedded in porous nitrogen-doped bulk carbon materials to form micro-nanostructures.The carbon source-biological material egg yolk was environmental friendly,low cost and simple synthesis,and so can achieve large-scale production.This special structure inhibits the agglomeration of MoS2 during charge and discharge.Nano-MoS2 can exposes more active sites,and the introduction of carbon materials can improve the conductivity of the material.The results show that the cycling stability and magnification performance of pure MoS2 were far away from MoS2/N-C composites.The capacity of MoS2/N-C composites can maintained 805 mAh g-1 at the current density 100 mAg-1after 100 cycles,while the pure MoS2 were less than100 mA g-1.The capacity of MoS2 /N-C composites can be maintaned as high as 630 mAh g-1 after 500 cycles at a current density of 500 mA g-1.The MoS2 /N-C composites exhibit excellent cycle performance.(2)Co3O4/rGO composites were synthesized by one-step hydrolysis of cobalt salt at high temperature.The preparation of the material is simple and safe.The obtained Co3O4 nanoparticles were deposited on the reduced graphene sheet.The specific surface area of the material is large and the size of the active material is small,increasing the active site in the electrolyte.The composite material with graphene can alleviate the volume expansion of the material during cycles andincrease the conductivity of the materials.Co3O4 /rGO composites show excellent electrochemical performance when applied to lithium ion battery anode material.The results show that the composites of Co3O4 / rGO have been greatly improved in cyclic stability,magnification performance and capacity compared with pure Co3O4.The first discharge capacity of Co3O4/rGO composites is 1217.3 mAh g-1,and the charge capacity is 805.6 mAh g-1 at a current density of 100 mA g-1.The Coulomb efficiency is 66.2%,and the capacity can be maintained at 1095 mAh g-1after 130 cycles.The performance Co3O4/rGO composite were much better than pure Co3O4.