Studies On Synthesis And Performance Of Lithium Vanadium Oxide/conducting Polymer Composites As Cathode Materials

Lithium ion batteries (LIBs), with the advantages of higher working voltage,larger capacities, and longer cycle life, have attracted extensive attention. LayeredLiV3O8is well-known as one of the most promising cathode materials for LIBs due toits high-specific capacity, low cost, good structural stability and superior safetyfeatures. However, because of the dissolution of small quantity of LiV3O8in theelectrolyte and phase transformation during cycling, bare LiV3O8shows poor ratecapability and rapid capacity fade during cycling, which seriously restricts itsapplication in rechargeable LIBs. In this thesis, we focus on enhancing theelectrochemical performance of LiV3O8for LIBs. Firstly, monoclinic crystal LiV3O8is synthesized by a classic sol-gel method. Secondly, LiV3O8/conducting polymer(PANI and PTh) composites are synthesized by chemical oxidative polymerizationmethod. Finally, the physical properties and electrochemical performance of thesematerials are systematically studied.LiV3O8is synthesized by a classical peroxide sol-gel method. And then,LiV3O8/PANI composite is synthesized by oxidative polymerization of anilinemonomer on the LiV3O8. The electrochemical performance of the composite ischaracterized by discharge-charge measurements, CV, EIS and GITT. TheLiV3O8/PANI composite shows much better cycling behavior and rate capability. Theinitial discharge capacity of LiV3O8/PANI composite is197mAh/g, then its increasesto208mAh/g at the30th cycles, and retains at201.6mAh/g after55cycles at1C.And at the current density of3C, the initial discharge capacity of LiV3O8/PANIcomposite is165.4mAh/g and remains135.2mAh/g after55cycles. The superiorelectrochemical characterization can be attributed to the PANI conductive networkand the well-crystallized area and amorphous-like area co-exist in the LiV3O8/PANIcomposite, promoting the lithium ion chemical diffusion coefficients and transport ofelectrons.As-prepared LiV3O8serves as precursor, LiV3O8/PTh composite is successfullysynthesized by an in-situ chemical oxidative polymerization method. The structureand morphology of the LiV3O8/PTh composite are characterized. An uniform red-brown PTh shell with a thickness of3~5nm covered on the surface of LiV3O8hasbeen confirmed by HRTEM. The PTh layer could restrict the aggregation of LiV3O8 particle and form a good network of electrically conductive paths among the LiV3O8particles, which ensures LiV3O8nanofibers to get electrons from all directions.LiV3O8/PTh composite exhibits superior electrochemical performance compared withLiV3O8. The discharge capacities of LiV3O8/PTh composite at the first discharge are213.3mAh/g and200.3mAh/g at1C and3C, and they reduce to216.7mAh/g and197.9mAh/g after50cycles respectively. The excellent electrochemicalcharacterization of LiV3O8/PTh composite could be ascribed to PTh coating, whichcould enhance electrical conductivity, reduce the charge transfer resistance andimprove the lithium diffusion coefficient, as are confirmed by CV, EIS and GITTanalyses.