| Lithium ion battery, as a new generation of energy device for energy storage andconversion, has been widely applied in the filed of portable electronic products such asMP3, mobile phone, ipad, digital camera and laptop; it also has the potential for vehicleand aircraft in the future. As for Li-ion batteries, energy density is the main bottlenecksduring the research development, which is restricted by cathode material. Thereby,lithium ion battery with higher energy density is the inevitable trend for new energydevelopment. Cathode material Li3V2(PO4)3(LVP) with a Na superionic conductor(NASICON) framework is of especial interest for its higher plateau (3.0~4.8V), highertheoretical capacity (197mAh g-1), crystal structure stability and thermostability. But thecommercialization of LVP is seriously hindered by its inherent low Li+diffusioncoefficient, low electronic conductivity and multiple plateau property (three plateauduring charge/discharge process between3.0and4.3V). Based on the survey of currentresearch about LVP, in this thesis, we have employed three kinds of methods tosynthesize this cathode material for the improvement of electrochemical performance andsettlement of multiple plateaux problem.(1) The hrdrothermal method is employed to fabricate monoclinic LVP with3Dnano-network structure. The network structure is consist of LVP nanofibers and thediameter of these nanofibers is between60and80nm. From the result we speculate thatPEG-400influence the morpholory of LVP precursor. This3D nano-network structureeffectively shorten the Li+transport pathways and greatly facilitate the electrolytesinfiltrating. Therefore, this LVP sample demonstrates excellent electrochemicalperformance. The discharge capacity is up to126mAh g-1at0.1C. At higher currentdensity like2C and5C, the disharge capacity still remain102mAh g-1and90mAh g-1,respectively. Even tested at1C for500cycles, the capacity retention of the sample is upto94.8%. (2) Electrospinning method has been employed to prepare flexible film of carbonnanofibers (CNFs) attached with Li3V2(PO4)3(LVP) nanospheres composite forimproving the electrochemical performance of Monoclinic Li3V2(PO4)3. The morphologyof nanofibers precursor could be controlled by adjusting polyvinylpyrrolidone (PVP)concentration. These LVP-CNFs intercross each other to form a3D net-work hybridfilms and could be applied as self-supported electrodes directly. Benefiting from theintriguing nanostructure, the binder-free LVP-CNFs hybrid film electrodes exhibitexcellent rate capability (110mAh g-1at5C) and cyclability of500cycles at2C (about95%of the initial capacity is retained). These results demonstrate that furtherimprovement or optimization of electrochemical performance could be realized bydesigning of3D net-work structure with unique morphologies.(3) A single rhombohedral NASICON-Structured cathode material Li2NaV2(PO4)3has been synthesized through a facile sol-gel route for the first time. The results suggestthat the Na+sources, i.e. anionic surfactants including sodium dodecyl benzene sulfonate(SDBS), sodium dodecyl sulfonate (SDS) and sodium oleate (NaOL) can induce Na+ionsto occupy all of the A1sites in formula unit of [V2(PO4)3]3-. Almost all monoclinic phaseof Li3V2(PO4)3can be transformed into rhombohedral phase of Li2NaV2(PO4)3with Li+partially substitutied by Na+ions offered by SDBS, SDS or NaOL. In the absence ofmonoclinic Li3V2(PO4)3, the rhombohedral Li2NaV2(PO4)3exhibits only one voltageplateau around3.76V versus lithium metal during charge/discharge process, which isconsistent with the one couple of V4+/V3+redox peaks in the cyclic voltammetry (CV)curves. Furthermore, the Li2NaV2(PO4)3obtained by using SDBS as Na+source displaysremarkable high-rate capability (80mAh g-1at5C and68mAh g-1at10C) and anexcellent cyclability of500cycles at2C (about93%of the initial capacity is retained)due to no fundamental change occurring in the host three-dimensional framework and theporous nanosheets structure with the carbon coating. |
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