Porous Vanadium-based Nanomaterials: Synthesis,Electrochemical Energy Storage And Optimization Mechanism

In this dissertation,we focus on the strategies to increase the electron/ion conductivity of vanadium based nanomaterials through the vanadium doping and the construction of prous electrode,thus improve the electrochemical performance in the applications of lithium ion battery.We carefuly investigate the material design,synthesis,and the electrochemical performance of the obtained electrode,and seeking a better way to improve the energy storage capacity.The details are summarized briefly as follows:?1?In order to improve the electron/ion conductivity of Li₃V₂?PO₄?₃,we designed and synthesized nanoporous Li₃V₂?PO₄?₃/C microspheres and three-dimensional hierarchical carbon decorated Li₃V₂?PO₄?₃.Both of them have the porous structure and consisted by the primary particles.And the single-crystalline primary nanoparticles are coated with amorphous carbon layer,which are embedded in the carbon matrix.As cathode for lithium ion battery,both of them could accommodate the volumetric changes during rapid ion insertion/deinsertion,resulting in long-life and high-rate performance.It is worth noting that three-dimensional biocontinuous Li₃V₂?PO₄?₃/C cathode material also exhibits a good temperature adaptability.When measured under the temperature of 60°C,an initial capacity of130 mAh/g can be delivered for the first cycle at 5 C,and the capacity retention is 85%after 1000 cycles.When the half cell was operated at-20°C,it still exhibited a high initial capacity of 106 mAh/g at 1 C charge/5 C discharge,and capacity retention of88%after 450 cycles at-20°C.The unique three-dimensional nanoporous structure is favorable for improving the cyclability and rate capability in energy storage applications,which has a guidance for the development of new type lithium ion battery electrode material with high rate and long life electrochemical performance.?2?To improve both the ionic and electronic conductivities of LiMnPO₄,a series of carbon decorated nLiMnPO₄·?1-n?Li₃V₂?PO₄?₃/C?n=1,0.8,0.6,0.4,0.2?nanocomposites are synthesized by a facile sol-gel method combining with the conventional solid-state method.The electrochemical mechanism under high rate and low rate are carefully investigated through the comparison of the discharge capacity and cycle performance under different current rates.When cycled at 20 C,0.6LiMnPO₄·0.4Li₃V₂?PO₄?₃/C exhibits an initial specific capacity of 103 mA h/g with a capacity retentionof 68%after 3000 cycles,corresponding to a capacity fading of 0.013%per cycle.The stable capacity and excellent rate capability make this carbon decorated 0.6LiMnPO₄·0.4Li₃V₂?PO₄?₃ nanocomposite a promising cathode for lithium-ion batteries.?3?To address the rapid capacity degradation of metal oxide during the electrochemical processs,we develop hierarchical Co₂V₂O₇ nanosheets and Co₃V₂O₈interconnected hollow microsphere as anode materials,which can potentially synergistically enhance the electronic/ionic conductivity,reversible capacity,and mechanical stability.Furthermore,the electrochemical reaction mechanism of cobalt vanadium oxide is thoroughly investigated by in-situ XRD.In order to explore the growth mechanism,time-dependent experiments were carried out to track the phase and morphology transformation during the synthetic process.Both of the electrode materials present high rate performance and stability.The Co₂V₂O₇ nanosheet displays a high reversible capacity of 677 mAh/g at 5 A/g,and a high capacity of 441mAh/g can be retained after 900 cycles.Co₃V₂O₈ interconnected hollow microspheres are able to deliver a high discharge capacity of 470 mAh/g at the current density of 10 A/g and obtain a stable capacity of 423 mAh/g after 300 cycle.Such a remarkable rate capability and cycling performance make the hierarchical Co₂V₂O₇ nanosheets and Co₃V₂O₈ interconnected hollow microsphere a promising anode material for lithium-ion batteries.This work also brings a new idea to optimize the electrochemical performance of binary metal oxides.