Synthesis Of Carbonous Molybdenum-Based And Vanadium-based Composites And Their Electrochemical Performances

The electrochemical properties of electrode materials can be improved via carbon composite as carbon can improve the conductivity and alleviate volume change and pulverization during Li insertion/desertion. Goodenough and others introduced carbon into LiFePO₄ materials to synthesize LiFePO4/C composite which showed enhanced specific capacity and cycling stability compared to bare LiFePO4 thus prompting its commercial application. In this thesis, we introduce carbon into Mo, V-based electrode materials to obtain corresponding carbon composites and investigate the effect of carbon on the electrochemical performance of the composites. The main contents of this paper are summarized as follows:1. A Mo₂C/C composite with three-dimensional porous framework is prepared by calcining the mixture of self-made MoO₃ and citric acid. The product micron-sized Mo₂C/C has a hierarchical porous structure, which is assembled of Mo₂C/C nanoparticles （the size is only about 20nm）. Lithium ion and electron of the inner of the product can transport fast and efficiently, thus improving the rate capability and cycling performance. At a current density of 200 mA g-1, a discharge capacity of 200 mAh g-1 can be obtained after 130 cycles.2. We synthesized a unique carbon coated SnO₂/MoO₃ one-dimensional nano-structure by calcining hydrothermal made SnO₂/MoO₃ precursor in acetylene gas. To explore the forming process of the precursor, the samples synthesized with different reaction time are characterized and a possible formation mechanism is proposed. The SnO₂/MoO₃ nano-heterostructures is formed due to the similar d-spacing of the （101） plane of rutile SnO₂ and the （140） plane of α-MoO₃. With the extension of the hydrothermal reaction time, α-MoO₃ nanoribbons are slowly dissolved to form the amorphous MoO₃ precipitating among SnO₂ nanosheets. Finally 1-D SnO₂/MoO₃ rodlike structure in which SnO₂ nanosheets are stacked along a direction and distributing in an amorphous matrix of MoO₃ is obtained. In order to improve the electrochemical properties of the product, carbon is coated on the SnO₂/MoO₃ nano-heterostructures. The product obtained has a outstanding cycling performance. When cycled at a current density of 200 mA g-1 after 120 times, the cell still delivers a discharge capacity of more than 560 mAh g-1. Related research was published on the RSC journal Journal of Materials Chemistry A.3. A Na₃V₂（PO₄）₃/C composite is prepared by calcining the mixture of Na₂CO₃, NH₄H₂PO₄å’ŒV₂O₅ at 700℃ in acetylene atmosphere. During the reaction, vanadium is reduced from V5+ to V3+. As a cathode material for sodium-ion battery, the composite can get a specific capacity approaching the theoretical capacity （118 mAh g-1） at a current rate of 1/20 C in a voltage range of 2.5-3.7 V （vs. Na+/Na）. At 1 C,70.9% of the initial charge capacity and 69.6% of the initial discharge capacity can still maintain after 300 cycles. And at a higher rate of 2 C cycling for 700 times,65% of the initial capacity （61 mAh g-1） still can be obtained. Besides the high sodium-ion conductivity of Na₃V₂（PO₄）₃, the continuous carbon layers can connect the composites and improve the electron conductivity, which is favorable for the superior electrochemical performance. The work has been published on Journal of Power Sources.