| Summary: | 碩士 === 明志科技大學 === 化學工程系碩士班 === 102 === The work reports the preparation of the Li3V2(PO4)3/C (named as LVP/C) composite material by the hydrothermal method and the post-sintering treatment. By using polystyrene (PS) polymer and polystyrene sphere (PS sphere) the carbon additives as the carbon sources and extra carbon sphere (CS) and graphene used as additives; the hydrothermal process and post-thermal treatment were used in sequence to prepare LVP/C composite material. It was found that the poor electron conductivity problem of the LVP/C materials can be solved by adding those composite carbon sources. In addition, by the micro-Raman spectroscopy analysis result, we found the R value (R = ID / IG) of the residual carbon is 1.01, which illustrates that the graphite-like degree of residual carbon material is quite high. It was also found that the composite carbon precursors of polystyrene sphere (PS sphere, PSS), graphene (Graphene nanosheet, GNS) showed good carbon coverage. From SEM morphology analysis results revealed LVP/C materials with 1D needle-like structure when PS sphere carbon precusor was added. LVP/C with 1D morphology can greatly help to improve the electron transport rate. TEM analysis results displayed that the surface of the LVP/C cathode material has a uniform carbon layer of ca. 5nm; it will help to improve the electrochemical properties. The particle size analysis results revealed that the average secondary particle size is about 3.85μm, and the smaller particles also helps to improve electrochemical performance. Furthermore, adding the graphene in LVP/C can significantly improve the high-rate discharge performance and long-term charge/discharge stability. The experimental results showed that when 2wt.% graphene (GNS) and 5wt.% polystyrene spheres (PSS) as the composite carbon precursor were added into LVP/C, the as-prepared Li3V2(PO4)3/C cathode material showed the best electrochemical performance among the various carbon precusors.
When only 5wt.% PSS carbon source was added into LVP/C material, the discharge capacities were 180~185 mAh g-1 and 20~25 mAh g-1 at 0.1C and 10C rates, respectively. While 5wt.% PSS +2wt.% CS composite carbon source were added, the discharge capacities were 195 mAh g-1 and 70~75 mAh g-1 at 0.1C and 10C rates, respectively. Furthermore, when 5wt.% PSS +2wt.% graphene composite carbon source were added, the discharge capacities were greatly enhanced to 190 mAh g-1 and 90~ 95 mAh g-1 at 0.1C and 10C rates, respectively.
In condusion, it was found that a suitable amount of 2wt.% grapheme or nano carbon sphere (CS) can markedly improve long-term cycle/high rate discharge performance. The hydrothermal preparation method demonstrated that the as-prepared LVP/C cathode materials had the uniform particle size and chemical compositions. The hydrothermal preparation method is simple and easy to carry out process at a low temperature (<200℃).
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