Study On The Modification Of High Capacity Lithium Vanadium Phosphate And Sulfur Cathode Materials

Lithium-ion batteries are promising green electrochemical energy storage devices that have been widely used in the range of portable electronic products and electric vehicles. The theory energy density of commercialization lithium-ion batter is up to 250 Wh kg^-1. However, the actual energy density is only about 150¹80 Wh kg^-1 which is hard to meet people's requirement of high energy density because of the low specific capacity, high expensive and complicated preparation technology of cathode materials. In this work, lithium vanadium phosphate?Li₃V₂?PO₄?₃? and sulfur?S8? were selected as research objects which have been studied systematically. The relationship among the preparation process, microstructure and electrochemical properties has been well discussed. This work will provide essential experimental and theoretical perspectives for the exploring new electrodes of lithium-ion batteries. The main contents are as follows:?1? A series of Sn-doped Li₃V_2-xSn_x?PO₄?₃/C?x = 0, 0.1, 0.15, 0.2? samples were synthesized via a citric acid assisted sol-gel method. The results indicated that all the Li₃V_2-xSn_x?PO₄?₃/C samples were pure single phase with a monoclinic structure?space group P21/n? and well crystallized. The structural refinement results, HRTEM analysis and SEM results indicated that Sn-doping did not alter the lattice structure, morphology and particle size of Li₃V₂?PO₄?₃, but increased the unit cell volume. A thin carbon layer about 5 nm was coated on the surface of Li₃V_2-xSn_x?PO₄?₃/C samples with a similar carbon content and good quality. The electrochemical performance was evaluated using coin-type half cells. Li₃V_1.85Sn_0.15?PO₄?₃/C exhibited the electrochemical performance in terms of specific capacity, rate capability, cycling performance. CV and EIS results implied that optimizing Sn-doping contents with x = 0.15 could greatly enhance the structural stability of Li₃V₂?PO₄?₃ during the charge-discharge processes, as well as increase electrical conductivity with a lower charge transfer resistance and a higher Li⁺ diffusion coefficient.?2? The supercritical fluid CO₂ was served as solvent and reaction medium to the controllable fabrication of sulfur-carbon composites. Morpholgy, microstructure,chemical composition, specific surface area, pore size and distribution and surface states of sulfur-carbon composites have been investigated by XRD?SEM?TEM?BET. The results showed that the supercritical fluid CO₂ not only have a decorative effect on the microstructure and interface of the carbon materials, but also can dissolve sulfur into the internal structure of carbons along with the expansion of the carbon layers. Comparing with the conventional preparation, the supercritical fluid technique not only can simplize the preparation process, but also can make the high utilization, loading capacity and dispersibility of sulfur. The electrochemical performance of the sulfur-carbon composities as cathode materials was studied as well. The results demonstrated that the Coulombic efficiency has been improved and the “polysulphide-shuttle” problem has been ameliorated. And activated carbon/S sample showed the optimal electrochemical performance among the as-prepared samples.?3? Porous silica?RHs-SiO₂? was synthesized by using rice husks as both biotempates and silicon sources. RHs-SiO₂ doped mesoporous carbon foams were synthesized via a citric acid assisted sol-gel method. XRD results showed that the composites were all existed in an amorphous form?SiOC?. SEM images showed that the pore strutrue of samples were different at different calcination temperatures. Among them, the sample synthesized at 800 oC with a porous structure can be an ideal carrier for sulfur. Sulfur was permeated into the conductive carbon porous network uniformly by a thermal diffusion method. The electrochemical performance clealy demonstrated that the RHsSiO₂/mesoporous carbon foams with 10% addition of RHs-SiO₂ had a good cycling stability.