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Research On The Doping Modification Of Li3V2?PO4?3/C Cathode Material For Lithium-ion Batteries

Posted on:2017-03-07Degree:MasterType:Thesis
Country:ChinaCandidate:T T ZengFull Text:PDF
GTID:2311330488476123Subject:Materials engineering
Abstract/Summary:PDF Full Text Request
The lithium vanadium phosphate cathode material with monoclinic system and NASION structure has become one of the main candidate materials of high performance lithium-ion batteries in the future. This is due to it posseses the advantages of stable structure, excellent electrochemical performance, high safty and environment friendly. However, its widely application has been blocked by its low electronic conductivity. In this paper, the electrochemical performances of Li3V2?PO4?3 has been improved by doping with metal ions. The relationships between the doped ions, doping amounts, carbon sources, preparation mehods and ion valences with the structure, morphology and electrochemical performance have been investigated systematically. Through characterized of X-ray diffraction, scanning electron microscopy, constant-current charge-discharge, cyclic voltammetry and electrochemical impedance, etc. The results show that:Firstly, phenolic resin and PVDF for carbon source has been prepared by using Li3V2-4/3XTix?PO4?3/C materials with Ti4+doping by sol-gel method. The results show that the doping materials using PVDF as carbon source possess greater diffusion of litium ions, better electronic conductivity, and possess has higher specific discharge capacity, high capacity retention, and better cyclic reversibility. When Ti4+doping amount is reaches 5wt.%, the electrochemical performance reaches the best. It shows that using PVDF as carbon source and moderate addition of Ti4+can improve the electrochemical performance of materials.Secondly, sol-gel method and solid-state method were used to prepare Li3V1.95Cr0.05?PO4?3/C materials by using PVDF for carbon source. After doping Cr3+, these samples prepared by two methods can get impure phases and monoclinic system. The doping amount of Cr3+in the material does not change the structure and morphology, however, the electrochemical performance of material is enhanced. The specific discharge capacities of Cr3+doping sample which prepared by sol-gel method has reached 122.1 mAhg-1 and 109.8 mAh-g"1 at 0.2 C and 5 C rates. The corresponding capacity retention ratio are 99.1% and 98.5% after 100 times tests, which its cyclic performances is excellent provided that using sol-gel method is an effective way to improve the electrochemical performance of Li3V2?PO4?3.Finally, Mn2+?MnCl2? and Mn4+?MnO2? doping Li3V2?PO4?3 materials have been synthesised by sol-gel method. Mn2+ and Mn4+ doping samples can maintain its monoclinic system and good crystallinity. The small amount of Mn doping also can minize the size of grain. The Mn4+ doping Li3V2-4X/3Mnx?PO4?3/C materials possess higher specific discharge capacity and capacity retention and better cyclic reversibility. When the Mn4+doping amount reached 7wt.%, the electrochemical performance of sample is the best. Compared comprehensively, using the high valent of Mn4+for the doping ion can improve the electrochemical performance of Li3V2?PO4?3.
Keywords/Search Tags:Lithium ion batteries, Cathode materials, Lithium vanadium phosphate, Doping modification
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