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Study On Preparation And Performance Of Li3V2?PO4?3/C Cathode Materials For Lithium-ion Batteries

Posted on:2019-03-13Degree:MasterType:Thesis
Country:ChinaCandidate:X Y LiuFull Text:PDF
GTID:2321330545499117Subject:Analytical Chemistry
Abstract/Summary:PDF Full Text Request
Monoclinic Li3V2(PO4)3 has been regarded as a promising cathode material due to its advantages of high specific capacity(197 m Ah g-1),high operating voltage and stable crystal structure.However,the low bulk electronic conductivity(?10-8 S cm-1)limits its charge/discharge performance at high rates.In this paper,the Li3V2(PO4)3/C composites were synthesized via sol-gel method,solid phase method and spray-drying method with carbon sources of different molecular structures.The effects of carbon sources on the morphology,microstructure,particle size and electrochemical property were discussed in detail.The contents are as follows:1.The Li3V2(PO4)3/C materials were synthesized by solid-state reaction using polyacrylonitrile,tannic acid,gallic acid,and gluconic acid lactone as carbon source,respectively.The effect of carbon source on structure,morphology and electrochemical property of Li3V2(PO4)3 was analyzed.The Li3V2(PO4)3/C particles with small size and good dispersion were synthesized via using gallic acid and gluconic acid lactone as single carbon source,along with higher carbon graphitization degree.Two kinds of materials deliver the discharge capacities of 105.7 m Ah g-1 and114.2 m Ah g-1 at 20 C in the voltage range of 3.0-4.3 V,respectively,and show excellent cycling stability.2.In order to overcome the disadvantages of single organic carbon source,the Li3V2(PO4)3/C/CNT and Li3V2(PO4)3/C/r GO composites were synthesized by using carbon nanotubes(CNTs)and reduced graphene oxide(r GO)with high conductivity as carbon sources.The Li3V2(PO4)3/C/CNT composite delivers a discharge capacity of121.8 m Ah g-1 at 10 C.The capacity retention reaches 92.3% after 300 cycles at 1 C in the voltage range of 3.0-4.3 V.The Li3V2(PO4)3/C/r GO composite deliveres the discharge capacity of 128.9 m Ah g-1 with capacity retention of 94.3% over 300 cyclesat 0.5 C in the voltage range of 3.0-4.3 V.The charge/discharge tests show that the electrochemical activity and cycle life of Li3V2(PO4)3 could be improved by the synergistic effect of double carbon sources.3.The Li3V2(PO4)3/C was obtained by a combination of two-step ball-milling and subsequent-sintering process,using lithium lactate as both lithium and carbon sources.The Li3V2(PO4)3/C delivers a discharge capacity of 132.4 m Ah g-1 at 0.2 C in the voltage range of 3.0-4.3 V.Furthermore,the capacity retention is up to 93.3%after 200 cycles at 5 C in the voltage range of 3.0-4.8 V.The excellent electrochemical performance could be ascribed to the small particle size and uniform carbon coating.4.High conductive and porous Li3V2(PO4)3/C composite was prepared by using starch with short carbon chain and poly(acrylic acid)with long carbon chain as double carbon sources.The porous Li3V2(PO4)3/C composite possesses the surface area of24.24 m2 g-1 with particle size of 80-250 nm.The effect of molecular structure of carbon source on the morphology and microstructure of the composite material was discussed.The porous Li3V2(PO4)3/C composite delivers the discharge capacities of121.5 m Ah g-1 at 10 C and 106.4 m Ah g-1 at 40 C in the voltage range of 3.0-4.3 V.The discharge capacity remains 90.0% of the initial cycle after 1000 cycles at 10 C.The porous Li3V2(PO4)3/C exhibits excellent rate performance and cyclic reversibility.5.The porous Li3V2(PO4)3/C microspheres made of nanoparticles were obtained by a combination of spray-drying and sol-gel process.Beta-cyclodextrin serves as both chelating reagent and carbon source to obtain carbon coated Li3V2(PO4)3 grains with a size of ca.30-50 nm and with the microspheres diameter of ca.3-9 ?m.When charging at 4.3 V,a discharge capacity of 117.7 m Ah g-1 is retained at 2 C after 1000 cycles.The capacity retention and coulombic efficiency reach at ca.96.0% and 99.5%,respectively,displaying excellent cycling stability and high electrochemical reversibility.The unique porous micro/nanostructure facilitates the fast transport of electron and lithium ion.Moreover,the Li3V2(PO4)3/C microspheres also give considerable cycling stability and high-rate reversible capacity at a higher end-of-charge voltage of 4.8 V.6.Na-doped Li3V2(PO4)3/C cathode material for Li-ion batteries was synthesizedvia a sol-gel method.The effect of Na-doping at Li site on crystalline structure and electrochemical performance of Li3V2(PO4)3/C was investigated.The Li2.96Na0.04V2(PO4)3/C presents the discharge capacities of 128.8 m Ah g-1 at 5 C and110.0 m Ah g-1 at 10 C,respectively,around 1.26 and 1.53 times of undoped Li3V2(PO4)3/C.Na-doping in Li site would be beneficial to expand the lattices and facilitate the fast Li+ diffusion,resulting in the improved electrochemical performance.
Keywords/Search Tags:Lithium-ion battery, Li3V2?PO4?3, carbon source, composite materials, electrochemical performance
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