Design,Synthesis And Electrochemical Properties Of The Lithium Vanadium Phosphate Cathode Material For Lithium Ion Batteries

Recently,the NASICON-type monoclinic lithium vanadium phosphate?Li₃V₂?PO₄?₃?has been considered as a promising cathode material for high-power lithium ion batteries?LIBs?due to its high operating potential?up to 4.8 V?,high theoretical capacity(197 mAh g^-1)and safety performance.Unfortunately,Li₃V₂?PO₄?₃ has an intrinsic low electronic conductivity and ion diffusion coefficient,which limit its practical implementation.In order to overcome such problems,in this thesis,Li₃V₂?PO₄?₃ materials with mesoporous,macroporous or nano-sphere morphology have been successfully prepared.Moreover,the effects of the structure on the electrochemical performances of Li₃V₂?PO₄?₃ materials have also been investigated.The main contents of this thesis are summarized as following:The mesoporous Li₃V₂?PO₄?₃@CMK-3 nanocomposite has been synthesized by a sol-gel method.Li₃V₂?PO₄?₃@CMK-3 nanocomposite exhibits the pure monoclinic structure and mesoporous morphology.Li₃V₂?PO₄?₃ has particle sizes of<50 nm,and is embedded in the mesoporous channels as well as well dispersed on the CMK-3surface.Li₃V₂?PO₄?₃@CMK-3 nanocomposite shows significantly better rate capability and cycling performance than the bulk Li₃V₂?PO₄?₃/C.In the potential range of 3.0–4.3 V,the Li₃V₂?PO₄?₃@CMK-3 nanocomposite delivers high initial discharge capacity of 130.0 mAh g^-1 at 0.2 C.After 300 cycles,it can still retain a discharge capacity of 95.4 and 73.5 mAh g^-1 at 5 C and 10 C,respectively.The good electrochemical performance for the Li₃V₂?PO₄?₃@CMK-3 nanocomposite are related to the special mesoporous structure,nanosized particles,and the existence of conductive carbon matrix,thus leading to improvement in electron and lithium ion diffusivity.A series of ordered macroporous Li₃V₂?PO₄?₃/C cathode materials with three different pore sizes?65 nm,120 nm and 210 nm?have been prepared via a templating method using poly?methyl methacrylate??PMMA?colloidal crystals as templates.The three Li₃V₂?PO₄?₃/C samples all show pure monoclinic structure and ordered macroporous morphology.The Li₃V₂?PO₄?₃/C sample with pore size of 210 nm shows the best electrochemical performance.In the potential range of 3.0-4.8 V,it delivers a high initial discharge capacity of 189.4 mAh g^-1 at 0.1 C,which is close to the theoretical capacity(197 mAh g^-1).Moreover,at 0.5 C,1 C and 5 C,it exhibits initial discharge capacities of 170.5,166.0 and 145.9 mAh g^-1,and can still retain 77.3%,71.2%and 77.1%of the initial discharge capacity after 100 cycles,respectively.Ce³⁺doped ordered macroporous Li₃V_2-xCe_x?PO₄?₃/C?x=0,0.01,0.03,0.05?samples have been fabricated via a colloidal crystal template method.The XRD and SEM analysis demonstrate that the Ce element doping does not affect the original monoclinic structure and macroporous morphology of the pristine Li₃V₂?PO₄?₃/C sample.Electrochemical measurement results prove that the Li₃V_1.97Ce_0.03?PO₄?₃/C sample presents the best electrochemical performances as the cathode material for lithium ion batteries among the as-prepared samples.In the potential range of 3.0-4.8V,it can still deliver discharge capacities of 147.9 mAh g^-1 with 87.5%capacity retention and 124.7 mAh g^-11 with 84.0%capacity retention at 1 C and 5 C after 100cycles,respectively.Furthermore,between 3.0 and 4.3 V,it achieves highest initial capacity(131.3 mAh g^-1)at the rate of 0.2 C.After 100 cycles,the capacity retention is 99.2%,98.5%,96.5%,and 81.0%at 0.5 C,1 C,2 C and 5 C rates,respectively.The substitution of V³⁺with an appropriate amount of Ce³⁺could increase the Li⁺diffusion coefficient based on the EIS results,which is mainly responsible for the excellent electrochemical performance.Nanostructured Li₃V₂?PO₄?₃/C composite has been synthesized by a facile microemulsion method.XRD,XPS,Raman spectra,SEM and TEM results confirm that the as-prepared Li₃V₂?PO₄?₃/C sample shows the pure monoclinic structure and nanosphere morphology.Li₃V₂?PO₄?₃/C has a particle size of about 100 nm,and these nanoparticles are connected each other by the conductive carbon network.Electrochemical measurements demonstrate that the Li₃V₂?PO₄?₃/C nanoparticles can deliver discharge capacities of 129.8,126.1,118.0,116.1 and 110.1 mAh g^-1 between3.0 and 4.3 V,and 171.4,163.1,153.2,144.0 and 133.4 mAh g^-1 between 3.0 and 4.8V at 1,2,5,10 and 20 C,respectively.Even at 20 C rate,it still can presents a reversible discharge capacity of 101.8 mAh g^-1 with 92.5%capacity retention after1400 cycles and 96.4 mAh g^-1 with 72.3%capacity retention after 1000 cycles in the potential ranges of 3.0-4.3 V and 3.0-4.8 V,respectively.The excellent rate capability and long-term cycle performances demonstrate that the Li₃V₂?PO₄?₃/C materials prepared in this work have great potential for application as cathode materials in high power lithium ion batteries.