Study On Synthesis And Electrochemical Performance Of Polyanion-type Ti-based Electrode Materials

Polyanion-type Ti-based electrode materials,such as lithium titanium phosphate and sodium titanium phosphate,should be a potential and attractive electrode material candidate in lithium ion batteries(LIBs)and sodium ion batteries(SIBs),which are attributed to its stable crystal structure,flexible charging and discharging platform and good cycling performance.Nevertheless,this kind of material possesses the defects,mainly containing the excessively low intrinsic electronic conductivity,preventing them from being extensively wielding in large-scale applications.In this thesis,electronic conductivity of materials is improved by carbon-covering and designing special structure.First,LiTi2(PO4)3/C composite has been synthesized by simple one-step solvothermal method using tetrabutyl titanate as titanium source and carbon source simultaneously.We have explored the effect of porous structure and carbon coating on the titanium phosphate materials.After a series of physical characterization and electrochemical performance testing of the material,the results confirmed that the abundant pores coupled with thin and uniform carbon-coating layer improve the lithium ion diffusion and electronic conductivity remarkably.The porous structure is beneficial to the infiltration of electrolyte,improving the migration efficiency of lithium ion.Hence,though the carbon content is as low as 0.495 wt.%,LTP/C composite reveals the excellent rate and cycle property.Such as from 0.5 C to 5 C,the specific discharge capacity of LTP/C is successively as high as about 110.2,103.8,101.2,96.9 mAh g-1.It could deliver reversible discharge capacity as high as 84.4 mAh g-1 at 20 C(1 C = 138 mA g-1).Moreover,the initial discharge capacity of LTP/C composite is 98.6 mAh g-1 and the capacity retention reach up to 85%after 2000 cycles at 10C.Second,NaTi2(PO4)3/C composite nanofibers(NTP/C-F)have been prepared with success by simple and efficient electrospinning.NTP/C-F is composed of NTP primary nanoparticles and 1D carbon matrix.NTP primary nanoparticles provides very short Na-ion and electron diffusion pathway.Besides,the 1D carbon matrix form good electro-conductive network,which can interconnect the NaTi2(PO4)3 primary nanoparticles.And its small nanosized NTP primary particles not only further favors fast transport of Na+ and electrons to active material but also augment active sites for Na.Therefore,the NaTi2(POd)3/C composite nanofibers have outstanding electrochemical performance.The discharge capacity of NTP/C-F at the first cycle is 104.5 mAh g"1,and it reserves as high as 96.7 mAh g-1 after 500 cycles.The capacity decay is only 7.5%.Moreover,it has good rate property.NTP/C-F delivers the discharge capacities,which are about 130,123,122,119,114,and 103 mAh g-1 at the current densities of 0.1 C,0.2 C,0.5 C,1 C,2 C,and 5 C,respectively.