Synthesis And Electron Microscopy Study Of Sodium Tungsten Bronzes

Sodium ion batteries(SIBs)have recently attracted remarkable attention in the rechargeable battery market due to the abundance and low-cost of sodium.However,in spite of the fact that SIBs exhibited very similar physical and chemical processes with those of LIBs,the large size of sodium ion relative to lithium ion hindered the applications.In terms of SIBs,more and more tungsten bronzes were applied as novel electrode materials due to their kinetically favorable open frameworks.In order to improve the performance of tungsten bronzes in SIBs,it is of necessity to select the suitable synthesis method and research the structures of tungsten bronzes.This thesis includes two major parts shown as follows:1.Sodium tungsten bronzes with channel structures were successfully synthesized by a simple heating method.After heating at 1073 K for 1 h,three products were observed and identified to be a mixture of Na2W4O13,Na5W14O44 and pseudo-hexagonal NaxWO3+y by applying the X-ray diffractometry,scanning and transmission electron microscopy.Moreover,NaxWO3+y was never reported before and proved to be triclinic(cell parameters:aⅢ=7.274 A,bⅢ=7.291 A,cⅢ=110.31 A,αⅢ=90.71°,βⅢ= 90.69°,γⅢ=119.65°),according to the structural relationship between Na5W14O44 and NaxWO3+y.2.The same heating method was applied for the synthesis of various sodium tungsten bronzes.After heating at 873 K,the final products were identified as Na2W2O7 and Na2W4O 13,Na2W4O13 and WO3,WO3 under different annealing time of 4 h,10 h and 30 h,respectively.It was revealed that the Na content decreases with the increasing annealing time.This research serve as a possible guidance for the synthesis of sodium tungsten bronzes with controlled structures.