Study on Synthesis and Modification of Lithium Zinc Titanate as A Ti-based Anode Material for Li-ion Batteries

Author:Chen Chi

Supervisor:ma xin bin


Degree Year:2017





Due to the merits of high energy density,long cycle life and good safety,Li-ion batteries have been widely used in portable electronic devices and are also expected to be used in large scale in other energy storage devices such as electric vehicles in the future,while designing and developing high-performance electrode materials is an important way to promote its development.Based on the bottleneck problem of lithium zinc titanate(Li2ZnTi3O8)as an electrode material applied in Li-ion batteries,its modification research was carried out in this work,which aimed at improving its electronic conductivity and high-rate performance,and providing more choices for the anode materials of high power Li-ion batteries.First of all,Li2ZnTi3O8 was synthesized by a solid-phase reaction method using Li2CO3,TiO2 and Zn(CH3COO)2·2H2O as raw materials.It can be found from the investigation of the optimum reaction conditions that Li2ZnTi3O8 synthesized at750°C for 4 h presents good crystallinity and excellent electrochemical performance.And it also shows good cycle stability when it is assembled into a full battery with LiFePO4.Li2ZnTi3O8 was doped by Y3+.Among all the samples,Li2ZnTi3-xYxO8(x=0.03)shows the best electrochemical performance.After 10 cycles at the current density of0.114,0.454,1.135,2.270,3.405 and 4.540 A g-1,the discharge specific capacity of Li2ZnTi3-xYxO8(x=0.03)reached 231.2,217.6,209,200.9,197.5 and 192.9 mAh g-1,respectively.The trivalent Y3+doping can not only affect the lattice spacing of Li2ZnTi3O8 to some extent and enhance its ionic conductivity,but also induce its formation of Ti3+.In addition,the excess Y3+can form Y2O3 scattering between the Li2ZnTi3O8 particles and effectively hinder their growth/agglomeration.To highlight the effect of Ti3+on Li2ZnTi3O8,the reductive C6H8O6 was selected as the carbon source to reduce Li2ZnTi3O8 by a carbothermal reduction.In the experiment,the amount of residual carbon in the finally formed product Ti3+self-doped Li2ZnTi3O8 could be negligible(0.59 wt%)by strictly controlling the input of C6H8O6.After 1000 cycles at the current density of 1.5 A g-1,Ti3+self-doped Li2ZnTi3O8 could present a discharge specific capacity of 122.9 mAh g-1,showing an excellent long cycle life.Based on the traditional carbon coating,the core-shell Li2ZnTi3O8@C composites were synthesized by using C6H5Li3O7·4H2O as both lithium source and carbon source.During the reaction,the Li2O generated by the pyrolysis of C6H5Li3O7·4H2O was encapsulated by the carbon shell formed by its simultaneous carbonization.Therefore,the synthesis of Li2ZnTi3O8 was carried out in a relatively limited environment and the growth/agglomeration of the particles was effectively suppressed.When the current density was increased to 1.135 A g-1,Li2ZnTi3O8@C could deliver a discharge specific capacity of 226.4 mAh g-1,which was higher than185.4 mAh g-1 of pristine Li2ZnTi3O8.Finally,N-doped carbon coated Li2ZnTi3O8 was prepared by using C8H15NO6 as the N-containing carbon source.The results show that the NC-LZTO-2 sample with the carbon content of7 wt%has the best electrochemical performance.It could contribute a reversible specific capacity of 128.4 mAh g-1 after 1000 cycles at the current density of 2.270 A g-1,higher than Li2ZnTi3O8 coated by pure carbon(77.5mAh g-1).The N-doped carbon layer coating can not only regulate the size of Li2ZnTi3O8 particles,but also improve its electronic conductivity.Additionally,its N atoms can form various chemical bonds with the atoms in Li2ZnTi3O8,which can increase the stability of the coating layer and further enhance the electrochemical performance of Li2ZnTi3O8.