| Rechargeable lithium-ion batteries,characterized by long life cycle and high energy density,have been widely used in portable electronic devices,electric vehicles,hybrid electric vehicles and many other life applications.The main reason is that they can be used as an environmentally sustainable power source.Li2ZnTi3O8(LZTO)is a"zero-strain" material with a cubic spinel structure and a large theoretical capacity(229 mAh g-1),which is considered as a commercially promising negative electrode material.However,LZTO,as a titanium-base anode material,has low conductivity,and its magnification performance at high current density is greatly reduced.In this paper,the structure and electrochemical properties of LZTO were studied by ion doping and surface coating.(1)Li2ZnTi3O8 anode material was prepared by three synthesis methods:solid phase method,sol-gel method and molten salt method.The electrochemical test results of LZTO materials prepared by these three methods show that the initial discharge capacity of LZTO-2 sample is 155.84 mAh g-1 at the high current density of 1.0 A g-1.After 500 cycles,the specific discharge capacity of LZTO-2 material was still maintained at 126.21 mAh g-1,and the material showed better magnification performance under different current densities.EIS tests show that the LZTO-2 material has the smallest impedance and the largest lithium ion diffusion coefficient.(2)In order to improve the electronic and ionic conductivity of Li2ZnTi3O8 anode material.We designed Li2ZnTi3O8 and Li2-xZnTi3-xNbxO8(x=0.2,0.4 and 0.5)series negative electrode materials.Different amounts of high-valence Nb5+dopants not only reduce the content of lithium in Li2ZnTi3O8,but also change the inherent properties of the composites,thus optimizing the electrochemical performance of the doped composites.The results of XRD,SEM,TEM and XPS tests showed that Nb dopants were successfully introduced,but a large amount of Nb5+ dopants(x=0.4,0.5)formed ZnNb2O6 and TiO2..LZTNO-2 has the best electrochemical performance due to its small particle size,improved structural stability,and the charge-discharge capacity of 182.7/181.2 mAh g-1 after 500 cycles of 1.0 A g-1,much higher than that of LZTO(100.41/100.41 mAh g-1).The experiment shows that the stoichiometry and lithium content of cathode materials can be adjusted effectively by introducing high dopants,and the available vacancy for subsequent lithium insertion can be expanded significantly.The EIS test also confirmed that LZTNO-2 material has the lowest electrochemical impedance and the highest lithium ion diffusion rate.(3)It is urgent to improve the interfacial stability of Li2ZnTi3O8 material and lithium ion diffusion kinetics,so we use sol-gel method to prepare LZTO-PDA composite.PDA is a new type of environmentally friendly N-doped C material,and its structural characteristics can effectively increase the specific capacity of materials.At the same time,surface coating can also enhance the interfacial stability of the material and protect the surface of LZTO particles.The experimental results show that the LZTO-PDA(5 wt.%)sample exhibits excellent electrochemical performance,and the discharge capacity of the material is 162.09 mAh g-1 after 500 cycles at the high current density of 1.0 A g-1.This is much larger than the discharge capacity of the LZTO(114.40 mAh g-1)at the same current density.EIS also demonstrated that the coating of 5 wt.%PDA achieved a reduction in charge transfer impedance,an increase in electron conductivity and an increase in lithium ion diffusion capacity. |
