Surface Modification Of TiNb₂O₇ Anode Material And Its Electrochemical Performance For Lithium-ion Battery

Lithium ion batteries have already been widely used in portable electronic devices for their high energy and power density,high voltage,low self-discharge,wide working temperature range and long cycle life.But there are still many challenges when they are used in hybrid electric vehicles(HEV)and electric vehicles(EV),such as the safety issue and and high power density performance.Ti Nb2O7 has been regarded as an ideal candidate for anode material of lithium ion battery due to its high theoretical capacity of 387.6 m Ah g-1 and excellent electrochemical performance.The high Li+ intercalation potential of 1.64 V can avoid the formation of lithium dendrtite,and therefore ensure an outstanding safety property.Previous research of our group indicated that an unstable solid electrolyte interface(SEI)can be formed on the surface of TNO particles when it is cycled between 1.0~3.0V,which may lead to the decomposition of electrolyte.In the thesis,the formation of SEI film and the surface modification were investigated,and the effect on electrochemical performance was also studied.TNO materials were synthesized by the solid state method and the SEI on TNO anode was confrimed by the electrochemical impedance spectroscopy(EIS)results of TNO/TNO battery.TEM images also showed that a thin SEI film about 1~2nm was formed on the surface of TNO after long cycling.During the activation cycle,the discharge voltages were set to 0V,0.7V,0.8V,0.9V respectively.The electrolyte would be decomposed at low voltage and a stable SEI would be formed on the surface of TNO anode.The TNO/Li battery showed good electrochemistry performances after being activated between 0.8~3V or 0.9~3V once,but thick SEI would lead to low capacity and poor cycle performance.In order to form a stable SEI on the surface of TNO anode,PTSI additive was added into the electrolyte.Then the effect of temperature on the battery performance was studied when the batteries were cycled at 30? and 55?.The results show that 0.2wt% and 0.5wt% PTSI in the electrolytes could help to keep a good cyclability of TNO/Li battery.TNO@C composites were synthesized via a simple solid-state method and high temperature pyrolysis of sugar.All the TNO@C composites showed better cyclability than pure TNO and TNO@C with 1.62wt% pyrolytic carbon showed the highest reversible capacity.Cyclic voltammetry(CV)and EIS tests indicated that carbon layer enhanced the electronic conductivity of TNO particles.By the investigations of surface composition of TNO@C and TNO anode at various lithiated /delithiated states,a stable SEI was proved to be formed on the surface of TNO@C anode and thus to enhance the cycling stability of TNO@C mate rial.To further verify the effect of PTSI and carbon layer on the electrochemical performance of TNO,Ti Nb2O7/Li Fe PO4 pouch cells were assembled and the gas volumes during cyclic process were measured.The results indicated that all the TNO/LFP full cell would generate a small amount of gas during the activation process.But during following cycling,TNO/LFP cell with PTSI additive and TNO@C/LFP cell produced less gas than pristine TNO/LFP cell.