Modified Li₄Ti₅O₁₂ As Anode Materials For Lithium-Ion Batteries And The Degradation Mechanism Involved

As a promising energy storage device,lithium ion battery has been attracted widespread concern.Anode material is an important part of lithium ion battery,which could directly determine the electrochemical performance of lithium ion battery.However,the traditional graphite anode material has poor high-rate performance,which cannot meet the requirements for fast charge and discharge.Therefore,it is significant to seek an anode material with outstanding high-rate performance.In this paper,the surface modification of Li4Ti5O12 anode material was synthesised to improve its electrochemical performance.Meanwhile,the degredation mechanism of Li4Ti5O12 anode is also investigated.The main research contents are as followed:(1)Li4Ti5O12 cells were cycled at a current density of 500 mA/g,and the changes in structure,composition,cyclic voltammogram and electrochemical impedance with cycling were investigated systematically to collect comprehensive information on the capacity decay.Besides the performance degradation resulted from the electrolyte decomposition and gassing effect during cycling,the transition from spinel Li4Ti5O12 to rock-salt Li7Ti5O12 accompanying with structure and composition variation also leads to capacity fading of Li4Ti5O12,namely,the repetitively discharging/charging gives rise to more and more residual Li+ and Ti3+ in the delithiatGd Li4Ti5O12,structure disordering,gradually decreased ionic and electronic conductivities,and escalated polarization,thus aggravating the performance decay.Therefore,enhancing the structure and composition stability by modification is necessary to achieve more stable performance for Li4Ti5O12 applicable in Lithium ion batteries anode.(2)Li4Ti5O12 was modified by LiNaAl22O34 via a simple reaction between LiNO3,Na2CO3,AI(NO3)3·9H2O and LTO at suitable sintering temperatures.The product with a LiNaAl22O34/Li4Ti5O12 mass ratio of 0.0106 and calcined at 600 0C achieved reversible capacities of 163.8,160.6,156.5,150.9,132.9 and 163.4 mAh/g at the current rates of 100,200,400,800,1600 and 100 mA/g,respectively.Even cycled 800 times at 500 mA/g,a capacity of 147.9 mAh/g was retained.The outstanding cycling and rate performance is attributable to the simultaneously formed LiNaAl22O34 coating on the Li4Ti5O12 particles and superficial Al3+ doping in the Li4Ti5O12,achieving combined improvement in the ionic and electronic conductivities of Li4Ti5O12 and thus boosting the comprehensive electrochemical performance of Li4Ti5O12.(3)By virtue of the good absorbability,adhensivity,suspensibility,cation exchangeability and rapid ion diffusion channels,lithium magnesium silicate(LMSO)is utilized to modify Li4Ti5O12(LTO)by simply and uniformly coating LMSO on LTO particles and sintering at suitable temperatures.The LMSO-modified LTO with a LMSO/LTO mass ratio of 0.015 and calcined at 700 ? achieved reversible capacities of 160.2,157.8,155.8,151.3,145.5 and 162.9 mAh/g at diverse current rates of 100,200,400,800,1600 and 100 mA/g,respectively.Even cycled at 500 mA/g for 900 cycles,a capacity of 140.6 mAh/g is still maintained,markedly higher than that of the pristine LTO.The appreciable performance is attributable to the uniform LMSO coating for suppressing grain aggregation and growth,the optimization of ionic and electronic conductivities of electrolyte by the interaction with LMSO,the enhanced electronic conductivity of LTO resulted from superficial Mg2+ and Si4+ co-doping,the meliorated ionic conductivity introduced by the Li-ion diffusion channels in LMSO and cation exchangeability of LMSO,and the weakened polarization.