Synthesis And Modification And Electrochemical Performance Of Lithium Titanate Anode Material For Lithium Ion Battery

Spinel Li₄Ti₅O₁₂has been considered as one of the most promising alternativesfor carbon due to its cycling stability, zero-strain structural change in thecharge-discharge process, good stability in organic electrolyte, good safetyperformance and so on. However, low electronic conductivity and poor high-rateperformance are the main obstacles preventing Li₄Ti₅O₁₂to be put intocommercialization. It is significant for fastening the commercialization of Li₄Ti₅O₁₂to solve the above problems and improve its electrochemical property.The optimal technological parameters of solution method determined by theorthogonal experiment were as follows: sintering temperature was750℃, sinteringtime was8h, the lithium resource was LiOH and the mole ration of Li to Ti was0.85. Li₄Ti₅O₁₂prepared by the method exhibited better electrochemicalperformances, the discharge plateau was about1.54V and the initial reversiblespecific capacity was178.59mAhg^-1 at0.2C rate. The discharge plateau reduced to1.32V, the initial reversible specific capacity was92.73mAhg^-1 at5C rate. CV andEIS tests were applied to study the electrode kinetics of Li₄Ti₅O₁₂. It was found thatthe relationship of peak current value and the square root of the scanning rate waslinear and the electrode process accorded with half-infinite diffusion model from theresults of CV. Exchange current density and lithium ion diffusion coefficient ofLi₄Ti₅O₁₂at different state of charge（SOC） were calculated by EIS analysis. Whenthe SOC was100%, the exchange current density reached a maximum of0.3034mA·cm-2. Lithium ion diffusion coefficient was between1.83×10^-12cm²·s^-1 and8.94×10^-13cm²·s^-1, which changed as the SOC decreased from100%to15.5%.Al and K were chosen to dope into Li site and Li_4-xAl_xTi₅O₁₂（x=0.03,0.05,0.10）and Li_4-xK_xTi₅O₁₂（x=0.02,0.04,0.06） series compound were prepared. XRDresults showed that the lattice structure was not changed after Al and K doping,while the lattice volume of doping materials shrank and inflated respectively. Theoptimal doping amount was determined by investigating the electrochemicalproperties of different doping amounts. The reversible specific capacity ofLi_3.95Al_0.05Ti₅O₁₂and Li_3.96K_0.04Ti₅O₁₂were147.23mAh·g^-1 and157.13mAh·g^-1 respectively at0.5C rate after30cycles. Li_3.96K_0.04Ti₅O₁₂showed the optimumhigh-rate performance at3C rate.Although doping at Li site improved the rate performance, the reversiblespecific capacity declined. So Al and K were chosen to dope into Ti site. Li₄Ti_5-xAl_xO₁₂（x=0.03,0.05,0.10） and Li4Ti5-xZrxO12（x=0.03,0.05,0.10） seriescompounds were prepared. XRD results showed that the lattice structure was notchanged after doping Al and Zr, but leading to the lattice parameter largen andlessen respectively. ICP test proved that the actual molecular formula of differentdoping amount was similar as the initial ration. Charge-discharge tests determinedthat the doped material of x=0.05had the best electrochemical properties for bothAl and Zr doping. The reversible specific capacity of Li₄Ti_4.95Al_0.05O₁₂andLi₄Ti_4.95Zr_0.05O₁₂were168.35mAh·g^-1 and157.14mAh·g^-1,159.50mAh·g^-1 and153.49mAh·g^-1,149.14mAh·g^-1 and143.92mAh·g^-1 at0.5C，1C，3C, respectively.The value of Rsand Rctof Li₄Ti_4.95Zr_0.05O₁₂was minimum after EIS analysis andfitting. The electrochemical performance of Li₄Ti_4.95Zr_0.05O₁₂was the best.Conductivity of four materials of optimal doping amount were tested. Conductivityof Li₄Ti_4.95Zr_0.05O₁₂was maximum, it was8.910-6S·cm^-1. The conductivity wasincreased two orders of magnitude compared with Li₄Ti₅O₁₂.The coating modification of Li₄Ti₅O₁₂was also studied. Sugar and oleic acidwere chosen as carbon source. The optimal amount of carbon source wasdetermined by comparing structure and properties. Structure, morphology andelectrochemical performances of Li₄Ti₅O₁₂/C using oleic acid as carbon sourcewere investigated in detail and the coating mechanism was studied considering thehigh carbon content, consumption, no pollution of oleic acid.On the basis of the above research, Li₄Ti_4.95Zr_0.05O₁₂/C was further prepared.XRD results showed that the lattice volume of doping materials inflated. SEM testshowed that the particles size of doping materials was slightly decreased.Charge-discharge performance was tested at0～2.5V and1～2.5V voltage range.The reversible specific capacity reached289.03mAh·g^-1 and264.03mAh·g^-1 forLi₄Ti_4.95Zr_0.05O₄/C and Li₄Ti₅O₁₂/C respectively at the0.2C discharge to0V after50cycles, and the discharge capacities was212.6and137.07mAh·g^-1 at5Crespectively. The results of conductivity, CV and EIS tests indicated thatLi₄Ti_4.95Zr_0.05O₄/C significantly improved conductivity, lowered the polarization ofhigh rate and reduced the value of Rct. The electrochemical performances weredramatically improved especially at low voltage interval.