Preptaration And Electrochemical Performance Of Titanium-based Oxides As Anode Materials For Lithium-Ion Batteries

Currently,the development of electric vehicle mainly replies on the battery pack.Although lithium ion batteries(LIBs)exhibit superior merits including high energy density,high operating voltage,long lifespan,wide operating working potential and no memory effect compared with traditional lead-acid batteries and nickel-cadmium batteries,the current LIBs are unable to meet the requirement for high performance and safety.Anode material is the key component to determine the performance of LIBs.Due to the poor rate capability and safety issue possessed by conventional graphite anode,seeking for alternative candidates becomes more and more urgent.Titanium-based materials,mainly including lithium titanium(LTO)and titanium dioxides(TiO2),exhibit high cycling stability which could be applied in power battery field.Both materials have little volume variation during the lithiation/delithiation process,making them serve as promising candidates for power battery.However,the inherent low electronic conductivity hinders their practical application.In order to solve the problem mentioned above,surface coating and bulk doping have been verified to be very effective methods.Coating a layer on titanium-based oxides nanoparticles for instance conductive carbon or solid state electrolyte layer,could not only enhance the electronic conductivity or lithium ion diffusion rate,but inhibit the growth of particles and eliminate particles aggegration as well.With doping,the electronic conductivity of titanium-based oxides is highly improved.In this paper,we focus on the modification of LTO and Ti02 via surface engineering and doping involvement,to obtain several modified titanium-based materials.(1)Copper(Cu)-modified LTO with various Cu-doping contents was prepared via simple hydrolysis of tetrabutyl titanate in the water solution of LiOH·H2O and the subsequent drip of Cu(NO3)2·3H2O,followed by sintering the dried mixture at 600 ℃for 5 h.The modified products exhibit outstanding rate performance at 0.1-40 C.The product with a Cu-doping content of 0.08 exhibits reversible capacities of 176.9,153.3,150.7,148.0,145.3,144.7 and 138.5 mAh g-1 at 0.1,1,2,5,10,20 and 40 C,respectively,and also excellent long-term cycling stability with reversible capacity of 132.3 mAh g-1 after 2500 cycles at a high current rate of 10 C.The modified products exhibit outstanding rate performance at 0.1-40 C.By means of the characterization of structure and specific surface area as well as the EIS analysis,the modified product exhibits higher specific surface area,refined grains,slightly increased lattice constant,and improved electronic and ionic conductivities,leading to the improvement of electrochemical performance.(2)Ionic conductor of Li2SiO3(LSO)was used as an effective modifier to fabricate surface-modified LTO anode material via simply mixing followed by sintering at 750 ℃ for 5 h in air.The electrochemical performance of LTO was enhanced by merely adjusting the mass ratio of LTO/LSO,and the LTO/LSO composite with 0.51 wt%LSO exhibited outstanding rate capabilities(achieving reversible capacities of 163.8,157.6,153.1,147.0,and 137.9 mAh g-1 at 100,200,400,800,and 1600 mA g-1,respectively)and remarkable long-term cycling stability(120.2 mAh g-1 after 2700 cycles with a capacity fading rate of only 0.0074%per cycle even at 500 mA g-1).With the help of multiple strcture charateriazation methods,slight doping effect adjacent to the LTO surface occurs,favorable to the enhancement of electronic conductivity.And EIS also veifies that the ionic diffusion rate increases after coating LSO.The LSO coating coupled with the slight doping effect contributes to the enhancement of both electronic and ionic conductivities of LTO,contributing to the superior electrochemical properties.(3)TiO2/C nanocomposites were fabricated by simple hydrolysis of tetrabutyl titanate to yield Ti02 nanoparticles followed by carbonizing the mixture of glucose and TiO2 at 600 ℃.The particle size after coating reduces remarkably which is conductive to increase the contact area between electrolyte and active material,shortening the lithium ion transmission path.By varying the weight ratio of glucose:TiO2,the electrochemical performance of the composites could be optimized significantly.At a ratio of 0.8:1,the composite exhibits a high reversible capacity of 283.7 mAh g-1 after cycling 100 times at a current density of 100 mA g-1,as well as the capacities of 245.1,213.6,179.9 and 136.6 mAh g-1 at the corresponding densities of 200,400,800 and 1600 mA g-1.After cycling 1000 times at 500 mA g-1,the capacity of 122.8 mAh g-1 was retained for the composite with a ratio of 0.8:1,and even a capacity of 149.1 mAh g-1 for the composite with a ratio of 0.7:1.The enhanced performance is ascribed to the carbon-coated Ti02 nanoparticles uniformly embedding in the carbon matrix with appropriate carbon content which could improve the ionic conductivity without harming the ionic diffusion rate.(4)Nitrogen-doped Ti02 nanoparticles coated with N-doped carbon are prepared by simply hydrolyzing tetrabutyl titanate to obtain TiO2 nanoparticles followed by heating the mixture of nanoparticles and urea in an autoclave at 550 ℃ for 5 h.N-doping and coating carbon are achieved simultaneously in one-step reaction.The modified TiO2 exhibits reduced particle size and EIS spectra show the interface resistance decreases significantly.The modified TiO2 exhibits outstanding reversible capacities of 227.7,204.0,186.4,160.6,113.1 mAh g-1 corresponding to current densities of 100,200,400,800 and 1600 mA g-1.The modified Ti02 could deliver an excellent long-term cycling capacity of 106.4 mAh g-1 even cycled 2500 times at a high density of 500 mA g-1 with an average capacity loss of only 0.016%per cycle.The combination of simultaneously N-doping with coating carbon results in the enhancement in electronic and ionic conductivities.