Synthesis And Electrochemical Performance Of Highly-Packed Nano-Micro Spherical Li₄Ti₅O₁₂ Anode Materials

Li₄Ti₅O₁₂?LTO?,as an anode material for Li-ion batteries?LIBs?,has many merits such as high?dis?charge voltage plateau?1.55 V vs Li⁺/Li?,three dimensional transporting network for lithium ion,almost no volume change?<0.2%?during the?dis?charge process,and etc..Threreore,the application of LTOcan dramatically improve the cyclic reversibility,security and fast charge performance of the electrode,exhibiting a bright future in the field of energy storage devices and high power LIBs.However,one of the key challenges towards high power LIBs is to develop cheap easy to prepare materials that combine high volumetric and gravimetric energy density with high power densities and a long cycle life.This requires electrode materials with large tap densities,which generally compromises the charge transport and hence the power density.Herein in this dissertation,we developed a facile one-pot hydrolysis synthetic method to produce compact spheres of LTO materials through the size control of the precursors and the maintaining of the spherical morphology.The as prepared materials combined high tap densities and low carbon with long-term high rate-cycling performance.It is of great significanceto the fabrication of high volumetric andhigh power densities LIBs.Firstly,a robust strategy for the synthesis of uniform LTO nanosphereswith an average diameter of 120 nmwas developed.The precursorscomposed of uniform TiO₂/Li⁺ nanospheres were formed in a stable alkaline environment during the course of heating of the solution,and the OH-was found to effectively retard the hydrolysis of peroxo-titanium complex as well as the aggregation of Ti O₂/Li⁺ nanoparticlesprecursors.Intriguingly,a uniform polyvinyl pyrrolidone?PVP?layer formed in-situ on the surface of TiO₂/Li⁺ nanospheres rendered LTO to retain the monodisperse spherical morphology after annealing.The as-prepared monodisperse LTO nanospheres uniformly coated by a carbon layer exhibited a high tap density?1.1 g cm-3?and an outstanding rate-cycling capability.The charge specific capacities at 50 and 80 C were 128.8 and 108.9 mAh g-1,respectively.More importantly,the capacity retention after 500 cycles at 10 C was as high as 92.6 %.In addition,in order to further improve the tap density,densely-packed Li₄Ti₅O₁₂ sub-microspheres?0.5 ?m?are prepared viaa simple and easily up-scalableself-assembly process with the tetrabutyl titanate?TBT?and the hexadecylamine?HDA?,manifesting very high tap densities?1.2 g·cm-3?and exceptionally stable long-term high rate cyclic performance.The specific capacities at a charge rate of 10 and 20 C reach 148.6 and 130.1 mAhg-1,respectively.Moreover,the capacity retention ratio is 97.3% after 500 cycles at 10 C in a half cell and no obvious capacity reduction is found even after 20000 cycles at 30 C in a full LiFePO4/LTO battery.The excellent performance is explained by the abundant presence of grain boundaries between the nano-crystallites in the sub-micron spheres creating a 3D interconnected network,which allows very fast Li-ion and electron transport as indicated by the unusual large Li-ion diffusion coefficients and electronic conductivity at 52% SOC?6.2×10-12 cm2 s-1 and 3.8×10-6 S cm-1,respectively?.At last,based on the synthesis of nano and sub-micron of spherical LTO matersials,a facile method was developed for the synthesis of highly-packed microspheres with TiN,which have high tap density?1.5 g cm-3?and excellent cyclic stability,with an average diameter of 1.5 ?m.Micro-sized TiO₂/Li⁺ spheres were obtained by regulating the amount of TiN nanopowers.Monodispersed uniform LTO microspherescan be obtained bycalcining the TiO₂/Li⁺ spheres at 700 oC for 10 h.The specific capacity at a charge rate of 10 C reaches 99.5 mAh g-1 and no obvious capacity reduction is found after 350 cycles in a half cell.