Studies Of Ti-Based Oxide Anode Materials For Lithium Ion Battery

Comparing with graphite anode,Ti-based oxide anodes are considered as one of the most potential electrode materials for lithium ion batteries（LIBs）owing to its superior safe lithium insertion/extraction voltage（1.55 V vs.Li⁺/Li）and negligible volume change during its discharge/charge process.However,its poor lithium ion diffusion coefficient(10^-9-10^-13 cm² s^-1)and low electronic conductivity(10^-13 S cm^-1)greatly lower the rate performance of electrode materials and restrict their wide-spread applications in electric vehicles（EVs）or hybrid electric vehicles（HEVs）.In this paper,the micro-nanostructured Ti-based anode materials with different morphologies and Li₄Ti₅O₁₂/C or TiO₂/C composites are prepared.Therefore,the lithium ion diffusion rate of electrode materials is improved and the electronic conductivity is enhanced,leading to the improved rate capability and cycling performance.The Li₄Ti₅O₁₂ materials with different morphologies are synthesized by using solvothermal reaction.To shorten the diffusion paths of lithium ions,the ultrathin mesoporous Li₄Ti₅O₁₂ nanosheets with thickness of only 1 nm are prepared by using the tert-butanol as reaction solvent.The formation mechanism of Li₄Ti₅O₁₂ nanosheet is investigated in detail using a series of time-dependent and control experiments,which suggests that the water/alcohol ratio,surfactant and reaction time are key factors to grow ultrathin nanosheets.Owing to the unique structures including ultra-small thickness,large surface areas and uniform mesoporous structures,optimal Li₄Ti₅O₁₂ nanosheets show the superior rate capability.In order to increase the crystal structure of Li₄Ti₅O₁₂,the novel cylindroid-like Li₄Ti₅O₁₂ mesocrystals are prepared by employing the methanol as reaction solvent and lithium acetate as lithium source.The unique features,including the mesoporous structures and single crystal nature endow the Li₄Ti₅O₁₂ mesocrystals with convenient lithium ion and electron diffusion channels,which results in the excellent cycling performance.The nanosheet-assembled Li₄Ti₅O₁₂ hierarchical microspheres are prepared by using glycerol as reaction solvent and correctly controlling the solvothermal temperature,which can simultaneously achieve high tap density,short diffusion path of lithium ions and good structural stability.The formation mechanism of Li₄Ti₅O₁₂microspheres is systemically investigated which suggests that the water/alcohol ratio,concentration of surfactant and reaction time are key factors determining their morphology and crystal phase.The Li₄Ti₅O₁₂/LiNi_0.5Mn_1.5O₄ full cells show a 93.4%of capacity retention after 1000 cycles at current density of 3 C.In order to increase the electronic conductivity of Li₄Ti₅O₁₂ material,the Li₄Ti₅O₁₂/activated graphene nanoplatelets composites are prepared via a low-cost,green,and fast hydrothermal process,which the preparation of Li₄Ti₅O₁₂ and the activation of graphene nanoplatelets are realized simultaneously.The three-dimensional porous graphene network structures endow the composites with convenient lithium ion diffusion channels and efficiently conductive structures for electron and ion transport,leading to the enhanced rate performance.To further improve the rate capability,the Li₄Ti₅O₁₂/nitrogen,sulfur co-doped porous graphene composites are prepared via one-step facile calcination and activation process by using Li OH as lithium source and activating agent.Mechanisms underlying the beneficial effects of N,S co-doped graphene for Li₄Ti₅O₁₂ anode are substantiated by the Density Functional Theory calculations,revealing that co-doped N and S atoms not only significantly facilitate lithium ion adsorption,but also ameliorate lithium ion diffusion in graphene.Therefore,rate performance of Li₄Ti₅O₁₂ is greatly enhanced(123 m A h g^-1 at 100 C).In order to enhance the structural stability of TiO₂ materials,the hedgehog-like hierarchically mesoporous rutile TiO₂ is prepared via one-step solvothermal process without any surfactants.The reaction solvent,time and temperature are systematically investigated to adjust the morphology of TiO₂ materials.Given its unique hierarchical mesoporous nanostructures,the TiO₂ shows high specific capacity and long cycling life.Based on this,the hierarchically micro-nanostructured TiO₂/micron carbon fibers composites were prepared via an easy impregnation-calcination process.The electronic conductivity is greatly enhanced,and the excellent high-rate cycling performance is obtained for TiO₂/micron carbon fibers composites(92%capacity retention after 5000 cycles at current density of 2000 m A g^-1).