Synthesis And Electrochemical Performance Of Titanium-based Oxide Anodes For High-power Lithium Ion Batteries

Lithium ion battery（LIB）is the optimum electrochemical energy storage device to power the electric vehicles due to its superior overall performance compared with the traditional rechargeable batteries.Developing high-power LIBs capable of fast charging/discharging is now a research hotspot,in which anode material is the critical component.However,current commercial graphite anode material suffers safety issues and poor cycle stability at high-rate operation.By comparison,titanium-based oxides(e.g.,TiO₂,Li₄Ti₅O₁₂ and layered titanate)are the ideal alternative anode materials for high-power LIBs because of their better safety and longer cycle life.But the inherent low electronic conductivity of titanium-based oxides results in relatively poor rate performance,limiting their practical application.Previous reports show that nanostructuring design and hybrid-composite design are two efficient strategies to improve the rate capability of titanium-based oxides,and the eco-friendly hydrothermal method is considered suitable to synthesize nanostructured or hybrid titanium-based oxides in large-scale.Nevertheless,several difficult problems are still needed to be dissolved in the scalable hydrothermal synthesis of nanostructured titanium-based oxides,such as long period,low productivity,high cost,and ambiguous growth mechanism.This dissertation studied how to realize efficient and low-cost synthesis of high-rate titanium-based oxide nanomaterials by hydrothermal reaction.The preparation efficiency of titanium-based oxide nanomaterials was improved by developing new process;the effect law and function mechanism of experimental parameters on the structural features of the nanomaterials was revealed;the relationship between the structural features and electrochemical performance of the nanomaterials was discussed;and the electrochemical kinetics of these as-prepared nanomaterials was analyzed to illuminate why they exhibited excellent rate performance.The detailed research contents and conclusions are as follows:1.Rapid synthesis and electrochemical performance of layered titanate hierarchical microspheres.Low-cost industrial H₂TiO₃ powder was dissolved in NaOH solution with the help of H₂O₂ to form Na-containing peroxotitanate complex solution,which was finally transformed into 3D layered titanate(H_0.7Ti_1.825□_0.175O_4.0·xH₂O)hierarchical spheres（W-THSs）assembled by nanowire components through rapid supercritical hydrothermal reaction.High solution pH value and supercritical temperature were found two determinant factors to rapidly synthesize well-defined W-THSs;reducing pH value or hydrothermal temperature could significantly change the morphology and crystal structure of the products.Based on this,the rapid synthesis of W-THSs was proposed maily following a dissolution-nucleation-growth mechanism.Benefiting from the open interlayer spacing（⁰.9nm）that facilitates fast Li⁺movement to the inner electrochemically active sites,W-THSs exhibited an excellent electrochemical performance,giving a high capacitance of 123.6 mAh g^-1 even at a current density of 5.0 A g^-1,much higher than 64.5 mAh g^-1 of the compared anatase TiO₂ hierarchical spheres（AHSs）obtained by calcining W-THSs at 450°C for 2 h.In addition,kinetics analysis based on cyclic voltammetry measurements revealed that surface-controlled pseudocapacitive process occurs during the Li⁺storage in W-THSs,significantly contributing to the excellent rate capability.2.Scalable synthesis and electrochemical performance of Li₄Ti₅O₁₂ hierarchical hollow microspheres.H₂TiO₃ powder was further dissolved in the LiOH solution with the help of H₂O₂ to form high-concentration Li-containing peroxotitanate complex solution,which was successfully transformed into 3D Li₄Ti₅O₁₂ hierarchical hollow microspheres（HLTOMs）assembled by zigzag-like nanosheets by hydrothermal treatment and followed calcination.Systematic research found that the peroxotitanate complex solution concentration,Li/Ti ratio,hydrothermal temperature and duration were correlative and could be adjusted to obtain pure Li₄Ti₅O₁₂,or dual phase Li₄Ti₅O₁₂-TiO₂ and Li₂TiO₃-Li₄Ti₅O₁₂ products.Based on this,a dissolution-crystallization-growth mechanism was proposed and a high yield of HLTOMs up to 120 g L^-1 was achieved.The HLTOMs delivered an ourstanding rate capability of 108 mAh g^-1 at 30 C,much higher than 24 mAh g^-1 of the compared commercial Li₄Ti₅O₁₂ particles.Moreover,electrochemical kinetics analysis showed that the Li⁺diffusion coefficient of HLTOMs was 1.31×10^-10 cm² s^-1,6 times larger than 2.16×10^-11 cm² s^-1 of commercial Li₄Ti₅O₁₂ particles,illuminating why the HLTOMs exhibited better rate performance.3.Synthesis and electrochemical performance of dual phase Li₄Ti₅O₁₂-TiO₂ hierarchical hollow microspheres.Dual phase Li₄Ti₅O₁₂-TiO₂ hierarchical hollow microspheres were synthesized to further investigate the effect of compositing TiO₂ on the rate performance of Li₄Ti₅O₁₂.The as-prepared dual phase Li₄Ti₅O₁₂-TiO₂ hierarchical hollow microspheres delivered excellent rate capability of 134 mAh g^-1 at 25 C,higher than 116 mAh g^-1 of pure HLTOMs,demonstrating that introducing TiO₂ could improve the rate capability of Li₄Ti₅O₁₂.The performance improvement of dual phase Li₄Ti₅O₁₂-TiO₂ composites benefits from the synergy effect:on one hand,numerous Li₄Ti₅O₁₂/TiO₂ grain boundaries provide diffusion channels and extra storage sites for Li⁺;on the other hand,the Li_xTiO₂ with high conductivity induced by Li-insertion in TiO₂ exists throughout the whole Li insertion/extraction processes of Li₄Ti₅O₁₂ and thus improves its electrochemical kinetics.4.Synthesis and electrochemical performance of free-standing Li₄Ti₅O₁₂-rGO composite film electrode.A flexible free-standing Li₄Ti₅O₁₂-rGO composite film electrode was prepared by a three-step approach including hydrothermal reation,vaccum filtration,and calcination from Li-containing peroxotitanate complex solution and graphene oxide（GO）.The 3D rGO network in the film electrode serves as not only a conductive path but also a flexible mechanical support,making the film electrode possess good conductivity and mechanical strength,while the Li₄Ti₅O₁₂ nanoparticles（wt.73.9%）are sandwiched between the rGO layers,making the film electrode possess mesoporous channels that facilate electrolyte penetration.As a result,The obtained Li₄Ti₅O₁₂-rGO film electrode exhibited an excellent rate capability of 135.4 mAh g^-1 at 40 C,much higher than 99.2 mAh g^-1 of conventional Li₄Ti₅O₁₂-rGO powder electrode containing metal substrate and polymeric binder.It was also found that when the total rGO content in the Li₄Ti₅O₁₂-rGO film electrode was set,more GO addition during hyrothermal reaction resulted in larger conductivity,better mechanical strength,and better electrochemical performance.Moreover,kinetics analysis based on EIS measurements showed that the Li⁺diffusion coefficient of free-standing Li₄Ti₅O₁₂-rGO film electrode was 8 times larger than that of binder-containing Li₄Ti₅O₁₂-rGO powder electrode,illuminating why the Li₄Ti₅O₁₂-rGO film electrode exhibited better rate performance.