The Niobium-Titanium Oxide For Lithium Ion Battery Anode And Its Mechanism

Intercalation type transition metal oxides（niobium/titanium-based compounds）are regarded as one of the most promising anode materials for next generation lithium ion batteries due to limited volume change induced by lithiation,high theoretical specific capacity,high operating safety,stable cycling,huge abundance,and environmental friendliness.Compared to the traditional titanium dioxide and niobium oxide anode materials,Ti_0.667Nb_1.333O₄ inherited with a narrow band gap is expected to have better electrochemical performance.However,Ti_0.667Nb_1.333O₄ has a few drawbacks such as poor lithium ion diffusion coefficient,leading to moderate electrochemical performance.In addition,very few studies about the Ti_0.667Nb_1.333O₄ anode materials have been reported so far.Therefore,studies on the controllable preparation,nanostructure construction,carbon composition,and lithium storage mechanism of the Ti_0.667Nb_1.333O₄ anode have been addressed in this thesis.The main research contents of the thesis include the following aspects:Firstly,bare Ti_0.667Nb_1.333O₄ particles were synthesized by solid-phase reaction at high temperature.Compared with Ti O₂ and Nb O₂,the Ti_0.667Nb_1.333O₄ particles had a low band gap value of 0.38 e V and a higher lithium ion diffusion coefficient.After 100cycles at a current density of 0.2 C(1 C=247 m Ah g^-1),the reversible specific capacity was significantly higher than those of the Ti O₂ and Nb O₂.Better rate performance was also exhibited by Ti_0.667Nb_1.333O₄ at high current densities,Secondly,to increase the actual capacity of Ti_0.667Nb_1.333O₄,Ti_0.667Nb_1.333O₄/C micro-/nano composite materials were synthesized using a bifunctional methacrylic resin monomer as a carbon source and solvent,and different metal oxide precursors as solutes.Due to nanoscale structure and carbon coating,the electronic conductivity and lithium ion diffusion coefficient of the Ti_0.667Nb_1.333O₄/C composite were further increased,and the electrochemical performance was greatly boosted.Compared to the bare Ti_0.667Nb_1.333O₄ anode material prepared by the solid phase method,the Ti_0.667Nb_1.333O₄/C composite exhibited high reversible specific capacity after 100 cycles at a current density of 0.2 C(1 C=247 m Ah g^-1).After 300 cycles at a current density of 1.0 C,the Ti_0.667Nb_1.333O₄/C still had a high reversible specific capacity.Finally,based on the controllable preparation and electrochemical performance studies of the Ti_0.667Nb_1.333O₄/C composite materials,in-situ characterization methods and density functional calculations were further applied to gain insights about the lithium storage mechanism.It was confirmed that the lithium de/intercalation process of Ti_0.667Nb_1.333O₄ was of single-phase process along the c-axis similar to the rutile Ti O₂.After complete lithiation,the volume change of Ti_0.667Nb_1.333O₄ was approximately of7.5%.Density functional theory（DFT）calculations showed that Ti_0.667Nb_1.333O₄ had the narrowest theoretical band gap compared to Ti O₂ and Nb O₂,which was consistent with the experimental results.More importantly,Ti_0.667Nb_1.333O₄had a slightly larger diffusion energy barrier for lithium ion transportation than Ti O₂ and Nb O₂,thereby preventing lithium ion agglomeration and enhancing the kinetics of the lithium intercalation process,thus increasing the reversible specific capacity.At last,the in-situ TEM test under different voltages confirmed that the overall volume change was 14.3%of Ti_0.667Nb_1.333O₄/C after complete lithiation.An irreversible volume change of 6.66%was exhibited after delithiation,where no further volume change was observed with high voltage.