| Lithium-ion batteries are widely used in various fields,due to their advantages of high energy density,long lifespan,and good cycling stability.The research on the safety and high performance of lithium-ion batteries has received close attention from both academia and industry.In the practical application of lithium-ion batteries,graphite is still the main anode for commercial lithium-ion batteries,despite it has poor specific capacity and inferior safety.The low potential(<0.1 V)lithium intercalation process of graphite anode is accompanied by the precipitation of metallic lithium,which easily leads to the formation of dendritic lithium,further leading to spontaneous combustion and explosion of lithium ion batteries.Therefore,avoiding low potential(<0.1 V)lithium intercalation process is the key to effectively avoiding the formation of lithium dendrites and improving the safety of lithium ion batteries.In this work,titanium dioxide(TiO2)nanotube array with high lithium intercalation potential(～1.7 V)is used as the negative electrode to prepare high safety lithium ion batteries.Furthermore,the improvement of Li ion storage performance of TiO2 nanotube anodes is realized via morphology optimization,crystal structure adjustment,oxygen vacancy/Ti3+ doping,and material recombination.The specific research results are as follows:1.This work realized the morphology regulation of TiO2 nanotube,via adjusting the inherent characteristics of the titanium substrate.So far,most researches have focused on the regulation of TiO2 nanotubes’ morphology by changing the anodic oxidation conditions(including voltage,current and oxidation time,etc.)and electrolyte.However,few studies have focused on the effect of titanium foils on nanotube morphology during electrochemical anodization.In this work,TiO2 nanotube arrays with different morphologies were prepared by using five different titanium foils as the working electrodes during electrochemical anodization process.And in detail,we find that the morphology of nanotubes is affected by the purity,electrical conductivity,and mechanical properties of the Ti foils.While the purity of the titanium foil is as high as 99.7%,the as-prepared TiO2 nanotubes have the largest aspect ratio.In addition,the smaller the residual stress of the titanium foil,the higher the adhesion of the nanotube array to the titanium substrate.Finally,we find that TiO2 nanotubes prepared from higher purity Ti substrates exhibit more obvious color changes after the electrochemical cathodic self-doping process.The implementation of morphology regulation lays the foundation for the TiO2 nanotubes’ application in lithium batteries.2.Self-doped TiO2 nanotubes were prepared via an adjustable electrochemical process,and the principle of electrochemical cathodic self-doping was proposed by us.In addition,the reasons for the high capacity and excellent rate performance of self-doped TiO2 were also studied in this work.One-dimensional TiO2 nanotubes have large specific surface area and short ion diffusion distance,making them ideal binder-free lithium battery anodes.However,the wide band gap of TiO2 leads to its poor conductivity,which severely limits the application of nanotubes in lithium batteries.To address this issue,we performed cathodic electrochemical self-doping of the amorphous as-anodized TiO2 nanotubes.After self-doping,as-anodized yellow TiO2 nanotubes turn into black appearance.And this black TiO2 possesses abundant oxygen vacancies/Ti3+ and hydroxyl functional groups,which significantly improves TiO2’s conductivity and provides more paths and sites for lithium intercalation of nanotubes.Therefore,when self-doped TiO2 nanotube array is used as the anode in lithium battery,the initial discharge capacity is as high as 1355 μAh cm-2,which is 4 times that of the undoped TiO2(338 μAh cm-2).Moreover,by adjusting the number and voltage of cathode doping pulses during self-doping process,we also achieve a controllable adjustment of self-doped TiO2 anodes’ lithium capacity.After 100 cycles,the capacity of the self-doped TiO2 is still much higher than that of the undoped one.The good capacity retention of self-doped TiO2 is mainly attributed to the good tubular morphology and the secondary growth of nanotubes after cycling.The perfect retention and secondary growth of nanotube morphology significantly increase the specific surface area of self-doped TiO2 nanotubes,which not only provides sufficient sites for lithium intercalation and deintercalation,but also make it possible to reuse anode materials.The cycled self-doped TiO2 anode is also used as a catalyst to photocatalytically degrade the organic dye methylene blue,showing an excellent photocatalytic degradation effect.The reuse of cycled anodes may play an important role in resource conservation and environmental protection.3.Binary and ternary TiO2 nanotube-matrix composites were prepared,and the reasons for the formation of high capacity and good stability of composites were analyzed through energy band diagrams and morphological changes before and after cycling.TiO2 nanotube arrays with honeycomb morphology can be prepared via two-step electrochemical anodization.Such honeycomb TiO2 nanotube array has a clean top surface and highly ordered nanotube length,providing a uniform morphology and a large specific surface area for the preparation of composites.Using the unique honeycomb TiO2 nanotube as the matrix,binary(TiO2 nanotube@Au nanoparticle,TiO2 nanotube@MoS2 nanosheets)and ternary(TiO2 nanotube@MoS2 nanosheet@Au nanoparticle)composites are prepared in this work.Especially,the as-designed ternary TiO2 nanotube@MoS2 nanosheet@Au nanoparticle anode exhibits a high initial discharge specific capacity(487.4 μAh cm-2,2.6 times the capacity of pristine TiO2 nanotube),and an excellent capacity retention rate(81.0%).4.The study of self-doped TiO2 and the preparation of ternary TiO2 NT@MoS2 NS@Au NP composite paved the way for the preparation of defective TiO2@Co@NC.Based on the analysis of energy band diagrams and molecular dynamics calculations,the defective TiO2-supported dualSchottky heterostructure anode was designed and prepared.In addition,the presence of isolated Co nanodots significantly improved the reversibility of lithium intercalation in defective TiO2@Co@NC composite.Black TiO2 with abundant oxygen vacancies,have received extensive attention due to their good electrical conductivity and large specific capacity.However,the lithium intercalation irreversibility of oxygen vacancies leaded to the capacity decay of defective TiO2 anodes during cycling.In order to overcome the disadvantage of poor stability of defective TiO2,we prepared defective TiO2@Co-MOF derived 3D hierarchical defective TiO2@Co@NC composites.Molecular dynamics calculations show that isolated cobalt nanoparticles can stably exist in defective TiO2@Co@NC composite.These Co nanodots can not only bridge the defective TiO2 and the Co embedded N-doped carbon coating to form a dual-Schottky heterostructure,promoting rapid reaction kinetics,but also effectively inhibit the irreversibility of Li ion insertion in defective TiO2@Co@NC anode,improving the cycle stability of lithium batteries.Correspondingly,defective TiO2@Co@NC electrode was successfully prepared and it exhibits extremely high area specific capacity(1191.2 μAh cm-2/490.9 mAh g-1 at 100 μA cm-2/41 mA g-1,2.8 times of the pristine anatase TiO2 anode),excellent cycle stability(a capacity fading rate of 0.026%per cycle,at 500 μA cm-2/206.0 mA g-1 for 600 cycles)and remarkable rate properties(405.0 μAh cm-2/166.9 mAh g-1 at 1000 μA cm-2/412.0 mA g-1).Defective TiO2@Co3O4 anode has also been prepared by annealing defective TiO2@Co-MOF precursor in air,and it delivers an ultra-high specific capacity(2065.1 μAh cm-2,4.8 times the capacity of pristine anatase TiO2 anode)in Li-ion battery.We also achieved the preparation of the Co-MOF coating with controllable thickness by adjusting the growth time of the Co-MOF in the defective TiO2@CoMOF precursor,which facilitated the preparation of defective TiO2@Co@NC derivatives with tailorable capacity. |
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