Synthesis And Electrochemical Properties Of Doped TiO₂Nano Materials

Lithium-ion batteries are widely used in portable electronic products,implantable medical devices and power or energy storage batteries due to their highenergy density, long cycle life, and environmental friendly. Graphite is now applied asthe dominant anode in the commercial Li-ion batteries, however, the lower capacity,poor rate performance and big security risk limit the utilization of the material. Basedon facing the challenge of graphite, developing new alternative anode materials seemslike a significant task for the research Li-ion batteries. Among variety of novelcandidates, titanium dioxide (TiO2) has attracted great interest, due to its high safety,non-pollution etc. However, the electronic conductivity of titanium dioxide material istoo low, limiting its application, especially at high power conditions. Conventionally,it is considerably effective to prepare nanostructured TiO2combining with highlyconductive material, such as carbon nanotubes (CNT), carbon fibers (CNF), andgraphene etc., in order to improve the high rate charge and discharge performance andcycle stability of the materials. However, the conductive additives can only improvethe contact conductance between adjacent TiO2particles, while the intrinsic electricalconductivity inside of the TiO2bulk is still unimproved. To solve these problems, weprepared a series of doped titanium dioxide anode materials, and enhance the intrinsicconductivity by modulating the electronic structure and tailoring the crystal structureof the titanium dioxide anode materials in order to achieve excellent electrochemicalproperties. First, we synthesized anatase titanium dioxide nanoparticles by solvothermalmethod, and adopt Potentiostatic Intermittent Titration Technology and ACElectrochemical Impedance Spectroscopy to investigate the lithium ion diffusioncoefficient of the materials. According to the results, DLivalue of TiO2is1.3×108cm2s1at the beginning of discharge, and then rapidly decreases to a minimum valueof9.77×1012cm2s1at the center of the discharge plateau. Afterward, DLiincreasesto7.68×1010cm2s1and then slightly decreases again to1.7×1010cm2s1. Finally,DLirises to2.98×1010cm2s1at the end of the discharge progress. The changesclosely relate to the charge-discharge mechanism: the electrochemical process ofanatase TiO2is the two-phase transition mechanism, with the increased of Li+, thematerial gradually transforms from single-phase to two-phase coexistence region, sothat DLireaches a minimum value at the center of the discharge plateau. When thenumber of lithium-ion is large enough, the active material gradually shifts to asingle-phase region, therefore, the lithium ion diffusion coefficient reboundedsignificantly.Subsequently, TiO2and N doped TiO2nano particles were prepared by thesolvethermal method. According to the results of XPS etching techniques, N anionsnot only exist on the surface of the materials, but also access to the inside of thematerial bulk and reside on the interstitial and substitutional sites, which slightlyexpands the crystal lattice of TiO2. The N doped TiO2shows better rate capabilitythan the pure TiO2. A discharge capacity of45mAh g1is obtained at the15C rate,which is80%higher than that of the pure TiO2. The results of GalvanostaticIntermittent Titration Technology demonstrate that the lithium ion diffusioncoefficient is only1.65×1012cm2s1at the center of discharge plateau, whereas thediffusion coefficient of N doped materials is13times higher than that of the pureTiO2, about2.14×1011cm2s1, which benefit to a higher specific capacity and betterrate performance.Then, N doped TiO2-B nanowires with different content of N were prepared by the solvothermal method using intermediate materials. N-doped TiO2nanoparticlesare prepared by calcination treatment using commercial TiN nanoparticles asintermediate materials. XPS and Raman tests indicate that the N dopantspreferentially occupy the interstitial sites of TiO2-B, and the maximum content ofinterstitial N in TiO2-B is about0.55at.%, above which the N dopants substitute theoxygen atoms at the O1and/or O3positions. The interstitial N has little impact on thestructure and physical properties of TiO2-B, whereas the substituted N plays a keyrole in improving the electronic conductivity and local structural stability of TiO2-B.According to the electrochemical data, the electrochemical properties of the TiO2-Bnano wires are significantly improved by the substituted N dopants. The materialshows a discharge capacity of153mAh g1at the20C rate with a capacity retentionability of76%after1000cycles. In addition, it can deliver a discharge capacity of100mAh g1at an extremely high rate of100C.Finally, we have improved the preparation technology of TiO2, ultrafine TiO2-Bnanowires were prepared by the microwave-assisted hydrothermal synthesis method,and we also synthesized Cu doped TiO2-B nanowires. According to the investigation,a portion of replacement of Ti4+by Cu2+results in the generation of oxygen vacanciesin the materials, and makes the band gap fall down from3.15eV to2.97eV.Furthermore, the electron density on the conduction band increases, therebyimproving the electronic conductivity of the materials. Meanwhile, ElectrochemicalImpedance Spectroscopy demonstrates that the charge transfer resistance of theCu doped materials also reduces from155Ω to92Ω. Therefore, copper dopedmaterials achieve excellent rate properties with a discharge capacity of150mAh g1at60C, which is~50mAh g1higher than that of the pure materials.Above all, we increase the electron conductivity, lithium-ion diffusioncoefficients and structural stability of titanium dioxide by doping aliovalent ions.Therefore, the electrochemical properties are improved considerably, such ascharge-discharge capacity, rate performance and cycling stability. This works provide us novel ideas for doping modification research of TiO2anode materials, and promotethe application of TiO2anode materials in lithium-ion batteries.