Novel Structures Of Titanium-based Na-Insertion Materials And Their Electrochemical Applications

In recent years,sodium-ion batteries have received particular attention in the field of large-scale electric energy storage application,due to its natural abundance of sodium resources,low cost and environmental friendliness.Under the new trend of ecological and environmental protection,it is an important task to develop high performance,low price and green new materials to construct high-performance sodium ion batteries.In general,titanium-based materials,with its low price,environmental friendliness,and structural stability,has become attractive materials for sodium ion batteries.This thesis was aimed at exploring high-performance anode and cathode electrodes by designing new structure and optimizing the synthetic method,thus recealing the mechanism of sodium storage,and testing the feasibility of titanium-based sodium ion full cell.The main contents and conclusions are as follows:?1?Generally,the realizable capacity of TiO₂ is still far from its theoretical value,possibly due to the intrinsic low conductivity of pure TiO₂.To address this issue,graphene-supported TiO₂ nanospheres?rGO-TiO₂?were synthesized by the method of morphological control.Further more,one-dimensional TiO₂/C nanofibers were designed by embedding the TiO₂ nanoparticle in the conductive carbon network to improve the electrochemical performance.The as-prepared rGO-TiO₂ electrode demonstrates a high capacity of 300 mA h g-1 and a cycling stability over 300 cycles.Also,this rGO-TiO₂ electrode exhibits a high rate capability of 123.1 mA h g-1 at a very high rate of 4.0 A g-1 The TiO₂/C anode shows a high redox capacity of?302.4 mA h g-1 and can be cycled with nearly 100%capacity retention over 1000 cycles at 200 mA g-1,showing a sufficiently high capacity utilization and long cycle life for battery applications.In addition,the tailoring of lattice structure was also used to enhance the bulk conductance and lattice size of TiO₂ by doping niobium?Nb?in the lattice.Higher capacity was obtained by combine the doped-TiO₂ with one-dimensional conductive carbon network and the oxidation/reduction of the doping element.The capacities of the Nb doped-TiO₂ nanofibers were 338 mA h g-1,exhibiting a remarkably long-term cycling stability with nearly 100%capacity retention over hundreds of charge/discharge cycles.In order to obtain materials with higher specific capacity,we synthesized the Sb/TiO₂/C composite to improve the conductivity and enhance the capacity at the same time.The capacities of the Sb/TiO₂/C nanofibers were 452 mA h g-1,exhibiting a remarkably long-term cycling stability.?2?Cathode-active NaNi_0.5Ti_0.5O₂ was synthesized by sol-gel method.Compared with the traditional mechanical ball milling method,the sol-gel method has the advantages of making the material more evenly dispersed on the molecular scale with mild reaction conditions and faster reaction rate.The first charge/discharge specific capacity of NaNi_0.5Ti_0.5O₂ cathode was 126.6/102.6 mA h g-1.Nevertheless,after the carbon caption process,the conductivity of NaNi_0.5Ti_0.5O₂ could be further improved,producing the more stable NaNi_0.5Ti_0.5O₂/C materials with better performances.At 1C and 2C rate,the reversible specific capacity was 84.7 mA h g-1 and 81.1 mA h g-1 respectively,showing a strong high rate tolerance.Further more,ex-situ XRD characterization proved that multi-level phase transition could be considerably inhibited during charge/discharge process of NaNi_0.5Ti_0.5O₂/C,while reversible phase transition of 03-P3 structure was discovered instead.?3?Ti-based full battery was constructed for the first time based on the previous researches on above-mentioned Ti-oxide cathode and anode.Using the TiO₂/C as anode coupled with Na2FeFe?CN?6,Na3V2?PO4?3 and NaNi_0.5Ti_0.5O₂ cathodes,three types of full Na-ion batteries(TiO₂//Na2FeFe?CN?6,TiO₂//Na3V2?PO4?3,TiO₂//NaNi_0.5Ti_0.5O₂)were successfully constructed.It is found that these three types of batteries could realize reversible charge/discharge capability with satisfactory specific capacity.Specifically,the TiO₂//Na3V2?PO4?3 full battery system showed best stability of the three systems,showing a promising future for battery application.