Catalytic Mechanism And Application Of Modified Titanium Dioxide In Lithium Air Battery

Lithium-air?Li-air? batteries have been proposed as the next-generation power sources for portable electronic devices and electric vehicles due to their ultra-high theoretical energy density of 11140 W·h·kg-1 at charged state?excluding oxygen? and 3458 W·h·kg-1 at discharged state?including oxygen?. Among various types of Li-air batteries, the one using a non-aqueous electrolyte is of greatest interests since the pioneered works by Abraham and Jiang. Although promising, the non-aqueous Li-air battery is still far from practical applications. A number of scientific and practical issues have impeded its development. The main challenge is the sluggish kinetics of oxygen reduction reaction?ORR? and oxygen evolution reaction?OER? on the cathode, which results in poor discharge performance and cyclability. Thus, the development of efficient electrocatalysts to accelerate the ORR and OER kinetics is urgently needed. Up to now, a variety of electrocatalysts, including carbon materials, noble metals, perovskites and transition metal oxides have been investigated in non-aqueous Li-air batteries. Among them, transition metal oxides have attracted considerable interests as promising catalysts for their defective structure, low cost and outstanding catalytic activity. In the present work, two kinds of modified titanium dioxide?TiO₂? were syntheisizd and their physical and chemical characteristics as well as the electrochemical performance in non-aqueous Li–air batteries were studied systematically.Firstly, rutile TiO₂ with oxygen vacancies?H-TiO₂? was synthesized by a facile heat treatment method and a variety of characterization methods were used to prove the existence of oxygen vacancies on the surface of H-TiO₂. It is shown that H-TiO₂ exhibited a high catalytic activity toward ORR in non-aqueous electrolyte. Compared to the TiO₂ catalyst, the discharged specific capacity of Li-air batteries with H-TiO₂ catalyst was increased 39% and 75% respectively at the current densities of 0.3 and 0.5 mA·cm-2. Moreover, the cell with H-TiO₂/NCNTs could undergo over 400 cycles before the discharge voltage fell to 2.0 V at a current density of 0.3 mA·cm-2, which was much larger than those with the NCNTs?155 cycles? and TiO₂/NCNTs?225 cycles? electrodes under the same conditions. Due to the presence of oxygen vacancies on H-TiO₂, oxygen was believed to be more easily absorbed on the surface of H-TiO₂, creating more active sites binding to O2 for ORR thus resulted in a larger discharge capacity and better cyclability.Secondly, in order to further enhance the catalytic effect of TiO₂, Mn doped TiO₂ nanosheets as a bifunctional catalyst?ORR and OER? was synthesized by hydrothermal method. It is shown that synergic catalytic effect was generated through the combaition of special material surface morphology and Mn doping effect. On one hand, the catalyst active sites were appreciably increased by the morphology of nanosheets. Furthermote, the morphology of nanosheets is beneficial to the transmission of O2 and the storage of Li2O2. On the other hand, The Mn dopant converts TiO₂ into P-type semiconductor, and therefore changes the original TiO₂ conducting mechanism. More specifically, the presence of conduction holes favors capturing electrons and speeds up the decomposition of discharge product Li2O2 so as to accelerate the OER process. It is shown that Mn-doped TiO₂ catalyst can increase the ORR onset potential but decrease OER onset potential, indicating the bifunctional effect of Mn doping TiO₂. Moreover, the overvoltage during discharging and charging process was reduced appreciably by adding Mn-doped TiO₂ nanosheets and the stability of lithium-air batteries was also enhanced.In summary, two kinds of modified TiO₂ were synthesized successfully and their catalytic mechanism was analyzed through materials characterization and electrochemical testing.