The Study Of Mesoporous Carbon Materials With Nitrogen And Oxygen Dual-doped As The Cathode Catalysts For Lithium Air Batteries

As the demand for storage device with high energy density becomes more and more intense in modern society,the nonaqueous Lithium-air Batteries?LABs?with high theoretical energy density have attracted researchers' attention all around the world.However,there are still a lot of problems to be solved to realize the practical application of this promising novel battery,such as low energy efficiency,serious parasitic reactions,poor cycle performance and so on.Thus,based on the design and optimization of cathode materials,porous carbon materials with different structure and surface chemical properties were fabricated and the effects of structure and surface chemical properties on the performance of lithium air batteries were explored.In addition,through the research on catalyst RuO₂/carbon composites,the effects of carbon matrix on the electrochemical properties of composite materials were investigated,and finally lithium air batteries with high energy efficiency and long cycle life were achieved.Initially,through hydrothermal polymerization and the subsequent high temperature calcination,single template induced meseporous carbon foam?MCF?with N and O dual doped were obtained,where the nitrogen riched melamine and formaldehyde were used as carbon sources and the SiO₂ nanoparticles?fumed silica: Aerosil-200?with different mass as hard template.When the mass ratio of melamine and SiO₂ was 7:4,the obtained MCF-H7 had the highest specific surface?1028 m2 g-1?and the largest pore vulome?2.577 cm3 g-1?.Results from batteries tests showed that large pore volume and high surface area are beneficial to obtain a higher discharge capacity and good catalytic activity for oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?,and finally the MCF-H7 electrode exhibited the highest discharge specific capacity of 13100 m Ah g-1 and lower overpotential of 1.08 V at a current density of 100 m A g-1,which were all superior than that of commercial carbon black Super P?capacity: 4600 m Ah g-1,overpotential: 1.55 V?.However,the cycling performance of the MCF-H7 electrode did not show a significant improvement.For enhancing the comprehensive performance of material furtherly,different qualities of soft template F127 were introduced based on the synthetic process of MCF-H7 to obtain the double templates induced mesoporous carbon foam.Results from batteries tests showed that the MCF-H7-S2.5 sample had the best comprehensive performance,which exhibited good cycle performance as well as high specific capacity and high ORR/OER catalytic activity.In the end,the MCF-H7-S2.5 electrodes could run with no loss of capacity up to 25 cycles at the current density of 200 m A g-1 and limited discharge specific capacity of 500 m Ah g-1,which were much more than that of MCF-H7 electrodes?15 cysles?.The performance differences of materials were analyzed in detail from the structure characteristics and the impurity atoms doping.The results demonstrated that higher O doping is beneficial to obtain the higher ORR and OER catalytic activity,but also can accelerate the decomposition of electrolyte and corrosion of carbon materials,which result in poor cycle performance;In contrast,lower O and higher N doping maintain the carbon foam high catalytic activity,moreover,inhibit the parasitic reactions,so as to enhance the cycle performance.These results also demonstrated that in parallel with the reasonable design of porous architecture,the chemical nature modification is another effective way to promote the comprehensive performance of Li-O₂ batteries.Finally,RuO₂/carbon composite materials were synthesized by hydrothermal reaction based on the preapared mesoporous carbon foams.The batteries tests showed the composite materials had a high ORR and OER by-functional catalytic activity with a low overpotential of 1.02 V at 200 m A g-1.When cycled at 400 m A g-1 with limited capacity of 500 m Ah g-1,the RuO₂/MCF-H7 and RuO₂/MCF-H7-S2.5 electrodes could run with no loss of capacity up to 102 and 160 cycles respectively,which indicated excellent cycle stability of composite materials and confirmed that the stability of carbon substrate materials influenced the performance of Li-O2 batteries even after the composition with other catalysts of highly catalytic activity.