Novel Bifunctional Catalysts For Lithium-O₂ Batteries

Lithium-oxygen batteries have attacted ’ attention in recent years due to their much higher specific capacity compared with those of the lithium-ion batteries.During discharging process,the O2 molecule is reduced by an electron（e-）at the surface of the electrode and combines with Li⁺ to form lithium oxide species（LiOx）.During charging process,the LiOx are converted back into Li⁺ and O₂.Thus,the cathode materials should function for both oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）.One of the hot topics for lithium-O₂ batteries is how to improve the round-trip efficiency or reduce charge/discharge polarization by rational designing cathode materials.Though noble metals exhibit excellent catalysis performances in lithium-oxygen batteries,they are limited by high cost and complicate preparation procedures in practical applications.Compared with precious metal catalysts,Co and Fe based cathode materials have great advantages due to low cost and simple synthesis procedures.In this dissertation,we mainly focus on Co,Fe-based spinel and perovskite structure materials as cathode materials for lithium-oxygen batteries.By controlling the solvent,reaction temperature and reaction time,we prepared Co₃O₄ nanocrystals mainly exposed with {100} and {111} crystal planes via hydrothermal method.They were used as cathode materials for lithium-oxygen batteries.The electrochemical performances testing shows that Co₃O₄ octahedron exhibits better performances than Co₃O₄ cubes as a catalyst for lithium-oxygen batteries,in terms of voltage gaps of discharge/charge process,specific capacity and cyclability.Layered montmorillonite was taken as template for infiltrating aniline,which contains C and N.Montmorillonite was removed after carbonization under nitrogen in order to get N-doped graphene.A kind of Co,Fe-based oxides with an inverse spinel structure,Co and Fe occupying octahedron sites simultaneously,was synthesised through a hydrothermal method and dispersed on N-G uniformly.Cells with Co[Co,Fe]O₄/N-G as a cathode material exhibits lower charging polarization than those of Super P and Pt/C catalysts,the specific capacity in the 1st cycle is 13312 mA h g-1 and it can stably run for 110 cycles,indicating an outstanding composite catalyst for lithium-oxygen batteries.1D La_0.6Sr_0.4Co_0.2Fe_0.8O₃ nanofibers decorated with 7.4% RuO₂ nanoparticles was prepared by electrospinning combined with a facile method for RuO₂ nanoparticles decoration and used as an efficient cathode material for Li-O₂ batteries.The cell with the LSCF-NFs catalyst degrades with cycling for 70 cycles.The charge voltage around 500 mA h g-1 increases about 306 mV.In contrast,the cell with RuO₂@LSCF-NFs catalyst performs better.The charge voltage around the middle of the limited capacity increases only about 98 mV.Cells with RuO₂@LSCF-NFs cathode exhibit excellent cyclability for 120 cycles and outperform the cells with LSCF-NFs cathode（82 cycles）.Ba_0.9Co_0.7Fe_0.2Nb_0.1O3-δ(B_0.9CFN)powder with A-site deficiency was synthesised via a solid-state reaction.The ratio of atoms at A and B sites less than 1:1 can cause valence change and oxygen vacancy increase.The cell with B_0.9CFN as a cathode catalyst shows a specific capacity of 12970 mA h g-1,2.66 V in discharge plateau and a higher voltage in charge process.IrO₂ nanoparticles were loaded on surface of B_0.9CFN particles by atomic layer deposition.The cell with IrO₂@ B_0.9CFN as a cathode catalyst exhibits a much lower charge potential of 560 mV at 6000 mA h g-1,suggesting decreasing charging polarization.Temperature-dependent neutron powder diffraction was carried out to reveal a "breathing" features of B_0.9CFN.After 350 h test,Ba_0.9CFN keeps ORR activity with a slight increase in polarization of about ⁰.04 Ω cm².All these results suggest that Ba_0.9CFN is a promising catalyst material for ORR at high temperature.