Perovskite Iron Oxide-based Composites: Design, Synthesis And Their Application In Lithium-Oxygen Battery

Lithium-oxygen battery is regarded as one of the most promising energy storage devices to replace the current lithium ion battery because of its high theoretical energy density.Although the research of lithium-oxygen battery has made great progress in recent years,its high overpotential,poor stability and limited capacity still prevent its practical application.Because cathode is the main reaction position of lithium-oxygen battery,the cathode catalyst is the key factor to improve the performance of lithium-oxygen battery.Therefore,it is very important to develop a cheap,efficient and stable cathode catalyst for lithium-oxygen battery.Perovskite-oxides have been widely studied because of their low price,flexible structure,wide element distribution,obvious catalytic performance and high thermal stability.However,the poor conductivity,low surface sites activity,limited specific surface area and low cycling stability restrict further application.Composite materials are composed of two or more solid materials through a physical or chemical methods,which often show enhanced properties or even new functions.Therefore,construct perovskite-oxide-based composite is an efficient method to develop cathode catalysts.The improvement of perovskite-oxide-based composites’properties can be attributed to three parts,which are special interface structure,electron transfer and distorted material structure.On the other hand,chemical reactions of lithium-oxygen battery often occur on the surface or interface of the catalysts,so modulating the electronic states of the catalyst is another important method to promote performance.In this paper,we aim to optimize the structure and electronic state of materials at atom and chemical bond scale,and synthesize a variety of efficient cathode catalysts for lithium-oxygen battery.1.α-Fe₂O₃ decorated LaFeO_3-x composite:Perovskite La_0.85FeO_3-δprecursor was synthesized through a sol-gel method.After calcination at 900 ^oC,perovskite LaFeO_3-x decorated withα-Fe₂O₃ was obtained.Because of the in-situ growth ofα-Fe₂O₃ on LaFeO_3-x,the two phases combine tightly and exhibit a stronger interaction,which promote the absorption of oxygen on the cathode,and then improve the lithium-oxygen battery performance.The overpotential ofα-Fe₂O₃-LaFeO_3-x based lithium-oxygen battery decrease to 1.0 V.What’s more,the battery can stabled cycle for 108 cycles,which is twice of the perovskite orα-Fe₂O₃phase.We also carefully characterize the structure of the composite,and then discusse the generation mechanism ofα-Fe₂O₃.We also analyze the influence of high temperature annealing for catalyst structure and activity.2.Decoration of Ag on La_0.9FeO_3-δ:After discussing of the influence of interaction on the performance improvement of catalyst,we synthesized Ag decorated La_0.9FeO_3-δcomposite（Ag@LFO）through a one-step electrospin method.In Ag@LFO,Ag exist as both Ag metal and Ag⁺which doped into the perovskite lattice.We also synthesized Ag decorated La_0.9FeO_3-δcomposite（Ag/LFO）through post-loading method as comparison.Compared to Ag/LFO,the existence of Ag⁺modulate the electronic state of composite surface,and improve the generation of Fe⁴⁺.Therefore the lithium-oxygen property obviously promote.At a current density of 100 mA?g^-1,the specific capacity of Ag@LFO based lithium-oxygen battery is8476 mAh?g^-1.And the overpotential of the battery decrease to 0.7 V.What’s more,the battery can maintain cycling for 174 cycles.Moreover,we also characterized the structure and electronic state of Ag@LFO and Ag/LFO,and analyze the relation between structure and lithium-oxygen property.At last,we discuss the formation mechanism of Li₂O₂ during discharge.3.Exsolution of CoFe alloy and Co in La_0.8Fe_0.9Co_0.1O_3-δ:After deeply studying of the influence of interaction and electronic state,we synthesize La_0.8Fe_0.9Co_0.1O_3-δthrough a simple sol-gel method.After annealing in 5%H₂/Ar,three kinds of composites are obtained,which are CoFe alloy decorated perovskite（LFCO-600H）,CoFe alloy and Co metal co-decorated perovskite（LFCO-750H）,and Co metal decorated perovskite（LFCO-900H）.We prove that there is interaction between the exsolution nanoparticles and parent perovskite,and after exsolution,all the perovskites have structure distortion which can increase the active sites.Because of the most obvious structure distortion and the synergetic effects of the three phases,LFCO-750H has better lithium-oxygen battery performance.The overpotential decrease to 0.7 V,and can cycle fot 205 cycles without obvious degradation.We also analyze the structure and electronic state of the catalysts,and also discuss their influence for the morphology of Li₂O₂.In this paper,a series of perovskite oxide composites are constructed by one-step synthesis or in-situ growth method.Because of the enhancement of the interaction,adjustment of the surface electronic state and increment of the active sites,the performance of lithium air battery promote obviously.This paper provide a new perspective for the evelopment of perovskite oxide composites used in lithium air battery.