Rational Design And Intensive Study Of High-efficiency Catalysts For Lithium-oxygen Batteries Pplication

Rechargeable Li-O2 or Li-Air batteries have greatly captured worldwide attention,because of their high theoretical energy density comparable to that of a gasoline engine.And this system has also considered as one of the most promising next-generation energy storage devices.In the past decade,lots of reserchers have conducted an intensive study on the related fundamental theory of aprotic Li-O2 batteries,and achieved considerable progress.However,numerous critical technical problems remain to be solved for the practical use of this state-of-the-art technology.The biggest challenge is that the high discharge/charge overpotentials,especially in the charge process,resulting from the insulating nature of lithium peroxide and the sluggish kinetics of ORR/OER.This will directly result in low energy efficiency,detrimental degradation of the electrolyte,and poor cycle life.A main strategy to counter this severe drawback is to explore various cathode catalysts,which can help to reduce the polarization and lower the overpotentials upon discharging and charging,and thus improve the round-trip energy efficiency and cycling stability.So far,the main task in this filed is to design and develop durable catalysts with high efficiency.So,an optimized design of the catalysts' type and structure will help to achieve the Li-O2 batteries system with both a high energy efficiency and an excellent cycle stability.In order to deal with all the critical issues of heterogeneous catalysts,we have proposed and demonstrated that the incorporation of soluble redox mediators(RMs)can facilitate the oxidation of Li2O2 at the liquid-solid interface through a chemical reaction,which can help to significantly boost the energy efficiency,reversibility and cycling stability.Furthermore,a sandwich-typed Fe2O3/GNS catalyst and a free-standing carbon-free Ru@UNF catalyst have been designed and testified to lower the discharge/charge overpotentials and alleviate or avoid the carbon corrosion,improving the stability of Li-O2 batteries.In this dissertation,the electrochemical performance of Li-O2 cells with different high-efficiency catalysts have been investigated,including soluble mediators(for example,MPT and LiI),a sandwich-typed Fe2O3/GNS and a free-standing Ru@UNF carbon-free catalyst.The major innovations in this work can be summarized as follow:1.MPT as an effective soluble mediator in Li-O2 batteries:In the initial stage of the research,we systematically summarized the fundamental criteria for a suitable redox mediator as soluble catalysts,and preliminary screening of the redox potential(below 4.0 V)of some possible redox mediator candidates.Conducting the experimental research on the electrochemical properties and physical properties of MPT,we considered that MPT with the proper redox potential and faster molecular diffusion rate,is a potential redox shuttle catalyst for Li-O2 cells.The Li-O2 cell with MPT additive delivers a dramatic reduction in charge overpotential to 0.67 V and an improved round-trip energy efficiency close to 76%,and a better reversibility.Moreover,the catalytic mechanism of MPT in Li-O2 cells was further investigated by various ex and in situ characterizations.On charging,MPT+ cations are first generated electrochemically at the cathode surface(2MPT = 2MPT+ + 2e-),and subsequently oxidize the solid discharge products Li2O2 through a chemical reaction(2MPT+ + Li2O2=2MPT + 2Li+ + O2?).Furthermore,the presence of MPT has been demonstrated to improve the cycling stability of the cells and suppress side reactions arising from carbon and electrolytes at high potentials.At this point,we continued the further study of MPT to research its catalytic efficiency influenced by some factors,thus finding some useful methods to improve the catalytic efficiency of these soluble catalysts.Finally,a pouch-typed Li-O2 battery using MPT catalyst has demonstrated high energy efficiency and excellent stability.2.LiI as an effective soluble mediator in Li-O2 batteries:LiI has demonstrated to be suitable as an effective soluble catalyst in Li-O2 batteries owing to its proper redox potential and fast diffusion of I' anions.And the Li-o2 cell with LiI as a soluble catalyst provides a dramatic reduction in charge potential to 3.5 V.And a corresponding round-trip energy efficiency of 74.3%is obtained,which is much higher than that of the Li-02 cell without LiI.Moreover,the Li-O2 cell with LiI also exhibits the excellent property on the reversible formation and decomposition of Li2O2 by virtue of ex-characterizations.Furthermore,the combination of the electrochemical quartz crystal microbalance(EQCM)and CV tests can be described for the quantitative detection of the mass change on the electrode surface during the discharge and charge process,testifying the catalytic mechanism of LiI.On discharging,the presence of LiI has little effect on the formation of Li2O2.And on charging,I3-anions are first generated electrochemically at 3.2 V on the cathode surface,then continued being oxidized to I2 at 3.5 V.And subsequently I2 oxidize the solid discharge products Li2O2 through a chemical reaction(I2 + Li2O2 = 2Li+ + 2I-+O2 ?).In addition,the incorporation of LiI has also substantially improved the cycling stability of the cell.3.Sandwich-structured Fe2O3/GNS cathode catalyst for Li-O2 batteries:A multi-layered Fe2O3/GNS composite with sandwich structure was synthesized by an easy thermal casting method,and served as a cathodic catalyst for aprotic Li-O2 batteries.The aprotic Li-O2 cell with Fe2O3/GNS catalyst demonstrates a better reversibility,a reduced overpotential for oxygen evolution,and a higher energy efficiency than those of pure GNS.The excellent rate performance and good cycle stability were also confirmed.Combined with MPT as a soluble mediator,the studied Li-O2 cell possesses a lower charge potential of 3.7 V and a better cycling stability.The results characterized by the ex-and in-situ methods revealed that the dominant discharge product Li2O2 was decomposed below 4.35 V.This superior electrochemical performance is mainly attributed to the unique sandwich structure of Fe2O3/GNS catalyst with mesopores,which can substantially provide more catalytic sites and prevent direct contact between carbon and Li2O2.4.Free-standing Ru@UNF as carbon-free catalyst for Li-O2 cell:A free-standing ultra-light nickel foam(UNF)as a substrate is designed and fabricated by the Cu temple method,then electrodepositing Ru nanoparticles on UNF substrate to obtain the final carbon-free catalyst Ru@UNF.The aprotic Li-O2 cell with Ru@UNF catalyst exhibits a reversible capacity of 2410 mAh g-1 at a current density of 150 mA g-1.Moreover,the battery also demonstrates a high energy efficiency of 74.7%(Edischarge=2.66 V,Echarge=3.56 V,respectively),and an excellent cycling stability(over 100 cycles of limited capacity).Furthermore,the charging reaction mechanism of Li-O2 batteries with Ru@UNF catalyst is also studied by the in situ DEMS technique.This superior electrochemical performance is mainly attributed to the following two points:(1)the excellent catalytic performance of Ru can dramatically reduce the discharge/charge overpotentials,thus improving the round-trip energy efficiency;(2)the use of carbon-free cathode can avoid the carbon corrosion,thus boosting the cycling stability.All these results highlight the significance of the rational design of catalysts,while indicating a new direction and strategy in seeking a breakthrough to deal with the challenges of large discharge/charge overpotentials,low energy efficiency and poor cycling stability in aprotic Li-O2 batteries.Therefore,it represents an important progress in the development of high performance and practical Li-O2 batteries.