| Lithium-oxygen(Li-O2)batteries have attracted much attention due to their theoretical energy density of 3500 Wh kg-1.Low actual energy density,poor rate performance and short cycle life seriously restrict its commercialization process,which is mainly caused by the sluggish kinetics of oxygen reduction reaction in the non-aqueous electrolyte and decomposition reaction kinetics of Li2O2 as well as side reaction.Additionally,due to the side products would cover the catalyst during the charging and discharging process,and thus reducing the reaction rate.Generally,the slow reaction kinetics is suitable for producing Li2O2 with good crystallinity,which is hard to be decomposed.Obviously,it is imperative to explore the restriction relationship between side-products and Li2O2 structure.Therefore,it is important to reveal the relationship between the electrochemical environment around the cathode region and the structure of Li2O2.The previous results have demonstrated that amorphous Li2O2 shows more advantages in lithium ion and electron transport than crystalline Li2O2.Conventional catalysts for oxygen reduction and water splitting can improve the electrochemical reaction kinetics to a certain extent in Li-O2 batteries,but most of them can hardly change the crystal structure of Li2O2,which is the biggest challenge facing the structure regulation of Li2O2.Thus,the key approach to break through the performance bottleneck of Li-O2 battery is to develop a reasonable catalyst to regulate the structure of Li2O2.In this dissertation,to overcome the above problems of Li-O2 batteries,separator adsorption,the local surface plasmon resonance effect(LSPR)and the cathode with oxygen vacancy and metal/semiconductor heterostructure was used to regulate the properties of Li2O2.The main innovations and results are stated as follows:1.Amino modified Ti O2/Si O2 flexible separator with a Si/Ti ratio of6,fiber diameter of about 150 nm and porosity of 84.8%(STON WF)is prepared by electrospinning method.Nuclear magnetic resonance spectra,penetration experiment and DFT theoretical calculation proved that STON WF can effectively adsorb the side-products HCOOLi,CH3COOLi and Li2CO3 produced during the charging and discharging process.The interface of the cathode protected by STON WF separator is conducive to change the final discharge product Li2O2 presented a loose structure,and thus,the cycle life of the battery with STON WF is twice more in comparation with that of the commercial PP separator.Furthermore,in the case of commercial 5%Pt/C cathode,the STON WF separator also displayed two-fold increase life expectancy in comparison with PP separator,demonstrating the universal adaptability of the strategy.In this chapter,the relationship between the separator and the cathode was first established around Li2O2,and it was observed for the first time that the separator could not only play a conventional insulating role,but also serve as a collection site for side-products on the cathode,thus affecting the structure of Li2O2.The relationship between the electrocatalytic activity of the cathode and separator discovered has important theoretical significance for understanding the formation and decomposition of the shaggy Li2O2.2.The LSPR effect can introduce light energy into non-aqeous Li-O2 cells,and the obtained catalytic effect is far better than that of the conventional catalysts.Under the UV light of 395 nm,the Ag nano cubic(Ag NC)cathode with a side length of about 70 nm can achieve a 146 h cycling stablility in the voltage range higher than 2.96 V.LSPR effect also shows high catalytic efficiency in Zn-air battery.At a current density of 1 m A cm-2,the charge and discharge overpotential of Ag NW with a diameter of about 150 nm and a length of about 3?m is only 0.15V,which is far better than the 0.92 V of commercial 5%Pt/C catalyst.Infrared thermal camera and time-current curve proved that the synergistic effect of the photothermal effect and the driven catalysis of the hot carriers(electron–hole)is beneficial to the enhanced catalytic activity of the catalyst,resulting in the poor crystalline spherical structure of Li2O2,which was suitable for the improvement of electrochemical reaction kinetics of Li-O2 batteries.The presented charge transport mechanism of LSPR is of guiding significance for further understanding the formation and decomposition mechanism of Li2O2 with poor crystallinability.3.For the first time,the Ag/Bi2Mo O6(AB-OV/CC)photocathode with staggered nanosheet structure of metal/semiconductor heterojunction containing oxygen vacancy was prepared.The size of Ag sheet is about 300 nm,and the irregular hexagon is dominant,while the size of Bi2Mo O6 nanosheet is about 400-500 nm,and the thickness is20-30 nm.The photocathode has strong absorption of light in the range of 200 nm?800 nm.The results revealed that the kinetics of ORR and OER can be improved synergistically due to the metal/semiconductor heterojunction effect and the catalytic activity of oxygen vacancy.At a current density of 50 m A g-1,the discharge and charge platesu is 3.05 V and 3.25 V,with a round-trip efficiency of 93.8%and retention of 70%over 500 hours,respectively.It is demonstrated that amorphous Li2O2can be obtained by AB-OV/CC cathode in the range of UV to visible light,which has been confirmed by Raman,XPS,in situ XRD and in situ DEMS.Comparison experiments show that amorphous Li2O2 is the fundamental factor of improving the kinetics of product formation and decomposition during charge and discharge.This chapter proposes a new way to design the cathode microstructures of high-efficiency Li-O2batteries and deepens the understanding of the reaction kinetics of photo-assisted batteries.It has important theoretical guiding significance for establishing the structure-activity relationship between the properties of Li2O2 and catalysts. |
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