Preparation And Electrochemical Performance Of Nanofiber Like Free-standing Cathode In Lithium-oxygen Batteries

Lithium-oxygen?Li-O₂? batteries with a theoretical energy density close to that of gasoline,due to the obstacles,such as high overpotential,poor cyclability and unsatisfied energy efficiency,need to be solved before the practical application of Li-O₂ battery.The above problems are closely related to oxygen reduction reaction?ORR? and oxygen precipitation reaction?OER? on the surface of cathode materials.The results show that the catalytic activity of the cathode can be effectively improved by the designing of reasonable structure and the developing of catalyst with high catalytic activity,so as to improve the comprehensive performance of the Li-O₂ battery.The electrocatalytic activity of cathode materials depends largely on their surface properties and the number of surface active sites,the properties and number of active sites on the cathode surface are closely related to the design of surface morphology.The effect of morphological feature on battery performance plays an important role in controlling the diffusion of oxygen species and thus the discharge capacity.Besides the morphology designing,electronic structure engineering on cathode surfaces is considered to not only optimize the catalytic activity of OER/ORR,but also have great potential to release regulated band structure,thus improving the intrinsic conductivity of catalytic materials.For example,heteroatom doping can improve the catalytic activity of the cathode materials by adjusting the electronic structure and optimizing the adsorption energy of the intermediate products.Magnetron sputtering and plasma etching can cause partial reduction of cation in catalyst to produce anion vacancy,which can form a large number of exposed active sites on the material surface and accelerate REDOX kinetics.Therefore,studying on the modification of cathode surface is crucial to the development of lithium oxygen batteries.In this paper,heterogeneous atoms were doped on transition metal cobalt phosphide and cobalt selenide that are grown on the carbon cloth as free-standing cathodes for Li-O₂ batteries.The nanofiber like transition metal nickel selenide was grown on nickel foam by hydrothermal synthesis and Ar plasma etching,and directly used as free-standing cathode for Li-O₂ batteries.The morphology and phase of the electrode were characterized by scanning electron microscope?SEM?,transmission electron microscope?TEM?,X-ray diffraction?XRD?and X-ray photoelectron spectroscopy?XPS?.The effect of electrodes on discharge/charge and cycle property of batteries were studied by cyclic voltammetry and constant current charge-discharge method.The results show that:?1?A novel free-standing Fe doped CoP nanowire on carbon cloth?Fe-CoP@CC?cathode with high catalytic activity for Li-O₂ batteries was fabricated via hydrothermal method.Thanks to the special morphology electrode's outstanding catalytic activity for oxygen electrode reactions,abundant space for Li₂O₂ accommodation and abundant channels for O₂ transfer,the Fe-Co P@CC based Li-O₂ battery demonstrates good reversibility as well as excellent cycling stability.The electrochemical performance test results show that the Fe-Co P@CC electrode based Li-O₂ battery exhibits an outstanding cycle stability of over 500 cycles with no significant voltage attenuation and a high discharge capacity of around 9600 mA h g^-1at a current density of 100 mA g^-1.?2?A facile hydrothermal method combining with Ar plasma etching was adapted to create Se vacancy rich Ni Se nanorods on the Ni foam framework(V_Se-NiSe@NF) and directly used as the cathode of Li-O₂ battery.The initial charge/discharge overpotential of the V_Se-NiSe@NF based Li-O₂ battery is markedly reduced compared with that of pristine Ni Se@NF or bare NF based Li-O₂ battery?1.01 V?;and the cycling life is also significantly improved after introducing Se vacancy in NiSe@NF?over 950 h?.The study shows Se vacancies on the NiSe surface not only improve the electronic conductivity for rapid electron transport,but also serve as active sites to bind oxygen species for electrochemistry reactions.?3?The basal material of nitrogen doped carbon cloth was fabricated through magnetron sputtering,then the hydrothermal method combined with Ar plasma etching technology in situ growth selenium vacancy rich sisal like cobalt selenide(V_Se-CoSe₂@N-CC) and used as the free-standing cathode of Li-O₂ battery.The results show that the V_Se-CoSe₂@N-CC based electrode showed a performance comparable to that of precious metals in improving the overpotential??0.81v?.At the same time,the Li-O₂ battery based on V_Se-CoSe₂@N-CC electrode showed good cycle stability?>134 th?and durability?>955 h?,and effectively enhanced the battery rate performance(discharge capacity up to 3122 mA h g^-1 at a current density of 500 mA g^-1).