Research On The Preparation And Application Of CoO@Ni For Lithium-air Batteries

A non-aqueous lithium-air battery as a high-energy density secondary battery is a new type of battery in the post-lithium ion era.Due to its high energy density,it has received extensive attention and research worldwide.At the same time,there are still many problems in lithium-air batteries.For example,the insolubility of the discharge product Li₂O₂ makes it accumulate on the surface of the positive electrode after discharge and easily blocks the positive electrode channel,resulting in blunt positive electrode.The effect of mass transfer on the battery life is reduced;the insulating properties of Li₂O₂ itself require a higher voltage for the charging process to decompose,resulting in a high charging overpotential.Although various catalysts have been developed to be applied to conventional carbon-based positive electrodes to improve such problems,carbon-based positive electrodes have their own drawbacks.Conventional carbon-based positive electrodes generate undesirable by-products due to their vulnerability to discharge products and discharge intermediates(O₂^2-and O₂^-).These by-products have a high decomposition voltage and are easily accumulated on the surface of the positive electrode to accelerate the deterioration of battery performance.Therefore,there is a need to develop a carbon-free positive electrode structure to replace the carbon-based positive electrode currently under study.In order to avoid the problems caused by carbon materials,a carbon-free lithium air battery cathode was prepared by directly growing the catalyst on the surface of the current collector by in-situ growth.And applied it to a lithium air battery cathode,showing a good overall battery performance.Then,through the exploration and improvement of the catalyst preparation process,the surface of the original CoO catalyst is structured with oxygen vacancies,thereby further improving the electrochemical performance of the positive electrode structure and the overall performance of the lithium air battery.The main contents are as follows:1.A carbon-free positive electrode CoO@Ni is prepared by simple hydrothermal and heat treatment under an inert atmosphere.The characterization of the positive electrode materials by XRD,FESEM,TEM,Raman,XPS and other characterization methods proved that the porous structure positive CoO@Ni was successfully prepared.The surface topography is a porous structure composed of an array of nanowires having a diameter of about 80 nm.Finally,the electrochemical related performance tests were carried out on the assembled lithium air battery.The carbon-free positive electrode exhibits better cycle performance and lower charge overpotential than conventional carbon-based cathode.At a current density of 200 and 300mA/g and a fixed discharge capacity of 500 mAh/g,the CoO@Ni can be stably cycled for 60 and 90 cycles,which is significantly better than the 38 and 40 cycles of the carbon-based positive electrode.This is related to the non-carbon positive electrode CoO@Ni avoiding the addition of carbon material and reducing the by-products which are difficult to decompose,which is beneficial to reduce the charging overpotential and ensure the reversible decomposition of the discharge product Li₂O₂.However,the carbon-free positive electrode has a smaller discharge capacitance than the carbon-based positive electrode.The non-carbon positive electrode CoO@Ni first cycle discharge capacity is 4952 mAh/g at a current density of200 mA/g.Far less than the first discharge capacity of the carbon-based positive electrode?8881.4 mAh/g?.This is due to the poor specific surface area and electrical conductivity of the carbon-free positive electrode compared to the carbon-based positive electrode.Therefore,it is chosen to optimize the original preparation method.2.The method of two-step treatment of the hydrothermal sample?heat treatment in a reducing atmosphere after calcination in air?was used to improve the preparation process of the original CoO@Ni,and the reduced CoO@Ni-R cathode was successfully prepared.And to explore the effect of the treatment time of the material under the reducing atmosphere on the positive electrode structure,three kinds of positive electrodes were obtained:CoO@Ni-R1,CoO@Ni-R2 and CoO@Ni-R3.After 1 and 2 hours of heat treatment in a reducing atmosphere,a nano-cluster structure with smaller size was obtained.The nanostructure of the positive electrode began to be destroyed with the extension of the reduction time to 3 hours.The test of EDS and XPS proved that the reduction can generate a certain oxygen vacancy on the surface of the CoO catalyst crystal,and the excessive reduction time will also destroy the original structure of the original crystal.The overall performance of the reduced positive electrode applied to the lithium air battery is improved compared to CoO@Ni?CoO@Ni-R1h and CoO@Ni-R2h are boosted to nearly 6041 and 8286 mAh/g at a current density of 200 mA/g and can be cycled 112 and 180 cycles at a current density of 300 mA/g?.It is proved that the CoO@Ni-R cathode material obtained after the two-step treatment may generate oxygen vacancies and thus enhance the catalytic activity of the CoO catalyst.At the same time,if the reduction treatment time is too long,the structure of the original positive electrode is destroyed,and the catalytic performance of the catalyst is lowered,so that the overall performance of the battery is deteriorated.3.In-depth study of the carbon-free cathode CoO@Ni-R and compare it with the CoO@Ni cathode.It has been confirmed by Raman,EDS,TEM,XPS and EPR that oxygen vacancies exist in the carbon-free positive electrode CoO@Ni-R.Compared to the CoO@Ni positive electrode,the CoO@Ni-R positive electrode exhibits better cycle performance?Increased from90 cycles to 180 cycles?,higher discharge capacity?Increased from 4592 mAh/g to 8286mAh/g?,and lower charge and discharge overpotential?The first cycle charge and discharge over-current is reduced by 0.99V?.This is because the presence of oxygen vacancies can effectively enhance the migration rate of electrons and lithium ions,and is advantageous for increasing the adsorption capacity for O₂ and providing more reactive sites.Thereby improving the overall performance of the battery.In summary,this work has prepared a porous nanostructured CoO@Ni carbon-free cathode,which exhibits superior electrochemical performance compared to the conventional carbon-based cathode.Through the optimization and exploration of the preparation method,the CoO@Ni-R cathode with oxygen vacancies was successfully prepared by two-step heat treatment,and the physical and chemical properties and electrochemistry performance between CoO@Ni and CoO@Ni-R were compared in depth.It was confirmed that the CoO@Ni-R cathode significantly improved the discharge capacity and effectively reduced the charge and discharge overpotential of the battery and improved the cycle performance of the battery compared to the CoO@Ni cathode.