Preparation And Application Of Composite Cathode NiO/MnCo₂O₄@Ni For Lithium-O₂ Batteries

Lithium-oxygen battery has superhigh theoretical energy density,which can even be comparable to the gasoline.In recent years,Li-O₂ battery has become the focus of research and achieved a lot of meaningful progress.However,the serious electrochemical polarization problem results in the poor energy efficiency and cycle life of Li-O₂ battery.Among them,the polarization problem of the positive electrode is particularly prominent.Therefore,the research and development of positive electrodes has become the focus of this article.In order to avoid side reactions of carbon-based materials and adhesives,and improve the energy efficiency and cycle life of Li-O₂ battery,carbon-free MnCo₂O₄@Ni was selected as the research object.Nanowire-like bundles of MnCo₂O₄were successfully synthesized on the glossy nickel foam through a simple hydrothermal and annealing process.NiO was prepared onto MnCo₂O₄@Ni,in order to decompose the discharge product effectively and increase the cycle life of battery.The main contents of this article are as follows:?1?This article studied the hydrothermal and annealing conditions for preparation of carbon-free MnCo₂O₄@Ni.The temperature and time in the hydrothermal and annealing process is important for carbon-free positive electrodes.By the single-variable comparison experiments,the precursor could be completely converted to MnCo₂O₄ by two processes:one was the hydrothermal method at the temperature of120?for 12 h,the other was that the temperature rose from the room temperature to450?with a rate of 2 K/min,and the annealing kept at 450?for 2 h.Compared with other preparation conditions,this positive electrode has the highest specific surface area,pore volume and specific capacity?11243 mAh/g?.Subsequent electrodes would be prepared under this hydrothermal and annealing condition.?2?NiO can be successfully prepared onto MnCo₂O₄@Ni through a secondary hydrothermal and annealing process.In addition,from the analysis of LSV,CV,XPS and Raman,it is confirmed that NiO can promote the decomposition of by-products and discharge products.Therefore,at a fixed capacity of 300 mAh/g and a current density of 200 mA/g,the composite electrode operated 218 cycles stably for Li-O₂battery.The cycle life of the composite electrode is 1.6 times higher than that of MnCo₂O₄@Ni for Li-O₂ battery.?3?XPS,O₂-TPD and HRTEM also confirmed that both electrodes contain oxygen vacancies.In the early stage of discharging of Li-O₂ battery,oxygen vacancies promoted the growth of Li₂O₂ by following the surface mechanism,which formed film-like discharge products on the catalyst nanowires.As the discharge proceeded,the oxygen vacancies were gradually depleted,and Li₂O₂ formed flower-like structure by following the solution mechanism.However,NiO hindered the surface mechanism process.Compared with MnCo₂O₄@Ni,the surface mechanism process of the composite electrode ended prematurely,and then flower-like discharge products appeared earlier.To sum up,the research of carbon-free MnCo₂O₄@Ni and NiO is focused in this article.Moreover,it systematically studied the oxygen vacancies structure inside materials,and studied the impact of oxygen vacancies and the structure of cathodes in the discharge process.The composite electrode has a better decomposition effect on discharge products and by-products.The composition of NiO can optimize cycle life of Li-O₂ battery,which provides a new choice for the optimization of positive electrodes.