Research On The Preparation And Application Of Co₃O₄ Nanoarrys For Lithium-air Batteries

With ultra-high theoretical energy density,rechargeable non-aqueous lithium-air?Li-air?battery has been widely studied as a promising power source for electric vehicles and other high-energy devices.However,the commercialization of Li-air batteries still faced with many challenges,such as high discharge/charge overpotential,poor cycling life and low energy efficiency.All of these are closely related to the insulation and insolubility of Li₂O₂,the main discharge product during battery operation,which will inevitably block the pores and passivate the cathode,leading to the poor electrochemical performances.In addition,discharge products Li₂O₂?or Li₂O?show high reactivity toward the electrolyte,carbon materials and binder,which will bring out irreversible decomposition by-products?Li₂CO₃?.Obviously,the accumulation of Li₂CO₃ will influence the electrochemical performance and finally lead to the battery deactivation.Therefore,optimizing the structure of oxygen electrode combined with stable catalyst is an effective way to enhance the performance of Li-O₂ battery.In this work,with the idea of carbon-,binder-free cathode,a series of Co₃O₄ nanoarrays with different morphologies grown on the surface of nickel foam were prepared by a facile hydrothermal method.Compared the performance of Li-O₂ battery with different cathode and then selected the optimal structure.Finally,based on the Co₃O₄ nanoarrays,the modification of noble metal and the construction of oxygen vacancies are further promoted to improve the performance of the Li-O₂ battery.The main contents are as follows:?1?Three types of Co₃O₄ nanoarrays were prepared by changing the NH₄F content of reactants,which were nanowires,nanorods and nanosheets,respectively.The materials were characterized by SEM,TEM,XRD and BET.The successful preparation of Co₃O₄ and the porous structure of nanoarrays were confirmed.Finally,Co₃O₄ nanoarrys were assembled into batteries for electrochemical performance test.Compared with the other two electrodes,Co₃O₄nanowires exhibited the best electrochemical performance with a discharge specific capacity of2459 mAh/g at a current density of 50mA/g and a stable cyclability of 47 cycles at a current density of 200 mA/g,which was higher than that of the nanorods?25 cycles?and the nanosheets?10 cycles?.It was closely related to the specific morphology of the nanowires,provided a larger surface area and pore volume,which is conducive to the formation and decomposition of the discharge products.Therefore,Co₃O₄ nanowires was selected for subsequent optimization of free-standing cathode.?2?On the basis of the original Co₃O₄ nanowires,the secondary hydrothermal was carried out to decorate Co₃O₄ with noble metal RuO₂.The SEM,TEM and XPS characterizitions were used to prove that the composite RuO₂/Co₃O₄@Ni electrode was successfully prepared and the nanoarrays structure was completely preserved.At the current density of 50 mA/g,RuO₂/Co₃O₄@Ni electrode displayed a discharge capacity of 3506 mAh/g and decreased the overpotential over 350 mV.At the current density of 200 mA/g,RuO₂/Co₃O₄@Ni electrode showed excellent cycling performance of 109 cycles,which is more than twice of Co₃O₄@Ni electrode.The presence of RuO₂ nanoparticles promoted the mass transfer and increased the catalyst active sites,thus facilitated the ORR/OER process.Meanwhile,SEM,XPS and Raman characterization showed that the main discharge product Li₂O₂ was loosely dendritic or flower-like and kept close contact with the catalyst,which can be decomposed at lower voltage,reflecting the excellent catalytic active of RuO₂/Co₃O₄@Ni cathode.?3?The nanostructure of Co₃O₄ nanowires had no significant change after NaBH₄reduction treatment.With the extension of the reduction time,the surface of nanowires become rough and the specific surface area increased.Meanwhile,grain refinement and crystallinity gradually become weak.The presence of oxygen vacancies was demonstrated by XPS,EDS,and O₂-TPD characterizations.The performance of Li-O₂ battery with reduced Co₃O₄@Ni cathode were all better than that of pristine Co₃O₄@Ni and r-Co₃O₄@Ni-1 cathode exhibited the best discharge capacity?4448 mAh/g?as well as cycling performance?162 cycles?.We can conclude that the increase of oxygen vacancies enhanced the cathode catalytic activity,but longer reduction time would destroy the original structure and instead lead to poor battery performance.As the main discharge product,Li₂O₂ displayed a film form.The abundant oxygen vacancies generated during Na BH₄ treatment promote Li₂O₂ surface growth and the nanoarray structure with a high specific surface area provide sufficient space to accommodate more Li₂O₂.As a result,the Li-O₂ battery with r-Co₃O₄@Ni-1 cathode displayed excellent performance.In conclusion,we prepared the structure-optimized carbon-and binder-free cathode with Co₃O₄ nanoarrays for Li-O₂ battery.Based on the nanoarrys structure,two aspects of modification were conducted to enhance the battery performance:RuO₂ surface modification and construction of oxygen vacancies by NaBH₄ treatment.The surface characteristics and physicochemical properties of Co₃O₄ nanoarrays before and after the modification were compared.Combined with their electrochemical performance in Li-O₂ batteries and the appearance of discharge products,the catalytic mechanism was further explored.