Research On Composite Cathodes Construction And Discharge Products Manipulation For High-performance Li-O₂ Batteries

The lithium-oxygen?Li-O₂?battery is deemed as a promising candidate for the next generation energy storage system due to its ultrahigh theoretical energy density(3505 Wh kg^-1).Especially,the aprotic Li-O₂ battery has attracted intensive investigations for its wide application prospect.The Li-O₂ battery based on the reversible decomposion of Li₂O₂ shows low energy efficiency and inferior cycle stability,which is induced by the sluggish kinetics of charge transfer in discharge products.In addition,by-product lithium carbonates?Li₂CO₃?are also generated when the carbon-based cathodes are charged above 3.5 V,sharply degrading the battery cycle stability.Consequently,in this work,we have attempted to manipulate the discharge products in Li-O₂ batteries by modifying cathode surfaces to lower the charge overpotential and improve the energy effieiency.Major results are listed as follows:?1?Tin dioxide?SnO₂?nanoparticles decorated carbon nanotubes?SnO₂/CNTs?have been constructed as composite cathodes to manipulate the morphology and component of discharge products in Li-O₂ batteries.Owing to the strong oxygen adsorption of SnO₂,oxygen reduction reactions tend to occur on composite cathode surfaces,resulting in formation of flake-like discharge products of Li_2-xO₂ below 10nm in thickness rather than toroidal particles of several hundred nanometers.Such homogeneous nanosized discharge products with lithium vacancies markedly enhance the electrode kinetics and charge transfer in discharge products.Consequently,the Li-O₂ batteries based on the SnO₂/CNT cathodes show the small polarization voltage gap?from 1.45 V to 0.72 V?,which leads to the superior energy efficiency?80%? compared with that based on the pristine CNT cathodes.?2?Different-type Si nanoparticles with different conductivity?n-Si/CNTs,p-Si/CNTs and i-Si/CNTs?decorated carbon nanotubes have been constructed as composite cathodes to manipulate the morphology and component of discharge products in Li-O₂ batteries.For n-Si decoration with the highest conductivity,the rapid reaction kinetics are conducive to the generation of massive LiO₂,which tends to adsorb on the cathode surfaces due to the strong adsorption of Si nanoparticles and be disproportionate to the film-like Li_2-xO₂ less than 10 nm.For i-Si with the lowest conductivity,only CNTs provides active sites,and LiO₂ is prone to dissolving in the electrolyte and forming the toroidal Li₂O₂ with 200 nm.For p-Si with moderate conductivity,both the decoration and CNTs can provide active sites,thus forming the disc-like discharge products with a diameter of 100 nm entangled on the CNTs.Correspondingly,the charge voltage increases in the order of V_n-si<V_p-si<V_i-si.Therefore,the formation of discharge products of Si/CNTs-based Li-O₂ battery attributes to the synergistic function of decorations and CNTs.