Research On Catalysis And Stabilization Of Ceria/Graphene Based Cathode For Non-aqueous Lithium-air Battery

With the rapid development of electric vehicles in recent years,the requirement of high mileage has focused us to seek for state-of-the-art cells beyond lithium-ion batteries.Lithium-air batteries?LABs?have attracted much attention because of their extremely high specific energy density.However,the research about LABs is still on the early stage.Many problems should be resolved before practical application,such as the large overpotential,limited capacity during discharge/charge processes,and the poor stability during cycle test.As the main reaction zone,the cathode is vital to the batteries,of which the catalyst application plays the most important roles.However,Li₂O₂ is insulating at room temperature,and the aggregation during discharge process would limit the catalytic efficiency.So appropriate catalysts should be introduced to anchor the formation of Li₂O₂during discharge process,and promote its decomposition during the following charge process.Our research is focused on ceria and reduced graphene oxide,and then applied as the cathode catalysts of non-aqueous LABs to improve the electrochemical performance.As the traditional rare-earth oxide,CeO₂ has been confirmed with strong adsorption towards O₂^-and O₂^2-,which could delay the aggregation of Li₂O₂ during discharge process;as the novel two-dimension materials,graphene could further promote the electrical performance because of its high electrical conductivity and catalyst activity.Based on the systems,the catalytic mechanism is to be analyzed,and finally to realize its application in flexible batteries.The conclusions are summarized as follows:?1?Research on catalytic mechanism for CeO₂@rGO cathodeCeO₂@rGO cathodes with different CeO₂ contents are synthesized through hydrothermal methods.The morphology and electrochemical tests confirms that the 45wt%CeO₂@rGO cathode has the best electrochemical performance.With the uniform distribution of CeO₂ particles,the cathode could efficiently prevent the catalytic loss during electrochemical tests.Compared with that of pure rGO,the cathodes show high discharge capacity of 11878 mAh·g^-1 with low charge overpotential at the current density of 400 mA·g^-1,and can cycle up to 40 times without obvious degradation with the capacity limitation of 1000 mAh·g^-1.The mechanism is studied as follows:the dissolved discharge intermediate LiO₂ in DMSO would preserve the active sites of the cathode,which contributes to its high capacity during discharge process.Besides,the discharge product Li₂O₂ could be anchored formation without serious self-aggregation with the strong adsorption between CeO₂ and O₂^-/O₂^2-.The limited Li₂O₂ size of 50 nm with better contact on rGO substrate would reduce the overpotential during the following charge process.?2?Research on stabilization of CeO₂@rGO cathodeThe degradation of CeO₂@rGO is serious during long-term test based on the reason that CeO₂ nanocrystal would aggregate and grow up with the strong adsorption between CeO₂ and Li₂O₂,the batteries can only cycle up to 60 times at the capacity limitation of600 mAh·g^-1.So Gd³⁺is chosen to dope in CeO₂ lattice(Ce_0.8Gd_0.2O_2-?@rGO,GDC@rGO)to enhance the stabilization during cycle processes.Abundant oxygen vacancy would be introduced during the doping process,which could prevent the migration and diffusion of GDC nanoparticles based on the strong adsorption with oxygen-functional groups on rGO,so the batteries show better cycle stability for up to 100 times at the same test condition.Also,the abundant oxygen vacancy on GDC surface would promote the adsorption of O₂,and serve as the catalytic active site for oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?,which contributes to 0.22 V lower overpotentials during cycle processes.?3?Synthesis and application of 3D flexible cathode3D inter-connected porous flexible cathode?3D-rGO?is synthesized with nickel foam as the hard temples.As the characterization of sufficient macropore and mesopore structure,3D-rGO cathode could promote the diffusion of Li⁺and O₂ before reaction,and provide enough storage for Li₂O₂ after reaction.CeO₂ nanospheres could be in-situ grown on 3D-rGO cathode?C-CeO₂@3D-rGO?.With the anchoring effect of CeO₂ nanospheres,the batteries display stable discharge potential at 2.68 V and can cycle up to 80 times with capacity limitation of 600 mAh·g^-1.Flexible batteries are also assembled based on C-CeO₂@3D-rGO,and the cathode could be cycled up to 80 times with the stable discharge potential over 2.5 V in ambient air based on the capacity limitation of 600mAh·g^-1.The batteries could also discharge and charge smoothly during bending progress,and shows stable cycle performance even after 1000 bendings.