| Lithium-oxygen(Li-O2)batteries are considered as a promising electrochemical energy storage technology due to their high theoretical energy density,which has attracted more and more scientific researchers’ attention.Despite the broad prospect,Li-O2 batteries still face many challenges in the practical applications,such as poor reversibility,pitiful rate performance and unsatisfactory cycling life,this is mainly attributed to the insulating and insoluble property of the discharge product lithium peroxide(Li2O2)itself,which can inactivate the active site of the air cathode,thus blocking the oxygen and electrolyte transport channel in the oxygen reduction reaction(ORR)process,and thus slowing down the kinetics of the oxygen evolution reaction(OER)process.Therefore,the development of high efficiency cathode catalyst is the key to the realization of highly performance Li-O2 battery.In this paper,the design of cathode catalyst and the morphology control of discharge products are mainly studied.The factors affecting the morphology of discharge products and the influence on the electrochemical performances of the battery are explored and studied,and the discharge product morphology that is beneficial to the battery operation is induced in the Li-O2 battery.The main research contents are as follows:1.Using apple as precursor,N-doped porous activated carbon was synthesized by simple pyrolysis and carbonization.On this basis,a free-standing cathode of RuO2 nanoparticles(RuO2 NPs)anchored on as-prepared N-HMACs is obtained(NHMACs-RuO2)via subsequent solvothermal process.The self-supporting cathode not only avoids the parasitic reaction caused by the binder,but also facilitates electron transfer.The porous structure of the cathode not only provides sufficient transport channels for the reaction of reactants,but also affords storage space for the discharge products.The anchored RuO2 NPS can not only serve as a catalytic site to accelerate the ORR/OER kinetics,but also optimize the reaction process,so that the discharge product Li2O2 is more likely to form a sheet structure through the surface adsorption pathway,which is conducive to the charging process of Li-O2 battery.First-principles calculations(DFT)showed that the addition of RuO2 regulated the growth mechanism of intermediate products(LiO2),optimized the deposition behavior of discharge products,and made the distribution of LiO2 on the cathode surface more uniform.Thanks to the above synergistic effect,N-HMACS-RuO2 as Li-O2 battery cathode catalyst exhibited excellent electrochemical performances,including low overpotential(0.97 V),high discharge specific capacity(current density:200 mA g-1,cut-off voltage:2V,discharge capacity:13400 mAh g-1)and satisfying cycling stability(current density:200 mA g-1,cutoff capacity:1000 mAh g-1,215 cycles,more than 2000 h).2.The layered porous carbon shells(UPCS)were synthesized by a simple thermal reduction method,and then MnO2-x/UPCS cathode was obtained by the redox reaction of KMnO4 and UPCS.On this basis,the OER performance was accelerated by introducing a small amount of cerium dioxide(CeO2)nanoparticles into the UPCS/MnO2-x matrix.When used as a cathode catalyst for Li-O2 batteries,excellent electrochemical performances were obtained,including low overpotential(0.58 V),high discharge capacity(current density:200 mA g-1,cut-off voltage of 2V,discharge capacity:15322 mAh g-1)and good cycle stability(current density:200 mA g-1,302 cycles).DFT calculations show that the d-f electron coupling established by the interaction between MnO2-x and CeO2 significantly promotes the high-speed transfer of electrons.The experimental results combined with theoretical calculation show that the UPCS/MnO2-x@CeO2 heterostructure can not only promote the interfacial electron transfer,regulate the adsorption energy of the reaction intermediate,but also accelerate the breakup of O-O bond,thus promoting the process of OER.This work provides a new way for the design of non-noble metal ORR/OER electrocatalysts with excellent catalytic activity using rare earth oxides. |
