Microstructure Regulation And Electrochemical Performance Of Cathodes For Lithium-Oxygen Batteries

Safety,performance,cost and environmental environmental friendliness are the central pursuits for electrochemical energy storage devices,which play an effective role in realizing the goal of peaking carbon dioxide emissions for China.Due to the ultra-high theoretical energy density close to that of fuel oil,the newly emerged lithium-oxygen battery demonstrates outstanding application prospect of surmounting the state-of-the-art lithium-ion counterparts.However due to the multi-step and multi-electron redox chemistry during the oxygen reduction reaction(ORR)and oxygen evolution reaction(OER),both the mass transfer and surface reaction kinetics are usually sluggish in lithium-oxygen battery.Furthermore,the intrinsic insulation characteristics and inferior solubility of the discharge products,namely Li2O2 in electrolyte induce more severe ORR/OER overpotential and undesirable parasitic reaction derived from electrolyte decomposition.These factors exert unfavorable influence on the discharge/recharge reversibility,energy conversion efficiency,rate capability as well as cycling stability.In this case,constructing hierarchical porous cathodes with ORR/OER bifunctional catalytic activity is of vital importance to amerliorate the overall electrochemical performance of Li-O2 battery.Hence,considering the the root cause of the discharge/recharge polarization and side reaction during cycling,the elaborate designs of high efficiency catalysts are carried out around the main guideline of enhancing interface charge transfer dynamics and modulating the growth and decomposition behaviors of Li2O2 products.First,in view of optimizing three-phase interfacial chemistry and guaranteeing sufficient accommodation space for Li2O2,various hierarchical porous catalysts with huge specific surface area are designed.On the other hand,considering modulation the type/distribution of the active sites,heightening the utilization efficiency of catalytic centers and enhancing the intrinsic catalytic activity,a series of catalysts are optimized based on heterostructure and defect strategies.The main research contents are as follows.1.N-doped carbon nanotubes hybrid with Co2P and Ru nanoparticles(Co2P/Ru/CNT)were fabricated by a simple one-step calcination route.In this design,the N-doped CNT matrix with open-frame construction offer fast charge transfer channels and rich three-phase active sites.Specifically,the homogenously distributed Ru ultrafine nanoparticles and Co2P nanoparticles synergistically tune the nucleation and growth behaviors of Li2O2 during discharge process,inducing the formation of Li2O2 nanosheets with poor-crystallized and quasi-amorphous structure.The Li2O2 nanosheets homogeneously grow along the whole CNT matrix,constructing three-dimensional porous coating layer,which efficiently reduce Li2O2/catalyst interface impedance.Therefore,the Li2O2 species on the Co2P/Ru/CNT cathode are more easily decomposed during OER.As a result,the optimized catalyst demonstrates superior discharge/recharge capacity and cycling stability,achieving a high capacity of 18048 mAh g-1 and superior cycle life for 185 cycles at 100 mA g-1.2.Hierarchical NiCo2S4/NiO heterostructure cathode was fabricated via hydrothermal method and low temperature annealing procedure.Specifically,benefiting from the unique dendritic nanoarray network,multidimensional passageways for electrolyte impregnation and O2 transfer are built up.In addition,the built-in electric field effect between NiCo2S4 and NiO conspicuously improve the interfacial charge transportation kinetics.Furthermore,based on the density functional theory(DFT)results,NiCo2S4 and NiO active components manifest different LiO2 intermediate adsorption capabilities,synergistically altering the nucleation and growth patterns of the final discharge products and thus leading to the formation of lots of peasecod-like Li2O2 species.The special Li2O2 accommodations are evenly and tightly wound around the NiCo2S4/NiO electrode surface,which effectively reduce mass transfer resistance and hasten the decomposition kinetics during OER.Therefore,the NiCo2S4/NiO heterostructure demonstrates much lower overpotential and enhanced ORR/OER reversibility.The Li-O2 batteries based on NiCo2S4/NiO output impresive rate capability of 6150 mAh g-1 at 1000 mA g-1,and can steadily operate for 300 cycles at 200 mA g-1.3.The cobalt single atoms were stabilized within ultrathin N-rich carbon nanosheets by a "gas-migration-trapping" strategy,achieving the Co-SAs/N-C catalyst.The isolated Co atoms and the graphene-like carbon skeleton help maximumly expose active sites.High-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM)and X-ray absorption fine structure(XAFS)certify that Co-SAs and pyridine N atoms are bonded by robust covalent bonds,establishing abundant unsaturatedly coordinated Co-N4 catalytic centers.According to DFT results,Co-N4 moieties remarkably reinforce the intrinsic LiO2-affinity,thus realizing the government of the morphology,size and distribution of Li2O2 species during ORR.Then,numerious nano-sized Li2O2 ultrafine nanoparticles are homogenously covered on the whole surface of the Co-SAs/N-C catalyst during ORR.During OER,the Li2O2 species with small dimensions can make full use of the strong catalytic capability of the surrounding Co-N4 sites and shorten the mass transmission path,thus distinctly enhancing the decomposition efficiency of Li2O2.As a result,the Co-SAs/N-C based Li-O2 battery demonsrates much-improved electrochemical performance.Finally,an ultra-low overpotential of 0.4 V and a superior life of 260 cycles at 200 mA g-1 can be achieved.4.The Ti3C2 MXene quantum dot clusters confined on N-doped carbon nanosheets(Ti3C2 QDC/N-C)were fabricated by a hydrothermal thermal-shearing reaction and the following electrostatic self-assembly procedure.The HAADF-STEM results clarify the existence of abundant grain boundary and edge defects within Ti3C2 QDC.The XAFS results reveal that the rich crystal defects induce the reduction of Ti-C coordination number and shrinkage of bonding length,which govern the valence state and electronic structure of Ti3C2 QDC/N-C.The theoretical simulations disclose that the electron delocaliztion surrounding the central unsaturatedly coordinated Ti metal centers regulates the electronic structure optimization of Ti3C2 QDC/N-C.Additionally,active O2 and LiO2 species are inclined to preferentially adsorb around the grain boundary and edge defects within Ti3C2 QDC during ORR,which significantly strengthen the interaction between the catalyst and intermediates.Finally,many well-ordered porous flower-like Li2O2 assemblies are homogenously distributed on the surface of Ti3C2 QDC/N-C during ORR,which are feasibly oxided under lower decomposition energy barriers during OER,thus dramatically ameliorating the dual functional electrochemical kinetics.As expected,the Li-O2 batteries based on Ti3C2 QDC/N-C hybrid achieve a lower voltage gap of 0.62 V at 200 mA g-1,and a more durable lifespan of 240 cycles under a limited capacity of 1000 mAh g-1 at 200 mA g-1.5.The In-MoS2/Ru hybrid was fabricated by hydrothermal reaction and hydrogen thermal reduction strategies.In atoms are precisely doped at the Mo sites,inducing local phase transition of MoS2 from semiconductor 2H phase to metal 1T phase.The epitaxial growth of Ru nanoclusters is realized at the edge of MoS2,demonstrating "lattice grafting" orientation.The heterogeneous atom doping effect and strong metal-support interaction synergistically induce the local charge rearrangement around the metal sites,fully modulating the microstructure of the catalytic centers at atomic scale and thus activating the inert base plane of MoS2 matrix.During ORR process,In-MoS2/Ru induces the formation of weak crystalline Li2O2 films with a thickness of only about 3 nm,which are more easily decomposed during OER.Finally,a low ORR/OER overpotential of 0.65 V and an improved life of 175 cycles at 200 mA g-1 can be realized for In-MoS2/Ru.