| Among the emerging energy storage devices,Li-O2 batteries have extremely high energy density,which is much higher than Li-ion batteries,and is expected to be used in new energy vehicles and large-scale power grid energy storage devices.However,Li-O2 battery is faced with many problems,which limit its further practical application.As the core of electrocatalytic reaction,the cathode material is very important for the high-performance Li-O2 battery.In recent years,with the emergence of various catalyst materials.the performance of the battery has been greatly improved.This work focuses on studying the performance of novel transition metal sulfides(Pd4S)and optimizing transition metal oxides(FeCo2O4),and investigating the catalytic mechanism of these materials in Li-O2 batteries.The core-shell Pd@Pd4S heterostructures decorated on the porous carbon matrix are successfully fabricated by adopting hypo(Na2S2O3·5H2O)as a sulfidation reactant in a simple method,which is used as an oxygen cathode for LOBs for the first time.The porous carbon materials with good conductivity were designed by hard template method and compounded with Pd@Pd4S heterogeneous nanoparticles to obtain Pd@Pd4S-C composites.Porous carbon as the matrix not only reduces the amount of precious metals,but also makes the Pd@Pd4S nanoparticles disperse uniformly,which can give full play to the excellent catalytic activities of nanoparticles.The hierarchical regular pore structure in Pd@Pd4S-C with a high surface area offers sufficient Li2O2 accommodations,which can facilitate a higher capacity and achieve a longer cycle life.The introduction of the Pd4S shell greatly boosts decomposition of Li2O2,which effectively decreases the overpotentials,and contributes to the outstanding cycling stability of Pd@Pd4S-C cathode.It could maintain for 176 cycles at a current density of 500 mA g-1 with capacity limitation of 500 mAh g-1,while the Pd-C composite can only deliver 90 cycles under the same conditions.This novel heterostructure fabrication method highlights the importance of efficient O2 electrode designs and provides a facile way to optimize the cathode materials for LOBs.Although precious metal sulfidation can achieve more stable cycling performance for cathode materials,the composite itself still has many problems.The carbon materials used in cathode material brings about more the carbon-involving side reactions during the charge-discharge process.Besides,the binder will inevitably block some pores,and the catalysts will be unevenly distributed on the carbon paper,resulting in part of the Li2O2 decomposed on its surface rather than on the contact surface with the catalyst.Considering the above problems,FeCo2O4 inverse spinel nanowires grown on Ni foam are fabricated as the carbon-free and binder-free cathodes for Li-O2 batteries.Superior high rate cycle durability and deep charge capability are obtained.For example,300 cycles with a low overpotential under a fixed capacity of 500 mAh g-1 are achieved at a high current density of 500 mA g-1.In deep discharge/charge mode at 500 mA g-1,the optimized FeCo2O4 cathode can stable worked over 30 cycles with the capacity maintained at about 2100 mAh g-1.Owing to the moderate incorporation of Fe3+ in the surface of stable inverse spinel oxides,the regulated FeCo2O4 cathodes possess a more stable and higher ratio of Co3+/Co2+,which offers improved adsorption ability of reactive oxygen intermediates and thus achieves the enhanced electrocatalytic performance in the higher current density.In addition,the morphology evolution from array to pyramid-like structure of nanowires further provide insurance on the superior cycle capability.Coupling pyramid-shaped nanowires with binary inverse spinel,the obtained Fe-Co oxide brings a promising material for the practical application of Li-O2 batteries.This work offers a general strategy to design efficient mixed metal oxide based electrode for the critical energy storage fields. |
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