Metal–organic framework-derived MnO/CoMn2O4@N-C nanorods with nanoparticle interstitial decoration in core@shell structure as improved bifunctional electrocatalytic cathodes for Li–O2 batteries

Abstract

Core@shell-structured, hierarchically porous manganese oxide/cobalt manganite@nitrogen-doped carbon (MnO/CoMn2O4@N–C) nanorods with interstitially decorated CoMn2O4 nanoparticles are synthesized via one-step carbonization of metal–organic framework (MOF)-coated α˗manganese oxide (α-MnO2@ZIF-67) nanorods and are evaluated as bifunctional electrocatalytic cathodes for Li–O2 batteries (LOBs) to improve the bifunctionality, specific discharge capacity, and cyclability of α˗MnO2 nanorod cathode-based LOBs. The MnO/CoMn2O4@N–C nanorods feature a MnO nanorod core with CoMn2O4 nanoparticle interstitial decoration, both coated by an N–C conductive shell. The MnO core renders Mn active sites and oxygen vacancies, while the CoMn2O4 interstitial decoration gives additional Mn, Co active sites, thereby enhancing bifunctional electrocatalytic ORR–OER. The N–C shell increases electronic conductivity, hierarchical porosity, specific surface area, and protects the core and interstitial decoration against lithium peroxide (Li2O2) passivation. The improved structural features allow the MnO/CoMn2O4@N–C nanorod cathode-based LOB cells to exhibit superior full specific discharge capacity of 8,625 mAh·g−1 and cyclability of 48 discharge–charge cycles at 200 mA·g−1 specific current and 2000 mAhg−1 limited specific discharge capacity compared to their α˗MnO2 nanorod counterparts. An ORR–OER mechanism is proposed to describe the interesting formation of particle- and film-type Li2O2 deposits at different cycles for the MnO/CoMn2O4@N–C nanorod cathodes. Such MOF-derived, interstitial nanoparticle-decorated nanoarchitectures can lead to high-performance tunable bifunctional electrocatalysts.