Electrochemical Performance On Metal Organic Frameworks Derived Electrodes

The variability for renewable energies requires ever-increasing demand for low-cost and stable large-scale energy storage technologies.Among various energy storage devices,secondary batteries are deemed as the most promising devices to store electrical energy at large scale for the high energy conversion efficiency.Therefore,it is urgent to develop rechargeable batteries with high energy density,long cycle life and high stability.In the past decades,lithium-ion batteries(LIBs)are the major power source due to the long cycle life,large energy density and no memory effect.Besides,sodium-ion batteries(SIBs)are considered as the most potential candidate for large-scale electrical energy storage devices due to the similar working principle to LIBs,abundant sodium resources.In addition,metal-chalcogens batteries spring up for the high theoretical energy density and eco-friendliness,which also are deemed as the reasonable alternative for storing electricity on large scale.However,those above mentioned batteries suffer from significant challenges for further development.The huge volume expansion and sluggish ion diffusion kinetics can create tremendous obstacles in developing high reversible and stable batteries.Metal-organic frameworks(MOFs)with tunable porous structure and large surface area are deemed as the promising precursors to synthesize the desired electrode materials.Herein,MOFs derived materials have been prepared to enhance the electrochemical kinetics and structural stability by the surface functionalization,heterogeneous construction and active sites introduction.The mechanism for reaction and enhanced kinetics have been systematically investigated by the experiment and theoretical calculations.The main contents are described as following:1.Porous ZnO/ZnCo2O4/C core@shell nanospheres composed of ZnCo2O4 as the shell,ZnO as the core,are fabricated via directly carbonizing the core@shell structured ZnCo-MOFs precursor.The hierarchically porous core/shell structure offers abundant active sites,enhances the electrode/electrolyte contact area,provides abundant channels for electrolyte penetration,and also alleviates the structure decomposition induced by Li+insertion/extraction.The carbon layers effectively improve the conductivity of the hybrids and thus enhance the electron transfer rate,efficiently prevent ZnCo2O4 from aggregation and disintegration as well as partially buffer the stress induced by the volume change during cycles.When evaluated as the anode materials for LIBs,the as-synthesized ZnO/ZnCo2O4/C hybrids exhibit a large reversible capacity,excellent rate capability,and superior cycling stability.It can retain a capacity of 669 mAh g-1 after 250 cycles at a current density of 0.5 A g-1 and deliver 715 mAh g-1 at a high rate of 1.6 A g-1.2.ZIF-67 derived core/shell Co@C structures are in-situ transformed to porous core/shell structured CoP@C polyhedrons via a low temperature phosphidation process.The porous core/shell CoP@C nanostructures are anchored on 3D reduced graphene oxide(RGO)networks,which are supported by 3D nickel porous foam(NF)skeleton.The unique CoP@C-RGO-NF binder-free anode exhibits a remarkable electrochemical performance with outstanding cycling stability and high rate capability,delivering a specific capacity of 473.1 mAh g-1 at a current density of 100 mA g-1 after 100 cycles.The excellent properties can be attributed to the synergistic effects between core/shell CoP@C polyhedrons and RGO networks.The unique core/shell CoP@C polyhedrons can offer more electrode/electrolyte contact area and reduce the diffusion distance of Na+,while carbon layer shell can enhance electronic conductivity and buffer volume change,and prevent CoP from pulverization and aggregation.Furthermore,3D RGO networks can provide adequate surface areas for a high loading content of CoP and enhance charge transfer kinetics.Meanwhile,RGO/NF can efficiently act as a binder and electrical conductor to interconnect the separate CoP@C polyhedrons.3.The rational and comprehensive route to tune polarity and porous structure of ZIF-67 has been proposed.Tannic acid(TA)tuned ZIF-67 could create uniformly polar sites for in-situ polysulfide absorption,and C-S and Co-S abundantly functional groups for chemically trapping polysulfides to achieve stable cycling performance.Furthermore,TA tuned ZIF-67 polyhedrons can be sculptured to a core/shell hierarchical porous structure.Foremost,the functionalized polar hydroxyl groups can undergo redox reaction with long polysulfides to form thiosulfate and short insoluble polysulfides,relieving the shuttle effect ZIF-67-5-S cathode exhibits superior cycling stability and rate capability,showing a steady capacity of 521 mAh g-1 after 550 cycles under a current density of 500 mA g-1,still maintaining a capacity of 510 mAh g-1 even at a current density of 1600 mAh g-1.4.N-doped Co nanoparticles inlaid in porous N-doped carbon polyhedrons are successfully prepared via an original low-temperature pyrolysis approach.For maximizing the utilization of catalyst,Co nanoparticles have been tuned from 7 nm to homogenously distributed clusters about 2-3 nm.Specifically,compared with single Co nanoparticles,the homogenously distributed Co nanoclusters with Co-N bonds can significantly enhance the migration and coupling of K+and electrons,lower the energy barriers,promise a fast reaction kinetics of S/polysulfides,thus remarkably inhibiting the dissolution of long-chain polysulfides.The hierarchical porous N-doped carbon matrix guarantees more reasonable space to store or capture S and redundant space to buffer the volume expansion.Theoretical calculations result further confirm that N-doped Co nanoclusters can provide much stronger chemical adsorption than pure Co,promising an enhanced conversions kinetics of polysulfdes and thus inhibiting their dissolution.Optimized combination of the abovementioned superiorities,the S-N-Cos-C cathode delivers a high initial columbic efficiency,reaching 74.7%,a superior reversible capacity of 452.7 mAh g-1 at 50 mA g-1 after 50 cycles,a dramatic rate capacity of 415.2 mAh g-1 at 400 mA g-1 and a long cycling stability.5.The S-doped Te nanorods with optimal S doping amount are successfully prepared as Li-Te cathodes without any carbonous matrix.It is revealed from experimental results and theoretical calculations that S-doping can effectively enlarge the d-spacing and induce the redistribution of electrons from low electronegative Te to high electronegative S,forming internal electrical fields,which can greatly facilitate the transport of electrons and Li+.DFT calculations ascertain that S-doping can significantly boost the reaction kinetics,lower the energy barriers,and further relieve the redox reaction polarization.Taking advantage of heteroatomic S doping,the optimal Te0.92S0.08 cathode without any carbonous matrix exhibits high volumetric capacity of 1615 mAh cm-3 over 100 cycles under a current density of 0.1 C,a dramatic rate capacity of 562 mAh cm-3 at 2 C and long cycling stability.