Design And Preparation Of ZIF-67-based Derived Nanomaterials And Their Application In Lithium-sulfur Batteries

With the rapid development of the economy and the gradual rise of electric vehicles,the new electrode materials of lithium-ion batteries have made great progress,but the energy density of lithium-ion batteries remains gradually approached its bottleneck,energy storage systems with reliability safety,rapid charge and discharge capability and long cycle life need to be developed urgently to meet peoples increasing energy demands.Lithium-sulfur battery(Li-S)greatly meets the requirements of cleanliness and availability,due to its high energy density(2600 Wh kg^-1),sulfur as the cathode active material with low cost,environmental friendliness and abundant reserves.The requirements for the development of renewable energy make Li-S batteries competitive to become the next generation of clean energy.However,sulfur,exists a poor conductivity,a volume expansion of about 80%during the charging and discharging period of the battery,and reacts with lithium ions to form lithium polysulfides(Lithium Polysulfurs,Li PSs),and causes the shuttle effect,these disadvantages lead to poor cycle life,low Coulombic efficiency and loss of active material.ZIF-67 is a precursor and converted into cobalt sulfide by solid-phase reaction and hydrothermal sulfidation.Cobalt sulfide improves the conductivity of the cathode material and inhibits the shuttle effect of polysulfides.This paper also further explores the inhibition effect of cobalt sulfide with the doping of carbon nanotubes(CNTs)and bimetallic sulfide on polysulfides,the follows are main research and conclusions:1.The CNTs is acidified to have functional groups such as sulfonic acid group and nitro group on the surface,and the precursor material of ZIF-67 is added to grow nano boxes on the CNTs surface.Finally,the Co₃S₄/CNT is vulcanized by the hydrothermal method.Due to the CNTs strengthens the electrical conductivity of the material,and the Co₃S₄ has adsorption and catalytic effects on polysulfides,the Co₃S₄/CNT material has the functions of fast electron conduction,strong adsorption of polysulfides,and rapid transformation of polysulfides.In Li-S batteries,the initial specific capacity of Co₃S₄/CNT/S is 1015 m Ah g^-1 at a current density of 1 C with a sulfur areal loading of around 1 mg cm^-2,after 300 to 500 cycles,the capacity drops to about 439.1 m Ah g^-1 with a capacity decay rate of 0.069%per cycles.2.Conversion of ZIF-67 into a hollow Co-Mo hydrotalcite(layered double hydroxide,Co-Mo LDH)structure by hydrolysis of ammonium molybdate in ethanol and aqueous solution with ZIF-67.LDH@PDA was synthesized by the polymerization coating of dopamine,and the elemental sulfur and LDH@PDA were reacted at high temperature under nitrogen atmosphere,converted into Mo S₂/Co₄S₃/C in the end.In this material,Co₄S₃ quickly adsorbs polysulfides and promotes the conversion of polysulfides;Mo S₂ improves the diffusion rate of lithium ions,and C framework acts as a shell.On the one hand,it enhances the conductivity of the material.On the other hand,it buffers the volume of sulfur during charge and discharge period.As a cathode material host for Li-S batteries,Mo S₂/Co₄S₃/C/S exists a high initial discharge capacity of 651 m Ah g^-1,the discharge capacities after 1000 cycles maintained 394m Ah g^-1,and the capacity decay rate is only 0.04%after 1000 cycles and high reversible specific capacities at 0.1 C maintained 906.7 m Ah g^-1 after 30 cycles with high sulfur loading of 4.1 mg cm^-2.