Preparation Of MOFs Derivatives And Their Application In Lithium-Sulfur Batteries

The depletion of fossil energy and the degradation of environment have gradually become two major challenges that limit the sustainable development of today’s society.Therefore,it is important to develop efficient and economical electrochemical energy storage technologies to rationally utilize the electricity generated from renewable energy sources.Lithium-sulfur batteries(LSBs)have been considered as promising next-generation energy storage systems due to their excellent theoretical specific capacity(1675 mAh g-1)and energy density(2600 Wh kg-1),as well as abundant and environmentally friendly raw material sulfur.However,the poor ion/electron conductivity of sulfur and its discharge products,the "shuttle effect" of liquid-phase polysulfides(LiPSs)during charge-discharge,and the slow sulfur reaction kinetics have led to significant challenges in the practical application of LSBs.In view of the above problems,it is urgent to design and develop electrode materials with excellent electrochemical properties for high-performance LSBs.In this paper,a series of threedimensional polar high-performance composites with controlled morphology are prepared as LSBs sulfur hosts using Metal-organic frameworks(MOFs)as precursors.The main research is as follows:1.Two different materials,Co-CNT and Co-C,were obtained by calcination of Cobalt-Prussian blue analogue(Co-PBA)as precursors using a single-ended porcelain tube reactor with a large aspect ratio and an open ceramic boat reactor,respectively.The results show that the single-ended porcelain tube reactor is capable of storing the reaction gas released during the pyrolysis of Co-PBA,and the cathode functionalized material(Co-CNT)with in situ grown CNT can be prepared by one-step pyrolysis without the addition of reducing gas and additional carbon source.The three-dimensional CNT conductive network can effectively improve the electrical conductivity of sulfur hosts,facilitate electron/ion transport.The Co nanoparticles can also accelerate the conversion of LiPSs.At 0.1 C current density,Co-CNT/S provides an initial high discharge capacity of 1238.5 mAh g-1 and maintains its high reversible discharge capacity after 350 cycles at 0.5 C current density with an average capacity decay rate of 0.14%per cycle and a stable Coulomb efficiency of about 99%.2.Co-PBA polymerized by dopamine as the precursor(Co-PBA@PDA),carboncoated CNT and Co carbon cage(Co-CNT@C)were prepared for the first time by a one-step process using the single-ended porcelain tube reactor,followed by phosphorylation to obtain the cathode functionalized material(CoP-CNT@C).During calcination,the presence of PDA reduces the aggregation of Co nanoparticles,allowing CoP and CNT to be uniformly distributed in each carbon cage.The three-dimensional CNT conductive network inside CoP-CNT@C can effectively improve the electron/ion conductivity.The uniformly distributed CoP nanoparticles can chemically adsorb LiPSs and reduce their "shuttle effect".While CoP can also accelerate the conversion kinetics of LiPSs.In addition,the porous structure of the carbon cage of CoP-CNT@C can physically slow down the diffusion of LiPSs into the electrolyte and provide storage space for sulfur.Therefore,CoP-CNT@C/S cathode exhibits a high initial discharge capacity of 1456.8 mAh g-1 at 0.1 C,Even at 3 C,it still provides a high discharge capacity of 663.3 mAh g-1.After 750 charge-discharge cycles at 0.5 C,CoP-CNT@C/S still exhibits a reversible discharge capacity of 473.9 mAh g-1,and the average capacity decay rate is only 0.075%per cycle.In addition,the high-load sulfur cathode(CoP-CNT@C/S=1:6 and CoP-CNT@C/S=1:9)can achieve 400 stable chargedischarge cycles at 1 C current density with an average capacity decay rate of 0.039%and 0.055%per cycle,respectively.