Study On Preparation Of MOF-Derived Composites And Their Properties In Lithium-Sulfur Batteries

With the rapid development of a large number of portable electronic devices and new energy electric vehicles,the demand for high energy density battery systems is also growing.The development of high energy density batteries for portable energy storage devices and storage technologies has become an urgent problem.The theoretical specific capacity of lithium-sulfur batteries（LSBs）is 1675 m Ah g^-1,which is nearly five times that of lithium-ion batteries（LIBs）.In addition,as an active substance,elemental sulfur has abundant resource reserves and has the advantages of environmental protection and low cost.Therefore,LSBs have become one of the most promising alternatives to LIBs.Although LSBs has bright development prospect,it has encountered many bottlenecks in practical application.The charge-discharge process of LSBs is carried out by a series of soluble lithium polysulfide intermediates（Li₂S_n,2≤n≤8）undergoing reversible conversion reactions between sulfur and lithium sulfide（Li₂S）.The low electronic conductivity of sulfur itself(5×10^-30S cm^-1)leads to the slow REDOX kinetics of lithium and sulfur in liquid electrolyte.In addition,the shuttle of polysulfide（Li PSs）in the process of charge and discharge will lead to a rapid decline in the capacity of LSBs,the utilization rate of active substances is reduced,and the cycle retention rate is poor,which seriously hinders the practical application of LSBs.In order to solve the above problems,this article designs and optimizes the sulfur carrier and separator for LSBs to improve the inhibition effect on shuttle effect and improve the battery cycle performance.（1）In this study,a unique metal-organic frameworks（MOFs）-derived CoS₂/NC@1T MoS₂structure is designed as a Li PSs inhibitor,and applied to the coating material of modified separator for LSBs.The nitrogen-rich MOFs are ideal sources of microporous nitrogen-doped carbon（NC）,which can improve the conductivity of separator coating and inhibition ability to Li PSs.In CoS₂/NC@1T MoS₂composite prepared by one-step hydrothermal method,the layered configuration composed of NC,inner CoS₂nanoparticles and outer modified MoS₂nanosheets co-promotes the REDOX kinetics of Li PSs and provides abundant active sites for the inhibition of shuttle effect during battery charging and discharge with a strong anchoring ability for Li PSs.The first discharge capacity of LSBs with CoS₂/NC@1T MoS₂separator reaches to 1421 m Ah g^-1at 0.1 C.And after 100 cycles,a reversible capacity of 972 m Ah g^-1is obtained.A discharge capacity of 616 m Ah g^-1is still maintained after 500 cycles at 1 C,and the average capacity attenuation rate per cycle is only 0.07%.（2）The exploration of a high efficiency,stable,strong polarity,porous conductive cathode material is a key challenge in the development of advanced LSBs.In this study,a Ce ion doped MIL-88A derived three-dimensional（3D）hollow encapsulated Ni-Fe bimetallic layered hydroxide structure（Ce-Ni/Fe LDH）is proposed,and a conductive structure with rapid ion/electron transfer is constructed.The high efficiency synergistic interaction between the 3D hollow capsules of Ni/Fe LDH and doped Ce ion significantly enhances the chemical affinity for Li PSs.In addition,Ce-Ni/Fe LDH also shows strong catalytic action which can improve the REDXO kinetics of Li PSs.Combining the above advantages,S@Ce-Ni/Fe LDH composite material is used as the cathode of LSBs.The initial discharge capacity is as high as 1207 m Ah g^-1at 0.1 C,and the capacity retention rate is as high as 94.5%after 100 cycles at 0.2 C,showing good stability.The capacity decay rate is 0.067%per cycle after 1000 cycles at 1 C.This work provides a novel and simple method for the design of LSBs cathode sulfur carrier.The research can also be applied to more energy storage fields such as room temperature Na-S batteries with excellent performance.