Research On The Design Of Functional Interlayers And The Electrochemical Properties Of Cathode Materials In Lithium-Sulfur Batteries

Lithium-sulfur(Li-S)batteries are considered to be one of the most promising next-generation electrochemical energy storage devices due to their huge advantages in energy density,specific capacity,and material cost.However,the commercialization of Li-S batteries is impeded by their inherent defects,which lead to low discharge capacity and cycling life.The main challenges of cathodes currently are as below: the low electrical conductivity of sulfur and its discharge product Li2 S,as well as their torpid oxidation/reduction processes;a low sulfur utilization caused by the severe shuttle effect that is originated from the diffusion of soluble polysulfides lithium(Li PSs);the significant volume change of the cathode during cell cycling.It is of huge benefit to resolve these problems for realizing high-performance Li-S batteries.Focusing on enhancing the overall electrical conductivity,suppressing the shuttle effect,and improving the kinetics of redox reactions in Li-S batteries,in this dissertation,multi-functional interlayers and cathode materials are designed to improve the properties of Li-S batteries.The operational principles and mechanisms of the prepared functional interlayers and cathode materials are further investigated.The main contents and research findings are as follows.(1)A flexible Ni/SS-PI interlayer with flame-retardant properties is designed based on the coating and solid phase separation methods.The Ni/SS-PI interlayer is inserted between the separator and the cathode,which is used to improve the performance of Li-S batteries.The results show that the Ni/SS-PI interlayer can effectively adsorb the soluble Li PSs in the electrolyte.The multi-walled carbon nanotubes(MCNTs)and the metal mesh enhanced the electrical conductivity of the cathode.The nickel layer on the surface of the metal mesh serves as an efficient catalyst to accelerate the redox reaction of sulfur species.In addition,the Ni/SS-PI interlayer is resistant to flame,which can be used as a barrier to improve battery safety.The Li-S battery with a Ni/SS-PI interlayer shows high initial discharge capacities of 1275.3 m Ah/g and 1190.9 m Ah/g at 0.2 C(1C=1672 m A/g)and 0.5 C,respectively.After 600 cycles,reversible capacities of 1023.5m Ah/g and 935.7 m Ah/g are maintained at the corresponding current densities,and the average Coloumbic efficiencies in the whole cycling process reach 99.1% and 98.2%.The excellent electrochemical performance of the battery confirms the advantages of the Ni/SS-PI interlayer.(2)To deal with the serious shuttle effect and the sluggish kinetics of the sulfur redox reactions,highly catalytic high-entropy alloy nanoparticles(HEA NP)are employed as cathode materials.A hierarchically porous and honeycomb-like Mg Cr Mn Fe Co Ni high-entropy alloy material is prepared based on a template method.A thin oxidation layer is introduced on the metallic surface of Mg Cr Mn Fe Co Ni to enhance the affinity with Li PSs.The resultant Mg Cr Mn Fe Co Ni-O shows good conductivity,Li PSs adsorbability,and high catalytic properties in enhancing the electrochemical conversions of sulfur species.The Li-S battery using Mg Cr Mn Fe Co Ni-O as the cathode material exhibits a high initial capacity of 1396.9m Ah/g at 0.5 C,with a reversible discharge capacity of about 1100 m Ah/g and an average Coloumbic efficiency over 99% even after 1200 cycles.The excellent performance of the Li-S battery demonstrates the feasibility of using Mg Cr Mn Fe Co Ni-O as the cathode material.The study on the Mg Cr Mn Fe Co Ni-O provides a valuable reference for the innovative application of HEA NP in the field of Li-S batteries.(3)The Ni S/NiO heterostructure is in-situ constructed by thermal bath synthesis and the Co doped heterostructured Co0.12Ni1.88S2/NiO is synthesized as the cathode material on the basis of Ni S/NiO.The better electrochemical performance of the Co0.12Ni1.88S2/NiO heterostructure is ascertained by regulating the contents of the components and Co element.In addition,the relationship between the electrochemical impedance and the conversions of sulfur species is investigated by in-situ EIS measurements.The results of theoretical calculations and measurements indicate that the polar surface of Co0.12Ni1.88S2/NiO heterostructure has a good affinity with Li PSs,which can effectively adsorb the Li PSs in the electrolyte.Additionally,Co0.12Ni1.88S2/NiO can significantly reduce the Gibbs free energies of the electrochemical conversions of sulfur species,which promotes the reactions.As a result,the Li-S battery with Co0.12Ni1.88S2/NiO delivered an excellent electrochemical performance,including an impressive initial discharge capacity of 1424.8 m Ah/g and a stable reversible discharge capacity of about 1000 m Ah/g at the current density of 1 C.After 1000 times of repeated charging and discharging,the battery still retains a high discharge capacity of 958.2 m Ah/g,demonstrating the advantages of the Co0.12Ni1.88S2/NiO heterostructure.The investigation of the Co0.12Ni1.88S2/NiO provides a new perspective for the design and development of heterostructure materials applied in Li-S batteries.