Preparation And Properties Of Functional Organic-Inorganic Hybrid Separators For Lithium-based Rechargeable Batteries

With the ever-increasing demands for energy,the development of renewable energy source has attracted unparalleled attention.Among these,rechargeable electrochemical energy storages such as lithium-ion battery and lithium-sulfur battery are one of the most promising candidates.Separators,placed between cathodes and anodes,significantly affect the battery performance,including safety,energy density,and cycle life.Commonly used polyolefin-based separator stands out for its good chemical stability,proper porosity,and low cost.However,poor thermal stability and low affinity to organic liquid electrolyte limit its further application in high-performance lithium-based battery.Moreover,the open pore structure of polyolefin-based separator gives rise to the shuttling of soluble polysulfides,leading to rapid capacity fading and severe self-discharge of lithium-sulfur battery.Herein,we propose a series of advanced functional separators by varying the material compositions and structures to meet the requirements of high performance lithium-ion batteries and lithium-sulfur batteries.The main research works are described as follows:A nanocomposite separator with high electrolyte absorption and enhanced thermal stability is prepared by electrospinning and subsequent thermal cross-linking method.PC4SA-co-PMMA-co-PMPS,synthesized by free radical solution polymerization,is combined with PVDF and ZSM-5 to form a highly interconnected macro-porous structure.Molecular sieves with special channel structures and strong Lewis acidity are proposed to contribute to the disassociation of lithium salt and the transportation of lithium ion.Factors affecting thermal stability,electrolyte affinity,and lithium transport are investigated in detail.As a result,the three-dimensional cross-linked separator integrates advantages of ZSM-5,copolymers and PVDF,exhibiting enhanced electrolyte uptake,thermal stability and ionic conductivity(1.72 mS cm-1,25?).In addition,the cells with nanocomposite membranes possess better cycling performance(93.8%capacity retention after 200 cycles at 0.5 C)and improved C-rate capability compared with commercial separators.The practical implementation of lithium-sulfur batteries is seriously hampered by the notorious shuttling of polysulfides.Herein a symmetric composite PMIA-based separator is developed by in-situ phase inversion to suppress the shuttle effect and improve the electrochemical property of lithium-sulfur batteries.Thermal stability and electrolyte affinity of separators are studied.Also,the Li-ion diffusion and the conversion reactions of polysulfides are systematic discussed.The upper layer with MWCNT/MoO3/ZSM-5 could significantly improve the electrolyte affinity.The high porous ZSM-5,polar metallic oxide,as well as the conductive MWCNT could provide easy access to ion and electron,meanwhile,suppress the polysulfides diffusion through-physical and chemical interaction.Compared with the PP separator,this asymmetric separator could greatly enhance the rate capability and cycling stability(0.2 C,initial capacity 1201 mAh g-1)of the incorporated lithium-sulfur battery.Unfortunately,materials suppressing polysulfides shuttling by physical or chemical adsorption are merely palliative solutions and cannot address the fundamental problems of lithium-sulfur batteries.Electrocatalytically accelerating the conversion of polysulfides is an alternative strategy to restrain the shuttle effect and improve the sulfur utilization of Li-S batteries.Herein,we report a facile and efficient approach for fabrication of a functional bilayer a functional bilayer separator to decrease the accumulation of sulfur species and minimize their diffusion.The top layer comprises CeO2 nanocrystals spatially besieged by carbon nanofibers,serving as electrocatalysts to accelerate the reduction of polysulfides,meanwhile,acting as a dual-conducti ve upper current collector and a polysulfide inhibitor.The support layer maintains the structural integrity of the separator and allows the safe operation of lithium-sulfur batteries at high temperature.The chemical composition,morphology,thermal stability,and Li-ion diffusion of separators are investigated.The shuttle effect and conversion reactions of polysulfides as well as their mechanisms are systematic discussed.Compared with other separators,the bilayer separator possesses the highest Li-ion diffusion coefficients,lower charge-transfer resistance,and better dimensional stability.Consequently,the Li-S batteries with bilayer separators exhibit higher rate capability,superior cycling stability(with a capacity decay rate of 0.04%per cycle at 0.5 C over 300 cycles),and ultralow self-discharge.To obtain a thinner and lighter separator with high comprehensive performance,PMIA is used to prepare a nanofiber membrane.CeO2@MWCNT layer is easily deposited on the PMIA membrane by simple filtration to form a functional separator.The structure,chemical composition,morphology,and thermal stability of the composite separator are investigated.Thanks to the high conductive CeO2@MWCNT layer,a thin separator can improve the electrochemical performance of battery.The 0D(nanocrystals)/1D(MWCNT)composite mats shows synergistic absorption-electrocatalysis behavior for polysulfides.A higher voltage retention and a smaller loss rate of capacity(8%)are achieved for cells with CeO2@MWCNT/PMIA separators after 48 h rest,whose self-discharge issue are significantly suppressed.This novel asymmetric separator with high electrolyte uptake ability,excellent thermal stability and enhanced electrochemical property provides a promising strategy in dealing with multiple challenges of lithium-sulfur batteries.