Application Of Metal Organic Frameworks-Derived Hollow Nanocages For High Performance Lithium-Sulfur Batteries

Efficient storage of intermittent electricity via rechargeable battery systems is considered one of the most important solutions to alleviate the energy crisis and environmental pollution.Lithium-sulfur（Li-S）battery has emerged as a promising next-generation energy storage technology due to its superior theoretical specific capacity(1672 mAh g^-1)and energy density(2600 Wh kg^-1),as well as the natural abundance and environmental benignity of sulfur（S₈）.However,Li-S batteries still confront grand challenges when advancing to practical applications,such as the poor conductivity of S₈ and its discharge solid products,the large volume expansion during charging and discharging,the"shuttle effect"of liquid polysulfides（Li PSs）,and the dendritic growth of lithium negative electrodes.In view of the above problems,it is urgent to develop and design electrode materials with excellent electrochemical performance for high-performance Li-S batteries.In this paper,metal-organic frameworks（MOFs）are used to prepare morphologically controllable three-dimensional polar hollow nanocages electrode materials,which can significantly improve the electrochemical performance of lithium batteries.The main research contents are as follows:（1）Zeolite imidazole framework-8（ZIF-8）as a precursor,the hybrid of hollow ZIF-8 polyhedron functionalized with carbon nanotube（HZIF/CNT）cathode is fabricated via facile tannic acid（TA）etching and low-temperature treatment.The conductive HZIF/CNT structure with ordered hollow ZIF-8 morphology,rich heteroatoms functionalities,and hydrophilic surface endow strong adsorption of Li PSs,boosted Li PSs conversion kinetics,and regulated Li₂S deposition.As a result,S@HZIF/CNT cathode delivers a superior initial capacity of 1371 mAh g^-1 at 0.1 C and excellent rate retention at 3.0 C(696 mAh g^-1).The remarkable cycling stability is evaluated for 500 cycles at 0.5 C with a 0.073%capacity decay per cycle.Moreover,the cathode with a high sulfur loading of 3.41 mg cm^-2 delivers an initial capacity of696.4 mAh g^-1 at 1.0 C and stable cycling performance over 200 cycles.（2）Nickel-Cobalt Prussian blue analogue（Ni-Co-PBA）as the precursor,implanting nickel and cobalt phosphide into well-defined carbon nanocages through simple dopamine coating and phosphating treatment to obtain the nickel-cobalt bimetallic phosphide hollow nanocage（Ni-Co-P@C）cathode electrode carrier for Li-S battery.Transition metal phosphide has excellent electronic conductivity,polar adsorption and catalytic conversion capacity for Li PSs.The outer carbon layer regulates the formation of hollow nanocages and provides physical confinement to Li PSs diffusion.Meanwhile,the embedded polar Ni-Co-P nanoparticles can effectively trap the Li PSs and are the catalytically active sites for Li PSs conversions.The highly conductive nature of carbon and TMPs endow superior electron transportability and enhances the redox reaction kinetics.Therefore,the S@Ni-Co-P@C cathode exhibits a higher initial discharge specific capacity of 1317.2 mAh g^-1 at 0.1 C,and excellent rate retention at 3 C(694 mAh g^-1).（3）Ni,Co bimetal phosphides are in-situ implanted into hollow carbon nanocages（Ni-Co-P@C）and utilized as the multifunctional separator interlayer.Benefiting from the unique hollow nanocage and abundant synergistic adsorption-electrocatalytic sites,the Li-S battery with Ni-Co-P@C separator exhibit ultra-fast Li⁺transfer rates,which greatly promotes activity kinetics of the catalytic conversion of Li PSs.After incorporation of Ni-Co-P@C on the PP separator,the assembled Li-S cell exhibits superior Li⁺ion diffusion rate,outstanding rate performance(654.5 mAh g^-1 at 5 C)and remarkable cycling stability（only 0.056%capacity decay per cycle over 1000cycles at 0.5 C）.Besides,the Li-S batteries with a high sulfur loading of 4.5 mg cm^-2can still maintain a high areal capacity of 3.7 mAh cm^-2 after 85 cycles.