Preparation And Electrochemical Properties Study Of Cobalt Ferrite Based Cathode Materials For Lithium-sulfur Batteries

Lithium-sulfur（Li-S）batteries are considered as a promising candidate for advanced electrochemical energy storage systems,owing to their high theoretical energy density of2600 Wh kg^-1,abundant sulfur natural resources and low material cost.However,the practical application of sulfur cathode is limited to the insulation characteristic of sulfur,huge volume change during charge/discharge processes,the notorious“shuttle effect”arising from the dissolution of lithium polysulfides（Li PS）and the slow redox reation kinetics of sulfur species.Three brilliant strategies to fabricate the effective sulfur host materials have been proposed in this paper combining with high conductivity,adequate accommodation space,chemical adsorption and electrocatalytic activity.The host materials can be achieved by the surface modification or composite treatment of Co Fe₂O₄,and the bimetallic oxides are obtained from prussian blue analog（Co Fe-PBA）precursor.Various material measurement techniques,electrochemical measurements and theoretical calculation are used to systematically investigate the electrochemical performances and improvement effect mechanism of sulfur cathode.A strategy of carbon-coated bimetallic oxide as host material for sulfur cathode is proposed by combining the high electrical conductivity of carbon and strong chemisorption capability of bimetallic oxide.The hollow structure of Co Fe₂O₄ is obtained from Co Fe-PBA precursor via a one-step heat treatment process.Afterward,the Co Fe₂O₄@C nanocages are acquired as efficient host material with the polydopamine-derived surface carbon coating.The hollow material acted as nano-reactor not only can provide adequate space for sulfur to prevent the aggregation of insulating sulfur particles,but also can buffer the enormous volumetric expansion during lithiation to avoid cathode pulverization phenomenon.Co Fe₂O₄ can effectively restrict the soluble Li PS through chemical bonding to reduce“shuttle effect”.The surface carbon coating layer can significantly improve the electrical conductivity of metal oxide host,accelerate electron transfer in the sulfur electrode.Benefiting from the hollow structure,excellent electrical conductivity and strong Li PS affinity of host material,the S/Co Fe₂O₄@C cathode still can exhibit the discharge capacity of 557 m Ah g^-1 after 500 cycles at 2 C.A strategy of bimetallic oxide decorated with conductive polymer as host material for sulfur cathode is proposed because the conductive polymer possesses the essential characteristics of high electrical conductivity and polar functional group compared with the non-polar carbon material.The Co Fe-PBA precursor can transform into Co Fe₂O₄nanotubes via a facile heat treatment and Co Fe₂O₄@PANI nanotubes are obtained through decorating with conductive polyaniline.The excellent conductivity of polyaniline can make up the poor conductivity drawback of metal oxide,accelerate the electron transfer in electrochemical reaction.Meanwhile,surface polyaniline modification layer can enhance the chemical interactions between nanotube host and Li PS.Moreover,the soft and flexible modification layer will enable the nanotube host to withstand the internal stress caused by volume expansion/shrinkage of active substance during the charge/discharge cycles,avoid pulverization of nanotube host.In addition,the Co Fe₂O₄@PANI host exhibits high catalytic activity for the redox process of Li PS,which can accelerate the electrochemical reaction kinetics of sulfur species.Benefiting from the unique structure and properties of host material,the S/Co Fe₂O₄@PANI cathode delivers high specific discharge capacity,good rate performance and long-term cycling stability and the reversible capacity of 583 m Ah g^-1 is still retained after 500 cycles at 2 C.In order to avoid the conductive additives,polymer binder and metal foil current collector in the traditional electrode production procedure,Co Fe-PBA precursor is embedded in the carbon nanotube paper,and three-dimensional free-standing Co Fe₂O₄/CNTP host is subsequently acquired via a facile annealing treatment.The abundant porous space in free-standing host not only can facilitate the uniform loading of elemental sulfur,but also can buffer the huge volume change of active substance in charge/discharge cycles.The cross-linked carbon nanotubes offer a continuous electron transport network for sulfur cathode,which can reduce the electrode polarization phenomenon.The Co Fe₂O₄ nanoparticles embedded in carbon nanotube paper can effectively suppress the“shuttle effect”of Li PS and accelerate sulfur species conversion to improve the redox kinetics of sulfur cathode through adsorption-catalysis synergistic effect.The S/Co Fe₂O₄@/CNTP free-standing paper cathode exhibits excellent electrochemical performances,demonstrating a high discharge capacity of 643 m Ah g^-1after 500 cycles at 2 C.More importantly,the free-standing sulfur electrode with high loading of 9.4 mg cm^-2 and lean electrolyte/sulfur ratio of 6μL mg^-1 can deliver a high areal reversible capacity of 4.1 mAh cm^-2 after 100 cycles.