Electrocatalytic Effects Of Iron Enhancing Lithium Storage Performances For Lithium Secondary Batteries

Lithium secondary batteries become the most promising candidate to new energy,because of its environmentally friendly and renewable advantages.However,lithium secondary batteries still face great challenges in their practical applications.During the charge and discharge process,the depth and reversibility of redox reactions seriously affect the electrochemical performance of the lithium secondary battery.Many studies have found that the introduction of electrocatalytic effect during the charge and discharge process could not only increase the capacity by catalyzing other the occurrence of other electrochemical reactions other than the electrode material,but also deepen the depth of redox reaction and enhance reversibility of the reaction with accelerated reaction kinetics.In addition,introducing high conductivity carbon materials into the active materials could improve the utilization of the active materials and rate performances by improving the interface electron transfer.Therefore,in order to solve the above two problems,we have studied the application of electrocatalytic effect of iron in the anode material of lithium-ion batteries and the interlayer of lithium sulfur batteries,to meet the high energy density and high power density requirements of lithium secondary batteries.1.The electrocatalytic effect of iron enhancing lithium storage performances for sandwich Fe3S4/reduced graphene oxide anodes:Increasing demands for lithium-ion batteries(LIBs)with high energy density and high power density requirehighly reversible electrochemical reactions to enhance the cyclability and capacities of electrodes.As the reversible formation/decomposition of the solid electrolyte interface(SEI)film during the lithiation/delithiation process of Fe3S4 could bring about a higher capacity than its theoretical value.Therefore,synthesized Fe3S4 nanoparticles are sandwich-wrapped with reduced graphene oxide(RGO)to fabricate highly reversible and long cycling life anode materials for high-performance lithium-ion batteries.The micron-sized long slit between sandwiched RGO sheets could effectively prevents the aggregation of active materials during the discharge/charge process.Furthermore,the RGO sheets interconnect with each other by a face-to-face mode to construct a more efficiently conductive network,and the maximum interfacial oxygen bridge bonds benefit the fast electron transfer from RGO to Fe3S4,improving the depth of the electrochemical reactions and facilitating the highly reversible lithiation/delithiation of Fe3S4.The Fe nanoparticles generated in situ during the cycle could catalyze the reversible formation and decomposition of the SEI film,thereby increasing the reversible capacity.Thus,the resultant Fe3S4/RGO shows a highly reversible charge capacity of 1324 mA h g-1 over 275 cycles at a current density of 100 mA g-1,even retains 480 mA h g-1 over 500 cycles at 1000 mA g-1.2.The electrocatalytic effects of iron enhancing ultralong lithium storage for dual-carbon-confined Fe7S8 anodes:Although the electrochemical catalytic conversion process is effective in increasing the reversible capacity of lithium-ion batteries,the low contact efficiency between metal catalyst and substrate and pulverization of the SEI film without protection are not beneficial for the electrochemical reactions.In this work,in order to further enhance the electrocatalytic effect of metal Fe,we synthesized a dual-carbon-confined in which RGO and in-situ-formed amorphous carbon(C)confine the Fe7S8 nanoparticles.During the charge and discharge process,the dual-carbon-confined structure provides a confined space,which not only prevent pulverization of the SEI film,but also increases the local concentration of intermediate phases of the active materials,improving the depth and reversibility of the electrochemical reactions.Moreover,the confinement structure increases the contact between the Fe catalyst formed in situ and the substrate and accelerates the electrochemical catalytic conversion reaction,resulting in a higher reversible capacity.In addition,the dual-carbon-confined structure ensures fast transfer of electrons and deepens the depth of reaction due to the highly conductive dual-carbon shell.Thus,the Fe7S8/C/RGO anode delivers an excellent rate performance and long cycling stability.At current densities of 2000 and 5000 mA g-1,the reversible capacities are 520 mA h g-1 over 1500 cycles and 294 mA h g-1 over 2000 cycles,respectively.3.The electrocatalytic effects of iron accelerating reaction kinetics for suppressing shuttle effect of lithium-sulfur batteries:An irreversible capacity fading is owing to the loss of soluble polysulfide and sluggish kinetic of polysulfides in electrochemical processes,thus seriously reducing the utilization of sulfur cathode in lithium-sulfur batteries(LSBs).The introduction of a catalytic interlayer between the separator and the cathode is an effective approach to solve the abovementioned problems.However,most reported catalysts are not conductive and could not benefit the electron transfer during the electrocatalytic process.Herein,we reasonably synthesize a graphited carbon interlayer decorated with Fe nanoparticles by thermal treatment.During the preparation process,the carbon interlayer effectively helps confine Fe nanoparticles,while Fe nanoparticles boost the graphitization of carbon interlayer.The well dispersed Fe nanoparticles could catalytically accelerate electrochemical conversions of soluble polysulfides by accelerating reaction kinetics,while the improved conductive carbon network is benefit for the electrons transfer during the electrocatalytic process to increase the utilization of polysulfides.Thus,the composite interlayer could not only physically block soluble intermediate polysulfides,but also further catalyze the conversions of these captured polysulfides to decrease their residence time.The cells with the optimal interlayer could deliver an excellent cyclability and rate performance.At the current densities of 0.2C(1C=1675 mA g-1)and 2C,the specific capacities are close to 1000 and 735 mA h g-1,respectively.Even after 200 cycles at 1C,the reversible specific capacity is still 772 mA h g-1.Such a synergistic catalytic interlayer with an enhanced conversion kinetic towards polysulfides provides a new approach for improving electrochemical performances of LSBs.