Synthesis Of Dual Carbon Protected Metal Sulfides As Anode Materials For Electrochemical Sodium Storage

Over the past three decades,lithium-ion batteries have dominated portable devices and electric vehicles as energy storage devices.The"rocking chair"energy storage mechanism of sodium-ion batteries are similar to that of lithium-ion batteries,and the crustal abundance of sodium element?2.3%?is thousands of times higher than that of lithium?0.0017%?.Therefore,more and more attention has been paid to the application value of sodium-ion batteries in large-scale energy storage fields such as stationary storage for renewable energies and distributed energy storage stations.However,sodium ions cannot effectively intercalate or react with commercial lithium anode materials such as graphite and silicon.Therefore,search for new anode materials with low cost,high capacity,long cycle life,and good rate capability is the key to practical sodium ion batteries.The metal sulfides based on the conversion reaction mechanism have the advantages of high capacity,good reversibility,and abundant sources.However,their sodium storage performances are still poor,mainly due to intrinstically low conductivity,huge volume change during the charge-discharge processes,and electrochemical shuttling of generated polysulfides.In this paper,the metal sulfide nanophases are limited to a dually protected structure constructed by amorphous carbon and low-dimensional conductive carbon by appropriate methods,so as to improve its structural stability and interfacial charge transfer,thus improving its electrochemical sodium storage performance.The main research contents and results are as follows:1.A method for the preparation of nitrogen-doped carbon-coated Fe1-xS nanophase/carbon nanotube composites(Fe_1-xS@NC/CNT)through in-situ transformation of sulfur-containing metal complexes is developed.The materials are prepared by ball-mill mixing of ferric diethyldithiocarbamate?III?with carbon nanotubes followed by annealing.The synthesis is simple and scalable.The electrochemical sodium storage test shows that the initial reversible specific capacity was 415 mAh g^-1 at a current density of 0.2 A g^-1.After 100 discharge-charge cycles,the reversible specific capacity is 408 mAh g^-1,representing a high capacity retention of 98.3%.Importantly,the reversible capacity of 233 mAh g^-1 can still be maintained even when the current density is up to 20 A g^-1,indicative of outstanding rate capability.Moreover,coupled with a Na3?VO0.5PO4?2F2@GO cathode,the full sodium ion battery exhibits a capacity retention rate of 96.5%after 80 cycles under the voltage window of 1.0-4.5 V.The excellent cycling stability and rate capability of the Fe1-xS composites protected by the dual carbon phase may be related to the construction of the three-dimensional conductive network composed by nitrogen-doped carbon shells and carbon nanotubes,which not only provides robust electron/ion transport pathways but also effectively buffers the volume effect of the active material during the electrochemical process.In addition,since most metal ions can interact with diethyl dithiocarbamate ligands to form stable complexes,this method is also applicable to preparing diverse other metal sulfide-based composites.2.A method for the preparation of carbon-coated Ni₃S₂/reduced graphene oxide?rGO?composites based on the“hydrothermal-adsorption-sulfide”conversion process is developed.Carbon nanolayers enriched with oxygen-containing functional groups are first deposited on the surface of GO via hydrothermal carbonization?HTC?of glucose,which subsequently capture nickel ions by coordination adsorption.Upon annealing,Ni₃S₂@HTC/rGO composites with a sandwich structure are obtained.The electrochemical sodium storage test shows that the initial reversible specific capacity is 387 mAh g^-1 at a current density of 0.2 A g^-1,and the specific capacity is maintained to be 281 mAh g^-1 after 400 cycles,with a tiny capacity decay rate of 0.08%per cycle.Since the HTC/rGO precursor has strong adsorption to many metal ions,this method is a universal method for the preparation of dual carbon-protected metal sulfide anode materials.