Design And Sodium Storage Properties Of Bimetallic Sulfides Derived From MOFs

Sodium ion batteries(SIBs)have been considered as potential substitute for lithium ion batteries(LIBs),for the sodium possessing the advantages of abundant reserves in nature as well as cost-effective in exploitation,furtherly,can reduce the development costs and obtain higher safety in sodium ion batteries.Which reflecting an ever more boundless development potential and economic effectiveness in the field of large-scale energy storage.In order to further meet the demands of market,in recent years,exploring and developing electrode materials with high energy density and long cycle characteristics,has become one of the research hotspots for field of energy storage.Transition metal sulfide(TMS),owing to alloy-conversion mechanism,as anode material with a high theoretical specific capacity,which can form stable covalent bonds during sodium storage,thereby achieving high strength combination with sodium.However,due to the poor electrical conductivity and structural stability,as well as the distinct agglomeration and volume expansion,which hindered the further development and application of TMS in sodium ion batteries.Therefore,exploring a composite structure electrode with high structural stability and high electrical conductivity,so that can provide better ion transmission path and electron diffusion rate,then achieve a high discharge specific capacity,becomes one of research hotpot nowadays.In addition,metal-organic framework(MOF),a periodic network crystal formed by self-assembly of metal ions and organic functional groups,can provide metal ion ligands and highstrength organic structural framework,as well as good electrical conductivity.In this paper,we use different metal organic frameworks as self-templates,synergistically introduces high specific capacity transition metal sulfides-SnS2 and SnS,designed and synthesized bimetallic sulfides with different hierarchical structures by a simple hydrothermal method,furtherly the sodium storage ability and diffusion dynamics was tested and analyzed.The main research contents are shown as follows:(1)Using the typical ZIF-8 as the raw material,first,carbon layer was covered and stabled outside the polyhedral structure by polymerizing the phenolic resin.Drived by the low-temperature vulcanization strategy,Zn-based MOF was vulcanized to the precursor of ZnS-NC.Afterwards,tin and sulfur source were introduced for one-step hydrothermal reaction,to obtain ZnS-NC/SnS2 nanocage composite material.Thanks to the unique MOF-derived nanocage morphology and carbon-coated structure,this composite material exhibits good structural stability and electrical conductivity as anode electrode for SIBs,effectively improving the sodium storage performance of SnS2.At a current density of 200 mA g-1,the specific discharge and charge capacities of the ZnS-NC/SnS2 electrode can reach 894 mAh g-1 and 693 mAh g-1,respectively,and the initial coulomb efficiency was 75%.Especially at current density of 1 A g-1,which still maintain the reversible specific capacity of 375.7 mAh g-1 after 500 cycles,and the capacity decay rate per cycle is less than 0.08%,showing excellent structural stability and sodium storage performance.Based on excellent electrochemical performance,furtherly we performed diffusion kinetic analysis by cyclic voltammetry and galvanostatic intermittent titration technique(GITT).The bimetallic sulfide electrode shows as high as 96.11%pseudo-capacitance contribution under the scannning rate of 2 mV s-1,and apparent sodium ion diffusion coefficient is significantly better than that of pure sulfide,which strongly explained the reason of excellent electrochemical performance.(1)Using urchin-like ZIF-67 hollow nanotube as a precursor,by introducing sulfur and tin source synergistically,urchin-shaped Co4S3-NC/SnS hollow nanotubes were successfully constructed through one-step method.This kind of Co4S3-NC/SnS primary nanomaterials are oriented and assembled into hollow fibers with a diameter of 120 nm.When applied as anode materials of SIBs.The special structure can significantly increase the contact surface area of the electrolyte and electrode materials,further promote the apparent diffusion rate of Na+and shorten the electron transport channels.More importantly,can effectively buffer the huge volume expansion of the alloy-conversion reaction.Therefore,the bimetallic composite Co4S3-NC/SnS shows good sodium storage performance.When at the current density of 1 A g-1,the bimetallic sulfide has a reversible specific discharge capacity of 354.9 mAh g-1 after 100 cycles.It was significantly higher than the sodium storage performance of Co4S3/SnS nanofiber composites,which result from the high electronic conductivity and ion transport rate,provided by the introduction of ZIF-67 with high nitrogen content.What’s more,the Co atoms in ZIF-67 can improve more active sites and catalyze the effects of alloyconversion reaction.All of the above further promote the excellent sodium storage performance of Co4S3-NC/SnS bimetallic sulfide.