Construction And Sodium Storage Properties Of Cobalt-based Sulfide Heterostructures

With the wide application of lithium ion batteries（LIBs）in the field of electric vehicles,the disadvantages of insufficient resources and high cost have gradually emerged,which has prompted researchers to find a substitute for lithium as soon as possible.Considering the physical and chemical properties similar to lithium,the earth’s rich and low-cost sodium is back in the researchers’sight.However,the commercially mature LIBs materials have not achieved the expected results after being used in sodium ion batteries（SIBs）.Therefore,it is urgent to develop new material systems more suitable for SIBs according to the characteristics of sodium ions.Cobalt-based sulfides,due to their high cost performance,high safety,abundant natural reserves and high theoretical capacity,become a research hotspot.However,its poor cycle,rate performance and low reversible capacity at high current density have become a major problem.The modification of cobalt-based sulfides by nanocrystallization,defect engineering and other modification methods can greatly improve their sodium storage performance.In this paper,cobalt-based sulfides are taken as the research object,and cobalt-based sulfide heterostructures with nanoscale distribution of different components are constructed to improve their sodium storage performance.The reasons for the improvement of electrochemical performance are further explored by means of reaction kinetics analysis and theoretical simulation calculation.Through a two-step hydrothermal method,the precursor cobalt-iron layered double hydroxide（Co Fe-LDH）was first synthesized,and then the Co S_1.097/Fe S micro-intercalated layered heterostructure was obtained.The as-prepared sample exhibits a hydrotalcite-like intercalated structure and heterogeneous composition,which boosts the electrochemical reaction kinetics by shortening the transport distance,unifying move direction and accelerating mobility of Na⁺.Meanwhile,benefited from such advantages as additional active sites at surfaces,enhanced internal structure stability and relaxed stress from volume expansion etc.,SIBs with as-prepared Co S_1.097/Fe S heterojunction anodes shows excellent sodium storage performances,including high rate capability(564.1 m Ah g^-1at 10 A g^-1),good the initial coulombic efficiency(80.65%at 1 A g^-1)as well as great cyclability(543.9 m Ah g^-1at 2 A g^-1after 450 cycles).Theoretical calculations based on the first principles are carried out to clarify the possible sodium storage mechanisms.V₃S₄was introduced into Co S_1.097by one-step hydrothermal method to synthesize two-dimensional nanoflower-like Co S_1.097/V₃S₄heterostructure.In the well-designed nanoscale double active component heterostructure,the doping of V₃S₄with excellent structural stability improves the volume expansion of pure Co S_1.097material.At the same time,the synergistic effect between the two components of the heterostructure makes the Co S_1.097/V₃S₄composite has better performance than the two single component sulfides.In addition,due to the charge rearrangement at the heterostructure interface due to the doping of vanadium sulfide and the formation of self-built electric field at the interface,the Co S_1.097/V₃S₄heterostructure electrode exhibits fast Na⁺reaction kinetics,low ion diffusion barrier and excellent structural stability,thereby improving its Na⁺storage capacity.Based on this,the Co S_1.097/V₃S₄heterostructure electrode exhibits excellent electrochemical performance when used as an anode material for sodium ion batteries.It provides a high initial discharge capacity of 833.8m Ah g^-1at a current density of 0.5 A g^-1.It is worth mentioning that the reversible capacity of 330.8 m Ah g^-1can be maintained after 1000 cycles at a high current density of 5 A g^-1,exhibiting a favorable cycling stability.