Preparation And Performance Study Of Core-shell Pyrite FeS₂ Composite For Sodium Ion Batteries

Metal sodium is rich in resources,it is low in price and is an environment-friendly material.Its physical and chemical properties are similar to metal lithium.The sodium-ion battery has the same working principle as the lithium ion battery,and its production equipment and process are similar.Therefore,sodium-ion battery is expected to become the next generation of secondary battery.However,the radius of Na⁺is larger than Li⁺,resulting in less electrode materials suitable for sodium storage.Wherefore finding and developing high-performance electrode materials for sodium storage has became the key to the research of sodium-ion battery.As an electrode material of sodium-ion battery,pyrite FeS₂ has the advantages of abundant resources,non-toxic environmental friendly and high theoretical capacity,which has became a hot spot in current research.However,during the cycling of the battery,its poor conductivity,excessive volume expansion and the shuttling of soluble polysulfide,which limit its application in sodium-ion battery.In order to solve these problems,FeS₂ was modified by various strategies to improve its electrochemical sodium storage performance in this paper.The research work includes the following parts:?1?FeS₂ regular octahedral particles with a side length of about 200 nm were prepared by a simple solvothermal reaction.And FeS₂ was carbon-coated with glucose as a carbon source.After high temperature annealing and pickling,the target product FeS₂@C is obtained,which has a very thin carbon-encapsulated?about 4 nm?structure of a regular octahedral core-shell porous nanoparticle.When tested as the sodium ion electrode material at a voltage window of0.8-2.8 V,the initial discharge capacity of FeS₂@C at 0.1C is 417 mA h g^-1.When teasted in different rates of 0.1C-10C,compared with the pure FeS₂,FeS₂@C exhibits excellent rate performance.After being activated by 0.3C,the batteries exhibit stable cycling performance at10C for 2000 cycles,and the capacity retention rate was about 89%.This indicates that the modified carbon layer can not only improve the conductivity of the material,but also alleviate the pulverization phenomenon during the cycles,further improving its electrochemical performance in the sodium ion battery.?2?Based on the above carbon modification,the carbon-coated product before annealing is further hydrothermally reacted with graphene.The high temperature annealing treatment of the material can not only reduce the graphene,but also improve the crystallinity of the material,and then pickling to get the target product FeS₂@rGO.It has a three-dimensional network core-shell structure.Test results at a wide voltage window of 0.01-2.9 V indicate:the electrochemical test of the material at a voltage window of 0.01-2.9 V is available.The initial discharge capacity at 0.1C is as high as 1090 mA h g^-1,and the composite exhibits good rate performance in the range of 0.1-4C.The reversible capacity rises back to 802 mA h g^-1 after returning to 0.1C.After 1000 cycles at 4C,the cycling curve of the FeS₂@rGO electrode eventually tends to a horizontal straight line,and the coulomb efficiency?CE?remains at about 99.9%except for the first cycle.Both the rate performance and the cycle performance are much higher than the pure FeS₂,which prove that the electrochemical performance of the FeS₂@rGO electrode is excellent.This indicates that the FeS₂@rGO electrode still has excellent electrochemical performance when subjected to a conversion reaction of 0.8 V or less,indicating that the double effect of graphene and carbon encapsulation can alleviate the problem of volume expansion of the material caused by the conversion reaction.?3?Taking FeS₂ regular octahedral particles as a hard template and ammonium molybdate as a molybdenum source,a one-pot hydrothermal reaction is carried out.A nano-sheet MoS₂-coated FeS₂@MoS₂ core-shell composite was obtained,and the thickness of the encapsulated nanosheet-like MoS₂ was about 50 nm.The electrochemical performance of the composite was tested in the voltage window of 0.8-2.8 V.The initial discharge capacity at 0.1C reaches to 424mA h g^-1.When teasted in 0.1-4C,the composite shows good rate performance.The reversible capacity is 228 mA h g^-1 after back to 1C.The reversible capacity after 600 cycles at 4C is 129mA h g^-1.Compared with the electrochemical performance of pure FeS₂,its specific capacity,rate performance and cycle stability are improved,which fully demonstrates that the nano-sheet MoS₂ modified composite FeS₂@MoS₂ has better compatibility with the electrolyte system,its material utilization and stability have also been improved.