Nanostructure Designing Of FeS₂ Electrode For Better Sodium-ion Batteries

As the development of commercial lithium ion batteries,the price of lithium raw materials has been rising year by year due to the shortage of lithium resources and uneven geographical distribution.Thus,it's urgently necessary to develop new energy storage systems to satisfy efficient energy storage and utilization.Sodium ion batteries have the advantages of low cost,non-toxicity and high safety,so they have attracted the attention of researchers.However,due to the large radius of Na⁺,electrode materials that adapt to the characteristics of sodium ion batteries must be developed to realize their excellent energy storage potential.FeS₂ not only has a high theoretical specific capacity(894 m Ah/g),but also is environmentally friendly and cheap,which is considered as a very promising sodium ion electrode material.However,FeS₂ features poor electrical conductivity and severe structural collapse during redox reaction with Na⁺,resulting in low reversible capacity and poor cycling stability in sodium ion batteries.In view of the above problems,FeS₂ electrode with notable microstructure were designed,which could improve the diffusion efficiency of Na⁺,increase the specific surface area and buffer volume expansion.In addition,by introducing defects,dual phase interfaces,and element doping,the electrode conductivity is improved and the activation sites are increased.The specific content includes the following aspects:(1)One-dimensional FeS₂@C nanorods were prepared by a hydrothermal process and annealing process.The carbon coating on the outer surface can not only improve the conductivity of the material,but also buffer the volume expansion caused by Na⁺insertion and ensure the structural integrity.The electrochemical performance of FeS₂@C in different electrolyte were investigated by electrochemical test.FeS₂@C obtained a cycle life of 9000 cycles,a reversible specific capacity of 506.9 m Ah/g at 500 m A/g,and a high-power performance of140 m Ah/g at 20A/g.The energy storage mechanism of FeS₂@C in SIBs is analyzed by CV curves,the proportion of capacitance-controlled capacity is calculated quantitatively,and it is concluded that at the sweep speed of 1.5 m V/s,the contribution rate of capacitance-controlled capacity to the total capacity is up to 81%.(2)F-MIL nanorods,a metal organic skeleton material,were prepared by hydrothermal method.In the subsequent vulcanization annealing process,sea-cucumber like FeS₂@C nanorods finally were formed.The structure is wrapped by a carbon shell with tiny FeS₂ nanoparticles inside,which can effectively shorten the Na⁺transmission distance,and FeS₂@C nanosheets are distributed on the outer surface,increasing the contact area between electrode materials and electrolyte.The carbon shell can not only improve the electrical conductivity of FeS₂@C,but also prevent structure collapse and facilitate the formation of stable SEI film.The FeS₂@C,like a sea-cucumber,maintained a specific capacity of160 m Ah/g after circulating for 10000 cycles at a high current density of 20 A/g,demonstrating excellent rate performance and cycle life.After kinetic analysis of the FeS₂@C electrode in the shape of a sea cucumber,it is proved that,at the scanning speed of 2 m V/s,the contribution rate of capacitance-controlled capacity is as high as 91%.(3)On the basis of the sea-cucumber like FeS₂@C nanorods electrode,the redox graphene(GO)was added to form the composite structure of FeS₂@C/G.In this structure,sea-cucumber like FeS₂@C is wrapped in layered GO.The flexible graphene can not only avoid the agglomeration of FeS₂@C nanorods,but also increase the contact area between electrode material and electrolyte to accelerate Na⁺/e^-conduction.The composite structure combines the advantages of the 0D/1D/2D structure to improve the reaction kenitic of FeS₂ and Na⁺,thus improving the capacity and rate performance of the sodium ion battery.FeS₂@C/G was tested as the electrode of the SIBs.When the cut-off voltage was0.01-3V,the material maintained a specific capacity of 615.1 m Ah/g after 300cycles under the current intensity of 0.5 A/g,with capacity retention of 83.3%.Specific capacity of 479.4 m Ah/g was obtained after 2000 cycles under the current intensity of 5 A/g.According to the kinetic analysis,the contribution ratio of the capacitance-controlled sodium storage mechanism to the total capacity can reach87.3%at the scanning speed of 10 m V/s.(4)Fe³⁺/PVP fibers were prepared by means of electrospinning,and then were annealed to form a structure of porous,N,S co-doped carbon fiber skeleton combined with FeS₂.In this structure,FeS₂ is distributed in the interior of carbon fiber in the form of nano particles,and embedded in the exterior surface of carbon fiber in the form of nanosheets.N,S doped carbon skeleton accelerates Na⁺/e^-transport,and can effectively buffer the volume changes caused by Na⁺diffusion.FeS₂ nanoparticles can shorten the transport distance of Na⁺and improve the reaction rate of FeS₂ and Na⁺.For the FeS₂@CF-NS electrode,at the current intensity of 1 A/g,the specific capacity of 637.1 m Ah/g is still maintained after400 cycles.Under the current intensity of 5 A/g,the multiplier performance of431.1 m Ah/g was obtained.Kinetic analysis shows that the composite structure is beneficial to the capacitive mechanism sodium storage,and the capacitance contribution rate of the capacitive control can reach 92.7%.The FeS₂@CF-NS negative electrode and Na₃V₂(PO₄)₃ positive electrode were combined for full battery performance test.The full battery obtained a high specific capacity of561.1 m Ah/g at 1 A/g,and was capable of stable charge and discharge for 5000cycles at 5 A/g current intensity,and maintained a specific capacity of 338.6m Ah/g.(5)With butyltitanate as titanium source,a composite structure containing anatase TiO₂ and rutile TiO₂ was formed,covering FeS₂ nanorods.The coating layer of biphase TiO₂ in this structure is more c a core-shell structure,FeS₂nanoparticles encapsulated in biphase TiO₂ shell(FeS₂@TiO₂),is developed towards the improvement of sodium storage.The diphase TiO₂ coating supplies abundant anatase/rutile interface and oxygen vacancies which will enhance the charge transfer,and avoid severe volume variation of FeS₂ caused by the Na⁺insertion.The FeS₂ core will deliver high theoretical capacity through its conversion reaction mechanism.Consequently,the FeS₂@TiO₂ nanorods display notable performance as anode for SIBs including long-term cycling performance(637.8 m Ah/g at 0.2 A/g after 300 cycles,374.9 m Ah/g at 5.0 A/g after 600 cycles)and outstanding rate capability(222.2 m Ah/g at 10 A/g).Furthermore,the synthesized FeS₂@TiO₂ demonstrates significant capacitive Na⁺storage behavior which accounts for 90.7%of the Na⁺storage,and efficiently boosts the rate capability.(6)Sn O₂ was hydrothermal coated on the outer surface of the?-Fe OOH nanorods,which were then transformed into a composite structure of two-dimentional Sn S₂ coated with FeS₂ through high-temperature vulcanization.The Sn S₂ has a high theoretical specific capacity of 1136 m Ah/g.In addition,the two-dimentional Sn S₂ could increase the connecting area between electrode and electrolyte,which could facilitate the reversible embedding/stripping of Na⁺.As the electrode material of sodium ion battery,FeS₂@Sn S₂ maintains the specific capacity of 526.9 m Ah/g after 300 cycles at 1 A/g,and exhibits the multiplier performance of 347.0 m Ah/g at 10 A/g.