Synthesis And Sodium Storage Properties Of Bimetallic Sulfide Composites

As new secondary batteries,lithium-ion batteries?LIBs? and sodium-ion batteries?SIBs? have attracted wide attention in the field of new energy storage devices in recent years.Although LIBs have many advantages,lithium resources were facing serious resource shortages and uneven distribution on the earth,which leads to the high price of lithium metal.And,SIBs have the same electrochemical mechanism as LIBs.SIBs are considered as one of the most promising alternatives to LIBs in various large-scale energy storage applications because of their abundant sodium reserves,low cost,environmental friendly and high system security.However,the development of sodium ion batteries is restricted by the volume expansion,slow reaction kinetics and low electrochemical activity caused by large radius of sodium ions.Therefore,researchers urgently need to develop new negative electrode materials for SIBs with high specific capacity and stable cycle performance.Bimetallic sulfides have been widely used in the study of negative electrode materials for sodium ion batteries due to their abundant stoichiometric ratio,low activation energy of electron transfer and good conductivity.However,bimetallic sulfides are also affected by volume expansion and shrinkage during charging and discharging,resulting in low cycle life and rate performance.In this paper,the electrochemical properties of the composites were improved by compounding CoNi₂S₄ with reduced graphene oxide?rGO? and MOF-74 as precursor to carbonize,respectively.The detailed research contents were as follows:1.CoNi₂S₄ were successfully grown on rGO by a simple one-pot solvothermal process,and applied to anode materials for SIBs for the first time.The structural features and components of the obtained CoNi₂S₄/rGO were characterized by a series of testing methods.The electrochemical performances testing results suggested that the sodium storage performances of the CoNi₂S₄/rGO nanocomposites were enhanced dramatically compared with pure Co Ni₂S4.The CoNi₂S₄/rGO anode expresses an excellent reversible capacity of 430 mAh g^-1after 50 cycles at 100 mA g^-1.The CoNi₂S₄/rGO electrode still shows a reversible capacity of 521 mAh g^-1when the current density grows to 1000 mA g^-1.The enhancement of electrochemical performances is due to the addition of rGO,which can effectively buffer volume variations and improve electrical conductivity during charge-discharge process.The CoNi₂S₄/rGO nanocomposites can be a promising anode material for SIBs,which provides a research idea for the application of CoNi₂S₄ related composites in SIBs.2.In this work,CoNi₂S₄@C was successfully prepared with Co/Ni-MOF-74 as template.MOF-74 is a class of isostructured MOFs compound based on 2,5-dihydroxyterephthalic acid?H4Dobdc?.H4Dobdc as connectors connects metal nodes,forming an open hexagonal channel.After heat treatment,porous carbon structure will be formed.The SEM and TEM images indicate that CoNi₂S₄ nanoparticles with a diameter of about 8 nm were uniformly distributed in the porous carbon structure.Compared with the pure CoNi₂S₄,CoNi₂S₄@C exhibits excellent electrochemical reversibility as an anode material for sodium ion batteries?SIBs?.The specific capacity of the CoNi₂S₄@C electrode could be maintained 438 mAh g^-1after 50 cycles at 100 mA g^-1.After the CoNi₂S₄@C electrodes were charged/discharged at the different current density of 50,100,200,500,800,1000,2000 mA g^-1,the specific capacity could be restored to the original level when the current density returns to 50 mA g^-1.This is due to the unique porous carbon structure in the composite material provides more sites for the storage of Na⁺and reduces the effect of volume expansion.3.A solvothermal method was designed to prepare CoNi₂S₄/C composites derived from MOF-74 as excellent anode electrode materials for sodium ion batteries.Compared with CoNi₂S₄,CoNi₂S₄/C has higher specific capacity,better cycle stability and better rate performance.In addition,compared with gas phase vulcanization by calcination,solvothermal method can reduce the instability of material formation and reduce the introduction of impurities.This method can be applied to the preparation of other metal composites using MOFs as precursor,which has a broad application prospect in electrochemical devices such as sodium ion battery,lithium ion battery and supercapacitor.