Studies On The Controllable Preparation And Electrochemical Properties Of Non-stoichiometric Cobalt Sulfide Co_1-xS

The exploration and development of anode materials with high energy density and low cost are of great importance,owing to the widespread applications of lithium ion batteries(LIBs)in people's daily life.Anyway,as secondary energy storage devices,presently commercial LIBs cannot meet the needs of electric vehicles and smart grids for higher energy density and power density,and thus,research and development of high-performance LIB electrode materials are hot topics.Compared to the LIB anode of graphite with a theoretical specific capacity of 372 mAh g-1,transition metal sulfides have relatively high theoretical capacities(500-800 mAh g-1)when serving as anode materials for LIBs.In transition metal sulfides,cobalt sulfides with different stoichiometric ratios(Co1-xS,0 ?x ?0.5),CoS,CoS2,Co3S4 or Co9S8)are considered as potentially valuable anode materials because of their higher electrical conductivity.However,the actual capacity of cobalt sulfide is usually higher than its theoretical capacity as the negative electrode of LIBs,and the source of reversible lithium storage mechanism and additional specific capacity is not clearIn this thesis,the controlling synthesis and electrochemical performances of CoS2 spherical aggregates are first studied.On this basis,this dissertation focuses on the controllable preparation and electrochemical performances of non-stoichiometric ratio cobalt sulfide Co1-xS(x?0.19),and the main aspects of this thesis are listed as the following:1.Synthesis of CoS2 spherical aggregates or compositeCNT-CoS2,and Co1-xS@C mesoporous nanospheres or composite CNT-Co1-xS@CHydrothermal method is used to synthesize intermediate nanospherical CoS2 or its composite CNT-CoS2 using solid-state CoCl2·6H2O or CNT-Co as cobalt source,and therein additives of polyvinylpyrrolidone(PVP)serve as the nucleating and morphology-controlling agents of intermediate CoS2.Subsequently,heat-treatment of intermediates was used to obtain Co1-xS@C mesoporous nanospheres or their composite of CNT-Co1-xS@CShape and particle size of the four samples are relatively uniform,and the order of their average particles'size is CoS2>CNT-CoS2>Co1-xS@C>CNT-Co1-xS@C Particles of intermediate CoS2(or CoS2 in composite CNT-CoS2)are actually spherical aggregates,being composed of tiny building blocks manipulated/protected by the surface-anchored PVP molecules.Composite Co1-xS@C has an integral structure,and the contamination carbon in Co1-xS@C mesoporous nanospheres(or in composite CNT-Co1-xS@C)originates from thermal decomposition of residual PVP molecules.Mesoporous nature of Co1-xS@C or CNT-Co1-xS@C comes from both the transformation of CoS2 to Co1-xS and the degradation of PVP to contamination carbon 2.Electrochemical reaction mechanism of Co1-xS@CAccording to literature reports,the conversion reaction of CNT-Co1-xS@C is Co1-xS+2Li(?)Li2S+1-xCo,and based on the two-electron transfer mechanism of Com+(?)Co0(m=2),the theoretical capacity of Co1-xS is 672 mAh g-1 at x?0.19 Herein,the PVP-derived carbons and/or the doped CNTs functionalize to trap electrolyte-soluble intermediate lithium polysulfides(Li2Sn,3 ?n?8)for the reversible reaction of Sn2-(?)S2-.These are the reasons why in each cycle the reversible specific capacity of CNT-Co1-xS@C,calculated according to the mass of component Co1-xS or to the overall quality of composite,is always higher than the theoretical capacity of phase-pure Co1-xS.Therefore,the reversible lithium storage mechanism of Co1-xS@C may be explained by both Com+(?)Co0 and Sn2-(?)S2-3.Performance-improved mechanism of CNT-Co1-xS@CCompared to anode Co1-xS@C,CNT-Co1-xS@C composite electrode presents an enhancement in lithium-storage capability,and the performance-improved mechanism may be assigned to the following aspects:(1)mesoporous nanostructure of Co1-xS@C and/or the doping of CNTs can buffer volume changes and facilitates a closer contact of active substance with electrolyte to cut down the pathway of Li+ transfer;(2)the doping of CNTs modifies the conductivities of electrons and Li+-ions,and promotes the electrochemical reaction activity and reversibility of CNT-Co1-xS@C by capturing a more amount of intermediate Li2Sn(3?n?8)within the working electrode;(3)contribution of interfacial lithium storage to the high capacity of CNT-Co1-xS@C composite is obvious,especially for the gradual increase trend of interfacial lithium storage capacity with the increasing discharge-charge cycle number.