Modification And Lithiation Properties Of Cobalt-/Copper-Sulfides

Since the commercialization of the lithium ion batteries(LIBs)in 1990s,the LIBs have replaced the Ni/Cr and Ni/H batteries,becoming the common energy storage system for portable electronic devices.With the increasing energy demand of electrical vehicles and large-scale energy storage systems,the research and development of LIBs with high energy/power density become urgently important.The energy/power density of LIBs is mainly determined by the intrinsic properties of cathode/anode materials.Among these,one of the commercial anode materials is graphite,which shows advantages of abundance,environmental-friendliness and high cycling stability.Nevertheless,the biggest disadvantage of graphite is the easy formation of lithium dendrites under large current density cycling,which greatly prevents it from being the anode material for the high-performance LIBs.Therefore,the development and investigation of new-type anode materials with high energy density and high safety show much more significance.In recent years,the transition metal sulfides(TMSs)attract widely attentions due to the merits of high theoretical specific capacities,high lithiation potential,low-cost,environmental-friendliness and so on.Although displaying such advantages,the TMSs still face many problems,impeding their further use as anode materials in LIBs.The problems such as slow lithium ions diffusion in the sulfides,large volume change,low electronic conductivity of the reduction product(Li2S)and the dissolution/diffusion of sulfur species cause the irreversible capacities and low coulombic efficiency.Focusing on the aforementioned problems,this thesis proposes different strategies such as nano-structure,carbon coating and complexing to improve the electrochemical properties of TMSs.Besides,the displacement reaction mechanism of Cu2S is well investigated by synchrotron radiation-based operando analysis.The research content of this thesis is shown below:1.The first part is about the improvement in the lithiation performances of Co-S compounds(CoS and Co9S8).Initially,the electrochemical performances of Co9S8 are enhanced via the hierarchical micro/nano structure.This work takes advantage of the H2S,from the decomposition of thiourea,to sulfidate the Co3O4 to obtain cubic Co9S8(C-Co9S8)and spherical Co9S8(S-Co9S8)with high phase purity.To find the relation between stability and the structure,the electrochemical performances are tested.The results show that although both exhibit high reversible capacities firstly,the capacities quickly fade after 50 cycles,in which the capacity retention of S-Co9S8 is superior to the other.After analyzing the morphologies of both cases after 300 cycles,it can be found that the spherical structure is maintained well while the cubic structure is significantly destroyed.The electrochemical impedance spectra also indicate the larger impedance of C-Co9S8 after cycling.To further improve the performance of Co-S compounds,the physical confinement and chemical absorption strategy is adopted to optimize the lithiation properties of Co-S compounds.The mesoporous CoS microspheres and Co9S8 plum-like microspheres are obtained via a spray-drying and calcination method.The cobalt sulfide nanoparticles are embedded in the mesoporous amorphous carbon matrix,which are penetrated by numerous twisted CNTs.Besides,the C-S bonds are formed,which can effectively adsorb the sulfur intermediates.The electrochemical tests show excellent cycling stability and rate capability in both cases,which is ascribed to:(a)the side reactions with electrolytes and loss of sulfur intermediates are inhibited by the protection of carbon matrix and the C-S bonds,respectively;(b)the volume change is well addressed by the mechanical strong carbon matrix;(c)the twisted CNT conductive network functions as the effective electron transport/transfer network,greatly improving the utilization of active materials inside and the rate capability.2.The second part mainly focuses on the modification of Cu-S compounds(CuS and Cu2S)and investigation in the mechanism of Cu2S in carbonate-/ether-based electrolytes.Firstly,the electrochemical performances of CuS are improved by introducing the three-dimensional CNTs conductive network.This work proposes a facile microwave-assisted method to construct CuS/CNT composite,in which the intercalated CNTs penetrate the interior of CuS nanoparticles and act as a 3D conductive skeleton.The electrochemical tests show the greatly improved lithiation properties of CuS/CNT,which is related to reasons:(a)the interior 3D CNTs skeleton is beneficial to the utilization of CuS,effectively decreasing the capacity loss in the first few cycles;(b)the 3D CNTs skeleton,transporting/transferring electrons,is deemed as the active sites,which makes the dissociative active material re-utilize;(c)the reaction kinetics is improved by introducing the 3D CNTs skeleton.Secondly,the lithiation properties of Cu2-xS are improved by combining the nanostructure and the coating strategy.Via a self-template method,different coating Cu2-xS hollow spheres are obtained.All composites display excellent cycling stability,especially the carbon coating one.The improved performances are ascribed to the effective inhibition of side reactions and loss of active material by the coating layer,the well accommodation of volume expansion.Finally,the unique displacement reaction mechanism and the compatibility with ether/carbonate-based electrolytes of Cu2S are well investigated via the synchrotron radiation-based XRD and XAS operando analysis techniques,trying to figure out the relation between capacity loss and the phase change of Cu2S.The results show the different reaction pathways of Cu2S in carbonate and ether electrolytes.In carbonate electrolytes,the Cu2S suffers from a more irreversible reaction way,leading to capacity loss.On the contrary,the reaction of Cu2S in the ether electrolytes is more reversible,insuring the excellent cycling stability.Based on the result,a modified strategy is adopted to separate the Cu2S and carbonate electrolytes,which also maintains the highly reversible reaction pathway.Thus,the compatibility with carbonate electrolytes is solved and make it possible for Cu2S to match commercial LiCoO2 cathode to assemble a full cell.3.The final part proposes an ultrafast solution combustion method to prepare series of TMSs and TMSs/CNT.Different from the conventional combustion method,this new strategy aviods the use of inert gas as the protection gas,in which sulfides and their carbon composites can be easily obtained in several minutes.The electrochemical measurement results show the excellent cycling stability and rate capability of TMSs/CNT,which is similar with that of CuS/CNT above.