Synthesis,Characterization And Li-storage Performance Of Cobalt-based Carbonates

Cost-efficiency transition metal carbonates?TMCs?have attracted considerable attentions as an anode material for lithium-ion batteries?LIBs?thanks to their large capacities and easy preparation.However,the capacity of the battery is difficult to reach the theoretical capacity practically,which is limited by the structure and conductivity of the electrode material.Therefore,this thesis focuses on designing reasonable microstructure and hybridizing with graphene to improve the structural stability and electronic conductivity of cobalt-based carbonates,which finally improve the electrochemical properties,and its lithium storage mechanism and electrochemical behavior have been deeply discussed and studied.The main research contents are shown as below:?1?Monodisperse CoCO₃ sub-microspheroids assembled from ultra-thin nanosheet was synthesized by one-step hydrothermal method and the mechanism of lithium storage was also studied.At the current density of 500 mA g^-1,the initial Coulomb efficiency was as high as 75.2%,and after 500 cycles,the discharge specific capacity was 1475 mAh g^-1 without attenuation,and the capacity retention rate reached 138%.When the current density reached2000 mA g^-1,its reversible specific capacity remained 663 mAh g^-1.We also used in-situ XRD,ex-situ XRD and XPS to analyze the lithium storage mechanism of CoCO₃ sub-microspheroids.Results showed that there are not only the previously reported transform reaction of Co²⁺and CO₃²⁺?CoCO₃+2Li?Co+Li₂CO₃,Li₂CO₃+5Li?3Li₂O+0.5Li₂C₂?,but also reversible transformation between Co²⁺and Co³⁺during the charge and discharge.?2?Three kinds of Co_2/3Ni_1/3CO₃ with different morphologies were synthesized by adjusting the content of NH₄HCO₃ by simple hydrothermal method.The electrochemical analysis showed that the Co_2/3Ni_1/3CO₃ spheres?CNCO-20?exhibited better cycling stability and rate performance than Co_2/3Ni_1/3CO₃ spindles?CNCO-15?and Co_2/3Ni_1/3CO₃ cubes?CNCO-30?thanks to finer substructure,smaller size and larger specific surface area.Especially,after 500 cycles under the current density of 1000 mA g^-1,CNCO-20 still had the reversible capacity of 740.9 mAh g^-1,which is much larger than that of the other two samples.In addition,the reversible specific capacity of CNCO-20 was 827 mAh g^-1 at the high current density of 2000 mA g^-1,which is better than other reported carbonate materials.?3?CoCO₃ microrods and CoCO₃/rGO composites were respectively synthesized via one-step hydrothermal method without precipitant.In the hydrothermal process,reduced GO uniformly encapsulated CoCO₃,which limited the deposition of CoCO₃,finally the smaller spindle CoCO₃/rGO composites were obtained.The rGO can effectively improve the conductivity,prevent the agglomeration of electrode materials,thereby improving the cycling stability and rate performance.At the current density of 0.5 A g^-1,CoCO₃/rGO composites showed better cycling stability than CoCO₃ microrod and higher capacity(the reversible specific capacity of 924.6 mAh g^-1)after 600 cycles.Especially,when the current density is 2 A g^-1,the average specific capacity of CoCO₃/rGO composites was 803 mAh g^-1,which is also significantly better than CoCO₃.This is due to the conductive network provided by rGO,which enables faster transfer of lithium ions and electrons.