Design And Synthesis Of Transition Metal Compounds And Their Enhanced Performances For Lithium Ion Batteries

Resulting from the high energy density,low self-discharge rate,long life span,safe and pollution-free,lithium ion batteries have dominated the market of various batteries,such as nickel-metal hydride battery and lead-acid cell,and they are the alternative devices to fossil energy.Among the large number of negative materials,the transition metal compounds,which can reversibly react with Li metal,addressing the reversible reaction as "conversion reaction",have attracted researchers' attention,because that the theoretical capacity of transition metal compounds through conversion reaction can be extraordinary high?eg 1232 mAh/g for MnO2,but 372 mAh/g for graphite?,and the unique excess of the theoretical capacity phenomenon will occur.Nevertheless,the low conductivity and the severe capacity fading,result in inferior rate performance and cycling performance,which hamper the practical application of the transition metal oxides in the field of lithium ion batteries.In this thesis,we elucidated the unique excess of the theoretical capacity phenomenon in detail in terms of the two proposed theories in state-of-art,namely,electrolyte decomposition and interfacial storage.We selectively prepared the hierarchical structured MnCO₃ microdumbbells and combined the Pechini method with the participation of surfactant to synthesize Ca₉Co₁₂O₂₈ nanobricks,proposed a conceivable crystal growth mechanism,and systematically investigated their electrochemical performances.The main works were as follows:1.Hierarchical architectured MnCO₃ microdumbbells:facile synthesis and enhanced performance for lithium ion batteriesWe synthesized the hierarchical structured MnCO₃ and subsequent lamellar architectured MnCO₃ via refluxing and succeeding heating under low temperature.The hierarchical structtured MnCO₃,which was composed of a large number of nanoparticles,combined the features of nanosized materials with the features of microsized materials,namely,shorting the diffusion distance,buffering the volume change,and sustaining the structure of the electrode materials,which resulting from the a great deal of channels among the nanoparticles.The reasonable mechanism was proposed according to the existing experiment data,and the PVP reactant played a vital role in terms of the mechanism..The electrochemical performances showed that the hierarchical structured MnCO₃ released a capacity of 583,463,397,399,262 mAh/g at a series of current rate,and a reverted capacity of 697 mAh/g at 0.2 C,which exhibited an excellent rate performance.2.The synthesis of Ca₉Co₁₂O₂₈ nanobricks and the investigation of their electrochemical performances as negative materialswe combined the Pechini method with the participation of surfactant to synthesize Ca₉Co₁₂O₂₈ nanobricks.The particles' size would be influenced with the help of surfactant.Meanwhile,the materials exposed active facets with suitable percentage could provide fast and efficient delithiation/lithiation processes,which resulted in superior reaction kinetic and rate performance.The crystallization phase of as-obtained materials was characterized by XRD,and the SEM,TEM coupled with the EDX results illustrated the morphologies and structure of Ca₉Co₁₂O₂₈ nanobricks.In addition,we investigated the electrochemical performances in detail,exhibiting inferior rate performance and cycling performance.The Ca₉Co₁₂O₂₈/Li half cell delivered a stable capacity of about 210 mAh/g after 200 cycles at a current rate of 3 C.we proposed a reasonable mechanism based on the experimental data,which shed light on enriching the growth mechanism of Ca₉Co₁₂O₂₈,and improving the electrochemical performances for lithium ion batteries.