Preparation and Lithium Storage Properties Regulation of Transition Metai Oxides Composites

Author:Hou Chuan Xin

Supervisor:dang feng


Degree Year:2019





The global environmental issues and energy shortage have led to continuous demands for efficient green energy and energy storage devices to utilize it.Lithium ion batteries(LIBs)have been widely used as energy storages devices for portable electronic devices and electrical vehicles due to its advantage of no memory effect,high energy density,little self-discharge and environmental-friendly.However,the conventional graphite anode with a low theoretical capacity of 372 mAh g-1 is far from satisfying the tremendous ever-growing application.Designing and meliorating anode materials with higher capacities are required for next generation LIBs.Transition metal oxides have been investigated as attractive anode electrodes due to their high capacity,high abundance,eco-friendliness and loderate price.Nevertheless,their wider practical applications are challenged by three main aspects,which include relative low conductivity,huge volume changes and capacity changes during cycling performance.To address the mentioned issues,the electrochemical performance are enhanced by introducing oxygen vacancies,doping elements,regulating the morphology,optimizing the composition,compounding with conductive carbon materials and constructing heterogeneous structure in our work.Meanwhile,the local built-in electric field,the thermodynamic calculation of phase diagram,density functional theory(DFT)and density of sates(DOS)calculations were applied to clarify the mechanism of the improved electrochemical performance.Our work will shed light on improving electrochemical properties of all kinds for other transition metal oxides.The specific content of our research is listed as followings:(1)High performance Mno@C microcages with hierarchical structure as anode electrode for LIBs.A bio-gel derived process was designed to obtain unique multi-structured MnO@C microcages as anode for LIBs.The MnO@C microcage exhibites a double-carbon coating hierarchical structure,in which the MnO units with a micrometer size are coated with a carbon network inside the thick carbon shell.This unique structure is forecast to offer multiple advantages to the microscale MnO units during the Li-ion insertion/extraction process.The MnO@C microcages with micrometer-sized MnO units can enable remarkable high specific capacity and ultra-long stable cycling performance as an anode for LIBs without addition of conductivity additive(2)The carbon shell and electrochemical performance of MnO@C microcages were tuned by the addition of graphene.The Mn0@C-G was built by the addition of graphene on the basis of the work(1).The carbon shell and electrochemical performance of MnO@C were optimized by the content of graphene at the same time We found that the thinner thickness of the carbon shell tuned by graphene provide more rapid and efficient electron transport tunnels of Li ions passing though the carbon shell,which is favorable for improving the surface-controlled capacitance according to the results of CV tests at various scan rates.The capacity changes during cycling are relieved by the addition of graphene(3)Insight of optimized ultra-stable long-life cycling properties for grape-shell MnO-Ni@C composites with superior Li storage performance.A Ni protected strategy was introduced to stable the long cycle performance of MnO@C composites with superior Li storage properties,a grape-shell structured MnO-Ni@C composites was fabricated.The unique structure with double carbon decoration and embedded Ni nanoparticles not only relieves the huge volume changes during cycling process,but also facilitates the diffusion of Li+ and electrons.The ultra-stable long-life cycling performance and superior electrochemical properties are thus obtained Electrochemical analysis indicates that metallic Ni nanoparticles also involve in the charge/discharge process,and the oxidation of Ni protects the Mn2+ from being oxidized to higher valence states.The thermodynamic calculations were applied to clarify the mechanism of the ultra-stable long-life cycle performance.The grape-shell structured MnO/Ni-C composites were simplified to a Ni-Mn-O system.The thermodynamic simulation proved that Ni-Mn-O system can provide a large buffer composition region to stable the further oxidation of MnO under electrochemical process,and s phase diagram is supposed.(4)Agaric-like nanocomposites of content optimized porous carbon decorated with MoO2 nanoparticles for LIBs.The hybrid versions of agaric-like carbon matrix and nanosized MoO2 electrodes(MoO2/C)are prepared by the bio-inspired method accompanied with carbonization process.The carbon matrix originated from the agar and the optimum carbon concentration in the MoO2/C composites is explored.With the specially designed unique structure and the optimized carbon content,the as-prepared MoO2/C hybrids displayed excellent battery behaviors,presenting a beyond theoretical capacity,a stable prolong cycling performance and displaying good rate capability.For a further understanding the superior electrochemical behaviors of the MoO2/C electrodes,the kinetic analyses of the electrochemical behaviors of the as-prepared electrodes were carried out to disclose the function of capacitive contribution in the LIBs(5)Molybdenum dioxide-molybdenum carbide heterostructures coupled with 3D holey carbon nanosheets for highly efficient and ultrastable cycling LIBs.MoO2/Mo2C heterostructures anchored on 3D holey carbon substrates were simultaneously formed by one-pot carbonizing a bio-inspired gel.Well matched interfaces between the(101)plane of MoO2 and the(100)plane of Mo2C were identified from these heterostructures.The huge capacity changes were successfully avoided due to the introduction of unique heterostructures in MoO2/Mo2C/C composites.Both a local in-built driving force in the electrodes and synergistically induced more efficient mass transport in the interface were predicted by the Density Functional Theory(DFT)and Density of States(DOS)calculations from the formed interfacial electric field,yielding the fast reaction kinetics and outstanding lithium storage of the MoO2/Mo2C/C electrodes(6)Oxygen vacancy derived local build-in electric field in mesoporous hollow Co3O4 microspheres promotes high-performance Li-ion batteries.The urchin-like hollow Co3O4 microspheres were prepared and oxygen vacancies were introduced into this material.To reveal the influence of oxygen vacancies in this unique structure,the content of oxygen vacancies was tuned by adjusting the calcination temperature and the mechanism of a built-in electric field at the atomic level was proposed to identify the effect of oxygen vacancies.It is worth noting that the morphology and structure of the as-prepared materials were controlled via changing the calcination temperature,resulting in the variation of oxygen vacancy concentration.As expected,the anodes containing the urchin-like hollow Co3O4 microspheres presented outstanding cycling stability,high rate capability and large specific capacity for LIBs.