Design And Synthesis Of High Capacity Electrode Materials And Study Of Their Magnesium/Lithium Storage

Lithium ion batteries are widely used in portable mobile devices,electric vehicles,energy storage power station and other fields.However,due to the continuous development of human and increasingly prominent of environment problem,people take more and more attentions to the requirement of chemical power source.The scientists kept on exploring and put forward some primary batteries with high energy,such as metal-air battery（such as Zn-O₂,Mg-O₂,Al-O₂,etc.） and Li-SOCl₂ battery,and some new secondary batteries,such as rechargeable magnesium batteries.Whether for primary or secondary batteries,finding the ideal electrode materials is the important task.For primary battery,the main focus is on the capacities of the positive and negative materials,output voltage and the self-discharge behavior.For secondary battery,in addition to the above factors,more attention should be paid to the reversibility of the electrode reactions.So far,a variety of positive and negative materials have been developed,among which multi-valent transition metal oxides are favored because of their high theoretical capacity.However,there are still many problems for transition metal oxide as electrode materials for the secondary batteries,such as low initial coulomb efficency,poor cycle performance and poor rate performance.For these problems,we prepared a series of new transition metal oxides with high energy density through a variety of controllable synthesis methods,and studied the impacts of the morphologies,particle size and micro-nano structure on the electrochemical performance as cathode for the rechargeable Mg-Li hybrid-ion batteries and as anode for lithium ion battery,respectively.The detailed work and achievements are summarized as follows:（1）CF_0.8 as a cathode material for primary magnesium battery is demonstrated for the first time.It is verified by the experiment that layered CF_0.8 has the advantages of high specific capacity and no self-discharge.The CF_0.8 cathode exhibits a high discharge capacity of 813.4 mAh g^-1 after discharging to 0.5 V at 20 mA g^-1 in 0.4 M（PhMgCl）₂-AlCl₃/THF electrolyte（named as APC）.The discharge product of MgF₂ is confirmed by EDX and XPS measurements.（2）Rose-like Cr₂Mo₃O₁₂/Graphene（named as CMO/G） composite is synthesized by hydrothermal method as cathode material for rechargeable Mg batteries and Mg-Li hybrid-ion batteries,respectively.The effects of hydrothermal time and modification of graphene on the morphology,structure and electrochemical properties of CMO are discussed,respectively.The results show that CMO/G composite delivers a high initial discharge capacity of 238.6 mAh g^-1 at 10 mA g^-1 in APC/1.0 M LiCl electrolyte.The initial coulombic efficiency is 73.62% and it presents good cycling and rate performance.XPS analysis reveals that both Mg²⁺ and Li⁺ ions take part in the intercalation/deintercalation reactions in cathode material during the discharge process.（3）A new type of layered multivariate metal oxide―V₂MoO₈（named as VMO） is synthesized firstly by a facile electrospinning method,and used as a cathode material for rechargeable Mg batteries and Mg-Li hybrid-ion batteries,respectively.The effects of ammonium molybdate content,calcining temperature and time on the morphology,structure and electrochemical properties of VMO material are discussed,respectively.In this thesis,Hastelloy is used for current collector and corrosion-resistant self-designed mould battery with APC/LiCl as electrolyte and Mg as anode towards a relatively high voltage.The results indicate that Mg-VMO cell delivers an initial discharge capacity of 199.1 mAh g^-1 with a coulombic efficiency of 33%in APC electrolyte at room temperature and presents poor cycling behavior.In APC/1.0 M LiCl electrolyte,it delivers a high initial discharge capacity of 312 mAh g^-1 at 20 mA g^-1 with the initial coulombic efficiency of 87.2%.Moreover,the discharge voltage platform（～1.5 V）,rate performance and cyclic stability of the batteries are significantly improved.XPS analysis reveals that both Mg²⁺ and Li⁺ ions take part in the cathode intercalation reaction,along with the valence changes of Mo/V ions.（4）A free-standing ZnFe₂O₄@Fe₃C/CNF anode for LIB has been synthesized by a facile foaming-assisted electrospinning strategy with tea saponin.The core of the composite is ZnFe₂O₄ nanoparticle（about 5-10 nm）,which is modified by discrete Fe₃C layer and further encapsulated within porous carbon nanofiber（named as CNF）.Owing to the buffer structure of CNF and modification of conductive Fe₃C,ZnFe₂O₄@Fe₃C/CNF composite exhibits superior rate performance and outstanding prolonged cycling stability.In LIBs,it delivers a large reversible capacity of ca.1000 mAh g^-1（based on the mass of the composite,the same as the following） at 0.1 A g^-1 for 200 cycles.Moreover,it delivers 970.9 mAh g^-1 at 1.0 A g^-1 after 800 cycles and 440.6 mAh g^-1 at 10 A g^-1 even after 1000 cycles.The excellent electrochemical performance of ZnFe₂O₄@Fe₃C/CNF composite is mainly attributed to the synergistic effect of its nanostructure and conductive material.