Tuning Lithium-storage Behavior Of High-capacity Anodes By Fabricating Micro/nano Multi-phase Structure And Alloying

Clean energy is an inevitable choice to reduce the reliance on fossil fuels,to alleviate environmental damage and to achieve a sustainable economy.Although there are abundant clean energy resources for power generation,their large-scale applications are still impeded by the difficulty of energy storage.Currently,Lithium-ion battery is one of the most advantageous method of energy storage.However,it is still facing daunting challenges brought by the critical requirements of large-scale applications,for example,electric vehicles.Improving the performance of anode material is of great significance for improving the energy storage density of lithium ion batteries.Iron oxide and silicon are high-energy-density conversion reaction type anode materials and alloying type anode material,respectively.This thesis focuses on the improvement of electrochemical properties of these two anode materials,by synthesizing iron oxides or silicon based composites with micro/nano structure,tunable multi-phase and alloying via conventional ball-milling,dielectric barrier discharge plasma assisted ball milling（P-milling）and hydrothermal reactions,to obtain high reversible capacity,long cycle life and high rate performance,eventually to meet the demands of industrialization.Fe₂O₃-graphite（Fe₂O₃-C）composites was synthesized via ball milling.Its mechanical effect can facilely and effectively destroy the preferred orientation of graphitic structure and create amorphization.We studied the time effect of ball milling and found that the average size of graphite in[001]direction was reduced to 51-47 nm,and Fe₂O₃ to 32-19 nm.With an appropriate carbon content（20%-30%）and milling time（5 h）,the composites formed spherical secondary particles of around 10μm in diameter,which could significantly benefit the capacity,cycle stability and coulombic efficiency because of the reduction on surface area and the buffer effect on volume change during cycling.The initial reversible charge capacity of Fe₂O₃-20%C developed 550 mAh/g in the first discharge,equivalent to 61%of its theoretical capacity.This structure shows the highest initial coulombic efficiency（72%）among all the Fe₂O₃-x%C composites.After 50 cycles,90%of the first reversible capacity was reserved.Considering the tendency of amorphization of graphite when using conventional ball,we need to optimize the process to achieve our target structure of original two-dimension graphite with less-layers.P-milling can effectively peel off the graphite to produce few layered graphene（FLG）and further synthesize the FLG covering Fe₂O₃ composites structure.The thickness of FLG could be reduced to 3-8 nm,corresponding to 10-24 graphitic layers.Meanwhile,the average size of Fe₂O₃ particles was refined from 60 nm to 12 nm.The Fe₂O₃-FLG sample treated by P-milling for 20 h（P20）delivered an initial reversible capacity of 729 mAh/g,which is equivalent to 87%of its theoretical capacity.P20 also showed a reversible capacity of 695 mAh/g after 200 cycles,which is equivalent to 95%of the initial capacity.After systematic investigation,it is concluded that wrapping of FLG is favorable to preserve the integrity of the Fe₂O₃-FLG composite electrode,resulting in the leap of cycle performance.To further explore the potential of iron-based oxides,this chapter focused on low cost Fe₃O₄ based composites.Fe₃O₄-SnO₂-rGO（FSO）composites were prepared via a one-step hydrothermal method.The Fe₃O₄ nanoparticles have an average grain size of 57 nm and Sn O₂34 nm.Both of them were uniformly loaded on reduced graphene oxide matrixes,forming a porous structure with an average pore size of 4 nm.The initial discharge of FSO composite delivered a reversible capacity of 947 mAh/g in the first cycle with a coulumbic efficiency of70%.It maintained 831 mAh/g after 200 cycles,which exhibited an outstanding combination of high capacity and good cycle life.It’s evident that the combination of SnO₂ and Fe₃O₄ can enormously enhance the kinetics of lithiation and delithiation.In the meantime,the successive reactions of Fe₃O₄ and SnO₂ can buffer volume expansion/contraction during discharge/charge process.FSO composite also showed an excellent rate capacity of 513mAh/g under a high current density of 2000 mA/g.The Li⁺diffusion kinetics of metal oxide-rGO composites is carefully studied and it is learned from the calculation that the Li⁺diffusion coefficiency of FSO is one to two orders higher than its counterparts.Thus it is evident that the porosity of FSO is significantly favorable for the Li⁺diffusion kinetics.The effect of adding Ti on the structure and electrochemical properties of Si-based anode materials was also investigated by ball milling.An Si_1-xTi_x alloy mixture of Si and C49-TiSi₂metastable phases was obtained by ball milling of Ti and Si powders.As the value of x increases,Si phase gradually becomes disordered.The electrochemical properties of Si_1-xTi_x alloys are closely related to the changes of the microstructure.With the increasing Ti,the amorphization of Si and the formation of C49-TiSi₂ phase became more obvious.At low potential,amorphous silicon normally reacts with lithium ions and results in a Li₁₅Si₄ phase,which would deteriorate the stability of cyclic capacity.When 0.15≤x≤0.30,the formation of Li₁₅Si₄ phase in Si_1-xTi_x alloy was completely suppressed,and the cycle performance of the alloy is significantly improved.SEM analysis of the surface morphology of the electrodes after cycling showed that the adhesion between electrode material and current collector is directly related to the suppression of the Li₁₅Si₄ phase formation.The cell test results indicated that the capacity of Si_1-xTi_x（x>0）alloy was mainly determined by the electrochemical active silicon phase,while C49-TiSi₂ alloy phase showed no electrochemical activity towards lithium.The reversible capacity of Si-Ti alloy negative electrodes with Si_0.85Ti_0.15 in composition were 1372 mAh/g or 1710 Ah/L.The initial coulombic efficiency was as high as 81%and the reversible capacity retention after about 50 cycles was about 96%.Together with advantages of abundant reserve and low cost,the Si-Ti alloy has great potential to become the next generation Li-ion anode materials.