Electrochemical Behavior Of Defected-TiO₂-coated Si Nanoparticles And Investigation On Growth Mechanism Of CuO Nanowire Anode

Because of the irreversible loss of energy resources and serious environmental pollution caused by huge consumption of fossil fuels,development and innovation of the technology of electrochemical power devices?batteries?and exploration of the related advanced material have attracted worldwide research interest.High volume?Wh/L?and weight energy density?Wh/kg?,long cycle life and high safety are necessary for the actual applications of batteries.Due to the high energy density,rechargeable lithium-ion batteries have been considered to be promising energy storage devices for next-generation portable electronics,electric vehicles and grid-storage.Silicon?Si?anode has an ultra-high theoretical specific capacity(Li₂₂Si₅ is 4200mAh g^-1),a low lithium storage potential?less than 0.34V vs Li/Li⁺?.In addition,Si is abundant in nature and has no pollution to the environment.Therefore,utilizing Si as anode material of lithium-ion batteries has received great attention.However,Si anode electrode suffers severely from a large volume expansion?about 300%?in the processes of intercalation and deintercalation during the battery cycle.It results in large mechanical stress,pulverization and electrical contact loss of the electrode.In addition,the low conductivity of Si and low ion diffusivity in Si inhibit the realization of high capacity at high cell rates.Compared with Si materials,titanium dioxide?TiO₂?has good safety and high stability under high pressure,small volume change during charging and discharging?about 4%?,and low production cost.However,the low capacity of TiO₂ limits its applications in energy storage devices.It is highly expected that high-performance material can be developed by combining Si nanoparticles and TiO₂ to take advantages of these the two materials.In this thesis,amorphous TiO₂ and anatase TiO₂ were prepared by sol-gel method and their electrochemical properties were studied.It is found that anatase TiO₂ exhibits better electrochemical performance.However,the ionic conductivity in TiO₂ is low,which is not favorable for lithium ions transportation.As a result,the battery capacity is low.With a heat treatment of TiO₂ in hydrogen?to create oxygen vacancies?and ammonia atmosphere?to realize nitrogen doping?,additional Li-ions transport channels are created.Accordingly,cycle performance and rate performance of the corresponding batteries are effectively improved.Then,we coat Si nanoparticles with a uniform layer of TiO₂ as a buffer layer to buffer and confine the volume expansion of Si.In comparison with pure Si,the TiO₂-coated Si?Si@TiO₂ core-shell structure?presents improved stability and the first Coulomb efficiency is increased from 60%to 69%.However,the battery capacity is still low.We then post-treated Si@TiO₂ in hydrogen and ammonia atmosphere to introduce oxygen vacancies and nitrogen doping to the coating layer of TiO₂.The defects can provide transport channels for lithium ions and thus improve the cycle and rate performance of the batterry.Also,the first Coulomb efficiency is increased to 72%and 86%,respectively.The results indicate that the irreversible loss of lithium ions decreases during the first cycle.Copper oxide?CuO?is another outstanding anode material for lithium-ion batteries.It has a high specific capacity up to 670 mAh g^-1.Using thermal oxidation of copper?Cu?in air to obtain CuO nanowires?NWs?is a feasible and low-cost method.The growth mechanism of CuO NWs is yet to be fully understood.Although the stress-driven grain boundary?GB?diffusion mechanism is the most widely acceptable one to explain the growth of CuO nanostructures,it failed to include the effect of oxygen partial pressure on the growth of CuO NWs and explain the long single crystal CuO NWs of uniform diameter.Here,we attempt to develop a better understanding about the growth of CuO NWs by considering a synerric effect of the Kirkendall effect,stress-induced GB diffusion,surface free energy difference and CuO surface polarization.