Synthesis And The Electrocatalytic Study Of Magnetite Nanoparticles And Composites In The Air Electrode For Lithium–air Battery

With the urgency requirement of clean and secure energy across the world, sustainable development and the concept of low-carbon economy were put forward. Batteries are the most promising electrical energy storage systems for many applications, from portable electronics to electric vehicles. As a new generation of high capacity battery, rechargeable lithium–air battery become one of the most prospective clean energy in foreseeable future because of its high energy density（11000 Wh kg-1）, portable and environmental friendliness. For a lithium–air battery, additional difficulty include much lower practical energy density than the theoretical energy density, the problem is due primarily to the inefficiency of the O2-breathing electrode, including transport of oxygen through the pores.Air electrode is the core part of the lithium–air batteries, and the oxygen reduction catalyst is a key factor affecting the performance of the air electrode. As a result, finding an efficient catalyst and designing an air electrode with special structure becomes a hot scientific research field, in order to reduce the electrochemical polarization of oxygen reduction reaction and improve the discharge capacity and recycling performance.Compared with other catalytic materials, Fe₃O₄ as the transition-metal oxide has attracted widespread attention due to significant advantages of its abundance, having superior electrical conductivity, high catalytic activity, low cost, being eco-friendly. However, the volume and appearance changes, which occurs in the charge/discharge process, and consequently, resulting in poor capability. The most attractive and effective strategies to enhance the electrochemical performance of Fe₃O₄ anodes are designing Fe₃O₄ with special morphologies or synthesising of iron oxides/carbon composites.In this dissertation, Fe₃O₄ nanostructures with special morphologies and iron oxides/carbon composites were successfully synthesized. This paper aims at finding the simple and environmental method for the preparation and modification of the special morphology nanostructures as efficient oxygen reduction catalysts. High performance nanostructured catalysts were developed. Electrochemical properties of the prepared nanostructured materials were studied. The major research contents and results are presented as follows:（1） Mace-like Fe₃O₄ nanostructures were successfully synthesized via a microemulsion-mediated solvothermal method and used as an electrode catalyst for Li–air battery. The results showed that the mace-like Fe₃O₄ nanostructures were obtained by adjusting the Na OH concentrations, reaction temperatures, reaction time, and the addition of PEG-1000. The growth and assembly mechanism of the mace-like Fe₃O₄ nanostructures were also discussed. Polyethylene glycol-1000 not only acted as a soft template to form Fe₃O₄ nanorods, but also assisted in the assembly of the Triton X-100-decorated Fe₃O₄ nanoparticles onto the nanorods. The results of charge/discharge tests showed that the lithium–air battery based on mace-like Fe₃O₄ nanostructures exhibited a high discharge capacity of 1427 m Ah g–1 at a current density of 50 m A g–1 in ambient air, lower overpotential, ?E=1.2 V. Unique one-dimensional mace-like Fe₃O₄ nanostructures was highly active to air electrode of lithium–air battery, which is effective for improving the electron transport rate. FT-IR analysis of the cathode both before and after discharge state were carried out. The results showed that the discharge products were undesirable Li2CO3 and expected Li2O2.（2） This paper reported the growth of Fe₃O₄ nanoflakes and novel pagoda-like Fe₃O₄ particles via a facile microemulsion-mediated solvothermal procedure. The Na OH concentration and polyethylene glycol（PEG）-2000 had key effects on the formation of the final product. The morphologies of the as-prepared pagoda-like Fe₃O₄ particles evolved from pagoda-like to pinwheel-like to flower-like shapes with increasing reaction time. The electrocatalytic properties of the prepared Fe₃O₄ nanoflakes and pagoda-like micro-Fe₃O₄, as catalytic materials for a lithium-air battery, were further evaluated by galvanostatic charge/discharge cycling and electrochemical impedance spectrometry. Results of pagoda-like micro-Fe₃O₄ showed that the cell displayed an initial discharge capacity of 1429 m Ah g-1 at a voltage of 1.5–4.5 V at 100 m A g-1. Results of Fe₃O₄ nanoflakes showed that the cell displayed an initial discharge capacity of 1332 m Ah g-1 at a voltage of 2.0–4.1 V at 50 m A g-1. The two-dimensional structure of Fe₃O₄ nanoflakes with a higher specific surface area, which can be fully electrolyte infiltrated, meanwhile the two-dimensional structure has a short vertical distance, in favor of electronic spreading rapidly.（3） Hollow spherical and cubic porous Fe₃O₄ nanoparticle catalysts were prepared by microemulsion-solvothermal respectively. The chemical compositions and morphologies of the as-grown Fe₃O₄ particles were characterized by X-ray diffraction（XRD）, energy dispersive X-ray spectroscopy（EDX）, and field emission scanning electron microscopy（FE-SEM）. The electrocatalytic properties of the prepared Fe₃O₄ particles, as catalytic materials for a lithium–air battery, were further evaluated by galvanostatic charge/discharge cycling and electrochemical impedance spectrometry. Results of Hollow spherical Fe₃O₄ showed that the cell displayed an initial discharge capacity of 1602 m Ah g-1 at a voltage of 2.0–4.1 V at 50 m A g-1. The reversible lithium storage capacity only about 74.2 m Ah g-1 over 3 cycles. By limiting the depth of diseharge, the operation time of rechargeable lithium air battery could be prolonged. Results of cubic porous Fe₃O₄ showed that the cell displayed an initial discharge capacity of 1263 m Ah g-1 at a voltage of 2.0–4.1 V at 50 m A g-1. The porous structure of the material is necessary for the smooth transport and ion conductivity, and it also provides high electrocatalytic active sites and a good place for the reduction/oxide reaction occurring in the interface of the phase.（4） The Fe₃O₄/C composite as a cathode material of lithium–air batteries was successfully synthesized by an effective solvothermal method using glucose as carbon source. The cell based on Fe₃O₄/C composite delivers high discharge capacity of 1516 m Ah g–1 at 50 m A g–1. The discharge plateau and the charging potential plateau of the lithium–air battery based on Fe₃O₄/C composite were 0.01 V higher and 0.04 V lower than that of the lithium–air battery based on Fe₃O₄ NPs, respectively. This was because the carbon can not only improve the conductivity and reduce the polarization phenomenon during charge/discharge, but also can buffer the volume expansion of the material and increase the structural stability of the composite.（5） A novel litchi-like Fe₃O₄/graphene composite as a cathode material of lithium–air batteries was successfully synthesized by an effective solvothermal method. The cell based on Fe₃O₄/graphene composite delivers higher discharge capacity of 1638 m Ah g–1 at 50 m A g–1. The discharge plateau of the lithium–air battery based on Fe₃O₄/graphene composite was flat at 2.72 V and the charging potential plateau was flat at 3.76 V with high reversible capacity. It was 0.09 V higher and 0.02 V lower than that of the lithium–air battery based on Fe₃O₄ NPs, respectively. The difference between the charge and discharge voltage（?V） was 1.04 V, the energy efficiency of 72% was obtained for Li–air cells using Fe₃O₄/graphene composite as a catalyst. This was because the unique litchi-like Fe₃O₄ within the Fe₃O₄/graphene composite could provide a larger electrochemical reaction surface and the synergistic effect between the Fe₃O₄ NPs and the graphene sheets. Graphene can buffer the volume change of Fe₃O₄ nanoparticles during electrochemical cycling, enhanced the structure stability and conductivity of the material.