Developing Solid-State Na-Air batteries Based on Nasicon (Na3Zr2Si2PO12) Solid Electrolytes

DoctorRechargeable Li-ion batteries (LIBs), comprised of organic liquid electrolytes, a LiCoO2 cathode, and a graphite anode, are widely used for portable devices, electric vehicles (EVs) and grid energy storage systems. As the devices that require the LIBs are more sophisticated, greater cell performances are increasingly demanded. Theoretically, Na-Air batteries are considered as a promising energy storage due to high energy density and low energy cost. Also, utilizing inorganic solid electrolytes in Na-Air batteries instead of organic liquid electrolytes can prevent several problems such as flammability, limitations of electrochemical potential window and evaporation. In this context, new strategies for improving ionic conductivities ("σ" _"ion" ) of Nasicon (Na3Zr2Si2PO12), a representative Na-ion conductor, were developed and solid-state Na-Air batteries based on the Nasicon materials were firstly operated. Adding excess Na and modifying phase transition behavior can be new methods to increase "σ" _"ion" of Nasicon. Also, electrochemical redox processes of the solid-state Na-Air batteries were investigated. General introductions for the thesis are included in chapter 1. Principles of Li-ion and Na-Air batteries and necessity of solid electrolytes is mentioned in this chapter. Then, material information of Nasicon (Na3Zr2Si2PO12) solid electrolytes is introduced. Chapter 2 and 3 deal with the new strategies to improve "σ" _"ion" of Nasicon. A study of increased "σ" _"ion" of Nasicon by adding excess-Na is shown in chapter 2. The excess Na positively affected to "σ" _"ion" in both grain and grain boundary, leading to increase in total conductivity of Nasicon. A bottleneck, which definitely determines activation energies for ion conduction, was expanded by the incorporation of excess Na into Nasicon lattice. Also, excess Na probably compensated deficiency of Na-ion concentration in grain boundary. Another method to increase "σ" _"ion" of Nasicon by modifying a phase transition behavior is introduced in chapter 3. Nasicon has two polymorphs: rhombohedral which stabilizes at high temperature and monoclinic at low temperature. The rhombohedral phase can have higher "σ" _"ion" than monoclinic because the rhombohedral phase has lower activation energy. When Ge element was added to Nasicon, the phase transition temperature of Nasicon was lowered and more rhombohedral phase was found in the Ge-added samples than in bare sample, resulting in the increase of "σ" _"ion" . Chapter 4 includes preparation processes of the solid-state Na-Air batteries based on the Nasicon solid electrolyte and electrochemical redox processes of the system. A porous air electrode, comprised of Nasicon and Ni metal, and a gold (Au) buffer layer were attached to each side of the dense Nasicon pellet. Then, sodium (Na) metal anode was laminated on the top of Au buffer layer, and the cell was operated in ambient air. The cell was exhibited an unexpected electrochemical performances and its electrochemical redox mechanism was investigated