Developing a new class of cation-disordered cathode compounds for Li/Na ion batteries

DoctorLithium ion batteries (LIBs) are one of the most advanced energy conversion and storage technologies. Since the commercialization of LIBs, their use has been widely extended. Recently, they have been utilized for powering large-scale applications such as electric vehicles (EV) and energy storage system (ESS). To meet this surge of new applications, developing novel electrode materials that simultaneously exhibit high energy density and excellent structural stability for long cycle life is in great demand. In this regard, triplite LiFeSO4F can be the most promising candidate as cathode materials for Li ion batteries. Triplite LiFeSO4F achieves the highest voltage (3.9 V) among Fe2+/Fe3+ redox couples, and therefore can have higher energy density than LiFePO4. However, full utilization of Triplite LiFeSO4F is severely limited by its non-scalable synthesis process and poor electrochemical activity. In the chapter 2 and chapter 3, we focused on the development of a new scalable solid-state synthetic process for triplite LiFeSO4F. Based on the understanding the thermodynamics of triplite, we developed a new solid-state synthetic process that can be able to achieve triplite within just 1h. In the chapter 4, we focused on the understanding the electrochemical reaction mechanism of triplite LiFeSO4F, and we observe that the arrangement of Li/Fe ions can be a key parameters which governs the electrochemical properties of triplite LiFeSO4F. Based on the understandings on chapter 4, in the chapter 5, we tried to understand the electrochemical properties depending on the arrangement of Li/Fe ions, and thereby defined the best arrangement to fully activate its electrochemical performance. Consequently, we developed a new solid-state synthetic strategy to control the arrangement of Li/Fe ions for the best one, and developed a fully-activated triplite LiFeSO4F. These understanding further expanded to Na cathode materials. In the chapter 6, we also focused on the development a triplite-type Na cathode materials, triplite NaFeSO4F, for Na ion batteries. The developed triplite NaFeSO4F can show high redox potential of ~3.7 V versus Na+/Na0, leading to ~430 W·h/kg of practical energy density. Therefore, triplite NaFeSO4F can be one of candidate positive electrode material which can meet the requirements for high energy density Na ion batteries