High-capacity layered oxides and organic compounds as cathode materials for lithium rechargeable batteries

Abstract

Lithium rechargeable batteries power most portable electronic devices mainly due to their capability of offering the high energy density compared to other energy storage systems. Recently, the large-scale applications of lithium rechargeable batteries such as electric vehicles (EVs) and stationary power backups have moved in the center of interests coinciding with the large demands on the sustainable energy storage technologies. However, the currently available lithium rechargeable batteries are not sufficient for the large-scale applications in terms of the energy density, cost, life time, and safety. To address this issue, intensive scientific effects have been made in exploring new cathode materials that can substitute current cathode materials including LiCoO2 and LiFePO4. Among various cathode materials reported so far, layered Li-excess transition metal oxides and organic compounds have received wide attention of the materials scientists due to their capability of reversibly storing and delivering large quantity of energy in the small weight of the compounds. Despite their high energy density, the energy storage mechanisms of the layered Li-excess nickel-manganese oxides and organic compounds are not clearly understood yet. The lack of understanding on the materials makes the origin of poor electrochemical performances such as power capability and cycle stability more ambiguous and limits their practical applications. In this respect, here, in-depth study is performed to understand the energy storage mechanisms of the high-capacity electrode materials by using the combination of experimental analyses and DFT calculations. Furthermore, the strategies to improve their battery performances are demonstrated. The layered Li-excess transition metal oxides exhibit large capacity (~230 mAh g-1) which exceeds the amount of available redox couples in the pristine compounds when they are charged to high voltage beyond the plateau at 4.5~4.6 V accompanied with oxygen gas evolution. Moreover, this class of materials suffers gradual voltage decay during repeated electrochemical cycling. However, the detailed reactions of oxygen in the batteries after the first charge process and the origins of the voltage decay have not been clearly revealed. In this thesis, the role and effects of oxygen evolved from the layered Li-excess transition metal oxides during electrochemical cycling are proposed and the strategy to protect the surface of electrode materials from the acidic byproducts was briefly introduced. Furthermore, it is proposed that the compositional optimization of the Li-excess compounds effectively suppress the voltage decay increasing the energy storage efficiency. The organic compounds have a great potential to achieve large specific capacity due to the absence of the heavy transition metal ions in their structure and the capability of multi-electron transfer. Inspired by the biologically occurring redox-active organic cofactors incorporated in the sequential energy transduction reactions, here I propose a new class organic electrode compounds. The organic compounds containing pteridine redox center are demonstrated as sustainable cathode materials: flavin-derivatives. Through the computational material design, I have discovered the simper compounds containing pteridine redox moiety which are alloxazine and lumazine. Furthermore, a strategy is proposed for optimizing the electrochemical performances of these biological redox units achieving excellent rate capability and cycle stability.