Nanostructured Transition Metal Oxide-based Electrodes for Electrochemical Energy Storage

Abstract

Layered transition-metal oxides have been intensively investigated as one of the most promising electrode materials for electrochemical energy storage systems, e.g., electrochemical capacitors (ECs) and Li-ion batteries (LIBs), due to their facile ion diffusion kinetics and suitable electrical conductivity. However, layered-structure electrodes still require further improvement of their electrochemical performances. Limited mass and charge transport kinetics under the high mass loading of electrodes lead to a low utilization degree and ratio of measured/theoretical capacitance or capacity, and insufficient power performances. In addition, irreversible side reactions, including phase transitions, changes in composition of electrodes, and undesired electrochemical or chemical reactions with electrolytes, are considered to be negative factors for stable cycling performances. The objective of this thesis is to identify the energy storage mechanism in ECs and LIBs and to provide guidelines for electrode design with respect to the crystallographic phase and structure of electrodes and interfacial properties between the electrodes and electrolytes. First, 1-D nanostructured MnO2-electrodes for ECs were developed using the electrospinning method followed by electrochemical oxidation to precisely control the phase of MnO2. The main advantage of nanofibers (NFs) is their ability to overcome the poor cation diffusivity and electrical conductivity of Mn oxides that result in a low degree of utilization. To date, the phase of electrospun MnOx NFs after thermal calcination has been limited to the low oxidation state of Mn (x < 2), which has resulted in insufficient specific capacitance. The organic contents in the as-spun MnOx NFs, which are essential for forming the NF structure, make it difficult to obtain the optimum phase to achieve high electrochemical performance. δ-MnO2 NFs, which were obtained by galvanostatic oxidation of thermally calcined MnOx NFs, were successfully fabricated while maintaining the 1-D nanoscale structure and inhibiting the loss of active materials. The galvanostatically oxidized Mn3O4 exhibited an outstanding performance of 380 F/g under a mass loading of 1.2 mg/cm2. From a comparison study on the effect of the initial phases of MnOx (Mn3O4 and Mn2O3), it was confirmed that galvanostatic oxidation-induced phase transformation to δ-MnO2 occurred thorough 2 step-wise processes, and each step was strongly dependent on the concentration of Mn2+ and the energetic stability of the Mn3+ ions in the MnOx phases. The second focus is to develop a novel interfacial engineering method and to improve the performances of Li(Ni1/3Co1/3Mn1/3)O2 cathodes by introducing amorphous ZrO2 surface layers. We developed a novel sputtering system that was modified to induce continuous movement of the powders of electrode materials to secure a uniform coating. The electrochemical performance of a ZrO2-coated Li(Ni1/3Co1/3Mn1/3)O2 cathode was investigated using two types of electrolyte materials: conventional organic liquid and sulfide-based glassy solid-state electrolytes. In the cells with the liquid-state electrolytes, the cycling stability (capacity retention) was enhanced by the surface ZrO2 layer: 94.6 and 71.3 % for the coated and pristine cathodes, respectively. The effect of the surface layer on improving the stability was analyzed with respect to the electronic conduction and Li ion diffusion kinetics. The superior power performance of the ZrO2-coated cathodes was also investigated by considering the interaction between ZrO2 and Li ions during cycling and quantitatively measuring the diffusion coefficient of Li ions through the surface layers. In the all-solid-state cell based on the composite solid electrolyte of Li2S and P2S5, the improved specific capacity and retention were also confirmed in coated electrodes, while the responsible mechanism was distinguished from that effect in liquid-state cells. Based on understanding the mechanism for stabilizing the interface by introducing surface coating layers, we suggest an appropriate coating material to improve LIBs using both liquid- and solid-state electrolytes. This study provided useful information for understanding the energy storage mechanism in layered electrode materials and suggested versatile design methods to improve the electrochemical energy storage performances.