A Study of Morphological Effects on Electrochemical Reactions in Li-based Energy Materials

Abstract

The sustainability of our current society is undermined by a range of environmental issues stemming from the long-standing reliance on a fossil fuel-based energy system. It is imperative to instigate a fundamental shift in our energy paradigm to address these challenges. A key aspect of this transition involves the adoption of eco-friendly energy technologies, such as energy storage and hydrogen production/utilization, which heavily rely on various electrochemical reactions. Given that these electrochemical reactions, including electrocatalysis and charge/discharge processes, often transpire under intricate and harsh conditions, multiple factors can influence the overall performance of energy applications. Among these factors, optimizing the properties of electrode materials emerges as a fundamental strategy. However, the intricate nature of these reactions in complex environments poses a challenge in translating the exceptional characteristics of materials to practical devices. Consequently, morphological control, such as nanostructurization, becomes crucial to elicit the best intrinsic features. Such structural modifications typically result in enhanced electrical conductivity, surface area (pore volume), and improved mass transfer behavior. In this context, the objective is to comprehend the relationship between the morphology of electrode materials and their physicochemical attributes. This understanding aims to enhance the performance of Li-based energy system. In Chapter 2, polygonal-stacked Cu2O (PCO) are prepared by modified Benedict’s reaction by simple controlling concentration of cetyltrimethylammonium bromide (CTAB). The PCO materials synthesized with a sharp tip structure demonstrated superior capacity compared to those with a round-edge structure. The electrochemical performance of PCO materials appears to be influenced significantly by surface area and structural differences. Among the three distinct Cu2O materials with varying structures, the PCO electrode featuring a sharp tip shape exhibited a notable discharge capacity of approximately 402 mAh g-1 during the second cycle, which is larger than commercial graphite anode (372 mAh g-1). This discharge capacity gradually increased to around 506 mAh g-1 over the course of the first 100 cycles. Additionally, the diffusion coefficient of the sharp-tip-shaped PCO was observed to be larger than that of PCO with a round-structured morphology. The uncomplicated synthesis approach used to achieve this distinctive electrode morphology provides valuable insights into the effects of morphology on anode materials for lithium-ion batteries. In Chapter 3, a straightforward electrospinning method for creating self- supporting bundles of lithium titanium oxide (LTO) nanowires are presented, featuring easily adjustable electrode thickness. These bundles, termed LTO nanowire sheet bundles (LNSBs), exhibit a remarkably high areal capacity as an anode. This is attributed to their microscale layer-by-layer arrangement, where nanoscale LTO nanowires form a network within each microscale layer. The distinctive structure incorporates interspaces between stacked sheet layers, facilitating efficient electrolyte penetration through the thick electrode layer. The assembly of nanoscale wires enhances ion and electron transfer rates during lithiation/delithiation processes. As a result, the produced LNSB electrode demonstrates an exceptionally high areal capacity, reaching approximately 14.2 mAh cm-2 for the initial cycle and around 6.5 mAh cm-2 for the 500th cycle at a current density of 0.2 C rate. This surpasses the areal capacity of commercial graphite anodes, which is approximately 3.5 mAh cm-2. The achievement of such a substantial areal discharge capacity in a novel free- standing electrode design holds promise for advancements in energy storage applications.