Operando studies of functional materials for energy storage and conversion [electronic resource]
Keywords: Materials Science and Engineering.

As the world's demand for energy increases, there is a need to explore alternative sources of energy that would not only alleviate our dependence on crude oil, but also reduce carbon emissions and make the environment a greener place to live in. Two of the most promising energy storage and conversion devices are rechargeable lithium-ion batteries and solar cells. To achieve breakthroughs in device performance, it is important to develop a fundamental understanding of the processes taking place in the active materials during device operation, so that targeted strategies can be developed to improve their efficiencies. In this dissertation, I will show two bottoms-up approaches to achieve the above: operando x ray studies of energy storage materials and improving theoretical calculation methods through correlating experimental results with theoretical predictions. Overall, significant improvements in materials for clean energy applications can be made in the future through bottoms-up investigations. In the first part of my dissertation, I discuss the development, execution, and insights from operando x ray diffraction and x ray absorption of crystalline germanium-based (c-Ge) anode materials for rechargeable lithium-ion batteries. These two probes track both crystalline (XRD) and amorphous (XAS) phase transformations with potential, which allows detailed information on the Ge anode to be obtained. Through conducting operando XRD and XAS, I mapped out the entire reaction mechanisms for c-Ge cycled at C/15 (discharging battery in 15 hours). The combined operando XRD and XAS results present new insights into the reaction mechanism of Ge as anodes in LIBs, and demonstrate the importance of correlating electrochemical results with operando studies. Next, I discuss how different conductive additives affect the phase transformations of crystalline Ge anodes during cycling. I find that Ge electrodes using carbon nanotubes (CNT) as conductive additives exhibit higher structural and electrochemical reversibility compared to those with carbon black additives and thus gives rise to better stability in subsequent cycling. Based on these findings, a proposed strategy to prolong the cycle life of crystalline Ge anodes is presented. The cycling rate of the battery is also found to affect the phase transformation, cycling stability, and storage capacity of germanium electrodes. It has been discovered that the formation of crystalline Li15Ge4 (c-Li15Ge4) during lithiation is suppressed beyond a certain cycling rate. A very stable and reversible high capacity of ~ 1800 mAh g-1 can be attained up to 100 cycles at a slow C-rate of C/21 when there is complete conversion of Ge anode into c-Li15Ge4. When the C-rate is increased to ~ C/10, the lithiation reaction is more heterogeneous and a relatively high capacity of ~ 1000 mAhg-1 is achieved with poorer electrochemical reversibility. An increase in C-rate to C/5 and faster reduces the capacity (~ 500 mAhg-1) due to an impeded transformation from amorphous LixGe to c-Li15Ge4, and yet it improves the electrochemical reversibility. In the final chapter of the first part of my dissertation, I discuss how by alloying tin (Sn) with Ge, the cycling stability can be improved further. Fabrication of new Sn-Ge alloy as anodes for high capacity lithium-ion batteries was conducted using melt-spinning. High, reversible and stable capacities of over 1000 mAh g-1 are maintained over 60 cycles at C/10. Good rate capability of 500 mAh g-1 at 5C is also achieved, making it extremely attractive for very fast charge/discharge applications. More remarkably, it has a tap (or bulk) density of 2.05 gcm-3 and thus high volumetric capacities of 2050 mAh cm-3 at C/10 and 1025 mAh cm-3 at 5 C. Sn-Ge alloy is observed to undergo a transformation from crystalline Sn-Ge alloy to phase separated nanocrystalline Sn in an amorphous Ge matrix. The excellent lithium storage properties exhibited by Sn-Ge are attributed to the synergistic effect between the phases. In the second part of my dissertation, I discuss the background of theoretical bandstructure calculations for semiconducting oxides for energy applications, and the approach to improve its accuracy in predicting materials' properties. Here, I use zinc oxide (ZnO) as the prototype for semiconducting oxides. ZnO contains occupied d10 bands that interact with the anion p states and is thus challenging for electronic structure theories. Within the context of these theories, incomplete cancellation of the self-interaction energy results in a Zn d band that is too high in energy, leading to the upwards repulsion of the valence band maximum (VBM) states, and an unphysical reduction of the band gap. Methods such as GW should significantly reduce the self-interaction error. In order to evaluate such calculations, I measured high resolution and resonant angle-resolved photoemission spectroscopy (ARPES) and compared these to several electronic structure calculations. I find that in a standard GW calculation, the d bands remain too high in energy by more than 1 eV irrespective of the Hamiltonian used for generating the input wavefunctions, causing a slight underestimation of the band gap due to the p-d repulsion. I show that a good agreement with the ARPES data over the full valence band spectrum is obtained when the Zn-d band energy is shifted down by applying an on-site potential Vd for Zn-d states during the GW calculations to match the measured d band position. The magnitude of the GW quasi-particle energy shift relative to the initial density functional calculation is of importance for the prediction of charged defect formation energies, band-offsets and ionization potentials.