<p> The progressive advancements in communication and transportation has changed human daily life to a great extent. While important advancements in battery technology has come since its first demonstration, the high energy demands needed to electrify the automotive industry have not yet been met with the current technology. One considerable bottleneck is the cathode energy density, the Li-rich layered oxide compounds xLi₂MnO₃.(1-x)LiMO₂ (M= Ni, Mn, Co) (0.5= Co) (0.5=discharge capacities greater than 280 mAh g^-1 (almost twice the practical capacity of LiCoO₂).</p><p> In this work, neutron diffraction under <i>operando</i> battery cycling is developed to study the lithium and oxygen dynamics of Li-rich compounds that exhibits oxygen activation at high voltage. The measured lattice parameter changes and oxygen position show movement of oxygen and lattice contractions during the high voltage plateau until the end of charge. Lithium migration kinetics for the Li-rich material is observed under operando conditions for the first time to reveal the rate of lithium extraction from the lithium layer and transition metal layer are related to the different charge and discharge characteristics.</p><p> In the second part, a combination of multi-modality surface sensitive tools was applied in an attempt to obtain a complete picture to understand the role of NH4F and Al₂O₃ surface co-modification on Li-rich. The enhanced discharge capacity of the modified material can be primary assigned to three aspects: decreased irreversible oxygen loss, the activation of cathode material was facilitated with pre-activated Mn³⁺ on the surface, and stabilization of the Ni redox pair. These insights will provide guidance for the surface modification in high voltage cathode battery materials of the future.</p><p> In the last part, the idea of Li-rich has transferred to the Na-ion battery cathode. A new O3 - Na_0.78Li_0.18Ni_0.25Mn_0.583O_w is prepared as the cathode material for Na-ion batteries, delivering exceptionally high energy density and superior rate performance. The single-slope voltage profile and ex situ synchrotron X-ray diffraction data demonstrate that no phase transformation happens through a wide range of sodium concentrations (0.8 Na removed). Further optimization could be realized by tuning the combination and ratio of transition metals.</p>

<p>Solar photovoltaics (PV), the technology that converts sunlight into electricity, has immense potential to become a significant electricity source. Nevertheless, the laws of economics dictate that to grow from the current 2% of U.S. electricity generation and to achieve large scale adoption of solar PV, the cost needs to be reduced to the point where it achieves grid parity. For silicon solar cells, which form 90% of the PV market, a significant and slowly declining component of the cost is due to the high-temperature (> 900 &deg;C) processing required to form p-n junctions. In this thesis, the replacement of the high-temperature p-n junction with a low-temperature amorphous titanium dioxide (TiO₂)/silicon heterojunction is investigated. The TiO₂/Si heterojunction forms an electron-selective, hole-blocking contact. A chemical vapor deposition method using only one precursor is utilized, leading to a maximum deposition condition of 100 &deg;C. High-quality passivation of the TiO₂/Si interface is achieved, with a minimum surface recombination velocity of 28 cm/s. This passivated TiO₂ is used in a double-sided PEDOT/n-Si/TiO₂ solar cell, demonstrating an open-circuit voltage increase of 45 mV. Further, a heterojunction bipolar transistor (HBT) method is developed to investigate the current mechanisms across the TiO₂/p-Si heterojunction, leading to the determination that 4nm of TiO₂ provides the optimal thickness. And finally, an analytical model is developed to explain the current mechanisms observed across the TiO₂/Si interface. From this model, it is determined that once &#916;E_V (TiO₂/Si) is large enough (400 meV), the two key parameters are the Schottky barrier height (resulting in band-bending in silicon) and the recombination velocity at the TiO₂/Si interface. Data corroborates this, indicating the hole-blocking mechanism is due to band-bending induced by the unpinning of the Al/Si interface and TiO₂ charge, as opposed to due to the TiO₂ valence band edge.