Browsing by Author "Lin, Feng"

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    Atomistic Modeling of Defect Energetics and Kinetics at Interfaces and Surfaces in Metals and Alloys
    Alcocer Seoane, Axel Emanuel (Virginia Tech, 2024-01-02)
    Planar defects such as free surfaces and grain boundaries in metals and alloys play important roles affecting many material properties such as fracture toughness, corrosion resistance, wetting, and catalysis. Their interactions with point defects and solute elements also play critical roles on governing the microstructural evolution and associated property changes in materials. This work seeks to use atomistic modeling to obtain a fundamental understanding of many surface and interface related properties and phenomena, namely: orientation-dependent surface energy of elemental metals and alloys, segregation of solute elements at grain boundaries and their impact on grain boundary cohesive strength, and the controversial sluggish diffusion in both the bulk and grain boundaries of high entropy alloys. First, an analytical formula is derived, which can predict the surface energy of any arbitrary (h k l) crystallographic orientation in both body-centered-cubic (BCC) and face-centered-cubic (FCC) pure metals, using only two or three low-index (e.g., (100), (110), (111)) surface energies as input. This analytical formula is validated against 4357 independent single element surface energies reported in literature or calculated by the present author, and it proves to be highly accurate but easy to use. This formula is then expanded to include the simple-cubic (SC) structure and tested against 4542 surface energies of metallic alloys of different cubic structures, and good agreement is achieved for most cases. Second, the effect of segregation of substitutional solute elements on grain boundary cohesive strength in BCC Fe is studied. It is found that the bulk substitution energy can be used as an effective indicator to predict the embrittlement or strengthening potency induced by the solute segregation at grain boundaries. Third, the controversial vacancy-mediated sluggish diffusion in an equiatomic FeNiCrCoCu FCC high entropy alloy is studied. Many literature studies have postulated that the compositional complexity in high entropy alloys could lead to sluggish diffusion. To test this hypothesis, this work compares the vacancy-mediated self-diffusion in this model high entropy alloy with a hypothetical single-element material (called average-atom material) that has similar average properties as the high entropy alloy but without the compositional complexity. The results show that the self-diffusivities in the two bulk systems are very similar, suggesting that the compositional complexity in the high entropy alloy may not be sufficient to induce sluggish diffusion in bulk high entropy alloys. Based on the knowledge learned from the bulk alloy, the exploration of the possible sluggish diffusion has been extended to grain boundaries, using a similar approach as in the study of self-diffusion in bulk. Interestingly, the results show that sluggish diffusion is evident at a Σ5(210) grain boundary in the high entropy alloy due to the compositional complexity, especially in the low temperature regime, which is different from the bulk diffusion. The underlying mechanisms for the sluggish diffusion at this grain boundary is discussed.
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    Block Copolymer Derived Porous Carbon Fiber for Energy and Environmental Science
    Serrano, Joel Marcos (Virginia Tech, 2022-04-26)
    As the world population grows, a persistent pressure on natural resources remains. Resource requirements have extensively expanded due to industrialization. Several technological advancements continually aim to alleviate these resource shortages by targeting existing shortcomings in effective and efficient material design. Practical, high-performing, and economical materials are needed in several key application areas, including energy storage, energy harvesting, electronics, catalysis, and water purification. Further development into high-performing and economical materials remain imperative. Innovators must seek to develop technologies that overcome fundamental limitations by designing materials and devices which address resource challenges. Carbon serves as a versatile material for a wide range of applications including purification, separation, and energy storage owing to excellent electrical, physical, and mechanical properties. One-dimensional (1D) carbon fiber in particular is renowned for excellent strength with high surface-to-volume ratio and is widely commercially available. Although an exceptional candidate to address current energy and environmental needs, carbon fibers require further investigation to be used to their full potential. Emerging strategies for carbon fiber design rely on developing facile synthetic routes for controlled carbon structures. The scientific community has shown extensive interest in porous carbon fabrication owing to the excellent performance enhancement in separation, filtration, energy storage, energy conversion, and several other applications. This dissertation both reviews and contributes to the recent works of porous carbon and their applications in energy and environmental sciences. The background section shows recent development in porous carbon and the processing methods under investigation and current synthetic methods for designing porous carbon fibers (PCF). Later sections focus on original research. A controlled radical polymerization method, reversible addition-fragmentation chain transfer (RAFT), enabled a synthetic design for a block copolymer precursor, poly(methyl methacrylate) (PMMA) and polyacrylonitrile (PAN). The block copolymer (PMMA-b-PAN) possesses a unique microphase separation when electrospun and develop narrowly disperse mesopores upon carbonization. The PMMA and PAN domains self-assemble in a kinetically trapped disordered network whereby PMMA decomposes and PAN cross-links into PCF. The initial investigation highlights the block copolymer molecular weight and compositional design control for tuning the physical and electrochemical properties of PCF. Based on this study, mesopore (2 – 50 nm) size can be tuned between 10 – 25 nm while maintaining large surface areas, and the PAN-derived micropores (< 2 nm). The mesopores and micropores both contribute to the development of the unique hierarchical porous carbon structure which brings unprecedented architectural control. The pore control greatly contributes to the carbon field as the nano-scale architecture significantly influences performance and functionality. The next section uses PCF to clean water sources that are often tainted with undesirable ions such as salts and pollutants. Deionization or electrosorption is an electrochemical method for water purification via ion removal. I employed the PCFs as an electrode for deionization because of their high surface area and tunable pore size. Important for deionization, the adsorption isotherms and kinetics highlight the capacity and speed for purification of water. I studied PCF capacitive filtration on charged organic salts. Because PCF have both micropores and mesopores, they were able to adsorb ions at masses exceeding their own weight. The PFC adsorption efficiency was attributed to the diffusion kinetics within the hierarchical porous system and the double layer capacitance development on the PCF surface. In addition, based on the mechanism of adsorption, the PCFs showed high stability and reusability for future adsorption/desorption applications. The PCF performance as an electrosorption material highlights the rational design for efficient electrodes by hierarchical interconnected porosity. Another application of PFCs is updating evaporative desalination methods for water purification. Currently distillation is not widely used as a source of potable water owing to the high cost and energy requirement. Solar desalination could serve as a low-cost method for desalination; however, the evaporation enthalpy of water severely limits practical implementation. Here I apply the pore design of PCF as a method for water nano-confinement. Confinement effects reduce water density and lowers evaporation enthalpy. Desalination in PCF were studied in pores < 2 nm to 22 nm. The PCF pore size of ~ 10 nm was found to be the peak efficiency and resulted in a ~ 46% reduction in enthalpy. Interestingly, the PCF nano-confinement also contributed to the understanding in competing desorption energy for evaporation in micropores. The pore design in PCF also shows confinement effects that can be implemented in other environmental applications. Lastly, the block copolymer microphase morphology was explored in a vapor induced phase separation system. The resulting PCF properties showed a direct influence from the phase separation caused by nonsolvent. At low nonsolvent vapor, a disordered microphase separation occurred, however upon application of nonsolvent vapor, the polymer chains reorganized. The reorganization initially improved mechanical properties by developing more long-range ordered graphic chains in the PAN-derived carbon. However, at higher nonsolvent vapor concentrations, the fibers experienced polymer precipitation which resulted in bead and clump formation in the fiber mats. The beads and clumps lowered both mechanical properties and electrochemical performance. The vapor induced phase separation showed a method for enhancing mechanical properties without compromising electrochemical performance in flexible carbon fibers.
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    Charge distribution guided by grain crystallographic orientations in polycrystalline battery materials
    Xu, Zhengrui; Jiang, Zhisen; Kuai, Chunguang; Xu, Rong; Qin, Changdong; Zhang, Yan; Rahman, Muhammad Mominur; Wei, Chenxi; Nordlund, Dennis; Sun, Cheng-Jun; Xiao, Xianghui; Du, Xi-Wen; Zhao, Kejie; Yan, Pengfei; Liu, Yijin; Lin, Feng (2020-01-08)
    Architecting grain crystallographic orientation can modulate charge distribution and chemomechanical properties for enhancing the performance of polycrystalline battery materials. However, probing the interplay between charge distribution, grain crystallographic orientation, and performance remains a daunting challenge. Herein, we elucidate the spatially resolved charge distribution in lithium layered oxides with different grain crystallographic arrangements and establish a model to quantify their charge distributions. While the holistic "surface-to-bulk" charge distribution prevails in polycrystalline particles, the crystallographic orientation-guided redox reaction governs the charge distribution in the local charged nanodomains. Compared to the randomly oriented grains, the radially aligned grains exhibit a lower cell polarization and higher capacity retention upon battery cycling. The radially aligned grains create less tortuous lithium ion pathways, thus improving the charge homogeneity as statistically quantified from over 20 million nanodomains in polycrystalline particles. This study provides an improved understanding of the charge distribution and chemomechanical properties of polycrystalline battery materials.
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    Computational Modeling of Heterogeneity of Stress, Charge, and Cyclic Damage in Composite Electrodes of Li-Ion Batteries
    Liu, Pengfei; Xu, Rong; Liu, Yijin; Lin, Feng; Zhao, Kejie (IOP Publishing Limited, 2020-03-05)
    Charge heterogeneity is a prevalent feature in many electrochemical systems. In a commercial cathode of Li-ion batteries, the composite is hierarchically structured across multiple length scales including the sub-micron single-crystal primary-particle domains up to the macroscopic particle ensembles. The redox kinetics of charge transfer and mass transport strongly couples with mechanical stresses. This interplay catalyzes substantial heterogeneity in the charge (re)distribution, stresses, and mechanical damage in the composite electrode during charging and discharging. We assess the heterogeneous electrochemistry and mechanics in a LiNixMnyCozO₂ (NMC) cathode using a fully coupled electro-chemo-mechanics model at the cell level. A microstructureresolved model is constructed based on the synchrotron X-ray tomography data. We calculate the stress field in the composite and then quantitatively evaluate the kinetics of surface charge transfer and Li transport biased by mechanical stresses. We further model the cyclic behavior of the cell. The repetitive deformation of the active particles and the weakening of the interfacial strength cause gradual increase of the interfacial debonding. The mechanical damage impedes electron transfer, incurs more charge heterogeneity, and results in the capacity degradation in batteries over cycles.
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    Defect and structural evolution under high-energy ion irradiation informs battery materials design for extreme environments
    Rahman, Muhammad Mominur; Chen, Wei-Ying; Mu, Linqin; Xu, Zhengrui; Xiao, Ziqi; Li, Meimei; Bai, Xianming; Lin, Feng (Nature Research, 2020-09-11)
    Understanding defect evolution and structural transformations constitutes a prominent research frontier for ultimately controlling the electrochemical properties of advanced battery materials. Herein, for the first time, we utilize in situ high-energy Kr ion irradiation with transmission electron microscopy to monitor how defects and microstructures evolve in Na- and Li-layered cathodes with 3d transition metals. Our experimental and theoretical analyses reveal that Li-layered cathodes are more resistant to radiation-induced structural transformations, such as amorphization than Na-layered cathodes. The underlying mechanism is the facile formation of Li-transition metal antisite defects in Li-layered cathodes. The quantitative mathematical analysis of the dynamic bright-field imaging shows that defect clusters preferentially align along the Na/Li ion diffusion channels (a-b planes), which is likely governed by the formation of dislocation loops. Our study provides critical insights into designing battery materials for extreme irradiation environments and understanding fundamental defect dynamics in layered oxides.
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    Design and synthesis of Ni-rich and low/no-Co layered oxide cathodes for Li-ion batteries
    Yang, Zhijie (Virginia Tech, 2023-02-23)
    Li-ion batteries (LIBs) have achieved remarkable success in electric vehicles (EVs), consumer electronics, grid energy storage, and other applications thanks to a wide range of electrode materials that meet the performance requirements of different application scenarios. Cathodes are an essential component of LIBs, which governs the performance of commercial LIBs. Layered transition metal oxide, i.e., LiNixCoyMn1-x-yO2 (NMC), is one family of cathodes that are widely applied in the prevailing commercial LIBs. With increasing demand for high energy density, the development of layered oxide cathodes is towards high Ni content because Ni redox couples majorly contribute to the battery capacity. Meanwhile, the battery community has been making tremendous efforts to eliminate Co in layered cathodes due to its high cost, high toxicity, and child labor issues during Co mining. However, these Ni-rich Co-free cathodes usually suffer from low electrochemical and structural stability. Several strategies are adopted to enhance the stability of Ni-rich Co-free cathodes, such as doping, coating, and synthesizing single crystal particles. However, the design principles and synthesis mechanisms of these approaches have not been fully understood. Herein, we design and synthesize stable Ni-rich and low/no-Co layered oxide cathodes by manipulating the chemical and structural properties of cathode particles. Our studies reveal the cathode formation mechanisms and shed light on the cathode design through complementary synchrotron microscopic and spectroscopic characterization methods. In Chapter 1, the motivation for LIB research is introduced from the perspective of its indispensable role in achieving carbon neutrality. We then comprehensively introduce the status of LIBs at present, including assessing their sustainability, worldwide supply chain and manufacturing, and cathode materials. Subsequently, we focus on the Co-free layered oxide cathodes and discuss their structure, limitations, and strategies to address the challenges. Finally, we discuss single crystal Ni-rich layered oxide cathodes and the challenges and strategies associated with their synthesis. In Chapter 2, we investigate the dopant redistribution, phase propagation, and local chemical changes of layered oxides at multiple length scales using a multielement-doped LiNi0.96Mg0.02Ti0.02O2 (Mg/Ti-LNO) as a model platform. We observed that dopants Mg and Ti diffuse from the surface to the bulk of cathode particles below 300 °C long before the formation of any layered phase, using a range of synchrotron spectroscopic and imaging diagnostic tools. After calcination, Ti is still enriched at the cathode particle surface, while Mg has a relatively uniform distribution throughout cathode particles. Our findings provide experimental guidance for manipulating the dopant distribution upon cathode synthesis. In Chapter 3, we synthesized Mn(OH)2-coated single crystal LiNiO2 (LNO) and used it as the platform to monitor the Mn redistribution and the structural and chemical evolution of the LNO cathode. We use in situ transmission X-ray microscopy (TXM) to track the Mn tomography inside the LNO particle and Ni oxidation state evolution at various temperatures below 700 °C. We further reveal chemical and structural changes induced by different extents of Mn diffusion at ensemble-averaged scale, which validates the results at the single particle scale. The ion diffusion behavior in the cathode is highly temperature dependent. Our study provides guidance for ion distribution manipulation during cathode modification. In Chapter 4, we successfully fabricated a surface passivation layer for NMC particles via a feasible quenching approach. A combination of bulk and surface structural characterization methods show the correlation of surface layer with bulk chemistry including valence state and charge distribution. Our design enables high interfacial stability and homogeneous charge distribution, impelling superior electrochemical performance of NMC cathode materials. This study provides insights into the cathode surface layer design for modifying other high-capacity cathodes in LIBs. In Chapter 5, we use statistical tools to identify the significance of multiple synthetic parameters in the molten salt synthesis of single crystal Ni-rich NMC cathodes. We also create a prediction model to forecast the performance of synthesized single crystal Ni-rich NMC cathodes from the input of synthetic parameters with relatively high prediction accuracy. Guided by the models, we synthesize single crystal LiNi0.9Co0.05Mn0.05O2 (SC-N90) with different particle sizes. We find large single crystals show worse capacity and cycle life than small single crystals especially at high current rates due to slower Li kinetics. However, large single crystal has higher thermal stability potentially because of smaller specific surface area. The findings of particle size effect on the performance provide insights into size engineering while developing next-generation single crystal Ni-rich NMC cathodes. The statistical and prediction models developed in this study can guide the molten salt synthesis of Ni rich cathodes and simplify the optimization process of synthetic parameters. Chapter 6 summarizes our efforts on the novel design and fundamental understanding of the state-of-the-art cathodes. We also provide our future perspectives for the development of LIBs.
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    Effect of the grain arrangements on the thermal stability of polycrystalline nickel-rich lithium-based battery cathodes
    Hou, Dong; Xu, Zhengrui; Yang, Zhijie; Kuai, Chunguang; Du, Zhijia; Sun, Cheng-Jun; Ren, Yang; Liu, Jue; Xiao, Xianghui; Lin, Feng (Springer, 2022-06-15)
    One of the most challenging aspects of developing high-energy lithium-based batteries is the structural and (electro)chemical stability of Ni-rich active cathode materials at thermally-abused and prolonged cell cycling conditions. Here, we report in situ physicochemical characterizations to improve the fundamental understanding of the degradation mechanism of charged polycrystalline Ni-rich cathodes at elevated temperatures (e.g., ≥ 40 °C). Using multiple microscopy, scattering, thermal, and electrochemical probes, we decouple the major contributors for the thermal instability from intertwined factors. Our research work demonstrates that the grain microstructures play an essential role in the thermal stability of polycrystalline lithium-based positive battery electrodes. We also show that the oxygen release, a crucial process during battery thermal runaway, can be regulated by engineering grain arrangements. Furthermore, the grain arrangements can also modulate the macroscopic crystallographic transformation pattern and oxygen diffusion length in layered oxide cathode materials.
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    Electrochemical Carbon Dioxide Reduction for Renewable Carbonaceous Fuels and Chemicals
    Han, Xue (Virginia Tech, 2023-03-15)
    Electrochemical CO2 reduction reaction (ECO2RR) powered by renewable electricity possesses the potential to store intermittent energy in chemical bonds while producing sustainable chemicals and fuels. Unfortunately, it is hard to achieve low overpotential, high selectivity, and activity simultaneously of ECO2RR. Developing efficient electrocatalysts is the most promising strategy to enhance electrocatalytic activity in CO2 reduction. Herein, we designed novel Bi-Cu2S heterostructures by a one-pot wet-chemistry method. The epitaxial growth of Cu2S on Bi results in abundant interfacial sites and these heterostructured nanocrystals demonstrated high electrocatalytic performance of ECO2RR with high current density, largely reduced overpotential, near-unity FE for formate production (Chapter 2). Meanwhile, we see a lot of opportunities for catalysis in a confined space due to their tunable microenvironment and active sites on the surface, leading to a broad spectrum of electrochemical conversion schemes. Herein, we reveal fundamental concepts of confined catalysis by summarizing recent experimental investigations. We mainly focus on carbon nanotubes (CNTs) encapsulated metal-based materials and summarize their applications in emerging electrochemical reactions, including ECO2RR and more (Chapter 3). Although we were able to obtain high activity and selectivity toward C1 products, it is more attractive to go beyond C1 chemicals to produce C2 products due to their high industrial value. Herein, we designed Ag-modified Cu alloy catalysts that can create a CO-rich local environment for enhancing C-C coupling on Cu for C2 formation. Moreover, Ag corporate in Cu can chemically improve the structural stability of Cu lattice. (Chapter 4) Nevertheless, advanced electrocatalytic platforms cannot be developed without a fundamental understanding of binding configurations of the surface-adsorbed intermediates and adsorbate-adsorbate interaction on the local environment in electrochemical CO2 reduction. In this case, we make discussions of recent developments of machine learning based models of adsorbate-adsorbate interactions, including the oversimplified linear analytic relationships, the cluster expansion models parameterized by machine learning algorithms, and the highly nonlinear deep learning models. We also discuss the challenges of the field, particularly overcoming the limitations of pure data driven models with the integration of computational theory and machine learning of lateral interactions for catalyst materials design. (Chapter 5).
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    Enhancing surface oxygen retention through theory-guided doping selection in Li1-xNiO2 for next-generation lithium-ion batteries
    Cheng, Jianli; Mu, Linqin; Wang, Chunyang; Yang, Zhijie; Xin, Huolin L.; Lin, Feng; Persson, Kristin A. (2020-11-28)
    Layered lithium metal oxides have become the cathode of choice for state-of-the-art Li-ion batteries (LIBs), particularly those with high Ni content. However, the Ni-rich cathode materials suffer from extensive oxygen evolution, which contributes to the formation of surface rocksalt phases as well as thermal instability. Using first-principles calculations, we systematically evaluate the effectiveness of doping elements to enhance surface oxygen retention of Li1-xNiO2. The evaluation process includes (i) choosing the most stable surface facet from the perspective of equilibrium surface stability analysis of as-synthesized LiNiO2, (ii) determining the preferable atomic site and segregation behavior for each dopant, and (iii) evaluating the surface oxygen retention ability of doped-Li1-xNiO2 (0.25 <= x <= 1) compared to the pristine material. We also discuss and rationalize the ability of these elements to enhance surface oxygen retention based on local environment descriptors such as dopant-oxygen bond strength. Overall, W, Sb, Ta and Ti are predicted as the most promising surface dopants due to their strong oxygen bonds and robust surface segregation behavior. Finally, Sb-doped LiNiO2 is synthesized and shown to present a surface enrichment of Sb and a significantly improved electrochemical performance, comparing with pristine LiNiO2. This work provides a generic approach that can lead to the greatly enhanced stabilization of all high-energy cathode materials, particularly the high Ni and low Co oxides.
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    Fundamental Investigations into the Metal-Organic Framework Redox-Hopping Charge Transport——Mechanisms and Improvement Strategies
    Yan, Minliang (Virginia Tech, 2024-10-16)
    Redox hopping is the dominant charge transport mechanism in many catalyst-modified metal-organic frameworks (MOFs). Previous studies have shown that ion diffusion is the rate-determining step of redox hopping, but the realization regarding to the fundamental mechanism of redox hopping in MOF is still infantile. In this dissertation, we will discuss the redox hopping process in MOFs from multiple perspectives, including how to use the Scholz model to analyze the coefficients in redox hopping, the influence of the type of carrier, the influence of electrolyte concentration, and the influence of temperature on redox hopping, so as to try to reveal the mechanism of the redox hopping process and make some constructive suggestions for the future design and application of MOF based on this topic.
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    Heterogeneous Redox Chemistries in Layered Oxide Materials for Lithium-Ion Batteries
    Xu, Zhengrui (Virginia Tech, 2022-01-05)
    The invention of the lithium-ion battery has revolutionized the passenger transportation field in recent years, and it has emerged as one of the state-of-the-art solutions to address greenhouse gases emission and air pollution issues. Layered oxide lithium-ion battery cathode materials have become commercially successful in the past few decades due to their high energy density, high power density, long cycle life, and low cost. Yet, with the increasing demand for battery performance, it is crucial to understand the material fading mechanisms further to improve layered oxide materials' performance. A heterogeneous redox reaction is a dominant fading mechanism, which limits the utilization percentage of a battery materials' redox capability and leads to adverse effects such as detrimental interfacial reactions, lattice oxygen release, and chemomechanical breakdown. Crystallographic defects, such as dislocations and grain boundaries, are rich in battery materials. These crystallographic defects change the local lithium-ion diffusivity and have a dramatic effect on the redox reactions. To date, there is still a knowledge gap on how various crystallographic defects affect electrochemistry at the microscopic scale. Herein, we adopted synchrotron-based diffraction, imaging, and spectroscopic techniques to systematically study the correlation between crystallographic defects and redox chemistries in the nanodomain. Our studies shed light on design principles of next-generation battery materials. In Chapter 1, we first provide a comprehensive background introduction on the battery chemistry at various length scales. We then introduce the heterogeneous redox reactions in layered oxide cathode materials, including a discussion on the impacts of heterogeneous redox reactions. Finally, we present the different categories of crystallographic defects in layered oxide materials and how these crystallographic defects affect electrochemical performance. In Chapter 2, we use LiCoO2, a representative layered oxide cathode material, as the material platform to quantify the categories and densities of various crystallographic defects. We then focus on geometrically necessary dislocations as they represent a major class of crystallographic defects in LiCoO2. Combining synchrotron-based X-ray fluorescence mapping, micro-diffraction, and spectroscopic techniques, we reveal that geometrically necessary dislocations can facilitate the charging reactions in LiCoO2 grains. Our study illustrates that the heterogeneous redox chemistries can be potentially mitigated by precisely controlling the defects. In Chapter 3, we systematically investigated how grain boundaries affect redox reactions. We reveal that grain boundaries can guide redox reactions in LiNixMnyCo1-x-yO2 (NMC) materials. Specifically, NMC materials with radially aligned grains have a more uniform charge distribution, less stress mismatch, and better cycling performance. NMC materials with randomly orientated grains have a more heterogeneous redox reaction. These heterogeneous redox reactions are related to the lattice strain mismatch and worse cycling performance. Our study emphasizes the importance of tuning grain orientations to achieve improved performance. Chapter 4 systematically investigated how the grain boundaries and crystallographic orientations affect the thermal stability of layered oxide cathode materials. Combining diffraction, spectroscopic, and imaging techniques, we reveal that a cathode materials' microstructure plays a significant role in determining the lattice oxygen release behavior and, therefore, determines cathode materials' thermal stability. Our study provides a fundamental understanding of how the grain boundaries and crystallographic orientations can be tuned to develop better cathode materials for the next-generation Li-ion batteries. Chapter 5 summarizes the contributions of our work and provides our perspective on future research directions.
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    High-performance inertial impaction filters for particulate matter removal
    Zhang, Xiaowei; Zhang, Wei; Yi, Mingqiang; Wang, Yingjie; Wang, Pengjun; Xu, Jun; Niu, Fenglei; Lin, Feng (Springer Nature, 2018-03-19)
    Airborne particulate matter (PM) is causing more and more serious air pollution and threatening the public health. However, existing air filter technologies with the easy-to-block manner can rarely meet the requirements of high-performance PM filters. Here we propose a conceptually new type of inertial impaction filters for rapidly high-efficiency PM removal. Under the airflow velocity of 8.0 m/s, the real inertial impaction filters show high PM removal efficiencies of up to 97.77 +/- 1.53% and 99.47 +/- 0.45% for PM2.5 and PM10, respectively. Compared with the traditional air filters reported previously, the inertia impaction filters exhibit extremely low pressure drop of 5-10 Pa and high quality factor (QF) values of 0.380 Pa-1 and 0.524 Pa-1 for PM2.5 and PM10, respectively. These greatly improved QF values are achieved through a series of inertial separation processes. The feature dimension of filtration channel is dozens of times larger than PM average size, which greatly decreases airflow resistance. Particularly, this inertial structure can be made of various types of materials, which shows great potential for low-cost fabrication of large-area devices. As a stand-alone device or incorporated with the existing PM air filter, this inertial impaction filter will bring great benefits to the public health.
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    Intrinsic and Extrinsic Catalysis in Zirconium-based Metal-Organic Frameworks
    Gibbons, Bradley James (Virginia Tech, 2022-05-31)
    Metal-organic frameworks (MOFs) are a class of hybrid materials that offer a promising platform for a range of catalytic reactions. Due to their complex structure, MOFs offer unique opportunities to serve as novel catalysts, or as host to improve the properties of previously studied species. However, while other catalytic approaches have been studied for many decades, the recency of their discovery means that significant work is still needed to develop MOFs as a viable option for large scale application. Herein, we aimed to advance the field of MOFs as both novel catalysts, and as host platforms for other catalytic species. To this end, we studied synthetic pathways to produce favorable MOF properties such as higher porosity and active site concentration through introduction of defects and macromorphological control, as well as utilization of molecular catalysts imbedded in the MOF structure for multicomponent, light driven reactivity. Chapter 1 introduces the history MOFs and the pursuit of the stable structures commonly associated with MOF chemistry. The synthesis process for zirconium-based MOFs will be discussed, with specific attention given to the modulated synthesis process which can harnessed to change MOF properties and improve catalysis. Two specific reactions will be introduced which serve as a basis for study in this work. First, the hydrolysis of organophosphate nerve agents by MOFs acting as novel catalysts will be introduced. The mechanism of reaction, as well as previous work in this field will be discussed. Finally, water oxidation as part of artificial photosynthesis through incorporated molecular catalysts will be introduced. Chapter 2 presents a modulator screening study on a zirconium-based MOF, UiO-66. One of the most commonly studied MOFs, UiO-66 provides an excellent platform for synthetic modulation. Particle size and defect level were measured of 26 synthetic variations and synthetic conditions were found to isolate changes in defect level and particle size, which typically change coincident with each other. Hydrolysis of the organophosphate compound dimethyl 4-nitrophenylphosphate (DMNP) was used to study the impact of particle size and defect level on reactivity. The reaction was found to be surface limited, even at high levels of missing linker defects. In Chapter 3, the macromorphology of three zirconium-based MOFs were tuned through synthesis modification. MOF powders and xerogels were prepared and characterized to highlight the desirable properties obtained through the gelation process. The materials were compared in the hydrolysis of DMNP and significant enhancement was observed for UiO-66 and NU-1000 xerogels. This was largely attributed to the introduction of mesoporosity and nanocrystalline particle sizes, which significantly increase the number of reactive sites easily accessible for catalysis. In Chapter 4 the authors examine MOFs as a host for molecular catalysts for use in photoelectrochemical water oxidation. A ruthenium-based catalyst [Ru(tpy)(dcbpy)]2+ was incorporated into UiO-67 through a mixed linker synthesis and grown on a WO3 substrate (Ru-UiO-67/WO3). Previous work from our group demonstrated Ru-UiO-67 retained the catalytic activity as the molecular species, while improving the recyclability of the material. In this work, addition of WO3 as a light harvester allowed for the reaction to be driven at a photoelectrochemical underpotential, a first for MOF-based water oxidation. Finally, Chapter 5 offers a perspective of the field of MOF-based artificial photosynthesis. Particular attention is given to issues of diffusion, selectivity, stability, and moving towards integration of multiple components rather than the study of half-reactions.
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    Investigating Cathode–Electrolyte Interfacial Degradation Mechanism to Enhance the Performance of Rechargeable Aqueous Batteries
    Zhang, Yuxin (Virginia Tech, 2023-12-04)
    The invention of Li-ion batteries (LIBs) marks a new era of energy storage and allows for the large-scale industrialization of electric vehicles. However, the flammable organic electrolyte in LIBs raises significant safety concerns and has resulted in numerous fires and explosion accidents. In the pursuit of more reliable and stable battery solutions, interests in aqueous batteries composed of high-energy cathodes and water-based electrolytes are surging. Limited by the narrow electrochemical stability window (ESW) of water, conventional aqueous batteries only achieve inferior energy densities. Current development mainly focuses on manipulating the properties of aqueous electrolytes through introducing excessive salts or secondary solvents, which enables an unprecedentedly broad ESW and more selections of electrode materials while also resulting in some compromises. On the other hand, the interaction between electrodes and aqueous electrolytes and associated electrode failure mechanism, as the key factors that govern cell performance, are of vital importance yet not fully understood. Owing to the high-temperature calcination synthesis, most electrode materials are intrinsically moisture-free and sensitive to the water-rich environment. Therefore, compared to the degradation behaviors in conventional LIBs, such as cracking and structure collapse, the electrode may suffer more severe damage during cycling and lead to rapid capacity decay. Herein, we adopted multi-scale characterization techniques to identify the failure modes at cathode–electrolyte interface and provide strategies for improving the cell capacity and life during prolonged cycling. In Chapter 1, we first provide a background introduction of conventional non-aqueous and aqueous batteries. We then show the current development of modern aqueous batteries through electrolyte modification and their merits and drawbacks. Finally, we present typical electrode failure mechanism in non-aqueous electrolytes and discuss how water can further impact the degradation behaviors. In Chapter 2, we prepare three types of aqueous electrolytes and systematically evaluate the electrochemical performance of LiNixMnyCo1-x-yO2, LiMn2O4 and LiFePO4 in the aqueous electrolytes. Combing surface- and bulk-sensitive techniques, we identify the roles played by surface exfoliation, structure degradation, transition metal dissolution and interface formation in terms of the capacity decay in different cathode materials. We also provide fundamental insights into the materials selection and electrolyte design in the aqueous batteries. In Chapter 3, we select LiMn2O4 as the material platform to study the transition metal dissolution behavior. Relying on the spatially resolved X-ray fluorescence microscopy, we discover a voltage-dependent Mn dissolution/redeposition (D/R) process during electrochemical cycling, which is confirmed to be related to the Jahn–Teller distortion and surface reconstruction at different voltages. Inspired by the findings, we propose an approach to stabilize the material performance through coating sulfonated tetrafluoroethylene (i.e., Nafion) on the particle, which can regulate the proton diffusion and Mn dissolution behavior. Our study discovers the dynamic Mn D/R process and highlights the impact of coating strategy in the performance of aqueous batteries. In Chapter 4, we investigate the diffusion layer formed by transition metals at the electrode–electrolyte interface. With the help of customized cells and XFM technique, we successfully track the spatiotemporal evolution of the diffusion layer during soaking and electrochemical cycling. The thickness of diffusion layer is determined to be at micron level, which can be readily diminished when gas is generated on the electrode surface. Our approach can be further expanded to study the phase transformation and particle agglomeration at the interfacial region and provide insights into the reactive complexes. In Chapter 5, we reveal the correlation between the electrolytic water decomposition and ion intercalation behaviors in aqueous batteries. In the Na-deficient system, we discover that overcharging in the formation process can introduce more cyclable Na ions into the full cell and allows for a boosted performance from 58 mAh/g to 124 mAh/g. The mechanism can be attributed to the water oxidation on the cathode and Na-ion intercalation on the anode when the charging voltage exceeds the normal oxidation potential of cathode. We emphasize the importance of unique formation process in terms of the cell performance and cycle life of aqueous batteries. In Chapter 6, we summarize the results of our work and propose perspectives of future research directions.
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    Investigating Chemical and Structural Heterogeneities of High-Voltage Spinel Cathode Material for Li-ion Batteries
    Spence, Stephanie Leigh (Virginia Tech, 2023-03-20)
    Li-ion battery technologies have transformed the consumer electronics and electric vehicles landscape over the last few decades. Single-crystal cathode materials with controllable physical properties including size, morphology, and crystal facets can aid researchers in developing relationships between physical characteristics, chemical properties, and electrochemical performance. High-voltage LiNi0.5Mn1.5O4 (LNMO) materials are desirable as cathodes due to their low cost, low toxicity, and high capacity and energy density making them promising to meet increasing consumer demands for battery materials. However, transition metal dissolution, interfacial instability, and capacity fading plague these materials when paired with graphite, limiting their commercial capability. Furthermore, variation in Ni/Mn ordering can lead to complex multiphase co-existence and changes in Mn oxidation state and electrochemical performance. These properties can be adjusted during synthesis using a facile and tunable molten salt synthesis method. This dissertation focuses on the investigation of chemical and structural heterogeneities of LNMO prepared under different synthetic conditions at different length scales. In Chapter 2, the influences of molten salt synthesis parameters on LNMO particle size, morphology, bulk uniformity, and performance are evaluated revealing the difficulty of reproducible cathode synthesis. We utilize the X-ray nanodiffraction technique throughout this work, which provides high-resolution structural information. We develop a method to measure and relate lattice strain to phase distribution at the tens of nanometers scale. In Chapter 3, mapping lattice distortions of LNMO particles with varying global Mn oxidation states reveals inherent structural defects and distortion heterogeneities. In Chapter 4, we examine lattice distortion evolution upon chemical delithiation, Mn dissolution behaviors, and evaluate the chemical delithiation method as a means to replicate electrochemical cycling conditions. We further investigate lattice distortion spatially via in situ nanodiffraction during battery cycling in Chapter 5, illustrating the capabilities of the measurement to provide practical understanding of cathode transformations. From intra-particle to electrode materials level, heterogeneities that arise in cathode materials can dictate performance properties and degradation mechanisms and are necessary for researchers to understand for the improvement of Li-ion battery systems. The development of the nanodiffraction measurements aids in our understanding of inherent and dynamic materials chemical and structural heterogeneities.
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    Investigating Particle Size-Dependent Redox Kinetics and Charge Distribution in Disordered Rocksalt Cathodes
    Zhang, Yuxin; Hu, Anyang; Liu, Jue; Xu, Zhengrui; Mu, Linqin; Sainio, Sami; Nordlund, Dennis; Li, Luxi; Sun, Cheng-Jun; Xiao, Xianghui; Liu, Yijin; Lin, Feng (Wiley-V C H Verlag, 2022-04)
    Understanding how various redox activities evolve and distribute in disordered rocksalt oxides (DRX) can advance insights into manipulating materials properties for achieving stable, high-energy batteries. Herein, the authors present how the reaction kinetics and spatial distribution of redox activities are governed by the particle size of DRX materials. The size-dependent electrochemical performance is attributed to the distinct cationic and anionic reaction kinetics at different sizes, which can be tailored to achieve optimal capacity and stability. Overall, the local charged domains in DRX particles display random heterogeneity caused by the isotropic delithiation pathways. Owing to the kinetic limitation, the micron-sized particles exhibit a holistic "core-shell" charge distribution, whereas sub-micron particles show more uniform redox reactions throughout the particles and ensembles. Sub-micron DRX particles exhibit increasing anionic redox activities yet inferior cycling stability. In summary, engineering particle size can effectively modulate how cationic and anionic redox activities evolve and distribute in DRX materials.
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    Investigating the interfacial process and bulk electrode chemistry in tungsten oxide electrochromic materials
    Hu, Anyang (Virginia Tech, 2020)
    The growing need for high-performance electrode materials in electrochemical conversion and storage applications requires further fundamental investigation on the working and degradation mechanisms of these materials. Among various functional materials, transition metal oxides are still one of the main choices due to their tunable chemical compositions and diverse crystal structures in most aqueous and organic electrolytes. The charge transfer process mainly occurs at the electrode-electrolyte interface, and controlling the electrochemical interfacial stability represents a key challenge in developing sustainable and cost-effective electrochromic materials. The present thesis focuses on classical tungsten trioxide (WO3) materials as the platform to uncover the previously unknown interrelationship between phase transformation, morphological evolution, nanoscale color heterogeneity, and performance degradation in these materials during 3,000 cyclic voltammetry cycles. Through the application of novel cell design, synchrotron/electron spectroscopic, and imaging analyses, we observe that the interface between the WO3 electrode and 0.5 M sulfuric acid electrolyte undergoes constant changes due to the tungsten oxide dissolution and redeposition. The redeposition of dissolved tungsten species provokes in situ crystal growth, which ultimately leads to phase transformation from the semicrystalline WO3 to a nanoflake-shaped, proton-trapped tungsten trioxide dihydrate (HxWO3·2H2O). The multidimensional (surface and bulk) quantification of the electronic structure with X-ray measurements reveals that the tungsten reduction caused by proton trapping is heterogeneous at the nanometric scale and is responsible for the nanoscale color heterogeneity. The Coulombic efficiency, optical modulation, apparent diffusion coefficients, and switching kinetics are gradually diminished during 3,000 cyclic voltammetry cycles, resulting from the structural and chemical changes of the WO3 electrode. We hypothesize that the high interfacial reactivity in the electrode-electrolyte interfacial region could be the universal underlying mechanism leading to undesired bulk structural changes of inorganic electrochromic materials.
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    Investigation of Alkali Metal-Host Interactions and Electrode-Electrolyte Interfacial Chemistries for Lean Lithium and Sodium Metal Batteries
    Kautz Jr, David Joseph (Virginia Tech, 2021-06-21)
    The development and commercialization of alkali ion secondary batteries has played a critical role in the development of personal electronics and electric vehicles. The recent increase in demand for electric vehicles has pushed for lighter batteries with a higher energy density to reduce the weight of the vehicle while with an emphasis on improving the mile range. A resurgence has occurred in lithium, and sodium, metal anode research due to their high theoretical capacities, low densities, and low redox potentials. However, Li and Na metal anodes suffer from major safety issues and long-term cycling stability. This dissertation focuses on the investigation of the interfacial chemistries between alkali metal-carbon host interactions and the electrode-electrolyte interactions of the cathode and anode with boron-based electrolytes to establish design rules for "lean" alkali metal composite anodes and improve long-term stability to enable alkali metal batteries for practical electrochemical applications. Chapter 2 of this thesis focuses on the design and preliminary investigation of "lean" lithium-carbon nanofiber (<5 mAh cm-2) composite anodes in full cell testing using a LiNi0.6Mn0.2Co0.2O2 (NMC 622) cathode. We used the electrodeposition method to synthesize the Li-CNF composite anodes with a range of electrodeposition capacities and current densities and electrolyte formulations. Increasing the electrodeposition capacity improved the cycle life with 3 mAh cm-2 areal capacity and 2% vinylene carbonate (VC) electrolyte additive gave the best cycle life before reaching a state of "rapid cell failure". Increasing the electrodeposition rate reduced cycling stability and had a faster fade in capacity. The electrodeposition of lithium metal into a 2D graphite anode significantly improved cycle life, implying the increased crystallinity of the carbon substrate promotes improved anode stability and cycling capabilities. As the increased crystallinity of the carbon anode was shown to improve the "lean" composite anode's performance, Chapter 3 focuses on utilizing a CNF electrode designed with a higher degree of graphitization and probing the interacting mechanism of Li and Na with the CNF host. Characterization of the CNF properties found the material to be more reminiscent of hard carbon materials. Electrochemical analysis showed better long-term performance for Na-CNF symmetric cells. Kinetic analysis, using cyclic voltammetry (CV), revealed that Na ions successfully (de)intercalated within the CNF crystalline interlayers, while Li ions were limited to surface adsorption. A change in mechanism was quickly observed in the Na-CNF symmetric cycling from metal stripping/plating to ion intercalation/deintercalation, enabling the superior cycling stability of the composite anode. Improving the Na metal stability is necessary for enabling Na-CNF improved long-term performance. Sodium batteries have begun to garner more attention for grid storage applications due to their overall lower cost and less volumetric constraint required. However, sodium cathodes have poor electrode-electrolyte stability, leading to nanocracks in the cathode particles and transition metal dissolution. Chapter 4 focuses on electrolyte engineering with the boron salts sodium difluoro(oxolato)borate (NaDFOB) and sodium tetrafluoroborate (NaBF4) mixed together with sodium hexafluorophosphate (NaPF6) to improve the electrode-electrolyte compatibility and cathode particle stability. The electrolytes containing NaDFOB showed improved electrochemical stability at various temperatures, the formation of a more robust electrode-electrolyte interphase, and suppression in transition metal (TM) reduction and dissolution of the cathode particles measured after cycling. In Chapter 5, we focus on the electrochemical properties and the anode-electrolyte interfacial chemistry properties of the sodium borate salt electrolytes. Similar to Chapter 4, the NaDFOB containing electrolytes have improved electrochemical performance and stability. Following the same electrodeposition parameters as Chapter 2, we find the NaDFOB electrolytes improves the stability of electrodeposited Na metal and the "lean" composite anode's cyclability. This study suggests the great potential for the NaDFOB electrolytes for Na ion battery applications.
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    Is Nonflammability of Electrolyte Overrated in the Overall Safety Performance of Lithium Ion Batteries? A Sobering Revelation from a Completely Nonflammable Electrolyte
    Jia, Hao; Yang, Zhijie; Xu, Yaobin; Gao, Peiyuan; Zhong, Lirong; Kautz, David J. J.; Wu, Dengguo; Fliegler, Ben; Engelhard, Mark H. H.; Matthews, Bethany E. E.; Broekhuis, Benjamin; Cao, Xia; Fan, Jiang; Wang, Chongmin; Lin, Feng; Xu, Wu (Wiley-VCH, 2023-01)
    It has been widely assumed that the flammability of the liquid electrolyte is one of the most influential factors that determine the safety of lithium-ion batteries (LIBs). Following this consideration, a completely nonflammable electrolyte is designed and adopted for graphite||LiFePO4 (Gr||LFP) batteries. Contrary to the conventional understanding, the completely nonflammable electrolyte with phosphorus-containing solvents exhibits inferior safety performance in commercial Gr||LFP batteries, in comparison to the flammable conventional LiPF6-organocarbonate electrolyte. Mechanistic studies identify the exothermic reactions between the electrolyte (especially the salt LiFSI) and the charged electrodes as the "culprit" behind this counterintuitive phenomenon. The discovery emphasizes the importance of reducing the electrolyte reactivity when designing safe electrolytes, as well as the necessity of evaluating safety performance of electrolytes on a battery level.
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    Junction Based Gallium Nitride Power Devices
    Ma, Yunwei (Virginia Tech, 2023-09-05)
    Power electronics plays an important role in many energy conversion applications in modern society including consumer electronics, data centers, electric vehicles, and power grids, etc. The key components of power electronic circuits are power semiconductor devices including diodes and transistors, which determine the performance of power electronics circuits. Traditional power devices are based on the semiconductor silicon (Si), which have already reached the silicon's material limit. Gallium nitride (GaN) is a wide bandgap semiconductor with high electron mobility and high critical electric field. GaN-based power devices promise superior device performance over the Si-based counterpart. The primary design target of a unipolar power device is to achieve low on-resistance and high breakdown voltage. Although GaN high electron mobility transistor (HEMT) is commercially available in a voltage class from 15 V to 900 V, the performance of GaN devices is still far below the GaN material limit, due to several reasons: 1) To achieve the normally-off operation in a GaN HEMT, the density of two-dimensional electron gas (2DEG) channel cannot be too high; this limits the on-resistance reduction in the access region. 2) The gate capacitance of GaN HEMT is usually low so that the carrier concentration in the channel underneath the gate is relatively low, limiting the on-resistance reduction in the gated channel region. 3) The electric-field distribution in the drift region is not uniform, resulting in a limited breakdown voltage. We proposed to use the junction-based structure in GaN power devices to address the above problems and fully exploit GaN's material properties. The first part of this dissertation characterizes nickel oxide (NiO) as a p-type material to construct the junction-based GaN power devices. Although the homogenous p-GaN/n-GaN junction is preferred in many devices, the selective-area, p-GaN regrowth can lead to excessive leakage current; in comparison, the p-NiO/n-GaN junction is stable without leakage. This section describes the optimization of NiO deposition as well as the NiO characterization. Although acceptor in NiO is not generated by impurity doping, the acceptor concentration modulation is realized by tuning the O2 partial pressure during the sputtering process. Practical breakdown electric field is also characterized and confirmed to be higher than GaN. These results provide the design guidelines for NiO-GaN junction-based power devices. The second part of this dissertation demonstrates the 3D NiO-GaN junction gate to improve the GaN HEMT's on-resistance. The 3D junction gate structure enables a high carrier concentration under the gate region in the device on-state. Meanwhile, the strong depletion effect of the junction-based gate allows for a robust normally-off operation; as a result, the GaN wafer with a higher 2DEG concentration can be used to achieve both normally-off and low on-state resistance in HEMT devices. Simulation is also performed to project the performance space of trigate GaN junction HEMTs using the p-GaN instead of NiO. The third part of this dissertation presents the application of the p-GaN/n-GaN junction in the drift region of the multi-channel lateral devices to achieve the high breakdown voltage. Here p-GaN is grown in-situ with the multi-channel AlGaN/GaN structure, and there is no leakage problem. The structure is designed to achieve charge balance between the acceptor in p-GaN and the net donor in the multichannel AlGaN/GaN. This design enables a uniform electric field distribution and breakdown voltage over 10 kV. The fourth part of this dissertation presents the application of the p-NiO/n-GaN junction in vertical superjunction (SJ) devices. We show the design and simulation of this heterojunction structure in a SJ and confirm the uniform electric field and high breakdown voltage under the charge balance. Then the device fabrication is presented in detail, which mainly comprises the deep GaN trench etch, NiO self-aligned lift off, and photoresist trench planarization. The optimized device shows a trade-off between its drift region specific on-resistance versus breakdown that exceeds the 1D GaN's limit. The last part of this dissertation is exploring the design and fabrication of p-GaN/n-GaN based SJ devices. First, the challenges in p-GaN regrowth especially the introduction of interface impurities are discussed, followed by device simulation and modeling to optimize the SJ performance considering these interface impurities. The activation of regrown p-GaN in deep trenches is more difficult than planar p-GaN, and we present the characterization and physical model for the activation of the deep buried p-GaN. Last, the results of p-GaN filling regrowth and the acceptor concentration calibration in the lightly doped p-GaN are presented and discussed. In summary, our work combines experimental device fabrication and characterization, TCAD simulation, and device modeling to demonstrate the benefit of multi-dimensional, junction-based GaN power devices as compared to the traditional GaN power devices. The junction-based structure at gate region can provides stable normally-off operation and low on-resistance. When being applied to the drift region, the multidimensional junction structure can push the device specific on-resistance versus breakdown voltage trade-off near or even exceeding the material limit. These results will advance the performance and application spaces of GaN power devices.