The adsorption and activation energetics underpinning small molecule conversion on heterogeneous (photo)-electrocatalysts are intrinsically tied to catalyst surface properties. Absent compositional characterization techniques with sufficient interface sensitivity, however, (photo)-electrochemical performance can be misinterpreted in the context of bulk or near-surface material properties. Here we provide a fundamental investigation of the convoluting role of near-surface compositional heterogeneity in the interpretation of (photo)-electrochemical alkaline oxygen evolution reaction (OER) activity, highlighting challenges in correlating composition measured by surface- and near-surface-sensitive probes. TiO2 thin films grown by air-annealing the superelastic alloy Nitinol (TiO2/NiTi) crack under tensile mechanical strain, increasing the number of electrochemically active Ni sites (Ni site density) that are probed via voltammetric features corresponding to Ni3+/Ni2+ redox events. (Photo)-electrochemical OER kinetics trend with Ni site density, with overpotentials and Tafel slopes decreasing for Ni site densities < 1013 Ni/cm2 geo and asymptotically approaching the performance of the base NiTi substrate for Ni site densities > 1013 Ni/cm2 geo. Photoelectrochemical fill factors follow similar Ni site density dependent trends. When probing unstrained TiO2/NiTi, Ni site densities are two orders of magnitude higher when comparing near-surface-sensitive techniques (e.g., X-ray photoelectron spectroscopy (XPS), time of flight secondary ion mass spectrometry (TOF-SIMS), and scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS)) to surface-sensitive electrochemical measurements. This result highlights the challenge of correlating kinetic performance with intrinsic surface properties of electrochemical interfaces in the presence of near-surface compositional heterogeneity. Further, it reinforces the importance of fundamental investigations of surfaces with well-controlled composition and structure and the need for physically grounded and self-consistent interpretation of multiple near-surface characterization techniques.

A substantial number of electric vehicle batteries are poised to reach end-of-life conditions in the next decade. Direct recycling has advantages over typical recycling processes because it preserves the chemical structure of the material. One key step of direct recycling materials like nickel manganese cobalt oxide (NMC) cathodes is relithiation, which includes replacing the depleted lithium inventory in the cathode and annealing the material to fix crystallographic degradation. This work uses multiple characterization techniques (synchrotron X-ray diffraction, Ni X-ray absorption near edge structure, scanning transmission electron microscopy) to understand the lithiation mechanism of degraded and chemically relithiated NMC 622. Despite the necessary reconstruction after relithiation being limited to the surface of the degraded NMC 622, a high annealing temperature of 720 °C is still necessary to restore the NMC 622 structure back to pristine condition after relithiation. This work also finds that treating degraded NMC 622 with an annealing step is sufficient to restore key electrochemical and structural properties, other essential features of pristine NMC 622 material such as particle porosity and morphology are largely unaffected. Understanding the effect of this annealing step has important implications on defining the degree of success of any given direct recycling strategy.

Widespread adoption of electric vehicles is still limited as the incumbent anode material, graphite, is too low energy-density and too high cost to make electric vehicle batteries more affordable and longer range. Silicon anodes have the potential to solve these issues as silicon is earth-abundant and can provide much higher theoretical energy density than graphite. However, high content silicon anodes have not yet been commercialized because of silicon’s large volumetric expansion (~200-300%, compared to 10% for graphite) which causes electrode mechanical degradation, and silicon’s reactive surface which causes surface passivation difficulties. It is well known that silicon particles smaller than ~150 nm will not rupture, so many researchers have turned to silicon nanoparticles (Si NPs) as a solution to silicon’s mechanical problems [1]. As such, previous research has demonstrated success using plasma-enhanced chemical vapor deposition (PECVD) silicon nanoparticles with a hydrophilic coating such as polyethylene oxide (PEO) [2-4]. A correlation with smaller nanoparticles and better cycle life was found, even though the PECVD particles are all well below the 150 nm threshold. One explanation for this is that while the silicon itself will not crack under this threshold, the large volume expansion of the particle still transmits stress to the surrounding electrode matrix, which could lead to mechanical degradation of the composite and affect cycling. The radius of the particle is directly correlated with the stress felt by the surrounding electrode composite; as such, smaller particles will have better cycle life as damage to the electrode is limited [5]. This research investigates the cyclability and mechanical integrity PEO-coated PECVD Si electrodes with nanoparticles ranging from 6 to 27 nm in diameter and find that smaller nanoparticles have better cycle lifetimes because they damage the electrode less. We also find that the calendar life is unaffected by nanoparticle size, despite theoretical increases in active material surface area, as the PEO-coated Si NPs form dense electrodes that only allow electrolyte contact at the top surface of the electrode. This is significant as the choice of ultra-small Si NPs to improve cycle life does not exacerbate silicon’s surface passivation issues which control calendar life. References: [1] X. H. Liu, L. Zhong, S. Huang, S. X. Mao, T. Zhu and J. Y. Huang, "Size-Dependent Fracture of Silicon Nanoparticles During Lithiation," ACS Nano, vol. 6, no. 2, pp. 1522-1531, 2012. [2] G. M. Carroll, M. C. Schulze, T. R. Martin, G. F. Pach, J. E. Coyle, G. Teeter and N. R. Neale, "SiO2 Is Wasted Space in Single-Nanometer-Scale Silicon Nanoparticle-Based Composite Anodes for Li-Ion Electrochemical Energy Storage," ACS Applied Energy Materials, vol. 3, no. 11, pp. 10993-11001, 2020. [3] S. Jiang, B. Hu, R. Sahore, H. Liu, G. F. Pach, G. M. Carroll, L. Zhang, B. Zhao, N. R. Neale and Z. Zhang, "Tailoring the Surface of Silicon Nanoparticles for Enhanced Chemical and Electrochemical Stability for Li-Ion Batteries," ACS Applied Energy Materials, vol. 2, no. 9, pp. 6176-6183, 2019. [4] M. C. Schulze, G. M. Carroll, T. R. Martin, K. Sanchez-Rivera, F. Urias and N. R. Neale, "Hydrophobic versus Hydrophilic Interfacial Coatings on Silicon Nanoparticles Teach Us How to Design the Solid Electrolyte Interphase in Silicon-Based Li-Ion Battery Anodes," ACS Applied Energy Materials, vol. 4, no. 2, pp. 1628-1636, 2021. [5] C. T. Herakovich, "Thin-Walled Pressure Vessels," in A Concise Introduction to Elastic Solids , Springer, 2017, pp. 77-81.

Silicon anodes offer high energy density but face challenges like volume expansion and reactivity. This study shows ultra-small nanoparticles (∼6 nm) improve cycle life without reducing calendar life, as dense electrodes limit exposed surface area.

Calendering is a technique used to maximize the volumetric energy density of battery electrodes. However, higher amounts of calendering result in increased tortuosity and particle cracking. We propose a novel packing structure of electrode particles to maximize calendering benefits while minimizing particle fracture. Cobalt-free layered oxide cathode LiNi0.92Mn0.04Al0.04O2 (NMA) particles are pulverized through ball-milling and coated with lithium phosphate. Pulverized and pristine NMA are fabricated into “bimodal” electrodes, whereas “unimodal” electrodes consist of only pristine NMA. Each electrode type was made into 30 Calendering is an essential manufacturing process for Li-ion batteries (LIBs). Despite calendering increasing the energy density of electrodes and improving ionic and electronic connections, it also enhances particle cracking and hinders Li-ion migration. In this article, we investigate a novel “bimodal” packing configuration of a Co-free cathode material and analyze the configuration’s impact on particle cracking at different levels of calendering. There is an increasing effort to eliminate Co in LIB cathode materials due to its expensive and controversial mining conditions. Through electrochemical and structural characterization techniques, we probe particle cracking at different levels of calendering in bimodal and control electrodes. We report the ability of our bimodal configuration to decrease particle cracking and enhance capacity retention. We believe this work is highly impactful because it suggests a direct application to manufacture design of next-generation, Co-free cathodes which have accelerated intragranular cracking compared to current LIB cathodes.

Cathode active material is the most valuable component of spent lithium-ion batteries, accounting for ≈30% of their overall value. Direct recycling of cathode materials involves recovering, regenerating, and reusing them without breaking down their chemical structure. This approach maximizes the added value of the cathode compound and reduces manufacturing costs by avoiding the need for virgin material production. However, one key challenge in scaling direct recycling from lab to industry is the requirement for highly purified cathode materials, contrasting with the low purity of black mass generated from battery shredding. No efficient separation process currently exists to isolate different lithium-nickel-manganese-cobalt oxides (NMCs) from each other. Thus, direct recycling technologies that can operate with mixtures of multiple NMC stoichiometries will be best-suited for industrial adoption. This study explores the direct recycling of NMC mixtures into NMC 622 using a "reciprocal ternary molten salts (RTMS)" system. Ionothermal relithiation and upcycling within the RTMS system successfully restore the layered structure, lithium content, and electrochemical performance of degraded NMCs, yielding results comparable to pristine NMC 622 (P-NMC 622).

Analysis of the performance evolution and failure mechanisms of commercial Li-ion batteries is crucial for improving testing methods, accurately modeling battery performance, and ensuring safe battery operation. Here, we present the results of a 2-year aging study conducted on commercial large-format LiNiMnCoO2-graphite pouch cells. Loss of lithium inventory (LLI) and loss of positive electrode active material (LAMPE) are shown to dominate capacity fade, as quantified by differential voltage-capacity analysis; only a small amount of LAMPE was measured in extracted electrode material, indicating that LAMPE in full cells was due to electrode dry-out. Resistance evolution, observed by direct current pulses and electrochemical impedance spectroscopy analyzed using the distribution of relaxation times, shows complex trends. Particle cracking and electrode expansion is theorized to cause most changes to resistance. Post-mortem measurements reveal a 10% increase in electrode stack thickness and substantial gas generation, with lithium plating observed in extreme cycling conditions, causing large resistance increases. Circumstantial evidence for self-discharge via redox shuttle, which decomposes the electrolyte, is shown. Overall, electrolyte stability was determined to be the limiting factor for cell lifetime. The impacts of many degradation mechanisms on diagnostic signals substantially overlap, making it challenging to monitor cell health and safety.

We report the investigation of silicon nanoparticle composite anodes for Li-ion batteries, using a combination of two nm-scale atomic force microscopy-based techniques: scanning spreading resistance microscopy for electrical conduction mapping and contact resonance and force volume for elastic modulus mapping, along with scanning electron microscopy-based energy dispersion spectroscopy, nanoindentation, and electrochemical analysis. Thermally curing the composite anode—made of polyethylene oxide-treated Si nanoparticles, carbon black, and polyimide binder—reportedly improves the anode electrochemical performance significantly. This work demonstrates phase segregation resulting from thermal curing, where alternating bands of carbon and silicon active material are observed. This electrode morphology is retained after extensive cycling, where the electrical conduction of the carbon-rich bands remains relatively unchanged, but the mechanical modulus of the bands decreases distinctly. These electrical and mechanical factors may contribute to performance improvement, with carbon bands serving as a mechanical buffer for Si deformation and providing electrical conduction pathways. This work motivates future efforts to engineer similar morphologies for mitigating capacity loss in silicon electrodes.

Despite the record-high efficiency of GaAs solar cells, their terrestrial application is limited due to both the particularly high costs related to the required single-crystal substrates and epitaxial growth. A water-soluble lift-off layer could reduce costs by avoiding the need for toxic and dangerous etchants, substrate repolishing, and expensive process steps. Sr3Al2O6 (SAO) is a water-soluble cubic oxide, and SrTiO3 (STO) is a perovskite oxide, where a SAO ≈ 4 × a STO ≈ (2√2)a GaAs. Here, the pulsed laser-deposited epitaxial growth of SrTiO3/Sr3Al2O6 templates on STO and Ge substrates for epitaxial GaAs growth was investigated, where SAO works as a sacrificial layer and STO protects the hygroscopic SAO during substrate transfer between deposition chambers. We identified that the SAO film quality is strongly dependent on the growth temperature and the O2 partial pressure, where either a high T or a high P(O2) improves the quality. XRD spectra of the films with optimized deposition parameters showed an epitaxial STO/SAO stack aligned to the STO (100) substrate, and TEM analysis revealed that the grown films were epitaxially crystalline throughout the thickness. The STO/SAO growth on Ge substrates at a high T with no intentional O2 flow resulted in some nonepitaxial grains and surface pits, likely due to partial Ge oxidation. GaAs was grown by metalorganic vapor-phase epitaxy (MOVPE) on STO/SAO/STO templates. Lift-off after dissolving the sacrificial SAO in water resulted in free-standing ⟨001⟩ preferentially oriented polycrystalline GaAs.

The goal of this project was to demonstrate the feasibility of employing an epitaxial NaCl thin film as a water-soluble release layer for III-V photovoltaic devices and GaAs substrate reuse. Over the course of this project efforts were focused on: achieving crystalline NaCl thin films on GaAs (100) substrate, exploring the growth parameters for subsequent GaAs on NaCl thin films in effort to improve crystallinity of the semiconductor layer, deposition and removal of single crystalline solar cell devices from the parent substrate, and improving morphology and reducing large scale defects in the removed layers to fabricate a working a solar cell device. There was no previous work on direct integration of GaAs/NaCl/GaAs heterostructures at the time of this study. We used molecular beam epitaxy (MBE) to deposit both the alkali halide salt and subsequent III-V material in the same chamber, with no vacuum break. Single crystalline GaAs films were achieved on NaCl layers through careful tuning of the growth parameters and exposure to the reflection high energy electron diffraction (RHEED) beam. Dissolution of the NaCl layer in water provided rapid release of the semiconductor layer from the substrate. Monocrystalline solar cells were grown on the III-V templates using MBE and dynamic hydride vapor phase epitaxy (HVPE). However, the extended time at elevated temperatures required for the cell growth resulted in large area defects from fusion of the semiconductor overlayer to the substrate, causing shorts in fabricated devices. By changing the way that the template layer was grown, the density of these defects could be reduced. Unfortunately, the defects were not reduced to a level allowing for fabrication of a working device.

Identification and evaluation of structural heterogeneity in spent cathode materials is crucial to developing appropriate remediation strategies for novel recycling processes. Native heterogeneities may be exacerbated during the cell's operational lifetime, as sub-particle-scale variations induce anisotropic expansion and contraction upon cycling. Structural transformations resulting from repeated cycling and calendar aging predominantly occur at the secondary particle surface and at the grain boundaries (GBs) between primary particles. However, the diffusion and stress build up around and across GBs are poorly understood. In this study, electron backscatter diffraction (EBSD) is employed to track sub-grain lattice structure across a statistically relevant number of Li(Ni-0.33 Mn-0.33 Co-0.33)O-2 (NMC-111) particles. Specifically, differences in lattice misorientation - measured as the deviation from the grain's average orientation - are tracked as a function of position within the electrode (near current collector, middle, and near separator) and as a function of electrochemical cycling. Further, a novel method of structural analysis is developed, offering insight into sub-grain diffusion behavior by comparing lattice misorientation near the grain boundary versus in the grain bulk. The present results suggest that electrode-scale spatial heterogeneity in lattice structure is induced by initial manufacturing conditions, and that radial gradients in lattice misorientation evolve at the primary particle scale with repeated electrochemical cycling.

Laser ablation is a scalable technique for decreasing the effective tortuosity of electrodes by selectively removing material with high precision. Applied to asymptotic to 110 mu m thick electrode coatings, this work focuses on understanding the impact of laser ablation on electrode material properties at the beginning of life and synergistic impacts of ablated channels on cell performance throughout their cycle life. Post laser ablation, local changes in chemistry, crystallography, and morphology of the laser-impacted electrode regions are investigated. It is shown that femtosecond pulsed laser ablation can achieve high-rate material removal with minor material damage locally at the interface of the impacted zones. The capacity achieved during a 6C (10 min) constant current constant-voltage charge to 4.2 V improved from 1 mAh cm(-2) for the non-ablated electrodes to almost 2 mAh cm(-2) for the ablated electrodes. This benefit is attributed to a synergistic effect of enhanced wetting and decreased electrode tortuosity. The benefit was maintained for over 120 cycles, and upon disassembly decreased Li-plating on the graphite anode was observed. Finally, multi-physics modeling in conjunction with wetting analyses showed that laser ablating either one of the electrodes led to substantial improvements in wetting and rate capability, indicating that substantial performance benefits can be achieved by ablating only the graphite anode as apposed to both electrodes.

Expensive III-V substrates are cost limiting for the adoption of many technologies. Thus, being able to reuse the original substrate is highly desirable. Existing substrate reuse techniques have significant drawbacks, but this work discusses a new method using molecular beam epitaxy deposition of water soluble NaCl thin films on commonly employed (0 0 1) GaAs substrates. Single-crystal GaAs templates are grown on continuous NaCl layers utilizing careful exposure of the NaCl to an in-situ electron beam and a low temperature nucleation layer. The template layers can be quickly removed from the substrate via dissolution of the NaCl. After liftoff, the original wafer shows an increase in rms surface roughness of only 0.2 nm.

Exploiting the extraordinary transport and optical properties of 3D topological semimetals for device applications requires epitaxial integration with semiconductors to carefully control carrier transport, yet no studies have established heteroepitaxy on top of any topological semimetals to date. Here, a novel approach toward fabricating heterostructures is demonstrated by epitaxially incorporating the Dirac semimetal Cd 3 As 2 between Zn x Cd 1‐x Te and CdTe layers via molecular beam epitaxy on GaAs (001) substrates. The approach utilizes the higher energy (001) surface of Cd 3 As 2 to stabilize 2D epitaxy of zinc blende semiconductors. To demonstrate the impact heterostructure formation offers to device performance, an all‐epitaxial, barrier‐type vertical photodetector is fabricated that accesses a different carrier separation mechanism than previously reported non‐epitaxial junctions and consequently exhibits significantly reduced dark currents. The results highlight the important role that epitaxial integration can play in accessing advanced architectures for topological semimetal‐based devices.

The high cost of substrates for III-V growth can be cost limiting for technologies that require large semiconductor areas. Thus, being able to separate device layers and reuse the original substrate is highly desirable, but existing techniques to lift a film from a substrate have substantial drawbacks. This work discusses some of the complexities with the growth of a water-soluble, alkali halide salt thin film between a III-V substrate and overlayer. Much of the difficulty stems from the growth of GaAs on an actively decomposing NaCl surface at elevated temperatures. Interestingly, the presence of an in situ electron beam incident on the NaCl surface, prior to and during GaAs deposition, affects the crystallinity and morphology of the III-V overlayer. Here, we investigate a wide range of growth temperatures and the timing of the impinging flux of both elemental sources and high energy electrons at different points during the growth. We show that an assortment of morphologies (discrete islands, porous material, and fully dense layers with sharp interfaces) and crystallinity (amorphous, crystalline, and highly textured) occur depending on the specific growth conditions, driven largely by changes in GaAs nucleation which is greatly affected by the presence of the reflection high energy electron diffraction beam.

The three-dimensional Dirac semimetal Cd3As2 exhibits ultrahigh electron mobilities that are attractive for optoelectronic devices. However, its strong propensity to grow in the (112) orientation limits the feasibility to epitaxially integrate it into semiconductor structures that are conventionally grown in the (001) orientation. Here, we demonstrate a route to epitaxially growing high mobility Cd3As2(112) layers on GaAs(001) substrates, opening up possibilities for device design. The (001) crystallographic orientation of the GaAs substrate is switched to the (111) orientation through a strain-driven process at a CdTe/GaAs interface, resulting in a CdTe(111) buffer layer on top of which Cd3As2 can be grown. Although the CdTe(111) buffer layer templates Cd3As2 in the (112) orientation, it is not sufficient for producing Cd3As2 with high electron mobility. We therefore demonstrate additional buffer layer design principles for realizing Cd3As2(112) epilayers with similar electron mobilities to those grown on lattice-mismatched III-V (111) substrates. Finally, we outline a pathway to use this approach to grow Cd3As2(112) epilayers on Si(001) substrates, further expanding the potential to integrate Cd3As2 into electronic devices.

Two generations of multiwire modules were studied under dynamic mechanical loading (DML) with in-situ differential conductance (dG) and electroluminescence (EL) imaging. Energy-dispersive x-ray spectroscopy (EDS) was used to identity the solder alloys. The earlier generation module was found to use an In-based solder alloy, and the current generation a Bi-based alloy. The earlier generation module degraded significantly under DML with increasing resistance, while the current generation module did not demonstrate degradation under DML. Atomic force microscopy scratch testing was used to probe the wear resistance of each solder alloy. These results indicate that current multiwire designs may have higher mechanical durability than earlier generations.

Beta-gallium oxide (β-Ga2O3) is an ultrawide bandgap semiconductor that has potential for power electronic applications and devices operating at high temperatures. Particularly important for these applications are its 4.9 eV bandgap, facile electron doping, and the ability to grow β-Ga2O3 crystals from the melt. In this work, vertical β-Ga2O3 Schottky barrier diodes were fabricated using Pt Schottky and Ti-based Ohmic contacts and Au contact pads on unintentionally doped n-type, (2¯01)-oriented single crystal substrates. The diode’s temperature-dependent electrical properties up to 400 °C were investigated, and the Pt/Ga2O3 Schottky barrier height was determined to be close to 1.2 eV. The degradation of the contacts over multiple cycles up to 400 °C was observed, resulting in a significant increase in series resistance of the diodes by 1000× at ambient temperature after they were cycled. According to electron microscopy measurements, this degradation is likely due in part to the migration and oxidation of Ti at the top surface of the Au contact pads. This degradation highlights the need for further research and development to ensure stable Ohmic and Schottky contacts to Ga2O3 at temperatures above 400 °C.