Toward Improving Ambient Volta Potential Measurements with SKPFM for Corrosion Studies

Scanning Kelvin probe force microscopy (SKPFM) is used in corrosion studies to quantify the relative nobility of different microstructural features present within complex metallic systems and thereby elucidate possible corrosion initiation sites. However, Volta potential differences (VPDs) measured via SKPFM in the literature for metal alloys exhibit large variability, making interpretation and application for corrosion studies difficult. We have developed an improved method for referencing SKPFM VPDs by quantifying the closely related work function of the probe relative to an inert gold standard whose modified work function is calculated via density functional theory (DFT). By measuring and tracking changes in the probe vs. gold VPD, this method compensates for some of the complex effects that cause changes in an individual probe\u27s work function. Furthermore, it provides a path toward direct, quantitative comparison of SKPFM results obtained by different researchers. Application of this method to a Cu-Ag-Ti eutectic braze of a steel sample imaged with multiple SKPFM probes of differing compositions led to enhanced repeatability both within and among probe types, as well as enabled the calculation of modified work function values for each of the microstructural constituents present

A Comparison of Solid Electrolyte Interphase Formation and Evolution on Highly Oriented Pyrolytic and Disordered Graphite Negative Electrodes in Lithium-Ion Batteries

The presence and stability of solid electrolyte interphase (SEI) on graphitic electrodes is vital to the performance of lithium-ion batteries (LIBs). However, the formation and evolution of SEI remain the least understood area in LIBs due to its dynamic nature, complexity in chemical composition, heterogeneity in morphology, as well as lack of reliable in situ/operando techniques for accurate characterization. In addition, chemical composition and morphology of SEI are not only affected by the choice of electrolyte, but also by the nature of the electrode surface. While introduction of defects into graphitic electrodes has promoted their electrochemical properties, how such structural defects influence SEI formation and evolution remains an open question. Here, utilizing nondestructive operando electrochemical atomic force microscopy (EChem-AFM) the dynamic SEI formation and evolution on a pair of representative graphitic materials with and without defects, namely, highly oriented pyrolytic and disordered graphite electrodes, are systematically monitored and compared. Complementary to the characterization of SEI topographical and mechanical changes during electrochemical cycling by EChem-AFM, chemical analysis and theoretical calculations are conducted to provide mechanistic insights underlying SEI formation and evolution. The results provide guidance to engineer functional SEIs through design of carbon materials with defects for LIBs and beyond

Localized Deformation in Ni-Mn-Ga Single Crystals

The magnetomechanical behavior of ferromagnetic shape memory alloys such as Ni-Mn-Ga, and hence the relationship between structure and nanoscale magnetomechanical properties, is of interest for their potential applications in actuators. Furthermore, due to its crystal structure, the behavior of Ni-Mn-Ga is anisotropic. Accordingly, nanoindentation and magnetic force microscopy were used to probe the nanoscale mechanical and magnetic properties of electropolished single crystalline 10M martensitic Ni-Mn-Ga as a function of the crystallographic c-axis (easy magnetization) direction relative to the indentation surface (i.e., c-axis in-plane versus out-of-plane). Load-displacement curves from 5–10 mN indentations on in-plane regions exhibited pop-in during loading, whereas this phenomenon was absent in out-of-plane regions. Additionally, the reduced elastic modulus measured for the c-axis out-of-plane orientation was ∼50% greater than for in-plane. Although heating above the transition temperature to the austenitic phase followed by cooling to the room temperature martensitic phase led to partial recovery of the indentation deformation, the magnitude and direction of recovery depended on the original relative orientation of the crystallographic c-axis: positive recovery for the in-plane orientation versus negative recovery (i.e., increased indent depth) for out-of-plane. Moreover, the c-axis orientation for out-of-plane regions switched to in-plane upon thermal cycling, whereas the number of twins in the in-plane regions increased. We hypothesize that dislocation plasticity contributes to the permanent deformation, while pseudoelastic twinning causes pop-in during loading and large recovery during unloading in the c-axis in-plane case. Minimization of indent strain energy accounts for the observed changes in twin orientation and number following thermal cycling

Localized High-Concentration Electrolytes Get More Localized Through Micelle-Like Structures

Liquid electrolytes in batteries are typically treated as macroscopically homogeneous ionic transport media despite having complex chemical composition and atomistic solvation structures, leaving a knowledge gap of microstructural characteristics. Here, we reveal a unique micelle-like structure in a localized high-concentration electrolyte (LHCE), in which the solvent acts as a surfactant between an insoluble salt in diluent. The miscibility of the solvent with the diluent and simultaneous solubility of the salt results in a micelle-like structure with a smeared interface and an increased salt concentration at the centre of the salt-solvent clusters that extends the salt solubility. These intermingling miscibility effects have temperature dependencies, wherein an exemplified LHCE peaks in localized cluster salt concentration near room temperature and is utilized to form a stable solid-electrolyte interphase (SEI) on Li-metal anode. These findings serve as a guide to predicting a stable ternary phase diagram and connecting the electrolyte microstructure with electrolyte formulation and formation protocols to form stable SEI for enhanced battery cyclability

A Closed-Host Bi-Layer Dense/Porous Solid Electrolyte Interphase for Enhanced Lithium-Metal Anode Stability

Thanks to its high specific capacity and low electrochemical potential, lithium metal is an ideal anode for next-generation high-energy batteries. However, the unstable heterogeneous surface of lithium gives rise to safety and efficiency concerns that prevent it from being utilized in practical applications. In this work, the formation of a closed-host bi-layer solid electrolyte interphase (SEI) improves the stability of lithium metal anode. This is successfully realized by forming an interconnected porous LiF-rich artificial SEI in contact with Li metal, and a dense, stable in-situ formed upper layer SEI. The porous layer increases the number of Li/LiF interfaces, which reduces local volume fluctuations and improves Li+ diffusion along these interfaces. Additionally, the tortuous porous structure guides uniform Li+ flux distribution and mechanically suppresses dendrite propagation. The dense upper layer of the SEI accomplishes a closed-host design, preventing continuous consumption of active materials. The duality of a dense top layer with porous bottom layer led to extended cycle life and improved rate performance, evidenced with symmetric cell testing, as well as full cell testing paired with sulfur and LiFePO4 (LFP) cathodes. This work is a good example of a rational design of the SEI, based on comprehensive consideration of various critical factors to improve Li-metal anode stability, and highlights a new pathway to improve cycling and rate performances of Li metal batteries

Microgalvanic Corrosion Behavior of Cu-Ag Active Braze Alloys Investigated with SKPFM

The nature of microgalvanic couple driven corrosion of brazed joints was investigated. 316L stainless steel samples were joined using Cu-Ag-Ti and Cu-Ag-In-Ti braze alloys. Phase and elemental composition across each braze and parent metal interface was characterized and scanning Kelvin probe force microscopy (SKPFM) was used to map the Volta potential differences. Co-localization of SKPFM with Energy Dispersive Spectroscopy (EDS) measurements enabled spatially resolved correlation of potential differences with composition and subsequent galvanic corrosion behavior. Following exposure to the aggressive solution, corrosion damage morphology was characterized to determine the mode of attack and likely initiation areas. When exposed to 0.6 M NaCl, corrosion occurred at the braze-316L interface preceded by preferential dissolution of the Cu-rich phase within the braze alloy. Braze corrosion was driven by galvanic couples between the braze alloys and stainless steel as well as between different phases within the braze microstructure. Microgalvanic corrosion between phases of the braze alloys was investigated via SKPFM to determine how corrosion of the brazed joints developed

Oxidation Behavior of Zr-Alloy Cladding Candidates for the TrEat Reactor

The TREAT facility, located at the Idaho National Laboratory, is an air-cooled test facility designed to simulate rapid reactor transient conditions up to 600 °C. Zirconium and its alloys are known to undergo a transition from parabolic to linear oxidation kinetics, termed breakaway. After breakaway, the oxide formed is porous and non-protective which significantly decreases its mechanical stability as a cladding material. In this work, the oxidation behavior of Zircaloy-3, Zircaloy-4, and Zr-1Nb is evaluated in isothermal and rapid transient conditions (air at ≤ 820 °C). Separate effects studies were done on plate samples in air (80% N2+O2) and oxygen (80% Ar+O2) to characterize the effect of nitrogen on the oxidation behavior during isothermal oxidation using thermogravimetric analysis. It was observed that the three alloys exhibit decreased resistance to breakaway in air. In addition, the effects of chamfering and welding on the oxidation behavior were investigated during isothermal and rapid transient oxidation. All alloys were affected by welding, however only the Fe/Sn containing alloys were affected by plastic deformation. Advanced characterization techniques (scanning Kelvin probe force microscopy and Raman spectroscopy) were performed on cross sections of oxidized specimens to further investigate the effects of oxide phase and composition on breakaway

Characterization of Zirconium Oxides Part II: New Insights on the Growth of Zirconia Revealed Through Complementary High-Resolution Mapping Techniques

Raman mapping, scanning Kelvin probe force microscopy (SKPFM), and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS) were combined to investigate oxidized zirconium alloys. Raman provided spatially resolved phase composition and relative stress state. When coupled with SKPFM, phase composition was correlated to Volta potentials differences. The potential of tetragonal zirconia was lower than the metal zirconium, making the tetragonal phase favorable for reaction with diffusing species, thus hindering further oxidation of the relatively cathodic metal. This provides new insight to the theory of the tetragonal phase being an oxidation barrier

First-Principles Surface Interaction Studies of Aluminum-Copper and Aluminum-Copper-Magnesium Secondary Phases in Aluminum Alloys

First-principles density functional theory-based calculations were performed to study θ-phase Al2Cu, S-phase Al2CuMg surface stability, as well as their interactions with water molecules and chloride (Cl−) ions. These secondary phases are commonly found in aluminum-based alloys and are initiation points for localized corrosion. Density functional theory (DFT)-based simulations provide insight into the origins of localized (pitting) corrosion processes of aluminum-based alloys. For both phases studied, Cl− ions cause atomic distortions on the surface layers. The nature of the distortions could be a factor to weaken the interlayer bonds in the Al2Cu and Al2CuMg secondary phases, facilitating the corrosion process. Electronic structure calculations revealed not only electron charge transfer from Cl− ions to alloy surface but also electron sharing, suggesting ionic and covalent bonding features, respectively. The S-phase Al2CuMg structure has a more active surface than the θ-phase Al2Cu. We also found a higher tendency of formation of new species, such as Al3+, Al(OH)2+, HCl, AlCl2+, Al(OH)Cl+, and Cl2 on the S-phase Al2CuMg surface. Surface chemical reactions and resultant species present contribute to establishment of local surface chemistry that influences the corrosion behavior of aluminum alloys