Bulk Ion Research Articles

High‐Density, Crosstalk‐Free, Flexible Electrolyte‐Gated Synaptic Transistors Array via All‐Photolithography for Multimodal Neuromorphic Computing

AbstractHigh‐density bio‐electrolyte‐gated synaptic transistors (BEGTs) array are promising for constructing neuromorphic computing architectures. Due to the bulk ion conductivity and the crack sensitivity of the electrolyte film, patterning the electrolyte is an indispensable route to prevent spatial crosstalk and improve the flexibility of the device array. However, the susceptibility of bio‐electrolyte to organic solvents poses challenges in developing reliable all‐photolithography techniques for fabricating scalable, patterned, and high‐density BEGTs array. This study introduces an all‐photolithography method that adopts a photo‐crosslinker‐enabled electrolyte to create a high‐density (11846 devices per cm2) multimodal BEGTs array. This array demonstrates essential neuromorphic behaviors without inter‐device crosstalk and maintains its flexibility, enduring 200 bending cycles at a 6 mm radius without significant performance degradation. Meanwhile, the BEGTs array exhibits multimodal synaptic behavior, not only successfully mimicking the biological visual memory system for sensing and processing images but also proving highly accurate in classifying handwritten digits, making it suitable for constructing neuromorphic computing systems. This work offers a dependable strategy for the scalable and stable fabrication of BEGTs array, providing valuable insights for advancing artificial neuromorphic systems.

Bulk, overlap and surface effects of swift heavy ions in CeO2

Formation of tracks of swift heavy ions decelerating in the electronic stopping regime in CeO2 was studied, combining the Monte Carlo code TREKIS with molecular dynamics. We show that strong lattice disordering (melting) followed by structure recovery form finally a damaged ion track consisting of a discontinuous crystalline region in CeO2. Normal ion impacts result in appearance of spherical crystalline hillocks on CeO2 surface. The solid-vacuum interface strongly suppresses the recrystallization of the near-surface layers, forming conically shaped tracks with several tens of nanometers lengths. Grazing ion irradiation induces intensive material expulsion from the surface forming finally grooves surrounded by nanohillocks. The processes of surface nanostructures formation is similar to those observed previously in CaF2 which has the similar crystalline structure, however requires much longer recrystallization time. Recent experimental data confirm the simulation results.

Dynamic electroosmotic flow and solute dispersion through a nanochannel filled with an electrolyte surrounded by a layer of a dielectric and immiscible liquid.

The present article deals with the modulation of oscillatory electroosmotic flow (EOF) and solute dispersion across a nanochannel filled with an electrolyte solution surrounded by a layer of a dielectric liquid. The dielectric permittivity of the liquid layer adjacent to supporting rigid walls is taken to be lower than that of the electrolyte solution. Besides, the aforesaid liquid layer may bear additional mobile charges, e.g., free lipid molecules, charged surfactant molecules etc., which in turn lead to a nonzero charge along the liquid-liquid interface. Such a layer of a dielectric liquid resembles the membrane of various biological cells. An AC voltage is applied to generate the fluid motion. Note that among others, the major advantage of AC voltage is that it can suppress the formation of gas bubbles that are often very detrimental in flow through microdevices. Considering the combined impact of ion partitioning and ion steric effects, we have studied the EOF modulation and its impact on the dispersion of the solute band of given width placed initially at the middle of the channel. The full scale numerical results for flow modulation induced by an AC electric field and its impact on the solute transport are presented considering a wide range of pertinent parameters. It is observed that the molar concentration of additional charge present in the dielectric liquid layer and its thickness, interfacial charge, and concentration of the bulk electrolyte, ion partitioning and ion steric effects, frequency of oscillatory electric field, channel height etc., have a substantial impact on the flow modulation, effective dispersion coefficient as well as broadening of the solute band across the channel. We have further highlighted the impact of the Péclet number on the transport and dispersion of solutes. Along with the numerical results, several benchmark analytical results under various limits are deduced for electrostatic potential, flow velocity and various quantities associated with the dispersion process.

Suppression of Collisionless Magnetic Reconnection in the High Ion β, Strong Guide Field Limit

In magnetic reconnection, the ion bulk outflow speed and ion heating have been shown to be set by the available reconnecting magnetic energy, i.e., the energy stored in the reconnecting magnetic field (B r ). However, recent simulations, observations, and theoretical works have shown that the released magnetic energy is inhibited by upstream ion plasma beta β i —the relative ion thermal pressure normalized to magnetic pressure based on the reconnecting field—for antiparallel magnetic field configurations. Using kinetic theory and hybrid particle-in-cell simulations, we investigate the effects of β i on guide field reconnection. While previous works have suggested that guide field reconnection is uninfluenced by β i , we demonstrate that the reconnection process is modified and the outflow is reduced for sufficiently large βi>(Br2+Bg2)/Br2 . We develop a theoretical framework that shows that this reduction is consistent with an enhanced exhaust pressure gradient, which reduces the outflow speed as vout∝1/βi . These results apply to systems in which guide field reconnection is embedded in hot plasmas, such as reconnection at the boundary of eddies in fully developed turbulence like the solar wind or the magnetosheath as well as downstream of shocks such as the heliosheath or the mergers of galaxy clusters.

Application of a Scale Normalization Technique for High Resolution Analysis of the Magnetosheath at Mars

AbstractIn order to study spatial distributions of global magnetosheath structures, physicists often rely upon spatial binning, whereby space is divided into cells, each filled with the average value of all spacecraft measurements within that cell. The traditional binning schema utilizes a fixed Cartesian grid of cube bins. The morphology of the magnetosheath's boundaries are not fixed, however, but driven by upstream and planetary conditions. Therefore, the spatial structures are not fixed in Cartesian space, and thus a Cartesian binning technique will produce a highly coarse grained spatial distribution. We propose an alternative binning technique utilizing a scale normalized dimensionless coordinate system defined in terms of magnetosheath morphology. To demonstrate the efficacy of this technique, we apply a basic implementation to the Martian system. We are thereby able to achieve a high‐resolution spatial mapping of bow shock and magnetosheath processes and resolve spatial structures that are washed out when binned traditionally. In particular, we can resolve the shock overshoot, analyze the dominant forces acting at the shock, and obtain fine‐scale distributions of the bulk ion plasma magnetosheath forces and thermalization mechanisms. Magnetic tension and magnetic pressure gradient are compared. The ion pressure divergence at the shock is found to significantly vary in line with the solar wind temperature anisotropy. The dependency of the mirror mode instability on location and Mach number, and its implications for thermalization processes in the small Martian magnetosheath are investigated.

Co-Ion Identity Affects Double Layer Structure and Reactivity Trends in the Hydrogen Evolution Reaction

The formation of an electric double layer is ubiquitous in electrolytes undergoing electrochemical reactions. Currently, models for the double layer rely on the assumptions of dilute electrolyte theory where ions in solution do not interact. Dilute electrolyte theory predicts that solely hydrated counterions, or ions opposing the surface charge, occupy the near surface region and impact reactivity. Since this theory does not account for conditions like high applied potentials and high ion concentrations, understanding the relationship between double layer structure and electrochemical reactivity is critical for opening new domains for electrochemical modulation.Notably, our recent work shows that co-ions, or ions with the same charge as the surface, can also have a definitive influence on reaction rates, particularly under conditions of high applied potential and large bulk ion concentrations. In this talk, I will discuss how to use the hydrogen evolution reaction as a model system to explore co-ion effects in electrocatalytic reactions. In our work, we systematically vary co-ion identity and track hydrogen evolution reactivity as a function of concentration to highlight how the co-ion becomes increasingly relevant in the concentrated regime. In particular, we highlight the unique ability of the tetrafluoroborate co-ion to undergo dynamic ligand exchange between hydroxide and fluoride. This allows tetrafluoroborate to act as a hydroxide sink to stabilize the hydrogen evolution byproduct. Furthermore, I will discuss our use of Raman spectroscopy to describe how co-ions impact interfacial hydrogen bonding networks, which can have a significant impact on aqueous electrochemistry. Our results highlight how co-ions influence double layer structures and have significant implications for tuning other aqueous reactions such as carbon dioxide and nitrate electroreduction.

Micrometer Scale X-Ray CT Assisted Cathode Pore Space Designs for High Energy Fast Discharge Rate Lithium-Ion Battery

The increasing demand for hybrid electric vehicles (HEVs) necessitates batteries capable of higher discharge rates. This study focuses on the design of cathode pore structures in lithium-ion batteries, aiming to enhance bulk ion transport and thereby achieving higher spatial energy density at higher discharge rates, a critical requirement for hybrid EVs. We report here novel cathode pore space designs using well understood NCM 811 high energy density cathode material to address this need. These designs include a double layer structure with varying porosity, a femto-second laser etched directional pore structure, and how different electrolyte interacts with each of these systems. These tailored structures are hypothesized to facilitate improved ion transport, reducing local stress, and thus contributing to enhanced battery performance and extended battery lifespan. To validate our designs, we employed micrometer-scale X-ray computed tomography (CT) and pore network modelling for structural characterization, quantifying the pore structure mathematically. The performance of these cathodes was evaluated using a suite of electrochemical techniques, including cyclic voltammetry and Electrochemical Impedance Spectroscopy (EIS), which reveals their impact on cell level performance. Our findings contribute to the ongoing efforts to optimize lithium-ion battery technology for hybrid EVs, offering insights into the potential of tailored cathode structures to improve battery performance. Further research is needed to refine these designs and assess their scalability and feasibility for commercial applications.

Enhancing Interfacial Capacitance Using Anionic Amphiphiles in Ionic Liquid Electrolyte Blends

The acceleration of climate change due to carbon emissions has generated an imminent need to transition towards renewable energy sources. However, the intermittent nature of renewable sources like wind or solar energy necessitates breakthroughs in energy storage devices that can rapidly charge and discharge. This demand creates a new role for high power density supercapacitors. While supercapacitors can quickly discharge electricity, they are limited by low energy densities. This limitation in energy density has generated research interest into charge storage mechanisms at interfaces and new types of supercapacitor electrolytes. An ideal supercapacitor electrolyte has a wide electrochemical stability window and high capacitance to maximize the energy density. Ionic liquids (ILs) show potential for this role with their wide electrochemical stability windows, nonflammability, and nonvolatility. Problematically, strong ion correlations present in ILs result in low capacitances. I will discuss our work aimed at tuning ionic correlations in binary IL blends to provide pathways towards higher energy density capacitive interfaces. We show how the addition of surface-active amphiphilic anions weakens these correlations increasing the interfacial charge density and capacitance of these IL blends. Notably, we find that mixing surface-active amphiphilic anions with nonamphiphilic ILs doubles the capacitance at negative surfaces. We hypothesize that polarity-dictated anion-anion van der Waals interactions serve to weaken IL cation-anion interactions and ultimately drive molecular assembly at negative surfaces, increasing capacitance. I will also present our complementary investigations of non-amphiphilic binary IL mixtures and provide spectroscopic insight into bulk ion correlations within these systems. Our study ultimately paves the path towards fundamental investigations into the chemical nature of additives in ionic liquid blends to boost performance within next-generation supercapacitors.

Molecular insights into the degradation of organic matter from secondary swine wastewater effluent: A comparative study of advanced oxidation processes

Advanced oxidation processes (AOPs) are promising for the treatment of secondary swine wastewater effluents, however, the molecular-level understanding of effluent organic matter (EfOM) removal and transformation during AOPs is limited. This study employed molecular-level characterization based on Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and bulk characterizations to investigate these processes in various AOPs, including Cu-Fenton, UV-Cu-Fenton, Fenton, UV-Fenton, and UV/H2O2 treatments. Our findings revealed that the removal rates of dissolved organic carbon and EfOM molecules follow the sequence of UV-Fenton > Fenton > UV-Cu-Fenton > UV/H2O2 > Cu-Fenton, correlating with the rates of H2O2 decomposition during reactions. AOPs with faster H2O2 decomposition, indicative of higher reactive oxygen species generation, predominantly mineralize rather than transform EfOM. Linkage analysis highlighted oxygen addition and deamination as the primary transformation reactions, with variations in the dominance levels of these reactions across different AOPs. Recalcitrant molecule, particularly CHNO and CHO types, including low-molecular-weight carboxyl-rich alicyclic molecules, pose challenges in treatment. To enhance the efficacy of secondary effluent treatment, strategies focusing on the targeted removal of such recalcitrant EfOM should be developed. This study provided new insights into the selection and optimization of AOPs for secondary swine wastewater effluent treatment.

Neutron imaging of the deuterium-tritium tamping gas volume in an inertial confinement fusion hohlraum.

To benchmark the accuracy of the models and improve the predictive capability of future experiments, the National Ignition Facility requires measurements of the physical conditions inside inertial confinement fusion hohlraums. The ion temperature and bulk motion velocity of the gas-filled regions of the hohlraum can be obtained by replacing the helium tamping gas in the hohlraum with deuterium-tritium (DT) gas and measuring the Doppler broadening and Doppler shift of the neutron spectrum produced by nuclear reactions in the hohlraum. To understand the spatial distribution of the neutron production inside the hohlraum, we have developed a new penumbral neutron imager with a 12mm diameter field of view using a simple tungsten alloy spindle. We performed the first experiment using this imager on a DT gas-filled hohlraum and successfully obtained the spatial distribution of neutron production in the hohlraum plasma. We will report on the design of the spindle, characterization of the detectors, and methodology of the image reconstruction.

Recent Progress of Thin Crystal Engineering for Perovskite Solar Cells.

Metal halide perovskite single crystals hold promise for photovoltaics with high efficiency and stability due to their superior optoelectronic properties and weak bulk ion migration. The past several years have witnessed rapid development of single-crystal perovskite solar cells (PSCs) with efficiency rocketed from 6.5 % to 24.3 %, however, which still lags behind their polycrystalline counterparts. Moreover, the poor device stability under light illumination is contrary to the high ion migration barrier of perovskite single crystals. The key limiting factors should be the low crystalline quality and high surface defect density of solution-grown thin single crystals. Under this circumstance, a review paper summarizing the recent progress and challenges will be instructive for future development of this emerging field. In this manuscript, the crystal engineering used to enhance carrier transport and suppress carrier recombination in vertical single-crystal PSCs will be summarized initially, including crystal growth, component control, surface and interface modification. Subsequently, the application of perovskite single crystals in lateral single-crystal PSCs will be discussed and compared with the conventionally vertical structure. Finally, the challenges and proposed strategies for the development of single-crystal PSCs are provided.

Concurrent oxygen evolution reaction pathways revealed by high-speed compressive Raman imaging.

Transition metal oxides are state-of-the-art materials for catalysing the oxygen evolution reaction (OER), whose slow kinetics currently limit the efficiency of water electrolysis. However, microscale physicochemical heterogeneity between particles, dynamic reactions both in the bulk and at the surface, and an interplay between particle reactivity and electrolyte makes probing the OER challenging. Here, we overcome these limitations by applying state-of-the-art compressive Raman imaging to uncover concurrent bias-dependent pathways for the OER in a dense, crystalline electrocatalyst, α-Li2IrO3. By spatially and temporally tracking changes in stretching modes we follow catalytic activation and charge accumulation following ion exchange under various electrolytes and cycling conditions, comparing our observations with other crystalline catalysts (IrO2, LiCoO2). We demonstrate that at low overpotentials the reaction between water and the oxidized catalyst surface is compensated by bulk ion exchange, as usually only found for amorphous, electrolyte permeable, catalysts. At high overpotentials the charge is compensated by surface redox active sites, as in other crystalline catalysts such as IrO2. Hence, our work reveals charge compensation can extend beyond the surface in crystalline catalysts. More generally, the results highlight the power of compressive Raman imaging for chemically specific tracking of microscale reaction dynamics in catalysts, battery materials, or memristors.

Leveraging Ion Pairing and Transport in Localized High-Concentration Electrolytes for Reversible Lithium Metal Anodes at Low Temperatures.

Coulombic efficiency of over 99 % is rarely achieved for Li metal anode below -40 °C, hindering the practical application of high-energy-density Li metal batteries under extreme conditions. Herein, limiting factors for Li metal reversibility are investigated utilizing ether-based localized high-concentration electrolytes of different solvent-diluent combinations. We find that along with the desolvation barrier, bulk ion transport properties including ionic conductivity, transference number, and diffusivity are also crucial factors for low-temperature Li deposition behavior. Superior Li metal reversibility was observed within the combination of the solvent with moderately weak solvating power and the diluent with minimal viscosity, highlighting the role of ion transport and the necessity for a trade-off with desolvation. The optimized electrolyte composed of lithium bis(fluorosulfonyl)imide, methyl n-propyl ether, and 1,1,2,2-tetrafluoroethyl methyl ether delivers exceptional Coulombic efficiency of 99.34 % at -40 °C and 98.96 % at -60 °C under a current density of 0.5 mA cm-2. Furthermore, Li||LiCoO2 (2.7 mAh cm-2) cells demonstrate impressive reversible capacity and cycling stability at these temperatures. This work sheds light on the less-recognized relevance of bulk ion transport to low-temperature performance and provides guidelines for the electrolyte design of Li metal batteries operating in cold environments.

Electrolytes in conducting nanopores: Revisiting constant charge and constant potential simulations.

Simulating electrolyte-electrode systems poses challenges due to the need to account for the electrode's response to ion movements in order to maintain a constant electrode potential, which slows down the simulations. To circumvent this, computationally more efficient constant charge (CC) simulations are sometimes employed. However, the accuracy of CC simulations in capturing the behavior of electrolyte-electrode systems remains unclear, especially for microporous electrodes. Herein, we consider electrolyte-filled slit nanopores and systematically analyze the in-pore ion structure and diffusivity using CC and constant potential simulations. Our results indicate that CC simulations provide comparable pore occupancies at high bulk ion densities and for highly charged pores, but they fail to accurately describe the ion structure and dynamics, particularly in quasi-2D (single-layer) pores and at low ion densities. We attribute these results to the superionic state emerging in conducting nanoconfinement and its interplay with excluded volume interactions.

Lower hybrid and solitary waves in dusk flank region of the Earth’s magnetosphere

A variety of plasma waves have been detected in the vicinity of the magnetopause by various spacecraft missions. In this paper, utilizing high-resolution data from the Magnetospheric Multiscale (MMS) mission, we present new observations of simultaneous lower hybrid and ion solitary waves in the dusk flank region of Earth’s magnetosphere. All four MMS spacecraft consistently observed this wave activity during their traversal from Earth’s magnetosphere to the magnetosheath. Our analysis suggests that the lower hybrid drift waves, driven by lower-hybrid drift instability, were observed in correlation with density gradients. Furthermore, the analysis indicates that the entire ion bulk population drifts, which drives ion solitary waves. It is found that these waves play a crucial role in particle heating in the dusk flank region of Earth’s magnetosphere.

Quantification of Charge Transport and Mass Deprivation in Solid Electrolyte Interphase for Kinetically‐Stable Low‐Temperature Lithium‐Ion Batteries

AbstractGraphite (Gr)‐based lithium‐ion batteries with admirable electrochemical performance below −20 °C are desired but are hindered by sluggish interfacial charge transport and desolvation process. Li salt dissociation via Li+‐solvent interaction enables mobile Li+ liberation and contributes to bulk ion transport, while is contradictory to fast interfacial desolvation. Designing kinetically‐stable solid electrolyte interphase (SEI) without compromising strong Li+‐solvent interaction is expected to compatibly improve interfacial charge transport and desolvation kinetics. However, the relationship between physicochemical features and temperature‐dependent kinetics properties of SEI remains vague. Herein, we propose four key thermodynamics parameters of SEI potentially influencing low‐temperature electrochemistry, including electron work function, Li+ transfer barrier, surface energy, and desolvation energy. Based on the above parameters, we further define a novel descriptor, separation factor of SEI (SSEI), to quantitatively depict charge (Li+/e−) transport and solvent deprivation processes at Gr/electrolyte interface. A Li3PO4‐based, inorganics‐enriched SEI derived by Li difluorophosphate (LiDFP) additive exhibits the highest SSEI (4.89×103) to enable efficient Li+ conduction, e− blocking and rapid desolvation, and as a result, much suppressed Li‐metal precipitation, electrolyte decomposition and Gr sheets exfoliation, thus improving low‐temperature battery performances. Overall, our work originally provides visualized guides to improve low‐temperature reaction kinetics/thermodynamics by constructing desirable SEI chemistry.

Quantification of Charge Transport and Mass Deprivation in Solid Electrolyte Interphase for Kinetically-Stable Low-Temperature Lithium-Ion Batteries.

Graphite (Gr)-based lithium-ion batteries with admirable electrochemical performance below -20 °C are desired but are hindered by sluggish interfacial charge transport and desolvation process. Li salt dissociation via Li+-solvent interaction enables mobile Li+ liberation and contributes to bulk ion transport, while is contradictory to fast interfacial desolvation. Designing kinetically-stable solid electrolyte interphase (SEI) without compromising strong Li+-solvent interaction is expected to compatibly improve interfacial charge transport and desolvation kinetics. However, the relationship between physicochemical features and temperature-dependent kinetics properties of SEI remains vague. Herein, we propose four key thermodynamics parameters of SEI potentially influencing low-temperature electrochemistry, including electron work function, Li+ transfer barrier, surface energy, and desolvation energy. Based on the above parameters, we further define a novel descriptor, separation factor of SEI (SSEI), to quantitatively depict charge (Li+/e-) transport and solvent deprivation processes at Gr/electrolyte interface. A Li3PO4-based, inorganics-enriched SEI derived by Li difluorophosphate (LiDFP) additive exhibits the highest SSEI (4.89×103) to enable efficient Li+ conduction, e- blocking and rapid desolvation, and as a result, much suppressed Li-metal precipitation, electrolyte decomposition and Gr sheets exfoliation, thus improving low-temperature battery performances. Overall, our work originally provides visualized guides to improve low-temperature reaction kinetics/thermodynamics by constructing desirable SEI chemistry.

Counterion Distribution in the Stern Layer on Charged Surfaces.

Counterion adsorption at the solid-liquid interface affects numerous applications. However, the counterion adsorption density in the Stern layer has remained poorly evaluated. Here we report the direct determination of surface charge density at the shear plane between the Stern layer and the diffuse layer. By the Grahame equation extension and streaming current measurements for different solid surfaces in different aqueous electrolytes, we are able to obtain the counterion adsorption density in the Stern layer, which is mainly related to the surface charge density but is less affected by the bulk ion concentration. The charge inversion concentration is further found to be sensitive to the ion type and ion valence rather than to the charged surface, which is attributed to the ionic competitive adsorption and ion-ion correlations. Our findings offer a framework for understanding ion distribution in many physical and chemical processes where the Stern layer is ubiquitous.

Azimuthal ion dynamics at the inner pole of an axisymmetric Hall thruster

The azimuthal dynamics of ions along the inner pole of a Hall thruster with a centrally mounted cathode and a magnetic shielding topography are experimentally investigated. A time-averaged laser-induced fluorescence diagnostic is implemented to characterize the azimuthal ion velocity distribution, and its moments are computed numerically to infer bulk rotation speed and ion temperature. It is found that the time-averaged ion swirl velocity grows to 2 km/s in the near-pole region, and the cathode ions exhibit ion temperatures in the azimuthal direction approaching 8 eV. Both of these quantities exceed the speeds and temperatures anticipated from classical acceleration and heating. Time-resolved laser-induced fluorescence is then employed to investigate the role of plasma fluctuations in driving the time-averaged ion properties. Semicoherent fluctuations at 90 kHz are observed in the ion velocity distribution and its associated moments. These oscillations are correlated with the gradient-driven anti-drift wave, which propagates azimuthally in the near-field cathode plume. Quasilinear theory is used to construct a 1D model for acceleration and heating of the ion population as a result of the anti-drift mode. This approach demonstrates qualitative agreement with the time-averaged ion velocity and temperature, suggesting that the anti-drift mode may be a dominant driver of azimuthal ion acceleration and heating in front of the cathode keeper and the inner half of the inner front pole cover. These results are discussed in terms of their relevance to the erosion of thruster surfaces in the near-field cathode plume.

Scaling of Ion Bulk Heating in Magnetic Reconnection Outflows for the High-Alfvén-speed and Low-β Regime in Earth’s Magnetotail

We survey 20 reconnection outflow events observed by Magnetospheric MultiScale in the low-β and high-Alfvén-speed regime of the Earth’s magnetotail to investigate the scaling of ion bulk heating produced by reconnection. The range of inflow Alfvén speeds (800–4000 km s−1) and inflow ion β (0.002–1) covered by this study is in a plasma regime that could be applicable to the solar corona and flare environments. We find that the observed ion heating increases with increasing inflow (upstream) Alfvén speed, V A, based on the reconnecting magnetic field and the upstream plasma density. However, ion heating does not increase linearly as a function of available magnetic energy per particle, miVA2 . Instead, the heating increases progressively less as miVA2 rises. This is in contrast to a previous study using the same data set, which found that electron heating in this high-Alfvén-speed and low-β regime scales linearly with miVA2 , with a scaling factor nearly identical to that found for the low-V A and high-β magnetopause. Consequently, the ion-to-electron heating ratio in reconnection exhausts decreases with increasing upstream V A, suggesting that the energy partition between ions and electrons in reconnection exhausts could be a function of the available magnetic energy per particle. Finally, we find that the observed difference in ion and electron heating scaling may be consistent with the predicted effects of a trapping potential in the exhaust, which enhances electron heating, but reduces ion heating.