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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Elucidating Electric Field-Induced Rate Promotion of Brønsted Acid-Catalyzed Alcohol Dehydration

Applied potentials have been demonstrated as a powerful tool to promote heterogeneous Brønsted acid catalysis by orders of magnitude, leveraging interfacial electric fields to stabilize protonated intermediates. However, the use of flat two-dimensional electrodes with inherently low active site densities limits the application of conventional thermochemical characterization techniques that can probe the nature of catalytic active sites. Here, we use kinetic analyses with an electrostatics-based model to elucidate the intricacies of potential-induced rate promotion, employing liquid-phase dehydration of 1-methylcyclopentanol catalyzed by carboxylic acid groups on carbon nanotubes as a probe system. By using a basket electrode to directly polarize catalyst powder, we demonstrate that thermocatalytic reaction rates can be promoted by 100,000-fold, exhibiting a log–linear dependence on applied potential with rate-potential scalings as high as 125 ± 4 mV per 10-fold rate increase. In agreement with model predictions, we show that lower ionic strengths attenuate potential sensitivity, resulting from a weakening of the interfacial electric field that interacts with the acidic proton. Furthermore, we experimentally confirm the model-predicted “isokinetic potential” (at ∼0.6 V vs Ag/AgCl)─the potential at which all rate scaling lines at various ionic strengths intersect, making the rate independent of ionic strength. Base titrations reveal that only ∼8% of the carboxylic acid sites are catalytically active, yet these same active sites are operational at the highest and lowest potentials. Collectively, our results provide a key methodology for modeling catalytic effects of electric fields, quantifying active sites under applied potential, and demonstrating fundamental principles of electric field-induced rate promotion.

Catalysts↗
DOE OSTI2022

Perspectives on Microfluidics for the Study of Asphaltenes in Upstream Hydrocarbon Production: A Minireview

The utilization of microfluidics has generated deep insights into asphaltene precipitation mechanisms and oil–water emulsion stabilization. Agglomeration and precipitation of asphaltenes can cause flow assurance problems during the extraction and transportation of crude oil. Change in temperature, pressure, reservoir conditions, and solvents can change the local environment, leading to asphaltene precipitation. Understanding asphaltene properties and precipitation pathways becomes critical in devising mitigation methods, demulsifiers, and suitable conditions during hydrocarbon processing. Microfluidics has helped in high throughput measurement studies, understanding critical processing conditions, fast demulsifier screening, and the effect of solvent concentration on deposition, generating useful information for utilization at the point of resource extraction facilitating improved resource management. It has become possible to capture the porous, complex nature of reservoir formations and the interaction of chemicals during precipitation through integrated analytics and visualization studies available only through microfluidics. The use of droplet microfluidics, with optical microscopy and high-speed imaging to study the oil–water interface, has resulted in greater understanding of the role of asphaltenes in interfacial properties and emulsion stabilization. Here, this minireview highlights the crucial aspects of microfluidics that have been used to understand physicochemical behavior and dynamics of asphaltene deposition. Some of the unique devices have been presented focusing on the key elements of microfluidics design, fabrication, and analysis, as the insight obtained from microfluidics strongly depends on the device design and the controllability of the experimental parameters. Successful implementation of microfluidics for efficient and controlled experiments, short analysis time scales and rapid screening, and generation of high-quality, reliable data that convey asphaltene deposition issues and interface behavior in emulsions shows the importance of microsystems for advancing knowledge in hydrocarbon production and processing.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Suppressing Phase Segregation and Improving Stability in Mixed-Halide Perovskites through Spinel Oxide-Directed Epitaxy

Mixed-cation mixed-halide perovskite compositions are essential for achieving the required bandgaps for high-efficiency multijunction photovoltaics, yet their stability remains limited by interfacial defects, phase segregation, and degradation. Here, we introduce spinel oxides as a new family of lattice-matched substrates that enable crystalline, phase-pure, compositionally-uniform, bromide-rich perovskite film growth. The effect of spinel oxides is two-fold: reducing defects at the bottom interface by templating film growth and inducing beneficial compressive strain through mismatch-dependent substrate-perovskite lattice coupling. Spinel oxide substrates facilitate growth of highly crystalline films and eliminate detrimental secondary phases across thicknesses. Using grazing incidence X-ray diffraction, X-ray fluorescence, cathodoluminescence–scanning electron microscopy, cryogenic photoluminescence, and density functional theory, we reveal that Mg-halide bonds at the bottom interface induce lattice mismatch-dependent compressive strain that suppresses halide segregation and further reduces defect formation. In addition, films grown on spinel oxides maintain over 87% of the perovskite phase after 12 h under 100% relative humidity, as monitored by in situ grazing incidence wide-angle X-ray scattering (GIWAXS), compared to less than 70% for control samples. This work extends lattice matching from vapor-deposited epitaxial semiconductors to solution-processed halide perovskites to establish a broadly applicable strategy for defect suppression, phase homogenization, and long-term stability. Based on the fundamental science explored here, we set the stage for the development of lattice-matched spinel oxide charge transport layers to be integrated into perovskite solar cells and other optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Migration of inclusions in a matrix due to a spatially varying interface energy

An interfacial energy can be a function of a bulk field such as temperature or electric field. Here we find that in a system with a gradient in temperature or electric potential, the resulting variation in interfacial energy can induce a particle to migrate by either surface or bulk diffusion. For a circular particle under a constant unidirectional gradient in the bulk field, the field dependence of the interfacial energy induces a circular particle to move as a circle at a constant velocity along the direction from higher to lower interfacial energy. A linear stability analysis of this steady state migration suggests that perturbations will damp out as time evolves, and thus under these conditions a migrating circular particle is morphologically stable. Other spatial distributions of interface energy can lead to the distortion of an initially circular shaped particle during migration. A phase field model is developed that captures these distortions and verifies the theoretical results mentioned above.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Computational Insights into Phase Equilibria Between Wide-Gap Semiconductors and Contact Materials

Novel wide-band-gap semiconductors are needed for next-generation power electronics, but there is a gap between a promising material and a functional device. Finding stable (metal) contacts is one of the major challenges that is currently dealt with mainly via trial and error. Herein, we computationally investigate the thermochemistry and phase coexistence at the junction between three wide-gap semiconductors, ..beta..-Ga2O3, GeO2, and GaN, and possible contact materials. The pool of possible contacts includes 47 elemental metals and a set of 4 common, n-type transparent conducting oxides (ZnO, TiO2, SnO2, and In2O3). We use first-principles thermodynamics to model the Gibbs free energies of chemical reactions as a function of gas pressure (pO2/pN2) and equilibrium temperature. We deduce whether a semiconductor/contact interface will be stable at relevant conditions or a chemical reaction between them is to be expected, possibly influencing the long-term reliability and performance of devices. We generally find that most elemental metals tend to oxidize or nitridize and form various interface oxide/nitride layers. Exceptions include select late- and post-transition metals and, in the case of GaN, also the alkali metals, which are predicted to exhibit stable coexistence, although in many cases at relatively low gas partial pressures. Similar is true for the transparent conducting oxides, for which, in most cases, we predict a preference toward forming ternary oxides when in contact with ..beta..-Ga2O3 and GeO2. The only exception is SnO2, which we find to form stable contacts with both oxides. Finally, we show how the same approach can be used to predict gas partial pressure vs temperature phase diagrams to help direct synthesis of ternary compounds. We believe these results provide a valuable guidance in selecting contact materials to wide-gap semiconductors and suitable growth conditions.

contact materials↗
DOE OSTI2021

Computation-guided optimization of split protein systems

Splitting bioactive proteins into conditionally reconstituting fragments is a powerful strategy for building tools to study and control biological systems. However, split proteins often exhibit a high propensity to reconstitute, even without the conditional trigger, limiting their utility. Current approaches for tuning reconstitution propensity are laborious, context-specific or often ineffective. Here, we report a computational design strategy grounded in fundamental protein biophysics to guide experimental evaluation of a sparse set of mutants to identify an optimal functional window. We hypothesized that testing a limited set of mutants would direct subsequent mutagenesis efforts by predicting desirable mutant combinations from a vast mutational landscape. This strategy varies the degree of interfacial destabilization while preserving stability and catalytic activity. We validate our method by solving two distinct split protein design challenges, generating both design and mechanistic insights. Furthermore, this new technology will streamline the generation and use of split protein systems for diverse applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Site Density Functional Theory and Structural Bioinformatics Analysis of the SARS-CoV Spike Protein and hACE2 Complex

The entry of the SARS-CoV-2, a causative agent of COVID-19, into human host cells is mediated by the SARS-CoV-2 spike (S) glycoprotein, which critically depends on the formation of complexes involving the spike protein receptor-binding domain (RBD) and the human cellular membrane receptor angiotensin-converting enzyme 2 (hACE2). Using classical site density functional theory (SDFT) and structural bioinformatics methods, we investigate binding and conformational properties of these complexes and study the overlooked role of water-mediated interactions. Analysis of the three-dimensional reference interaction site model (3DRISM) of SDFT indicates that water mediated interactions in the form of additional water bridges strongly increases the binding between SARS-CoV-2 spike protein and hACE2 compared to SARS-CoV-1-hACE2 complex. By analyzing structures of SARS-CoV-2 and SARS-CoV-1, we find that the homotrimer SARS-CoV-2 S receptor-binding domain (RBD) has expanded in size, indicating large conformational change relative to SARS-CoV-1 S protein. Protomer with the up-conformational form of RBD, which binds with hACE2, exhibits stronger intermolecular interactions at the RBD-ACE2 interface, with differential distributions and the inclusion of specific H-bonds in the CoV-2 complex. Further interface analysis has shown that interfacial water promotes and stabilizes the formation of CoV-2/hACE2 complex. This interaction causes a significant structural rigidification of the spike protein, favoring proteolytic processing of the S protein for the fusion of the viral and cellular membrane. Moreover, conformational dynamics simulations of RBD motions in SARS-CoV-2 and SARS-CoV-1 point to the role in modification of the RBD dynamics and their impact on infectivity.

3DRISM↗
DOE OSTI2026

Mechanism-Resolved PFM of Ferroionic and Ferroelectric Responses in Thickness-Gradient Hf 0.5 Zr 0.5 O 2 Libraries

Resolving growth mechanisms and thickness evolution of functional properties is one of the key tasks in materials discovery and optimization involving thin-film materials, traditionally requiring significant experimental budgets. Here we introduce the combination of thickness-gradient libraries and automated scanning probe microscopy as a systematic pathway to elucidate growth modes and disentangle ferroelectric and electrochemical contributions in ferroelectric thin films. As a model system, we explore the Hf 0.5 Zr 0.5 O 2 (HZO) gradient thin films grown on La x Sr 1-x MnO 3 (LSMO) bottom electrode thin films. Automated piezoresponse force microscopy, spectroscopy, and lithography reveals that irreversible topographic deformation arises from electrochemical activity at the LSMO surface, whereas reversible phase inversion in HZO reflects ferroelectric switching. Automated topography height-map scans are used to further quantify nucleation density, particle-size evolution, and roughness correlations across the thickness-gradient, demonstrating that improved plume stabilization during growth suppresses interfacial reactions and promotes dense, fine-grained HZO conducive to ferroelectric phase formation. This combined materials-engineering and automated-SPM framework establishes a platform for high-throughput, mechanism-resolved characterization of ferroionic and ferroelectric responses in complex oxide films.

FOS: Physical sciences↗
DOE OSTI2026

Mitigating electrochemical degradation in CsPbBr{sub 3} gamma detectors by organic and inorganic encapsulation.

CsPbBr3 perovskite semiconductors have emerged as a leading candidate for nextgeneration radiation detectors because of their exceptional charge transport properties, defect tolerance, and record-breaking sensitivity and energy resolution. Their long-term stability, however, is hindered by electrode-driven electrochemical decomposition, which is accelerated by moisture- and oxygen-assisted ion migration during operation. Here, we investigated organic and inorganic encapsulation strategies as both environmental barriers and means to suppress interfacial degradation pathways. Atomic layer deposition (ALD) of Al2O3 provided a conformal passivation layer that blocked environmental ingress, suppressed ionic diffusion, reduced leakage current, enhanced energy resolution and expanded the operational electric-field window beyond 5 kV∙cm1 . By contrast, organic encapsulants such as paraffin wax and polystyrene slowed moisture diffusion but did not suppress interfacial reactions, with wax extending stability to over 90 days. These results show that ALD-Al2O3 suppresses dominant interfacial degradation pathways, enabling stable, high-field operation and advancing the practical deployment of CsPbBr3 γ-ray detectors.

Unal, Mustafa↗
DOE OSTI2021

Water Interplays during Dysprosium Electrodeposition in Pyrrolidinium Ionic Liquid: Deconvoluting the Pros and Cons for Rare Earth Metallization

The electrochemical production of rare earth metals (REMs) in ionic liquids (ILs) has received much attention as a promising, sustainable and replacement to the molten salt electrolysis. Water additives have been recently suggested as a strategy to overcome the limitations of the ionic liquid process, but without complete assessment of its overall viability for the REMs production. In this regard, a full investigation of water impact on dysprosium (Dy) electrodeposition in pyrrolidinium triflate (BMPyOTf) ionic liquid was carried out. Water introduction was revealed to involve an interplay of implications on the electrodeposition process, including coordination, speciation, reduction pathways, interfacial dynamics, nucleation and metal stability and purity. In highly dry conditions, the reduction occurs at very negative potentials (-3.3V) in a consecutive pathway, resulting in negligible metal electrodeposition (low rate and efficiency) at the electrode surface. Initial water introduction (<30% molar fraction) leads to partitioning of the Dy complex between water and IL coordinated speciation, giving rise to an additional wave at more positive potentials (-2.4V). Probing and quantification of the heterogeneous Dy speciation by spectroscopic analyses enabled the uncovering of the reduction mechanism and accurate evaluation of the mass transport properties. While lowering the reduction thermodynamics, water addition also improved the nucleation, deposition rate and faradaic efficiency. In spite of these benefits, stripping voltammetric analysis also indicated substantial chemical reactivity of the deposited Dy metal with water additives and/or electrolyte components, under extended timescales. Surface characterization of the obtained product confirmed limited Dy metal stability (mostly oxidized/fluorinated) and purity (~60%) in the studied system. Moreover, high water introduction (>30% molar fraction) was shown to trigger a fast hydrogen evolution reaction (HER), downgrading the robustness of system efficiency. We report the overall impact of water additives seems to engender both promoting and mitigating effects on the Dy electrodeposition process, making the case for more innovative strategies to be sought.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Natural nanoparticle complexes at water-water interfaces

Aqueous two-phase systems (ATPSs) with multifunctional attributes have significant promise as biomimetic materials, but current approaches do not harness their full potential. Here, we show that ATPSs can be stabilized to form microcapsules by the interfacial assembly of chitin nanofibers (ChNF) and rod-like cellulose nanocrystals (CNC). The high structural stability of the ChNF/CNC complexes integrates permeability and transport across the membrane at the liquid/liquid interface. Driven by density and osmotic stress gradients, the microcapsules show switchable motility, including cyclic meniscus-climbing and subsurface transport. These observations demonstrate a self-regulating system with potential for cargo transfer, cell biomimicry, as well as ATPSs-based microreactors and microrobots.

Bioinspired materials↗
DOE OSTI2022

Compact Peptoid Molecular Brushes for Nanoparticle Stabilization

Controlling the interfaces and interactions of colloidal nanoparticles (NPs) via tethered molecular moieties is crucial for NP applications in engineered nanomaterials, optics, catalysis, and nanomedicine. Despite a broad range of molecular types explored, there is a need for a flexible approach to rationally vary the chemistry and structure of these interfacial molecules for controlling NP stability in diverse environments, while maintaining a small size of the NP molecular shell. Here, we demonstrate that low-molecular-weight, bifunctional comb-shaped, and sequence-defined peptoids can effectively stabilize gold NPs (AuNPs). The generality of this robust functionalization strategy was also demonstrated by coating of silver, platinum, and iron oxide NPs with designed peptoids. Each peptoid (PE) is designed with varied arrangements of a multivalent AuNP-binding domain and a solvation domain consisting of oligo-ethylene glycol (EG) branches. Among designs, a peptoid (PE5) with a diblock structure is demonstrated to provide a superior nanocolloidal stability in diverse aqueous solutions while forming a compact shell (similar to 1.5 nm) on the AuNP surface. We demonstrate by experiments and molecular dynamics simulations that PE5-coated AuNPs (PE5/AuNPs) are stable in select organic solvents owing to the strong PE5 (amine)-Au binding and solubility of the oligo-EG motifs. At the vapor-aqueous interface, we show that PE5/AuNPs remain stable and can self-assemble into ordered 2D lattices. The NP films exhibit strong near-field plasmonic coupling when transferred to solid substrates.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Interfacial Void Formation and Self-Healing in Oxide Scales on Al-containing High-Entropy Alloy

The exceptional high-temperature oxidation resistance of Al-containing high-entropy alloys (HEAs) is often attributed to the formation of a protective α-Al 2 O 3 scale. However, the dynamic, atomic-scale mechanisms governing the stability of this scale—including interfacial void formation and the often-postulated but rarely visualized “self-healing” capacity—remain poorly understood. Herein, we reveal the complex evolution of the triple-layer oxide scale on an Al 10 CoCrFeNi HEA through combined electron microscopy and diffraction study. We show that interfacial voids are an inherent consequence of the scaling process, originating from two distinct mechanisms: the Kirkendall effect at the interface between the γ-Al 2 O 3 /α-Al 2 O 3 and alloy driven by cationic diffusion imbalance and volumetric contraction due to phase transformations at the spinel/Cr 2 O 3 interface. Crucially, we provide microstructural evidence consistent with an intrinsic self-healing response. This process is driven by coupled inward diffusion of oxygen and outward diffusion of metal cations, leading to the in-situ formation of transient θ-Al 2 O 3 and spinel phases that partially fill and seal the voids. Here, these results provide atomic-scale insights into the phase evolution, defect formation, and self-repair of oxide scales in HEAs—highlighting pathways to enhance their oxidation resistance in extreme environments.

High-entropy alloy↗
DOE OSTI2021

Enabling stable and high-rate cycling of a Ni-rich layered oxide cathode for lithium-ion batteries by modification with an artificial Li⁺-conducting cathode-electrolyte interphase

Ni-rich LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) cathodes are investigated to realize high energy density Li ion batteries for long life electric vehicle applications. However, capacity decay and thermal instability due to cathode-electrolyte interfacial degradation remain challenges that require sophisticated surface stabilization methods to address. Here, we propose a strategy, for the first time, to form an artificial Li + -conducting cathode-electrolyte interphase (ALCEI) on the NCM811 cathode surface using a nucleophilic reaction between polysulfides and vinylene carbonate (VC). Furthemore, the as-formed ALCEI layer simultaneously protects the NCM particles from electrolyte corrosion and facilitates Li + ion transport, thus enabling stable and high rate cycling of NCM811. As a result, the ALCEI-modified NCM811 cathode exhibits a high capacity (211.6 mA h g -1 at 0.1C), notable rate capability (134 mA h g -1 at 10C), and superior cycle stability (94.2% over 200 cycles at 1C). These results underscore the use of interfacial engineering in high voltage cathode material development and provide a feasible strategy for stabilizing Ni-rich cathode interfaces in practical Li ion battery applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Chemical factors controlling the behaviour of oxide cathodes in batteries

Oxide cathodes enable high-energy lithium-ion and sodium-ion batteries, with their performances fundamentally governed by three interrelated chemical factors: electronic configuration, chemical bonding, and chemical reactivity. Here, we illustrate how these factors dictate the redox energy, structural stability, ionic and electronic transport, and interfacial behavior in both layered oxide and polyanion oxide cathodes. We discuss how crystal-field effects and octahedral-site stabilization energies influence cation migration, and how inductive effects tune bond covalency and operating voltages. We also explain how chemical bonding governs thermal stability, gas evolution, and first-cycle capacity loss, and how alignment of transition-metal redox band with the oxygen 2p band determines electrolyte reactivity. Comparison between lithium and sodium layered oxides further reveals how differences in Li-O and Na-O bond ionicity affect chemical reactivity. Finally, we outline strategies including compositional tuning, surface doping, and electrolyte optimization, and emphasize how high-throughput, data-driven approaches in guiding the design of next-generation oxide cathodes.

25 ENERGY STORAGE↗
DOE OSTI2025

Ab initio prediction of FeCr sigma (001)/FCC Fe(111) interfacial energy: Effect of interfacial doping of C, B, and N

Interfacial energy plays a crucial role for high-temperature material systems in determining their microstructure evolution and mechanical properties. Optimizing the interfacial energy through doping can improve the stability, performance, and overall functionality of these materials. Here, the present study delivers theoretical investigation on the sigma FeCr (001)//FCC Fe (111) interfacial energy using ab initio methods, focusing on the implications of interfacial doping with important interstitial elements carbon (C), boron (B), and nitrogen (N). The calculated interfacial energy without doping is 0.183J/m 2 . Upon doping, notable reductions in interfacial energy were observed. Doping with B decreased the interfacial energy to 0.046J/m 2 , with single C doping to 0.126J/m 2 , with two C doping to 0.098J/m 2 , and with a single N doping to 0.071J/m 2 . These findings not only shed light on the atomistic origin for the effect of stabilizing the microstructure of stainless steel at elevated temperatures by interstitial doping, but also present a systematic, thorough ab initio approach to predict interfacial doping properties and guide alloy design to enhance the performance and stability.

FeCr sigma phase↗
DOE OSTI2024

A Fluorinated Lewis Acidic Organoboron Tunes Polysulfide Complex Structure for High–Performance Lithium–Sulfur Batteries

Many challenges in lithium-sulfur (Li–S) batteries are associated with the radical change in lithium polysulfide (LPS) solubility during cycling, but chemical approaches to address such inconsistency are still lacking. Here, the use of a strong Lewis acidic fluorinated organoboron, tri(2,2,2-trifluoroethyl) borate (TFEB), is reported as a multi-functional mediator to simultaneously overcome multiple technical barriers in practical Li–S batteries. TFEB acts as an anion acceptor and forms strong molecular complexes with Lewis basic LPS. The TFEB-LPS complexes have consistent solubility across the full polysulfide spectrum and deliver several times improved better redox kinetics, unlocking a true redox catalytic mechanism that covers the majority of redox events in thick sulfur cathodes. As a result, Li–S batteries evaluated under practical conditions exhibit significantly improved discharge capacity, rate capability, and cycling stability with the addition of the TFEB additive. More importantly, TFEB also contributes to the stabilization of lithium anode in the presence of polysulfides by generating strong interfacial film. These attributes significantly improve the cycling stability of practical Li–S pouch cells, which are assembled with a unit energy density of 219 Wh kg –1 . Finally, the results provide new molecular insights on the design of unlocking solvation networks of practical Li–S systems.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Improving the Comprehensive Performance of Na 0.7 MnO 2 for Sodium Ion Batteries by ZrO 2 Atomic Layer Deposition

Sodium ion batteries with Na-Mn-O compounds as cathode have been widely studied as substitutes for lithium ion batteries due to their abundant resources. Still, the relatively poor cycling stability and low capacity of Na-Mn-O compounds significantly limit their applications. Different approaches, including element substitution and surface modification, have been applied to improve the electrochemical performance of those cathode materials. Herein, element doping and coating of ZrO 2 on Na 0.7 MnO 2 particles have been achieved by atomic layer deposition (ALD) followed by post-annealing. The rate capability and cycling stability of the modified material were significantly improved, and the mechanism of performance enhancement was revealed. The ZrO 2 coatings acted as a stable interfacial layer to enhance the cycling stability of Na 0.7 MnO 2 by suppressing side reactions between the electrode and electrolyte. The doping of transition metal ions reduced energy barriers for sodium ion insertion and deintercalation during cycling, further improving the charge and discharge capacity and rate performance of Na 0.7 MnO 2 .

25 ENERGY STORAGE↗