Publications

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The present study has used a variety of characterization techniques to determine the products and reaction pathways involved in the rechargeable Li-FeS2 system. We revisit both the initial lithiation and subsequent cycling of FeS2 employing an ionic liquid electrolyte to investigate the intermediate and final charge products formed under varying thermal conditions (room temperature to 100 °C). The detection of Li2S and hexagonal FeS as the intermediate phases in the initial lithiation and the electrochemical formation of greigite, Fe3S4, as a charge product in the rechargeable reaction differ significantly from previous reports. The conditions for Fe3S4 formation are shown to be dependent on both the temperature (∼60 °C) and the availability of sulfur to drive a FeS to Fe3S4 transformation. Upon further cycling, Fe3S4 transforms to a lower sulfur content iron sulfide phase, a process which coincides with the loss of sulfur based on the new reaction pathways established in this work. The connection between sulfur loss, capacity fade, and charge product composition highlights the critical need to retain sulfur in the active material upon cycling.

Conversion cathodes represent a viable route to improve rechargeable Li+battery energy densities, but their poor electrochemical stability and power density have impeded their practical implementation. Here, we explore the impact cell fabrication, electrolyte interaction, and current density have on the electrochemical performance of FeS2/Li cells by deconvoluting the contributions of the various conversion and intercalation reactions to the overall capacity. By varying the slurry composition and applied pressure, we determine that the capacity loss is primarily due to the large volume changes during (de)lithiation, leading to a degradation of the conductive matrix. Through the application of an external pressure, the loss is minimized by maintaining the conductive matrix. We further determine that polysulfide loss can be minimized by increasing the current density (>C/10), thus reducing the sulfur formation period. Analysis of the kinetics determines that the conversion reactions are rate-limiting, specifically the formation of metallic iron at rates above C/8. While focused on FeS2, our findings on the influence of pressure, electrolyte interaction, and kinetics are broadly applicable to other conversion cathode systems.

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Zinc oxide is of great interest for advanced energy devices because of its low cost, wide direct bandgap, non-toxicity, and facile electrochemistry. In zinc alkaline batteries, ZnO plays a critical role in electrode passivation, a process that hinders commercialization and remains poorly understood. Here, novel observations of an electroactive type of ZnO formed in Zn-metal alkaline electrodes are disclosed. The electrical conductivity of battery-formed ZnO is measured and found to vary by factors of up to 104, which provides a first-principles-based understanding of Zn passivation in industrial alkaline batteries. Simultaneous with this conductivity change, protons are inserted into the crystal structure and electrons are inserted into the conduction band in quantities up to ≈1020 cm−3 and ≈1 mAh gZnO−1. Electron insertion causes blue electrochromic coloration with efficiencies and rates competitive with leading electrochromic materials. The electroactivity of ZnO is evidently enabled by rapid crystal growth, which forms defects that complex with inserted cations, charge-balanced by the increase of conduction band electrons. This property distinguishes electroactive ZnO from inactive classical ZnO. Knowledge of this phenomenon is applied to improve cycling performance of industrial-design electrodes at 50% zinc utilization and the authors propose other uses for ZnO such as electrochromic devices.

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In alkaline zinc–manganese dioxide batteries, there is a need for selective polymeric separators that have good hydroxide ion conductivity but that prevent the transport of zincate (Zn(OH)₄)^2-. Here we investigate the nanoscale structure and hydroxide transport in two cationic polysulfones that are promising for these separators. We present the synthesis and characterization for a tetraethylammonium-functionalized polysulfone (TEA-PSU) and compare it to our previous work on an N-butylimidazolium-functionalized polysulfone (NBI-PSU). We perform atomistic molecular dynamics (MD) simulations of both polymers at experimentally relevant water contents. The MD simulations show that both polymers develop well phase separated nanoscale water domains that percolate through the polymer. Calculation of the total scattering intensity from the MD simulations reveal weak or nonexistent ionomer peaks at low wave vectors. The lack of an ionomer peak is due to a loss of contrast in the scattering. The small water domains in both polymers, with median diameters on the order of 0.5–0.7 nm, lead to hydroxide and water diffusion constants that are 1–2 orders of magnitude smaller than their values in bulk water. This confinement lowers the conductivity but also may explain the strong exclusion of zincate from the PSU membranes seen experimentally.

Lithium/fluorinated graphite (Li/CFx) primary batteries show great promise for applications in a wide range of energy storage systems due to their high energy density (>2100 Wh kg^–1) and low self-discharge rate (<0.5% per year at 25 °C). While the electrochemical performance of the CF_x cathode is indeed promising, the discharge reaction mechanism is not thoroughly understood to date. In this article, a multiscale investigation of the CF_x discharge mechanism is performed using a novel cathode structure to minimize the carbon and fluorine additives for precise cathode characterizations. Titration gas chromatography, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, cross-sectional focused ion beam, high-resolution transmission electron microscopy, and scanning transmission electron microscopy with electron energy loss spectroscopy are utilized to investigate this system. Results show no metallic lithium deposition or intercalation during the discharge reaction. Crystalline lithium fluoride particles uniformly distributed with <10 nm sizes into the CF_x layers, and carbon with lower sp2 content similar to the hard-carbon structure are the products during discharge. This article deepens the understanding of CF_x as a high energy density cathode material and highlights the need for future investigations on primary battery materials to advance performance.

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The galvanostatic intermittent titration technique (GITT) is widely used to evaluate solid-state diffusion coefficients in electrochemical systems. However, the existing analysis methods for GITT data require numerous assumptions, and the derived diffusion coefficients typically are not independently validated. To investigate the validity of the assumptions and derived diffusion coefficients, we employ a direct-pulse fitting method for interpreting the GITT data that involves numerically fitting an electrochemical pulse and subsequent relaxation to a one-dimensional, single-particle, electrochemical model coupled with non-ideal transport to directly evaluate diffusion coefficients. Our non-ideal diffusion coefficients, which are extracted from GITT measurements of the intercalation regime of FeS2 and independently verified through discharge predictions, prove to be 2 orders of magnitude more accurate than ideal diffusion coefficients extracted using conventional methods. We further extend our model to a polydisperse set of particles to show the validity of a single-particle approach when the modeled radius is proportional to the total volume-to-surface-area ratio of the system.

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