Publications
My first-author and collaborative publications.
My first-author and collaborative publications.
Dec 2024
Aluminum anodes and quinone-based organic cathodes couple as earth abundant, sustainable, and safe battery electrodes. However, different numbers of galvanostatic discharge plateaus have been observed in aluminum-quinone cells depending on the experimental conditions, a phenomenon that has not yet been explained and furthermore affects cell energy density. Here, in aluminum-anthraquinone batteries, we show that ion mass transport affects the electrochemical reaction pathway and controls the occurrence of either one or two reduction plateaus during galvanostatic discharge. The effects of electrode mass loading, porosity, rest periods, and cycling rates were analyzed on the reduction potential and relative specific capacity of the galvanostatic plateaus. When AlCl2+ cations charge compensate the electrochemically reduced carbonyl groups concurrently, a single reduction plateau is observed. When ion mass transport is limiting, sequential reduction and charge compensation of each carbonyl group results in two distinct reduction plateaus. The second plateau occurs at a potential approximately 0.3 V lower than the first, thus decreasing the cell energy density. The potential of the electrochemical oxidation reaction where AlCl2+ cations dissociate, however, is not affected. Ion mass transport is thus shown to be a critical variable that can control the electrochemical reaction pathway, redox potentials, and practical energy densities of aluminum-quinone batteries.
Jul 2024
Rechargeable aluminum (Al) metal batteries are enticing for the coming generation of electrochemical energy storage systems due to the earth abundance, high energy density, inherent safety, and recyclability of Al metal. However, few electrolytes can reversibly electrodeposit Al metal, especially at low temperatures. In this study, Al electroplating and stripping were investigated from 25 \,^∘C to −40 \,^∘C in mixtures of aluminum chloride (AlCl3), 1-ethyl-3-methyl-imidazolium chloride ([EMIm]Cl), and urea. The ternary ionic liquid analogue (ILA) consisting of AlCl3-urea-[EMIm]Cl in a molar ratio of 1.3:0.25:0.75 enabled reversible Al electrodeposition at temperatures as low as −40 \,^∘C while exhibiting the highest current density and the lowest overpotential among all of the electrolyte mixtures at 25 \,^∘C, including the AlCl3-[EMIm]Cl binary mixture. The ILA electrolyte was further tested in a rechargeable Al-graphite battery system down to −40 \,^∘C. The addition of urea to AlCl3-[EMIm]Cl binary mixtures can improve the Al electrodeposition, extend the liquid temperature window, and reduce the cost.
May 2024
Selenium (Se) is an attractive positive electrode material for rechargeable aluminum (Al) batteries due to its high theoretical capacity of 2037 mA h g–1 and its higher electronic conductivity compared to sulfur. Selenium can undergo a series of electrochemical reactions between Se(−II) and Se(IV), resulting in a six-electron capacity per Se atom. However, existing Al−Se battery literature is inconsistent regarding the different electrochemical reactions possible, while the conditions enabling the electrochemical reduction of Se to Al2Se3 are not well understood. Here, we demonstrate that this electrochemical reduction is achievable using amorphous selenium but is suppressed for crystalline selenium. We further show that the electrochemical oxidation of Se to SeCl4, which occurs at higher potentials, reduces the long-range order of crystalline Se and enables its discharge to Al2Se3. Solid-state 77Se nuclear magnetic resonance (NMR) measurements further establish that the local Se helical structures are maintained upon the loss of crystallinity.
May 2024
Next-generation batteries demand solid polymer electrolytes (SPEs) with rapid ion transport and robust mechanical properties. However, many SPEs with liquid-like Li+ transport mechanisms suffer a fundamental trade-off between conductivity and strength. Dynamic polymer networks can improve bulk mechanics with minimal impact to segmental relaxation or ionic conductivity. This study demonstrates a system where a single polymer-bound ligand simultaneously dissociates Li+ and forms long-lived Ni2+ networks. The polymer comprises an ethylene oxide backbone and imidazole (Im) ligands, blended with Li+ and Ni2+ salts. Ni2+–Im dynamic cross-links result in the formation of a rubbery plateau resulting in, consequently, storage modulus improvement by a factor of 133× with the introduction of Ni2+ at rNi = 0.08, from 0.014 to 1.907 MPa. Even with Ni2+ loading, the high Li+ conductivity of 3.7 × 10–6 S/cm is retained at 90 \,^∘C. This work demonstrates that decoupling of ion transport and bulk mechanics can be readily achieved by the addition of multivalent metal cations to polymers with chelating ligands.
Apr 2024
Fifty years after its introduction, the lithium-carbon monofluoride (Li-CFx) battery still has the highest cell-level specific energy demonstrated in a practical cell format. However, few studies have analyzed how the main electrochemical discharge product, LiF, evolves during the discharge and cell rest periods. To fill this gap in understanding, we investigated molecular-level and interfacial changes in CFx electrodes upon the discharge and aging of Li-CFx cells, revealing the role of LiF beyond that of a simple discharge product. We reveal that electrochemically formed LiF deposits on the surface of the CFx electrode and subsequently partially disperses into the electrolyte to form a colloidal suspension during cell aging, as determined from galvanostatic electrochemical impedance spectroscopy (EIS), solid-state 19F nuclear magnetic resonance (NMR), dynamic light scattering (DLS), and operando optical light microscopy measurements. Electrochemical LiF formation and LiF dispersion into the electrolyte are distinct competing rate processes that each affect the cell impedance differently. Using knowledge of LiF dispersion and saturation, an in-line EIS method was developed to compute the depth of discharge of CFx cells beyond coulomb counting. Solid-state 19F NMR measurements quantitatively revealed how LiF and CF moieties evolved with discharge. Covalent CF bonds react first, followed by a combination of covalent and ionic CF bonds. Quantitively correlating NMR and electrochemical measurements reveals not only how LiF formation affects cell impedance but also that CF bonds with the most ionic character remain unreacted, which limits realization of the full theoretical specific capacity of the CFx electrode. The results reveal new insights into the electrochemical discharge mechanism of Li-CFx cells and the unique role of LiF in cell discharge and aging, which suggest pretreatment strategies and methods to improve and measure the performance of Li-CFx batteries.
Jan 2024
Abstract Chloroaluminate ionic liquids are commonly used electrolytes in rechargeable aluminum batteries due to their ability to reversibly electrodeposit aluminum at room temperature. Progress in aluminum batteries is currently hindered by the limited electrochemical stability, corrosivity, and moisture sensitivity of these ionic liquids. Here, a solid polymer electrolyte based on 1-ethyl-3-methylimidazolium chloride-aluminum chloride, polyethylene oxide, and fumed silica is developed, exhibiting increased electrochemical stability over the ionic liquid while maintaining a high ionic conductivity of ≈13 mS cm–1. In aluminum–graphite cells, the solid polymer electrolytes enable charging to 2.8 V, achieving a maximum specific capacity of 194 mA h g–1 at 66 mA g–1. Long-term cycling at 2.7 V showed a reversible capacity of 123 mA h g–1 at 360 mA g–1 and 98.4% coulombic efficiency after 1000 cycles. Solid-state nuclear magnetic resonance spectroscopy measurements reveal the formation of five-coordinate aluminum species that crosslink the polymer network to enable a high ionic liquid loading in the solid electrolyte. This study provides new insights into the molecular-level design and understanding of polymer electrolytes for high-capacity aluminum batteries with extended potential limits.
Feb 2023
Rechargeable zinc (Zn) metal batteries are attractive for use as electrochemical energy storage systems on a global scale because of the low cost, high energy density, inherent safety, and strategic resource security of Zn metal. However, at low temperatures, Zn batteries typically suffer from high electrolyte viscosity and unfavorable ion transport properties. Here, we studied reversible Zn electrodeposition in mixtures of 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ([EMIm]TFSI) ionic liquid, γ-butyrolactone (GBL) organic solvent, and Zn(TFSI)2 zinc salt. The electrolyte mixtures enabled reversible Zn electrodeposition at temperatures as low as −60 °C. An electrolyte composed of 0.1 M Zn(TFSI)2 in [EMIm]TFSI:GBL with a volume ratio of 1:3 formed a deep eutectic solvent that optimized electrolyte conductivity, viscosity, and the zinc diffusion coefficient. Liquid-state 1H and 13C nuclear magnetic resonance (NMR) spectroscopy and molecular dynamic (MD) simulations indicate increased formation of contact ion pairs and the reduction of ion aggregates are responsible for the optimal composition.
Mar 2023
Rechargeable aluminum-organic batteries are composed of earth-abundant, sustainable electrode materials while the molecular structures of the organic materials can be controlled to tune their electrochemical properties. Aluminum metal batteries typically use electrolytes based on chloroaluminate ionic liquids or deep eutectic solvents that are comprised of polyatomic aluminum-containing ions. Quinone-based organic electrodes store charge when chloroaluminous cations (AlCl2+) charge compensate their electrochemically reduced carbonyl groups, even when such cations are not natively present in the electrolyte. However, how ion speciation in the electrolyte affects the ion charge storage mechanism, and resultant battery performance, is not well understood. Here, we couple solid-state NMR spectroscopy with electrochemical and computational methods to show for the first time that electrolyte-dependent ion speciation significantly alters the molecular-level environments of the charge-compensating cations, which in turn influences battery properties. Using 1,5-dichloroanthraquinone (DCQ) for the first time as an organic electrode material, we utilize dipolar-mediated solid-state NMR experiments to elucidate distinct aluminum coordination environments upon discharge that depend significantly on electrolyte speciation. We relate DFT-calculated NMR parameters to experimentally determined quantities, revealing insights into their origins. The results establish that electrolyte ion speciation impacts the local environments of charge-compensating chloroaluminous cations and is a crucial design parameter for rechargeable aluminum-organic batteries.
Nov 2022
Hybrid electric storage systems that combine capacitive and faradaic materials need to be well designed to benefit from the advantages of batteries and supercapacitors. The ultimate capacitive material is graphite (GR), yet high capacitance is usually not achieved due to restacking of its sheets. Therefore, an appealing approach to achieve high power and energy systems is to embed a faradaic 2D material in between the graphite sheets. Here, a simple one-step approach was developed, whereby a faradaic material [layered double hydroxide (LDH)] was electrochemically formed inside electrochemically exfoliated graphite. Specifically, GR was exfoliated under negative potentials by Co(II) and, in the presence of Mn(II), formed GR-CoMn-LDH, which exhibited a high areal capacitance and energy density. The high areal capacitance was attributed to the exfoliation of the graphite at very negative potentials to form a 3D foam-like structure driven by hydrogen evolution as well as the deposition of CoMn-LDH due to hydroxide ion generation inside the GR sheets. The ratio between the Co(II) and Mn(II) in the CoMn-LDH was optimized and analyzed, and the electrochemical performance was studied. Analysis of a cross-section of the GR-CoMn-LDH confirmed the deposition of LDH inside the GR layers. The areal capacitance of the electrode was 186 mF cm–2 at a scan rate of 2 mV s–1. Finally, an asymmetric supercapacitor was assembled with GR-CoMn-LDH and exfoliated graphite as the positive and negative electrodes, respectively, yielding an energy density of 96.1 μWh cm–3 and a power density of 5 mW cm–3.
Aug 2022
Rechargeable aluminum–organic batteries are of great interest as a next-generation energy storage technology because of the earth abundance, high theoretical capacity, and inherent safety of aluminum metal, coupled with the sustainability, availability, and tunabilty of organic molecules. However, the ionic charge storage mechanisms occurring in aluminum–organic batteries are currently not well understood, in part because of the diversity of possible charge-balancing cations, coupled with a wide array of possible binding modes. For the first time, we use multidimensional solid-state NMR spectroscopy in conjunction with electrochemical methods to elucidate experimentally the ionic and electronic charge storage mechanism in an aluminum–organic battery up from the atomic length scale. In doing so, we present indanthrone quinone (INDQ) as a positive electrode material for rechargeable aluminum batteries, capable of reversibly achieving specific capacities of ca. 200 mAh g–1 at 0.12 A g–1 and 100 mAh g–1 at 2.4 A g–1 . We demonstrate that INDQ stores charge via reversible electrochemical enolization reactions, which are charge compensated in chloroaluminate ionic liquid electrolytes by cationic chloroaluminous (AlCl2+ ) species in tetrahedral geometries. The results are generalizable to the charge storage mechanisms underpinning anthraquinone-based aluminum batteries. Lastly, the solid-state dipolar-mediated NMR experiments used here establish molecular-level interactions between electroactive ions and organic frameworks while filtering mobile electrolyte species, a methodology applicable to many multiphase host–guest systems.
Aug 2022
Phosphorus pentoxide (P2O5) is investigated as an acid scavenger to remove the acidic impurities in a commercial lithium hexafluorophosphate (LiPF6) carbonate electrolyte to improve the electrochemical properties of Li metal batteries. Nuclear magnetic resonance (NMR) measurements reveal the detailed reaction mechanisms of P2O5 with the LiPF6 electrolyte and its impurities, which removes hydrogen fluoride (HF) and difluorophosphoric acid (HPO2F2) and produces phosphorus oxyfluoride (POF3), OF2P–O–PF5– anions, and ethyl difluorophosphate (C2H5OPOF2) as new electrolyte species. The P2O5-modified LiPF6 electrolyte is chemically compatible with a Li metal anode and LiNi0.6Mn0.2Co0.2O2 (NMC622) cathode, generating a POxFy-rich solid electrolyte interphase (SEI) that leads to highly reversible Li electrodeposition, while eliminating transition metal dissolution and cathode particle cracking. The excellent electrochemical properties of the P2O5-modified LiPF6 electrolytes are demonstrated on Li||NMC622 pouch cells with 0.4 Ah capacity, 50 μm Li anode, 3 mAh cm–2 NMC622 cathode, and 3 g Ah–1 electrolyte/capacity ratio. The pouch cells can be galvanostatically cycled at C/3 for 230 cycles with 87.7% retention.
May 2022
Rechargeable aluminum-sulfur (Al-S) batteries have recently garnered significant interest to the low cost, earth abundance, safety, and high theoretical capacity of the electrode materials. However, Al-S batteries exhibit many challenges that plague other metal-sulfur battery systems, including significant capacity fade of the sulfur electrode due to the formation of electrolyte-soluble reaction intermediates. Here, Al-S cells using chloroaluminate-containing ionic liquid electrolytes were investigated up from the molecular level using multidimensional solid-state 27Al MAS NMR spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and electrochemical measurements. Solid-state 27Al single-pulse NMR measurements acquired on cycled sulfur electrodes containing electrolyte-soaked separator revealed multiple discharge products, which were distinguished into liquid- and solid-phase products based on 27Al chemical exchange and nutation NMR experiments. During discharge, electrolyte-soluble sulfide species form that coordinate with the AlCl4– chloroaluminate anions, resulting in (SxAlCl4)y– electrolyte complexes. These electrolyte-coordinated sulfide species persist upon charge, resulting in the loss of active mass that explains the significant capacity fade observed upon galvanostatic cycling. XPS, XRD, and solid-state 27Al NMR measurements reveal that solid amorphous Al2S3 forms reversibly upon discharge. The results highlight the technological importance of understanding how electrolyte-soluble sulfide species coordinate with the complex electroactive species used in multivalent metal-sulfur batteries, which can affect their reversibility and electrochemical properties.
Apr 2022
Today’s electrochemical energy storage technologies aim to combine high specific energy and power, as well as long cycle life, into one system to meet increasing demands in performance. These properties, however, are often characteristic of either batteries (high specific energy) or capacitors (high specific power and cyclability). To merge battery- and capacitor-like properties in a hybrid energy storage system, researchers must understand and control the co-existence of multiple charge storage mechanisms. Charge storage mechanisms can be classified as faradaic, capacitive, or pseudocapacitive, where their relative contributions determine the operating principles and electrochemical performance of the system. Hybrid electrochemical energy storage systems can be better understood and analyzed if the primary charge storage mechanism is identified correctly. This tutorial review first defines faradaic and capacitive charge storage mechanisms and then clarifies the definition of pseudocapacitance using a physically intuitive framework. Then, we discuss strategies that enable these charge storage mechanisms to be quantitatively disentangled using common electrochemical techniques. Finally, we outline representative hybrid energy storage systems that combine the electrochemical characteristics of batteries, capacitors and pseudocapacitors. Modern examples are analyzed while step-by-step guides are provided for all mentioned experimental methods in the Supplementary Information.