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.
Revealing impacts of electrolyte speciation on ionic charge storage in aluminum-quinone batteries by NMR spectroscopy
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.
Formation of a CoMn‐Layered Double Hydroxide/Graphite Supercapacitor by a Single Electrochemical Step
Atanu Roy, Theresa Schoetz,
Leo W. Gordon, Hung‐Ju Yen, Qingli Hao, and Daniel Mandler
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 CoII and, in the presence of MnII, 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 CoII and MnII 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 \muWh cm−3 and a power density of 5 mW cm−3.
Molecular-Scale Elucidation of Ionic Charge Storage Mechanisms in Rechargeable Aluminum–Quinone Batteries
Leo W. Gordon, Ankur L. Jadhav, Mikhail Miroshnikov, Theresa Schoetz, George John, and Robert J. Messinger
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.
Performance Leap of Lithium Metal Batteries in LiPF6 Carbonate Electrolyte by a Phosphorus Pentoxide Acid Scavenger
Jian Zhang, Jiayan Shi,
Leo W. Gordon, Nastaran Shojarazavi, Xiaoyu Wen, Yifan Zhao, Jianjun Chen, Chi-Cheung Su, Robert J. Messinger, and Juchen Guo
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.
Soluble Electrolyte-Coordinated Sulfide Species Revealed in Al–S Batteries by Nuclear Magnetic Resonance Spectroscopy
Rahul Jay, Ankur L. Jadhav,
Leo W. Gordon, and Robert J. Messinger
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.
Disentangling Faradaic, Pseudocapacitive, and Capacitive Charge Storage: A Tutorial for the Characterization of Batteries, Supercapacitors, and Hybrid Systems
L.W. Gordon, S. Ivanov, A. Bund, D. Mandler, and R.J. Messinger
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.