Achieving practical, high-energy-density calcium batteries requires controlling the stability of Ca2+electrolytes during calcium metal cycling. Because of the highly reactive nature of calcium, most typical electrolyte constituents are unstable, leading to electrode passivation and low Coulombic efficiency. Among various commercially available salts, calcium bis(trifluoromethylsulfonyl)imide (Ca(TFSI)2) is attractive because of its oxidative stability and high solubility in a variety of solvents. However, this salt does not allow for calcium metal plating, and it has been proposed that TFSI-instability induced by Ca2+coordination is to blame. In this work, we test the ability of strongly coordinating Ca2+cosalts such as halides and borohydrides to displace TFSI-from the first coordination shell of Ca2+and thereby stabilize TFSI-based electrolytes to enable calcium plating. Through spectroscopic analysis, we find that the effectiveness of these cosalts at displacing the TFSI-anion is dependent on the solvent's coordination strength toward Ca2+. Surprisingly, electrochemical calcium deposition behavior is not correlated to the population of bound or free TFSI-. Instead, the nature of the coordination interaction between Ca2+and the cosalt anion is more important for determining stability. Our findings indicate that TFSI-anions are inherently unstable during calcium deposition even in the nominally free state. Therefore, strategies aimed at eliminating the interactions of these anions with the electrode surface via interface/interphase design are required.
Proceedings of the National Academy of Sciences of the United States of America
Ma, Lin M.; Vatamanu, Jenel V.; Hahn, Nathan H.; Pollard, Travis P.; Borodin, Oleg B.; Petkov, Valeri P.; Schroeder, Marshall S.; Ren, Yang R.; Ding, Michael D.; Luo, Chao L.; Allen, Jan A.; Wang, Chungsheng W.; Xu, Kang X.
Rechargeable Zn metal batteries (RZMBs) may provide a more sustainable and lower-cost alternative to established battery technologies in meeting energy storage applications of the future. However, the most promising electrolytes for RZMBs are generally aqueous and require high concentrations of salt(s) to bring efficiencies toward commercially viable levels and mitigate water-originated parasitic reactions including hydrogen evolution and corrosion. Electrolytes based on nonaqueous solvents are promising for avoiding these issues, although full cell performance demonstrations with solvents other than water have been very limited. To address these challenges, we investigated MeOH as an alternative electrolyte solvent. These MeOH-based electrolytes exhibited exceptional Zn reversibility over a wide temperature range, with a Coulombic efficiency > 99.5% at 50% Zn utilization without cell short-circuit behavior for > 1,800 h. More important, this remarkable performance translates well to Zn || metal-free organic cathode full cells, supporting < 6% capacity decay after > 800 cycles at –40°C.
Pastel, Glenn P.; Chen, Ying C.; Pollard, Travis P.; Schroeder, Marshall S.; Bowden, Mark E.; Zheng, Allen Z.; Hahn, Nathan H.; Ma, Lin M.; Murugesan, Vijayakumar M.; Ho, Janet H.; Garaga, Mounesha G.; Borodin, Oleg B.; Mueller, Karl T.; Greenbaum, Steven G.; Xu, Kang X.
Aqueous aluminum (Al) batteries are posited to be a cheap and energy dense alternative to conventional Li-ion chemistries, but an aqueous electrolyte mediating trivalent aluminum cations (Al3+) warrants greater scrutiny. This study provides a rigorous examination of aqueous Al electrolytes, with the first compelling evidence for a dynamic octahedral solvation structure around the Al3+, without Al–OTf contact ion pairs, even at high concentrations. This solvation behavior and the concomitant, transient electrostatic hydrolysis of Al–OH2 ligands contrasts strongly with previously reported water-in-salt electrolytes, and occurs due to the high charge density of the Lewis acidic Al3+. Nuclear magnetic resonance spectroscopy and other physicochemical measurements quantitatively reveal how species activity evolves with concentration and temperature. Additionally, this new understanding exposes practical concerns related to the corrosiveness of the acidic aqueous solutions, the degree of hydration of aluminum trifluoromethanesulfonate (Al(OTf)3) salt, and the grossly insufficient reductive stability of the proposed electrolytes (>1 V between HER onset and Al3+/Al). Collectively, these factors constitute multiple fundamental barriers to the feasibility of rechargeable aqueous Al batteries.
Hahn, Nathan H.; Self, Julian; Driscoll, Darren M.; Dandu, Naveen; Han, Kee S.; Murugesan, Vijayakumar; Mueller, Karl T.; Curtiss, Larry A.; Balasubramanian, Mahalingam; Persson, Kristin A.; Zavadil, Kevin R.
Ion interactions strongly determine the solvation environments of multivalent electrolytes even at concentrations below that required for practical battery-based energy storage. This statement is particularly true of electrolytes utilizing ethereal solvents due to their low dielectric constants. These solvents are among the most commonly used for multivalent batteries based on reactive metals (Mg, Ca) due to their reductive stability. Recent developments in multivalent electrolyte design have produced a variety of new salts for Mg2+ and Ca2+ that test the limits of weak coordination strength and oxidative stability. Such electrolytes have great potential for enabling full-cell cycling of batteries based on these working ions. However, the ion interactions in these electrolytes exhibit significant and non-intuitive concentration relationships. In this work, we investigate a promising exemplar, calcium tetrakis(hexafluoroisopropoxy)borate (Ca(BHFIP)2), in the ethereal solvents 1,2-dimethoxyethane (DME) and tetrahydrofuran (THF) across a concentration range of several orders of magnitude. Surprisingly, we find that effective salt dissociation is lower at relatively dilute concentrations (e.g. 0.01 M) than at higher concentrations (e.g. 0.2 M). Combined experimental and computational dielectric and X-ray spectroscopic analyses of the changes occurring in the Ca2+ solvation environment across these concentration regimes reveals a progressive transition from well-defined solvent-separated ion pairs to de-correlated free ions. This transition in ion correlation results in improvements in both conductivity and calcium cycling stability with increased salt concentration. Comparison with previous findings involving more strongly associating salts highlights the generality of this phenomenon, leading to important insight into controlling ion interactions in ether-based multivalent battery electrolytes.
Dereka, Bogdan; Lewis, Nicholas H.C.; Zhang, Yong; Hahn, Nathan H.; Keim, Jonathan H.; Snyder, Scott A.; Maginn, Edward J.; Tokmakoff, Andrei
Understanding the mechanisms of charge transport in batteries is important for the rational design of new electrolyte formulations. Persistent questions about ion transport mechanisms in battery electrolytes are often framed in terms of vehicular diffusion by persistent ion-solvent complexes versus structural diffusion through the breaking and reformation of ion-solvent contacts, i.e., solvent exchange events. Ultrafast two-dimensional (2D) IR spectroscopy can probe exchange processes directly via the evolution of the cross-peaks on picosecond time scales. However, vibrational energy transfer in the absence of solvent exchange gives rise to the same spectral signatures, hiding the desired processes. We employ 2D IR on solvent resonances of a mixture of acetonitrile isotopologues to differentiate chemical exchange and energy-transfer dynamics in a comprehensive series of Li+, Mg2+, Zn2+, Ca2+, and Ba2+ bis(trifluoromethylsulfonyl)imide electrolytes from the dilute to the superconcentrated regime. No exchange phenomena occur within at least 100 ps, regardless of the ion identity, salt concentration, and presence of water. All of the observed spectral dynamics originate from the intermolecular energy transfer. These results place the lower experimental boundary on the ion-solvent residence times to several hundred picoseconds, much slower than previously suggested. With the help of MD simulations and conductivity measurements on the Li+ and Zn2+ systems, we discuss these results as a continuum of vehicular and structural modalities that vary with concentration and emphasize the importance of collective electrolyte motions to ion transport. These results hold broadly applicable to many battery-relevant ions and solvents.
Driscoll, Darren M.; Dandu, Naveen K.; Hahn, Nathan H.; Seguin, Trevor J.; Persson, Kristin A.; Zavadil, Kevin R.; Curtiss, Larry A.; Balasubramanian, Mahalingam
Ca-ion electrochemical systems have been pushed to the forefront of recent multivalent energy storage advances due to their use of earth-abundant redox materials and their high theoretical specific densities in relation to monovalent or even other more widely explored multivalent-charge carriers. However, significant pitfalls in metal plating and stripping arise from electrolyte decomposition and can be related to the coordination environment around Ca2+ with both the negatively charged anion and the organic-aprotic solvent. In this study, we apply multiple spectroscopic techniques in conjunction with density functional theory to evaluate the coordination environment of Ca2+ across a class of ethereal solvents. Through the combination of X-ray absorption fine structure and time-dependent density functional theory, descriptive measures of the local geometry, coordination, and electronic structure of Ca-ethereal complexes provide distinct structural trends depending on the extent of the Ca2+-solvent interaction. Finally, we correlate these findings with electrochemical measurements of calcium tetrakis(hexafluoroisopropoxy)borate (CaBHFIP2) salts dissolved within this class of solvents to provide insight into the preferred structural configuration of Ca2+ electrolytic solutions for optimized electrochemical plating and stripping.
Hahn, Nathan H.; Driscoll, Darren M.; Yu, Zhou; Sterbinsky, George E.; Cheng, Lei; Balasubramanian, Mahalingam; Zavadil, Kevin R.
The emergence of magnesium and calcium batteries as potential beyond Li ion energy storage technologies has generated significant interest into the fundamental aspects of alkaline earth metal cation coordination in multivalent electrolytes and the impact of coordination on application-critical electrolyte properties such as solubility, transport, and electrochemical stability. Understanding these details in calcium electrolytes is of immediate importance due to recent, unprecedented demonstrations of reversible calcium metal electrodeposition in a limited number of ethereal solvent-based systems. In this work, we provide insight connecting Ca2+ coordination tendencies to important calcium battery electrolyte properties. Our results demonstrate a clear solvent:Ca2+ coordination strength trend across a series of cyclic ether and linear glyme solvents that controls the extent of ion association in solutions of "weakly"coordinating salts. We apply understanding gained from these results to rationalize relative anion:Ca2+ coordination tendencies and attendant Ca2+ coordination structures using two oxidatively stable anions of particular interest for current battery electrolytes. Armed with this understanding of solvent and anion interactions with Ca2+, we demonstrate and interpret differences in electrochemical calcium deposition behavior across several electrolyte exemplars with varying solvent and anion coordination strengths. Our findings demonstrate that solvents exhibiting especially strong coordination to Ca2+, such as triglyme, can inhibit reversible calcium deposition despite effective elimination of anion:Ca2+ coordination while solvents exhibiting more modest coordination strength, such as 1,2-dimethoxyethane, may enable deposition provided anion:Ca2+ coordination is substantially limited. These results reveal that the strength of coordination of both anion and solvent should be considered in the design of electrolytes for calcium batteries.
Proceedings of the National Academy of Sciences of the United States of America
Trahey, Lynn; Brushett, Fikile R.; Balsara, Nitash P.; Ceder, Gerbrand; Cheng, Lei; Chiang, Yet M.; Hahn, Nathan H.; Ingram, Brian J.; Minteer, Shelley D.; Moore, Jeffrey S.; Mueller, Karl T.; Nazar, Linda F.; Persson, Kristin A.; Siegel, Donald J.; Xu, Kang; Zavadil, Kevin R.; Srinivasan, Venkat; Crabtree, George W.
Energy storage is an integral part of modern society. A contemporary example is the lithium (Li)-ion battery, which enabled the launch of the personal electronics revolution in 1991 and the first commercial electric vehicles in 2010. Most recently, Li-ion batteries have expanded into the electricity grid to firm variable renewable generation, increasing the efficiency and effectiveness of transmission and distribution. Important applications continue to emerge including decarbonization of heavy-duty vehicles, rail, maritime shipping, and aviation and the growth of renewable electricity and storage on the grid. This perspective compares energy storage needs and priorities in 2010 with those now and those emerging over the next few decades. The diversity of demands for energy storage requires a diversity of purpose-built batteries designed to meet disparate applications. Advances in the frontier of battery research to achieve transformative performance spanning energy and power density, capacity, charge/discharge times, cost, lifetime, and safety are highlighted, along with strategic research refinements made by the Joint Center for Energy Storage Research (JCESR) and the broader community to accommodate the changing storage needs and priorities. Innovative experimental tools with higher spatial and temporal resolution, in situ and operando characterization, first-principles simulation, high throughput computation, machine learning, and artificial intelligence work collectively to reveal the origins of the electrochemical phenomena that enable new means of energy storage. This knowledge allows a constructionist approach to materials, chemistries, and architectures, where each atom or molecule plays a prescribed role in realizing batteries with unique performance profiles suitable for emergent demands.
Detailed speciation of electrolytes as a function of chemical system and concentration provides the foundation for understanding bulk transport as well as possible decomposition mechanisms. In particular, multivalent electrolytes have shown a strong coupling between anodic stability and solvation structure. Furthermore, solvents that are found to exhibit reasonable stability against alkaline-earth metals generally exhibit low permittivity, which typically increases the complexity of the electrolyte species. To improve our understanding of ionic population and associated transport in these important classes of electrolytes, the speciation of Mg(TFSI)2 in monoglyme and diglyme systems is studied via a multiscale thermodynamic model using first-principles calculations for ion association and molecular dynamics simulations for dielectric properties. The results are then compared to Raman and dielectric relaxation spectroscopies, which independently confirm the modeling insights. We find that the significant presence of free ions in the low-permittivity glymes in the concentration range from 0.02 to 0.6 M is well-explained by the low-permittivity redissociation hypothesis. Here, salt speciation is largely dictated by long-range electrostatics, which includes permittivity increases due to polar contact ion pairs. The present results suggest that other low-permittivity multivalent electrolytes may also reach high conductivities as a result of redissociation.