Lithium-ion batteries have been optimized for a limited temperature range and experience rapid capacity fade at elevated temperature (> 50 °C). Cycling data and design of experiment (DOE) studies established that the commonly used polyolefin-based separator was an important factor contributing to poor battery performance at elevated temperatures upwards of 100 °C. Understanding this failure mechanism is key to unlocking solutions that broaden the operating temperature range, enabling new applications in space exploration, drilling equipment, and automotive technologies. Here, an in-depth analysis was undertaken of the high-temperature failure mechanisms for cells containing a trilayer polypropylene/polyethylene/polypropylene (PP/PE/PP) separator with NMC cathode, graphite anode, and conventional electrolyte. Electrochemical methods consisting of cycling and electrochemical impedance spectroscopy (EIS) measurements revealed capacity loss, reduced coulombic efficiencies, and large resistance increases during exposure at 100 °C for trilayer separators relative to polyimide separators. Spectroscopic measurements showed inhomogeneities within the heated, trilayer separator with localized deformations, indicating that increasing temperatures caused the porosity to decrease until complete continuity was observed. Tensile and puncture testing revealed that a 100 °C environment was the dominant factor decreasing tensile strength and increasing variability. Overall, this study identified multiple failure mechanisms associated with polyolefin separators in cells cycling at 100 °C.

Lithium-carbon monofluoride (Li/CFx) batteries are among the most energy-dense electrochemical power sources known, making them ideal for applications ranging from implantable medical devices to space exploration. Li/CFx batteries may be subject to gamma radiation during sterilization or use in space, though its effects on battery components and electrochemical interphases remain poorly understood. Here, we investigate the impact of radiation on interfacial and molecular structures of CFx and Li metal electrodes. Electrochemical impedance spectroscopy (EIS) revealed delayed, non-monotonic impedance behavior, characterized by a significant post-exposure increase, indicating electrode film restructuring, particularly at the anode. X-ray photoelectron spectroscopy (XPS) identified LiF as the dominant chemical component in the anode film, while XPS and Raman spectroscopy detected radiation-induced defects in the electronic structure of LiF. Solid-state 19F nuclear magnetic resonance (NMR) measurements established the remarkable stability of the CFx active material and composite electrode under radiation. However, 2D 19F{7Li} dipolar-mediated NMR of discharged CFx electrodes revealed a new fluorine environment in the LiF discharge product with reduced electronic shielding upon radiation exposure. Overall, these findings provide new insights into the impacts of radiation on Li/CFx cells, demonstrating that radiation forms defects in LiF within the Li metal film and the CFx discharge product.

Rechargeable aluminum–sulfur (Al–S) batteries are considered promising candidates for next-generation energy storage owing to the natural abundance, intrinsic safety, low cost, and high gravimetric capacities of aluminum and sulfur (2980 mAh g⁻¹ and 1675 mAh g⁻¹, respectively). However, their practical deployment has been hindered by the scarcity of electrolytes that can simultaneously support reversible aluminum electrodeposition and sulfur electroreduction, while also minimizing polarization losses. Alkali chloroaluminate molten salts (AlCl₃–NaCl–KCl) were recently used as electrolytes for Al–S batteries instead of conventional AlCl₃–EMImCl (1-ethyl-3-methylimidazolium chloride) ionic liquids, demonstrating significantly reduced polarization at elevated temperature (e.g., 110 °C), though its origins are poorly understood. In this work, we investigate how electrolyte speciation, ion mobility, and soluble intermediates in these two electrolyte systems influence polarization losses in rechargeable Al–S batteries. Variable-temperature liquid-state 27 Al, 23 Na, and 1 H single-pulse and relaxation nuclear magnetic resonance (NMR) measurements were conducted under quantitative conditions to probe electrolyte speciation and ion dynamics. 23 Na and 1 H pulsed-field-gradient (PFG) NMR experiments were also performed to quantify cation diffusion coefficients and their activation energies for diffusion. Additionally, integrated 27 Al and 23 Na NMR signal intensities of the molten salt electrolyte enabled accurate measurements of its freezing and melting temperature range, which coupled with changes in ion mobility, yielded insights into the optimum operating temperature. Differential scanning calorimetry (DSC) was also performed to measure these thermodynamic phase transitions and analyzed with respect to the NMR data. Solid-state 27 Al and 23 Na single-pulse NMR measurements of both pristine and cycled molten salt electrolytes provided mechanistic insights into the electrochemical conversion reaction, including the formation of electrolyte soluble polysulfide-like intermediates. In addition, we formulated quaternary electrolyte mixtures comprising AlCl₃, NaCl, KCl, and a fourth component (e.g., [EMIm]Cl, urea, or LiCl) with the objective of lowering the operating temperature and understanding the effects on cell polarization. Al-S batteries were prepared using the different electrolytes to compare their overpotentials, as well as specific capacities, cycle life, and cell impedance. Overall, the roles of electrolyte speciation, ion mobility and soluble intermediates in governing the overpotential in Al-S batteries were elucidated, providing new insights for the rational design of low cost, energy dense electrolytes for Al-S batteries.

Lanthanide-binding tag (LBT) peptides selectively complex lanthanide cations (Ln3+) in their binding pockets and are promising for lanthanide separation. However, designing LBTs that selectively target specific Ln3+ cations remains a challenge due to limited molecular-level understanding and control of interactions within the lanthanide-binding pocket. In this study, we reveal that the N5 asparagine residue acts as a gatekeeper in the binding pocket, resulting in a 100-fold selectivity for smaller Lu3+ over larger La3+ cations. Nuclear magnetic resonance spectroscopy and molecular dynamics simulations show that the N5 residue weakly binds to the larger La3+ cation, permitting H2O molecules inside the pocket. For the smaller Lu3+ cations, the N5 residue forms an inter-arm hydrogen bond with the E14 glutamic acid residue, locking the Lu3+ cation in the pocket and preventing H2O infiltration. Mutating the N5 asparagine to a D5 aspartic acid prevents such a hydrogen bond, eliminating the gatekeeping mechanism and precipitously reducing selectivity. The resulting binding affinity to Ln3+ cations is non-monotonic but generally increases with cation size. These results suggest a molecular design paradigm: the reduced affinity for larger lanthanides is due to open pocket conformations, while the selectivity of smaller Ln3+ cations over larger ones is due to the gatekeeping hydrogen bond.

Low-temperature and fast charging of lithium (Li)-ion batteries remains a challenge due to the undesirable Li plating that occurs on graphite anodes under these conditions. Here, we yield new insights into the mechanistic processes underpinning electrochemical Li-ion intercalation and Li metal plating reactions on graphite anodes at low temperatures and fast rates. Variable-temperature (-40 to 30 °C) galvanostatic measurements were conducted on three-electrode cells comprised of Li metal counter, graphite working, and Li metal reference electrodes, as well as two-electrode cells without the reference electrode. The results establish that the local minima in the voltage profiles, often associated with the nucleation overpotential for Li metal plating on graphite, must be disentangled from contributions from Li metal stripping at the counter electrode. Differential capacity analysis enables intercalation and plating processes to be further distinguished, revealing temperature regimes where the reactions either occur sequentially (e.g., near ambient temperature) or simultaneously (e.g., below -20 °C). The temperature dependence of overpotentials and characteristic reaction time scales were analyzed, suggesting that a two-step, pre-equilibration mechanism occurs prior to either intercalation or plating, wherein Li + cations reversibly adsorb on the graphite surface followed by irreversible charge transfer to form either Li metal or Li x C 6 . The results yield mechanistic understanding into how Li + cations electrochemically intercalate and plate onto graphite electrodes, as well as their competition at low temperatures and fast rates. We extend these results from Li plating on graphite to zinc (Zn) metal to demonstrate the universal behavior of metal electroplating on an ion-intercalation electrode. Two- and three-electrode cells with Zn metal as counter/reference electrode and chevrel-phase Mo 6 Se 8 as working electrode were used to demonstrate for the first time that Zn can electrochemically plate on Mo 6 Se 8 in aqueous 1 M ZnSO 4 electrolyte. Under a constant-current, Zn 2+ cations can either electrochemically intercalate into the chevrel-phase or reduce to form metallic Zn, as evidenced by the measurement of the nucleation overpotential. The objective is to develop a better understanding of the competition between electrochemical intercalation and plating of these metal cations into a model battery electrode (e.g., chevrel-phase Mo 6 Se 8 ). The results will be analyzed with respect to lithium cation intercalation versus lithium metal plating on graphite anodes to yield a general understanding of how ion charge density and size affect these competing electrochemical processes at variable-temperature and current density.

Ionic liquids are tunable solvents composed entirely of ions that have properties desirable as electrolytes for lithium batteries such as nonflammability and a large electrochemical stability window. Solvate ionic liquids are a subclass of ionic liquids that consist of a glyme-based solvent and lithium salt in an equimolar ratio, where Li+ cation-glyme solvation interactions result in ionic liquid-like properties. LiG4TFSI is a well-studied solvate ionic liquid consisting of equimolar amounts of lithium bis-(trifluoromethylsulfonyl)-imide (LiTFSI) and tetraglyme (G4). In this work, pyrrolidinium ionic liquids with ether-functionalized side chains were synthesized, containing either one ether (EO1) moiety or three ether (EO3) moieties and mixed with LiG4TFSI to form a new class of electrolyte mixtures. Their physical and transport properties, as well as ion solvation structures, were characterized by electrochemical, thermal, rheological, and spectroscopic measurements. The conductivity of the electrolyte mixture composed of EO1:LiTFSI:G4 in a 1:1:1 molar ratio is 2.54 mS/cm at 30 °C, compared to 1.53 mS/cm for LiG4TFSI, an increase of 67%. A significant decrease in the conductivity to 0.279 mS/cm is observed for the EO3:LiTFSI:G4 mixture in a 1:1:0.4 molar ratio. Pulsed-field gradient nuclear magnetic resonance (PFG-NMR) measurements revealed that the EO1 cation diffuses significantly faster than the EO3 cation in their respective mixtures. Liquid-state 13C NMR experiments indicate that Li+ cations preferentially coordinate with tetraglyme. Li+ cations do not coordinate with the EO1 cation and coordinate with the EO3 ether side chains only at lower concentrations of tetraglyme. We hypothesize that the oligoether EO3 cation competes with G4 and TFSI- for lithium cation solvation in G4-deficient compositions, leading to a largely adverse effect on the mass transport properties of the electrolyte.

One possible failure mechanism for ion intercalation-based energy storage systems (e.g., lithium (Li)-ion batteries) is uncontrolled metal electrodeposition (plating), where metal ions from the electrolyte reduce on the electrode surface to deposit metal instead of intercalating. We studied zinc (Zn) and copper (Cu)-ion intercalation and metal plating in two-electrode coin cells comprised of either Zn or Cu metal counter electrodes and Mo 6 Se 8 working electrodes in aqueous 1 M ZnSO 4 or 1 M CuSO 4 , respectively. We demonstrate for the first time that Zn and Cu metal can electrochemically plate on Mo 6 Se 8 cathodes. The presence of both metallic Zn and Cu metal deposits was determined using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy, and X-ray diffraction on galvanostatically discharged Mo 6 Se 8 cathodes. Zn- and Cu-ion intercalation and metal plating on Mo 6 Se 8 was investigated further through variable-temperature (+50 C down -10 C), constant-current (10 mA/g) discharge of Zn/Cu-Mo 6 Se 8 coin cells. Specifically, the results show that the discharge of Zn-Mo 6 Se 8 cells reveal a temperature-dependent interplay between Zn 2+ intercalation, hydrogen evolution reaction (HER), and Zn plating on the Mo 6 Se 8 electrode surface. However, the discharge of Cu-Mo 6 Se 8 suggests that an interplay between Cu-ion intercalation and Cu metal plating occurs in the absence of HER even at +20 C. Arrhenius analysis on an empirically-defined rate constant for Zn metal plating shows that the Zn plating process possesses a negative apparent activation energy, indicative of a multi-step plating process that includes Zn 2+ adsorption on the Mo 6 Se 8 electrode surface. This work establishes a connection between Zn/Cu metal plating as an analog to Li plating on graphite anodes in Li-ion batteries. Building a mechanistic understanding of Zn, Cu, and Li plating as failure modes could improve each metal’s respective battery chemistries in more demanding cycling conditions including fast- and low-temperature charge/discharge.

This chapter aims to distinguish batteries and supercapacitors, as well as hybrid electrodes and devices, by their root cause of storing charge. Faradaic and capacitive charge storage mechanisms are analyzed, including how the interplay between mass transfer and electrochemical kinetics affects them. The special case of pseudocapacitive charge storage is elucidated and discussed in terms of different schools of thought from the current literature. Common electrochemical techniques are then analyzed to identify—both qualitatively and quantitively—the different charge storage mechanisms possible in hybrid electrochemical materials and devices. Practical guidelines are given to enable researchers to disentangle charge storage mechanisms and define hybrid electrochemical energy storage systems. Lastly, different hybrid cell chemistries and commercial devices are discussed, including open challenges.

Low-temperature and fast-charging lithium (Li)-ion batteries remain challenging due to the undesirable Li plating on graphite anodes under these conditions. Here, we present a kinetic mechanism that underpins electrochemical Li+ cation intercalation and Li metal plating reactions on graphite electrodes at low temperatures and fast rates. Variable-temperature (30 °C to -40 °C) and variable-rate (0.1 to 10 mA/cm2) constant-current measurements were conducted on three-electrode cells comprised of Li metal counter, graphite working, and Li metal reference electrodes, as well as two-electrode cells. The local minima in the potential profiles, often associated with the nucleation overpotential for Li metal plating on graphite, must be disentangled from contributions from Li metal stripping at the counter electrode. Differential capacity analyses of three-electrode measurements of graphite potential show that the extent of electrochemical Li+ cation intercalation drops precipitously as temperature decreases below -20 °C. The temperature dependence of empirically defined rate constants for Li+ cation intercalation and Li plating determined from constant-current measurements revealed non-Arrhenius behavior for Li+ cation intercalation that suggests a two-step pre-equilibration mechanism, while typical Arrhenius behavior for Li plating suggests a unimolecular single-step process. A kinetic model based on Langmuir adsorption shows that the interfacial concentration of Li+ cations adsorbed on graphite active sites is critical in dictating the kinetics of the charging process. We show that rate limitations, either adsorption-limited or surface reaction-limited, manifest at different temperatures and rates during the charging process. The results yield new mechanistic understanding of how Li+ cations electrochemically compete for intercalation into and plating on graphite electrodes, as a function of temperature and charge rate.

Ionic liquids are an appealing material for lithium battery electrolytes due to their low flammability, large electrochemical window and tunability through a diverse selection of cation-anion pairing and modifiable functionalities. Solvate ionic liquids are equimolar mixtures of lithium salts and glyme molecules, commonly tetragylme (G4), which lead to formation of a Li[G4] + cation complex and similar properties to ionic liquids. The thermophysical and electrochemical properties of solvate ionic liquids can be tuned through addition of ionic liquids. Both the cation and anion of the ionic liquid can have a significant influence on the lithium solvation structures and electrolyte properties. This work investigated G4 mixtures with various lithium perfluoroalkyl(sulfonyl)imide salts and ether functionalized pyrrolidinium (EO1) perfluoroalkyl(sulfonyl)imide ionic liquids to better understand the role of the anion on the properties on the Li[G4] + cation complex. With the addition of ionic liquids to the G4 lithium salt mixtures we observed suppression of the onset of G4 ether oxidation and an increase in the oxidation limit of each composition . Raman spectroscopy was used to probe changes in the anion solvation environment. Both the Raman bands for the S-N and S-O vibrations shifted to higher wavenumber, indicating the addition of ionic liquids resulted in more Li + -anion interactions. Single-pulse 13 C NMR was used to probe the interactions between the Li + -G4 and Li + -EO1 with various perfluoroalkyl(sulfonyl)imide by tracking a significant shift to lower frequencies upon Li + coordination. Pulsed field gradient 1 H, 19 F, and 7 Li NMR to determined diffusion coefficients of individual nuclei helped inform on species mobilities in the electrolyte and in tandem with other techniques can assist in elucidating interactions between various components of the electrolyte.

Water-in-salt electrolytes (WiSE) have been successfully used to expand the operating potential of aqueous electrolytes for zinc metal batteries, increasing their energy density. Acetate-based concentrated electrolytes are low cost and safe, yet their technological development has been hindered by insufficient understanding and control of zinc electrochemistry, morphology, and reversibility, particularly at realistic current densities and high zinc utilization. In this work, zinc stability and discharge behavior were studied in concentrated and dilute potassium acetate (KOAc)-based electrolytes using electrochemical, computational, and spectroscopic tools. The impact of high salt concentrations on the working electrochemical windows of the electrolytes were measured, where the largest electrochemical window of 3.15 V (vs. Zn/Zn2+) was observed for 27 m KOAc. However, more dilute acetate electrolytes provided greater zinc utilization with minimal reduction of electrolyte stability. Zinc foil was discharged in acetate-based electrolytes with different compositions, revealing that mild acidification of 5 m KOAc prevents inactive zinc oxide formation and enables up to 80% zinc utilization at current densities of 5 mA cm-2. This study lays the groundwork for understanding and developing acetate-based electrolytes in the pursuit of high energy density, aqueous zinc batteries.

Phase change materials (PCMs) are latent heat storage materials that can store or release thermal energy while undergoing thermodynamic phase transitions. Organic PCMs can be emulsified in water in the presence of surfactants to enhance thermal conductivity and enable applications as heat transfer fluids. However, PCM nanoemulsions often become unstable during thermal cycling. To better understand the molecular origins of phase stability in PCM nanoemulsions, we designed a model PCM nanoemulsion system and studied how the molecular-level environments and dynamics of the surfactants and oil phase changed upon thermal cycling using liquid-state nuclear magnetic resonance (NMR) spectroscopy. The model system used octadecane as the oil phase, stearic acid as the surfactant, and aqueous NaOH as the continuous phase. The liquid fraction of octadecane within the nanoemulsions was quantified noninvasively during thermal cycling by liquid-state H-1 single-pulse NMR measurements, revealing the extent of octadecane supercooling as a function of temperature. The mean droplet size of the PCM nanoemulsions, measured by dynamic light scattering (DLS), was correlated with the liquid content of octadecane to explain phase instability in the solid-liquid coexistence region. Quantitative C-13 single-pulse NMR experiments established that the carbonyl surfactant head groups were present in multiple distinct environments during thermal cycling. After repeated thermal cycling, the C-13 signal intensity of the carbonyl surfactant head groups decreased, indicating that the surfactant head groups lost molecular mobility. The results explain, in part, the origin of phase instability of PCM nanoemulsions upon thermal cycling.

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 Se-77 nuclear magnetic resonance (NMR) measurements further establish that the local Se helical structures are maintained upon the loss of crystallinity.

Rechargeable aluminum-sulfur (Al-S) batteries hold great potential due to the abundance, safety, low cost, and high gravimetric capacity of Al metal and elemental sulfur (2980 mAh g-1 and 1675 mAh g-1, respectively). Despite these advantages, technological development of Al-S batteries faces challenges due to the limited number of electrolytes that not only enable the reversible electrostripping of Al metal and concomitant electroreduction of sulfur, but also yield small polarization losses. Currently, Lewis acidic mixtures of AlCl3 in 1-ethyl-3-methylimidazolium chloride ([EMIm]Cl) are the state-of-the-art electrolytes used in Al metal batteries, including Al-S batteries, though polarization losses in Al-S cells are large at room temperature. Recently, alkali chloroaluminate molten salts (NaCl-KCl-AlCl3) were tested in Al-S batteries at 110 °C, resulting in a large decrease in polarization compared to AlCl3-[EMIm]Cl electrolytes, even at elevated temperatures [1]. However, the link between chloroaluminate anion speciation and dynamics, temperature, and overpotential for sulfur reduction and oxidation is poorly understood. Here, we prepare quaternary electrolyte mixtures composed of AlCl3, NaCl, KCl, and either [EMIm]Cl, urea, or LiCl to reduce the liquid temperature window and study how ion speciation and dynamics affect polarization losses in rechargeable Al-S batteries as a function of temperature. All mixtures were compared to AlCl3-[EMIm]Cl (1.5:1 molar ratio) and NaCl-KCl-AlCl3 (26:13:61) molar ratio mixtures as baseline electrolytes. Differential scanning calorimetry (DSC) was performed to measure thermodynamic phase transitions and the liquid temperature windows of these electrolytes. Liquid-state 27Al single-pulse and relaxation nuclear magnetic resonance (NMR) measurements were performed to characterize chloroaluminate anion speciation environments, and dynamics up to 110 °C. In addition, liquid-state 1H single-pulse, relaxation, and pulsed-field-gradient (PFG) NMR experiments were performed to measure the local environments, dynamics, and diffusion coefficients of organic species (e.g., [EMIm]Cl, urea). The electrolytes were tested electrochemically in rechargeable Al-S batteries by performing galvanostatic cycling and cyclic voltammetry up to 110 °C, revealing the effects on cell polarization as well as specific capacity and cyclability. Furthermore, solid-state 27Al NMR measurements on discharged sulfur electrodes containing residual electrolyte [2] revealed how electrolyte composition affects the average environments of rapidly exchanging electrolyte-soluble sulfur species (SxAlCl4)y−). Overall, the results yield design strategies for electrolyte design in rechargeable Al-S batteries to reduce the operating temperature, reduce the polarization loss, and enhance energy efficiency. References “Fast-Charging Aluminium–Chalcogen Batteries Resistant to Dendritic Shorting.,” Quanquan Pang, Jiashen Meng, Saransh Gupta, Xufeng Hong, Chun Yeun Kwok, Ji Zhao, Yingxia Jin, Like Xu, Ozlem Karahan, Ziqi Wang, Spencer Toll, Liqiang Mai, Linda Nazar, Mahalingam Balasubramanian, Badri Narayanan, Donald Sadoway, Nature., 2022, 608, 704-711. “Soluble electrolyte-coordinated sulfide species revealed in Al-S Batteries by nuclear magnetic resonance spectroscopy,” Rahul Jay, Ankur L. Jadhav, Leo W. Gordon, Robert J. Messinger, Chem. Mater., 2022, 34, 4486-4495.

We are developing an all-aqueous, bio-inspired approach using peptides as surfactants that selectively bind rare earth element (REE) cations and adsorb at the air/water interface to enable green capture and separation. REEs are essential components in modern electronic devices and clean energy technologies which must be separated from feedstocks of aqueous mixtures. Their selective capture is particularly challenging owing to their similarity in size and charge. Lanthanide binding tags (LBTs) are amphiphilic peptide sequences based on the binding loop in the evolutionarily conserved EF-hand metal binding motif. We study LBTs optimized for coordination to Tb$^{3+}$ using a suite of experimental methods including luminescence spectroscopy, surface tensiometry, x-ray reflectivity and x-ray fluorescence near total reflection, and find that these LBTs capture Tb$^{3+}$ in bulk and adsorb at the interface. Molecular dynamics show that the binding pocket remains intact upon adsorption. We find that, if the net negative charge on the peptide results in a negatively charged complex, excess cations are recruited to the interface by non-selective Coulombic interactions that compromise selective REE capture. If, however, the net negative charge on the binding loop is -3, resulting in a neutral complex, a 1:1 surface ratio of cation to peptide is achieved. We demonstrate selective interfacial extraction from an equimolar mixture of Tb$^{3+}$ and La$^{3+}$, validating an LBT-mediated interfacial separation of REEs.

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 degrees C to -40 degrees 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 degrees C while exhibiting the highest current density and the lowest overpotential among all of the electrolyte mixtures at 25 degrees C, including the AlCl3-[EMIm]Cl binary mixture. The ILA electrolyte was further tested in a rechargeable Al-graphite battery system down to -40 degrees 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.

Researchers developing the next generation of energy storage systems are challenged to understand and analyze the different charge storage mechanisms, subsequently, use this understanding to design and control materials and devices that bridge the gap between high specific energy and power at a target cycle life. Correctly identifying and quantifying the prominent charge storage mechanism, which is either faradaic diffusion-limited, faradaic non-diffusion-limited (or pseudocapacitive), or capacitive, is of the utmost importance for understanding how the system functions and tuning material properties for specific applications. The different charge storage mechanisms are defined by a characteristic current-time scaling, that has been expressed for faradaic diffusion-limited (Cottrell relationship) and true capacitive charge storage and has been used to disentangle and determine the prominent mechanisms. However, the characteristic current-time scaling for faradaic non-diffusion-limited (or pseudocapacitive) charge storage remains unelucidated despite to date many energy storage devices, particularly those having ionic liquids, deep eutectic solvents or highly concentrated electrolytes, exhibit electrochemical interfaces with faradaic non-diffusion-limited charge storage. This talk presents the work on a theoretical intuitive framework for pseudocapacitive charge storage taking into consideration the faradaic nature of this mechanism and deviation from the classic electrochemical interface structure, which is the root cause for pseudocapacitance1. The theory will be corroborated with experimental current-time scaling using conventional variable-rate cyclic voltammetry and variable-rate AC cyclic voltammetry at higher harmonics. The results emphasize the distinct current-time scaling of pseudocapacitive charge storage mechanism and shed light on interfacial kinetics and mass transport processes in multivalent batteries with ionic liquid or deep eutectic solvents. The insights inform electrode and electrolyte material design, e.g., for fast-charging2,3and low-temperature batteries4,5. 1 Schoetz, T., Gordon, L. W., Ivanov, S., Bund, A., Mandler, D. & Messinger, R. J. Disentangling faradaic, pseudocapacitive, and capacitive charge storage: A tutorial for the characterization of batteries, supercapacitors, and hybrid systems. Electrochimica Acta 412, 140072 (2022). 2 Xu, J. H., Schoetz, T., McManus, J. R., Subramanian, V. R., Fields, P. W. & Messinger, R. J. Tunable Pseudocapacitive Intercalation of Chloroaluminate Anions into Graphite Electrodes for Rechargeable Aluminum Batteries. Journal of The Electrochemical Society 168, 060514 (2021). 3 Leung, O. M., Gordon, L. W., Messinger, R. J., Prodromakis, T., Wharton, J. A., Ponce de León, C. & Schoetz, T. Solid Polymer Electrolytes with Enhanced Electrochemical Stability for High-Capacity Aluminum Batteries. Advanced Energy Materials 14, 2303285 (2024). 4 Hawkins, B. E., Schoetz, T., Gordon, L. W., Kt, S., Wang, J. & Messinger, R. J. Reversible Zinc Electrodeposition at −60 °C Using a Deep Eutectic Electrolyte for Low-Temperature Zinc Metal Batteries. The Journal of Physical Chemistry Letters 14, 2378-2386 (2023). 5 Schoetz, T., Xu, J. H. & Messinger, R. J. Ionic Liquid Electrolytes with Mixed Organic Cations for Low-Temperature Rechargeable Aluminum–Graphite Batteries. ACS Applied Energy Materials 6, 2845-2854 (2023).

The lithium-carbon monofluoride (Li-CFx) couple has the highest specific energy of any practical battery chemistry. However, the large polarization associated with the CFx electrode (>1.5 V loss) limits it from achieving its full discharge energy, motivating the search for new CFx reaction mechanisms with reduced overpotential. Here, using a liquid fluoride (F)-ion conducting electrolyte at room temperature, we demonstrate for the first time the electrochemical defluorination of CFx cathodes, where metal fluorides form at a metal anode instead of the CFx cathode. F-ion primary cells were developed by pairing CFx cathodes with either lead (Pb) or tin (Sn) metal anodes, which achieved specific capacities of over 700 mAh g(-1) and over 400 mAh g(-1), respectively. Solid-state F-19 and Sn-119{F-19} nuclear magnetic resonance (NMR), X-ray diffraction (XRD), Raman, inductively coupled plasma (ICP), and X-ray fluorescence (XRF) measurements establish that upon discharge, the CFx cathode defluorinates while Pb forms PbF2 and Sn forms both SnF4 and SnF2. Technological development of F-ion metal-CFx cells based on this concept represents a promising avenue for realizing primary batteries with high specific energy.

Li-ion batteries are commonly used as electrochemical energy storage systems due to their high energy density. However, few Li-ion batteries can reliably function at elevated temperatures, which is necessary for space and defense applications. In this study, Li-ion electrolytes were prepared and investigated for use at 100 degrees C. The previously developed baseline electrolyte, 1.0 M lithium hexafluorophosphate (LiPF6) in 1:1 ethylene carbonate (EC):ethyl-methyl carbonate (v/v) with 2 wt% vinylene carbonate (VC), was altered to observe the effects of the lithium salts lithium difluoro(oxalato)borate (LiDFOB) and lithium difluorophosphate (LiDFP), and the fluorinated co-solvent 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE). The resulting formulations showed significantly improved capacity retention at 100 degrees C in multiple cell configurations. X-ray photoelectron spectroscopy characterization of the electrodes following cycling at high temperatures with the improved electrolyte revealed the cathode-electrolyte-interface to be boron-rich, while the graphite anodes were found to have little boron but were more fluorine rich compared to the baseline anodes. Raman spectroscopy determined notable changes in solvation structure upon addition of the TTE diluent. Overall, the use of various Li salts as well as the TTE co-solvent improved specific capacity retention at 100 degrees C in Li-ion cells.

Sulfur‐tuned advanced carbons (STACs) with high mass loadings of sulfur are synthesized using an environmentally benign and scalable steam‐assisted sulfur insertion (SASI) method. While steam provides the pressure necessary to promote deep and rapid sulfur insertion into a carbon porous structure, a strong affinity between melted sulfur and carbon excludes water from pore penetration. The resulting STACs exhibit sulfur mass loadings up to 85% and the electrical conductivity of the carbon framework is largely preserved. The sulfur penetration can be tuned to fill specific pore sizes, enabling pore‐size‐dependent allocation of sulfur and controllable porosity, while sulfur lines the carbon pore surfaces. A significant amount of sulfur is in the monoclinic γ phase. To demonstrate their energy and environmental applications, the STACs are used as cathode materials in rechargeable aluminum‐sulfur batteries and as adsorption materials for spilled oil removal.