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Few materials are known to electrochemically intercalate trivalent aluminum cations, a charge storage mechanism central to rechargeable aluminum-ion battery electrodes. Here, using the chevrel phas...

Rechargeable aluminum–graphite batteries are an emerging energy storage technology with great promise: they exhibit high rate performance, cyclability, and a discharge potential of approximately 2 V, while both electrodes are globally abundant, low cost, and inherently safe. The batteries use chloroaluminate-containing electrolytes and store charge in the graphite electrodes when molecular AlCl4− anions electrochemically intercalate within them. However, much remains to be understood regarding the ion intercalation mechanism, in part due to challenges associated with characterizing the chloroaluminate anions themselves. Here, we use solid-state 27Al nuclear magnetic resonance (NMR) spectroscopy to probe the molecular-level electronic and magnetic environments of intercalated chloroaluminate anions at different states-of-charge. The results reveal broad 27Al NMR signals associated with intercalated AlCl4− anions, reflecting high extents of local disorder. The intercalated anions experience a diversity of local environments, many of which are far from the ideal crystalline-like structures often depicted in graphite staging models. Density functional theory (DFT) calculations of the total 27Al isotropic shifts enable the contributions of chemical shift, ring-current effects, and electric quadrupolar interactions to be disentangled quantitatively. In combination, the solid-state NMR and DFT results reveal the molecular geometries and environments of intercalated AlCl4− anions and capture the significant disorder present in intercalated graphite battery electrodes.

Rechargeable aluminum metal batteries are promising next-generation electrochemical energy storage devices due to the high energy density, inherent safety, low cost, earth abundance, and recyclability of aluminum. Aluminum batteries using graphite cathodes and chloroaluminate-containing electrolytes have recently been the focus of intense research in the energy storage and electroplating community, particularly due to their fast-charging capabilities. Engineered graphite cathodes have been shown to exhibit supercapacitor-like rate performance since chloroaluminate anions intercalate into graphite with significant pseudocapacitive characteristics1,2. Batteries with high considerable pseudocapacitive charge storage3 are especially attractive for energy storage systems that need to function at low temperatures and ideally exhibit high specific power. For example, robotic spacecraft that embark on planetary science missions often have typical mission temperature targets of -40 to -60 °C. However, state-of-the-art lithium-ion batteries with predominant faradaic diffusion-limited charge storage suffer from lithium plating, dendritic growth, and severe capacity loss at low temperatures. While organic electrolyte mixtures have been developed that enable moderate low-temperature performance, new battery concepts, including those using ionic liquid electrolytes, are needed to enable new paradigms for low-temperature space missions and electromobility. Herein we present, for the first time, results towards rechargeable aluminum-graphite batteries designed specifically for low-temperature applications that demonstrate high capacity retention and favorable electrochemical kinetics at temperatures down to -40 °C. We first disentangle quantitatively the faradaic diffusion-limited, pseudocapacitive, and capacitive contributions to the overall charge storage for different graphite materials. Moreover, we shed light on the relationships between graphite structure, ion mass transport, and the overall rates of electrochemical aluminum deposition/dissolution and chloroaluminate anion intercalation. We then develop ionic liquid electrolytes with mixed anion-cation compositions to impart disorder and disrupt crystallization at low temperatures. The resulting aluminum-graphite batteries were characterized by variable-temperature and rate cyclic voltammetry (CV), galvanostatic cycling, electrochemical impedance spectroscopy (EIS), and solid-state nuclear magnetic resonance (NMR) spectroscopy to understand their reaction mechanisms and factors limiting their rate performance. The ionic liquid electrolyte mixtures were also characterized by a combination of differential scanning calorimetry (DSC) and electrochemical methods to understand their freezing points and electrochemical stability. The results provide fundamental insights into the design of aluminum-graphite batteries for low-temperature space and electromobility applications. References [1] J.H. Xu, T. Schoetz, J.R. McManus, V.R. Subramanian, P.W. Fields, R.J. Messinger, Tunable pseudocapacitive intercalation of chloroaluminate anions into graphite electrodes for rechargeable aluminum batteries, J. Electrochem. Soc. 168 (6) (2021), 060514. [2] J.H. Xu, D.E. Turney, A.L. Jadhav, R.J. Messinger, Effects of graphite structure and ion transport on the electrochemical properties of rechargeable aluminum-graphite batteries, ACS Appl. Energy Mater. 2 (11) (2019) 7799–7810. [3] T.Schoetz, L.W.Gordon, S.Ivanov, A.Bund, D.Mandler, R.J.Messinger, Disentangling faradaic, pseudocapacitive, and capacitive charge storage: A tutorial for the characterization of batteries, supercapacitors, and hybrid systems, Electrochimica Acta 412 (2022), 140072.

Rechargeable aluminum–graphite batteries using chloroaluminate-containing ionic liquid electrolytes store charge when molecular chloroaluminate anions intercalate into graphite. However, the relati...

To demonstrate the importance of electrode/electrolyte stability in rechargeable aluminum (Al) batteries, we investigate the chemical compatibility between vanadium pentoxide (V2O5), a proposed pos...

International audience; Magic‐angle‐spinning (MAS) enhances sensitivity and resolution in solid‐state nuclear magnetic resonance (NMR) measurements. MAS is obtained by aerodynamic levitation and drive of a rotor, which results in large centrifugal forces that may affect the physical state of soft materials, such as polymers, and subsequent solid‐state NMR measurements. Here, we investigate the effects of MAS on the solid‐state NMR measurements of a polymer electrolyte for lithium‐ion battery applications, poly(ethylene oxide) (PEO) doped with the lithium salt LiTFSI. We show that MAS induces local chain ordering, which manifests itself as characteristic lineshapes with doublet‐like splittings in subsequent solid‐state 1H, 7Li, and 19F static NMR spectra characterizing the PEO chains and solvated ions. MAS results in distributions of stresses and hence local chain orientations within the rotor, yielding distributions in the local magnetic susceptibility tensor that give rise to the observed NMR anisotropy and lineshapes. The effects of MAS were investigated on solid‐state 7Li and 19F pulsed‐field‐gradient (PFG) diffusion and 7Li longitudinal relaxation NMR measurements. Activation energies for ion diffusion were affected modestly by MAS. 7Li longitudinal relaxation rates, which are sensitive to lithium‐ion dynamics in the nanosecond regime, were essentially unchanged by MAS. We recommend that NMR researchers studying soft polymeric materials use only the spin rates necessary to achieve the desired enhancements in sensitivity and resolution, as well as acquire static NMR spectra after MAS experiments to reveal any signs of stress‐induced local ordering.

Mesostructured MFI zeolite nanosheets are established to crystallize non-topotactically through a nanolayered silicate intermediate during hydrothermal synthesis. Solid-state 2D NMR analyses, with sensitivity enhanced by dynamic nuclear polarization (DNP), provide direct evidence of shared covalent 29 Si-O-29 Si bonds between intermediate nanolayered silicate moieties and the crystallizing MFI zeolite nanosheet framework.

New and novel systems for the reversible electrochemical storage of energy are required to sustain the increasing global energy demands. Indeed the high cost, the intrinsic energy limits and the strategic aspects of lithium and lithium technology-related metals (e.g., Co) are prompting the research towards the development of battery systems based on novel chemistries. In this regard, light multivalent metals are considered the holy grail. Magnesium and aluminum, for example, can be considered one of the best choices due to their low reduction potential (-1.66 and -2.37 V vs. SHE for Al3+/Al and Mg2+/Mg, respectively), high volumetric and gravimetric theoretical capacities (2980 Ah∙kg-1 and 8046 Ah∙dm-3 for Al, and 2205 Ah∙kg-1 and 3832 Ah∙dm-3 for Mg), high abundance in the Earth’s crust and low cost (1’925 and 2’700 $∙ton-1 for Al and Mg, respectively) [1, 2]. The main roadblock to the development of these technologies is the discovery of efficient, stable and high-performing electrolytes. Ionic liquids have been demonstrated to be a good choice to be used as solvents in multivalent metal electrolytes, thanks to the low volatility and high solvating powers [3, 4]. Despite large efforts, there are no examples in the literature that demonstrate the advantage of the synergic effect of the coupling of Mg2+/Al3+ in the deposition and stripping processes. Moreover, the fundamental understanding of ion coordination in this novel class of electrolytes would be a cornerstone information necessary to reveal the conduction mechanism. In this work we study the properties of a family of hybrid Al/Mg electrolytes with formula [Pyr14Cl/(AlCl3)1.5]/(δ-MgCl2)x (x = 0, 0.056, 0.091 and 0.146), to foster the synergic effect of these two metals for application in multivalent metal secondary batteries. In this study we combine thermal analyses, vibrational, electric and nuclear magnetic spectroscopies to achieve a clear picture of the ionic coordination occurring in the proposed electrolytes and how ions are able to rearrange and form ionic coordination nanodomains. It is demonstrated that AlCl4 - and Al2Cl7 - species are formed, which act as ligands for δ-MgCl2 units. Thus, the Mg2+/Al3+ conduction occurs through the exchange of anionic species between the different ionic clusters. These phenomena are significantly correlated to the host medium dynamics of the IL matrix. The rise of the temperature results in an increased degree of disorder in the nanodomains, which modifies the long-range charge migration process. Finally, the electrochemical activity in the synergic Al/Mg alloy deposition and stripping is demonstrated. Acknowledgements This project has received funding from the program “Budget Integrato per la Ricerca Interdipartimentale - BIRD 2018” of the University of Padova (protocol BIRD187913). References [1] R. Dominko, J. Bitenc, R. Berthelot, M. Gauthier, G. Pagot, V. Di Noto, Magnesium batteries: Current picture and missing pieces of the puzzle, Journal of Power Sources, 478 (2020) 229027. [2] D.J. Kim, D.-J. Yoo, M.T. Otley, A. Prokofjevs, C. Pezzato, M. Owczarek, S.J. Lee, J.W. Choi, J.F. Stoddart, Rechargeable aluminium organic batteries, Nature Energy, 4 (2019) 51-59. [3] F. Bertasi, C. Hettige, F. Sepehr, X. Bogle, G. Pagot, K. Vezzù, E. Negro, S.J. Paddison, S.G. Greenbaum, M. Vittadello, V. Di Noto, A Key concept in Magnesium Secondary Battery Electrolytes, ChemSusChem, 8 (2015) 3069-3076. [4] F. Bertasi, F. Sepehr, G. Pagot, S.J. Paddison, V. Di Noto, Toward a Magnesium-Iodine Battery, Adv. Funct. Mater., 26 (2016) 4860-4865.

Bulk thermodynamic and kinetic properties of mixtures are generally composition dependent, often in complicated ways, especially for partially miscible and multicomponent systems. Combined 1H chemical shift, 1H diffusion NMR, and surface forces analyses establish the compositional dependences of water solubility and self-diffusion in ternary polymeric polydimethylsiloxane–water–tetrahydrofuran (THF) mixtures. The addition of THF significantly increases the solubility of water, while decreasing its diffusivity, in hydrophobic polydimethylsiloxane. Minimum values for the self-diffusivities of both water and THF coincide with a minimum in the hydrophobic adhesion energy between silicone polymer thin films near the same binary composition of 0.20 mole fraction THF. Such interrelated diffusivities, solubilities, and hydrophobic interactions are analyzed with respect to hydrogen bonding among the constituent species to account for the bulk physical properties of technologically important mixtures of silicone po...

The disruption in next-generation batteries can only be realised by employing both new advanced materials and designs, eventually allowing to holistically achieve new levels of performance, safety and sustainability in a single battery system. To close the gap between high power and energy per unit weight, new materials are needed that can act as a battery and capacitor simultaneously. Conductive polymers, e.g. poly(3,4-ethylenedioxythiophene) (PEDOT), used with ionic liquids or semi-solid ionogels have recently attracted attention as hybrid battery-(pseudo)-capacitor systems, achieving a specific capacity, energy, and power of 40-180 Ah kg-1, 50-230 Wh kg-1, and 30-140 W kg-1 when used as positive electrodes in rechargeable aluminium batteries 1-4. In addition, the polymer composites are non-flammable, non-toxic, economically available and recyclable at low-cost. In comparison to other charge storage materials, like graphite, conductive polymers are less limited by their capacity as the electrode architecture can be redesigned from a conventional 2D design to an interconnected 3D electrode-electrolyte network, allowing a higher active surface, shorter charge transfer and ion diffusion paths for fast charging (up to 80C 4), and controllable electrode shapes/sizes (Figure A 3). The performance of conductive polymers can be tailored by their synthesis path and conditions 1,5. The relationship between electrode fabrication, underlying operating mechanisms, and performance was studied by in operando methods such as electrochemical atomic force microscopy (EC-AFM), and electrochemical microgravimetry (EQCM) 3. The main achievement is the visualisation of reversible morphological modifications at the nano-scale (Figure B and C 3) of the conductive polymer as a function of state-of-charge. Such changes influence significantly the viscoelastic material properties, which are linked directly to the electrochemical fabrication process 5. Overall, the findings provide an explanation why conductive polymers behave like a hybrid battery-(pseudo)-capacitor (Figure D), setting a new standard for how energy storage can be approached. [1] Heinze, J. et al. Chemical Reviews 2010, 110, 4724–4771. [2] Hudak, N.S. The Journal of Physical Chemistry 2014 , 118, 5203–5215. [3] Schoetz, T. et al. Journal of Materials Chemistry A 2018, 6, 17787–17799. [4] Schoetz, T. et al. Journal of the Electrochemical Society 2020, 28, 101176. [5] Schoetz, T. et al. Electrochimica Acta 2018 , 263, 176–183. Figure 1

Zinc-manganese dioxide (Zn|MnO2) alkaline batteries have been widely studied as the active materials exhibit high theoretical capacities (820 mAh/g for Zn and 620 mAh/g for MnO2), low cost, and are inherent safety. Furthermore, the active materials are electrochemically active in aqueous alkaline electrolytes, such as potassium hydroxide (KOH), which is beneficial from a cost and safety perspective. It is widely known that one of the issues with this system is active MnO2 poisoning by zincate anions during the 2nd electron reaction of MnO2. As a result, an electrochemically inert material, ZnMn2O4 (hetaerolite), is formed, which reduces the capacity and cycle life of Zn|MnO2 batteries. Another issue is associated with high depth-of-discharge (DOD) of each material. For example, at high DOD, Zn will undergo shape change, dendritic growth and passivation, while MnO2 will form inactive phases like Mn3O4. Therefore, the 1st electron reaction range of MnO2 is normally accessed in rechargeable Zn|MnO2 batteries with low DOD. We have observed that the use of alkaline-based hydrogel electrolytes can significantly increase the cycle life of Zn|MnO2 batteries. Large-scale Zn|MnO2 cells were tested with either hydrogel or conventional alkaline electrolytes, whose nameplate capacities were 95 Ah based on the 1st electron capacity of MnO2. The hydrogel electrolytes were formed by mixing KOH solution, acrylic acid, and potassium persulfate solution as an initiator. Galvanostatic tests were carried out at cycling rates of C/20 (4.75 A) with 20% DOD of the capacity (19 Ah). The single full discharge performance to 0 V with hydrogels was compared to that with KOH aqueous electrolytes. The cell with the KOH electrolyte showed two sigmoidal curves, which means that two steps of electrochemical reactions were undergone, achieving 131 Ah. After recharging it, the single full discharge was conducted again, but nevertheless it attained less than 4 Ah to 1 V onwards. On the contrary, the cell with the hydrogel presented one sigmoidal curve with 40 Ah to 0 V, and it was continuously able to extract discharge capacities ≥28 Ah to 1 V during 40 cycles. Thus, the hydrogel electrolytes significantly enhanced the electrochemical performance of Zn|MnO2 cells, compared to conventional KOH electrolytes, the reasons for which will be discussed. The results suggest that hydrogels could help preserve active materials without the aforementioned issues and make rechargeable Zn|MnO2 batteries achieve better performance while accessing the 1st electron reaction of MnO2.

Rechargeable aluminum batteries represent a beacon of possibilities for sustainable electrochemical energy storage. Aluminum metal is energy dense, exhibiting high volumetric and gravimetric capacities of 8.05 A h cm-3 and 2.98 A h g-1, respectively. These capacities approximate to 75% of the volumetric and 400% of the gravimetric capacities of lithium metal. Additionally, aluminum is the most abundant metal in the Earth’s crust (8.23 wt. %), non-flammable, and benefits from mature mining, refinement, and recycling industries, thus making aluminum a low-cost alternative to lithium anodes in batteries. However, rechargeable aluminum batteries currently suffer from a scarcity of positive electrode materials (cathodes) that are both high performance and compatible with commonly used ionic liquid electrolytes. Organic molecules are derived from renewable and sustainable sources and can be leveraged as cathode materials; when paired with aluminum metal anodes, they result in batteries with earth abundant electrodes that are environmentally viable at scale. Here, we demonstrate a rechargeable aluminum battery using the organic molecule tetrathiobarbituric acid (TTBA) for the positive electrode and investigate its charge storage mechanism up from the molecular level through nuclear magnetic resonance (NMR) and electrochemical methods. For the electrolyte, a non-flammable, non-volatile ionic liquid was used, composed of AlCl3 and [EMIm]Cl (molar ratio: 1.5:1). The electrochemical performance of these cells was investigated through cyclic voltammetry and galvanostatic cycling techniques to determine reaction reversibility, cell capacity, and rate capability. Chiefly, the electrochemical results display >100 mA h g-1 for over 600 cycles at 100 mA g-1 and approximately 70 mA h g-1 at 500 mA g-1. The Al-TTBA batteries demonstrate long cycle lifetimes, yielding hundreds of cycles with significant capacity retention. This long cycle lifetime is attributed to TTBA’s low solubility in the electrolyte, a result of TTBA’s tetrameric structure. Molecular-level understanding of the charge storage mechanism was probed through spectroscopic and diffraction analyses, particularly multi-dimensional solid-state NMR measurements of TTBA electrodes conducted at different stages-of-charge. Solid-state 1H single-pulse and 13C{1H} cross-polarization (CP) magic-angle-spinning (MAS) NMR measurements yielded insights into local chemical, electronic, and structural changes that the TTBA undergoes upon ion complexation, while solid-state 27Al single-pulse NMR was used to probe aluminum coordination environments. Two-dimensional (2D) 27Al{1H} dipolar-mediated NMR correlation techniques, which probe sub-nanometer through-space proximities between 27Al and 1H moieties, were used to unambiguously establish the presence of aluminum bound to the electrode structure. X-ray diffraction (XRD) studies demonstrated changes in crystalline structure upon cycling. Overall, the results establish TTBA as a promising positive electrode material for rechargeable aluminum batteries, as well as yield molecular-level insights into its unique charge storage mechanism that are expected to aid the design of organic structures for multivalent-ion batteries.

The continual rise in global energy demands coupled with recent incremental advances in the cost and energy density of lithium-ion batteries have given researchers an impetus to investigate high-energy-density batteries such as those that use multivalent-ions. Rechargeable Al metal batteries have recently garnered significant interest due to its low cost, earth abundance, inherent safety, and high volumetric (8040 mAh cm -3) and gravimetric (2980 mAh g -1) capacities due to the trivalent nature of Al3+ ions (3-electron redox). Commercialization of rechargeable aluminum battery systems has been limited, however, due to the small number of (i) electrolytes that enable reversible electrodeposition of Al metal at room temperature and (ii) positive electrode materials that are compatible with the electrolytes while providing high capacity and cycle life. Sulfur (S) and selenium (Se) are promising positive electrode materials for rechargeable battery systems as they exhibit very high specific capacities during electrochemical conversion reactions. Sulfur is a very low-cost material due to its earth abundance and generation as a byproduct of the petroleum industry, while it can provide a very high specific capacity up to 1675 mAh g-1. Commercialization of battery systems with sulfur electrodes has been hindered due to its low electrical conductivity, high volume expansion during galvanostatic cycling, and deleterious polysulfide shuttle reactions during charge and discharge1-3. Selenium, like sulfur, is also a chalcogen and is expected to exhibit similar electrochemical reactions. Although Se is a heavier element and consequently exhibits lower specific capacity (675 mAh g-1), it possesses significantly higher electronic conductivity compared to sulfur4 that would be beneficial in mitigating key challenges that faces sulfur electrodes, such as low active material utilization and high charge transfer overpotentials during galvanostatic cycling. To date, there are few reports of Al-S2,3, and Al-Se4,5 systems. There is no clear consensus on the electrochemical reaction mechanism during the charge/discharge process, while capacity fade plagues both systems during galvanostatic cycling. Here, aluminum-sulfur (Al-S) and aluminum-selenium (Al-Se) cells were investigated with a chloroaluminate ionic liquid electrolyte, AlCl3:[EMIm][Cl] (molar ratio of 1.5:1), whose electrochemical reaction mechanisms and ensuing reaction products were analyzed via a combination of bulk (XRD, NMR) and surface (XPS) analyzation techniques. Electrochemical experiments for Al-S batteries showed high initial capacities close to 1200 mAh g-1, which exhibited a inverse correlation with galvanostatic cycling rate. For Al-S batteries a higher capacity was observed upon charge, compared to discharge, pointing towards either an unwanted side reaction or polysulfide-shuttle-like behavior. This behavior occurred predominately at low current densities and affected the reversibility of the system. For the Al-Se batteries, the two different reaction mechanisms that have been previously reported were found to be dependent on (i) the applied current density and (ii) the crystallinity of the active material, as shown by XRD. We demonstrate that, with appropriate cycling conditions, both reaction mechanisms can occur during a single charge/discharge step, thus significantly increasing the capacity and the electrochemical window of the Al-Se cell. A significant capacity fade was also observed over a course of multiple cycles showing similar behavior compared to Al-S cells. To understand the reaction mechanisms of Al-S and Al-Se batteries at a molecular level, solid-state and liquid-state 27Al NMR and 77Se NMR measurements (on Al-Se systems) were acquired on the electrodes and electrolyte at different states-of-charge, respectively. XPS was also performed on cycled chalcogen electrodes to understand surface compositions. The combination of electrochemical, spectroscopic, and X-ray diffraction data yield insights into the reaction mechanisms and products, including the aluminum coordination environments and relative populations in these environments. This work establishes the potential of conversion-type aluminum-chalcogen batteries as high-energy-density energy storage systems and highlights the importance that molecular-level analytical tools like NMR spectroscopy can have in clarifying electrochemical mechanisms in emerging electrochemical systems. Reference L. Ellis, K. T. Lee, and L. F. Nazar, Chemistry of Materials, 22, 691 (2010). Gao et al., Angew. Chemie - Int. Ed., 55, 9898–9901 (2016). . X. Yu, M. J. Boyer, G. S. Hwang, and A. Manthiram, Chem, 4, 586–598 (2018). Liu et al., Nano Energy, 66, 104159 (2019) Li, J. Liu, X. Huo, J. Li, and F. Kang, ACS Appl. Mater. Interfaces, 11, 45709–45716 (2019).

Rechargeable alkaline batteries using zinc metal (Zn) electrodes are of significant interest for next-generation energy storage because of their high energy density, low cost, environmental friendliness, and inherent safety. Rechargeable Zn-alkaline batteries, however, have not been widely commercialized in part due to the low cyclability of Zn electrodes. At high depths of discharge, zinc oxide (ZnO) discharge product accumulates in the electrode, passivating the zinc-electrolyte interface and ultimately leading to cell failure. While the ZnO discharge product has been studied for decades, there is no general consensus on its local structure, composition, and electrochemical properties, nor on how these properties are affected by the electrode potential or cycling conditions. In this work, we used in operando analytical techniques including X-Ray diffraction (XRD), confocal Raman spectroscopy, electrochemical impedance spectroscopy (EIS), and ultraviolet-visible (UV-vis) spectroscopy to demonstrate that the discharge product is proton-doped ZnO with oxygen vacancies, and that the electrical conductivity, band gap, and color vary systematically as a function of electrode potential. To investigate local hydrogen and zinc environments within the ZnO discharge product, we performed ex situ solid-state 1H, 2H, and 67Zn magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. Solid-state 1H and 2H NMR measurements establish that hydrogen environments change as a function of potential, while solid-state 67Zn NMR spectra indicate that the local zinc environments are distorted compared to ZnO generated via conventional routes. We also performed 2D 1H-1H exchange (EXSY) NMR measurements to investigate proton mobility between defect environments and acquired 1D 1H-1H dipolar-mediated double quantum-filtered MAS NMR spectra to establish through-space molecular-level proximities between proton sites. This work provides insights into the dynamics of zinc electrodes in alkaline environments, as well as how to electrochemically control the defect structures and electronic and optical properties of ZnO in alkaline media, which are expected to aid the development of next-generation Zn alkaline batteries and electronic devices utilizing ZnO.

Metallic zinc (Zn) is an attractive negative electrode material for rechargeable batteries because it is a non-toxic, non-flammable, earth abundant metal that is compatible with aqueous electrolytes. Zinc metal is also energy dense, exhibiting a high gravimetric discharge capacity of 820 mAh/g, a high volumetric discharge capacity of 5850 mAh/mL, and a low equilibrium potential of -1.35 V vs. mercury-mercuric oxide (Hg|HgO) in concentrated alkaline electrolyte. Rechargeable aqueous Zn alkaline batteries have thus recently been the subject of intense investigations for safe, low-cost, stationary electrochemical energy storage. Our research aims to achieve Zn depth-of-discharge (DOD) of 20% that is sustainable for 500 or more charge-discharge cycles, corresponding to an overall battery energy density of 200 Wh/L at the cell level. However, the cycle life of the porous Zn electrode in concentrated alkaline electrolyte is observed to decrease strongly with increasing Zn DOD. For example, we recently demonstrated that metallic Zn electrodes cycled in the range of 1% to 16% Zn DOD had an exponential decrease in cycle life with Zn DOD, which was attributed mostly to the loss of active Zn material. Here, we study the effect of additives to increase the Zn DOD and cycle life of Zn electrodes. Zinc oxide (ZnO) was used as a starting material from which metallic Zn was formed in zinc-nickel (Zn-Ni) batteries. Additives were incorporated into the ZnO electrodes for the tests done on Zn-Ni batteries and these electrodes were cycled in the range of 15% to 30% Zn DOD. The potential values of the Zn electrodes vs. Hg|HgO reference electrodes were recorded to identify failure due to Zn. At 15% Zn DOD, ZnO electrodes with certain additives or combinations of additives achieved an increased cycle life compared to Zn electrodes without additives. This highlights the need to perform electrochemical and spectroscopic studies to determine the effects of the additives on the mechanisms of the Zn electrode. These additives, with further understanding, could be implemented in low-cost, energy-dense, rechargeable Zn alkaline batteries for grid-scale energy storage.

Rechargeable aluminum-graphite batteries using chloroaluminate-containing ionic liquid electrolytes are a promising beyond-lithium technology because they utilize electrodes that are globally abundant, low-cost, and inherently safe.1,2 Notably, these batteries have demonstrated ultrafast charge/discharge rates, which are critical for advancing high-power and rapid-charging applications such as electric transportation. Since Lin et al.’s pioneering report in 2015, several groups have engineered graphene-like materials to further improve the high-rate capability and capacity retention to as high as 120 mAh/g @ 400 A/g, on the cell-level.3 Recently, we elucidated the relationship between mesoscale graphite structure and macroscopic electrochemical properties such as high-rate performance.4 However, molecular-level understanding of the electrochemical charge storage mechanism upon cycling, which involves the intercalation of sterically bulky, molecular AlCl4 - anions into graphite, remains incomplete. To better understand what enables such rapid molecular-level ionic diffusion and consequently high-rate cell-level cycling, we combined electrochemical studies with solid-state 27Al magic-angle-spinning (MAS) NMR and density functional theory (DFT) to understand the molecular-level environments and ion transport properties of intercalated AlCl4 - anions. First, to directly observe local environments of chloroaluminate species intercalated within the graphite electrodes, solid-state 27Al MAS NMR measurements were acquired on intercalated graphite electrodes at various states-of-charge. DFT calculations were performed to simulate different molecular geometries of intercalated AlCl4 - anions. Correlation of experimental NMR and DFT results established that the high extents of local disorder observed in the experimental solid-state 27Al NMR spectra can be attributed to distributions of intercalated chloroaluminate anions in different molecular configurations. Second, to address the technological objective of further improving the high-rate capacity retention of graphite electrodes, we employed liquid-phase exfoliation and centrifugation to modify the c-axis thickness of the highly ordered pristine graphites. In full-cell electrochemical tests of the exfoliated-graphite electrodes, variable-rate galvanostatic cycling revealed increased capacity retention, while variable-rate cyclic voltammetry (CV) and galvanostatic intermittent titration technique (GITT) experiments revealed modified effective ionic diffusivities and ohmic resistances. Collectively, this multi-scale study of chloroaluminate intercalation into graphitic electrodes has revealed new insights that are key contributing factors to their high-rate capability, which can be adapted to engineer Al-graphite batteries with enhanced performance. (1) Lin, M.-C.; Gong, M.; Lu, B.; Wu, Y.; Wang, D.-Y.; Guan, M.; Angell, M.; Chen, C.; Yang, J.; Hwang, B.-J.; Dai, H. An Ultrafast Rechargeable Aluminium-Ion Battery. Nature 2015, 520, Pages: 324–328. (2) Sun, H.; Wang, W.; Yu, Z.; Yuan, Y.; Wang, S.; Jiao, S. A New Aluminium-Ion Battery with High Voltage, High Safety and Low Cost. Chem. Commun. 2015, 51, 11892–11895. (3) Chen, H.; Xu, H.; Wang, S.; Huang, T.; Xi, J.; Cai, S.; Guo, F.; Xu, Z.; Gao, W.; Gao, C. Ultrafast All-Climate Aluminum-Graphene Battery with Quarter-Million Cycle Life. Sci. Adv. 2017, 3. (4) Xu, J. H.; Turney, D. E.; Jadhav, A. L.; Messinger, R. J. Effects of Graphite Structure and Ion Transport on the Electrochemical Properties of Rechargeable Aluminum-Graphite Batteries. ACS Appl. Energy Mater. 2019, 2, 7799–7810.

Battery electrodes that enable electrochemical intercalation of multivalent ions result in multiple electron transfers per ion, respectively, yielding significant enhancements in electric charge storage capacity. When multivalent cathodes are paired with their corresponding metal anodes, potentially transformative gains in energy density are possible. However, few positive electrode materials exist that intercalate multivalent ions, in part due to the slow solid-state diffusion of highly charged multivalent ions. The chevrel phases Mo6X8, where X is a chalcogen atom (e.g., S or Se) are among the few materials known to reversibly and electrochemically intercalate both divalent (e.g. Mg2+, Zn 2+, etc.) and trivalent (Al3+) ions. Thus, understanding the ion intercalation and electron charge transfer mechanisms within these unique compounds may provide insights into the molecular-level design of new intercalation electrodes for rechargeable multivalent ion batteries. Here, we couple electrochemical and solid-state magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) measurements to reveal the concurrent aluminum-ion intercalation and electron charge transfer mechanisms in chevrel electrodes Mo6S8 and Mo6Se8 up from the molecular level. Solid-state 27Al single-pulse MAS NMR measurements were performed on the thio-chevrel Mo6S8 electrodes at different states-of-charge to observe changes in the environments and to quantify the local populations of aluminum ions within the host crystal structure, revealing both intercalated ions and additional aluminum species associated with surface layers and/or decomposition products. Al-Mo6Se8 batteries were made for the first time and their electrochemical properties were compared to the thio-chevrel Mo6S8. Solid-state 77Se and 95Mo MAS NMR measurements on the seleno-chevrel Mo6Se8 at charged and discharged states revealed that electrons are transferred preferentially to the anionic chalcogen framework upon aluminum ion intercalation, as opposed to the transition metals (Mo). Anionic redox was further confirmed by density functional theory (DFT) calculations and X-ray absorption near-edge structure (XANES) measurements. Lastly, we generalize the results to other ion valencies, demonstrating reversible electrochemical anionic redox in the Chevrel phase upon divalent Zn2+ cations in aqueous Zn batteries. Overall, for the first time, we demonstrate reversible electrochemical anionic redox as a charge storage mechanism for rechargeable batteries, which preserves the crystalline framework structure and does not involve the breaking and forming of chemical bonds. This charge storage mechanism is fundamentally different than that observed for lithium-ion intercalation into transition metal oxides, where the electrons are stored within the d-orbitals of the transition metal. The results suggest materials design principles aimed at designing multivalent-ion intercalation electrodes with improved electrochemical properties. Figure 1

Rechargeable zinc alkaline batteries are of great interest for stationary storage applications because they are inherently safe, energy dense, environmentally friendly, and low-cost. However, these batteries have not been commercialized on a large scale because of their low cycle life at high depths of discharge. One cause is the buildup of ZnO on the electrode. It has been shown recently that ZnO produced in zinc alkaline electrodes inserts protons and electrons and undergoes electrochromic color changes as a function of electrode potential. This dynamic behavior increases hydrogen catalysis, contributes capacity to the electrode, and changes oxide conductivity as a function of electrode potential. In this work we investigate the disorder and nonstoichiometry of ZnO that allow the material to become electrochemically active. We used 1H, 2H, and 67Zn solid-state MAS NMR to better understand structure, defects, order, and the hydrogen insertion mechanism of the ZnO. Understanding the physical properties of the ZnO will enable better design of electrodes and additives for zinc alkaline batteries in the future.

Rechargeable aluminum-graphite batteries using chloroaluminate-containing ionic liquid electrolytes store charge when chloroaluminate anions (e.g. AlCl4 -) intercalate into graphite1,2. Such batteries utilize electrodes that are globally abundant, low-cost, and safe, while exhibiting ultra-fast rate capabilities, high cycle life and discharge voltages of ca. 2 V. Recently, we elucidated the relationship between graphite structure and macroscopic electrochemical properties such as high-rate performance and the maximum intercalation composition.3 Variable-rate cyclic voltammetry (CV) revealed potential-dependent transport regimes that yield new insights into their ultra-fast rate capabilities. To directly observe the chloroaluminate species intercalated within the graphite electrodes, we performed solid-state 27Al MAS NMR measurements, which revealed their various local environments and populations. To aid the interpretation of the experimental results, DFT calculations were performed to compute the magnitude of 27Al NMR shift contributions due to varying graphite d-spacing, aromatic ring-current shielding, and quadrupolar interactions. With this increased understanding of the elusive transport mechanism of sterically-bulky chloroaluminates, the current technological objective is to improve the high-rate capacity retention of synthetic graphite electrodes. To tune the ionic diffusivity within the graphite framework, we employed liquid-phase exfoliation and centrifugation to modify the c-axis thickness of the highly-ordered pristine graphites, generating few-layered graphene structures that we characterized using Atomic Force Microscopy (AFM), Dynamic Light Scattering (DLS) and Transmission Electron Microscopy (TEM). In full-cell electrochemical tests of the exfoliated-graphite electrodes, variable-rate galvanostatic cycling revealed increased capacity retention (~75% @ current density of 960 mA/g, compared to ~43% for non-exfoliated synthetic graphite). Furthermore, galvanostatic intermittent titration technique (GITT) experiments revealed modified ionic diffusivity and ohmic resistances. These results are expected to aid the rational design of graphite electrodes for enhanced performance in aluminum-graphite batteries. (1) Lin, M.-C.; Gong, M.; Lu, B.; Wu, Y.; Wang, D.-Y.; Guan, M.; Angell, M.; Chen, C.; Yang, J.; Hwang, B.-J.; Dai, H. An Ultrafast Rechargeable Aluminium-Ion Battery. Nature 2015, 520, 324–328. (2) Sun, H.; Wang, W.; Yu, Z.; Yuan, Y.; Wang, S.; Jiao, S. A New Aluminium-Ion Battery with High Voltage, High Safety and Low Cost. Chem. Commun. 2015, 51, 11892–11895. (3) Xu, J. H.; Turney, D. E.; Jadhav, A. L.; Messinger, R. J. Effects of Graphite Structure and Ion Transport on the Electrochemical Properties of Rechargeable Aluminum-Graphite Batteries. ACS Appl. Energy Mater. 2019, 2, 7799–7810.

Rechargeable multivalent-ion batteries are safe and low-cost alternatives to traditional lithium-ion batteries. However, higher Coulombic charge density of the multivalent ions results into sluggish diffusion of their respective cations within crystalline transition metal compounds. The Chevrel phases Mo6X8, where X is a chalcogen atom (e.g., S or Se) are among the few materials known to reversibly and electrochemically intercalate both divalent (e.g. Mg2+, Zn 2+, etc.) and trivalent (Al3+) ions. Thus, understanding the ion intercalation and charge transfer mechanisms within these unique compounds may provide insights into the molecular-level design of new intercalation electrodes for rechargeable multivalent ion batteries. Here, we use multi-nuclear solid-state magic-angle-spinning (MAS) NMR spectroscopy to reveal the ion intercalation and charge transfer mechanisms of zinc and aluminum intercalation in Chevrel electrodes Mo6S8 and Mo6Se8. Ex situ single pulse solid-state 27Al MAS NMR measurements were performed on the thio-chevrel electrodes at different states-of-charge to observe changes in the environments and to quantify the local populations of aluminum ions within the host crystal structure, revealing both intercalated ions and additional aluminum species associated with surface layers and/or decomposition products. Al-Mo6Se8 batteries were made for the first time and their electrochemical properties were compared to the thio-chevrel Mo6S8. Ex situ 77Se and 95Mo measurements on the seleno-chevrel Mo6Se8 at charged and discharged states revealed that the electrons are transferred to the anionic chalcogen framework the electron charge transfer mechanism and its effects on the local crystalline framework environments upon aluminum ion intercalation. These results were then generalized for zinc intercalation into chevrel phase electrodes. Overall, the solid-state NMR measurements combined with the electrochemical measurements yield molecular-level insights into the coupled aluminum ion intercalation and electron charge transfer processes that occur in a model crystalline transition metal electrode. Unique reversible anionic redox reactions were observed for multivalent ion intercalation into chevrel, which is very different that typically observed for lithium-ion intercalation into transition metal oxides, where the electrons are stored into the d-orbitals of the transition metal. The results suggest materials design principles aimed at designing multivalent-ion intercalation electrodes with improved electrochemical properties. Figure 1

Rechargeable aluminum metal batteries are an emerging energy storage technology with great promise: aluminum has among the highest capacities of common metal electrodes and is low cost, earth abundant, environmentally friendly, and inherently safe. Despite these opportunities, their technological development has been hindered due to fundamental challenges associated with aluminum electrochemistry. Few electrolytes enable the reversible electrodeposition of aluminum metal at room temperature, while few positive electrode materials have been demonstrated that exhibit high energy density and cycle life in those electrolytes. Here, recent progress will be discussed in the development and characterization of positive electrode materials for rechargeable aluminum metal batteries, including graphites, organic materials, sulfur, and crystalline transition metal compounds. Molecular-scale understanding of their charge storage mechanisms will be elucidated, revealed by a combination of electrochemical, spectroscopic, diffraction, imaging, and theoretical methods, such as multi-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy, in operando X-ray diffraction (XRD), electron microscopy, and density functional theory (DFT). The diversity of electrochemical charge storage mechanisms possible for aluminum battery chemistries will be highlighted. Overall, the results are aimed at developing next-generation rechargeable aluminum metal batteries for diverse energy storage applications.

Molecular compositions, structures, interactions, polymer chain dynamics, and micron-scale cell structures of elastomeric organosiloxane foams have been analyzed and correlated with their macroscopic mechanical properties. Open-cell organosiloxane foams were synthesized within a narrow range of relative densities (±5% relative uncertainty) and with similar micron-scale pore structures, as determined from quantitative analyses of micro-X-ray computed tomography (MXCT) images. Network cross-linking densities, polymer molecular weights, organic side-chain moieties, and inorganic filler contents were varied systematically, resulting in materials with significantly different mechanical properties. Solid-state single-pulse 1H and 29Si magic-angle-spinning (MAS), two-dimensional (2D) 29Si{1H} hetereonuclear correlation (HETCOR), and transverse 1H relaxation (T2) nuclear magnetic resonance (NMR) spectroscopy measurements establish significant differences in molecular and polymer network characteristics that are c...

The intrinsic ionic conductivity of polyethylene oxide (PEO)-based block copolymer electrolytes is often assumed to be identical to the conductivity of the PEO homopolymer. Here, we use high-field 7Li nuclear magnetic resonance (NMR) relaxation and pulsed-field-gradient (PFG) NMR diffusion measurements to probe lithium ion dynamics over nanosecond and millisecond time scales in PEO and polystyrene (PS)-b-PEO-b-PS electrolytes containing the lithium salt LiTFSI. Variable-temperature longitudinal (T1) and transverse (T2) 7Li NMR relaxation rates were acquired at three magnetic field strengths and quantitatively analyzed for the first time at such fields, enabling us to distinguish two characteristic time scales that describe fluctuations of the 7Li nuclear electric quadrupolar interaction. Fast lithium motions [up to O(ns)] are essentially identical between the two polymer electrolytes, including sub-nanosecond vibrations and local fluctuations of the coordination polyhedra between lithium and nearby oxygen atoms. However, lithium dynamics over longer time scales [O(10 ns) and greater] are slower in the block copolymer compared to the homopolymer, as manifested experimentally by their different transverse 7Li NMR relaxation rates. Restricted dynamics and altered thermodynamic behavior of PEO chains anchored near PS domains likely explain these results.

International audience; In a recent study, we showed by solid-state NMR that LiVPO4F, which is a promising material as positive electrode for Li-ion batteries, often exhibits some defects that may affect its electrochemical behavior. In this paper, we use DFT calculations based on the projector augmented-wave (PAW) method in order to model possible defects in this (paramagnetic) material and to compute the Fermi contact shifts expected for Li nuclei located in their proximity. The advantage of the PAW approach versus FP-LAPW we have been previously using is that it allows considering large supercells suitable to model a diluted defect. In the first part of this paper, we aim to validate the Fermi contact shifts calculation using the PAW approach within the VASP code. Then we apply this strategy for modeling possible defects in LiVPO4F. By analogy with the already existing homeotypic LiVOPO4 phase, we first replace one fluoride ion, along the VO2F4 chains, by an oxygen one and consider, in a second step, an association with a lithium vacancy. As a result, the agreement between the calculated NMR spectra and the experimental one is satisfying. In both cases, the local electronic structure and the spin transfer mechanisms from V3+ or V4+ ions to the Li nuclei are analyzed.

Rechargeable zinc-alkaline batteries are attractive alternatives to established battery technologies such as lithium-ion and lead-acid in certain applications because of their inherent low cost, safety, and environmental friendliness. Commercialization of zinc-alkaline batteries has been largely unsuccessful because of the limited cycle life of zinc electrodes at high depth of discharge. At high depths of discharge, dissolved zinc precipitates from the electrolyte as solid zinc oxide. Zinc oxide buildup causes passivation of zinc metal particles, which increases resistance within the battery and results in eventual cell death. It can also electrically isolate metal particles from the current collector, resulting in loss of active zinc mass. Zinc oxide in alkaline batteries has been studied for decades, but an understanding of the properties of the material and effects on battery performance is still not complete. A complete understanding of the discharge products of zinc electrodes is a necessary step toward wide-spread commercialization of zinc metal electrodes. We demonstrate in this work that zinc oxide produced during discharge undergoes chemical changes as a function of electrode potential. In operando optical microscopy shows that the oxide reversibly changes color from blue at more negative zinc electrode potentials to white at more positive zinc electrode potentials. In operando Raman spectroscopy shows differences in Raman scattering between the blue and the white oxide species, indicating that additional bonds are present in the blue material, which are not observed in the white material. Solid-state 1H magic angle spinning NMR spectroscopy on extracted oxide material suggests that changes to electrode potential result in simultaneous insertion or disinsertion of protons and electrons into the oxide material. More immobile species are present in the blue material compared to the white, indicating inserted protons may be more abundant in the blue oxide. Additionally, two-dimensional 1H-1H exchange spectroscopy shows more chemical exchange between sites in the white material compared to the blue, indicating protons may be more strongly bound in the blue oxide. The behavior observed in these studies is similar to that observed for electrochromic materials. In electrochromic materials, inserted electrons can increase the electrical conductivity of the oxide by orders of magnitude. At more positive zinc potentials, an increase in the electrical resistance of the oxide may result in conductivity losses between zinc metal particles. Additionally, when protons are disinserted from the oxide, they result in a local pH decrease in the electrolyte surrounding the oxide, which affects the ionic conductivity of the electrolyte. The work outlined in this study demonstrates the effects of this behavior on electrode performance and explores avenues for controlling the behavior through electrode additives and operation controls. Figure 1

Energy dispersive X-ray diffraction (EDXRD) from a high energy source allows the evolution of materials to be tracked from deep within large specimens, due to (a) the use of highly penetrating X-rays and (b) the ability to define a well-controlled diffraction gauge volume in space.1,2 This non-destructive characterization tool lends itself to the study of batteries, whose electrodes are generally composites of compressed particles, with electrolyte filling the pore space, and a sealed containment surrounding the cell designed to be as compact as possible. The fundamental electrochemistry of the active material can be known from laboratory experiments, but reaction inhomogeneity that is a consequence of the system itself is best observed in an in situ fashion without disassembly, or in an operando fashion within the battery as it cycles. EDXRD allows tomographic-like observation of the bulk interior of electrodes and critical locations such as the electrode-separator interfaces.3,4 This talk will illustrate the technique for understanding complex electrochemical reactions in three diverse systems: (1) proton insertion into γ-MnO2 in alkaline electrolyte, (2) Al3+ intercalation into Mo6S8 chevrel in chloroaluminate electrolyte, and (3) Zn2+ intercalation into Mo6S8 in aqueous ZnSO4. Deeply cycling MnO2 (617 mAh/g-MnO2) has the potential to provide battery storage of high safety, high energy density, and low cost, for applications such as intermittent renewable generation backup at the scale of the power grid.5 Figure 1a shows the γ-MnO2 discharge curve, in which reduction to one electron equivalent per Mn (x = 1) proceeds through proton insertion. Reaction beyond this point involves new phase formation at relatively constant potential. Operando EDXRD data in Figure 1b reveals that the nominal discharge product Mn(OH)2 was found only in a relatively small region clustered at the Ni current collector, while a propagating front of Mn3O4 traversed the electrode (panel iii). Figure 1c correlates the spatial locations of the various crystalline phases with the potential data in 1a. This shows that the final potential plateau at x = 1.8 corresponded to sudden Mn(OH)2 formation throughout the electrode, a phenomenon not previously reported.6 Dopant strategies to provoke earlier Mn(OH)2 formation and suppress Mn3O4 hold the key to MnO2 rechargeability.7 EDXRD is also useful in situations when electrode discharge products are prone to damage or alteration by cell disassembly, casting doubt on material structures observed by ex situ analysis. The chevrel phase Mo6S8 has been shown to intercalate divalent and trivalent ions, making it an important material of study. It is known that two second order phase transitions are expected during intercalation of a guest ion A into the host chevrel: Mo6S8 → AMo6S8 → A2Mo6S8.8 However, a recent ex situ study of Zn2+ intercalation suggested that conversion to Zn2Mo6S8 may be incomplete.9 We demonstrate this was a consequence of material oxidation during cell disassembly, and that when observed by EDXRD conversion to Zn2Mo6S8 is complete. The corresponding Al3+ intercalation will be compared to show there is only one phase transition in this case. References M. Croft, V. Shukla, E. K. Akdogan, N. Jisrawi, Z. Zhong, R. Sadangi, A. Ignatov, L. Balarinni, K. Horvath and T. Tsakalakos, J Appl Phys, 105 (2009). M. Croft, V. Shukla, N. M. Jisrawi, Z. Zhong, R. K. Sadangi, R. L. Holtz, P. S. Pao, K. Horvath, K. Sadananda, A. Ignatov, J. Skaritka and T. Tsakalakos, Int J Fatigue, 31, 1669 (2009). E. S. Takeuchi, A. C. Marschilok, K. J. Takeuchi, A. Ignatov, Z. Zhong and M. Croft, Energ Environ Sci, 6, 1465 (2013). J. W. Gallaway, C. K. Erdonmez, Z. Zhong, M. Croft, L. A. Sviridov, T. Z. Sholklapper, D. E. Turney, S. Banerjee and D. A. Steingart, Journal of Materials Chemistry A, 2, 2757 (2014). N. D. Ingale, J. W. Gallaway, M. Nyce, A. Couzis and S. Banerjee, J Power Sources, 276, 7 (2015). J. W. Gallaway, G. G. Yadav, D. E. Turney, M. Nyce, J. Huang, Y.-c. K. Chen-Wiegart, G. Williams, J. Thieme, J. S. Okasinski and X. Wei, J Electrochem Soc, 165, A2935 (2018). G. G. Yadav, J. W. Gallaway, D. E. Turney, M. Nyce, J. C. Huang, X. Wei and S. Banerjee, Nature Communications, 8 (2017). E. Levi, E. Lancry, A. Mitelman, D. Aurbach, G. Ceder, D. Morgan and O. Isnard, Chem Mater, 18, 5492 (2006). M. S. Chae, J. W. Heo, S.-C. Lim and S.-T. Hong, Inorg Chem, 55, 3294 (2016). Figure 1

Solid-state nuclear magnetic resonance (NMR) spectroscopy is a powerful and quantitative characterization tool that can elucidate the local chemical, structural, and electronic changes that battery materials undergo upon electrochemical cycling. However, to date no solid-state NMR experiments have been reported that directly probe multivalent aluminum or zinc cations within intercalation electrodes, in part due to their challenging quadrupolar nature and in part due to the small number of structures that reversibly intercalate multivalent ions. Here, multi-nuclear solid-state magic-angle-spinning (MAS) experiments will be presented on rechargeable aluminum and zinc intercalation electrodes for the first time, revealing insights into their ion intercalation and charge transfer mechanisms. For aluminum or zinc batteries using the thio-Chevrel Mo6S8 and seleno-Chevrel Mo6Se8 as cathode materials, solid-state 27Al and 67Zn NMR experiments establish quantitatively the relative populations of intercalated aluminum or zinc cations in different local environments as a function of state-of-charge. For the seleno-chevrel electrodes, solid-state 77Se NMR experiments reveal the effects of aluminum- and zinc-ion intercalation on the local electronic structures of the Mo6Se8 frameworks. For comparison, aluminum-graphite batteries will also be analyzed, where solid-state 27Al NMR measurements yield insights into the local environments of monovalent chloroaluminate anions after intercalation into natural graphite. Opportunities and challenges will be discussed regarding the application of NMR spectroscopy to aluminum and zinc battery materials. Overall, the solid-state NMR and electrochemical results pave the way towards a better understanding of material design principles aimed at realizing multivalent intercalation electrodes with enhanced electrochemical properties.

Metallic zinc (Zn) is an attractive negative electrode material for rechargeable batteries because it is a non-toxic, earth abundant metal. It has a low equilibrium potential of -1.35 V vs. mercury-mercuric oxide (Hg|HgO) in concentrated alkaline electrolyte, a high gravimetric discharge capacity of 820 mAh/g, and a high volumetric discharge capacity of 5850 mAh/mL. During discharge, Zn oxidizes and forms a dissolved complex known as the zincate ion, Zn(OH)4 2-, from which zinc oxide (ZnO) precipitates as a solid. On charge, Zn(OH)4 2- is reduced to metallic Zn. Motivation to study the Zn electrode in rechargeable batteries is driven by a target overall battery energy density of 200 Wh/L that is sustainable for 500 or more charge-discharge cycles. This corresponds to a Zn electrode volumetric capacity of 650 mAh/mL. The cycle life of the porous Zn electrode in concentrated alkaline electrolyte is observed to decrease with increasing percentage of discharge capacity of Zn accessed. The percentage of discharge capacity of Zn accessed is known as the depth of discharge (DOD). Zn electrodes were cycled in the range of 1% to 15% Zn DOD in zinc-manganese dioxide (Zn-MnO2) batteries and in the range of 15% to 30% Zn DOD for zinc-nickel (Zn-Ni) batteries. Additives were incorporated into the zinc electrodes for the tests done on Zn-Ni batteries. Additives used were surfactant cetyltrimethylammonium bromide (CTAB), bismuth oxide (Bi2O3), synthetic layered silicate Laponite, and calcium hydroxide (Ca(OH)2). The potential values of the Zn electrodes vs. Hg|HgO reference electrodes were recorded. During healthy galvanostatic discharge, Zn electrodes displayed a nearly constant potential vs. Hg|HgO at approximately -1.35 V. Tests that failed due to Zn exhibited a shift from this equilibrium potential that corresponded to a loss of cell voltage. This shift may be attributed to a concentration overpotential as ZnO is precipitated, thereby inhibiting the hydroxide ions from reaching the reacting metal interface. The shift may also be due to formation of a passivated layer on the electrode which effectively blocks the remaining active material. There are multiple regimes observed in the potential curves of the Zn electrode vs. Hg|HgO prior to cell failure. Beyond the healthy plateau at -1.35 V, a sloping potential is observed. This is followed by an additional plateau. The second plateau shifts with increasing cycle number and precedes cell death. See the attached figure for discharge curves of a Zn electrode cycled at 11.3% Zn DOD. After the second plateau a steep slope is observed in the potential for later cycles, which is believed to be due to passivation of the Zn electrode. The additives were observed to modify the discharge potential curve of the Zn electrode vs. Hg|HgO. Figure 1

Rechargeable zinc-alkaline electrodes are attractive forms of electrochemical energy storage for commercial applications because of the high theoretical energy density of zinc metal and the low cost, safety, and environmentally friendliness of aqueous zinc systems. The rechargeable zinc electrode has been historically limited in depth-of-discharge and cycle life, which has prevented it from being widely used on a commercial scale. This limitation is partly due to irreversible reactions occurring during discharge, which lead to passivation of the zinc electrode. Despite extensive study, the properties of these oxide species and the conditions leading to their formation are not fully understood. In this work, we identify different compounds that form on zinc electrodes using in operando optical microscopy, in operando X-ray diffraction, and scanning electron microscopy, as well as chemically identify these species and the conditions at which they form using in operando confocal Raman spectroscopy. Identification of zinc species that detrimentally affect zinc reachargeability and understanding their formation will promote the design of better electrodes and electrolytes, leading to improved cycle life and depth-of-discharge in rechargeable zinc-alkaline batteries. Figure 1

Rechargeable aluminum batteries are a promising alternative to lithium-ion batteries due to the high volumetric energy density (8040 mAh/cm3), high abundance in the earth’s crust, and low cost of aluminum metal. However, aluminum ion intercalation into crystalline electrode materials is challenging, as the high coulombic charge of trivalent aluminum ions significantly increases the activation energy of diffusion compared to monovalent ions. Chevrel phase Mo6S8 is one of the most well-studied and successful cathode materials for magnesium-ion batteries, partly due to the facile transport of divalent magnesium ions within its crystal structure. Reversible intercalation of aluminum ions within chevrel phase has been reported, though the intercalation mechanism is not well understood at a molecular level. Here, we report enhanced understanding of aluminum-ion intercalation in chevrel using a combination of spectroscopic, diffraction, and imaging techniques. Ex situ solid-state 27Al MAS NMR measurements were performed as a function of state-of-charge to observe changes in the environments and populations of aluminum ions within the host crystal structure, revealing both intercalated ions and additional aluminum species associated with surface layers and/or decomposition products. In situ diffraction methods were performed to observe changes in the crystal structure during the cycling process, while transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were conducted to characterize surface structures and compositions. Variable-temperature electrochemical measurements including cyclic voltammetry (CV), galvanostatic cycling, and electrochemical impedance spectroscopy (EIS) were performed to study the effects of temperature on the diffusion of Al3+ ions, electrochemical performance, and capacity retention. In combination, the results yield molecular-level insights into the aluminum-ion intercalation mechanism in the chevrel phase as well as changes of surface compositions and structures upon cycling in the ionic liquid electrolyte. Figure 1

Aluminum metal has long been envisioned as an electrode for high-performance rechargeable batteries due to its high global abundance, low cost, non-flammability, and high volumetric capacity. Graphite has recently been shown to pair as a cathode with aluminum anodes when using chloroaluminate-containing ionic liquid electrolytes, exhibiting capacities that are stable at high charge/discharge rates and over long cycle lifetimes. However, there are presently unanswered questions related to the intercalation of sterically bulky chloroaluminate anions (e.g. AlCl4 -), structural changes of the cathode after prolonged cycling, and more generally, the relationship between the local structures of different graphitic materials and their electrochemical properties. Here, we report on the investigation of charge storage and ion transport mechanisms in graphitic cathodes and their implications on bulk electrochemical behavior. Three types of graphites (pyrolytic, natural and synthetic) were studied. Solid-state 27Al magic-angle-spinning (MAS) NMR measurements on intercalated graphitic cathodes reveal the presence of chloroaluminate anions in different local environments and with different mobilities that evolve as a function of state-of-charge. Cyclic voltammetry experiments at varying scan rates were conducted to yield insights into ion transport phenomena and to disentangle the Faradaic and capacitive contributions to the total discharge capacity. Additionally, the surface area and porosity of the cathode materials, which affect the number of accessible sites for ion electrosorption, were quantified and correlated to the observed capacitance and overall energy density. Our findings reveal molecular-level insights into the intercalation processes and redox mechanisms of graphitic cathodes that can be collectively used to better understand and optimize aluminum-graphite battery performance. Figure 1

Rechargeable aluminum batteries are an emerging energy storage technology with great potential due to the high capacity, global abundance, low cost, and inherent safety of aluminum metal. However, few cathode materials have been demonstrated that exhibit high energy density and cycle life, in part due to the challenges associated with the high charge density of trivalent aluminum cations and the highly reactive nature of the chloroaluminate-containing ionic liquids commonly used as electrolytes. Here, investigations of selected intercalation and conversion cathodes for rechargeable aluminum metal batteries will be presented, including crystalline transition metal compounds, graphite, and sulfur-containing materials. Molecular-scale understanding of their charge storage mechanisms will be discussed, revealed by a combination of electrochemical methods, solid-state magic-angle-spinning (MAS) NMR spectroscopy, in situ diffraction methods, and electron microscopy. Intrinsic challenges—and opportunities—associated with intercalation and conversion cathode materials will be discussed. Overall, the results yield insights into possible rational design strategies for developing next-generation cathode materials for rechargeable aluminum metal batteries with improved energy densities, rate capabilities, and cycle lifetimes.

The commercial prevalence of energy-storage devices, including secondary batteries, has necessitated development of higher capacity and more energy dense systems.1 However, even as the battery components are modified to accommodate these needs, the flammable electrolytes found in certain batteries pose an inherent safety risk that has threatened their continued use. Furthermore, current safety mechanisms often irreversibly shut down the battery should thermal runaway occur. Reversible ionic liquids (RevILs) are a viable alternative to the electrolyte solvents currently used. These RevILs switch between an ionically-insulating molecular liquid (ML) state to an ionically-conducting state (IL) via external stimuli such as carbon dioxide (CO2) or temperature.2 Thus, the battery would operate under the premise that the electrolyte would be in its IL form during normal battery operation and in its ML form when a thermal safety limit is triggered. This would address the underlying safety concerns. More interestingly, with the application of an external stimulus, the battery can be revived simply by switching from the ML form back to the IL form and battery cycling may resume as normal. From our previous study, silylamine-based RevILs have demonstrated promise as potential electrolytes based on their ionicity.3 . Additionally, the work was expanded to determine the effect of different organic co-solvents on the conductivity. We found that the conductivity of trialkylsilylamines, in particular, increased up to 40 times in their RevIL state (~0.5 mS/cm) compared to their ML state (~0.01 mS/cm), thereby indicating that the silylamine could act as a safety switch via a conductivity change. With relevance towards formulating a silylamine-based electrolyte, we found that a tripropylsilylamine-based solution had an enhanced conductivity of 2.16 mS/cm upon the addition of a metallic salt (LiPF6). However, there are current difficulties associated with their use, including (1) ion aggregation, (2) low solubility of metallic salts, and (3) high viscosity. Towards this end, structure functionalization is one avenue of exploration that has been shown to simultaneously maintain the switchability while enabling other properties of the silylamine system to be tuned.4 However, these experiments should be guided by fundamental understanding of the ion interactions that underlie the observed phenomena. This study focuses on the elucidation of the ion-ion and ion-solvent interactions that dictate ion aggregation behavior and low salt solubility. These interactions were probed via two-dimensional nuclear magnetic resonance (NMR) spectroscopy experiments and electrochemical methods. By isolating the structural effects that result in the aforementioned difficulties, the silylamine structures can be tuned accordingly to address these challenges, resulting in improved RevIL-based electrolytes for battery applications. References: (1) Scrosati, B.; Garche, J. Lithium batteries: Status, prospects and future. J. Power Sources 2010, 195, 2419-2430. (2) Blasucci, V.; Dilek, C.; Huttenhower, H.; John, E.; Llopis-Mestre, V.; Pollet, P.; Eckert, C. A.; Liotta, C. L. One-component, switchable ionic liquids derived from siloxylated amines. Chemical Communications 2009, 116-118. (3) Jimenez, J. D.; Jung, S.; Biddinger, E. J. Ionicity analysis of silylamine-type reversible ionic liquids as a model switchable electrolyte. J. Electrochem. Soc. 2015, 162, H460-H465. (4) Switzer, J. R.; Ethier, A. L.; Hart, E. C.; Flack, K. M.; Rumple, A. C.; Donaldson, J. C.; Bembry, A. T.; Scott, O. M.; Biddinger, E. J.; Talreja, M.; Song, M.-G.; Pollet, P.; Eckert, C. A.; Liotta, C. L. Design, synthesis, and evaluation of nonaqueous silylamines for efficient CO2 capture. ChemSusChem 2014, 7, 299-307.