Functional additives for AlCl3/EMIC ionic liquid electrolyte of rechargeable aluminum batteries: advancements and challenges

Rechargeable aluminum battery is considered as a promising battery system used in energy storage devices, due to its abundant natural resources and high capacity. However, fabrication of this battery working at room temperature didn’t succeed until haloaluminate contained ionic liquids were used as electrolytes. Therefore, electrolytes are expected to have a great effect on performance of rechargeable aluminum battery. For ionic liquid electrolytes, anions play a major role in its electrochemical properties. Anion-effect of haloaluminate contained ionic liquids prepared with different halogenated imidazole salt and AlCl3/imidazolium chloride mole ratio are studied. Electrochemical window is found narrowing with reducibility of halide ion and AlCl3 mole fraction. When used as electrolyte in rechargeable aluminum battery with V2O5 nanowire cathode, AlCl3/[BMIM]Cl ionic liquid with the mole ration of 1.1:1 shows the best performance. The as-assembled cell exhibits a high discharge voltage platform (1 V) and capacity (288 mAh/g) at the first cycle. Concentration of Al2Cl7 - is considered as a key factor in chloroaluminate ionic liquids when used as electrolyte. A slightly corrosion is found on the surface of Al metal foil immerged in acid AlCl3/[BMIM]Cl ionic liquids after 24 h. It means this kind of ionic liquids may help to remove the oxide film on Al metal foil. The oxide film is considered as a mainly hinder to performance of aluminum battery. Performance of battery using ionic liquid with other anions (such as BF4 -, PF6 - and CF3SO3 -) has not been reported, which is probably due to the dense Al2O3 film formed on surface of Al anode. It prevents the oxidation of Al in discharge process of battery and causes a failure. Figure 1

Batteries operating reliably at low temperatures are required to power robotic spacecraft, particularly those that embark on planetary science missions1. For example, Mars surface missions have typical temperature targets of -40 to -60 °C, while surface mission concepts to the outer planets or Ocean Worlds, such as Europa, have targets down to -180 °C. At low temperatures, state-of-the-art lithium-ion batteries suffer from lithium plating and dendritic growth on the graphite anodes, severe capacity loss associated with sluggish ion diffusion in crystalline transition metal cathodes, and kinetic limitations2. 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 in low or ultra-low temperature space missions. Recently, rechargeable aluminum batteries have been developed that use chloroaluminate-containing ionic liquid electrolytes, which may be suitable alternatives to lithium-based batteries when used under extreme temperatures. Aluminum metal is also distinguished by its high energy density, safety, low cost, and sustainability, which are also key properties for space applications.Herein we present, for the first time, results towards rechargeable Al-batteries designed specifically for low-temperature space missions that demonstrate high capacity retention, long-term cyclability, and favorable electrochemical kinetics at low temperatures down to -40 °C. We focus on the development of ionic liquid electrolytes with mixed anion-cation compositions to impart disorder and disrupt crystallization, while pairing them with functionalized graphite and conducting polymer cathodes.3 , 4 The resulting aluminum 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 positive electrode materials and ionic liquid electrolyte mixtures in novel batteries for low-temperature space applications.References Bugga, R. V.; Brandon, E. J., Energy Storage for the Next Generation of Robotic Space Exploration. Electrochemical Society Interface 2020, 29 (1), 59-63.Jones, J.-P.; Smart, M. C.; Krause, F. C.; Bugga, R. V., The Effect of Electrolyte Additives upon Lithium Plating during Low Temperature Charging of Graphite-LiNiCoAlO2 Lithium-Ion Three Electrode Cells. Journal of The Electrochemical Society 2020, 167 (2), 020536.Schoetz, T.; Kurniawan, M.; Stich, M.; Peipmann, R.; Efimov, I.; Ispas, A.; Bund, A.; Ponce de Leon, C.; Ueda, M., Understanding the charge storage mechanism of conductive polymers as hybrid battery-capacitor materials in ionic liquids by in situ atomic force microscopy and electrochemical quartz crystal microbalance studies. Journal of Materials Chemistry A 2018, 6 (36), 17787-17799.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 Applied Energy Materials 2019, 2 (11), 7799-7810.

Rechargeable aluminum battery system is very intriguing due to the following reasons: First of all, aluminum has high capacity due to its trivalency. And aluminum is very cheap since it is the most abundant metal element in earth’s crust.1 As a result, rechargeable aluminum battery can be very promising in large scale energy storage application. One of the main reasons that hinders the development of rechargeable aluminum battery is the lacking of electrolyte that can enable facile deposition and dissolution of aluminum in the anode side. On the other hand, facile electrochemical deposition and dissolution of aluminum can be achieved in room temperature ionic liquid (molten salt) synthesized by mixing aluminum chloride (AlCl3) with organic salts such as 1-butylpyridinium chloride, 1-ethyl-3-methylimidazolium chloride, etc. at a certain ratio.2,3 In a previous research, our group proposed Chevrel phase Mo6S8 as the first conventional intercalation type cathode material.4 The logic of choosing transition metal sulfide instead of transition metal oxide as cathode material for aluminum ion battery is very important. Due to the strong coulombic effect, the energy barrier of multivalent ions transportation in the crystal structure is very high.5 Thus, a softer anionic framework is needed. Sulfide has a much lower electronegativity than oxide, which makes transition metal sulfide a very promising cathode candidate for rechargeable aluminum ion battery. To further validate this assumption, we synthesized nano sized layered TiS2 and cubic Ti2S4 as well as investigated their electrochemical properties as cathode materials for ionic liquid electrolyte based rechargeable aluminum ion battery at both room temperature and 50 °C. We further confirmed the aluminum intercalation in the TiS2 and Ti2S4 crystal structure using ex-situ XRD and XPS. The proposed titanium sulfide cathode materials showed decent reversible capacity and a higher working potential. In order to further understand the mechanism of Al intercalation into Chevrel Phase Mo6S8, high resolution TEM is used to directly visualize the crystal structure of AlxMo6S8 from different discharged and charged stages. Powder XRD Rietveld refinement is also undertaken to provide support from a theoretical simulation perspective. 1. Li, Q.; Bjerrum, N. J. J. Power Sources 2002, 110, 1. 2. Endres, F. ChemPhysChem 2002, 3, 144. 3. Jiang, T.; Chollier Brym, M. J.; Dube, G.; Lasia, A.; Brisard, G. M. Surf. Coat. Technol. 2006, 201, 1. 4. Geng, L.; Lv, G.; Xing, X. Guo, J. 2015, 27, 4926−4929. 5. Rong, Z.; Malik, R.; Canepa, P.; Sai Gautam, G.; Liu, M.; Jain, A. Persson, K.; Ceder, G. Chem. Mater., 2015, 27 (17), pp 6016–6021.

Limited lithium resources in the Earth’s crust alongside the ever-growing demand for rechargeable batteries for portable electronics and electric vehicles (EVs) have led to extensive research on rechargeable battery systems beyond lithium-ion (Li-ion) technology. Among various possible technologies, multivalent-ion batteries offer the potential for cost-effective and safe energy storage devices with high energy density. However, the realization of multivalent-ion batteries depends on the development of electrolytes and cathode materials for these systems. Rechargeable aluminum batteries are promising alternative energy storage devices due to their low cost, abundance of Aluminum, and the potential for three-electron redox reaction leading to higher capacities. Aluminum has a theoretical volumetric capacity of 8040 mAh cm-3 (four times higher than that of lithium). Also, it can be easily handled in open air resulting in an enhanced cell fabrication options and elimination of some of the safety issues associated with lithium and sodium batteries [1]. Various materials have been studied as potential cathodes for Aluminum batteries, however, most suffer from problems such as low capacity, lack of distinct voltage plateaus, and low cycle life with significant capacity decay over 100 cycles. Herein, we are presenting several members of two-dimensional (2D) transition metal carbides (called MXenes) as potential cathode materials for rechargeable Aluminum batteries utilizing a mixture of AlCl3 and [EMIm]Cl as the electrolyte. MXenes are a family of 2D transition metal carbides and/or carbonitrides that are produced by selective removal of the A layer (i.e. Al) from MAX phases (i.e. Ti2AlC), a large family of hexagonal layered ternary carbides and carbonitrides [2, 3]. The MXene based Aluminum batteries work by electrochemical deposition and dissolution of aluminum at the anode and intercalation/de-intercalation of Al3+ ions between the layers of 2D MXene nanosheets. Among different studied MXene materials, Ti2CTx (Tx represents different functional groups such as O, OH, and F on the surface of MXene sheets) and V2CTx showed distinct charge and discharge plateaus with first discharge cycle capacity of as high as 200 mAh/g and 400 mAh/g, respectively at current density of 100 mA/g. The capacity of Ti2CTx dropped to around 60 mAh/g over 50 cycles, however, the V2CTxmaintained a reversible capacity of 200 mAh/g over 50 cycles with a high coulombic efficiency of ~95%. The MXene based Al-ion batteries were cycled in 1.7 V potential window and showed a good rate-capability. Our results show the high potential of MXenes as aluminum battery cathodes and open a new direction in search for high capacity cathode materials for these battery systems. Keywords: 2D, Transition Metal Carbides, MXenes, Aluminum battery

A facile in-situ polymerization of cross-linked Poly(ethyl acrylate)-Based gel polymer electrolytes for rechargeable aluminum batteries

Lacking viable electrolytes is one of the fundamental obstacles preventing the realization of rechargeable aluminum batteries. To date, the only electrolytes which can enable allowing reversible Al deposition-stripping with excellent chemical and electrochemical stability are the chloroaluminate ionic liquids (ILs). However, these kind of ILs are extremely corrosive due to the high concentration of chloride. To demonstrate the importance of the cathode/electrolyte interfacial stability in emerging rechargeable aluminum (Al) batteries, chemical compatibility between vanadium(V) oxide (V2O5), a widely studied cathode material for Al batteries, and the most common chloroaluminate ionic liquid electrolyte are studied. The potential reactions between V2O5 and Lewis acidic species (Al2Cl7 -) and Lewis neutral species (AlCl4 -), respectively, and the resulting electrochemical properties are investigated with electrochemical analysis, spectroscopic characterizations including Raman and nuclear magnetic resonance (NMR) spectroscopy, supported by computational studies using methods based on density functional theory (DFT). Our studies clearly demonstrate that V2O5 reacts to both Al2Cl7 - and AlCl4 -, and the reaction mechanisms are proposed and validated. We also developed new organic electrolyte for Al deposition at room temperature based on AlCl3/g-butyrolactone (GBL) mixture in benzene. It is found that the solubility of AlCl3 in GBL can be significantly enhanced by adding benzene as diluent. Besides, the coordination structure between AlCl3 and GBL would change when the molar ratio of AlCl3/GBL increased above 1:1, which lead to the generation of active species for Al deposition.

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.

Recently, rechargeable aluminum batteries have received much attention due to their low cost, easy operation, and high safety. As the research into rechargeable aluminum batteries with a room-temperature ionic liquid electrolyte is relatively new, research efforts have focused on finding suitable electrode materials. An understanding of the environmental aspects of electrode materials is essential to make informed and conscious decisions in aluminum battery development. The purpose of this study was to evaluate and compare the relative environmental performance of electrode material candidates for rechargeable aluminum batteries with an AlCl3/EMIMCl (1-ethyl-3-methylimidazolium chloride) room-temperature ionic liquid electrolyte. To this end, we used a lifecycle environmental screening framework to evaluate 12 candidate electrode materials. We found that all of the studied materials are associated with one or more drawbacks and therefore do not represent a “silver bullet” for the aluminum battery. Even so, some materials appeared more promising than others did. We also found that aluminum battery technology is likely to face some of the same environmental challenges as Li-ion technology but also offers an opportunity to avoid others. The insights provided here can aid aluminum battery development in an environmentally sustainable direction.

Rechargeable aluminum (Al) batteries are promising technologies for large scale energy storage applications. One of the main challenges hindering the development of rechargeable aluminum batteries is the lacking of understanding of cathode materials. In this study, we investigate a number of transition metal sulfides as the intercalation-type cathode materials for rechargeable Al batteries. Our selection of metal sulfides instead of oxides as Al-ion cathode materials is crucial: Due to the strong coulombic effect, the energy barrier for multivalent ion transport in the oxide crystal structure is very high. Thus, a more polarizable (softer) anionic framework is needed for facile Al-ion intercalation-extraction. The sulfide anion is more polarizable than the oxide anion due to the larger size, making transition metal sulfides promising candidates as Al-ion cathode materials. We hereby demonstrate Chevrel phase molybdenum sulfide (Mo6S8) as the intercalation-type cathode material with aluminum chloride/1-butyl-3-methyimidazolium chloride (AlCl3-[BmIm]Cl) ionic liquid as the electrolyte. Electrochemical characterizations including cyclic voltammetry and galvanostatic charge-discharge demonstrate unambiguous Al intercalation-extraction in the Mo6S8. The X-ray diffraction analysis further indicates the crystal structure change of Mo6S8 after Al intercalation. In addition, we report the synthesis and electrochemical properties of layered TiS2 and cubic Ti2S4 as intercalation-type cathode materials for rechargeable aluminum battery at both room temperature and 50 °C. We further confirmed the aluminum intercalation in the TiS2 and Ti2S4 crystal structure using combined electrochemical analysis and ex-situ XRD. The proposed titanium sulfide cathode materials showed decent reversible capacity and a higher working potential than Mo6S8. Furthermore, our study suggests the low Al ion diffusivity in the titanium sulfide framework induced by strong coulombic intercalation is the key obstacle for fast discharge-charge. This finding could provide guideline for design and synthesis of future titanium sulfides electrode materials.

Introduction The use of fossil fuels and the resulting greenhouse gas emissions, particularly CO2, have caused significant environmental damage. Rechargeable batteries are attracting attention as a storage application for renewable energy, and the emergence of next-generation rechargeable batteries that exceed the energy density of lithium-ion batteries is eagerly awaited. Aluminum is a promising material for batteries due to its cost-effectiveness, chemical stability in air, safety, and ease of handling. The oxidation of Al is a 3-electron reaction, resulting in a high theoretical volumetric capacity of 8046 mAh cm-3. Therefore, rechargeable aluminum batteries (RABs), which use aluminum as the negative electrode, are considered one of next-generation rechargeable batteries.Graphite has attracted considerable attention as the positive electrode active material in RABs due to its low cost, high stability, excellent electrical conductivity, and high redox potential. As for electrolyte, room temperature ionic liquids (RTILs) such as 1-ethyl-3-methylimidazolium chloride (EMImCl) offer several advantages, including high ionic conductivity, low vapor pressure, non-flammability, a wide potential window. It is known that AlCl3, a Lewis acid, forms a stoichiometric complex, AlCl4 -, with Cl-, a Lewis base, and AlCl4 - forms the dinuclear complex, Al2Cl7 -, with excess Cl-. So, the mixing ratio of AlCl3 to EMImCl (AlCl3/EMImCl molar ratio) will have a significant effect on the composition of RTILs, and the reaction kinetics of the graphite positive electrode. In this study, we investigated the effect of AlCl3/EMImCl ratio on the positive electrode properties. Experimental A slurry was prepared by mixing graphite powder and polyvinylidene fluoride (PVDF) as a binder with N-methylpyrrolidone (NMP), followed by ultrasonication. An appropriate amount of prepared slurry was applied to a Mo foil and dried under vacuum conditions to prepare a graphite positive electrode. In a glove box, AlCl3 and EMImCl were mixed in different molar ratios and stirred until a light-yellow liquid was obtained. Polished aluminum foil was used as the negative electrode. Results and Discussion Figure 1 shows Raman spectra of ionic liquids with AlCl3/EMImCl ratios from 1.1 to 1.9. In each spectrum, peaks assigned to AlCl4 - and Al2Cl7 - in addition to EMIm+ were observed. As the AlCl3/EMImCl ratio increased, the intensity of the AlCl4 - peak at 350 cm-1 decreased and that of the Al2Cl7 - peaks at 310 cm-1 and 430 cm-1 increased. The intensity of AlCl4 - and Al2Cl7 - linearly decreased and increased with an increase in the AlCl3/EMImCl ratio, respectively, indicating the reaction, AlCl4 - + AlCl3 → Al2Cl7 -, proceeds quantitatively.The cyclic voltammogram of the graphite electrode suggest that some couples of redox peaks of graphite were observed in a potential range between 0.5 V and 2.5 V. Each redox peak shifted negatively as the AlCl3/EMImCl ratio increased. Galvanostatic charge-discharge tests at 500 mA g-1 between 0.5 and 2.5 V were performed using Al/graphite cells containing electrolytes with different AlCl3/EMImCl ratios. In each AlCl3/EMImCl ratio, discharge curve with two plateaus was observed. The discharge capacity at the higher plateau voltage for the electrolyte with AlCl3/EMImCl = 1.1 was lower than that for the other electrolytes, while the discharge capacity at the lower plateau voltage was similar to each other. Irrespective of AlCl3/EMImCl ratio, discharge capacity hardly decreased over 100 charge-discharge cycles, while coulombic efficiency decreased with increasing the AlCl3/EMImCl ratio. This study compared the electrochemical properties of graphite in different molar ratios of electrolytes, which may help with future research and practical applications of RABs. Figure 1

A reliable gel polymer electrolyte enables stable cycling of rechargeable aluminum batteries in a wide-temperature range

An interface-reconstruction effect for rechargeable aluminum battery in ionic liquid electrolyte to enhance cycling performances

As a promising post-lithium battery, rechargeable aluminum battery has the potential to achieve a three-electron reaction with fully use of metal aluminum. Alternative electrolytes are strongly needed for further development of rechargeable aluminum batteries, because typical AlCl3-contained imidazole-based ionic liquids are moisture sensitive, corrosive, and with low oxidation voltage. In this letter, a kind of noncorrosive and water-stable ionic liquid obtained by mixing 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM]OTF) with the corresponding aluminum salt (Al(OTF)3) is studied. This ionic liquid electrolyte has a high oxidation voltage (3.25 V vs Al3+/Al) and high ionic conductivity, and a good electrochemical performance is also achieved. A new strategy, which first used corrosive AlCl3-based electrolyte to construct a suitable passageway on the Al anode for Al3+, and then use noncorrosive Al(OTF)3-based electrolyte to get stable Al/electrolyte interface, is put forward.

Renewable energy technologies, such as wind power and solar energy, have become great importance in modern society [1]. Unfortunately, the intermittent nature of these energies leads to a huge barrier in energy generation and stabilization in electricity supply [2]. The rechargeable batteries received a lot of attention due to high energy density and satisfactory cycle life. Rechargeable aluminum batteries (RABs) gain a great amount of research interest in recent times due to the low cost of aluminum compared to that of lithium. Aluminum, being the most abundant metal in the earth crust, has three-electron transfer characteristic, high volumetric capacity, and great safety, which make RABs suitable for large-scale energy storage [3]. However, problems like low discharge voltage, high corrosion of electrolyte, serious self-discharge impede the growth and development of RABs. Jiao’s group demonstrated self-discharge phenomena for Ah-level RABs, where self-discharge rate was about 6%/day [4]. Their research only presented the self-discharge phenomena, but not studied the factors affecting self-discharge behavior. Self-discharge is an important issue for practical applications. The self-discharge rate of RABs is much higher than that of commercial lithium-ion batteries, whose self-discharge rate is approximately 5%/month. It is thus necessary to investigate the factors affecting self-discharge properties of RABs. In this work, effects of various types of graphite cathodes on charge-discharge properties and self-discharge behavior in ionic liquid electrolyte are studied. The results demonstrate that the changes in d-spacing and morphology of the graphite are crucial. The phase transformation of the graphite cathode during charging/discharging and self-discharge is examined using in-situ X-ray diffraction (performed at National Synchrotron Radiation Research Centre, Taiwan). The reaction mechanism is discussed in details.

Electrolytes for rechargeable aluminum batteries