Aluminum electrolytes for Al dual-ion batteries

Abstract

In the search for sustainable energy storage systems, aluminum dual-ion batteries have recently attracted considerable attention due to their low cost, safety, high energy density (up to 70 kWh kg−1), energy efficiency (80–90%) and long cycling life (thousands of cycles and potentially more), which are needed attributes for grid-level stationary energy storage. Overall, such batteries are composed of aluminum foil as the anode and various types of carbonaceous and organic substances as the cathode, which are immersed in an aluminum electrolyte that supports efficient and dendrite-free aluminum electroplating/stripping upon cycling. Here, we review current research pursuits and present the limitations of aluminum electrolytes for aluminum dual-ion batteries. Particular emphasis is given to the aluminum plating/stripping mechanism in aluminum electrolytes, and its contribution to the total charge storage electrolyte capacity. To this end, we survey the prospects of these stationary storage systems, emphasizing the practical hurdles of aluminum electrolytes that remain to be addressed.

Historical aspects of the development of Al electrolytes for ADIBs

Although the research on Al electrolytes for ADIBs may appear to be a new subject, this is a misconception. The employment of Al electrolytes in ADIBs based on molten salts was assessed for the first time in the 1970s by Fouletier et al.19 Fouletier’s ADIB was composed of metallic aluminum and graphite as negative and positive electrodes, respectively, which were immersed into molten LiCl/AlCl3 salts (ca. 140 °C). In 1988, Gifford et al.20 extended research on ADIBs towards the use of room-temperature ionic liquids (RTILs) based on imidazolium chemistry (AlCl3:1,2-dimethyl-3-propylimidazolium chloride). In 2015, Al electrolytes received a great deal of attention since the publication of Dai et al.21 on ADIBs employing a metallic Al anode, two synthetic forms of graphite as the cathode (CVD-grown graphitic foam and pyrolytic graphite) and 1-ethyl-methylimidazolium ionic liquid electrolyte (AlCl3:EMIMCl)21. Dai’s ADIB showed a high reversibility over thousands of cycles and graphite cathodic capacities of up to 67 mAh g–1. Those first publications have initiated the exploration of deep eutectic solvents (DESs) as Al electrolytes for ADIBs by Dai et al.22,23, Jiao et al.24,25,26, and others27,28,29,30,31. A schematic illustration of the different classes of Al electrolytes presently used in ADIBs is shown in Fig. 1a. A comparison of the various Al electrolytes with respect to their density, viscosity, ionic conductivity, theoretical charge storage capacity, and cost is given in Table 1.

Working principle of ADIBs and Al electrolytes based on RTILs

The basic configuration of ADIBs comprises a carbonaceous (graphite, zeolite-templated carbon (ZTC), or graphene) or organic positive electrode, chloroaluminate ionic liquid electrolyte and metallic aluminum anode as demonstrated in Fig. 1b. ADIBs operate as an electrochemical energy storage system employing reversible intercalation/insertion of the AlCl4 anion species into the positive electrode upon charge (oxidation). Concomitantly, the electroplating of aluminum occurs at the negative electrode of ADIBs. The working principle of ADIBs can be represented by the following cathodic and anodic half-reactions during charge:

$${\mathrm{On}}\,{\mathrm{the}}\,{\mathrm{negative}}\,{\mathrm{electrode}}:4{\rm{Al}}_2{\rm{Cl}}_7^ - + 3{\rm{e}}^ - \leftrightarrow 7{\rm{Al}}{\rm{Cl}}_4^ - + {\rm{Al}},$$
(1)
$${\mathrm{On}}\,{\mathrm{the}}\,{\mathrm{positive}}\,{\mathrm{electrode}}:x{\rm{C}} + {\rm{Al}}{\rm{Cl}}_4^ - \leftrightarrow {\rm{C}}_x\left( {{\rm{Al}}{\rm{Cl}}_4^ - } \right) + {\rm{e}}^ -,$$
(2)

where C is graphite32, ZTC33, graphene34 or organic active material35,36. Consequently, the mechanism of ADIBs is significantly different from the “rocking-chair” metal-ion batteries. There is no one-directional motion of Al3+ ions from the positive to the negative electrodes. Al species are depleted from the chloroaluminate ionic liquid during the charge of ADIBs and are consumed by both electrodes. The Al electrolytes that are used in ADIBs play a double function: they support the Al plating/stripping process and act as the source of AlCl4 ions being needed for the intercalation/insertion into the positive electrode during charge. Consequently, ADIB term is applied for batteries, were electrolyte acts as a source of two types of ions (i.e., Al2Cl7 and AlCl4) required for electrochemical energy storage. Thus, the composition of the electrolyte is changing upon charge and discharge, contrary to “rocking-chair” Al-ion battery systems, where electrolyte acts exclusively as a transmitter of the ions. The most conventional example of an Al electrolyte is a mixture of aluminum chloride and other chlorides comprising an organic cation, for instance, 1-butyl-3-methylimidazolium chloride (BMIM) and 1-ethyl-3-methylimidazolium chloride (EMIM). As a consequence of the acid–base interactions between AlCl3 (Lewis acid) and Cl- (Lewis base), the salt mixture becomes a liquid at room temperature, forming an RTIL. The latter is composed of AlCl4 anions that are charge-balanced with organic cations. The RTIL with an excess of Lewis acid AlCl3 over Lewis base EMIMCl is comprised of both AlCl4 and Al2Cl7 ions. Importantly, ADIBs are operational only in acidic formulations. Solely Al2Cl7 ions enable the electroplating of aluminum, which therefore, occurs only in chloroaluminate melts with an excess of AlCl337,38,39,40,41,42,43,44,45,46. As a result, the charge storage capacity of the chloroaluminate melt is a function of the concentration of Al2Cl7 ions in the RTIL. Electroplating, and therefore, the charging process stops when no Al2Cl7 ions are left in the ionic liquid, which results in the formation of the neutral melt (AlCl3:EMIMCl = 1). The highest molar ratio (r) between AlCl3 and EMIMCl that forms an RTIL is ca. 2:1. AlCl3 does not dissolve at higher molar ratios. Apart from the ionic melts based on AlCl3/EMIMCl, other RTILs have also been recently reported to be composed of 1-methyl-3-propylimidazolium chloride (MPIMCl)47, benzyltriethylammonium chloride (TEBACl)48, and 1,2-dimethyl-3-propylimidazolium chloroaluminate (DMPIMCl)49.

Figure 2 illustrates the impact of acidity r on the charge storage capacity of the chloroaluminate ionic liquid electrolytes, which can be, in fact, called anolytes. The theoretical gravimetric and volumetric capacities of the ionic liquid Can can be calculated as follows:

$${\mathrm{Gravimetric}}\,C_{{\rm{an}}} = \frac{{Fx\left( {r - 1} \right)}}{{rM_{{\rm{AlCl}}_3} + M_{{\rm{ACl}}}}}({\mathrm{mAh}}\,{\mathrm{g}}^{ - 1}),$$
(3)
$${\mathrm{Volumetric}}\,C_{{\rm{an}}} = \frac{{Fx\left( {r - 1} \right)\rho }}{{rM_{{\rm{AlCl}}_3} + M_{{\rm{ACl}}}}}\,({\mathrm{mAh}}\,{\mathrm{g}}^{ - 1}),$$
(4)

where F = 26.8 × 103 mAh mol−1 (the Faraday constant), $$x = \frac{3}{4}$$ (number of electrons that are used to reduce 1 mol of the Al2Cl7 ions), MAlCl3 is the molar mass of AlCl3 (g mol−1), MACl is the molar mass of the Cl salt (g mol−1), r is the AlCl3:ACl molar ratio, and ρ is density of the chloroaluminate melt (g mL−1). A detailed description of the derivation of Eqs. 3 and 4 can be found in ref. 50.

For instance, the gravimetric charge storage capacities of the AlCl3:EMIMCl ionic liquid are equal to 19 mAh g−1 and 48 mAh g−1 for r = 1.3 and r = 2, accordingly. Notably, these capacities define the overall energy density of ADIBs14,18,50,51,52,53,54,55,56,57. Moreover, it should be pointed out that these theoretical capacities are not always achievable experimentally, i.e., they depend on practically relevant experimental conditions and on whether Al2Cl7 ions can be fully depleted for Al electroplating. To figure out this point, recently, Kravchyk et al.58 assembled the anolyte-limited cell in a three-electrode configuration with a significant excess of graphite cathode (anolyte-limited cell). Upon these measurements (Fig. 3a), in addition to the voltage profile of the full cell, the profiles for both positive and negative electrodes were recorded. It was demonstrated that the voltage profile at the negative electrode (ECE) remained relatively stable during charging for 15 min. Upon further charging, however, the voltage dropped sharply indicating the end of the Al plating process caused by the depletion of Al2Cl7 ions at the negative electrode. Importantly, the voltage profile at the negative electrode (ECE) for the graphite-limited cell was constant during the entire charge, with a small overpotential of <50 mV (Fig. 3b) pointing to the access of Al2Cl7 ions at the negative electrode. Using this approach, Kravchyk et al.58 performed rate capability measurements of anolyte-limited full cells at different current densities ranging from 5 to 20 mA g−1 for the AlCl3:EMIMCl ionic liquid formulations with r = 1.3–2.0 (see Fig. 3c, d). These experiments revealed two main points. First, as expected, the higher capacities of chloroaluminate melts could be obtained only using highly acidic formulations. For instance, the charge storage capacity of the AlCl3:EMIMCl anolyte with r = 1.3 was measured to be ca. 21 mAh g−1 at a current density of 20 mA g−1. In contrast, at r = 2, the capacity was ca. 46 mAh g−1. These results indicate that the highest energy density of the ADIBs can be obtained using chloroaluminate ionic liquids with r = 2, and, therefore, future works on ADIBs should be focused on the most acidic formulations. Second, the applied current density affects the charge storage capacity of the anolyte. This is reflected in the significant deviation in the voltage profiles of the negative electrode at high currents (Fig. 3a). As a result, low charge storage capacities (ca. 10–14% from theoretical values) were obtained at very high current densities of 1 A g−1. These results suggest that the frequent statements regarding the high power density of ADIBs are not fully correct. Specifically, at high current densities, a significant drop in the energy density of ADIBs is foreseen. The latter is caused by the rate capability limitations of both the chloroaluminate ionic liquid anolytes and the cathodes of ADIB. In fact, these observations show that the charge storage capacities of the anolyte significantly deviate from the theoretical value at charge current densities higher than 20 mA g−1.

Apart from the charge storage capacity, the acidity of the chloroaluminate ionic liquid electrolytes strongly influences their ionic conductivity. For instance, as revealed by Ferrera et al.59 for the AlCl3–EMIMCl chloroaluminate ionic liquid, its conductivity gradually decreases from 20 to 10 mS cm−1 at room temperature upon increasing the AlCl3/EMIMCl molar ratio from 1.1 to 1.7, respectively. Furthermore, it was shown by Wang et al.60, that the ionic conductivity increases with decreasing halide ionic radius (from iodide to bromide and chloride anions). Moreover, the acidity variations strongly influence the electrochemical stability window of the electrolyte. Specifically, it has been determined that the oxidation of the AlCl3: 1-ethyl-buthyl-imidazolium ionic liquid with mole ratios of 1, 1.1, 1.5 and 2 takes place at approximately 2.6 V vs. Al3+/Al, while for the 0.8 mole ratio, the oxidation starts at 1.75 V vs. Al3+/Al. The oxidation stability limit is associated with the chlorine evolution at the cathode side41,61. It should be noted that Wang et al.62 assessed the Al3+/Al redox potential in the AlCl3–EMIMCl ionic liquid vs. the standard hydrogen electrode (SHE) using a specially made cell composed of a β-alumina solid-state electrolyte and Na reference electrode. It was stated that plating/stripping of aluminum in AlCl3–EMIMCl ionic liquid occurs at −0.7 V vs. SHE and 2.3 V vs. Li+/Li. This Al3+/Al redox potential is ca. 1 V higher than that observed in aqueous solutions.

Inorganic molten salts

Another type of electrolyte capable of reversible aluminum plating/stripping electrochemistry are low-cost molten salt eutectics. One of the first investigations of aluminum electrodeposition in molten salt electrolytes was undertaken by Del Duca in 197163. This work on AlCl3–NaCl and AlCl3–(LiCl–KCl) mixtures elucidated the kinetics of aluminum electroplating. Gale et al.64 examined the subvalent ion effect during aluminum anodization in molten AlCl3–NaCl. The melting point of a chloroaluminate eutectic mixture is dependent on its precursors and composition. The typical binary AlCl3/NaCl and AlCl3/KCl systems melt at above 108 and 128 °C, respectively65. The addition of a third/fourth salt to the binary electrolyte can further reduce the melting point. For example, a ternary AlCl3/NaCl/KCl electrolyte (molar ratio: 61/26/13) has been reported to melt below 100 °C65, and the AlCl3/LiCl/KCl system with a molar ratio of 59/29/12 turns entirely liquid at approximately 95 °C. Recently, the quaternary AlCl3/NaCl/LiCl/KCl inorganic molten salt with the lowest eutectic temperature of less than 75 °C was also systematically studied as the electrolyte in an ADIB66. Apart from the low cost, among the advantages of inorganic molten salts as electrolytes for ADIBs are their low viscosity and high ionic conductivity that facilitate the kinetics of AlCl4 insertion/deinsertion reactions. Additionally, with respect to the charge storage capacity, the lower molar mass of alkali chlorides yields higher values of gravimetric capacity over imidazolium or pyridinium chlorides at the same acidity (Fig. 2).

Deep eutectic solvents

In the search for a more economical and environmentally sustainable alternative to imidazolium-based RTILs, a new class of ionic liquids, namely, DESs, were recently employed as Al electrolytes for ADIBs43,44,67. They are also known as ionic liquid analogs (ILAs)67. DESs can be defined as a mixture of a strongly Lewis acidic metal halide and an oxygen donor amide ligand, such as urea, acting as a Lewis base. Similar to the AlCl3–EMIMCl system41, AlCl3–urea forms through the exothermic reaction between AlCl3 and urea according to the following equation:

$$2{\rm{AlCl}}_3 + 2\,{\rm{urea}} \to {\rm{AlCl}}_4^ - + [{\rm{AlCl}}_2\left( {{\rm{urea}}} \right)_2]^ +.$$
(5)

The comprehensive characterization of DESs by both Raman22,24,27,68 and NMR22,23,68 spectroscopies have revealed that only AlCl4 are present in the AlCl3–urea DESs at an AlCl3/urea ratio of 1.0 (neutral). However, as the acidity of the melt increases through the addition of AlCl3, the concentration of the Al2Cl7 species gradually increases when compared to the concentration of the AlCl4 species (Fig. 4a). The highest molar ratio (r) between the AlCl3 and urea that forms an ionic liquid is ca. 1.5:122. Importantly, as revealed by Ng et al.27, Al electroplating takes place only in the acidic melts (AlCl3/urea > 1.1). Similar observations have also been reported in other AlCl3–amide systems69. Aiming to identify which species are responsible for Al electrodeposition, Dai et al.22 performed operando Raman spectroscopy measurements of AlCl3–urea DESs (AlCl3/urea = 1.4) during Al deposition. Upon Al plating (0 to −0.5 V vs. Al3+/Al), a large decrease in the intensity of the Al2Cl7 peak (313 cm−1) and a corresponding increase in the AlCl4 peaks (350, 445 cm−1) were observed (Fig. 4b). Upon Al stripping (0–0.5 V vs. Al3+/Al), the exact opposite processes were observed, namely, there was a large intensity increase and decrease in the Al2Cl7 and the AlCl4 peaks, respectively; no changes to any other peaks were detected. According to Dai’s observations, the Al electrodeposition/stripping processes in AlCl3–urea DESs can be described as follows: $$4{\rm{Al}}_2{\rm{Cl}}_7^ - + 3{\rm{e}}^ - \leftrightarrow 7{\rm{Al}}{\rm{Cl}}_4^ - + {\rm{Al}}$$. We note that previously23,26, it had been assumed that the cationic aluminum species $$[{\rm{Al}}{\rm{Cl}}_2\left( {{\rm{area}}} \right)_2]^ +$$ were responsible for the Al electroplating reaction. This conclusion is in line with the work of Chu et al.70, whereby the Al plating pathways were investigated by density functional theory (DFT) calculations (Fig. 4c, d). Considering the difference in the dissociation energy barriers of Al2Cl7- and $$[{\rm{Al}}{\rm{Cl}}_2\left( {{\rm{urea}}} \right)_2]^ +$$ is 0.28 eV, it has been assumed that the dissociation of $$[{\rm{Al}}{\rm{Cl}}_2\left( {{\rm{urea}}} \right)_2]^ +$$ is less kinetically preferred, therefore, favoring the Al plating through the Al2Cl7- reduction. From this perspective, taking into consideration the concentration of Al2Cl7 in the acidic AlCl3–urea DESs, the charge storage capacity equals ca. 21 mAh kg−1 (for AlCl3/urea = 1.4). This value can be derived from Eq. 5 (see Supplementary Note 1) using the concentration of Al2Cl7 ions in AlCl3:Urea DES reported in the ref. 22. For AlCl3:Me–urea and AlCl3:Et-urea anolytes, one can obtain similar capacities of ca. 16 mAh g−1 and 18 mAh g−1, respectively.

In addition to AlCl3–urea DESs, several articles have been published reporting the ability of acetamide (AcAm) to form eutectic mixtures when mixed with AlCl3, yielding the reversible electrodeposition of aluminum28,71. As stated in ref. 28, the AlCl3–AcAm DES comprises the anionic AlCl4 and Al2Cl7 species as well as complexed aluminum cationic species, such as [AlCl2(AcAm)2]+. Another eutectic AlCl3–organic solvent system with a coordination behavior similar to that of AlCl3–amide is the AlCl3–diglyme electrolyte, which also supports reversible Al deposition/dissolution72,73. The active species in this electrolyte are AlCl2(diglyme)2+ cations, which are paired with the AlCl4 anions.

Considering the low cost of DESs and their environmental friendliness, energy storage systems that utilize DES electrolytes will have significant economic and environmental cost advantages over conventional RTILs. However, such systems have a relatively low electrochemical stability window of 2.2 V vs. Al3+/Al in comparison with that of dialkylimidazolium chloride-based RTILs22,23,27. The latter are stable up to 2.45 V vs. Al3+/Al62. Apart from this, the conductivity of the AlCl3–amide systems is rather poor at room temperature (see Table 1)25,43,74. The lower conductivity and the sluggish kinetics of the DES electrolytes have been ascribed to their relatively low concentration of active ionic species as well as strong coordination interactions. Atomistic simulations demonstrated that aluminum ions in the RTILs have a weaker coordination and form Al–Cl complexes with a low stability compared to those in some molecular solvents, and this contributes to the facile ion transport and dissociation75.

Corrosion and current collectors

One of the main practical issues to address for the realization of cell prototypes of ADIBs is the high reactivity and corrosivity of aluminum anolytes. For instance, the coin-type cells composed of stainless steel corrode in chloroaluminate melts, requiring the employment of corrosion-free battery cases. Furthermore, contrary to the LIBs, where the Al foil is considered as an established current collector, the current collector for ADIBs at the positive electrode is still under development. Earth-abundant metals, such as aluminum and iron, are easily oxidized in aluminum anolytes at the high voltages of 4.5–5.25 V vs. Li+/Li being employed during positive electrode operation76,77,78. Therefore, oxidatively stable conductive materials such as tungsten, molybdenum, and glassy carbon are typically utilized in ADIBs79,80. Of note, it has recently been suggested to use titanium nitride (TiN) as a compelling current collector for ADIBs. As reported by Wang et al.62, TiN coated on a stainless steel or flexible polyimide substrate can be fabricated by a low-cost and scalable method, such as magnetron sputtering. The TiN oxidative stability in the AlCl3:EMIMCl ionic liquid is on par with that of W and Mo current collectors. Notably, graphitic electrodes without a current collector were also proposed by Di-Yan Wang et al.81 as an alternative approach to address the oxidation issues. So-called free-standing natural graphite films were fabricated by graphite slurry tape casting on the Cu foil following its etching in an iron chloride (FeCl3) solution.

Moreover, the research on the chemical stability of Al foil on the negative side of the AGDIB in acidic RTILs was performed recently by Tak et al.82,83. It has been demonstrated that Al surface corrodes in chloroaluminate melts. Furthermore, the corrosion enhances significantly upon increasing their acidity. Importantly, the works of Tak et al.82,83, pointed to the fact that additional research efforts should be carried out to mitigate or entirely suppress the reaction of Al with chloroaluminate electrolytes. The quest for non-corrosive electrolyte formulations, therefore, continues.

Outlook

Although significant advances were accomplished recently on Al electrolytes for ADIBs, substantial room remains for the improvement in their gravimetric/volumetric charge storage capacity and the efficiency of Al plating/stripping. We note that ionic melts used in ADIBs are not just electrolytes (ion-conductors), but represent an electrochemically active, capacity- and rate-limiting battery component. In this context, further research should be focused on finding the practical amounts of the electrolyte needed for the operation of ADIBs. Notably, most of the studies presented in the literature have employed a 2–10-fold excess of the electrolyte required to match the charge storage capacity of the positive electrode (cathode-limited cell). Such tests are acceptable for research purposes, but they do not provide correct and practically relevant information on achievable energy/power densities or the cycling stability of ADIBs. It is also apparent that future research should focus on the decrease in the redox potential of the Al plating/stripping being ca. 1 V higher (~−0.7 V vs. SHE) than that in aqueous solutions (−1.66 V vs. SHE). Additionally, we suggest that subsequent work should stress the other issues associated with ADIB technology, one being the incompatibility of most metallic current collectors with the corrosive Al electrolytes. For instance, aluminum and stainless steel slowly oxidize in the AlCl3:EMIMCl ionic liquid when electrochemically polarized up to 2.5 V vs. Al3+/Al. Thus far, only tungsten, molybdenum, glassy carbon, chromium, and titanium nitrides have been identified as electrochemically stable current collectors in such batteries. In this framework, we note that any oxidation stability issues in the current collectors that cause a severe decrease in the coulombic efficiency of ADIBs will have a profound effect on their electrochemical performance, which is almost ignored in scientific publications. Towards this end, we state that all these advancements on Al electrolytes should be achieved at a low cost to keep the overall cost-competitiveness of ADIBs.

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Acknowledgements

This research is part of the activities of SCCER HeE, which is financially supported by Innosuisse - Swiss Innovation Agency.

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K.V.K. conceptualized and wrote the original draft. K.V.K. and M.V.K. discussed and revised the manuscript.

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Correspondence to Kostiantyn V. Kravchyk or Maksym V. Kovalenko.

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Kravchyk, K.V., Kovalenko, M.V. Aluminum electrolytes for Al dual-ion batteries. Commun Chem 3, 120 (2020). https://doi.org/10.1038/s42004-020-00365-2

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