natural graphite battery utilizing urea-based ionic liquid analog

Journal Pre-proof A low-cost rechargeable aluminum/natural graphite battery utilizing urea-based ionic liquid analog Kok Long Ng, Monu Malik, Elizaveta Buch, Tobias Glossmann, Andreas Hintennach, Gisele Azimi PII:

S0013-4686(19)31902-4

DOI:

https://doi.org/10.1016/j.electacta.2019.135031

Reference:

EA 135031

To appear in:

Electrochimica Acta

Received Date: 27 May 2019 Revised Date:

7 October 2019

Accepted Date: 8 October 2019

Please cite this article as: K.L. Ng, M. Malik, E. Buch, T. Glossmann, A. Hintennach, G. Azimi, A low-cost rechargeable aluminum/natural graphite battery utilizing urea-based ionic liquid analog, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.135031. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

A Low-Cost Rechargeable Aluminum/Natural Graphite Battery Utilizing Urea-based Ionic Liquid Analog

Kok Long Nga, Monu Malikb, Elizaveta Buchc, Tobias Glossmannc, Andreas Hintennachd, Gisele Azimia,b,* a

b

Department of Materials Science and Engineering, University of Toronto, 184 College Street, Toronto, Ontario M5S3E4 Canada

Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario M5S3E5 Canada c

Mercedes-Benz Research & Development North America, Inc., 12120 Telegraph Road, Redford, MI 48239, USA

d

Daimler AG (Mercedes-Benz Cars), Group Research, HPC G012-BB, 71034 Boeblingen, Germany *Corresponding Author: [email protected]

1

ABSTRACT The exponentially rising demand for rechargeable energy storage systems along with the market-driven increase of raw materials prices have led to extensive research and development on post lithium-ion battery systems. Among emerging candidates, aluminum-based batteries are particularly appealing due to the abundance of material, low cost, ease of handling in an ambient environment, and high theoretical capacities. To further maximize the benefits of utilizing aluminum, an economical and non-toxic ionic liquid analog, deriving from a mixture of AlCl3 and urea, was employed in an aluminum/natural graphite battery operating at ambient temperature. An average specific capacity of 50 mAh g−1 at 600 mA g−1 (~12 C) with an average Coulombic efficiency of 96% across 1,000 cycles was achieved with ultrasonicated natural graphite flakes. A new electrodeposition mechanism of Al in acidic AlCl3-urea electrolyte is proposed for the first time by taking the contribution of both electroactive cationic (AlCl2·(urea)2+) and anionic (Al2Cl7−) species into consideration. The concentration of Al2Cl7− in the electrolyte is suggested to be the limiting factor to the cell-level capacity of AlCl3-urea/graphite battery systems. The results of this study indicate that the practically attainable cell-level specific energy density of AlCl3-urea/graphite battery systems is around 50-60 Wh kg−1.

2

1.

Introduction Among existing energy storage technologies, electrochemically rechargeable energy storage

and generation systems are the most prominent because of their excellent energy conversion efficiency, outstanding performance, compactness, reliability, and swift on-demand responses [1–3]. In particular, the development of lithium-ion batteries (LIBs) has brought a paradigm shift to the electrochemical storage systems by outperforming conventional lead-acid batteries in several areas, such as energy density, cycle life, efficiency, and maintenance. LIBs have become the most mature energy storage technology used in a wide range of applications ranging from portable devices to transportation to large scale energy grids [4–6]. Despite spectacular commercial penetration in the last 25 years and being the frontrunner among a variety of rechargeable batteries, the future of LIBs is debatable [7]. Considering a possible future shortfall of the raw materials used in LIBs such as nickel (Ni) and cobalt (Co), exceptional research studies and discussions at both academic and industry levels are endeavoring to find an alternative and sustainable battery chemistry using more earthabundant materials than lithium. Among post-lithium rechargeable battery chemistries, systems utilizing sodium (Na)[8,9], magnesium (Mg) [10–12], potassium (K) [13–15], calcium (Ca) [16,17], zinc (Zn) [18,19], and aluminum (Al) [20,21] have demonstrated encouraging results. Among all, batteries employing metallic Al as the anode material are particularly promising. Several desirable properties of Al, such as high abundance, low cost, ease of handling in an ambient environment, high theoretical gravimetric (2,981 mAh g−1) and volumetric capacity (8,046 mAh cm−3), are ideal for energy storage systems. Since the cornerstone report by Dai et al. in 2015 [22], Al batteries utilizing dialkylimidazolium chloride-based chloroaluminate room temperature ionic liquids (RTILs) have drawn tremendous attention to the subject. A specific capacity of 60 mAh g−1 at 4,000 mA g−1 for over 7,500 cycles with a Coulombic efficiency (CE) of ~97% was achieved by employing metallic Al as the anode, acidic aluminum chloride-1-ethyl-3-methylimidazolium chloride (AlCl3-[EMIm]Cl) as the electrolyte, and chemical vapour deposition (CVD) grown 3-dimensional graphene foam as the cathode. Successive studies by the same and other research groups further improved the capacities, cyclability, and rate capability of Al batteries by employing numerous graphitic/carbonaceous materials, such as Ar+ etched and exfoliated graphitic foam [23,24], graphene [25–27], and carbon nanoscrolls [28]. Despite the excellent performance delivered by such systems, drawbacks such as complicated and energy-intensive preparation processes of the cathode materials [22–28], as well as the high cost of dialkylimidazolium chloride, greatly diminish the various benefits of employing metallic Al as the anode materials.

3

Serving as a cost-effective alternative, the most abundant and inexpensive allotropes of carbon, natural graphite (NG) has been the subject of research in several studies [29–32]. In a study by Wang et al. in which an Al/AlCl3-[EMIm]Cl/NG cell was employed, a stable specific capacity of 60 mAh g–1 at 660 mA g−1 across 6,000 cycles with ~98% CE was achieved [30]. To further increase the capacity of Al/NG batteries, it has been demonstrated that with a simple processing technique of NG through ultrasonication, a significant improvement of specific capacity from 95 to 132 mAh g–1 (at 100 mA g–1) can be achieved [32]. On the other hand, in the search for a more economical and environmentally sustainable alternative to dialkylimidazolium chlorides, a new class of ionic liquids, namely ionic liquid analogs (ILAs) also known as deep eutectic solvents (DESs), which can be derived from a mixture of AlCl3 and urea are considered. Similar to AlCl3-[EMIm]Cl system [33], AlCl3-urea relies on exothermic reactions between the complexation of urea with AlCl3 near the eutectic compositions of the mixture to form liquid at room temperature. Considering its large production rate (~190 million tons per annum) [34] and its environmental friendliness as a commercial fertilizer, energy storage systems that utilize urea-based electrolyte will have significant economic and environmental cost advantages over conventional RTILs. Several studies have shown the feasibility of the electrodeposition of aluminum at room temperature in AlCl3-urea ILA [35–37] and a few research groups have since investigated its application in battery systems [31,38,39]. However, only the study by Angell et al. focused on ambient temperature application and the resulting cell displayed a discharge capacity of 73 mAh g–1 at 100 mA g–1 with a limited number of cycles (~ 200 cycles) reported [31]. Inspired by the preceding reports and publications mentioned, herein, we investigated the performance of a rechargeable Al battery utilizing widely available NG flakes as the cathode material, combining with acidic AlCl3-urea ILA (AlCl3/urea = 1.3 by mole) as the electrolyte. Through a simple ultrasonication process of NG flakes for 30 min, the battery exhibited an average specific capacity of 50 mA g–1 at a high current density of 600 mA g–1 (~12 C) with ~96% CE across 1,000 cycles with two distinct discharge voltage plateaus at 2.0-1.8 and 1.6-1.3 V, respectively. In addition, the effect of several parameters, such as electrolyte compositions, sonication time, and active material loadings were systematically investigated. Through electron microscopy and Raman Spectroscopy, we confirmed that ultrasonication was an effective method to reduce the size of NG flakes without significantly introducing crystalline defects to the microstructures, which is an important criterion of graphitic cathode materials in chloroaluminate melt systems. On the anode side, our proposed mechanism suggested that both electroactive Al-containing cations (AlCl2·(urea)2+) and anions (Al2Cl7−) were essential to Al electroplating. The concentration of Al2Cl7− in AlCl3-urea ILA was predicted to be the limiting factor in determining the anodic capacity. Lastly, the theoretical cell-level capacity and specific energy density of Al/natural graphite system utilizing AlCl3-urea ILA were 4

evaluated. The attainable specific energy of such systems was estimated to be in the range of 50-60 Wh kg–1.

2.

Experimental section

2.1 Chemicals Anhydrous aluminum chloride (99.985%), aluminum shots (99.999%), aluminum rod (99.95%), and natural graphite flakes (–10 mesh; 99.9%) were supplied by Alfa Aesar (USA). Aluminum foil (thickness of 50 µm, 99.999%) and molybdenum sheet (thickness of 130 µm, 99.95%) were purchased from Beijing Loyaltarget Tech. Co., Ltd. (China). Glass microfiber separators (Whatman GF/A) was obtained from Sigma-Aldrich Co. (USA). Urea (≥ 99.5%) and sodium alginate were acquired from Bioshop Canada Inc. (Canada) and Landor Trading Co. Ltd. (Canada), respectively.

2.2 Purification methods Prior to usage, Al metals (including Al foil, rod, and shots) were ultrasonicated in anhydrous ethanol to remove surface impurities. Al metals were then dipped in 8 M HNO3 followed by rinsing in distilled water (until pH reached ~7) and acetone prior to transferring into the glovebox for use. Urea was vacuum dried at 100 ºC for 24 h and transferred to the glovebox immediately after discharging from the vacuum oven.

2.3 Preparation of AlCl3-urea electrolyte The AlCl3-urea electrolyte was prepared by slowly mixing urea and anhydrous AlCl3 (with respect to the designated composition) in a glass beaker under constant magnetic stirring in an argonfilled glove box (O2 and H2O < 1 ppm). To prevent decomposition of the electrolyte, the beaker was wrapped in ice gel patches during mixing to regulate the temperature of the mixture. A transparent, yellowish, and viscous liquid was obtained after stirring the mixture overnight at ambient temperature. To remove impurities, such as HCl (formed as a result of residue H2O) and colored organic impurities in the electrolyte, Al shots were added, and the mixture was heated to 60 °C for 30 min and kept under vacuum for 10 min. This heating plus vacuum treatment procedure was repeated twice.

5

2.4 Ultrasonication of natural graphite 1.0 g of pristine natural graphite (PNG) was placed into a 50 mL test tube filled with 20 mL of anhydrous ethanol and ultrasonicated for 10-60 min at a 10-min step using Misonix ultrasonic liquid processor (XL-2000 series) operated at a power output of 15 W. During ultrasonication, the test tube was immersed in an ice bath to minimize the temperature increase of the suspension. The resulting suspension was centrifuged for 1 h to isolate graphite particles from ethanol. Ethanol was then removed, and the remaining solids were dried under vacuum at 60 °C overnight. The resulting graphite is named as sonicated natural graphite. 2.5 Preparation of natural graphite electrodes The binder was prepared by mixing 2.5 ml of distilled water with 0.1 g of sodium alginate in a 20 mL glass vial and the resulting mixture was magnetically stirred overnight at room temperature until a homogeneous viscous mixture was obtained. 0.9 g of either pristine or sonicated NG was added into the sodium alginate binder and was stirred overnight to yield a viscous slurry. The resulting slurry was doctor–bladed onto a piece of Mo tab (1.0 × 5.5 cm; width × length (w × l)) with a known mass and dried at 80

overnight. The area of the active materials was approximately 1.0

2

cm . The loading of active materials was determined by dividing the weight difference between coated- and uncoated Mo with the area of graphite slurry (determined using Image-J software).

2.6 Materials characterization Electrolytes (AlCl3/urea = 1.0-1.5 by mole) were injected into quartz cuvette cells and the spectra were acquired (40-1,540 cm–1) by a Dispersive Raman Microscope (Bruker) using Ar+ laser (532 nm) at a resolution of 0.5 cm–1. For each electrolyte composition, a total of 5 spectra were acquired and an averaged value of the spectra was presented. The spectra of pristine and sonicated NG were acquired using the same microscope. The particle size distributions of pristine and sonicated NG were determined using a particle size analyzer (LA-950, Horiba). The morphologies and elemental mappings were investigated by scanning electron microscope (SEM) (SU 3500, Hitachi) equipped with an energy dispersive spectrometer (EDS). X-ray diffraction (XRD) patterns were obtained through a Rigaku Miniflex 600 X-ray diffractometer with Cu Kα1 radiation (1.5405 Å) in the range of 10–50°. A coaxial insert along with 5 mm NMR tube (Wilmad-LabGlass) is used to prepare the sample for

27

Al nuclear magnetic resonance (NMR) spectra, in which chloroform with 0.05%

tetramethylsilane is used as deuterium. All spectra were acquired at 25 ºC (unless otherwise noted) on an Agilent DD2 spectrometer equipped with a 5 mm HFX probe (Agilent Technologies, Santa Clara, 6

USA). X-ray photoelectron spectroscopy (XPS) analysis was collected on an Escalab250Xi spectrometer with an Mg (Kα) source. As for ex-situ XPS and EDS characterization, Al/NG-30 pouch cells were fully charged/discharged at a current density of 100 mA g–1 for 20 cycles. After that, the cells were transferred to a glove box and were disassembled to extract the cathodes. The extracted cathodes were then washed in 1,2-dichloroethane (DCE, soaked in 3

molecular sieves for days) to remove the

residual AlCl3-urea electrolyte. The as-rinsed cathodes were dried at 80

overnight to remove

residual DCE. Prior to measurements, the cathodes were contained in an air-tight plastic pouch to minimize contamination by reactions with atmospheric moisture and oxygen before XPS and EDS characterization. For XRD measurements, the disassembled cathodes (without rinsing) were attached to a glass slide using double-sided tape. The cathodes were then covered with a piece of Kapton film and were immediately discharged from the glove box for measurements.

2.7 Cyclic voltammetry and electrochemical impedance spectroscopy Cyclic voltammetry (CV) experiments were carried out using a potentiostat in a threeelectrode configuration. Mo tab, pristine, and sonicated NG were used as the working electrodes in separate experiments. Al foil was used as both the auxiliary and the reference electrode in all experiments. The electrochemical cell was assembled in a quartz cuvette cell containing AlCl3/urea of 1.3 by mole in a glovebox and was closed with a PTFE cap. The electrochemical cell was then sealed with PTFE thread seal tape. The scanning voltage range was set from 0.50 to 2.40 V (vs. Al) for Mo tab, pristine, and sonicated NG at 1.0 mV s–1. Electrochemical impedance spectroscopy (EIS) of an Al/NG-30 cell was carried out in a frequency range from 0.1 Hz to 100 kHz at a ±10 mV amplitude versus open-circuit potential.

2.8 Galvanostatic charge-discharge tests The as-prepared cathode and a piece of glass fiber membrane (3.0×3.0 cm; w × l) were placed inside a partially heat-sealed aluminum-laminated film pouch (8.5×5.0 cm; w × l) with two open ends. The partially fabricated pouch assembly was heated at 80

overnight under vacuum and was swiftly

transported to the glovebox right after heating. In the glovebox, a piece of L-shape Al foil (area facing the cathode: 2.5×2.0 cm; w × l) was inserted into the pouch and the assembly was removed from the glovebox. The open end with current collectors was immediately heat-sealed and swiftly readmitted into the glovebox. Approximately 1.5 mL of the electrolyte was injected using a glass pasteur pipette. 7

The cell was then removed from the glovebox and the remaining open end was instantly heat-sealed. The specific charge/discharge currents (mA g–1) and the specific capacities (mAh g–1) throughout this report are referred to the mass of natural graphite (without binder). To study the effects of sonication on the rate capability of the Al/PNG and Al/sonicated NG cells, the current density was varied from 50 to 800 mA g–1 (10 cycles). To investigate the change in the electrolyte composition and the loading of active materials on the rate performance of the Al/NG-30 min cell, the current density was varied from 100 to 1,000 mA g–1 (25 cycles). All cells were charged to 2.20 V and discharged to a cut-off voltage of 1.00 V using a multichannel battery tester (CT-4008, Neware). The aluminum symmetric cell was constructed using Al rods as the working electrode and counter electrode in a home-made PTFE Swagelok cell. The cell was cycled at 0.1 mA cm−2 for 1 h across 100 cycles.

3. Results and discussion 3.1 Speciation in AlCl3-urea ILA and the proposed operation mechanism of Al/AlCl3-urea/NG battery Raman spectroscopy has been widely applied to study chloroaluminate anionic species in RTILs [40,41] and ILAs [42]. Figure 1a presents the Raman spectra obtained for AlCl3/urea: 1.0-1.5 (by mole). With an AlCl3/urea ratio of 1.0 (neutral), only AlCl4– (348 cm–1) was present in the electrolyte. However, as the acidity of the melts increased through the addition of AlCl3, the Al2Cl7– peak at 314 cm–1 gradually intensified relative to AlCl4– peak, indicating an increase in Al2Cl7– concentration at higher AlCl3/urea ratio. It has been demonstrated that mole fractions of Al2Cl7– and AlCl4– can be approximated to the intensity of the strongest bands for the respective species [43]. In predicting the relative concentration of chloroaluminate anions species, previous studies have demonstrated that the ratio of the peak Raman intensity obtained for Al2Cl7–/AlCl4– in chloroaluminate ILs is directly proportional to the relative concentration of Al2Cl7–/AlCl4– (calculated through assumed stoichiometry and mass balance consideration) [44,45]. The coefficient of proportionality, K, is the Raman scattering cross-section ratio between Al2Cl7– and AlCl4–, which indicates the relative light-scattering capacity of the two species. By relating the intensity of the Al2Cl7– and AlCl4– peaks obtained with the K of Al2Cl7–/AlCl4– derived for AlCl3-1-butyl-3methylimidazolium chloride system (K = 0.87) [44], the relative concentration of Al2Cl7–/AlCl4– was estimated to increase from 0.43 to 1.24 for AlCl3/urea = 1.1, and 1.5, respectively (Figure S1a). To further investigate the speciation in the melts, –

–

27

Al NMR spectroscopy was performed and the

+

presence of AlCl4 , Al2Cl7 , AlCl2·(urea)2 , AlCl3·(urea), and AlCl3·(urea)2 (Figure S1b) were confirmed in the AlCl3/urea = 1.1 melt (peaks assigned based on the work by Coleman at al. [42]). Unlike chloroaluminate RTILs in which all speciation in acidic melts is ionic [46], a significant 8

proportion of neutral species (particularly AlCl3·(urea)) was present in the AlCl3/urea = 1.1 melt. As the acidity of the melts increased (Figure S1b), the peaks merged and became less defined (potentially due to dynamic equilibria between various species in the melts), making attempts to deconvolute this broadened signal difficult. Despite this setback, when overall charge neutrality of the system was considered (calculation shown in the Supporting Information, Calculation 1), it was estimated that the relative concentration of AlCl2·(urea)2+ was higher than Al2Cl7− across the acidic compositions evaluated (Figure S1a). These estimated relative concentrations agree reasonably well with the values determined by in-situ IR titration and NMR methods for similar ILA systems [47]. Previous studies [31,39] suggested that in AlCl3/urea ≥ 1.0 (molar ratio), the electrodeposition/stripping of aluminum would likely be dominated by AlCl2·(urea)2+ and subsequently by Al2Cl7− through the following pathways: 2AlCl2·(urea)2+ + 3e− ⇌ Al + AlCl4− + 4(urea)

(1)

4Al2Cl7− + 3e− ⇌ Al + 7AlCl4–

(2)

As presented in the linear sweep voltammogram in Figure S1c, our experimental results indicate that appreciable electrochemical activity is observed only in the acidic melt (for instance, AlCl3/urea = 1.1 by mole) but not in the neutral melt (AlCl3/urea = 1.0). Similar behaviors have also been reported in other AlCl3-amide systems even when a different working electrode (tungsten) was utilized [48]. Since no electrochemical activity is observed in neutral AlCl3-melts, we believe that AlCl2·(urea)2+ alone cannot be responsible for Al deposition as originally proposed [35]. It is known that Al2Cl7– is the only electroactive species responsible for reversible electrodeposition of Al (Eq. 2) due to the more negative reduction potential of AlCl4– (AlCl4– + 3e– ⇌ Al + 4Cl–) relative to that of organic cations in acidic chloroaluminate RTILs [49]. Upon considering that other Al-containing electroactive cations can be responsible for Al deposition [50–53], we propose that the electrodeposition/stripping of Al at the anode in acidic AlCl3/urea melts follows the following mechanism: AlCl2·(urea)2+ + 2Al2Cl7− + 3e− ⇌ Al + 4AlCl4− + 2(urea)

(3)

Essentially, Eq. 3 combines Eq. 1. and 2 by taking both electroactive cationic and anionic species (AlCl2·(urea)2+ and Al2Cl7−) into consideration. As shown in Eq. 3, the insignificant electrochemical activity observed in neutral AlCl3-amide melts during Al electrostripping/plating can be easily explained by the absence of Al2Cl7− in the neutral melt (as evidenced by the Raman spectrum presented in Figure 1a). Such behavior is different from dialkylimidazolium chloride-aluminum chloride RTILs, in which the anionic chloroaluminate

9

species

(AlCl4–

and

Al2Cl7–

;

Eq.

(2))

are

mainly

responsible

for

the

reversible

electrodeposition/stripping of Al during charging and discharging [22]. On

the other hand, when

graphitic

materials

are

used as the

cathode, the

intercalation/deintercalation reactions of AlCl4– into/from graphitic structures is given as [22,30]: Cn + AlCl4− → Cn[AlCl4] + e−

(4)

Hence, the overall reaction mechanism of Al/AlCl3-urea/graphitic material systems is expected to follow: AlCl2·(urea)2+ + 2Al2Cl7− + 3Cn ⇌ Al + 3Cn[AlCl4] + AlCl4− + 2(urea)

(5)

A schematic diagram of the reaction mechanism in an Al/NG cell using AlCl3-urea electrolytes during the discharging process is illustrated in Figure 1b. During discharging, deintercalation of AlCl4– from natural graphite occurs at the cathode and the resulting AlCl4– anions combine with dissolved urea and metallic Al at the anode to produce AlCl2·(urea)2+ and Al2Cl7−. The reverse reaction takes place during charging.

10

Figure 1. (a) Raman spectra of AlCl3/urea = 1.0, 1.1, 1.3, 1.4, and 1.5 ILAs (by mole), (b) Schematic representation of the system during discharging.

3.2 Effects of ultrasonication on structural properties of natural graphite flakes It is known that the introduction of defect to graphitic structure can negatively affect the electrochemical performances of Al batteries, and several studies have since emphasized the importance of low defect intensity towards enhancing the performance of Al batteries [23,25– 27,30,32,54]. Among the techniques utilized for size reduction of graphite, ultrasonication [32,55] is known to produce graphite particles with lower defect intensity relative to conventional techniques such as planetary ball milling [56,57]. As presented in Figure 2a and b, the typical plate-like morphology of pristine NG was preserved even after a sonication time of 30 min. With increasing sonication time above 30 min, a transition towards a more spherical morphology was observed (Figure S2a). Particles size distributions in Figure S2b revealed the occurrence of a bimodal distribution of particles. Such distribution was particularly more evident for sonication time above 30 min. The mean and median particle size of pristine NG reduced from 486 and 473 µm to 140 and 60 µm (after 30-min sonication), and further decreased to 92 and 48 µm after 60-min sonication, respectively (Figure S2b). The characteristic features in the Raman spectra of carbonaceous materials are the G and D peak/bands, which lie at around 1,560 and 1,360 cm–1, respectively [58]. The G peak is associated with the bond stretching of the C-C bond (sp2 hybridization) in both carbon rings and chains of graphitic materials, while the D peak is associated with defect-induced double-resonance Raman scattering processes involving the electronic π-π* transitions [58]. The Raman spectrum of pristine NG exhibits negligible D peak/band intensity, indicating low defect density in the starting material (Figure 2c). As for the sonicated NG flakes, a slight increase in the D-peak intensity was observed, likely because of the increasing proportions of edges as a results of the size decrease of NG (Figure S2a and b). Nevertheless, the intensity ratio of the D band to that of the G band (ID/IG) remained low (ID/IG < 0.22 on average) and relatively invariant even with increasing sonication time. This indicates that sonication is a facile way to reduce the size of NG particles while introducing minimal structural defects to the materials.

11

Figure 2. Secondary electron images of (a) pristine, and (b) 30-min sonicated NG flakes, (c) Raman spectra and the respective ID/IG ratio of pristine and sonicated NG flakes.

3.3 Electrochemical testing To study the electrochemical activity of pristine and sonicated NG in AlCl3/urea = 1.3 (by mole), cyclic voltammetry (CV) of pristine (labeled as PNG), 30- min (NG-30), and 60-min sonicated natural graphite (NG-60) were performed and the results are presented in Figure 3a. Both pristine and sonicated NG demonstrated similar electrochemical behaviors in which oxidation peaks appeared in the voltage range of 1.50 to 2.40 (V vs. Al), while reduction occurred in the range of 0.70 to 2.15 V. The observed oxidation shoulder (~2.00 V) and the reduction peaks (~1.65 and 1.20 V) can be attributed to multistage intercalation/de-intercalation of AlCl4−into/from the graphene layers of NG [59,60]. Theoretical DFT calculations have suggested that prior to triggering a higher-stage intercalation, the intercalation of AlCl4− remains at one single gallery until the maximum occupancy of that particular stage is achieved [60]. If the Coulombic repulsion among intercalated AlCl4− is small compared to the total energy required for the graphitic interlayer-spacing expansion, AlCl4− intercalation will happen within the same gallery prior to triggering intercalation into other unoccupied galleries [60]. The distinct redox peaks that were observed in Figure 3a can be attributed 12

to the subsequent deintercalation of AlCl4− from the higher stage followed by deintercalation from the lower stages. In studying electrochemistry at the anode, we have previously investigated the electroplating/stripping of Al in AlCl3-urea ILA through CV, and the main cathodic and anodic peak were observed at –0.6 to –0.1 V and –0.1 to 0.3 V (at 1 mV s–1), respectively [61]. To further investigate the long-term stability of Al plating/stripping in AlCl3-urea ILA, galvanostatic cycling of a symmetric Al cell in AlCl3/urea = 1.3 (by mole) was performed and the results are given in Figure S3. After the initial cycling/stabilization period, the symmetrical Al cell exhibited a relatively constant and stable hysteresis (±0.22 V) across the time frame of the experiment (Figure S3a). For instance, the apparent Coulombic and energy efficiency were both above 99% at nearly 100 h (the 63rd cycle) of the electrostripping/plating process (Figure S3b). Among the AlCl3/urea molar ratios evaluated, Al/NG batteries employing AlCl3/urea = 1.3 (by mole) electrolyte were determined to deliver the optimum results in terms of rate capability, retained capacity, and Coulombic efficiency (Figure S4a and b). To maintain the CE above ~99%, the charging cut-off voltage of the Al/NG cell was set at 2.20 V because significant reduction in CE was observed at higher voltages (Figure S5a). This observation is attributed to side reactions involving oxidation of the electrolyte at higher potentials, as indicated in the CV curve of Mo against Al (Figure S5b). In addition to the molar ratio of AlCl3/urea ILA, the effects of sonication time on cell performance were investigated (Figure S4c and d). In terms of rate capability performance, the three graphitic materials demonstrated the following order: NG-60 ≈ NG-30 > PNG. Both sonicated NG electrodes can retain about ~64% (~44 mAh g–1 at 600 mA g–1) of their capacity (~69 mAh g–1 at 50 mA g–1), while PNG cathode retained only ~31% (~17 mAh g–1 at 600 mA g–1) of its capacity (~54 mAh g–1 at 50 mA g–1) under the same charge-discharge condition. The diffusion and intercalation/deintercalation of AlCl4– into/from graphite is considered to be facilitated by a combinational effect of particle size reduction coupled with open structure around the edges of sonicated NG (Figure S2a), resulting in enhanced performance of the cells. Interestingly, when the charging/discharging rate was reversed, the capacity and CE obtained by the NG-30 electrode were higher than that of the NG-60 electrode (Figure S4c and d). Given the mean and median particle size of NG-30 is larger than that of NG-60 (Figure S2b), it is hence suggested that the NG-30 cathode with larger average particle size exhibited higher stability upon electrochemical cycling. A separate study by Zhang et al. also reported that the larger-size graphitic cathode demonstrate better capacity retention and cycling performance compared with smaller size graphite and graphene [25]. They attributed the poorer performance of Al batteries utilizing smaller size graphitic cathode to increasing structural defects caused by enrichment of functionalized groups such as carboxyl or hydroxyl groups near the edges. Because of these

13

defects, a higher activation energy is required to overcome the repulsion force between edge defects and AlCl4− during the intercalation process [25]. To evaluate the specific capacity obtained under different graphite loadings (0.5 – 2.0 mg cm– 2

), Al/NG-30 cells with the loading of ~1.0 mg cm–2 were found to perform better in terms of specific

capacity and coulombic efficiency across different rates (Figure S4e and f). Figure 3b presents the rate performance of the Al/NG-30 cell with cathode loading of ~1.1 mg cm–2. At the higher chargedischarge rate, the capacity decreased while CE increased. Even at a high rate of 1,000 mA g–1 (~20 C), the Al/NG-30 cell could still deliver a discharge capacity of ~32 mAh g–1 with ~98% CE. The corresponding voltage profiles at different charge-discharge rates are provided in Figure S6a. The long-term electrochemical cycling performance of the cell is presented in Figure 3c. Prior to charging/discharging at a high rate of 600 mA g–1, initial cycling (~20 cycles) at a lower rate of 100 mA g–1 was necessary. The assembled Al/NG-30 cell delivered an average specific capacity of 50 mAh g–1 over 1,000 cycles at an average CE of 96% (Figure 3c). As can be seen in Fig. 3d, despite suffering a slight drop in specific capacity from ~51 (100th cycle) to ~48 mAh g-1 (500th cycle), the voltage profiles of the cell recorded at the 500th and 1,000th cycle were nearly identical. Furthermore, the two distinct discharge plateaus observed at 1.8-2.0 and 1.3-1.6 V, are consistent with the CV results presented in Figure 3a. To investigate the change in electrochemical behavior of the cathode in the initial cycling period, electrochemical impedance spectroscopy (EIS) analysis of the NG-30 cathode was performed at different cycles and the results are presented in Figure 4a and b. The EIS data were analyzed based on the equivalent circuit inset in Figure 4b. The parameter Rs is the electrolyte resistance, Cdl is the double-layer capacitance between the electrolyte and the current collector. CPE (constant phase element) and Rct represent the capacitance and charge-transfer resistance of electrolyte/NG interface, respectively, and W is the Warburg impedance related to the diffusion of AlCl4– anions into the bulk of the electrode. The parameters of the equivalent circuit corresponding to the impedance plots are provided in Table S1. We found that every Nyquist plot consists of only one semicircle (confirmed by phase angle vs. frequency plot presented in Figure S6a) at high/medium-frequency and a linear section in the low-frequency region. The charge-transfer resistance of uncycled cell was the highest (Rct ≈ 6690 Ω) and decreased remarkably down to 313 Ω after the first cycle, and further decreased to 151 Ω in the 20th cycle (Figure S6b). The significant decrease in charge-transfer resistance upon cycling indicates an activation process of NG electrode in AlCl3-urea ILA, as it allows for more effective permeation of electrolyte and increases ion migration channels into the cathode upon cycling. In addition to the diffusion of AlCl4– in the electrolyte, potential source of AlCl4– can originate from the dissociation of Al2Cl7– (Al2Cl7– + Cl– ⇌ 2AlCl4–) near the cathode surface. 14

However, due to the low concentrations of Cl– present in acidic chloroaluminate melt [62,63], the dissociation of Al2Cl7– to form AlCl4– is considered to have negligible impact on the concentration of AlCl4– near the graphitic cathode. As will be discussed in the next section, the increase in d-spacing of the graphitic structure with cycles is responsible for the necessity of such activation process prior to charge/discharge at higher rates.

Figure 3. Electrochemical performance of the Al/natural graphite cell based on the AlCl3/urea electrolyte at a mole ratio of 1.3, a) Cyclic voltammogram curves of pristine (PNG), 30-min sonicated (NG-30), and 60-min sonicated (NG-60) natural graphite cathodes at a scan rate of 1 mV s−1, (b) Rate capability at different charge/discharge current densities from 100 to 1,000 mA g−1, (c) Long-term stability test of an Al/NG-30 cell at 600 mA g−1 (initial 20 cycles at 100 mA g−1), (d) Chargedischarge voltage profiles at the 100th, 500th, and 1,000th cycle. The specific capacity of all cells (with a loading of 1.0 ± 0.1 mg cm−2) was recorded between charging and discharging cut-off voltages of 2.20 and 1.00 V.

15

Figure 4. (a) Nyquist plots of the Al/NG-30 cell in AlCl3/urea electrolyte after different cycle numbers of charge-discharge processes, (b) Close-up views in the high/medium-frequency region of (a), the inset is the equivalent circuit; Rs and Rct represent the electrolyte resistance, and the charge-transfer resistance, respectively. Cdl is the double layer capacitance between the electrolyte and the current collector, CPE is the constant phase element and W is the Warburg impedance.

3.4 AlCl4− anions intercalation in graphitic materials To investigate the chloroaluminate anions intercalation/deintercalation mechanism, ex-situ measurements employing different techniques were performed and the results are illustrated in Figure 5. As can be seen in XRD spectrum in Figure 5a, the (002) peak of uncycled NG-30 electrode was observed at 2θ = 26.4° (d = 3.37

). When the cell was charged to 2.20 V, the (002) peak disappeared

and two distinct peaks emerged at 2θ = 18.6° (d = 4.76

) and 28.0° (d = 3.18

), suggesting the

intercalation of chloroaluminate anions into the graphitic structure [22,30]. Upon fully discharging to 0.01 V, the two X-ray diffraction peaks returned to the initial peak position (2θ ≈ 26.4°), with a broadened shoulder at higher angle (Figure 5a). This is likely due to the strain and disorderness in the graphitic structure as a result of AlCl4− intercalation-deintercalation, as well as trapping of AlCl4− as a result of partial irreversibility of the anion intercalation/deintercalation process [22,64–66]. This is further evidenced by the distinct initial charging curve as compared with subsequent charge-discharge curves (Figure S6b). The CE of the cell increased from 79% in the first cycle to 88% (in the 10th cycle) and stabilized at around 87% (in the 20th cycle). As presented in the XPS spectra in Figure 5b, the uncharged material demonstrated a typical graphite sp2 hybridization at binding energy around 284.1 eV with a π-π* shake-up feature at ~290.7 eV. Upon charging, a slightly broadened peak at lower binding energy of ~283.5 eV, corresponding to 16

sp3 hybridization of carbon. Evidencing by the intensified Al 2p peak (~73.5 eV) and Cl 2p peak (~197.6 eV) observed in the charged electrode (Figure 5c and d), the results clearly suggested the electrochemical oxidation of the electrode through intercalation of AlCl4– anions in the graphitic structure. In addition, two peaks associated to C-N and C-O bonding were also observed in the charged specimens. This is likely due to the presence of some trapped electrolyte (particularly urea) or partial oxidation of the charged cathode material in the interaction volume studied. In the fully discharged electrode, a large proportion of the C 1s peak retracted to that of the uncharged electrode with a small proportion of graphitic carbon remained in an oxidized form. This can be attributed to some trapped/adsorbed Al and Cl in the graphitic structure (Figure 5c and d), which well correspond to the XRD pattern presented in Figure 5a. Furthermore, elemental mappings of fully charged electrode revealed the uniform distribution of Al and Cl signals throughout the observed NG particle (Figure 5e), thus further confirming AlCl4− intercalation into the graphitic structure during charging. In addition, judging from the absence of nitrogen signal in the spectrum obtained, it is unlikely that the cationic AlCl2·(urea)2+ species is involved in the graphite intercalation reaction. When the cell was fully discharged, a significant reduction of Al and Cl signals (indicating de-intercalation of AlCl4− from the graphitic structure) was observed. The above spectroscopic evidence clearly verifies the intercalation/de-intercalation of chloroaluminate ions into/from the graphitic structures during the charging/discharging process.

17

Figure 5. (a) Ex situ X-ray diffraction patterns, (b) XPS data of the C 1s peak, (c) XPS data of the Al 2p peak, (d) XPS data of the Cl 2p peak of the cathode material in uncharged, fully charged, and fully discharged states. Secondary electron image and the corresponding elemental mapping for C, Al, and Cl, along with the spectrum of the boxed region in (e) fully charged state, and in (f) fully discharged state. These results were based on the 20th cycle of Al/NG-30 cells charged-discharged at a current density of 100 mA g−1. The cut-off voltages of fully charged and fully discharged states were 2.20 and 0.01 V, respectively.

3.5 Estimation of cell-level capacity and energy density of AlCl3-urea/graphite batteries As shown in Figure 1a and Figure S1a, the concentration of ionic species in the system depends on the molar ratio of AlCl3 to urea. Since all ionic species originating from the electrolyte are electrochemically involved in the reactions, the electrolyte is not only as a media for ion transport but also as an active material. In evaluating the performance of Al batteries on a cell level, it has been demonstrated that the theoretical cell capacity and energy density are predominantly limited by the concentration of the electroactive anionic species (particularly, Al2Cl7−) in dialkylimidazolium chloride-based RTILs [32,67]. On the other hand, in AlCl3-amide systems, the contribution from the 18

electroactive cationic (AlCl2·(amide)n+) species to the overall theoretical cell capacity and energy density cannot be neglected. In general, the total cell capacity (QTotal) can be expressed in terms of anode and cathode capacity as follows [68]:  =









     



(6)

where QAnode and Qcathode are the theoretical specific capacities of the cathode and anode materials, respectively. 1/QM is the contribution from other electrochemically inactive cell components (such as separator, current collectors, packaging, etc.) in g mAh−1. Since 1/QM varies significantly with cell design, its contribution to  is neglected for simplification purposes (1/QM ≈ 0). Therefore, Eq. 6 can be simplified to  =

















  

  

=

(7)

Judging from the stoichiometry of the anodic reaction proposed in Eq.3, as well as the relative concentration of AlCl2·(urea)2+ to Al2Cl7− (Figure S1a), it is estimated that  (hence,  ) of AlCl3-urea system is limited by the concentration of Al2Cl7− in the melt. This Al2Cl7−-concentration dependent effect is further evidenced by the experimental results provided in Figure S4a, which shows the cathodic capacity decreases with decreasing AlCl3/urea molar ratio. Owing to a limited thermodynamic database of the speciation equilibria in AlCl3-urea system, an empirical model of QAnode by considering the charge and material balance of ionic species in AlCl3urea system with (AlCl3/urea = 1.0-1.5 molar ratio) is proposed as:  =

  (" #.% & ' ().)# & * " +.,(& -  #+.++& . "/).,/,&) &1 22- 134

(8)

where F = 26801.4 mAh mol−1 (Faraday constant), 5 = 3/2 (number of electrons required to reduce 1 mol of Al2Cl7− in Eq. 3), r is the AlCl3/urea molar ratio, 67- and 68& are the molar

mass of AlCl3 (133.34 g mo1−1) and urea (60.04 g mo1−1), respectively. A detailed derivation of the empirical equation is provided in the Supporting Information (Calculation 2). Given the cathodic reaction of chloroaluminate Al/graphite battery is also a one-electron transfer process (Eq. 4), Qcathode is calculated assuming one AlCl4− intercalation per 6 atoms of carbon. This assumption is made based on similar assumption made in the intercalation of Li+ (Li+ + 6C + e− = LiC6). Based on Eq. 4, 9: can be calculated using Faraday’s law: 19

9: =

  1

= 372 >?ℎ A"

(9)

where 5 = 1/6 (number of electrons required to oxidize 1 mol of C in Eq. 4), and 67 is the molar

mass of carbon (12.011 g mol−1). By taking Eq. 7-9, we evaluated the effects of r on  and the

overall discharge energy density (B , determined using the average discharge voltage CDE. at 100 mA g−1 extracted from the 20th cycle presented in Figure S4a where B =  CDE. ) of AlCl3-

urea/graphite system and the result is presented in Figure 6. The cell-level capacity of the system is relatively invariant of the molar ratio of AlCl3 to urea, particularly at molar ratios larger than 1.1. These results are different from the ones obtained in dialkylimidazolium chloride-AlCl3/graphite systems in which a more remarkable effect of r on the theoretical cell-level capacity was reported [32,67]. The primary reason for such discrepancy can be due to the lack of thermodynamic data of the equilibria between ionic and neutral species in AlCl3-amide ILAs across various compositions [42] since they are required to accurately determine the actual concentration of each ionic species in the melt. Nevertheless, in terms of magnitude, the  and B determined in this study agree reasonably well with the values reported for dialkylimidazolium chloride-AlCl3/graphite systems [32,67]. As indicated in Figure 6, the predicted  and B in the compositional range studied here lie in the range of 20-25 mAh g−1 and 30-45 Wh kg−1 at a graphite capacity of ~70 mAh g−1 (specific capacity obtained in this study at 100 mA g−1; Figure 3b). These values are on the same level of the practical specific energy density of lead-acid (25-55 Wh kg−1) battery, but lower than that of nickel-metal hydride (50-70 Wh kg−1) and lithium-ion batteries (150-210 Wh kg−1) [69]. Increasing graphite intercalation capacity will lead to an increase in overall cell capacity and energy density. However, such effects become less significant for graphite capacities larger than 125 mAh g−1, suggesting the practically attainable specific energy density of aluminum chloride-urea/natural graphite system to be around 50-60 Wh kg−1.

20

Figure 6 Calculated cell-level capacities and energy densities for 1.1-1.5 AlCl3/urea molar ratio.

4. Conclusions In summary, we developed an ambient temperature Al/natural graphite cell using an economic and environmentally friendly ionic liquid analog deriving from a mixture of AlCl3 and urea. Through a series of optimization processes involving electrolyte composition, sonication time, and active material loading, an average specific capacity of 50 mAh g−1 at 600 mA g-1 (~12 C) across 1,000 cycles with ~96% CE was achieved. The cell could also sustain a high current density of 1,000 mA g-1 (~20 C) while delivering an appreciable capacity of ~32 mAh g-1 with ~98% CE. This comprehensive study is the first to report an ambient temperature Al battery system utilizing AlCl3-urea electrolyte across various current densities, in particular at high current densities (600 mA g−1 and above) with long term stability. Previous studies have focused on high temperature systems and low current densities and did not report long term stability. Ultrasonication was found to be an effective technique to reduce the size of natural graphite while maintaining the overall structural integrity and crystallographic quality of graphite particles. At the cathode, the charge-discharge mechanism of Al/NG batteries in AlCl3-urea ILA is similar to other rechargeable batteries, which relies on the intercalation and deintercalation of chloroaluminate anions (AlCl4−) during charging and discharging, respectively.

21

In this study, we also usher in a new insight into the electrodeposition/stripping mechanism of Al in AlCl3-urea system. Based on the experimental results obtained in this study and the results from other AlCl3-amide systems, we indicate that in the contrary to what was suggested in the literature, (AlCl2·(urea)2+) alone cannot be responsible for Al deposition. On the basis of fundamental investigations,

we

propose

that

both

Al2Cl7−

and

AlCl2·(urea)2+

contribute

in

the

electrostripping/deposition of Al. On the basis of the proposed mechanism, the practically attainable cell-level energy density of Al/AlCl3-urea/graphitic material system was calculated to be in the range of 50 to 60 Wh kg−1. The determination of cell-level energy density provides an insight into the application of Al batteries with urea-based electrolyte. Despite the promising results rendered by the above Al/AlCl3-urea/NG cells in terms of rate capabilities and cyclability, there is still considerable room for improvement. Herein, we suggest that future work should focus on improving the electrochemical stability window of AlCl3-urea ILA. Owing to the narrower electrochemical stability window of AlCl3-urea ILA compared with dialkylimidazolium chloride-based RTILs, the charging cut-off voltage of graphitic cathodes is limited to 2.20 V to avoid excessive oxidation of the electrolyte (versus ~2.45 V in AlCl3-[EMIm]Cl systems). An increase in electrochemical stability window of AlCl3-urea ILA enables a higher charging cut-off, which would lead to higher specific capacity without triggering unintended side reactions. Furthermore, the electrical/ionic conductivities of AlCl3-urea ILA remain low as compared with dialkylimidazolium chloride-based RTILs. To achieve a better rate capability, future studies should also focus on improving the conductivities of AlCl3-urea ILA.

Acknowledgements The authors acknowledge the financial support provided by Potent Group (No. 503355), Ontario Centres of Excellence (No. 503760) and Mercedes-Benz Research and Development North America (MBRDNA) (No. 208133). We thank Mr. Nick Di Pede, Mr. Dwight Gomes, Mr. Tony Alonzi, and Mr. Minos Lam for their collaboration throughout this project. We thank Dr. Peter Broderson for assistance with XPS characterization and analysis and Dr. Raiden Acosta for help with XRD. Access to the electron microscopy facility in the Canada Foundation for Innovation (CFI) funded Ontario Centre for the Characterization of Advanced Materials and the Walter Curlook Materials Characterization & Processing Laboratory is acknowledged.

22

Supplemental file description: Additional electrochemical, relative concentration, NMR, SEM/PSA of sonicated natural graphite, galvanostatic charge-discharge of symmetrical Al cell, EIS of the cell used for electrochemical measurements, prediction of the relative concentration of ionic species in acidic AlCl3-urea ILA, and the derivation of  .

23

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Graphical Abstract

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: