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Cite This: ACS Appl. Mater. Interfaces 2017, 9, 38381-38389

Polyacrylonitrile Separator for High-Performance Aluminum Batteries with Improved Interface Stability Giuseppe Antonio Elia,*,† Jean-Baptiste Ducros,‡ Dane Sotta,‡ Virginie Delhorbe,‡ Agnès Brun,‡ Krystan Marquardt,† and Robert Hahn§ †

Research Center of Microperipheric Technologies, Technische Universität Berlin, Gustav-Meyer-Allee 25, D-13355 Berlin, Germany Commissariat à l’Énergie Atomique et aux Énergies Alternatives CEA, LITEN, DEHT, STB, Université Grenoble Alpes, F-38000 Grenoble, France § Fraunhofer-Institut für Zuverlässigkeit und Mikrointegration, Gustav-Meyer-Allee 25, D-13355 Berlin, Germany ‡

S Supporting Information *

ABSTRACT: Herein we report, for the first time, an overall evaluation of commercially available battery separators to be used for aluminum batteries, revealing that most of them are not stable in the highly reactive 1-ethyl-3-methylimidazolium chloride:aluminum trichloride (EMIMCl:AlCl3) electrolyte conventionally employed in rechargeable aluminum batteries. Subsequently, a novel highly stable polyacrylonitrile (PAN) separator obtained by the electrospinning technique for application in high-performance aluminum batteries has been prepared. The developed PAN separator has been fully characterized in terms of morphology, thermal stability, and air permeability, revealing its suitability as a separator for battery applications. Furthermore, extremely good compatibility and improved aluminum interface stability in the highly reactive EMIMCl:AlCl3 electrolyte were discovered. The use of the PAN separator strongly affects the aluminum dissolution/deposition process, leading to a quite homogeneous deposition compared to that of a glass fiber separator. Finally, the applicability of the PAN separator has been demonstrated in aluminum/graphite cells. The electrochemical tests evidence the full compatibility of the PAN separator in aluminum cells. Furthermore, the aluminum/ graphite cells employing the PAN separator are characterized by a slightly higher delivered capacity compared to those employing glass fiber separators, confirming the superior characteristics of the PAN separator as a more reliable separator for the emerging aluminum battery technology. KEYWORDS: aluminum batteries, separator, electrospinning, polyacrylonitrile, PAN, ionic liquid

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INTRODUCTION Electrochemical storage systems are fundamental to the worldwide energy policy. In particular, batteries are considered to be the power source of choice for electric-powered vehicles (EVs) and of strategic importance in the stationary storage required for electricity produced by renewable sources like wind and solar. Due to their excellent performance, lithium ion batteries (LIBs) are the power source of choice for portable electronic devices and are nowadays considered the best option for powering EVs.1,2 However, the increased lithium demand and its rising price trigger concerns and debate on the longterm feasibility and cost impact of utilizing lithium-based batteries as the main power source for electromobility.3 Accordingly, in recent years, an increased interest has grown toward recycling processes for LIBs4 and the development of alternative electrochemical storage systems employing more abundant elements, such as sodium,5−9 potassium,10−12 calcium,13,14 magnesium,15,16 and aluminum.17,18 The use of such elements as raw materials for cell components is expected to reduce the cost of the electrochemical storage systems and © 2017 American Chemical Society

enable long-term sustainability. Among them, aluminum represents an interesting alternative due to its extremely high volumetric capacity of 8040 mAh cm−3, which is 4 times higher than that of lithium, and its favorable gravimetric capacity of 2980 mAh g−1. Furthermore, aluminum is the third most abundant element in the earth’s crust and can be handled in ambient atmosphere, leading to enormous advantages for the cell manufacture. Recently, several aluminum battery configurations characterized by extremely interesting performances have been reported; in particular, those involving an anion intercalation process18−29 are considered suitable for practical applications due to their long cycle life and remarkable rate capability. Recently, Jiao et al.30 reported an industrial scale prototype of an aluminum graphite battery demonstrating the practical applicability of such an aluminum/graphite battery. In addition to the graphite-based cathodes, several other materials Received: June 29, 2017 Accepted: October 18, 2017 Published: October 18, 2017 38381

DOI: 10.1021/acsami.7b09378 ACS Appl. Mater. Interfaces 2017, 9, 38381−38389

Research Article

ACS Applied Materials & Interfaces that intercalate trivalent Al3+ ions,23,31−33 or involving electrochemical conversion reactions,34−37 appear interesting for the realization of aluminum batteries. One of the main limitations for practical applications of aluminum batteries is associated with the extremely high reactivity and corrosivity of the electrolyte conventionally employed in such systems.17 The most commonly used 1-ethyl-3-methylimidazolium chloride:aluminum trichloride electrolyte (EMIMCl:AlCl3)17−29 is characterized by high reactivity. In fact, stainless steel and even titanium corrode33,38 in that environment, and the conventional polyvinylidene difluoride (PVDF) binder dramatically reacts with this electrolyte;39 thus, the electrolyte strongly limits the practical development of such systems. New lessreactive electrolyte compositions can help to alleviate the issue.22,40−44 Unfortunately, a certain reactivity of the electrolyte is required to enable the reversible electrochemical process of aluminum dissolution and deposition, as well as to consume the Al2O3 native passivation layer on the starting aluminum metal’s surface. For that reason, any suitable electrolyte for aluminum batteries must always have a certain corrosivity and reactivity. In order to forge ahead with the development of aluminum batteries, it will be necessary to design and select proper cell components. With respect to separators, fundamental component of a battery cell, no investigation has been performed so far, to evaluate which one is suitable for application in aluminum battery cells. The electrochemical tests reported in the literature have been carried out mainly with glass fiber separators that, due to their high thickness and limited mechanical properties, cannot be used for the realization of practical cells.18−29,31−37 In this paper, we report an extensive stability screening of the most widely used commercial separators for battery application in the EMIMCl:AlCl3 electrolyte. Our investigation evidenced that the widely used commercial separators are characterized by a limited stability in the EMIMCl:AlCl3 electrolyte due to the reactivity of the polymeric materials used for their preparation, i.e., polypropylene (PP), cellulose, polyethylene (PE), poly(vinyl alcohol) (PVA), or polyimide. On the contrary, our results indicate that polyacrylonitrile (PAN) is stable in the EMIMCl:AlCl3 electrolyte and, hence, very suitable as a separator for the realization of aluminum battery cells.45 To function as a high-quality separator, some important parameters need to be fulfilled: (i) homogeneity in terms of porosity and thickness are important to avoid inhomogeneous cycling of the battery (overheating, shorting, dendrites, etc.); (ii) the electrolyte wettability must be as high as possible; (iii) it should also be characterized by a good thermal stability (at least 90 °C); (iv) its mechanical strength must be high enough to withstand industrial-scale processing; (v) its thickness must be limited to 25−30 μm to avoid loss of energy density in the whole battery; and (vi) it should be dimensionally stable for easy handling.46 Accordingly, an easily scalable electrospinning process for the preparation of PAN separators has been developed. Electrospinning has been widely used for the preparation of membranes from a lot of different polymers synthetic, natural, biodegradable, or nondegradableor even their blends. This preparation technique is well-established for the preparation of polymer separators for advanced lithium batteries.47,48 Furthermore, thanks to the versatility of the process, it is also used for the synthesis of advanced anode materials for lithium ion battery applications.49,50 It uses electrostatic forces to generate very fine polymer fibers. The generated fibers are then deposited on the collector and a

nonwoven mat membrane, generally characterized by high porosity (60−80%) and a large pore size (20−50 μm), is obtained.51−55 The obtained PAN membrane characteristics, i.e., thickness, porosity, and bendability, are fully in line with the requirements of commercial separators. Furthermore, the PAN separator has been fully electrochemically characterized for application in aluminum batteries, in terms of interface stability against aluminum metal, suitability to favor stripping/ deposition, as well as suitability to operate as a separator in aluminum/graphite cells. The obtained results clearly evidence the excellent compatibility of the developed PAN separator with the aluminum battery electrolyte, its superior interface stability against aluminum, and its suitability for the realization of aluminum battery prototypes. The improved electrochemical performances can be ascribed to the influence of the PAN separator on the aluminum dissolution/deposition process, resulting in extremely homogeneous and efficient depositions. Furthermore, thanks to the improved interface stability, the aluminum/graphite cycling tests evidence that the cells employing the PAN separator can deliver a slightly higher capacity with respect to those employing the conventional glass fiber separator, indicating the superior electrochemical proprieties of the batteries using PAN as a separator.

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EXPERIMENTAL SECTION

The electrolyte 1-ethyl-3-methylimidazolium chloride:aluminum trichloride EMIMCl:AlCl3 in a 1:1.5 mol ratio has been provided by Solvionic; the water content of the electrolyte is lower than 100 ppm. The screening of the membrane’s chemical stability with different compositions has been performed employing the following separators: #1 = polypropylene (PP) monolayer, #2 = glass fibers, #3 = cellulose/ polyacrylonitrile (PAN) mixed fibers, #4 = polyethylene/polypropylene (PE/PP) + poly(vinyl alcohol) (PVA) micro- and nanofibers, #5 = PP (open structure, micro- and nanofibers), #6 = polyimide microfibers. The chemical stability of the separators has been evaluated in an argon-filled glovebox (with water and oxygen contents lower than 0.1 ppm). Disk samples 14 mm in diameter have been fully immersed in 1 mL of EMICl:AlCl3 electrolyte, in polytetrafluoroethylene (PTFE) containers. After 7 days, their thickness and mass loss have been assessed. Furthermore, a simple visual observation showed that most of the membranes were deteriorated [Table S1, Supporting Information (SI)]. The electrospinning process has been performed with a Fluidnatek LE-10 (BioInicia). Polyacrylonitrile (PAN; Mw = 150 000 g mol−1, Sigma-Aldrich) and dimethylformamide (DMF; anhydrous, 99.8%, Sigma-Aldrich) were used to prepare the polymer solution. First, PAN was dissolved in DMF with different concentrations, namely, 5 and 10 wt %, to obtain the polymer solution. Complete dissolution was obtained at T = 60 °C during 2 days in a closed vessel. The viscosity of the 5% solution was measured to be around 0.05 Pa s at T = 25 °C, while the viscosity of the 10% solution was measured to be around 0.6 Pa s at T = 25 °C. For both compositions, the deposition process was performed at room temperature (≈ 23 °C), setting the high voltage to 25 kV, the distance emitter−collector to 15 cm, and the rotational speed to 200 rpm. Due to the different viscosity values, different injection flow rates have been used for the two solutions, i.e., a flow rate of 3 and 10 mL h−1 for the 5% PAN and the 10% PAN solution, respectively. The membrane was deposited on an Al foil (dimensions = 30 × 20 cm) and stuck onto the cylindrical collector. As the injection flow rate decreased, the duration of the electrospinning deposition increased because the volume of polymer was around 6 mL in both case. Consequently, it took 2 h for complete deposition with 5% PAN solution and less than 40 min with 10% PAN solution. The thicknesses of the membranes were measured by the Lhomargy setup. It was respectively around 10 ± 3 and 30 ± 5 μm for the membranes prepared from 5% and 10% PAN solution in DMF. The morphology of the obtained PAN separator has been investigated by scanning 38382


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the separator. Also, the #6 polyimide separator is characterized by a mass loss of 7.4%, indicating that it is also not completely stable in this electrolyte. Furthermore, #3 and #6 do not show any relevant optical changes in the structure stability of the separators. Finally, the glass fiber separator (#2) reveals only a minor mass loss of 1.2%. Besides the negligible mass loss, no structural modification of the morphology of the separator can be evidenced optically, indicating that this separator can be considered suitable for the realization of aluminum batteries, as already demonstrated by tens of papers employing it as a separator for testing aluminum batteries.18−29,31−37 Unfortunately, even if the glass fiber separator can be considered suitable for the realization of lab-scale prototypes, due to its elevated thickness and limited bendability (or processability), it cannot be considered an appropriate candidate for the realization of commercial aluminum battery prototypes. Hence, due to its exceptional stability in the EMIMCl:AlCl3 electrolyte, PAN has been selected for the realization of a battery separator. The separator has been prepared by electrospinning. A scheme of the preparation process is presented in Figure 1. A syringe linked to a metallic needle is

electron microscopy (SEM) employing a Zeiss Leo 1530. The thermal shrinkage of the separator has been determined on a 16 cm2 separator sheet (Figure S3, SI) that was placed in an oven at 90 and 150 °C in air for 1 h. The air permeability of the separator has been measured using a Gurley setup. The mechanical strength of the separator has been measured using a universal tensile machine from Shimadzu (AGX plus series). For comparison, the same measurements have been performed on PP monolayer and PP−PE−PP trilayer commercial separators since they are the most widely used separators for commercial lithium ion batteries. For that reason, those separators have been selected for benchmarking. The electrochemical measurements were performed using Teflon Swagelok type T cells.29 All potentials quoted in this paper refer to the quasi-reference Al/Al3+ electrode. The cycling stability of the aluminum metal was evaluated by continuous stripping/deposition tests on symmetrical Al/Al cells (Al 99.99%, Alfa Aesar) employing the PAN separator soaked in the electrolyte, with a current of 0.1 mA cm−2 and a stripping/deposition time of 1 h, by means of a Maccor 4000 battery test system. The morphology of the aluminum metal, upon 50 stripping/deposition cycles, has been investigated by employing a Zeiss Leo 1530 scanning electron microscope. The interface stability of the Al/electrolyte interface employing the PAN separator has been evaluated by impedance measurements (using Solartron PARSTAT MC potentiostat) in the frequency range from 75 kHz to 10 mHz at 10 mV sinusoidal amplitude, with a symmetrical Al/Al cell at different storage times. For comparison, the same tests have been performed by employing a conventional glass fiber separator (thickness 270 μm). The electrochemical performance of the PAN separator in the aluminum battery has been evaluated with pyrolytic graphite (PG) with a thickness of 25 μm and a loading of 4.71 mg cm−2 as the cathode material.18,29 The cycling tests of Al/EMIMCl:AlCl3(PAN)/ PG cells were carried out by applying increasing specific currents (from 25 to 200 mA g−1) in the voltage range 0.4−2.4 V. For the longterm cycling test, current rates of 25 mA g−1 for the first five cycles and of 75 mA g−1 for the subsequent cycles were used. The impedance measurements on the cells were performed with the same parameters as described above. For comparison, the cycling tests performed with increasing specific currents have been carried out also on Al/ EMIMCl:AlCl3/PG cells employing a glass fiber separator. All the electrochemical measurements were carried out at 25 °C in a thermostatic climatic chamber (with a possible deviation of ±1 °C).

Figure 1. Scheme of the preparation of the PAN separator.

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connected to a high-voltage power supply and a grounded metallic collector (in our case, a rotating cylindrical metallic collector) is placed at a specific distance. The applied voltage forces the fibers expelled by the metallic needle to deposit on the metal collector. The rotating metal cylinder collects the fibers, leading to the formation of the membrane. Figure S2a,b (SI) shows the photographic images of the membrane obtained employing the 5 wt % PAN solution (Figure S2a) and the 10 wt % PAN solution (Figure S2b); in both cases, 30 × 20 cm homogeneous membranes were obtained. The thicknesses of the membranes prepared from 5 and 10 wt % PAN solutions in DMF were around 10 ± 3 and 30 ± 5 μm, respectively. Figure 2a,b shows the SEM images of the separators prepared with the 5 wt % (Figure 2a) and the 10 wt % (Figure 2b) PAN solutions. As expected, both membranes were constituted of nonwoven fibers, and the average diameters of the fibers were ∼300 and ∼500 nm, respectively, for the 5 and 10 wt % PAN membranes. While the membrane prepared with 10 wt % PAN solution shows a very homogeneous fiber diameter with less agglomeration and negligible formation of spherical particles, the membrane prepared with 5 wt % PAN solution shows a relatively high inhomogeneity in the diameter of the fibers, a large number of fiber agglomerates, and the formation of spherical particles. Generally, more homogeneous fiber diameter and less polymer agglomeration are often observed in electrospun membranes prepared with concen-

RESULTS AND DISCUSSION Table S1 (SI) reports the comparison of the thickness and of the chemical stability against EMIMCl:AlCl3 electrolyte of some of the available commercial separators for battery applications. The measurements evidence that the investigated separators, representative of those most conventionally used for the realization of commercial batteries as well as for research purpose, are not compatible with the highly reactive electrolyte used in aluminum batteries. In particular, those made of pure PP#1 and #5are characterized by an extreme instability in the aluminum battery environment, as revealed by the photographic image reported in Table S1 (SI). A large amount of polymer dissolution was observed in the electrolyte, and mass loss was as high as 20 and 50%, respectively, for #1 and #5. The open structure of #5 probably offered a larger surface area of contact between the PP and the electrolyte than #1, as observed in the SEM pictures (see Figure S1, SI). On the other hand, separator #4, composed of a mixture of PVA and PP/PE, did not show any mass loss and kept its dimensions after aging in EMIMCl:AlCl3. Its morphology and/or the presence of PVA probably prevents polyolefin from dissolution in the electrolyte, but the color change after exposition to the electrolyte solution clearly indicates the instability of the separator. Separator #3 (cellulose/PAN) is characterized by a mass loss of 11%, most likely due to the decomposition of the cellulose component of 38383


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Figure 2. SEM pictures of PAN electrospun membranes prepared from 5 wt % (a) and 10 wt % (b) PAN solutions in DMF.

Figure 3. Electrochemical impedance spectroscopy (EIS) Nyquist plots of a symmetrical Al/EMIMCl:AlCl3/Al cell at different storage times, employing (a) the glass fiber separator and (b) the PAN separator. (c) Voltage vs time signature of the stripping/deposition measurement performed on a symmetrical Al/EMIMCl:AlCl3/Al cell, employing the glass fiber separator (red curve) and the PAN separator (blue curve).

trated polymer solutions.56 Due to its inhomogeneity and the large number of agglomerates, the membrane obtained with the 5 wt % PAN solution was not considered suitable for testing in electrochemical cells. On the contrary, the membrane obtained with the 10 wt % PAN solution, which has a very homogeneous fiber diameter with less agglomeration, can, indeed, be suitable as a separator for the aluminum batteries. The thermal stability of the separator has been evaluated by a shrinkage test. Figure S3 (SI) shows the photographic image of a 16 cm2 square PAN membranes before (Figure S3a) and after (Figure S3b) heat treatment at 150 °C during 1 h under air. The shrinkage of the membrane was calculated using the formula 1: shrinkage (%) = 100 ×

(ST − Sinit) Sinit

membranes. Table S3 (SI) shows the Gurley values obtained for different membranes (#1−#3) and compared them to that of the electrospun PAN membrane. The Gurley value corresponds to the time required for a specified amount of air (here 100 mL) to pass through a specified area of membrane, using a specified pressure. A higher Gurley value generally means that the membrane possesses a higher resistivity.57 The low Gurley value obtained for the electrospun PAN membrane indicates that when it is used as a separator and soaked with an electrolyte, it is not going to significantly increase the resistance of the “pure” electrolyte. Finally, the mechanical strength of the separator has been evaluated to be in the range of 4.5 N/mm2. This value indicates that, most likely, the membrane will not be strong enough to withstand the tension of the winding operation during industrial-scale battery assembly. Further experiments are currently running in our laboratories to improve the membrane’s mechanical strength. Nevertheless, the obtained membrane can fulfill most of the requirements needed for commercial separators. Accordingly, the PAN separator has been further characterized for application in aluminum batteries. Figure 3 reports the Nyquist plots of the impedance spectra performed on symmetrical Al/EMIMCl:AlCl3/Al cells employing the glass fiber separator (Figure 3a) and the PAN separator

(1)

where Sinit and ST represent, respectively, the initial surface and the surface after heat treatment. The values of the shrinkage of the PAN membranes in comparison with commercial polyolefin (PP monolayer and PP−PE−PP trilayer) separators are presented in Table S2 (SI). Even if acceptable, the shrinkage of the electrospun PAN membranes at 90 °C is slightly higher with respect to that of the polyolefin membranes. At a higher temperature (150 °C), the shrinkage of the membranes did not increase for either the reference or the electrospun PAN 38384


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Figure 4. SEM images of the pristine aluminum anode (left) and of the aluminum anode subjected to 50 stripping/deposition cycles employing the glass fiber separator (middle) or the electrospun PAN separator (right).

Figure 5. Comparison of the steady-state voltage signature of the Al/EMIMCl:AlCl3/PG cell at (a) 25 mA g−1 and (b) 100 mA g−1. (c) Cycling behavior with the Coulombic efficiency of the Al/EMIMCl:AlCl3/PG cell galvanostatically measured at increasing currents, i.e., 25, 50, 75, 100, and 200 mA g−1.

better characteristics of the PAN separator compared to those of the glass fiber. The reason for the higher stability of the PAN separator with respect to the glass fiber is not completely clear; most likely, the phenomena can be ascribed to an interaction between the PAN polymer and the EMIMCl:AlCl3 electrolyte that reduces the reactivity at the electrode/electrolyte interface, leading to an improved stability. In order to characterize the stability of the separator under dynamic conditions, an aluminum stripping/deposition measurement has been performed. Figure 3c shows the comparison of the stripping/ deposition measurement employing a current of 0.1 mA cm−2 and a stripping/deposition time of 1 h on a symmetrical Al/ EMIMCl:AlCl3/Al cell, using the PAN separator (blue line) and, for comparison, the glass fiber separator (red line). The cell employing the PAN separator reveals a lower voltage polarization value with respect to the cell employing a conventional glass fiber separator, indicating that a more

(Figure 3b), upon different storage times. For both cells, a relatively high decrease of the interface resistance is evidenced during the first 24 h of storage. This behavior can be, most likely, associated with the dissolution of the Al2O3 passivation layer by the electrolyte solution. After the first day of measurement, the interface resistance of the cell employing the glass fiber separator decreased to about 5 kΩ, following the cell interface resistance values, which evidence a slight oscillation over time, increasing to about 6−7 kΩ to then decreasing to 5 kΩ. This behavior indicates processes occurring at the aluminum/electrolyte interface; the nature and the mechanism of these processes are not clear, and further studies are needed to clarify them. On the contrary, the cell employing the PAN separator shows a lower interface resistance of about 3 kΩ after the first day of measurement; this value remained stable over time with a negligible oscillation. The lower interface stability and the relatively higher stability indicate the 38385


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with the glass fiber separator. Galvanostatic cycling was performed using a current density of 25 mA g−1. The measurements, performed to have a statistical evaluation on the delivered capacity of the two systems, clearly confirm that, on average, the cells employing the PAN separator deliver a slightly higher capacity with respect to those employing the glass fiber separator. The stability of the PAN separator has been further evaluated at a higher temperature, namely, 50 °C. Figure S5a (SI) shows the cycling behavior of the Al/ EMIMCl:AlCl3/PG cell employing the PAN separator; the cell was galvanostatically cycled at 100 mA g−1 for the first five cycles and at 200 mA g−1 for the subsequent cycles. The obtained good electrochemical results clearly demonstrate the stability of the investigated separator at moderate operation temperatures. Moreover, from the voltage profiles shown in Figure S5b (SI) (current density of 200 mA g−1), the performance of the PAN-based Al/EMIMCl:AlCl3/PG cell at 50 °C (red line) is far better than at 25 °C (blue line); this behavior can be substantiated by the fact that increased temperature leads to lower cell polarization, better kinetics, and hence higher delivered capacity. Finally, Figure 6a shows the long-term cycling behavior of the Al/EMIMCl:AlCl3/graphite cells employing the PAN separator; the cell was galvanostatically cycled at a current rate of 25 mA g−1 for the first five cycles and 75 mA g−1 for the subsequent cycles. The measurement confirms the excellent electrochemical stability of the Al/EMIMCl:AlCl3/graphite cells employing the PAN separator, as no capacity decay can be noticed until the 300th cycle. The excellent cyclability of the system is further demonstrated by the perfect overlapping of the voltage profiles (see Figure 6b). The interface stability of the cell has been evaluated by measuring the cell impedance after cycling. Figure 6c shows the Nyquist plots of the Al/ EMIMCl:AlCl3(PAN)/graphite cell at different cycles. The measurement evidences a huge decrease of the interface resistance in the initial cycles of the tests; this, most likely, can be associated with the removal of the Al2O3 passivation layer at the anode side as well as to a structural reorganization of the graphite cathode upon cycling, as reported in a previous paper.29

efficient stripping/deposition process takes place in the cell employing the PAN separator compared to the cell employing the conventional glass fiber separator; this truly agrees with the electrochemical behavior presented in Figure 3a,b. Figure 4 shows the SEM images of the pristine aluminum anode (left image) and of the aluminum anode subjected to 50 stripping/deposition cycles employing the glass fiber separator (middle image) or the electrospun PAN separator (right image). The picture of the aluminum cycled using the glass fiber separator evidences an increase of the roughness of the aluminum surface; on the contrary, the surface of the aluminum cycled employing the electrospun PAN separator appears smooth and flat. The measurement evidences that the PAN separator strongly affects the aluminum dissolution/deposition process, leading to a more homogeneous deposition with respect to the glass fiber separator. The improved dissolution/ deposition process positively affects the electrochemical performances of the aluminum cell, as evidenced by the electrochemical measurements (Figure 3a−c). In order to characterize the electrochemical behavior of the electrospun PAN separator in an aluminum battery, aluminum/graphite cells have been assembled with pyrolytic graphite as the cathode material.29 Figure 5a,b shows the voltage signature of the galvanostatic cycling test performed on the Al/EMIMCl:AlCl3/graphite cell employing the PAN separator (in blue) and the glass fiber separator (in red) at currents of 25 mA g−1 (Figure 5a) and 100 mA g−1 (Figure 5b). The graphs reveal the expected multiplateau voltage profile characteristic of the multistage anion intercalation process in between the graphite layers.29 The measurement evidences that the cell employing the electrospun PAN separator delivers a slightly higher capacity; this is in line with the electrochemical results presented in Figure 3. Figure 5c shows the evolution of discharge capacity versus cycle number (cycling trend) at increasing currents (25, 50, 75, 100, and 200 mA g−1) for the Al/EMIMCl:AlCl3/ graphite cells employing the PAN separator (in blue) and the glass fiber separator (in red). It is obvious that the cell employing the PAN separator is characterized by a slightly higher capacity compared to the one employing the glass fiber separator; this is also in agreement with the electrochemical behavior shown in Figure 3. At a higher current density of 200 mA g−1, the two cells show the same delivered capacity, indicating that the improvement of the cycling behavior employing the PAN separator is effective only at low/medium current regimes. After the high current density (200 mA g−1), cycling was continued at a low current density (25 mA g−1) for 10 additional cycles (cycle 50−60). Both PAN and glass fiber separators showed good capacity retention. The specific capacity was respectively 73.5 and 68.4 mAh g−1 for PAN and glass fiber systems, confirming the higher electrochemical performance of the Al battery using PAN instead of glass fiber. The coulumbic efficiency of the Al/EMIMCl:AlCl3/graphite cell is in the range of 95% if cycled at relatively low current value, i.e., 25 mA g−1. The slightly inefficiency is most likely associated with the decomposition of the electrolyte;17,58 in fact, the oxidation of the electrolyte and the upper cutoff voltage used in the cycling test are extremely close. At the higher current rate, the amount of parasitic reactions are reduced due to their low kinetics, thus leading to an increase of the cell coulumbic efficiency. Figure S4 (SI) shows the cycling behavior of six Al/graphite cells for five cycles: three of the cells were assembled with the PAN separator and the other three

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CONCLUSIONS Electrospinning has been successfully utilized to develop PAN as a separator for application in aluminum batteries, for the first time. The electrospinning technique, employing 10 wt % PAN in a DMF solution, allowed the preparation of a highly homogeneous membrane (S = 20 × 30 cm2). The developed PAN separator is very stable, compared to commercial battery separators, in the highly corrosive 1-ethyl-3-methylimidazolium chloride:aluminum trichloride (EMIMCl:AlCl3) electrolyte conventionally employed in aluminum batteries. The developed separator also revealed excellent aluminum stripping/deposition properties and very good aluminum/electrolyte interface stability, even superior with respect to the conventionally employed glass fiber separator. Ex situ SEM imaging of the aluminum surface upon cycling evidenced that the PAN separator strongly affects the aluminum dissolution/deposition process, leading to a more homogeneous deposition compared to that of the glass fiber separator. Furthermore, it has been demonstrated that due to the improved aluminum/electrolyte interface stability induced by the PAN separator, aluminum/ graphite cells delivered far better electrochemical performance compared to similar cells with a glass fiber separator. Finally, in 38386


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ACS Applied Materials & Interfaces ORCID

Giuseppe Antonio Elia: 0000-0001-6790-1143 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the European Commission in the H2020 ALION project under contract 646286 and the German Federal Ministry of Education and Research in the AlSiBat project under contract 03SF0486.

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Figure 6. (a) Cycling behavior with Coulombic efficiency and (b) 10th, 50th, 100th, and 200th cycle voltage signatures of the Al/ EMIMCl:AlCl3/PG cell employing the PAN separator, galvanostatically cycled at 25 mA g−1 for the first five cycles and 75 mA g−1 for the subsequent cycles. (c) Evolution of the interface resistance of the Al/ EMIMCl:AlCl3/PG cell upon cycling.

view of the excellent properties of the PAN separator demonstrated herein, as well as the relatively easy, scalable, and cost-effective electrospinning process, the developed PAN separator can be considered as an excellent candidate for the realization of commercial high-performance aluminum batteries.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b09378. Characterization of separator stability and more results of separator characterization (photographic images, SEM, shrinkage test, Gurley test) and separator performance at elevated temperature (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 38387


Research Article

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ACS Applied Materials & Interfaces lytes for Rechargeable Aluminum Batteries. J. Mater. Chem. A 2015, 3 (45), 22677−22686.

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