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An amorphous carbon-graphite composite cathode for long cycle life rechargeable aluminum ion batteries
Accepted Manuscript Title: An Amorphous Carbon-Graphite Composite Cathode for Long Cycle Life Rechargeable Aluminum Ion Batteries Authors: Jiang Wei, Wei Chen, Demin Chen, Ke Yang PII: DOI: Reference:
S1005-0302(17)30163-9 http://dx.doi.org/doi:10.1016/j.jmst.2017.06.012 JMST 1007
To appear in: Received date: Revised date: Accepted date:
12-3-2017 18-4-2017 9-6-2017
Please cite this article as: Jiang Wei, Wei Chen, Demin Chen, Ke Yang, An Amorphous Carbon-Graphite Composite Cathode for Long Cycle Life Rechargeable Aluminum Ion Batteries (2010), http://dx.doi.org/10.1016/j.jmst.2017.06.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
An Amorphous Carbon-Graphite Composite Cathode for Long Cycle Life Rechargeable Aluminum Ion Batteries Jiang Wei,1,2 Wei Chen,2 Demin Chen,2,* Ke Yang2
1
University of Chinese Academy of Sciences, Beijing 100049, China
2
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
* Corresponding author. Prof.; Tel.: +86 24 2397 1641.
E-mail address: [email protected] (Demin Chen).
[Received 12 March 2017; Received in revised form 18 April 2017; Accepted 9 June 2017]
Natural graphite is investigated as the cathode for aluminum ion batteries in recent years. However, some drawbacks of the natural graphite such as severe volume swelling shorten its lifetime. In this work, we prepared a composite material by depositing an amorphous carbon on the graphite paper. The composite was used as a cathode to study the electrochemical performance in aluminum ion batteries. The charge/discharge results showed that the composite could exhibit a longer cycle life than the graphite paper. Electrochemical analyses demonstrated that the interface between the amorphous carbon and the graphite paper made a major contribution to the improvement of the cycling stability.
Keywords: Amorphous carbon, Graphite paper, Aluminum ion batteries, Cathode, Ionic liquid, Interface, Phase equilibrium
1. Introduction Lithium ion batteries have been widely used in portable devices in the past few years, but the shortcomings such as limited power density, high cost and safety risk still restrict their further applications in electric vehicles or smart grid fields. For a sustainable modern society, it 1
is urgent to develop a better battery system in order to meet the demand of the increasing market[1]. Recently, multivalent ion batteries such as magnesium ion battery and aluminum ion battery are becoming more attractive due to their high theoretical capacities and abundant natural resources[2,3]. Among them, aluminum is considered as a promising electrode candidate for rechargeable batteries because of its high energy density, low cost and environmental benignity. Although aluminum presents these advantages, neither of the aluminum primary battery nor the secondary battery has been successfully commercialized in the market. On one hand, the aluminum primary battery exhibits lower voltage than the theoretical value due to a protective film on aluminum metal, which induces a delayed voltage recovery phenomenon. On the other hand, the aluminum secondary battery can only work at high temperature because electrolytes are subjected to the limitations of some high-temperature molten salts [4]. To make the aluminum battery work at room temperature, an ionic liquid electrolyte with some superior features such as high ionic conductivity, negligible volatility and flame retardancy has been introduced into the battery system [5,6]. Kinds of materials such as transition metal oxides, inorganic salts and organic polymers, especially carbonaceous materials, have been studied and achieved a certain progress so far[7-17]. Carbonaceous materials with novel frameworks are of great attractiveness for energy conversion and storage in Li-ion, Li-S, and supercapacitors[18-21]. Natural graphite is a widely used electrode material, which has been commercialized successfully in Li-ion battery due to its unique physical and chemical properties. However, the natural graphite used as cathode in Al-ion battery was found to be severely swollen after a few cycles, which could be a risk for the battery application [11,14]. In addition, the reaction mechanism for such an Al-ion battery seems more complex than the “rocking-chair” style in Li-ion battery. Therefore, it is necessary for us to improve the electrochemical properties of the widespread and low cost natural graphite used as the cathode for rechargeable Al-ion battery. In this study, we developed a long cycle life aluminum ion battery using a graphite/amorphous carbon composite material as the cathode. The composite material was prepared by depositing an amorphous carbon (a-C) film on a graphite paper (GP) by radio frequency (RF) magnetron sputtering method. The gavnolstatic charge/discharge measurements were conducted and the differential capacities versus voltage (dQ/dV) plots were used to evaluate the phase equilibrium changes during the entire cycling. 2. Experimental 2.1 Materials preparation 2
Firstly, a graphite paper (GP) with a thickness of 0.05 mm (Beijing JingLongTeTan Co.) was cut into a Φ12 mm (8 mg in weight) circle and then ultrasonically cleaned in acetone, ethanol and distilled water for 15 min, respectively. Then the GP was dried in vacuum for overnight. The deposition of a-C film on the GP was carried out by a radio frequency (RF) magnetron sputtering system (KYKY Co.). Prior to the deposition, the base pressure of the magnetron system was evacuated to 10 -4 Pa. Then a high purity argon gas (99.999%) was introduced to keep a working pressure of 1 Pa. A high purity graphite target (99.999%, China Material Tech. Co.) was used as the carbon source for deposition. Preliminary sputtering was performed to remove the surface oxide on the graphite target at a constant power of 1 W cm -2 for 10 min. This procedure was shielded by a shutter to avoid contamination of the GP substrate. Then the depositing power was increased to 4 W cm -2 and maintained for 1 h at room temperature. Finally, the a-C/GP composite material was obtained and transferred to a glove box (Mikrouna) filled with high purity argon gas for battery assembly through a door connecting the chamber and the glove box directly to avoid the oxygen and moisture contamination. The areal loading of the a-C on the GP is about 0.38 mg cm -2. A contrastive composite was also prepared by depositing the amorphous carbon on a glassy carbon (Tianjin AIDA HengSheng Co.) by the same procedure. 2.2 Electrolyte synthesis The ionic liquid electrolyte was composed of 1-ethyl-3-methylimidazolium chloride (EMImCl, 97%, TCI Co.) and AlCl 3 (99%, Shanghai Aladdin Industrial Inc.). The 1-ethyl-3-methylimidazolium chloride was dried in vacuum at 120 °C for 72 h to remove residual water. In this experiment, the mole ratio of AlCl 3 to EMImCl was 1.3:1, which was suitable for the aluminum electro-deposition[11]. The ionic liquid was synthesized by slowly adding AlCl3 into EMImCl. All the processes were performed in a high purity argon gas filled glove box where both the oxygen and moisture were controlled under 1 ppm. Prior to usage, the synthesized ionic liquid electrolyte was well stirred by a magnetic stirrer till the liquid turned to light yellow. 2.3 Battery assembly A Swagelok type cell (Φ16 mm) was used to carry out the electrochemical measurements in the experiment. To avoid the corrosion of the battery components by the strongly acidic ionic liquid electrolyte, polytetrafluoroethylene (PTFE, Yangzhong GongZheng Fluid Controls Co.)
3
was chosen as the battery shell instead of stainless steel. Molybdenum metal (Hebei Qingyuan Metal Materials Co.) was used as the current collector rod since the influence of Mo on the electrode reaction in this battery system could be ignored [14]. Two pieces of rapidly filtering paper (Φ15 mm, Toyo Roshi Kaisha Ltd.) and a high purity electro-polished aluminum foil (99.999%, IMR, CAS) were used as the separator and the anode, respectively. The amount of the electrolyte used in this experiment was 28.125 g per mass of cathode. No other ad ditive or component was used in this experiment. All the assembling processes were performed in an argon gas filled glove box under the same circumstance as mentioned in electrolyte synthesis. 2.4 Characterization Raman spectra were obtained in a backscattering geometry (HORIBA LabRAM HR) with laser wavelength of 633 nm. The surface morphology of the samples was observed by scanning electron microscopy (SEM, FEI INSPECT F50) and atomic force microscopy (AFM, Bruker Dimension ICON). The surface chemical information was collected by the X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250). The galvanostatic charge/discharge test was carried out at a current density of 75 mA g -1 (1 C ≈ 66 mA g-1) on Arbin BT-2043 battery test system. The discharge capacity calculation was based on the total mass of the cathode
material.
Cyclic
voltammetric
(CV)
measurement
was
performed
on
an
electrochemical working station (Zahner Zennium). The scanning voltage range was from 0.3 to 2.5 V and the scanning rate was 1 mV s-1. 3. Results and Discussion Fig. 1 shows the SEM and AFM images of the surface and cross sectional morphology of the GP substrate and the GP after sputtering for 1 h. A flat and intact surface morphology of the pristine GP is demonstrated in Fig. 1(a), which is similar to that of the sputtered GP sample as shown in Fig. 1(b). To further study the morphology difference between the two samples, AFM was used and the corresponding images are shown in Fig. 1(c and d), respectively. The original GP shows a rough and sharp profile while the sputtered GP becomes smoother. In Fig. 1(e), the cross sectional SEM image of the sputtered GP shows a clear interface between the deposited layer and the GP substrate. It can also be found that the thickness of the deposited film is about 3±1 μm. The Raman spectra of the two samples are shown in Fig. 2, in which some differences can
4
be clearly found. In the spectra, the sharp G-band peak and D-band peak at 1325 cm -1 and 1580 cm-1 are found for the GP sample, respectively, while a relatively broad peak is shown for the sputtered graphite paper. The D-band peak and G-band peak represent the C-C bending (A1g) and the stretching (E2g) modes, which can reflect the carbon atom alignment in graphite -based material. The two sharp peaks confirm a well-defined graphite material, while the broad peak profile presented in the spectrum of the sputtered GP indicates that the deposited film is an amorphous carbon[22]. Cycling stability is one of the most important performance indexes for an electrode material in rechargeable batteries. In this work, 1,000 charge/discharge cycles were conducted to observe the electrochemical performance changes. It is clear to see in Fig. 3(a) that the cycling curves of the two samples can be divided into three stages. During the first stage (1-150 cycles), the specific capacity of the a-C/GP composite increased faster than that of the GP although both underwent a gradual increase after the initial cycle. In the present study, the electrodes we used are bulk materials, which have small specific areas for electrolyte saturating. The compact installation of the Swagelok cell restricted the electrode expansion, which is cause by and necessary for the [AlCl4]- anion intercalation. Therefore, the batteries showed low specific capacities in the initial cycles. Then both of them performed a stable discharge behavior during the second stage (150-650 cycles). After that, the specific capacity of the GP decayed while the a-C/GP still maintained a stable charge/discharge performance. Three charge/discharge curves which correspond to the 10th, 100th and 200th of the two samples are shown in Fig. 3(b and c), respectively. The number of potential plateau increased from one in the 10th cycle to three in the 200th cycle as can be seen in the charge/discharge curves. In addition, the a-C/GP delivered a discharge capacity of about 27.5 mAh g-1 on the 10th cycle and increased to 48 mAh g -1 after 200th cycle, and the GP delivered 25 mAh g-1 and 46 mAh g-1 after the same cycles. Previous studies have investigated the charge/discharge reaction in the graphitic cathode Al-ion battery by CV test and different results were obtained by different investigators. For example, Yu et al.[13] obtained a typical CV curve containing one oxidation peak and two reduction peaks in an Al-ion battery using graphene nanoribbons as the cathode. They explained that the two reduction peaks are all assigned to the [AlCl 4]- deintercalation. However, Sun et al.[14] obtained a CV curve consisting of three oxidation peaks and three reduction peaks in an Al-ion battery with a graphite paper cathode without a clear explanation. The different 5
results indicate a complicated battery reaction mechanism, which is needed to make further investigations. So far, it is prevailing that the intercalate/deintercalate active material in such a imidazolium chloride (EMImCl) /AlCl 3 ionic liquid based Al-ion battery is the [AlCl 4]anion[11,16,17,31,32]. Recently, Jung et al.[23] used first-principles method to calculate the structural characteristic of graphitic foam with [AlCl 4]- intercalation. They found that [AlCl 4]- anions were stored by forming doubly stacked ionic layers in the interlayer space between graphene sheets. It means that more than one phase transition occurs when the concentration of the intercalated [AlCl 4]- anions increases during charging and discharging time. This is quite helpful for understanding the reactions in such an Al-ion battery with graphite cathode. In this work, the dQ/dV plots were used to illuminate the phase equilibrium changes during the entire cycling since the peaks in dQ/dV plot can not only indicate the battery potential plateau in charge/discharge curves but also reflect the phase equilibrium of the active materials [24]. Some dQ/dV curves are shown in Fig. 4(a and b) and summarized in Fig. S1. In Fig. 4, both electrodes exhibit only one pair of dQ/dV peak in the 10th cycle and the peak number increased to two and three pairs when the cycling increased to the 100th and 200th cycle, respectively. This means that the reaction occurred in the graphite cathode Al-ion battery follows an evolving process which is quite similar to the phase transitions in lithium-graphite secondary battery[25,26]. The phase equilibrium of the two samples becomes stable after the activation stage. It is also worth noting that the value of delivered capacity strongly depends on the number of peaks in dQ/dV plots. That is to say that the capacity is strongly dependent on the accumulation of [AlCl 4]-. Therefore, it is necessary to point out that the full capacity of the two electrodes can not be compared directly unless they are in the same reaction stages. At present, we do not aquire these graphite intercalation compounds in atomic scale due to the absence of in-situ measurement. Nevertheless, the dQ/dV analyses are helpful for understanding the phase transition reactions in such an Al-ion battery using graphite as the cathode. Recent studies have reported that some amorphous materials can be used as the host material to accommodate metal ions [9,27,28.] In order to clarify the role of the a-C/GP electrode, CV measurements were carried out. As shown in Fig. S2(a), there is no redox couple in the CV curves of the amorphous carbon, which means that it had no electrochemical activity in the EMImCl ionic liquid electrolyte. The charge/discharge curve of the a-C coated glassy carbon also showed almost no capacity, as can be seen in Fig. S2(b). In other words, the amorphous carbon cannot be the host material for the [AlCl4]- anions by itself. Thus, the better performance could only be attributed to the interface between the amorphous film and the
6
graphite. In order to confirm this point, the a-C/GP composite was annealed at 2900 °C for 1 h to recrystallize the amorphous carbon. The Raman spectrum and charge/discharge result are shown in Fig. 5(a and b), respectively. It can be seen that the D band and G band peaks of the sample became sharper and narrower, which means that the amorphous carbon had been recrystallized into graphite. The charge/discharge performance of the annealed a-C/GP electrode is inferior to that of the as-prepared one by comparing Figs. 5(b) and 3(a). This is a solid evidence for the performance promotion from the a-C film. Previous studies have also pointed out that the graphene layer would exhibit not only sp2 but also sp3 hybrid characters when the graphite host material was fully charged [11,23]. This means that the formation of graphite intercalation compounds needs to overcome the energy of graphene deformation. Interestingly, the amorphous carbon has sp2 and sp3 hybridized C atoms intrinsically as seen in Fig. S3. Therefore, the [AlCl 4]- can diffuse faster in the interface between the amorphous carbon and the graphite paper by overcoming less energy barrier than in the two graphene layers as illustrated in Fig. 6. Considering the electrochemical inertness of the amorphous carbon, the most possible cause for the electrochemical promotion should be ascribed to the unique interface between the a-C and the GP. To further verify this viewpoint, the rate capability was performed at different high current densities since the intercalation/deintercalation of [AlCl 4]- in the electrode is dominanted by the diffusion. The results are shown in Fig. 7. It is clearly to see that the a-C/GP electrode has high charge/discharge specific capacities than the GP at high current rates. This result means that the a-C/GP can greatly facilitate the [AlCl 4]- diffusion. The
post-cycled
electrodes
were
examined
by
SEM
to
investigate
their
micro-morphological changes. The surface morphology of the GP changed from smooth plane to separated bulks with some valleys across the surface, as shown in Fig. 8(a), while the film of the a-C/GP electrode almost kept integrated with some wrinkles, as shown in Fig. 8(b). We believe that the formation of disintegrated surface of the GP is due to the volume swelling, though others’ work did not report an analogous phenomenon to the post cycled graphite [14]. This is because the electrode was placed in a Swagelok type cell in the present work. The bulgy graphite was squeezed into the gaps between the fibers of the filter paper as evidenced in Fig. 8(c). The valleys shown in Fig. 8(a) are actually the indentations of the filter fibers. On the contrary, there was no indentation on the a-C/GP electrode surface possibly because 1) the amorphous carbon was harder and stiffer than graphite; 2) the a-C/GP electrode underwent a 7
smaller scale of expansion/contraction upon charge/discharge. Although some cracks and peeled layers (Fig. 8(d)) appeared in the GP electrode after cycling, the capacity decay may not only result from the above causes. It may also be caused by the electrolyte irreversible reaction and it should be further investigated [30]. In the present work, all these drawbacks did not occurred in the a-C/GP composite. Based on the above discussion, we believe that the longer cycle life of the a-C/GP electrode resulted from the amorphous carbon film which confined the large volume change of the GP substrate. 4. Conclusion In summary, the electrochemical tests showed that the a-C/GP electrode could exhibit a better performance in terms of galvanostatic charge/discharge characteristics and cycling life compared with the GP electrode. The amorphous carbon film played a key role in protecting the graphite from disintegration, which could result in a capacity decay. Also the interface between the a-C and GP promoted a fast diffusion for [AlCl 4]-. This study may provide a facile and simple approach for developing more reliable and durable ionic liquid electrolyte based rechargeable Al-ion batteries. Acknowledgements The authors gratefully acknowledge Dr. Min Cheng for supplying Raman spectrum measurements, Dr. Zhenguo Zhu for amorphous carbon recrystallization and Dr. Guoqing Duan for supporting electrochemical analysis.
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Figure and table captions Fig. 1. SEM images of the top surface morphology of the pristine GP (a) and the GP (b) after sputtering for 1 h, respectively. AFM images of the top surface morphology of the pristine GP (c) and the GP after sputtering (d). (e) SEM image of the cross-sectional morphology of the sputtered GP.
Fig. 2. Raman spectra records of the GP and sputtered GP.
Fig. 3. (a) Cycling performance of the a-C/GP and GP electrodes. (b) and (c) charge/discharge curves for 10th, 100th and 200th cycle of the a-C/GP electrode and GP electrode, respectively. Fig. 4. dQ/dV plots for 10th, 100th and 200th cycle of the a-C/GP electrode (a) and the GP electrode (b), respectively.
Fig. 5. (a) Raman spectrum of the annealed a-C/GP sample and (b) cycling performance of the annealed sample. Fig. 6. Schematic illustration of the [AlCl4]- diffusion in the interface between a single a-C layer and a graphene layer. Fig. 7. Rate capability performance of the a-C/GP and GP electrodes.
Fig. 8. Morphology comparison of the GP (a) and the a-C/GP (b) electrodes after cycling test, (c) SEM image of the part of the GP which penetrated into the gaps between the fibers of filter paper, (d) enlarged SEM image of the GP with cracks and peeled layers
Figure list:
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S1005-0302(17)30163-9 http://dx.doi.org/doi:10.1016/j.jmst.2017.06.012 JMST 1007
To appear in: Received date: Revised date: Accepted date:
12-3-2017 18-4-2017 9-6-2017
Please cite this article as: Jiang Wei, Wei Chen, Demin Chen, Ke Yang, An Amorphous Carbon-Graphite Composite Cathode for Long Cycle Life Rechargeable Aluminum Ion Batteries (2010), http://dx.doi.org/10.1016/j.jmst.2017.06.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
An Amorphous Carbon-Graphite Composite Cathode for Long Cycle Life Rechargeable Aluminum Ion Batteries Jiang Wei,1,2 Wei Chen,2 Demin Chen,2,* Ke Yang2
1
University of Chinese Academy of Sciences, Beijing 100049, China
2
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
* Corresponding author. Prof.; Tel.: +86 24 2397 1641.
E-mail address: [email protected] (Demin Chen).
[Received 12 March 2017; Received in revised form 18 April 2017; Accepted 9 June 2017]
Natural graphite is investigated as the cathode for aluminum ion batteries in recent years. However, some drawbacks of the natural graphite such as severe volume swelling shorten its lifetime. In this work, we prepared a composite material by depositing an amorphous carbon on the graphite paper. The composite was used as a cathode to study the electrochemical performance in aluminum ion batteries. The charge/discharge results showed that the composite could exhibit a longer cycle life than the graphite paper. Electrochemical analyses demonstrated that the interface between the amorphous carbon and the graphite paper made a major contribution to the improvement of the cycling stability.
Keywords: Amorphous carbon, Graphite paper, Aluminum ion batteries, Cathode, Ionic liquid, Interface, Phase equilibrium
1. Introduction Lithium ion batteries have been widely used in portable devices in the past few years, but the shortcomings such as limited power density, high cost and safety risk still restrict their further applications in electric vehicles or smart grid fields. For a sustainable modern society, it 1
is urgent to develop a better battery system in order to meet the demand of the increasing market[1]. Recently, multivalent ion batteries such as magnesium ion battery and aluminum ion battery are becoming more attractive due to their high theoretical capacities and abundant natural resources[2,3]. Among them, aluminum is considered as a promising electrode candidate for rechargeable batteries because of its high energy density, low cost and environmental benignity. Although aluminum presents these advantages, neither of the aluminum primary battery nor the secondary battery has been successfully commercialized in the market. On one hand, the aluminum primary battery exhibits lower voltage than the theoretical value due to a protective film on aluminum metal, which induces a delayed voltage recovery phenomenon. On the other hand, the aluminum secondary battery can only work at high temperature because electrolytes are subjected to the limitations of some high-temperature molten salts [4]. To make the aluminum battery work at room temperature, an ionic liquid electrolyte with some superior features such as high ionic conductivity, negligible volatility and flame retardancy has been introduced into the battery system [5,6]. Kinds of materials such as transition metal oxides, inorganic salts and organic polymers, especially carbonaceous materials, have been studied and achieved a certain progress so far[7-17]. Carbonaceous materials with novel frameworks are of great attractiveness for energy conversion and storage in Li-ion, Li-S, and supercapacitors[18-21]. Natural graphite is a widely used electrode material, which has been commercialized successfully in Li-ion battery due to its unique physical and chemical properties. However, the natural graphite used as cathode in Al-ion battery was found to be severely swollen after a few cycles, which could be a risk for the battery application [11,14]. In addition, the reaction mechanism for such an Al-ion battery seems more complex than the “rocking-chair” style in Li-ion battery. Therefore, it is necessary for us to improve the electrochemical properties of the widespread and low cost natural graphite used as the cathode for rechargeable Al-ion battery. In this study, we developed a long cycle life aluminum ion battery using a graphite/amorphous carbon composite material as the cathode. The composite material was prepared by depositing an amorphous carbon (a-C) film on a graphite paper (GP) by radio frequency (RF) magnetron sputtering method. The gavnolstatic charge/discharge measurements were conducted and the differential capacities versus voltage (dQ/dV) plots were used to evaluate the phase equilibrium changes during the entire cycling. 2. Experimental 2.1 Materials preparation 2
Firstly, a graphite paper (GP) with a thickness of 0.05 mm (Beijing JingLongTeTan Co.) was cut into a Φ12 mm (8 mg in weight) circle and then ultrasonically cleaned in acetone, ethanol and distilled water for 15 min, respectively. Then the GP was dried in vacuum for overnight. The deposition of a-C film on the GP was carried out by a radio frequency (RF) magnetron sputtering system (KYKY Co.). Prior to the deposition, the base pressure of the magnetron system was evacuated to 10 -4 Pa. Then a high purity argon gas (99.999%) was introduced to keep a working pressure of 1 Pa. A high purity graphite target (99.999%, China Material Tech. Co.) was used as the carbon source for deposition. Preliminary sputtering was performed to remove the surface oxide on the graphite target at a constant power of 1 W cm -2 for 10 min. This procedure was shielded by a shutter to avoid contamination of the GP substrate. Then the depositing power was increased to 4 W cm -2 and maintained for 1 h at room temperature. Finally, the a-C/GP composite material was obtained and transferred to a glove box (Mikrouna) filled with high purity argon gas for battery assembly through a door connecting the chamber and the glove box directly to avoid the oxygen and moisture contamination. The areal loading of the a-C on the GP is about 0.38 mg cm -2. A contrastive composite was also prepared by depositing the amorphous carbon on a glassy carbon (Tianjin AIDA HengSheng Co.) by the same procedure. 2.2 Electrolyte synthesis The ionic liquid electrolyte was composed of 1-ethyl-3-methylimidazolium chloride (EMImCl, 97%, TCI Co.) and AlCl 3 (99%, Shanghai Aladdin Industrial Inc.). The 1-ethyl-3-methylimidazolium chloride was dried in vacuum at 120 °C for 72 h to remove residual water. In this experiment, the mole ratio of AlCl 3 to EMImCl was 1.3:1, which was suitable for the aluminum electro-deposition[11]. The ionic liquid was synthesized by slowly adding AlCl3 into EMImCl. All the processes were performed in a high purity argon gas filled glove box where both the oxygen and moisture were controlled under 1 ppm. Prior to usage, the synthesized ionic liquid electrolyte was well stirred by a magnetic stirrer till the liquid turned to light yellow. 2.3 Battery assembly A Swagelok type cell (Φ16 mm) was used to carry out the electrochemical measurements in the experiment. To avoid the corrosion of the battery components by the strongly acidic ionic liquid electrolyte, polytetrafluoroethylene (PTFE, Yangzhong GongZheng Fluid Controls Co.)
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was chosen as the battery shell instead of stainless steel. Molybdenum metal (Hebei Qingyuan Metal Materials Co.) was used as the current collector rod since the influence of Mo on the electrode reaction in this battery system could be ignored [14]. Two pieces of rapidly filtering paper (Φ15 mm, Toyo Roshi Kaisha Ltd.) and a high purity electro-polished aluminum foil (99.999%, IMR, CAS) were used as the separator and the anode, respectively. The amount of the electrolyte used in this experiment was 28.125 g per mass of cathode. No other ad ditive or component was used in this experiment. All the assembling processes were performed in an argon gas filled glove box under the same circumstance as mentioned in electrolyte synthesis. 2.4 Characterization Raman spectra were obtained in a backscattering geometry (HORIBA LabRAM HR) with laser wavelength of 633 nm. The surface morphology of the samples was observed by scanning electron microscopy (SEM, FEI INSPECT F50) and atomic force microscopy (AFM, Bruker Dimension ICON). The surface chemical information was collected by the X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250). The galvanostatic charge/discharge test was carried out at a current density of 75 mA g -1 (1 C ≈ 66 mA g-1) on Arbin BT-2043 battery test system. The discharge capacity calculation was based on the total mass of the cathode
material.
Cyclic
voltammetric
(CV)
measurement
was
performed
on
an
electrochemical working station (Zahner Zennium). The scanning voltage range was from 0.3 to 2.5 V and the scanning rate was 1 mV s-1. 3. Results and Discussion Fig. 1 shows the SEM and AFM images of the surface and cross sectional morphology of the GP substrate and the GP after sputtering for 1 h. A flat and intact surface morphology of the pristine GP is demonstrated in Fig. 1(a), which is similar to that of the sputtered GP sample as shown in Fig. 1(b). To further study the morphology difference between the two samples, AFM was used and the corresponding images are shown in Fig. 1(c and d), respectively. The original GP shows a rough and sharp profile while the sputtered GP becomes smoother. In Fig. 1(e), the cross sectional SEM image of the sputtered GP shows a clear interface between the deposited layer and the GP substrate. It can also be found that the thickness of the deposited film is about 3±1 μm. The Raman spectra of the two samples are shown in Fig. 2, in which some differences can
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be clearly found. In the spectra, the sharp G-band peak and D-band peak at 1325 cm -1 and 1580 cm-1 are found for the GP sample, respectively, while a relatively broad peak is shown for the sputtered graphite paper. The D-band peak and G-band peak represent the C-C bending (A1g) and the stretching (E2g) modes, which can reflect the carbon atom alignment in graphite -based material. The two sharp peaks confirm a well-defined graphite material, while the broad peak profile presented in the spectrum of the sputtered GP indicates that the deposited film is an amorphous carbon[22]. Cycling stability is one of the most important performance indexes for an electrode material in rechargeable batteries. In this work, 1,000 charge/discharge cycles were conducted to observe the electrochemical performance changes. It is clear to see in Fig. 3(a) that the cycling curves of the two samples can be divided into three stages. During the first stage (1-150 cycles), the specific capacity of the a-C/GP composite increased faster than that of the GP although both underwent a gradual increase after the initial cycle. In the present study, the electrodes we used are bulk materials, which have small specific areas for electrolyte saturating. The compact installation of the Swagelok cell restricted the electrode expansion, which is cause by and necessary for the [AlCl4]- anion intercalation. Therefore, the batteries showed low specific capacities in the initial cycles. Then both of them performed a stable discharge behavior during the second stage (150-650 cycles). After that, the specific capacity of the GP decayed while the a-C/GP still maintained a stable charge/discharge performance. Three charge/discharge curves which correspond to the 10th, 100th and 200th of the two samples are shown in Fig. 3(b and c), respectively. The number of potential plateau increased from one in the 10th cycle to three in the 200th cycle as can be seen in the charge/discharge curves. In addition, the a-C/GP delivered a discharge capacity of about 27.5 mAh g-1 on the 10th cycle and increased to 48 mAh g -1 after 200th cycle, and the GP delivered 25 mAh g-1 and 46 mAh g-1 after the same cycles. Previous studies have investigated the charge/discharge reaction in the graphitic cathode Al-ion battery by CV test and different results were obtained by different investigators. For example, Yu et al.[13] obtained a typical CV curve containing one oxidation peak and two reduction peaks in an Al-ion battery using graphene nanoribbons as the cathode. They explained that the two reduction peaks are all assigned to the [AlCl 4]- deintercalation. However, Sun et al.[14] obtained a CV curve consisting of three oxidation peaks and three reduction peaks in an Al-ion battery with a graphite paper cathode without a clear explanation. The different 5
results indicate a complicated battery reaction mechanism, which is needed to make further investigations. So far, it is prevailing that the intercalate/deintercalate active material in such a imidazolium chloride (EMImCl) /AlCl 3 ionic liquid based Al-ion battery is the [AlCl 4]anion[11,16,17,31,32]. Recently, Jung et al.[23] used first-principles method to calculate the structural characteristic of graphitic foam with [AlCl 4]- intercalation. They found that [AlCl 4]- anions were stored by forming doubly stacked ionic layers in the interlayer space between graphene sheets. It means that more than one phase transition occurs when the concentration of the intercalated [AlCl 4]- anions increases during charging and discharging time. This is quite helpful for understanding the reactions in such an Al-ion battery with graphite cathode. In this work, the dQ/dV plots were used to illuminate the phase equilibrium changes during the entire cycling since the peaks in dQ/dV plot can not only indicate the battery potential plateau in charge/discharge curves but also reflect the phase equilibrium of the active materials [24]. Some dQ/dV curves are shown in Fig. 4(a and b) and summarized in Fig. S1. In Fig. 4, both electrodes exhibit only one pair of dQ/dV peak in the 10th cycle and the peak number increased to two and three pairs when the cycling increased to the 100th and 200th cycle, respectively. This means that the reaction occurred in the graphite cathode Al-ion battery follows an evolving process which is quite similar to the phase transitions in lithium-graphite secondary battery[25,26]. The phase equilibrium of the two samples becomes stable after the activation stage. It is also worth noting that the value of delivered capacity strongly depends on the number of peaks in dQ/dV plots. That is to say that the capacity is strongly dependent on the accumulation of [AlCl 4]-. Therefore, it is necessary to point out that the full capacity of the two electrodes can not be compared directly unless they are in the same reaction stages. At present, we do not aquire these graphite intercalation compounds in atomic scale due to the absence of in-situ measurement. Nevertheless, the dQ/dV analyses are helpful for understanding the phase transition reactions in such an Al-ion battery using graphite as the cathode. Recent studies have reported that some amorphous materials can be used as the host material to accommodate metal ions [9,27,28.] In order to clarify the role of the a-C/GP electrode, CV measurements were carried out. As shown in Fig. S2(a), there is no redox couple in the CV curves of the amorphous carbon, which means that it had no electrochemical activity in the EMImCl ionic liquid electrolyte. The charge/discharge curve of the a-C coated glassy carbon also showed almost no capacity, as can be seen in Fig. S2(b). In other words, the amorphous carbon cannot be the host material for the [AlCl4]- anions by itself. Thus, the better performance could only be attributed to the interface between the amorphous film and the
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graphite. In order to confirm this point, the a-C/GP composite was annealed at 2900 °C for 1 h to recrystallize the amorphous carbon. The Raman spectrum and charge/discharge result are shown in Fig. 5(a and b), respectively. It can be seen that the D band and G band peaks of the sample became sharper and narrower, which means that the amorphous carbon had been recrystallized into graphite. The charge/discharge performance of the annealed a-C/GP electrode is inferior to that of the as-prepared one by comparing Figs. 5(b) and 3(a). This is a solid evidence for the performance promotion from the a-C film. Previous studies have also pointed out that the graphene layer would exhibit not only sp2 but also sp3 hybrid characters when the graphite host material was fully charged [11,23]. This means that the formation of graphite intercalation compounds needs to overcome the energy of graphene deformation. Interestingly, the amorphous carbon has sp2 and sp3 hybridized C atoms intrinsically as seen in Fig. S3. Therefore, the [AlCl 4]- can diffuse faster in the interface between the amorphous carbon and the graphite paper by overcoming less energy barrier than in the two graphene layers as illustrated in Fig. 6. Considering the electrochemical inertness of the amorphous carbon, the most possible cause for the electrochemical promotion should be ascribed to the unique interface between the a-C and the GP. To further verify this viewpoint, the rate capability was performed at different high current densities since the intercalation/deintercalation of [AlCl 4]- in the electrode is dominanted by the diffusion. The results are shown in Fig. 7. It is clearly to see that the a-C/GP electrode has high charge/discharge specific capacities than the GP at high current rates. This result means that the a-C/GP can greatly facilitate the [AlCl 4]- diffusion. The
post-cycled
electrodes
were
examined
by
SEM
to
investigate
their
micro-morphological changes. The surface morphology of the GP changed from smooth plane to separated bulks with some valleys across the surface, as shown in Fig. 8(a), while the film of the a-C/GP electrode almost kept integrated with some wrinkles, as shown in Fig. 8(b). We believe that the formation of disintegrated surface of the GP is due to the volume swelling, though others’ work did not report an analogous phenomenon to the post cycled graphite [14]. This is because the electrode was placed in a Swagelok type cell in the present work. The bulgy graphite was squeezed into the gaps between the fibers of the filter paper as evidenced in Fig. 8(c). The valleys shown in Fig. 8(a) are actually the indentations of the filter fibers. On the contrary, there was no indentation on the a-C/GP electrode surface possibly because 1) the amorphous carbon was harder and stiffer than graphite; 2) the a-C/GP electrode underwent a 7
smaller scale of expansion/contraction upon charge/discharge. Although some cracks and peeled layers (Fig. 8(d)) appeared in the GP electrode after cycling, the capacity decay may not only result from the above causes. It may also be caused by the electrolyte irreversible reaction and it should be further investigated [30]. In the present work, all these drawbacks did not occurred in the a-C/GP composite. Based on the above discussion, we believe that the longer cycle life of the a-C/GP electrode resulted from the amorphous carbon film which confined the large volume change of the GP substrate. 4. Conclusion In summary, the electrochemical tests showed that the a-C/GP electrode could exhibit a better performance in terms of galvanostatic charge/discharge characteristics and cycling life compared with the GP electrode. The amorphous carbon film played a key role in protecting the graphite from disintegration, which could result in a capacity decay. Also the interface between the a-C and GP promoted a fast diffusion for [AlCl 4]-. This study may provide a facile and simple approach for developing more reliable and durable ionic liquid electrolyte based rechargeable Al-ion batteries. Acknowledgements The authors gratefully acknowledge Dr. Min Cheng for supplying Raman spectrum measurements, Dr. Zhenguo Zhu for amorphous carbon recrystallization and Dr. Guoqing Duan for supporting electrochemical analysis.
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Figure and table captions Fig. 1. SEM images of the top surface morphology of the pristine GP (a) and the GP (b) after sputtering for 1 h, respectively. AFM images of the top surface morphology of the pristine GP (c) and the GP after sputtering (d). (e) SEM image of the cross-sectional morphology of the sputtered GP.
Fig. 2. Raman spectra records of the GP and sputtered GP.
Fig. 3. (a) Cycling performance of the a-C/GP and GP electrodes. (b) and (c) charge/discharge curves for 10th, 100th and 200th cycle of the a-C/GP electrode and GP electrode, respectively. Fig. 4. dQ/dV plots for 10th, 100th and 200th cycle of the a-C/GP electrode (a) and the GP electrode (b), respectively.
Fig. 5. (a) Raman spectrum of the annealed a-C/GP sample and (b) cycling performance of the annealed sample. Fig. 6. Schematic illustration of the [AlCl4]- diffusion in the interface between a single a-C layer and a graphene layer. Fig. 7. Rate capability performance of the a-C/GP and GP electrodes.
Fig. 8. Morphology comparison of the GP (a) and the a-C/GP (b) electrodes after cycling test, (c) SEM image of the part of the GP which penetrated into the gaps between the fibers of filter paper, (d) enlarged SEM image of the GP with cracks and peeled layers
Figure list:
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Fig. 1.
Fig. 2.
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Fig. 3.
Fig. 4.
Fig. 5.
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Fig. 6.
Fig. 7.
Fig. 8.
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