Rechargeable Aluminum/Iodine Battery Redox Chemistry in Ionic Liquid Electrolyte Huajun Tian,*,†,⊥ Shunlong Zhang,†,‡,⊥ Zhen Meng,† Wei He,† and Wei-Qiang Han*,†,‡,§ †
Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo 315200, China School of Physical Science and Technology, Shanghai Tech University, Shanghai 201210, China § Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡
S Supporting Information *
ABSTRACT: Rechargeable aluminum ion batteries (RABs) have attracted much attention because of their high charge density, low cost, and low flammability. However, the traditional cathodes used in RABs had limited intercalation ability of Al3+ ion, leading to a low capacity. We report for the first time a rechargeable aluminum/iodine (Al/I2) battery. The unique conversion reaction mechanism of the Al/I2 battery chemistry avoids the cathode material disintegration during repeated charge−discharge processes, and this system successfully suppresses the shuttle of dissolved polyiodide in ionic liquid because of the hydrogen-bonding interaction, resulting in a robust rechargeable RAB system. The rechargeable Al/I2 battery based on the I3−/I− redox chemistry is demonstrated to be highly reversible in Al3+ ion storage, providing a high capacity of >200 mAh g−1 at 0.2C and high stability for even over 150 cycles at 1C. This work provides a new insight into designing a RAB system based on redox chemistry. he “beyond Li-ion” batteries (Mg2+, Al3+, Ca2+, Zn2+ multivalent ion batteries) have attracted much attention because of their low cost, environmental friendliness, and high charge density.1−9 Rechargeable aluminum ion batteries (RABs) have been considered one of the most promising next-generation rechargeable batteries because Al is the most abundant metal element in earth’s crust, and it can provide a remarkable volumetric capacity (8.05 Ah cm−3), which is four times higher than that of lithium. However, two major bottlenecks have hindered the development of RABs: (i) Difficulty surrounds the design of a proper electrolyte with wide electrochemical stability window which can enable highly reversible Al deposition and stripping. (ii) Only a limited number of cathode materials can reversibly intercalate and deintercalate Al3+ ion because of difficult intercalation and the disintegration of traditional cathode materials during the charge−discharge process. Graphite, one of the carbon-based materials, which was chosen to be a cathode for RABs for the first time by Michel Armand, has been proven to be a suitable candidate because of its effective electrochemical intercalation AlCl4− within graphite.10 Rani et al. employed fluorinated natural graphite as cathode in RABs, providing a stable performance (>40 cycles) but delivering only a low operating voltage (∼0.5 V).11 Recently, Lin et al. used a three-dimensional (3D) graphitic foam as cathode for RABs, showing a ∼ 2 V operating voltage. However, despite the high working voltage, graphitic foam/Al
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battery cannot satisfy the demand of high energy density application because of its low capacity (∼60 mAh/g).12 Therefore, it is urgent to find a high-capacity cathode in RABs with high safety and long cycle life to meet the increasing demands of energy storage and conversion systems. As a high-capacity cathode material, iodine (211 mAh g−1) has been proven to be a promising candidate in Li/I2,13−15 Zn/ I2,16 and Mg/I217,18 batteries. A rechargeable Al/I2 battery is highly attractive because its high energy density (∼240 Wh/ kg(total electrode)), reasonable theoretical operating voltage (∼1.2 V), and low cost (see Calculations in the Supporting Information). Xue et al. first reported a primary Al/I2 battery and a dye-sensitized solar cell (DSSC) combination based on AlI3 electrolyte.19 Unfortunately, this Al/I2 primary battery cannot be recharged. To the best of our knowledge, a rechargeable Al/I2 battery has not been reported mainly because of the obstacle of finding an appropriate iodine-based cathode and highly reversible Al deposition−stripping electrolyte. Herein, we report for the first time a rechargeable Al/I2 battery using PVP-I2 complex as cathode and room-temperature ionic liquid as electrolyte. The hydrogen bonds existing between the PVP and iodine element guarantee that the Received: February 25, 2017 Accepted: April 24, 2017 Published: April 24, 2017 1170
DOI: 10.1021/acsenergylett.7b00160 ACS Energy Lett. 2017, 2, 1170−1176
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http://pubs.acs.org/journal/aelccp
Letter
ACS Energy Letters conversion reaction of I3−/I− redox pair in the cathode is highly reversible, effectively mitigating the shuttle effect of polyiodide toward the Al anode. The iodine-based cathode undergoes a conversion reaction mechanism in the Al/I2 battery as illustrated in Figure 1a, prompting a kind of novel redox chemistry in RAB system design.
homogeneously (Figure S3) by the ball milling method to improve the active material utilization. The charge−discharge profile of Al/I2 battery shown in Figure 2a displayed the typical voltage profile at a constant charge and discharge current density of 1/5C with a single discharge plateau of ∼0.74 V and charge plateau (∼1.01 V). The discharge capacity of Al/I2 battery could reach 203.2 mAh g−1 (based on the mass of iodine) corresponding to an I2 utilization of ∼91.6%. The cell can provide an energy density of >150 Wh/kg (Figure 1b) at 0.2C. On the basis of the total mass of PVP-I2 cathode, the overall capacity can reach 110.3 mAh g−1 by increasing the iodine content in PVP-I2 cathode. Taking the total mass of the cathode and anode Al into consideration, it also could reach up to 71.7 Wh/kg (Figure S4). The battery has an advantage over lead-acid (40 Wh/kg), Ni-Cd(40 Wh/kg), and Ni-MH (50 Wh/kg) batteries in energy density, but it is a bit lower compared with the lithium battery.21 Further work on achieving high energy density of rechargeable Al/I2 battery is ongoing. The Coulombic efficiency of the cell was 94.7%. To understand the reaction mechanism, the Al3+ ion storage of PVP-I2 electrode was investigated by cyclic voltammetry (CV). The CV of Al/I2 battery was scanned at a low rate of 0.1 mV/s (Figure 2b), and a major single cathodic peak at ∼0.71 V in the first curve could be observed, while a single anodic peak at ∼1.05 V could be observed. Combined with the chemical formula of PVP-I2 (Figure S5), the single peak could correspond to the I3−/I− redox couple. The Al/I2 battery showed a good electrochemical stability in the potential range at 0.2C (Figure 2c and 2d), showing a good cycling performance with a high capacity of 189.0 mAh g−1 even after 35 cycles. In contrast, we characterized the performance using the iodine as cathode in RABs (Figure S6). The process of preparing iodine cathode was the same as that of the PVP-I2 cathode. The discharge plateau of iodine cathode disappeared quickly after the second cycle. The following sloping profile in Figure S6 could be ascribed to the capacitive storage of the conductive KB in the cathode (Figure S7). The capacity fading of only iodine cathode could be attributed to the iodine having high solution in the electrolyte (Figure S8), resulting in severely uncontrollable shuttle effect and then resulting in a quick capacity degradation. This shuttle effect of the polyiodide is very similar to that of sulfur in Li/S chemistry.22−27 Therefore, the polyiodide shuttle effect can also be substantially prevented through host and electrolyte optimization. Further work on improving the physical and chemical properties of the host for better iodine utilization and absorbability of polyiodide is ongoing. Moreover, Figure 2e showed the rate capability of PVP-I2 cathode in Al/I2 cell at different current rates. The capacity of the PVP-I2 cathode obtained at 0.2C, 0.5C, and 1C are 207, 160.8, and 79.2 mAh g−1, respectively. After the current density is reduced back to 0.5C, the capacity increased to 132.9 mAh g−1 again, indicating the excellent rate performance of this RABs system. The long cycling stability of the Al/I2 battery is shown in Figure 2f, indicating a stable electrochemical performance even after >150 cycles at 1C. To verify the effective suppression of the shuttle effect of polyiodide for the hydrogen-bonding interaction existing in the PVP-I2 complex, we did the solubility measurement as shown in Figure S8. The results indicated that the PVP-I2 would be easily saturated (2.4 g/100 mL). We also assembled a Li/PVP-I2 battery (Figure 3a), which could provide an obvious discharge plateau (∼3.0 V), which was a
Figure 1. Schematic illustration of the rechargeable Al/I2 battery. (a) Schematic of rechargeable Al/I2 batteries. (b) Capacity and voltage of the iodine cathode compared to reported rechargeable nonaqueous aluminum batteries (RABs) cathodes.
The Al/I2 battery was assembled by coupling a PVP-I2 complex cathode disk, a glass fiber separator, and an Al foil anode into a Swagelok cell instead of a coin cell. The electrolyte was made by mixing AlCl3 with 1-ethyl-3-methylimidazolium chloride (EMIC) in an molar ratio of 1.3:1. To be free of continuous corrosion caused by this ionic liquid electrolyte, the Swagelok cell adopted Inconel alloy as current collector to resist the corrosive electrolyte rather than stainless steel as current collector, which ensures kinetic stability of the cell system.20 The deposition−striping process of the electrolyte was measured by coin cell, and a Coulombic efficiency of close to 100% can be obtained (Figure S1), indicating that the Al deposition−striping process was highly reversible. The average diameter of typical PVP-I2 particles as shown in Figure S2 was around 50−120 μm. To improve the electron-transfer ability of the PVP-I2 cathode, the composites of PVP-I2 and Ketjenblack (KB) were ball milled for a half hour. After that, smaller particles of active materials were obtained (Figure S3). More importantly, the PVP-I2 and conductive KB could be mixed 1171
DOI: 10.1021/acsenergylett.7b00160 ACS Energy Lett. 2017, 2, 1170−1176
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ACS Energy Letters
Figure 2. (a) Typical galvanostatic charge−discharge curve of Al/I2 battery with PVP-I2 cathode and ionic liquid electrolyte. (b) Cyclic voltammogram of the Al/I2 battery. (c) Charge and discharge voltage profiles of the Al/I2 battery in 180 h. (d) Cycling performance of Al/I2 battery at 0.2C. (e) Rate capability of Al/I2 battery. (f) Cycling performance of Al/I2 battery at 1C.
dissolved polyiodide in electrolyte because of the hydrogenbonding interaction, resulting in a robust rechargeable RAB system. Figure 3c shows the high-loading (>1.5 mg/cm2) PVP-I2 cathode electrochemical performance. The high-loading PVP-I2 cathode also exhibited an Al3+ ion storage behavior. However, the capacity would decrease with the increase of active materials loading (Figures 3c and S9). When the loading of active material iodine is 200 mAh/g, corresponding to an energy density of >150 Wh/kg (based on the mass of iodine) (Figure 1b) at 0.2C, and excellent stability even after 150 cycles at 1C, which is one of the best RABs (Supplementary Table 1). The low cost, safe, and unique rechargeable Al/I2 battery makes it a promising candidate for rechargeable multivalent metal ion 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/acsenergylett.7b00160. Experimental details and characterization of the materials, Figures S1−S15, and Supplementary Table 1 (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Wei-Qiang Han: 0000-0001-5525-8277 Author Contributions ⊥
H.T. and S.Z. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Grants 51504234 and 51371186), Zhejiang Provincial Natural Science Foundation of China (Grant LY16E040001), the “Strategic Priority Research Program” of the Chinese Project Academy of Science (Grant XDA09010201), the Ningbo 3315 International Team of Advanced Energy Storage Materials, Zhejiang Province Key Science and Technology Innovation Team (Grant No. 2013TD16).
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