Letter pubs.acs.org/NanoLett
Rechargeable Lithium-Iodine Batteries with Iodine/Nanoporous Carbon Cathode Qing Zhao,†,‡ Yanying Lu,† Zhiqiang Zhu,† Zhanliang Tao,† and Jun Chen*,†,‡ †
Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) and ‡Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin 300071, China S Supporting Information *
ABSTRACT: Rechargeable Li-iodine batteries are attractive electrochemical energy storage systems because iodine cathode provides the possibility of high energy density, wide abundance and low cost. However, the safety risk caused by low thermostability of iodine and the self-discharge reaction due to high solvency of iodine in aprotic solvent are target issues to be considered. Herein, we designed a room-temperature “solution-adsorption” method to prepare a thermostable iodine−carbon cathode by utilizing the strong adsorption of nanoporous carbon. Meanwhile, Li-iodine batteries constructed by the as-prepared cathode and ether-based electrolyte with the addition of LiNO3 showed negligible self-discharge reaction, high rate and long cycling performance. The reversible reactions of I2/LiI3 and LiI3/LiI in Li-iodine batteries were also proved with in situ Raman measurement. For the demonstration of application, soft-package batteries with Al-plastic film were assembled, displaying energy densities of 475 Wh/kg by mass of Li and iodine, and 136 Wh/kg by total mass of the battery. The use of nanoporous carbon to adsorb iodine at room-temperature represents a new and promising direction for realizing high-performance cathode for rechargeable Li-iodine batteries. KEYWORDS: Li-iodine batteries, nanoporous carbon, adsorption, binder−free, electrolyte additives
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of liquid component. In addition, complex configuration was also needed to separate Li anode and aqueous cathode.7,8 It is therefore of great importance to develop aprotic Li-iodine batteries because aprotic Li-iodine batteries with iodine as cathode could achieve the whole theoretical capacity and also share the facile construction with commercial Li-ion battery. Carbon materials with nanopores have been widely applied in energy storage and conversion11,12 such as lithium battery,13−17 fuel cells,18 and supercapacitor.19,20 Specially, the powerful adsorption capacity of nanosized pores existing in carbon materials is effective to inhibit the dissolution and improve the stability of electrodes. This encourages us to introduce iodine into nanoporous carbon for enhancing the thermostability of electrodes and batteries. Moreover, to solve the self-discharge reaction, the protection of Li anode is fairly essential.21,22 As studied in Li-sulfur battery, LiNO3 is added in ether-based electrolyte to solve the self-discharge issue since the addition of LiNO3 with ether solvent would form a passive film on the surface of Li anode.23−25 Thus, this promotes us to adopt ether-based electrolyte with LiNO3 addition to conquer the self-discharge problem of Li-iodine batteries. Herein, we report a high-performance rechargeable Li-iodine battery with the combination of a thermostable iodine−carbon
eveloping rechargeable batteries with high energy density and low cost is an ongoing demand nowadays.1−5 Rechargeable Li-iodine batteries with metal Li as anode and iodine as cathode (reaction, 2Li + I2 ↔ 2LiI) are promising candidates.6−8 First, Li-iodine batteries exhibit a high capacity of 1040 mAh/cm3 and 211 mAh/g with an average operation voltage of 2.9 V (based on the mass of iodine). Moreover, the highly abundance and low cost of iodine resource in ocean (50−60 μgiodine/Locean) meet the needs for sustainable energy storage.9 In spite of these considerable advantages, the development of rechargeable Li-iodine batteries was slugged in the past decades, mainly due to the following two reasons. On one hand, the easy sublimation of iodine decreases the thermostability, making the preparation of the cathode uncontrollable. On the other hand, iodine is easily dissolved in aprotic electrolyte, which would spontaneously shuttle to anode and react with metal Li. This leads to serious selfdischarge phenomenon and low Coulombic efficiency of the whole battery system.10 To date, only a handful studies have focused on the challenge of Li-iodine batteries. An all solid-state rechargeable lithium− iodine battery was reported using LiI(3-hydroxypropionitrile)2 solid electrolyte to prevent the dissolution of iodine and this battery could work several cycles at low current density.6 The employment of an aqueous triiodide cathode could realize much better cycling performance and rate capability, but this would inevitably sacrifice the capacity due to the introduction © XXXX American Chemical Society
Received: May 29, 2015 Revised: July 9, 2015
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DOI: 10.1021/acs.nanolett.5b02116 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
Figure 1. (a) Structure of Li-iodine batteries with metal Li as anode, aprotic electrolyte, iodine on nanoporous carbon cloth as cathode. (b) Schematic diagram of preparing iodine/nanoporous carbon cloth, showing that iodine was adsorbed on carbon cloth in aqueous solution. (c) SEM image of front view of iodine/carbon cloth. (d) SEM image of section view on iodine/carbon cloth. (e) EDX mapping with element C and I of front view of iodine/carbon cloth. (f) EDX mapping with element C and I of section view of iodine/carbon cloth.
Figure 2. (a) Thermogravimetric analysis curves of pure carbon cloth, iodine/carbon cloth and pure iodine. (b) XPS (I3d) of iodine/carbon cloth with 5.6 mg iodine/cm2. (c) N2 adsorption−desorption isotherm of pure carbon cloth and iodine/carbon cloth. (d) Pore size distribution plots of pure carbon cloth and iodine/carbon cloth.
nearly 200 °C). More importantly, the as-prepared iodine cathode on carbon cloth was assembling batteries without any binder and conductive additives, which would largely save manufacturing and time cost of batteries. Furthermore, Liiodine batteries with selected electrolyte (1 M LiN(CF3SO2)2 (LiTFSI) in 1,3-dioxolane (DOL)/dimethoxyethane (DME) with LiNO3 addition) exhibited negligible self-discharge reaction for five days. The reaction mechanism of Li-iodine batteries with reversible reactions of I2/LiI3 and LiI3/LiI was
cathode prepared with easily adsorption of iodine into nanoporous carbon and an ether-based electrolyte with the addition of LiNO3. The iodine cathode was prepared under room-temperature with “solution-adsorption” process on commercial flexible carbon cloth with an average pore of 1 nm, which is energy-saving and easily large scale. The carbon cloth electrode owns both superior conductivity and strong adsorption ability, which could improve the electrochemical activities and restrain the sublimation of iodine (stable for B
DOI: 10.1021/acs.nanolett.5b02116 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
displays large specific surface area. After adsorbing iodine, the specific surface area gradually decreased from 1350 m2/g of pure carbon cloth to 57 m2/g of iodine/carbon cloth (with iodine mass of 16.9 mg/cm2). This proved that iodine was filled in the pores of carbon cloth. Therefore, the as-prepared iodine adsorbed on nanoporous carbon has shown much improved thermostability. The as-prepared iodine/nanoporous carbon cloth were directly cut into round size (diameter of 10 mm) as cathodes for assembling coin 2032 batteries without any binder and conductive additives. We applied three different electrolytes to test the electrochemical properties, namely, 1 M LiPF6 in ethylene carbonate (EC)/ethyl methyl carbonate (EMC)/ dimethyl carbonate (DMC) (1:1:1 by volume), 1 M LiTFSI in DOL/DME (1:1 by volume), 1 M LiTFSI in DOL/DME (1:1 by volume) with 1 wt % LiNO3 addition. The commercial ester-based electrolyte and ether-based electrolyte without LiNO3 addition exhibit both low Coulombic efficiency and serious self-discharge phenomenon (Figure 3a,b). This is owing
directly proved with in situ Raman analysis. The as-prepared iodine/carbon cloth cathode exhibited an initial discharge capacity of 299 mAh/g and a capacity of 195 mAh/g after 300 cycles as well as superior rate performance (169 mAh/g at 5 C, about 60% capacity retention of 0.5 C). Meanwhile, an Alplastic film package battery with the energy densities of 475 Wh/kg by mass of Li and iodine, and 136 Wh/kg by total mass of the battery further demonstrated the application prospects of Li-iodine batteries. Our work displayed a superfacile and energy-saving fabrication method for the cathode of rechargeable Li-iodine batteries with high performance (such as low selfdischarge reaction and high Coulombic efficiency). Typical rechargeable Li-iodine batteries mainly consist of Li anode, organic electrolyte, and iodine/carbon cathode (Figure 1a). We applied a room-temperature method to prepare iodine cathode on carbon cloth (Figure 1b). The shaped carbon cloth was put into the iodine aqueous solution and stirred for about 2 h. Then, iodine was fully adsorbed on the carbon cloth and the brown iodine aqueous solution turned clear. This process was named as “solution-adsorption”, as iodine first dissolved in water and then continuously adsorbed on carbon cloth (Supporting Information, Figure S1). The carbon cloth was knited with carbon fiber (Supporting Information, Figure S2) and no structure change was found on its surface after adsorbing iodine (Figure 1c−d). However, when we took EDX mapping on the selected area, C and I were homogeneous distributed (Figure 1e and 1f). These results could be explained that iodine is not existed on the surface of carbon cloth, but in the pores of the carbon. XRD spectra showed that no characteristic peaks of iodine were founded on the prepared iodine/carbon cloth cathode as iodine was adsorbed (Supporting Information, Figure S3). More importantly, comparing with high solubility of pure iodine in both carbonate and ether solvent, the solubility was largely trapped after the adsorption on carbon cloth (Supporting Information, Figure S4). This method could be also applied in other porous carbon materials such as active carbon and ordered-mesoporous carbon (CMK3). The iodine was uniformly adsorbed in the pores of the carbon materials (Supporting Information, Figure S5−7). Thermogravimetric analysis measurement was applied to measure the iodine content and stability of the as-prepared iodine/carbon cloth cathodes. Pure iodine exhibited most mass loss before 100 °C, showing low thermostability. After adsorbing on carbon cloth, there exists nearly no mass loss before 200 °C (Figure 2a), revealing that the thermostability was obviously improved. These results demonstrated the much higher stability of the as-prepared iodine/carbon cloth than that of pure iodine. The iodine mass in the four cathodes were calculated to be 2.5, 5.6, 11.3, and 16.9 mg/cm2, respectively. In addition, 16.9 mg iodine/cm2 was the max mass of iodine adsorbed on carbon cloth. X-ray photoelectron spectroscopy (XPS) analysis was also used to characterize the as-prepared iodine cathode on carbon cloth. The interaction between iodine and carbon host was thought to be mostly physical,26,27 which has been confirmed with the measurement of C1s XPS analysis (Supporting Information, Figure S8). This interaction improved the stability of iodine on carbon cloth. I3d spectra of iodine cathode displayed the typical iodine 3d3/2 peak with binding energy of 630.6 eV and 3d5/2 peak with binding energy of 619.2 eV (Figure 2b).28 N2 adsorption−desorption isotherm further characterized the reducing specific surface area of carbon cloth after adsorbing iodine (Figure 2c). The pure carbon cloth with typical micropore structures (Figure 2d)
Figure 3. Discharge and charge curves of Li-iodine batteries after standing different time at current density of 0.5 C (1 C = 211 mAh/ giodine) with three electrolytes. (a) 1 M LiPF6 in EC/EMC/DMC, (b) 1 M LiTFSI in DOL/DME, (c) 1 M LiTFSI in DOL/DME with 1 wt % LiNO3 addition. SEM images of Li anode in Li-iodine batteries after standing 12 h with three electrolytes. (d) 1 M LiPF6 in EC/EMC/ DMC, (e) 1 M LiTFSI in DOL/DME, (f) 1 M LiTFSI in DOL/DME with 1 wt % LiNO3 addition. The inserts are EDX I mapping of marked area.
to that the easily dissolved iodine species would spontaneously shuttle to the anode side and react with it. This is demonstrated by some microsized precipitate that was formed on the surface of metal Li after standing for 12 h (Figure 3d−e). From insert image of EDX mapping, we could judge that it belongs to iodine compounds. This microsized precipitate might be hard to decompose at charge stage, which causes low Coulombic efficiency. As comparisons, when we applied the ether-based electrolyte with the addition of 1 wt % LiNO3, negligible selfC
DOI: 10.1021/acs.nanolett.5b02116 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters discharge phenomenon was founded even after five days (Figure 3c), meaning the high stability of Li-iodine batteries. The surface of metal Li was relative plan and hardly iodine compounds were formed on the surface of Li anode (Figure 3f). In addition, a passive film on the surface of Li anode was formed (Supporting Information, Figure S9). Further detailed element analysis on metal Li anode demonstrated that elements of O, F, C, S, and N were the major element on the surface of metal Li, which is the main component of the passive film. The element I only took a small bit part on metal Li surface (Supporting Information, Table S1). Previous study showed that LiI can also stabilize lithium metal.21 However, as known in Li-iodine batteries, LiI ion can easily react with I2 to form LiI3, which will cause the side reaction at charging process and decrease the Coulombic efficiency of Li-iodine batteries (Supporting Information, Figure S10). Thus, LiI would unlikely stabilize lithium metal in Li-iodine batteries. The major reason was the passive film formed with the addition of LiNO3 in ether solvent. The initial discharge capacities of first cycles are all about 300 mAh/g (based on the mass of iodine). The capacity is higher than the theoretical capacity of Li-iodine batteries (211 mAh/g) for the reason that carbon cloth also contributes to a part of capacity for capacitance behavior (Supporting Information Figure S11).20 Meanwhile, the Li-iodine battery delivers two reduction plateaus at about 3.3 and 2.9 V. This could be explained that there are two steps of Li-iodine batteries, I2 was reduced to LiI3 at 3.3 V and LiI3 was further reduced to LiI at 2.9 V. The charging is a reversible process, in which LiI was oxidized to LiI3 at 3.0 V and LiI3 was furthered oxidized to I2 at 3.4 V. These two steps were also demonstrated by differential capacity plots and cyclic voltammetry curves (Supporting Information, Figure S12). In order to prove this reaction mechanism of rechargeable Liiodine batteries, we designed an in situ Raman test to record the reaction of discharge and charge on iodine cathode. In short, the cathode cap of normal coin Li-iodine battery was cut into a hole to capture the Raman signals of iodine cathode (the inset diagram in Figure 4a).29,30 About 30 points among discharge and charge were taken to collect Raman spectra (Figure 4a). The major signal we detected is symmetric stretching mode of I3−, which is between 120 and 110 cm−1 (Figure 4b).31−33 At discharge process, peak of I3− was emerged, corresponding to the reaction of I2 to LiI3. The peak of I3− then decayed little by little and disappeared after the full discharge, which indicated the transformation of LiI3 to LiI at the second stage. This was also proved by XRD patterns (Supporting Information, Figure S13). At charge process, the peaks of I3− was first increased and then disappeared. This proved the reversed reaction of charging process, meaning that LiI was first oxidized to LiI3 and LiI3 was further oxidized to I2 (as displayed in Figure 1a). The whole reaction of Li-iodine batteries is 2Li + I2 ↔ 2LiI with two-electron transformation. The dissolution and transport of iodine in the electrolyte could be visually observed by the color change of the separator (Supporting Information, Figure S14). First, the separator was white for fresh battery that stood 12 h. Second, it turned a little brown at 2.9 V for the formation of highly soluble LiI3 and then white again at 2.0 V for the formation of LiI. Because the charging process was reversible, the white separator turned brown at 3.0 V for the oxidation from LiI to LiI3, then turned white at 3.6 V for the oxidation from LiI to iodine. The above reaction mechanism of iodine species matched well with previous study on kinetics of I−/I3−/I2 redox reactions on
Figure 4. In-situ Raman tests of rechargeable Li-iodine batteries. (a) Discharge and charge curves of an in situ Raman cell, the inset is a schematic diagram of an in situ Raman cell. The marked points (from point a to γ) are collected to take Raman tests. (b) In-situ Raman analysis of rechargeable Li-iodine batteries at marked points in panel a.
platinum with LiAsF6/tetrahydrofuran solutions.34 This reversible reaction process was favorable for long cycling stability of rechargeable Li-iodine batteries. Further electrochemical tests of rechargeable Li-iodine batteries were applied to this ether-based electrolyte with the addition of 1 wt % LiNO3. The iodine cathodes with different iodine (2.5 mg/cm2, 5.6 mg/cm2, 11.3 mg/cm2, and 16.1 mg/ cm2) were tested at the same area current density of 0.6 mA/ cm2 (about 0.5 C of 5.6 mg iodine/cm2 cathode). All capacities are calculated by the mass of iodine. The cathode with 5.6 mg iodine/cm2 exhibited both high specific capacity of 299 mAh/g and area capacity of 1.7 mAh/cm2 (Supporting Information, Figure S15). Thus, we focus on the electrochemical tests of this cathode. Further long-cycle measurements showed the high capacity retention with this cathode. After 300 cycles, it still displayed a capacity of 195 mAh/g (Figure 5a). Moreover, the Coulombic efficiency is all above 90%. The decayed capacity is ascribed to that the dissolving iodine species might slowly react with the metal side with long cycling process. This will also result in the gradually increasing resistance (Supporting Information, Figure S16). It should be mentioned that the carbon cloth also contributes a stable capacity of about 19 mAh/g (calculated by carbon cloth, Supporting Information, Figure S17a). In addition to long cycling stability, the asprepared iodine cathode exhibited outstanding high-rate performance (Figure 5b). The capacities of 301, 273, 232, 169 mAh/g were obtained at 0.5 C, 1 C, 2 C, 5 C, respectively. When we gradually decreased the current density back to 0.5 C (30th cycle and 60th cycle), it recurs to higher capacity. The capacity of pure carbon cloth at high rate decays faster than that of iodine/carbon cloth cathode (Supporting Information, Figure S17b). As comparisons, after 100 cycles at 0.6 mA/ cm2, the cathodes with the iodine mass of 2.5 mg/cm2 and 11.3 mg/cm2 exhibited capacities of 307 mAh/g and 109 mAh/g, respectively (Supporting Information Figure S18). This optimized ether-based electrolyte was also appropriate in other iodine/carbon materials (Supporting Information Figure S19). We also assembled a soft-package Li-iodine battery to D
DOI: 10.1021/acs.nanolett.5b02116 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 5. Electrochemical performance of the as-prepared iodine cathodes and soft-package Li-iodine batteries. (a) Cycling performance with Coulombic efficiency of iodine cathode (iodine mass, 5.6 mg/cm2) at a current density of 105 mA/g (0.5 C). (b) Rate performance with equal mass cathode. (c) Digital photos of the as-prepared iodine cathode on nanoporous carbon cloth with excellent flexibility. The insert is Al-plastic film package Li-iodine battery with a lighten LED. (d) Cycling performance of soft package Li-iodine battery for initial 60 cycles at a current density of 20 mA/g (about 0.1 C rate).
ance rechargeable Li-iodine batteries. The cathode was prepared at room-temperature and aqueous solution with nearly zero energy consumption. Iodine was strongly absorbed on high specific surface carbon cloth and largely improved the thermal stability of the as-prepared cathode. Combined with ether-based electrolyte with LiNO3 addition, the assembled Liiodine batteries exhibited negligible self-discharge phenomenon, long cycling, and high-rate performance. In addition, in situ Raman test demonstrated the reversible redox couples of I2/ LiI3 and LiI3/LiI reaction mechanism of Li-iodine batteries. This easily preparing method with iodine adsorption on porous carbon would be directive for the cathode development of rechargeable Li-iodine batteries with high performance.
further manifest the application prospect of the as-prepared Liiodine batteries. Owing to excellent flexibility of the as-prepared iodine cathode on carbon cloth, it is facile to assemble a soft package Li-iodine battery (Figure 5c). In addition, this battery delivered an initial energy density of 475 Wh/kg by iodine and Li metal, which is higher than aqueous cathode of Li-iodine batteries.7 In addition, a total energy density of 136 Wh/kg was obtained for the whole battery. The energy density turned stable with cycling. After initial 60 cycles, a retentive energy density of 107 Wh/kg was obtained (Figure 5d). As known, the regular electrode preparation usually contains several steps such as mixing the active substrate with binder and conductive additives, then smearing the paste on to the current collector, and finally dried in the vacuum. In this case, a huge amount of time and manufacturing cost would be taken. The present iodine cathode was prepared in room-temperature with nearly zero emission and energy consumption. Meanwhile, no additional steps were needed in assembling batteries, which will be favorable in industry production. In addition, the theoretical energy density of Li-iodine battery is higher than that of commercial Li-ion batteries (Supporting Information, Table S2)35−37 and the practical soft-package battery also showed comparable energy density.38 The present study demonstrates the potential application of iodine/nanoporous carbon as the cathode of rechargeable Li-iodine batteries. In conclusion, we applied a “solution-adsorption” method to prepare iodine/nanoporous carbon cathode for high-perform-
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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/acs.nanolett.5b02116. Detailed experimental procedures and additional materials characterization. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. E
DOI: 10.1021/acs.nanolett.5b02116 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters Notes
(32) Stephanie, B. S.; Gregory, I. G. J. Phys. Chem. A 1997, 101, 2192−2197. (33) Štangar, U. L.; Orel, B.; Vuk, A. S.; Sagon, G.; Colomban, P.; Stathatos, E.; Lianos, P. J. Electrochem. Soc. 2002, 149, E413−E423. (34) Behl, W. K.; Chin, D. T. J. Electrochem. Soc. 1988, 135, 16−21. (35) Whittingham, M. S. Chem. Rev. 2004, 104, 4271−4302. (36) Chung, S. Y.; Bloking, J. T.; Chiang, Y. M. Nat. Mater. 2002, 1, 123−128. (37) Cheng, F.; Wang, H.; Zhu, Z.; Wang, Y.; Zhang, T.; Tao, Z.; Chen, J. Energy Environ. Sci. 2011, 4, 3668−3675. (38) Dunn, B.; Kamath, H.; Tarascon, J. M. Science 2011, 334, 928− 935.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Programs of National 973 (2011CB935900), NSFC (51231003 and 21322101), and MOE (B12015 and 113016A).
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REFERENCES
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DOI: 10.1021/acs.nanolett.5b02116 Nano Lett. XXXX, XXX, XXX−XXX
Rechargeable Lithium-Iodine Batteries with Iodine/Nanoporous Carbon Cathode Qing Zhao,†,‡ Yanying Lu,† Zhiqiang Zhu,† Zhanliang Tao,† and Jun Chen*,†,‡ †
Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) and ‡Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin 300071, China S Supporting Information *
ABSTRACT: Rechargeable Li-iodine batteries are attractive electrochemical energy storage systems because iodine cathode provides the possibility of high energy density, wide abundance and low cost. However, the safety risk caused by low thermostability of iodine and the self-discharge reaction due to high solvency of iodine in aprotic solvent are target issues to be considered. Herein, we designed a room-temperature “solution-adsorption” method to prepare a thermostable iodine−carbon cathode by utilizing the strong adsorption of nanoporous carbon. Meanwhile, Li-iodine batteries constructed by the as-prepared cathode and ether-based electrolyte with the addition of LiNO3 showed negligible self-discharge reaction, high rate and long cycling performance. The reversible reactions of I2/LiI3 and LiI3/LiI in Li-iodine batteries were also proved with in situ Raman measurement. For the demonstration of application, soft-package batteries with Al-plastic film were assembled, displaying energy densities of 475 Wh/kg by mass of Li and iodine, and 136 Wh/kg by total mass of the battery. The use of nanoporous carbon to adsorb iodine at room-temperature represents a new and promising direction for realizing high-performance cathode for rechargeable Li-iodine batteries. KEYWORDS: Li-iodine batteries, nanoporous carbon, adsorption, binder−free, electrolyte additives
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of liquid component. In addition, complex configuration was also needed to separate Li anode and aqueous cathode.7,8 It is therefore of great importance to develop aprotic Li-iodine batteries because aprotic Li-iodine batteries with iodine as cathode could achieve the whole theoretical capacity and also share the facile construction with commercial Li-ion battery. Carbon materials with nanopores have been widely applied in energy storage and conversion11,12 such as lithium battery,13−17 fuel cells,18 and supercapacitor.19,20 Specially, the powerful adsorption capacity of nanosized pores existing in carbon materials is effective to inhibit the dissolution and improve the stability of electrodes. This encourages us to introduce iodine into nanoporous carbon for enhancing the thermostability of electrodes and batteries. Moreover, to solve the self-discharge reaction, the protection of Li anode is fairly essential.21,22 As studied in Li-sulfur battery, LiNO3 is added in ether-based electrolyte to solve the self-discharge issue since the addition of LiNO3 with ether solvent would form a passive film on the surface of Li anode.23−25 Thus, this promotes us to adopt ether-based electrolyte with LiNO3 addition to conquer the self-discharge problem of Li-iodine batteries. Herein, we report a high-performance rechargeable Li-iodine battery with the combination of a thermostable iodine−carbon
eveloping rechargeable batteries with high energy density and low cost is an ongoing demand nowadays.1−5 Rechargeable Li-iodine batteries with metal Li as anode and iodine as cathode (reaction, 2Li + I2 ↔ 2LiI) are promising candidates.6−8 First, Li-iodine batteries exhibit a high capacity of 1040 mAh/cm3 and 211 mAh/g with an average operation voltage of 2.9 V (based on the mass of iodine). Moreover, the highly abundance and low cost of iodine resource in ocean (50−60 μgiodine/Locean) meet the needs for sustainable energy storage.9 In spite of these considerable advantages, the development of rechargeable Li-iodine batteries was slugged in the past decades, mainly due to the following two reasons. On one hand, the easy sublimation of iodine decreases the thermostability, making the preparation of the cathode uncontrollable. On the other hand, iodine is easily dissolved in aprotic electrolyte, which would spontaneously shuttle to anode and react with metal Li. This leads to serious selfdischarge phenomenon and low Coulombic efficiency of the whole battery system.10 To date, only a handful studies have focused on the challenge of Li-iodine batteries. An all solid-state rechargeable lithium− iodine battery was reported using LiI(3-hydroxypropionitrile)2 solid electrolyte to prevent the dissolution of iodine and this battery could work several cycles at low current density.6 The employment of an aqueous triiodide cathode could realize much better cycling performance and rate capability, but this would inevitably sacrifice the capacity due to the introduction © XXXX American Chemical Society
Received: May 29, 2015 Revised: July 9, 2015
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Nano Letters
Figure 1. (a) Structure of Li-iodine batteries with metal Li as anode, aprotic electrolyte, iodine on nanoporous carbon cloth as cathode. (b) Schematic diagram of preparing iodine/nanoporous carbon cloth, showing that iodine was adsorbed on carbon cloth in aqueous solution. (c) SEM image of front view of iodine/carbon cloth. (d) SEM image of section view on iodine/carbon cloth. (e) EDX mapping with element C and I of front view of iodine/carbon cloth. (f) EDX mapping with element C and I of section view of iodine/carbon cloth.
Figure 2. (a) Thermogravimetric analysis curves of pure carbon cloth, iodine/carbon cloth and pure iodine. (b) XPS (I3d) of iodine/carbon cloth with 5.6 mg iodine/cm2. (c) N2 adsorption−desorption isotherm of pure carbon cloth and iodine/carbon cloth. (d) Pore size distribution plots of pure carbon cloth and iodine/carbon cloth.
nearly 200 °C). More importantly, the as-prepared iodine cathode on carbon cloth was assembling batteries without any binder and conductive additives, which would largely save manufacturing and time cost of batteries. Furthermore, Liiodine batteries with selected electrolyte (1 M LiN(CF3SO2)2 (LiTFSI) in 1,3-dioxolane (DOL)/dimethoxyethane (DME) with LiNO3 addition) exhibited negligible self-discharge reaction for five days. The reaction mechanism of Li-iodine batteries with reversible reactions of I2/LiI3 and LiI3/LiI was
cathode prepared with easily adsorption of iodine into nanoporous carbon and an ether-based electrolyte with the addition of LiNO3. The iodine cathode was prepared under room-temperature with “solution-adsorption” process on commercial flexible carbon cloth with an average pore of 1 nm, which is energy-saving and easily large scale. The carbon cloth electrode owns both superior conductivity and strong adsorption ability, which could improve the electrochemical activities and restrain the sublimation of iodine (stable for B
DOI: 10.1021/acs.nanolett.5b02116 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
displays large specific surface area. After adsorbing iodine, the specific surface area gradually decreased from 1350 m2/g of pure carbon cloth to 57 m2/g of iodine/carbon cloth (with iodine mass of 16.9 mg/cm2). This proved that iodine was filled in the pores of carbon cloth. Therefore, the as-prepared iodine adsorbed on nanoporous carbon has shown much improved thermostability. The as-prepared iodine/nanoporous carbon cloth were directly cut into round size (diameter of 10 mm) as cathodes for assembling coin 2032 batteries without any binder and conductive additives. We applied three different electrolytes to test the electrochemical properties, namely, 1 M LiPF6 in ethylene carbonate (EC)/ethyl methyl carbonate (EMC)/ dimethyl carbonate (DMC) (1:1:1 by volume), 1 M LiTFSI in DOL/DME (1:1 by volume), 1 M LiTFSI in DOL/DME (1:1 by volume) with 1 wt % LiNO3 addition. The commercial ester-based electrolyte and ether-based electrolyte without LiNO3 addition exhibit both low Coulombic efficiency and serious self-discharge phenomenon (Figure 3a,b). This is owing
directly proved with in situ Raman analysis. The as-prepared iodine/carbon cloth cathode exhibited an initial discharge capacity of 299 mAh/g and a capacity of 195 mAh/g after 300 cycles as well as superior rate performance (169 mAh/g at 5 C, about 60% capacity retention of 0.5 C). Meanwhile, an Alplastic film package battery with the energy densities of 475 Wh/kg by mass of Li and iodine, and 136 Wh/kg by total mass of the battery further demonstrated the application prospects of Li-iodine batteries. Our work displayed a superfacile and energy-saving fabrication method for the cathode of rechargeable Li-iodine batteries with high performance (such as low selfdischarge reaction and high Coulombic efficiency). Typical rechargeable Li-iodine batteries mainly consist of Li anode, organic electrolyte, and iodine/carbon cathode (Figure 1a). We applied a room-temperature method to prepare iodine cathode on carbon cloth (Figure 1b). The shaped carbon cloth was put into the iodine aqueous solution and stirred for about 2 h. Then, iodine was fully adsorbed on the carbon cloth and the brown iodine aqueous solution turned clear. This process was named as “solution-adsorption”, as iodine first dissolved in water and then continuously adsorbed on carbon cloth (Supporting Information, Figure S1). The carbon cloth was knited with carbon fiber (Supporting Information, Figure S2) and no structure change was found on its surface after adsorbing iodine (Figure 1c−d). However, when we took EDX mapping on the selected area, C and I were homogeneous distributed (Figure 1e and 1f). These results could be explained that iodine is not existed on the surface of carbon cloth, but in the pores of the carbon. XRD spectra showed that no characteristic peaks of iodine were founded on the prepared iodine/carbon cloth cathode as iodine was adsorbed (Supporting Information, Figure S3). More importantly, comparing with high solubility of pure iodine in both carbonate and ether solvent, the solubility was largely trapped after the adsorption on carbon cloth (Supporting Information, Figure S4). This method could be also applied in other porous carbon materials such as active carbon and ordered-mesoporous carbon (CMK3). The iodine was uniformly adsorbed in the pores of the carbon materials (Supporting Information, Figure S5−7). Thermogravimetric analysis measurement was applied to measure the iodine content and stability of the as-prepared iodine/carbon cloth cathodes. Pure iodine exhibited most mass loss before 100 °C, showing low thermostability. After adsorbing on carbon cloth, there exists nearly no mass loss before 200 °C (Figure 2a), revealing that the thermostability was obviously improved. These results demonstrated the much higher stability of the as-prepared iodine/carbon cloth than that of pure iodine. The iodine mass in the four cathodes were calculated to be 2.5, 5.6, 11.3, and 16.9 mg/cm2, respectively. In addition, 16.9 mg iodine/cm2 was the max mass of iodine adsorbed on carbon cloth. X-ray photoelectron spectroscopy (XPS) analysis was also used to characterize the as-prepared iodine cathode on carbon cloth. The interaction between iodine and carbon host was thought to be mostly physical,26,27 which has been confirmed with the measurement of C1s XPS analysis (Supporting Information, Figure S8). This interaction improved the stability of iodine on carbon cloth. I3d spectra of iodine cathode displayed the typical iodine 3d3/2 peak with binding energy of 630.6 eV and 3d5/2 peak with binding energy of 619.2 eV (Figure 2b).28 N2 adsorption−desorption isotherm further characterized the reducing specific surface area of carbon cloth after adsorbing iodine (Figure 2c). The pure carbon cloth with typical micropore structures (Figure 2d)
Figure 3. Discharge and charge curves of Li-iodine batteries after standing different time at current density of 0.5 C (1 C = 211 mAh/ giodine) with three electrolytes. (a) 1 M LiPF6 in EC/EMC/DMC, (b) 1 M LiTFSI in DOL/DME, (c) 1 M LiTFSI in DOL/DME with 1 wt % LiNO3 addition. SEM images of Li anode in Li-iodine batteries after standing 12 h with three electrolytes. (d) 1 M LiPF6 in EC/EMC/ DMC, (e) 1 M LiTFSI in DOL/DME, (f) 1 M LiTFSI in DOL/DME with 1 wt % LiNO3 addition. The inserts are EDX I mapping of marked area.
to that the easily dissolved iodine species would spontaneously shuttle to the anode side and react with it. This is demonstrated by some microsized precipitate that was formed on the surface of metal Li after standing for 12 h (Figure 3d−e). From insert image of EDX mapping, we could judge that it belongs to iodine compounds. This microsized precipitate might be hard to decompose at charge stage, which causes low Coulombic efficiency. As comparisons, when we applied the ether-based electrolyte with the addition of 1 wt % LiNO3, negligible selfC
DOI: 10.1021/acs.nanolett.5b02116 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters discharge phenomenon was founded even after five days (Figure 3c), meaning the high stability of Li-iodine batteries. The surface of metal Li was relative plan and hardly iodine compounds were formed on the surface of Li anode (Figure 3f). In addition, a passive film on the surface of Li anode was formed (Supporting Information, Figure S9). Further detailed element analysis on metal Li anode demonstrated that elements of O, F, C, S, and N were the major element on the surface of metal Li, which is the main component of the passive film. The element I only took a small bit part on metal Li surface (Supporting Information, Table S1). Previous study showed that LiI can also stabilize lithium metal.21 However, as known in Li-iodine batteries, LiI ion can easily react with I2 to form LiI3, which will cause the side reaction at charging process and decrease the Coulombic efficiency of Li-iodine batteries (Supporting Information, Figure S10). Thus, LiI would unlikely stabilize lithium metal in Li-iodine batteries. The major reason was the passive film formed with the addition of LiNO3 in ether solvent. The initial discharge capacities of first cycles are all about 300 mAh/g (based on the mass of iodine). The capacity is higher than the theoretical capacity of Li-iodine batteries (211 mAh/g) for the reason that carbon cloth also contributes to a part of capacity for capacitance behavior (Supporting Information Figure S11).20 Meanwhile, the Li-iodine battery delivers two reduction plateaus at about 3.3 and 2.9 V. This could be explained that there are two steps of Li-iodine batteries, I2 was reduced to LiI3 at 3.3 V and LiI3 was further reduced to LiI at 2.9 V. The charging is a reversible process, in which LiI was oxidized to LiI3 at 3.0 V and LiI3 was furthered oxidized to I2 at 3.4 V. These two steps were also demonstrated by differential capacity plots and cyclic voltammetry curves (Supporting Information, Figure S12). In order to prove this reaction mechanism of rechargeable Liiodine batteries, we designed an in situ Raman test to record the reaction of discharge and charge on iodine cathode. In short, the cathode cap of normal coin Li-iodine battery was cut into a hole to capture the Raman signals of iodine cathode (the inset diagram in Figure 4a).29,30 About 30 points among discharge and charge were taken to collect Raman spectra (Figure 4a). The major signal we detected is symmetric stretching mode of I3−, which is between 120 and 110 cm−1 (Figure 4b).31−33 At discharge process, peak of I3− was emerged, corresponding to the reaction of I2 to LiI3. The peak of I3− then decayed little by little and disappeared after the full discharge, which indicated the transformation of LiI3 to LiI at the second stage. This was also proved by XRD patterns (Supporting Information, Figure S13). At charge process, the peaks of I3− was first increased and then disappeared. This proved the reversed reaction of charging process, meaning that LiI was first oxidized to LiI3 and LiI3 was further oxidized to I2 (as displayed in Figure 1a). The whole reaction of Li-iodine batteries is 2Li + I2 ↔ 2LiI with two-electron transformation. The dissolution and transport of iodine in the electrolyte could be visually observed by the color change of the separator (Supporting Information, Figure S14). First, the separator was white for fresh battery that stood 12 h. Second, it turned a little brown at 2.9 V for the formation of highly soluble LiI3 and then white again at 2.0 V for the formation of LiI. Because the charging process was reversible, the white separator turned brown at 3.0 V for the oxidation from LiI to LiI3, then turned white at 3.6 V for the oxidation from LiI to iodine. The above reaction mechanism of iodine species matched well with previous study on kinetics of I−/I3−/I2 redox reactions on
Figure 4. In-situ Raman tests of rechargeable Li-iodine batteries. (a) Discharge and charge curves of an in situ Raman cell, the inset is a schematic diagram of an in situ Raman cell. The marked points (from point a to γ) are collected to take Raman tests. (b) In-situ Raman analysis of rechargeable Li-iodine batteries at marked points in panel a.
platinum with LiAsF6/tetrahydrofuran solutions.34 This reversible reaction process was favorable for long cycling stability of rechargeable Li-iodine batteries. Further electrochemical tests of rechargeable Li-iodine batteries were applied to this ether-based electrolyte with the addition of 1 wt % LiNO3. The iodine cathodes with different iodine (2.5 mg/cm2, 5.6 mg/cm2, 11.3 mg/cm2, and 16.1 mg/ cm2) were tested at the same area current density of 0.6 mA/ cm2 (about 0.5 C of 5.6 mg iodine/cm2 cathode). All capacities are calculated by the mass of iodine. The cathode with 5.6 mg iodine/cm2 exhibited both high specific capacity of 299 mAh/g and area capacity of 1.7 mAh/cm2 (Supporting Information, Figure S15). Thus, we focus on the electrochemical tests of this cathode. Further long-cycle measurements showed the high capacity retention with this cathode. After 300 cycles, it still displayed a capacity of 195 mAh/g (Figure 5a). Moreover, the Coulombic efficiency is all above 90%. The decayed capacity is ascribed to that the dissolving iodine species might slowly react with the metal side with long cycling process. This will also result in the gradually increasing resistance (Supporting Information, Figure S16). It should be mentioned that the carbon cloth also contributes a stable capacity of about 19 mAh/g (calculated by carbon cloth, Supporting Information, Figure S17a). In addition to long cycling stability, the asprepared iodine cathode exhibited outstanding high-rate performance (Figure 5b). The capacities of 301, 273, 232, 169 mAh/g were obtained at 0.5 C, 1 C, 2 C, 5 C, respectively. When we gradually decreased the current density back to 0.5 C (30th cycle and 60th cycle), it recurs to higher capacity. The capacity of pure carbon cloth at high rate decays faster than that of iodine/carbon cloth cathode (Supporting Information, Figure S17b). As comparisons, after 100 cycles at 0.6 mA/ cm2, the cathodes with the iodine mass of 2.5 mg/cm2 and 11.3 mg/cm2 exhibited capacities of 307 mAh/g and 109 mAh/g, respectively (Supporting Information Figure S18). This optimized ether-based electrolyte was also appropriate in other iodine/carbon materials (Supporting Information Figure S19). We also assembled a soft-package Li-iodine battery to D
DOI: 10.1021/acs.nanolett.5b02116 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 5. Electrochemical performance of the as-prepared iodine cathodes and soft-package Li-iodine batteries. (a) Cycling performance with Coulombic efficiency of iodine cathode (iodine mass, 5.6 mg/cm2) at a current density of 105 mA/g (0.5 C). (b) Rate performance with equal mass cathode. (c) Digital photos of the as-prepared iodine cathode on nanoporous carbon cloth with excellent flexibility. The insert is Al-plastic film package Li-iodine battery with a lighten LED. (d) Cycling performance of soft package Li-iodine battery for initial 60 cycles at a current density of 20 mA/g (about 0.1 C rate).
ance rechargeable Li-iodine batteries. The cathode was prepared at room-temperature and aqueous solution with nearly zero energy consumption. Iodine was strongly absorbed on high specific surface carbon cloth and largely improved the thermal stability of the as-prepared cathode. Combined with ether-based electrolyte with LiNO3 addition, the assembled Liiodine batteries exhibited negligible self-discharge phenomenon, long cycling, and high-rate performance. In addition, in situ Raman test demonstrated the reversible redox couples of I2/ LiI3 and LiI3/LiI reaction mechanism of Li-iodine batteries. This easily preparing method with iodine adsorption on porous carbon would be directive for the cathode development of rechargeable Li-iodine batteries with high performance.
further manifest the application prospect of the as-prepared Liiodine batteries. Owing to excellent flexibility of the as-prepared iodine cathode on carbon cloth, it is facile to assemble a soft package Li-iodine battery (Figure 5c). In addition, this battery delivered an initial energy density of 475 Wh/kg by iodine and Li metal, which is higher than aqueous cathode of Li-iodine batteries.7 In addition, a total energy density of 136 Wh/kg was obtained for the whole battery. The energy density turned stable with cycling. After initial 60 cycles, a retentive energy density of 107 Wh/kg was obtained (Figure 5d). As known, the regular electrode preparation usually contains several steps such as mixing the active substrate with binder and conductive additives, then smearing the paste on to the current collector, and finally dried in the vacuum. In this case, a huge amount of time and manufacturing cost would be taken. The present iodine cathode was prepared in room-temperature with nearly zero emission and energy consumption. Meanwhile, no additional steps were needed in assembling batteries, which will be favorable in industry production. In addition, the theoretical energy density of Li-iodine battery is higher than that of commercial Li-ion batteries (Supporting Information, Table S2)35−37 and the practical soft-package battery also showed comparable energy density.38 The present study demonstrates the potential application of iodine/nanoporous carbon as the cathode of rechargeable Li-iodine batteries. In conclusion, we applied a “solution-adsorption” method to prepare iodine/nanoporous carbon cathode for high-perform-
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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/acs.nanolett.5b02116. Detailed experimental procedures and additional materials characterization. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. E
DOI: 10.1021/acs.nanolett.5b02116 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters Notes
(32) Stephanie, B. S.; Gregory, I. G. J. Phys. Chem. A 1997, 101, 2192−2197. (33) Štangar, U. L.; Orel, B.; Vuk, A. S.; Sagon, G.; Colomban, P.; Stathatos, E.; Lianos, P. J. Electrochem. Soc. 2002, 149, E413−E423. (34) Behl, W. K.; Chin, D. T. J. Electrochem. Soc. 1988, 135, 16−21. (35) Whittingham, M. S. Chem. Rev. 2004, 104, 4271−4302. (36) Chung, S. Y.; Bloking, J. T.; Chiang, Y. M. Nat. Mater. 2002, 1, 123−128. (37) Cheng, F.; Wang, H.; Zhu, Z.; Wang, Y.; Zhang, T.; Tao, Z.; Chen, J. Energy Environ. Sci. 2011, 4, 3668−3675. (38) Dunn, B.; Kamath, H.; Tarascon, J. M. Science 2011, 334, 928− 935.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Programs of National 973 (2011CB935900), NSFC (51231003 and 21322101), and MOE (B12015 and 113016A).
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DOI: 10.1021/acs.nanolett.5b02116 Nano Lett. XXXX, XXX, XXX−XXX
