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Metal–Organic Frameworks as Cathode Materials for Li–O2 Batteries Doufeng Wu, Ziyang Guo, Xinbo Yin, Qingqing Pang, Binbin Tu, Lijuan Zhang,* Yong-Gang Wang,* and Qiaowei Li* Recently, Li–O2 batteries have attracted extensive research owing to their higher theoretical gravimetric energy density than that of other chemical batteries.[1] In its most common configuration, the Li–O2 battery comprises a lithium metal anode, an electrolyte for Li+ conduction, and an O2 electrode consisting of an electrically conductive porous material, such as mesoporous carbon structures[2] and graphene.[3] The O2 electrode material provides channels for the transport of lithium and oxygen,[1b] space for the deposition of discharge products,[4] and in some cases, active sites for the reaction.[1f,5] However, there remain several challenges in the design and construction of O2 electrodes: i) the supply of O2 to the reaction is hampered by the weak interaction between O2 molecules and the electrode; ii) electrolyte and product deposition into the pores blocks the path for O2 diffusion,[6] and impedes the reaction from proceeding efficiently; and iii) without full structural details of electrode materials with low periodicity, some cells lack high repeatability in their electrochemical behavior, and are thus likely to complicate further improvement in performance. Reversibility of Li2O2 formation is also affected by parasitic reactions due to the decomposition of the electrolyte used.[1d-e,7] We sought to address these issues by using for the electrode material a composite that contains two types of pore. Our materials have well-defined metal–organic framework (MOF)[8] micropores that are lined with open metal sites onto which enhanced O2 binding can take place, and also larger carbon material mesopores ideally suited for electrolyte diffusion and product deposition. MOFs as the O2 electrode materials offer uniform pores with high surface areas and adjustable chemical environments for specific interactions with guests,[9] which lead to O2 enrichment in the electrode. These micropores are mainly for O2 transportation, which could not be fully occupied by the larger molecules during battery operation. Furthermore, the regularity, flexibility, and precision with which MOFs can be made and modified allows for further improvements in the battery performance.
D. Wu, Z. Guo, Q. Pang, B. Tu, Prof. Y.-G. Wang, Prof. Q. Li Department of Chemistry Fudan University 220 Handan Road, Shanghai 200433, China E-mail: [email protected]; [email protected] X. Yin, Dr. L. Zhang Laboratory of Advanced Materials Fudan University 2205 Songhu Road, Shanghai 200438, China E-mail: [email protected]
DOI: 10.1002/adma.201305492
Adv. Mater. 2014, DOI: 10.1002/adma.201305492
Although MOF open architectures have been shown to serve as reservoirs for polysulfide to further stabilize the sulfur cathodes in Li–S battery,[10] application of MOFs as the cathode materials for Li–O2 cells remains largely unexplored. Herein, we show that MOFs can increase the concentration of O2 in the micropores by up to eighteen times under ambient pressure at 273 K. We further show that combinations of MOFs and carbon black (Super P) as cathode materials produce higher discharge capacities than Super P alone. Specifically, the Mn-MOF-74– Super P combination delivers a primary capacity of 9420 mA h g−1 under 1 atm of O2 at room temperature, which is more than four times higher than the capacity in a corresponding cell without an MOF. We selected MOF-5,[11] HKUST-1,[12] and M-MOF-74 (M = Mg, Mn, Co)[13] as possible MOFs for the O2 electrode materials and assessed how their structural attributes impact performance (Figure 1 and Figure S9 in the Supporting Information). These MOFs provide a wide range of surface areas,
Figure 1. Crystal structures of a) MOF-5, b) HKUST-1, and c) the isostructural M-MOF-74, and d) a view of the 1D channel (yellow cylinder) in M-MOF-74. Blue polyhedra and spheres, red spheres, and gray spheres represent metal, oxygen, and carbon atoms, respectively. Hydrogen atoms are omitted for clarity.
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www.MaterialsViews.com Table 1. BET surface areas, oxygen adsorption data, and corresponding primary discharge capacities of Li–O2 batteries for MOFs and related materials. Materials MOF-5
BET [m2 g−1]
O2 uptakea) [mg g−1]
Capacityb) [mA h g−1]
3622
6.6
1780
[12b]
16.6
4170
Co-MOF-74
1446
18.4
3630
Mg-MOF-74
1582
15.1
4560
Mn-MOF-74
1213
15.0
9420
Super P
87.6
< 0.7
2170c)
Mn(OAc)2
—
—
1920
Mn2(DOBDC)(H2O)2
—
—
2820
HKUST-1
1944
a)At
273 K and 1 atm; b)O2 electrode: 40 wt% material, 40 wt% Super P, and 20 wt% PVDF, with a total loading around 0.7 mg cm−2. The primary discharge capacity is based on the combined weight of MOF and Super P. See the Supporting Information for the capacities reported under different standards; c)O2 electrode: 80 wt% Super P and 20 wt% PVDF. The discharge capacity is based on the weight of Super P.
diverse structural topologies, and uniquely accessible metal sites. MOF-5 [Zn4O(BDC)3, BDC = 1,4-benzenedicarboxylate] is an infinite cubic framework with a Brunauer–Emmett– Teller (BET) surface area of 3622 m2 g−1 (see the Supporting Information). Pores in guest-free HKUST-1 [Cu3(BTC)2, BTC = benzene-1,3,5-tricarboxylate] are decorated with unsaturated Cu(II) coordination centers, which provide binding sites for incoming guests. M-MOF-74 is a series of MOFs built from various divalent metal ions and 2,5-dihydroxyterephthalic acid (H4DOBDC), but with the same hexagonal, 1D pore structure. The M-MOF-74 series is ideal to study the role of various open metal sites in impacting the discharge capacities of the corresponding Li–O2 batteries. Low-pressure nitrogen adsorption isotherms of these prototype MOFs and Super P were performed at 77 K (Figure S6, Supporting Information), with their BET surface areas listed in Table 1. These values are in agreement with those already reported.[11–13] To investigate the O2-enrichment capabilities of these porous MOFs, their low-pressure O2 adsorption isotherms were examined at 273 K (Figure 2 and Table 1, and Figure S7, Supporting Information). For all five MOF materials, uptake of O2 increases linearly as a function of pressure, without reaching a plateau. The coordination-saturated MOF-5, which has the highest surface area in this study, has an O2 uptake of 6.6 mg g−1 at 1 atm. Both HKUST-1 and M-MOF-74, with lower surface areas than MOF-5, have pores lined with open metal sites, and therefore have higher uptake. Each gram of HKUST-1 takes up 16.6 mg O2 at 1 atm. Members of the M-MOF-74 series are isostructural, and exhibit similar adsorptive capacities (from 15.0–18.4 mg g−1 at 1 atm). By comparison, Super P, which has a surface area of 87.6 m2 g−1, takes up less than 0.7 mg g−1 of O2 under the same condition. For Mn-MOF-74, the high capacity (15.0 mg g−1) represents an O2 concentration in the pores eighteen times higher than pure oxygen at 1 atm (see the Supporting Information for calculation details). Open metal sites in these MOFs have been shown to bind gas molecules, such as CO2 and C2H2, and increase their uptake capacities.[14] Thus, the presence of an MOF acts to
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Figure 2. Low-pressure O2-adsorption isotherms for the MOFs at 273 K, with the adsorbed amount at 1 atm listed in parentheses. Filled and empty symbols represent adsorption and desorption, respectively.
compact O2 molecules within the pores, and thus significantly increases the amount of O2 available for use in the Li–O2 battery without a corresponding increase in the volume and weight of the cathode. To evaluate the performance of MOFs in the Li–O2 battery, we incorporated the guest-free MOFs as the cathode materials, along with Super P. Specifically, a mixture composed of MOF crystals, Super P, and polyvinylidene fluoride (PVDF) as binder was coated onto carbon paper as the O2 electrode. This mixture provides pores with distinct sizes (mesopores in Super P and micropores in MOFs (Figure S8, Supporting Information)) and moderate electrical conductivity from carbon. Tetraethylene glycol dimethyl ether (TEGDME) was employed as the electrolyte because of its relatively high stability towards superoxide (O2−).[1d] Electrochemical tests involving the discharge and charge of these assemblies (Figure 3a and Figure S1a, Supporting Information) were performed in an O2-atmosphere glove box (see the Supporting Information for battery assembly and electrochemical test details). Primary discharge profiles of Li–O2 batteries that use MOF–Super P composites as the O2 electrodes are shown in Figure 3b, with an applied current density of 50 mA g−1. All the cells show very stable operating voltages of between 2.6–2.7 V, similar to a wide range of Li–O2 cells reported.[15] The Li–O2 battery that uses Super P alone has a capacity of 2170 mA h g−1. The discharge capacities for batteries using HKUST-1, Mg-MOF-74, and Co-MOF-74 are 4170, 4560, and 3630 mA h g−1, respectively (Table 1). Moreover, the battery containing Mn-MOF-74 demonstrates the highest discharge capacity in this study, at 9420 mA h g−1; this is a 330% enhancement of the discharge performance compared to the cell without an MOF. Clearly, MOFs play a key role in capacity enhancement, perhaps due to the open metal sites in HKUST-1 and M-MOF-74. The carbon paper may also facilitate O2 reduction, and thus contribute some capacity on discharge. Our test shows the capacity of the carbon paper alone is quite limited, with lower
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Figure 3. a) Schematic illustration of a Li–O2 cell using MOF–Super P composite as the O2 electrode. Oxygen molecules relative sizes reduced for clarity. b) Discharge profiles of the Li–O2 cells using MOF–Super P composites or Super P only under O2 atmosphere with a current of 50 mA g−1 at room temperature.
before electrochemical tests, after discharge, and after charge are coincident. The MOF’s stability towards Li2O2 was further confirmed by the XRD pattern of a solid mixture prepared by blending Mn-MOF-74 and Li2O2 with grinding (Figure S14, Supporting Information). In Figure 4, besides diffraction peaks from Mn-MOF-74 and the carbon paper, the XRD pattern also indicates the formation of Li2O2 as the discharge product in the electrode, which is in accordance with observations in other similar Li–O2 systems.[1a] We also observed the presence of LiOH in some MOF-based electrodes after discharge, which is attributed to the fact that we recorded the XRD patterns in air (see the Supporting Information for details). Nevertheless, the stability of the MOFs is evident by the sharpness and high intensity of the diffraction lines. The robustness of MOFs avoids the problem of self-aggregation often encountered by nanostructural materials,[5c] and thus maintains stable electrode integrity. The cell with an MOF-5 electrode, which has the lowest O2 uptake among the MOFs investigated, delivers a capacity of 1780 mA h g−1. This value is still higher than that of Super P electrode in terms of the discharge capacity based on the weight of carbon material only.[16] The Li–O2 battery with nonporous anhydrous manganese(II) acetate delivers a capacity of 1920 mA h g−1 (Figure S4 in the Supporting Information and Table 1). Another MOF structure that shares the same backbone framework with Mn-MOF-74, Mn2(DOBDC)(H2O)2, was prepared by covering the open metal sites in Mn-MOF-74 with H2O coordination (see the Supporting Information for details). The Mn2(DOBDC)(H2O)2–Super P system displays a discharge capacity of 2820 mA h g−1, only 30% of the capacity obtained from Mn-MOF-74. These results clearly evidence the intrinsic importance of open metal sites accessibility in achieving higher capacity in an MOF-based Li–O2 battery.
operating voltage than that of our combined cell (Figure S5, Supporting Information). The discharge capacity values are reported here normalized with respect to the combined mass of MOF and Super P.[16] For the Mn-MOF-74 based Li–O2 cell, the discharge capacity is 1010 mA h g−1 with respect to the total mass including MOF, Super P, PVDF, and the formed Li2O2 after discharge, which corresponds to 756 mA h g−1 with respect to the total weight of the electrode including carbon paper, and 6.76 mA h cm−2 with respect to the area of the O2 electrode.[17] Table S1 summarizes the capacities at different standards for all the MOF-based Li–O2 cells. For the Mn-MOF-74 based cell, the volume fraction of Li2O2 generated on discharge is 12.0%, considering that the thickness of the electrode including the coating is 0.21 mm. These measurements were repeated multiple times to give similar capacities under the same test conditions, and the values reported above are the typical values obtained. The stability of the cathode materials was examined by measuring the powder X-ray diffraction (XRD) pattern of the disassembled electrodes. These results are shown in Figure 4. Powder XRD patterns of the simulated Mn-MOF-74 (blue), and the O2 electrodes Figure 4 and Figure S10–S13 in the Sup- before electrochemical test (orange), after discharge (green), and after a discharge/charge cycle porting Information, where the diffrac- (red). Inset shows the PXRD pattern of the O2 electrodes with longer scanning time, which tion peaks of the MOFs in the electrodes confirms the formation of Li2O2 during discharge.
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Figure 5. a) Cycling response of the Mn-MOF-74 based battery. Current: 200 mA g−1. b) In situ O2 and CO2 evolution measured by DEMS on charging of a Mn-MOF-74 based cell in response to a current of 300 mA g−1 at 0 min, with the voltage profile shown in purple. c) O2 and CO2 analysis of the accumulated evolved gas after charge.
A full discharge–charge cycle of the Mn-MOF-74 based cell is shown in Figure S2 in the Supporting Information. A cycling test of the Mn-MOF-74 based cell was performed in a voltage range of 2.0–4.5 V vs. Li+/Li at a current of 200 mA g−1 for six full cycles (Figure 5a). The reversibility of our Li–O2 cell could be impacted by the possible electrolyte decomposition;[1e] on the other hand, the gas–solid state conversion during the reaction may also affect electrode performance.[18] The discharge profiles at rates progressively increasing from 50–500 mA g−1 (Figure S3, Supporting Information) demonstrate that the cell can still operate efficiently with slightly lower voltage at high current rates. To rule out the possibility that the increased capacity results from parasitic reactions, in situ differential electrochemical mass spectrometry (DEMS) was employed to measure the gas evolution during charging[19] (Figure S1b, see the Supporting Information for more details). The result (Figure 5b) shows that O2 is evolved dominantly during the charge, and almost no CO2 evolves in response to a 300 mA g−1 current, which suggests the decomposition of Li2O2 during charge. Mass spectrometry analysis was also carried out to examine the accumulated evolved gases after charge (Figure 5c), which showed that O2, but not CO2, is evolved. Compared with Super P, MOFs with open metal sites provide a more polarized surface in tailored pores, and they are capable of interacting with small molecules and ions more strongly.[14] M-MOF-74 has a high density of 3.3 accessible open metal sites available per 100 Å2 on the inner surface. Furthermore, M-MOF-74 has uniform 1D channels (Figure 1d) with a diameter of 11.0 Å. This channel is large enough to permit
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entry of oxygen molecules (kinetic diameter of 3.46 Å), and acts as reservoirs of O2 to feed the reaction. Among its isostructural analogs with similar O2 adsorption behavior, Mn-MOF-74 achieved the highest discharge capacity, showing possible catalytic activity of the Mn sites, compared to the Mg and Cocontaining MOFs. In summary, we have shown that a primary capacity of 9420 mA h g−1 under 1 atm of oxygen was achieved in a Li–O2 battery based on the robust Mn-MOF-74; more than four times higher than the value obtained in a cell without an MOF. The accessible open metal sites in the uniform channels enhance the population of O2 molecules in the pores, and assist the reaction efficiently towards high capacity. These findings indicate that MOFs represent a viable option as cathode materials in Li–O2 batteries. The diversity of MOFs and the facility with which their organic links and multimetal building units can be varied should lend many advantages in crafting electrode materials.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors acknowledge funding support from the Creative Research Group of MOE (IRT1117), National Natural Science Foundation of China (21101030, 21071032), and Shanghai Chenguang Scholar Program (12CG01). Received: November 5, 2013 Revised: December 30, 2013 Published online: [1] a) K. M. Abraham, Z. Jiang, J. Electrochem. Soc. 1996, 143, 1–5; b) P. G. Bruce, S. A. Freunberger, L. J. Hardwick, J.-M. Tarascon, Nat. Mater. 2011, 11, 19–29; c) A. Kraytsberg, Y. Ein-Eli, J. Power Sources 2011, 196, 886–893; d) H.-G. Jung, J. Hassoun, J.-B. Park, Y.-K. Sun, B. Scrosati, Nat. Chem. 2012, 4, 579–585; e) Z. Peng, S. A. Freunberger, Y. Chen, P. G. Bruce, Science 2012, 337, 563–566; f) Y.-C. Lu, Z. Xu, H. A. Gasteiger, S. Chen, K. Hamad-Schifferli, Y. Shao-Horn, J. Am. Chem. Soc. 2010, 132, 12170–12171; g) J.-J. Xu, D. Xu, Z.-L. Wang, H.-G. Wang, L.-L. Zhang, X.-B. Zhang, Angew. Chem. Int. Ed. 2013, 52, 3887–3890; h) Y. Shao, F. Ding, J. Xiao, J. Zhang, W. Xu, S. Park, J.-G, Zhang, Y. Wang, J. Liu, Adv. Funct. Mater. 2013, 23, 987–1004. [2] a) J. Xiao, D. Wang, W. Xu, D. Wang, R. E. Williford, J. Liu, J.-G. Zhang, J. Electrochem. Soc. 2010, 157, A487–A492; b) Z. Guo, D. Zhou, X. Dong, Z. Qiu, Y. Wang, Y. Xia, Adv. Mater. 2013, 25, 5668–5672; c) H.-D. Lim, K.-Y. Park, H. Song, E. Y. Jang,
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H. Gwon, J. Kim, Y. H. Kim, M. D. Lima, R. O. Robles, X. Lepró, R. H. Baughman, K. Kang, Adv. Mater. 2013, 25, 1348–1352. J. Xiao, D. Mei, X. Li, W. Xu, D. Wang, G. L. Graff, W. D. Bennett, Z. Nie, L. V. Saraf, I. A. Aksay, J. Liu, J.-G. Zhang, Nano Lett. 2011, 11, 5071–5078. a) R. Black, S. H. Oh, J.-H. Lee, T. Yim, B. Adams, L. F. Nazar, J. Am. Chem. Soc. 2012, 134, 2902–2905; b) J.-J. Xu, Z.-L. Wang, D. Xu, L.-L. Zhang, X.-B. Zhang, Nat. Commun. DOI: 10.1038/ ncomms3438. a) S. H. Oh, L. F. Nazar, Adv. Energy. Mater. 2012, 2, 903–910; b) A. Débart, A. J. Paterson, J. Bao, P. G. Bruce, Angew. Chem. Int. Ed. 2008, 47, 4521–4524; c) Y. Zhao, L. Xu, L. Mai, C. Han, Q. An, X. Xu, X. Liu, Q. Zhang, Proc. Natl. Acad. Sci. USA 2012, 109, 19569–19574. S. S. Zhang, D. Foster, J. Read, J. Power Sources 2010, 195, 1235–1240. a) S. A. Freunberger, Y. Chen, Z. Peng, J. M. Griffin, L. J. Hardwick, F. Bardé, P. Novák, P. G. Bruce, J. Am. Chem. Soc. 2011, 133, 8040– 8047; b) S. A. Freunberger, Y. Chen, N. E. Drewett, L. J. Hardwick, F. Bardé, P. G. Bruce, Angew. Chem. Int. Ed. 2011, 50, 8609–8613. a) O. M. Yaghi, M. O’Keeffe, N. W. Ockwig, H. K. Chae, M. Eddaoudi, J. Kim, Nature 2003, 423, 705–714; b) S. Kitagawa, R. Kitaura, S. Noro, Angew. Chem. Int. Ed. 2004, 43, 2334–2375; c) G. Férey, Chem. Soc. Rev. 2008, 37, 191–214; d) H. Furukawa, K. E. Cordova, M. O’Keeffe, O. M. Yaghi, Science, 2013, 341, 1230444; e) S. T. Meek, J. A. Greathouse, M. D. Allendorf, Adv. Mater. 2011, 23, 249–267; f) H.-C. Zhou, J. R. Long, O. M. Yaghi, Chem. Rev. 2012, 112, 673–674. Q. Li, W. Zhang, O. Š. Miljanic´, C.-H. Sue, Y.-L. Zhao, L. Liu, C. B. Knobler, J. F. Stoddart, O. M. Yaghi, Science 2009, 325, 855–859.
[10] R. Demir-Cakan, M. Morcrette, F. Nouar, C. Davoisne, T. Devic, D. Gonbeau, R. Dominko, C. Serre, G. Férey, J.-M. Tarascon, J. Am. Chem. Soc. 2011, 133, 16154–16160. [11] H. Li, M. Eddaoudi, M. O’Keeffe, O. M. Yaghi, Nature 1999, 402, 276–279. [12] a) S. S.-Y. Chui, S. M.-F. Lo, J. P. H. Charmant, A. G. Orpen, I. D. Williams, Science 1999, 283, 1148–1150; b) A. G. Wong-Foy, A. J. Matzger, O. M. Yaghi, J. Am. Chem. Soc. 2006, 128, 3494– 3495. [13] a) S. R. Caskey, A. G. Wong-Foy, A. J. Matzger, J. Am. Chem. Soc. 2008, 130, 10870–10871; b) W. Zhou, H. Wu, T. Yildirim, J. Am. Chem. Soc. 2008, 130, 15268–15269; c) H. Wu, W. Zhou, T. Yildirim, J. Am. Chem. Soc. 2009, 131, 4995–5000; d) Y.-S. Bae, C. Y. Lee, K. C. Kim, O. K. Farha, P. Nickias, J. T. Hupp, S. T. Nguyen, R. Q. Snurr, Angew. Chem. Int. Ed. 2012, 51, 1857–1860; e) P. D. C. Dietzel, Y. Morita, R. Blom, H. Fjellvag, Angew. Chem. Int. Ed. 2005, 44, 6354–6358. [14] X. Kong, E. Scott, W. Ding, J. A. Mason, J. R. Long, J. A. Reimer, J. Am. Chem. Soc. 2012, 134, 14341–14344. [15] Y.-C. Lu, H. A. Gasteiger, M. C. Parent, V. Chiloyan, Y. Shao-Horn, Electrochem. Solid State Lett. 2010, 13, A69–A72. [16] Capacity values based on the mass of carbon materials have often been reported. In terms of the capacities based on the weight of carbon only, the values would be twice as high as those reported here for MOF-based systems. See the Supporting Information for more details. [17] Y. Gogotsi, P. Simon, Science 2011, 334, 917–918. [18] B. D. McCloskey, D. S. Bethune, R. M. Shelby, T. Mori, R. Scheffler, A. Speidel, M. Sherwood, A. C. Luntz, J. Phys. Chem. Lett. 2012, 3, 3043–3047. [19] H. Baltruschat, J. Am. Soc. Mass Spectrom. 2004, 15, 1693–1706.
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Metal–Organic Frameworks as Cathode Materials for Li–O2 Batteries Doufeng Wu, Ziyang Guo, Xinbo Yin, Qingqing Pang, Binbin Tu, Lijuan Zhang,* Yong-Gang Wang,* and Qiaowei Li* Recently, Li–O2 batteries have attracted extensive research owing to their higher theoretical gravimetric energy density than that of other chemical batteries.[1] In its most common configuration, the Li–O2 battery comprises a lithium metal anode, an electrolyte for Li+ conduction, and an O2 electrode consisting of an electrically conductive porous material, such as mesoporous carbon structures[2] and graphene.[3] The O2 electrode material provides channels for the transport of lithium and oxygen,[1b] space for the deposition of discharge products,[4] and in some cases, active sites for the reaction.[1f,5] However, there remain several challenges in the design and construction of O2 electrodes: i) the supply of O2 to the reaction is hampered by the weak interaction between O2 molecules and the electrode; ii) electrolyte and product deposition into the pores blocks the path for O2 diffusion,[6] and impedes the reaction from proceeding efficiently; and iii) without full structural details of electrode materials with low periodicity, some cells lack high repeatability in their electrochemical behavior, and are thus likely to complicate further improvement in performance. Reversibility of Li2O2 formation is also affected by parasitic reactions due to the decomposition of the electrolyte used.[1d-e,7] We sought to address these issues by using for the electrode material a composite that contains two types of pore. Our materials have well-defined metal–organic framework (MOF)[8] micropores that are lined with open metal sites onto which enhanced O2 binding can take place, and also larger carbon material mesopores ideally suited for electrolyte diffusion and product deposition. MOFs as the O2 electrode materials offer uniform pores with high surface areas and adjustable chemical environments for specific interactions with guests,[9] which lead to O2 enrichment in the electrode. These micropores are mainly for O2 transportation, which could not be fully occupied by the larger molecules during battery operation. Furthermore, the regularity, flexibility, and precision with which MOFs can be made and modified allows for further improvements in the battery performance.
D. Wu, Z. Guo, Q. Pang, B. Tu, Prof. Y.-G. Wang, Prof. Q. Li Department of Chemistry Fudan University 220 Handan Road, Shanghai 200433, China E-mail: [email protected]; [email protected] X. Yin, Dr. L. Zhang Laboratory of Advanced Materials Fudan University 2205 Songhu Road, Shanghai 200438, China E-mail: [email protected]
DOI: 10.1002/adma.201305492
Adv. Mater. 2014, DOI: 10.1002/adma.201305492
Although MOF open architectures have been shown to serve as reservoirs for polysulfide to further stabilize the sulfur cathodes in Li–S battery,[10] application of MOFs as the cathode materials for Li–O2 cells remains largely unexplored. Herein, we show that MOFs can increase the concentration of O2 in the micropores by up to eighteen times under ambient pressure at 273 K. We further show that combinations of MOFs and carbon black (Super P) as cathode materials produce higher discharge capacities than Super P alone. Specifically, the Mn-MOF-74– Super P combination delivers a primary capacity of 9420 mA h g−1 under 1 atm of O2 at room temperature, which is more than four times higher than the capacity in a corresponding cell without an MOF. We selected MOF-5,[11] HKUST-1,[12] and M-MOF-74 (M = Mg, Mn, Co)[13] as possible MOFs for the O2 electrode materials and assessed how their structural attributes impact performance (Figure 1 and Figure S9 in the Supporting Information). These MOFs provide a wide range of surface areas,
Figure 1. Crystal structures of a) MOF-5, b) HKUST-1, and c) the isostructural M-MOF-74, and d) a view of the 1D channel (yellow cylinder) in M-MOF-74. Blue polyhedra and spheres, red spheres, and gray spheres represent metal, oxygen, and carbon atoms, respectively. Hydrogen atoms are omitted for clarity.
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www.MaterialsViews.com Table 1. BET surface areas, oxygen adsorption data, and corresponding primary discharge capacities of Li–O2 batteries for MOFs and related materials. Materials MOF-5
BET [m2 g−1]
O2 uptakea) [mg g−1]
Capacityb) [mA h g−1]
3622
6.6
1780
[12b]
16.6
4170
Co-MOF-74
1446
18.4
3630
Mg-MOF-74
1582
15.1
4560
Mn-MOF-74
1213
15.0
9420
Super P
87.6
< 0.7
2170c)
Mn(OAc)2
—
—
1920
Mn2(DOBDC)(H2O)2
—
—
2820
HKUST-1
1944
a)At
273 K and 1 atm; b)O2 electrode: 40 wt% material, 40 wt% Super P, and 20 wt% PVDF, with a total loading around 0.7 mg cm−2. The primary discharge capacity is based on the combined weight of MOF and Super P. See the Supporting Information for the capacities reported under different standards; c)O2 electrode: 80 wt% Super P and 20 wt% PVDF. The discharge capacity is based on the weight of Super P.
diverse structural topologies, and uniquely accessible metal sites. MOF-5 [Zn4O(BDC)3, BDC = 1,4-benzenedicarboxylate] is an infinite cubic framework with a Brunauer–Emmett– Teller (BET) surface area of 3622 m2 g−1 (see the Supporting Information). Pores in guest-free HKUST-1 [Cu3(BTC)2, BTC = benzene-1,3,5-tricarboxylate] are decorated with unsaturated Cu(II) coordination centers, which provide binding sites for incoming guests. M-MOF-74 is a series of MOFs built from various divalent metal ions and 2,5-dihydroxyterephthalic acid (H4DOBDC), but with the same hexagonal, 1D pore structure. The M-MOF-74 series is ideal to study the role of various open metal sites in impacting the discharge capacities of the corresponding Li–O2 batteries. Low-pressure nitrogen adsorption isotherms of these prototype MOFs and Super P were performed at 77 K (Figure S6, Supporting Information), with their BET surface areas listed in Table 1. These values are in agreement with those already reported.[11–13] To investigate the O2-enrichment capabilities of these porous MOFs, their low-pressure O2 adsorption isotherms were examined at 273 K (Figure 2 and Table 1, and Figure S7, Supporting Information). For all five MOF materials, uptake of O2 increases linearly as a function of pressure, without reaching a plateau. The coordination-saturated MOF-5, which has the highest surface area in this study, has an O2 uptake of 6.6 mg g−1 at 1 atm. Both HKUST-1 and M-MOF-74, with lower surface areas than MOF-5, have pores lined with open metal sites, and therefore have higher uptake. Each gram of HKUST-1 takes up 16.6 mg O2 at 1 atm. Members of the M-MOF-74 series are isostructural, and exhibit similar adsorptive capacities (from 15.0–18.4 mg g−1 at 1 atm). By comparison, Super P, which has a surface area of 87.6 m2 g−1, takes up less than 0.7 mg g−1 of O2 under the same condition. For Mn-MOF-74, the high capacity (15.0 mg g−1) represents an O2 concentration in the pores eighteen times higher than pure oxygen at 1 atm (see the Supporting Information for calculation details). Open metal sites in these MOFs have been shown to bind gas molecules, such as CO2 and C2H2, and increase their uptake capacities.[14] Thus, the presence of an MOF acts to
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Figure 2. Low-pressure O2-adsorption isotherms for the MOFs at 273 K, with the adsorbed amount at 1 atm listed in parentheses. Filled and empty symbols represent adsorption and desorption, respectively.
compact O2 molecules within the pores, and thus significantly increases the amount of O2 available for use in the Li–O2 battery without a corresponding increase in the volume and weight of the cathode. To evaluate the performance of MOFs in the Li–O2 battery, we incorporated the guest-free MOFs as the cathode materials, along with Super P. Specifically, a mixture composed of MOF crystals, Super P, and polyvinylidene fluoride (PVDF) as binder was coated onto carbon paper as the O2 electrode. This mixture provides pores with distinct sizes (mesopores in Super P and micropores in MOFs (Figure S8, Supporting Information)) and moderate electrical conductivity from carbon. Tetraethylene glycol dimethyl ether (TEGDME) was employed as the electrolyte because of its relatively high stability towards superoxide (O2−).[1d] Electrochemical tests involving the discharge and charge of these assemblies (Figure 3a and Figure S1a, Supporting Information) were performed in an O2-atmosphere glove box (see the Supporting Information for battery assembly and electrochemical test details). Primary discharge profiles of Li–O2 batteries that use MOF–Super P composites as the O2 electrodes are shown in Figure 3b, with an applied current density of 50 mA g−1. All the cells show very stable operating voltages of between 2.6–2.7 V, similar to a wide range of Li–O2 cells reported.[15] The Li–O2 battery that uses Super P alone has a capacity of 2170 mA h g−1. The discharge capacities for batteries using HKUST-1, Mg-MOF-74, and Co-MOF-74 are 4170, 4560, and 3630 mA h g−1, respectively (Table 1). Moreover, the battery containing Mn-MOF-74 demonstrates the highest discharge capacity in this study, at 9420 mA h g−1; this is a 330% enhancement of the discharge performance compared to the cell without an MOF. Clearly, MOFs play a key role in capacity enhancement, perhaps due to the open metal sites in HKUST-1 and M-MOF-74. The carbon paper may also facilitate O2 reduction, and thus contribute some capacity on discharge. Our test shows the capacity of the carbon paper alone is quite limited, with lower
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Figure 3. a) Schematic illustration of a Li–O2 cell using MOF–Super P composite as the O2 electrode. Oxygen molecules relative sizes reduced for clarity. b) Discharge profiles of the Li–O2 cells using MOF–Super P composites or Super P only under O2 atmosphere with a current of 50 mA g−1 at room temperature.
before electrochemical tests, after discharge, and after charge are coincident. The MOF’s stability towards Li2O2 was further confirmed by the XRD pattern of a solid mixture prepared by blending Mn-MOF-74 and Li2O2 with grinding (Figure S14, Supporting Information). In Figure 4, besides diffraction peaks from Mn-MOF-74 and the carbon paper, the XRD pattern also indicates the formation of Li2O2 as the discharge product in the electrode, which is in accordance with observations in other similar Li–O2 systems.[1a] We also observed the presence of LiOH in some MOF-based electrodes after discharge, which is attributed to the fact that we recorded the XRD patterns in air (see the Supporting Information for details). Nevertheless, the stability of the MOFs is evident by the sharpness and high intensity of the diffraction lines. The robustness of MOFs avoids the problem of self-aggregation often encountered by nanostructural materials,[5c] and thus maintains stable electrode integrity. The cell with an MOF-5 electrode, which has the lowest O2 uptake among the MOFs investigated, delivers a capacity of 1780 mA h g−1. This value is still higher than that of Super P electrode in terms of the discharge capacity based on the weight of carbon material only.[16] The Li–O2 battery with nonporous anhydrous manganese(II) acetate delivers a capacity of 1920 mA h g−1 (Figure S4 in the Supporting Information and Table 1). Another MOF structure that shares the same backbone framework with Mn-MOF-74, Mn2(DOBDC)(H2O)2, was prepared by covering the open metal sites in Mn-MOF-74 with H2O coordination (see the Supporting Information for details). The Mn2(DOBDC)(H2O)2–Super P system displays a discharge capacity of 2820 mA h g−1, only 30% of the capacity obtained from Mn-MOF-74. These results clearly evidence the intrinsic importance of open metal sites accessibility in achieving higher capacity in an MOF-based Li–O2 battery.
operating voltage than that of our combined cell (Figure S5, Supporting Information). The discharge capacity values are reported here normalized with respect to the combined mass of MOF and Super P.[16] For the Mn-MOF-74 based Li–O2 cell, the discharge capacity is 1010 mA h g−1 with respect to the total mass including MOF, Super P, PVDF, and the formed Li2O2 after discharge, which corresponds to 756 mA h g−1 with respect to the total weight of the electrode including carbon paper, and 6.76 mA h cm−2 with respect to the area of the O2 electrode.[17] Table S1 summarizes the capacities at different standards for all the MOF-based Li–O2 cells. For the Mn-MOF-74 based cell, the volume fraction of Li2O2 generated on discharge is 12.0%, considering that the thickness of the electrode including the coating is 0.21 mm. These measurements were repeated multiple times to give similar capacities under the same test conditions, and the values reported above are the typical values obtained. The stability of the cathode materials was examined by measuring the powder X-ray diffraction (XRD) pattern of the disassembled electrodes. These results are shown in Figure 4. Powder XRD patterns of the simulated Mn-MOF-74 (blue), and the O2 electrodes Figure 4 and Figure S10–S13 in the Sup- before electrochemical test (orange), after discharge (green), and after a discharge/charge cycle porting Information, where the diffrac- (red). Inset shows the PXRD pattern of the O2 electrodes with longer scanning time, which tion peaks of the MOFs in the electrodes confirms the formation of Li2O2 during discharge.
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Figure 5. a) Cycling response of the Mn-MOF-74 based battery. Current: 200 mA g−1. b) In situ O2 and CO2 evolution measured by DEMS on charging of a Mn-MOF-74 based cell in response to a current of 300 mA g−1 at 0 min, with the voltage profile shown in purple. c) O2 and CO2 analysis of the accumulated evolved gas after charge.
A full discharge–charge cycle of the Mn-MOF-74 based cell is shown in Figure S2 in the Supporting Information. A cycling test of the Mn-MOF-74 based cell was performed in a voltage range of 2.0–4.5 V vs. Li+/Li at a current of 200 mA g−1 for six full cycles (Figure 5a). The reversibility of our Li–O2 cell could be impacted by the possible electrolyte decomposition;[1e] on the other hand, the gas–solid state conversion during the reaction may also affect electrode performance.[18] The discharge profiles at rates progressively increasing from 50–500 mA g−1 (Figure S3, Supporting Information) demonstrate that the cell can still operate efficiently with slightly lower voltage at high current rates. To rule out the possibility that the increased capacity results from parasitic reactions, in situ differential electrochemical mass spectrometry (DEMS) was employed to measure the gas evolution during charging[19] (Figure S1b, see the Supporting Information for more details). The result (Figure 5b) shows that O2 is evolved dominantly during the charge, and almost no CO2 evolves in response to a 300 mA g−1 current, which suggests the decomposition of Li2O2 during charge. Mass spectrometry analysis was also carried out to examine the accumulated evolved gases after charge (Figure 5c), which showed that O2, but not CO2, is evolved. Compared with Super P, MOFs with open metal sites provide a more polarized surface in tailored pores, and they are capable of interacting with small molecules and ions more strongly.[14] M-MOF-74 has a high density of 3.3 accessible open metal sites available per 100 Å2 on the inner surface. Furthermore, M-MOF-74 has uniform 1D channels (Figure 1d) with a diameter of 11.0 Å. This channel is large enough to permit
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entry of oxygen molecules (kinetic diameter of 3.46 Å), and acts as reservoirs of O2 to feed the reaction. Among its isostructural analogs with similar O2 adsorption behavior, Mn-MOF-74 achieved the highest discharge capacity, showing possible catalytic activity of the Mn sites, compared to the Mg and Cocontaining MOFs. In summary, we have shown that a primary capacity of 9420 mA h g−1 under 1 atm of oxygen was achieved in a Li–O2 battery based on the robust Mn-MOF-74; more than four times higher than the value obtained in a cell without an MOF. The accessible open metal sites in the uniform channels enhance the population of O2 molecules in the pores, and assist the reaction efficiently towards high capacity. These findings indicate that MOFs represent a viable option as cathode materials in Li–O2 batteries. The diversity of MOFs and the facility with which their organic links and multimetal building units can be varied should lend many advantages in crafting electrode materials.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors acknowledge funding support from the Creative Research Group of MOE (IRT1117), National Natural Science Foundation of China (21101030, 21071032), and Shanghai Chenguang Scholar Program (12CG01). Received: November 5, 2013 Revised: December 30, 2013 Published online: [1] a) K. M. Abraham, Z. Jiang, J. Electrochem. Soc. 1996, 143, 1–5; b) P. G. Bruce, S. A. Freunberger, L. J. Hardwick, J.-M. Tarascon, Nat. Mater. 2011, 11, 19–29; c) A. Kraytsberg, Y. Ein-Eli, J. Power Sources 2011, 196, 886–893; d) H.-G. Jung, J. Hassoun, J.-B. Park, Y.-K. Sun, B. Scrosati, Nat. Chem. 2012, 4, 579–585; e) Z. Peng, S. A. Freunberger, Y. Chen, P. G. Bruce, Science 2012, 337, 563–566; f) Y.-C. Lu, Z. Xu, H. A. Gasteiger, S. Chen, K. Hamad-Schifferli, Y. Shao-Horn, J. Am. Chem. Soc. 2010, 132, 12170–12171; g) J.-J. Xu, D. Xu, Z.-L. Wang, H.-G. Wang, L.-L. Zhang, X.-B. Zhang, Angew. Chem. Int. Ed. 2013, 52, 3887–3890; h) Y. Shao, F. Ding, J. Xiao, J. Zhang, W. Xu, S. Park, J.-G, Zhang, Y. Wang, J. Liu, Adv. Funct. Mater. 2013, 23, 987–1004. [2] a) J. Xiao, D. Wang, W. Xu, D. Wang, R. E. Williford, J. Liu, J.-G. Zhang, J. Electrochem. Soc. 2010, 157, A487–A492; b) Z. Guo, D. Zhou, X. Dong, Z. Qiu, Y. Wang, Y. Xia, Adv. Mater. 2013, 25, 5668–5672; c) H.-D. Lim, K.-Y. Park, H. Song, E. Y. Jang,
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