A Highly Concentrated Catholyte Enabled by a Low-Melting-Point Ferrocene Derivative Guangtao Cong, Yucun Zhou, Zhejun Li, and Yi-Chun Lu* Electrochemical Energy and Interfaces Laboratory, Department of Mechanical and Automation Engineering, The Chinese University of Hong Kong, Shatin, N.T. 999077, Hong Kong SAR, China S Supporting Information *
ABSTRACT: Nonaqueous redox flow batteries (NRFBs) exhibit a wide potential window (>3.0 V) but have been limited by the low solubility of the active materials. Here, we propose and demonstrate a high-energy-density nonaqueous redox flow battery based on a lowmelting-point (37−40 °C) ferrocene derivative, 1,1-dimethylferrocene (DMFc), operated at its liquid state. The liquid redox-active DMFc not only contributes to high capacity but also acts as a solvating medium to the ion-conducting salts. Taking advantage of DMFc’s high concentration (3 M), superior stability, and fast kinetics, the lithium/DMFc battery achieves a high volumetric density (∼68 Ah L−1catholyte) with a high Coulombic efficiency (>95%) and high cycling stability. Our work demonstrates that exploiting a low-meltingpoint redox-active species at its melting state is a promising direction for developing high-energy-density NRFBs for next-generation energy storage technologies.
R
pumping loss, and the operation efficiency of these systems still remains to be evaluated systematically. Second, Jia et al. proposed exploiting multiple mediators serving as charge carriers exchanging electrons between the electrodes (electrochemical reactions) and high-energy-density solid-phase active materials (chemical reactions) stored in the reservoir.20,22 This novel approach can provide high capacity (∼200 Ah/L) but has been limited by the low power output and the chemical compatibility and stability of the mediators needed in the system. Third, organic redox-active molecule-based Li-RFBs have been widely deemed a promising candidate4,8,26,27 owing to the large flexibility in manipulating the structure of the molecules. The solubility of the redox-active molecule can be improved by chemically substituting the undesired functional groups with functional groups that could improve the interactions between solvent molecules and the redox-active molecules.13,14,26 As promising candidates, ferrocene derivatives attract researchers with their excellent stability, superior reversibility, and fast kinetics. However, the solubility of these ferrocene derivatives reported to date is less than 1.0 M, which corresponds to a theoretical energy density of 26.8 Ah L−1.13,14,28−30 Quinones represent another compelling group of redox-active organic molecules because of their fast and reversible two-electron reactions; however, these molecules
echargeable electrochemical energy storage systems play a crucial role in the integration of intermittent renewable energy sources, such as solar or wind, into the modern smart grid.1−3 To date, redox flow batteries (RFBs) has been regarded one of the most promising energy storage technologies owing to their unique design flexibility of decoupling power and energy storage capacity.4−8 The most prominent bottleneck of RFBs resides in their low energy density. Generally, the energy density of RFBs is determined by both the cell voltage and the solubility of the active materials dissolved in the flowing electrolyte.9 Aqueous RFBs normally possess a lower energy density because of their low operation voltage (below 2.0 V) limited by water electrolysis. RFBs based on nonaqueous electrolytes can be operated at a much higher cell voltage (>2.0 V);9−12 however, the solubility of many reported active materials in most nonaqueous solvents is less than 1.0 M, which corresponds to a volumetric energy density less than 26.8 Ah L−1catholyte.13−15 Many methods have been studied and discussed by researchers to increase the concentration of active molecules of RFBs.13,14,16−24 For instance, Duduta et al.19 proposed employing a semisolid suspension with electronic conductive carbon network as electrolytes, and this approach was later applied in many battery systems.17−19,25 With this approach, Chen et al. demonstrated an energy density of 290 Ah L−1 catholyte using sulfur-impregnated Ketjen black composite suspension as catholyte.18 However, the cycling of the semisolid electrolyte requires much higher energy input because of larger © 2017 American Chemical Society
Received: February 9, 2017 Accepted: March 15, 2017 Published: March 15, 2017 869
DOI: 10.1021/acsenergylett.7b00115 ACS Energy Lett. 2017, 2, 869−875
Letter
http://pubs.acs.org/journal/aelccp
Letter
ACS Energy Letters
as high as ∼2.35 M, which corresponds to a theoretical volumetric energy density of 63 Ah L−1catholyte.24 Here we propose an alternative direction to increase the volumetric capacity of RFBs. Usually the catholyte compositions of most traditional nonaqueous RFBs contain three parts: (i) redox-active material (determines the capacity); (ii) salt (provides the ionic conductivity); and (iii) organic solvent (acts as the solvating medium to dissolve the active material and salt, while providing flowability; usually redox-inactive). Neither salt nor organic solvent involves the electrochemical reaction; hence, no additional capacity is contributed from these two components. By replacing redox-inactive salt (LiTFSI, LiClO4 etc.) with redox-active salt such as Lithium iodide (LiI), Chen et al. designed a multiredox semisolid flow battery and demonstrated a volumetric energy density of 550 Ah L−1catholyte.17 In this work, we propose to replace part or
suffer from low solubility and relatively low redox potentials (1.0 mA cm−2). To examine the redox properties of DMFc/DMFc+ when a large portion of the redox-inactive solvent was replaced by liquid-phase redox-active DMFc, CV tests of the melted DMFc cathode were performed at 50 °C using the static cell configuration (Figure 4a). The catholyte was prepared by dissolving 0.5 M LiClO4 in melted DMFc under constant stirring at 50 °C. The 1 M LiClO4 in EC:DEC (V:V = 1:1) was used as electrolyte in anode. A NASICON-type structured Li1.5Al 0.5Ge1.5(PO4)3 (LAGP) lithium ion ceramic conductor was synthesized and used as separator to reduce the shuttling effect of DMFc+ and prevent the mixing of anolyte and catholyte. The photographs and XRD patterns of the asprepared LAGP ceramic separator before and after cycling tests are presented in Figure S3. Neither color nor structural change of the LAGP ceramic separator was observed after cycling, indicating high compatibility of the as-prepared LAGP ceramic separator with the melted DMFc catholyte. The CV of the melted DMFc (Figure 4 a) reveals a pair of well-defined oxidation−reduction peaks associated with oxidation and reduction of the melted DMFc molecule. Note that the peak currents shown here are limited by the LiClO4 concentration rather than the diffusion of DMFc (excess). The reversible CV 873
DOI: 10.1021/acsenergylett.7b00115 ACS Energy Lett. 2017, 2, 869−875
Letter
ACS Energy Letters
(9) Li, B.; Nie, Z.; Vijayakumar, M.; Li, G.; Liu, J.; Sprenkle, V.; Wang, W. Ambipolar Zinc-Polyiodide Electrolyte for a High-Energy Density Aqueous Redox Flow Battery. Nat. Commun. 2015, 6, 6303. (10) Gong, K.; Fang, Q.; Gu, S.; Li, S. F. Y.; Yan, Y. Nonaqueous Redox-Flow Batteries: Organic Solvents, Supporting Electrolytes, and Redox Pairs. Energy Environ. Sci. 2015, 8, 3515−3530. (11) Huang, Y.; Gu, S.; Yan, Y.; Li, S. F. Y. Nonaqueous Redox-Flow Batteries: Features, Challenges, and Prospects. Curr. Opin. Chem. Eng. 2015, 8, 105−113. (12) Zhao, Y.; Ding, Y.; Li, Y.; Peng, L.; Byon, H. R.; Goodenough, J. B.; Yu, G. A Chemistry and Material Perspective on Lithium Redox Flow Batteries Towards High-Density Electrical Energy Storage. Chem. Soc. Rev. 2015, 44, 7968−7996. (13) Kim, H.-s.; Yoon, T.; Kim, Y.; Hwang, S.; Ryu, J. H.; Oh, S. M. Increase of Both Solubility and Working Voltage by Acetyl Substitution on Ferrocene for Non-Aqueous Flow Battery. Electrochem. Commun. 2016, 69, 72−75. (14) Wei, X.; Cosimbescu, L.; Xu, W.; Hu, J. Z.; Vijayakumar, M.; Feng, J.; Hu, M. Y.; Deng, X.; Xiao, J.; Liu, J.; et al. Towards HighPerformance Nonaqueous Redox Flow Electrolyte Via Ionic Modification of Active Species. Adv. Energy Mater. 2015, 5, 1400678. (15) Ding, Y.; Yu, G. A Bio-Inspired, Heavy-Metal-Free, DualElectrolyte Liquid Battery Towards Sustainable Energy Storage. Angew. Chem., Int. Ed. 2016, 55, 4772−4776. (16) Brushett, F. R.; Vaughey, J. T.; Jansen, A. N. An All-Organic Non-Aqueous Lithium-Ion Redox Flow Battery. Adv. Energy Mater. 2012, 2, 1390−1396. (17) Chen, H.; Lu, Y. C. A High-Energy-Density Multiple Redox Semi-Solid-Liquid Flow Battery. Adv. Energy Mater. 2016, 6, 1502183. (18) Chen, H.; Zou, Q.; Liang, Z.; Liu, H.; Li, Q.; Lu, Y.-C. SulphurImpregnated Flow Cathode to Enable High-Energy-Density Lithium Flow Batteries. Nat. Commun. 2015, 6, 5877. (19) Duduta, M.; Ho, B.; Wood, V. C.; Limthongkul, P.; Brunini, V. E.; Carter, W. C.; Chiang, Y. M. Semi-Solid Lithium Rechargeable Flow Battery. Adv. Energy Mater. 2011, 1, 511−516. (20) Huang, Q.; Li, H.; Grätzel, M.; Wang, Q. Reversible Chemical Delithiation/Lithiation of LiFePO4: Towards a Redox Flow LithiumIon Battery. Phys. Chem. Chem. Phys. 2013, 15, 1793−1797. (21) Janoschka, T.; Martin, N.; Martin, U.; Friebe, C.; Morgenstern, S.; Hiller, H.; Hager, M. D.; Schubert, U. S. An Aqueous, PolymerBased Redox-Flow Battery Using Non-Corrosive, Safe, and Low-Cost Materials. Nature 2015, 527, 78−81. (22) Jia, C.; Pan, F.; Zhu, Y. G.; Huang, Q.; Lu, L.; Wang, Q. HighEnergy Density Nonaqueous All Redox Flow Lithium Battery Enabled with a Polymeric Membrane. Sci. Adv. 2015, 1, e1500886. (23) Wei, X.; Xu, W.; Vijayakumar, M.; Cosimbescu, L.; Liu, T.; Sprenkle, V.; Wang, W. Tempo-Based Catholyte for High-Energy Density Nonaqueous Redox Flow Batteries. Adv. Mater. 2014, 26, 7649−7653. (24) Takechi, K.; Kato, Y.; Hase, Y. A Highly Concentrated Catholyte Based on a Solvate Ionic Liquid for Rechargeable Flow Batteries. Adv. Mater. 2015, 27, 2501−2506. (25) Hamelet, S.; Larcher, D.; Dupont, L.; Tarascon, J.-M. SiliconBased Non Aqueous Anolyte for Li Redox-Flow Batteries. J. Electrochem. Soc. 2013, 160, A516−A520. (26) Huskinson, B.; Marshak, M. P.; Suh, C.; Er, S.; Gerhardt, M. R.; Galvin, C. J.; Chen, X.; Aspuru-Guzik, A.; Gordon, R. G.; Aziz, M. J. A Metal-Free Organic-Inorganic Aqueous Flow Battery. Nature 2014, 505, 195−198. (27) Perry, M. L.; Weber, A. Z. Advanced Redox-Flow Batteries: A Perspective. J. Electrochem. Soc. 2016, 163, A5064−A5067. (28) Hwang, B.; Park, M. S.; Kim, K. Ferrocene and Cobaltocene Derivatives for Non-Aqueous Redox Flow Batteries. ChemSusChem 2015, 8, 310−314. (29) Ding, Y.; Zhao, Y.; Li, Y.; Goodenough, J. B.; Yu, G. A HighPerformance All-Metallocene-Based, Non-Aqueous Redox Flow Battery. Energy Environ. Sci. 2017, 10, 491−497. (30) Ding, Y.; Zhao, Y.; Yu, G. A Membrane-Free Ferrocene-Based High-Rate Semiliquid Battery. Nano Lett. 2015, 15, 4108−4113.
membranes or applying other effective DMFc solvating solvents. In summary, by replacing the redox-inactive solvent with redox-active liquid DMFc at 50 °C, we successfully demonstrated a highly concentrated catholyte and achieved a volumetric density of ∼68 Ah L−1catholyte, which represents one of the highest volumetric energy densities reported to date (Figure 5 and Table S3). The low-concentration Li/DMFc battery shows a high diffusion rate, excellent stability, and long cycle life (1000 cycles) with superior rate capability under both static and flow mode. Future works involving functional group optimization and better lithium metal protection will further improve the performance of the Li-DMFc battery. On the basis of this concept, other redox-active molecules with melting points lower than room temperature would be more favorable for the application of RFBs.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00115. Detailed methods regarding cell assembly and electrochemical characterization; methods used for DFT calculations and results; fabrication process and characterization of Li1.5Al0.5Ge1.5(PO4)3 (LAGP) lithium ion ceramic conductor (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yi-Chun Lu: 0000-0003-1607-1615 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The work described in this Letter was supported by a grant from the Research Grants Council (RGC) of the Hong Kong Special Administrative Region (HK SAR), China, under Theme-based Research Scheme through Project No. T23407/13-N and a RGC Project No. CUHK24200414.
■
REFERENCES
(1) Benson, S. M.; Orr, F. M. Sustainability and Energy Conversions. MRS Bull. 2008, 33, 297−302. (2) Barnhart, C. J.; Dale, M.; Brandt, A. R.; Benson, S. M. The Energetic Implications of Curtailing Versus Storing Solar-and WindGenerated Electricity. Energy Environ. Sci. 2013, 6, 2804−2810. (3) Dunn, B.; Kamath, H.; Tarascon, J.-M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928−935. (4) Soloveichik, G. L. Flow Batteries: Current Status and Trends. Chem. Rev. 2015, 115, 11533−11558. (5) Wang, W.; Luo, Q.; Li, B.; Wei, X.; Li, L.; Yang, Z. Recent Progress in Redox Flow Battery Research and Development. Adv. Funct. Mater. 2013, 23, 970−986. (6) Pan, F.; Wang, Q. Redox Species of Redox Flow Batteries: A Review. Molecules 2015, 20, 20499−20517. (7) Huang, Q.; Wang, Q. Next-Generation, High-Energy-Density Redox Flow Batteries. ChemPlusChem 2015, 80, 312−322. (8) Park, M.; Ryu, J.; Wang, W.; Cho, J. Material Design and Engineering of Next-Generation Flow-Battery Technologies. Nat. Rev. Mater. 2016, 2, 16080. 874
DOI: 10.1021/acsenergylett.7b00115 ACS Energy Lett. 2017, 2, 869−875
Letter
ACS Energy Letters (31) Yao, M.; Senoh, H.; Yamazaki, S.-i.; Siroma, Z.; Sakai, T.; Yasuda, K. High-Capacity Organic Positive-Electrode Material Based on a Benzoquinone Derivative for Use in Rechargeable Lithium Batteries. J. Power Sources 2010, 195, 8336−8340. (32) Senoh, H.; Yao, M.; Sakaebe, H.; Yasuda, K.; Siroma, Z. A TwoCompartment Cell for Using Soluble Benzoquinone Derivatives as Active Materials in Lithium Secondary Batteries. Electrochim. Acta 2011, 56, 10145−10150. (33) Ding, Y.; Li, Y.; Yu, G. Exploring Bio-Inspired Quinone-Based Organic Redox Flow Batteries: A Combined Experimental and Computational Study. Chem. 2016, 1, 790−801. (34) Huang, J.; Cheng, L.; Assary, R. S.; Wang, P.; Xue, Z.; Burrell, A. K.; Curtiss, L. A.; Zhang, L. Liquid Catholyte Molecules for Nonaqueous Redox Flow Batteries. Adv. Energy Mater. 2015, 5, 1401782. (35) Zhao, Y.; Ding, Y.; Song, J.; Li, G.; Dong, G.; Goodenough, J. B.; Yu, G. Sustainable Electrical Energy Storage through the Ferrocene/Ferrocenium Redox Reaction in Aprotic Electrolyte. Angew. Chem., Int. Ed. 2014, 53, 11036−11040. (36) Beh, E. S.; De Porcellinis, D.; Gracia, R.; Xia, K.; Gordon, R. G.; Aziz, M. A Neutral Ph Aqueous Organic-Organometallic Redox Flow Battery with Extremely High Capacity Retention. ACS Energy Letters 2017, 2, 639−644. (37) Slovokhotov, Y. L.; Neretin, I. S.; Howard, J. A. Symmetry of Van Der Waals Molecular Shape and Melting Points of Organic Compounds. New J. Chem. 2004, 28, 967−979. (38) Pinal, R. Effect of Molecular Symmetry on Melting Temperature and Solubility. Org. Biomol. Chem. 2004, 2, 2692−2699. (39) Brown, R.; Brown, R. Melting Point and Molecular Symmetry. J. Chem. Educ. 2000, 77, 724−731. (40) Zhao, Y.; Wang, L.; Byon, H. R. High-Performance Rechargeable Lithium-Iodine Batteries Using Triiodide/Iodide Redox Couples in an Aqueous Cathode. Nat. Commun. 2013, 4, 1896.
875
DOI: 10.1021/acsenergylett.7b00115 ACS Energy Lett. 2017, 2, 869−875
