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Enhanced cycle stability of rechargeable Li-O2 batteries using immobilized redox mediator on air cathode
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Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
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Enhanced cycle stability of rechargeable Li-O2 batteries using immobilized redox mediator on air cathode Ji-Hoon Baika,1, Su Young Leea,b,1, Kihyun Kimc, Seongjun Baea,b , Sangwan Kima , Soyoul Kwakd, Dong Gi Honga , Inho Namd,* , Jongheop Yia,b,* , Jong-Chan Leea,* a School of Chemical and Biological Engineering and Institute of Chemical Processes, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea b WCU Program of C2E2, ICP, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea c School of Materials Science and Engineering Polymer Science and Engineering, Gyeongsang National University, 501 Jinju-daero, Jinju, 660-701, Republic of Korea d School of Chemical Engineering & Materials Science, Institute of Energy Converting Soft Materials, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul, 06974, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 July 2019 Received in revised form 15 October 2019 Accepted 10 November 2019 Available online xxx
Overcoming the low round-trip energy efficiency and poor cycle stability of lithium-oxygen (Li-O2) batteries still remains a challenge. Here, we show that 2,2,6,6,-tetramethylpiperidinyl-1-oxyl (TEMPO)immobilized air cathode can effectively reduce the charge voltage and increase the cycle stability in Li-O2 batteries. The TEMPO-immobilized air cathode is prepared using a gas diffusion layer by a simple dip coating method, in which polydopamine is used as a linker. In this method, the immobilized TEMPO on the cathode does not crossover to the anode, and the consumption of TEMPO by side reactions is minimized. As a result, the redox mediation by TEMPO is well maintained in its immobilized state. This highlights that the use of an immobilized redox mediator can be a rational strategy for expanding the practical applications of Li-O2 batteries. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Lithium-oxygen batteries Redox mediator 2,2,6,6,-tetramethylpiperidinyl-1-oxyl Polydopamine Modified air cathode
Introduction Aprotic lithium-oxygen (Li-O2) batteries have recently attracted significant interests because of their high theoretical specific energy of up to 13,000 Wh kg1, which overcomes the limitation of intercalation electrodes [1–5]. However, the practical application of Li-O2 batteries requires overcoming several challenges such as the low round-trip energy efficiency and poor cycle stability [6,7]. Since insoluble and insulating polycrystalline lithium peroxide (Li2O2) is formed on the cathode surface during the discharge process, a high overpotential is required in the charging process to reduce Li2O2, leading to the extremely low round-trip efficiency [8–10]. Carbonaceous materials with high electric conductivity, light weight, and low fabrication cost have been used for cathodes of Li-O2 batteries. However, they are prone to parasitic reactions
* Corresponding authors. E-mail addresses: [email protected] (I. Nam), [email protected] (J. Yi), [email protected] (J.-C. Lee). 1 J.-H.B. and S.Y.L. contributed equally.
such as the oxidative decomposition of the cathode during the charging process at high potentials [11]. The decomposed products block the surface of the carbon cathode, and this is considered as the major reason for the poor cycle stability [7]. Therefore, reduction of the overpotential during charging is essential for the application of Li-O2 batteries, because it can provide higher energy efficiency and long term cycle stability. Heterogeneous catalysts including nanoparticles and metal oxides have been widely used to reduce the charge overpotential in Li-O2 batteries [12–15]. A redox mediator (RM) is used by dissolving it in the electrolyte, as the former can act as electron carriers between the electrode surface and Li2O2 via the reversible reaction such as RM Ð RM+ + e [16–18]. However, dissolution of the RM in the electrolyte results in the diffusion through the separator and reduction at the Li anode. As the RM is consumed during the electrochemical process, the redox mediation is not possible anymore [19]. Hence, the strategy of dissolving the RM in the electrolytes has a certain disadvantage. Several approaches using size-exclusive separators [20,21], negatively charged separators [22], and gel polymer electrolytes [23] have been tried to address the undesirable consumption of RM. However, no clear
https://doi.org/10.1016/j.jiec.2019.11.015 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: J.-H. Baik, et al., Enhanced cycle stability of rechargeable Li-O2 batteries using immobilized redox mediator on air cathode, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.11.015
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solutions have been arrived at as yet, primarily because of the selfdischarge process, which is an inherent disadvantage in the current configurations of Li–O2 batteries. In this study, we tried to immobilize the RM on a gas diffusion layer (GDL) cathode to prevent its diffusion to the Li metal anode. Polydopamine (PDA), a mussel-inspired functional material, was used as a linker to connect the RM to GDL, because PDA can be easily coated on various substrates [24,25] and has a catechol group that can react with the amino group in RM via Michael addition or Schiff base formation [26,27]. As 2,2,6,6,-tetramethylpiperidinyl-1-oxyl (TEMPO) and its derivatives have been applied as mobile RM in Li-O2 batteries owing to their appropriate redox potential, fast electrochemical kinetic, good redox reversibility, and high solubility in aprotic electrolytes [16,28,29], 4-amino2,2,6,6,-tetramethylpiperidinyl-1-oxyl (4-amino-TEMPO) was used as the RM for immobilization. TEMPO-attached graphene oxide has been used as a heterocatalyst for the oxidation of alcohols to aldehydes or ketones, demonstrating that the immobilized TEMPO can still participate in redox reactions [30,31]. Breton et al. reported that TEMPO-attached electrode exhibited electrocatalytic activity [32,33]; however, there are no report on immobilized RM on the cathode of Li-O2 batteries. The Li-O2 battery prepared using our modified air cathode exhibited an enhanced cycle stability with reduced charge overpotential, compared with those of the conventional Li-O2 batteries without any RM or with RM dissolved in electrolyte. Experimental TEMPO-immobilized cathode was prepared using the gas diffusion layer (GDL, JNT-20A, JNTG) as a carbon cathode and substrate on which the PDA layer was coated. The GDL was cut into 3 cm 3 cm pieces and washed with ethanol. The GDL was immersed in 20 mM precursor solution (dopamine hydrochloride, Sigma Aldrich) while maintaining the pH at 8.5 in 20 mM Tris buffer and then stirring for 30 min. The GDL treated with PDA was immersed overnight in 20 mM TEMPO-precursor solution (4amino-2,2,6,6-tetramethylpiperidine 1-oxyl, TCI chemicals) while maintaining the pH at 7.8 in 50 mM Tris buffer to induce Schiff base formation or Michael addition between 4-amino-2,2,6,6-tetramethylpiperidine 1-oxyl and the catechol group of PDA. The modified GDL was used as a cathode in a Li-O2 battery assembled in the Swagelok-type cell with lithium metal as an anode and tetraethylene glycol dimethyl ether (TEGDME, SigmaAldrich) containing 1 M lithium bis(trifluoromethanesulfonyl) imide (LiTFSI, Sigma-Aldrich) as the electrolyte. TEGDME was dried over molecular sieves (3 Å) prior to use. All the batteries were assembled in an argon-filled glove box, and 100 mL of electrolyte was used. The batteries were transferred to a homemade closed chamber to purge oxygen for 1 h before the electrochemical characterizations. Galvanostatic charge-discharge test and linear sweep voltammetry (LSV) test were conducted with a multichannel automatic battery cycler (WonATech, WBCS3000) at 30 C. The current density and scan rate for the electrochemical analyses were 0.1 mA cm2 and 1 mV s1, respectively. The voltage was
varied from 2.2 to 4.7 V, and the charge-discharge cycles were investigated at a fixed capacity of 0.5 mA h cm2 to minimize the contribution from parasitic reactions. The surface elemental composition was determined by X-ray photoelectron spectroscopy (XPS, Thermo Scientific, Sigma Probe). The binding energy was corrected with reference to the C 1s peak at 284.5 eV. Density functional theory (DFT) calculations were carried out with the Vienna ab initio simulation package (VASP) [34]. The generalized gradient approximation Perdew-Burke-Ernzerhof (GGA-PBE) exchange-correlation functional was applied to the calculation, and the projector augmented wave (PAW) method, as implemented in VASP, was employed [35–37]. A plane-wave energy cutoff of 400 eV and 1 1 1 Monkhorst-Pack k-point mesh were employed to sample the Brillouin zone. The Van der Waals interactions between the organic molecules were also considered using a DFT-D2 method established by Grimme. Grimme’s parameters were sourced from the previous work by Kresse [34]. The difference in the charge density, Dr, of TEMPO before and after binding with PDA is defined as
Dr ¼ rPDA=TEMPO ðrPDA þ rTEMPOÞ where r is the charge density of the molecular unit denoted in subscript. Results and discussion The GDL was readily functionalized via the consecutive coating by dopamine and grafting of TEMPO onto the PDA layer, as shown in Fig. 1. When dopamine hydrochloride was added into a weak alkaline buffer solution (pH 8.5), dopamine was immediately coated on the surface by oxidative polymerization, changing the color of the solution from colorless to brownish black. The selfpolymerization of dopamine produces an adhesive coating layer of PDA onto the GDL cathode. 4-Amino TEMPO was grafted onto the PDA-coated GDL cathode via Michael addition or Schiff’s base reaction. Considering that Schiff-base formation is facilitated in weak acid condition, the product through Michael addition is expected to be dominant in the immobilized TEMPO on cathode. Since these chemical reactions can occur by simply dipping in aqueous solutions, the GDL could be functionalized without any complex procedures or harsh treatment, which is advantageous for commercialization and mass production. Multifunctional electrochemical platforms can also be prepared in a similar manner using other compounds having amino, thiol, and imidazole functional groups, which can react with catechol groups in the PDA coated on the GDL [38,39]. The successful functionalization and grafting were confirmed by LSV from an open circuit voltage to 4.8 V (vs. Li/Li+) using the Li-O2 battery containing the modified cathode (Fig. 2). As a control system to observe the effect of the immobilized RM, another Li-O2 battery was prepared, where TEMPO was simply dissolved in the electrolyte (10 mM TEMPO and 1 M LiTFSI in TEGDME), rather than being immobilized. The LSV of the control system was performed. An appropriate RM in Li-O2 batteries should have a redox potential greater than that for the formation of Li2O2 (2.96 V vs. Li/Li+). The
Fig. 1. Schematic of the construction of cathode with immobilized TEMPO.
Please cite this article in press as: J.-H. Baik, et al., Enhanced cycle stability of rechargeable Li-O2 batteries using immobilized redox mediator on air cathode, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.11.015
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Fig. 2. Linear sweep voltammograms of the Li-O2 batteries with TEMPO dissolved in the electrolyte and immobilized TEMPO on the cathode. Scan rate is 1 mV s1.
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LSV curve of the Li-O2 battery with TEMPO dissolved in the electrolyte shows an anodic peak at 3.76 V (vs. Li/Li+), representing the electrochemical oxidation of TEMPO to TEMPO+. On the other hand, the Li-O2 battery with the immobilized TEMPO on cathode shows slightly larger anodic peak potential of 3.93 V (vs. Li/Li+), because the dangled structure can change the electron density of the reaction site toward an electronically depleted state. The amount of immobilized TEMPO on the cathode was estimated from the area of the anodic peak for TEMPO/TEMPO+ redox couple (Fig. 2). The area of the anodic peak for TEMPO dissolved in the electrolyte (A1) and that for immobilized TEMPO on the cathode (A2) were 0.0397 and 0.0315 C, respectively. Thus, the amounts of TEMPO dissolved in the electrolyte and immobilized TEMPO on the cathode are calculated to be 0.411 and 0.326 mmol/cell, respectively. It is noteworthy that the loaded TEMPO in the cell in which TEMPO was dissolved in the electrolyte was 1 mmol/cell, indicating that the dissolved TEMPO easily decomposed via the reaction with the lithium metal anode. The ratio of A1/A2 is 1.26, suggesting that the amount of immobilized TEMPO on the cathode is slightly smaller than that of the active TEMPO dissolved in the electrolyte. The Li-O2 battery with immobilized TEMPO on cathode exhibited
Fig. 3. Model structures obtained by the (a) Michael addition reaction and (c) Schiff-base formation of dopamine and TEMPO and (b, d) their corresponding plots of charge density difference obtained by DFT calculation (For interpretation of the references to color in the text, the reader is referred to the web version of this article).
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Fig. 4. (a) N 1 s peak in the XPS spectra of bare cathode, PDA-coated cathode, and TEMPO-immobilized cathode. Magnified N 1 s peak of (b) PDA-coated cathode and (c) TEMPO-immobilized cathode.
better cycle stability as compared with the case where TEMPO was dissolved in the electrolyte (discussed later). Therefore, it can be concluded that the RM is more effectively utilized in its immobilized state, and this immobilization strategy can guide the designing principles for electrodes of batteries containing RMs. The energy difference between the dissolved and immobilized TEMPO was estimated using density functional theory (DFT)
calculations. 4-Amino-TEMPO bonded to dopamine via Michael addition and Schiff-base formation at the ortho-position were chosen as the model structure (Fig. 3(a), (c)), and their charge density was compared with that of the unbound TEMPO and dopamine. The hydrogen atom of TEMPO has been hidden for easier visualization. The plots of the calculated charge density difference are shown in Fig. 3(b) and (d); blue and yellow lobes
Fig. 5. Cycle stability of Li-O2 batteries (a) with TEMPO dissolved in the electrolyte, (b) with immobilized TEMPO on the cathode, (c) without TEMPO, and (d) with PDA-coated cathode. Charge-discharge profiles of the 1st, 5th, 10th, and 20th cycles. (e) Round-trip efficiency of the Li-O2 batteries with TEMPO. (f) Full discharge profiles up to 2.2 V (vs. Li/ Li+) of the all prepared Li-O2 batteries. Current density is 0.1 mA cm–2.
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represent the charge depletion and accumulation after bonding, respectively. It is remarkable that the charge depletion occurs at N O bond of TEMPO (red arrows) when TEMPO is bonded to dopamine in the both cases. This could be attributed to the increased electron-withdrawing effect of the nitrogen atom in 4amino TEMPO after bonding, in which the formal charge is partially positive. After TEMPO loses an electron in the charging process, N O becomes N+¼O having a double-bond character and positive charge on the nitrogen atom [16]. The charge depletion on N and O atoms in the N O bond indicates that TEMPO bonded to PDA are partially oxidized, which would resist the additional electron loss during the charge process, resulting in the positive shift of the charge plateau. The immobilized TEMPO was also examined by XPS (Fig. 4). For PDA-coated cathode, the binding energy was found to be 400.1 eV, which corresponds to the nitrogen from PDA. For the TEMPO-immobilized cathode, the binding energies were 400.1 and 402.2 eV, which correspond to a nitrogen atom from PDA and an amino group from 4-amino-TEMPO and the nitroxide nitrogen atom in the TEMPO, respectively. It is to be noted that no high binding energy N 1s peak at 405 eV was observed. These results suggest that 4-amino-TEMPO was directly bound to PDA via covalent bonding rather than physical adsorption [40]. This is in good agreement with the computational simulations and electrochemical analyses. The electrochemical performances of the Li-O2 batteries without any RM, with TEMPO dissolved in the electrolyte, and with immobilized TEMPO on the cathode were evaluated via the galvanostatic discharge/charge test (Fig. 5). As a conventional LiO2 battery setup, a Swagelok cell with Li metal anode and a fully sealed gas reservoir were fabricated to enable high reproducibility. Fig. 5 shows that the discharge plateaus appear at approximately 2.7 V for the all the fabricated Li-O2 batteries, demonstrating that the discharge reaction is not affected by TEMPO. However, the charging voltage in the first cycle significantly decreases in the presence of TEMPO, as observed from the LSV data, resulting in a significant improvement of the round-trip efficiency. In the first cycle, the battery with TEMPO dissolved in the electrolyte shows a plateau-like charging curve at 3.6–3.7 V (vs. Li/Li+) (Fig. 5(a)) and the battery with immobilized TEMPO on the cathode shows the same at 3.8–3.9 V (vs. Li/Li+) (Fig. 5(b)). This is attributed to the electrochemical oxidation of TEMPO to TEMPO+, which is shown in the voltammogram from linear sweep voltammetry (Fig. 2). It is noteworthy that both the dissolved and immobilized TEMPO can successfully act as RMs, because the charge voltage of these batteries are evidently smaller than those of the batteries without TEMPO containing both the bare cathode and the PDA coated cathode without TEMPO (4.1 V) (Fig. 5(c–d)). The charge capacity of the battery without TEMPO did not reach to 0.5 mA h cm2 at 4.7 V in the 20th cycle due to the increased charge overpotential, demonstrating the unstable battery operation. For the battery with TEMPO dissolved in the electrolyte, a rapid increase in the charging voltage is observed immediately after the first cycle. This can be ascribed to the parasitic degradation reactions that may lead to the depletion of RM, resulting in electrode passivation and increased polarization during cycling. In contrast, much lesser change in the charging voltage is observed over 20 cycles for the battery with immobilized TEMPO on the cathode. The improved cycle stability in this case can be mainly attributed to the increased lifetime of TEMPO on the cathode. Consequently, the stable round-trip efficiency over cycles is achieved in the battery with immobilized TEMPO on cathode. (Fig. 5(e)) The full discharge capacities of the Li-O2 batteries without any RM, with TEMPO dissolved in the electrolyte, and with immobilized TEMPO on the cathode were also examined (current density of 0.1 mA cm2) (Fig. 5(f)). The increase in the discharge capacity upon the addition of TEMPO in the Li-O2 battery can be ascribed to the enhanced charge transfer. With immobilized
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TEMPO on the cathode, the discharge capacity further increases, clearly indicating the good stability of the immobilized TEMPO. Conclusions In summary, a nonaqueous Li-O2 battery with high performance was fabricated using a TEMPO-immobilized cathode that was prepared by a simple coating and grafting process. The effect of immobilized TEMPO on the cathode was verified by comparing the electrochemical performances of Li-O2 batteries with immobilized TEMPO on the cathode, TEMPO dissolved in the electrolyte, and in the absence of TEMPO. The immobilized TEMPO on the cathode suppressed the degradation of TEMPO at the Li metal anode and extended the redox mediated Li2O2 oxidation over repeated cycles. This study introduces a simple and effective strategy to modify the electrodes of advanced energy storage systems containing RMs. The insights from these studies will allow the utilization of RMs more efficiently, using a smaller amount, to prolong their lifespan and also enhance the battery performance. Declarations of interest None. Acknowledgments Funding: This work was supported by the National Research Foundation of Korea (NRF), Korea government (MSIT) (No. NRF-2018R1A5A1024127), Korea government (MEST) (NRF2016R1E1A1A01942936)), and the Supercomputing Center/Korea Institute of Science and Technology Information with supercomputing resources, which provided technical support (KSC-2017-C2-0029). References [1] K.M. Abraham, Z. Jiang, J. Electrochem. Soc. 143 (1996) 1. [2] S.D. Beattie, D.M. Manolescu, S.L. Blair, J. Electrochem. Soc. 156 (2009) A44. [3] S.A. Freunberger, Y. Chen, N.E. Drewett, L.J. Hardwick, F. Barde, P.G. Bruce, Angew. Chem. Int. Ed. 50 (2011) 8609. [4] T. Ogasawara, A. Debart, M. Holzapfel, P. Novak, P.G. Bruce, J. Am. Chem. Soc. 128 (2006) 1390. [5] J. Read, J. Electrochem. Soc. 149 (2002) A1190. [6] G. Girishkumar, B. McCloskey, A.C. Luntz, S. Swanson, W. Wilcke, J. Phys. Chem. Lett. 1 (2010) 2193. [7] B.D. McCloskey, A. Speidel, R. Scheffler, D.C. Miller, V. Viswanathan, J.S. Hummelshoj, J.K. Norskov, A.C. Luntz, J. Phys. Chem. Lett. 3 (2012) 997. [8] R. Black, S.H. Oh, J.H. Lee, T. Yim, B. Adams, L.F. Nazar, J. Am. Chem. Soc. 134 (2012) 2902. [9] C.O. Laoire, S. Mukerjee, E.J. Plichta, M.A. Hendrickson, K.M. Abraham, J. Electrochem. Soc. 158 (2011) A302. [10] V. Viswanathan, K.S. Thygesen, J.S. Hummelshoj, J.K. Norskov, G. Girishkumar, B.D. McCloskey, A.C. Luntz, J. Chem. Phys. 135 (2011)214704. [11] M.M. Ottakam Thotiyl, S.A. Freunberger, Z. Peng, P.G. Bruce, J. Am. Chem. Soc. 135 (2013) 494. [12] J.-H. Lee, R. Black, G. Popov, E. Pomerantseva, F. Nan, G.A. Botton, L.F. Nazar, Energy Environ. Sci. 5 (2012) 9558. [13] J. Lu, Y. Lei, K.C. Lau, X. Luo, P. Du, J. Wen, R.S. Assary, U. Das, D.J. Miller, J.W. Elam, H.M. Albishri, D.A. El-Hady, Y.-K. Sun, L.A. Curtiss, K. Amine, Nat. Commun. 4 (2013) 2383. [14] J.-J. Xu, Z.-L. Wang, D. Xu, L.-L. Zhang, X.-B. Zhang, Nat. Commun. 4 (2013) 2438. [15] J.-J. Xu, D. Xu, Z.-L. Wang, H.-G. Wang, L.-L. Zhang, X.-B. Zhang, Angew. Chem. Int. Ed. 52 (2013) 3887. [16] B.J. Bergner, A. Schurmann, K. Peppler, A. Garsuch, J. Janek, J. Am. Chem. Soc. 136 (2014) 15054. [17] Y. Chen, S.A. Freunberger, Z. Peng, O. Fontaine, P.G. Bruce, Nat. Chem. 5 (2013) 489. [18] Z. Liang, Y.C. Lu, J. Am. Chem. Soc. 138 (2016) 7574. [19] T. Zhang, K. Liao, P. He, H. Zhou, Energy Environ. Sci. 9 (2016) 1024. [20] S.H. Park, T.H. Lee, Y.J. Lee, H.B. Park, Y.J. Lee, Small 14 (2018)1801456. [21] Y. Qiao, Y. He, S. Wu, K. Jiang, X. Li, S. Guo, P. He, H. Zhou, ACS Energy Lett. 3 (2018) 463. [22] S.H. Lee, J.-B. Park, H.-S. Lim, Y.-K. Sun, Adv. Energy Mater. 7 (2017)1602417. [23] J. Zhang, B. Sun, A.M. McDonagh, Y. Zhao, K. Kretschmer, X. Guo, G. Wang, Energy Storage Mater. 7 (2017) 1.
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Enhanced cycle stability of rechargeable Li-O2 batteries using immobilized redox mediator on air cathode Ji-Hoon Baika,1, Su Young Leea,b,1, Kihyun Kimc, Seongjun Baea,b , Sangwan Kima , Soyoul Kwakd, Dong Gi Honga , Inho Namd,* , Jongheop Yia,b,* , Jong-Chan Leea,* a School of Chemical and Biological Engineering and Institute of Chemical Processes, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea b WCU Program of C2E2, ICP, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea c School of Materials Science and Engineering Polymer Science and Engineering, Gyeongsang National University, 501 Jinju-daero, Jinju, 660-701, Republic of Korea d School of Chemical Engineering & Materials Science, Institute of Energy Converting Soft Materials, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul, 06974, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 July 2019 Received in revised form 15 October 2019 Accepted 10 November 2019 Available online xxx
Overcoming the low round-trip energy efficiency and poor cycle stability of lithium-oxygen (Li-O2) batteries still remains a challenge. Here, we show that 2,2,6,6,-tetramethylpiperidinyl-1-oxyl (TEMPO)immobilized air cathode can effectively reduce the charge voltage and increase the cycle stability in Li-O2 batteries. The TEMPO-immobilized air cathode is prepared using a gas diffusion layer by a simple dip coating method, in which polydopamine is used as a linker. In this method, the immobilized TEMPO on the cathode does not crossover to the anode, and the consumption of TEMPO by side reactions is minimized. As a result, the redox mediation by TEMPO is well maintained in its immobilized state. This highlights that the use of an immobilized redox mediator can be a rational strategy for expanding the practical applications of Li-O2 batteries. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Lithium-oxygen batteries Redox mediator 2,2,6,6,-tetramethylpiperidinyl-1-oxyl Polydopamine Modified air cathode
Introduction Aprotic lithium-oxygen (Li-O2) batteries have recently attracted significant interests because of their high theoretical specific energy of up to 13,000 Wh kg1, which overcomes the limitation of intercalation electrodes [1–5]. However, the practical application of Li-O2 batteries requires overcoming several challenges such as the low round-trip energy efficiency and poor cycle stability [6,7]. Since insoluble and insulating polycrystalline lithium peroxide (Li2O2) is formed on the cathode surface during the discharge process, a high overpotential is required in the charging process to reduce Li2O2, leading to the extremely low round-trip efficiency [8–10]. Carbonaceous materials with high electric conductivity, light weight, and low fabrication cost have been used for cathodes of Li-O2 batteries. However, they are prone to parasitic reactions
* Corresponding authors. E-mail addresses: [email protected] (I. Nam), [email protected] (J. Yi), [email protected] (J.-C. Lee). 1 J.-H.B. and S.Y.L. contributed equally.
such as the oxidative decomposition of the cathode during the charging process at high potentials [11]. The decomposed products block the surface of the carbon cathode, and this is considered as the major reason for the poor cycle stability [7]. Therefore, reduction of the overpotential during charging is essential for the application of Li-O2 batteries, because it can provide higher energy efficiency and long term cycle stability. Heterogeneous catalysts including nanoparticles and metal oxides have been widely used to reduce the charge overpotential in Li-O2 batteries [12–15]. A redox mediator (RM) is used by dissolving it in the electrolyte, as the former can act as electron carriers between the electrode surface and Li2O2 via the reversible reaction such as RM Ð RM+ + e [16–18]. However, dissolution of the RM in the electrolyte results in the diffusion through the separator and reduction at the Li anode. As the RM is consumed during the electrochemical process, the redox mediation is not possible anymore [19]. Hence, the strategy of dissolving the RM in the electrolytes has a certain disadvantage. Several approaches using size-exclusive separators [20,21], negatively charged separators [22], and gel polymer electrolytes [23] have been tried to address the undesirable consumption of RM. However, no clear
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solutions have been arrived at as yet, primarily because of the selfdischarge process, which is an inherent disadvantage in the current configurations of Li–O2 batteries. In this study, we tried to immobilize the RM on a gas diffusion layer (GDL) cathode to prevent its diffusion to the Li metal anode. Polydopamine (PDA), a mussel-inspired functional material, was used as a linker to connect the RM to GDL, because PDA can be easily coated on various substrates [24,25] and has a catechol group that can react with the amino group in RM via Michael addition or Schiff base formation [26,27]. As 2,2,6,6,-tetramethylpiperidinyl-1-oxyl (TEMPO) and its derivatives have been applied as mobile RM in Li-O2 batteries owing to their appropriate redox potential, fast electrochemical kinetic, good redox reversibility, and high solubility in aprotic electrolytes [16,28,29], 4-amino2,2,6,6,-tetramethylpiperidinyl-1-oxyl (4-amino-TEMPO) was used as the RM for immobilization. TEMPO-attached graphene oxide has been used as a heterocatalyst for the oxidation of alcohols to aldehydes or ketones, demonstrating that the immobilized TEMPO can still participate in redox reactions [30,31]. Breton et al. reported that TEMPO-attached electrode exhibited electrocatalytic activity [32,33]; however, there are no report on immobilized RM on the cathode of Li-O2 batteries. The Li-O2 battery prepared using our modified air cathode exhibited an enhanced cycle stability with reduced charge overpotential, compared with those of the conventional Li-O2 batteries without any RM or with RM dissolved in electrolyte. Experimental TEMPO-immobilized cathode was prepared using the gas diffusion layer (GDL, JNT-20A, JNTG) as a carbon cathode and substrate on which the PDA layer was coated. The GDL was cut into 3 cm 3 cm pieces and washed with ethanol. The GDL was immersed in 20 mM precursor solution (dopamine hydrochloride, Sigma Aldrich) while maintaining the pH at 8.5 in 20 mM Tris buffer and then stirring for 30 min. The GDL treated with PDA was immersed overnight in 20 mM TEMPO-precursor solution (4amino-2,2,6,6-tetramethylpiperidine 1-oxyl, TCI chemicals) while maintaining the pH at 7.8 in 50 mM Tris buffer to induce Schiff base formation or Michael addition between 4-amino-2,2,6,6-tetramethylpiperidine 1-oxyl and the catechol group of PDA. The modified GDL was used as a cathode in a Li-O2 battery assembled in the Swagelok-type cell with lithium metal as an anode and tetraethylene glycol dimethyl ether (TEGDME, SigmaAldrich) containing 1 M lithium bis(trifluoromethanesulfonyl) imide (LiTFSI, Sigma-Aldrich) as the electrolyte. TEGDME was dried over molecular sieves (3 Å) prior to use. All the batteries were assembled in an argon-filled glove box, and 100 mL of electrolyte was used. The batteries were transferred to a homemade closed chamber to purge oxygen for 1 h before the electrochemical characterizations. Galvanostatic charge-discharge test and linear sweep voltammetry (LSV) test were conducted with a multichannel automatic battery cycler (WonATech, WBCS3000) at 30 C. The current density and scan rate for the electrochemical analyses were 0.1 mA cm2 and 1 mV s1, respectively. The voltage was
varied from 2.2 to 4.7 V, and the charge-discharge cycles were investigated at a fixed capacity of 0.5 mA h cm2 to minimize the contribution from parasitic reactions. The surface elemental composition was determined by X-ray photoelectron spectroscopy (XPS, Thermo Scientific, Sigma Probe). The binding energy was corrected with reference to the C 1s peak at 284.5 eV. Density functional theory (DFT) calculations were carried out with the Vienna ab initio simulation package (VASP) [34]. The generalized gradient approximation Perdew-Burke-Ernzerhof (GGA-PBE) exchange-correlation functional was applied to the calculation, and the projector augmented wave (PAW) method, as implemented in VASP, was employed [35–37]. A plane-wave energy cutoff of 400 eV and 1 1 1 Monkhorst-Pack k-point mesh were employed to sample the Brillouin zone. The Van der Waals interactions between the organic molecules were also considered using a DFT-D2 method established by Grimme. Grimme’s parameters were sourced from the previous work by Kresse [34]. The difference in the charge density, Dr, of TEMPO before and after binding with PDA is defined as
Dr ¼ rPDA=TEMPO ðrPDA þ rTEMPOÞ where r is the charge density of the molecular unit denoted in subscript. Results and discussion The GDL was readily functionalized via the consecutive coating by dopamine and grafting of TEMPO onto the PDA layer, as shown in Fig. 1. When dopamine hydrochloride was added into a weak alkaline buffer solution (pH 8.5), dopamine was immediately coated on the surface by oxidative polymerization, changing the color of the solution from colorless to brownish black. The selfpolymerization of dopamine produces an adhesive coating layer of PDA onto the GDL cathode. 4-Amino TEMPO was grafted onto the PDA-coated GDL cathode via Michael addition or Schiff’s base reaction. Considering that Schiff-base formation is facilitated in weak acid condition, the product through Michael addition is expected to be dominant in the immobilized TEMPO on cathode. Since these chemical reactions can occur by simply dipping in aqueous solutions, the GDL could be functionalized without any complex procedures or harsh treatment, which is advantageous for commercialization and mass production. Multifunctional electrochemical platforms can also be prepared in a similar manner using other compounds having amino, thiol, and imidazole functional groups, which can react with catechol groups in the PDA coated on the GDL [38,39]. The successful functionalization and grafting were confirmed by LSV from an open circuit voltage to 4.8 V (vs. Li/Li+) using the Li-O2 battery containing the modified cathode (Fig. 2). As a control system to observe the effect of the immobilized RM, another Li-O2 battery was prepared, where TEMPO was simply dissolved in the electrolyte (10 mM TEMPO and 1 M LiTFSI in TEGDME), rather than being immobilized. The LSV of the control system was performed. An appropriate RM in Li-O2 batteries should have a redox potential greater than that for the formation of Li2O2 (2.96 V vs. Li/Li+). The
Fig. 1. Schematic of the construction of cathode with immobilized TEMPO.
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Fig. 2. Linear sweep voltammograms of the Li-O2 batteries with TEMPO dissolved in the electrolyte and immobilized TEMPO on the cathode. Scan rate is 1 mV s1.
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LSV curve of the Li-O2 battery with TEMPO dissolved in the electrolyte shows an anodic peak at 3.76 V (vs. Li/Li+), representing the electrochemical oxidation of TEMPO to TEMPO+. On the other hand, the Li-O2 battery with the immobilized TEMPO on cathode shows slightly larger anodic peak potential of 3.93 V (vs. Li/Li+), because the dangled structure can change the electron density of the reaction site toward an electronically depleted state. The amount of immobilized TEMPO on the cathode was estimated from the area of the anodic peak for TEMPO/TEMPO+ redox couple (Fig. 2). The area of the anodic peak for TEMPO dissolved in the electrolyte (A1) and that for immobilized TEMPO on the cathode (A2) were 0.0397 and 0.0315 C, respectively. Thus, the amounts of TEMPO dissolved in the electrolyte and immobilized TEMPO on the cathode are calculated to be 0.411 and 0.326 mmol/cell, respectively. It is noteworthy that the loaded TEMPO in the cell in which TEMPO was dissolved in the electrolyte was 1 mmol/cell, indicating that the dissolved TEMPO easily decomposed via the reaction with the lithium metal anode. The ratio of A1/A2 is 1.26, suggesting that the amount of immobilized TEMPO on the cathode is slightly smaller than that of the active TEMPO dissolved in the electrolyte. The Li-O2 battery with immobilized TEMPO on cathode exhibited
Fig. 3. Model structures obtained by the (a) Michael addition reaction and (c) Schiff-base formation of dopamine and TEMPO and (b, d) their corresponding plots of charge density difference obtained by DFT calculation (For interpretation of the references to color in the text, the reader is referred to the web version of this article).
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Fig. 4. (a) N 1 s peak in the XPS spectra of bare cathode, PDA-coated cathode, and TEMPO-immobilized cathode. Magnified N 1 s peak of (b) PDA-coated cathode and (c) TEMPO-immobilized cathode.
better cycle stability as compared with the case where TEMPO was dissolved in the electrolyte (discussed later). Therefore, it can be concluded that the RM is more effectively utilized in its immobilized state, and this immobilization strategy can guide the designing principles for electrodes of batteries containing RMs. The energy difference between the dissolved and immobilized TEMPO was estimated using density functional theory (DFT)
calculations. 4-Amino-TEMPO bonded to dopamine via Michael addition and Schiff-base formation at the ortho-position were chosen as the model structure (Fig. 3(a), (c)), and their charge density was compared with that of the unbound TEMPO and dopamine. The hydrogen atom of TEMPO has been hidden for easier visualization. The plots of the calculated charge density difference are shown in Fig. 3(b) and (d); blue and yellow lobes
Fig. 5. Cycle stability of Li-O2 batteries (a) with TEMPO dissolved in the electrolyte, (b) with immobilized TEMPO on the cathode, (c) without TEMPO, and (d) with PDA-coated cathode. Charge-discharge profiles of the 1st, 5th, 10th, and 20th cycles. (e) Round-trip efficiency of the Li-O2 batteries with TEMPO. (f) Full discharge profiles up to 2.2 V (vs. Li/ Li+) of the all prepared Li-O2 batteries. Current density is 0.1 mA cm–2.
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represent the charge depletion and accumulation after bonding, respectively. It is remarkable that the charge depletion occurs at N O bond of TEMPO (red arrows) when TEMPO is bonded to dopamine in the both cases. This could be attributed to the increased electron-withdrawing effect of the nitrogen atom in 4amino TEMPO after bonding, in which the formal charge is partially positive. After TEMPO loses an electron in the charging process, N O becomes N+¼O having a double-bond character and positive charge on the nitrogen atom [16]. The charge depletion on N and O atoms in the N O bond indicates that TEMPO bonded to PDA are partially oxidized, which would resist the additional electron loss during the charge process, resulting in the positive shift of the charge plateau. The immobilized TEMPO was also examined by XPS (Fig. 4). For PDA-coated cathode, the binding energy was found to be 400.1 eV, which corresponds to the nitrogen from PDA. For the TEMPO-immobilized cathode, the binding energies were 400.1 and 402.2 eV, which correspond to a nitrogen atom from PDA and an amino group from 4-amino-TEMPO and the nitroxide nitrogen atom in the TEMPO, respectively. It is to be noted that no high binding energy N 1s peak at 405 eV was observed. These results suggest that 4-amino-TEMPO was directly bound to PDA via covalent bonding rather than physical adsorption [40]. This is in good agreement with the computational simulations and electrochemical analyses. The electrochemical performances of the Li-O2 batteries without any RM, with TEMPO dissolved in the electrolyte, and with immobilized TEMPO on the cathode were evaluated via the galvanostatic discharge/charge test (Fig. 5). As a conventional LiO2 battery setup, a Swagelok cell with Li metal anode and a fully sealed gas reservoir were fabricated to enable high reproducibility. Fig. 5 shows that the discharge plateaus appear at approximately 2.7 V for the all the fabricated Li-O2 batteries, demonstrating that the discharge reaction is not affected by TEMPO. However, the charging voltage in the first cycle significantly decreases in the presence of TEMPO, as observed from the LSV data, resulting in a significant improvement of the round-trip efficiency. In the first cycle, the battery with TEMPO dissolved in the electrolyte shows a plateau-like charging curve at 3.6–3.7 V (vs. Li/Li+) (Fig. 5(a)) and the battery with immobilized TEMPO on the cathode shows the same at 3.8–3.9 V (vs. Li/Li+) (Fig. 5(b)). This is attributed to the electrochemical oxidation of TEMPO to TEMPO+, which is shown in the voltammogram from linear sweep voltammetry (Fig. 2). It is noteworthy that both the dissolved and immobilized TEMPO can successfully act as RMs, because the charge voltage of these batteries are evidently smaller than those of the batteries without TEMPO containing both the bare cathode and the PDA coated cathode without TEMPO (4.1 V) (Fig. 5(c–d)). The charge capacity of the battery without TEMPO did not reach to 0.5 mA h cm2 at 4.7 V in the 20th cycle due to the increased charge overpotential, demonstrating the unstable battery operation. For the battery with TEMPO dissolved in the electrolyte, a rapid increase in the charging voltage is observed immediately after the first cycle. This can be ascribed to the parasitic degradation reactions that may lead to the depletion of RM, resulting in electrode passivation and increased polarization during cycling. In contrast, much lesser change in the charging voltage is observed over 20 cycles for the battery with immobilized TEMPO on the cathode. The improved cycle stability in this case can be mainly attributed to the increased lifetime of TEMPO on the cathode. Consequently, the stable round-trip efficiency over cycles is achieved in the battery with immobilized TEMPO on cathode. (Fig. 5(e)) The full discharge capacities of the Li-O2 batteries without any RM, with TEMPO dissolved in the electrolyte, and with immobilized TEMPO on the cathode were also examined (current density of 0.1 mA cm2) (Fig. 5(f)). The increase in the discharge capacity upon the addition of TEMPO in the Li-O2 battery can be ascribed to the enhanced charge transfer. With immobilized
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TEMPO on the cathode, the discharge capacity further increases, clearly indicating the good stability of the immobilized TEMPO. Conclusions In summary, a nonaqueous Li-O2 battery with high performance was fabricated using a TEMPO-immobilized cathode that was prepared by a simple coating and grafting process. The effect of immobilized TEMPO on the cathode was verified by comparing the electrochemical performances of Li-O2 batteries with immobilized TEMPO on the cathode, TEMPO dissolved in the electrolyte, and in the absence of TEMPO. The immobilized TEMPO on the cathode suppressed the degradation of TEMPO at the Li metal anode and extended the redox mediated Li2O2 oxidation over repeated cycles. This study introduces a simple and effective strategy to modify the electrodes of advanced energy storage systems containing RMs. The insights from these studies will allow the utilization of RMs more efficiently, using a smaller amount, to prolong their lifespan and also enhance the battery performance. Declarations of interest None. Acknowledgments Funding: This work was supported by the National Research Foundation of Korea (NRF), Korea government (MSIT) (No. NRF-2018R1A5A1024127), Korea government (MEST) (NRF2016R1E1A1A01942936)), and the Supercomputing Center/Korea Institute of Science and Technology Information with supercomputing resources, which provided technical support (KSC-2017-C2-0029). References [1] K.M. Abraham, Z. Jiang, J. Electrochem. Soc. 143 (1996) 1. [2] S.D. Beattie, D.M. Manolescu, S.L. Blair, J. Electrochem. Soc. 156 (2009) A44. [3] S.A. Freunberger, Y. Chen, N.E. Drewett, L.J. Hardwick, F. Barde, P.G. Bruce, Angew. Chem. Int. Ed. 50 (2011) 8609. [4] T. Ogasawara, A. Debart, M. Holzapfel, P. Novak, P.G. Bruce, J. Am. Chem. Soc. 128 (2006) 1390. [5] J. Read, J. Electrochem. Soc. 149 (2002) A1190. [6] G. Girishkumar, B. McCloskey, A.C. Luntz, S. Swanson, W. Wilcke, J. Phys. Chem. Lett. 1 (2010) 2193. [7] B.D. McCloskey, A. Speidel, R. Scheffler, D.C. Miller, V. Viswanathan, J.S. Hummelshoj, J.K. Norskov, A.C. Luntz, J. Phys. Chem. Lett. 3 (2012) 997. [8] R. Black, S.H. Oh, J.H. Lee, T. Yim, B. Adams, L.F. Nazar, J. Am. Chem. Soc. 134 (2012) 2902. [9] C.O. Laoire, S. Mukerjee, E.J. Plichta, M.A. Hendrickson, K.M. Abraham, J. Electrochem. Soc. 158 (2011) A302. [10] V. Viswanathan, K.S. Thygesen, J.S. Hummelshoj, J.K. Norskov, G. Girishkumar, B.D. McCloskey, A.C. Luntz, J. Chem. Phys. 135 (2011)214704. [11] M.M. Ottakam Thotiyl, S.A. Freunberger, Z. Peng, P.G. Bruce, J. Am. Chem. Soc. 135 (2013) 494. [12] J.-H. Lee, R. Black, G. Popov, E. Pomerantseva, F. Nan, G.A. Botton, L.F. Nazar, Energy Environ. Sci. 5 (2012) 9558. [13] J. Lu, Y. Lei, K.C. Lau, X. Luo, P. Du, J. Wen, R.S. Assary, U. Das, D.J. Miller, J.W. Elam, H.M. Albishri, D.A. El-Hady, Y.-K. Sun, L.A. Curtiss, K. Amine, Nat. Commun. 4 (2013) 2383. [14] J.-J. Xu, Z.-L. Wang, D. Xu, L.-L. Zhang, X.-B. Zhang, Nat. Commun. 4 (2013) 2438. [15] J.-J. Xu, D. Xu, Z.-L. Wang, H.-G. Wang, L.-L. Zhang, X.-B. Zhang, Angew. Chem. Int. Ed. 52 (2013) 3887. [16] B.J. Bergner, A. Schurmann, K. Peppler, A. Garsuch, J. Janek, J. Am. Chem. Soc. 136 (2014) 15054. [17] Y. Chen, S.A. Freunberger, Z. Peng, O. Fontaine, P.G. Bruce, Nat. Chem. 5 (2013) 489. [18] Z. Liang, Y.C. Lu, J. Am. Chem. Soc. 138 (2016) 7574. [19] T. Zhang, K. Liao, P. He, H. Zhou, Energy Environ. Sci. 9 (2016) 1024. [20] S.H. Park, T.H. Lee, Y.J. Lee, H.B. Park, Y.J. Lee, Small 14 (2018)1801456. [21] Y. Qiao, Y. He, S. Wu, K. Jiang, X. Li, S. Guo, P. He, H. Zhou, ACS Energy Lett. 3 (2018) 463. [22] S.H. Lee, J.-B. Park, H.-S. Lim, Y.-K. Sun, Adv. Energy Mater. 7 (2017)1602417. [23] J. Zhang, B. Sun, A.M. McDonagh, Y. Zhao, K. Kretschmer, X. Guo, G. Wang, Energy Storage Mater. 7 (2017) 1.
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