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Letter
MOF-Based Separator in Li-O2 Battery: An Effective Strategy to Restrain the Shuttling of Dual Redox Mediators Yu Qiao, Yibo He, Shichao Wu, Kezhu Jiang, XIANG LI, Shaohua Guo, Ping He, and Haoshen Zhou ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00014 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 23, 2018
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ACS Energy Letters
MOF-Based Separator in Li-O2 Battery: An Effective Strategy to Restrain the Shuttling of Dual Redox Mediators Yu Qiao1,3†, Yibo He1,3†, Shichao Wu1*, Kezhu Jiang2, Xiang Li1,3, Shaohua Guo2, Ping He2, and Haoshen Zhou1,2,3*
1
Energy Technology Research Institute, National Institute of Advanced Industrial
Science and Technology (AIST), 1-1-1, Umezono, Tsukuba 305-8568, Japan. 2
National Laboratory of Solid State Microstructures & Department of Energy Science
and Engineering, Nanjing University, Nanjing 210093, P. R. China. 3
Graduaste School of System and Information Engineering, University of Tsukuba, 1-
1-1, Tennoudai, Tsukuba 305-8573, Japan. *Correspondence to:
[email protected] (S. W.)
[email protected] &
[email protected] (H. Z.) †These authors contributed equally to this work.
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Abstract Tuning the electrochemical formation/decomposition of Li2O2 from the circumscribed surface pathways to the solution ones, dual redox mediators (RMs) strategy largely promotes the discharge capacity and reduces the charge overpotential in aprotic Li-O2 batteries, revealing a promising strategy to realize the anticipated high specific energy cycling. However, both RM-induced Li degradation and electron shuttling between cathode and anode become the short slab on the bucket, resulting a poor sustainability. Here, with a narrow pore size window, a metal-organic framework (MOF)-based separator has been proposed, which acts as a RM molecules sieve to restrain the shuttling. By maximizing the advantages of the dual RMs strategy, the LiO2 cell reveals a prolonged cycled life (100 cycles, 5000 mAh g-1) at high current rate (1000 mA g-1). Moreover, the Li-O2 pouch cell fabricated by the flexible MOF-based separator exhibits the potential for the development of large scale energy storage devices. Table of Contents Image (TOC)
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If the theoretical specific gravimetric energy (3500 Wh kg-1) and rechargeability can be achieved in practical, aprotic Li-O2 battery system would essentially reshapes the pattern of energy storage devices.1-2 However, due to the insulating nature of Li2O2, the dominate discharge product of Li-O2 battery, there exist several intrinsic obstacles severely inhibiting the practical achievement; two are particularly pressing: (1) during discharging, the buildup of passivating Li2O2 film on cathode leads to the early cell death with low capacity; (2) upon charging, the decomposition of Li2O2 requires high overpotential, which inevitably leads to the parasitic degradation of electrode and electrolyte.3-4 Recently, by decoupling the directly surface electrochemical reactions from the formation/decomposition of Li2O2,5-6 a dual mediators strategy has been rationally introduced against the pair of inherent defects above: (1) instead of oxygen, discharging RM (RMd) firstly undergoes reduction and carries the electron to further reduce the oxygen in the solution.7-8 The solution-based Li2O2 formation pathway largely extends the discharge capacity: 2RMd + 2e- → 2RMd- (Surface Pathway) 2Li+ + 2RMd- + O2 → Li2O2 + RMd (Solution Pathway) (2) instead of rigid solid-solid contact between Li2O2 and various catalytic cathode, the diffusible charging RM (RMc) is firstly oxidized and has a wet contact with Li2O2 particles, thus maximizing the available reactivity sites.9-14 The solution-based Li2O2 decomposition pathway largely reduces the charge potential: 2RMc - 2e- → 2RMc+ (Surface Pathway) Li2O2 + RMc+ → 2Li+ + O2 + 2RMc (Solution Pathway) Benefited from the tuning of equilibrium potential of the soluble redox mediator species, the RMs become the electron carriers, and the electrochemistry no longer being enslaved to the insulating and insoluble nature of Li2O2, highlighting the dual mediators strategy as a rational design for the sustainable cycling of promising Li-O2 battery 3
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system.15-16 However, the drawbacks of solution-based pathway focus on the shuttle effect of these diffusible RM species,17-20 which has also been systematically reviewed by Y. K. Sun et al.21 As strong reductants, both RMd and RMc+ can easily react with Li anode, resulting the redox shuttle and serious Li degradation. The unwanted chemical/electrochemical cross-talk between cathode and Li anode can be ascribed to the poor cycling stability, even with the employment of various mediators.16-17, 19 Thus, inhibiting the diffusion of RMs to the Li anode side becomes the essential way to solve the problem. However, few attentions were paid on this very key point so far,17, 22-24 while limited attempts focused on the use of solid electrolyte,5, 17, 24 which is obviously not suitable for developing practical large scale energy storage devices due to its poor flexibility, high cost and low ion conductivity. Herein, after systematically elucidating the shuttling-induced degradation mechanism by comprehensive spectroscopic investigations, we design a metal-organic framework (MOF)-based separator with narrow pore size window in the specific framework, which acts as a sieve and efficiently inhibits the crossover of RMs to the Li anode. After restraining the shuttling of RMs by MOF-based separator, the lifetime of Li-O2 cell with dual mediators strategy presents dramatically improvement (100 cycles with 5000 mAh g-1 capacity), especially at high current rate (1000 mA g-1). Besides, the pouch-type Li-O2 cell fabricated by flexible MOF-based separator would further meet the need for large scale energy storage devices. In this study, due to each of their suitable equilibrium redox potential windows (Figure S1), 2,5-di-tert-butyl-1,4-benszquinone (DBBQ, RMd)7 and tetrathiafulvalene (TTF, RMc)9 have been employed for the design of dual mediators strategy. We select Cu3(BTC)2 (HKUST-1, MOF) to fabricate the MOF-based separator because its 3D channel structure contains highly ordered micropores with a size window of approximately 6.9 to 9Å, which is smaller than the diameters of specific RM molecules, while the Li+ could fluently pass through (Figure 1a).25-28 After the MOF particles insitu growth onto the as-prepared Celgard separator, a certain amount of polymer solution is filtered through the MOF-layer, filling the void space between the boundary 4
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of MOF particles. After repeating this process for three times to achieve a compact fabrication, a flexible MOF@Celgard-based separator is harvested (Figure 1b), with a MOF-layer (thickness, ~15 μm) tightly adhered on the top of Celgard film (Figures 1c and S2). XRD patterns of the MOF-based separator further confirm the composition of HKUST-1 MOF particles (Figure S3). The permeation resistance towards the specific RM species is the core assessment factor for the feasibility of the MOF-based separator. In our homemade V-type device for permeation experiments (Figures 1d-1g), the left chambers (injected with pristine RMs) are isolated by the separator from the right one (injected with reduced/oxidized RMs). As a comparison, the typical Celgard separator cannot efficiently inhibit the permeation (Figures 1d and 1f), since the DBBQ- (orange) and TTF+ (dark blue) in the right chambers gradually diffuse to the left sides within 24 h aging, which is well consistent with the increasing UV-vis peak at 538 and 588 nm harvested from the left chambers, respectively (Figure S4). While RMs cannot pass through the MOF-based separator within a prolonged period of 48 h with a distinct boundary between the pair of chambers (Figures 1e, 1g and S5). With additional electrochemical demonstration (Figure S6), the obtained MOF-based separator reveals outstanding ability to block the penetration of RMs, and exhibits potential to restrain the relevant shuttling of RMs in Li-O2 cells. Turning to the Li-O2 coin cell (Scheme S1 and Figure S7), the MOF-based separator present good permeability with ether-based electrolyte (Figure S8). Note that, before transferring the separator into the glove-box, an additional degas procedure has been conducted to clean up guest molecules (Figure S9). As shown in Figure 2a, compared with typical condition without RM (red trace), the cells with dual RMs strategy (black and blue traces) exhibit both enlarged discharge capacity and high Coulombic Efficiency (CE). However, the formation/decomposition of Li2O2 demonstrated by conventional XRD and SEM results (Figure S10) cannot rationally explain the reduced charge potential by the employment of MOF-based separator (~0.5 V, blue trace, Figure 2a). Based on in-situ Raman results (Figure 2b), compared with 5
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the typical condition without RM (red traces), the accumulation of LiO2 intermediate (peaks at 1135 and 1496 cm-1)29 can be restrained by the addition of DBBQ due to the mediated oxygen reduction pathway (black and blue traces).5, 7 As a result, the parasitic formation of Li2CO3 (1080 cm-1), acetate (1039 cm-1) and other by-products are also effectively inhibited in DBBQ-involved cells, which is further confirmed by the NMR analysis (Figure S11). To our knowledge, this is the first direct evidence to prove that DBBQ can restrain the superoxide-induced parasitic reaction during discharging. Moreover, without fabricating MOF-based separator, Li2O2 cannot be fully decomposed after charging (without TTF, Figure S12), nor after a 100% CE charging with the help of TTF. Besides, due to the highly polarized charging potential (~4.25 V), parasitic decomposition of cell components is exacerbated,30 which are nearly invisible in the cell with MOF-based separator. Additionally, the difference can be further illustrated by the TiOSO4-based quantitative results (Figures 2c and S13), in which the remaining black bar and corresponding residual yellow species after charging is ascribed to the incomplete decomposition of Li2O2 product in the cell with typical separator. While the cell with MOF-based separator (blue bars) exhibits both higher Li2O2 formation efficiency (>95%) and lower Li2O2 residual rate (
