Decoupling Energy Storage from Electrochemical Reactions in Li−Air Batteries toward Achieving Continuous Discharge Kensuke Takechi,*,† Nikhilendra Singh,† Timothy S. Arthur,† and Fuminori Mizuno†,‡ †
Materials Research Department, Toyota Research Institute of North America, 1555 Woodridge Avenue, Ann Arbor, Michigan 48105, United States ‡ Toyota Motor Corporation, Higashifuji Technical Center, 1200 Mishuku, Susono, Shizuoka 410-1193, Japan S Supporting Information *
ABSTRACT: Intrinsic performance issues in Li−air batteries, such as poor reversibility and power, result from multiple functions at the cathode. Electrochemical reactions and discharge product storage are two such contradictory functions that interfere with each other. Here, we propose a new concept Li−air battery that can decouple those functions via simple yet drastic changes in the electrolyte composition, completely eliminating Li+ ions in the area around the cathode using an ionic liquid. The Li+-free environment for the cathode reaction realizes continuous discharge, in similar fashion to fuel cells, for more than 1000 h due to its unique concept of storing generated Li2O2 in the bulk electrolyte. The new concept opens an avenue to redesign Li−air battery systems with wide flexibility to balance power and capacity independently and efficiently.
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back to Li+ ions and oxygen again. However, one huge concern in realizing this concept is that Li2O2 is electronically insulating, and hence, the electrochemical growth of Li2O2 is not a straightforward process; even electrochemically generated Li2O2 in aprotic solvents has a unique growth mechanism and electronic conducting behavior.16−18 In other words, the aforementioned challenges are caused by the fact that ORR occurs in the same physical location as energy storage (precipitation of Li2O2). This fact is captured in Figure 1a. Additionally, we previously determined that one of the major causes of irreversible capacity was electrochemically isolated Li2O2 due to an insulating layer formed by interfacial side reactions.19 Thick Li2O2 layer formation also critically avoids smooth ion and electron transfers between the cathode and electrolyte, which also limits the rate capability of the battery. Therefore, the purpose of this report is to propose and prove a new concept Li−air battery that is free from the abovedescribed issues to realize drastic improvement of the performance of the battery.
i−air batteries are one of the promising battery technologies due to their ultrahigh capacity, and many research groups have strongly developed the technology from both scientific and engineering approaches toward practical application.1,2 In the past, we have also pointed out serious issues in that the electrolyte is capable of being easily decomposed by the discharge reaction intermediate, a superoxide radical.3,4 Then, many groups extensively studied the mechanism and effective countermeasures to overcome those issues and found that ionic liquid was one of the candidates that is able to enhance the reversibility of the battery.5−12 However, there still remain additional challenges, such as capacity limitation, capacity fading, and low rate capability. Here, we ask the question, “What is the root cause of these performance issues?” Before answering that question, the basic concept of the “original” Li−air battery has to be revisited. During discharge, the cathode interacts with oxygen from outside of the cell to reduce it electrochemically. During reduction (oxygen reduction reaction, ORR), the oxygen forms superoxide radical species and finally produces Li2O2 by coupling with Li+ ions provided by the electrolyte.13−15 Because Li2O2 is a solid-state compound under ambient conditions, it precipitates on top of the cathode surface (Figure 1a). During oxidation, Li2O2 is decomposed via the electrochemical charge process, ideally © XXXX American Chemical Society
Received: January 18, 2017 Accepted: February 14, 2017 Published: February 14, 2017 694
DOI: 10.1021/acsenergylett.7b00056 ACS Energy Lett. 2017, 2, 694−699
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ACS Energy Letters
Figure 1. Schematic illustrations of the difference between the (a) original and (b) new Li−air battery concepts and (c) the cross section of the new Li−air battery and designed reaction mechanism.
In this report, we focus on steps 1 and 2 during discharge. For discharge step 1, one strategy to realize continuous ORR is to form and keep a superoxide radical in the electrolyte without further reactions because the radical is soluble in the electrolyte. However, in the presence of Li+ in the electrolyte, it is quite difficult to keep the superoxide radical itself due to the very quick reaction between the radical and Li+ immediately following ORR (Figure 1a, “original”). Therefore, the simplest way to prevent the superoxide radical from reacting with Li+ is to remove Li+ from the electrolyte. This is especially important in the area around the cathode. There are two major ways to realize this concept: (i) replacing the Li-salt with another non-Li-salt dissolved in organic solvent or (ii) just simply removing the Li-salt in the electrolyte. In reality, option (i) is not realistic because most organic solvents have poor stability against the superoxide radical in the absence of Li+.4 For option (ii), in general, it is impossible to carry out any electrochemical reactions without any supporting salt in the electrolyte. However, one of the exceptions of the rule is when the solvent is an ionic liquid
Because the key point hinges on how we can decouple the discharge product storage from the electrochemical reaction in the battery, we propose a new Li−air battery concept consisting of separated discharge steps, as shown in Figure 1b. New Concept Discharge Step 1: Continuous ORR on the cathode: In this step, the ORR without any cathode passivation is required to keep a functional cathode surface. The discharge product of ORR, such as the superoxide radical, should be transported to a separate location from the cathode surface. New Concept Discharge Step 2: Storage of the discharge product in an electrolyte: The discharge product will be transported from the cathode surface to an electrolyte reservoir. The capacity of the battery is determined by how much of the product can be stored in the reservoir, not by the cathode structure or material. New Concept Charge: Chemical regeneration of the discharge product: The discharge product stored in the electrolyte should be decomposed via a charging process by mediators that are oxidized (charged) electrochemically, in advance. 695
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Figure 2. (a) Discharge profiles of the original (with Li+, black line) and the new concept Li−air batteries (without Li+, red and green lines with static and stirred electrolyte, respectively). The inset shows the initial stage of the same profiles. (b) Raman spectroscopy of the discharge product of the new Li−air battery (the differential spectrum subtracted by DEME-TFSI and the original DEME-TFSI spectrum are also shown). The inset shows a photo of the electrolyte after 14.1 mAh discharge.
control experiment, Li+-containing electrolyte (0.352 mol/kg LiTFSI dissolved in DEME-TFSI) was also used for the battery evaluation. The evaluations were performed by supplying highpurity O2(>99.99%) in the cell within a temperature chamber kept at 25 °C. Figure 2a shows the current during discharge with and without LiTFSI (black and red lines, respectively) in the electrolyte. At the initial stage of discharge, the current with LiTFSI was higher than that without LiTFSI due to effective consumption of the superoxide radical in the presence of Li+. Then, the current dramatically dropped before reaching 0.1 mAh of capacity due to the electrochemical deactivation of the carbon surface caused by Li2O2 deposition. On the other hand, in the absence of LiTFSI, the current became a constant value after the initial drop and the cell kept generating output current to more than 10 mAh of capacity (>1000 h), which is more than 100 times larger capacity as compared with that of the Liion containing system. This data clearly indicates that the nature of discharge product formation is totally different from that of the “original” Li−air battery system. Our new concept depends on free diffusion of the superoxide radical (O2•−, Figure 1b). However, the diffusion coefficient of the superoxide radical (O2•−) is almost 1 order of magnitude slower than that of O2 and becomes the rate-limiting step of the discharge reaction due to the equilibrium of these species (eq 1).20
because the ionic liquid itself carries functions of both solvent and salt (ionic liquid is also called “room-temperature molten salt”). Fortunately, through our development of unique electrolyte systems for the battery using ionic liquids, we are able to transfer the necessary materials to this new system. By utilizing a neat ionic liquid as an electrolyte, the superoxide radical formed on the cathode surface by ORR will diffuse into the bulk electrolyte because there is no driving force to form any precipitation in the absence of Li+. At the same time, Li+ should be released from the solid-state Li+ conductor to keep the electronic neutrality in the electrolyte. Then, Li2O2 particles will be formed in the bulk electrolyte, NOT on the cathode surface by a disproportionation reaction, when the superoxide radical and Li+ meet (Figure 1b). The following experiment is a trial to demonstrate the new Li−air battery to realize our concept. If the concept works properly as designed, we should find Li2O2 particles in the bulk electrolyte, which we never observed in the original Li−air battery systems. A cross-sectional structure of the Li−air battery cell used in this study is also shown in Figure 1c and described in detail in the Supporting Information. The cathode side is completely separated from the anode side by a solid-state Li-ion conductor (LIC-GC, Ohara Corp.) of 1 mm thickness to avoid direct contact between the discharge product and Li-metal anode. To ensure a Li-ion-free environment around the cathode, we located the cathode 5 mm away from the solid-state Li-ion conductor, which releases Li+ during discharging. The most important component of this experiment is the electrolyte for the cathode side. N,N-diethyl-N-(2-methoxyethyl)-N-methylammonium bis(trifluoromethylsulfonyl-imide (DEME-TFSI, Kanto corp.) was used as the electrolyte without addition of any salt. We have already shown that DEME-TFSI is quite stable against superoxide radical.9,10 As the cathode, to simplify the experiment, we used a single sheet of carbon paper (360 μm thick, TGP-H-120, Toray corp.) without any modification or treatment. Because the expected discharge current was much smaller than that of the typical Li−air batteries due to the small active surface area of the carbon paper cathode, we selected the CCCV (constant current/constant voltage) mode for discharge evaluation at 2.0 V and monitored changes in the resulting current. The initial current was 0.1 mA, and after reaching 2.0 V, the current was decreased to maintain the voltage. As a
O2 + e− → O2•−
(1)
Therefore, to boost the current generation capability, we introduced an electrolyte convection mechanism through stirring (see Figure S2 for structural details) to promote ionic diffusion in the system. The discharge behavior (Figure 2a, green line) clearly indicates an improved current generation capability, which was found to be almost four times higher than that of the nonstirred setup (43 μA), due to the accelerated superoxide diffusion into bulk electrolyte from the cathode. After the discharge of the Li−air battery without LiTFSI (Figure 2a, red line) at a capacity of 14.1 mAh, we stopped the evaluation and opened the cell to analyze the discharge product. The photo (Figure 2b, inset) of electrolyte after discharge clearly indicates that a large amount of white precipitate was generated and stored in the electrolyte. 696
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Figure 3. Detailed analyses of carbon paper cathodes. (a−c) SEM images of pristine and discharged cathodes with and without a Li-ion in the electrolytes. (d) Electrochemical impedance spectra of Li−air batteries after discharge. Discharge duration: 3 h.
observations are consistent with our expected mechanism and advantage of the new Li−air battery. To support the above discussion, electrochemical impedance spectra after the 3 h discharge were also measured and are shown in Figure 3d. While the spectrum in the presence of the initial Li+ indicated a capacitor-like steep rise in the lowfrequency region, which reflected quite high interfacial resistance, the spectrum without Li+ lacked such behavior, ensuring reversible electrode capability, which was like a fuel cell cathode. The semicircle corresponding to bulk transport in cathodes was also enlarged in the Li+-containing system due to slower ionic diffusion in the carbon paper cathode. For the charging process, a major concern in the original Li− air battery is electrochemical side reactions, which decompose electrolyte and carbon-based electrode materials.21,22 To avoid the side reactions, different types of redox mediator have been proposed recently not to directly decompose Li2O2 on the cathode.23,24 However, the issue has not been completely solved due to the other electrochemical side reactions caused by oxidation of mediators on the cathode. Because those side reactions are irreversible, the capacity of the battery is also hampered permanently during discharge/charge cycling. This fatal phenomenon is also another serious issue caused by the mechanistic challenge in the original Li−air concept, coupling electrochemical reaction and discharge product storage on the cathode. In the case of our new concept Li−air battery, the charging reaction can also be decoupled from the side reaction on the cathode. Li2O2 stored at a remote location from the cathode can be decomposed indirectly by a mediator-assisted “chemical regeneration” concept.25 Because the decomposition
Raman spectroscopy was utilized to identify the precipitate formed directly in the electrolyte. Because the raw spectrum included peaks attributed to the ionic liquid (Figure 2b “Discharge product”), we obtained a differential spectrum via subtraction of those peaks (Figure 2b “DEME-TFSI”) to emphasize peaks of the remaining compounds. The differential spectrum shows clear evidence of the existence of Li2O2 at 256 and 786 cm−1 (Figure 2b “Differential”). This observation matches our prediction and proves our new concept. A small amount of LiOH formation was also confirmed and suggested slight decomposition of the DEME cation as the cation is the only H-source for possible LiOH formation. This is most likely a result of exposure of DEME to the extended lifetime of the superoxide radical. Further investigations are necessary to improve the stability of this battery for extra long time operation. For the carbon paper cathode, we compared the nature of the surfaces with and without Li+ via scanning electron microscopy (SEM, Figure 3a−c) after a fixed duration of discharge (3 h). In the case of the Li+-containing system, the surface was fully covered by a thin layer of discharge product (Figure 3b), which can be emphasized by prolonged electron beam exposure to a limited area. The surface of the Li+-containing system carbon paper looked much like the pristine cathode after this prolonged electron beam exposure period (Figure 3a). On the other hand, the cathode after discharge in the Li+-free system (Figure 3c) maintained an active carbon surface that was able to carry out continuous electrochemical reaction even after the battery exhibited higher discharge capacity (160 μAh) than the above Li-ion-containing system (89 μAh). These 697
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stability in aprotic electrolyte Li−air batteries: nucleophilic substitution by the superoxide anion radical (O2•−). J. Phys. Chem. A 2011, 115, 12399−12409. (6) McCloskey, B. D.; Bethune, D. S.; Shelby, R. M.; Mori, T.; Scheffler, R.; Speidel, A.; Sherwood, M.; Luntz, A. C. Limitations in rechargeability of Li-O2 batteries and possible origins. J. Phys. Chem. Lett. 2012, 3, 3043−3047. (7) Mizuno, F.; Takechi, K.; Higashi, S.; Shiga, T.; Shiotsuki, T.; Takazawa, N.; Sakurabayashi, Y.; Okazaki, S.; Nitta, I.; Kodama, T.; et al. Cathode reaction mechanism of non-aqueous Li−O2 batteries with highly oxygen radical stable electrolyte solvent. J. Power Sources 2013, 228, 47−56. (8) Mizuno, F.; Arthur, T. A.; Takechi, K. Water in Ionic Liquid for Electrochemical Li Cycling. ACS Energy Letters 2016, 1, 542−547. (9) Nakamoto, H.; Suzuki, Y.; Shiotsuki, T.; Mizuno, F.; Higashi, S.; Takechi, K.; Asaoka, T.; Nishikoori, H.; Iba, H. Ether-functionalized ionic liquid electrolytes for lithium-air batteries. J. Power Sources 2013, 243, 19−23. (10) Higashi, S.; Kato, Y.; Takechi, K.; Nakamoto, H.; Mizuno, F.; Nishikoori, H.; Iba, H.; Asaoka, T. Evaluation and analysis of Li-air battery using ether-functionalized ionic liquid. J. Power Sources 2013, 240, 14−17. (11) Allen, C. J.; Mukerjee, S.; Plichta, E. J.; Hendrickson, M. A.; Abraham, K. M. Oxygen Electrode Rechargeability in an Ionic Liquid for the Li-Air Battery. J. Phys. Chem. Lett. 2011, 2, 2420−2424. (12) De Giorgio, F.; Soavi, F.; Mastragostino, M. Effect of lithium ions on oxygen reduction in ionic liquid-based electrolytes. Electrochem. Commun. 2011, 13, 1090−1093. (13) Laoire, C. O.; Mukerjee, S.; Abraham, K. M.; Plichta, E. J.; Hendrickson, M. A. Elucidating the mechanism of oxygen reduction for lithium-air battery applications. J. Phys. Chem. C 2009, 113, 20127− 20134. (14) Sawyer, D. T.; Valentine, J. S. How super is superoxide? Acc. Chem. Res. 1981, 14, 393−400. (15) Gibian, M. J.; Sawyer, D. T.; Ungermann, T.; Tangpoonpholvivat, R.; Morrison, M. M. Reactivity of superoxide ion with carbonyl compounds in aprotic solvents. J. Am. Chem. Soc. 1979, 101, 640−644. (16) Viswanathan, V.; Thygesen, K. S.; Hummelshoj, J. S.; Norskov, J. K.; Girishkumar, G.; McCloskey, B. D.; Luntz, A. C. Electrical conductivity in Li2O2 and its role in determining capacity limitations in non-aqueous Li-O2 batteries. J. Chem. Phys. 2011, 135, 214704. (17) Johnson, L.; Li, C.; Liu, Z.; Chen, Y.; Freunberger, S. A.; Ashok, P. C.; Praveen, B. B.; Dholakia, K.; Tarascon, J.-M.; Bruce, P. G. The role of LiO2 solubility in O2 reduction in aprotic solvents and its consequences for Li−O2 batteries. Nat. Chem. 2014, 6, 1091−1099. (18) Radin, M.; Siegel, D. Charge transport in lithium peroxide: relevance for rechargeable metal-air batteries. Energy Environ. Sci. 2013, 6, 2370−2379. (19) Hase, Y.; Ito, E.; Shiga, T.; Mizuno, F.; Nishikoori, H.; Iba, H.; Takechi, K. Quantitation of Li2O2 stored in Li−O2 batteries based on its reaction with an oxoammonium salt. Chem. Commun. 2013, 49, 8389−8391. (20) Herranz, J.; Garsuch, A.; Gasteiger, H. A. Using Rotating Ring Disc Electrode Voltammetry to Quantify the Superoxide Radical Stability of Aprotic Li-Air Battery Electrolytes. J. Phys. Chem. C 2012, 116, 19084−19094. (21) McCloskey, B. D.; Valery, A.; Luntz, A. C.; Gowda, S. R.; Wallraff, G. M.; Garcia, J. M.; Mori, T.; Krupp, L. E. Combining Accurate O2 and Li2O2 Assays to Separate Discharge and Charge Stability Limitations in Nonaqueous Li−O2 Batteries. J. Phys. Chem. Lett. 2013, 4, 2989−2993. (22) Ottakam Thotiyl, M. M.; Freunberger, S. A.; Peng, Z.; Bruce, P. G. The Carbon Electrode in Nonaqueous Li−O2 Cells. J. Am. Chem. Soc. 2013, 135, 494−500. (23) Chen, Y.; Freunberger, S. A.; Peng, Z.; Fontaine, O.; Bruce, P. G. Charging a Li−O2 battery using a redox mediator. Nat. Chem. 2013, 5, 489−494.
process is governed by chemical steps without any electrochemical reactions, the system can minimize irreversible side reaction and maintain good cyclability (>98%, initial cycle). In summary, we demonstrated a new concept Li−air battery with a small composition change in the electrolyte and proved that it had a huge impact on the discharge reaction mechanism, to realize continuous reaction of the battery. In this new battery system, the function of the electrochemical reaction on the cathode is successfully decoupled from the function of discharge product storage in the electrolyte. In other words, one can design these components separately to maximize the performance of each single function. For example, the cathode structure is now independent of the capacity of the battery because the capacity is defined by Li2O2 stored in the electrolyte reservoir. The theoretical capacity of the new Li− air battery was roughly calculated as more than 1000 Wh/L (see the Supporting Information) and can be tuned by changing the balance between the volume of the electrode assembly (reactor cell) and that of the electrolyte reservoir. Such flexibility on the battery design is similar to the advantage of the redox flow battery, in which power and capacity are defined by the reactor and dissolved species in the electrolytes, respectively. Conversely, the same strategy can also be applied on a new Li−air battery system utilizing flowing electrolyte to efficiently supply required chemical species and store discharge product appropriately. The movement (circulation) of the electrolyte also provides effective diffusion of the superoxide radical, which was proven in this study (Figure 2a, green line), and dramatically improves the power generation capability of the battery. This extended “flow-type Li−air battery concept” is currently under development.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00056. Experimental details and supporting figures, including the cell configuration and capacity estimation (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kensuke Takechi: 0000-0001-6364-0695 Notes
The authors declare no competing financial interest.
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REFERENCES
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