Redox catalysts for aprotic Li-O2 batteries: Toward a redox flow system

Nano Materials Science 1 (2019) 173–183

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Redox catalysts for aprotic Li-O2 batteries: Toward a redox flow system YunGuang Zhu, F.W. Thomas Goh, Qing Wang * Department of Materials Science and Engineering, Faculty of Engineering, National University of Singapore, 117576, Singapore

A R T I C L E I N F O

A B S T R A C T

Keywords: Lithium-air battery Redox catalysis Oxygen reduction reaction Oxygen evolution reaction Redox flow cell

Large-scale electrical energy storage with high energy density and round-trip efficiency is important to the resilience of power grids and the effective use of intermittent renewable energy such as solar and wind. Lithiumoxygen battery, due to its high energy density, is believed to be one of the most promising energy storage systems for the future. However, large overpotentials, poor cycling stability, and degradation of electrolytes and cathodes have been hindering the development of lithium-oxygen batteries. Numerous heterogeneous oxygen electrocatalysts have been investigated to lower the overpotentials and enhance the cycling stability of lithium-oxygen batteries. Unfortunately, the prevailing issues of electrode passivation and clogging remain. Over the past few years, redox mediators were explored as homogenous catalysts to address the issues, while only limited success has been achieved for these soluble catalysts. In conjunction with a flowing electrolyte system, a new redox flow lithium-oxygen battery (RFLOB) has been devised to tackle the aforementioned issues. The working mechanism and schematic processes will be elaborated in this review. In addition, the performance gap of RFLOB with respect to practical requirements will be analysed. With the above, we anticipate RFLOB would be a credible solution for the implementation of lithium-oxygen battery chemistry for the next generation energy storage.

1. Introduction

Anodic reaction: 2Li  2e ↔ 2Liþ

(1)

While lithium ion batteries (LIBs), the state-of-the-art power sources, are playing increasingly important roles in portable electronics, automotive and stationary (distributed and power grid) energy storage, their further development is limited by lithium–based intercalation chemistry to meet the increasing demands for high energy density [1,2]. Thus it is critical to explore alternative battery chemistries for energy storage systems in the “post-LIBs” era. Lithium-oxygen (Li-O2) battery, with 5–10 times higher theoretical energy density than that of LIBs, has caught considerable attention from all over the world in the past decades [3]. As most of the existing batteries (in the market) with enclosed configuration, the aprotic Li-O2 battery consists of anode (Li metal), separator, cathode, and electrolyte; except that the cathodic active material consisted of an externally fed oxygen (Fig. 1). During the discharge process, Li metal is electrochemically oxidized to Liþ ions which are released into the electrolyte, and electrons flow to the cathode via the external circuit. Meanwhile, oxygen is reduced to form toroid shape Li2O2 on the porous cathode surface, which is however generally impeded by sluggish interfacial charge transfer because of the insulating nature of Li2O2 [4–6]. The battery chemistry can be described based on the following equations (1)–(3):

Cathodic reaction: O2 þ 2Liþ þ 2e ↔ Li2 O2

(2)

Overall reaction: 2Li þ O2 ↔ Li2 O2

(3)

The aggregation of Li2O2 in the cathode will eventually block the transport channels for oxygen and Liþ ions, terminating the discharge process. Such a phenomenon of Li-O2 batteries has been investigated by simulation and experimental studies [7,8]. During the charging process, Li2O2 is electrochemically reduced back to Li metal releasing O2, with the assistance of oxygen evolution reaction (OER) catalysts. One challenge affecting the aprotic Li-O2 batteries is the instability of organic solvents. Currently, the commonly used solvents in Li-O2 batteries are dimethyl sulfoxide (DMSO), ether-, carbonate-, amide-based, or acetonitrile, just to name a few. However, none of the above solvents meets the requirement for long-term cycling [9,10]. For example, DMSO is a high donor number (DN) solvent which was known to promote the formation of Li2O2 in the electrolyte during the discharge process, and to extend the discharge capacity of Li-O2 batteries [11]. However, parasitic reactions, stemming from DMSO with the ORR products, result in the formation of dimethyl sulfone (DMSO2) and LiOH during long-term

* Corresponding author. E-mail address: [email protected] (Q. Wang). https://doi.org/10.1016/j.nanoms.2019.02.008 Received 21 November 2018; Accepted 18 February 2019 Available online 29 March 2019 2589-9651/© 2019 Chongqing University. Production and hosting by Elsevier B.V. on behalf of KeAi. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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2. Heterogeneous electrocatalysts The prevalent kinetics involved in the cathode of aprotic Li-O2 batteries are the ORR/OER reactions, as well as mass transport of Liþ and O2 [27]. Over the last decade, a large segment of research focused on the development of structured cathode materials with advanced electrocatalysts to facilitate the kinetics in Li-O2 batteries, including carbonaceous materials, metal oxides (including perovskites and spinels), metal nitrides, precious metals, etc. Carbon-based materials are usually designed to enhance the performance of Li-O2 batteries for which one dimensional (1D) (e.g. carbon nanotubes, fibers) [28,29], 2D (e.g. graphene) [30], and 3D carbons (e.g. porous carbon) have been investigated to provide ample reaction sites to accommodate the formed Li2O2. Besides the morphological and structural modifications, N, B, P, or S-doped carbon materials, have also been attempted to engineer the defects and edge plane sites in carbon-based materials for enhanced catalytic properties [31]. Although carbon-based materials exhibit significant advantages in Li-O2 batteries, the long-term stability of carbon is a critical concern for prolonged use. As shown in Fig. 2, carbonaceous materials tend to be oxidized when the applied voltage is higher than 3.50 V, to form Li2CO3 [32]. Thus, researchers have turned to carbon-free materials. One type of effective carbon-free oxygen electrocatalysts are noble metals, such as Pt, Pd, Au and their respective alloys. An example is a bifunctional catalyst made of Pt/Au nanoparticles, in which the surfaces of Au and Pt atoms are primarily responsible for the ORR and OER kinetics, respectively [33]. The most popular carbon-free materials are the transition metal oxides, which represent a large family of oxygen electrocatalysts, including perovskiteand spinel-types. Transition metal oxides possess many advantages, including low cost, facile syntheses, and environmental friendliness. In particular, MnOx-based catalysts were employed as the air cathode due to its bifunctional catalytic properties [34–41]. Apart from metal oxides, some other metal-based compounds have also been investigated in Li-O2 batteries, such as TiC, TiN, MoS2, Mo2C, etc. [42–45] Among these, TiC, reported by Bruce group, has attracted considerable attention among researchers due to its good stability towards aprotic Li-O2 batteries [42]. According to their work, TiC-based cathode significantly suppresses undesirable side reactions as compared to pure carbon cathode. In order to further enhance the performance of air electrodes, researchers have combined metal-based compounds with carbonaceous materials to boost the performance of Li-O2 batteries [46–52]. Carbon provides large surface area, high conductivity and ORR catalytic activity, while metal compounds usually have high stability and catalytic activity. Although such hybrid cathodes potentially possess merits of each composition, the inherent drawbacks of heterogeneous catalysts remain inevitable. Therefore, homogenous catalysts have been studied and employed in aprotic Li-O2 batteries by researchers in recent years [53, 54].

Fig. 1. Schematic illustration of an aprotic Li-O2 battery. It contains a Li metal anode, separator, electrolyte, and air cathode. The predominant product is Li2O2.

cycling [12,13]. Therefore, DMSO is yet ideal for Li-O2 batteries despite its advantages [14]. Another type of commonly used solvents are the ether-based solvents, which are stable in low reduction potentials (using lithium metal anode) and high oxidation potential (>4.5 V vs. Li/Liþ) [15]. However, McCloskey and co-workers found that 1,2-dimethoxyethane (DME) degrades with the decomposition of Li2O2 during the charging process [15]. Later, Fruenberger et al. investigated the stability of n-glymes (n ¼ 2, 4) in Li-O2 batteries and observed that they are, unfortunately, not stable during the discharge process, either [16]. It was proposed that the parasitic reaction mechanism was due to the superoxide radical anions removing a proton from a CH2 group from the glymes, subsequently forming an alkyl radical. Carbonate- and amide-based solvents were also investigated and have been proven to be unstable with oxygen reduction products [15,17]. Although acetonitrile is more stable towards the reduced oxygen products, its incompatibility towards Li metal presents another challenge [18]. Thereafter, researchers attempted to modify the solvent molecules to create solvents which are resistant to the parasitic reactions in Li-O2 batteries [19–21]. For example, Aurbach and coworkers designed a new aprotic solvent, 2, 4-dimethoxy-2,4-dimethylpentan-3-one (DMDMP), to prevent the nucleophilic attack and hydrogen abstraction by the oxygen reduction species in Li-O2 batteries [21]. Feng et al. have recently established a comprehensive framework to evaluate the stability of organic electrolyte in non-aqueous Li-O2 batteries, which is meaningful to guide the design of stable aprotic solvents [22]. Due to the kinetic limitations of oxygen reduction (ORR) and OER reactions, which impose a large overpotential (>1 V) on the cathode, the round-trip efficiency of Li-O2 batteries is relatively poor. Electrocatalysts, such as carbonaceous materials, precious metal (e.g. Au, Pt, Pd, etc.), transition metal oxides (e.g. MnO2, Co3O4, etc.) [23], are grafted on the cathode to promote the ORR and/or OER reactions. However, such heterogeneous electrocatalysts fail to circumvent the passivation and clogging issues on the cathode, especially at deep discharge state. Redox mediators were then explored as soluble catalysts to lower the overpotentials while alleviating the above issues [24–26]. Although the reported data seem promising, the discharge capacity of Li-O2 battery is still limited by the pore volume of the cathode, and the cathode clogging issue remains unresolved. To eliminate the clogging problems on the cathode, an approach which integrates the redox-mediated ORR/OER reactions with redox flow system has been proposed recently, in which the Li2O2 is formed in an external gas diffusion tank (GDT) but not within the cathode, hence elegantly obviates the above issues confronted by the conventional Li-O2 systems.

Fig. 2. Diagram illustrating the parasitic reactions from electrolyte and carbon electrode during the charge and discharge processes of aprotic Li-O2 battery. Reprint from Ref. 37 with permission from American Chemical Society, Copyright 2012.

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3. Early attempts of using redox catalysis for oxygen reduction reactions in fuel cells

catalysts. During ORR, O2 interacts and receives electrons from the metal center of macrocycles, in which two intermediate states were produced: (1) metal center is reduced to lower valence state via electron gain; (2) O2 adsorbs onto the metal center and electronically interacts with the macrocyclic molecule; (3) further interactions ensure the fourelectron reduction process from O2 to H2O via the cleavage of O–O bond [64]. Typical examples of metal macrocycle-based ORR catalysts are metalloporphyrins and metallophthalocyanines, of which metallophthalocyanine and its derivatives were also known to work as CO2 reduction catalysts [65]. Other homogenous catalysts have also been applied to promote oxygen reactions. For instance, a Mn-protein complex works as a catalyst for water oxidation via a complex directed four-electron oxidation of two H2O molecules to form one O2 molecule [66]. Recently, Preger et al. have developed a modified quinone species, showing excellent stability in the strong acid environment as redox mediator to promote the ORR process in the fuel cell [67]. Since these redox catalysts have been extensively investigated in fuel cells and some of them have exhibited interesting catalytic property as compared to the precious metals, researchers working on Li-O2 batteries can make use of them and learn from past in search of potential homogenous catalysts.

The use of redox catalysis for oxygen reduction reactions can be traced back to the 1960s when porphyrin derivatives were studied as oxygen reduction centre or oxygen carrier, due to their ability to transport oxygen in mammals during aerobic respiration process [55]. Later in the 1980s, redox catalysts were employed in microbial fuel cell [56]. All these studies on redox catalysts used in fuel cells are valuable for their application in today's Li-O2 batteries. Therefore, this section will introduce the applications of redox catalysts in previous fuel cells research. 3.1. Redox mediators (RMs) in microbial fuel cell Redox mediators (RMs) were introduced in the microbial fuel cell (MFC) in the 1980s, whereby the current density and power output were enhanced dramatically [57–59]. As shown in Fig. 3, MFC converts chemical energy (stored in biodegradable substances) through microorganism biocatalysts into electricity. With the assistance of RMs, electrons can be delivered to the terminal electron acceptors (e.g. oxygen or nitrate) in the cathode. In contrast to anaerobic digestion, MFC produces electrical current and an off-gas containing mainly carbon dioxide [60]. After many years’ studies, there are still critical issues hindering the scale-up development: either the intrinsic conversion rate of MFCs needs to be increased, or the engineering design needs to be simplified so that a more cost-effective, large-scale system can be developed. In previous studies, multiple RMs had been experimented in MFCs, some of which have potential applications for use in aprotic Li-O2 batteries. For example, phenothiazine and its derivatives reported more than 30 years ago could be interesting catalysts for Li-O2 batteries [61,62]. Compared with 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) used in the Li-O2 battery [63], 10-methyl-10H-phenothiazine (MPTZ) shows lower redox potential (3.67 V) and thus may potentially minimize the charging overpotential of a Li-O2 battery [62].

4. Redox catalysis for Li-O2 batteries 4.1. Working principle of redox mediators in Li-O2 batteries Redox catalysts are developed with the aim to lower the overpotentials of ORR and OER, and consequently to improve the round-trip efficiency of aprotic Li-O2 batteries. As shown in Fig. 4a, an ORR RM is first reduced on the electrode via an electrochemical reaction, and then the reduced ORR redox mediator reduces atmospheric O2, while regenerating itself. In contrast, an OER RM first releases electrons to the electrode, and then oxidizes Li2O2 back to Liþ ions and O2, as shown in Fig. 4b. The corresponding discharge and charge processes are described in the following equations (4)–(7): During the discharge process:

3.2. Redox catalysis in fuel cells Oxygen reduction reaction (ORR) is one of the dominating reactions in fuel cells, for which macrocycles are among the most studied redox

Fig. 3. Schematic diagram showing the structure and working principle of a microbial fuel cell (MFC). Biodegradable substrates are metabolized by bacteria, which transfers electrons to the anode. Mobile redox mediators are critical on the electron-transfer processes. M: redox mediator; Red oval: terminal electron shuttle in or on the bacterium. E and EH represent the enzyme in reduced and oxidized form, respectively.

Fig. 4. The working principle of redox mediated ORR and OER reactions in aprotic Li-O2 battery. RMOR and RMOE represent ORR and OER redox mediators, respectively. 175

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 RM ox → RM red OR þ e OR ðelectrochemical reduction on the electrodeÞ

due to a lack of electric contact. Therefore, the rechargeability of these Li-O2 batteries is poor. (3) When both the OER and ORR redox mediators are employed in the Li-O2 batteries, it is anticipated that both the discharge capacity and rechargeability will be enhanced. However, this only happens when the cathode is not fully covered by Li2O2 in order to ensure an effective charge transfer of OER redox mediators with the electrode. Along the above three approaches, substantial work has been done by researchers to improve the performance of aprotic Li-O2 batteries, which will be discussed in the following sections.

(4)

ox 2Liþ þ 2RM red OR þ O2 → Li2 O2 þ 2RM OR ðchemical reaction in electrolyteÞ (5)

During the charge process: ox  RM red OE → RM OE þ e ðelectrochemical oxidation on the electrodeÞ

(6)

þ þ O2 þ RM red Li2 O2 þ RM ox OE → Li OE ðchemical reaction in electrolyteÞ (7)

4.2. ORR redox mediators

The first work on the use of redox mediators in a metal-air battery was reported in a patent in 2012, in which the inventors introduced a liquid comprising an oxygen evolving catalyst (OEC) for the cathodic reaction [68]. The OEC is soluble in the liquid phase, allowing it to partially fill the cathode pores, hence enhancing the efficiency of OER reaction by oxidizing a discharge product (metal oxide, peroxide, or superoxide) from the metal-air battery. In this work, a number of OER RMs with their respective potentials higher than the equilibrium potentials of the various air cathodes were introduced. In the presence of redox catalysts, the formation/decomposition of Li2O2 may take place on electrode or in electrolyte, thus affecting the charge-discharge performances of Li-O2 batteries. Compared with the cells without ORR RMs, the mediated batteries exhibit much enhanced capacity. For example, Gao et al. demonstrated a Li-O2 battery using 2,5di-tert-butyl-1,4-benzoquinone (DTBBQ) as the ORR catalyst, which achieved an areal capacity of 4 mAh/cm2 at current densities of >1 mA/ cm2, 80–100-fold larger than the conventional ones [69]. As illustrated in Fig. 5, there have been three different scenarios which have been explored for redox-mediated Li-O2 batteries. (1) When only OER redox mediators are applied in Li-O2 batteries, the discharge process will not be affected, and Li2O2 would deposit on the cathode through electrochemical reactions and passivate it. As a result, the discharge capacity remains limited. Thus, the electrochemical oxidation of OER redox mediators on the electrode will be hindered during charging process. (2) When only ORR redox mediators are introduced in Li-O2 batteries, the formation of Li2O2 occurs not only on the electrode surface but also within the pores of the electrode due to the direct reaction between the dissolved ORR redox mediators and oxygen. Therefore, the discharge capacity would be enhanced. Nonetheless, the improved discharging process does not benefit to the decomposition of Li2O2 during the charging process because of the insulating nature of Li2O2 which impedes efficient charge transfer to the passivated electrode. And it is plausible that the Li2O2 formed in the electrolyte may be left intact during charging

A number of ORR redox mediators have been screened in order to enhance the discharge capacity of Li-O2 batteries. Owen group firstly reported ethyl viologen (EtV) as an efficient ORR catalyst for Li-O2 batteries [70]. Later, the same group studied the stability of Li-O2 batteries using EtV [71]. With the existence of superoxide and peroxide ions formed in the ORR process, some of the electrolyte components were observed to undergo parasitic reactions. The authors observed that the lifetime of superoxide drastically decreased in the presence of EtV, which provides an intriguing strategy to improve the stability of Li-O2 batteries by eliminating the superoxide species. Apart from its application in Li-O2 batteries, Frith et al. also used EtV to facilitate the discharge process of sodium-oxygen (Na-O2) batteries [72], whereby the predominant product is sodium superoxide (NaO2) after the discharge process. Compared with Li-O2 battery, EtV seems to exert a different reaction mechanism in Na-O2 system for the reduction of O2. While the authors didn't specifically comment on the distinctions, we postulate that the relative potential difference between EtV and the ORR products may play an important role in these two battery chemistries. And if the driving force is large enough, it may further promote the formation of sodium peroxide (Na2O2), which doubles the discharge capacity and enhances the energy density. Nakanishi et al. studied several quinone derivatives as ORR RMs in LiO2 batteries [73]. They found the semiquinone species were able to catalyze the aprotic ORR, resulting in the formation of Li2O2. Among the studied RMs, benzoquinone exhibited the best catalytic performance with a low overpotential of less than 100 mV. Thereafter, Bruce group reported the use of 2,5-di-tert-butyl-1,4-benzoquinone (DTBBQ) in Li-O2 batteries and observed an enhanced discharge capacity by 80–100 times [69]. Apart from capacity enhancement, the authors speculated, based on their calculations, that LiDTBBQO2 with slightly lower energy than O 2 is formed as an intermediate during the ORR process, which could stabilize superoxide and Li-O2 batteries as a whole. Recently, we have conducted in-situ spectroelectrochemical measurements to detect the intermediate

Fig. 5. Three different scenarios of redox mediators involved in Li-O2 batteries during discharge and charge. 176

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Table 1 A summary of ORR redox mediators for Li-O2 batteries reported in literature. Redox mediator

redox potential

concentration

Li-O2 battery discharge voltage and capacity

References

Ethyl viologen

2.65 V vs. Li/Liþ

10 mM

[70,71,75]

1,4-naphthoquinone (NQ)

2.60 V vs. Li/Liþ

1 mM

Discharge voltage ~2.7 V 11 mAh/cm2 at current density of 0.125 mA/cm2 Discharge voltage ~2.7 V at current density of 1 μA/cm2

2,5-di-tert-butyl-p-benzoquinone (DTBBQ)

2.63 V vs. Li/Liþ.

10 mM

Discharge voltage ~2.7 V 1.5 mAh/cm2 at current density of 0.125 mA/cm2 90-400 mAh/m2 BET at current densities from 0.2 to 2 mA/cm2

[69,76]

Duroquinone (DQ)

2.66 V vs. Li/Liþ

200 mM

Discharge voltage ~2.5 V 17.5 mAh/cm2 at current density of 15 mA/cm2 (with combination of 200 mM EV)

[74]

AQ ~2.65 V; Methyl-NQ: ~2.70 V; Methoxy-NQ ~2.75 V; Methyl-BQ ~2.73 V; Methoxy-BQ ~2.85 V. (vs. Li/Liþ)

1 mM

Discharge voltage ~2.7 V at current density of 1 μA/cm2

[73]

Other quinone species: Anthraquinone (AQ) 2-methyl-1,4-naphthoquinone (Methyl-NQ) 2-methoxy-1,4-naphthoquinone (Methoxy-NQ) benzoquinone (BQ) ~2.86, tetramethyl-1,4benzoquinone (Methyl-BQ) methoxy-benzoquinone (MethoxyBQ)

Molecule structure

–

[73]

þ TTFþCl x on Li2O2 surface [82]. In this way, side-reactions between TTF and lithium anode can be avoided by fixing the TTF mediator on the cathode. However, due to the labile electrochemical reaction of TTF, it may not be suitable for long-time cycling in Li-O2 battery, as indicated by the obvious redox activity decay in extended CV measurements [54]. In addition, this method may not be effective when ORR redox mediators are introduced in the catholyte, as Li2O2 is not preferentially formed on the electrode surface, but largely in the electrolyte instead. Thus, Li2O2 won't be effectively decomposed by the fixed OER mediators or catalysts on the electrode, as indicated in Fig. 5. Table 2 summarizes of the reported OER redox mediators in Li-O2 batteries. Not only in Li-O2 batteries, but a variety of OER redox mediators have also been investigated in other metal-air batteries, such as sodium-air, zinc-air, magnesium-air batteries [83–86].

state of the ORR process with quinone species [74]. Based on the experimental results and calculations, Li-quinone-O-2 complex was confirmed in the ORR process, which is distinct from the direct electron-transfer process in the presence of EtV. A summary of ORR redox mediators used in Li-O2 batteries is listed in Table 1. 4.3. OER redox mediators Since the first report in 2012, Bruce group studied tetrathiafulvalene (TTF) as an effective OER redox mediator for Li-O2 batteries [25]. In their work, ferrocene (Fc) and N,N,N0 ,N0 -tetramethyl-p-phenylenediamine (TMPD) were also investigated, which were found to be unstable in the OER process. Another interesting work was reported by Hase et al., in which an oxoammonium salt was used to analyze the quantity of Li2O2 from the discharge process [77]. Although the oxoammonium salt was not intended to promote the decomposition of Li2O2 in the charging process, it is still applicable to such an application in Li-O2 battery. In 2014, Lee et al. reported lithium iodide (LiI) as an OER redox mediator, which lowers the charging overpotential by ~0.25 V [24]. In this work, triiodide (I 3 ) was considered as an OER redox mediator to oxidize the Li2O2 in Li-O2 batteries, while some issues on the experimental design are to be addressed, as will be discussed in a later section. Some other groups reported a similar halide compound LiBr to enhance the stability upon charging while with low overpotentials [78,79]. Later, Lee et al. utilized soluble ruthenium bromide as a catalyst to promote the OER process, which exhibited much improved round-trip efficiency and rate capability [80]. Some other OER soluble redox catalysts demonstrated in the Li-O2 batteries include TEMPO [63], FePc [81], etc. Although the OER redox mediators have demonstrated proven effectiveness in promoting the decomposition of Li2O2 during the charging process, they also bring about parasitic reactions on the anode surface due to the “shuttle effect”. Therefore, the ion-selective membrane is generally required to segregate the two electrode compartments. Alternatively, one group tried to coordinate TTFþ with LiCl during the charging process which forms precipitate of an organic conductor

4.4. Bi-functional redox mediators Following our discussion in section 4.1, it seems recommendable to simultaneously introduce both OER and ORR redox mediators to the LiO2 battery. Huang and co-workers reported an iron phthalocyanine (FePc) as a redox shuttle for O 2 species and electrons between the electrode and insulating Li2O2 product concurrently for both the ORR and OER reactions [81], with which Li-O2 cell showed considerably better cycling stability than the one without soluble catalyst. For such a single RM-assisted ORR and OER reactions, the RM should share identical redox potential to that of Li2O2 so that it may promote both the formation and decomposition of Li2O2 driven by the Nernstian potential differences, as demonstrated by Wang and co-workers in a different battery system for the delithiation and lithiation of LiFePO4 by a single molecule [108,109]. In addition, Yu et al. found that 2,6-di-tert-butyl-hydroxytoluene (BHT) contained a quasi-reversible redox potential of 3.0 V vs. Li/Liþ, in which the formation and decomposition of Li2O2 were facilitated by BHT in Li-O2 battery [110]. Instead of using a single redox molecule, a more feasible approach is to employ two different RMs, one for the ORR reaction and the other for OER reaction. Fig. 6 illustrates the 177

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Table 2 A summary of OER redox catalysts for Li-O2 batteries reported in literature. Redox mediator

Molecule structure

Redox potential þ

Concentration

Li-O2 battery charge voltage

Reference

0.1 M

Average charge voltage is 3.4 V at current density of 1 mA/cm2

[25,82,87–90]

Tetrathiafulvalene (TTF)

~3.5 V vs. Li/Li

N,N,N0 ,N0 -tetramethyl-pphenylenediamine (TMPD)

~3.4 V vs. Li/Liþ

NA

NA

[25]

2,2,6,6tetramethylpiperidinyloxyl (TEMPO)

3.74 V vs. Li/Liþ

10 mM

Average charge voltage is 3.7 V at current density of 0.1 mA/cm2

[63,91,92]

10-Methyl-10-phenothiazine (MPT)

3.67 V vs. Li/Liþ

0.1 M

Average charge voltage is 3.6 V at current density of 150 mA/gcarbon

[93,94]

Tris{4-[2-(2-methoxyethoxy) ethoxy]phenyl}amine (TMPPA)

3.63 V vs. Li/Liþ

20 mM

Average charge voltage 3.7 V at current of 0.125 mA/cm2.

[76]

Tris[4-(diethylamino)phenyl] amine (TDPA)

3.1 and 3.5 V vs. Li/Liþ

50 mM

Average charge voltage 3.4 V at current of 0.1 mA/cm2.

[95]

Ferrocene

~3.6 V vs. Li/Liþ

NA

NA

[25]

Cobalt Bis(terpyridine) metal complex

3.34 V vs. Li/Liþ

50 mM

Average charging voltage is 3.8 V at 200 mA/gCNT.

[96]

Heme biomolecule

3.5 V vs. Li/Liþ

2.3 mM

Stable charging voltage is 4.2 V at a current density of 100 mA/gcarbon

[97]

Charge voltages are from 3.1 to 3.8 V in different measurement conditions Average charge voltage is 3.5 V at current density of 0.032 mA/cm2 Average charge voltage is 3.5 V at current density of 500–2000 mA/g

[24,26,75,98–106]

Lithium iodide

LiI

3.1 and 3.65 V vs. Li/Liþ

10–100 mM

LiI(3-hydroxypropionitrile)2 (LiI(HPN)2) Lithium bromide

LiI(HPN)2

3.0 and 3.5 V vs Li/Liþ

100 mM

LiBr

3.48 V vs. Li/Li

þ

10 mM

[107] [78,79]

et al. also utilized a pair of RMs (TEMPO and DBBQ) to verify that the Coulombic efficiency of a Li-O2 battery was significantly influenced by the RMs added in the electrolyte [111]. These results are consistent with the proposed mechanism illustrated in Fig. 5. Thermodynamic equilibrium potential is a critical criterion for the selection of suitable RMs. Moreover, the kinetics between RMs and oxygen or Li2O2 are of equal importance. One example is the sluggish ki  netics of Li2O2 oxidation by I 3 , for which although the potential of I /I3 is thermodynamically favorable, it requires a stronger oxidant such as I2 for a good reaction rate. Several works discussed the selection rules of RMs in Li-O2 batteries. Bergner et al. highlighted the concentration

working process of a Li-O2 battery using a pair of RMs for both OER and ORR processes. Our group was the first to use a pair of redox mediators (ORR and OER) to enhance the performance of Li-O2 batteries for both charging and discharging processes [75,76]. Going beyond that, we further combined the dual redox mediators with a flow system so that a new type of redox flow battery–redox flow lithium-oxygen battery (RFLOB) was developed. The working principle of RFLOB will be introduced in detail in a separate section. Later, Bruce group demonstrated a rechargeable Li-O2 battery system using a pair of redox mediators, 2, 5-Di-tert-butyl-1,4-benzoquinone (DBBQ) for discharging and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) for charging [92]. Recently, Kwak 178

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Table 3 A summary of bi-functional redox catalysts for Li-O2 batteries reported in literature. Redox mediator

Molecule structure

Performance I

II

Concentration

Li-O2 battery discharge/charge voltage

Reference

Iron phthalocyanine (FePc)

2.86 V (Fe /Fe ) and 3.65 V (FeII/FeIII) vs. Li/Liþ

2 mM

Discharge voltage is 2.69 V and average charge voltage is 3.6 V at current densities of 0.5 and 0.3 mA/cm2, respectively.

[81]

2,6-di-tert-butyl-hydroxytoluene (BHT)

3.0 V vs. Li/Liþ

NA

Discharge voltage is 2.60 V and average charge voltage is 3.2 V at current densities of 0.1 mA/cm2.

[110]

in aprotic Li-O2 battery, which leads to the formation of LiOH. As a result, good cycling performance has been reported in humid/humidified O2 [117]. Our group carefully investigated the battery chemistry of aprotic Li-O2 battery in the presence of a relatively large amount of water in the electrolyte when LiI is used as an OER redox mediator [105]. Lithium hydroperoxide (LiOOH⋅H2O) and LiOH⋅H2O were identified as the two main products when water is involved in the discharge process. It was noted that compared with Li2O2, the oxidation of LiOOH⋅H2O by I 3 has much faster kinetics. On the other hand, LiOH can only be oxidized by I2 in terms of titration and electrochemical experiments. The working mechanism is described as follows: Discharging process (ORR reaction): 2Liþ þ O2 þ 3H2 O þ 2e → LiOOH  H2 O þ LiOH  H2 O

Fig. 6. Schematic illustration of the working process of a Li-O2 battery using both OER and ORR redox mediators (RMOE and RMOR). The charge and discharge voltages of the Li-O2 battery are determined by the respective electrochemical redox potential of RMOE and RMOR against that of Li anode. The formation and decomposition of Li2O2 occur preferentially in the electrolyte and not on the electrode.

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Charging process (OER reaction):  3I  → I  3 þ 2e

effects which could influence the redox potentials of the RMs and their mass transport [112]. For the design of RMs, Lim et al. have shown that the ionization energy can be used as a descriptor to the redox potentials of RMs [113]. Vikram et al. emphasized the importance of quantitative analyses of discharge product formation/disappearance, oxygen consumption, and oxygen evolution, etc. as relevant criteria for the selection of RMs in non-aqueous Li-O2 batteries [114]. Moreover, the chemical stability of RMs should be critically considered especially there is only a low concentration of redox molecules used, as stated in the study by Kwak et al. on a range of redox mediators (e.g. TTF, TEMPO, LiI, LiBr, etc.) [54]. It has to be noted that, although RMs could work as effective catalysts to promote the ORR and OER reactions, they are unable to concomitantly address the problems associated with surface passivation and pore clogging of the air cathode in conventional Li-O2 batteries. The battery capacity remains limited by surface passivation, and the accessible volume of cathode considering that after fully discharging a Li-O2 battery, the pores in the cathode will be blocked by the produced Li2O2 particles. As such, radical approaches should be taken to disruptively address these critical issues.

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4LiOOH  H2 O þ 2LiI3 → 6LiI þ 3O2 þ 6H2 O

(9a)

 I 3 → 3=2I2 þ e

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4LiOH  H2 O þ 6I2 → 4LiI3 þ O2 þ 6H2 O

(10a) 

Zhou group also reported the formation of OOH in Li-O2 battery in the presence of water [118]. The authors stated in their work that the electrochemical reaction in Li-O2 battery with water is a single two-electron transfer process (O2 to O2 2 ), which can inhibit superoxide-related side reactions. During the charging process, water can act as a mediator to promote the decomposition of Li2O2 by shifting the chemical equilibrium between Li2O2 and soluble peroxides. Apart from water, phenol can also function as a proton source, to promote a solution-phase reaction by forming LiOOH in the discharge process [119]. Shortly after, Wu et al. reported a urea-hydrogen peroxide as a proton source to protect lithium metal and ensure good stability [120]. Recently, Tulodziecki et al. have further studied the role of iodide in the formation of lithium hydroxide in Li-O2 batteries [104]. In their work, the low H2O:LiI ratios (<5) in DME-based electrolytes rendered LiOH as the major product from the disproportionation reaction. At higher H2O:LiI ratios, the decrease in H2O acidity promotes the formation of LiOOH⋅H2O. LiOOH⋅H2O can further disproportionate to form LiOH⋅H2O, which involves the oxidation of I to I 3 . Most recently, Liu et al. have reported that both H2O and LiI played critical roles in the formation of LiOH [106]. Although these findings are interesting for the understanding of the complex reaction mechanism, the formation of LiOH inevitably compromises the rechargeability and cyclability of Li-O2 battery. Therefore, the study of redox mediators in Li-O2 battery should be carefully planned with the content of water or other proton sources strictly controlled, so as to prevent undesired reactions. Unfortunately, there still isn't a solution to eliminate the formation of LiOH in the

4.5. Redox-mediated Li-O2 battery chemistry in the presence of protons Despite redox mediators have been extensively investigated in Li-O2 batteries, the working mechanism is yet clearly understood because of the complexity of multiple electron transfer process. This is more so in the presence of trace amount of proton source, such as water, in the aprotic electrolyte. For instance, lithium hydroxide (LiOH) was, however, identified as the main discharge product in the presence of moisture [26,99], while disputes persist on the oxidation of LiOH by I 3 during the charging process [100–102,107,115,116]. Moreover, water itself was believed to serve as a homogeneous catalyst promoting the ORR reaction 179

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Fig. 7. Schematic illustration of the configuration and working process of a redox flow lithium–oxygen battery (RFLOB). The cell stack constitutes a lithium metal anode and a carbon felt cathode (2 cm  2 cm), separated by a membrane. A gas diffusion tank (GDT) is connected to the cathodic compartment through a pump. During the discharging process, oxygen flows into the tank and is reduced to form Li2O2 while the electrolyte fluid containing redox mediators and Liþ circulates between GDT and the cell. The photo at the lower right corner shows a RFLOB single cell powering three light-emitting diodes. Reprint from Ref. 75 with permission from Royal Society of Chemistry, Copyright 2015.

Chemical reaction in the GDT

presence of water. In addition, both the redox mediators and proton sources could impair the stability of lithium anode, for which Liþ-conducting solid electrolyte or ion-exchange membranes could be used to alleviate the issue.

þ Li2 O2 þ RM ox þ O2 þ RM red 2 → Li 2

In RFLOB system, as the formation of insulating Li2O2 takes place in GDT, the electrode surface is kept clean and not passivated and clogged. This brings considerable advantages for the improvement of capacity and power capability of the system [121]. Liþ-conducting membrane plays a critical role in RFLOB system in determining the stability and thus overall performance of the battery. Currently, the most reliable and commonly used membrane is the NafionPVDF composition membrane [75,122], sulfonated poly(ether ether ketone) (SPEEK) membrane [123], apart from the ceramic membranes [112]. However, these Liþ-conducting membranes are far from being practically useful due to their high internal resistance, which gives rise to large Ohmic loss of the batteries. It is of high importance to develop superior Liþ-conducting membranes for the redox flow and other redox-mediated Li-O2 systems.

5. The design of redox flow Li-O2 battery (RFLOB) As discussed above, employing a pair of redox mediators for both the ORR and OER reactions would be an ideal way for redox-mediated rechargeable Li-O2 batteries. However, issues pertaining to surface passivation and pore clogging are not entirely resolved and the capacity remains dependent on the accessible volume of the cathode. Therefore, innovative modifications should be made to the design of Li-O2 battery in order to overcome the above technical barriers. Fig. 7 schematically shows the configuration of a redox flow Li-O2 battery (RFLOB), which is designed by combining a redox-mediated Li-O2 cell with a flow system [75]. The electrolyte fluid is circulated between the gas diffusion tank (GDT) and the cell using a peristaltic pump, in which two different redox mediators are introduced to catalyze the O2 reduction and evolution reactions during discharging and charging processes, respectively. The GDT is filled with a porous material which allows easy access of the redox fluid and O2, whereby the O2 pressure is kept constant via a gas inlet and outlet. Upon operation, electrochemical and chemical reactions occur on the cathode and in the GDT, respectively. During the discharge process, the ORR redox mediator RM1 is reduced at the cathode, and subsequently flows into the GDT tank where it is oxidized by O2 gas in the presence of Liþ ion: Electrochemical reaction on the cathode:  RM ox → RM red 1 þe 1

6. Challenges and opportunities Although the aprotic Li-O2 battery has been extensively studied since it was firstly reported 20 years ago, there are still numerous technical barriers to overcome before it becomes a credible technology for advanced energy storage. While a great deal of research is focused on the development of air electrodes in Li-O2 batteries, the issues of electrode passivation and pore clogging have to be adequately tackled to improve the round-trip energy efficiency and capacity. However, there seem to be major obstacles for cells operated with conventional design, and disruptive approaches will have to be taken to circumvent the barriers. In this regard, homogenous redox catalysis provides a promising way which has shown great effectiveness to partially resolve the above issues. Despite extensive studies have been performed in searching and optimizing RMs for promoted ORR and OER reactions, there hasn't been a viable redox system discovered so far for sustained operation of Li-O2 batteries. Here, we recommend that the following be critically considered while further work is being carried on: The development of viable RMs for Li-O2 batteries faces the dilemma between ORR/OER reaction kinetics and voltage efficiency. On the one hand, fast ORR/OER reactions require larger driving force arising from the equilibrium potential difference between RMs and that of oxygen species; on the other hand, the disparity of redox potentials of ORR and OER RMs gives rise to lower voltage efficiency. An ideal scenario is to use a single RM for both the ORR and OER reactions which meanwhile has

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Chemical reaction in the GDT þ O2 → Li2 O2 þ RM ox Liþ þ RM red 1 1

(12)

During this process, Li2O2 is formed and deposited inside the GDT. Then, the regenerated RM1 flows back to the cell for a subsequent cycle of reactions. During the charge process, another OER redox mediator RM2 is oxidized at the cathode, and subsequently flows into the GDT tank where it decomposes Li2O2 and releases O2. Electrochemical reaction on the cathode:  RM red → RM ox 2 2 þ e

(14)

(13)

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“real” catalytic effect towards a specific oxygen species instead of being simply driven by the potential difference. This not only eliminates the free energy loss, simplifies the electrolyte composition beneficial to the durability of the system, but also retains a good reaction rate, without compromising one or another. The challenges here are the searching of RMs with an identical potential to that ORR/OER reactions, and more critically those have specific interactions with the oxygen species (or intermediates) for catalyzed reactions. For these, the macrocycles for MFCs and those of quinone derivatives could be interesting. Another critical issue is the robustness of RMs upon prolonged cycling. Redoxmediated ORR and OER reactions in Li-O2 batteries requires remarkable chemical and electrochemical stability of the redox mediators, considering the harsh environment and that the RM undergoes multiple cycles of reactions for each battery charge/discharge cycle. For instance, in the case that a redox mediator has 0.1% decay per electrochemical and/or chemical cycle, then it would have lost half of its activity after 70 cycles assuming a turnover number of 10 for each battery cycle. Therefore, the durability of RMs is one of the most critical factors in Li-O2 batteries, which should be properly evaluated. Apart from the stability of RMs, that of other electrolyte components especially those involved the reactions with RMs is equally important. In addition, for a reliable evaluation of the contribution of oxygen species in the battery reaction, the capacity of RMs added in the electrolyte and those caused by the parasitic reactions should be clearly determined and excluded, as those indicated in some recent studies [78,79]. Apart from the air cathode, Li metal anode represents another critical component dictating the operation of Li-O2 batteries. Besides those caused by the air electrode, the durability of Li-O2 batteries is largely constrained by the poor cycling stability of Li metal [124,125]. As a “compromise”, most of the work reported the cycling stability by limiting the cell capacity to rather shallow lithium utilizations. Zhou and co-workers rectified this issue and employed lithiated Si as anode material in Li-O2 batteries, and exhibited good cycling stability [125]. The solid electrolyte interface (SEI) film grown in glyme-based electrolyte displayed strong resistivity towards the attack of oxygen, endowing these batteries with stable cyclability. In the context of redox-mediated devices, besides the crossover of oxygen to Li anode, a notorious process is the “shuttling” of mobile RMs between the two electrodes, causing an adverse effect on the operation of Li-O2 battery, especially for the charging process. Therefore, Liþ-conducting membranes should be constantly employed to eliminate the parasitic reactions on Li anode [75, 76,112], which were however ignored in many of the early studies for redox-mediated Li-O2 batteries. One issue for these membranes is that they usually have rather low Liþ conductivity, which impacts on the power output of Li-O2 batteries. Some researchers attempted to mitigate the parasitic reactions between RMs and Li anode by other means. For instance, electrolyte additives‒vinylene carbonate (VC) [75], fluoroethylene carbonate (FEC) [126] and LiNO3 [127] have been used to promote the formation of robust SEI film on the Li anode. Al2O3/PVdF-HFP composite was designed as a composite protection layer for Li anode, and to prevent self-discharge of Li-O2 battery using TEMPO as the RM [128]. These artificially made Liþ-conducting films can effectively prevent RMs from attacking the Li anode. Apart from direct modifications with additives, some other researchers tried to use multi-functional RMs to engineer the anode surface. For example, Zhang et al. utilized InI3 to prevent the corrosion of Li anode by I 3 during the charging process [129], whereby In3þ was first reduced on the Li surface forming a layer of indium, which could hinder both the parasitic reaction of I 3 and growth of Li dendrite. In another study, by adding LiCl to the electrolyte and using TTF as an OER RM, TTFþ can be anchored onto the surface of cathode by the precipitation of TTFCl which is electrically conducting [82]. In fact, this work is similar to their previous work, in which a solid organic electrocatalyst‒dilithium quinone-1, 4-dicarboxylate (Li2C8O6), was employed [93]. Although multiple strategies have been employed by researchers to prevent the parasitic reactions with commendable results, to fulfil the demand for a practically

rechargeable energy storage system, a holistic consideration is highly desired for to achieve high-energy, high-efficiency and durable operation. As discussed above, the deployment of rechargeable Li-O2 batteries is critically hindered by the low round-trip efficiency, poor power density and instability. To address these issues, RFLOB provides an intriguing way to shift the reactions of oxygen species from the electrode to a separate GDT tank. This elegantly obviates the hurdles related to surface passivation and pore clogging of the cathode, particularly suitable for large-scale energy storage. However, the issue remains for searching viable redox mediators with minimal overpotential and good robustness, which is especially challenging for the OER process at relatively high potentials. In contrast, primary Li-O2 batteries appeared to be much easier to realize [23]. Recently, we have demonstrated that, based on a dual ORR redox molecule EtV and duroquinone, both the power density and lithium utilization of the RFLOB could be unprecedently enhanced [74]. With the help of an electrolyte spray system, the areal power of the cell can go up to 60 mW/cm2 when fed with oxygen, and 34 mW/cm2 when fed with dry air. Intriguingly, the reaction yield of the lithium anode is nearly unity even in the absence of Liþ-conducting membrane, thanking to the fast reaction kinetics of the two ORR molecules on the cathode. The cell could operate continuously with multiple Li metal feeding after it is consumed. The above features make the system interesting for various applications should a feasible refilling method of Li is developed. When the GDT is filled fully by the discharge product (Li2O2), it can be simply replaced by draining the tank. The discharge product can be easily collected by battery companies, which can be recycled and used as a Li source for other applications. This primary setup is not just a good compromise before a rechargeable system is in place, it also possesses high potential in the application of electric vehicles (EVs) as backup energy storage system due to its high energy density. Acknowledgements This research was supported by the National Research Foundation, Prime Minister's Office, Singapore, under its Competitive Research Program (CRP Awards No. NRF-CRP10-2012-06). References [1] D. Aurbach, B.D. McCloskey, L.F. Nazar, P.G. Bruce, Nature Energy 1 (2016) 16128. [2] L. Grande, E. Paillard, J. Hassoun, J.B. Park, Y.J. Lee, Y.K. Sun, S. Passerini, B. Scrosati, Adv. Mater. 27 (2015) 784–800. [3] K.M. Abraham, J. Phys. Chem. Lett. 6 (2015) 830–844. [4] R. Black, B. Adams, L.F. Nazar, Advanced Energy Materials 2 (2012) 801–815. [5] N.B. Aetukuri, B.D. McCloskey, J.M. García, L.E. Krupp, V. Viswanathan, A.C. Luntz, Nat. Chem. 7 (2015) 50–56. [6] B.M. Gallant, D.G. Kwabi, R.R. Mitchell, J. Zhou, C.V. Thompson, Y. Shao-Horn, Energy Environ. Sci. 6 (2013) 2518–2528. [7] M.D. Radin, C.W. Monroe, D.J. Siegel, J. Phys. Chem. Lett. 6 (2015) 3017–3022. [8] J. Højberg, B.D. McCloskey, J. Hjelm, T. Vegge, K. Johansen, P. Norby, A.C. Luntz, ACS Appl. Mater. Interfaces 7 (2015) 4039–4047. [9] B.D. McCloskey, R. Scheffler, A. Speidel, D.S. Bethune, R.M. Shelby, A.C. Luntz, J. Am. Chem. Soc. 133 (2011) 18038–18041. [10] Y. Li, X. Wang, S. Dong, X. Chen, G. Cui, Advanced Energy Materials 6 (2016) 1600751, n/a. [11] L. Johnson, C. Li, Z. Liu, Y. Chen, S.A. Freunberger, P.C. Ashok, B.B. Praveen, K. Dholakia, J.-M. Tarascon, P.G. Bruce, Nat. Chem. 6 (2014) 1091–1099. [12] D. Sharon, M. Afri, M. Noked, A. Garsuch, A.A. Frimer, D. Aurbach, J. Phys. Chem. Lett. 4 (2013) 3115–3119. [13] D.G. Kwabi, T.P. Batcho, C.V. Amanchukwu, N. Ortiz-Vitoriano, P. Hammond, C.V. Thompson, Y. Shao-Horn, J. Phys. Chem. Lett. 5 (2014) 2850–2856. [14] Z. Peng, S.A. Freunberger, Y. Chen, P.G. Bruce, Science 337 (2012) 563–566. [15] B. McCloskey, D. Bethune, R. Shelby, G. Girishkumar, A. Luntz, J. Phys. Chem. Lett. 2 (2011) 1161–1166. [16] S.A. Freunberger, Y. Chen, N.E. Drewett, L.J. Hardwick, F. Bard
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