Perspectives and challenges of rechargeable lithium–air batteries

Materials Today Advances 4 (2019) 100031

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Perspectives and challenges of rechargeable lithiumeair batteries N. Imanishi*, O. Yamamoto** Graduate School of Engineering, Mie University, Tsu, Mie, 514-8507, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 August 2019 Received in revised form 25 October 2019 Accepted 25 October 2019 Available online xxx

Rechargeable lithiumeair batteries have a far higher theoretical energy density than lithium-ion batteries, and are, thus, expected to become a possible power source for electric vehicles (EVs). Three types of rechargeable lithiumeair batteries have been developed: non-aqueous, aqueous, and solid. The majority of research efforts have been devoted to the non-aqueous battery in the past two decades. However, non-aqueous lithiumeair batteries still have critical issues to be addressed to realize the practical use for EVs, such as a low practical areal capacity, low round-trip energy efficiency, and air purification. The aqueous and solid lithiumeair systems do not have the critical issues observed in the non-aqueous system; however, they have not shown capacity for high power density and extended deep cycling. In this short review, our emphasis is on the progress made with respect to cell performance, such as capacity at high current density and cycle life, and we identify lithiumeair battery prospects for EVs and key technologies. © 2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Rechargeable battery Lithium air battery High energy density Electric vehicle

1. Introduction Electric vehicles (EVs) are becoming a realistic prospect to reduce the amount of greenhouse gases released and petroleumderived fuels used by the transportation sector. However, even the most promising lithium-ion technologies fall well short of delivering the energy density required to give EVs ranges that are comparable to those of internal combustion engine-powered cars [1]. Therefore, there is a real incentive to develop advanced battery types that exceed the energy storage performance of present lithium-ion batteries. Three main types of lithiumeair (or oxygen) batteries, that is, non-aqueous, aqueous, and solid, have been developed. The non-aqueous rechargeable lithiumeair battery proposed by Abraham and Jiang in 1996 [2] consists of a lithium metal anode, a porous cathode, and a non-aqueous electrolyte and has theoretically high specific energy density of 3,505 Wh/kg (3,436 Wh/L) [3] based on the following cell reaction: 2 Li þ O2 ¼ Li2O2.

(1)

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (N. Imanishi), Yamamoto@chem. mie-u.ac.jp (O. Yamamoto).

The mass and volume-specific energy densities based on lithium and oxygen are, respectively, around 10 times and three times higher than those of the conventional lithium-ion battery. The calculated specific energy density of gasoline automotive applications is approximately 1,700 Wh/kg using the energy density of gasoline of 13,000 Wh/kg and the tank-to-wheel efficiency of the U.S. fleet of 12.6% [4]. The mass theoretical energy density of nonaqueous lithiumeair batteries is also higher than that of the conventional gasoline engine. Non-aqueous lithiumeair batteries are now attractive growing attention and R&D efforts as possible power sources for EVs. Many researchers have studied this system extensively over the last two decades [5e7]. However, no technology basis exists to support the highly optimistic energy densities that have been claimed as possible for non-aqueous lithiumeair batteries at high power density and high specific areal capacity. Many challengeable research items have remained, including the stability of the electrolyte with the reaction products of superoxides, a high overpotential during the charge and discharge processes, poor cycle performance, and the elimination of water and CO2 in the air [8e12]. An alternative rechargeable aqueous lithiumeair battery was proposed by Visco et al. in 2004 [13], which consisted of a lithium metal anode, a porous cathode, and an aqueous electrolyte separated from the lithium anode by a water-stable lithium-ion-conducting solid electrolyte. The theoretical energy density of the aqueous lithiumeair battery based on the reaction:

https://doi.org/10.1016/j.mtadv.2019.100031 2590-0498/© 2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

2

4 Li þ O2 þ 6H2O ¼ 4(LiOHH2O),

N. Imanishi, O. Yamamoto / Materials Today Advances 4 (2019) 100031

(2)

is 1,910 Wh/kg (2,004 Wh/L) [14]. The energy density is lower than that of the non-aqueous lithiumeair system, but higher than that of the internal combustion engine. Severe problems in non-aqueous systems, such as decomposition of the electrolyte, clogging of the porous air cathode by the insoluble discharge product of Li2O2, and contamination by water from the air, are not appreciable in the aqueous lithiumeair system. The reaction product of LiOH is soluble in the catholyte, which improves the high overpotentials observed for the reduction of the reaction product in the nonaqueous lithiumeair system. The separator between the lithium anode and the aqueous catholyte should be a water-stable and water-impermeable solid electrolyte with high lithium-ion conductivity and excellent mechanical properties. The NASICON-type solid electrolytes of Li1þxAlxTi2-x(PO4)3 (LATP) [15] have been mainly used for the separator, the electrical conductivity of which is in a range of 1e7  104 S/cm at room temperature [16,17]. However, the NASICON-type solid electrolyte is unstable in contact with lithium metal [18], and a lithium-stable lithium conducting electrolyte should be used as a buffer layer between a lithium anode and the solid electrolyte. The disadvantage of the aqueous system with a low specific area capacity is that the weight and volume of the solid electrolyte separator can reduce the specific energy density of cell significantly. The third type of lithiumeair battery with a solid electrolyte was proposed by Kumar et al. in 2010 [19], which consisted of a lithium anode, a lithium ion conducting solid electrolyte, and carbon air electrode. This type of cell was expected to mitigate fire and explosion problems due to non-aqueous electrolyte. The operation temperature is generally higher than room temperature to pass a high current density because the electrical conductivity of the solid electrolyte is lower than that of a liquid electrolyte [20]. Many excellent reviews and a book have been published in this decade [5,21e26], in which the development of the lithiumeair batteries is reported to still be in the initial stage with many challenges remaining; however, significant progress has also been achieved. In this short review, we focus on the direction of the lithiumeair battery research for practical application in EVs. 2. Challenge for lithium-air battery

However, Luntz et al. considered that, at present, there was no electrolyte that was sufficiently stable to provide acceptable cyclability. Bruce and co-workers demonstrated that Li2O2 could only be obtained during the first discharge, and that little or no Li2O2 remained after the fifth discharge process when tetraethylene glycol dimethyl ether (TEGDME) was used [9]. Bruce and coworkers also concluded that ethers, amides, ionic liquids, and dimethyl sulfoxide (DMSO) are more stable than the organic carbonates, but not all are equally stable. The instability of the nonaqueous electrolytes is due to nucleophilic attack by reduced oxygen species (LiO2) produced in the discharge process at the cathode [22,33]: Liþ þ e þ O2 ¼ LiO2

(3)

Liþ þ e þ LiO2 ¼ Li2O2

(4)

2LiO2 ¼ Li2O2 þ O2

(5)

In 2012, Chase et al. [34] proposed a new concept of a highly soluble redox shuttle that can be reduced at the electrode, which is now known as a redox mediator (RM). The oxygen reduction mediator (orRMn) accepts an electron at the electrode surface and chemically reacts with O2: RMn þ e ¼

or

or

RMn1

2orRMn1 þ O2 þ 2Liþ ¼ 2orRMn þ Li2O2

(6) (7)

The oxygen evolution mediator (oeRMn) is electro-oxidized at the electrode surface and chemically oxidizes the Li2O2 deposited on the air electrode: oe

RMn ¼

oe

RMnþ1 þ e

2oeRXnþ1 þ Li2O2 ¼ 2oeRMn þ O2 þ 2Liþ

(8) (9)

These reaction mechanisms produce no superoxide; therefore, we could expect stable charge and discharge performance of the lithium oxygen cells. Fig. 1 shows typical charge and discharge performance for a lithiumeoxygen cell with N-methylphenothiazine (MPT) as the RM with 1 M LiCF3SO3 in TEGDME

2.1. Non-aqueous lithiumeair battery In the 10 years following the first report of a lithiumeair battery by Abraham and Jiang [2] in 1996, research activity into lithiumeair batteries was limited. Since Bruce and co-workers revisited lithiumeair batteries in 2006 [27], research activity on this type battery has grown significantly. Over that period, the demand for high energy density rechargeable batteries for EV applications has been accelerated. The lithiumeair batteries are one candidate of energy source beyond conventional lithium-ion batteries for EV applications. The cell reported by Bruce and co-workers with an electrolyte of LiPF6 in propylene carbonate (PC) and a cathode of carbon with electrolytic MnO2 showed good cyclic performance compared with the previous report by Read [28]. However, in 2010, Mizuno et al. [29] reported similar cyclic behavior of the cell, but concluded that the PC electrolyte was decomposed. Bruce and co-workers [30] also confirmed the decomposition of an alkali carbonate electrolyte by the discharge process. Subsequently, there were many studies that explored electrolyte stability with various aprotic electrolytes. Luntz and co-workers [10,31] and Bruce and co-workers [32] explored various solvent and salt systems using in situ differential electrochemical mass spectrometry. Luntz et al. claimed the best solvents were ethers, especially dimethoxyethane (DME) [31].

Fig. 1. Initial charge and discharge profiles of Li/1.0 M LiCF3SO3 in TEGDME with and without 0.1 M MPT at 150 mA/g (0.022 mA cm2). Adapted from Feng et al. 2017 [35].

N. Imanishi, O. Yamamoto / Materials Today Advances 4 (2019) 100031

reported by Feng et al. [35]. The charging potential was considerably decreased to around 3.5 V versus Liþ/Li at 150 mA/g by addition of the RM. The round-trip energy efficiency of the cell without the RM of around 50% was improved to 76% with the RM. High energy efficiency is one of the important requirements for application as an energy source in EVs. The low charge potential is also acceptable for the carbon electrode because carbon is not stable during the charge process at a high charging voltage. Bruce and coworkers reported carbon may be relatively stable at or below 3.5 V versus Liþ/Li [36]. Several authors have reported outstanding cycle performance in lithiumeoxygen batteries assisted by RMs. However, most of them limit their galvanic cycles to a fixed small specific areal capacity and a low current density, as shown in Table 1. Bruce and co-workers [40] reported excellent cycle performance at 1.0 mA cm2 for 2 h, as shown in Fig. 2, where dual mediators on discharge 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ) and charge 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) were used. The lithium electrode and catholyte were separated by a lithium-ion conducting solid electrolyte of LATP. The round-trip energy efficiency was around 70% (excluding the ohmic loss by LATP). However, such performance is still far from that required for EV batteries. Review articles on RMs in lithiumeoxygen batteries have recently been published [42,43]. In 2005, Kokubo et al. [44] investigated the use of a hydrophobic ionic liquid of 1-alkyl-3-methyl imidazolium cation and perfluoroalkylsufonyl imide anion in a primary lithiumeair battery. The cell worked for 56 days in air, and the carbon cathode materials showed high capacity of 5,360 mAh/g. Mizuno et al. observed the stability of an ionic liquid of N-methyl-N-propylpiperidinum bis(trifluoromethansulfonyl)amide (PP13TFSI) with oxygen radicals [45]. Many researchers have since reported on rechargeable lithiumeair batteries with ionic liquids. Elia et al. [46] and Zhang and Zhou [47] demonstrated stable cyclic performance in lithiumeoxygen cells with ionic liquid electrolytes. However, the energy efficiencies at high current density were not so high, as shown in Table 1. More recently, Zhang et al. [48] reported excellent cycle performance for a cell with 1,2-dimethyl-3-(4-(2,2,6,6tetramethyl-1-oxyl-4-piperidoxyl)-pentyl)imidazolium bis(trifluoromethane)sulfonamide (IL-TEMPO), which served multiple functions as an RM, an oxygen shuttle, lithium anode protector, and as an electrolyte solvent. IL-TEMPO showed a highly reversible redox reaction at 3.0 and 3.75 V at 0.1 mA/cm, as shown in Fig. 3. The energy efficiency of 70% for the first cycle at 0.1 mA cm2 and 0.28 mAh cm2 was decreased to 60% at 50 cycles. This cell showed excellent rate performance; the energy efficiency at the first cycle at 0.5 mA cm2 was recorded to be 60%. The other challenging issue for non-aqueous lithiumeair batteries is lithium dendrite formation and growth during the charging process. Lithium metal is the best anode for a high energy density battery because it has a high theoretical specific capacity of 3,861 mAh/g and a low negative potential of 3.04 V

3

Fig. 2. Cyclic performance of a Li/0.3 M LiClO4 in TEGDME/LATP/25 mM DBBQ-25 mM TEMPO-0.3 M LiTFSI in DME/CNT, O2 at 1.0 mA cm2. Adapted from Gao et al. 2017 [40].

Fig. 3. Cyclic performance of a Li/IL-TEMPO in DEGDME/CNT, O2 cell at 0.1 mA cm2 and room temperature. Adapted from Zhang et al. 2019 [48].

versus normal hydrogen electrode . In the last four decades, lithium anodes for rechargeable batteries have been studied extensively [49,50]. However, the rechargeable batteries with lithium anodes have not yet been commercialized, because lithium deposition on a lithium electrode from conventional nonaqueous electrolytes results in lithium dendrite formation for a short period of polarization at a small current density. For example, lithium deposition on a lithium electrode from a conventional non-aqueous electrolyte such as 1 M LiPF6 in ethylene

Table 1 Performance of non-aqueous lithiumeair batteries. Electrolyte

Air electrode

Current density (mA cm2)

Area capacity (mAh cm2)

Energy efficiency (%)

Ref.

LiCF3SO3-TEGDME (1:4 mol) 10 mM TEMPO- 0.1 M LiTFSI in DEGDME 0.05 M LiI-0.25 M LiTFSI in DME 25 mM DBBQ-25 mM TEMPO-0.3 M LiClO4 in DME 0.5 M LiTFSI-30 mM TTF -30 mM LiCl in DEGDME PYR14TFSI-LiTFSI C2min LiTFSI IL-TEMPO in DEGDME

Super P KB GO CNT GO Porous carbon SWNT CNT

0.5 0.1 0.1 1.0 0.2 0.04 0.12 0.1

5 0.25 0.75 2.0 1.0 0.4 0.6 0.25

43 73 86 77 80 78 60 70

[37] [38] [39] [40] [41] [46] [47] [48]

(1st)-40 (30th) (1st)-47 (50th) (1st)-78 (300th) (1st)-77 (50th) (1st)-80 (60th) (1st)-067 (30th) (1st)-47 (10th) (1st)-60 (50th)

CNT: carbon nanotube; TTF: tetrathiafulvalene; GO: graphene oxide; PYR: N-butyl-N-methylpyrrolidinium; C2min: 1-ethyl-3-methylimidazolium; SWNT: single-walled carbon nanotube; DEGDME: diethylene glycol dimethyl ether.

4

N. Imanishi, O. Yamamoto / Materials Today Advances 4 (2019) 100031

Table 2 Lithium dendrite formation in various electrolytes. Electrolyte

Li-ion transpor numbert

1.85 M LiCF3SO3 in DME/DOL (1:1) with Kimwipe paper 4 M LiFSI in DME 1 M LiFSI in DX/DME 1 M LiPF6 in EC/DME with mesoporous epoxy

0.82

1 M LiPF6 in FEC/DMSO with AlPO4 1 M LiTFSI in EC/DME with BN

0.79 0.62

Current density (mA cm2)

Area capacity (mAh cm2)

Short circuit (h)

Reference

10 5 10 5 2 4 2 4 1

30 15 1.0 7.5 4.0 12 6 2 3

>400 >1,000 >600 >100 >300 >600 >1,000 >200 >1,940

[52] [53] [54] [55] [56] [57]

DX: 1,4 dioxane; FEC: fluoroethylene carbonate; LiFSI: Li(FSO2)2N.

carbonate (EC)-dimethyl carbonate (DMC)-ethyl methyl carbonate (EMC) with a Teflon separator was short-circuited after 650 s polarization at 1.0 mA cm2 [51]. The formation of lithium dendrites can cause internal short-circuiting in the cell, which represents a serious safety issue. Many researchers have recently attempted to suppress lithium dendrite formation for high energy density batteries such as lithiumeair and lithiumesulfur batteries. Table 2 summarizes the short-circuit period for Li/electrolyte/Li symmetrical cells at a high current density of more than 1 mA cm2. Some electrolyte systems were reported to have no dendrite short-circuit at a high current density as high as 10 mAh cm2. Fig.

4 shows the galvanostatic cycle performance of a Li/Kimwipe (KW) paper/1.85 M LiCF3SO3 in DME-1.3 dioxolane (DOL) (1:1 v/v) with a Celgard® separator/KW paper/Li cell at room temperature. No short-circuit was observed at 10 mA cm2 for 3 h polarization and more than 60 cycles [52]. These authors claimed that the celluloseefiber framework of the KW paper with abundant polar function groups, including eOH and CeOeC groups, serves as a lithium-ion redistributing layer to inhibit the inhomogeneous accretion of Li ions. Qian et al. [53] and Miao et al. [54] also reported a dendrite-free electrolyte at a high current density. However, these electrolytes are volatile and have a problem for

Fig. 4. (a) Configuration of symmetric cells for the control cell and the Kimwipe-protected (KW-protected) cell. Voltage versus time profiles for (b) the control cell at 2 mA cm2, and the KW- protected cell at (c) 2, (d) 5 and (e) 10 mA cm2. The inset in (c) shows the initial 400 h operation. Adapted from Chang et al. 2017 [52].

N. Imanishi, O. Yamamoto / Materials Today Advances 4 (2019) 100031

5

Fig. 5. Discharge and charge curves of the Li/1 M LiTFSI in DMSO/CNT/PTFE, air cells (a) with and (b) without PTFE protection at a current density of 0.05 mA cm2. (c) Comparison of the cyclic performance of the cell with and without PTFE protection. Adapted from Xie et al. 2019 [64].

the lithiumeair battery, which have open channels to the atmosphere. The effect of the KW paper should be examined for nonvolatile electrolytes. Wu et al. [58] and Takeda et al. [59] have presented reviews for the lithium metal anode. In batteries for EVs, operation should be under ambient air conditions to obtain a high specific energy density. Air contains carbon dioxide and water vapor. Water will penetrate into the battery from the air electrode to the lithium electrode, which leads to serious oxidation of the Li anode, and CO2 will gradually react with Li2O2 to form Li2CO3, which can only be decomposed at a high potential during the charge process. These side reactions considerably reduce the columbic efficiency and decrease the cycle life. Zhang et al. [12,60,61] proposed for the first time that an O2-selective membrane with high O2 permeability could be obtained by loading silicone oil into porous supports such as porous metal sheets and polytetrafluoroethylene (PTFE) films. Lithiumeair cells with membranes of the silicone oil in the porous PTFE film-enabled operation in ambient air (at 20% RH) for 16.3 days at a current density as low as 0.05 mA cm2 with a specific capacity of 789 mAh/ gcarbon. Cao et al. [62] proposed a new oxygen-selective membrane, which consisted of a metal organic framework of [Al4(OH)2(OCH3)4(H2Nbdc)3]xH2O and polymethylmethacrylate. The oxygen-selective membrane was effective to enhance the cyclic performance for operation in the ambient atmosphere; however, the cycle life was only slightly improved from 21 cycles to more than 61 cycles. Wang et al. [63] recently reported that low-density polyethylene film integrated with a gel electrolyte was effective to protect the cell from moisture. Xie et al. [64] more recently reported the cell performance with and without an oxygen-permeating waterproof membrane based on a PTFE-wetted Celgard® membrane. The cell performance was significantly improved under ambient air by the PTFE protection layer. Fig. 5 compares the cyclic performance for the Li/1 M Li(CF3SO2)2N (LiTFSI) in DMSO/PTFE/

carbon nanotube film/air and Li/1 M LiTFSI in DMSO/carbon nanotube film/air cells. The cell could operate for 58 days under ambient air. However, the charge and discharge current densities and the specific areal capacity were as low as 0.05 mA cm2 and 0.25 mAh cm2, respectively. Oxygen-selective membranes improve the cycle life of lithiumeair batteries; however, highly selective membranes should be developed to realize high power density batteries. Asadi et al. [65] recently proposed another idea to operate under ambient air. They protected the lithium electrode with Li2CO3/C, which was synthesized on a lithium anode in a custom-made

Fig. 6. Discharge and charge voltage profiles for the Li/LiCO3eC/0.1 M LiTFSI in (EMIMBF4)- DMSO (25:75 v/v)/MoS2, air cell at 0.05 mA cm2. Adapted from Asadi et al. 2018 [65].

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N. Imanishi, O. Yamamoto / Materials Today Advances 4 (2019) 100031

Fig. 7. Structure of the aqueous lithiumeair battery.

electrochemical lithiumecarbon dioxide cell filled with pure CO2. Fig. 6 shows the long-term charge and discharge performance at 0.05 mA cm2 and capacity of 500 mAh/g (0.05 mAh cm2) for the Li/Li2CO3eC/0.1 M LiTFSI in 1-ethyl-3-methylimidzolium tetrafluoroborate (EMIM-BF4) and DMSO/MoS2, air-like atmosphere. No failure of the battery during testing up to 500 cycles was observed. The potential gap of the initial charge and discharge potentials of 0.88 V was increased to 1.62 V after 550 cycles. This architecture is interesting; however, to realize this system, the cell performance should be further studied at a higher current density and specific areal capacity. 2.2. Aqueous lithiumeair battery The theoretical energy density of aqueous lithiumeair batteries is lower than that of non-aqueous lithiumeair batteries; however, the aqueous system does not have some of the severe problems that the non-aqueous system has, and that still need to be addressed, such as lithium corrosion with water vapor from the air and decomposition of the electrolyte by reaction products. Furthermore, as the reaction product of LiOH is soluble in the catholyte, a low overpotential for the oxygen evolution reaction and a high specific areal capacity (>100 mAh cm2) could be expected [24]. Fig. 7 shows the aqueous lithiumeair cell configuration. The lithium anode and the aqueous electrolyte should be separated by a water-stable lithium-conducting solid electrolyte because lithium metal reacts violently with water. The NASICON-type LATP solid

electrolytes, which have been extensively used for aqueous lithiumeair batteries, was reported to have a high lithium-ion conductivity of 7  104 at 25  C [15] and to be stable in a saturated LiCl aqueous solution [16], but unstable in contact with lithium metal [18]. Therefore, a buffer layer should be used between the lithium anode and LATP. As buffer layers, Li3N [13], lithium phosphorous oxynitride glass film (LiPON) [66], a polyethylene oxide-based (PEO-based) polymer electrolyte [67] and a conventional nonaqueous liquid electrolyte for lithium-ion batteries [68] have been used. The formation of Li3N and LiPON on a lithium anode is somewhat complicated, and these interlayers are not acceptable for large size cells. The polymer electrolyte should be operated at a higher temperature because of its low conductivity at room temperature. Lithium dendrite-free non-aqueous liquid electrolytes have been proposed recently, as shown in Table 2; therefore, a liquid electrolyte could be acceptable for the interlayer electrolyte. The garnet-type solid lithium-ion conductors, Li7La3Zr2O12 (LLZ), with high room temperature conductivity and a wide electrochemical window suffer from proton exchange in water, which makes them unstable in an aqueous catholyte solution [69]. Two types of aqueous lithiumeair batteries have been proposed, that is, alkaline catholyte and acid catholyte types. The electrode reactions of these cells are as follows:

anode:Li%Liþ þe E ¼  3:04V vs: NHE

(10)

cathode:ðalkalineÞ O2 þ 2H2 O þ 4e %4OH E ¼ 0:401V vs: NHE

(11)

ðacidÞ O2 þ 4Hþ þ 4e % 2H2 O E ¼ 1:229V vs: NHE

(12)

Acid catholytes provide higher voltage than alkaline catholytes. Furthermore, the contamination of CO2 from air is not so serious with acid catholytes. However, the water-stable lithium-ion-conducting LATP solid electrolyte is unstable in a low pH aqueous solution. For the alkaline or neutral catholytes, the reaction product is LiOH, and the pH value increases with reaction depth. LATP is also unstable in high pH aqueous solution. An additional supporting salt, such as LiCl, is required to provide lower and high pH catholytes [16]. The oxygen reduction reaction (ORR) has been extensively studied for fuel cells, where the cells could operate at more than several tens of milliamp hours per square centimeter [70]. The oxygen evolution reaction (OER) was also studied for rechargeable zinceair batteries, where a current density of 1.42 mA cm2 at an overpotential of 100 mV was observed in a concentrated alkaline solution [71]. Table 3 summarizes the performance of various aqueous lithiumeair batteries. Fig. 8 shows the electrode potential (porous carbon Ketjenblack electrode) versus current density

Table 3 Performance of aqueous lithiumeair batteries. Solid electrolyte Anolyte

Catholyte

Air electrode Current density (mA cm2) Area capacity (mAh cm2) Energy efficiency (%) Ref.

LATP LATP LATP LATP LANTP LAGTP LLTO LATP LATP LATP

sat. LiOH, sat. LiOH 10 M LiCl 1 M LiNO3- 0.5 M LiOH sat. LiOH- 10 M LiCl 10 M LiCl-1.5 M LiOH 10 M LiCl 1.5 M LiOH 1 M LiNO3 0.1 M H3PO4 1 M LiH2PO3 0.1 M H3PO4 -1 M LiH2PO4 (HAc(90%) BaTiO3eH2O)-LiAc

KB/RuO2 GNS KB/RuO2 MnO2 MnO2 MnO2 C/Pt Pt/CeIrO2, KB

LiPON, 1 M LiClO4 in EC/DEC 1 M LiClO4 in EC/DEC LiFSI -2G4 4 M LiFSI in DME 4.5 M LiFSI LiPON 0.1 M LiPF6 in EC/DEC 1 M LiPF6 in EC/DEC PEO18LiTFSI 10 wt%

KB: Ketjenblack, GNS: graphene nano sheet.

0.1 0.64 0.5 0.64 0.25 1.0 0.5 0.5 1.0 0.5

0.2 5.8 1.0 12 3.2 1.0 0.5 0.24 0.5 0.25

90 (1st)-90 (100th) 60 (1st)-1.53 (8th) 70 (1st)-60 (50th) 70 (1st)-50 (12th) 63 (1st)-51 (30th) 85 (1st)-85 (10th) 79 (1st)-54 (40th) 72 (1st)-67 (20th) 82 (1st) 0 (1st)-70 (15th)

[72] [73] [74] [75] [17] [76] [77] [78] [79] [80]

N. Imanishi, O. Yamamoto / Materials Today Advances 4 (2019) 100031

Fig. 8. Current density versus oxygen evolution and reduction polarization curves for KB electrode in saturated LiOH with 10 M LiCl aqueous solution at room temperature. Adapted from Sunahiro et al. 2014 [73].

curves for the ORR and OER in a 10 M LiCl aqueous solution at 25  C reported by Sunahiro et al. [73]. The overpotentials for the ORR and OER at 7.8 mA cm2 were 560 and 295 mV, respectively. The overpotentials for the ORR and OER in aqueous lithiumeair batteries are considerably lower than those in the non-aqueous lithiumeair batteries. Li and Manthiram [79] reported that a Pt/C and IrO2 composite air electrode reduced the overpotential for the OER in an acid catholyte. The maximum power density and energy loss by round-trip overpotential at 2 mA cm2 and 40  C was recorded to be 40 mW cm2 and 20%, respectively. The power density is comparable with that of a polymer electrolyte membrane (PEM) fuel cell at an energy loss of 20% [81]. Of course, the PEM fuel cell can operate at more higher current densities, such as 1 A cm2; however, the energy loss is very high at high current density. The low overpotentials are an important advantage of the aqueous lithiumeair system compared to the non-aqueous system. More recently, Bai et al. [76] reported a water-stable and high lithium-ion conductivity solid electrolyte thin film of Li1.4Al0.4Ge0.2Ti1.4(PO4)3-10 wt% TiO2 (LAGTP) (90 mm thick and lithiumion conductivity of 4.45  104 S/cm at 25  C) with excellent mechanical properties (bending strength of 200 N mm2). They examined the performance of an aqueous lithiumeair cell using this thin film at room temperature and found a low energy loss of around 15% at 1.0 mA cm2 for a round-trip charge and discharge process at room temperature, as shown in Fig. 9. However, to maintain the low energy loss for long cycling, the lithium electrode performance should be improved. Park et al. [82] compared the specific energy density of graphite/LiCoO2 lithium-ion, lithiumesulfur, and non-aqueous lithiumeair batteries based on the weights of the cell components without packaging as a function of the specific areal capacity, where the lithiumesulfur and lithiumeair batteries had a 30 mm thick lithium-ion-conducting LATP solid electrolyte and three times excess lithium anode to the cathode capacity. Fig. 10 shows the dependence of the specific energy densities of these battery

7

Fig. 9. Cyclic performance of the Li/4.5 M LiFSI in DME/LAGTP/1.5 LiOH with 10 M LiCl aqueous solution/MnO2, air cell at 1.0 mA cm2 and room temperature. Adapted from Bai et al. 2019 [76].

Fig. 10. Dependence of the energy density and estimated driving distance of Li-ion, LieS and lithiumeair batteries as a function of the specific areal capacity. Adapted from Park et al. 2014 [82].

systems and the estimated driving distance for an EV. A lithiumeair battery with a specific energy density of higher than 500 Wh/kg should be developed in a system with a specific areal capacity of higher than 10 mAh cm2. At present, the reported highest specific capacity is less than 12 mAh cm2 for the aqueous system and 5.0 mAh cm2 for non-aqueous system, as shown in Tables 1 and 3. 2.3. Rechargeable solid-state and molten salt lithiumeair batteries The serious problems of lithiumeair batteries with liquid electrolytes are leakage and evaporation of the electrolyte over long operation period of more than 10 years for EVs and stationary use

Table 4 Performance of solid-state lithiumeair batteries. Solid electrolyte

Anolyte

Air electrode

Temperature ( C)

Current density (mA cm2)

Area capacity (mAh cm2)

Energy efficiency (%)

Ref.

LATP LATP

PEO LiPF6 in EC/DMC

Carbon LATP-silicone oil-coated carbon KB-LLZ-LiTFSI -PPC LLTO-C-CoO

85 25

0.1 0.3

0.2 0.6

43 (15th) 61 (50th)

[19] [85]

80 50

0.02 0.3

0.31 0.5

56 (50th) [86] 53 (1st) 0 (135th) [88]

LLZ LLTO

PVDF-HFP PYR14TFSI -LiTFSI

PPC: polypropylene carbonate; PVDF: poly(vinylidene)fluoride; HFP: hexafluoropropylene; PYR: butyl-N-methylpyrrolidinium .

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Fig. 11. (a) Cycle performance and (b) coulombic efficiency and energy efficiency change during cycling for the Li/LiPF6 in EC/DME/LATP/carbon in 19 mm thick LATP-silicone oil/air cell at 0.3 mA cm2 and room temperature. Adapted from Zhu et al. 2015 [85].

under open air. To address these problems, a solid-state lithiumeair battery system has been developed [83,84] Table 4 summarizes the performance of various solid-stat lithium-air batteries. In 2010, Kumar et al. [19] firstly reported this type of lithiumeair cell, which consisted of a lithium metal anode, LATP sandwiched with a PEObased polymer electrolyte and a carbon black and LATP composite air electrode. The cell was charged and discharged at 0.1 mA cm2 for 2 h at 85  C. The discharge potential was gradually decreased with cycling to 2.5 V, and the charge potential was gradually increased with cycling to 4.2 V after 15 cycles. Zhu et al. [85] presented a high performance solid-state lithiumeair battery. The system consisted of a lithium anode, a solid electrolyte of LATP, and a carbon-LATP composite cathode. A 19 mm thick porous LATP layer (75% porosity) was prepared on the LATP electrolyte by a spincoating technique, and carbon nanoparticles were coated on the LATP surface by infiltration of a sucrose solution and firing at 650  C. The carbon nanoparticles were covered with a silicone oil film (around 50 nm) to protect from the diffusion of water and CO2 from the air. Fig. 11 shows the cycle performance of the cell at 0.3 mA cm2 and room temperature under air with a relative humidity of 50% at a fixed capacity of 5,000 mAh per gram of carbon, which corresponds to around 550 mAh/gcathode and to around 0.60 mAh cm2. After 50 cycles, the energy efficiency during cycling decreased from 65% to 61%. The energy efficiency is comparable to that for the aqueous lithiumeair cell reported by Nemori et al. [17],

as shown in Table 3. Kumar et al. [19] and Zhu et al. [85] both used LATP for the lithium-ion-conducting solid electrolyte. However, this electrolyte is unstable in contact with lithium metal [18]. Kumar et al. used a PEO-based polymer electrolyte, and Zhu et al. used a conventional lithium-ion-conducting non-aqueous liquid electrolyte for the interlayer electrolyte between lithium metal and LATP. Sun et al. [86] used a lithium-stable lithium-ion-conducting solid electrolyte of a garnet-type electrolyte, Li6.4La3Zr1.4Ta0.6O12 (LLZ), for a solid-state lithiumeair battery. The lithium-ion conductivity of LLZ is around 103 S/cm at room temperature [87]. The proposed cell consisted of a lithium anode, an LLZ separator, and a KB, LLZ, LiTFSI, and polypropylene carbonate composite cathode. The cycle performance of the cell is shown in Fig. 12. The cell could be run for more than 50 cycles at 20 mA cm2 and 80  C under ambient air when the discharge capacity was approximately 316 mAh/gcathode (0.316 mAh cm2). The energy efficiency of 72% at the initial cycle decreased to 56% after 50 cycles. However, LLZ undergoes surface reactions with moisture and carbon dioxide in ambient air [88]. To operate this cell at room temperature, the composite cathode should be improved as with the unique cathode proposed by Zhu et al. [85]. More recently, Le et al. [89] reported a solid-state lithiumeair cell with a solid electrolyte of the perovskite-type (Li0,33La0.56)1.005Ti0.99Al0.01O3 (LLTO). This type of high solid lithium-ion conductor was firstly reported by Inaguma et al. [90].

Fig. 12. (a) Cycle performance and (b) specific capacity change during cycling for the Li/LLZ/KB-LLZ-PPC-LiTFSI/air cell at 20 mA cm2 and 80  C. Adapted from Sun et al. [86].

N. Imanishi, O. Yamamoto / Materials Today Advances 4 (2019) 100031

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Fig. 13. (a) Cycle performance and (b) the correspondence change of discharge (red), charge (black), and coulombic efficiency (blue) during cycling for the Li/LiNO3eKNO3/LAGT/Ni(LiNO3eKNO3) cell at 0.2 mA cm2 and 150  C. Adapted from Xia et al. [92].

The cathode was prepared by sintering a mixture of lithium lanthanum titanate (LLTO) and a starch pore former. A cobalt oxide catalyst was formed on the surface of the cathode. The cell was cycled for more than 100 cycles at 50  C and 0.3 mA cm2 in a limited capacity mode of 500 mAh/g (0.5 mAh cm2). The energy efficiency of 53% at the initial cycle decreased to 0% at 135 cycles. The air electrode is, thus, a key component to develop a high performance solid-state lithiumeair cell. Giordani et al. [91] demonstrated a new type lithiumeair system using a nitrate molten salt electrolyte of LiNO3eKNO3 and a carbonbased oxygen electrode. The cell showed a high energy efficiency of 96% at 150  C and 0.64 mA cm2 (2.8 mAh cm2) for 50 cycles. However, the uncontrolled diffusion and precipitation of soluble Li2O2 is a major case of capacity loss. Nazar and co-workers [92] recently proposed an improved molten salt lithiumeair cell that consisted of a lithium anode, a molten nitrate electrolyte of LiNO3eKNO3, a solid electrolyte of Li1.5Al0.5Ge1.5(PO4)3 (LAGP) as a separator and a nickel oxide nanoparticle, and LiNO3eKNO3 composite cathode. The LAGP separator protects from the crossover of the soluble reaction product. A four-electron conversion to lithium oxide at 150  C was confirmed. The theoretical energy density of this system is 5.2 kWh/kg. Fig. 13 shows the cycle performance of the Li/LiNO3eKNO3/LAGT/NiO-(LiNO3eKNO3), air cell at 0.2 mA cm2 and 150  C. This cell showed a high energy efficiency of 70% after 150 cycles at 0.2 mA cm2 and 0.5 mAh cm2. A medium temperature lithiumeair cell with a high energy density and high energy efficiency could, thus, be applied for energy storage with a green grid [93]. 3. Conclusions and perspectives In this short review, we have introduced the state of the art of the three types of lithiumeair batteries. In the last two decades, the research activity on lithiumeair batteries has been concentrated on the non-aqueous system. However, the non-aqueous system with high energy and high power densities is still in the initial stage, although significant progress has been achieved. The cell performance for almost all of the non-aqueous systems was measured at a low current density of less than 0.2 mA cm2 and a low specific areal capacity of less than 1.0 mAh cm2. Only in an exceptional case, Bruce and co-workers [40] reported excellent cycle performance at 1.0 mA cm2 and 2 mAh cm2 for the Li/0.3 M LiClO4 in TEGDME/LATP/25 mM DBBQ-25 mM TEMPO-0.3 M LiClO4 in DME/ CNT, O2 cell, where a dual mediator of DBBQ and TEMPO, and an

interlayer of LATP between the lithium anode and catholyte were used. The interlayer protects the lithium metal anode from the catholyte solution containing the RM. The round-trip energy efficiency with a capacity of 2 mAh cm2 at 1.0 mA cm2 was 77% after 50 cycles. The capacity was limited to 3 mAh cm2 because of the limited utilization of the porous cathode. As shown in Fig. 10, the specific energy density of the lithiumeair battery increases with the specific areal energy, and the specific areal capacity should be more than 10 mAh cm2 to obtain a specific energy density of 500 Wh/kg (including 30 mm thick LATP and excluding the package). The development of a high specific areal capacity cathode architecture is an important research target for non-aqueous lithiumeair batteries because the highly resistive Li2O2 reaction product is deposited on the air electrode surface [93]. The nonaqueous system requires an air purification system to reduce the water and CO2 contents. However, no acceptable purification system for EVs has been developed to date [94]. Asadi et al. [65] recently proposed a unique concept for protection of the anode from water and CO2 in air by the formation of a thin Li2CO3/C layer on the lithium anode. The cell exhibited excellent cycle performance, but the current density and areal capacity were as low as 0.05 mA cm2 and 0.05 mAh cm2, respectively. A high lithiumion-conducting solid electrolyte interlayer between the lithium anode and the catholyte is effective to prevent reaction of the lithium anode with water and CO2 in air and also protect from shuttle reactions of RMs with lithium [95]. In the next step, we hope that a lithium or lithium alloy electrode with a high lithiumion conducting or a high lithium-ion and electron mixedconducting solid thin layer will be developed. The aqueous lithiumeair system can operate in the atmosphere, and the discharge products are highly soluble in the catholyte, which improves the cycle efficiency, power performance, and energy density. At present, the highest specific capacity of 12 mAh cm2 at 0.64 mA cm2 was reported for the Li/LiFSI-2G4/LATP/ sat.LiOH-10 M LiCl/KB, RuO2, air cell by Imanishi and co-workers [73]. The key material for the aqueous system is the lithium-ionconducting solid electrolyte. Recently, Bai et al. [76] prepared a high lithium-ion-conducting solid electrolyte film (ca. 90 mm thick) of Li1.4Al0.4Ge0.2Ti1.4(PO4)3-10 wt%TiO2 (LAGTP) by a tape-casting method. The lithium-ion conductivity of the water-impermeable film with epoxy resin was 4.45  104 S/cm at 25  C and the three-point bending strength was around 200 N mm2. The calculated energy density of an aqueous lithiumeair battery with a 90 mm thick LAGTP layer and a LiCl-saturated catholyte was 370

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N. Imanishi, O. Yamamoto / Materials Today Advances 4 (2019) 100031

Wh/kg (550 Wh/L) based on the calculation method reported by Park et al. [82], where the utilization of the 10 M LiCl catholyte was estimated to be 50%. The specific energy density is around two times higher than that of the lithium-ion battery with a LiCoO2 cathode. The cycle performance shown in Fig. 9 was measured for a cell with a third electrode for the OER, and the utilization of the catholyte was not so high. Therefore, a cell with a stable air electrode for the OER and a high utilization catholyte should be developed. The aqueous lithiumeair cell with an acid aqueous catholyte is attractive to reduce contamination by CO2 from the air. The Li/CH3COOH (HAc)/C air cell showed excellent cycle performance at 60  C and 0.5 mA cm2 under 3 atm air to suppress evaporation of the catholyte. The discharge capacity was calculated as 250 mAh/gHAc (0.25 mAh cm2) with consumption of 56% HAc, which corresponded to an energy density of 779 Wh/kg against the weight of lithium and HAc. The cell performance of the liquid catholyte cell is not dependent on the specific areal capacity, where the reaction product is soluble in the catholyte [96]. The specific energy density of the Li/HAc system with a specific areal capacity of 12 mAh cm2, including the cell component (excluding the package) is estimated to be 379 Wh/kg (465 Wh/L), which is comparable to that for the Li/aqueous LiCl system. The Li/HAc lithiumeair system is attractive as a battery for EV applications. However, one problem for this system is vaporization of the catholyte. Therefore, an oxygen-selective membrane such as a silicone oil-wetted PVDFHFP film [97] and Celgard® with perfluoropolyether [64] should be used to prevent catholyte evaporation. The solid-state lithiumeair battery is attractive with respect to the safety issues and long-term stability. However, lithium-stable and lithium dendrite formation-free high lithium-ion-conducting solid electrolytes have yet to be developed. Therefore, a lithiumstable interlayer such as a liquid or polymer electrolyte between the lithium anode and the solid electrolyte is required. Furthermore, the cell capacity is dependent on the structure of the air electrode. The molten electrolyte lithiumeair battery has the potential to be a compact battery for electricity storage because it has an extremely high theoretical volume-specific energy density (as high as 10.5 kWh/L), which is three times higher than that of the non-aqueous lithiumeair battery and 10 times higher than that of the lithium-ion battery. The molten electrolyte system exhibited a very high discharge capacity of 11 mAh cm2, which is more than 20 times higher than that of the non-aqueous system [92]. However, cycle performance at a specific areal capacity of 0.5 mAh cm2 was reported, and that at a high capacity was questionable. High power density and extended deep cycling capabilities should, thus, be examined.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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