Lithium–Air Batteries with Hybrid Electrolytes - The Journal of Physical

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Lithium−Air Batteries with Hybrid Electrolytes Ping He,*,†,§ Tao Zhang,‡,§ Jie Jiang,† and Haoshen Zhou*,†,‡ †

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Center of Energy Storage Materials & Technology, College of Engineering and Applied Sciences, National Laboratory of Solid State Microstructures, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China ‡ Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1, Umezono, Tsukuba 305-8568, Japan ABSTRACT: During the past decade, Li−air batteries with hybrid electrolytes have attracted a great deal of attention because of their exceptionally high capacity. Introducing aqueous solutions and ceramic lithium superionic conductors to Li−air batteries can circumvent some of the drawbacks of conventional Li−O2 batteries such as decomposition of organic electrolytes, corrosion of Li metal from humidity, and insoluble discharge product blocking the air electrode. The performance of this smart design battery depends essentially on the property and structure of the cell components (i.e., hybrid electrolyte, Li anode, and air cathode). In recent years, extensive efforts toward aqueous electrolytebased Li−air batteries have been dedicated to developing the high catalytic activity of the cathode as well as enhancing the conductivity and stability of the hybrid electrolyte. Herein, the progress of all aspects of Li−air batteries with hybrid electrolytes is reviewed. Moreover, some suggestions and concepts for tailored design that are expected to promote research in this field are provided.

S

Li−air battery, 18,19 and solid-state Li−air battery.20−25 Obviously, the main discrepancy among them is the electrolyte component. In nonaqueous electrolyte-based Li−air batteries, a nonaqueous electrolyte is employed to separate the air electrode and metallic Li anode. Generally, the nonaqueous electrolytes to be considered here contain Li+ salt and organic solvents such as alkene-ester,17,26−28 ether,29−35 dimethyl sulfoxide (DMSO),36−40 tetra(ethylene) glycol dimethyl ether,41−52 and so on. The essential advantage of this system is the use of inexhaustible oxygen in air as reagent, rather than carrying the necessary chemicals around inside the battery. A fresh Li−air battery, just assembled, is in a charged state. Air electrode, which usually consists of catalyst and porous carbon, provides a place for the electrochemical reaction as follows:

ince the naissance of lead-acid batteries in the 1860s, numerous technical innovations in energy storage have been fostered to make life better. Nowadays, because of the impacts of global warming and energy shortages, it is more imperative than ever before for us to explore advanced energy storage systems. From the first lead-acid battery (∼40 Wh· kg−1) to the present Li-ion battery (∼150 Wh·kg−1), in a period of more than 150 years, the practical specific energy of commercial batteries has increased only 4-fold. Although the state-of-the-art Li-ion battery has achieved great triumphs in the portable electronic devices market,1−6 the energy storage is still too low to meet residential and long-distance transportation requirements. In the routine procedure for battery research, people are always trying to redesign active materials in electrodes that can deliver more capacity and provide higher operating voltage.7−10 Thus, the Li−air battery, strictly speaking the Li−O2 battery, was brought forward in 1996.11 The greatest strength of the Li−air battery lies in its Li metal anode, which has the largest capacity of 3800 mAh·g−1 and lowest potential of 0 V vs Li/Li+.12,13 Furthermore, low-cost O2 from the atmosphere as cathode also possesses very high capacity of 1670 mAh·g−1, while most Li-ion battery cathodes deliver capacity less than 300 mAh·g−1.14 The theoretical specific energy of the Li−air battery can achieve 3500 Wh·kg−1 based on the product mass of Li2O2, far exceeding that of an internalcombustion engine based on gasoline (700 Wh·kg−1).15,16 Consequently, the significant increase in specific energy provided by a Li−air battery opens a promising way to meet the demands of high energy storage. Most efforts have been focused on three different chemical designs classified by their inner structures: nonaqueous electrolyte-based Li−air battery,17 hybrid electrolyte-based © 2016 American Chemical Society

2Li+ + O2 + 2e− = Li 2O2

(R1)

In addition to Li2O2, LiO2 has been demonstrated as one component of the discharge product along with Li2O2 by both experimental methods and theoretical calculations.47,53−56 Recently, Lu et al. have successfully achieved a one-electron discharge process by forming only LiO2, which is stable enough to be repeatedly charged and discharged with a very low charge overpotential in a Li−O2 battery.57 Both the catalyst and porous carbon within the air cathode are electrochemically inert, which is different from the cathode in Li-ion battery. To some extent, during the discharge process, the air electrode in nonaqueous electrolyte-based Li−air battery provides only an oxygen diffusion path, reductive reaction site, and accomReceived: January 13, 2016 Accepted: March 15, 2016 Published: March 15, 2016 1267

DOI: 10.1021/acs.jpclett.6b00080 J. Phys. Chem. Lett. 2016, 7, 1267−1280

The Journal of Physical Chemistry Letters

Perspective

modation for product. It is considered by some researchers as an analogous to the air catalyst cathode of a H2/O2 fuel cell (FC). It is necessary to clarify that the reaction mechanism in the cathode of nonaqueous electrolyte-based Li−air battery is distinct from that in a FC. In a FC, the discharging product H2O, the only discharge process for FC, can be conducted out from membrane electrode assemble (MEA). As for the Li−air battery, insulated and insoluble Li2O2 locates in air electrode, which limits the discharge capacity of the battery. After becoming fully clogged by the resultant Li2O2 deposits, the air electrode becomes incapable of reducing O2 from air. Thus, the capacity of the cathode is influenced by the ability for accommodation of Li2O2. This is also quite different from the O2 reduction that takes place in a Zn−air battery. The O2 reduction product (OH−) is soluble in an aqueous electrolyte. On the other hand, the nonaqueous Li−air batteries are hypersensitive to H2O, N2, and CO2 entering the nonaqueous electrolyte from the surrounding air, resulting in corrosion of the lithium anode.58 In addition, LiOH and Li2CO3 would form during discharge when the battery was operated in air.59−61 Lim et al. investigated the reaction mechanisms in the Li−O2/CO2 cell under various electrolyte conditions and found the reversible formation and decomposition of Li2CO3 in high dielectric electrolytes.62 As such, proper operation of these batteries is dependent upon preventing the ingress of H2O, N2, and CO2 into the nonaqueous electrolyte. A novel concept of hybrid electrolyte was proposed by Zhou and Wang, which integrated the O2 catalytic reduction in an aqueous solution and the lithium-anode in an organic electrolyte.18 This is done using a ceramic lithium superionic conductor film (LISICON). By using this design, a new type of rechargeable lithium−air battery has been developed, as shown in Figure 1. The reaction in two electrodes can be expressed as follows:

Anode reaction Li = Li+ + e−

E θ = −3.04 V

(R4)

Total reaction 4Li + O2 + 2H 2O = 4LiOH

(R5)

During the discharge process of the proposed lithium−air battery, metallic lithium at the anode surface is converted into Li ions; simultaneously, Li ions in the organic electrolyte solution pass across the LISICON film and then combine with O2 from air to form LiOH within the aqueous electrolyte. The charge process of the proposed lithium−air battery reverses its discharge process. The water-stable ceramic LISICON film protects Li metal from corrosive damage and allows the battery to breathe in atmosphere directly with an air diffusion electrode. This is because the H2O and O2 are incapable of passing through the LISICON film; thus, they cannot react with the lithium-anode. Furthermore, in an aqueous solution, the soluble hydroxide product will not clog the porous catalytic electrode, which can facilitate continues discharge. All these intrinsic advantages make this kind of Li−air battery a promising energy storage device.63 The reaction mechanism with four-electron transfer process based on oxygen−water redox couple shown in reactions R2−R5 above was accepted widely. Obviously, the oxygen reduction reaction/oxygen evolution reaction (ORR/OER) process through reaction R2 in alkaline solution is equivalent to that in acidic solution (reaction R3). Therefore, they can be represented as an oblique line that is considered to be an upper edge of the water stability window (Figure 2). In addition, a two-electron transfer process was proposed and is described in the following (reactions R6 and R7). Acidic solution O2 + 2e− + 2H+ = H 2O2

E θ = 0.682 V

(R6)

Basic solution O2 + H 2O + 2e− = HO2− + OH−

E θ = −0.067 V (R7)

In acidic aqueous solution, the H2O2 molecule is a product of the ORR process, while both HO2−and OH− are yielded electrochemically at the interface of cathode and basic catholytes. Considering the chemical stability of solid-state LISICON film, an aqueous catholyte with mild alkalinity (pH 11) often employed for aqueous Li−air battery. As shown in Figure 2, the potential of reaction R7 at pH 11 is 3.054 V, which is lower than that of reaction R2 at the same alkalinity. In succession, HO2−and OH−, the ORR products, will precipitate with a Li ion to form Li2O2 chemically according to reaction R8: Figure 1. Schematic representation of the Li−air battery with the hybrid electrolyte.

2Li+ + HO2− + OH− = Li 2O2 ↓ + H 2O

Combining reactions R7 and R8, we can obtain a total reaction in the cathode that is identical to reaction R1 in the aprotic Li−O2 battery mentioned above:

Cathode reaction

2Li+ + O2 + 2e− = Li 2O2 ↓

Basic solutions O2 + 4e− + 2H 2O = 4OH−

E θ = 0.402 V

E θ = 1.229 V

(R1)

This two-electron reaction mechanism leads to a rockingchair type battery. The oxygen−peroxide system has a lot of advantages such as high reversibility, low polarization between the discharging and charging processes, and high energy density benefiting from needless excess of catholytes. The only

(R2)

Acidic solutions O2 + 4e− + 4H+ = 2H 2O

(R8)

(R3) 1268



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according to reaction R2. The increase of charge voltage will lead to inferior energy conversion efficiency. As stated above, the Li−air battery with aqueous electrolyte comprises four key components: ceramic lithium superionic conductor film, Li anode, porous catalytic electrode, and electrolyte. All battery performance indicators, such as lifetime, specific energy, and power as well as safety, depend on the character of these components. In this Perspective, we review the recent progress of Li−air batteries with aqueous electrolytes mainly based on our sustained work. Some new configurations, properties, and improvements of all components are specifically introduced. We also analyze the challenges of this system in fundamental research and future applications. Protective Layer. A creditably protective layer is fundamental to aqueous Li−air batteries. The protective layer is responsible for completely separating the Li metal anode from aqueous electrolytes and ambient air. Therefore, the primary requirement for a protective layer is the stability against water. With this requirement as a screener, the candidates of the protective layer at present include mainly the LISICON-type (Li analogues of NASICON) and garnet-type Li-ion conducting materials. Among those, a Li−Ti−Al−P−Si−O (LTAP) LISICON glass ceramic (developed by Ohara Inc.)65 has received the most attention because of its integrated reliability on water stability and ionic conductivity. Li-Ion Conductivity. The LTAP glass ceramic has a general formula of Li1+x+yTi2−xAlxP3−ySiyO12 with x = 0−0.25 and y = 0−0.3. It contains a main phase of rhombohedral LiTi2(PO4)3 (91 mass%) and an AlPO4 impurity phase (9 mass%).66,67 The crystalline LiTi2(PO4)3 consists of TiO6 octahedra and PO4 tetrahedra units, which are corner-sharing, to construct a threedimensional network for Li-ion transport. There are two different Li-ion sites in the structure. When containing exclusively tetravalent Ti4+ ions, the I sites are fully occupied, while the II sites are completely vacant.68,69 Partial substitution of Ti4+ ions by trivalent Al3+ can introduce additional Li ions to occupy the II sites and thus enhance the ionic conductivity by more than 2 orders of magnitude.70 The ionic conductivity of the LTAP plate (260 μm) can reach 3.5 × 10−4 S cm−1 at 25 °C.71 A value above 10−3 S cm−1 is desirable. Actually, the Li-ion conductivity of crystalline grains with LISICON-type structure is in the range of 10−3 S cm−1. Unfortunately, the total conductivity is reduced by a relatively high grain boundary resistance.67,72 A sketch of the LTAP microstructure deduced from high-resolution transmission electron microscopy (HR-TEM) images is shown in Figure 4.73 The major part consists of large crystalline grains attached to each other (I), and minor parts are amorphous (III) or consist of small crystalline grains embedded in amorphous material (II). Two types of grain boundaries are suggested: Type A is a thin amorphous layer between two grains with strongly dissimilar lattice orientations. Type B is a thicker transition layer between two grains with similar lattice orientation. Li ions are able to transport through the type B, but they would be blocked by the type A. Stability to Lithium Metal. The LTAP glass ceramic reacts with lithium metal when contacting directly, leading to a rapid increase of interface resistance.72 The reason lies in the easy reduction of Ti4+ by Li metal (or Li-metal alloy). Considerable efforts have been made toward synthesizing lithium-stable conductors, including mainly the following:

Figure 2. H2O stability window and ORR/OER process potential versus pH value in aqueous solution. (The upper oblique line represents four-electron reaction process based on oxygen−water redox couple. The red and blue lines represent two-electron mechanism processes based on oxygen−peroxide redox couple in acidic and alkaline aqueous solution. The yellow horizontal line describes the formation/dissolution reaction of Li metal anode in Li− air battery. All the referred potentials are in the normal state.)

This two-electron reaction mechanism leads to a rocking-chair type battery. requirement for this system is preventing hydrolysis of Li2O2 in high concentration of Li ion solution saturated with LiOH. The Li−air battery following the two-electron transfer mechanism based on the oxygen−peroxide redox couple will be a promising development orientation in this field because it provides lower charging potential and higher energy transformation efficiency. Matsui and collaborators first realized the two-electron reaction based on oxygen−peroxide in high concentration Li ion solution saturated with LiOH.64 As seen in Figure 3a, they

Figure 3. (a) Galvanostatic charging profiles of the Li−O2 cell operated at 1.0 mA cm−2. (b) An SEM image of the discharged electrode. Reprinted with permission from ref 64. Copyright 2015 Royal Society of Chemistry.

prepared an aqueous Li−air battery with a charge voltage plateau of 3.2 V, which is a little higher than the thermodynamical potential of 3.054 V due to polarization. The discharge product of Li2O2 was also characterized with the similar morphology observed in the aprotic system (Figure 3b). It is found from Figure 3a that not all the Li2O2 formed during the discharging process decomposes reversibly and another portion of charge capacity comes from the oxidation of water

1 Ti-free LISICON of Li1+xAlxGe2−x(PO4)3 (LAGP).67,74,75 1269



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4 LTAP can retain stable Li-ion conductivity in LiClsaturated LiOH solution because of the suppression of LiOH dissociation.86 Based on the third conclusion, a novel acidic Li−air battery has been developed in which the discharge product is LiOAc, not the conventional LiOH.87 Given the fourth conclusion, to develop a long-lived Li−air battery with LiOH product is possible. In addition, aqueous Li−air batteries with H2SO4-based88 and H3PO4-based89,90 electrolytes were also reported. These batteries provided more evidence for the relative stability of LTAP in acidic electrolytes. Zhang and colleagues investigated H+ diffusion in LTAP glass and their interactions with the glass surface using both experimental and modeling approaches. The results reveal the apparent adsorption of H+ on the LTAP glass rather than the bulk diffusion of H+, and H+ conductivity in LTAP glass is negligible at room temperature.91 As proof-ofconcept studies they are alluring, but the long-term stability requires additional investigation. LAGP was stable in the saturated LiCl and LiOH aqueous solution, as well as the LTAP. 9 2 A garnet-type Li7−xLa3Zr2O12−1/2x pellet was reported to be stable in saturated solution of LiCl.93 Unfortunately, there are still no protective layers stable in strong acidic and alkaline solutions. Inaguma and Nakashima prepared the perovskite-type La2/3−xLi3xTiO3 (LLTO) ceramic electrolytes with high total lithium ion conductivity of 3−5 × 10−4 S cm−1 at 300 K, which is attributed to the elimination of resistive grain boundary by the grain growth. Furthermore, the chemical stability of LLTO to alkaline LiOH aqueous solution was confirmed in the weak range. Unfortunately, there are still no protective layers stable in strong acidic and alkaline solutions.94 Zhou and Wang proposed a cycle operating model to separate the discharge product of LiOH from aqueous electrolytes.18 The advantages of this method lie in (1) maintaining the LTAP from corroding by the generated LiOH and (2) the separated LiOH can hopefully be used to extract Li metal for reuse. They further develop this concept to a Li−air cell with two subunits: an energy conversion unit and a product recycling unit, as shown in Figure 5.95 This system realized the continuous reduction of O2 from ambient air and improved the stability of the LTAP plate. Note that this system can be regarded as a fuel cell rather than a secondary battery.

Figure 4. Sketch of the LTAP microstructure. Li ions migrate along conduction pathways (blue) in the crystalline grains through type-B grain boundaries. Reprinted from ref 73. Copyright 2012 American Chemical Society.

2 Garnet-type Li7La3Zr2O12 (LLZ).76−80 Both of the conductors contain no lithium-reducible titanium and other metal ions, such as niobium, tungsten, molybdenum, or vanadium.76 The main phase of LAGP, LiGe2(PO4)3, belongs to the same NASICON-type structure of LiM2(PO4)3 (M4+ = Ge, Ti, Hf, and Zr).5 Because of the different lattice size for Li+ migration, the activation energies for the crystalline grain are 0.38 and 0.30 eV for the Ge and Ti systems, respectively.69 Therefore, the LTAP exhibits grain conductivity higher than that of the LAGP. However, the total conductivity of LTAP is lower because of a grain boundary resistance that is larger than that in LAGP.67 Garnet-like Li-ion conductors were reported by Weppner and colleagues, as Li5La3M2O12 (M = Nb, Ta) initially,76,77 in which the Ta system (less reducible than the Nb one) is stable against reaction with molten lithium.76 Among the further investigated garnet-like compounds, Li6BaLa2Ta2O12 exhibited the highest Li-ion conductivity of 4 × 10−5 S cm−1 at 22 °C with an activation energy of 0.40 eV. 78 Although Li6BaLa2Ta2O12 is stable against reaction with metallic lithium and moisture in air, its Li-ion conductivity is still much lower than the desirable ∼10−3 S cm−1. Note that the above compounds are just structurally related to the garnet structure. Recently, Li7La3Zr2O12 has been developed with more closed garnet-type structure and much high Li-ion conductivity (3 × 10−4 S cm−1 at 25 °C).79 In addition, Ta-doped Li7La3Zr2O12 was prepared by using a sol−gel precursor and is proposed as the protective layer for a water-stable lithium electrode for lithium−air rechargeable batteries. The total and bulk conductivities of the sintered Li6.75La3Zr1.75Ta0.25O12 pellet were 5.20 × 10−4 and 6.55 × 10−4 S cm−1 at 25 °C, respectively.81 Stability in Aqueous Electrolytes. The stability of LTAP glass ceramics in a variety of aqueous solutions has been investigated in detail. The results have been summarized in two reviews.80,82 The primary conclusions are the following: 1 LTAP is stable in water and the neutral aqueous solutions like 1 M LiNO3 and 1 M LiCl.66,83 2 LTAP suffers surface decomposition in strong acidic and alkaline solutions, for instance, 1 M LiOH and 0.1 M HCl.83 3 LTAP can retain stable Li-ion conductivity in a weak acidic acetic solution: saturated LiOAc in HOAc−H2O solution.84,85

Figure 5. A schematic representation of the developed Li−air fuel cell system with energy conversion unit and product recycle unit. Reprinted with permission from ref 95. Copyright 2010 Elsevier. 1270



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fabricated, and they are beneficial to improve the mechanical stability because of their flexibility. A typical polymer electrolyte is poly(ethylene oxide) (PEO) with Li(CF3SO2)N (LiTFSI) salt. The addition of nanoscale ceramic fillers or ionic liquids into PEO electrolytes reduces both their bulk resistance and the Li/PEO interfacial resistance.98,99 Nevertheless, the PLE with polymer buffer layer still requires further optimization for reducing resistance. Liquid-type aprotic electrolytes are extensively adopted because they are the most obtainable ones.83,100,101 Compared to the term “buffer layer”, Zhou et al. proposed a concept of “hybrid electrolyte” to define the separated aprotic electrolyte and aqueous electrolyte by the LTAP plate.102 Primarily, all of the aprotic electrolytes used for Li-ion batteries are able to be employed, typically like 1 M LiPF6 in 1:1 EC/DEC (vol:vol). The key problem to be addressed in developing a hybrid electrolyte is suppressing the relatively severe formation of Li dendrites during repeated cycling. In addition, the possible leakage of liquid-type aprotic electrolytes should be given careful attention. When LAGP and LLZ are used as the protective layer, the buffer layer is not necessary. In this case, constructing a closed Li/LAGP and Li/LLZ interface and reducing the interfacial resistance become the main requirements. Li Dendrite Formation. Dendritic growth on Li metal is a main stumbling block to developing the batteries with a Li metal anode. With growing, Li dendrites may divorce from the Li metal bulk, which are unable to join the discharge−charge process, resulting in limited cyclability of the Li electrode.103 If the Li dendrites grow more severely, they may pierce through the separator to short-circuit the cell, leading to fire or explosion hazards.1,104 With a protective layer, although the dendritic growth is still present, the short-circuit touch of Li dendrite with cathode can be prevented completely, as shown in Figure 6a. It is what to be really important for safety issue. Note that the Li dendrites formed in the buffer layers of organic or polymer electrolytes may still divorce from Li metal or contact the LTAP plate. To suppress the dendrite formation in PEO buffer layer and improve the cyclability of Li electrode, the influence of additives on Li striping and plating was investigated.105 Initially, the dendrite formation was observed after 15 h of polarization. The dendrite onset time was prolonged significantly to 46 h after the addition of nano-SiO2 filler and ionic liquid (PP13TFSI). The dendrite onset time is proportional to the diffusion constant of Li-ions in electrolyte and hence is greatly associated with the resistance of the solid electrolyte interface (SEI) between Li metal and PEO. It is suggested that the effect of the additives on reducing the interfacial resistance is an important factor, but tackling the dendrite problem is still more complex.

In summary, LTAP glass ceramics are still the major protective layer for aqueous Li−air batteries. The Li-ion

The Li-ion conductivity of the LTAP glass ceramics must be further improved by finely controlling the configuration of the grain boundary. conductivity of the LTAP glass ceramics must be further improved by finely controlling the configuration of the grain boundary. All of the protective layers studied, including LTAP, LAGP, and LLZ, do not have sufficient stability in strong acidic and alkaline solutions. More evidence should be provided to demonstrate their sustainable operation in the weak acids and saturated Li salt-supported LiOH solutions. Protected Lithium Electrode. The protective layer enables Li metal to release and recover electrons stably in aqueous Li−air batteries. There are two features related to the protective function: 1 The spontaneous chemical reaction between Li metal and water is avoided. 2 The narrow electrochemical window of aqueous electrolytes is expanded, that is, the chemically reactive Li/H2O interface is replaced with an electrochemically stable Li/ electrolyte interface. The functions have been explained clearly by analyzing the change of Pourbaix diagrams and interface potentials related to the protected lithium electrode (PLE) in aqueous electrolytes.80 Conf iguration and Resistance. A buffer layer is necessary when using LTAP as the protective layer. The buffer layer is stable with Li metal. Three types of buffer layers have been attempted, including solid-state Li-ion conductors, polymer electrolytes, and aprotic liquid electrolytes. Visco et al. proposed the deposition or evaporation of solid Li3N or Li3(P,N)O3 (LiPON) onto the LTAP by radio frequency sputtering, e-beam, or thermal evaporation.96 Puech et al. prepared a thin membrane with a thickness of down to 40 μm by a tape-casting of a Li1.3Ti1.7Al0.3(PO4)3-AlPO4 (LTAPAP) based slip followed by a sintering step. One side of the membrane was coated with a LiPON thin film, and lithium metal was electrochemically deposited on the LiPON surface from a saturated aqueous solution of LiOH.97 The advantage is the high Li-ion conductivity of Li3N and LiPON (∼10−3 S cm−1 at room temperature), but the preparation cost is expensive for scale up. Imanishi et al. developed composite polymer electrolytes as a buffer layer.72 Polymer electrolytes are easily

Figure 6. Schematic illustrations of (a) dendrite formation in buffer layer between Li metal and LTAP plate, (b) separation of Li metal and LAGP or LLZ under deep discharge, and (c) inert Al as a matrix to maintain contact with LAGP or LLZ under deep discharge. 1271



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operating voltage of 3.3 V at a low current density; organic electrolyte-based Li−air batteries generally provide a lower voltage of only 2.8 V. It is also worth noting that discharging voltage plays an important role of output energy. Introducing acid aqueous solution to catholyte is a feasible way to enhance output voltage. As we know, the reactions that take place in aqueous electrolyte vary with different alkalinity, which can be expressed by reactions R1 and R2. Considering these two cathodic reactions, a Li−air battery with an acidic aqueous catholyte can provide a voltage higher than that under basic conditions. However, almost all oxide and transition-metal alloy catalysts, including Mn oxide and Co- and Fe-based alloys, are unsuitable for use in acidic aqueous solutions. On the other hand, the high cost of noble metal and alloys is a major drawback that prohibits practical applications of these highly effective catalysts. Thus, discovering a substitute for noble metals in acidic conditions should be a task of primary importance. TiN particles with high electron conduction were employed by Zhou and co-workers to prepare a catalytic layer. This TiNbased cathode is also used in a Li−air battery with a weak acidic electrolyte (LiAc saturated 90 vol % HAc aqueous solution). The electrochemical polarization curves demonstrated that the TiN-based catalytic layer cathode has an onset potential of 3.8 V vs Li/Li+, which is higher than that of carbon (3.4 V); they are both lower than that of Pt plate (4.0 V). This suggests that although slightly inferior to Pt, TiN still has considerable electrochemical catalytic activity for Li−air batteries in weak acidic solution. Furthermore, particle size of TiN seriously affects its catalytic activity. According to the results from Zhou’s group, microsized TiN particles demonstrate evident electrocatalytic activities toward ORR in the aqueous Li−air battery, with the nanosized TiN showing a much better catalytic activity, which is comparable to that of the nanosized Mn3O4.107 Manthiram and colleagues employed nanocrystalline IrO2, adding Pt to fabricate air electrode. The catholyte consisted of a buffer solution of 0.1 M H3PO4 and 1 M LiH2PO4. The prepared Li−air battery with the weak acid catholyte in their work can provide a maximum power density of 40 mW cm−2 and 80% conversion efficiency at 2 mA cm−2 and 40 °C.108 To reduce the cost of noble metal catalyst, the same group developed the composite catalyst of mesoporous NiCo2O4 nanoflakes on Ni foam. They report a favorable cycle life and efficiency retention rate.109 Carbon Materials. The carbon family attracts enduring attention in the field of battery materials because of its high electron-conductivity, low cost, and satisfactory catalytic activity. As the catalyst in the catholyte, carbon materials should possess attributions such as the following:

In the case of using LAGP and LLZ as the protective layer, the dendritic growth is greatly confined by the solid-state conductors. However, the uniform Li stripping and plating happened generally at low current density.106 If the nonuniform Li plating can not be eliminated completely at higher current density, it is deleterious to the tight contact of Li metal and the solid-state conductor. Moreover, the Li utilization is low because the Li metal is prone to separate from the solid-state conductor under deep discharge, as shown in Figure 6b. These problems have been encountered constantly in the studies of thin-film Li batteries. From these points of view, we suggest replacing the Li metal with Li alloys, such as Li−Al and Li−Si. The Al and Si can act as the matrix to keep contact with the protective layer during Li striping and plating, as shown in Figure 6c. Electrode Performance. Electrode resistance, SEI, and Li dendrite are all directly correlated to the cycling stability of the PLE in aqueous electrolytes. The anodic reaction occurring on the PLE is clearly Li+ + e− ↔ Li.72 The polarization overpotentials of the PLE on discharge−charge are proportional to the current densities, suggesting that the electrode resistance is the key factor to improve the limiting current on the PLE.98 The PLE with PEO18LiTFSI-1.44PP13TFSI buffer layer exhibited a relatively low electrode resistance of 113 Ω cm−2 at 60 °C.71 Nevertheless, it is still much higher than the Li metal anode itself. The changes of electrode resistance are observed during repeated cycling for 100 cycles.71 The interfacial and chargetransfer resistances increase significantly after the initial 25 cycles; afterward, they decrease and remain stable until the 100th cycle. The resistance changes reflect the interface information related to SEI and Li dendrites, which should be further explored. In summary, the interfacial resistance of PLE must be further decreased to improve the limiting current. The related SEI and Li dendrite information should be elucidated in more detail. To simplify the multilayer configuration from the current Li/buffer layer/LTAP to Li/LAGP or Li/LLZ, the Li metal may be replaced by Li alloys to improve Li utilization. Catalytic Porous Electrode. Catalytic porous electrode, as shown in Figure 1, consists of a catalytic layer, an air diffusion layer, and a current collector. The air diffusion layer made of hydrophobic materials has a porous structure to facilitate fast passing of oxygen from the atmosphere and prevent water seepage. The catalytic layer including catalyst, conductive agent, and bonder mainly takes charge of catalytic reaction for oxygen reduction and revolution on its surface. There are three kinds of catalyst materials used in the catalytic layer for an aqueous Li− air battery, such as noble metal, transition-metal derivatives, and carbon materials. Despite marvelous catalytic performance, noble metal is not a preferred choice of catalyst because of its high cost. Transition-Metal Compounds. Herein, transition-metal compounds were employed in aqueous Li−air batteries mainly involving oxide and nitride. The first catalytic porous electrodes for Li−air batteries with hybrid electrolytes including a Mn3O4/ C-based catalytic layer was developed by Zhou’s group, which could continuously reduce O2 from air to provide capacity.18 The prepared catalytic porous electrode delivered a specific capacity of 50 Ah g−1 based on total mass of catalytic layer, which is much higher than conventional Li−air batteries using organic electrolyte. Another superior characteristic of the Mn3O4/C-based catalytic layer in aqueous electrolyte is high

1. High specific surface area that can bring more active sites for ORR and OER in aqueous electrolyte. 2. Chemical and electrochemical stability of carbon materials with O2 and moisture to facilitate long-term life and high Coulombic efficiency of the battery. Some mesoporous carbon has been used as the support for metal oxide or noble metal nanoparticles in Li−air batteries. Note that most carbon materials themselves usually exhibit quite moderate performance and just act as a supporting substrate, providing an electron conductivity path and lowering the cost of the noble metal component. Carbon allotropes possessing dislocated graphene stacking with sp3-bonded carbon atom, such as carbon nanotubes (CNTs) and graphene 1272



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rich graphene edges exposed and N doping. The ORR mechanism proposed by this work is illustrated in Figure 8b.

nanosheets (GNSs), were considered to be promising catalysts for ORR in Li−air batteries. Carbon black (CB) and CNTs, two common carbon allotropes, were used as the supporting substrate for Pt. Xing and colleagues compared the catalytic activity of these two composite catalysts in hybrid electrolyte Li−air batteries. They found that the cell with Pt/CNTs buckypaper showed a much better performance than the cell with Pt/CB. However, the cycling performance of Pt/CNTs buckypaper catalyst in sulfuric acid electrolyte is not satisfactory, which is considered by the authors to be due to corrosion of Pt during OER.88 Zhou and Yoo first demonstrated that metal-free graphene nanosheets (GNSs) present good performance as catalyst for ORR in Li− air batteries with a hybrid electrolyte, even though it exhibits a marginally poor cycling performance. They assumed that the low overpotential of GNSs employed in Li−air batteries derives from both the presence of dangling σ-bonds at the edges and defects of GNSs. They also reported that calcination of GNSs was a useful method for enhancing the cycling stability.110 To further improve the performance of carbon allotropes, foreign atoms, such as nitrogen and oxygen atoms, were introduced to the structure of graphene stacking. This strategy has been widely adopted to enhance the electrochemical activity of GNSs and CNTs.37,111−114 Zhou’s group creatively proposed a concept of metal-free GNSs and developed Ndoped GNSs that were prepared by heating GNSs in flowing NH3. As shown in Figure 7, the N-doped GNSs were first

Figure 8. (a) SEM images of CNTAs grown on a backing carbon paper. The high-magnification inset shows tip structure of the CNTAs. (b) ORR mechanisms on N-doped CNTs enduring. Reprinted with permission from ref 113. Copyright 2013 Royal Society of Chemistry.

As seen in Figure 8b, O2 in alkaline media is electrochemically reduced to form HO2− intermediate which is chemically disproportionated to yield OH− and O2. Xing and co-workers agreed with the insight of Zhou that both quaternary and pyridinic-N play the role of active sites on the surface of carbon.113 The combination of CNTs and graphene oxide with oxygen-containing groups was developed by Cui and colleagues.115 These hybrid materials were employed for ORR toward Li−air batteries with aqueous electrolyte. This work also followed the strategy of metal-free catalyst. Through electrochemical analysis, the hybrid materials exhibited a very low voltage gap of 0.17 V at 0.1 mA cm−2 between charge and discharge. They believed the superiority of catalytic activity was due to the easy absorption of O2 onto graphene oxide with the oxygen-containing group. Composite Catalyst. In addition to N-doping and introducing oxygen, combination of GNSs with other catalytic compounds is considered to be an effective strategy to improve performance of GNSs. Zhou and co-workers prepared Fe phthalocyanine (FePc) supported by carbon materials such as GNSs, multiwall carbon nanutubes (MWCNTs), and acetylene black (AB). They subsequently researched extensively the electrochemical profile of these composite catalysts in alkaline solution. The results show that ORR on the surface of all the FePc/carbon composites has a synergic effect on the electrocatalytic activity and endures the 4-electron process. The Li−air battery using FePc/CNTs as catalyst layer exhibited the most stable cycling performance.116 The same group associated the CNTs with imidazolium ion-based 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)-imide to prepare a gel cathode.117 The structure and performance of a hybrid electrolyte-based Li−air battery with sustainable gel/solid interface is illustrated in Figure 9. As shown in Figure 9, during the complete cycle, the proposed battery delivered a

Figure 7. Schematic representation of Li−air fuel cell based on Ndoped GNSs with hybrid electrolyte. Reprinted with permission from ref 112. Copyright 2012 Royal Society of Chemistry.

employed as catalytic electrode in Li−air fuel cell with acidic solution. The prepared N-doped GNSs exhibit a high discharge voltage plateau, which is near to that of commercial 20 wt % Pt/carbon black usually used in H2−O2 fuel cells. They attributed the excellent catalytic performance of N-doped GNSs in ORR under acidic conditions to a large proportion of edge sites and pyridine-type N sites. However, further efforts to clarify the mechanism of electrocatalysis were not made.112 One year later, an analogous mechanism called “pseudo” fourelectron pathway for ORR on N-doped CNTs with dislocated graphene stacking was proposed by Xing and co-workers.113 They prepared the nitrogen-doped carbon nanotube arrays (CNTAs) on carbon paper, which is used as hierarchical air cathodes in Li−air batteries with alkaline solution. The morphology picture of the air cathode is presented in Figure 8a. The obtained high activity of the CNTAs is also due to the 1273



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solution (saturated LiOH with 10 M LiCl).118 The results show that among these carbonaceous materials KB presents the best performance for ORR and OER due to its highest surface area. Through the analysis of gas chromatography data, CO was observed for OER on the cathode. They alleged that the CO gas comes from carbon materials based on the following anodic reaction, which resulted in a loss of catalytic activity for OER: C + 2OH− = CO + H 2O + 2e−

(R9)

Addition of a perovskite oxide such as La0.6Sr0.4Co0.2Fe0.8O3 to the above carbon electrode proved to be an effective method for confining carbon corrosion.119 The needs for ORR and OER catalysts are so different that it is almost impossible for one material to catalyze ORR and OER processes. Therefore, it is considered that the catalysts for charge and discharge should be designed individually.18 Manthiram and co-workers109 improved this strategy to construct a hybrid Li−air battery with two nanostructured catalyst electrodes countering the Li anode. As seen in Figure 10a, OER and ORR air electrodes were prepared separately and incorporated in an aqueous Li−air battery. Mesoporous NiCo2O4 nanoflakes grown onto the Ni foam act as a positive electrode for the charging process (see Figure 10b). N-doped mesoporous carbon loaded onto a hydrophobic carbon paper was used for discharging, offering high OER activity and good electrochemical activity. This elaborately designed Li−air battery with alkaline catholytes batteries shows good cycling performance (100 cycles, 400 h). After that, they also reported a series of nanostructured catalysts that are beneficial for both the ORR and OER processes in the double air electrode system, such as ordered Pd3Fe intermetallic (for ORR)120 as seen in Figure 11a,b and Co3O4 microtrepangs directly grown on nickel foam (for OER)121 as seen in Figure 11c,d. Thus, the double air electrode system indeed enhances the catalytic activity of OER and ORR processes at the same time. The overpotential of charge and discharge can be lowered markedly, resulting in high energy conversion efficiency. Therefore, development of aqueous Li− air batteries with double air electrodes should be a promising avenue for practical application. Hybrid Electrolyte. In all battery configurations, the electrolytes must be stable, with both the anodic and cathodic active materials remaining at their redox electrode potentials. This is a requirement that is critical in the design and synthesis of electrolytes. Usually, an organic solution such as LiPF6/ carbonic ester was serviced as electrolyte for Li-ion batteries

Figure 9. (a) Schematic illustration of the proposed Li−air battery with gel electrode. (b) Discharge−charge performance and XRD patterns of gel electrode. Reprinted with permission from ref 117. Copyright 2013 Macmillan Publishers Limited.

high capacity of 56 800 mAh g−1 for discharge as well as a high Coulombic efficiency of 95% for recharging. They considered that the discharge reaction is based on the Li2O2 formation and the recharge process involved the decomposition of Li2O2 and LiOH as well as Li2CO3. The OER of the latter reactant presents much higher overpotential, which results in a lower electrical energy efficiency. This work realized a long-term reversible discharge−discharge behavior over 100 cycles in ambient air for the first time, which has great significance in theory and application. Besides CNTs and GNSs, other carbon allotropes were also attempted to serve as the catalyst for Li−air batteries with aqueous electrolyte. However, several major obstacles arising from the carbonaceous air electrode, such as carbon’s oxidation in both charge and discharge processes. Yamamoto and colleagues compared the electrochemical property of various carbon materials such as Ketjen black (KB), acethlene black, vulcan X-72R, and vapor grown carbon fiber in an aqueous

Figure 10. (a) Schematic representation of the hybrid Li−air cell with mesoporous nanocatalysts. (b) SEM and TEM images showing the mesoporous NiCo2O4 nanoflakes directly grown onto nickel foam as the OER catalyst. (c) SEM and TEM images showing the N-doped mesoporous carbon as the ORR catalyst. (d) Cycling performance of the hybrid Li−air batteries with double air electrode with 2 h cycle period for 100 cycles. Reprinted with permission from ref 109. Copyright 2014 Royal Society of Chemistry. 1274



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Figure 11. (a) TEM image of ordered Pd3Fe/C. (b) Cycling performance of the hybrid Li−air batteries with ordered Pd3Fe/C as ORR air electrode. (c) SEM image showing the nanostructure of Co3O4 microtrepangs on Ni foam. (d) Discharge and charge voltage profiles of hybrid Li−air batteries with Co3O4 @Ni as OER air electrode. Reprinted from ref 120. Copyright 2015 American Chemical Society.

provide a larger power at elevated temperature due to the decrease of all resistance of elements.122 Obviously, there are two contradictory aspects: increase of LiOH concentration lower both electrolyte resistance and operating voltage. To decrease the internal resistance of the battery, LiClO4 salt was added to the basic catholyte by Chen’s group, which also kept the discharge voltage because of the unchangeable pH value of catholyte.123 A Li−air fuel cell with an acidic aqueous electrolyte can provide a voltage higher than that under basic and neutral conditions. In batteries with acidic catholytes, the total battery reaction is

because of its stability against Li anode and oxide cathode. As for hybrid electrolyte-based Li−air batteries, few effects have been focused on the anolyte. The same organic solution as that in Li-ion battery was used as anolyte. However, in this open system, corrosion of Li metal with moisture and O2 and N2 entering the organic electrolyte from the ambient atmosphere aggravates the performance fading of the battery. The ceramic plate called LISICOM film here avoids accessing of H2O and air to anolyte, which was thoroughly reviewed in previous paragraphs. Besides organic solution and LISICON plate, an aqueous solution also was employed as catholyte of hybrid electrolyte. Aqueous catholyte possesses the merits of high ion conductivity, low cost, environmental friendliness, and solubility for discharge products. There are two kinds of aqueous catholytes classified by the alkalinity for hybrid electrolyte-based Li−air batteries, such as neutral or basic solution and acidic solution. Differing from the exact mechanism of Li2O2 formation (shown in reaction R1) in aprotic electrolytes, ORR in aqueous catholyte follows alternative ways according to the alkalinity. In cells with basic or neutral catholytes, the battery reaction can be expressed by reaction R5. The excess product LiOH will be deposited and clog the catalytic porous cathode. Moreover, the LiOH is intensely corrosive to the ceramic plate and reacts with CO2 from ambient air to form inactive Li2CO3. Considering the battery reaction R2, the OH− concentration in catholyte can influence the energy and power density of lithium−air battery directly through the following three aspects: (i) the thermodynamic potential of oxygen reduction, (ii) the catalytic activity of air catalytic electrode, and (iii) solution conductivity. Zhou and colleagues investigated the performance of hybrid electrolyte-based lithium−air batteries under the mixed control of alkalinity and temperature by means of galvanostatic measurement and the analyses of ac EIS. The results showed that the electromotive force and inner resistance of the battery decrease with the increase of LiOH concentration in aqueous electrolyte. As for the power performance, 1.0 M could be the suitable parameter for the LiOH concentration of aqueous electrolyte. They also found that the battery can

4Li + O2 + 4HA = 4LiA + 2H 2O

(R10)

where A is a weak acid anion such as H2PO4− and CH3COO− in most Li−air batteries with acidic aqueous electrolyte.87,90,124 Due to its intense corrosive property to LISICON plate, a strongly acidic solution can not be employed as catholytes, although it facilitates increasing the operating voltage of Li−air batteries with hybrid electrolyte. Imanishi and co-workers proposed a stable cell employing a buffer catholyte prepared from a LiOAc-saturated 90 vol % HOAc aqueous solution. The schematic diagram of this cell is shown in Figure 12a. The ceramic LISICON plate used in this cell is stable in the buffer solution. As shown in Figure 12b, the prototype Li−air battery can retain a rechargeable capacity of 250 mAh g−1. They reported that this new type of battery can deliver an expected energy density of more than 400 W h kg−1, which is superior to that of conventional graphite/LiCoO2 battery.87 It is an obvious contrast that the aqueous solution including solute and water in a dual-electrolyte battery plays the two roles of electrolyte and active reactant, whereas the solution in a nonaqueous battery acts as only the Li-ion conductive media. The extra weight of the aqueous solution involved in the battery reaction inevitably limits the energy density of the battery. To increase battery energy, an active solute that can react with more Li and O2 based on reaction R6 should be considered. As we know, weak acids usually have low solubility 1275



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based on the concept of an aqueous Li−air battery, the Li-ion redox flow battery, was designed by the groups of Goodenough and Zhou. In these approches, an aqueous electrolyte containing Fe2+/Fe3+ or Fe(CN6)3−/Fe(CN6)4− redox couple was separated from Li metal in nonaqueous electrolyte by a LISICON plate and was pumped to flow through a tank in a loop.100,126−128 The electrochemical reactions during the discharge process can be described as follows: Anode: Cathode:

discharge

n Li ⎯⎯⎯⎯⎯⎯⎯⎯→ n Li+ + e− discharge

Mn +(aq) + ne− ⎯⎯⎯⎯⎯⎯⎯⎯→ M(z − n) +(aq)

where M represents a metal element; Mn+ is a metal ion in an oxidation state, while discharge product M(z−n)+ is in a reduction state. As demonstrated by Figure 13, in ambient air, O2 can also be employed to chemically recharge the battery by oxidizing

Figure 12. (a) Schematic diagram of the proposed lithium−air cell. (b) Charge−discharge profiles of Li/PEO/LTAP/HOAc-H2OLiOAc/carbon air cell. Reprinted with permission from ref 87. Copyright 2010 Royal Society of Chemistry.

It is an obvious contrast that the aqueous solution including solute and water in a dual-electrolyte battery plays the two roles of electrolyte and active reactant, whereas the solution in a nonaqueous battery acts as only the Li-ion conductive media.

Figure 13. Concept image for a new Li−Air (O2) battery containing Mz+/M(z−n)+ ion pairs as catalyst of the O2 reduction reaction. Reprinted with permission from ref 129. Copyright 2012 John Wiley & Sons, Inc.

discharge product M(z−n)+ into Mn+. Thus, the redox couple M(z−n)+/Mn+ can be considered as a catalyst for ORR. This battery actually appears as a new concept Li−air battery and was named “Li redox flow air battery”. The detailed introduction can be found in our previous review article.129 In this Perspective, we introduced in detail the hybrid electrolyte-based Li−air battery in terms of construction, reaction mechanism, and materials. To some extent, these system can be considered as a combination of a Li-ion battery and an aqueous fuel cell. Benefiting from the smart design, Li− air batteries with hybrid electrolytes can easily circumvent Li metal corrosion, Li2O2 clogging, and comparative low voltage, all which are the main disadvantages of nonaqueous Li−air batteries. Therefore, it is certain that aqueous Li−air batteries are worthy of further study and will play an increasingly important role in future research of Li−air batteries. However, the inherent drawbacks of aqueous Li−air batteries should receive sufficient consideration as well. First, the chemical and mechanical stability of LISICON plate is a bottleneck to the realization of practical application of aqueous Li−air batteries. A plastic LISICON plate with high ion conductivity should be developed, which also has low chemical reactivity with the aqueous solution. Second, design and preparation of a carbon-based composite catalyst with low cost and high efficiency should be one of the most promising ways to improve the performance of hybrid electrolyte-based Li−air

in water, which restricts the practical capacity of the catholytes. The strong acid with relatively high concentration is considered to be effective to enhance capacity of this type of Li−air battery. However, the strong acid system suffers from corrosion of LISICON. Manthiram’s group proposed an imidazole buffered to maintain a mild pH of ∼5.0 in HCl solution, which facilitates increasing the battery capacity.125 In addition to one-proton acid CH3COOH and HCl solution, polyprotic acid H3PO4 in a Li2SO4 supporting electrolyte also has been used as a catholyte in aqueous Li−air batteries.90 H3PO4 endures a multistep reaction with oxygen and Li metal and facilitates providing larger capacity. On the other hand, the ORR in a weak acid solution takes place at an average voltage of 3.3 V. The moderate pH value of phosphate buffer solution leads to less corrosion of LISICON plate. The experimental data suggested that the cell offered 740 mA h g−1 and good rechargeability. Despite these achievement on proton acid solution system, the chemical stability of LISICON plate can not yet meet the requirements of prolonged use. Introducing aqueous catholyte with redox metal element, a smart and flexible configuration 1276



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batteries. The promotion of a catalyst for a two-electron transfer reaction based on the oxygen−peroxide redox couple should receive significant attention. Finally, a weak acid or redox couple electrolyte is recommended to be used in aqueous catholytes. To further protect the LISICON plate and recover the Li resource, a recycle electrolyte system similar to a fuel cell is considered an effective solution. Accordingly, the advantages and challenges coexist in hybrid electrolyte-based Li−air batteries. We hope this Perspective will lead to new developments in the field.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions §

P.H. and T.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest. Biographies Ping He is an Associate Professor in the College of Engineering and Applied Sciences at Nanjing University, China. He received his Ph.D. from Fudan University in 2009 and then worked at AIST of Japan. His research interests focus on electrochemical functional materials and energy storage systems such as lithium-ion batteries and lithium−air batteries. He has published over 40 papers in prestigious journals. http://energy.nju.edu.cn Tao Zhang received his Ph.D. from Fudan University and was then an assistant professor at Shanghai University. Since 2008, he has worked at Mie University and AIST of Japan on Li−air batteries, all-solid-state Li-ion batteries, and Li-ion conducting ceramics. He is currently a professor at Shanghai Institute of Ceramics, Chinese Academy of Sciences. Jie Jiang is a doctoral candidate under the supervision of Prof. Haoshen Zhou and Prof. Ping He in the College of Engineering and Applied Sciences at Nanjing University, China. He received his bachelor degree in Materials Chemistry from Nanjing University in 2011. His research interests focus on cathode materials and reaction mechanisms for Li−air batteries. Haoshen Zhou is a prime senior researcher of Energy Technology Research Institute at National Institute of Advanced Industrial Science and Technology in Japan. He is also a joint professor at both Nanjing University and the University of Tokyo. His research interests include the synthesis of functional materials and their applications in lithium ion batteries, metal−air batteries, and new types of batteries and cells.

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ACKNOWLEDGMENTS This research was partially supported financially by the National Basic Research Program of China (2014CB932302, 2014CB932303), National Natural Science Foundation of China (21403107, 21373111), Natural Science Foundation of Jiangsu Province of China (BK20140055), Specialized Research Fund for the Doctoral Program of Higher Education of China (20120091120022), PAPD of Jiangsu Higher Education Institutions, the Project on Union of Industry-Study-Research of Jiangsu Province (BY2015069-01), and Open Fund of Jiangsu Key Laboratory of Materials and Technology for Energy Conversion (MTEC-2015M02). 1277



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