LithiumâAir Batteries with Hybrid Electrolytes - American Chemical

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Lithium Air Batteries with Hybrid Electrolytes Ping He, Tao Zhang, Jie Jiang, and Haoshen Zhou J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00080 • Publication Date (Web): 15 Mar 2016 Downloaded from http://pubs.acs.org on March 16, 2016

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The Journal of Physical Chemistry Letters

Lithium Air Batteries with Hybrid Electrolytes Ping He,*1 Tao Zhang2, Jie Jiang1 and Haoshen Zhou*1, 2

1, 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. 2, 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 electrolyte have attracted a deal of attentions due to their superiorly high capacity. Introducing the aqueous solutions and ceramic lithium super-ionic conductor to Li-air batteries can circumvent some drawbacks of conventional Li-O2 batteries such as decomposition of organic electrolytes, corrosion of Li metal from humidity, and insoluble discharge product blocking air electrode. The performance of this smart design battery depends essentially on property and structure of the cell components (i.e., hybrid electrolyte, Li anode, and air cathode). In recent years, towards aqueous electrolyte based Li-air batteries, extensive efforts have been dedicated to developing high catalytic activity of cathode as well as enhancing conductivity and stability of hybrid electrolyte. Herein, the progress of all aspects of Li-air batteries with hybrid electrolytes is reviewed. Moreover, some suggestions and concepts are provided for tailored design that is expected to promote the research of this field.

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TOC


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Since the naissance of lead-acid batteries in 1860s, numerous technical innovations about energy storage had been fostered to make life better. Nowadays, due to the impact of global warming and energy shortage, it is more imperative for us to explore advanced energy storage system as never before in our history. 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 by only 4 times. Although the state-of-the-art Li-ion battery achieves great triumphs in the market of portable electronic devices,1-6 the energy storage is still too low to meet the requirement of residence and long distance transportation. As a routine 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, Li-air battery, strictly speaking Li-O2 battery, was brought forward in 1996.11 The greatest strength of 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 atmosphere as cathode also possess very high capacity of 1670 mAh·g-1, while most of Li-ion battery cathode only 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 internal-combustion 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 Li-air battery,18,19 and solid state Li-air battery.20-25 Obviously, the main discrepancy among them is the component of electrolyte. In nonaqueous based electrolyte Li-air batteries, a nonaqueous electrolyte is employed to separate air electrode and metallic Li anode. Generally the nonaqueous electrolytes to be considered here contains Li+ salt and organic solvents such as alkene-ester,17,26-28 ether,29-35 dimethyl sulphoxide (DMSO),36-40 tetra(ethylene) glycol dimethyl ether41-52 and so on. The essential advantage of this system is to use 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 charged state. Air electrode, which usually consists of catalyst and porous carbon, provides a place for electrochemical reaction as follows:



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2Li + +O 2 + 2e − = Li 2O 2

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(R1)

Besides 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 Anyway, both catalyst and porous carbon within air cathode are electrochemically inert, which is different from cathode in Li-ion battery. To some extent, during discharge process, the air electrode in nonaqueous electrolyte based Li-air battery only provides oxygen diffusion path, reductive reaction site and accommodation for product. It is considered by some researchers as an analogy with air catalyst cathode of H2/O2 fuel cell (FC). It is necessary to clarify that the reaction mechanism in cathode of nonaqueous electrolyte based Li-air battery is distinct from that in FC. In FC, the discharging product H2O, just only discharge process for FC, can be conducted out from Membrane Electrode Assemble (MEA). As to Li-air battery, insulated and insoluble Li2O2 locates in air electrode, which limits discharge capacity of battery. After becoming fully clogged by the resultant Li2O2 deposits, the air electrode becomes incapable of reducing O2 from air. Thus, capacity of cathode is influenced by 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.


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e-

eLi+ Aprotic Electrolyte

Li+Aqueous Electrolyte

Li+

Li+

LISICON Film

Li+

Li+

Li+

Li+

+

Catalyst layer Air diffusion layer

Li Metal

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O2

Air

Figure 1. A schematic representation of the Li-air battery with the hybrid electrolyte.

A novel concept of hybrid electrolyte was proposed by Zhou and his colleagues, 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 super-ionic conductor film (shorten as 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: Cathode reaction: Basic solutions:

O2 + 4e- + 2H2O = 4OH-

Eθ = 0.402 V

(R2)

Acidic solutions:

O2 + 4e- + 4H+ = 2H2O

Eθ = 1.229 V

(R3)

Eθ = -3.04 V

(R4)

Anode reaction:

Li = Li+ + e-

Total reaction:

4Li + O2 + 2H2O = 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.



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The water stable ceramic LISICON film protects Li metal from corrosive damage and allows the battery breathes in atmosphere directly with an air diffusion electrode. This is because the H2O or 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 to be a promising energy storage device.63

Figure 2. The H2O stability window and ORR/OER process potential vs. 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 process 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 potentials referred are in normal state.) The reaction mechanism with four-electron transfer process based on oxygen-water redox couple shown in R2 to R5 above was accepted widely. Obviously, ORR/OER process through R2 in alkaline solution is equivalent to that in acidic solution (R3). So, they can be represented as an oblique line that is considered as an upper edge of water stability window, which are shown in Figure 2. Besides, a two-electron transfer process was proposed described in following equations


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(R6~R7). Acidic solution:

O2 + 2e- + 2H+ = H2O2

Eθ = 0. 682 V

(R6)

Basic solution:

O2 + H2O + 2e- = HO2- + OH-

Eθ = -0.067 V

(R7)

In acidic aqueous solution H2O2 molecule is product of ORR process, while both HO2-and OHyield electrochemically at interface of cathode and basic catholytes. Considering the chemical stability of solid state LISICON film, an aqueous catholytes with mild alkalinity (pH=11) often employed for aqueous Li-air battery. As shown in Figure 2, the potential of R7 at pH=11 is 3.054 V, which is lower that that of R2 at same alkalinity. In succession, HO2-and OH-, the ORR products, will precipitate with Li ion to form Li2O2 chemically according to R8: 2Li+ + HO2- + OH- = Li2O2↓ + H2O

(R8)

Combining R7 and R8, we can obtain a total reaction in cathode that is identical to R1 in aprotic Li-O2 battery mentioned above: 2Li+ + O2 + 2e- = Li2O2↓

(R1)

This two-electron reaction mechanism leads a rocked-chair 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 benefitting from needless of excess catholytes. Only one requirement for this system is to prevent 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 they provide lower charging potential and higher energy transformation efficiency. Matsui and his collaborators first realized two-electron reaction based on oxygen-peroxide in high concentration Li ion solution saturated with LiOH.64 As seen in Figure 3a, they prepared an aqueous Li-air with a charge voltage plateau of 3.2 V that is a litter 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 aprotic system (Figure 3b). It is found from Figure 3a that not all the Li2O2 formed during discharging process decompose reversibly and



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another portion of charge capacity comes from the oxidation of water according to R2. The increase of charge voltage will lead to inferior energy conversion efficiency.

(a)

(b)

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. As the statement above, the Li-air battery with aqueous electrolyte is comprised of four key components: ceramic lithium super-ionic conductor film, Li anode, porous catalytic electrode and electrolyte. All performances of battery, such as life-time, specific energy and power as well as safety, depends on the character of these components. In this paper, we review the recent progress of Li-air battery with aqueous electrolytes mainly based on our sustained works. Some new configuration, property, and improvement of all components are specifically introduced. We also analyzed 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 separating Li metal anode from aqueous electrolytes and ambient air completely. 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 current include mainly the LISICON-type (Li analogues of NASICON) and garnet-type Li-ion conducting materials. Among that, a Li-Ti-Al-P-Si-O (LTAP) LISICON glass ceramics (developed by Ohara Inc.)65 has received most attentions due to its integrated reliability on water-stability and ionic conductivity.

Li-ion conductivity ACS Paragon Plus Environment

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The LTAP glass ceramics 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 three-dimensional 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 two orders of magnitude.70 The ionic conductivity of LTAP plate (260 µm) can reach 3.5 × 10-4 S cm-1 at 25 oC.71 A value above 10-3 S cm-1 is further 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 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 boundary are suggested: type A is a thin amorphous layer between two grains with strongly dissimilar lattice orientation; type B is a thicker transition layer between two grains with similar lattice orientation. Li-ions are able to transport through the type B, while would be blocked by the type A.

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

Stability to lithium metal LTAP glass ceramics reacts with lithium metal when contacting directly, leading to an increase of interface resistance rapidly.72 The reason lies in the easy reduction of Ti4+ by Li metal (or Li-metal ACS Paragon Plus Environment


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alloy). Considerably efforts have been made on synthesizing lithium-stability conductors, including mainly: 1 Ti-free LISICON of Li1+xAlxGe2-x(PO4)3 (LAGP).67,74,75 2 Garnet-type Li7La3Zr2O12 (LLZ).76-80 Both of the conductors contain no the 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 Owing to 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 higher grain conductivity than the LAGP. However, the total conductivity of LTAP is lower due to a larger grain boundary resistance than that in LAGP.67 Garnet-like Li-ion conductors were reported by Weppner and his 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 oC 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 oC).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 oC, respectively.81

Stability in aqueous electrolytes The stability of LTAP glass ceramics in variety of aqueous solutions has been investigated in detailed. The results have been summarized in two reviews.80,82 The primary conclusions are: 1 LTAP is stable in water and the neutral aqueous solutions like 1M LiNO3 and 1M LiCl.66,83 2 LTAP suffers surface decomposition in strong acidic and alkaline solutions, for instance, 1M 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


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4 LTAP can retain stable Li-ion conductivity in LiCl-saturated LiOH solution due to 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 his 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-of-concept studies they are alluring but the long-term stability still requires more investigations. LAGP was stable in the saturated LiCl and LiOH aqueous solution, as well as the LTAP.92 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 until now. Inaguma and Nakashima prepared the perovskite-type La2/3-xLi3xTiO3 (LLTO) ceramics 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. Futhermore, the chemical stability of LLTO to alkaline LiOH aqueous solution was confirmed in the week range. Unfortunately, there are still no protective layers stable in strong acidic and alkaline solutions until now.94



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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 2011 Elsevier. Zhou and He proposed a cycle operating model to separate the discharge product of LiOH from aqueous electrolytes.18 The advantages of this method lies in: (1) maintain the LTAP from corroded by the generated LiOH; (2) the seperated LiOH is hopeful to extract Li metal and 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. In summary, LTAP glass ceramics is still the major protective layer for aqueous Li-air batteries. Its Li-ion conductivity requires to be further improved by finely controlling the configuration of grain boundary. All of the protective layers studied, including LTAP, LAGP and LLZ, have no 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 Li 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 electrochemically stable Li/electrolyte one. The functions have been explained clearly by analyzing the change of pourbaix diagram and interface potentials related to the PLE in aqueous electrolytes.80 Configuration 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 layer 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 RF sputtering, e-beam or thermal evaporation.96 Puech et al. prepared a thin membrane


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with a thickness of down to 40 µm by a tape-casting of a Li1.3Ti1.7Al0.3(PO4)3-AlPO4 (LTAP-AP) based slip followed by a sintering step. One side of the membrane was coated with a LiPON thin film and lihium 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 buffer layer.72 Polymer electrolytes are easily fabricated and they are beneficial to improve the mechanical stability of the due to their flexibility. A typical polymer electrolyte is poly(ethylene oxide) (PEO) with Li(CF3SO2)N (LiTFSI) salt. The addition of nano-scale ceramic fillers or ionic liquids into PEO electrolytes reduce both their bulk resistance and the Li/PEO interfacial resistance.98,99 Nevertheless, the PLE with polymer buffer layer still requires further optimum on reducing resistance. Liquid-type aprotic electrolytes are extensively adopted because they are the most obtainable ones.83,100,101 Compared to the term of “buffer layer”, Zhou et al. proposed a concept of “hybrid electrolyte” to define the separated aprotic electrolyte and aqueous electrolyte by LTAP plate.102 Primarily, all of the aprotic electrolytes using for Li-ion batteries are able to be employed, typically like 1M LiPF6 in 1:1 EC/DEC (vol:vol). The key problem to be addressed in developing hybrid electrolyte is suppressing the relatively severe formation of Li dendrite during repeated cycling. In addition, the possible leakage of liquid-type aprotic electrolytes should be paid attentions carefully. 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 main requirements. Li dendrite formation Dendritic growth on Li metal is a main stumbling block to developing the batteries with 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 burn or explosion hazards.1,104



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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; (c) inert Al as a matrix to keep contact with LAGP or LLZ under deep discharge. With a protective layer, although the dendritic growth is still be presence, 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 with LTAP plate. To suppress the dendrite formation in PEO buffer layer and improve the cycleability of Li electrode, the influence of additives on Li striping and plating was investigated.105 Initially, the dendrite formation was observed after 15 hours polarization. The dendrite onset time was prolonged significantly to 46 hours 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 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 non-uniform 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 probe 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 to replace 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 the cycling stability of the PLE in aqueous electrolytes. The anodic reaction happened on PLE is clearly the Li++e-↔Li.72 The polarization overpotentials of 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 PLE.98 The PLE with PEO18LiTFSI-1.44PP13TFSI buffer layer exhibited an relatively low electrode resistance of 113 Ω ACS Paragon Plus Environment

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cm-2 at 60 oC.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 charge transfer resistance have significant increase after the initial 25 cycles, afterwards, decrease and keep stable until the 100th cycle. The resistance changes reflects the interface information related to SEI and Li dendrite, which should be further revealed. Summarily, the interfacial resistance of PLE must be further decreased to improve the limiting current. The related SEI and Li dendrite information should be elucidated more detailed. 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 the Li utilization.

Catalytic porous electrode Catalytic porous electrode as shown in Figure 1 is consisted of catalytic layer, air diffusion layer and current collector. The air diffusion layer made of hydrophobic materials has porosity structure to facilitate fast pass of oxygen from atmosphere and prevent water seepage. 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 catalytic layer for aqueous Li-air battery, such as noble metal, transition metal derivatives and carbon materials. Despite marvelous catalytic performance, considering its high cost noble metal is not preferred choice of catalyst. transition metal compounds Herein transition metal compounds were employed in aqueous Li-air battery mainly involving oxide and nitride. First catalytic porous electrode for Li-air battery with hybrid electrolyte including Mn3O4/C based catalytic layer was developed by Zhou’s group, which realized continuously reduce O2 from air to provide capacity. 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 battery using organic electrolyte. Another superiority of the Mn3O4/C based catalytic layer in aqueous electrolyte is high operating voltage of 3.3 V at a low current density. While organic electrolyte based Li-air battery generally only provide a lower voltage of 2.8 V. It is also worthy to note that discharging voltage play 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 two formula R1 and R2 in previous paragraph. Considering these two cathodic reaction, a Li–air battery with an acidic aqueous catholyte can provide a voltage higher than that ACS Paragon Plus Environment


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under basic conditions. However, almost all oxide and transition metal alloy catalysts, including Mn oxide, Co-based, and Fe-based alloys are unsuitable for use in acidic aqueous solutions. On the other hand, the high cost of noble metal and alloy 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 his co-workers to prepare an catalytic layer. This TiN based 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 a higher onset potential of 3.8 V vs. Li/Li+ than that of carbon (3.4 V), while they are both lower that thiat of Pt plate (4.0 V). It suggests that although slightly inferior to Pt, TiN still has considerable electrochemically catalytic activity for Li-air battery in weak acidic solution. Furthermore, particle size of TiN seriously affect its catalytic activity. According to the results from Zhou’s group, micro-sized TiN particles demonstrate evident electrocatalytic activities toward ORR in the aqueous Li-air battery, with the nano-sized TiN showing a much better catalytic activity, which is comparable to that of the nano-sized Mn3O4.107 Manthiram and his colleagues employed nanocrystalline IrO2 adding Pt to fabricate air electrode. The catholyte consisted of 0.1 M H3PO4 + 1 M LiH2PO4 buffer solution. The prepared Li-air battery with the weak acid catholyte in their work can provide the maximum power density of 40 mW cm-2 and 80% conversion efficiency at 2 mA cm-2 and 40 ℃.108 To reduce the cost of noble metal catalyst, the same group developed the composite catalyst of mesoporous NiCo2O4 nanoflakes on Ni foam. They state a favourable cycle life and efficiency retention rate.109 Carbon materials Carbon family attracts enduring attentions in the field of battery materials due to its high electron-conductivity, low cost and satisfied catalytic activity. As the catalyst in catholyte, cabon materials should possess attributions as follow: 1、 High specific surface area can bring more active site for ORR and OER in aqueous electrolyte. 2、 Long-term life and high coulombic efficiency of battery requires chemical and electrochemical stability of carbon materials with O2 and moisture. Some mesoporous carbon has been used as the support for the metal oxide or noble metal nanoparticles in Li-air battery. Note that most carbon materials themselves usually exhibite quite moderate performance and just act as a supporting substrate providing electron conductivity path and lowering the cost of noble metal component. Carbon allotropes possessing dislocated graphene


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stacking with sp3-bonded carbon atom, such as cabon nanotubes (CNTs) and graphene nanosheets (GNSs), were considered to be promising catalyst for ORR in Li-air battery. Carbon black (CB) and CNTs, two common carbon allotropes, were used as the surpporting substrate for Pt. Xing and his colleagues compared the catalytic activity of these two composite catalyst in hybrid electrolyte Li-air batteries. They found that the cell with Pt/CNTs buckypaper showed a much better performance than with Pt/CB. However, the cycling performance of Pt/CNTs buckypaper catalyst in sulfuric acid electrolyte is not satisfied, which is considered by authors due to corrosion of Pt during OER.88 Zhou and Yoo first demonstrated that metal-free graphene nanosheets (GNSs) presents a good performance as catalyst for ORR in Li-air battery with hybrid electrolyte, even though it shows a little poor cycling performance. They assumed that the low overpotential of GNSs employed in Li-air battery derives from both the presence of dangling σ-bonds at the edges and defects of GNSs. They also pointed out calcination of GNSs was a useful methode to enhance the cycling stability.110

Figure 7. Schematic representation of Li-air fuel cell based on N-doped GNSs with hybrid electrolyte. Reprinted with permission from ref 112. Copyright 2011 Royal Society of Chemistry. To further improve the performance of carbon allotropes, foreign atoms, such as nitrigen 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 and developped N-doped GNSs that was prepared by heating GNSs in flowing NH3. As shown in Figure 7, the N-doped GNSs was first 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 cell. They attributed the excellently catalytic perfomance of N-doped GNSs in ORR under acidic condition to a large proportion of edge sites and pyridine-type N sites. However, further efforts to clarify the machanism of electrocatalysis were not made.112 One year later, an



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analogous mechanism called “pseudo” four electron pathway for ORR on N-doped CNTs with dislocated graphene stacking was proposed by Xing and co-workers. They prepared the nitrogen doped carbon nanotube arays (CNTAs) on carbon paper which is used as a hierarchical air cathodes in Li-air battery 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 rich graphene edges exposed and N doping. The ORR machanism proposed by this work is illustrated in Figure 8b. (A)

(B)

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 HO2intermediate which chemically disproportionated to yield OH- and O2. They were agreed with the insight of Zhou that both quaternary and pyridinic-N play a role of active sites on surface of carbon.113 The combination of CNTs and graphene oxide with oxygen-containing groups was developed by Cui and his colleagues.115 These hybrid materials were employed for ORR towards Li-air battery with aqueous electrolyte. This work also kept to 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 easy absorption of O2 onto graphene oxide with oxygen-containing group. Composite catalyst Besides N-doping and introducing Oxygen, combination of GNSs with other catalytic compound


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is considered as a effective strategy to improve performance of GNSs. Zhou prepared Fe phthalocyanine (FePc) supported by cabon materials such as GNSs, multi-wall 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 surface of all the FePc/carbon composites has a synergic effect of electrocatalytic activity and endure 4-electron process. The Li-air battery using FePc/CNTs as catalyst layer exhibited the most stable cycling perfomance.116 The same group associated the CNTs with imidazolium ion based 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)-imide to prepar a gel cathode.117 The structure and performance of hybrid electrolyte based Li-air battery with sustainable gel/solid interface is illustrated in Figure 9. As shown in Figure 9 During complete cycle, the proposed battery delivered a high capacity of 56,800 mAh g-1 for discharge as well as a high columbic eficiency 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, LiOH as well as Li2CO3. OER of the latter reactant presents much higher overpotential, which result in a lower electrical energy efficiency. This work realized a long-term reversible discharge-discharge behaviour over 100 cycles in ambient air for the first time, which has great significance in theory and application. (A)

(B)

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. Besides CNTs and GNSs, other carbon allotropes were also attempted to service as the catalyst for Li-air with aqueous electrolyte. However, several major obstacles arising from the cabonaceous air electrode such as carbon’s oxidation in both charge and discharge processes. Yamamoto and his colleagues compared the electrochemical property of various cabon materials such as Ketjen balack ACS Paragon Plus Environment


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(KB), acethlene black, vulcan X-72R, and vapor grown cabon fiber in an aqueous solution (saturated LiOH with 10 M LiCl).118 The results show that among these cabonaceous materials KB presents the best performance for ORR and OER due to its highest surface area. Through the analysis of gas chromatograph, CO was observed for OER on the cathode. They alleged that the CO gas come from carbon materials based on the following anodic reaction, which resulted in a loss of cataltic activity for OER:

C+2OH - =CO+H 2O+2e −

(R9)

Addtion of Perovskite-oxide such as La0.6Sr0.4Co0.2Fe0.8O3 to above cabon electrode was proved to be an effective method to confine carbon corrosion.119 The needs for ORR and OER catalysts are different that it is almost impossible for one material to catalyze ORR and OER processes. So, it is considered that the catalysts for charge and discharge should be designed individually.18 Manthiram and his colabrators109 imporved this strategy to construct a hybrid Li-air battery with two nanostructured catalyst electrodes countering Li anode . As seen in Figure 10a, OER and ORR air electrodes were prepared separately and incorperated in a aqueous Li-air battery. Thereinto, mesoporous NiCo2O4 nanofakes grown onto the Ni foam acts as a positive electrode for 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. This elaborately designed Li–air battery with alkaline catholytes batteries show good cycling perfomance (100 cycles, 400 hours)

(d)

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 ACS Paragon Plus Environment

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with 2 h cycle period for 100 cycles. Reprinted with permission from ref 109. Copyright 2014 Royal Society of Chemistry. Afte that, they also reported a serious of nanostructured catalysts that benificial for both the ORR and OER processes in the duouble air electrode system, such as ordered Pd3Fe intermetallic (for ORR)120 as seen in Fig. 11(a, b), Co3O4 microtrepangs directly grown on nickel foam (for OER)121 as seen in Fig. 11(c, d). Thus the double air electrodes system indeed enhance the catalytic activity of OER and ORR processes at the same time. The overpotential of charge and discharge can be lowered markedly, resulting to high energy conversion effecicency. So, development of aqueous Li-air battery with double air electrodes should be a promissing oriention for practical application.

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 microtrepans on Ni foam. (d)The discharge and charge voltage profiles of hybrid Li–air batteries with Co3O4 @Ni as OER air electrode. Reprinted with permission from ref 120. Copyright 2015 American Chemical Society.

Hybrid Electrolyte In all cases of battery, 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, organic solution such as LiPF6/carbonic ester was serviced as electrolyte for Li-ion battery due to its stability against Li anode and oxide cathode. As to hybrid electrolyte based Li-air battery, few effects have been focused on anolyte. The same organic solution as that in Li-ion battery was used as anolyte. However, in this open system,



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corrosion of Li metal with moisture, O2 and N2 entering organic electrolyte from ambient atmosphere aggravates the performance fading of battery. The ceramic plate called LISICOM film here avoids accessing of H2O and air to anolyte, which was deeply reviewed in previously paragraph. Besides organic solution and LISICON plate, aqueous solution also was employed as catholyte of hybrid electrolyte. Aqueous catholyte possesses the merits of high ion conductivity, low cost, friendly environment and solubility for discharge products. There are two kinds of aqueous catholytes classified by the alkalinity for hybrid electrolyte based Li-air battery, such as neutral or basic solution and acidic solution. Differing from exact mechanism of Li2O2 formation (shown in R1) in aprotic electrolyte, 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 R5. The excess product LiOH will be deposited and clogging the catalytic porous cathode. Moreover, the LiOH is intensely corrosive to the ceramic plat 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 following three aspects: (i) the thermodynamic potential of oxygen reduction; (ii) the catalytic activity of air catalytic electrode; (iii) solution conductivity. Zhou and his colleagues had investigated the performance of hybrid electrolyte based lithium-air battery under the mixed control of alkalinity and temperature by means of galvanistatic 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 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 both lower electrolyte resistance and operating voltage. To decrease the internal resistance of battery, LiClO4 salt was added to basic catholyte by Chen’s group, which also kept the discharge voltage due to 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 battery with acidic catholytes, the total battery reaction is:

4Li +O 2 +4HA= 4LiA+2H 2O (R10)


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Where A is a weak acid anion such as H2PO4- and CH3COO- in most Li-air battery with acidic aqueous electrolyte.87,90,124 Due to its intensely corrosive to LISICON plate, strongly acidic solution can not be employed as catholytes, although it facilitates hightening operating voltage of Li-air battery with hybrid electrolyte. Imanishi and his co-workers proposed a stable cell employing a buffer catholyte prepared from LiOAc saturated 90 vol% HOAc aqueous solution. The schematic diagram of this cell was 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 battrey can retain a rechargeable capactiy of 250 mAh g-1. They alleged that this new type of battery can deliver an exceped energy density of more than 400 W h kg-1, which is superior than conventional graphite/LiCoO2 battery.87

(a)

Figure 12. (a) Schematic diagram of the proposed lithium-air cell. (b) Charge-discharge profiles of Li/PEO/LTAP/HOAc-H2O-LiOAc/carbon air cell. Reprinted with permission from ref 87. Copyright 2010 Royal Society of Chemistry.

It is obvious contrast that the aqueous solution including solute and water in dual-electrolyte battery plays two roles of electrolyte and active reactant, while the solution in nonaqueous battery only acts as Li-ion conductive media. The extra weight of aqueous solution involving in the battery reaction inevitably limits the energy density of battery. To increase battery energy, active solute that can react with more Li and O2 based on R6 formula should be considered. As we know, weak acids usually have low solubility in water, which restricts ACS Paragon Plus Environment


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the practical capacity of the catholytes. So the strong acid with relatively high concentration is considered to be effective to enhance capacity of this type 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 facillitates increasing the battery capacity.125 Besides 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 battery.90 H3PO4 endures multistep reaction with oxygen and Li metal, and facilitates providing larger capacity. On the other hand, the ORR in 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 date suggested that the cell offered 740 mA h g-1 and good rechargeablity. Despite these achievement on proton acid solution system, the chemical stability of LISICON plate can not yet meet the requirement for prolonged use. Introducing aqueous catholyte with redox metal element, a smart and flexible configuration based on the conception of aqueous Li-air battery, Li-ion redox flow battery, was designed by Goodenough’group and Zhou’s group respetively. 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 discharge process can be described as follows: Anode: Cathode:

discharge nLi  → nLi + + e − discharge M n + (aq)+ ne −  → M ( z-n )+ (aq)

Where M represents a metal element, Mn+ is metal ion in oxidation state while discharge product M(z-n)+ is in reduction state.


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Figure 13. A concept image for a new Li-Air (O2) Battery containing Mz+/M(z-n)+ ion pairs as catalyst of O2’s reduction reaction. Reprinted with permission from ref 129. Copyright 2012 John Wiley & Sons, Inc. As demonstrated by Figure 13, in ambient air, O2 can also be employed to chemically recharge the battery by oxidizing 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 refered in our previous review article.129 In this review, we detailedly introduced the hybrid electrolyte based Li-air battery in constructure, reaction machanism and materials. To some extent, these system can be considered as the combination between Li-ion battery and aqueous fuel cell. Benefitting from the smart design, Li-air battery with hybrid electrolyte can easily circumvent Li metal corrosion, Li2O2 cloging and comparative low voltage all which are main disadvantages of nonaqueous Li-air battery. So, it is undoubted that aquesous Li-air battery is worth to be paid attentions and will play a more and more important role in future research of Li-air batteries. However, the inherent drawbacks of aqueous Li-air battery should be attracted enough importance to. Firstly, the chemical and mechanical stability of LISICON plate is bottleneck to realize practical application of aqueous Li-air battery. A plastic LISICON plate with high ion conductivity should be developped, which also has low chemical reactivity with aqueous solution. Secondly, design and preparation of carbon based composite catalyst with low cost and high effeciency should be one of the most promising ways to improve the performance of hybrid electrolyte based Li-air batteries. Especially, the promotion of catalyst for two-electron transfer reaction based on oxygen-peroxide redox couple should be paid more attentions. Last, weak acid or redox couple electrolyte is recommended to be used in aqueous catholytes. To further protect LISICON plate and recover Li resource, recycle electrolyte system like a fuel cell is considered as an effective solution. Accordingly, the advantages and challenges are coexistence in hybrid electrolyte based Li-air battery. We hope our review will draw on new developments in the field.

Acknowledgement



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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).

AUTHOR INFORMATION Corresponding Authors *Email: [email protected]; [email protected] Author Contributions P. H. and T. Z. contributed equally to this work.

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 system 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 assistant professor in Shanghai University. Since 2008, he has worked at Mie University and AIST of Japan on the research of Li-air batteries, all-solid-state Li-ion batteries, and Li-ion conducting ceramics. He is currently Professor of 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 of 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 the University of Tokyo and. His research interests include the synthesis of functional materials and their applications in lithium ion batteries, metal–air batteries and new type


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batteries/cells.

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Quotes to highlight in paper 1，， This two-electron reaction mechanism leads a rocked-chair type battery. 2，， Its Li-ion conductivity requires to be further improved by finely controlling the configuration of grain boundary. 3，， It is obvious contrast that the aqueous solution including solute and water in dual-electrolyte battery plays two roles of electrolyte and active reactant, while the solution in nonaqueous battery only acts as Li-ion conductive media.


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LithiumâAir Batteries with Hybrid Electrolytes - American Chemical

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LithiumâAir Batteries with Hybrid Electrolytes - American Chemical