Aqueous Lithium-air Batteries Research Articles

Recent progresses and challenges in aqueous lithium-air batteries relating to the solid electrolyte separator: A mini-review.

The lithium–air (Li–air) battery utilizes infinite oxygen in the air to store or release energy through a semi-open cathode structure and bears an ultra-high theoretical energy density of more than 1,000 Wh/kg. Therefore, it has been denoted as the candidate for next-generation energy storage in versatile fields such as electric vehicles, telecommunications, and special power supply. Among all types of Li–air batteries, an aqueous Li–air battery bears the advantages of a high theoretical energy density of more than 1,700 Wh/kg and does not have the critical pure oxygen atmosphere issues in a non-aqueous lithium–air battery system, which is more promising for the actual application. To date, great achievements have been made in materials’ design and cell configurations, but critical challenges still remain in the field of the solid electrolyte separator, its related lithium stripping/plating at the lithium anode, and catholyte design. In this mini-review, we summarized recent progress related to the solid electrolyte in aqueous Li–air batteries focusing on both material and battery device development. Moreover, we proposed a discussion and unique outlook on improving solid electrolyte compatibility and battery performance, thus designing an aqueous Li–air battery with higher energy density and better cycle performance in the future.

A Rechargeable Aqueous Lithium-Air Battery with an Acetic Acid Catholyte Operated at High Pressure

Acidic aqueous lithium-air batteries are attractive candidates for use as energy sources in electric vehicles because of their high energy and power densities and ability to operate under ambient conditions. An aqueous lithium-air battery with an acetic acid catholyte has a high theoretical energy density of 1,478 Wh kg–1, and the reaction product is soluble in the catholyte. In this study, we have studied the cell performance of a Li/interlayer electrolyte/NASICON-type solid lithium-ion conductor/acetic acid catholyte/air electrode cell at high pressure and room temperature under air. The cell was successfully operated at 0.5 mA cm–2 and 0.2 MPa.

Aqueous Lithium--Air Batteries with High Power Density at Room Temperature under Air Atmosphere

Rechargeable batteries with higher energy and power density exceeding the performance of the currently available lithium-ion batteries are suitable for application as the power source in electric vehicles (EVs). Aqueous lithium-air batteries are candidates for various EV applications due to their high energy density of 1910 Wh kg-1. The present study reports a rechargeable aqueous lithium-air battery with high power density at room temperature. The battery cell comprised a lithium anode, a non-aqueous anode electrolyte, a water-stable lithium-ion-conducting NASICON type separator, an aqueous catholyte, and an air electrode. The non-aqueous electrolyte served as an interlayer between the lithium anode and the solid electrolyte because the solid electrolyte in contact with lithium was unstable. The mixed separator comprised a Kimwipe paper and a Celgard polypropylene membrane for the interlayer electrolyte, which was used for preventing the formation of lithium dendrites at a high current density. The proposed aqueous lithium-air battery was successfully cycled at 2 mA cm-2 for 6 h at room temperature under an air atmosphere.

Water-Stable High Lithium-Ion Conducting Solid Electrolyte of Li1.4Al0.4Ge0.2Ti1.4(PO4)3–LiCl for Aqueous Lithium-Air Batteries

Aqueous lithium-air batteries are one of the most promising batteries for electric vehicles because of its high energy and power density. The battery system consists of a lithium anode and an aqueous solution catholyte, which are separated by a water-stable lithium-ion conducting solid electrolyte, and an air electrode. The theoretical energy density of this system is 1910 Wh kg-1, which is around 5 times higher than that of conventional lithium-ion batteries. A key component of this system is the water-stable lithium-ion conducting solid electrolyte. In this work, we have developed a water-stable and water-impermeable solid electrolyte with a high lithium-ion conductivity of around 10-3 S cm-1 at room temperature by the addition of epoxy resin and LiCl into a tape-cast NASICON-type Li1.4Al0.4Ge0.2Ti1.4(PO4)3 film. The aqueous lithium-air battery with the solid electrolyte separator was successfully cycled at 0.5 mA cm-2 and 25 ℃ in an air atmosphere.

Ambient Air Operation Rechargeable Lithium-Air Battery with Acetic Acid Catholyte

Lithium-air batteries are expected as next-generation secondary batteries for electric vehicles because of their high energy density. In particular, an aqueous lithium-air battery that uses an aqueous electrolyte has advantages such as a high power density and availability of operation under an air atmosphere. Here, we show the feasibility of an acidic aqueous lithium-air battery that consists of a lithium anode, a lithium-ion conducting liquid interlayer, a solid lithium-ion conductor separator of Li1.4Al0.4Ge0.2Ti1.4(PO4)3, an acetic acid catholyte, and a fuel cell-analogous air electrode. The theoretical energy density of this system based on the masses of the lithium anode, oxygen, and acetic acid is 1340 Wh kg−1. This system was successfully cycled at 0.2 mA cm−2 for 5 h polarization and room temperature under an air atmosphere for 30 cycles.

Recovery Li/Co from spent LiCoO2 electrode based on an aqueous dual-ion lithium-air battery

While lithium-ion batteries dominate the field of electronic devices and vehicles, recovery of LiCoO2 from spent batteries becomes a significant process. Aqueous lithium-air battery and dual-ion battery were blended into a novel electrochemical system as dual-ion lithium-air battery (DLAB) system to recover Li and Co elements from the spent LiCoO2 (acid solution). In the system, oxygen, LiCoO2 acid solution and Li2Ti2(PO4)3 (LTPO), which has Na+ superionic conductor structure and is synthesized by sol-gel method, were used as cathode, dual-ion electrolyte and anode. Relatively low voltage plateaus, stability and insolubility of LTPO make it to be a feasible candidate for this aqueous DLAB. By applying Pt/C as a catalyst for cathode, the full discharged system exhibits a Co ion recovering rate of up to 96.2%. Moreover, this system presents positive discharge plateaus at around 0.47 V and 0.10 V during recovering, demonstrating an output energy density of 43.88 Wh kg−1. The system shows a green and promising way for the recovery of spent LiCoO2 battery.

Electrical and mechanical properties of water-stable NASICON-type Li1+xAlxGe0.2Ti1.8-x(PO4)3

Water-stable and high lithium-ion conductivity solid electrolytes have potential for application as the separator in aqueous lithium-air batteries and as the electrolyte in all-solid-state batteries. Herein we have systematically examined the electrical and mechanical properties of water-stable NASICON-type Li1+xAlxGe0.2Ti1.8-x(PO4)3 (LAGTP; x = 0.25–0.55) sintered at 900–1000 °C. The highest lithium-ion conductivity of 1.58 × 10−3 S cm−1 at 25 °C was observed for Li1.35Al0.35Ge0.2Ti1.45(PO4)3 sintered at 950 °C, the conductivity of which is the highest in the NASICON-type lithium-ion conductivity solid electrolytes reported previously. The high conductivity could be explained by the high grain boundary conductivity of 6.58 × 10−3 S cm−1 at 25 °C. The conductivity showed no effect of aging after more than one month immersed in saturated LiCl aqueous solution. A 3-point bending strength of 110 N mm−2 was also recorded for Li1.35Al0.35Ge0.2Ti1.45(PO4)3 sintered at 950 °C.

Lithium-ion conduction of Li1.4Al0.4Ti1.6(PO4)3-GeO2 composite solid electrolyte

The NASICON-type lithium-ion conducting Li1+xAlxTi2−x(PO4)3 solid electrolytes are attractive candidates as the separator for high energy density aqueous lithium-air batteries and all-solid-state batteries because they are stable in aqueous solutions with a high content of lithium ions. To improve the lithium-ion conductivity of 6.0 × 10−4 S cm−1 at 25 °C for Li1.4Al0.4Ti1.6(PO4)3 (LATP), the additions of metal oxides of GeO2, TiO2, and Al2O3 were examined. The highest lithium-ion conductivity of 1.15 × 10−3 S cm−1 at 25 °C was observed for LATP-3 wt% GeO2 sintered at 950 °C. This enhancement is explained by enhancements in both bulk and grain boundary conductivity.

Aqueous lithium-air batteries with a lithium-ion conducting solid electrolyte Li1.3Al0.5Nb0.2Ti1.3(PO4)3

A rechargeable aqueous lithium-air battery with a water-stable high lithium-ion conductivity and an excellent bending strength solid electrolyte as a separator between the lithium metal anode and aqueous electrolyte was developed. The cell consists of a lithium metal anode, a buffer layer of 4 M lithium bis(fluorosulfonyl)imide in ethylene glycol dimethyl ether, a lithium-ion conducting solid electrolyte separator of Li1.3Al0.5Nb0.2Ti1.3(PO4)3 (LANTP), a 10 M LiCl aqueous solution with 1.5 M LiOH, and an activated MnO2 air electrode. The tape-cast water-impermeable LANTP film with a small amount of epoxy resin was used as the separator between the lithium anode and aqueous electrolyte. The conductivity and 3-point bending strength of the ca 0.2 mm thick film was 9.1 × 10−4 S cm−1 at 25 °C and 100 MPa, respectively. The cell with the LANTP film was operated successfully at 0.26 mA cm−2 for more than 30 cycles at room temperature in air.

Study of the effect of positive electrode structure in an aqueous lithium air battery on its performance

Study of the effect of positive electrode structure in an aqueous lithium air battery on its performance

Oxygen Concentration Measurement in the Porous Cathode of Aqueous Lithium–Air Battery By Fine Optical Fiber

Spatial and temporal oxygen concentration distribution in the porous cathode of discharging aqueous lithium-air secondary battery was successfully measured. Lithium-air battery attracts great attention because of its high energy density. In this study, we focus on the aqueous lithium–air secondary battery. The aqueous electrolyte realizes cost reduction and ensures the safe use of a battery. On the other hand, low solubility of oxygen in aqueous electrolyte limits oxygen transport to the reaction site. To realize a more powerful aqueous lithium–air battery, it is necessary to clarify and improve the oxygen transport phenomena in the porous cathode. In this study, we developed the measurement technique of oxygen concentration in the porous cathode under operating condition by inserting a fine optical fiber oxygen sensor. In this study, platinum tetrakis pentrafluoropheny porphine (PtTFPP) was employed as oxygen indicator [1]. When the PtTFPP is exposed to excitation light (λ = 405 nm), phosphorescence emission (λ= 650 nm) is produced, and its intensity depends on oxygen partial pressure. Thus, quantitative oxygen concentration can be obtained by measuring phosphorescence intensity and using calibration data. The PtTFPP was painted at the edge of optical fiber of which diameter was 110 μm. In order to measure the phosphorescence intensity, bifurcated optical fibers were connected to excitation light source and spectrometer, respectively. As shown in Figure 1, phosphorescence intensity changed at 650 nm depending on oxygen concentration. The experimental setup is shown in Figure 2. The porous cathode (SIGRACET® 10AA, SGL Group, USA) was fixed near electrolyte interface horizontally. The tiny optical fiber sensor was moved up and inserted in the cathode. The temporal oxygen concentration change from the start of discharge was measured at arbitrary positions in the porous cathode. The composite anode was water-stable and had a multilayer structure for the aqueous lithium air battery [2]. Measurement of spatial and temporal distribution of oxygen concentration has been conducted under the assumption that discharge experiment had reproducibility. Initial position (z = 0 μm) was defined arbitrary, and the sensor was moved up each 70 μm. Time variation of oxygen concentration was measured at each point. Finally, spatiotemporal distribution of oxygen concentration for each current density was obtained. As shown in Figure 3, the spatial and temporal oxygen concentration distribution at 0.10 mA/cm2 was successfully observed. When the discharge was started, the oxygen concentration gradually decreased in the cathode, and finally, oxygen concentration reached considerably low value even in the condition of 0.10 mA/cm2. In this experiment, oxygen concentration gradients are present at both ends of the atmosphere side (0-140 μm) and electrolyte side (490-910 μm) of the cathode. Flux for both side of cathode, upper side and lower side, was discussed and comprehensible interpretation was obtained. Although dissolved oxygen that forms flux from the lower side of cathode has an effective supply to the porous electrode, electrolyte layer formed at between the atmosphere and the cathode causes oxygen transport resistance. Acknowledgement: This work was supported by JSPS KAKENHI Grant Number 15K13881, 15H02347. Reference: [1] CS Chu., CA Lin, Sensors and Actuators B: Chemica, 2014, 195, 259-265. [2] T. Zhang, N. Imanishi, S. Hasegawa, A. Hirano, J. Xie, Y. Takeda, O. Yamamoto, N. Sammes, J. Electrochem. Soc., 2008, 155, A965. Figure 1

Hybrid Li Ion Conducting Membrane as Protection for the Li Anode in an Aqueous Li-Air Battery: Coupling Sol-Gel Chemistry and Electrospinning.

Aqueous lithium-air batteries have very high theoretical energy densities, which potentially makes this technology very interesting for energy storage in electric mobility applications. However, the aqueous electrolyte requires the use of a watertight layer to protect the lithium metal, typically a thick NASICON glass-ceramic layer, which adds ohmic resistance and penalizes performance. This article deals with the replacement of this ceramic electrolyte by a hybrid organic-inorganic membrane. This new membrane combines an ionically conducting inorganic phase for Li ion transport (Li1.3Al0.3Ti1.7(PO4)3 (LATP) and a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) polymer for water tightness and mechanical properties. The Li ion transport through the membrane is ensured by an interconnected 3-D network of crystalline LATP fibers obtained by coupling an electrospinning process with the sol-gel synthesis followed by thermal treatment. After an impregnation step with PVDF-HFP, hybrid membranes with different volumetric fractions of PVDF-HFP were synthesized. These membranes are watertight and have Li ion conductivities ranging from 10-5 to 10-4 mS/cm. The conductivity depends on the PVDF-HFP volume fraction and the fibers' alignment in the membrane thickness, which in turn can be tuned by adjusting the water content in the electrospinning chamber during the process. The alignment of fibers parallel to the membrane surface is conductive to poor conductivity values whereas a disordered fiber mat leads to interesting conductivity values (1 × 10-4 mS/cm) at ambient temperature.

High lithium-ion conducting NASICON-type Li1 + x − yAlxNbyTi2 – x − y(PO4)3 solid electrolytes

A water-stable lithium-ion conducting solid electrolyte with high conductivity and excellent mechanical properties has potential application for aqueous lithium-air batteries. In this study, a water-stable NASICON-type lithium-ion conducting solid electrolyte with a nominal composition of Li1+x−yAlxNbyTi2−x−y(PO4)3 was synthesized with variation of the Al and Nb contents using a conventional solid-state reaction method, and the electrical conductivity and mechanical properties were examined. The highest lithium-ion conductivity of 7.5×10−4Scm−1 at 25°C and the highest three-point bending strength of 110Nmm−2 at room temperature were observed for Li1.30Al0.5Nb0.2Ti1.30(PO4)3 and Li1.35Al0.55Nb0.2Ti1.25(PO4)3, respectively. Li1.30Al0.5Nb0.2Ti1.30(PO4)3 was stable in a saturated LiOH aqueous solution with saturated LiCl.

Electronic Coupling of Cobalt Nanoparticles to Nitrogen-Doped Graphene for Oxygen Reduction and Evolution Reactions.

The rational design of nonprecious-metal electrocatalysts with activities comparable to or greater than that of platinum is extremely valuable to the development of high energy density metal-air batteries. Herein, the two-step preparation of a highly active oxygen electrocatalyst based on ultrasmall cobalt nanoparticles stabilized in a nitrogen-doped graphene matrix is reported. The catalyst performs as well as the commercial Pt/C catalyst in the oxygen reduction reaction, and better than the Pt/C catalyst in the oxygen evolution reaction. This particular electrocatalyst could significantly lower the overpotentials of oxygen electrochemical reactions in aqueous lithium-air batteries to attain a round-trip efficiency of about 79.0 % at a current density of 0.1 mA cm-2 , thereby surpassing the performance of the commercial Pt/C catalyst. The good performance may be attributed to strong metal-support interactions, maximized by a high dispersion of ultrasmall cobalt nanocrystals in a nitrogen-doped graphene matrix, which yields electrocatalytic properties greater than the sum of its parts.

(Battery Division Research Award) Aqueous Lithium-Air Batteries with Protected Lithium Electrode

Lithium-air battery system consists of a lithium metal anode, an electrolyte, and a carbon-air cathode. Two types of lithium-air batteries have been developed based on the electrolyte type; non-aqueous and aqueous. The aqueous system was firstly proposed by Visco et al in 2004. This system consists of a lithium metal protected by a water-stable lithium conducting solid electrolyte, aqueous electrolyte, and a carbon air electrode. The cell reaction is 4 Li + 6H2O + O2 = 4(LiOH•H2O) Water molecules are involved in the reaction and the theoretical cell voltage in an alkaline electrolyte is ca. 3.0 V, which is dependent on the pH value of the electrolyte. The energy density of the aqueous system is 1910 Wh kg-1 including oxygen. Materials development is critical to accomplish aqueous lithium-air rechargeable batteries. One requirement is to develop a protected lithium electrode, which can perform a lithium stripping/deposition with a long lifespan in aqueous electrolyte. The key point of the electrode is in adopting a NASICON-type lithium conducting solid electrolyte, Li1+x+yAlxTi2-xP3-ySiyO12, (LATP), as the protect layer which covers and isolates lithium metal from contacting directly with aqueous electrolytes. LATP is unstable in a direct contact with lithium metal, so that a buffer layer should be placed between lithium metal and LATP. In the early stage of our study, polymer electrolyte was adopted for the buffer layer. The interfacial resistance between lithium metal and polymer electrolytes has been demonstrated a key factor in initiating lithium dendrite formation by addition of ionic liquids, oligomer ethers and/or nanofillers. All these additives can reduce the interfacial resistance substantially and prolong the onset time of lithium dendrite initiation. One main approach to enhance electrode kinetics is to use ether-oligomer solvent without polymeric counterpart. A novel electrolyte prepared by the complexation of lithium cations supplied by lithium bis(fluorosulfonyl)imide and tetraethylene glycol dimethyl ether with the co-solvent of 1,3-dioxolane give metallic lithium anodes with high specific areal capacity in the absence of lithium dendrite formation. Along with this buffer material, lithium ion conducting solid electrolyte that is stable in aqueous solution is another key component of aqueous lithium-air batteries. Two types of solid electrolytes, NASICON-type LATP and garnet-type Li7La3Zr2O12, have been a candidate for the protecting material, since they show high lithium ion conductivity at room temperature and chemical stability in a highly concentrated aqueous solution of lithium ions. For its practical applications the solid electrolyte film thickness must be minimized to obtain a high gravimetric and volumetric energy density, while higher ionic conductivity is required to reduce ohmic resistance. We have developed a solid electrolyte thin film using a tape casting method. In addition to the detailed studies of those materials, other important materials about carbon air cathode, mechanisms, and the direction of future development will be mentioned in this presentation.

Rechargeable Aqueous Lithium-Air Batteries

The lithium-air battery is a promising power source for electric vehicles because of its high theoretical specific energy density. Two types of the lithium-air battery are proposed, namely, non-aqueous and aqueous systems [1]. The theoretical specific energy densities of the non-aqueous and aqueous systems are 3458 and 1910 Wh kg-1, respectively. However there are many changeable research drawbacks that must still be addressed. At present, non-aqueous lithium air battery is mainly focused, but this system has severe problems such as lithium corrosion by water, high polarization during charge process, etc. The aqueous lithium-air system could avoid these problems by using water stable lithium electrode [2] and aqueous electrolyte. In this study, we demonstrate the cell performance of the aqueous lithium-air system. The laminate type cell was tested at room temperature. The cell was consisted of Li/interlayer/water stable lithium conducting solid electrolyte/aqueous electrolyte/air electrode. As the interlayer, a non-aqueous electrolyte such as Li(FSO2)2N in glyme [3] was used, because the water stable lithium conducting solid electrolyte of Li1+xAlxTi2-x(PO4)3 (LATP) is unstable in contact with lithium. The LATP plate of ca. 0.15 mm in thickness was supplied by Ohara Inc. Japan. The platinum catalyst coated carbon sheet for a large size cell and the composite electrode of carbon black (KB) supported MnO2 catalyst for a small size cell were used as the air electrodes, respectively. The cells were operated under open air. The impedance profiles of the test cells with different interlayer electrolytes of glyme were examined. The profiles showed two semi-circles. The high frequency semi-circle showed no dependence on the character of the interlayers, but the low frequency semi-circle showed dependence on it. The high frequency semi-circle corresponds to the grain boundary resistance of the LATP plate and the second semi-circle to the interlayer resistance between lithium and interlayer electrolyte. The lowest cell resistance of ca. 270 Ω cm2 at room temperature was observed for the Li/DME-LiTFSI/LTAP/saturated LiCl aqueous solution/C cell. A large size cell of 6.2 Ah was successfully discharged at 50 °C with 2 mA cm-2 for 150 h. The initial cell voltage of 3.0 V was gradually decreased to 2.7 V after 150 h polarization. A cycle performance of the small size cell was examined at room temperature. For a single cycle, the charge-discharge test was performed with 2 mA cm-2 for 4 h. The discharge and charge voltages were not changed significantly for 100 cycle and 400 h. In conclusion, the aqueous lithium air battery was operated with high current density at room temperature under open air. These results suggest that aqueous lithium-air rechargeable batteries have a possibility for further development with high specific energy and power density.

High Lithium-Ion Conducting Nasicon-Type Li1+x-YAlxNbyTi2-x-Y(PO4)3 Solid Electrolyte

Recently, the demand of a high energy density battery for electric vehicles (EVs) has been significantly increased, because the driving range of EVs with conventional lithium-ion batteries is too short compared with vehicles with internal conversion engines. Lithium-air batteries are attractive candidate for the high specific energy density batteries, because of their high theoretical energy density as 3560 Wh kg-1 for the non-aqueous and 1910 Wh kg-1 for the aqueous system. Non-aqueous systems have some severe problems that still need to be addressed such as decomposition of the electrolyte by the reaction product, lithium corrosion by water, and high polarization for reduction of the reaction product of Li2O2. While, some of these problems are not appreciable to the aqueous system. The aqueous system cannot operate without a water stable lithium metal electrode, because lithium reacts violently with water. The idea of the water stable lithium electrode was proposed by Visco et al. in 2004 [1]. The lithium metal was separated with an aqueous electrolyte by the water stable NASICON-type lithium conducting solid electrolyte of Li1+XAlxTi2-x(PO4)3. In the last half century, many types of high lithium-ion conducting solid electrolytes have been reported, the conductivities of which are in a range of 10-4-10-2 S/cm at room temperature. Of them, Li1+xAxTi2-x(PO4)4 and Li7La3Zr2O12 were reported to be stable in the LiCl saturated aqueous solution. The stability in the aqueous solutions is essential for the lithium protect solid electrolyte in aqueous lithium-air batteries. The Ta doped Li7La3Zr2O12 exhibited a high lithium ion conductivity of 1x10-3 S cm-1 at 25 °C and is stable in contact with lithium metal, but it is difficult to prepare a thin film, because of lithium evaporation during the high temperature sintering. The thin film of Li1+xAxTi2-x(PO4)4 was prepared by a tape casting method and the ion conductivity of the Li1.4Al0.4Ti1.6(PO4)3-3wt% TiO2 film was 7.7 x10-4 S cm-1at 25 °C [2]. . Since the report by Aono et al. in 1989 [3], the NASICON-type lithium-ion conducting solid electrolytes have been extensively examined. Zhang et al. reported that the electrical conductivity of Li1+xAlxTi2-x(PO4)3 was enhanced by a partial substitution of Ge, Co, and Fe for Ti. The highest ion conductivity of 1.3×10-3 S cm-1 at 25 °C was observed in Li1.4Al0.4Ge0.2Ti1.4(PO4)3. In this study, we have examined dependence of the substitution of Nb5+ for Ti4+ in Li1+xAlxTi2-x(PO4)3 on the electrical conductivity and mechanical properties. Ionic radius of Nb5+(0.064 nm) is slightly higher than that of Ti4+ (0.061 nm) and higher than that of Ge4+ (0.053 nm). Nb2O5 is considerably cheaper than GeO2. Li1+x-yAlxNbyTi2-x-y(PO4)3 was synthesized in a range of x=0-0.60 and y=0-0.4 using a conventional solid-state reaction method. The lithium-ion conductivity of 7.5×10-4 S cm-1 and three-point bending strength of 67 N mm-2 for Li1.3Al0.5Nb0.2Ti1.3(PO4)3 and the lithium-ion conductivity of 3x10-4 S cm-1 and the three-point bending strength of 120 N mm-2 for Li1.35Al0.55Nb0.2Ti1.25(PO4)3at room temperature were observed. The water-stable high lithium-ion conducting solid electrolytes with excellent mechanical properties have potential applications for lithium-air batteries.

Effects of O2 Concentration and Precipitates on the Lithium Air Battery

Lithium-air battery attracts great attention because of its high energy density. In this study, we focus on the aqueous lithium–air secondary battery. The aqueous electrolyte realizes cost reduction and ensures the safe use of a battery. However, to realize a more powerful lithium–air battery, it is necessary to clarify and improve the reaction and mass transport phenomena in the cathode [1]. Low solubility of oxygen in aqueous electrolyte limits oxygen transport to the reaction site. Discharge reaction products in cathode electrode cause cell performance degradation. In this study, effect of dissolved oxygen concentration in the electrolyte on cathode performance is investigated. Low energy X-ray CT was employed to observe discharge reaction products in a porous cathode electrode.Figure 1 shows experimental apparatus, which is composed of two main parts: a beaker cell and control section for oxygen concentration in the electrolyte, respectively. The cell is consisted from composite anode, cathode, and reference electrode. The composite anode was water-stable and had a multilayer structure for the aqueous lithium air battery [2]. LATP [Li1+x+yAlxTi2-xSiyP3-y] (Ohara Co., Kanagawa, Japan), which is a water-stable lithium-ion-conducting glass ceramic, was used, along with PEO18LiTFSI [poly (ethylene oxide) with lithium bis (trifluoromethanesulfonyl) imide] as the organic electrolyte. Because the cell resistance was drastically decreased [3], beaker cell was heated in the constant temperature bath to keep the cell temperature at 60°C. For the cathode, SIGRACET® 10AA (SGL Group, USA) was used as the carbon porous layer with no catalyst. Reaction area of anode/cathode was f4 mm. In order to control the dissolved oxygen concentration in electrolyte (1M LiCl aq.), a micro-bubble generator (Nagoya Oshima machinery Co.,Ltd.) was employed. Acceleration of dissolution rate and the overconcentration condition were achieved by generating oxygen micro-bubbles. Normal dissolved oxygen concentration was approximately 6.5 mg/L under 60 °C heating condition. On the other hand, the generator realized high concentration up to 15 mg/L.Figure 2 shows cathodic polarization curves. The cathode performance was improved by using high oxygen concentration electrolyte. However, performance degradation was still caused under high current density condition. When the cathode potential excesses approximately -1.0 V, plot trend of all results changes and shows same plateau. It means that the discharge reaction on the cathode was shifted from oxygen reduction to hydrogen evolution reaction under high current density condition [4].In order to observe precipitate of discharge products in the cathode, low energy X-ray CT and visualization cell as shown in Figure 3. By using closed type cell and aqueous solution (saturated LiOH with 10 M LiCl), experimental condition mimicked less oxygen condition involving precipitation of discharge products. As shown in Figure 4, emergence of bubble and solid precipitate were clearly visualized by low energy X-ray CT. It is considered that emergence of bubble and solid precipitate depended on oxygen concentration. Solid precipitate (Li2O2 [5]) was emerged at the bottom part of cathode where oxygen was able to reach by diffusion. On the other hand, oxygen was not transported efficiently to the top part of cathode, and discharge reaction was shifted. As result, electrolysis of water (H2bubble generation) has occurred.In order to realize higher performance under high current density condition, electrolyte was flowed through the porous cathode. As shown Figure 5, cathode performance was significantly improved. It is considered that oxygen concentration overpotential was reduced by supplying oxygen rich electrolyte. AcknowledgementThis work was supported by JSPS KAKENHI Grant Number 15K13881, 15H02347. Reference[1] K. Shibata, S. Uemura, S. Tsushima, S. Hirai, ECS Trans. 2015, 64(19), pp 39-46.[2] T. Zhang, N. Imanishi, S. Hasegawa, A. Hirano, J. Xie, Y. Takeda, O. Yamamoto, N. Sammes, J. Electrochem. Soc., 2008, 155, A965.[3] T. Zhang, N. Imanishi, A. Hirano, Y. Takeda, and O. Yamamoto, Electrochem. Solid-State Lett., 2011, 14, A45.[4] H. He, W. Niu, N. M. Asl, J. Salim, R. Chen, Y. Kim, Electrochimica Acta, 2012, 67, 87.[5] M.Matsui, A. Wada, Y. Maeda, H. Ohkuma, O. Yamamoto, N. Imanihsi, 224th ECS meeting abstract, 2013, MA2013-02, 411. Figure 1

Development of Materials for Aqueous Lithium Batteries

A reversible high-areal-capacity metallic lithium electrode (MLE) is pivotal to develop protected lithium electrodes (PLE) for aqueous lithium-air batteries, whose practical specific energy strongly depends on the realization of the high-areal-capacity of reversible MLEs as well as the sustainable unclosed air-breathing cathode/catholyte couple. Recent advances of reviving MLE studies further shed new light on the key role of electrolytes (i.e., the anolyte in the PLE part). [1] In this study, a novel electrolyte has been prepared by the complexation of lithium cations supplied by lithium bis(fluorosulfonyl)imide (LiFSI) and tetraethylene glycol dimethyl ether (G4) with the additive of 1,3-dioxolane (DOL). [2] The matrix has been disclosed to enable MLE to deliver a high-areal-capacity at elevated temperature in our latest publication. The composition of LiFSI-2G4 (the molar ratio of G4 to Li+ is 2) is found to be a solvate ionic liquid (SIL) with appealing attributes of suppressing lithium dendrite formation at 60 oC, possibly due to the fact that the high fraction of surfactant-role G4 in the solid-electrolyte interphase (SEI) film and elevated temperature could lower the Ehrlich-Schwoebel barrier to facilitate adatoms migration down the descending step that favor two-dimensional layer-by-layer lithium deposition mode.[3] To make this SIL work at ambient temperature, decreases in the viscosity and interfacial resistance are of great importance. The addition of DOL into LiFSI-2G4 has been demonstrated to lead to the decrease in the viscosity but the absence of alteration of the solvation manner of Li+.[4] Herein, we first unveil the significant impact of the introduction of cyclic voltammetry (CV) modulation, prior to the lithium plating onto Cu or Li working electrode, on lithium plating process and surface film chemistry. With CV modulations, a lithium/lithium cell can be cycled at 5.0 mA cm-2 for 100 cycles, and a copper/lithium cell can be cycled at 5.0 mA cm-2 for 80 cycles with a coulombic efficiency (CE) of 100%, where both cells exhibit the exceptional high-areal-capacity of 12 mAh cm-2 at 25 oC. The combination of a novel SIL and CV modulations paves a new avenue to developing practical MLEs and thus PLEs. Figure (a) shows the CV measurements of Li/LiFSI-2G4-50 vol% DOL/Cu exhibited a sudden change in the 6th scan, where the onset potential for lithium deposition (Eld) fell from the initial -0.153 V to -0.083 V and substantial increases in the peak current density for lithium deposition and stripping. The decrease of Eld continued with the sweeping but got into a steady value of -0.042 V until the 23rd scan as shown in Figure (b). Current density of 5 mA cm-2 occurred at -0.283 V, -0.139 V, -0.070 V in the 5th, 6th, 30th cathodic sweep, respectively, which suggests that the lithium deposition onto Cu substrate is supposed to evolve faster with the cycle number. Other electrolytes (e.g. LiFSI-2G4, LiFSI-G4, LiTFSI-2G4, LiFSI-2G4-50 vol% G4, 1M LiPF6-EC-DEC, LiFSI-5G4) were also investigated in the same condition. Only the electrolyte of LiFSI-2G4 was found to exhibit the comparable descending trend of Eld but much lower degree of the increase in the Ip. Under the CV pre-modulation, the CE measurement of Li/LiFSI-2G4-50 vol%/Cu cell was performed at 5.0 mA cm-2 and 25 oC with a constant lithium deposition capacity of 12 mAh cm-2 as shown in Figure (c). Surprisingly, the CE of 100% can be sustained up to 80 cycles. In the same condition, the cell of Li/Li can be cycled for 100 cycles without short-circuit by the CV pre-modulation and modulation during the intervals after each half-cycle. To the best of our knowledge, MLE is first reported to achieve a high specific areal-capacity of 12 mAh cm-2 at 5 mA cm-2 and ambient temperature by the combination of a novel SIL and the CV modulation technique. Details of this study will be presented in a paper under the submission. [1] J. Qian, J.-G. Zhang et al., Nat Commun 2015, 6; b) Y. Lu, L. A. Archer et al., Nat Mater., 2014, 13, 961-969; c) W. Li, Y. Cui et al., Nat Commun., 2015, 6. [2] D. Aurbach, J. Power Sources., 2000, 89, 206-218. [3] a) H. Wang, N. Imanishi et al., ChemElectroChem 2015, 2, 1144-1151; b) Q. Chen, K. Geng, K. Sieradzki, J. Electrochem. Soc., 2015, 162, A2004-A2007. [4] H. Wang, N. Imanishi et al., in The 56th Battery Symposium in Japan, Nagoya, 2015, 3A03. Figure 1

High Lithium-Ion-Conducting NASICON-Type Li1+xAlxGeyTi2−x−y(PO4)3 Solid Electrolyte

A water-stable solid electrolyte is a key material without which aqueous lithium-air batteries could not be operated. In this study, we have examined the electrical conductivity and mechanical properties of a water-stable lithium ion conducting solid electrolyte, Li1+xAlxGeyTi2-x-y(PO4)3 with the NASICON-type structure, as a function of the Al and Ge content. Li1+xAlxGeyTi2-x-y(PO4)3 was synthesized by the conventional solid-state reaction method. The highest lithium-ion conductivity of 1.0×10-3 S cm-1 at 25 °C and the highest three-point bending strength of 90 N mm-2 at room temperature were observed for a pellet of Li1.45Al0.45Ge0.2Ti1.35(PO4)3 sintered at 900 °C.