Lithium Air Secondary Batteries Research Articles

Electrochemical Characteristics of Lithium-Air Secondary Battery Using Amide-Based Ionic Liquids

Electrochemical Characteristics of Lithium-Air Secondary Battery Using Amide-Based Ionic Liquids

Electrochemical Properties of Lithium-Air Secondary Batteries with Catalyst-Modified Carbon Electrodes

Introduction Lithium-air batteries (LABs) are one of the promising post lithium-ion battery technologies owing to its remarkably high theoretical specific energy density. Low energy efficiency and poor cyclability due to large charging overpotential should be addressed for realizing practical LABs. Recently, it was reported that the overpotential especially during charging is greatly reduced and thus the cyclability is significantly improved by modification of air electrodes with a noble metal catalyst such as Pt or Ru 1). An appropriate base metal-based electrode catalyst, e.g. Co3O4 and MnOx 2), is also capable to effectively reduce the overpotential. In this study, the base metal-based ternary (Fe-Co-Ni) and binary (Fe- Ni, Fe-Co, and Co-Ni) alloy catalysts were prepared. Their catalytic activities on the discharge-charge reactions were evaluated and compared with the RuO2 catalyst. Experimental The base metal alloy-modified carbon, Fe-Co-Ni/KB, Fe-Ni/KB, Fe-Co/KB, and Co-Ni/KB (KB: Ketjen black), were prepared via the impregnation process. RuO2/KB was also prepared by a similar method. The catalyst-modified carbon and polyvinylidene fluoride were mixed together with N-methyl-2-pyrrolidone solution. The obtained slurry was coated on the carbon paper and dried in vacuum at 80 oC overnight. The coin-type cell of CR2032 consisting of the carbon cathode, lithium metal anode, and 1 mol dm-3 LiTFSA/tetraglyme electrolyte was assembled in an Ar filled glovebox. For the electrochemical measurement, the discharge-charge tests were carried out in dry air atmosphere at 25 oC in the voltage range of 2.0 - 4.3 V at the current density of 50 μA cm-2. Results and Discussion Figure 1 shows the 1st and 20th discharge-charge curves of the coin cells with the prepared catalyst-modified carbon electrodes. The discharge behavior was similar, ca. 2.7 V of stable plateau observed, regardless of the carbon electrodes used. On the other hand, a remarkable difference was observed in the charging process, depending on the type of the catalysts. The RuO2 catalyst effectively suppresses the charging overpotential over 20 cycles. The overpotential of the Fe-Ni/KB electrode was also small during the initial charging stage at the 1st cycle, while the catalytic activity of this catalyst would be almost completely diminished after 20 cycles. On the other hand, the charging overpotential of the other electrodes with Co was very large irrespective of the cycle number. As the ternary Fe-Co-Ni catalyst is found to be not contributed to decreasing charging overpotential, the catalytic activity of Co would be considerably small and/or Co would impede the catalytic activity of other metals. Unfortunately, the air electrodes are rather spoiled by the presence of the base-metal based catalysts, probably due to oxidation of the catalysts during charging process. Improvement in catalyst durability is pivotal to achieve the cost-effective base metal-based electrode catalysts for LABs. Reference s 1) F. Wu, et al., J. Power Sources, 332, 96 (2016). 2) D. Oh, et al., J. Phys. Chem. C, 121, 1407 (2017). Acknowledgments This research was partly supported by ALCA-SPRING project of Japan Science and Technology Agency (JST). Fig.1. Discharge-charge curves at the 1st (left) and 20th (right) cycles of the metal-modified carbon electrodes. current density: 50 μA cm-2, loading density of active materials: 0.5 mg cm-2. voltage range: 2.0 - 4.3 V, discharge-charge time: 2 h, temperature: 25 ° C, atmosphere: dry air. Figure 1

Electrochemical Activities for Reduction and Evolution of Oxygen in Nonaqueous Electrolyte Solutions Containing Manganese Salen Complex As an Additive

Introduction Lithium air secondary batteries (LABs) have attracted attention due to their high theoretical energy density. One of the major problems is their poor cycle characteristics, which is caused by the decomposition of electrolytes, the strength degradation of air electrodes, and so on. These are related to the large resistivity of discharge products Li2O2 deposited in the air electrodes. To solve these problems, the air electrodes of LABs have been developed by using various catalysts [1]. However, the catalysts become inactivated gradually after a large number of cycles because undecomposed Li2O2 remains on the active catalytic sites in the air electrodes even after charging processes [2]. Since additives are dissolved in an electrolyte solution, they were predicted to promote the deposition and the decomposition of Li2O2 without deactivation due to the undecomposed Li2O2. The additives are expected to overcome the problems associated with the catalyst for the air electrode. We are focusing on (R,R)-(-)-N,N-Bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) chloride (MnSl) as an electrolyte additive. In our previous work, the LAB cells with 1 mol/l LiTFSA/TEGDME containing 2 wt% MnSl exhibited large first discharge capacity of 4903 mAh/g and good cycle retention of 61% at the 10th cycle [3]. However, the details of the reaction mechanisms were still unclear. In this study, we investigated the fundamental mechanisms by evaluating the electrochemical activities for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in an electrolyte containing 0.005 wt% MnSl additives. Experimental A 0.005 wt% of MnSl (Sigma-Aldrich) vacuum-dried at 90°C for 12 h was added to 1.0 mol/l LiTFSA/TEGDME. Ketjenblack EC-600JD (KB, 80 wt%) and polyvinylidene difluoride (PVdF, 20 wt%) coated on carbon paper (Toray, TGP-H-120) were used as an air electrode. A Li metal sheet was used as a counter electrode. These components were installed in a commercial two-electrode LAB cell (ECC-Air, EL-Cell GmbH). Electrochemical experiments were performed at a current density of 200 mA/g under dry air atmosphere with a dew point of less than -50°C. Cyclic voltammetry was performed at a scan rate of 100 mV/s under O2atmosphere using the three-electrode ECC-Air cell. Platinum disks were used as working and counter electrodes. A Li metal was used as a reference electrode. Results and discussion Figure 1 shows the first discharge /charge curves of LAB cells without and with MnSl. In the discharging process, the cell with MnSl shows a higher average voltage of 2.4 V and a larger capacity of 893 mAh/g than the other cell. In the charging process, the cell with MnSl shows a lower voltage during the early process and a larger capacity of 676 mAh/g than the other cell. These results indicate that MnSl promotes electrochemical activities for both ORR and OER. Electrochemical measurements were carried out to investigate how the MnSl additives work in the electrolyte solution during ORR/OER processes. Figure 2 shows cyclic voltammograms (CVs) of the three–electrode cells without and with MnSl under O2 atmosphere. The cells without and with MnSl show a cathodic ORR peak at around 2.0 V and an anodic OER peak at around 4.4 V. The cell with MnSl shows higher current densities in the whole potential range than the cell without MnSl. In the cell with MnSl, the cathodic peak around 2.0 V for the ORR becomes larger, and the onset of that peak shifts significantly toward more positive potential. This might indicate that MnSl promoted the electrochemical activity for ORR. As a result, the average discharge voltage of the cell with MnSl becomes higher compared with the cell without MnSl, as shown in Fig. 1. In the cell with MnSl, the anodic peak for the OER also becomes larger compared with the cell without MnSl and exhibits a new peak at around 4.0 V. This peak was not observed in the CV result for the cell without MnSl. This might indicate that the MnSl interacted with Li2O2at around 4.0 V and such an interaction promoted the activities for OER at around 4.4 V The plateau voltage around 4.0 V shown in Fig. 1 might reflect the electrochemical behavior for the anodic process in the CV result of Fig. 2. Consequently, these results show a tendency similar to the charge and discharge properties shown in Fig. 1 indicating the effectiveness of MnSl as an electrolyte additive.

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

Influence of Surface Area of Mesopore on Discharge Characteristics for Lithium-Air Secondary Battery

We prepared the carbon electrodes using the CNovel carbons (CNovel P(2)010, CNovel P(4)050, and CNovel MH) as the new carbon materials and investigated the discharge characteristics of carbon electrodes for lithium-air secondary battery. The discharge tests showed that the CNovel P(2)010 electrode had the highest first discharge capacity (2.116 mAh cm-2). From the relationship between the surface area of mesopores (2-50 nm) and the first discharge capacity, the first discharge capacity increased with increasing the surface area of mesopores. The first discharge capacity was more strongly related to the surface area of mesopores than the mesopore volume. From the electrochemical impedance spectroscopy (EIS), the CNovel P(2)010 electrode showed the lowest charge transfer resistance. It was suggested that an increase in the thickness of Li2O2 layer enhanced the overpotential. It was found that the surface area of mesopores had an influence on the discharge characteristics.

Relationship Between Discharge Profile and Pore Structure for Lithium-Air Secondary Battery

We prepared the carbon electrodes using the three carbon materials (EC300J (Ketjen Black), ECP600JD (Ketjen Black), and CNovel MH) as a positive electrode, analyzed the discharge products, and investigated the factor affecting the electrochemical characteristics. The discharge tests showed that the discharge capacities of ECP600JD and CNovel MH electrodes were higher than that of EC300J electrode. The FE-SEM and EDX showed that the Li2O2 aggregation generated on the EC300JD electrode. On the other hand, the Li2O2 is highly dispersed on the ECP600JD and CNovel MH electrodes. From the nitrogen adsorption-desorption measurements, strong relationship between the discharge capacity and surface area of mesopore (3-10 nm) before discharge were confirmed. It was suggested that surface area of mesopore (3-10 nm) of carbon electrode had an influence on the discharge profile.

Highly Ordered Mesoporous Carbon Support Materials for Air Electrode of Lithium Air Secondary Batteries

Highly Ordered Mesoporous Carbon Support Materials for Air Electrode of Lithium Air Secondary Batteries

Electrochemical properties of lithium air batteries with Pt 100−x Ru x (0 ≤ x ≤ 100) electrocatalysts for air electrodes

Abstract Electrochemical properties of lithium air secondary battery cells with Pt 100− x Ru x (0 ≤ x ≤ 100) electrocatalysts, prepared by the formic acid reduction method and loaded into air electrodes were examined in 1 mol/l LiTFSA/TEGDME electrolyte solution. Among the cells, the one with the Pt 10 Ru 90 ( x = 90)/carbon sample showed the largest discharge capacity of 1014 mAh/g and the lowest average charge voltage of 3.74 V. In addition, the x = 90 sample showed comparatively good cycle stability with discharge capacity of over 800 mAh/g at the 8th cycle. As a result, x = 90 was confirmed to be the optimized composition as the electrocatalyst for the air electrode.

Preparation and Electrochemical Properties of Pt–Ru/Mn<SUB>3</SUB>O<SUB>4</SUB>/C Bifunctional Catalysts for Lithium-Air Secondary Battery

Preparation and Electrochemical Properties of Pt–Ru/Mn<SUB>3</SUB>O<SUB>4</SUB>/C Bifunctional Catalysts for Lithium-Air Secondary Battery

Bacterial-Cellulose-Derived Carbon As Air Electrode Material for Lithium Air Secondary Batteries

Introduction Lithium air secondary batteries exhibit higher theoretical energy density than lithium ion batteries and are expected as next-generation secondary batteries. However, there remain technical challenges related to their poor cycle properties.Since the first report by K. M. Abraham et al. [1], various oxygen reduction/evolution catalyst [1-3] and electrolyte [1, 4] materials have been intensively investigated for air batteries to improve their electrochemical properties, such as their cyclability. However, there have only been a few studies on support materials for air electrodes, such as nanoporous gold [5].We consider that one of the reasons for the poor cyclability is a decrease in the electrical contact of the air electrode during Li2O2 deposition (discharge) and oxygen generation (charge), which results from large the volume change. The purpose of our work is to improve the cyclability by using a new air electrode structure and material. As an elastic support material for air electrodes, we focus on carbon fabricated from bacterial cellulose (BC), which has a continuous interconnected network of carbon. Such an elastic carbon support would prevent the large volume change of the air electrodes and improve the cycle performance. Here we report the preparation conditions for BC-derived carbon supports materials and the performance of lithium air secondary batteries incorporating them. Experimental A BC sheet (Fujicco Co., Ltd.) was frozen at -40 °C and then freeze-dried [6]. The freeze-dried BC sheet was carbonized in N2 atmosphere at 600, 900, and 1200 °C to obtain BC-derived carbon. It was cut into a circle (diameter: 5 mm) to obtain the binder-free air electrode. The lithium air secondary battery consisted of the air electrode, a lithium metal sheet, and 1.0 mol/l lithium bis(trifluoromethanesulfonyl)amide (LiTFSA)/tetraethylen glycol dimethyl ether (TEGDME) as the positive electrode, negative electrode, and electrolyte solution, respectively. The battery was assembled with an ECC-Air Cell (EL-CELL GmbH). Electrochemical measurements were carried out under a galvanostatic condition of 0.1 mA/cm2 in an O2 atmosphere. The discharge and charge capacities were normalized by the weight of the BC-derived carbon (the air electrode). Results and discussion Figure 1 shows an SEM image of the BC-derived carbon prepared at 1200 °C. The continuous interconnected network of the carbon fibers with diameter of less than 10 nm can be seen in this image. These carbon fibers build a 3-D disordered macroporous framework. The BC-derived carbon prepared at 600 and 900 °C was confirmed to have morphologies similar to that at 1200 °C. Moreover, in a preliminary compression test, the BC-derived carbon showed a highly elastic compressibility and almost completely recovered its original volume after the compression was released.Figure 2 shows the first discharge/charge curves of the air batteries incorporating the BC-derived carbon prepared at 600, 900, and 1200 °C. The air batteries with the carbon prepared at 900 °C showed the largest discharge capacities of 8800 mAh/g and the average discharge voltage of 2.68 V. This is because it has the largest number of active sites among the carbons tested. The batteries incorporating BC-derived carbon prepared at 900 and 1200 °C showed almost the same charge capacities of about 8000 mAh/g. These charge properties have a tendency similar to the discharge properties even though the air electrodes have no catalysts. These results indicate that the BC-derived carbon can be used as support material for binder-free air electrodes. Figure 3 shows the correlation between the discharge capacities, D/G ratio, and carbonization temperature. In the Raman spectra of the carbon as shown in the inset, the peaks of D- and G-bands are derived from the edge-plane and basal-plane in the crystal structure of carbon, respectively. The D/G ratio was obtained from the results of wave analyses with the spectra in the inset. It correlates roughly with the discharge capacities as shown in Fig. 3. In particular, the battery incorporating the carbon prepared at 900 °C, which has the largest discharge capacity, has the highest D/G ratio. This indicates the importance of the number of carbon edge planes in relation to the discharge property, because the D/G ratio corresponds to a ratio of number of carbon edge planes to that of basal planes. These results indicate that the carbon edge planes would play an important role as active sites in the reaction mechanism with the air electrode.

Influence of Surface Area of Mesopore Volume on Discharge Characteristics for Lithium-Air Secondary Battery

Introduction Lithium-air secondary batteries have been expected to the next generation secondary battery due to extremely high theoretical energy density. The carbon material is generally used as a positive electrode material because of low cost and high conductivity. However, the overpotential is high because Li2O2 is deposited on the positive carbon electrode during discharge process and then blocks the diffusion path for lithium ion, oxygen, and electrolyte. Therefore, the structure of carbon material affects the electrochemical performance of lithium-air secondary battery. The relationship between the mesopore sutructure of the carbon material and the electrochemical performance is not clear. In this study, we prepared the various carbon electrodes using the porous carbons (CNovel) having larger surface area and pore volume and the conventional carbon material (acetylene black (AB)), and compared their electrochemical performances. Experimental CNovel MH (MH), CNovel P(2)010 (P(2)010), CNovel P(4)050 (P(4)050), and AB were used as the carbon materials. The carbon materials and polyvinylidene fluoride were mixed in an N-methyl-2-pyrrolidone solution. The obtained slurry was coated on the carbon paper and dried in vacuum at 80 oC overnight. After drying, the loading of the carbon material on the carbon paper was about 0.5 mg cm-2. The carbon electrodes were transferred into an argon-filled glovebox. The CR2032 coin type cell was assembled in the glovebox under argon atmosphere by using the obtained carbon electrodes (16 mm in diameter) as a positive electrode, lithium metal foil (16 mm in diameter) as a negative electrode, 1 mol dm-3 LiCF3SO3 dissolved in tetraethylene glycol dimethyl ether (TEGDME) as an electrolyte, and glass fiber filter (Whatman GF/A) as a separator. The separator was saturated with 1 mol dm-3 LiCF3SO3/TEGDME. For the electrochemical measurements, the discharge-charge tests were carried out in dry air atmosphere at 25 oC in the 2.0-4.5 V voltage range at the current density of 100 mA g-1. The specific capacity was calculated by the loading of the carbon material. Results and Discussion Fromthe pore size distribution of various carbon materials, the modal pore size of MH and P(2)010 was about 4 nm, while that of P(4)050 was about 50 nm. From the nitrogen adsorption-desorption isotherms, the order of the surface area of mesopore (mesopore range: 2-50 nm) was as follows: P(2)010 > MH > P(4)050 > AB. From the first discharge curves of various carbon electrodes (current density: 200 mA g-1), the discharge plateau voltages of P(2)010, MH, P(4)050, and AB electrodes were 2.69, 2.67, 2.68, and 2.58 V, respectively. Fig. 1 shows the relationship between the pore structure (the surface area of mesopore and the mesopore volume) of carbon materials and the first discharge capacity. The first discharge capacities of P(2)010, MH, P(4)050, and AB electrodes are 4231, 3652, 3552, and 2484 mAh g-1, respectively. The CNovel electrodes show higher first discharge voltage and capacity than the AB electrode. From Fig. 1a, the first discharge capacity increases with increasing the surface area of mesopore. The first discharge capacity is more strongly related to the surface area of mesopore than the mesopore volume. It was suggested that the increase in the thickness of the Li2O2 layer enhanced the overpotential. Therefore, the discharge reaction was cut off at the certain thickness of the Li2O2 layer. Based on these results, it was found out that the surface area of mesopore of the carbon material is more important parameter than volume for lithium-air secondary battery, since the thickness of the Li2O2 layer directly affects overpotential. References 1) J. Xiao et al., J. Electrochem. Soc., 157,A487-A492 (2010). 2) L. Wang et al., J. Power Sources, 234,8-15 (2013). 3) P. Guan et al., Electrochim. Acta, 129,326 (2014). 4) I. Landa-Medrano et al., J. Electrochem. Soc., 162,A3126-3132 (2015). Acknowledgments This research was partially supported by ALCA-SPRING, JST. Figure 1

Relationship Between Discharge Profile and Pore Structure for Lithium-Air Secondary Battery

Introduction High-capacity next-generation secondary batteries have been desired to spread plug-in hybrid vehicles and electric vehicles. Lithium-air secondary batteries have attracted much attention as the large storage secondary batteries due to their high theoretical capacity (5200 Wh kg-1). However, the reversible capacity is limited because the discharge products block the oxygen diffusion path in air electrode. The influence of the pore structure of carbon materials on the deterioration of electrochemical performance is not clear, although the carbon materials are widely studied as air electrode for lithium-air secondary batteries. In this study, we have prepared the carbon electrodes using carbon materials (CNovel MH (MH) and ketjen black (KB)) with different pore volume, analyzed the discharge products and change of pore structure by discharging. Experimental The carbon materials (MH and KB) were mixed with PVdF (carbon : PVdF = 93 : 7 wt. %). After coating on the carbon paper as a current collector (the loading of carbon material 1 mg, electrode area 2.0 cm2), it was dried overnight at 80 oC in a vacuum to obtain carbon electrodes. The CR2032 coin type cell was assembled in an Ar-filled glovebox. It consisted of the prepared carbon electrode as a cathode, lithium foil as an anode, 1 mol dm-3 LiCF3SO3/TEGDME as an electrolyte, and Whatman glass filter GF/A as a separator. The carbon electrodes were analyzed by using FE-SEM, EDX, XRD, and the nitrogen adsorption method. For the electrochemical measurements, the discharge-charge tests were carried out (current value: 100 mA, voltage range: 2.0-4.5 V, measured temperature: 25 oC, and atmosphere: dry air). Results and Discussion From the discharge-charge tests, the MH electrode showed higher discharge capacity than the KB electrode. Fig. 1 shows SEM images of MH and KB electrodes before and after 6 h discharge. The MH electrode has much larger volume of micropore than the KB electrode, as shown in the pore size distribution. The spherical Li2O2 particles is deposited on the surface of the KB electrode after 6 h discharge, while they are not observed on the MH electrode. The micropore structures prevent growth of Li2O2 particles for MH. The mesopore structure of the MH electrode does not change by 6 h discharge, while mesopore volume of the KB electrode clearly decreases by 6 h discharge. From these results, it was found that the deposited spherical Li2O2 particles changed not only micropore structure but also mesopore structure of carbon electrode to affect the discharge profile. References 1) Y. Gao et al., Int. J. Hydrog. Energy, 37, 12725 (2012). 2) T. Cetinkaya et al., J. Power Sources, 267, 140 (2014). Acknowledgments This research was partially supported by ALCA-SPRING, JST. Figure 1

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

Electrochemical Properties of Lithium Air Secondary Batteries with Nonaqueous Electrolyte Solution Containing Manganese-Based Organic Complexes As Solution-Phase Catalysts

Introduction Lithium air secondary batteries (LABs) have the highest theoretical energy density among secondary batteries reported so far. However, major problems are poor cycle characteristics and large discharge/charge overpotential. To improve these properties, various kinds of solid-phase catalysts loaded into air electrodes have been reported [1]. One of the significant problems is that the solid-phase catalysts become inactivated gradually because discharge product Li2O2 accumulats on the surface of the air electrode as a result of imperfect decomposition during the charging process after a large number of cycles [2]. Since a solution-phase catalyst dissolved in the electrolyte solution could work stably during the cycles, it would overcome these issues associated with the solid-phase catalyst. Recently, as solution-phase catalysts, transition metal complexes such as manganese phthalocyanine (MnPc) [3], cobalt phthalocyanine (CoPc) [4] have been reported [3-5]. It is necessary to investigate the electrocatalytic activities for various kinds of solution-phase catalysts because the catalytic mechanism is unclear. We focused on manganese-containing salen-type complexes (MnSl) as a new solution-phase catalyst (Fig. 1). The electrochemical properties would be improved by involving oxygen adsorbed on the central Mn-ion site with the reaction over the air electrode [6]. Here, we report discharge/charge properties in LABs using MnSl as solution-phase catalyst. Experimental MnSl [(R,R)-(−)-N,N-Bis(3,5–di–tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III)chloride] and MnPc [manganese(II) phthalocyanine] as a reference catalyst were purchased from Sigma-Aldrich Co. LLC. A 2.0 wt% of both catalysts vacuum-dried at 90 °C for 12 h was added to an electrolyte solution of 1.0 mol/l LiTFSA / TEGDME (tetraglyme). Ketjenblack EC-600JD (KB, 80 wt%) and polyvinylidene difluoride (PVdF, 20 wt%) coated on carbon paper (Toray, TGP-H-120) was used as the air electrode. A Li metal sheet was used as the negative electrode. The experimental LAB cell was assembled using a commercial cell (ECC-Air, EL-Cell GmbH). Electrochemical experiments were conducted under galvanostatic condition at 200 mA/g in dry air atmosphere. Current density and discharge/charge capacity were normalized by the mass of the reaction layer (KB + PVdF) in the air electrode. Results and discussion Figure 2 shows the first discharge/charge curves of LAB cells with MnSl, with MnPc catalyst, and whithout the solution-phase catalysts. The cells with MnSl catalyst and MnPc show higher average discharge voltages of 2.64 and 2.55 V and larger discharge capacities of 4903 and 3309 mAh/g, respectively, compared to the cell without the catalysts. In addition, the cell with MnSl catalyst shows lower average charge voltage of 3.93 V and larger charge capacity of 6390 mAh/g than the other cells. These results suggest that the solution-phase catalyst could have electrocatalytic activities. Moreover, the MnSl catalyst exhibits rather higher activity than the conventional catalyst, MnPc. However, with the MnSl catalyst, the first charge capacity was larger than the discharge capacity by about 2000 mAh/g. This behavior suggests that some side reactions might co-occur with the charge (oxygen evolution) reaction in the high-voltage region above 4.0 V. Figure 3 shows SEM images of the air electrodes as prepared and after the first discharge without and with MnSl catalyst. Compared to the electrode surface as prepared shown in Fig. 3(a), the fine sub-micrometer discharge products with similar structures were deposited in a part of the electrode surface both without and with MnSl catalyst as shown in Fig. 3(b) and (c). In particular, there were spherical micrometer structures of the discharge products only with MnSl as shown in Fig. 3(c). MnSl catalyst three-dimensionally catalyzed deposition and decomposition of Li2O2 over the air electrode, which led to significant increase in the capacity. According to the above results, the MnSl catalyst exhibited higher activities than MnPc. One of the reasons might be that the oxygen adsorption on the central metal Mn containing N-Mn-N bonds is stronger in MnSl than in MnPc. At the IMLB meeting, we will also discuss the cycle characteristics of LABs with the MnSl catalyst.

Electrochemical Properties of Lithium Air Secondary Batteries with Pt100-XRux Electrocatalysts for Air Electrodes: Optimization of Pt/Ru Composition Ratio

Introduction Lithium air secondary batteries (LABs) exhibit much higher theoretical energy density than lithium ion batteries and are anticipated as one of the most promising candidates for the next generation of batteries. However, problems with LABs include their electrochemical properties, such as cyclability, overpotential, and round-trip efficiency need further improvement[1-2]. Various kinds of electrocatalysts have been studied to overcome these drawbacks [1-2]. We have reported that a battery incorporationg Pt33Ru67 electrocatalyst shows discharge capacities of more than 400 mAh/g for over 30 cycles in TEGDME-based electrolyte solution [2]. In this study, to further improve the cyclability, we prepared Pt100-x Ru x /carbon electrocatalysts with wide composition range of 0 ≤ x ≤ 100 by the soft liquid process and examined the electrochemical properties of the cells loaded with the electrocatalysts into air electrodes. Experimental Pt100-x Ru x /carbon was prepared by the formic acid reduction method [3]. KetjenBlack EC600JD (KB) was used as the carbon support material. The KB powder was dispersed in formic acid solution by sonication, and H2PtCl6¥6H2O and/or RuCl3 solution was dropped into the solution, which was then stirred overnight. The Ru content, x, in Pt100-x Ru x was 0 (Pt alone) 30, 60, 75, 83, 90, or 100 (Ru alone). Then, dried Pt100-x Ru x /KB powder was obtained by heat-treating the mixture at 300°C for 12 h in Ar. The air electrodes were prepared by coating the mixture of Pt-Ru/KB powder, and PVdF in N-methylpyrrolidone solvent, and drying it at 90°C. The composition ratio of an air electrode with a diameter of 5 mm was KB/Pt100-x Ru x /PVdF = 80: 10: 10. The LAB cell (ECC-Air, EL-Cell) was assembled, incorporating the air electrode loaded with Pt100-x Ru x /KB, an electrolyte solution (1 mol/l LiTFSA/TEGDME), a glass separator, and Li metal sheets. Electrochemical measurements were carried out under a galvanostatic condition of 0.1 mA/cm2 in a dry air atmosphere. The discharge and charge capacities were normalized by the weight of Pt100-x Ru x /KB powder and PVdF in the air electrodes. Results and discussion Figure 1 shows XRD patterns of samples with various composition ratios of Pt100-x Ru x /KB. The patterns of x = 0 and 30 corresponded to a single phase of Pt (PDF #00-004-0802), and the ones of x = 83, 90, 100 corresponded to a single phase of Ru (PDF#00-006-0663). The sample with x = 60 and 75 was identified as a mixture of the Pt and Ru phase. In addition, the XRD peaks became broad by increasing the content of Ru. This suggests that the added Ru is effective in reducing the particle size of Pt100-x Ru x alloy, and then it should enhance the electrocatalytic activities. Figure 2 shows the typical first discharge-charge curves of the cells incorporating the air electrodes loaded with Pt100-x Ru x /KB catalysts. First discharge capacities are 496, 671, 905, 1014, and 909 mAh/g in the cells with samples with x = 0, 30, 75, 90, and 100, respectively. The discharge capacities clearly increased with increasing Ru content, indicating the maximum capacity in the x = 90 sample. Moreover, the cell for the x = 90 shows the highest average voltage of 2.58 V, while the one for x = 0 shows rather low voltage of 2.43 V. In the charging process, the average charge voltages decreased with increasing Ru content and, accordingly, the charge capacities increased as a result of the smaller charge overpotentals, indicating the maximum capacity of 867 mAh/g in the x = 90 sample. Figure 3 shows the cycle properties of cells incorporating the air electrodes loaded with Pt100-x Ru x /KB catalysts. The discharge capacities of all cells gradually decreased. In particular, the cell for x = 0 shows very small capacity of less than 80 mAh/g at the 8th cycle. This result indicates the effectiveness of Ru addition for improving the cyclability. Among the cells tested, the one with the x = 90 sample showed comparatively better cycle stability with discharge capacity of over 800 mAh/g at the 8th cycle. These behaviors are very similar to the above discharge/charge properties. As a result, the x= 90 sample was confirmed to be the optimized composition as the electrocatalyst for the air electrode. From the above results, it was found that the higher Ru content leads to a higher surface area and higher dispersion state of Pt100-x Ru x electrocatalyst accompanied by inhibition of particle growth, which result in good electrochemical performance of cells incorporating Pt10Ru90/KB. [1] A. Debart et al., Angew.Chem., 120, 4597 (2008). [2] M. Hayashi et al., The 56th Battery Symposium in Japan, 3G02, (2015). [3] J. Prabhuram et al., J. Power Sources, 134, 1 (2006). Figure 1

Stability of garnet-type solid electrolyte LixLa3A2 -yByO12 (A = Nb or Ta, B = Sc or Zr)

Garnet-type solid electrolytes LixLa3A2-yByO12 (A=Nb5+ or Ta5+, B=Sc3+ or Zr4+) were examined with respect to their stability in contact with lithium metal, water, and a liquid electrolyte for application in an aqueous lithium-air secondary battery. Electrolyte samples were synthesized by a solid state reaction method using a magnesia rather than alumina crucible to avoid aluminum contamination. The garnet-type electrolytes including Zr4+ ions exhibited higher ionic conductivity than those with Sc3+ ions. The electrolytes including Ta5+ were stable in contact with lithium metal, whereas those with Nb5+ were not. Lithium ions in all the samples were partly proton-exchanged in distilled water; however, the garnet-like structure remained unchanged as reported in many papers. In contrast, all the electrolyte samples were stable in the aqueous solution of 10M LiCl and 2M LiOH (suitable electrolyte for aqueous Li-air battery system). The garnet-type lithium ion conductors comprising Zr4+ and Ta5+ ions were revealed to be suitable for use as the protective membrane between the lithium metal electrode and aqueous electrolyte in rechargeable lithium-air batteries.

Preparation Conditions of Porous Carbon Monolith Support Materials for Air Electrodes and Their Application to Lithium Air Secondary Batteries

Introduction Lithium air secondary batteries exhibit higher theoretical energy density than lithium ion batteries and are expected to be used as the next generation of secondary batteries. However, there remain technical challenges related to their poor cycle properties. Since the first report by K. M. Abraham et al. [1], various oxygen reduction/evolution catalysts [1-4] and electrolyte [1, 5] materials have been intensively investigated for air batteries to improve their electrochemical properties, such as the their cyclability. However, there have been a few studies on support materials for air electrodes, for example, nanoporous gold [6]. We think that a decrease in the electrical contact of the air electrode during Li2O2deposition at discharge and oxygen generation at charge is one of the reasons for the poor cyclability. The purpose of our research is to improve the cyclability by using a new air electrode structure. We are focusing on a porous carbon monolith, which has a continuous interconnected network of carbon, as support material for air electrodes. We have already fabricated porous-carbon-monolith supports to replace the current carbon-powder supports. Here we report the preparation conditions for porous-carbon-monolith support materials and the performance of air batteries incorporating these supports. Experimantal A porous polyacrylonitrile (PAN) monolith was prepared as the precursor of the porous-carbon-monolith support. PAN was dispersed in dimethyl sulfoxide (DMSO). After the solution with a concentration of 94 g/l was dissolved, water was sprayed on the solution. The porous PAN monolith film was deposited from the solution due to water, which is poor solvent for PAN. The monolith was washed with methanol and dried in vacuum at room temperature. Then it was carbonized in Ar atmosphere at 1300 °C to obtain a porous carbon monolith. In the next step, for activation treatment, the porous carbon monolith was heat treated in CO2 atmosphere at 900 °C for 1 h [7]. The lithium air secondary battery consisted of the air electrode, a lithium metal sheet, and 1.0-mol/l lithium bis(trifluoromethanesulfonyl)amide (LiTFSA)/propylene carbonate (PC) as the positive electrode, negative electrode, and electrolyte solution, respectively. The battery preparation is described in detail in our previous paper [4]. Electrochemical measurements were carried out under a galvanostatic condition of 0.05 mA/cm2 in an O2atmosphere. The discharge and charge capacities were normalized by the weight of the air electrodes. Results and discussion In the SEM image of the porous carbon monolith in Fig. 1, we see the continuous interconnected network of the carbon. These particles build a 3-D disordered macroporous framework, with sizes in the range of 2-5 mm. In XRD measurements of the porous carbon monolith, all of the peaks in the XRD patterns corresponded to the ICDD data for carbon (#01-077-7164). These results indicate that our method can produce porous carbon monoliths. Figure 2 shows the pore distributions of the porous carbon monolith measured using mercury intrusion porosimetry. The porous carbon monolith has mesopores with size of ~10 nm, which correlated with first discharge capacities [8]. Figure 3 shows the first discharge/charge curves of the air batteries incorporating the as-synthesized and activation-treated porous carbon monolith. The air batteries, showed higher discharge and charge capacities, with the activation-treated porous carbon monolith. This is because activation-treated porous carbon monolith has many more active sites than the as-synthesized one. In particular, the air batteries with the activation-treated porous carbon monolith show the capacity of 12 mAh/g and the average discharge voltage of 2.6 V. This indicates that the porous carbon monolith can be used as support material for air electrodes. However, compared with Ketjen Black EC600JD (KB) powder [9], the air batteries with the porous carbon monolith show lower discharge capacities. It seems that KB-powder has many more active sites than the porous carbon monolith because its BET surface area of 1300 m2/g is larger than that of the porous carbon monolith, which is 4.3 m2/g.

Stability of carbon electrodes for aqueous lithium-air secondary batteries

The air electrode performance of various carbon materials, such as Ketjen black (KB), acetylene black (AB and AB-S), Vulcan XC-72R (VX), and vapor grown carbon fiber (VGCF) with and without La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) catalyst were examined in an aqueous solution of saturated LiOH with 10 M LiCl in the current density range 0.2–2.0 mA cm−2. The best performance for oxygen reduction and evolution reactions was observed for the KB electrode, which has the highest surface area among the carbon materials examined. A steady over-potential of 0.2 V was obtained for the oxygen reduction reaction using the KB electrode without the catalyst, while the over-potential was 0.15 V for KB with the LSCF catalyst at 2.0 mA cm−2. The over-potentials for the oxygen evolution reaction were slightly higher than those for the oxygen reduction reaction, and gradually increased with the polarization period. Analysis of the gas in the cell after polarization above 0.4 V revealed the evolution of a small amount of CO during the oxygen evolution reaction by the decomposition of carbon in the electrode. The amount of CO evolved was significantly decreased by the addition of LSCF to the carbon electrode.

Carbon electrode with perovskite-oxide catalyst for aqueous electrolyte lithium-air secondary batteries

Carbon electrodes with perovskite-type oxide catalysts were investigated as air electrodes for aqueous lithium-air secondary batteries. The system consisted of a water-stable lithium electrode, an aqueous solution of saturated LiOH with 10 M LiCl as electrolyte, and a carbon black air electrode with perovskite catalyst additive. The stability, oxygen reduction and oxygen evolution catalytic activity of the La0.6Ca0.4Co0.8Fe0.2O3, La0.8Sr0.2Fe0.8Mn0.2O3, and La0.6Sr0.4Co0.2Fe0.8O3 perovskite-type oxides were examined in the LiOH–LiCl electrolyte. The perovskite-type oxides were stable in this solution and the oxygen reduction and evolution overpotential on the carbon black substrate was decreased by addition of these perovskite-type oxides. No chlorine evolution was observed during the oxidation reaction on carbon black with the perovskite-type oxides in the LiOH–LiCl electrolyte. The results indicate that carbon black with perovskite-type oxide additives is a potential candidate for the air electrode in the aqueous electrolyte lithium-air rechargeable batteries.

TFSI-Based Ionic Liquids as Electrolytes for Lithium-Air Secondary Batteries

not Available.