Katsuya Hayashi’s research while affiliated with Nippon Telegraph and Telephone and other places

Discharge–charge properties of Sn-Co anode materials with various kinds of binders [polyvinylidene difluoride (PVdF), polyacrylic acid (PAA), sodium polyacrylate (PAANa), sodium carboxymethyl cellulose (CMC), and polyimide (PI)] for sodium ion batteries were investigated to determine the correlation between cycle performance and the properties of electrode with binder. Sn-Co electrodes with PAA or CMC binders exhibited better cycle properties (discharge capacities of more than 400 mAh/g up to 20 cycles) than with PVdF, PAANa, or PI. These cycle properties with PAA or CMC were due to smaller changes in the electrode volume that occurred during cycling, as revealed by in situ light microscopy.

Sodium-ion batteries (SIBs) are anticipated as promising alternatives to lithium-ion batteries (LIBs), and many studies have been conducted on electrodes for SIBs using half-cells. In general, sodium metals are used for the counter electrode of half-cells. However, there are few reports [1] on sodium dissolution/deposition, knowledge of which an understanding of which is important for advancing the development of SIBs. In this study, we focused on sodium metal and observed morphologies of sodium dissolution/deposition reactions in propylene carbonate-based electrolyte solution using in situ light microscopy. Morphologies of sodium depositions were observed by using in-situ light microscopy (Lasertec Corp., ECCS B310). The semicircular cell for in-situ light microscopy examination was a stack consisting of a sodium sheet as a counter electrode (0.2 mm thick), an electrolyte solution (1 mol/l NaPF 6 /PC)-soaked polypropylene separator (19 mm in diameter), and a Cu sheet (0.01 mm thick) or sodium sheet (0.2 mm thick) as a working electrode. Electrochemical measurements were performed by using an automatic galvanostatic discharge-charge system (Hokuto HJ1001SD8) at a constant current of 65 mA/cm ² at room temperature. The sodium deposition reaction on the sodium sheet was observed, and the morphologies were granular and needle-like (not shown here). Figure 1 shows cross-sectional images of sodium deposition on the Cu sheet, separator, and sodium metal. The morphologies of sodium deposition showed no difference from that on sodium sheet. Then, needle-like sodium led to away from the Cu sheet and became “dead sodium”. This suggests that the sodium metal dissolution/deposition efficiency is low. [1] K. Matsumoto et al., J. Power Sources , 265 (2014) 36. Figure 1

Lithium air batteries incorporating RuO2(10wt%)/Ketjen Black EC600JD catalyst heat-treated at 110°C showed a rather low charge voltage of about 3.6 V in an electrolyte solution of 1 mol/l lithium bis(trifluoromethanesulfonyl)amide/propylene carbonate. The charge overpotentials decreased greatly as a result of the improved dispersion state of the oxide realized by employing the supporting-method process, indicating a low charge voltage of about 3.6 V even at a small oxide loading of 1 wt%. A battery incorporating a dimethyl sulfoxide-based solution exhibited high discharge and low charge voltages of about 2.8 and 3.2 V, respectively. As a result, a battery incorporating RuO2/Ketjen Black EC600JD and the dimethyl sulfoxide-based electrolyte solution exhibited a high round-trip energy efficiency of more than 80%.

Prussian blue, KxFey[Fe(CN)6]z/nH2O, was investigated as a positive electrode material for sodium-ion batteries. A Na cell with a Prussian blue positive electrode exhibited a first discharge capacity of 57 mAh/g. However, the discharge capacity rapidly decreased. It appears that the lattice parameter of Prussian blue changes and electron conductivity is gradually lost. Then, we attempted to improve the cycle performance of the cell with a positive electrode using high conductivity carbon, ketjen black EC600JD. As a result, a positive electrode of Prussian blue and ketjen black exhibited a first discharge capacity of 67 mAh/g and exhibited a retention of more than 80% at the 40th cycle. Moreover, the positive electrode was improved owing to dehydration caused by the thermal treatment of the Prussian blue. Prussian blue showed a reversible Na+ intercalation potential around 2.5 V vs. Na, which corresponds to the redox of Fe2 +/Fe3 +.

Li2CuO2 substituted by various cations, such as Co, Ni, Fe, Mn, and Ti, was examined to decrease the irreversible capacity of unsubstituted one. Among several kinds of cations tested, Ni and Co were found to be effective in improving the irreversible capacity. Li2Cu0.7Ni0.3O2 exhibited a reversible capacity of about 150 mAh/g, while the unsubstituted one exhibited a capacity of only about 100 mAh/g. The structural changes in the Cu-based oxides that occurred during the electrochemical reaction were investigated with ex-situ XRD measurements. The results indicated that the structural transformation behavior changed as a result of the cation substitution.

Introduction Lithium air batteries exhibit higher theoretical energy density than lithium ion batteries and are expected to be used in the next generation of secondary batteries. However, there are many problems, such as a decrease in discharge capacities after some cycles and a large difference between discharge and charge voltages, ΔV. A large/small ΔV means low/high round-trip (discharge-charge) energy efficiency, respectively. High-performance secondary batteries show small ΔV, i.e., high round-trip energy efficiency. Since the first report by K. M. Abraham et al. [1], various kinds of 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 cycleability and the round-trip energy efficiency of the air batteries. The purpose of this research is to improve the round-trip energy efficiency by using highly-active catalysts for air electrodes. We are focusing on RuO 2 as the catalyst and here report the performance of air batteries incorporating this oxide catalyst. Experimental Precursor powder of RuO 2 , Ru(OH) n , was prepared neutralizing 0.1 mol/l RuCl 3 aq with 0.1 mol/l NaOH aq. RuO 2 was obtained by heat-treating the hydroxide powder at 110, 200, and 500 ◦ C. An air electrode was prepared by rolling a mixture of RuO 2 , KetjenBlack EC600JD (KB), and PTFE (10:54:36 in weight ratio) into a sheet about 0.5-mm thick. The lithium air battery consisted of the air electrode, a lithium metal shee,t and 1.0 mol/l lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)/aprotic organic solvent as positive electrode, negative electrode, and electrolyte solution, respectively. Solvents used were propylene carbonate (PC), tetraethylene glycol dimethyl ether (TEGDME), and dimethyl sulfoxide (DMSO). The preparation of the battery is described in detail in our previous paper [4]. Electrochemical measurements were carried out under a galvanostatic condition of 0.1 mA/cm ² in a dry air atmosphere with a dew point of less than -50 ◦ C at room temperature. The discharge/charge capacities were normalized by the weight of the air electrodes. Results and discussion Figure 1 shows XRD patterns of RuO 2 heat-treated at 110, 200 and 500 ◦ C. All the peaks correspond to the PDF data for RuO 2 (#01-070-2662). The peaks become sharper as the heat-treatment temperature increases. This indicates that the particle size of RuO 2 became larger. It is notable that RuO 2 is crystallized even at a low temperature of 110 ◦ C. These RuO 2 particles would be very fine and suitable as catalyst for the air electrode. Figure 2 shows the first discharge/charge curves of the air batteries incorporating the RuO 2 catalyst heat-treated at 110, 200, and 500 ◦ C. Compared with KB only, the air batteries with RuO 2 catalyst showed larger charge capacities. The charge capacities became larger with RuO 2 prepared at lower heat-treatment temperature. As for the cell voltage, the RuO 2 catalyst reduced the charge overvoltage in particular, even though there were no changes in the discharge overvoltages. The charge overvoltages became smaller for RuO 2 with the lower heat-treatment temperature. The tendencies observed in the capacities and voltages with the heat-treatment temperature were almost the same. These results indicate that the fine-powder RuO 2 catalyst prepared at the lower temperature had great effects on not the discharge but on the charge capacity/overvoltage. Figure 3 shows the first discharge/charge curves of the air batteries incorporating the RuO 2 catalyst in the electrolyte solution of 1.0 mol/l LiTFSI/PC, TEGDME, and DMSO. The air batteries with the TEGDME solution showed smaller discharge and charge capacities and larger discharge and charge overvoltages compared with the PC solution. On the other hand, the air batteries with the DMSO solution showed the largest discharge and charge capacities and the smallest discharge and charge overvoltages. In particular, the air batteries with the DMSO solution showed rather low average charge voltage of about 3.1 V. Such low charge voltage greatly improves the round-trip energy efficiency. This superior battery performance would be due to some properties of the DMSO solution such as its stability under the operation condition of the air battery. In conclusion, the fine RuO 2 powder prepared at the lower temperature reduced the charge overvoltage. Moreover, the use of the DMSO-containing solution led to the great improvement in the round-trip efficiency due to the decrease in charge overvoltage. References [1] K. M. Abraham et al., J. Electrochem. Soc., 143 (1996) 1. [2] T. Ogasawara et al., J. Am. Chem. Soc., 128 (2006) 1390. [3] A. K. Thapa et al., Electrochem. Solid-State Lett., 13 (2010) A165. [4] H. Minowa et al., Electrochemistry, 78 (2010) 353. [5] N.-S. Choi et al., J. Power Sources, 225 (2013) 95.

Sodium-ion batteries (SIBs) are anticipated as promising alternatives to replace lithium-ion batteries. There are many reports on anode materials for SIBs, such as hard carbon and tin [1,2]. However, there are hardly any reports on materials that provide both high capacity and stable cycle property. In this study, we focused on Sn-Co as an anode for SIBs. Sn-Co has been reported as a good anode for lithium ion batteries [3]. We investigated electrochemical properties of Sn-Co, and examined the correlation between the cycle performance and the binders of electrode component materials. A working electrode was prepared by mixing Sn-Co, Ketjen Black EC600JD, and polyvinylidene di fluoride (PVdF) or polyacrylic acid (PAA). Sodium metal was used for the counter electrodes. Figure 1(a) shows the first discharge-charge curves of Na/Sn-Co cells incorporating PAA or PVdF as binder. The first capacities of electrode incorporating PAA and PVdF were 505 and 569 mAh/g, respectively. Na/Sn-Co cells using both binders showed two distinct plateaus (about 0.6 and 0.2 V), and the plateau regions were similar to a two phase equilibrium in Na-Sn alloy [2]. Figure 1(b) shows cycle properties of Na/Sn-Co cells incorporating PAA or PVdF as binder. The electrode incorporating PAA showed a better cycle property than the one incorporating PVdF. The discharge capacity of the former reached about 300 mAh/g after 30 cycles. This good cycle performance is attributed to buffering of the volume change during insertion and extraction of sodium ions because of the porous structure of PAA [4]. In addition, a large irreversible capacity loss in the first cycle in Na/Sn-Co cells (binder: PAA or PVdF) was observed. We tried sodium pre-doping to reduce the irreversible capacity and found that the pre-doping technique greatly reduced it. The detailed results will be shown at the meeting. References [1] S. Komaba et al., Electrochem. Commun., 21 , 65 (2012). [2] L. D. Ellis et al., J. Electrochem. Soc., 159 , A1801 (2012). [3] N. Tamura et al., Electrochim. Acta, 49 , 1949 (2004). [4] Y.-S. Park et al., J. Power Sources , 248 , 1191 (2014). Acknowledgement We are grateful to Mitsubishi Materials Corp. for supplying Sn-Co.

We studied the effect of carbon treatment on improving the electrochemical properties (capacity and cyclability) of NASICON-type iron sulfate (Fe2(SO4)3) for the cathodes of lithium ion batteries (LIBs). An optimized ball milling technique that was applied to a mixture of as-synthesized iron sulfate and conductive carbon ketjen black EC-600JD (KB) resulted a high capacity of 117 mA h g−1, which is close to its theoretical capacity of 134 mA h g−1. Moreover, the cyclability of coin-type test cell was doubled to 58 cycles by adding a polymer followed by heat treatment to the mixture of conductive carbon and Fe2(SO4)3. The smaller particle size and more uniform distribution in the mixture of conductive carbon and Fe2(SO4)3 particles accounted for the improved electrochemical properties after treatment, as confirmed by FE-SEM observations.

The electrochemical properties of lithium-air batteries incorporating air electrodes loaded with MnOx, which had different Mn valences as the electrocatalysts were examined in an organic electrolyte solution consisting of 1 mol l-1 lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)/propylene carbonate (PC). Furthermore, we attempted to improve battery performance by substituting the Mn site in MnOx with Fe, Ni, or Co. The batteries using Mn2-xFexO3 showed rather large 1st discharge capacities of 230 mAh g-1 at a current density of 0.25 mA cm-2 in a dry air atmosphere. The discharge and charge overpotentials were both greatly reduced by loading Mn2-xFexO3 catalysts. However, almost all of these oxides exhibited poor cycle performance. Of the oxides that we examined, Mn1.8Fe0.2O3 had comparatively stable cycle characteristics with a capacity loss of only 25% after 10 cycles.

We fabricated electrolyte supported single cells with three types of cathodes that consisted of a Ce0.1Gd0.1O1.95 (GDC) buffer layer, a LaNi0.6Fe0.4O3 (LNF) cathode and an active layer. The only difference between the three cathodes was that each had a different active layer, namely a GDC-LNF composite active layer (our conventional cathode), a PrxCe1-xO2-δ (x = 0.1, 0.3, 1.0)-LNF composite active layer and a Pr6O11 (PrxCe1-xO2-δ (x = 1.0)) active layer. The interface resistance, Rinf, and overvoltage, ηc, of the cathodes were investigated. At 800 °C, the Rinf of the cathode with the Pr6O11 active layer was reduced to 1/30 that of the cathode with the GDC-LNF composite active layer. The Rinf at 800 °C for the cathode with the Pr6O11-LNF composite active layer was reduced to 1/8 that of the cathode with the GDC-LNF composite active layer. The Rinf values of the cathode with an active layer between 650 and 750 °C were also much better than those of the cathode with the GDC-LNF composite active layer. By using the cathode with the Pr6O11 active layer, the operating temperature can be reduced to 700 °C while retaining the same performance (same overvoltage at 254 mA/cm2) as a cathode with a GDC-LNF composite active layer at 800 °C.