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Li-air batteries (LAB) have been attracting much attention because of their theoretical energy density of 3500 Wh kg−1, which is five times as high as that of Li-ion batteries. However, many problems remain to be challenged in every component of the battery: air electrodes, electrolytic solutions, Li electrodes, etc. As for the Li electrode, we have reported some results on the reversibility of the Li dissolution/deposition reaction by using Li | Li symmetric cells with tetraglyme (G4) -based electrolytic solution [1, 2]. In particular, it has been confirmed that in the 1.0 M LiNO3/G4 electrolyte under an O2 atmosphere, a Li2O protective layer is efficiently formed on the Li electrode at a current density of 0.4 mA cm-2, and Li dendrite formation is suppressed. In this study, we expanded test conditions in current densities up to 2.0 mA cm-2 and temperatures at 10 °C to 50 °C to reveal whole picture of dissolution/deposition behavior of the Li electrode. A Li | Li symmetric cell was assembled using a pair of Li foils inserted by a separator (Celgard 2400) and 1.0 M LiNO3/G4 or 1.0 M LiTFSI / G4 as the electrolytic solutions in an Ar-filled glove box. The Li dissolution/deposition cycle tests by the cells were carried out at 10, 30 and 50℃ at a constant current density between 0.2 to 2.0 mA cm-2 regulated by a constant capacity of 0.5 mAh cm− 2. After the tests, the Li electrodes were taken out of the cells followed by rinsing in G4 in the glove box, the surfaces of which were observed by SEM and analyzed by XPS. Figure 1 showed Li | Li cell test results at 0.4 mA cm−2 and 30 ℃ in different electrolytic solutions. Fig ure 1a for that in 1.0 M LiTFSI/G4 showed gradual decrease in overvoltages and unstable polarization curves. These two features suggested that the Li electrode protection by the Li2O layer from O2 gas could not withstand the current density and the exposed Li metal reacted with the solvent and electrolyte, resulting in growing Li dendrite deposition. On the other hand, Figure 1B for that in the 1.0 M LiNO3 / G4 overvoltages, suggesting suppressed Li-electrolytic solution showed very flat polarization plateaus and slightly growing dendrite formation. The difference between Figure 1a and 1b demonstrated the effect of Li2O protective layer of Li surface formed by the reaction between Li and NO3 −. Figure 2 showed the SEM image of the Li electrode after the dissolution/deposition test for 15 cycles and the surface analysis by C1s XPS. Figure 2a for LiTFSI/G4 electrolytic solution showed bulky surface layer as thick as 30 µm accompanied by a large dent. Figure 2b suggested various kind of C-containing deposits on the Li surface, which was presumably decomposed products of electrolytic solution. Therefore, Figure 2a and 2b were well consistent with Figure 1a which suggested successive surface reactions. Figure 2c and 2d for LiNO3/G4 electrolytic solution, on the contrary, showed thin surface layer and poor amount of surface products, even though the current density, 0.6 mA cm−2, was higher than that in Figure 2a, 0.4 mA cm−2. These results suggested steady surface protected by Li2O layer and were also consistent with Figure 1b. Other results presenting the effect of current densities and temperatures will be reported in our presentation to clarify the whole picture of Li dissolution/deposition reaction mechanism. References [1] M. Saito et al. J. Electrochem. Soc., 164, 2872–2880 (2017). [2] M. Saito et al. J. Electrochem. Soc., 168, 010520 (2021). Acknowledgement This study was supported by JST Project ALCA-SPRING (JPMLAL1301) and NIMS Joint Research Hub Program, Japan Figure 1

The cycle stability of lithium negative electrodes for Li–air secondary batteries was studied under oxygen atmosphere using Li∣Li symmetric cells with three organic electrolyte solutions: 1.0 M LiCF3SO3/tetraglyme (G4), 1.0 M LiN(SO2CF3)2/G4, and 1.0 M LiNO3/G4. Of these, 1.0 M LiNO3/G4 showed excellent stability without dendrite deposition, even for increased dissolution/deposition capacity from 0.50 to 2.0 mAh cm−2. These results are considered to be due to the stable Li2O passivation layer that was formed, not only by the direct reaction with oxygen, but also by the action of NO3 − as an oxidant, which released NO2 − as a redox mediator. Li–O2 cells with 1.0 M LiNO3/G4 showed a clear charging voltage plateau at 3.7 V, which evidenced the redox mediator effect of NO2 −, and cell cycleability was enhanced to 25 cycles.

In recent years, rechargeable Li-air batteries (LAB) have attracted much attention because of their potential high energy density over 5 times larger than that of the conventional Li-ion batteries [1]. However, there are many problems still to be solved for all the cell components: the cathode, anode and electrolyte solution. As for the anode, Li dendrite growth during discharge/charge cycles should be suppressed to stabilize the cycle life of the LAB. In order to improve the reversibility of the Li electrode, we have been evaluating tetraglyme (G4)-based electrolyte solutions by using a Li|Li symmetric cell [2]. We confirmed that a 1.0 M LiNO3/G4 electrolyte solution containing O2 formed a Li2O protective layer on the Li surface effectively, which suppressed the formation of Li dendrite at the current density of 0.2 mA cm-2. In this study, high rate tests up to 2.0 mA cm-2 above our target of 0.4 mA cm-2 were applied to verify and analyze the reversibility of Li electrode reaction. LiN(SO2CF3)2 (LiTFSI, 99.9%, KISHIDA) and LiNO3 (99.9% purity, Wako) were vacuum- dried at 110 °C and dissolved in tetraglyme (G4, Japan Advanced Chemicals, H2O < 30 ppm) solvent to prepare 1.0 M glyme electrolyte solution of each electrolyte salt. Li|Li symmetric cells with 1.0 M LiNO3/G4 or 1.0 M LiTFSI/G4 electrolyte solution were assembled in an Ar-filled glove box to be used for Li dissolution/deposition tests in an isothermal chamber set at 30°C at a constant current from 0.2 to 2.0 mA cm-2. The surface of the Li electrodes after the dissolution/deposition test and its cross-section were observed by SEM-EDS and analyzed by elementary analysis. In addition, the outermost surface was analysed by using XPS. The flatness of the polarization curve for dissolution/deposition tests in the 1.0 M LiNO3/G4 electrolyte suggests that Li dendrite growth and electrolyte decomposition are suppressed at an applied current of 0.6 mA cm-2 or less. Fig. 1 shows the SEM image of the Li electrode surface after the dissolution/deposition test. Although no large dendrites were observed in the LiNO3/G4 electrolyte, the entire surface was covered with fine Li dendrites of about 100 nm in diameter and about 800 nm long in the LiTFSI one (Fig. 1a & c). In addition, the cross-sectional SEM image of the Li surface tested in the LiNO3/G4 electrolyte shows some pits, while that in the LiTFSI/G4 electrolyte solution exhibits bulky decomposition product layer of about 20 μm thick accompanying a larger crevices (Fig. 1b & d). C 1s spectra of XPS showed a dominant signal assigned to Li2CO3 on the electrodes tested at higher rates, which suggested the formation of the protective layer of Li2CO3 and Li2O. Moreover, weak signals such as C-C and C-O detected on the Li surface in LiNO3/G4 electrolyte solution suggested the suppressed degradation. The details of the mechanism will be discussed on our presentation. This study was supported by JST “Next Generation Batteries Area in Advanced Low Carbon Technology Research and Development Program (ALCA)” from MEXT, Japan. [1] P. G. Bruce, S. A. Freunber, L. J. Hardwick, J-M. Tarascon, Nature materials, 11, 19 (2012). [2] M. Saito, S. Yamada, T. Fujinami, S. Kosaka, Y. Tachikawa, K. Ito, Y. Kubo, ECS Trans., 75(22), 53 (2017). Figure 1

Juveniles of bivalves Mactra veneriformis and Ruditapes philippinarum, and ghost shrimps Nihonotrypaea japonica and N. harmandi were reared on a microalga of a constant isotopic value to quantify their diet-tissue isotopic fractionation. The weights of the animals increased by >7-fold, resulting in isotopic equilibria with their diet. Fractionation for bivalve soft tissues was 0.6 to 0.9‰ for carbon and 3.4 to 3.6‰ for nitrogen, which fell within the range of the currently accepted fractionation values (0 to 1 and 3 to 4‰). Examinations of acid-treated or untreated whole body, mus- cle and exoskeleton of the ghost shrimps showed (1) large variations in δ 13 C for untreated exoskele- tons, (2) reduced δ 13 C for acid-treated exoskeletons by 3.5 to 6.2‰, (3) confined ranges in 13 C and 15 N fractionations for muscles (2.0 to 2.2 and 3.6 to 4.0‰), (4) only slight effects of acid treatment on 13 C and 15 N fractionation for muscles (≤0.3‰ differences), (5) a significant difference in 13 C fractionation for acid-treated whole bodies between N. japonica (-0.3‰) and N. harmandi (-1.7‰), and (6) 2.3 to 3.0‰ of 15 N fractionation for whole bodies, which were smaller than for muscles due to negative frac- tionation for exoskeletons (-3.0 to -1.9‰). These findings suggest that carbonates in exoskeletons should be removed by acid and that muscle is the most appropriate tissue for isotopic analysis. Although 15 N fractionation for ghost-shrimp muscle was within the above-mentioned accepted range, 13 C fractionation was outside this range. The present study highlights that fractionation is species- and tissue-specific, and that the accepted fractionation values may not be universally applicable.

To delineate a soft-shore trophic structure, macrobenthos was collected from an intertidal sandflat at the mouth of the Shirakawa River in the central part of Ariake Sound, Kyushu, Japan, on 3 occasions in 2003 (January, March, and June to July during the rainy season), and the isotopic compositions of animals and their potential food sources were analyzed. Food sources of animals were assessed based on the diet - tissue isotopic fractionation (δ 13 C = 0.6 to 2.0‰, δ 15 N = 3.4 to 3.9‰) of 3 main constituent species (the bivalves Mactra veneriformis and Ruditapes philippi- narum and the ghost shrimp Nihonotrypaea japonica), which had been determined by an earlier laboratory experiment. The results revealed that (1) consumers were classified into 3 groups; a group consisting of 51 species including 4 main species (the above-mentioned species and the mud shrimp Upogebia major) that derived nutrition mainly from coastal phytoplankton, one consisting of 7 species that incorporated more 13 C-enriched material (e.g. benthic microalgae) and a group con- sisting of 4 species that incorporated more 13 C-reduced material (e.g. riverine organic matter); (2) the phytoplankton-based trophic structure was composed of 3 trophic levels; (3) M. veneriformis juve- niles collected during the rainy season appeared to incorporate riverine organic matter; and (4) reeds from the flood plain, detrital terrestrial plant material, sewage effluents and seaweeds were not food sources for most consumers. We concluded that the high phytoplankton biomass and scarcity of benthic microalgae on this sandflat account for its phytoplankton-based trophic structure.