Analysis of Li Deposition/Dissolution Behavior on the Interphase between Li Metal Anode and Glyme-Based Electrolytes for Li-Air Batteries

Abstract

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