Liquid Structure of Super-Concentrated Aqueous Electrolyte Solutions with Various Alkali Metal Salts and Its Effect on Battery Performance

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Super-concentrated electrolyte aqueous solution (SCES) has been attracting attention as an electrolyte solution for aqueous lithium-ion secondary battery (ALIB) because of its wide potential window. However, further improvement is needed in the currently reported SCES because the SCES contains several tens of percent of bulk water that is not coordinated with Li ions1). It is known that Alkali metal ions with a large ionic radius interact with surrounding water molecules to destroy the hydrogen bonding network in water, which may promote the coordination of free water molecules to Li ions. In the present work, to improve the battery performance, we developed new SCESs with alkali metal ions with a large ionic radius as additives. SCES (aqueous SCES + X+) was prepared by mixing LiTFSA (TFSA- : (CF3SO2)2N-), XTFSA (X = Cs, K, Li) and H2O. Raman and attenuated total reflectance infrared spectroscopy (ATR-IR) were performed using a laser Raman spectrophotometer NRS-4500 and Fourier transform infrared spectrophotometer FT/IR-6000, respectively. Linear sweep voltammetry (LSV) measurements were conducted using Pt (anodic scan) and Al (cathodic scan) electrodes as working electrodes. Pt wire and saturated KCl silver chloride silver electrode (SSE) were used as a counter electrode and a reference electrode, respectively. The surface of the Al electrode after the measurement was observed with X-ray photoelectron spectroscopy (XPS). ALIB charge-discharge tests were performed by the in-situ electrochemical impedance spectroscopy2)3). LCO and LTO electrodes (16 mmφ) were used as positive and negative electrodes, respectively. The CR2032 coin cells were assembled with these electrode, GA-55 glass separator (17 mmφ) and the electrolyte solution. The in-situ EIS measurements were carried out at 45°C. The DC current was 1.6 mA (C-rate of 0.5C) and a small AC current was superimposed on DC current adjusted so that the response voltage amplitude was less than 5 mV. Electrochemical measurements were performed using an electrochemical measurement system (SP-150, Bio-Logic). According to the previous study4), Aqueous SCES has a unique liquid structure that coordinates not only water but also TFSA- coordinates to Li ions, which improves the oxidation resistance of water and widens the potential window on the reducing side due to SEI film formation. This indicates that the liquid structure plays a key role in driving ALIB. The broad OH stretching bands of water appeared at approximately 3500 cm-1 with the small shoulder at 3200 cm-1 in IR spectra for these aqueous SCES + X+. The intensity at 3200 cm-1 decreased with the addition of alkali salt, suggesting that the hydration structure and hydrogen bonding changed despite the addition of a small amount of alkali salt. The Raman band of TFSA- at 752 cm-1 was shifted to lower wavenumber as the Cs or K salt were added, suggesting that the addition of Cs+ and K+ metal salts changes not only the water molecules but also the structure around TFSA-. According to the LSV measurement results, there is no difference in oxidation stability among these aqueous SCES + X+. In contrast, the peaks assigned to SEI formation were observed in the cathodic polarization, and their peak positions differed depending on the sample. KF and CsF were formed on the Al electrode surface after the polarization measurements in the aqueous SCES + K+ and Cs+, respectively. In addition, a C-C bond peak was observed on the Al electrode surface measured in the aqueous SCES + Cs+, suggesting that a carbon-derived film is also formed. We believe that the difference in these films affects the battery performance. The coulombic efficiency of ALIB using the aqueous SCES with alkali metal ions after 50 cycles was higher than that of ALIB using the aqueous SCES, remaining above 70%. References1) N. Arai, H. Watanabe, E. Nozaki, et. al. J. Phys. Chem. Lett. 11, 4517−4523 (2020).2) Z. B. Stoynov, B. S. Savova-Stoynov, J. Electroanal. Chem., 183, 133 (1985).3) M. Itagaki, N. Kobari, S. Yotsuda, et, al. J. Power Sources, 135, 255 (2004).4) L. Suo, O. Borodin, T. Gao, et, al. Science. 350, 938-943, (2015).