EQCM and EIS Investigation of Lithium Anode Reaction in Various Electrolyte Solutions

Abstract

Introduction With the increasing concern for energy problems, the demand for secondary battery with high safety, low cost, long life, and high energy density has been rising. The lithium metal is attracting worldwide attention as an anode material for next-generation lithium-based secondary battery such as a lithium air battery (LAB) because of its high capacity (3860 mA h g-1) and negative reduction potential (-3.04 V vs. SHE). However, the formation of lithium dendrites, which cause short circuit, low Coulombic efficiency, and short cycling lifespan, on the electrode surface is one of the biggest issues to practically use the LAB. During lithium deposition, moreover, solid electrolyte interphase (SEI), which is produced by the decomposition of solvents and/or electrolytes, is also accumulated on electrode. In order to suppress lithium dendrite formation [1], we are trying to produce the ideal SEI, which has high lithium ion conductivity, low electron conductivity, and uniform composition with an appropriated thickness [2]. In this study, electrochemical quartz microbalance (EQCM) technique, which provides us with the mass change (Δm) [3,4] and the resonance resistance change (ΔR) [5,6], and electrochemical impedance spectroscopy EIS), which provides us with the resistance at the interface, were employed to investigate SEI formation, lithium deposition, and charge/discharge cycles on Cu electrode. Tetraglyme (G4) which has relatively high flash point and wide potential window with Li[N(SO2CF3)2] (LiTFSI), Li[N(SO2F)2] (LiFSI), or LiOSO2CF3 (LiTFS) was utilized as an electrolyte solution. Experimental After ultrasonically cleaning with ultra-pure water and drying with nitrogen flow, a Cu EQCM electrode (surface area: 0.196 cm2) as a working electrode was set to EQCM electrochemical cell with lithium sheet (99.999%) as a counter electrode in argon gas filled glove box (moisture < 0.5 ppm, oxygen concentration < 1 ppm) at room temperature, followed by input of electrolyte solution such as 1 M LiTFSI in G4, 1 M LiFSI in G4, and 1 M LiTFS in G4. Native-SEI formation, lithium deposition, and charge/discharge cycles were conducted as described below in the thermostatic chamber kept at 15 °C. After native-SEI formation and lithium deposition, electrochemical impedance spectroscopy (EIS) was measured.Native-SEI formation: After 3 h at open circuit potential (~3 V vs. Li/Li+), constant current electrolysis was performed at -0.01 mA cm-2 or -1 mA cm-2 with a cutoff potential of -0.04 V. During electrolysis, the potential, Δm, and ΔR were recorded.Lithium deposition: Lithium was deposited on the native-SEI covered Cu electrode at a constant current density of -0.1 mA cm-2for 1 h.Charge/discharge cycles: A charge/discharge measurements at ±0.1 mA cm-2 for 6 min was performed with a 6-min rest at 0 mA cm-2. Results and Discussion According to the EQCM results, native-SEI was hardly formed at a constant current of -1.0 mA cm-2 in all three electrolyte solutions. In these cases, relatively small amounts of lithium was deposited/dissolved in the charge/discharge cycle measurements. On the other hand, at -0.01 mA cm-2, suitable amounts of the native-SEI was formed in all solutions and ideal behaviors were observed in the charge/discharge cycle measurements. Based on the EIS results, we found that there are few differences of the formed SEIs’ resistance value among three electrolyte solutions formed at -0.01 mA cm-2. However, purer lithium was accumulated during lithium deposition and longer charge/discharge cycles were observed, especially in 1 M LiFSI in G4.