Improvement of the Li Dissolution/Deposition Reversibility Under Oxygen Atmosphere By Employment of LiNO3⁻based Electrolyte

Abstract

Introduction Rechargeable non-aqueous Li-air batteries (LABs) have attracted much attention as one of the next-generation batteries because of their potential for possessing large specific energy (theoretical value: 3.5 kWh kg-1) more than 5 times of those of Li-ion batteries [1]. For practical use, some technical problems have to be overcome; during discharging, the choking of air electrode by the Li2O2 generated; during charging, the requirement of applying high overpotential for the electrochemical oxidation of Li2O2 or the short circuit of LAB cells caused by Li dendrite growth on Li electrode. Li dissolution/deposition reactions using various Li salts, solvents, and additives have been investigated under oxygen free condition, which is far from practical use, to find that the repetition of the reaction provides Li deposition on Li electrode non-uniformly with clear boundary. This is probably caused by the chemical composition and uniformity of the solid electrolyte interphase (SEI) films formed on Li electrode. Such Li deposition can raise the problems mentioned above. These remaining problems, moreover, restrict the Li dissolution/deposition reactions only at low depth of charge/discharge though LABs are promising due to their large specific energy. We have reported that the presence of oxygen improved the Li dissolution/deposition reversibility, due to the formation of proper SEI films [2]. In this study, focusing on not only presence of oxygen gas but also LiNO3-based electrolyte [3], we evaluated the reversibility of Li | Li symmetric cell and Li-air (Li | O2) cell at relatively high state of charge/discharge. Experimental Li | Li symmetric cells was constructed with Li metal foils (thickness：0.5 mm) used for both electrodes, a separator (Celgard 2400), and 1.0 M glyme-based electrolytes containing LiSO3CF3 (LiOTf), LiN(SO2CF3)2 (LiTFSI) or LiNO3 in an Ar-filled dry box. Li-air (Li | O2) cells were constructed in the similar method but Ketjen Black-loaded carbon paper was used as an air electrode, the positive electrode. Li dissolution/deposition tests were conducted under Ar or O2 atmosphere. The applied current density and maximum discharge/charge capacity were 0.20 mA cm−2 and 2.0 mAh cm−2 (corresponding to 10 μm of Li dissolution/deposition), respectively, in the cut off voltage range of −2.0 to 2.0 V. The Li metal electrodes after 15 cycles of the tests were rinsed with tetraglyme to remove Li salts, and the obtained electrodes were analyzed using scanning electron micrograph (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) to characterize the resulting SEI films. Results and Discussion Fig. 1 showed the surface and cross-sectional SEM images of Li electrodes of Li | Li symmetric cells after 15 cycles of Li dissolution/deposition reactions at 2.0 mAh cm−2. In the case of LiTFSI-based electrolyte, oxygen suppressed crack and Li dendrite growth (thick deposition) compared to oxygen free condition. In addition, oxygen suppressed the decomposition of the electrolyte, LiOTf tetraglyme solution, and enhanced the deposition of Li2O, which works as a protective layer, though rather non-uniformly. These are indicated by chemical compositions determined by EDS (F and S contents are lower under oxygen atmosphere) and XPS spectra (O1s, Li1s, C1s, and F1s spectra). In the case of LiNO3-based electrolyte under oxygen, larger overpotential was observed than that in the case of LiOTf and LiTFSI but the suppression of dendrite growth and the uniform Li2O deposition were observed on the Li electrodes. These must contribute to improve the reversibility. The similar tendency was also observed for Li-air (Li | O2) cells. The cell using LiNO3 tetraglyme solution as an electrolyte showed the suppression of Li dendrite growth and the uniform deposition of Li2O, though the charge/discharge reversibility was slightly improved in this experimental condition. These results can be associated with the formation of uniform Li2O protective layer on Li negative electrode, probably promoted by NO3 -: NO3 -+ 2Li →NO2 -+ Li2O [4]. Of particular note is that these Li dissolution/deposition reactions using LiNO3-based electrolyte, which showed the suppression of Li dendrite growth and the uniform deposition of Li2O, were performed at high depth of charge/discharge, where the thickness of dissolved/deposited layer reached about 10 μm, comparable to practical use.