Lithium- and manganese-rich layered oxides, also referred to as high energy NCM (HE‑NCM), are a promising class of materials for applications in future Li-ion batteries. Due to their high specific discharge capacity (~250 mAh/g) and the high operating voltage, the energy density of next-generation batteries could be increased using these materials [1]. However, using ethylene-carbonate (EC) based electrolytes with Li- and Mn-rich layered oxides (HE-NCM) leads to a rapid capacity decay, and in order to improve the lifetime of HE‑/NCM//Graphite full-cells, EC has to be replaced by a different cyclic carbonate [2].

Thus, fluoroethylene carbonate (FEC) is commonly used as cyclic carbonate to replace EC when using HE‑NCM. While FEC was shown to significantly improve the cycling stability of HE-NCM, it is suggested that residual FEC, used as co-solvent, reacts with LiPF₆ conductive salt, leading to detrimental side reactions within the electrolyte [2-4]. This reaction was shown to be significantly enhanced at high temperature cycling, while the exact mechanism is still not clear [3].

As a first step, we will use on-line electrochemical mass spectrometry (OEMS [5]) to examine the chemical FEC degradation in presence of LiPF₆, studying the gas evolution of LiPF₆ (1.5 M) containing solutions of EC and FEC at different temperatures. Figure 1 shows the gas evolution of EC + 1.5M LiPF₆ and FEC + 1.5M LiPF₆, with the aim to quantify the thermal stability of the conductive salt with different cyclic carbonates. While POF₃ evolution in EC starts only at temperatures >60°C, it can be clearly seen that POF₃ evolution in an FEC solvent starts already at 25°C, indicating the decomposition of LiPF₆ already at room temperature.

By analyzing the gases evolved from these reactions we aim to get mechanistic insights on the degradation mechanisms of LiPF₆ in combination with FEC. Furthermore, we will use full cell cycling and electrochemical impedance spectroscopy in order to get evidence about the practical effect of these reactions, which is investigated at 25°C and 45°C. For the analysis of the decomposition products, X-ray photoelectron spectroscopy (XPS) will be used to analyze products on the anode and the cathode, produced in EC and FEC containing electrolyte.

Literature:

[1] M. M. Thackeray, S.-H. Kang, C. S. Johnson, J. T. Vaughey, R. Benedek and S. A. Hackney, J. Mater. Chem. 17, 3112-3125 (2007).

[2] T. Teufl, D. Pritzl, B. Strehle, H.A. Gasteiger and M. A. Mendez, manuscript in preparation.

[3] K. Kim, I. Park, S.-Y. Ha, Y. Kim, M.-H. Woo, M.-H. Jeong, W.C. Shin, M. Ue, S.Y. Hong and N.-S. Choi, Electrochimica Acta 225, 358-368 (2017).

[4] B. Aktekin, R. Younesi, W. Zipprich, C. Tengstedt, D. Brandell and K. Edström, J. Electrochem. Soc., 164(4), A942-A948 (2017).

[5] N. Tsiouvaras, S. Meini, I. Buchberger and H. A. Gasteiger, J. Electrochem. Soc., 160(3), A471-A477 (2013).

[6] M. Metzger, M. Egawa, S. Solchenbach, and H. A. Gasteiger, Meeting Abstracts, MA2017-02 (4), 272 (2017).

Acknowledgement:

The authors want to thank BASF SE for their financial support of this work.

Figure 1: OEMS cell containing 240 μl electrolyte, without separator or electrodes. The temperature applied to the cell is shown in the upper panel and was increased from 25°C up to 80°C. The lower panel shows the signal measured for m/z = 85, which can be correlated to POF₃, as well as PF₅ [6].