Anodic Oxidation of Carbon and Electrolyte with Different Conducting Salts in High-Voltage Lithium-Ion Batteries Studied By Online Electrochemical Mass Spectrometry

Abstract

Recently, many research activities have been devoted to the development of near 5 V cathode materials, e.g. the LiMn1.5Ni0.5O4 spinel, in order to raise the energy density of lithium-ion batteries and to allow for longer driving ranges of battery electric vehicles.1 However, the enhanced degradation of carbon and electrolyte by the use of these high-voltage cathodes could not be mitigated so far. It was demonstrated recently by On-line Electrochemical Mass Spectrometry (OEMS),2 that the anodic oxidation of conductive carbon, carbon coatings, and electrolyte at ≈5.0 V can be substantial at high temperature and in the presence of trace water, posing significant challenges for the implementation of 5 V cathode materials.3,4 While these studies were done with LiClO4 as conducting salt in order to study the effect of H2O addition on oxidation without side reactions of salt and water, e.g. HF formation, we want to investigate now to which extent the lithium salt can influence gas generation at high voltage. We employ our newly developed two-compartment cell in which anode and cathode compartments are separated by a Li+-ion conducting solid electrolyte (Ohara glass) laminated with aluminum and polypropylene foil, so that the gas evolution from degradation processes at high voltage can be studied selectively for the positive electrode without cross-diffusion of reaction products and gas generation from the counter-electrode, thereby enabling a more detailed analysis of the decomposition pathways.5 This is a major advance over conventional cells, where the gases come from both electrodes, and thus do not allow a deconvolution of the simultaneously occurring reactions from anode and cathode. Furthermore, OEMS is used to compare three types of lithium salts in terms of their influence on the anodic stability (close to 5 V) of electrolyte and conductive carbon in the battery cell. These are the commercially used salt LiPF6, the sulfur- and nitrogen-containing LiTFSI, and the fluorine-free and oxygen-containing compound LiClO4. The salts are mixed with ethylene carbonate (EC) at a concentration of 1.5 M, so that linear carbonates like EMC or DMC which have a much higher vapor pressure than EC can be avoided, allowing for precise signal quantification in OEMS.3 The comparison of the salts will be done on the basis of the CO/CO2 gas evolution monitored by OEMS at various temperatures between 25 and 60°C. We employ a fully 13C-labeled carbon electrode to deconvolute the CO/CO2 evolution from electrolyte oxidation (12C) from that of the conductive carbon oxidation (13C). We quantify our OEMS results using a calibration gas, and give both, quantitative and mechanistic insights into the effect of the conducting salt on gas evolution in high-voltage lithium-ion batteries. By quantification of both CO/CO2 isotopes we determine the molar oxidation rate and the weight loss of electrolyte and carbon due to anodic oxidation. In summary, this study elucidates to which extent the lithium salt can influence gas generation at high voltage and might allow to deduce design principles for the synthesis of novel electrolyte salts. Figure 1 shows in the upper panel the current-potential profiles of 13C-carbon//lithium half-cells with the three different conducting salts at 1.5 M in EC upon a linear potential sweep from OCV to 5.5 V vs. Li/Li+. The corresponding evolution of both isotopes of CO2 (solid lines) and CO (dotted lines) for the electrolyte oxidation and the 13C-carbon oxidation are shown in the middle panel and the lower panel, respectively. References O. Gröger et al., J. Electrochem. Soc., 162, A2605 (2015).N. Tsiouvaras et al., J. Electrochem. Soc., 160, A471 (2013).M. Metzger et al., J. Electrochem. Soc., 162, A1123 (2015).M. Metzger et al., J. Electrochem. Soc., 162, A1227 (2015).M. Metzger et al., J. Electrochem. Soc., 163, A798 (2016). Acknowledgement The authors gratefully acknowledge BASF SE for financial support of this research through the framework of its Scientific Network on Electrochemistry and Batteries. Figure 1. Carbon and electrolyte oxidation upon linear potential sweep from OCV to 5.5 V vs. Li/Li+ at 0.1 mV/s with a 13C-carbon working-electrode and a metallic lithium counter-electrode for an EC-based electrolyte with 1.5 M LiClO4, LiPF6, or LiTFSI, respectively. (a) Current-potential profile, (b) 12CO/12CO2 from electrolyte oxidation, (c) 13CO/13CO2 from carbon oxidation. Figure 1