Noticeable Role of TFSI- Anion in the Carbon Cathode Degradation of Li-O2 Cells

Daniela Giacco; Marco Carboni; Sergio Brutti; Andrea G Marrani

doi:10.1021/acsami.7b05153

Noticeable Role of TFSI^- Anion in the Carbon Cathode Degradation of Li-O₂ Cells

ACS Appl Mater Interfaces. 2017 Sep 20;9(37):31710-31720. doi: 10.1021/acsami.7b05153. Epub 2017 Sep 7.

Authors

Daniela Giacco¹, Marco Carboni¹, Sergio Brutti^{2

3}, Andrea G Marrani¹

Affiliations

¹ Department of Chemistry, Sapienza University of Rome , P.le Aldo Moro 5, 00185 Rome, Italy.
² Department of Science, University of Basilicata , V.le Ateneo Lucano 10, 85100 Potenza, Italy.
³ Istituto dei Sistemi Complessi, Consiglio Nazionale delle Ricerche (ISC-CNR), Via dei Taurini, 00185 Rome, Italy.

PMID: 28853551
DOI: 10.1021/acsami.7b05153

Abstract

In this work we address the phenomena at the basis of the performance loss in a Li-O₂ cell operating in the presence of a lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)/tetraethylene glycol dimethyl ether (TEGDME) salt/solvent couple and a porous carbonaceous cathode. The cell was discharged/charged applying both voltage and capacity limits, and the effects of repeated galvanostatic cycling were addressed. The ex situ characterization of carbonaceous cathodes corresponding to different cutoff voltages was based on vibrational spectroscopies, transmission electron microscopy, and X-ray photoelectron spectroscopy. The reversible precipitation/decomposition of undesired products deriving from degradation of both carbon cathode and ethereal solvent is pointed out within a single voltage limited (2.0-4.6 V) discharge/charge cycle, whereas their irreversible accumulation on the surface of the electrode results after 100 capacity limited cycles. At the same time, the presence of polar degradation products (carbonates and carboxylates) at the cathode surface is accompanied by the buildup of a surface electric potential gradient, as revealed by differential binding energy shifts resulting from C 1s photoelectron spectra. This effect, seldom reported for Li-ion batteries, is for the first time put in evidence for a Li-O₂ cell. Furthermore, the use of TFSI^- anion is shown to lead to carbonate-based degradation products not involving the formation of Li₂CO₃. The peculiar occurrence of such degradation phenomena are attributed to the intrinsic low-donor number characteristic of the TFSI^- anion.

Keywords: Fourier transform infrared spectroscopy; LiTFSI/TEGDME electrolyte; Li−O2 battery; X-ray photoelectron spectroscopy; carbon cathode surface reactivity; transmission electron microscopy.