Single-Solvent Electrolytes for Lithium-Ion Batteries Studied By Operando Diffusive Reflectance Infrared Fourier Transform Spectroscopy

Abstract

Stable cycling of Lithium-Ion batteries with graphite anodes is only possible due to the formation of a stable solid-electrolyte interphase (SEI). Thus, there is a big interest in surface characterization techniques such as X-ray photoelectron spectroscopy (XPS) or infrared spectroscopy (IR) in order to characterize the electrode decomposition products on the electrode surface. At the same time, gassing phenomena are characteristic for electrolyte decomposition processes [1, 2], whereby on-line gas analysis can provide insight into the corresponding reaction mechanisms [3, 4]. In the present work, we developed a novel cell design for operando Diffusive Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). Using this setup, the working electrode is monitored spectroscopically during electrochemical cycling. Compared to the setup described in the literature [5, 6], our operando DRIFTS has two major advantages. First, with our cell we do not only observe the electrode surface and electrolyte, but also gas evolution during electrochemical cycling. Second, for our DRIFTS studies we use half-cell stacks with separator and working electrode materials that are closer to commercial batteries than the setup described in the literature. Operando measurements were performed with lithium half-cells. To facilitate spectral interpretation, single solvent electrolytes such as solutions of LiPF6 in pure ethylene carbonate (EC) or vinylene carbonate (VC) were applied. Inside our setup, the infrared radiation is diffusively scattered at the working electrode surface. We chose materials such as LiFePO4 and Li5Ti4O12 for electrode preparation. They were found to have better infrared reflective properties compared to highly absorbing graphite or carbon electrodes. Due to the fact that the infrared beam penetrates the gas volume in the head space of the cell, formation of gases is detected as well. For instance, we were able to detect gaseous POF3 and CO2 evolution during electrolyte oxidation at high potentials (see Figure 1). We will present EC and VC oxidation and reduction studies using operando DRIFTS. Acknowledgements: We gratefully acknowledge BASF SE for financial support of this research through the framework of its Scientific Network on Electrochemistry and Batteries.