Reaction Mechanism Analysis of a Li-O2 Battery: Structure of Electrode/Electrolyte Interface Probed Via Soft-x-Ray Absorption Spectroscopy, Hard x-Ray Photoelectron Spectroscopy, and in-Situ Hard x-Ray Diffraction

Rechargeable Li-air batteries have been attracting much attention because of their extraordinary high energy capacity, viz, theoretically it reaches more than ten-times than those of existing Li-ion secondary battery. Recent materials development, such as use of glyme-series electrolytes, effective electrodes (metal based catalysts and/or high surface area carbon materials with nitrogen doping), or use of Redox-mediators, is about to find realistic solutions to overcome long-term crucial issues of Li-air battery, such as large over-potential at charge process, electrolyte degradation due oxygen radicals, and low cycleabilty presumably caused by unstable electrode properties at both anodes and cathodes.

Thus, additional deeper information on the reaction mechanism, involving unwanted side reactions, could be expected to accelerate such improvement. In this context, we performed reaction mechanism analysis by utilizing various experimental techniques, such as in-situ time-resolved x-ray diffraction, soft x-ray absorption spectroscopy at Li, C, O, and S K-edges, and hard x-ray photoelectron spectroscopy (HAXPES), combined with other conventional laboratory analysis methods.

Charge/discharge tests of Li-O₂ battery were performed by using standard coin-type or electrochemical cells under various conditions including impact of moisture to the reaction. Soft x-ray absorption spectroscopy measurements were carried out at SR center of Ritsumeican University (BL-11) and Aichi synchrotron radiation center (BL6N1). Hard x-ray photoelectron spectroscopy and in-situ hard x-ray diffraction measurements were performed at SPring-8 on beamlines BL46XU and BL19B2.

We observed formation and dissolution processes of Li₂O₂ via in-situ time-resolved hard x-ray diffraction, which exhibited non-linear behaviors reflecting difference of nature of deposited Li₂O₂ particles. Also we analyzed the structure and chemical compositions of interface between Li₂O₂ and/or carbon electrodes and electrolyte solutions via soft x-ray and hard x-ray spectroscopy. The result indicates the structure of interface is significantly affected by the operation condition, and consists of various species decomposed from electrolyte and/or Li-salts. More detailed discussion will be presented at the Meeting.