| Popis: |
Non-aqueous Li-air (Li-O2) batteries have been emerging as one of the most promising high-energy storage systems, e.g., to meet the requirements for electric vehicle applications, owing to their high theoretical energy density (3505 Wh kg-1 for Li/Li2O2 cells). One of the major challenges to develop high-performance Li-O2 battery is to understand the mechanism how Li2O2 is formed and decomposed during battery cycling. Although many cathode catalysts have proven to be effective to ORR/OER in Li-O2 batteries, little is known on such reversibility at fundamental level at present. Different from Li-ion batteries, the rechargebility cannot be easily determined by Coulombic efficiency in Li-O2 batteries because of extensive parasitic chemistry/electrochemistry. In most of the studies, the evaluations of Li-O2 battery rechargebility have been based on galvanostatic charge-discharge measurements, for which many Coulombic cycles are possible, and each cycle is possibly dominated by solvent decomposition1-3. Consequently, it is impossible to accurately judge the rechargeability of the system without any quantitative evidence to show which kind of and how much electrochemical reactions contribute to discharge or charge stage. Accurate quantification of the amount of Li2O2 that forms and decomposes in each cycle is a direct and simple way to determine the rechargebility of a Li-O2 battery. In this study, we have investigated this important fundamental issue by operando synchrotron X-ray diffraction. We modified the previous operando cell design, to achieve quantitative tracking of Li2O2 during both discharge (ORR) and charge (OER) in a binder-free CNT electrode. We present, for the first time, an approach that combines ex situ spectroscopic analysis and state-of-the-art operando X-ray diffraction with the ability of tracking real-time Li2O2 evaluation, as a means to better understanding of the mechanisms underlying electrochemical reactions. The evolutions of Li2O2 formation and decomposition related to its morphology and structure were observed under the actual electrochemical conditions in real time. Our operando X-ray diffraction results also reveal a two-step growth of Li2O2 on ORR and also a two-step oxidation of Li2O2 on OER, which can be associated with the different mechanisms on forming and decomposing Li2O2. Reference 1. W. Xu, K. Xu, V. V. Viswanathan, S. A. Towne, J. S. Hardy, J. Xiao, Z. Nie, D. Hu, D. Wang and J.-G. Zhang, J Power Sources, 2011, 196, 9631-9639. 2. S. A. Freunberger, Y. Chen, Z. Peng, J. M. Griffin, L. J. Hardwick, F. Barde, P. Novak and P. G. Bruce, J Am Chem Soc, 2011, 133, 8040-8047. 3. F. Mizuno, S. Nakanishi, Y. Kotani, S. Yokoishi and H. Iba, Electrochem Commun, 2010, 78, 3. |
