Fundamental Studies of Interfacial Phenomena at LiNi0.5Mn1.5O4/Graphite Electrodes in Organic Carbonate Electrolytes

Abstract

The LiNi0.5Mn1.5O4 spinel exhibits a charge capacity of 148 mAh/g and operates at potential ca. 4.7 V vs. Li+/Li i.e., at the edge of the electrochemical stability window of standard carbonate-based electrolytes [1,2]. These conditions can result in electrolyte decomposition, electrode surface film formation, which can lead to impedance rise and overall degradation of the battery cell. A variety of spectroscopy and microscopy techniques have been used to investigate basic properties of LiNi0.5Mn1.5O4 and its interfacial behavior in Li-ion cells. In fact, a thin surface layer of electrolyte decomposition products has been detected on LiNi0.5Mn1.5O4 composite electrodes by FTIR, XPS, and TEM [3-6].Interestingly, Raman signal from the electrode/electrolyte interface of cycled cathodes is often obscured by a strong fluorescent signal from the fluorescent products of electrolyte decomposition reactions. It has been also observed in LiNi0.8Co0.15Al0.05O2/graphite Li-ion cells after aging at 55 °C [7]. These fluorescent species, which originate from electrolyte side-reactions tend to spread out through the cell and interfere with the cell chemistryInterfacial reactions at a LiNi0.5Mn1.5O4 spinel electrode in LiPF6-based organic carbonate-based electrolyte were investigated using FTIR, Raman and fluorescence spectroscopies. In situ fluorescence spectroscopy measurements at a carbon- and binder-free LiNi0.5Mn1.5O4 electrode showed formation of fluorescent species that coincides with the oxidation of Ni2+ in LiNi0.5Mn1.5O4. The majority of these electrolyte oxidation products tend to diffuse away into the electrolyte but some remain at the surface of the electrode and form an unstable layer. The formation of photoluminescent active species is not only influenced by the electrode material and the electrolyte but also by the crystalline orientation of the particle surface. The mechanism of these side reactions and their impact on the interfacial chemistry and electrochemical behavior of the graphite electrode will be discussed. Acknowledgments This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy, under contract no. DE-AC02-05CH11231.