Diagnostic Examination of NCM523//Graphite Cells Cycled at High Voltages

Abstract

High energy materials are needed to increase the energy density of Li-ion batteries for vehicular applications. Layered-oxides are promising high energy positive-electrode materials; however, the performance of cells containing these oxides degrades faster when cycled to high potentials (> 4.3 V). In order to understand the origin of this performance loss, we conducted extensive tests with 2032-type coin cells and reference electrode (RE) cells; the results of these tests will be reported during this presentation. The coin cells in this study contain 1.6 cm2 area Li1.03(Ni0.5Co0.2Mn0.3)0.97O2-based (NCM523) positive and a graphite-based negative electrode (Gr); the electrolyte is 1.2M LiPF6 in EC:EMC (3:7 w/w) electrolyte (Gen 2). The reference electrode cells contain 20.3 cm2 area NCM523-positive and Gr-negative electrodes, two Celgard 2325 separators enveloping a LixSn RE, and a Li-metal RE external to the electrode sandwich. Cell cycling includes formation cycles followed by aging cycles; pulse-power and AC impedance measurements are made periodically throughout the cycling.The Li-metal RE provides information on the positive and negative electrode potentials during full cell cycling; such information cannot be obtained from the coin cell data. For example, during cell formation, when the full cell is cycled between 3.0 and 4.4 V, the positive electrode cycles between 3.67 and 4.46 V vs. Li/Li+ and the negative electrode cycles between 0.67 and 0.06 V vs. Li/Li+. The electrode cycling potential ranges change during cell aging. Furthermore, electrode impedances measured with the LixSn RE also change during aging. We will highlight the reasons for these changes during the presentation. In addition to the electrochemistry results we will discuss data obtained by various diagnostic techniques that include the following: Scanning electron microscopy, Raman spectroscopy, and X-ray diffraction. Our mechanistic understanding of the capacity fade and impedance rise of the NCM523//Gr cells will be discussed through these various data.