High-energy electrochemically-active materials are urgently needed to increase the energy density of lithium-ion batteries for vehicular applications. Layered-oxides are promising high-energy positive electrode materials. However, the performance of full cells containing these oxides degrades faster when cycled to high potentials (> 4.3 V w/graphite negative electrodes). In order to understand the origin of this performance loss, we conducted extensive aging tests in 2032-type coin cells and reference electrode (RE) cells. The observations and conclusions of our various tests will be highlighted during this presentation.

The coin cells in this study contain 1.6 cm² area Li_1.03(Ni_0.5Co_0.2Mn_0.3)_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 Li_xSn 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 Li_xSn 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.

Acknowledgments

This document has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. Financial support from the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, is gratefully acknowledged. Use of the Center for Nanoscale Materials and the Advanced Photon Source was supported by the U. S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.