Performance Degradation Characteristics of Layered Oxide//Silicon-Graphite Full Cells

Abstract

High-energy and high-capacity positive and negative electrode materials are needed to reduce the cost and weight per kWh of Li-ion batteries. The use of silicon, mixed with various amounts and types of carbon, can greatly enhance the specific capacity of the negative electrode. Lithium-rich layered oxides can provide high capacities, as well as the high voltages, required to increase energy densities. The performance degradation of these material types has been shown to be influenced by cell cycling conditions. However, detailed investigations of electrode performance in a full cell using this combination of materials are rarely reported, and the degradation behavior is harder to explain because of the difficulty of interpreting full cell cycling and impedance data. Furthermore, side reactions cause a relative state-of-charge shift of the positive and negative electrodes in a full cell, so that electrochemical cycling between given potential limits in reality exerts varying conditions on the electrodes themselves as the cell ages. In order to understand the degradation behavior of this combination of high-energy high-capacity materials, we conducted extensive tests with 2032-type coin cells and reference electrode (RE) cells; the results of these tests will be reported. The high-capacity negative electrode is a mixed silicon-graphite (SiGr) electrode, and the positive electrode has 90 wt% Li1.03(Ni0.5Co0.2Mn0.3)0.97O2 (NCM523). The coin cells contain 1.58 cm2 area electrodes with 1.2 M LiPF6 in EC:EMC (3:7 w/w) electrolyte (Gen 2). The RE cells contain 20.3 cm2 area NCM523-positive and SiGr-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 effects of various system variables on cell performance and aging have been explored and will be discussed. These variables include upper and lower cut-off potentials for cell cycling, electrode coatings, and electrolyte additives. In addition to the electrochemical results, diagnostic techniques such as X-ray photoelectron spectroscopy and scanning electron microscopy were performed on the electrodes in order to gain a mechanistic understanding into the capacity fade and impedance rise of the NCM523//SiGr cells. Our results provide insight into the performance and limitations of various cell chemistries that are being considered for use in high-energy high-capacity cells. Acknowledgements: Financial support from the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, is gratefully acknowledged. 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.