High-Energy Li-Ion Batteries: Full Cell and Electrode Monitoring for Evaluating Cycling and Impedance Performance of Layered Oxide//Si-Graphite Cells

High-energy battery materials for both positive and negative electrodes are needed to reduce the cost and weight per kWh for 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, but suffers from irreversible capacity losses related to large volume expansion during cycling [1]. On the positive electrode side, lithium-rich layered oxides can provide high capacities, as well as high voltages, required to increased energy densities. The performance degradation of these material types has separately been shown to be influenced by the cycling conditions [1-3]. However, detailed investigations of the electrode performance in full cells using this combination of materials are rarely reported, and the degradation behavior is often hard to rationalize because of the difficulty of interpreting full cell cycling and impedance data. One issue is that side reactions in the cell cause the operational state-of-charge (SOC) curves of the positive and negative electrodes to shift relative each other. Electrochemical cycling in a given potential range can hence in reality translate to varying electrode conditions as the cell ages.

We have previously used reference electrode measurements for the successful distinction and evaluation of positive and negative electrode contributions to the full cell impedance in aging studies [4, 5]. In the present work, full cell cycling and impedance behavior of a silicon-graphite and layered oxide system is explored in detail by simultaneous monitoring of the positive and negative electrodes. The negative electrode is a mixed silicon-graphite electrode (15 wt% Si, NanoAmor, 73 wt% graphite, Hitachi MAGE, 2wt% carbon, C45 Timcal), and the positive electrode has 90 wt% Li_1.03 (Ni_0.5Co_0.2Mn_0.3)_0.97O₂ (Toda NCM523). Both were obtained from the Cell Analysis, Modeling, and Prototyping (CAMP) Facility, Argonne National Laboratory. A new cell set-up is used that combines both a Li metal reference electrode to examine cycling behavior and a Li_~4.4Sn micro-reference electrode for impedance measurements at different SOC, in order to account for electrode utilization and impedance in relation to cell performance.

The effect of various system variables on performance and aging are explored and will be discussed in the presentation, including the effect of cut-off potentials and electrolyte compositions. Our results provide insight into the performance and characteristics of electrode materials that are being considered for high energy and high voltage cells, and contribute to the development of strategies for their utilization when combined.  

Acknowledgements: Financial support from the Gålö Foundation, the Swedish Royal Academy of Engineering Sciences (IVA), and the U.S. Department of Energy’s Office of Vehicle Technologies is gratefully acknowledged.

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