Effect of overcharge on lithium-ion cells: Silicon/graphite anodes
Introduction
With the increased adoption of pure electric vehicles (EVs) by consumers, there is increasing interest in making their total experience with the EV as close as possible to what is familiar to them in an internal combustion vehicle, including driving range. Since driving range is closely associated with the energy that the power supply, that is, the battery pack, can store, several strategies have been proposed. Among them are increasing the number of parallel cell strings in the pack and adopting new, energy-dense materials. The former, as can be easily seen, adds weight, and possibly complexity, to the power supply. The latter increases the intrinsic ability of the electrochemical cell to store energy, and thus may be a more practical solution.
Advanced anode materials have been proposed as a method to store more energy in the cell. These include metal oxides, such as those made by Ostwald ripening of tin oxide [1] to make nanospheres, and novel oxides, such as Bi4Ge3O12 [2]. Another group of materials is based on silicon/graphite composites, which, in principle, take advantage of the high capacity of silicon and the cyclability of graphite. Some investigators have evaluated novel fabrication approaches [[3], [4], [5]] to realize the potential of silicon, 4200 mAh g−1 [6].
From a safety standpoint, using the new materials may also change the abuse response of the cells. Very little information is currently in the literature on the abuse response of silicon/graphite-containing cells. Thus, understanding how these cells respond to, for example, overcharge would be of keen interest to EV developers as the new materials are adopted for use in practical battery packs. Three National Laboratories—Argonne, Oak Ridge (ORNL), and Sandia (SNL)—characterized the response of these cells to overcharging in a systematic fashion.
For this work, 1.2-Ah pouch cells containing Li(Ni0.5Mn0.3Co0.2)O2 (NMC532) cathodes and silicon/graphite composite anodes were charged to 100, 120, 140, 160, 180, and 250% state of charge (SOC). The cell components were then characterized to determine the changes caused by the overcharge.
Section snippets
Experimental
Most of the equipment, techniques, and procedures used in this work were described in earlier papers [7,8]. They are given below for the reader's convenience.
Results
The voltage vs. time curves from these cells as a function of time are given as Fig. S1 in the Supplemental Material. As expected, as more capacity is add, the charging process takes longer and yields higher cell voltages. In general, though, the charging curves have the same basic shape: charge to a capacity level, rest, discharge and rest.
Fig. 1 shows images of the anode immediately after disassembly. In general, not much change was seen on the anode or separator until cell failure. Changes
Discussion
Not surprisingly, the microstructural features and the surface chemistry of the Si/graphite electrode surface were very similar to those found on graphite. In the composite electrode, graphite is the major component and silicon the minor one. Both types of electrodes display dendrite-like features on their surfaces as the extent of overcharge increases. Indeed, even at the limiting SOC, the anode surface from both types of cells displays layered features, however more layers were observed in
Conclusions
Small pouch cells containing Si/graphite electrodes were systematically charged to 100, 120, 140, 160, 180 and 250% SOC. Characterization of the Si/graphite electrode showed attributes similar to those found in a pure graphite electrode. In part, this was not unexpected, since silicon represented a relatively small fraction of the total electrode, ∼15 wt%. The effect of silicon was seen in the composition of the SEI layer and the trends in two components of the SEI layer. One of the SEI
Acknowledgments
The authors thank Dr. Javier Bareño for providing the XPS data and some interpretation. We gratefully acknowledge support from the U.S. Department of Energy, Vehicle Technologies Office. Argonne National Laboratory is operated for DOE Office of Science by UChicago Argonne, LLC, under contract number DE-AC02-06CH11357.
The work at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725, was sponsored by the Office of Energy
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