![]() In principle, neutron diffraction is well suited for in-situ studies of Li-ion batteries 17, 18, 19, 20, 21, 22. Because degradation and failure are spatially heterogeneous, it is also important to conduct spatially resolved measurements. Thus, in-situ study of large format pouch cells is necessary in order to determine the principal factors controlling the degradation in batteries for high power applications. For example, Li + transport in a coin cell, which has excess electrolyte, can be substantially different from that in a pouch cell. As a battery's performance and service life strongly depend upon its design and packaging 16, the degradation mechanisms in large format pouch cells, representative of what is in use in the newest electrified vehicles, are expected to be very different from those in small or coin cells. However, most of these experiments focus on small format batteries, often in half-cell form, in order to allow access to the anode or cathode material for detailed studies. More recently, transmission electron microscopy 13 and nuclear magnetic resonance 14, 15 have been used for in-situ observations. There is a long history of experimental characterization of the charge-discharge process in Li-ion batteries using, for example, impedance measurements 6, laboratory X-ray diffraction 10, 11, and synchrotron X-ray diffraction 12. (The curved white lines in the images are artifacts from collecting the samples.) They may have become electrically disconnected from the current collector. Black indicates the presence of graphite particles that are not lithiated. In (b), taken close to the end cap (about 20 mm away from (a)) the electrode is only partially gold. In (a), taken far from the end caps, the entire region becomes gold, indicating that all of the graphite in this region became lithiated. Full lithiation (LiC 6) turns graphite to a gold color. These regions were shorted to metallic Li and thereby lithiated to the maximum extent possible. The images (a) and (b), each showing a region approximately 2×2 mm 2 on the graphite electrode, were taken from different locations (shown as red squares). Optical micrographs taken from a failed commercial 18650 battery cells to illustrate the nature of heterogeneous failure. Similar results can be expected for large format pouch cell batteries. The deteriorated region, which cannot be fully lithiated, lost 2/3 of its capacity. Lithiation of graphite electrodes – the LiC 6 phase – turns them to a golden color 9 that can be readily observed. The optical micrographs of samples from each region are contrasted in Figure 1. Close examination shows that the central region of the electrode tape is largely homogeneous, while the edge areas appear to be highly fractured with a substantial loss of capacity. The graphite anode suffered damage near each end cap. Figure 1 highlights the spatial inhomogeneity in the anode of a commercial 18650 battery after it lost a significant amount of capacity under cycling. In many cases, degradation and failure in large format batteries start locally at inhomogeneities or weak points, rather than uniformly across the entire battery 3, 7, 8. Analysis of specific degradation mechanisms can in some cases provide a rationale for experimentally observed phenomena, but without comprehensive in-situ experimental data, quantitative cause-and-effect relationships between observation and degradation pathways are difficult to establish. Factors contributing to the durability of large format Li-ion batteries are complex and vary widely with different electrode materials, battery manufacturing processes, cycling rate, temperature and other operating conditions. While the electrochemical performance of a battery can be reasonably well described within the model of Newman 5, our ability to predict battery life is extremely limited because many seemingly unrelated degradation mechanisms have been identified 6. ![]() These requirements are about one order of magnitude greater than what current technology provides in small format cells (as in, for example, laptop computers and cell phones). High power Li-ion batteries require an operating lifetime of over 5,000 cycles and 10–15 years or more in order to achieve the economic viability in battery-powered vehicles and in the future electricity infrastructure 4. High capacity batteries are an important component in the overall strategy for a secured energy future 1, 2, 3. ![]()
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