The researchers observed the inside of the battery as it charged and discharged over 15 hours. They gathered unique, three-dimensional movies that capture the degradation mechanisms occurring in the battery and quantified the processes occurring within every particle for the thousands of particles in the electrode. The results of this study will be published in the journal Science; a pre-print version is available online at Science Express.
Irreversible structural changes
The data illustrate that tin oxide (SnO) particles expand during charging due to the influx of lithium ions causing an increase in particle volume. The scientists demonstrate that material lithiation happens as a core-shell process, progressing uniformly from the particle surface to the core. The material undergoing this reaction expands linearly with the stored charge. The x-ray images show that charging destroys the particle structure irreversibly with cracks forming within the particles. "This crack-formation is not random," emphasizes Ebner. Cracks grow at locations where the crystal lattice contains preexisting defects. During discharge, the particle volume decreases; however, the material does not reach its original state again; the process is therefore not completely reversible.
The volume change of the individual particles drives expansion of the entire electrode from 50 micrometers to 120 micrometers. However, during discharge, the electrode contracts only to 80 micrometers. This permanen
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