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Scanning Acoustic Microscopy of Side Reactions During Initial Cycles of Commercial Lithium-Ion Cells

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In lithium-ion batteries, internal mechanical defects can be caused by a variety of production processes. Extensive research has shown that these defects are accompanied by a massive loss of capacity, are vastly limiting the fast charging capability, or can even lead to a safety-critical state. Therefore, it is of great interest to avoid unwanted artifacts in production processes or quantify whether these will have a detrimental impact on the lifetime or performance of the cell. Unfortunately, this is usually done by tedious and destructive post-mortem analyses in the chemistry laboratory.
In this work, we present an open-hardware apparatus that non-destructively images the internal condition of the battery. This is achieved with a method where ultrasonic waves are emitted and received at each point of the surface of a battery cell with ultrasound transducers (also called scanning acoustic microscopy). The ultrasonic waves change some of their core properties like velocity or amplitude depending on the medium they are traveling through. This effect is used to detect the mechanical properties of the cell components at each position approached by the transducers. Combined with a robust signal processing toolchain, this method produces an image of the inside of the battery in less than 5 minutes with an image resolution in the micrometer range. The significant advantage of this technology compared to other imaging techniques is that it is non-destructive, cost-effective and fast. This makes it suitable for integration into existing production and battery testing processes.
Using the developed apparatus, we demonstrate the effect of a locally clogged separator on battery aging. For this purpose, three cells with known locally clogged separators were clamped and cycled down to 80% residual capacity. The apparatus was able to non-destructively image both the clogged separators and the resulting local covering layer formation and gassing in all investigated cells. Post-mortem analysis revealed that the covering layer was lithium plating, which was plausibly caused by local increased current density at the edge of the clogged separator sites.
The developed apparatus is published as open hardware to increase the availability of scanning acoustic microscopy for further research activities in academia and industry.

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