Gas evolution in lithium-ion batteries due to non-reversible aging phenomena is one of the most problematic side reactions in battery technology. Besides the fact that it is usually accompanied by high capacity loss, gas evolution limits the safety of lithium-ion batteries, and the risk of thermal runaway increases dramatically.
In this work, we present an open-hardware setup for the non-destructive detection of specifically this gas formation with an ultrasonic imaging technique. This is achieved with a method where ultrasonic waves are generated 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. We use this effect to detect the mechanical properties of the cell components at the position of the transducers. Combined with a robust signal processing toolchain, this produces an image of the inside of the battery in less than 5 minutes and 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.
With the developed apparatus, we demonstrate how gas is locally generated due to the electrolyte decomposition in the first 16 cycles of a 7.5 Ah N1M1C1/graphite pouch cell. This may have occurred as a result of incomplete formation at the battery manufacturer or due to effects that occurred during transport or storage of the cell. In the following cycles, the amount of gas is seen to disappear strongly, which can be explained by dissolving in the subsequent electrolyte reduction at the anode surface. Along with the ultrasonic measurements, electrochemical impedance spectroscopy measurements were performed. An increase in the ohmic resistance and a decrease in the charge transfer resistance were observed, confirming the presumptions made in the ultrasonic images.
The developed apparatus is published as open hardware to increase the availability of scanning acoustic microscopy for further research facilities.