Higher capacitive active materials, thicker electrode coatings, and/or overall larger sized cells are current trends to increase the energy density in lithium-ion cells. Thereby, safe operation over a long battery lifetime needs to be maintained or even improved. Regarding safety incidents, short-circuits are often the cause for critical states leading to irreversible cell damage or total failure (e.g. thermal runaway). A better understanding of the short-circuit behavior helps to improve safety systems, to mitigate or even prevent the actual shorting scenario, e.g. by applying specified cooling procedures. The correlation of the experimental and the electrochemical simulation studies results show that the appearing plateau and transition regions of current flux, cell voltage, and heat generation originate by transport limitations in the electrolyte and in the active material particles, as well as by inhibition of reaction kinetics. Recent work compares and correlates the external (i.e. triggered via the current collector tabs) and local (i.e. triggered via nail penetration in the center of the active electrode area) short-circuits. Hereby graphite/NMC-111 pouch-type lithium-ion cells with moderate electrode loadings of ≈ 2 mAh cm-2 were investigated.
This presentation extends the short-circuit investigations to custom-built, silicon-graphite/NMC-811 and silicon-graphite/NCA cells at higher electrode loadings (i.e. > 2.0 mAh cm-2) considering the trend towards high-energy cells. For the short-circuit measurements, a quasi-isothermal, calorimetric test bench embeds the single-layered pouch-type cells between two copper bars in a thermally insulated case. Both the test bench and the measurement equipment are placed in a custom-built climate chamber at a constant temperature of 25°C. The cell under test is either externally shorted via the current collector tabs using a 0 V-condition applied by a potentiostat, or shorted in the center of the cell using a nail-penetration technique with a stainless-steel needle (Ø 1 mm). Three digital multi-meters, a source measurement unit, and the potentiostat measure the current flux, cell voltage, and heat rate/cell temperature during an external short-circuit. The same is possible for the local short-circuit except for the current flux. However, the electrical potential in the penetration spot vs. the cell’s negative tab is measured via an electrically conductive needle. Based on the measurement results, external and local short-circuits are investigated towards similarities and differences appearing in the electrical-thermal behavior. The differences from common graphite/NMC-111 to silicon-graphite/nickel-rich cells will be discussed. Finally, the results of the post-mortem analysis regarding degradation on a local scale over the electrode width and length (i.e. 3 cm x 5.45 cm) for the tested cells are presented.
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