Volume and Thickness Change of NMC811|SiOx Graphite large format Lithium Ion Cells: from Pouch Cell to Active Material Level

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The reversible thickness change of a large-format lithium-ion automotive pouch cell was investigated under precisely monitored cell pressure and temperature using an in-house built actively controlled pneumatic cell press. The quantitative and qualitative contribution of the state-of-the-art NMC811 cathode and the SiOx-graphite composite anode to the total pouch cell expansion was resolved by electrochemical dilatometry and validated. At the beginning of discharge, the pouch cell thickness first decreased and suddenly increased again between 90% and 80% SOC. Electrochemical dilatometry revealed that this increase in cell thickness was caused by a steep increase in cathode thickness at the beginning of discharge. In this case, the decrease of NMC811 volume counteracted the thickness change of the SiOx-Gr composite anode by 21.8%, which is interesting for automotive battery and cell design decisions. Overall, a highly nonlinear expansion behavior for the cathode was observed which was linked to the change of the lattice parameters of the crystal structure. Pouch cell thickness showed large hysteresis which was caused by the voltage and volume hysteresis of SiOx inside the anode. For closer inspection, the capacities of both anode active materials were estimated using DVA. The sum of both active material voltage curves showed great agreement with the actual half-cell measurements of the composite anode and allowed to calculate the SOC and the volume expansion of each active material as a function of the anode SOC. By introducing fitting parameters and applying theories about the interaction of SiOx with the surrounding morphology, the correlation between the volume expansion of the active materials and the thickness change of the SiOx-Gr composite anode was investigated. The findings suggest that there is significant nonlinear reduction of pore volume at low state of charge. Here, the impact of the microscopic SiOx-carbon-black-binder-gaps on the thickness change of large-format Li-ion cells were quantified for the first time. Overall, about 52.40% of the total 7.85% thickness change of the anode was caused by graphite and 47.60 % was caused by SiOx.

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