Analysis of Transient Current and Heat Flow during Voltage Holds: For 70 wt% Silicon Anode Lithium-Ion Cells

This study compares the impact of conducting voltage hold measurements with Si/NCA Lithium-ion cells containing 70 wt% silicon in the anode and G/NMC811 cells. Silicon is known to present significant volume changes and hysteretic behavior, which has a big impact on the electrochemical behavior of the cells. Isothermal microcalorimetry is combined with voltage hold measurements to investigate the heat flow signals in relaxed (with 30 days storage) and unrelaxed (without storage) conditions. In addition, the influence of relaxation on the voltage hold experiments was investigated in different cell formats: coins, T-cells, and multi-layer pouch cells. The calorimetry results indicate that the equalization of inhomogeneities, and relaxation hysteresis, are more pronounced electrically than thermally, showing a pronounced impact from silicon´s intrinsic material characteristics. When 720 h of storage is introduced, the transient heat flow for silicon and graphite is similar and stabilizes much earlier for coin cells. Moreover, when conducting voltage holds with positive and negative charging currents, the intrinsic hysteresis of silicon impacts the amount of the measured leakage current. For graphite, the leakage currents are very similar.