Measurements of ageing and thermal conductivity in a secondary NMC-hard carbon Li-ion battery and the impact on internal temperature profiles

The ageing of 75 commercial Li-ion secondary batteries with LiNiMnCoO2| hard carbon chemistry was studied up to 4 years. The nominal capacity was 17.5 Ah. The batteries were cycled at different current rates and between different states of charge. Shelf studies were carried out at different temperatures and at different states of charge. The ageing temperature varied from 18-55°C. The specific ohmic resistance was obtained as a function of state of health, ageing temperature, and ageing time. We found that the cell tolerated less cycles at higher temperatures. The loss of capacity also increased for storage at higher temperatures, in a predictable manner. We observed that the state of charge at the moment of storage was very important for the loss of discharge capacity. Thermal conductivities of pristine and aged electrodes were measured in the presence and absence of electrolyte solvent and under different compaction pressures. The thermal conductivity was found to range from 0.14–0.41WK−1m−1 for dry electrode active material and from 0.52–0.73 WK−1m−1 for electrolyte solvent-soaked electrode active material. The thermal conductivity of the electrode materials did not change significantly with ageing, but a strong correlation was seen between remaining battery capacity and increasing ohmic resistance. To assess the impact of these changes, the measured results were used in a one-dimensional model to compute the battery internal temperature. Temperature profiles were computed as a function of discharging rate (2C - 10C) and ageing time (0 - 4 years). The model showed that the internal temperature can raise by a factor about 2.5 during ageing from the pristine state of health at 100 % to 58 % capacity.