Aging Behaviour of Ni-rich Li-Ion Battery Material in Winter- and Summer-Like Humid Atmosphere

In recent years, Ni-rich cathode active materials (CAMs) with Ni contents above 80% have gained great importance and are already used in automotive Lithium batteries, such in e.g. Mercedes EQS and EQE models. However, the higher the Ni content in the CAM, the more sensitive it is to water resulting in a high reactivity even in humid atmospheres. The formation of surface species by reaction with water can lead to a performance decay and fast aging of the battery cells.^1–3

We present a systematic study on LiNi_0.83Co_0.12Mn_0.05O₂
in form of a powder or a positive electrode aged under different relative humidities and analyzed subsequently by various analytical methods. First, the powder and electrodes were stored during winter and summer (southwest of Germany) with average relative humidities of 20% and 45% respectively. Both sets of storage were performed at room temperature for 14 days. Samples were taken after 1, 3, 7 and 14 days to examine the progression of aging.

Next, Karl Fischer titration was used to analyze the water uptake after storage and a subsequent drying step. Hydroxide and carbonate species present on the CAM after storage were determined by acid-base titration and compared to the initial values of the pristine powder. Electrical resistance measurements were performed on the electrode composite and electrochemical impedance measurements were carried out to determine changes in charge-transfer resistance. Finally, all electrodes were characterized in pouch cells to determine if and to what extent the storage in humid air impacted the cell performance.

The results demonstrate that the level of humidity during storage has a significant impact on the performance of the materials. The most severe performance impact is observed after storage under 45% humidity while 20% humidity does not significantly alter the samples and their electrochemical performance. This also implies that Ni-rich electrodes do not necessarily have to be produced or processed under dry room atmosphere with very low dew point (_50 to _60°C) as often postulated today.

References

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2. R. Jung, R. Morasch, P. Karayaylali, K. Phillips, F. Maglia, C. Stinner, Y. Shao-Horn and H. A. Gasteiger, J. Electrochem. Soc., 165(2), A132-A141 (2018).
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