Formation Strategies for Over-Lithiated NCMs Suitable for Large-Scale Cells

Abstract

As a potential next generation cathode material, Li- and Mn-rich layered oxides, also referred to as High-Energy NCM (HE-NCM), offer high reversible capacities of ≈250 mAh/g. Despite the high capacity and relatively low material cost compared to state-of-the-art cathode materials, the use of HE-NCM is still challenged by issues such as voltage fading, a relatively large charge-discharge voltage hysteresis, and high impedance. Gassing, especially oxygen and carbon dioxide release during the initial formation cycles with HE-NCM, has been studied before [1] and a recent publication by Teufl et al. [2] correlated the extent of gassing to the extent of surface degradation and impedance gain. An up to now neglected issue of the extensive gassing of HE-NCM concerns the associated difficulty during the formation of large-scale cells. The roughly one order of magnitude higher quantity of gas released from HE-NCM compared to regular NCMs results in either a large volume expansion in pouch-cells or a large pressure increase in hardcase-cells. The amount of the released gases produced by the activation of the HE-NCM during the first cycle and during cycles 2-5 depends significantly on the formation temperature, as shown by on-line electrochemical mass spectrometry (OEMS) data, given in Fig.1. For example, increasing the formation temperature from 15 to 45°C does not change the total amount of gas evolved over 5 cycles, but shifts a large fraction of the total amount of released gas into the first formation cycle. This is beneficial for industrial cell production, as degassing steps are usually placed in the first cycle. On the other hand, Erickson et. al. [3] have shown that an increased formation temperature has a negative impact on the cycling performance, leading to lower capacities. By combining OEMS analysis with operando pressure measurements, we will show the effect of different formation procedures (variation of temperature, C-rate, and number of formation cycles) on the amount of evolved gas and the long-term cycling performance. In parallel, we correlate the formation temperature with the impedance of the cell at different stages of cycling to connect the cycling performance with possible generated resistance due to the formation. Finally, the OEMS based gassing data will be used to project the behavior of 7.5 Ah pouch-cells (as done in Fig. 1) in order to evaluate the different formation procedures with regards to their feasibility for the formation of 7.5 Ah pouch-cells.