High Energy Electrodes Based on Nickel-Rich Mixed Oxides: What Are High Operational Voltages Good for and How Do They Affect Lifetime?

摘要

Nickel-rich layered mixed oxides such as NCM622 or NCM811 exhibit in general a considerable higher specific energy compared to NCM111 which is widely used in many commercial cells today. In addition, they promise cost reduction due to a reduced cobalt content. However, it seems that the nickel-rich materials show an inferior cyclic stability and are therefore mostly used within a relatively narrow cell voltage window today. Although these materials can be operated at higher operational voltages the end-of-charge voltage is typically limited to 4.2 - 4.3 V (vs. graphite anode). To considerably increase the energy- and power density and to bring down costs per kWh at the same time, people strive to increase the operational voltage of the cells up to 4.4 V or even higher. To do so, it is essential to better understand the ageing behaviour of the cells and the underlying mechanisms. The poster should contribute to close this lack of knowledge. We will present results comparing the electrochemical performance and cycle stability of cells operated at different voltages using NCM622 cathodes and accordingly balanced graphite based anodes. Beside detailed electrochemical tests we performed extensive post-mortem analysis using different techniques such as scanning electron microscopy (SEM), focussed ion beam preparation (FIB), energy dispersive x-ray analysis (EDS) and x-ray diffraction (XRD) but also electrochemical tests with rebuilt (aged) cells. In doing so we were able to detect crucial ageing mechanisms on various length scales on both electrodes that correlate with the operational voltage.