Assessing key issues contributing to the degradation of NCM-622 || Cu cells. Competition between transition metal dissolution and "dead Li" formation

Combining LiNi${}_x$Co${}_y$Mn${}_{1-x-y}$O${}_2$ (NCM) as cathode with bare Cu as anode will potentially lead to next-generation batteries that are smaller, lighter, and can run for longer periods on a single charge. However, maintaining high performance and a long lifespan of NCM || Cu cells is challenging as it can be affected by various factors from both the cathode and the anode. From the cathode, it is well-known that transition metal (TM) dissolution accelerates cell degradation. From the anode, one of the main challenges is the formation of high surface area Li deposits which later transform into "inactive Li" or "dead Li". In this study, a comprehensive assessment regarding these competing factors (i.e., TM deposits and "dead Li") is discussed. Accelerated TM dissolution is accomplished by introducing TM-containing additives into the electrolyte. The effects of these competing factors and their degradation mechanism are studied quantitatively and qualitatively through inductively coupled plasma, i.e., optical emission spectroscopy and mass spectrometry. The "dead Li" influence is analyzed quantitatively using gas chromatography. The results demonstrate the obvious deleterious impact of dissolved TM ions on cell performance. At the same time, "dead Li" has also become a notable factor for a sudden capacity drop. (© 2024 Wiley‐VCH GmbH)