A Comparative Study of Structural Changes during Long-Term Cycling of NCM-811 at Ambient and Elevated Temperatures

Lithium-ion batteries operate predominantly at room temperature, but some applications such as electric vehicles also demand operation at higher temperature. This is especially challenging for cathode active materials (CAMs), which undergo an accelerated failure at elevated temperature. Here, we systematically compare the capacity fading of the Ni-rich NCM-811 at two different temperatures. The first dataset over 1000 cycles at 22 °C stems from a former study, while the NCM-811/graphite full-cells are investigated now under similar conditions at 45 °C for 700 cycles. We focus on the CAM by using pre-lithiated graphite anodes. The capacity loss due to NCM-811 degradation at 45 °C is more than doubled compared to 22 °C. The underlying mechanisms related to the bulk and the surface of the CAM are quantified by several ex situ techniques such as X-ray powder diffraction, half-cell cycling with impedance spectroscopy, and Kr-BET. The aging happens mainly at the surface of the primary particles, forming a resistive, disordered surface layer, whose thickness is estimated to reach ≈6 nm at 22 °C and ≈12-14 nm at 45 °C by the end-of-test. Furthermore, the Li-Ni mixing in the bulk increases by ≈1%-2% at elevated temperature, but its contribution to the capacity loss remains elusive.