Origins of Capacity Fade and Material Degradation in Ni-Rich NMC Li-Ion Batteries

Abstract

Nickel-rich layered oxide cathode materials are promising candidates for automotive lithium-ion batteries due to their low cost and high reversible capacity.1 However, paired with graphite anodes these materials exhibit faster voltage and capacity fading compared to isostructural LiNixMnyCozO2 (NMC) cathodes with lower nickel content.2 In this work we will explore the origins of capacity fade and material degradation in graphite/LiNi0.8Mn0.1Co0.1O2 (NMC811) full cells. With a focus on strategically designed electrochemical protocols (see Fig. 1), the role of several key cycling parameters (e.g. upper cut-off voltage, time at high voltage, phase transitions) on the degradation are decoupled. The aging processes are quantified in terms of the full cell capacity retention, impedance rise, electrode slippage, and NMC cathode deterioration. We find that most full cell capacity loss and impedance rise is caused by increasing the upper cut-off voltage to >4.2 V, with the impedance increase largely driven by concomitant full state-of-charge (SOC) cycling. Time spent at high voltage (voltage holds) has less impact on these parameters. These findings are supported by electron microscopy and spectroscopy experiments (SEM, high-resolution TEM, and STEM-EELS) on cycled electrodes, which show clear evidence for cycle history dependent degradation including particle cracking, shearing, and surface reconstruction. Figure 1