Oxygen loss at high voltages in Ni-rich NMC//graphite Li-ion batteries promotes degradation but increasing evidence from full cells has shown the depth of discharge choice can further accelerate aging i.e. synergistic degradation. In this letter, we employ cycling protocols of single crystal LiNi0.834Mn0.095Co0.071O2//graphite pouch cells to examine the origin of the synergistic degradation in terms of material degradation mechanisms. In regimes where oxygen loss is not promoted (V < 4.3 V), discharging to a lower cutoff voltage improves capacity retention despite significant graphite expansion occurring. In contrast, when NMC surface oxygen loss is induced (V > 4.3 V), the deeper depths of discharge lead to pronounced faster aging. Using a combination of post-mortem analysis and density functional theory we present a mechanistic description of evolution of the surface densification as a function of voltage and its impact on lithium-ion kinetics to explain the observed cycling results.
Supp. Info.: Synergistic degradation mechanism in single crystal Ni-rich NMC//graphite cells
Supporting Information: Methodology and details on cells and active materials, normalized capacity graph to show SEI dominated degradation, dQ/dV and the crystal structure, SEM Ni-rich single crystal particle morphology, CEI and surface species,