Mechanisms of Degradation and Necessity of ALD Coatings for High Voltage NMC532, NMC622, and NMC811 Li-Ion Cells

Lithium-ion battery-based energy storage is widely regarded as the best technology to realize affordable electrification of automobiles (hybrids and EVs), buses, and ships. However strategies to realize higher energy density and lower cost come at the expense of cell lifetime and safety. For example, 10-20% higher energy density can be achieved by charging cells with layered cathodes (e.g., NMC and NCA) to voltages higher than 4.2V, however this is well known to accelerate capacity fade, resistance growth, SEI growth, and gas evolution. Although studies increasingly show the sites of degradation are active material particle surfaces and the active material-electrolyte interface, the specific mechanisms of these phenomena remain unclear and difficult to quantify. Without clear understanding of the specific degradation mechanisms that dominate cell failure during higher voltage cycling, solutions to enable high voltage operation will have limited success. We have isolated key mechanisms occurring at higher voltages in NMC532, NMC622, and NMC811 cells including positive electrode resistance growth (R ct ) as measured by Electrochemical Impedance Spectroscopy (EIS), and Mn dissolution as measured by Inductively Coupled Plasma (ICP) of cycled negative electrodes, and shown the correspondence to cycle life of 95mm x 64mm pouch cells using graphite anodes. Foremost, we have found that Al 2 O 3 ALD coatings dramatically diminish these degradation mechanisms and enable significantly better cycling performance in pouch cells when charged to higher voltage.