Meaning
Lithium-ion cells utilizing nickel-rich layered oxide cathodes lose storage capacity and experience impedance rise over prolonged cycling and storage. This performance decay, known as NMC-811 aging, is driven by the chemical instability of the highly de-lithiated cathode surface. It occurs because high nickel content increases the reactivity of the electrode toward the liquid electrolyte at high states of charge.
The boundary of this decay process is accelerated by exposure to elevated temperatures and high cut-off voltages.
Degradation Chemistry
Reactions between the active cathode and the organic solvents lead to the formation of a thick surface layer that blocks ion flow. During NMC-811 aging, transition metal ions can dissolve into the electrolyte and migrate to the anode, where they disrupt the solid electrolyte interphase. This cross-talk increases the rate of lithium consumption.
It leads to accelerated self-discharge.
Structural Collapse
Microcracking occurs along the grain boundaries of the cathode particles due to anisotropic lattice strain during lithium insertion and extraction. This physical cracking, which is a symptom of NMC-811 aging, isolates regions of the active material from the conductive network. The isolated particles can no longer participate in the electrochemical process.
This breakdown is irreversible.
Commercial Consequence
Procurement contracts for energy storage systems establish strict warranty limits on the acceptable rate of capacity loss. Minimizing NMC-811 aging is a primary objective for battery management systems, which limit the upper voltage to extend operating life. This restriction protects the return on investment for the system operator.
