Degradation Mechanisms

A new lithium-ion battery rated at 100 kWh will hold only 70-80 kWh after 8-10 years of typical use. That capacity loss is not a manufacturing defect. It is chemistry: four distinct mechanisms slowly consume the materials that store lithium.

SEI growth is the dominant degradation mechanism. The solid electrolyte interphase (SEI) is a thin layer that forms on the anode surface during the first few charges, stabilizing the interface between graphite and electrolyte. But the SEI continues to grow slowly over time, consuming lithium ions and electrolyte. Each lithium ion trapped in the SEI is one that can no longer shuttle between electrodes. Lithium plating occurs during fast charging or cold-temperature charging, when lithium deposits as metallic lithium on the anode surface instead of intercalating into graphite. Plated lithium is lost from cycling and can form dendrites. Electrode cracking results from mechanical stress as electrodes expand and contract with each charge/discharge cycle. Cracks expose fresh surfaces that form new SEI, accelerating lithium loss. Calendar aging degrades batteries even when idle, especially at high temperature and high state of charge. A fully charged battery stored at 40 degrees C loses capacity faster than one stored at 50% charge and 20 degrees C.

Quantify the timeline. NMC cells typically retain 80% capacity after 1,000-2,000 cycles. LFP cells retain 80% after 3,000-5,000 cycles.

Why does LFP degrade more slowly than NMC?

Structural stability. LFP's iron phosphate crystal structure is more stable during lithiation/delithiation, producing less electrode cracking and less SEI growth. NMC's layered oxide structure experiences more strain.

This is why grid storage (which prioritizes long life over energy density) overwhelmingly uses LFP.

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