Section 5 · Topic 16
Degradation Mechanisms and the SEI
Cell aging is the sum of chemical and mechanical processes that share two endpoints — they consume cyclable lithium and they raise internal resistance.
Three mechanisms drain the cell’s finite lithium inventory and stiffen its impedance, each through a different driver. Telling them apart matters: each leaves a distinct field signature, developed in Section 7.
SEI growth
Formed on the first charge when the carbonate electrolyte reduces at the graphite surface, a stable SEI is electronically insulating — which stops further electrolyte reduction — and ionically conductive, which admits desolvated lithium. The film does not stay fixed: high temperature and high electrode potential drive it to thicken, each increment consuming more lithium and adding resistance, producing the calendar-clock fade of Section 3.
Lithium plating
Plating is a kinetic failure, not a thermodynamic one. When charging at low temperature or excessive rate pushes lithium toward the anode faster than it can intercalate, the lithium deposits as metal instead of inserting. Some re-intercalates on rest — the basis of the diagnostic signature in Section 3 — but much remains as electrically isolated dead lithium, and the deposits can grow as separator-threatening dendrites.
Mechanical fade
Each cycle expands and contracts the electrode lattices, and over thousands of cycles that breathing fractures active-material particles — severing the electronic path to the current collector — and delaminates the active coating from the foils, leaving dead zones that no longer store charge.
Particle cracking and SEI consumption are different failures: cracking severs contact and strands capacity mechanically, while SEI growth consumes lithium and raises resistance chemically. A post-mortem that conflates them misreads the dominant failure — Section 7 separates the signatures.
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