Section 7 · Topic 22
Failure Analysis and Diagnostic Techniques
When a pack fails or degrades faster than expected, diagnosis proceeds from non-destructive telemetry to destructive teardown — and each failure mode of the earlier sections leaves a distinct signature.
Diagnostic signatures (non-destructive)
Lithium plating reveals itself through the relaxation behaviour after charge: as plated metallic lithium chemically strips and re-intercalates during rest, it produces a characteristic voltage plateau, and the same event leaves a high-voltage feature early in the subsequent discharge that resolves in the dV/dQ trace. An internal short shows as rapid, localised self-discharge — a cell that drops to near zero overnight while its neighbours hold charge has suffered a separator breach or the copper-dendrite shorting of Section 3. SEI growth and electrolyte dry-out present as the opposite of a short: a steady, linear rise in DCIR with no corresponding capacity loss — the cell still holds energy but can no longer deliver it efficiently.
The tell for plating is not a sudden voltage drop during constant-current charging. It is the voltage plateau during the rest/relaxation after charge — plated lithium stripping back — plus the high-voltage dV/dQ feature early in the next discharge. Looking for a CC-charge drop misses it.
Post-mortem (destructive)
Where telemetry cannot resolve the cause, physical teardown in a dry room separates the mechanisms. Visual inspection reads the chemistry directly: dendritic plated lithium appears as a grey-silver metallic film on the graphite anode, while the reddish-brown deposits of copper dissolution on the separator or cathode confirm the over-discharge pathway. Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDS) resolves particle cracking in the cathode lattice — distinguished from SEI consumption by where the damage sits and what it contains — and identifies trace contaminants that may have seeded the degradation.
Comments
Post a Comment