Formation must establish stable interphases while revealing poor wetting, moisture, side reactions, and local shorts. Final capacity is only one output; process histories matter equally.
Remove equipment error first
Contact resistance and temperature differences can make good cells look bad
Before formation confirm polarity, initial OCV, mass, thickness, and leak result and bind barcode to tray and channel. Dirty probes, weak fixture force, or loose wiring add voltage drop and heat; temperature-position differences also alter voltage and capacity.
A practical approach uses standards or channel swaps to check bias and retains calibration and contact-drop data. If faults cluster in one channel or tray position, investigate equipment and temperature before rejecting the lot.
| Pre-load check | Why it matters | Abnormal signal |
|---|---|---|
| Barcode—tray—channel | Keeps each history tied to one cell | Mismatch, duplicate, or missing |
| Channel contact resistance | Avoids extra drop and local heat | Fault follows channel after swap |
| Temperature and fixture pressure | Makes interphase and swelling conditions comparable | Position-related capacity, heat, or thickness difference |
Understanding the protocol
Low-rate and staged protocols control interphase reactions and expose faults early
Hard carbon consumes sodium to form SEI in the first cycle, while the cathode high-voltage region can drive electrolyte oxidation and CEI formation. Current, cutoffs, rest, and temperature jointly set reaction rate. Cathode, hard carbon, electrolyte, N/P, and voltage window cannot share one universal formation protocol.
Published work on commercial sodium-ion cells includes low-rate formation followed by capacity measurement, but that is a sample-specific experiment. Each plant must choose stages and rates using voltage history, first-cycle efficiency, heat, gas/thickness, and long-term cycling.
- —Store protocol ID with data
- —Interpret first-cycle efficiency with cell balance
- —Quarantine abnormal heat first
Reading the curves
Use lot distribution and timing before assigning material or assembly causes
High voltage can come from resistance, contact, poor wetting, protocol, or temperature. Longer constant-voltage time can signal polarization. Low first-cycle efficiency can involve hard-carbon surface, side reactions, moisture, balance, or electrolyte. One endpoint cannot separate them.
Mark where each anomaly begins, how long it lasts, and whether heat or thickness rises, then compare with the lot and upstream fill, weld, and electrode-roll IDs. Repeated faults in one channel point first to equipment; clustering by material or assembly lot points upstream.
| Signal | Check first | Then correlate |
|---|---|---|
| Sudden voltage jump | Contact, wiring, missing samples | Cell soft short or protection |
| Persistently high voltage at same stage | Temperature, contact, wetting, resistance | Compaction, weld, material lot |
| Low efficiency with gas/heat | Moisture, electrolyte, wetting, protocol | Material surface and balance |
Degassing and aging
After degassing recheck loss, leak, and thickness; aging observes slow variables
Pouch and some formats require post-formation degassing and resealing. Record mass and thickness before/after to ensure gas, not excessive electrolyte, is removed; recheck seal, leak, and insulation. Abnormal gas is not solved by degassing alone and requires moisture, electrolyte, interphase, and protocol review.
Aging observes slow changes in OCV, resistance, thickness, mass, and leak. Time cells at comparable SOC and temperature; OCV before thermal equilibration is not directly comparable. Longer aging can detect some slow faults but increases inventory and space, so source particle and microleak control remain more important.
Formation protocol, degassing condition, and aging time are product-specific and should not be copied directly from papers or other plants.
Bibliography
References
- 01Excessive sodiation and desodiation of commercial NFM-hard carbon sodium-ion batteries induced by low-rate cycling
- 02Areal capacity balance to maximize the lifetime of layered oxide/hard carbon sodium-ion batteries
- 03Modeling battery formation: boosted SEI growth, multi-species reactions, and irreversible expansion
- 04Challenges and opportunities for high-quality battery production at scale
Updated: 2026-08-26