Topic 11 - Mastering the Factors that Impact Li-ion Cell Performance

Section 3 · Topic 11

Operational Stress Factors and Copper Dissolution

Cell aging responds to five stressors, and the engineering hazard is that they multiply rather than sum — and that one of them opens a silent, delayed failure.

The five stress factors

Temperature sets the baseline reaction rate and, below 0 °C, shifts the dominant risk to lithium plating. C-rate acts on both electrodes — discharge current generates I²R heat, charge current plates lithium. Depth of discharge drives mechanical fatigue, age advances SEI and electrolyte decomposition, and cycle count accumulates lattice damage. Their joint effect exceeds the sum.

Compounding

Charging at 2C in a 45 °C environment combines high charge current with elevated temperature, thickening the SEI and gassing the electrolyte fast enough to destroy a cell in dozens of cycles rather than thousands. Deep discharge at sub-zero temperature combines maximum resistance with maximum strain, stranding energy through voltage sag — and if it pushes a cell into reversal, it opens the failure below.

Over-discharge and copper dissolution

Over-discharge is the most insidious abuse mode because it produces no immediate event — the damage is latent and detonates on the next charge. The mechanism is electrochemical and quantitatively fixed by the materials.

Fig. 3.11 — Latent failure in two stages. Stage 1: as the anode fully delithiates, its potential climbs past 3.38 V vs Li/Li+ and the copper collector oxidises and dissolves. Stage 2: on recharge the anode potential drops, the dissolved Cu²+ reduces to metallic copper, and a conductive dendrite bridges the separator — a permanent internal short.

The 3.38 V threshold is simply the standard potential of the Cu/Cu²+ couple expressed against the lithium reference. A parallel pathway runs alongside: the same excessive delithiation breaks down the SEI, generating CO, CO₂, and methane that swell the cell and delaminate the electrode coatings. In some cells, forced reversal beyond roughly −12% SOC produces permanent internal shorts — though the exact threshold is cell- and chemistry-specific.

Why the cutoff is a hard limit

A cell carrying copper-seeded micro-shorts and delamination, if recharged and returned to service, presents an unpredictable thermal-runaway risk. This is why a BMS lower-voltage cutoff is a safety boundary, not a convenience — and why cell balancing (Section 4), which prevents series-string mismatch, is the upstream control that keeps any single cell from being driven into reversal.

Continue the series

• Standard charging protocols and C-rates (Section 4)
• BMS architecture and cell balancing (Section 4)
• Failure analysis and diagnostic signatures (Section 7)

Section 3 · Performance & DegradationTopic 11 / 23