Coulombic Efficiency and Cycle Life
Coulombic efficiency (CE) is the discharge charge divided by the charge charge in one cycle:
$$CE = \frac{Q_\text{discharge}}{Q_\text{charge}} = \frac{Q_d}{Q_c}$$
An ideal, side-reaction-free cell returns exactly what it took in, so CE = 1. Any process that adds counted charge on charge, or removes it on discharge, pushes CE below 1 — SEI growth on the negative electrode consuming active lithium, and electrolyte oxidation at the positive electrode adding counted charge, are two typical sources.
The 0.01% resolution threshold
A commercial cell usually sits above 99.9% CE, so the useful information is in the third and fourth decimals. A cell at 99.90% loses 0.10% per cycle; one at 99.99% loses 0.01% — a tenfold difference. To separate the two, the instrument's current resolution has to reach ~1×10⁻⁴ (0.01%); a device accurate to only ±0.05% reads them as identical. Over a few hundred cycles, that tenfold per-cycle gap opens a clear gap in capacity retention.
Decomposing CE: fade and slippage
CE sums every side reaction into one number and drifts with temperature, rate, and SOC window. It only says a side reaction is present — not whether it is on the negative side (lithium loss) or the positive side (self-discharge-like slippage). To point at a specific fix, decompose CE into capacity fade and charge-endpoint slippage.
Hourly inefficiency rate (CIE/h)
Coulombic inefficiency is CIE = 1 − CE. Dividing by the cycle time gives an hourly rate, CIE/h = (1 − CE) / t_cycle. At the same CE = 99.95%, a 1C cycle (~1 h) and a C/10 cycle (~10 h) give CIE/h values a factor of ten apart: this separates reactions that scale with time (similar per-hour rate) from those that scale with cycle count (similar per-cycle), and flags rate-specific mechanisms such as lithium plating.