Coulombic Efficiency and Cycle Life
Coulombic efficiency (CE) is the discharge charge divided by the charge charge in one cycle:
$$CE = \frac{Q_\text{discharge}}{Q_\text{charge}} = \frac{Q_d}{Q_c}$$
A perfect, side-reaction-free cell returns exactly what it took in, so CE = 1. Any process that adds counted charge on the way in, or removes it on the way out, pushes CE below 1.
Why a tiny number matters
CE sits close to 1, so the informative digits are far to the right. A cell at 99.90% loses ten times as much per cycle as one at 99.99%. Over 500 cycles that difference dominates retention. This is why the resolution of the measurement, not just its average, decides what you can conclude.
CE is a summary, not a mechanism
CE aggregates every side reaction, and it shifts with temperature, rate, and voltage window. It tells you that something parasitic is happening, not which mechanism. To act on it, CE is decomposed into capacity fade and charge-endpoint slippage.
A rate-normalised view: CIE/h
Coulombic inefficiency is CIE = 1 − CE. Dividing by the cycle time gives an hourly rate, CIE/h = (1 − CE) / t_cycle, which makes cells run at different rates comparable and helps separate reactions that scale with time from those that scale with cycle count.