Coulombic efficiency is the ratio between charge capacity and discharge capacity (see Cell capacity) of a cell, even with very long relaxation. Due to parasitic reactions (incl. SEI layer on anode formation and Lithium plating, at least in classical Li-ion cells with carbon anode), discharged capacity (𝑄𝑑𝑖𝑠) is lower than charged capacity (𝑄𝑐ℎ𝑎). Coulombic efficiency should be higher than 0.99 after several initial cycles (i. e., after Cell formation).
This is not the same as the difference described in Precise cell capacity measurement, which is about cell relaxation effect.
Coulombic efficiency should not be confused with ‣.
Coulombic efficiency depends on the discharge temperature and the discharge rate:
SEI layer on anode grows faster (hence, the coulombic efficiency is higher) when the state-of-charge is higher:
Measuring Reversible and Irreversible Capacity Losses on Lithium-ion Batteries (2016)