Technical guide
Coulombic Efficiency in Lithium-Ion Cells
Coulombic efficiency is one of the most quoted and most easily misread numbers in cell testing. It says how much of the charge put into a cell comes back out — and over hundreds of cycles, a value that looks close to perfect can still be the reason a cell fails early.
In short
Coulombic efficiency is the discharge capacity of a cycle divided by the charge capacity of the same cycle, expressed as a percentage. The first-cycle value is dominated by interphase formation and irreversible lithium loss; the steady-state value in later cycles reflects ongoing side reactions and is a sensitive indicator of cycle life.
Definition and calculation
For a single cycle, coulombic efficiency is the charge extracted on discharge divided by the charge inserted on charge, taken from the same cycle. It is a charge balance, not an energy balance: it says nothing about the voltage difference between charge and discharge, and therefore nothing about round-trip energy efficiency.
A value below 100% means charge was consumed by something other than reversible storage — interphase formation, electrolyte decomposition, or another parasitic reaction. Values reported above 100% normally indicate a measurement or protocol artefact, or lithium released from a reservoir formed in an earlier cycle, rather than a gain.
- Coulombic efficiency = discharge capacity / charge capacity of the same cycle
- It is a charge balance, not an energy or round-trip efficiency
- First-cycle value: dominated by interphase formation
- Later cycles: dominated by ongoing parasitic reactions
Why small deviations compound
Cycle life is a cumulative effect. A per-cycle loss that seems marginal is applied again on every cycle, so the difference between a steady-state efficiency of 99.9% and 99.5% is not a rounding detail — it is the difference between a cell that still performs after several hundred cycles and one that has visibly faded.
This is why the steady-state value is treated as an early indicator: it is measurable within a modest number of cycles and points to degradation trends long before a capacity-retention curve makes them obvious.
Measuring it so the number means something
Coulombic efficiency is sensitive to how it is measured. Rate, temperature, voltage limits, rest steps and the accuracy of the cycler all shift the result, and at high efficiencies the required measurement resolution becomes demanding.
A value quoted without its conditions is not comparable to another value. Reporting the protocol alongside the number is what allows two electrodes, two dosages or two prelithiation methods to be judged against each other.
- State the cycle number the value refers to
- State rate, temperature and cut-off voltages
- Distinguish half-cell from full-cell results
- Use the same cycler and protocol when comparing variants
The link to prelithiation
First-cycle coulombic efficiency is the metric prelithiation is dimensioned against: the irreversible share it exposes is what the lithium dose has to compensate. After treatment, the first-cycle value of a full cell is also the most direct check that the dose was right.
Steady-state efficiency answers a different question. It shows whether the electrode, binder and electrolyte package keeps parasitic reactions under control once formation is over. A prelithiated cell with an excellent first cycle and a poor steady-state efficiency has an electrode-stability problem, not a dosage problem — which is why both values are read together.
Key points
- Coulombic efficiency is a per-cycle charge balance, not an energy efficiency.
- The first-cycle value quantifies irreversible loss; the steady-state value indicates degradation.
- Small steady-state deviations compound over hundreds of cycles.
- In prelithiation work, the first-cycle value checks the dose and the steady-state value checks electrode stability.