For an electrochemical cell or battery, the ratio of the total electric charge extracted during a discharge to the total charge supplied during the preceding charge, usually expressed as a percentage; the measure is based on charge (coulombs or ampere‑hours) and does not include voltage‑dependent energy differences.

Principle

Coulombic efficiency (CE) equals Q_out ÷ Q_in for the measured cycle; CE below 100% implies net charge loss to side reactions, parasitic currents or irreversible processes that remove electroactive species from the reversible cycle.

Demonstration

Situation: A cell is charged with 100 Ah and subsequently discharged delivering 98 Ah. Recognition: Measured CE = (98 Ah ÷ 100 Ah) × 100% = 98%. Action: Diagnose and mitigate causes (e.g., electrolyte decomposition, self‑discharge) if high CE is required. Consequence: The 2% per‑cycle coulombic loss requires extra charging throughput to deliver cumulative energy and may correlate with capacity fade mechanisms.

Misapplication

Equating coulombic efficiency with round‑trip energy efficiency or assuming CE ≈ 100% guarantees absence of capacity fade; the error ignores the role of voltage profiles and other degradation modes that affect energy and capacity independently of charge balance.

Consequence

Persistent CE <100% causes cumulative loss of active charge over many cycles, increasing required charge throughput and accelerating capacity degradation pathways; it also reduces accuracy of SoC estimation when based solely on coulomb counting without correction.

Reversal

In some technologies a measured CE near 100% does not preclude capacity loss caused by mechanical isolation of active material, volume change, or structural failure; conversely, certain ancillary processes (formation cycles) temporarily lower CE while improving long‑term reversibility.

Boundary

Clearly Within: A Li‑ion cell whose charge and discharge are measured at the same C‑rate and temperature with coulomb counting instrumentation. Boundary Case: Measurements that include charge consumed by battery management electronics or balancing circuits — inclusion or exclusion alters reported CE. Clearly Outside: Energy efficiency (RTE) or capacity retention metrics, which incorporate voltage and energy considerations.

Semantic Tension

Coulombic Efficiency ↔ Energy Efficiency: CE isolates conservation of charge but omits voltage effects that determine energy; optimizing for CE alone may miss voltage‑dependent losses that dominate delivered energy, so both measures are needed for full diagnostics.

Synthesis

Coulombic efficiency is a focused diagnostic of charge conservation and parasitic charge losses; interpreted together with voltage profiles and capacity retention, it helps distinguish between chemical side reactions and mechanical or structural degradation mechanisms.