Coulombic efficiency (CE) has been widely used in battery research as a quantifiable indicator for the reversibility of batteries. While CE helps to predict the lifespan of a lithium-ion battery, the prediction is not necessarily accurate in a rechargeable lithium metal battery. Here, we discuss the fundamental definition of CE and unravel its true meaning in lithium-ion batteries and a few representative configurations of lithium metal batteries. Through examining the similarities and differences of CE in lithium-ion batteries and lithium metal batteries, we establish a CE measuring protocol with the aim of developing high-energy long-lasting practical lithium metal batteries. The understanding of CE and the CE protocol are broadly applicable in other rechargeable metal batteries including Zn, Mg and Na batteries.

库仑效率（CE）已被广泛用于电池研究，作为电池可逆性的可量化指标。尽管CE有助于预测锂离子电池的寿命，但在可充电锂金属电池中该预测不一定准确。在这里，我们讨论了CE的基本定义，并阐明了其在锂离子电池和锂金属电池的一些代表性配置中的真实含义。通过研究锂离子电池和锂金属电池中CE的异同，我们建立了CE测量协议，旨在开发高能量持久耐用的实用锂金属电池。对CE和CE协议的理解可广泛应用于其他可充电金属电池，包括Zn，Mg和Na电池。