Half-Cell Cumulative Efficiency Forecasts Full-Cell Capacity Retention in Lithium-Ion Batteries

A Li-ion battery's Coulombic efficiency (CE) is defined as the quotient of the discharge capacity and its antecedent charge capacity for a given set of operating conditions. It is a measure of how reversible the electrochemical energy storing reactions are, with any value less than unity indicating non-productive, often irreversible, reactions. While non-productive reactions can be reversible and result only in self-discharge of the battery, many are irreversible and have more severe consequences. The importance of CE is appreciated when considering that rechargeable Li-ion batteries are chemically isolated systems with a limited inventory of reactants (Li+ and anode/cathode active materials) that need to remain electrochemically active for hundreds of charge-discharge cycles over many years to ensure minimal energy or power loss. Such requirements demand near-perfect chemical reactions: Even a 99.9% CE reflecting one non-productive reaction in every 1000 reactions will result in irreversible losses of reactants in each cycle that compound to great consequence over hundreds of cycles. In this Viewpoint, we highlight the importance of CE and recommend that the battery community adopt reporting practices where advancements can be readily evaluated.