The Electrodes Whose Names Change When Charged: Rethinking Battery Terminology

The common terms 'anode' and 'cathode' in battery technology, often translated as 'positive electrode' and 'negative electrode' respectively, are a source of significant confusion. This stems from their definition by the International Union of Pure and Applied Chemistry (IUPAC) as electrodes where oxidation and reduction occur, respectively. Crucially, these reactions can reverse depending on whether the battery is charging or discharging.

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anode is the electrode where oxidation occurs, and cathode is the electrode where reduction occurs. The name of the electrode is determined not by whether it is (+) or (−), but by the reaction occurring on the electrode surface.

In a typical lithium-ion battery, the graphite electrode undergoes oxidation during discharge, making it the anode. Simultaneously, the lithium transition metal oxide electrode undergoes reduction, becoming the cathode. However, during charging, these roles reverse: the graphite electrode becomes the cathode (reduction), and the oxide electrode becomes the anode (oxidation). This dynamic switching means that 'anode' and 'cathode' are not fixed labels for physical components but rather descriptions of their function at a given moment.

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During discharge, oxidation occurs at the graphite electrode, making it the anode, and reduction occurs at the oxide electrode, making it the cathode. But when charging, the reaction direction is reversed.

This functional definition clashes with the more static concepts of 'positive electrode' and 'negative electrode,' which refer to the relative electrical potential and do not change with the battery's state of charge. For instance, the graphite electrode is always the negative electrode, regardless of whether it's acting as the anode or cathode. The article argues that conflating these different criteria, reaction type versus electrical potential, leads to logical contradictions, such as a 'positive electrode material' becoming an 'anode' during charging.

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If we adhere to the industry's equation that 'positive electrode material is cathode material,' we reach an absurd conclusion. Because the moment it charges, the positive electrode material becomes the anode.

To resolve this, the author proposes a clearer system of nomenclature. Functional names like 'anode' (oxidation electrode) and 'cathode' (reduction electrode) should reflect the reaction occurring. Simultaneously, 'positive electrode' and 'negative electrode' should consistently denote the electrodes based on their potential, irrespective of the operating mode. This separation would align with Michael Faraday's original intent when coining 'anode' and 'cathode' to describe the process rather than a fixed sign, thereby ending long-standing confusion in the field and facilitating better understanding across different scientific and engineering disciplines.

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The solution is not to invent new terms. It is to arrange the existing words according to different criteria.