Redox couple

Example

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Cell voltage

The difference between the positive and negative electrode potentials at equilibrium: for NMC against graphite about 3.7 V. It is set by the chemistry, and the difference in Gibbs free energy of the cell reaction per unit charge.

Electrode potential

The potential of an electrode relative to a reference, which for lithium-ion work is lithium metal in the same electrolyte, written as V vs Li/Li⁺. Graphite sits near 0.1 V and layered oxides near 3.7–4.3 V on this scale.

Half-reaction

The oxidation or reduction occurring at one electrode, written on its own with electrons shown explicitly. The cell reaction is the sum of its two half-reactions, and the cell voltage is the difference of their potentials.

LFP

A positive electrode material, LiFePO₄, giving cells a nominal voltage around 3.2 V, long cycle life and good thermal stability, but less energy per kilogram than nickel-rich oxides. Its flat voltage curve makes state of charge hard to read from voltage alone.

Redox reaction

A reaction in which electrons move from one species to another: one is oxidised, the other reduced. A battery separates the two halves onto two electrodes so the electrons must travel through the external circuit.

Standard electrode potential

An electrode’s potential under standard conditions, tabulated against the standard hydrogen electrode. Lithium’s −3.04 V is the most negative of any metal, which is why lithium cells have the highest voltages.

References & further reading

Further reading on this topic

These sources provide broader context; they are not presented as citations for every statement in this entry.

1. Quantities, Units and Symbols in Physical Chemistry, 3rd edition, 2nd printing

   IUPAC · 2011

   Topic background · Physical-chemistry notation, thermodynamics, electrochemistry and transport quantities.