Although it is commonplace to assess changes in 𝐸 A with changes in chemical composition, this unfortunately convolutes the effects of changes in phonon frequencies with other coinciding structural changes. In order to investigate the phonon dependence of 𝐸 A directly, it is possible to use isotopic substitutions to change the vibrational frequencies without significantly altering the composition or structure of the material. This is possible because the vibrational frequencies of the ions inherently depend on isotope mass, whereas the bonding interactions are expected to remain approximately constant since the electronic structure and coordination environment are nominally the same. Changes to the activation barrier due to isotopic substitution are one of the phenomena referred to as isotope effects, or kinetic isotope effects when corresponding changes in the reaction rate (e.g. diffusion coefficient) are considered. In solid-state ion transport, kinetic isotope effects have been investigated when protons H + are replaced by a deuterium ion or tritium ion. Interestingly, the heavier isotopes tend to have lower vibrational frequencies and higher activation barriers for ionic transport. At face value, none of the aforementioned phonon theories for 𝐸 A predict this frequency dependence. It has been proposed that the isotope effect on 𝐸 A occurs because the heavier isotopes have a lower zero-point energy (vibrational ground state) due to their higher mass, which results in a higher effective energetic barrier. This explanation was first put forth to explain isotopic effects on chemical reaction rates and has also been used to explain changes in the magnitude of 𝐸 A in solid-state proton conductors. Although the diffusion of Li + isotopes in various solid electrolytes has been investigated, the magnitude of isotope effects in battery materials has, so far, received little attention.