From lattice dynamics, the frequencies of phonons in a material can be understood from a springmass perspective, where atoms are the masses whose interactions can be associated with springlike forces. The harmonic description of phonons allows the angular frequency 𝜔 of the oscillator to be defined by an effective force constant 𝐾 and the atomic mass 𝑚 as 𝜔 = √(𝐾/𝑚). The force constant K describes the interaction of the mobile ion with the host lattice and, therefore, depends on the structure. If isotopic substitutions change the atomic mass, but not 𝐾, then the phonon frequencies are expected to change according to the change in mass. Changes in 𝐾 are not expected since the electronic structure and coordination environments are negligibly affected by isotopic substitution. Experimentally and computationally, the unit cell volume did not change (within experimental uncertainty of ~0.5%, see Supplemental Note 1) between the 6 Li and Li structures, which confirms that 𝐾 is constant. In the case of Li and Li, the theoretical ratio of the average lithium vibrational frequencies 𝜔 ̅ can be calculated as