From transition state theory, the activation barrier is defined as the energetic distance from the zero-point energy 𝐸 ZPE to the transition state (or saddle point energy 𝐸 SP ) as 𝐸 A = 𝐸 SP -𝐸 ZPE . The zero-point energy is the vibrational ground state, having no thermally excited phonons. Since the zero-point energy has an explicit frequency dependence (𝐸 ZPE = ℏ𝜔/2), this theory does predict that Δ𝐸 A should increase when Li is substituted for Li when 𝐸 SP remains constant or increases. Assuming that 𝐸 SP remains constant, the magnitude of Δ𝐸 A ≈ 0.001 eV calculated using Eq. 2 is about an order of magnitude lower than what is observed experimentally herein (Figure ).