where x is the number of Li atoms per Si atom, E 𝑓 is the formation energy per Si atom of c-LixSi/a-LixSi. 𝐸 𝐿𝑖 𝑥 𝑆𝑖 is the total energy per Si atom of c-LixSi/a-LixSi, 𝐸 𝐿𝑖 and 𝐸 𝑆𝑖 are the total energies of BCC Li and amorphous Si per atom, respectively. Here, the a-Si before lithiation is taken as the reference state for Si. For the lithiation/delithiation processes, the voltage relative to Li/Li + can be expressed in electron volts (eV) as follows :