Hybrid density functional theory modeling of Ca, Zn, and Al ion batteries using the Chevrel phase Mo₆S₈ cathode

Hybrid density functional theory (DFT) is used to study the Chevrel phase Mo₆X₈ (X = S, Se, Te) as a promising cathode material intercalated with various metal ions (M = Li, Na, Be, Mg, Ca, Sr, Ba, Zn, Al). Electronic properties and voltages are calculated for each case. Ca ions are predicted to produce a voltage output ranging from 1.8-2.1 V, comparable to the voltage calculated for Li ions while providing two electrons per transferred ion. The highest voltage is determined to result when the chalcogen X in Mo₆X₈ is S, over Se or Te. Additionally, a comparison of the local-density approximation (LDA), the Perdew-Burke-Ernzerhof (PBE), the Hubbard U corrected GGA-PBE (PBE+U), the meta-GGA modified Becke-Johnson (mBJ), and the hybrid Heyd-Scuseria-Ernzerhof (HSE) functionals are made. The electronic structure determined with HSE is taken as the most reliable, and PBE and LDA can provide reasonable approximations. The PBE+U approach yields an erroneous band gap and should be avoided. The voltages calculated with HSE are in excellent agreement with available experimental data.