Comparative study on the electronic structures and redox reactions in LiCrX₂ and NaCrX₂ (X = O and S)

LiCrO₂ and NaCrO₂ have been well-studied as cathode materials in lithium and sodium ion batteries, while the studies on LiCrS₂ and NaCrS₂ are relatively rare. In this work, a comparative study on the electronic structures and redox reactions in oxides (LiCrO₂, NaCrO₂) and sulfides (LiCrS₂, NaCrS₂) is performed. A first-principles method has been used to calculate the Bader charge transfer, the electronic structures and the magnetic moments during the entire delithiation or desodiation process. The Bader charge analysis suggests that all the S, O and Cr ions in LiCrX₂ and NaCrX₂ participate in the redox reactions, where the loss of electrons of S ions is clearly larger than that of O ions. Besides, the redox processes of Cr ions are of much less significance. It is noted that, in the sulfides, Cr ions even gain a small portion of electrons rather than losing electrons during the extraction of Li/Na ions. All the charge transfer happens between the S-3p/O-2p and the Cr-3d bands. The redox reactions of O or S ions originate from the energy levels of O/S being pushed towards/across the Fermi surface due to the strong p-d hybridization.