Electrochemical Behaviors and Doping Rules of NaRhO₂ Cathode Materials for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have great advantages for energy storage and conversion due to their low cost and large storage capacity. Currently, NaRhO₂ is used as an electrode material for sodium-ion batteries. Doping first- and second-row transition metals has been carried out to comprehensively assess NaRhO₂ as a cathode material. The geometric and electronic structures and electrochemical and doping behaviors of NaRhO₂ cathode materials for SIBs have been investigated using density functional theory calculations. The results show that the bond lengths of Rh-O in NaRhO₂ decrease during sodium deintercalation. The band gap of NaRhO₂ with sodium extraction gradually reduces. The density of states of NaRhO₂ shows that the interaction between the Rh-4d and O-2p orbitals increases and the orbitals shift toward the right. The average intercalation voltage of NaRhO₂ cathode material increased from 2.7 to 3.9 eV. After doping with first- and second-row transition metal elements from Sc to Zn and Y to Cd, the changes in the band gaps of the doped NaRhO₂ materials exhibit a W-type rule. In contrast, their magnetic moments show a reverse W-type rule. These findings on the pristine and doped NaRhO₂ can provide theoretical guidance for the preparation of novel electrode materials suitable for sodium-ion batteries.