First-Principles Insights into the Role of 3d Transition-Metal Substituents in Intercalating LixFeF3 Cathode Materials for Lithium-Ion Batteries

Despite the high theoretical capacity and operating voltage of FeF₃ cathode materials, their widespread application is impeded mainly by low electronic conductivity, resulting in substantial ohmic polarization. Although numerous studies have explored composite engineering using conductive agents, studies on atomic substitutions aimed at tuning the intrinsic properties of FeF₃ remain relatively scarce. Hence, this study systematically investigates the role of transition-metal substituents (Ti, V, Mn, Cr, Co, and Ni) on the crystallographic and electronic properties of lithiated Li_xFeF₃, as well as the intercalation voltages and thermodynamic stabilities. Based on first-principles calculations, the V, Mn, and Co substituents are predicted to effectively reduce the bandgap. However, the Cr substituent is responsible for unstable cyclability owing to Jahn–Teller distortion in the lithiated phase. Moreover, Ti and V significantly decrease the discharge potential, whereas Ni thermodynamically facilitates the formation of fluorine vacancies in the lattice. Based on the screening factors and results, Mn and Co are identified as the most promising substituents for enhancing the electrochemical performance of Li_xFeF₃. The findings provide theoretical guidelines for the rational design of high-performance trirutile Li_xFeF₃ cathode materials.