First-Principles Insights into the Role of 3d Transition-Metal Substituents in Intercalating Li_<i>x</i>FeF₃ 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 LixFeF3, 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 LixFeF3. The findings provide theoretical guidelines for the rational design of high-performance trirutile LixFeF3 cathode materials.