Fe and Ti co-doped LiCoPO₄ as High-voltage Cathode Materials for Lithium-ion Batteries

Lithium cobalt phosphate (LiCoPO₄) is a high-voltage cathode material, offering a high theoretical capacity of ∼167 mA h g^-1 and a cobalt redox potential up to ∼4.8 V, which enable high energy density. However, LiCoPO₄ suffers from low ionic conductivity owing to the structural limitation of one-dimensional [0 1 0] lithium diffusion pathway. In this study, both Fe and Ti are introduced to substitute the lattice site of Co, to investigate the synergistic effects of Ti co-doping in 10 at.% Fe-substituted LiCoPO₄. While Fe substitution has been widely reported to enhance electrochemical stability, Ti, with a higher valence than Co, is expected to generate cation vacancies that facilitate lithiation and delithiation. The electrochemical, kinetic, and structural properties of LiCo_0.9-2xFe_0.1Ti_xPO₄ were systematically evaluated. The Fe and Ti co-doped sample exhibited an initial discharge capacity of 121 mA h g^-1 at 0.1C and a capacity retention of 85 % after 200 cycles at 1C. Moreover, lithium-ion diffusion coefficients derived from PITT and GITT methods were in the range of 10^-11-10^-10 cm² S^-1, with higher values observed for Fe and Ti co-doping compared to Fe-only doping, contributing to improved rate performance. In addition, operando XRD indicated that Ti co-doping transformed the two-phase transition into a solid-solution behavior, demonstrating faster reaction kinetics. Overall co-doping with Fe and Ti enhanced both the structural and electrochemical properties LiCoPO₄ as a high-voltage cathode in lithium-ion battery.