Effect of Fluorine Doping on LiCoO2 Cathode Material: a DFT Study

This study employs density functional theory (DFT) in order to investigate the fluorine doping effects on the structural, electrochemical and electrical, properties of LiCoO2 cathode materials for LIBs. The research reveals that fluorine substitution with oxygen can significantly enhance the performance and stability of LiCoO2 in multiple aspects. Investigation show that the fluorine doping results in n-type doping and Fermi level increases in electron density of states, which may enhance the electrical conduction. The substitution of fluorine modifying the cycling life of battery and improves the structural stability by suppressing the expansion rate of volume and increasing the lattice parameter along the c-axis during full delithiation. The findings demonstrate that the fluorine doping improves the structural stability of LiCoO2 by decreasing volume shrinkage during the delithiation. Accordingly, the study identifies the most stable fluorine substitution site, located furthest from the lithium layer. Consistent results from calculations using different approaches (internal and Fermi energy) and methods (GGA and GGA+U) confirm that LiCoO2-xFx exhibits lower voltage compared to LiCoO2, making it desirable for electrolyte tolerance and prolonged battery lifetime. Evaluation of electrical properties demonstrates that the fluorine doping enhances the electrical conductivity of LiCoO2 by reducing the band gap and charge carrier transfer barrier (CCTB). The examination of intrinsic and extrinsic band gaps, as well as Delta and CCTB approaches, consistently reveals that LiCoO2-xFx exhibits a lower band gap and CCTB, indicating improved rate-capability and electrical conductivity.