Defect-engineering accelerates the conduction of lithium ions in the cathode materials of lithium-ion batteries. However, the effects of defect-engineering on ion conduction and its mechanisms in humid environments remain unclear in the academic discourse. Here, we report on the effect of vacancy defects on the electronic properties of and Li-ion diffusion in a LiFePO4 material in humid environments. The research findings indicate that vacancy defects reduce the lattice constant and unit cell volume of LiFePO4. Additionally, the water molecules occupy the Li-ion vacancies, leading to an increase in the lattice constant of LiFePO4. The computational results of the electronic properties show that the introduction of water molecules induces a transition in LiFePO4 from a semiconductor to a metallic behavior, with a transfer of 0.38 e of charge from the water molecules to LiFePO4. Additionally, the migration barrier for Li ions in the H2O + LiFePO4 system is found to be 0.50 eV, representing an 11.1% increase compared to the pristine LiFePO4 migration barrier. Our findings suggest that water molecules impede the migration of Li ions and provide important insights into the effect of defect-engineering on electronic properties and ion conduction under humid conditions.
