There are growing efforts towards the control of thermal transport in the lithium-ion (Li-ion) battery, where thermal conductivity of cathode material is one of the most important material parameters that determine the safety. Despite playing critical role, fundamental understanding of thermal conductivity and how delithiation changes thermal conductivity are still an uncharted territory. To this end, we investigate the effect of delithiation on thermal conductivity of LiCoO₂ , a common Li-ion cathode material, by using non-equilibrium molecular dynamics simulations and lattice dynamics analysis. A significant reduction (exponential decay behavior) in thermal conductivity is observed in the delithiation (charging) process. For instance, thermal conductivity is only 30% of pristine cathode comparing with 20% of delithiation. The calculation results of heat energy transmission coefficient and spectral energy density together suggest that the delithiation induces orderedto- disordered transition, which results in the non-propagating heat transport. Furthermore, the propagating phonon modes are strongly scattered by Li-vacancy. These are responsible for the remarkable reduction of thermal conductivity and the weakened temperaturedependence. Our work establishes a fundamental understanding of the thermal energy transport in cathode materials of Li-ion battery, and emphasizes the important role of delithiation effect in the thermal management in electrochemical energy storage devices.