Lithium Layers Govern Wave-like Cross-Plane Transport and Thermal Anisotropy in LiCoO2

Abstract

LiCoO2, a layered crystalline material characterized by pronounced bonding heterogeneity between stiff covalent CoO2 layers and soft ionic Li layers, remains a dominant cathode material for lithium-ion batteries. Using a theoretical framework that combines ab initio anharmonic lattice dynamics with equilibrium molecular dynamics based on machine-learning interatomic potentials, we reveal a dual thermal role of the Li layers: while contributing negligibly to heat transport directly, they fundamentally reshape the mechanism of cross-plane vibrational energy transfer. First, the ab initio calculations uncover a giant lattice thermal conductivity anisotropy, with κL∥/κL⊥ ≈ 21.6 at room temperature. This disparity is accompanied by a striking contrast in temperature dependence: the in-plane conductivity follows a conventional crystalline trend (κL∥ ∝ T–0.89), characteristic of particle-like phonon transport limited by anharmonic scattering, whereas the cross-plane conductivity exhibits a much weaker temperature dependence (κL⊥ ∝ T–0.22). The soft Li layers strongly suppress the particle-like phonon across the layers while preserving sufficient interlayer vibrational coupling for wave-like heat transfer through intermode coherences to become significant. Second, the role of Li-ion diffusion is evaluated using molecular dynamics simulations with a developed Deep Potential model. After validating the model in the diffusive regime, the thermal contributions of Li-ion diffusion and its coupling with lattice vibrations are quantified using the Green–Kubo formalism. Heat transport remains dominated by lattice vibrations, demonstrating that high Li-ion mobility does not necessarily translate into a significant ionic contribution to thermal transport. The negligible ion–phonon cross-correlation further demonstrates that Li-ion motion does not form a coupled heat-transport channel with lattice vibrations.