Effect of Ionic Radius and Resultant Two-Dimensionality of Phonons on Thermal Conductivity in M x CoO2 (M = Li, Na, K) by Perturbed Molecular Dynamics

Phonon thermal conductivity calculations for Li (x) CoO2, Na (x) CoO2, and K (x) CoO2 (x = 1, 0.5) have been carried out by perturbed molecular dynamics to clarify the dependence of thermal conductivity on alkali-metal vacancy concentration in these materials. While thermal conductivity decreased for all compounds upon introduction of alkali-metal vacancies, the magnitude of the decrease is strongly dependent on the size of the alkali-metal ion. Further numerical analyses using fictitious physical parameters reveal that, with increasing ionic radius, the two-dimensionality of the phonons in the CoO2 layers, which are responsible for overall thermal conductivity, is enhanced, resulting in lower thermal conductivity in vacancy-free compounds as well as ineffectiveness of alkali-metal vacancies in lowering thermal conductivity. In contrast, for systems with smaller alkali-metal ionic radius, even though higher thermal conductivity is predicted when no vacancies are present, vacancies are quite effective in significantly lowering thermal conductivity by modifying phonon states in the CoO2 layers, more so than in systems with larger alkali-metal vacancies.