Lithium electrochemical deintercalation from O2-LiCoO₂: structural study and first principles calculations

Abstract

We present a detailed study of the O2-LiCoO₂ phase used as positive electrode in lithium batteries. This phase is a metastable form of LiCoO₂ and is prepared by ionic exchange from P2-Na_0.70CoO₂. The O2-LiCoO₂ system presents interesting fundamental problems as it exhibits several phase transformations upon lithium deintercalation that imply either CoO₂ sheet gliding or lithium/vacancy ordering. Two unusual structures are observed: T^#2 and O6. The T^#2 phase was characterized by X-ray, neutron and electron diffraction, whereas the O6 phase was only characterized by XRD.

In order to better understand the structures and the driving forces responsible for the phase transformations involved in lithium deintercalation, we combine our experimental study of this system with a theoretical approach. The voltage-composition curve at room temperature is calculated using Density Functional Theory combined with Monte Carlo simulations, and is qualitatively in good agreement with the experimental voltage curve over the complete lithium composition range. Pseudopotential and thermodynamic calculations both show that two tetrahedral sites have to be considered for Li in the T^#2 structure. The calculated voltage curve thus exhibits a two-phase O2/T^#2 region that indicates that this phase transformation is driven by the entropy maximization and not by a non-metal to metal transition. We also predict two ordered phases for Li_1/4CoO₂ (O2) and Li_1/3CoO₂ (O6) and show that the formation of the O6 phase is not related to Li staging or Co³⁺/Co⁴⁺ charge ordering.