Atomic insight into electrochemical inactivity of lithium chromate (LiCrO2): Irreversible migration of chromium into lithium layers in surface regions

Abstract

Cr-based cathode materials for Li-ion batteries have attracted significant attentions due to the feature of multiple electron transfer. The origin of the poor electrochemical inactivity of LiCrO2 has not been clarified for decades. Here an irreversible phase transformation from the layered to the rock-salt structure is observed at atomic scale in partially electrochemical delithiated LiCrO2: Cr ions migrate from Cr layers into Li layers in the surface regions. The Cr ions at Li layers in the surface regions could block extraction of lithium from the interior regions. Density functional theory (DFT) calculations confirm that Cr ions in Li layers can stabilize the structure in the Li-poor area, but the diffusion energy barrier of Li ions will be greatly increased. It is proposed accordingly that the surface phase transformation and the blocking of diffusion channel are the main origin for the poor electrochemical reactivity of LiCrO2. Such a surface blocking phenomenon may be a common origin for inactivity of some cathode materials, in which cation mixing become significant after initial delithiation. In addition, Cr ions in LiCrO2 are oxidized only from Cr3+ to Cr4+ during electrochemical delithiation, instead of Cr6+ as usually expected, based on synchrotron X-ray absorption spectra (XAS) studies.