Dependence of Initial Capacity Irreversibility on Oxygen Framework Chemistry in Li-Rich Layered Cathode Oxides

The undesirable capacity loss after first cycle is universal among layered cathode materials, which results in the capacity and energy decay. The key to resolving this obstacle lies in understanding the effect and origin of specific active Li sites during discharge process. In this study, focusing on Ah-level pouch cells for reliability, an ultrahigh initial Coulombic efficiency (96.1%) is achieved in an archetypical Li-rich layered oxide material. Combining the structure and electrochemistry analysis, we demonstrate that the achievement of high-capacity reversibility is a kinetic effect, primarily related to the sluggish Li mobility during oxygen reduction. Activating oxygen reduction through small density would induce the oxygen framework contraction, which, according to Pauli repulsion, imposes a great repulsive force to hinder the transport of tetrahedral Li. The tetrahedral Li storage upon deep oxygen reduction is experimentally visualized and, more importantly, contributes to 6% Coulombic efficiency enhancement as well as 10% energy density improvement for pouch cells, which shows great potentials breaking through the capacity and energy limitation imposed by intercalation chemistry.