Abstract
The structural stability and the redox mechanism of Li-rich layered oxides (LLOs) are two very important aspectsfor high energy density. The former is related to the irreversible loss of lattice oxygen and capacity fading duringcycling, while the latter determines the overall capacity of the materials. This paper aims at clarifying the factorsgoverning the structural stability, the extra capacity and the redox mechanism of LLOs upon Li-removal. Theresults show that the structural stability against oxygen vacancy formation is improved with increasing M–Ocovalency, while it decreases with increasing d-shell electron number and with electrochemical extraction oflithium from the lattice. The redox mechanism of Li2-xMO 3 electrodes formed by 3d metals or by heavier metals0with a d electronic configuration is related to the electron depletion from the oxygen lone-pairs (localized non-bonding O(2p) states) leading to an irreversible anionic redox ending with the reductive elimination of O 2 uponcycling. For these phases, long-term cycling is predicted to be very unlikely due to the irreversible loss of latticenoxygen upon charging. For the electrodes formed by 4d and 5d metals with intermediate d electronicconfigurations, reversible cationic and anionic redox activities are predicted, therefore enabling reversible extra-capacities. The very different redox mechanisms exhibited by Li2-x MO3 electrodes are then linked to the delicatebalance between the Coulomb repulsions (U term) and the M–O bond covalency (D term) through the generaldescription of charge-transfer vs. Mott–Hubbard insulators. The present findings will provide a uniformguideline for tuning the band structures of Li2MO 3 phases and thus activating desired redox mechanisms,being beneficial for the design of high-energy density electrode materials for Li-ion battery applications