What happens to Li-rich Layered oxides when we cation disorder them?
 
Susim Sabuj Sarkara, Subham Ghosha, Urmimala Maitraa*
School of Materials Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata, India
*Correspondence email: urmimala.maitra@iacs.res.in

 Abstract
 
Honey-comb ordered Li-rich layered oxide (HLO) cathodes that are being studied extensively over the past two decades owing to their ability to exploit high-voltage oxygen redox in addition to transition metal (TM) redox delivering high capacity and high-energy density. While large-energy density afforded by O-redox makes a compelling case, voltage hysteresis, capacity decay and voltage fade issues plague their practical significance. Activation of oxygen redox for all HLO phases is also accompanied by a major structural reorganization of the layered structure which results in the HLO-cathode completely losing its first-cycle plateau (i.e. loses its two-phase behaviour) and adopting a single-phase so called the “S”-shape deintercalation. Several studies have established that this behaviour represents a structural change in the cathode from the ordered HLO-phase changing to a disordered phase followed by formation of rocksalt to a spinel-phases as the cathode cycles over long duration. It has been proposed that TM-migrations and resultant densification of the surface is the sole reason behind O-loss from lattice. Based on this postulation, a disordered crystal lattice with no preference for TM-migration and vacancy clustering should be an ideal lattice to stop TM-migrations and O-loss. We therefore prepared a cation-disordered rock-salt (DRS) phase of Li-rich oxide cathode and compared the O-redox behaviour of the DRS and HLO phases. Quite contrary to our hypothesis, we find that the oxidized oxygen species seems to remain “trapped” in the lattice for HLO-phase over many weeks, while in the DRS crystal the oxidized O-species are mobile and are lost from the lattice in less than a day. DFT studies point at TM-migration induced formation of under-coordinated On- (0<n<2) species in DRS cathodes as the reason behind easy O-loss from DRS cathodes. The finding clearly relates O-loss to rock-salt phase formation and resolves a long-standing ambiguity in the field.
 
Keywords: Li-excess, Oxygen redox, honey-comb-layered, disordered-rocksalt.