The role of Ni and Co in suppressing O-loss in Li-rich layered cathodes

Research by the Peter Bruce Group, as reported in Advanced Functional Materials, explains that Lithium-rich transition metal cathodes can deliver higher capacities than stoichiometric materials by exploiting redox reactions on oxygen.  Oxidation of 0^2- on charging, however, often results in loss of oxygen from the lattice.  In the case of Li₂MnO₃ all the capacity arises from oxygen loss, whereas doping with Ni and/or Co leads to the archetypal O-redox cathodes Li(Li_0.2Ni_0.2Mn_0.6]_O2 and Li[Li_0.2Ni_0.13Co_0.13Mn_0.54]O₂, which exhibit much reduced oxygen loss.

Understanding the factors that determine the degree of reversible O-redox versus irreversible O-loss is important if Li-rich cathodes are to be exploited in next generation lithium-ion batteries.  Here it is shown that the almost complete eradication of O-loss with Ni substitution is due to the presence of a less Li-rich, more Ni-rich (nearer stoichiometric) rocksalt shell at the surface of the particles compared with the bulk, which acts as a self-protecting layer against O-loss.

In the case of Ni and Co co-substitution, a thinner rocksalt shell forms, and the O-loss is more abundant.  In contrast, Co doping does not result in a surface shell yet it still suppresses O-loss, although less so than Ni and Ni-Co doping, indicating that doping without shell formation is effective and that two mechanisms exist for O-loss suppression.