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 02- 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(Li0.2Ni0.2Mn0.6]O2 and Li[Li0.2Ni0.13Co0.13Mn0.54]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.