The Role of Ni and Co in Suppressing O-Loss in Li-Rich Layered Cathodes

Lithium-rich transition metal cathodes can deliver higher capacities than stoichiometric materials by exploiting redox reactions on oxygen. However, oxidation of O(2-)on charging often results in loss of oxygen from the lattice. In the case of Li(2)MnO(3)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]O(2)and Li[Li0.2Ni0.13Co0.13Mn0.54]O-2, 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.

Automated summary

It is shown that almost complete eradication of oxygen loss with Ni substitution is due to the presence of a less lithium-rich, more nickel-rich rocksalt shell at the surface of the particles compared with the bulk. Thinner rocksalt shell forms in the case of Ni and Co co-substitution, leading to more abundant oxygen loss. Co doping does not result in a surface shell yet it still suppresses oxygen loss, indicating that two mechanisms exist for oxygen loss suppression.