Breaking the energy density limit of LiNiO2: Li2NiO3 or Li2NiO2?

The development of next-generation layered oxide cathodes for high-energy-density electrical vehicle Li-ion batteries (LIBs) is an urgent topic. The existing method is achieved by continuously increasing the Ni contents of Ni-based layered oxides, but it has been limited to LiNiO2. To break this limit and attain increased energy densities, a promising strategy, which involves the introduction of excess Li ions into transition metal (TM) layers to form Li-excess compounds Li2MO3 (M is a TM cation), has attracted enormous interest recently. However, another strategy, which has been neglected in recent years, involves the insertion of an extra layer of Li ions between the TM and original Li layers to form Li2MO2. In this study, typical reversible Li2NiO3 and 1T-Li2NiO2 were selected as two representative cathodes to break the limit of LiNiO2, thereby availing comprehensive comparison with LiNiO2 regarding their overall properties as cathodes from a theoretical perspective. Interestingly, dissimilar to the Ni3+/Ni4+ monoelectron cationic redox associated with LiNiO2, a polaronic anionic redox reaction occurs in Li2NiO3, while a reversible Ni2+/Ni4+ double-electron redox reaction accompanied by insulator-metal transition occurs in Li2NiO2. Owing to this double-electron cationic activity, Li2NiO2 exhibits absolute advantages over the other two materials (LiNiO2 and Li2NiO3) as cathodes for LIBs in terms of the capacity, energy density, electronic conductivity, and thermal stability, thus rendering it the most promising candidate for next-generation layered oxide cathodes with high energy densities to break the limit of LiNiO2.