(549c) Understanding the Cation-Mixing Mechanism in Ni-Rich Cathodes from First Principles

The growing demand for high energy density Li-ion batteries requires a move away from conventional Co-containing cathode materials. Although LiNiO₂ is chemically similar to LiCoO₂ and offers the same theoretical capacity, LiNiO₂ and related Co-free Ni-rich cathode materials suffer from degradation during electrochemical cycling that has hindered commercialization. One complication affecting LiNiO₂ is the difficulty of synthesizing the material in its ideal stoichiometric composition. In practice, off-stoichiometries (Li_1-xNi_1+xO₂) cannot be avoided. In addition, the formation of structural defects via Li/Ni cation mixing reduces cyclability and leads to poor capacity retention.

We determined the Ni migration mechanism in ideal and defected LiNiO₂ using first-principles calculations and identified the energetically most probable pathways for Ni migration and for the phase transition to the spinel structure. Our results show that off-stoichiometries promote the formation of Li/Ni defects. Furthermore, we find that the formation of the spinel phase occurs via a concerted mechanism, which explains why the phase transition is usually not observed during cycling. Our study is a first step towards understanding degradation in Co-free cathodes and underlines the importance of considering defects in materials modeling.