(Invited) Understanding the Impact of the Stoichiometry on Cation-Mixing in Ni-Rich Cathodes

Abstract

Although LiNiO2 is chemically similar to LiCoO2 and offers a nearly identical theoretical capacity, LiNiO2 and related Co-free Ni-rich cathode materials suffer from degradation during electrochemical cycling. Especially the formation of Li/Ni anti-site defects can lead to a reduced cyclability and poor capacity retention. We report evidence that such cation mixing in LiNiO2 is linked to the difficulty of synthesizing the material in its ideal stoichiometric composition. In practice, off-stoichiometries (Li1-xNi1+xO2) can hardly be avoided. We determined the Ni migration mechanism in ideal and off-stoichiometric 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 is a highly concerted mechanism, explaining why the phase transition is usually not observed during cycling at room temperature. Our study is a first step towards understanding degradation in Co-free cathodes and underlines the importance of considering defects in materials modeling.