  Nickel-based layered oxides offer an attractive platform for the development
of energy-dense cobalt-free cathodes for lithium-ion batteries but suffer from
degradation via oxygen gas release during electrochemical cycling. While such
degradation has previously been characterized phenomenologically with
experiments, an atomic-scale understanding of the reactions that take place at
the cathode surface has been lacking. Here, we develop a first-principles
methodology for the prediction of the surface reconstructions of intercalation
electrode particles as a function of the temperature and state of charge. We
report the surface phase diagrams of the LiNiO2(001) and (104) surfaces and
identify surface structures that are likely visited during the first charge and
discharge. Our calculations indicate that both surfaces experience oxygen loss
during the first charge, resulting in irreversible changes to the surface
structures. At the end of charge, the surface Ni atoms migrate into tetrahedral
sites, from which they further migrate into Li vacancies during discharge,
leading to Li/Ni mixed discharged surface phases. Further, the impact of the
temperature and voltage range during cycling on the charge/discharge mechanism
is discussed. The present study thus provides insight into the initial stages
of cathode surface degradation and lies the foundation for the computational
design of cathode materials that are stable against oxygen release.
