Understanding the onset of surface degradation in LiNiO2 cathodes

TL;DR

This study uses first-principles calculations to understand how LiNiO2 cathodes degrade at the atomic level during battery cycling, focusing on surface reconstructions and oxygen loss.

Contribution

It introduces a computational methodology to predict surface reconstructions and degradation mechanisms of LiNiO2 cathodes at different states of charge and temperatures.

Findings

Oxygen loss occurs during the first charge, causing irreversible surface changes.

Ni atoms migrate into tetrahedral sites and into Li vacancies during cycling.

Surface degradation mechanisms depend on temperature and voltage range.

Abstract

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.