On Proton Defects and the Phase Transformation of NiO₂

Abstract

One approach to high-capacity Li-ion battery cathodes is to increase the Ni content. Unfortunately, Ni-rich materials undergo a phase transition and volume collapse at a high state of charge (SOC), which degrades battery performance. At a high SOC, NiO2 undergoes a phase transition from the O3 to the O1 phase. The O1 phase appears to be more thermodynamically stable than the O3 phase. However, density functional theory (DFT) often predicts higher stability for O3, depending on the DFT approximation. The energy difference between the O1 and O3 phases is on the order of 10 meV/atom. We find that proton defects are more stable in the O1 phase than in the O3 phase by about 90 meV per proton. Defects in NiO2 energetically favor the O3–O1 phase transition observed in a high SOC in Li x NiO2.