On Proton Defects and the Phase Transformation of NiO2

One approach to high-capacity Li-ion battery cathodesis to increasethe Ni content. Unfortunately, Ni-rich materials undergo a phase transitionand volume collapse at a high state of charge (SOC), which degradesbattery performance. At a high SOC, NiO2 undergoes a phasetransition from the O3 to the O1 phase. The O1 phase appears to bemore thermodynamically stable than the O3 phase. However, densityfunctional theory (DFT) often predicts higher stability for O3, dependingon the DFT approximation. The energy difference between the O1 andO3 phases is on the order of 10 meV/atom. We find that proton defectsare more stable in the O1 phase than in the O3 phase by about 90 meVper proton. Defects in NiO2 energetically favor the O3-O1phase transition observed in a high SOC in Li x NiO2.