Mitigating the P2–O2 phase transition-induced structural instability in P2-Na x [Ni 1/3 Mn 2/3 ]O 2 Na-ion battery cathodes: to dope or not to dope, that is the question

To achieve high specific capacities in sodium-ion batteries (SIBs) containing P2-type Na_2/3[Ni_1/3Mn_2/3]O₂ (NNMO) cathodes, researchers have increasingly explored cycling batteries beyond the conventional 2-4 V voltage range. However, crossing this voltage boundary at either the upper or lower limits causes phase transitions that result in structural instability. This opinion article primarily focuses on the performance of NNMO in the higher voltage region, where P2-O2 phase transition occurs (≥4.2 V vs. Na/Na⁺). This transition is accompanied by metal oxide layer gliding and lattice oxygen loss, leading to structural degradation. Among the various strategies to mitigate this effect, NNMO doping has been shown to minimize structural degradation and reduce capacity fading to a certain extent. In this opinion article, we argue that regardless of the doping elements, doped-NNMO does not yield satisfactory results for practical applications. This raises the question of whether doping NNMO is the right approach to mitigate structural instability issues during the P2-O2 phase transition between 2 and 4.5 V. In this opinion article, we clarify some misconceptions about NNMO doping strategies in mitigating the P2-O2 phase transition. Finally, we discuss a strategy beyond doping that might help address the aforementioned issues.