Assessing Atomic-Phase Transitions And Ion Transport In Layered Naxnio2 (X <= 0.67) Cathode Materials

Ni-based layered transition-metal oxides are advanced cathode materials used in rechargeable Na-ion batteries due to their high specific capacity; however, their applications are blocked by irreversible phase transitions and rapid capacity decay in high-voltage cycles. In this study, we explore the structural phase transitions and their effects on the performance degradation of layered NaxNiO2 (x <= 0.67) cathode materials at the atomic level. When x values between 0.16 and 0.22 in NaxNiO2, a phase transition occurs from P2 to O2. The diffusion of Na ions differs in the two phases: (1) in the P2 phase, Na ions tend to migrate from Na1 to Na2, whereas Na ions diffuse to the nearest neighbor sites in the O2 phase. (2) The diffusion coefficient of Na ions in the P2 phase is approximately 9 times greater than that in the O2 phase, and the diffusion time of the former is nearly 10 times faster than the latter. In particular, oxygen vacancies, which more easily form in the O2 phase than in the P2 phase, can significantly inhibit Na-ion migration because they shorten the Na intralayer spacing and promote Ni ions into the Na layer to block the Na-ion diffusion channel. As a result, the P2-O2 phase transitions and presence of oxygen vacancies degrade the performance of NaxNiO2 cathode materials. Understanding the degradation mechanism at the atomic level can facilitate the future development of high-performance cathodes for Na-ion batteries.