Assessing Atomic-Phase Transitions and Ion Transport in Layered Na_xNiO₂ (x ≤ 0.67) Cathode Materials

Abstract

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 Na x NiO 2 (x ≤ 0.67) cathode materials at the atomic level. When x values between 0.16 and 0.22 in Na x NiO 2 , 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 Na x NiO 2 cathode materials. Understanding the degradation mechanism at the atomic level can facilitate the future development of high-performance cathodes for Na-ion batteries.

Magnesium Mitigation Behavior in P2-Layered Sodium-Ion Battery Cathode

“…For the case of dilute Mg doping, there are two most likely diffusion paths: Path a and Path b� the Mg ion in the center of the octahedron will migrate to the first and second nearest neighbor Na vacancies, respectively. The calculated diffusion energy barriers of Path a and Path b are 0.32 and 0.89 eV, respectively (Figure1e), indicating that the Mg ion is more inclined to migrate to the first nearest neighbor Na vacancy 8. However, for the case of heavy Mg doping (such as Na 0.25 NMM 8 ), there are various possible diffusion paths.…”

“…The sodium-ion battery (NIB) is a promising large-scale energy storage technology for electric vehicles and stationary storage due to its low cost and earth-abundant sodium over the lithium-ion battery (LIB). − Among the reported NIB cathode materials, the layered transition metal (TM) oxide (Na x TMO 2 ) has attracted increasing attention owing to its high specific capacity. − Na x TMO 2 has three main phases: triangular prism-coordinated P2 phase and octahedral-coordinated O2 and O3 phases. , Among them, the P2 structure can provide a wider diffusion channel for Na ions and exhibits better electrochemical performance due to its higher phase stability. For example, in P2-Na 0.67 Ni 0.33 Mn 0.67 O 2 (P2-NM), Na ions could all be reversibly shuttled through Ni 2+ /Ni 4+ redox reactions between 2.0 and 4.5 V, resulting in an average discharge voltage of about 3.7 V .…”

“…4−6 Na x TMO 2 has three main phases: triangular prism-coordinated P2 phase and octahedral-coordinated O2 and O3 phases. 7,8 Among them, the P2 structure can provide a wider diffusion channel for Na ions and exhibits better electrochemical performance due to its higher phase stability. For example, in P2-Na 0.67 Ni 0.33 Mn 0.67 O 2 (P2-NM), Na ions could all be reversibly shuttled through Ni 2+ /Ni 4+ redox reactions between 2.0 and 4.5 V, resulting in an average discharge voltage of about 3.7 V. 9 Under highvoltage cycling, however, the TM layer in P2-NM is prone to slip and the P2−O2 phase transition occurs, which not only hinders the migration of Na in the structure but also changes the volume significantly, resulting in intragranular cracks, and significantly affecting the stability and life of battery.…”