Insights into the Effects of Zinc Doping on Structural Phase Transition of P2-Type Sodium Nickel Manganese Oxide Cathodes for High-Energy Sodium Ion Batteries

Abstract

P2-type sodium nickel manganese oxide-based cathode materials with higher energy densities are prime candidates for applications in rechargeable sodium ion batteries. A systematic study combining in situ high energy X-ray diffraction (HEXRD), ex situ X-ray absorption fine spectroscopy (XAFS), transmission electron microscopy (TEM), and solid-state nuclear magnetic resonance (SS-NMR) techniques was carried out to gain a deep insight into the structural evolution of P2-Na0.66Ni0.33-xZnxMn0.67O2 (x = 0, 0.07) during cycling. In situ HEXRD and ex situ TEM measurements indicate that an irreversible phase transition occurs upon sodium insertion-extraction of Na0.66Ni0.33Mn0.67O2. Zinc doping of this system results in a high structural reversibility. XAFS measurements indicate that both materials are almost completely dependent on the Ni(4+)/Ni(3+)/Ni(2+) redox couple to provide charge/discharge capacity. SS-NMR measurements indicate that both reversible and irreversible migration of transition metal ions into the sodium layer occurs in the material at the fully charged state. The irreversible migration of transition metal ions triggers a structural distortion, leading to the observed capacity and voltage fading. Our results allow a new understanding of the importance of improving the stability of transition metal layers.

Hierarchical Engineering of Porous P2‐Na2/3Ni1/3Mn2/3O2 Nanofibers Assembled by Nanoparticles Enables Superior Sodium‐Ion Storage Cathodes

“…The electrochemical impedance spectra (EIS) of the above three samples are compared in Figure S9 (Supporting Information). The CV curves and charge/discharge profiles featuring smoother peaks or plateaus than those of the bulk materials, [28,36,41,43,44,46] along with the well-maintained voltage plateau at ≈4.2 V during cycling, suggest that the hierarchical nanofibers could effectively alleviate the Na + /vacancy ordering and P2-O2 phase transition occurring for the pristine materials. In addition, the electrochemical performance of the P3-type Na 2/3 Ni 1/3 Mn 2/3 O 2 (annealed at 700 °C for 6 h) has also been evaluated ( Figure S10 in the Supporting Information), which shows a much lower specific capacity with inferior cycling stability relative to the P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 nanofibers.…”

“…[36,37] Dahn's group first investigated the Na-intercalation/deintercalation of P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 , [38] revealing that the rapid capacity decay caused by the undesirable P2-O2 phase transition (with a large volume change of ≈23%) when charged to above 4.2 V was the main challenge hindering the wide application of P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 . doping to stabilize the crystal structure, [28,[41][42][43] surface coating to suppress the unfavorable side reactions, [44][45][46] and lowering the charge cut-off voltage to eliminate the phase transformation, [36,47,48] have been adopted for modifying P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 . doping to stabilize the crystal structure, [28,[41][42][43] surface coating to suppress the unfavorable side reactions, [44][45][46] and lowering the charge cut-off voltage to eliminate the phase transformation, [36,47,48] have been adopted for modifying P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 .…”

Recent Progress of Layered Transition Metal Oxide Cathodes for Sodium‐Ion Batteries

“…Cu or Zn doping is used for P2‐Na x (Ni, Mn)O 2 to reduce the occurrence of the P2–O2 transformation at high voltage . In the case of Na 2/3 Ni 1/3 Mn 2/3 O 2 electrode, the Ni 2+ /Ni 3+ /Ni 4+ redox reactions endow it with high specific capacity and high operating voltage.…”

“…The presence of Cu(II) can stabilize the P2 phase, and at the same time contribute to capacity due to the high potential of the Cu 2+ /Cu 3+ redox couple . Similarly, the Zn 2+ doping reduces the degree of distortion of Ni–O octahedra during the charge/discharge processes of Na 0.66 Ni 0.33 Mn 0.66 O 2 while improving the reversibility of the distortion, endowing it with better voltage and capacity retention than Na 0.66 Ni 0.33 Mn 0.66 O 2 (Figure b) . Li substitution is also confirmed to delay the P2–O2 phase transformation that takes place in Na x Ni 1/3 Mn 2/3 O 2 and even greatly improves its air stability .…”

“…b) In situ XRD patterns of Na 0.66 Ni 0.33 Mn 0.67 O 2 (top) and Na 0.66 Ni 0.26 Zn 0.07 Mn 0.67 O 2 (bottom). Reproduced with permission . Copyright 2016, American Chemical Society.…”