O2-P2 Phase Transitions in Na-Ion Cathodes

• O2-P2 phase transitions are reconstructive structural changes in layered sodium-ion oxides that alter oxygen stacking and Na-ion coordination, crucial for battery function.
• The mechanism involves rigid gliding of transition-metal–oxygen slabs with variable energy barriers modulated by sodium content and characterized by DFT and MD simulations.
• Advanced operando techniques and materials design strategies reveal that these transitions significantly influence Na-ion mobility, voltage hysteresis, and capacity retention.

O2–P2 phase transitions in layered sodium-ion oxides are reconstructive structural transformations involving a shift in stacking sequence and sodium-ion (Na⁺) coordination environment, with direct implications for electrochemical performance in Na-ion battery cathodes. These transitions, which occur via rigid gliding of transition-metal–oxygen (TM–O) slabs (notably CoO₂, Ni₁/₃Mn₂/₃O₂), are central to understanding capacity fading, voltage hysteresis, and the kinetic (de)stabilization of active domains during cycling. Here, the crystallography, atomistic mechanisms, energetics, transport consequences, advanced operando characterization, and materials design strategies for managing the O2→P2 and P2→O2 transitions are detailed.

1. Structural Motifs and Stacking Sequences

The O2 and P2 phases in layered NaₓTMO₂ cathodes (TM = transition metal) are distinguished by both the oxygen stacking and the Na-ion coordination geometry.

• O2 Phase: Exhibits an AB AB... oxygen stacking, with Na⁺ occupying octahedral (O) sites—each coordinated by six O atoms, arranged as a pair of 60°-rotated O₃ triangles. For example, in NaₓCoO₂, the O2 slab is characterized by Na in octahedral interslab sites and a hexagonal arrangement (commonly R3̅m or monoclinic subgroup) (Köster et al., 8 Dec 2025).
• P2 Phase: Defined by AA′ AA′... stacking (a shift of every second TM–O slab by 1/3 of the in-plane lattice parameter, a₀), with Na⁺ in trigonal prismatic (P) sites—still sixfold coordinated, but with a 0° O₃–O₃ triangle orientation. This stacking motif is usually assigned to hexagonal P6₃/mmc symmetry (Huang et al., 2021, Weinstock et al., 2021).

The transformation is realized by a rigid translation (glide) of a TM–O slab:

$\mathbf{t} = (1/3, 0, 0)a_0$

tracking the reaction coordinate $\xi \in [0,1]$ as $\mathbf{R}(\xi) = \mathbf{R}_{O2} + \xi \mathbf{t}$ in DFT–NEB simulations (Köster et al., 8 Dec 2025).

2. Atomistic Mechanism and Energetics

At the atomistic level, the O2→P2 and reverse transitions are reconstructive, involving breaking and reforming of inter-slab registry rather than simple lattice relaxations.

• Energy barrier dependence: Static DFT NEB calculations for NaₓCoO₂ show that the O2→P2 transition barrier, $\Delta E_{O2\rightarrow P2}(x)$, depends strongly on sodium content $x$:

| $x$ | $\Delta E_{O2\rightarrow P2}^{DFT}$ [eV] | |----------|------------------------------------------| | 1.00 | 0.90 | | 0.83 | 1.08 | | 0.67 | 0.67 | | 0.50 | 0.61 | | 0.33 | 0.50 | | 0.17 | 0.45 | | 0.00 | 0.30 |

The barrier rises slightly between $x=1.00$ to $x=0.83$ but then decreases monotonically with further desodiation, approaching $0.3$ eV for $x \rightarrow 0$ (Köster et al., 8 Dec 2025).

• Dynamic lowering: $\upmu$s-long MD runs (using a classical Coulomb–Buckingham potential fit to $>40,000$ DFT data points) reveal further reduction of the barrier due to full ionic relaxations and entropic effects, with the MD-derived activation barrier at $x=0.67$, $E_a^{MD} = 0.32$ eV, approximately half the static DFT barrier (0.67 eV) (Köster et al., 8 Dec 2025).
• Coordination tracking: The O-to-P phase pathway can be diagnosed by the rotation angle θ between O₃ triangles:
  • θ = 60° → O2 (octahedral)
  • θ = 0° → P2 (prismatic)

Energy analysis shows that short-range Na–O repulsion dominates the transition barrier, with a minimal Na–O Buckingham + electrostatics model reproducing ~80% of $\Delta E$.

3. Intermediate OPₙ Phases and Transition Pathways

Large-scale MD simulations reveal that the O2→P2 transformation is gradual, proceeding through a sequence of intermediate “OPₙ” phases. For a prototypical 12-layer cell:

• The trajectory is O₁₂ → O₁₀P₂ → O₆P₆ → O₂P₁₀ → P₁₂.
• Each intermediate (OPₙ) corresponds to discrete intergrowths with both O and P coordination in neighboring layers.
• OPₙ domains persist with characteristic lifetimes (e.g., O₁₀P₂ is stable for ≈250 ns), and no evidence is found for tetrahedral-Co transition states (Co–O remains ~6-coordinated) (Köster et al., 8 Dec 2025).

These metastable intergrowths, or "Z-phases" (editor's term), are implicated in Na⁺ trapping phenomena, contributing to macroscopic voltage hysteresis during cycling.

4. Charge Compensation, Redox Coupling, and Nucleation

Operando spectroscopic and diffraction studies on Na₂/₃Ni₁/₃Mn₂/₃O₂ and related materials elucidate the interplay between local redox and lattice transformation.

• Redox–structure lock: P2 Ni ions accommodate oxidation only up to +3; once Na content falls below $x \approx 1/3$, further extraction would require Ni⁴⁺, which is energetically unfavorable in the P2 lattice. At this point, nucleation of the O2 phase becomes preferred, evidenced by the halting of P2 Ni redox activity and the concurrent growth of O2 domains (Weinstock et al., 2021).
• Phase fractions: P2 and O2 fractions evolve approximately linearly during phase coexistence:
  • For $x \geq 1/3$: $f_{P2} = 1$
  • For $0 \leq x \leq 1/3$: $f_{P2}(x) = 3x$, $f_{O2}(x) = 1 - 3x$ (Weinstock et al., 2021)
• Jahn–Teller correlations: In Na₂/₃[Ni₁/₃Mn₂/₃]O₂, the P2–O2 transition is preceded by enhanced layer alignment and a small additional voltage plateau, linked to a transition from localized to correlated Jahn–Teller distortions among Ni³⁺ ions (Huang et al., 2021). This results in a temporary increase in long-range order before stacking-fault proliferation upon O2 nucleation.

5. Na-Ion Transport and Kinetic Effects

The phase transition has a pronounced impact on Na⁺ mobility, as quantified by mean-squared displacement (MSD) analysis in MD simulations:

Phase	$D_{Na}$ [cm²/s] at 300K ($x=0.67$)
O₁₂	$1.9 \times 10^{-8}$
O₁₀P₂	$6.0 \times 10^{-8}$
O₆P₆	$1.8 \times 10^{-7}$
P₁₂	$3.9 \times 10^{-7}$

A fully-converted P₂ structure displays ~20× higher Na mobility than the pristine O₂, aligning with single-crystal experimental measurements ($1.2\times10^{-7}$ cm²/s for Na₀.₆₇CoO₂) (Köster et al., 8 Dec 2025).

This dramatic jump in diffusive transport upon gliding underlines the sensitivity of rate capability and capacity retention to the precise phase microstructure and domain proportions.

6. Advanced Operando Characterization Methodologies

• Structure-selective operando x-ray spectroscopy (oREXS): Enables resolution of redox events specific to the P2 phase by tuning to the (002) Bragg reflection, cleanly separating the contributions of coexisting structural domains (Weinstock et al., 2021).
• Operando single-particle diffraction (oSP-XRD): Resolves intra-particle lattice rotation, interlayer spacing, and layer misorientation (quantified by parameters α(x) and S(x)) in real time, allowing identification of disorder–order–disorder sequences and correlation with voltage features (Huang et al., 2021).

Key experimental signatures include shifts and broadening of diffraction peaks, direct measurement of misorientation angles, and the emergence of voltage plateaus preceding bulk phase transformation.

7. Implications for Cathode Materials Engineering

• Kinetic stabilization: Higher transition barriers at high Na content kinetically stabilize O-type domains, explaining the persistence of mixed phases and delayed transitions under moderate cycling (Köster et al., 8 Dec 2025).
• Materials strategies: Approaches to suppress or delay detrimental phase gliding include cation substitution (e.g., Ti⁴⁺/Mg²⁺ doping), particle size minimization (sub–100nm), and engineered strain-accommodation layers (Köster et al., 8 Dec 2025, Weinstock et al., 2021, Huang et al., 2021).
• Avoiding premature transition: Disrupting correlated Jahn–Teller effects and maintaining structural/mosaic disorder can extend the solid-solution regime and minimize damage from sudden phase reconstruction (Huang et al., 2021).

Additional strategies under consideration involve surface coatings and the design of graded or core/shell architectures to mitigate domain nucleation and mechanical degradation.

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References:

• "Computational Studies on O2-P2 Phase-Transition Dynamics in Layered-Oxide Sodium-Ion Cathode Materials" (Köster et al., 8 Dec 2025)
• "Structure-selective operando x-ray spectroscopy" (Weinstock et al., 2021)
• "Disorder Dynamics in Battery Nanoparticles During Phase Transitions Revealed by Operando Single-Particle Diffraction" (Huang et al., 2021)