Advanced Materials: IEST SPFT2000 Quantitatively Stress-Chemical Coupling Drives Multi-Step Phase Transition Degradation in Single-Crystal Ultra-High-Nickel Cathodes

1. Background: Chemo-Mechanical Degradation in Ni-Rich Cathodes

Ultra-high-Ni layered oxide cathodes (LiNiₓCoᵧMn₁₋ₓ₋ᵧO₂, x ≥ 0.92) deliver exceptional specific capacity but suffer from accelerated capacity fade when operated at elevated cut-off voltages (≥4.3 V). Previous explanations have largely focused on the reconstruction of the layered surface into spinel- and rock-salt-like structures. The relationships among intermediate phases, lattice deformation, local stress, and crack formation, however, have remained less clearly defined.

Previous studies have faced two fundamental limitations: (1) polycrystalline samples introduce grain-boundary cracking as a confounding failure mode, obscuring the intrinsic lattice-level mechanisms; and (2) ex-situ characterization captures only the final degradation state, missing the transient intermediate phases that reveal the thermodynamic and kinetic pathway. The multi-step phase transition pathway in single-crystal NCM cathode materials has therefore remained incompletely defined — until this study.

2. Experimental Design and Characterization Methods

The study combined electrochemical testing, structural characterization, particle-level mechanical measurements, spectroscopy, and multiscale simulation:

A multi-scale characterization suite was deployed:

• Electrochemical testing: Coin-type half- and full cells were evaluated at upper cutoff voltages of 4.3–4.6 V, while graphite-based pouch full cells were tested at 4.3, 4.4, and 4.5 V. EIS, GITT, dQ/dV, rate testing, and long-term cycling were used to evaluate impedance, Li⁺ transport, polarization, and capacity retention.
• Structural characterization: FIB-SEM and atomic-resolution HAADF-STEM were used to examine particle cracking and phase reconstruction. GPA was applied to map local lattice strain.
• Mechanical testing: The IEST SPFT2000 Single-Particle Mechanical Testing System was used to compare the force response and fracture behavior of particles from the 4.3 and 4.5 V groups, and establish the correlation between lattice distortion severity and mechanical strength.
• In-situ characterization: In-situ XRD tracked the evolution of the (003) diffraction peak and the corresponding c-axis lattice parameter during cycling over voltage windows of 2.75–4.3, 2.75–4.5, and 2.75–4.6 V.
• Modeling and simulation: DFT calculations examined Li⁺ migration, differential charge density, and electronic states at representative phase boundaries. Molecular dynamics simulations compared Li⁺ transport in the reconstructed structures, while COMSOL modeled Li⁺ concentration and stress distributions within particles.
• Chemical and interfacial analysis: TOF-SIMS depth profiling, XPS etching, XANES/EXAFS, and LA-ICP-MS to trace electrolyte decomposition products, transition metal dissolution, and cross-contamination from cathode to anode.

Figure 1. Electrochemical data in half/fullcells for SC-N92 at 45°C. (a) Cycling performance in half cells, (b) Rate performance. (c) EIS Nyquist plots before cycling and after 200 cycles within the voltage range of 4.3-4.6V. (d) Cycling performance in pouch-type full cells, (e) Corresponding energy density, GITT diagrams during (f) charge and (g) discharge processes, (h-j) dQ/dV contour diagrams with a cut-off voltage window of 4.3-4.5V at 45°C.

3. Phase Reconstruction and Intracrystalline Cracking

3.1 Lattice Bending and Local Strain Distribution

Figure 2. Morphology and crystal structure for cathode in pouch-type full cells at 45°C after 300 cycles.

Atomic-resolution HAADF-STEM imaging combined with GPA strain analysis revealed that lattice bending induced stress concentration is the primary trigger — not a secondary consequence — of phase degradation. Bending regions exhibit concentrated elastic strain that: (1) distorts the oxygen framework, facilitating transition-metal migration from octahedral to tetrahedral sites; (2) locally increases the c-axis lattice parameter, destabilizing the layered O3 structure; and (3) creates a positive-feedback loop where initial phase transformation further amplifies local stress, accelerating subsequent transformation steps.

3.2 Phase Evolution from O3 to Rock-Salt Structures

The O3→O1→LiNi₂O₄→Ni₃O₄→rock-salt degradation chain uncovered in this study represents the first complete mapping of the transformation pathway in ultra-high-Ni cathodes.

3.3 Ni₃O₄ as an Intermediate Phase and Multiphase Structures near Cracks

The microscopy results showed that crack-associated reconstruction cannot be described solely as rock-salt formation. LiNi₂O₄ (Fd-3m), Ni₃O₄ (Cmmm), and ordered or disordered rock-salt structures (Fm-3m) were identified in different surface and internal crack regions. EDS line scans across crack walls show a Ni/O ratio gradient consistent with progressive oxygen loss along the degradation chain, with Ni₃O₄ representing an intermediate stoichiometry between LiNi₂O₄ and NiO.

DFT and MD simulation for cathode phase transition kinetics provided the quantitative rationale: Ni₃O₄ exhibits a Li⁺ migration barrier that is approximately 3× higher than that of the O3 phase, effectively blocking the two-dimensional diffusion channels. The Cmmm orthorhombic structure lacks the continuous octahedral-vacancy network required for facile Li⁺ hopping, making even a thin Ni₃O₄ layer at crack walls a potent barrier to lithium transport.

Figure 3. XRD poles and molecular dynamics simulation for cathode in pouch-type full cells at 45°C after 300 cycles.

4. The Stress-Chemical Coupling Feedback Loop

4.1 c-Axis Contraction and Simulated Stress Distribution

Figure 4. In-situ XRD analysis in the first cycle and COMSOL modeling in pouch-type full cells at 45°C after 300 cycles.

In-situ XRD monitoring during charge/discharge (Figure 4) quantified the c-axis lattice contraction and anisotropic strain in Ni-rich cathodes as a function of voltage. At a cut-off of 4.3V, the maximum c-axis contraction was limited to 3.91%, and the H2→H3 phase transition remained fully reversible — upon relithiation, the lattice recovered its original dimensions with no detectable residual strain. At 4.6V, contraction reached 6.41% — a 64% increase — and the H2-H3 irreversible phase transition in high-voltage NCM was triggered: the (003) diffraction peak showed permanent broadening and position shift after the first delithiation/lithiation cycle, indicating that the layered structure could not fully recover its original orientation and interlayer spacing.

This irreversibility is the structural root cause of the observed voltage and capacity fade: each high-voltage cycle introduces a small but permanent lattice disordering that accumulates over hundreds of cycles, progressively degrading the crystallographic template required for facile two-dimensional Li⁺ diffusion.

COMSOL Multiphysics Simulation provided the engineering-scale validation of these atomic-scale findings. The model coupled lithium concentration diffusion with mechanical stress generation, simulating the evolution of Li⁺ distribution and von Mises equivalent stress within an SC-N92 particle after extended cycling. At 4.3V, the simulation showed a relatively uniform Li⁺ concentration profile from the particle surface to the core, with moderate and evenly distributed von Mises stress. At 4.5V, a dramatically different picture emerged: a steep Li⁺ concentration gradient developed between the particle surface (near-fully delithiated) and the interior (partially lithiated). This gradient generated a correspondingly non-uniform stress field with stress concentrations at the phase-boundary regions where lattice parameters differed most sharply. The simulation directly demonstrates how the high-voltage phase transitions create self-amplifying mechanical stress that drives the progressive particle cracking observed in FIB-SEM — perfectly consistent with the stress-chemical coupling induced cathode particle fracture model.

4.2 DFT First-Principles and XANES/EXAFS: Electronic Origin of Kinetic Degradation

Figure 5. (a–d) DFT first-principles calculations: density of states (DOS), charge density difference, Li⁺ migration energy barriers, and COHP bond analysis across O3, O1, LiNi₂O₄, Ni₃O₄, and rock-salt phase interfaces. (e–g) XANES, EXAFS, and wavelet transform (WT) of the Ni K-edge for SC-N92 cycled at 4.3V and 4.6V.

DFT and MD simulation for cathode phase transition kinetics provided the electronic-structure explanation for the observed degradation. Across the five-phase chain, DFT calculations revealed a progressive deterioration of electronic and ionic transport properties:

• Density of States (DOS): The O3 phase exhibits a clear metallic character with substantial electron density at the Fermi level. As the phase transformation progresses to O1, LiNi₂O₄, and Ni₃O₄, the DOS near the Fermi level systematically decreases — Ni₃O₄ and rock-salt show a distinct band gap opening, indicating the transition from metallic conductor to electronic insulator.
• Charge Density Difference: Electron redistribution maps across phase boundaries show that at high-voltage O3/O1 and O1/LiNi₂O₄ interfaces, charge is substantially depleted from the oxygen framework and accumulates on transition-metal sites — weakening TM–O bonds and facilitating further TM migration into lithium sites (cation mixing).
• Li⁺ Migration Barriers: Nudged elastic band (NEB) calculations quantify the progressively increasing activation energy for Li⁺ hopping: O3 (~0.3 eV) → O1 (~0.5 eV) → LiNi₂O₄ (~0.7 eV) → Ni₃O₄ (~0.9 eV, ~3× the O3 barrier) → rock-salt (>1.2 eV, effectively blocked). This trend confirms Ni₃O₄ as the kinetic bottleneck predicted by the structural analysis.
• COHP Bond Analysis: Crystal orbital Hamilton population analysis across phase interfaces reveals that at 4.5V+, the excessively strong interfacial bonding at multi-phase boundaries (particularly LiNi₂O₄/Ni₃O₄ and Ni₃O₄/rock-salt interfaces) contracts the interlayer spacing, further elevating the Li⁺ migration barrier. This “over-bonding” effect at high voltage is the electronic-structure origin of the observed kinetic degradation.

XANES and EXAFS spectroscopy at the Ni K-edge (Figure 5e–g) provided the experimental validation. After cycling at 4.3V, the XANES white-line position and EXAFS Fourier transform confirm that a high proportion of electrochemically active Ni³⁺/Ni⁴⁺ is maintained, with Ni–O bond lengths close to the pristine values. In stark contrast, the 4.6V-cycled sample shows: (1) a pronounced shift of the absorption edge to lower energy — indicating substantial reduction of Ni³⁺/Ni⁴⁺ to inactive Ni²⁺; (2) EXAFS fitting reveals shortened Ni–O bond lengths in the first coordination shell, consistent with the formation of rock-salt-like local environments; and (3) wavelet transform analysis shows the disappearance of the characteristic second-shell Ni–TM scattering peak, indicating severe disruption of the layered ordering. These spectroscopic signatures directly corroborate the DFT prediction that high-voltage cycling drives irreversible electronic and structural degradation through the multi-stage phase transition cascade.

4.3 Chain Interfacial Failure: From Cathode CEI to Anode SEI

Figure 6. Characterization of the morphology in pouch-type full cells at 45°C after 300 cycles.

The degradation initiated by lattice bending and phase transition does not remain confined to the cathode. TOF-SIMS depth profiling combined with XPS etching (Figure 6) provided a layer-by-layer chemical analysis from the SC-N92 particle surface into the bulk interior. Three key findings emerged:

• Electrolyte penetration depth: At 4.5V, electrolyte decomposition products — including LiF, LiₓPOᵧF₂, and organic carbonate oligomers — were detected at depths exceeding 200 nm from the particle surface, indicating that the crack network formed by phase-transition-driven mechanical failure provides continuous pathways for electrolyte infiltration deep into the particle interior. At 4.3V, these species were confined to the outermost ~50 nm.
• CEI thickness and uniformity: The 4.5V-cycled cathode developed a thick (≥100 nm), chemically heterogeneous CEI with alternating fluoride-rich and organic-rich layers — a signature of repeated cracking/exposure/passivation cycles. The 4.3V cathode formed a thin (~20 nm), uniform, and chemically stable CEI dominated by a single LiF-rich layer — consistent with controlled surface passivation that inhibits further side reactions.
• Transition metal dissolution: XPS quantification showed that the CEI on 4.5V cathodes contained substantial Ni, Mn, and Co — direct evidence of active-material dissolution into the electrolyte, driven by the acidic decomposition products (HF from LiPF₆ hydrolysis) concentrated in the crack-confined electrolyte volume.

LA-ICP-MS and cross-sectional TEM (Figure 6) traced the downstream consequences of this cathode degradation. Quantitative LA-ICP-MS analysis of the cycled graphite anode revealed significant Ni deposition — the dissolved transition metals migrate through the separator and electrochemically deposit on the anode surface. The corresponding anode TEM images showed a thick, irregular SEI on the 4.5V-paired anode, with Ni-rich inclusions that act as catalytic sites for further electrolyte decomposition. This creates the chain interfacial failure from cathode to anode in Li-ion batteries: cathode cracking → electrolyte penetration → TM dissolution → Ni deposition on anode → catalytic SEI growth → active lithium trapped in anode SEI → accelerated full-cell capacity fade. The bidirectional consumption of active lithium — at both the cathode CEI and the anode SEI — explains why the full-cell capacity fade rate at 4.5V is substantially faster than what half-cell data alone would predict.

4.4 Effect of Cutoff Voltage on Degradation Severity

Figure 7. Comprehensive degradation mechanism schematic during long-term cycling.

Voltage window optimization for single-crystal NCM cathodes is not merely a matter of trading capacity for cycle life — it represents a fundamental binary choice between two degradation regimes:

• 4.3V “mild high-voltage” regime: All degradation drivers are partially but significantly suppressed. Lattice bending is limited to isolated regions; the c-axis contraction of 3.91% remains within the elastic recovery limit; the H2-H3 transition is fully reversible; phase degradation is confined to the early O3→O1 stage without progressing to spinel or rock-salt; Li⁺ diffusion remains uniform across the particle; internal stress is low and evenly distributed; and single-particle mechanical strength, measured by IEST SPFT2000, shows only moderate reduction.
• ≥4.5V “ultra-high-voltage” regime: The degradation cascade is fully activated. Lattice bending becomes pervasive; c-axis contraction reaches 6.41% — beyond the reversible limit; the H2-H3 transition becomes irreversible; the complete O3→O1→LiNi₂O₄→Ni₃O₄→rock-salt chain is triggered; ion channels are progressively blocked by Ni₃O₄ and rock-salt; a steep Li⁺ concentration gradient develops between surface and bulk; internal stress accumulates continuously and non-uniformly; and particle mechanical integrity drops precipitously — directly linked by IEST SPFT2000 data to the severity of lattice distortion.

Figure 7 provides the complete mechanistic schematic — the full-chain chemo-mechanical failure loop — synthesizing all characterization data into a single degradation model that connects lattice-scale phenomena (bending, phase transition) to particle-scale outcomes (cracking, strength loss) to cell-scale failure (CEI/SEI growth, TM crossover, capacity fade). This model fills the theoretical gap in understanding high-voltage cycling failure mechanism of single-crystal Ni-rich cathodes and provides atomic-scale guidance for practical engineering strategies including voltage window control, lattice-strain buffer doping, and conformal surface coating.

5. Implications for Cutoff-Voltage Selection and Materials Design

These results support the use of cutoff-voltage control as one route to improving the durability of single-crystal Ni-rich cathodes. Materials strategies that reduce lattice bending, stabilize layered-to-reconstructed interfaces, or limit electrolyte access to cracked regions may provide additional benefits. Their effectiveness, however, should be evaluated under practical loading, temperature, and full-cell conditions.

6. Conclusions

This study linked phase transformation, lattice deformation, Li⁺ transport, and particle cracking in single-crystal SC-N92 cathodes cycled at elevated temperature and cutoff voltage. HAADF-STEM and GPA identified bent, strained phase boundaries containing O1, LiNi₂O₄, Ni₃O₄, and ordered or disordered rock-salt structures. The results showed that crack regions can contain different combinations of reconstructed phases. DFT, molecular dynamics, and COMSOL simulations further indicated that advanced reconstruction restricts Li⁺ transport, increases concentration gradients, and produces a more heterogeneous stress distribution.

Increasing the cutoff voltage from 4.3 to 4.5 V intensified phase reconstruction, interfacial reactions, transition-metal crossover, and particle damage. Single-particle force testing with the IEST SPFT2000 showed better mechanical integrity in particles cycled to 4.3 V than in those cycled to 4.5 V. Taken together, the findings support cutoff-voltage control and structural or interfacial stabilization as practical directions for improving the long-term durability of single-crystal Ni-rich cathodes.

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