Why NCM Surfaces Collapse at High Voltage

Surface phase transformation in NCM cathodes is the irreversible collapse of the layered R3̄m crystal structure into electrochemically inactive rock-salt and disordered spinel phases, occurring at the particle surface under deep delithiation above 4.3 V vs. Li/Li⁺. This transformation is not a gradual fade — it is a thermodynamically driven structural failure that simultaneously triggers oxygen evolution, transition metal dissolution, electrolyte decomposition, and impedance growth, each of which compounds upon further cycling to produce accelerating capacity loss.

Research from South China University of Technology (2022) established the cascade of failure modes at high voltage: cation mixing, adverse phase transitions, gas release (O₂ and CO₂), and the formation of a high-resistance solid interface film. The H2–H3 phase transition — associated with a large c-axis contraction — is identified as the primary structural instability, driving anisotropic lattice strain and crack propagation that exposes fresh, unprotected surface to the electrolyte.

At the atomic scale, work from the University of California (2021) used in situ electron microscopy and first-principles modelling to clarify the surface-specific degradation pathway. The O1 phase formed at high voltages acts as a preferential nucleation site for rock-salt transformation via a two-step pathway: cation mixing followed by shear along (003) planes, with planar cracks simultaneously propagating from both the particle interior and surface. Columbia University (2022) used first-principles surface phase diagrams of the (001) and (104) LiNiO₂ facets to confirm that oxygen loss during the first charge causes irreversible changes — surface reconstruction is thermodynamically driven and facet-dependent, meaning different crystal faces are vulnerable to different degrees.

The cathode–electrolyte interface (CEI) instability compounds structural collapse. At voltages above 4.3 V, surface phase reconstruction, stress-induced cracking, transition metal dissolution, electrolyte decomposition, and oxygen redox reactions are all interlinked. Surface reconstruction from layered to disordered spinel/rock-salt structures is specifically responsible for impedance growth and capacity loss even in single-crystal NCM, as confirmed by EMPA Switzerland (2023) in studies of single-crystal LiNi₀.₈₃Co₀.₁₁Mn₀.₀₆O₂. This is the problem that oxide coatings — and high-entropy oxide coatings in particular — are designed to solve.

The Three Mechanisms by Which Oxide Coatings Suppress Phase Transformation

Oxide coatings on NCM cathode particles suppress surface phase transformation through three distinct but interacting mechanisms: physical barrier formation, coating-induced surface doping, and surface acidity-controlled CEI chemistry. Each mechanism has been independently validated, and their combination — achievable in multi-element and high-entropy oxide coatings — produces stabilisation effects that exceed any single mechanism in isolation.

Physical Barrier and Electrolyte Exclusion

The most extensively studied mechanism is the formation of a conformal physical barrier that prevents direct contact between the NCM surface and the reactive electrolyte, thereby limiting corrosive attack and electrolyte-driven cation extraction. The University of Michigan (2016) demonstrated this explicitly: ALD-deposited TiO₂ and Al₂O₃ coatings on Ni-rich FCG NMC and NCA particles substantially improved high-voltage cycling performance and allowed increased upper cutoff voltage by preventing surface phase transitions. High-resolution TEM and selected-area electron diffraction (SAED) analysis confirmed that the layered structure was retained at the particle surface after extended cycling — the most direct evidence that the coating physically blocks the structural collapse. Al₂O₃ coating improved NMC cycling performance by 40% and NCA by 34% at their respective high cutoff voltages in 2 Ah pouch cells.

Surface Doping and Lattice Stabilisation

Beyond simple barrier function, oxide coatings suppress surface phase transformation through partial cation diffusion into the NCM lattice, stabilising the near-surface structure. Pacific Northwest National Laboratory (2019) revealed that aluminium dopants in NCA are enriched near the primary particle surface and partially exist as Al₂O₃ nano-islands epitaxially dressed on the surface. These aluminium-concentrated surface regions lower the transition metal redox energy levels, stabilising oxygen and suppressing the onset of surface reconstruction. The epitaxial relationship between the coating nano-islands and the layered oxide substrate allows the coating to exert structural constraint on the underlying lattice, resisting the shear distortions associated with phase transformation.

Songshan Lake Materials Laboratory (2022) extended this concept to facet-selectivity: ALD-deposited WO₃ preferentially accumulated on the (003) facet of LiNi₀.₆Co₀.₂Mn₀.₂O₂ particles, with partial migration of W⁶⁺ ions into this specific plane. The (003) facet is the plane most susceptible to the H2–H3 transition and c-axis contraction; its selective stabilisation directly suppressed the anisotropic structural collapse responsible for crack formation and phase transformation. This work demonstrates that the crystallographic selectivity of coating deposition is as important as the coating chemistry itself — a principle that informs the design of high-entropy oxide coatings toward the most vulnerable NCM surface orientations.

Surface Acidity and CEI Chemistry Control

The University of Tennessee (2020) systematically investigated how the surface acidity of the oxide coating modulates the cathode–electrolyte interface. Thin films of TiO₂, ZnO, and Cr₂O₃ — with differing surface acidities and basicities — produced markedly different CEI compositions and impedance behaviours under 4.5 V cycling. More acidic oxide surfaces provided higher initial specific capacity and better capacity retention, while basic surfaces generated greater impedance growth. These differences were attributed to differential LiPF₆ salt decomposition at the interface, controlled by surface acid-base chemistry. This finding is critical for understanding why multi-element oxide coatings — which combine cations of differing electronegativities and Lewis acidities — can tune the interfacial chemistry in ways that single-component coatings cannot.

“More acidic oxide surfaces provided higher initial specific capacity and better capacity retention under 4.5 V cycling, while basic surfaces generated greater impedance growth — differences controlled by surface acid-base chemistry at the cathode–electrolyte interface.”

Oxygen Anchoring: The Most Advanced Suppression Strategy

Oxygen anchoring — the electronic stabilisation of lattice oxygen at the cathode surface to prevent oxygen evolution — represents the most mechanistically sophisticated approach to suppressing surface phase transformation identified in the research dataset. Oxygen evolution is the thermodynamic driving force for surface rock-salt formation; preventing it at source eliminates the primary trigger for the entire cascade of degradation events.

Beijing Institute of Technology (2023) demonstrated this principle with a La₂Mo₂O₉ coating engineered to contain approximately 41% oxygen vacancies on NCM811. The introduced La and Mo ions transferred electrons to enhance surface oxygen electronegativities, creating an “oxygen anchor” effect that alleviated oxygen evolution at high voltages. The oxygen-vacancy-rich inert phase was shown to stabilise not only NCM811 but also LiCoO₂ and Li-rich cathodes, establishing a generalizable principle: a coating phase with controllable oxygen vacancies and multi-valent cations can electronically passivate the surface oxygen, suppressing the redox-driven cation migration that initiates phase transformation.

The oxygen anchoring concept is directly relevant to understanding the mechanistic attractiveness of high-entropy oxide (HEO) coatings. In HEO coatings, five or more principal cations with differing oxidation states and electronegativities are simultaneously present. The resulting “cocktail effect” of mixed cation–oxygen bonding environments creates a surface with heterogeneous local electronic structures that collectively resist oxygen loss across a broader range of delithiation states than any single-cation oxide. The multi-cation coordination also tends to produce amorphous or nanocrystalline coating phases with high configurational entropy, which stabilise against crystalline phase transitions in the coating itself — an important consideration for coatings operating at high voltages where coating crystallinity may itself become a liability.

From Single-Element to High-Entropy Oxide: Engineering the Coating

The engineering methodology for delivering oxide coatings on NCM particles has evolved substantially across the research dataset, with key trade-offs between conformality, thickness control, scalability, and functional performance. Three principal deposition routes have been validated: atomic layer deposition (ALD), wet chemical routes, and room-temperature liquid-phase processes.

Atomic Layer Deposition

ALD has emerged as the standard for conformality and thickness control. Soochow University (2019) demonstrated ALD at a growth rate of 1.12 Å/cycle, producing a ~2 nm Al₂O₃ layer with high conformality on NCM811 particles. The optimised 20-cycle ALD coating delivered an initial discharge capacity of 212.8 mAh/g at 2.7–4.6 V and substantially improved coulombic efficiency, suppressing surface phase transformation by maintaining layered structure integrity at the particle surface during extended high-voltage cycling. The University of Michigan (2016) applied the same ALD approach to FCG NMC and NCA, confirming by TEM/SAED that the layered structure was retained after extended cycling — the most direct structural evidence for phase transformation suppression in the dataset.

Wet Chemical and Sol-Gel Routes

Wet chemical approaches provide scalability with some sacrifice of conformality. Harbin Institute of Technology (2022) demonstrated that wet impregnation of Al₂O₃ on NCM523 produced an ultra-thin coating with strong interaction with the NCM surface, resulting in surface Al-doping that simultaneously reduced lithium diffusion resistance and stabilised the CEI at 4.5 V cutoff. KAUST (2019) validated a polyvinyl alcohol-aided sol-gel method that produced a microporous γ-Al₂O₃ layer on NCM622, protecting the particle surface while permitting fast Li⁺ transport through engineered pores — demonstrating that coating microstructure, not just composition, can be engineered to balance protection and ionic transport.

Room-Temperature Liquid-Phase and Multi-Element Deposition

KIT (2019) reported that solution-based trimethylaluminum treatment of NCM811 achieved drying and coating in a single step, improving cycling performance without vacuum processing. This scalability advantage is significant for industrial implementation of multi-element oxide coatings, where the simultaneous deposition of multiple precursors in solution represents an accessible path to high-entropy oxide surface layers. KIT (2021) extended this approach to a hybrid Al–Si oxide coating produced by reaction of trimethylaluminum (TMA) and tetraethyl orthosilicate (TEOS) with surface moisture on LiNi₀.₈₅Co₀.₁₀Mn₀.₀₅O₂ and LiNiO₂. The multi-element coating outperformed either single-element coating alone through a fluorine-scavenging mechanism and structural passivation — confirmed in long-term pouch full-cell studies, electron microscopy, XPS, and differential electrochemical mass spectrometry.

The conceptual framework for multi-element coating design, articulated by Illinois Institute of Technology (2019), identifies five coating functionalities for NCM materials: physical barrier against electrolyte attack, HF scavenging, structural support against phase transformation, ionic conductivity enhancement, and surface doping. High-entropy oxide coatings are uniquely positioned to deliver all five simultaneously by incorporating cations with diverse functionalities — Lewis acidic cations (Al³⁺, Ti⁴⁺) for HF scavenging, high-field-strength cations (W⁶⁺, Mo⁶⁺) for structural stabilisation, and redox-buffering cations (Ce⁴⁺/³⁺, La³⁺) for oxygen anchoring — within a single conformal coating phase, as described in the literature reviewed by Nature-indexed journals.

Research Landscape and Innovation Trajectory Toward High-Entropy Oxide Coatings

The research landscape for NCM surface coating spans more than 50 sources from academic institutions, national laboratories, and industrial research centres across 2015–2024. The innovation trajectory is clear: from single-element binary oxide coatings, through dual and multi-element hybrids, toward coatings that deliberately combine multiple cations to achieve oxygen stabilisation, structural pinning, and CEI control simultaneously — the defining characteristics of high-entropy oxide design.

Karlsruhe Institute of Technology (KIT) contributes multiple high-impact studies on room-temperature liquid-phase coating, multi-element hybrid coatings, and long-term pouch cell validation. KIT’s emphasis on scalable, industrially relevant coating processes positions it as a leader in translating multi-element coating concepts toward manufacturable high-entropy oxide solutions. Their 2021 work on Al–Si hybrid coatings on LiNi₀.₈₅Co₀.₁₀Mn₀.₀₅O₂ and LiNiO₂ provides the most direct precedent for high-entropy oxide coating in the dataset.

University of Michigan and Pacific Northwest National Laboratory pioneered the explicit demonstration that ALD oxide coatings prevent surface phase transitions at high voltage. The University of Michigan’s 2016 study established the fundamental mechanistic link between surface coating composition and phase transformation suppression, while PNNL’s 2019 work on lattice doping via Al₂O₃ nano-islands revealed the dual mechanism of barrier function and structural doping.

Beijing Institute of Technology introduced defective oxygen-inert phase coatings that serve as oxygen anchors, representing the most mechanistically advanced approach to surface phase transformation suppression identified in this dataset. The La₂Mo₂O₉ coating with ~41% oxygen vacancies is a direct conceptual precursor to high-entropy oxide coating design, establishing that controllable oxygen vacancies and multi-valent cations in a coating phase can electronically passivate surface oxygen.

Songshan Lake Materials Laboratory demonstrated facet-selective ALD coating of WO₃ on the (003) plane of NCM, directly addressing the crystallographic origin of the H2–H3 phase transition. This facet-selective approach is a key design principle for high-entropy oxide coatings: the multi-cation composition provides additional degrees of freedom to tune the coating’s preferential wetting and adhesion to specific NCM surface facets.

According to WIPO, battery materials represent one of the fastest-growing patent categories globally, with cathode surface engineering among the most contested technical areas. The transition from single-component to multi-element oxide coatings is mechanistically consistent with the high-entropy oxide design principle of maximising configurational entropy to suppress crystalline phase transitions in the coating itself while delivering multi-functionality at the NCM interface. Standards bodies including IEC and research programmes tracked by the U.S. Department of Energy have identified high-voltage NCM stabilisation as a critical pathway to next-generation lithium-ion battery performance.

“High-entropy oxide coatings are uniquely positioned to deliver all five NCM coating functions simultaneously — physical barrier, HF scavenging, structural support, ionic conductivity enhancement, and surface doping — by incorporating cations with diverse functionalities within a single conformal coating phase.”