Irreversible loss of active lithium drives NCM cathodes from their ideal stoichiometric composition (LiMO₂) toward a lithium-deficient state (Li_1-xMO₂), with x increasing over repeated cycling, and is a major cause of performance decay [Figure 2A]^[25-27]. Meanwhile, the disordered accumulation of lithium vacancies represents another critical factor triggering structural instability^[28]. These vacancies are not uniformly distributed; instead, their localized enrichment provides both pathways and driving forces for the migration of transition-metal (TM) ions with similar ionic radii (including Ni, Co, and Mn), particularly Ni²⁺ (0.69 Å), into the lithium layers. This results in the formation of Li/TM anti-site defects, which physically block the two-dimensional lithium-ion diffusion channels and thereby exacerbate electrode polarization [Figure 2B]^[29-32]. The formation and progressive aggravation of Li/TM anti-site defects signify a structural transition of the material from an ordered layered framework (α-NaFeO₂ type) toward a more disordered configuration. These defects destabilize lattice oxygen, promote oxygen vacancies and O-O dimer formation, and trigger the release of molecular O₂^[33,34]. The emergence of oxygen vacancies further lowers the migration barrier of TM ions, thereby establishing a deleterious feedback loop of anti-site formation and oxygen loss that progressively weakens the lattice and can ultimately lead to structural collapse^[35].

Cathode–electrolyte interface instability arises from multiple, interrelated factors, including electrolyte oxidation, Jahn-Teller distortions, and phase transitions, which collectively promote the dissolution of TM ions from the lattice^[36]. Dissolved TM ions not only lead to direct loss of active material but also migrate toward the anode under the electric field and deposit there [Figure 2C]^[37,38]. Such deposition catalyzes further electrolyte decomposition, exacerbates active lithium loss, and promotes the continuous growth of the solid-electrolyte interphase (SEI), ultimately contributing to capacity fade^[39-41].

The electronic conduction capability of NCM cathode materials is intrinsically limited by the electronic structure of their TM-O framework, while variations in the structural continuity at the particle scale further modulate the practical realization of this intrinsic conductivity in actual materials^[42,43]. During cycling, structural evolution readily occurs within particle interiors and at interfaces, such as the formation of microcracks and the development of surface insulating layers. These local structural changes introduce barriers to electron transport, weaken the continuity of electron migration across different regions, and consequently reduce the electrochemical participation of certain active areas^[44]. A systematic understanding of the degradation behavior of the electronic conduction network at the particle scale is therefore essential to elucidate the mechanisms underlying performance decay in NCM cathodes.

The formation of microcracks disrupts particle structural continuity and introduces additional local barriers to electron transport, thereby weakening the continuity of electronic pathways^[45,46]. Although multiple factors contribute to microcrack formation, lattice stress induced by lithium intercalation/deintercalation during cycling is considered a primary driving force^[47,48]. During deep charging, NCM materials, particularly Ni-rich compositions, undergo a pronounced H2 to H3 phase transition, accompanied by significant c-axis contraction and comparatively limited a-axis shrinkage, resulting in marked anisotropic volumetric changes^[27,49]. In secondary particles formed from agglomerated primary particles with differing crystallographic orientations, non-uniform expansion and contraction generate concentrated internal stresses at vulnerable regions such as grain boundaries. Once the local stress exceeds the material’s cohesive strength, microcracks nucleate and propagate along primary particle boundaries, gradually forming crack networks spanning entire secondary particles [Figure 3A]^[44,50]. From the perspective of electronic conduction, the emergence of microcracks within particles alters the originally continuous electron transport pathways. This not only reduces the effective contact area between primary particles but also introduces additional interfacial barriers at the local scale, making electron migration between different regions within a particle increasingly dependent on the limited residual contact pathways^[51]. As cycling progresses, conduction pathways become increasingly heterogeneous, placing certain regions at a kinetic disadvantage, limiting their electrochemical participation, and progressively degrading overall material performance^[52].

Insulating phases formed on the particle surface constitute an additional barrier to electronic conduction in NCM cathodes. A critical underlying factor is the reconstruction of the crystal structure at the particle surface. During prolonged cycling, particularly under high-voltage conditions, the layered structure at the particle surface irreversibly transforms into thermodynamically more stable rock-salt or spinel phases due to lattice oxygen loss and the migration of TM ions, primarily Ni²⁺ [Figure 3B]^[53-56]. These reconstructed surface phases are electronically insulating and form an intrinsic barrier that hinders electron transport from the bulk to the particle surface^[55]. In parallel with structural reconstruction, electrochemical side reactions at the cathode/electrolyte interface further exacerbate interfacial resistance^[57]. Under high-voltage cycling conditions, a series of complex side reactions readily occur at the interface, and the oxidative decomposition of organic electrolytes on the cathode surface gradually leads to the formation of a cathode-electrolyte interphase (CEI) covering the particle surface [Figure 3C]^[58-60]. The CEI initially consists primarily of organic lithium salts but evolves into a composite layer dominated by highly ion- and electron-insulating inorganic species (such as LiF and Li₂CO₃) and polymeric species^[59]. As both the reconstructed surface phases and the CEI layer thicken and become more continuous, their combined impediment to interfacial charge transfer becomes increasingly significant, leading to sluggish reaction kinetics, increased polarization, and pronounced performance degradation, particularly under high-rate conditions^[59].

The degradation of the electronic conduction network within NCM particles proceeds hierarchically, from physical structural damage to interfacial chemical passivation. Microcrack formation disrupts the continuity of intrinsic electron pathways, while surface insulating phases further impede interfacial charge transfer. These processes are strongly coupled: microcracks allow electrolyte penetration into particle interiors, promoting side reactions and local structural evolution at newly exposed surfaces, while surface reconstruction and the associated volumetric changes and stress accumulation can trigger further crack initiation and propagation^[32,61]. The interplay of these physical and chemical processes progressively weakens particle-scale electronic conduction, limiting the electrochemical activity of certain regions during cycling. This behavior is particularly pronounced in Ni-rich NCM materials.

Cathode materials in LIBs are composite systems composed of active material particles, conductive additives, binders, and metallic current collectors. The macroscopic electronic conduction of such electrodes relies on the integrity and stability of the three-dimensional conductive network formed by the synergistic interaction of these components^[62-64]. During electrochemical cycling, this network can undergo structural and functional evolution under the combined influence of electric fields, mechanical stresses, and chemical reactions, which directly influence electron transport at the electrode scale. Unlike particle-level failures, which are primarily governed by local structural continuity, electrode-scale conduction degradation involves the coordinated evolution of multiple component interfaces, spatial scales, and transport pathways. This complexity leads to more intricate failure modes and exerts a pronounced impact on overall electrode performance.

At the electrode scale, the evolution of physical contacts between active material particles and conductive additives plays a critical role in the stability of the macroscopic electronic conduction network. Conductive additives, such as carbon black or carbon nanotubes, are generally dispersed among active particles through point contacts, bridging, or localized networks rather than forming continuous coatings, facilitating electron transport between the current collector and individual particles^[63]. During cycling, NCM particles undergo repeated volumetric changes, which are further amplified under deep charge-discharge conditions, thereby introducing cyclic shear and tensile stresses at the interfaces between particles and the conductive network^[49]. Concurrently, microcracks propagating to particle surfaces alter particle geometry and modes of contact. Over extended cycling, these structural and mechanical effects may lead to the gradual deterioration of interfacial contact between active particles and conductive additives^[38,65]. Reduced contact areas and redistributed or agglomerated conductive additives increase pathway tortuosity and spatial heterogeneity within the network, manifesting as diminished macroscopic electron transport^[66]. Cumulatively, this degradation elevates internal electrode resistance and limits the kinetic accessibility of active material, contributing to a gradual decline in reversible capacity.

The chemical and mechanical stability of the binder is critical for maintaining the structural integrity of the electrode, and its degradation can amplify failures in the electronic conduction network at the electrode scale. Polyvinylidene fluoride (PVDF), the most commonly used binder, is particularly susceptible under high-voltage operation and in cathodes with highly active lattice oxygen^[67]. On one hand, residual basic lithium salts on the cathode surface increase the pH of the electrode slurry, which induces PVDF crosslinking via a dehydrofluorination reaction, thereby leading to polymer chain degradation^[60]; on the other hand, highly reactive oxygen species generated under high-voltage operation may attack the polymer backbone^[68]. These chemical processes can induce chain scission, cross-linking, or chemical modification of the PVDF molecules, leading to changes in their mechanical properties, such as reduced flexibility and diminished adhesive strength [Figure 3D]^[66,67]. As a key component connecting active material particles, conductive additives, and the current collector, binder degradation weakens cohesion among electrode constituents^[69]. Under the combined influence of electrolyte-induced swelling and mechanical stresses generated during cycling, the electrode structure becomes more prone to local loosening, interfacial delamination, and even separation between the active layer and the current collector^[66]. Such structural degradation significantly increases the impedance of electron transport from the current collector into the electrode interior, adversely affecting the evolution of cell internal resistance and capacity retention. Because binder failure often involves abrupt structural collapse, its impact on electrode performance can be highly nonlinear.

In contrast to conventional analyses that treat ionic and electronic degradation as independent processes, we propose that the performance decay of spent NCM cathodes originates from the strong coupling and mutual reinforcement between ionic transport limitations and electronic conduction deterioration. Although numerous previous reviews have summarized degradation phenomena from either ion-transport or electronic-conduction perspectives individually, the intrinsic interdependence and co-evolution mechanisms between these two transport processes remain insufficiently clarified in a systematic manner^[32,44,70]. Clarifying this coupled behavior is essential for connecting isolated mechanistic interpretations with the complex, multiscale degradation observed in practical NCM systems.

During prolonged electrochemical cycling, the lattice of Ni-rich cathodes undergoes continuous structural evolution, including TM migration, oxygen loss, and the accumulation of lithium vacancies. These changes gradually distort the lattice and can eventually drive phase transitions, leading to the formation of rock-salt or spinel-like domains^[53,55]. As a result, Li⁺ diffusion pathways become increasingly constricted and, in some cases, partially blocked, which significantly raises ionic transport resistance^[71]. At the same time, the evolving structural instability introduces mechanical stress within particles. This stress facilitates the formation and propagation of both intragranular and intergranular microcracks^[47]. Once formed, such cracks interrupt the originally continuous electronic conduction network and expose fresh surfaces to the electrolyte, thereby intensifying TM dissolution and parasitic surface reactions. The dissolved TM species may subsequently redeposit on particle surfaces or at interfaces, where they further hinder Li⁺ transport and increase interfacial resistance.

Lithium loss further intensifies this coupled degradation behavior. Irreversible Li depletion disrupts the Li/TM ratio and facilitates Li/Ni cation mixing, which in turn retards Li⁺ diffusion kinetics^[30]. Meanwhile, increased local polarization promotes electrolyte decomposition and contributes to the continuous formation of the CEI layer^[59]. As cycling proceeds, the CEI progressively thickens and becomes spatially heterogeneous. This change leads to higher ionic and electronic resistance and, more importantly, induces uneven current distribution across the electrode, ultimately aggravating localized over-delithiation and lattice distortion.

Restricted Li⁺ diffusion often leads to reaction heterogeneity, causing uneven lithiation and delithiation within individual particles. Over-delithiated regions tend to accumulate stress and structural defects, which can promote crack growth and weaken electronic connectivity over time^[51]. As these changes develop, charge distribution within the particle becomes increasingly uneven. In practice, reactions tend to concentrate in certain regions, further aggravating local transport limitations. The interplay between ionic and electronic processes therefore evolves into a coupled degradation loop, extending from particle interiors to interfacial regions.

The interaction between ionic and electronic transport can evolve into a self-reinforcing degradation process, accelerating structural instability and progressively impairing transport properties. As this coupled behavior develops, the reversible recovery of spent NCM cathodes becomes more challenging, and long-term cycling stability is gradually compromised. Understanding how these processes interact is important for the development of direct regeneration strategies. In practice, approaches capable of restoring both lithium transport pathways and electronic conduction tend to be more effective in mitigating structural damage and functional loss.

At the particle scale, degradation of electronic conduction mainly arises from two coupled factors: physical disruption of electron pathways and chemical passivation at particle surfaces. Effective repair strategies must therefore address both mechanisms in a coordinated manner. Surface reconstruction and surface coating have emerged as central approaches for restoring particle-scale electronic conduction networks. Surface reconstruction focuses on activating and transforming electrochemically inactive interfacial phases. Through controlled oxidation or mild chemical/electrochemical treatments, lithium can be replenished into surface-depleted, highly defective rock-salt layers, triggering relithiation and partial recrystallization of the interfacial region^[75,92]. Notably, complete restoration of the original layered structure is not always required. In some cases, the formation of spinel phases with three-dimensional lithium-ion diffusion pathways, or layered/rock-salt coexistence structures, can effectively enhance interfacial transport kinetics^[93]. In essence, surface reconstruction converts electronically insulating surface layers into mixed-conductive interphases, thereby reducing charge-transfer resistance and re-establishing continuous electron transport pathways between the particle bulk and the electrode interface. For example, Jia et al.^[2] employed hydrothermal treatment combined with ammonia regulation to transform the surface rock-salt phase of LiNi_0.5Co_0.2Mn_0.3O₂ into a Ni_0.5Co_0.2Mn_0.3(OH)₂ precursor, enabling a transition from high-barrier 2-TM lithium-migration channels to lower-barrier 1-TM channels. Alternatively, Huang et al.^[71] used citric acid etching to selectively remove surface rock-salt phases while preserving internal spinel or layered domains that retain viable lithium-ion pathways, thereby improving interfacial transport properties [Figure 5A].

In addition to surface reconstruction, constructing uniform coating layers on particle surfaces offers a complementary, synergistic route to enhance the performance of regenerated NCM cathodes. Appropriately designed coatings simultaneously improve electronic conduction, suppress interfacial side reactions, and mitigate cycling-induced mechanical stress. From an electronic transport perspective, ultrathin coatings can facilitate electron transfer via tunneling, while intrinsically conductive coatings introduce supplementary conductive pathways. Both mechanisms partially compensate for electron-transport losses arising from disrupted interparticle contacts and grain-boundary resistance. For instance, Wang et al.^[94] employed phosphate etching to achieve a Li₃PO₄ surface coating, which reduced interfacial polarization and preserved continuous charge-transfer pathways, thereby enabling improved high-rate performance. Coatings also function as effective chemical barriers, physically isolating the active material from direct electrolyte contact. This isolation suppresses oxidative electrolyte decomposition at high potentials and inhibits transition-metal dissolution driven by corrosive species such as HF, thereby slowing the formation and evolution of the CEI. Chang et al.^[53] demonstrated that a spandex-based polymer coating, owing to its high elasticity and strong adhesion, uniformly covered active particles and markedly suppressed surface phase reconstruction and transition-metal leaching [Figure 5B]. Beyond chemical stabilization, coating layers can act as structural buffers that accommodate volume changes during repeated lithiation/delithiation. By redistributing local stress, such coatings mitigate crack initiation and propagation at the particle surface. Sun et al.^[95] applied molecular layer deposition (MLD) to fabricate a conformal nanoscale HPU coating on NCM94, effectively constraining particle-level volume fluctuations and suppressing microcrack formation during cycling [Figure 5C]. Notably, the uniformity of the coating directly determines its protective effectiveness, as non-uniform coatings can induce localized stress concentration during cycling, thereby accelerating crack propagation and ultimately leading to coating failure^[96]. To enable a more systematic and intuitive comparison of the above‑discussed regeneration strategies targeting bulk defects and particle‑scale conduction repair, recent advances are summarized in Table 1.

To further enhance the practical relevance of this review, we have also conducted a comparative analysis of surface reconstruction and coating strategies^[32,70]. These two approaches serve different functional purposes. Surface reconstruction primarily aims to recover degraded surface structures and can improve interfacial charge-transfer kinetics. In contrast, surface coating is more effective in enhancing chemical stability and mitigating side reactions during cycling. From an industrial standpoint, coating strategies are generally easier to integrate into existing cathode manufacturing processes. By comparison, reconstruction-based methods tend to rely on tighter control over processing conditions, which currently limits their scalability.

At the electrode scale, regeneration challenges extend beyond individual particles to the reconstruction of the composite electrode architecture, which comprises active materials, conductive additives, binders, and the current collector^[55,58]. The primary objectives are to restore electrical contact between active particles and the conductive network disrupted during cycling, and to repair interfacial delamination between the electrode layer and the current collector. Achieving durable electrode-scale regeneration therefore requires the coordinated regulation of conductive-network architecture and binder stability, ensuring mechanical integrity and continuous electron transport under repeated electrochemical cycling.

Reconstruction of the conductive network is fundamental to restoring macroscopic electronic conduction. During electrode re-fabrication of regenerated cathode materials, the intrinsic limitations of conventional single-component carbon black networks can be overcome by designing multiscale, multidimensional conductive architectures^[97,98]. Carbon black mainly provides localized point contacts between particles, whereas carbon nanotubes, with their high aspect ratio, bridge multiple particles to form continuous one-dimensional conduction pathways. Graphene sheets further introduce two-dimensional conductive planes on or between particles, offering low-resistance channels for electron transport. The synergistic integration of these point-line-plane components enables the construction of a three-dimensional conductive network that simultaneously enhances electron transport efficiency and electrode structural stability. For example, Wang et al.^[99] optimized a hybrid system of zero-dimensional acetylene black and two-dimensional graphene, establishing efficient point-to-plane conductive pathways and markedly improving electron transport within the cathode.

Restoration of electrode structural integrity also depends critically on optimizing and reconstructing the binder system. Conventional PVDF binders are susceptible to polymer chain scission and cross-linking under high-voltage operation and in the presence of active lattice oxygen, leading to embrittlement and reduced adhesion. In contrast, advanced binders such as polyacrylic acid and sodium alginate have attracted significant attention for their superior chemical adhesion and mechanical resilience^[100,101]. These binders are rich in polar functional groups, which enable them to form strong chemical bonds with transition metals on active particle surfaces^[100]. Their intrinsic elasticity and flexibility also buffer the repeated volume changes of active particles during cycling, suppressing microcrack formation and preventing electrode-scale delamination. Together, these properties maintain structural and mechanical integrity, supporting stable macroscopic electronic conduction over prolonged cycling.