In Situ‐Constructed Elastomeric Interphase for Grain Boundary Stabilization in High‐Voltage Layered Oxide Cathodes

ABSTRACT

Layered oxide cathodes are a leading class of high‐energy‐density electrode materials for lithium‐ion batteries, but their long‐term stability at high voltages is compromised by chemical corrosion and mechanical stress at grain boundaries. Here, we report the construction of an elastic and voltage‐tolerant poly(urea‐siloxane) (PUSi) coating via the in situ polymerization of amine‐terminated polydimethylsiloxane and toluene diisocyanate (TDI). This polymer acted as a conformal artificial interphase that stabilized the secondary‐particle surface and internal primary‐particle surfaces. The PUSi layer acted as a stable chemical barrier that isolated cathode interfaces from direct contact with the electrolyte. This suppressed interfacial side reactions and inhibited the layered‐to‐spinel phase transformation. The tailorable elasticity of the PUSi layer allowed it to accommodate cyclic volumetric variations and suppress intergranular cracking during high‐voltage cycling. When applied to LiNi_0.6Co_0.2Mn_0.2O₂, the cathode showed a capacity retention of 81.2% after 450 cycles in coin cells and 82.4% after 200 cycles in Li‐metal pouch cells under a cut‐off voltage of 4.5 V. The generality of this approach was demonstrated by using it to coat LiNi_0.8Co_0.1Mn_0.1O₂ and lithium‐rich layered oxides. This work provides a scalable and eco‐friendly route for engineering grain boundaries and interfacial stability in high‐energy cathode systems.

This study introduces a universal strategy for stabilizing layered oxide cathodes during high‐voltage operation by developing a highly elastic and voltage‐tolerant poly(urea‐siloxane) (PUSi) artificial interphase through in situ polymerization.

1. Introduction

The pursuit of lithium‐ion batteries with higher energy densities has led to the development of layered oxides LiNi_xCo_yMn_zO₂ (NCM, x ≥ 0.6) as leading candidate materials for cathodes operating at high voltages [1, 2, 3, 4]. Operating these cathodes at voltages ≥ 4.5 V versus Li⁺/Li is essential for utilizing their high theoretical specific capacity, but this exacerbates interfacial and structural degradation [5, 6, 7, 8]. Under such extreme conditions, the materials experience severe chemical and mechanical failure. This typically manifests as accelerated surface‐side reactions, irreversible phase transitions from layered to rock‐salt or spinel structures, and pervasive microcracking driven by cyclic strain [9, 10, 11]. Together, these failures accelerate rapid capacity fading and voltage decay in high‐energy battery systems. Grain boundaries (GBs) within secondary particles of cathodes are a critical factor leading to this degradation [12, 13, 14]. Their disordered atomic arrangement and high interfacial energy make GBs intrinsically more vulnerable to electrochemical corrosion than the grain interior [15, 16, 17]. Thus, they serve as preferential pathways for interphase growth and localized side reactions, as well as stress hotspots during repeated lithium intercalation/deintercalation. Accumulated strain initiates microcracks that accelerate the disintegration of particles [18, 19, 20]. These two failure modes are tightly coupled, wherein electrochemical attack at GBs weakens local cohesion to facilitate crack initiation. Similarly, the formation of microcracks enables additional electrolyte infiltration and exposes fresh GB‐rich interfaces, which further accelerate both degradation and crack propagation. This forms a self‐reinforcing feedback loop that drives rapid structural collapse and severe performance decay.

Tailoring the GB structure and chemistry is a core design approach for optimizing the functional properties of materials [21, 22, 23]. Considerable research efforts have been devoted to stabilizing GBs, including heteroatom doping [24, 25, 26, 27], electrolyte modification [28, 29, 30, 31], structural engineering [32, 33], and surface coating [34, 35, 36, 37]. Among these, artificial coatings provide a direct method to impede electrolyte corrosion and suppress interfacial degradation [38, 39], but they often suffer from intrinsic limitations. First, the mechanical properties of the coating layer are usually mismatched with the dynamic nature of high‐capacity cathodes [40]. Inorganic coatings tend to be brittle and fracture under repeated lattice strain, and while organic coatings can better accommodate volume changes, few polymeric materials combine sufficient elasticity with electrochemical stability during high voltage operation [41, 42, 43]. Second, most polymer coating processes utilize solvents and generally involve two key steps: polymer synthesis and evaporation‐induced deposition [44]. Each of these processes emits volatile organic compounds during solvent removal, and the resulting coatings suffer from poor interfacial adhesion due to defects produced during solvent evaporation. High processing costs and low efficiencies due to complex post‑processing treatment steps further complicate these methods. Third, the high viscosity of polymer solutions often prevents wetting and penetration into the pores and grain boundaries within secondary particles [45]. Thus, coatings may only cover the surfaces of secondary particles, leaving internal GBs unprotected and reducing the coating's effectiveness. These challenges highlight a design direction for artificial coating layers, where an ideal polymeric coating should be introduced as solvent‑free, low‑viscosity monomers. They should be able to permeate the internal voids of secondary particles and reach intergranular regions between primary particles to completely wet precursors. Subsequent in situ polymerization would then form a uniform, adherent, and mechanically compliant elastomeric interphase to conformally protect both the external surface of secondary particles and internal GB‑rich interfaces.

Herein, we report a general, solvent‐free strategy for constructing an elastic and ion‐conductive poly(urea‐siloxane) (PUSi) coating to conformally stabilize both the surface and grain boundaries of high‐voltage layered cathodes. The PUSi interphase was synthesized in situ via the polymerization of amine‐terminated polydimethylsiloxane and toluene diisocyanate (TDI). It served as a physicochemical barrier that suppressed electrolyte penetration, interfacial side reactions, and the layered‐to‐spinel phase transition. Its tailored elasticity allowed it to accommodate cyclic volume variations and also mitigated intergranular cracking. When applied to LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622), the PUSi‐coated cathode exhibited improved cycling stability and retained 81.2% of its initial capacity after 450 cycles in coin cells and 82.4% after 200 cycles in a practical pouch cell at a high cut‐off voltage of 4.5 V. The approach was also extended to LiNi_0.8Co_0.1Mn_0.1O₂ and lithium‐rich manganese‐based oxides, demonstrating its potential commercial applications for high‐energy‐density batteries.

2. Results and Discussion

Poly(urea‐siloxane) (PUSi) was used as the polymeric coating and was synthesized via a spontaneous step‐growth polymerization between amino‐terminated polydimethylsiloxane (NH₂‐PDMS‐NH₂) and TDI, which formed urea bonds (─NH─CO─NH─) (Figure S1). Figure 1a illustrates the integration of PUSi with the cathode particles, where the monomers first infiltrated gaps between primary particles and then underwent in situ polymerization. PUSi demonstrated a characteristic microphase‐separated structure, comprised of soft PDMS segments and hard TDI‐derived spherical domains. Small‐angle x‐ray scattering (SAXS) confirmed the presence of periodic hard domains with a spacing of 82.7 nm, calculated from the scattering peak (q _max) using the equation d = 2π/q _max (Figure 1b). Atomic force microscopy (AFM) also revealed hard domains with lateral sizes below 100 nm (Figure S2), in agreement with the SAXS results. In this biphasic system, the hard domains acted as reinforcing units, while the soft PDMS matrix provided elastomeric properties, which are expected to endow PUSi with exceptional mechanical properties.

FIGURE 1.

(a) Schematic illustration of solvent‐free polymer coating on NCM622 particles, alongside the polymerization reaction formula between the two monomers. (b) Small‐angle x‐ray scattering (SAXS) profile of the synthesized PUSi. (c) Stress‐strain curve of the synthesized PUSi. (d) Electrostatic potential distribution of the PDMS fragment. (e) Comparative analysis of HOMO energy levels between PDMS and conventional elastomer fragments. (f) High‐resolution transmission electron microscopy (HRTEM) image of NCM622@PUSi. (g) TEM image of the cross‐section of NCM622@PUSi and the corresponding energy dispersive spectroscopy (EDS) element maps.

The uniaxial tensile stress‐strain curve of PUSi indicated an ultimate strain of 114.8% (Figure 1c). Under repeated cyclic stretching to 80% strain, PUSi maintained its mechanical strength even after 100 cycles, demonstrating its fatigue resistance (Figure S3). These mechanical properties are expected to allow PUSi to accommodate the cyclic volume changes of high‐voltage cathodes during battery operation, thereby ensuring durable electrochemical and mechanical protection. Unlike the plastic properties of polyvinylidene fluoride, PUSi exhibited a low glass transition temperature (T_g ) of −122.7°C, making it an elastomer (Figure S4). The PDMS fragment exhibited a uniform electrostatic potential distribution (Figure 1d) and remarkably low HOMO of −7.379 eV (Figure 1e; Figure S5). These electronic properties made it inherently nonpolar and gave it superior oxidation resistance, making it suitable for high‐voltage cathode protection. Linear sweep voltammetry (LSV) measurements of the PUSi further confirmed that the PUSi coating exhibits favorable oxidative stability above 5 V (Figure S6), demonstrating its suitability for high‐voltage cathode applications. The ionic conductivity of electrolyte‐saturated PUSi reached 2.0 × 10⁻⁵ S cm⁻¹ (Figure S7), which facilitates the transport of Li⁺ ions through the protective layer.

The low‐viscosity NH₂‐PDMS‐NH₂ and TDI monomers readily wet NCM622 secondary particles throughout their interior and to their exterior, enabling rapid polymerization to form a homogeneous, elastic PUSi coating with intimate interfacial contact with the cathode (Figure S8). The spatial distribution of the PUSi coating was characterized by high‐resolution transmission electron microscopy (HRTEM) and energy‐dispersive xX‐ray spectroscopy (EDS) element maps. Because silicon (Si) was present in PUSi and absent in the NCM622 particles, we used it to precisely map the spatial distribution of the PUSi coating. The HRTEM image showed a continuous and amorphous PUSi layer that conformed to the surface of the NCM622 secondary particles, with a uniform thickness of approximately 3.5 nm (Figure 1f). EDS element maps revealed significant Si enrichment both at the surface and within the interior of NCM622 particles (Figure S9; Figure 1g). This confirmed that the NH₂‐PDMS‐NH₂ and TDI monomers penetrated the interior of the secondary particles and generated the PUSi coating via in situ polymerization, as schematically shown in Figure 1a. Atomic force microscopy (AFM) demonstrated a pronounced reduction in surface stiffness, with the NCM622@PUSi composite exhibiting a Young's modulus of approximately 0.1 GPa, compared with 0.8 GPa for the unmodified NCM622 (Figure S10). This mechanical softening confirmed the formation of a PUSi layer. X‐ray diffraction (XRD) indicated that the crystal structure of NCM622 was unchanged after coating (Figure S11), while Fourier‐transform infrared spectroscopy (FTIR) showed characteristic functional groups of PUSi on the modified cathode, further confirming an interphase formation (Figure S12). The higher contact angle on the NCM622@PUSi compared to pristine NCM622 helps minimize the exposure of the cathode material to trace moisture in the ambient environment, thereby suppressing undesirable reactions with atmospheric components (Figure S13). In addition, compared with pristine NCM622, the NCM622@PUSi cathode shows improved wettability toward the liquid electrolyte, thereby promoting more efficient Li‐ion transport (Figure S14).

The PUSi content in the NCM622@PUSi composite is critical for the performance of the composite cathodes and was controlled by varying the amounts of NH₂‐PDMS‐NH₂ and TDI monomers (Figures S15–S17). The PUSi coating formed on the 0.5 wt% sample was highly uneven, with some grain surfaces remaining entirely uncovered. This incomplete coverage leaves exposed regions at the cathode‐electrolyte interface that allow side reactions to proceed more readily, thereby compromising cycling stability. In contrast, the coating layer formed on the 2.0 wt% sample become excessively thick, with local thickness exceeding 10 nm. While this ensures full interfacial protection, the overly thick layer introduces additional resistance to electron/ion transport, which in turn impairs rate capability and cycling performance. When a PUSi content of 1.0 wt% was used, a uniform and complete PUSi interphase was obtained. As shown in Figure S18, the NCM622 cathode coated with 1.0 wt% PUSi retained 83.0% of its initial capacity after 600 cycles at 0.5 C, which markedly outperformed the unmodified NCM622 (49.5%), as well as the samples coated with 0.5 wt% (71.6%) and 2.0 wt% PUSi (64.5%). These results highlight the essential role of a uniform and continuous PUSi coating in preventing direct contact between the cathode and electrolyte, thereby extending its cycling life. Based on the above analysis, the 1 wt% content of PUSi was optimal, so 1 wt% PUSi‐coated NCM622 (denoted as NCM622@PUSi) was adopted in all remaining experiments.

The electrochemical performance of NCM622 and NCM622@PUSi was evaluated by using them in coin‐type half cells, where they served as cathodes for lithium‐ion batteries. When cycled at 1 C between 2.5 V and 4.3 V versus Li⁺/Li at 25°C, the capacity of the unmodified NCM622 cell rapidly declined from 152.3 to 81.3 mA h g^{− 1} after 200 cycles, resulting in a capacity retention of 53.4% (Figure S19). In contrast, NCM622@PUSi delivered an initial capacity of 146.9 mA h g^{− 1} and maintained 139.8 mA h g^{− 1} after 200 cycles, corresponding to a 95.1% capacity retention. Moreover, NCM622@PUSi exhibited improved rate capability, delivering 121.5 mA h g^{− 1} at 5 C compared to 98.5 mA h g^{− 1} for unmodified NCM622 (Figure S20). This confirms the enhanced kinetics of NCM622@PUSi imparted by the PUSi coating. To further clarify the kinetic effect of the PUSi coating, GITT and CV measurements were performed (Figures S21 and S22). Although NCM622@PUSi exhibits slightly higher ohmic polarization, consistent with the electronically insulating nature of the coating, both GITT and CV indicate faster apparent Li⁺ transport kinetics after coating. This result suggests that the ultrathin PUSi layer does not block Li⁺ migration. Instead, by suppressing interfacial side reactions and maintaining sufficient Li⁺ transport capability across the interface, the PUSi coating outweighs the modest polarization penalty and leads to an overall enhancement in electrochemical kinetics. The NCM622@PUSi cathode retained 86.9% of its capacity after 120 cycles at 60°C and 64.8% after 200 cycles at –20°C (Figures S23 and S24), demonstrating its ability to operate even in extreme environments.

Based on the high‐voltage tolerance of PUSi, we assessed the performance of NCM622@PUSi over a voltage range of 2.5 V to 4.5 V. The coated cathode retained 81.2% of its initial capacity after 450 cycles at 0.5 C, whereas the unmodified NCM622 underwent rapid degradation with only 43.1% capacity retention (Figure 2a; Figure S25). This improvement was attributed to the ability of the PUSi coating layer to suppress parasitic reactions. Electrochemical impedance spectroscopy (EIS) was also carried out to evaluate the stabilizing effect of the PUSi coating. Although the impedance of both electrodes increased after cycling, the rise was mostly mitigated for NCM622@PUSi (169.0 Ω to 323.2 Ω) compared with unmodified NCM622 (160.2 Ω to 386.3 Ω) (Figure S26). Distribution of relaxation times (DRT) analysis indicated that this difference primarily originated from cathode‐electrolyte interphase (CEI) resistance. NCM622@PUSi maintained lower R _CEI values (Figure 2b; Figure S27), which indicates suppressed interfacial side reactions by PUSi. When the areal mass loading was increased to 41.2 mg cm⁻², the NCM622@PUSi delivered an areal capacity of approximately 7.6 mAh cm⁻² at 0.2 C and maintained about 7.0 mAh cm⁻² at 0.5 C, accompanied by stable charge‐discharge voltage profiles (Figure S28). These results demonstrate that the PUSi interphase maintains stable operation under high loading, highlighting the practical potential of this strategy. The practical viability of this coating was further validated in a Li‐metal pouch cell, where NCM622@PUSi showed 82.4% capacity retention during operation at 4.5 V while reliably powering LED arrays (Figure 2c). As shown in Table S1, the PUSi coating demonstrated a protective effect on NCM622 cathodes competitive with previously reported materials. Similar improvements were achieved when applying PUSi to NCM811 and Li‐rich Li_1.2Ni_0.13Co_0.13Mn_0.5O₂ (LRMO) cathodes and then operating at high voltages (Figure 2d–f). This underscores its broad application potential for high‐energy‐density lithium‐ion batteries.

FIGURE 2.

(a) Long‐term cyclic performance of pure NCM622 and NCM622@PUSi under 4.5 V cut‐off voltage at 0.5 C. (b) DRT profiles of NCM622 and NCM622@PUSi after 100 cycles in the voltage range of 2.5‐4.5 V at 0.5 C. (c) Long‐term cyclic performance of pouch cells employing NCM622@PUSi cathode in the voltage range of 2.5‐4.5 V at 0.5 C. (d) Long‐term cyclic performance of pure NCM811 and NCM811@PUSi in the voltage range of 2.5‐4.5 V at 0.5 C. (e) Long‐term cyclic performance and (f) charge/discharge profiles of pure LRMO and LRMO@PUSi in the voltage range of 2.0‐4.6 V at 0.5 C.

Building on these kinetic insights, we employed time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) to analyze the compositional evolution of the electrode surface and help determine the mechanism by which the PUSi coating mitigated interfacial side reactions. 3D spatial distribution maps were acquired for both pristine NCM622 and NCM622@PUSi cathodes after 450 charge‐discharge cycles (2.5–4.5 V). The signal intensity of C₂HO⁻, which represents organic solvent decomposition, was much stronger in the spectrum of the unmodified NCM622 electrode (Figure 3a) than in the NCM622@PUSi (Figure 3b). This indicates more severe degradation of ethylene carbonate (EC) and diethyl carbonate (DEC) in the absence of the coating [46]. Similarly, the PO₂ ⁻ mapping showed greater decomposition of LiPF₆ in the uncoated NCM622 cathode (Figure 3c,d).

FIGURE 3.

(a–f) 3D spatial distribution images of secondary ions (TOF‐SIMS) from the pure NCM622 and NCM622@PUSi cathodes after 450 cycles at 2.5‐4.5 V cut‐off voltage. (g–i) TOF‐SIMS intensity distributions of C₂HO⁻, PO₂ ⁻, and Li₂F₃ ⁻. (j,k) XPS spectra and fitting for C1s and F1s element of two cathodes after 450 cycles at 0.5 C in the voltage range of 2.5–4.5 V.

Notably, the spatial distribution of LiF (detected as Li₂F₃ ⁻) revealed a critical distinction between the two electrodes. In the pristine NCM622 cathode, LiF accumulated not only on the outermost surface but also within the secondary particle interior (Figure 3e), indicating that LiPF₆ decomposition had propagated along internal grain boundaries. Such pervasive LiF penetration reflects uncontrolled and widespread side reactions that consume active lithium and electrolyte, ultimately degrading cycling performance. In contrast, the NCM622@PUSi cathode exhibited LiF enrichment mainly at the outermost surface, with negligible internal accumulation (Figure 3f). This localized yet LiF‐enriched surface chemistry demonstrates that the PUSi coating effectively confines interfacial degradation to a limited outer region, preventing the spread of side reactions into the secondary particle interior.

Quantitative analysis of the TOF‐SIMS intensity distributions (Figure 3g–i) further corroborate these spatially resolved observations, showing that the interphase‐related species in NCM622@PUSi are mainly confined to the outer surface region, whereas those in unmodified NCM622 extend more deeply into the secondary particle interior. These results demonstrate that the PUSi coating suppresses interfacial side reactions by preventing the decomposition of both the organic solvent and lithium salts. Moreover, it maintains a stable and protective LiF‐containing CEI confined to the outermost surface, a distribution that avoids the deleterious effects of excessive LiF penetration and contributes to superior cycling stability.

We employed xX‐ray photoelectron spectroscopy (XPS) to analyze the composition and structure of the CEI after 450 cycles at 4.5 V and 0.5 C. The C 1s spectrum (Figure 3j) contained characteristic peaks at 284.8 eV, 286.3 eV, and 288.8 eV, which correspond to C─C, C─O, and C═O/O─C═C bonds, respectively [47]. In the F 1s spectrum (Figure 3k), two peaks were assigned to LiF and C‐F₂. Quantitative analysis of surface components (Figure S29) showed a higher C─O content in unmodified NCM622 than in NCM622@PUSi, indicating accelerated solvent decomposition in the uncoated sample. Together with the TOF‐SIMS data, the higher LiF content of unmodified NCM622 provides evidence for its more extensive electrolyte decomposition.

To further determine the structural influence of PUSi infusion along grain boundaries and its impact on electrochemical performance, in situ x‐ray diffraction (XRD) was conducted for both pristine NCM622 and NCM622@PUSi. Upon charging to 4.5 V, the (003) peak shifted to much higher angles [48]. Such large lattice parameter changes often induce the formation of intragranular and intergranular microcracks, which compromise particles’ structural integrity. Figure 4a shows that pristine NCM622 underwent significant structural changes during delithiation/lithiation, with its (003), (101), and (104) diffraction peaks shifting by 0.60°, 0.81°, and 0.96°, respectively. These shifts were larger than those of NCM622@PUSi during charging (0.47°, 0.69°, and 0.79°), suggesting a more pronounced increase in interlayer spacing due to enhanced electrostatic repulsion [49]. Moreover, most diffraction peaks in the pristine material did not fully return to their original 2θ positions after the first cycle. This implies an irreversible structural change, which likely accumulated during repeated cycling and ultimately led to progressive structural degradation and capacity fading.

FIGURE 4.

In situ x‐ray diffraction (XRD) patterns of the (a) pure NCM622 and (b) NCM622@PUSi. HR‐TEM images of the (c) pure NCM622 and (h) NCM622@PUSi cathodes charge to 4.5 V after 450 cycles. (d‐g, i‐l) Part of electron diffraction patterns from the regions marked (I), (II), (III), and (IV). SEM images of the (m) NCM622 and (o) NCM622@PUSi representative particles. Cross‐section SEM morphology images of the (n) NCM622 and (p) NCM622@PUSi after 450 cycles at 0.5 C in the voltage range of 2.5–4.5 V.

HRTEM and electron diffraction were adopted to assess how PUSi enhanced the cycling stability after 450 cycles within the voltage range of 2.5–4.5 V. The infused PUSi prevented the liquid electrolyte from penetrating the grain boundaries of the secondary particles. This suppressed the phase transformation from the original layered structure (R‐3m) to spinel/rock‐salt phases (Fm‐3m), which is usually initiated at the particle surface and propagates inward upon cycling in pristine NCM cathodes. This mechanism was directly evidenced by the images in Figure 4c–l. At the atomic scale, the uncoated particles transitioned from a layered to a rock‐salt‐type structure (Figure 4c–g), whereas the PUSi‐infused particles largely retained their original crystallographic forms even after 450 cycles (Figure 4h–l). Cross‐sectional and surface SEM images revealed very different structural evolutions between the two samples after prolonged cycling. The uncoated secondary particles developed extensive intergranular cracks (Figure 4m,n), which are a major degradation pathway contributing to cathode failure. In contrast, the PUSi‐infused samples exhibited no such cracks (Figure 4o,p), suggesting that PUSi infiltration into the grain boundaries inhibited the formation of intergranular cracks and suppressed structural degradation.

Chemo‐mechanical simulations were conducted to further clarify how the PUSi coating protected NCM622 particles from damage during delithiation/lithiation. Figure 5 shows time series snapshots of the normalized Li concentration (c _Li), equivalent stress (σ_eq), hydrostatic pressure (σ_h), and damage field (d) of pure NCM622 and NCM622@PUSi particles caused by Li extraction and insertion. Under identical delithiation/lithiation conditions, the simulated Li concentration profiles of the unmodified and PUSi‐coated particles are nearly identical (Figure 5a). This similarity arises because the simulation primarily captures the intrinsic Li⁺ distribution governed by the random crystallographic orientations of primary grains within the secondary particle. The anisotropic volume changes associated with randomly oriented grains generate substantial mismatch stress at grain boundaries (Figure 5b,c), which serves as the primary driver for subsequent grain boundary damage and crack initiation (Figure 5d). In the case of unmodified NCM622, direct electrolyte contact induces interfacial side reactions that progressively degrade grain boundary strength, ultimately leading to particle cracking. Once microcracks develop, the constraint between primary particles is released, allowing local deformation. In contrast, the PUSi coating applied to the outer surface of secondary particles and infiltrated into internal voids and grain boundaries suppresses these side reactions, thereby preserving grain boundary cohesion and mitigating mechanical damage. The simulated damage evolution thus supports the conclusion that the mechanical confinement provided by the PUSi coating contributes to improved cycling stability.

FIGURE 5.

Chemo‐mechanical simulated time evolutions of the (a) normalized Li concentration, (b) equivalent stress, (c) hydrostatic pressure, and (d) damage field during the delithiation/lithiation process of the pure NCM and NCM@PUSi particles.

3. Conclusion

In summary, we developed a solvent‐free in situ polymerization strategy to construct a multifunctional poly(urea‐siloxane) (PUSi) coating for high‐voltage layered oxide cathodes. Our approach eliminated volatile organic emissions while enabling the uniform, conformal, and scalable encapsulation of secondary particle surfaces and internal primary particle surfaces. The elastomeric PUSi network integrating soft PDMS segments and rigid TDI domains formed an adherent and voltage‐tolerant artificial interphase that accommodated cyclic volumetric strain during lithiation/delithiation. This coating also served as a stable chemical barrier that isolated cathode interfaces from the electrolyte, thereby suppressing interfacial side reactions and inhibiting the detrimental layered‐to‐spinel phase transformation. By simultaneously mitigating chemical corrosion and mechanical stress at grain boundaries, this grain‐boundary‐targeted protection framework enhanced the structural integrity of cathodes during operation at high voltages. The PUSi‐coated NCM622 cathode achieved a capacity retention of 81.2% after 450 cycles in coin cells and 82.4% after 200 cycles in Li‐metal pouch cells under a cut‐off voltage of 4.5 V. The application scope of this strategy was demonstrated using other high‐energy‐density cathodes, including NCM811 and lithium‐rich layered oxides, highlighting its generalizability for grain‐boundary engineering in lithium‐ion batteries.

Author Contributions

Haoqi Fan: investigation, visualization. Zi‐Jian Zheng: conceptualization, data curation, supervision, funding acquisition, writing – review and editing. Xu‐Dong Zhang: data curation, writing – review and editing. Changhui Ke: software. Ziyun Wang: investigation. Fan Lin: investigation, validation, visualization. Huan Ye: writing – review and editing, formal analysis. Qin Chen: software. Hui Yang: software, methodology.

Conflicts of Interest

All authors declared that there are no conflicts of interest.

Supporting information

Supporting File: adma74129‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.