Enhanced Cycling Stability of NCM811 Cathodes at High C‐Rates and Voltages via LiMTFSI‐Based Polymer Coating

Abstract

Improving the cycling stability in Ni‐rich LiNi_xCo_yMn_1−x−yO₂ (NCM) cathodes, particularly under high C‐rates and elevated voltages, remains a significant challenge in lithium battery technology. A novel polymer coating based on lithium sulfonyl(trifluoromethane sulfonyl)imide methacrylate (LiMTFSI), a material commonly used in solid polymer electrolytes (SPEs), is applied to LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) cathodes. This coating improves electrochemical stability at high C‐rates (2C and 4C) and voltages up to 4.5 V, compared to uncoated cathodes, enabling reduced charging times (e.g., 1 h at 1C to 15 min at 4C) while maintaining relatively enhanced cycling performance. Mechanistically, the coating helps suppress surface phase transitions to the rock‐salt phase, mitigates transition metal dissolution, and facilitates lithium‐ion transport at the cathode–electrolyte interface. These combined effects contribute to enhanced cycling durability under demanding conditions. Galvanostatic intermittent titration technique (GITT) analysis further supports that the coating promotes interfacial lithium‐ion conduction without acting as an insulating barrier. Additionally, the coated NCM811 electrodes exhibit improved rate performance. This study shows that repurposing SPE‐derived monomers as cathode surface modifiers provides a practical route to improving rapid‐charging capability, energy utilization, and long‐term operational stability in lithium batteries.

The use of LiMTFSI‐based polymer, commonly found in solid polymer electrolytes (SPEs), as a coating material for LiNi_xCo_yMn_1‐x‐yO₂ (NCM) cathodes, enhances cycling stability at high C‐rates and voltages by suppressing phase transitions and metal dissolution while improving lithium‐ion conduction. Extending the utility of SPEs to liquid electrolyte systems offers a promising pathway for the advancement of fast‐charging and high‐energy‐density lithium batteries.

1. Introduction

Electric vehicles (EVs) have gained increasing attention as a sustainable alternative to fossil fuel‐powered vehicles over the past decade. Lithium‐based batteries, including lithium‐ion batteries (LIBs) or lithium metal batteries (LMBs), are extensively studied for their potential to enable efficient energy storage systems in EVs. Cathodes are the primary determinant of the capacity and cost of batteries. Ni‐rich layered oxides LiNi_xCo_yMn_1‐x‐yO₂ (NCM, x > 0.5) are considered among the most competitive cathodes, meeting both high capacity (200 mAh g⁻¹) and low cost requirements.^{[ 1} , ^{2 ]} High nickel content enhances capacity by increasing the contribution of Ni redox reactions (Ni²⁺→Ni³⁺, Ni³⁺→Ni⁴⁺).^{[ 3 ]} Moreover, Ni‐rich NCM exhibits significant potential as a high‐performance cathode material, particularly under demanding conditions such as high C‐rates (e.g., 2C for 30 min charging and 4C for 15 min charging) and elevated cut‐off voltages up to 4.5 V. These conditions enable faster charging and higher energy densities, as energy density is determined by the integral of capacity over the voltage range, which can increase under high‐voltage conditions. The combined benefits of high C‐rate capability and increased energy density at elevated voltages highlight the cathode's potential for next‐generation lithium‐ion batteries, particularly in fast‐charging applications. However, maintaining cycling stability under these harsh conditions remains a significant hurdle, as structural instability often leads to performance degradation.^{[ 4} , ⁵ , ^{6 ]} The loss of cycling stability under these conditions can be attributed to several interconnected factors: 1) the high reactivity of Ni⁴⁺ with the carbonate electrolyte, leading to severe side reactions, including oxygen loss,^{[ 7} , ^{8 ]} 2) dissolution of transition metals (TM), followed by their migration and deposition onto the anode, which disrupts the solid electrolyte interphase (SEI),^{[ 9} , ^{10 ]} 3) Li⁺/Ni²⁺ cation mixing due to their similar ionic radii (Li⁺: 0.76 Å, Ni²⁺: 0.69 Å),^{[ 11 ]} 4) anisotropic volume changes associated with the phase transition from the second hexagonal (H2) to the third hexagonal (H3) phase at 4.2 V during delithiation,^{[ 12 ]} 5) HF attack,^{[ 13 ]} 6) random and uneven formation of the cathode‐electrolyte interphase (CEI).^{[ 14 ]} These degradation pathways are interrelated, often triggering a cascade effect that accelerates the structural and electrochemical deterioration of Ni‐rich NCM under demanding conditions. For instance, TM dissolution can occur as a consequence of oxygen loss, which results from the structural instability of Ni‐rich NCM at high voltages. Additionally, the HF attack accelerates TM dissolution by etching the cathode surface, further compromising its integrity.^{[ 15} , ^{16 ]} As a result, the layered structure of NCM gradually converts to a rock‐salt structure, leading to a loss of both capacity and cycling stability.

To address the structural instability challenges of NCM, various surface modification techniques, such as elemental doping,^{[ 17} , ^{18 ]} inorganic,^{[ 19} , ^{20 ]} polymer coatings,^{[ 21} , ²² , ^{23 ]} combination of inorganic and polymer coatings,^{[ 24 ]} and the introduction of electrolyte additives,^{[ 25} , ²⁶ , ^{27 ]} have been extensively explored. Among various strategies, polymer coatings can accommodate volume changes during charge and discharge^{[ 28 ]} and offer significant advantages for industrial applications due to their cost‐effectiveness and ease of manufacture. Polymer coatings, especially those applied through wet processes, offer a scalable and facile approach compared to techniques such as calcination or atomic layer deposition. Despite the advantages of polymer coatings, key challenges remain, including achieving uniform coating^{[ 29} , ^{30 ]} and optimizing Li⁺ conduction within the coating.^{[ 31} , ^{32 ]} Additionally, although substantial research has addressed high cut‐off voltages for polymer‐coated NCM, there is a notable lack of investigation into cycling performance at high C‐rates. Addressing this gap is critical because poor cycling performance at high charge rates limits rapid battery charging, a key requirement for the broader adoption of EVs.^{[ 33 ]} Moreover, overheating of lithium batteries, a growing concern is closely linked to the high charge/discharge rates typical of high C‐rate conditions.^{[ 34 ]}

In this study, we propose a novel and practical approach to enhance interfacial stability by applying a thin, in situ polymerized coating derived from lithium sulfonyl(trifluoromethane sulfonyl)imide methacrylate (LiMTFSI) onto NCM secondary particles (denoted as NCM@PLiMTFSI). LiMTFSI, a lithium salt with polymerizable functionality, has been primarily studied in solid polymer electrolyte (SPE) systems for its ability to improve lithium‐ion conductivity and increase the lithium transference number (t _Li⁺) through anion immobilization.^{[ 35} , ³⁶ , ^{37 ]} For instance, Choi et al. demonstrated that integrating LiMTFSI into a porous silica aerogel‐reinforced nanocomposite SPE enhanced Li⁺ transport by immobilizing the salt onto functionalized silica frameworks via its vinyl groups.^{[ 38 ]} In contrast, our work introduces LiMTFSI into a liquid carbonate‐based electrolyte system and utilizes it as a surface‐functionalizing monomer rather than as an SPE component.

Specifically, we exploit the sulfonyl functional groups of LiMTFSI to establish favorable interactions with the NCM surface, while the methacrylate (vinyl) group remains available for polymerization. This interfacial interaction results in a uniform PLiMTFSI coating layer that conformally wraps around the NCM particles. By adapting this single‐ion conductor chemistry to the cathode–electrolyte interface, our strategy aims to regulate Li⁺ flux and promote more efficient ion transport under demanding electrochemical conditions.

We find that NCM@PLiMTFSI significantly enhances cycling performance by mitigating capacity decay under both high C‐rates (2C, 4C) and high‐voltage conditions (4.5 V) by reducing TM dissolution and stabilizing phase transitions (Figure 1 ). Furthermore, we demonstrate that the rate capability of NCM@PLiMTFSI is dramatically improved due to the facile Li⁺ transport enabled by the PLiMTFSI coating. This approach not only addresses critical challenges in NCM stability but also represents a notable advancement in cathode material design, with profound implications for the development of fast‐charging, high energy‐density, and durable lithium battery technologies.

Figure 1.

Schematic illustration of the effectiveness of PLiMTFSI coating on NCM secondary particles (NCM@PLiMTFSI).

2. Results and Discussion

2.1. Verification of PLiMTFSI Coating on NCM

PLiMTFSI coating on NCM811 was achieved via a simple one‐pot wet process by mixing NCM811, LiMTFSI, and AIBN in one flask (Figure S1, Supporting Information). To investigate the nature of the coating at the molecular level, we performed density functional theory (DFT) calculations using LiNiO₂ as a representative model for Ni‐rich layered oxides. The results revealed that the electron‐rich sulfonyl (S═O) groups of MTFSI⁻ preferentially interact with surface hydroxyl groups on hydrogen‐terminated NCM surfaces,^{[ 39} , ⁴⁰ , ^{41 ]} forming ion–dipole interactions driven by localized charge separation and electrostatic attraction (Figure S3, Supporting Information). Charge density difference plots further illustrated charge accumulation around the sulfonyl oxygen of MTFSI⁻ and partial charge depletion near the hydrogen atoms of surface hydroxyl groups, highlighting the strength of these ion–dipole interactions. This chemically anchored configuration, supported by negative adsorption energies and observable charge redistribution, facilitates the formation of a robust and uniform polymer layer. Concurrently, the methacrylate groups in LiMTFSI underwent in situ polymerization initiated by thermally decomposed AIBN radicals, forming a conformal PLiMTFSI coating that uniformly covers the NCM surface.

The presence of such interfacial interactions was further supported by Raman spectroscopy (Figure S4, Supporting Information). Compared to pristine NCM811, the PLiMTFSI‐coated sample exhibited significant broadening and suppression of the TM–O vibrational mode ≈600 cm⁻¹. This change is indicative of a perturbed local bonding environment, which we attribute to ion–dipole interactions between the sulfonyl (─SO₂─) groups of MTFSI⁻ and localized hydroxyl functionalities on the NCM surface, leading to distortion of the TM–O coordination environment. These interactions modify the surface lattice structure without forming new phases, providing spectroscopic evidence for the chemically anchored polymer layer.

FT‐IR analysis confirmed the presence of PLiMTFSI on the NCM surface, with NCM@PLiMTFSI showing characteristic peaks at 1400, 1250, and 1060 cm⁻¹. These peaks correspond to C─F, C─O, and S═O stretches in PLiMTFSI. Specifically, the peak at 1400 cm⁻¹ can be attributed to either C─F or S═O stretching, the peak at 1250 cm⁻¹ to C─F or C─O stretching, and the peak at 1060 cm⁻¹ to C─O or S═O stretching (Figure 2a). These findings validated the successful incorporation of PLiMTFSI onto the NCM surface. Moreover, the broadening of the NCM@PLiMTFSI peaks, compared to those of PLiMTFSI alone, suggests ion–dipole interactions between the sulfonyl groups of LiMTFSI and hydroxyl groups on the NCM surface, which facilitate the stable anchoring of the polymer onto the cathode interface. X‐ray diffraction (XRD) patterns of pristine NCM and NCM@PLiMTFSI exhibited similar peaks between 10° and 80°, attributed to the low content and amorphous nature of the polymer coating (Figure 2b). However, the ratio of I(003)/I(104), an indicator of Li⁺/Ni²⁺ disordering,^{[ 42 ]} was 1.81 for NCM@PLiMTFSI compared to 1.72 for pristine NCM, demonstrating that cation mixing was reduced by the PLiMTFSI coating. Additionally, distinct splitting of the (006)/(012) and (018)/(110) peaks was observed after surface modification with PLiMTFSI, indicating that the coating did not affect the layered structure (Figure 2c).^{[ 43 ]} Scanning electron microscopy (SEM) images revealed similar morphologies for pristine and coated NCM, with both exhibiting particle diameters of ≈11 µm. However, the coated NCM appeared to have a smoother surface (Figure S2, Supporting Information). Focused ion beam‐transmission electron microscopy (FIB‐TEM) was used to confirm the uniform and homogeneous PLiMTFSI layer. To enable cross‐sectional observation with FIB and prevent the PLiMTFSI layer from peeling off during FIB milling, a carbon coating (30 nm) and a Pt coating (200 nm) were applied. FIB‐TEM images revealed an amorphous PLiMTFSI coating layer (light gray) on the NCM surface, with a heterogeneous carbon and Pt layer (dark gray) observed above it. The PLiMTFSI layer, which adhered uniformly, covered the NCM surface with a thickness of ≈20 nm (Figure 2d). Low magnification of FIB‐TEM images further confirmed the uniformity of the PLiMTFSI layer (Figure S5, Supporting Information). Energy‐dispersive X‐ray spectroscopy (EDS) results confirmed the presence of the PLiMTFSI‐based polymer coating. In the carbon elemental map (Figure 2e), the polymer‐coated region showed a stronger carbon signal compared to the protective carbon and platinum layers added during FIB‐TEM sample preparation. This contrast clearly indicates the presence of the polymer coating, independent of the artificial layers introduced during processing. X‐ray photoelectron spectroscopy (XPS) analyses confirmed the chemical composition of the coating. The wide scan showed that NCM@PLiMTFSI contained F, N, and S, indicative of LiMTFSI, whereas pristine NCM did not exhibit peaks for these elements, suggesting the existence of the coating material (Figure 3a). Additionally, the S 2p spectra provided further evidence of the coating, revealing distinct SO₂ and C─S peaks in NCM@PLiMTFSI that were absent in pristine NCM, thus confirming the presence of LiMTFSI coating on the NCM surface (Figure S6, Supporting Information). The area ratio of Ni²⁺/Ni³⁺ in the Ni 2p spectra decreased from 1.37 in pristine NCM to 0.19 in NCM@PLiMTFSI (Figure 3b), indicating a substantial increase in the Ni³⁺ fraction after coating. This transformation is attributed to ion–dipole interactions between the sulfonyl groups of MTFSI⁻ and the surface hydroxyl groups on hydrogen‐terminated NCM, as supported by our DFT calculations. These interactions perturb the oxygen ligand field surrounding the transition metal sites, stabilizing Ni in a higher oxidation state (Ni³⁺) and suppressing the formation of Ni²⁺ species associated with Li⁺/Ni²⁺ cation disordering.^{[ 44 ]} This finding was supported by XRD data, which shows an increased I(003)/I(104) ratio after coating. Since the I(003)/I(104) ratio is inversely proportional to Li⁺/Ni²⁺ disordering, the increase in this ratio reflects improved structural stability. Additionally, the C 1s spectra revealed that the deconvoluted CO₃^{2 −} peak for NCM@PLiMTFSI was 8.78%, significantly lower than the 12.26% observed in pristine NCM, indicating that residual lithium species such as Li₂CO₃ were effectively removed by the surface modification process (Figure 3c).^{[ 45 ]}

Figure 2.

a) FT‐IR spectra of NCM, NCM@PLiMTFSI, and PLiMTFSI. b) XRD spectra between 10° and 80°. c) XRD spectra: splitting of (006)/(012) and (018)/(110). d) FIB‐TEM image of NCM@PLiMTFSI and e) EDS elemental mapping of the cross‐section of NCM@PLiMTFSI (bright‐field, Ni, C).

Figure 3.

XPS spectra of NCM and NCM@PLiMTFSI: a) wide scan. b) Ni 2p spectra. c) C 1s spectra. d) O 1s spectra.

Figure 3d of the O 1s XPS spectra shows that NCM@PLiMTFSI exhibited a 100.7% increase in lattice oxygen content compared to pristine NCM, accompanied by a decrease in surface oxygen species. This increase in lattice oxygen suggests enhanced structural stability, aligning with previous findings that associate lattice oxygen preservation with reduced surface degradation and improved integrity in Ni‐rich layered oxides.^{[ 46 ]} This enhancement is attributed to the stabilization effects of MTFSI⁻ on the hydrogen‐terminated surface oxygen of NCM. These interactions effectively passivate reactive sites and suppress the formation of oxygen vacancies, contributing to the stability of the material. Furthermore, both the surface oxygen and lattice oxygen peaks in the O 1s spectra shifted to lower binding energies, indicating electronic redistribution in the oxygen ligand field around transition metal–oxygen complexes. This shift reflects stronger metal–oxygen interactions and improved passivation, supporting the enhanced structural stability of NCM@PLiMTFSI.

2.2. Electrochemical Performance of NCM@PLiMTFSI

Cyclic voltammetry (CV) was conducted for three cycles between 2.7 and 4.4 V at a scan rate of 0.1 mV s⁻¹ (Figure 4a,b). The potential difference between the anodic peaks of the first and third cycles (ΔE _1‐3) serves as a measure of electrode polarization. For NCM@PLiMTFSI, ΔE _1‐3 was 0.24 V, substantially lower than the 0.38 V observed for pristine NCM, indicating a more rapid electrochemical reaction and improved reversibility.^{[ 47 ]} Additionally, the potential difference between the anodic and cathodic peaks was 0.28 V for NCM@PLiMTFSI, smaller than the 0.39 V observed for pristine NCM, indicating a decrease in interfacial resistance.^{[ 48 ]} Consistent with this smaller potential difference compared to pristine NCM, the anodic current of NCM@PLiMTFSI increased with each third cycle, indicating superior electrochemical kinetics.

Figure 4.

Electrochemical performance of NCM and NCM@PLiMTFSI. CV curves of three cycles at a scan rate of 0.1 mV s⁻¹ of a) NCM and b) NCM@PLiMTFSI. c) Rate performance at 2.8–4.3 V. d) GITT curves obtained after two cycles at 0.1C, with Li⁺ diffusion coefficients calculated during e) charge and f) discharge steps.

In the rate performance test, NCM@PLiMTFSI showed higher discharge capacities at 0.2C, 0.6C, 1C, 4C, and 8C, revealing better kinetic performance (Figure 4c). Notably, NCM@PLiMTFSI exhibited an average discharge capacity of 206.82 mAh g⁻¹ at 0.2C, significantly outperforming the pristine NCM's capacity of 183.15 mAh g⁻¹. Additionally, at a high C‐rate of 8C, PLiMTFSI‐coated NCM retained a discharge capacity of 103.08 mAh g⁻¹, surpassing the 81.54 mAh g⁻¹ observed for pristine NCM. Both NCM and NCM@PLiMTFSI also showed reversible properties when cycling from 8C back to 0.2C.

The galvanostatic intermittent titration technique (GITT) was tested during 20 min charging/discharging, followed by 2 h of rest after 2 cycles at 0.1C in the voltage range of 2.8–4.3 V (Figure 4d). The Li⁺ diffusion coefficients ($D_{\mathrm{L}{\mathrm{i}}^{+}}$) were calculated from Equation (1), assuming the secondary particles of NCM and NCM@PLiMTFSI are spherical with a diameter of 11 µm (Figure 4e,f).^{[ 49} , ^{50 ]}

$$D_{\mathrm{L}{\mathrm{i}}^{+}}=\frac{4}{\pi \tau }\times {\left(\frac{R_s}{3}\right)}^2\times {\left(\frac{\mathrm{\Delta }{E}_s}{\mathrm{\Delta }{E}_t}\right)}^2$$ (1)

where τ denotes the duration time of the charging or discharging current pulse, while R _s represents the radius of the secondary particle of the active material. ΔE_s refers to the difference between the open‐circuit voltage before the pulse and the voltage at the end of relaxation after the pulse. ΔE_t indicates the change in voltage during the pulse, excluding the drop caused by internal resistance.

The average $D_{\mathrm{L}{\mathrm{i}}^{+}}$ values were 1.81 × 10⁻¹¹ and 2.73 × 10⁻¹¹ cm² s⁻¹ during charge, and 1.63 × 10⁻¹¹ and 2.13 × 10⁻¹¹ cm² s⁻¹ during discharge for pristine and PLiMTFSI‐treated NCM, respectively. The higher $D_{\mathrm{L}{\mathrm{i}}^{+}}$ values for NCM@PLiMTFSI correlated with improved rate performance, indicating that the PLiMTFSI coating layer facilitated more efficient Li⁺ diffusion during both extraction and insertion on the NCM surface, contributing to superior discharge capacities at various C‐rates. This finding was consistent with the well‐established role of LiMTFSI as a single‐ion conductor, enabled by its immobile anion (MTFSI⁻), which promoted efficient Li⁺ transport.^{[ 51 ]} In this study, LiMTFSI was utilized not as a solid electrolyte but as a coating material for secondary particles of the NCM cathode, enhancing lithium‐ion kinetics during both charges (as Li⁺ moves from the bulk of the cathode particles to the surface and toward the anode) and discharge (as Li⁺ returns from the anode to the cathode). This improvement in Li⁺ diffusion kinetics, compared to pristine NCM, highlighted the critical role of the PLiMTFSI layer in facilitating efficient ion transport.

Galvanostatic cycling performance was tested under 2.8–4.3 V at 4C and under 2.8–4.5 V at 2C. As shown in Figure 5a, NCM@PLiMTFSI consistently delivered overall higher discharge capacities compared to pristine NCM throughout the cycling test, demonstrating superior performance under high current density conditions. At a high C‐rate of 4C, NCM@PLiMTFSI exhibited significantly more stable cycling performance, with a capacity retention of 81.8% after 150 cycles, whereas pristine NCM retained only 49.7% of its capacity. This enhanced stability can be attributed to the robust structure of NCM@PLiMTFSI, which maintains a gradual capacity fade over time. In contrast, pristine NCM exhibited a pronounced decline in capacity, especially during the initial and final stages of cycling. Figure 5b shows that NCM@PLiMTFSI exhibits significantly higher capacity retention (63.5%) compared to pristine NCM (41.6%) after 200 cycles at 2.8–4.5 V and 2C. Although NCM@PLiMTFSI initially shows lower discharge capacities than pristine NCM, a reversal occurs after 5 cycles, resulting in higher discharge capacities. By 200 cycles, NCM@PLiMTFSI achieved a capacity of 120.51 mAh g⁻¹, ≈1.5 times that of pristine NCM. The 4.5 V high cut‐off voltage, while beneficial for increasing energy density, typically accelerates surface reconstruction and structural instability in Ni‐rich NCM due to lithium depletion and TM migration into Li vacancies, disrupting the structure.^{[ 52 ]} However, the PLiMTFSI coating enhanced stability by mitigating TM migration and surface degradation, relatively preserving the layered structure under these demanding conditions. This improved interfacial stability from the PLiMTFSI coating limited side reactions, HF attack, and TM dissolution, thereby slowing the formation of the rock‐salt phase that impedes Li‐ion transport. These results highlighted the PLiMTFSI coating's role in comparatively maintaining structural integrity and sustaining efficient Li‐ion conductivity, enabling better cycling stability and electrochemical performance under high‐voltage and high C‐rate conditions.

Figure 5.

Cycling performance a) 2.8–4.3 V at 4C and b) 2.8–4.5 V at 2C. Charge/discharge profiles at 4C between 2.8 and 4.3 V for c) pristine NCM and d) NCM@PLiMTFSI, shown after two formation cycles at 0.1C, and at the 50th, 100th, and 150th cycles under 2.8–4.3 V at 4C.

However, the polymer coating layer was not effective in retarding capacity decay at 1C (Figure S7, Supporting Information). Nevertheless, comparable capacity retentions of 87.4 and 86.0%, along with similar discharge capacities of 162.88 and 162.49 mAh g⁻¹ after 100 cycles, were achieved for uncoated and coated NCM, respectively. This result implied that the PLiMTFSI layer was particularly effective at high C‐rates, specifically at 2C and 4C, suggesting that the PLiMTFSI layer on NCM contributes to enabling fast‐charging lithium batteries. Figure 5c,d present the voltage profiles of pristine NCM and NCM@PLiMTFSI during the initial two formation cycles at 2.8–4.3 V and 4C, as well as at the 50th, 100th, and 150th cycles. The first and second formation cycles were similar for both uncoated and coated NCM, indicating that the coating did not introduce additional redox reactions. After two initial formation cycles at 0.1C, both the pristine and NCM@PLiMTFSI samples, excluding the first formation cycle and the initial cycle at 4C during 150 cycles, exhibited symmetric charge/discharge voltage profiles across all current densities. This symmetry suggested a reversible electrochemical reaction with stabilization occurring after the first cycle. In the first cycle at 4C, pristine NCM demonstrated a discharge capacity of 159.66 mAh g⁻¹, while NCM@PLiMTFSI exhibited a slightly lower capacity of 156.42 mAh g⁻¹. The capacity decrease from the second formation cycle was 26.4% for pristine NCM and 26.6% for NCM@PLiMTFSI, both indicating significant loss due to the high current density of 4C. Although the decrease ratio for NCM@PLiMTFSI was slightly higher, both samples showed comparable performance initially. However, at the 50th, 100th, and 150th cycles, the discharge capacities of NCM@PLiMTFSI were consistently higher than those of pristine NCM. The decrease in discharge capacity from the 100th to the 150th cycle was much greater for pristine NCM at 29.2%, while NCM@PLiMTFSI showed a significantly lower reduction of 7.8%. This comparison underscored the positive effect of the PLiMTFSI coating on cycling stability. Electrochemical impedance spectroscopy (EIS) was carried out to investigate resistance before and after delithiation/lithiation of Li⁺, indicating the kinetics of electrochemical reactions. As known, three resistances can be calculated from the Nyquist plots. 1) Ohmic resistance (R _s), originating from the bulk electrolyte and observed at high frequency; 2) Surface film resistance (R _sf), related to the CEI and observed in the first semicircle; 3) Charge transfer resistance (R _ct), which is the main factor determining interfacial kinetics and is observed in the second semicircle.^{[ 53 ]} Before cycling (Figure S8a, Table S1, Supporting Information), the R _ct​ of fresh NCM@PLiMTFSI significantly decreased to 400.3 Ω compared to 655.4 Ω for fresh pristine NCM, indicating that the coating enhances charge transfer kinetics. This improvement aligned with the results from GITT and rate performance tests. Subsequently, EIS was further performed after both the two formation cycles at 0.1C and 15 cycles at 4C in the voltage range of 2.8–4.3 V. After the formation cycles, the formation of the CEI layer was confirmed by the appearance of an additional semicircle in the Nyquist plot, corresponding to the R _sf, which appears before the semicircle associated with R _ct​. However, at this stage, the resistance values of pristine NCM and NCM@PLiMTFSI remained similar, indicating that the activation process had a negligible effect on resistance (Figure S8b, Table S1, Supporting Information). The EIS analysis after 15 cycles at 4C was conducted due to the sharp decline in cycling stability, as shown in Figure 5a. The R _sf + R _ct value for NCM@PLiMTFSI was 52.06 Ω, lower than the 58.04 Ω of pristine NCM, primarily due to its reduced R _ct, which compensated for the slightly higher R _sf (Figure S8c, Table S1, Supporting Information). This decrease in R _ct suggested that the polymer coating on pristine NCM's secondary particles enhanced interfacial kinetics upon current application, facilitating Li‐ion transport while maintaining capacity retention with minimal loss compared to pristine NCM.

2.3. Postmortem Analysis

To observe and validate the changes within the cell after cycling under harsh conditions, a thorough series of material characterizations was conducted, comparing the effects of uncoated samples and PLiMTFSI‐coated samples. One such change was TM dissolution, which typically occurs when NCM is exposed to HF,^{[ 54 ]} leading to the migration and deposition of these metals onto the Li metal anode, negatively impacting SEI formation.^{[ 55 ]} Postmortem SEM analysis of the Li metal after 200 cycles revealed that the anode cycled with NCM@PLiMTFSI had a smoother surface than the anode cycled with pristine NCM, which exhibited multiple particles and aggregates (Figure 6a,b). This observation suggested a reduction in transition metal dissolution with the NCM@PLiMTFSI coating. The inductively coupled plasma mass spectrometry (ICP‐MS) results revealed significantly lower concentrations of deposited Ni, Co, and Mn on the Li metal anode paired with NCM@PLiMTFSI after 200 cycles. This observation was consistent with the SEM images, suggesting that the S═O groups in LiMTFSI had adsorbed onto transition metal ions, thereby inhibiting TM leaching (Figure 6c). Consistently, inductively coupled plasma atomic emission spectroscopy (ICP‐AES) analysis of the electrolyte stored with NCM@PLiMTFSI for 23 days showed reduced transition metal dissolution, with lower Ni levels and undetectable Co and Mn, compared to the electrolyte stored with pristine NCM. This suggested that the PLiMTFSI coating played a protective role, preventing the leaching of transition metals into the electrolyte (Table S2, Supporting Information). XRD analysis of the cathode slurry after cycling was also conducted to examine the effect of charging/discharging on the structure of NCM (Figure 6d). The PLiMTFSI layer contributed to reducing cation mixing during Li⁺ extraction/insertion, resulting in an I(003)/I(104) ratio of 0.76, which was higher than the 0.70 ratio observed for pristine NCM after 200 cycles. The postmortem O 1s spectra revealed a significant reduction in C–O and Li_xPF_yO_z species for NCM@PLiMTFSI, indicating effective suppression of electrolyte‐induced side reactions (Figure 6e). This suggests that the PLiMTFSI coating acts as a protective barrier against electrolyte penetration, mitigating surface degradation during cycling. An increase in C═O content was also observed for NCM@PLiMTFSI. This enhancement can be partially attributed to the intrinsic carbonyl groups present in LiMTFSI, which contribute to the C═O signal. Additionally, the stabilized interfacial environment provided by the polymer coating may lead to the preferential formation of C═O‐rich organic species during cycling. These species may form due to reduced parasitic reactions and enhanced surface passivation, resulting in a more controlled reconfiguration of surface layers. Moreover, a lattice oxygen peak at 529.2 eV appeared exclusively in NCM@PLiMTFSI, absent in pristine NCM. This peak indicates preservation of the oxygen framework, preventing oxygen loss and enhancing the structural integrity of the cathode material. These findings confirm that the LiMTFSI‐based polymer coating effectively prevents surface decomposition, promotes controlled surface reconfiguration, and maintains the bulk structure, contributing to improved cycling stability under demanding electrochemical conditions. The F 1s spectra indicated that LiF formation at the cathode was suppressed by the PLiMTFSI layer after 200 cycles (Figure 6f). The 13.9% reduction in LiF content, known for its electron‐insulating properties, indicated that the NCM@PLiMTFSI coating enhanced electronic conductivity compared to pristine NCM.^{[ 56 ]} Generally, the OCO₂, C═O, and C─O peaks identified in the XPS spectra of the Li metal anode result from electrolyte decomposition on its surface. The combined intensity of these peaks in the C 1s XPS spectrum of the Li metal anode paired with NCM@PLiMTFSI accounted for 35.6%, significantly lower than the 52.8% observed for the Li metal anode paired with pristine NCM (Figure S10a, Supporting Information). This significant reduction indicated a mitigation of electrolyte decomposition during cycling on the anode. Additionally, LiF on the Li metal anode plays a contrasting role compared to the cathode, as it helps prevent Li dendrite formation and enhances cycling stability.^{[ 57 ]} The LiF peak on the Li metal anode paired with coated NCM after 200 cycles was slightly higher (66.0%) than that paired with pristine NCM (64.2%) (Figure S10b, Supporting Information).^{[ 58 ]} These results suggested that the PLiMTFSI coating prevented direct contact between NCM and the electrolyte, thereby improving the electrolyte's chemical stability and facilitating the formation of chemical components that contributed to a relatively more stable SEI layer on the lithium metal surface.

Figure 6.

Analysis of anode and cathode after 200 cycles at 2C: SEM of Li metal anode a) NCM and b) NCM@PLiMTFSI. c) ICP‐MS of Li metal anode. d) XRD spectra of cathodes. e) O 1s and f) F 1s XPS spectra of cathodes.

Scanning transmission electron microscopy (STEM) analysis was performed to observe phase transitions near the surface of NCM (Figure 7a,b). The STEM images reveal a layered phase in the NCM bulk and a rock‐salt phase at the near‐surface, both confirmed by fast Fourier transform (FFT) analysis.^{[ 59 ]} For PLiMTFSI‐coated NCM after 200 cycles, the average thickness of the rock‐salt phase was reduced to ≈2 nm, compared to 4 nm in the uncoated NCM.^{[ 60 ]} This reduction illustrated the effectiveness of the polymer coating in inhibiting the detrimental transition from a layered to a rock‐salt structure, a change that can produce oxygen vacancies and obstruct Li⁺ and electron diffusion pathways, thereby impairing the electrochemical performance of Ni‐rich materials. STEM was used to structurally compare the thickness of the rock‐salt phase, while FIB‐SEM was subsequently employed to observe the post‐cycling changes in the cross‐section of the cathode. FIB‐SEM analysis was conducted after cycling under severe conditions: at a high current density of 4C for 150 cycles and at a high cutoff voltage of 4.5 V for 200 cycles (Figures S11 and S12, Supporting Information). In the voltage range of 2.8–4.3 V at 4C, pristine NCM shows significant degradation, with a high concentration of large voids forming primarily in the central regions of the secondary particles. In contrast, NCM@PLiMTFSI retained a more intact structure, with many fewer voids, demonstrating significantly better stability under these high‐current conditions. At 4.5 V, where high delithiation occurs, both pristine NCM and NCM@PLiMTFSI exhibited structural degradation, with cracks and voids forming in both. However, NCM@PLiMTFSI showed fewer and smaller cracks, maintaining better structural integrity under these severe delithiation conditions. Furthermore, time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) was used to perform a detailed analysis of the chemical species on the cycled cathode surface, including the CEI (Figure 7c). TOF‐SIMS depth profiling revealed the distribution of chemical components from the surface to the bulk phase. The observed compounds included those produced from electrolyte decomposition (C₂HO⁻, PO₂ ⁻, POF⁻) and from released transition metals due to HF attack (NiF₃ ⁻, CoF₃ ⁻, MnF₃ ⁻).^{[ 61 ]} For the C₂HO⁻ signal, both NCM and NCM@PLiMTFSI showed a sharp decrease at 55 s. However, the surface‐coated NCM exhibited significantly reduced intensity and stabilized more rapidly, suggesting that the PLiMTFSI layer suppressed the formation of organic by‐products. Similarly, the NiF₃ ⁻ signal in NCM@PLiMTFSI, which was primarily formed by TM migration, did not show a notable peak during early sputtering (surface), while pristine NCM exhibited a maximum NiF₃ ⁻ peak at 35 s. This demonstrated that the polymer coating layer also inhibited transition metal dissolution. Other TOF‐SIMS results showed a similar trend with C₂HO⁻and NiF₃ ⁻, indicating improved quality of the CEI and surface species on NCM@PLiMTFSI, as evidenced by the reduced peaks of secondary ions from side reactions or TM dissolution, and the faster attainment of the plateau. (Figure S9, Supporting Information).

Figure 7.

Analysis after 200 cycles at 2C: STEM and FFT analyses of a) Pristine NCM b) NCM@PLiMTFSI. c) TOF‐SIMS spectra of NCM and NCM@PLiMTFSI.

3. Conclusion

In this study, we successfully incorporated LiMTFSI, a material commonly used in SPE, into liquid electrolyte systems to coat NCM811 cathodes. This approach yielded significantly enhanced rate capabilities compared to pristine NCM cathodes. Moreover, the NCM@PLiMTFSI material exhibited notable improvements in cycling stability, particularly at high C‐rates (2C and 4C) and elevated cut‐off voltages (4.5 V). These advancements enabled both fast‐charging capabilities and high energy density in lithium batteries. Although the polymer coating did not entirely eliminate capacity fade, it substantially reduced the rate of degradation compared to untreated NCM. The uniform PLiMTFSI coating enhances Li⁺ transport, reduces TM dissolution, and mitigates side reactions. These combined effects improve rate capabilities, preserve the cathode's layered structure, and enhance lifespan stability under harsh conditions. Extensive electrochemical testing, including cycling performance under various conditions, along with comprehensive material characterization using techniques such as STEM, XPS, XRD, and TOF‐SIMS, provides strong evidence for the effectiveness of the PLiMTFSI coating in decelerating capacity loss. This work demonstrates the successful application of a material traditionally used in SPE to liquid electrolyte systems, highlighting its potential to enhance lithium battery performance under demanding conditions.

4. Experimental Section

Fabrication of PLiMTFSI Coating Layer on NCM

To fabricate a poly LiMTFSI layer on NCM secondary particles, a wet chemical coating method in solution was used. In the N₂ atmosphere, 400 mg of NCM and 20 mg of lithium sulfonyl(trifluoromethane sulfonyl)imide methacrylate (LiMTFSI) monomer were blended with 3 mL anhydrous acetonitrile for 1 h. AIBN solution was added, and the mixture was stirred vigorously at 60 °C. After being agitated for 4 h, it was filtered with methanol and then dried in a vacuum oven overnight.

Material Characterizations

FT‐IR spectroscopy (TENSOR 27, Bruker, Germany) was employed to investigate chemical bonds. Raman measurements were performed using a DXR2xi Raman spectrometer (Thermo Scientific, USA) equipped with a 532 nm excitation laser. Density functional theory (DFT) calculations were performed using the VASP package with the PBE functional and GGA+U approach.^{[ 62 ]} A 520 eV plane‐wave cutoff and a Γ‐centered k‐point mesh were used. Structural optimization was conducted until residual forces were below 0.02 eV Å⁻¹. LiNiO₂ slabs with different surface terminations were modeled, and the MTFSI⁻ anion was initially placed 2 Å above the surface to evaluate interfacial interactions. The crystal structure of the as‐prepared samples was analyzed using X‐ray diffraction (XRD) with a Smart lab instrument (Rigaku, Japan) over a 2θ range of 10–80° at a scanning rate of 5° min⁻¹. Elemental composition and chemical valence states were characterized using X‐ray photoelectron spectroscopy (XPS) with an AXIS SUPRA system (Kratos, UK), calibrating all spectra to the C 1s peak at 284.8 eV. Surface morphologies of the samples were observed using field emission scanning electron microscopy (FE‐SEM, JSM‐7800F Prime). Detailed lattice fringes were analyzed with field emission transmission electron microscopy (FE‐TEM, JEM‐F200(TFEG)) and Cs‐corrected scanning transmission electron microscopy (Cs‐STEM, JEM‐ARM200F, JEOL, Japan). During FE‐TEM analysis, energy‐dispersive spectroscopy (EDS) was utilized to map elemental distributions. Fast Fourier transform (FFT) patterns were obtained using Digital Micrograph software GMS 3.2 (Gatan Inc., USA). A focused ion beam (FIB, Helios 650) was employed to investigate the cross‐sections of active materials after cycling and to perform pre‐treatment for Cs‐STEM analysis. Electrode surface analysis of decomposition products with electrolytes was conducted using time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS, ION‐TOF, Germany) with a Bi⁺ primary ion source (30 keV, 1 pA) and a Cs⁺ etching ion source (2 keV, 120 nA). Trace concentrations of transition metals deposited on the Li metal anode were detected using inductively coupled plasma mass spectrometry (ICP‐MS, NexION 350D, Perkin‐Elmer SCIEX), while ICP‐AES (OPTIMA 8300, Perkin‐Elmer, USA) was used to analyze transition metals dissolved in the electrolyte from the cathode.

For postmortem analysis, the battery cell was disassembled in an Ar‐filled glove box. The disassembled cathode and anode were washed three times with DMC and then dried. All characterizations were performed at the National Center for Inter‐University Research Facilities (NCIRF) at Seoul National University.

Electrochemical Measurements

The cathodes consisted of active material (NCM or NCM@PLiMTFSI), binder (PVDF), and conducting agent (Super P) at a ratio of 8:1:1. The obtained slurry with N‐methyl‐2‐pyrrolidone (NMP) was cast on aluminum (Al) foil with loading targeted at 4.0 ± 0.3 mg cm⁻² (diameter: 14 mm). To isolate the effect of the polymer coating, all electrodes were prepared under identical conditions—including composition, mass loading, and processing parameters—with the only variable being the presence or absence of the LiMTFSI‐derived polymer layer on the NCM particles. The coin cell was fabricated with NCM‐based cathode, polyethylene (PE) separator, Li metal anode, and 1.25 m LiPF₆ in EC: EMC: DMC (20:5:75, vol%) electrolyte using a CR2032 crimper. The coin cell assembly was conducted in an Ar glove box (O₂ < 0.3 ppm, H₂O < 0.3 ppm). The cycling performance and rate capability tests of the as‐assembled coin cells were conducted using a WonATech battery cycler (WBCS3000Le) at a temperature of 30 °C. The cycling tests were performed within a voltage range of 2.8–4.3 V and 2.8–4.5 V versus Li/Li⁺, corresponding to current densities of 4C and 2C, respectively (1C = 186.2 mAh g⁻¹). Prior to cycling, all cells were pre‐cycled at 0.1C for two cycles to activate the internal cell system. For the GITT test, measurements were conducted after two initial cycles at 0.1C within the 2.8–4.3 V range, with each 20 min charge/discharge step followed by a 2 h rest period. Electrochemical impedance spectroscopy (EIS) was conducted using an electrochemical workstation (IviumStat, HS Technologies, Korea) over a frequency range of 100 kHz–10 mHz with an amplitude of 10 mV at 30 °C. Cyclic voltammetry (CV) was performed on the same workstation with a scan rate of 0.1 mV s⁻¹ in the potential range of 2.7–4.4 V at 30 °C to analyze the redox behavior.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Kim H., Jeong M.‐K., Kim H.‐J., Kim Y., Kang K., Oh J. H., Enhanced Cycling Stability of NCM811 Cathodes at High C‐Rates and Voltages via LiMTFSI‐Based Polymer Coating. Small 2025, 21, 2502816. 10.1002/smll.202502816

Data Availability Statement

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