Method for coating cathode material using two-dimensional material wrapping, coated cathode material, and secondary battery including the coated cathode material

Abstract

A method for coating a cathode material using a two-dimensional material wrapping method, a coated cathode material, and a secondary battery including the coated cathode material are disclosed. A cathode material wrapping method according to one embodiment may include the step of manufacturing a cathode material including a MoS2 wrapping layer through a self-assembly process in which MoS2 nanoflakes with induced positive surface charges surround a cathode particle by electrostatic attraction.

Description

Technology Field

[0001] The following description relates to a method for coating a cathode material using a two-dimensional material wrapping method, a coated cathode material, and a secondary battery including the coated cathode material. Background Technology

[0002] Currently, lithium-ion battery technology is gradually expanding from small electronic devices such as laptops and smartphones to devices requiring high electrochemical performance, such as electric vehicles, smart grids, and large-capacity energy storage systems, driven by industrial advancements. Consequently, there is a demand for various characteristics in cathode active materials, including high energy, high density, lifespan, and improved stability. In particular, high-nickel transition metal oxide cathode active materials possess high energy density and offer excellent cost competitiveness. However, repeated charging and discharging cycles cause shrinkage and expansion of the active material. This leads to phase changes, detachment of the active material, and adverse reactions with the electrolyte, resulting in degraded battery performance. Furthermore, problems such as structural instability of the material itself and reduced lifespan characteristics arise.

[0003] Accordingly, it is essential to develop new coating materials for cathode materials that can suppress phase changes and improve structural stability, as well as achieve high energy density and lifespan characteristics, by introducing surface modification and coating materials of the active material.

[0004] [Prior Art Literature]

[0005] Korean Patent Publication No. 10-2022-0072302 The problem to be solved

[0006] A method for coating a cathode material using a two-dimensional material wrapping method, a coated cathode material, and a secondary battery including the coated cathode material are provided. means of solving the problem

[0007] A cathode material including a MoS2 wrapping layer is provided.

[0008] According to one aspect, the MoS2 wrapping layer may be characterized by being formed by wrapping MoS2 nanoflakes with induced positive surface charges around positive particles through electrostatic attraction.

[0009] According to another aspect, the anode particles may be characterized by comprising stacked anode particles containing negatively charged nickel (Ni).

[0010] According to another aspect, the MoS2 nanoflakes may be characterized by being functionalized with a cetyltrimethylammonium bromide (CTAB) surfactant during the exfoliation process, thereby inducing a positive surface charge.

[0011] According to another aspect, the anode particles may be characterized by being manufactured through a wet chemical coating process by mixing the MoS2 nanoflakes and a solution containing the cetyltrimethylammonium bromide surfactant.

[0012] A method for wrapping an anode material comprises the step of manufacturing an anode material including a MoS2 wrapping layer through a self-assembly process in which MoS2 nanoflakes with positive surface charges induced surround an anode particle by electrostatic attraction.

[0013] A secondary battery is provided comprising: a positive electrode comprising a positive electrode including a positive electrode material including a MoS2 wrapping layer; a negative electrode; and an electrolyte that transfers ions between the positive electrode and the negative electrode. Effects of the invention

[0014] A method for coating a cathode material using a two-dimensional material wrapping method, a coated cathode material, and a secondary battery including the coated cathode material can be provided. Brief explanation of the drawing

[0015] FIG. 1 is a diagram illustrating an example of a procedure for preparing an anode wrapped with MoS2 in one embodiment of the present invention. FIG. 2 is a diagram illustrating an example of the performance of a MoS2@NCM anode according to an embodiment of the present invention. FIG. 3 is a diagram showing images for comparison between Pristine NCM and MoS2@NCM according to an embodiment of the present invention. Figure 4 is a diagram illustrating an example of electrochemical performance between Pristine NCM and MoS2@NCM according to an embodiment of the present invention. FIG. 5 is a diagram showing cross-sectional SEM images of Pristine NCM and MoS2@NCM according to an embodiment of the present invention before and after cycling. FIG. 6 is a figure showing the ex-situ XPS spectra of Pristine NCM and MoS2@NCM according to an embodiment of the present invention before and after cycling. FIG. 7 is a figure illustrating an example of in situ differential electrochemical mass spectrometry and voltage profiles during primary charging for Pristine NCM and MoS2@NCM according to an embodiment of the present invention. FIG. 8 is a diagram illustrating an example of the ex-situ XRD pattern of Pristine NCM and MoS2@NCM according to an embodiment of the present invention after 50 cycles. FIG. 9 is a diagram schematically illustrating the functional effects of a MoS2@NCM anode according to an embodiment of the present invention. Specific details for implementing the invention

[0016] The present invention is capable of various modifications and may have various embodiments; therefore, specific embodiments will be described in detail below based on the attached drawings.

[0017] In describing the present invention, if it is determined that a detailed description of related known technology may obscure the essence of the present invention, such detailed description is omitted.

[0018] Nickel (Ni)-rich layered cathode materials have been intensively studied to maximize the energy density of lithium-ion batteries (LIBs). However, performance is degraded due to unstable surface reactions in high voltage regions, leading to detrimental changes in the surface structure. In the embodiments of the present invention, the long-term stability of the LIB can be improved by conformally wrapping two-dimensional (2D) molybdenum disulfide (MoS2) nanoflakes onto a Ni-rich layered cathode through electrostatic interactions between positively surface-treated molybdenum disulfide (MoS2) and negatively charged cathode particles. The formation of the MoS2 wrapping layer prevents direct interfacial contact between the cathode and the electrolyte, thereby preventing structural degradation and unwanted side reactions caused by the surface. This can provide improved cell stability through advantages such as reduced gas generation, easy lithium ion transport, and mitigation of microcrack propagation. Through this 2D material wrapping, the anode surface can be covered thinly and effectively, which can consequently improve the safety and extend the lifespan of the LIB.

[0019] introduction

[0020] As the lithium-ion battery industry develops rapidly, there is an increasing focus on formulating new strategies for implementing batteries that meet various requirements, including (i) high energy density, (ii) longer lifespan, (iii) enhanced safety, (iv) cost-effectiveness, and (v) environmental friendliness. As part of efforts to achieve these goals, new materials were investigated by controlling the transition metal components and their ratios, including layered LiMO2 cathode materials (M = transition metal). For example, Ni-rich transition metal oxides (LiNi), known as NCM x Co y Mn zO2, x ≥ 0.8, x+y+z = 1) class has excellent capacity (>200 mAh g -1 It has been intensively studied due to its excellent operating voltage (>4V), relatively low cost, and reduced use of harmful Co species. Increasing the Ni content in NCM cathodes generally results in Ni 2+ / Ni 3+ wa Ni 3+ / Ni 4+ The capacity increases because a two-step redox reaction between them becomes possible.

[0021] However, Ni-rich cathodes in a highly delithiated state often face surface instability problems caused by (i) structural and thermal decomposition, (ii) unwanted surface reactions with electrolyte species, and (iii) gas release from the cathode. From the perspective of cathode structure, Li + (0.76 ) and Ni 2+ (0.69 Due to the similar ionic radii of ), undesirable cation mixing occurs between Li and M sites. This results in the crystal structure collapsing from a layered state to an inert, disordered spinel or rock salt phase, particularly at the anode surface. Structural degradation on the surface easily propagates into the internal bulk grains. At high voltages, Ni 4+ The reduction of [substance] and the subsequent O2 release from the anode are accelerated, leading to the accumulation of byproducts such as Ni-O-like phases and residual lithium carbonate. Additionally, the highly reactive anode surface at high voltages tends to decompose electrolyte species and form a thick anode electrolyte interphase layer. Hydrogen fluoride (HF), an electrolyte decomposition product resulting from the reaction of PF5 with H2O impurities, corrodes the NCM surface and subsequently dissolves transition metal species. Furthermore, residual Li species in the NCM anode, such as LiOH and Li2CO3, can unexpectedly react with the electrolyte to generate gases (e.g., O2, CO2, CO, etc.), potentially causing safety issues.

[0022] Various approaches, including surface treatments and coatings, have been developed to address the unwanted structural and chemical degradation of nickel-rich NCM cathode materials at high voltages. Metal oxides such as Al2O3, TiO2, ZrO2, and AlPO4 have been successfully used to protect the surfaces of Ni-rich NCM cathodes. However, these valve metal-based oxides exhibit limitations in terms of kinetic properties for cell operation due to their low ionic and electronic conductivity. Furthermore, these brittle metal oxide materials tend to make point contact with the cathode particles, failing to provide adequate surface covering and protection. As an alternative, two-dimensional (2D) materials can offer a promising pathway to maximize surface contact and enhance the protective coverage as a protective layer. For example, the effectiveness of conductive graphene coatings has been demonstrated in improving the high-voltage cycle life and Coulomb efficiency of NCM cathodes by enhancing mechanical resistance to volume changes and promoting charge transfer reactions. Similarly, it has been shown that using WSe2 grown via Chemical Vapor Deposition (CVD) as a functional 2D coating material improves the interfacial stability and electrochemical performance of NCM anodes. This is achieved by mitigating transition metal dissolution, structural instability, and microcrack formation. Although there have been several reported attempts, the development of a simple and cost-effective wrapping strategy utilizing robust and conductive 2D materials to address the unstable surface reaction issues of Ni-rich NCM anodes remains a challenge.

[0023] Embodiments of the present invention provide a method and system for protection and conformal self-assembled anode wrapping utilizing electrostatic attraction between a negatively charged Ni-rich stacked anode and a positively charged 2D MoS2 nanoflake. This strategy allows for mitigating unwanted surface reactions while maintaining high ionic conductivity.

[0024] FIG. 1 illustrates an example of a procedure for preparing an anode wrapped with MoS2 in one embodiment of the present invention. To ensure dispersion stability and induce a positive surface charge, MoS2 nanoflakes were functionalized with a cetyltrimethylammonium bromide (CTAB) surfactant during the exfoliation process. Subsequently, the charged MoS2 nanoflakes automatically and completely wrap around the Ni-rich stacked anode due to electrostatic attraction.

[0025] Enhanced structural stability and improved electrochemical performance were observed through the application of surface-modified NCM cathodes featuring a uniform, ultra-thin MoS2 wrapping layer, which can effectively mitigate unwanted side reactions on the cathode surface. To verify the efficacy of the MoS2-wrapped NCM cathode (MoS2@NCM), in-situ Differential Electrochemical Mass Spectrometry (DEMS) measurements were performed to confirm a reduction in gas generation. Additionally, ex-situ characterization was conducted to investigate structural evolution and electrochemical reaction mechanisms.

[0026] Preparation of dispersion of positively charged MoS2 nanoflakes

[0027] To perform liquid exfoliation, 0.2 g of MoS2 powder was used and exfoliated in a 45 ml aqueous solution of 10 wt% CTAB. The exfoliation process was carried out using a 40 W tip sonicator in an ice bath for 2 hours to maintain a low temperature. Subsequently, the resulting MoS2 dispersion was centrifuged at 5000 rpm for 10 minutes to remove unexfoliated bulk powder.

[0028] MoS 2 Fabrication of a wrapped NCM anode

[0029] The MoS2@NCM anode was prepared via a wet chemical coating process. The active material is Li 0.98 Ni 0.83 Co 0.11 Mn 0.05 O2 was mixed with a 10 wt% CTAB-MoS2 solution and stirred at room temperature for 1 hour. Subsequently, the mixture was freeze-dried for 72 hours to form MoS2@NCM powder.

[0030] Material Characterization

[0031] The crystalline phase of the sample is Cu-Kα (λ = 1.54 The sample was characterized using X-ray Diffraction (XRD) with radiation. Surface morphology and microstructure were characterized using Scanning Electron Microscopy (SEM). Crystal structure and elemental distribution were analyzed by Transmission Electron Microscopy (TEM), and elemental composition was measured by Energy Dispersive X-ray Spectroscopy (EDS). After cycling, cross-sectional images of the sample were obtained using Focused Ion Beam Scanning Electron Microscopy (FIB-SEM). Surface atomic chemical composition was analyzed using X-ray Photoelectron Spectroscopy (XPS). Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) was performed to quantify the Li, Ni, Co, and Mn content of the active material. Thermogravimetry analysis (TGA) was used to analyze the thermal decomposition of the sample. UV-visible spectra were obtained by measuring the MoS2 dispersion in a quartz cuvette, and Raman spectra were acquired using a 532 nm laser source (Alpho300R spectrometer). Zeta potential measurements of MoS2 and NCM were performed using a Zetasizer analyzer.

[0032] Electrochemical characterization

[0033] A slurry composed of 85 wt% active material, 8 wt% conductive material (Super-P), and 7 wt% polyvinylidene difluoride (PVDF) was prepared. Electrodes were fabricated by coating an Al foil to a thickness of 200 mm using a doctor blade. After coating, the electrodes were dried in a vacuum oven at 110 °C for 12 hours, followed by cell assembly. Half cells were assembled in glove boxes filled with Ar at O2 and H2O concentrations of less than 0.1 ppm, using a perforated electrode (14 mm diameter) as the working electrode. Li metal foil was used as the counter electrode. The electrolyte consisted of 1 M LiPF6 in a mixture of ethylene carbonate and diethyl carbonate (1:1 v / v) and 5 wt% fluoroethylene carbonate, and Celgard 2500 polypropylene (19 mm diameter) was used as the separator. Electrochemical performance measurements were performed using the Li / Li ratio. + For 25mA g within a voltage range of 2.5~4.3V -1 It was performed using a potentiometric rectifier (WBCS3000) at a constant current density. Electrochemical Impedance Spectroscopy (EIS) was performed by measuring frequencies from 100 kHz to 0.01 Hz with an amplitude of 5.0 mV.

[0034] Ex-situ characterization

[0035] Cross-sectional images of the electrode were acquired using FIB-SEM after 20 cycles. The electrode was 125 mA g -1 It was prepared after constant current charging and discharging in a voltage range of 2.5–4.3 V at a current density. Structural changes and surface characteristics of the electrode were evaluated using XRD and XPS, and the electrode was 25 mA g -1 It was prepared after constant current charging and discharging in a voltage range of 2.5-4.3 V at a current density.

[0036] In-situ DEMS

[0037] In-situ DEMS analysis was performed to measure the amounts of O2 and CO2 gases generated during the first charge-discharge cycle. The DEMS cell was assembled in a glove box filled with Ar, and a perforated plate was placed on top of the cell and a glass fiber membrane was used as a separator to detect gas generation in the coin cell system. The coin cell was sealed using top and bottom SUS plates with leak-free Si rings. The DEMS analysis line consisted of an Ar carrier gas, a flow controller, a mass spectrometer, and inlet / outlet gas lines for cell connection. To minimize unwanted components in the tube lines, the lines were emptied for 30 minutes and the cell was purged with Ar. After connecting the cell and lines, 10 cm was used with high-purity Ar (99.999%) as the carrier gas. 3 min -1 It was made to flow at a speed of . The cell is Li / Li + For 25mA g within a voltage range of 2.5~4.5V -1 It underwent charging and discharging at a current density.

[0038] result

[0039] To achieve effective and complete self-assembly, negatively charged NCM (Li 0.98 Ni 0.83 Mn 0.05 Co 0.11 O2) The repulsive force between the anode and the MoS2 nanoflakes must be overcome. To this end, in one embodiment, a CTAB surfactant was introduced to modify the surface charge of MoS2 to generate a positive intrinsic charge. This modification can promote electrostatic attraction between the two materials, as described in Fig. 1. The negative surface charge of the NCM particles was successfully offset by the attachment of positively functionalized MoS2 (f-MoS2) nanoflakes. In this approach, CTAB was selected as the surfactant due to its ability to induce a positive surface charge on MoS2 while maintaining the high dispersion stability of the nanoflakes in aqueous solution.

[0040] FIG. 2 is a diagram illustrating an example of the performance of a MoS2@NCM anode according to an embodiment of the present invention.

[0041] Figure 2(a) shows an example of the UV-vis spectrum of a surface-functionalized MoS2 dispersion. The UV-vis spectrum of the f-MoS2 dispersion showed a clear exciton peak, which indicates that the intrinsic properties of MoS2 were maintained even after the inclusion of CTAB.

[0042] Figure 2(b) shows examples of the zeta potential of MoS2 nanoflakes before and after surface functionalization and examples of optical images of NCM. Zeta potential measurements showed that the exfoliated MoS2 nanoflakes had a peak value of -20.1 mV. This is attributed to unintended electron doping due to sulfur defects during exfoliation. In contrast, f-MoS2 exhibited a positive zeta potential value of 42.3 mV due to the presence of the cationic surfactant CTAB. Through a simple mixing process involving the tuning of surface properties, the NCM anode surface can be spontaneously wrapped with f-MoS2 nanoflakes via hydrogen bonding and / or electrostatic interactions, making it easy to fabricate 2D wrapped NCM. Unlike conventional approaches using reduced graphene oxide (rGO) or CVD-grown graphene that require harsh conditions, such as high process temperatures (>300°C) or severe pH ranges using H2SO4, these embodiments provide a simpler and more effective strategy under intermediate process conditions. This strategy does not cause serious damage or contamination to the cathode material.

[0043] Figure 2(c) shows an example of the Raman spectrum of NCM particles wrapped in f-MoS2. Raman spectroscopy can confirm the presence of f-MoS2 on the wrapped NCM surface. 377.8 cm⁻¹ -1 and 402.9 cm -1 The two peaks are the representative E of MoS2, respectively. 1 2gand A 1g It is a Raman mode.

[0044] Figure 2(d) shows examples of XRD patterns of NCM anodes before (NCM) and after (CTAB-MoS2) wrapping with surface-functionalized MoS2, respectively. XRD measurements were performed to investigate the crystal structures of NCM and MoS2@NCM. Both NCM and MoS2@NCM exhibited a layered structure with an R3m space group without noticeable impurities. The intensity ratio (I) of the (003) and (104) peaks. 003 / I 104 ) indicates the degree of cation mixing and structural ordering and shows similar values ​​for NCM (1.13) and MoS2@NCM (1.33). Peak splitting in the (110) / (108) and (006) / (102) planes also implies a well-ordered layered structure.

[0045] The results shown in Fig. 2 indicate that the original structure was maintained without significant shifts in diffraction patterns or deformations, despite surface deformation using f-MoS2 nanoflakes. The absence of MoS2 signals in the XRD pattern is due to the very thin packing of a small amount of MoS2 nanoflakes on the NCM surface. However, XPS analysis clearly verified the presence and chemical composition of the MoS2 wrapping layer. The thermal decomposition of NCM and MoS2@NCM was investigated using TGA-DTG (thermogravimetry-derivative thermogravimetry). The DTG curve is a first-order derivative of the TG (thermogravimetry) curve and provides information regarding sample mass loss. Weight loss in the low-temperature range (30–200 °C) is due to the desorption of physically absorbed moisture (LiOH·H2O). While no additional weight loss was observed in NCM, several decomposition stages were present, which were confirmed for MoS2@NCM in the 250–600 °C range, associated with the decomposition of CTAB and MoS2 species. A slightly lower mass reduction was observed in MoS2@NCM compared to NCM because MoS2 flakes effectively wrap the NCM cathode material, mitigating oxygen release and improving structural stability.

[0046] Figure 3 shows images for comparison between Pristine NCM and MoS2@NCM according to an embodiment of the present invention. Figures 3 (a, b) show SEM images of Pristine NCM, Figures 3 (e, f) show SEM images of MoS2@NCM according to an embodiment of the present invention, Figures 3 (c, d) show TEM images of Pristine NCM, Figure 3 (g) shows a TEM image of MoS2@NCM according to an embodiment of the present invention, and Figure 3 (h) shows an elemental mapping image of molybdenum and sulfur of MoS2@NCM.

[0047] Microscopic analysis, including SEM and TEM, was performed to investigate the surface morphology and the presence of the MoS2 wrapping layer. Figures 3(a) and 3(e) show the morphological characteristics of the NCM particle surface before and after MoS2 wrapping, respectively. The NCM anode consists of secondary particles densely packed with small primary NCM crystals. The TEM image of pristine NCM without surface deformation shown in Figure 3(c) reveals a lattice distance of 0.47 nm corresponding to the (003) plane of the layered NCM anode, as shown in Figure 3(d). In contrast, as shown in Figures 3(e) and 3(f), MoS2@NCM exhibits smoother surface characteristics without structural degradation because the protruding NCM particles are covered by the f-MoS2 wrapping layer. As shown in Figure 3(g), the ultrathin f-MoS2 wrapping layer was homogeneously and clearly coated along the surface of the NCM particles with a thickness of approximately 2 nm without noticeable cracks or breakage. The layered structure of MoS2@NCM could be maintained due to the uniform and very thin nature of the f-MoS2 wrapping layer on the NCM anode. The wrapping layer can improve structural stability by preventing the formation of microcracks inside the anode particles, which typically occur due to volume expansion and contraction during repetitive cycling. To investigate the atomic distribution of MoS2@NCM, Scanning Transmission Electron Microscopy-Energy Dispersive X-ray Spectroscopy (STEM-EDS) elemental mapping was performed as shown in Fig. 3 (h), which revealed a uniform distribution of Mo and S throughout the NCM, indicating that a MoS2 wrapping layer was formed on the surface of the NCM particles.

[0048] FIG. 4 illustrates an example of electrochemical performance between Pristine NCM and MoS2@NCM according to an embodiment of the present invention. FIG. 4(a) shows the initial and 50th charge-discharge voltage curves of NCM and MoS2@NCM, FIG. 4(b) shows the cycle performance of NCM and MoS2@NCM, FIG. 4(c) and (d) show the dQ / dV versus voltage curves of NCM and MoS2@NCM, and FIG. 4(e) and (f) show the EIS spectra of NCM and MoS2@NCM before and after 100 cycles, respectively. FIG. 4 evaluates the electrochemical performance of Pristine NCM and MoS2@NCM to demonstrate the protective effect of the wrapping layer against anode degradation.

[0049] Figure 4(a) shows the 1st and 50th charge / discharge profiles. The primary discharge capacity of Pristine NCM is 200.2 mAh g⁻¹. -1 On the other hand, the primary discharge capacity of the MoS2@NCM anode is 191.3 mAh g -1 It was slightly lower. This is expected to be because Li ion insertion is hindered by the presence of the electrochemically inert MoS2 wrapping layer. After 50 cycles, the discharge capacity of Pristine NCM (106.9 mAh g⁻¹) -1 ) showed a significant decrease and reached a capacity retention rate of 53.4% ​​compared to the first cycle. This may be due to unwanted surface reactions caused by the high Ni content. On the other hand, the MoS2@NCM cathode was 138.1 mAh g after 50 cycles. -1 The discharge capacity value was shown, which indicates a lower capacity degradation than the original NCM (72.2% capacity retention).

[0050] Figure 4(b) shows the long-term cycling performance of Pristine NCM and MoS2@NCM. In the initial cycles, MoS2@NCM exhibited a lower capacity than the original NCM. However, a capacity crossover was observed after 20 cycles. After 100 cycles, Pristine NCM maintained a capacity of 83.6 mAh g⁻¹ with 41.8% capacity retention. -1 It maintained only . This capacity reduction is largely attributed to a phase transition in which the surface undergoes phase decomposition into a rock salt structure, causing cation mixing and irreversible oxygen loss. Additionally, side reactions at the NCM cathode-electrolyte interface and the dissolution of transition metals also act as factors contributing to capacity degradation. This is caused by electrolyte decomposition products, such as HF resulting from the reaction of PF5 with H2O. In contrast, MoS2@NCM maintained 116.3 mAh g even after 100 cycles. -1 It maintained a higher discharge capacity. This enhanced stability can be attributed to the presence of a MoS2 wrapping layer that prevents undesirable surface degradation of the anode. Furthermore, MoS2 nanoflakes, characterized by small lateral dimensions (<500 nm), did not hinder Li ion migration during charging and discharging even after being attached to the NCM surface. Rate capability tests were evaluated at high current densities of 0.2 C and 0.4 C. Although MoS2@NCM exhibited a slightly lower capacity retention rate, it maintained a higher discharge capacity than the original NCM during cycling. These results also demonstrate that the MoS2 wrapping layer not only mitigates NCM degradation but also enables efficient charging and discharging.

[0051] To evaluate the electrochemical mechanisms and phase transitions during charging and discharging, differential capacity (dQ / dV) curves for various cycles were obtained, as shown in Figures 4(c) and 4(d). The four prominent redox peaks are associated with multiple phase transitions during the lithiation and delithiation of the cathode material. The major redox peaks can be assigned to phase transitions from the hexagonal layer structure (H1) to the monoclinic phase (M), from M to the second hexagonal phase (H2), and from H2 to the third hexagonal phase (H3). In nickel-rich anodes, the irreversible phase transition of H2+H3 can lead to severe polarization and cause rapid volume shrinkage of the unit cell along the c-axis and collapse of the bulk structure. In Figure 4(c), the curve of the H2+H3 peak of the NCM during the second cycle shows a gradual decrease in intensity and a shift to high voltage in subsequent cycles, indicating that polarization increases and capacity fading is severe during cycling. In contrast, the H2+H3 phase transition of MoS2@NCM was effectively suppressed to lower polarization after the introduction of the MoS2 wrapping layer, as shown in Fig. 4(d). The dQ / dV curve of MoS2@NCM was maintained during cycling, suggesting that enhanced structural stability and reversible transition were achieved by using an ultrathin MoS2 wrapping layer for surface protection.

[0052] EIS analysis was performed to evaluate the charge transfer resistance of cells by comparing Pristine NCM and MoS2@NCM. Figures 4(e) and 4(f) show the EIS results around the 100th cycle. The semicircle at high frequencies is mostly related to the resistance effects caused by the presence of the surface film and double layer formation, as well as the charge transfer process between the electrolyte and the electrode. The straight line at low frequencies represents the Warburg impedance associated with the diffusion of Li ions. Before cycling, the charge transfer resistances of NCM and MoS2@NCM were 78.82 and 174.98 Ω, respectively. MoS2@NCM exhibited a higher charge resistance than NCM, as shown in Figure 4(e), because the charge transfer reaction was slightly hindered by the MoS2 wrapping layer of the anode particles during the initial cycles. However, after 100 cycles, the semicircle of MoS2@NCM (8.01 Ω) in the high-frequency region became smaller than that of NCM (16.74 Ω). The results of Figure 4 (f) indicate that the MoS2 wrapping layer effectively suppressed surface degradation of the stacked NCM structure and consequently mitigated the increase in charge transfer resistance during cycling.

[0053] Figure 5 shows cross-sectional SEM images of Pristine NCM and MoS2@NCM according to an embodiment of the present invention before and after cycling. Cross-sectional observation of the circulating electrode was performed using FIB-SEM to investigate the structural stability and degradation mechanism of NCM and MoS2@NCM.

[0054] Figures 5(a) and 5(b) show cross-sectional SEM images of the NCM and MoS2@NCM electrodes, respectively, before cycling. Both NCM and MoS2@NCM exhibited dense secondary particles and well-structured features without noticeable microcracks.

[0055] Figure 5(c) shows an example in which intergranular cracks were observed in the original NCM after 20 cycles at high current density due to repeated irregular structural stress and changes in unit cell volume. These microcracks, caused by mechanical stress at the interface, can lead to capacity reduction and structural degradation. Additionally, the electrolyte may penetrate into the interior of the secondary particles through the cracks, leading to electrolyte decomposition, phase transitions, oxygen evolution, and transition metal dissolution.

[0056] In contrast, in Fig. 5(d), MoS2@NCM exhibited negligible intergranular cracks compared to the original NCM, indicating that the original structure did not change noticeably even after 20 cycles. These observations demonstrate the efficacy of the MoS2 wrapping layer in improving electrochemical performance by ensuring structural stability.

[0057] Figure 6 shows the ex-situ XPS spectra of Pristine NCM and MoS2@NCM according to an embodiment of the present invention before and after cycling. To further investigate surface features and chemical structures after cycling, ex-situ XPS analysis of the NCM and MoS2@NCM anodes was performed around 50 cycles.

[0058] Figures 6(a) to 6(d) show the C 1s, O 2s, F 1s, and Ni 2p spectra of the NCM and MoS2@NCM anodes before cycling. In the C 1s spectrum (Figure 6(a)), C=CC, CH, CO, C=O, and CF2 peaks were observed in both the NCM and MoS2@NCM samples. The CC peak is associated with conductive carbon, while the CH and CF2 peaks originate from PVDF. In the O 1s spectrum (Figure 6(b)), peaks at 529.5, 531.6, and 533.1 eV are assigned to lattice oxygen (MO), impurity oxygen (Li2CO3 / LiOH), and CO, respectively. In the F 1s spectrum (Figure 6(c)), peaks at 684.9 and 687.8 eV are attributed to LiF and CF (PVDF), respectively. LiF can be generated by the dehydrofluorination reaction of PVDF during the preparation of the electrode slurry. The peaks observed at 854.3 and 856.60 eV in the Ni 2p spectrum (Fig. 6 (d)) are Ni, respectively. 2+ wa Ni 3+ This corresponds to, and the satellite peak is also observed around 861.6 eV due to the shake-up process. The spectral peaks of the NCM and MoS2@NCM samples appeared similar.

[0059] Figures 6(e) to 6(h) show the C 1s, O 1s, F 1s, and Ni 2p spectra of the NCM and MoS2@NCM anodes after 50 cycles. In the C 1s spectrum (Figure 6(e)), the C=O and CO peaks of NCM were much higher than those of MoS2@NCM due to the formation of Li2CO3 impurities and organic products (R-OCO2-Li and R-COOLi) resulting from side reactions with the electrolyte. By-products can be derived from carbonates and organic groups resulting from electrolyte oxidation. In the case of O 1s (Figure 6(f)), the peak intensity of CO and CO3 / OC=O species increased for the cycled NCM sample, which correspond to R-COOLi, R-OLi, or Li2CO3 species resulting from the decomposition of the carbonate solvent. On the other hand, MoS2@NCM after cycling exhibited smaller peaks for CO and CO3 / OC=O species compared to NCM after cycling, which may imply that the MoS2 wrapping layer effectively reduced the oxidation of electrolyte / surface Li residues and improved surface stability. x PO y F z The formation of and LiF was observed in the F 1s spectra of both samples and is associated with the decomposition of LiPF6 salt and electrolyte decomposition (Fig. 6(g)). However, [Li of the MoS2@NCM electrode after cycling x PO y F z The peak ratio between [+ LiF] and [CF] was smaller than the peak ratio of the recycled NCM. Ni 2p 3 / 2 The peak is Ni 2+ (855.6 eV) and Ni 3+ It separates into a peak at (856.6 eV) (Fig. 6 (h)). Ni 2+ If the ratio is high, not only does severe cation mixing occur, but a rock salt phase may also form on the surface. Additionally, the presence of NiO (858.5 eV) may imply that a rock salt phase grew during cycling. 2+ / Ni 3+The ratio was determined by calculating the peak area. Ni of NCM and MoS2@NCM 2+ / Ni 3+ The peak ratios are 0.69 and 0.25, respectively. In this regard, the Ni of MoS2@NCM 2+ The content was lower than the amount of NCM, which is Li + wa Ni 2+ Reduced cation mixing between them results in relatively less Ni on the MoS2@NCM surface. 2+ It confirms the presence of species. These spectroscopic results demonstrate that the MoS2 wrapping layer plays a crucial role not only in reducing surface-induced structural degradation of NCM but also in mitigating side reactions with the electrolyte, gas generation, and polarization during cycling.

[0060] Figure 7 illustrates an example of in-situ differential electrochemical mass spectrometry and voltage profiles during primary charging for Pristine NCM and MoS2@NCM according to an embodiment of the present invention. In-situ DEMS analysis was performed to investigate gases generated at the NCM and MoS2@NCM anodes during initial charge / discharge. Gases can be generated due to the oxidative decomposition of the electrolyte and severe side reactions between lattice oxygen released from Ni-rich layered anodes and the electrolyte in a high charge state. Such gas generation is O 2- 2p band and Ni 3+ / 4+ e g It occurs due to the overlapping region of the bands, and Ni 3+ / 4+ Band and O 2- The overlap at the top of the 2p band is Co 3+ / 4+ t 2g It is smaller than the band overlap. Electronic delocalization, which provides high capacity during the delithiation process in Ni-rich anodes, is Ni 3+ / 4+ It is observed in. However, due to the continuous depletion of lithium, Ni 3+ Ga Ni 4+ The energy band of Ni and O when oxidized 2-The overlap of the 2p energy bands gradually strengthens, weakening the binding strength of oxygen and oxidizing lattice oxygen. This weakening of oxygen binding forces leads to gas generation during repeated charge / discharge cycles. This weakening of oxygen binding forces inevitably results in the release of oxygen. Gas generated in Ni-rich stacked cathodes has a critical impact on safety issues, electrode performance degradation, cycle life, and cell impedance. Surface impurities associated with parasitic reactions and gas generation in Ni-rich stacked cathodes also lead to unwanted cell degradation. Therefore, we investigated whether a MoS2 wrapping layer could stabilize the electrode surface by preventing the formation of residual lithium and reducing gas generation.

[0061] Cells with NCM and MoS2@NCM cathodes were charged to 4.5V, and the CO2 and O2 emissions from the cathode materials were verified and quantified. During the first charging process, the degree of O2 generation between the NCM and MoS2@NCM cathodes showed a negligible difference. However, there was a noticeable difference in CO2 generation between NCM and MoS2@NCM. CO2 generation is known to occur due to the decomposition of organic solvents and the direct electrochemical oxidation of carbonate molecules. Figures 7(a) and 7(b) show that a small amount of CO2 was generated during charging with MoS2@NCM compared to NCM. The reduced gas generation behavior of MoS2@NCM indicates that the MoS2 wrapping layer successfully stabilizes the surface of the NCM cathode and reduces undesirable interfacial side reactions between the electrode and the electrolyte during high-voltage charge / discharge cycling, thereby improving safety and electrochemical performance.

[0062] FIG. 8 illustrates examples of ex-situ XRD patterns of Pristine NCM and MoS2@NCM according to an embodiment of the present invention after 50 cycles. Ex-situ characterization of the NCM and MoS2@NCM electrodes was performed after 50 cycles to investigate structural changes and electrochemical mechanisms during cycling. In FIG. 8, which shows the ex-situ XRD results for the Pristine NCM and MoS2@NCM electrodes, NCM and MoS2@NCM maintain their original structures without peak shape or shift even after 50 cycles. While the 50-cycle NCM shows a low-intensity (003) peak, the (003) peak of the 50-cycle MoS2@NCM shows a relatively sharp and high intensity. Generally, I 003 / I 004 The peak intensity ratio of the Li+ / Ni of the stacked cathode material 2+ It is known to indicate the degree of cation mixing and structural order. A peak intensity ratio of less than 1.2 indicates a significant amount of cation mixing, whereas a peak ratio higher than 1.2 indicates a well-ordered structure or negligible cation mixing. I of Pristine NCM 003 / I 004 While the value is 0.13, MoS2@NCM's I 003 / I 004 The value is 2.38, indicating that the stacked structure of MoS2@NCM remains stable even after 50 cycles. Peak splitting in the (110) / (108) and (006) / (102) planes also indicates a well-layered hexagonal structure. The MoS2 layer surrounding the NCM cathode contains Li + / Ni 2+ It effectively suppresses cation mixing and stabilizes the electrode surface, allowing the original layered NCM structure to be maintained during repeated charging and discharging.

[0063] FIG. 9 is a schematic diagram illustrating the functional effects of a MoS2@NCM anode according to an embodiment of the present invention. FIG. 9 schematically illustrates the functional effects of a MoS2 wrapping layer that improves the structural stability and electrochemical properties of a Ni-rich stacked anode. The improved electrochemical properties of MoS2@NCM as an anode material can be attributed to the following reasons: (i) Structural stability was enhanced by forming a thin and uniform MoS2 wrapping layer that acts as a protective layer without negatively affecting the anode material. (ii) Irreversible phase transitions and TM dissolution were suppressed during cycling due to the MoS2 wrapping layer. (iii) Gas generation (O2 and CO2) during charging (>4.5V) was effectively suppressed by reducing direct contact between the electrolyte and the cathode surface.

[0064] In summary, embodiments of the present invention can provide a cathode material wrapping method that simply and effectively wraps the surface of a cathode material using a two-dimensional material (2D MoS2 nanoflakes). Applying this cathode material wrapping method can improve the long-term stability and operational safety of Ni-rich stacked cathode particles in advanced electrode designs for LIBs. By using positively functionalized MoS2 nanoflakes, the embodiments of the present invention can achieve conformal coverage of the negatively charged NCM cathode surface through self-assembly. A very thin and uniform MoS2 wrapping layer can provide various advantages. These advantages may include mitigating surface-induced structural degradation of the NCM anode during cycling, improving interfacial motion characteristics, enhancing cell stability, and preventing the formation of microcracks due to mechanical failure and the consequent infiltration of electrolyte into the anode. Additionally, the MoS2 wrapping layer can contribute to improving structural stability by effectively mitigating gas generation. Furthermore, the introduction of a MoS2 wrapping layer can reduce unwanted interfacial side reactions between the electrode and the electrolyte. This can reduce the accumulation of byproducts on the cathode surface, such as Li2CO3 impurities and organic compounds resulting from electrolyte decomposition at high voltages. This not only highlights the potential of effective packaging strategies for various electrode materials but also offers opportunities for application in other areas requiring ultra-thin and compact surface protection.

[0065] Thus, according to embodiments of the present invention, a method for coating a cathode material using a two-dimensional material wrapping method, a coated cathode material, and a secondary battery including the coated cathode material can be provided.

[0066] The foregoing description is merely an illustrative explanation of the technical concept of the present invention, and those skilled in the art to which the present invention pertains will be able to make various modifications and variations within the scope of the essential characteristics of the present invention. Accordingly, the embodiments described in the present invention are intended to explain, not limit, the technical concept of the present invention, and are not limited to such embodiments. The scope of protection of the present invention shall be interpreted by the claims below, and all technical concepts within an equivalent scope shall be interpreted as being included within the scope of rights of the present invention.

Claims

Claim 1 An anode material comprising an electrostatically self-assembled MoS2 wrapping layer formed by self-assembling MoS2 nanoflakes, in which positive surface charges are induced by an ionic surfactant, around the surface of an anode particle containing negatively charged nickel (Ni) through electrostatic attraction. Claim 2 delete Claim 3 delete Claim 4 The cathode material according to claim 1, characterized in that the MoS2 nanoflakes are functionalized with a cetyltrimethylammonium bromide (CTAB) surfactant during the exfoliation process to induce a positive surface charge. Claim 5 A cathode material according to claim 4, characterized in that the cathode particles are mixed in a solution containing the MoS2 nanoflakes and the cetyltrimethylammonium bromide surfactant and manufactured through a wet chemical coating process. Claim 6 A secondary battery comprising: a positive electrode including a positive electrode material including an electrostatically self-assembled MoS2 wrapping layer; a negative electrode; and an electrolyte that transfers ions between the positive electrode and the negative electrode, wherein the electrostatically self-assembled MoS2 wrapping layer is formed by self-assembling MoS2 nanoflakes, in which a positive surface charge is induced by an ionic surfactant, to conformally wrap the surface of a positive electrode active material particle containing negatively charged nickel (Ni) through electrostatic attraction. Claim 7 delete Claim 8 delete Claim 9 A secondary battery according to claim 6, characterized in that the MoS2 nanoflakes are functionalized with a cetyltrimethylammonium bromide (CTAB) surfactant during the exfoliation process to induce a positive surface charge. Claim 10 A secondary battery according to claim 9, characterized in that the anode particles are mixed with a solution containing the MoS2 nanoflakes and the cetyltrimethylammonium bromide surfactant, and the anode material including the MoS2 wrapping layer is manufactured through a wet chemical coating process. Claim 11 A method for wrapping an anode material, comprising the step of manufacturing an anode material including a MoS2 wrapping layer through a self-assembly process in which MoS2 nanoflakes, in which a positive surface charge is induced by an ionic surfactant, conformally wrap the surface of an anode active material particle containing negatively charged nickel (Ni) by electrostatic attraction. Claim 12 delete Claim 13 A method for wrapping an anode material according to claim 11, wherein the manufacturing step comprises the step of peeling off the MoS2 nanoflakes in an aqueous solution containing a cetyltrimethylammonium bromide (CTAB) surfactant. Claim 14 A method for wrapping an anode material according to claim 13, wherein the peeling step is performed in an ice bath using a tip ultrasonic processor. Claim 15 A method for wrapping an anode material according to claim 13, wherein the manufacturing step comprises mixing the anode particles in a solution containing the MoS2 nanoflakes and the cetyltrimethylammonium bromide surfactant to produce an anode material including the MoS2 wrapping layer through a wet chemical coating process.

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