WO2024177351A1 - Lithium metal secondary battery comprising electrolyte-swellable polymer film - Google Patents

Abstract

The present invention relates to a lithium metal secondary battery comprising an electrolyte-swellable polymer thin film that inhibits the high reactivity of lithium metal, the formation of lithium dendrites and dead lithium by forming a special polymer film capable of swelling in a liquid electrolyte, to increase the cycle life of lithium metal symmetrical batteries and secondary batteries.

Description

Lithium metal secondary battery comprising electrolyte swellable polymer thin film

Cross-citation with related application(s)

This application claims the benefit of priority to Korean Patent Application No. 10-2023-0022353, filed Feb. 20, 2023, and Korean Patent Application No. 10-2024-0024293, filed Feb. 20, 2024, the entire contents of which are incorporated herein by reference.

This specification discloses a lithium metal secondary battery including an electrolyte swellable polymer thin film.

Lithium metal has been recognized as an anode material for high-energy lithium batteries due to its high theoretical specific capacity (3860 mAh g ^-1 ) and low redox potential (-3.040 V vs. standard hydrogen electrode). However, despite the development of lithium batteries for over 100 years, lithium metal anodes have not been easily applied to actual battery systems due to their unstable surface chemistry, which induces dendrite formation during the charge/discharge process. Metal Li itself is a strong reducing agent with high hydration enthalpy (-520 kJ mol ^-1 ) and can easily react with electrolyte to form a solid-electrolyte interphase (SEI) layer without applying a potential. However, the SEI layer has a mechanical weakness because it has heterogeneous polycrystalline grain boundaries that easily crack depending on the volume change of the lithium metal anode, and as lithium grows through the cracks, it is exposed to the electrolyte, and as an additional SEI layer is formed spontaneously on the surface, the liquid electrolyte is consumed (Fig. 6). As the lithium metal and electrolyte are consumed as dead lithium, lithium dendrites grow over subsequent cycles, which leads to long-term low Coulombic efficiency, short battery cycle life, and safety issues due to the risk of lithium metal explosion.

To solve these problems, one method is to introduce an ‘artificial SEI layer’ that stabilizes the electrochemical reaction by modifying the lithium surface through a functional outer layer. For example, the Li metal surface can be coated with various materials, such as an etched polymer layer, an organic/inorganic framework, and a nanosheet, which promote lithium ion transport while blocking lithium dendrites. In addition, there is a method to create a lithium-based composite film as an artificial SEI layer by treating the outermost lithium surface with liquid and gaseous chemicals to stabilize the lithium surface. However, the native SEI layer begins to grow preferentially between the Li surfaces adjacent to the artificial SEI layer, which is because the native SEI layer has low interfacial resistance with respect to the Li surface, whereas the artificial SEI layer, which does not have good contact with the Li surface, exhibits high resistance. Due to these problems, the introduction of an artificial SEI layer is limited.

Meanwhile, a method of directly modifying the native SEI layer by utilizing the electrolyte decomposed by reacting with the electrode material has also been proposed. The native SEI layer can be modified to promote or hinder the electrochemical reaction at the anode and cathode by adjusting the electrolyte composition with additives or by adjusting the cycling conditions. In this case, the contact between the SEI layer and the lithium surface can be improved, but the directly modified SEI layer has polycrystalline characteristics with randomly distributed particles and different elemental compositions, and these structural characteristics weaken the mechanical stability, making it particularly prone to cracking, and the native SEI layer is vulnerable to expansion by various electrolytes, which reduces the mechanical strength. Therefore, it is necessary to develop a solution strategy that can promote the formation of a structurally strengthened native SEI layer.

Lithium (Li) metal anodes have the problems of instability and reactivity toward electrolytes, and form a brittle SEI layer with non-uniform distribution, which ultimately forms Li dendrites, which deteriorate the battery life. Therefore, in an embodiment of the present invention, an electrolyte-swellable polymer nanolayer can be monolithically deposited using initiator-assisted chemical vapor deposition (iCVD) to strengthen the SEI layer and stabilize the interface to passivate lithium metal. For example, a 100 nm thick iCVD poly(dimethylaminomethylstyrene) (pDMAMS) layer can be constructed to swell about 264% in a carbonate electrolyte to form a soft scaffold filled with the electrolyte for lithium ion transport. In this case, a native SEI layer that is uniform, free of Li ₂ O, and rich in Li ₂ CO ₃ can be provided.

In this specification, a novel functional polymer scaffold capable of structurally accommodating and reinforcing an SEI layer to improve mechanical properties and stabilize electrochemical reactions is disclosed. Specifically, the functional polymer scaffold requires the following properties: 1) it should be able to transport lithium ions and electrolyte through the matrix, 2) it should be compatible and miscible with the native SEI layer to enable the integration of lithium ions and electrolyte into the matrix, 3) it should provide sufficient mechanical strength, and 4) it should not be structurally damaged upon electrochemical decomposition. Ideally, a functional polymer layer capable of being swelled by an electrolyte present in a battery system while completely accommodating the native SEI layer in a large free space should satisfy the above-mentioned criteria. Accordingly, an embodiment of the present invention applies a polymer protective film layer including an electrolyte-swellable polymer (FIG. 1a).

Meanwhile, inductive chemical vapor deposition (iCVD), a method for coating a functional polymer layer on a lithium metal surface, can precisely fabricate a uniform polymer layer with a thickness of less than 10 nm, which can be particularly useful for minimizing cell resistance, improving lithium ion diffusion, and investigating interfacial phenomena (Fig. 1b). In particular, the iCVD method can modify the lithium surface without a solvent or a high-temperature process, so that various types of high-purity functional polymers can be directly applied while preventing potential damage to lithium. In addition, the iCVD method can control the composition of copolymers with opposite chemical properties (e.g., hydrophilic-hydrophobic).

In one embodiment of the present invention, an electrode for a lithium metal battery is provided, comprising: a negative electrode current collector; a lithium metal layer formed on the negative electrode current collector; and a polymer protective film layer formed on the lithium metal layer, wherein the polymer protective film layer includes an electrolyte swellable polymer, and the electrolyte swellable polymer has a swelling ratio (%) expressed by the following mathematical formula 1 of 15% or more.

[Mathematical formula 1]

Here, d _pristine is the thickness of the polymer protective film layer before swelling, and d _swollen is the thickness of the polymer protective film layer after swelling.

In an exemplary embodiment, the lithium metal layer can have a thickness of 1 to 200 μm.

In an exemplary embodiment, the electrolyte swellable polymer can be a polymer comprising one or more monomers selected from the group consisting of dimethylaminomethyl styrene, ethylene glycol dimethacrylate, acrylic acid, 2-(perfluorohexyl) ethyl acrylate, and divinylbenzene.

For example, the electrolyte swellable polymer may include one or more polymers selected from the group consisting of pDMAMS [poly(dimethylaminomethyl styrene)], pDVB (polydivinylbenzne), pC6FA [poly{2-(perfluorohexyl) ethyl acrylate}], pEGDMA [poly(ethylene glycol dimethacrylate)], and pAA [poly(acrylic acid)].

Meanwhile, the present applicants selected amine-rich poly(dimethylaminomethylstyrene) (pDMAMS), fluorine-rich poly[2-(perfluorohexyl) ethyl acrylate] (pC6FA), oxygen-rich poly(ethylene glycol dimethacrylate) (pEGDMA), anionic poly(acrylic acid) (pAA), and aromatic polydivinylbenzene (pDVB) to investigate their interactions with the electrolyte (Fig. 1c). These polymers have been widely studied as additives or binders for lithium-ion batteries due to their properties such as compatibility with electrolytes, high energy levels of LUMO, and excellent electrical insulation. In particular, the iCVD technique, which is not affected by oxygen and moisture at all, can directly coat the polymers on lithium metal with a uniform and controlled thickness because they can be free from the effects of oxygen and moisture. Specifically, a surface-growth polymerization process can be performed to promote stable and continuous adsorption of monomers and radicals vaporized in iCVD on lithium metal (Table S1). For example, the cross-sectional SEM image in Fig. 1d shows that a uniform 100 nm pDMAMS coating is formed on the Li metal surface (Fig. 1d). With a constant iCVD deposition rate (2 nm/min ^-1 ~ 12 nm/min ^-1 ), precise nanolayer thicknesses can be obtained by simply adjusting the deposition time (Fig. 7 ). This one-step surface treatment process, which is performed at a relatively low treatment temperature of less than 40 °C, protects key functional groups of each monomer, as confirmed by in situ Fourier transform infrared (FT-IR) analysis (Fig. 8).

Table 1 shows the iCVD process conditions and composition of poly(DMAMS-co-C6FA). Here, a) the flow rate of TBPO in the copolymerization is 0.59 sccm, and b) the composition ratio of DMAMS and C6FA present in the polymer film.

In an exemplary embodiment, the polymer protective film layer may be provided to cover all or part of the surface where the negative electrode current collector and the lithium metal layer come into contact. Specifically, the iCVD technique is particularly useful for forming a conformal coating on a rough surface, since the vaporized monomer and initiator can easily access even the deep part of the concave surface. As a result, the lithium surface coated with pDMAMS showed almost the same topology as the bare lithium surface (Figs. 1e and 1f). Such smooth contact and sufficient adhesion of the polymer nanolayer and the Li surface are very important for stabilizing the interface to withstand mechanical stress during cell fabrication and electrochemical cell operation. In particular, when the coating thickness is less than 100 nm, the iCVD-treated surface can be effectively planarized, as can be seen from the reduction in sharp edges (Fig. 1f) and the flattening of the upper boundary (Fig. 7b) compared to the untreated region at the bottom. This planarization effect evenly induces an electric field throughout the cell for uniform lithium ion transport and electrochemical reaction.

In an exemplary embodiment, the electrolyte swellable polymer can swell in an electrolyte solution to form a solid-electrolyte layer complex.

Specifically, the polymer protective film layer should have very high permeability to the battery electrolyte to ensure high lithium ion conductivity. Lithium ion permeability can be achieved by diffusing through the open polymer network via a hopping mechanism or through the expanded polymer network swollen by the electrolyte solution. For reference, all the polymer candidates in the unswollen state are essentially composed of multiple open channels that allow the passage of oxygen molecules (diameter 0.35 nm) and water vapor (diameter 0.26 nm) as evidenced by the oxidation of the polymer-coated lithium when exposed to ambient air (Figure 9). Nevertheless, the pores of the scaffold should be larger (diameter ≒ 0.9 nm) to ensure that the free space within the polymer protective film layer is completely filled with the battery electrolyte to facilitate the movement of lithium ions. Accordingly, the surface and swelling properties of the prospective polymer layers were comprehensively evaluated using an electrolyte solvent (EC: DEC = 30:70, v/v mixture). As a result of the contact angle measurement, the one with the best interfacial affinity with the electrolyte solvent was pEGDMA (3.52°), followed by pDMAMS (11.89°), pDVB (13.30°), pAA (27.69°), and pC6FA (43.72°) (Fig. 1g). Based on the measured contact angle values, pEGDMA, pDMAMS, and pDVB can be identified as electrolyte hydrophilic materials that facilitate the interfacial transport of electrolyte.

Ellipsometric analysis was used to compare the thickness changes of the iCVD films before and after immersion in electrolyte solutions to characterize their swelling properties. In contrast to the trend of interfacial properties, the amine-rich pDMAMS layer exhibited the highest swelling ability with a remarkable swelling ratio of 164%, followed by pAA (16%), pC6FA (12%), pDVB (6%), and pEGDMA (2%) (Figures 1h, 1i, and S10). This unusual fast (<3 min) swelling behavior of pDMAMS in carbonate solvents is probably due to the abundant amine functionalities in the side chains, which is confirmed by the similar Hansen solubility parameters of DEC (20.2 MPa ^1/2 ) and dimethylaminomethyl-containing polymers (20.4 MPa ^1/2 ~ 21.0 MPa ^1/2 ). In addition, the electrolyte-swollen pDMAMS layer can maintain its layer thickness even after 3 min, which can be resistant to further dissolution and structurally intact, all of which is due to the high degree of entanglement of the pDMAMS polymer chains. In addition, the polyionic amine can dynamically stabilize the structure by binding to lithium ions and achieve strong adhesion to the lithium surface even at a highly swollen state of 164%. In contrast, the cross-linked molecular structure of pEGDMA did not contribute to the swelling characteristics, as in the case of pDVB. pC6FA showed the worst interfacial contact with the battery electrolyte solvent and its swelling characteristics were also below par. Although pC6FA may appear to be swollen by the solvent, the slight change in refractive index indicates that the thickness change is actually due to the partial rearrangement of the ethyl groups of pC6FA. In particular, pDMAMS showed the best swelling performance among other iCVD polymers, which may lead to its high lithium ion transport properties.

Additionally, the lithium ion transport properties of the iCVD polymer layer were evaluated by lithium stripping/plating measurements in symmetrical Li-Li coin cells. Figure 2a shows the galvanostatic voltage profiles over time at high current density (1 mA cm ^-2 ) versus actual lithium capacity (1 mAh cm ^-2 ) in each discharge/charge cycle. The bare Li cell lasts only for 120 h, at which high cell polarization of 150 mV occurs, mainly due to the formation of Li dendrites, dead Li fragments, and depleted electrolyte, which are identified by voltage fluctuations, respectively.

On the other hand, the iCVD polymer-coated Li cells showed very different electrochemical performances depending on the type of polymer used due to their different chemical structures (Fig. 2a). As predicted from the swelling test results, the electrolyte-swellable pDMAMS-Li exhibited the longest cycling performance with a low polarization of 50 mV and a cumulative capacity of 410 mAh cm ^-2 for 820 h, followed by pAA-Li (430 h), pDVB-Li (270 h), pEGDMA-Li (130 h), and pC6FA-Li (20 h). All the iCVD polymer-Li cells showed high cell polarization in the initial cycles (<50 h), which decreased thereafter due to the formation of a low-impedance SEI layer. However, pC6FA lasted only for 14 h with a rapidly increasing cell polarization, indicating that the electrolyte-hating fluorine-rich polymer is insulating toward lithium ions. For other polymers, the cycle life is inversely proportional to the mass transfer control potential (μmtc, Figure 2b), indicating that it is difficult to transport Li ions through the polymer layer without swelling. pDMAMS-Li exhibited the lowest μmtc and tip potential (μtip,begin; μtip,end), which are correlated with the formation of Li dendrites and dead Li, respectively. The low polarization degree of the pDMAMS-Li cell could be attributed to the uniform and close adhesion of pDMAMS to Li metal, which significantly reduces the interfacial resistance and induces a uniform Li ion flux. As evidenced by comparative analysis of the micrographs and SEM images of pDMAMS-Li and bare Li after the 1st, 10th, and 100th cycles at 1 mA cm ^-2 , the pDMAMS-Li cell showed a significant reduction in the formation of Li dendrites and dead Li (Figures 2a and S11). In particular, the bare lithium cell exhibited irregular and mossy dendrite formation within a few cycles, which then gradually transformed into isolated dead lithium, which led to a deterioration of the battery life. In contrast, the pDMAMS-Li cathode still exhibited a densely packed and well-preserved topology without dendrites even after 100 cycles (200 h), indicating uniform lithium transport through the layer (Figure 11).

In an exemplary embodiment, the electrolyte may have a lithium ion transfer number (tLi+) of 0.2 to 1 as measured by the Bruce-Vincent method.

Specifically, the lithium ion transfer numbers (tLi; the contribution of cations to the overall ion conduction performance) of bare-Li and pDMAMS-Li cells were measured using the Bruce–Vincent method (Figs. 2c, 2d, and S12). As a result, pDMAMS-Li exhibited a tLi+ value of 0.95, which is the highest value reported for any kind of electrolyte additive, artificial SEI, or solid electrolyte known so far, compared with that of the conventional organic liquid electrolyte (LiPF6 EC/DEC = 3:7 (v/v), tLi ≈ 0.4). This high tLi+ value indicates that the electrolyte-swollen pDMAMS matrix provides favorable diffusion paths for lithium ions without a dissolution shell, where the coordination complexes of lithium and polymeric polydisaccharide amines can contribute to the lithium ion selectivity. Additionally, the 100 nm pDMAMS layer exhibits excellent ionic conductivity of 6.54 mS cm ^-1 due to the electrolyte-swollen pDMAMS matrix (Figure 13).

To separate the contributions of the interfacial effect and the swelling phenomenon of pDMAMS-Li to stabilize the electrochemical reaction, poly(DMAMS-co-C6FA)-Li with different ratios of electrolyte-swellable DMAMS and electrolyte-hating C6FA moieties were prepared (Figs. 1g and 2e). To this end, three copolymer nanolayer samples with different ratios of DMAMS and C6FA were prepared by adjusting the flow rates of each monomer during the iCVD process, and the mixing ratios were confirmed by FT-IR and X-ray photoelectron spectroscopy (XPS) analyses (Figs. 2f, 14, and 15, Table S1). The cycle life of poly(DMAMS-co-C6FA)-Li was found to be proportional to the ratio of DMAMS/C6FA (Fig. 2g). Incorporation of C6FA moieties can modulate three properties of the polymer-Li layer, namely, the degree of electrolyte swelling, the adhesion, and the mechanical robustness of the lithium surface and the poly(DMAMS-co-C6FA) layer. The μmtc value of poly(DMAMS-co-C6FA)-Li showed a positive correlation with the C6FA content, but was lower than that of the bare Li cell. Considering that the μmtc values (44 mV–54 mV) were relatively similar among all types of pDMAMS-containing polymers evaluated ( Figure 2h), the incorporation of C6FA moieties appears to have a minimal effect on the Li ion transport properties because Li ions can bypass the electrolyte-hating C6FA domains through the electrolyte-swollen DMAMS domains ( Figures 1h and 1i). In addition, the two peaks in the initial (dendrite formation) and terminal (dead Li formation) sections of the voltage profile of the C6FA-containing polymer become stronger with increasing C6FA content (Fig. 2h), indicating that C6FA weakens the interfacial contact and adhesion with lithium (Fig. 16). The C6FA domains at the poly(DMAMS-co-C6FA)-Li interface act as impurities on which lithium dendrites and dead lithium grow preferably (Fig. 17). Finally, the mechanical robustness of poly(DMAMS-co-C6FA) weakens with increasing C6FA content because the number of DMAMS moieties with rigid aromatic rings decreases. In addition, the long fluoroalkyl chains of C6FA facilitate chain mobility and enlarge the free volume in the copolymer network. Therefore, the C6FA domains are vulnerable to dendritic lithium growth, and the cycle life of poly(DMAMS-co-C6FA) is inversely proportional to the content of C6FA (Figs. 2g and 17a). In summary, we could confirm that proper adhesion between the 3S layer and the lithium surface is as important as proper lithium ion transport to promote stable electrochemical reactions.

Furthermore, the electrochemical performance of the full cell assembled with the poly(DMAMS-co-C6FA)-Li cathode and LiNi _0.6 Mn _0.2 Co _0.2 O ₂ (NMC-622) cathode was investigated. Conventionally, the performance of symmetric lithium cells was not sufficiently reflected in the evaluation of the whole cell because of the different operating voltage windows, especially for high-voltage cathodes operating above 4.2 V. Therefore, before evaluating the battery performance, electrochemical floating experiments on the iCVD layers were performed to determine the voltage window that can completely avoid electrochemical degradation. Compared with bare Li, most of the iCVD polymer layers, except for pC6FA, were electrochemically stable below 4.8 V (Figure 18). In particular, the leakage current of the pDMAMS-Li cell at 4.8 V was 0.5 μA lower than that of the bare Li. Moreover, cyclic voltammetry data collected within the entire cell voltage window (3.0 V to 4.2 V) support that other iCVD polymers except pC6FA do not undergo electrochemical degradation in the operating cell (Fig. 19). Owing to this excellent electrochemical stability, the 3S layer can also be applied to NMC-622 full cells using a high voltage of 4.2 V (Fig. 3a). It maintained a discharge capacity of 159 mAh g ^-1 at 1 °C (2 mA cm ^-2 ) without any capacity fading (0.08% per cycle) for more than 650 cycles at 20 °C, whereas a significant capacity fading of 0.29% per cycle occurred in the bare lithium (Figs. 3b and 3c). Furthermore, galvanostatic intermittent titration technique (GITT) and in situ XPS data supported that the pDMAMS coating on lithium metal had no detrimental side effects on the anode and instead slightly enhanced the lithium ion diffusion of the NMC anode compared to the bare lithium (Fig. 20).

To analyze the interfacial behavior of pDMAMS-Li in the absence of cathode influence, impedance spectroscopy of Li–Li symmetric cells was performed (Figures 3d, 3e, and 21, Table 2 ). The interfacial resistance of pDMAMS-Li (~ 190 Ω) under non-cycled conditions was two times lower than that of bare Li (~ 440 Ω) and other artificial SEI layers (Table S3). Interestingly, the addition of an insulating polymer layer reduced the interfacial resistance to less than half of that of bare Li. This indicates that the bare Li/electrolyte interface has intrinsic electrochemical instability, which can be dramatically improved by reducing the interfacial resistance with the electrolyte-swollen polymer layer. Furthermore, as the cell was cycled, the interfacial resistance of pDMAMS-Li further decreased to 60 Ω due to the formation of a low-impedance SEI layer, which was similar to that of bare Li (55 Ω) (Figures 3d and 3e). These differences in interfacial resistance imply distinct differences in the composition of the SEI layer with and without the pDMAMS layer, suggesting that a higher resistivity SEI layer may be more advantageous in stabilizing cell performance.

Table 2 shows the corresponding fitting parameters for bare lithium and pDMAMS-Li(100 nm) cells obtained by fitting the impedance data of Figs. 3d and 3e to the equivalent circuit model of Fig. 21.

Table 3 shows the resistance measured in a symmetric cell and the Nyquist plot of an embodiment of the present invention compared to prior studies.

Typically, bare Li forms an additional SEI layer on the surface, so the single semicircle of the electrical impedance spectroscopy (EIS) data is split into two parts for analysis after the first cycle. In particular, pDMAMS-Li retains the initial two interfaces (i.e., two semicircles for Li and pDMAMS, respectively) after cycling, which demonstrates that the native SEI layer formed by the LiPF6 carbonate electrolyte is compatible with the pDMAMS layer. This is possible because the native SEI layer evolves inside the electrolyte-swollen structure to form a single thermodynamic phase that is electrochemically indistinguishable. The unique interfacial design according to the present invention that accommodates the native SEI layer opens up another chemically transformable route toward practical lithium metal anodes.

After 10 cycles, the pDMAMS-Li in the NMC-622 full cell was examined, and it was confirmed that it maintained a much brighter surface compared to the bare Li and pC6FA-Li cases (Fig. 3f). Among the poly(DMAMS-co-C6FA)-Li samples, the pure pDMAMS-Li cell exhibited the highest capacity of 130 mAh g ^-1 at a rate of 2 °C (70% of the capacity at a rate of 0.1 °C), demonstrating reasonable rate capability (Fig. 3g). According to the symmetric cell test, the critical current density of the pDMAMS solvogel was about 2 mA cm ^-2 , after which the capacity and cycle life started to degrade (Fig. 22). Lithium dendrites started to appear only in the region above the critical current density because the supersaturated Li ions accumulated at the pDMAMS/Li interface, providing space for the dendrite growth. Owing to the flexibility of the swollen pDMAMS layer, the further growth of Li dendrites was effectively mitigated even in the full cell operated at 2 °C (Fig. 3h). Overall, the overall performance of the pDMAMS-Li full cell was significantly superior to that of other types of iCVD thin films and bare lithium (Fig. 23).

To clarify the chemical composition of the 3S/SEI layer, the pDMAMS-Li cathode was analyzed before and after lithiation (i.e., after the first cycle using the NMC cathode in a full cell) using time-of-flight secondary ion mass spectrometry (TOF-SIMS) and XPS with depth profiling. In TOF-SIMS analysis, the pDMAMS was characterized by the C ₁₂ H ₁₆ ⁺ fragments released from the backbone and the amine-containing C ₃ H ₈ N ⁺ and C ₈ H ₇ N ⁺ that were well retained in the pDMAMS-Li (Figures 4a, 4b, and 24). Since the pDMAMS was swollen and accommodated by the SEI layer and electrolyte, the intensity of the ionized pDMAMS species decreased compared to that of the uncycled pDMAMS-Li after one cycle of lithiation. On the other hand, the decreased portion seems to be replaced by the native SEI layer (Figures 4b and 4c). The coexistence of the native SEI layer components and the pDMAMS structure confirms the miscibility and compatibility of the two structures, and is in good agreement with the results of the EIS data analysis (Fig. 3e). In addition, the mechanical stability of the composite was improved like reinforced concrete (Fig. 3h) (wherein pDMAMS can be described as the 'reinforcement' and the native SEI layer as the 'concrete body', Fig. 4g). Although the native SEI layer is usually composed of a multi-granular structure with an inorganic layer adjacent to the electrode and an organic-rich layer going outward, the SEI layer formed in the pDMAMS structure was more homogeneous, and it was confirmed that the outer layer was richer in inorganic species, i.e., LiF ⁺ and PF2 ⁺ , instead of CH ₂ Li ⁺ and other organic components.

The structural composition of the SEI layer was further investigated using XPS in depth profiling mode. As a result, the native SEI layer in bare lithium showed a typical composition of an organic outer layer and an inorganic inner layer with noticeably enriched Li ₂ O (Figures 4d, 4h, and 25). However, the organic and inorganic components of the composite layer composed of native SEI and pDMAMS were largely homogenized, which is because the pDMAMS and native SEI layers have excellent miscibility (Figures 4e, 4f, 4i, 26, 27, and 28).

In particular, the Li ₂ O component completely disappeared in the SEI layer formed on the pDMAMS structure, which is because the formation of Li ₂ O is inhibited due to the low miscibility of pDMAMS and Li ₂ O and the fact that pDMAMS prevents further decomposition of lithium carbonate into Li ₂ O and LiF (Fig. 4i). Another mechanism may involve that some tertiary amine groups at the ends of the pDMAMS polymer branches electrochemically react with DEC to form quaternary ammonium cations and carbonate ions (Figs. 27, 28, and 29). The resulting quaternary ammonium cation polymer (poly(vinylbenzyl trimethylammonium carbonate), pVBTMAC) constitutes one of the important precursors for the synthesis of selective ion-exchange polymers with side-chain functionalities, and it can effectively trap and preserve carbonate anions and other beneficial SEI components. In addition, the partially tetrameric poly(DMAMS-co-VBTMAC) layer selectively filters out lithium ions from the molten shell, allowing only the 3S layer to pass through, which results in the high tLi+ value (0.95). It should be noted that the quaternization reaction is always accompanied by an electrochemical reaction (Figure 30). The slightly higher interfacial resistance of pDMAMS-Li (60 Ω) than that of bare Li (55 Ω, Figures 3d and 3e) can be confirmed by the fact that Li ₂ CO ₃ is a higher insulating material than LiF and Li ₂ O, which is also consistent with the Li ₂ CO ₃ -rich composition of the SEI layer formed around the pDMAMS layer. Furthermore, these results suggest that inorganic and crystalline Li ₂ O may be the main factors that deteriorate the mechanical and chemical stabilities by giving the SEI layer a heterogeneous multicomponent nature with rich grain boundaries, which is a different perspective from the previously known role of Li ₂ O in the SEI layer. The homogeneous and Li ₂ O-free pDMAMS-SEI composite layer provides flexibility and durability to the Li/SEI interface to withstand large volume changes induced by Li metal and dendrite formation (Figure 3h). In addition, the Li ₂ O-free homogeneous SEI layer improves the adhesion between the Li surface and the pDMAMS layer, which imparts mechanical properties and electrochemical stability to the Li/SEI/pDMAMS interface. The SEI layer structurally reinforces the 3S layer similar to how steel bars reinforce concrete, thereby physically suppressing the expansion of the SEI layer, and this reinforcing mechanism can explain the low polarization in Li–Li symmetric cells, low interfacial resistance in the whole cell, and extended battery cycle.

In an exemplary embodiment, the polymer protective film layer can have a thickness in the range of 10 to 500 nm. Although it has been confirmed that the 3S layer is permeable to lithium ions, the thickness needs to be optimized for practical considerations. If it is too thick, it will reduce the lithium flux, whereas if it is too thin, it is more likely to cause physical damage (e.g., bruising, tearing, punctures, creases, etc.). Therefore, cells made with symmetric and full pDMAMS-Li with thicknesses ranging from 10 nm to 500 nm were evaluated, and it was confirmed that a 3S layer of 100 nm in particular provided the optimal cycle performance (Figs. 5a, 5b, 5c). For thinner 3S layers (less than 50 nm), the pDMAMS layer could not fully accommodate the native SEI layer, which led to the formation of Li dendrites and dead Li, as can be seen from the reduced double peak potential compared to bare Li (Fig. 5c). On the other hand, when the 3S layer was much thicker than 100 nm, the interfacial resistance increased, which decreased the initial specific capacity (Figs. 5b, 5d, 5e, and 5f). In addition, since complete swelling was difficult as the layer was thicker, the non-swollen region was located at the bottom of the layer, which reduced the utilization of the 3S layer (Figs. 5c and 31). This increased the overall cell resistance, which deteriorated the cell performance. In addition, the transition number of 300 nm pDMAMS-Li (0.63) was significantly reduced compared to that of 100 nm pDMAMS-Li (0.95), indicating that complete swelling of the pDMAMS layer is required to provide high lithium ion flux (Fig. 32).

In conclusion, we report a novel 3S nanolayer directly formed on Li metal anode by iCVD method. The 3S layer prepared by pDMAMS shows satisfactory swelling behavior in the presence of carbonate electrolyte. The formed electrolyte solvogel can host a homogeneous and _Li2O -free native SEI layer in the swollen structure, which provides low DC ionic conductivity (Fig. 5g, Table S4) and high lithium ion transfer value with low interfacial resistance through strong interfacial adhesion, thereby exhibiting mechanical stability and stable electrochemical reactions. In particular, pDMAMS-Li with an optimal thickness of 100 nm showed a cycle life extension of 550% in a symmetrical cell and 600% in a full cell using a highly loaded NMC anode, compared with that of bare Li metal. Table S4 summarizes the ionic conductivity, lithium transfer number, and cycling performance data.

Based on the results of the study, the inventors set the ideal criteria required for the swollen soft scaffold layer (Fig. 5h). First, the 3S layer should be adequately swollen by the battery electrolyte to provide sufficient free volume to facilitate lithium ion transport. Second, the electrolyte solvogel should be thermodynamically miscible with the native SEI layer. Third, the thickness of the 3S layer should be approximately 100 nm to minimize cell resistance and completely accommodate the native SEI layer. Fourth, the 3S layer should be chemically and electrochemically inert over the large voltage range of high-voltage cathode batteries. Fifth, the layer should be flexible and mechanically strong to withstand the large volume change of lithium metal. Finally, the adhesion of the 3S layer to the Li surface should be strong enough to suppress the formation of Li dendrites or dead Li. In particular, it is not desirable to completely strip the Li metal to the copper because the 3S layer and Li should maintain a close bond. The 3S layer formation method using iCVD can be widely applied to other metal battery systems (e.g., sodium, potassium, zinc, magnesium, aluminum, etc.), especially because it supports roll-to-roll process that supports commercial-scale production.

In another embodiment of the present invention, a secondary battery is provided, which comprises: an anode, which is an electrode for a lithium metal battery as described above; an anode; and an electrolyte layer interposed between the anode and the anode; wherein the anode includes a solid electrolyte interphase formed on a surface thereof.

In an exemplary embodiment, the solid-electrolyte interfacial layer may be a Li ₂ O free-SEI layer. Here, the Li ₂ O free-SEI layer may not substantially contain Li ₂ O on the SEI layer. For example, when the electrolyte swellable polymer is pDMAMS, the miscibility of pDMAMS and Li ₂ O is low, and the pDMAMS may prevent lithium carbonate from further decomposing into Li ₂ O and LiF, thereby forming a Li ₂ O free-SEI layer. By substantially not containing Li ₂ O on the SEI layer, flexibility and durability may be maintained against volume changes due to the formation of dendrites. Meanwhile, in the lithium secondary battery of the other embodiment, the positive electrode described above may be manufactured by, for example, dispersing and mixing the positive electrode active material, the binder, and the conductive agent in a dispersion medium (solvent) to make a slurry, applying the slurry on a positive electrode current collector, and then drying and rolling. At this time, NMP (N-methyl-2-pyrrolidone), DMF (Dimethyl formamide), DMSO (Dimethyl sulfoxide), ethanol, isopropanol, water, and mixtures thereof can be used as the dispersion medium, but are not necessarily limited thereto.

The above positive electrode active material is not particularly limited as long as it is a material capable of reversible insertion and de-insertion of lithium ions, and may include, for example, a lithium metal composite oxide including one or more metal elements selected from the group consisting of Co, Mn, Ni, W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, and Mo.

More specifically, as the positive electrode active material, a compound represented by any one of the following chemical formulas can be used. Li _a A _1-b R _b D ₂ (wherein 0.90 ≤ a ≤ 1.8 and 0 ≤ b ≤ 0.5); Li _a E _1-b R _b O _2-c D _c (wherein 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, and 0 ≤ c ≤ 0.05); LiE _2-b R _b O _4-c D _c (wherein 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05); Li _a Ni _1-bc Co _b R _c D _α (in the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05 and 0 < α ≤ 2); Li _a Ni _1-bc Co _b R _c O _2-α Z _α (in the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05 and 0 < α <2); Li _a Ni _1-bc Co _b R _c O _2-α Z ₂ (in the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05 and 0 < α <2); Li _a Ni _1-bc Mn _b R _c D _α (in the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05 and 0 < α ≤ 2); Li _a Ni _1-bc Mn _b R _c O _2-α Z _α (in the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05 and 0 < α <2); Li _a Ni _1-bc Mn _b R _c O _2-α Z ₂ (in the above formula, 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05 and 0 < α <2); Li _a Ni _b E _c G _d O ₂ (wherein 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, and 0.001 ≤ d ≤ 0.1); Li _a Ni _b Co _c Mn _d GeO ₂ (wherein 0.90 ≤ a ≤ 1.8, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, 0 ≤ d ≤ 0.5, and 0.001 ≤ e ≤ 0.1); Li _a NiG _b O ₂ (wherein 0.90 ≤ a ≤ 1.8 and 0.001 ≤ b ≤ 0.1); Li _a CoG _b O ₂ (wherein 0.90 ≤ a ≤ 1.8 and 0.001 ≤ b ≤ 0.1); Li _a MnG _b O ₂ (wherein 0.90 ≤ a ≤ 1.8 and 0.001 ≤ b ≤ 0.1); Li _a Mn ₂ G _b O ₄ (wherein 0.90 ≤ a ≤ 1.8 and 0.001 ≤ b ≤ 0.1); QO ₂ ; QS ₂ ; LiQS ₂ ; V ₂ O ₅ ; LiV ₂ O ₅ ; LiTO ₂ ; LiNiVO ₄ ; and Li _(3-f) J ₂ (PO ₄ ) ₃ (0 ≤ f ≤ 2).

In the chemical formula, A is Ni, Co, Mn or a combination thereof; R is Al, Ni, Co, Mn, Cr, Fe, Mg, V or a combination thereof; D is O, F, S, P or a combination thereof; E is Co, Mn or a combination thereof; Z is F, S, P or a combination thereof; G is Al, Cr, Mn, Fe, Mg, La, Ce, V or a combination thereof; Q is Ti, Mo, Mn or a combination thereof; T is Cr, V, Fe, Sc, Y or a combination thereof; J is V, Cr, Mn, Co, Ni, Cu or a combination thereof.

In addition, the positive electrode may further include a binder and a conductive material in addition to the positive electrode active material described above. The above binder is a component that assists in the bonding of the positive electrode active material and the conductive agent and the bonding to the current collector, and may be, for example, polyvinylidene fluoride (PVdF), polyvinylidene fluoride-polyhexafluoropropylene copolymer (PVdF/HFP), polyvinylacetate, polyvinyl alcohol, polyvinyl ether, polyethylene, polyethylene oxide, alkylated polyethylene oxide, polypropylene, polymethyl(meth)acrylate, polyethyl(meth)acrylate, polytetrafluoroethylene (PTFE), polyvinyl chloride, polyacrylonitrile, polyvinylpyridine, polyvinylpyrrolidone, styrene-butadiene rubber, acrylonitrile-butadiene rubber, ethylene-propylene-diene monomer (EPDM) rubber, sulfonated EPDM rubber, styrene-butylene rubber, fluorine rubber, carboxymethyl cellulose (CMC), starch, One or more selected from the group consisting of hydroxypropyl cellulose, regenerated cellulose, and mixtures thereof may be used, but is not necessarily limited thereto.

The above binder can be used in an amount of 1 to 50 parts by weight, or 3 to 15 parts by weight, based on 100 parts by weight of the total weight of the positive electrode. As a result, the adhesive strength between the positive electrode active material and the current collector and the capacity characteristics of the secondary battery can be excellently maintained.

In addition, the conductive material included in the positive electrode is not particularly limited as long as it has excellent electrical conductivity without causing side reactions in the internal environment of the lithium secondary battery and without causing chemical changes in the battery, and graphite or conductive carbon can be used as representative examples thereof, and for example, graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, Ketjen black, Denka black, thermal black, channel black, furnace black, lamp black, etc.; carbon-based materials having a crystal structure of graphene or graphite; conductive fibers such as carbon fibers or metal fibers; fluorinated carbon; metal powders such as aluminum powder and nickel powder; conductive whiskey such as zinc oxide or potassium titanate; conductive oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives; may be used alone or in combination of two or more thereof, but is not necessarily limited thereto.

The above-mentioned conductive agent can be used in an amount of 0.5 to 50 parts by weight, or 1 to 30 parts by weight, based on 100 parts by weight of the total weight of the positive electrode. As a result, the electrochemical properties such as conductivity and capacity of the positive electrode and the lithium secondary battery can be excellently maintained.

In addition, a filler may be optionally added to the positive electrode as a component that suppresses its expansion. The filler is not particularly limited as long as it can suppress the expansion of the electrode without causing a chemical change in the battery, and for example, an olefin polymer such as polyethylene or polypropylene; a fibrous material such as glass fiber or carbon fiber; and the like may be used.

In addition, as the positive electrode current collector, platinum (Pt), gold (Au), palladium (Pd), iridium (Ir), silver (Ag), ruthenium (Ru), nickel (Ni), stainless steel (STS), aluminum (Al), molybdenum (Mo), chromium (Cr), carbon (C), titanium (Ti), tungsten (W), ITO (In doped SnO ₂ ), FTO (F doped SnO ₂ ), and alloys thereof, as well as aluminum (Al) or stainless steel surface-treated with carbon (C), nickel (Ni), titanium (Ti), or silver (Ag), but the present invention is not necessarily limited thereto. The form of the positive electrode current collector may be in the form of a foil, a film, a sheet, a punched body, a porous body, a foam, or the like.

Meanwhile, the liquid electrolyte included in the electrolyte layer may include a non-aqueous organic solvent and a lithium salt. At this time, the type of non-aqueous organic solvent that can be used is not particularly limited, and any organic solvent that has been previously known to be applicable to electrolytes of lithium ion batteries, etc. may be used. Examples of such organic solvents include at least one selected from the group consisting of carbonate solvents, ether solvents, nitrile solvents, phosphate solvents, and sulfone solvents.

More specifically, as the carbonate solvent, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate, methyl propyl carbonate, ethyl methyl carbonate, ethyl propyl carbonate or methyl (2,2,2-trifluoroethyl) carbonate can be used, and as the phosphate solvent, trimethyl phosphate, triethyl phosphate or 2-(2,2,2-trifluoroethoxy)-1,3,2-dioxaphosphorane 2-oxide can be used.

In addition, as the ether solvent, tetrahydrofuran derivatives such as dibutyl ether, tetraglyme, diglyme, dimethoxy ethane or 2-methyl tetrahydrofuran can be used, and as the nitrile solvent, succinonitrile, adiponitrile, sebaconitrile, acetonitrile or propionitrile can be used. In addition, as the sulfone solvent, dimethyl sulfone, ethylmethyl sulfone or sulforane can be used.

In an exemplary embodiment, the electrolyte can be a liquid electrolyte or a solid electrolyte, for example, the liquid electrolyte can include one or a mixture of two or more selected from the group consisting of cyclic carbonate compounds of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, vinylethylene carbonate, and fluoroethylene carbonate (FEC), and linear carbonate compounds of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methylpropyl carbonate, and ethylpropyl carbonate. Additionally, the solid-state electrolyte may include Li ₁₀ GeP ₂ S ₁₂ (LGPS), Li ₂ SiP ₂ S ₅ Cl (LSPSCl), LPS, LiLaTiO ₄ (LLTO), Li ₇ La ₃ Zr ₂ O ₁₂ (LLZO), Li _2+2x Zn _1-x GeO ₄ (LISICON), Na _1+x Zr ₂ Si _x P _3-x O ₁₂ (NASICON, 0<x<3) and combinations thereof.

However, in terms of the superior mechanical properties and safety of the composite electrolyte membrane, it is preferable to use, as the organic solvent, a carbonate solvent, a sulfone solvent, or a phosphate solvent, which can be cured at least in part together with the crosslinked polymer and exhibit flame retardancy. In addition, it is more preferable to use, as the organic solvent, a solvent which exhibits low volatility under curing conditions for forming the crosslinked polymer, for example, under thermal curing conditions of 60 to 80° C.

Meanwhile, the lithium salt dissolved or dispersed in the organic solvent may be any lithium salt that has been previously known to be applicable to the electrolyte of a lithium secondary battery, for example, LiFSI (lithium bis(fluorosulfonyl)imide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiPF ₆ , LiB ₁₀ Cl ₁₀ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiC ₄ BO ₈ , LiAsF ₆ , LiSbF ₆ , LiAlCl ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, (C ₂ F ₅ SO ₂ ) ₂ NLi, (CF ₃ SO ₂ ) ₃ CLi, lithium chloroborane, lithium lower aliphatic carboxylic acid having 4 or fewer carbon atoms, lithium 4-phenylborate, and lithium imide. One or more of the following may be used:

Such lithium salts may be included in the organic solvent of the liquid electrolyte at a concentration of 0.8 M to 4.0 M, or 1.0 M to 2.0 M, thereby enabling the composite electrolyte membrane of one embodiment to exhibit excellent thermal stability and ionic conductivity.

Meanwhile, in the lithium secondary battery of the other embodiment, an electrolyte layer may be interposed between the positive and negative electrodes, for example, in the form of a layered membrane or film. In this case, the electrolyte layer may also function as a separator (i.e., electrically insulating the negative and positive electrodes while allowing lithium ions to pass through). At this time, the electrolyte layer may be included in the secondary battery by being coated and attached in the form of a thin film on one surface of the positive or negative electrode. In addition, the electrolyte layer may be independently interposed between the positive and negative electrodes. In addition, the lithium secondary battery of the other embodiment may be a semi-solid battery that uses a liquid electrolyte and a solid electrolyte together.

In addition, when a porous separator is added to the electrolyte layer in the lithium secondary battery, the separator can be used in the form of a sheet, a multi-membrane, a microporous film, a woven fabric, a non-woven fabric, etc., such as an olefin polymer such as polyethylene or polypropylene, glass fiber, etc., but is not necessarily limited thereto. However, it may be preferable to apply porous polyethylene or porous glass fiber non-woven fabric (glass filter) as the separator, and it may be more preferable to apply porous glass fiber non-woven fabric as the separator. The separator may be an insulating thin film having high ion permeability and mechanical strength, and the pore diameter of the separator may generally be in the range of 0.01 to 10 ㎛, and the thickness may generally be in the range of 5 to 300 ㎛, but is not limited thereto.

Meanwhile, the lithium secondary battery of the above other embodiment can be manufactured according to a conventional method in the art. For example, it can be manufactured by forming a composite electrolyte membrane, etc. between the positive electrode and the negative electrode, and optionally adding a porous separator, etc.

These lithium secondary batteries can be applied to battery cells used as power sources for small devices, and are particularly suitable for use as unit cells for battery modules that are power sources for medium- to large-sized devices.

In another embodiment of the present invention, a method for manufacturing an electrode for a lithium metal battery is provided, comprising: forming a polymer protective film layer on a negative electrode current collector by an initiated Chemical Vapor Deposition (iCVD) process using an initiator, wherein the polymer protective film layer includes an electrolyte swellable polymer, and the electrolyte swellable polymer is formed from at least one monomer selected from the group consisting of dimethylaminomethyl styrene, ethylene glycol dimethacrylate, acrylic acid, 2-(perfluorohexyl) ethyl acrylate, and divinylbenzene.

In an exemplary embodiment, a chemical vapor deposition process using the initiator may be performed using an initiator including tert-butyl peroxide (TBPO).

In an exemplary embodiment, the chemical vapor deposition process using the initiator may be performed at a deposition rate of 2 nm/min ^-1 to 14 nm/min ^-1 . For example, the deposition rate may be in the range of 2 to 12 nm/min ^-1 .

Figure 1 illustrates the schematic of the electrolyte-expandable iCVD polymer nanolayers formed on Li metal: a) a schematic of the expandable-soft scaffold formed on Li metal anode for strengthening the SEI layer, b) a schematic of the formation of the iCVD polymer nanolayers directly on Li metal. c) chemical structures of pDMAMS, pC6FA, pDVB, pAA, and pEGDMA. d) cross-sectional focused ion beam scanning electron microscopy (FIB-SEM) image of Li metal coated with a 100 nm thick pDMAMS layer (scale bar is 500 nm). e) SEM image of bare Li. f) SEM image of 100 nm pDMAMS-Li. g) Contact angle of the sample coated with iCVD polymer on Si substrate for EC:DEC (3:7, v/v) mixture. h) Ellipsometry analysis results of 100 nm pDMAMS-Si swollen with electrolyte solvent (wherein the refractive index (n) of the EC:DEC mixture is 1.39) i) Refractive index difference and swelling ratio of iCVD polymer film before/after swelling with electrolyte solvent are shown.

Figure 2 shows voltage profiles of symmetric bare-Li and 100 nm iCVD polymer-Li symmetric cells. a) The inset images are the Li cathode images after the 10th and 50th cycles of pDMAMS-Li. b) Magnified views of the voltage profiles of the 2nd, 10th, 100th, and 200th cycles in a). c) and d) Steady-state current measurements of symmetric bare-Li and pDMAMS-Li(100 nm) cells under 10 mV polarization for 1 h in 1 cycle. e) Molecular structure of poly(DMAMS-co-C6FA). f) Content ratio (DMAMS:C6FA) of poly(DMAMS-co-C6FA)-Li samples. g) Voltage profile of the symmetric poly(DMAMS-co-C6FA)-Li(100 nm) cell. h) Enlarged view of the voltage profile of the 10th cycle of Fig. 2g.

Figure 3. Electrochemical behaviors of 3S-Li||NMC batteries, a) Cycling performance and Coulombic efficiency of 100 nm poly(DMAMS-co-C6FA)-Li||NMC batteries with different DMAMS/C6FA ratios (charge and discharge current densities were fixed at 2 mA cm ^-2 after the formation cycle of 0.1 mA cm ^-2 at 20 °C). b) and c) Charge/discharge profiles of bare lithium and 100 nm pDMAMS-Li||NMC batteries from the 1st to the 100th cycle. d) and e) Nyquist plots of symmetric bare lithium and 100 nm pDMAMS-Li cells from the 0th to the 100th cycle. f) Images of bare lithium, pC6FA-Li, and pDMAMS-Li after the 10th cycle. g) Rate performance of 100 nm poly(DMAMS-co-C6FA)-Li||NMC battery (1C=2 mA cm ^-2 ). h) SEM image of the pDMAMS layer showing inhibition of propagation of Li dendrites (scale bar, 50 μm).

Figure 4 illustrates TOF-SIMS analysis results for a) to c) non-cycled 100 nm pDMAMS-Li, resourced 100 nm pDMAMS-Li, and resourced bare-Li cathodes with depth profiling; d) to f) XPS analysis results for non-cycled 100 nm pDMAMS-Li, lithiated 100 nm pDMAMS-Li, and lithiated bare-Li cathodes with depth profiling; g) Schematic illustration of native SEI layer accommodated in swollen pDMAMS layer; h) and i) XPS spectra for C1s and O1s data of lithiated bare Li and 100 nm pDMAMS-Li with depth profiling.

Figure 5 shows the electrochemical performances of 3S-Li batteries with different layer thicknesses: a) voltage profiles of symmetric pDMAMS-Li batteries with pDMAMS layers ranging from 10 nm to 500 nm; b) cycling performances and Coulombic efficiencies of pDMAMS-Li||NMC batteries with different layer thicknesses (charge and discharge current densities were fixed at 2 mA cm ^-2 after the formation cycle at 0.1 mA cm ^-2 ). c) Magnified view of the voltage profile of a). d) Initial charge/discharge profiles of bare-Li and pDMAMS-Li||NMC batteries with different pDMAMS layer thicknesses; e) and f) Nyquist plots of pDMAMS-Li||NMC batteries with different thicknesses at the initial and 50th cycle; (g) summary of comparison of lithium ion transfer numbers and ionic conductivities in recent studies; (h) schematic diagram of the 3S strategy.

Figure 6 is a schematic diagram of the challenges of lithium metal anodes, a) schematic diagram showing the general interfacial problems of lithium metal anodes; b) schematic diagram showing the limitations of the artificial SEI layer approach.

Figure 7 shows the cross-sectional focused ion beam scanning electron microscopy (FIB-SEM) analysis results. a) FIB-SEM images of Li foils each coated with a 50 nm thick poly(dimethylaminomethylstyrene) (pDMAMS) layer (scale bar, 500 nm). b) FIB-SEM images of Li foils each coated with a 200 nm thick pDMAMS layer (scale bar, 500 nm).

Figure 8 shows Fourier transform infrared (FT-IR) spectroscopy results of monomers and iCVD-polymers in embodiments of the present invention.

Figure 9 shows images of iCVD polymer-coated lithium metal exposed to ambient air at 25°C and relative humidity (RH) of 20% to 35% for 0 h, 1 h, 2 h, and 24 h.

Figure 10 shows the results of ellipsometry analysis of pAA-Si, pEGDMA-Si, pDVB-Si, and pC6FA-Si swollen by battery electrolyte solvents (a) to d) respectively.

Figure 11 shows the topologies of bare and iCVD-coated lithium anodes after cycling, a) SEM images of bare lithium after the 1st, 10th, and 100th cycle; b) SEM images of pDMAMS-Li after the 1st, 10th, and 100th cycle.

Figure 12 shows a) the steady-state current measurement of a symmetric bare-Li cell under 10 mV polarization for 1 hour in the 50th cycle. b) the steady-state current measurement of a symmetric pDMAMS-Li(100 nm) cell under 10 mV polarization for 1 hour in the 50th cycle. c) the electrochemical impedance spectroscopy (EIS) of a symmetric bare-Li cell measured at an open-circuit voltage range of 100 mHz to 1 MHz with an amplitude of 10 mV. d) the EIS of a symmetric pDMAMS-Li(100 nm) cell measured at an open-circuit voltage range of 100 mHz to 1 MHz with an amplitude of 10 mV.

Figure 13 shows the ionic conductivity of iCVD polymer nanolayers, a) comparing various 100 nm iCVD polymer layers as measured by Electromagnetic Scattering (EIS); b) measuring (EIS) data from assembled cells of stainless steel/iCVD polymer/stainless steel.

Figure 14 shows a) FT-IR spectroscopy results; b) X-ray photoelectron spectroscopy (XPS) survey spectra; and c) high-resolution (N1s) XPS spectra of pDMAMS, poly(DMAMS-co-C6FA) and pC6FA according to embodiments of the present invention.

Figure 15 shows the topologies of pDMAMS and pC6FA homopolymers and poly(DMAMS-co-pC6FA) copolymers, comparing SEM images of pDMAMS-Li, pD2F1-Li, and pC6FA-Li (a) to c, respectively), scale bar, 50 μm.

Figure 16 shows a FIB-SEM image of pC6FA-Li (scale bar, 400 nm).

Figure 17 shows the topological evaluation of pD2F1 after cycling, showing SEM images of pD2F1-Li after the 1st, 10th, and 100th cycles (a) to c, respectively), scale bar, 50 μm.

Figure 18 shows the electrochemical floating experiments of bare-Li, pC6FA _- Li, pEGDMA-Li, pDVB-Li, pAA-Li, and pDMAMS-Li full cells using _{LiNi0.6Mn0.2Co0.2O2} (NMC-622) _cathodes (a) to f) ( _the cells were charged to 4.0 V at 0.2°C and then gradually increased to 4.9 V for 10 h).

Figure 19 shows cyclic voltammetry experiments of bare lithium, pC6FA-Li, pEGDMA-Li, pDVB-Li, pAA-Li, and pDMAMS-Li cells at scan rates of 0.1, 0.2, 0.3, 0.5, and 1.0 mV s ^-1 (a) to f, respectively).

Figure 20 shows the galvanic intermittent titration technique (GITT) and in situ XPS analysis of NMC cathodes, where a-b) typical GITT plots of bare lithium and pDMAMS-Li (100 nm) full cells using NMC 622, respectively; c-d) reaction resistances of bare-lithium and pDMAMS-Li (100 nm) cells in a) and b), respectively; e-f) high-resolution N1s XPS spectra of NMC cathodes assembled with bare-lithium and pDMAMS-Li cathodes.

Figure 21 shows the equivalent circuit for EIS analysis.

Figure 22 shows the rate capacity of a symmetric cell, a) voltage profiles of a symmetric bare lithium cell at various current densities of 1, 2, 5, and 10 mA cm ^-2 (1 mAh cm ^-2 ). b) voltage profiles of a symmetric pDMAMS-Li(100 nm) cell at various current densities of 1, 2, 5, and 10 mA cm ^-2 (1 mAh cm ^-2 ).

Figure 23 shows the full-cell performances of iCVD polymers with NMC anode, a) Cycling performance and coulombic efficiency of 100 nm pC6FA-Li, pEGDMA-Li, pDVB-Li, pAA-Li, and pDMAMS-Li full cells using NMC 622 anode (charge and discharge current densities were fixed at 0.1 mA cm ^-2 and 2 mA cm ^-2 after the formation cycle at 25°C). b) to d) Charge/discharge profiles of 100 nm pAA-Li, pEGDMA-Li, and pDVB-Li full cells using NMC from the 1st to the 100th cycle. e) to h) Charge/discharge profiles of 100 nm pC6FA-Li and poly(DMAMS-co-C6FA)-Li full cells using NMC anode from the 1st to the 100th cycle.

Figure 24 shows the TOF-SIMS analysis results of a) uncirculated 300 nm pDMAMS-Li as a time-of-flight secondary ion mass spectrometry (TOF-SIMS). b) TOF-SIMS analysis results of cycled 300 nm pDMAMS-Li.

Figure 25 shows the XPS analysis results of a non-cycled bare lithium negative electrode through depth profiling.

Figure 26 shows the XPS analysis results of the pDMAMS-Li cathode with depth profiling, a) XPS analysis results of the non-cycled 300 nm pDMAMS-Li cathode with depth profiling; b) XPS analysis results of the cycled 300 nm pDMAMS-Li cathode with depth profiling.

Figure 27 shows the N1s XPS spectra of pDMAMS-Li with depth profiling, where the XPS spectra of N1s data of lithiated bare lithium, lithiated pDMAMS-Li, and non-cycled pDMAMS-Li with depth profiling are shown (a) to c) respectively.

Figure 28 shows N1s XPS spectra of pDMAMS-Li with depth profiling, where a) a survey XPS spectrum of 100 nm pDMAMS-Li is shown. b) a high-resolution N1s XPS spectrum of 100 nm pDMAMS-Li is shown. c) a high-resolution C1s XPS spectrum of 100 nm pDMAMS-Li is shown. d) an XPS spectrum of N1s in pDMAMS-Li after the 50th cycle is shown. e) an XPS spectrum of C1s in pDMAMS-Li after the 50th cycle is shown.

Figure 29 illustrates a schematic diagram of the proposed quaternary reaction of pDMAMS and diethyl carbonate (DEC) to form poly(DMAMS-co-VBTMAC).

Figure 30 shows the NMR analysis results of pDMAMS cultured in various solvents, a) NMR analysis results of pDMAMS, DEC, and pDMAMS cultured in DEC solvent. b) NMR analysis results of pDMAMS, DEC, and pDMAMS cultured in DEC solvent at 3.0 ppm to 3.5 ppm and 2.0 ppm to 2.5 ppm. c) FT-IR analysis results for pDMAMS, DEC, ethylene carbonate (EC):DEC (3:7, v/v), and pDMAMS cultured in solvents (*: tertiary amine (2764 cm ^-1 ), **: CN (CH ₃ ) ₃ (930 cm ^-1 to 920 cm ^-1 ).

Figure 31 shows the results of a) ellipsometry analysis of 300 nm pDMAMS-Li swollen by electrolyte solvent; b) Nyquist plot of 300 nm pDMAMS-Li||NMC battery after 50 cycles; and c) Nyquist plot of 500 nm pDMAMS-Li||NMC battery after 50 cycles.

Figure 32 shows the results of steady-state current measurements of a) a symmetric 300 nm pDMAMS-Li cell under 10 mV polarization for 1 hour at 1 cycle, as a function of the transition number of the thick pDMAMS layer; b) a symmetric 300 nm pDMAMS-Li cell under 10 mV polarization for 1 hour at 50 cycles; and c) an EIS measurement result of a symmetric 300 nm (10 nm) cell in the range of 100 mHz to 1 MHz at an open-circuit voltage of 10 mV amplitude.

Analysis method

FT-IR Characterization

The monomers and polymers synthesized by iCVD were characterized by Fourier transform infrared (FT-IR) spectroscopy. In Fig. 8, the peaks in the blue region from 1637 cm ^-1 to 1627 cm ^-1 were derived only from the vinyl moieties of each monomer, and the peak intensities of the iCVD polymers were reduced compared to those of the monomers. The decrease in the vinyl peak in the spectrum indicates successful polymerization through the iCVD process. In contrast, the copolymerization of DMAMS and C6FA was confirmed by the tertiary amino-methyl peak at 2764 cm ^-1 in the red region and the carbonyl peak at 1735 cm ^-1 in the green region in Fig. 14a. As the DMAMS content of poly(DMAMS-co-C6FA) increased, the peak intensity in the red region gradually strengthened, and the peak intensity in the green region weakened. The composition of each copolymer could be appropriately controlled by precisely controlling the flow rate of the monomers injected into the iCVD chamber (Table S1).

XPS peak deconvolution

The SEI components of the pDMAMS layers were investigated by X-ray photoelectron spectroscopy (XPS) depth profiling using an Ar cluster gun (10 keV). For the deposited bare Li and pDMAMS-Li, C1, O1, and N1, high-resolution spectra at various depths were obtained by Ar sputtering. Survey scans and high-resolution XPS deconvolution of the electrolyte-swollen pDMAMS layers were performed on a flat Si substrate for more accurate XPS deconvolution. As shown in Figures 28a, 28b, and 28c, the peak at 399.25 eV in the N1s spectrum corresponds to CN, and the peaks at 285.6 eV and 284.7 eV in the C1s spectrum correspond to CN and CC/C=C, respectively. After deposition with bare lithium, the C1s spectrum is decomposed into Li ₂ CO ₃ (290 eV), COC (288.5 eV), CO (286.8 eV), CC (284.8 eV), and lithium carbide (R-Li) (283.5 eV) derived from organic components in the electrolyte, especially ethylene carbonate (EC) (left side of Fig. 4h). In contrast, the O1s spectrum shows peaks related to COC of ROCO ₂ Li (533.5 eV), Li ₂ CO ₃ (532 eV), LiOR (531.5 eV) associated with the carbonate electrolyte, and inorganic Li ₂ O (528 eV) of LiPF ₆ salt (right side of Fig. 4h). As the etching time increased, the organic SEI component rapidly disappeared, while at the same time, the intensity of the inorganic SEI component increased. For the precipitated pDMAMS-Li, Fig. 4i shows the peaks of pDMAMS and the organic electrolyte except for the reduced R-Li and Li ₂ O of the salt by EC. In addition, despite the gradual surface etching, the precipitated pDMAMS-Li exhibited a uniform organic SEI component without a significant decrease in the intensity of the existing peaks or the formation of other inorganic peaks. After the 50th cycle, the quaternized pDMAMS was characterized by a peak at 402.5 eV corresponding to the quaternary ammonium salt (CN ⁺ (CH ₃ ) ₃ ) in the N1s spectrum (Fig. 28d).

NMR characterization

To investigate the reaction between pDMAMS and the electrolyte, 1H nuclear magnetic resonance (NMR) spectra were recorded using a Bruker Avance Neo Nanobay 400 MHz NMR spectrometer at T = 298 K and analyzed with TopSpin 4.1.3 software in CentOS. For NMR sampling, the pDMAMS film deposited on the substrate was peeled off using a razor blade and collected in a vial. Deuterated chloroform (CDCl ₃ ) containing 0.03% tetramethylsilane (TMS) was used as the NMR solvent. The ratio of pDMAMS to electrolyte was adjusted to 1:10, and the NMR solution was prepared with a mixture ratio of 5 mg per 1 mL of CDCl ₃ , and approximately 0.6 mL of the solution was transferred to a 5 mm NMR tube.

Experimental method

ingredient

Dimethylaminomethylstyrene (DMAMS, 90%, Acros, Belgium), 2-(perfluorohexyl)ethyl acrylate (C6FA, 98%, Shanghai Qinba Chemical, China), ethylene glycol dimethacrylate (EGDMA, 98%, Sigma-Aldrich, USA), divinylbenzene (DVB, 80%, Sigma-Aldrich, USA), and acrylic acid (AA, 99%, TCI, Japan) were used as monomers, respectively, and butyl peroxide (TBPO, 98%, Sigma-Aldrich, USA) was used as a thermal initiator.

iCVD polymer-Li fabrication

For iCVD polymer-Li, Li metal foil was punched and then functionalized using a custom iCVD reactor installed in an Ar atmosphere glove box. Poly(dimethylaminomethylstyrene) (pDMAMS), poly[2-(perfluorohexyl) ethyl acrylate] (pC6FA), poly(ethylene glycol dimethacrylate) (pEGDMA), polydivinylbenzene (pDVB), and poly(acrylic acid) (pAA), poly(DMAMS-co-C6FA) were synthesized and conformally coated onto the Li metal cathode by iCVD process. Monomers, DMAMS, C6FA, EGDMA, DVB, AA, and initiator, TBPO, were vaporized by heating to 50, 50, 65, 45, 35, and 25 °C, respectively, and then fed into the iCVD reactor. For the polymerizations using a single monomer in iCVD, the flow rates of DMAMS, C6FA, EGDMA, DVB, and AA were fixed at 0.82, 0.55, 0.33, 2.50, and 1.43 sccm, respectively, and that of TBPO was fixed at 0.59, 0.30, 0.44, 0.89, and 0.74 sccm, respectively. The substrate temperature was changed from 30 to 40 °C, and the corresponding chamber pressure was adjusted from 100 to 300 mTorr to prevent excessive adsorption of reactants, such as lithium metal surface condensation. The deposition rates of pDMAMS, pC6FA, pEGDMA, pDVB, and pAA are 2.4, 12.5, 4.0, 6.1, and 8.0 nm/min ^-1 , respectively. For the copolymerization of poly(DMAMS-co-C6FA), the Pm/Psat and the surface concentration of each comonomer were controlled by the flow rates of DMAMS and C6FA at fixed chamber pressure and substrate temperature. In all iCVD processes, the filament temperature was maintained at 140°C to initiate vapor phase polymerization.

Characterization of iCVD polymer films and SEI layers

The polymerization in each monomer was chemically confirmed by absorbance mode of Fourier transform infrared (FT-IR) spectroscopy (ALPHA FT-IR, Bruker Optics). The structural compositions including the DMAMS ratio of the DMAMS and C6FA copolymers were obtained by multipurpose X-ray photoelectron spectroscopy (Sigma Probe, Thermo VG Scientific) using a microfocus monochromated Al source (1486.7 eV). The electrolyte contact angles of the iCVD-coated polymers were measured by a contact angle analyzer (Phoenix 150, SEO). Cross-sectional images of the iCVD-coated polymers on lithium metal were obtained using a high-resolution focused ion beam (FIB, Helios G4 FX, Thermo Fisher). The surface morphology of the lithium metal including dendrites and SEI layer was observed using an ultrahigh-resolution field emission scanning electron microscope (UHR FE-SEM, SU8230, Hitachi). The iCVD polymer films coated on Si wafers and the lithium metal oxidation process were imaged optically using a digital camera (ILCE-7M3, SONY). The lithium metal oxidation was photographed every hour for the first 6 h, and then every 6 h for the first day after air exposure. In-situ X-ray photoelectron spectroscopy (In-situ XPS, Axis-Supra, Kratos) and time-of-flight secondary ion mass spectrometry (TOF-SIMS, TOF.SIMS 5, ION-TOF) were used to perform depth profiling to identify the pDMAMS-SEI layer. The components and bonding states of the SEI layer were investigated using in-situ XPS with an Al Kα radiation source at 15 kV operation under 1 × 10 ^-9 torr. XPS depth profiling was performed by etching with Ar clusters (10 keV). TOF-SIMS was also performed to analyze the chemical components of the SEI layer in a vacuum chamber at 5 × 10 ^-9 mbar. The etching area of 300 μm × 300 μm and the analysis area of 100 μm × 100 μm ^were sputtered and profiled with Ar cluster (5 keV) and Bi ₃₊ (60 keV) ion beams, respectively. To prevent oxidation by air, the lithium metal was sealed in a welded aluminum pouch inside an Ar-filled glove box prior to all characterizations.

Ellipsometric Characteristics of iCVD Polymer Films

The refractive indices (n) and thicknesses (d) of all iCVD polymer films on Si substrates were obtained by spectroscopic ellipsometry (M2000U with auto-angle ESM-300 base, JA Woollam). The swelling ratios of the polymer films in organic electrolytes were measured in a liquid cell (5 mL Heated Liquid Cell ^TM , JA Woollam) with an optical attachment to the ellipsometer at a constant temperature of 25 °C, where d _pristine and d _swollen are the thicknesses before and after introducing the electrolyte into the liquid cell, respectively.

Specifically, spectroscopic ellipsometry was started at the air/film interface with a nominal angle of 75°, and the thickness and refractive index of the deposited films in ambient air were measured at optical wavelengths from 400 nm to 800 nm. All profile data were fitted using the Cauchy model. The refractive index dispersion as a function of wavelength is described as follows:

Here, λ is the wavelength of the beam, and A, B, and C are optical constants derived through data fitting. To avoid physical deviation from the fitting, the range of the constants is set to A > 0, 0 < B < 2, and C = 0.

A 25 nm SiO ₂ calibration Si wafer fixed in an electrolyte-filled cell was used to verify the liquid-surrounding model at the liquid/solid interface to observe the swelling behavior. The optical constants of the EC:DEC (3:7, v/v) electrolyte were found to be in agreement with previously reported literature values. After the electrolyte was infiltrated into the thin film, it was assumed that the corresponding refractive index gradient occurs in the direction perpendicular to the plane of the polymer film when the film swells. The graded layer was divided into five layers and analyzed as each layer having an individual refractive index. All model fits were performed within a mean square error (MSE) of 5 or less using CompleteEASE 6 software (JA Woollam).

Electrochemical measurements

In all electrochemical tests, 2032-type coin cells containing 75 μL of 1.0 M LiPF ₆ in EC:DEC (3:7, v/v) electrolyte (Panax Etec) were used. The amount of electrolyte required to swell the entire electrolyte was less than 1.2% (less than 700 nm of pDMAMS), which is small for cell operation and within the experimental error tolerance. Polypropylene-polyethylene-polypropylene (25 μm, Celgard) was used as the separator. High-purity Li foil (200 μm, 99.95% (Shinhyung E&T)) and LiNi _0.6 Mn _0.2 Co _0.2 O ₂ (NMC 622) sheets (94% active material, Welcos) were punched into 1 cm ² and used as bare-Li symmetric cells and bare Li||NMC full cells. Galvanostatic cycling measurements of Li-Li symmetric cells were performed at a current density of 1 mA cm ^-2 (1 mAh cm ^-2 ). The Li||NMC full cells were cycled at 1C (2 mA cm ^-2 , 177 mA g ^-1 ) within a voltage window of 3.0 V to 4.2 V after the first activation cycle at C/20 charge/discharge. Electrical impedance spectroscopy (EIS) measurements were performed on a ZIVE MP1 system. The impedance data of the lithium electrode were calculated by fitting the Nyquist plot to an equivalent circuit consisting of a resistor (R) and a positive-phase element (CPE) using ZIVE's ZMAN software. Cycling tests on symmetric cells and full cells were performed on a Wonatech battery system in an environmental chamber. EIS measurements were performed in the frequency range of 100 mHz to 1 MHz with an amplitude of 10 mV. The ionic conductivity of the iCVD-coated polymer film was measured using the same device by assembling the stainless steel/iCVD-coated polymer film/stainless steel cell. The ionic conductivity was calculated from the Nyquist plot using the following formula.

Here, σ is the ionic conductivity, t is the thickness, R is the resistance, and A is the area of the iCVD-coated polymer film. For the transfer number measurement, a 10 mV constant voltage bias was applied to the lithium symmetric cell for 1 h, and the lithium ion transfer number (tLi+) was obtained using the following Bruce–Vincent–Evans equation.

Here, I _s and I ₀ are the steady-state current and initial current, respectively, ΔV is the applied potential, and R _s and R ₀ are the steady-state resistance and initial resistance before and after polarization, respectively.

In addition, cyclic voltammetry, galvanostatic intermittent titration (GITT), and floating experiments were performed to confirm the stability of the NMC cathode and the interface between the lithium metal anode and the deposited polymer. The cyclic voltammetry test was performed for 3 cycles within the voltage range of 0.1 mV to 1 V. In the GITT experiment, the entire cell was repeatedly discharged and charged at C/2 for 5 min, and then rested for 1 h within the window voltage. Lithium ion diffusivity was measured using the Wepner–Huggins method considering the planar shape of the electrode. In the floating test, the entire cell was charged to 4.0 V at 0.05°C, and then applied up to 4.9 V for 10 h.

Example 1: 100 nm pDMAMS

A cathode was used (2 mAhcm ^-2 ) coated on an aluminum current collector by mixing 94% of NMC-based cathode active material, 3% of conductive agent, and 3% of binder by weight ratio. A 25 μm thick separator (Celgard 2325) was interposed between the cathode and lithium metal, which was protected by covering with a pDMAMS [poly(dimethylaminomethyl styrene)] polymer, which was about 100 nm thick by the iCVD process, and the electrolyte was injected to manufacture a 2032 type coin cell as a lithium metal secondary battery. The electrolyte used was an organic solvent consisting of EC: DEC in a volume ratio of 3:7 and a salt of 1 M LiPF ₆ dissolved therein. Among the above manufacturing methods, a 2032 type coin cell manufactured by placing the same lithium metal anode in the cathode position without using the NMC-based cathode was used as a lithium metal symmetric battery. Specifically, the pDMAMS polymer thin film was manufactured using the iCVD process. The substrate was placed in the iCVD chamber, and the temperature of the substrate was maintained at 40°C. Thereafter, dimethylaminomethyl styrene (DMAMS) and tert-butyl peroxide (TBPO), an initiator, were vaporized at rates of 0.82 and 0.59 sccm, respectively, and transferred to the chemical vapor deposition chamber. The pressure within the chamber was maintained as a vacuum state of 200 mTorr, and at the same time, the filament was heated to 140°C to radically polymerize the monomers adsorbed on the substrate to produce a pDMAMS homopolymer. Here, the desired thickness of the polymer thin film was obtained at a deposition rate of 2.4 nm/min.

Example 2: 10 nm pDMAMS

Coin cells were manufactured in the same manner as in Example 1, except that the thickness of the pDMAMS polymer protective layer in the cathode of Example 1 was about 10 nm.

Example 3: 500 nm pDMAMS

Coin cells were manufactured in the same manner as in Example 1, except that the thickness of the pDMAMS polymer protective layer in the cathode of Example 1 was 500 nm.

Example 4: 100 nm pDVB

Except that the thickness of the pDVB (polydivinylbenzne) polymer protective layer in the cathode of Example 1 was about 100 nm, a coin cell was manufactured in the same manner as in Example 1. Specifically, a pDVB polymer thin film was manufactured using an iCVD process. The substrate was placed in an iCVD chamber, and the temperature of the substrate was maintained at 30°C. Thereafter, divinylbenzene and tert-butyl peroxide (TBPO), an initiator, were vaporized at a ratio of 2.50 and 0.89 sccm, respectively, and transferred to the chemical vapor deposition chamber. The pressure in the chamber was maintained in a vacuum state of 300 mTorr, and at the same time, the filament was heated to 140°C to radically polymerize the monomers adsorbed on the substrate to manufacture a pDVB homopolymer. Here, the desired thickness of the polymer thin film was obtained at a deposition rate of 6.1 nm/min.

Example 5: 100 nm pC6FA

Coin cells were manufactured in the same manner as in Example 1, except that the thickness of the pC6FA [poly{2-(perfluorohexyl) ethyl acrylate}] polymer protective layer on the cathode of Example 1 was about 100 nm. Specifically, a pC6FA polymer thin film was manufactured using an iCVD process. The substrate was placed in an iCVD chamber, and the temperature of the substrate was maintained at 30°C. After that, 2-perfluorohexyl ethyl acrylate [2-(perfluorohexyl) ethyl acrylate, C6FA] and tert-butyl peroxide (TBPO), an initiator, were vaporized at a ratio of 0.55 and 0.30 sccm, respectively, and transferred to the chemical vapor deposition chamber. The pressure inside the chamber was maintained as a vacuum state of 100 mTorr, and at the same time, the filament was heated to 140°C to radically polymerize the monomers adsorbed on the substrate to produce a pC6FA homopolymer. Here, the desired thickness of the polymer thin film was obtained at a deposition rate of 12.5 nm/min.

Example 6: 100 nm pEGDMA

Except that the thickness of the pEGDMA [poly(ethylene glycol dimethacrylate)] polymer in the cathode of Example 1 was about 100 nm, a coin cell was manufactured in the same manner as in Example 1. Specifically, a pEGDMA polymer thin film was manufactured using an iCVD process. The substrate was placed in an iCVD chamber, and the temperature of the substrate was maintained at 30°C. Thereafter, ethylene glycol dimethacrylate (EGDMA) and tert-butyl peroxide (TBPO), an initiator, were vaporized at a ratio of 0.33 and 0.44 sccm, respectively, and transferred to the chemical vapor deposition chamber. The pressure in the chamber was maintained in a vacuum state of 150 mTorr, and at the same time, the filament was heated to 140°C to radically polymerize the monomers adsorbed on the substrate to manufacture a pEGDMA homopolymer. Here, the desired thickness of the polymer thin film was obtained at a deposition rate of 4.0 nm/min.

Example 7: 100 nm pAA

Except that the thickness of the pAA polymer in the cathode of Example 1 was about 100 nm, a coin cell was manufactured in the same manner as in Example 1. Specifically, a pAA [poly(acrylic acid)] polymer thin film was manufactured using an iCVD process. The substrate was placed in an iCVD chamber, and the temperature of the substrate was maintained at 30°C. Thereafter, acrylic acid and tert-butyl peroxide (TBPO), an initiator, were vaporized at a ratio of 1.43 and 0.74 sccm, respectively, and transferred to a chemical vapor deposition chamber. The pressure in the chamber was maintained in a vacuum state of 200 mTorr, and at the same time, the filament was heated to 140°C to radically polymerize the monomers adsorbed on the substrate to manufacture a pAA homopolymer. Here, the desired thickness of the polymer thin film was obtained at a deposition rate of 8.0 nm/min.

Comparative Example 1: 200 μm pure lithium metal

In Example 1, a 2032 type coin cell of the same size was fabricated using pure lithium metal of about 200 μm thickness without a polymer protective film.

Experimental Example 1: Refractive Index and Swelling Rate Analysis

The refractive index and swelling ratio of the thin film obtained by the ellipsometer liquid cell according to the embodiment of the present invention were measured. The polymer thin film deposited with a thickness of 100 nm on a silicon wafer (Si wafer) substrate was prepared in the liquid cell, and the change in the refractive index and swelling ratio of the polymer thin film before and after the electrolyte was injected into the liquid cell were recorded in Table 4.

Experimental Example 2: Cycle Life

The lithium metal symmetrical batteries manufactured in Examples 1 to 7 and Comparative Example 1 were charged and discharged at a current density of 1 mAcm-2 for 1 hour each, and the number of cycles in which an overvoltage of 5 V or more occurred was recorded in Table 5.

It was confirmed that the cycle life of the lithium symmetric battery of Example 1 protected by the pDMAMS polymer thin film was significantly improved compared to the lithium symmetric battery of Comparative Example 1 without the polymer thin film. In addition, it was confirmed that the cycle life was significantly improved compared to the Comparative Example in Example 7 and Example 1, where the swelling ratio was 15% or higher.

Experimental Example 2: Charge/Discharge Test

The lithium metal secondary batteries manufactured in the above Examples 1 to 7 and Comparative Example 1 were evaluated by the following charge/discharge test. In the first cycle of the charge/discharge test, the secondary battery was charged at a constant current of 0.5 C until the voltage became 4.2 V, and then discharged at a constant current of 0.5 C until the voltage became 3.0 V. After that, the lithium metal secondary battery was charged under the CC/CV conditions of 1 C, 4.2 V, and then a charge/discharge test was performed under the discharge conditions of 1 C, CC 3 V. The number of cycles at which the initial specific capacity reached 80% is recorded in Table 6 below.

The cycle number of the lithium metal secondary battery of Example 1 was significantly improved compared to that of the lithium metal secondary battery of Comparative Example 1, indicating that the pDMAMS polymer thin film suppresses the formation of lithium metal dendrites and dead lithium. In addition, a significant improvement in cycle life was confirmed in Example 7 and Example 1, where the swelling ratio was 15% or higher, compared to that of the Comparative Example.

Claims (14)

1. Negative current collector;

   A lithium metal layer formed on the negative electrode current collector; and

   A polymer protective film layer formed on the lithium metal layer;

   An electrode for a lithium metal battery, wherein the polymer protective film layer comprises an electrolyte-swellable polymer, and the electrolyte-swellable polymer has a swelling ratio (%) expressed by the following mathematical formula 1 of 15% or more.

   [Mathematical formula 1]

   

   Here, d _pristine is the thickness of the polymer protective film layer before swelling, and d _swollen is the thickness of the polymer protective film layer after swelling.
2. In the first paragraph,

   An electrode for a lithium metal battery, wherein the electrolyte swellable polymer is a polymer including at least one monomer selected from the group consisting of dimethylaminomethyl styrene, ethylene glycol dimethacrylate, acrylic acid, 2-(perfluorohexyl) ethyl acrylate, and divinylbenzene.
3. In the first paragraph,

   An electrode for a lithium metal battery, wherein the electrolyte swellable polymer comprises at least one polymer selected from the group consisting of pDMAMS [poly(dimethylaminomethyl styrene)], pDVB (polydivinylbenzne), pC6FA [poly{2-(perfluorohexyl) ethyl acrylate}], pEGDMA [poly(ethylene glycol dimethacrylate)], and pAA [poly(acrylic acid)].
4. In the first paragraph,

   An electrode for a lithium metal battery, wherein the polymer protective film layer is provided to cover all or part of the surface where the negative electrode current collector and the lithium metal layer come into contact.
5. In the first paragraph,

   An electrode for a lithium metal battery, wherein the polymer protective film layer has a thickness in the range of 10 to 500 nm.
6. In the first paragraph,

   An electrode for a lithium metal battery, wherein the polymer protective film layer is formed based on an iCVD process using an initiator, and the initiator comprises tert-butyl peroxide (TBPO).
7. In the first paragraph,

   An electrode for a lithium metal battery, wherein the above electrolyte swellable polymer swells in an electrolyte solution to form a solid-electrolyte layer composite.
8. In Article 7,

   The above electrolyte is an electrode for a lithium metal battery, wherein the lithium ion transfer number (tLi+) measured using the Bruce-Vincent method is in the range of 0.2 to 1.
9. In the first paragraph,

   An electrode for a lithium metal battery, wherein the lithium metal layer has a thickness of 1 to 200 ㎛.
10. A lithium metal battery electrode according to any one of claims 1 to 9, comprising: an anode; an anode; and an electrolyte layer interposed between the anode and the cathode;

    A secondary battery, wherein the cathode includes a solid electrolyte interphase formed on the surface.
11. In Article 10,

    A secondary battery, wherein the solid-electrolyte interface layer is a Li ₂ O free-SEI layer.
12. A method for manufacturing an electrode for a lithium metal battery according to any one of claims 1 to 9,

    It comprises forming a polymer protective thin film layer on a negative electrode current collector by an initiated Chemical Vapor Deposition (iCVD) process using an initiator,

    A method for manufacturing an electrode for a lithium metal battery, wherein the polymer protective film layer comprises an electrolyte swellable polymer, and the electrolyte swellable polymer is formed from one or more monomers selected from the group consisting of dimethylaminomethyl styrene, ethylene glycol dimethacrylate, acrylic acid, 2-(perfluorohexyl) ethyl acrylate, and divinylbenzene.
13. In Article 12,

    A method for manufacturing an electrode for a lithium metal battery, wherein the method is performed using an initiator comprising tert-butyl peroxide (TBPO) in a chemical vapor deposition process using the above initiator.
14. In Article 12,

    A method for manufacturing an electrode for a lithium metal battery, wherein the chemical vapor deposition process using the above initiator is performed at a deposition rate of 2 nm/min ^-1 to 14 nm/min ^-1 .

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