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. 2025 Aug 20;10(34):39012–39021. doi: 10.1021/acsomega.5c05199

Development of Thermally Stable Ionic Liquid-Based Composite Polymer Electrolytes Enabled by In Situ Polymerization for Lithium-Ion Rechargeable Batteries

Wookil Chae 1, Taeshik Earmme 1,*
PMCID: PMC12409686  PMID: 40918355

Abstract

Commercial lithium-ion batteries using organic solvent-based liquid electrolytes (LEs) face safety issues, including risks of fire and explosion. As a safer alternative, solid-state electrolytes are being extensively explored to replace these organic solvent-based LEs. Among various solid electrolyte options, polymer electrolytes offer advantages such as flexibility and ease of processing. However, they present challenges like low ionic conductivity at room temperature and reduced stability at elevated temperatures compared with other solid electrolytes. In this study, ionic liquid-based composite polymer electrolytes (CPEs) with exceptional thermal stability are synthesized via in situ polymerization. The in situ polymerized CPEs exhibit high ionic conductivity, reaching up to 1.38 mS cm–1 at 25 °C, and show improved interfacial contact with the electrode. These CPEs demonstrate robust thermal stability, withstanding thermal decomposition at 350 °C, and maintaining nearly the same initial cell capacity, even after being stored at elevated temperatures above 120 °C. The cell using CPEs also had a broad electrochemical stability window of 5 V, making it suitable for high-voltage electrode applications. Additionally, CPEs showed a high lithium-ion transference number (t Li+ = 0.5), and the NCM 811/CPE/Li cell showed a substantial discharge capacity of 210 mA h g–1 at 0.1 C at 25 °C. Furthermore, the battery cell retained a 66% capacity after 100 cycles at a 0.3 C-rate compared to the initial value. The results suggest that the developed CPE has promising thermal stability, cell performance, and cycle life for future applications in lithium-ion rechargeable batteries.

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1. Introduction

Lithium-ion batteries (LIBs) play a crucial role in our daily lives, finding applications in various devices ranging from small portable electronics to large-scale energy storage systems. Due to their notable attributes, including high energy density and prolonged cycle life, LIBs are increasingly utilized in electric vehicles, drones, and energy storage systems (ESS). , The constituent components of LIBs encompass a cathode, an anode, a separator, and a liquid electrolyte. The liquid electrolyte demonstrates a commendable ionic conductivity of 10–2 S cm–1 and exhibits favorable compatibility with the electrodes. − Nevertheless, the organic solvents employed in liquid electrolytes (LEs), such as ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC), pose significant safety concerns due to their high flammability, thereby giving rise to potential risks such as fire, explosion, and leakage. , In addressing these safety issues, research efforts have focused on solid-state electrolytes as alternatives to liquid electrolytes based on organic solvents. Solid-state electrolytes can be categorized into two major types: inorganic solid electrolytes and polymer electrolytes. Among these, polymer electrolytes (PEs) exhibit promising potential for replacing liquid electrolytes owing to their low flammability, excellent processability, cost-effectiveness in terms of raw materials, and inherent flexibility. − Moreover, PEs offer distinct advantages over inorganic solid electrolytes, including heightened resistance to electrode volume changes during the charge–discharge process, as well as enhanced safety features, superior flexibility, and processability. Notably, under specific conditions, the growth of dendrites, a common issue in battery technology, can be mitigated in polymer electrolytes.

PEs within batteries can be synthesized through either ex situ or in situ methodologies. In the ex situ approach, a solution casting technique is employed, wherein a precursor comprising dissolved organic solvent, polymer, and lithium salt is cast onto a flat substrate and subsequently dried. The resulting ex situ PEs are incorporated between the cathode and anode during the cell assembly. However, this approach faces practical limitations as it encounters difficulties in achieving intimate contact with internal active materials, leading to high interfacial resistance between the electrode and electrolyte due to imperfect impregnation of PEs into the electrode. On the other hand, the in situ approach addresses the challenges associated with the ex situ method. In this method, PEs are synthesized through free-radical polymerization upon thermal or UV treatment of a precursor consisting of a vinyl monomer, liquid electrolytes (LEs), and an initiator. − Employing in situ polymerization in cell fabrication offers the advantage of facile impregnation of the precursor into the electrode, ensuring effective interfacial contact between the electrode and electrolyte. −

Among these PEs, gel polymer electrolytes (GPEs) are formulated by entrapping liquid electrolytes within a polymer matrix. The Li+ transport mechanism primarily relies on the diffusion of Li+ through liquid electrolytes confined in a polymer matrix. Consequently, GPEs exhibit a trade-off relationship where a higher ratio of liquid electrolytes results in increased ionic conductivity but diminished mechanical strength, rendering them susceptible to external pressure generated between the anode and cathode. Furthermore, GPEs are not immune to ignition issues stemming from the use of liquid electrolytes. To address these challenges, extensive studies have explored composite polymer electrolytes (CPEs) by incorporating additives into GPEs, such as inorganic fillers, ionic liquids (IL), and plasticizers. , This addition enhances the amorphous area, facilitates Li+ migration, and enhances thermal and electrochemical stability by reducing the crystallinity of the polymer matrix.

Ionic liquid (IL)-based electrolytes have recently gained interest as alternatives to organic solvent-based electrolytes in GPE systems. ILs exhibit a range of properties, such as high thermal stability, nonflammability, and a wide electrochemical window. These advantages contribute to enhanced cell safety compared to cells using organic solvent-based electrolytes. , Typically, imidazolium-, pyrrolidinium-, and ammonium-based ILs are used in many fields, with imidazolium- and pyrrolidinium-based ILs often incorporated into electrolytes to enhance ionic conductivity and mechanical properties. However, pyrrolidinium-based ILs are known to have higher viscosity, which can reduce the mobility of lithium ions, while imidazolium-based ILs have relatively lower value for processability. ,

CPE typically incorporates inorganic fillers without ion-conducting properties, including SiO2, TiO2, and Al2O3. Among these inert inorganic fillers, SiO2 demonstrated the ability to augment ionic conductivity, electrochemical stability, and mechanical strength of polymer electrolytes, which has been extensively employed in many previous reports. − CPE incorporating SiO2 as an inorganic filler demonstrates an electrochemical stability of 4.8 V, surpassing that of the conventional carbonate-based organic solvent electrolyte (4.2 V) when combined with a polyacrylonitrile (PAN) membrane. Furthermore, CPE-based electrolytes exhibited good resistance to thermal shrinkage when exposed at 150 °C for 1 h and maintained thermal stability up to 230 °C. ,

In this study, we report CPE based on imidazolium-based IL and SiO2 inorganic filler to achieve very high stability as well as enhanced performance of battery cells. The fabricated CPE exhibited a surprisingly high thermal stability up to 350 °C with wide electrochemical windows above 5 V. CPE was synthesized by in situ polymerization with UV irradiation, which made it possible to obtain a polymeric electrolyte matrix instantly and obtain an improved interface between active electrode materials and the electrolytes. 1 M lithium bis­(trifluoromethanesulfonyl)­imide (LiTFSI) salt in 1-butyl-3-methylimidazolium bis­(trifluoromethanesulfonyl)­imide ([BMIM] [TFSI]) was used as the electrolyte, and trimethylolpropane ethoxylate triacrylate (ETPTA) was used as a monomer later to form a polymeric matrix to achieve good mechanical strength due to its three-bridged acrylate functional groups, whereas poly­(ethylene glycol) dimethyl ether (PEGDME) was used to add flexible property to CPE by generating a semi-interpenetrating polymer network (Semi-IPN). The cells with CPE showed a notably high ionic conductivity, reaching 1.38 mS cm–1 at 25 °C and good mechanical flexibility as well as thermal stability.

2. Experimental Methods

2.1. Materials

Lithium bis­(trifluoromethanesulfonyl)­imide (LiTFSI, ≥99%), 1-butyl-3-methylimidazolium bis­(trifluoromethanesulfonyl)­imide ([BMIM] [TFSI], ≥98.0%), silicon dioxide (SiO2, 500 nm size), poly­(ethylene glycol) dimethyl ether (PEGDME, M n ∼ 500), trimethylolpropane ethoxylate triacrylate (ETPTA, M n ∼ 428), and 2-hydroxy-2-methylpropiophenone (HMPP, 97%) are all purchased from Merck Co. and were used without further purification (Figure ).

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Chemical structures of the materials used in the electrolyte.

2.2. Preparation of Polymer Electrolyte Precursor Solution

Initially, an IL-based electrolyte was prepared by dissolving 1 M LiTFSI powder in [BMIM] [TFSI] and stirring overnight. IL-based electrolyte and ETPTA were dissolved at a ratio of 90:10 (w/w), and HMPP initiator was also dissolved in the electrolyte at a ratio of 0.1 wt % of ETPTA. For the preparation of the CPE precursor solution, SiO2 was introduced to the above electrolyte solution with 30 min of ultrasonication to ensure a homogeneous dispersion of particles. For the polymer matrix, ETPTA:PEGDME was fixed at 85:15 (w/w), and the ionic liquid-based electrolyte and the above total polymer matrix (ETPTA:PEGDME) ratio were 90:10 (w/w). All precursor solutions underwent overnight stirring at room temperature for thorough homogenization under inert conditions.

2.3. Fabrication of Battery Cells

The slurry for cathode was obtained by mixing LiNi0.8Co0.1Mn0.1O2 (NCM811, Umicore Korea Co.), carbon black (Super P, Timcal Ltd.), and poly­(vinylidene fluoride) (PVDF, Solef 5130, Solvay S.A.) in an N-methyl-2-pyrrolidine (NMP, Merck Co.) solvent at a weight ratio of 8:1:1, respectively, followed by blade coating onto aluminum foil (mass loading = 1.62 ± 0.5 mg cm–2), and then subsequently dried at 100 °C in a vacuum oven for 8 h. The coin cells were assembled by slowly adding 200 μL of the precursor solution into CR2032-type coin cell cases using a micropipette, ensuring that all internal components were fully covered and submerged. The solution was added to the coated cathode, which was then covered with a polypropylene (PP) separator (Celgard 2400, CELGARD). After electrolyte injection, the coin cells were rested for 2 h to allow full impregnation. After resting, the precursor electrolyte was cured under the UV-LED lamp (INNOCURE HQ-100, Lichtzen) for 20 s, and then, the cells were fully assembled by clamping with lithium metal, spacer, spring, and cap.

2.4. Characterization of Polymer Electrolytes

The UV curing response of CPE was analyzed using attenuated total reflectance Fourier-transform infrared spectroscopy (ATR FT-IR, Spectrum 3, PerkinElmer) in the range of 650–4000 cm–1.

The cross-sectional morphology and elemental mapping of NCM811/CPE were analyzed by Field Emission Scanning Electron Microscope (FE-SEM) with Energy-Dispersive X-ray Spectroscopy (EDS) (JSM-7610F-Plus, JEOL).

The crystallinity of CPE was analyzed by X-ray diffraction (XRD, SmartLab, Rigaku) at a scanning rate of 5° min–1 in the range of 10–90 deg. The crystallinity value was calculated using the following equation.

Crystallinity=AaAa+Ac 1

where A a and A c are the areas corresponding to the amorphous and crystalline peaks.

The thermal stability of CPE was investigated using thermogravimetric analysis (TGA, TG 209 F3 Tarsus, NETZCH) under a N2 atmosphere at a heating rate of 10 °C min–1 in the range of 40–700 °C.

Differential scanning calorimetry (DSC, DSC 25, TA Instruments) under an N2 atmosphere at a heating rate of 10 °C min–1 in the range of −90 to 350 °C.

2.5. Electrochemical Measurements

The ionic conductivity (σ) of GPE and CPE was measured by Electrochemical Impedance Spectroscopy (EIS) (VersaSTAT 3, AMTEK) from 106 to 1 Hz at 10 mV amplitude with electrolyte sandwiched between the stainless-steel (SS) cell and calculated using below equation.

σ=t(A×Rb) 2

where A is the area of the stainless-steel cell, R b is the bulk resistance of the polymer electrolyte, and t is the thickness of the polymer electrolyte.

The lithium transference number (t Li+) was obtained by EIS measurement and chronoamperometry with a Li symmetric cell.

tLi+=ls(ΔV−loRo)lo(ΔV−lsRs) 3

where l o and l s are the initial and steady-state current values, respectively, and R o and R s are the initial and steady-state interfacial resistance, respectively. ΔV (10 mV) is the applied voltage.

Linear sweep voltammetry (LSV) was conducted by an electrochemical analyzer (VersaSTAT 3, AMTEK) to measure the electrochemical oxidation stability of CPE with SS/CPE/Li asymmetric cell in a voltage range of 0–6 V under a scanning rate of 1 mV s–1.

3. Results and Discussion

FT-IR analysis was conducted to verify the successful reaction and polymerization of the acrylate functional group in the monomer induced by UV irradiation. As illustrated in Figure a, the peak corresponding to the acrylic −(C=C)– bond, ranging from 1610 to 1625 cm–1, completely vanished after exposure to UV light. The initiation of UV irradiation gives rise to radicals from the initiator, in turn, cleaving the −(C=C)– bonds of the acrylate, facilitating the creation of a polymer electrolyte matrix through cross-linking. This affirms that a reaction of carbon double bonds completely takes place, resulting in the effective formation of a polymeric matrix. ETPTA was selected as the polymer backbone due to its possession of three reactive sites, surpassing other similar monomers with lithium-ion conducting chains (−O–, −COO−), thereby enabling the formation of a denser polymer matrix. The unique properties of ETPTA enable the fabrication of a polymer electrolyte with free-standing characteristics, even with a minimal amount of approximately 10 wt % of the total precursor solution.

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(a) FT-IR spectra indicating the corresponding acrylic −(C=C)– bond of polymer matrix before and after UV illumination; (b) photos of flexible and bendable composite polymer electrolyte film.

Furthermore, the CPE with a semi-IPN structure demonstrates remarkable flexibility by incorporating PEGDME entangled in cross-linked ETPTA. As illustrated in Figure b, CPE showed an absence of cracks after bending and returned to its original state upon straightening. This electrolyte endures repeated bending and unbending without sustaining damage, highlighting its exceptional flexibility, as demonstrated in Supporting Video 1. ETPTA possesses nine oxygen atoms that could facilitate enhanced lithium-ion migration through the polymer chain, consequently improving the ionic conductivity. Owing to these advantageous properties, ETPTA is presently employed in many studies on polymer electrolytes.

As depicted in the cross-sectional SEM-EDS images in Figure , the CPE produced through in situ polymerization exhibited favorable interfacial contact between the electrode and CPE. Figure a illustrates a continuous phase throughout the composite polymeric electrolyte extending toward the upper electrode side. The precursor in the liquid state is injected into the electrode, and subsequent polymerization ensures enhanced interfacial contact between the electrode and polymer electrolyte. This effective interfacial contact is crucial for enhancing cell performance, as it facilitates the easy transport of lithium ions at the interface between the electrode and polymer electrolyte. The impregnation of the polymer electrolyte into the electrode is verified through EDS mapping. Figure b–g presents images corresponding to the elements carbon, oxygen, fluorine, sulfur, silicon, and nickel, respectively, as from the same image of Figure a. Common elements, such as carbon and oxygen atoms, which are integral to both the polymer electrolyte and the electrode, are uniformly distributed. These elements originated from the electrolyte, cathode active material, carbon black, and PVDF binder. In contrast, sulfur and silicon are specifically associated with the LiTFSI salt and SiO2 filler, respectively, and they also exhibited a uniform distribution in both the electrode and electrolyte sides within the electrolyte. The results indicate that the polymeric precursor liquid permeated easily into the gaps inside the electrode and then effectively polymerized under UV illumination, forming a well-dispersed polymeric electrolyte matrix throughout the electrode. However, an exclusive atom such as nickel, which solely comes from the cathode active material, showed a concentrated distribution in the electrode. Oxygen atoms are also observed in the concentrated form in Figure c, which presumably originated from lithium metal oxides. These cross-sectional SEM-EDS images demonstrate that the created polymeric electrolyte by in situ polymerization filled the inner gaps of the electrode, effectively forming a good interfacial contact that could provide a facilitated pathway for lithium-ion transport between the electrode and electrolyte during charging and discharging cycles.

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(a) Cross-sectional SEM images of electrode and polymer electrolyte; cross-sectional EDS element mapping of (b) carbon; (c) oxygen; (d) fluorine; (e) sulfur; (f) silicon; and (g) nickel.

The ionic conductivity of the GPE with ETPTA and an ionic liquid electrolyte varies with the addition of PEGDME (Figure a). PEGDME incorporated into the cross-linked ETPTA matrix enhances lithium-ion mobility and acts as an ion-conducting plasticizer. The plasticizer improves the solvation of lithium salt and facilitates the transport of lithium ions along the ethylene oxide (EO) chains. − In the absence of PEGDME, the polymer electrolyte exhibited an ionic conductivity of 0.61 mS cm–1. This conductivity increased to 0.64 and 0.73 mS cm–1 with the addition of 5 and 10 wt % PEGDME, respectively. The highest ionic conductivity of 1.03 mS cm–1 was achieved with 15 wt % PEGDME. However, when PEGDME is added in amounts ranging from 20 to 25 wt %, the ionic conductivity decreases to 0.97 and 0.95 mS cm–1, respectively. The reduction is likely due to the excess PEGDME becoming entangled in the cross-linked ETPTA, which increases the crystallinity of the polymer matrix and thus could restrict the movement of the lithium ions and create trapping sites.

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(a) Ionic conductivities of GPEs at different PEGDME ratios at 25 °C; (b) ionic conductivities of CPEs at various SiO2 ratios with 15 wt % PEGDME at 25 °C; (c) comparison of average ionic conductivity between GPE and CPE at 25 °C; and (d) temperature-dependent ionic conductivity of GPE and CPE.

Figure b shows how the ionic conductivity of CPEs varies with the addition of SiO2 to a GPE containing 15 wt % PEGDME. When 1 and 2 wt % SiO2 is added, the GPE exhibited higher ionic conductivities of 1.18 and 1.22 mS cm–1 compared to the GPE without SiO2 (1.03 mS cm–1), respectively. The highest ionic conductivity of 1.38 mS cm–1 was achieved with the incorporation of 3 wt % SiO2. This increase in the ionic conductivity is due to the Lewis acid–base interaction of SiO2 with the polymer matrix. The Lewis acid–base interaction can reduce the crystallinity of the polymer matrix and cause more segmental motion in the polymer matrix to aid in lithium-ion transport. Additionally, nanosized SiO2 particles can disrupt the crystalline regions of the polymer matrix, increasing the amorphous phase where ionic conductivity is higher. This enhances the segmental mobility of polymer chains, facilitating lithium-ion transport. However, with the higher ratios of 4 and 5 wt % SiO2, the ionic conductivity decreased to 1.03 and 0.75 mS cm–1, respectively. These results can be explained by the fact that intermolecular interactions between nanosized SiO2 particles are more dominant than interactions between the polymer matrix and SiO2, leading to the aggregation of SiO2 particles. These SiO2 agglomerates are ionically nonconductive and act as obstacles in the CPEs, making the lithium-ion transport pathways longer and more complex. In addition, the excessive SiO2 reduces the effective contact surface area between the electrode and the CPE, which in turn lowers the mobility of lithium ions.

Figure c compares the ionic conductivities of GPE and CPE. CPE (1.38 mS cm–1) exhibits nearly twice the ionic conductivity of GPE (0.61 mS cm–1), demonstrating that the ionic conductivity increases with the addition of the composites. The temperature-dependent ionic conductivities of GPE and CPE are also shown in Figure d. It is noted that CPE consistently possesses higher ionic conductivity than GPE across the entire temperature range (25–65 °C). The calculated values of the activation energy (E a) from an Arrhenius plot, GPE had the value of 0.14 eV compared to the lower value of CPE at 0.08 eV. The lower activation energy of CPE is definitely attributed to the addition of SiO2, which increases the portion of amorphous regions in the polymer matrix and thus enhances chain mobility, which facilitates lithium-ion transport. The overall ionic conductivity measurement suggests that the facile lithium-ion transport occurs in GPE by adding PEGDME, and further enhancement was observed by the incorporation of composite SiO2 up to 3 wt %.

In CPE systems, lithium ions are transported through the diffusion of entrapped liquid electrolytes and the segmental motion of the amorphous regions in the polymer matrix. More amorphous regions are formed with the decreased crystallinity of the polymer matrix, which facilitates Li+ transport and results in enhanced ionic conductivity. XRD analysis was conducted to assess the crystallinity of the polymer matrix. As shown in Figure a, CPE showed a reduced crystallinity of 32.7% while GPE had a crystallinity of 52.9%, confirming the decrease in crystallinity of the polymer matrix of CPE by the addition of the SiO2 composite. This indicates that CPE potentially has better ion transport properties than GPE.

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(a) XRD analysis of GPE and CPE matrix; (b) DSC thermal analysis with melting temperatures (T m) of GPE and CPE indicated; (c) DSC profile of GPE and CPE with the glass transition temperatures (T g) specified.

To investigate the thermal stabilities of the polymeric electrolytes, thermal characteristics of GPE and CPE were measured by using DSC. Figure b displays the melting temperatures (T m) of both GPE and CPE. GPE exhibits a T m of 192 °C while CPE shows a slightly lower T m of 185 °C. This lower in T m of CPE suggests that its polymeric matrix is less crystalline compared to that of GPE. The shift in the glass transition temperature (T g) for both materials is illustrated in Figure c. The T gs of the cross-linked ETPTA in GPE and CPE is 53 and 45 °C, respectively. This indicates that SiO2 is positioned at the binding sites of ETPTA and hinders long-range polymerization, which in turn limits the crystallization of the polymer and lowers the T g. Consequently, this leads to the increased chain flexibility and the formation of ion-conducting amorphous regions. ,

To ensure stability under extremely elevated thermal conditions, the thermal decomposition temperature (corresponding to 5% weight loss compared to the original mass) of GPE and CPE was measured as shown in Figure a. GPE begins to decompose at 300 °C, while CPE decomposes at a higher temperature of 350 °C. Both electrolytes demonstrate robust thermal stability as the decomposition begins above 300 °C. The high decomposition temperatures of GPE and CPE are presumably attributed to the excellent thermal stability of the room temperature ionic liquid (RTIL) contained in the polymeric matrix used in the electrolytes. , Initially, C–C and C–H bonds in the polymer electrolytes break as the temperature rises, forming gaseous compounds such as CO2, CO, and H2O, which results in weight loss. The higher decomposition temperature of CPE compared with GPE can be explained by the stronger bond strength of the Si–O bond, originating from the added SiO2 composite. This results in decomposition beginning at a higher temperature.

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Thermal stability of GPE and CPE: (a) decomposition temperature of GPE and CPE; (b) flammability test photos of CPE films fabricated (left: before ignition, middle: ignition, right: after ignition)

To ensure the fire retardancy of CPE with its excellent thermal stability, direct contact with the flame was conducted as shown in Figure b. CPE did not ignite when exposed to flame, unlike the immediate burning behavior typically observed in organic solvent-based electrolytes. Upon removal of the flame, only soot was observed on the surface of CPE, which still retained its ability to form a free-standing film (Supporting Video 2). The superior flame retardancy of the IL-based CPE demonstrates its potential as a much safer electrolyte for lithium-ion batteries, reducing the risk of fire and explosion in the event of a malfunction.

The electrochemical stability of GPE and CPE was measured by using LSV with an SS/Li asymmetric cell. As shown in Figure a, GPE undergoes electrochemical decomposition at 4.5 V, whereas CPE decomposes above 5.0 V. With an electrochemical window of 5.0 V, CPE is suitable for commercial LIBs and high-voltage active materials, such as high-nickel compositions and spinel structures.

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(a) Linear sweep voltammograms of GPE and CPE measured by SS/Li asymmetric cell; chronoamperometry and EIS (inset) spectra of (b) GPE and (c) CPE using a Li/Li symmetric cell to calculate lithium transference number.

The lithium transference number (t Li+), which represents the fraction of the total current carried by lithium ions, was determined by using the Bruce–Vincent method. EIS was conducted before and after polarization, along with measuring current variation over time using Li/Li symmetric cells, to calculate t Li+. As shown in Figure b,c, GPE had a t Li+ value of 0.27, while CPE exhibited a higher value of 0.50. This high t Li+ of CPE indicates that a larger portion of the current during charge/discharge is carried by lithium ions. The high electrochemical stability above 5 V and the t Li+ of 0.50 in CPE are attributed to the presence of SiO2, which immobilizes the TFSI-anion at the Lewis-acid site. , It is commonly accepted that the anodic decomposition of anions causes electrochemical instability of electrolytes at high voltages. In contrast to the case for GPE, anions are strongly adsorbed on the Lewis acid sites at SiO2 in CPE, which effectively suppresses the anodic decomposition of anions. As a result, CPE can exhibit good electrochemical stability at high voltage. The high t Li+ of CPE is due to the large number of free Li+ cations in the electrolyte, which demonstrates more possible ionic conduction in the system.

NCM811/Li coin cells were fabricated to evaluate the performance of cells using GPE and CPE. The NCM 811/GPE/Li and NCM 811/CPE/Li cells were first charged to 4.5 V and discharged to 2.7 V at 0.1 C (1 C = 210 mA). As shown in Figure a, the rate performance of the cells was measured at various C-rates ranging from 0.1 to 1 C at 25 °C, with the corresponding discharge profiles shown in Figure b. Both the GPE and CPE cells exhibited an initial discharge capacity of 210 mA h g–1 at 0.1 C. However, as the C-rate increased from 0.2 to 1 C, all the NCM 811/CPE/Li cells showed higher discharge capacities compared to the NCM 811/GPE/Li cells. Specifically, GPE had discharge capacities of 172, 79.7, and 31.8 mA h g–1 at 0.2, 0.5, and 1 C, respectively, while CPE exhibited 195, 142, and 74.9 mA h g–1 at the same rates. The higher discharge capacities of CPE compared to GPE at all different C-rates demonstrate the superior cell performance with composite polymer electrolyte.

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Cell performance of the NCM 811/Li cells: (a) rate performance of GPE and CPE from 0.1 to 1.0 C at 25 °C; (b) discharge profile of GPE and CPE from 0.1 to 1 C at 25 °C; (c) cycle performance of GPE and CPE at 0.3 and 25 °C; (d) discharge profile of LE and CPE before thermal shock; (e) discharge profile of CPE after thermal shock.

After 20 cycles, the NCM 811/CPE/Li cell retained a capacity of 215 mA h g–1, similar to its initial discharge capacity at 0.1 C, while the NCM 811/GPE/Li cell exhibited decreased capacity of 189 mA h g–1, which shows a capacity retention of 90%. To further evaluate cycling performance, the NCM 811/CPE/Li and NCM 811/GPE/Li cells were charged to 4.5 V and discharged to 2.7 V at 0.3 and 25 °C (Figure c). The NCM 811/GPE/Li cell demonstrated an initial discharge capacity of 154 mA h g–1 with a Coulombic efficiency of 98.8%, while the NCM 811/CPE/Li cell exhibited a higher capacity of 161 mA h g–1 with a Coulombic efficiency of 99.6%. After 40 cycles, the GPE cell retained 80% of its initial capacity (123 mA h g–1), whereas the CPE-based cell maintained 80% of its initial capacity up to 64 cycles (131 mA h g–1). After 100 cycles, the NCM 811/GPE/Li cell decreased to a capacity of 45.8 mA h g–1 (30% of initial capacity), while the NCM 811/CPE/Li cell held a capacity of 107 mA h g–1 (66% of initial capacity), demonstrating significantly more stable cycling performance compared to GPE.

While the thermal stability and flame retardancy of CPE had been confirmed, the direct heat exposure of the battery cell is also essential to ensure safety. A thermal shock test was conducted to validate the heat resistance of CPE. As shown in Figure d, the cell with commercial carbonate-based liquid electrolyte (LE, 1 M LiPF6 in EC/DEC = 50/50 v/v) before thermal shock exhibited a capacity of 199 mA h g–1 while CPE showed 178 mA h g–1. After both the NCM 811/CPE/Li and NCM 811/LE/Li cells were exposed to heat in a 120 °C chamber for 6 h, the CPE-based cell remarkably maintained a discharge capacity of 169 mA h g–1, showing only a 5% capacity loss (Figure e). However, the LE-based cell exploded and thus disassembled due to the swelling by instant gas formed by the organic electrolyte, as shown in supporting data, Figure S1. The thermal shock test perfectly demonstrates the excellent thermal stability of CPE and its potential for applications in safer lithium-ion rechargeable batteries.

4. Conclusions

A CPE with excellent electrochemical and high-temperature stability was developed and investigated. The CPE consisted of ETPTA and PEGDME unit precursors with an ionic liquid-based electrolyte of 1 M LiTFSI in [BMIM]­[TFSI], and incorporated SiO2 as an inorganic filler was fabricated using a unique in situ polymerization method to form a polymeric electrolyte matrix. The use of an ionic liquid and polymeric matrix rather than a carbonate-based organic solvent provided superior thermal stability of the battery cells. This was confirmed by the flame retardancy of the CPE and thermal shock tests. The addition of PEGDME as an ion-conductive plasticizer facilitates lithium-ion transport, while SiO2 reduces the crystallinity of the polymer matrix, further enhancing ion mobility along the polymer chains, resulting in increased ionic conductivity. The submicrosized SiO2 also immobilizes TFSI- anions, contributing to improved electrochemical stability and a higher lithium transference number.

The CPE also demonstrated excellent electrochemical stability, including stable charge and discharge at high voltages up to 4.5 V, and exhibited more stable cycling performance compared to the GPE. Robust electrochemical stability enables operation at high voltages, suggesting the possibility of increased energy density in lithium-ion batteries. Furthermore, the outstanding thermal stability of CPE offers a potential solution for preventing thermal runaway during abnormal operation of cells.

Supplementary Material

ao5c05199_si_001.pdf (286.4KB, pdf)
Download video file (10.9MB, mp4)
Download video file (2.7MB, mp4)

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Ministry of Science and ICT (MSIT) RS-2025-02653344 and supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government (MOTIE) (RS-2024-00349003, support for on-site practical training linked to public enterprises). The authors would also like to acknowledge the 2025 Hongik University Innovation Support Program Fund as well as Mr. Jaehyeon Kim and Ms. Yeri Joo for their discussions.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c05199.

  • Thermal shock experiment (PDF)

  • Supporting Video 1: Flexibility of the GPE (MP4)

  • Supporting Video 2: Flame retardancy of the IL-based CPE (MP4)

The manuscript was written through contributions of all authors. The authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

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