Electrolyte-as-binder strategy using in situ-formed PDOL gel for binder-free cathodes in flexible quasi-solid-state Li–S batteries

Abstract

Quasi-solid-state electrolytes (QSSEs) have attracted significant attention as a promising solution to safety and shuttle-effect issues in lithium-sulfur batteries (LSBs) owing to their high lithium-ion transference numbers (t_Li⁺), which suppress lithium dendrite formation and enhance safety and electrochemical stability. In this study, a binder-free (BF) cathode with a QSSE is fabricated via in situ polymerization of 1,3-dioxolane into poly(1,3-dioxolane) (PDOL). The QSSE serves simultaneously as an electrolyte and a binder. Despite being BF, the electrode exhibits stable electrochemical performance and mechanical strength, even under deformation. The polyvinylidene fluoride (PVDF) cathode shows a lower initial capacity of 885.8 mAh g⁻¹ because the PVDF binder impedes capillary absorption, preventing deep electrolyte infiltration and generating voids that hinder charge transport and reduce coulombic efficiency. The BF cathode achieves 1059.3 mAh g⁻¹ at 0.2C owing to infiltration of polymerized PDOL into the porous structure, enhancing interfacial integration and wettability. This study is the first to employ PDOL as a bifunctional binder–solid polymer electrolyte in LSBs, exploiting its strong adhesion and high lithium-ion conductivity. The BF@QSSE pouch cell is exceptionally flexible and safe under cycling and mechanical abuse, demonstrating the potential of combining BF cathodes with in situ-formed PDOL to fabricate flexible LSBs.

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1 Introduction

The rapid advancement of wearable electronics and unmanned aerial vehicles has created a high demand for lightweight and flexible energy-storage devices.¹ Electric vehicles and energy-storage systems also require rechargeable batteries with high energy and power density.^2,3 Lithium-sulfur batteries (LSBs) have emerged as one of the most promising next-generation energy storage systems, with a high theoretical specific capacity of 1675 mAh g⁻¹, nearly 8–10 times greater than that of commercialized cathodes such as layered transition metal oxides or lithium iron phosphate.⁴ This exceptional capacity enables the construction of LSBs with a specific energy density of up to 2567 Wh kg⁻¹, which is nearly 5 times higher than that of conventional lithium-ion batteries (≈420 Wh kg⁻¹), with the added benefits of low cost and high natural abundance of elemental sulfur.^5,6 However, despite these advantages, the practical application of LSBs remains limited owing to several critical challenges. First, the insulating nature of sulfur limits the active material utilization, necessitating carbon-hosting materials, which in turn reduce the energy density of the electrode.⁷ Second, the dissolution and migration of polysulfide intermediates lead to the continuous loss of cathode-active materials and the degradation of the Li metal anode, a phenomenon known as the “shuttle effect”.⁸ Third, the substantial volumetric expansion (≈80%) of sulfur during the lithiation/delithiation process compromises the mechanical integrity of the cathode.⁹ Finally, the uncontrolled growth of Li dendrites during plating/stripping not only deteriorates the cycling performance but also raises serious safety concerns.^10,11 To address these challenges, various strategies have been proposed, including the development of solid-state and quasi-solid-state electrolytes,¹² functional additives,^13,14 and advanced separator designs.^15,16 Among them, solid-state electrolytes, despite offering improved safety and mechanical stability, often suffer from low ionic conductivity,^17,18 high interfacial resistance owing to poor solid–solid contact between the electrode and electrolyte,¹⁹ and a limited ability to accommodate electrode volume changes during cycling.^20,21 Quasi-solid-state electrolytes (QSSEs) more effectively mitigate interfacial resistance, which is commonly attributed to poor electrode–electrolyte interfacial compatibility.²² In particular, poly(1,3-dioxolane) (PDOL) is a promising candidate because it forms flexible polymer networks and is compatible with lithium metal batteries.^23–25 As a polyether-based material, PDOL exhibits strong Li-salt dissociation and excellent interfacial stability.²⁶ Its high electron affinity supports efficient charge transfer, whereas the residual polymerization of 1,3-dioxolane (DOL) enhances ion transport and interfacial wettability.^27,28 However, no research has simultaneously exploited the strong adhesion of PDOL and the bifunctionality of polymer solid electrolytes for applications in LSBs.

In this study, a quasi-solid-state LSB was developed via the in situ polymerization of DOL to form bifunctional PDOL. By exploiting the strong adhesion of the in situ-polymerized PDOL, binder-free (BF) cathodes were fabricated and evaluated without the use of insulating polyvinylidene fluoride (PVDF) binders, enabling improved electrode flexibility and enhanced electrochemical performance. The BF cathode utilizing bifunctional PDOL achieved an initial capacity of 1059.3 mAh g⁻¹ and 700.4 mAh g⁻¹ after 100 cycles at 0.2C. In comparison, the PVDF-based cathode delivered 885.8 mAh g⁻¹ initially and 370.9 mAh g⁻¹ after 100 cycles. These results demonstrate that the BF cathode combined with QSSE provides enhanced cycling stability. Additionally, the solid form of the QSSE not only facilitates efficient ion transport in solution but also effectively suppresses the polysulfide shuttle effect, mitigating lithium anode corrosion.¹⁸ This approach offers a promising design pathway for BF configurations in LSBs, particularly when integrated with QSSEs, enabling both mechanical cohesion and enhanced interfacial stability. Notably, our investigation revealed that the use of a PVDF binder obstructed the electrolyte infiltration pathways within the electrode, in turn suppressing capillary absorption. This blockage led to the formation of internal voids, which were identified as critical factors contributing to the reduced electrochemical performance of PVDF cathodes. By contrast, the porous structure of the BF cathode allows the polymerized PDOL to infiltrate deeply, fill internal gaps, and improve interfacial integration and wettability, consequently enhancing the electrochemical performance. Furthermore, in cycling and abuse tests, the pouch cell exhibited exceptional flexibility and reliable safety without any internal short circuits, highlighting its strong potential for practical applications.

2 Experimental

2.1 Materials

Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI; 99%, Sigma-Aldrich), DOL (99.5%, Thermo Fisher Scientific), 1,2-dimethoxyethane (DME; 99%, Thermo Fisher Scientific), silicon tetrachloride (SiCl₄, 99%, Sigma-Aldrich), lithium nitrate (LiNO₃, 1 wt%, Thermo Fisher Scientific), and lithium sulfide (Li₂S, 99.5%, Aladdin) were all used as received without further purification.

2.2 Preparation of electrolytes

For the in situ polymerization of the QSSE, LiTFSI (1.0 M) and DOL were used as the lithium salt and solvent, respectively. Subsequently, 10 mM SiCl₄ was added as a Lewis acid initiator to trigger the ring-opening polymerization of the DOL monomer. The prepared solution was drop-cast onto a PE separator during cell assembly and polymerized in situ to form the QSSE.

The LE was prepared by dissolving LiTFSI (1.0 M) in a mixture of DOL and DME (1 : 1 v/v), followed by the addition of LiNO₃ (1 wt%, Thermo Fisher Scientific).

To prepare the Li₂S₆ solution, S and Li₂S were dissolved in DOL at a mass ratio of 5 : 1 and stirred at 50 °C for 24 h. All procedures were conducted in an argon-filled glovebox.

2.3 BF, PVDF cathode, and cell preparation

Sulfur (99.95%, Duksan) and Ketjen black (KB; Lion) were mixed in a weight ratio of 7 : 3 to prepare the S/KB composite material. Subsequently, the mixture was melt-diffusion treated at 155 °C for 12 h to ensure uniform infiltration of sulfur into the carbon matrix. For the BF and PVDF cathodes, the S/KB composite and conductive carbon (Super-P) were mixed at weight ratios of 80 : 10 and 80 : 10 : 10 (S/KB : Super-P : PVDF), respectively. The PVDF binder was added to the PVDF cathode formulation. In both cases, 1-methyl-2-pyrrolidinone (Sigma-Aldrich) was used as the solvent to form a uniform slurry. The coin cells were assembled using a BF@QSSE configuration, in which the BF cathode was first placed into the cell. The QSSE precursor was then dropped onto the PE separator, followed by placement of the Li foil. After assembly, the cells were left to stand at room temperature for 24 h, during which DOL polymerized in situ, forming a QSSE. For comparison, PVDF@QSSE cells with PVDF cathodes were assembled and gelled using the same procedure. As an additional control, BF@LE cells filled with the LE were also assembled. For the pouch-cell assembly, a cell with a total area of 33 cm² was assembled using the same design as that of the coin cell.

2.4 Material characterization

FTIR spectroscopy (FT/IR-4700, JASCO) was used to analyze the changes before and after polymerization by comparing the spectra of the monomer (DOL) and resulting polymer (PDOL). The thermal stabilities of the lithium salt and polymer were determined using TGA (TGA550, TA), which was performed over a temperature range of 30–450 °C under N₂ at a heating rate of 10 °C min⁻¹. The specific surface area and pore size distribution of different cathodes were measured and calculated by BET (BELSORP-max II, MicrotracBEL). The electrodes, including Li metal anodes after cycling and cathodes, were examined using FE-SEM (SU3800, Hitachi); the microscope was fitted with an energy-dispersive spectrometer (SU3800, Hitachi). AFM (MFP-3D Origin⁺, OXFORD Instruments) was used to analyze the surface morphology of the lithium metal. Polysulfide diffusion was analyzed using a UV–Vis spectrophotometer (V-650, JASCO) in the wavelength range of 250–600 nm.

2.5 Electrochemical characterization

EIS was used to measure the ionic conductivities of both the QSSE and LE at varying temperatures (VSP-300, BioLogic). Stainless steel/electrolyte/stainless steel cells (SS//LE//SS or SS//QSSE//SS) were assembled to obtain the impedance. The conductivity (σ) was calculated using eqn (1):where R is the bulk resistance, and L and A are the thickness and effective area of the QSSE, respectively. The electrochemical operating windows of the quasi-solid-state batteries were estimated in an LSV test (VSP-300, BioLogic) adopting a Li/QSSE/SS configuration in the potential range of 2–5.5 V. A QSSE-based symmetric cell was assembled to estimate the transference number of the Li ions (t_Li⁺) can be deduced using eqn (2):where V, I₀, I_s, R₀, and R_s are the applied voltage, initial and steady-state currents, and impedances before and after the test, respectively. The redox behavior of the cells was investigated using CV (VSP-300, BioLogic) in the voltage range of 1.8–2.8 V at sweep rates of 0.1 and 0.4 mV s⁻¹. The Li⁺ diffusion coefficient (D^app_Li) was subsequently calculated using eqn (3)

Charge–discharge tests were carried out from 1.8 to 2.8 V (SERIES 4000, Maccor). The lithium plating/stripping behavior was examined using Li/Li symmetric cells (VSP-300, BioLogic); cells assembled with QSSE or LE were tested at current densities of 0.1, 0.2, 0.5, 1.0, and 1.5 mA cm⁻².

3 Results and discussion

Fig. 1a presents a schematic illustration of the in situ polymerization process used to fabricate the QSSE and BF@QSSE configurations. Upon infiltration, the Lewis acid initiator, SiCl₄ (10 mM), induces the ring-opening polymerization of DOL in the presence of 1 M LiTFSI, leading to the formation of PDOL electrolyte matrix uniformly distributed within the cathode. The optical photographs in Fig. 1b show the QSSE precursor solutions before and after addition of silicon tetrachloride (SiCl₄). Before polymerization, the solution was clear and free flowing. After adding SiCl₄, polymerization occurs, forming a quasi-solid gel. As shown in Fig. 1c, Fourier-transform infrared (FTIR) spectroscopy was employed to investigate the changes in the functional groups of DOL before and after polymerization. The disappearance of the C–H peak at 915.1 cm⁻¹, the shifting of the C–O–C peak from 1030 to 1014 cm⁻¹, and the emergence of a new band at 845 cm⁻¹ provide clear evidence that SiCl₄-initiated ring-opening polymerization of DOL leads to the formation of extended PDOL chains. In addition, the DOL peaks at 663 cm⁻¹ and 1081 cm⁻¹ exhibit noticeable spectral changes after polymerization. The band originally located at 663 cm⁻¹ in DOL shifts to 632 cm⁻¹ in PDOL with increased intensity, which is attributed to C–O in-plane bending vibrations, indicating the transformation of the constrained five-membered cyclic C–O–C structure into more flexible linear ether environments along the polymer backbone. Meanwhile, the C–O stretching vibration peak at 1081 cm⁻¹ in DOL shifts to 1105 cm⁻¹ in PDOL with decreased intensity, reflecting the conversion of cyclic ether linkages in DOL to linear ether linkages in PDOL.^29–31Fig. 1d shows the thermogravimetric analysis (TGA) curves of the QSSE and liquid electrolyte (LE). Thermal stability is a critical factor for ensuring battery safety under elevated-temperature conditions. In the PDOL-based electrolyte, thermal decomposition initiates at ≈58 °C, with ≈50 wt% of the polymer remaining at 121 °C, whereas the LE is nearly fully decomposed at the same temperature. These results indicate that the QSSE exhibits enhanced thermal stability and may effectively increase the thermal runaway threshold of the battery.³² This improvement was presumably attributable to the increased crystallinity induced by the ring-opening polymerization of DOL.³³ In addition to thermal analysis, a flammability test was performed (Fig. S1). LE ignited instantly and burned for ≈5 s, confirming its flammability, whereas QSSE did not ignite under the same conditions. These results highlight the superior flame retardancy of the QSSE, which enhances the thermal safety of lithium metal batteries.³⁴

Fig. 1 (a) Schematic of the ring-opening polymerization of DOL and the in situ formation of the BF@QSSE cathode via electrolyte infiltration. (b) Optical images of the QSSE precursor solution (before) and the solidified QSSE after polymerization (after). (c) FTIR spectra of liquid DOL and PDOL. (d) TGA curves of QSSE and LE.

To assess the morphological changes induced by QSSE injection, scanning electron microscopy (SEM) was used to analyze three cathode configurations: BF cathode (BF), PVDF cathode (PVDF), and QSSE on a BF cathode (BF@QSSE). As shown in Fig. 2a and b, the BF cathode exhibits noticeable interparticle gaps and surface cracks, indicating weak cohesion and poor mechanical integrity. By contrast, the PVDF-based cathode (Fig. 2c and d) shows reduced cracking and improved structural stability. Notably, the BF@QSSE cathode (Fig. 2e and f) presents a smooth and uniform surface, indicating homogeneous integration and structural integrity. To evaluate whether the PDOL-based electrolyte can function as a binder in the cathode, three mechanical tests (adhesion strength, tape peeling, and folding) were performed. As shown in Fig. S2, both PDOL and PVDF supported a 100 g weight without detachment, indicating a comparable adhesion strength. Given PVDF's established role as a binder, the similar performance of PDOL indicates that it can also ensure structural integrity of the cathode. To further examine the mechanical stability, a peel test was conducted using Scotch tape. As shown in Fig. S3, the PVDF-based and QSSE-coated cathodes exhibited moderate adhesion with partial detachment, whereas the BF cathode completely delaminated, exposing the Al foil. These results underscore the importance of binders in maintaining cathode integrity. Additionally, a folding test was conducted to assess mechanical robustness (Fig. S4). The QSSE-coated and PVDF-based cathodes maintain their structural integrity after folding, whereas the BF cathode exhibits delamination and surface cracking. Thus, the QSSE acts effectively as a binder, improving particle cohesion and interfacial adhesion, which is expected to enhance cycling stability and flexibility.

Fig. 2 Surface SEM images of (a and b) BF cathode; (c and d) PVDF cathode; and (e and f) BF@QSSE.

The ionic conductivities of the LE and QSSE were measured over the temperature range of 30–60 °C, with values of 7.05 × 10⁻³ and 2.07 × 10⁻³ S cm⁻¹ at 30 °C, respectively. Although the QSSE exhibited lower conductivity than the LE, as shown in Fig. 3a, it still demonstrated a remarkably high ionic conductivity at room temperature, which is considered excellent for solid-state electrolytes. As shown in Fig. 3b and S5, the electrochemical stability windows of the QSSE and LE were evaluated using linear sweep voltammetry (LSV). The onset of a distinct oxidation current is defined as the upper voltage limit of the electrochemical stability window. As illustrated in the magnified view in Fig. S5, compared with the LE, which exhibited oxidative decomposition at 3.9 V, the QSSE remained stable up to 4.15 V, indicating improved oxidative resistance. This enhancement is attributed to PDOL, as the ring-opening polymerization initiated by Lewis acid catalysts improves the oxidative stability of DOL by reducing the number of free solvent molecules that are otherwise easily oxidized under high-voltage conditions.^35,36 A higher lithium-ion transference number (t_Li⁺) can effectively mitigate concentration polarization during the charge–discharge process of lithium–sulfur batteries, thereby promoting uniform Li⁺ deposition and suppressing the formation of Li dendrites.³⁷ As shown in Fig. 3c and d, t_Li⁺ of the QSSE-based cell (0.74) was higher than that of the LE-based cell (0.34), demonstrating improved Li⁺ transport. This enhancement may be attributed to the favorable interaction between the PDOL-based quasi-solid matrix and the Lewis acid initiator (SiCl₄), which likely coordinates with the TFSI⁻ anions and restricts their mobility, thereby increasing the effective t_Li⁺.³⁸ Additional measurements were conducted for various electrolyte compositions (Fig. S6, t_Li⁺ values of 0.62 for the 1 M, 20 mM system; 0.19 for the 2 M, 10 mM system; and 0.40 for the 2 M, 20 mM system). Based on these results, the 1 M, 10 mM precursor formulation, exhibited the best performance, was selected for all subsequent QSSE preparations in this study.

Fig. 3 (a) Ion conductivity of QSSE and LE. (b) LSV curves of QSSE and LE. (c and d) Steady-state polarization curve of the Li/Li symmetric batteries assembled from 1 M 10 mM QSSE and LE; insets: EIS curves before and after polarization.

Fig. 4a shows the cyclic voltammetry (CV) curves of the LSB with BF@QSSE measured at a sweep rate of 0.1 mV s⁻¹. The CV curves of the BF@QSSE cell in the initial three cycles exhibited high repeatability, indicating good electrochemical reversibility. Subsequent CV measurements were carried out at scan rates ranging from 0.1 to 0.4 mV s⁻¹ to evaluate the kinetic features of the redox reactions (Fig. 4b). Two distinct cathodic peaks were observed during the discharge process. The first cathodic peak (C1) at around 2.3 V corresponds to the initial reduction of elemental sulfur (S₈) into soluble long-chain polysulfides (Li₂S₈ and Li₂S₆) through S–S bond cleavage and multi-electron transfer. The resulting Li₂S_x species (x ≥ 6) dissolve into the electrolyte, forming active intermediates that enable further redox reactions. The second cathodic peak (C2) near 2.0 V is attributed to the subsequent reduction of these soluble polysulfides into short-chain and insoluble species (Li₂S₂/Li₂S), which nucleate and deposit on the cathode surface, completing the conversion of sulfur to Li₂S.³⁹ Two distinct anodic peaks (A1 and A2) were observed during the charging process at approximately 2.3 V and 2.4 V, respectively. The first anodic peak (A1) near 2.3 V corresponds to the oxidation of insoluble discharge products(Li₂S₂/Li₂S) into soluble higher-order polysulfides such as Li₂S₄, representing the reverse of the low-voltage reduction process. During this step, the solid Li–S species are reoxidized and dissolved into the electrolyte, regenerating electrochemically active intermediates. The second anodic peak (A2) at around 2.4 V is attributed to the further oxidation of these polysulfides into long-chain species (Li₂S₆,/Li₂S₈) and elemental sulfur (S₈).⁴⁰ As shown in Fig. 4c, the linear relationship between the anodic and cathodic peak currents and the square root of the scan rate indicates that the redox processes were governed by a diffusion-controlled mechanism.⁴¹

Fig. 4 (a and b) CV curves BF@QSSE at a scan rate of 0.1 mV s⁻¹ and increasing scan rates of 0.1–0.4 mV s⁻¹. (c) Peak current values versus the square root of the sweep rates of BF@QSSE. (d) Li⁺ diffusion coefficients at a scan rate of 0.1 mV s⁻¹.

To quantitatively evaluate the effect of BF@QSSE on Li⁺ diffusion kinetics, diffusion coefficients were derived from the CV curves obtained at varying scan rates using the Randles–Sevcik equation, as described in eqn (3). The coin cell constructed with BF@QSSE exhibited Li⁺ diffusion coefficients of 5.63 × 10⁻¹⁰, 9.38 × 10⁻¹⁰, 3.45 × 10⁻¹⁰, and 6.35 × 10⁻¹⁰ cm² s⁻¹ for the A1, A2, C1, and C2 peaks, respectively, at a scan rate of 0.1 mV s⁻¹ (Fig. 4d). A full overview is provided in Table S1, which summarizes the diffusion coefficients corresponding to A1, A2, C1, and C2 at scan rates of 0.1, 0.2, 0.3, and 0.4 mV s⁻¹, showing comparatively favorable values.

Fig. 5a–c show the galvanostatic charge–discharge profiles of the BF@QSSE, BF cathode with LE (BF@LE), and PVDF cathode with QSSE (PVDF@QSSE) cells recorded at the 1st, 25th, 50th, 75th, and 100th cycles at a current rate of 0.2C (1C = 1675 mAh g⁻¹). During the discharge process, two distinct voltage plateaus are observed, which correspond to the typical stepwise redox reactions of lithium–sulfur batteries. The first plateau at around 2.3–2.2 V arises from the electrochemical reduction of solid sulfur (S₈) to soluble long-chain polysulfides (Li₂S₈), while the subsequent plateau at 2.1–1.9 V is associated with the further reduction of these intermediates into short-chain and insoluble lithium sulfides (Li₂S₂/Li₂S). This multistep conversion involves solid–liquid and liquid–solid phase transitions, consistent with the CV results. Among the three configurations, the BF@QSSE cell exhibited the highest initial discharge capacity of 1059.3 mAh g⁻¹, outperforming the BF@LE (917.2 mAh g⁻¹) and PVDF@QSSE (885.8 mAh g⁻¹) cells, confirming its superior sulfur utilization and more efficient redox kinetics. As shown in Fig. 5a, the BF@QSSE cell displays two well-defined discharge plateaus at ≈2.4 and 2.1 V, characteristic of a catholyte-mediated redox mechanism. The first charge curve is the key feature to focus on. The overpotential observed during the initial charge process reflects the energy barrier associated with Li₂S oxidation (Fig. S7). Notably, the lower overpotential of the QSSE-based cell indicates superior reaction kinetics during this conversion step.^42,43 Moreover, QSSE-based cells displayed a lower polarization potential than LE-based cells (Fig. S8), as determined by the voltage gap, ΔE (measured at 50% discharge capacity); specifically, the BF@QSSE cell showed a lower ΔE of 150 mV than the BF@LE cell (183 mV). This improvement is attributed to the higher Li⁺ transference number of the QSSE, which enhances Li⁺ transfer dynamics and reduces concentration polarization during cycling.

Fig. 5 Galvanostatic charge–discharge curves of LSB assembled using (a) BF@QSSE, (b) PVDF@QSSE, and (c) BF@LE at 0.2C for 100 cycles. (d) Cyclability of the LSB for 100 cycles. (e) C-rate performance at varying C-rates from 0.1C to 2.0C. (f) Cyclability of the LSB for 300 cycles and 400 cycles at 1–2C.

However, from the 75th cycle onward, the plateaus in the BF@QSSE cell gradually faded and shortened, likely owing to the increased solution resistance stemming from the elevated viscosity caused by high concentrations of dissolved polysulfides.^44–46 Nevertheless, the BF@QSSE cell, benefiting from the high t_Li⁺ of the QSSE and its reversible redox behavior, maintained superior performance, delivering an initial discharge capacity of 1059.3 mAh g⁻¹ and retaining 700.4 mAh g⁻¹ after 100 cycles (66.12% retention, Fig. 5d). The BF@LE cell showed rapid capacity fading, likely owing to inadequate suppression of the polysulfide shuttle and the formation of unstable SEI and Li dendrites.^18,47 Interestingly, although PVDF was expected to provide a stable and highly reversible electrochemical performance, the PVDF@QSSE electrode exhibited poor reversibility and a low capacity retention of only 41.88%. This unexpected result indicates that the use of PVDF binders must be carefully reconsidered when combined with in situ-polymerized solid electrolytes.

Fig. 5e compares the rate capabilities of Li–S cells under varying C-rates. As the current density increases from 0.1C to 2.0C, the BF@QSSE cell delivers discharge capacities of 1072, 851.9, 767.4, 712.6, 624.9, and 522.8 mAh g⁻¹, respectively. These values are consistently higher than those of the BF@LE cell (979.8, 476.7, 406.5, 360.5, 297.1, and 248.4 mAh g⁻¹) and the PVDF@QSSE cell (797.4, 626.5, 551.9, 489.2, and 320.9 mAh g⁻¹). When the current rate is returned to 0.1C, the discharge capacities of the BF@QSSE, BF@LE, and PVDF@QSSE cells recover to 686.8, 455.7, and 529.6 mAh g⁻¹, respectively. Notably, the BF@QSSE cell exhibited excellent capacity recovery, indicating excellent structural resilience and rate reversibility, in agreement with the CV test results and confirming its highly reversible redox behavior. By contrast, the rapid capacity loss of the BF@LE cell at 0.1C is likely attributed not only to reduced structural stability caused by the absence of a binder but also to additional capacity fading induced by LiPS dissolution. Furthermore, a comparison of BF@QSSE and PVDF@QSSE across all current densities reveals a pronounced capacity gap, which is likely due to enhanced electrolyte infiltration within the BF cathode, thereby increasing the efficiency of charge transport.

The long-term cycling performances at 1C and 2C were also evaluated (Fig. 5f). At 1C, the BF@QSSE cell delivered an initial discharge capacity of 1048.7 mAh g⁻¹ and retained 418.8 mAh g⁻¹ after 300 cycles, outperforming BF@LE (initial: 873.3 mAh g⁻¹, 300th: 254.5 mAh g⁻¹) and PVDF@QSSE (initial: 859.5 mAh g⁻¹, 300th: 225.3 mAh g⁻¹). At 2C, the BF@QSSE cell still exhibited stable cycling, retaining 352.4 mAh g⁻¹ after 400 cycles, whereas BF@LE and PVDF@QSSE showed much lower capacities of 159.43 and 199.41 mAh g⁻¹, respectively. The superior performance of BF@QSSE was attributed to the enhanced electrolyte wettability and Li⁺ transport enabled by the porous BF structure, which effectively accommodated volume changes during cycling.⁴⁸ Although the BF@LE cell exhibited a comparable initial capacity (542.2 mAh g⁻¹) to the BF@QSSE cell (610 mAh g⁻¹) at 2C, its capacity declined more severely after 400 cycles. This degradation is likely attributable to the formation of lithium dendrites, as inferred from the previously observed higher overpotential and polarization potential of the BF@LE cell. By contrast, the PVDF@QSSE cell showed a significantly lower capacity at high current densities, likely owing to the use of the PVDF binder, which can block active pores and hinder ion transport. This pore blockage increases the interfacial resistance and impairs electrode–electrolyte interactions, ultimately limiting the electrochemical performance under high current density conditions.⁴⁹

Electrochemical impedance spectroscopy (EIS) was performed to evaluate the kinetics and resistance behavior of the LSB containing BF@QSSE, BF@LE, and PVDF@QSSE before and after 1C cycling (Fig. S9, Table S2). The high-frequency intercept, high- and medium-frequency semicircles, and low-frequency slope correspond to bulk resistance (R_b), SEI resistance (R_SEI), charge transfer resistance (R_CT), and Warburg impedance (W_o), respectively.^50–52 First, a comparison between BF@QSSE and BF@LE, both employing the same BF cathode, indicates that the initially higher R_SEI and R_CT observed for BF@LE could be attributed to the reduced mechanical stability owing to the absence of a binder. Notably, BF@QSSE exhibited a stable R_SEI, whereas the R_SEI of BF@LE significantly changed from 100.93 Ω to 34.14 Ω after 300 cycles. This stability can be attributed to the improved interfacial contact between the QSSE and lithium anode, achieved via in situ polymerization, which facilitates uniform lithium deposition and efficient charge transfer.⁵³ Consequently, a robust SEI film was formed during cycling, contributing to shuttle suppression and lithium anode protection.^54,55

Similarly, when comparing BF@QSSE and PVDF@QSSE, both employing the same QSSE, the BF@QSSE cell exhibited an increase in R_b from 2.48 Ω to 9.12 Ω and in R_SEI from 18.17 Ω to 21.4 Ω after prolonged cycling. These increases align well with the earlier observation in Fig. 5a, supporting our explanation that the fading and shortening of the discharge plateaus originate from the viscosity-induced rise in solution and interfacial resistances caused by the accumulation of dissolved polysulfide. Nevertheless, the BF@QSSE cell still demonstrated a markedly lower R_CT than the PVDF@QSSE counterpart even after extended cycling. The elevated R_CT in the PVDF-based cathode is attributed to the insulating nature of PVDF, which forms resistive polymer layers and, owing to its strong dipole moment (2.1 D), induces Li⁺ dipole interactions that hinder both electron and ion transport within the cathode.^48,56 Therefore, despite the gradual rise in R_b and R_SEI, the BF@QSSE configuration maintained superior interfacial kinetics and overall electrochemical stability compared with PVDF@QSSE.

Contrary to our initial expectation that a quasi-solid-state battery with a PVDF-based cathode would exhibit the most stable electrochemical characteristics, this battery was found to be poorly reversible. As mentioned earlier, this indicates that the use of PVDF binders may be detrimental when combined with in situ-polymerized quasi-solid-state electrolytes. To validate this assumption, cross-sectional analyses of the electrodes were conducted using FE-SEM, as shown in Fig. 6.

Fig. 6 (a and b) Cross-sectional SEM images of BF@QSSE and PVDF@QSSE and sulfur/oxygen elemental mapping (EDS) of (a-i, a-ii) BF@QSSE; (b-i, b-ii) PVDF@QSSE. (c) Schematic illustration of void formation in the cathode during in situ polymerization of the electrolyte.

Fig. 6a and b display cross-sectional SEM images of the BF@QSSE and PVDF@QSSE cathodes, respectively, and Fig. 6a-i,ii and b-i,ii show the corresponding energy-dispersive X-ray spectroscopy (EDS) mapping images. In BF@QSSE, the polymerized PDOL effectively infiltrates the porous structure, penetrating deep into the cracks and reaching the Al foil, highlighting its superior wettability and interfacial integration. The porous structure of the BF cathode facilitated electrolyte penetration and active material utilization. By contrast, in a PVDF cathode, the PVDF binder is expected to occupy a significant fraction of the pore volume of the carbon host, thereby reducing the accessible surface area and limiting electrolyte infiltration. This blockage hinders effective charge transport and prevents deep electrolyte penetration, leading to the formation of voids within the cathode. The sulfur confined in these voids remained electrochemically inactive, essentially becoming dead space, which explains the reduced capacity and inferior cycling performance of the PVDF@QSSE cell.^57–60 This behavior is closely related to the capillary effect.⁶¹ To quantitatively substantiate the hypothesis that the PVDF cathode restricts electrolyte infiltration by limiting the accessible surface area, Brunauer–Emmett–Teller (BET) analysis was performed for both the BF and PVDF cathodes. As shown in Fig. S10a and b, the BF cathode exhibited a higher specific surface area of 56.6 m² g⁻¹, compared with 45.7 m² g⁻¹ for the PVDF cathode. In addition, the pore size distribution results (Fig. S10c) revealed that the BF cathode possessed a larger average pore width (75.9 nm) than the PVDF cathode (40.8 nm), indicating a more open porous framework. These differences in surface area and pore size quantitatively confirm that the PVDF binder significantly reduces the accessible pore volume of the carbon host, thereby impeding electrolyte infiltration. The restricted penetration likely leads to the formation of voids within the cathode. To clarify the correlation between electrode and electrolyte uptake, the contact angle was measured (Fig. S11). The BF cathode (17°) exhibited a smaller contact angle than the PVDF cathode (28°), indicating better electrolyte wettability. Additionally, the electrolyte absorption was evaluated by immersing the electrodes (1 × 4 cm) into the electrolyte for controlled times of 5, 10, and 15 s; the weight change is shown in Fig. S12. The PVDF-based electrode rapidly absorbed the electrolyte within the first 5 s, but barely increased afterward, whereas the BF electrode continued to absorb the electrolyte over time. These results indicate that in the PVDF electrodes, the DOL solution could not penetrate deeply during in situ polymerization, leading to the formation of internal voids and degraded electrode performance (Fig. 6c).

Li-polysulfide (0.1 M Li₂S₆) diffusion tests were conducted using H-type cells equipped with either a QSSE-coated polyethylene (PE) membrane or a bare PE membrane, as shown in Fig. 7a. In the H-cell with the QSSE-coated PE, no visible color change occurred on the right side, even after 4 h, indicating effective suppression of Li-polysulfide permeation. By contrast, the H-cell with bare PE showed gradual yellowing of the solution on the right side within 4 h owing to the rapid diffusion of Li₂S₆ from the left chamber through the membrane. This result indicates that the QSSE-coated membrane can effectively mitigate the shuttle effect in LSB by blocking the migration of soluble polysulfides. After the H-type cell experiment, the variation in the concentration of polysulfides in the electrolyte on the right side of the cell was analyzed using UV-vis spectroscopy. As shown in Fig. 7b, the polysulfide solutions exhibit broad absorption peaks ranging from 250 to 600 nm. The QSSE-coated PE significantly reduced the Li₂S₆ absorbance intensity compared with that of the bare PE membrane, indicating that only a trace amount of Li₂S₆ permeated the QSSE-coated PE. This result confirms that the BF@QSSE cell suppressed the shuttle effect more effectively than the BF@LE cell.

Fig. 7 (a) Digital images of LiPS dissolution test in H-type cells with the PE + QSSE and PE for 4 h. (b) UV-vis spectra of Li₂S₆ solutions after diffusion through different separators (PE + QSSE and PE).

The interfacial stability between the electrolyte and lithium metal anode is critical for the reliable operation of lithium-based batteries.⁶² To investigate the Li⁺ deposition behavior, the cycling performances of Li symmetric cells were evaluated at current densities of 0.1, 0.2, 0.5, 1.0, and 1.5 mA cm⁻², as shown in Fig. 8a. Symmetric Li cells were assembled using QSSE, whereas the control cells were treated with LE. The QSSE-based cell exhibited lower voltage polarization across all current densities, indicating superior compatibility and interfacial stability with lithium metal compared with the LE-based cell. This can be attributed to the faster ion transport and enhanced interfacial stability of the QSSE, even at high current densities, which ultimately led to superior electrochemical performance. The slight asymmetry of the overpotential around 0 V originates from the different kinetics of Li plating and stripping. During plating, nucleation and growth of new Li deposits induce a higher interfacial resistance, whereas stripping occurs from pre-formed Li, leading to a lower overpotential.⁶³ SEM was used to analyze the Li plating/stripping behavior by examining the surface morphology of the Li metal after the Li plating/stripping test of the Li/Li symmetric cells. As shown in Fig. 8b and c, the Li surface of the QSSE-based symmetric cell exhibits a smooth, silver-white morphology, indicating uniform Li plating/stripping and suppressed dendrite formation. By contrast, the LE-based cell exhibited a rough, dark-gray surface with byproducts and a powdery texture, implying uncontrolled deposition and parasitic reactions. These SEM and optical observations (inset images) confirmed that the QSSE promotes interfacial stability by mitigating dendrite growth and inactive Li accumulation during cycling.^39,64

Fig. 8 (a) Rate performance for Li plating/stripping of the Li/Li symmetric cells with QSSE and LE. (b and c) SEM images of lithium surfaces after rate performance cycling with QSSE and LE. (d and e) AFM images and roughness of lithium surfaces after rate performance cycling with QSSE and LE.

Atomic force microscopy (AFM) was employed to further investigate the Li plating/stripping behavior by analyzing the surface morphology and roughness of the Li metal electrodes after cycling. As shown in Fig. 8d, the QSSE-based cell exhibited a significantly smoother and denser lithium surface with a reduced surface roughness (S_q) of 227.7 nm. This uniform topography indicates more stable Li plating/stripping behavior and effective suppression of dendrite growth. By contrast, the LE-based cells (Fig. 8e) displayed a rough and irregular surface characterized by coarse lithium particles and branch-shaped dendritic structures, resulting in a hilly and uneven topography with an S_q of 596 nm. This morphology indicates uncontrolled Li deposition, which can accelerate electrolyte consumption and promote undesirable side reactions owing to the increased surface area.^65,66 These findings highlight the beneficial role of the QSSE in stabilizing the Li interface and promoting homogeneous Li⁺ transport during cycling.

The electrochemical performance of the flexible battery under repeated deformation is shown in Fig. 9a. An initial high discharge capacity of 952.8 mAh g⁻¹ was maintained. The excellent mechanical flexibility and stable cycling performance of the Li–S cells were primarily attributed to the highly conductive and free-standing BF sulfur cathodes, along with the QSSE, which provided enhanced electrochemical performance and mechanical integrity. Moreover, a well-packaged pouch battery can steadily drive a red light-emitting diode (LED) under various deformation states (e.g., bending and folding) in a fully charged state (Fig. 9b). Even after cutting, as shown in Fig. S13, the pouch cell continued to light an LED lamp without any flame, smoke, or liquid leakage, clearly demonstrating the excellent safety and flexibility of the BF@QSSE-based cell.

Fig. 9 (a) Cyclic performance of the BF@QSSE pouch cell under flattening, 60° bending, and 180° folding at 0.2C; the insets show the voltage variation of the BF@QSSE pouch cell under different states. (b) Optical photographs of the flexible BF@QSSE pouch cell lighting up LEDs in series under different states.

4 Conclusion

In summary, a QSSE with high ionic conductivity was successfully synthesized via in situ polymerization, serving as both a QSSE and a binder in LSBs. The integration of a QSSE with a BF cathode enables a robust electrode–electrolyte interface and enhances the overall structural stability. Electrochemical evaluations revealed that the BF@QSSE cell delivered a high initial discharge capacity of 1059.3 mAh g⁻¹ at 0.2C and retained 700.4 mAh g⁻¹ after 100 cycles, corresponding to a capacity retention of 66.12% and an average capacity fading rate of only 0.34% per cycle. The high initial capacity is primarily attributed to the excellent electrical conductivity of the BF cathode, which, unlike PVDF-based electrodes with poor capillary absorption and low reversibility, enables effective in situ polymerization through superior wettability and the formation of a void-free structure. However, complete suppression of capacity fading was not achieved, likely due to increased R_b and R_SEI caused by electrolyte viscosity growth and slight cathode volume expansion during cycling. Future efforts will focus on enhancing the mechanical robustness of the BF cathode to mitigate resistance buildup and improve long-term stability. Nevertheless, QSSE played a critical role in inhibiting lithium dendrite growth, enabling superior electrochemical performance compared with both the BF@LE and PVDF@QSSE cells, as evidenced by the lower voltage polarization, higher capacity, and enhanced rate and cycling stabilities. This study not only demonstrates the feasibility of integrating a QSSE with a BF architecture in quasi-solid-state LSBs but also highlights that the QSSE, serving as a binder for the cathode, exhibits superior adhesive capability and enables the formation of flexible and stretchable electrodes suitable for pouch cell applications. These findings indicate that the strategy is promising for advancing high-performance, flexible, next-generation energy storage systems.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

Raw data that support the findings of this study are available from the corresponding author, upon reasonable request.

The authors confirm that the data supporting the findings of this study are available within the article and its supplementary information (SI). The supplementary information contains LSV and EIS data, BET analysis, and additional supporting figures and tables. See DOI: https://doi.org/10.1039/d5ta08211k.

Acknowledgements

This research was supported by the National Research Council of Science & Technology (NST) grant from the Korean government (MSIT) (No. GTL24012-000).

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Footnote

† These authors contributed equally to this work.

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