Eco-Friendly Gallic Acid-Tailored Binder with Synergistic Polarity Sites for High-Loading Lithium–Sulfur Batteries

Abstract

The development of polymer binders with tailored functionalities and green manufacturing processes is highly needed for high-performance lithium–sulfur batteries. In this study, a readily hydrolyzable 3,9-divinyl-2,4,8,10-tetraoxaspiro-[5.5]-undecane is utilized to prepare a water-based binder. Specifically, the acrolein produced by hydrolysis undergoes in situ polymerization to form a linear polymer, while the other hydrolyzed product, pentaerythritol, physically crosslinks these polymer chains via hydrogen bonding, generating a network polymer (BTU). Additionally, gallic acid (GA), a substance derived from waste wood, is further introduced into BTU during slurry preparation, forming a biphenol-containing binder (BG) with a multi-hydrogen-bonded structure. This resilience and robust cathode framework effectively accommodate volumetric changes during cycling while maintaining efficient ion and electron transport pathways. Furthermore, the abundant polar groups in BG enable strong polysulfide adsorption. As a result, sulfur cathode with a high mass loading of 5.3 mg cm⁻² employing the BG (7:3) binder still retains an areal capacity of 4.7 mA h cm⁻² after 50 cycles at 0.1 C. This work presents a sustainable strategy for battery manufacturing by integrating renewable biomass-derived materials and eco-friendly aqueous processing to develop polymer binders, offering a green pathway to high-performance lithium–sulfur batteries.

1. Introduction

Lithium-ion batteries (LIBs) have revolutionized energy storage for portable electronics and play an increasingly vital role in electric vehicles (EVs) and distributed energy storage systems [1,2,3,4]. However, conventional LIBs based on intercalation chemistry and transition metal materials face challenges in meeting the growing demand for higher energy density and lower cost [5,6]. In contrast, lithium–sulfur (Li-S) batteries offer a promising alternative due to their exceptionally high theoretical specific capacity (1675 mAh g⁻¹) and energy density (2600 Wh kg⁻¹), coupled with low material costs and environmental friendliness [7,8,9,10,11].

Despite these advantages, several challenges hinder the large-scale commercialization of Li-S batteries, primarily due to issues related to the sulfur cathode. The significant volume change (~80%) between sulfur (2.03 g cm⁻³) and lithium sulfide (1.66 g cm⁻³) can lead to cathode structural collapse and rapid capacity decay [12,13,14,15]. Additionally, the shuttle effect of polysulfides results in reduced Coulombic efficiency and increased internal resistance, while sulfur’s low conductivity limits reaction kinetics [16,17]. Therefore, stabilizing the sulfur cathode structure and effectively mitigating polysulfide dissolution are key to advancing Li-S battery technology [18].

To address these challenges, extensive research has explored strategies for improving cathode materials [19,20,21,22]. Highly conductive and porous carbon materials, such as carbon nanotubes [23,24] and nanospheres [25,26,27], are widely applied as sulfur hosts. To further accelerate polysulfide conversion, various metal catalysts have been combined with carbon materials in cathode or coated on the surface of separator, significantly improving cycling stability and rate performance [28]. Recently, rational electrolyte engineering, such as the introduction of redox mediators and selective anion screening, has been reported to effectively regulate the redox reactions of dissolved LiPSs [29,30,31,32]. Despite these advances, the substantial volume expansion of the sulfur cathode remains problematic, leading to microcrack formation and progressive electrode degradation over cycling [33,34,35]. If left unaddressed, these microcracks expand, eventually compromising structural integrity and battery performance [36,37].

Thus, developing innovative strategies to mitigate microcracks and suppress the polysulfide shuttle effect is critical for advancing the commercialization of lithium–sulfur (Li-S) batteries. As essential components of cathode fabrication, polymer binders play a pivotal role in maintaining electrode integrity and inhibiting polysulfide diffusion [9,38,39]. For instance, a self-healing binder was engineered through ring-opening polymerization of lipoic acid (with inherent disulfide bonds) combined with a thiol radical–polyphenol Michael addition reaction using gallic acid, enabling intrinsic crack repair and polysulfide anchoring [40]. Similarly, Liu et al. proposed a multifunctional binder where flame-retardant properties were introduced through the incorporation of hexachlorocyclotriphosphazene, while dynamic covalent bonds and polar groups provided self-healing capability and lithium polysulfide adsorption, respectively [41]. Although these binders address critical challenges in electrode stability and shuttle effects, scalability, cycling durability, and industrial processing compatibility remain unresolved.

Here, we present an environmentally friendly binder (BG) for sulfur cathodes, synthesized via acid-catalyzed hydrolysis and polymerization of 3,9-divinyl-2,4,8,10-tetraoxaspiro-[5.5]-undecane [42] (Figure 1a). This process yields a polar polymer (BTU) with dense hydrogen-bonding sites for lithium polysulfide (LiPS) adsorption. When composited with gallic acid (GA), the resulting BG binder further strengthens LiPS retention through synergistic hydrogen-bonding and π-π interactions (Figure 1b). Compared to conventional polyvinylidene fluoride (PVDF)-based binders, the BG-based sulfur cathode demonstrates superior LiPS confinement and cycling stability, as confirmed by ultraviolet–visible (UV-Vis) spectroscopy analysis of the binder–LiPS mixture, X-ray photoelectron spectroscopy (XPS) analysis of Li anode surfaces, and cyclic voltammetry (CV) kinetics studies. The hydrogen-bond-rich 3D network dynamically accommodates sulfur’s volumetric expansion during cycling, preserving long-term electrode integrity, as evidenced by field-emission scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS) mapping. Systematic optimization confirms that a BTU-to-GA mass ratio of 7:3 achieves optimal electrochemical performance by balancing polar site density and structural flexibility. Finally, we assessed the applicability of the BG (7:3) binder under high-sulfur loadings and elevated current densities, exploring its viability for practical Li-S battery applications.

2. Materials and Methods

2.1. Experimental Materials

The 3,9-divinyl-2,4,8,10-tetraoxaspiro-[5.5]-undecane, gallic acid (GA), ammonium persulfate (APS), and sodium hydroxide (NaOH) used in this study were all purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). N-methylpyrrolidone (NMP) and sublimed sulfur (S, purity 99.95%) were provided by Aladdin (Shanghai, China). The Super P conductive agent was supplied by Timcal (Baudieu, Switzerland). The Ketjen black (KB) conductive agent was purchased from Suzhou Jilong Energy Technology Co., Ltd. (Suzhou, China).

2.2. Synthesis of BG (7:3) Binder

First, dissolve 3,9-divinyl-2,4,8,10-tetraoxaspiro-[5.5]-undecane (1 g) and APS (0.05 g) in deionized water (35 g), adjust the solution pH to 5 with NaOH, and react at 60 °C for 48 h under argon protection. The resulting product was freeze-dried and denoted as BTU. Secondly, when preparing the electrode slurry, dissolve BTU and GA in a 7:3 mass ratio in deionized water, stir for 3 min in a homogenizer, and use the resulting solution directly as a binder, denoted as BG (7:3).

2.3. Preparation of Electrodes

To prepare the sulfur cathode, sublimed sulfur and conductive agent (KB) were uniformly mixed in a ratio of 7:3, and then heated at 155 °C for 12 h to obtain the S/KB composite material. Next, a slurry containing a mass ratio of 8:1:1 of S/KB composite, Super P, and binder was cast onto carbon-coated aluminum foil and vacuum-dried at 60 °C for 12 h. The resulting electrodes were punched into 12 mm diameter disks for use. Each cathode contained 1.0–1.2 mg of sulfur per cm². To prepare electrodes with a high active material loading, nickel foam was used as the current collector.

2.4. Characterization

Fourier transform infrared spectroscopy (FTIR-850) was employed to analyze the chemical structure of 3,9-divinyl-2,4,8,10-tetraoxaspiro-[5.5]-undecane before and after hydrolysis. The surface morphology of the sulfur cathode and lithium anode was observed using a field emission scanning electron microscope (SEM, model Nano SEM430) (Product of Carl Zeiss Optics Co., Ltd. Guangzhou, China). The elemental composition of the sulfur cathode was determined using an X-ray photoelectron spectrometer (XPS, product of Thermo Scientific, Massachusetts, USA) with an Al X-ray source. The measurements of Nuclear Magnetic Resonance (NMR) spectra were carried out on a Mercury VX-300 spectrometer (Bruker (Beijing) Scientific Technology Co., Ltd. (Beijing, China)).

2.5. Electrochemical Measurement

Using the Neware CT-4008 battery testing system, constant–current charge–discharge cycling tests were conducted on half-cells, with the test voltage range set at 1.7–2.8 V. CR2025 coin cells were assembled in an argon-filled glove box, using Celgard 2400 as the separator material. The half-cell electrolyte was composed of 1 M LiTFSI dissolved in a 1:1 (volume ratio) mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) solvents, with an additional 1% lithium nitrate (LiNO₃) added as an auxiliary additive. For the electrochemical cycling performance of the low-loading sulfur cathode, the 0.5 C battery was activated under 0.3 C conditions for the first two cycles, the 1 C battery was activated under 0.3 C conditions for the first three cycles, and the 2 C battery was activated under 0.3 C conditions for the first five cycles. For the cycling performance of the high-loading sulfur cathode, the first cycle of the 0.1 C and 0.2 C batteries was activated under 0.03 C conditions. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) tests were both performed using the CHI760E electrochemical workstation. CV tests were conducted within a voltage range of 1.7–2.8 V, at scanning rates of 0.05, 0.10, 0.15, 0.2, and 0.25 mV s⁻¹. The diffusion coefficient of lithium ions (Li⁺) was calculated using the classical Randles–Sevcik equation:

$${\mathrm{I}}_{\mathrm{p}}=(2.69 \times {10}^5{)\mathrm{n}}^{1.5}\mathrm{A}{{\mathrm{D}}_{{\mathrm{Li}}^{+}}}^{0.5}{\mathrm{v}}^{0.5}{\mathrm{C}}_{{\mathrm{Li}}^{+}}$$

(1)

where I_p represents the peak current, ${\mathrm{C}}_{{\mathrm{Li}}^{+}}$ represents the concentration of lithium ions in the electrolyte, A represents the electrode surface area, v represents the scanning rate (V s⁻¹), n represents the number of electrons in the reaction (n = 2), and ${\mathrm{D}}_{{\mathrm{Li}}^{+}}$ represents the diffusion coefficient of lithium ions (cm² s⁻¹). EIS tests were conducted at open-circuit voltage, with a frequency range of 1 MHz to 1 Hz, and an AC voltage amplitude of 10 mV. All tests were conducted at room temperature.

2.6. Preparation of Li₂S₆ Solution

Sulfur powder (S) and lithium sulfide (Li₂S) were added to a solution of 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) in a 1:1 volume ratio, with a molar ratio of 5:1 for S and Li₂S. The mixture was then reacted at 70 °C for 24 h to obtain a lithium polysulfide (LiPS) solution with an average molecular formula of Li₂S₆.

3. Results

3.1. Structure of BG Binder

Using 3,9-divinyl-2,4,8,10-tetraoxaspiro-[5.5]-undecane as the raw material, the BTU binder was synthesized via one-step radical polymerization [43,44]. Subsequently, BTU and GA were mixed in a specific ratio to prepare the BG (7:3) binder. As shown in Figure 2a,b, ¹H NMR spectroscopy confirmed the complete hydrolysis of 3,9-divinyl-2,4,8,10-tetraoxaspiro-[5.5]-undecane, as evidenced by the disappearance of proton peaks corresponding to C=C double bonds at 4.2~5.8 ppm. In the ¹H NMR spectrum of BTU, new proton peaks appeared at 1 ppm, attributed to protons on the terminal carbon of the BTU chain, while a characteristic –CHO proton peak was detected at 9.7 ppm. Additionally, the emergence of an –OH proton peak at 1.7 ppm further supported the successful synthesis of BTU. The appearance of the C=O bond and the disappearance of the C=C bond in Fourier transform infrared (FTIR) spectroscopy further indicated the successful preparation of the BTU binder (Figure 2c). Moreover, a broad stretching vibration peak at 3423 cm⁻¹, attributed to –OH groups from pentaerythritol, confirmed the presence of hydrogen-bonding interactions.

The solubility of GA in water is significantly affected by temperature, and generally, its solubility is lower in cold water. As shown in Figure 3a,b, we added GA (0.01 g) into water (1 mL) with BTU (0.023 g) and without BTU, respectively. It was observed that GA could not completely dissolve at room temperature, appearing as a white opaque suspension. After placing it in a 50 °C oven for 10 min, GA completely dissolved. Subsequently, when placed in a 3 °C environment, GA precipitated. At room temperature, GA dissolved in water containing BTU, forming a transparent liquid. After placing it in a 3 °C environment for 10 min, GA did not precipitate. The proportion of GA was gradually increased by mass. When the mass ratio of GA to BTU reacheds 7:3, partial precipitation of GA occurred at 3 °C, indicating that GA had reached its solubility limit. This phenomenon is attributed to hydrogen-bonding interactions between GA and the BTU network, which prevent GA from precipitating in aqueous solution. These findings confirm the strong interactions between BTU and GA, enabling the formation of an effective binder for Li-S batteries through synergistic hydrogen-bonding interactions.

3.2. Electrochemical Performance

To figure out the optimal BTU-to-GA ratio, electrochemical cycling tests were conducted on BG binders with different compositions. Figure 4a presents the cycling performance of batteries with various binders at 0.5 C. After 150 cycles, the battery with the BG (7:3) binder delivers a reversible capacity of 928.9 mAh g⁻¹, outperforming BG (8:2), BTU, BG (6:4), and PVDF. This enhancement is attributed to the ability of GA, at a 7:3 ratio, to effectively regulate the polar sites of BTU, leading to superior LiPS adsorption and accelerated Li₂S redox reactions. Even at higher current densities, the BG (7:3) binder enables excellent performance, as shown in Figure 4b,c. At 1 C, the battery achieves a reversible capacity of 845.4 mAh g⁻¹ after 150 cycles, while at 2 C, it retains 727.1 mAh g⁻¹, after 200 cycles. In contrast, the PVDF-based battery suffers from rapid capacity decay, reaching only 449.3 mAh g⁻¹ and 455.7 mAh g⁻¹ at 1 C and 2 C, respectively. Figure 4d–f illustrate the typical charge–discharge profiles of Li-S batteries, where BG (7:3) exhibits lower polarization and extended discharge plateaus compared to BTU and PVDF. This suggests enhanced redox kinetics, improved lithium-ion conductivity, and higher active material utilization. Therefore, subsequent analyses primarily focus on the BG (7:3) binder.

As shown in Figure 4g–j, the BG (7:3) battery delivers higher reversible discharge capacities across various current densities (0.1, 0.3, 0.5, 1, and 2 C), reaching 1158.2, 1065.8, 1028.1, 968.8, and 861.7 mAh g⁻¹, respectively, and surpassing both BTU and PVDF. When the current density is returned to 0.1 C, the BG (7:3) battery can still achieve a reversible discharge capacity of 1063.9 mAh g⁻¹. Notably, as the current rate increases, the capacity disparity between BG (7:3) and PVDF widens, signifying faster sulfur redox kinetics in BG (7:3)-based cells.

The practical application of the BG (7:3) binder in high-energy-density Li-S batteries with high-sulfur–mass loadings was further validated. As shown in Figure 4k, at a S loading of 3 mg cm⁻², the battery maintains a capacity of 813.2 mAh g⁻¹ after 100 cycles at a current density of 0.2 C. When the S loading increases to 3.7 and 5.3 mg cm⁻², the capacities remain high at 916.6 and 696.9 mAh g⁻¹, respectively, after 100 cycles at 0.1 C. Figure 4l reveals that the charge–discharge profiles for a S loading of 3 mg cm⁻², remain nearly identical between the 10th and 30th cycles, indicating stable redox kinetics and long-term cycling stability. This superior performance is attributed to the BG (7:3) binder’s strong LiPS adsorption capability and its ability to preserve electrode integrity, even under high-sulfur loading conditions. Additionally, the electrochemical performance of the Li-S batteries in this work was compared with that reported in the relevant literature, as shown as

Table 1. Electrochemical performance of lithium–sulfur batteries with different binders.

Binder	Cathode Material	Sulfur Loading (mg cm−2)	Electrochemical Performance	Ref.
PIL	Super/S	3.6	686.7 mAh g−1 at 0.1 C after 100 cycles	[6]
HMM/PAA	Ketjen black/S	4.2	813.6 mAh g−1 at 0.1 C after 90 cycles	[7]
PVA-BA0.07	Acetylene–carbon black/S	3.5	947 mAh g−1 at 0.2 C after 300 cycles	[10]
PNAVS	Ketjen black/S	1.0	647.8 mAh g−1 at 1 C after 500 cycles	[14]
CPS	Ketjen black/S	8.5	627 mAh g−1 at 0.5 C after 100 cycles	[16]
CSEG	Ketjen black/S	4.1	641.4 mAh g−1 at 0.2 C after 100 cycles	[45]
LiPAA/PVA	Carbon/S	1.2	654.2 mAh g−1 at 0.5 C after 200 cycles	[46]
PLM	Ketjen black/S	1.0	741.1 mAh g−1 at 1 C after 500 cycles	[47]
BG(7:3)	Ketjen black/S	3.0	813.2 mAh g−1 at 0.1 C after 100 cycles	This work
BG(7:3)	Ketjen black/S	1.2	928.9 mAh g−1 at 0.5 C after 150 cycles	This work

.

The lithium-ion (Li⁺) diffusion rate is a critical parameter governing polysulfide (LiPS) conversion kinetics in Li-S batteries [45]. To evaluate this, CV tests were performed at scan rates ranging from 0.05 to 0.25 mV s⁻¹, as shown in Figure 5a–c. The two reduction peaks observed at 2.3 V (C₁) and 2.04 V (C₂) signify the stepwise reduction of S₈ to long-chain lithium polysulfides (Li₂S_x, 4 ≤ x ≤ 6), culminating in the formation of Li₂S₂/Li₂S. Conversely, the oxidation peak at 2.33 V (A) corresponds to the reverse process, starting from the oxidation of Li₂S₂/Li₂S to Li₂S_x and culminating in a reversion to S₈. All cathodes exhibited increasing peak current densities and well-defined redox peaks at higher scan rates, confirming robust electrochemical stability. The linear correlation between peak current (Ip) and the square root of the scan rate (v^0.5) indicates a diffusion-limited redox process. As shown in Figure 5d–f, the BG(7:3)-based battery showed the steepest slope across the three peaks, indicative of superior Li⁺ diffusivity. Using the Randles–Sevcik equation [48], Li⁺ diffusion coefficients (${\mathrm{D}}_{{\mathrm{Li}}^{+}}$) were calculated.

Table 2. List of calculated lithium-ion diffusion coefficients.

Binders	${{\mathbf{D}}_{\mathbf{Li}}}^{{\mathbf{I}}_{\mathbf{p}}\mathbf{A}}$	${{\mathbf{D}}_{\mathbf{Li}}}^{{\mathbf{I}}_{\mathbf{p}}{\mathbf{C}}_1}$	${{\mathbf{D}}_{\mathbf{Li}}}^{{\mathbf{I}}_{\mathbf{p}}{\mathbf{C}}_2}$
BG (7:3)	1.60 × 10−7	3.13 × 10−8	9.74 × 10−8
BTU	8.68 × 10−8	1.88 × 10−8	6.81 × 10−8
PVDF	4.72 × 10−8	1.55 × 10−8	2.36 × 10−8

further illustrated that the characteristic peak values (${\mathrm{D}}_{{\mathrm{Li}}^{+}}$) for the BG(7:3)/S battery surpassed those of the BTU/S and PVDF/S batteries.

The BG (7:3) cathode demonstrated significantly higher ${\mathrm{D}}_{{\mathrm{Li}}^{+}}$ values than those of BTU and PVDF, which we attribute to its enhanced LiPS adsorption capability. This adsorption shortens the electron transfer pathway and mitigates polysulfide shuttling, thereby improving reaction kinetics. Furthermore, the CV curves of the BG (7:3) cathode during initial cycles (Figure 5g–i) showed negligible shifts in peak positions (<50 mV) and retained >95% of their original intensity, underscoring exceptional electrochemical reversibility and cycling stability. In contrast, BTU and PVDF cathodes exhibited progressive polarization and capacity fade, consistent with their lower ${\mathrm{D}}_{{\mathrm{Li}}^{+}}$ values.

Notably, the BG (7:3)-based cathode exhibits a positive shift in reduction peaks and a negative shift in oxidation peaks compared to its BTU and PVDF counterparts, indicating reduced polarization and enhanced reversibility (Figure 6a). The BG (7:3)-based cathode exhibits a negative shift in peak as compared to BTU and PVDF counterparts, which we attribute to its better ionic conductivity. In addition, its network structure effectively enables a compact electrode structure, which shortens the electron transfer pathway, thereby improving reaction kinetics. The strong polysulfides absorbability of BG (7:3) leads to a much suppressed shuttling effect, resulting in a higher response current and larger integrated area in the CV profile. Furthermore, the BG (7:3) cathode shows sharper and more intense redox peaks, reflecting faster reaction kinetics and improved lithium-ion diffusion. This is further corroborated by Tafel analysis (Figure 6b–d), where the BG (7:3) binder demonstrates significantly lower polarization slopes at critical peaks: (1) peak C₁ (Li₂S_n to Li₂S/Li₂S): 45.8 mV dec⁻¹, (2) peak C₂ (S₈ to Li₂S_n): 21.1 mV dec⁻¹, and (3) peak A (Li₂S to S₈): 88.26 mV dec⁻¹. These values are markedly lower than those of BTU (51.57, 41.61, 109.37 mV dec⁻¹) and PVDF (51.92, 43.67, 126.62 mV dec⁻¹), confirming the superior conversion kinetics enabled by the BG (7:3) binder. The reduced Tafel slopes align with its enhanced polysulfide adsorption capability and efficient charge transfer, which collectively suppress the shuttle effect and stabilize the sulfur cathode.

3.3. Absorbability of Binder to Polysulfide Lithium

To further elucidate the superior adsorption capability of the BG (7:3) binder toward lithium polysulfides (LiPSs) and its ability to maintain cathode integrity, in situ electrochemical impedance spectroscopy (EIS) and UV-Vis spectroscopy comparisons were conducted, along with a post-cycling cathode morphology analysis. As shown in Figure 7a, in situ EIS measurements were performed on Li-S batteries with the BG (7:3) binder during a 0.1 C charge–discharge process. The Nyquist plots consist of three components: electrolyte resistance (R₀), interface resistance (R_S), and Warburg impedance (W₀). During the discharge process, a slight increase in the high-frequency semicircle was observed, attributed to electrolyte viscosity changes caused by partial LiPS dissolution. As the discharge depth increases, R₀ decreases due to LiPS oxidation into insoluble Li₂S, accompanied by a reduction in the semicircle diameter. Simultaneously, a new semicircle emerged at lower frequencies, reflecting an increase in Rs due to Li₂S deposition. These changes were fully reversible during charging, demonstrating the BG (7:3) binder’s efficient LiPS adsorption and rapid redox kinetics. In contrast, PVDF-based batteries exhibited irreversible increases in R₀ and R_S during cycling (Figure 7b), indicative of poor LiPS confinement and sluggish reaction dynamics, likely due to extended charge transfer paths and higher energy barriers.

The LiPS absorbability of the BG (7:3) binder was evaluated using UV-Vis spectroscopy (Figure 7c). After 8 h of static adsorption with the Super P-BG (7:3) mixture, the LiPS solution turned a faint yellow color, whereas the Super P-BTU-containing solution remained a darker yellow, and the Super P-PVDF-containing solution showed no visible change. The BG (7:3) sample also displayed attenuated absorbance near 400 nm, characteristic of LiPS, further verifying its strong chemisorption capability via polar functional groups. Further investigation of the Li anode surface composition was conducted via XPS characterization (Figure 7d). The S 2p spectrum of the BG (7:3)-based Li anode displayed four peaks at 162.0, 163.4, 167.1, and 169.1 eV, which can be attributed to polysulfides, sulfates, and thiosulfates, respectively. Compared to BTU and PVDF binders, the BG (7:3) binder showed significantly weaker sulfate/thiosulfate peaks, confirming its ability to suppress LiPS shuttling.

Furthermore, the PVDF-based batteries exhibited rapid capacity decay during cycling. Post-cycling SEM and EDS analyses of the cathodes revealed the underlying cause. As shown in Figure 7e, the PVDF-based cathode surface displayed severe protrusions and cracks after 100 cycles, resulting from active material disconnection during volume changes. This led to rapid capacity fade and poor cycling stability. In contrast, the BG (7:3) binder, with its hydrogen-bond-rich structure, effectively buffered sulfur volume expansion, preserving cathode integrity without significant structural damage. EDS mapping (Figure 7e–h) confirmed a more uniform sulfur distribution and higher sulfur content in the BG (7:3) cathode, attributed to polar site-mediated LiPS adsorption and shuttle effect suppression.

4. Conclusions

In summary, we developed an environmentally friendly BG (7:3) binder, which greatly improved the electrochemical performance and cycling stability of Li-S batteries compared to conventional binders. By tuning the polarity sites and polymer network structure through the synergistic integration of BTU and GA, the BG (7:3) binder effectively mitigates sulfur volume expansion-induced cathode cracking while optimizing lithium polysulfide (LiPS) adsorption and redox kinetics, as evidenced by post-cycling SEM morphology, CV measurements, and XPS analyses. These advancements give Li-S batteries exceptional cycling stability and capacity retention. Specifically, the BG (7:3)-based battery delivers a reversible capacity of 727.1 mAh g⁻¹ after 200 cycles at 2 C, and even under a sulfur loading of 5.3 mg cm⁻², it retains 890 mAh g⁻¹ after 50 cycles at 0.1 C. This work not only addresses critical challenges in Li-S battery commercialization but also underscores the pivotal role of sustainable material design in advancing next-generation energy storage technologies.