Structure‐Tunable Fluorinated Polyester Electrolytes with Enhanced Interfacial Stability for Recyclable Solid‐State Lithium Metal Batteries

Abstract

Polyesters, with their tunable chemical structures and environmental sustainability, have drawn growing attention as solid polymer electrolytes for next‐generation solid‐state lithium metal batteries (SSLMBs). Through a comprehensive experimental and theoretical study involving the systematic variation of carbon chain lengths in the flexible (diol) and coordinating (diacid) segments, along with selective fluorination at distinct positions along the polymer backbone, 18 types of polyester are fabricated and demonstrate that fluorination at the coordinating segment significantly enhances ionic conductivity by suppressing crystallinity. In contrast, fluorination at the flexible segment reduces ionic migration barriers by providing more homogeneous coordinating sites, thereby improving the lithium‐ion transference number, despite increasing chain rigidity and a reduction in overall ionic conductivity. The optimized fluorinated polyesters exhibit excellent interfacial stability with lithium metal by facilitating the formation of a LiF‐rich solid electrolyte interphase, as supported by experimental and theoretical simulation analysis. More importantly, to enhance cost‐effectiveness and sustainability, a solvent‐based recycling route has been developed, achieving substantial recovery yields and notable environmental benefits. This work provides a versatile molecular design and closed‐loop recovery strategy for fluorinated polyester electrolytes, advancing their practical application in high‐energy and sustainable SSLMBs.

Fluorinated polyesters are shown to regulate lithium‐ion coordination and promote stable electrode–electrolyte interfaces. The rational structural design enables uniform lithium deposition, suppressed dendrite growth, and enhanced cycling performance in lithium‐metal batteries. This work highlights the importance of molecular engineering in developing high‐performance and sustainable electrolytes.

1. Introduction

Solid‐state polymer electrolytes are widely recognized as promising candidates for large‐scale implementation in next‐generation solid‐state lithium metal batteries (SSLMBs), owing to their intrinsic advantages, including high flexibility, low interfacial resistance, lightweight nature, cost‐effectiveness, and facile processability.^{[ 1 ]} Poly(ethylene oxide) (PEO) is widely recognized for use as a polymer electrolyte due to its good ability to coordinate Li⁺ via the ether group (C─O─C), enabling ionic conduction in polymer electrolytes.^{[ 2 ]} However, its high crystallinity, low ionic conductivity (<10⁻⁶ S cm⁻¹), limited Li⁺ transference number (≈0.2), and narrow electrochemical stability window (ESW) (≈3.6 V) have increasingly limited its application in high‐energy SSLMBs.^{[ 3 ]} In contrast, polyesters, with their tunable backbone structures and environmental benignity, have emerged as promising alternatives, offering improved ionic transport properties and better electrochemical stability for next‐generation solid polymer electrolytes.^{[ 4 ]}

In polyester‐based electrolytes, Li⁺ coordination primarily occurs through carbonyl (C═O) and C─O─C, with the former providing stronger electron donation. The structural tunability of polyesters offers a distinct advantage, as variations in molecular structure lead to diverse electrochemical properties, thereby garnering significant research interest. Prior studies suggest three main strategies for enhancing electrochemical performance. First, shortening the length of the diacid unit improves the electron supply capacity of the carbonyl oxygen and increases the dissociation of the lithium salt.^{[ 5 ]} Second, increasing the length of the diol unit enhances chain flexibility and promotes the migration rate of Li⁺ within the polymer.^{[ 5} , ^{6 ]} Third, fluorinating the end groups of the polyester effectively improves the interfacial compatibility between the polymer and the lithium metal, thereby enhancing electrochemical stability.^{[ 7 ]} Despite these efforts, room‐temperature ionic conductivities remain below the critical 10⁻⁴ S cm⁻¹ threshold.^{[ 8 ]} This gap limits the practical deployment of polyester electrolytes in high‐energy SSLMBs. Considering the prominent role of fluorinated compounds in lithium‐ion batteries (LIBs),^{[ 9 ]} we hypothesize that fluorination of the polyester backbone may also serve as an effective strategy for enhancing performance. Incorporating fluorine atoms into the main chain is expected to modulate the local polarity and Li⁺ coordination environment, thereby synergistically improving salt dissociation, ion transport efficiency, and interfacial compatibility.

Herein, we successfully prepared 18 types of polyesters through the strategic manipulation of carbon chain lengths in the flexible (diol) segment and the coordinating (diacid) segment (Figure 1a). These polyesters can be designated as Cx‐Cy, where x and y represent the carbon number in the polymerized monomer, with Cx denoting the flexible segment and Cy the coordinating segment. Polyesters with fluorinated flexible segments and fluorinated coordinating segments are defined as CxF‐Cy and Cx‐CyF, respectively. An in‐depth investigation was conducted to elucidate how the molecular structures of these polyesters influence their electrochemical properties and the underlying mechanisms. The finding revealed that fluorination of coordinating segments can lead to a reduction in the crystallinity of polyesters (Figure 1b), and the coordinating structure and transport mode of Li⁺ are influenced by the distinct polyester structures (Figure 1c). Additionally, the fluorine atoms in the fluorinated polyester can react with lithium atoms on the surface of the lithium metal anode (LMA), forming a LiF‐rich solid‐state electrolyte interface (SEI) during the charging and discharging processes (Figure 1d), thereby enhancing cycle stability. Finally, given the high cost of fluorinated polyester and the environmental risks associated with the direct pyrolysis of spent LIBs during recycling, a strategic recovery approach was introduced for fluorinated polyester electrolytes, as shown in Figure 1e. This work offers novel insights into the development of high‐performance, environmentally friendly, and sustainable polyester electrolytes.

Figure 1.

Project design and research strategy. a) Structural design of different polyesters. b) Effect of fluorinated polyester on crystallinity. c) Different Li⁺ coordinating structures and transport mechanisms induced by different structural polyesters. d) Effect of fluorinated polyester on SEI and lithium dendrite formation. e) Recycling route of fluorinated polyester electrolyte.

2. Results and Discussion

2.1. Influence of Polyester Structure on Physicochemical Properties

By systematically tuning the carbon chain length of diol and diacid monomers and introducing fluorinated groups into specific polymer segments, 18 types of polyesters were successfully synthesized via a condensation polymerization approach, as illustrated in Figures 2a and S1 (Supporting Information). The ¹H and ¹⁹F nuclear magnetic resonance (NMR) spectra of these polyesters are presented in Figures S2 and S3 (Supporting Information), and their molecular weights were calculated based on the ¹H NMR results, ranging from 1294 to 6190 g mol⁻¹ (Figure S4, Supporting Information). Since Li⁺ primarily coordinates with the C═O groups derived from the diacid units, the diacid segment is herein referred to as the coordinating segment. Meanwhile, increasing the carbon chain length (i.e., the number of ─CH₂─ groups) in the diol units enhances the flexibility of the polymer backbone by facilitating adjustments in chain conformation. Thus, the diol segment is referred to as the flexible segment in this context. The transport of Li⁺ in polymer electrolytes is governed by a combination of Li⁺ coordination/decoordination with polar groups and the segment motion of the polymer chains.^{[ 1c ]} Therefore, both the coordinating and flexible segments play essential roles in determining ion transport behavior. Based on this understanding, a systematic investigation was conducted to explore how the structures of these 18 polyesters influence their electrochemical properties.

Figure 2.

Influence of polyester structure on physicochemical properties. a) Molecular structures of different polyesters. b) Melting temperature and glass transition temperature. c) Electrostatic potential distribution. d) HOMO–LUMO energy levels.

As ion transport in polymer electrolytes predominantly occurs in the amorphous phase of the polymer, the degree of crystallinity of the polyester plays a critical role.^{[ 1c ]} Figures S5 and S6 (Supporting Information) display the melting temperatures of all designed polyesters. Fluorination of the coordinating segment markedly alters the physical state of the polyesters. C6‐C4F, C8‐C4F, C4‐C5F, and C4‐C6F become viscous melts at room temperature, unlike their solid, non‐fluorinated counterparts (C6‐C4, C8‐C4, C4‐C5, and C4‐C6). This indicates a complete suppression of crystallinity due to the strong electronegativity and steric hindrance of the fluorinated coordinating segments, which disrupt chain packing. Figure 2b and Figure S7 (Supporting Information) show the crystallization temperatures of different polyesters. All of the polyesters with fluorinated coordinating segments exhibited extremely slow crystallization kinetics or no crystallization upon cooling from the melt, confirming that fluorination effectively inhibits polyester crystallization. In contrast, polyesters with fluorinated flexible segments exhibit an even‐odd effect in crystallization behavior, wherein those containing an even number of carbon atoms demonstrate faster crystallization rates. The “even–odd effect” refers to the systematic variation in physicochemical properties (such as crystallinity, melting point) depending on whether the number of methylene units in the polymer chain is even or odd. This effect originates from differences in chain packing symmetry: even‐numbered chains typically adopt more favorable packing arrangements, leading to higher crystallinity, whereas odd‐numbered chains disrupt packing, reducing crystallinity.^{[ 10 ]}

Furthermore, even in the non‐fluorinated polyester, the C4‐C3 also exhibits a fully amorphous structure at room temperature. This phenomenon can also be primarily attributed to the even‐odd effect.^{[ 10 ]} Figure 2b and Figure S8 (Supporting Information) present the glass transition temperatures (T _g) of the 18 polyesters. The polyesters containing fluorinated coordinating segments exhibit an extremely low T _gs, indicating that the polymer chains possess high segmental mobility. In contrast, the non‐fluorinated polyesters, due to the rapid crystallization, do not display a distinct T _g behavior during in the DSC analysis, suggesting restricted chain mobility. Furthermore, the polyesters with an odd‐carbon‐number fluorinated flexible segments, which exhibit slower crystallization rates, also show a lower T _gs, reflecting enhanced segmental dynamics. These results collectively demonstrate that fluorination and parity control in specific chain segments not only regulate crystallization behavior but also significantly influence the thermal and dynamic properties of the resulted polyesters. These structural and thermal modifications have a profound impact on the ionic conductivity of polyester‐based electrolytes, as enhanced segmental mobility and suppressed crystallinity are key factors in facilitating efficient ion transport.

Moreover, the structural variations of the polyesters also exert a profound influence on their electrostatic potential (ESP) distributions, which in turn modulate the coordination environments and transport behaviors of Li⁺. As illustrated in Figure 2c, when maintaining consistent lengths for both the flexible and coordinating segments, the introduction of fluorine atoms into the coordinating segment results in a pronounced reduction in the ESP values localized in that region. This indicates an enhanced electron‐donating capability of the coordinating sites, which may strengthen their affinity toward Li⁺. Despite fluorination, it is noteworthy that the regions with the most negative ESP values remain associated with the C═O groups, highlighting their most significant potential to coordinate with Li⁺ binding. In contrast, fluorination at the flexible segment results in a more evenly distributed negative ESP across the segment, which could facilitate a smoother electrostatic environment for Li⁺ migration. These distinct ESP distributions arising from different fluorination strategies suggest that the site‐specific introduction of electron‐donating groups can be leveraged to finely tune the balance between Li⁺ coordination strength and segmental mobility. Consequently, such ESP modulation provides a molecular‐level insight into how polyester structural design would govern ion transport mechanisms, which is critical for optimizing their performance in SSLMBs.

The redox stability of polyesters is directly linked to the ESW of the corresponding polyester electrolytes. During battery charging, the rising cathode potential can extract electrons from the electrolyte if its highest occupied molecular orbital (HOMO) level is too high, causing oxidative decomposition. Conversely, during discharge or with LMA (close to or below 0 V), a low‐lying lowest unoccupied molecular orbital (LUMO) can result in undesired electron uptake and reductive degradation.^{[ 11 ]} Therefore, the HOMO and LUMO levels of the 18 types of polyesters were analyzed systematically (Figure 2d). Increasing the carbon chain length of the flexible segment elevates both HOMO and LUMO energies, while elongation of the coordinating segment increases the HOMO but decreases the LUMO level. Accordingly, for improved redox stability, the coordinating segment should be kept as short as possible. Fluorination serves as a powerful tool for tuning the frontier orbital energies: the strong electronegativity of fluorine lowers both the HOMO and LUMO levels, enhancing oxidative stability at the cost of reduced reductive stability. This implies that fluorinated polyesters are more suitable for high‐voltage cathode applications and may also participate in SEI formation. Further analysis reveals that for polyesters with identical fluorine content and chain lengths, fluorination on the coordinating segment results in better redox stability compared to fluorination on the flexible segment, indicating the positional distribution of fluorine plays a critical role in electronic structure modulation.

Additionally, increasing the fluorine content in either segment results in a decreasing trend in LUMO energy, indicating reduced reductive stability. However, the HOMO level exhibits a structure‐dependent behavior. Increasing the number of fluorine atoms in the coordinating segment slightly elevates the HOMO level, thereby weakening the oxidative stability of the compound. In contrast, increasing fluorination in the flexible segment continues to lower the HOMO, resulting in enhanced oxidative robustness.

2.2. Electrochemical Properties of Polyester Electrolytes

Ionic conductivity, t_Li ⁺, and ESW are three fundamental electrochemical parameters for polymer electrolytes, and modifications in polyester structures can lead to significant variations in these properties. As shown in Figures 3a and S9 (Supporting Information), the linear sweep voltammetry (LSV) curves of different polyester electrolytes were recorded to assess their ESW. The results indicate that all non‐fluorinated polyester‐based electrolytes exhibit decomposition voltages exceeding 4.0 V versus Li/Li⁺, which is significantly higher than the ≈3.6 V ESW typically observed for conventional PEO‐based electrolytes.^{[ 3b ]} This suggests that the polyester systems possess superior electrochemical compatibility under high‐voltage cathode conditions. Further analysis of the oxidative stability of different polyester structures reveals that the incorporation of fluorinated functional groups markedly enhances electrochemical stability, as evidenced by oxidation onset voltages exceeding 4.5 V. This trend is consistent with the decrease in HOMO energy levels predicted by density functional theory (DFT) calculations, suggesting that fluorination reduces the likelihood of electron transfer from the polymer backbone, thereby improving oxidative stability. These findings validate the feasibility and effectiveness of fluorination strategies for enhancing the electrochemical performance of polymer electrolytes.

Figure 3.

Electrochemical performance of polyester electrolytes. Summary of a) ESW, b) ionic conductivities, and c) t_Li ⁺ of different polyester electrolytes.

Figure 3b and Figure S10 (Supporting Information) present the ionic conductivity of different polyesters, revealing several key trends. Firstly, when the coordinating segment monomer is succinic acid, an increase in the carbon chain length of the flexible segment leads to a rise in the ionic conductivity of the polyester electrolyte. This is attributed to the enhanced flexibility of the polyester main chain, which reduces spatial hindrance of chain segments’ motion and facilitates the migration of Li⁺ within the polymer matrix. This result suggests that extending the chain length of flexible segments can effectively modulate the dynamic properties of the polymer, thereby enhancing its ionic conductivity. Notably, when the flexible segment is fixed as butanediol, the ionic conductivity of the polyester electrolytes displays a non‐monotonic trend with increasing carbon number of the coordinating segment, showing an inflection at the C4‐C4 composition (Figure 3b). This phenomenon can be rationalized by the odd–even effect commonly observed in aliphatic polyesters.^{[ 10 ]} Polyesters with even–even combinations of carbon atoms in both the diol and diacid segments (e.g., C4‐C4) tend to pack more regularly due to enhanced symmetry, thereby facilitating crystallization.^{[ 12 ]} This higher crystallinity restricts polymer segmental motion and diminishes the number of amorphous pathways available for Li⁺ transport, leading to reduced ionic conductivity. In contrast, odd–even or even–odd carbon combinations (e.g., C4‐C3 or C4‐C5) introduce irregularities in chain packing, suppress crystallization, and promote segmental mobility, thereby enhancing ionic conductivity. Furthermore, when the coordinating segment monomer is switched to TFSA, the ionic conductivity is further improved. This can be attributed to the increased number of amorphous regions in the fluorinated polyesters, providing more free chain segments that can participate in Li⁺ transport. In addition, the introduction of fluorine atoms into the coordinating segments may also influence the coordination behavior and transport of Li⁺. However, it is noteworthy that not all fluorinated coordinating segments enhance ionic conductivity. For example, although C4‐C5F and C4‐C6F are fully amorphous at room temperature, their ionic conductivity is lower than that of C4‐C5 and C4‐C6. This may be due to the increased rigidity of the fluorinated polyester coordinating segments as their carbon length increases. The introduction of fluorine atoms reduces the flexibility of these segments, thereby limiting their free motion and, consequently, the migration of Li⁺ in the fluorinated polyester. This suggests a balance between polarity and flexibility.

Additionally, this phenomenon is also observed in polyester with fluorinated flexible segments, where an increase in the length of the fluorinated flexible segment leads to a decrease in ionic conductivity. Therefore, the flexibility of the molecular chain plays a crucial role in ion transport within polyester electrolytes, and it is sometimes even more critical than crystallinity. Interestingly, when comparing the ionic conductivity of different fluorination sites under the condition of the same carbon chain length, it was found that polyesters with fluorinated coordinating segments generally exhibited higher ionic conductivity than those with fluorinated flexible segments. This phenomenon is likely due to the local polarity environment provided by the fluorinated coordination segment structure, which is more favorable for lithium salt dissociation, leading to higher ionic migration efficiency.^{[ 13 ]} Therefore, the fine‐tuning of the flexible and coordinating segment structures in polyesters, particularly optimizing the carbon chain length and fluorination sites, plays a significant role in enhancing the ionic conductivity of polyester electrolytes.

To further investigate the impact of polyester structure on Li⁺ migration behavior, this study systematically analyzes the regulation of t_Li ⁺ by different structural parameters, as shown in Figure 3c and Figure S11 (Supporting Information). The results indicate that as the length of the flexible and coordinating segments increases, the t_Li ⁺ of the system gradually decreases. This phenomenon is likely due to the introduction of longer carbon chains, which enhances the flexibility of the polymer chains, making the movement of solvated Li⁺ more reliant on the cooperative migration of the polymer backbone. This results in an increased migration of anions, thereby reducing the t_Li ⁺.^{[ 14 ]} In addition, the introduction of fluorine atoms enhances the polarity and solubility of lithium salts in the system. However, excessive fluorine content significantly increases the rigidity and polarity of the polymer chains. This may cause the over‐accumulation of Li⁺ at the polar sites, further reducing the t_Li ⁺, as shown in Figure S12 (Supporting Information).

Experimental results confirm that with the increase of fluorine content in the polyester, the t_Li ⁺ exhibits a noticeable decreasing trend. It is noteworthy that the position of fluorination also significantly influences the t_Li ⁺ value. When fluorine atoms are introduced into the flexible segment, they exhibit a relatively higher t_Li ⁺ compared to those incorporated into the coordinating segment. This is because the fluorination of the flexible segment helps introduce polar centers, which facilitates the formation of stable coordinating structures with Li⁺ at the center in the flexible segment, thereby enhancing t_Li ⁺. On the other hand, fluorination of the coordinating segment increases the number of coordinating sites. Furthermore, it enhances the attraction of the coordinating segment to Li⁺, leading to an excessive accumulation of Li⁺ at these sites. This prevents the easy dissociation of Li⁺ to the following polar site, thus reducing the migration ratio of Li⁺.^{[ 3} , ^{15 ]} In conclusion, the t_Li ⁺ is subject to dual regulation by the length of the carbon chains and the positions of fluorination in the polyester structure. Rational design of segmental structure and fluorination strategy is crucial for achieving high t_Li ⁺ in polymer electrolytes.

2.3. Influence of Polyester Structure on Li⁺ Transport

Polyesters with varying structures not only modulate the coordinating interactions between Li⁺ and the polymer backbone at the molecular level but also exert a significant influence on the solvation structure and transport behavior of Li⁺. Therefore, a systematic understanding of the relationship between polyester structure and Li⁺ transport properties is crucial for the molecular design and performance optimization of polyester electrolytes. DFT calculations were further performed for electrolyte systems with different polyester structures to assist in verifying the above explanations for the variations in ionic conductivity and t_Li ⁺.

To elucidate how the polyester structure governs the local chemical environment of Li⁺, molecular dynamics (MD) simulations was first conducted to investigate the solvation structures of Li⁺ in polyesters with different structures. As illustrated in Figure 4a–c and Figures S13–S17 (Supporting Information), the radial distribution functions (RDFs) and corresponding coordination numbers (CNs) between Li⁺ and the main coordinating atoms in the polyesters, including carbonyl oxygen, ether oxygen, and fluorine, were analyzed. The simulation results show that, whether in non‐fluorinated polyesters or fluorinated polyesters, the carbonyl oxygen is the main coordination atom of Li⁺. Taking C4‐C4 and C4‐C4F as representative examples (Figure 4d,e), the coordination peak of Li⁺ with C═O appears at the first peak position of ≈2.1 Å of the RDF of Li─O, confirming that carbonyl oxygen is the nearest and most strongly coordinating donor atom for Li⁺, suggesting that fluorine atoms do not participate directly in the first solvation shell of Li⁺. Despite the absence of direct Li─F interactions in the first coordination shell, fluorination still indirectly impacts the Li─O coordination behavior. Statistical analysis reveals a slight decrease in the average oxygen coordination number (CN) of Li⁺ with increasing fluorine content. This reduction can be attributed to the strong electron‐withdrawing nature of fluorine, which decreases the electron density on the carbonyl oxygen, thereby weakening its electrostatic interaction with Li⁺.^{[ 15} , ^{16 ]}

Figure 4.

Influence of polyester structure on Li⁺ transport. Li─O RDF and CN of a) non‐fluorinated polyester electrolyte and b) fluorinated polyester electrolyte. c) Li─F RDF and CN of fluorinated polyester electrolyte. Diagram of Li coordination environment in d) C4‐C4 and e) C4‐C4F polyester electrolyte. f) The binding energy of C═O and Li⁺ of different polyesters. g) Diagram of Li⁺ transport and Li⁺ migration energy barrier in polyester with different fluorination sites. h) Transference energy barriers of Li⁺ between different fluorinated polyester chains.

To further elucidate the influence of polyester structure on Li⁺ coordination behavior beyond the structural and solvation characteristics, quantum chemical calculations was performed to quantify the coordination energies between Li⁺ and various polyester segments. As shown in Figure 4f, the coordination structures and corresponding coordination energies between Li⁺ and polyesters with different segmental structures were systematically analyzed. The results reveal that increasing the flexible segment length slightly enhances Li⁺–carbonyl coordination, which is due to conformational relaxation that mitigates electronic perturbation and structural strain on the carbonyl group, thereby strengthening their electrostatic interactions with Li⁺.^{[ 17 ]} Fluorination of coordinating segments generally weakens Li⁺–carbonyl coordination due to the electron‐withdrawing effect of fluorine, which lowers the electron density on carbonyl oxygens and reduces their Coulombic attraction to Li⁺.^{[ 18 ]} However, in TFSA‐based units, fluorine unexpectedly induces alignment of two carbonyls on the same side via conformational restriction and dipole‐driven folding, forming a favorable conformation that enhances Li⁺ coordination.^{[ 19 ]}

Further analysis reveals that coordination energy decreases with increasing methylene units in the coordinating segment, due to larger carbonyl separation. However, this trend shows an odd–even oscillation: odd‐carbon chains adopt cis‐like conformations with co‐aligned carbonyls, enhancing cooperative coordination with Li⁺. In contrast, even‐carbon chains form trans‐like conformations, weakening synergistic binding. Fourier transform infrared spectroscopy (Figures S18–S20, Supporting Information) confirms this effect, showing stronger C═O─Li⁺ interactions in odd‐carbon systems. Notably, C4‐C3 and C4‐C3F polyesters form stable six‐membered chelates with Li⁺, boosting local binding energy and suppressing Li⁺ decoordination, thus reducing t_Li ⁺. Beyond coordination, the odd–even effect also affects polyester crystallinity, as mentioned before. These findings highlight the structural sensitivity of ion transport to chain parity, underscoring the importance of precise monomer design for high‐performance polyester electrolytes.

Moreover, introducing fluorine atoms into the flexible segments leads to a consistent decrease in Li⁺–carbonyl coordination strength with increasing fluorine content. This results from a combination of electron density redistribution and enhanced chain rigidity caused by fluorination, which together reduce the polyester backbone's ability to form effective coordination environments around Li⁺.^{[ 20 ]} Comparative analysis reveals that, at equal chain lengths, fluorination at the coordinating segment results in a more pronounced reduction in Li⁺ binding compared to the flexible segment, underscoring the critical role of local electronic structure and chemical environment in Li⁺ solvation and transport.

To clarify the fluoridation impact on Li⁺ transport, comparisons were made among the inter‐segmental migration energy barriers under identical carbon chain lengths and fluorine content. Figure 4g shows the model of Li⁺ transport between different fluorinated polyester chains, and Figure 4h shows its corresponding Li⁺ transference energy barrier. Simulations indicate that fluorination at the coordinating segment results in higher energy barriers, as Li⁺ must traverse non‐coordinating, flexible regions lacking polar groups. In contrast, fluorination of the flexible segment lowers migration barriers by introducing weak but favorable electrostatic interactions that transiently stabilize Li⁺ during migration. Additionally, fluorine's inductive effect enhances local dipole moments, improving electrostatic guidance. This also induces a conformational shift toward a more rigid and aligned backbone, facilitating more uniform Li⁺ pathways.^{[ 21 ]} This effect is most notable in systems with short‐chain fluorinated coordinating segments, where non‐uniform coordination site spacing imposes higher migration barriers. Under such conditions, fluorinated flexible segments act as auxiliary facilitators, reducing energy barriers by stabilizing Li⁺ during transitions. However, as the length of the coordinating segment increases, the spacing between carbonyl groups becomes more uniform, forming continuous coordination channels. In these systems, the added benefit of flexible‐segment fluorination diminishes as primary coordination alone supports efficient Li⁺ transport.^{[ 3} , ¹⁵ , ^{22 ]} Experimentally, polyesters with fluorinated flexible segments exhibit higher t_Li ⁺ but lower total ionic conductivity. This trade‐off arises because increased rigidity from fluorination suppresses chain dynamics and bulk ion mobility, despite enhanced local polarity and Li⁺ selectivity.^{[ 23 ]}

2.4. Battery Performance of Polyester Electrolytes

Since C4‐C4F electrolyte maintains solid‐state form while exhibiting good electrochemical properties—such as an ionic conductivity of 1.15 × 10⁻⁴ S cm⁻¹, an ESW of 4.75 V, and a Li⁺ transference number of 0.63—it was selected as a representative fluorinated polyester electrolyte. To further validate the effectiveness of the fluorination strategy under practical battery conditions, LMBs were assembled using C4‐C4F, and their performance was systematically evaluated. Given that polymer electrolyte systems typically suffer from low ionic conductivity at room temperature compared to liquid electrolytes, many polymer‐based battery performance tests are often conducted at elevated temperatures.^{[ 24 ]} Based on this, the ionic conductivities of C4‐C4F and C4‐C4 electrolytes at different temperatures were tested, as shown in Figure S22 (Supporting Information). The temperature dependence of ionic conductivity follows the Arrhenius equation. The calculated activation energy for ion transport in C4‐C4F electrolyte is 0.39 eV, significantly lower than that of C4‐C4, which reaches 0.67 eV. This result indicates that fluorination of the coordinating segment effectively reduces the energy barrier for ion migration.

Following this, the charge–discharge behavior of the C4‐C4F and C4‐C4 electrolytes at various temperatures was further investigated with LiFePO₄ (LFP)||Li batteries during a voltage testing range of 2.5–4.2 V. Figure S23 (Supporting Information) shows the charge–discharge curves of the C4‐C4F‐based battery during the second cycle at different temperatures. The LFP||Li battery constructed with C4‐C4F electrolyte exhibited a discharge capacity of 124.2 mAh g⁻¹ at 0.1 and 20 °C, with a relatively high polarization voltage of ≈0.4 V during the charge‐discharge process. Upon increasing the temperature to 30 °C, the discharge capacity increased to 146.3 mAh g⁻¹, and the polarization voltage significantly decreased to ≈0.2 V. Further increasing the temperature to 40 °C resulted in a discharge capacity of 150.3 mAh g⁻¹, and the polarization voltage was further reduced to ≈0.1 V. In contrast, as shown in Figure S24 (Supporting Information), the C4‐C4 electrolyte exhibits no charge/discharge capability at 20 °C and delivers a very low discharge specific capacity along with a high polarization voltage (≈0.9 V) at 30 °C. Although the polarization voltage decreases to ≈0.2 V at 40 °C, the discharge specific capacity remains as low as 99.7 mAh g⁻¹ at 0.1 C, and lost performance after 15 cycles (Figure S25, Supporting Information), which is still insufficient to meet the performance requirements of modern LIBs. In comparison, the C4‐C4F electrolyte demonstrates stable Li⁺ transport and electrochemical reactions at moderate temperatures (20–40 °C), providing a clear performance advantage over typical polymer systems that require high testing temperature.

Given the high discharge capacity exhibited by C4‐C4F electrolyte at 30 °C, the rate and cycle performance of C4‐C4F at 30 °C was further tested. As shown in Figure 5a, this system achieved discharge capacities of 146.3 and 108.0 mAh g⁻¹ at 0.1 and 0.5 C rates, respectively. After 30 cycles at varying rates, the discharge capacity returned to 136.0 mAh g⁻¹ at 0.1 C, maintaining 93.0% of its initial capacity and indicating excellent rate adaptability. Subsequently, a 200‐cycle long‐cycle test was performed at 0.2 C, showing an initial discharge capacity of 131.4 mAh g⁻¹ (Figure 5b). It remained at 89.6% of its capacity after 200 cycles, demonstrating the superior cycling stability of the C4‐C4F electrolyte at lower to moderate temperatures. It is worth noting that the polarization voltage at 30 °C has not been fully optimized and still impacts capacity retention, which is likely related to interface impedance and limitations in Li⁺ migration. To further support this view, electrochemical impedance spectroscopy measurements were conducted on Li||Li symmetric cells at 20, 30, and 40 °C (Figure S26, Supporting Information). The results reveal a significant decrease in interfacial resistance with increasing temperature, indicating that the relatively high polarization voltage observed at 30 °C is indeed associated with elevated interfacial impedance. This trend is consistent with the temperature‐dependent polarization behavior observed in the charge/discharge curves (Figure S23, Supporting Information). To further assess its high‐rate performance, the battery was tested at 40 °C for both rate and long‐cycle performance. As shown in Figure 5c, the battery achieved an initial capacity of 152.3 mAh g⁻¹ at 0.1 C, and maintained 68.1 mAh g⁻¹ at a high rate of 2C, exhibiting excellent high‐rate charge–discharge capability. After 30 cycles at various rates, the capacity returned to 152.9 mAh g⁻¹ at 0.1 C with no degradation, which represents a noticeable improvement over the performance at 30 °C. Furthermore, the 200‐cycle long‐term test at 40 °C and 0.5 C showed an initial capacity of 134.4 mAh g⁻¹ and a final capacity retention of 87.0% (Figure 5d). It should be noted that while high temperature can enhance ionic conductivity, it may also increase the polarization of the electrode during charge and discharge, leading to a faster capacity fade.^{[ 25 ]}

Figure 5.

Battery performance of fluorinated polyester electrolyte. a) Rate performance and b) cycle performance at 0.2 C of C4‐C4F electrolyte battery at 30 °C. c) Rate performance and d) cycle performance at 0.5 C of C4‐C4F electrolyte battery at 40 °C. e) Cycling performance of Li||Li symmetric battery with C4‐C4F electrolyte.

Considering the key challenges faced by LMBs in practical applications, such as lithium dendrite growth, interface instability, and uneven deposition/stripping behavior, evaluating the performance of polyester electrolytes in lithium metal symmetric cells (Li||Li) is critical. Particularly in SSB systems, which pursue high energy densities, whether polyester electrolytes can form stable and low‐resistance lithium interfaces is a crucial factor determining their practical feasibility and serves as a key prerequisite for their application in high‐energy LMBs.^{[ 26 ]} Based on this, C4‐C4 and C4‐C4F polyester electrolytes were selected to conduct Li||Li symmetric cell tests, comparing the impact of fluorination on the stability of the lithium interface. As shown in Figure 5e, the C4‐C4 system exhibited a sharp increase in polarization voltage during the initial charging process, quickly reaching the voltage limit of 5 V and indicating severe interface mismatch or ion migration limitations at the lithium interface. As the cycling progressed, the polarization voltage decreased, likely due to a gradual improvement in interface contact; however, the overall fluctuation remained significant. Eventually, the battery experiences a short‐circuit failure after 63 h, highlighting the lack of sufficient chemical stability and mechanical blocking capacity at the lithium interface. In contrast, the C4‐C4F system, under the same testing current conditions (0.1 mA cm⁻² × 1 h), exhibited lower polarization voltage (≈0.2 V) and was able to cycle stably for over 1000 h without noticeable voltage fluctuations or short‐circuiting. This demonstrates that the fluorination modification significantly enhanced the stability and dendrite resistance of the polyester electrolyte at the lithium metal interface. The performance enhancement is primarily attributed to the excellent interface compatibility and smaller Li⁺ migration energy barriers of the fluorinated polyester,^{[ 27 ]} confirming its potential application in high‐energy LMBs.

2.5. Effect of Fluorinated Polyesters on the Mechanism of SEI Formation

As previously discussed, fluorinated polyester electrolytes exhibit significant advantages in enhancing the stability of LMA, a critical feature for their practical application in high‐energy‐density LMBs. To explore the underlying mechanisms by which fluorinated polyesters improve the stability of the lithium interface, a systematic analysis of the relevant samples was conducted. Firstly, the surface morphology of the LMA after 50 h of cycling in a Li||Li symmetric cell was observed using scanning electron microscopy (SEM). As shown in Figures 6a and S27 (Supporting Information), in the non‐fluorinated C4‐C4 electrolyte system, a large number of lithium dendrites were observed on the LMA surface, with a rough interface, indicating significant uneven lithium deposition behavior, which could easily lead to a battery short circuit. In contrast, the fluorinated C4‐C4F system, as shown in Figure 6b and Figure S28 (Supporting Information), exhibited a notably smoother and more compact LMA surface, with no significant dendrite formation. This suggests that the electrolyte promotes more uniform lithium deposition at the lithium interface, thereby greatly enhancing interface stability. Moreover, cross‐sectional SEM images of LMAs cycled in C4‐C4 and C4‐C4F electrolytes were further compared (Figures S29 and S30, Supporting Information). The LMA paired with C4‐C4 showed evident porosity and increased volume expansion, while that with C4‐C4F exhibited a significantly denser and smoother morphology. This result demonstrates that the fluorinated polyester contributes to a more stable interfacial environment, effectively alleviating lithium dendrite formation and suppressing mechanical degradation of the LMAs. Furthermore, the Focused ion beam‐SEM (FIB‐SEM) image of the LMA after cycling in the C4‐C4 system (Figure S31, Supporting Information) reveals a severely pulverized surface with pronounced particle‐like features and a lack of structural continuity. In addition, the internal morphology exhibits cracks, voids, and non‐dense regions, which compromise the mechanical stability of the electrode and induce localized current density fluctuations, thereby accelerating dendrite growth and interfacial degradation. Such structural defects ultimately undermine the long‐term cycling stability of the battery. In sharp contrast, the FIB‐SEM image of the LMA cycled in the C4‐C4F system (Figure S32, Supporting Information) displays a uniform, dense, and smooth surface, while the internal structure remains compact and continuous without apparent cracks or voids. This enhanced integrity and densification effectively suppress dendrite initiation and propagation, ensuring stable electrode–electrolyte contact and thereby markedly improving the cycling stability and safety of the cell.

Figure 6.

Analysis of SEI formation mechanism. SEM image of the lithium metal surface of the Li||Li symmetric cell with a) C4‐C4 and b) C4‐C4F electrolytes after cycling for 50 h. F1s spectrum of the lithium metal surface corresponding to the Li||Li symmetric cell with c) C4‐C4 and d) C4‐C4F electrolytes. ELF analysis of e) C4‐C4 and f) C4‐C4F molecules. Bond order analysis of g) C4‐C4 and h) C4‐C4F molecules. Molecular dynamics snapshots of the decomposition process of i) C4‐C4 and j) C4‐C4F electrolytes on the lithium metal surface.

To further investigate the electrolyte‐lithium interface reaction behavior, X‐ray photoelectron spectroscopy (XPS) analysis on the LMA after cycling was performed. The F 1s spectra shown in Figure 6c and Figure S33 (Supporting Information) reveal that, in the C4‐C4 system, the lithium surface was predominantly enriched with ─CF_x from residual lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and LiF resulting from LiTFSI decomposition. In the C4‐C4F system, as shown in Figure 6d and Figure S34 (Supporting Information), however, due to the presence of residual C4‐C4F, the LMA surface accumulated more CF_x. Notably, in addition to the ─CF_x component, there was also a significant increase in LiF content. LiF, an inorganic compound that is electronically insulating but Li⁺ conductive, forms a dense and stable SEI layer on the lithium surface, effectively suppressing dendrite growth and improving interface stability, which is critical for the safety and cycle performance of high‐energy LMBs.^{[ 28 ]}

Building on the previous analysis of the electronic structural modulation of the polyester upon fluorination, it was observed that the LUMO energy level of the fluorinated polyester significantly decreases, making the polymer molecules more prone to electron acceptance at the lithium metal interface, thus enhancing its interfacial reactivity. Based on this, it is hypothesized that the fluorinated polyester not only serves as an ion‐conducting medium but may also directly participate in the interfacial reaction process, providing an additional fluorine source for the formation of LiF in the SEI, thereby leading to the significant increase in LiF content. To verify this hypothesis, DFT calculations and analyses on the electronic structures and localization behaviors of both fluorinated and non‐fluorinated polyester molecules were conducted. By comparing their electron localization functions (ELF), as shown in Figure 6e,f, it was found that in the fluorinated C4‐C4F system, the ELF of certain bonds significantly decreased, particularly in the C‐F related regions, where the ELF value was lower, indicating relatively weaker bond stability and a greater propensity for bond rupture at the interface.^{[ 29 ]} Moreover, bond order analysis (Figure 6g,h) further confirmed this trend: in the C4‐C4 system, the lowest bond order was observed at the ester bond C─O─C (0.25 and 0.5), whereas the C─F bond in the C4‐C4F system had a bond order of only 0.15, indicating that this bond is the most vulnerable and suggesting a higher tendency for decomposition at the lithium metal surface. Therefore, it can be concluded that the fluorinated polyester not only improves the electrochemical performance of the electrolyte itself but also facilitates the formation of a stable LiF‐rich interface membrane through structural modifications induced by fluorination, providing clear molecular‐level support for the design of polymer electrolytes for high‐performance LMBs.

To more intuitively simulate the decomposition pathways of fluorinated polyester electrolytes on the lithium metal surface, molecular dynamics models (Figures S35 and S36, Supporting Information) were constructed, and 100 fs simulations were performed. As shown in Figure 6i, in the C4‐C4 system, the polyester main chain remained stable throughout the process, with no significant dissociation reactions observed. At the same time, LiTFSI released a fluorine atom at 17 fs, which combined with lithium to form LiF. At 24 fs, an N─S bond rupture occurred, and at 30 and 49 fs, CF₃SO₂NLi and CF₂SOOLi products were formed. In contrast, in the C4‐C4F system (Figure 6j), not only did LiTFSI release a fluorine atom at 17 fs to form LiF, but the polyester main chain itself also released three fluorine atoms at 7, 15, and 67 fs, contributing to the formation of LiF. These results clearly indicate that the fluorinated polyester played the role of the “first source” in the interface reaction process, providing an additional fluorine source for LiF generation and significantly promoting the formation of LiF in the interface SEI. Thus, the fluorinated polyester electrolyte, through the modulation of the polymer's electronic structure and interfacial reactivity, not only optimizes the morphology of lithium deposition at the physical level but also chemically participates in constructing a high‐quality SEI layer, thereby significantly enhancing the interface stability of the LMA. This provides strong support for the development of high‐energy‐density LMBs.

2.6. Recovery of Fluorinated Polyester Electrolyte

As previously discussed, fluorinated polyester electrolytes exhibit great potential for application in LMBs due to their excellent electrochemical stability and interface regulation capabilities. However, with the rapid increase in the number of spent LIBs in recent years, improper disposal without effective recycling may not only lead to substantial resource wastage but also pose serious environmental threats.^{[ 30 ]} In particular, the incorporation of fluorinated monomers in polyester electrolytes significantly elevates the production cost. Moreover, conventional recycling strategies—especially direct pyrolysis—tend to generate large quantities of toxic and greenhouse gases, thereby exacerbating both environmental and economic burdens associated with their disposal.^{[ 31 ]} Therefore, the development of an efficient and environmentally friendly recycling strategy is imperative. Such a strategy would not only enable the closed‐loop reuse of critical materials but also significantly reduce greenhouse gas emissions, thus contributing to the sustainable development of fluorinated polyester electrolytes.

Recent studies have explored chemical and solvent‐based recycling methods for polyester or fluorinated polymers. For instance, Xie et al. separated LiTFSI and polyester from polyester electrolyte by stepwise dissolution based on solvent distribution, in which the recovery rate of LiTFSI could reach 90% and the regeneration rate of polyester could reach 86%.^{[ 7a ]} Dai et al. decomposed polyester into diol monomer and sodium dibasic acid monomer, which could be used for regeneration of polyester through an easy chemical alkaline washing step, which is a simple, fast and efficient method.^{[ 32 ]} Taken together, these reports support the broader applicability of a solvent‐based recycling protocol for fluorinated polyester electrolytes. In this work, a green solvent‐based recycling process was proposed for fluorinated polyesters, as illustrated in Figures 7a and S37 (Supporting Information). Spent LMBs are first dismantled to isolate integrated cell units comprising the cathode, electrolyte, and LMA. These cell units are then subjected to a two‐step dissolution process involving chloroform and dimethoxyethane (DME), yielding a chloroform solution containing the fluorinated polyester and a DME solution containing the LiTFSI. The recycled fluorinated polyester (r‐C4‐C4F) and LiTFSI (r‐LiTFSI) are subsequently recovered via methanol precipitation and solvent evaporation, achieving recovery yields of 91.2% and 87.8%, respectively.

Figure 7.

Recycling of Fluorinated Polyester and its LCA. a) Recycling roadmap of fluorinated polyester. Cost analysis of b) C4‐C4F preparation, C4‐C4F electrolyte preparation, and C4‐C4F electrolyte recovery. c) Variation curves of the total quantity and cost of C4‐C4F with recovery times. d) ¹H NMR and e) ¹⁹F NMR spectra of r‐C4‐C4F and C4‐C4F. f) ¹³C NMR and g) ¹⁹F NMR spectra of r‐LiTFSI and LiTFSI. h) Cycle performance at 30 °C and 0.2 C, and i) rate performance at 40 °C of the battery with r‐C4‐C4F electrolyte. j) LCA of r‐C4‐C4F and C4‐C4F production. Comparison of k) CO₂ and l) SO₂ emissions during the production of r‐C4‐C4F and C4‐C4F.

Building on this, a comprehensive cost analysis that encompassed the synthesis of C4‐C4F, the preparation of the C4‐C4F electrolyte, and the recovery of the C4‐C4F electrolyte was conducted. As shown in Figure 7b, the synthesis of C4‐C4F costs ≈$7424.8/kg, with the fluorinated monomer, TFSA, accounting for 99.45% of the total cost. Furthermore, the cost of preparing the C4‐C4F‐based electrolyte is ≈$ 5114.80/kg, with the fluorinated polyester accounting for 98.72% of this cost. In stark contrast, the cost of recycling the fluorinated polyester electrolyte is only ≈$5.6/kg, with solvents and electricity accounting for 61.31% and 3.69% of this value, respectively. These results underscore the economic incentive for recycling, as the high cost of fluorinated monomers and the low cost of recovery collectively make recovery an economically compelling choice. Additionally, Figure 7c illustrates the effect of repeated recycling on the cost per kilogram of the polyester. Table S1 (Supporting Information) lists the cost of materials and electricity. Assuming a recovery yield of ≈90% per cycle, up to ≈10 kg of product can be cumulatively obtained from an initial 1 kg of fluorinated polyester. Under these conditions and considering the recycling cost, the theoretical minimum cost of the polyester can be reduced to ≈$750.6/kg, indicating that a closed‐loop recycling approach can significantly mitigate the high cost associated with fluorinated polyesters. To verify the chemical integrity of the recovered materials, NMR spectroscopy was performed on both r‐C4‐C4F and r‐LiTFSI. As shown in Figure 7d–g, the ¹H and ¹⁹F NMR spectra of r‐C4‐C4F and the ¹³C and ¹⁹F NMR spectra of r‐LiTFSI closely match those of the pristine materials, confirming the successful recovery and high chemical purity of the products.

The recovered r‐C4‐C4F and r‐LiTFSI were further used to reconstitute fluorinated polyester electrolytes, which were assembled into full cells for electrochemical testing. As depicted in Figure 7h, the initial discharge capacity of the cell with recovered C4‐C4F electrolyte at 30 °C and 0.2 C was 131.7  mAh g⁻¹, and it maintained a capacity of 134.8  mAh g⁻¹ after 100 cycles. Moreover, the rate performance at 40 °C (Figure 7i) demonstrates a capacity of 150.0  mAh g⁻¹ at 0.1 C and 103.7  mAh g⁻¹ even at 2 C, indicating high electrochemical reversibility and practical feasibility of the recycled electrolyte components. Notably, the r‐C4‐C4F electrolyte exhibits superior rate capability compared to its pristine counterpart. This enhancement is mainly attributed to partial degradation of the polyester chains during the battery operation and the chemical recovery process, which leads to the formation of shorter polymer chains. These shorter chains exhibit increased segmental mobility, thereby accelerating polymer relaxation and promoting more efficient Li⁺ transport.^{[ 33 ]}

Finally, a comprehensive life cycle assessment (LCA) was conducted to compare the environmental impacts associated with producing 1 kg of C4‐C4F via either new synthesis or the proposed recycling route, as shown in Figures S38 and S39 (Supporting Information). As summarized in Figure 7j, the recycling process consistently exhibits lower environmental burdens across ten impact categories, including climate change, acidification, eutrophication, toxicity, and resource depletion. Notably, as shown in Figure 7k,l, CO₂ and SO₂ emissions were reduced by 71.4% and 52.9%, respectively, highlighting the substantial reductions in both carbon footprint and acidifying emissions achieved via recovery. In summary, the solvent‐based recycling pathway for fluorinated polyester electrolytes not only enables efficient material recovery and significant cost reduction but also aligns with green chemistry principles by minimizing environmental impacts. These findings underscore the potential of recycling strategies to address both the economic and ecological challenges posed by high‐performance fluorinated polymer electrolytes, paving the way for more sustainable practices in next‐generation LMB technologies.

3. Conclusion

This study presents a comprehensive strategy for developing high‐performance, sustainable polyester electrolytes through precise structural design and functional fluorination. By tuning the carbon chain lengths of diol and diacid monomers, fine control over polymer flexibility and lithium‐coordinating environments was achieved. Fluorination at the coordinating segment enhances ionic conductivity by increasing local polarity and promoting the dissociation of lithium salts. In contrast, fluorination at the flexible segment favors Li⁺ migration by introducing transient dipole stabilization and reducing energy barriers. However, excessive fluorination may lead to increased rigidity and reduced bulk ion conductivity, underscoring the need for a balance between polarity and flexibility. Mechanistic studies based on molecular dynamics and quantum chemical calculations unveiled that Li⁺ primarily coordinates with carbonyl oxygens, and fluorination modulates both coordination strength and migration pathways. Particularly, the chain‐parity effects and fluorine‐induced conformational changes were found to have a critical influence on binding energy and ion transport dynamics. Battery tests further validated that fluorinated polyesters, represented by C4‐C4F, deliver excellent capacity retention and cycle stability across a range of temperatures. Furthermore, fluorination facilitates the formation of LiF‐rich SEI layers via direct chemical participation in interfacial reactions, significantly enhancing LMA compatibility and suppressing dendrite formation. To address the cost and environmental impact of fluorinated polyesters, a green solvent‐based recycling process was developed that effectively recovers both fluorinated polyester and lithium salt with high chemical purity. LCA confirmed substantial reductions in CO₂ and SO₂ emissions, offering a scalable and eco‐friendly end‐of‐life solution. Overall, this work provides a multifaceted reference for the rational design, performance optimization, and sustainable deployment of fluorinated polyester electrolytes in advanced LMBs.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Dai X., Ye H.‐M., Huang G., and Li G., “Structure‐Tunable Fluorinated Polyester Electrolytes with Enhanced Interfacial Stability for Recyclable Solid‐State Lithium Metal Batteries.” Adv. Mater. 38, no. 1 (2026): e11556. 10.1002/adma.202511556

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.