Stereochemically Tailored Polyester Electrolytes Enable Stable Binder-Free Solid Lithium Batteries

Scientists have developed an advanced solid-state battery architecture by tailoring the molecular stereochemistry of crosslinkable aliphatic polyester electrolytes, resolving the trade-off between mechanical strength and ionic conductivity while eliminating conventional binders and separators. Conventional lithium-ion batteries rely on flammable liquid electrolytes that present severe thermal runaway hazards and restrict achievable energy density in electric transportation. To overcome these constraints, researchers introduced a stereo-configured polymer design based on cis-configured poly(n-pentyl maleate), functionalized with fluorinated end-groups and unified directly into the cathode microstructure, enabling all-solid-state lithium metal cells to achieve exceptional cycling longevity and high-rate discharge capability.

The development of solid polymer electrolytes has long been constrained by polyether hosts, most notably poly(ethylene oxide). Poly(ethylene oxide) exhibits strong room-temperature crystallization that suppresses segmental motion, confining ionic conductivity between 10⁻⁸ and 10⁻⁶ siemens per centimeter. Furthermore, strong coordination between lithium cations and ether oxygens leads to a low transference number of 0.2 or less, inducing severe concentration polarization and high interfacial resistance during cell operation. Polyether hosts also possess a narrow electrochemical stability window below 4.0 volts versus the lithium reference electrode, precluding integration with high-voltage cathodes. While earlier investigations showed that carbonyl-coordinating aliphatic polyesters construct multichain solvation cages, lower cation-anion correlations, and improve oxidative stability, these post-polyether formulations still suffered from dendrite penetration, insufficient mechanical stiffness, and mobile anions that accelerate cell degradation.

To address these interrelated challenges, the research team synthesized aliphatic polyesters via transesterification polycondensation coupling 1,5-pentanediol with selected dimethyl esters. By comparing saturated dimethyl succinate against unsaturated stereoisomeric precursors—trans-configured dimethyl fumarate and cis-configured dimethyl maleate—the authors generated poly(n-pentyl succinate), poly(n-pentyl fumarate), and poly(n-pentyl maleate). Nuclear magnetic resonance and size-exclusion chromatography verified that all synthesized polyesters shared a comparable degree of polymerization between 33 and 35 repeating units, ensuring that observed property variations originated from macromolecular architecture rather than molar mass. Subsequently, reactive hydroxyl terminals on the hemitelechelic polymer chains were modified using trifluoroacetic anhydride to attach trifluoroacetate end-groups. Fluorine nuclear magnetic resonance spectroscopy confirmed that the conversion of terminal hydroxyl functions exceeded 70 percent, yielding fluorinated macromolecular hosts designated with a three-fluorine suffix for salt-in-polymer formulation and photochemical crosslinking.

The distinct spatial orientations of the stereoisomeric monomers produced radically different macromolecular conformations and interchain packing within the electrolyte matrices. Density functional theory calculations and molecular dynamics simulations revealed that trans-derived poly(n-pentyl fumarate) adopts an extended, linear backbone geometry that facilitates dense, ordered interchain packing. In sharp contrast, the cis-configuration of poly(n-pentyl maleate) introduces severe backbone bending and conformational distortion along the polymer chain. This geometry disrupts close interchain aggregation and substantially increases the fractional free volume distributed throughout the polymeric host. Quantitative computational evaluations confirmed that this elevated free volume provides accessible, interconnected pathways for ion migration, resulting in calculated lithium-ion diffusion coefficients that significantly exceed those of saturated poly(n-pentyl succinate) and linear poly(n-pentyl fumarate). Consequently, the cis-isomer inherently facilitates accelerated cation migration through its disordered packing arrangement, mitigating steric congestion.

The spatial consequences of stereoisomerism proved equally decisive during interchain photo-crosslinking, which proceeds through ultraviolet-induced cycloaddition across internal carbon-carbon double bonds. In linear poly(n-pentyl fumarate) chains, close interchain packing places crosslinkable double bonds in continuous proximity, producing high crosslinking probability and lower crosslinking formation energy. As a consequence, poly(n-pentyl fumarate) forms a densely crosslinked, brittle network with a crosslinking content of 97.9 percent and a Young’s modulus of 123 megapascals, severely constricting polymer mobility and suffocating ionic transport. Conversely, the distorted cis-backbone of poly(n-pentyl maleate) increases the spatial separation between neighboring chains, reducing junction-forming events under identical ultraviolet exposure. This yielded a loosely connected, sparsely crosslinked network with a crosslinking content of 57.5 percent for the unmodified polymer and 60.1 percent for the fluorinated derivative, preserving open ion channels while providing a Young’s modulus of 19.2 to 32.0 megapascals.

Extensive thermal and spectroscopic investigations corroborated the structural evolution driven by stereochemistry and network formation. Differential scanning calorimetry established that uncrosslinked poly(n-pentyl succinate) possesses a glass transition temperature of minus 51 degrees Celsius and a melting point of 34 degrees Celsius, which shifts to an amorphous state upon incorporating 25 weight percent lithium bis(trifluoromethanesulfonyl)imide. Both uncrosslinked and crosslinked poly(n-pentyl maleate) formulations were completely amorphous, exhibiting only glass transitions without melting endotherms. In contrast, poly(n-pentyl fumarate) displayed semi-crystalline characteristics with a distinct melting point around 66 degrees Celsius, and its glass transition shifted upward by 20 degrees Celsius after crosslinking, twice the shift observed for maleate systems. Solid-state carbon-13 nuclear magnetic resonance of crosslinked poly(n-pentyl fumarate) revealed a new aliphatic carbon resonance at 45 parts per million and a shifted carbonyl peak at 171 parts per million, while infrared spectroscopy confirmed sharp attenuation of the carbon double bond stretching band at 1645 reciprocal centimeters.

Density functional theory calculations established that poly(n-pentyl fumarate) exhibited high lithium-salt dissociation energies exceeding 4.4 electron-volts, indicating strong cation-anion association that impedes charge separation. Conversely, poly(n-pentyl maleate) and its fluorinated analog demonstrated lower dissociation barriers, ranging from 3.2 to 4.6 electron-volts for the base maleate and dropping to 2.2 to 3.8 electron-volts for the fluorinated derivative. The strongly electron-withdrawing trifluoroacetate end-groups polarize adjacent carbonyl oxygens, facilitating spontaneous salt dissociation and generating more free charge carriers. Consequently, the optimized crosslinked fluorinated maleate electrolyte achieved a lithium-ion conductivity of approximately 0.1 millisiemens per centimeter at 80 degrees Celsius. Furthermore, pulsed-field-gradient nuclear magnetic resonance confirmed that network crosslinking preferentially hindered anion diffusion, elevating the lithium transference number to 0.74 in the fluorinated crosslinked maleate and 0.87 in the unfluorinated counterpart.

Linear sweep voltammetry demonstrated that all synthesized polyester electrolytes exhibited oxidation onset potentials 1.5 to 2.3 volts higher than conventional poly(ethylene oxide), ensuring superior high-voltage compatibility. Unexpectedly, saturated poly(n-pentyl succinate) displayed the lowest oxidative breakdown threshold among the tested polyesters, contradicting assumptions regarding double bond vulnerability. Post-oxidation oscillatory rheology and infrared spectroscopy elucidated this behavior: under anodic polarization, saturated succinate suffered oxidative ester cleavage and chain scission, decreasing its storage modulus and generating degradation products identified by carboxylate stretching vibrations at 1580, 1540, and 1450 reciprocal centimeters. In contrast, unsaturated double bonds within poly(n-pentyl fumarate) and maleate derivatives underwent localized electrochemical crosslinking initiated by radical cation formation and intermolecular coupling. This in-situ partial crosslinking increased the dynamic storage modulus, suppressed radical diffusion by restricting chain mobility, and arrested continuous chemical decomposition. End-group fluorination and photochemical crosslinking further reinforced chemical resilience under high-voltage operating conditions.

The interfacial stability of the electrolyte formulations against metallic lithium was evaluated through galvanostatic plating and stripping in symmetric cells operated at 60 degrees Celsius under 0.1 milliampere per square centimeter. Cells assembled with saturated succinate and semi-crystalline fumarate failed within 170 to 300 hours due to rapid interfacial deterioration. In contrast, uncrosslinked poly(n-pentyl maleate) sustained over 1000 hours of continuous cycling with a 200-millivolt overpotential increase, while the fluorinated maleate restricted overpotential growth to merely 50 millivolts. X-ray photoelectron spectroscopy confirmed that terminal trifluoroacetate groups underwent electrochemical reduction at the anode interface, producing a dense, lithium-fluoride-enriched solid electrolyte interphase with characteristic signals at 57 electron-volts for lithium and 684.8 electron-volts for fluorine. Cross-sectional electron microscopy confirmed that while succinate surfaces developed irregular moss-like dendrites, the crosslinked fluorinated maleate maintained a pristine, dendrite-free lithium interface, sustaining stable cycling over 300 hours at 0.4 milliamperes per square centimeter.

Leveraging the balanced elasticity, mechanical robustness, and inherent adhesive qualities of the crosslinked fluorinated maleate network, the researchers engineered a monolithic architecture that unified the functions of binder and separator. In conventional solid-state configurations, pre-formed electrolyte membranes are stacked onto dry composite cathodes, leaving microscopic voids that inflate interfacial resistance and obstruct ion transfer. In the unified fabrication protocol, a precursor solution of poly(n-pentyl maleate) containing lithium salt and photo-initiator was directly cast onto a calendered cathode containing active materials and conductive carbon, followed by drying and ultraviolet curing. Cross-sectional electron microscopy verified complete conformal wetting throughout the electrode microstructure without voids. Peel tests demonstrated that the unified monolith achieved a 90-degree adhesion strength of 1.7 newtons per millimeter, matching commercial electrodes containing 10 weight percent poly(vinylidene fluoride) binder. Depth-profiling and infrared spectroscopy confirmed uniform network formation across the electrode thickness, while full-cell impedance measurements documented dramatic decreases in bulk and charge-transfer resistances.

Full-cell evaluations validated the performance advantages of the unified monolithic configuration in solid-state lithium metal batteries. When paired with lithium iron phosphate cathodes and cycled at 60 degrees Celsius under a 0.3 C rate, standard stacked cells with crosslinked fluorinated maleate retained 86.6 percent capacity after 300 cycles. Under elevated 0.5 C rates, conventional stacks experienced severe capacity decay, whereas the unified monolith retained 93.6 percent capacity after 100 cycles with average Coulombic efficiency exceeding 99.8 percent. The unified cell delivered 65 percent of its reversible capacity at a high discharge rate of 2 C, and at 40 degrees Celsius under an areal loading of 5 milligrams per square centimeter, it delivered 90 percent, 79 percent, and 65 percent capacity at 0.2 C, 0.3 C, and 0.5 C, respectively. When integrated with nickel-cobalt-manganese oxide cathodes, the unified monolith achieved 80.3 percent capacity retention after 100 cycles, outperforming conventional stacked cells at 61.4 percent. These results demonstrate how stereochemically tuned polyesters provide a viable pathway for high-energy all-solid-state energy storage.

Subject of Research: Stereochemically tailored crosslinkable polyester solid electrolytes for all-solid-state lithium metal batteries

Article Title: Tailored Stereo‐Configuration of Crosslinkable Polyester‐Based Solid Polymer Electrolytes for All‐Solid‐State and Binder‐Free Li Metal Batteries

Article References: Song, H. J., Kim, J. Y., Hong, S., Kim, Y. H., Nam, M. G., Ho, J.-W., Kang, M. G., Kim, H. S., Ha, C., Lee, S. U., Yoo, P. J., & Rannou, P. (2026). Tailored Stereo‐Configuration of Crosslinkable Polyester‐Based Solid Polymer Electrolytes for All‐Solid‐State and Binder‐Free Li Metal Batteries. Advanced Science, 13(63), Article e00063. https://doi.org/10.1002/advs.202600063

Image Credits: AI Generated

DOI: 10.1002/advs.202600063

Keywords: Solid polymer electrolytes, All-solid-state lithium batteries, Poly(n-pentyl maleate), Poly(n-pentyl fumarate), Stereoisomerism, Interchain photo-crosslinking, Lithium bis(trifluoromethanesulfonyl)imide, Transference number, Trifluoroacetate end-groups, Solid electrolyte interphase, Lithium dendrite suppression, Binder-free electrode monolith, Linear sweep voltammetry, Electrochemical stability window, Lithium iron phosphate cathode, Monolithic cell architecture, Differential scanning calorimetry, Ionic conductivity