Nanofiber Production Equipment| Deep Eutectic Solvent Gel Electrolytes Reinforced withCellulose Nanofibers for High-Performance Flexible Solid-StateSupercapacitors

Professors Wen Jialong and Yuan Tongqi from Beijing Forestry University AFM: Preparing Deep Eutectic Solvent Gel Electrolytes Enhanced with Nanocellulose for High-Performance Flexible Solid-State Supercapacitors

With the rapid development of wearable electronic devices and smart flexible equipment, solid-state supercapacitors (SCs) that combine high energy density, excellent cycle stability, and fast charge/discharge capabilities have become a research hotspot in the field of energy storage. As one of the core functional components, the comprehensive performance of gel electrolytes directly determines the safety, stability, and application scope of devices. However, traditional hydrogel electrolytes still face numerous challenges such as poor conductivity, insufficient mechanical strength, easy drying and freezing, unstable interfaces, and lack of self-healing capabilities. Additionally, material sustainability has gradually become an important criterion that cannot be ignored. Therefore, how to construct a multifunctional gel electrolyte with high conductivity, flexibility, self-healing properties, and environmental friendliness has become a key issue in the material design of flexible energy storage devices.

Recently, Professors Wen Jialong and Yuan Tongqi from Beijing Forestry University published their latest research titled "Deep Eutectic Solvent Gel Electrolytes Reinforced with Cellulose Nanofibers for High-Performance Flexible Solid-State Supercapacitors" in the journal Advanced Functional Materials. The researchers used a nanocellulose reinforcement strategy to prepare multifunctional ion gel electrolytes with a wide temperature range, high ionic conductivity, excellent mechanical toughness, self-healing, and strong adhesion properties. Flexible solid-state supercapacitors assembled based on this gel electrolyte exhibited outstanding electrochemical performance and environmental tolerance.

Figure 1: Preparation process of multifunctional ionic gel electrolyte and flexible supercapacitor.

By optimizing the molar ratio of the choline chloride:urea system (1:2), polyacrylic acid (PAA), DES, and CNF were compounded to form a stable three-dimensional porous network structure. Due to the low volatility of DES and the hydrogen bond interactions between CNF and water molecules, the material exhibited excellent water retention (80% water retention rate) and antifreeze properties (freezing point -26.0°C). XRD analysis showed that the hydrogen bond interactions of CNF itself were weakened, which facilitated the improvement of ion mobility (ionic conductivity reached 45.5 mS cm⁻¹, a 116% increase compared to pure water-based ion gels).

 Figure 2: a) Water retention of ionic gel electrolytes at 20℃ and b) 60℃ for 7 days; c) SEM image of PAA/DES/CNF ionic gel (DES3C0.5); d) FTIR spectra, e) DSC curves, f) XRD patterns, g) TGA curves, and h) DTG curves of samples with different DES and CNF mass fractions; i) Ionic conductivity of ionic gel electrolytes.

CNF with a rigid skeleton structure and abundant hydroxyl groups could form dense and dynamic hydrogen bond networks with the PAA main chain and DES, contributing to the high uniformity and compactness of the gel microstructure. MicroCT three-dimensional imaging showed that the gel reinforced with CNF exhibited a denser and more uniform internal network structure.Under optimized conditions (CNF/DES mass ratio of 1:1), the tensile strength of the gel electrolyte reached 142 kPa, with a maximum strain of 322%, and demonstrated good tensile and compression recovery capabilities. Cyclic tensile tests showed significant hysteresis loops, reflecting the system's excellent energy dissipation capacity. 

Figure 3: Mechanical properties of the ionic gel electrolyte: a) MicroCT images of DES3H2O and b) DES3C0.5; c) Photos of the gel in its original state, twisted state, and stretched to 300% of its length; d) Photos of the gel after puncture and recovery; e) Tensile curves of different gel samples; f) Corresponding Young's modulus and toughness; g) Tensile cycle curves of DES3C0.5 gel under different strains; h) Tensile cycle loading-unloading curves of DES3C0.5 gel; i) Compressive load-unloading curves of DES3C0.5 samples under different strain ranges; j) Compressive stress-strain curves of DES3C0.5 ionic gel electrolyte at 50% strain for different holding times.

The gel material exhibited outstanding interfacial adhesion performance, firmly adhering to various substrates such as metal sheets, carbon cloth, and nickel foam, with a maximum adhesion strength of 83.1 kPa, effectively avoiding the delamination problem between the electrode and electrolyte. Meanwhile, the large number of dynamic hydrogen bonds and ionic interactions introduced in the gel endowed it with excellent self-healing properties. After 24 hours of self-healing at room temperature without external force, the structural integrity and good ionic conductivity were restored (ionic conductivity recovered to over 90% of the original state), significantly improving device reliability and service life.

Figure 4: Self-adhesion and self-healing properties of the ionic gel: a) Schematic diagram of the lap shear test process; b) Adhesion strength-displacement curves and c) adhesion strength of DES3C0.5 samples on different substrates; d) Photos of the adhesion tests of DES3C0.5 samples on various materials and their adhesion stability under low-temperature conditions; e) Schematic diagram of the adhesion mechanism between DES3C0.5 hydrogel and substrates; f) Photos of the cutting and self-healing process of DES3C0.5 samples, along with optical microscopic images before and after self-healing; g) Nyquist plots (illustration: ionic conductivity before and after self-healing) of DES3C0.5 before and after self-healing; h) Performance of the ionic gel electrolyte as a conductor under different deformations.

Flexible solid-state supercapacitors assembled with PAA/DES/CNF ion gel as the electrolyte achieved a high specific capacitance of 94.4 F g⁻¹ at a current density of 1 A g⁻¹, along with excellent cycle stability, maintaining over 90% capacity retention after 5,000 charge/discharge cycles. The device could operate stably under extreme cold (-20°C), high temperature (60°C), and various mechanical deformations (compression and bending), demonstrating outstanding environmental adaptability and mechanical flexibility. After series and parallel expansion, it could stably power LED loads, further verifying its practical potential in flexible energy storage systems.

Figure 5: Electrochemical performance of flexible supercapacitors assembled based on ionic gel electrolyte.

Figure 6: Electrochemical performance of flexible supercapacitors assembled based on ionic gel electrolyte.

By leveraging the designability of DES and the natural green attributes and high mechanical flexibility of CNF, a novel ion gel electrolyte integrating high ionic conductivity, excellent mechanical properties, adhesion, room-temperature self-healing, wide voltage window, and broad temperature tolerance was successfully developed, with ideal electrochemical performance. This gel electrolyte has significant application potential in wearable electronics and flexible energy systems, promoting the development of flexible energy device materials toward green, intelligent, and high-performance directions while providing a feasible approach for the multifunctional synergistic optimization of gel electrolytes.

Xu Linghua, a doctoral student at Beijing Forestry University, is the lead author of the paper. Professors Wen Jialong and Yuan Tongqi from Beijing Forestry University are co-corresponding authors. This research was supported by the National Natural Science Foundation of China's General Program and the 5+5 Project Research and Innovation Team Program of Beijing Forestry University.

Paper link: https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202501263