High-Throughput Electrospinning System| Porosity and Conductivity Dual-Gradient Design onUltrathin 3D Nanofbrous Anode for Flexible Zn-lonBatteries

Prof. Jian Fang (Soochow University) & Prof. Yong Du (Shanghai Institute of Technology) AFM: Ultrathin 3D Nanofiber-Based Zinc Anode Boosts Development of Flexible Zinc-Ion Batteries

Aqueous zinc-ion batteries (AZIBs) with high energy density, low cost and excellent safety are considered one of the most promising flexible energy storage units. However, their flexible anode design faces two major challenges: (1) dendrite growth caused by non-uniform zinc deposition affecting battery stability; (2) conventional zinc foil's shape memory and metal fatigue characteristics making it unsuitable for flexible batteries. Therefore, there's an urgent need to develop new strategies for preparing highly flexible zinc anodes while effectively regulating zinc deposition behavior to suppress dendrite growth and maintain AZIBs' long-term cycling stability.

Recently, Prof. Jian Fang and lecturer Zhe Sun from Soochow University, together with Prof. Yong Du's team from Shanghai Institute of Technology, prepared a 3D nanofiber Zn anode (DG-Zn) with dual-gradient porosity and conductivity design using electrospinning and electrospraying techniques, employing TPU nanofibers as the skeleton and silver nanowires (AgNWs) as conductive materials. Experiments and finite element simulations revealed the Zn deposition mechanism: the conductivity gradient promotes Zn deposition at the anode bottom first, while the porosity gradient facilitates Zn2+ migration into the anode interior, effectively suppressing top dendrite formation. The symmetric cell demonstrated stable cycling for over 410 hours at 1mA cm-2 and 1 mAh cm-2, significantly outperforming single-gradient or non-gradient control samples. Moreover, the flexible quasi-solid-state battery maintained stable operation under various mechanical deformations (bending, twisting, shearing and puncturing), showing great potential for flexible energy storage applications.The research was published in Advanced Functional Materials under the title "Porosity and Conductivity Dual-Gradient Design on Ultrathin 3D Nanofibrous Anode for Flexible Zn-Ion Batteries".

Using a synergistic strategy of gradient-concentration polymer spinning solutions and AgNWs dispersions, the team successfully prepared a 3D flexible nanofiber zinc anode with dual-gradient conductivity and pore size design. The nanofiber membrane exhibited gradually decreasing pore size and increasing conductivity from top to bottom along the thickness direction. The 3D structure released internal anode space, avoiding deformation during Zn deposition with negligible thickness change before/after plating (88 μm). Additionally, the dual-network structure of polymer nanofibers and AgNWs endowed DG-Zn with excellent flexibility and conductivity, showing only 10% resistance change after 15,000 bending cycles.

Fig. 1 (a) DG-Zn preparation flowchart; SEM images of (b-d) top/middle/bottom sections; Cross-sectional SEM (e) before and (f) after Zn plating; (g) Water contact angle; (h) Flexibility demonstration; (i) Resistance change after bending test.

To verify the dual-gradient anode's effectiveness, COMSOL simulations compared DG-Zn with single-gradient (PG-Zn, CG-Zn) and non-gradient (NG-Zn) models, revealing that the dual-gradient design with 3D structure concentrated high current density at recessed areas, guiding preferential Zn deposition and forming flat surfaces with ordered deposition behavior - confirmed by SEM observations.

Fig. 2 Finite element simulations of current density and Zn deposition locations for (a-c) NG-Zn, (d-f) PG-Zn, (g-i) CG-Zn and (j-l) DG-Zn at different deposition times.

Fig. 3 (a) Schematic of bottom-up Zn deposition in DG-Zn vs NG-Zn; Cross-sectional SEM of (b-d) DG-Zn and (e-g) NG-Zn at 2/3/4 mAh cm-2 capacity (10 mA cm-2); (h-k) SEM images after 50 cycles (top fibers removed).

The asymmetric cell achieved 300 stable cycles at 10 mA cm⁻² with 99.5% average Coulombic efficiency. The symmetric cell operated stably for 410 hours at 1 mA cm-2 and 1 mAh cm-2 with smooth voltage polarization curves, outperforming most reported 3D flexible Zn anodes. Full cells exhibited excellent rate performance, low charge transfer resistance (2 Ω) and long cycling stability, delivering 140 mAh g-1 reversible capacity at 2 A g-1 with 83% retention after 350 cycles. The quasi-solid-state flexible battery maintained stable current output and reversible capacity under various mechanical deformations (0°-180° bending, rolling), demonstrating wearable application potential.

Fig. 4 (a) CE curves at 10 mA cm-2; Symmetric cell cycling at (b) 5 mA cm-2, 1 mAh cm-2 and (c) 1 mA cm-2, 1 mAh cm-2; (d) Rate performance at 0.5-10 mA cm−2; (e) Cycling performance comparison with recent 3D Zn anodes.

Fig. 5 Full cell electrochemical performance: (a) CV curves; (b) EIS; (c) Rate capability; (d) Cycling stability at 2 A g-1; (e) Flexible quasi-solid-state ZIB assembly; (f) Practical application demonstration; (g) Cycling stability under mechanical deformations.

Summary:
This study constructed a 3D nanofiber Zn anode through combined electrospinning and electrospraying, achieving bottom-up Zn deposition behavior via dual-gradient porosity/conductivity design to effectively prevent dendrite growth while enhancing electrode flexibility and electrochemical stability. The design demonstrated exceptional cycling life and rate performance in both symmetric and full cells, along with outstanding deformation tolerance in flexible quasi-solid-state batteries, proving its potential for flexible energy storage devices. Future research could further optimize gradient combinations to improve Zn2+ migration efficiency while exploring compatibility with other flexible anode materials to advance commercial development of flexible zinc-ion batteries.