Abstract:

Objective The performance and safety limitations of lithium-ion batteries, as well as the environmental concerns of commercial separators necessitates development of novel high-performance separators is crucial. Cellulose-based materials offer distinct advantages over other polymers, including low cost, eco-friendliness, and excellent electrolyte wettability, making them promising candidates for battery separators. Nanocellulose (NC) further boasts a high aspect ratio, large specific surface area, robust mechanical strength, and an entangled network structure. In this study, NC was applied to modify an electrospun polyacrylonitrile (PAN) nanofiber membrane, resulting in a high-performance NC/PAN composite separator.

Method NC was synthesized via a chemical-mechanical method. PAN nanofibrous separators were fabricated by electrospinning technology. Subsequently, NC was deposited onto the PAN nanofibrous separators through spraying, followed by thermal pressing of the resulting NC/PAN composite separators. The surface morphology of NC was characterized using scanning electron microscopy and transmission electron microscopy. Its crystalline structure was investigated via X-ray diffraction, while its functional groups were analyzed using Fourier-transform infrared spectroscopy. Additionally, the separators were evaluated for their mechanical (e.g., tensile strength), thermal, and wetting properties (contact angle), as well as pore size distribution. Finally, the assembled batteries underwent comprehensive electrochemical performance tests, including ionic conductivity, lithium-ion transference number, cycling stability, and rate capability.

Results Characterization showed that the NC/PAN composite separator had superior mechanical and electrochemical properties compared to Celgard and PAN separators. It featured a uniform pore size of 216 nm, which is smaller and more consistent than that of PAN (306 nm), and exhibited a markedly higher mechanical strength of 22.6 MPa versus 15.2 MPa for PAN. The reduced and uniform pore structure contributes to efficient electrolyte infiltration, while the enhanced mechanical strength effectively prevented lithium dendrite penetration and internal short circuits. The NC/PAN separator showed significantly greater thermostability than Celgard. Furthermore, the NC/PAN separator demonstrated exceptional electrolyte wettability with a contact angle of 19°, surpassing that of PAN (31°) and Celgard (54°). The incorporation of NC substantially improved the lithium-ion conductivity to 1.89 mS/cm and the Li⁺ transference number to 0.65. The Cu|Li asymmetric cell equipped with the NC/PAN separator maintained a high coulombic efficiency of 98% after 70 cycles at 1.0 mA/cm², whereas cells with Celgard and PAN separators showed performance decay after only 45 and 53 cycles, respectively. Moreover, the Li|Li symmetric cell with the NC/PAN separator achieved stable cycling for 1 000 h at an ultra-low overpotential of 22 mV (1 mA/cm²), with less lithium dendrite growth observed post-cycling. In contrast, cells with Celgard and PAN separators experienced micro-short circuits after approximately 750 hours. The superior cycling stability is further evidenced in LiFePO₄|Li full cells, where the NC/PAN-based cell retained over 90% of its initial capacity after 700 cycles at a 2C rate. This comprehensive performance enhancement is attributed to the uniform Li⁺ flux regulated by the NC/PAN separator, promoting homogeneous lithium deposition and effectively suppresses dendrite formation and growth.

Conclusion This study employed a spraying method to deposit NC onto a PAN separator. The subsequent hydrogen bonding between NC and PAN nanofibers formed a robust cross-linked network, resulting in the successful fabrication of an NC/PAN composite separator. The NC coating markedly enhances the mechanical properties and electrolyte wettability of the separator while reducing the average pore size. As a result, the Li⁺ flux across the separator becomes more uniform. These improvements collectively promote homogeneous lithium deposition on the anode surface and effectively inhibit lithium dendrite growth. The findings underscore the considerable promise of bio-based nanocellulose and its derived materials for applications in next-generation energy technologies.

Key words: lithium battery separator, nanocellulose, polyacrylonitrile, uniform deposition, lithium dendrites, electrochemical performance