GAO Li ^{1 ,2}, LIU Dingrong ^{1, 2}, XU Rongrong ¹, LI Xinxin ¹, ZHANG Wenzhan ²

(1. School of Medical Science, Nanchang Institute of Technology, Nanchang 330044, Jiangxi, China; 2. Graphene and Advanced Materials Laboratory, Nanchang Institute of Technology, Nanchang 330044, Jiangxi, China)

Extended abstract:[Background and purposes] Sodium metal boasts the advantages of low cost, wide resource distribution and abundant reserves. Meanwhile, it exhibits an extremely low redox potential (−2.71 V relative to the standard hydrogen electrode) and an ultra-high theoretical specific capacity (1166 mAh·g⁻¹). Therefore, sodium-ion batteries have become one of the potential alternative systems to lithium-ion batteries. The composition of sodium-ion batteries is similar to that of lithium-ion batteries, mainly including cathode, anode, separator, current collector and electrolyte. Among them, the separator, as one of the key components of the battery, has the core function of achieving electrical insulation and separation between the cathode and the anode, while ensuring the efficiency of ion transport in between the two parts. However, commercial polyethylene (PE) separators have poor compatibility with ester-based electrolytes, thus being prone to pore blockage when heated, which in turn leads to short circuits of the batteries and hence safety accidents. Currently, the most promising solution in practical applications is to modify the surface of the separator to enhance its wettability with the electrolyte, while simultaneously improving the mechanical strength and electrochemical stability of the separator. In this study, ceramic ionic conductor NZSP (sodium superionic conductor) was used to modify the surface of commercial PE separators, thus forming NZSP@PE composite separators. Microstructure of the composite separators was characterized in detail. Meanwhile, thermal stability of the composite separators and their compatibility with electrolytes were systematically studied. Finally, by preparing NZSP@PE composite separators with different thicknesses, the influence of separator thickness on cycle performance of the sodium-ion batteries was further examined.[Methods] With a Xifeng (Suzhou) ultrasonic sprayer, homogeneous NZSP dispersion was coated onto a commercial PE separator, yielding a multifunctional NZSP@PE composite membrane. Phase identification of both the pristine NZSP powder and the coated separator was carried out with a Haoyuan DX-27mini X-ray diffractometer [2θ=5°–90°, scan rate 5 (°)·min⁻¹]. Surface and cross-sectional morphologies were visualized using a Thermo Scientific Apreo 2S HiVac field-emission SEM, while elemental mapping was simultaneously acquired with the integrated EDS system. Electrolyte wettability was quantified by measuring the contact angle on the composite membrane with a ZJ-6900 goniometer.[Results] According to the XRD results, the NZSP@PE composite separator exhibited characteristic diffraction peaks of the monoclinic phase of NZSP, without interference from impurity phase peaks. This confirms that the NZSP layer was successfully coated on surface of the PE separator. The surface SEM images of the composite separator clearly showed a uniform and dense NZSP coating, with no obvious cracks or agglomeration in the coating. Cross-sectional SEM images further indicated that the thickness of the NZSP coating on one side of the separator was approximately 10 μm. This demonstrates that the adopted ultrasonic spraying process enables effective loading of the NZSP powder on surface of the PE separator and the formation of a uniform film. EDS spectroscopy testing was performed on the NZSP coating. Characteristic peaks of five elements (Na, Zr, Si, P and O) were clearly observed in the spectrum, while their elemental composition completely matched the theoretical composition of NZSP. Combined with the elemental mapping images, it was found that the aforementioned elements were uniformly distributed on the PE separator surface, with no obvious elemental enrichment or deficiency. This verifies the compositional uniformity of the NZSP coating.[Conclusions] By combining the characterization conclusions mentioned earlier and the battery performance data, it can be concluded that the NZSP@PE composite separator exhibits excellent comprehensive performance. On one hand, EDS elemental analysis has confirmed the uniform distribution of NZSP particles on surface of the PE separator. This microstructural advantage lays a foundation for the improvement of separator performance, which not only significantly enhances the wettability of the composite separator with the electrolyte (as verified by the contact angle test mentioned earlier) but also effectively improves its thermal stability. On the other hand, after applying this composite separator to the sodium-ion battery, outstanding electrochemical performance is achieved. In the Na|NZSP@PE|Cu half-cell, the average Coulombic efficiency during the battery cycling process can reach 97%, while the number of stable cycles exceeds 300 times. This indicates that the composite separator can effectively inhibit the growth of sodium dendrites and improve the reversibility of the battery. In the Na|NZSP@PE|Na symmetric cell, the continuous and stable cycling time of the battery is as long as 650 hours, which further verifies the optimizing effect of the composite separator on the interfacial stability of sodium-metal batteries. In addition, the technical ideas and research conclusions regarding the preparation process, structure regulation and performance optimization of the NZSP@PE composite separator not only provide a feasible solution for improving the performance of separators for sodium-ion batteries but also hold important reference value and significance for the development of separator materials for other types of advanced energy storage devices.

Key words: sodium metal battery; separator; NZSP ion conductor; cycling performance