Hierarchically structured hydroxyapatite nanowire–cellulose composite separator with exceptional thermal stability and efficient ion transport for safer lithium-ion batteries

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Lithium-ion batteries are widely used because of their high energy density, but the thermal shrinkage of conventional polyolefin separators remains a safety concern. In this study, hydroxyapatite nanowires (HA NWs) were combined with ramie-derived cellulose fibers (SS) to prepare composite separators. Among the tested compositions, HA NW/SS-3 (30:70) showed the most uniform membrane morphology and was selected for detailed evaluation. In the dimensional-stability test, HA NW/SS-3 showed no observable shrinkage up to 200 °C, whereas the polypropylene (PP) separator underwent severe shrinkage. The composite showed an apparent porosity of 95.3% by the n-butanol uptake method, an ionic conductivity of 2.67 mS cm −1 , and a lithium-ion transference number of 0.55. In LiFePO 4 |Li cells, additional drying enabled one complete charge-discharge cycle with a discharge capacity of 138.37 mAh g −1 at 0.075 C; however, voltage clamping reappeared in the following cycle. These results support the high thermal dimensional stability and ion-conducting ability of the HA NW/SS-3 composite membrane, while the present cycling data remain insufficient to demonstrate long-term separator performance.