A bamboo-carbon-coated separator with biomimetic ion channels for high-loading sodium metal anodes

Sodium, due to its abundance in the Earth’s crust (2.74%), is 421 times that of lithium (0.0065%), making sodium-metal batteries (SMBs) a promising supplement to lithium-based systems. The intrinsic properties of sodium metal, including kinetic and thermodynamic instabilities during plating and stripping, however, lead to dendrite growth, internal shorts, and serious safety issues. In this work, a functional polyethylene (PE) based separator decorated with a uniform layer of bamboo carbon (BC) is prepared using a simple blade casting method. Compared with the pristine PE separator, the BC-modified separator shows better electrolyte wettability and uniform biomimetic channels. The coated BC promotes uniform sodium-ion flux and even sodium deposition, effectively suppressing dendrite formation and greatly enhancing cycling stability. The BC-modified PE separator mitigates the tip effect at sodium metal defects, enabling stable performance of symmetric cells. In electrochemical tests, sodium symmetric cells with this separator operated stably for over 450 h at 0.5 mA·cm⁻² and 0.5 mAh·cm⁻². Moreover, Na|PE-BC|sodium iron pyrophosphate phosphate (NFPP) full cells retained over 98% capacity after 1000 cycles at 1 C (1 C = 129 mA·g⁻¹), over 90% after 2000 cycles at 5 C, and over 70% after 6000 cycles at a high rate of 10 C. Even with a high cathode loading of 7.1 mg·cm⁻², the full cells maintained 90% capacity for more than 300 cycles. This work offers a simple and effective modification of commercial PE separators using renewable, low-cost bamboo carbon, providing a practical and sustainable route toward highly stable, high-loading sodium-metal batteries.