Hydrophobic Protective Layer Suppresses Rehydration and Structural Degradation of Carbon Nanotube-Enhanced Prussian Blue Cathodes

Prussian blue analogs (PBAs) have garnered considerable attention as cathode materials for sodium-ion batteries due to their open three-dimensional frameworks, large ionic channels, tunable redox-active sites, and facile room-temperature solution synthesis. However, their practical application has been hindered by intrinsic low electronic conductivity and structural instability under humid conditions. In this work, a fluorinated acrylic copolymer emulsion was employed to impart surface hydrophobicity to self-dispersing nanoscale carbon nanotube-enhanced PBA composites (PB-MCNT), resulting in HPB-MCNT with a water contact angle increasing from 58.2° to 123.7°, demonstrating pronounced hydrophobicity. After exposure to 84% relative humidity for 7 days, scanning electron microscopy revealed severe particle fragmentation and volume expansion in unmodified PB-MCNT, while X-ray diffraction detected impurity phases indicative of decomposition side reactions, and thermogravimetric analysis showed that the water content rising markedly from 8.84% to 14.95%. In contrast, HPB-MCNT retained its morphology, crystal structure, and water content under identical conditions. Electrochemical testing indicated that although initial capacity was slightly reduced by the hydrophobic coating, HPB-MCNT-7D exhibited significantly superior rate capability and cycling stability compared to PB-MCNT-7D, with a capacity retention of 84.85% after 500 cycles, far exceeding the 73.15% of the unmodified sample. These results demonstrate that coupling a nanoscale carbon nanotube conductive network with hydrophobic surface protection effectively suppresses moisture ingress, rehydration, and high-humidity-induced structural degradation, thereby enhancing the environmental stability of PBA cathodes.