The hydrophobic surface characteristics of carbon nanotube fibers (CNTFs) limit their usage in aqueous systems. Herein, we report a surface treatment method for preparing hydrophilic CNTFs by attaching a sulfonic acid group onto the surface of CNTF. The introduction of a sulfonic acid group was examined by x-ray photoelectron spectroscopy. The contact angle between the CNTF and water droplets decreased after a surface treatment to 22.9°, implying enhanced hydrophilicity. The nanostructure of the CNTF is rarely changed during surface treatment, so the surface characteristics can be controlled without losing the advantages of direct-spun CNTFs. Water infiltration was examined by measuring the electric double-layer capacitance of CNTFs. The capacitance of surface-treated CNTFs (140 μF cm−1) was even higher than the as-prepared CNTFs (21 μF cm−1), meaning that the interfacial area between CNTFs and water electrolytes significantly increased, and water can infiltrate effectively into the nanopores of CNTFs as a result of the surface treatment. The sulfonic acid group is more efficient than the carboxyl group in improving hydrophilicity and water infiltration. The surface treatment also enhances the specific strength of the CNTFs by 50%, which is beneficial for preparing CNTF-based devices. The hydrophilic CNTFs prepared in this work will be useful in preparing CNTF-possessing functional nanoparticles or in use in CNTFs for electrochemical devices.

Carbon nanotube fiber (CNTF) is a highly conductive and porous platform to grow active materials of lithium-ion batteries (LIB). Here, we prepared SnO2@CNTF based on sulfonic acid-functionalized CNTF to be used in LIB anodes without binder, conductive agent, and current collector. The SnO2 nanoparticles were grown on the CNTF in an aqueous system without a hydrothermal method. The functionalized CNTF exhibited higher conductivity and effective water infiltration compared to the raw CNTF. Due to the enhanced water infiltration, the functionalized CNTF became SnO2@CNTF with an ideal core–shell structure coated with a thin SnO2 layer. The specific capacity and rate capability of SnO2@-functionalized CNTF were superior to those of SnO2@raw CNTF. Since the SnO2@CNTF-based anode was free of a binder, conductive agent, and current collector, the specific capacity of the anode studied in this work was higher than that of conventional anodes.