Carbon Nanotube-Loaded Electrospun LiFePO 4 /Carbon Composite Nanofibers As Stable and Binder-Free Cathodes for Rechargeable Lithium-Ion Batteries Ozan Toprakci, Hatice A.K. Toprakci, Liwen Ji, Guanjie Xu, Zhan Lin, and Xiangwu Zhang* Fiber and Polymer Science Program, Department of Textile Engineering, Chemistry and Science, North Carolina State University, 2401 Research Drive, Raleigh, NC, 27695-8301, United States ABSTRACT: LiFePO 4 /CNT/C composite nanofibers were synthe- sized by using a combination of electrospinning and sol-gel techniques. Polyacrylonitrile (PAN) was used as the electrospinning media and carbon source. Functionalized CNTs were used to increase the conductivity of the composite. LiFePO 4 precursor materials, PAN and functionalized CNTs were dissolved or dispersed in N,N- dimethylformamide separately and they were mixed before electro- spinning. LiFePO 4 precursor/CNT/PAN composite nanofibers were then heat-treated to obtain LiFePO 4 /CNT/C composite nanofibers. Fourier transform infrared spectroscopy measurements were done to demonstrate the functionalization of CNTs. The structure of LiFePO 4 /CNT/C composite nanofibers was determined by X-ray diffraction analysis. The surface morphology and microstructure of LiFePO 4 /CNT/C composite nanofibers were characterized using scanning electron microscopy and transmission electron microscopy. Electrochemical performance of LiFePO 4 /CNT/C composite nanofibers was evaluated in coin-type cells. Functionalized CNTs were found to be well-dispersed in the carbonaceous matrix and increased the electrochemical performance of the composite nanofibers. As a result, cells using LiFePO 4 /CNT/C composite nanofibers have good performance, in terms of large capacity, extended cycle life, and good rate capability. KEYWORDS: lithium-ion batteries, cathodes, LiFePO 4 , carbon nanofibers, carbon nanotubes 1. INTRODUCTION Rapidly increasing demand for lithium-ion batteries opened a new area in the cathode material research. Among various cathode materials, lithium iron phosphate (LiFePO 4 ) comes into prominence because of its high discharge potential, excellent cycling performance, good thermal stability, low toxicity, relatively low cost, and safe nature. However, LiFePO 4 has low conductivity (∼1 × 10 -9 S cm -1 ), which causes poor rate capability and high impedance. 1 To increase the efficiency of LiFePO 4 , researchers proposed various structural and morphological modifications such as doping LiFePO 4 with metal ions, 1-3 reducing the particle size, 4,5 coating with conductive materials, 6-8 and fabrication of conductive LiFePO 4 composites. 9-15 In all these methods, conductive LiFePO 4 composites are of increasing importance for their contribution to electrochemical performance. These materials are typically prepared by mixing LiFePO 4 or its precursors with a polymer, followed by a heat treatment procedure to convert the polymer matrix into a conductive carbon. The conductivity of these composites can be further improved by adding additional electrical conductors. Although many materials can be used to increase the electrical conductivity of the system, 14,16,17 carbon nanotubes (CNTs) are one of the most promising materials because of their high electrical conductivity, large surface area, and high aspect ratio. 18,19 LiFePO 4 is typically produced by both solid-state and solution-based methods, which have been reviewed else- where. 20-23 In this work, LiFePO 4 /CNT/C composite nano- fibers were synthesized via the combination of electrospinning and sol-gel techniques. The novelty of this study mainly stems from the unique electrospinning process. Electrospun pre- cursor/CNT/polyacrylonitrile (PAN) nanofibers can be easily converted into LiFePO 4 /CNT/C composite nanofibers by heat treatment. After heat treatment, the resultant LiFePO 4 /CNT/ C composite nanofibers form free-standing and flexible mats that not only show increased electrical conductivity but also eliminate the use of polymer binders. The unique composite nanofiber structure also restricts the growth of LiFePO 4 particles during heat treatment. Because the transformation of PAN to carbon and formation of LiFePO 4 particles take place simultaneously in composite nanofibers, the carbon nanofiber matrix behaves as an inhibitor between the LiFePO 4 particles and prevents the particle growth. This further shortens the charge transfer distance and leads to increased lithium diffusion coefficient. Thus, LiFePO 4 /CNT/C composite nanofibers can not only have large capacity and good cycling performance but also possess high rate capability. In addition, chemically modified CNTs are incorporated into the composite nanofibers in order to enhance the electrochemical performance and the stability of the LiFePO 4 cathodes. As schematically presented in Received: November 2, 2011 Accepted: February 2, 2012 Published: February 2, 2012 Research Article www.acsami.org © 2012 American Chemical Society 1273 dx.doi.org/10.1021/am201527r | ACS Appl. Mater. Interfaces 2012, 4, 1273-1280