Electrospinning of "Welded" Carbon Fiber Paper for Lithium Ion Battery Current Collectors

An approach for electrospinning interconnected (“welded”) carbon fiber mats for improved electronic conduction in 3D porous positive electrode current collectors for lithium ion batteries is presented. Although individual carbon fibers have high axial electronic conductivity, when used as current collectors in lithium ion batteries and other electrochemical storage/conversion systems, their electronic conductivity is largely limited by the interconnectivity of the carbon fibers. Active electrode materials for lithium ion batteries have low electronic conductivity and can block the carbon fiber interconnectivity when loaded onto the fibers. Interconnectivity in commercial carbon fiber paper (CFP) (d ≈ 10μm) is achieved by bonding of individual carbon fibers using additional polymer precursor and subsequent carbonization. In some reports of CFP current collectors in electrochemical devices additional carbon additives are mixed with the active material slurry and then pressed into the CFP in order to improve interconnectivity and electronic conductivity [1]. When carbon fiber paper is used directly in an electrode without an addition of carbon additives, the electrode is reported to suffer from low electronic conductivity due to low interconnectivity of carbon fibers [2]. Additionally, for application in lithium ion battery electrodes, CFP should be composed of small diameter fibers. Decreasing carbon fiber diameter can improve mass loadings for active material coated carbon fiber current collectors, and reduces the path length for ionic conductivity, however this also increases the number of junction points of carbon fibers in the CFP and decreases interconnectivity. In a typical carbon fiber paper (CFP) electrode, hundreds of fiber-to-fiber contact points are encountered along the path of the electron to the external circuit. A multitude of junction points creates a tortuous electronic pathway, decreasing the interconnectivity of the fibers. In order to improve electronic conductivity in small diameter CFP based LIB electrodes we have used an electrospinning method that allows us to weld the carbon fiber junctions together, increasing the conductivity and maintaining the integrity of the connections during coating of ceramic active materials. In this study, welded CFP paper was created by electrospinning polyacyrlonitrile (PAN) precursor solution into fiber mats followed by carbonization to create carbon fiber paper. LiMn₂O₄ active materials were grown directly on the welded CFP using a hydrothermal synthesis technique. The individual fibers were approximately 1 μm diameter and the fibers were loaded with approximately 60wt% active material. The LiMn₂O₄ loaded electrodes were used directly as positive electrodes in lithium ion batteries without additional binder or carbon additive.  The improvement in electronic conductivity of the welded fibers dramatically improved the performance of the lithium ion battery (Li metal / 1 M LiPF₆ in 1:1 EC:DMC / LiMn₂O₄) at intermediate rates.

[1] Kercher, Andrew K., J. O. Kiggans, and Nancy J. Dudney. "Carbon Fiber Paper Cathodes for Lithium Ion Batteries." Journal of The Electrochemical Society157.12 (2010): A1323-A1327.

[2] Kim, Chan, et al. "Fabrication of Electrospinning‐Derived Carbon Nanofiber Webs for the Anode Material of Lithium‐Ion Secondary Batteries." Advanced Functional Materials 16.18 (2006): 2393-2397.