High Precision Coulometry of Commercial PAN-Based Carbon Fibers as Electrodes in Structural Batteries

Abstract

Carbon fibers have the combined mechanical and electrochemical properties needed to make them particularly well suited for usage as electrodes in a structural lithium-ion battery, a material that simultaneously works as a battery and a structural composite. Presented in this paper is an evaluation of commercial polyacrylonitrile-based carbon fibers in terms of capacity and coulombic efficiency, as well as a microstructural and surface evaluation. Some polyacrylonitrile based carbon fibers intercalate lithium ions, resulting in a similar capacity as state-of-the-art graphite based electrodes, presently the most commonly used negative electrode material. Using high precision coulometry, we found a capacity of around 250-350 mAh/g and a very high coulombic efficiency of over 99.9% after ten cycles, which is even higher than a commercial state-of-the art graphitic electrode evaluated as reference. The high coulombic efficiency is attributed to the very low surface area of the carbon fibers, resulting in a small and stable solid-electrolyte interface layer. A highly graphitized ultra high modulus carbon fiber was evaluated as well and, compared to the other fibers, less lithium was inserted (corresponding to approximately 150 mAh/g). We show that the use of carbon fibers as an electrode material in a structural composite battery is indeed viable. The demand for new and improved battery technologies is continuously rising. In particular the electrification of the transportation sector needs innovative new solutions to develop and meet new demands. Much research is dedicated to pushing the limits of lithium (Li)-ion batteries in terms of energy, power, safety, cost, lifetime and environmental aspects.Improving graphite and other carbon based electrodes has been extensively researched since the beginning of the 90s since they are the dominating type of negative electrodes in Li-ion batteries.1-5 Another way of improving the effectiveness at a system level is through so called multifunctional battery technologies. The idea is to let the battery perform several functions, in particular carry a mechanical load and still maintain the energy storage function. The weight at a system level can be reduced, and a performance enhancement can be achieved, even if the battery function is not on par with conventional state of the art Li-ion batteries. Batteries could be included as an integral part of the structure in different kinds of vehicles, such as cars or even airplanes. Multifunctionality of materials for battery and capacitor applications has been researched in various ways for a number of years. [6][7][8][9][10][11][12][13]

Flexible and Lightweight Lithium-Ion Batteries Based on Cellulose Nanofibrils and Carbon Fibers

“…The relatively large interlayer spacing and the low values of L c for the CF indicate a largely disordered and amorphous structure with small crystal sizes, which is consistent with previous work [23]. The BET surface area for the chopped CF is found to be 0.65 m 2 g −1 , slightly higher than the unchopped CF measured previously (0.42 m 2 g −1 [23]), which is due to the extra area from the chopped cross-sections. It is still very low, however, which is beneficial for a small solid-electrolyte interface (SEI)-layer and stable cycling performance.…”

“…This is to be expected, since the fibers are chopped here which, in addition to the added CNF binder, lowers the conductivity of the electrodes (the conductivity of the non-chopped carbon fibers is 694 S cm −1 ). The capacity retention is~83% after 13 cycles, which is similar to previous studies [23]. The electrode maintains a relatively stable specific capacity for the following cycles, with a specific capacity retention of~80% after 110 cycles.…”

“…When 2 wt% SPC is added to CF/CNF, increasing the conductivity between the CF, even better electrochemical properties are achieved, including higher specific capacity and lower polarization (see Figure 5a,b). The initial specific capacity of the electrode is~300 mAh g −1 , but drops rapidly to~250 mAh g −1 after 13 cycles, which is ascribed to the solid electrolyte interface (SEI) layer stabilization for CF electrodes [23]. A slightly lower specific capacity is observed for these flexible electrodes compared to free standing, unchopped, electrodes of the same type of CF in previous work [23] (300 mAh g −1 versus 358 mAh g −1 ).…”

“…The initial specific capacity of the electrode is~300 mAh g −1 , but drops rapidly to~250 mAh g −1 after 13 cycles, which is ascribed to the solid electrolyte interface (SEI) layer stabilization for CF electrodes [23]. A slightly lower specific capacity is observed for these flexible electrodes compared to free standing, unchopped, electrodes of the same type of CF in previous work [23] (300 mAh g −1 versus 358 mAh g −1 ). This is to be expected, since the fibers are chopped here which, in addition to the added CNF binder, lowers the conductivity of the electrodes (the conductivity of the non-chopped carbon fibers is 694 S cm −1 ).…”

“…Carbon fibers (CF) have been investigated as an alternative current collector for LIB due to their high conductivity and good mechanical properties [18][19][20]. Furthermore, polyacrylonitrile (PAN)-based CF can be used as active material for the negative electrode in LIB [21][22][23], and Kjell et al reported that CF can be used as both active material and current collector in LIB [22]. Jabbour et al showed that carbon/cellulose fibers composite sheets could be used as current collector-free negative electrodes [19].…”