Carbon nanotube conductive additives for improved electrical and mechanical properties of flexible battery electrodes

Flexible electronics are being pursued as replacements for rigid consumer electronic products such as smartphones and tablets, as well as for wearable electronics, implantable medical devices, and RFIDs. Such devices require flexible batteries with electrodes that maintain their electro-chemical performance during multiple bending cycles. These electrodes typically consist of an active battery material blend with a conductive additive and a binder. Whilst the choice of active battery material is typically dictated by the desired battery power and energy requirements, there is more freedom in changing the conductive additives to cope with strain induced during the bending of flexible batteries. Here we compare the mechanical and electrical properties of free standing cathodes using lithium cobalt oxide (LiCoO2) as the active material and 10-20 wt% of amorphous carbon powder (CP) or carbon nanotubes (CNTs) as conductive additives. We found that the CNT based electrodes showed less crack formation during bending and have a Young's modulus up to 30 times higher than CP electrodes (10 wt% loading). Further, the electrical resistance of pristine CNT electrodes is 10 times lower than CP electrodes (20 wt% loading). This difference further increases to a 28 times lower resistance for CNT films after 2000 bending cycles. These superior properties of CNT films are reflected in the electrochemical tests, which show that after bending, only the electrodes with 20 wt% of CNTs remain operational. This study therefore highlights the importance of the conductive additives for developing reliable flexible batteries.