CNT Fibrous Network Derived Free-Standing Film Electrodes for Pragmatic Flexible Li-ion Batteries

Abstract

As the development of wearable and flexible devices such as foldable smartphones, smart watches, and organic light-emitting diode (OLED) patches for light therapy accelerates, an interest in flexible batteries with high capacity is rapidly increasing. In conventional lithium-ion battery (LIB) systems, a metal current collector is the critical factor preventing the development of flexible and light-weight batteries. To replace the problematic metal current collector, we introduced multi-walled carbon nanotubes (MWCNTs) having low density, high mechanical strength, and superior electrical conductivity as a current collector in this study. Through the use of cellulose acetate (CA) as an intermediate dispersion agent, free-standing film electrodes (CA-CNT film electrodes) with entangled structure between MWCNTs and active material particles were successfully prepared. In addition, porous solid polymer electrolyte (PSPE) was introduced to improve the safety for the application to wearable devices. The prepared porous solid polymer showed higher thermal stability and electrolyte wettability than a conventional polyolefin separator. The CA-CNT film electrode showed superior electrochemical performance compared to the conventional electrode at high current density due to three-dimensional fibrous current networks of MWCNTs surrounding active material particles. The CA-CNT film electrodes also maintained high flexibility even at high loading for high areal capacity of 1 mAh/cm2. In addition, a stacked electrode with four sheets of CA-CNT film achieved high areal capacity of 3.7 mAh/cm2. A pouch flexible battery assembled with the CA-CNT film electrodes and PSPE was stable even under extreme deformation states such as rolling and folding. The CA-CNT film electrodes developed in this study could be applied to practical wearable devices because they have low production costs, high flexibility, and superior electrochemical performance.