Flexibility and High-Rate Discharge Properties of Organic Radical Batteries with Gel-State Electrodes

Abstract

Technology related to Internet of Things (IoT) and wearable devices has just begun to be actively developed. In that field, the demand for thinness and flexibility has increased in secondary batteries due to the structural compatibility of IoT or wearable devices such as vital sensors. We have been developing organic radical batteries (ORBs) with cathodes consisting of nitroxyl radical polymers such as poly(2,2,6,6-tetramethylpiperidinyl-N-oxyl-4-yl methacrylate) (PTMA). Our previous work showed that ORBs with PTMA cathodes have high rate capabilities. PTMA is also known to be changed into a gel-like state due to the absorption of organic electrolytes. The PTMA gel in the electrodes is certain to be suitable for thin and flexible batteries because it is hardly damaged when it is repeatedly bent and unbent. However, the state of the PTMA cathodes in the battery has not been characterized, and the flexibility of ORBs with the PTMA cathodes has not been investigated so far. The objective of this work is to clarify the state of the cross-linked PTMA cathodes in the battery and flexibility of the ORBs. We also report the electrode preparation method of cross-linked PTMA cathodes to obtain practical ORBs with both high rate discharge properties and excellent cyclability. Cross-linked poly(2,2,6,6-tetramethylpiperidinyl-N-oxyl-4-yl methacrylate) (PTMA) gel used in organic radical batteries (ORBs) is characterized by rheological measurements, and the micro structure of gel-state cathodes is observed using low vacuum SEM. The storage elastic modulus of the cross-linked PTMA gel is under 10 kPa which is in the range of soft gel. It suggests that the PTMA cathodes are fairy flexible in the battery. The ORBs with the cathodes made from cross-linked PTMA/vapor grown carbon fiber (VGCF) composite show excellent discharge rate properties (20C/1C capacity 91%). The structure of needle-like VGCF penetrating PTMA particles in the composite results in excellent rate capabilities. In the PTMA gel cathode, the cross-link structure of the PTMA has the effect of maintaining homogeneous dispersion of VGCF, which gives improvement in cyclability. The capacity and resistance of the ORBs are maintained in the initial state after repetitive bending 250 times. We demonstrate that the gel state cross-linked PTMA is a promising electrode material for practical flexible batteries.