Organic Radical-Boosted Ionic Conductivity in Redox Polymer Electrolyte for Advanced Fiber-Shaped Energy Storage Devices

Abstract

Fiber-shaped energy storage devices (FSESDs) with exceptional flexibility for wearable power sources should be applied with solid electrolytes over liquid electrolytes due to short circuits and leakage issue during deformation. Among the solid options, polymer electrolytes are particularly preferred due to their robustness and flexibility, although their low ionic conductivity remains a significant challenge. Here, we present a redox polymer electrolyte (HT_RPE) with 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (HT) as a multi-functional additive. HT acts as a plasticizer that transforms the glassy state into the rubbery state for improved chain mobility and provides distinctive ion conduction pathway by the self-exchange reaction between radical and oxidized species. These synergetic effects lead to high ionic conductivity (73.5 mS cm−1) based on a lower activation energy of 0.13 eV than other redox additives. Moreover, HT_RPE with a pseudocapacitive characteristic by HT enables an outstanding electrochemical performance of the symmetric FSESDs using carbon-based fiber electrodes (energy density of 25.4 W h kg−1 at a power density of 25,000 W kg−1) without typical active materials, along with excellent stability (capacitance retention of 91.2% after 8,000 bending cycles). This work highlights a versatile HT_RPE that utilizes the unique functionality of HT for both the high ionic conductivity and improved energy storage capability, providing a promising pathway for next-generation flexible energy storage devices.

1. Introduction

As the global market for state-of-the-art flexible electronics has grown over the past few years, fiber-shaped energy storage devices (FSESDs) that feature an exceptional flexibility and/or compatibility with textiles/fabrics have become a peerless class in the field of wearable power sources [1–6]. The electrolyte, which is one of the components for FSESDs, should preferably be a solid type rather than a liquid type to inhibit short circuits and electrolytes leakage issues during deformation in wearable application [6]. Solid electrolytes can be classified into three main materials: sulfides, oxides, and polymers [7]. Among these, polymer electrolytes are particularly suitable for loading FSESDs due to their strong adhesive properties, outstanding physical flexibility and stability [8, 9]. However, enhancing ionic conductivity remains a significant challenge for polymer electrolytes in FSESDs applications.

One of the innovative insights for high ionic conductivity of polymer electrolytes is to integrate redox additives into the polymer matrix [10–13]. It has been known that the additives can promote the ionic conduction by transforming a semicrystalline phase of the matrix (i.e., polyvinyl alcohol, PVA) into an amorphous phase [10, 14, 15]. Importantly, by integrating redox materials into the matrix to replace the pseudocapacitive materials in the composite fiber electrode, they can minimize the interfacial complexity that the fiber electrode often encounters. The most widely studied redox additives are inorganic species such as halogens (Br, I) or metal ions (Cu, Fe, etc.). However, these are prone to side reactions, such as water decomposition or the inverse growth of the additives on the electrode surface [16]. Another example of such redox additive is that of small molecular hydroquinone, which displays very high redox reversibility and thus the electrochemical performance [15, 17, 18]. Despite the initial success, low solubility of hydroquinone in an aqueous electrolyte and activation only in acidic environments has been notable limitations [12, 15, 19, 20]. It is thus highly desirable that one develops a redox-based additive platform including higher hydrophilicity and operation in a wide range of chemical environment.