Flexible solvent-free supercapacitors with high energy density enabled by electrical-ionic hybrid polymer nanocomposites

Abstract

High-performance flexible solvent-free supercapacitors are promising for next-generation energy storage with excellent reliability, high energy density, and improved safety. However, the low ionic conductivities of solvent-free polymer solid electrolytes (PSEs) and poor electrolyte/electrode contacts impede their applications. Here, we report a PVDF-HFP/LiBOB PSE and novel electrical-ionic hybrid polymer nanocomposite (EIHPN) electrode to address these challenges. With the large-sized BOB⁻ acting as a solid plasticizer, the PVDF-HFP/LiBOB PSE shows a high ion conductivity up to 6.1 × 10⁻⁵ S cm⁻¹ at 25 °C and 5.7 × 10⁻⁴ S cm⁻¹ at 80 °C. This PSE is adopted as both the host of graphene oxide/carbon nanotube scaffold to form GO/CNT/PSE nanocomposite electrodes and separator to form flexible solvent-free lithium-ion symmetric supercapacitors. The EIHPN electrodes facilitate the access of ions to capacitive material surfaces and their monolithic integration with PSE remarkably improves the electrode/electrolyte contacts, leading to a high double-layer specific capacitance of 267 F g⁻¹ at 1 A g⁻¹, comparable to the records of carbon electrodes in liquid electrolytes. The EIHPN structure also enables a nearly thickness-independent performance and an ultrahigh areal capacitance of 590 mF cm⁻² at 1 mA cm⁻² has been achieved with 100 μm-thick electrodes. The supercapacitors show a low self-discharge characteristic, an ultrahigh specific energy of 30 W h kg⁻¹ at 1 A g⁻¹ and a high energy density of 6.64 mW h cm⁻³ at 1 mA cm⁻² and excellent cyclic reliability (over 88% capacitance retention after 20 000 charge–discharge cycles) and flexibility (negligible capacitance decay after 10 000 bending tests), demonstrating great potential for flexible energy storage.