Gradient-Layered MXene/Hollow Lignin Nanospheres Architecture Design for Flexible and Stretchable Supercapacitors

Abstract

With the rapid development of flexible wearable electronics, the demand for stretchable energy storage devices has surged. In this work, a novel gradient-layered architecture was design based on single-pore hollow lignin nanospheres (HLNPs)-intercalated two-dimensional transition metal carbide (Ti3C2Tx MXene) for fabricating highly stretchable and durable supercapacitors. By depositing and inserting HLNPs in the MXene layers with a bottom-up decreasing gradient, a multilayered porous MXene structure with smooth ion channels was constructed by reducing the overstacking of MXene lamella. Moreover, the micro-chamber architecture of thin-walled lignin nanospheres effectively extended the contact area between lignin and MXene to improve ion and electron accessibility, thus better utilizing the pseudocapacitive property of lignin. All these strategies effectively enhanced the capacitive performance of the electrodes. In addition, HLNPs, which acted as a protective phase for MXene layer, enhanced mechanical properties of the wrinkled stretchable electrodes by releasing stress through slip and deformation during the stretch-release cycling and greatly improved the structural integrity and capacitive stability of the electrodes. Flexible electrodes and symmetric flexible all-solid-state supercapacitors capable of enduring 600% uniaxial tensile strain were developed with high specific capacitances of 1273 mF cm−2 (241 F g−1) and 514 mF cm−2 (95 F g−1), respectively. Moreover, their capacitances were well preserved after 1000 times of 600% stretch-release cycling. This study showcased new possibilities of incorporating biobased lignin nanospheres in energy storage devices to fabricate stretchable devices leveraging synergies among various two-dimensional nanomaterials.

1. Introduction

In recent years, flexible electronics have experienced rapid development in various fields such as wearable multifunctional sensor [1–3], electronic skin [4], human–machine interface [5], soft robot [6], and flexible display. This advancement has led to an increasing interest in developing energy storage devices that are not only flexible but also mechanically compatible with these emerging devices. Among various options, stretchable supercapacitors are seen as the ideal candidates for powering flexible electronics due to their fast charging/discharging process, excellent cycling stability and easy fabrication procedure [7, 8].

MXene is regarded as a promising electrode material for flexible electrochemical energy storage devices owing to its desirable properties, such as metal-like electronic conductivity (exceeding 10,000 S cm−1), solution processability, and high volumetric capacitance (up to 1500 F cm−3) [9–14]. Few-layer MXene nanosheets after well dispersed in solution were able to easily assemble into flexible films through vacuum filtration as supercapacitor electrodes [15]. The intrinsic layer structure of parallel stacked MXene nanosheet formed flexible MXene films that were able to withstand bending. But these films were prone to fracture during tensile deformation and could not be used directly in stretchable devices [16–18]. An interesting strategy to address this limitation was to build the wrinkled structure of the MXene film by adhering the MXene film to a pre-stretched elastomer, followed by strain release. The wrinkled structure provided reserved space for the subsequent tensile deformation [19–22]. This clever structural design leveraged MXene film’s bending resilience while effectively circumventing its actual strain in the parallel direction of the MXene layer, providing the electrodes with good stretchability and electrochemical stability [23]. Nonetheless, the mechanical strength of the pure MXene film was not sufficient to withstand the compressive effect during strain.