Synergising hard-carbon and soft-carbon interactions for delivering metal-free microscale supercapacitors with high specific energy and rapid power delivery

Abstract

Overcoming the limitations of low energy density and efficiency is a pertinent challenge for the continued development of micro-supercapacitive (MSC) energy storage devices. Traditional metal-thin film-based current collectors suffer from improper interfacial contact with the active material leading to rapid decay in cyclability, while carbon based current collectors underperform in energy density and energy efficiency. In this submission, we design multiscale hierarchical carbon nanostructures and achieve synergistic interactions between them that overcome these limitations. Specifically, the highly conductive (0.2 S/cm) laser-induced-carbon (LIC) acts as an effective current collector with negligible iR drop and simultaneously establishes robust interface with multidimensional nanostructured carbons made-up of (a) porous nanostructured hardcarbon florets (NCF) with high surface area (948 m2/g), open-ended framework and graded porosity and (b) conventional soft-nanocarbons such as one-dimensional single wall carbon nanotubes (CNT) and two-dimensional-graphene (Gr). Consequently, the binder-free MSC resulting from the combination of LIC, NCF and CNT exhibits outstanding high specific capacitance (18 mF/cm2), energy density (10 μWh/cm2), and relaxation time constant (67 ms) without sacrificing power density (1.66 mW/cm2). This is clearly reflected in the unique solitary position achieved by the MSC in the Ragone plot. Fundamentally, the addition of CNT and its integration with non-graphitizable, hard-carbon NCF mimics a ball-net structure that facilitates the ion-migration pathways and thereby delivers the combination of highest energy density and scan rate capability (12,000 mV/s) among all MSCs reported.