Functionalized Metallic 2D Transition Metal Dichalcogenide-Based Solid-State Electrolyte for Flexible All-Solid-State Supercapacitors

Abstract

Highly efficient and durable flexible solid-state supercapacitors (FSSSCs) are emerging as low-cost devices for portable and wearable electronics due to the elimination of leakage of toxic/corrosive liquid electrolytes and their capability to withstand elevated mechanical stresses. Nevertheless, the spread of FSSSCs requires the development of durable and highly conductive solid-state electrolytes, whose electrochemical characteristics must be competitive with those of traditional liquid electrolytes. Here, we propose an innovative composite solid-state electrolyte prepared by incorporating metallic two-dimensional group-5 transition metal dichalcogenides, namely, liquid-phase exfoliated functionalized niobium disulfide (f-NbS₂) nanoflakes, into a sulfonated poly(ether ether ketone) (SPEEK) polymeric matrix. The terminal sulfonate groups in f-NbS₂ nanoflakes interact with the sulfonic acid groups of SPEEK by forming a robust hydrogen bonding network. Consequently, the composite solid-state electrolyte is mechanically/dimensionally stable even at a degree of sulfonation of SPEEK as high as 70.2%. At this degree of sulfonation, the mechanical strength is 38.3 MPa, and thanks to an efficient proton transport through the Grotthuss mechanism, the proton conductivity is as high as 94.4 mS cm^-1 at room temperature. To elucidate the importance of the interaction between the electrode materials (including active materials and binders) and the solid-state electrolyte, solid-state supercapacitors were produced using SPEEK and poly(vinylidene fluoride) as proton conducting and nonconducting binders, respectively. The use of our solid-state electrolyte in combination with proton-conducting SPEEK binder and carbonaceous electrode materials (mixture of activated carbon, single/few-layer graphene, and carbon black) results in a solid-state supercapacitor with a specific capacitance of 116 F g^-1 at 0.02 A g^-1, optimal rate capability (76 F g^-1 at 10 A g^-1), and electrochemical stability during galvanostatic charge/discharge cycling and folding/bending stresses.

Keywords: flexibility; functionalization; niobium disulfide; solid-state supercapacitors; transition metal dichalcogenides.

Figure 1

(a) Sketch of the functionalization of NbS₂ nanoflakes. The thiol group of SMPS molecules was linked to NbS₂ via S–S bonds or S-vacancy passivation. (b) Sketch of the preparation of the flexible FSSSC electrodes. (c) Sketch of the preparation of the solid-state electrolyte via the incorporation of f-NbS₂ nanoflakes into the SPEEK matrix.

Figure 2

(a) TEM and (b) AFM images of representative f-NbS₂ nanoflakes. (c) Lateral size and (d) thickness statistical analyses for f-NbS₂ nanoflakes. (e) XRD patterns and (f) Raman spectra of NbS₂ bulk crystals, exfoliated NbS₂, and f-NbS₂ nanoflakes. The XRD and Raman peaks assigned to the 2H- and 3R-NbS₂ phases are also shown.

Figure 3

Cross-sectional SEM images of (a) SPEEK and (b) 2.5%-f-NbS₂:SPEEK, respectively. (c) EDX map of Nb (M line at 2.18 keV) for 2.5%-f-NbS₂:SPEEK.

Figure 4

Adhesion force maps measured by AFM for (a) SPEEK and (b) 2.5%-f-NbS₂:SPEEK in humid ambient air, respectively, and the corresponding (c) detachment work and (d) adhesion force distributions. (e) FTIR spectra of SPEEK and 2.5%-f-NbS₂:SPEEK electrolytes. (f) WU, MS, and σ of the prepared solid-state electrolytes.

Scheme 1. Ion Transport Mechanisms through the Composite Solid-State Electrolyte: Grotthuss (Primary) Mechanism and Vehicle (Secondary) Mechanisms

Figure 5

Electrochemical characterization of the investigated solid-state supercapacitors and 1 M H₂SO₄-based EDLC reference. CV curves measured for the (a) PVDF-SPEEK, (b) SPEEK-SPEEK, and (c) SPEEK-2.5%-f-NbS₂:SPEEK, acquired at voltage scan rates ranging from 5 to 1500 mV s^–1. (d) CV curves of the investigated EDLCs acquired at 100 mV s^–1 voltage scan rate. (e) Electrode C_g of the investigated EDLCs as a function of the voltage scan rate, extrapolated from the CV analysis. (f) Nyquist plots of the investigated EDLCs. The inset panel shows the enlargement of the high-frequency regions of the Nyquist plots.

Figure 6

Electrochemical characterization of the prepared solid-state supercapacitors. GCD curves acquired at specific currents ranging from 0.02 to 50 A g^–1 measured for (a) PVDF-SPEEK, (b) SPEEK-SPEEK, and (c) SPEEK-2.5%-f-NbS₂:SPEEK. (d) GCD curves measured for the device at 2 A g^–1. (e) V_drop measured from GCD curves at specific current of 10 A g^–1. (f) Electrode specific C_g and (g) Coulombic efficiency vs specific current plots and (h) Ragone plots measured for the investigated solid-state supercapacitor electrodes. (i) Stability of SPEEK-2.5%-f-NbS₂:SPEEK and SPEEK-SPEEK over 30 000 charge/discharge cycles (inset: charge/discharge cycles at 10 A g^–1).

Figure 7

Schematic illustration of the prepared FSSSCs, based on SPEEK-2.5%-f-NbS₂:SPEEK composite electrolyte, proton-conducting SPEEK as electrode binder, and flexible current collectors (carbon cloths). The FSSSCs were protected by a Kapton-based packaging.

Figure 8

Electrochemical characterization of the prepared FSSSC. GCD curves at specific currents ranging from 0.02 to 50 A g^–1 for (a) the FSSSC in normal state, (b) folded at 180°, and (c) after 1000 bending cycles at a curvature radius of 2 cm. (d) GCD curves of the FSSSC measured after 100, 150, 300, 500, 750, and 1000 bending cycles at 1 A g^–1. (e) Capacitance retention and Coulombic efficiency (red, right y-axis) of the FSSSC over 1000 bending cycles. (f) Capacitance retention and CE (red, right y-axis) of the FSSSC folded at 0°, 90°, and 180°.