Solid-state 3D micro-supercapacitors based on ionogel electrolyte: Influence of adding lithium and sodium salts to the ionic liquid

Highlights

• Interdigitated 3D-scaffold MnO₂ electrodes are shrewdly combined with an ionogel electrolyte formulation to provide an all-solid-state, leakage free microsupercapacitor.

• The addition of LiTFI or NaTFSI to 1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonate)imide based ionogel drastically improves the electrochemical performance of the microsupercapacitor.

• Energy densities of ∼10 µWh.cm⁻² were obtained at 1.0 mW.cm⁻² over more than 50,000 charge/discharge cycles.

Abstract

The ever-increasing interest in miniaturized Internet of Things devices and embedded electronics has given rise to a host of inquiries surrounding the need for safe, high performance energy storage devices. Solid-state 3D micro-supercapacitors based on ionogels provide a promising response to many of these pressing questions. Herein, leakage-free solid-state-like 3D micro-supercapacitors incorporating lithium and sodium salts added to 1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide-based ionogels were investigated. The resulting micro-supercapacitors containing these lithium and sodium ions displayed energy densities of 10.2 and 9.5 µWh.cm⁻² at power densities of 1.1 and 1.0 mW.cm⁻², respectively. In those devoid of these alkaline ions, however, the energy density reached a mere 3 µWh.cm⁻² at the same power density, thereby validating the proposed strategy. The 3D interdigitated MnO₂ // MnO₂ micro-supercapacitors were cycled 50 000 times at 1.75 mA.cm⁻² with good capacitance retention (∼ 85 %). While performing under high temperatures (100°C), there was no evidence of electrolyte degradation, capacitance fading or electrolyte leakage.

Introduction

The Internet of Things (IoT) is a revolutionary technology aimed at streamlining the connection and communication between machines and devices. These chiefly include internet servers, personal computers, smartphones and, most notably, sensors [1], which are required to be autonomous and miniaturized. Several energy harvesting and scavenging technologies are currently being investigated as a means of supplying the energy required by these sensors [2], [3], [4], and since these energy sources are often intermittent, energy storage is critical when it comes to ensuring a stable power supply for electronic devices.

A viable approach to fulfilling this requirement involves the miniaturization of electrochemical energy storage systems (EESSs). Since the use of liquid electrolytes occasions packaging and safety issues [5], solid electrolytes are preferred. Polymer electrolytes were thus developed for use in all-solid-state lithium or Li-ion micro-batteries (µBats) [6]. These, however, display limited performance due to their planar configuration, having thereby led to current studies centered on µBats with 3D electrodes [[7], [8]].

µBats can deliver a constant power supply for several hours, although such faradic devices show limited power capability and cycle life. In this context, therefore, if micro-supercapacitors (µSCs) were to be combined with µBats then the requirements for sustaining power pulses, together with steady energy delivery while providing high cycling stability, would simultaneously be met.

Indeed, µSCs have a long lifespan and are able to provide high power density due to the capacitive charge storage within the electrical double-layer capacitance [9]. On account of this, the performance of µSCs is greatly dependent upon the surface of the electrodes, for which the planar configuration used for µBats is not feasible. There are two main options that are currently being considered for use as electrode materials in µSCs in order to improve their energy density: the first one entails using ultra-porous nano-engineered carbon materials, whereas the second one makes use of 3D electrodes [10]. In both cases, the use of pseudocapacitive materials is also an effective means of improving µSC performance. Pseudocapacitance refers to surface-located redox reactions providing a capacitive-like signature owing to their fast kinetics and reversibility [11].

Aside from the use of porous carbon materials, 3D scaffolds are of great interest since bottom-up and top-down microfabrication techniques allow for a significant improvement in the surface of the electrodes while maintaining a limited footprint area [12]. These 3D scaffolds can be combined with pseudocapacitive materials [13].

Among the various pseudocapacitive materials used for µSCs, such as ruthenium dioxide [14], [15], [16], vanadium nitride [17,18] and conducting polymers, manganese dioxide (MnO₂) stands out as a good compromise between availability, price and performance [19], [20], [21]. However, MnO₂ is known to be at a disadvantage due to low electronic conductivity. MnO₂ thin film deposition on 3D current collectors for µSC electrodes is thus a reasonable trade-off providing high areal capacitance and fair electronic conductivity [22].

The performance of µSCs depends not only on the choice of electrodes but also on the choice of electrolyte. The latter must possess a high ionic conductivity in order to cope with the fast redox reactions occurring in this pseudocapacitive-based µSC, as well as being able to respond to the thermal and chemical constraints. Nowadays, the electrolytes used in commercial devices are based on salts dissolved in organic solvents, owing to their wide operating potential window and good ionic conductivity, although these are plagued by safety issues and limited thermal stability [23]. Aqueous electrolytes provide an answer to the safety concerns, showing great ionic conductivity, but they severely limit the potential window of the µSC [24] and the thermal operating range. While many µSC studies are focused on using aqueous- or organic-based liquid electrolytes, real-world implementation of these µSCs isn't feasible due to packaging issues that are liable to engender evaporation and leakage. Another packaging issue concerns the fact that the µSCs need to be integrated into a printed circuit board and must be able to withstand the reflow soldering process occurring at 250°C.

Another possibility is the use of ionic liquids (ILs), which are presenting negligible vapor pressure and showing no degradation below ∼200°C [25]. Their ionic conductivity is comparable to that of organic electrolytes and they display wide electrochemical windows, as well as excellent thermal and chemical stability [26]. However, leakage is still an issue as regards their use in µSCs.

When it comes to safety concerns, stability requirements and ease of fabrication as an electrolyte for µSCs, solid electrolytes are indeed an appealing option. Inorganic and polymer electrolytes, often employed as thin films in µBats, are not suitable for µSCs due to their very limited ionic conductivity [21]. The confinement of known liquid electrolytes presents an alternative solution: gels made from aqueous electrolytes (i.e., hydrogels) [27], [28], [29] or from ILs (i.e., ionogels) are being explored as possible µSC electrolytes. Although the latter display a lower ionic conductivity than that of aqueous gels, they are not subject to evaporation. Moreover, they can be operated using a wider potential window. The desirable properties that using ionogels as electrolytes for µSCs provides are a high energy density and a long lifespan for the related microdevice [30], [31], [32].

In this study, lithium and sodium salts were added in various concentrations to the chosen IL. These IL mixtures were confined following the same synthesis route that was described in a previous study [33]. The impact of the salts on confined and non-confined ILs was initially evaluated through conductivity measurements. The impact of the 3D microstructures and MnO₂ thin film when used as µSC electrodes was investigated by comparing these to equivalent solid-state µSCs. The IL mixtures were confined and cast onto the 3D electrodes to form solid-state MnO₂ // MnO₂ µSCs so as to study the influence of the added salts concentration. Interdigitated µSCs were then fabricated and tested at high temperatures.