Water-in-Salt Electrolytes for High-Voltage Supercapacitors
Authors: Thangavel Naresh kumar , Mahankali Kiran , Ding Yi , Arava Leela 
Published: February 2020

The rising ecological problems, fossil fuel curtail and global concerns dramatically demand the development of sustainable energy storage systems. Though lithium-ion batteries are capable of providing adequate energy density, their deficient performance upon fast charging limits its applications in many high-power storage systems. On the other hand, supercapacitors (SCs) are a special class of energy storage devices that fill the gap between traditional dielectric capacitors (1-100 kW kg−1) and batteries (~100 Wh kg−1). However, SCs are power devices that can be fully charged or discharged in seconds; consequently, their energy density (~ 5 Wh kg−1) is lower than batteries. To mitigate this issue, several types of device configurations were constructed with various micro/nanoelectrodes with larger surface area which cause high power and energy density supercapacitors1,2. Among several other strategies, enlarging the voltage window for a device is given utmost priority, since the energy density is proportional to the voltage squared. Today, the state of the art technique is the usage of organic electrolyte solutions such as, acetonitrile (ACN) or propylene carbonate (PC), with an optimized salt content. Despite these solvents offering excellent ionic conductivity and wide voltage window; low boiling point and high viscosity poses safety concerns and critically slows down the device performance upon extended cycling. Considerable research has been carried out with ionic liquids which are well known for wide voltage window operations, but their high viscosity falloff the ionic conductivity at room temperature. The balance of low viscosity, extended electrochemical stability window, and high dielectric constants can encounter all sorts of problems with existing electrolytes while maintaining high energy output3.
In this context, aqueous based electrolytes hold significant advantages of low cost, high safety, and high power; it is green and a key component for sustainable electrochemical energy storage system. However, early decomposition of water sets operation of these devices in a narrow voltage window (<1.5 V) which is a substantial drawback to obtain high energy density output for its application in supercapacitor’s. Recent studies on developing high voltage aqueous electrolytes for Li-ion battery application4,5 though provides safety solution, has shown reasonable energy density in the context of rapidly evolving battery power technologies. Herein, we report a supercapacitor application of “Water-in-Salt” electrolytes in comparison with conventional aqueous (low voltage < 1.5 V) and no-aqueous electrolytes systems. Extensive electrochemical studies including cyclic and linear voltammetry demonstrate that these “Water-in-Salt” electrolytes are stable up to 3 V without gas evolution reactions. The stability mechanism can be ascribed to the high salt concentration which strongly perturb the ionic clouds at the interface and forms a solid electrolyte interphase layer that postpones the evolution reactions. The extended electrochemical stability window allows the operation of SCs at voltages significantly above the thermodynamic stability limit of water. To realize efficacies of electrolyte, a symmetrical SC was fabricated with nano-porous carbon and activated carbon as anode and cathodes respectively with a glass fiber separator. The symmetric supercapacitor with ‘Water-in-Salt’ electrolytes allowed a maximum operating voltage of 2.9 V and delivered 120 F g-1 of specific capacitance. Benefitting from the large potential window, the SCs showed a high energy density of 12.5 Wh kg-1. In addition, its excellent performance at different current densities (0.1 A g-1 to 10 A g-1) and prolonged cycle life paves a way for the development of high-voltage aqueous supercapacitors. Hence, this class of electrolytes can remarkably enhance the energy density and safety performance of next-generation high-energy supercapacitors.

References:

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