Systematic Approach to Constructing Safe and High-Performance Supercapacitors with Nonflammable Electrolytes

The successful implementation of supercapacitors in large-scale applications necessitates careful consideration of safety aspects, along with factors such as energy density, power density, working voltage, and cycling performance. One effective method for ensuring safety is the utilization of nonflammable trimethyl phosphate (TMP) electrolyte in supercapacitors. However, this approach suffers from the drawback of low power density due to its low ionic conductivity. To overcome this limitation, we propose the addition of co-solvents, namely propylene carbonate, acetonitrile, and propionitrile (PN), to enhance limited electrochemical properties of TMP-based electrolytes. We systematically investigate the impact of incorporating TMP into various organic solvents on physical and electrochemical properties. Binary electrolytes show improved ionic conductivity, capacitance, power density, energy density, and working voltage compared to the single TMP-based electrolyte. Notably, our result highlights that the carbon-based supercapacitors using TMP-PN with 70 : 30 volume ratio electrolyte provide the best compromise between ionic conductivity (13.5 mS cm^-1 ), capacitance (24.0 F g^-1 ), energy density (13.2 Wh kg^-1 ), power density (2.3 kW kg^-1 ), working voltage (3.5 V), and safety. By combining the nonflammable properties of TMP with co-solvents, we can overcome the trade-off between safety and electrochemical performances, presenting advancement in the development of supercapacitors for energy storage applications.