Using Hmpa As an Additive in Sodium Supercapacitor Electrolyte to Increase Thermal Stability

Abstract

The development of hybrid electric vehicles (HEV) and portable electronic devices has caused greater demand for high energy and power density devices typically in the form of batteries, but supercapacitors are being integrated into these devices. Supercapacitors boast high power densities (104 W/kg), long cycle lives (>100,000), and simplicity in fabrication. One issue that arises in the practical use of supercapacitors in HEVs is the instability of the electrolyte at elevated temperatures (above 60° C) in the working environment. A number of these sytems use electrolyte systems are unstable at elevated temperaturesr have high vapor pressures that can lead to dangerous conditions for devices. Another concern of supercapacitors is their narrow voltage window that directly affects the overall power density. One novel electrolyte has been discovered that contains sodium hexafluorophosphate (NaPF6) in dimethoxyethane (DME) that boasts a 4V voltage window. The issue arises when this electrolyte is placed at elevated temperatures. PF6 -based salts are well known for quick and irreversible thermal degradation that leads to device failure. This degradation is due to an equilibrium that forms between PF6 - and PF5 - that occurs at elevated temperatures. PF5 - is unstable and will begin to decompose and react with the solvent, impurities, or the electrolyte itself which leads to an autocatalytic degradation reaction. In lithium ion batteries, this issue can be alleviated by the addition of hexamethylphosphoramide (HMPA). HMPA acts as a complexing agent with PF5 -, the thermal decomposition product of NaPF6. This complexing inhibits further decomposition of PF6 - which ultimately increases the thermal stability of the electrolyte. Our work investigates the limitations and advantages of using HMPA as an additive in NAPF6 in DME. A combination of electrochemical impedance spectroscopy (EIS) and Fourier transform infrared spectroscopy (FTIR) provides enough evidence to prove that NaPF6 in DME does not degrade at elevated temperatures.