Anion coordination improves high-temperature performance and stability of NaPF6-based electrolytes for supercapacitors

Electrolyte stability can be improved by incorporating complexing agents that bind keydecomposition intermediates and slow down decomposition. We show that hexamethyl-phosphoramide(HMPA) extends both the thermal stability threshold of sodium hexafluorophosphate(NaPF6) in dimethoxyethane (DME) electrolyte and the cycle life of double-layer capacitors. HMPAforms a stable complex with PF5, an intermediate in PF6 anion thermal degradation. Unbound, thisintermediate leads to autocatalytic degradation of the electrolyte solution. The results of electrochemicalimpedance spectroscopy (EIS) and galvanostatic cycling measurements show largechanges in the cell without the presence of HMPA at higher temperatures (≥60 °C). Fourier transforminfrared spectroscopy (FTIR) on the liquid and gas phase of the electrolyte shows withoutHMPA the formation of measurable amounts of PF5 and HF. The complimentary results of thesemeasurements proved the usefulness of using Lewis bases such as HMPA to inhibit the degradationof the electrolyte solution at elevated temperatures and potentially lead to improve cycle life of anonaqueous capacitor. The results showed a large increase in capacitance retention during cycling(72% retention after 750,000 cycles). The results also provide evidence of major decomposition processes(0% capacitance retention after 100,000 cycles) that take place at higher temperatures withoutthe additive of a thermal stability additive such as HMPA.