Abstract
Electrolyte stability can be improved by incorporating complexing agents that bind key decomposition 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. HMPA forms a stable complex with PF5, an intermediate in PF6 anion thermal degradation. Unbound, this intermediate leads to autocatalytic degradation of the electrolyte solution. The results of electrochemical impedance spectroscopy (EIS) and galvanostatic cycling measurements show large changes in the cell without the presence of HMPA at higher temperatures (≥60 °C). Fourier transform infrared spectroscopy (FTIR) on the liquid and gas phase of the electrolyte shows without HMPA the formation of measurable amounts of PF5 and HF. The complimentary results of these measurements proved the usefulness of using Lewis bases such as HMPA to inhibit the degradation of the electrolyte solution at elevated temperatures and potentially lead to improve cycle life of a nonaqueous 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 without the additive of a thermal stability additive such as HMPA.
Highlights
Energy storage devices have taken on a monotony of various forms to satisfy numerous operating conditions: energy density, voltage windows, cycle rate, power density, high temperature ranges, et cetra [1,2,3,4,5]
Commercial supercapacitors or electric double-layer capacitors (EDLC) generally have lower energy densities but higher power densities compared to Li-ion batteries, 10 vs. 200 Wh kg−1, and 104 vs. 102 W kg−1 [1,6,7,8,9,10,11], respectively
We propose a simplistic pathway on how the HMPA–PF5 complex inhibits the autocatalytic decomposition reaction as reported in other systems [15,28] and improves cycle life and performance
Summary
Energy storage devices have taken on a monotony of various forms (fuel cells, capacitors, batteries, etc.) to satisfy numerous operating conditions: energy density, voltage windows, cycle rate, power density, high temperature ranges, et cetra [1,2,3,4,5]. Commercial supercapacitors or electric double-layer capacitors (EDLC) generally have lower energy densities but higher power densities compared to Li-ion batteries, 10 vs 200 Wh kg−1, and 104 vs 102 W kg−1 [1,6,7,8,9,10,11], respectively. Double-layer charging is fast, which enables supercapacitors to have excellent power (rate) performance and long cycle life (>106 cycles) at the cost of poor energy density. The maximum energy stored (W) within capacitors is directly related to the specific capacitance (C) and the voltage window of the system (V) as follows:
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