Ionic Liquid Electrolytes in Supercapacitor Applications

Ionic liquids (ILs) have emerged as versatile electrolytes for supercapacitors owing to their intrinsically wide electrochemical stability windows, negligible vapour pressure and excellent thermal resilience. Composed entirely of organic cations and inorganic or organic anions, ILs enable devices to operate at elevated voltages—often exceeding 3 V—thereby enhancing energy density without compromising the rapid charge–discharge capability characteristic of electric double-layer capacitors. The tunability of ILs through cation–anion selection and functionalisation (for example via ether or surface-active groups) allows optimisation of viscosity and ionic conductivity, critical parameters that govern power density. Protic ILs can introduce pseudocapacitive contributions via hydrogen bonding or proton transfer, while aprotic systems tend to rely solely on double-layer storage. Key challenges include ensuring ion-pore size matching at electrode interfaces, mitigating high viscosities at ambient temperature and integrating sustainable or non-halogenated chemistries. Recent advances focus on composite electrolyte architectures, biomass-derived ILs and ionogel frameworks to balance safety, cost and performance. Such developments are accelerating the deployment of supercapacitors in electric vehicles, grid stabilisation and harsh-environment electronics, emphasising the global significance of IL-based energy storage solutions.

Research from Nature Portfolio

Recent studies have demonstrated solid-state supercapacitors employing a proton-conducting phosphate composite electrolyte that retains high conductivity (0.02 S cm⁻¹) and a ±2 V stability window at 200 °C, delivering an energy density of 32 Wh kg⁻¹ with robust cyclability over thousands of cycles. In parallel, the incorporation of imidazolium-based IL co-salts into conventional organic electrolytes has been shown to suppress decomposition, extend the electrochemical stability potential window to above 3.5 V and improve long-term capacitance retention to within a few per cent loss after 10 000 cycles, highlighting a straightforward strategy to boost voltage and durability.

Ionic Liquid Electrolytes in Supercapacitor Applications publication trend

The graph below shows the total number of articles in ionic liquid electrolytes in supercapacitor applications across all publications each year (not limited to Nature Index journals).

Technical terms

Ionic liquid: A salt composed entirely of ions that remains liquid below 100 °C, offering low volatility and high thermal stability.

Electric double-layer capacitor (EDLC): A supercapacitor that stores charge via accumulation of ions at the electrode–electrolyte interface without faradaic reactions.

Pseudocapacitance: Charge storage arising from fast, reversible faradaic reactions at the electrode surface, contributing additional capacitance beyond the double layer.

Electrochemical stability window: The voltage range over which an electrolyte remains stable, beyond which decomposition occurs.

Specific capacitance: The capacitance normalised to mass of active material (F g⁻¹), indicating energy storage capability per unit mass.

Ionic conductivity: A measure of an electrolyte’s ability to transport charged species (S cm⁻¹), influencing power delivery.

References

1. Recognition of Ionic Liquids as High-Voltage Electrolytes for Supercapacitors. Frontiers in Chemistry (2020).
2. Ether-Bond-Containing Ionic Liquids as Supercapacitor Electrolytes. The Journal of Physical Chemistry Letters (2013).
3. Nonhalogenated Surface-Active Ionic Liquid as an Electrolyte for Supercapacitors. ACS Applied Energy Materials (2021).
4. Charge storage mechanism of α-MnO2 in protic and aprotic ionic liquid electrolytes. Journal of Power Sources (2020).
5. Effect of structural variation in biomass-derived nonfluorinated ionic liquids electrolytes on the performance of supercapacitors. Journal of Energy Chemistry (2022).
6. Prospects and Design Insights of Neat Ionic Liquids as Supercapacitor Electrolytes. Frontiers in Energy Research (2021).
7. High-temperature supercapacitor with a proton-conducting metal pyrophosphate electrolyte. Scientific Reports (2015).
8. The effect of ILs as co-salts in electrolytes for high voltage supercapacitors. Scientific Reports (2019).