# High-performance solid-state ceramic supercapacitors based on novel NASICON-ionic liquid composite electrolyte

## TL;DR

This paper introduces a new composite electrolyte for solid-state supercapacitors that significantly improves performance and stability.

## Contribution

The study presents a novel NASICON-ionic liquid composite electrolyte that reduces grain-boundary impedance and enables high-performance solid-state supercapacitors.

## Key findings

- The composite electrolyte with 12 wt% EMIMBF4 in NZSP achieves an ionic conductivity of ∼2.2 × 10−3 Ω−1 cm−1.
- The supercapacitor retains 75% of its capacitance after 15,000 charge–discharge cycles at 2 V.
- The device delivers 1970 W kg−1 of specific power and 15 Wh kg−1 of specific energy at 2 V.

## Abstract

A high grain-boundary impedance of sodium superionic conductors (NASICONs) is one of the main obstacles to their use as electrolytes in solid-state energy storage devices, whether batteries or supercapacitors. The present study delves into the use of Na+-ion-conducting NZSP (Na3.45Zr2Si2PO12.225) in combination with the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) in solid-state supercapacitors (SSCs). The optimal composition with ∼12 wt% EMIMBF4 in NZSP exhibits a high ionic conductivity of ∼2.2 × 10−3 Ω−1 cm−1, which is nearly three orders of magnitude higher than that of pristine NZSP. Rietveld refinement confirms the formation of the monoclinic phase of NZSP. In situ high-temperature X-ray diffraction further confirms the stability of the composite over a wide temperature range. An optimised electrolyte composition was used to assemble SSCs with ∼1800 m2 g−1 surface area activated carbon in a lamination cell geometry. The SSC showed outstanding stability, retaining approximately 75% of its capacitance after 15 000 galvanostatic charge–discharge cycles at 2 V and a charge–discharge current density of 1.33 A g−1 (2 mA). A typical cell at 2 V/1 mA exhibited a specific capacitance of ∼216 F g−1 (50 °C). Further, at 2 V/8 mA discharge current, the symmetric supercapacitor delivers approximately 1970 W kg−1 of specific power and about 15 Wh kg−1 of specific energy. Supercapacitors exhibited electric double-layer capacitive behaviour at operating potentials ≤2 V. Furthermore, the fabricated devices demonstrated excellent high-temperature operational stability, as evidenced by their reliable operation at 50 °C and 100 °C over 15 thermal cycles. The practical applicability of the fabricated devices is tested by connecting 2 such cells (2 V each) in series, which power a 4 V blue LED for more than 30 minutes.

A high grain-boundary impedance of sodium superionic conductors (NASICONs) is one of the main obstacles to their use as electrolytes in solid-state energy storage devices, whether batteries or supercapacitors.