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

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 (Na_3.45Zr₂Si₂PO_12.225) 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.